This work presents a study of the adsorption properties of defective nanostructures. The calculations have quantum mechanical detail and are based on a semi-empirical Hamiltonian, which is applied to the evaluation of both the electronic structure and of the conductance. The material considered in this study, ie SnO2, has a widespread use as gas sensor and oxygen vacancies are known to act as active catalytic sites for the adsorption of small molecules. In the following calculations crystalline SnO2 nanograins, with a size and ...