We investigated the functionalization of photonic crystals slabs with aptamers for the label-free detection of biomolecules. Photonic crystal slabs are slab waveguides with a periodic nanostructure that supports quasi guided modes coupling to far field radiation. Measurements were performed with light at normal incidence to the photonic crystal slab. The influence of the nanostructure duty cycle as well as the high index layer thickness is investigated for the surface and the bulk sensitivity. Nanoimprint lithography with subsequent deposition of a high index layer and biofunctionlization is presented as one approach for fabricating photonic crystal slab biosensors. In comparison photonic crystal slabs fabricated by injection-molding are considered. Oblique angle layer deposition is introduced as a means to achieve structured high index layers. Results obtained with wavelength dependent measurements are compared to simple intensity measurements in a particular wavelength range. Both methods are applicable for label-free detection. In a spectrometer-free setup we measured the binding kinetics of a 250 nM solution of the protein thrombin to the photonic crystal surface functionalized with aptamers.