*2.1. Materials*

The hydrochloroauric acid trihydrate (HAuCl4·3H2O, 99.9% trace metals basis), hexadecyltrimethylammonium bromide (CTAB, AR, 99%), glutathione (Mw = 307.32, ≥98%), 4-aminothiophenol (4-ATP, GC, ≥98%), nitric acid (HNO3, AR), and hydrochloric acid (HCl, AR) were purchased from Aladdin. Silver nanospheres (diameter: 80 nm; purity: >99% concentration: 0.1 mg/mL, XFJ63) were purchased from Nanjing/Jiangsu XFNANO Materials Tec Co., Let. Fentanyl hydrochloride standard product was purchased from the Shanghai Institute of Criminal Science and Technology (Shanghai, China). All reagents were used directly, without further processing. All of the water used in the experiment was ultrapure water (18.2 M·cm) purified using a Milli-Q Lab System (Nihon Millipore Ltd., Hangzhou, China). The flasks and glass slides used in the experiment were first washed with aqua regia (HCl:HNO<sup>3</sup> = 3:1) for 30 min, then washed with water (18.2 MΩ·cm) and absolute ethanol twice, and finally dried for use.
