Covalent functionalization of CNTs

From an atomic configuration point of view, a nanotube can be divided into two parts that are generated by curvatures the end caps and sidewall. The end caps are close to the hemispherical fullerene and are curved in 2D, and the sidewall contains less-distorted carbon atoms and is curved in ID (Polizu et al., 2006). Owing to their specific curvatures, the chemical reactivity at the sidewall is significantly lower than that at the end caps The sidewall is thought to be inert and highly reactive agents are required for the covalent functionalization of CNT sidewalls (Wei et al., 2007). 

The use of CNTs in composites for optical, mechanical, electronic, biological and medical applications, etc., requires the chemical modification of their surface in order to meet specific requirements depending on the application [140]. While searching for how to perform the covalent functionalization of CNTs, it was found that the tips of CNTs were more reactive than their sidewalls [142,143]. 

There are several chemical reactions that can be used as an alternative to achieve covalent functionalization of CNTs. Two of them are amidation and/or esterification reactions. Both reactions take advantage of the carboxylic groups sitting on the side-walls and tips of CNTs. In particular, they are converted to acyl chloride groups (-C0-C1) via a reaction with thionyl (SO) or oxalyl chloride before adding an alcohol or an amine. This procedure is very versatile and allows the functionalization of CNTs with different entities such as biomolecules [154-156], polymers [157], and organic compounds [158,159] among others. 

Although the approach of covalent functionalization of CNT surface is an effective means to obtain a homogeneous dispersion of CNTs in polymer matrix and a strong interfacial interaction with the polymer, it inevitably destroys the intrinsic properties of CNTs such as the unique ji-electron system of pristine CNTs is affected due to formation of covalent bonds and shortening of length of CNTs during chemical treatments (70). 

Functionalization of CNTs by covalent chemistry. Covalent functionalization of CNTs has attracted a great interest for biosensors development. This t5T)e of functionalization is expected to play a crucial role in tailoring the properties of materials and the engineering of CNT biosensing devices. The first step in a covalent functionalization process involves a chemical treatment of CNTs under oxidizing conditions, such as sonication in a mixture of sulfuric and nitric acids or treatment with piranha... 

From these results, it can be seen that the direct mixing of PVK with CNTs results in PVK noncovalently functionalized CNTs, while chemical and electrochemical polymerization lead to covalent functionalization of CNTs with PVK in the undoped and doped... 

A final remark regarding the vibrational spectra of CNTs covalentiy functionahzed with CPs, is that depending on the type of eovalent bond formed between the two constituents (CP and CNT), e.g. C-C, and C-N or C-S, composites with flagellene- and bracelet-type moleeular struetures, respectively, are obtained. All composites involving covalent functionalization of CNTs with CPs are eharacterized by steric hindrance effects. 

Electron transfer reactions are key processes responsible for the maintenance of fife. Certainly, supramolecular principles can help our understanding of the mechanisms of many biological processes such as photosynthetic reactions, oxidative phosphorylation, and many other events such those observed in the respiratory chain [1, 4]. Non-covalent functionalization of CNT has attracted investigation in technological applications as photovoltaic cells and light-emitting diodes (LEDs). 

Figure 6.1. Strategies for covalent functionalization of CNTs. Scheme A direct sidewall functionalization. Scheme B defect functionalization Reproduced from [26] with permission from Elsevier...

Carbon nanotubes (CNTs) are unique one-dimensional (1-D) nanomaterials composed entirely of sp hybridized carbon atoms. Unlike other 1-D nanomaterials, every atom in a CNT is located on the surface, which gives rise to unique properties desirable for many applications. In order to utilize this nanomaterial in most applications, CNTs must be chemically functionalized. Covalent functionalization of CNTs represents a vibrant field of research. Often in covalent modification, the sidewalls or the end groups are subject to functionalization (Figure 1) the primary problem with this approach, however, is that the physical properties of the nanotube are impaired. As this chapter does not cover this topic, interested readers are referred to high-quality review articles. In order to chemically functionalize CNTs while preserving their physical properties, supramolecniar chemistry of CNTs needs to be developed. 

Overall, covalent functionalization of CNTs has diverse mechanical and electrical attributes caused by the intervention of the attached moieties and the modification of the structural p-network (Wang et al. 2010). This structural alteration occurred at the termini of the tubes and/or at the sidewalls. Moreover, the direct sidewall functionalization associated with rehybrization of one or more sp C ate of C network into a sp configuration and concurrent loss of conjugation (Lee et al. 2013). 

In addition to interfacial interactions between the CNT and the polymer matrix, the dispersion of CNTs in the polymer has signifieant influence on the performance of a CNT-polymer nanocomposite. Many different approaches have been used by researchers in an attempt to disperse CNT in polymer matrix such as physical sonication and chemical modification of CNT surface [124-126]. Functionalization of CNT surface can lead to the construction of chemical bonds between the nanotube and polymer matrix and offers the most efficient solution for the formation of strong interface. A strong interface between the coupled CNT-polymer creates an efficient stress transfer [31]. It should be noted that covalent functionalization of CNT may disrupt the grapheme sheet bonding, and thereby reduce the mechanical properties of the final product. However, noncovalent treatment of CNT can improve the CNT-polymer (Fig. 23.10) composite properties through various specific interactions [127]. 

The presence of active functional groups such as carboxylic acids or amines allows for further covalent functionalization with polymer molecules (polymer grafting). Two main approaches for the covalent functionalization of CNTs with polymers have been reported grafting from and grafting to [64]. 

This review focuses mainly on non-covalent functionalization of CNTs [34] with heterocyclic molecules including artificial and bio-originated molecules. The functionalization is performed not only on the tube (exohedral), but also in the tube (endohedral). CNTs were found to encapsulate a variety of materials for many purposes, which will be described in Sect. 3. 

CNTs lend themselves to a range of chemical modifications. Both covalent and non-covalent functionalizations are possible at intact CNT sidewalls, at defect sites on sidewalls or at the tip of the nanotubes. The most common modification is the formation of carboxyl residues [39, 40]. The non-covalent functionalization of CNTs can be carried out by coating CNTs with amphiphilic surfactant molecules or polymers (poly-ethylene-glycol). 

Covalent functionalization of CNTs is based on the formation of a covalent bond between functional entities and the carbon backbones of CNTs, conducted at the termini of the CNTs or at their sidewalls. It could also be divided into direct covalent sidewall functionalization and indirect covalent functionalization (defect functionalization). Direct covalent sidewall functionalization is associated with a change of hybridization from sp to sp and a simultaneous loss of conjugation. The latter takes advantage of chemical transformations of the already present defect sites, which can be the open... 

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