Electrodes, modified, uses

Similar results have been obtained at a gold electrode modified using (3-thiopropionate as promoter.15... 

A systematic study by Kennedy and coworkers was performed to find a catalyst that promoted the oxidation of insulin at physiological pH but was more stable than mvRuOx in these solutions [13]. Polynuclear hexacyanometallates, binary metal oxides, and metallophthalocyanines with various metal centers were tested as electrode modifiers using Ru, Cr, Fe, Co, and Os alone or in combination with Pb, Pd, and Ir. Of the modifiers tested, only with films originating by oxidation of RuClj or (NH4)RuClg was oxidation of insulin promoted (an extension of this work, detailed later, described an iridium-based catalyst for the oxidation of insulin [14]). Voltammetric deposition of a ruthenium oxide film from a 0.2 mM RuClj, 10 mM HCIO, mixture onto glassy carbon yielded a modified... 

Let us pass to other cytochromes. Cytochrome f (or cytochrome C552) (FW = 15 000), the crystal structure of which is known,11 is an electron carrier present in the photosynthetic chain and also possesses a positive overall charge. It exhibits a reversible Fe(III)/Fe(II) reduction at a gold electrode modified with 4,4/-dithiopyridine,12 Figure 10. 

The examples described in this chapter clearly show the potential of modified electrodes based on redox-active osmium-containing polymers. The redox potentials of these materials can be manipulated by varying the nature of the polymer-bound redox couple, which allows us to tailor polymers to particular application, especially in the sensors area. Furthermore changes can also be made in the polymer composition both with respect to polymer loading and the nature of the polymer backbone. This will allow control of such parameters as substrate diffiision and charge transport through the layer. This flexibility allows the systematic investigation of electrochemical properties of electrodes modified with such materials. 

A discussion of the charge transfer reaction on the polymer-modified electrode should consider not only the interaction of the mediator with the electrode and a solution species (as with chemically modified electrodes), but also the transport processes across the film. Let us assume that a solution species S reacts with the mediator Red/Ox couple as depicted in Fig. 5.32. Besides the simple charge transfer reaction with the mediator at the interface film/solution, we have also to include diffusion of species S in the polymer film (the diffusion coefficient DSp, which is usually much lower than in solution), and also charge propagation via immobilized redox centres in the film. This can formally be described by a diffusion coefficient Dp which is dependent on the concentration of the redox sites and their mutual distance (cf. Eq. (2.6.33). 

Ferrocene containing condensation polymers have been utilized by us to modify the surfaces of electrodes.Materials of this type that incorporate organo-iron compounds into a polymer matrix, either through chemical bonding or by formation of blends, have the potential of being thermally processed to yield iron oxides. 

An effective means of achieving chemical selectivity is to modify the electrode with either a synthetic catalyst or an enzyme. Let us first consider the ampero-... 

Although at present their use has been restricted to redox-active sensors in solution, it should be possible to immobilise these receptors at an electrode, and we may then have a simple redox-active electrode whose behaviour in solution is modified by the presence of ions. The high sensitivity of electrochemical techniques would then give us a sensitive and selective method of anion detection. 

In the second chapter, Anil Agiral and Han J.G.E. Gardeniers take us to a fascinating world wherein "chemistry and electricity meet in narrow alleys." They claim that microreactor systems with integrated electrodes provide excellent platforms to investigate and exploit electrical principles as a means to control, activate, or modify chemical reactions, or even preparative separations. Their example of microplasmas shows that the chemistry can take place at moderate temperatures where the reacting species still have a high reactivity. Several electrical concepts are presented and novel principles to control adsorption and desorption, as well as the activity and orientation of adsorbed molecules are described. The relevance of these principles for the development of new reactor concepts and new chemistry is discussed. 

The results obtained enable us to conclude that in the DNA-modified glassy carbon electrode, although the groups that undergo electrochemical... 

Despite its beneficial effects on electroanalytical techniques, which include avoiding electrode passivation, enhancing mass transport and current intensity, and the ability to modify process kinetics, US assistance has not yet gained widespread acceptance in routine analytical laboratories [132,133]. 

Let us note that until now electrochemistry has not been able to yield chiral sulfoxides from the starting prochiral thioethers even though the use of chiral (chemically modified) electrodes was considered. 

If the pressure of H2 is maintained at 1 bar, apphcation of the Nernst equation (equation 7.21) allows us to calculate E over a range of values of [H ]. For neutral water (pH 7),, = -0.41V, and at pH 14, °[qh = i = -0.83V. Whether or not the water (pH 7) or molar aqueous alkali (pH 14) is reduced by a species present in solution depends upon the reduction potential of that species relative to that of the 2H /H2 couple. Bear in mind that we might be considering the reduction of H2O to H2 as a competitive process which could occur in preference to the desired reduction. The potential of —0.83 V for the 2H /H2 electrode in molar alkali is of hmited importance in isolation. Many M /M systems that should reduce water under these conditions are prevented from doing so by the formation of a coating of hydroxide or hydrated oxide. Others, which are less powerfully reducing, bring about reduction because they are modified by complex formation. An example is the formation of [Zn(OH)4] in alkahne solution (equation 7.26). The value of E° = —0.76 V for the Zn jZn half-cell (Table 7.1) applies only to hydrated Zn ions. When they are in the form of the stable hydroxo complex [Zn(OH)4] , °[oh 1 = 1 = -1.20 V (equation 7.27). 

Let us now turn our attention to the electronic requirements of microelectrode experiments where the main difficulty presented is the measurement of the inherently low current levels (often less than 10-10A). Many measurements have been successfully made using conventional three-electrode techniques with a potentiostatic system that has been modified to increase the gain of the current follower. Potentiostats are, however, inherently... 

Summary. Structures and properties of iodine adlayers on Pt(l 11), Au(l 11), and other sLugle-crystal electrodes are described, based mainly on our recent STM studies. The imderpotential deposition of Ag on Pt(l 11) in the absence and presence of the iodine adlayer is also briefly described. It is shown that the iodine-modified electrodes are promising substrates for the investigation of the adsorption of organic molecules. High-resolution STM allows us to determine molecular arrangements and internal structures of molecules adsorbed on the iodine-modified electrodes in solution. 

UTCFC has modified the carbothermal synthesis process (U.S. Patent 4,677,092, US 4,806,515, US 5,013,618, US 4,880,711, US 4,373,014, etc.) to prepare 40 wt% ternary Pt alloy catalysts. Various high-concentration Pt catalyst systems were synthesized and the electrochemical surface area (EGA) and electrochemical activity values compared to commercially available catalysts (see Table 3). The UTCFC catalysts showed EGA and activity values comparable to the commercial catalysts. A rotating disk electrode technique for catalyst activity measurements has been developed and is currently being debugged at UTCFC. 

Let us modify the difference of potential between electrode and electrolyte from a value A< ). The energy of final state does not vary, whereas the initial state energy is modified from the quantity -FA (j). The conditions for the reaction are the same as before (curve 3), however the summit of the energy curve only increases from a value -aFA( >, because the shape of the energy curve. 

Let us now consider the V 1 limit. In this case the expression for the modified electrode rate constant k E is given by Eqn. 53. Again this expression is rather complex. There are two terms in the numerator on the rhs of Eqn. 53. The first of these describes mediated electron transfer and the second, the kinetics for direct reaction at the electrode surface. The denominator on the rhs of Eqn. 53 describes the concentration polarization of 5 in the layer, where it may be consumed at the electrode surface by direct unmediated reaction represented by the heterogeneous rate constant ks, or in a homogeneous reaction layer of dimension Xq. Let us now assume that the direct unmediated process can be neglected. If this is true, then we simplify Eqn. 53 as follows ... 

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