Constant potential coulometric readout of ion-selective electrode is known to improve sensitivity by producing a current signal that is easier to identify than that of direct potentiometric measurement. The current spike is measured and integrated, giving charge information that is proportional to logarithmic of ion activity. The general coulometric readout utilizes an ion-to-electron transducing material as a capacitive layer for solid-contact ion selective electrode (SC-ISE) [1]. In this case, the charge transfer process may be limited by the capacitive behavior of the transducer, resulting in a long response time and current drifting form zero baseline. We presented recently for the first-time the use of an electronic capacitor instead of a solid-contact material to overcome the main drawbacks of constant potential capacitive readout [2]. The capacitor can be simply adapted to amplify the current signal so that it is optimally suitable for each application. Importantly, the current baseline drift is minimized owing to the ideally capacitive behavior of electronic capacitor. In that work, the electronic capacitor is discharged after each sample measurement by sequentially introducing a reference solution, which is required for a certain time to acquire again a zero-potential difference.

To improve this method for practical use, we report the use of an electronic circuit to automate the control the capacitive readout [3]. The open-circuit potential (OCP) of the reference solution is measured and stored by the potentiostat. In this method, the OCP value can be applied directly to the sample solution resulting in a sharp current spike. Discharging the capacitor is executed by short circuiting after each chronoamperometric measurement. Hence, the reference solution is no longer needed, resulting in shorter response time. The capacitive readout method with a 10 µF capacitor was used for sodium measurements in serum. The precision is found to be better than direct potentiometry, corresponding to 0.11 mmol L^-1 NaCl and 0.13 mmol L^-1 NaCl for standard solutions and pooled serum sample, respectively. Quantitative analysis of ion-selective electrode by this type of electronic circuit make the system become more robust and reliable.

[1]   E. Hupa, U. Vanamo, J. Bobacka, Electroanalysis, 2015, 27, 591-4.
[2]   P. Kraikaew, S. Jeanneret, Y. Soda, T. Cherubini, E. Bakker, ACS Sensors, 2020, 5, 650-4.
[3]   P. Kraikaew, S. K. Sailapu, E. Bakker, Sensors and Actuators B: Chemical, 2021.