Tailoring carbon nanomaterials for specificap-plicationsis of great importance in the quest to improve the properties of these materials and hence increase their functionality. Using a simple and easy to apply technique Zinc,Zinc oxide and Iodine were encapsulated and con-finedwithin double walled carbon nanotubes whose internal diameter ranges from 1.2to2.5 nm.The simultaneous confinement of Zinc and Iodine was shown to improve the sensitiv-ity by 100 times whileat the same time en-hancing the selectivity of DWNTs towards formaldehyde. By exploiting the p-doping effect of Iodine, CNTs networks were engineered to differentiate formaldehyde from ethanol and acetone, some of the common volatile organic compounds.The ability to tune the chemical selectivity and sensitivity of carbon nanotubes based sensors through inner encapsulation of a specific material thus appears as a new possible route compared to more conventional outer surface functionalization