Coulometric Transduction Method for Solid-Contact Ion-Selective Electrodes

As one of the pioneering works in the field of solid-contact ion-selective electrodes (SCISEs), a new coulometric transduction method was proposed by Bobacka’s group from Åbo Akademi University. This thesis work was carried out to deepen the understanding of the coulometric transduction method for SCISEs.

In this thesis, cation-selective SCISEs (K+, H+, Ca2+ and Pb2+) and anion-sensitive SCISEs (Cl-, NO 3 , ClO 4 and SO2- 4 ) were studied utilizing chronoamperometry, coulometry, and electrochemical impedance spectroscopy (EIS). The conducting polymer poly(3,4-ethylenedioxythiophene) PEDOT was used as ion-to-electron transducer in the fabrication of SCISEs. Cyclic voltammetry was performed for characterizing PEDOT-based films. In the coulometric transduction method, instead of measuring the potential between the SCISE and reference electrode, the potential is held constant and the current between the SCISE and counter electrode is measured. A transient current is obtained whenever the primary ion activity changes in the solution. Integration of the current-time curve results in the cumulated charge that is used as the analytical signal. The cumulated charge is linearly proportional to the logarithmic activity of the primary ion, and inversely proportional to the charge of the detected ion. The current and cumulated charge responses of cation-selective SCISEs and anion-sensitive SCISEs are of opposite sign under the same experimental protocol, in analogy with potentiometry. A thicker solid-contact film, i.e. larger redox capacitance of conducting polymer, leads to amplification of the cumulated charge, at the price of a longer response time.

The amperometric and coulometric response can be improved by increasing the geometrical area of the SCISE surface and by applying a thinner ion-selective membrane (ISM) deposited using a spin-coating technique. A larger electrode area and a thinner ISM lower the resistance of the ISM to facilitate the ion transport through the SCISE, i.e. the ISM and solid-contact film.

Under optimized conditions, the high sensitivity of the coulometric method for the K+-selective SCISE made it possible to detect 5 µM concentration changes at the starting solution with 5 mM concentration, i.e. 0.1% change in activity was detectable. Theoretical modeling of the coulometric method was done with Cl-sensitive SCISEs. A 0.2% change in activity was detectable in Cl-sensitive SCISEs utilizing the coulometric method. The coulometric method was also applied for detecting pH changes in seawater and concentration changes of K+ ions in control serum samples.

Some interesting features were observed from coulometric results of divalent cation (Pb2+- and Ca2+-) SCISEs. The coulometric response of the Pb2+-SCISEs was mainly limited by ion transport in the poly(3,4 ethylenedioxythiophene) poly(styrene sulfonate) PEDOT(PSS) solid-contact layer, while the coulometric response of Ca2+-SCISEs was mainly dependent on the ion transport through the ISM. These results are in qualitative agreement with the obtained electrochemical impedance spectra.

Furthermore, anion sensors based on PEDOT doped with different counterions were studied using the coulometric transduction method and cyclic voltammetry. The obtained results indicate that the counter ions in the electrolyte solution used during electropolymerization play a role in the formation of the PEDOT solid-contact film, as well as influence the cyclic voltammograms when cycling in the electrolyte solution. This may be related to anion affinity following the Hofmeister series and the size (hydrated radius) of the anions. For a given polymerization charge, the nitrate counterion was found to give a higher yield of PEDOT, compared to chloride.