*2.2. Synthesis and Characterization of AgNPs*

The AgNPs used in this study were synthesized following a modified Lee and Miesel protocol [46]. In brief, AgNO3 (19 mg) was added to 100 mL of deionized water. The solution was heated to boiling point. Sodium citrate solution (2 mL, 10 g L<sup>−</sup>1) was poured into the AgNO3 solution under vigorous stirring. The reaction solution was kept at gentle boiling for 30 min under continuous heating. The color of the colloid turned from light yellow to gray during the reaction. After that, the obtained Ag colloid was cooled at room temperature and stored in a refrigerator. All glassware was cleaned and rinsed with deionized water multiple times before use.

Transmission electron microscopy (TEM) images and energy-dispersive X-ray spectrum of the synthesized AgNPs were recorded with a transmission electron microscope (Tecnai G2 F30, FEI Co., Ltd., Hillsboro, OR, USA). The UV-Vis absorption spectrum was collected using a multimode plate reader (EnVision, PerkinElmer Inc., Waltham, MA, USA).
