Improving the Sensitivity of Solid-Contact Ion-Selective Electrodes by Using Coulometric Signal Transduction

Abstract

A fundamental limitation of potentiometric ion sensors is their relatively low sensitivity due to the logarithmic dependence between potential and activity. Here we address this issue by exploring a recently developed coulometric transduction method for solid-contact ion-selective electrodes (SCISEs). Spin-coated thin-layer ion-selective membranes are used to lower the membrane resistance and shorten the response time of the SCISEs. When using coulometric transduction, an optimized design of the K + -SCISE is able to detect a concentration change of 5 μM at a concentration level of 5 mM, corresponding to a 0.1% change in K + activity. This indicates that SCISEs can provide extremely high sensitivity when employing coulometric transduction. Impedance measurements show that the coulometric transduction process for the K + -SCISE is limited by diffusion even for very thin ion-selective membranes. On the other hand, the H + -SCISE shows a low impedance and a fast coulometric response that is related to the high mobility of H + in the H + -selective polymeric membrane as well as in the solid contact layer. The coulometric transduction method was used to detect small changes of pH in seawater and found to improve the sensitivity compared to classical potentiometry. The coulometric method was briefly tested also for determining activity changes of K + in a serum sample.

Coulometric response characteristics of solid contact ion-selective electrodes for divalent cations

“…Potentiometric calibrations were performed to verify the proper functioning of the Pb 2+ -and the Ca 2+ -selective SCISEs. Both potentiometric and coulometric calibrations were done following the protocol from our previous work [26]. In these measurements, Pb 2+ -and Ca 2+ -SCISEs with variable loading of the PEDOT(PSS) (polymerization charge 1, 5, and 10 mC) covered either with spin-coated thin-layer ISM or dropcast thick-layer ISM were studied.…”

“…The analytical signal is thus a transient current that is measured and integrated to obtain the charge corresponding to the change in the logarithm of ion activity in solution [23,24]. The method was investigated using PEDOT as solid contact in SCISEs for the detection of monovalent ions, such as K + [24][25][26], Cl − [27] and H + (pH) [26]. The unique feature of this coulometric method is that the analytical signal, i.e., charge (for a given change in ion activity) is proportional to the redox capacitance of the solid contact, meaning that the signal can be amplified by increasing the thickness (loading) of the solid contact layer [24].…”

“…An initial drawback of the coulometric method was a relatively long response time [23,24]. However, by lowering the bulk resistance of the ion-selective membrane, either by increasing the geometrical surface area of the electrode [25] or by using spincoated thin-layer ISMs [26,27], it was possible to significantly shorten the response time of the coulometric signal readout method. Based on a capacitance model used earlier for SCISEs [8], the theory of the coulometric readout was developed for Cl − -SCISEs [27].…”

“…Based on a capacitance model used earlier for SCISEs [8], the theory of the coulometric readout was developed for Cl − -SCISEs [27]. Furthermore, coulometric SCISEs were evaluated for measurement of pH in seawater [26] and K + in serum [26,28].…”

“…A theoretical capacitive model based on a series RC circuit gave an excellent explanation of the coulometric response of a Cl − -SCISE [27]. However, impedance measurements showed that the coulometric response for K + -SCISEs was limited by diffusion even for thin-layer ISMs [26]. Mikhelson et al presented recently an extended theoretical model for the amperometric and coulometric response of K + -SCISEs by considering also concentration polarization in the ISM [28].…”