Biosensors receptors

Schuck P 1996 Kinetics of iigand binding to receptors immobiiized in a poiymer matrix, as detected with an evanescent wave biosensor, i. A computer simuiation of the influence of mass transport Biophys. J. 70 1230-49... 

Potcntiomctric Biosensors Potentiometric electrodes for the analysis of molecules of biochemical importance can be constructed in a fashion similar to that used for gas-sensing electrodes. The most common class of potentiometric biosensors are the so-called enzyme electrodes, in which an enzyme is trapped or immobilized at the surface of an ion-selective electrode. Reaction of the analyte with the enzyme produces a product whose concentration is monitored by the ion-selective electrode. Potentiometric biosensors have also been designed around other biologically active species, including antibodies, bacterial particles, tissue, and hormone receptors. 

The low detection limit, high sensitivity, and fast response times of chemoreceptor-based biosensors result primarily from the extremely high binding constants of the receptor R for the target substrate S. The receptor—substrate binding may be described... 

In the enterocyte as it enters the absorptive zone near to the villus tips, dietary iron is absorbed either directly as Fe(II) after reduction in the gastrointestinal tract by reductants like ascorbate, or after reduction of Fe(III) by the apical membrane ferrireductase Dcytb, via the divalent transporter Nramp2 (DCT1). Alternatively, haem is taken up at the apical surface, perhaps via a receptor, and is degraded by haem oxygenase to release Fe(II) into the same intracellular pool. The setting of IRPs (which are assumed to act as iron biosensors) determines the amount of iron that is retained within the enterocyte as ferritin, and that which is transferred to the circulation. This latter process is presumed to involve IREG 1 (ferroportin) and the GPI-linked hephaestin at the basolateral membrane with incorporation of iron into apotransferrin. (b) A representation of iron absorption in HFE-related haemochromatosis. 

Figure 1. The specific binding of analytes to immobilized receptors induces a response of the optical biosensor due to changes in the evanescent wave at the sensor surface.

The selectivity and sensitivity of biosensors are governed by the biocatalyst. Commonly used biocatalysts in biosensors are enzymes [10, 11], antibodies [12, 13], whole cells [14], and artificial receptors such as molecularly imprinted polymers [15, 16],... 

It is worth noting that TTF-based sensors for species other than metals, particularly organic species, have been well studied. The principle is, of course, the same a host group capable of recognising a guest molecule is tethered to the signaller TTF, which displays an altered physical response when a guest is bound and when the receptor is free. Systems of this type are of particular interest as biosensors, for example, in the detection of saccharides for disease... 

Zourob, M. Elwary, S. Turner A. A. F., Principles of Bacterial Detection Biosensors, Recognition Receptors and Microsystems Springer science + business media, ISBN 978 0 387 75112 2,2008... 

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. 

In recent years many efforts have been made to develop immunochemical techniques integrating the recognition elements and the detection components, in order to obtain small devices with the ability to carry out direct, selective, and continuous measurements of one or several analytes present in the sample. In this context biosensors can fulfill these requirements. Biosensors are analytical devices consisting of a biological component (enzyme, receptor, DNA, cell, Ab, etc.) in intimate contact with a physical transducer that converts the biorecognition process into a measurable signal (electrical or optical) (see Fig. 4). In... 

Immunosensors have been developed commercially mostly for medical purposes but would appear to have considerable potential for food analysis. The Pharmacia company has developed an optical biosensor, which is a fully automated continuous-flow system which exploits the phenomenon of surface plasmon resonance (SPR) to detect and measure biomolecular interactions. The technique has been validated for determination of folic acid and biotin in fortified foods (Indyk, 2000 Bostrom and Lindeberg, 2000), and more recently for vitamin Bi2. This type of technique has great potential for application to a wide range of food additives but its advance will be linked to the availability of specific antibodies or other receptors for the various additives. It should be possible to analyse a whole range of additives by multi-channel continuous flow systems with further miniaturisation. 

Certain biomolecules can be added into the bead or be attached to its surface. These mostly include such recognition elements as antibodies, oligonucleotides or other receptors such as conconovalin A. Enzymes can be used to design biosensors (e.g., for glucose) on a microscale but this research is still in its infancy. Finally, fluorescent proteins can be used as alternative to the dyes. The same refers to quantum dots which can also be used in principle. 

None of the involved species are fluorescent. Therefore, for fluorescence signaling of citrate recognition, carboxyfluorescein is first added to the medium because binding to the receptor in the absence of citrate is possible and causes deprotonation of carboxyfluorescein, which results in high fluorescence. Citrate is then added, and because it has a better affinity for the receptor than carboxyfluorescein, it replaces the latter, which emits less fluorescence in the bulk solvent as a result of protonation. Note that this molecular sensor operates in a similar fashion to antibody-based biosensors in immunoassays. It was succes-fully tested on a variety of soft drinks. 

An NIR biosensor coupled with an NIR fluorescent sandwich immunoassay has been developed. 109 The capture antibody was immobilized on the distal end of an optical fiber sensor. The probe was incubated in the corresponding antigen with consecutive incubation in an NIR-labeled sandwich antibody. The resulting NIR-labeled antibody sandwich was excited with the NIR beam of a laser diode, and a fluorescent signal that was directly proportional to the bound antigen was emitted. The sensitivity of the technique increased with increasing amounts of immobilized receptor. There are several factors involved in the preparation of the sandwich type biosensor. A schematic preparation of the sandwich optical fiber is shown in Figure 7.14. 

C. L. Poglitsch and N. L. Thompson, Substrate-supported planar membranes containing murine antibody Fc receptors A total internal reflection fluorescence microscopy study, in Biosensor Technology, Fundamentals and Applications (R. P. Buck, W. E. Hatfield, M. Umafiia, and E. F. Bowden, eds.), pp. 375-382, Marcel Dekker, New York (1990). 

Morton, T.A., D.B. Bennett, E.R. Appelbaum, D.M. Cusimano, K.O. Johanson, R.E. Matico, RR. Young, M. Doyle, and I.M. Chaiken. 1994. Analysis of the interaction between human interleukin-5 and the soluble domain of its receptor using a surface plasmon resonance biosensor. J Mol Recognit 7 47-55. 

Biosensor devices must operate in liquids as they measure effects at a liquid-solid interface. Then, the immobilization of the receptor molecule on the sensor surface is a key step for the efficient performance of the sensor. When the complementary analytes are flowing over the surface, they can be directly recognized by the receptor through a change in the physico-chemical properties of the sensor. In this way, the interacting components do not need to be labeled and complex samples can be analyzed without purification. 

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