Highly selective and sensitive ratiometric detection of Hg2+ ions with NFS co-doped carbon dots: Real sample analysis, antibacterial properties, and cellular imaging applications.

A simple, low-cost hydrothermal method was employed to synthesize highly fluorescent nitrogen-, fluorine-, and sulfur-co-doped carbon dots (NFS-CDs) using flufenamic acid and L-cysteine as precursors. The synthesized NFS-CDs exhibited dual emission peaks at 490 and 580 nm with a quantum yield of 24.7 %. They exhibit excellent stability, excitation-dependent fluorescent, and particle sizes ranging from 2 to 8 nm. The fluorescent chemosensor probe, NFS-CDs, showed strong selectivity and sensitivity for Hg²⁺ over other metal ions investigated in aqueous solutions (pH ∼ 7.4). Strong fluorescent enhancement at 490 nm and considerable quenching at 580 nm was observed in the presence of Hg²⁺ ions. The stoichiometric ratio of the NFS-CDs/Hg²⁺ complex was optimized to 1:1 according to the Benesi-Hildebrand and Stern-Volmer plot methods. The NFS-CDs exhibited a linear dynamic detection range from 0 to 10 × 10^-6 M for Hg²⁺ ions with a lower detection limit of 18.0 and 67.5 × 10^-9 M, respectively, at 490 and 580 nm. Practical applications of NFS-CDs in detecting Hg²⁺ ions in natural water samples showed high recovery rates (98.9-104.6 %) and low relative standard deviation (RSD ≤ 2.47 %). The NFS-CDs/Hg²⁺ achieved 78.7 ± 2.6 % and 83.4 ± 2.3 % antibacterial activity against E. coli and S. aureus as NFS-CDs/Hg²⁺ could damage the bacterial walls when they entered the bacteria. Furthermore, the NFS-CDs were used to detect Hg²⁺ ions intracellularly in HCT116 cells with low toxicity using live cell imaging.