Effects of Viscosity and Salt Interference for Planar Iridium Oxide and Silver Chloride pH Sensing Electrodes on Flexible Substrate

Abstract

The equivalency of pH measurements between aqueous and non-aqueous or viscous solutions is of great interest in biomedical applications as well for processing food and pharmaceuticals. Commercial glass-type electrodes have practical limitations, such as bulky sizes and membrane clogging in viscous environments. In this study, planar and flexible electrochemical pH sensors with iridium oxide as the sensing film have been developed by sol-gel and oxidation processes. A reference electrode was prepared by screen printing Ag/AgCl ink on the same polyimide substrate. The small form factors of the planar flexible electrodes provide an advantage in small volume or conformal surface measurements. Cyclic voltammetry was performed in different pH solutions. The electrode originally produced a response of −70.1 mV/pH at room temperature in aqueous solutions. The sensitivities were reduced when salt was added into the buffer solutions, although output potentials were increased. Sensing performances in a wide range of viscous conditions with various concentrations of added salt have been analyzed to study their effects on pH-sensing responses. Suitable calibration techniques using aqueous buffer solutions were studied for output potentials and their respective pH readings in viscous salt-added solutions. The mechanisms affecting output potentials are explained and results matched well for two different thickening agents. Specificity to pH changes measured by the planar IrOx-Ag/AgCl pH electrodes showed how the potential-pH calibration should consider the interference effect of salt. The viscosity effects on pH reading errors became more pronounced as solution viscosity increased. Comparisons of pH readings to those from a commercial glass-bodied pH meter indicated that the planar electrodes provided predictable pH deviations that were confined to a limited range. The planar IrOx-Ag/AgCl electrodes on flexible polyimide substrates have mostly been demonstrated with aqueous solutions in various diagnostic and environmental monitoring applications. This work provides more insights into pH sensing performance when the fluid is viscous and contains salt, which often is the case in biomedical and food-processing applications.