Poly(N-isopropylacrylamide)/Gold Hybrid Hydrogels Prepared by Catechol Redox Chemistry. Characterization and Smart Tunable Catalytic Activity

PNIPAM hydrogels functionalized with gold nanoparticles were prepared by making use of catechol redox chemistry. For this purpose catechol groups were introduced in the PNIPAM network during the cross-linking polymerization process (PNIPAM–catechol x hydrogel). These groups act as reducing agents of HAuCl 4, which enables the functionalization of the PNIPAM with gold nanoparticles (PNIPAM–catechol x @Au hydrogel). The rheological study shows that catechol groups reinforce the hydrogel structure. A stronger effect was observed after the functionalization of the hydrogel with gold nanoparticles. This influence could be easily observed since the variation of G ′ with the MBA mole fraction was fitted to a power-law expression G ′ ∼ x MBA a, with a = 1.6 for PNIPAM x hydrogels, a = 2.3 for PNIMPAM–catechol x hydrogels and a = 3.2 for PNIMPAM–catechol x @Au hydrogels. The capability of the PNIPAM–catechol x @Au hydrogel to act as a tunable catalyst was demonstrated with a model reduction reaction. The half-lifetime at 25 °C was 10.5 min; however, at 38 °C the half-lifetime was 133 min.