For the ZnO QDs/graphene composite sensor, its unique sensing performance is attributed to the decorated ZnO QDs, which play a role as catalytically active centers. The chemisorbed oxygen ion (O2-) from the atmosphere at the surface of ZnO QDs reacts with the detecting molecule (formaldehyde) and forms a metastable intermediate product (formate species, –HCOO-), releasing electrons and changing the electrical conductivity. When the detecting gas is evacuated, the oxidation process will reverse with fast recovery for the metastability of the intermediate product (formate species, –HCOO-).