Noncovalent functionalization of CNTs

At the present time in Raman spectroscopy studies, noncovalent functionalization of CNTs with CPs has been seen to induce only variations in the RBM range. An example is the case of SWNTs noncovalently functionalized with PPV. Thus, in Figure 5.16, Raman spectra... 

CNTs can be further functionalized through noncovalent or covalent interactions for better dispersibility and stability in polymer matrix. The noncovalent functionalization of CNTs normally involves van der Waals, n-n, CH-n, or electrostatic interactions between polymer molecules and CNT surfaces. The advantage of noncovalent functionalization is that it does not destroy the conjugated system of the CNT sidewall, and therefore it does not affect their final properties (Spitalsky et al., 2010). It should be noted that, the graphitic sidewalls of CNTs provide the possibility for tt-stacking interactions with conjugated and aromatic... 

Table 3.3 Noncovalent functionalization of CNTs comparison of different types of dispersing agents and their general applications in electrochemistry. 

Thionine was employed as a fimctional molecule for the noncovalent functionalization of CNTs because it shows a strong interaction with either SWCNTs or MWCNTs... 

The solubility of derivatives of CS has extended its utilization in noncovalent functionalization of CNTs to improve the properties of CNTs by making them individually dispersible. It has been reported that carboxymethyl CS was able to produce CNT dispersion in acidic media. On the other hand, PEG-functionalized carboxymethyl CS was found as an excellent candidate to produce highly effective debundUng and dispersion of CNTs in neutral aqueous solution [91], In an innovative method, the interaction between the functional groups of CS and CNTs was considered and the dispersibihty of CNTs was studied by Fourier transform infrared (FTIR) spectroscopy (Fig. 2.13). The authors found that the free electron pair of amine group of carboxymethyl CS and PEG-functionalized carboxymethyl CS plays a crucial role in dispersing CNTs via n-% interactions. 

At the primitive stage, nearly all noncovalent functionalization of CNTs focused on sorting out and dispersing them by chemical oxidation in acidic media, where the acid not only breaks up any residual metal catalyst but also eliminates the CNT caps, leaving behind COOH residue (Yin et al. 2014 Jerez et al. 2014). The oxidized CNTs are easily dispersible in a variety of NH-R organic solvents, under the impact of an ultrasonic force field (Lertrojanachusit et al. 2013 Parveen et al. 

Polymers, particularly conjugated polymers, have proved to serve as exceptional wrapping materials for the noncovalent functionalization of CNTs due to n-n stacking and van der Waals interactions between the conjugated polymer chains... 

In addition to the aforementioned noncovalent functionalization of CNTs, a photochemical method has also been developed to functionalize CNTs with photoreactive reagents (e.g., azirido-thymidine, AZT), followed by the coupling of single-strand DNA (ssDNA) chains onto the CNTs... 

On the basis of the above examples, we may conclude that the noncovalent functionalization of CNTs could fecilely and effectively equip CNTs with specified functional groups with high density. These surface groups function as active sites to anchor metal precursors or metal nanopartides, resulting in well-... 

Metallophthalocyanines can be used to functionalized SWCNT, covalently [70], Fig. 24a, or noncovalent (jt-Jt interaction). Fig. 24b [108], in order to improve electron transfer process in electrocatalysis. The main advantage of the noncovalent functionalization of CNTs is that it fiiUy preserves the electronic network of these tubular stmctures. This approach of functionalization is useful and important for developing new nanomaterials. The immobilization of MPc molecules onto CNTs may give rise to novel nanodevices where the photophysical and conducting properties of the Pcs are coupled to the unspoiled electronic properties of the nanotubes. 

The vast majority of functionalization methods of carbon nanotubes belong to two broad categories (a) covalent and (b) noncovalent functionalization of the external CNT surface. The former is achieved by covalent attachment of functional groups to the C-C double bond of the n-conjugated framework. The latter is based on the adsorption through van der Waals type bonds of various functional entities. 

Noncovalent functional strategies to modify the outer surface of CNTs in order to preserve the sp2 network of carbon nanotubes are attractive and represent an effective alternative for sidewall functionalization. Some molecules, including small gas molecules [195], anthracene derivatives [196-198] and polymer molecules [118, 199], have been found liable to absorb to or wrap around CNTs. Nanotubes can be transferred to the aqueous phase through noncovalent functionalization of surface-active molecules such as SDS or benzylalkonium chloride for purification [200-202]. With the surfactant Triton X-100 [203], the surfaces of the CNTs were changed from hydrophobic to hydrophilic, thus allowing the hydrophilic surface of the conjugate to interact with the hydrophilic surface of biliverdin reductase to create a water-soluble complex of the immobilized enzyme [203]. 

A nonwrapping approach to noncovalent engineering of CNT surfaces by short and rigid conjugated polyarylenethynylenes was reported [199]. This technique allowed for the dissolution of various types of CNTs in organic solvents and the introduction of numerous neutral and ionic functionalities onto the CNT surfaces... 

Several strategies for immobilizing proteins on CNTs modified electrodes have been proposed. The step of immobilization is critical, since the enzyme has to remain as much active as possible in order to perform an efficient biorecognition of the substrate. The other aspect to consider is that the transducer where the protein will be immobilized has to allow a fast charge transfer to ensure a rapid and sensitive response. Therefore, it is important to take into account that the noncovalent functionalization of the sidewalls of SWCNTs is the best way to preserve the sp nanotube structure and their electronic characteristics. 

Dai et al. [90] reported a very interesting work introducing a simple and general approach for noncovalent functionalization of the sidewalls of CNTs for further immobilization of ferritin, streptavidin and biotinyl-3,6-dioxaoetanediamine in a very efficient way. The first step was the noncovalent funetionalization of SWCNTs by irreversible adsorption of a bifimetional moleeule, 1-pyrenebutanoic acid, succinimidyl ester onto the hydrophobie surfaees of SWCNTs dispersed in DMF or methanol. 

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... 

Finally, we note that for composites based on PPV-type polymers and CNTs, a noncovalent functionalization of the tube surface with CPs is systematically reported. Unlike other composites, the PPV/CNT hybrid material can be obtained only in the thin-film form. 

Characterization of functionalized CNTs takes a big part of work to convince the achievement of the chemical linkage. Purifying functionalized CNTs is essential to obtain trustable results because the agents used for the functionalization of CNTs can be physically attached to CNTs. Removal of these noncovalently attached... 

CNTs are very difficult to dissolve or disperse in most organic or aqueous solutions. Thus, it is necessary to modify or functionalize CNTs. The functionalization of CNTs can be carried out by covalent functionalization or by noncovalent functionalization. 

As is shown in Section 3.3.2, biomolecules such as proteins and DNA bind to CNTs by nonspecific interactions. Dai and coworkers sought to develop a chemical method for the noncovalent functionalization of SWNTs with biomolecules.To this end, 1-pyrenebutanoic acid, succinimidyl ester was employed this molecule adsorbs irreversibly to the wall of pristine SWNTs in organic solvents such as A,A-dimethylformamide (DMF) or methanol. The pyrenyl group of 1-pyrenebutanoic acid, succinimidyl ester, which is aromatic, is known to strongly bind to the... 

Some aromatic molecules, such as pyrene, porphyrin, and their derivatives, have strong affinity with the basal plan of graphene sheets via n-n interactions. Noncovalent functionalization has been used in the functionalization of CNTs, and as the rise of graphene, it has been used for functionalization of graphene. 

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