Miniaturizable Ion-Selective Electrode System: Solid Contact Electrode and Liquid Junction Free Reference Electrode

Abstract

Ion-selective electrodes (ISEs) are widely used as important, analytical tools to determine the concentration of a broad range of ions for clinical analysis. As a potentiometric sensor, an ISE is comprised of two major parts: an ion-selective electrode and a reference electrode, both of which are vital in guaranteeing accurate and reliable measurement results. Conventional ISEs have an interior solution that forms a liquid contact with a selective membrane. However, complications due to evaporation and freezing in certain circumstances lead to irreproducibility, instability of the signals, and shortened life expectancy. In addition, miniaturization is an obstacle for this type of electrode. A universal method for a solid contact electrode that is calibration-free, durable, reproducible, and inexpensive to fabricate. A plasticized poly(vinylchloride) doped with tris(1,10-phenanthroline) cobalt(2+) tetrakispentafluorophenylborate and tris(1,10-phenanthroline) cobalt(3+) tetrakispentafluorophenylborate has been developed as a transducer layer. This layer has shown very reproducible potential in potassium chloride solutions with a potentil of standard deviation of 0.5 mV and response slope of 61mV / decade to varying concentrations. A redox buffer platform, based on the more lipophilic redox buffer consisting of the Co(III) and Co(II) complexes of 4,4'-dinonyl-2,2'-bipyridyl, was developed and applied to K<super>+</super>, Na<super>+</super>, Ca<super>2+</super>, H<super>+</super>, and CO₃<super>2-</super> with emf values of an electrode-to-electrode standard deviation as low as 0.7 mV (2% error in concentration) The reference electrode must maintain a constant potential over long periods of continuous measurements. Used in real life samples, salt bridges for conventional reference electrodes clog with proteins and lipids, suffer from contamination by sample components. A new method was developed that involves the application of a current pulse to a hydrophobic ion-doped membrane, thereby controlling transmembrane ion fluxes to obtain a sample independent reference electrode potential. The concentration of the ions released into the sample depended on the current amplitude and length and can be explained quantitatively by diffusion theory. The observable stability of the potential exhibited in this study by reference electrodes exposed to serum was particularly promising in view of biological and medical applications that require long term monitoring.