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theranostic purposes in preclinical research. Non-covalent functionalization can be categorized according to the derivatization process like: small molecule adsorption or – stacking attachments, use of surfactants, and interaction with biomolecules.

2.2.1  Small Molecule Adsorption or π–π Stacking

Aromatic derivatives, such as pyrene, porphyrins, and aromatic macrocycles, can functionalize the sidewalls of CNT through – stacking interactions [88] or donor–acceptor systems [90].

2.2.1.1  Pyrene Derivatives  Pyrene is the smallest peri-fused planar hydrocarbon. It is highly symmetrical, having 16 electron and aromatic character despite not following Hückel’s 4n + 2 rule [91]. Pyrene-containing molecules have been extensively used as bridges to functionalize nanotubes’ surface with di erent biomolecules, polymers, dendrimers, and DNA derivatives, among others [92]. Zhu and coworkers anchored CNTs onto oxide surfaces using bifunctional molecules, with succinimidyl ester and pyrene groups as a bridge [93]. Chen et al. employed 1-pyrenebutanoic acid, succinimidyl ester to functionalize carbon nanotubes with proteins [90]. Calle et al. used 1-amine pyrene to obtain homogenous – stacking adducts with MWCNT [88]. Once the pyrenyl group is anchored to CNTs, they can be further decorated with ester groups that are highly reactive to primary and secondary amines ubiquitous on the surface of proteins.

2.2.1.2  Porphyrin Derivatives  Porphyrin is a -conjugated system in a planar arrangement with a total of 18 electrons and aromatic character. Using this structure, Zhang and coworkers [94] prepared polymeric porphyrin-functionalized CNTs by the condensation of terephthaldehyde and pyrrole in the presence of carbon nanotubes. Subsequent metalation with ­Ru3(CO)12 incorporated ruthenium to the composite, which showed excellent catalytic performance toward hydrogenation of biomassrelated compounds. Derivatized porphyrins can also be employed [95].

2.2.2 Surfactants

Amphiphilic molecules interact with carbon nanotubes in aqueous media through their hydrophobic parts while their hydrophilic ends face outwards [96]. Surfactants have been widely used to suspend CNTs in aqueous solutions, increasing their dispersibility [97] and reducing their cytotoxicity [98]. Examples of this type of molecule include deoxycholic acid, sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, and sodium dodecyl sulfate, among others [96, 99, 100]. This procedure is very e ective in dispersing carbon nanotubes, allowing studying CNTs individual properties, and can be used as the starting point for further functionalization. Furthermore, if the hydrophobic tails of the surfactant present aromatic moieties, particularly strong – stacking interaction will be established with the CNT, as with sodium dodecyl benzene sulfonate (SDBS) [101].

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