Hollow nanospheres of cobalt oxide have been investigated as materials for gas sensor electrodes, for the detection of toluene, acetone, and other organic vapors.  
Cobalt oxide nanoparticles anchored on single-walled carbon nanotubes have been investigated for sensing nitrogen oxides  and hydrogen. This application takes advantage of the reactivity between the gas and the oxide, as well as the electrical connection with the substrate (both being p-type semiconductors).  Nitrogen oxides react with the oxide as electron acceptors, reducing the electrode's resistance; whereas hydrogen acts as an electron donor, increasing the resistance.