Abstract: A method is disclosed for coating the surface of an optical structure, such as a diffraction grating, useful for the detection of a ligand, which method comprises forming on the surface of the optical structure a thin, uniform layer of a polymerizable material, particularly one which polymerizes on exposure to light or heat, and subsequently exposing said material to polymerizing conditions. A specific binding partner is subsequently absorbed on or bound to the cured polymer layer, either directly or indirectly. Complex formation between the specific binding partner and ligand present in the sample to be analyzed alters the optical properties of the device and the change forms the basis of an assay.