Surface Modified Electrodes

Electrodes are often surface modified in an attempt to improve their electroanalytical responses Comment critically on the implications of modifying a macroelectrode in each of the following ways  

SO that the measured effect of accelerating the electrode kinetics is a maximum of 30%, although, of course, the peak potential moves to lower overpotentials which may be analytically useful in removing the effects of some interfering species. Note, however, that if pulse techniques are used rather than LSV, then the acceleration of electrode kinetics from irreversible to reversible behaviour can lead to a greater improvement in analytical signal. 

As the coverage increases, there is increasing overlap of the diffusion fields (Fig. 9.17) of the nanoparticles and eventually a signal is observed that is very close to that expected if the entire geometric area of the electrode were active. Note that not all of the electrode needs to be covered by nanoparticles for this to occur. 

The different possible diffusional cases are summarised in Fig. 9.17 and are discussed in Chapters 6 and 11 in more detail. In Case 4 there is effectively linear diffusion to the entire geometric area covered by the nanoparticles, and so the peak-shaped voltammetry reflects the area of the imderlying electrode 

The above discussion assumes implicitly that the nanoparticles have similar electrochemical behaviour to the corresponding bulk metal. In fact, the changing surface and electronic structures between the bulk material and nanoparticle means that there may be qualitatively altered electroreactivity at least in principle some reactions may not proceed at the nanoscale, or vice versa (see also Chapter 11). 

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Surface-modified electrodes were used for prevention of high overpotentials with direct oxidation or reduction of the cofactor, electrode fouling, and dimerization of the cofactor [7cj. Membrane electrochemical reactors were designed. The regeneration of the cofactor NADH was ensured electrochemically, using a rhodium complex as electrochemical mediator. A semipermeable membrane (dialysis or ultrafiltration) was integrated in the filter-press electrochemical reactor to confine... 

Light-emitting devices Surface-modified electrodes... 

COORDINATION COMPOUNDS AND ELECTRODE PHENOMENA SURFACE MODIFIED ELECTRODES... 

The electroactivity of a redox polymer may depend upon the solvent with which the surface modified electrode is in contact. Thus, for example, PVP containing an EDTA complex of ruthenium(III) is electroactive in contact with aqueous media, but inactive in contact with solvents such as dimethyl sulfoxide. 

Considerable potential exists to design surface modified electrodes which can mimic the behaviour of electronic components. For example, a rectifying interface can be produced by using two-layer polymer films on electrodes. The electroactive species in the layers have different redox potentials. Thus electron transfer between the electrode (e.g. platinum) and the outer electroactive layer is forced to occur catalytically by electron transfer mediation through the inner electroactive layer. 

Electroanalytical sensors based on amperometric measurements at chemically modified electrodes are in the early stages of development. The modes of modification can take many forms, but the most common approach at the present time is the immobilization of ions and molecules in polymer films which are applied to bare metal, semiconductor, and carbon electrodes. Such surface-modified electrodes exhibit unique electrochemical behavior which has been exploited for a variety of applications. 

XPS of Polymer Surfaces Analytical Applications Radiation Damage Surface Modified Electrodes Interfacial Chemistry and Adhesion Surface Chemistry and Catalysis Mineralogy and Geochemistry Microelectronics Processing... 

The mediator is linked to the electrode surface, forming a surface-modified electrode, and the biological molecule links itself to the mediator layer by heterogeneous electron transfer. 

Permselectivity — According to IUPAC A term used to define the preferential permeation of certain ionic species through - ion-exchange membranes. (See also surface-modified electrodes). Discrimination is based on the size or ion charge of the ionic species studied. Permselectivity prevents electrode surface fouling by sample matrix components, e.g., by proteins in biological fluids. 

Since the appearance of the redox [ii, iii] and conducting [iv] polymer-modified electrodes much effort has been made concerning the development and characterization of electrodes modified with electroactive polymeric materials, as well as their application in various fields such as -> sensors, actuators, ion exchangers, -> batteries, -> supercapacitors, -> photovoltaic devices, -> corrosion protection, -> electrocatalysis, -> elec-trochromic devices, electroluminescent devices (- electroluminescence) [i, v-viii]. See also -> electrochemically stimulated conformational relaxation (ESCR) model, and -> surface-modified electrodes. 

See also - bifunctional mediator, - biofuel cells, -> catalytic current, - catalytic hydrogen evolution, - dye cell, -> enzyme electrodes, -> ferrocene, - glucose sensor, -> indirect and direct electrolysis, and - surface-modified electrodes. 

Solid-state electrochemistry — is traditionally seen as that branch of electrochemistry which concerns (a) the -> charge transport processes in -> solid electrolytes, and (b) the electrode processes in - insertion electrodes (see also -> insertion electrochemistry). More recently, also any other electrochemical reactions of solid compounds and materials are considered as part of solid state electrochemistry. Solid-state electrochemical systems are of great importance in many fields of science and technology including -> batteries, - fuel cells, - electrocatalysis, -> photoelectrochemistry, - sensors, and - corrosion. There are many different experimental approaches and types of applicable compounds. In general, solid-state electrochemical studies can be performed on thin solid films (- surface-modified electrodes), microparticles (-> voltammetry of immobilized microparticles), and even with millimeter-size bulk materials immobilized on electrode surfaces or investigated with use of ultramicroelectrodes. The actual measurements can be performed with liquid or solid electrolytes. 

Surface complexation — is complexation of metal ions by ligands immobilized on the electrode surface (-> electrode surface area). The ligands may be incorporated in the structure of a -> carbon paste electrode, covalently bound to the surface of a chemically modified electrode (-> surface-modified electrodes), or adsorbed (-> adsorption) on the electrode surface etc. Surface complexation is not confined to electrodes. It can occur on many surfaces, e.g., minerals, when in contact with metal ion solutions or solutions containing complexing ions (in the first case, the surface provides the ligand and the solution the metal ion, whereas in the second case, the surface provides the metal ion and the solution the ligand). Surface complexation can be an important step in the dissolution of solid phases [ii]. 

Thin-film electrode — An electrode covered with a thin film of a given substance. The purpose of placing a thin film on the electrode surface is to obtain desired electrode properties. Many different substances have been used to prepare film electrodes they include among others mercury (see - thin mercury film electrodes) gold, boron-doped diamond (see - boron-doped diamond electrode), conductive polymers (see - polymer-modified electrode), and alkanethiols. The film thickness can vary from several micrometers (mercury) to monomolecular layers (thiols). In some cases (e.g., for - spectroelectrochemistry purposes) very thin layers of either gold or tin oxide are vapor-deposited onto glass plates. Thin film electrodes are often called - surface-modified electrodes. 

Electrocatalysis is a type of electrosynthesis that uses surface modified electrodes, or mediators/electrocatalysts to facilitate the redox reaction. Meyer reported the design and synthesis of a chemically modified electrode that consists of a thin polymer film with covalently attached redox sites,designed to facilitate rapid electron transport for electrocatalysis. Complexes of Fe, Ru, Os, Re, and Co were synthesized in such a way that when electrochemically reduced, they reacted to form smooth electroactive polymer films that adhered well to the working electrode to form a chemically modified electrode designed for electrocatalysis. 

Surface Modified Electrodes 57.3.2,1 Nafton modijied electrodes... 

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