Elemental Zinc to Zinc Nanoparticles: Is Zno Nps Crucial for Life?
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Nanotechnol Rev 2018; 7(5): 413–441 Review Attarad Ali, Abdul-Rehman Phull and Muhammad Zia* Elemental zinc to zinc nanoparticles: is ZnO NPs crucial for life? Synthesis, toxicological, and environmental concerns https://doi.org/10.1515/ntrev-2018-0067 Received June 21, 2018; accepted July 21, 2018; previously published 1 Introduction online September 18, 2018 Over the last few decades, nanotechnology has wit- Abstract: The semi-conducting material zinc is one of nessed an incredible development in the fastest-grow- the essential trace elements for humans, is a co-factor of ing domain of science and technology because metal more than 300 enzymes, and plays an important role in oxide nanoparticles (NPs) are progressively being used maintaining vital cellular functions. Deficiency of zinc in many industrial applications. Among the different may lead to cancer initiation; however, a high concentra- metals, zinc attracts more attention because of its strong tion also has toxic effects that might be life threatening. reducing potential, moderate reactivity, and having five The toxicity can be addressed by the disequilibrium of stable isotopes. Among the different zinc-based nano- zinc-mediated proteins and oxidative stress that produce structures like sulfide, ferrite, phosphide, selenide, nascent oxygen, hydroxyl radicals, and other reactive oxy- and telluride, zinc oxide (ZnO) is most attractive due to gen species. Zinc-based nanoparticles (NPs) are among its vast applicability, being eco-friendly, and its diverse the most important and multifunctional compounds. physiochemical properties. A general description of zinc, Zinc oxide (ZnO) NPs exhibit attractive antimicrobial and its importance and NPs, are described in Figure 1. ZnO photocatalytic properties due to the smaller particle size NPs have been reported as being of different shapes, and increased particle surface reactivity. Further, these sizes, and structures (Figure 2). They have also been are more biocompatible compared to other metallic NPs, reported in one-dimensional (1D), 2D, and 3D structures. easily synthesizable, and have high selectivity, enhanced The 1D structures constitute the major group, containing cytotoxicity, and are a promising anticancer agent. How- nanorods, tubes, needles, wires, ribbons, belts, combs, ever, some of the pertinent concerns regarding nano-zinc helixes, springs, and rings. The 2D structures of ZnO still needs to be clarified. Current research also demon- are found as nanosheets/nanoplates and nanopellets, strates their usage in wastewater treatment, textile, med- whereas flower, snowflakes, dandelion, and coniferous icine, etc. This review covers the importance of zinc for urchin-like, etc., are examples of 3D structures [1]. living systems and its NPs, with more emphasis on ZnO ZnO is a bio-safe material that possesses photocataly- NPs. A comprehensive overview of ZnO NPs, their synthe- sis and photo-oxidizing influences on chemical and biolog- sis, characterization techniques, crystal structure, proper- ical species, respectively [2–4]. Bulk ZnO, compared to ZnO ties, and brief industrial applications are presented. NPs, is a bio-safe material. Indeed, various studies have already revealed the potential toxicity of ZnO NPs based on Keywords: biologics; cytotoxicity; nanoparticles (NPs); their size, shape, and concentration, etc. [5, 6]. Although photocatalysis; zinc oxide (ZnO). 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 nano- *Corresponding author: Muhammad Zia, Department of materials is, thus far, not fully understood [7]. Besides, Biotechnology, Quaid-i-Azam University, Islamabad 45320, ZnO NPs are also reported to have a good biocompatibility Pakistan, e-mail: [email protected] with human cells, and the ZnO has been documented as a Attarad Ali: Department of Biotechnology, Quaid-i-Azam University, Islamabad 45320, Pakistan safe material by the FDA [8, 9], although the intrinsic cyto- Abdul-Rehman Phull: Department of Biochemistry, Shah Abdul Latif toxicity of ZnO NPs against certain human cell lines has University, Khairpur, Sindh 66020, Pakistan raised some apprehension concerning potential health 414 A. Ali et al.: Elemental zinc to zinc nanoparticles Figure 1: General description of zinc metal and zinc NPs concerning synthesis, applications, and toxicity. A B C D E F G H 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 [1–3]. A. Ali et al.: Elemental zinc to zinc nanoparticles 415 hazards [10]. On the other hand, their inherent cytotoxic- state of +2 and having an atomic number 30, and is bluish ity indicates a necessary quality against pathogenic cells if white in color. It is hard and brittle and a good conductor the properties are precisely tailored [7, 11]. Certainly, recent of electricity. It has low melting and boiling points and studies have revealed that ZnO NPs have cytotoxic effects is dissolvable in both acids and alkalis. Zinc is ecologi- toward cancerous cells, while at the same concentration, cally pervasive and present in the earth’s crust at about ZnO NPs have negligible effects on normal cells, leading 70 mg/1000 mg [21]. Likewise, according to proportion, it to speculation that they can be used in cancer treatment. is found abundantly (0.02%) in the earth’s crust carrying However, for more widespread applicability, ZnO NPs an elemental ranking of 24 in the periodic table. It has are currently being used in modified forms, for example, five stable isotopes and never occurs as a free element on through appropriate surface coatings with complementary earth [22]. The zinc metal remained unknown for a long polymeric materials [12, 13] or green synthesis approaches time because it boils or vaporizes easily from its compos- (capping with phytochemicals) and bio-inspired manufac- ites on heating. In nature, some of the prominent ores of turing [14, 15] that could boost their biocompatibility. zinc are (1) zincite or zinc oxide (ZnO); (2) sphalerite or Biodegradability and low toxicity are the most impor- zinc blende or zinc sulfide (ZnS); (3) smithsonite or zinc tant features of these nanomaterials. Zinc (Zn) is a crucial spar or zinc carbonate (ZnCO3); (4) willemite or zinc sili- 2+ trace element for mature individuals (Zn ~10 mg/day cate (ZnSiO3), and (5) franklinite [(Zn, Mn, Fe)O(Fe,Mn2) is allowed), and it is involved in numerous parameters O3] [23]. of metabolism. The surface of ZnO is chemically rich The zinc metal is now prominently used in galvaniz- with -OH groups that can easily be modified by several ing other metals by laying down a thin layer of zinc on the surface- adorning molecules. ZnO NPs have various dis- surface of a second metal because it does not corrode as tinctive properties such as semiconducting, deodorizing, easily as iron or other metals. Furthermore, the zinc metal photocatalytic, piezoelectric, pyroelectric, and required sublimates easily during the formation process of zinc biocompatibility [16]. Therefore, they have gained many from its ore. applications in industry, for instance as transducers and Zinc ions are absorbed deeply into soils at pH 5 or sensors, in ceramic processing, and in medical care [10, 17]. more and are likely to have slight mobility [24]. In natural They are extensively used in several products, such as wall waters, several forms of zinc can be found like metal- paints, toothpaste, sunscreens, beauty agents, textiles, and organic complexes, hydrated ions, or metal-inorganic in building materials. Because of their distinctive proper- complexes. Hydrated Zn+ may be hydrolyzed to form ZnO ties, i.e. reduced size and high surface area ratio, the bio- or Zn(OH)2. However, ZnS may be formed in anaerobic logical safety of NPs has received global attention from the environments. In aquatic organisms the bio-concentra- scientific world [10]. ZnO NPs can easily enter water when tion factor (BCF) values of zinc is 1000 and 2000 for fresh- they are immersed in water through bathing, washing, and water fish and marine fish, respectively [25]. 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 environ- 1.2 Zinc in the human body mental risk assessment of NPs, although ZnO NPs have been reported to lack excess toxicity for various organisms Zinc is organically known as a helper molecule that assists [18–20]. about 300 enzymes, which are involved in various bodily The objective of this article is to review the semi- functions. After iron, zinc is the most vital and richest trace conducting material zinc and its chemical compounds element in the human body and has been estimated to be and nanostructures focusing on ZnO as biologics. The 30 mm (2–4 g) of the total body zinc content. The maximum potential applications of ZnO in industry, agriculture, the quantity of zinc occurs in parts of the eye, liver, kidney, environment, and toxicological threats as related to envi- bones, muscles, prostate, and brain [26]. Weak vision like ronmental release and possible effects on plants, humans, cloudy cataracts and poor night vision have been con- and animals are reviewed. nected to zinc deficiency. A low zinc concentration in puts the human body at risk for alopecia (hair loss from eye- lashes and eyebrows), greater vulnerability to infection, 1.1 Zinc and mental lethargy. Almost 15 mg/day (zinc) of trace quan- tities is required in the human diet to fulfill the demands of Zinc, also called as spelter, is a trace mineral. It is a dia- all body fluids and tissues. Zinc also plays an essential role magnetic fairly active transition metal with an oxidation in the immune system along with the maintenance and 416 A. Ali et al.: Elemental zinc to zinc nanoparticles integrity of cellular components (molecules and or mem- development during childhood, adolescence, and in brane stability, etc.) [22].