**4. Discussion**

The present study demonstrated the development of a sensitive and reliable method for the quantitative detection of Cr6+ in tea samples. The method relied on the redox reaction between Cr6+ and carbimazole, leading to a decrease in the intensity of the Raman peak at 595 cm−1. This study provided valuable insights into the potential application of SERS technology as a rapid and label-free detection technique for Cr6+ in tea. The results indicated that the Raman intensity of the characteristic peak at 595 cm−<sup>1</sup> was inversely correlated with the concentration of Cr6+. Moreover, the relationship between the Raman intensity at 595 cm−<sup>1</sup> and the logarithm of the concentration of Cr6+ was linear. The LOD (3.78 μg/L) of Cr6+ calculated using this approach indicated the sensitivity of the developed method. Compared with the previous study [29] using photoelectrochemical to detect Cr6+ (LOD = 0.01 μM) in the environment, the proposed method exhibited similar sensitivity and simpler detection operation.

Another key aspect investigated in this study was the selectivity of carbimazole towards Cr6+ in the presence of other metal ions. A specific selection of Cr6+ by carbimazole was demonstrated, highlighting the potential applicability of the developed method for real tea samples, which may contain various metal ions as contaminants. To validate the accuracy and reliability of the developed SERS method, a comparison was carried out with the standard method (ICP-MS). The reproducibility, accuracy, and recovery rate of the SERS technique were thoroughly analyzed. The results showed that the SERS method exhibited comparable performance to the ICP-MS method in terms of accuracy and recovery rate, indicating its suitability for practical applications.

The findings of this study exhibited the potential of SERS technology for the sensitive and label-free detection of Cr6+ in tea samples. The advantages of SERS, such as its high sensitivity, non-destructivity, and minimal sample preparation requirements, make it an attractive alternative to traditional analytical techniques. The established SERS method offers a convenient and sensitive approach for the determination of Cr6+, which is of great importance considering the potential health hazards associated with its presence in tea. The results obtained in this study contribute to the growing interest in the application of SERS technology for the detection of contaminants in food, and it paves the way for further research.

#### **5. Conclusions**

In this study, a highly sensitive nano-SERS substrate was developed for the detection of Cr6+ in tea samples. The Au@AgNPs with uniform particle size distribution and good enhancement effect was synthesized using tannin. The combination of the specific redox reaction of carbimazole and Cr6+ and NaCl-induced aggregation of nanoparticles enhanced the EF value of Au@AgNPs to 3.56 × 105. Compared with AuNPs, the EF of Au@AgNPs was two orders of magnitude higher, thus improving the sensitivity of Cr6+ detection. Quantitative analysis showed a linear relationship between the Raman intensity of the characteristic peak at 595 cm−<sup>1</sup> and the logarithm of the Cr6+ concentration. The LOD of this proposed SERS method was 3.78 μg/L, and the detection range was established to be 5~100 μg/L. Due to the 250-fold dilution during tea sample processing, the detection range of Cr6+ in tea sample was 1.25~25 mg/kg, with a LOD of 0.945 mg/kg. Additionally, the proposed method showed high specificity, even in the presence of other metal ions, and good reproducibility in detecting Cr6+ in the tea sample. In particular, it showed good accuracy (recovery rates ranged from 91.62% to 104.84%) and precision (RSD ≤ 3.07%) in the recovery experiment, and the obtained results were validated against ICP-MS. Conclusively, this method has great potential for rapid and label-free detection of Cr6+ in tea.

**Author Contributions:** Conceptualization, Z.G.; methodology, L.Y.; software, J.C.; validation, L.Y. and H.J.; formal analysis, L.Y.; investigation, L.Y.; resources, J.C. and X.Z.; data curation, L.Y.; writing—original draft preparation, L.Y. and H.J.; writing—review and editing, H.R.E.-S.; visualization, L.Y.; supervision, Z.G.; project administration, Z.G.; funding acquisition, Z.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China (31972151), the Postgraduate Research and Practice Innovation Program of Jiangsu Province (KYCX21\_3383), the Outstanding Young Teachers of Blue Project in Jiangsu province, Key R&D Project of Jiangsu Province (BE2022363), Jiangsu Agriculture Science and Technology Innovation Fund (CX(22)3069), the Open Fund of Key Laboratory of Modern Agricultural Equipment and Technology of Ministry of Education (MAET202117), and the Youth Project of Faculty of Agricultural Equipment of Jiangsu University (NZXB20210205).

**Data Availability Statement:** The data presented in this study are available on request from the corresponding authors.

**Conflicts of Interest:** The authors declare no conflict of interest.
