Nitrogen and sulfur co-doped carbon quantum dots for highly selective and sensitive fluorescent detection of Fe(III) ions and L-cysteine

Nitrogen and sulfur co-doped carbon quantum dots (N,S-CQDs) with a high quantum yield (69%) and excellent photoluminescent properties were synthesized via a facile hydrothermal method using citric acid and cysteine as precursors. The cytotoxicity of N,S-CQDs was evaluated through the MTT assays using HepG2 cells as the target. The cell viability is still >95% after a 24 h incubation with particles in a concentration of up to 300 μg·mL−1, thereby indicating low cytotoxicity and excellent biocompatibility of N,S-CQDs. The emission spectra of the N,S-CQDs are nearly independent of the excitation wavelengths in the range from 280 to 380 nm and have the strongest fluorescence emission centered at 415 nm. Co-doping with N and S promotes the electron-transfer rate and coordination interaction between N,S-CQDs and Fe(III) ions which acts as a quencher of fluorescence. Fluorescence can be recovered, however, by addition of L-Cys. Hence, the N,S-CQDs act as a highly sensitive and selective “turn-off-on” probe for the determination of Fe(III) or Cys. The limits of detection are 14 nM for Fe(III) and 0.54 μM for Cys. The interferences by 17 common metal ions and heavy metal ions, 12 other quenchers and 16 amino acids were investigated and found to be tolerable. The probe was successfully applied for cellular imaging of Fe(III) in HepG2 cells by using fluorescence microscopy.