Electrochemical studies at modified carbon electrodes

Abstract

Electrochemistry finds widespread applications in the field of chemical analysis, so-called electroanalysis, as well as in electrosynthesis. The results obtained can be highly dependent on the chemical nature of the electrode used, and chemically modified electrodes are often employed to fine tune the electrochemistry to suit particular applications. This thesis is concerned with the investigation of the use of carbon materials, both as electrode substrates and electrode modifiers, primarily for electrochemical analysis. The work has been carried out using a range of electrochemical voltammetric techniques, as well as impedance measurements. A number of physical methods, including electron microscopy and X-ray photoelectron spectroscopy, have also been used to determine the physical and chemical structures of the materials employed. The use of boron-doped diamond electrode (BDDE) for the electrochemical detection of H₂O₂ was explored. Although BDDE shows no useful electrochemical response to H₂O₂, a good electrochemical signal can be obtained if the electrode is modified with silver nanoparticles or haemoglobin. The best results are obtained using electrode interfaces fabricated by binding haemoglobin in an active form to silver nanoparticles prepared by electrodeposition on the BDDE in the presence of the surfactant CH₃(CH₂)₁₅Br, permitting state of the art performance with a limit of detection (LOD) < 0.5μM. The presence of haemoglobin at the BDDE surface is also capable of catalysing the electrochemical reduction from Ag⁺(aq) to silver particles. However it reduces their adhesion to the electrode surface, hence they are lost to solution. This observation was used to demonstrate a viable process for the electrosynthesis of Ag nanoparticles, producing particles of about 10 nm diameter at a yield of approximately 50%. The effects of modifying a glassy carbon electrode with various forms of nanocarbon material for the electrochemical detection of phenolic compounds including hydroquinone (HQ) and dihydroxybenzene (DHB) were studied. The nanocarbons considered included carbon black, graphene nanoplatelets and nanodiamond, for which the former two materials were found to show a large increase in the detection sensitivity. It was shown that the simultaneous detection of HQ and DHB was possible using these electrodes, and in 'real' samples such as river water and green tea. Additional modification of the electrodes with tyrosine also permitted detection of phenol and p-cresol. The nanodiamond and carbon black modified electrodes were also employed for the electrochemical detection of Bisphenol A (BPA), which can be severely hampered by electropolymerisation of the oxidation products of BPA, producing rapid electrode fouling. However because of the inert nature of diamond surfaces, it is shown that this fouling process can be minimised by modifying the glassy carbon electrode with nanodiamond. Alternatively, it was also observed that for the carbon black modified electrode, a strong electrochemical response could be seen associated with the quinone forms produced by BPA oxidation. The associated electrochemical signal is also found to be relatively insensitive to electrode fouling, opening up an alternative strategy for the detection of BPA. Finally the use of a carbon black modified glassy carbon electrode for the detection of dopamine in the presence of the interfering compounds, ascorbic and uric acids, was studied. The carbon black modifier is shown to increase detection sensitivity, and help separate the electrochemical signals of the differing redox active species present in the solution.