Biocatalytic-based biosensors

Various types of biocatalytic materials, such as isolated enzymes, bacterial cells, and intact mammalian and plant tissue sections, are available for the preparation of biocatalytic-based biosensors (3-7), An enzyme, or a group of enzymes, provides the required biocatalytic activity. For the bacterial cell and tissue based systems, the required enzyme is housed in these biocatalytic materials which can help stabilize the enzyme and prolong the biocatalytic activity. 

Redox-based biosensors. Noble metals (platinum and gold) and carbon electrodes may be functionalized by oxidation procedures leaving oxidized surfaces. In fact, the potentiometric response of solid electrodes is strongly determined by the surface state [147]. Various enzymes have been attached (whether physically or chemically) to these pretreated electrodes and the biocatalytic reaction that takes place at the sensor tip may create potential shifts proportional to the amount of reactant present. Some products of the enzyme reaction that may alter the redox state of the surface e.g. hydrogen peroxide and protons) are suspected to play a major role in the observed potential shifts [147]. 

In addition to the development of absorbance-based fiber-optic biosensors, we have recently demonstrated the feasibility of fluorescence-based biosensors (9, 10). Our initial work in this area has focused on the development of fiber-optic biosensors based on the fluorometric detection of reduced nicotinamide adenine dinucleotide (NADH). Here, a dehydrogenase enzyme supplies the biocatalytic activity, and either the generation or consumption of NADH is oionitored. 

While enzymatic biosensors have gained increasing popularity over the past decades, researchers also focused on the development of whole ceU- and tissue-based biosensors. One of the first studies that reported the use of living cells as biocatalytic elements for biosensors was published in 1978 by G.A. Rechnitz... 

The amperometric biosensor based on carbon paste electrode ensures proximity at the molecular level between the catalytic and electrochemical sites because the carbon electrode is both the biocatalytic phase and the electrode sensor (Table 17.2). The tissue containing carbon paste can be incorporated in various electrode configurations and these have very rapid response times, extended lifetimes, high rigidity, mechanical stability and very low cost. 

Aizawa presented an overview on the principles and applications of the electrochemical and optical biosensors [61]. The current development in the biocatalytic and bioaffinity bensensor and the applications of these sensors were given. The optical enzyme sensor for acetylcholine was based on use of an optical pH fiber with thin polyaniline film. 

Amperometric sensors — A class of electrochemical sensors based on amperometry. A - diffusion-limited current is measured which is proportional to the concentration of an electrochemically active analyte. Preferred technique for - biosensors with or without immobilized enzymes (biocatalytic sensors). The diffusion layer thickness must be kept constant, either by continuous stirring or by means of an external diffusion barrier. Alternatively, micro electrodes can be... 

Up to now the practical application of biosensors has been almost entirely limited to oxidase-based amperometric monoenzyme electrodes and pH-shifting by hydrolases. However, the internal coupling of different biocatalytic reactions in biosensors will lead to a greatly extended applicability and a substantial improvement of the analytical performance characteristics. Exciting results might be achieved by applying the concepts of chemical and genetic modification of enzymes. Further, the site-to-site directed fixation of artificially coupled enzymes could improve the speed and practicability of coupled substrate conversions. 

SCHEME 3 Schematic diagram illustrating the concept of the third-generation 02 biosensors based on the biocatalytic activity inherent in SODs toward the dismutation of O2. (Reprinted from [150], with permission from the Royal Society of Chemistry.)... 

The interplay between nanopartides and biological systems is of spedal relevance for semiconductor nanopartides, known simply also as quantum dots (QDs). In recent years, these have emerged as ideal systems for molecular sensors and biosensors, based largely on their sizewide variety of chemical functionalities with which QDs can be equipped that makes them ideal partners for different biosystems. In contrast to former passive optical labds, specifically functionalized QDs can operate as optical labds so as to observe the dynamics of biocatalytic transformations and conformational transitions of proteins. This development will surely open a wide variety of doors in modem nanobiotechnology. 

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