Electron Relays

Much work has been done on exploration and development of redox polymers that can rapidly and efftciendy shutde electrons. In several instances an enzyme has been attached to the electrode using a long-chain polymer having a dense array of electron relays. The polymer which penetrates and binds the enzyme is also bound to the electrode. 

A compound which is a good choice for an artificial electron relay is one which can reach the reduced FADH2 active site, undergo fast electron transfer, and then transport the electrons to the electrodes as rapidly as possible. Electron-transport rate studies have been done for an enzyme electrode for glucose (G) using interdigitated array electrodes (41). The following mechanism for redox reactions in osmium polymer—GOD biosensor films has... 

This photoinitiator (or photocatalyst) is composed of an electron relay system in which ruthenium bipyridyl... 

FIGURE 6-8 Composition of an electron-relaying redox polymer used for wiring enzymes to electrode transducer. (Reproduced with permission from reference 14.)... 

With respect to using methyl viologen as electron relay, it might be of interest to note tlmt MV " can be oxidized by positive holes produced in illuminated colloidal semiconductors such as Ti02 Two oxidation products of MV are 1, 2 -di-hydro-l,r-dimethyl-2 -oxo-4,4 -bipyridylium chloride and 3,4-dihydro-l,r-dime-thyl-3-oxo-4,4 -bipyridylium chloride, which can readily be detected by their strong fluorescences at 516 nm and 528 nm, respectively. These products are also produced in the direct photolysis of MV " solutions and in the reaction of MV "" with OH radicals in homogeneous solution... 

Similar structural features are present in the [FeFe]-hydrogenases (FeS electron relay centers, CO and CN-ligands at the active site, and channels for gas access). 

Heering HA, Wiertz FGM, Dekker C, de Vries S. 2004. Direct immobilization of native yeast Iso-1 cytochrome c on bare gold Fast electron relay to redox enzymes and zeptomole protein-film voltammetry. J Am Chem Soc 126 11103-11112. 

A novel polysiloxane, containing the isocyanide group pendent to the backbone, has been synthesized. It is observed to react with the metal vapors of chromium, iron and nickel to afford binary metal complexes of the type M(CN-[P])n, where n = 6, 5, 4 respectively, in which the polymer-attached isocyanide group provides the stabilization for the metal center. The product obtained from the reaction with Fe was found to be photosensitive yielding the Fe2(CN-[P])q species and extensive cross-linking of the polymer. The Cr and Ni products were able to be oxidized on exposure of thin films to the air, or electrochemically in the presence of an electron relay. The availability of different oxidation states for the metals in these new materials gives hope that novel redox-active polymers may be accessible. 

A series of nonlinear optical (NLO) donor-aceptor chromophores containing a fused dithienothiophene (DTT) as electron relay have been synthesized and investigated. The compounds Dj-OTT-Aj 75, D -DTT-A2 76, and D -DTT-A3 77 have shown high thermal stability, which is significant for their use as the active components in optoelectronic devices <2004CEJ3805, 1999JMC2227>. 

Using the tri-iodide/iodide redox couple and the sensitizers (22) and (56), several groups have reported up to 8-10% solar cell efficiency where the potential mismatch between the sensitizer and the redox couple is around 0.5 V vs. SCE. If one develops a suitable redox couple that decreases the potential difference between the sensitizer and the redox couple, then the cell efficiency could increase by 30%, i.e., from the present value of 10% up to 13%. Towards this goal, Oskam et al. have employed pseudohalogens in place of the triiodide/iodide redox couples, where the equilibrium potential is 0.43 V more positive than that of the iodide/iodide redox couple.17 Yamada and co-workers have used cobalt tris-phenanthroline complexes as electron relays (based on the CoII/m couple) in dye-sensitized solar cells.95... 

Y. Degani and A. Heller, Direct electrical communication between chemically modified enzymes and metal electrodes. I. Electron transfer from glucose oxidase to metal electrodes via electron relays, bound covalently to the enzyme. J. Phys. Chem. 91, 1285-1289 (1987). 

Arsenic and nitrogen compete for the electrons participating in the natural electron-relay cycles in the body. The number of electrons transferred by nitrogen depends on the number of arsenic atoms competing for them. 

In most cases the electronic connection between an immobilized redox enzyme and the electrode requires a mediator to shuttle the electrons to the prosthetic group or some type of wiring that plays the same role. There are cases, however, especially those involving relatively small enzymes, where direct electron transfer takes place between the electrode and the prosthetic group or some electronic relay in the enzyme. Analysis of the catalysis responses then follows the principles described and illustrated in Section 4.3.2. Somewhat more complicated schemes are treated in references7, where illustrative experimental examples can also be found. 

A mthenium-containing melanoidin (a condensation product of amino acids and carbohydrates) was found to photocatalyze hydrogen production from water under light irradiation with 1. < 320 nm, in the presence of EDTA as an electron donor and methyl viologen as an electron relay. The Ru-melanoidin photocatalyst is stable nnder prolonged irradiation times and, due to its insolubility in water, is suitable for... 

Maruthamuthu, R, Ashokkumar, M., Gurunathan, K., Subramanian, E., Sastri, M.V.C. 1989. Hydrogen evolution from water with visible radiation in presence of CulliyWO, and electron relay. Int J Hydrogen Energy 14 525-528. 

Utilizing DCA as an electron relay is proposed. As shown in Scheme 47 electron transfer from PhaP to excited DCA leads to PhsP and DCA". Consequent back-electron transfer from DCA to 241 yields the radical anion 242, which undergoes cyclization to form 243 in good yields. DCA is regenerated. 

Scliemc 4.1. Dissociative electron transfer using an aromatic radical-anion as electron relay... 

Metal oxide, e.g. RUO2 [35,36] and WO3 [151], loaded Cd8 colloidal suspensions with and without Ft functionalization were investigated for water decomposition under visible light. WO3-Pt/Cd8, in an aqueous solution of methyl viologen (MV ) which serves as an electron relay, has been found most efficient to date in terms of water splitting, see reactions (7.4.1) to (7.4.8) [151] ... 

Some of the materials highlighted in this review offer novel redox-active cavities, which are candidates for studies on chemistry within cavities, especially processes which involve molecular recognition by donor-acceptor ii-Jt interactions, or by electron transfer mechanisms, e.g. coordination of a lone pair to a metal center, or formation of radical cation/radical anion pairs by charge transfer. The attachment of redox-active dendrimers to electrode surfaces (by chemical bonding, physical deposition, or screen printing) to form modified electrodes should provide interesting novel electron relay systems. 

In many cases, although and M are both readily reduced by radiolytic radicals, a further electron transfer from the more electronegative atoms (for example, M ) to the more noble ions ( °(M /M )electron transfer is also possible between the low valencies of both metals, so increasing the probability of segregation [174]. The intermetal electron transfer has been observe directly by pulse techniques for some systems [66,175,176], and the transient cluster (MM ) sometimes identified such as (AgTl) or (AgCo) [176]. The less noble metal ions act as an electron relay toward the precious metal ions, so long as all are not reduced. Thus, monometallic clusters M are formed first and M ions are reduced afterward in situ when adsorbed at the surface... 

Similarly, at moderate dose rate for the couple Au Cl4, Ag, gold initially appears at 520 nm. Therefore, Ag ions essentially act as an electron scavenger, and as an electron relay toward more noble gold ions as far as gold ions are not totally reduced. Then silver-coated gold clusters are formed and the maximum is shifted to 400 nm, which is that of silver (Fig. 11) [102]. But at higher y- or EB dose rate (irradiation time of a few seconds), the electron transfer is too slow to compete with coalescence and the spectrum of alloyed clusters... 

Figure 8. Electron relay of glucose sensors mediated by ferrocene LB film.

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