l-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 μM, with a detection limit of 0.23 μM and a binding constant of 9.83 × 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.