Chemically modified electrode surfaces

We see this line of research as eventually leading to a wide array of chemically modified electrode surfaces with unusual analytical, chemical, catalytic and optical properties. 

This paper will survey the current status of surface analysis in the examination of chemically modified electrode surfaces. In doing so, we shall take selected examples from our laboratory and the literature to illustrate some of the methods that have been employed to answer questions about surface topography, atomic and molecular speciation, and molecular orientation and bonding. 

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]. 

The electron transfer in biology usually involves initial protein-protein complex formation based on the complementarity of the docking sites. Efficient protein-electrode reactions appear to have some similarities to the way in which proteins act with their natural redox partner [22]. Therefore, methods for chemically modifying electrode surfaces as to mimic the biological situation were developed. The heterogeneous electron transfer between proteins and electrodes may be coupled with other reactions where the proteins act as vectorial mediators [25,26]. 

An electrode covered with several molecular layers of dye could be made to adsorb all of the visible light, and obviate the need for the multielectrode stack. Very thick dye layers have tended not to be conductive or highly photoconductive so that their photoelectrochemical efficiencies are no better and perhaps worse than those seen on electrodes modified with very thin dye films. Molecular disorder of the dye appears to be the dominant reason for lack of conductivity in thick films of fluorescein-type, cyanine-type, and phthalocyanine-type dyes (12). It has been shown however that ordered molecular systems (mainly conjugated, highly unsaturated hydrocarbons) have considerable potential as conductive media, and that these ordered systems may be used to chemically modify electrode surfaces (12, 15). 

Voltammetry is widely used by analytical, inorganic, physical, and biological chemists for fundamental studies of( 1) o.xidation and reduction processes in various media, (2) adsorption processes on surfaces, and (3) electron transfer mechanisms at chemically modified electrode surfaces. For analytical purposes, several fonns of... 

Silylation Researchers at the University of North Carolina first developed the practice of chemically modifying electrode surfaces through the coupling of alkyl silane compounds to surface hydroxyl groups, as in eq. 9.7 (Moses et al, 1975). 

An exciting prospect in synthetic organic electrochemistry is the selective synthesis of specific optical isomers by taking advantage of the asymmetry afforded by certain types of surface sites achieved with chemically modified electrode surfaces on substrates such as carbon. 

D. H. Karweik, C. W. Miller, M. D. Porter, T. Knwana, Prospects in the Analysis of Chemically Modified Electrodes , Surface Analysis, 1982, 89-119, American Chemical Society Conference Proceedings. 

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. 

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... 

Some porous ceramic structures of oxides on titanium (CT2O3, RuOj, MnOj, VOJ obtained by baking films of metal complexes like acetylacetonates on titanium surfaces can also be regarded as chemically modified electrodes Applications... 

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... 

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]. 

The indicated formal potential E° n of the corresponding monomer (-1.17V) in solution is very near that of the surface film (-1.13V vs. SSCE). That formal potentials of surface films on chemically modified electrodes are near those of their corresponding dissolved monomers (13,18) is actually a common, and quite useful, observation. In the present case, it demonstrates that the electronic structures of the porphyrin rings embedded in the polymer film are not seriously perturbed from that of the monomer. 

In Situ Surface Extended X-ray Absorption Fine Structure at Chemically Modified Electrodes... 

A way to circumvent the first problem is to ensure that all of the active material is present at the electrode surface. That is, employ a chemically modified electrode where a precursor to the active electrocatalyst is incorporated. The field of chemically modified electrodes Q) is approaching a more mature state and there are now numerous methodologies for the incorporation of materials that exhibit electrocatalytic activity. Furthermore, some of these synthetic procedures allow for the precise control of the coverage so that electrodes modified with a few monolayers of redox active material can be reproducibly prepared. Q)... 

Figure 1. Electrochemical cell for in situ surface EXAFS at chemically modified electrodes.

While chemically modified electrodes are excellent for stopping side reactions, they tend not to possess smooth continuous layers but, rather, they are often porous or so rough as to be virtually three-dimensional (Figure 5.5). The electrode surface is often said to be fractal for this reason. 

The current volume addresses issues of chemically modified electrodes. Whenever bare surfaces do not fulfill the needs required, their chemical modification is a most promising vay out of the dilemma. Purposeful attachment of atoms, molecules or even vhole (nano)particles to the surface allo vs one to tailor the electronic and structural properties of a surface and hence, its functionality over a vide range. In the five chapters of this volume, internationally renovmed scientists describe, how to modify a surface and what to do with it. 

An improved adsorption of DNA bases has been observed at a chemically modified electrode based on a Nafion/ruthenium oxide pyrochlore (Pb2Ru2-x FhxOj-y modified GC (CME). Nafion is a polyanionic perfiuorosulfonated ionomer with selective permeability due to accumulation of large hydrophobic cations rather than small hydrophilic ones. The Nafion coating was demonstrated to improve the accumulation of DNA bases, while the ruthenium oxide pyrochlore proved to have electrocatalytic effects towards the oxidation of G and A. The inherent catalytic activity of the CME results from the Nafion-bound oxide surface being hydrated. The catalytically active centers are the hydrated surface-boimd oxy-metal groups which act as binding centers for substrates [50]. 

A biological compound (an enzyme, usually) intended to improve the response of an electrode can be incorporated into it in two ways, namely (a) by altering the sensing surface in order to accommodate the biocatalysts [i.e. by constructing a (bio)chemically modified electrode] and (h) by using a membrane place in front of the surface electrode in order to trap the enzyme. The enzyme can be used in isolation (most often in a commercially available form) or be part of a tissue material or bacterial cells. 

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