Negatively Charged AgNPs. These AgNPs were synthesized through the reduction of silver nitrate using sodium borohydride, according to the method proposed by Mulfinger et al. [19] For this purpose, initially, all glassware were placed in a 1 : 3 of HCl/HNO3 solution and then rinsed three times with triply distilled water. After that, by using ice and stirring, the sodium borohydride was chilled to roughly 0°C. A 1.0 mL portion of 0.01 mM AgNO3 was added to 20 mL of the stirring 6.2 mM NaBH4 solution using a dropper (one drop per second). Stirring was stopped after complete addition of AgNO3.