Electrode surfaces chemical modification

Optical properties light-emitting diodes, resonance absorption of near IR-radiation Physical and chemical properties large specific surface and possibihty of surface chemical modification, adsorbents, catalysts, chemical sensors, materials for electrodes, chemical batteries, fuel elements and super condensers. 

Inhibitor (of an electrode reaction) — is a substance that added to the electrolyte solution causes a decrease in the rate of an electrochemical process by a physical, physicochemical, or chemical action and, generally, by modifying an electrode surface. This modification is due to adsorption of the inhibitor. The inhibitor may play no direct role in the electrochemical reaction or it can be a reaction intermediate. 

Figure 8.3 shows the voltammetric responses of an amine-modified opal electrode in aqueous solutions of either 5.1 mM Ru(NH3)6 ", 5.2 mM Fe(CN)6" , or 1.6 mM hydroxymethylferrocene (Fc(CH20H)2) and 0.1 M KCl as supporting electrolyte. To separate the effects of the lattice tortuosity and surface chemistry on the molecular flux, voltammograms were recorded for (i) the bare electrodes, (ii) the electrodes after the film self-assembly, and (iii) the electrodes after chemical modification of the film with 3-aminopropyltriethoxysilane. 

A qualitatively new approach to the surface pretreatment of solid electrodes is their chemical modification, which means a controlled attachment of suitable redox-active molecules to the electrode surface. The anchored surface molecules act as charge mediators between the elctrode and a substance in the electrolyte. A great effort in this respect was triggered in 1975 when Miller et al. attached the optically active methylester of phenylalanine by covalent bonding to a carbon electrode via the surface oxygen functionalities (cf. Fig. 5.27). Thus prepared, so-called chiral electrode showed stereospecific reduction of 4-acetylpyridine and ethylph-enylglyoxylate (but the product actually contained only a slight excess of one enantiomer). 

Micro- (and even nano-) electrode arrays are commonly produced with photolithography and electronic beam techniques by insulating of macro-electrode surface with subsequent drilling micro-holes in an insulating layer [136, 137], Physical methods are, however, expensive and, besides that, unsuitable for sensor development in certain cases (for instance, for modification of the lateral surface of needle electrodes). That s why an increasing interest is being applied to chemical approaches of material nanostructuring on solid supports [140, 141],... 

In addition to the development of new methods, new applications of molecular dynamics computer simulation are also needed in order to make comparisons with experimental results. In particular, more complicated chemical reactions, beyond the relatively simple electron transfer reaction, could be studied. Examples include the study of chemical adsorption, hydrogen evolution reactions, and chemical modification of the electrode surface. All of the above directions and opportunities promise to keep this area of research very active ... 

In the last 30 years considerable progress has been made in the development of tailor-made electrode surfaces by chemical modification [4-12] of electrodes surfaces with electroactive polymer films. A comprehensive description of electroactive polymer-modified electrodes can be found in the book edited by M. Lyons [13]. 

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. 

On the other hand, the activation of the electrode can take place by its chemical modification. In this case the redox catalyst is fixed to the electrode surface either by adsorption, polymer coating, or covalent binding. This aspect has been treated in several reviews covered separately within this series... 

If the surface of a metal or carbon electrode is covered with a layer of some functional material, the electrode often shows characteristics that are completely different from those of the bare electrode. Electrodes of this sort are generally called modified electrodes [9] and various types have been developed. Some have a mono-molecular layer that is prepared by chemical bonding (chemical modification). Some have a polymer coat that is prepared either by dipping the bare electrode in a solution of the polymer, by evaporating the solvent (ethanol, acetone, etc.) of the polymer solution placed on the electrode surface, or by electrolytic polymerization of the monomer in solution. The polymers of the polymer-modified electrodes are either conducting polymers, redox polymers, or ion-exchange polymers, and can perform various functions. The applications of modified electrodes are really limit-... 

In the previous edition of this book, Dryhurst and McAllister described carbon electrodes in common use at the time, with particular emphasis on fabrication and potential limits [1]. There have been two extensive reviews since the previous edition, one emphasizing electrode kinetics at carbon [2] and one on more general physical and electrochemical properties [3]. In addition to greater popularity of carbon as an electrode, the major developments since 1984 have been an improved understanding of surface properties and structure, and extensive efforts on chemical modification. In the context of electroanalytical applications, the current chapter stresses the relationship between surface structure and reproducibility, plus the variety of carbon materials and pretreatments. Since the intent of the chapter is to guide the reader in using commonly available materials and procedures, many interesting but less common approaches from the literature are not addressed. A particularly active area that is not discussed is the wide variety of carbon electrodes with chemically modified surfaces. 

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... 

Chemical modification of electrode surfaces by polymer films offers the advantages of inherent chemical and physical stability, incorporation of large numbers of electroactive sites, and relatively facile electron transport across the film. Since th% polymer films usually contain the equivalent of one to more than 10 monolayers of electroactive sites, the resulting electrochemical responses are generally larger and thus more easily observed than those of immobilized monomolecular layers. Also, the concentration of sites in the film can be as high as 5 mol/L and may influence the reactivity of the sites because their solvent and ionic environments differ considerably from dilute homogeneous solutions [9]. 

Electrode modification by the attachment of various types of biocomponents holds considerable promise as a novel approach for electrochemical (potentiometric, conductometric, and amperometric) biosensors. Potentiometric sensors based on coupled biochemical processes have already demonstrated considerable analytical success [26,27]. More recently, amperometric biosensors have received increasing attention [27,28] partially as a result of advances made in the chemical modification of electrode surfaces. Systems based on... 

After use, the electrode surface can be renewed by a simple polishing procedure for further uses, highlighting a clear advantage of this new material with respect to surface-modified approaches such as classical biosensors and other common biological assays. Biotinylated antiatrazine antibodies can be easily immobilized on the surface of the avidin-modified transducer through the avidin-biotin reaction since antibodies can be readily linked to biotin without serious effects on their biological, chemical, or physical properties. Moreover, antiatrazine antibodies can be easily immobilized on the surface of the Protein A-modified transducer without any modification of the antibodies. 

Stability of Prussian blue modified screen-printed electrodes The operational stability of all the PB-modified sensors is a critical point, especially at neutral and alkaline pH. A possible explanation for reduced stability could be the presence of hydroxyl ions at the electrode surface as a product of the H202 reduction. Hydroxyl ions are known to be able to break the Fe-CN-Fe bond, hence solubilising the PB [21]. An increased stability of PB at alkaline pH was first observed by our group after adopting a chemical deposition method for the modification of graphite particles with PB for the assembling of carbon paste electrodes [48]. 

As already pointed out in our previous papers [48-50], the high stability is probably the result of the newly developed chemical modification procedure which may lead to a stronger adsorption of the PB particles on the electrode surface. In contrast to the PB layer obtained with the more commonly used electrochemical procedures, these modified electrodes are in fact more stable at basic pH and their continuous use is possible with a minimal loss of activity after several hours. Moreover, with respect to the electrochemical procedure, our chemical deposition is much more suitable for mass production since no electrochemical steps are required and a highly automated process could be adopted (see Procedure 17 in CD accompanying this book). 

The chemical deposition of PB leads to an effective modification of the electrode surface, which then shows positive features as to hydrogen peroxide detection with an effective rate constant very similar to that measured for the peroxidase enzyme (2 x 104 M-1 s-1) [1]. Another major advantage of the chemical deposition is that it involves a more convenient and shorter procedure that avoids long electrochemical procedures during modification [2,3]. Moreover, the chemical deposition gives a more stable PB layer, which is only slightly affected by alkaline pH. 

---