Nano zinc oxide – Physical, chemical, biological synthesis methods and applications

Nanotechnology is the most innovative field of the 21st century. Extensive research is underway to commercialize nano products around the world. Due to their unique properties, nanoparticles have gained considerable importance over bulk particles. Among other metal nanoparticles, zinc oxide nanoparticles are important as they are used in gas sensors, biosensors, cosmetics, drug delivery systems, etc. Nano zinc oxide (ZnO NPs) also have remarkable optical, physical and antimicrobial properties and thus have great potential in agriculture. Regarding the formation method, ZnO NPs can be synthesized by several chemical methods such as precipitation method, vapor transport method and hydrothermal process. Nowadays, biosynthesis of ZnO NPs using various plant extracts is also very popular. This green synthesis is quite safe and environmentally friendly compared to chemical synthesis. This paper presents in detail the synthesis, properties and applications of nano zinc oxide.

(Copyright by NanoCMM Technology)

1. INTRODUCE

1.1. Nanotechnology

Nanotechnology is an emerging technology that can lead to a new revolution in all fields of science [ 1 ]. This technology is used in conjunction with optics, electronics, biomedical and materials science. Research in this field has gained momentum in recent years by providing innovative solutions in various scientific fields.

Nanotechnology deals with nanoparticles that are aggregates of atoms or molecules that are characterized by sizes less than 100 nm. This is actually a modified form of the basic elements derived by changing the atomic as well as molecular properties of the elements [ 2 , 3 ]. Nanoparticles gain considerable traction because of their unusual and intriguing properties, with a wide variety of applications, compared with their bulk counterparts.

1.2. Nano zinc oxide

Zinc oxide is an inorganic compound with the molecular formula ZnO. It appears as a white powder and is almost insoluble in water. ZnO powder is widely used as an additive in many materials and products including ceramics, glass, cement, rubber (e.g. car tires), lubricants, paints, ointments , adhesives, plastics, sealants, pigments, foodstuffs (source of Zn nutrients), batteries, iron and flame retardants. In the Earth’s crust, ZnO exists as the mineral zincite, but most of the ZnO used commercially is produced synthetically. ZnO is often referred to as II-VI semiconductor in materials science because zinc and oxygen belong to the 2nd and 6th groups of the periodic table. ZnO semiconductors have some unique properties such as good transparency, high electron mobility, wide bandgap, and strong luminescence at room temperature. These properties account for its applications in transparent electrodes in liquid crystal displays and in energy saving or thermal protection windows and other electronic applications. Zinc oxide (wurtzite, p63m) is known to be a wide band gap semiconductor with a band gap energy of 3.3 eV at room temperature (RT). Today, the unique properties of nanomaterials have prompted researchers to develop many simple and inexpensive techniques for producing nanostructures of technologically important materials. Several metal oxide nanoparticles are produced with possible future applications. Among them zinc oxide is considered as one of the best exploited at the nanoscale. The wide band gap and large excitonic binding energy have made zinc oxide important for both scientific and industrial applications [4 ].

1.2.1. Crystal structure of ZnO

The ZnO crystal has a wurtzite (B4) crystal structure, which has a hexagonal unit cell with two lattice parameters  a and  c. In this wurtzite hexagonal structure, each anion is surrounded by four cations at the corners of the tetrahedron, which exhibits tetrahedral coordination and thus sp3 covalent bonding. The tetrahedral configuration of ZnO results in an asymmetrical structure [ 5 – 7 ] (Fig. 1 ).

2. Chemical synthesis of nano zinc oxide

Nanomaterials or nanostructures can be synthesized by various techniques such as spray pyrolysis, thermal decomposition, molecular beam epitaxial, chemical vapor deposition, and laser ablation.

2.1. Advantages of chemical synthesis

Chemical synthesis is one of the most important techniques that can be performed using a variety of precursors and different conditions such as temperature, time, concentration of reactants, etc. The variation of these parameters leads to morphological differences in the size and shape of the resulting nanoparticles.

The different chemical methods used to synthesize ZnO NPs are listed below.

2.2. Chemical reaction of zinc metal with alcohol

Most alcohol-containing media such as ethanol, methanol or propanol are used for the chemical synthesis of ZnO nanoparticles. Typically in this synthesis, 5 mg of metallic zinc powder is added to 10 mL of ethanol. Furthermore, this reaction mixture was sonicated for 20 min and transferred to a stainless steel autoclave and sealed under inert conditions. The reaction mixture was slowly heated (2°C to 200°C per minute) and maintained at this temperature for 24 to 48 hours. Then, the resulting suspension will be centrifuged to get the product, washed, and finally vacuum dried. In alcohol media, the growth of oxide particles is slow and controllable [ 8 ].

2.3. Synthesized by vapor transport

Vapor transport is the most common method for the synthesis of ZnO nanostructures. During this process, zinc and oxygen or slightly mixed oxygen are transported and react with each other resulting in the formation of ZnO nanostructures. There are many ways to generate Zn vapor and oxygen. ZnO decomposition is an easier, direct and simple method; however, it is limited to very high temperatures like ~1400°C.

Another direct method involves heating zinc powder under a stream of oxygen. It involves a relatively low growth temperature (500~700°C), but the ratio between Zn vapor pressure and oxygen pressure must be carefully controlled to obtain the desired ZnO nanostructure. It has been observed that this scale change leads to a large variation in the morphology (size and geometry) of the [ 9 ] nanostructures.

2.4. Hydrothermal Engineering

Hydrothermal engineering is an effective alternative synthesis method because of the low process temperature; Very easy to control the particle size. This process has a number of advantages such as simple equipment use, catalyst-free production, low cost, uniform production, environmental friendliness, and less danger than other processes. This method is very attractive for microelectronics and plastic electronics due to its low reaction temperature. This technique has been successfully used to prepare ZnO NPs and other luminescent materials. The morphology and size of the particles can be controlled through the hydrothermal process by adjusting the reaction temperature, time and precursor concentration.

To synthesize ZnO nanoparticles, a stock solution of Zn(CH3 COO)2 .2H2O (0.1 M) was prepared and then 25 mL of NaOH solution was added to this stock solution (from 0.2). M to 0.5 M) was prepared in methanol. Add during stirring to obtain a pH value of the reactants from 8 to 11. Furthermore, these solutions are transferred to a Teflon-lined sealed stainless steel autoclave and maintained at different temperatures over a range of 100–200°C for 6 to 12 hours under natural pressure. The obtained white solid product will be washed with methanol, filtered and air-dried in a laboratory oven at 60°C. Then characterization of the synthesized samples will be carried out for determination. Their structures by X-ray diffraction [ 10 ].

2.5. Precipitation method

In this method, ZnO can be synthesized using zinc nitrate and urea as precursors. In a typical synthesis, 0.5 M (4.735 gm) zinc nitrate (Zn(NO3)2 .6H 2O) is dissolved in 50 mL distilled water and stirred continuously for 30 min for complete dissolution . 1 M urea (3.002 gm) was also prepared in 50 mL of distilled water, stirring continuously for 30 min; This urea solution acts as a precipitate. This urea solution was added dropwise to the zinc nitrate solution and stirred vigorously at 70 °C for 2 h to allow complete formation of the nanoparticles. Finally, the precipitated solution turned opalescent. This white precursor product was centrifuged at 8000 rpm for 10 min and washed with distilled water to remove any impurities or absorbed ions if present. The resulting product calcination will be carried out at 500 °C in air for 3 h using a muffle furnace [ 11]. All the chemical reactions that occur in this process are shown in the diagram (Figure 2 ).

2.6. Chemical reactions of zinc acetate dihydrate and NaOH

During this process 0.02 M aq. zinc acetate dihydrate was dissolved in 50 mL of distilled water and stirred vigorously. Then add 2.0 M NaOH aqueous drop by drop to reach pH 12 at room temperature; then the whole solution was placed in a magnetic stirrer for 2 h. After the reaction was completed, the white precipitate obtained was washed thoroughly with distilled water followed by ethanol to remove impurities if present. The precipitate is then dried in a hot air oven overnight at 60 °C and during drying complete conversion of Zn(OH) 2  to ZnO NP [ 12 ].

2.7. Disadvantages of chemical synthesis of nanoparticles

Chemical synthesis methods of ZnO NPs such as chemical precipitation, hydrothermal method, pyrolysis, chemical vapor deposition, etc. leads to the presence of some toxic chemicals that absorb on the surface which can have adverse effects in medical applications. There are some reactions in these chemical processes that require high temperature and high pressure to start while some require vacuum or inert conditions. Some chemical techniques also involve the use of certain toxic substances such as H2S, toxic templates, and metal precursors [ 13 ]. The chemicals used to synthesize nanoparticles and to stabilize them are toxic and lead to unfriendly by-products [ 14 ].

3. Green synthesis of nano zinc oxide

The green synthesis process involves the synthesis of plant-based nanoparticles. Green synthesis technique uses non-polluting chemicals to synthesize nanostructures. It includes the use of environmentally friendly and safe solvents such as water, natural extracts.

Therefore, biological approaches using microorganisms and plants or plant extracts to synthesize metal nanoparticles have been proposed as safe alternatives to chemical methods. . In the biosynthesis of nanoparticles, several biological systems including bacteria, fungi and yeasts have been safely used [ 15 ]. But synthesizing nanoparticles using microorganisms is a bit difficult because it involves complex process to maintain cell culture, intracellular synthesis and many purification steps.

3.1. Advantages of green nanoparticle synthesis

Currently, the “green” method of synthesizing nanoparticles has become a topic of great interest because conventional chemical methods are expensive and require the use of chemical compounds/organic solvents. as well as toxic reducing agents [ 16 ].

Green chemistry reduces the risk of pollution at the source level and it is enhanced to contain waste rather than treat or clean up the waste after it has been formed. The principle focuses on the selection of environmentally friendly reagents. Although physical and chemical methods are quicker and easier to synthesize nanoparticles, biological techniques are better and environmentally friendly [ 17 , 18 ].

3.2. By using leaf extract of Coriandrum sativum

Nano zinc oxide can be synthesized using the leaf extract of the plant  Coriandrum sativum . In this procedure, take 50 mL of distilled water and add 0.02 M zinc acetate dihydrate solution to it under constant stirring. Then, after 10 min of stirring, aqueous  Coriandrum  leaf extract was put into different sets (0.25, 0.5, 1 mL) into the above solution. Add 2.0 M NaOH to give pH 12, giving a off-white aqueous solution. It was then placed in a magnetic stirrer for 2 h.

After stirring, the off-white precipitate was removed and washed several times with distilled water followed by ethanol to remove impurities. Then, after drying at 60°C in a vacuum oven overnight, a off-white powder of ZnO nanoparticles [ 19 , 20 ] was obtained.

3.3. By using the leaf extract of the plant Calotropis gigantea

Plants Calotropis gigantea have the potential to be used to synthesize ZnO NPs. 50 mL of leaf extract Calotropis gigantea  is taken and boiled to a temperature of 60–80 degrees using a stirrer or electric stove. Then, 5 grams of zinc nitrate was added to the solution when the temperature reached 60 degrees Celsius. The whole mixture was then boiled until it turned into a dark yellow mixture. The mixture was then collected in a porcelain crucible and calcined in an oven at 400 °C for 2 h. A pale yellow powder is obtained and it is carefully collected and packaged for further characterization purposes. The material is ground in a mortar and pestle to obtain a finer natural product for specific purposes [ 20 ].

3.4. By using the leaf extract of the plant Acalypha indica

In this procedure, all fresh leaves of  Acalypha indica  were first taken and washed methodically with double distilled water, then milled and the extracts filtered through Whatman filter paper. Zinc acetate dihydrate (99% purity) and sodium hydroxide (99% pellets) were used as precursor materials. Zinc acetate dihydrate was added to distilled water under vigorous stirring and after 10 min of stirring, aqueous leaf extract Acalypha was added to the above solution followed by 2.0 M NaOH in water; it produces a white aqueous solution at pH 12. The pH of the medium greatly affects the size of the ZnO nanoparticles. The solution is then placed in a magnetic stirrer for 2 hours. Finally, the precipitate was removed and washed again with distilled water followed by ethanol to remove impurities of the resulting product. A white powder composed of ZnO nanoparticles will be obtained after drying at 60°C in an overnight vacuum oven [ 12 ].

The schematic representation of this process is as shown in Figure  3.

3.5. By using Milky Latex of Calotropis procera

The milky sap of the plant  Calotropis procera (AK, Maddar) is useful for the synthesis of zinc oxide nanoparticles. In this process 0.02 M aq. Dehydrated zinc acetate solution was mixed in 50 mL of distilled water and stirred continuously. After stirring for 10 min, the latex of  Calotropis procera0.25, 0.5 mL and 1.0 mL will be added in three sets to the above solution, and after adding the milk latex, the 2.0 M NaOH aqueous solution will also be added to the solution. is added to the above aqueous solution; it will produce a white aqueous solution of pH 12 which is then placed on a magnetic stirrer to stir continuously for 2 hours. Finally, the precipitate is removed and washed with distilled water 2 or 3 times, followed by ethanol to remove impurities present in the final product. A white powder will then be obtained after drying at 60°C in an overnight vacuum oven [ 20 ].

3.6. By using Biotemplate softened rice

Oryza sativa rice is a renewable and abundant biosource with unique properties that can be used as a biofilter for the synthesis of various functional nanomaterials. ZnO particles can be synthesized via hydrothermal-biotemplate method using zinc acetate, sodium hydroxide and uncooked rice flour in different proportions used as precursors at 120 °C for 18 h. This biotemplate sheet also affects the morphology and size of ZnO [ 21 ] NPs.

4. Distinguishing properties of zinc oxide nanoparticles

Zinc oxide nanoparticles have the following distinguishing properties.

4.1. Physical properties of nano zinc oxide

Zinc oxide nanoparticles have tremendous physical properties. It is worth noting that as the size of semiconductor materials shrinks continuously down to the nanometer scale or even smaller than this reduction, some of their physical properties undergo changes known as “efficiency”. quantum size response”. For example, quantum confinement increases the band gap energy of ZnO almost one-way (Q1D), which has been confirmed by photoluminescence [ 22 ] (Table 1 ).

4.2. Optical properties of ZnO.NPs

The internal optical properties of ZnO nanostructures are being intensively studied for the realization of photonic devices. The photoluminescence (PL) spectrum of ZnO nanostructures has been widely reported, [ 23 ]. From the optical conductivity measurements of the ZnO nanowires, it was found that the presence of O2  had an important influence on the optical response. It was found that the desorption of O 2  affects the optical response of ZnO nanowires. When illuminated, the photovoltaic holes ejected to the surface absorbed O 2  through electron-hole recombination on the surface, while the photoelectrons significantly increased the electrical conductivity. When the light is turned off, O2 molecules are absorbed onto the nanowire surface and reduce the conductivity [ 24 , 25 ].

4.3. Antibacterial properties of nano zinc oxide

The antibacterial activity of metal oxides (ZnO NPs) against  Staphylococcus aureus,  Escherichia coli, or fungi was evaluated quantitatively in the culture medium. It was observed that growth inhibition was only higher in biosynthetic ZnO compared with chemical ZnO nanoparticles as well as other conventional antibacterial agents. The enhanced biological activity of these smaller particles is attributed to the higher surface area to volume ratio. ZnO nanoparticles form an effective antibacterial agent against pathogenic microorganisms. Essentially, the reactive oxygen species detected generated by these metal oxide particles may be the main mechanism of their antimicrobial activity.

The antibacterial mechanism of ZnO NPs involves a direct interaction between ZnO nanoparticles and the cell surface that affects the permeability of the cell membrane; then these nanoparticles penetrate and cause oxidative stress in bacterial cells, which leads to inhibition of cell growth and eventually to cell death; The proven antibacterial activity of ZnO NP suggests its possible application in the field of food preservation. It can be applied as a powerful disinfectant to disinfect and disinfect equipment and containers in the food industry against attack and contamination by foodborne pathogens. ZnO NPs show both toxicity against pathogenic bacteria (e.g.,  Escherichia coli  and  Staphylococcus aureus ) and beneficial effects against bacteria, such as Pseudomonas putida , which have bioremediation and biocide ability. strong roots [ 26 ].

5. Applications and uses of nano zinc oxide

ZnO NPs have attracted intensive research efforts because of their unique properties and versatile applications in transparent electronics, ultraviolet (UV) emitters, piezoelectric devices, sensors chemical variables and spin electrons [ 5 , 27 ].

ZnO is non-toxic; It can be used as photocatalytic decomposition material of environmental pollutants. The thin films and bulk of ZnO have demonstrated high sensitivity to a variety of toxic gases [ 28 ].

ZnO is currently listed as a “generally recognized as safe (GRAS)” material by the Food and Drug Administration and is also used as a food additive. The ZnO nanostructure exhibits high catalytic efficiency, as well as strong adsorption capacity, and is used more often in the production of sunscreens. Most preferred, among various metal oxide nanoparticles, zinc oxide (ZnO) nanoparticles are of particular importance due to their wide range of applications, for example, gas sensors, biosensors, cosmetics, etc. products, storage, optical devices, window materials for displays, solar cells, and drug delivery [ 29 – 32 ].

5.1. Therapeutic use of nano zinc oxide

ZnO NPs play several potential roles in the CNS and perhaps in disease progression through mediated neuronal excitability or even neurotransmitter release. Several studies have shown that ZnO NPs affect the function of different cells or tissues, biocompatibility and neural tissue engineering [ 6 , 33 , 34 ] but little information is available. on the effects on diseases of the central nervous system and the central nervous system. ZnO NPs have been proposed to modulate synaptic transmission in vitro and alter spatial cognition through long-term potentiation (LTP) enhancement in mice. It has also been suggested that exposure to ZnO NPs leads to genotoxicity mediated by lipid peroxidation and oxidative stress [ 35 , 36]. However, because of its targeting ability, ZnO NPs have the potential to be useful in cancer and/or autoimmune [ 37 ].

6. ROLE OF ZNO NPs IN AGRICULTURE

Agriculture is the mainstay of the third world economy but unfortunately today, the agricultural sector is facing various global challenges such as climate change, urbanization, sustainable use of resources, etc. resources and environmental problems such as runoff, accumulation of pesticides and fertilizers; population is increasing day by day and food demand is increasing rapidly and it is estimated that world population will increase from current level of 6 billion to 9 billion by 2050. So we must apply efficient techniques results to make agriculture more sustainable [ 38 ].

Nanotechnology has a pivotal place in transforming agriculture and food production. Nanotechnology has great potential to modify conventional agricultural practices. Most agrochemicals used for crops are lost and do not reach the target site due to a number of factors including leaching, drift, hydrolysis, photolysis and microbial degradation. Nanoparticles and nanocapsules provide an efficient means of delivering pesticides and fertilizers in a controlled manner with high local specificity, thereby reducing property damage. The application of nanotechnology on the farm is gaining attention for its effective control and precise release of pesticides, herbicides and fertilizers. The development of nanosensors could help determine the amount of required farm inputs such as fertilizers and pesticides. The nanosensor for pesticide residue detection has high sensitivity, low detection limit, super selectivity, fast response and small size. They can also detect soil moisture levels and soil nutrients. Plants can quickly absorb nano fertilizer. Nano-encapsulated slow release fertilizer can save fertilizer consumption and reduce environmental pollution.

Zinc oxide National Park has the potential to promote the yield and growth of food crops. Peanut seeds were treated with different concentrations of zinc oxide nanoparticles. Zinc oxide nanoscale treatment (average particle size 25 nm) at 1000 ppm has been used to promote seed germination, seedling vigor, and plant and seed growth. This zinc oxide nanoparticle was also shown to be effective in stem and root growth in peanut [ 39 ].

Colloidal solutions of zinc oxide nanoparticles are used as fertilizers. This is a type of nano fertilizer that plays an important role in agriculture. Nano fertilizer is a plant nutrient that is not only a fertilizer as it not only provides nutrients to the plants but also restores the soil to an organic state without the harmful agents of fertilizers. chemistry. One of the advantages of nanofertilizers is that they can be used in very small amounts. An adult plant only needs 40–50 kg of fertilizer while the usual amount of fertilizer is 150 kg. Nanodrugs can also be successfully used as fertilizers and pesticides [ 40 , 41 ]. Yields of wheat plants grown from seeds treated with metal nanoparticles increased by an average of 20–25% [ 42].

6.1. Effects of ZnO NPs on plants

Nanoparticles of various metal oxides may play an important role to promote plant growth and yield but currently investigations into the toxicological effects of NPs continue to increase over time. and only a few studies have been performed to determine the effect of ZnO NPs on plants [ 43 – 46 ]. A study was performed on seed germination and root development of six higher plant species (radish, canola, rye, lettuce, corn and cucumber); The toxicity of five types of NPs (multi-walled carbon nanotubes, aluminum, alumina, zinc and zinc oxide) showed that seed germination was generally unaffected in most cases while root elongation was inhibition. The IC50 of ZnO NPs was estimated to be about 50 mg/L for radish and about 20 mg/L for weed and rye grass [ 16].

The toxicity studies of ZnO nanoparticles on rye grass showed that, in the presence of ZnO NPNs, the biomass of rye grass was significantly reduced, the root tips were atrophied, the root epidermal cells were also reduced. As the shell becomes vacuumed and collapses. The majority of ZnO NPNs remained attached to the root surface and individual NPs were observed to be present in the stem and protoplasm of the root and target endoderm. No or little or no dissociated zinc ions were translocated in rye grass exposed to ZnO [ 44 ] nanoparticles.

7. NEGATIVE OR TOXIC EFFECTS OF nano zinc oxide

Although ZnO NPNs are of great commercial importance and are present in various commercial products, it is clear that the public is increasingly concerned about the toxic and environmental effects of ZnO NPNs. Unfortunately, toxicological studies performed on zinc oxide nanoparticles over the past ten years suggest that ZnO NPNs pose potential health as well as environmental risks. ZnO NPs can cause severe toxicity to bacteria, Daphnia magna, freshwater microalga, mice and even human cells [ 36 , 47 – 50 ].

7.1. Deep skin

ZnO NPs are particularly useful in sunscreens because they have intrinsic UVA as well as UVB filters. Because of this remarkable property, they offer broader protection than any other sunscreen. But these nanoparticles have the ability to penetrate the skin and reach the surviving cells, leading to the potential toxicity caused by them. A comparative analysis of skin permeability between different animals was performed, ranking them in the order rabbit > rat > pig > monkey > human and noted that pig and rat skin were permeable. 4 and 9–11 times more penetrating than human skin, respectively. Overall,36 , 50 ].

8. CONCLUSION

Nanotechnology is the emerging technology of the present century operating in all areas of science. Nano zinc oxide stand out as one of the most versatile materials, due to their diverse properties, functions and applications. The NPs of ZnO have tremendous physical and optical properties. They also have antibacterial effects against certain bacteria and fungi. In terms of synthesis of zinc oxide nanoparticles, they can be synthesized by chemical methods but in recent times due to the development of green chemistry, it is also possible to biosynthesize nano zinc oxide by how to use different plant extracts. Green synthesis of ZnO NPs is much safer and more environmentally friendly than chemical synthesis as it does not lead to the formation of toxic by-products. According to their uses, nanoparticles play an important role in agriculture, where colloidal solutions of ZnO NPs are used in nanofertilizers. The application of these nanoparticles to plants increases their growth rate and yield. Due to the increasing demand for food, the productivity of major food crops is very low. So it takes hours to commercialize metal nanoparticles for sustainable agriculture.

Reference source:

Zinc Oxide Nanoparticles for Revolutionizing Agriculture: Synthesis and Applications

Sidra Sabir,¹ Muhammad Arshad,¹ and Sunbal Khalil Chaudhari¹