*6.1. Silver Nanoparticles (AgNPs)*

The silver nanoparticles (AgNPs), show a substantial biochemical and catalytic activity, which can be attributed to their significantly large surface area, as compared with other particles with similar chemical structures [134]. The AgNPs synthesis occurs via two steps, at first, Ag<sup>+</sup> ions are reduced to Ag◦, followed by the agglomeration of colloidal silver nanoparticles, to form the oligomeric clusters which are finally stabilized [134]. Biological catalysts (enzymes) are required to reduce the Ag<sup>+</sup> ions, and the production of AgNPs with characteristic antioxidant properties can be achieved with a variety of plant extracts, as shown in Table 3. Patra et al. (2016) extracted the aqueous portion of watermelon rind (WRA), and used it to synthesize the AgNPs under light-exposed conditions at room temperature [92].

In recent studies, actinobacteria have been identified from various ecosystems that are potential natural producers of AgNPs and the whole-cell biomass and cell free extract of *Streptomyces naganishii* MA7 and *Streptomyces griseorubens* AU2, respectively, were used to synthesize silver nanoparticles with potential antioxidant properties [108,111]. On the other hand, fungal species like *Aspergillus versicolor* ENT7, *Cladosporium cladosporioides* and *Pestalotiopsis microspore* have also been used for the production of antioxidant functionalized AgNPs [117,119,120]. Various researchers have also reported *Ganoderma lucidium* as a potential source of antioxidant functionalized AgNPs [126,128,129]. Venkatesan et al. (2016) prepared the extracts of *Ecklonia cava,* a marine alga known to be a reservoir of phenolic compounds that can act as capping and reducing agents, and used it for the synthesis of silver nanoparticles [59]. Several lichens have also been identified with potential antioxidant properties, and extracts of *Parmeliopsis ambigua*, *Punctelia subrudecta, Evernia mesomorpha,* and *Xanthoparmelia plitti* were used to synthesize AgNPs [132].

The AgNPs synthesized using the leaves extract of *Clerodendrum phlomidis,* are found to have ferric reducing power of 1.63 AU, which is higher than the leaves extract used alone [87]. Whereas, on comparing the IC50 value of the extract (i.e., 1920 μg/mL), the AgNPs synthesized using the leaves extract of *Clerodendrum phlomidis* showed higher scavenging activity as confirmed by IC50 value (i.e.,55.86 μg/mL). Furthermore, the DPPH radical scavenging activity of AgNPs was found to be dose-dependent, showing the maximum inhibition (85.74%) greater than that of the extract alone. The AgNPs IC50 value (9.12 μg/mL) was found to be significantly lower, as compared to the extract (IC50 value 388.4 μg/mL) and standard ferulic acid (182.8 μg/mL), which confirmed the AgNPs with high antioxidant potential. In 2019, Das and his colleagues reported that AgNPs synthesized using *Morus alba* leaves extract increased the DPPH scavenging activity to 47.81% against the ~56% of the activity shown by standard ascorbic acid at the same concentration. However, plant extract mediated AgNPs showed 95.08% ABTS<sup>+</sup> scavenging activity, which is the equivalent to that which was shown by the BTH standard i.e., 95.51% at 100μg/mL, therefore indicating its strong scavenging potential. On the other hand, for nitric oxide scavenging activity, AgNPs displayed 64.04% scavenging, as compared to 45.72% and 88.62% in plant extract and gallic acid standard, respectively, at 100 μg/mL. Similarly, AgNPs at 100μg/mL concentration have shown a significant superoxide scavenging activity of 81.92%, as compared to the 85.35% present in the tocopherol standard [90].

### *6.2. Gold Nanoparticles (AuNPs)*

Gold nanoparticles (AuNPs) have attained significant attention, owing to their physical characteristics (size and shape), optical properties and biocompatibility [134]. Gold nanoparticles of varied sizes and diverse morphologies have been employed in the medical sector for various purposes, such as the detection of tumors, and as a carrier for drugs etc. (e.g., Paclitaxel) [134]. Antioxidant AuNPs are mostly derived from extracts of plant parts like leaves and fruits, as shown in Table 3. Markus et al. (2016) reported the identification of *Lactobacillus kimchicus* DCY51T, a novel probiotic from Korean kimchi, using an intracellular membrane-bound approach, and synthesized antioxidant functionalized AuNPs from these bacteria [107]. It was observed that the AuNPs formed a capping layer which comprised of several amino acids, while the presence of surface-bound proteins rendered

them harmless to cancerous cell lines i.e., human colorectal adenocarcinoma (HT29) and murine macrophage (RAW264.7). As compared with gold salts, biological produced gold nanoparticles were found to be better scavengers of free radicals, especially DPPH. A food inhabiting *Enterococcus* species was identified and used for the production of gold nanoparticles, and was further confirmed through techniques like Fourier transform infrared spectroscopy (FT-IR), UV-vis absorption spectroscopy and transmission electron microscopy (TEM) [109].

Veeraapandian et al. (2012) utilized extracellular proteins of *Escherichia coli* to synthesize gold nanoparticles of anisotropic nature [110]. The size and shape of AuNPs are greatly influenced by the quantity of protein, as extracellular proteins significantly increases the shelf life of nanoparticles by acting as a capping agent, and thereby conferring them with stability. Manjunath et al. (2017) in their study, used a fungal endophyte, *Cladosporium cladosporioides*, obtained from *Sargassum wightii*, seaweed, to synthesize gold nanoparticles [118]. The process of reduction of gold metal salts into gold nanoparticles involved the use of phenolic compounds, and was found to be mediated through the activity of NADPH-dependent reductase enzyme. Lee et al. (2015) used the extracts of a mushroom species, *Inonotus obliquus*, to develop AuNPs without application of deleterious agents [127]. In another study, AuNPs were synthesized from the extracts of *Gracilaria corticate,* a red alga inhabiting in the marine environment and using it as a reducing agent [130]. Sharma et al. (2014) used the desiccated biomass of *Lemanea fluviatilis* to reduce and stabilize the AuNPs [131]. Debnath et al. (2016) developed the gold nanoparticles without the use of extraneous stabilizing and reducing agents by using desiccated biomass of lichens inhabiting in the alpine region of Eastern Himalayas (India) at high altitudes [133]. The gold nanoparticles obtained from lichen *Acroscyphus* sp. comprised of prismatic and multiply twinned quasi-spherical shapes, while those synthesized from *Sticta* sp. were simply multiply twinned and, both of these exhibited antioxidant activity.
