Redox-sensitive ferrocene functionalised double cross-linked supramolecular hydrogels

Responsive double cross-linked hydrogels have proven to be a powerful approach to create smart polymer networks but unfold even greater potential if combined with supramolecular chemistry. A novel redox-sensitive ferrocene functionalised double cross-linked hydrogel is the core of this work. The explored network is based on poly( N -isopropylacrylamide) (PNiPAAm) and poly(2-methyl-2-oxazoline) (PMOXA) grafted chains with a β-cyclodextrin/ferrocene supramolecular host–guest system. A defined responsive behaviour was achieved using modified PMOXA macromonomers from a controlled synthesis, functionalised with β-cyclodextrin (CD-PMOXA) or with ferrocene (Fer-PMOXA), which were characterised thoroughly by 1 H NMR, GPC, FTIR and Raman spectroscopy. The association affinity between host–guest macromonomers (CD-PMOXA and Fer-PMOXA) was investigated by DLS and 2D NOESY and ROESY NMR analysis. Additional redox responsive supramolecular host–guest complexes were included as bismacromonomers and acted as additional cross-linking points in the hydrogel. These reversible changes of the degree of cross-linking in the polymer network allowed for reversible swelling and shrinkage by dissociation and re-formation of the complex, which also influenced the rheology of the hydrogel. The reproducible swelling and rheological changes could be tailored by the number of supramolecular cross-linking points in the network as well as the chain length of PMOXA macromonomers. Connecting the changes in swelling behaviour with the mechanical characteristics allowed for deeper structural insights into the network. The adjustable supramolecular redox-sensitive double cross-linked hydrogel provides an opportunity for future applications, e.g. in microfluidics and diagnostics.