Miroslava Varničić

Dynamic and Steady State 1-D Model of Mediated Electron Transfer in a Porous Enzymatic Electrode

A 1-D mathematical model of a porous enzymatic electrode exhibiting the mediated electron transfer (MET) mechanism has been developed. As a model system, glucose oxidation catalyzed by immobilized glucose oxidase (GOx) in the presence of a co-immobilized tetrathiafulvalene (TTF) mediator in the porous electrode matrix has been selected. The balance equations for potential fields in the electron- and ion-conducting phases as well as concentration field have been formulated, solved numerically and validated experimentally under steady state conditions. The relevant kinetic parameters of the lumped reaction kinetics have been obtained by global optimization. The confidence intervals (CIs) of each parameter have been extracted from the respective likelihood. The parameter study has shown that the parameters related to mediator consumption/regeneration steps can be responsible for the shift of the reaction onset potential. Additionally, the model has shown that diffusion of the oxidized mediator out of the catalyst layer (CL) plays a significant role only at more positive potentials and low glucose concentrations. Only concentration profiles in different layers influence the electrode performance while other state fields like potential distributions in different phases have no impact on the performance. The concentration profiles reveal that all electrodes work through; the observed limiting currents are diffusion-reaction limiting. The normalized electrode activity decreases with an increase of enzyme loading. According to the model, the reason for this observation is glucose depletion along the CL at higher enzyme loadings. Comparison with experiments advices a decrease of enzyme utilization at higher enzyme loadings.

Combined electrochemical and microscopic study of porous enzymatic electrodes with direct electron transfer mechanism

In the present work electrochemical and microscopic methods have been utilized to get more insight into the complex relationship between the preparation route, structure and activity of porous enzymatic electrodes. Enzymatic electrodes have been prepared following two procedures. In one procedure enzymes were physically entrapped into a porous conductive matrix stabilized by “inert” binder (Vulcan-PVDF), while in the second one (Vulcan-Gelatin) gelatin has been used as a binder and the electrodes were cross-linked. Vulcan-PVDF electrodes show exceptionally high activity (up to 1.2 mA cm−2) compared to Vulcan-Gelatin electrodes (0.3 mA cm−2) at nominally lower enzyme loading. The scanning electron microscopy cross-sections of these electrodes revealed similar thicknesses, but a higher level of Vulcan nanomaterial agglomeration, somewhat reduced porosity and formation of gelatin film on top in the case of Vulcan-Gelatin electrodes. Additionally, fluorescence microscopy studies provided evidence of a higher level of enzyme agglomeration in the case of cross-linking. Although the gelatin matrix and the reduced catalyst layer porosity might slow down hydrogen peroxide diffusion, Vulcan-Gelatin electrodes are less affected by mass transfer conditions than Vulcan-PVDF electrodes. A plausible cause of the Vulcan-Gelatin electrode inferior performance is a lower number of active enzymes (lower enzyme utilization) compared to the Vulcan-PVDF electrode caused by a higher level of enzyme agglomeration in former case.