FeN@CDs nanozyme as a GSH-mediated "turn OFF-ON" colorimetric sensor for Hg²⁺ detection

Nanozyme-based colorimetric sensors hold great promise for environmental monitoring and water quality assessment. However, developing nanozymes with high catalytic activity, sensitivity, and selectivity remains challenging. This study successfully synthesized Fe-doped N-containing carbon dots (FeN@CDs) via a one-step hydrothermal method. Efficient Fe doping endowed FeN@CDs with excellent peroxidase-like (POD) activity. The Michaelis constants (K_m) for H₂O₂ and TMB were 11.7 µM and 105.4 µM, respectively. These values indicate high substrate affinity. More importantly, the sulfhydryl groups (-SH) in GSH potentially interact with FeN@CDs and inhibit enzyme activity (OFF). Hg²⁺ competitively interacts with GSH, potentially restoring enzyme activity (ON). Leveraging this proposed competitive mechanism, we developed a novel "turn OFF-ON" FeN@CDs colorimetric sensor for Hg²⁺ detection. The linear range and limit of detection (LOD) were 0-20 µM and 6.9 nM, respectively. Notably, this sensor achieved satisfactory performance in real water sample analysis. This study provides a powerful tool for sensitive detection of Hg²⁺ in environmental samples.