N-doped carbon dots for highly sensitive and selective sensing of copper ion and sulfide anion in lake water

Water contamination by copper ions (Cu2+) and sulfide anions (S2-) has been a longstanding human health hazard. The limitations of current detection strategies have created a high demand for effectively monitoring the concentrations of these ions in water systems. A facile fluorescent bioassay for Cu2+ and S2- was developed in this study based on "on-off-on" fluorescent nitrogen-doped carbon dots (N-CDs). N-CDs were synthesized for the first time via thermal treatment using hexamethylene-tetramine and ammonium citrate precursors. The obtained CDs were highly stable in aqueous solution, with a strong green emission and a 51.2% quantum yield. The maximum fluorescence emission peak was observed at 478 nm for a 360-nm excitation. Notably, the fluorescence intensity of N-CDs can be quenched by a factor of 6 by Cu2+, which can be applied to Cu2+ assays ranging from 0.05 to 5 mu M with a limit of detection (LOD) of 25 nM. Interestingly, further addition of S2recovered the fluorescence of the N-CD@Cu2+ nanocomplex, which can be used to quantify S2in a broad range from 0.05 to 10 mu M with an LOD of 32 nM. The N-CD and N-CD@Cu2+ nanocomplexes were also demonstrated to be highly specific towards Cu2+ and S2-, respectively. Finally, the sensor was successfully applied to lake water for detecting both Cu2+ and S2-, demonstrating excellent application prospects for analytical chemistry and environmental remediation.