An apparatus for sensing at least one property of a fluid is described. A first photonic crystal structure and a second photonic crystal structure are defined in a dielectric slab. The first and second photonic crystal structures comprise differently patterned arrays of channels extending through the dielectric slab. The apparatus further comprises a fluid introduction device configured to introduce a common volume of the fluid into the channels of the first and second photonic crystal structures. The at least one property of the fluid can be sensed by measuring the propagation of radiation through the first and second photonic crystal structures.