A reversible fluorescence chemosensor for sequentially quantitative monitoring copper and sulfide in living cells

We report a novel, selective and sensitive strategy for the sequentially "ON-OFF-ON" fluorescent detection of Cu²⁺ and S^2- based on a fluorescein derivative, FL. The specific binding of FL towards Cu²⁺ in aqueous and biological media led to the intensive green fluorescence quenching and a notable increase of the absorbance maximum at 480 nm. In the presence of S^2-, the intensity and overall pattern of the fluorescence emission and UV-vis spectra of FL-Cu²⁺ ensemble were recovered since the abolishment of paramagnetic Cu²⁺. This displacement approach exhibited highly specificity, and sensitivity with detection limits of 3 nM for Cu²⁺ and 150 nM for S^2-. The fluorescence "ON-OFF-ON" circle can be repeated to a minimum of 5 times by the alternative addition of Cu²⁺ and S^2-, implying that FL is a renewable dual-functional chemosensor. The biocompatibility of FL toward breast carcinoma cells, MDA-MB-231 was confirmed by MTT assay. The reversible "ON-OFF-ON" fluorescent response of FL to Cu²⁺ and S^2- in living system was further confirmed by confocal fluorescence imaging of living cells. The quantification of Cu²⁺ and S^2- in single intact cell was realized by the flow cytometry analysis.