Nanotubes, covalent functionalization

S.S. Wong, E. Joselevich, A.T. Woolley, C.L. Cheung, and C.M. Lieber, Covalently functionalized nanotubes as nanometre-sized probes in chemistry and biology. Nature 394, 52—55 (1998). 

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). 

It has also been demonstrated that CNT sidewalls can be covalently fluorinated [148 150], or they can be derivatized with certain highly reactive chemicals such as dichlorocarbene [142], In this context, Chen et al. applied derivatization chemistry with thionychloride and octadecylamine in order to obtain organic soluble SWCNTs and later they performed a reaction with dichlorocarbene that led to the covalent functionalization of the nanotube walls. 

A. Ghosh, K.V. Rao, R. Voggu, S. J. George, Non-covalent functionalization, solubilization of graphene and single-walled carbon nanotubes with aromatic donor and acceptor molecules, Chemical Physics Letters, vol. 488, pp. 198-201, 2010. 

Covalent functionalization of the nanotubes also affects the voltammetric features the richer curve morphology observed in the case of pristine nanotubes reflects the more complex electronic structure of the pristine materials, compared to functionalized ones (Scheme 9.17), in agreement with recent calculations that suggested that functionalization significantly affects the low-lying electronic states of the nanotubes.61... 

In many cases the potential application of single-walled carbon nanotubes is associated with solubility of this nanomaterial in different solvents. Unfortunately, nanotubes are poorly soluble in the most of organic solvents and are insoluble in water, and this fact especially hinders biological using SWNT. Weak solubility of SWNT is a result of substantial van der Waals attractions between nanotubes aggregated in bundles. To solve nanotubes in water without any covalent functionalization, a surfactant would be added into aqueous solution, and then this mixture is suspended by sonication. It is supposed that the sound wave splits bundles in aqueous solution. A surfactant in suspension adsorbed onto the nanotube surfaces precludes aggregation of nanotubes in bundles. 

The same amidation reaction was employed by Joshi et al. to covalently functionalize MWNTs with a specific peptide sequence, which specifically binds to the heptameric receptor-binding subunit of anthrax toxin.33 These nanotube-peptide conjugates were found to selectively destroy anthrax toxin with the reactive oxygen species generated by the nanotubes upon near-IR radiation.33... 

The oxidative introduction of carboxylic functions to nanotubes provides a large number of CNT-functional exploitations and permits covalent functionalization by the formation of amide and ester linkages and other carboxyl derivatives [24]. Bifunctional molecules (diamines, diols, etc.) are often utilized as linkers. More illustrative examples are nanotubes decorated with amino-functionalized dendrimers, nucleic acids, enzymes, etc., and the formation of bioconjugates of CNTs [96]. 

Hou et al. (43) reported the poly(vinyl alcohol) nanocomposites using single walled (SWNT), few walled (FWNT) and multi walled (MWNT) nanotubes. The nanotubes were covalently functionalized to generate acid functionalities on the sidewalls. The incorporation... 

Besides covalent functionalization of carbon nanotubes, non-covalent interaction between CNTs and polyurethane can also help fabricate uniform CNT dispersion nanocomposites. A dominant improvement in the dispersion of MWNTs in hyperbranched polyurethane (HBPU) matrix was found, and good solubility of... 

Another approach to covalently attach carbon nanotubes is based on the chemical modification of a PEEK matrix which allows further covalent interaction with functionalized carbon nanotubes. In 2007 Babaa et al. (40) proposed a route to covalently graft commercial MWCNTs by using this approach. The process initiates by dissolution of PEEK in concentrated H2S04, leading to functionalization yields of 70%. MWCNTs covalently functionalized with... 

Most of PVA/CNT composites are processed under the form of films. Generally, films are casted and dried from water-based PVA and nanotube dispersions. Different types of water-based dispersions have been used. Carbon nanotubes come from various production sources and can be covalently functionalized. The PVA molecular weight and hydrolysis rate can also be varied as well as the nanotube fraction. This is why comparisons between all the contributions in the literature can sometimes be difficult. Nevertheless some general and important features can still be deduced from all the studies reported on this topic. 

Nanotube functionalization is performed in other examples listed in Table 11.1. In 1999, Shaffer et al. used carboxylated MWNTs. By contrast to studies described in the followings, this allows to the achievement of homogeneous composites with a large nanotube fraction of about 50 wt%. However, the improvement of the Young s modulus is still weak, particularly if we consider the large amount of CNTs included in the matrix. Liu et al. (47) reported an improvement of mechanical properties with SWNTs which are covalently functionalized by OH groups, compared to the same neat nanotubes simply dispersed with SDS. 

More recently, Liu et al. (47), compared OH- covalently functionalized SWNTs to SDS stabilized SWNTs. The authors used Raman spectroscopy to understand why the improvements of the Young s modulus and strength are greater for covalently functionalized nanotubes. The Raman shift was plotted versus the tensile strain in the two composites PVA/OH-SWNTs and PVA/ SDS dispersed SWNTs, in the elastic regime (for strain values below 1.2%). The shift is linear in this regime, and a larger slope is measured... 

Figure 31 Representative reaction products obtained in the covalent functionalization of the carbon nanotubes. The preciusor acid functionality is distributed all around the tubes and concentrated at the open ends ( 1 - 3 mol%). For clarity, one substitution with n multiples is illustrated...

Recently, we have developed a relatively green process for covalently functionalizing nanotubes. Versatile functional groups were covalently anchored onto MWCNTs by one-step free radical addition of water-soluble azo initiators in water, and their coverage densities were easily controlled by adjusting feed ratio of initiators to... 

Secondly, however, the carbon nanotube may be covalently functionalized with monomer units, initiator molecules or crosslinking groups, which leads to a significantly better interaction with the matrix. 

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