CAPACITANCE MEASUREMENTS OF ANTIBODY-ANTIGEN INTERACTIONS IN A FLOW SYSTEM

Capacitive immunosensors were made by coupling monoclonal antibodies t o thioctic acid, which had self-assembled on a gold electrode. Surface areas that were not covered were plugged with 1-dodecanethiol to make the layer dense and insulating. Cyclic voltammetry showed that the he xacyanoferrate redox reactions were blocked by this procedure. The cap acitance of the electrode was evaluated from the current transients ob tained when a potentiostatic step was applied. The immunosensor was pl aced in a now system, and a capacitance decrease could be observed aft er injection of an unlabeled antigen. It was linear over almost three decades when plotted vs the logarithm of the antigen concentration. Hu man chorionic gonadotropin hormone could be determined in the range 1 pg/mL-1 ng/mL, with a detection limit of 0.5 pg/mL (15 10(-15) M). A s imilar response was obtained with immobilized F(ab')(2) fragments. No cross-reactivity was observed with the thyrotropic hormone, which has one chain in common with gonadotropin. Monoclonal antibodies toward in terleukin-2 immobilized on the immunosensor gave also a response over 1 pg/mL-1 ng/mL, with a detection limit of 1 pg/mL. An immunosensor wi th monoclonal antibodies toward human albumin gave a calibration curve with lower slope than the other proteins but still with a detection l imit of 1 pg/mL.