## 3 Results and Discussion
### Antioxydant Activity of AuNPs  
Oxidative stress is associated with many diseases, including cancer, high blood pressure, diabetes, and atherosclerosis. Interestingly, the oxidation of substrates can be slowed or prevented by low concentrations of antioxidants (organic chemical compounds) [40]. Metal nanoparticles have been reported as an efficient antioxidant against DPPH (prevalent nitrogen-centered free radical) [6]. Accordingly, a DPPH assay was used herein to examine the prospective properties of _Z. officinal_-derived AuNPs as antioxidants. According to the results summarized in Figure 6, the DPPH scavenging activity of AuNPs has values significantly higher than the levels recorded for standard ascorbic acid (\(p\leq 0.05\)). Moreover, the free radical-scavenging potential of the phytoderived AuNPs is dose-dependent; antioxidant activity increases with concentration. At high concentrations (i.e., 1000 ug mL\({}^{-1}\)), synthesized AuNPs displayed scavenging activity with an efficiency of 87.6 \(\pm\) 0.5% compared to ascorbic acid as a control (which showed scavenging activity percentages of 97.3 \(\pm\) 0.2%, Figure 6). Analysis of variance clearly shows that the scavenging activity decreases with reduced concentration. For instance, the scavenging activity percentages were recorded at values of 82.5 \(\pm\) 1.4, 77.2 \(\pm\) 2.2, 62.7 \(\pm\) 2.9, and 31.8 \(\pm\) 3.1% at concentrations of 500, 250, 62.5, and 1.9 ug mL\({}^{-1}\) respectively, as compared to a control, with percentages of 94.1 \(\pm\) 0.2, 87.8 \(\pm\) 0.1, 75.1 \(\pm\) 0.2, and 37.5 \(\pm\) 1.3% at the same concentration for ascorbic acid. The IC\({}_{50}\) values for phytosynthesized AuNPs that correspond to a scavenging efficiency of 50% were calculated. Data analysis shows that the IC\({}_{50}\) value for AuNPs was close to 16 ug mL\({}^{-1}\), compared with a value of 12.8 ug mL\({}^{-1}\) for ascorbic acid. In agreement with our results, _Sargassum longifolium_-mediated AuNPs displayed considerable antioxidant activity against H2O2 (25 ug mL\({}^{-1}\)) and DPPH free radicals (20-30 ug mL\({}^{-1}\)) [44]. Similarly, AuNPs prepared from plant extract (_Physalis minima_) showed efficient scavenging activity against DPPH in a dose-dependent manner; this activity was attributed to their catalytic properties and the large surface area of AuNPs, which enhances their antioxidant capacity [25]. It was proposed that the antioxidant activity of biologically manufactured AuNPs may be due to their ability to transfer hydrogen or an electron (at the atomic level) to the free radical (DPPH\(\bullet\)) and convert it to stable DPPH-H [97]. Similarly, the phytoengineered AuNPs from the aqueous extract of _Acalypha indica_ showed efficient DPPH scavenging potency, and the IC\({}_{50}\) value was evaluated as 16.25 ug mL\({}^{-1}\)[98], which is consistent with the current study. Apart from the contribution of AuNPs, the presence of plant secondary metabolites, such as sesquiterpenes, phenolic, and flavonoid compounds (which play a role in the synthesis and the capping of AuNPs), may also assist the antioxidant activity of nanoparticles [99]. Gold nanoparticles have been reported to have a cytotoxic effect through induction of oxidative stress [100,101]. However, in this study, green synthesized AuNPs showed high antioxidant activity. This difference is associated with the role of mitochondria in the induced production of free radicals. The oxidative stress production by the action of nanoparticles is influenced by many factors, including non-cellular factors, such as the presence of metals, the size and surface area of the nanoparticles, concentrations, and method of synthesis, in addition to cellular factors, such as the interaction of the nanoparticles with cells [102,103]. Figure 5: Antimicrobial activity of _Z.__official_-mediated green synthesis of AuNPs against **(A)** Gram-positive and Gram-negative bacteria, and **(B)** unicellular fungi, and multicellular fungi. Data are presented as average value \(\pm\) SD (**n** = 3).