Electron-transfer relay systems

Electron-transfer relay systems binding electron transfer relays to enzyme proteins... 

Unfortunately, the response of sensors containing glucose oxidase and ferrocene modified electron-transfer relay system began to deviate fi om linearity below 10 mM of glucose concentration. Additionally, sensor sensitivity toward oxygen and oxidation of interferent species at operating potential has yet to be solved. 

When ferrocene-containing polysiloxane proved to be an efficient electron-transfer relay system, further modification of this type redox pol3uner was investigated to develop optimal enzyme biosensors. Attempts were made to synthesize redox polymers with different mediators and/or different polymer backbones and/or different side chains through which mediators are attached to the polymer backbone. Resulting redox poisoners were tested to construct different types of enzyme sensors. 

Pofy(ether amine quinone)s as Electron-Transfer Relay Systems in Amperometric Glucose Sensors... 

Amperometric glucose electrodes based on glucose oxidase, chemical or electrochemical steps, 124-125 Amperometric glucose sensors glucose oxidase and nonphysiological redox mediators, electron-transfer mechanism, 169-170 poly(etheramine-quinone)s as electron transfer relay systems, 124-135 viologen derivative containing polysiloxane as electron-transfer mediator, 169-178... 

Enzyme biosensors containing pol3mieric electron transfer systems have been studied for more than a decade. One of the earlier systems was first reported by Degani and Heller [1,2] using electron transfer relays to improve electrochemical assay of substrates. Soon after Okamoto, Skotheim, Hale and co-workers reported various flexible polymeric electron transfer systems appUed to amperometric enz5une biosensors [3-16], Heller and co-workers further developed a concept of wired amperometric enzyme electrodes [17—27] to increase sensor accuracy and stability. 

Iron can be said to be the most chemically versatile of all the elements used by nature. It is essential for dioxygen uptake and transport in the vast majority of living systems, is ubiquitous in electron transfer relays and oxygen metabolism, is used widely as the catalytic center in enzymes catalyzing chemical changes as diverse as dinitrogen fixation, nitrate reduction, and isopenicillin-N-synthase, and is vital for DNA... 

In most electrochemical systems used for glucose determination, a common element is the use of an electron transfer "shuttle" for transport of electrons between glucose oxidase and the electrode surface. This species can be a monomeric, freely diffusing mediator that is soluble in the analyte sample, or polymeric in nature, and immobilized on the electrode surface along with the enzyme in the form of a thin film or hydrogel. An example of the latter type has been pioneered by Gregg and Heller whereby a polymeric electron-transfer relay based on an osmium-substituted polyvinylpyridine polymer is co-immobilized with glucose oxidase on an electrode... 

They found almost complete quenching of the emission from the ruthenium complex and in addition the covalently linked compound considerably enhanced electron transfer to relay systems of aligned viologen units on micelles and polymers. 

Natural photosynthesis applies electron transfer systems, where a relay of electron-transfer reactions evolves among chlorophyll and quinone moieties embed-... 

More recently, nanotechnology has faciUtated progress in miniaturizing redox enzyme electrodes and extending their application. In order to achieve contact between the active site of the redox enzyme where electron transfer takes place, usually buried within the protein structure, and the electrode electrical contact, cofactor-functionaUzed nanomaterials have been developed [75]. Diffusible cofactors such as FAD can be used as the relay system for carrying electrons to electrical... 

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... 

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... 

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