Enzyme systems, sensor materials

Enzyme-free urea sensor Nano carbon-containing transducer, covered with chemical catalytic system, is used as electrochemical enzyme-free sensor for urea determination in biological materials. 

Microencapsulated enzyme/dye systems Nanostructured materials Polyelectrolyte multilayer capsules Sensor system... 

The high specificity required for the analysis of physiological fluids often necessitates the incorporation of permselective membranes between the sample and the sensor. A typical configuration is presented in Fig. 7, where the membrane system comprises three distinct layers. The outer membrane. A, which encounters the sample solution is indicated by the dashed lines. It most commonly serves to eliminate high molecular weight interferences, such as other enzymes and proteins. The substrate, S, and other small molecules are allowed to enter the enzyme layer, B, which typically consist of a gelatinous material or a porous solid support. The immobilized enzyme catalyzes the conversion of substrate, S, to product, P. The substrate, product or a cofactor may be the species detected electrochemically. In many cases the electrochemical sensor may be prone to interferences and a permselective membrane, C, is required. The response time and sensitivity of the enzyme electrode will depend on the rate of permeation through layers A, B and C the kinetics of enzymatic conversion as well as the charac-... 

Applications exploiting porous silica to encapsulate sensor molecules, enzymes and many other compounds are developing rapidly. Nowadays, sol-gel technology is being used in various fields of modem technology, as for example the basis for optodes, integrated systems, fiber optics, lasers, and new materials. 

A further improvement in sensor response is obtained when the ferrocene-siloxane-ethylene oxide polymers (H and I) are used as the electron relay system, as shown in Figures 9 and 10. These materials are based on the hydrophobic siloxane backbone, yet the hydrophilic ethylene oxide side chains, onto which the ferrocene moieties are attached, allow the electron relays to achieve a close interaction with the enzyme molecules. 

Enzymes play a central role in biotechnological processes in the production of useful materials other than those presented above, and enzyme sensors in which enzyme reaction is coupled with an electrochemical monitoring system are used clinical, process and environmental analyses. It is, however, out of the scope of this section to review these topics and other reviews should be consulted.81 ... 

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]. 

Several implanted biosensors have been developed and evaluated in both animals and humans (see Chapter 4). Detection systems are based on enzymes, electrodes, or fluorescence. The most widely studied method is an electrochemical sensor that uses glucose oxidase. This sensor can be implanted intravenously or subcutaneously. Intravenous implantation in dogs for up to 3 months has demonstrated the feasibility of this approach. Alternatives to enzymes are being developed, including artificial glucose receptors. Less success has been achieved with subcutaneous implants. Implantation of a needle type of sensor into the subcutaneous tissue induces a host of inflammatory responses that alters the sensitivity of the device. Microdialysis with hoUow fibers or ultrafiltration with biologically inert material can decrease this problem. 

Beside these disposable systems enzyme-membrane-based devices (first-generation enzyme sensors) are working with a biological component that can be repeatedly used for thousands of measurements. The functional stability depends on the quality of the enzyme membrane material used. The autoanalyzer Stat-Profile 5 (Nova Biomedical, USA) and the lonometer (Fresenius, Germany) permit the analysis of metabolites, such as glucose and lactate, in addition to electrolytes and blood gases (see table 17.2). 

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