*2.3. Synthesis of SERS Substrate Au@AgNPs*

Synthesis of AuNPs: Gold nanoparticles were prepared by utilizing tannins as a reducing agent [18]. Briefly, 1 mL of 10 mM tannin was added to a 50 mL beaker containing 18.5 mL of ultrapure water under stirring conditions (500 rpm), followed by the addition of 500 μL of 1% HAuCl4·4H2O. The total volume of the reaction mixture was about 20 mL, and the reaction was allowed to continue for 20 min. The pH of the solution was measured at 6 due to the acidic nature of the tannin. Another batch of nanoparticles was also prepared by adding HAuCl4·4H2O (1%) after adjusting the pH of the solution to 7 with K2CO3 solution (0.2 M).

Synthesis of Au@AgNPs: The prepared AuNPs were coated adding a silver layer by reducing AgNO3 using tannins as a reducing agent [19]. In a 25 mL round-bottom flask, 9.5 mL of prepared AuNPs colloidal solution was stirred at 500 rpm. Then, 250 μL of 10 mM tannin solution and 250 μL of 10 mM AgNO3 solution were sequentially added to the solution with constant stirring for 30 min at room temperature. The formation of the core–shell structure was indicated by the change in colour of the solution from wine red to orange.

Synthesis optimization: It was found that the volume of HAuCl4 and AgNO3 during the preparation of Au@AgNPs greatly affects the size and concentration of nanoparticles formed in the solution, which will, in turn, affect the enhancement capabilities of the prepared substrate. In order to obtain high enhancement abilities, the volume of the two reactants (HAuCl4 and AgNO3) was optimized. The synthesis of AuNPs involved varying the volume of HAuCl4 from 100 to 600 μL with an increment of 100 μL. The enhancement of the AuNPs was evaluated using 4-MBA (10−<sup>3</sup> M) as a Raman signal probe. Meanwhile, the synthesis of Au@AgNPs involved varying the volume of AgNO3 from 100 to 600 μL with an increment of 100 μL. The UV–Visible spectra of the synthesized Au@AgNPs substrates were collected to better understand the difference in surface plasmon resonance. Further, the enhancement of the Au@AgNPs was evaluated using 4-MBA (10−<sup>5</sup> M) as a Raman signal probe.

### *2.4. Tea Sample Preparation*

The black tea sample was purchased from the supermarket in Zhenjiang, Jiangsu. After grinding it into powder, the tea powder (0.2 g) was weighed and added to the microwave digestion tank. Concentrated HNO3 (65%, 8 mL) and the Cr6+ standard solution was added to the tea sample before digestion (1 h). The digestion tanks were put in an acid extractor (130 ◦C, 60 min), cooled, and then digested in a microwave digester following the standard operating procedure with some modifications. Finally, a light green clarified solution was obtained without any solid residue [20]. The digested samples were degassed by sonication (100 ◦C, 10 min), and the inner cap was rinsed with a little water. Then the resulting digestion solution was filtered into 50 mL volumetric flasks with syringe filter (diameter 33 mm, pore size 0.22 μm) and filled up to the mark with ultrapure water to obtain the final solution to be assayed. A total of 8 samples spiked with various amounts of Cr6+, including 100, 80, 60, 40, 20, 10, 5 μg L<sup>−</sup>1, and blank, were used for the analysis. The prepared samples were also used for ICP-MS and SERS analysis.
