Competitive immunoassay antibody-coated surface

FIGURE 12.3 Competitive immunoassay using (A) an antibody-coated surface, and (B) an... 

Competitive immunoassays may also be used to determine small chemical substances [10, 11]. An electrochemical immunosensor based on a competitive immunoassay for the small molecule estradiol has recently been reported [11]. A schematic diagram of this immunoassay is depicted in Fig. 5.3. In this system, anti-mouse IgG was physisorbed onto the surface of an SPCE. This was used to bind monoclonal mouse anti-estradiol antibody. The antibody coated SPCE was then exposed to a standard solution of estradiol (E2), followed by a solution of AP-labeled estradiol (AP-E2). The E2 and AP-E2 competed for a limited number of antigen binding sites of the immobilized anti-estradiol antibody. Quantitative analysis was based on differential pulse voltammetry of 1-naphthol, which is produced from the enzymatic hydrolysis of the enzyme substrate 1-naphthyl phosphate by AP-E2. The analytical range of this sensor was between 25 and 500pg ml. 1 of E2. 

The future use of coated piezoelectric devices as immunochemical sensors, even directly in the liquid phase, is very promising and could be considered a very competitive alternative to other types of immunoassay. Only this technique and that of surface plasmon resonance provide labelless methods for the direct study of antigen-antibody reactions, and their analytical possibilities. The devices can be easily automated or combined with flow injection systems, extending their capability of continuous and repeated assays. This raises an exciting possibility of using crystal arrays to assay for different analytes in complex samples with an on-line display of the results. 

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