Electrochemical modified electrodes

In view of the overwhelming success of PS in surface science, it is not surprising that XPS has been used rather early for the study of electrochemically modified electrode surfaces. Winograd et al. [10-12] were the first to use this spectroscopy for the study of oxide formation on Pt electrodes and also for the investigation of metal underpotential deposition (UPD) on Pt. Although a standard surface analytical tool, XPS has not found a corresponding consideration in electrochemistry. 

In this chapter, the effects and roles of secondary materials on platinum were investigated using electrochemically modified electrodes with ruthenimn, tin and molybdates. 

SERS is used to study monolayers of materials adsorbed on metal substrates. Although the substrates are often used as electrodes, a wide variety of substrate formats have been found to exhibit SERS electrochemically modified electrodes [64], colloids, island films, particles grafted on silanized glasses [65-67], and more... 

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

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... 

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 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. 

Electropolymerization is also an attractive method for the preparation of modified electrodes. In this case it is necessary that the forming film is conductive or permeable for supporting electrolyte and substrates. Film formation of nonelectroactive polymers can proceed until diffusion of electroactive species to the electrode surface becomes negligible. Thus, a variety of nonconducting thin films have been obtained by electrochemical oxidation of aromatic phenols and amines Some of these polymers have ligand properties and can be made electroactive by subsequent inincorporation of transition metal ions... 

For the in situ characterization of modified electrodes, the method of choice is electrochemical analysis by cyclic voltammetry, ac voltammetry, chronoamperometry or chronocoulometry, or rotating disk voltametry. Cyclic voltammograms are easy to interpret from a qualitative point of view (Fig, 1). The other methods are less direct but they can yield quantitative data more readily. 

The reduction of carbon dioxide is another of the basic electrochemical reactions that has been studied at modified electrodes. The reduction at Co or Ni phthalocyanine in acidic solution yields formic acid or carbon monoxide A very high selectiv-... 

Reference electrodes for non-aqueous solvents are always troublesome because the necessary salt bridge may add considerable errors by undefined junction potentials. Leakage of components of the reference compartment, water in particular, into the working electrode compartment is a further problem. Whenever electrochemical cells of very small dimensions have to be designed, the construction of a suitable reference electrode system may be very difficult. Thus, an ideal reference electrode would be a simple wire introduced into the test cell. The usefulness of redox modified electrodes as reference electrodes in this respect has been studied in some detail... 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

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... 

Chemical and electrochemical techniques have been applied for the dimensionally controlled fabrication of a wide variety of materials, such as metals, semiconductors, and conductive polymers, within glass, oxide, and polymer matrices (e.g., [135-137]). Topologically complex structures like zeolites have been used also as 3D matrices [138, 139]. Quantum dots/wires of metals and semiconductors can be grown electrochemically in matrices bound on an electrode surface or being modified electrodes themselves. In these processes, the chemical stability of the template in the working environment, its electronic properties, the uniformity and minimal diameter of the pores, and the pore density are critical factors. Typical templates used in electrochemical synthesis are as follows ... 

The oxidation of N ADH has been mediated with chemically modified electrodes whose surface contains synthetic electron transfer mediators. The reduced form of the mediator is detected as it is recycled electrochemically. Systems based on quinones 173-175) dopamine chloranil 3-P-napthoyl-Nile Blue phenazine metho-sulphatemeldola blue and similar phenoxazineshave been described. Conducting salt electrodes consisting of the radical salt of 7,7,8,8-trtra-cyanoquinodimethane and the N-methylphenazium ion have been reported to show catalytic effects The main drawback to this approach is the limited stability... 

Figure 17.7 Electrocatalysis of O2 reduction by Pycnoporus cinnabarinus laccase on a 2-aminoanthracene-modified pyrolytic graphite edge (PGE) electrode and an unmodified PGE electrode at 25 °C in sodium citrate buffer (200 mM, pH 4). Red curves were recorded immediately after spotting laccase solution onto the electrode, while black curves were recorded after exchanging the electrochemical cell solution for enzyme-fiiee buffer solution. Insets show the long-term percentage change in limiting current (at 0.44 V vs. SHE) for electrocatalytic O2 reduction by laccase on an unmodified PGE electrode ( ) or a 2-aminoanthracene modified electrode ( ) after storage at 4 °C, and a cartoon representation of the probable route for electron transfer through the anthracene (shown in blue) to the blue Cu center of laccase. Reproduced by permission of The Royal Society of Chemistry fi om Blanford et al., 2007. (See color insert.)...

The initial hurdle to overcome in the biosensor application of a nucleic acid is that involving its stable attachment on a transducing element which commonly includes a metallic electrode. In the first part of this chapter, we wish to introduce our approach for DNA immobilization (Scheme 1). A detailed characterization of the immobilization chemistry is also presented. In the second part, we follow the development of work from our laboratory on chemical sensor applications of the DNA-modified electrode involving a biosensor for DNA-binding molecules and an electrochemical gene sensor. 

The use of non-inert and chemically modified electrodes and other strategies for the detection of species that are difficult to analyze with the normal electrode materials have been reviewed.55 Photosensitization prior to amperometric detection is another tactic that has proved useful for the analysis of substances that are normally considered to be electrochemically inert.56 The use of pulsed amperometry has recently been reviewed.57... 

Special electrochemical sensors that operate on the principle of the voltammetric cell have been developed. The area of chemically modified solid electrodes (CMSEs) is a rapidly growing field, giving rise to the development of new electroanalytical methods with increased selectivity and sensitivity for the determination of a wide variety of analytes [490]. CMSEs are typically used to preconcentrate the electroactive target analyte(s) from the solution. The use of polymer coatings showing electrocatalytic activity to modify electrode surfaces constitutes an interesting approach to fabricate sensing surfaces useful for analytical purposes [491]. 

A.R. Hillman, in Reactions and Applications of Polymer Modified Electrodes, Electrochemical Science and Technology of Polymers (R.G. Linford, ed.), Elsevier, London (1987), p. 241. 

The new edition of Principles of Electrochemistry has been considerably extended by a number of new sections, particularly dealing with electrochemical material science (ion and electron conducting polymers, chemically modified electrodes), photoelectrochemistry, stochastic processes, new aspects of ion transfer across biological membranes, biosensors, etc. In view of this extension of the book we asked Dr Ladislav Kavan (the author of the section on non-electrochemical methods in the first edition) to contribute as a co-author discussing many of these topics. On the other hand it has been necessary to become less concerned with some of the classical topics the details of which are of limited importance for the reader. 

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