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Study of redox activity at modified liquid/liquid interfaces using electrochemical and physicochemical methods

Georganopoulou, Dimitra; (2001) Study of redox activity at modified liquid/liquid interfaces using electrochemical and physicochemical methods. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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The aim of this work was to establish an experimental basis for exploration of the reactivity of cell membrane-bound redox enzymes, by using electrochemistry at an aqueous/organic interface. The study presented in this thesis consisted of two parts. The first concerned the effect of surfactant adsorption on interfacial electron transfer. The second involved the measurement of activity of an enzyme within a surfactant-modified interface, catalysing the interfacial electron transfer. Surfactant adsorption at the interface inhibited electron transfer across an aqueous/organic interface, between ferro/ferricyanide and dimethyl (DiMFc) or decamethyl (DcMFc) ferrocene, directly in proportion to the fraction of the surface covered. Interfacial capacitance and interfacial tension measurements gave the surface coverage of the non-ionic surfactant, sorbitan monostearate, and SECM approach curves gave the overall reaction rate constant. Four-electrode voltammetry revealed an asymmetry of the interface geometry due to surfactant adsorption. Oxidation of the neutral ferrocene was inhibited, but the reduction of ferricenium was not. From the limiting current values for DiMFc oxidation, a model of uncovered micro-holes was proposed and their diameter within the surfactant layer at nominal full coverage was calculated to be ca. 200 nm. The adsorption of glucose oxidase (GOx) at the H2O/dichloroethane interface was also investigated, using capacitance and surface tension techniques. Both methods indicated that the adsorption process was time and concentration dependent. The adsorption isotherm obtained from the double layer capacitance suggested that there are two different adsorption states and that the interface saturated at a bulk GOx concentration of 500 nM. This behaviour was further investigated with specular neutron reflection at the water/air interface, assuming that the behaviour of the enzyme adsorption at the two interfaces is similar. This assumption was strengthened by surface tension results at the water/air interface that indicated similar behaviour to that at the water/oil interface. For low concentrations the neutron data were fitted for a uniform layer of small thickness and large area/molecule (molecules flattened after adsorption). For higher enzyme concentrations and high ionic strength the area/molecule became comparable with that expected from the dimensions of the enzyme (molecules retained rigidity). For the even higher concentration range (>400 nM) the fit needed to include a second layer of flattened molecules, adsorbing underneath the first layer. The reactivity of glucose oxidase adsorbed at the dichloroethane-water interface was studied using scanning electrochemical microscopy, with dimethylferricenium as the mediator, electrogenerated in the organic phase. The feedback current caused by recycling of the mediator as the generator electrode approached close to the interface from the organic side was interpreted in terms of an enzyme-catalysed oxidation of the aqueous substrate, glucose, within an interfacial protein-surfactant film.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Study of redox activity at modified liquid/liquid interfaces using electrochemical and physicochemical methods
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Pure sciences; Redox activity
URI: https://discovery.ucl.ac.uk/id/eprint/10097539
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