Rapid screening of toxic free arsenic in water and traditional Chinese medicine beverages via signal-off SERS tagging

A “signal-off” surface-enhanced Raman scattering (SERS) strategy is developed for the rapid and ultrasensitive detection of total free arsenic in water and traditional Chinese medicine (TCM) beverages. The method employs methimazole (MMI) as a multifunctional probe that first reduces As(V) to As(III) and subsequently forms a As-MMI complex, leading to concentration-dependent attenuation of the characteristic MMI SERS signal. The assay achieves rapid equilibration within 2 min under optimized conditions and exhibits exceptional selectivity for free arsenic over common interfering ions, attributed to the combined redox and As-S complexation mechanisms under acidic pH. Ultrasensitive detection is achieved with a limit of detection of 0.6 fmol/L, more than eight orders of magnitude below the WHO safety guideline for drinking water. Practical applicability is demonstrated through quantitative recovery (93.8–106%) of spiked arsenic in real water and TCM beverage samples with relative standard deviations below 6.1%. This work provides a rapid, selective, and ultra-sensitive platform for free arsenic monitoring in environmental and pharmaceutical quality control applications.