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Without modification of its periphery, cucurbit[7]uril (a macrocyclic cavitand comprising seven glycoluril units) forms a hydrogel. The gelation is thermoreversible and sensitive to pH, but is inhibited by the presence of alkali-metal ions and shows guest-induced stimuli-responsive behavior: upon addition of a small amount of a guest, it undergoes a reversible gel–sol transition with alternating UV irradiation and heat treatment (see picture).
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Hwang et al. (2006) studied this question.
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