The difference interferometer: application as a direct affinity sensor

This paper describes the application of a novel sensor technology for determining the concentration of biologically and diagnostically relevant analytes with high sensitivity and investigating biological recognition reactions such as antibody-antigen reactions. On the basis of the interaction between an anti-human IgG and a human IgG, a broad dynamic concentration range extending over six orders of magnitude and a detection limit of 1 × 10 −11 m dissolved antibody are demonstreated to be relevant for the determination of concentrations with this sensor system. The effective surface concentration obtained at this dilution is 19 pg mm −2, which corresponds to less than 4 parts per thousand of a closely packed monolayer of these molecules. Measurements of the clinically relevant hepatitis B surface antigen show the same high sensitivity: 2 X 10 −13 m antigen could be detected even in undiluted human serum. The most interesting feature of biospecific interaction analysis with this sensor system is the fact that one can observe the course of a binding reaction between two molecules in real time. On the basis of the anti-human IgG/human IgG model system, we demonstrate the determination of equilibrium as well as rate constants of such reactions directly from the binding data.