An optically sensitive membrane for pH based on swellable polymer microspheres in a hydrogel

A membrane that is optically sensitive to pH has been prepared by suspending aminated polystyrene microspheres in a hydrogel. Light crosslinked poly(vinylbenzyl chloride) microspheres with diameters about 1 µm were prepared by dispersion polymerization and aminated with diethanolamine. These microspheres were suspended in a solution of hydroxyethylmethacrylate, which was then polymerized to form a hydrogel. The resulting membranes are turbid because the refractive index of the microspheres is greater than the refractive index of the hydrogel. Turbidity decreases with increasing wavelength. The turbidity of the membranes is greater in a base than in an acid. In acid, protonation of the amine group causes the polymer microspheres to swell. Swelling affects turbidity, both by increasing microsphere diameter and by reducing the microsphere refractive index so that it is closer to the refractive index of the hydrogel. The latter effect dominates in the membranes described here. A simplified theory to describe this behavior yields values that are consistent with observations. These membranes can be used for optical sensing in the visible and near-infrared regions, including wavelengths used for fiber optics telecommunications. They are expected to have excellent long-term stability. However, the microspheres prepared for this study respond very slowly because they are not sufficiently porous to allow easy analyte access to the interior of the polymer.