Highly selective and sensitive fluorescence probe based on thymine-modified carbon dots for Hg2+ and L-cysteine detection

Herein, an on-off-on fluorescence probe for mercury ions and L-cysteine was designed and fabricated with thymine (T) covalently modified on the surface of carbon dots (CDs) by a simple, economical and effective strategy. The prepared thymine-coated CDs (CDs-T) could detect Hg2+ ions with good selectivity and sensitivity by forming the specific thymine-Hg2+-thymine (T-Hg2+-T) structure, which induced aggregation of CDs-T and resulted in significant decrease of CDs's fluorescence intensity (on-off). In the presence of L-Cys, the fluorescence was obviously recovered as the result of the disaggregation of CDs-T (off-on). In this process, Hg2+ ions were removed from the comparatively weaker connected complex of T-Hg2+-T by the formation of stronger Hg2+-S bond between Hg2+ and L-Cys, thus the aggregates were broken and the fluorescence could be turned on. Under the optimal conditions, the concentration linear range for detecting Hg2+ and L-Cys were 0.03-8 mu M (R-2 = 0.9978) and 0.003-7 mu M (R-2 = 0.9957), respectively. The limits of detection (LOD) were as low as 0.93 nM and 0.88 nM, respectively. This method was successfully used to analyze Hg2+ ions in real water samples and determine L-Cys in human urine and blood serum samples.