Carbon nanotube carboxylated

Fig. 1.14 (A) Single-wall carbon nanotubes wrapped by glyco-conjugate polymer with bioactive sugars. (B) Modification of carboxyl-functionalized single-walled carbon nanotubes with biocompatible, water-soluble phosphorylcholine and sugar-based polymers. (A) adapted from [195] with permission from Elsevier, and (B) from [35] reproduced by permission of Wiley-VCH.

An important route to solubilization of carbon nanotubes is to functionalize their surface to form groups that are more soluble in the desired solvent environment. It has been shown that acid treatment of nanotube bundles, particularly with HC1 or HNO3 at elevated temperatures, opens up the aggregate structure, reduces nanotube length, and facilitates dispersion (An et al., 2004 Kordas et al., 2006). Nitric acid treatment oxidizes the nanotubes at the defect sites of the outer graphene sheet, especially at the open ends (Hirsch, 2002 Alvaro et al., 2004), and creates carbonyl, carboxyl, and hydroxyl groups, which aid in their solubility in polar solvents. 

Figure 15.15 An aldehyde derivative of pyrene can be used to couple a hydrophilic amino-PEG-carboxylate spacer by reductive amination. The resultant derivative then can be used to coat a carbon nanotube through pyrene ring adsorption and result in a water-soluble derivative containing terminal carboxylates for coupling amine-containing ligands.

Many chromatographic methods such as permeation chromatography, column chromatography, and size exclusion chromatography have been used to purify CNTs. The size exclusion chromatography (SEC) is the only carbon nanotube purification method in the literature that is not subjected to the acid treatments which tend to create the carboxylic functionality on CNTs. 

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). 

Newkome-type dendrons were attached to the carbon scaffold of SWCNTs and MWCNTs by defect group functionalization [108], First- and second-generation amine dendrons such as those depicted in Fig. 1.5 were condensed with the carboxyl groups of purified and opened SWCNTs and MWCNTs according to the car-bodiimide technique [108], These CNTderivatives can be expected to combine the characteristics of carbon nanotubes with those of dendrimers, potential building blocks for supramolecular, self-assembling and interphase systems. 

Preparation of Multi-walled Carbon Nanotube-g-Carboxylic Acid... 

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