Mediators, enzymes, sensor materials

PHC-based sensors have also been proposed, especially gas sensors. PP has been indicated as a gas-sensing material for SO2, NO2, L, etc. [335] on the basis of the increase of conductivity of the neutral polymer in the presence of oxidizing gases. On the other hand, exposure of PT films to NO or NO2 increases their conductivity, whereas HjS and NH3 have the opposite effect [336,337]. Enzyme sensors based on PHCs have arouse great interest. The enzyme glucose oxidase can be immobilized in polypyrroles [338,339]. The electrical communication between the enzyme and the electrode requires the presence of mediators such as... 

The sensor materials in question are organic in nature, and with notable exceptions, such as the naturally occurring actins and valinomycin, and the ubiquitous enzymes, the neutral carrier and ion-exchanger materials, like their associated solvent mediators, must be synthesized. 

So far, it has been reported that redox polymers, such as polysiloxane or polyarrylamide on which ferrocene is introduced as a side chain, are effective mediators [15-25]. Other effective mediators include poly(vinyl pyridine) or poly(vinyl imidazole) on which osmium complex is introduced in the side chain. However, the structure-mediator fimction correlation has been poorly understood. Here, the glucose sensor was used as an example of enzyme sensors, and as shown in Table 1, sensors using enzymes other than glucose oxidase have also been actively studied. It is now possible to detect electrochemically materials in the body using an electrode with an enzyme and a polymer gel. 

By changing the enzyme and mediator, the amperometric sensor in Figure 11.39 is easily extended to the analysis of other substrates. Other bioselective materials may be incorporated into amperometric sensors. For example, a CO2 sensor has been developed using an amperometric O2 sensor with a two-layer membrane, one of which contains an immobilized preparation of autotrophic bacteria. As CO2 diffuses through the membranes, it is converted to O2 by the bacteria, increasing the concentration of O2 at the Pt cathode. 

Researchers turned their attention to applications of silica gel as a new electrode material. Silica gel, which has a three-dimensional structure with high specific surface area and is electroinactive in an aqueous medimn can be used as a support for electroactive species during their formation and/or enzymes by adsorption or entrapment [92,93]. Patel et al. recently reported application of poljwinyl ferrocene immobilized on silica gel particles to construct glucose sensors. Efficiency of carbon paste electrodes prepared with these polymeric electron mediators and GOx was comparable to electrodes constructed with other ferrocene based polymeric electron transfer systems. The fact that 70% of initial anodic current was retained after a month when electrodes were kept in the buffer at room temperature shows that polymerization of monomer vinylferrocene in the pores of silica gel and entrapping GOx in the matrix of poljwinyl ferrocene appears to have added stability to the sensors [94]. 

Other applications of conductive polymers in chemical and biological sensors include a building of electrical connection between different elements of these devices6 or electrical connection and mediators of electron transfer between enzyme and electrode26,27 or between electrocatalytically active functional groups or incorporated nanoparticles and electrode,28 as a filter,29 as matrix for immobilization of enzymes or even as a combination of many of these functions.26 The requirements for materials for these applications are very specific and will not be analyzed here. 

The discovery of new materials such as nanoparticles and nanotubes has led to their use in electrochemical sensors in combination with ferrocene mediators and redox enzymes. Davis and co-workers modified single-wall carbon nanotubes (SWCNTs) by adsorbing glucose oxidase to them and then used a soluble ferrocene derivative as a... 

The wealth of natural examples provides immense inspiration for the molecular design of novel peptide-based materials that can be potentially applied as devices, sensors, and biomaterials for medical applications. In addition to hierarchical self-assembly, nature uses other mechanisms, for example, enzyme-mediated covalent cross-linking, to build up structural proteins and higher-ordered structures. In the following sections we will focus on manmade peptide-based materials that belong to the three classes listed below. They will be split with respect to the underlying design concept into materials formed by ... 

Tliis work demonstrates the potential for application of potentiometric enzyme electrodes based on mediatorless enzyme electrocatalysis for fast and sensitive assay of organophosphorus pesticides. The sensing element based on screen-printed carbon material pomits mass fabrication of the electrodes at a low cost which is essential for the disposable sensor concept. The biosensor does not require any low-molecular weight mediator and can be arranged as an all-solid-state device. Such electrodes. 

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