Fluorescent probes for “off–on” highly sensitive detection of Hg2+ and L-cysteine based on nitrogen-doped carbon dots

Fluorescent nitrogen-doped carbon dots (NCDs) were synthesized by a facile, and low-cost one-step hydrothermal strategy using citric acid as carbon source and ammonia solution as nitrogen source for the first time. The obtained NCDs show stable blue fluorescence with a high quantum yield of 35.4%, along with the fluorescence lifetime of ca. 6.75   ns. Most importantly, Hg 2+ can completely quench the fluorescence of NCDs as a result of the formation of a non-fluorescent stable NCDs–Hg 2+ complex. Static fluorescence quenching towards Hg 2+ is proved by the Stern–Volmer equation, ultraviolet–visible absorption spectra, temperature dependent quenching and fluorescence lifetime measurements. Subsequently, the fluorescence of the NCDs–Hg 2+ system is completely recovered with the addition L-cysteine (L-Cys) owing to the dissociation of NCDs–Hg 2+ complex to form a more stable Hg 2+ -L-Cys complex by Hg 2+ –S bonding. Therefore, such NCDs can be used as an effective fluorescent “turn-off” probe for rapid, rather highly selective and sensitive detection of Hg 2+ , with a limit of detection (LOD) as low as 1.48   nM and a linear detection range of 0–10   μM. Interestingly, NCDs–Hg 2+ system can be conveniently employed as a fluorescent “turn-on” sensor for highly selective and sensitive detection of L-Cys with a low LOD of 0.79   nM and a wide linear detection range of 0–50   μM. Further, the sensitivity of NCDs to Hg 2+ is preserved in tap water with a LOD of 1.65   nM and a linear detection range of 0–10   μM.