Abstract
The semi-conducting material zinc is one of the essential trace elements for humans, is a co-factor of more than 300 enzymes, and plays an important role in maintaining vital cellular functions. Deficiency of zinc may lead to cancer initiation; however, a high concentration also has toxic effects that might be life threatening. The toxicity can be addressed by the disequilibrium of zinc-mediated proteins and oxidative stress that produce nascent oxygen, hydroxyl radicals, and other reactive oxygen species. Zinc-based nanoparticles (NPs) are among the most important and multifunctional compounds. Zinc oxide (ZnO) NPs exhibit attractive antimicrobial and photocatalytic properties due to the smaller particle size and increased particle surface reactivity. Further, these are more biocompatible compared to other metallic NPs, easily synthesizable, and have high selectivity, enhanced cytotoxicity, and are a promising anticancer agent. However, some of the pertinent concerns regarding nano-zinc still needs to be clarified. Current research also demonstrates their usage in wastewater treatment, textile, medicine, etc. This review covers the importance of zinc for living systems and its NPs, with more emphasis on ZnO NPs. A comprehensive overview of ZnO NPs, their synthesis, characterization techniques, crystal structure, properties, and brief industrial applications are presented.
1 Introduction
Over the last few decades, nanotechnology has witnessed an incredible development in the fastest-growing domain of science and technology because metal oxide nanoparticles (NPs) are progressively being used in many industrial applications. Among the different metals, zinc attracts more attention because of its strong reducing potential, moderate reactivity, and having five stable isotopes. Among the different zinc-based nanostructures like sulfide, ferrite, phosphide, selenide, and telluride, zinc oxide (ZnO) is most attractive due to its vast applicability, being eco-friendly, and its diverse physiochemical properties. A general description of zinc, its importance and NPs, are described in Figure 1. ZnO NPs have been reported as being of different shapes, sizes, and structures (Figure 2). They have also been reported in one-dimensional (1D), 2D, and 3D structures. The 1D structures constitute the major group, containing nanorods, tubes, needles, wires, ribbons, belts, combs, helixes, springs, and rings. The 2D structures of ZnO are found as nanosheets/nanoplates and nanopellets, whereas flower, snowflakes, dandelion, and coniferous urchin-like, etc., are examples of 3D structures [1].
General description of zinc metal and zinc NPs concerning synthesis, applications, and toxicity.
![Figure 2: ZnO electron microscopic nanostructures: (A) needles, rods, and wires; (B) helixes and springs; (C) nanopellets/nanocapsules; (D) flower, snowflakes, and dandelion; (E) peanut-like; (F) interwoven particle hierarchy; (g) raspberry, nanosheet/nanoplate; (H) circular/round or sphere shaped [1], [2], [3].](/document/doi/10.1515/ntrev-2018-0067/asset/graphic/j_ntrev-2018-0067_fig_002.jpg)
ZnO electron microscopic nanostructures: (A) needles, rods, and wires; (B) helixes and springs; (C) nanopellets/nanocapsules; (D) flower, snowflakes, and dandelion; (E) peanut-like; (F) interwoven particle hierarchy; (g) raspberry, nanosheet/nanoplate; (H) circular/round or sphere shaped [1], [2], [3].
ZnO is a bio-safe material that possesses photocatalysis and photo-oxidizing influences on chemical and biological species, respectively [2], [3], [4]. Bulk ZnO, compared to ZnO NPs, is a bio-safe material. Indeed, various studies have already revealed the potential toxicity of ZnO NPs based on their size, shape, and concentration, etc. [5], [6]. Although ZnO has been permitted for cosmetic uses by the U.S. FDA (Food and Drug Administration), the detailed toxicological profile and the mechanism of cytotoxicity for ZnO nanomaterials is, thus far, not fully understood [7]. Besides, ZnO NPs are also reported to have a good biocompatibility with human cells, and the ZnO has been documented as a safe material by the FDA [8], [9], although the intrinsic cytotoxicity of ZnO NPs against certain human cell lines has raised some apprehension concerning potential health hazards [10]. On the other hand, their inherent cytotoxicity indicates a necessary quality against pathogenic cells if the properties are precisely tailored [7], [11]. Certainly, recent studies have revealed that ZnO NPs have cytotoxic effects toward cancerous cells, while at the same concentration, ZnO NPs have negligible effects on normal cells, leading to speculation that they can be used in cancer treatment. However, for more widespread applicability, ZnO NPs are currently being used in modified forms, for example, through appropriate surface coatings with complementary polymeric materials [12], [13] or green synthesis approaches (capping with phytochemicals) and bio-inspired manufacturing [14], [15] that could boost their biocompatibility.
Biodegradability and low toxicity are the most important features of these nanomaterials. Zinc (Zn) is a crucial trace element for mature individuals (Zn2+ ~10 mg/day is allowed), and it is involved in numerous parameters of metabolism. The surface of ZnO is chemically rich with -OH groups that can easily be modified by several surface-adorning molecules. ZnO NPs have various distinctive properties such as semiconducting, deodorizing, photocatalytic, piezoelectric, pyroelectric, and required biocompatibility [16]. Therefore, they have gained many applications in industry, for instance as transducers and sensors, in ceramic processing, and in medical care [10], [17]. They are extensively used in several products, such as wall paints, toothpaste, sunscreens, beauty agents, textiles, and in building materials. Because of their distinctive properties, i.e. reduced size and high surface area ratio, the biological safety of NPs has received global attention from the scientific world [10]. ZnO NPs can easily enter water when they are immersed in water through bathing, washing, and swimming as they are a common constituent of sunscreens [16]. The consideration and investigation of nano-specific toxicity are significant for the safe design and environmental risk assessment of NPs, although ZnO NPs have been reported to lack excess toxicity for various organisms [18], [19], [20].
The objective of this article is to review the semiconducting material zinc and its chemical compounds and nanostructures focusing on ZnO as biologics. The potential applications of ZnO in industry, agriculture, the environment, and toxicological threats as related to environmental release and possible effects on plants, humans, and animals are reviewed.
1.1 Zinc
Zinc, also called as spelter, is a trace mineral. It is a diamagnetic fairly active transition metal with an oxidation state of +2 and having an atomic number 30, and is bluish white in color. It is hard and brittle and a good conductor of electricity. It has low melting and boiling points and is dissolvable in both acids and alkalis. Zinc is ecologically pervasive and present in the earth’s crust at about 70 mg/1000 mg [21]. Likewise, according to proportion, it is found abundantly (0.02%) in the earth’s crust carrying an elemental ranking of 24 in the periodic table. It has five stable isotopes and never occurs as a free element on earth [22]. The zinc metal remained unknown for a long time because it boils or vaporizes easily from its composites on heating. In nature, some of the prominent ores of zinc are (1) zincite or zinc oxide (ZnO); (2) sphalerite or zinc blende or zinc sulfide (ZnS); (3) smithsonite or zinc spar or zinc carbonate (ZnCO3); (4) willemite or zinc silicate (ZnSiO3), and (5) franklinite [(Zn, Mn, Fe)O(Fe,Mn2)O3] [23].
The zinc metal is now prominently used in galvanizing other metals by laying down a thin layer of zinc on the surface of a second metal because it does not corrode as easily as iron or other metals. Furthermore, the zinc metal sublimates easily during the formation process of zinc from its ore.
Zinc ions are absorbed deeply into soils at pH 5 or more and are likely to have slight mobility [24]. In natural waters, several forms of zinc can be found like metal-organic complexes, hydrated ions, or metal-inorganic complexes. Hydrated Zn+ may be hydrolyzed to form ZnO or Zn(OH)2. However, ZnS may be formed in anaerobic environments. In aquatic organisms the bio-concentration factor (BCF) values of zinc is 1000 and 2000 for freshwater fish and marine fish, respectively [25].
1.2 Zinc in the human body
Zinc is organically known as a helper molecule that assists about 300 enzymes, which are involved in various bodily functions. After iron, zinc is the most vital and richest trace element in the human body and has been estimated to be 30 mm (2–4 g) of the total body zinc content. The maximum quantity of zinc occurs in parts of the eye, liver, kidney, bones, muscles, prostate, and brain [26]. Weak vision like cloudy cataracts and poor night vision have been connected to zinc deficiency. A low zinc concentration in puts the human body at risk for alopecia (hair loss from eyelashes and eyebrows), greater vulnerability to infection, and mental lethargy. Almost 15 mg/day (zinc) of trace quantities is required in the human diet to fulfill the demands of all body fluids and tissues. Zinc also plays an essential role in the immune system along with the maintenance and integrity of cellular components (molecules and or membrane stability, etc.) [22]. A number of enzymes are regulated by zinc that are involved in homeostasis and other up-keeping bodily functions. It also participates in controlling polynucleotide transcription by playing a crucial role in genetic expression. The volume-dependent absorption of zinc occurs throughout the small intestine. However, the loss of zinc from the body occurs through the skin, kidney, and intestines [27]. Zinc deficiency in humans causes cell impairment and malignancy development that eventually may lead to cancer. Thus, in the prevention and treatment of various cancers, the zinc-accelerated cancer chemoprevention is effective.
Zinc deficiency usually occurs due to insufficient zinc absorption or intake, greater losses of body zinc, or more requirements for zinc [28]. Zinc scarcity is manifested by the loss of appetite, growth retardation, and impaired immune function [29], along with hair loss, malabsorption, hypogonadism in males, sickle cell disease, delayed sexual maturation, acrodermatitis enteropathica, impotence, malignancy, chronic renal disease, diarrhea, diabetes, chronic liver disease, skin/eye lesions, and other chronic illnesses particularly in severe cases. Taste abnormalities, weight loss, altered cognition, impaired appetite, and delayed healing of wounds can also take place [30], [31].
Numerous zinc passages sustain an essential stability as regards life and cell death, regulating the intracellular movements of zinc and unrestricted quantity of the metal. Although a smaller concentration of zinc could result in the origination and development of cancer, a greater concentration of zinc also has a damaging effect on health. A greater zinc quantity surpasses the capability of the zinc homeostasis system causing breakdown of zinc transferring assemblage of the plasma membrane and subsequently an enhanced intracellular zinc concentration that eventually triggers apoptosis leading to cell death [32].
1.3 Biological role of Zn
The key functional role of zinc in cellular metabolism involves DNA synthesis, protein synthesis, cell division, immune function, wound healing, and its being essential for the catalytic activity of over 200 enzymes [33]. Zinc is potentially bonded with almost 10% of the human proteins, besides hundreds that transport and circulate zinc. Furthermore, it is a necessary element for sensing smell and taste [30], supporting ordinary growth and development during childhood, adolescence, and in pregnancy [31]. Various studies of zinc have shown antioxidant properties (vary as to its effectiveness) that may defend against earlier aging and aids in expediting the healing process after an injury [34], [35]. Gastroenteritis is intensely reduced by the absorption of zinc, and this effect might be owing to the direct antimicrobial action of Zn2+ in the gastrointestinal tract or to the ingestion of zinc and reproduce from immune cells [36]. Zinc signaling is used by the cells of the immune system, prostrate, salivary gland, and intestines as one way to communicate with other cells [37]. Zinc is stored in specific synaptic vesicles in the brain by glutamatergic neurons and can control the excitability of the brain [38]. It has neurotoxicity, signifying zinc homeostasis and, thus, has implications in the functioning of the brain and the central nervous system and also plays a vital role in learning activity by its involvement in synaptic plasticity [38].
Zinc is commonly bonded and transported by albumin (60%, less affinity) and 10% remains freely available in the blood plasma [39]. Iron is also transported by transferring, and too much iron can lessen the absorption of zinc and vice versa [40]. Irrespective of zinc consumption, its concentration stays moderately constant in the blood plasma. Zinc may be stored in metallothionein reserves and also transported in metal carriers of ZnT and ZIP assemblage transporter proteins [26]. Metallothioneins are competent in regulating and ingesting zinc by 15–40% in the intestinal cells [41]. Furthermore, concentration of zinc mostly harms copper absorption as metallothioneins absorb both metals [42].
Carboxypeptidase and carbonic anhydrase are two zinc-containing enzymes that are important for the digestion of proteins and processes of carbon dioxide (CO2) regulation, respectively [43]. During the digestion of proteins in vertebrate blood, carboxypeptidase cuts peptide linkages in which a coordinate covalent bond is formed between the C=O group attached to zinc and the terminal peptide that results in a positive carbon charge. The resulting composite develops a hydrophobic pocket on the enzyme near the zinc attracting the non-polar part of the protein being digested [26]. The carbonic-anhydrase transforms CO2 into bicarbonate, and the similar enzyme converts bicarbonate again into CO2 for exhalation via the lungs (Figure 3) [44]. This conversion takes place almost 1 million times slower when this enzyme is lacking at pH 7, the normal pH of blood, or it would need a pH 10 or greater [45].
(A) Zn atom observable in the middle of the human carbonic anhydrase. (B) The two histidine side chains in Zn fingers, which coordinate with the Zn ion (green).
In zinc fingers (Figure 3B), the zinc works a virtuously structural role [46], and these zinc fingers are proteins that produce fragments of some transcription factors. Such proteins identify the DNA base sequences throughout the transcription and replication of the DNA. The structure of the finger is assisted and maintained by each of the 9 or 10 Zn2+ ions in a zinc finger, which are coordinately bonded to four amino acids in the transcription factor [47]. The transcription factor uses the fingers to precisely bind with the DNA sequence by wrapping around the DNA helix [48]. The zinc ion coordinates with cysteine, aspartic acid, histidine, and glutamic acid amino acid side chains [49]. The flexible coordination geometry of metal also allows proteins to consume it, to promptly shift conformations to execute biological reactions [50].
1.4 Zinc compounds
Zinc does not exist freely in nature. It occurs in a +2 oxidation state. The predominant zinc minerals are zincite (zinc oxide), smithsonite (zinc carbonate), and sphalerite (zinc sulfide). Almost 55 zinc minerals exist in nature. Some most common zinc minerals with their basic information are listed in Table 1.
Zinc compounds, its percentage, and uses.
| Zn compounds | Formula | Mineral name | % Zn | Uses |
|---|---|---|---|---|
| Zinc oxide | ZnO | Zincite | 80.3 | Paints, ointments, cosmetics, cement, glass, automobile tires, fabricated rubber products, plumbing fixtures, glue, matches, tiles, ceramics, and porcelains, feed additives, seed treatment, inks, zinc green, electrostatic copying paper and color photography, flame retardant, semiconductor manufacturing, and as an ultraviolet absorber in plastics |
| Zinc sulfide | ZnS | Sphalerite | 67.0 | Bleaching agent for textiles, straw, vegetable oils, and other products; brightening agent for paper and beet and cane sugar juice |
| Zinc silicate | Zn2SiO4 | Willemite | 58.5 | Hardening agent for concrete |
| Zinc silicate hydroxylate | Zn4Si2O7(OH)2·H2O | Hemimorphite | 54.2 | Mothproofing agent; hardener for concrete |
| Zinc hydroxide carbonate | Zn5(OH)6(CO3)2 | Hydrozincite | 56.0 | Textile, wood, and food industries |
| Zinc carbonate | ZnCO3 | Smithsonite | 52.0 | Manufacture of rayon; supplement in animal feeds; dyeing of textiles; and wood preservative |
| Zinc ferromanganate | (Zn,Fe,Mn)(Fe,Mn)2O4 | Franklinite | 15–20 | Wood preservatives |
| Zinc acetate | (Zn(C2H3O2)2) | Acetic acid, galzin | 29–36 | Wood preservative; dye for textiles; additive for animal feed; glazing for ceramics |
| Zinc arsenate | (Zn3(AsO4)2) | Arsenic acid | 41.4 | Wood preservative; insecticide |
| Zinc borate | (ZnB4O7) | Trizinc diborate, zinc borate | 45–63 | Fireproofing of textiles; prevents the growth of fungus and mildew |
| Zinc chloride | (ZnCl2) | Zinc butter | 46–48 | Solder (for welding metals); fireproofing; food preservative; additive in antiseptics and deodorants; treatment of textiles; adhesives; dental cement; petroleum refining; and embalming and taxidermy products |
| Zinc phosphide | (Zn3P2) | Arrex, Denkarin Grains, Deviphos | 76 | Rodenticide (rat killer) |
Other important zinc compounds include zinc fluorosilicate (ZnSiF6), zinc hydrosulfite (ZnS2O4), zinc sulfate (ZnSO4), zinc fluoride, zinc hydride, zinc nitrate, zinc ammonium nitrite, zinc cyanide, zinc permanganate, and zinc chlorate, etc.
2 Zinc nanoparticles
There are various types of zinc NPs, and some that are important are discussed here. Furthermore, a short description of zinc NP characteristics are presented in Table 2.
Zinc NPs, synthesis, characteristics, and properties.
| Zn NP | Synthesis precursor | Size | Shape variation | Characteristic applications | References |
|---|---|---|---|---|---|
| Zinc sulfide NPs | Zinc acetate, sodium sulfide, EDTA | ~50 nm, 4–7 nm | Zinc-blended structure of ZnS cubic nanocrystals, powder | Optical coating, electro-optic modulator, photocatalyst, photoconductors, optical sensors, dielectric filter | [51], [52], [53] |
| Zinc ferrite NPs | Ferric nitrate, zinc nitrate, poly (vinyl pyrrolidone) (PVP) | 17–31 nm | spinel-structured ferrite NPs | gas sensor, semiconductor photocatalysis, high-density magnetic recording media, radar-absorbent materials | [54], [55], [56] |
| Zinc phosphide NPs | tri-n-octylphosphine, methylzinc | ~8 nm | Tetragonal phased tree-shaped nanostructures as nanobelts, and nanowires | Solar cells, infrared (IR) and ultraviolet (UV) sensors, lasers, light polarization step indicators | [57], [58], [59] |
| Zinc selenide NPs | Zinc acetate, sodium selenite | 20–60 nm | Spherical shape, nano size and smooth surface | Blue diode lasers, supersonic materials, photovoltaic solar cells, optical filters, optical recording materials, solar cells | [60], [61] |
| Zinc telluride NPs | Zinc acetate, sodium telluride | 2.6 nm, ~50–70 nm | Spherical shape, Small spheres or faceted particles, Uniform hexagonal shape | Optoelectronics and photonics, nano-sized semiconductor particles | [61], [62] |
2.1 Zinc sulfide nanoparticles
Zinc sulfide (ZnS) is well known due to its use as phosphors, in field emission display, as optical coating, as dielectric filters, electro-optic modulators, window material, as photoconductors, reflectors, optical sensors, its use in other light-emitting materials, and most important, its use as a direct gap semiconductor [51]. Beside these, the chemical stability of ZnS is more promising than that of other chalcogenides.
ZnS synthesis has been reported through the chemical precipitation method, the spray-based method, the sol-gel method, the mechano-chemical route, the electrospinning technique, and the ultrasonic radiation method [52], [63], [64], [65]. ZnS responds to ultraviolet radiation if the band gap energy is 3.68 eV for optical interband transition. ZnS produces visible luminescence when the transition state localizes in the band gap state with emission and absorption peaks at around 420 nm [53]. The band gap energy plays an important role for ZnS to be applied as a photocatalyst for the removal of organic pollutants and toxic water pollutants from the environment [51]. ZnS nanomaterials were applied for the photodegradation of organic pollutants including benzene derivatives, halogenated derivatives, dyes, and p-nitrophenol in wastewater treatment [34].
However, the applications of ZnS NPs are limited because of the high cost of large-scale production, difficulties in separation, recovery, and recycling.
2.2 Zinc ferrite nanoparticles
The unique physical and chemical properties of spinel-structured ferrite NPs along with technological applications in biomedicine, ferrofluids, radar absorbent, gas sensors, high-density magnetic recording media, and photocatalysis caught the interest of scientists in recent years. Change in properties with the change in particle size of ZnFe2O4 also makes them attractive [54], [66].
Zinc ferrite nanopowders in a broader size range (5–45 nm in size, depending on the annealing temperature) were prepared by the co-precipitation method from the corresponding nitrate precursors and thermal treating of the obtained precursor at different temperatures [67]. ZnFe2O4 was successfully used as a photocatalyst for phenol degradation and oxidative dehydrogenation of n-butane to butane [55].
2.3 Zinc phosphide nanoparticles
Zn3P2 is a novel optoelectronic material containing a direct band gap of 1.4–1.6 eV, the optimum range for solar energy conversion. The long minority diffusion length and large optical absorption coefficient permit Zn3P2 to be used as a high current collection efficiency [57]. The presence of Zn and P, the most abundant, inexpensive, and nontoxic materials make them more promising [58]. These properties also favor the deployment of this material in devices such as solar cells, light polarization indicators, infrared (IR), lasers, and ultraviolet (UV) sensors [55]. Based on wide properties, many kinds of heterojunctions, such as In ZnO/Zn3P2, ITO/Zn3P2, Zn3P2/ZnSe, Mg/Zn3P2, and P/Zn3P2, were designed [57]. Zn3P2 are mostly prepared as thin films and synthesis of other NPs [68] and nanotrumpets are prepared with ZnO layer coated on the surface [57]. Zn3P2 exhibits a pronounced quantum size effect because of the large excitonic radii [68]. Zn3P2 has been synthesized using the thermal-assisted pulsed laser ablation process in a single-zone horizontal tube furnace [57], [69].
2.4 Zinc selenide (ZnSe) nanoparticles
Selenides are widely used as optical recording materials, optical filters, laser materials, thermoelectric supersonic materials, cooling materials, solar cells, and sensors. Because of the diverse properties and industrial applications, selenide is a material of current interest [70].
ZnSe has a band gap of 2.7 eV [71] and is used as a semiconductor material for photovoltaic solar cells and blue diode lasers. ZnSe NPs are normally prepared by physical methods [69] though the chemical synthesis route, which is economical, effective, and in bulk mode [60]. Beside these, hydrothermal and solvo-thermal synthesis for hollow ZnSe microspheres has also been established [72]. Reiss et al. [73] reported on the synthesis of ZnSe NPs in octadene (non-coordinating solvent) by zinc stearate with a selenium reaction. The synthesized ZnSe NPs demonstrate absorption features in the range of 390–440 nm [72].
2.5 Zinc telluride nanoparticles
Zinc telluride is a Group II–VI semiconductor compound with a band gap of 2.26 eV [60] at room temperature. ZnTe usually has a cubic crystal structure (zinc blende or sphalerite), but can also be synthesized as hexagonal crystals (wurtzite structure) [74], [75]. ZnTe has potential applications in the fields of light-emitting diodes, solar cells, high-efficiency multi-junction solar cells, photodetectors, optoelectronic devices [62], terahertz (THz) devices [76], and electronic devices [77]. All those are dependent on the crystal structure and particle size. Several researchers employed various techniques for synthesizing ZnTe NPs such as the electrodeposition method, chemical synthesis, thermal evaporation, microwave irradiation, a sublimation technique, spray pyrolysis, microwave plasma, and electrical conduction [75].
3 Zinc oxide, nanomaterial of the present
Zinc oxide NPs are used as an additive in various industrial products and materials including glass, ceramics, cement, rubbers, ointments, lubricants, paints, foods, fire retardants, plastics, pigments, adhesives, ferrites, sealants, batteries, first-aid tapes, etc. [78]. It is a wide-band gap semiconductor having numerous other favorable properties with good transparency, photochemical stability, strong room temperature luminescence, and high electron mobility. The intrinsic doping of this semiconductor is the n-type because of zinc interstitials or oxygen vacancies [79]. Such properties are valued in emergent applications for heat-protecting windows, transparent electrodes in energy saving, liquid crystal displays, light-emitting diodes, and thin-film transistors in electronics. ZnO has a potential biocompatibility over many other metal oxides and has explored many pronounced applications in current antiviral, antimicrobial, biomedical, and environmental areas [17], [80].
3.1 ZnO crystal structure
The chemical co-precipitation method for ZnO NPs synthesis has been proven as a versatile and powerful technique for growing 1D nanomaterials [81]. The process is normally carried out in aqueous medium of zinc nitrate or zinc acetate and hexamine at about 90°C, followed by the provision of a basic environment. Some additives, for example, polyethylenimine (PEI) or polyethylene glycol (PEG), can develop the aspect ratio of the ZnO nanowires [80]. ZnO nanowire doping was attained by the addition of extra metal nitrates or other salts to the growth solution [82]. The morphology of the resulting nanostructures can be modified by changing the environmental conditions, precursor concentration, and others (e.g. the pH and zinc concentration) or the thermal treatment (e.g. heating rate and temperature) [83]. During the synthesis, ZnO with pre-seeding substrates create sites for homogeneous nucleation of ZnO crystals. Common pre-seeding techniques include spin coating of ZnO NPs, in situ thermal decomposition of zinc acetate crystallites, and the use of physical vapor deposition methods to deposit thin films of ZnO [84]. Pre-seeding can be achieved in combination with top down patterning approaches like prior to growth, nano-sphere lithography, and electron beam lithography to designate nucleation sites. Such interlinked ZnO nanowires can be used in field emission devices and dye-sensitized solar cells [85].
ZnO shows three main crystalline structures: the hexagonal wurtzite, zinc blende, and periodically observed rock salt [86]. Wurtzite is the most common and thermodynamically stable ZnO structure at ambient conditions. It has a lattice parameter spacing a=0.325 nm and c=0.521 nm, the ratio c/a~1.6, i.e. close to the perfect value for the hexagonal cell c/a=1.633. Four oxygen atoms are bounded with each tetrahedral Zn atom and vice versa [4]. It is generally demonstrated schematically as various alternating planes of zinc and oxygen ions that are arranged along the c-axis. The tetrahedral coordination in ZnO results in a non-central symmetric structure resulting in pyroelectricity and piezoelectricity. Further, the polar surfaces of ZnO are its essential characteristic, and the basal plane is its most common polar surface [87].
A spontaneous polarization along the c-axis and the normal dipole moment as well as the divergence in surface energy take place by the production of positively charged Zn-(0001) and negatively charged O-(000ī) surfaces due to the oppositely charged ions. Commonly, the polar surfaces have facets or show huge surface reconstructions to maintain a stable structure, but ZnO-(0001) are exceptions: they are atomically flat, stable, and without reconstruction. The other two most generally observed facets for ZnO are {2īī0} and {01ī0}, which have a lower energy than the {0001} facets and are non-polar surfaces [88]. The structure of zinc blende is metastable that could be stabilized by growing ZnO on substrates with a cubic lattice structure. Such crystal structures with tetrahedral geometry are shown in Figure 4, where the black and whitish-gray circles indicate Zn and oxygen atoms, respectively, for Wurtzite, while the black and whitish-gray circles show O and Zn atoms, respectively, for the zinc blende and rock salt structures [89]. At a comparatively high pressure of about 10 GPa, ZnO transforms to the rock salt design [90].
![Figure 4: ZnO crystal structures of wurtzite (tetrahedral coordination of ZnO), zinc blende, and rock salt. Revised from Sirelkhatim et al. [4].](/document/doi/10.1515/ntrev-2018-0067/asset/graphic/j_ntrev-2018-0067_fig_004.jpg)
ZnO crystal structures of wurtzite (tetrahedral coordination of ZnO), zinc blende, and rock salt. Revised from Sirelkhatim et al. [4].
3.2 Growth directions of ZnO structures
ZnO has structurally three types of fast growth directions: (2īī0) (±[2īī0], ±[ī2ī0], ±[īī20]); (01ī0) (±[01ī0], ±[10ī0], ±[1ī00]); and ±[0001]. Together with the polar surfaces, ZnO shows a wide range of novel structures that can be grown by tuning the growth rates along these directions due to the atomic terminations [2]. Relative surface activities of various growth facets are one of the most promising factors determining the particle morphology under given conditions. Under controlled growth conditions, a crystal has macroscopically different kinetic parameters for different crystal planes. Therefore, a crystallite will develop generally into a 3D object with a low index, well-defined, and crystallographic faces after an early period of nucleation and incubation. Figure 5A–D displays some characteristic growth morphologies of ZnO (1D) nanostructures. Because of the lower energy, these structures are likely to maximize the areas of the {2īī0} and {01ī0} facets. The shape displayed in Figure 4 is ruled by polar surfaces that grow by introducing planar defects parallel to the polar surfaces [90]. Twins and planar defects are identified infrequently parallel to the (0001) plane; however, dislocations are occasionally observed.
![Figure 5: Typical growth morphologies of 1D ZnO nanostructures and the corresponding facets [2].](/document/doi/10.1515/ntrev-2018-0067/asset/graphic/j_ntrev-2018-0067_fig_005.jpg)
Typical growth morphologies of 1D ZnO nanostructures and the corresponding facets [2].
ZnO NPs are the key technological materials that have gained a vast consideration owing to their distinguished performance in optics, electronics, biologics, environment, photonics, etc. These particles have currently a wide range of mainly biological (drug/gene delivery, biosensing, cancer therapy, biomaterials for shape memory polymers like molecular switches, tissue engineering, nanomachines that can act as biological mimetic, antimicrobial actions, environmental applications, etc. [17], [80], [91]. Synthesis of ZnO thin films as catalysts, sensors, and transducers has been applicable since the 1960s. At room temperature, the ZnO band gap is 3.37 eV. The UV emission is an attribute of direct exciton transition that recombines with holes in the valence band or in traps near the valence bond with detection at 370 nm. For this reason, many point defects have been suggested, i.e. oxygen interstitials, zinc vacancies, oxygen vacancies, zinc interstitials, antisite oxygen, and surface states [92]. The band structure and charge transfer pathway of the ZnO nanocrystal with a band gap (3.2 eV) for various applications are shown in Figure 6.
![Figure 6: Band structure and charge transfer pathways of ZnO nanocrystal with oxygen defects [93].](/document/doi/10.1515/ntrev-2018-0067/asset/graphic/j_ntrev-2018-0067_fig_006.jpg)
Band structure and charge transfer pathways of ZnO nanocrystal with oxygen defects [93].
3.3 Easy synthesis of ZnO NPs
ZnO NPs can easily be synthesized by numerous techniques, such as green, metallurgical, solid, liquid (i.e. chemical), and gaseous. In the metallurgical approach, the ZnO NPs could be achieved by the roasting of a suitable zinc ore through a direct or indirect process [3]. Thanks to the development of the methods of obtaining ZnO NPs that enables precise control of the ZnO NPs’ size that extensive scientific research is possible today. It is also worth mentioning that such properties as band gap, conductivity, or magnetic properties can be controlled by doping ZnO NPs with ions of transition metals (e.g. Co, Mn, Cr, Ni, Fe, V). Nevertheless, the chemical techniques are the most reliable, economical, and environmentally friendly and also offer flexibility for controlling the shape and size of prepared NPs. There are a variety of chemical techniques (Table 3), for example, the precipitation process, the hydrothermal, mechanochemical process, physical vapor, solvo-thermal, sol-gel, micro-emulsion methods, etc. Some of the important synthesis techniques are briefly discussed in the following.
Synthesis techniques, precursors, preferred solvents, conditions, and the resultant morphology of ZnO NPs.
| Technique | Precursor | Medium/solvent | Conditions during preparation | Size (nm) | Morphology | References |
|---|---|---|---|---|---|---|
| Mechano-chemical | Zinc chloride, Na2CO3 and NaCl | De-ionized water | Calcination: 2 h, 400–800°C | 20–30 nm | Hexagonal, regular | [94] |
| Physical vapor | Solid ore | Plasma, gas, vapor, diethylzinc (DEZ), oxygen | Heat, reactive carrier gas (helium) | 8–75 nm | Rod, sphere | [95], [96] |
| Simple precipitation | Zinc acetate, zinc nitrate, zinc sulfate, etc | Double distilled water | Drying: overnight, 100°C calcination: 300–500°C (may change with precursor types) | 80 (l), 30–60 (d) | Nanorod, nano-flakes | [34], [97], [98], [99] |
| Microwave decomposition | Zinc acetate dehydrate, zinc acetyl-acetonate | 1-Butyl-3-methylimidazolium bis (tri-fluoro-methyl-sulfonyl) imide (BMIM) (NTf2) | Precursor concentration: 2.5–10 wt%; microwave heating: 800 W, 4 min; drying: 75°C in air | 37–47 | Sphere, average crystallite size | [100] |
| Hydrothermal | Zinc acetate dehydrate | Poly-vinyl-pyrrolidone (PVP) | Reaction: 180°C; drying: 80°C in vacuum oven; calcinations: 400–600°C | 5 μm (l), 50–200 (d) | Nanorod, hexagonal prismatic | [101], [102] |
| Microwave hydrothermal | zinc nitrate, zinc acetyl-acetonate | Ethanol, imidazolium tetrafluoroborate | Microwave heating: 2 min, 90°C; drying: 2 h, 60°C (may change with precursor types) | 10–80 nm | Hexagonal wurtzite, nanorod and nanowire, mulberry-like, hollow | [103] |
| Wet chemical | Zinc nitrate hexahydrate | Sodium hydroxide (NaOH) as precursors and soluble starch as stabilizing agent | Solution incubation: 50–55 min, 101°C | 20–30 | Wurtzite, acicular, rod, flowers, dumbbell, rice flakes, and rings | [104], [105] |
| Sol-gel (gelatin media) | Zinc nitrate | Distilled water and gelatin as substrate | Aging: 96 h, ambient temperature, calcination: 2 h, 500 °C | 30–60 | Circular and hexagonal | [106] |
| Solvo-thermal | Zinc acetate, zinc nitrate | Ethanol, imidazolium tetrafluoroborate ionic liquid | Reaction: 10–48 h, 120–250°C | 60–100 nm | Hexagonal (wurtzite) structure, hollow spheres, nano-flowers, nanorods | [107] |
| Micro-emulsion | Zinc acetate, zinc nitrate, zinc chloride, etc | Heptane, hexanol, triton, benzene, ethanol, diethyl ether, chloroform, acetone, methanol, glycerol, etc | Reaction: 24 h, 60–70°C; drying: 1 h, 100°C; calcination: 3 h, 300–500°C (may change with precursor types) | 15–24 nm | Hexagonal wurtzite, spherical | [1], [108] |
| Deposition process | Zinc acetate, zinc nitrate, CdS | P3HT (poly(3-hexylthiophene) | Atmospheric O2, pressure 1.3 Pa, pulsed laser, drying: 2 h, 400–700°C | 20–60 nm | Dumbbell, nanofibers, rod | [108], [109] |
| Microwave irradiation | Zinc nitrate | Deionized water, HMT (hexamethylene-tetramine) | Ultrasonic irradiation: 30 min, 80°C; drying: 2 h, 60°C | 80–100 nm | Hexagonal wurtzite, rod, nanowire | [110] |
| Solution combustion | Zinc acetate di-hydrate | Ethanol: ethylene-glycol (volume ratio of 60/40) | Ultrasonication and centrifugation, drying: 50°C in air, 5 h | 60–80 nm | Spherical | [111] |
| Microwave solvo-thermal | Zinc acetate dihydra | Water, 2-propanol solution, ethylene glycol | Using a closed vessel as reactor in conventional microwave oven, heating for 3 min, centrifugation, washing, and drying | 25–50 | Irregular and spheroidal nanostructures | [112], [113], [114] |
3.3.1 Mechanochemical processing (MCP)
MCP is a novel, simple, and economical technique of achieving large-scale detached NPs. A large diversity of other crystalline NPs, such as ZnO, ZnS, CdS, CeO2, and SiO2 can also be synthesized by this simple approach [93], [94]. This method entails two processes of conventional ball milling for the reduction of physical size and chemical reactions that are activated mechanically during grinding at the nanoscale. The precursors used in this procedure are sodium carbonate (Na2CO3) and zinc chloride (ZnCl2) that are milled simultaneously in a ball mill to generate sodium chloride (NaCl) and zinc carbonate (ZnCO3) through a chemical exchange reaction and ball-powder collisions. On heating fine zinc carbonate converts into ZnO NPs (Figure 6). The size of the obtained ZnO NPs (Table 3) depends on the milling time and the heat treatment [94], [115]. Therefore, there is an optimum grinding time to obtain ZnO NPs with the minimum average size. Conversely, increasing the temperature of the heat treatment process causes an increase in the size of the ZnO NPs [116]. MCP is typically an appropriate method for large-scale ZnO NP production owing to its low cost and simplicity. Furthermore, this technique is promising from an environmental point of view as the reactions involve eviction of organic solvents [117]. Besides, drawbacks of this method during milling may be particle agglomeration that could be minimized by the existence of a salt matrix in the reaction, i.e. eventually separated by a simple washing process before calcination.
3.3.2 Physical vapor synthesis (PVS)
Plasma arc energy is employed to a solid precursor to produce vapor at a high temperature in the PVS method. When the precursor is inserted into the plasma to induce reactions, the plasma arc provides the desired energy leading to super saturation and particle nucleation. This reaction process causes complete decomposition into atoms that again react or condense to create particles on cooling (via mixing with cool gas) by expansion through a nozzle [95]. A reactant gas is provided to the vapor in this process, i.e. cooled at a controlled rate and condensed to produce NPs. Fully dense and discrete particles with average sizes ranging from 8 to 75 nm of defined crystallinity were prepared by the PVS method [96].
3.3.3 Other miscellaneous chemical methods
The distinctive versatility and properties of ZnO paved the way to use numerous chemical methods to produce more promisingly functional ZnO nanostructures. Compared to other nano-metal oxides, ZnO nanostructures reflect the richest nano-configuration assembly [2], [4]. Each nanostructure has specific physicochemical, electrical, optical, and structural properties, allowing significant applications. Some important characteristics of these techniques such as functioning precursor, solvents, reaction conditions along with size and morphology of the created NP are presented in Table 3.
The selected approach typically depends on the desired application, as various techniques produce different morphologies and sizes of particles. A diversity of ZnO NPs with different growth morphologies were successfully manufactured by adjusting the growth conditions [2]. The most adopted synthesis techniques include the hydrothermal and thermal evaporation of ZnO powders at 1400°C, sol-gel technique, double-jet precipitation, solution synthesis, self-combustion, vapor-liquid-solid technique, polymerized complex method, and simple thermal sublimation [118]. Numerous researchers also used the solution process system to fabricate ZnO nanostructures. Wahab et al. [119] synthesized flower-shaped ZnO nanostructures using dihydrated zinc acetate and NaOH at 90°C by the solution process approach. The scientists also synthesized prickly sphere-like and prism-like ZnO through a decomposition process at 100°C for 13 h [4]. Most of the nanostructures are imperative factors for the biological and environmental applications, as each morphology accounts for a certain mechanism of action [80], [120]. Therefore, a greater number of scientists were encouraged to attain selective nanostructured ZnO for different methodical experiments.
3.4 Capping/doping of ZnO NPs with foreign materials
The implanting of foreign metals on ZnO nanostructures and the doping methods to improve the functional bioactive agent became a topic among various scientists. Different shapes of doped and undoped ZnO NPs were prepared by the simple wet chemical method and were annealed [121]. The resulting ZnO samples were examined against different microbial strains, and consequently, the ZnO-doped samples showed considerable significant activity (37% higher than undoped ZnO). Such results are medically valuable particularly in controlling different bacterial infections such as in skin creams/lotions and or in UV protection. Some antibacterial tests are typically carried out in cell culture media or aqueous media. ZnO is almost insoluble in water, and during synthesis, it agglomerates rapidly with water owing to high polarity that leads to the deposition. Issues of non-dissolution, aggregation, settling, or re-precipitation inhibit the fabrication processes. At this point, most of the scientists resolve this issue by adding certain additives that have no substantial effect on bioactivity. Polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), poly (a, c, l-glutamic acid) (PGA), poly (a, c, l-glutamic acid), and PVA were used as doping agents and as stabilizers to boost ZnO morphology and modify the sizes for numerous biological activities [122], [123], [124]. Some investigators used suitable deflocculants or capping agents [99], [125], [126] such as sodium carbonate (Na2CO3) or sodium silicate (Na2-SiO3). After doping or adding dispersants, such blends were treated with a vigorous vortex (likely for 5 min) or set aside overnight with magnetic stirring and then ultrasonicated (for 20–30 min) to avoid deposition and aggregation of particles. Last, the characterization of ZnO NPs is needed to recognize effect-impacted stability such as morphology, structural, pH solution, surface properties, and particle size; plus, these factors sequentially have influences on the bioactivities.
3.4.1 Green synthesis of ZnO NPs
Currently, ZnO NPs are fastidiously studied due to their large bandwidth, simplicity, non-toxic, high exciton-binding energy, easy fabrication, biocompatible, biosafe, environment friendly, and they have potential applications like antibacterial, antioxidant, antifungal, photocatalysts, anti-inflammatory, wound healing, cosmetic, food, anti-diabetic, and optic properties [127], [128]. Because of the large rate of toxic chemicals and extreme environment employed in the conventional physio-chemical manufacturing approaches of these NPs, green methods employing the use of plants, fungus, bacteria, and algae were adopted. The synthesis conditions could further be optimized for maximal and narrow size range preparation of ZnO NPs. In recent years, due to low-cost, variety in shape and size, stability, large-scale production, eco-friendliness, and methodological easiness of NPs, there is a great stress on the utility of plant materials (the most preferred source) such as the leaf, root, or shoot powders and flowers in the form of solvent-based extracts as capping and stabilizing agents for the synthesis of pure ZnO NPs [127], [129]. These natural strains and plant extracts secrete some phytochemicals that act as both a reducing agent and capping or stabilization agent; e.g. synthesis of ZnO nanoflowers of uniform size from cell soluble proteins of Bacillus licheniformis exhibited enhanced photocatalytic activity and so there was a degradation of methylene blue (MB) pollutant dye, which clearly showed the photo-stability of ZnO NPs. Biologically reducing means the reduction of metal or metal oxide ions to zero valence metal NPs by the action of phytochemicals such as vitamins, polysaccharides, amino acids, alkaloids, polyphenolic compounds, and terpenoids secreted from plants [129], [130]. ZnO NPs are readily soluble in biological fluids and tend to amass easily under different physiological conditions. However, the physicochemical properties of these NPs have an impact on the bioavailability [128]. Extensive research is needed to perform on ZnO NP pharmacokinetics (drug movement) and bioavailability, which are still looked-for understanding and required to establish the exact mechanism in human beings.
3.5 Characterization techniques
The synthesized ZnO NPs prepared through numerous methods were further studied using various characterization techniques according to their analytical parameters (such as morphology, size, agglomeration, etc.), as shown in Table 4, and furthermore, characterization techniques are reviewed in detail in various recent papers [13], [53], [98], [106], [118], [133].
Summary of analytical techniques to conduct physicochemical characterization and monitoring of ZnO NPs (in living systems mostly).
| Technique | Analysis | Parameters studied | References |
|---|---|---|---|
| Fourier transform infrared spectroscopy (FTIR) | Structural analysis | Chemical changes in polymers after NP incorporation, photostability measurements | [131] |
| Scanning electron microscopy (SEM) | Structural analysis | Morphological characterization | [118], [131] |
| Transmission electron microscope (TEM) | Structural analysis | Physical examination and dispersion quality of NPs in the polymeric matrix | [131] |
| X ray diffraction (XRD) Electron microscopy | Shape and structure | Crystallinity and visualization of NP structure and size | [131], [132], [133] |
| Selected-area electron diffraction (SED) | Shape perkiness | Confirmation of the preferential orientation of nanocrystals instead of irregular shapes/rings | [118], [133] |
| UV-Vis spectroscopy | Polymers/matrices/protein binding affinity | Transmission optical spectra | [134] (125) |
| Atomic force microscopy | Polymers/matrices/protein binding affinity | Topographic characterization and surface morphology of nanomaterial | [135] (126) |
| Thermogravimetric analysis (TGA) | Property measurements | Weight variation measurement of samples as a function of temperature/time, with controlled temperature programming | [131] |
| Dynamic light scattering (DLS) | Size and charge Analytical ultracentrifugation | Changes in the hydrodynamic diameter of NP | [17], [136] |
| Ultra-sonication | De-agglomeration | Uses sound energy to disrupt large aggregates of NPs | [137] (128) |
| Inductively coupled mass spectrometry | Dissolution | For detecting elemental composition of the nanomaterial | [138] (129) |
| Photoluminescence spectroscopy (PLS) | Physical properties (spectrofluorimeter measures PL spectrum dispersed in water) | Measurement of quantum size effects on increasing and reducing band gap energy | [133] (124) |
| Fluorescence spectroscopy | Polymers/organics/protein binding affinity | Measures change in fluorescence spectra due to NP-protein interaction | [139] (130) |
| Braunauer-Emmet-Teller method | Surface area | Measures specific surface area using adsorption of gas on the surface | [134] (123) |
| Nuclear magnetic resonance | Polymers/organics/protein structural changes after binding | Depends on magnetic properties of atomic nuclei to predict structure | [140] (131) |
| Confocal microscopy | NP uptake | Visualization of fluorescent NPs in vitro | [141] (132) |
3.6 Biocompatibility of ZnO NPs
ZnO NPs exhibit reasonable biocompatibility. The FDA generally recognized their bulkier form as a harmless (GRAS) material. As discussed earlier, zinc is an essential co-factor in several cellular mechanisms; therefore, zinc NPs may also demonstrate biocompatibility. ZnO could simply be biodegraded, or such particles can take part in the active nutritional cycle of the human body [36], [142]. Furthermore, ZnO NPs have several biomedical applications, for example, in biomedical imaging (consists of fluorescence, positron emission tomography, magnetic resonance along with dual-modality imaging), biosensing, gene delivery, drug delivery, and various arrays of molecules of interest. Over the next era, a lot of scientific investigations in this field will flourish, and more research exertion is required to progress biodegradable/biocompatible ZnO nano-daisies for potential clinical translation [143].
The physicochemical interaction between the surface of biological constituents and nanomaterial surface deals with the kinetic and thermodynamic exchanges among the interfaces [144]. It comprises the interaction of biological membranes with NPs and the interaction among NPs themselves. Several forces, for example, electrostatic forces, van der Waals forces, solvo-phobic, solvation, and depletion forces, applied on such interactions in media, affect the agglomeration rate of NPs with biomolecules [122]. The knowledge of such interactions is imperative for the suitable dispersion of NPs with slight agglomeration in media. Moreover, by studying these interactions, the usage of optimal polymers and surfactants to stabilize NPs can be observed. Identification and isolation of organic polymers (OP) constituting the NP-OP are important to recognize the bio-reactivity of NPs. The interactions of NP surfaces with separate organics were studied using a range of analytical techniques (Table 5). Electrostatic force acts as a repulsive force, while depletion and Van der Waals forces are attractive in nature. The changes in the dipole moment of electrons give rise to Van der Waals forces that induce a dipole moment in the contiguous atoms. Thus, the quantum mechanical dance of electrons arises from these kinds of diverse forces that play a significant role in NP interaction with the cellular surface and their passive attainment inside the cell [149], [150]. This bonding interaction with the cell surface is preferred by the improved and adjustable NPs’ surface area rendering to cell surface receptor and the relative NP size with biomolecules/ligands that causes passive uptake and adhesive interactions inside the cell, which boycotts the phagocytic process [151]. NPs can directly interact with cell organelles and cytoplasmic proteins through this passive uptake that leads to a greater cytotoxicity. They can be confined at any place inside the cell comprising the cytoplasm, outer membrane, mitochondria, DNA, lipid vesicles, nuclear membrane, nucleus, etc., which is detrimental to these cell organelles and eventually leads to cell death [152], [153].
Brief description of properties of ZnO NPs and effect capping/doping agents on size.
| Capping/doping agent | Size before capping (nm) | Size after capping (nm) | Properties | Synthesis methodology | References |
|---|---|---|---|---|---|
| Ethylene glycol (EG) | 65±1.2 | 91±6.3 | Photocatalytic, antibacterial, antibiofilm activities | Chemical co-precipitation method | [99], [126] |
| Polyethylene glycol (PEG) | 34 | 26 | Antidiabetic, antioxidant, antibacterial activity | Co-precipitation method | [123], [124] |
| Polyethylene glycol (PEG) | 90 | 10.70 | Antibacterial activity | Precipitation method | [123], [145] |
| Polyvinyl alcohol (PVA) | 65±1.2 | 60±1.9 | Photocatalytic, antibacterial, antibiofilm activities | Chemical co-precipitation method | [123], [124], [126] |
| Ascorbic acid (AsA) | 90 | 13.20 | Antibacterial activity | Precipitation method | [99], [123], [145] |
| Polyvinyl pyrrolidone (PVP) | 65±1.2 | 52±2.3 | Photocatalytic, antibacterial, antibiofilm activities | Chemical co-precipitation method | [123], [126] |
| Polyvinyl pyrrolidone (PVP) | 30–50 | 25–40 | Antidiabetic assay, antioxidant, antibacterial activity | Co-precipitation method | [123], [124] |
| Polysorbate 80 (T-80) | 90 | 7.58 | Antibacterial activity | Precipitation method | [126], [145] |
| Gelatin (GE) | 65±1.2 | 134±6.2 | Photocatalytic, antibacterial, antibiofilm activities | Chemical co-precipitation method | [99], [126] |
| Mercaptoacetic acid (MAA) | 90 | 8.84 | Antibacterial activity | Precipitation method | [99], [145] |
| Cobolt (Co) | 32 | 45 | – | Freeze-drying route | [99], [146] |
| Magnesium (Mg) | 26 | 22 | Antibacterial activity | Co-precipitation technique | [126], [147] |
| Citrus cellulose | 50 | – | Photocatalytic, antibacterial activity | Chemical co-precipitation | [34], [97], [148] |
3.6.1 Antimicrobial activity of ZnO nanoparticles
Antimicrobial agents are drugs with the ability to inhibit or damage bacterial growth, whereas they are not detrimental to the host. Such compounds perform as chemotherapeutic agents for the prevention or treatment of bacterial infections [4]. Several researchers studied the antibacterial activity of ZnO NPs (Table 6) using the colony-forming unit test, culture turbidity, and cell viability. Nair et al. [152] concluded that antibacterial activity is enhanced by reducing the initial number of bacterial cells having 102 against 106 CFUs, although there are some variations in the established laboratory techniques and protocols in the estimation of bactericidal activity [160].
Anticancer and antibacterial activities of the ZnO NPs and composites.
| NPs/material | size (nm) | Cancer cells/bacterial strains | Effect | References |
|---|---|---|---|---|
| ZnO NPs | 21.59 | HepG2 | Viab. decr 39%, 15 μg/ml | [13] |
| ZnO NPs | 21.59 | A549 | Viab. decr 47%, 15 μg/ml | [13] |
| ZnO NPs | 21.59 | BEAS-2B | Viab. decr 33%, 15 μg/ml | [13] |
| ZnO NPs | – | Caco-2 cells | Sign. 89 mg/ml | [154] |
| ZnO NPs | 50–70 | LoVo | Sign. viab. decr. at 10 μg/ml | [155] |
| ZnO NPs | 19 | K. pneumoniae | Sign. 500 mg/l | [156] |
| ZnO NPs | 50–70 | B. subtilis | Sign. 85.8 mg/l | [157] |
| ZnO NPs | 50–70 | S. aureus | Sign. >125 mg/l | [157] |
| CdO-ZnO composite | ~27 | P. vulgaris | Sign. 20 μg·ml−1 | [158] |
| CdO-ZnO composite | ~27 | P. aeruginosa | Sign. 15 μg·ml−1 | [158] |
| ZnO NPs | 30 | E. coli | MIC: 0.4 mg/ml | [159] |
| ZnO NPs | 50 | S. aureus | Sign. 10 mg/ml | [34] |
| ZnO NPs | 50 | E. coli | Sign. 10 mg/ml | [34] |
| ZnO-Cel composite | – | S. aureus | Sign. 10 mg/ml | [34] |
| ZnO-Cel composite | – | E. coli | Sign. 10 mg/ml | [34] |
K. pneumoniae, Klebsiella pneumoniae; B. subtilis, Bacillus subtilis; S. aureus, Staphylococcus aureus; P. vulgaris, Proteus vulgaris; P. aeruginosa, Pseudomonas aeruginosa; E. coli, Escherichia coli; decr., decrease; Sign., significant; MIC, minimal inhibitory concentration; Viab, viability.
Researchers investigated the morphology of bacterial variations induced by ZnO NPs, but several concerns were referred to the antibacterial activity. The precise toxicity mechanism is still controversial, is not fully explained, and requires deep explanations of queries within the spectrum of antibacterial activity [27]. In the literature, the distinct mechanisms that were proposed are documented as follows: ZnO NPs’ direct interaction with the cell walls causing damage to the integrity of the bacterial cell [122], releasing antimicrobial ions mainly Zn2+ ions and formation of reactive oxygen species (ROS) [161], [162], NP penetration inside the cell, membrane dysfunction, and even ROS produced on the surface of the NPs can also damage the cell [150], [162].
The antibacterial activity of ZnO NPs varies with the variation in particle size, and an inverse relationship exists between the size and efficacy of ZnO NPs [122], [163]. The effects of ZnO NPs and Zn ion in eukaryotes and prokaryotes are depicted in Figure 7.
![Figure 7: ROS generation on eukaryotic (A) and prokaryotic (B) cells by ZnO NPs [164].](/document/doi/10.1515/ntrev-2018-0067/asset/graphic/j_ntrev-2018-0067_fig_007.jpg)
ROS generation on eukaryotic (A) and prokaryotic (B) cells by ZnO NPs [164].
3.6.2 Anticancer activity
Numerous in vitro studies showed that ZnO NPs exhibit selective cytotoxicity toward cancerous cells (Table 6). Hanley recommended that compared to normal cells, ZnO NPs show 28–35 times more selective toxicity toward cancerous cells [165], [166], which can also additionally be exploited in the in vivo condition by selectively targeting ZnO NPs toward cancer cells [154].
3.6.3 Biosensor and bioimaging
Fast electron transfer kinetics and biocompatibility of ZnO make this material favorable to modify biomolecules and immobilization of biomimic membranes. The dominant property of ZnO nanowire for sensing pH in a liquid medium has made their applications in electrical sensors for biological detection. Beside this, ZnO nanostructures replaced florescent dyes because of their better photoluminescent properties. The other properties that favor bioimaging and photoluminescence are a tunable emission wavelength based on quantum size effects, including a broad absorption, weak self-absorption, narrow and symmetric emission band, high stability against photo-bleaching, and large Stokes shifts [155], [156], [157], [158], [159], [163], [164], [167], [168], [169]. This might be a good replacement of quantum dots (QDs) because they exhibit toxicity to biological systems, and their release may cause environmental pollution [170]. In comparison to QDs, ZnO NPs are of high safety, lack polluting effects, have a low price, and have good stability [171], although synthesis of luminescent ZnO is a challenge. The aqueous medium may destroy the surface defects that are a source of visible fluorescence; however, surface modification may resolve this problem [155]. ZnO with polymer core-shell NPs exhibit stable luminescence in aqueous solutions and are successfully applied in cell imaging (Figure 8).
![Figure 8: Photoluminescence of ZnO NPs under UV light (D), luminescence of ZnO inside the cell (E, F), and mice model (G–I) (adopted from Xiong) [169].](/document/doi/10.1515/ntrev-2018-0067/asset/graphic/j_ntrev-2018-0067_fig_008.jpg)
Photoluminescence of ZnO NPs under UV light (D), luminescence of ZnO inside the cell (E, F), and mice model (G–I) (adopted from Xiong) [169].
3.6.4 Drug delivery
Drug delivery to a diseased cell is practiced using Fe3O4 NPs, carbon nanotubes, mesoporous silica NPs, and organic polymer nanobeads. The mechanism of entering the cell is simple: through an intracellular endocytic pathway [171], [172] and releasing the drug inside especially when the drug, itself, cannot cross the cell membrane. However, a controlled release, i.e. pH, temperature, and light dependence, is a prerequisite for a focused target [173]. pH sensitivity plays a critical role in this aspect as pH in tumors and inflammatory tissues remains appreciably lower than in normal tissues. ZnO having both properties (drug carrier and pH sensitivity) was first suggested in 2010 [155], [174]. Thereafter, many researchers developed ZnO nanostructured carriers and designed a system to release the drug at the target tissue, i.e. release of doxorubicin (DOX) to HeLa cells in vitro by ZnO and QDs (Figure 8), where ZnO QDs are stable at neutral pH, however, rapidly disintegrating at pH < 6 [175]. The ZnO QD florescent composite can also be used to observe drug deliverance [168].
3.6.4.1 Diagnostic, therapeutic, and dentistry
ZnO NPs have fantastic luminescent properties that are good for biocompatibility, less expensive, and have low toxicity that have made these nanomaterials into one of the key contenders for bio-imaging. The detection of other required characteristics like their capability to form damaging ROS, strong adsorption capability, enhanced catalytic efficiency, and greater isoelectric point also upgrades them (ZnO NPs) for diagnostic and therapeutic functions [176]. Moreover, the impregnation of ZnO NPs onto various biomaterials (polymers) particularly denture-based polymers such as polymethyl methacrylate can greatly affect the microbial deposition on their surface. It improves the hydrophilicity and hardness, and even after this modification, the roughness parameter does not change. Thus, it achieves the requirements of the ISO (International Organization for Standardization) standards. Because of the hardness with absorbability (within the normal range), enhanced hydrophilicity and lack of substantial deterioration, such nanocomposites (based on ZnO NPs) can demonstrate a reduced microorganism growth on the denture base and, therefore, meet the requirements for clinical (dentistry) use [177].
3.7 UV lighting effect and photocatalysis
ZnO is more biocompatible than TiO2, and among all the inorganic photocatalytic materials, it has great photocatalytic efficiency [172], [175]. ZnO has good absorbance [169] as well as a better response to UV light; hence, its conductivity vividly improves, and this feature considerably triggers the interaction with biological (e.g. microorganisms) and environmental (e.g. dyes) parameters [90], [119]. After turning off the UV light, the photoconductivity of ZnO persists for a long time, and it has been attributed to the state of surface electron depletion, i.e. intensely linked to negative O2 species (O2−; O22−), adsorbed on the surface [4]. UV radiance quickly starts the desorption of insecurely bound O2 from the surface prompting enhanced photoconductivity and reducing the surface electron depletion area [170].
Photo-induced oxidation that impairs and inactivates organisms is the process of photocatalysis [171]. ROS, for example, superoxide ions (O2−) and hydrogen peroxide (H2O2), are produced by ZnO NPs in aqueous solution under UV radiation that has phototoxic effects, and these species are highly significant in biologics [168]. This process encouraged the use of photocatalytic activity of ZnO NPs in bio-nanomedicine, bionanotechnology, environmental nanotechnology, and for various antimicrobial applications. A comprehensive mechanism of reaction that describes this process was suggested by Padmavathy and Vijayaraghavan [125] and Seven et al. [178] as follows. As a semiconductor material, the ZnO electronic band structure comprises a valence band (VB) and a conduction band (CB). Incident radiation with energy more than 3.3 eV rapidly absorbs and, consequently, the electrons transfer to the CB from the VB. Thus, possible photoreactions are started by this transmission of electrons, and subsequently, free electrons are generated within the CB, whereas positive holes (h+) are created in the VB [115]. In the photocatalytic system, such a positive hole (h+), a direct oxidant, and imperative for the formation of reactive hydroxyl radicals (OH), act as the principal oxidants. On the other hand, the electrons of the CB reduce oxygen that is adsorbed by the photocatalyst [175], [179]. In the meantime, Padmavathy and Vijayaraghavan [125] suggested a link between the antibacterial activity and photon reaction in a series (of reactions) that results in the formation of hydrogen peroxide (H2O2) molecules, which cause lethal damage by penetrating the membrane. Because of photocatalytically triggered H2O2, various researchers also confirmed the cell membrane distraction to unsaturated phospholipid peroxidation [178], [180], [181], [182], [183], [184].
3.8 Mechanism of Zn ion toxicity
The intracellular release of zinc ions and subsequent generation of ROS are the basic processes behind the cytotoxicity of zinc NPs. Figure 9 exhibits the whole mechanism of the cytotoxicity of ZnO NPs. Zinc-dependent protein activity imbalance or disequilibrium and greater ROS generation causes cytotoxicity. This occurrence results in zinc-created oxidative stress and the imbalance of the protein activity that ultimately destroy the cell. A less significant cytotoxicity is presented by soluble extracellular zinc. Certain studies demonstrate the formation of not very soluble amorphous zinc-carbonate phosphate precipitates (phosphate due to media) on the exposure of extracellular soluble zinc to cell culture and media. Such precipitates are thought to defend the cell from zinc cytotoxicity [185]. Conversely, a cascade of passages interconnected to each other takes place on the releasing intracellular soluble zinc ions, i.e. they are responsible for the cytotoxic reaction of zinc NPs. These activities are further explained in detail by Bisht and Rayamajhi [166] in three major themes of zinc-mediated protein activity disequilibrium, ROS production, oxidative stress, and DNA damage and apoptosis.
A chart depiction of the whole cytotoxicity of zinc NPs, causing cell death.
4 Other applications of ZnO NPs
Because of the various physical and chemical properties, ZnO NPs are widely used in many dimensions (Table 7). They play an important role in a very wide range of applications [1], [16], ranging from types of ceramics, pharmaceuticals to agriculture, environment to human health, and paints to chemicals.
Summarized applications of ZnO NPs in nearly all fields.
| Field or industry | Fields/practices | References |
|---|---|---|
| Rubber | Fillers, activator of rubber compounds, polymer matrix, elastomers, etc. As an activator and accelerator in vulcanized rubber tires having longer working life | [15], [186], [187], [188], [189] |
| Pharmaceutical and cosmetics | Medicines, dental pastes, absorber of UV radiations, component of creams, ointments, powders (inorganic ZnO as photocatalyst perform better than organic photocatalyst), etc. | [190], [191], [192] |
| Textile | Absorber of UV radiations in cotton and wool fabrics mostly, self-cleaning and water repellent, super-hydrophobic nature, to impart sunscreen activity to the treated textiles, dye degradation, etc. To enhance the wash fastness, by dipping fabrics in a solution having a specific binder | [2], [15], [192], [193] |
| Electronics | Sensors, field emitters, photoelectronics, solar cells, photovoltaic and electroluminescent equipment, UV lasers, etc. Electrical devices, image recorder, attenuation of light, high temperature lubricant gas turbine engines | [15], [194], [195], [196], [197] |
| Environment Photocatalysis | ZnO NPs are luminescent materials because of their unique properties like wide band gap (3.37 eV), greater binding energy (60 meV), and radiation hardness nature Photo-degradation and UV degradation of various organic/inorganic pollutants, etc. | [15], [16], [80], [198], [199] |
| Antimicrobial action | Antifungal, antibacterial, antiviral actions, etc. | |
| Wastewater treatment | Pollutant removal and disinfectants, due to high chemical stability, oxidation-reduction capability, and toxic-less characteristics. Techniques may include nanomembrane, nanoabsorbent and nanocatalyst technology, pesticide detection, desalination, oxidation of organic pollutants, etc. | |
| Biologics | Solar cell window, piezo actuator devices, surface acoustic devices, and in gas sensors. Biocompatible material with antiseptic properties DNA and RNA damage has been raising industrial and academic concerns for the safe use of ZnO as an effective UV-shielding agent Exhibiting various catalytic antibacterial, anticorrosive, antifungal, and UV shielding/filtering properties As a demilitarization of chemical biological warfare agent | [19] |
| Agriculture | Nanofertilizers, pesticide detection, soil moisture, nutrient detection, better-quality crops, etc. | [150], [200] |
| Plants | Seed germination, growth of stem, roots, and shoots, enhanced antioxidant activities, enzymatic, non-enzymatic molecules, enriched phenolic and flavonoid contents, etc. | |
| Biomedical | Micronutrients for humans, plants, and animals, cancer treatment (enhanced permeability and retention effect and electrostatic interaction and selective cytotoxicity due to increased ROS present in cancer cells), drug delivery and in therapeutic interventions, etc. (such treatments are in clinical use or the development pipeline) | [166] |
| Chemistry | Apart from the scorch problems for carboxylated elastomers, ZnO is effectively used and common cross linking agent As a catalyst for the synthesis of coumarins, for carbon-carbon formation in fine chemical hetero Diels-Alder reaction Production of vulcanisates with high tensile strength, tear resistance, and hardness | [15] |
| Human health | Preventing diarrhea in infants and children, antioxidant and anti-inflammatory agent, effective therapeutic agent, Wilson’s disease and development of age-related macular degeneration (AMD) and its complications and blindness in the elderly age. Zn deficiency causes growth retardation, testicular hypofunction, immune dysfunctions, increased oxidative stress, and increased generation of inflammatory cytokines | [142] |
| Other applications and/or industries | Cement and concrete production (construction processes), ceramic varistors, and piezoelectric transducer, methanol production, typographical and offset links, biosensors, production of zinc silicates, vegetable products, Brass, zinc pyrithione in antidandruff shampoos, zinc is used against sunburns and on baby diapers to avoid rashes, production process and packing meat, etc. In auto industry, in hydrogen fuel engine, in engine oil, reduces friction, ZnO LEDs used in headlight (more efficient, less power usage, and longer life) In other imperative processes such as catalysis, in Gratzel type solar cells, and short-wavelength light-emitting devices, etc. | [15], [131], [151], [201], [202] |
| Water treatment | ZnO NPs have photodegradation potential, which can also be enhanced by coupling with other semiconductors including CdO, SnO2, and TiO2, as feasible approach. ZnO NPs are environment friendly and compatible with organisms, which makes them suitable for the treatment of water and wastewater | [34], [203] |
4.1 Morphological effect of ZnO
Several studies reported that the toxicity and naiveté of ZnO NPs are considerably affected by their different morphologies [19]. Therefore, enviably created ZnO NP structures for different biological and environmental applications could be achieved by controlling solvents, precursor types, physicochemical settings, such as pH and temperature, etc. [115], along with shape-directing agents [81], [92]. Furthermore, the surface morphology could be determined through surface activity in controlled growth conditions.
The shape-dependent activities were clarified in terms of the active facet percentage in the ZnO NPs. Synthesis and growth methods hold several active facets in NPs. Spherical nanostructures predominantly have (100) facets, while the rod structures of ZnO have (111) and (100) facets. High-atom-density facets with (111) facets show effective biological activities [197]. The facet-dependent ZnO activities were assessed by limited studies [93], [105], [204]. In this regard, the ZnO nanostructures can efficiently perform functions and affect their mechanism of internalization, for example, nanowires, nanospherical, nanoplates, nanoflowers, nanorods, etc., penetrating into the microscopic domains of organisms (cells/organelles) or environmental matrices (water/soil/air, etc.) more easily than their general macroscopic conditions [94], [186], [204]. Moreover, in the improvement of internalization, it was proposed, concerning the involvement of polar facets of nanostructured ZnO to various activities, that the greater number of polar surfaces has greater vacancies of oxygen. Oxygen vacancies are recognized to intensify the production of ROS and, thus, affect the ZnO photocatalysis. At present, it was found that ZnO morphologies of greatly exposed (0001)-Zn terminated polar facets obtained the best antimicrobial and photocatalytic results [187], [188], [189].
4.2 Influence of Zn and ZnO particle size and concentration
The lack of zinc stunts development and growth, and causes system dysfunction in animals, plants, and microbial organisms. The biotic functions of zinc involve structural, catalytic, and regulatory functions. It plays an important role in regulating gene expression and maintenance of proteins’ structural integrity. Its deficiency in various parts of the world is not explicitly common in humans; however, a wide range of mild deficiency signs are noticed because of the involvement of zinc abundance in metabolic processes. The recommended and estimated (via factorial analysis) dietary allowance for adults is 11 mg/day for men and 8 mg/day for women, whereas, the maximum (tolerable) intake level for adults has been established as 40 mg/day in the USA [28], [29], [30], [33], [205].
In the nanoscale range of NPs, the zinc concentrations play important roles in various biological activities. Higher concentration and larger surface area are responsible for the applications of ZnO NPs [89], [94], [123], [190], [205]. Several investigations show that the size of the NPs is directly proportional to toxicity; furthermore, size handling is important to fabricate a greater enhanced permeation and retention (EPR) effect to enhance an intra-tumor concentration of NPs [191], [205]. Smaller-sized ZnO NPs can easily penetrate even into the cell membranes owing to their greater interfacial area, so there is an increase in antimicrobial efficacy. The dissolution of Zn2+ from ZnO NPs is considered size dependent. Padmavathy and Vijayaraghavan [125] described the generation of H2O2 to be most probably based on the surface area of ZnO. Greater surface area and high concentration of ROS can be obtained from bioactive NPs. However, most of the studies also endorse that the decrease in particle size will increase the bioactivities [162], [193], [206].
4.3 Surface defects
The molecular structure of ZnO NPs depicts surface defects and surface charges making the surface a potential reactive site. Though ZnO has simple structure and formula, it is very rich in chemistry defects [4]. These defects and charges play a vital role in bioactivities. These defects deliberately change the particle boundary properties and characteristics [26]. The spatial configuration of ZnO arranged randomly enhances the biocidal activity compared with the regularly arranged structures [192]. Some researchers [4], [105], [167] also referred to the toxicity of the nanostructured ZnO to orientation, whereas no reference was found for crystallographic orientation [195], [207].
4.4 Zinc ion (Zn2+) release
ZnO internalization is controlled by fictionalization, surface chemistry, particle size, and defects. One of the main proposed antimicrobial and photocatalytic mechanisms for ZnO NPs is the release of zinc ions in the medium [24]. The release of Zn2+ significantly affects enzyme system disruption, amino acid metabolism, and active transport inhibition. Despite this, the leaked Zn2+ in media is responsible for ZnO toxicity, and the release is size dependent. Consequently, ZnO NP toxicity can be modified by size decrease and low dissolution rate [10], [124], [196], [197], [198], [199]. Zn2+ release is affected by two main reasons: (i) the morphology of the NP, concentration, particle size, porosity and (ii) the chemistry of the media [142], [150], [200], [201], [202], [203], 205], [208], [209], [210], [211], [212], [213], [214]. Peng et al. [207] observed the release of Zn2+ ions at a higher level from spherical structures than from rod structures. Leung et al. [215] demonstrated that such characteristics are influenced by surface modifications including the liberation of Zn2+ ions and ROS generation on the NP surface.
5 Negative impacts of ZnO NPs
The widespread use of ZnO NPs raised great concerns about their occupational and biological safety [216], [217], [218]. In various spheres of nanoscience research, they were frequently found to be more toxic compared to some of the other NPs [26]. Released Zn2+ is an important contributor to this NP toxicity [204]. The mechanism of ZnO NPs toxicity involves oxidative stress, i.e. revealed by evaluating the human bronchial epithelial cells [217]. These NPs are ingested by living organisms from aquatic and terrestrial environments where they accumulate, before being eliminated. Because of their small size, NPs occur as foreign elements inside the organisms with their own physicochemical properties and, thus, the chances of interference are increased with normal physiological mechanisms of the embryos, growing animals, and adults, and it is crucial to recognize their potentially direct or indirect detrimental effects on living organisms. The ZnO NPs may also be intermingled in the diets of animals that must be considered. The presence of more than one pollutant under natural conditions could have a synergistic effect on their toxicity [218]. Moreover, they could also be toxic to algae, invertebrates, and vertebrates [219]. Mechanisms of NP interaction with living cells are unknown. However, it was established that they can bind with membranes, proteins, and DNA, and are able to produce oxidative stress [220]. Highly engineered ZnO NPs (in greater quantity) will lead to human contact and exposures (ingestion, inhalational, and dermal) [204]. They can penetrate the skin (particularly through sunscreens) resulting in possible toxicity and infections [199]. A comparative analysis of dermal penetration among various animals was performed, rating them in the order of rabbit>rat>pig>monkey>humans. Further, it was also observed that pig and rat skin were up to 4 and 9–11 times more permeable than human skin, respectively [221].
Various research has been carried out to determine the effects of ZnO NPs on plant species [150], [216], [218]. They can penetrate soil through accidental or intentional release. Some NPs are well recognized to affect crop development, yield, and accumulate in the edible parts of plants with other tissues as well. The behavior of various NPs in plants is not totally clear. However, NPs (on exposing with plant tissue) penetrate into the cell membrane and cell wall of the epidermis, cortex of the root together with a complex series of actions to enter the plant vascular bundle (xylem), and passage to the stele. The xylem acts as the most important vehicle in the circulation and translocation of NPs to the leaves. The epidermis, cortex, endodermis, cambium, and xylem accumulate more NPs than the other plant tissues. The NP uptake mechanism is normally considered as an active transport mechanism that includes several other cellular processes like recycling, signaling, and the regulation of plasma membrane [216], [218]. Toxicological studies of ZnO NPs on numerous plants such as on rye grass exhibited the presence of these NPs’ shrunk root tip and epidermis, reduced biomass, and cortical cells turned out to be extremely vacuolated and collapsed [216]. Boonyanitipong et al. [222] observed the reduction of the number of roots and stunted the length of rice seedlings (Oryza sativa L), while Raskar and Laware [222] observed the inhibition of chlorophyll biosynthesis along with the efficiency of photosynthesis in Arabidopsis. Another study by Zafar et al. [150] showed the effects of ZnO NPs on germination and shoot growth of Brassica nigra and so on.
ZnO NPs can impose serious toxicity to microbial populations (e.g. Daphnia magna, bacteria, etc.), mice, freshwater microalga, and human cells [4], [20], [104], [221], [223], [224]. The toxic nature of ZnO NPs for different bacterial systems leads to biomedical and antibacterial applications. Their exposure causes changes in cellular morphology and eventually death of the bacteria due to which, they can be used extensively in environmental remediation and as an antibacterial agent. ZnO NPs can induce modifications in microbial enzymatic activities that were observed in numerous in vitro studies. The cell viability is dependent on both the concentration of the particles and exposure time [218]. The effect of ZnO NPs is being observed to be relatively higher than that of many other metal-based NPs such as TiO2 NPs, as reflected by the lower DNA content and stronger shifts in the bacterial community composition at the same exposure concentration [218], [225].
Being very reactive, the ZnO NPs form complexes in the environment. Study of such complexes is obligatory to evaluate the potential threats [220]. These NPs have more toxicity than dissolved Zn2+, and these particles disintegrate comparatively faster, and released Zn2+ is the primary source of toxicity. Although both ZnO NPs and Zn2+ are biologically and ecologically noxious, they have different modes of action [61], [150]. Currently, the discharging of NPs into the environment, e.g. through the effluent of wastewater treatment plant (WWTP) can upsurge the exposure of the ecosystem, which is difficult to quantify. However, through modeling exertions, there may currently be the higher environmental concentrations of ZnO (0.432 μg/l in Europe, 0.3 μg/l in the USA) in the effluent of WWTP that can pose a toxicological hazard to aquatic organisms [16], [61], [226]. The industrial applications of ZnO also have serious health and environmental concerns particularly in the rubber industry where after the expiry time of the rubber products, various types of hazardous compounds are finally released into the lithosphere during their misuse for degradation [1]. The effects of excess zinc on aquatic organisms are further considered as one of the great ecological concerns [43], which can be overwhelmed by reducing zinc levels in rubber compounds [227], for example, by replacing the commonly used bulk ZnO material with granular nanoscale ZnO [1], [10].
6 Environmental fate and biological toxicity of ZnO NPs
The release of synthesized ZnO NPs into the environment and exposure to organisms are considered toxic, although it is unclear whether this toxicity is caused by such particles, dissolution to Zn2+, or some amalgamation thereof [10], [61]. Investigations were carried out to determine the relative solubility of ZnO biological toxicity studies or in matrices used for environmental fate and transport. Even the dissolution of ZnO is observed in nanopure water (5.0–7.40 mg/l of dissolved zinc, as determined by filtration), but much more dissolution was observed in the dissolved Zn concentration that exceeded 34 mg/l in a different medium. Hardwater moderately exhibits low Zn solubility, probably due to the precipitation of a zinc carbonate solid phase. Even after more than 1000 h of dissolution, the balanced circumstances according to the ZnO solubility was not observed in these matrices. Such findings recommend the exertion of a strong influence of solution chemistry on ZnO dissolution and, thus, causing a limitation on zinc solubility from the precipitation of slightly soluble solid phases [90], [204].
George et al. [226] recommended that iron doping of ZnO NPs caused reduced dissolution, thus, there is less cytotoxicity and prospect for synthesizing safer nanomaterials. However, another study investigated the effects of Fe-doped ZnO on microbial toxicity [224], [228], and it exhibited that water chemistry influenced toxicity greater than doping.
7 Conclusion
Zinc is an essential element, necessary for the function of more than 300 enzymes. This review elaborates the ephemeral outline of zinc and its multifunctional compounds as ZnO and their applications. Zinc compounds and nanostructures possess various interesting properties (piezo- and pyroelectric) including high photostability, biodegradability, biocompatibility, a wide range of UV absorption, and ZnO has different ranges of nanostructures. Furthermore, the current progresses in electrochemical bio-sensing, based on a diversity of nanostructures like nanotubes, nanowires, nanoflowers, nanopores, etc., attracted great interest in environmental and agricultural and biomedical applications. Additionally, bio-safety, easy fabrication, non-toxicity, improved analytical performance, biological compatibility, high-electron transfer rates, greater sensitivity, low cost, and having a wide range of antimicrobial activities (e.g. against foodborne pathogens) and high resistance to severe processing conditions are prominent features. The synthesis of ZnO NPs could be carried out by green, metallurgical, and chemical methods. The formation of ZnO NPs in different solvents could be modified and extended by making the fabrication techniques faster than formerly reported without significant changes in their numerous properties. ZnO nanomaterials can retain their antimicrobial activity even on incorporation with polymeric matrices, which may indicate their use in antimicrobial food packaging, thermally and mechanically resistant. The anticipated mechanism of the antimicrobial action of such particles is attributed to three leading mechanisms, i.e. releasing antimicrobial ions, microorganisms’ interaction with NPs that consequently damage the bacterial cell integrity, and ultimate ROS formation by the effect of light radiation. ZnO NPs can also be applied for the disinfection purposes of industrial wastewater treatment, such as improved inactivation of viruses and biofouling control. Beside all the applications, domestic to industrial, the toxicological impact of ZnO NPs must also be assessed to determine their positive or negative effects on food, health, and environmental safety. Controversies regarding toxicological impact are gaining attention so there is a requirement to formulate legislations. Though zinc and its NPs are considered safer and falls in the category of micro-essential elements, still, the NPs performing their function as the zinc individually is still questionable. For example, ZnO NPs may act as a smart weapon toward multidrug-resistant microbes and a talented substitutional tactic to antibiotics. The in vivo experiments show that ZnO NP exposure via inhalation poses the most significant hazard compared to the other itineraries of exposure such as on the skin due to limited uptake and the absence of local effects. There is a dire need for risk assessment by the dose-response relationships. Most of the research works propose that such influences are due to Zn2+ causing NP dissolution outside the cell. Nano-biotechnologists recommend that cells take up the NPs, after which, dissolution takes place inside the cell. Beside all the controversies, we are thankful to the scientific community who devoted themselves for the betterment of humankind in the field of nanotechnology. Still, there are a lot of areas that need exploration, and we hope that material science will flourish in the field of biological science for sustainable health, agriculture, and environment.
About the authors
Attarad Ali did his BS in Environmental Sciences in 2011 at the University of Punjab, Lahore, Pakistan. He completed his MPhil degree in Environmental Sciences in 2013 at Quaid-i-Azam University (QAU) Islamabad, Pakistan. He is pursuing his PhD degree (2014–2018) at QAU. During his PhD, he also worked at the University of Florida, USA, under the HEC sponsored IRSIP (scholarship) program. His area of interest is nanoparticle synthesis (doped with polymeric materials), characterization, and their applications in biomedical and environmental domains.
Abdul Rehman Phull obtained his Master’s degree in Biochemistry/Molecular Biology from Quaid-i-Azam University, Islamabad, Pakistan, and his PhD in Biology from Kongju National University, Republic of Korea. Currently, Dr. Abdul Rehman Phull is working as an Assistant Professor of Biochemistry at Shah Abdul Latif University, Khairpur, Sindh, Pakistan. His research interests include cell biology, enzymology, therapeutic applications of nano-materials, and natural and synthetic molecules.
Muhammad Zia obtained his PhD degree in Biotechnology from Quaid-i-Azam University, Islamabad Pakistan. His current focus area of research is nanotechnology, nanobiotechnology, and synthesis, characterization, and applications of nanomaterials.
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