Application of Hydrophobic Solid Contacts in Ion-Selective Electrodes

Ionophore-based ion-selective electrodes (ISEs) are attractive tools for environmental monitoring, process control and clinical analysis. Recent developments in ISE research, such as single-use disposable sensors, require a cost-effective miniaturized electrode design. Replacing the internal filling solution of conventional ISEs by a suitable ion-to-electron transducer is one of the main challenges in fabricating robust, driftless and maintenance-free potentiometric solid-contact ion-selective electrodes (SC-ISEs). Due to their cost-effectiveness and easy fabrication, electrically conducting polymers (ECPs) applied as the ion-to-electron transducer for ISEs offer a promising solution to fabricate SC-ISEs. However, the water layer formation at the interface between the ion-selective membrane (ISM) and the solid-contact (SC) and the SC | electrode substrate interface results in potential instability of the ECP-based SC-ISEs. To eliminate the water layer formation, the use of hydrophobic ISMs and SCs is recommended.

In this work, the hydrophobicity and water uptake properties of the ISMs were studied by the Fourier transform infrared spectroscopy attenuated total reflection (FTIR-ATR) technique and the oven-based coulometric Karl Fischer titrations. The water uptake of plasticized polyvinyl chloride (PVC) and silicone rubber (SR) based ISMs was compared. Furthermore, the hydrophobic polyazulene (PAz) was introduced for the first time as the SC layer in SC-ISEs, because it was found that PAz had a relatively high water contact angle (CA) in its semiconducting form (98±11º). The K⁺-selective ISEs with pre-polarized PAz (at E ≤ 0.2 V) applied as the SC showed significantly improved potential stability when they were for the first time contacted with electrolyte solution. The reproducibility of their standard potential, E⁰, (SD ≤ ±7.9 mV, n=4) was also improved by the pre-polarization. Moreover, the hydrophobic perfluorinated anion-doped polypyrrole (PPy-PFOS) was chosen as the SC in K⁺-selective electrodes in further studies since PPy-PFOS is hydrophobic in its conducting form, contrary to most other ECPs, which are hydrophilic due to the charged polymer backbone. The pre-polarized PPy-PFOS based K⁺-SC-ISEs had a very small standard deviation of E⁰ (±0.7 mV, n=4), which is the best E⁰ reproducibility reported for ECP-based SC-ISEs. For both the PAz and PPy based SC-ISEs, no evidence of water layer formation was found, which is beneficial for their long-term stability.