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.