Supramolecular hydrogelation via host-guest anion recognition: Lamellar hydrogel materials for the release of cationic cargo

Summary Hydrogelation triggered by host-guest anion recognition is difficult to achieve due to the challenging anion recognition in water and de novo gelator design. Herein, we report such a hydrogel system, based on bambus[6]uril anion receptor, BU. Albeit not a gelator per se, BU formed hydrogels selectively in the presence of iodides or perchlorates. Anion recognition by BU governed the hydrogelation, indicated by the NMR, ATR-IR, and single-crystal X-ray. Gelation mechanism and lamellar nature of the network structure were elucidated by SAXS, cryoSEM, and holotomography methods. Rheological characterization revealed the hydrogels were stable and relatively strong. In physiological saline solutions, the hydrogels released cationic cargo by cation metathesis, leading to a gel-to-gel transformation. Choline derivatives, as model drugs, exhibited slow release from the hydrogels. Our simple hydrogel system may inspire the design of smart materials based on host-guest anion recognition, where the release or sequestration of specific charged species are desirable.