Glucose-specific electrode

Other applications of the technique include the use of immobilized glucose isomerase for the enrichment in fructose of corn starch hydrolysates and the use of immobilized pectinases for the clarification of fruit juice and wine. Immobilized enzymes have also been used as sensors for particular substrates. For example, a glucose-specific electrode has been developed which consists of glucose oxidase (page 94) entrapped in a polyacrylamide gel which is layered over a conventional polarographic oxygen electrode. 

Both the sensitivity and the specificity of a glucose microenzyme electrode have been improved by using voltage pulses (Ikariyama et al., 1988). 

The chemical specificity of enzymes is increasingly being exploited in a range of electrochemical sensors. Simple enzyme electrodes result from the combination of an amperometric electrochemical sensor with a thin (10-250 pm) layer of enzyme. The most developed of these is the glucose oxidase electrode for glucose... 

Homo- and copolymers of 1-alkyl-4-vinylpyridinium ions, 1, are well-known for their ability to form complexes with a wide variety of materials [1, 2]. For example, the interaction of poly(l-alkyl-4-vinylpyridiniums), 2, 3, and 4 with natural and synthetic polyanions has been used to form stable inter-polymer complexes [3-10]. Such complexes have been successfully used to attach redox enzymes or electroactive systems of small molecules to working electrodes. When 2 or 3 is used as a matrix for an anionic enzyme such as glucose oxidase, at pH > pKi the enzyme can be effectively wired to an electrode and can contribute to a variety of electroanalytical applications. Heller and associates [11] have utilized such interpolymer complexes to provide a glucose-specific... 

Finally, the phosphate-specific electrode relies on the analysis of enzymatic inhibition. The phosphate ions inhibit one enzyme, alkaline phosphatase, thereby preventing the production of detectable glucose by a second enzyme, glucose oxidase [ 185]. 

For nonvolatile or thermally labile samples, a solution of the substance to be examined is applied to the emitter electrode by means of a microsyringe outside the ion source. After evaporation of the solvent, the emitter is put into the ion source and the ionizing voltage is applied. By this means, thermally labile substances, such as peptides, sugars, nucleosides, and so on, can be examined easily and provide excellent molecular mass information. Although still FI, this last ionization is referred to specifically as field desorption (FD). A comparison of FI and FD spectra of D-glucose is shown in Figure 5.6. 

Entrapment of biochemically reactive molecules into conductive polymer substrates is being used to develop electrochemical biosensors (212). This has proven especially useful for the incorporation of enzymes that retain their specific chemical reactivity. Electropolymerization of pyrrole in an aqueous solution containing glucose oxidase (GO) leads to a polypyrrole in which the GO enzyme is co-deposited with the polymer. These polymer-entrapped GO electrodes have been used as glucose sensors. A direct relationship is seen between the electrode response and the glucose concentration in the solution which was analyzed with a typical measurement taking between 20 to 40 s. 

The demand for monitoring common metabolites of diagnostic utility such as glucose, urea and creatinine continue to provide the impetus for a staggering research effort towards more perfect enzyme electrodes. The inherent specificity of an enzyme for a given substrate, coupled with the ability to electrochemically detect many of the products of enzymatic reactions initiated the search for molecule-selective electrodes. 

There are also RMs which are prepared for a specific application and are used for validation of relevant methods. Cobbaert et al. (1999) made use of Ion Selective Electrode (ISE)-protein-based materials when evaluating a procedure which used an electrode with an enzyme-linked biosensor to determine glucose and lactate in blood. Chance et al. (1999) are involved with the diagnosis of inherited disorders in newborn children and they prepared a series of reference materials consisting of blood spotted onto filter paper and dried, from which amino-acids can be eluted and... 

Bioelectrocatalysis involves the coupling of redox enzymes with electrochemical reactions [44]. Thus, oxidizing enzymes can be incorporated into redox systems applied in bioreactors, biosensors and biofuel cells. While biosensors and enzyme electrodes are not synthetic systems, they are, essentially, biocatalytic in nature (Scheme 3.5) and are therefore worthy of mention here. Oxidases are frequently used as the biological agent in biosensors, in combinations designed to detect specific target molecules. Enzyme electrodes are possibly one of the more common applications of oxidase biocatalysts. Enzymes such as glucose oxidase or cholesterol oxidase can be combined with a peroxidase such as horseradish peroxidase. 

Most ultramicrobiosensors use differential measurement to overcome the problems of interferences and electrode fouling. The practical use of these biosensors for direct measurement is limited by interferents, such as ascorbic acid, acetaminophen (paracetamol), uric acid, etc., which are present in complex matricies such as serum. The specificity of the biochemical system is compromised by the partial selectivity of the electrode. The electrode not only oxidizes the desired product (e.g., H2O2 formed in the enzymatic oxidation of glucose by glucose oxidase), but also any other species oxidizable at the working potential. This produces a larger current response and a positive error. 

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