Mercury(II) Recognition and Fluorescence Imaging <i>in Vitro</i> through a 3D-Complexation Structure

8,8′-(1,4,10,13-Tetrathia-7,16-diazacyclooctadecane-7,16-diyl)-bis(methylene)diquinolin-7-ol (TTBQ) was synthesized and proved to selectively recognize Hg2+, forming a Hg2+/TTBQ complex with an association constant (Ka) as large as ∼1.3 × 104 ± 520 M−1 in aqueous solution. Single crystals of TTBQ and a TTBQ-Hg2+ complex have also been successfully grown, in which the latter unambiguously revealed a cagelike configuration consisting of thiol-crown and dual 7-hydroxyquinoline moieties to firmly trap Hg2+. This 3D-complexation structure accounts for ∼25-fold luminescence enhancement and a detection limit of sub-μM in water for sensing Hg2+. Great selectivity toward Hg2+ has been exhibited over alkali- and alkaline-earth metal ions, first-row transition-metal ions, and other cations studied. This chemosensor is particularly suited for the detection of Hg2+ in a pH range of 5.5 to 7.5. This, in combination with its fine biocompatibility, leads to the success toward in vitro mercury recognition based on fluorescence imaging.