Supramolecular polymer hydrogels from bolaamphiphilic L-histidine and benzene dicarboxylic acids: thixotropy and significant enhancement of Eu(III) fluorescence.

Supramolecular polymers from the bolaamphiphilic L-histidine (BolaHis) and benzene dicarboxylic acids (o-phthalic acid, OPA; isophthalic acid, IPA and terephthalic acid, TPA) were found to form hydrogels although neither of the single components could gel water. It was suggested that the hydrogen bond and ionic interactions among different imidazole and carboxylic acid groups are responsible for the formation of the supramolecular polymer as well as the hydrogel formation. Depending on the structures of the dicarboxylic acids, different behaviors of the gels were observed. The hydrogels from OPA/BolaHis and IPA/BolaHis showed thixotropic properties, that is, the hydrogel was destroyed by hand shaking and then slowly gelated again at room temperature. However, the hydrogels of TPA/BolaHis could not. Interestingly, when Eu(III) was doped into IPA/BolaHis supramolecular polymers, very strong luminescence enhancement was observed. FT-IR spectroscopies and XRD analysis revealed that the strong luminescence enhancement could be attributed to the matched supramolecular nanostructures, which render the correct binding and a good dispersion of Eu(III) ions. The work offers a new approach for fabricating functional hydrogels through the supramolecular polymers.