*4.2. Antioxidant Activity*

In the human body, excessive reactive free radicals are formed from various sources, such as low diet, mental stress, smoking, and other ailments [95]. Metal NPs exhibited profound antioxidant activities in both intracellular and extracellular environments, as summarized in Figure 4.

**Figure 4.** Schematic diagram of the antioxidant mechanism.

α α β β In the year 2012, Adebayo et al. synthesized metabolite derived from *P. pulmonarius* extract and evaluated their radical scavenging ability via α, α-diphenyl-β-picrylhydrazyl (DPPH) radical scavenging assay and the β-carotene-linoleate model method. The metabolite derived from *P. pulmonarius* extract showed dose-dependent radical scavenging activity. It was found that the existence of glutathione, ascorbic acid, cysteine, tocopherol, polyhydroxy compounds, and aromatic

α

β

amines in metabolite reduces and decolorizes the violet color of DPPH via hydrogen transferability. Authors claimed that, at a concentration of 2 mg/mL, metabolite showed butylated hydroxyanisole (BHA) (75%), LAU 09 (80%), and α-tocopherol (90%) of inhibition, which attributes to of the presence of phenolic compounds in the extract [14]. A few years later, in 2017, Madhanraj et al. synthesized gold (Au) and silver (Ag) nanoparticles derived from edible mushroom (basidiomycetes) and studied their antioxidant activity via various radical scavenging assays. Both the prepared NPs (Au & Ag) showed significant antioxidant activity in a cell-free system [39]. Acay and Baran (2020), synthesized *P. eryngii* AgNPs and evaluated their radical scavenging ability via DPPH, chelation of ferrous ions reducing power, and the β-carotene-linoleate model method, and found that, at a concentration of 10 mg/mL, antioxidant activities were 85%, 82%, and 77%, respectively [96]. Zinc plays a role in protecting cells from oxidative stress and acts as an antioxidant. The ZnONPs derived with the help of *P. djamor* possess strong antioxidant properties (DPPH 59%, H2O<sup>2</sup> 59.65%, and ABTS 59.30%), with IC<sup>50</sup> values of 428.35 µg/mL, 417.22 lg/mL, and 500 lg/mL), respectively [43].

Two possible primary mechanisms for the antioxidant activity are; (i) hydrogen atom transfer, and (ii) single electron transfer [92]. Excessive free radicals could be neutralized or terminated via donating a hydrogen atom that includes total oxyradical scavenging capacity assay, inhibition of induced low-density lipoprotein oxidation, oxygen radical absorbance capability, and radical-trapping antioxidant parameters [78]. On the other hand, the single-electron transfer involves the reduction of compounds, such as radicals, metals and carbonyls by transferring one electron, including change in the color when the compound is reduced, such as Ferric Reducing Antioxidant Potential (FRAP), 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) [92]. However, in the intracellular level, metal NPs enter inside the cells via endocytosis and decrease the ROS levels generated by any probe; for example, 2, 7′ -dichlorodihydrofluorescein diacetate (DCFDA) [92].
