Biosensors biocatalytic sensors

Biosensors may be classified into two categories biocatalytic biosensors and bioaffinity biosensors. Biocatalytic sensors contain a biocatalyst such as an enzyme to recognize the analytic selectively. Bioaflinity biosensors, on the other hand, may involve antibody, binding protein or receptor protein, which form stable complexes with the corresponding ligand. An immunosensor in which antibody is used as the receptor may represent a bioaflinity biosensor. 

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... 

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]. 

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. 

Biocatalytic membrane electrodes significantly expand the scope of direct potentiometry by enabling biosensors that respond to a whole host of organic substrates to be made. The selectivity of these sensors is a combination of the selectivity of the biocatalyst for the substrate and the ISE for other constituents in the sample that might reach the ISE surface membrane. Thus, the selectivity with respect to other organic constituents in the sample is determined by biocatalyst... 

Fitting into the trend towards improvement of the availability and simplification the preparation of biocatalytic layers for biosensors, the use of crude materials has been explored. Arnold and coworkers investigated the feasibility of employing Jack bean meal in a urea sensor (Arnold and Glaizer, 1984) and rabbit muscle acetone powder in a sensor for adenosine monophosphate (Fiocchi and Arnold, 1984). Both sensors turned out to be serious contenders with the appropriate enzyme electrodes with respect to lifetime and slope of the calibration curves. Other parameters, such as response time and linear range, were quite similar. 

Biosensors using higher integrated biocatalytic phases, i.e. cell organelles, intact cells, and tissue material, are compared with isolated enzyme sensors. The merits of the former in the determination of complex variables , such as mutagenicity and nutrient content, are outlined. 

The enzyme loading to a major extent determines the stability of a biosensor. An enzyme reserve is built up by employing more enzyme activity in front of the electrochemical probe than is minimally required to achieve diffusion control. As long as this reserve lasts, the sensitivity will remain essentially constant. This is only significant, however, for sensors for substrate determinations. If effectors of the biocatalytic sensing reactions are to be measured, kinetic control is desired, which permits the enzyme loading to be varied only in a relatively narrow range. 

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. 

The beauty of this reaction from a sensor development point of view is that the thermodynamically favored direction for the reaction can be adjusted simply by setting the solution pH. At high pH, pyruvate is the favored product, and at low pH, lactate is favored. The biosensor can be configured for lactate measurements by adjusting the solution pH to 8.6 and adding NAD to the solution. As lactate enters the biocatalytic layer, the NAD is converted to HASH and an increase in the fluorescence intensity is measured. For a pyruvate biosensor, the solution pH is adjusted to 7.4 and HASH is added to the solution. Pyruvate from the sample enters the biocatalytic layer and a decrease in HADH is measured as the reaction converts the HADH to HAD . A decrease in fluorescence intensity is measured. 

Figure 8 shows a preliminary response curve for a glutamate biosensor. For this sensor, glutamate dehydrogenase has been added to the biocatalytic layer in a sensor like that shown in Figure 6. 

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