The present invention relates to a method and apparatus for detecting analytes in a medium, and more particularly the present invention relates to an assay based on light diffraction which appears or changes upon the binding of analytes to their specific receptors laid out in patterns on a substrate, which has high sensitivity due to the appropriate choice of such patterns. The present invention is based on the principle that the pattern of recognition elements, which gives rise to the diffraction of the incident light in a diffraction-based assay, can be chosen in such a way so as to facilitate detection, and to enhance the signal to be detected compared to known gratings such as parallel straight lines. In one aspect the substrate itself has a surface topography designed to enhance the diffraction pattern signals. In another aspect the substrate is a diffractive optic element having the analyte-specific receptors affixed to the optic element. In another aspect the diffractive optic element is used as a master stamp for producing patterns of analyte-specific receptors which give the signal enhancements.