A metal oxide sensor is provided on a semiconductor substrate to provide on-chip sensing of gases. The sensor may include a metal layer that may have pores formed by lithography to be of a certain width. The top metal layer may be oxidized resulting in a narrowing of the pores. Another metal layer may be formed over the oxidized layer and electrical contacts may be formed on the metal layer. The contacts may be coupled to a monitoring system that receives electrical signals indicative of gases sensed by the metal oxide sensor.