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Functionalized Multiwalled Carbon Nanotubes

Functionalized Multiwalled Carbon Nanotubes (MWCNTs) are made of many graphene layers and the diameter can be up to 100 nm. Two main models describe MWCNTs: (i) the Russian doll model or coaxial cylindrically curved model in which many graphene layers are rolled up and arranged in a cylindrical fashion, and (ii) the parchment model in which a single graphene layer is scrolled as a parchment (Figure 5a and c respectively). A further evolution of the coaxial cylindrically curved model is the coaxial cylindrically poligonized.

 Functionalized Multiwalled Carbon Nanotubes

The electronic structure of a MWCNT is considered as the sum of the electronic structures of the constituent nanotubes. Thereby, we state that in the limit of the large nanotube diameter, all the nanotubes properties correspond to those of graphite. The interlayer distance in MWCNTs is equal to the interlayer distance in graphite (3.35 Å). These interactions are not sufficient to correlate the chirality of near tubes and MWCNTs are a mix of achiral shells. The result is that the lattice structures of the inner and outer layers are generally incommensurate.

 
Functionalized Multiwalled Carbon Nanotubes

Solubilization

Pristine nanotubes are insoluble in aqueous media and this has been a major technical roadblock for their biological and biomedical applications. However, recent advances in chemical modification and functionalization methods have lead to Solubilization and dispersion of CNTs in water, thereby facilitating their handling and processing in physiological environments. Generally, dispersion or Solubilization of carbon nanotubes can be achieved by three methods: (i) dispersion upon oxidative acid treatments; (ii) non covalent functionalization iii) covalent functionalization.

Functionalization

Functionalized carbon nanotubes (fCNT) can play an important role in many applications, especially in biomedical -applications of CNTs. Recently; they have been functionalized with several bioactive moieties like proteins, carbohydrates, nucleic acids, anticancer drugs, antibodies and enzymes. Functionalizing the nanotubes makes them soluble in water and various solvents facilitating dispersion, manipulation, sorting and separation. Functionalization with organic, inorganic or bioactive molecules also helps in the interfacing of nanotubes with other materials to form bioconjugates.

Nanotubes can be functionalized by both covalent and non-covalent methods.

Non-covalent functionalization is based on van der Waals, hydrophobic or ?-? interactions, and it is particularly interesting because it does not perturb the electronic structure of CNTs and of SWCNTs in particular. This functionalization involves weak forces, and so some applications are prevented. The systems, in fact, are not only difficult to control but also difficult to characterize. The amount of weakly bound molecules is not always calculable; moreover, especially in solution, other interactions can occur and these molecules can be replaced by solvent. Anyway, many examples of non covalent interaction are reported: surfactants are widely used to exfoliate bundles of SWCNTs, as well as ionic liquids in order to facilitate CNTs manipulation and further reactions. CNTs wrapping by polymers, including DNA, have been studied; also proteins are able to non covalently interact with CNTs and these are often used for bio-sensor applications.

Covalent functionalization, of CNTs can be achieved either by oxidation to develop carboxylic groups on their surface or by the attachment of hydrophilic moieties using addition reactions. The covalent bond ensures the stability of the moieties during manipulation as well as allows the further derivatization of the different functional groups on the nanotubes.

CNTs are functionalized by not reversible attachment of appendage on the sidewalls and on the tips. Also in this case, many different approaches are reported. Briefly, reactions can be performed at the sidewall site (sidewall functionalization) or at the defect sites (defect functionalization), usually localized on the tips.

 
Functionalized Multiwalled Carbon Nanotubes

Functionalized -MWCNTs are not modified in their electronic structure and new properties can be added by means of functionalization. Instead, electronic properties of SWCNTs are perturbed by covalent functionalization and double bonds are irreversibly lost. This may affect conductive property, preventing further CNT applications.

 

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