*3.3. In Situ Detection of PAHs on Fruit and Vegetable Surfaces*

The SERS spectra of the spiked samples prepared by spraying BaP, Pyr, and Nap onto the surfaces of apples, tomatoes, and peaches are shown in Figure 4. When BaP concentrations on the surfaces of apples (Figure 4(A1)), tomatoes (Figure 4(B1)) and peaches (Figure 4(C1)) are as low as 0.1, 0.25, and 0.25 μg/cm2, respectively, there is still a weak SERS signal at 607 cm<sup>−</sup>1, especially at 0.25 μg/cm2, which can be attributed to the C-C and C-H bending vibration modes. The characteristic peaks at 587 cm−<sup>1</sup> and 1233 cm−<sup>1</sup> can still be observed at Pyr concentrations as low as 0.25, 0.5, and 0.25 μg/cm<sup>2</sup> on the surfaces of apples (Figure 4(A2)), tomatoes (Figure 4(B2)), and peaches (Figure 4(C2)), corresponding to C-C stretching and C-H bending vibration modes. The difference in spectral signal intensity between the different peels may be related to the roughness and chemical composition of the peels. When the concentration of Nap on the surface of the apple (Figure 4(A3)), tomato (Figure 4(B3)), and peach (Figure 4(C3)) is as low as 0.5, 1, and 0.5 μg/cm2, respectively, the characteristic peak at 1372 cm−<sup>1</sup> has a weak signal, while the signal at 505 cm−<sup>1</sup> is almost invisible, which is related to the stretching and bending vibrations of the C-C bond.

**Figure 4.** Detection of different concentrations of PAHs on fruit and vegetable surfaces by using a flexible β-CD@AuNP/PTFE substrate. (**A1**–**A3**), (**B1**–**B3**), and (**C1**–**C3**) are the SERS spectra of BaP, Pyr, and Nap with different concentrations on the surface of apple, tomato, and peach, respectively.

Although sensitive SERS detection of Nap on the fruit and vegetable surfaces is realised, the sensitivity is still weaker than that of BaP and Pyr because the C-H bond bending vibration mode of BaP and Pyr has higher polarisability and exhibits stronger peak intensity compared with the C-C bending vibration mode of Nap. The above results indicate that using β-CD@AuNP/PTFE as a flexible SERS substrate can realise the sensitive and in situ detection of PAH residues on the surface of fruits and vegetables, which is of great significance for food safety assessment.
