Electron transfer metal electrodes

Electrochemical template-controlled sjmthesis of metallic nanoparticles consists of two steps (i) preparation of template and (ii) electrochemical reduction of metals. The template is prepared as a nano structured insulating mono-layer with homogeneously distributed planar molecules. This is a crucial step in the whole technology. The insulating monolayer has to possess perfect insulating properties while the template has to provide electron transfer between electrode and solution. Probably, the mixed nano-structured monolayer consisting of alkylthiol with cavities which are stabilized by the spreader-bar approach [19] is the only known system which meets these requirements. 

Fig. 4-18. Electron levels of an electronic electrode in equilibrium of redox electron transfer eojenox s> = redox electron at equilibrium e ) = electrons in metal electrode .q = electrode potential in equilibrium of electron transfer.

Fig. 8-2. Electron state draisity in a metal electrode and in hydrated redox particles on both sides of an electrode interface in equilibrium with redox electron transfer I>m = state density of electrons in metal electrode Oo Dhbd)=state density of redox electrons in hydrated oxidant (reductant) particle cfcredox) = Fermi level of redox electrons ...

Willner and coworkers have extended this approach to electron relay systems where core-based materials facilitate the electron transfer from redox enzymes in the bulk solution to the electrode.56 Enzymes usually lack direct electrical communication with electrodes due to the fact that the active centers of enzymes are surrounded by a thick insulating protein shell that blocks electron transfer. Metallic NPs act as electron mediators or wires that enhance electrical communication between enzyme and electrode due to their inherent conductive properties.47 Bridging redox enzymes with electrodes by electron relay systems provides enzyme electrode hybrid systems that have bioelectronic applications, such as biosensors and biofuel cell elements.57... 

Much use has also been made of tunneling concepts in work on electron transfer from electrode to enzymes in solution, some of which adsorb on the electrode (Tarasevich, 1983). Most enzymes are huge in size (r > 50 A) compared with the hydrated ions usually considered. They contain a heme group that is a metal ion that must be reached by an electron if reduction is to occur. This would seem to introduce a hindrance to the development of the theoiy of enzyme electrochemistry. An electron... 

Some novel NPs, particularly metal NPs, can facilely act as enhancing agents for effectively accelerating the electron transfer between electrode and probe molecules, which will lead to a more rapid current response. 

Refs. [i] Born M (1920) Z Phys 1 45 [ii] Marcus Y (1997) Ion properties. Marcel Dekker, New York [iii] Rashin AA, Honig B (1985) ] Phys Chem 89 5588 [iv] Stilly WC, TempczykA, Hawley RC, Hendrickson TA (1990) ] Am Chem Soc 112 6127 [v] Rashin AA (1990) / Phys Chem 94 1725 [vi] Shoichet BK, Leach AR, Kuntz ID (1999) Proteins 34 4 [vii] Marcus RA (1977) Theory and application of electron transfer at electrodes and in solutions. In Rock PA (ed) Special topics in electrochemistry. Elsevier, Amsterdam, pp 61 [viii] Millery C/ (1995) Heterogeneous electron transfer kinetics at metallic electrodes. In Rubinstein I (ed) Physical electrochemistry. Marcel Dekker, New York, pp 46-47... 

Many of the classical electrolytic reactions occur at a potential which is either more negative (reduction) or more positive (oxidation) than the decomposition potentials of the media. The mechanism of such reactions must be investigated in each case, but it can usually be classified as one of the following three cases (1) a direct electron transfer from electrode to substrate (A2), (2) a formation of solvated electrons which, in turn, reduoe the substrate (Bl), or (3) a formation of an active species in the electrochemical step (adsorbed hydrogen, active metals, hydrogen peroxide, hydroxyl radicals, halogens, etc.) which reacts chemically with the substrate (B2). 

Direct electrical communication between enzyme aetive sites and electrodes may also be facilitated by the nanoscale morphology of the electrode. The modification of electrodes with metal nanoparticles allows the tailoring of surfaees with features that can penetrate close enough to the enzyme aetive site to make non-mediated electron transfer possible. Electrodes modified by unaggregated 12 nm diameter gold nanoparticles have been found to have the eorrect morphology to allow direct electron transfer between the cytochrome c active site and the eleetrode [41]. Elec-... 

This section presents phenomenological and theoretical features of mechanistically simple electrochemical processes in a parallel manner to the corresponding treatment for homogeneous electron transfer in 12.2.3. Discussion, therefore, will be limited to the elementary single-electron transfer process itself and restricted to thermal-electron transfer at metal-solution interfaces, although some aspects are common to all types of interfacial charge-transfer processes. Although narrow in scope, this approach serves to illustrate the relationship between, and the common features of, electron transfer at electrodes and in bulk solution. 

Fig. 12. Energy diagram for the process of electron transfer from electrode into solution during electrochemical generation of solvated electrons. 1 thermoemission 2 dissolution of electrons ej delocalized electron e solvated electron Fermi level in metal. Dashed line shows the solvated electron potential well in solution...

All parameters appearing in Equations (2.71) and (2.72) are herewith defined. In applying these equations to electron transfer at electrodes, the difference in the density of states distribution between metals and semiconductors suggests a separate treatment of these cases. 

Nitrate reductases (NaR) with an iron-sulfur center are used for nitrate conversion. Generally, nitrate is enzymatically reduced and NaR is in the oxidized form, which can be electrochemically reduced. However, the direct electron transfer between an enzyme and an electrode is strongly limited due to the fact that (1) the distance between the electrode surface and the redox active site of the enzyme, which is normally inside the globular protein, is large and (2) the orientation of donor to acceptor sites depends on the method of the immobilization of the enzyme at the electrode.Thus, low molar mass redox mediators including qui-nones, metal complexes, ferricyanide, derivatives of ferrocene, and organic redox dyes " have been used to facilitate the electron transfer between electrode and enzyme (Fig. 11.5). 

Electrochemistry, also defined as oxidation/reduction reactions involving electron transfer between electrodes (usually metals, conductors of electricity) and ionic solutions (or electrolytes), was founded by John Daniell and Michael Faraday in the 1830s. 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

Electron donor molecules are oxidized in solution easily. Eor example, for TTE is 0.33V vs SCE in acetonitrile. Similarly, electron acceptors such as TCNQ are reduced easily. TCNQ exhibits a reduction wave at — 0.06V vs SCE in acetonitrile. The redox potentials can be adjusted by derivatizing the donor and acceptor molecules, and this tuning of HOMO and LUMO levels can be used to tailor charge-transfer and conductivity properties of the material. Knowledge of HOMO and LUMO levels can also be used to choose materials for efficient charge injection from metallic electrodes. 

This difference is a measure of the free-energy driving force for the development reaction. If the development mechanism is treated as an electrode reaction such that the developing silver center functions as an electrode, then the electron-transfer step is first order in the concentration of D and first order in the surface area of the developing silver center (280) (Fig. 13). Phenomenologically, the rate of formation of metallic silver is given in equation 17,... 

Metal oxide electrodes have been coated with a monolayer of this same diaminosilane (Table 3, No. 5) by contacting the electrodes with a benzene solution of the silane at room temperature (30). Electroactive moieties attached to such silane-treated electrodes undergo electron-transfer reactions with the underlying metal oxide (31). Dye molecules attached to sdylated electrodes absorb light coincident with the absorption spectmm of the dye, which is a first step toward simple production of photoelectrochemical devices (32) (see Photovoltaic cells). 

The standard electrode potentials , or the standard chemical potentials /X , may be used to calculate the free energy decrease —AG and the equilibrium constant /T of a corrosion reaction (see Appendix 20.2). Any corrosion reaction in aqueous solution must involve oxidation of the metal and reduction of a species in solution (an electron acceptor) with consequent electron transfer between the two reactants. Thus the corrosion of zinc ( In +zzn = —0-76 V) in a reducing acid of pH = 4 (a = 10 ) may be represented by the reaction ... 

Consider now the transfer of electrons from electrode II to electrode I by means of an external source of e.m.f. and a variable resistance (Fig.. 20b). Prior to this transfer the electrodes are both at equilibrium, and the equilibrium potentials of the metal/solution interfaces will therefore be the same, i.e. Ey — Ell = E, where E, is the reversible or equilibrium potential. When transfer of electrons at a slow rate is made to take place by means of the external e.m.f., the equilibrium is disturbed and Uie rat of the charge transfer processes become unequal. At electrode I, /ai.i > - ai.i. 3nd there is... 

A salt bridge serves as an ionconducting connection between the two half-cells. When the external circuit is closed, the oxidation reaction starts with the dissolution of the zinc electrode and the formation of zinc ions in half-cell I. In half-cell II copper ions are reduced and metallic copper is deposited. The sulfate ions remain unchanged in the aqueous solution. The overall cell reaction consists of an electron transfer between zinc and copper ions ... 

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

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