Photonic clystals are an exciting component in the field of nanophotonics. They allow the control, confinement and manipulation of light at the nanometre scale. The ability to fabricate photonic clystals with semiconductor fabrication technology makes them a suitable building block of photonic integrated circuits. Photonic clystals offer sensitivity to surrounding materials and they can enhance light-matter interaction. This has motivated considerable research into their application in the area of chemical and biological sensing. Photonic clystals provide a versatile platform for lab-on-a-chip applications and the prospect of high integration density could benefit cost-sensitive applications such point-of-care diagnostics or environmental sensing. This thesis investigates the feasibility of creating photonic crystal sensors on gallium nitride. The maturity of GaN-based photonic devices, such as LEDs, makes it an ideal platform for lab-on-a-chip applications. Two types of GaN photonic clystals sensors are designed, fabricated and characterised in this work. The first type is a ID grating, which supports guided mode resonances. These are fabricated by electronbeam lithography and dlY etching on GaN membranes and on GaN-on-sapphire. The ability of membrane gratings to sense the refractive index of a liquid that is present at one side of the membrane is verified experimentally. GaN-on-sapphire gratings are presented as a method of enhancing fluorescence emission from molecules placed on the gratings through the guided mode resonances. The second structure analysed is a modified 2D photonic clystal L3 cavity. This novel structure possesses a central hole, which allows the positioning of fluorescent molecules in a region of high electric field density. It is shown by finite difference time domain calculations, that the resonant modes of the cavity significantly enhance the absorption and emission of the molecules. The fabrication and characterisation of those cavities, along with coupling to ridge waveguides, are shown as a first step towards an integrated sensor.