Performance improvement of solid contact ion-selective electrodes: towards sensors with high reproducibility and stability

Abstract

This dissertation is focused on two aspects important aspects of the performance of ion-selective electrodes: device-to-device reproducibility and long-term stability. Both of these performance characteristics are particularly important considerations for devices intended to be used in applications where minimal maintenance and calibration, both initially and later recalibration, are highly desirable. Solid-contact ion-selective electrodes have been an area of significant research in these efforts to develop sensors which can fulfill these needs of high reproducibility and stability. One category of solid-contact ion-selective electrodes are those prepared with redox buffers, which can result in a high device-to-device reproducibility. New redox buffers were developed using ruthenium-based organometallic complexes, which were expected to provide high reproducibility due to the high electron transfer rate coefficients known to be an attribute of ruthenium complexes. However, the complexes tested proved to be too unstable to provide high reproducibility, with the best observed being ± 7.1 mV standard deviation. Additionally, a hydrophilic redox buffer was implemented with miniaturized, gel-supported inner filling solutions for the first time. Using a ferricyanide/ferrocyanide redox buffer with plasticized PVC selective membranes, the reproducibility was ± 1.1 mV, a value that was reproducible across different batches of devices. In addition to the high reproducibility, these electrodes also proved to have a relatively small long-term drift on the order of 40 µV/h, allowing for long time periods between required recalibrations. The long-term stability of solid-contact ion-selective electrodes with porous carbon solid contacts was also assessed through measurement of the capacitance of these electrodes. Experiments were undertaken to gain a clear understanding of the aspects of the ion-selective membrane which effect the measured capacitance. It was shown that the identity of the ions in the membrane, as well as the concentration of those ions, and the plasticizer used to prepare the membrane all affected the capacitance as measured using chronopotentiometry. Differential water uptake in the membrane with different ions did not explain the observations. These ion-selective electrodes were also studied at varying applied potentials, confirming a high degree of drift after charging, and revealing a change in capacitance across different applied potentials.