Solid-contact ion-selective electrodes with high sensitivity : comparison of potentiometric and coulometric signal transduction

Abstract

In this research work, the novel coulometric signal transduction method, recently developed for improving the precision of ion sensing, was further investigated. Potassium-, hydrogen-, and chloride- solid-contact ion-selective electrodes (SCISEs) were prepared and studied in this work. The conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT), doped either with poly(styrene sulfonate) (PSS), i.e. PEDOT:PSS , or chloride (Cl-), i.e. PEDOT:Cl-, was used as ion-to-electron transducer in the prepared SCISEs. By using sequential addition-dilution procedure, the electrodes were tested using the two-electrode potentiometric, and three-electrode coulometric (chronoamperometric) signal transduction methods. The measurements were carried out in solutions with and without interfering ions. The results for potassium SCISEs show that small changes of potassium ion concentration cannot be monitored by potentiometry, while clear current (charge) peaks are observed when using the coulometric signal transduction method. Electrochemical impedance spectroscopy (EIS) was used to probe the theoretical concept of RC circuit that was recently developed for chloride SCISEs in coulometric signal transduction. With capacitance of the solid-contact and the thickness of the ion-selective membrane (ISM) kept constant, all three types of SCISEs imply the presence of correlation between the equilibration times and the resistances of the ISMs. To test this further, the electrodes with PEDOT:PSS and PEDOT:Cl- solid-contacts, without ISMs, were exposed to changes in potassium, hydrogen and chloride concentrations. In this case, the PEDOT:PSS electrodes behaved according to the ionic mobilities of hydrogen and potassium ions, with the equilibration times for potassium ions being five times higher than those for hydrogen ions. On the contrary, equilibration times with PEDOT:Cl- electrodes were comparably fast, almost instantaneous, for chloride ions, clearly showing a different ionic transduction in PEDOT:Cl- compared to PEDOT:PSS. By performing water-layer test, it was observed that having thin ISMs makes the SCISEs susceptible to water-layer formation.