Historical perspectiveZinc oxide nanoparticles: Synthesis, antiseptic activity and toxicity mechanism
Graphical abstract
Introduction
Nanotechnology has attracted immense interest during the last few decades. Since 1931, when Knoll and Ruska invented the electron microscope, and the famous lecture (“There's Plenty of Room at the Bottom”) by Richard P. Feynman in 1959 [1], observation and manipulation of nanoscale materials have become possible and, what is more, nanotechnology has provided numerous innovative solutions in the field of biomedicine, materials science, optics and electronics [2].
Nanoparticles (NPs) are particles at the atomic level (1–100 nm), where at least one dimension of NPs should be smaller than 1 μm [3]. Nanoparticles can be made of a broad range of materials, so they are classified as 1) metallic nanoparticles (e.g. Au, Ag, Cu, Fe, Zn); 2) metal and non-metal oxides (e.g. FeO, VO, AlO, ZnO); 3) semiconductor nanoparticles (ZnS, CdSе, ZnSe, CdS, etc.) and 4) carbon nanoparticles [4], [5], [6]. Based on the structure and shape of nanoparticles, we can distinguish quantum dots [7], nanotubes, nanowires, nanorods and nanobelts [8], [9], [10].
In comparison to bulk materials, nanoparticles show different or improved properties such as size, distribution and morphology [11]. Quantum effect, surface and its heterogeneity (e.g. (bio)capping, coating) play an important role in chemical reactivity and in mechanical, optical, electric and even magnetic properties of nanomaterials. Specific surface heterogeneity and its area are also relevant with regard to other related properties such as antimicrobial activity [4], [5], [11].
One of the most interesting and promising metallic nanomaterials is zinc (Zn) and its oxide (ZnO). Zinc is a fairly active element and simultaneously a strong reducing agent; according to its reduction potential it can easily oxidize, forming zinc oxide [12], which is very helpful in preparation of zinc oxide nanoparticles. Zinc plays an important role in human organisms as one of the most essential microelements [13]. It is found in all body tissues, i.e. in muscle and bone (85% of the whole body zinc content), in the skin (11%) and in all the other tissues; it is intracellular, located mainly in the nucleus, cytoplasm and cell membrane [14]. Zinc has been shown to be crucial for proper functioning of a large number of macromolecules and enzymes, where it has both catalytic (as an active center of enzymes) and structural roles. Zinc finger motifs provide a unique scaffolding which makes it possible for protein subdomains to interact with either DNA or other proteins [15]. Zinc is also critical for the functioning of metalloproteins. Although zinc lacks redox activity and is regarded as relatively non-toxic, there is an increasing amount of evidence that free zinc ions may cause e.g. degradation of neurons [16]. Therefore, in order to eliminate its cytotoxic effect, binding zinc cations with bioactive ligands (e.g. proteins) [17] and synthesis of zinc oxide nanoparticles are performed. Zinc oxide holds unique optical, chemical sensing, semiconducting, electric conductivity, and piezoelectric properties [18]. It is characterized by a direct wide band gap (3.3 eV) in the near-UV spectrum, high excitonic binding energy (60 meV) at room temperature [19], [20], [21], and natural n-type electrical conductivity [21], [22]. Zinc oxide exists in two main forms - hexagonal wurtzite and cubic zinc blende. The wurtzite structure is most stable at ambient conditions and thus most common [23]. Wurtzite zinc oxide is a hexagonal crystal with lattice parameters a = 0.325 nm and c = 0.521 nm and with three primary growth directions – {1010}, {1120} and {0001} [19], [24], [25]. Each tetrahedral Zn atom is surrounded by four oxygen atoms and vice versa (Fig. 1) [26]. Nanostructures of wurtzite ZnO can present different growth morphology including nanocombs, nanorings, nanohelixes, nanobelts, nanowires and nanocages [19], [26], [27], [28]. Those zinc oxide nanostructures may be formed using different chemical, physical and biological methods such as thermal evaporation technique, chemical reduction, and synthesis with plant extracts [29], [30], [31].
With a variety of morphological forms and properties, nanostructured ZnO can be used in many ways. Due to unique semiconducting, optical, and piezoelectric properties [18], zinc oxide has been studied with a view to using it as nano-electronic and nano-optical devices, energy storage or nanosensors [33], [34], [35], [36], [37]. The optical properties of ZnO nanomaterials can be tuned by doping them with appropriate elements, and the nanomaterials are used for bioimaging [38], [39], [40]; they are also good nanoplatforms for drug or gene delivery [41], [42]. What is more, ZnO can be found in sunscreens where it plays the role of an UV-ray blocker [43], [44]. Applications of ZnO nanomaterials are summarized in Table 1.
The objective of this paper is to review the methods of synthesizing Zn/ZnO nanoparticles, the methods of analysis of the obtained nanomaterials, their antimicrobial and antitumor properties and the cytotoxicity of the reviewed nanoparticles.
Section snippets
Methods of zinc oxide nanomaterials synthesis
There are two general types of strategies used in synthesis of nanomaterials – the bottom-up and top-down approach [32]. The first one uses atoms and molecules to create nanostructures which can be obtained by chemical synthesis, biological methods or controlled deposition and growth. Conversely, the top-down approach refers to slicing or cutting of bulk material to get nano-sized particles [52]. Both methods are used to prepare nanostructures on a large, industrial scale. It is obvious that
Physicochemical characterization of nanoparticles
Various techniques are used to characterize nanoparticles and predict their ultimate properties including size, shape, surface charge and modifications, purity of the sample or antibacterial activity. Each of them often needs separate and precise tools which are at the same time simple and inexpensive. Some of the analytical methods used for NP characterization can be divided into static and dynamic approaches; their descriptions can be found below.
Antimicrobial activity
Investigations on antibacterial nanoagents are very important in medicine, food, textiles, packaging and construction industries [172], [173]. In comparison with traditionally used compounds [172], [174], metal oxide nanoparticles are more stable in extreme conditions, exhibit antimicrobial activity at low concentrations and are considered non-toxic for humans [174], [175], [176], [177], [178]. Among metal oxide powders, ZnO is very strong antibacterial agent [179]. Zinc oxide nanoparticles
Conclusion
Considering their unique properties, the applications of zinc oxide nanoparticles is on the increase not only in industry but also in medicine as antimicrobial and antitumor agents. This review has mainly focused on ZnO NPs synthesis including physical, chemical and biological methods, the antimicrobial activity of nanoparticles and their toxic effects against both cancer and normal cells.
The synthesis route determines the later properties of the nanomaterial, so selection of the best
Acknowledgements
This work was supported by Opus 11 No. 2016/21/B/ST4/02130 (2017–2020) from the National Science Centre, Poland, by Plantarum No. BIOSTRATEG2/298205/9/NCBR/2016 (2016–2019) from the National Center for Research and Development, Poland and by Foundation for Polish Science “START” No. 068.2017; subsidy (2017/2018).
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