Electron relay system

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

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. 

Oxidation of water to evolve 02 is an important reaction for water photolysis as described in Sect. 2. Oxygen evolution from water by Ru(bpy)f+ oxidation with Ru02 catalyst was studied 41). The authors established an electron relay system for water oxidation (Scheme 3) by the polymer Ru(bpy) + complex and Ru02 catalyst with Pb02 as an oxidant40). 

A photoinduced electron relay system at solid-liquid interface is constructed also by utilizing polymer pendant Ru(bpy)2 +. The irradiation of a mixture of EDTA and water-insoluble polymer complex (Ru(PSt-bpy)(bpy) +, prepared by Eq. (15)) deposited as solid phase in methanol containing MV2+ induced MV 7 formation in the liquid phase 9). The rate of MV formation was 4 pM min-1. As shown in Fig. 14, photoinduced electron transfer occurs from EDTA in the solid to MV2+ in the liquid via Ru(bpy)2 +. The protons and Pt catalyst in the liquid phase brought about H2 evolution. One hour s irradiation of the system gave 9.32 pi H2 after standing 12 h and the turnover number of the Ru complex was 7.6 under this condition. The apparent rate constant of the electron transfer from Ru(bpy)2+ in the solid phase to MV2 + in the liquid was estimated to be higher than that of the entire solution system. The photochemical reduction and oxidation products, i.e., H2 and EDTAox were thus formed separately in different phases. Photoinduced electron relay did not occur in the system where a film of polymer pendant Ru complex separates two aqueous phases of EDTA and MV2 9) (see Fig. 15c). 

The photoinduced electron relay systems in the solid phase and at the solid-liquid interface containing Ru(bpy) + in the solid phase are summarized in Fig. 15. The electron transfer from Ru(bpy)2 + to MV2+ is very facile and occurs even at the solid-liquid interface. The back electron transfer of the products (Ru(bpy) + + MV--> Ru(bpy) + + MV2+) is so rapid, however, that MVt can be accumulated only when Ru(bpy)3+ is rapidly reduced by a reducing agent such as EDTA. If the reduction of Ru(bpy)j+ by EDTA shall compete with that by MV"t, both EDTA and Ru complex, or at least both EDTA and MV2+ should exist in the phase. The systems (a) and (b) of Fig. 15 reduce Ru(bpy) + by EDTA with the Ru complex... 

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... 

Mother nature has resolved the various limitations involved in multi-electron processes. Unique assemblies composed of cofactors and enzymes provide the microscopic catalytic environments capable of activating the substrates, acting as multi-electron relay systems and inducing selectivity and specificity. Artificially tailored heterogeneous and homogeneous catalysts as well as biocatalysts (enzymes and cofactors) are, thus, essential ingredients of artificial photosynthetic devices. 

Figure 1. Structures of redox polymers used as electron relay systems in flavoenzyme-based biosensors. Shown are siloxane (top), ethylene oxide (middle), and branched siloxane-ethylene oxide (bottom) polymers.

A further improvement in sensor response is obtained when the ferrocene-siloxane-ethylene oxide polymers (H and I) are used as the electron relay system, as shown in Figures 9 and 10. These materials are based on the hydrophobic siloxane backbone, yet the hydrophilic ethylene oxide side chains, onto which the ferrocene moieties are attached, allow the electron relays to achieve a close interaction with the enzyme molecules. 

Figure 11. Schematic representation of an amperometric biosensor for acetylcholine based on polymeric electron relay systems.

Figures 12-14 show the steady-state current dependence of the acetylcholine sensors on substrate concentration these sensors contained polymers C, F, and I, respectively, as the electron relay systems. For an applied potential of +250 mV vs. SCE, the time required to reach 95% of the steady-state current was typically 10-15 sec after addition of the acetylcholine sample. At lower potentials, the response time was slightly slower. For these systems, a detection limit (as defined by a signal-to-noise ratio of approximately 2) of approximately 0.5 to 1.0 /xM was achieved under N2-saturated conditions. The response of the sensors to choline was nearly identical to the acetylcholine response, which demonstrates the efficient conversion of acetylcholine to choline by acetylcholinesterase.

An elegant electron relay system has been developed which uses triphenylphos-phine as the electron source and dicyanoanthracene as PET-sensitizer. The DCA radical anion subsequently transfers an electron to a Michael system which eventually undergoes radical cyclization chemistry [146]. 

Figure 6. Schematic diagram of a photoelectrochemical cell based on a sensitized semiconductor electrode. M = metal-polypyridine sensitizer, R = reversible electron-relay system, e.g. h/lj -...

The ratio of ferrocene-modified siloxane subunits to unsubstituted siloxane subunits rrv.n ratio) was varied as was the length (a ) of the alkyl side chain onto which the ferrocene moiety was attached as shown in Fig. 3.3. The electrode containing co-polymer with m n ratio of 1 1 or 1 2 was the more efficient electron relay systems. The ferrocene-modified homopolymer on the other hand loses flexibility due to steric hindrance caused by the side chain substitution by ferrocene, preventing efficient electron transfer from the enzyme to the electrode. The length of the alkyl side chain onto which the ferrocene moiety is attached was also found to influence the electron transfer efficiency of the electron relay system. Maximal current density was measured... 

DEVELOPMENT OF ENZYME BIOSENSORS CONTAINING POLYMERIC ELECTRON RELAY SYSTEMS... 

Other ferrocene containing polymeric electron relay systems were investigated by Casado et al. and Watanabe et al. for application to glucose... 

Fig. 3.7. Redox cycles for bilayer-bienzyme electrodes incorporating a flavoenzyme, horseradish peroxidase and a polsmieric electron relay system.

The photochemical cleavage of water can be achieved by coupling a sensitizer to an electron relay system as shown in Figure 2. A critical feature of cyclic reaction... 

Figure 2 General scheme for the cyclic photodecomposition of water. S is a sensitizer and R is an electron relay system...

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