Diamond sensor

A sensor assembly comprising single crystal diamond material, the single crystal diamond material comprising a surface which is configured in use to contact a fluid, the surface comprising a sensing surface region, wherein the single crystal diamond material comprises a plurality of quantum spin defects in proximity to the sensing surface region. The sensor assembly further comprises a first electrode and a second electrode. The electrodes are located such that, in use, they generate an electric field proximate to a surface of the sensing surface region, the electric field being sufficient to generate oxidising species in the fluid for removing contaminant material from the sensing surface region. The containment material may comprise organic material. The first electrode may be located at a first electrode surface region of the single crystal diamond material and may comprise any of electrically conducting boron doped diamond; graphite; and a metal material deposited on the surface of the sensor. Also claimed are a single crystal diamond with a surface configured to contact a fluid, and a method of using a sensor assembly comprising a single crystal diamond.