Functionalization of carbon nanotubes

Because of their diverse structure, one-third of the tubes are expected to possess metallic character and the remaining two-thirds to behave as semiconductors [2, 3]. CNTs represent potential candidates to be used in field emission [4-6] and nanoelectrical devices [6-10], components of electrochemical energy [11, 12] and hydrogen storage systems [13, 14] and as components in composite materials [15-17]. They represent the ultimate carbon fiber, exhibiting exceptional mechanical properties [18-21] by being up to 100 times stronger than steel [22]. 

Since their discovery by Iijima in 1991 [23], carbon nanotubes have become the subject of intense research activities [2, 3], Today they are widely recognized as the essential contributors to nanotechnology. However, their lack of solubility and their multidisperse dimensions present a considerable barrier towards processing and usage of their promising property profile for technological applications. 

Attaching chemical functionalities to CNTs can improve their solubility and allow for their manipulation and processability [24]. The chemical functionalization can tailor the interactions of nanotubes with solvents, polymers and biopolymer matrices. Modified tubes may have physical or mechanical properties different from those of the original nanotubes and thus allow tuning of the chemistry and physics of carbon nanotubes. Chemical functionalization can be performed selectively, the metallic SWCNTs reacting faster than semiconducting tubes [25]. 

The CNT/polymer nanocomposites can be fabricated by means of solution blending, in situ polymerization and melt compounding [33-39]. The properties of CNT /polymer nanocomposites are directly related to their hierarchical microstructures. The processing conditions and polymers selected dictate the morphology, structure, electrical and mechanical properties of CNT/polymer nanocomposites. In addition, exfoliation and homogeneous dispersion of CNTs in the polymer matrix also play important roles in electrical properties of the composites. The agglomeration of nanotubes is detrimental to the formation of a conductive path network through the matrix of percolative CNT/polymer nanocomposites. 

Despite much interest in CNs, manipulation and processing of these materials has been limited by their lack of solubility in most common solvents. Many applications of CNs (mainly SWNTs) require chemical modification of the materials to make them soluble and more amenable to manipulation. Understanding the chemistry of SWNTs is critical for rational modification of their properties, and several different procedures for chemical derivatization of CNs have been described in the last four years. These methods have been developed in an effort to understand the chemical derivatization and to control the properties of these systems. There is substantial interest in studying the photophysical properties of single-walled carbon nanotube (SWNT) derivatives obtained by covalent [82] and noncovalent [83] functionalization, with the overall objective of obtaining materials with new properties [84]. Functionalization of SWNTs by covalent bonding can be achieved by two different approaches - the bonds can be formed either at the tube opening or on the lateral walls. 

Initial attempts to functionalize these compounds were limited to oxidation reactions [85] that resulted in shortened nanotubes with carboxylic acid groups on the open edges. Haddon and coworkers first reported use of these acid groups to attach long alkyl chains by means of amide linkages [86]. Later, Sun and coworkers showed that esterification can also be used to functionalize SWNTs [87]. These procedures afforded solubilized SWNTs, which enabled characterization and further solution-based investigations. 

These two examples are based on functionalization of SWNTs at the tube opening but microwave irradiation has also been applied to the synthesis of SWNTs functionalized at the lateral walls. 

3-Dipolar cycloaddition reactions of azomethine ylides are probably the most widely used reactions for functionalization of [60]fullerene. This reaction has also been used to obtain SWNT derivatives and, occasionally, microwave irradiation has been used [89]. Pyrrolidino-SWNT 76 was synthesized by reaction of pristine 

As described above, 1,3-dipolar cydoaddition reactions of nitrile oxides have also been used for preparation of fullerene derivatives. Theoretical calculations per-ormed for this type of reaction with SWNTs predict that the 1,3-dipolar cydoaddition reaction of nitrile oxides to the sidewall of the nanotubes is possible but not as 

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Hirsch A, Vostrowsky O (2001) Dendrimers with Carbon Rich-Cores. 217 51-93 Hirsch A, Vostrowsky O (2005) Functionalization of Carbon Nanotubes. 245 193-237 Hissler M, Dyer PW, Reau R (2005) The Rise of Organophosphorus Derivatives in p-Conjugated Materials Chemistry. 250 127-163 Hiyama T, Shirakawa E (2002) Organosilicon Compounds. 219 61-85 Holmberg K, see Hager M (2003) 227 53-74... 

Kam NWS, Liu Z, Dai HJ (2005a) Functionalization of carbon nanotubes via cleavable disulfide bonds for efficient intracellular delivery of siRNA and potent gene silencing. J. Am. Chem. Soc. 127 12492-12493. 

Functionalization of carbon nanotubes with metals can be achieved by different techniques exploiting either the covalent or the noncovalent approach. This topic, which is important for many applications, will be briefly discussed in a separate section after the description of the two methods. 

The second type of functionalization of carbon nanotubes is based on noncovalent interactions, such as CH-n, n-n stacking, van der Waals and electrostatic forces. 

Functionalization of carbon nanotubes becomes essential for multiple reasons. Firstly, chemical modification can allow debundling and therefore solubilization of the tubes, which is an important feature for their processability. Secondly, insertion of functional groups enables attachment of more complex moieties that find applications in several fields. 

H. Dai, Noncovalent functionalization of carbon nanotubes for highly specific electronic biosensors, Proceeding of the National Academic of Science of the United States of America, vol. 48, pp. 4984-4989, 2003. 

Palacin, T., et al., Efficient Functionalization of Carbon Nanotubes with Porphyrin Dendrons via Click Chemistry. Journal of the American Chemical Society, 2009.131(42) p. 15394-15402. 

Dyke, C.A., et al., Diazonium-based functionalization of carbon nanotubes XPS and GC-MS analysis and mechanistic implications. Synlett, 2004. 2004 p. 155-160. 

Roquelet, C., et al., U-Stacking functionalization of carbon nanotubes through micelle swelling. ChemPhysChem, 2010.11(8) p. 1667-1672. 

Wu, B., et al., Functionalization of carbon nanotubes by an ionic-liquid polymer Dispersion ofPt and PtRu nanoparticles on carbon nanotubes and their electrocatalytic oxidation of methanol. Angewandte Chemie International Edition, 2009. 48(26) p. 4751-4754. 

Dai et al. reported on the functionalization of carbon nanotubes. Vinyl-and norborn-2-ene-functionalized pyrenes, respectively, were adsorbed onto the surfaces of the nanotubes and used to graft norborn-2-ene [131]. RuCl2(PCy3)2(CHPh) was used throughout. A polymer film thickness of between 5 and 20 nm was achieved through this approach. 

Krajcik R, Jung A, Hirsch A, Neuhuber W, Zolk O (2008) Functionalization of carbon nanotubes enables non-covalent binding and intracellular delivery of small interfering RNA for efficient knock-down of genes. Biochem Biophys Res Commun 369(2) 595-602... 

B.N. Khare et al., Functionalization of carbon nanotubes using atomic hydrogen from a glow discharge. Nano Lett. 2, 73 (2002)... 

Besides synthetic polymers and small molecules, biological or bioactive species are used in the functionalization of carbon nanotubes not only for water solubility but also enhanced biocompatibilities and biorecognition capabilities. Various proteins, DNAs, and carbohydrates have been covalently or noncovalently functionalized with carbon nanotubes, producing highly aqueous stable and biocompatible... 

Scheme 6.4 Schematic illustration of addressable biomolecular functionalization of carbon nanotubes.51 (Reprinted with permission from C.-S. Lee et al,Nano Lett. 2004,4, 1713-1716. Copyright 2004 American Chemical Society.)...

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