THE BIDIFFRACTIVE GRATING COUPLER - APPLICATION TO IMMUNOSENSING

We report on the application of the bidiffractive grating coupler to t he sensitive detection of immunoreactions. The thyroid-stimulating hor mone (TSH) assay is used as a model system. The capture antibodies are randomly coupled to the functionalized TiO2 surface using two approac hes: direct coupling to the waveguide surface via a self-assembled mon olayer of an amino-reactive silane; and coupling via a thin hydrophili c polymer interlayer. All steps of the surface modification are charac terized by the bidiffractive grating coupler and the properties of the two surfaces are compared with respect to non-specific binding, amoun t of antibody immobilization and antigen binding capacity. A direct TS H assay (label free) shows a detection limit of 1 X 10(-9) mol l(-1), which corresponds to a surface coverage of 24 pg mm(-2). In a sandwich -type assay the sensitivity can be improved by two to three decades by the use of latex particles.