8,8'-(1,4,10,13-Tetrathia-7,16-diazacyclooctadecane-7,16-diyl)-bis(methy lene)diquinolin-7-ol (TTBQ) was synthesized and proved to selectively recognize Hg2+, forming a Hg2+/TTBQ complex with an association constant (K-a) as large as similar to 1.3 x 10(4) +/- 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 similar to 25-fold luminescence enhancement and a detection limit of sub-mu M 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.