Carbon nanotubes functionalization reactions

This is mainly due to their laborious purification procedures and their required chemical modification for solubilization. Only recently, Prato et al. reported the electrochemistry of carbon nanotubes functionalized using the 1,3-dipolar cycloaddition reaction.120 The cyclic voltammogram obtained is shown in Fig. 8.9. 

As for the fullerenes, the development of methods for nanotube functionalization began very soon after their discovery. After first successes in opening the tubes and attaching functional groups to their ends, the nejct attempts were made in applying the common reactions of fullerene modification to the side-waU functionalization of the structurally related carbon nanotubes. Many reactions performed on fullerenes can indeed be applied to nanotubes as expected. However, the latter are generally observed to be less reactive, which has already been discussed in Section 3.5.1. 

Figure 3.70 Halogenation of carbon nanotubes. The reaction at least partly takes place as 1,4-addition. Fluorinated nanotubes are good starting materials for further functionalization (bottom).

In particular, the unique properties of polypyrrole-carbon nanotubes allowed the detection of hybridization reactions with complementary deoxyribonucleic acid sequences via a decrease in impedance [115], Alternatively, similar deoxyribonucleic acid sensors have been created from a composite of polypyrrole and carbon nanotube functionalized with carbon groups to covalently immobilize deoxyribonucleic acid into carbon nanotubes [116, 117]. Carbon nanotubes have also been incorporated into biosensors as nanotube arrays into which enzymes can be immobilized, along with a conducting polymer [118] and a polypyrrole dopan [119]. In general, the presence of carbon nanotubes tends to increase the overall sensitivity and selectivity of biosensors. 

Due to their moderate specific surface area, carbon nanotubes alone demonstrate small capacitance values. However, the presence of heteroatoms can be a source of pseudocapacitance effects. It has been already proven that oxygenated functional groups can significantly enhance the capacitance values through redox reactions [11]. Lately, it was discovered that nitrogen, which is present in carbon affects also the capacitance properties [12]. 

C.A. Dyke and J.M. Tour, Unbundled and highly functionalized carbon nanotubes from aqueous reactions. Nano Lett. 3, 1215—1218 (2003). 

As with fullerenes, carbon nanotubes are also hydrophobic and must be made soluble for suspension in aqueous media. Nanotubes are commonly functionalized to make them water soluble although they can also be non-covalently wrapped with polymers, polysaccharides, surfactants, and DNA to aid in solubilization (Casey et al., 2005 Kam et al., 2005 Sinani et al., 2005 Torti et al., 2007). Functionalization usually begins by formation of carboxylic acid groups on the exterior of the nanotubes by oxidative treatments such as sonication in acids, followed by secondary chemical reactions to attach functional molecules to the carboxyl groups. For example, polyethylene glycol has been attached to SWNT to aid in solubility (Zhao et al., 2005). DNA has also been added onto SWNT for efficient delivery into cells (Kam et al., 2005). 

One problem associated with An + 2n cyclizations of carbon nanotubes is the reversibility of the process, and for this reason Diels-Alder reactions have been a less used synthetic route. One of the most representative examples of Diels Alder functionalization was reported by Langa and co-workers, who performed MW-assisted addition of o-quinodimethane on pentanol-ester functionalized SWCNTs [34]. 

Encapsulation of different entities inside the CNT channel stands alone as an alternative noncovalent functionalization approach. Many studies on the filling of carbon nanotubes with ions or molecules focus on how the presence of these fillers affects the physical properties of the tubes. From a different point of view, confinement of materials inside the cylindrical structure could be regarded as a way to protect such materials from the external environment, with the tubes acting as a nanoreactor or a nanotransporter. It is fascinating to envision specific reactions between molecules occurring inside the aromatic cylindrical framework, tailored by CNT characteristic parameters such as diameter, affinity towards specific molecules, etc. 

In general, on chemical modification carbon nanotubes exhibit much less toxicity or nontoxicity to living cell lines that have been investigated so far.117,118 For instance, Dumortier et al. conducted an in vitro cell uptake study of the functionalized SWNTs with B and T lymphocytes and macrophages.117 Two types of functionalized SWNTs were used, one prepared via 1,3-dipolar cycloaddition reaction and the other obtained through oxidation/amidation treatment. Both types of the functionalized nanotubes were rapidly taken up by lymphocytes and macrophages without affecting the overall... 

Acyl chloride-functionalized SWCNTs are also susceptible to reactions with other nucleophiles, e.g. alcohols. Haddorfs group reported the preparation of soluble ester-functionalized carbon nanotubes SWCNT-COO(CH2)17CH3 (Fig. 1.6a) obtained by esterification with octadecanol [134]. The syntheses of soluble polymer-bound and dendritic ester-functionalized SWCNTs have been reported by Riggs et al. by attaching poly(vinyl acetate-co-vinyl alcohol) (Fig. 1.6b) [135] and hydrophilic and lipophilic dendron-type benzyl alcohols [119], respectively, to SWCNT-COC1 (Fig. 1.6c). These functional groups could be removed under basic and acidic hydrolysis conditions and thus additional evidence for the nature of the attachment was provided [119, 136]. 

The amine function served also as the starting point for the first covalent linkage of Pcs to single-walled carbon nanotubes (SWNTs) [94], The pipes with open-end and surface-bound acyl chloride moieties were used to prepare the Pc-SWNTs system by amide-bond formation (Fig. 14). Accordingly, statistical reaction of 4-aminophthalonitrile with 4-tcr/-bu(yIph111alonitrile in the presence of zinc ions delivered the monoamino Pc that was then employed in the conjugation with the acid chloride modified carbon nanotubes (CNTs). Here, it should also be mentioned that other functions have been applied to the covalent modification of CNTs, i.e., amide [95], ester [96,97], or click chemistry [98],... 

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