Nitrogen and sulfur co-doped graphene quantum dots for the highly sensitive and selective detection of mercury ion in living cells.

In this work, a novel nitrogen and sulfur co-doped graphene quantum dots (N,S/GQDs) were synthesized by a simple and green pyrolysis method with citric acid as carbon source, d‑penicillamine as doped molecule. The obtained N,S/GQDs shows good water solubility, low cytotoxicity and high quantum yield of 56.4%. The doping of N and S can efficient promoted the coordination between the residual group of N,S/GQDs and Hg2+, thus, the fluorescence of N,S/GQDs can be dramatically and selectively quenched by Hg2+. Under the optimum conditions, the quenching effect of Hg2+ to N,S/GQDs was lined with its concentrations in the range of 0.9 to 30 nM with a detection limit of 0.69 nM. 100 fold of common metals had no influence for the determination of Hg2+. When Hela cells were incubated into the N,S/GQDs solutions for 2 h, more than 85% Cells still kept living even at a high concentration of 400 μg/mL suggesting the good biocompatibility and low toxicity of the obtained N,S/GQDs. Besides this, the N,S/GQDs also showed good cell permeability and could enter into the cell and showed bright blue light. Thus, they were successfully applied as a powerful fluorescent nano-sensor for imaging detection of mercury ions in the living cells.