Reduced texaphyrin: A ratiometric optical sensor for heavy metals in aqueous solution

摘要

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 com-plexation, 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 response-producing 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 of texaphyrin metal complexes may be prepared. The formation of metal cation complexes under the present aqueous conditions was confirmed by reversed phase high-performance liquid chromatography, ultra-violet-visible absorption and fluorescence spectroscopies, and high-resolution mass spectrometry.