Tuning of distinguished fluorescent responses to methylmercury and Hg2+ ions: Selective detection of methylmercury over Hg2+ ions by fluorescent sensor using micelle system

Abstract

Although CH3Hg+ is highly toxic, reversible fluorescent sensors for detecting CH3Hg+ over Hg2+ are rare. In this study, we synthesized a reversible fluorescent sensor (1) based on tryptophan amino acid and 6-membered cyclic amine as an electron-donating unit of the fluorophore for turn-on monitoring of Hg2+ in H2O and developed a fluorescent sensing method for selective sensing CH3Hg+ over Hg2+ using 1 and the micelles with a different surface charge. 1 in triton X-100 (TX-100) micelles differentiated between CH3Hg+ and Hg2+ by fluorescent response types (turn-on vs red-shifted emission), whereas 1 in cetyltrimethylammonium bromide (CTAB) micelles detected only CH3Hg+ among 16 metal ions by turn-on response. 1 in the micelles exhibited remarkable detection ability for CH3Hg+ such as ultrafast response (<30 s), high sensitivity and/or selectivity for CH3Hg+, low detection limits (107 nM), and large enhancement of the fluorescence. We proposed the plausible binding mode of 1 in the micelles for selective sensing CH3Hg+ based on fluorescence and NMR and IR study. The 6-membered cyclic amine group of the fluorophore part of 1 might play a critical role to distinguish between CH3Hg+ and Hg2+ by fluorescent response types in TX-100 micelles. 1 detected sensitively Hg2+ and CH3Hg+ in live cells through significant enhancement of green and red fluorescence. 1 with and without the micelles successfully quantified CH3Hg+ and Hg2+ in groundwater and tap water. The combination of the sensor with the micelles provided a selective detection of CH3Hg+ over Hg2+ or distinguished between CH3Hg+ and Hg2+ by fluorescent response types.