(a) The response curve of ZnO QDs/graphene sensors and pure graphene sensors to the formaldehyde gas at different concentrations; (b) the response of ZnO QDs/graphene sensors to formaldehyde at different concentrations; (c) the response of ZnO QDs/graphene sensors to several gases at the concentration of 100 ppm; (d) the cycling response curve of ZnO QDs/graphene sensors to formaldehyde at the concentration of 100 ppm.