Capacitance measurements of antibody-antigen interactions in a flow system

Capacitive immunosensors were made by coupling monoclonal antibodies to 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 hexacyanoferrate redox reactions were blocked by this procedure. The capacitance of the electrode was evaluated from the current transients obtained when a potentiostatic step was applied. The immunosensor was placed in a flow system, and a capacitance decrease could be observed after injection of an unlabeled antigen. It was linear over almost three decades when plotted vs the logarithm of the antigen concentration. Human 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 similar 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 interleukin-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 with monoclonal antibodies toward human albumin gave a calibration curve with lower slope than the other proteins but still with a detection limit of 1 pg/mL.