<sc>l</sc>-Cysteine-Functionalized Boron-Doped Graphitic Carbon Nitride Quantum Dots: A Biocompatible Fluorescent Sensor for Cadmium Detection in Water

Boron-doped graphitic carbon nitride quantum dots (B-gC3N4 QDs) are synthesized through a simple thermal process and functionalized with l-Cysteine (l-Cys) via an EDC/NHS coupling reaction. The resulting l-Cys/B-gC3N4 QDs demonstrate a high quantum yield of 28%, excellent water solubility, and resistance to photobleaching and ionic strength. These quantum dots are employed as fluorescent probes for detecting Cd2+ ions at trace levels in water. They exhibit a fluorescence signal enhancement in response to Cd2+ ions, attributed to a chelation-enhanced fluorescence (CHEF) mechanism. The sensor detects Cd2+ ions within a linear range of 0.1-0.7 mu M, with a detection limit of 0.23 mu M and a binding constant of 9.83 x 105 M-1. Cytotoxicity assays reveal that l-Cys/B-gC3N4 QDs, both alone and in the presence of Cd2+, show no DNA damage or cell membrane disruption, confirming their nontoxic nature. Furthermore, the sensor achieves high accuracy in detecting Cd2+ in real water samples, with recovery rates ranging from 95 to 106%. This work presents a sustainable, biocompatible, and cost-effective fluorescent probe for real-time monitoring of cadmium ions in environmental water sources.