Solvothermal preparation of N,S-doped carbon quantum dots from Leucas aspera for sensing and bioimaging applications

Carbon quantum dots (CQDs) were synthesised from Leucas aspera leaf extract via a green solvothermal method, yielding ultrasmall nanoparticles with an average size below 7 nm and high crystallinity. UV-visible and FTIR spectroscopy studies confirmed the surface functional groups such as hydroxyl, amine, and carboxyl groups, essential for fluorescence and biocompatibility. The CQDs exhibited strong photoluminescence with a maximum excitation at 397 nm and emission at 483 nm, and a quantum yield of 23.2%. Fluorescence quenching studies demonstrated sensitive and selective detection of toxic Cd(II) ions, with a detection limit of 3.02 ng/L. The Stern-Volmer quenching constant was found to be 0.0023 nM−1, indicating efficient quenching likely due to static complex formation between Cd(II) and surface functional groups on CQDs. Cytotoxicity assays on MDA-MB-231 breast cancer cells showed good biocompatibility at concentrations up to 40 µg/mL, maintaining over 80% cell viability. The CQDs also served as effective fluorescent probes for live-cell bioimaging, displaying multicolour emission. These findings confirm that Leucas aspera-derived CQDs are promising nanomaterials for environmental monitoring of heavy metals and biomedical imaging applications, combining sustainability, sensitivity, and safety.