*3.2. Characterization of Au@AgNPs*

The morphology and distribution of nanoparticles were observed and measured by TEM. The size and morphology of the prepared nanoparticles changed based on the pH of the solution as shown in Figure 3a,b. The nanoparticles synthesized under pH 7 were aggregated and non-uniform compared to AuNPs synthesized under pH 6. The colloidal solution contained triangular nanoparticles, polygonal nanoparticles, nanorods, and irregular particles; however, the round nanoparticles account for the majority. Figure 3c shows the TEM image of Au@AgNPs synthesized under pH 6, which indicated that the formed particle had a particle size of around 10–30 nm. Since the shape and size of the nanoparticles formed under pH 6 conditions showed more uniformity, pH 6 was selected for further synthesis and applications.

**Figure 3.** TEM image of AuNPs synthesized under pH 7 (**a**) and pH 6 (**b**); (**c**) TEM image of Au@AgNPs synthesized under pH 6; (**d**) FT-IR spectra of AuNPs and Au@AgNPs; (**e**) SERS enhancement of Au@AgNPs synthesized under pH 6 and pH 7 conditions.

For the FTIR characterization, spectra were collected from a 10 μL of drop cast nanoparticle solution, and the spectra displayed a characteristic peak at 1400 cm−<sup>1</sup> for AuNPs (Figure 3d). Further, the silver coating caused a decrease in the peak intensity at 1400 cm−<sup>1</sup> and a slight shift. The enhancement effects of Au@AgNPs synthesized under pH 6 and pH 7 conditions were also compared, and the obtained spectra are shown in Figure 3e. The calculated enhancement factors for Au@Ag nanoparticles synthesized under pH 6 and pH 7 conditions were 3.56 × 105 and 2.39 × <sup>10</sup>5, respectively. The Au@Ag further prepared by AuNPs synthesized at pH 6 had a better enhancement effect, which was closely related to the better uniform dispersion of AuNPs under this condition.
