Electrode surfaces pretreated

Many dehydrogenase enzymes catalyze oxidation/reduction reactions with the aid of nicotinamide cofactors. The electrochemical oxidation of nicotinamide adeniiw dinucleotide, NADH, has been studied in depthThe direct oxidation of NADH has been used to determine concentration of ethanol i s-isv, i62) lactate 157,160,162,163) pyTuvate 1 ), glucose-6-phosphate lactate dehydrogenase 159,161) alanine The direct oxidation often entails such complications as electrode surface pretreatment, interferences due to electrode operation at very positive potentials, and electrode fouling due to adsorption. Subsequent reaction of the NADH with peroxidase allows quantitation via the well established Clark electrode. 

Schreurs, J., Van den Berg, J., Wonders, A., Barendrecht, E., Characterization of a Glassy-Carbon-Electrode Surface Pretreated with rf-Plasma , Reel. Thav. Chim. Pays-Bas 103 (1984) 251-259. 

Since membrane fording could quickly render the system inefficient, very careful and thorough feedwater pretreatment similar to that described in the section on RO, is required. Some pretreatment needs, and operational problems of scaling are diminished in the electro dialysis reversal (EDR) process, in which the electric current flow direction is periodically (eg, 3—4 times/h) reversed, with simultaneous switching of the water-flow connections. This also reverses the salt concentration buildup at the membrane and electrode surfaces, and prevents concentrations that cause the precipitation of salts and scale deposition. A schematic and photograph of a typical ED plant ate shown in Eigure 16. 

Hence, it is important to remember that the products, reaction mechanism and the rate of the process may depend on the history and pretreatment of the electrode and that, indeed, the activity of the electrode may change during the timescale of a preparative electrolysis. Certainly, the mechanism and products may depend on the solution conditions and the electrode potential, purely because of the effect of these parameters on the state of the electrode surface. 

Electrochemical reactions at semiconductor electrodes have a number of special features relative to reactions at metal electrodes these arise from the electronic structure found in the bulk and at the surface of semiconductors. The electronic structure of metals is mainly a function only of their chemical nature. That of semiconductors is also a function of other factors acceptor- or donor-type impurities present in bulk, the character of surface states (which in turn is determined largely by surface pretreatment), the action of light, and so on. Therefore, the electronic structure of semiconductors having a particular chemical composition can vary widely. This is part of the explanation for the appreciable scatter of experimental data obtained by different workers. For reproducible results one must clearly define all factors that may influence the state of the semiconductor. 

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). 

The study of metal ion/metal(s) interfaces has been limited because of the excessive adsorption of the reactants and impurities at the electrode surface and due to the inseparability of the faradaic and nonfaradaic impedances. For obtaining reproducible results with solid electrodes, the important factors to be considered are the fabrication, the smoothness of the surface (by polishing), and the pretreatment of the electrodes, the treatment of the solution with activated charcoal, the use of an inert atmosphere, and the constancy of the equilibrium potential for the duration of the experiment. It is appropriate to deal with some of these details from a practical point of view. 

Among the main goals of electrochemical research are the design, characterization and understanding of electrocatalytic systems, (1-2) both in solution and on electrode surfaces. (3.) Of particular importance are the nature and structure of reactive intermediates involved in the electrocatalytic reactions.(A) The nature of an electrocatalytic system can be quite varied and can include activation of the electrode surface by specific pretreatments (5-9) to generate active sites, deposition or adsorption of metallic adlayers (10-111 or transition metal complexes. (12-161 In addition the electrode can act as a simple electron shuttle to an active species in solution such as a metallo-porphyrin or phthalocyanine. 

It is shown that the rate-limiting step in the photoelectrochemical evolution of hydrogen in an HF electrolyte is linearly dependent on the excess electron concentration at the surface of the p-type silicon electrode. The rate of this step does not depend on the electrode potential and the H+ concentration in the solution, but is sensitive to the surface pretreatment [Sell]. The plateau in the I-V curve, slightly... 

The current transients observed upon immersion of n-type Si electrodes in HF of low concentration (2%) depend on the sample pretreatments. Immersion in 50% HF prior to anodization, for example, leads to current densities of up to 30 pA cm-2, which decreases within 1 s to the steady-state dark current value, producing a charge density of 24 pC cnrf 2. This effect has been ascribed to the density of Si-F bonds present on the electrode surface electrode prior to anodization [Be22],... 

Figure 3 Cyclic voltammograms recorded at different scan rates in aqueous solution (pH 7) of cytochrome c, under the following experimental conditions (a) protein adsorbed on the Sn02 electrode surface (b) Au electrode pretreated with bipyridyl protein in solution...

In summary, the overall successful effect has been assigned to the fact that any pretreatment of either the electrode surface (use of promoters, use of specific carbon electrodes, with the eventual generation of functional COO groups) or the solutions (addition of multicharged cationic species, proper choice of pH) creates at the bare electroinactive surface more and more specific microscopic active sites able to favour the exchange of electrons with proteins.10 This means that, in the absence of proper pretreatments, the electrode surface does not possess specific sites... 

The changes in reorientation of surface atoms were explained using the dynamic model of the crystal space lattice. It was assumed that during anodic polarization, when the oxidation of adsorbed water is taking place, atoms oscillate mainly in a direction perpendicular to the electrode surface. This process leads to periodic separation of atoms in the first surface layer. Thus, the location of atoms in different orientations is possible. It was stated that various techniques of electrode pretreatment used for... 

Owing to these characteristics, PG has been extensively used for the adsorption of DNA and its derivatives. DNA was successfully adsorbed on PG by dry-adsorption at 100 °C [67]. The electrodes were stored in TriS buffer at 4 °C without loss of DNA, showing that DNA was firmly adsorbed on PG. It was demonstrated that the adsorbed ODN was also able to be hybridized with its complementary strand, suggesting that although DNA bases are compromised in the adsorption, they are still available for hybridization [67]. A composite film of DNA and the polyanionic perfluorosulfonated ionomer Nation was cast on PG by the layer-by-layer procedure performed by dry-adsorption [68]. In another approach, the PG surface was electrochemically pretreated at - 1.7 V for 60 s. DNA was then wet-adsorbed at the pretreated electrode surface from solutions containing 0.2 M NaCl, 10 mM Tris- HCl, pH 7.4, for 1 min followed by rinsing the electrode with distilled water [69,70]. 

Among the surface-modified CNTs materials, a bulk-modified CNT paste (CNTP) has also been reported [126]. The new composite electrode combined the ability of CNTs to promote adsorption and electron-transfer reactions with the attractive properties of the composite materials. The CNTP was prepared by mixing MWCNTs powder (diameter 20-50 nm, length 1-5 jim) and mineral oil in a 60 30 ratio. The oxidation pretreatment [performed in ABS (pH 5.0) for 20 s at 1.30 V, vs Ag/AgCl] proved to be critical in the state of the CNTP surface. Pretreatments improved the adsorption and electrooxidation of both DNA and DNA bases, probably due to the increase in the density of oxygenated groups. 

In recent years, Burke and coworkers have found [373] that severe cathodization of pc-Au in acid solution resulted in the appearance of faradaic responses in the double-layer region. Such anomalous behavior may be explained by the presence of active gold atoms on the electrode surface. These active atoms appear as a result of the pretreatment process, when the part of inserted energy into the gold sample is retained mosdy by the surface atoms and atoms of the outer layers, in the form of various types of defects, for example, adatoms, vacancies, grain boundaries, and others. 

Solid electrode performance can be affected by the electrode s previous history. A freshly polished electrode surface is virtually free of functional groups. To what extent its electrochemical behaviour changes in use depends very much on the electrode material and electrochemical pretreatment procedures [98]. 

Figure 12. Photocurrent-voltage curves for a ZnO electrode after two different surface pretreatments (electrolyte 7M KCl)...

Figure 13. Luminescence from a ZnO electrode into which holes are injected by SOi radicals (25) (spectra for two different surface pretreatments)...

The pretreatment of the carbon disk UMEs (like a greater part of plane electrodes) is commonly based on mechanical procedures, i.e. polishing [118]. Initially, carbon disk UMEs are grounded down on emery paper (600 grit). Then, the electrode surface is polished with successively finer grades of alumina slurries (0.1- and 0.05-gm diameter particles) on respective cloths. Usually, that is enough to obtain a clean and adequate electroactive electrode surface. 

Surface pretreatment The electrode surface was activated in ABS applying a constant potential +1.6Y (vs. Ag/AgCl pseudoreference electrode) for 2 min and +1.8Y for lmin to oxidise eventually occurring impurities. All procedures were executed in stirring solution. 

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