Biosensors general principle

The enzyme can be incorporated into an amperometric sensor in a thick gel layer, in which case the depletion region due to the electrochemical reaction is usually confined within this layer. Alternatively, enzyme can be immobilized at the surface of the electrode or even within the electrode material itself, in which case the depletion region extends into the solution in the same way as it would for an unmodified electrode. In the latter case, the enzyme can then be seen as a true electrocatalyst that facilitates the interfacial electron transfer, which would otherwise be too slow. The general principles of the design and operation of these biosensors is illustrated on the example of the most studied enzymatic sensor, the glucose electrode (Fig. 2.14, Section 2.3.1). 

K. Cammann, Selectivity Modulation of Ion-selective Membranes - The General Principle of a New Class of Immunosensors , Biosensors, 90, Elsevier Scientific, London, 1991. 

The wealth of information obtained on the general principles of crystalline bacterial cell surface layers, particularly on their structure, assembly, surface, and molecular sieving properties have revealed a broad application potential. Above all, the repetitive physicochemical properties down to the subnanometer-scale make S-layer lattices unique self-assembly structures for functionalization of surfaces and interfaces down to the ultimate resolution limit. S-layers that have been recrystallized on solid substrates can be used as immobilization matrices for a great variety of functional molecules or as templates for the fabrication of ordered and precisely located nanometer-scale particles as required for the production of biosensors, diagnostics, molecular electronics, and nonlinear optics [2,3,6]. 

Rapid equivalent methods - Generally based on a principle different from that of the corresponding laboratory method, alternative or surrogate systems are used more and more often for on-line and off-line monitoring. For example, the spectrophotometric methods or biosensors proposed for the measurement of organic compounds or electrochemical techniques for metals must be considered as alternative methods. 

In this chapter, a general introduction to fiber-optic sensors is presented, followed by sections on the principle of sensors design and sensors development and processing, as well as on sensors characterization and optimization. The technical feasibility and viability of fiber optics in chemical and biosensors applications have been demonstrated with a number of examples and a list of references on successfully reported research. Also, an overview on state-of-the-art research is presented, which is still under development and requires more work before the ultimate limits imposed by fiber optics science and technologies are reached. 

Optoelectronic biosensors based on immobilized dyes have been developed for the determination of glucose, urea, penicillin, and human serum albumin (Lowe et al., 1983). Other promising approaches use immobilized luciferase or horseradish peroxidase to assay ATP or NADH or, when coupled with oxidases, to measure uric acid or cholesterol. These principles have not yet been generally accepted for use in routine analysis. Most probably, the first commercial optical biosensors will be those for immunological assays. 

In general, electroanalytical detection principles can be divided into three potentiometry, amperometry, and conductometry (or impedometry). Potentiostats used for electrochemical biosensors are mostly equipped with amperometric and... 

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