## 3 Results and Discussions
### Antioxidant Activity
For example, at 500 \(\upmu\)g/mL of ZS-ZnO-NPs synthesized using ZnSO\({}_{4}\cdot\)H\({}_{2}\)O, the antioxidant activity was 91.3% after 12 h of incubation but increased to 93.2% at 1000 \(\upmu\)g/mL after the same time of incubation. These findings suggest that the antioxidant activity of ZS-ZnO-NPs is concentration- and time-dependent and can be modulated by varying these parameters. The observed differences in antioxidant activity among different zinc salts used in the synthesis, such as ZnSO\({}_{4}\cdot\)H\({}_{2}\)O, ZnCl\({}_{2}\), Zn(CH\({}_{3}\)COO)\({}_{2}\cdot\)2H\({}_{2}\)O, as well as Zn(NO\({}_{3}\))\({}_{2}\cdot\)6H\({}_{2}\)O, could be attributed to their varying chemical properties. For example, ZnSO\({}_{4}\cdot\)H\({}_{2}\)O has been reported to have higher antioxidant activity compared to other zinc salts, and this property may have been transferred to the synthesized ZS-ZnO-NPs [86]. Furthermore, the stabilizing agents inherent in the _Phoenix_ extract utilized during the synthesis might play a role in the discerned variations in antioxidant activity. The phytochemicals within the extract may possess functional groups capable of interacting with reactive oxygen species (ROS), such as superoxide anion (O\({}^{2-}\)), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), and hydroxyl radical (OH\({}^{\prime}\)). These ROS can engage with functional proteins through both specific and non-specific binding mechanisms on the surface of the nanoparticles. This intricate interaction potentially contributes to an augmented antioxidant activity [87; 88; 89; 90]. Additionally, another possible justification for this observation could be the unique morphology of ZnO nanoparticles synthesized from ZnSO\({}_{4}\cdot\)H\({}_{2}\)O, which were reported to exhibit hollow spherical, nanorings, and spheroidal morphology (See Section 3.4). These morphological characteristics could provide a larger surface area for interaction with free radicals, leading to enhanced antioxidant activity. Moreover, the size and morphology of the green-synthesized ZS-ZnO-NPs could also play a role in their antioxidant activity, with smaller nanoparticles generally exhibiting higher antioxidant activity due to their increased surface area and reactivity. Therefore, the smaller particle size of ZnCl\({}_{2}\)-synthesized ZS-ZnO-NPs compared to the other salts could have contributed to their relatively higher antioxidant activity [91; 92]. Furthermore, the incubation time of the nanoparticles with the DPPH radical may also play a role in their antioxidant activity. Longer incubation times could allow for more efficient scavenging of free radicals by ZS-ZnO-NPs, leading to higher antioxidant activity. The concentration-dependent and time-dependent trends in antioxidant activity observed in this study are consistent with previous reports on the antioxidant properties of ZS-ZnO-NPs [93; 94; 95]. The results suggest that the synthesized ZS-ZnO-NPs exhibit concentration- and time-dependent antioxidant activity, and this property can be modulated by varying the concentration of ZS-ZnO-NPs and the incubation time. It is worth mentioning that the antioxidant activity of nanoparticles is a complex phenomenon influenced by various factors, including their size, shape, surface chemistry, and stabilizing agents used in the synthesis. The differences in the chemical properties and solubility of the zinc salts used in the synthesis, such as ZnCl\({}_{2}\), ZnSO\({}_{4}\cdot\)H\({}_{2}\)O, Zn(CH\({}_{3}\)COO)\({}_{2}\cdot\)2H\({}_{2}\)O, as well as Zn(NO\({}_{3}\))\({}_{2}\cdot\)6H\({}_{2}\)O, have resulted in variations in the size, morphology, and surface chemistry of the synthesized ZS-ZnO-NPs, which in turn have influenced their antioxidant activity.