  Graphene, being one-atom thick, is extremely sensitive to the presence of
adsorbed atoms and molecules and, more generally, to defects such as vacancies,
holes and/or substitutional dopants. This property, apart from being directly
usable in molecular sensor devices, can also be employed to tune graphene
electronic properties. Here we briefly review the basic features of
atomic-scale defects that can be useful for material design. After a brief
introduction on isolated $p_z$ defects, we analyse the electronic structure of
multiple defective graphene substrates, and show how to predict the presence of
microscopically ordered magnetic structures. Subsequently, we analyse the more
complicated situation where the electronic structure, as modified by the
presence of some defects, affects chemical reactivity of the substrate towards
adsorption (chemisorption) of atomic/molecular species, leading to preferential
sticking on specific lattice positions. Then, we consider the reverse problem,
that is how to use defects to engineer graphene electronic properties. In this
context, we show that arranging defects to form honeycomb-shaped superlattices
(what we may call "supergraphenes") a sizeable gap opens in the band structure
and new Dirac cones are created right close to the gapped region. Similarly, we
show that substitutional dopants such as group IIIA/VA elements may have gapped
quasi-conical structures corresponding to massive Dirac carriers. All these
possible structures might find important technological applications in the
development of graphene-based logic transistors.
