A sterilisation device (200, 300) for generating a flow of hydroxyl radicals, the sterilisation device (200, 300) comprising: a housing defining a water mist fluid flow path between an inlet for receiving water mist and an outlet for directing a flow of hydroxyl radicals towards a region to be sterilised; an energy delivery tip (205, 305) configured to produce a thermal or non-thermal plasma; a coaxial transmission line mounted in the housing and arranged to convey radiofrequency and/or microwave frequency electromagnetic energy to the energy delivery tip (205, 305); and a gas conduit mounted in the housing and arranged to deliver gas to the energy delivery tip (205, 305), wherein the coaxial transmission line comprises an inner conductor, an outer conductor, and a dielectric material separating the inner conductor from the outer conductor, wherein the energy delivery tip (205, 305) extends from a distal end of the coaxial transmission line, and wherein the energy delivery tip (205, 305) comprises: a first electrode (214, 302) electrically connected to the inner conductor of the coaxial transmission line; and a second electrode (216, 304) electrically connected to the outer conductor of the coaxial transmission line, wherein the second electrode (216, 304) is configured to define an internal volume of the energy delivery tip, wherein the first electrode (214, 305) extends longitudinally within the internal volume, wherein the first electrode (214, 302) and second electrode (216, 304) are configured to: strike a plasma in the gas delivered to the energy delivery tip (205, 305), and direct the plasma into the water mist fluid flow path to form hydroxyl radicals therein. a housing defining a water mist fluid flow path between an inlet for receiving water mist and an outlet for directing a flow of hydroxyl radicals towards a region to be sterilised; an energy delivery tip (205, 305) configured to produce a thermal or non-thermal plasma; a coaxial transmission line mounted in the housing and arranged to convey radiofrequency and/or microwave frequency electromagnetic energy to the energy delivery tip (205, 305); and a gas conduit mounted in the housing and arranged to deliver gas to the energy delivery tip (205, 305), wherein the coaxial transmission line comprises an inner conductor, an outer conductor, and a dielectric material separating the inner conductor from the outer conductor, wherein the energy delivery tip (205, 305) extends from a distal end of the coaxial transmission line, and wherein the energy delivery tip (205, 305) comprises: a first electrode (214, 302) electrically connected to the inner conductor of the coaxial transmission line; and a second electrode (216, 304) electrically connected to the outer conductor of the coaxial transmission line, wherein the second electrode (216, 304) is configured to define an internal volume of the energy delivery tip, wherein the first electrode (214, 305) extends longitudinally within the internal volume, a first electrode (214, 302) electrically connected to the inner conductor of the coaxial transmission line; and a second electrode (216, 304) electrically connected to the outer conductor of the coaxial transmission line, wherein the second electrode (216, 304) is configured to define an internal volume of the energy delivery tip, wherein the first electrode (214, 305) extends longitudinally within the internal volume, wherein the first electrode (214, 302) and second electrode (216, 304) are configured to: strike a plasma in the gas delivered to the energy delivery tip (205, 305), and direct the plasma into the water mist fluid flow path to form hydroxyl radicals therein. strike a plasma in the gas delivered to the energy delivery tip (205, 305), and direct the plasma into the water mist fluid flow path to form hydroxyl radicals therein.