Aconitic acid derived carbon dots as recyclable “on–off–on” fluorescent nanoprobes for sensitive detection of mercury(ii) ions, cysteine and cellular imaging

Abstract

In this study, aconitic acid (AA) was applied as a new carbon precursor for the fabrication of fluorescent carbon dots (CDs) using a facile one-step microwave-assisted synthesis approach with ethylenediamine as the co-doping reagent. The AA derived CDs (AA–CDs) were water-soluble and the aqueous solution of AA–CDs exhibited excitation-independent, bright blue fluorescence with an absolute quantum yield of 45.1%. Without further surface modification, the intrinsic fluorescence can be gradually quenched by the incremental addition of Hg²⁺ ions through an effective electron or energy transfer process due to the high affinity of Hg²⁺ to AA–CDs. By introducing cysteine (Cys) into the AA–CDs–Hg²⁺ system, the quenched fluorescence will be recovered via the formation of Hg–S bonds, which can be utilized to further enhance the selectivity for sensing Hg²⁺ ions. Based on the “on–off–on” system, we established a sensitive and selective fluorescence sensor for turn-off detection of Hg²⁺ ions and turn-on detection of Cys in aqueous solution, with detection limits of 5.5 and 30 nM, respectively. The fluorescence of AA–CDs could be repeatedly turned off and on for over 10 cycles by alternately adding Hg²⁺ ions and Cys, suggesting excellent reversibility. Furthermore, the prepared AA–CDs were successfully applied for the determination of Hg²⁺ in environmental water samples, and also as fluorescent nanoprobes for imaging Hg²⁺ ions and Cys in living cells.