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A miniature conductimetric pH sensor has been developed, based on the measurement of the conductivity of a pH-responsive hydrogel. The microfabricated sensor is constructed by coating planar interdigitated electrode arrays with a photolithographically patterned hydrogel membrane. The hydrogel sensing layer swells or shrinks to a hydration determined by the pH of the solution in which it is immersed. This leads to a corresponding increase or decrease in the mobility of the ions partitioned by the gel. The sensitivity of ion mobility to small changes in hydration leads to sensor resistance changes as large as 45% per pH unit at physiological pH. Since the sensor operation is based on changes in ion mobility, the sensor operates in high ionic strength solutions, whereas most conductimetric sensors to date have been limited to use in solutions with low mobile ion content. The sensor response time is approximately five minutes, and appears to be limited by the protonation/deprotonation of the polybasic hydrogel. Devices have operated for more than six weeks with a 50% loss of sensitivity over three weeks.
Sheppard et al. (Tue,) studied this question.