*3.1. Influence of Different Flexible Substrates on SERS Activity*

Owing to the interference of several inherent flexible-film characteristics—such as transparency, fluorescent background, viscosity, and impurities—the morphology of the nanoarray structure transferred to its surface was affected, greatly affecting the SERS activity. To select an ideal flexible substrate and realise the in situ detection of trace PAH residues on irregular surfaces in a real-world environment, the influence of three flexible substrates on SERS activity was explored (Figure 2). Figure 2a, c, and e shows the SERS spectra of β-CD@AuNP/PDMS, β-CD@AuNP/tape, and β-CD@AuNP/PTFE, respectively, while the SERS spectra of 10 μg/mL BaP detected by them are shown in Figure 2b, d, and f. As is evident from the figure, β-CD@AuNP/PTFE obtained the best SERS activity (Figure 2f), the characteristic peak of BaP at 524, 607, 1231, and 1376 cm−<sup>1</sup> being greatly improved. The vibration modes corresponding to each characteristic peak are listed in Table S2 in the Supporting Information. The SERS activity of the β-CD@AuNP/tape is the weakest (Figure 2d), owing to the viscosity and surface roughness of the tape, which easily destroys the two-dimensional nanoarray structure of the β-CD@AuNPs. The PDMS surface is not affected by the viscosity, but the PDMS hydrophobicity is lower than that of the PTFE, so the SERS activity of β-CD@AuNP/PDMS (Figure 2b) is also weaker. The surface hydrophobicity of the PTFE film coated with the perfluorinated liquid limits the diffusion of NPs, which is conducive to reducing the gap between the particles and generating a large number of hot spots. This result can be demonstrated using the SEM image of β-CD@AuNP/PTFE in Figure S6 of the Supporting Information, the gap between the aggregated β-CD@AuNPs being less than 10 nm, which generates abundant hot spots, resulting in a large SERS enhancement with Enhancement Factor of 106~107 (Figure S7 in Supporting Information). Therefore, β-CD@AuNP/PTFE was used as the flexible SERS substrate in subsequent experiments.

**Figure 2.** SERS spectra of 10 μg/mL BaP detected based on different flexible substrates: (a) β-CD@AuNP/PDMS, (b) 10 μg/mL BaP detected by β-CD@AuNP/PDMS; (c) β-CD@AuNP/tape, (d) 10 μg/mL BaP detected by β-CD@AuNP/tape; (e) β-CD@AuNP/PTFE, (f) 10 μg/mL BaP detected by β-CD@AuNP/PTFE.
