*2.2. Synthesis of Ag@Au Nanoparticles*

First, we mixed the silver nanoball seeds with 100 µL of 1 mM glutathione solution in a 1.5 mL centrifuge tube at room temperature; using an IKA shaker, we then vortexed it for 1 min, and incubated it for 1 h to make glutathione modified into silver nanospheres by the Ag–S bond. At the same time, we added 4.0 mL ultrapure water to a 20 mL glass flask, followed by 200 µL of 10 mM HAuCl4·3H2O solution and 0.8 mL of 0.1 M CTAB solution. The mixed solution was stirred at 500 rpm for 30 min at room temperature using an IKA magnetic stirring device, mixed well, and then 475 µL of 0.1 M ascorbic acid solution was added, stirring was continued at room temperature for 5 min, and the mixed solution was used as a growth solution. Finally, the silver nanospheres modified with glutathione were added to the growth solution, and the reaction was carried out at room temperature under magnetic stirring for 2 h. The resulting product was centrifuged at 5000 rpm for 10 min; the supernatant was removed, and washed twice with water and ethanol, respectively. In order to explore the effect of the amount of silver nanoball seeds added on the produced Ag@Au NPs, we added 40 µL, 80 µL, 160 µL, and 500 µL of silver nanosphere seeds to the reaction, without changing any of the other reaction conditions.
