*3.4. Specific Selectivity and Recovery for Cr6+ Detection in Spiked Tea*

For evaluating the specificity of the SERS method for Cr6+ analysis, some metal ions commonly found in tea, including Hg2+, Mn2+, Ni2+, Pb2+, Cu2+, Cd2+, Fe3+, As3+, Na+, and Cr6, were selected as interferences [10]. The Raman intensity of the peak at 595 cm−<sup>1</sup> was shown in Figure 5a, and the results showed that only the Cr6+ (100 μg/L) have a specific redox reaction with the hydrolysate of the carbimazole, resulting in the decrease in the Raman intensity at the 595 cm−<sup>1</sup> peak. Therefore, the presence of other ions does not interfere with the quantitative detection of Cr6+.

**Figure 5.** (**a**) Specificity of SERS detection for Cr6+ in tea when the concentration of all the ions was 100 μg/L; (**b**) relative standard deviation (RSD) of three repeated experiments and 15 random points in the detection solution at 595 cm<sup>−</sup>1.

The experiment was repeated three times at the same concentration (Cr6+ = 5 μg/L) to verify the reproducibility. For each detection solution, 15 random points were chosen to obtain the Raman intensity at peak 595 cm−1. As shown in Figure 5b, in each experiment, the RSD value of 15 points were all less than 5%, indicating that the detection solution had good uniformity. Similarly, the RSD value of the three repeated experiments was less than 5%, indicating that the detection method had a good reproducibility.

Further the quantitative results obtained from SERS detection were validated against standard ICP-MS, and the results are shown in Table 2. The recovery rates for the SERS method ranged from 91.62% to 104.84% (RSD ≤ 3.07%), while the recovery rates of the ICP-MS method ranged from 97.65% to 103.86% (RSD ≤ 1.53%). Comparative analysis indicates that the SERS method demonstrates higher accuracy and good reproducibility in detecting Cr6+ content in tea leaves.

**Table 2.** Comparison between the SERS method (this study) and ICP-MS quantitative detection of Cr6+.

