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pH sensor based on swellable poly(3-nitro-4-hydroxystrene) microspheres in a hydrogel

A new polymer material for pH sensing is described in this work. The optically sensitive membranes consists of lightly crosslinked poly(3-nitro-4-hydroxystyrene) microspheres embedded in a hydrogel matrix of polyvinyl alcohol (PVA). The microspheres are designed to swell and shrink as a function of hydrogen ion concentration accompanying the deprotonation and protonation of the hydroxyl group on the polymer backbone. The optical properties of the membranes are affected by both the change in size of the microspheres and the change in refractive index that accompanies swelling. These properties can be measured as a change in membrane turbidity at the interface between the polymer particle and the hydrogel matrix by a spectrophotometer or fiber optic sensor device. Microspheres are prepared by seeded emulsion polymerization of 4-acetoxystyrene followed by hydrolysis and nitration to introduce the nitro-phenol functional groups. The magnitude of the change in turbidity with pH changes were studies. Other parameters such as loading of the particles and ionic strength effects were also studied.