The aim of this study was to find a conducting polymer-based solid contact (SC) for ion-selective electrodes (ISEs) that could become the ultimate, generally applicable SC, which in combination with all kinds of ion-selective membranes (ISMs) would match the performance characteristics of conventional ISEs. We present data collected with electrodes utilizing PEDOT-C, a highly hydrophobic derivative of poly(3,4-ethylenedioxythiophene), PEDOT, as SC and compare its performance characteristics with PEDOT-based SC ISEs. PEDOT-C has not been used in SC ISEs previously. The PEDOT-C-based solid contact (SC) ion-selective electrodes (ISEs) (H, K, and Na) have outstanding performance characteristics (theoretical response slope, short equilibration time, excellent potential stability, etc.). Most importantly, PEDOT-C-based SC pH sensors have no CO interference, an essential pH sensors property when aimed for whole-blood analysis. The superhydrophobic properties (water contact angle: 136 ± 5°) of the PEDOT-C SC prevent the detachment of the ion-selective membrane (ISM) from its SC and the accumulation of an aqueous film between the ISM and the SC. The accumulation of an aqueous film between the ISM and its SC has a detrimental effect on the sensor performance. Although there is a test for the presence of an undesirable water layer, if the conditions for this test are not selected properly, it does not provide an unambiguous answer. On the other hand, recording the potential drifts of SC electrodes with pH-sensitive membranes in samples with different CO levels can effectively prove the presence or absence of a water layer in a short time period.

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.

To understand the rate determining processes during the equilibration of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate-based (PEDOT(PSS)-based) solid contact (SC) ion-selective electrodes (ISEs), the surfaces of Pt, Au, and GC electrodes were coated with 0.1, 1.0, 2.0, and 4.0 μm thick galvanostatically deposited PEDOT(PSS) films. Next, potential vs time transients were recorded with these electrodes, with and without an additional potassium ion-selective membrane (ISM) coating, following their first contact with 0.1 M KCl solutions. The transients were significantly different when the multilayered sensor structures were assembled on Au or GC compared to Pt. The differences in the rate of equilibration were interpreted as a consequence of differences in the hydrophilicity of PEDOT(PSS) in contact with the substrate electrode surfaces based on X-ray photoelectron spectroscopy (XPS) and synchrotron radiation-XPS (SR-XPS) analysis of 10-100 nm thick PEDOT(PSS) films. The influence of the layer thickness of the electrochemically deposited PEDOT(PSS)-films on the hydrophilicity of these films has been documented by contact angle measurements over PEDOT(PSS)-coated Au, GC, and Pt electrode surfaces. This study demonstrates that it is possible to minimize the equilibration (conditioning) time of SC ISEs with aqueous solutions before usage by optimizing the thickness of the SC layer with a controlled ISM thickness. PEDOT(PSS)-coated Au and GC electrodes exhibit a significant negative potential drift during their equilibration in an aqueous solution. By coating the PEDOT(PSS) surface with an ISM, the negative potential drift is compensated by a positive potential drift related to the hydration of the ISM and activity changes at the PEDOT(PSS)|ISM interface. The potential drifts related to activity changes in the ISM have been determined by a novel adaptation of the "sandwich membrane" method.

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Studying the responses of potassium ion‐selective electrodes with poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate), PEDOT(PSS), as solid contact (SC) revealed significant differences in the equilibration times and standard potentials of the electrodes fabricated on Au or Pt as substrate electrodes. To trace the source of these differences, PEDOT(PSS) films were deposited under the same conditions onto Au and Pt electrodes on the surface of 10 MHz quartz crystals. During the galvanostatic polymerization, the frequency decrease of the quartz crystal was monitored by an electrochemical quartz crystal microbalance (EQCM). In the initial 15 seconds of the electrochemical deposition, the rate of PEDOT(PSS) polymer growth was significantly faster on Au compared to Pt although the current density used for the deposition was the same. Consequently, the total frequency change after a given electrolysis time was always larger on Au compared to Pt, indicating a larger deposited mass or PEDOT(PSS) layer thickness. The differences in the thicknesses of the PEDOT(PSS) films on Au and Pt could be quantitatively confirmed by X‐ray photoelectron spectroscopy (XPS) etching studies. Scanning electron microscopy (SEM) analysis of PEDOT(PSS) films on Au and Pt also showed characteristic differences.