Equilibration Time of Solid Contact Ion-Selective Electrodes

Papers published on ion-selective electrodes (ISEs) generally report on the performance characteristics of these devices after long, extensive conditioning. Conditioning refers to the equilibration of the ion-selective electrode in an aqueous solution before the measurement of the sample. The requirement for long and repeated conditioning is a significant burden in a variety of applications, for example, single-use sensors aimed for in vivo or field applications and solid contact (SC) ISEs, which were developed to provide simple, mass-produced sensors that have the potential to be implemented without calibration and extensive conditioning. In this study we recorded the potential of SC K(+), Na(+), and H(+) ISEs as a function of time following their first contact with an aqueous electrolyte solution and used these transients to determine their equilibration times. The SC electrodes were built on Au, Pt, and glassy carbon (GC) substrates using galvanostatically deposited conductive polymer (PEDOT(PSS(-)), poly(3,4-ethylenedioxythiophene) polystyrenesulfonate) as ion-to-electron transducer (solid internal contact) between the ion-selective membrane and the substrate. The SC electrodes built on GC and Au had significantly shorter equilibration times (between 5 and 13 min) than the SC electrodes built on Pt substrates (>60 min). Such significant differences in the equilibration times of SC ISEs built on different substrate electrodes are reported here for the first time. These unexpected findings suggest that the interface between the conductive polymer and the electron-conducting substrate (EC) has significant influence on the long-term dynamic behavior of SC ISEs.