Electronic devices surface modified electrodes

Techniques for attaching such ruthenium electrocatalysts to the electrode surface, and thereby realizing some of the advantages of the modified electrode devices, have been developed.512-521 The electrocatalytic activity of these films have been evaluated and some preparative scale experiments performed. The modified electrodes are active and selective catalysts for oxidation of alcohols.5 6-521 However, the kinetics of the catalysis is markedly slower with films compared to bulk solution. This is a consequence of the slowness of the access to highest oxidation states of the complex and of the chemical reactions coupled with the electron transfer in films. In compensation, the stability of catalysts is dramatically improved in films, especially with complexes sensitive to bpy ligand loss like [Ru(bpy)2(0)2]2 + 51, 519 521... 

It would appear certain that the most important need in LCEC is the development of improved electrode materials. It may be possible in the near future to design an electrode that will give superior performance for certain classes of compounds. Modifying electrode surfaces by covalent attachment of various ligands or electron-transfer catalysts (including enzymes) can provide the key to better amperometric devices for all sorts of analytical purposes. Research in the area of chemically modified electrodes (CMEs) has been reviewed (see Chap. 13) [6,11]. Those interested in improving the performance of electrochemical detectors would do well to study these developments in detail. 

In recent years, considerable effort has gone into the development of a new class of electrochemical devices called chemically modified electrodes. While conventional electrodes are typified by generally nonspecific electrochemical behavior, i.e., they serve primarily as sites for heterogeneous electron transfer, the redox (reduction-oxidation) characteristics of chemically modified electrodes may be tailored to enhance desired redox processes over others. Thus, the chemical modification of an electrode surface can lead to a wide variety of effects including the retardation or acceleration of electrochemical reaction rates, protection of electrodes, electro-optical phenomena, and enhancement of electroanalytical specificity and sensitivity. As a result of the importance of these effects, a relatively new field of research has developed in which the... 

The discussion above must be modified if the acceptor molecules adsorb onto the electrode surface. Under these circumstances, the electrons are captured by the acceptor directly rather than through an intermediary solvated electron, and such direct photoassisted electron transfer has been much studied recently with the advent of dye-sensitized solar cells and molecular electronic devices. The normal approach is to use two-photon excitation both to probe the existence of localized states at the surface and to explore their dynamics. In experiments of this nature, which have been mostly carried out on ad-... 

Semiconductor electrodes modified with reagents I-III exhibit properties that are fairly well predicted from the properties associated with the naked semiconductors in contact with ferrocene or Mv2+. Strongly interacting modifiers may alter the interface energetics and surface state distribution in useful ways.(11-14) A classic example of altering surface state distribution comes from electronic devices based on Si.(48) The semiconducting Si has a large density of surface states situated between the valence band and the conduction band. Oxidation of... 

Hemoproteins are a broad class of redox-proteins that act as cofactors, e.g. cytochrome c, or as biocatalysts, e.g. peroxidases. Direct ET between peroxidases such as horseradish peroxidase, lactoperoxidase," or chloropcroxidasc"" and electrode surfaces, mainly carbonaceous materials, were extensively studied. The mechanistic aspects related with the immobilized peroxidases on electrode surfaces and their utilization in developing biosensor devices were reviewed in detail. The direct electrical contact of peroxidases with electrodes was attributed to the location of the heme site at the exterior of the protein that yields close contact with the electrode surface even though the biocatalyst is randomly deposited on the electrode. For example, it was reported " that non-oriented randomly deposited horseradish peroxidase on a graphite electrode resulted in 40-50% of the adsorbed biocatalyst in an electrically contacted configuration. For other hemoproteins such as cytochrome c it was found that the surface modification of the electrodes with promoter units such as pyridine units induced the binding of the hemoproteins in an orientation that facilitated direct electron transfer. By this method, the promoter sites induce a binding-ET process-desorption mechanism at the modified electrode. Alternatively, the site-specific covalent attachment of hemoproteins such as cytochrome c resulted in the orientation of the protein on the electrode surfaces and direct ET communication. ... 

Use of electropolymerised conducting films in the development of small sensing devices is found to be important because electropolymerisation allows control over the thickness and spatial location of electrode modification. These interfacing electroactive materials have inherent oxidation-reduction abilities. These materials are important for direct and rapid electron transfer at the electrode surface [73,119,161-163]. Physical and chemical properties can be modified by appropriate polarisation and doping with counter ions. 

Electroanalytical chemistry entered a new era in 1978 with the publication of the first paper on polymer-modified electrodes by Miller and Van De Mark (j ). In the decade that followed, a foundation was provided for the creation of devices in which electron transfer will be controlled on a molecular level and selectivity and sensitivity will rival that of redox enzymes Jji vivo Electroanalytical chemists have begun to think differently. Measurement of micromoles and microcoulombs using classical microelectrodes has been replaced by thoughts of detecting tens of molecules, ions, and electrons with "organized molecular assemblies at the surfaces of ultramicroelectrodes. 

What does the future hold for modified electrodes Three major areas of electrochemistry will continue to make use of modified electrodes. First, specialized microstructures can be prepared to gain increased understanding of electron transfer in solutions and polymers, along surfaces, and in biological systems. Second, electroanalysis will benefit as devices are created for detection of minute amounts of species in various environments in the field and at the outflow of miniaturized devices used for separations. ... 

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