Sensors forpH based on swelling and reflectivity of derivatized polystyrene beads and membranes

Kraton-modified amine-derivatized poly(vinylbenzyl chloride) beads prepared by suspension polymerization are sensitive to pH. They swell in acid due to protonation of amine groups and shrink in base. Polymer swelling generates a measurable swelling signal that is related to the concentration of hydrogen ions in solution. The sensor responded to pH in the range from 6.0 to 10.0. Bead size, level of porogenic solvents and Kraton G1652 affected the magnitude of sensor response and response time. Diethanolamine derivatized poly(VBC) membranes formulated with a small amount of Kraton G1652 reflect light. At low pH membranes turn clear. The observed optical change is caused by a decrease in the refractive index of the bulk polymer as it swells. A commercially available optical reflective device was used to measure changes in reflectivity of a swell/shrink membrane as a function of pH. The optical pH sensor responded to pH in the range from 6.0 to 8.0 with response times on the order of 10 seconds. Parameters which were important to optimizing sensor sensitivity, such as composition, membrane thickness, stability, reproducibility and effects of temperature and ionic strength, were evaluated. Another reflective membrane, poly-4-hydroxy-3-nitrostyrene, was also formulated and coupled with an ORD. The apparent pKa of the membrane is 8.5 and the membrane responded to pH in the range from 8.0 to 10.0, which is suitable for pH measurements of seawater. However, the sensor was subject to poor adhesion of the membrane to the surface of ORD. Monodisperse poly-4-hydroxy-3-nitro styrene microspheres were prepared by dispersion polymerization. These microspheres were embedded in a hydrogel membrane that was coupled to an ORD. Due to different refractive indices between hydrogel membrane and polymer microspheres, light is reflected. As polymer particles swell in base, a decrease in refractive index of polymer particles caused a decrease in reflected light at the hydrogel/polymer particle interface. The approach led to improved adhesion of the sensing element on the ORD surface and allowed maximum swelling of the polymer microspheres