Ferriprotoporphyrin IX chloride (haemin)-functionalized Al-doped zinc oxide (ZnO) is used for nitric oxide (NO) sensing at room temperature. The devices show high selectivity to NO against CO 2 and O 2 . There is no such selectivity without haemin. Porous ZnO film made by a polystyrene sphere template is used to provide a high surface area. In comparison with non-porous ZnO film with low surface area, both the sensitivity and the response speed of the sensor made by porous ZnO are clearly improved. NO concentration down to 1 ppm can be detected. The response time is 20 s. The sensor is reversible after purging with pure nitrogen for about 100 s. Such a device is able to detect real-time variation of NO which is a vital physiological signalling molecule.