Pre-Polarized Hydrophobic Conducting Polymer Solid-Contact Ion-Selective Electrodes with Improved Potential Reproducibility

Electrically conducting polymers (ECPs) are one of the most popular types of materials to interface ion-selective membranes (ISMs) with electron-conducting substrates to construct solid-contact ion-selective electrodes (SCISEs). For optimal ion-to-electron transduction and potential stability, the p-doped ECPs with low oxidation potentials such as PPy need to be generally in their conducting form along with providing asufficiently hydrophobic interface to counteract the aqueous layer formation. The first criterion requires that the ECPs are in their oxidized state, but the high charge density of this state is detrimental for the prevention of the aqueous layer formation. We offer here a solution to this paradox by implementing a highly hydrophobic perfluorinated anion (perfluorooctanesulfonate, PFOS⁻) as doping ion by which the oxidized form of the ECP becomes hydrophobic. The proof of concept is shown by using polypyrrole (PPy) films doped with PFOS⁻ (PPy-PFOS) as the solid contact in K⁺-selective SCISEs (K⁺-SCISE). Prior to applying the plasticized poly(vinyl chloride) ISM, the oxidation state of the electrodeposited PPy-PFOS was adjusted by polarization to the known open-circuit potential of the solid contact in 0.1 M KCl. We show that the prepolarization results in a hydrophobic PPy-PFOS film with a water contact angle of 97 ± 5°, which effectively prevents the aqueous layer formation under the ISM. Under optimal conditions the K⁺-SCISEs had a very low standard deviation of E⁰ of only 501.0 ± 0.7 mV that is the best E⁰ reproducibility reported for ECP-based SCISEs.