Electrically contacted enzyme electrodes applications

Besides the broad applications of electrically contacted enzyme electrodes as amperometric biosensors, substantial recent research efforts are directed to the integration of these functional electrodes as biofuel cell devices. The biofuel cell consists of an electrically contacted enzyme electrode acting as anode, where the oxidation of the fuel occurs, and an electrically wired cathode, where the biocatalyzed reduction of the oxidizer proceeds (Fig. 12.4a). The biocatalytic transformations occurring at the anode and the cathode lead to the oxidation of the fuel substrate and the reduction of the oxidizer, with the concomitant generation of a current through the external circuit. Such biofuel cells can, in principle, transform chemical energy stored in biomass into electrical energy. Also, the use... 

Figure 8. (A) The assembly of an electrically contacted glutathione reductase monolayer. (B) The rate of bioelectrocatalyzed reduction of oxidized glutathione (GSSG) by the electrically contacted enzyme electrode using various connecting chain lengths (a) n = 2, (b) n = 5, (c) = 11. Application of -0.72 V vs. SCE to the enzyme electrode in the presence of GSSG (10 mM). The experiments were performed in 0.1 M phosphate buffer, pH 7.2, under Ar.

Refs. [i] Clark LC, Lyons C (1962) Ann NY Acad Sci 102 29 [ii] Turner APT, Karuhe I, Wilson GS (1987) Biosensors fundamentals and applications. Oxford University Press, Oxford [in] Scheller FS, Schubert F (1992) Biosensors. Elsevier, Amsterdam [iv] Heller A (1990) Acc Chem Res 23 128 [v] Scheller F, Wollenberger U (2002) Enzyme electrodes. Ire Bard AJ, Stratmann M, Wilson GS (eds) Bioelectrochemistry. Encyclopedia of electrochemistry, vol. 9. Wiley-VCH, Weinheim [vi] Scheller F, LisdatF, Wollenberger U (2005) Application of electrically contacted enzymes for biosensors. In Willnerl, KatzE (eds) Bioelectronics from theory to applications. Wiley-VCH, Weinheim [vii] Cass AEG (ed) (1990) Biosensors a practical approach. Oxford University Press, Oxford... 

The following sections will concentrate on the analytical application of redox proteins and redox enzymes for biosensing. The biomolecule will be briefly introduced, the major route for its direct electric contact to electrodes outlined and the analytical application discussed. The bioelectrochemical studies on structure-function relationship and their role in biological redox processes will not be covered in detail in this review. 

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... 

Methods to electrically wire redox proteins with electrodes by the reconstitution of apo-proteins on relay-cofactor units were discussed. Similarly, the application of conductive nanoelements, such as metallic nanoparticles or carbon nanotubes, provided an effective means to communicate the redox centers of proteins with electrodes, and to electrically activate their biocatalytic functions. These fundamental paradigms for the electrical contact of redox enzymes with electrodes were used to develop amperometric sensors and biofuel cells as bioelectronic devices. 

Figure 1. The direct, non-mediated electrical contacting of enzymes (A) direct electron transfer between an electrode and an enzyme molecule located in close vicinity to the surface (B) application of the direct electrical wiring of HRP for the detection of H2O2 produced by an oxidase in response to a primary substrate.

Photoswitchable electrical communication between enzymes and electrodes has also been achieved by the application of photoisomerizable electron-transfer mediators [195, 199]. DilTusional electron mediators (viologen or ferrocene derivatives) were functionalized with photoisomerizable spiropyran/merocyanine units. These mediators can be reversibly photoisomerized from the spiropyran state to the merocyanine state (360 < A < 380 nm) and back (A > 475 nm). An enzyme multilayer array composed of glutathione reductase or glucose oxidase was electrically contacted only when the photoactive group linked to the redox relay (viologen or ferrocene derivative, respectively) was in the spiropyran state. 

FIG. 7.19 Electronic transduction of phocoswitchable bioelectrocacalytic functions of enzymesf proteins by the application of a photoisomerizable command Interfece that controis the electrical contact between die redox enzyme/protain and the electrode. 

Recent advances and progress in electrically contacted, layered enzyme electrodes in the potential applications for amperometric biosensors, sensor... 

Yan, Y.-M., Yehezkeli, O., Willner, 1. Integrated, electrically contacted NAD(P) + -dependent enzyme-carbon nanotube electrodes for biosensors and biofuel cell applications. Chem. Eur. J. 13(36), 10168-10175 (2007). doi 10.1002/chem.200700806... 

The application of nanomaterials in a glucose sensor is an active area of research. Nanomaterials, such as AuNPs and SWNTs, are used to facilitate the electron contact between the redox center of the enzyme and electrodes, which paves the way for the development of reagent-free glucose sensors. In Figure 37, Willner s group has applied AuNPs as an electron relay for electrical wiring of the redox-active center of the enzyme. AuNPs with the... 

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