Suprarmolecular hydrogels driven by the dual host-guest interactions between α-cyclodextrin and ferrocene-modified poly(ethylene glycol) with low-molecular-weight

In general, α-cyclodextrin (α-CD) and low-molecular weight poly(ethylene glycol) (low-MW PEG) (Mw = 400-10,000) cannot construct supramolecular hydrogels but easily form crystalline precipitates. In this study, low-MW PEG (Mn = 2000, PEG-2000) was functionalized by ferrocene as mono-end-group. The obtained ferrocene-modified PEG-2000 (FcPEG-2000) further self-assembled into supramolecular hydrogel with α-CD even at low concentration (CFcPEG-2000 = 17 mg/ml), driven by dual host-guest interaction between α-CD and FcPEG-2000. Interestingly, the hydrogel was still observed even when hydrophobic Fc group was oxidized to hydrophilic ferrocenium (Fc+) or included into the cavity of β-CD. In the former case, the existence of Fc+ end groups is considered to decrease the probability of PEG de-penetration from α-CD cavity, so that α-CDs have more location and opportunities to aggregate into more channel-type crystalline domains as physical cross-linking points. While in the later case, the synergistic effect of host-guest interaction between β-CD and ferrocenyl groups and host-guest interaction between α-CD and PEG chains are considered to be the main reason. The resultant FcPEG-2000 based hydrogels showed the property of shear-thinning.