Effect of Biofouling on Electrochemical Impedance Parameters and Sensitivity of Polyvinyl Alcohol-Coated Iridium Oxide-Based pH Biosensors

Implantable or minimally invasive biosensors suffer from biofouling, which limits their functionality and lifetime postimplantation. To address this issue, hydrophilic coating of hydrogels and self-assembled monolayers have been used to slow down the process of biofouling. This, in addition to a regular manual calibration, can lead to reliable sensor functionality and lifetime at the cost of user/patient compliance and comfort. To address the problem of repeated calibrations and to study the effect of biofouling on the electrode–electrolyte interactions, we explore the link between the degradation of sensitivity and the changes in impedance parameters of iridium oxide-based pH sensors coated with polyvinyl alcohol (PVA) hydrogel layers. Results indicate that coating IrOx-based pH sensors with PVA hydrogel layers leads to reduced open-circuit potential (OCP) drift which, in turn, leads to easier and more accurate measurements. It also leads to reduced degradation in sensitivity over time. In the presence of a PVA hydrogel layer, the sensitivity degradation follows an approximately linear trend compared to the sigmoidal degradation curve in the absence of PVA hydrogel. Furthermore, it was also observed that with the reduction in sensitivity, the change in internal resistance and double-layer capacitance readings from in situ electrochemical impedance spectroscopy (EIS) measurements over days also reduced, establishing a link between sensitivity and EIS parameters in metal-oxide-based pH sensors. In addition to highlighting the efficacy of PVA layers in reducing biofouling, the impact of PVA on the sensitivity and EIS parameters of the sensor and the relationship between sensitivity and EIS parameters are also presented, which may aid in in situ autocalibration of sensors.