PEDOT(PSS) as Solid Contact for Ion-Selective Electrodes: The Influence of the PEDOT(PSS) Film Thickness on the Equilibration Times

Abstract

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.

Differences in Electrochemically Deposited PEDOT(PSS) Films on Au and Pt Substrate Electrodes: A Quartz Crystal Microbalance Study

“…In our earlier studies, the significant differences in the equilibration times and standard potentials of PEDOT(PSS)‐based K + ISEs built on Au and Pt substrate electrodes were explained by the more hydrophilic properties of PEDOT(PSS) films on Pt . Here, to investigate the source of the differences in PEDOT(PSS) film chemistries deposited with the same protocol onto Au and Pt, we used EQCM to follow the change in frequency during electrochemical deposition of the polymer.…”

“…While a statistically significant difference was recorded in the contact angles over thin, 0.1 μm PEDOT(PSS) films (30°±3° on Pt and 35°±4° on Au), the difference in the water contact angles over thicker, 4 μm films faded (63°±1° on Pt and 63°±4° on Au). In our previous publication, based on photon energy dependent SR‐XPS measurements, the hydrophilicity of the PEDOT(PSS) films was related to the PSS − /PEDOT ratio . The contact angle data are in agreement with the SR‐XPS findings, i. e., thin PEDOT(PSS) films are more hydrophilic on Pt than on Au.…”

“…To confirm the SR‐XPS results, PSS − /PEDOT ratios in this study were calculated from XPS spectra collected before the first etch cycle in order to obtain ratios on the surface of pristine PEDOT(PSS)‐coated Au and Pt electrodes. The PSS − /PEDOT ratios on Au and Pt in this study (0.43 and 0.91) closely match those obtained with SR‐XPS (0.24 and 1.16) on both Au and Pt, respectively .…”

“…These findings were supported by synchrotron radiation X‐ray photoelectron spectroscopy (SR‐XPS) analysis, which indicated that the PSS − to PEDOT ratio was significantly larger on Pt than on Au. These data suggested that the PEDOT(PSS) film is more hydrophilic on Pt compared to Au . To trace the source of the differences in the PEDOT(PSS) film chemistries and electrochemical behaviors on Au and Pt substrates, PEDOT(PSS) was electrochemically deposited onto Au and Pt‐coated quartz crystals while frequencies were recorded with an electrochemical quartz crystal microbalance (EQCM).…”

Comparison of Different Commercial Conducting Materials as Ion-to-Electron Transducer Layers in Low-Cost Selective Solid-Contact Electrodes

“…Among the large variety of the proposed solid-contact (SC) compositions used to establish a stable and reversible contact between an ion-selective membrane and metal, conducting polymers with mixed electron-ion conductivity have received the most attention [23][24][25][26][27]. Incorporation of nanostructured materials into the conducting polymer layer permits to obtain high double layer capacitance, thus enhancing potential stability of SCISEs and lowering detection limits [28,29].…”