Fig. 4.

Schematic depiction of the direct immunoassay. The conjugate of alkaline phosphatase and a monoclonal antibody was prepared and coupled to the activated SAM on the NPG surface. These modified NPG coated electrodes were incubated with varying concentrations of antigen and then the enzymatic activity of alkaline phosphatase with and without antigen were compared. The presence of antigen on the antibody creates a steric hindrance to the substrate access to the active site of the enzyme, and hence the enzymatic activity is reduced with an increase in the concentration of the antigen bound to antibody on the surface.