Fluorine-doped g-C3N5 quantum dots for detection of heavy metal ions

Heavy metal contamination is toxic at low concentrations and leads to considerable environmental and health issues. Detection of heavy metal ions by photoluminescence quenching attracts significant attention. This study demonstrates the synthesis of fluorine-doped triazole-based g-C3N5 quantum dots (FQDs) with reduced photoluminescence emission intensities and enhanced excited charge lifetimes. The excited electrons of these FQDs are withdrawn by fluorine ions with high electronegativity, thereby inhibiting photoluminescence emission. FQDs are used for the detection of Fe2+, Fe3+, and Hg2+ ions through photoluminescence (PL) quenching, and the excited electrons are transferred between the F-doped QDs and these transition metal ions, reducing the PL emission. The LOD (limit of detection) values of Fe2+ and Hg2+ ions for CNQD were 0.423 and 0.438 ppm, respectively, whereas 3FQD exhibited an LOD value of 0.262 ppm for Fe2+, and 10FQD had an LOD value of 0.218 ppm for Hg2+ ions. Computational calculations suggest that the divalent cations contribute considerably to the lowest unoccupied molecular orbitals (LUMOs) of C3N5-Fe2+, C3N5-Zn2+, and C3N5–Hg2+, and the low-lying excitations of these C3N5−M2+ (M = Fe and Hg) complexes show strong charge transfer features. Most of the excited electrons are situated around the Fe2+ and Hg2+ ions in the C3N5-Fe2+ and C3N5–Hg2+ complexes. As a result, Fe2+ and Hg2+ ions induce extensive PL quenching of the triazole-based C3N5 QDs.