Redox label immunoassay, enzyme

Le Gal La Salle, A. Limoges, B. Rapicault, S. Degrand, C. Brossier, P. New immunoassay techniques using Nafion-modified electrodes and cationic redox labels or enzyme labels. Anal. Chim. Acta 1995, 311, 301-308. 

Immunoassays, electrochemical — A quantitative or qualitative assay based on the highly selective antibody-antigen binding and electrochemical detection. Poten-tiometric, capacitive, and voltammetric methods are used to detect the immunoreaction, either directly without a label or indirectly with a label compound. The majority of electrochemical immunoassays are based on -> voltammetry (-> amperometry) and detection of redox-active or enzyme labels of one of the immunochemical reaction partners. The assay formats are competitive and noncompetitive (see also -> ELISA). 

Figure 14.2. Some detection principies used in the double-surface DNA hybridization techniques. (A) Label-free detection of target DNA (tDNA). [B] Labeling of tDNA. Redox labels are covalently attached to the tDNA strand outside the segment or on a secondary DNA stiund recognized by the capture probe. After hybridization and separation, the electroactive tags are determined electrochemically [e.g., hy ex situ adsorptive stripping voltammetry [a]. Alternatively, electrochemical enzyme-linked immunoassay can be used for detection of labeled tDNA at the MB surface (b).

Scheme 2. Reaction pathways utilising a redox enzyme linked amperometric measurement via a redox mediator, (b) assay for glucose (a)+(b) assay for creatine, creatine kinase or hexokinase (c) competitive immunoassay using Fc-labelled antigen.

The attachment of the ferrocene groups to small molecules has been used in two main classes of biosensor in the first, the conjugate is an enzyme substrate such that a shift in its redox potential occurs following enzymatic modification. An example is ferrocenylethyl phosphate (Figure 9) that facilitates the electrochemical assay of alkaline phosphatase, a common enzyme label for immunoassays. The redox potential of the alcohol product is lower than the phosphate ester and hence by poising the potential, the product can be selectively measured and so the alkaline phosphatase activity measured. The sensitivity of this method was further enhanced by the use of stripping voltammetry to detect the product. ... 

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