Nitrogen and sulfur Co-doped carbon dots as selective and visual sensors for monitoring cobalt ions

Using l-cysteine as the precursor, carbon dots with nitrogen and sulfur heteroatoms co-doped (N, S-CDs) were synthesized via a simple hydrothermal method. The doped N and S atoms enhance the fluorescence intensity, and the modified functional group on N, S-CDs facilitate the coordination interaction between N, S-CDs and Co2+. Thus, the prepared N, S-CDs can be used as fluorescence probe to detect Co2+ with excellent selectivity and sensitivity. Moreover, various interferences displayed unobvious effect on the sensing of N, S-CDs to Co2+. This probe provided a rapid, visual method to monitor Co2+ with a wide monitoring range to Co2+ (1–50 μM) and a low detection limit of 26 nM. More interestingly, the quenched fluorescence (turn-off) of N, S-CDs induced by Co2+ could be recovered (turn-on) with the addition of H2O2 or EDTA. The fluorescence quenching mechanism is a static quenching, accompanied with electron transfer by forming the complex between Co2+ and functional groups of N, S-CDs.