Microgravimetric immunoassay with piezoelectric crystals

Abstract

A new immunoassay technique has been developed using piezoelectric quartz plates, which undergo physical deformation in response to electrical charge. An oscillating electrical circuit causes the crystal to alternately deform and recover at a frequency dependent on properties of the crystal. Detection is based on the change in frequency of the crystal in response to mass changes on its surface due to adsorption of analyte. In order to prepare the quartz surfaces for detection of protein, they were modified using organosilane coupling agents which are known to bridge organic/inorganic interfaces. Aminosilanes and epoxy-functional alkoxysilanes were bonded to the surfaces, followed by oxidation of the epoxide to an aldehyde group. The amine function of the protein to be bound to the surface was then reacted with the aldehyde and the resulting Schiff base was reduced to a stable amine linkage. Surface modification was characterized using ESCA and FTIR. Analysis was performed on immunoglobulin and viral samples by immobilizing their complements on the surface of a quartz device and exposing it to solution. Sensitivity was defined using radiolabeled antibody. Detection limit was initially found to be 1.2 mg/ml. By improving surface modification procedures and using immobilized monoclonal antibody, detection limit was lowered to 0.018 (mu)g/ml.