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.

Binary RuO2–CuO Electrodes Outperform RuO2 Electrodes in Measuring the pH in Food Samples

“…In this study, we attempted to measure the pH of the product and not homogenate, thus, setting a challenging task. The challenges of measuring the pH in viscous samples correlate well with the findings of Chawang et al, where authors have demonstrated that the viscosity of starch significantly influenced the measured potential of the iridium oxide electrode.…”

“… 30 Furthermore, the viscosity of the samples can negatively affect the potential of a metal oxide electrode. 33 A more detailed study of this phenomenon is necessary and will be addressed in our future work.…”

“…Fruit juices contain ascorbic acid (reducing agent) that negatively affects the performance of metal oxide electrodes . Furthermore, the viscosity of the samples can negatively affect the potential of a metal oxide electrode . A more detailed study of this phenomenon is necessary and will be addressed in our future work.…”

Timing matters: the overlooked issue of response time mismatch in pH-dependent analyte sensing using multiple sensors

“…15,16 These response times depend on factors such as electrode design, manufacturer, sample conditions, and are influenced by temperature, ionic strength, viscosity, and the presence of interfering chemical species (refer to Table S1 in the ESI † for the response times of some commercially available sensors). [17][18][19][20][21] Recent work by our research group addressed these problems by modifying amperometric microsensors with an acidic conversion chamber to measure the total pool pH-dependent analytes. This principle allowed for in situ conversion of the total pool to a single measurable species close to the sensing unit.…”

pH Monitoring in Biological Environment: An Overview of Conventional to Cutting-Edge Sensing Platforms

“…Other characterizations such as hysteresis, drift, fluctuation, and deviation test were performed to evaluate the stability as well as the repeatability of IrOx-based pH sensor. Further, the fabricated electrodes presented a suitable data from pH 4 to 9 when tested against a synthetic sweat [124]. Similarly, PEDOT:PSS and IrOx NMs provided the pH sensing matrix due to the redox chemistry of IrOx-complex as it exhibited Ir-III/Ir-IV redox couple, which ultimately triggered PEDOT:PSS chain that extracted electrons from the polymeric (PEDOT:PSS) film, causing to current flow across the film (Figure 13 (B)).…”