L-cysteine-driven in situ Synthesis of Silver Nanoparticles for Mercury Ion Detection

Mercury contamination in aquatic systems poses a serious environmental and public health concern, necessitating the development of simple and sensitive detection methods. In this work, a colorimetric sensing approach based on the in-situ formation of silver nanoparticles (AgNPs) stabilized by L-cysteine is presented for the detection of Hg²⁺ in aqueous media. The synthesis conditions were systematically optimized to regulate nanoparticle nucleation, growth, and stability. The presence of Hg²⁺ interferes with Ag–S coordination by preferentially binding to the thiol groups of L-cysteine, leading to nanoparticle aggregation and a corresponding change in optical properties. This interaction results in a visible color transition and a measurable decrease in surface plasmon resonance intensity. The sensing system exhibited a concentration-dependent response toward Hg²⁺ with good linearity and a limit of detection of 0.1 nM. Selectivity studies confirmed minimal interference from other metal ions. The practical utility of the method was demonstrated through successful Hg²⁺ detection in groundwater, tap water, and seawater samples, showing satisfactory recovery and reproducibility. Owing to its straightforward design, low reagent consumption, and rapid response, the proposed sensing strategy provides a promising alternative for mercury monitoring in environmental water samples.