Selective response of cyclodextrin-dye hydrogel to metal ions

The design and preparation of novel hydrogels have gained much interest recently, and in spite of the promising results to date, increasing pressure remains to develop novel gels based on stimuli-responsive systems. Here we present an anisotropic supramolecular gel of gamma-cyclodextrin and a simple azo dye in which physical gelation is completed through specific host-guest interaction. The obtained hydrogel exhibits respective morphological transitions based on supramolecular assembly and dissociation, leading to either precipitation or a sol-to-gel transition. It can identify different classes of metal ions, and, among them, naked-eye differentiation of lead ion is possible due to the coordination-induced unthreading of dye molecules. Gel-to-sol transitions can also be realized using chromium ions as a denaturing reagent. Accompanying structural changes were identified by various characterization techniques, including 2D-ROESY, HR-MAS, UV-Visible absorption, small-angle X-ray scattering, and induced circular dichroism measurements. The hydrogel properties discussed here will be useful in analytical applications such as metal ion sensing and removal applications.