Rapid, Selective, and Ultrasensitive Detection of Cr(VI) in Complex Samples by a Redox-Mediated Surface-enhanced Raman Spectroscopy Method

Surface-enhanced Raman spectroscopy (SERS) often suffers from poor selectivity, sensitivity, and anti-interference capabilities in metallic ions detection. Herein, a redox-mediated SERS method is presented for on-site detecting Cr(VI) in complex samples with ultrahigh sensitivity (2.6 fmol/L), selectivity, and rapidity (within 5   min). 2-Mercaptoimidazole was selected as the SERS probe due to its strong SERS activity and the ability of its thiol group to reduce Cr(VI) to Cr(III). The redox reaction between Cr(VI) and thiol group of 2-mercaptoimidazole under acid condition was investigated, including pH, mixing ratio, temperature, and concentration. Remarkably, the reaction equilibrium was achieved within 1   min. Furthermore, the method exhibited high selectivity for Cr(VI) over other ions due to its strong oxidizing properties, and excellent anti-interference capabilities attributed to the robust sulfur-metal bond formation. The redox-mediated SERS method with ultrahigh sensitivity and a detection limit of 2.6 fmol/L, which is 12 orders of magnitude below the safety level (0.1   mg/L) set by United States Environmental Protection Agency for drinking water. Finally, the method was successfully applied to detect Cr(VI) residues in environmental water and traditional Chinese medicine drinking samples (Codonopsis pilosula and Astragalus membranaceus), achieving satisfactory recoveries (85.5–110%) and low RSDs (0.1–4.2%). This work not only advances the analytical performance for on-site detection of Cr(VI) residue but also offers a versatile tool with substantial implications for public safety monitoring and pharmaceutical quality control.