Driving Quick and Large Amplitude Contraction of Viologen-Incorporated Poly-l-Lysine-Based Hydrogel by Reduction

A glutaraldehyde-cross-linked poly-l-lysine-based hydrogel with pendant viologens was synthesized with various [viologen unit]/[Lys unit] ratios. The hydrogel with the ratio of 25% was extensively characterized. Characterization of the hydrogel revealed that (i) water content reaches 95.8%, (ii) unbound viologen unit is absent, and (iii) lyophilized gel, showing porous structure, is rewettable. The hydrogel in contact with a gold, glassy carbon, pyrolytic graphite, or ITO electrode was electroactive, showing voltammetric responses involving diffusion process. Occurrence of electron transfer between viologen sites was verified by ESR measurements. The electroreflectance spectrum demonstrated significant dimerization of one-electron-reduced forms, viologen radical cations. Most remarkably, reduction of the hydrogel by dithionite showed a 93% volume decrease, being near to the water content, at 100 s and finally 97% contraction at 380 s. The initial rate of contraction was 0.02 s^-1, and the swelling weight ratio was over 17. The contraction rate was much faster than that observed when adding of redox-inactive salts in the outer medium. Recovery by reoxidation was relatively sluggish even when O₂-saturated water was used. The quick and large-amplitude contraction with removal of water from the gel body should be the consequence of permeability of reductant, interchain π-stacking of one-electron-reduced forms (viologen monoradical monocation), desolvation of viologen upon reduction, electron hopping between viologen units and its acceleration with contraction, and effective osmotic pressure difference.