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 Fe 3+ (turn-off) and then recovered by glutathione (turn-on). Moreover, N-CDs applied as an “on-off-on” fluorescent sensor for the detection of Fe 3+ 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 μM. Furthermore, the stable N-CDs/Fe 3+ exhibited a high sensitivity detection for GSH with a LoD of 0.226 μM. 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 Fe 3+ rather than other metal ions, and the N-CDs/Fe 3+ 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.