Ultrasensitive carbon nanotube-based biosensors using antibody-binding fragments.

We report a method to build ultrasensitive carbon nanotube-based biosensors using immune binding reaction. Here carbon nanotube-field effect transistors (CNT-FETs) were functionalized with antibody-binding fragments as a receptor, and the binding event of target immunoglobulin G (IgG) onto the fragments was detected by monitoring the gating effect caused by the charges of the target IgG. Because the biosensors were used in buffer solution, it was crucial to use small-size receptors so that the charged target IgG could approach the CNT surface within the Debye length distance to give a large gating effect. The results show that CNT-FET biosensors using whole antibody had very low sensitivity (detection limit approximately 1000 ng/ml), whereas those based on small Fab fragments could detect 1 pg/ml (approximately 7 fM level). Moreover, our Fab-modified CNT-FET could successfully block the nontarget proteins and could selectively detect the target protein in an environment similar to that of human serum electrolyte. Significantly, this strategy can be applied to general antibody-based detection schemes, and it should enable the production of label-free ultrasensitive electronic biosensors to detect clinically important biomarkers for disease diagnosis.