Characterization of Na-selective solid-contact ion-selective electrodes using coulometric signal transduction

In a world where receiving, sending and interpreting information has become faster than ever, it is critical to develop analytical methods which perform in accordance with the society’s ever growing needs. With the requirement of on-site analyses increasing, this need has to be met in a fast, portable and straightforward way. Ion-selective electrodes (ISE) have proven their worth as reliable, fast and rugged analysis tools, as is shown by their extensive use in clinical and industrial analyses. With the increasing demand, the ion-selective electrode field keeps on extending - coming up with new solutions to make electrodes easier to manufacture, easier to calibrate and to improve their accuracy. Solid-contact ISEs (SC-ISE) are of interest because they are easy to construct and miniaturize, because they lack an internal solution. With the appearance of solid-contact electrodes, new methods to characterize them have also emerged. The coulometric signal read-out method has been developed due to interest in measuring smaller changes of analyte concentrations and characterization of solid-contact ion-selective electrodes. The method has been tested for SC-ISEs selective towards K+, H+ and Cl-. This work covers the creation and characterization of solid-contact sodium selective electrodes (Na+-SCISEs), made with three different membrane thicknesses. The electrodes are characterized with potentiometry and with the relatively new coulometric signal transduction method. In order to study the influence of different membrane thicknesses, a spin-coating protocol for thin membranes was developed and optimized. Three thicknesses were used in this study – two sets of electrodes with spin-coated membranes and one set with drop-casted membranes. The coulometric signal read-out method was used to calibrate the electrodes and check the performance for small changes in the Na+ ion concentration. The method involves keeping the working electrode potential constant against the reference electrode, so when a change in the activity of the primary ion in the sample solution takes place, a difference in the potential is caused at the membrane-solution interface. A transient current forms to compensate for that difference, resulting from a change in the redox state of the conducting polymer solid-contact poly(3,4–ethyledioxythiophene) poly(styrene-sulphonate) (PEDOT(PSS)). The current is integrated over time to obtain the charge, which can be related to the change in the activity of the analyte and therefore change in concentration of the analyte ions. The Na+-SCISE in this work were also characterized with potentiometry to confirm their functionality as well as to compare the two methods with each other. The three sets of Na+-selective SC-ISEs with three different membrane thicknesses were compared both according to their performance in potentiometric and coulometric signal read-out method. Based on the results, it can be concluded that spin-coated electrodes having thinner membranes perform better with the signal read-out method, while drop-casted thick membrane electrodes have lower detection limits and slopes closer to the Nernstian ideal in potentiometry. Drop-casted electrode membranes had longer response time in the coulometric signal read-out method (over 5 minutes) so the equilibrium was not reached for drop-casted electrode measurements with coulometry in the experiments of this study. Medium-membrane electrodes exhibited steeper slopes than thin-membrane electrodes, possibly because the resistance of the membrane is high enough to keep unwanted side-reactions from happening