Discriminative dual-mode sensing of heavy metal ions using defect-engineered carbon nanodots

Heavy metal contamination poses severe risks to human health and the environment, demanding probes that are not only sensitive and selective but also capable of identifying multiple pollutants. Here, we report the synthesis of defect-engineered carbon nanodots (CNDs) that achieve an ultrahigh photoluminescence quantum yield of 61.3% and excellent thermal stability. These CNDs serve as a versatile, masking-agent-free platform for the discriminative dual-mode (fluorometric/colorimetric) sensing of heavy metal ions, specifically demonstrating single-probe detection of Co2+ and Ni2+. The probe distinguishes between these ions through unique optical signatures: both ions cause significant fluorescence quenching, but each induces a distinct visible color change—to light orange for Co2+ and light green for Ni2+—under ambient light. This discriminative response originates from specific metal-ligand interactions with heteroatom-functionalized surface groups. The sensor achieves low detection limits of 0.85 μM for Ni2+ and 1.02 μM for Co2+ fluorometrically, and below 6 μM colorimetrically, while maintaining high selectivity in complex ionic environments. Practical utility was confirmed through the accurate analysis of real water samples, yielding recovery rates of 92.2–102.4%. This work establishes a simple, cost-effective, and robust strategy for the simultaneous and discriminative detection of heavy metal ions, offering a powerful tool for environmental monitoring and biosensing.