Phyto-Nanofabrication: Plant-Mediated Synthesis of Metal and Metal Oxide Nanoparticles

Introduction And Background

Utilization of metal and metal oxide nanoparticles (NPs) in various areas such as cosmetic, pharmaceutical, health care, drug delivery, food processing and packaging, photonics, surface coatings, biosensors, agricultural sectors, fuel cells, piezoelectric components, microelectronic fabrication, and toxic elements elimination and many other environmental applications has become a foreseeable tendency over the last decade (Hussain et al., 2016; Iravani, 2011; M. Rai & Ingle, 2012; Vijayaraghavan & Ashokkumar, 2017). This is due to the fact that they possess nano-scale structure with a high proportion of surface area as well as exhibit enhanced catalytic, morphological, physiochemical, electrical, magnetic, optical, mechanical, and biological characteristics, which differ from those of bulk materials (Ishak et al., 2019; Kamat, 2002). Because of possessing these unique and fascinating properties, metal and metal oxide nanoparticles have become the focus of intensive studies over the last few years, and many studies have been accompanied synthesizing these nanoparticles applying different approaches.

Two routes are mainly followed for the production of nanoparticles, i.e., the ‘top-down’ approach where NPs are formed by size reduction and decomposing of larger macro particles into fine particles (Ahmed et al., 2016), and ‘bottom-up’ approach that involves the assembly of small substances to synthesize nanoparticles through the oxidation of metallic ions into zero-valent metal particles (Zheng et al., 2016). Top-down process consists of physical and chemical methods whereas bottom-up approach can be achieved through wet chemical methods (Figure 1).

Nevertheless, these approaches require a significant amount of harmful and toxic ingredients, prolonged reaction times, enormous consumption of energy, expensive instrumentations, and also produce non-eco-friendly toxic by-products, which increase the cost of the experiment as well as intensify environmental toxicity (Shah et al., 2015; Soundarrajan et al., 2012; B. Zheng et al., 2013). Therefore, green chemistry by using environment-friendly materials for nanoparticle production has become more acceptable and has been established as a superior, advantageous, and alternative synthesis approach over other conventional techniques (Chandra et al., 2020). Green synthesis of metallic nanoparticles is in the category of bottom-up approach, and the nanoparticle formation mechanisms are similar to the chemical reduction process (reduction of metal ions to nanoparticles). Major emphases of this approach are i) utilizing harmless solvents/reagents for synthesis, ii) modeling, adapting, and development of an inexpensive, facile, and energy-saving synthesis protocol, iii) involving diverse biological entities available in natural surroundings, and iv) fabrications of nontoxic and biocompatible nanoparticles for further applications (Jahan et al., 2019; Malik et al., 2014). To evaluate and differentiate the biocompatibility of synthesized silver nanoparticles, comparisons were conducted using both chemical reduction (conventional wet-chemistry) and plant-mediated green synthesis of AgNPs, and comparatively lower phytotoxicity and cytotoxicity were observed in AgNPs from green synthesis (Kummara et al., 2016).