Voltammetric Ion Sensing with Ionophore-Based Ion-Selective Electrodes Containing Internal Aqueous Solution, Improving Lifetime of Sensors

The possibility of voltammetric ion sensing is demonstrated, for the first time, for ion-selective electrodes (ISEs) containing an internal aqueous solution. ISEs selective to calcium, lithium and potassium ions are used as model systems. The internal solution of the ISEs contains a chloride salt of the respective cation and a ferrocenemethanol or ferrocyanide/ferricyanide redox couple. A platinum wire is used as the internal reference electrode. It is shown, theoretically and experimentally, that the dependence of oxidation and reduction peak potentials on the sample composition obeys the Nernst law, while the peak currents virtually do not depend on the sample composition. Thus, the electrode behavior is similar to that reported by Bakker's group for solid contact ISEs with ultra-thin membranes (200-300 nm). It is shown that the use of classical ISEs with relatively thick membranes (100-300 µm) and internal aqueous solution allows for the sensor lifetime of about one month. It is also shown that use of a suitable background electrolyte allows for improvement of the detection limits in voltammetric measurements with ISEs.

Ionophore-Based Ion-Selective Electrodes in Non-Zero Current Modes: Mechanistic Studies and the Possibilities of the Analytical Application

Abstract

This mini review briefly describes (i) literature data on the non-zero current measurements with ionophore-based ion-selective electrodes (ISEs) aimed at fundamental studies of the mechanism of their potentiometric response, and (ii) the data on the possibilities of analytical applications of ISEs in voltametric and constant potential chronoamperometric/coulometric modes, in particular the K+ ion assay in blood serum with the sensitivity of 0.1%. A special attention is paid to the basics of voltammetry and chronoamperometry/coulometry with the ionophore-based ISEs, and to how and why these methods differ from the classical voltammetry and coulometry.

Plasticized PVC Membrane Modified Electrodes: Voltammetry of Highly Hydrophobic Compounds

In the last 50 years, plasticized polyvinyl chloride (PVC) membranes have gained unique importance in chemical sensor development. Originally, these membranes separated two solutions in conventional ion-selective electrodes. Later, the same membranes were applied over a variety of supporting electrodes and used in both potentiometric and voltammetric measurements of ions and electrically charged molecules. The focus of this paper is to demonstrate the utility of the plasticized PVC membrane modified working electrode for the voltammetric measurement of highly lipophilic molecules. The plasticized PVC membrane prevents electrode fouling, extends the detection limit of the voltammetric methods to sub-micromolar concentrations, and minimizes interference by electrochemically active hydrophilic analytes.

Quantitative Analysis of Redox-Inactive Ions by AC Voltammetry at a Polarized Interface between Two Immiscible Electrolyte Solutions

The interface between two immiscible electrolyte solutions (ITIES) is ideally suited to detect redox-inactive ions by their ion transfer. Such electroanalysis, based on the Nernst-Donnan equation, has been predominantly performed using amperometry, cyclic voltammetry, or differential pulse voltammetry. Here, we introduce a new electroanalytical method based on alternating-current (AC) voltammetry with inherent advantages over traditional approaches such as avoidance of positive feedback iR compensation, a major issue for liquid|liquid electrochemical cells containing resistive organic media and interfacial areas in the cm² and mm² range. A theoretical background outlining the generation of the analytical signal is provided and based on extracting the component that depends on the Warburg impedance from the total impedance. The quantitative detection of a series of model redox-inactive tetraalkylammonium cations is demonstrated, with evidence provided of the transient adsorption of these cations at the interface during the course of ion transfer. Since ion transfer is diffusion-limited, by changing the voltage excitation frequency during AC voltammetry, the intensity of the Faradaic response can be enhanced at low frequencies (1 Hz) or made to disappear completely at higher frequencies (99 Hz). The latter produces an AC voltammogram equivalent to a "blank" measurement in the absence of analyte and is ideal for background subtraction. Therefore, major opportunities exist for the sensitive detection of ionic analyte when a "blank" measurement in the absence of analyte is impossible. This approach is particularly useful to deconvolute signals related to reversible electrochemical reactions from those due to irreversible processes, which do not give AC signals.