A sensitive and selective, in-line method to measure and validate the sulfur content at ppb levels in both the liquid and gas phase of fuel. The method includes etching graphene, for example to form a mesa structure comprising horizontal or vertical lines or an array of multidentate star features; functionalizing the etched graphene and attaching metal oxide nanoparticles to the functionalized graphene to form a device; exposing the device to a fuel in the gas or liquid phase; detecting a change in conductivity when sulfur is present in the fuel; and recovering the device for future use. Also disclosed is the related in-line graphene-based ppb level sulfur detector for fuels.