*3.1. Synthesis of Au@AgNPs and Optimization*

Tannins are naturally occurring polyphenols that can act as an ideal reductant in the synthesis of nanoparticles. Tannins were responsible for the reduction of HAuCl4, resulting in the formation of stable Au NPs which then serve as a seed to induce Ag NPs synthesis. The change in colour of the colloidal nanoparticle from wine red to orange indicated the formation of the core–shell structure. The surface plasmon resonance of the prepared nanoparticles with different amounts of precursors exhibited a change in surface plasmon resonance, as shown in Figure 2a. As the volume of HAuCl4 increased, the colour of the nanoparticle solution gradually changed, and when the volume of HAuCl4 reached 600 μL, the synthesized nanoparticle solution appeared turbid and exhibited aggregation and precipitation after 3 days of storage [26]. The UV–Vis spectra of AuNPs were shown in Figure 2a, and the peak at 520 cm−<sup>1</sup> increased gradually and a slight red shift occurred as the volume of HAuCl4 increased. This observation suggested that the particle size of Au NPs increased [27]. The enhancement effect of AuNPs on the Raman reporter molecule 4-MBA (10−<sup>3</sup> M) was also used to optimize the amount of HAuCl4. As shown in Figure 2c, the Raman intensity at 1074 cm−<sup>1</sup> was the greatest when the amount of HAuCl4 was 500 μL. It was attributed to AuNPs with larger particle size can produce strong localized surface plasmon resonance. However, as the amount of HAuCl4 continued to increase (600 μL), the Raman intensity decreased. This is because the larger particle size leads to the instability of the Au NPs. Considering the stability of storage and Raman enhancement effect, the optimal volume of HAuCl4 was selected as 500 μL.

To increase the Raman intensity, the growth of the Ag shell on the Au NPs was carried out. The amount of AgNO3 was optimized because it affected the thickness of the Ag shell and thus affected the Raman intensity. As shown in Figure 2b, as the amount of AgNO3 increased, the Ag absorption peak around 400 nm gradually intensified, while the Au absorption peak near 520 nm weakened rapidly or even vanished entirely. The Raman intensity of 4-MBA (10−<sup>6</sup> M) at 1584 cm−<sup>1</sup> was also used to determine the optimal amount of AgNO3. As shown in Figure 2d, the Raman intensity reached its maximum when the volume of AgNO3 was 500 μL. However, the Raman intensity weakened slightly when the amount of AgNO3 increased to 600 μL, which was due to the 4-MBA signal transmission being hindered by a thicker Ag shell. Therefore, the optimal amount of AgNO3 was 500 μL.

**Figure 2.** (**a**) UV spectra of AuNPs synthesized by tannin with different volumes of HAuCl4; (**b**) UV spectra of Au@AgNPs reduced by tannin with different volumes of AgNO3; (**c**) SERS enhancement of AuNPs synthesized using different volumes of HAuCl4; (**d**) SERS enhancement of Au@AgNPs synthesized using different volumes of AgNO3.
