Reversible photoswitch of viscosity and photoresponsive binding of protein has been achieved in water solution of azobenzene-modified polyacrylate. We synthesized polymers with a few mol % hydrophobic side groups, including azobenzene groups with different spacers between the photochrome and the backbone. The binding of the protein (BSA) onto the polymers was investigated by capillary electrophoresis in dilute solution and rheology (viscosity, dynamic moduli, and stress relaxation) in semidilute solution of polymer. In the dilute regime, BSA/polymer complexes are formed in equilibrium with unbound BSA. Both the length of the hydrophobic spacer on the azobenzene side group and the presence of additional n-alkyl side groups significantly affect the affinity of BSA for the polymer. In the semidilute regime, viscosity enhancement by several decades is obtained upon addition of BSA in polymer solutions and ascribed to physical cross-linking involving BSA. In the two regimes, light was shown to modify the binding properties. Reversible release of BSA (by up to 80% of the protein) was obtained by exposure to UV. Reversible viscosity swings by up to 40-fold were cycled for hours by alternative exposure to UV/vis light. Light-induced cis-trans isomerization of the azobenzene together with low concentration of photochrome in the samples made it possible to obtain rapid responses (half-time ∼20 s) in solutions or in gels having thickness of the order of centimeters. An unprecedented degree of sensitivity is achieved thanks to the amplification provided by properties of optimal modified polymers. These properties are analyzed in term of the response of cross-links density, chain dynamics, and binding affinity.

In aqueous solutions, beta-cyclodextrin (CD) and cyclodextrin-containing polymers (PolCD) associate with azobenzene-modified polyacrylate (AMP). Inclusion complexes in solution of CD (or PolCD) and AMP, and the viscosity of these mixtures, have been studied as a function of the composition of AMP and concentrations of samples. AMPs are random copolymers containing a low fraction of a light-responsive hydrophobic moieties (<10 mol % of 6-[4-alkylamido]phenylazobenzene acrylamide), and a charged hydrophilic unit, sodium acrylate. PolCDs are beta-cyclodextrin randomly conjugated with epichlorohydrin and fractionated to yield copolymers of average number of CD per chain equal to 50. In dilute solutions, the composition of complexes has been investigated by capillary electrophoresis and UV-vis spectrometry. Association between PolCD and AMP appears more complex than the conventional Benesi-Hildebrand scheme. We identified a tight (quantitative) binding regime followed by a gradual increase of the density of AMP-bound PolCD upon increasing the concentration of PolCD. At higher concentrations, the formation of large clusters has been characterized by the increase of viscosity by several decades. Light-triggered trans-conformation of the azobenzene moieties of AMPs leads to a marked photoswitch of viscosity. Reversible viscosity swings by up to 6-fold were achieved by alternative exposure to UV and visible lights. In contrast, the composition of PolCD/AMP complexes in dilute regime does not respond to light, though subtle modifications of the structures of complexes are reflected by variation of electrophoretic mobilities and UV spectra. The properties of interpolymer clusters and photoviscosity are accordingly the result of modification of the dynamics of association. In practice, the low concentration of photochrome makes it possible to obtain rapid responses in samples having a thickness of the order of cm. The data reported provide guidelines for the formulations of CD/polymer systems, specifically, viscosity enhancers, which should show promising developments in pharmaceuticals or cosmetics.