Scheme 1.

Illustration of detection principles of SWCNT immunosensors. On the bottom left is a tapping mode atomic force microscope image of a SWCNT forest that serves as the immunosensor platform. On the right is a cartoon of a SWCNT immunosensor platform used for capture antibody (Ab₁) bioconjugation and subsequent antibody–antigen sandwich immunoassay (HRP is the enzyme label). Picture (A) on the right shows the immunosensor after treating with Ab₂–Polybead–HRP to obtain amplification by providing numerous enzyme labels per binding event. Picture (B) on the right shows the immunosensor after treating with a conventional HRP–Ab₂ (biotinylated Ab₂ followed by streptavidin modified HRP) providing 14–16 labels per binding event. The final detection step involves immersing the immunosensor after secondary antibody attachment into a buffer containing mediator in an electrochemical cell, applying voltage, and injecting a small amount of hydrogen peroxide.