Nitrogen-doped carbon dots used as an on-off-on fluorescent sensor for Fe3+ and glutathione detection

A novel kind of nitrogen-doped carbon dots (N-CDs) with strong blue fluorescence emission was synthesized via microwave pyrolysis method using humic acid, urea, and citric acid as the precursor. We found that the fluorescence emission of N-CDs can be firstly quenched by Fe3+ (turn-off) and then recovered by glutathione (turnon). Moreover, N-CDs applied as an on-off-on fluorescent sensor for the detection of Fe3+ and glutathione (GSH) was further studied in this work. N-CDs showed good selectivity and sensitivity towards Fe(3+ )than other metal ions, and the limit of detection (LoD) is lower to 0.102 tiM. Furthermore, the stable N-CDs/Fe3+ exhibited a high sensitivity detection for GSH with a LoD of 0.226 mu M. In addition, density functional theory calculations were further performed to reveal the mechanism of the turn-off and turn-on process. According to geometries and frontier molecular orbital (FMO) results, N-CDs exhibited strong interaction with Fe3+ rather than other metal ions, and the N-CDs/Fe3+ complex exhibited the highest stability among other complexes. Furthermore, this on-off-on fluorescent sensor was successfully applied as a stain-screening detector for Streptococcus thermophilus with high production of GSH.