Nitrogen and phosphorus co-doped carbon dots as a "turn-off-on" fluorescence probe for the detection of Hg2+ and GSH and cell imaging

Nitrogen and phosphorus co-doped carbon dots (NPCDs) were synthesized by a microwave method, using citric acid monohydrate and diammonium hydrogen phosphate as raw materials. The morphology, structures and elemental compositions of NPCDs were characterized by transmission electron microscopy, X-ray diffractometry, and X-ray photoelectron spectroscopy. It was found that NPCDs show good selectivity to Hg2+ over other metal ions. And the fluorescence of the NPCDs-Hg2+ system can be recovered in the presence of glutathione (GSH). As such, a "turn-off-on" fluorescence probe can be constructed for the detection of Hg2+ and GSH. Under the optimum working conditions, the linear response of NPCDs to Hg2+ was in the range 0.10-30 mu M with a detection limit of 9.9 nM. The linear range of GSH was 0.50-32 mu M with a limit of detection of 11.2 nM. The proposed method has been successfully applied to the detection of Hg2+ and GSH in various biological samples. What is more, NPCDs have low cytotoxicity verified by the MTT assay. Using NPCDs as a probe, fluorescence images were obtained for HepG2 cells, which indicated that it could monitor the levels of Hg2+ and GSH in biological systems.