Toxin detection using capacitance measurements on immunospecies grafted onto a semiconductor substrate

Abstract

The feasibility of direct immunosensors based on capacitance measurements on silicon-silica heterostructures is shown for an immunosystem based on G type immunoglobulin. The first results obtained for the detection of enterotoxin B from Staphylococcus Aureus are presented. Capacitance measurements on immunosystems succeed only if the successive biomolecular layers grafted onto the heterostructures are electrically insulating, so they are strongly dependent on the coupling procedure used for antibody fixation on the silanized silica layer. Two coupling procedures were investigated. One is based on a cyanosilane [γ-cyanodecyldimethyl(dimethylamino)silane]; in this instance antibody fixation proceeds through a non-covalent strong interaction between the cyano groups and residual hydroxyl groups of the glycoproteins. However, in spite of the high fixation rate of antibodies, checked by ELISA tests, no capacitive response was obtained. The second procedure involved covalent grafting of the antibody onto an amino-silanized heterostructure (δ-aminobutyldimethylmethoxysilane) through a special glutaraldehyde coupling technique, allowing good electrical characteristics of the biomolecular layers. A typical capacitance decrease of 400 pF cm⁻² was obtained for a toxin concentration of 10μg ml⁻¹.