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 relev ant analytes with high sensitivity and investigating biological recogn ition reactions such as antibody-antigen reactions. On the basis of th e interaction between an anti-human IgG and a human IgG, a broad dynam ic concentration range extending over six orders of magnitude and a de tection limit of 1 X 10(-11) M dissolved antibody are demonstrated to be relevant for the determination of concentrations with this sensor s ystem. The effective surface concentration obtained at this dilution i s 19 pg mm-2, which corresponds to less than 4 parts per thousand of a closely packed monolayer of these molecules. Measurements of the clin ically relevant hepatitis B surface antigen show the same high sensiti vity: 2 x 10(-13) M antigen could be detected even in undiluted human serum. The most interesting feature of biospecific interaction analysi s 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 dete rmination of equilibrium as well as rate constants of such reactions d irectly from the binding data.