A single-wall nanotube (SWNT) is a rolled-up sheet of graphene that can be metallic or semiconducting depending on its chiral vector (N, M), where N and M are two integers. The rule is that a metallic or a semiconducting nanotube is obtained when the difference N-M is not a multiple of 3, respectively. The resulting tubes have a perfectly straight shape, but in reality demonstrate to us that according to the synthesis methods used, SWNTs also consistently display structural defects. Theory states that the most common defects consist of pentagon and heptagon inclusions in the perfect hexagonal lattice. This kind of defects mostly affect the local electronic density of states (LDOS), near the Fermi level of the system in a relatively large neighbourhood. Such non-hexagonal inclusions observed by scanning tunneling microscopy (STM) images will be presented. Considering the perturbation created in the electronic structure of those quasi one-dimensional heterojunctions, we shall propose several possible applications based on these interesting properties that may be foreseen.