DIRECT IMMUNOSENSING WITH AN INTEGRATED-OPTICAL OUTPUT GRATING COUPLER

We have used an output grating coupler for direct immunosensing. The a ntigen, a rabbit-IgG, is selectively detected in subnanomolar concentr ations in a few minutes by its binding to the corresponding antibodies (goat-anti-rabbit-IgG) immobilized on the waveguide surface. The foll owing immobilization technique of the antibody, which requires no surf ace chemistry, is used: the waveguide is first coated (by adsorption) with avidin which adheres well on it; in a second step, the biotinylat ed antibodies are bound irreversibly to the avidin layer by the strong avidin-biotin affinity binding. In order to improve the accuracy of t he determined surface mass densities and thus to lower the detection l imit, we introduce a new drift-correction method which is based on the simultaneous measurement of both effective refractive indices N-TE0 a nd N-TM0 of the guided TE0 and TM0 modes. The drift effect is a small and slow but persistent increase of the effective refractive indices N -TE0 and N-TM0 of the somewhat microporous (SiO2-TiO2) waveguides in c ontact with an aqueous buffer solution with time.