*2.3. Preparation of SERS Sample*

A solution of 100 μg/mL was obtained by dissolving 10 μg BaP, Pyr, and Nap solid powders in a 0.1 L ethanol solution. Standard solutions of BaP, Pyr, and Nap at different concentrations (10, 8, 5, 2.5, 1, 0.5, 0.1, and 0.05 μg/mL) were prepared by diluting the 100 μg/mL solution with ethanol. Twenty samples were prepared at each concentration. A standard solution was then used to evaluate the effect of flexible β-CD@AuNP/PTFE on SERS detection. To simulate the actual environment, the spiked samples were prepared by spraying 10 μL of BaP, Pyr, and Nap standard solutions with different concentrations on a fixed area (1 × 1 cm2) of the fruit and vegetable surfaces, after which they were air-dried at 25–30 ◦C. With 20 samples at each concentration, 5 spectra were collected for each.

The PAH samples comprised four classes—that is, BaP + Pyr, BaP + Nap, Pyr + Nap, and BaP + Pyr + Nap—with 20 samples in each class, covering a concentration range of 10 μg/mL to 0.05 μg/mL. Similarly, the 10 μL sample solution was sprayed onto a fixed area of the fruit and vegetable surfaces before being air-dried, followed by 10 μL ethanol being sprayed onto the fixed area to dissolve and extract various PAHs. Finally, the prepared SERS substrate was pasted onto the fruit and vegetable surfaces, gently pressed and lifted, the process being repeated two to three times to realise peel surface sampling. After sampling, the substrate was placed on a slide for SERS detection. Five spectra were collected for each sample.
