Rhodol Derivatives as Selective Fluorescent Probes for the Detection of HgII Ions and the Bioimaging of Hypochlorous Acid

Two sensors, 1 with a spirolactone group and 2 with a spirolactam group containing a phenyl isothiocyanate moiety, based on rhodol, were designed and synthesized in order to obtain materials with excellent optical properties for the detection of environmentally and biologically important Hg2+ and hypochlorous acid (HClO) ions. The crystal structure of 1 revealed two moieties, a rhodamine-like portion with a spirolactone and a fluorescein-like portion without a spirolactone. In the absence of analyte, 1 produced an optical output with a maximum absorption and emission at 475 and 570nm, respectively, which was attributed to the fluorescein-like moiety without a spirolactone. In contrast, the rhodamine-like moiety containing a spirolactone was activated by the addition of H+ or Hg2+ ions, and 1 yielded new absorption and emission peaks at 530 and 612nm, respectively. Further functionalization with a phenyl isothiocyanate group afforded 2, a fluorescent probe for HClO. High selectivity and sensitivity towards the hypochlorite ion were anticipated, owing to the stoichiometric and irreversible formation of a thiosemicarbazide group, which led to dramatic fluorescence responses. With good functionality at physiological pH, probe 2 was successfully used to image HClO in HeLa cells.