Electrodes, modified

A large amount of research has been done on chemical modification of electrodes. The authoritative treatment of this subject can be found in Bard and Faulkner (2001). Because it is a very active area of electrochemistry, this subject is being periodically reviewed. From the sensing point of view, the motivation for electrode modification has been to introduce additional flexibility in the design of, and additional control over, the electrochemical processes taking place at the electrodes. We have seen one example of such a modification already (Section 7.3 Soukharev et al., 2004). 

In general, traditional electrode materials are substituted by electrode superstructures designed to facilitate a specific task. Thus, various modifiers have been attached to the electrode that lower the overall activation energy of the electron transfer for specific species, increase or decrease the mass transport, or selectively accumulate the analyte. These approaches are the key issues in the design of chemical selectivity of amperometric sensors. The long-term chemical and functional stability of the electrode, although important for chemical sensors as well, is typically focused on the use of modified electrodes in energy conversion devices. Examples of electroactive modifiers are shown in Table 7.2. 

To this table we can also add nonconducting blocking layers that selectively hinder access of certain species, for example, interferants to the electroactive part of the electrode. 

Any additional layer that modifies the metal electrode has a certain thickness. That thickness can range from 1-2 nm to thousands of nanometers. This opens up 

We must remember that with amperometric sensors, the analytical information is obtained from the mass transport limiting current. One important consequence of the current-voltage equation is that one can always apply a potential high enough in order to transfer electrons to or from the electrode to a given species of interest. 

There are several ways of preparing different types of modified electrodes  

Chemical modification ( chemical bonding). An electroactive species is immobilized on the electrode surface by chemical reaction. Normally the fact that the electrode is covered by hydroxyl groups owing to the oxygen in the atmosphere is used. For example, the silanization process is 

Adsorption. Adsorption can be reversible or irreversible. This method has been used particularly for the preparation of polymer-modified electrodes. A solution of polymer is either painted on the electrode and the solvent evaporated, or the electrode is immersed in a solution of the polymer. Relevant examples are polymers that let charge pass through the film polyvinylpyridine (PVP), polyvinylferrocene (PVF), porphyrins, and phthalocyanines. Direct deposition in the gas phase or sputtering are also possible. 

Electroadsorption—adsorption carried out with an applied electrode potential. The quantity deposited is a function of deposition time, multilayer formation being possible, as is the case with thionine. On the other hand, application of a potential, in the correct conditions, in the presence of a molecule susceptible to polymerization, can produce radicals, initiating polymerization and subsequent electrode modification. Examples of these conducting polymer monomers are pyrrole, N-phenylpyrrole and W-methylpyrrole, aniline, and thiophene. 

Plasma. A plasma is used to clean the electrode surface, leaving unbonded surface atoms and, thus, an activated surface. Carbon is much used for this subsequent exposure to amines or ethenes, for example, results in chemical bond formation. Plasma discharge in the presence of radical monomers in solution, leading to polymer formation on the surface, is equivalent to chemical activation. The use of lasers in this area may be interesting, but has been little exploited as yet. 

Attention in what follows, however, will be focused on systems in which the adsorbate is irreversibly confined to the electrode surface (at least within a time scale longer than that required for electrocatalytic studies), even in the absence of solution phase material. Various aspects of the preparation and characterization of such chemically modified electrodes will be presented next. 

The general concept of chemical modification [55, 56] of electrode surfaces can be attributed by and large to the seminal work of Hubbard and co workers [57] in the early 1970s. Since then, electrode surfaces modified by a wide variety of adsorbed species, including polymers and other types of films have received wide attention not only in electrocatalysis, but also in sensor technology and other applications. 

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Flavin adenine dinucleotide (FAD) has been electropolymerized using cyclic voltammetry. Cyclic voltammograms of poly (FAD) modified electrode were demonstrated dramatic anodic current increasing when the electrolyte solution contained NADH compare with the absence of pyridine nucleotide. 

Solar energy, 6, 488 surface modified electrodes, 6, 30 Sol-Gel process fast reactor fuel, 6, 924 Solid state reactions, 1, 463-471 fraction of reaction, 1, 464 geometric, 1, 464 growth, 1, 464 nucleation, 1, 464 rate laws, 1,464 Solochrome black T metallochromic indicators, 1,555 Solubility... 

Measurements of the double-layer capacitance provide valuable insights into adsorption and desorption processes, as well as into the structure of film-modified electrodes (6). 

Chemically modified electrodes (CMEs) represent a modem approach to electrode systems. These rely on the placement of a reagent onto the surface, to impart the behavior of that reagent to the modified surface. Such deliberate alteration of electrode surfaces can thus meet the needs of many electroanalytical problems, and may form the basis for new analytical applications and different sensing devices. 

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

Explain clearly how chemically modified electrodes can benefit electrochemical measurements ... 

Propose a modified electrode surface suitable for detecting in-situ micromolar concentrations of ferric ion in an industrial stream. What are the challenges of such in-situ monitoring ... 

To circumvent high overvoltage and fouling problems encountered with the direct oxidation of NADH at conventional electrode (equation 6-11), much work has been devoted to the development of modified electrodes with catalytic properties for... 

Chemically modified electrodes, 39, 118 Chemometrics, 197 Chemoreceptor, 187 Chip, 194, 195 Chloramphenicol, 70 Chloride electrode, 159 Chlorpromazine, 34 Cholesterol, 182 Cholinesterase, 182 Chromium, 85, 86 Chronoahsorptometry, 42 Chronoamperometry, 21, 60, 130, 135, 132, 177... 

Miniaturization, 128, 163, 193 Minigrid electrode, 41, 52 Mixed-salt electrodes, 159 Modified electrodes, 118, 121 Monensin, 155 Monolayers, 117, 118, 173 Multichannel electrodes, 93, 94 Multipotentiostat, 106, 198 Mutation detection, 185... 

The electrochemistry of conducting polymers has been the subject of several reviews2-8 and has been included in articles on chemically modified electrodes.9-14 The primary purpose of this chapter is to review fundamental aspects of the electrochemistry of conducting polymer films. Applications, the diversity of materials available, and synthetic methods are not covered in any detail. No attempt has been made at a comprehensive coverage of the relevant literature and the materials that have been studied. Specific examples have been selected to illustrate general principles, and so it can often be assumed that other materials will behave similarly. 

The electrochemistry of a polymer-modified electrode is determined by a combination of thermodynamics and the kinetics of charge-transfer and transport processes. Thermodynamic aspects are highlighted by cyclic voltammetry, while kinetic aspects are best studied by other methods. These methods will be introduced here, with the emphasis on how they are used to measure the rates of electron and ion transport in conducting polymer films. Charge transport in electroactive films in general has recently been reviewed elsewhere.9,11... 

There has therefore been much interest in the mediation of redox reactions in solution by conducting polymer-modified electrodes. 

If the film is nonconductive, the ion must diffuse to the electrode surface before it can be oxidized or reduced, or electrons must diffuse (hop) through the film by self-exchange, as in regular ionomer-modified electrodes.9 Cyclic voltammograms have the characteristic shape for diffusion control, and peak currents are proportional to the square root of the scan speed, as seen for species in solution. This is illustrated in Fig. 21 (A) for [Fe(CN)6]3 /4 in polypyrrole with a pyridinium substituent at the 1-position.243 This N-substituted polypyrrole does not become conductive until potentials significantly above the formal potential of the [Fe(CN)6]3"/4 couple. In contrast, a similar polymer with a pyridinium substituent at the 3-position is conductive at this potential. The polymer can therefore mediate electron transport to and from the immobilized ions, and their voltammetry becomes characteristic of thin-layer electrochemistry [Fig. 21(B)], with sharp symmetrical peaks that increase linearly with increasing scan speed. 

The ratio of the rate constant as observed with the modified electrode at a selected electrode potential is divided by the rate constant observed rmder the same conditions with an unmodified electrode. 

It was also observed that, with the exception of polyacetylene, all important conducting polymers can be electrochemically produced by anodic oxidation moreover, in contrast to chemical methoconducting films are formed directly on the electrode. This stimulated research teams in the field of electrochemistry to study the electrosynthesis of these materials. Most recently, new fields of application, ranging from anti-corrosives through modified electrodes to microelectronic devices, have aroused electrochemists interest in this class of compounds... 

Intensive research on the electrocatalytic properties of polymer-modified electrodes has been going on for many years Until recently, most known coatings were redox polymers. Combining redox polymers with conducting polymers should, in principle, further improve the electrocatalytic activity of such systems, as the conducting polymers are, in addition, electron carriers and reservoirs. One possibility of intercalating electroactive redox centres in the conducting polymer is to incorporate redoxactive anions — which act as dopants — into the polymer. Most research has been done on PPy, doped with inter alia Co 96) RyQ- 297) (--q. and Fe-phthalocyanines 298,299) Co-porphyrines Evidently, in these... 

The historical development of chemically electrodes is briefly outlined. Following recent trends, the manufacturing of modified electrodes is reviewed with emphasis on the more recent methods of electrochemical polymerization and on new ion exchanging materials. Surface derivatized electrodes are not treated in detail. The catalysis of electrochemical reactions is treated from the view of theory and of practical application. Promising experimental results are given in detail. Finally, recent advances of chemically modified electrodes in sensor techniques and in the construction of molecular electronics are given. 

In 1975, the fabrication of a chiral electrode by permanent attachment of amino acid residues to pendant groups on a graphite surface was reported At the same time, stimulated by the development of bonded phases on silica and aluminia surfaces the first example of derivatized metal surfaces for use as chemically modified electrodes was presented. A silanization technique was used for covalently binding redox species to hydroxy groups of SnOj or Pt surfaces. Before that time, some successful attemps to create electrode surfaces with deliberate chemical properties made use of specific adsorption techniques... 

In the first part of the present review, new techniques of preparation of modified electrodes and their electrochemical properties are presented. The second part is devoted to applications based on electrochemical reactions of solute species at modified electrodes. Special focus is given to the general requirements for the use of modified electrodes in synthetic and analytical organic electrochemistry. The subject has been reviewed several times Besides the latest general review by Murray a number of more recent overview articles have specialized on certain aspects macro-molecular electronics theoretical aspects of electrocatalysis organic applicationssensor electrodes and applications in biological and medicinal chemistry. 

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