Carbon nanostructures, especially carbon nanotubes, are of high interest for bio-nanotechnology, and especially medical applications. They have been considered as carrier systems, transporting drugs or other medical devices, such as contrast agents for improved diagnosis, to spatially well-defined areas of the body. Low-weight, high-chemical, mechanical, and thermal stability properties combined with a large surface area are but a few properties that make many carbon nanostructures pertinent to the purposes of biomedicine. Before employing a certain class of carbon nanostructures for clinical treatment, however, issues of its biocompatibility have to be resolved. In more detail, carbon allotropes tend to be hydrophobic, implying low solubility in an aqueous environment. As non-polar materials, pristine carbon nanostructures display only weak interactions with strongly polar molecules such as H2O. This feature has consequences for the metabolic pathway of these materials, as it promotes their aggregation, which in turn may prevent them from being excreted by the body and thus cause them to accumulate in vital organs. Laboratory studies of CNTs point further at the risk of carcinogenesis, associated with ingestion of long, rigid nanotubes that cannot be eliminated by macrophage cells and may thus act like toxic fibers, in analogy to asbestos [631]. Another concern about administering carbon nanostructures in vivo is that they may harbor metal impurities that can be hostile to the body [632].

An expedient way to enhance the solubility of carbon nanostructures is modifying their surfaces by depositing hydrophilic ligand molecules on them. The toxicity of adequateley functionalized CNTs, for instance, has been found to be very low [633]. Surface modificationmay proceed through covalent or non-covalent bonding of ligand species [634]. In the former case, functional groups may bind to carbon atoms, or they may attach to oxygen groups that are already bound to the carbon surface. The covalent connections have a significant effect on the electronic structure of graphitic surfaces, as they change sp2 into sp3 configurations. Noncovalent interactions, on the other hand, leave the sp2 network intact. They involve electrostatic, dispersive or inductive forces.