Nitrated poly(4 -hydroxystyrene) microspheres for optical pH and potassium ion sensing based on turbidity changes accompanying polymer sweller

Porous poly(4-acetoxystyrene) swellable microspheres with diameters approximately 1∼2 mum were prepared by seeded emulsion polymerization. Toluene was used as the porogenic solvent and divinylbenzene was used as the crosslinker. The seed particles with diameters approximately 0.5∼1 mum were prepared by dispersion polymerization without adding porogenic solvent and crosslinker. Functionality was introduced by two derivatization reactions, hydrolysis and nitration, to form nitrated poly(4-hydroxystyrene). These polymer microspheres swell at high pH due to the deprotonation of the hydroxyl group on the polymer backbone. Swelling is accompanied by an increase in water content which causes the polymer refractive index to decrease. These microspheres, were embedded in a hydrogel for pH sensing. When either dibenzo-18-crown-6 or valinomycin was co-immobilized on the polymer, they were then used to sense potassium ion. Poly(2-hydroxyethylmethacrylate) or poly(vinylalcohol) hydrogel membranes with embedded nitrated poly(4-hydroxystyrene) microspheres were prepared by photopolymerization. These membranes possess desirable optical properties for pH sensing. The refractive index of the hydrated hydrogel is constant and not affected by pH, but the refractive index of the microspheres does vary with pH. When the membrane is in contact with a buffer at high pH, the membrane turbidity decreases because the refractive index difference between the microspheres and the hydrogel decreases. The apparent pKa values can be adjusted by varying the nitrogen percentage of the microspheres, by controlling the conditions of the nitration reaction. The observed pK a value can be as low as 5.6 or as high as 10.2. The response time of the membrane with microspheres prepared by seeded emulsion polymerization utilizing PVA as the membrane matrix was 10∼15 seconds. Response times were longer for the poly(HEMA) matrix with embedded microspheres synthesized by dispersion polymerization. A very small amount of particles, 0.1 wt%, in a 127 mum thickness membrane was needed to obtain significant changes in turbidity. Stability tests showed that the poly(HEMA) hydrogel membranes were mechanically stable when they were stored in pH 4, pH 10, deionized water, and dry under room temperature, 80°C, or in sunlight for about a month. Membranes consisting of ionophore modified nitrated poly(4-hydroxystyrene) microspheres in a hydrogel were prepared for potassium ion sensing. The sensing concept can be modeled as a cation-exchange system. When the membrane is immersed in a solution of potassium ions, the neutral ionophore, DB18C6 or valinomycin, in the polymer will selectively bind the potassium ion to form the cation complex. The ion binding is accompanied by release of a proton to maintain electroneutrality. This introduces charged sites into the polymer causing it to swell. The observed detection limit was as low as 10-4 M potassium ion. The response time of the PVA membrane with microspheres prepared by seeded emulsion polymerization was ∼2 minutes