Chelation Mechanisms and Surface Coordination Chemistry of N-GQDs for Trace Heavy Metal Ion Sensing

Chelation mechanisms and surface coordination chemistry of nitrogen-doped graphene quantum dots (N-GQDs) play a central role in governing their performance as fluorescent probes for trace heavy metal ion sensing. This review systematically elucidates how surface functional groups, heteroatom doping, and edge-defect structures collectively define metal–ion binding behavior and subsequent signal transduction. In particular, nitrogen incorporation in pyridinic, pyrrolic, and graphitic configurations modulates local electron density, enhancing coordination affinity toward metal ions such as Hg 2+ , Pb 2+ , and Cd 2+ . The sensing response is shown to arise from a complex interplay between chelation-driven electronic perturbation and multiple quenching pathways, including photoinduced electron transfer, exciton trapping, and interfacial charge redistribution. Importantly, the analysis highlights that selectivity is not solely determined by thermodynamic binding strength but is strongly influenced by kinetic competition, surface heterogeneity, and hydration effects in multi-ion environments. Furthermore, synthesis-dependent structural variability and lack of site-specific characterization remain key barriers to mechanistic predictability. By integrating coordination chemistry concepts with nanoscale photophysics, this work provides a unified framework for understanding structure–function relationships in N-GQD-based sensing systems and outlines challenges for advancing toward rationally engineered, high-performance heavy metal ion sensors.

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