Electrode hybrid-enzyme

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... 

Kavanagh P, Leech D (2006) Redox polymer and probe DNA tethered to gold electrodes for enzyme-amplified amperometric detection of DNA hybridization. Anal Chem 78 2710-2716... 

Several protein assemblies have successfully been fabricated on the solid surfaces sifter the bioinformation transduction. These include the following molecular systems molecularly interfaced redox enzymes on the electrode surfaces, calmodulin / protein hybrides, and ordered antibody array on protein A. These protein assemblies find a wider application in various fields such as biosensors, bioreactors, and intelligent materials. 

Bacterial electrodes [11, 31, 33, 46, 48, 49, 60] In this type of electrode, a suspension of suitable bacteria is placed between the sensor proper and a dialysis membrane that prevents passage of high-molecular substances (see fig. 8.3). The sensor is usually a gas probe. In the simple types of bacterial electrode, the determinand is converted by a suitable strain of bacteria into a product sensed by the gas probe. Thus it is possible to determine arginine [46], glutamine [48],/.-aspartic acid [31],/.-histidine [60] and nitrate [33]. Hybrid bacterial - enzyme electrodes contain both a bacterial strain and a suitable enzyme. For example, an extract from ivingas Neurospora chossa can be used as a source of NAD nucleosidase and an Escherichia coli culture as a source of nicotinamide deaminase, so that the electrode responds to NAD [49] as a result of the series of reactions... 

A disposable electrochemical enzyme-amplified genosensor was described for specific detection of Salmonella (Del Giallo et al., 2005). A DNA probe specific for Salmonella was immobilized onto screen-printed carbon electrodes and allowed to hybridize with a biotinylated PCR-amplified product of Salmonella. The hybridization reaction was detected using streptavidin conjugated-AP where the enzyme catalyzed the conversion of electroinactive a-naphthyl phosphate to electroactive a-naphthol, which was detected by differential pulse voltammetry. 

Metallic nanoparticles and single-walled carbon nanotubes (SWCNTs) exhibit nanoscale dimensions comparable with the dimensions of redox proteins. This enables the construction of NP-enzyme or SWCNT-enzyme hybrids that combine the unique conductivity features of the nanoelements with the biocatalytic redox properties of the enzymes, to yield wired bioelectrocatalyts with large electrode surface areas. Indeed, substantial advances in nanobiotechnology were achieved by the integration of redox enzymes with nanoelements and the use of the hybrid systems in different bioelectronic devices.35... 

Albeit the substantial progress in the bioelectrochemical activation of enzymes, one could identify two important future challenges in the field (i) The active relay units wiring the redox centers of the enzymes with the electrodes could be generated by photoinduced electron transfer. This could pave the way to the photochemical wiring of enzymes and to the development of photobiofuel cells, (ii) DNA scaffolds provide unique templates for the ordered self-assembly of molecular or biomolecular units through dictated hybridization. The ordering of relay units and enzymes, or of relay units photosystems, on DNA templates associated with electrodes may yield attractive new supramolecular nanostructures for bioelectronics and optobioelec tronic s. 

In the field of biosensor technology, immobilized enzyme electrode development occupies a place of prominence due to the attractive performance of this hybrid device. Coupling an immobilized enzyme layer with an electrochemical sensor combines the advantages of using an insolubilized enzyme system (see below) with the sensitivity of readily available potentiometric and amperometric electrodes. The resulting biosensor enables direct, reliable, and reproducible... 

Enzyme DNA hybridization assays with electrochemical detection can offer enhanced sensitivity and reduced instrumentation costs in comparison with their optical counterparts. Efforts to prevent non-specific binding of the codissolved enzyme and to avoid fouling problems by selecting conditions suitable to amplify the electrode response have been reported by Heller and co-workers [107]. A disposable electrochemical sensor based on an ion-exchange film-coated screen-printed electrode was described by Limoges and co-workers for an enzyme nucleic acid hybridization assay using alkaline phosphatase [108] or horseradish peroxidase [109]. In another methodology to improve sensitivity, a carbon paste electrode with an immobilized nucleotide on the electrode surface and methylene blue as hybridization indicator was coupled, by Mascini and co-workers [110], with PGR amplification of DNA extracted from human blood for the electrochemical detection of virus. 

---