Nanostructures and molecular force bases of a highly sensitive capacitive immunosensor.
While biosensors have been constructed using various strategies, there is no report describing nanostructures of antibody-immobilized electrode interface in an immunosensor. Here, atomic force microscopy (AFM) and electrochemistry analyses were employed to construct and characterize the nanostructures and electrochemistry of biosensing surface that was created by a sequential self-assembling of bioactive aminobenzenthiol oligomer (o-ABT), glutareldehyde and anti-transferrin (anti-Tf) antibody on the electrode gold surface. Under AFM, a complete coverage of bioactive o-ABT interface could be achieved by anti-Tf antibody at an optimal concentration. The anti-Tf antibody immobilized on electrode surface of the immunosensor exhibited globular-shape topography with some degree of aggregation. Extensive force-curve analysis allowed mapping the functional spots of the anti-Tf immunosensor. Surprisingly, although immunosensing surface was fully covered by anti-Tf antibodies at the optimal concentration, only about 52% of coated anti-Tf antibody molecules (spots) on the electrode surface were able to specifically capture or bind Tf antigen under AFM. Despite limited functional spots, however, the anti-Tf immunosensor was highly specific and sensitive for sensitizing Tf antigen in solution. The anti-Tf molecules on the immunosensor exhibited a greater molecular force bound to holo-Tf (iron-containing form of Tf) than that to apo-Tf (iron-absent form of Tf). Consistently, the anti-Tf immunosensor had a greater electrochemical capacity to sensitize apo-Tf than holo-Tf, supporting the molecular force-based finding by AFM. Thus, the present study elucidated the nanostructures and molecular force bases for the immunosensing capacity of a highly sensitive capacitive immunosensor.