On-off-on nanosensors of carbon quantum dots derived from coal tar pitch for the detection of Cu2+, Fe3+, and L-ascorbic acid

Novel nitrogen-doped carbon quantum dots (N-CQDs) were synthesized by a chemical oxidation method using medium-low temperature coal tar pitch as the raw material. Such quantum dots were developed as a highly sensitive fluorescent “on-off-on” switch sensor for the selective and simultaneous sensing of Cu2+ and Fe3+. The as-prepared N-CQDs, which emit blue light, were characterized by TEM images, FT-IR spectra, Raman spectroscopy, XPS analysis, fluorescence spectra, and UV–vis absorption spectra. The results showed that the N-CQDs exhibit outstanding optical properties and high optical stability within the pH range of 4–10, with a quantum yield of approximately 7%. Additionally, the material performed as an “on-off” sensor which can be dramatically extinguished by Cu2+ and Fe3+. A linear relationship between Cu2+ and Fe3+ ion concentration and fluorescence intensity was observed in the range from 0 to 50 μM. The limits of detection of the fluorescent sensor toward Cu2+ and Fe3+ were 0.16 μM and 0.173 μM, respectively. The Fe3+-quenched N-CQDs were restored after adding L-ascorbic acid, due to the redox reaction between Fe3+ and L-ascorbic acid, which resulted in the detachment of Fe3+ from the surface of the N-CQDs. The linear range for the detection of L-ascorbic acid was 0–28 μM. Therefore, the amount of L-ascorbic acid can be measured by using the sensing system consisting of Fe3+ and N-CQDs. In consequence, N-CQDs are considered an important material for the detection of Cu2+ and Fe3+ in water samples or L-ascorbic acid in drugs.