Ligand-to-Metal Charge Transfer Quenching of Carbon Dots for Highly Selective Hg2+ Detection in Microfluidic Devices

Carbon dots have emerged as highly efficient and versatile fluorescent probes for detecting and quantifying Hg²⁺. This study reports a method to prepare nitrogen-doped carbon dots (N-CDs) via ligand-to-metal charge transfer fluorescent quenching for Hg²⁺ detection. Besides the excitation-independent emission feature, the synthesized N-CDs demonstrate a high selectivity of Hg²⁺ over 23 potentially interfering environmental ions. The structures of 2-hydroxy-N,N-dimethylbenzamide and amide functional groups are identified as the determinants of the relatively high quantum yield (53.33%). N-CDs also exhibit a linear relationship with Hg²⁺ concentration in the range of 0.007–120 μM, with a limit of detection of 7.1 nM. The produced N-CDs can be employed in a customized microfluidic real-time monitoring platform, demonstrating the robustness in receiving efficient fluorescence signals with a satisfactory limit of detection. The acceptable accuracy and recovery of Hg²⁺ detection in tap water and river water highlight the potential practical applications of the synthesized N-CDs. Results in this study can facilitate the development of a portable device for early warning of water pollution.