Biosensor applications of enzymes

Practical Applications of Enzymes on Electrodes. There is a growing field of biosensors, in which electrochemistry is used in biological situations. For example, ultramicroelectiodes (Section 7.5.4.4) can be used to monitor electroencephalogmphic activity in the brain. 

Process control applications of enzyme electrochemical biosensors... 

Kauffmann, J.M. Guilbault, G.G. Enzyme electrode biosensors theory and applications. In Bioanalytical Application of Enzymes Suetler, C.H., Ed. Wiley New York, 1992 Vol. 36, 63-113. 

The most important and most studied applications of enzyme biosensors are to detect and monitor blood glucose, followed by lactate, because of the medical applications of such sensors. Thus, by initially detailing the development of glucose biosensors we can better understand and trace the general development of enzyme biosensors containing polymeric electron transfer systems. 

The first applications of enzymes in bioanalytical chemistry can be dated back to the middle of nineteenth century, and they were also used for design of first biosensors. These enzymes, which have proved particularly useful in development of biosensors, are able to stabilize the transition state between substrate and its products at the active sites. Enzymes are classified regarding their functions, and the classes of enzymes are relevant to different types of biosensors. The increase in reaction rate that occurs in enzyme-catalyzed reactions may range from several up to e.g. 13 orders of magnitude observed for hydrolysis of urea in the presence of urease. Kinetic properties of enzymes are most commonly expressed by Michaelis constant Ku that corresponds to concentration of substrate required to achieve half of the maximum rate of enzyme-catalyzed reaction. When enzyme is saturated, the reaction rate depends only on the turnover number, i.e., number of substrate molecules reacting per second. 

The application of enzyme electrodes in a flow system enables two biosensors to be placed in a tubular flow-cell facing each other to measure two analytes simultaneously, e.g., glucose and urea [318]. 

A very specialized application of enzyme-based biosensor arrays has been reported for the resolution of pesticide mixtures containing dichlorvos and methylparaoxon, with a three-element array and a flow injection system [41]. The screen-printed, amperometric electrode array was modified with three acetylcholinesterase enzyme variants, one from electric eel and two from Drosophila (fruit fly) mutants, and were used to measure signal inhibition in conjunction with an artificial neural network. Good results down to the low uM range of pesticide concentrations were reported. 

Particularly attractive for numerous bioanalytical applications are colloidal metal (e.g., gold) and semiconductor quantum dot nanoparticles. The conductivity and catalytic properties of such systems have been employed for developing electrochemical gas sensors, electrochemical sensors based on molecular- or polymer-functionalized nanoparticle sensing interfaces, and for the construction of different biosensors including enzyme-based electrodes, immunosensors, and DNA sensors. Advances in the application of molecular and biomolecular functionalized metal, semiconductor, and magnetic particles for electroanalytical and bio-electroanalytical applications have been reviewed by Katz et al. [142]. 

Enzyme electrodes belong to the family of biosensors. These also include systems with tissue sections or immobilized microorganism suspensions playing an analogous role as immobilized enzyme layers in enzyme electrodes. While the stability of enzyme electrode systems is the most difficult problem connected with their practical application, this is still more true with the bacteria and tissue electrodes. 

Fig. 1.11 Applications of LDHs as (A) non-viral vector in gene therapy for transfection of DNA to the cell nucleus, and (B) as matrix for enzymes immobilization in the development of biosensors.

For application of protein-immobilized porous materials to sensor fields, use of an electroactive substance as the framework material is important. DeLouise and Miller demonstrated the immobilization of glutathione-S-transferase in electrochemically etched porous silicon films [134], which are attractive materials for the construction of biosensors and may also have utility for the production of immobilized enzyme bioreactors. Not limited to this case, practical applications of nanohybrids from biomolecules and mesoporous materials have been paid much attention. Examples of the application of such hybrids are summarized in a later section of this chapter. 

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