Direct immunosensors

Fig. 9. Immunosensor approaches where A is the analyte, is the labeled analyte, and Y is the antibody, (a) Direct immunosensors where the actual antigen—antibody interaction is measured (b) indirect immunosensors 1 and 2 which utilize formats similar to competitive and displacement...

There are mainly three types of transducers used in immunosensors electrochemical, optical, and microgravimetric transducers. The immunosensors may operate either as direct immunosensors or as indirect ones. For direct immunosensors, the transducers directly detect the physical or chemical effects resulting from the immunocomplex formation at the interfaces, with no additional labels used. The direct immunosensors detect the analytes in real time. For indirect immunosensors, one or multiple labeled bio-reagents are commonly used during the detection processes, and the transducers should detect the signals from the labels. These indirect detections used to need several washing and separation steps and are sometimes called immunoassays. Compared with the direct immunosensors, the indirect immunosensors may have higher sensitivity and better ability to defend interference from non-specific adsorption. 

Direct hydration, of ethylene, 10 538 Direct hydrogenation, 6 827 Direct immunosensors, 14 154 Direct ingot (dingot) method, 25 409 Direct initiation, 14 270 Direct injection (DI) diesel engines, 12 421 Direct inlet injection, gas chromatography, 6 383, 415-416 Directional couplers, 17 446 Directional drilling techniques, in sulfur extraction, 23 572 Directive 89/107/EEC (EU), 12 36 Direct liquefaction, 6 827 Direct marketing, technical service personnel and, 24 343 Direct metal nitridation, 17 211-213 aerosol flow reactor, 17 211-212 Direct methanol fuel cells (DMFC),... 

Yamamoto et al. developed a direct immunosensor (Y3). The immu-noelectrode was made of titanium wire, on which antigen or antibody was chemically fixed. The electric potential changes when an antigen-antibody reaction occurs. For example, the potential of the electrodes sensitized with anti-hCG antibody shifted to positive polarity when hCG was added. Aizawa et al. also reported a direct immunosensor (Al). The sensitivity is insufficient for clinical use but it is possible that the methods will be improved in the near future. 

Lukosz W, Clerc D, Nellen PM, Stamm C, Weiss P (1991) Output grating couplers on planar optical waveguides as direct immunosensors. Biosens Bioelectron 6 227-232... 

A. Brecht, J. Piehler, G. Lang, G. Gauglitz, A Direct Immunosensor for Atrazine , Anal. Chim. Acta, 311, 289-300 (1995). 

In contrast to direct immunosensors, indirect immunosensors, using as a label an enzyme [307-309] or an ionophore [310], have been more developed. This technique is derived from well established and widely utilized enzyme immunoassays evaluated by photometric methods. Enzyme immunoassay with electrochemical detection (EEIA) represents an important innovation of EIA by the replacement of optical detection with voltammetric or potentiometric measurements. It may also be adapted to flow-through systems [311, 312]. 

Direct immunosensors register immunochemical complex formation at the transducer surface via electrochemical, mass or optical changes. The advantage of a direct immuno-sensor is that measurement of the antigen-an-... 

Among direct immunosensors the most favored are optical sensors, because they incorporate the advantages of fiber optics or optoelectronic transducers. The nonelectrical properties of optical systems offer important safety advantages for... 

D. Schlatter et al. "The Difference Interferometer Application as a Direct Immunosensor," in The Second World Congress on Biosensors. Oxford. Elsevier Advanced Technology. 1992. pp. 347 - 355. 

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