Functionalization of CNTs

CNTs own excellent materials properties. DNA is an excellent molecule to construct macromolecular networks because it is easy to synthesize, with a high specificity of interaction, and is conformationally flexible. The complementary base-paring properties of DNA molecules have been used to make two-dimensional crystals and prototypes of DNA computers and electronic circuits (Yan et al., 2002 Batalia et al., 2002). Therefore functionalization of CNTs with DNA molecules has great potential for applications such as developing nanodevices or nanosystems, biosensors, electronic sequencing, and gene transporters. 

CNTs have recently received extensive attention due to their nanoscale dimensions and outstanding materials properties. Integration of CNTs and proteins can gain more advantages over CNTs or proteins and can reahze novel, specific functions. Therefore, functionalization of CNTs with proteins own broad application prospects. 

Functionalization of CNTs with Proteins via Covalent or Non-Covalent Bond... 

Functionalization of CNTs with biomolecules provides novel opportunities for developing the CNT-based bio-nanotechnology. CNTs in the Polymer/CNT composites can improve mechanical, electric, thermal, electrochemical, optical,... 

From an atomic configuration point of view, a nanotube can be divided into two parts that are generated by curvatures the end caps and sidewall. The end caps are close to the hemispherical fullerene and are curved in 2D, and the sidewall contains less-distorted carbon atoms and is curved in ID (Polizu et al., 2006). Owing to their specific curvatures, the chemical reactivity at the sidewall is significantly lower than that at the end caps The sidewall is thought to be inert and highly reactive agents are required for the covalent functionalization of CNT sidewalls (Wei et al., 2007). 

Guo et al. (2007) have used water-soluble MWNTs labelled with 99l"Tc for biodistribution study in mice. There was no sign of renal or other severe acute toxicity responses consistent with the results of Singh et al. (2006). This work also suggests that functionalization of CNTs might improve their biocompatibility of CNTs. 

The surface properties of CNTs are paramount for their hybridization with other components. The formation of large bundles due to van der Waals interactions between hydrophobic CNT walls further limits the accessibility of individual tubes. Functionalization of CNTs can enhance their dispersion in aqueous solvent mixtures and provide a means for tailoring the interfacial interactions in hybrid and composite materials. Functionalization techniques can be divided in covalent and non-covalent routes, which will be described in greater detail in Chapter 3. 

Finally, it is worth mentioning that functionalization of CNTs generally results in a lowered toxicity of the tubes, this being of paramount importance in nanomedicine applications. 

The use of CNTs in composites for optical, mechanical, electronic, biological and medical applications, etc., requires the chemical modification of their surface in order to meet specific requirements depending on the application [140]. While searching for how to perform the covalent functionalization of CNTs, it was found that the tips of CNTs were more reactive than their sidewalls [142,143]. 

There are several chemical reactions that can be used as an alternative to achieve covalent functionalization of CNTs. Two of them are amidation and/or esterification reactions. Both reactions take advantage of the carboxylic groups sitting on the side-walls and tips of CNTs. In particular, they are converted to acyl chloride groups (-C0-C1) via a reaction with thionyl (SO) or oxalyl chloride before adding an alcohol or an amine. This procedure is very versatile and allows the functionalization of CNTs with different entities such as biomolecules [154-156], polymers [157], and organic compounds [158,159] among others. 

Pristine CNTs are chemically inert and metal nanoparticles cannot be attached [111]. Hence, research is focused on the functionalization of CNTs in order to incorporate oxygen groups on their surface that will increase their hydrophilicity and improve the catalyst support interaction (see Chapter 3) [111]. These experimental methods include impregnation [113,114], ultrasound [115], acid treatment (such as H2S04) [116— 119], polyol processing [120,121], ion-exchange [122,123] and electrochemical deposition [120,124,125]. Acid-functionalized CNTs provide better dispersion and distribution of the catalysts nanoparticles [117-120],... 

In this chapter, we will focus on CNTs as advanced materials for the design of electrochemical devices. The next section vdll be devoted to review the structure, electronic, chemical and electrochemical properties of CNTs. Section 3.3 will comprise an overview of the synthesis, purification and (bio)functionalization of CNT, as well as the modification of substrates with CNT. In Section 3.4, we will address the electrochemical applications of functionalized CNT electrodes... 

Studying toxicity and biocompatibility of CNT is very important. Smart et al. [9] outlined directions of future research in this field. It includes pulmonary toxicity, skin irritability, macrophage response, interrelation of CNT with their toxicity, absorption, distribution and excretion, and influence of chemical functionalization of CNT on their biocompatibility. 

Nanotubes are functionalised to improve their solubility in water or to attach to their surface biologically active substances such as peptides and drugs. The ability to attach biological substances has raised an interest in using nanotubes as carriers for delivery of drugs and vaccines. A number of researchers performed functionalization of CNT with physiologically active molecules and macro-objects. These results are summarized in Table 2.3 [30 0]. 

Perfluorinated alkyl radicals, generated by photoinduction from heptadecafluoro-octyl iodide, were added to SWCNTs and the perfluorooctyl-derivatized CNTs obtained (Scheme 1.14). No difference in the solubility of the fluoroalkyl-substituted nanotubes and the starting materials was observed [148]. A pathway to the radical functionalization of CNTs sidewalls was predicted by classical molecular dy-... 

In the course of a study on organic functionalization of CNTs, Haddon s group discovered in 1998 that dichlorocarbene was covalently bound to soluble SWCNTs (Scheme 1.18) [97]. Originally, the carbene was generated from chloroform with potassium hydroxide [79a] and later from phenyl(bromodichloromethyl)mercury [97]. However, the degree of functionalization was as low as 1.6 at. % of chlorine only, determined by XPS [153],... 

The first Diels-Alder [4+2]-cydoaddition functionalization of CNTs has been reported recently [171]. Ester-functionalized SWCNTs were reacted with o-quinodi-methane, generated in situ from benzo-l,2-oxathiin-2-oxide under microwave irradiation (Scheme 1.23). This technique opened up a new access to a novel family... 

Endohedral functionalization of CNTs is the filling of the tubes with various atoms or small molecules [254]. The internal cavity of CNTs (1-2 nm in diameter) provides space for the accommodation of guest molecules [254-260], Even small proteins and other biomolecules [207, 208, 261] and oligonucleotides [262] have been trapped in the nanotube cavity. In 1998, Luzzi s group [263] for the first... 

The question about functionalization of CNTs in nature has a close connection with their bio-application. Functionalization is closely associated with the ability to disperse and perhaps dissolve the nanotubes, which would greatly improve processability.21,22 Due to the fact that the majority of popular synthetic methods produce samples yielding a mixture of many different diameters and chiralities of nanotubes, post-synthesis chemical processing protocols23 are the most popular among the methods of chemical modification. 

Figure 3. Functionalization of CNTs (a) non-covalent interacions with polymers and biomolecules (b) covalent surface chemical modification (end-functionalization and side-wall functionalization).

Figure 4.5. (a) Tensile strength of chitosan/clay nanocomposite as a function of clay content (b) tensile strength of chitosan/CNTs nanocomposite as a function of CNT content (c) stress-strain behavior for neat chitosan, chitosan/0.4% CNTs, chitosan/3% clay, and chitosan/3%clay/0.4% CNTs composites (d) tensile strength of chitosan/clay/CNTs nanocomposite as a function of clay content. Reprinted with permission from ref (42). 

A number of studies on CNT-polymer composites have focused on improving the dispersion and load transfer efficiency in other words the compatibility between the CNTs and polymer matrix through covalent chemical functionalization of CNT surface (12,40). Many of the studies reported above have used acid-functionalized CNTs to fabricate MWCNT-PMMA composites with improved mechanical properties using different processing methods (24,25,27,62). Yang et. al (68) modified the acid functionalized CNTs with octadecylam-ine (ODA) to obtain ODA-functionalized CNTs. These CNTs were reinforced in a copolymer P(MMA-co-EMA) to form composites with improved dispersion and mechanical properties. 

Although the approach of covalent functionalization of CNT surface is an effective means to obtain a homogeneous dispersion of CNTs in polymer matrix and a strong interfacial interaction with the polymer, it inevitably destroys the intrinsic properties of CNTs such as the unique ji-electron system of pristine CNTs is affected due to formation of covalent bonds and shortening of length of CNTs during chemical treatments (70). 

Figure 7.13. Electrical conductivity of MWCNT-PMMA composites as a function of CNT loading. Reprinted with permission from John Wiley Sons, Inc (24).

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