Monoenzyme Sensors

Of all types of biosensors, metabolism sensors based on the molecular analyte recognition and conversion have been most intensively studied. According to the degree of integration of the biocomponents they can be classified into monoenzyme sensors, biosensors using coupled enzyme reactions, organelle, microbial, and tissue-based sensors. The sequence of the following sections corresponds to this classification. 

This sensor type initiated biosensor research and has reached the highest state of practical application. The enzyme sensors described and used up to now have been mainly based on oxidoreductases and hydrolases. 

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The third chapter concentrates on metabolism sensors, which are arranged according to the degree of biocatalyst integration. The various different ways of coupling enzymes with transducers in monoenzyme sensors are exemplified by the determination of glucose and urea. The current state of the art is shown for monoenzyme sensors for some further 25 analytes and classes of analytes. Coupled enzyme reactions are shown to provide expansion of the biosensor concept to new analytes and to multiparameter assays as well as to an improvement of such analytical parameters as specificity and sensitivity. This chapter offers for the first time a complete overview of the potentials of coupled enzyme reactions in biosensors. 

Since not all enzyme-catalyzed reactions involve transducer-active compounds such as H, oxygen, or hydrogen peroxide, only a limited number of substances can be determined by using monoenzyme sensors. Owing to their high susceptibility to nonspecific effects, transducers in-... 

If the given classification is used, the simplest model of an amperometric sensor is that of a stationary single-layer monoenzyme electrode with linear reaction kinetics and electrode-active product, P (Schulmeister and Scheller, 1985a). 

The response time under steady state measurement conditions was 5 min, which was double that of a COD monoenzyme electrode (see also Section 3.1.8). The upper limit of the linear current-concentration dependence was 100 (jmol/1. Owing to the low initial activity of CEH and its rapid inactivation after immobilization the sensor was stable for only 1 day. This is insufficient for practical application. 

For a given substrate, e.g. hypoxanthine, the sensitivity decrased with increasing number of enzymes, i.e. from the monoenzyme electrode to the four-enzyme electrode. The sensitivity of each enzyme sensor decreased likewise in the order HX, HXR, IMP, AMP. This indicates kinetic control by several enzyme reactions. In order to measure the concentrations of all four of the analytes in a sample, the sensitivity of each sensor for each substrate must be known because the current changes are coupled with each other according to ... 

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