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Title:Relationships between electron transport and redox center concentrations within molecular networks
Author(s):Fritsch-Faules, Ingrid
Doctoral Committee Chair(s):Faulkner, Larry R.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Analytical
Abstract:A microscopic model was developed to describe electron diffusion among redox centers in a rigid network. The model describes electron diffusion coefficients, $D\sb E$, that are based on a random walk, and includes a hard-sphere approximation for the redox molecules, the electronic structure of redox centers, and intervening matrix effects. In dilute systems, $D\sb E$ rises sharply with redox center concentration, $C$; however at large C's, the hard sphere $D\sb E$ reaches a limiting value due to packing. Hard-sphere $D\sb E$ values for metalloprotein kinetics and a redox molecular diameter of 13 A (corresponding to Ru(bpy)$\sb3\sp{2+}$) are 1.9 $\times$ 10$\sp{-10}$ cm$\sp2$s$\sp{-1}$ and 5.0 $\times$ 10$\sp{-9}$ at 0.10 $M$ and 1.07 $M$, respectively.
The dependence of $D\sb E$ on the oxidation state of a redox network was determined by measuring C-profiles of oxidized, O, and reduced, R, species under steady-state conditions. Microlithographically-defined microelectrode arrays were coated with quarternized poly(4-vinylpyridine), containing electrostatically-bound Fe(CN)$\sb6\sp{3-/4-}$, in either 0.1 M KNO$\sb3$ or 0.1 M potassium $p$-toluenesulfonate (KOTs) supporting electrolyte. Steady state conditions were established laterally in the film across 188 $\mu$m, between two, strongly polarized 50 $\mu$m-wide gold, "generating" electrodes. Fifteen, 4 $\mu$m-wide inner electrodes, separated by 8 $\mu$m gaps, probed the resulting potential, E, profile.
The E-profiles were converted into C-profiles with calibration curves that were obtained from long pulse width chronocoulometry. The calibration curves demonstrate nernstian behavior, and show that R and O partition between the film and surrounding electrolyte. The total $C$ of redox centers in the film ($C\sb{O}\ +\ C\sb{R}$) changes with oxidation state.
The steady-state C-profiles for the KNO$\sb3$ system are linear with respect to the position between generating electrodes. Agitation of the solution over the film significantly disturbed the profile. The KOTs system displayed slightly curved steady-state C-profiles. Curved steady-state C-profiles might be characteristic of structural changes in the film that may change diffusion of O and R within the electric field. These ideas were supported with digital simulations.
The experimental results are consistent with a model that involves (1) partitioning of redox centers into and out of the film, (2) diffusion of the centers mostly outside of the film across the array, and, perhaps, (3) some lateral diffusion within the film. The difference in profiles between the KNO$\sb3$ and KOTs electrolytes is not known, but might be explained by different extents of solvation.
Issue Date:1990
Rights Information:Copyright 1990 Fritsch-Faules, Ingrid
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9026185
OCLC Identifier:(UMI)AAI9026185

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