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, Hg2+ can completely quench the fluorescence of NCDs as a result of the formation of a non-fluorescent stable NCDs-Hg2+ complex. Static fluorescence quenching towards Hg2+ is proved by the Stern-Volmer equation, ultraviolet-visible absorption spectra, temperature dependent quenching and fluorescence lifetime measurements. Subsequently, the fluorescence of the NCDs-Hg2+ system is completely recovered with the addition L-cysteine (L-Cys) owing to the dissociation of NCDs-Hg2+ complex to form a more stable Hg2+-L-Cys complex by Hg2+-S bonding. Therefore, such NCDs can be used as an effective fluorescent "turn-off" probe for rapid, rather highly selective and sensitive detection of Hg2+, with a limit of detection (LOD) as low as 1.48 nM and a linear detection range of 0-10 M. Interestingly, NCDs-Hg2+ 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 M. Further, the sensitivity of NCDs to Hg2+ is preserved in tap water with a LOD of 1.65 nM and a linear detection range of 0-10 M. (C) 2016 Elsevier B.V. All rights reserved.