A supramolecular photonic crystal hydrogel based on host–guest interactions for organic molecule recognition

Abstract

The host component β-cyclodextrin (β-CD) and the guest component tertiary-butyl (t-Bu) were introduced into the polymeric backbone material of an inverse opal hydrogel. The association between β-CD and t-Bu functions as an openable cross-linker (“lock”) in the hydrogel. Meeting a competitive guest as the target molecule, 1-adamantanecarboxylic acid sodium (AdCANa) salt acts as a “key” to dissociate the pair of β-CD and t-Bu due to its stronger combination capacity with β-CD than t-Bu. At the same time, another cross-linker, N,N-methylenebisacrylamide (MBAAm), provides covalent bonds to the backbone material, ensuring that the subsequent association/dissociation of the host–guest pair only results in an expansion–contraction in volume, rather than a sol–gel phase transition. In this way, the target molecule acting as a competitive guest can cause an expansion-induced structural color change of the inverse opal hydrogel. Following the same strategy, the L- and D-enantiomers of tryptophan (Try) can also be recognized. The combination between the supramolecular host–guest pair and photonic crystals promises applicability in molecular recognition and other interesting optical devices.