A novel nitrogen-doped carbon dots as “on-off-on” fluorescent sensor for ultrasensitive and visual quantitative detection of mercuric (II) and glutathione

A novel nitrogen-doped carbon dots (NCDs) was prepared composed of anhydrous citric acid and 2-amino-1-H-benzimidazole with solid state method. The XPS and FT-IR spectra indicate that lot of chelating groups such as -OH, -COOH and -NH2 are attached on the surface of NCDs. NCDs exhibited the characteristic fluorescence property and its fluorescence spectra gradually red-shifted with the increase of excitation wavelengths. The prepared NCDs in water solution has highly selective “turn-off” fluorescence response towards Hg2+ over the other metal ions and common anions with a low detection limit of 3.50 μM. Furthermore, NCDs presented the successive identification and detection capability, NCDs@Hg2+ displayed highly selective “turn-on” fluorescence response towards the bioactive glutathione (GSH) over the other amino acids with a low detection limit of 3.43 μM. The “turn-off” fluorescence of NCDs was caused by the selective coordination with Hg2+ through both static and dynamic quenching mechanisms, and the fluorescence recovery can be attributed to the removal of Hg2+ from the complex NCDs@Hg2+ by GSH. The hydrogel films containing NCDs and NCDs@Hg2+ can semi-quantitatively detect Hg2+ and GSH in micromolar level with naked eyes under UV lamp, respectively. In addition, the prepared NCDs have satisfactory recovery in detection of Hg2+ and GSH in practical water samples and human serum samples. This work proves the N-doped carbon NCDs are efficient material as fluorescence chemosensors to identify and detect metal ions and bioactive substances in practical samples.