Nano-scale carbon onions produced by laser photolysis of toluene for detection of optical, humidity, acetone, methanol and ethanol stimuli

•Laser liquid medium pyrolysis synthesis of onion likes carbon (OLC) and growth mechanism.•The semiconductor switching characteristic (from p- to n-type) of OLC hierarchical structure with relative humidity.•Highly selective to the alcohol than acetone and insensitive to the dry gases like CH4, CO, etc.•The OLCs reveals a photoelectric effect as well.