This work focuses on the effects of custom-designed, two-dimensional grating structures on the sensitivity of optical waveguides biosensors in the input grating coupler configuration. Calculations suggest that suitably designed diffractive structures with optimum pitch in two orthogonal directions can increase the sensitivity of devices when compared to a conventional one-dimensional grating under the same conditions. A set of six diffractive structures designed for 1550 nm wavelength were fabricated by thermal nano-imprint lithography on silicon oxynitride waveguides; the silicon master stamp was patterned by deep UV stepper lithography. Preliminary experimental results indicate a sensitivity enhancement of a factor two due to the 2D diffractive couplers.