Direct immunosensing with an integrated-optical output grating coupler

We have used an output grating coupler for direct immunosensing. The antigen, a rabbit-IgG, is selectively detected in subnanomolar concentrations in a few minutes by its binding to the corresponding antibodies (goat-anti-rabbit-IgG) immobilized on the waveguide surface. The following immobilization technique of the antibody, which requires no surface chemistry, is used: the waveguide is first coated (by adsorption) with avidin which adheres well on it; in a second step, the biotinylated antibodies are bound irreversibly to the avidin layer by the strong avidin-biotin affinity binding. In order to improve the accuracy of the determined surface mass densities and thus to lower the detection limit, we introduce a new drift-correction method which is based on the simultaneous measurement of both effective refractive indices N-TE0 and 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 contact with an aqueous buffer solution with time.