Figure 3.

Schematic illustration for the general immunosensor formats. (A) Sandwich-type immunoassay with a labeled-secondary antibody. After the binding of the target antigen to the primary antibody-modified surface, the secondary antibody with a label is introduced to the form the sandwich-type complex. Various molecules such an enzyme or a nanoparticle can act as the source of electrochemical responses. (B) In a competitive-type immunosensor, the target molecule competes with its labeled form to attach on the primary antibodies on the surface. The electrochemical responses obtained from the labeled antigens are inversely proportional to the concentration of the target antigen. High sensitivity levels can be reached using competitive immunosensing strategies.