Carbon nanotubes, covalent modification

The covalent methods previously discussed for fullerene modification using cycloaddition reactions also can be applied to carbon nanotubes. This strategy results in chemically linking molecules to the graphene rings on the outer surface of the cylinder, resulting in stable... 

An other interesting strategy is the modification of the surface of the electrodes with multiwalled carbon nanotubes (MWNTs) or single-walled carbon nanotubes (SWNTs) [13,32]. The MWNTs are grown on the electrodes covered with a nickel catalyst film by plasma-enhanced chemical vapour deposition and encapsulated in Si02 dielectrics with only the end exposed at the surface to form an inlaid nanoelectrode array [13]. In the other case, commercial SWNTs are deposited on SPE surface by evaporation [32], The carbon nanotubes are functionalised with ssDNA probes by covalent attachment. This kind of modification shows a very efficient hybridisation and, moreover, the carbon nanotubes improve the analytical signal. 

The amine function served also as the starting point for the first covalent linkage of Pcs to single-walled carbon nanotubes (SWNTs) [94], The pipes with open-end and surface-bound acyl chloride moieties were used to prepare the Pc-SWNTs system by amide-bond formation (Fig. 14). Accordingly, statistical reaction of 4-aminophthalonitrile with 4-tcr/-bu(yIph111alonitrile in the presence of zinc ions delivered the monoamino Pc that was then employed in the conjugation with the acid chloride modified carbon nanotubes (CNTs). Here, it should also be mentioned that other functions have been applied to the covalent modification of CNTs, i.e., amide [95], ester [96,97], or click chemistry [98],... 

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... 

Park MJ, Lee JK, Choi IS et al (2006) Covalent modification of multiwaUed carbon nanotubes with imidazolium-based ionic liquids effect of anions on solubility. Chem Mater 18 1546-1551... 

It has already been mentioned in the introduction on general chemical properties that the central cavity of carbon nanotubes is another conceivable site for chemical modification. Normally, however, covalent interactions with potential bonding partners cannot evolve inside of a nanotube as due to the surface curvature the bonding orbital lobes are only little in size and intensity (Figure 3.94). StiU this effect is less pronounced than in the fuUerenes, and there is indication, for instance, of nanotubes with large diameters being at least partially coated also on the inside upon reaction with for example, ammonia (see below). 

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. 

Tseng, C. H., Wang, C. C., and Chen, C. Y, Functionalizing carbon nanotubes by plasma modification for the preparation of covalent-integrated epoxy composites, Chem. Mater., 19, 308-315 (2007). 

The design must take into account the fact that a good redox mediator should have fast electrode kinetics, be stable and have a low redox potential in order to be easily regenerated in a biosensor context. Other modifications have concerned the use of carbon nanotubes, either immobilized by electrostatic adsorption or covalently immobilized in chitosan matrices and layer-by-layer self-assembly of sensing structures. Some of these strategies will be addressed in the following sections. 

Abiman, P, Wildgoose, G., and Compton, R. (2008) Investigating the mechanism for the covalent chemical modification of multiwalled carbon nanotubes using aryl diazonium salts. Int.. Electrochem. Sci, 3 (2), 104-117. 

Pristine carbon nanotubes are extremely hydrophobic and insoluble in aqueous solutions, which is necessary for biological applications. Many efforts were put forth to increase the aqueous solubility of carbon nanotubes using sidewall covalent modification or dispersion with surfactants. In order to solubilize carbon nanotubes in aqueous solution, it is necessary to attach a hydrophilic coating. This can be done in two fundamental... 

It has been reported that cell uptake of carbon nanotubes is independent of cell type as well as the functionalization of the nanotube. This study used CNTs functionalized with several different small molecules with increasing molecular complexity via reagent-mediated covalent modification. Confocal microscopy was used to track uptake of the functionalized CNTs. However, DNA was not included among the included functionalizations. The authors discussed uptake of functionalized carbon nanotubes... 

Carbon nanotubes (CNTs) are unique one-dimensional (1-D) nanomaterials composed entirely of sp hybridized carbon atoms. Unlike other 1-D nanomaterials, every atom in a CNT is located on the surface, which gives rise to unique properties desirable for many applications. In order to utilize this nanomaterial in most applications, CNTs must be chemically functionalized. Covalent functionalization of CNTs represents a vibrant field of research. Often in covalent modification, the sidewalls or the end groups are subject to functionalization (Figure 1) the primary problem with this approach, however, is that the physical properties of the nanotube are impaired. As this chapter does not cover this topic, interested readers are referred to high-quality review articles. In order to chemically functionalize CNTs while preserving their physical properties, supramolecniar chemistry of CNTs needs to be developed. 

Although outer-surface modification of carbon nanotubes has been developed for nearly two decades, interior modification via covalent chemistry is still challenging due to the low reactivity of the inner-surface. Specifically, forming covalent bonds at inner walls of carbon nanotubes requires a transformation from sp to sp hybridization. The formation of sp carbon is energetically unfavorable for concave surfaces. Hence, the interior functionalization of carbon nanotubes remains a challenge. 

Use of surfactants is an effective way for dispersing CNTs [39]. Reports show that the outer most nanotubes in a bundle are treated more than the innermost tubes and the nanotube remains predominantly btmdled even after surfactant treatment. But mechanical methods like ultrasonication can debundle the nanotubes by steric or electrostatic repulsions [40]. On sonication the high local shear will unravel the outer carbon nanotubes in a bundle and expose other sites for additional surfactant adsorption, thus the surfactant molecules gradually exfoliate the bundle in an unzippering mechanism [41]. Some of the common surfactants used for the dispersion of carbon nanotubes are sodium dodecyl benzene sulfonate (SDBS) [42], dodecyl trimethyl ammonium bromide (DTAB) [43], hexadecyl trimethyl ammonium bromide (CTAB) [44], octylphenol ethoxylate (Triton X-100) [45] and sodium dodecyl sulfate (SDS) [46]. Covalent modification is another way to solubilize the CNTs in different solvents and to improve the interaction with the matrix in composites [47]. 

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