Industrial Manufacturing Applications of Zinc Oxide Nanomaterials: A Comprehensive Study

Abstract

Nanomaterials (NMs) that are created with zinc oxide are very valuable for a wide variety of applications. There is a present interest in ZnO nanoparticles in a wide range of industries. This interest may be attributed to the fact that ZnO NPs have many important features. It will be necessary for ZnO NPs to possess certain qualities in order for them to rapidly find uses in industry and for these applications to have an effect on the expansion of the economy. A large surface area, a large bandgap, photocatalytic property, biosensing, bioimaging, and other qualities are included in this list. In this article, the extraordinary characteristics of ZnO NPs, as well as their novel applications in industrial settings and the challenges that come along with their utilization, will be discussed.

1. Introduction

Nanomaterials have been employed in a wide range of applications, including medical, food processing, fuel engineering, cosmetic products, textile industries, agricultural, electrical devices, automobile manufacturing, and aerospace engineering [1,2,3,4]. NPs are utilized both in academics and in industry for their suitable characteristics, unique design capabilities, ecofriendliness, ease of manufacturing, and low price [5]. NPs incorporated into a matrix of specific materials (polymer, metal, or ceramics) introduce additional novel characteristics, such as excellent mechanical stability (in terms of stability, toughness, strength, dimension, flexibility, and so on), good flame retardancy, optical features, high electro-thermal conductivity, and low water/gas permeability [6]. Due to their potential to be employed in a number of downstream applications, ZnO nanoparticles are one of the most investigated materials [7]. After iron, ZnO NPs are the second most common metal oxide, and they are cheap, safe, and simple to produce [8]. Modifying the shape of ZnO NPs and utilizing various synthesis methods, precursors, or materials to create NPs may readily change their physical and chemical characteristics [9]. ZnO NPs are used in analytical sensing because they are inorganic Group II–IV semiconductors [10]. ZnO NPs seem to be white particles that are not water soluble. Their outstanding chemical, electrical, and thermal stabilities are provided by the ZnO nanoparticle’s 3.37 eV energy band and 60 meV bonding energy [11]. The optical and photocatalytic characteristics of ZnO NPs are also promising [12]. ZnO NPs are used in solar cells [13,14], photocatalysis [15,16], and chemical sensors [17]. ZnO NPs are also renowned for their low toxicity and strong UV-absorption, making them an excellent option for biological applications [18]. The strong and stiff structure of ZnO NPs makes them useful in the ceramic industry. When used as a surface material, ZnO NPs have many beneficial applications in the biomedical sector. Microbes naturally have a high resistance to ZnO NPs [19]. ZnO NPs are widely utilized in biological sensing, biological labeling, gene delivery, drug delivery, and also in nanomedicine [9]. ZnO has been certified by the Food and Drug Administration as a safe compound. ZnO may also solubilize in acidic medium, which opens up the possibility of using the material as multipurpose nanocarriers to help in medication delivery and releasing [20].

Biosynthesis of ZnO NPs reduces the production cost, whereas the conventional methods are expensive and use hazardous precursors [21]. Nonbiosynthetic methods can also be cheap and facile, such as synthesis through solid-state reactions [22]. The biosynthesis of ZnO NPs using leaf extraction from P. pinnata and rambutan peel extract is inexpensive [23,24]. It can also be produced economically from the fungus A. potronii [25]. Rice is one of the most widely consumed products worldwide [26], and its unconsumed biproduct is rice bran. Therefore, production of ZnO NPs using rice bran extract can be cost effective and easier [27]. The NPs of ZnO were predominantly rectangular, and their average size was approximately 50 nm. This environmentally friendly approach to synthesis is more promising than traditional chemical synthesis methods. A straightforward solvothermal technique was used to cultivate nanostructures of ZnO with several distinct morphologies, including nanopyramids, nanosheets, and NPs. Solution pretreatment has an effect on the shape of ZnO nanostructures as well as their optical characteristics [26]. Nanoscale ZnO may take on a number of different shapes and forms in natural settings. Needles, helices, nanorods, belts, wires, ribbons, and combs are all examples of one-dimensional structures. ZnO may exist in the form of two-dimensional structures, such as nanosheets/nanoplates, and nanopellets. Further, ZnO may be used in the production of a wide variety of three-dimensional formations, such as flowers, snowflakes, dandelion seeds, and other shapes [7]. The unusual blend of physiochemical properties that ZnO has is partly responsible for its increasing popularity. These characteristics include resistance to chemical and mechanical deterioration, broad radiation absorption, great catalytic activity, a high electrochemical coupling coefficient, and a nontoxic composition. Other characteristics include wide radiation absorption and a high electrochemical coupling coefficient. There is some degree of correlation between ZnO structure and its activity and properties. For example, the ZnO quantum dot’s photocatalytic activity is quite high when compared to that of the other morphologies. Due to their high surface area, rapid adsorption rate, substantial charge separation, and low rates of electron and hole recombination, dots are the preferred form of photocatalysis. The photocatalytic potential, quantum confinement of photoinduced carriers, and specific (active) surface area are all impacted by the shape of the material [27]. Photocatalysis results indicated that disk-shaped hexagonal ZnO NPs displayed more photocatalytic activity than rod-shaped ZnO NPs. There has been an interest in semiconductor-based photocatalysts because of the hope that they offer for resolving environmental problems. ZnO’s exceptional electrical and optical properties have garnered it a lot of attention, making it important because of its bandgap semiconductor properties [24]. The most frequent types of gram-positive bacteria are Staphylococcus aureus and Escherichia coli, and it was shown that ZnO NPs with a flower-like shape were more likely to be helpful in moderating the impacts of these bacteria. ZnO nanoflowers were discovered to have more sites capable of absorbing gas molecules than ZnO nanoplates, making them more suitable for use in gas sensing applications [22]. This article discusses many striking and state-of-the-art applications of ZnO NPs in a diversity of industrial settings.

2. Multi-Disciplinary Industrial Applications of ZnO NMs

Due to their unique properties, nanoparticles may be used for a variety of purposes. ZnO nanoparticles are the most widely utilized kinds of metal NPs, and their uses span a wide variety of fields, including medicine, food, agriculture, gas sensors, cosmetics, and electronics.

3. Antibacterial and Anti-Fungal Applications of ZnO NPs

Industries use antimicrobial and antifungal agents to prevent contamination and preserve their products by inhibiting micro-organisms [28]. It is, thus, crucial to develop more effective antimicrobial and antifungal agents for their auspicious applications in the industrial sectors. ZnO NPs exhibit promising antibacterial and antifungal properties (Figure 1). It shows excellent effectiveness against both gram-negative and gram-positive bacteria (Table 1). ZnO NPs have been demonstrated to be efficient against a broad spectrum of micro-organisms. The biocidal activity of Bacillus subtilis and Escherichia coli rose from silicon dioxide to titanium dioxide to ZnO [29]. ZnO is more bactericidal against B. subtilis than E. coli [29,30]. The MIC (minimum inhibitory concentration) ranged from 2000 to 12,500 ppm for B. subtilis and from 50,000 to 100,000 ppm for E. coli. The antimicrobial activity of Ta-doped ZnO NPs against Staphylococcus aureus, Pseudomonas aeruginosa, E. coli, and B. subtilis has been studied, and the findings indicate that adding Ta⁵⁺ ions to ZnO improves the bacteriostasis impact of ZnO on bacteria in the dark [29]. ZnO nanoparticle powders and suspensions demonstrated antibacterial efficacy against E. coli and S. aureus. It is highly active against S. aureus [31]. ZnO NPs showed prevention against P. aeruginosa, E. coli, S. aureus, B. cereus, Salmonella typhimurium, P. aeruginosa, Enterococcus faecalis, B. subtilis, Staphylococcus epidermidis, P. aeruginosa, Klebsiella pneumoniae, Candida albicans, and many other bacteria [32]. The concentration of ZnO NPs has an effect on the antibacterial activity [33]. Another study showed that ZnO NPs have unique characteristics and a long life compared to organic disinfectants, which prompted their usage as an antibacterial agent. As a consequence of their high surface area-to-volume ratio, they may be used as new antibacterial agents. ZnO NPs have a high toxicity to organisms, making them a good option for antimicrobial chemistry and antimicrobio-anatomy [34].

The antibacterial activity of ZnO NPs may be related to their small size, which makes them easily adhere to the cell wall and break down the bacterial membrane, killing the cell. It has also been suggested that NPs exert their effects by releasing various levels of H₂O₂ and reactive oxygen species (ROS) (Figure 1). Results showed that ZnO NPs were more toxic to gram-positive bacteria than to gram-negative bacteria. This could be because gram-positive bacteria have a lower antioxidant cellular capacity, rendering them more susceptible to ROS. However, it has been suggested that the Zn²⁺ ions released from the dissolution of ZnO NPs can cling to the bacterial membrane and prolong the growth phase of the bacteria [34].

Antibacterial activity of ZnO quantum dots (practically spherical and 3–7 nm) against E. coli was shown to be dependent on the surface-adsorbed anionic species. The antimicrobial potential of the acetate-adsorbed ZnO QDs was greater than that of the nitrate-adsorbed ZnO quantum dots under light. ZnO QDs that were nitrate-adsorbed were less effective in killing bacteria than those that were acetate-adsorbed. ZnO QDs may be able to trap electrons through the adsorption of anions on their surface. The semiconductor nature of ZnO QDs causes electron-hole pairs to form when they are exposed to light. They produce superoxide, H₂O₂, and hydroxyl radicals (by reactions I and II) that hinder the growth of bacteria.

O₂ +e⁻ → O₂⁻; H⁺ + O₂⁻ → •HO₂; •HO₂ + H⁺ +e⁻ → H₂O₂

(1)

H⁺ + OH⁻ → •OH; •OH⁺ •OH → H₂O₂

(2)

Weaker nucleophile acetate can impact the functional groups of the cell membrane of the bacteria in low light conditions, and this allows the stored electron to be released and create superoxide or other ROS to kill the bacteria. As nitrate is not a strong nucleophile, it is able to strongly trap the electron at the lattice defect sites because of its low nucleophilicity. ZnO NPs were functionalized with 0, 1.5, 3, and 6% of polymeric fibers constructed of recycled polyethylene terephthalate (r-PET) from postconsumer water bottles. These fibers were created in order to test their effectiveness against germs and fungi [35]. In both the light and the dark, ZnO-Ac QDs exerted a stronger inhibitory effect [36]. In recent years, researchers have explored the impact of ZnO NPs in borosiloxane on E. coli bacteria development and growth. E. coli’s growth and development were unaffected by the presence of NPs in borosiloxane. The density of bacterial cultures cultivated on the composite was lowered by 58, 90, and 96% when ZnO NPs were added to the polymer at concentrations of 0.001, 0.010, and 0.10%, respectively. Bacteriostatic characteristics of aqueous colloidal solution of ZnO NPs were tested in another set of tests. Each of the concentrations investigated was 0.0001, 0.001, 0.01, and 0.10%. Nanoparticle doses of 0.001, 0.011, and 1.0% protected bacterial growth and development. The intensity of the bacterial culture is decreased by 93%, with a concentration of 0.0001% [37,38]. The ZnO NPs’ ability to inhibit bacterial growth is proportional to their average particle size. ZnO NPs are proven to be more effective against bacterial activities at higher concentrations when their size is reduced [39].

Table 1. Antimicrobial properties of ZnO NPs.

Product	Size (Nanometer)	Species of Bacteria	Mechanism	Ref.
ZnO NPs	30	E. coli	Damage the membrane’s integrity and the production of ROS.	[40]
	8	S. aureus, E. coli, and B. subtilis	Due to the release of free Zn2+ ions formed in the ZnO suspension for significant growth inhibition of bacteria.	[39]
	10	L. Plantarum	By reaction between the surface of ZnO and cell surface enzymes of bacteria	[41]
	12, 45	E. coli	ZnO involves disrupting the membrane of bacteria	[42]
	∼20	E. coli 11,634	H2O2 generation	[43]
		S. aureus, E. coli	Release of Zn2+ ion	[44]
	∼80	V. cholera	Depolarization of the membrane structure, enhanced permeabilization, DNA damage, and ROS production	[45]
	40	S. pyogenes (MTCC1926), S. mutans (MTCC497), S. flexneri (MTCC1457), V. cholerae (MTCC3906), S. typhi (MTCC1252)	Zn2+ release and ROS production	[46]
	90–100	enterotoxin E. coli (ETEC), V. cholerae	adenylyl cyclase function inhibition, cAMP levels are reduction	[47]
Ag-ZnO nanocomposite	64	GFP E. coli, S. aureus	Release of Ag+ and Zn2+ and ROS production	[48]
Phβ-GBP-coated ZnO NPs (Phβ-GBP- ZnO NPs)	20–50	P. vulgaris, S. aureus	Changes in the permeability of bacterial cell membranes and a high quantity of reactive oxygen species (ROS)	[49]
ZnO nanocatalyst	∼18	E. coli, B. subtilis, S. typhimurium, K. pneumonia	OH−, H2O2 generation, ROS generation	[50]
ZnO-CdO nanocomposite	27	P. aeruginosa, E. coli, K. pneumonia, P. vulgaris, S. aureus, B. spp.	Release Cd2+ and Zn2+ and generate ROS (H2O2, OH−, and O22−)	[51]
ZnO QDs	4	C. metallidurans CH34, E. coli MG1655	Released Zn2+ ion generated toxicity	[52]
Kaoline-ZnO nanocomposites		E. coli, S. aureus, P. aeruginosa, E. faecalis	Zn2+ release, subsequent diffusion of ions into cytoplasm	[53]
ZnO nanostructures (ZnO NSs)	70–80	S. aureus, P. vulgaris, K. pneumoniae, S. typhimurium	Damage to cell membranes by reactive oxygen species (ROS)	[54]
ZnO-Ge NPs	20	E. faecalis, P. aeruginosa	Bacterial cell death triggered by cell penetration	[55]
ZnO-SA composites		S. aureus, E. coli	Reactive Oxygen Species production	[56]
ZnO@GA NPs	11.5 ± 4.4	S. aureus, E. coli	Due to GA’s strong affinity for the bacterial cell membrane and the increased lipophilicity that results from its addition.	[57]

Table 1. Antimicrobial properties of ZnO NPs.

Product	Size (Nanometer)	Species of Bacteria	Mechanism	Ref.
ZnO NPs	30	E. coli	Damage the membrane’s integrity and the production of ROS.	[40]
	8	S. aureus, E. coli, and B. subtilis	Due to the release of free Zn2+ ions formed in the ZnO suspension for significant growth inhibition of bacteria.	[39]
	10	L. Plantarum	By reaction between the surface of ZnO and cell surface enzymes of bacteria	[41]
	12, 45	E. coli	ZnO involves disrupting the membrane of bacteria	[42]
	∼20	E. coli 11,634	H2O2 generation	[43]
		S. aureus, E. coli	Release of Zn2+ ion	[44]
	∼80	V. cholera	Depolarization of the membrane structure, enhanced permeabilization, DNA damage, and ROS production	[45]
	40	S. pyogenes (MTCC1926), S. mutans (MTCC497), S. flexneri (MTCC1457), V. cholerae (MTCC3906), S. typhi (MTCC1252)	Zn2+ release and ROS production	[46]
	90–100	enterotoxin E. coli (ETEC), V. cholerae	adenylyl cyclase function inhibition, cAMP levels are reduction	[47]
Ag-ZnO nanocomposite	64	GFP E. coli, S. aureus	Release of Ag+ and Zn2+ and ROS production	[48]
Phβ-GBP-coated ZnO NPs (Phβ-GBP- ZnO NPs)	20–50	P. vulgaris, S. aureus	Changes in the permeability of bacterial cell membranes and a high quantity of reactive oxygen species (ROS)	[49]
ZnO nanocatalyst	∼18	E. coli, B. subtilis, S. typhimurium, K. pneumonia	OH−, H2O2 generation, ROS generation	[50]
ZnO-CdO nanocomposite	27	P. aeruginosa, E. coli, K. pneumonia, P. vulgaris, S. aureus, B. spp.	Release Cd2+ and Zn2+ and generate ROS (H2O2, OH−, and O22−)	[51]
ZnO QDs	4	C. metallidurans CH34, E. coli MG1655	Released Zn2+ ion generated toxicity	[52]
Kaoline-ZnO nanocomposites		E. coli, S. aureus, P. aeruginosa, E. faecalis	Zn2+ release, subsequent diffusion of ions into cytoplasm	[53]
ZnO nanostructures (ZnO NSs)	70–80	S. aureus, P. vulgaris, K. pneumoniae, S. typhimurium	Damage to cell membranes by reactive oxygen species (ROS)	[54]
ZnO-Ge NPs	20	E. faecalis, P. aeruginosa	Bacterial cell death triggered by cell penetration	[55]
ZnO-SA composites		S. aureus, E. coli	Reactive Oxygen Species production	[56]
ZnO@GA NPs	11.5 ± 4.4	S. aureus, E. coli	Due to GA’s strong affinity for the bacterial cell membrane and the increased lipophilicity that results from its addition.	[57]

Due to the small particle size and the large surface area, ZnO NPs show enhanced antimicrobial and antifungal activities [58]. ZnO NPs inhibited the growth of the plant pathogen F. graminearum [59]. Antifungal activity that has been found for ZnO NPs was effectively evaluated against A. niger, P. expansum, A. alternata, B. cinerea, and F. oxysporum [60]. It also showed great effectiveness against E. salmonicolor, which is a coffee fungus [61], B. cinerea [62] and P. expansum [63], and phytopathogenic fungi species; Fusarium oxysporum f.sp. lycopersici is a fungal plant pathogen. It is a big pathogen in the tomato plant. It has a violet-to-white color on most media but does not produce a pigment on King’s B medium. It has been spread to tomato seeds by the hands of contaminated workers, such as F. solani, and C. gloeosporioids [64]. Thus, the ZnO QDs have promising antifungal activities (

Table 2. Applications of ZnO QDs against Fungi.

Type of Device	Type of Fungi Inhibited	Reference
ZnO QDs	B. cinerea, P. expansum	[64]
	A. saloni, S. rolfii	[65]
	R. stolonifera, A. nidulans, A. flavus, T. harzianum	[66]
	E. salmonicolor	[61]
	A. fumigatus, C. albicans	[67]
	R. stolonifera, P. expansum	[68]
	C. krusei	[69]
Zn/Mg Oxide QDs	A. niger, Paraconiothyrium sp., P. oxalicum, P. maculans	[70]
CS-LiA ZnO QDs	C. albicans	[71]

). Therefore, ZnO-based NPs are crucial for their applications as disinfectants and sterilizing agents in the biomedical and healthcare industries. Moreover, ZnO NPs may be promising for inhibiting fungi and pathogens in agriculture- and food-based industries.

4. Photocatalytic Applications

Major contaminants in textile dyeing wastewater include unfixed dyes and inorganic salts. Existing treatment techniques are ineffective and have limitations. Wastewater from textile dyeing processes is poorly treated and immediately released into the environment. Long-term dumping endangers the ecosystem [72]. The high photocatalytic activities of ZnO NPs for various azo dyes were likewise extremely high, indicating a promising use in organic pollution remediation [73]. Since they are non-toxic, low-cost, and much more effective at absorbing over a wide portion of the solar spectrum, ZnO nanostructures have been demonstrated to be promising photocatalyst candidates for application in photodegradation. The bandgap energy of ZnO is also a significant element in influencing its photocatalytic activity in use [74]. ZnO NPs degraded reactive orange (RO) and methylene blue (MB) (Figure 2) dyes under blacklight irradiation [75]. It also degraded methyl orange, methyl red, reactive blue 21 (RB-21) [76], and rhodamine B dye [77]. Moreover, modified ZnO NPs showed outstanding photocatalytic effects in the decomposition of methylene blue [78]. Manganese-doped ZnO NPs also exhibited excellent performance in the decomposition of methylene blue in the presence of UV and visible light [79]. Ce-doped ZnO NPs showed effective photocatalytic decomposition of harmful dyes, such as direct red-23(DR-23) under UV irradiation [80], direct blue-86 (DB-86), methyl orange (MO), and food black-2 (FB-2) dyes under ultraviolet and visible light irradiation [81]. ZnO-based MOF photocatalyst, ZnO@MOF-46(Zn), has been investigated for methylene blue (MB) degradation, which showed promising results in degrading more than 90% of the dyes (Figure 2) [82]. In a photocatalytic system, the catalyst’s surface is where the photoinduced atomic or molecular change or reaction occurs. When a photocatalyst is excited by photons with energies that are equivalent to a level that is equal to or higher than its bandgap energy, electrons absorb the energy from the photons. Hole formation in the valance band (VB), which has the ability to produce hydroxide radicals, occurs when the energy level exceeds the bandgap energy level. It is possible for a hole to start a dye molecule’s disintegration process by reacting with the target molecule, which causes the loss of one of the molecule’s electrons. ZnO granules are only occasionally used in applications that offer UV protection due to the material’s photocatalytic characteristics. A photocatalyst ZnO produces reactive free radicals when it is exposed to UV light. UV radiation energies, electrons in ZnO are ultimately responsible for producing electron-hole pairs (e-h) [73]. The photocatalytic properties of ZnO produced by thermal evaporation and chemical deposition, in UV-induced degradation of dyes, such as methyl orange, depended on the particle size, shape, and production method of ZnO. Modifying the ZnO photocatalysts’ particle size, shape, and as well as the manufacturing conditions and methodologies, may result in an improvement in the overall effectiveness of the photocatalytic process [75]. Therefore, ZnO-based photocatalysts are highly efficient in the degradation of dyes in industrial effluent to protect the environment.

5. Biosensing Applications

Several ZnO NPs were used for sensing different biomolecules (

Table 3. Applications of ZnO NPs in biosensing.

ZnO NPs	Biological Compound Sensing	Ref.
ZnO NRs/TNs	Acetylcholinesterase	[104]
ZnO/chitosan-graft-poly(vinyl alcohol)core-shell nanocomposite	Glucose	[102]
ZnO/chitosan/MCNT/polyaniline composite film	Xanthine	[90]
Ionic liquid/ZnO/chitosan/gold electrode	DNA	[91]
ZnO NPs decorated multi-walled carbon nanotubes (MWCNT)	Lactate	[92]
MWCNT–ZnO NPs	Cholesterol	[93]
Cysteamine functionalized ZnO NPs	N-Acyl Homoserine Lactone	[96]
Enzyme electrode modified by ZnO NPs	Uric Acid	[97]
ZnO nanoparticle/1, 3-dipropylimidazolium bromide ionic liquid-modified carbon paste electrode	Epinephrine	[98]
Nanostructured ZnO film for urea sensor.	Urea	[99]

). In electrochemical biosensor development, nanostructures of ZnO were used. Analytes, such as uric acid, cholesterol, dopamine, and DNA can all be distinguished using ZnO-based electrochemical biosensors [83,84,85,86]. With a high isoelectric point, ZnO can be used in electrochemical biosensors because it has high biocompatibility, a fast charge transfer property, and an easy and more grounded attachment of diverse proteins on its surface. The fluorescence and piezoelectric characteristics of ZnO make it a delicate optical and piezoelectric biosensor. A consistent and repeatable method of developing ZnO nanostructures directly on electrode surfaces is vital for biosensor innovation. To develop electrochemical biosensors for the sensing of biologically relevant analytes, such as DNA, metabolites, and cancer indicators, ZnO-based platforms can be employed as an immobilization matrix [87]. Acetylcholinesterase [88], glucose [89], xanthine [90], DNA [91], lactate [92], cholesterol [93,94,95], N-acyl homoserine lactone [96], uric acid [97], epinephrine [98], and urea [99] have all been shown to be successfully sensed by ZnO NPs. ZnO NPs-modified carbon paste electrode [100], ZnO NPs film [101], ZnO/chitosan-graft-poly(vinyl alcohol) core-shell nanocomposite [102] etc. are utilized to detect glucose. When constructing a hemoglobin biosensor, ZnO NPs–polypyrrole film has been used [103]. For the detection of acetylcholinesterase, ZnO nanocomposite ZnO NRs/NWs were produced and utilized [104].

In the development of enzymatic biosensors, ZnO NPs are employed. A new L-lactate sensor has been created using an enzyme electrode customized with ZnO NPs and MWCNT [105]. Through electrostatic interaction, ZnO can immobilize elements with low isoelectric points [106,107]. As an example, MWCNT/ZnO nanofiber-based biosensors can be used to detect malarial parasites [93]. ZnO thin films are also ideal for biosensing systems that require extreme sensitivity while maintaining a low cost. These films were manufactured using a flexographic printing process, which resulted in a nanotextured surface. These surface nanostructures have excellent execution for increasing surface functionalization [108]. ZnO nanorods were grown hydrothermally, and ZnO thin films were deposited electrochemically. Furthermore, ZnO nanorods and thin films were also investigated for their potential use in chemical and biological sensing applications. In addition, a ZnO nanorod-based strontium ion sensor was created. ZnO nanorods showed better response than ZnO thin films. ZnO nanorods are advantageous because their surface area-to-volume ratio is much higher than that of ZnO thin films [109]. ZnO and nanowires (NWs) have been studied for use as sensors. Interest in III-nitride nanowires, such as GaN and AlN, increased their prospective applications. NWs’ high surface-to-volume ratio should increase sensor sensitivity and selectivity. High-performance NW transistors can be used to build a generic sensing device. Attaching a recognition group to the nanowire’s surface enables specialized sensing. An amide link between the amine group of APTES (3-Aminopropyl)triethoxysilane) and the carboxylic group in mercapto propionic acid (MPA) allows MPA to be immobilized on ZnO surfaces. APTES and ODTMS (octadecyltrimethoxysilane) monolayers can covalently functionalize hydroxylated GaN and AlN surfaces. Schiff-base selectively immobilizes label-free oligonucleotides. Fluorescence microscopy allowed base synthesis of APTES-functionalized III-nitride films and hybridization with a fluorescently tagged oligonucleotide [110]. cTnT (cardiac troponin T)-spiked human serum was detected by a ZnO nanostructured biosensor. ZnO electrodes and serum buffer charge distribution helped achieve excellent detection. Electrochemical imbalances from cTnT binding and polarization were revealed by electrochemical impedance spectroscopy and Mott–Schottky. Binding creates ZnO’s space charge layer (SCL) and the electrolyte’s Helmholtz layer. Label-free and sensitive methods can detect cTnT at 0.1 pg/mL. Multiplexed ZnO sensors detected cTnT and cTnI. Nonspecific and cross-reactive antibodies reacted with BSA and cardiac isoforms (-cTnT and -cTnI) [111].

Further, copper-doped ZnO NPs, Cu_xZn_1−xO (where x is 0, 0.01, 0.02, 0.03, or 0.04), were utilized as a nonenzymatic electrochemical sensor to detect glucose [112]. Glucose sensing may be performed using pure ZnO and Zn_1−xCo_xO (where x is 0.05, 0.10, or 0.15) NPs. With other interfering chemicals, such as uric acid, ascorbic acid, l-dopa, and hydrogen peroxide, the sensor preferentially oxidizes glucose [113]. An electrode constructed of glassy carbon (GC) that has been doped with Fe-doped ZnO nanoparticles (Fe@ZnO NPs) exhibited good glucose sensing capability electrochemically [114]. ZnO NPs may boost acetylcholinesterase activity and so increase the efficacy of enzyme electrodes [115]. ZnO NPs were evenly distributed in a chitosan matrix and utilized to build a hybrid nanocomposite coating over indium tin oxide to quantify cholesterol in blood samples [95]. Cholesterol oxidase (ChOx) mounted on ZnO nanoporous thin films developed on gold surfaces are sensitive to cholesterol detection [116]. Due to its high surface area, large excitation binding energy, wide band, nontoxicity, chemical stability, good biocompatibility, and high electron communication property, a nano-ZnO film was fabricated on indium tin oxide ITO using the sol-gel technique. This film was used to immobilize cholesterol oxidase (ChOx) [117]. Biosensors that are capable of identifying l-lactate and other biological and chemical species could be developed by using gold nanoparticles coated on ZnO nanorods as a suitable matrix. These biosensors well recognized the l-lactate [118]. The MnAl₂O₄ ZnAl₂O₄ NM/GCE/Nafion combination is a potential sensor probe for the specific diagnosis of 3-CP on a wide scale in healthcare domains [119]. Therefore, ZnO NP-based biosensors are attractive for their promising applications.

6. Bioimaging Applications

ZnO NPs have demonstrated remarkable promise in bioimaging owing to their superior biocompatibility and inexpensive cost. The UV-blue light emitted by ZnO NPs fabricated using the gas evaporation process is intense. Therefore, it is compatible for bio-conjugation and can be used for bioimaging [120]. The ZnO surface was capped with covalently bound SiO₂ and TiO₂ layers with a 0.5 nm capping thickness, resulting in strong PL emission in the visible region. There were strong bioimaging capabilities in plant cells. Due to their greater quantum yields than the fluorescein standard material, uncapped ZnO and ZnO with a TiO₂ cap were suitable for bioimaging applications [121]. Enhanced yellow emissive Gd-doped ZnO quantum dots, had less cytotoxicity and held promise for magnetic resonance imaging (MRI) [122]. ZnO@Gd₂O₃ multimodal hybrid nanostructures might be useful as contrast agents in T2-weighted MRI [123]. Doped ZnO exhibited improved CT (computerized tomography) imaging and MRI in vitro for great tissue penetration depth with high-resolution three-dimensional visual reconstruction due to its minimal toxicity in vivo [124]. Electrochemical investigation of ZnO NPs revealed it to be a safe alternative contrast medium for use in CT scanning [125]. Cu-doped ZnO NPs have the ability to be employed in positron emission tomography (PET) imaging studies of cancer [126]. Using silica-coated Ga(III)-doped ZnO: Yb³⁺, Tm³⁺ for near-infrared optical imaging is possible due to its low in vivo toxicity and ability to induce photon avalanche processes (OI) [127]. ZnO NPs are an innovative form of a viable material for fluorescence-based imaging. In order for ZnO fluorescence to take place, the ZnO bandgap has to be activated by UV light. Since ultraviolet light can only penetrate the skin a few millimeters deep, it cannot be used for the majority of in vivo research. It was proposed that ZnO NPs should be doped with magnetic elements in order to generate unique binary probes that are capable of fluorescence; magnetism would make it possible for MRI to detect tagged tissues even when they are located deep inside the body. Recent research has resulted in the development of Fe₃O₄@ZnO core-shell nanoparticles with the goal of transferring carcinoembryonic antigen (CEA) into dendritic cells (DCs) for the treatment of cancer in mice via immunotherapy. These Fe₃O₄@ZnO core-shell nanoparticles were efficient nanocarriers for antigen distribution, and they were also able to be identified by confocal laser scanning microscopy (CLSM) in vitro and MRI in vivo [111]. The ZnO.Fe₃O₄@ZnO Chitosan (CS) NPs have important biological applications. Fe₃O₄@ZnO CS NPs have been the subject of significant research for their possible utility in drug delivery, photocatalysis, magnetic separation, and other domains.

The anti-hepatocellular carcinoma effects of transferrin receptor-functionalized and DOX-loaded Fe₃O₄@ZnO nanocomposites were investigated. After X-ray irradiation, Fe₃O₄@ZnO nanocomposites showed improved chemotherapeutic efficacy and radiosensitizer characteristics [124]. Two-photon emission from ZnO NPs can be distinguished from SHG (second harmonic generation) emission by linewidth, emission frequency relative to input light, and emission duration. Two-photon emission lasts longer than SHG emission. If the SHG signal emits light above the ZnO bandgap, weak autofluorescence may result. At 745 nm, when the pump laser’s two-photon energy doubles and exceeds ZnO’s bandgap, two-photon fluorescence can be observed alongside SHG. Two-photon fluorescence begins at 385 nm, near the ZnO band edge, and SHG at 372.5 nm. SHG was studied using zebrafish blood and a Ti:Sapphire laser. The author performed all tests at laser excitation wavelengths above 750 nm to rule out two-photon absorption. Blood from ZnO-injected zebrafish demonstrated enhanced thrombocyte uptake [125]. ZnO quantum dots (QDs) and nanocrystals are safer alternatives to other substances in this class. ZnO quantum dots have a high efficiency of UV-blue emission. As a direct consequence of this, ZnO nanocrystals represent an intriguing possibility for use in the area of bioimaging [126]. Thus, imaging modalities utilizing ZnO-based NPs are crucial for their promise in medicine and healthcare.

7. Gas Sensing Using ZnO

When it comes to biosensors and chemical sensors, ZnO is a fascinating substance because it can simultaneously detect changes in mass, field effect, and surface resistivity. This semiconducting metal oxide has the potential to be a multisensing sensor platform. ZnO NPs may be utilized as sensing systems for gas sensors because of their increased sensitivity and long-term durability [128]. ZnO NPs produced by annealing zinc carbonate hydroxide at 400 °C displayed outstanding NO₂ sensing properties. It takes 30 s to react to NO₂ and another 120 s for it to make a recovery [129]. At ambient temperatures, ZnO NPs produced from zinc hydroxide utilizing a trisodium citrate-assisted hydrothermal method showed a response to 5 ppm NO₂ gas. At 400 °C, it also demonstrated a strong response to CO, ethanol, and acetaldehyde [130]. It has been demonstrated that ZnO is a potential material for application in sensors. This semiconducting metal oxide offers application as a substrate for integrated multisensing sensors since it can simultaneously detect changes in field effect, surface resistivity, and mass. When ZnO NPs were exposed to oxygen, they reacted significantly more strongly than the films did. Additionally, selectivity tests were carried out against additional gases that can be found in the exhaust of vehicles or chimneys [128]. ZnO nanostructures having a porous network structure with oxygen vacancies, synthesized by electrospinning technique, resulted in a greater and faster sensitivity to acetone vapor, as well as superior selectivity. The sensitivity and selectivity of polyvinylpyrrolidone (PVP)-modified ZnO nanoparticle-based gas sensors for trimethylamine (TMA) are quite remarkable. The sensor has a reaction time of 10 s and a recovery time of 150 s [131]. Nanosized ZnO powder created using the sol-spray combustion process has highly sensitive gas sensing characteristics to ethanol, and it responds and recovers in 10 s and 40 s, respectively, for different concentrations [132]. Ag-doped ZnO NPs and MoS₂ nanosheets coated with ZnO NPs also display sensitivity to C₂H₅OH [133]. At the optimal operation temperature of 150 °C, In₂O₃ hollow microtubes decorated with ZnO NPs produced from metal-organic frameworks (MOFs) demonstrated a significant gain in ozone gas sensing ability [134]. The detection features of doped ZnO NPs for n-butanol gas demonstrate a high-performance gas detecting capability at an operating temperature of 300 °C, including strong gas response, excellent response/recovery time, selectivity, stability, and repeatability [135]. RF magnetron sputtering has a low cost, can generate uniform films across a large surface area, and is easy to manage. The sputtering method can adjust the base pressure, substrate temperature, RF power, deposition duration, and target-to-substrate distance. These parameters optimize gas sensing sensitivity. Low sputtering power (100 W) and high substrate temperatures (300–400 °C) yield accurate ZnO growth outcomes. Recent studies have focused on determining the room-temperature gas reaction. So far, this operation cannot be performed at room temperature. This study investigated deposited films’ gas sensing characteristics at 205 °C, managed parameters to speed up reaction and recovery. Researchers increased annealing temperature, adjusted deposition time and substrate, added doping materials, and executed synthesis to improve gas sensing characteristics [136]. The bandgaps and work functions of ZnO and Fe₂O₃ are quite different from one another, despite the fact that both of these materials are n-type semiconductors. At the point where the Fe₂O₃ core and the ZnO shell meet, a heterojunction can develop, which results in the formation of potential barriers. Since ZnO has a larger work function, the electrons in its Fermi level flow into the lower Fermi level of Fe₂O₃, and this process continues until the Fermi levels of both materials are equal. This process is repeated multiple times until the work functions of ZnO and Fe₂O₃ are equivalent to one another. After the requirement of equilibrium has been satisfied, an electron depletion layer begins to form at the interface of the heterojunction. The transmission of electrons is extremely important for the surface [137]. ZnO NPs demonstrate the potential for applications in gas sensors. Furthermore, the ZnO NPs-based semiconducting metal oxide offers a platform for integrated multisensing sensors.

8. Medicinal Applications of ZnO NPs

Lipid-coated ZnO NPs have potential and can be a novel photosensitizer for cancer detection. The photogeneration of short-chain carbon-centered free radicals is induced by lipid bilayer coating, indicating that surface chemistry is important for different kinds of photogenerated free radicals. ZnO semiconductors convert UV energy into visible light. This suggests bioimaging or a LED reporter. When ZnO creates ROS within cells, it can destroy tumor cells. Too many ROS disturb the cell cycle and increase the risk of apoptosis or autophagy. ROS can damage membranes, proteins, and DNA through lipid peroxidation [138]. ZnO QDs nanoprobes in vitro may be utilized as a fluoroprobe alternative for cancer cell targeting. An innovative nanoprobe based on ZnO for in vitro imaging as a sensitive bioassay is promising. ZnO is a versatile material that may be used in a wide variety of medicinal contexts thanks to the fact that it is nontoxic and biocompatible. ZnO is an environmentally friendly compound that has potential use in cancer diagnostics and live cell imaging. In this particular research endeavor, transferrin served as the tumor-targeting ligand, and a ZnO nanocrystal bioconjugate that was coated in silane and had an amino functional group was used for the ligand conjugation process. The capability of the nanoprobes to specifically target breast cancer cells was tested [139]. Due to their biocompatibility, excellent selectivity, increased cytotoxicity, and ease of production, ZnO NPs may be utilized as an anticancer therapy [140]. ZnO nanocomposites are likewise applied in theranostics and drug delivery systems (Figure 3). Various ZnO NPs, for example, ZnO-quercetin [141], Fe₃O₄@ZnO:Er³⁺,Yb³⁺@(β-CD) nanoparticles [142], ZnO@PNIPAM hybrid NPs [143], ZnO-gated porMOF-AS1411 [144], biopolymer K-carrageenan wrapped ZnO NPs [145], ZnO quantum dots-conjugated Au nanoparticle [146], ZnO-GO (graphene oxide) nanocomposites [147], chitosan/ZnO bionanocomposite [148], chitosan-encapsulated ZnO quantum dots [149], Fe₃O₄@ZnO@mSiO₂ nanocarrier [150], ZnO-gated hollow mesoporous silica [151], ZnO Quantum Dots-Doxorubicin NPs [152,153], ZnO-DOX@ZIF-8 Core-Shell NPs [154] etc. are utilized for effective drug delivery for different ailments. For example, β-CD-modified Fe₃O₄@ ZnO: Er³⁺ and Yb³⁺ nanocarriers are effective for antitumor drug delivery and microwave-triggered drug release; a ZnO-gated porphyrinic metal organic framework-based drug delivery system is efficient for targeted bimodal cancer therapy, and biopolymer K-carrageenan-wrapped ZnO NPs are excellent drug delivery vehicles for anti-MRSA (Methicillin-resistant Staphylococcus aureus) therapy [142,144,145]. Furthermore, ZnO quantum dots have shown promise as a multifunctional anticancer agent, and DOX-FA-ZnO NPs have been implicated in the effective treatment of breast cancer (Figure 3) [155,156,157]. As a result, ZnO-based NPs are efficient as anticancer and antibacterial drugs, as well as therapeutics for other diseases.

9. ZnO in Food Industry

ZnO NPs have many prospective applications in food-based industries. ZnO NPs are potentially used in active packaging. Prior to the broad industrial use of nanoparticles in food packaging, it was essential to perform research on the regulatory concerns that must be addressed in light of the efficacy of NMs in protecting the chemical, physical, sensory, and microbiological quality of food. Modern packaging solutions that are lighter, more durable, and more practical may also substantially reduce shipping costs and environmental impacts [158]. ZnO nanoparticles included in polystyrene film or other suitable matrixes are suitable for several food packages and related uses [159]. ZnO NPs coated on PVC (polyvinyl chloride) films are examples of metal or metal oxide nanoparticles integrated in polymer nanocomposites, used as disinfectants, for food packaging, etc. [160]. Antimicrobial properties of a variety of metal and metal oxide NPs have been promising. Nanocomposite films composed of polystyrene, polyvinyl chloride, and polyvinylpyrrolidone have also been shown to bind to ZnO NPs and inactivate food pathogens [159]. Lactate in foods could be detected using an Au/NanoZnO/lactate dehydrogenase (LDH) bioelectrode biosensor [161]. The electrochemical biosensor based on ZnO nanowires has a lot of promise for detecting L-lactic acid in real samples [162]. ZnO NP has investigated several bacteria, the majority of which were foodborne pathogens. ZnO NP has been shown to be more effective than powder as an antibacterial agent in the food sector. The findings suggested that it might be employed as a food preservation agent. Salmonella species are widely suspected to be the primary pathogens responsible for many different types of food poisoning. The presence of Salmonella species has been confirmed, causing widespread alarm in the food service industry. Staphylococcus bacteria are another major cause of food poisoning. In immunocompromised people, it is most often connected with nosocomial infections but can also spread from person to person in the community. Within the genus Staphylococcus, the species S. aureus is regarded as the most significant. They live on the skins of people and other animals, but they rarely spread to other organs and cause diseases there. The potential of ZnO NP as an antibacterial agent, the majority of which are foodborne pathogens, has been perspective. Further, micrographs of S. typhimurium and S. aureus cells treated with ZnO NP were used to examine the effect at the cellular level [163]. Chitosan with ZnO NP-infused gallic acid films (CS-ZnO@gal) showed extraordinary antibacterial capability and great antioxidant activity when compared to pure chitosan, which may be explored for active food packaging applications [164]. ZnO NPs loaded on starch-coated polyethylene film were efficiently examined for their biocidal activity against model bacteria using killing kinetics and zone inhibition of bacterial growth. Nano ZnO could disinfect food dyes and water-grown bacterial cultures. Thus, nano ZnO may be utilized to remove rapid green dye and bacterial toxins, such as E. coli and B. subtilis. Histology and cytology laboratories utilize quick green dye despite accusations that it may promote cancer, mutagenesis, and irritation. Furthermore, fast green dye is immunotoxic. Multiple efforts have been undertaken to remove this dangerous watercolor.

B. subtilis can cause allergic reactions, is prevalent everywhere, is conveyed by water, and can spread quickly. E. coli causes urethritis, gastroenteritis, and meningitis in infants [165]. As a consequence, the nano ZnO material has the opportunities to be utilized as a food packaging material to avoid food contamination due to micro-organisms. ZnO nanorods produced using the hydrothermal method also showed antibacterial activity. ZnO’s performance against E. coli and B. atrophaeus was effective. Both species were found to have damaged cell membranes. Polyethylene-based films feature exceptional mechanical strength, hydrophobicity, and moisture barrier characteristics, which are some of the most crucial features of a high-quality packaging film and may be further improved by incorporating ZnO NPs [166]. Nanofiltration is a good technology for clarifying and concentrating raw juices, in which the elements are separated depending on their molecular size. ZnO NPs are often utilized in the construction of nanofibrous membranes or as filtration aids in food processing. ZnO NPs have been observed to significantly enhance the quality of pear juice by reducing cloudiness [167]. Thus, ZnO-based nano- and polymer composites have shown promising applications in the food industry to improve food quality.

10. Applications of ZnO in Environmental Industry

Outstanding photocatalytic activity was demonstrated by ZnO NPs generated from the microalgae chlorella in the destruction of the pollutant known as dibenzothiophene (DBT) [168]. The degradation of the organic pollutants, methyl orange and methyl blue, by ZnFe₂O₄/ZnO NPs has shown outstanding performance [169]. Metal oxide-based photocatalysts created from ZnO@In₂O₃ core-shell hollow tubes have the potential to speed up the degradation of organic dyes as well as antibiotics in wastewater [170]. ZnO NPs are among the semiconductor nanostructures that are investigated most frequently for environmental uses. This is because they have high photocatalytic activity. They have the potential to break down antimicrobial agents as well as organic contaminants found in water and air [171]. The ZnO nanocomposites have a high rate of textile effluent degradation. Nanocomposites of ZnO/CdO [172], ZnO/Ag [173], CuO-ZnO [174], CuAu–ZnO–graphene [175], and ZnO/zinc tin oxide nanocomposites (ZnO/ZTO) have demonstrated 50% photocatalytic degradation efficiency and 77% COD removal from textile wastewater when exposed to sunlight [176]. Photodegradation of rhodamine B (Figure 4), methylene blue, and neutral red may be utilized to clean wastewater using artemia eggshell-ZnO nanocomposites [177]. Hybrid Au/ZnO nanostructures may be utilized in water treatment for heterogeneous photocatalytic applications [178]. ZnO/Fe₃O₄-sepiolite nanostructured materials work well as a photocatalyst in water for pollutant removal [179]. The application of nano-ZnO as a remedy for water tainted with fast green dye has been shown to be effective [165]. ZnO NPs were able to inhibit the growth of waterborne pathogens, such as E. coli, S. epidermidis, Proteus, and K. pneumoniae, in municipal wastewater, demonstrating their enhanced antibacterial activity [180]. Inorganic UV blockers are semiconductor oxides, such as ZnO, TiO₂, Al₂O₃, and SiO₂. It has been established that nanosized titanium dioxide and ZnO are superior to larger sizes in both their capacity to absorb and scatter ultraviolet light as well as their efficacy in blocking UV radiation [181]. Coating a fabric with a nanoparticulate layer or covering the surface of the fabric with three-dimensional surface structures are important methods for making a fabric water-resistant. It has been demonstrated that ZnO NPs are capable of achieving antistatic characteristics [182]. The employment of NMs, particularly ZnO, displays higher photocatalytic activity in comparison to their bulk counterparts [183]. Therefore, ZnO-based NPs are suitable candidates for their significant applications in environmental industries.

11. ZnO in Cosmetics and Toiletries Industry

Using excisions of human skin placed in diffusion cells, it was possible to explore the skin absorption of ZnO NPs in a transparent sunscreen formulation. Only 0.03 percent of the zinc given to the epidermis was able to permeate into the receptor fluid under the skin. According to electron microscopy results, no ZnO NPs were discovered in the stratum corneum or in the viable epidermis of the mice. In vivo testing with human volunteers revealed that there was no substantial penetration of ZnO NPs into the skin. Skin surface ZnO NPs were found after application to the skin for 4 h and after removal from the skin. In skin folds and at the apex of hair follicle shafts, ZnO seemed to be restricted to a small area and did not spread to surrounding cells. At 24 h, no NPs were seen on the skin’s surface, which was most likely due to washing the object. There was no indication of ZnO entering through the stratum corneum into the viable epidermis [184]. ZnO NPs are used in sunscreen. Nanosized ZnO cleans polluted regions due to their photocatalytic activity [185]. ZnO NPs/Zn²⁺ are employed in a variety of eye makeup/eye shadow formulations [186]. ZnO NPs provide tremendous defense against sunburn caused by UV radiation and are proved to be safe [187]. There are many ZnO NM-based cosmetic products available in market, such as ‘Proctor and gamble’ manufacturers “Olay complete UV protective moisture lotion”, ‘Boots’ manufacturers “Soltan facial sun defense cream”, ‘Image skincare’ manufacturers “Solar defense organic moisturizer”, ‘Dermatone’ manufactures “Moisturizing dermatone lips ‘n’ face protection crème”, ‘ColoreScience’ manufacturers “Sunforgettable corrector colores SPF 20, sunforgettable SPF 30 brush range, wild to mild skin bronzer” [185]. Studies showed that four cosmetic products (commercial sunscreens) out of six contained ZnO NPs [188]. Senna alata methanol leaf extract ZnO NPs (SaZnO NPs) showed promising antibacterial properties in cold cream [189]. Adhatoda vasica leaf extract created with ZnO NPs showed promise in cold cream formulation. ZnO consumption is safe for cosmetic production. This chemical inhibits the bacteria’s thiol peroxidases, glutathione reductases, and dehydrogenases, which kills them. ZnO destroys fungal hyphae and inhibits conidiophore growth. ZnO NPs are a cosmetic-friendly antioxidant. Its ability to permeate the stratum corneum protects against free radicals and other skin-damaging ROS. Particle size, degree of modification, and polydispersity of the mixture can further affect efficiency in their use in the cosmetic industry. Encapsulating vitamins, unsaturated fatty acids, and antioxidants in NPs makes them more stable and effective topically. ZnO NPs could improve healthcare and beauty. Their flexibility and small size make them promising cosmetic and dermatological tools. NPs will replace traditional preservatives in many cutting-edge cosmetics. The ZnO NPs are long-lasting, prevent disease-causing micro-organisms, and protect skin from UV rays [190]. In addition, ZnO NPs are also used in mouthwash, toothpaste, and root canal flings [191]. Studies are also investigating their toxicity in humans. Another study found that mouthwash comprising Ag/ZnO, 10 milligrams of NPs, and 100 mL of base material has the best antibacterial activity against S. mutans and is safe for cells; therefore, it may be used as an alternate mouthwash to chlorhexidine 0.2 percent in plaque control following in vivo testing. Over-the-counter mouthwashes can have cationic, anionic, or nonionic active components, all of which are known to affect bacterial membrane function. Chlorhexidine, Cu²⁺, Zn²⁺, and Sn²⁺ are some of the most commonly used cationic components. It is well established that metal ions can affect bacterial membrane function and enzyme activity. It has been shown that zinc chloride mouthwashes are antibacterially active against Streptococcus bacteria, and that ZnO nanoparticles are active against E. coli. Therefore, it is clear that modified zinc salts and their derivatives are effective at preventing plaque. ZnO NPs are commonly found in sunscreen creams and act as both a nutritional supplement and a protective ingredient against UVA and UVB rays. There is conclusive evidence that silver and zinc NPs kill S. mutans bacteria while being almost entirely nontoxic to humans [192]. Therefore, ZnO NPs could easily be employed in cosmetic fabrication, including soap, face wash, hand wash, and many other cosmetic and toiletry industries. However, industries must evaluate the toxicity profile of ZnO-based products implicated in various cosmetic products for daily life use.

12. Applications of ZnO in Oil and Gas Industry

ZnO NPs with a high surface tension are able to lower the interfacial tension between water and oil in the context of increased oil recovery. Nano-enhanced oil recovery has attracted a lot of attention since modern NPs have substantial interfacial properties that can be used to adjust capillary forces in favor of easily accessing oil. Since ZnO NPs can lower interfacial tension, they have drawn attention as a possible tool for improving oil recovery. Nanostructured ZnO has tremendous adsorption capacity, a low growth temperature, excellent chemical stability, and high catalytic effectiveness, making it a promising material for increased oil recovery (EOR). The increased surface area makes it suitable for use as a catalytic agent. More specifically, this phenomenon would result in increased catalytic activity. The material’s structure and synthesis process could be modified to alter its chemical and physical properties [193]. ZnO NPs with sizes ranging from 14 to 25 nm are effective scavengers for eliminating H₂S and soluble sulfides from drilling fluids. Due to the higher performance of NPs in the elimination of H₂S, the usage of bulk ZnO will be reduced, resulting in less pollution in the environment and less use of nature’s resources [194]. Microwave-synthesized ZnO NP claims to be a good candidate to be applied as an enhanced oil recovery agent [195]. To improve the repeatability of the H₂S gas sensor, thermal evaporation and spray pyrolysis techniques were used to produce ZnO NPs coated with chromium (III) oxide (Cr₂O₃). The ZnO nanoparticle improved the responsiveness at high concentrations without altering the operating temperature, according to gas sensing studies [196]. In-doped ZnO nanoparticles showed better gas sensitivity towards volatile organic compounds (VOCs), acetone, benzene, ethyl alcohol, xylene, and toluene than undoped ZnO NPs [197]. As sensing materials, reduced graphene oxide-ZnO NPs (ZnO-rGO) hybrids were used to create an NO₂ gas sensor. Most significantly, the sensor has greater sensitivity, faster reaction time, and faster recovery time than a sensor based on rGO, suggesting that adding ZnO NPs to the rGO matrix improved the sensing performance for NO₂ sensing at ambient temperatures [198]. Dopped ZnO NMs showed better sensitivity towards VOCs, such as acetone, benzene, ethyl alcohol, xylene, and toluene, than undoped ZnO NPs [199]. Oil and gas firms must understand their water consumption, the demands and constraints of their operational sites, and the possibilities of investing in nanomaterial research to preserve and boost future profitability. While using steam-assisted gravity drainage (SAG), ZnO NPs may reduce viscosity and enhance heavy oil recovery while lowering the ratio of water to oil (WCUT %). The catalytic chemical reaction of NPs cracking carbon-sulfur bonds is assumed to be the root cause of lower viscosity. The lowest nanoparticle concentration lowers viscosity the most (0.2–0.5 percent wt). The viscosity reduction characteristics of ZnO NPs have a lower residual oil saturation (SOR) and WCUT percent [200]. The gas hydrogen sulfide is very corrosive, poisonous, and hazardous. During the drilling of gas and oil wells, it may seep into drilling fluid from rock formations. To limit pollution, preserve drilling employees’ health, and avoid pipeline and equipment corrosion, H₂S should be eliminated from the mud. The following chemical process was used to extract H₂S from water-based drilling fluid using 14–25 nm ZnO with 44–56 m² per gram of specific surface area.

ZnO + H₂S → ZnS + H₂O

In approximately 15 min, synthesized ZnO NPs can entirely remove H₂S from water-based drilling mud, but bulk ZnO may take up to 90 min to eliminate 2.5 percent H₂S [194]. ZnO NPs have a lot of promise for reducing viscosity since they can change wettability to water and wet through thick oil. However, after the addition of ZnO NPs, the surface tension decreased, reducing capillary forces and raising the relative permeability of the oil, which ultimately overpowered the gravitational forces to remove the oil. The initial amount of oil in place, or OOIP, was zero because the carbonate core was oily and wet. The results for sandstone core recovery ranged from 17.3 to 2 and 15% OOIP without NPs then to 20.68, 17.57, and 36.2% OOIP with NPs, respectively [201]. Bacteria that degrade petroleum are perhaps observed in Kuwait’s and Bahrain’s oil fields. Isolated bacteria are a potential bioaugmentation of petroleum-contaminated Arabian Gulf soils as well as biosurfactant manufacturing because of their numerous traits (crude oil breakdown and biosurfactant production). The biodegradation process may be slowed down by ZnO NPs. NPs have a significant influence on environmental micro-organisms’ ability to develop and degrade. Some bacteria may be able to degrade crude oil without producing biosurfactants [202]. Thus, ZnO NPs are highly promising due to their crucial applications in the oil and gas industries, including viscosity reducers, inhibition of biodegradation, removal of the poisonous gas H₂S, and as gas sensors.

13. ZnO in Electronics Industry

Using ZnO NPs for light control, transparent and flexible C₂H₅OH gas sensors can be created for wearable devices. The ability to detect dirty, toxic, and combustible gases is a key characteristic of today’s wearable electronic devices. Semiconducting metal oxide nanostructures, such as ZnO nanorods, are attractive possibilities for highly sensitive and reliable gas sensors due to their high surface-to-volume ratio. Wearable electronics require gas sensors that are flexible, transparent, and operable at room temperature, however, conventional gas sensors do not meet these criteria. As a result, it is both an exciting and hard task to create a new generation of gas sensors that are able to run at room temperature while remaining flexible and transparent [203,204]. ZnO NPs can be utilized in dye-sensitized solar cells [205]. Organic compounds incorporating ZnO NPs have a significant potential for supercapacitors [206]. High-performance supercapacitor electrodes created from graphene-ZnO (G-ZnO) nanocomposite hybrid materials are promising [207]. In comparison with the pure graphene electrode, the graphene/ZnO nanocomposite electrodes showed enhanced electrochemical performance as a capacitor [208]. PVA/ZnO nanocomposite exhibited a modification in the dielectric constant, thus, its capacitance is widely used for device applications [209]. ZnO-loaded porous carbons (CMK-3 or CMK-8) are produced as a new anode component for lithium-ion batteries (LIBs) and have superior electrochemical characteristics than pure ZnO particles [210]. ZnO@NC@CNT showed high potentiality as an anode in Li-ion batteries for its high specific capacity, excellent rate capability, and cycling stability [211]. The capacity, rate performance, and cycling behavior of ZnO nanorods/reduced graphene oxide (ZnO/RGO) composite as an anode for sodium-ion batteries (SIBs) are significantly greater than those of pure ZnO substances [212]. Nanoparticle-based thin film transistors (TFT) display nonvolatile memory features, such as a conductance ratio of more than 10⁵, making them ideal for applications, such as data storage. Due to the voltage-controlled trapping and release of positive charges at the rough semiconductor/dielectric interface, there is a memory effect. More than 6 months of shelf life and useful endurance features are provided by the memory transistors operating in ambient air [213]. ZnO ink has the potential to be used in flexible printed electronics, which is a growing sector. Even at a relatively low processing temperature of 250 °C, the ZnO TFTs (thin film transistors) created from the suggested ZnO mixture ink showed considerably increased field effect mobility of 1.75 cm² V⁻¹ s⁻¹ and an on/off ratio of 5.89 × 10⁸. The impact of ZnO NPs’ incorporation into the thin film nanostructure on the structural, chemical, and electrical features of ZnO TFTs was investigated using a variety of structural studies [214]. ZnO NPs may also be applied to memory devices (Figure 5). Memory devices developed employing ZnO NPs encapsulated in a polystyrene layer exhibited promise for applications in write-once-read-many-times (WORM) memories [215,216]. ITO/PVP:ZnO/Al device constructed using ZnO has a higher on/off current ratio and better memory performance than the ITO/PVP/Al device (Figure 5) [217]. Figure 5 displays a comparison between the energy level diagrams of the PVP-based device and that of the ITO/PVP:ZnO/Al device and the mechanism of enhanced memory performance in the ITO/PVP:ZnO/Al device. Additionally, Zn_1−xCu_xO particles exhibit great potential to be employed in microwave semiconductor devices. Due to its remarkable ferroelectric, photoelectric, piezoelectric, catalytic, and dielectric qualities, ZnO is a semiconductor material that is both environmentally safe and highly desirable for usage in a wide variety of microelectronics applications. Numerous studies have focused on ZnO NPs because of their potential uses in the electronic field, such as luminescence, photodetection, gas sensors, and metal oxide semiconductors (MOS). Scientists in the field of semiconductor-based microelectronics have been interested in ZnO NPs due to their unique dielectric properties. Efforts are currently being made to improve these characteristics so that the materials can be used in microelectronics [218]. Accordingly, ZnO NPs are promising for the development of next-generation smart wearables and memory devices.

14. Conclusions

ZnO NMs have vast applications in various sectors. Industry uses ZnO NMs to advance the products that are delivered to customers. These ZnO NMs have great antibacterial function, in degradation of organic pollutants and air pollutants, in gas sensing, and in solar cells. Some of these NMs can purify both wastewater and groundwater. ZnO NPs demonstrate free radical scavenging activity by efficiently donating electrons to highly reactive free radicals, indicating their potential for application as antioxidants. Due to the antioxidant and antibacterial properties of ZnO NPs, they are useful for tissue regeneration, and as sterilizers and disinfectants. In the food industry, ZnO NPs are utilized profoundly in packaging to avoid food contamination due to micro-organisms, inhibiting foodborne pathogens, and may be used as food preservative agents. These NMs are also used in biosensors, for glucose metabolism and homeostasis, accelerating insulin secretion, etc. ZnO NPs are effective in biological fluorescent imaging and are efficient and safe contrast agents in CT, PET, and MRI. ZnO-based NPs are prospective anticancer agents, novel antibacterial drugs, drug delivery platforms, and theranostic agents. ZnO NPs are promising for their applications in the cosmetic industry as skin care and UV-protectors, the oil and gas industries as purifiers and antimicrobial agents, and the electronic industry as gas sensors for wearable devices and memory devices. Finally, utilization of ZnO NPs has shown great prospect and promise in a wide range of industrial applications.

15. Future Perspectives

Metal oxide NPs, such as ZnO, have gained popularity as a way to treat antibiotic-resistant bacteria. ZnO NPs’ many features are helpful for treating fatal infections [219,220]. It has been suggested that the green method, which involves obtaining NPs from a plant source rather than a chemical or physical one, is more cost-effective and less harmful to the environment. Significant in vitro antioxidant and free radical scavenging capabilities were also seen in ZnO NPs produced through the green technique employing plant extracts. Furthermore, synthetic routes may be expanded to include ZnO NPs sourced from a wide range of medicinal plants for biomedical applications [221,222,223,224]. As a result, more innovative green synthetic approaches may be helpful for fabricating highly efficient ZnO NPs for different industrial manufacturing applications.

The capacity of ZnO NPs to suppress microbial growth relies on their ability to form reactive oxygen species, which can damage biomolecules, release cations, interact with membranes, and deplete ATP. To enhance the effectiveness of novel ZnO-based NPs in inhibiting microbial growth with a suitable toxicity profile and to prevent microbial resistance, further studies are required [225,226]. ZnO NPs are involved in the production and secretion of insulin, as well as glucose metabolism and homeostasis; further investigation may be helpful in utilizing ZnO as a prospective insulin mimetic agent and enhanced mediator of glucose homeostasis. ZnO NPs are promising as anticancer agents, drug delivery systems, and theranostic agents. Thus, the development of novel ZnO-based NPs will further accelerate their use in medicine, tumor targeting, tumor imaging, and diagnosis.

Research on the use of nanostructures as agrochemicals (fertilizers or insecticides) to foster plant growth and protect crops is ongoing using ZnO NPs, with good prospects for ZnO NPs in agroeconomics. There has been a growing focus in recent funding and planned research on creating environmentally friendly ZnO NMs for effective solutions. Research on the use of ZnO in farming is quickly progressing. To ensure the safe use of nanoscale agrochemicals, regulatory frameworks must be established, and for this to happen, knowledge of how nanofertilizers function is crucial [227,228].

Applications of ZnO-based NPs are highly effective in electronic industries. Further investigations may reveal more state-of-the-art electronic devices and high-performance memory devices. Thus, the implications of ZnO NPs’ use in different industrial settings are highly prospective and innovative as well.