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

Abstract

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 μM (R² = 0.9978) and 0.003–7 μM (R² = 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.