Carbon nanotubes functionalizing carboxylic acid functionalities

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. 

The acid reflux procedure was first described by Rinzler el al. [28], in which raw nanotube materials are refluxed in nitric acid to oxidize the metals and carbon impurities. Acid-treated CNTs are considered to have carboxylic acid groups at the tube ends and, possibly, at defects on the side walls. The functionalized SWNTs have considerably different properties from those of the pristine tubes. 

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). 

Another technique to increase the dispersability and the biocompatibility of the carbon nanotubes is the insertion of terminal groups on the surface of CNTs with a procedure called functionalization [104, 105, 118, 123, 136]. The creation of free carboxylic acid and amino acid moieties on the surface are the most commonly used functionalization for enzyme immobilization and stabilization [104, 105, 118,123, 136]. Apart from these two functional groups, other groups are also used, for various purposes. For example, Jeykumari and Narayanan functionalized MWCNTs with toluidine blue, in order to prevent the leakage of the redox mediator from their bienzymic biosensor [114]. 

In general, functionalization reactions of SWNTs are very slow and take several days to proceed. In this respect, microwave irradiation seems to be a potentially powerful tool to functionalize SWNTs but only a few such reactions have been described to date. One example of the application of microwaves was described by Della Negra et al. [88]. Soluble single-walled carbon nanotubes were synthesized by grafting poly(ethylene glycol) (PEG) chains on to SWNTs. Use of microwave irradiation enhanced reaction rates in comparison with similar syntheses using conventional heating. An amidation reaction has also been performed, in two steps, under microwave irradiation conditions (Scheme 21.23) [89]. Amide-SWNT derivative 68 was synthesized by reaction of 2,6-dinitroaniline and the carboxylic acid-... 

Figure 21.8 Methods for oxidizing carbon nanotubes. Oxidation results in surface functional hydroxyl, carbonyl, and/or carboxylic acid groups. (Reproduced with permission from V. H. Grassian, Nanoscience and Nanotechnology Environmental and Health Impacts. Copyright 2008 John Wiley Sons, Inc.)...

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