Solid-contact Potentiometric Anion-selective Electrodes - Receptors, Challenges, and Applications

An ion-selective electrode (ISE) is a type of chemical sensor, which produces an electrochemical signal that is in the ideal case specific to the activity of a certain type of ion, the primary ion. In practice, the signal is affected to some extent by the activities of other, interfering ions, but the selectivity can be enhanced by, e.g., incorporating a receptor molecule into the ion-selective membrane (ISM) of the ISE. ISEs for different inorganic ions, both cations and anions, are used routinely in various applications ranging from clinical analysis to environmental monitoring.
The development of ISEs that are highly selective for organic anions is made challenging by several factors. For example, most ISMs consist mostly of organic materials such as a polymer and a plasticizer, and organic anions can be strong interferents due to their lipophilicity. Designing a suitable receptor that is capable of discriminating anions with structural similarities presents its own challenges.
Several receptors for carboxylates were designed, synthesized, and characterized by collaborators at the University of Tartu, Estonia. A subset of these receptors were used to prepare solid-contact ion-selective electrodes (SC-ISEs) as sensor prototypes at Åbo Akademi University, Finland. The sensors were characterized as acetate-selective electrodes and found to be modestly selective. Anomalous selectivities were observed during this time and the phenomenon appeared to depend on the electrode bodies that were used to prepare the ISEs. Additional ISEs were prepared and characterized both to optimize the ISM composition and to investigate the issue affecting the selectivities.
An ISE incorporating one of the carboxylate receptors was successfully applied to the determination of benzoate in sample solutions prepared from frozen cranberries and lingonberries. Ion chromatography (IC) was used as a reference method to compare against. The results obtained with IC and the ISE were in agreement with each other and the literature.
The investigation into the anomalous selectivities was continued with the preparation of cation- and anion-selective electrodes. Both conventional ISEs with internal filling solution and SC-ISEs were prepared. The latter were prepared with electrode bodies made out of three different materials. Only the anion-selective electrodes prepared with electrode bodies made out of poly(vinyl chloride) (PVC) exhibited significantly different selectivities compared to the conventional ISE and the SC-ISE with an electrode body made of poly(tetrafluoroethylene). The selectivities of the SC-ISEs prepared from two different batches of PVC electrode bodies also differed from each other. Based on the results obtained with nuclear magnetic resonance spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy, additives present in the PVC electrode bodies are suspected to have affected the selectivities. However, the exact mechanism is not yet known and further investigation is required.