Electrical communication with

Provided that the required enzymes can be immobilized at, and electrically communicated with, the surface of an electrode, with retention of their high catalytic properties and there is no electrolysis of fuel at the cathode or oxidant at the anode, or a solution redox reaction between fuel and oxidant, the biocatalytic fuel cell then simply... 

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... 

The electrical contact of redox proteins is one of the most fundamental concepts of bioelectronics. Redox proteins usually lack direct electrical communication with electrodes. This can be explained by the Marcus theory16 that formulates the electron transfer (ET) rate, ket, between a donor-acceptor pair (Eq. 12.1), where d0 and d are the van der Waals and actual distances separating the donor-acceptor pair, respectively, and AG° and X correspond to the free energy change and the reorganization enery accompanying the electron transfer process, respectively. 

A further approach to controlling electrical communication between redox proteins and their electrode support through a photo-command interface includes photo stimulated electrostatic control over the electrical contact between the redox enzyme and the electrode in the presence of a diffusional electron mediator (Scheme 12).[58] A mixed monolayer, consisting of the photoisomerizable thiolated nitrospiropyran units 30 and the semi-synthetic FAD cofactor 25, was assembled on an Au electrode. Apo-glucose oxidase was reconstituted onto the surface FAD sites to yield an aligned enzyme-layered electrode. The surface-reconstituted enzyme (2 x 10-12 mole cm-2) by itself lacked electrical communication with the electrode. In the presence of the positively charged, protonated diffusional electron mediator l-[l-(dimethylamino)ethyl]ferrocene 29, however, the bioelectrocatalytic functions of the enzyme-layered electrode could be activated and controlled by the photoisomerizable component co-immobilized in the monolayer assembly (Figure 12). In the... 

The direct electron transfer between the redox-active sites of proteins and electrodes is normally prohibited as a consequence of steric insulation by the protein matrix. Early studies demonstrated, however, that certain enzymes or redox proteins can exhibit electrical communication with electrode supports, and that electrically stimulated biocatalytic transformations can be driven by that process (Figure lA)... 

The electrical contacting of redox enzymes that defy direct electrical communication with electrodes can be established by mediated electron transfer using synthetic or biologically active charge carriers. Mediated electron transfer (MET) can be effected by a diffusional mechanism (Figure 2), where the electron relay is either oxidized or reduced at the electrode surface. Diffusional penetration of the oxidized or... 

Electrical communication with electrodes and the amperometric transduction of biorecognition... 

Recent advances in the development of artificial photosynthetic systems, where native enzymes are coupled to photoinduced ET products, have been reviewed. Chemical means to modify bioactive proteins and to establish electrical communication with photoexcited entities have been addressed. Such functionalized proteins can substitute complex cofactor-enzyme assemblies. Finally, the use of heterogeneous and homogeneous catalysts, and their functions as cofactor-enzyme models in activation of the substrates and accumulation of electrical charges, have been discussed. 

As corrosion protection by active coatings such as Zn-rich [3], Mg-rich [11], or CP-coatings relies on electrical communication with the underlying metal, the nature of any intervening oxide layer will likely play an important role. As discussed in Section 15.6.3, the electrodeposition of CP films on oxideforming metals is also greatly influenced by the electrical properties of the oxide layer. A detailed understanding of oxide films requires aspects of materials science, solid-state physics, and electrochemistry, and such a discussion is beyond the scope of this chapter. For a detailed discussion of oxide films and their properties, the reader is referred to Ref. [16]. In this section, we provide a brief overview of the... 

The ideal immobilization method should place the biological layer in close proximity to the electrode, but should also maintain its biological activity after its immobilization at the interface. In order to achieve a high efficiency of transduction, or the catalytic process, the immobilization procedures should lead to a high density of biomolecules on the electrode surface and even establish an electrical communication with these biomolecules. 

The electrochemical insulation of the enzyme-active site by its protein or glycoprotein shell usually precludes the possibility of any direct electron-transfer with bulk electrodes [15]. However, under carefully controlled conditions, some enzymes can exhibit direct, nonmediated electrical communication with electrode supports, and biocatalytic transformations can be driven by these processes [16, 17]. For example, the direct electroreduction of O2 and H2O2 biocatalyzed by laccase [18] and horseradish peroxidase (HRP) [19], respectively, have been demonstrated. This unusually facile electronic contacting is believed to be the consequence of incompletely encapsulated redox centers. When these enzymes are properly orientated at the electrode surface, the electrodeactive site distance is short enough for the electron-transfer to proceed relatively unencumbered. Direct electron communication between enzyme-active 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 surfaces with features that can penetrate close enough to the enzyme active site to make direct electron-transfer possible [20, 21]. 

The electrical contacting of redox enzymes that defy direct electrical communication with electrodes can be estabhshed... 

The intermediate location of a redox-relay between the electrode surface and the cofactor unit embedded in the en2yme is of key importance for the establishment of electrical contact between the enTyme and the electrode. For example, a PQQ monolayer assembled onto an Au-electrode was employed to reconstitute the PQQ-dependent apo-GDH [164, 165]. In this case, the PQQ plays the role of the embedded cofactor, and since no additional electron-relay was immobilized between PQQreconstituted enzyme lacks the electrical contact with the electrode. The electrochemical oxidation of glucose by the reconstituted biocatalyst was only stimulated in the presence of a diffusional electron-transfer mediator. In other cases, however, the orientation of the protein with respect to the electrode is sufficient to promote electron-transfer vrithout the need for a mediator. An Fe(111 )-protoporphyrin IX complex was assembled as a monolayer on an Au-electrode and apo-Mb was reconstituted with the heme-cofactor monolayer [166]. Although native Mb usually lacks direct electrical communication with electrode supports as a result of insulation of the heme center. 

Potentiometric biosensors based in both ISE and ISFET for water analysis have been widely developed in the last few years, with recent research leading to nanomaterial-based devices. New nanoparticle (NP)-based signal amplification and coding strategies for bioaffinity assays are in use, along with molecular carbon-nanotube (CNT) wires for achieving efficient electrical communication with redox-enzyme and nanowire-based label-free DNA sensors. ... 

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