Enzymatic Formation of pH Gradients within Polyelectrolyte Gels with Immobilized Urease

Thermally responsive cationic gels with immobilized urease, in the shape of a small cylinder with a diameter of 290−640 μm, were prepared via gelation of an aqueous monomer solution containing the enzyme. N-Isopropylacrylamide and N-vinylimidazole were used as thermosensitive and pH-sensitive monomers, respectively. Diameters at different positions of the cylinder were microscopically measured in a cell through which substrate solution (pH 4; 35 °C) was passed at a constant flow rate; thus, both substrate concentration and pH at the gel surface were maintained at a constant level throughout the experimental period. It was found that the gel undergoes a shrinking change due to an enzymatically induced increase in pH within the gel phase. There was a marked position dependence of the shrinking degree; the diameter at the center of the cylinder was smaller than that at either the top or the bottom, but the diameters at the top and bottom were identical with each other. This trend was observed during the period of shrinking of gel and after the establishment of swelling equilibrium of gel. To account for these results in connection with a pH gradient, which would be enzymatically formed within the gel phase, mathematical simulations were conducted with a reaction−diffusion model. The central part of the gel was then taken as an infinitely long circular cylinder. There was a good agreement between the results of simulations and experiments after equilibrium swelling was reached. Therefore, it is reasonable to conclude that (i) a charge distribution depending on the pH gradient would appear in the gel and (ii) this distribution affects the overall swelling ratio of the gel. These can no longer be explained in terms of the concept of osmotic pressure arising from mobile counterions within the gel phase.