Fluorescent turn-on detection of inorganic mercury and methylmercury in aqueous solution with PC micelles and live cells using fluorescent chemosensors based on Trp amino acid and cycloalkylamine-7-sulfonyl-2,1,3-benzoxadiazole

Mercury species are well-known to be toxic to humans. Particularly, methylmercury is a highly toxic substance that can readily penetrate lipid membranes, accumulating in human body and causing lethal effects. Thus, the sensitive detection of methylmercury in aquatic environments and live cells is crucial. In this study, we synthesized fluorescent chemosensors (1–5), primarily composed of a tryptophan amino acid (Trp) and cycloalkylamine-7-sulfonyl-2,1,3-benzoxadiazole fluorophores bearing a different size of cyclic amine moiety and developed new sensing systems for Hg²⁺ and CH₃Hg⁺ by combining the fluorescent chemosensors with lyso-phosphatidylcholine (PC) micelles to resemble cytoplasmic lipid membranes of human cells. Fluorescent chemosensors (1–5) with visible light excitation (>450 nm) exhibited significant enhanced emissions to Hg²⁺ among various metal ions in purely aqueous solutions. In PC micelles, the chemosensors (1–5) showed significant enhanced green and red emissions (500 ∼ 670 nm) to CH₃Hg⁺ and Hg²⁺ among various metal ions. Interestingly, the chemosensors have different binding affinities and fluorescent signal changes for CH₃Hg⁺ depending on the size of cyclic amine moiety. Among them, chemosensor 5 with 7-membered cyclic amino group, exhibited the largest fluorescence change for CH₃Hg⁺ and demonstrated the highest binding affinity for CH₃Hg⁺ (K_d = 24.1 μM) and Hg²⁺ (K_d = 15.8 nM). Chemosensor 5 exhibited a significant enhancement of emission as well as an extremely fast response (<20 sec) for methylmercury in aqueous solution with PC micelles. Thus, we investigated the binding mode of chemosensor 5 with the mercury species in PC micelles using UV/visible absorbance titration, pH titration, and NMR titration experiments. Furthermore, chemosensor 5 was successfully applied to detect sensitively CH₃Hg⁺ and Hg²⁺ in live cells through increase in both green and red emissions. Fluorescent chemosensors in combination with PC micelles might serve as a promising new fluorescent sensing method for methylmercury and inorganic mercury ions in aqueous solutions and live cells.