*2.3. Characterization and SERS Test*

The morphology and structure of the prepared nanoparticles were passed through a field emission scanning electron microscope (SEM, 3.0 kV, JEOL, JSM-6700F, Japan), transmission electron microscope (TEM, 100.0 kV, JEOL-2100f, Japan), high-resolution TEM (HRTEM, 200 kV, JEOL-2100f, Japan), and selected-area electron diffraction (SAED, JEOL-2100f, Japan) for characterization. The SERS test was conducted using a Raman spectrometer (Thermo Fisher DXR2xi, America) under 633 nm laser excitation; the laser power was 6 mW, and the acquisition time was 10 s. For surface-enhanced Raman spectroscopy testing, we first added 10 µL of Ag@Au nanoparticles to the centrifuge tube, and then added 50 µL of 4-ATP solutions of different concentrations, mixing well. The mixed solution was stored at room temperature for 30 min, and then 5 µL was dropped on a

glass slide, dried at room temperature, and used for surface-enhanced Raman spectroscopy (SERS) testing. The SERS test used a laser Raman spectrometer (Thermo Fisher DXR2xi, America) equipped with a microscope (50× objective lens) and a CCD detector; the laser source was 633 nm, the acquisition time was 10 s, and the laser power was 6 mW. We used the supporting software to analyze the Raman peak spectrum. The SERS test for the drug fentanyl followed the same process.
