Development of a generic IC compatible silicon transducer for immunoreactions

There is widespread interest in developing a detection system for direct de tection of antibody antigen reactions which can be used to replace existing labour intensive, time consuming and expensive techniques. This paper outl ines a silicon transducer that can be used to measure a number of different of immunoreactions. Four device types were examined and characterised; sta ndard silicon-silicon dioxide-silicon nitride devices (Si-SiO2-Si3N4), sili con-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 ant i-human transferrin antibody was immobilised, by passive adsorption, on the surface of the devices. On addition of analyte (transferrin), only the mec hanically degraded devices detected an immunoreaction, as indicated by a de crease 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 tes ted and all showed a characteristic decrease in capacitance on addition of antigen. The immunoreactions tested were polyclonal rabbit anti-goat antibo dy 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 an ti-goat IgG. Scanning electron microscopy investigation showed a rough surf ace for the mechanically degraded devices. The sharp peaks may have an effe ct on the electric field distribution, making them more sensitive to detect ing immunoreactions;. The mechanically degraded devices were characterised electrochemically. Cyclic voltammograms show current decreasing as time inc reases, indicating the growth of an insulating oxide layer on the silicon s urface. Using 0.1 M ferrous cyanide [Fe(CN)(6)](4-/3-) as a probe for elect roactivity of a surface, a peak occurred at the same voltage for the gold a s for the mechanically degraded silicon devices, 265 mV. The mechanically d egraded devices appear to be acting as an electroactive surface. Oxidation reactions which occur in electrolyte are modulated by the binding of antige n to immobilised antibody. This binding blocks the path of ions to the oxid ation reaction and the effective electric double layer increases in thickne ss causing a decrease in capacitance. (C) 1999 Elsevier Science S.A. All ri ghts reserved.