Diffusion within film, modified electrodes

Figure 3.9 Schematic illustration of the processes that can occur at a modified electrode, where P represents a reducible substance in a film on the electrode surface and A a species in solution. The processes shown are as follows (1) heterogeneous electron transfer to P to produce the reduced form Q (2) electron transfer from Q to another P in the film (electron diffusion or electron hopping in the film) (3) electron transfer from Q to A at the film/solution interface (4) penetration of A into the film (where it can also react with Q or at the substrate/film interface) (5) movement (mass transfer) of Q within the film (6) movement of A through a pinhole or channel in the film to the substrate, where it can be reduced. From A.J. Bard and L.R. Faulkner, Electrochemical Methods Fundamentals and Applications, 2nd Edition, Wiley, 2001. Reprinted by permission of John Wiley Sons, Inc...

The modelling of kinetics at modified electrodes has received much attention over the last 10 years [1-11], mainly due to the interest in the potential uses of chemically modified electrodes in analytical applications. The first treatment published by Andrieux et al. [5] was closely followed by a complimentary treatment by Albery and Hillman [1, 2]. Both deal with the simplest basic case, that is, the coupled effects of diffusion and reaction for a second-order reaction between a species freely diffusing in the bulk solution and a redox mediator species trapped within the film at the modified electrode surface. The results obtained by the two treatments are essentially identical, although the two approaches are slightly different. 

De diffusion coefficient for electrons within the film at a modified electrode crrPfs 14.4.2... 

The fundamental processes involved in the mediating process are identified as charge introduction at the modifying/electrode interface, charge introduction at the layer/electrolyte interface, and reaction of the target analyte with the modifying layer. Coupled to these reactions, one may observe substrate diffusion into the film as dictated by the partition coefficient, K. If the substrate is capable of penetrating the film, then the diffusion rate of the substrate, Dy, within the layer will in all but a few cases be considerably less than the solution value Z). ... 

Figure 8.16 Illustration of four electron-transfer modes in polymer modified electrodes. Polymers containing redox molecules (where A + e = D) can mediate electrons to a solution-based analyte (where O + e = R) through an electron hopping mechanism (la), or by diffusion of free redox molecules within the film (lb). Permeable polymer layers, on the other hand, provide a suitable chemical environment for diffusion of O through the layer (2a) or contain channel/pinholes where electron-transfer occurs (2b). This material is adapted with permission from A. J. Bard, Integrated Chemical Systems A Chemical Approach to Nanotechnology, John Wiley Sons, Inc. New York, 1994. Copyright 1994 John Wiley Sons, Inc.

Abstract Clay-modified electrodes can provide an efficient method for studying the porosity of negatively charged, layered stuctures, i.e., swollen clay films. Diffusion transport processes of electroactive solute probes within hydrophilic and hy-drophobized montmorillonite clay films have been studied. Cyclic voltammetry was performed in a three-electrode cell. Results regarding film permeability, the structure of the porous aerogel-hydrogel, the effect of layer thickness as well as the role... 

S.3.2 Sol-Gel Encapsulation of Reactive Species Another new and attractive route for tailoring electrode surfaces involves the low-temperature encapsulation of recognition species within sol-gel films (41,42). Such ceramic films are prepared by the hydrolysis of an alkoxide precursor such as, Si(OCH3)4 under acidic or basic condensation, followed by polycondensation of the hydroxylated monomer to form a three-dimensional interconnected porous network. The resulting porous glass-like material can physically retain the desired modifier but permits its interaction with the analyte that diffuses into the matrix. Besides their ability to entrap the modifier, sol-gel processes offer tunability of the physical characteristics... 

The most diffused actuating configuration, in which these materials are used, is represented by the so-called unimorph bilayer bender. This kind of actuator consists of a film of active material coupled to a passive supporting layer. The bilayer structure is operated within an electrochemical cell, having a liquid electrolyte in which the device is immersed. The active polymeric layer of the actuator works as one electrode of the cell, while a counter electrode and a third reference electrode are separately immersed in the electrolyte. One end of the bilayer is constrained, while the other is free. The potential difference applied between the electrodes causes red-ox reactions of the conducting polymer. Since the CP and the passive layers are mechanically interlocked, when the polymer swells/shrinks the passive layer, which can not modify its dimensions, transforms the CP linear displacement into a bending movement of the structure [238-242]. Very similar is the bimorph structure. In this case the passive layer is substituted by a second CP film and they work in opposition of phase. 

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