Application of perturbation methods to voltammetry: from novel insights to `smart' voltage excitation protocol

Electrochemical sensing methodologies are used in a wide range of applications, from understanding the physics of electron transfer (ET) to process monitoring. From a plethora of electrochemical techniques, voltammetry, where the electrode voltage is excited in a predetermined manner, has been heavily applied for various chemical, biological, environmental and industrial measurements. For instance, the widely used cyclic voltammetry, where the voltage excitation is a ramp, has provided new insights in phenomena as diverse as neurotransmitter dynamics, protein ET or fuel cell technology. Recently, more complicated voltage inputs such as ac voltammetry have been applied in order to probe the electrochemical system under investigation on different timescales, explore the kinetics and thermodynamics of different processes or selectively target specific process dynamics, such as parallel reactions, leading to comparisons with NMR or impedance spectroscopy but with the advantage of including in vivo applications. Despite the obvious advantages of such voltammetric methodologies their application is demanding. The major challenge lies in the interpretation of the current response signal. Whilst previous work has revealed how the shape of the current response is related to different processes such as kinetic- or mass transport- control, it did not offer direct information about the relationship between the applied voltage and the resulting patterns in the current response. This is due to the highly nonlinear relation between the applied voltage and the transient current response which renders a direct association non-intuitive.How do the parameters of the applied voltage influence the electrochemical current response? Indeed, how could the applied voltage waveform be manipulated in such a manner to quantify the underlying dynamics even more efficiently? Using voltammetry the experimentalist can apply a wide variety of voltage waveforms that can be used to analyse the electrochemical process. Hitherto, such possibilities have remained unexplored due to the mainly empirical knowledge regarding such processes. For instance, cyclic voltammetry or square wave voltammetry, the two most popular voltammetric methods, were developed over 50 years ago and the techniques used to analyse them, mostly empirical, have remained essentially the same for the past two decades. The research proposed herein will enhance our understanding of the underlying phenomena and the governing parameters of such processes. Based on this knowledge we will design more efficient excitations and propose rules of extracting the information sought. For instance, the findings of this analysis could be used to enable harmonic time-dependent amplitude and frequency excitations, so-called chirps, to be used to provide fast and accurate information about various processes occurring on different timescales. This would be a significant step towards the use and application of 'tailored' voltage waveforms to interrogate electrochemical systems. In this project we propose to study these effects theoretically using well-established analytical tools for four model cases: (a) an electrochemical species undergoing heterogeneous ET and 1-dimensional diffusion as in macroelectrode experiments; (b) apply the findings of (a) for very slow diffusion (D?0, where D is the diffusivity) in order to include cases for permanently adsorbed species on the electrode surface; (c) include uncompensated resistance in (a) and quantify its effect on the overall functionalities; and (d) an electrochemical species undergoing heterogeneous ET and 2-dimensional axis-symmetric diffusion as in microelectrode experiments. In order to conduct this research, we combine Dr Siggers', Dr Parker's and Dr Stone's expertise in perturbations methods, asymptotic analysis, fluid mechanics and biofluidics with Dr O'Hare's and Dr Anastassiou's background on theoretical and experimental aspects of voltammetry.