Doctor of Philosophy
Department of Chemistry
Moulton, Simon E., Investigation of the electrochemical behaviour of metal electrodes in biological media, Doctor of Philosophy thesis, Department of Chemistry, University of Wollongong, 2002. http://ro.uow.edu.au/theses/2919
The development of an optoelectric device (TruScan™) for the detection of cervical cancer provides the framework for this thesis. TruScan™ stimulates the cervical tissue with low energy electrical pulses producing a tissue response. This response is recorded at the three electrode tip and the signal is interpreted for a tissue classification. The aim of this work was to measure the extent of protein adsorption onto metal electrodes under various conditions, and then investigate the effects this protein layer had on the electrochemical behaviour measured at the electrode/solution interface.
The adsorption trends of proteins, and the effect that adsorption has on electrochemical responses of metal electrodes has been studied using various experimental techniques. The effect of pH and electrode potential on adsorption trends has been investigated using the proteins human serum albumin (HSA) and human immunoglobulin G (Ig.G) at both gold (Au) and titanium dioxide (Ti02) metal surfaces.
This work showed that protein adsorbs spontaneously at metal electrodes at neutral pH. The protein adsorption trends followed electrostatic interactions, with the greatest amount of adsorption occurring when the electrode and protein had opposite charges, and the least amount when they possessed the same charge. The extent of protein adsorption also appeared to be influenced by the waveform of the applied potential. Only a small percentage of the total adsorbed protein could be removed. This was attributed to the denaturing of the protein allowing it to form multiple binding sites, which increased its binding strength. This result lead to the hypothesis of two types of bound protein, namely weakly and strongly bound. Spectroscopic studies showed that the presence of phosphate in the protein solution hinders adsorption, resulting in lower adsorbed amounts. Phosphate was also shown to partially displace preadsorbed protein from a Ti02 surface. The fact that not all of the preadsorbed protein was displaced strengthened the hypothesis of two types of bound protein.
The effect of protein adsorption on the electrochemical behaviour of metal electrodes was studied using electrochemical impedance spectroscopy and cyclic voltammetry. Upon adsorption of protein at Au and Ti02 the capacitance decreased, which was attributed to the formation of a proteinaceous layer on the electrode surface. This decrease in capacitance was accentuated when the electrode surface was treated in such a way that the surface area and roughness increased. The formation of a proteinaceous layer on the electrode surface had the effect of increasing the charge transfer resistance. The resistance to charge transfer was shown to increase as the amount of adsorbed protein increased due to the increase in proteinaceous layer thickness. The electrode/electrolyte interface, for both electrode materials, was successfully modelled using a modified Randies circuit.