Carbon nanotubes functionalizing covalent functionalization

Maehashi et al. (2007) used pyrene adsorption to make carbon nanotubes labeled with DNA aptamers and incorporated them into a field effect transistor constructed to produce a label-free biosensor. The biosensor could measure the concentration of IgE in samples down to 250 pM, as the antibody molecules bound to the aptamers on the nanotubes. Felekis and Tagmatarchis (2005) used a positively charged pyrene compound to prepare water-soluble SWNTs and then electrostatically adsorb porphyrin rings to study electron transfer interactions. Pyrene derivatives also have been used successfully to add a chromophore to carbon nanotubes using covalent coupling to an oxidized SWNT (Alvaro et al., 2004). In this case, the pyrene ring structure was not used to adsorb directly to the nanotube surface, but a side-chain functional group was used to link it covalently to modified SWNTs. 

Direct consumption sugar, 23 450-451 Direct contact heat exchangers, 13 268 Direct cooler evaporators, 21 537 Direct-coupled plasma (DCF), 25 370 Direct covalent carbon nanotube functionalization, 17 54-55 Direct current (dc) diode sputtering, 24 730-731. See also dc sensing current... 

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... 

In particular, the unique properties of polypyrrole-carbon nanotubes allowed the detection of hybridization reactions with complementary deoxyribonucleic acid sequences via a decrease in impedance [115], Alternatively, similar deoxyribonucleic acid sensors have been created from a composite of polypyrrole and carbon nanotube functionalized with carbon groups to covalently immobilize deoxyribonucleic acid into carbon nanotubes [116, 117]. Carbon nanotubes have also been incorporated into biosensors as nanotube arrays into which enzymes can be immobilized, along with a conducting polymer [118] and a polypyrrole dopan [119]. In general, the presence of carbon nanotubes tends to increase the overall sensitivity and selectivity of biosensors. 

Huang, H.M. Liu, I.C. Chang, C.Y. Tsai, H.C. Hsu, C.H. Tsiang, R.C.C. (2004). Preparing a polystyrene-functionalized multiple-walled carbon nanotubes via covalently linking acyl chloride functionalities with living polystyryllithium. Journal of Polymer Science, Part A Polymer Chemistry, 42, 5802-5810. 

Zhang, Y., Shen, Y.F., Han, D.X., Wang, Z.J, Song, JJC, and Niu, L. (2006) Reinforcement of sihca with single-walled carbon nanotubes through covalent functionalization. /. Mater. Chan., 16, 4592-4597. 

Zhao, J., Park, H., Han, J. et al. 2004. Electronic properties of carbon nanotubes with covalent sidewall functionalization. J. Phys. Chem. B 108 4227-4230. 

Amide bond is an effective anchor to connect CNTs to substrate surfaces. Lan et al. [52] covalently assembled shortened multi-walled carbon nanotubes (s-MWNT) on polyelectrolyte films. The shortened MWNT is functionalized with acyl chloride in thionyl chloride (SOCl2) before self-assembling. The FTIR spectrum of self-assem-bled MWNT (SA-MWNT) adsorbed on a CaF2 plate modified with PEI/(PSS/PEI)2 shows two characteristic absorption peaks at 1646cm-1 (amide I bond) and 1524cm-1 (amide II bond) resulting from the amide bond formed between the polyelectrolyte films and s-MWNTs. 

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). 

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. 

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. 

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. 

Peng, X. and S.S. Wong, Functional covalent chemistry of carbon nanotube surfaces. Advanced Materials, 2009. 21(6) p. 625-642. 

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... 

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. 

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... 

Although the noncovalent binding is a versatile way to functionalize carbon nanotubes with proteins, the interaction is rather weak in comparison to covalent binding. For example, it has been reported that the nanotube-adsorbed proteins were largely... 

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