Coconut husk lignin-derived and heteroatom-doped carbon dots as a dual emissive fluorescent probe for ultrasensitive detection of Hg2+ ions with antibacterial and bioimaging applications

In this study, lignin-derived and heteroatom-doped carbon dots (Lig-NCDs) were synthesized from coconut husk and o -phenylenediamine as a nitrogen source through a facile hydrothermal method. It exhibits excellent optical performance, hydrophilicity, and physicochemical stability, as characterized by UV–Vis, FL, and FT-IR analyses. TEM and XRD studies revealed that the nanosized spherical particles (3.87 ± 0.12 nm) have with partially graphite core and an amorphous carbon matrix. Moreover, XPS studies demonstrated the presence of surface-rich oxygen and nitrogen atoms in the carbon core, which facilitated Hg 2+ interactions. The sensing study displayed an ultrasensitive “turn-off” response to Hg 2+ ions with a limit of detection (LOD) of 69.6 nM with linear range from 0 to 20 μM. According to the WHO and USEPA guidelines, the detection limit achieved in this study is lower than the permissible level of Hg 2+ in drinking water, demonstrating its suitability for environmental monitoring and water quality analysis. Interference studies confirmed the high selectivity of Hg 2+ in the presence of other metal ions, while the EDTA results showed a good reversible mechanism. In addition to sensing studies, Lig-NCDs and Lig-NCDs/Hg 2+ exhibit strong antibacterial activity against E. coli and S. aureus . They were successfully applied in bioimaging studies in HCT116 cell lines with Lig-NCDs, displaying bright intracellular fluorescence and biocompatibility. This result highlights that the Lig-NCDs act as fluorescent probes for the rapid detection of Hg 2+ ions, and antibacterial and bioimaging are other advantages for environmental and biomedical applications.