Colorimetric metal ion binding of catechol-based coatings inspired by melanin and molecular imprinting

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Abstract
Catechol-containing biomolecules and related synthetic materials are of interest for a broad range of applications. Our group has been interested in the potential of eumelanin, a catechol-containing biomaterial and the black-to-brown pigment in humans, and synthetic eumelanin analogues for water purification applications. In an attempt to apply the strategy of molecular imprinting towards lead-binding synthetic eumelanin coatings, we developed a PbO2-mediated synthesis that was subsequently tested with a number of catechols in addition to the eumelanin and polydopamine monomers l-dopa and dopamine. Although this strategy did not provide significant improvements in affinity or selectivity, we found that the PbO2-mediated oxidative polymerisation of 1,2-dihydroxybenzene generates a coating that darkens visibly upon binding Pb2+ and other metal ions. Oxidative polymerisation of catechol with sodium periodate also yields a coating with this colorimetric response. Of eight metal ions tested, the coatings are most responsive to Pb2+ and Cu2+, as analysed by quantitative colorimetry. With further optimisation, these coatings could prove useful for the colorimetric sensing of metal ions.

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