*8.3. Time of Exposure*

Wei et al. studied the effect of reaction time (0, 1, 2, 3, and 4 h) on the yield of Ag-NPs using berry extract of sea buckthorn [63]. As the reaction time increased, the absorption intensity of NPs increased steadily, reaching a maximum at 4 h, which was indicative of a high concentration of Ag-NPs. The UV spectra also showed a slight blue shift from 415 to 413 nm with the increase in time of exposure, indicating the formation of smaller-sized NPs.

#### *8.4. Concentration of Natural Reductants, Stabilizing Agents, and Bulk Materials*

The concentration of biological entities and the salts used for the synthesis of NPs influence the size and shape of the NPs. Hamouda et al. revealed that a surge in the amount of *Oscillatoria limnetica* extract during synthesis of Ag-NPs shifted the UV-spectra peak of the Ag-NPs from 420 to 430 nm, which reflected an increase in the size of the NPs [176]. Similarly, Vellora et al. synthesized copper oxide NPs using different concentrations (1, 2, and 3 mM) of copper chloride (CuCl2·H2O) and a constant concentration of Gum karya (10 mg/mL) with incubation at 75 ◦C and 250 rpm for 1 h in an orbital shaker [177]. The increase in the concentration of precursors promoted the generation of NPs of increasing sizes; the nanosizes were 4.8, 5.5, and 7.8 nm, respectively.
