*3.2. SERS Detection of PAHs Based on Flexible β-CD@AuNP/PTFE*

Nap, Pyr, and BaP were selected as targets because they are the most representative and widely distributed of the two-, four-, and five-aromatic PAHs, respectively; their structures are shown in Figure 3. SERS detection of BaP, Nap, and Pyr was conducted based on flexible β-CD@AuNP/PTFE, and its sensitivity, reproducibility, and stability were explored. The SERS spectra of BaP, Nap, and Pyr at different concentrations are shown in Figure 3A–C, respectively. The intensities of the characteristic peaks (highlighted in purple) decrease with decreasing concentration. The vibration modes corresponding to the characteristic peaks are listed in Table S2. When the concentration is as low as 0.05, 0.5, and 0.1 μg/mL, BaP at 607 cm−1, Nap at 1372 cm−1, and Pyr at 587 cm−<sup>1</sup> still have weak Raman signals, indicating that the flexible β-CD@AuNP/PTFE substrate has strong sensitivity and good universality.

**Figure 3.** Detection of PAHs by β-CD@AuNP/PTFE; SERS spectra of (**A**) BaP, (**B**) Nap, and (**C**) Pyr standard samples, insets showing their molecular structures; (**D**) RSD values of characteristic peak intensities of 10 μg/mL BaP, Pyr, and Nap at 607, 587, and 1372 cm<sup>−</sup>1, respectively.

The stability of β-CD@AuNP/PTFE was further evaluated by calculating the relative standard deviation (RSD) of the characteristic peak intensities of the 10 SERS spectra for 10 μg/mL PAHs, as shown in Figure 3D. The RSDs of BaP at 607 cm<sup>−</sup>1, Nap at 1372 cm−1, and Pyr at 587 cm−<sup>1</sup> are 7.9%, 9.8%, and 7.2%, respectively. All RSD values are less than 10%, indicating that the SERS detection of PAHs based on the flexible β-CD@AuNP/PTFE constructed in this study exhibits good stability.
