Raman spectra were collected with a ThermoFisher Scientific DXR3 Raman microscope using laser excitation with a wavelength of 785 nm and an output power of 1 mW. This instrument was equipped with an Olympus BX41 optical microscope and a thermoelectrically cooled charge-coupled detector (ThermoFisher front-illuminated CCD system) with 1024 × 256 pixel format, operating at -70 °C. The signal was calibrated by an internal polystyrene standard and a 10× objective. The spot size was about 3.8 µm. 200 SERS spectra were collected with an exposure time of 1 s for 5 accumulations at different spots for each sample.