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We report here a water-soluble metal cation sensor system based on the as-prepared or reduced form of an expanded porphyrin,texaphyrin.Upon metal complexation,a change in the redox state of the ligand occurs that is accompanied by a color change from red to green.Although long employed for synthesis in organic media,we have now found that this complexation-driven redox behavior may be used to achieve the naked eye detectable colorimetric sensing of several number of less-common metal ions in aqueous media.Exposure to In(Ⅲ),Hg(Ⅱ),Cd(Ⅱ),Mn(Ⅱ),Bi(Ⅲ),Co(Ⅱ),and Pb(Ⅱ) cations leads to a colorimetric response within 10 min.This process is selective for Hg(Ⅱ) under conditions of competitive analysis.Furthermore,among the subset of responseproducing cations,In(Ⅲ) proved unique in giving rise to a ratiometric change in the ligand-based fluorescence features,including an overall increase in intensity.The cation selectivity observed in aqueous media stands in contrast to what is seen in organic solvents,where a wide range oftexaphyrin metal complexes may be prepared.The formation of metal cation complexes under the present aqueous conditions was confirmed by reversed phase highperformance liquid chromatography,ultra-violet-visible absorption and fluorescence spectroscopies,and highresolution mass spectrometry.