This invention relates to an optical sensor element comprising a photonic crystal constituted by a membrane of a chosen transparent material, the membrane being provided with a number of defined openings in a chosen pattern, the pattern being adapted to provide resonance at a chosen wavelength or range of wavelengths, wherein said openings are provided with a reactive material acting as a receptor for a chosen type of molecules, e.g. proteins, the presence of which alters the resonance and/or scattering conditions in the sensor element thus altering the amount of light propagating out of the membrane plane.