Development of a generic IC compatible silicon transducer for immunoreactions

There is widespread interest in developing a detection system for direct detection of antibody antigen reactions which can be used to replace existing labour intensive, time consuming and expensive techniques. This paper outlines a silicon transducer that can be used to measure a number of different of immunoreactions. Four device types were examined and characterised; standard silicon-silicon dioxide-silicon nitride devices (Si-SiO2-Si3N4), silicon-silicon dioxide devices (Si-SiO2), Si-SiO2-Si3N4 devices which-have had an area of nitride and oxide laser ablated and Si-SiO2-Si3N4 devices that had an area of nitride and oxide mechanically degraded. Polyclonal goat anti-human transferrin antibody was immobilised, by passive adsorption, on the surface of the devices. On addition of analyte (transferrin), only the mechanically degraded devices detected an immunoreaction, as indicated by a decrease in measured capacitance. A standard curve of percentage capacitance drop vs. transferrin concentration was obtained. The linear portion of this plot was from 25 to 200 mu g ml(-l). Other antigen antibody pairs were tested and all showed a characteristic decrease in capacitance on addition of antigen. The immunoreactions tested were polyclonal rabbit anti-goat antibody binding to goat anti-mouse immunoglobulins (Igs), polyclonal rabbit anti-goat antibody binding to goat anti-human transferrin antibody and protein A binding to goat anti-human transferrin, goat anti-human Igs and rabbit anti-goat IgG. Scanning electron microscopy investigation showed a rough surface for the mechanically degraded devices. The sharp peaks may have an effect on the electric field distribution, making them more sensitive to detecting immunoreactions;. The mechanically degraded devices were characterised electrochemically. Cyclic voltammograms show current decreasing as time increases, indicating the growth of an insulating oxide layer on the silicon surface. Using 0.1 M ferrous cyanide [Fe(CN)(6)](4-/3-) as a probe for electroactivity of a surface, a peak occurred at the same voltage for the gold as for the mechanically degraded silicon devices, 265 mV. The mechanically degraded devices appear to be acting as an electroactive surface. Oxidation reactions which occur in electrolyte are modulated by the binding of antigen to immobilised antibody. This binding blocks the path of ions to the oxidation reaction and the effective electric double layer increases in thickness causing a decrease in capacitance. (C) 1999 Elsevier Science S.A. All rights reserved.