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tendency of nanotubes to agglomerate uncontrollably due to the high surface energy, and the stabilization of the bundles by van der Waals forces and – electron interactions among them. This phenomenon entails weak dispersibility and makes CNTs insoluble in most biocompatible solvents, highly limiting their biomedical applications. Functionalization solves many of these problems by modifying the CNT surface properties. Through functionalization, CNTs can be purified achieving high homogeneity, decreased toxicity, increased dispersibility, and solubility [34]. It also allows specific CNT decoration for di erent purposes and applications. This review presents the main functionalization pathways, with specific examples based on preparing nanotubes for preclinical research.

There are multiple strategies for functionalization of CNTs with applications in biomedicine [35]. Traditionally, they have been categorized according to the functionalization approaches into covalent and non-covalent, but a higher hierarchy factor can be introduced: the location of the functionalization. Endohedral functionalization accounts for changes in the inner face of the CNTs, whereas exohedral functionalization aims to add new functionalities in the outer face. Endohedral functionalization does not involve the formation of bonds between CNT and functional groups, but filling the inner part of the CNT with host material [36]. The first described endohedral functionalization was the filling with water through capillarity [37], but soon other solvents followed [38]. Hydrophobic molecules can also be successfully entrapped in the inner space of CNTs by simple incubation [39, 40]. Some other examples of endohedral functionalization by fullerene encapsulation were reported by Karousis et al. [41]. Nevertheless, this synthetic approach is limited due to the small diameter of the inner cavity of the CNTs, greatly restricting the size of the encapsulated elements.

Exohedral functionalization, on the other hand, aims to decorate the CNTs in their outer face. In principle, it has no size limitations and can potentially attach almost any type of functional group. The covalent strategies imply the formation of new chemical bonds altering the original CNT structure, and the non-covalent ones are based on the interaction through – stacking forces and/or van der Waals preserving the CNT skeleton intact. We present here the di erent types of functionalization categorized for clarity purposes, but it is important to notice that most of the synthetic procedures employed nowadays use a combination of several of the methodologies described here.

2.1 Covalent Functionalization

Covalent functionalization implies the formation of new chemical bonds, altering the original CNT structure. This reaction will change the carbon hybridization from sp2 to sp3, causing the loss of the -conjugation system of the graphene layer [42] responsible for most of the optical, electrical, and thermal properties of CNTs [43]. Despite this drawback, covalent functionalization presents many characteristics that make this approach highly attractive: it provides strong and very stable attachment of functional groups, entails higher selectivity, is more robust and better controlled

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