Immunosensors electrochemical detection

There are many other examples of competitive electrochemical immunoassays and immunosensors for detecting clinically important analytes [12-14], Despite simplicity, a disadvantage of competitive immunoassays is that labeling the analyte may reduce, or totally remove, its binding affinity for antibody. This would occur if the analyte were labeled at a site that is closely associated with an epitope. 

L. Yang, Y. Li, and G.F. Erf, Interdigitated array microelectrode-based electrochemical impedance immunosensor for detection of Escherichia coli 0157 H7. Anal. Chem. 76, 1107—1113 (2004). 

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. 

C. Valat, B. Limoges, D. Huet and J.-L. Romette, A disposable protein A-based immunosensor for flow-injection assay with electrochemical detection, Anal. Chim. Acta, 404 (2000) 187-194. 

Most electrochemical immunosensors use antibodies or antigens labelled with an enzyme that generates an electroactive product which can be detected at the electrochemical transducer surface. The combination of high enzyme activity and selectivity with the sensitive methods of electrochemical detection provides a basis for the development of immunosensors. Horse radish peroxidase (HRP) and alkaline phosphatase (AP) are popular enzyme labels and can be used with a variety of substrates. 

This configuration based on the use of two surfaces, magnetic beads for immunoassay and screen-printed electrodes for electrochemical detection, allows to obtain a faster and a more sensitive detection of the immunoreaction than using a unique surface (screen-printed electrode) in this case it is possible to perform the electrochemical measurement in faster times (less then 30 min) and improve the sensitivity (around two magnitude orders). For this reason, this approach is advised in the development of an electrochemical immunosensor specific to any analyte. 

The conjugate of protein A labeled with silver was used in electrochemical immunosensor for detecting the forest-spring encephalitis antibodies, which is described in Procedure 38 in CD accompanying this book. 

The development of immunosensors for the detection of diseases has received much attention lately and this has largely been driven by the need to develop hand-held devices for point-of-care measurements [67,68]. Immunosensors can incorporate either the antigen or the antibody onto the sensor surface, although the latter approach has been used most often [67]. Optical [69,70] and electrochemical [70] detection methods are most frequently used in immunosensors [67]. Detection by... 

The development of electrochemical genosensors and immunosen-sors based on labelling with NPs has registered an important growth, principally for clinical and environmental applications. The electrochemical detection of NP labels in affinity biosensors using stripping methods allows the detailed study of DNA hybridisation as well as immunoreactions with interest in genosensor or immunosensor applications. 

The analysis of trace substances in environmental science, pharmaceutical and food industries is a challenge since many of these applications demand a continuous monitoring mode. The use of immunosensors based on AuNPs in these applications should also be appropriate. Although there are many recent developments in the immunosensor field, which have potential impacts [36], nevertheless there are few papers concerning environmental analysis with electrochemical detection based on AuNPs. The application of some developed clinical immunosensors can also be extended to the environmental field. 

Antibody-based, fully automated immunosensors for small molecules have been used to detect explosives (see Refs. [11,12]) and biological warfare agents (see Refs. [13,14]), and can be used to analyse drinking water and extracts at hazardous waste sites (for examples, see Refs. [15-17]). In the following section, we will discuss how the addition of electrochemical detection has broadened the capabilities of such devices. 

Labeled immunosensors typically make use of a reporter molecule attached to the respective antibody. Apart from the electrochemical detection schemes discussed below, immunosensors employ fluorescent labels, radioactive labels, or nanoparticles, among other reporter systems [407—410]. Labeled immunosensors normally operate using either a direct or indirect sandwich procedure or a competitive format as depicted in Figure 1.18. 

A model immunosensor based on a labeling method using gold nanoparticles (AuNPs) and electrochemical detection is developed. Microparamagnetic beads (MB) as primary antibody immobilization platforms and AuNPs modified with a secondary antibody as high sensible electrochemical labels have been used. The carbon electrode used as transducer incorporates a magnet that allows the collection/ immobilization on its surface of the immunological sandwich attached to the MB. 

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