Dramatically Promoted Swelling of a Hydrogel by Pillar[6]arene–Ferrocene Complexation with Multistimuli Responsiveness

The swelling–shrinking transition of hydrogels is crucial for their wide applications such as actuators and drug delivery. We hereby fabricated a smart hydrogel with ferrocene groups on pendant of polymer networks. While it was immersed in the water-soluble pillar[6]arene (WP6) aqueous solution, the hydrogel was dramatically swollen, which was an approximately 11-fold promotion in weight compared with that in pure water, due to the formation of the inclusion complexes between WP6 and ferrocene groups in the hydrogel. In particular, the well-swollen hydrogel exhibited good responsiveness to multistimuli including temperature, pH, redox, and competitive guests by tuning the dissociation/formation of WP6–ferrocene inclusion complexes or the strength of their charges. Meanwhile, potential application of such a smart hydrogel in pH-responsive drug release was demonstrated as well.