Fluorescent nitrogen and sulfur co-doped carbon dots from casein and their applications for sensitive detection of Hg2+ and biothiols and cellular imaging.

Fluorescent nitrogen and sulfur co-doped carbon dots (NSCDs) were synthesized by a one-step pyrolysis strategy using casein as carbon, nitrogen and sulfur sources, and characterized by UV-vis spectrum, fluorescent spectrum, X-ray photoelectron spectroscopy (XPS) and FT-IR, etc. The synthesized NSCDs displayed a blue emission under ultraviolet illumination with a quantum yield of 31.8%, and a good aqueous solubility, photostability and biocompatibility. It was found that the fluorescence intensity of NSCDs could be selectively quenched by Hg2+, so NSCDs was used as an effective probe for the detection of Hg2+. The linear range and the limit of detection (LOD) of the fluorescent sensor based on NSCDs for the detection of Hg2+ were 0.01-0.25 μM and 6.5 nM, respectively. Spiked water samples were detected by the sensor with the recovery of 95.4-106.3% and relative standard deviation (RSD) of 3.6-8.6%. It was also observed that the quenched NSCDs-Hg2+ system could be restored by the addition of biothiols such as l-cysteine (Lcy), homocysteine (Hcy) and glutathione (GSH), thus NSCDs-Hg2+ system was employed as a fluorescent sensor for the detection of biothiols. The linear range and LOD of the NSCDs-Hg2+ system were 1-10 μM and 23.6 nM for Lcy, 0.2-2.5 μM and 12.3 nM for Hcy, and 0.1-2.0 μM and 16.8 nM for GSH, respectively. The NSCDs-Hg2+ system was applied for the detection of biothiols in serum samples with satisfied results. In addition, the study in vitro imaging HeLa cells revealed that the synthesized NSCDs could be used as effective fluorescent probes in cellular imaging without noticeable cytotoxicity.