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Tailoring zinc oxide nanoparticles via microwave-assisted hydrothermal synthesis for enhanced antibacterial properties.

1. Introduction

2. materials and methods, 2.1. sample preparation, 2.2. sample characterization, 4. discussion, 5. conclusions, author contributions, data availability statement, conflicts of interest.

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Click here to enlarge figure

Sample Name	Zn Precursor Concentration (mol/L)	Reaction Time (min)
S1_15	0.3488	15
S1_30	0.3488	30
S1_60	0.3488	60
S2_15	0.1744	15
S2_30	0.1744	30
S2_60	0.1744	60
S3_15	0.0872	15
S4_15	0.0436	15
S5_15	0.0218	15

Sample Name	BET Specific Surface Area (m /g)	Average Pore Size (nm)
S1_15	18.0301 ± 0.0694	4.6153 ± 0.0265
S1_30	14.8443 ± 0.0467	4.6759 ± 0.0135
S1_60	14.6759 ± 0.1356	5.006 ± 0.0201
S2_15	18.2113 ± 0.2296	3.8649 ± 0.0098
S2_30	15.7782 ± 0.1468	5.0625 ± 0.0354
S2_60	16.5817 ± 0.4052	5.341 ± 0.0333
S3_15	23.3261 ± 0.1918	4.5723 ± 0.0238
S4_15	25.7858 ± 0.3421	5.0674 ± 0.0117
S5_15	22.0476 ± 0.2824	4.9104 ± 0.0258

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Doicin, I.E.; Preda, M.D.; Neacsu, I.A.; Ene, V.L.; Birca, A.C.; Vasile, B.S.; Andronescu, E. Tailoring Zinc Oxide Nanoparticles via Microwave-Assisted Hydrothermal Synthesis for Enhanced Antibacterial Properties. Appl. Sci. 2024 , 14 , 7854. https://doi.org/10.3390/app14177854

Doicin IE, Preda MD, Neacsu IA, Ene VL, Birca AC, Vasile BS, Andronescu E. Tailoring Zinc Oxide Nanoparticles via Microwave-Assisted Hydrothermal Synthesis for Enhanced Antibacterial Properties. Applied Sciences . 2024; 14(17):7854. https://doi.org/10.3390/app14177854

Doicin, Irina Elena, Manuela Daniela Preda, Ionela Andreea Neacsu, Vladimir Lucian Ene, Alexandra Catalina Birca, Bogdan Stefan Vasile, and Ecaterina Andronescu. 2024. "Tailoring Zinc Oxide Nanoparticles via Microwave-Assisted Hydrothermal Synthesis for Enhanced Antibacterial Properties" Applied Sciences 14, no. 17: 7854. https://doi.org/10.3390/app14177854

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Zinc oxide nanoparticles: A comprehensive review on its synthesis, anticancer and drug delivery applications as well as health risks

Affiliations.

1 Advance School of Chemical Sciences, Faculty of Basic Sciences, Shoolini University, Solan, Himachal Pradesh 173212, India.
2 Advance School of Chemical Sciences, Faculty of Basic Sciences, Shoolini University, Solan, Himachal Pradesh 173212, India. Electronic address: [email protected].
3 Division of Molecular Medicine, Bose Institute, P-1/12, CIT Scheme VII M, Kolkata 700054, India. Electronic address: [email protected].
PMID: 33212389
DOI: 10.1016/j.cis.2020.102317

In recent years, zinc oxide nanoparticles (ZnONPs) emerged as an excellent candidate in the field of optical, electrical, food packaging and particularly in biomedical research. ZnONPs show cancer cell specific toxicity via the pH-dependent (low pH) dissolution into Zn 2+ ions, which generate reactive oxygen species and induce cytotoxicity in cancer cells. Further, ZnONPs have also been used as an effective carrier for the targeted delivery of several anticancer drugs into tumor cells. The increasing focus on ZnONPs resulted in the development of various synthesis approaches including chemical, pHysical, and green or biological for the manufacturing of ZnONPs. In this article, at first we have discussed the various synthesis methods of ZnONPs and secondly its biomedical applications. We have extensively reviewed the anticancer mechanism of ZnONPs on different types of cancers considering its size, shape and surface charge dependent cytotoxicity. Photoirradiation with UV light or NIR laser further increase its anticancer activity via synergistic chemo-photodynamic effect. The drug delivery applications of ZnONPs with special emphasis on drug loading mechanism, stimuli-responsive controlled release and therapeutic effects have also been discussed in this review. Finally, its side effects to vital body organs with mechanism via different exposure routes, the future direction of the ZnONPs research and application are also discussed.

Keywords: Adverse effects; Anticancer; Drug delivery; Synthesis, zinc oxide nanoparticles.

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Properties of Zinc Oxide Nanoparticles and Their Activity Against Microbes

Nano Review
Open access
Published: 08 May 2018
Volume 13 , article number 141 , ( 2018 )

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Khwaja Salahuddin Siddiqi 1 ,
Aziz ur Rahman 2 ,
Tajuddin 2 &
Azamal Husen ORCID: orcid.org/0000-0002-9120-5540 3

80k Accesses

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Zinc oxide is an essential ingredient of many enzymes, sun screens, and ointments for pain and itch relief. Its microcrystals are very efficient light absorbers in the UVA and UVB region of spectra due to wide bandgap. Impact of zinc oxide on biological functions depends on its morphology, particle size, exposure time, concentration, pH, and biocompatibility. They are more effective against microorganisms such as Bacillus subtilis , Bacillus megaterium , Staphylococcus aureus , Sarcina lutea , Escherichia coli , Pseudomonas aeruginosa , Klebsiella pneumonia , Pseudomonas vulgaris , Candida albicans , and Aspergillus niger . Mechanism of action has been ascribed to the activation of zinc oxide nanoparticles by light, which penetrate the bacterial cell wall via diffusion. It has been confirmed from SEM and TEM images of the bacterial cells that zinc oxide nanoparticles disintegrate the cell membrane and accumulate in the cytoplasm where they interact with biomolecules causing cell apoptosis leading to cell death.

Effect of Zn salts and hydrolyzing agents on the morphology and antibacterial activity of zinc oxide nanoparticles

Zinc Oxide Nanoparticles with Mangiferin: Optical Properties, In Vitro Release Studies, and Antibacterial Activity

Review on zinc oxide nanoparticles: antibacterial activity and toxicity mechanism.

Avoid common mistakes on your manuscript.

Nanotechnology deals with the manufacture and application of materials with size of up to 100 nm. They are widely used in a number of processes that include material science, agriculture, food industry, cosmetic, medical, and diagnostic applications [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ]. Nanosize inorganic compounds have shown remarkable antibacterial activity at very low concentration due to their high surface area to volume ratio and unique chemical and physical features [ 11 ]. In addition, these particles are also more stable at high temperature and pressure [ 12 ]. Some of them are recognized as nontoxic and even contain mineral elements which are vital for human body [ 13 ]. It has been reported that the most antibacterial inorganic materials are metallic nanoparticles and metal oxide nanoparticles such as silver, gold, copper, titanium oxide, and zinc oxide [ 14 , 15 ].

Zinc is an essential trace element for human system without which many enzymes such as carbonic anhydrase, carboxypeptidase, and alcohol dehydrogenase become inactive, while the other two members, cadmium and mercury belonging to the same group of elements having the same electronic configuration, are toxic. It is essential for eukaryotes because it modulates many physiological functions [ 16 , 17 ]. Bamboo salt, containing zinc, is used as herbal medicine for the treatment of inflammation by regulating caspase-1 activity. Zinc oxide nanoparticles have been shown to reduce mRNA expression of inflammatory cytokines by inhibiting the activation of NF-kB (nuclear factor kappa B cells) [ 18 ].

Globally, bacterial infections are recognized as serious health issue. New bacterial mutation, antibiotic resistance, outbreaks of pathogenic strains, etc. are increasing, and thus, development of more efficient antibacterial agents is demand of the time. Zinc oxide is known for its antibacterial properties from the time immemorial [ 19 ]. It had been in use during the regime of Pharaohs, and historical records show that zinc oxide was used in many ointments for the treatment of injuries and boils even in 2000 BC [ 20 ]. It is still used in sun screen lotion, as a supplement, photoconductive material, LED, transparent transistors, solar cells, memory devices [ 21 , 22 ], cosmetics [ 23 , 24 ], and catalysis [ 25 ]. Although considerable amount of ZnO is produced every year, very small quantity is used as medicine [ 26 ]. The US Food and Drug Administration has recognized (21 CFR 182.8991) zinc oxide as safe [ 27 ]. It is characterized by photocatalytic and photooxidizing properties against biochemicals [ 28 ].

Zinc oxide has been classified by EU hazard classification as N; R50-53 (ecotoxic). Compounds of zinc are ecotoxic for mammals and plants in traces [ 29 , 30 ]. Human body contains about 2–3 g of zinc, and the daily requirement is 10–15 mg [ 29 , 31 ]. No report has demonstrated carcinogenicity, genotoxicity, and reproduction toxicity in humans [ 29 , 32 ]. However, zinc powder inhaled or ingested may produce a condition called zinc fever, which is followed by chill, fever, cough, etc.

Morphology of zinc oxide nanoparticles depends on the process of synthesis. They may be nanorods, nanoplates [ 33 , 34 , 35 ], nanospheres [ 36 ], nanoboxes [ 35 ], hexagonal, tripods [ 37 ], tetrapods [ 38 ], nanowires, nanotubes, nanorings [ 39 , 40 , 41 ], nanocages, and nanoflowers [ 42 , 43 ]. Zinc oxide nanoparticles are more active against gram-positive bacteria relative to other NPs of the same group of elements. Ready to eat food is more prone to infection by Salmonella , Staphylococcus aureus , and E. coli which pose a great challenge to food safety and quality. The antimicrobial compounds are incorporated in the packed food to prevent them from damage. Antimicrobial packaging contains a nontoxic material which inhibits or slows down the growth of microbes present in food or packaging material [ 44 ]. An antimicrobial substance for human consumption must possess the following properties.

It should be nontoxic.

It should not react with food or container.

It should be of good taste or tasteless.

It should not have disagreeable smell.

Zinc oxide nanoparticle is one such inorganic metal oxide which fulfills all the above requirements, and hence, it can safely be used as medicine, preservative in packaging, and an antimicrobial agent [ 45 , 46 ]. It easily diffuses into the food material, kill the microbes, and prevent human being from falling ill. In accordance with the regulations 1935/2004/EC and 450/2009/EC of the European Union, active packaging is defined as active material in contact with food with ability to change the composition of the food or the atmosphere around it [ 47 ]. Therefore, it is commonly used as preservative and incorporated in polymeric packaging material to prevent food material from damage by microbes [ 48 ]. Zinc oxide nanoparticles have been used as an antibacterial substance against Salmonella typhi and S. aureus in vitro. Of all the metal oxide nanoparticles studied thus far, zinc oxide nanoparticles exhibited the highest toxicity against microorganisms [ 49 ]. It has also been demonstrated from SEM and TEM images that zinc oxide nanoparticles first damage the bacterial cell wall, then penetrate, and finally accumulate in the cell membrane. They interfere with metabolic functions of the microbes causing their death. All the characteristics of the zinc oxide nanoparticles depend on their particle size, shape, concentration, and exposure time to the bacterial cell. Further, biodistribution studies of zinc oxide nanoparticles have also been examined. For instance, Wang et al. [ 50 ] have investigated the effect of long-term exposure of zinc oxide nanoparticle on biodistribution and zinc metabolism in mice over 3 to 35 weeks. Their results showed minimum toxicity to mice when they were exposed to 50 and 500 mg/kg zinc oxide nanoparticle in diet. At higher dose of 5000 mg/kg, zinc oxide nanoparticle decreased body weight but increased the weight of the pancreas, brain, and lung. Also, it increased the serum glutamic-pyruvic transaminase activity and mRNA expression of zinc metabolism-related genes such as metallothionein. Biodistribution studies showed the accumulation of sufficient quantity of zinc in the liver, pancreas, kidney, and bones. Absorption and distribution of zinc oxide nanoparticle/zinc oxide microparticles are largely dependent on the particle size. Li et al. [ 51 ] have studied biodistribution of zinc oxide nanoparticles fed orally or through intraperitoneal injection to 6 weeks old mice. No obvious adverse effect was detected in zinc oxide nanoparticles orally treated mice in 14 days study. However, intraperitoneal injection of 2.5 g/kg body weight given to mice showed accumulation of zinc in the heart, liver, spleen, lung, kidney, and testes. Nearly ninefold increase in zinc oxide nanoparticle in the liver was observed after 72 h. Zinc oxide nanoparticles have been shown to have better efficiency in liver, spleen, and kidney biodistribution than in orally fed mice. Since zinc oxide nanoparticles are innocuous in low concentrations, they stimulate certain enzymes in man and plants and suppress diseases. Singh et al. [ 52 ] have also been recently reviewed the biosynthesis of zinc oxide nanoparticle, their uptake, translocation, and biotransformation in plant system.

In this review, we have attempted to consolidate all the information regarding zinc oxide nanoparticles as antibacterial agent. The mechanism of interaction of zinc oxide nanoparticles against a variety of microbes has also been discussed in detail.

Antimicrobial Activity of Zinc Oxide Nanoparticles

It is universally known that zinc oxide nanoparticles are antibacterial and inhibit the growth of microorganisms by permeating into the cell membrane. The oxidative stress damages lipids, carbohydrates, proteins, and DNA [ 53 ]. Lipid peroxidation is obviously the most crucial that leads to alteration in cell membrane which eventually disrupt vital cellular functions [ 54 ]. It has been supported by oxidative stress mechanism involving zinc oxide nanoparticle in Escherichia coli [ 55 ]. However, for bulk zinc oxide suspension, external generation of H 2 O 2 has been suggested to describe the anti-bacterial properties [ 56 ]. Also, the toxicity of nanoparticles, releasing toxic ions, has been considered. Since zinc oxide is amphoteric in nature, it reacts with both acids and alkalis giving Zn 2+ ions.

The free Zn 2+ ions immediately bind with the biomolecules such as proteins and carbohydrates, and all vital functions of bacteria cease to continue. The toxicity of zinc oxide, zinc nanoparticles, and ZnSO 4 ·7H 2 O has been tested (Table 1 ) against Vibrio fischeri . It was found that ZnSO 4 ·7H 2 O is six times more toxic than zinc oxide nanoparticles and zinc oxide. The nanoparticles are actually dispersed in the solvent, not dissolved, and therefore, they cannot release Zn 2+ ions. The bioavailability of Zn 2+ ions is not always 100% and may invariably change with physiological pH, redox potential, and the anions associated with it such as Cl − or SO 4 2− .

Solubility of zinc oxide (1.6–5.0 mg/L) in aqueous medium is higher than that of zinc oxide nanoparticles (0.3–3.6 mg/L) in the same medium [ 57 ] which is toxic to algae and crustaceans. Both nano-zinc oxide and bulk zinc oxide are 40–80-fold less toxic than ZnSO 4 against V. fischeri . The higher antibacterial activity of ZnSO 4 is directly proportional to its solubility releasing Zn 2+ ions, which has higher mobility and greater affinity [ 58 ] toward biomolecules in the bacterial cell due to positive charge on the Zn 2+ and negative charge on the biomolecules.

Since zinc oxide and its nanoparticles have limited solubility, they are less toxic to the microbes than highly soluble ZnSO 4 ·7H 2 O. However, it is not essential for metal oxide nanoparticles to enter the bacterial cell to cause toxicity [ 59 ]. Contact between nanoparticles and the cell wall is sufficient to cause toxicity. If it is correct, then large amounts of metal nanoparticles are required so that the bacterial cells are completely enveloped and shielded from its environment leaving no chance for nutrition to be absorbed to continue life process. Since nanoparticles and metal ions are smaller than the bacterial cells, it is more likely that they disrupt the cell membrane and inhibit their growth.

A number of nanosized metal oxides such as ZnO, CuO, Al 2 O 3 , La 2 O 3 , Fe 2 O 3 , SnO 2 , and TiO 2 have been shown to exhibit the highest toxicity against E. coli [ 49 ]. Zinc oxide nanoparticles are externally used for the treatment of mild bacterial infections, but the zinc ion is an essential trace element for some viruses and human beings which increase enzymatic activity of viral integrase [ 45 , 60 , 61 ]. It has also been supported by an increase in the infectious pancreatic necrosis virus by 69.6% when treated with 10 mg/L of Zn [ 46 ]. It may be due to greater solubility of Zn ions relative to ZnO alone. The SEM and TEM images have shown that zinc oxide nanoparticles damage the bacterial cell wall [ 55 , 62 ] and increase permeability followed by their accumulation in E. coli preventing their multiplication [ 63 ].

In the recent past, antibacterial activity of zinc oxide nanoparticle has been investigated against four known gram-positive and gram-negative bacteria, namely Staphylococcus aureus , E. coli , Salmonella typhimurium , and Klebsiella pneumoniae . It was observed that the growth-inhibiting dose of the zinc oxide nanoparticles was 15 μg/ml, although in the case of K. pneumoniae , it was as low as 5 μg/ml [ 63 , 64 ]. It has been noticed that with increasing concentration of nanoparticles, growth inhibition of microbes increases. When they were incubated over a period of 4–5 h with a maximum concentration of zinc oxide nanoparticles of 45 μg/ml, the growth was strongly inhibited. It is expected that if the incubation time is increased, the growth inhibition would also increase without much alteration in the mechanism of action [ 63 ].

It has been reported that the metal oxide nanoparticles first damage the bacterial cell membrane and then permeate into it [ 64 ]. It has also been proposed that the release of H 2 O 2 may be an alternative to anti-bacterial activity [ 65 ]. This proposal, however, requires experimental proof because the mere presence of zinc oxide nanoparticle is not enough to produce H 2 O 2 . Zinc nanoparticles or zinc oxide nanoparticles of extremely low concentration cannot cause toxicity in human system. Daily intake of zinc via food is needed to carry out the regular metabolic functions. Zinc oxide is known to protect the stomach and intestinal tract from damage by E. coli [ 65 ]. The pH in the stomach varies between 2 to 5, and hence, zinc oxide in the stomach can react with acid to produce Zn 2+ ions. They can help in activating the enzyme carboxy peptidase, carbonic anhydrase, and alcohol dehydrogenase which help in the digestion of carbohydrate and alcohol. Premanathan et al. [ 66 ] have reported the toxicity of zinc oxide nanoparticles against prokaryotic and eukaryotic cells. The MIC of zinc oxide nanoparticles against E. coli , Pseudomonas aeruginosa , and S. aureus were found to be 500 and 125 μg/ml, respectively. Two mechanisms of action have been proposed for the toxicity of zinc oxide nanoparticles, namely (1) generation of ROS and (2) induction of apoptosis. Metal oxide nanoparticles induce ROS production and put the cells under oxidative stress causing damage to cellular components, i.e., lipids, proteins, and DNA [ 67 , 68 , 69 ]. Zinc oxide nanoparticles, therefore, induce toxicity through apoptosis. They are relatively more toxic to cancer cells than normal cells, although they cannot distinguish between them.

Recently, Pati et al. [ 70 ] have shown that zinc oxide nanoparticles disrupt bacterial cell membrane integrity, reduce cell surface hydrophobicity, and downregulate the transcription of oxidative stress-resistance genes in bacteria. They enhance intracellular bacterial killing by inducing ROS production. These nanoparticles disrupt biofilm formation and inhibit hemolysis by hemolysin toxin produced by pathogens. Intradermal administration of zinc oxide nanoparticles was found to significantly reduce the skin infection and inflammation in mice and also improved infected skin architecture.

Solubility and Concentration-Dependent Activity of Zinc Oxide Nanoparticle

Nanoparticles have also been used as a carrier to deliver therapeutic agents to treat bacterial infection [ 1 , 9 ]. Since zinc oxide nanoparticles up to a concentration of 100 μg/ml are harmless to normal body cells, they can be used as an alternative to antibiotics. It was found that 90% bacterial colonies perished after exposing them to a dose of 500–1000 μg/ml of zinc oxide nanoparticles only for 6 h. Even the drug-resistant S. aureus , Mycobacterium smegmatis , and Mycobacterium bovis when treated with zinc oxide nanoparticles in combination with a low dose of anti-tuberculosis drug, rifampicin (0.7 μg/ml), a significant reduction in their growth was observed. These pathogens were completely destroyed when incubated for 24 h with 1000 μg/ml of zinc oxide nanoparticles. It is, therefore, concluded that if the same dose is repeated, the patient with such infective diseases may be completely cured. It was also noted that the size of zinc oxide nanoparticles ranging between 50 and 500 nm have identical effect on bacterial growth inhibition.

Cytotoxicity of zinc oxide has been studied by many researchers in a variety of microbes and plant systems [ 71 , 72 , 73 , 74 ]. Toxicity of zinc oxide nanoparticles is concentration and solubility dependent. It has been shown that maximum exposure concentration of zinc oxide (125 mg/l) suspension released 6.8 mg/l of Zn 2+ ions. Toxicity is a combined effect of zinc oxide nanoparticles and Zn 2+ ions released in the aqueous medium. However, minimal effect of metal ions was detected which suggests that the bacterial growth inhibition is mainly due to interaction of zinc oxide nanoparticles with microorganisms. The cytotoxic effect of a particular metal oxide nanoparticle is species sensitive which is reflected by the growth inhibition zone for several bacteria [ 75 ].

It has been suggested that growth inhibition of bacterial cells occurs mainly by Zn 2+ ions which are produced by extracellular dissolution of zinc oxide nanoparticles [ 76 ]. Cho et al. [ 77 ] have concluded from their studies on rats that zinc oxide nanoparticles remain intact at around neutral or biological pH but rapidly dissolve under acidic conditions (pH 4.5) in the lysosome of the microbes leading to their death. This is true because in acidic condition, zinc oxide dissolves and Zn 2+ ions are produced, which bind to the biomolecules inside the bacterial cell inhibiting their growth.

The zinc oxide nanoparticles have been shown to be cytotoxic to different primary immune-competent cells. The transcriptomics analysis showed that nanoparticles had a common gene signature with upregulation of metallothionein genes ascribed to the dissolution of the nanoparticles [ 78 ]. However, it could not be ascertained if the absorbed zinc was Zn 2+ or zinc oxide or both, although smaller sized zinc oxide nanoparticles have greater concentration in the blood than larger ones (19 and > 100 nm). The efficiency of zinc oxide nanoparticles depends mainly on the medium of reaction to form Zn 2+ and their penetration into the cell.

Chiang et al. [ 79 ] have reported that dissociation of zinc oxide nanoparticles results in destruction of cellular Zn homeostasis. The characteristic properties of nanoparticles and their impact on biological functions are entirely different from those of the bulk material [ 80 ]. Aggregation of nanoparticles influences cytotoxicity of macrophages, and their concentration helps in modulation of nanoparticle aggregation. Low concentration of zinc oxide nanoparticles is ineffective, but at higher concentration (100 μg/ml), they exhibited cytotoxicity which varies from one pathogen to another.

The inadvertent use of zinc oxide nanoparticles may sometime adversely affect the living system. Their apoptosis and genotoxic potential in human liver cells and cellular toxicity has been studied. It was found that a decrease in liver cell viability occurs when they are exposed to 14–20 μg/ml of zinc oxide nanoparticles for 12 h. It also induced DNA damage by oxidative stress. Sawai et al. [ 56 ] have demonstrated that ROS generation is directly proportional to the concentration of zinc oxide powder. ROS triggered a decrease in mitochondria membrane potential leading to apoptosis [ 81 ]. Cellular uptake of nanoparticles is not mandatory for cytotoxicity to occur.

Size-Dependent Antibacterial Activity of Zinc Oxide Nanoparticles

In a study, Azam et al. [ 82 ] have reported that the antimicrobial activity against both gram-negative ( E. coli and P. aeruginosa ) and gram-positive ( S. and Bacillus subtilis ) bacteria increased with increase in surface-to-volume ratio due to a decrease in particle size of zinc oxide nanoparticles. Moreover, in this investigation, zinc oxide nanoparticles have shown maximum (25 mm) bacterial growth inhibition against B. subtilis (Fig. 1 ).

Antibacterial activity and/or zone of inhibition produced by zinc oxide nanoparticles against gram-positive and gram-negative bacterial strains namely a Escherichia coli , b Staphylococcus aureus , c Pseudomonas aeruginosa , and d Bacillus subtilis [ 82 ]

It has been reported that the smaller size of zinc oxide nanoparticles exhibits greater antibacterial activity than microscale particles [ 83 ]. For instance, Au 55 nanoparticles of 1.4-nm size have been demonstrated to interact with the major grooves of DNA which accounts for its toxicity [ 84 ]. Although contradictory results have been reported, many workers showed positive effect of zinc oxide nanoparticles on bacterial cells. However, Brayner et al. [ 63 ] from TEM images have shown that zinc oxide nanoparticle of 10–14 nm were internalized (when exposed to microbes) and damaged the bacterial cell membrane. It is also essential that the zinc/zinc oxide nanoparticles must not be toxic to human being since they are toxic to T cells above 5 mM [ 85 ] and to neuroblastoma cells above 1.2 mM [ 86 ]. Nair et al. [ 87 ] have exclusively explored the size effect of zinc oxide nanoparticles on bacterial and human cell toxicity. They have studied the influence of zinc oxide nanoparticles on both gram-positive and gram-negative bacteria and osteoblast cancer cell lines (MG-63).

It is known that antibacterial activity of zinc oxide nanoparticle is inversely proportional to their size and directly proportional to their concentration [ 88 ]. It has also been noticed that it does not require UV light for activation; it functions under normal or even diffused sunlight. Cytotoxic activity perhaps involves both the production of ROS and accumulation of nanoparticles in the cytoplasm or on the outer cell membrane. However, the production of H 2 O 2 and its involvement in the activation of nanoparticles cannot be ignored. Raghupathi et al. [ 88 ] have synthesized zinc oxide nanoparticles from different zinc salts and observed that nanoparticles obtained from Zn(NO 3 ) 2 were smallest in size (12 nm) and largest in surface area (90.4). Authors have shown that the growth inhibition of S. aureus at a concentration of 6 mM of zinc oxide nanoparticles is size dependent. It has also been indicated from the viable cell determination during the exposure of bacterial cells to zinc oxide nanoparticles that the number of cells recovered decreased significantly with decrease in size of zinc oxide nanoparticles. Jones et al. [ 89 ] have shown that zinc oxide nanoparticles of 8-nm diameter inhibited the growth of S. aureus , E. coli , and B. subtilis. Zinc oxide nanoparticles ranging between 12 and 307 nm were selected and confirmed the relationship between antibacterial activity and their size. Their toxicity to microbes has been ascribed to the formation of Zn 2+ ions from zinc oxide when it is suspended in water and also to some extent to a slight change in pH. Since Zn 2+ ions are scarcely released from zinc oxide nanoparticles, the antibacterial activity is mainly owing to smaller zinc oxide nanoparticles. When the size is 12 nm, it inhibits the growth of S. aureus , but when the size exceeds 100 nm, the inhibitory effect is minimal [ 89 ].

Shape, Composition, and Cytotoxicity of Zinc Oxide Nanoparticles

Zinc oxide nanoparticles have shown cytotoxicity in concentration-dependent manner and type of cells exposed due to different sensitivity [ 90 , 91 ]. Sahu et al. [ 90 ] have highlighted the difference of cytotoxicity between particle size and different sensitivity of cells toward the particles of the same composition. In another recent study, Ng et al. [ 91 ] examined the concentration-dependent cytotoxicity in human lung MRC5 cells. Authors have reported the uptake and internalization of zinc oxide nanoparticles into the human lung MRC5 cells by using TEM investigation. These particles were noticed in the cytoplasm of the cells in the form of electron dense clusters, which are further observed to be enclosed by vesicles, while zinc oxide nanoparticles were not found in untreated control cells. Papavlassopoulos et al. [ 92 ] have synthesized zinc oxide nanoparticle tetrapods by entirely a novel route known as “Flame transport synthesis approach”. Tetrapods have different morphology compared to the conventionally synthesized zinc oxide nanoparticles. Their interaction with mammalian fibroblast cells in vitro has indicated that their toxicity is significantly lower than those of the spherical zinc oxide nanoparticles. Tetrapods exhibited hexagonal wurtzite crystal structure with alternating Zn 2+ and O 2− ions with three-dimensional geometry. They block the entry of viruses into living cells which is further enhanced by precisely illuminating them with UV radiation. Since zinc oxide tetrapods have oxygen vacancies in their structure, the Herpes simplex viruses are attached via heparan sulfate and denied entry into body cells. Thus, they prevent HSV-1 and HSV-2 infection in vitro. Zinc oxide tetrapods may therefore be used as prophylactic agent against these viral infections. The cytotoxicity of zinc oxide nanoparticles also depends on the proliferation rate of mammalian cells [ 66 , 93 ]. The surface reactivity and toxicity may also be varied by controlling the oxygen vacancy in zinc oxide tetrapods. When they are exposed to UV light, the oxygen vacancy in tetrapods is readily increased. Alternatively, the oxygen vacancy can be decreased by heating them in oxygen-rich environment. Thus, it is the unique property of zinc oxide tetrapods that can be changed at will which consequently alter their antimicrobial efficiency.

Animal studies have indicated an increase in pulmonary inflammation, oxidative stress, etc. on respiratory exposure to nanoparticles [ 94 ]. Yang et al. [ 95 ] have investigated the cytotoxicity, genotoxicity, and oxidative stress of zinc oxide nanoparticles on primary mouse embryo fibroblast cells. It was observed that zinc oxide nanoparticles induced significantly greater cytotoxicity than that induced by carbon and SiO 2 nanoparticles. It was further confirmed by measuring glutathione depletion, malondialdehyde production, superoxide dismutase inhibition, and ROS generation. The potential cytotoxic effects of different nanoparticles have been attributed to their shape.

Polymer-Coated Nanoparticles

Many bacterial infections are transmitted by contact with door knobs, key boards, water taps, bath tubs, and telephones; therefore, it is essential to develop and coat such surfaces with inexpensive advanced antibacterial substances so that their growth is inhibited. It is important to use such concentrations of antibacterial substances that they may kill the pathogens but spare the human beings. It may happen only if they are coated with a biocompatible hydrophilic polymer of low cost. Schwartz et al. [ 96 ] have reported the preparation of a novel antimicrobial composite material hydrogel by mixing a biocompatible poly ( N -isopropylacrylamide) with zinc oxide nanoparticles. The SEM image of the composite film showed uniform distribution of zinc oxide nanoparticles. It exhibited antibacterial activity against E. coli at a very low zinc oxide concentration (1.33 mM). Also, the coating was found to be nontoxic toward mammalian cell line (N1H/3T3) for a period of 1 week. Zinc oxide/hydrogel nanocomposite may safely be used as biomedical coating to prevent people from contracting bacterial infections.

Although zinc oxide nanoparticles are stable, they have been further stabilized by coating them with different polymers such as polyvinyl pyrolidone (PVP), polyvinyl alcohol ( PVA ), poly (α, γ, l -glutamic acid) (PGA), polyethylene glycol (PEG), chitosan, and dextran [ 97 , 98 ]. The antibacterial activity of engineered zinc oxide nanoparticles was examined against gram-negative and gram-positive pathogens, namely E. coli and S. aureus and compared with commercial zinc oxide powder. The polymer-coated spherical zinc oxide nanoparticles showed maximum bacterial cell destruction compared to bulk zinc oxide powder [ 99 ]. Since nanoparticles coated with polymers are less toxic due to their low solubility and sustained release, their cytotoxicity can be controlled by coating them with a suitable polymer.

Effect of Particle Size and Shape of Polymer-Coated Nanoparticles on Antibacterial Activity

E. coli and S. aureus exposed to different concentrations of poly ethylene glycol (PEG)-coated zinc oxide nanoparticles (1–7 mM) of varying size (401 nm–1.2 μm) showed that the antimicrobial activity increases with decreasing size and increasing concentration of nanoparticles. However, the effective concentration in all these cases was above 5 mM. There occurs a drastic change in cell morphology of E. coli surface which can be seen from the SEM images of bacteria before and after their exposure to zinc oxide nanoparticles [ 84 ]. It has been nicely demonstrated by Nair et al. [ 87 ] that PEG-capped zinc oxide particles and zinc oxide nanorods are toxic to human osteoblast cancer cells (MG-63) at concentration above 100 μM. The PEG starch-coated nanorods/nanoparticles do not damage the healthy cells.

In Vivo and In Vitro Antimicrobial Activity for Wound Dressing

Of all natural and synthetic wound dressing materials, the chitosan hydrogel microporous bandages laced with zinc oxide nanoparticles developed by Kumar et al. [ 100 ] are highly effective in treating burns, wounds, and diabetic foot ulcers. The nanoparticles of approximately 70–120 nm are dispersed on the surface of the bandage. The degradation products of chitosan were identified as d -glucosamine and glycosamine glycan. They are nontoxic to the cells because they are already present in our body for the healing of injury. The wound generally contains P. aeruginosa , S. intermedicus , and S. hyicus which were also identified from the swab of mice wound and successfully treated with chitosan zinc oxide bandage in about 3 weeks [ 100 ].

Effect of Doping on Toxicity of Zinc Oxide Nanoparticles

Doping of zinc oxide nanoparticles with iron reduces the toxicity. The concentration of Zn 2+ and zinc oxide nanoparticles is also an important factor for toxicity. The concentration that reduced 50% viability in microbial cells exposed to nano- and microsize zinc oxide is very close to the concentration of Zn 2+ that induced 50% reduction in viability in Zn 2+ -treated cells [ 101 , 102 ].

Coating of zinc oxide nanoparticles with mercaptopropyl trimethoxysilane or SiO 2 reduces their cytotoxicity [ 103 ]. On the contrary, Gilbert et al. [ 104 ] showed that in BEAS-2B cells, uptake of zinc oxide nanoparticles is the main mechanism of zinc accumulation. Also, they have suggested that zinc oxide nanoparticles dissolve completely generating Zn 2+ ions which are bonded to biomolecules of the target cells. However, the toxicity of zinc oxide nanoparticles depends on the uptake and their subsequent interaction with target cells.

Interaction Mechanism of Zinc Oxide Nanoparticles

Nanoparticles may be toxic to some microorganisms, but they may be essential nutrients to some of them [ 55 , 105 ]. Nanotoxicity is essentially related to the microbial cell membrane damage leading to the entry of nanoparticles into the cytoplasm and their accumulation [ 55 ]. The impact of nanoparticles on the growth of bacteria and viruses largely depends on particle size, shape, concentration, agglomeration, colloidal formulation, and pH of the media [ 106 , 107 , 108 ]. The mechanism of antimicrobial activity of zinc oxide nanoparticles has been depicted in Fig. 2 .

Mechanisms of zinc oxide nanoparticle antimicrobial activity

Zinc oxide nanoparticles are generally less toxic than silver nanoparticles in a broad range of concentrations (20 to 100 mg/l) with average particle size of 480 nm [ 55 , 62 , 63 ]. Metal oxide nanoparticles damage the cell membrane and DNA [ 63 , 109 , 110 , 111 ] of microbes via diffusion. However, the production of ROS through photocatalysis causing bacterial cell death cannot be ignored [ 112 ]. UV-Vis spectrum of zinc oxide nanoparticle suspension in aqueous medium exhibits peaks between 370 and 385 nm [ 113 ]. It has been shown that it produces ROS (hydroxyl radicals, superoxides, and hydrogen peroxide) in the presence of moisture which ostensibly react with bacterial cell material such as protein, lipids, and DNA, eventually causing apoptosis. Xie et al. [ 114 ] have examined the influence of zinc oxide nanoparticles on Campylobacter jejuni cell morphology using SEM images (Fig. 3 ). After a 12-h treatment (0.5 mg/ml), C. jejuni was found to be extremely sensitive and cells transformed from spiral shape to coccoid forms. SEM studies showed the ascendency of coccoid forms in the treated cells and display the formation of irregular cell surfaces and cell wall blebs (Fig. 3a ). Moreover, these coccoid cells remained intact and possessed sheathed polar flagella. However, SEM image of the untreated cells clearly showed spiral shapes (Fig. 3b ). In general, it has been demonstrated from SEM and TEM images of bacterial cells treated with zinc oxide nanoparticles that they get ruptured and, in many cases, the nanoparticles damage the cell wall forcing their entry into it [ 114 , 115 ].

SEM images of Campylobacter jejuni . a Untreated cells from the same growth conditions were used as a control. b C. jejuni cells in the mid-log phase of growth were treated with 0.5 mg/ml of zinc oxide nanoparticles for 12 h under microaerobic conditions [ 114 ]

Zinc oxide nanoparticles have high impact on the cell surface and may be activated when exposed to UV-Vis light to generate ROS (H 2 O 2 ) which permeate into the cell body while the negatively charged ROS species such as O 2 2− remain on the cell surface and affect their integrity [ 116 , 117 ]. Anti-bacterial activity of zinc oxide nanoparticles against many other bacteria has also been reported [ 1 , 5 , 114 , 115 ]. It has been shown from TEM images that the nanoparticles have high impact on the cell surface (Fig. 4 ).

a TEM images of untreated normal Salmonella typhimurium cells. b Effects of nanoparticles on the cells (marked with arrows). c , d Micrograph of deteriorated and ruptured S. typhimurium cells treated with zinc oxide nanoparticles [ 115 ]

Sinha et al. [ 118 ] have also shown the influence of zinc oxide nanoparticles and silver nanoparticles on the growth, membrane structure, and their accumulation in cytoplasm of (a) mesophiles: Enterobacter sp. (gram negative) and B. subtilis (gram positive) and (b) halophiles: halophilic bacterium sp. (gram positive) and Marinobacter sp. (gram negative). Nanotoxicity of zinc oxide nanoparticles against halophilic gram-negative Marinobacter species and gram-positive halophilic bacterial species showed 80% growth inhibition. It was demonstrated that zinc oxide nanoparticles below 5 mM concentration are ineffective against bacteria. The bulk zinc oxide also did not affect the growth rate and viable counts, although they showed substantial decrease in these parameters. Enterobacter species showed dramatic alterations in cell morphology and reduction in size when treated with zinc oxide.

TEM images shown by Akbar and Anal [ 115 ] revealed the disrupted cell membrane and accumulation of zinc oxide nanoparticles in the cytoplasm (Fig. 4 ) which was further confirmed by FTIR, XRD, and SEM. It has been suggested that Zn 2+ ions are attached to the biomolecules in the bacterial cell via electrostatic forces. They are actually coordinated with the protein molecules through the lone pair of electrons on the nitrogen atom of protein part. Although there is significant impact of zinc oxide nanoparticles on both the aquatic and terrestrial microorganisms and human system, it is yet to be established whether it is due to nanoparticles alone or is a combined effect of the zinc oxide nanoparticles and Zn 2+ ions [ 55 , 106 , 109 , 119 ]. Antibacterial influence of metal oxide nanoparticles includes its diffusion into the bacterial cell, followed by release of metal ions and DNA damage leading to cell death [ 63 , 109 , 110 , 111 ]. The generation of ROS through photocatalysis is also a reason of antibacterial activity [ 62 , 112 ]. Wahab et al. [ 120 ] have shown that when zinc oxide nanoparticles are ingested, their surface area is increased followed by increased absorption and interaction with both the pathogens and the enzymes. Zinc oxide nanoparticles can therefore be used in preventing the biological system from infections. It is clear from TEM images (Fig. 5a, b ) of E. coli incubated for 18 h with MIC of zinc oxide nanoparticles that they had adhered to the bacterial cell wall. The outer cell membrane was ruptured leading to cell lysis. In some cases, the cell cleavage of the microbes has not been noticed, but the zinc oxide nanoparticles can yet be seen entering the inner cell wall (Fig. 5c, d ). As a consequence of it, the intracellular material leaks out leading to cell death, regardless of the thickness of bacterial cell wall.

TEM images of Escherichia coli ( a ), zinc oxide nanoparticles with E. coli at different stages ( b and inset), Klebsiella pneumoniae ( c ), and zinc oxide nanoparticles with K. pneumoniae ( d and inset) [ 120 ]

Mechanism of interaction of zinc oxide nanoparticles with bacterial cells has been outlined below [ 120 ]. Zinc oxide absorbs UV-Vis light from the sun and splits the elements of water.

Dissolved oxygen molecules are transformed into superoxide, O 2 − , which in turn reacts with H + to generate HO 2 radical and after collision with electrons produces hydrogen peroxide anion, HO 2 − . They subsequently react with H + ions to produce H 2 O 2 .

It has been suggested that negatively charged hydroxyl radicals and superoxide ions cannot penetrate into the cell membrane. The free radicals are so reactive that they cannot stay in free and, therefore, they can either form a molecule or react with a counter ion to give another molecule. However, it is true that zinc oxide can absorb sun light and help in cleaving water molecules which may combine in many ways to give oxygen. Mechanism of oxygen production in the presence of zinc oxide nanoparticles still needs experimental evidence.

Zinc oxide at a dose of 5 μg/ml has been found to be highly effective for all the microorganisms which can be taken as minimum inhibitory dose.

Conclusions

Zinc is an indispensable inorganic element universally used in medicine, biology, and industry. Its daily intake in an adult is 8–15 mg/day, of which approximately 5–6 mg/day is lost through urine and sweat. Also, it is an essential constituent of bones, teeth, enzymes, and many functional proteins. Zinc metal is an essential trace element for man, animal, plant, and bacterial growth while zinc oxide nanoparticles are toxic to many fungi, viruses, and bacteria. People with inherent genetic deficiency of soluble zinc-binding protein suffer from acrodermatitis enteropathica, a genetic disease indicated by python like rough and scaly skin. Although conflicting reports have been received about nanoparticles due to their inadvertent use and disposal, some metal oxide nanoparticles are useful to men, animals, and plants. The essential nutrients become harmful when they are taken in excess. Mutagenic potential of zinc oxide has not been thoroughly studied in bacteria even though DNA-damaging potential has been reported. It is true that zinc oxide nanoparticles are activated by absorption of UV light without disturbing the other rays. If zinc oxide nanoparticles produce ROS, they can damage the skin and cannot be used as sun screen. Antibacterial activity may be catalyzed by sunlight, but hopefully, it can prevent the formation of ROS. Zinc oxide nanoparticles and zinc nanoparticles coated with soluble polymeric material may be used for treating wounds, ulcers, and many microbial infections besides being used as drug carrier in cancer therapy. It has great potential as a safe antibacterial drug which may replace antibiotics in future. Application of zinc oxide nanoparticles in different areas of science, medicine, and technology suggests that it is an indispensable substance which is equally important to man and animals. However, longtime exposure with higher concentration may be harmful to living system.

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Khwaja Salahuddin Siddiqi

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Siddiqi, K.S., ur Rahman, A., Tajuddin et al. Properties of Zinc Oxide Nanoparticles and Their Activity Against Microbes. Nanoscale Res Lett 13 , 141 (2018). https://doi.org/10.1186/s11671-018-2532-3

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Nanoscale Res Lett

Properties of Zinc Oxide Nanoparticles and Their Activity Against Microbes

Khwaja salahuddin siddiqi.

1 Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh 202002 India

Aziz ur Rahman

2 Department of Saidla (Unani Pharmacy), Aligarh Muslim University, Aligarh, Uttar Pradesh 202002 India

Azamal Husen

3 Department of Biology, College of Natural and Computational Sciences, University of Gondar, P.O. Box #196, Gondar, Ethiopia

Nanotechnology deals with the manufacture and application of materials with size of up to 100 nm. They are widely used in a number of processes that include material science, agriculture, food industry, cosmetic, medical, and diagnostic applications [ 1 – 10 ]. Nanosize inorganic compounds have shown remarkable antibacterial activity at very low concentration due to their high surface area to volume ratio and unique chemical and physical features [ 11 ]. In addition, these particles are also more stable at high temperature and pressure [ 12 ]. Some of them are recognized as nontoxic and even contain mineral elements which are vital for human body [ 13 ]. It has been reported that the most antibacterial inorganic materials are metallic nanoparticles and metal oxide nanoparticles such as silver, gold, copper, titanium oxide, and zinc oxide [ 14 , 15 ].

Morphology of zinc oxide nanoparticles depends on the process of synthesis. They may be nanorods, nanoplates [ 33 – 35 ], nanospheres [ 36 ], nanoboxes [ 35 ], hexagonal, tripods [ 37 ], tetrapods [ 38 ], nanowires, nanotubes, nanorings [ 39 – 41 ], nanocages, and nanoflowers [ 42 , 43 ]. Zinc oxide nanoparticles are more active against gram-positive bacteria relative to other NPs of the same group of elements. Ready to eat food is more prone to infection by Salmonella , Staphylococcus aureus , and E. coli which pose a great challenge to food safety and quality. The antimicrobial compounds are incorporated in the packed food to prevent them from damage. Antimicrobial packaging contains a nontoxic material which inhibits or slows down the growth of microbes present in food or packaging material [ 44 ]. An antimicrobial substance for human consumption must possess the following properties.

It should be nontoxic.
It should not react with food or container.
It should be of good taste or tasteless.
It should not have disagreeable smell.

Antimicrobial Activity of Zinc Oxide Nanoparticles

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The toxicity (30-min EC 50 , EC 20 and NOEC, and MIC) of metal oxide aqueous suspensions CuSO 4 and ZnSO 4 ·7H 2 O to bacteria Vibrio fischeri [ 59 ]

Chemical	Toxicity to , EC , EC , NOEC, and MIC (mg l )
Chemical	EC ± SD	EC ± SD	NOEC	MIC
ZnO	1.8 ± 0.1 (1.4 ± 0.08)	1.0 ± 0.4 (0.8 ± 0.3)	1.0 (0.8)	200 (160)
Nano-ZnO	1.9 ± 0.2 (1.5 ± 0.16)	0.9 ± 0.4 (0.7 ± 0.3)	0.75 (0.6)	100 (80)
ZnSO ·7H O	1.1 ± 0.25 (0.25 ± 0.06)	0.8 ± 0.3 (0.2 ± 0.1)	0.5 (0.11)	10 (2.0)
CuO	3811 ± 1012 (3049 ± 819)	903 ± 457 (722 ± 366)	313 (250)	20,000 (16,000)
Nano-CuO	79 ± 27 (63 ± 22)	24 ± 5 (19 ± 4)	16 (12)	200 (160)
CuSO	1.6 ± 0.29 (0.64 ± 0.12)	0.9 ± 0.3 (0.36 ± 0.12)	0.63 (0.25)	2.5 (1.0)
TiO	> 20,000	> 20,000	> 20,000	> 20,000
Nano-TiO	> 20,000	> 20,000	> 20,000	> 20,000

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It has been reported that the metal oxide nanoparticles first damage the bacterial cell membrane and then permeate into it [ 64 ]. It has also been proposed that the release of H 2 O 2 may be an alternative to anti-bacterial activity [ 65 ]. This proposal, however, requires experimental proof because the mere presence of zinc oxide nanoparticle is not enough to produce H 2 O 2 . Zinc nanoparticles or zinc oxide nanoparticles of extremely low concentration cannot cause toxicity in human system. Daily intake of zinc via food is needed to carry out the regular metabolic functions. Zinc oxide is known to protect the stomach and intestinal tract from damage by E. coli [ 65 ]. The pH in the stomach varies between 2 to 5, and hence, zinc oxide in the stomach can react with acid to produce Zn 2+ ions. They can help in activating the enzyme carboxy peptidase, carbonic anhydrase, and alcohol dehydrogenase which help in the digestion of carbohydrate and alcohol. Premanathan et al. [ 66 ] have reported the toxicity of zinc oxide nanoparticles against prokaryotic and eukaryotic cells. The MIC of zinc oxide nanoparticles against E. coli , Pseudomonas aeruginosa , and S. aureus were found to be 500 and 125 μg/ml, respectively. Two mechanisms of action have been proposed for the toxicity of zinc oxide nanoparticles, namely (1) generation of ROS and (2) induction of apoptosis. Metal oxide nanoparticles induce ROS production and put the cells under oxidative stress causing damage to cellular components, i.e., lipids, proteins, and DNA [ 67 – 69 ]. Zinc oxide nanoparticles, therefore, induce toxicity through apoptosis. They are relatively more toxic to cancer cells than normal cells, although they cannot distinguish between them.

Solubility and Concentration-Dependent Activity of Zinc Oxide Nanoparticle

Cytotoxicity of zinc oxide has been studied by many researchers in a variety of microbes and plant systems [ 71 – 74 ]. Toxicity of zinc oxide nanoparticles is concentration and solubility dependent. It has been shown that maximum exposure concentration of zinc oxide (125 mg/l) suspension released 6.8 mg/l of Zn 2+ ions. Toxicity is a combined effect of zinc oxide nanoparticles and Zn 2+ ions released in the aqueous medium. However, minimal effect of metal ions was detected which suggests that the bacterial growth inhibition is mainly due to interaction of zinc oxide nanoparticles with microorganisms. The cytotoxic effect of a particular metal oxide nanoparticle is species sensitive which is reflected by the growth inhibition zone for several bacteria [ 75 ].

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Size-Dependent Antibacterial Activity of Zinc Oxide Nanoparticles

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Shape, Composition, and Cytotoxicity of Zinc Oxide Nanoparticles

Polymer-Coated Nanoparticles

Effect of Particle Size and Shape of Polymer-Coated Nanoparticles on Antibacterial Activity

In Vivo and In Vitro Antimicrobial Activity for Wound Dressing

Effect of Doping on Toxicity of Zinc Oxide Nanoparticles

Interaction Mechanism of Zinc Oxide Nanoparticles

Nanoparticles may be toxic to some microorganisms, but they may be essential nutrients to some of them [ 55 , 105 ]. Nanotoxicity is essentially related to the microbial cell membrane damage leading to the entry of nanoparticles into the cytoplasm and their accumulation [ 55 ]. The impact of nanoparticles on the growth of bacteria and viruses largely depends on particle size, shape, concentration, agglomeration, colloidal formulation, and pH of the media [ 106 – 108 ]. The mechanism of antimicrobial activity of zinc oxide nanoparticles has been depicted in Fig. 2 .

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Mechanisms of zinc oxide nanoparticle antimicrobial activity

Zinc oxide nanoparticles are generally less toxic than silver nanoparticles in a broad range of concentrations (20 to 100 mg/l) with average particle size of 480 nm [ 55 , 62 , 63 ]. Metal oxide nanoparticles damage the cell membrane and DNA [ 63 , 109 – 111 ] of microbes via diffusion. However, the production of ROS through photocatalysis causing bacterial cell death cannot be ignored [ 112 ]. UV-Vis spectrum of zinc oxide nanoparticle suspension in aqueous medium exhibits peaks between 370 and 385 nm [ 113 ]. It has been shown that it produces ROS (hydroxyl radicals, superoxides, and hydrogen peroxide) in the presence of moisture which ostensibly react with bacterial cell material such as protein, lipids, and DNA, eventually causing apoptosis. Xie et al. [ 114 ] have examined the influence of zinc oxide nanoparticles on Campylobacter jejuni cell morphology using SEM images (Fig. 3 ). After a 12-h treatment (0.5 mg/ml), C. jejuni was found to be extremely sensitive and cells transformed from spiral shape to coccoid forms. SEM studies showed the ascendency of coccoid forms in the treated cells and display the formation of irregular cell surfaces and cell wall blebs (Fig. 3a ). Moreover, these coccoid cells remained intact and possessed sheathed polar flagella. However, SEM image of the untreated cells clearly showed spiral shapes (Fig. 3b ). In general, it has been demonstrated from SEM and TEM images of bacterial cells treated with zinc oxide nanoparticles that they get ruptured and, in many cases, the nanoparticles damage the cell wall forcing their entry into it [ 114 , 115 ].

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TEM images shown by Akbar and Anal [ 115 ] revealed the disrupted cell membrane and accumulation of zinc oxide nanoparticles in the cytoplasm (Fig. 4 ) which was further confirmed by FTIR, XRD, and SEM. It has been suggested that Zn 2+ ions are attached to the biomolecules in the bacterial cell via electrostatic forces. They are actually coordinated with the protein molecules through the lone pair of electrons on the nitrogen atom of protein part. Although there is significant impact of zinc oxide nanoparticles on both the aquatic and terrestrial microorganisms and human system, it is yet to be established whether it is due to nanoparticles alone or is a combined effect of the zinc oxide nanoparticles and Zn 2+ ions [ 55 , 106 , 109 , 119 ]. Antibacterial influence of metal oxide nanoparticles includes its diffusion into the bacterial cell, followed by release of metal ions and DNA damage leading to cell death [ 63 , 109 – 111 ]. The generation of ROS through photocatalysis is also a reason of antibacterial activity [ 62 , 112 ]. Wahab et al. [ 120 ] have shown that when zinc oxide nanoparticles are ingested, their surface area is increased followed by increased absorption and interaction with both the pathogens and the enzymes. Zinc oxide nanoparticles can therefore be used in preventing the biological system from infections. It is clear from TEM images (Fig. 5a, b ) of E. coli incubated for 18 h with MIC of zinc oxide nanoparticles that they had adhered to the bacterial cell wall. The outer cell membrane was ruptured leading to cell lysis. In some cases, the cell cleavage of the microbes has not been noticed, but the zinc oxide nanoparticles can yet be seen entering the inner cell wall (Fig. 5c, d ). As a consequence of it, the intracellular material leaks out leading to cell death, regardless of the thickness of bacterial cell wall.

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Mechanism of interaction of zinc oxide nanoparticles with bacterial cells has been outlined below [ 120 ]. Zinc oxide absorbs UV-Vis light from the sun and splits the elements of water.

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Zinc oxide at a dose of 5 μg/ml has been found to be highly effective for all the microorganisms which can be taken as minimum inhibitory dose.

Conclusions

Acknowledgements

The authors are thankful to publishers for the permission to adopt the table and figures in this review.

Authors’ contributions

AH, KSS, AR, and T gathered the research data. AH and KSS analyzed these data and wrote this review paper. All the authors read and approved the final manuscript.

Competing interests

The authors declare that they have no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Contributor Information

Khwaja Salahuddin Siddiqi, Email: ni.oc.oohay@iqiddis_sk .

Aziz ur Rahman, Email: [email protected] .

Tajuddin, Email: moc.liamg@umaniddujatrd .

Azamal Husen, Email: ni.oc.oohay@29toorda .

ZnO nanostructured materials and their potential applications: progress, challenges and perspectives

First published on 9th March 2022

Extensive research in nanotechnology has been conducted to investigate new behaviours and properties of materials with nanoscale dimensions. ZnO NPs owing to their distinct physical and chemical properties have gained considerable importance and are hence investigated to a detailed degree for exploitation of these properties. This communication, at the outset, elaborates the various chemical methods of preparation of ZnO NPs, viz. , the mechanochemical process, controlled precipitation, sol–gel method, vapour transport method, solvothermal and hydrothermal methods, and methods using emulsion and micro-emulsion environments. The paper further describes the green methods employing the use of plant extracts, in particular, for the synthesis of ZnO NPs. The modifications of ZnO with organic (carboxylic acid, silanes) and inorganic (metal oxides) compounds and polymer matrices have then been described. The multitudinous applications of ZnO NPs across a variety of fields such as the rubber industry, pharmaceutical industry, cosmetics, textile industry, opto-electronics and agriculture have been presented. Elaborative narratives on the photocatalytic and a variety of biomedical applications of ZnO have also been included. The ecotoxic impacts of ZnO NPs have additionally been briefly highlighted. Finally, efforts have been made to examine the current challenges and future scope of the synthetic modes and applications of ZnO NPs.

1. Introduction

ZnO has a slew of unique chemical and physical properties, viz. , high chemical stability, high electrochemical coupling coefficient, broad range of radiation absorption and high photostability, which make it among all metal oxides a key technological material and confer upon it its wide applications in varied fields. ZnO is categorized as a group II–VI semiconductor in materials science because zinc belongs to the 2 nd group while oxygen belongs to the 6 th group of the periodic table. Its covalence is on the borderline demarcating ionic and covalent semiconductors. Besides, it has good transparency, high electron mobility, an outsized exciton binding energy (60 meV), wide band gap (3.37 eV), 1 strong room temperature luminescence, high thermal and mechanical stability at room temperature, broad range of radiation absorption and high photostability that make ZnO the most favorite multitasking material. 2,3,5,6 As a result of its distinctive optical and electrical properties 4 it is considered to be a possible material in electronic applications, optoelectronic applications and laser technology. ZnO among nano-sized metal oxides has also been further extensively exploited to derive possible benefits from its antimicrobial and antitumor activities. 7 Because of its blocking and absorbing capabilities ZnO finds inclusion in some cosmetic lotions. 8 ZnO can also be used in human medicine as an astringent (for wound healing), and to treat hemorrhoids, eczema and excoriation. 9 ZnO nanoparticles have recently attracted attention owing to their unique features. There are numerous promising applications of ZnO nanoparticles in veterinary science due to their wound healing, antibacterial, antineoplastic and antigenic properties. Recently, many research studies and experimental analyses have improved the efficiency of zinc oxide (ZnO) materials by producing nano-structures where each nano-dimension is reduced to generate nanowires, thin films and other structures for plenty of applications including defense against intracellular pathogens and brain tumors. 10 One-dimensional structures include nanorods, 11–13 nanoneedles, 14 nanohelixes, nanosprings, nanorings, 1 nanoribbons, 15 nanotubes, 16–18 nanobelts, 19 nanowires 20–22 and nanocombs. 23 Nanoplates/nanosheets and nanopellets 24,25 are their two-dimensional forms while flowers, dandelions, snowflakes, coniferous urchin-like structures, etc. 26–29 count as the three-dimensional morphologies of ZnO nanoparticles. Nevertheless, the challenges in terms of the potential toxic effects of ZnO nanoparticles do require special attention.

2. Chemical methods for synthesis of zinc oxide nanoparticles


	Various strategies for the fabrication of ZnO NPs.

Chemical methods of synthesis	Precursors	Synthesis conditions	Experimental variables	Main mechanisms	Properties and applications	Advantages
Mechanochemical process	ZnCl , Na CO and NaCl	Calcination, 2 h, 600 °C, milling for 2–6 h	Milling time and heat-treatment temperature on ZnO nanocrystallite sizes	ZnO nanocrystallite growth is homogeneous, crystal nuclei were formed with decomposition of ZnCO and grew by emergence of the secondary formed ZnO. The driving force of the interfacial reaction came from the activation energy. Higher activation energy above 600 °C leads to a higher growth rate for the ZnO nanocrystallite	Hexagonal structure; particle diameter: 21–25 nm	Simplicity, relatively low-cost equipment, large-scale production, and applicability for a variety of materials. Operates at room temperature, which increases safety and reduces energy utilization. Induces not only morphological and structural changes of the particles but also modifies their optical and electrical properties and prevents the agglomeration of the synthesized particles
	ZnCl and oxalic acid	Calcination, 1 h, 400 °C, milling for 0.5–4 h	Oxalic acid and wet-milling conditions on the ZnO average particle size and morphology	—	Hexagonal structure; particle diameter: 1 μm to 50–90 nm
	ZnCl , NaCl and Na CO	Calcination, 0.5 h, 300–450 °C, milling for 9 h	Calcination temperature on particle size and structural properties of ZnO nanoparticles	—	Hexagonal structure; particle diameter: 27.7–56.3 nm
Precipitation process	ZnSO and NH OH	Reaction: 50–60 °C; drying: 60 °C, 8 h	—	—	Hexagonal structure; flakes, particle diameter: 30 nm	The precipitation method is an unsophisticated method. High quality of production typifies the method. The method further has the advantage of being monetarily cheap with high production yield
	Zn(OAc) ·2H O and NaOH	Reaction: 30 min, 75 °C; drying: Room temperature, overnight	—	On heating the solution of zincate ions, the molecules start to rearrange into hexagonal ZnO nanorods after growing along the 〈0001〉 direction. When the molecules got saturated, the ZnO nuclei grew to give rod shaped ZnO. Over time, these freshly formed nanorods deposited on the surface of formerly formed crystalline nanorods resulting in a leaf-like structure first and a number of such leaves came together in an ordered array which appeared as flower shaped ZnNSs	Hexagonal structure; flower shape (length of each petal did not exceed 800 nm); application: antimicrobial activity
	Zn(OAc) ·2H O, (NH ) CO , and polyethylene glycol	Drying: 100 °C, 12 h. ZnO (A): the dried precipitate was ball-milled for 1 h followed by calcination at 450 °C for 3 h to produce ZnO powder which was further ball-milled for 3 h. ZnO (B): the precipitate was ball-milled for an hour and then a 1	Reaction temperature and time, concentration of oleic acid		Hexagonal structure; ZnO (A): particle diameter is 40 nm. ZnO (B): particle diameter is 40 nm. Photocatalytic degradation of methyl orange dye
	ZnCl , NH OH, and CTAB	Aging: 96 h, ambient temperature, calcination: 2 h, 500 °C		Particle formation is a very complex process and involves nucleation, growth, coagulation and flocculation. Addition of surfactant CTAB affects the nucleation during the crystallization process. After nucleation, the surfactant can influence particle growth, coagulation and flocculation	Zincite structure; particle diameter: 54–60 nm, BET = ∼17 m g
	Zn(NO ) , NaOH, SDS, and TEA (triethanolamine)	Precipitation: 50–55 min, 101 °C	Addition of sodium dodecyl sulfate (SDS) and triethanolamine (TEA)	Dissolution–reprecipitation mechanism	Wurtzite structure, rod-like shape (L: 3.6 μm, D: 400–500 nm), nut-like and rice-like shapes, size: 1.2–1.5 μm
Sol–gel	Zn(OAc) ·2H O, polyvinyl pyrrolidone (PVP) and NaOH	Reaction: 60 °C; vigorous stirring for 1 h. Calcination: 600 °C, 1 h			Wurtzite structure; platelet-like ZnO with a grain size of 150 nm transformed into rod-shaped ZnO with a diameter of 100 nm at 3 × 10 M PVP	Sol–gel shows many advantages over other techniques such as its simplicity and low equipment cost
	Zn(OAc) ·2H O and oxalic acid	Reaction: refluxed at 50 °C, 1 h; drying at 80 °C for 20 h; calcination: 650 °C, 4 h			Wurtzite structure; uniform, spherically shaped ZnO nanoparticles with a crystallite size of 20 nm; BET surface area of 10 m g ; 69.75% degradation of phenol and 67.98% degradation of benzoic acid in 120 min under UV light
	Zinc 2-ethylhexanoate, 2-propanol, and tetramethylammonium (TMAH)	Reaction: room temperature; aging: 30 min; drying: 60 °C	Weight ratio of 2-propanol and tetramethylammonium (TMAH)		Cylinder-shaped crystallites, diameter: 25–30 nm; height: 35–45 nm
Vapour transport method	Zn and water vapour or oxygen	Heating: 1 h, 800 °C, pressure: 0.03–0.05 MPa, cooling rate: 7 °C per minute	Influence of the atmosphere	For crystal growth, after initial nucleation, the subsequent growth stage strongly governs the final morphology of the crystal. In O gas, the growth of ZnO is simply along the 〈001〉 direction due to the fastest growth kinetics in this direction and absence of side or reverse reactions	With H O: nanoflowers constructed by tens of ZnO nanosheets with random orientations. With O : hexagonal nanorod arrays, non-uniform sized nanorods	The vapour transport method has been emphasised because of the easy control of thicknesses, morphologies and crystal structures of ZnO films and nanostructures by varying the precursor gas, substrate temperature and substrate materials
	ZnO powder	Heating in a horizontal tube furnace: 1350 °C, 30 min; deposition: 400–500 °C under an Ar pressure of 250 Torr		Due to the small thickness of the nanobelts, spontaneous polarization normal to the nanobelt leads to the growth of helical nanostructures. The mechanism for the helical growth is attributed to the consequence of minimizing the total energy contributed by spontaneous polarization and elasticity	Wurtzite; nanobelts with widths of 10–60 nm, thickness of 5–20 nm and lengths up to several hundreds of micrometers
	Zn powder and O	Heating in a furnace at 450 °C, 550 °C, and 650 °C at a rate of 10 °C min , feeding O into the reaction zone at a rate of 5 mL min for 30, 45, and 60 min after reaching a furnace temperature of 450 °C	Growth temperature and growth time	The growth mechanism of 1D ZnO nanostructures can be divided into three stages, as follows: first, the Zn vapor and catalytic Cu form liquid alloy droplets during the heating process at a certain temperature, representing the initial stage of the nucleation process. Second, crystal nucleation occurs upon gaseous species adsorption until supersaturation is reached, and the formed sites serve as nucleation sites on the substrate. Finally, the axial growth of the nanorods begins from these sites	450 °C: ZnO nanorods with a diameter and length of 19–27 nm and 2.8 μm, respectively. 550 °C: ZnO nanorods with a diameter and length of 85 nm and 3.8 μm, respectively. 650 °C: ZnO nanorods with a diameter and length of 190–350 nm and 3.9 μm, respectively, covered with short nanorods with a diameter of 95 nm and length of 900 nm at the tips. 30 min growth time: ZnO nanorods with a diameter of 19–27 nm and a length of 2.8 μm. 45 min growth time: ZnO nanorods with a diameter of 65–190 nm and a length of 3.2 μm. 60 min growth time: ZnO nanorods with a diameter of 80–250 nm and a length of 3.8 μm
	Zn and O	Heating in a furnace at 750 °C, feeding O into the reaction zone at a rate of 50 mL min for 15 min	Gas flow rate, growth temperature, position from the zinc source, and reaction time can affect the size, morphology, and density of the zinc oxide nanostructures	The growth mechanism of zinc oxide tetrapods is believed to occur by growth of four wurtzitic arms from an octahedral zinc-blende embryo, each at a 109.5° angle from the adjacent one. The tapered ends of some of the tetrapod arms indicate continued growth of zinc oxide when the oxygen flow had been turned off but residual oxygen remains in the growth chamber	ZnO nanotetrapods: arm lengths, 0.5–3.5 μm and diameters of 120–350 nm
	Zn and O	Heating in a furnace at 700 °C, 800 °C and 900 °C, 50 sccm of oxygen flow for 2 h	Different evaporation temperatures		Wurtzite; ZnO tetrapods with an arm diameter of 22 nm and length of 90 nm. ZnO tetrapods have excellent supercapacitive performance. The maximum capacitance is 160.4 F g at a current density of 1.0 A g . Excellent capacitance retention of 94.3% over 1000 cycles
Hydrothermal method	ZnCl and NaOH	pH 5–8	Reaction temperature and template agents (organic compounds)		As temperature was increased, the ZnO particle morphologies changed	The hydrothermal technique is a promising alternative synthetic method because of the low process temperature and great ease of controlling the particle size. The hydrothermal process has several advantages over other growth processes such as use of simple equipment, catalyst-free growth, low cost, large area uniform production, environment friendliness and less hazardous nature. The low reaction temperatures make this method an attractive one for microelectronics and plastic electronics. This method has also been successfully employed to prepare nanoscale ZnO and other luminescent materials. The particle properties such as morphology and size can be controlled via the hydrothermal process by adjusting the reaction temperature, time and concentration of precursors
		100 °C	10 h		Bullet-like; 100–200 nm
		160 °C	6 h		Rod-like; 100–200 nm
		180 °C	6 h		Sheets; 50–200 nm
		200 °C	6 h		Polyhedra; 200–400 nm
		220 °C	5 h		Crushed stone-like; 50–200 nm
	Zn(OAc) ·2H O, NaOH and methanol	100–200 °C; 6–12 h; 0.2–0.5 M NaOH	Concentration of precursors (NaOH), reaction temperature and growth time		With 0.3 M NaOH and employing a growth time of 6 h the grain size was found to increase from 7 nm to 16 nm with temperature rise from 100 °C to 200 °C. The average grain size of ZnO synthesized at 200 °C for 12 h revealed an increase from 12 nm to 24 nm with elevation in concentration of NaOH from 0.2 M to 0.5 M
Solvothermal method	ZnSO , NaOH, Na CO and stearic acid; using the resulting ZnO nanoparticles as precursors	Reaction temperature: 60 °C; water–ethanol medium in an autoclave at 180 °C for 72–186 h			The ZnO wurtzite phase was formed. Average grain diameter of 27 nm using the Scherrer formula
		180 °C: 72 h	Precursors and time	The process appears to occur via an agglomeration/melting mechanism and leads to nanoneedles of relatively large dimensions	Nanoneedles with a diameter of 450–900 nm, length of 8–20 nm and aspect ratio of 0.05
		180 °C: 168 h		The formation mechanism of one-dimensional nanostructures does appear to be related more to a rolling-up/surfactant-segregation process than with the characteristic ZnO crystallite growth	Nanorods with a diameter of 40–160 nm, length of 5–8 nm and aspect ratio of 0.014
		180 °C: 168 h			Nanowires with a diameter of 30–50 nm, length of 0.8–1 nm and aspect ratio of 0.04. Photocatalytic activities with respect to the degradation of methylene blue
	Zn powder, trimethylamine N-oxide and 4-picoline N-oxide in organic solvents	Reaction: 24–100 h, 180 °C	Oxidants and solvents, trace amount of water in solvent		ZnO rod-like and particle-like nanostructures with diameters ranging in between 24 and 185 nm
Emulsion or microemulsion method	Zinc oleate in decane and NaOH in water or ethanol	Stirring: 2 h, room temperature or 90 °C, and maintaining the decane/water interface during stirring			Morphologies obtained: spherical agglomerates, needle shapes, near-hexagonal shapes, near-spherical shapes and irregular agglomerates. Diameters obtained: 2–10 μm, 90–600 nm, 100–230 nm and ∼150 nm
Emulsion or microemulsion method	Zinc acetate and KOH or NaOH. Cyclohexane as an organic phase, and nonylphenyl polyoxyethylene glycol ethers as a mixture of emulsifiers in emulsion formation	Stirring: 9000 rpm; destabilization: 80 °C; drying at 120 °C	Concentration of Zn(CH COO) solution. Precipitating agent. Amount of zinc acetate/cyclohexane (cm ). Dosing rate of KOH (or NaOH) to Zn(CH COO) (cm min )		Morphologies such as solids (Z1), ellipsoids (Z2), rods (Z3) and flakes (Z4) with modal diameters of ∼396 nm, ∼396 nm, ∼1110 nm and ∼615 nm. Values of 8 m g , 10.6 m g , 12 m g and 23 m g could be respectively assigned to samples Z1, Z2, Z3, and Z4

2.1 Mechanochemical process

ZnCl + Na CO → Zn CO + 2NaCl

ZnCO → ZnO + CO

Ao et al. 32 carried out a mechanochemical process of synthesizing ZnO NPs by exploiting the reaction between ZnCl 2 and Na 2 CO 3 and using NaCl as a diluent. 32 The pure nanocrystalline ZnO was obtained by removing the by-product NaCl and finally drying in a vacuum. TEM images showed moderately aggregated ZnO nanoparticles of size less than 100 nm which were prepared by a 6 h milling followed by a thermal treatment at 600 °C for 2 h. The effect of milling time and annealing was carefully investigated in the study. A decrease in nanocrystallite size from 25 nm to 21.5 nm was observed as the milling time increased from 2 to 6 h after which it attained steadiness. This phenomenon was chalked up to a critical effect prevailing in the course of milling. The crystal size, however, was found to increase with temperature with the rise being steep after 600 °C. The activation energies for nanocrystallite growth in different temperature ranges were calculated using the Scott equation. The activation energy was found to be 3.99 for growth in between 400 and 600 °C while it reached 20.75 kJ mol −1 beyond 600 °C. The higher growth rate at higher temperatures was thus attributed to extensive interfacial reactions driven by greater activation energy.

ZnCl + H C O ·2H O → ZnC O ·2H O + 2HCl

ZnC O ·2H O + 0.5O → ZnO + 2H O + 2CO

While the XRD analysis substantiated a perfect long-range order and a pure wurtzite structure of the synthesized ZnO powders regardless of the milling time, Raman spectroscopy revealed that lattice defects and impurities were introduced into ZnO powders at the middle-range scale depending on milling duration. Extended milling was found to reduce crystal defects but introduce impurities. The SEM images suggested that the milling duration of the reactant mixture positively regulated the morphology of the particles irrespective of the additional thermal treatment.

ZnO NPs were also prepared through a mechanochemical method by using ZnCl 2 , NaCl and Na 2 CO 3 as starting materials. 34 A solid phase reaction triggered by milling the starting powders led to the isolation of ZnCO 3 in the NaCl matrix. The ZnCO 3 was finally subjected to a thermal treatment at 400 °C which induced its decomposition to ZnO. The anatomization of TEM results indicated a mean particle size of 26.2 nm. The mean nanocrystallite size evaluated from the XRD peak width at 2 θ = 36° using the Scherrer equation was found to be 28.7 nm. Meanwhile, the surface area of the ZnO nanopowder evaluated from BET analysis was 47.3 m 2 g −1 corresponding to a spherical particle size of 27 nm.

Another study on the optical properties of ZnO NPs synthesized through mechanochemical means and using ZnCl 2 , NaCl and Na 2 CO 3 as raw materials was conducted by Moballegh et al. 35 The XRD and TEM results revealed that particle size increased with calcination temperature. The work proposed improved optical properties as a result of the decrease in particle size owing to the enhanced ratio of surface to volume in ZnO NPs. In another study 36 a mixture of starting powders (anhydrous ZnCl 2 , Na 2 CO 3 and NaCl) was milled at 250 rpm and then calcined at 450 °C for 0.5 h to yield ZnO NPs with a crystallite size of 28.5 nm as estimated from subsequent XRD analysis. The particle size that emerged from TEM and SEM analysis ranged in between 20 and 30 nm. The incongruent particle size estimated from BET analysis was ascribed to an agglomeration of nanoparticles in the course of drying.

The foremost shortcoming of the procedure exists in its fundamental difficulty encountered in the homogeneous grinding of the powder and controlled minimization of the particles to the required size. Note that the particle size reduces with increasing time and intensity of milling. However, if the powder is subjected to milling for longer periods of time, the chances of contamination increase. A highly shrunk size of nanoparticles is the prime advantage that can be extracted from the method apart from the benefit of a significantly low cost of generation coupled with diminished agglomeration of particles and pronouncedly homogeneous crystallite morphology and architecture. The mechanochemical process is particularly desirable for large-scale production of ZnO NPs.

2.2 Controlled precipitation

Kumar et al. 38 used zinc acetate (Zn(OAc) 2 ·2H 2 O) and NaOH as reagents, and the settled white powder was separated followed by washing with deionized water thrice and dried overnight under dust-free conditions at room temperature. XRD revealed the formation of hexagonal ZnO nanostructures. SEM and TEM analyses revealed the formation of crystalline ZnO flowers in which a bunch of ZnO nanorods assembled together to form a leaf-like structure followed by flower-shaped ZnO nanostructures. The ZnO nanoflowers were each formed by the combination of 8–10 leaf-like petals as shown. The length of each petal did not exceed 800 nm. The as-synthesized ZnO nanostructures showed good antimicrobial activity towards Gram-positive bacteria Staphylococcus aureus as well as Gram-negative bacteria Escherichia coli with a MIC/MBC of 25 mg L −1 . Zn(CH 3 COO) 2 ·2H 2 O and (NH 4 ) 2 CO 3 were employed as reagents by Hong et al. 39 in their method of synthesizing ZnO NPs. XRD and TEM tests revealed particle sizes of 40 and 30 nm. Heterogeneous azeotropic distillation thoroughly prevents agglomeration and reduces the size of ZnO NPs.

In the precipitation method of synthesizing nanopowders, it is more or less a ritual these days to use surfactants that would enable control over the growth of particles with the simultaneous prevention of coagulation and flocculation of particles thereby preventing an appreciable reduction in the final yield. The surfactants act as chelates encapsulating the metal ions in an aqueous medium. Wang et al. 41 used ZnCl 2 and NH 4 OH and a cationic surfactant, CTAB (cetyltrimethyl-ammonium bromide), for the generation of ZnO NPs. The formation of sharply crystalline ZnO NPs with a wurtzite structure and crystallite size of 40.4 nm was confirmed by XRD data, while TEM examination of the powder bore out the formation of spherical nanoparticles of size 50 nm.

2.3 Sol–gel method

Suwanboon et al. 43 using Zn(CH 3 COO) 2 ·2H 2 O, polyvinyl pyrrolidone (PVP) and NaOH prepared nano-structured ZnO crystallites via the sol–gel method. The XRD characterization revealed a wurtzite structure having an average crystallite size of about 45 nm. The role of PVP at its different concentrations on the morphology was checked. There occurred a shift from a platelet-like to a rod shape with an increase in PVP concentration. TEM images bore out the grain size of platelet-like ZnO to be 150 nm while the diameter of the rod-shaped ZnO was likewise determined to be 100 nm. In another sol–gel method-based synthesis by Benhebal et al. 44 zinc acetate dihydrate and oxalic acid were used to generate ZnO nanopowder with ethanol as a solvent which showed a hexagonal wurtzite structure. The crystallite size obtained from the Scherrer equation was found to be 20 nm. The SEM micrograph confirmed the formation of uniform, spherically shaped ZnO nanoparticles. BET analysis revealed a surface area of 10 m 2 g −1 . This was characteristic of a material with low porosity, or a crystallized material.

Sharma 45 obtained ZnO NPs with outstanding antibacterial properties using the sol–gel method. Zinc acetate, oxalic acid and water were employed as raw materials in this process. A white gel precipitate was first obtained. It was then thermally treated at 87 °C for 5 h, and then at 600 °C for 2 h. The ZnO NPs exhibited high crystallinity as borne out by XRD data. A diameter of 2 μm was obtained for the ZnO nano-aggregates from SEM analysis.

In a study conducted by Ristic et al. 46 nano-structured ZnO crystallites were obtained using the sol–gel route. From XRD examination and using the Scherrer formula, the average value of the basal diameter of the cylinder-shaped crystallites was found to be 25–30 nm, while the height of the crystallites was 35–45 nm. The sol–gel method presents a host of advantages in comparison with the previously mentioned methods. Prime amongst its merits are the low cost of the apparatus and raw materials, reproducibility and flexibility of generating nanoparticles. 47

2.4 Vapour transport method

Zn + H O → ZnO + H

2Zn + O → 2ZnO

In water vapour, ZnO nanoflowers were synthesized. The nanoflowers were constructed from tens of ZnO nanosheets with random orientations. In oxygen gas, ZnO hexagonal nanorods were obtained. The size of the nanorods was not uniform. It was argued that the size of the Au catalyst underneath might have influenced the size of the ZnO nanorods. Both the samples, however, exhibited a hexagonal wurtzite structure. Though the samples showed different morphologies and crystal structures, surprisingly, they had almost the same optical properties. The PL spectra revealed only one UV peak close to 389 nm wavelength for both samples, indicating the high quality of the synthesized ZnO samples.

Novel one-dimensional single-crystalline ZnO nanorod and nanoneedle arrays on a Cu catalyst layer-coated glass substrate were investigated by Alsultany et al. 50 via a simple physical vapour deposition method by thermal evaporation of Zn powder in the presence of O 2 gas. The ZnO nanorods and nanoneedles were synthesized along the c -axis growth direction of the hexagonal crystal structure. The diameter and growth rate of the high-quality and well oriented one-dimensional ZnO nanostructures were achieved as a function of varying growth temperature and growth time. At 450 °C, ZnO nanorods were uniformly distributed at a high density on the entire substrate surface and quasi-aligned, and small average diameters were obtained. The diameters and lengths of the obtained nanorods were in the range of 19–27 nm and 2.8 μm, respectively. When the temperature was increased to 550 °C, ZnO nanorods grew perpendicular to the substrate, uniformly throughout their length, and with more consistent shape and dimensions, with approximately 85 nm width and 3.8 μm length. The morphological change and distribution occurred at a growth temperature of 650 °C, and ZnO nanorods with a hexagonal shape at the tips of rods of hexagonal hierarchical structures were formed. These rods possessed a typical hierarchical structure with lengths and diameters of approximately 190–350 nm and 3.9 μm, respectively, whereas short nanorods with a diameter of 95 nm and length of 900 nm were observed on the tip of each rod of hexagonal hierarchical structures. As Cu metal catalysts were used in the study, the growth mechanism of 1D ZnO nanostructures presented therein followed the VLS method. This method could be divided into three stages, as follows: first, the Zn vapor and catalytic Cu formed liquid alloy droplets during the heating process at a certain temperature, representing the initial stage of the nucleation process. Second, crystal nucleation occurred upon gaseous species adsorption until supersaturation was reached, and the formed sites served as nucleation sites on the substrate. Finally, the axial growth of the nanorods began from these sites. Based on this study of the mechanism in the presence of Cu metal catalysts at different growth temperatures and according to the nucleation theory of the VLS growth mechanism, the Cu catalyst nanoclusters formed because of capillarity, which caused beading of the Cu layer at high growth temperature. Consequently, the Cu–Zn alloy process reached a certain solubility depending on the temperature; then, the Zn vapor began to precipitate out at the interface between the surface and droplet. That in turn determined the diameter and size of the nanostructures depending on the size of the liquid alloy droplets. Notably, large-scale ZnO nanorods with a lower diameter were formed at a low growth temperature of 450 °C. The Zn metal powder (melting point of 419 °C) vapor pressure at 450 °C was sufficiently high to investigate the growth of ZnO nanorods on the glass substrate via the VLS method, and the decrease in Zn vapor as a result of the decrease in the growth temperature led to a low lateral growth rate compared with the axial growth rate of the 1D nanostructure. In contrast, the higher growth temperature could also lead to the formation of hierarchical nanostructures. In addition, at high growth temperature along with the consumption of the Zn vapor during growth, the diameter of the nanorods markedly decreased. This condition consequently caused the production of rods with a typical hierarchical structure. At a growth time of 30 min, ZnO nanorods were obtained with a diameter of 19–27 nm and a length of 2.8 μm. When the growth time increased to 45 min, nanoneedles were obtained. The needles exhibited mean diameters of 65–190 nm and length of 3.2 μm. On the other hand, nanoneedles grown at 60 min were approximately 80–250 nm in diameter and 3.8 μm in length.

Diep and Armani 51 designed a flexible light-emitting nanocomposite based on ZnO nanotetrapods (NTPs) which they prepared using a vapour transport technique. The CVT synthesis of the ZnO NTPs was self-catalyzed. In the TEM images, the lattice fringes were clearly visible, indicating the single-crystalline nature of the nanostructures. The lattice spacing was found to be 2.6 Å, indicating growth in the [0001] direction. X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) analysis were also performed to confirm the crystal structure and elemental composition of the NTPs. Based on an analysis of the TEM and SEM images, the ZnO NTP arm lengths ranged from 0.5 μm to 3.5 μm and the diameters varied from 120 nm to 350 nm.

Luo et al. 52 also constructed ZnO tetrapods as potential electrode materials for low-cost and effective electrochemical supercapacitors using an oxidative-metal-vapor-transport method. The SEM images of the ZnO tetrapods collected at different temperatures showed that the products obtained were pure and uniform, and the tetrapods consist of four arms branching from one center, and the angles between the arms were nearly the same, analogous to the spatial structure of the methane molecule. As for the size variation with collected temperatures, it transpired that smaller size tetrapods were obtained with lower evaporation temperature. This demonstrated the power of the technique for controlling the size of the tetrapods. ZnO tetrapods with arms as thin as about 170 nm and shorter than 4000 nm were revealed by SEM analysis. The XRD pattern of the ZnO tetrapods showed that all the diffraction peaks could be indexed to a wurtzite 5 structure with lattice constants of a = 0.324 nm and c = 0.519 nm. The TEM and high resolution TEM (HRTEM) images of the ZnO tetrapods revealed that the arm diameter and length of the tetrapods are, on average, about 22 nm and 90 nm, respectively. The HRTEM image of a single arm revealed clear fringes perpendicular to the arm axis and these fringes were spaced by about 0.25 nm consistent with the interplanar spacing of (0002) suggesting that the nanowire growth direction was along [0001].

2.5 Hydrothermal method

Aneesh et al. 54 carried out an experiment in which they used Zn(CH 3 COO) 2 ·2H 2 O, NaOH and methanol as reagents. The ZnO NPs thus formed had a hexagonal wurtzite structure. XRD analysis demonstrated an enhancement in average grain size with rising temperature and concentration of the substrates. The average grain size of ZnO NPs prepared from 0.3 M NaOH employing a growth time of 6 h was found to increase from 7 to 16 nm with temperature rise from 100 to 200 °C. The average grain size of ZnO synthesized at 200 °C for 12 h revealed an increase from 12 to 24 nm with elevation in concentration of NaOH from 0.2 M to 0.5 M.

This process has many advantages over other methods. Organic solvents do not find use in this process. This coupled with the omission of supplementary processes like grinding and calcination within the ambit of the method endows it with the much sought after eco-friendly character. Low operating temperatures, the diversified morphologies and sizes of the resulting nano-crystals depending on the composition of the starting mixture and the process temperature and pressure, the greatly pronounced crystallinity of the nanoparticles and their high purity are factors that surely make the process more advantageous than others. 54,55

2.6 Solvothermal method

Chen et al. 57 also used a solvothermal route to generate ZnO NPs. They eventually prepared nano-structured ZnO crystals that were devoid of hydroxyl groups. They carried out a reaction of zinc powder with trimethylamine N -oxide (Me 3 N→O) and 4-picoline N -oxide (4-pic→NO). The medium for the reaction was a mixture of organic solvents (toluene, ethylenediamine (EDA) and N , N , N ′, N ′-tetramethylenediamine (TMEDA)) contained in an autoclave which was kept at 180 °C. It was observed that the size and morphology of the ZnO nanoparticles/nanowires were greatly influenced by the oxidants used and the ligating capacities of the solvents. The ramifications of the presence of water in the system were additionally investigated. It emerged that the presence of traces of water catalyzed the zinc/4-picN→O reaction and exerted an effect on the size of the nano-structured ZnO crystallites thus obtained. Depending on the reaction conditions, the ZnO nanostructures had diameters ranging in between 24 and 185 nm. The solvothermal synthesis method has many advantages. Foremost among them is the fact that reactions can be carried out under determined conditions. As a result, nano-structured ZnO with a range of architectures can be generated by exercising due control over the reaction conditions.

2.7 Method using an emulsion or microemulsion environment

Zn(C H COO) (decane) + 2 NaOH → ZnO (water and ethanol) + H O + 2NaC H COO

SEM and XRD analysis showed that the particle size and phase location were both dependent upon the conditions (ratio of two-phase components, substrates and temperature) employed for the accomplishment of the process. Depending on the process conditions, ZnO NPs with different particle morphologies were obtained. The morphologies that formed during the process included spherical agglomerates, needle shapes, near-hexagonal shapes, near-spherical shapes and irregular agglomerates. These NPs further had a wide range of diameters. Some had diameters ranging in between 2 and 10 μm, while the diameters of others ranged from 90 to 600 nm, some others had diameters in between 100 and 230 nm and yet others were characterized by diameters hovering around 150 nm.

Kołodziejczak-Radzimska et al. 59 used zinc acetate and KOH or NaOH in an emulsion system. For the generation of an emulsion, cyclohexane was utilized. Cyclohexane was held to have furnished a ready organic phase, and also essayed the role of a surfactant that wasn't ionic. In this method for emulsion formation cyclohexane was used as an organic phase, and nonylphenyl polyoxyethylene glycol ethers NP3 and NP6 were used as a mixture of emulsifiers. By tailoring the ZnO precipitation process by way of altering the precipitating agent, substrate ingredients and the tempo of substrate dosing, an amazing variety of ZnO nanostructures were designed. Four samples were obtained, labelled Z1, Z2, Z3, and Z4, composed of particles of different shapes. Morphologies such as solids (Z1), ellipsoids (Z2), rods (Z3) and flakes (Z4) with modal diameters of ∼396 nm, ∼396 nm, ∼1110 nm and ∼615 nm were obtained. They were further characterized by their considerable surface areas. Values of 8 m 2 g −1 , 10.6 m 2 g −1 , 12 m 2 g −1 and 23 m 2 g −1 could be respectively assigned to samples Z1, Z2, Z3, and Z4.

If a surfactant possessing balanced hydrophilic and lipophilic properties is used in the right proportion, a different oil and water system will be produced. The system remains an emulsion, but exhibits some characteristics that are different from emulsions. These new systems are “microemulsions”. The drop size in a microemulsion is significantly smaller than in an emulsion, and lies in the range 0.0015–0.15 μm. 60,61 In contrast to emulsions, microemulsions form spontaneously under appropriate conditions. This synthesis method does not require any complex preparation procedure, sophisticated equipment or rigorous experimental conditions, but still provides possibilities in controlling the size and morphology of the ZnO powders in a size scale approaching nanometers. Even though the product yield is low, the narrow size distribution due to well-dispersed cage-like small reactors (5–100 nm) formed under uniform nucleation conditions is the superior aspect of the ZnO nanoparticles obtained by microemulsion routes. Such low-dimensional uniform ZnO nanostructures offering size and morphology dependent tunable electrical and optical properties are of particular technological interest for applications such as quantum dots, UV-emission optoelectronic and lasing devices, and transparent conducting thin films.

Yildirim and Durucan 63 also synthesized ZnO NPs through the use of microemulsions. They made an endeavour to reshape the microemulsion modus operandi with an eye to generate monodisperse ZnO nanostructures. They subjected the zinc complex precipitate obtained in the course of the microemulsion method to thermal decomposition. Subsequent calcination was adopted. The use of glycerol as the internal phase of a reverse microemulsion imparted the intended modification. The synthesized ZnO NPs had spherical shapes. They were monodisperse and their diameter measured in between 15 and 24 nm.

All the procedures involving chemical synthesis of ZnO NPs generate a few toxic chemicals and their adsorption on the surface increases the likelihood of harmful effects being wielded in medical applications. Further, these approaches include reactions requiring high temperature and intense pressure for their commencement while some reactions require operations in an inert atmosphere or under inert conditions. Toxic materials such as metallic precursors, toxic templates and capping agents and even H 2 S find application in quite a few chemical routes. 64 Very often toxic substances are employed for the generation of nano-structured particles and for their stabilization as well. This in turn produces secondary products and residues that are detrimental to the ecosystem. 65,66

3. Green methods for the synthesis of ZnO nanoparticles

Source	Synthesis conditions	Experimental variables	Shape/morphology	Mechanism and applications	Size
Carom-Trachyspermum ammi seed extract	2 mL of the extract was slowly added dropwise to a 25 mL solution of 0.05 M ZnNO . Magnetic stirring for 2 h at 50 °C. Centrifugation and drying at room temperature at 35 °C	—	Uniform hexagonal plates, irregular and highly aggregated nanoparticles with a rough surface	Anti-bacterial activities on both Gram-positive (Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa) bacteria	∼41 nm
Nyctanthes arbor-tristis flower extract	0.01 M solution of zinc acetate and flower extract were added at a pH of 12 and the solution was stirred for 2 h. A white precipitate was obtained and dried at 60 °C overnight	Concentration of zinc acetate, pH, and temperature	Aggregate of nanoparticles	Nanoparticles were tested for their antifungal potential and were found to be active against all five tested phytopathogens with the lowest MIC value recorded being 16 μg mL	∼12–32 nm
Ulva lactuca seaweed extract	U. lactuca extract was added into 1 mM zinc acetate and kept under magnetic stirring at 70 °C for 3–4 h. The mixture was centrifuged at 4000 rpm for 10 min and the solid product was collected and heated at 450 °C for 4 h	—	Agglomeration of asymmetrically shaped nanoparticles	Excellent photocatalytic activity on methylene blue. High antibiofilm activity on 4 species of Gram-positive and -negative bacteria	∼15 nm
Muraya koenigii seed extract	20 mL of Murraya koenigii seed extract was mixed with 80 mL of zinc nitrate (ZnNO ) and 2.0 M NaOH solution was added with vigorous stirring for 3–5 h, and incubated overnight at room temperature. Zinc oxide nanoparticles (white precipitate) were washed with distilled water and ethanol and dried at room temperature	—	Spherical, triangle, radial, hexagonal, rod and rectangle shaped	ZnO nanoparticles used for antimicrobial activities using human pathogenic bacterial and fungal species	∼100 nm
Calotropis procera leaf extract	Leaf extract was added to 50 mL distilled water and heated up to 70 °C and 6 g of Zn(NO ) ·6H O was added and evaporated. Calcined at 400 °C for 3 h	—	Spherical	When Zn(NO ) ·6H O is mixed with C. procera leaf extract, the Zn ions dispense consistently and form a complex with active sites of hydroxyl groups. Polyphenolic molecules that interact with divalent Zn cations forming a bridge between two hydroxyl groups from two different chains come from the polyphenolic groups in close contact with Zn . The divalent cations keep the molecules together and form various structures of zinc complex. Photocatalytic degradation of methyl orange with an efficiency of 81% within 100 min under UV light	∼15–25 nm
Artocarpus heterophyllus leaf extract	5 g of zinc nitrate hexahydrate was added to 150 mL leaf extract and heated at 80 °C and calcined in a muffle furnace at 400, 600 and 800 °C for 1 h	Calcination temperature	Spherical	Photo-degradation of Congo red dye	∼10–15 nm at 400 °C, ∼15–25 nm at 600 °C and ∼25–30 at 800 °C
Moringa oleifera	2.97 g of zinc nitrate hexahydrate was dissolved in Moringa oleifera natural extract and heated on a hot plate with a stirrer to form a gel kind of product and kept in a muffle furnace maintained at 400 °C	Concentration of Moringa oleifera natural extract and time	Clusters of spherical nanoparticles	ZnO nanoparticles with smaller size show better H evolution rates up to 360 μmol h g . It is noteworthy that ZnO nanoparticles prepared via novel green synthesis exhibit oxygen vacancies and register enhanced photocatalytic activity as well as good photostability	100–200 nm
Carica papaya leaf extract	To zinc acetate dihydrate (5 mmol) papaya leaf extract was added and the mixture heated at a temperature of 60 °C for 2 h under stirring at a pH of 8. Finally, it was washed with a water and ethanol mixture and dried at 80 °C for 12 h	—	Spherical	Photocatalyst for methylene blue dye degradation (complete degradation within 180 min in the presence of UV) and photo-anode with an energy conversion efficiency of 1.6% with a current density of 8.1 mA cm in dye sensitized solar cells	∼50 nm
Nephelium lappaceum L. fruit peel extract	A volume of 50 mL was prepared and then 10 mL of rambutan peel extract was added to 0.1 M Zn(NO ) ·6H O with heating at a temperature of 80 °C for 2 h and then incubated at room temperature for 1 day to form zinc-ellagate and dried in an oven at 40 °C for 8 h. ZnO nanoparticles were obtained on direct decomposition of the zinc-ellagate complexes in a muffle furnace at 450 °C	—	Multidimensional chain-like structures in which spherical nanoparticles were intertwined with each other	—	∼20–50 nm
Moringa oleifera leaf extract	50 mL of Moringa oleifera extract was added to Zn(NO ) ·6H O at room temperature with a pH of 5 and subjected to heat treatment in air at 500 °C for 1 h	Concentration of zinc salt and calcination temperature	Drying at 100 °C: agglomerates of spherical particles; annealing at 500 °C: nanorods in addition to the clusters of spherical nanostructures	Three chemical reactions of the solvated Zn ions are considered with the phytochemicals of Moringa oleifera, i.e. with a phenolic acid, a flavonoid and vitamin based compounds. An altered chemical behavior of L-ascorbic acid and zinc nitrate, probable oxidation of biological compound i.e.L-ascorbic acid to L-dehydroascorbic acid via free radicals, followed by electrostatic attraction between the free radical and cation of the precursors. Electrochemical investigations by cyclic and square wave voltammetry	∼12.27–30.51 nm
Catharanthus roseus leaf extract	An aqueous leaf extract of C. roseus was added to 0.025 M aqueous zinc acetate and pH adjusted to 12 and the solution was dried in a vacuum	—	Spherical	Antibacterial activity was evaluated. Among the four bacterial species tested, Pseudomonas aeruginosa is more susceptible when compared with the other three species and may be used for the preparation of antibacterial formulations against Pseudomonas aeruginosa	∼23–57 nm
Camellia sinensis leaf extract	ZnO NPs using the aqueous extract of green tea leaves. In the prepared extract zinc acetate was dissolved by way of magnetic stirring. Intense stirring was eventually applied on this solution for 5–6 h; a temperature of about 150 °C was maintained during this time. The solid mass thus obtained subsequently underwent a 4500 rpm centrifugation for 15 min; this act was repeated again. Finally washing and drying at 80 °C for 7 to 8 h yielded agglomerates of irregularly shaped ZnO NPs. UV spectroscopy analysis showed maximum absorption at about 330 nm. The size of the particles was determined using a particles size analyzer. The average diameter of the particles was found to be 853 nm	—	Agglomerates of irregularly shaped nanoparticles	These nano-sized ZnO demonstrated remarkable antimicrobial properties against Gram-positive and Gram-negative bacteria as well as against a fungal strain	∼853 nm
Citrus aurantifolia fruit extract	50 mL of aqueous Citrus aurantifolia extract was boiled to 60–80 °C. It was followed by the addition of a specific amount (5 g) of Zn(NO ) to the solution as its temperature rose to 60 °C. The reaction mixture so prepared was then boiled until a deep yellow coloured paste was left. This paste was then collected and heated in a furnace in the presence of air at 400 °C for 2 h to eventually yield a powder. This powder bearing a faint white colour was further ground in a mortar-pestle. The synthesized nanoparticles were characterized by moderate stability. They had near spherical shapes with the most probable particle-size in the range of 9–10 nm	—	Near spherical shaped nanoparticles	—	∼9–10 nm
Oryza sativa rice extract	ZnO NPs were prepared by the hydrothermal method. The method involved the use of zinc acetate, sodium hydroxide, and uncooked rice flour at several ratios at 120 °C for 18 h. The rice bio-template was found to exert considerable influences upon the size and morphology of ZnO NPs	—	Flake-, flower-, star-, toothed-edge flake-like, rose- and rod-like structures for 0.25 g, 0.50 g, 1.0 g, 2.0 g, 4.0 g and 8.0 g uncooked rice, respectively	—	∼200–800 nm, ∼800–2000 nm, ∼200–1000 nm, ∼250–700 nm, ∼200–700 nm, ∼150–700 nm and ∼40–100 nm for 0.25 g, 0.50 g, 1.0 g, 2.0 g, 4.0 g and 8.0 g uncooked rice, respectively
Passiflora caerulea. L. leaf extract	The leaf extract was prepared by maintaining a temperature of 70 °C for 8 min. 50 mL of aqueous 1 mM zinc acetate [Zn(O CCH ) ·(H O) ] was prepared and subjected to stirring for 1 h. Subsequently, to this solution, a 20 mL of NaOH solution was slowly added. This was followed by a slow addition of 25 mL of plant extract. As a consequence, the color of the reaction mixture was found to change after incubation for an hour. This solution was again subjected to stirring for 3 h. The subsequent appearance of a yellow color confirmed the generation of ZnO NPs. The precipitate so obtained was centrifuged at 8000 rpm at 60 °C for 15 min. Thereafter, the pellets that resulted were dried in a hot air oven at a temperature of 80 °C for 2 h	—	Spherical	—	∼30–50 nm
Sucrose (as a capping agent)	Zinc acetate (Zn(CH COO) ·2H O) and sucrose (C H O ) served as the precursor and capping agent, respectively. The precursor was prepared by dissolving 4.3900 g of zinc acetate in 50 mL of double distilled water and stirring for 30 min at 60 °C. During the process, 3.4229 g of sucrose solution was slowly added. The resultant solution was stirred for 2 h at the same temperature. The solution was then bone dried at 80 °C and was calcined in an atmosphere of air at 400 °C for an hour. The end product was finely ground using an agate mortar to obtain the required ZnO/C nanocomposite. Similarly, without sucrose we synthesized pure ZnO nanoparticles	—	Granular	Carbon coated ZnO nanoparticles are used for symmetric supercapacitor device fabrication. The symmetric device yields a specific cell capacitance of 92 F g at a specific current of 2.5 A g	∼10–100 nm
Whey (as a chelating agent)	Firstly, zinc citrate was obtained by mixing Zn(NO ) ·6H O with citric acid (CA), previously dissolved in distilled water (0.1 g mL ), at a molar ratio of 1	Calcination temperature	Spherical	—	With an increase in calcination temperature from 400 to 1000 °C, the size of nanoparticles increased from 18.3 to 88.6 nm
Citrus sinensis fruit peel extract	An aqueous extract of orange peel was used as the biological reducing agent for the synthesis of ZnO NPs from zinc acetate dihydrate. The ZnO NPs were synthesized by mixing 2 g of zinc nitrate with 42.5 mL of the extracts. These mixtures were then stirred for 60 minutes and then placed in a water bath at 60 °C for 60 minutes. Subsequently, the mixtures were dried at 150 °C and then heat-treated at 400 °C for 1 hour	Annealing temperature and synthesis pH	Spherical	Ligation takes place between the functional components of the orange peel and the zinc precursor. The organic substances (flavonoids, limonoids, and carotenoids) in orange peel extract act as ligand agents. These hydroxyl aromatic ring groups, one of the extract components, form complex ligands with zinc ions. Through the process of nucleation and shaping, nanoparticles are stabilized and formed. The mixture of the organic solution is then decomposed directly upon calcination at 400 °C resulting in the release of ZnO nanoparticles. Antibacterial activities toward E. coli and S. aureus: without UV light, the bactericidal rate towards E. coli was over 99.9%, while the bactericidal rate towards S. aureus varied in the relatively wide range of 89–98%	400 °C, 700 °C and 900 °C: 35–60 nm, 70–100 nm and 200–230 nm, respectively. pH values of 6.0 and 8.0: 10–20 nm and 400 nm. pH value of 10.0: Agglomerates of blocks with lengths of ∼370 nm and widths of ∼160 nm

An extract prepared from Ajwain ( Carom-Trachyspermum ammi ) seeds has also been used to synthesize ZnO NPs. 70 The work boasts of its operation under ambient temperature conditions. The ZnO NPs were found to have a wurtzite structure. The synthesized ZnO nanostructures were morphologically characterized by FE-SEM images. The ZnO nanostructure showed uniform hexagonal plates, as well as irregular and highly aggregated nanoparticles with a rough surface. The average diameter of the nano-sized ZnO clusters has been observed to be ∼41 nm. XRD results showed an increase in interplanar spacing with an increase in the extract volume from 0.2474 nm to 0.2765 nm with a simultaneous decrease in crystallite size from 39.51 nm to 28.112 nm. The band gap also fell from 3.592 eV to 3.383 eV as the amount of extract increased. Phytoconstituents in the extract thus evidently played a key role of reductants and furthermore acted as capping agents in the generation and stabilization of ZnO NPs.

Jamdagni et al. 72 used an aqueous flower extract of Nyctanthes arbortristis for making ZnO NPs. The starting materials consisted of zinc acetate dihydrate and sodium hydroxide. XRD results showed an average crystallite size of 16.58 nm while TEM analysis revealed that the individual particle size ranged within 12–32 nm and the nanoparticles were obtained in the form of aggregates. In a very recent study, 73 Ulva lactuca seaweed extract was used to prepare ZnO nanoparticles. XRD analysis revealed strong characteristic peaks of ZnO suggesting high crystallinity of the synthesized material. Further, the average crystallite size thus calculated was found to range in between 5 and 15 nm. TEM micrographs revealed an agglomeration of asymmetrically shaped NPs bearing an average crystallite size of 15 nm.

Muraya koenigii seed extract was also recently reported to have been used as a stabilizer as well as a reductant in the preparation of ZnO NPs. 74 Sharp diffraction peaks in XRD results indicated remarkable crystallinity of the NPs whose average crystallite size was calculated to be 70–100 nm. Both SEM and TEM micrographs revealed nanoparticles with an average size of about 100 nm and bearing a wide range of morphologies – spherical, triangular, radial, hexagonal, rod-like and rectangle-shaped.

One recent experiment used Calotropis procera leaf extract and Zn(NO 3 ) 2 ·6H 2 O to synthesize ZnO NPs. 75 An XRD test confirmed a hexagonal wurtzite structure of the nanoparticles with marked crystallinity. The average crystallite size was calculated using the Scherrer equation and found to be 24 nm. Diffuse Reflectance Spectroscopy (DRS) revealed a band gap of 3.1 eV for the synthesized nanoparticles. In the FT-IR analysis of the synthesized ZnO NPs, a peak attributed to the metal–oxygen bond of ZnO appeared in between 500 and 700 cm −1 . Further, a conspicuous shift and broadening of peaks corresponding to functional groups like hydroxyl, aldehyde, amine, ketone, and carboxylic acid suggests their participation in the stabilization of ZnO by the extract. Surface attachment of groups like aldehyde, amine, phenol and terpenoid enhances stabilization additionally allowing the extract to function as a bio-template thereby preventing aggregation of ZnO NPs. TEM images revealed an average particle size of 15–25 nm, while SAED and HR-TEM further confirmed the high crystallinity of the material prepared.

The effects of Artocarpus heterophyllus leaf extract and varying temperatures on the morphology and properties of the ZnO NPs thus prepared were studied by Vidya C. et al. 76 XRD results show an increase in crystallinity and average crystallite size with temperature, the diffraction peaks being increasingly sharper and narrower with temperature. The particles were all spherical and a grain size of 50 nm was obtained from SEM images. SEM analysis also shows similar trends of size and morphology upon temperature variation. TEM analysis revealed a particle size of ∼10–15 nm at 400 °C, ∼15–25 nm at 600 °C and ∼25–30 nm at 800 °C. This further corroborated the results of XRD and SEM tests. Diffuse Reflectance Spectroscopy (DRS) showed a decrease in the calculated band gaps with increasing calcination temperatures.

Archana et al. 77 used Moringa oleifera natural extract and Zn(NO 3 ) 2 ·6H 2 O for the preparation of ZnO NPs. They took different volumes of the extract, viz. 2, 6, 10 and 14 mL, to prepare ZnO NPs which were accordingly labeled ZnO-2, ZnO-6, ZnO-10 and ZnO-14. The PXRD results of all the samples showed great crystallinity. They had a hexagonal wurtzite structure. And the average crystallite size was found to be 21.6 nm. Field Emission Scanning Electron Microscopy (FE-SEM) analysis showed highly crystalline ZnO-10 and ZnO-14 having a spherical shape and average crystallite size of 20–150 nm. HR-TEM micrographs revealed d -spacing of 0.28 and 0.19 nm for the (001) and (101) planes of wurtzite ZnO. The band gaps calculated using the results from Diffuse Reflectance Spectroscopy (DRS) had values of 2.92 eV for ZnO-2, 3.05 eV for ZnO-6, 3.12 eV for ZnO-10 and 3.10 eV for ZnO-14. The increase in band gap with the amount of fuel was attributed to quantum size effects.

In their research work, Rajeswari Rathnasamy et al. 78 used papaya leaf extract for the synthesis of ZnO NPs. Both FESEM and TEM data revealed an average size of ∼50 nm for the individual nanoparticles. The extract of Nephelium lappaceum L. (rambutan) peels (a natural ligation agent) was put into use for the preparation of ZnO NPs in another investigation. 79 The bio-mediated ZnO NPs were found to be spherical in shape. They were characterized by diameters between 20 and 50 nm. Some of the particles were found in agglomerated form. After a day, multi-dimensional chain-like structures formed. In these chains spherical nanoparticles were found intertwined to each other.

An investigation conducted by Matinise et al. 80 used Moringa oleifera extract as a remarkably operative chelating agent to prepare ZnO nanoparticles. The ZnO NPs eventually obtained were characterized by a particle size in between 12.27 and 30.51 nm. The sample obtained just after drying at 100 °C consisted of agglomerates of spherical particles while that obtained after annealing at 500 °C also had nanorods in addition to the clusters of spherical nanostructures.

The biocomponents of leaves of Catharanthus roseus have also been utilized to prepare ZnO NPs with zinc acetate and sodium hydroxide as reagents. 81 SEM micrographs revealed that in addition to the individual ZnO-NPs, aggregates were also formed and they were spherical with diameter ranging from 23 to 57 nm. Sharp and clear XRD peaks confirmed high purity and excellent crystallinity. Shah et al. 82 generated ZnO NPs using the aqueous extract of green tea ( Camellia sinensis ) leaves. The size of the particles was determined using a particle size analyzer. The average diameter of the particles was found to be 853 nm. These nano-sized ZnO particles demonstrated remarkable antimicrobial properties against Gram-positive and Gram-negative bacteria as well as against a fungal strain.

In another experiment, 50 mL of aqueous Citrus aurantifolia extract was boiled to 60–80 °C. 83 It was followed by the addition of a specific amount (5 g) of Zn(NO 3 ) 2 to the solution as its temperature rose to 60 °C. The synthesized nanoparticles were characterized by moderate stability. They had near-spherical shapes with the most probable particle size in the range of 9–10 nm. The extract of Oryza sativa rice 84 was also used to generate ZnO NPs. The extract has been considered a renewable bio-resource. Its abundance adds to its list of merits. The extract has also been cited as a source of bio-template that typically assists the generation of a variety of multifunctional nano-structured materials. ZnO NPs were prepared using the hydrothermal method. The method involved the use of zinc acetate, sodium hydroxide, and uncooked rice flour at several ratios at 120 °C for 18 h. The rice bio-template was found to exert considerable influences upon the size and morphology of ZnO NPs. Fig. 2 shows field emission scanning electron microscopy (FESEM) images of the samples synthesized at different concentrations of uncooked rice (UR). To investigate the effects of raw rice on the resulting ZnO morphology, FESEM was conducted on ZnO synthesized without UR ( Fig. 2a and b ). As seen in Fig. 2c and d , the ZnO structures were mostly flake-like structures assembling together. They were much more ordered in contrast to the one synthesized without UR (as a control) ( Fig. 2a and b ). The diameter of ZnO flakes dramatically decreased after adding 0.25 g UR. This was proposed to have occurred due to the inhibition of lateral growth of ZnO crystals. It was further proposed that the accessibility of the zinc ions to the ZnO crystal seeds was controlled by a bio-template. However, the size of particles seemed to increase when the synthesis was done using 0.25 g UR. Different morphologies of the as-synthesized ZnO were observed with increasing the amount of uncooked rice to 0.5 g. Particles with a very small flower-like shape could be observed ( Fig. 2e and f ). A lower magnification FESEM image indicated that the mentioned structure showed denticulated petals aggregated and form larger flowers of particles. Notably the size of the ZnO particles had been obviously decreased for the sample prepared using 0.5 g UR. In addition, the tooth-like flakes were more dominant for the ZnO sample prepared using 0.5 g UR compared to the one synthesized using 0.25 g UR. Fig. 1g and h indicate the FESEM images of the ZnO sample synthesized using 1 g UR. A very unique star-like structure could be clearly observed at low to high magnification. The star-like structure contained small flakes with denticulated edges which attach to other similar flakes in the center. A closer look showed that the lateral flake acted as a substrate for other flakes to grow on the surface and form a star-like structure. It was therefore argued that the branched pattern for soft templates of starch revealed that the semicrystalline granules of starch were made from concentric rings in which the amylose and amylopectin basic components were aligned perpendicularly to the growth rings and to the granule surface. Fig. 2g and h show that the size of the star-like ZnO particles decreased in comparison with the previous lower amount of uncooked rice. In the case of ZnO crystals synthesized at 2 g UR, increasing the amount of bio-template resulted in different morphologies of ZnO particles being produced. It formed lots of agglomerated toothed-edge flakes which became a secondary unit for larger particles. The star-like shape of the particles could be perceived in some areas but aggregation seemed to be dominant and prevented clearer observation of the particles as they really are. Fig. 2k and l show the FESEM images of the as-synthesized ZnO particles synthesized using 4 g UR. The ZnO morphology changed to flower-like structures, mostly rose-like shapes. A detailed view of the flower-like particles revealed that their flakes had the largest diameter compared to other samples. In the case of ZnO synthesized using 8 g UR, a new morphology, different from other and control samples, was observed. The ZnO crystals appear mostly as rods with around 100 nm size. Moreover, agglomerated without any specific shape, particles coexisted with nanorods in the structure of ZnO synthesized using 8 g UR. Fig. 3 shows the particle size distribution of the ZnO samples synthesized using 0.25, 0.5, 1, 2, 4, and 8 g UR. The particle size distribution of ZnO synthesized without rice is also given for comparison. As shown in Fig. 3 , the range of particle size for ZnO synthesized without UR lies between 200 and 800 nm. When 0.25 g UR was used in the synthesis, the size of particles increased dramatically to 800–2000 nm. Notably the size of ZnO synthesized using 0.5 g UR considerably decreased to the 200–1000 nm range. The decreasing trend continued for the sample synthesized using 1 g UR and with a size range of 250–700 nm. Although this distribution was quite similar to that of ZnO synthesized without a bio-template, it was slightly narrower. On the basis of the particle size distribution for the samples synthesized using 2 and 4 g UR, it could be clearly observed that the size of particles decreased to 200–700 nm and 150–700 nm, respectively. In the case of the ZnO sample synthesized using 8 g UR, the size of particles was within the nano regime, between 40 and 100 nm. As mentioned in the growth mechanism suggested by the study, adding a bio-template, which presumably acts as a flocculant, forces aggregation. Therefore, the surface-active sites of the template might influence the size and state of aggregation during the particle growth process and ultimately the resulting ZnO particle size distribution. Another procedure used the aqueous leaf extract of Passiflora caerulea. L. (Passifloraceae). 85 The SEM analysis revealed that the ZnO NPs had diameters ranging in between 30 and 50 nm.


	FESEM images of ZnO prepared using different concentrations of uncooked rice (g): 0 (a and b), 0.25 (c and d), 0.5 (e and f), 1 (g and h), 2 (i and j), 4 (k and l), and 8 (m and n) (reproduced from ref. with permission from Springer).


	Particle size distribution of ZnO samples synthesized using various concentrations of UR (g); 0, 0.25, 0.5, 1, 2, 4, and 8 (w/w%) (reproduced from ref. with permission from Springer).

Sucrose was used in a study as the capping agent to synthesize a ZnO/C nanocomposite adapting the sol–gel method. 86 The presence of carbon in the prepared ZnO/C was confirmed through EDAX. SEM images of the ZnO/C samples indicate a wide distribution of particles ranging from 10 to 100 nm and exhibit only an irregular granular feature. This kind of surface morphology was argued to be more suitable for supercapacitor electrode materials. Electrochemical investigations of the ZnO/C electrode were carried out using cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. The ZnO/C electrode exhibits a maximum specific capacitance of 820 F g −1 at a constant specific current of 1 A g −1 . The symmetric aqueous supercapacitor device exhibits a specific cell capacitance of 92 F g −1 at a specific current of 2.5 A g −1 . The aqueous symmetric supercapacitor device achieved an energy density of 32.61 W h kg −1 and a power density of approximately 1 kW kg −1 at a discharge current of 1.0 A g −1 . It has been found that the cells have an excellent electrochemical reversibility (92% after 400 continuous cycles) and capacitive characteristics in 1 M Na 2 SO 4 electrolyte.

Zinc oxide (ZnO) nanoparticles were successfully synthesized using a whey-assisted sol–gel method. 87 X-ray diffraction (XRD) and Raman spectroscopy analysis revealed a wurtzite crystalline structure for ZnO nanoparticles with no impurities present. Transmission electron microscopy (TEM), XRD observations, and UV-vis absorption spectroscopy results showed that with an increase in calcination temperature from 400 to 1000 °C, the size of the spherical nanoparticles increased from 18.3 to 88.6 nm, while their optical band gap energy decreased to ∼3.25 eV. The whey-assisted sol–gel method proved to be highly efficient for the synthesis of crystalline ZnO nanoparticles whose applications are of great interest in materials science technology. Eryngium foetidum L. leaf extract was also used for the nontoxic, cost-effective biosynthesis of ZnO nanoparticles (NPs) following the hydrothermal route. 88 The biosynthesized ZnO NPs served as an excellent antibacterial agent against pathogenic bacteria like Escherichia coli , Pseudomonas aeruginosa , Staphylococcus aureus susp. aureus and Streptococcus pneumoniae . The maximum zone of inhibition in ZnO NPs is 32.23 ± 0.62 and 28.77 ± 1.30 mm for P. aeruginosa and E. coli , respectively.

Another report presented an efficient, environmentally friendly, and simple approach for the green synthesis of ZnO nanoparticles (ZnO NPs) using orange fruit peel extract. 89 The approach aimed to both minimize the use of toxic chemicals in nanoparticle fabrication and enhance the antibacterial activity and biomedical applications of ZnO nanoparticles. The sample obtained without annealing exhibited relatively small spherical particles (10–20 nm) which were coagulated in large clusters on a matrix of residual organic material from the reducing agents. In the samples annealed at 400 °C and 700 °C, the particle sizes were randomly distributed and ranged from 35 to 60 nm and 70 to 100 nm, respectively. For an annealing temperature of 900 °C, the particle size increased intensively in the range of 200–230 nm. It was thus found that the morphology and size of the ZnO NPs depended on the annealing temperature. Specifically, with increasing annealing temperature, the particle size tended to increase and shape larger particles due to crystal growth. For pH values of 4.0 and 6.0, the particles were sphere-like in shape, and were distorted with distinct grain boundaries and low coagulation. At pH = 6, the particle size was in the 10–20 nm range and exhibited relative separation. Meanwhile, for a pH of 8.0, the particles had a variable shape and were coagulated in large clusters around 400 nm in size with indistinct grain boundaries. For a pH of 10.0, the particles were coagulated into large blocks with lengths of ∼370 nm and widths of ∼160 nm. The ZnO NPs exhibited strong antibacterial activity toward Escherichia coli ( E. coli ) and Staphylococcus aureus ( S. aureus ) without UV illumination at an NP concentration of 0.025 mg mL −1 after 8 h of incubation. In particular, the bactericidal activity towards S. aureus varied extensively with the synthesis parameters. This study presents an efficient green synthesis route for ZnO NPs with a wide range of potential applications, especially in the biomedical field.

4. Modification of zinc oxide nanoparticles

Cao et al. 90 used silica and trimethyl siloxane (TMS) for modifying ZnO in order to achieve a two-fold benefit: enhancing the compatibility of ZnO and cutting down on its agglomeration in the organic phase. A chemical precipitation method using zinc sulfate heptahydrate (ZnSO 4 ·7H 2 O), ammonium solution (NH 4 OH) and ammonium bicarbonate (NH 4 HCO 3 ) was adopted to first obtain the precursor, zinc carbonate hydroxide (ZCH). The surface of the ZCH was then successively modified by an in situ method using TEOS and hexamethyldisilazane (HMDS) in water. The functionalized ZHC was subjected to calcination, to yield extremely fine nanoparticles of ZnO. Reduced agglomeration was thereby effected through such functionalization of the surfaces of ZnO NPs although a lowered photocatalytic activity of the oxide was observed. Nevertheless, a marked increase in the compatibility of ZnO with the organic matrix lent credence to the method. Further, the greater shielding capacity of UV radiation renders the synthesized nanomaterial an excellent candidate for use in cosmetics. Below is a schematic representation ( Fig. 4 ) of the synthesis of surface-modified ZnO ultrafine particles using an in situ modification method.


	Schematic representation of the synthesis of surface-modified ZnO ultrafine particles using an in situ modification method (reproduced from ref. with permission from Elsevier).

ZnO(OH) + yOHC(CH ) CH ) CH → ZnO(OH) −y[OOC(CH ) CH ] + yH O

The FTIR spectra for the SiO 2 -modified ZnO revealed interphase bonds between ZnO and SiO 2 . A thin film coating of SiO 2 on the ZnO surface resulted in enhanced dispersion and reduced agglomeration of nanoparticles, a fact fairly well corroborated by HR-TEM data. The photocatalytic activity of SiO 2 -modified ZnO however suffered a setback in comparison with that demonstrated by uncoated ZnO. The work further demonstrated that the thorough reduction of the crystallinity of ZnO achieved through heterogeneous azeotropic distillation of the zinc oxide precursor not only precludes aggregation but also brings about a decline in the average particle size.

Yuan et al. 93 modified ZnO using Al 2 O 3 . A basic carbonate of zinc was obtained from the reaction between zinc sulfate and ammonium bicarbonate followed by precipitating aluminum hydroxide over it. The resulting compound-precipitate was then calcined at 400–600 °C to obtain ZnO NPs coated with Al 2 O 3 . It was discovered from TEM analysis that as the Al 2 O 3 -coating content rose from 3 to 5%, agglomeration decreased significantly and correspondingly the particle size decreased from an average value of 100 nm to 30–80 nm. The coating thus designed was 5 nm thick and was highly uniform. The coating-core interphase possibly had the structure of ZnAl 2 O 4 . Zeta potential data clearly confirm modifications on the ZnO surface by Al 2 O 3 deposition. The change in pH at the isoelectric point for ZnO NPs upon coating with Al 2 O 3 from around 10 to a value of 6 might have assisted a greater degree of dispersion of ZnO NPs.

In a study by Hu et al. , 94 nano-sized ZnO rods doped with transition metals such as Mn, Ni, Cu, and Co were designed by a plasma enhanced chemical vapor deposition method. The ZnO thus modified had a greater amount of crystal defects within its structure. This led to its greater sensitivity towards formaldehyde. When the 1.0 mol% Mn doped ZnO nanorods were activated by 10 mol% CdO, a maximum sensing of ∼25 ppm was obtained and the corresponding response and recovery time were found to be appreciably short.

Wysokowski et al. 95 developed a β-chitin/ZnO nanocomposite material. The β-chitin used in the synthesis was derived from Sepia officinalis , a cephalopod mollusk. This nanocomposite was found to exhibit remarkable anti-bacterial activity and was touted as an excellent ingredient for the making of wound-dressing materials.

Ong et al. 96 in their work synthesized a heterogeneous photocatalytic material by loading ZnO on solvent exfoliated graphene sheets. For anchoring ZnO onto the graphene sheet, they used poly(vinyl pyrrolidone) as an inter-linker which was also found to enhance the functionalization of the acid treated graphene sheets. The thermal stability of the decorated ZnO was found to be higher than that of the undecorated oxide. The modified ZnO proved to be an outstanding photocatalyst being able to cause 97% degradation of Reactive Black 5 under visible light. This improvement was attributed to a host of favourable parameters achieved through the modification, namely, an enhancement of light absorption intensity, widening of the light absorption range, suppression of charge carrier recombination, improvement of surface active sites and rise in the chemical stability of the designed photocatalyst.

Tang et al. 97 demonstrated a way to tackle the agglomeration tendency of ZnO NPs. They prepared ZnO/polystyrene nanocomposites via a mini-emulsion polymerization method. For this, a silane coupling agent, namely γ-glycidoxypropyl trimethoxysilane (KH-560, AR), was first allowed to cling to ZnO NPs via reaction between its Si–OCH 3 groups and the hydroxyl groups on the surface of the nanoparticles followed by anchoring of 4,4′-azobis(4-cyanovaleric acid) (ACVA) onto their surface through reaction of its carboxyl groups with the terminal epoxy groups of the aforementioned coupling agent. Subsequently, polymerization of the styrene monomer was initiated using the azo group of ACVA for designing the final nanocomposites. The monomer droplet of the mini-emulsion polymerization system thus obtained contained well dispersed ZnO/polystyrene nanocomposites with a high grafting efficiency of 85% as calculated from TGA. It was evident from scanning electron microscopy (SEM) that while pure ZnO NPs suffered considerable agglomeration in poly(vinyl chloride) (PVC) film, the ZnO/polystyrene nanocomposite particles underwent homogeneous dispersion in the PVC matrix. The scheme depicted in Fig. 5 explains the mechanism of the mini-emulsion polymerization method to construct ZnO/polystyrene nanocomposites adopted by Tang and his research group. From SEM micrographs ( Fig. 5 ), it was observed that functionalized ZnO (f-ZnO) nanoparticles had been well dispersed in the polymer matrix because the f-ZnO nanofiller had outstanding adhesion and strong interfacial bonding to PEA. As was observed, f-ZnO nanoparticles were homogeneously dispersed in the polymer matrix and their sizes were estimated to be between 20 and 50 nm.


	A schematic diagram showing the synthesis of ZnO/polystyrene nanocomposites by anchoring 4,4′-azobis(4-cyanovaleric acid) (ACVA) onto the surface of ZnO nanoparticles to initiate styrene polymerization (reproduced from ref. with permission from Elsevier).

Cyclodextrins (CDs) make up a class of cyclic torus-shaped oligosaccharides. CD has a hydrophilic external surface and a hydrophobic internal cavity. CDs have been extensively used as eco-friendly coupling agents. 98,99 Among the derivatives of CDs, monochlorotriazinyl-β-cyclodextrin (MCT-β-CD) with a monochlorotriazinyl group as a reactive anchor was found to possess the ability to form covalent bonds with substituents of the nucleophilic type, viz. , –OH or –NH 2 groups. 100–103 Therefore, MCT-β-CD provides an interesting way of surface modification for inorganic nanomaterials. Abdolmaleki et al. 104 accomplished surface modification of ZnO NPs by covalently grafting MCT-β-CD onto the surfaces of ZnO NPs through a facile and single-step procedure. In the next step, f-ZnO nanoparticles were employed for construction of a new series of poly(ester-amide)/ZnO bionanocomposites (PEA/ZnO BNCs) whose TEM image is shown in Fig. 6 . MCT-β-CD has monochloro-triazinyl groups that react with –OH groups on the surfaces of ZnO NPs through nucleophilic reaction ( Fig. 7 ). After the incorporation of MCT-β-CD on the surfaces of ZnO NPs, polymer/ZnO bionanocomposites (BNCs) were designed using a biodegradable amino acid containing poly(ester-amide) (PEA). ZnO NPs with β-CD functional groups incorporated on their surfaces exhibited a near-complete suppression of their tendencies towards agglomeration while simultaneously displaying enhanced compatibility with the polymer matrix. Scores of functional groups on the surfaces of ZnO NPs enable possible interactions with PEA chains that lead to excellent dispersion and compatibility with the polymer matrix. FE-SEM and TEM results bore out a reduction of agglomeration that can be safely attributed to the steric hindrance induced by the organic chains of MCT-β-CD between the inorganic nanoparticles. The dispersibility, surface morphology and particle dimensions of functionalized ZnO (f-ZnO) with β-CD are shown in Fig. 8 .


	FESEM of pure ZnO NPs (a) and grafted ZnO/polystyrene nanocomposite particles (b) dispersed in PVC matrices (reproduced from ref. with permission from Elsevier).


	Modification of ZnO nanoparticles with MCT-β-CD (reproduced from ref. with permission from Elsevier).


	(A) Photograph of aqueous dispersions of pure ZnO (left) and f-ZnO (right), and (B) FESEM and (C) TEM micrographs of f-ZnO (reproduced from ref. with permission from Elsevier).

5. Potential applications


	Applications of ZnO NPs.

5.1 Concrete and rubber industries

In their attempt to enhance the interactions between the nano-sized ZnO particles and the polymer, Yuan et al. 110 by incorporating vinyl silane groups on the surfaces of ZnO NPs using vinyl triethoxysilane through a procedure premised on the hydrosilylation reaction during curing carried out their surface modification. The vinyl silane groups on the ZnO surface enabled improved cross-linking with the rubber matrix. In order to solve this problem, surface modification techniques are applied to improve the interaction between the nanoparticles prepared by the sol–gel method and the polymer. In comparison with the nanocomposites of silicone rubber with ZnO, the nanocomposites of silicone rubber with vinyl triethoxysilane modified ZnO possessing extensive cross-linking and a higher degree of dispersion with the rubber matrix exhibited superior mechanical properties and enhanced thermal conductivity.

ZnO NPs have been widely used as an efficient material for the enrichment of cross-linking in elastic polymers. 111,112 The cured polymer produced through incorporation of ZnO NPs exhibited high ultimate tensile strength, tear strength, toughness and hysteresis. The slippage of polymer chains on the surfaces of ionic clusters and the renewal of ionic bonds when the sample gets externally deformed give rise to enhanced capacity of the ionic elastic polymer for stress relaxation which in turn results in its upgraded mechanical properties. Furthermore, the thermoplastic properties of such polymers enable their processing in a fused state in a manner akin to a thermoplastic polymer. 113 Nevertheless, carboxylic elastic polymers with ZnO as a cross-linker suffer from a few drawbacks prominent among which are their tendency to get scorched, feeble flex properties and high value of compression set. The tendency to get scorched is gotten rid of by the incorporation of either zinc peroxide (ZnO 2 ) or ZnO 2 /ZnO cross-linkers. ZnO 2 serves to not only create ionic cross-links but also generate covalent cross-links as a result of peroxide action. However, prolonged curing is needed to obtain elastomers with an ultimate strength and cross-link density comparable to that of ZnO-cross-linked elastomers. The three vital processes that amount to the curing of XNBR by ZnO 2 /ZnO cross-linkers are rapid creation of ionic crosslinks due to the initial ZnO present, covalent links resulting from peroxide cross-links and further ionic cross-linking due to the generation of ZnO from the decomposition of ZnO 2 . Leaving aside the problem of scorching, ZnO NPs make good and therefore widely used cross-linkers in carboxylated nitrile rubbers.

The prime factors affecting the involvement of ZnO in the formation of ionic cross-links with the carboxylic groups of the elastic polymers are its particle size, surface area and morphology. They are also found to govern the dimensions of the interphase between the cross-linkers and elastomer chains. 114 With a view to ascertain the correlation between the characteristics of ZnO NPs and their roles in the curing of elastic polymers, Przybyszewska et al. 115 employed a variety of ZnO NPs with different morphological characteristics (spheres, whiskers, and snowflakes) as cross-linkers in a carboxylated nitrile elastomer. It emerged from their investigation that ZnO NPs as cross-linkers imparted improved mechanical properties to vulcanizates than commercially used ZnO micro-particles. The ultimate tensile strength of vulcanizates with ZnO NPs was found to be four times higher than that of ZnO micro-particles containing vulcanizates. As a result, there is a 40% reduction of the quantity of ZnO that is put to such use. Since ZnO is known to have deleterious effects on aquatic life, an approach that reduces its usage is highly commendable from the point of view of eco-friendliness. However, ZnO cross-linked XNBR undergoes shrinkage on prolonged exposure to heat.

Among all the aforesaid morphologies, it was observed that ZnO snowflakes with a surface of approximately 24 m 2 g −1 had the highest activity. However, surface area and particle size exerted little influence on the activity of ZnO cross-linkers. It was also observed that the ZnO NPs exhibited a minimum tendency to agglomerate in the rubber matrix. There gathered smaller agglomerates with ZnO NPs as cross-linkers upon sample deformation as compared to the large agglomerates observed with ZnO microparticles.

The usage of ZnO as a cross-linker in rubber has an adverse impact on the environment, particularly when it is discharged into the surroundings upon degradation of rubber. 116 Zinc is known to cause great harm to aquatic species 117 and efforts to cut down on the content of ZnO in rubber are hence being made. 118 Bringing down the ZnO level in rubber, therefore, may follow any of the following three fundamental procedures:

(i) substituting the commonly used micro-dimensional ZnO of surface area 4–10 m 2 g −1 with nano-structured ZnO with surface area of up to 40 m 2 g;

(ii) carrying out surface modifications of ZnO with carboxylic acids ( viz. , stearic acid, maleic acid and the like);

(iii) using additional activators. 119

In order to get over the eco-toxicity associated with the usage of ZnO in large quantities, Thomas et al. 120 designed a few unique accelerators, namely, N -benzylimine aminothioformamide (BIAT)-capped-stearic acid-coated nano-ZnO (ZOBS), BIAT-capped ZnO (ZOB), and stearic acid-coated nano-zinc phosphate (ZPS), to probe their effects on the curing of natural rubber (NR) and thereby its mechanical properties. ZnO NPs prepared by the sol–gel route were surface-decorated using accelerators such as BIAT and fatty acids such as stearic acid. The capping agents functioned to reduce the size of agglomerates leading to an improvement of vulcanization and physicochemical properties of NR. Capping of ZnO further ensured a decline in the time and energy required for dispersion in the rubber matrix. As a result, there happened a further enhancement of the acceleration of vulcanization and a remarkable upgrade of the mechanical properties of the emerging vulcanizates. The rubber vulcanized with an optimal dose of BIAT-capped-stearic acid-coated zinc oxide (ZOBS) was found to possess superlative curing and mechanical properties in comparison with other countertypes and the reference polymer containing pristine ZnO NPs. The rigidity of vulcanizates containing ZPS was found to increase as a result of an enhanced cross-link density. The vulcanizates exhibited reduced tendency to get scorched as a result of incorporation of capped ZnO NPs and this was attributed to the delayed release of BIAT from the capped ZnO into the rubber matrix for interaction with CBS (conventional accelerator). Sabura et al. 121 adopted a solid-phase pyrolytic procedure to synthesize ZnO NPs of particle size in between 15 and 30 nm and surface area in the range 12–30 m 2 g −1 for use in neoprene rubber as cross-linkers. Two findings emerged from this study. One, the optimal content of ZnO required was found to be low in comparison with commercially used ZnO. Two, the cure characteristic and mechanical properties of the rubber showed a marked improvement when compared with those containing conventional ZnO.

5.2 Opto-electronic industry

The last decade has seen an upsurge in the fabrication of ZnO-based perovskite solar cells (PSCs). Although the conventional choice for an electron transport layer has been TiO 2 , ZnO with higher electron mobility is increasingly replacing it as an efficient and low-cost material for electron transport in PSCs. Additionally, the power conversion efficiency of PSCs at large has exceeded 20% of late giving the necessary impetus to delve deep into the fabrication of ZnO electron transport layers (ETLs) for yet more brilliant perovskite solar devices. Bi et al. 134 fabricated a PSC with ZnO nanorods aligned vertically over the substrate. With the length of nanorods, the J sc (short-circuit current density), FF (fill factor) and PCE of solar cells were found to increase. They however reported a decrease in V oc with nanorod length. They reasoned that nanorod length has a bearing on the electron transport time and lifetime that in turn influence the performance of the solar cell. They achieved a maximum overall cell efficiency of 5%. Son et al. 135 substituted the single-step method used by Bi et al. by a two-step coating procedure. Such a treatment generated a fully filled perovskite film that covered all ZnO nanorods of varying lengths without voids and formed an overlayer on the surface of nanorods. As a further consideration, the two-step coating treatment induced optimization of the cuboid size of MAPbI 3 and reduced the series resistance of the solar cell. 136 As a result, a maximum PCE of 11.13% was obtained. Tang et al. designed ZnO nanowall ETLs. 137 The best performance PSC based on ZnO nanowalls produced a J sc of 18.9 mA cm −2 , V oc of 1.0 V, FF of 72.1%, and PCE of 13.6%. Meanwhile, the control device shows a J sc of 18.6 mA cm −2 , V oc of 0.98 V, FF of 62%, and PCE of 11.3%. The introduction of ZnO nanowalls led to an evident boost in the FF and PCE of the PSCs and this can be ascribed to the greater contact area between ZnO and perovskite offered by the ZnO nanowalls in comparison with the planar ZnO film which improves not only the electron collection but also transportation efficiency at the interface of the ZnO nanowalls and perovskite. Moreover, the decomposition of ZnO by perovskite triggered by the alkaline nature of the ZnO surface leads to the formation of PbI 2 on the perovskite/ZnO interface. The presence of PbI 2 can suppress the surface recombination and improve the FF. 138

5.3 Gas-sensing

NO (gas) + e (CB) → NO (adsorption)

NO (gas) + O (adsorption) + 2e → NO (adsorption) + 2O (adsorption)

CO + O → CO + e


	Sensing responses of pure ZnO, ZnO–MoS , Pt–ZnO–MoS and Ag–ZnO–MoS film sensors towards 100 ppm CO gas (reproduced from ref. with permission from Elsevier).


	Selectivity of the Ag–ZnO–MoS nanocomposite sensor towards 100 ppm gas species of H , CH , CO, C H , C H and C H (reproduced from ref. with permission from Elsevier).


	(a) Schematic diagram of the Ag–ZnO–MoS nanocomposite sensor towards CO gas; (b) energy band diagram of the Ag–ZnO–MoS nanocomposite (reproduced from ref. with permission from Elsevier).


	Response of the Al-loaded and unloaded ZnO samples towards 50 ppm CO at different operating temperatures (reproduced from ref. with permission from Elsevier).


	Response of AZO nanoparticles as a function of CO concentration at a temperature of 300 °C (reproduced from ref. with permission from Elsevier).


	Response of the A3ZO sensor as a function of CO concentration at 300 °C (reproduced from ref. with permission from Elsevier).

The electrons injected into the conduction band lower the resistance of the Al-doped ZnO gas sensors. The response and recovery times observed for all Al-loaded ZnO samples were 6–8 s sand 16–30 s, respectively. The unloaded ZnO sample was marked by longer response and recovery times of 30 s and 70 s, respectively. The sensing films exhibited excellent thermo-mechanical and electrical stability.

C H OH (g) → CH CHO (g) + H (g) (basic oxide)

CH CHO (ad) + 5O → 2CO + 2H O + 5e

Therefore, the ZnO/SnO 2 nanocomposite gas sensor demonstrated a sharper response to ethanol gas than the pristine SnO 2 sensor. Moreover, a possible increase in the effective barrier height of the n–n heterojunction enabled better engagement with adsorbed oxygen causing greater depletion of electrons from the conduction band eventually leading to an enhanced gas sensing response by the system. Additionally, remarkable detection at a lower (ppb) limit was shown by the heterostructured sensor.

5.4 Cosmetic industry

In a study by Reinosa et al. , 149 it was brought to light that a nano/micro-composite comprising nanosized TiO 2 dispersed on ZnO micro-particles showed a higher sun protection factor (SPF) than individual TiO 2 and ZnO particles. The SPF of the synthesized nano-sized TiO 2 was found to be higher than that of its micro-sized counterpart with the former showing maximum absorption at 319 nm while the latter showed maximum absorption at 360 nm. The synthesized micro-sized ZnO had a higher SPF than its nano equivalent. Both exhibited maximum absorption at 368 nm. These data suggested that ZnO has a higher critical wavelength because it covers the entire UV range and has a higher UVA/UVB ratio since the maximum of the SPF curve lies in the UVA region ( Fig. 16a ). Additionally, it was observed that TiO 2 , with a lower UVA/UVB ratio owing to the presence of the SPF maximum in the lower wavelength region, has a lower critical wavelength ( Fig. 16b ). Therefore, to boost the SPF output, a suitable combination of the two oxides was thought out. A dry dispersion procedure was adopted to prepare the composite consisting of 15 wt% TiO 2 NPs and 85 wt% ZnO micro-structured particles. The results obtained from this composite were compared with those obtained by the standard procedure. Raman spectroscopy revealed a superior dispersion of the NPs and their anchoring with higher quantum confinement resulting from dry dispersion by using ZnO micro-structures as host particles. The SPF output was found to be higher for the sunscreen with the filter prepared by the dry dispersion method than the one with the filter synthesized following the standard method ( Fig. 17 ). This observation was chiefly attributed by the authors to the correct dispersion of TiO 2 NPs over the host ZnO micro-sized particles.


	SPF curves of COLIPA sunscreen incorporating an inorganic UV filter: nanometric (dashed lines) and micrometric (solid lines) (a) TiO and (b) ZnO particles (reproduced from ref. with permission from Elsevier).


	SPF curves of sunscreens with micro–nanocomposite filters. The solid line represents the SPF curve of the new micro–nano composite obtained by a nano-dispersion method and the dashed line represents the curve obtained by a standard method (reproduced from ref. with permission from Elsevier).

5.5 Textile industry

It has been shown in many research investigations that the use of ZnO in the processing of fabrics promotes their anti-bacterial and self-cleaning properties apart from upgrading their UV absorption capacity. 158 Moreover, in textile applications, coatings of ZnO in the nano-dimensions aside from being bio-compatible are found to exhibit air-permeability and UV-blocking ability far greater than their bulk equivalents. 159 Therefore, ZnO nanostructures have become very attractive as UV-protective textile coatings. Different methods have been reported for the production of UV-protective textiles utilizing ZnO nanostructures. For instance, hydrothermally grown ZnO nanoparticles in SiO 2 -coated cotton fabric showed excellent UV-blocking properties. 160 Synthesis of ZnO nanoparticles elsewhere through a homogeneous phase reaction at high temperatures followed by their deposition on cotton and wool fabrics resulted in a significant improvement in UV-absorbing activity. 161 Similarly, ZnO nanorod arrays that were grown onto a fibrous substrate by a low-temperature growth technique provided excellent UV protection. 162

Zinc oxide nanowires were grown on cotton fabric by Ates et al. 163 to impart self-cleaning, superhydrophobicity and ultraviolet (UV) blocking properties. The ZnO nanowires were grown by a microwave-assisted hydrothermal method and subsequently functionalized with stearic acid to obtain a water contact angle of 150°, demonstrating their superhydrophobic nature, which is found to be stable for up to four washings. The UV protection offered by the resulting cotton fabric was also examined, and a significant decrease in transmission of radiation in the UV range was observed. The self-cleaning activity of the ZnO nanowire-coated cotton fabric was also studied, and this showed considerable degradation of methylene blue under UV irradiation. These results suggest that ZnO nanowires could serve as ideal multifunctional coatings for textiles.

Research on the use of zinc oxide in polyester fibres has also been carried out at Poznan University of Technology and the Textile Institute in Lodz. 164 Zinc oxide was obtained by an emulsion method, with particles measuring approximately 350 nm and with a surface area of 8.6 m 2 g −1 . These results indicate the product's favourable dispersive/morphological and adsorption properties. Analysis of the microstructure and properties of unmodified textile products and those modified with zinc oxide showed that the modified product could be classed as providing protection against UV radiation and bacteria.

5.6 Antibacterial activity

Epidemic disease cholera mainly affects populations in developing countries. 169,180 It is a serious diarrheal disease caused by the intestinal infection of Gram-negative bacterium V. cholerae. The effective antibacterial activity of ZnO NPs and their mechanism of toxicity were explored against Vibrio cholerae (two biotypes of cholera bacteria (classical and El Tor)) by Sarwar et al. 176 Strong arguments and detailed justifications of the toxicity mechanism emerged as a result of this rigorous investigation. The bacterial membrane bears an overall negative charge that can be ascribed to the acidic phospholipids and lipopolysaccharides in it while ZnO NPs possess a positive charge in water suspension. An initial NP–membrane interaction via electrostatic attraction may result from this charge difference following which membrane disruption occurs. As the membrane plays an essential role by maintaining the vital function of the cell, such damage induces depolarization of the membrane, increased membrane permeabilization – loss in membrane potential and protein leakage and denaturation upon subsequent contact with ZnO NPs. Besides, ZnO NPs also have the ability of interacting with DNA as well as forming abrasions on it. Significant oxidative stress was also noticed inside the bacteria cells. They thus arrived at a conclusion that binding of ZnO NPs with the bacterial cell surface induces membrane damage followed by internalization of NPs into the cells, leakage of cytoplasmic content, DNA damage and cell death. Disruption of the membrane by ZnO NPs would additionally give easy access of antibiotics into the cell. Their findings further corroborated a synergic effect produced by the actions of ZnO NPs and antibiotics. They also encountered the antibacterial activity of the ZnO NPs in cholera toxin (CT) mouse models. It emerged that ZnO NPs could induce the CT secondary structure collapse gradually and interact with CT by interrupting CT binding with the GM1 ganglioside receptor. 181

In bacteria treated with NPs of ZnO, it was observed that the damage to cell membranes was an inevitable phenomenon. The pathways of the antibacterial activity of ZnO NPs were investigated using Escherichia coli ( E. coli ) as a prototype organism. 182 As was evident from the SEM images of E. coli obtained after treatment with ZnO NPs, a greater number of cell damage sites were noted at higher doses of ZnO NPs. This cell damage has been ascribed to pathways involving both the presence and absence of ROS. In the absence of ROS, the interaction of ZnO NPs with bacterial membranes would lead to damage to the molecular structure of phospholipids culminating in cell membrane damage.

Jiang et al. 183 studied the potential antibacterial mechanisms of ZnO NPs against E. coli . They reported that ZnO NPs with an average size of about 30 nm caused cell death by coming into direct contact with the phospholipid bilayer of the membrane and destroying the membrane integrity. The significant role of ROS production in the antibacterial properties of ZnO NPs surfaced when it emerged that the addition of radical scavengers such as mannitol, vitamin E, and glutathione could block the bactericidal action of ZnO NPs. However, the antibacterial effect triggered by Zn 2+ released from ZnO NP suspensions was not apparent. Reddy synthesized ZnO NPs with sizes of ∼13 nm and investigated their antibacterial ( E. coli and S. aureus ) activities. 168 It was discovered that ZnO NPs effected complete cessation of the growth of E. coli at concentrations of about 3.4 mM but induced growth inhibition of S. aureus at much lower concentrations (≥1 mM). Besides, Ohira and Yamamoto 184 also discovered that the antibacterial ( E. coli and S. aureus ) activity of ZnO NPs with small crystallite sizes was far more pronounced than for those with large crystallite sizes. From ICP-AES measurement, it emerged that the amount of Zn 2+ released from the small ZnO NPs was much higher than from the large ZnO powder sample and E. coli was more sensitive to Zn 2+ than S. aureus . This is a further confirmation that eluted Zn 2+ ions from ZnO NPs also play a key role in antibacterial action.

Iswarya et al. , 185 having extracted crustacean immune molecule β-1,3-glucan binding protein (Phβ-GBP) from the haemolymph of Paratelphusa hydrodromus , successfully designed Phβ-GBP-coated ZnO NPs. The Phβ-GBP-ZnO NPs were spherical shaped having a particle size of 20–50 nm and halted the growth of S. aureus and P. vulgaris . S. aureus was found to be more prone to the bactericidal action of Phβ-GBP-ZnO NPs than P. vulgaris . In addition, Phβ-GBP-ZnO NPs could induce drastic modification in cell membrane permeability and set off outrageous levels of ROS formation both in S. aureus and P. vulgaris . This work was thus pivotal in bringing to the forefront the immensely great antibacterial hallmark of Phβ-GBP-ZnO NPs.

The mechanism of breaking into bacterial cells by membrane disruption and then inducing oxidative stress in bacterial cells, thereby stalling cell growth and eventually causing cell death has been reported in many recent research studies. 186–191 Important bacterial biomolecules can also adsorb on ZnO NPs. Bacterial toxicity, in the recent past, has been heavily reported to have resulted from structural changes in proteins and molecular damage to phospholipids. 192 The antibacterial activity of ZnO NPs thus finds its apt application in the discipline of food preservation. As a formidable sanitizing agent, it can be used for disinfecting and sterilizing food industry equipment and containers against attack and contamination by food-borne pathogenic bacteria. ZnO NPs showed both toxicity on pathogenic bacteria ( e.g. , Escherichia coli and Staphylococcus aureus ) and beneficial effects on microbes, such as Pseudomonas putida , which has bioremediation potential and is a strong root colonizer. 193

Investigations into the antibacterial activities of ZnO micro-sized particles, ZnO NPs, and ZnO NPs capped with oxalic acid against S. aureus were carried out in the presence and absence of light. 194 It was observed that the efficiency of ZnO NPs was just 17% in the dark. However, their antibacterial properties saw a surge up to 80% upon application of light. The antibacterial behaviour was greatest for ZnO NPs while it was minimum for ZnO micro-sized particles, suggesting a higher release of Zn 2+ ions from ZnO NPs than ZnO micro-sized particles. The examination revealed that surface defects of the ZnO NPs boosted ROS production in the presence as well as absence of light. Additionally, it was also found that capping lowers the amount of superoxide radicals generated because capping blocks the oxygen vacancies that are chiefly accountable for the generation of superoxide radicals. In another investigation, the influence of NP size on bacterial growth inhibition by ZnO NPs and the mechanistic routes of their action were demonstrated. 195 ZnO NPs with diameters ranging from 12 nm to 307 nm were first generated. Thereafter, they were administered to Gram-positive and Gram-negative microorganisms ( Fig. 18 ). The results clearly illustrated the greater bactericidal efficacy of smaller ZnO NPs under dark conditions. The use of UV light resulted in an enhanced antibacterial behaviour of ZnO NPs owing to the enhanced formation of ROS from them. The antibacterial properties were rooted in the generation of ROS and the build-up of ZnO nano-sized particles in the cytoplasm and on the external membranes.


	The influence of different sizes of ZnO NPs on the growth of a methicillin sensitive S. aureus strain. (A) Growth analysis curves obtained by tracking the optical density at 600 nm. (B) Percentage of viable S. aureus recovered after treatment with ZnO NPs of different sizes (reproduced from ref. with permission from the American Chemical Society).

In another intriguing investigation, the toxicity induced in antibiotic resistant nosocomial pathogens such as Acinetobacter baumannii ( A. baumannii ) and Klebsiella pneumoniae ( K. pneumoniae ) by photocatalytic ZnO NPs was studied. 196 It was seen that A. baumannii and K. pneumoniae were significantly destroyed by 0.1 mg mL −1 of ZnO nano-structures with 10.8 J cm −2 of blue light. Further, the mechanistic pathway of the antibacterial activity of photocatalytic ZnO NPs against antibiotic defiant A. baumannii was investigated. While cytoplasm leakage and membrane disruption of A. baumannii were evident after treatment with ZnO NPs under blue light exposure, there was no sign of plasmid DNA fragmentation. Therefore, membrane disruption could be associated with the mechanistic route via which the photocatalytic ZnO NPs demonstrated antibacterial activity. The possibility of the role of DNA damage therein was categorically ruled out.

A novel approach comprising a combined application of ultrasonication and light irradiation to ZnO NPs has been developed to boost their antibacterial properties. 197 The sono-photocatalytic activity of ZnO nanofluids against E. coli was tested. The results revealed a 20% rise in the antibacterial efficacy of ZnO nanofluids. Further, ROS generation by ZnO nanofluids played a crucial role in bacterial elimination. The sono-photocatalysis of ZnO nanofluids also enhanced the permeability of bacterial membranes, inducing more efficacious penetration of ZnO NPs into the bacteria.

Although ZnO NPs make a promising antibacterial agent owing to their wide-ranging activities against Gram-positive as well as Gram-negative bacteria, the exact antibacterial pathway of ZnO NPs has not been adequately established. Hence, deep investigations into it hold a lot of important theoretical and practical value. In the future, ZnO NPs can be explored as antibacterial agents, such as ointments, lotions, and mouthwashes. Additionally, they can be overlayed on various substrates to prevent bacteria from adhering, spreading, and breeding in medical devices.

5.7 Drug delivery

Reports bearing evidence of the applications of ZnO NPs in the delivery of chemotherapeutic agents to treat cancers have emerged prolifically in the last few years. For instance, a porous ZnO nanorod based DDS (ZnO-FA-DOX), enclosing folic acid (FA) as a targeting agent and doxorubicin (DOX) as a chemotherapeutic drug, was fabricated by Mitra et al. 204 The ZnO-FA-DOX nano-apparatus was found to exhibit pH-triggered release of DOX and potent cytotoxicity in MDA-MD-231 breast cancer cells. The biocompatible nature of the ZnO-FA material, as observed from the acute toxicity study in a murine model also emerged from the investigation. In another research study by Zeng et al. , 205 a lymphatic-targeted DDS with lipid-coated ZnO-NPs (L-ZnO-NPs) enclosing 6-mercaptopurine (6-MP) as an anticancer agent was designed. The L-ZnO-NP apparatus demonstrated pH-susceptive drug release and remarkable cytotoxicity to cancer cells as a result of the generation of intracellular reactive oxygen species (ROS). Liu et al. 206 also reported the fabrication of DOX-loaded ZnO-NPs. The researchers encountered a pH-susceptive drug release from the DDS and diminished drug efflux with enhanced cytotoxicity in drug defiant breast cancer cells (MCF-7R). Likewise, Li et al. 207 fabricated a novel DDS enclosing hollow silica nanoparticles (HSNPs) embedded with ZnO quantum dots to co-deliver DOX and camptothecin. The nano-apparatus evinced pH-susceptive drug release and cytotoxicity to drug defiant cancer cells. A research study used ZnO NPs as caps to cover the pores of mesoporous silica NPs (MSNs), and when the designed drug delivery apparatus came into contact with acids, there took place a decomposition of ZnO NPs followed by a release of doxorubicin (DOX) molecules from the MSN nanostructures. 208 One major drawback of such an apparatus was that it had difficulty in degradation thereby resulting in an incomplete release of drugs. 209,210 Another scheme employed the technique of loading drugs onto the ZnO NPs directly. 211 Upon contact with acids, the drug molecules are released following the complete decomposition of ZnO NPs. In another investigation, a liposome-incorporated ZnO-NP based DDS (ZNPs-liposome-DNR) enclosing anticancer drug daunorubicin (DNR) was designed by Tripathy et al. 212 The incorporation of ZnO-NPs in the DDS was observed to prevent the premature release of DNR, which could be prompted only in acidic medium, thereby efficiently exerting an anticancer effect on A549 cells. The study of intracellular release in cancer cells with confocal laser scanning microscopy (CLSM) revealed that treatment with ZNPs-liposome-DNR induced a marked DNR release, causing greater cytotoxicity to cancer cells, compared to pure DNR and DNR-conjugated liposomes (liposome-DNR), as evidenced by the green fluorescence intensity. For the treatment of lung cancer, Cai et al. accomplished the construction of ZnO quantum dot-based drug delivery apparatus that was conjugated with a targeting agent (hyaluronic acid) and an anticancer agent (DOX). 213 The nano-apparatus demonstrated CD44 receptor-specific uptake and pH-driven drug release in lysosomal compartments of the cancer cells. Kumar et al. 214 also designed sub-micron sized self-assembled spherical capsules of ZnO nanorods that successfully effected the delivery of anticancer agent DOX to K562 cancer cells. Furthermore, Han et al. 215 also synthesized ZnO NPs conjugated with an aptamer as a functionalization agent and DOX as an anticancer agent and demonstrated the effect of combined chemo- and radiation therapy in MCF-7 breast cancer cells employing the nano-apparatus. Recently, Zhang et al. 216 as a part of their investigation devised a new scheme to restrain the proliferation of human hepatocarcinoma cells (SMMC-7721) via a combined application of ZnO nanorod based DNR in photodynamic therapy (PDT), where ROS generation had the possibility to play a key role in the net anticancer behaviour of the hybrid nano-apparatus ( Fig. 19 ). The researchers further discovered that ZnO NPs were able to transport a larger quantity of DNR via internalization into SMMC-7721 cells, thereby inducing outstanding restraint on the multiplication of these cancerous cells. Besides, UV irradiation on this drug delivery nano-apparatus further reinforced the arrest of cell proliferation through photocatalysis of ZnO nanorods. To look into the signaling pathway of anticancer activity of the DDS in PDT, the researchers monitored the caspase-3 activity, which is a hallmark of apoptosis. The results of immunocytochemistry study confirmed that upon treatment with a DNR–ZnO nanocomposite under UV irradiation, the cells demonstrated far more pronounced activation of caspase-3 molecules in cancer-afflicted cells. It was consequently proposed that ZnO nanorods could raise the drug's targeting efficiency and minimize the associated toxicity. Therefore, the DNR–ZnO hybrid nano-apparatus with UV irradiation was claimed to have the potential of a fruitful scheme for the treatment of cancers ( Fig. 20 and 21 ). In another study, Ye et al. 217 using a copolymerization process also prepared water soluble ZnO–polymer core–shell quantum dots, and designed a drug delivery apparatus based on these quantum dots containing Gd 3+ ions and anticancer drug DOX. The ZnO-Gd-DOX nano-system was found to be biocompatible, pH-responsive and led to a marked release of DOX into the acidic environment of cancer-afflicted cells and tumors. When administered to human pancreatic cancer (BxPC-3) tumor containing nude mice, this polymer-modified drug delivery nano-apparatus was found to display higher therapeutic efficiency compared to the FDA-approved liposomal DOX formulation DOXIL at 2 mg kg −1 DOX concentration. The histopathology study and ICP-AES analysis of the vital organs further confirmed that this ZnO-Gd-DOX nano-apparatus could substantially bring about growth-inhibition of tumors without exerting any toxic effects 36 days post administration. Additionally, the histopathology study of tumor sections also demonstrated severe damage to the tumor cells caused by the administration of the DDS, compared to the control, DOX and DOXIL groups.


	Schematic illustration of possible processes of ZnO nanorods encapsulating chemotherapeutic agents for anticancer therapy (reproduced from ref. with permission from Elsevier).


	Possible mechanism of ROS production by ZnO nanorods under UV irradiation (reproduced from ref. with permission from Elsevier).


	Cytotoxicity of DNR or the DNR–ZnO nanocomposite in the absence or presence of UV irradiation against SMMC-7721 cells. The inset graph shows the IC of DNR and the DNR–ZnO nanocomposite in the absence or presence of UV irradiation for SMMC-7721 cells (reproduced from ref. with permission from Elsevier).

5.8 Anti-cancer activity

Several studies have thus suggested the cytotoxic effects of ZnO NPs on cancer cells. The cancer cell viability percentage on the MCF7 cell line, A549 cell line, HL60 cell line and VERO cell line has been studied at various concentrations of ZnO. Results show that the cell viability of the above cell lines exhibits a marked decrease with a rise in ZnO concentration 221,222 with minimal damage to healthy cells.

The mitochondrial electron transport chain is known to be closely linked to intracellular ROS generation, and anticancer agents accessing cancer cells could impair the electron transport chain and release huge amounts of ROS. 223,224 However, an inordinate amount of ROS brings about mitochondrial damage thereby resulting in the loss of protein activity balance that eventually induces cell apoptosis. 225 ZnO NPs introduce certain cytotoxicity in cancer cells chiefly by a mechanism that involves a higher intracellular release of dissolved Zn 2+ ions, followed by enhanced ROS induction and induced cancer cell death by way of the apoptosis signaling pathway. The effects of ZnO NPs on human liver cancer HepG2 cells and their possible pharmacological mechanism were investigated by Sharma et al. 226 They observed that ZnO NP-exposed HepG2 cells exhibited higher cytotoxicity and genotoxicity, which were related to cell apoptosis conciliated by the ROS triggered mitochondrial route. The loss of the mitochondrial membrane potential led to the opening of outer membrane pores following which some related apoptotic proteins including cytochrome c were released into the cytosol thereby activating the caspase in due course. Mechanistic studies had proved that the loss of mitochondrial membrane potential-mediated HepG2 cell apoptosis was mainly due to the decrease in mitochondrial membrane potential and Bcl-2/Bax ratios as well as accompanying the activation of caspase-9. Besides, ZnO NPs could noticeably activate p38 and JNK and induce and attract p53 ser15 phosphorylation but this was not dependent on JNK and p38 pathways ( Fig. 21 ). These results afforded valuable insights into the mechanism of ZnO NP-induced apoptosis in human liver HepG2 cells. Moghaddam et al. 227 took recourse to biogenic synthesis and successfully generated ZnO NPs using a new strain of yeast ( Pichia kudriavzevii GY1) and examined their anticancer activity in breast cancer MCF-7 cells. ZnO NPs have been observed to exhibit powerful cytotoxicity against MCF-7 cells. This cytotoxicity is affected more likely via apoptosis than cell cycle arrest. The apoptosis induced by ZnO NPs was largely by way of both extrinsic and intrinsic apoptotic pathways. A few antiapoptotic genes of Bcl-2, AKT1, and JERK/2 were subjected to downregulation, while upregulation of some proapoptotic genes of p21, p53, JNK, and Bax was prompted. ZnO NPs have been widely employed in cancer therapy and reported to promote a selective cytotoxic effect on cancer cell proliferation. Chandrasekaran and Pandurangan evaluated the cytotoxicity of ZnO nanoparticles against cocultured C2C12 myoblastoma cancer cells and 3T3-L1 adipocytes. The study revealed that ZnO NPs could be more cytotoxic to C2C12 myoblastoma cancer cells than 3T3-L1 cells. Compared to 3T3-L1 cells, it emerged that ZnO NPs stalled C2C12 cell proliferation and brought about a more pronounced apoptosis by way of a ROS-conciliated mitochondrial intrinsic apoptotic route, an upregulation of p53, tempered Bax/Bcl-2 ratio, and caspase-3 routes. 228

In a study, biogenic zinc oxide nanoparticles (ZnO NPs) were developed from aqueous Pandanus odorifer leaf extract (POLE) with spherical morphology and approximately 90 nm size. 229 The anticancer activity of the ZnO NPs was evaluated by MTT assay and neutral red uptake (NRU) assays in MCF-7, HepG2 and A-549 cells at different doses (1, 2, 5, 10, 25, 50, and 100 μg mL −1 ). Moreover, the morphology of the treated cancer cells was examined by phase contrast microscopy. The results suggest that the synthesized ZnO NPs inhibited the growth of the cells when applying a concentration from 50–100 μg mL −1 . Overall, the study demonstrated that POLE derived biogenic ZnO NPs could serve as a significant anticancer agent. Phytomediated synthesis of metal oxide nanoparticles have become a key research area in nanotechnology due to its wide applicability in various biomedical fields. The work by Kanagamani et al. 230 explored the biosynthesis of zinc oxide nanoparticles (ZnO-NPs) using Leucaena leucocephala leaf extract. Biosynthesized ZnO-NPs were found to have a wurtzite hexagonal structure with particles distributed in the range of 50–200 nm as confirmed by TEM studies. The anticancer activity of ZnO-NPs against MCF-7 (breast cancer) and PC-3 (human prostate cancer) cell lines was evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. From the assay, biosynthesized ZnO-NPs were found to have better cytotoxic activity on PC-3 cell lines than MCF-7 cell lines. The in vitro cytotoxicity studies of biosynthesized ZnO-NPs against Dalton lymphoma ascites (DLA) cells revealed better antitumor activity with 92% inhibition at a ZnO-NP concentration of 200 μg mL −1 , and as the concentration increased, the anticancer efficiency also increased. These results suggested that ZnO NPs could selectively induce cancer cell apoptosis making them a bright candidate for cancer therapy.

Photodynamic therapy requires the administration of a photosensitizing agent that is subjected to activation by light of a specific wavelength thereby generating ROS. The application of ZnO NPs as effective photosensitizers can be ascribed to their capability to generate ROS in response to visible light or UV light. Recent studies exhibited that photo-triggered toxicity of ZnO NPs renders them aptly suitable for targeted PDT in a spatiotemporal manner, providing a surer way to selectively terminate cancerous cells. 231–234 An attempt was made to utilize the synergic effects of anticancer drugs with ZnO NPs in PDT to induce cell-death in cancer cells. 231 The cytotoxic effects of daunorubicin (DNR), an anti-cancer drug, on drug defiant leukemia K562/A02 cancer cells were put to the test in combination with ZnO NPs. The combination of DNR and ZnO NPs under UV irradiation could appreciably check the proliferation of drug-defiant cancer cells in a dose-dependent manner. Additionally, ZnO NPs were found to induce an enhanced cellular uptake of DNR.

An anticancer treatment using DNR-conjugated ZnO nanorods in PDT was investigated with human hepatocarcinoma cells (SMMC-7721) ( Fig. 19 ). 216 The fabrication of photo-excited ZnO nanorods with DNR displayed an outstanding boost in the anticancer properties of the ZnO nanorods ( Fig. 20 ). The ZnO nanorods raised the intracellular concentration of DNR and augmented the anticancer efficiency. This is further evidence of the drug carrying capacity of ZnO nanorods into target cancer cells. UV irradiation additionally reinforced the growth inhibition of cancerous cells via photocatalytic activity of ZnO nanorods. In this study, the promoted mortality of cancer cells indicates that ZnO nanorods under UV irradiation could efficiently induce the formation of ROS and further attack the cell membrane (mainly by lipid peroxidation), nucleic acids, and proteins (such as enzyme deactivation). The mechanism of ROS generation of ZnO nanorods under UV irradiation is displayed in Fig. 21 . ZnO is a direct band gap semiconductor with a band gap energy of 3.36 eV at room temperature, high exciton binding energy of 60 meV and high dielectric constant, which under UV irradiation will produce a hole (h + ) in the valence band and an electron (e − ) in the conduction band, namely electron/hole pairs. These electron/hole pairs will induce a series of photochemical reactions in an aqueous suspension of colloidal ZnO nanorods, generating ROS. Generally, at the surface of the excited ZnO nanorods, the valence band holes abstract electrons from water and/or hydroxyl ions, generating hydroxyl radicals (˙OH). Electrons reduce O 2 to produce the superoxide anion O 2 − ˙. ZnO nanorods can be one of the promising nanomaterials for PDT in cancer.

The size of ZnO NPs has been reported to have a strong association with their anticancer activities. The UV light-activated anti-cancer effects of various ZnO NPs with different sizes have been examined against human hepatocarcinoma cells (SMMC-7721). 232 To achieve synergetic cytotoxicity, a combination of ZnO NPs and an anticancer agent, DNR, was subjected to investigation. A schematic illustration of the anticancer effect of DNR-conjugated ZnO NPs under UV irradiation is shown in Fig. 22 . The outcome showed higher cytotoxicity of smaller NPs. UV irradiation greatly boosted the cytotoxic effect on SMMC-7721 cells treated with ZnO NPs via generation of ROS and a consequent cell apoptosis. Additionally, when the ZnO NPs were conjugated with DNR, their cytotoxicity against the cancer cells further increased by leaps and bounds.


	The schematic image of ZnO nanoparticle cytotoxicity and the PDT process in cooperation with daunorubicin in vitro (reproduced from ref. with permission from SpringerOpen).

To secure concomitant intracellular drug delivery and PDT for cancer treatment, poly(ethylene glycol) (PEG)-capped ZnO NPs enclosing DOX were fabricated. 233 It was found that DOX-loaded PEG-ZnO NPs on exposure to UV irradiation achieved significantly enhanced cell cytotoxicity through light-driven ROS production from the NPs. The synergistic anticancer activity of a combined treatment with PEG-ZnO NPs and DOX under UV irradiation came to the fore as a result of this investigation.

Likewise, poly(vinylpyrrolidone) (PVP)-capped ZnO nanorods (PVP-ZnO nanorods) were designed as a drug carrying nano-apparatus for the delivery of daunorubicin (DNR), as well as a photosensitizer for PDT. 234 The DNR-loaded PVP-ZnO nanorods (DNR-PVP-ZnO) encouraged an exceptional upswing in the anticancer activity of DNR due to elevated cellular uptake of the DNR delivered by the nanorods. The DNR-PVP-ZnO nanorods also demonstrated efficient PDT under UV light irradiation. It has been demonstrated that NPs can furnish solutions to confront the acute demerits of conventional photosensitizers. 235 By a dramatic enhancement of the solubility of photosensitizers, NPs can facilitate their increased cellular internalization. They also upgrade the target-specificity of photosensitizers by way of passive targeting to tumor tissues through the enhanced permeability and retention (EPR) effect. Further, cell-specificity of photosensitizers can be remarkably increased by surface modification of the NPs to bind active targeting components. Complexation of ZnO NPs with other photosensitizers has been widely researched to increase the efficacy of ZnO NPs in PDT by synergistically enhancing the ROS generation. 234,235 Meso -tetra( o -aminophenyl)porphyrin (MTAP)-conjugated ZnO nanocomposites were fabricated and examined for synergistic PDT against ovarian cancer cells. 236 The MTAP-ZnO NPs induced generation of ROS upon UV irradiation, the controlling parameters being concentration and light intensity. It emerged that 30 μM MTAP-ZnO NPs wielded high light-induced toxic effects in cancer-afflicted ovarian cells under UV illumination, while they remained inactive in the dark. The cytotoxic activity of MTAP-ZnO NPs under UV illumination was markedly boosted weighed against that of porphyrin alone. 235 This study elucidated the targeted and synergistic PDT by nanoparticles of ZnO loaded with photosensitizing substances. ZnO NPs were combined with protoporphyrin IX (PpIX) as a drug delivery nano-apparatus for photosensitizers. 237 Simple ZnO NPs and PEG-capped ZnO NPs were synthesized and examined for their cancer-eliminating effect against human muscle carcinoma cells. In the absence of laser light, ZnO NPs at 1 mM concentration were found to exert very low cytotoxicity (98% viability). In the presence of 630 nm laser light, PEG-capped ZnO NPs loaded with PpIX exhibited outstanding cytotoxicity owing to the increased ROS generation. Additionally, a high build-up of PpIX in the tumor area was observed when it was delivered by ZnO NPs, exhibiting the potency of ZnO NPs as a tumor-selective drug delivery system for photosensitizers.

5.9 Anti-diabetic activity

A natural extract of red sandalwood (RSW) as an effective anti-diabetic agent in conjugation with ZnO NPs has been tested by Kitture et al. 246 The anti-diabetic activity was evaluated with the help of α-amylase and α-glucosidase inhibition assay with murine pancreatic and small intestinal extracts. Results revealed that the ZnO–RSW conjugate effected a moderately higher percentage of inhibition (20%) against porcine pancreatic α-amylase and proved more effective against the crude murine pancreatic glucosidase than either of the two components alone (RSW and ZnO NPs). The conjugated ZnO–RSW induced 61.93% inhibition in glucosidase while the bare ZnO NPs and RSW exhibited 21.48 and 5.90% inhibition, respectively.

In an investigation conducted to compare the anti-diabetic activity and oxidative stress of ZnO NPs and ZnSO 4 in diabetic rats it was observed that ZnO NPs with small dimensions at higher doses (3 and 10 mg kg −1 ) had a much greater antidiabetic effect compared to ZnSO 4 (30 mg kg −1 ). The observation was backed up by a marvelous decline in the blood glucose level, a steep rise in the insulin level and a refinement of the serum zinc status in a time- and dose-dependent manner. However, it was finally inferred in the study that ZnO nanoparticles severely elicited oxidative stress particularly at higher doses corroborated by the altered erythrocyte antioxidant enzyme activity, enhancement in malondialdehyde (MDA) production, and remarkable drop in serum total antioxidant capacity. 240 Hyperglycemia can squarely trigger off an inflammatory state via activation of C-reactive protein (CRP) and cytokines, such as interleukins, eventually resulting in the development of cardiovascular diseases. Hussein et al. designed ZnO NPs using hydroxyl ethyl cellulose as a stabilizing agent to alleviate diabetic complications. 247 The study demonstrated that ZnO NPs could significantly decrease malondialdehyde (MDA), fast blood sugar and asymmetric dimethylarginine (ADMA) levels. The inflammatory markers, interleukin-1 (IL-1α) and CRP, were also notably lowered after ZnO NP treatment, concomitant with a rise in nitric oxide (NO) and serum antioxidant enzyme (PON-1) levels in diabetic rats.

An investigation was conducted in 2014 into the anti-diabetic potential of ZnO NPs in streptozotocin-induced diabetic albino (Sprague-Dawley) rats. 243 The researchers inferred that the administration of ZnO NPs in diabetic rats brought about a marked lowering of the blood glucose level, boosted the serum insulin level, and elicited the expression of insulin receptor and GLUT-2 proteins, suggesting the inherent capacity of ZnO NPs for diabetic remedy. The anti-diabetic activity of ZnO NPs in streptozotocin-induced diabetic (types 1 and 2) Wistar rats was also demonstrated by Umrani et al. in their research work. 248 The research revealed that ZnO NPs raised the levels of parameters like glucose, insulin, and lipid in rats attesting to the efficient anti-diabetic activity of ZnO NPs. The same research group recently undertook an enquiry into the mechanistic pathway behind the anti-diabetic properties of ZnO NPs in vitro . 249 They demonstrated that ZnO NPs led to protein kinase B (PKB) activation, enhanced glucose transporter 4 (GLUT-4) translocation and uptake of glucose, reduced glucose 6 phosphatase expression, proliferation of pancreatic beta cells, etc. , which were critically responsible for the anti-diabetic behaviour of ZnO NPs. The antidiabetic effectiveness of ZnO nanoparticles prepared using U. diocia leaf extract for treating alloxan-caused diabetic rats was evaluated. 250 From the characterization of the samples, the envelopment of extract over the ZnO-extract sample resulted in individual particles with enhanced properties compared to bulk ZnO. The occurrence of the nettle phytochemicals linked to the ZnO-extract sample was verified by various techniques, especially using TGA, FT-IR, and GC-MS analyses. Among all the employed treatments, the ZnO-extract performed the best for controlling the common complications accompanying diabetes. This biologically produced sample significantly lowered the levels of Fasting Blood Sugar (FBS), Total Triglycerides (TG), and Total Cholesterol (TC) and enhanced the high-density lipoprotein cholesterol (HDLC) and insulin levels in the diabetic rats when compared to the rest of the remedies. The results confirmed the synergistic relationship between ZnO and U. diocia leaf extract where ZnO-extract performed the best compared with the only extract and ZnO. From the results, the as-prepared ZnO-extract sample can be introduced as a non-toxic, applicable, and active phyto-nanotherapeutic agent for controlling diabetes complications.

ZnO nanoparticles were synthesized using a microwave-assisted method in the presence of Vaccinium arctostaphylos L. fruit extract. 251 A decrease in crystallite size was observed for the biologically synthesized ZnO compared to the chemically synthesized sample. Furthermore, the existence of organic moieties over the biologically synthesized ZnO NPs was approved using characterizing methods. Then, the alloxan-induced diabetic rats were divided into an untreated diabetic control group and a normal healthy control group, and the groups received insulin, chemically synthesized ZnO, plant extract, and biologically synthesized ZnO. After treatment, fasting blood glucose (FBS), high-density lipoprotein (HDL), total triglyceride (TG), total cholesterol (TC) and insulin were measured. Analysis showed a significant decrease in FBS and increase in HDL levels in all groups under treatment. However, the results for cholesterol reduction were only significant for the group treated with biologically synthesized ZnO. Despite the changes in the triglyceride and insulin levels, the results were not significant. For all the studied parameters, bio-mediated ZnO NPs were found to be the most effective in treating the alloxan-diabetic rats compared to the other studied treatment agents. All reports of ZnO NPs for diabetes treatment indicated that ZnO NPs could be employed as a promising agent in treating diabetes as well as attenuating its complications.

5.10 Anti-inflammatory activity

5.11 immunotherapy.


	Tumor growth and survival of immunized mice. (A) Tumor volume (left) and survival rate (right) of mice (five mice per group) injected with MC38/CEA cells. (B) Tumor growth in human CEA-transgenic mice (five mice per group) inoculated with MC38/CEA cells (reproduced from ref. with permission from Nature Publishing Group).

5.12 Wound healing


	Photographs of the wounds treated with (1) CO/CS-ZnO (5.0 wt%), (2) CO, and (3) gauze as a control at (A) day 0, (B) day 5, and (C) day 14 (reproduced from ref. with permission from the American Chemical Society).

The ensuing results revealed that, two weeks after administration, the synthesized nanocomposite induced a 90% reduction of wound area, while mere 70% wound repair was noted in the control experiment thereby bearing evidence of its commanding wound-fixing capacity. Augustine et al. 270 also fabricated ZnO NP decorated polycaprolactone (PCL) scaffolds and demonstrated that their implantation was able to boost faster wound-fixing by elevating the proliferation and migration of fibroblasts in an in vivo model (wound-healing model of American satin guinea pigs), without showing any marked signs of inflammation. Similarly, Bellare et al. 271 designed biocompatible ZnO NP based scaffolds of gelatin and poly(methyl vinyl ether)/maleic anhydride (PMVE/MA) with remarkable antibacterial effects. Their report threw light on the ability of the scaffolds for endothelial progenitor cell (EPC) adhesion and proliferation. Further, the topical application of the scaffolds on wounds of Swiss/alb mice displayed the potential to expedite the process of wound-fixing. Modern wound care materials suffer from several serious shortcomings that include inadequate porosity, inferior mechanical strength, lessened flexibility, lack of antibacterial properties, etc. Given this backdrop, a CS hydrogel/nanoparticulate ZnO-based bandage which exerted antibacterial effects against both Gram-negative ( E. coli ) and Gram-positive ( S. aureus ) bacteria was fabricated by Kumar et al. 272 The nanocomposite bandage characterized by biodegradability, microporosity and biocompatibility produced elevated wound healing in Sprague-Dawley rats and boosted re-epithelialization and collagen deposition at a remarkable pace. Taking into account the crucial factors of biocompatibility, antibacterial effects, and wound-fixing capacity, the researchers held that the hybrid nanomaterial-based bandage could be valuable for the healing of chronic wounds, burn wounds, diabetic foot ulcers, etc. Likewise, a porous bandage consisting of ZnO NPs conjugated with alginate hydrogel and exhibiting blood clotting capacity and bactericidal effects against E. coli , S. aureus , Candida albicans , and methicillin resistant S. aureus was fabricated by Mohandas et al. 261 The bandage made from the nanocomposite was observed to exhibit biocompatibility at a lower concentration of ZnO-NPs. Further, an ex vivo re-epithelialization investigation with porcine ear skin demonstrated that faster wound-fixing was effected by the hybrid nanomaterial-based bandage than only alginate control bandage. This was ascribed to the release of zinc ions that would enhance the proliferation and migration of keratinocyte cells to the wound area. Nair et al. 273 also developed a bandage consisting of a biocompatible nanocomposite of ZnO NPs conjugated with β-chitin hydrogel. The bandage showed efficient antibacterial activity (against S. aureus and E. coli ) and had the ability of blood clotting and activation of platelets. It was elucidated that the application of the bandage on wounds of Sprague Dawley rats led to faster healing, with enhanced collagen deposition and a reduced number of bacterial colonies than in the control experiment, indicating the remarkable wound repairing capacity of the hybrid nanomaterial-based bandage. A novel, biocompatible ZnO QDs@GO-CS hydrogel was constructed by Liang et al. 274 through the simple assembly of ZnO quantum dots (QDs) with GO sheets and via a simple electrostatic interaction with the loaded CS hydrogel. The antibacterial efficacy could reach 98.90% and 99.50% against S. aureus and E. coli bacteria, respectively, with a low-cost, rapid, and effective treatment. ZnO QDs in antibacterial nanoplatforms could immediately produce ROS and Zn 2+ under acidic intracellular conditions. In parallel, when exposed to 808 nm laser irradiation, hyperthermia from GO sheets could simultaneously kill bacteria. Thus, the excellent performance of the material stems from the combined effects of hyperthermia produced under the near-infrared irradiation of GO sheets, reactive oxygen species, the release of Zn 2+ from ZnO QDs under an acidic environment, and the antibacterial activity of the hydrogel. This work demonstrated that the synergy of antibacterial nanoplatforms could be used for wound anti-inflammatory activity in vivo indicated by the wound healing results. The hybrid hydrogel caused no evident side effects on major organs in mice during wound healing. Therefore, the biocompatible multimodal therapeutic nanoplatforms were proposed to possess great potential for antibacterial activity and wound healing. In a study by Dodero et al. , 275 the possibility of using for biomedical purposes alginate-based membranes embedding ZnO nanoparticles that were prepared via an electrospinning technique was extensively evaluated. The morphological investigation showed that the prepared mats were characterized by thin and homogeneous nanofibers (diameter of 100 ± 30 nm), creating a highly porous structure; moreover, EDX spectroscopy proved ZnO-NPs to be well dispersed within the samples, confirming the efficiency of the electrospinning technique to prepare nanocomposite membranes. Mouse fibroblast and human keratinocyte cell lines were used to assess the biological response of the prepared mats; cytotoxicity tests evidenced the safety of all the samples, which overall showed very promising outcomes in terms of keratinocyte adhesion and proliferation. In particular, the strontium- and barium-cross-linked mats were characterized by similar cell viability results to those obtained with a commercial porcine collagen membrane used as a control; moreover, except for the calcium-cross-linked sample, the prepared mats exhibited a good stability over a period of 10 days under physiological conditions. Antibacterial assays confirmed the proficiency of using ZnO nanoparticles against E. coli without compromising the biocompatibility of the membranes. The mechanical properties of the strontium cross-linked mats were similar to those of human skin ( i.e. , Young's modulus and tensile strength in the range 280–470 MPa and 15–21 MPa for the samples with and without nanoparticles, respectively), as well as the water vapor permeability ( i.e. , 3.8–4.7 × 10 −12 g m −1 Pa −1 s −1 ), which was held to be extremely important in order to ensure gas exchange and exudate removal; furthermore, due to the low moisture content ( i.e. , 11%), the prepared mats could be easily and safely stored for quite a long period without any negative effect on their properties. Consequently, the achieved results demonstrated that the prepared mats could be successfully employed for the preparation of surgical patches and wound healing products by using alginate as an economic and safer alternative to the commonly employed commercial animal collagen-derived membranes.

Ahmed et al. 276 fabricated chitosan/PVA/ZnO nanofiber membranes by using the electrospinning technique. The samples of chitosan/PVA and chitosan/PVA/ZnO tested for antibacterial efficacy and antioxidant potential demonstrated very encouraging results in diabetic wound healing. The nanofiber mats displayed outstanding antibacterial properties against various strains of bacteria. The samples of chitosan/PVA and chitosan/PVA/ZnO nanofiber membranes also manifested higher antioxidant properties which made them promising candidates for applications in diabetic wounds. In experiments involving diabetic rabbits, chitosan/PVA and chitosan/PVA/ZnO nanofiber mats exhibited increased performance of wound contractions in a time interval of 12 days. It was thus concluded in the study that the chitosan/PVA/ZnO nanofibrous membranes could serve as useful dressing materials for diabetic wounds, a major problem for type-2 diabetic patients worldwide.

5.13 Agriculture

5.14 photodegradation.

Ishwarya et al. 73 reported the degradation of methylene blue dye in the presence of ZnO NPs prepared using Ulva lactuca seaweed extract and solar irradiation in their study. With an optimum initial dye concentration of 25 ppm and an optimum catalyst loading of 200 mg, the dye present in 100 mL of water got degraded to 90.4% in 120 min.

Gawade et al. 75 carried out photocatalytic degradation of methyl rrange dye using green fabricated ZnO NPs. 81% of the dye (20 ppm) was degraded after 100 min exposure to UV light. This they carried out after the dye solution was stirred with the catalyst for 30 min in the dark for complete equilibrium of the adsorption–desorption phenomenon when 2% of the dye was found to be adsorbed. The optimum catalyst dose was observed to be 1.5 g dm −3 after the dose had been varied in between 0.1 and 2.0 g dm −3 . The increase in degradation efficiency is ascribed to two favourable factors: (a) an increase in the number of active sites and (b) an increase in the number of photons absorbed by the catalyst. Beyond the optimal quantity of the catalyst, aggregation of the catalyst results in the active sites on the catalyst surface becoming unavailable for light absorption. The turbidity of the suspension leading to the inhibition of photon absorption on the catalytic surface of ZnO NPs because of the scattering effect was cited as an additional cause for the lowered degradation efficiency after the optimal catalyst dose.

Enhanced photocatalytic activity of the Mg doped ZnO/reduced graphene oxide nanocomposite has been recently reported by Nithiyadevi et al. 286 They investigated photodegradation of cationic dyes Methylene Blue (MB) and Malachite Green (MG) under visible light irradiation. They achieved a 94.41% degradation of MB and a 99.56% degradation of MG after exposure to visible light for 75 min in each case. Both the photocatalytic degradations showed a marked increase in efficiency in comparison with that effected by bare ZnO NPs. They obeyed pseudo-first order kinetics and the rate constant assumed values of 0.0391 and 0.0493 min −1 respectively in the case of MB and MG. They cited the following reasons for the enhanced photocatalytic ability of the nanocomposite: (a) the introduction of reduced graphene oxide (RGO) enabled better adsorption of dye molecules through π–π conjugation between the dye and aromatic compounds of RGO, (b) the ability of RGO to facilitate the growth of the ZnO particles on RGO sheets, (c) the availability of a large reactive surface area, (d) the greater interfacial contact between ZnO and RGO, (e) increase in the lifetime of charge carriers most probably attributed to RGO, (f) narrowing of the energy bands of ZnO due to Mg 2+ substitution, (g) presence of oxygen vacancies and (h) the reduction of particle size.

Photodegradation of Methylene Blue (MB) was performed by Debasmita Sardar et al. 287 with an Ag-doped-ZnO nanocatalyst. On increasing the percentage of loaded Ag the rate of photocatalytic decomposition gradually increased and reached the maximum for 20% Ag loading on ZnO. The rate constant was found to be 0.0087 min −1 with a fairly high degradation efficiency of 55.87%. However, a sharp fall in the value of rate constant was observed for 25% Ag loading which remained almost the same on further increasing the Ag content, i.e. for 30 and 35% loading. It was also observed from TEM analysis that too much loading of Ag led to agglomeration and thus covered up the surface of ZnO preventing light absorption. Moreover, there were large numbers of unattached Ag nanoparticles which could be oxidized in the presence of reactive oxygen species. Oxidized silver would not initiate any charge separation in the system. It was thus assumed that silver up to this optimum amount could act as an electron–hole separation centre. Beyond the optimum amount, it could help in charge carrier recombination. In fact, a large number of negatively charged Ag particles (which had already accumulated electrons) on ZnO could capture holes and thus would start acting as a recombination site itself essentially by forming a bridge between an electron and a hole. Thus, the efficiency of charge separation and hence the photocatalytic capability declined to an appreciably large extent.

Very recently, Vaianoa et al. 288 too tried photo-catalytically favourable modification of ZnO by Ag. They too achieved similar results with regard to removal of phenol from water. The loading of Ag responded favourably in the range of 0.14–0.88 wt% but backfired beyond 1.28 wt%. Similar reasons as mentioned above were cited for the trends observed. A photocatalytic test was thus performed by using 0.15 g of the optimized catalyst (1% Ag/ZnO) to treat drinking water containing phenol with an optimized initial concentration of 50 mg L −1 in 100 mL aqueous solution. Near-complete mineralization was accomplished within 180 min of exposure to UV irradiation. Photoreaction was found to fit in the pseudo-first order kinetic model. Another investigation 289 reported a facile microwave assisted synthesis of two-dimensional ZnO nano-triangles with a band gap of around 3.33 eV. The as-synthesized ZnO nano-triangles were applied for the reduction of noxious p -nitroaniline within 50 min. They were further used for the effective elimination of Rose Bengal dye within 150 min.

Likewise, ZnO-nanorods were synthesized 290 by adopting a facile microwave assisted green route of synthesis for the complete reduction of nitro compounds. Lauric acid was used as a complexing and capping agent in the ethanol phase. The nanorods had an average diameter of 5.5–10.0 nm with a hexagonal crystal structure and further demonstrated unusual luminescence properties wherein high intensity UV and yellow emission bands were observed along with negligible blue and green emission bands. Toxic nitro-compounds p -nitrophenol, p -nitroaniline and 2,4,6-trinitrophenol were completely reduced into amino derivatives by NaBH 4 in the presence of these nanorods within 120, 45, and 18 min, respectively.

Chidambaram et al. 291 effectively constructed a ZnO/g-C 3 N 4 heterojunction using a facile, economically viable pyrolysis synthetic route for the photodegradation of methylene blue under visible light illumination. The nanocomposites prepared using 0.1, 0.2 and 0.3 molar ratios of zinc nitrate precursor are labeled 0.1ZnO/GCN, 0.2ZnO/GCN and 0.3ZnO/GCN, respectively. The nanocomposites are found to exhibit a fall in charge recombination corroborated by their photoluminescence spectra that showed a fall in the intensity of the concerned emission peak ( Fig. 25 ). A maximum photodegradation of 86% was achieved with 0.2ZnO/GCN in 60 min following a pseudo-first order kinetic rate constant of 0.032 min −1 while graphitic carbon nitride, 0.1ZnO/GCN and 0.3ZnO/GCN attained 44%, 73% and 76% degradation of methylene blue dye in the same time with lower rate constants. The loading of ZnO over g-C 3 N 4 sheets created a heterojunction ( Fig. 26 ). The excitation of electrons by visible light occurs from the valence band to the conduction band of g-C 3 N 4 . The excited electrons are transferred to the conduction band of ZnO while there occurs a simultaneous movement of holes from the valence band of ZnO to the valence band of g-C 3 N 4 via the smooth interface of the heterostructure. This enabled the generation of the superoxide anion radical and hydroxyl radicals that effected improved mineralization of the dye. An excess of Zn was deemed to cause recombination of photo-induced charges that led to decreased photocatalytic efficiency. In a recent investigation by the authors of the current work, 292 a destructive photocatalyst made up of ZnO nanorods/Fe 3 O 4 nanoparticles anchored onto g-C 3 N 4 sheets was synthesized using hydrothermal synthesis and ultrasonication techniques. HRTEM micrographs shed light on the coupling of Fe 3 O 4 nanoparticles with ZnO nanorods and the successful formation of the intended ternary heterojunction. The g-C 3 N 4 sheets fostered close contact between ZnO nanorods and Fe 3 O 4 nanoparticles thereby inducing a mellowed agglomeration of nanostructured ZnO/Fe 3 O 4 particles. The Tauc plot derived from UV-visible absorbance data showed that the ZnO/Fe 3 O 4 /g-C 3 N 4 nano-hybrid had a band gap of 2.48 eV. PL studies further confirmed the successful development of a staggered type II heterojunction with wide separation between light-induced charge carriers ( Fig. 27 ). The hybrid catalyst showed remarkable photocatalytic activity under visible light, as evident from the efficient degradation of pantoprazole, a pharmaceutical drug widely known as a recalcitrant organic water pollutant. This could be attributed to the synergistic interactions between ZnO, Fe 3 O 4 and g-C 3 N 4 . A degradation efficiency of 97.09% was achieved within 90 min with a remarkable pseudo-first order rate constant of 0.0433 min −1 . The incorporation of Fe 3 O 4 expectedly facilitated the ready recovery of the catalyst and the degradation efficiency displayed fair consistency up to 4 cycles. The work thus offered a cost-efficient strategy for tackling organic water pollutants.


	Photoluminescence spectra of GCN and ZnO/GCN nanocomposites (the inset shows the enlarged PL spectra in the wavelength region of 350–450 nm) (reproduced from ref. with permission from IOP Publishing).


	Schematic depiction of the photocatalytic degradation mechanism of the ZnO/GCN heterojunction (reproduced from ref. with permission from IOP Publishing).


	Schematic depiction of the photocatalytic degradation mechanism of the g-C N /ZnO/Fe O heterojunction (reproduced from ref. with permission from Elsevier).

In another study, 293 a facile generation of a quaternary nano-structured hybrid photocatalyst, g-C 3 N 4 /NiO/ZnO/Fe 3 O 4 , was proposed for photodegradation of an ecotoxic pharmaceutical drug, esomeprazole, in aqueous solution. The photocatalytic annihilation of esomeprazole as a prototypical organic contaminant was executed under LED irradiation. By itself the designed ternary heterojunction accomplished a maximum 95.05% photodegradation of esomeprazole and a TOC removal of 81.66% and COD reduction up to 70.68% under optimum conditions of catalyst dose, esomeprazole concentration and pH within 70 min at a superior pseudo-first order kinetic rate constant of 0.06616 min −1 . This actually implied an improvement of degradation over NiO/ZnO, g-C 3 N 4 /NiO and g-C 3 N 4 /ZnO up to ∼74, ∼57, and ∼42%, respectively. The specific reaction rate also went up remarkably by almost ∼3.8, ∼3.18, and ∼2.85 times in comparison with the values obtained for NiO/ZnO, g-C 3 N 4 /NiO and g-C 3 N 4 /ZnO, respectively. The remarkable photocatalytic potential of the heterostructured photocatalyst in practical applications was evident from its reconcilable performances under varying initial concentrations of esomeprazole and initial pH of the solution. The effect of the addition of H 2 O 2 was also put under scrutiny and it was found that the photocatalytic degradation, TOC removal and COD reduction increased to 98.43, 84.72, and 73.86%, respectively, upon addition of an optimum quantity of H 2 O 2 over the same time span. The impacts made by inorganic and organic species on photodegradation and the associated reaction kinetics were investigated and the results were reported. The inhibiting influence of water matrices on esomeprazole degradation was also evaluated for better assessment of the performance of the designed photocatalyst in a real aqueous environment.

CdS/ZnO photocatalysts were prepared by two steps via hydrothermal and photochemical methods for the photodegradation of rhodamine B (RhB) dye. 294 The UV/Vis absorption spectra revealed that the absorption performance of the heterostructure is extended toward the visible light region. The photocatalytic activities of both ZnO nanorod and CdS/ZnO heterostructures were investigated for the photodegradation of RhB dye. It was found that the CdS/ZnO heterostructure prepared with 30 min light illumination shows the best photocatalytic efficiency compared to the one at 15 min and pure ZnO nanorods. The better and enhanced photocatalytic efficiency of the CdS/ZnO heterostructure was ascribed to the high charge separation efficiency. The maximum photocatalytic efficiency of 85% was achieved within 8 h with the CdS/ZnO-30 min photocatalyst.

The photocatalytic degradation of rhodamine B (RhB) over chlorophyll-Cu co-modified ZnO catalysts (Chl-Cu/ZnO) was studied under visible-light irradiation by Worathitanon et al. 295 It was found that chlorophyll as an electron donor and copper in Cu 2+ form help inhibit the recombination of electron–hole pairs and improve the photoactivity of the catalyst. The synergistic effect between chlorophyll and Cu was found to improve the visible-light response of ZnO nanoparticles, resulting in excellent performance in photodegradation of RhB. The appropriate ratio of chlorophyll and Cu loadings over ZnO was 0.5Chl-0.10Cu/ZnO. At this ratio, under visible-light irradiation for 2 h, the degradation efficiency was approximately 99% (60 mg L −1 of RhB solution), of which 18% of RhB adsorption occurred under dark conditions. Moreover, outstanding reusability of Chl-Cu/ZnO, for up to six cycles, was found, with more than 80% degradation efficiency.

In yet another investigation, 296 ZnO nanowires (NWs) were successfully synthesized onto commercially available civil engineering materials using a hydrothermal synthesis method. This easy and low-cost method allowed obtaining an almost homogeneous repartition of nanostructures on the entirety of the surface of the substrates. The measured gap values were similar to those of the ZnO NWs grown on typical substrates, i.e. , ∼3.18 eV and 3.20 eV for concrete and tiling, respectively. The excellent photocatalytic efficiency of our samples was demonstrated on three commonly used dyes, namely, Methyl Orange (MO), Methylene Blue (MB) and Acid Red 14 (AR 14). All of the dyes were fully degraded in less than 2 h for MB and AR 14, and less than 3 h for the more difficult to degrade MO. Investigating the durability of the samples so prepared, very promising results were found, as they showed no loss of efficiency after four experiment cycles. The ability of implementing ZnO NWs on civil engineering materials, their good photocatalytic properties, and the possibility to re-use samples with minimal efficiency losses, even after several months, were found very promising for the use of the nanostructures as road surfaces for air or water depollution.

6. Toxic impacts and mechanisms of ZnO NPs

The toxicity mechanism of ZnO-NPs in zebrafish was investigated by Yu et al. 315 The toxicity caused by ZnO is primarily because of the release of Zn 2+ ions and through mechanical damage in zebrafish. ZnO-NPs induced elevation of intracellular Zn 2+ concentration, leading to over-generation of intracellular reactive oxygen species, leakage of plasma membrane, dysfunction of mitochondria, and ultimately cell death. 316 Therefore, it is demonstrated that cell uptake, intracellular dissolution and release of Zn 2+ are the inherent causes for high toxicity of ZnO-NPs. However, there are some disagreements regarding the role of dissolved Zn 2+ in the toxicity mechanisms of ZnO-NPs. Several researchers suggested that dissolved Zn 2+ from ZnO-NPs played a minor role in the toxicity of ZnO-NPs, 317,318 while other investigations indicated that most of the toxicity of ZnO-NPs is due to the dissolved Zn 2+ . 315,316 This discrepancy may be ascribed to the sensitivities of different organisms to dissolved Zn 2+ , such as single tissue cells, bacteria, zebrafish and so on. In the study of Stella et al. , 319 dissolved Zn 2+ from nZnO was considered to play the vital role in the toxicological mechanisms, which was inferred from the levels of the biomarkers of metallothionein (MT) and heat shock protein 70 (HSP70) in the body of O. melastigma larvae, but this dissolved Zn 2+ was obtained by filtering the ZnO-NP suspensions with a 0.1 μm sterile syringe filter and it might include ZnO-NPs whose diameters were smaller than 100 nm.

The dissolution of Zn 2+ ions from ZnO was also suggested to be the main mechanism for the toxicity of ZnO-NPs as claimed recently. 320,321 Li et al. 322 also reported the same mechanism for the toxicity of ZnO-NPs. They have studied the toxicity of ZnO-NPs with various initial concentrations to E. coli in ultrapure water, NaCl and PBS solutions. For higher concentrations of ZnO-NPs, although a few ZnO particles may attach to the bacterial cells, it was difficult to determine the contribution of nano-ZnO itself considering the high toxicity of co-existing Zn 2+ ions. In addition, bacteria could also release the solutes in response to osmotic down-shock in ultrapure water, resulting in damage to the normal physiological functions and the decrease of tolerance of bacteria to toxicants. 323 Therefore, the toxicity of nano-ZnO at 1 mg L −1 in ultrapure water was much higher than that in 0.85% NaCl solution. To confirm the toxicity mechanism of ZnO-NPs, the ultrastructural characteristics of normal E. coli cells and those treated with ultrapure water, ZnO-NPs, and Zn 2+ ions were investigated with TEM by Li and his research group. The morphologies of E. coli cells treated with ZnO-NPs or Zn 2+ ions were significantly different from those of normal E. coli cells. The cytoplasmic membranes were deformed, wherein some cells swelled and the intracellular substances leaked out under both Zn stress and osmotic stress. Combined with the toxicity results of nano-ZnO, bulk-ZnO, and Zn 2+ ions in ultrapure water, Li and co-workers concluded that the toxicity of nano-ZnO to E. coli was mainly attributed to the released Zn 2+ ions.

7. Challenges and prospects

With higher electron diffusivity than TiO 2 , high electron mobility, exceptionally large exciton binding energy, low cost and considerable stability against photo-corrosion, ZnO has been widely considered a perfect substitute for TiO 2 as the electron transport material in DSSCs and PSCs. However, ineffective surface passivation, interfacial charge recombination and long-term stability have collectively yielded poor electron injection efficiency and thereby low current density and efficiency of the ZnO based photovoltaic device. Probable remedies involve incorporation of organic and inorganic dopants for effective surface passivation and effecting surface modification for marked electronic contact. Poor control of the properties of individual building blocks and low device-to-device reproducibility are further areas that require investigative attention. As a yet further consideration, adequate studies devoted to the impact of facet selectivity, structure and morphology of ZnO nano-structures on the overall efficiency of solar cells and the associated mechanism have to be conducted.

ZnO nanostructured particles have revolutionized the field of photocatalysis. And their efficacy in water splitting and degradation of recalcitrant organic water pollutants has been widely investigated and taken advantage of. However, a few concerning aspects about their photocatalytic activity still need to be dealt with through possible corrective measures. First, the photodegrading ability of a prepared ZnO nano-catalyst needs to be checked by taking the pollutant of interest in lieu of a representative substance which in usual cases is a dye. This is because dye-degradation is relatively plain sailing while removal of pharmaceutical wastes, pesticides, insecticides or other endocrine disruptors offers greater challenges and complicacies. Moreover, the archives of scientific literature are brimming with thorough investigative reports concerning degradation of dyes. Furthermore, waste water contains a mix of different contaminants with varying ranges of pH and ionic strength. Few photodegradation studies have been conducted on organic pollutants in such a simulated sample of water while taking into account the effect of the presence of other contaminants, varying pH and ionic strength on the degradation kinetics. Second, a detailed insight into the mechanistic routes of the degradation of these compounds and their interaction with ZnO based nano-catalysts is elusive as of now and its development remains imperative and will unfold approaches to tackle other emerging contaminants. Third, many improvements in the very architecture of ZnO nanostructures are due specifically in areas such as surface area, particle size, separation and lifespan of charge carriers and so forth. Fourth, since band positions and band gaps are dependent on particle size, it becomes difficult to create heterojunctions able enough to achieve effective charge separation and thereby efficient photocatalytic activity. Systemic studies with a focus on discovering specific synthesis protocols for the achievement of ZnO based nanostructures with desired band positions and band gaps have to be embarked on. Also, there are a few difficulties associated with the operating procedures, such as loss and recovery of nano-structured photocatalysts in the course of post-synthesis treatment and photocatalytic activity. Furthermore, more sweeping research investigations are required to develop and verify the mathematical models for photocatalytic operations/systems for water/wastewater treatment in order to predict the quantum yield, kinetics and optimum conditions of the process.

ZnO nanomaterials may be outstanding candidates as biocompatible and biodegradable nanoplatforms for cancer targeted imaging and therapy. For in vivo imaging and therapy applications, the future of nanomedicine lies in multifunctional nanoplatforms combining both therapeutic components and multimodality imaging. Biocompatibility is also a concern for the applications of nanomaterials in biomedicine. Surface modification of nanomaterials plays a vital role in this context. Biocompatibility of ZnO nanomaterials might be enhanced by slowing down the dissolution rate through Fe doping 324 or surface capping. 325 Therefore, surface coating of ZnO NPs with biocompatible macromolecules, such as poly(lactic) acid, PEG, PEI and chitosan, was attempted to increase their suitability for further clinical usage. Another idea is the synthesis of ZnO nanoplatforms using the biodegradable and biocompatible materials already proven clinically. Some biocompatible polymers, such as liposomes and dendrimers, have been clinically approved for various pharmaceutical applications. Hence, the modification or conjugation of already approved therapeutic formulations or materials with functional ligands which will improve their diagnostic index could be essential. Much effort is needed for long-term in vivo toxicology studies to pave the way for future biomedical applications of these intriguing nanomaterials. Facile conjugation of various biocompatible polymers, imaging labels, and drugs to ZnO nanomaterials can be achieved because of the versatile surface chemistry.

Some other issues of ZnO NPs concerning their biomedical application and their impact on biological systems still need further meticulous inspection. Following are a few such concerns: (a) lack of comparative analysis of the biological advantages of ZnO NPs to other metal nanoparticles, (b) the limitations imposed by the toxicity of ZnO NPs toward biological systems continue to remain a hot potato in recent research, (c) limitations of biocompatible/biodegradable ZnO nanoplatforms for tumor targeted drug/gene delivery, (d) lack of evidence-based research carrying out as its focal point a thorough survey of the therapeutic roles of ZnO NPs in improving anticancer, antibacterial, anti-inflammatory, and anti-diabetic activities, and (e) lack of extensive in vivo investigations into the anticancer, antibacterial, anti-inflammatory, and anti-diabetic activities of ZnO. Fresh studies focused on the abovementioned issues would bring forth further elucidation and comprehension of the potential use of ZnO nanoparticles in biomedical diagnostic and therapeutic fields.

8. Conclusion

Author contributions, conflicts of interest, acknowledgements.

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ORIGINAL RESEARCH article

Synthesis of zinc oxide nanoparticles by ecofriendly routes: adsorbent for copper removal from wastewater.

$\nJulia de O. Primo$

1 Laboratório de Materiais e Compostos Inorgânicos (LabMat), Departamento de Química, Universidade Estadual Do Centro-Oeste, Guarapuava, Brazil
2 Chimie des Interactions Plasma-Surface (ChIPS), Research Institute for Materials Science and Engineering, Université de Mons, Mons, Belgium
3 Research Group on Carbon Nanostructures (CARBONNAGe), Université de Namur, Namur, Belgium
4 Laboratório Nacional de Luz Síncrotron (LNLS), Centro Nacional de Pesquisa em Energia e Materiais (CNPEM), Campinas, Brazil

Zinc Oxide nanoparticles have been synthesized by two simple routes using Aloe vera (green synthesis, route I) or Cassava starch (gelatinization, route II). The XRD patterns and Raman spectra show that both synthesis routes lead to single-phase ZnO. XPS results indicate the presence of zinc atoms with oxidation state Zn 2+ . SEM images of the ZnO nanoparticles synthesized using Cassava starch show the presence of pseudo-spherical nanoparticles and nanosheets, while just pseudo-spherical nanoparticles were observed when Aloe vera was used. The UV-Vis spectra showed a slight difference in the absorption edge of the ZnO particles obtained using Aloe vera (3.18 eV) and Cassava starch (3.24 eV). The ZnO nanoparticles were tested as adsorbents for the removal of copper in wastewater, it is shown that at low Cu 2+ ion concentration (~40 mg/L) the nanoparticles synthesized by both routes have the same removal efficiency, however, increasing the absorbate concentration (> 80 mg/L) the ZnO nanoparticles synthesized using Aloe vera have a higher removal efficiency. The synthesized ZnO nanoparticles can be used as effective and environmental-friendly metal trace absorbers in wastewater.

Introduction

The fast growth of the human population and the further development of industries have direct consequences on the environment, leading to the depletion of natural resources, with an emphasis on freshwater resources. The disposal of industrial, agricultural and domestic waste often contains heavy metals that are toxic to humans and other living species with long-term intake. Among these, copper is one of the most abundant pollutants in wastewater ( Ali et al., 2016 ), widely used in electroplating industries ( Rafiq et al., 2014 ), welding processes, agricultural processes, plumbing material, and electrical wiring ( Ali et al., 2016 ), its high consumption results in the presence of high amounts of this element in wasterwater. The toxic effects of this heavy metal, caused by bioaccumulation, can cause lung cancer, brain, liver, and kidney health problems, among others ( Aksu and Işoǧlu, 2005 ; Saleh, 2017 ). Therefore, it is crucial for the protection of the environment and for human health to remove this metal from industrial wasterwaters before it is disposed of.

Different techniques for the removal of copper from wastewaters have being proposed ( Fu and Wang, 2011 ), among them we can cite precipitation ( Negrea et al., 2008 ), electrocoagulation ( Dermentzis et al., 2011 ), filtration ( Kebria et al., 2015 ) and ion exchange ( Da̧browski et al., 2004 ). However, most of these methods are expensive and prove ineffective in removing heavy metals in trace concentrations. In this context, the adsorption method has stood out, due to its low-cost, ease of use ( Pan et al., 2003 ; Rafiq et al., 2014 ; Ali et al., 2016 ) and the possibility of recycling the adsorbent. Considering the importance of treating wastewater with ion removal at trace levels, in this work, ZnO (Zinc oxide) nanoparticles were used as an adsorbent. ZnO nanoparticles have been reported as good adsorbent of positive metal ions in wastewater ( Singh et al., 2011 ; Wang et al., 2013a ). The Zinc oxide is a type-n material belonging to the semiconductor group of II-VI, has a band-gap of 3.37 eV. It is one of the most widely studied oxides due to its singular physicochemical properties, that include high chemical stability, and wide light absorption range. Zinc oxide has been announced as active material in a myriad of applications such as antifungal ( Kavyashree et al., 2015 ; Sharma and Ghose, 2015 ), drug delivery ( Yuan et al., 2010 ; Chen et al., 2013 ), antibacterial ( Jones et al., 2008 ; Applerot et al., 2009 ), photocatalysts ( Banerjee et al., 2012 ; Lee et al., 2016 ), gas sensors ( Rai and Yu, 2012 ; Waclawik et al., 2012 ) and antioxidant ( Kumar et al., 2014 ). Due to its interesting properties and high applicability, various techniques have been reported for the ZnO synthesis ( Kolodziejczak-Radzimska and Jesionowski, 2014 ).

In this work, in addition to the use of ZnO nanoparticles as a copper ion adsorbent, it is described two low-toxicity routes to synthesize ZnO particles, which are easy to reproduce. The route I uses Aloe vera as an additive while route II uses Cassava starch. The use of these natural additives makes the synthesis more environmentally friendly, due to their high chemical reactivity and high combustion power, reducing the calcination temperature often used in the synthesis of the oxide, in addition, to act as complexing gelling. Aloe vera (Aloe barbadensis Miller) is a perennial plant belonging to the Liliaceae family, it consists mainly of glycoproteins, anthraquinones, saccharides, and others low-molecular-weight substances ( Choi and Chung, 2003 ); inside the leaves, there is a mucilaginous gel produced by the parenchymatous cells. Cassava starch, however, is a polysaccharide of biological, non-toxic, inexhaustible biocompatible, and biodegradable source ( Visinescu et al., 2011 ). The use of Starch and Aloe vera in the synthesis of ZnO nanoparticles has been reported ( Sangeetha et al., 2011 ; Khorsand Zak et al., 2013 ; Thirumavalavan et al., 2013 ; Carp et al., 2015 ; Kavyashree et al., 2015 ), however, here we propose simple routes with fewer steps for the synthesis of ZnO nanoparticles.

Experimental

All the chemicals used were of analytical grade. Zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O, 98%) was purchased from Dynamic and copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O, 99%) was purchased from Vetec (Sigma-Aldrich). All solutions were prepared with deionized water. Natural Cassava starch in the form of colloidal suspension was used as fuel. Aloe vera leaves were harvested in the São José region of Parana-Brazil. To obtain the extract of Aloe gel, about 200 g of Aloe vera leaves were washed with deionized water and the internal mucilaginous gel was extracted. Afterward, the mucilaginous gel was crushed using a pistil and a ceramic mortar to obtain the complete extract. Finally, the solution was washed, filtered and the resulting Aloe vera gel broth extract was stored under refrigeration (2°C).

Synthesis of Zinc Oxide

Two different routes, both easy to reproduce, were used for synthesizing Zinc oxide nanoparticles. In the route I (green synthesis), adapted from Sangeetha et al. (2011) , Aloe vera (AL) gel broth extracts at the concentration (90%) were prepared with distilled water, the volume was made up to 100 ml. Subsequently, zinc nitrate (9.40 g) was dissolved in the aloe extract solution under constant magnetic stirring (120 min.) and left at rest for 12 h. The suspension was calcined in a muffle furnace at temperatures (750 °C) for 1 h. In the route II (gelatinization method): first starch (ST) was extracted from 100 g of natural Cassava starch in 300 ml of distilled water under mechanical stirring for 2 h. It was then sieved, and in the colloidal starch suspension was added 9.40 g of zinc nitrate. After 60 min of mechanical stirring (600 rpm), the suspension was calcined in a muffle furnace at a temperature of 750 °C for 1 h ( Primo et al., 2019 ). The ZnO nanoparticles obtained were named Zn-AL (route I) and Zn-ST (route II).

Characterization Techniques

X-ray powder diffraction profile was performed at the Brazilian Synchrotron Light Laboratory (LNLS, using XRD1 beamline, 12 keV energy, λ = 1.033 Å, 2θ of 0°-80°) ( Carvalho et al., 2016 ). Scanning electron microscopy images were recorded using a JEOL-JSM-7500F Field Emission Scanning Electron Microscope operated 15 kV, the spatial resolution was 2.5 nm. The Raman spectra were recorded using a Micro-Raman system, Senterra Bruker Optik GmbH), λ = 532 nm, laser power 5 mW, time 10 s, resolution 4 cm −1 . The optical diffuse reflectance was measured (UV-VIS-NIR Spectrophotometer CARYb5G, Varian) in the range of 300–800 nm. Zeta potential was recorded using ZETASIZER NANO ZS90 (MALVERN), model ZEN 1,010 at 25°C. The zeta potentials of the nanoparticles were determined from their electrophoretic mobilities according to Smoluchowski's equation ( O'Brien and Hunter, 1981 ); the pH of these nanoparticles was adjusted between 3 and 11 using HCl or NaOH solutions. The chemical composition was evaluated by X-ray photoelectron spectroscopy (XPS) (Versaprobe PHI 5,000 from Physical Electronics, equipped with a monochromatic Al Kα X-ray source). The XPS spectra were collected at a take-off angle of 45° with respect to the electron energy analyzer and the spot size was 200 μm. Pass energy (PE) of 20 eV was used for the high-energy resolution spectra (Zn 2p, O 1s, and C 1s). The spectra were analyzed using the CASA-XPS software.

The metal ion solutions were analyzed using a Varian TM SpectrAA® 220 Flame Atomic Absorption Spectrometer (FAAS). The FAAS was equipped with an air-acetylene burner. The hollow cathode lamp was set at 4 mA and the analytical wavelength was adjusted at 324.8 nm. The slit size was adjusted to 0.2 nm. The standard curve was drawn by using copper standard solutions. After the adsorption, the ZnO nanoparticles were characterized concerning their composition by energy dispersive X-ray spectrometer (EDX) from Shimadzu, model EDX-7000, containing a Rh tube, operating at 50 and 15 W. The crystalline phases were identified by powder X-ray diffraction (XRD) performed on a Bruker model D2 Phaser with Cu Kα radiation (λ = 1.5418 Å), with scan in 2θ from 10° to 90° and step rate of 0.2°/s. The zeta potential was recorded using ZETASIZER NANO ZS90 from MALVERN, model ZEN 1010. The electronic spectra of the powdered pigments samples were measured on the range of 400–900 nm with a UV-Vis Ocean Optics spectrophotometer model USB-2000.

Adsorption Measurements

To investigate the efficiency of the ZnO nanoparticles as adsorbents for the removal of copper metal ions from water, an adsorption test was performed. The parameters: contact time; initial pH and initial metal ion concentration were investigated. The adsorption experiments were carried out in conical flasks containing 25 mL of copper solution with an initial concentration ranging from 40 to 120 mg L −1 . To this end, 250 mg of the ZnO particles were added, and the solutions were kept under continuous shaking for 240 min in a heating bath at 25°C. To study the adsorption kinetics and the pH parameters, 50 mg L −1 of a solution containing Cu (II) and the same amount of ZnO particles was prepared; its pH was adjusted using 0.1 HCl and 0.1 NaOH solutions. The resulting solutions were centrifuged at 1,200 rpm for 15 min. The ion concentration measurements were performed before the adsorption test without the presence of the adsorbent and after 4 h of adsorption in a flame atomic absorption spectrometer (FAAS).

The amount of Cu 2+ ion adsorbed at the end of the adsorption experiment and the ion percentage removal (%) by the ZnO nanoparticles were calculated applying Equations (1, 2), respectively:

where q is the amount of ion adsorbed by the adsorbent in mg g −1 , C o is the initial ion concentration in contact with the adsorbent (mg.L −1 ), C f is the ion concentration (mg.L −1 ) after the batch adsorption process, m (g) is the mass of adsorbent and V (L) is the volume of ion solution.

Test Leaching of Nanoparticles

To check the stability of the nanoparticles a method adapted from ( Rafiq et al., 2014 ) was used. Thus, 50 mL of simulated sample was treated separately with 250 mg of ZnO synthesized. The initial pH of the experiment was 4 or 6 and the contents were allowed to remain in contact for 240 min while maintaining the temperature at 25°C. After centrifugation and filtration, the residue was washed with deionized water followed by oven drying at 60°C.

Results and Discussion

Characterization of the zinc oxides nanoparticles.

Figure 1A presents the X-ray diffractograms of the zinc oxides nanoparticles obtained after heat treatment at 750°C for 60 min. The crystalline phase is identified by the presence of the characteristic peaks of the Wurtzite ZnO phase ( Kisi and Elcombe, 1989 ), belonging to the compact hexagonal system with a space group P63mc to the crystallographic chart [JCPDS, #PDF01-070-8070]. Additional peaks were not detected, evidencing that the single-phase ZnO was successfully obtained regardless of the synthesis route used and the precursors were completely decomposed. The XRD patterns allowed to determine the average crystallite size of the ZnO nanoparticles, estimated by Scherrer's equation {D = 0.9λ/(B cosθ)} ( Hedayati et al., 2015 ), with the average size of 43.3 nm for Zn-AL and 44.9 nm for Zn-ST. According to these results, the crystalline size is affected by the polysaccharide used in the synthesis, at the same calcination temperature.

Figure 1. (A) XRD pattern and (B) Raman spectrum of the ZnO samples.

Figure 1B shows the Raman spectra of the samples indicating the characteristic wurtzite phase peaks, corroborating with the XRD patterns. The predominant bands are at 99 cm −1 (mode E 2 low) and 437 cm −1 (mode E 2 high). The E 2 low mode is attributed to the vibrations of zinc sublattice in ZnO and E 2 high mode is assigned to the oxygen vibration ( Cuscó et al., 2007 ; Stanković et al., 2012 ), the strong E 2 high mode indicates the high crystallinity of the oxide ( Jothilakshmi et al., 2009 ), the same vibrational mode has been identified for the zinc oxide nanoparticles obtained via the Starch-assisted synthetic route, reported by Carp et al. (2015) . The bands at 380 and 408 cm −1 correspond to the first-order optical modes A 1 (TO) and E 1 (TO), bands at 202 and 330 cm −1 are characteristic of second-order modes 2E 2 low and E 2 high—E 2 low, caused by multiphonon processes. The bands located at 573 and 584 cm-1 are assigned to A1(LO) and E1(LO) modes, these bands are associated to the presence of structural defects in the ZnO structure, being the E 1 (LO) mode strongly affected ( Cuscó et al., 2007 ).

Figures 2A,B shows the SEM images for Zn-AL, which consists of pseudo-spheres, and non-uniform hexagonal particles. For Zn-ST, uniform spherical particles are formed ( Figures 2C,D ). The two samples show particle aggregation, related to the self-assembly effect ( Khorsand Zak et al., 2013 ). The Zn-AL particles tend to agglomerate in plaques ( Figure 2A ), this was attributed to the Aloe vera gel acting as a sacrifice complexant in the formation of the ZnO nanoparticles during the combustion ( Kavyashree et al., 2015 ). The two synthesis routes (Aloe vera and Cassava starch) have polysaccharides as fuel for the formation of ZnO nanoparticles; their formation mechanism can be described by the “egg-box” model ( Kavyashree et al., 2015 ). Therefore, their difference in morphology can be associated with the complex polymeric network of each polysaccharide. Aloe vera gel consists of a combination of organic chains, such as soluble polysaccharides, monosaccharides, proteins, amino acids, among others ( Choi and Chung, 2003 ). The colloidal suspension of Cassava starch is more homogeneous and less complex because it consists basically of amylopectin and amylose leading to the formation of uniform particles, since the Zn (II) ions occupy the “egg-box” more efficiently, with more regular distance. The Aloe vera gel presents a greater variation in its natural components than Cassava starch, affecting directly the shape and reproducibility of ZnO nanoparticles.

Figure 2 . SEM image (A) 4,000x (B) 8,000x Zn-AL; and (C) 4,000x (D) 8,000x Zn-ST.

The chemical environment of the zinc and oxygen atoms were analyzed using X-ray photoelectron spectroscopy (XPS). The O 1s and Zn 2p XPS core-level spectra are shown in Figure 3 . The binding energy of the XPS data was calibrated using the C 1s peak at 284.6 eV ( Das et al., 2010 ). The O 1s spectrum was fitted with three components centered on 530.2 ± 0.1, 531.4 ± 0.6, and 532.3 ± 0.7 eV, for both samples ( Figure 3A ). The low binding energy component located at 530.2 ± 0.1 eV is attributed to O 2− ions participating in the Zn-O bond in the wurtzite structure of the hexagonal Zn 2+ ions of ZnO ( Chen et al., 2000 ; Al-Gaashani et al., 2013 ). The component centered at 531.4 ± 0.6 is associated with photoelectrons emitted from O 2− ions in oxygen-deficient regions in the matrix of ZnO ( Chen et al., 2000 ). The high binding energy component located at 532.7 ± 0.7 is reported to be associated with oxygen species adsorbed on the surface of the ZnO, such a -CO 3 , H 2 O, or O 2 ( Sangeetha et al., 2011 ; Visinescu et al., 2011 ). The Zn 2p high-resolution XPS spectra show the 2p doublet ( Figure 3B ) with components centered at 1020.6 eV (Zn 2p 3/2 ) and 1043.5 eV (Zn 2p 1/2 ). For both samples, the binding energy difference between these core levels is 23.0 eV, reference value denoting the presence of zinc in Zn 2+ oxidation state ( Chen et al., 2000 ; Das et al., 2010 ), the chemical state is confirmed by the Zn LMM Auger data ( Figure 3C ).

Figure 3 . XPS spectra of (A) O 1s. (B) Zn 2p and (C) Zn LMM Auger of the ZnO samples.

Figure 4 shows the optical characterization of the ZnO nanoparticles synthesized using Aloe vera (Route I) and Cassava starch (Route II). It can be observed in Figure 4A an increase in the reflectance at wavelengths larger than 380 nm, this can be attributed to the direct band-gap of ZnO due to the electron transitions from the valence band to the conduction band (O 2p Zn 3d ) ( Kavyashree et al., 2015 ), with a lower percentage of reflectance for Zn-AL (~65%). The band energy gaps (E GAP ) of the samples were calculated using the Kubelka-Munk method ( Cuscó et al., 2007 ), the E GAP were determined by linear extrapolation of the curve [F(R) x E]2 vs. energy (E) in ( Figure 4B ), with values: 3.24 eV (Zn-ST) and 3.18 eV (Zn-AL), similar values have been reported in ( Khorsand Zak et al., 2013 ; Carp et al., 2015 ) for zinc oxides obtained with Starch. The variation in the optical gap of the ZnO nanoparticles can be associated with a variation in the average particle size and morphology. The synthesized ZnO nanoparticles exhibit a slight red shift in the absorption edge ( Figure 4A ), this increase in the response range toward the visible radiation region can be explored in the future as photocatalysts with visible light activity ( Stanković et al., 2012 ).

Figure 4. (A) Diffuse reflectance spectra and (B) Kubelka-Munk curves of ZnO samples.

Copper Ion Removal by ZnO Particles

Zeta potencial (ζ) vs. ph.

Figure 5 shows the obtained ζ-potential values as a function of pH for Zn-AL and Zn-ST. The zeta potential allows evaluating if the particles in the colloidal state show chemical stability. A high ζ-potential generates electrostatic repulsion, preventing particles flocculation and aggregation ( Rodrigues et al., 2020 ); this range of ζ-potential is located below −30 mV or above +30 mV. When the pH <6, the ZnO surface charge shows a strongly positive ζ potential value equal to + 30 ± 2 mV for Zn-ST and +24 ± 2 mV for Zn-ST. Increasing the pH, the point of zero charge (pH PZC ) is reached at 8.8 and 9.4 for Zn-AL and Zn-ST, respectively. These values are in accordance with the values of the literature pH PZC for ZnO ( Adair et al., 2001 ; Tso et al., 2010 ). By further increasing the pH ≥ pH PZC the ZnO nanoparticles exhibit negative surface charge values.

Figure 5 . Zeta potential as a function of pH and point of zero charge (pH PZC ) for the Zn-AL and Zn-ST samples.

Effect of pH on Adsorption

In the absorption study, the pH is an important factor that affects the surface charge of the adsorbent and the degree of ionization of the ions affects the adsorption capacity. The adsorption study was carried at different pHs (2–6), because the copper ion precipitates as Cu(OH) 2 at pH ≥ 6 ( Bagheri et al., 2014 ). The effect of the pH in the adsorption of the heavy metal by the ZnO is shown in Figure 6 . The removal of the Cu (II) ions is strongly dependent on pH, with percentage of removal > 95% for all evaluated pH, reaching the maximum adsorption at pH of 4 for Zn-AL.

Figure 6 . Effect of the pH on the adsorption of the Cu (II) ions by the Zn-AL and Zn-ST oxides.

The removal of Cu (II) on surface of ZnO, can be explained in terms of the adsorbent pH PZC (point zero charge), for values of the pH < pH PZC the adsorbent surface is protonated and positively charged, while for pH > pH PZC the active sites are deprotonated and the charge is negative ( Kikuchi et al., 2006 ; Bagheri et al., 2014 ). Thus, increasing the pH, the competition between H + and Cu 2+ decreases by the reduction of the repulsive force. However, in this study, the adsorption at pH < pH PZC was observed ( Figure 5 ), indicating that, the adsorption of metallic ions on the surface of ZnO may occur via non-electrostatic interaction. When the ZnO particles are exposed in water, hydroxyl groups will be formed ( Wang et al., 2010 ; Le et al., 2019 ), becoming adsorptive active sites removing metal ions by reacting with OH − on the ZnO surface ( Bagheri et al., 2014 ). Thus, the mechanism of ion adsorption can be explained by the model of complexing of ion adsorption in hydrated solids ( Faur-Brasquet et al., 2002 ; Bagheri et al., 2014 ), in which the Cu (II) ions interact with the active groups (OH − ) on the oxide surface:

Therefore, with an increase in the pH of the solution, the amount of active sites on the ZnO surface increases, becoming more favorable to the adsorption of the metallic ion, thus resulting in a greater removal efficiency. The decrease in the removal of Cu (II) ions at pH 6 ( Figure 6 ), can be associated to their precipitation occurring from pH ≥ 6, and thus forming complexes that are not adsorbed by the ZnO adsorbents.

The isotherm and adsorption kinetics studies were performed without pH (pH ~ 6) adjustment. The stability of ZnO nanoparticle was checked after the adsorption experiment at pH 4 and 6. For pH4 the dried samples weight 253.2 mg and 252.8 mg, and for pH6 they weight 252.4 mg and 252.1 mg for Zn-AL and Zn-ST, respectively. The small increase in the weight compared to the initial one (250 mg) can be associated to the absence of adsorbent leaching.

Effect of Initial Metal Ion Concentration

Figure 7 shows the effect of the initial metal concentration on the percentage of the Cu (II) removal. The studies were carried out at optimized contact time and temperature at 25 o C. The results show that the percentage of removal decreases for increasing the initial concentration for both Zn-AL and Zn-ST. At low concentrations the metal ions are adsorbed by specific sites, with the increase in the concentration of Cu (II) ion occurs a saturation of these active sites, and the exchange sites are filled ( Rafiq et al., 2014 ). Conversely, the amount of copper adsorbed per gram of adsorbent (q e ) increased with the increase in the initial concentration of Cu ions, due to the Cu (II) ion adsorption capacity on the available adsorbent.

Figure 7 . Effect of initial concentration on adsorption and equilibrium amount adsorbed of Cu (II) ion in Zn-AL and Zn-ST.

Adsorption Isotherms

The Cu (II) ion adsorption isotherms of Zn-AL and Zn-ST are presented in Figure 8 . The results show that in the Zn-AL the saturation was not effectively reached, while the Zn-ST shows saturation when the initial concentration of 100 mg L −1 and 120 mg L −1 were investigated.

Figure 8 . Adsorption of Cu (II) ions into ZnO samples at 25 °C.

Adsorption of Cu (II) ions into Zn-AL and Zn-ST data were adjusted according to the Langmuir and the Freundlich isotherm models and their correlation parameter are presented in Table 1 . The Langmuir model is applicable in systems with ideal homogeneous surface adsorption ( Azizian and Bagheri, 2014 ; Jing et al., 2018 ). This isothermal model is generally defined as monolayer saturation capacity and the maximum adsorption capacity of the adsorbent for a particular adsorbate ( Jaerger et al., 2015 ). The Freundlich model, on the other hand, reproduces better a heterogeneous system ( Jaerger et al., 2015 ). The Langmuir isotherm in the linear form is given as (Equation 3):

where q e (mg g −1 ) is the amount of ions adsorbed per unit mass of ZnO at equilibrium; K L (L mg −1 ) is the Langmuir constant related to the affinity of the binding sites; q max (mg g −1 ) is a parameter related to the maximum amount of Cu (II) per unit weight of ZnO.

Table 1 . Parameters of the Langmuir isotherms and the Freundlich for adsorption of Cu (II) ions into ZnO samples.

The Freundlich isotherm is an empirical model and is commonly used for low concentrations of adsorbate ( Jaerger et al., 2015 ). The linearized form of the Freundlich isotherm is given as (Equation 4):

where K F (mg L −1 ) is the Freundlich constant; n is a parameter related to the intensity of adsorption and the system heterogeneity. K F and n are the Freundlich constants determined from the intercept and slope of the straight line of the plot ln q e vs. ln C e .

The correlation coefficients ( r 2 ) obtained by the Langmuir isothermal model were well-fitted as shown in Table 1 . The adsorption process consists of monolayer adsorption of Cu (II) ions at the ZnO nanoparticles surface, this is observed for the nanoparticle synthesized by both routes. Another important property obtained analyzing the Langmuir isothermal is the maximum adsorption capacity (q max ). The values of q max for Zn-AL and Zn-ST were 20.42 and 10.95 mg g −1 , respectively. The maximum adsorption values obtained in this study are relatively low compared with data reported in the literature ( Table 2 ) ( Azizian and Bagheri, 2014 ; Jing et al., 2018 ). However, in the present study, the interest is in the removal of low concentrations, and saturation was not observed for both Zn-AL and Zn-ST as adsorbent. The ZnO nanoparticles produced by both routes have good characteristics as Cu (II) ion adsorbent showing a percentage of removal near to 98% for low metal concentration ( Figure 7 ) indicating that the synthesized nanoparticles are potential adsorbent of metal traces in wastewater.

Table 2 . Reported adsorption capacities (mg g −1 ) of copper using Zinc Oxide as adsorbent.

Effect of Contact Time

Figure 9 shows the effect of contact time on the Cu (II) adsorption. The adsorption of copper by ZnO nanoparticles was investigated as a function of contact time in the range between 5 min and 240 min with 50 mg L −1 initial ZnO concentration. The value of the copper removal gradually increases with the time until the equilibrium is reached within 120 min. No significant increase occurred between 180 and 240 min until the adsorption equilibrium was reached. Both Zn-AL and Zn-ST reached the Cu (II) ion adsorption equilibrium at 150 min. The pseudo-first-order model is widely used in solute adsorption in a liquid solution and is represented by Equation (5):

where q t (mg g −1 ) is the amount of Cu (II) adsorbed at time t (min) and k 1 is the rate constant of the pseudo-first-order adsorption (min −1 ). The pseudo-second-order kinetics equation is based on the adsorption capacity and is represented in Equation (6):

where k 2 (g mg −1 min −1 ) is the pseudo-second-order adsorption rate constant.

Figure 9 . Progressive removal of Cu (II) ions from aqueous solutions using Zn-AL and Zn-ST as adsorbent.

Table 3 shows the values of the kinetic parameters obtained for the removal of Cu (II) ions in Zn-AL and Zn-ST. As observed ( Figure 10 and Table 3 ), the adsorption data adjusted better to the pseudo-second-order kinetic model, since the linear correlation coefficients r 2 2 are above 0.99 for all Cu (II) ion solutions at 25 o C. The q e data obtained experimentally are closer to those obtained by the pseudo-second-order model, this fact indicates that the adsorption process is dependent on both the quantity of Cu (II) ions and ZnO sites available ( Almeida et al., 2010 ; Jaerger et al., 2015 ). These results agreed with the report on the adsorption of metals in ZnO by Rafiq et al. (2014) and Kumar et al. (2013) .

Table 3 . Kinetic parameters for Cu (II) removal using Zn-AL and Zn-ST as adsorbent.

Figure 10 . Kinetic parameters for Cu (II) removal using Zn-AL and Zn-ST as adsorbent.

Equation (7) describes a model of the effect of the intraparticle diffusion on adsorption based on the theory proposed by Weber and Morris:

where k i is the rate constant (mg g −1 t −0.5 ) and values of C i give information regarding the thickness of the boundary layer.

When the adsorption mechanism follows the intraparticle diffusion process k i can be obtained from q t vs. t 0.5 plot. In this study, the data displayed multilinear graphs, governed by two steps as shown in Figure 11 . This fact indicates that the adsorption process involves more than one mode in the adsorption of the Cu (II) ion by the ZnO nanoparticles. The linear segment of the adsorption curve is attributed to the immediate adsorption occurring at sites available on the oxide surface. While the second linear portion refers to the adsorption in the final stages of the adsorption equilibrium, where the intraparticle diffusion process begins to decrease and reach a plateau due to the low concentration of remaining Cu (II) ions or because the maximum adsorption by the adsorbate is achieved ( Rafiq et al., 2014 ; Jaerger et al., 2015 ). The results of the intraparticle diffusion for Cu (II) ions in both Zn-AL and Zn-ST oxides suggest that the adsorption is controlled initially by the external mass transfer, followed by the mass transfer by the intraparticle diffusion until reaching equilibrium ( Almeida et al., 2010 ; Jaerger et al., 2015 ). These steps agree with the decrease in the diffusion rate going from k i1 > k i2 corroborating with the increase in the thickness of the limit layer C i1 < C i2 ( Rafiq et al., 2014 ) as observed in Table 4 .

Figure 11 . An intraparticle diffusion model for Cu (II) removal using Zn-AL and Zn-ST as adsorbent.

Table 4 . Intra-particle diffusion constants for Cu (II) removal using ZnO as adsorbent.

Characterization of Cu/ZnO Nanoparticles

After the adsorption assay, the samples at pH 4 and 6 were dried at 60 °C in an oven-dry for 12 h and characterized. The chemical composition data (EDXRF) evince the incorporation of the Cu (II) ions in both Zn-AL and Zn-ST nanoparticles following their surface adsorption ( Table 5 ).

Table 5 . Compositional chemical analysis data by EDXRF (% element).

Figure 12 shows the XRD patterns recorded on the ZnO nanoparticles before and after the Cu (II) metal ions removal. It can be seen that the diffraction peaks recorded on the samples after the adsorption correspond to the majority of peaks of the ZnO hexagonal Wurtzite crystal phase (XRD recorded on the Zn-AL and Zn-ST samples before adsorption). However, in the XRD patterns recorded after adsorption, the CuO phase can be observed in both routes (I and II) samples and for different pHs. The diffraction peaks of CuO were indexed to the monoclinic Tenorite crystal phase of CuO (JCPDS, #PDF 96-901-6327). Moreover, similar to the CuO Bragg peaks the ZnO peaks slightly shift to lower diffraction angles compared to that of the ZnO recorded before adsorption, indicating the substitution of Zn 2+ by Cu 2+ ions in the crystal lattice ( Mukhtar et al., 2012 ). According to ( Wang et al., 2010 ), the hydrated Cu (II) or Cu(H 2 O) 6 2+ can react with the OH − groups and form Cu-O weak bounds through a Lewis interaction, in addition, the adsorbed copper ions can partially hydrolyze leading to the formation of Cu-OH and, consequently, the formation of Cu-O-Cu on the surface of ZnO, thus denoting the Tenorite phase of CuO formed on the surface of ZnO. The crystallite sizes after the adsorption were calculated using Scherrer's equation ( Hedayati et al., 2015 ), the obtained value were: 16.31 nm (Zn-AL, pH 4), and 36.22 nm (Zn-AL, pH 6); and 20.18 nm (Zn-ST, pH 4), and 31.89 nm (Zn-ST, pH 6).

Figure 12 . XRD pattern before adsorption of copper solution 50 mg L −1 , pH 4, and 6: (A) Zn-AL and (B) Zn-ST.

Figure 13 shows the UV-Vis absorbance spectra of ZnO samples before and after Cu (II) adsorption. A broad peak in the visible region centered at 720 nm can be observed after the adsorption of copper. This peak can be assigned to the Cu (II) d-d transition ( Li et al., 2013 ), indicating, the adsorption of Cu (II) ions by the ZnO nanoparticles (Zn-ST and Zn-AL), verifying the XRD results.

Figure 13 . Electronic absorption spectra (visible) of ZnO before the adsorption of copper solution 50 mg L −1 , pH 4 and 6: (A) Zn-AL and (B) Zn-ST.

Conclusions

ZnO nanoparticles have been successfully synthesized by an eco-friendly procedure based on a polysaccharide, without using any surfactant, organic solvent and at low calcination temperature. The type of polysaccharide used as fuel influences on the morphology and optical property of the synthesized nanoparticles (Zn-AL and Zn-ST). Cu (II) adsorption tests showed a low experimental (q max ) value, however saturation was not observed on both synthesized (route I and route II) ZnO adsorbents in the 4 h study period. For both samples (Zn-AL and Zn-ST) at a concentration of 40 mg L −1 of copper ions, there was a high removal value of R%>95% indicating that the synthesized nanoparticles have the potential to be used in the treatment of wastewater, especially in the removal of metal ions at low concentrations. The XRD analysis of the samples after Cu (II) adsorption indicates the formation of the Tenorite phase on the ZnO nanoparticles surface regardless of the pH used in the adsorption experiment, denoting the formation of a secondary phase in the ZnO structure. Accordingly, the two ZnO synthesis routes favor controlling the surface charge, phase, crystallite size, modulating solids for specific applications (photocatalysis, sensor, pigments).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

JP performed the methodology, conceptualization, investigation, and wrote the manuscript with input from CB, FA, and SJ. CB, SA, and JP performed XPS measurement and analysis. AS-C and J-FC performed the SEM measurement. VT investigation and formal data analysis of XRD. SJ performed of adsorption tests and discussion. J-FC, CB, and FA supervision. VT, CB, and FA funding acquisition and project administration. All authors contributed to the article and approved the submitted version.

This work was supported by CNPq, CAPES, Finep, Fundação Araucária, and Founds de la Recherche Scientifique (FNRS—No. 2019/V 6/5/006—JG/JN−296). This research used resources of the Brazilian Synchrotron Light Laboratory (LNLS), proposal implemented XRD1 #20190063, line XRD1/LNLS/CNPEM.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

JP thanks CAPES for a graduate scholarship. CB is a Research Associate of the FRS-FNRS, Belgium. The authors are grateful to Ketlyn W. Borth for XRD measurements, Dr. Sueli P. Quináia, and Ms. Mariane Butik for FAAS measurements, and Dr. Rafael Marangoni for the adsorption tests (UNICENTRO).

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Keywords: zinc oxide, starch, aloe vera, copper ion, water treatment

Citation: Primo JdO, Bittencourt C, Acosta S, Sierra-Castillo A, Colomer J-F, Jaerger S, Teixeira VC and Anaissi FJ (2020) Synthesis of Zinc Oxide Nanoparticles by Ecofriendly Routes: Adsorbent for Copper Removal From Wastewater. Front. Chem. 8:571790. doi: 10.3389/fchem.2020.571790

Received: 11 June 2020; Accepted: 26 October 2020; Published: 27 November 2020.

Reviewed by:

Copyright © 2020 Primo, Bittencourt, Acosta, Sierra-Castillo, Colomer, Jaerger, Teixeira and Anaissi. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Carla Bittencourt, carla.bittencourt@umons.ac.be

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Published: 04 September 2024

CTAB-crafted ZnO nanostructures for environmental remediation and pathogen control

Jyoti Gaur 1 ,
Sanjeev Kumar 2 ,
Mhamed Zineddine 3 ,
Harpreet Kaur 2 ,
Mohinder Pal 1 ,
Kanchan Bala 4 ,
Vanish Kumar 5 ,
Gurmeet Singh Lotey 6 ,
Mustapha Musa 3 &
Omar El Outassi 7

Scientific Reports volume 14 , Article number: 20561 ( 2024 ) Cite this article

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Nanoscale materials
Pollution remediation

This study addresses the critical need for efficient and sustainable methods to tackle organic pollutants and microbial contamination in water. The present work aim was to investigate the potential of multi-structured zinc oxide nanoparticles (ZnO NPs) for the combined photocatalytic degradation of organic pollutants and antimicrobial activity. A unique fusion of precipitation-cum-hydrothermal approaches was precisely employed to synthesize the ZnO NPs, resulting in remarkable outcomes. The synthesized CTAB/ZnO NPs demonstrated exceptional properties: they were multi-structured and crystalline with a size of 40 nm and possessed a narrow band gap energy of 2.82 eV, enhancing light absorption for photocatalysis. These nanoparticles achieved an impressive degradation efficiency of 91.75% for Reactive Blue-81 dye within 105 min under UV irradiation. Furthermore, their photocatalytic performance metrics were outstanding, including a quantum yield of 1.73 × 10 –4 Φ, a kinetic reaction rate of 3.89 × 10 2 µmol g –1 h –1 , a space–time yield of 8.64 × 10 –6 molecules photon –1 mg –1 , and a figure-of-merit of 1.03 × 10 –9 mol L J –1 g –1 h –1 . Notably, the energy consumption was low at 1.73 × 10 –4 J mol –1 , compared to other systems. Additionally, the ZnO NPs exhibited effective antimicrobial activity against S. aureus and P. aeruginosa . This research underscores the potential of tailored ZnO NPs as a versatile solution for addressing both organic pollution and microbial contamination in water treatment processes. The low energy consumption further enhances its attractiveness as a sustainable solution.

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Introduction.

The continuously increasing industrial discharge into lakes, oceans, rivers, and other water bodies severely affected the environment health 1 , 2 . It should be noted that pollutants like heavy metals, dyes, pesticides, and industrial and pharmaceutical waste are very common in wastewater. These pollutants may or may not be directly toxic; however, a few of them can produce hazardous by-products through diverse processes, e.g., hydrolysis, chemical reactions, and oxidation. The above-mentioned processes is suspected to increase the chemical oxygen demand (COD) and biological oxygen demand (BOD), which is threatening not only for the environment (e.g., for microorganisms, plants, animals, and aquatic life) 3 , 4 , 5 .In particular, the increased use of dyes in several industries (e.g., textiles, plastic, leather, medical, automobiles, and paper industries) has contributed significantly to the environmental degradation 6 , 7 , 8 . In recent years, rapid progress has been made in the dye business. Interestingly, as per United States Colour Index, till now tens of thousands commercial dyes have been reported. The world's annual dye waste is > 60,000 tonnes and ~ 80% of which consist of azo dyes 9 . Notably, a few of azo dyes and their metabolites are toxic and carcinogenic in nature. In general, the textile effluent can have 10–800 mg L −1 of dyes, which may vary from factory to factory 10 . Thus, the treatment of wastewater or pollutant containing water is necessary to make our environment healthy. Till now several research efforts have been put forward to develop efficient pollutant removal system. Most of the developed pollutants (especially organic pollutants) removal techniques can be categorized in chemical, physical, and biological techniques. The mythology behind these techniques can include sedimentation, ultrafiltration, adsorption, ion exchange chemical oxidation, anaerobic, photocatalytic, and reverse osmosis 11 , 12 , 13 .However, some of the above-said techniques are associated with limitations such as costly equipment, operational problems, secondary waste generation, low degradation efficiency, and slow process 14 . Consequently, advanced oxidation processes such as photocatalysis, and ultraviolet treatment are introduced as progressive treatment techniques for the removal/ degradation of organic dyes. Among them, photocatalysis one of the best, most explored, and cost-effective process to remove dyes 15 .

A paradigm shift has been observed in the photocatalysis process after the inception of nanomaterials. Till now, a wide variety of nanomaterials have been tested for the removal of diverse hazardous dyes 16 , 17 , 18 . A good number of semiconductor materials-based photocatalysts, e.g., TiO 2 , SnO 2 , Bi 2 O 3 , CdS, CdSe, WO 3 , and ZnO, have been investigated for the removal of organic pollutants from the water. Out of above-mentioned photocatalytic materials, nanometer-sized ZnO is of special interest to the scientific community and researchers because of their high exciton binding energy (60 meV), wide direct band-gap (3.37 eV), non-toxicity, long-term stability, cheap cost, and high electron mobility 19 , 20 . ZnO NPs are an efficient and promising material for the photocatalytic degradation of industrial dyes due to their high efficiency, biocompatibility, cheap manufacturing cost, and improved control of physicochemical parameters. On the other hand, ZnO nanoparticles are becoming increasingly popular among researchers due to its anti-microbial properties. In particular, the physicochemical properties of ZnO NPs make them a promising material for photocatalytic and anti-microbial applications.

Till now, a good number of synthesis techniques have been explored effectively to create and modify ZnO nanostructures, including hydrothermal, solvothermal, hydrolysis, sol–gel, co-precipitation, thermal decomposition, microwave, and solid-state reaction methods 21 , 22 , 23 , 24 .Among all these methods, the hydrothermal and chemical precipitation methods are found to be the most convenient as these offer various advantages, such as low-temperature processing, cost-effectiveness, easy adoptability, and the ability to produce in large quantities. Moreover, one can easily control the characteristics of obtained nanomaterials via altering the reaction conditions (such as reaction temperature, solvent, surfactant and precursors) in hydrothermal and chemical precipitation methods 25 .

Herein we report an effective strategy to synthesized multi-structured ZnO functionalized with CTAB through a streamlined precipitation-cum-hydrothermal process, marking a novel approach in the facile and one-pot fabrication of distinctive nanostructured ZnO, which demonstrated exceptional efficacy in the efficient degradation of industrial reactive blue-81 (RB-81) dye 26 , 27 , 28 , 29 . Beyond its prowess in organic molecule removal, these fabricated structures exhibited remarkable antimicrobial activity against two bacterial species, namely Staphylococcus aureus (S. aureus) and Pseudomonas aeruginosa (P. aeruginosa). Our research highlights the successful achievement of multi-structured ZnO, showcasing extraordinary capabilities in both organic pollutant removal and antibacterial lethality. Importantly, we quantified key performance parameters, including quantum yield (QY), kinetic reaction rate, space–time yield (STY), and figure-of-merit (FOM), and conducted a comparative analysis with recent studies in the field. The results indicate that our developed multi-structured ZnO material outperforms its counterparts, emphasizing its significant contributions to advanced catalytic systems.

Materials and methods

Zinc acetate dihydrate ((CH 3 COO) 2 Zn.2H 2 O: ≥ 98% purity), sodium hydroxide (NaOH: ≥ 98% purity), ethanol (CH 3 CH 2 OH: ≥ 99.9% purity), and N -cetyl- N , N , N -trimethyl ammonium bromide (C 19 H 42 BrN, ≥ 99% purity) were purchased from Sigma-Aldrich (St. Louis, Missouri, United States of America (USA)). All the above-mentioned chemicals were of analytical reagent (AR) grade and used as received without any further purification. The RB-81 dye (C 25 H 14 Cl 2 N 7 Na 3 O 10 S 3.3 Na) was procured from Parshwanath Dyestuff Industries, Ahmedabad, India. During the synthesis of ZnO NPs, distilled water (DW) and filter papers (grade 41, pore size 20 m, Whatman, England) were used.

Synthesis of CTAB-assisted ZnO

In this study, CTAB-assisted ZnO NPs were synthesized via a unique chemical precipitation-cum-hydrothermal method (Fig. 1 ). In addition, the CTAB-assisted ZnO nanoparticle synthesis method has been successfully optimized to achieve high yields through systematic investigation of various parameters. These parameters include: precursor concentration ((CH 3 COO) 2 Zn∙2H 2 O and CTAB), temperature (focusing on hydrothermal treatment), pH, and reaction times.

Schematic illustration of the precipitation-cum-hydrothermal technique for the synthesis of CTAB/ZnO.

In a typical experiment process, Zn(CH 3 CO 2 ) 2 (0.2 M, 3.5118 g) was added to the distilled water (50 mL) and stirred for 20 min. Then CTAB 30 mL (0.01 M) was added to the afore-prepared solution and stirred for another 15 min at a temperature of 60–70 °C. The pH of the solution was set to 8 and the solution was further stirred for an hour to achieve precipitation of the desired nanoparticles. Afterwards, the obtained solution was transferred to an autoclave and placed into a vacuum oven (temperature: 200 °C, time: 4 h). After that, the autoclave was allowed to cool down to room temperature. The precipitates were filtered and washed with distilled-water/ethanol repeatedly to remove any impurities. The precipitates were dried (temperature: 80 °C, time: 4 h) and crushed to obtain a fine powder of CTAB/ZnO. The synthesized particles were stored at dry and dark place for further characterizations and applications. During synthesis, CTAB is highly effective at stabilizing nanoparticles. Its amphiphilic nature, with a hydrophilic ammonium head and a hydrophobic cetyl tail, allows it to adsorb onto the nanoparticle surface and create a steric barrier. This barrier prevents agglomeration and ensures a stable dispersion of nanoparticles in the solution. Other capping agents might not provide the same level of stabilization, leading to larger or aggregated particles 30 . In addition, CTAB is superior due to its effective stabilization, controlled morphology, compatibility with various synthesis methods, enhanced solubility and dispersion, surface functionalization, and cost-effectiveness, leading to uniform, well-defined nanoparticles and making it ideal for large-scale synthesis.

Characterization techniques

The synthesized particles were characterized by X-ray diffractometer (X'Pert PRO, PANalytical, Japan) to know the interplanar spacing (d hkl ) among adjacent planes, Miller indices (h k l), crystallite sizes (D), and crystallite phases. The absorption band edge and electrical band gap energy (Eg) of ZnO NPs were examined using a UV–visible (UV–vis) spectrophotometer (UV-2600, Shimadzu, Japan). A field emission scanning electron microscope (FE-SEM; SUPRA 55-VP, Carl Zeiss, Germany) was used to record the size, shape, aggregation, agglomeration, and uniformity of particle distribution. The high-resolution transmission electron microscopy (HR-TEM; JEM-2100, JEOL, Japan) was used for the investigation of particle size distribution, shape, lattice parameters, and interplanar spacing for distinct planes of ZnO NPs. The elemental composition and functionality on synthesized ZnO NPs were confirmed by energy-dispersive X-ray (EDX) spectrometer (AZtec, Oxford, America) and Fourier-transform infrared (FTIR) spectrometer (Alpha II, Bruker, Germany), respectively. Moreover, the chemical state of ZnO was analyzed using X-ray photoelectron spectroscopy (XPS: PHI 5000 VersaProbe II, ULVAC-PHI, Inc., Japan).

Dye degradation studies using CTAB/ZnO

In current study, the effectiveness of the CTAB/ZnO has been evaluated on the RB-81 dye. The chemical composition of RB-81 dye is shown in Fig. 2 a. The photocatalytic dye degradation experiment has been conducted on a DW solution of 120 mg L −1 of RB-81 dye. To initiate the dye degradation experiment, 150 mg L −1 or 200 mg L −1 photocatalyst was ultrasonically disseminated in the RB-81 dye solution. To achieve the adsorption–desorption equilibrium, the suspension of ZnO nanomaterial and dye was stirred continuously while kept in the dark for sixty minutes. Following this, the suspension was moved to a photoreactor equipped with a UV (6 W) lamp (λ max = 254 nm) Fig. 2 b. After that, the suspension was irradiated with UV light for varying amounts of time (for example, 0–105 min) with continuous stirring. The dye degradation performance of the photocatalyst was monitored after every 10 min. Note that the photocatalyst was removed via centrifugation before measuring the absorbance of the testing solution. Equation ( 1 ) was used to perform the analysis necessary to determine the photocatalytic activity 31 .

Schematic illustration of the RB-81 dye and the experimental set-up for dye degradation: ( a ) chemical structure of RB-81, and ( b ) photoreactor along with various components.

The dye absorbance (λ max : 583 nm) is denoted by the symbols E 0 and E t at time = 0 and t min of UV exposure, respectively.

To conduct a control experiment, the RB-81 dye solution was subjected to UV light in absence of photocatalyst. The schematic of the photoreactor used in current study is shown in Fig. 2 b.

Importantly, the performance of the CTAB/ZnO photocatalytic on dye degradation was evaluated on four key performance metrics (1) QY (Eq. 2 ), (2) Photon flux (Eq. 3 ), (3) STY (Eq. 4 ), and (4) FOM (Eq. 5 ) 32 . In particular, the capability of photocatalyst to efficiently use the light energy can be estimated using QY. Likewise, the photocatalyst mass-based QY can be measured using STY, while the FOM gives a numerical description of the photocatalytic system that can be used on the industrial scale (by including energy consumption, time, catalyst mass, and product obtained into calculation) 32 , 33 .

Experimental design for antimicrobial test

The inhibitory effects of an aseptic ZnO solution on bacterial growth have also been investigated. The antibacterial properties of synthesized ZnO material were tested on gram-positive (e.g., S. aureus ) and gram-negative bacteria (e.g., P. aeruginosa ). The culture plates (3 mm deep) were kept at 35 °C for 18 h to simulate the optimal growth conditions for bacteria. The antibacterial potential of ZnO solution was examined by exposing bacteria to 10, 25, and 40 μL of ZnO solution. At the end of experiment, the centimetre-scale measurements of the inhibitory regions were taken to determine the extent of bacterial growth inhibition.

Results and discussion

Structural analysis of ctab/zno via powder x-ray diffraction (pxrd).

The phase structure analysis of the synthesized ZnO was conducted using powder X-ray diffraction (PXRD), a swift analytical technique widely employed for the phase identification of crystalline materials and the determination of unit cell dimensions. The XRD pattern of the CTAB-loaded ZnO developed in this study is presented in Fig. 3 . The data were collected within the range of 30°–70° (2θ), as shown in Fig. 3 a. Figure 3 a displays the Rietveld profile fitting for CTAB/ZnO NPs. The quality of this fitting was thoroughly assessed using the goodness of fit parameter (Chi 2 = 5.75). This statistical metric confirms that the theoretical model is in good agreement with the experimental results. During the analytical process, several parameters were modified, such as the background function, scale factors, profile functions, peak width parameters (u, v, w), and lattice parameters. These modifications are essential for optimising the alignment between the theoretical and actual XRD profiles, resulting in a more precise depiction of the crystal structure. The crystallinity of the ZnO nanostructure was clearly discerned in the XRD pattern, indicated by sharp peaks at specific 2θ values, such as 31.66°, 34.32°, 36.16°, 47.46°, 56.52°, 62.80°, 66.32°, 67.88°, and 69.00°, corresponding to the lattice planes (h k l) of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), and (2 0 1), respectively 34 . The diffraction pattern of the ZnO nanostructure was indexed to the hexagonal wurtzite crystal structure, as this result is in good agreement with the previous reports 35 . This analysis underscores the crystalline nature and structural integrity of the developed ZnO, providing valuable insights into its potential applications. The diffraction pattern of the ZnO nanostructure was indexed to the hexagonal wurtzite crystal structure, as this result is in good agreement with the previous reports 35 . In the earlier study, the structure of ZnO was described as several alternating planes composed of tetrahedrally coordinated O 2− and Zn 2+ ions, stacked alternately along the c-axis 36 . In another investigation, the XRD spectrum of Ni-doped ZnO NPs synthesized by the chemical co-precipitation method confirmed the single phase with a hexagonal wurtzite structure with space group P 6 3 mc .

X-ray diffraction spectrum of CTAB/ZnO powder: ( a ) Rietveld analysis, ( b ) standard ZnO, ( c ) W–H plot, and ( d ) and Modified Debye–Scherrer plot.

The average grain size and structural parameters of the synthesized ZnO were calculated using Debye–Scherrer’s formula 37 .

where D is the average crystallite size in Å, K is the shape factor (0.9), λ is the wavelength of X-ray, β is full-width at half-maximum intensity (FWHM), and θ is the Bragg’s angle. Inter-planar spacing between planes in the atomic lattice for each diffraction peak was calculated using Bragg’s law.

where d is the inter-planar spacing.

The structural and geometric parameters (e.g., FWHM, lattice planes, d, and D) are illustrated in Table 1 . The average crystallite size of synthesized ZnO NPs is ~ 28.22 nm. Based on highest peak (101) in XRD, the crystalline size was calculated as 32.24 nm. The deviation between the average size calculated from the maximum peak and average size suggested the presence of differentially shaped ZnO 38 . The Rietveld evaluation confirms that the positions and strengths of the diffraction peaks in the XRD spectrum of the nanoparticles are in line with the expected diffraction patterns of a hexagonal wurtzite crystal structure CTAB/ZnO with space group P6 3 mc and specific cell parameters (a = b = 2.8100 and c = 5.2200 Å, and V = 35.73 Å 3 ). The validity of this information is supported by the JCPDS card number 00-079-2205, as seen in Fig. 4 (b) and elaborated upon in Table 1 .

UV–vis energy band gap plots of CTAB/ZnO NPs: ( a ) UV–vis absorption spectrum and ( b ) Tauc’s plot.

The FWHM for all marked peaks was calculated using Lorentz curve fitting (R 2 > 0.99). Interplanar-spacing values of d experimental = 2.482 Å and d bulk = 2.481 Å (JCDPS card number #00-036-1451) for the longest peak (101) showed that the synthesised ZnO NPs crystallites were free of contaminants and crystallographic irregularities. It is worth noting that differences in d-space values between materials (photocatalysts) can result in different mass densities and electronic band structures. As a result, photocatalysts' absorption of light at specific energies alters their ability to degrade pollutants 39 . The absence of irregularities between synthesized and standard ZnO’s d-space values indicates the unchanged positions of Zn and O within the crystal structure. Furthermore, the considerably large peak intensities in the XRD spectrum (Fig. 3 a) represent that the prepared ZnO's phase evolved properly. Additionally, the prepared ZnO demonstrates a high degree of crystallinity, as evidenced by the sharp peaks in the XRD spectrum 40 . Interestingly, we did not notice any diffraction peaks corresponding to Zn, Zn(OH) 2 , or other ZnO phases, which indicate that pure ZnO with a hexagonal wurtzite phase was formed. The interaction of the zinc cluster and surfactant CTAB results in a chemical reaction in which the zinc cluster undergoes intense oxidation, resulting in the formation of Zn(OH) 2 , which thermally decomposes into ZnO 41 .

The CTAB/ZnO unit cell volume (35.73 A o ) differs significantly from the bulk ZnO volume (47.61 Å) (Table 1 ), indicating the existence of internal stresses. It was reported earlier that the analysis of the peak broadening in XRD can reveal the crystallite size as well as the internal stress of the nanocrystals. The Williamson–Hall (W–H) method was utilized to determine the relationship between the peak broadening in diffraction peaks and the nanocrystals’ internal stress, and crystallite size. Unlike the Scherrer equation, which is 1/cosθ dependent, the W–H technique is tanθ dependent 42 . The W–H plot between 4sinθ and β T cosθ is shown in Fig. 3 c. Equation ( 10 ) was used to compute the crystallite size from the plot.

Here β T denotes the overall expansion.

The W–H plot revealed a crystallite size of 44.00 nm with a strain (ε) value of + 3.15 × 10 –3 . The existence of tensile stress is indicated mostly by the positive value of internal strain. The lattice characteristics and unit cell volume of the CTAB/ZnO are also included in Table 1 . To get a more precise estimate of the crystallite size, it is possible to compute the systematic error in the Scherrer formula by subtracting the errors from the individual peaks 43 . Ln(1/cosθ) vs Lnβ is shown in Fig. 3 d, also known as the Monshi (Modified Debye–Scherrer) plot. From this figure, the crystallite size was determined using Eq. ( 11 ) as follows:

Here the intercept of a least-squares regression (ln = K λ /D) was used to figure out the sizes of the crystallites for each XRD peak. Table 2 lists the different lattice parameters estimated from the W–H and modified Debye–Scherrer plots. Based on the modified Debye-Scherer approach, the average crystallite size of CTAB/ZnO was determined to be 33.14 nm. The analysis of the crystallite size of CTAB/ZnO computed by various approaches yielded significantly distinct findings (Table 2 ). Small crystallite size CTAB/ZnO NPs may activate the ZnO surface for improved dye adsorption and photocatalytic activity.

The PXRD analysis highlights the successful synthesis of highly crystalline, pure ZnO nanoparticles with a hexagonal wurtzite structure. The detailed structural parameters obtained provide a solid foundation for further exploring the material’s applications in areas such as environmental remediation, sensor technology, and optoelectronics.

Analysis of the optical properties of CTAB/ZnO using UV–visible spectroscopy

The optical and electrical characteristics of semiconducting nanoparticles, particularly their excitonic and inter-transition properties, are crucial for various applications and are commonly studied using UV–vis absorption spectroscopy. The UV–vis absorption spectrum of CTAB/ZnO NPs is shown in Fig. 4 a. This spectrum, covering the range of 350–700 nm, reveals essential information about the optical properties and band structure of the synthesized nanoparticles. The excitonic absorption peak at 400 nm is a significant feature in the UV–vis absorption spectrum of CTAB/ZnO NPs. This peak is attributed to the intrinsic band-gap absorption of ZnO, which corresponds to electron transitions from the valence band to the conduction band (O 2p → Zn 3d ) 45 . Notably, this absorption peak is red-shifted compared to the absorption peak of bulk ZnO, typically observed at 365 nm. The red shift indicates a decrease in the optical band gap, which is often associated with the presence of shallow levels inside the band gap due to the incorporation of non-native atoms or defects such as oxygen vacancies within the ZnO lattice 46 .

The red shift observed in the absorption peak of CTAB/ZnO NPs suggests enhanced crystallinity and possibly larger average nanoparticle size compared to bulk ZnO 47 . The shift to a higher wavelength in the absorption spectrum can be linked to the quantum confinement effect, where the size of the nanoparticles influences their electronic properties. Larger nanoparticles tend to have a narrower band gap due to the reduced quantum confinement effect, leading to the observed red shift. The surfactant cetyltrimethylammonium bromide (CTAB) plays a critical role in the synthesis of ZnO nanoparticles. CTAB helps create a region of high surface energy around the nanoparticles, promoting aggregation and influencing the optical properties 48 . The surfactant defects introduced by CTAB may contribute to the red shift and affect the crystal size distribution of the nanoparticles.

Tauc’s method was used to derive the optical band gap energy (Eg) from the UV–vis spectrum, as shown in Fig. 4 b 49 , 50 . The Tauc plot involves plotting (αhν) 2 against photon energy (hv), where α is the optical absorbance coefficient, h is Planck’s constant, and ν is the frequency of the incident light. The optical band gap is determined by extrapolating the linear portion of the plot to the x-axis. The calculated Eg value for CTAB/ZnO NPs was found to be 2.82 eV, which is lower than the bulk ZnO value of 3.30 eV 47 . This reduction in band gap energy is indicative of the presence of crystal quantum defects and the progressive development of the nanoparticles. The narrowing of the optical band gap suggests that CTAB/ZnO NPs could be effective in applications requiring UV–vis light-responsive photocatalysts. The findings align with previous studies that have reported similar red shifts and band gap narrowing in ZnO nanoparticles due to defect states and size effects 51 , 52 , 53 , 54 . For instance, studies have shown that the presence of oxygen vacancies and other defects can introduce additional energy levels within the band gap, facilitating the red shift in absorption spectra.

In conclusion, the optical characterization of CTAB/ZnO NPs using UV–vis absorption spectroscopy revealed crucial insights into their band structure and defect states. The red shift in the absorption peak and the reduction in the optical band gap energy highlight the influence of nanoparticle size, surfactant effects, and defect states on the optical properties. These characteristics make CTAB/ZnO NPs promising candidates for applications in photocatalysis and other fields requiring efficient light absorption and electronic transitions.

Functional group analysis of CTAB/ZnO via Fourier transform infrared spectroscopy (FTIR)

The functionality on the CTAB/ZnO were determined using FTIR spectroscopy. The IR transmittance spectra (4000–500 cm −1 ) of CTAB, Zn acetate, and CTAB/ZnO NPs are shown in Fig. 5 a–c, respectively. The primary absorption band owing to O–H stretching of a hydroxyl group were observed at 3552 and 3406 cm −1 for CTAB (Fig. 5 a) 55 . In the spectrum of zinc acetate solution, bands at 1693 and 1547 cm −1 were suspected due to symmetric and antisymmetric stretching vibrations of the carboxyl groups, respectively. Interestingly, a few new peaks noticed in the FTIR spectra of CTAB loaded ZnO, which was generated due to the interactions between zinc acetate and CTAB during the synthesis of ZnO NPs (Fig. 5 c). In FTIR spectra of CTAB/ZnO NPs, the fundamental modes of vibration at 3666, 3614, and 1023 cm −1 were detected due to O–H stretching vibration, intermolecular bonds of water molecules, and hydrogen-bonded hydroxyl groups, respectively. The bands found at 2922 cm −1 correspond to the symmetric-asymmetric stretching vibration of the CH 2 group. The vibrations produced by the carboxyl group at 1690 and 1540 cm −1 were due to symmetric and asymmetric modes of vibration, respectively. The existence of absorption peaks at 738 cm −1 , and 679 cm −1 revealed the stretching vibrations of ZnO NPs and generally generated due to the creation of tetrahedral coordination of Zn 56 , 57 , 58 , 59 . The existence of the residual hydroxyl peak in CTAB/ZnO NPs is due to the hydroscopic nature of ZnO NPs. The peaks around 750–650 cm −1 can be assigned to the stretching of Zn–O during the formation of the ZnO NPs. A change in particle size and shape is responsible for the shift from the previous band at 645 cm −1 to 679 cm −1 . Our findings on the functionality of the synthesized ZnO NPs are in line with previously published report 60 .

FTIR spectra: ( a ) CTAB, ( b ) zinc acetate solution, and ( c ) CTAB-mediated ZnO NPs.

The FTIR analysis reveals significant interactions between CTAB and ZnO during the synthesis process, indicating successful functionalization of ZnO nanoparticles by CTAB. The appearance of new peaks and the shifting of existing peaks in the FTIR spectra confirm the chemical bonding and structural changes induced by CTAB. For instance, the detection of O–H stretching vibrations at multiple wavelengths indicates the presence of hydroxyl groups, which are crucial for the stability and reactivity of ZnO nanoparticles. Additionally, the presence of CH 2 stretching vibrations suggests the incorporation of the organic CTAB molecules into the ZnO matrix, which can enhance the dispersibility and prevent agglomeration of nanoparticles. Moreover, the observed peaks related to Zn–O stretching vibrations at 738 cm −1 and 679 cm −1 , along with the shift from 645 cm −1 , highlight the formation of ZnO nanoparticles with tetrahedral coordination. This shift is indicative of changes in particle size and shape, which are critical factors influencing the optical and electronic properties of ZnO nanoparticles. The residual hydroxyl peaks further suggest the hygroscopic nature of the synthesized nanoparticles, implying that they can readily interact with water molecules, which may be beneficial for certain catalytic and sensing applications.

Overall, the FTIR analysis provides evidence that the synthesis of CTAB/ZnO NPs was effective. It demonstrates notable interactions between CTAB and ZnO, which are supported by the existence of distinct functional groups and their unique vibrations. The identification of additional peaks and changes in the positions of existing ones suggests the creation of ZnO nanoparticles with unique structural characteristics, which are impacted by the capping and stabilising activities of CTAB. The results align with earlier studies and show that CTAB effectively alters the surface chemistry and enhances the stability of ZnO NPs 60 . The comprehensive spectrum analysis offers valuable information about the functional groups that are present and their significance in the process of synthesising nanoparticles. This study helps us get a deeper knowledge of the chemical characteristics of the material.

Topography analysis of CTAB/ZnO via field emission scanning electron microscopy (FESEM)

A FESEM analysis was carried out to investigate the morphological parameters of synthesised CTAB/ZnO NPs. Additionally, the FESEM was also utilized to understand the effect of CTAB on the topography of ZnO. The FESEM images of CTAB/ZnO NPs are shown in Fig. 6 a–d shows. The magnification of a specific location in the FESEM (Fig. 6 a) was raised from 25 to 65 KX (Fig. 6 b) in order to get an interior perspective of their aggregated shape. The nanocapsule-shaped nanostructure composed of spherical particle aggregates can be seen in the magnified FESEM images. Likewise, as evident from another magnified image in Fig. 6 c, diverse shapes, e.g., spherical, hexagonal, and rectangular of ZnO nanostructures were also present. Similarly, clusters of spherical and hexagonal particles were seen in Fig. 6 d. Notably, the homogeneity and shape of NPs play a crucial role in defining their applications. The use of CTAB during the synthesis of ZnO NPs significantly affected the morphology of CTAB/ZnO NPs. The presence of CTAB in CTAB/ZnO compelled the aggregation of ZnO into many different nano-structures (e.g., nano-capsules, hexagonal, spherical, and rectangular. It was postulated that due to positive electric charge of ZnO NPs and the cationic head groups of CTAB molecules repulsion between ZnO and CTAB occurred 61 , 62 . To overcome these electrostatic repulsive forces, CTAB molecules align in the form of a double layer on the ZnO surface, which caused the aggregation of ZnO particles.

FESEM images of CTAB/ZnO NPs at different resolutions: ( a ) 25 KX, ( b – d ) 65 KX, and ( e ) EDX spectrum.

Further, the capping of ZnO NPs with CTAB restricted the lateral development, which changed the topography of ZnO NPs from spherical to rectangular or hexagonal (Fig. 6 b–d). More specifically, the cationic part of CTAB interacts electrostatically with zinc species ([Zn(OH) 4 ] 2− ) 63 . On the basis of molar ratios of CTAB and zinc species, different topographies of the ZnO (such as hexagonal, rectangular or spherical) can be formed. The elemental analysis of CTAB/ZnO NPs was examined by energy dispersive X-ray spectroscopy (EDAX) (Fig. 6 e). In particular, the atomic and weight percentage of oxygen was 48.20% and 18.57%, respectively. On the other hand, the atomic and weight percentage of zinc was 51.80% and 81.43%, respectively. A small deviation in the Zn:O ratio (48.20:51.80) from the standard Zn:O ratio (50:50) may introduce defect states in CTAB/ZnO. These oxygen defects may enhance the light absorption capabilities of the ZnO and lead to high photocatalytic activity.

The investigation of CTAB/ZnO nanoparticles using FESEM demonstrates notable morphological changes caused by CTAB. CTAB’s presence induces the creation of various nanostructures, such as nanocapsules, spheres, hexagons, and rectangles. The differences occur as a consequence of electrostatic interactions between the positively charged ZnO nanoparticles and the cationic head groups of CTAB. This contact leads to the alignment of a double layer, which reduces repulsive forces and encourages the formation of different forms via aggregation. The action of CTAB limits the horizontal expansion of ZnO, causing a change in the surface structure from spherical to more intricate shapes. EDAX analysis provides further evidence of a little deviation in the Zn ratio, suggesting the existence of oxygen defects that increase the absorption of light and boost the photocatalytic activity.

Morphological analysis of CTAB/ZnO via high-resolution transmission electron microscopy (HRTEM)

The HRTEM microscopy technique is a useful tool for obtaining precise information about the structural properties of nanoscale materials such as nanoparticles, nanofibers, and nano-emulsions. In current study HRTEM is used for obtaining particle distribution and a SAED pattern (Fig. 7 ). The distribution pattern in the micrograph reveals that the particles were of varying sizes and forms (e.g., spherical, capsule, hexagonal, and rectangular) (Fig. 7 a). Higher resolution images were captured to better portray the particle morphologies (Fig. 7 b, c: 350 KX). The nanostructured findings that were seen in HRTEM agree with those from the FESEM analysis. The histogram for the spread of CTAB/ZnO NP size is shown in Fig. 7 d. The particle distributions yielded an average NP size of 31 nm.

Morphology of CTAB/ZnO NPs: ( a ) TEM image ( b , c ) different morphologies, ( d ) particle size distribution, ( e ) HRTEM image, and ( f ) SAED pattern.

Table 3 describes the various methods used to prepare ZnO NPs along with an assessment of their sizes and shapes. The ZnO NPs prepared with various capping agents (such as tri-n-octylphosphine oxide, triethanolamine, oleic acid, 1-thioglycerol, polyethylene glycol, polyvinyl pyrrolidone, histidine , etc.,) using different methods (such as hydrothermal, precipitation, solvothermal, simple-polyol, co-precipitation, etc.,), showed irregular particle shapes (relative to multi-structured CTAB/ZnO, Table 3 ). In the above discussion, which focused on the role of capping agents and the way the particles were synthesized, it was found that the photocatalytic activity of the present system (as CTAB/ZnO NPs) may be affected by the shape of the particles. The literature examples chosen for this study show that the type of capping agent and the synthetic method used to make ZnO NPs can have a big effect on how well they work as photocatalysts by changing their structure.

The fringes of CTAB/ZnO NPs were observed via HRTEM at high magnification (500 KX) (Fig. 7 e). A well-focused region is denoted by resolved fringes displaying the atomic planes’ d-spacing of 0.23 nm (compared to the usual d-spacing of 0.20–0.25 nm). The existence of a single isolated crystallite suggested that the CTAB/ZnO was mono-crystalline. The SAED pattern of CTAB/ZnO nanostructures was studied using TEM in dark field diffraction mode (Fig. 7 f). It shows a defect-free single crystalline pattern with first, second, and third-order facets of nanocrystals. The SAED pattern exhibits a typical diffraction of a ring pattern with some brighter and more defined spots in the rings. This showed the existence of some bigger crystallites, although the rings remained reasonably continuous. It implies that the crystallites were in the nm range and in a random orientation. The electron diffraction spots can be described by a hexagonal crystalline-structured ZnO with a space group P63mc and indices identical to the XRD spectrum of ZnO NPs, as illustrated in Fig. 7 f.

The HRTEM examination of CTAB/ZnO nanoparticles demonstrates a variety of morphologies, including spherical, capsule, hexagonal, and rectangular, with an average size of 31 nm, which supports the findings of the FESEM data. The nanoparticles have a monocrystalline nature with a hexagonal crystal structure, as shown by high-resolution pictures and SAED patterns. These images reveal well-defined atomic planes and continuous diffraction rings. Comparative analyses including various capping agents and synthesis techniques emphasise the distinctive capacity of CTAB to regulate the shape and improve the quality of ZnO nanoparticles. Having control over the morphology is essential for maximising the efficiency of photocatalytic activity and other uses.

Chemical analysis of CTAB/ZnO via X-ray photoelectron (XPS) spectroscopy

XPS is one of the most important technique to look at the complicated electronic structure of solids 64 . The valence status of elements on the surface of CTAB-mediated ZnO NPs was investigated using XPS (Fig. 8 ). The survey scan shows the presence of Zn, C, and O elements in ZnO NPs (Fig. 8 a). Figure 8 b–d shows the high-resolution XPS spectrum of Zn 2p, O 1s, and C 1s, respectively. The primary peak at 1020.91 eV corresponds to Zn 2p 3/2 , whereas the peak at 1044.00 eV corresponds to Zn 2p 1/2 Fig. 8 b. Zn atoms at the normal lattice location in ZnO are responsible for these emissions. Here, the difference between the binding energies of Zn 2P 3/2 and Zn 2P 1/2 emissions was found to be 23.09 eV, which is the typical value for ZnO 65 . As previously stated, the Zn 2P 3/2 peak form lacks an asymmetric characteristic, therefore, Zn LMM1 Auger peak analysis is employed to identify the chemical states of the Zn species 66 . This is due to the fact that Auger peaks usually exhibit larger shape changes than XPS peaks with different chemical states. In our study, an Auger Zn LMM1 emission centred at 472.95 eV is detected and attributed to the interstitial Zn–O bonds. It confirmed an oxygen-rich stoichiometry for the produced ZnO NPs and shows that they lack Zn flaws 67 .

XPS spectra of CTAB mediated ZnO NPs: ( a ) survey scan, ( b ) Zn 2p, ( c ) O 1s, and ( d ) C 1s.

Figure 8 c shows the high-resolution spectrum of O 1s (overall fit and deconvoluted O I , and O II ). The strong O peak at 529.71 eV is due to the overall fitting of the metal–oxygen bond of ZnO. The peak at the binding energy 529.51 eV is attributed to the O I of the Zn–O bond. The broad O II peak at a binding energy of 531.21 eV can be ascribed to the defect sites containing low oxygen coordination in ZnO. According to earlier research, the O 1 s state has three binding energy components, including a low binding energy peak at 530.15 eV, a moderate binding energy peak at 530.15 eV, and a high binding energy peak at 532.40 eV 68 . The low energy peak was caused by O 2− ions at oxygenated locations, whereas the middle peak was caused by O 2− ions in the oxygen-deficient zone. Chemisorbed oxygen was allocated the high peak. In the other study, three different O peaks were seen at 530.28 eV, 531.30 eV, and 532.36 eV, respectively, which were ascribed to O atoms at regular lattice sites, oxygen-deficient areas, and interstitial O. Figure 8 d depict a high-resolution spectrum of C 1s with a peak at 284.51 binding energy, which illustrates the C=O. It can be seen from the XPS analysis that no impurity peaks other than Zn, O, and C species are found.

An XPS examination of ZnO nanoparticles, mediated by CTAB, offers a thorough understanding of surface chemistry. The existence of zinc (Zn), oxygen (O), and carbon (C) components has been verified, and no contaminants have been discovered. The detected Zn 2p and O 1s peaks correspond to the expected values for ZnO, suggesting that the stoichiometry is correct and there is an abundance of oxygen in the bonding. The lack of Zn defects and the existence of well-defined lattice sites and regions with a lower amount of oxygen are verified. The existence of C=O bonds, as shown by the C 1s peak, provides further evidence of the purity of the synthesised nanoparticles.

Investigation of surface area and pore size via BET and BJH for CTAB/ZnO nanoparticles

CTAB-modified zinc oxide nanoparticles exhibit type IV isotherms with a type H3 hysteresis loop, as per the IUPAC classification (Fig. 9 a). This specific isotherm shape indicates the presence of mesoporous materials. Mesoporous materials are characterized by pores with diameters ranging from 2 to 50 nm. The observed hysteresis loop, occurring at a relative pressure range of 0.3–1.0, suggests capillary condensation within the mesopores and highlights their slit-like nature. The BET method was employed to calculate the surface area of CTAB/ZnO nanoparticles. Both multi-point and single-point approaches were utilized in the analysis of the isotherm data. The BET surface area of CTAB/ZnO was found to be approximately 102 m 2 g −1 —a remarkable value that surpasses the reported surface areas for both environmentally friendly and commercially produced ZnO nanoparticles. The superior surface characteristics of CTAB/ZnO make it an attractive material for various applications, especially in photocatalysis. The BJH method provides detailed insights into the mesoporous structure by evaluating the desorption branch of the isotherm (Fig. 9 b). For CTAB/ZnO nanoparticles, the pore volume is approximately 0.20 cubic centimetres per gram (c.c. g −1 ) and the average pore diameter: The average pore diameter is approximately 30 nm. These results confirm the mesoporous nature of CTAB/ZnO nanoparticles and indicate a uniform pore size distribution. Such characteristics are advantageous for applications requiring high surface area and specific pore properties, particularly in photocatalytic processes. The combination of a high BET surface area and the mesoporous structure suggests that CTAB/ZnO nanoparticles possess a highly porous architecture, making them ideal for photocatalytic applications.

BET analysis of ZnO nanoparticles: ( a ) nitrogen adsorption/desorption isotherm with corresponding BET surface area plot (inset), and ( b ) pore-size/volume distribution employing the BJH method.

Performance evaluation of CTAB/ZnO for the degradation of industrial RB-81 dye

Dye degradation.

To get insight into the photocatalytic activity of the produced CTAB/ZnO NPs, experiments were conducted to degrade RB-81 dye. Some reports indicate that the –N=N– chromophore group is mostly responsible for RB-81’s (an azo dye’s) predominant colour 69 . Due to the presence of two stable, complex aromatic rings linking the group, disassembly of RB-81 is a challenging task. The photostability of the commercial RB-81 dye was initially investigated as a function of time under UV irradiation, even without the use of a photocatalyst (Fig. 10 a). The concentration of RB-81 dye did not change after being exposed to UV light for 60 min. These results demonstrate that RB-81 maintains its physical and chemical characteristics when exposed to UV radiation. The photocatalytic activity of the prepared CTAB/ZnO NPs (150 mg L −1 ) towards RB-81 (120 mg L −1 ) in the dark and under UV light is shown in Fig. 10 b. There was a constant decrease in the absorption peak of RB-81 at 583 nm. In just 105 min under UV light, the absorption peak of RB-81 dye was almost diminished. Moreover, the change in the color of dye solution into translucence (due to degradation of chemical structure of dye) can also be observed to monitor the degradation of dye. The amount of RB-81 dye removed by the different CTAB/ZnO NP concentrations is displayed in Fig. 10 c. It has been observed that the amount of photocatalyst present in a reaction may have a significant impact on the total photocatalytic removal efficiency 70 . Here, the impact of photocatalyst dosage was evaluated at loading levels of 150 and 200 mg L −1 . When the photocatalyst dose was raised from 150 to 200 mg L −1 , the removal efficiency was increased from 87.20 to 91.75%, which is consistent with an increase in the number of active surface sites. Synthesized CTAB/ZnO was evaluated against a commercial TiO 2 P25 photocatalyst for its ability to degrade RB-81 (Fig. 10 d). Interestingly, the performance of synthesized CTAB/ZnO was found superior in comparison to the TiO 2 P25 especially in terms of photocatalytic activity. The small crystallite size, multi-structure shape, low band-gap energy, and functionalized surface of CTAB/ZnO can be attributed for the above-said better photocatalytic characteristics.

Photocatalytic degradation of the RB-81 dye via CTAB/ZnO NPs: ( a ) dye stability, ( b ) dye degradation (120 mg L −1 ), ( c ) removal efficiency, and ( d ) comparison of the efficiency.

The photocatalytic mechanism for the degradation of RB-81 dye by CTAB/ZnO is shown in Fig. 11 . Upon absorption of light with appropriate wavelength, an electron excited from the valence band to the conduction band of CTAB/ZnO NPs. Then the photogenerated holes and electrons react with the water and oxygen (adsorbed on the CTAB/ZnO surface) to generate hydroxyl (∙OH) and superoxide anion ( ${\text{O}}_{2}^{. - }$ ) radicals. The resulting ∙OH and ${\text{O}}_{2}^{. - }$ interacts with the RB-81 to degrade and decolourize it.

The photocatalytic dye (RB-81) degradation mechanism of the CTAB/ZnO NPs.

In addition, the degradation of Reactive Blue-81 by ZnO is highly dependent on the pH of the solution, as the pH affects both the surface charge of ZnO nanoparticles and the ionization state of the dye molecules. Reactive Blue-81, a synthetic dye with a sulfonate group (-SO3H) in its structure, is anionic in nature, especially at neutral to alkaline pH levels due to the deprotonation of the sulfonate group, resulting in a negatively charged dye molecule.

At lower pH values (acidic conditions), the surface of ZnO nanoparticles tends to be positively charged due to the adsorption of protons (H+ ions). This positive surface charge enhances the electrostatic attraction between the ZnO nanoparticles and the negatively charged Reactive Blue-81 molecules, leading to improved adsorption of the dye onto the catalyst surface. Enhanced adsorption facilitates closer interaction between the dye molecules and the reactive sites on the ZnO surface, promoting more effective photocatalytic degradation. In contrast, at higher pH values (alkaline conditions), the surface of ZnO nanoparticles becomes negatively charged due to the presence of hydroxide ions (OH−). The negative surface charge results in electrostatic repulsion between the ZnO nanoparticles and the negatively charged Reactive Blue-81 molecules, reducing the adsorption efficiency. Consequently, the decreased adsorption leads to less effective photocatalytic degradation of the dye under alkaline conditions. Moreover, the generation of hydroxyl radicals (∙OH), which are crucial oxidizing agents in the photocatalytic degradation process, is influenced by the pH. In alkaline conditions, the production of hydroxyl radicals is generally more favourable, which can enhance the photocatalytic activity. However, the overall degradation efficiency still depends on the balance between the enhanced radical generation and the reduced dye adsorption due to electrostatic repulsion.

Kinetic study and stability

This research utilized a comprehensive approach to understand the photodegradation process of RB-81 dye by the synthesized CTAB/ZnO nanoparticles. We employed a range of kinetic models, including zero-, first-, and second-order, each offering unique insights into the reaction dynamics. The equations used in our study are as follows:

In these equations, ‘A0’ represents the absorbance at the start of the reaction, while ‘Lt’ denotes the absorbance at subsequent time intervals. The symbols ‘x0’, ‘x1’, and ‘x2’ are the reaction kinetic rate constants, which provide a measure of the reaction speed.

We conducted an exhaustive study on the degradation of RB-81 dye by CTAB/ZnO nanoparticles. The kinetic fits, as shown in Fig. 12 , provide a visual representation of the reaction dynamics under zero-order (a), first-order (b), and second-order (c) conditions. The slope of these graphs corresponds to the reaction rate constant, with a steeper slope indicating a faster dye breakdown. Table 4 presents a summary of the kinetic rates of reactions determined by the various models for different systems. Interestingly, the first-order kinetic model showed the highest agreement with the experimental data, suggesting that this model most accurately describes the reaction dynamics in our system. We also explored the impact of varying the dose of CTAB/ZnO nanoparticles on the kinetic reaction rate for the photo-degradation of RB-81 dye. Our results showed that increasing the nanoparticle dose from 150 to 200 mg L −1 led to an increase in the reaction rate constants. At a dose of 200 mg L −1 , the dye degradation was particularly rapid, resulting in the highest rate constant (k1 = 0.00889 min −1 ) (Table 4 ).

Kinetics model of degradation of RB-81 dye: ( a ) pseudo-zero-order, ( b ) pseudo-first-order, and ( c ) pseudo-second-order.

Our findings underscore the effectiveness of CTAB/ZnO nanoparticles as a photocatalyst, with a high dye degradation rate of 91.75% achieved in just 105 min. We further tested the stability of CTAB/ZnO nanoparticles by conducting five runs of photodegradation at a dose of 200 mg L −1 . Impressively, the removal efficiency of the industrial RB-81 dye remained above 85% even after five cycles (Fig. 13 ). The slight decrease in photocatalytic efficiency over multiple cycles could be due to the blocking of some active sites on the CTAB/ZnO surface, a finding that could guide future efforts to improve the stability of the photocatalyst.

Reusability efficiency of CTAB/ZnO NPs for the RB-81 dye.

Assessment of photocatalytic performance metrics

Dye degradation kinetics and photocatalyst removal efficiency are common metrics used to compare the effectiveness of a photocatalytic systems. Since photocatalytic system performance is highly sensitive to fundamental process factors such as concentration levels of target pollutants or dose of catalysts. A simple calculation of removal efficiency is insufficient for meaningful evaluation. As a result, it is more crucial to concentrate on the impacts of factors that might directly influence the photoactivities of a particular system (e.g., factors such as UV or visible radiation dose, photoreactor geometry, power consumption per unit of pollutant degradation, and dye degradation efficiency). This emphasizes the requirement for additional precise performance measures to make an assessment in order to permit a comparison of the effectiveness of various photocatalytic systems. These performance parameters have thus been generated and are described in Table 5 . The photocatalytic activity of the produced ZnO NPs was also evaluated for different dyes and compared with certain similar photocatalytic systems.

Key considerations for implementing a specific photocatalytic system in the industry are the system's energy consumption and dye degradation rate. Calculating the amount of energy required for a photocatalytic process involves dividing the rate at which pollutant molecules (dye) are degraded by the rate at which incoming photons strike the reaction container 87 . Here, we looked at the relative photonic efficiency (how many dye molecules were destroyed per unit of photons at a certain wavelength) 87 . The minimal energy requirement for the RB-81 dye in the current photocatalytic system was 4.75 × 10 22 J mol −1 (Table 5 ).

QY is a useful indicator of a photocatalyst's ability to emit the photons after the absorption of photons in either the UV or visible spectrum 87 . QY values ranged from 10 4 to 10 7 (molecules photon −1 ) for a wide variety of nanomaterials, including ZnO, PEG/ZnO, CB[8] /ZnO, SnO 2 , TiO 2 , TiO 2 –Fe 3 O 4 , and S–TiO 2 (Table 5 ). Multiple factors, including the amount of dye employed, the number of photons utilized for degradation, reactor geometry, analytical methods, and the nature of the light source are crucial in deciding the QY values. CTAB/ZnO had the highest QY value of 1.73 × 10 –4 molecules photon −1 among the materials covered in Table 5 . Notably, CTAB/ZnO displayed multi-fold higher QY in comparison to the counterparts mentioned in Table 5 . The QY values calculated for other common photocatalyst, e.g., TiO 2 , ZnO/SnO 2 /TiO 2 -Fe 3 O 4 , and ZnO/S-TiO 2 are 1.14–1.85 × 10 –05 , 1.06–4.82 × 10 –06 , and 5.69–8.33 × 10 –07 molecules photon -1 , respectively.

Likewise, the kinetic reaction rate (e.g., the number of pollutants degraded by the photocatalyst in a certain period) of the CTAB/ZnO NPs was determined and compared to other photocatalytic systems to characterise their dye degradation kinetics. The as-synthesized CTAB/ZnO NPs had the greatest kinetic reaction rate (3.89 × 10 2 µmol g −1 h −1 ), followed by PEG/ZnO (1.37 × 10 1 µmol g −1 h −1 ), SnO 2 (4.97 × 10 1 µmol g −1 h −1 ), TiO 2 (8.54 µmol g −1 h −1 ), TiO 2 -Fe 3 O 4 (2.96 µmol g −1 h −1 ), and S-TiO 2 (2.87 × 10 1 µmol g −1 h −1 ). Prior studies have shown that the photocatalyst's surface morphology greatly affects the reaction rate 71 .

The other key performance metrics, e.g., STY and FOM was also calculated for CTAB/ZnO and compared with the other recently developed/common photocatalysts (Table 5 ). As in case of other performance metrics, the as-synthesized CTAB/ZnO NPs outperformed other counterparts in terms of STY with a value of, 8.64 × 10 –6 molecules photon −1 mg −1 for the degradation of RB-81 dye. This STY value is much greater than the STY values of other reported ZnO catalysts (9.35 × 10 –07 to 1.09 × 10 –08 molecules photon −1 mg −1 ), SnO 2 , TiO 2 , TiO 2 -Fe 3 O 4 , and S-TiO 2 (4.80 × 10 –07 to 1.14 × 10 –08 molecules photon −1 mg −1 ) (Table 5 ). Consequently, the excellent RB-81 dye degradation capacity of as-synthesized CTAB/ZnO NPs is established. The feasibility of our photocatalytic system’s to be used on an industrial scale was also assessed in terms of FOM (Table 5 ). The CTAB/ZnO NPs prepared in the present work exhibited the highest FOM value (1.03 × 10 –9 mol L J −1 g −1 h −1 ). The superior photocatalytic structural, optical, and chemical features of CTAB/ZnO NPs can be expected for its high FOM value.

Assessment of antimicrobial activity of CTAB/ZnO NPs

ZnO NPs have special physicochemical characteristics, which have led to extensive research into their possible antibacterial effects. In order to assess the antibacterial effectiveness of NPs, the agar well diffusion method is often used 72 . In the current study, the antibacterial activity of CTAB/ZnO NPs was evaluated using the agar well diffusion technique. Both were prepared and sterilised by heating the nutritional medium (NM) and agar media (nutritional broth; HiMedia Laboratories) to 120 °C for 20 min. By doing this, it was made sure that the medium was clear of any microbial contamination that would have impacted the experiment's outcomes. (15 mL) of the required medium was placed in clean petri dishes, and the mixture was then given time to settle (Fig. 14 ). Gram-positive (MRSA) and gram-negative ( P. aeruginosa ). Gram-negative ( P. aeruginosa ) and gram-positive ( S. aureus ) bacteria were injected, and the samples were incubated for 18 h at 40 °C and 140 rpm. After 18 h of growth, the bacterial culture was diluted with fresh NB media at a ratio of 1:100. Reduced pure strains of P. aeruginosa and S. aureus were spread on an NB agar plate. The ZnO solution samples (together with the control specimen) were pipetted into the wells of each plate using micropipettes with capacities of 10, 25, and 40 L. The samples were then incubated for 22 h at 40 °C. Table 6 shows the identification of inhibition zones of various sizes. The spores of gram-negative P. aeruginosa were greenish-blue with a pleasant scent, while the spores of gram-positive S. aureus were yellowish-white with a characteristic sour odour. Both bacteria's development habits have been thoroughly studied under an optical microscope and documented for future study. The experiment’s goal was to see how changing medium affected bacterial development and to evaluate the growth patterns of two distinct species of bacteria. The findings revealed that both microorganisms grew well in the given medium, however P. aeruginosa expanded quicker than S. aureus . These discoveries have important significance for microbiology investigation and may lead to new insights into the emergence of medications and other bacterial illness therapies.

Antibacterial activity of CTAB/ZnO NPs.

Antimicrobial action mechanism of CTAB -mediated ZnO NPs

Figure 15 depicts a simplified model of the bactericidal effects of CTAB/ZnO NPs against S. aureus and P. aeruginosa . Previous research has shown that the negative charge of microorganism cell walls influences how microbes engage with the NPs or electrons they create. This affects both gram-positive and gram-negative bacteria. The cell wall and membrane protect the microbe from its surroundings, enable equilibrium, and allow nutrition to enter the cell. The presence of CTAB/ZnO in water-based medium leads in the formation of Zn 2+ ions. The capacity of nanoparticles to generate reactive oxygen species makes them promising antimicrobial substances. Because of their electrostatic bonds with the cell's outermost section, Zn 2+ ions compromise the strength of the bacterial cell wall. When the membrane's stability is weakened, NPs may be allowed to enter the cell and cause oxidative haemorrhaging. Furthermore, the ZnO NPs interact with electrons inside the bacterial cell, amplifying the inhibition of cell development. Because of the breakdown of phosphate and hydrogen bonds, spindles are also destroyed. The interactions between the inorganic metal oxide nanoparticles and the bacterial cell modify the ability to leak of the membrane. This eventually leads to many cell deaths 73 , 74 , 75 .

Schematic of antibacterial action mechanism of CTAB-mediated ZnO NPs.

CTAB/ZnO NPs shown great effectiveness and perseverance, making them ideal for microbial eradication from the environment. As a result, our findings demonstrated the feasibility of producing ZnO NPs with significant antibacterial activity against both gram-positive and gram-negative bacteria.

In the current study, a unique chemical precipitation-cum-hydrothermal process was used to effectively synthesise multi-structured CTAB/ZnO. The synthesised CTAB/ZnO NPs’ exhibited an E g of 2.82 eV. FESEM and HRTEM images showed that the CTAB inhibits the ZnO NPs’ growth in particular directions and led to the creation of new multi-structures. The developed CTAB/ZnO was found excellent for the photocatalytic degradation of RB-81 under UV irradiation. The developed photocatalyst is tested with several key performance metrics, e.g., QY, STY, and FOM. Notably, the CTAB/ZnO NPs developed in current study outperform previously developed photocatalytic systems (in terms of performance metrics values). Also, the high activity of ZnO nanoparticles makes them beneficial for the eradication of bacteria. The advantageous characteristics of CTAB/ZnO NPs, including small crystallite size, high crystallinity, low E g , highly functionalized surface, and multi-structured morphologies can be suspected for their superior photocatalytic and antibacterial properties.

Data availability

All data generated or analysed during this study are included in this published article and its supplementary information files.

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The author K M Batoo would like to thank Researchers Supporting Project No. (RSP2023R148), King Saud University, Riyadh, Saudi Arabia for the financial support.

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Gaur, J., Kumar, S., Zineddine, M. et al. CTAB-crafted ZnO nanostructures for environmental remediation and pathogen control. Sci Rep 14 , 20561 (2024). https://doi.org/10.1038/s41598-024-65783-x

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Zanco Journal of Pure and Applied Sciences (Jun 2024)

Effects of zinc oxide nanoparticles (ZnO NPs) synthesized from different plant leaf extracts on mealworm larvae Tenebrio molitor L.,1758 (Tenebrionidae: Coleopetera)

Saher T Omar,

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Synthesize of zinc nanoparticles adopted through a simple, and eco-friendly biosynthesis process, utilizing the eucalyptus ( Eucalyptus camaldulensis), river oak (Casuarina cunninghamiana) and dill (Anethum graveolens ) leaves as sources and evaluate the insecticidal effects of the produced zinc oxide nanoparticles against the mealworms Tenebrio molitor L.,1758.The produced ZnO nanoparticles were characterized by X-ray diffraction (XRD), UV–visible, and transmission electron microscopy (TEM). The laboratory research was carried out with feeding method (the leaf immersion in ZnO NPs solution with different concentrations). The mortality effects of all the three synthesized ZnO nanoparticles against the studied larval stage was recorded in various period of time. the results of the statistical analysis showed that there were significant differences in the average mortality rate according to plant consisting of zinc nanoparticle, in which the highest average of the larval mortality was obtained (61.83%) for river oak, with an average percentage of adult emergence (27.50 %). Similarly the LC50 values of the ZnO NPs derived from the used plants was showed a varying effect on the larvae of mealworm with feeding method and that this effect varied according to the plant species in which for the River oak was (396.27 ppm ), Eucalyptus plant (3630.78 ppm )and Dill plant was( 6280.58 ppm). This result concluded that Zinc Oxide Nanoparticles from plant sources has larvicidal properties but the most effective one was from River oak plant and they serve as eco-friendly an alternative to synthetic insecticides for controlling insect stages. Hence the biogenic Zinc Oxide Nanoparticles can be used as potential insecticidal agent for the studied insect.

Zinc oxide nanoparticle, Mealworm , Eucalyptus , River oak ,Dill

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Published: 11 September 2024

Exploring the antifungal activities of green nanoparticles for sustainable agriculture: a research update

Muhammad Atif Irshad 1 ,
Azhar Hussain 1 ,
Iqra Nasim 1 ,
Rab Nawaz 1 , 2 ,
Aamal A. Al-Mutairi 3 ,
Shaheryar Azeem 1 ,
Muhammad Rizwan 4 ,
Sami A. Al-Hussain 3 ,
Ali Irfan 5 &
Magdi E. A. Zaki 3

Chemical and Biological Technologies in Agriculture volume 11 , Article number: 133 ( 2024 ) Cite this article

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Green nanotechnology has significant potential for use in agriculture particularly due to their antifungal properties, ability to control fungal diseases and reduce the reliance on chemical fungicides. Biotic stresses in agriculture have caused widespread damage worldwide, and green NPs provided eco-friendly alternatives to traditional chemical treatments, which are frequently toxic and harmful to the ecosystem. Green NPs could become an important tool in modern agricultural practices and environmental remediation if appropriate research is conducted to identify cost-effective production methods as well as safe and sustainable applications. In order to understand the potential of green NPs for sustainable agriculture and identify potential risks, research is ongoing into the effectiveness in agriculture sectors. Research update on green NPs is presented in this paper using data published on science direct over the last 15 to 20 years to clarify and understand the antifungal mechanisms of green metallic NPs, carbon and graphene nanotubes, nanocomposites as well as other type of nanomaterials. These green NPs are found to be more effective against pathogens on crops and humans than conventional fungicide approaches. They are very effective against fungi that affect cereal crops, including Fusarium oxysporum , Botrytis cinerea , and Candida species , etc. The green NPs developed using green synthesis methods are both cost-effective and environmentally friendly. Moreover, research is also required to identify the best methods for applying green NPs for crop production and sustainable agriculture. Furthermore, research should be undertaken to establish the most cost-effective methods of making and deploying green nanoparticles at a large field size study where there is fungal attack that diminishes agricultural output and affects global crop production.

Graphical Abstract

Green nanotechnology: an overview

There is an imbalance in the world's natural resources as a result of population growth and exploitation of ecosystem. Nanotechnology, according to researchers, can improve products by improving their performance, lowering manufacturing costs, and increasing resource efficiency. It is one of the most rapidly rising fields in science and technology. Nanotechnology can create nanoparticles (NPs) with higher surface-to-volume ratios and a variety of chemical and physical characteristics. Nanoparticle are widely employed in chemistry, energy, healthcare, and cosmetics for environmentally friendly applications. Metal and semiconductor NPs include oxides, nitrides, and sulphides [ 1 , 2 , 3 ]. The creation of nanoparticles by living cells, especially via plant resources is the subject of the newly developing scientific field known as “green nanotechnology”. Numerous sectors rely on this field, including electronics, biotechnology, nuclear energy, fuel and energy, and pharmaceuticals as well as for the remediation of various environmental ailments [ 4 , 5 , 6 ]. Since biological procedures using green synthesis tools are safer, more environmentally friendly, non-toxic, and more economical than other similar approaches, they are better suited for synthesizing nanoparticles between 1 and 100 nm. The top-down and bottom-up approaches use different physical, chemical, and biological processes to create the metal nanoparticles [ 7 , 8 ]. Following Fig. 1 explores the green synthesis routes along with the potential applications of green nanotechnology.

Mechanism of green synthesis of nanoparticles by using plant materials and its sustainable applications in various sectors

Greenly produced NPs have been shown to enhance the performance of solar cells, photocopiers, xerography, rectifiers, antioxidants, and photocatalysis [ 9 ]. According to Pansambal et al. [ 10 ], green-produced cerium oxide nanoparticles have antioxidant, antidiabetic, anticancer, antibacterial, and antifungal properties in addition to photocatalytic dye degradation. Potential photocatalytic, antioxidant, and antibacterial properties of green-produced stannic oxide nanoparticles make them useful for improving environmental and human health applications [ 11 ]. Applications in biomedicine and the environment are being developed with green-produced silver chloride nanoparticles [ 12 ]. Different plant parts are used to create green synthetic metal nanoparticles, which are also generated using economical, non-toxic, and environmentally beneficial processes. In contrast to different physical and chemical methods, environmentally friendly produced nanoparticles perform more actively in the removal of dyes, antibiotics, and metal ions from the soil and water media. The most effective way to make nanoparticles is by green synthesis, which also happens to be a cost-effective, environmentally friendly, and very stable process. In environmental and biological applications, green synthesis techniques respond more favorably [ 13 , 14 ]. Numerous phytochemical substances with oxidation–reduction properties, such as flavonoids, phenolics, terpenoids, and polysaccharides, are found in plants. For this reason, they are best used in the environmentally friendly creation of nanoparticles. The process of creating stable nanoparticles requires precise understanding of the phytochemical components, hence the synthesis of phytochemical compounds for nanoparticles is not a general process [ 15 , 16 ]. Most people feel that the important actors in the creation of environmentally friendly nanoparticle manufacturing are plant secondary metabolites, notably polyphenols, phenols, and other plant materials that participate in the synthesis process. According to [ 17 ], green synthesis approach is more sophisticated, repeatable, safe, and inexpensive. Comparing plant-based green manufacturing of nanoparticles to other comparable biological processes involving actinomycetes, bacteria, fungi, and algae reveals certain advantages [ 18 ]. The presence of considerable phytochemicals in these artificially manufactured green nanoparticles raises concerns for numerous plant parts, including the roots, stem, leaf, seed, and fruit [ 19 ]. In various plant portions, squeeze, wait, and apply salt solutions after cleaning with tap or distilled water to produce plant-synthesized nanoparticles. Using this method, metallic salts were added, and then the nanoparticles were eliminated using the required laboratory procedures. Among the industries that employ green nanoparticles are agriculture goods, food, aquaculture sciences, personal hygiene, medicine, and nano-enabled technology.

Green nanotechnology and agriculture

Green nanotechnology research has demonstrated a considerable potential to alleviate major barriers to reaching sustainable agricultural production objectives. Utilizing environmentally friendly materials has the potential to transform food systems and address the global food security issue of today. With the magic of nanotechnology, it has the power to change modern agriculture from the period of genetically modified crops to the exciting new era of atomically changed organisms [ 6 , 20 , 21 , 22 , 23 ]. The excessive use of chemical fertilizers for higher yields give rise to growth of insects and microbes causing fungal diseases in great numbers in the present day unsustainable agricultural practices. These fungal attacks can possibly impact both the crops growth and the crops yield imposing economic losses to the farmer’s community.

On the other hand, the innovative GNT is based upon the applications of nanotechnology principles and techniques applied in an eco-friendly mode for effective control of fungal activities of various pathogens in agriculture. The GNT involves the use of suitable materials on a microscopic scale of (0.1–100) nm size to effectively control the desired ailments from the start to the maturity of the plants. Hence, their applications in agriculture can enhance crop production and improve resource efficiency, offer innovative and eco-friendly approaches to control all possible antifungal activities in plants [ 24 , 25 , 26 ]. It is noticed during the research, that the use of GNT applications can greatly increase the stability of crops by reducing the losses due to abiotic and biotic stresses, producing higher crop yields by curtailing the production costs in agriculture [ 27 ]. The GNT involve the applications of following novel nanoparticles techniques in the field of emerging agriculture. The nano-coatings on seeds can speed up germination rates, protect against pests and diseases, and can provide controlled release of nutrients during early growth stages of plants in agriculture [ 28 ].

The nanotechnology can contribute to the development of efficient nano-based water filtration and adsorbent systems that ensure pathogen and toxic free clean water for irrigation purposes. A lot of inorganic materials, such as heavy metals, were present in the wastewater from the industries, posing serious health hazards to people. Nanobased filtration alleviates those effects. This technology can also be used to purify water from other sources, such as rivers and lakes. It can provide a cost-effective and efficient way to clean contaminated water and make it safe for consumption [ 29 , 30 , 31 , 32 , 33 , 34 ].

Numerous studies have demonstrated the potential of green nanotechnology to regulate stress-induced changes in plants. Moreover, the regulated and targeted release of nano-pesticides has been shown to be a highly successful method of removing biotic stressors in agriculture, particularly for wheat crops [ 35 , 36 ]. Some possible green NPs measures for the agriculture sector's sustainable farming practices are shown in Fig. 2 below. These actions include preserving water, enhancing soil health, and using fewer toxic pesticides and fertilizers. Using sustainable energy sources, including wind and solar energy, can also aid in lowering carbon emissions. These properties and futuristic approach of green nanotechnology can enhance the crop production in agriculture [ 37 , 38 ]. The development of nano-scale formulations for pesticides can improve their efficacy and reduce the amount of chemicals needed. Nano-encapsulation of active ingredients enhances targeted delivery and reduces the environmental contamination to the minimum level. The nano-based fertilizers aim to enhance nutrient uptake by plants, increasing the efficiency of nutrient utilization. Moreover, the controlled release mechanisms in nano-based fertilizers can supply necessary nutrients to plants for wider periods and hence reducing the need of their frequent supply to the plants. Moreover, the use of nano-carriers can improve the efficiency of delivering various agricultural inputs primarily pesticides, nutrients to the plants [ 39 , 40 , 41 , 42 ].

Role of green nanoparticles for the agricultural services as crop production, crop protection, crop monitoring, soil quality, and crop sensing

Antifungal activities of green nanoparticles

Techniques involving plants with nanotechnology called “green nanotechnology” offers an efficient, eco-friendly management and control of these fungal pathogens in agriculture. Plant-based nanotechnology is a cutting-edge approach that will undoubtedly bring an era of agricultural technology innovation to solve such issues. Nanoparticles based on phyto-extracts demonstrate the potential of antimicrobial activities for effective fungal pathogen control compared to conventional fungicides. In addition to ensuring plant health, nanoparticles satisfy agriculture's growing need for high output. The limits of chemicals and the potential of green nanoparticles, which provide fresh approaches to managing fungicides that cause fungal illnesses in agriculture, are the primary topics of this paper. Recent research showed that the rise of fungal diseases in plants resulted in economic losses in the agriculture. Chemical fungicide sprays are not an environmentally acceptable way to treat fungal illnesses since they pollute the environment and pose a risk to human health as well as other biotic life forms. However, these chemical fungicides appear overused due to their affordability and ease of application [ 43 ]. According to Moore et al. [ 44 ], fungi account for 70–80% of the losses brought on by microbial diseases. It is believed that there are around 1.5 million species of kingdom fungus, and most of these fungal pathogens cause plant illnesses and production losses. Fungal infections have been responsible for agricultural losses exceeding 200 billion euros annually [ 45 ]. Animal pests account for around 18% of agricultural crop losses, with microbiological diseases and weeds accounting for 16% and 34% of losses, respectively. However, it is utmost essential to consider potential environmental and health impacts while employing green nanotechnology in agriculture. The overall sustainability of agricultural practices requires careful development of GNT and essentially should be done thorough valid risk assessments before its field application.

A variety of fungal infections were effectively inhibited by the ZnO NPs made from Parthenium hysterophorus plant extracts. For example, ZnO NPs based on parthenium start to significantly slow down the growth of A. flavus and Aspergillus niger pathogens, respectively [ 46 ]. ZnO nanoparticles, which were produced with the help of Syzygium aromaticum bud extracts, shown efficacy against Fusarium graminearum , a pathogen that typically inhibits the growth of mycelial cells and the synthesis of mycotoxins such as zearalenone and deoxynivalenol. At the same time GNT treatments can raise lipid peroxidation, reactive oxygen species (ROS) production and lower ergosterol matters of fungal membrane which is highly damaging to established pathogens in agriculture [ 47 ].

Downy mildew, produced by Plasmopara viticola , reduces the crop production that effects the food security. This illness can be efficiently managed with a nano-composite of graphene oxide (GO) and iron oxide (Fe 3 O 4 ) known as GO-Fe 3 O 4 . Pretreating leaf discs with this nano-composite before inoculating with P. viticola sporangium significantly reduces spore germination, most likely by restricting water routes in the sporangia. While ordinary Fe3O4 and GO have limited control on spore germination, the nano-composite is far more powerful. Graphene oxide coated with silver nano-composite can cause antifungal effects by interacting with fungal cell membranes and disturbing their bonding severely as in case of pathogen F. graminearum [ 48 ].

The use of silver nanoparticles (Ag NPs) has demonstrated a favorable impact by establishing direct contact between the Ag ions and the pathogen’s spores and germ tubes, so stopping their negative influence. This confirms that Ag NPs are capable of curbing a variety of illnesses caused by plant pathogenic fungi. So AgNPs provide strong antifungal properties by disrupting fungal cell membranes and halting their cellular processes for further growth. They are effective against a broad spectrum of fungi, making them valuable in agriculture [ 49 ]. The Ag NPs are commonly utilized for sterilization processes, such as waste-water treatments and water sanitization, because of their antimicrobial qualities [ 50 ]. By employing the green chemistry approach, Ag NPs may be synthesized to regulate the detrimental effects of certain fungal diseases. For example, Krishnaraj et al. [ 49 ] tested the efficacy of Ag NPs at varying concentrations against a variety of fungal plant pathogens, such as Rhizoctonia solani , Macrophomina phaseolina , Alternaria alternata , Curvularia lunata , Botrytis Cinerea, and Sclerotinia Sclerotiorum , using green AgO NPs utilizing the leaf extract of Acalypha indica . Amazingly Ag NPs with a concentration of 15 mg showed a remarkable inhibitory activity against all above pathogens in the field of agriculture. In a different study, Ag NPs (30 ppm) prepared from AgNO 3 (5 mM) solution using Argemone mexicana leaf extract were shown to be extremely poisonous to the pathogenic fungus Aspergillus flavus [ 51 ]. Also, Ag NPs can be manufactured by using seeds extract of T. peruviana (10%) mixed in chemicals of AgNO 3 (1 mM) in the presence of sunlight or autoclave method or combination of both techniques. The performance of Ag NPs is much bigger by careful treatment which may inspire the direct contact of Ag ions with germ tubes and spores to control effectively pathogen and fungi activities in agriculture [ 49 ]. The following Table 1 illustrates the use of several NPs for successfully reducing fungal attacks on different crops. The NPs used are commercially available and have proven their effectiveness in reducing the attack of fungi on crops. These NPs are applied directly to the plants, where they act by inhibiting the growth of fungi. In some cases, the NPs can also be used to prevent future fungal attacks.

In the field of plant pathology in agriculture, the green NPs may be effectively utilized to treat a range of fungal infections [ 106 , 107 ]. Kumar et al. [ 108 ] reported on the use of Aloe Vera ( Aloe barbadensis Miller ) leaf extract for the production of Cu NPs, which shown antioxidant properties for plant diseases, including blackberry fruit. Also use of Citron juice (The Citrus Medica) for the biosynthesis of Cu NPs confirmed strong inhibitory properties against the pathogens of F. graminearum , Fusarium , culmorum , F. oxysporum and culmorum Fusarium , respectively. However, they proved less effective against pathogens of F. graminearum and F. oxysporum , respectively [ 109 ].

The study of green synthesis of Cu NPs with the stem extract of clove ( Syzygium aromaticum ) displays an outstanding antifungal action against pathogens Aspergillus niger , Aspergillus flavus and Penicillium spp., respectively [ 110 ]. Further successful control of fungal activities by green Cu NPs was reported against the harmful phytopathogens including Penicillium digitatum, Fusarium oxysporum, Phoma destructiva, Phytophthora cinnamon, Alternaria alternata, Pseudomonas, Curvularia lunata, syringae, and Alternaria alternata , respectively [ 111 ].

The gold nano particles (GNPs) can be successively synthesized by green method using variety of fresh leaves extract of Memecylon edule [ 112 ], Punica granatum [ 74 ], Capsicum annuum [ 113 ], Magnolia kobus and Artemisia dracunculus [ 114 ]. They are also synthesized by floral excerpts of Moringa oleifera [ 115 ]. These green nanoparticles have also an effective antifungal agent when mixed with GNPs [ 116 ]. GNPs were found more effective when used with suitable non-toxic reducing agents, especially sodium boro-hydride and sodium citrate, respectively [ 117 ]. According to Mittal et al. a range of stabilizing and reducing agents for the management of fungal infections in agriculture may be made from plants [ 118 ]. Therefore, to prevent fungal illnesses, non-toxic, healthful, and environmentally friendly sources must be developed [ 119 , 120 ]. Fungicides made of synthetic chemicals are poisonous and harmful to the environment, soil biodiversity, and human health. Accordingly, trends are changing in favor of using NPs to safely and effectively treat fungal infections in plants. It has been discovered that organic and inorganic NPs with a variety of biological purposes are effective against bacterial, viral, and fungal infections. Plant extracts are the most significant biological material bio-reductant for the creation of NPs [ 121 ]. Since phyto-extracts control fungal infections, encourage plant development, and successfully lower agricultural illnesses, they may be utilized to synthesize environmentally friendly NPs [ 122 ]. It is discovered that the several “green” produced NPs are lucrative, non-toxic, easy to use, and inexpensive. They are appropriate for curing agricultural plants of diseases. Compared to the several old approaches, the green synthesis using NPs produced more stable synthesized materials and is an essential component of agricultural sustainability [ 123 ].

Green nanoparticle manufacturing and use are likely to increase due to rising environmental consciousness, regulatory pressure to eliminate hazardous waste, and demand for sustainable solutions. The three categories of green nanoparticle synthesis are phytochemicals, extracellular, and intracellular. Due to the availability of phytochemical components in the extract, which can also function as reducing and stabilizing agents to turn metal ions into metal nanoparticles, the process of producing metal nanoparticles from plant extract is low cost and high yield [ 124 ]. Green nanoparticles, a fast-expanding sector, are experiencing substantial development due to increased demand for sustainable solutions across a variety of sectors. Global green nanotechnology market, estimated to increase from 2020, is expected to grow more by 2030, with high contributions from various countries of the world. The consumption and production of green NPs synthesized by eco-friendly methods are growing rapidly, as industries seek sustainable alternatives for their businesses. Green nanotechnology global market, estimated valued at $8.3 billion in 2020, will be projected to reach $26 billion till 2028, with significant contributions from the Asia-Pacific region. The main contributions of green NPs are mainly used in environmental remediation, agriculture, medicine, and with the healthcare sector driving substantial growth [ 125 , 126 , 127 ]. Environmental and agricultural applications are also expanding and reflect huge demands for sustainable solutions across these industries.

The global commercial production of green NPs faces challenges in scaling up despite growing interest in sustainable synthesis methods. Plant-based green synthesis has been proposed as an alternative, it has yet to achieve large-scale commercial viability [ 128 ]. Green nanoparticles are rapidly being employed in health, agriculture, and environmental remediation, with considerable market growth predicted in these sectors. Nanoparticles are found in both organic and inorganic modules, including ferritins, liposomes, micelles, dendrimers, and magnetic NPs, as well as metal and semiconductor NPs such as oxides, nitrides, and sulfides [ 129 ]. These green nanoparticles are sprayed as foliar treatments to the targeted crops to reduce disease. Overall, nanoparticle-based treatments are potential alternatives to traditional fungicides for controlling plant diseases in a variety of crops [ 130 ].

Antifungal activity of other nanocomposites synthesized by conventional methods

The overuse of pesticides and other chemicals, along with conventional methods for nanoparticle synthesis, has detrimental impacts on soil fertility, soil microorganisms, and the health of people, plants, and animals. By altering metabolic and physiological processes, the increasing use of conventional fertilizers has led to the emergence of pathogen strains that are resistant to them and delays the growth of photosynthetic pigments and plant reproductive organs. They also prevent plants from going through mitosis, forming microtubules, and respiring their cells. Engineered nanoparticles exhibit promise antifungal effectiveness against a variety of fungal species, including drug-resistant Candida albicans. Silver nanoparticles (Ag-NPs) have considerable antifungal activity, equivalent to traditional antifungal treatments [ 131 ]. Polyvinylpyrrolidone-coated Ag-NPs, when coupled with azole antifungals, have synergistic effects on resistant C. albicans , compromising cell membrane integrity and preventing budding processes. Amphotericin B-conjugated silica nanoparticles have fungicidal action against Candida sp. and may be reused repeatedly without losing efficacy [ 132 ]. Sub-lethal doses of different nanoparticles, such as Ag, SiO 2 , TiO 2 , and ZnO, might improve the antifungal activity of beneficial bacteria such as Pseudomonas protegens CHA0 by increasing the formation of antifungal chemicals [ 133 ]. These findings indicate that tailored nanoparticles may have significant benefits in fighting fungal infections and developing novel antifungal strategies. The research with carbon-nano tubes (CNTs) verified that multi-walled-carbon nanotubes (MWCNTs) can greatly enhances both the ability of seed germination and plant growth by control of antifungal activities. Furthermore, Tripathi and Sarkar [ 41 ] found that applying water-soluble CNTs helped wheat plants expand their roots and shoots in both light and dark environments. Additionally, it has been confirmed that industrial-grade MWCNTs (2560 mg kg −1 ) significantly increased crop germination and root elongation [ 42 ]. The following Table 2 explores the antifungal/antimicrobial action of various other nanomaterials that are being applied for antifungal activities on different crops.

Mechanism involved in the antifungal activity of green nanoparticles

Applying nanoparticles as a foliar spray on the cereal crops provide a multifaceted approach to fighting fungal infections, leveraging both direct antifungal properties and indirect benefits through soil and plant health improvement. When cells were exposed to NPs, they produced more ROS and OH radicals, reducing regulation of antioxidant machinery and oxidative enzymes, disrupting cellular integrity and osmotic balance, and decreasing pathogenicity. As a result, lipid peroxidation increased, inflammation developed, mitochondrial function declined, and cell death succeeded [ 141 , 142 ]. There was evidence that NPs caused cell death by a caspase-dependent pathway, suggesting they could induce apoptosis. As a result of NPs, ROS were generated more and antioxidant enzyme activity decreased. Antifungal effects of metallic nanoparticles are attributed to their electropositive surfaces, which oxidize plasma membranes and allow entry into the pathogen body [ 142 , 143 , 144 ]. The results of Zhang [ 145 ] provide more evidence for this, since they address the reversible conversion of Ce (III)/Ce (IV) between two valence states as a unique antibacterial mechanism. The role of ROS in the antibacterial activity of CeO 2 NPs is also highlighted by Kuang et al. [ 146 ] who found that exposure to these particles can increase intracellular ROS levels in E. coli . However, the specific mechanism of the antifungal activity of CeO 2 NPs and biochar is not fully elucidated and requires further research. It is possible that ROS generated by CeO 2 NPs are involved in the disruption of cell walls, leading to the death of fungal cells. It is also likely that ROS can activate the immune system, aiding in the fight against fungal infections. ROS may also damage the fungal membrane, preventing the transport of essential molecules such as oxygen and nutrients. Additionally, ROS can react with fungal enzymes, damaging their ability to catalyze important reactions. ROS can also damage the DNA of fungal cells, leading to mutations that prevent the cells from reproducing and spreading. Furthermore, in one of its foliar applications to wheat seedlings, ZnO NPs of nAl 2 O3 (< 50 nm) shown reducing the root length of the plants owing to oxidative stress activity of superoxide dismutase with catalase enzymes raising. The smaller concentration of ZnO NPs causes healthy impact on seed germination process. On the contrary, higher concentration of ZnO NPs can cause seed germination degradation as it is insoluble in water. The ZnO NPs display antifungal effects by inducing oxidative stress and damaging fungal cell membranes. Additionally, ZnO NPs created using phyto-extract of Eucalyptus beads were investigated to predict the fungal pathogen that causes illness in apple plants. Amazingly at 100 ppm concentrations, the highest reserve of 76.3% was noticed for pathogen Alternaria mali , 65.4% for Botryosphaeria dothidea and 55.2% for Diplodia seriata , respectively. Thus, it is possible to use these NPs to effectively control the aforementioned fungal infections in order to safeguard different fruit harvests in agriculture on time [ 74 ]. The silver-based chitosan Ag-Chit NPs possess antifungal properties due to their ability to bind to fungal cell walls, disrupting their body structure. They are found very fruitful, especially in its role as bio-fungicides in the field of agriculture. The Ag-Chit NPs were proved very effective in controlling the fungicides and pest communities of A. flavus present in the feed of livestock. These pest-suffered feed samples were collected and accordingly treated by Ag-Chit NPs composites of 30, 60 and 90 mg, respectively, for 10 days incubation at 10 °C producing successful results. Animal pests can cause agricultural harvests to drop by up to 18%, while microbiological illnesses and weeds caused losses of 16% and 34%, respectively. Fungal infections have been responsible for agricultural losses exceeding 200 billion euros annually [ 147 ]. Figure 3 provides the insightful mechanism against the fungus pathogen of wheat crop under the combined application of nano-biochar. Nanoparticles significantly increased the permeability of cells when exposed to them, resulting in alterations to their membranes.

Mechanism of antifungal activities of green nanoparticles

Toxicological effects of green nanoparticles

Green nanoparticles’ hazardous behavior toward the environment and its constituent parts has not been well examined. Nonetheless, a lot of research has been done on the harmful effects of the physicochemical characteristics of artificial nanoparticles. It has been discovered that the oxidation potential, DNA damaging potential, and pharmacological behavior of smaller particles are directly correlated. Almost all cell types are harmful to particles smaller than 50 nm [ 148 ]. According to Tran et al. [ 149 ], these green nanoparticles have the ability to stay suspended in water and the air for extended periods of time, exposing living things for longer and increasing their toxicity. According to reports, endocytosis and phagocytosis are influenced by the nanoparticle's form (triangular, star, tubular, or circular) [ 150 ]. It was also discovered that endocytosing circularly shaped nanoparticles was simple and they could be endocytosed quickly and efficiently even in the presence of other nanoparticles [ 151 ]. According to Gatoo et al. [ 150 ], a particle’s surface charge has a significant influence on its agglomeration behavior and, consequently, its toxicity may increase. Furthermore, surface coatings of organic compounds can affect the pore structure, surface charge, surface roughness, reactivity, and surface roughness of green nanoparticles, depending on the type of coating. The performance of green nanoparticles in a range of applications, such as biomedical, energy, and materials, can be strongly impacted by these features. Therefore, before green nanoparticles are applied, designed, or developed, their toxicological effects should be taken into account.

Emerging trends and technologies in agriculture sector

Emerging trends and technologies in agricultural arena make use of state-of-art data-driven decision-making through latest sensors and drones for fastidious farming based on green nanotechnology. In the future, farms will be factories to meet consumer needs. With the rise of capital-intensive industries and services, artificial intelligence (AI), robotics, and machine learning are replacing humans, saving them labor. The largest obstacle facing emerging nations is the lack of well-paying jobs in the agricultural sector, including secondary agriculture, processing, packaging, value chains, and value addition. Bio-based goods are finding increasing applications in the fields of alternative energy, building materials, chemicals, polymers, pharmaceuticals, cosmetics, fertilizers, nutrition, and insect/pest control. Biofuels are made from grains, oilseeds, and sugarcane [ 152 ]. Promotion of novel CRISPR–Cas9 and other gene editing tools can aid in the development of genetically engineered crops with increased resistance and nutrient value. The development of controlled-environment agriculture through the use of vertical farming (VF) is one of the finest strategies for ensuring year-round output with lower resource consumption, particularly in urban agriculture. The widespread adoption of GNT may improve global sustainable development, and the inclusion of a block-chain system for transparent supply chains can secure the agricultural requirements of both current and future generations [ 153 , 154 ]. At the same time, the GNT innovations mutually boost the energy efficiency, food sustainability by maintaining a fair balance between environmental resources, economy and social needs of the people as per the United Nations Sustainable Development Goals (SDGs). Despite having few commercially available products, green nanotechnology is still primarily in the research and development stage, despite its potential for sustainability and environmental benefits. Even though it claims to cut hazardous waste, greenhouse gas emissions, and energy consumption, its current influence on environmental protection is negligible. However, due to their special qualities, nanomaterials are useful for lowering environmental risks, improving energy efficiency, and producing long-lasting, environmentally friendly products. These technologies are anticipated to contribute significantly to energy challenges and climate change mitigation as they develop.

In order to address the present challenges of global warming, overconsumption of natural resources, and an ever-rising population, there is need of shifting from unsustainable traditional agricultural practices, causing the growth of chemical fungicides to eco-friendlier practices for ensuring global food security. The promotion of green nanotechnology is a suitable option for sustainable management of various plant pathogens without affecting the environment. This critical review profoundly calls for the large-scale production and use of green nanoparticles synthesized by plant extracts, which can greatly enhance the quality and yields of food by curtailing the harmful effects of chemicals and fungicides in the field of present-day agriculture. Green nanotechnology should be encouraged at large-scale due to its cost-effectiveness and environment friendly properties and its ability to ensure sustainable global food supplies to achieve some of the United Nations Sustainable Development Goals.

Futuristic scope and prospective of green nanotechnology for antifungal activity

Certain concern needs to be addressed before the large-scale application of nanotechnology practices in agriculture sector. For instance, the antifungal management requires nano-hybrid materials which are merged by various combinations of gold, silver, zinc, grapheme, copper, iron, polymers say chitosan and variety of organic molecules and chemicals which are highly expensive. At the same time the nano-hybrid construction techniques require expensive high-tech devices of higher energy inputs needs can raise the production cost of nano-hybrid materials. The use of nanohybrids is mostly very effective against phytopathogens control but in actual field conditions, they sometime display off-target movements and may damage the plants by entering into the vegetative parts of the plants.

Hence, the potential impacts of various nanoparticles for antimicrobial and fungicidal applications must be fully registered before evolving the particular nano-formulations and nanohybrids for agriculture purposes. It is also noticed that most metallic/metal oxide NPs may exercise negative impacts in plants and can also alter or reduce the soil microbial levels. Marketing of GNT products is another sky-high issue for their field use on broader levels possibly due to multifarious reasons including unclear technical benefits, high cost, lack of formers/public awareness and uncertainties in legislation about GNTs. Due to these reasons the applications of GNTs in agriculture sector is minimal as compared to other sectors of social and natural sciences. The sensors/kits built of nanomaterials should be used throughout the post-harvest phase in order to promptly identify the fungal infection. The concentration of nanocomposites used in a field determines how harmful they are. When compared to the concentrations of chemical-based insecticides and fungicides, the working concentrations of nanocomposites are relatively low. The capacity of biodegradable polymers to easily translocate inside plant tissues and to have antifungal properties in plants is the ultimate goal when creating nano-composite materials using different combinations of metal and metal oxide nanoparticles. Therefore, further research on biodegradable polymers should be encouraged due to their eco-friendly and biocompatible nature, which ensures sustained formation. The time, money, and resources required to produce GNT for agriculture can be recovered by establishing biological synthesis techniques. Additionally, it may successfully reduce the quantities of environmentally hazardous chemicals needed for the commercial synthesis of non-composite materials and nanomaterials using the most well-researched physical/chemical synthesis methods accessible worldwide.

Availability of data and materials

All the data of this study is contained in the manuscript. For any additional data or information needed regarding this review article, please contact the corresponding authors.

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Acknowledgements

The authors express their gratitude to the Deanship of Scientific Research, Imam Mohammad Ibn Saud Islamic University, Saudi Arabia to support this research work. The authors express sincere gratitude to the Department of Environmental Sciences, The University of Lahore, Pakistan for the provision of laboratory work and resources provided for this research.

This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU).

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Department of Environmental Sciences, The University of Lahore, Lahore, 54000, Pakistan

Muhammad Atif Irshad, Azhar Hussain, Iqra Nasim, Rab Nawaz & Shaheryar Azeem

Faculty of Engineering and Quantity Surveying, INTI International University, 71800, Nilai, Negeri Sembilan, Malaysia

Department of Chemistry, College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU), 11623, Riyadh, Saudi Arabia

Aamal A. Al-Mutairi, Sami A. Al-Hussain & Magdi E. A. Zaki

Department of Environmental Sciences, Government College University Faisalabad, Faisalabad, 38000, Pakistan

Muhammad Rizwan

Department of Chemistry, Government College University Faisalabad, Faisalabad, 38000, Pakistan

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Contributions

Muhammad Atif Irshad: conceptualization, supervision, writing—original draft, writing—review & editing. Azhar Hussain: resources, writing—review & editing. Iqra Nasim: writing—original draft, writing—review & editing. Rab Nawaz: conceptualization, writing—original draft. Aamal A. Al-Mutairi; data curation, writing—review & editing. Shaheryar Azeem: writing—original draft, visualization; Muhammad Rizwan; resources, writing—review & editing. Sami A. Al-Hussain; visualization, data curation. Writing—original draft; Ali Irfan: formal analysis (Literature Survey), visualization, funding acquisition, writing—review & editing. Magdi E. A. Zaki: funding acquisition, project administration, formal analysis (Literature Survey), writing—review & editing. All authors have read and agreed to the published version of the manuscript.

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Irshad, M.A., Hussain, A., Nasim, I. et al. Exploring the antifungal activities of green nanoparticles for sustainable agriculture: a research update. Chem. Biol. Technol. Agric. 11 , 133 (2024). https://doi.org/10.1186/s40538-024-00662-1

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DOI : https://doi.org/10.1186/s40538-024-00662-1

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Green nanotechnology
Metallic NPs
Antifungal activity
Biotic stresses
Sustainable agriculture
Crop production

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Zinc oxide nanoparticles: A comprehensive review on its synthesis, anticancer and drug delivery applications as well as health risks

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Properties of Zinc Oxide Nanoparticles and Their Activity Against Microbes

Aziz ur Rahman

Azamal Husen

Antimicrobial Activity of Zinc Oxide Nanoparticles

Solubility and Concentration-Dependent Activity of Zinc Oxide Nanoparticle

Size-Dependent Antibacterial Activity of Zinc Oxide Nanoparticles

Shape, Composition, and Cytotoxicity of Zinc Oxide Nanoparticles

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Effect of Particle Size and Shape of Polymer-Coated Nanoparticles on Antibacterial Activity

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Effect of Doping on Toxicity of Zinc Oxide Nanoparticles

Interaction Mechanism of Zinc Oxide Nanoparticles

Conclusions

Acknowledgements

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ZnO nanostructured materials and their potential applications: progress, challenges and perspectives

1. Introduction

2. Chemical methods for synthesis of zinc oxide nanoparticles

2.1 Mechanochemical process

2.2 Controlled precipitation

2.3 Sol–gel method

2.4 Vapour transport method

2.5 Hydrothermal method

2.6 Solvothermal method

2.7 Method using an emulsion or microemulsion environment

3. Green methods for the synthesis of ZnO nanoparticles

4. Modification of zinc oxide nanoparticles

5. Potential applications

5.1 Concrete and rubber industries

5.2 Opto-electronic industry

5.3 Gas-sensing

5.4 Cosmetic industry

5.5 Textile industry

5.6 Antibacterial activity

5.7 Drug delivery

5.8 Anti-cancer activity

5.9 Anti-diabetic activity