Functionalization of carbon nanotube devices is essential to tailor their affinity with other molecules, which is especially critical for applications as sensors. Among functionalization approaches, covalent chemistry produces stable moieties on nanotubes sidewalls, but the chemistry is invasive and alters the electronic properties of the material. In this presentation, I will describe recent efforts to control covalent diazonium chemistry on carbon nanotubes and understand its impact on electrical conductance. In particular, I will contrast experiments between extended and localized defect coverage of adducts on the electrical response of devices, and present applications in single-molecule sensors.