Nitrogen- and Sulfur-Codoped Carbon Dots for Highly Selective and Sensitive Fluorescent Detection of Hg2+ Ions and Sulfide in Environmental Water Samples

Nitrogen- and sulfur-codoped carbon dots (N,S-CDs) with a fluorescence quantum yield of 16.1% and good photoluminescent properties were synthesized by a simple hydrothermal method. Cytotoxicity of the N,S-CDs was evaluated by the MTT assay, and human hepatoma HepG2 cells were chosen as the target. The cell viability was more than 85% after 24 h of incubation when its concentration was up to 300 μg/mL, suggesting low cytotoxicity and good biocompatibility of the N,S-CDs. The fluorescence spectra of the N,S-CDs are excitation-dependent in the excitation-wavelength range of 295–400 nm, and it emits bright blue fluorescence centered at 435 nm. Its selective fluorescence recognition for Hg2+ ions was found. When Hg2+ ions were added to the N,S-CDs solution, its bright blue fluorescence was obviously quenched and could be recovered by the addition of sulfide. Accordingly, a new strategy based on N,S-CDs–Hg2+ system as a highly selective and ultrasensitive "turn off–on" fluorescence sensing for the detection of sulfide was fabricated. The limits of detection (S/N = 3) are 83 nM for Hg2+ ions and 11 nM for sulfide. Almost no statistically significant interference for Hg2+-ion and sulfide detection was observed among possible coexisting substances in the water samples, including 17 common metal ions and 11 anions. This probe was successfully applied for the cellular imaging of Hg2+ ions in HepG2 cells by fluorescence microscopy.