Nanotubes and nanotube-based devices are implemented in a variety of applications. According to an example embodiment of the present invention, a nanotube is doped with an impurity atom and used to detect the presence of a particular molecular species as a function of the particular molecular species bonding to the impurity atom. In one implementation, the doped nanotube responds electrically to the bonding of the particular molecular species to the impurity atom. With this approach, nanotubes such as single-walled carbon nanotubes can be doped to respond selectively to one or more types of molecular species.