Development of a capacitive immunosensor: a comparison of monoclonal and polyclonal capture antibodies as the primary...

There is widespread interest in capacitance immunosensor systems which directly detect antigen binding to immobilized antibody. Our system comprises an active biolayer of antibodies bound to a silicon--silicon dioxide--silicon nitride (Si-SiO2-Si3N4) surface. As with other groups, our system initially gave poorly reproducible responses on addition of antigen. We mechanically degraded the Si-SiO2-Si3N4 surface, and the responses on addition of transferrin were monitored. The mechanical degradation allowed the affinity reaction to be 'seen' capacitively. Once the system was established, a comparison of capture antibodies was performed to establish the most effective biolayer. Three affinity reactions were examined: (a) 1D2A4, monoclonal antibody (mAb) to human transferrin, as the capture layer; (b) polyclonal goat anti-human transferrin antibody (PcAb) as the capture layer; and (c) 1D2A4 with transferrin (Tf) prebound as the capture layer. There was no response to addition of transferrin where 1D2A4 was the capture layer. Addition of transferrin when the polyclonal antibody was used as the primary layer resulted in a drop in measured capacitance. Addition of goat anti-human transferrin antibody to a device with 1D2A4 plus transferrin as the capture layer also resulted in a measured capacitance decrease. There is a difference in dielectric/blocking effectiveness between the monoclonal and polyclonal antibodies.