Electrochemical sensors

This thesis focuses on the development of electrochemical sensors, in which three main perspectives are explored. First, bespoke pH sensors for near-neutral conditions are developed for both freshwater and seawater. Inspired by the importance and challenge of detection in seawater, bromide and chloride quantifications are subsequently studied. In addition to the optimisation of the working electrode, the potential of using a reference electrode based on a redox couple with soluble solution redox species in electrochemical equilibrium with a solid is analysed referring to the specific case of the Ag/AgBr/Br− reference electrode.
Chapter 1 serves as an introduction, providing essential background knowledge about fundamental electrochemistry and the electrochemical techniques employed throughout this thesis. Then, Chapter 2 offers a generic account of the chemicals, reagents and instrumentation employed with specific details being given in subsequent individual chapters.
Chapters 3 - 4 report the study of amperometric pH sensing using an iridium electrode for application under near-neutral conditions. In Chapter 3, the electrochemical behaviour of iridium under neutral conditions is first studied, providing insight information about the redox mechanism forming pH-sensitive iridium hydrous oxide, and the consequent in-situ electrochemical fabrication conditions required. Then, by using the square wave voltammetry, the pH sensing character of electrochemically generated material is revealed, and the methodology validated in freshwater. Building on the findings from the previous chapter, Chapter 4 presents a bespoke calibration-free pH sensor that utilises an in-situ modified iridium electrode for applications in seawater. The sensor is designed to be calibration-free by measuring the "super-Nernstian" response of Ir(III/IV) relative to the less sensitive upd H oxidation signal, with the pH reported on the total hydrogen ion scale. The optimized sensor can lead to a super-Nernstian response of high sensitivity in air-saturated seawater.
Chapters 5 and 6 focus on the chloride and bromide detection in seawater, of which the main challenges are the similar chemical properties between the two ions as halides and the presence of chloride at levels hundreds of times more concentrated than bromide, not to mention the complex matrix of the seawater. Analysis of bromide is first presented in Chapter 5. Noting the interference by chloride when present at high concentrations, traditional silver electrodes commonly used for amperometric halide measurements are seen to be not suitable. Instead, a bespoke reagent-free electrochemical bromide sensor is developed based on voltammetric oxidation at a macro-Pt electrode. By employing square wave voltammetry combined with the standard addition method, the proposed sensor is successfully validated in both artificial seawater and authentic natural seawater. In Chapter 6, three types of electrodes (Au, glassy carbon, and Pt) are further studied for the analysis of chloride and/or bromide in seawater. After studying their electrochemical behaviours in artificial seawater, we develop optimal voltammetric procedures for the detection using proper electrodes. Our findings indicate that the Au electrode is unsuitable for Cl−and/or Br−sensing due to its dissolution and passivation in ASW, while the use of glassy carbon results in poorly defined chloride and bromide signals. In contrast, platinum is identified as a favourable candidate for chloride detection in artificial seawater using square wave voltammetry. Based on the comprehensive analysis presented in Chapters 5 and 6, we recommend platinum electrodes for both bromide and chloride analysis in seawater due to their robust performance and reliable results.
In Chapter 7, we conduct an investigation of the voltammetry of a redox couple with soluble solution redox species in electrochemical equilibrium with a solid using the specific example of Ag/AgBr/Br−, using both experimental and computational approaches. Through the analysis of the voltammetric waveshape and the apparent transfer coefficient, we find that the process yields apparent transfer coefficients significantly exceeding unity, thus highlighting the advantage of employing a reference electrode with a 1:0 process.

Advisor: Compton, R