*3.1. Characterization of the Synthesized AgNPs*

In this study, TEM images, energy-dispersive X-ray, and UV-Vis spectroscopy were first used to characterize the synthesized AgNPs. As shown in Figure 1a, the AgNPs were well dispersed and the average size was measured as 70~80 nm. The energy-dispersive X-ray spectrum presented in Figure 1b confirmed the synthesized nanoparticles were composed of Ag. The UV-Vis absorption spectrum (see Figure 1b) showed a characteristic peak of AgNPs at 420 nm, which agreed well with previous literature reports [47]. The relatively narrow half band width of the absorption curve suggested the synthesized AgNPs had good size distribution [48].

**Figure 1.** TEM image (**a**), energy-dispersive X-ray spectrum (**b**), and UV-Vis absorption spectrum (**c**) of the synthesized AgNPs. Scale bar in (**a**): 100 nm.

The surface of PDMS film exhibits a hydrophobic effect, making it undesirable for coating with aqueous Ag colloid. Therefore, chemical modification was carried out by simply incubating the PDMS film in an APTES ethanol solution. Presumably, a small number of hydroxyl groups on the PDMS surface enabled the formation of chemical bonds with APTES, resulting in amino functionalization of PDMS and reversing the hydrophobic property. After dropping 5 μL concentrated Ag colloid and drying in a vacuum, a small circle with metallic appearance was observed, as presented in the inset of Figure 2c and S1.

**Figure 2.** SERS spectra of R6G on AgNPs-PDMS substrates with various surface densities of AgNPs (**a**). SERS spectra of R6G (0.1~100 μg L<sup>−</sup>1) on optimized AgNPs-PDMS substrate (**b**). SEM images of the optimized AgNPs-PDMS substrate under different scales (**c**,**d**). Inset of (**c**): photo of the prepared substrate. Inset of (**d**): statistical analysis of size distribution of the modified AgNPs.
