Biogenic Synthesis of Zinc Nanoparticles, Their Applications, and Toxicity Prospects

Biogenic Synthesis of Zinc Oxide Nanoparticles

The conventional methods of synthesizing ZnONPs include physical methods such as ultrasonification, laser ablation, irradiation, and chemical methods such as microwave, pyrolysis, solvothermal, chemical reduction, and photochemical (Jameel et al., 2020). While the former requires highly sophisticated instruments besides use of highly toxic chemicals and high energy consumption, leading to the rising cost of process, drawbacks of the latter are also very well known. Non-biodegradable nature and toxicity of the chemicals restrict the application of chemically synthesized NPs, especially in the biomedical area (Patel et al., 2015). Both physical and chemical methods imply more number of steps (Salem and Fouda, 2021). These limitations have paved the way for large-scale use of biogenic synthesis that is green, safe, more economical, and requires lesser number of steps (Khandel et al., 2018).

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Biogenic synthesis of metallic NPs is achieved through plants and microorganisms (Mukunthan and Balaji, 2012). The actual mechanism of synthesizing metallic nanoparticles remains the same in both. Neither it requires high-pressure/-temperature conditions nor use of toxic/hazardous chemicals. Other than being convenient, there is also no requirement of adding external reducing, stabilizing, and capping agents. Biogenic like chemical synthesis is facilitated by the bottom-up approach that involves assembling of the atoms into nuclei followed by growth into NPs. Top-down approach, on the other side, is driven by rupturing of bulk materials into fine particles as is the case with physical techniques (Salem and Fouda, 2021). In plant-mediated biogenic synthesis, the aqueous solution of metal salts consisting of metal ions is reduced by means of reducing agents that are present in the plant extract. The atoms thus obtained aggregate and form small clusters that further grow into particles (Ahmad et al., 2010; Bankar et al., 2010). Roots, shoots, flowers, seeds, fruits, bark, stems, and leaves act as different sources of biomass and secondary metabolites such as flavonoids, alkaloids, saponins, steroids, and tannins act as reducing and stabilizing agents (Abdel-Aziz et al., 2014). Plant extracts of Lemna minor, Parthenium hysterophorus, Satureja sahendica Bornm, and Carissa spinarum L. have been recently employed to synthesize ZnONPs (Del Buono et al., 2021; Umavathi et al., 2021; Chegini et al., 2022; Saka et al., 2022). Furthermore, agro wastes like coconut shell, red peanut skin extract, banana peel, and bagasse extract are being used to prepare a variety of metallic nanoparticles (Sinsinwar et al., 2018; Pan et al., 2020; Ruangtong et al., 2020; Ishak et al., 2021). However, this results in the production of polydispersed nanoparticles owing to the involvement of numerous phytochemicals that also can alter with the seasonal changes (Singh et al., 2013; Ovais et al., 2016).

Microbe-mediated synthesis involves reduction of metal ions into metal NPs with enzymes and other biomolecule compounds of the microbes, viz., bacteria, fungi, yeast, and algae (Mohd Yusof et al., 2021). Herein NPs are formed due to oxidation/reduction (O/R) of metallic ions by enzymes, proteins, and sugars (Figure 2). Each microbe interrelates with metallic ions using several pathways that, along with environmental conditions like temperature and pH, affects various characteristics of NPs such as size, shape, and morphology (Prabhu and Poulose, 2012; Makarov et al., 2014). Depending upon the type of microbe, NPs can be formed either intracellularly or extracellularly (Mohamed et al., 2019). Cofactors like NADH and NADPH-dependent enzymes, along with several compounds like naphthoquinones, anthraquinones, and hydroquinones, have been found to play a vital role in the reduction and production of metallic nanoparticles (Patra et al., 2014; Bose and Chatterjee, 2016). Intracellular mode presumes the interaction between the intracellular enzymes of microbe and the positively charged groups that leads to gripping of metal ions from the medium followed by its reduction into the cell (Dauthal and Mukhopadhyay, 2016). In microbe-mediated synthesis, it is possible to control and manipulate size and shape of NPs so as to produce the desired ones suitable for a particular application. Bacteria, entitled as the factory of NPs, are preferred for biogenic synthesis because of easy purification and requirement of mild conditions and higher yield. Filamentous bacteria, actinomycetes, have a unique advantage of secreting a wider range of secondary metabolites helping to synthesize NPs with diverse surface and size characteristics (Salem and Fouda, 2021). Attributes like highly efficient metabolites to fabricate various nanoparticles, ability to secrete well-built amounts of proteins, and easy to be traded in laboratory make fungi also to be used widely in the field of biogenic synthesis (Dhillon et al., 2012; Fouda et al., 2018; Mohamed et al., 2019). Also, fungal-mediated NPs are biologically more active compared to the other microorganisms (Yusof et al., 2019). Table 1 sums up recent studies for synthesizing of ZnONPs from different microorganisms.

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