Calibration-free ion-selective electrodes and theoretical insights and practical applications of ionic liquid references

Abstract

This dissertation focuses on three main research topics. One is the improvement of ion-selective electrodes (ISEs) for calibration-free and long-term stability (Ch. 2). The second is the development of reference electrodes by using ionic liquid doped polydimethylsiloxane (PDMS) reference membranes and colloid-imprinted mesoporous (CIM) carbon as solid contact (Ch. 3), and the third is the theoretical study of the limit of application of the ionic liquid reference electrodes (Ch. 4). Potentiometric sensors, including ISEs and reference electrodes, represent a significant segment of the field of electrochemical ion sensors. With the goal of developing cost-effective and portable analytical instruments, the use of all-solid-state ISEs and reference electrodes that comprise a solid interface for ion-to-electron conversion is particularly valuable. Compared to traditional ISEs, these all-solid-state alternatives demonstrate equivalent electrochemical functionality while offering the distinct benefits of easier maintenance and the possibility for miniaturization. With a view to potentiometric sensing with minimal calibration requirements and high long-term stability, this dissertation involves the development of CIM carbon with a covalently attached redox buffer (CIM-ph-Tpy-Co(III/II)-Tpy) as a novel solid-contact material for the fabrication of ISEs with minimal calibration requirements. The combined use of a high solid-contact surface area with the redox buffer results in a highly reproducible standard potential (E°), and it avoids leaching of the redox buffer from the ion-selective membrane (ISM) into the aqueous sample solution. Besides ISEs, in electrochemical sensing systems, a reference electrode serves to establish a constant potential as a reference point. To adapt them for implantable sensors, this dissertation also involves the construction of reference electrodes utilizing biocompatible PDMS doped with ionic liquids and incorporating CIM carbon as a solid contact. Size-exclusive sequestration of ionic liquid into CIM carbon pores was found but can be overcome by pre-saturating the CIM carbon with ionic liquid prior to silicone polymerization or by adding additional ionic liquid to the reference membrane. Ionic liquid-based reference electrodes have some advantages compared to conventional reference electrodes. However, one of the limitations of ionic liquid-based reference electrodes is the interference caused by the sample ions. This interference, relevant when sample ions enter the reference membrane, defines the electrode's limit of applicability (LOA). In the final project of this dissertation, it is demonstrated that the LOA for ionic liquid-based reference electrodes can be precisely predicted by considering the mobilities of the sample ions, the ionic liquid's cations and anions, as well as the thickness of the stagnant sample adhering to the reference membrane. This quantitative understanding is crucial for the informed selection of ionic liquids that maximize the LOA based on the expected concentrations and types of ions in the samples of interest.