Chemical functionalization of CNTs

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. 

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. 

The use of surfactants and the chemical functionalization of CNT surfaces have also been investigated in efforts to improve CNT dispersion as well as enhance the CNT-polymer interfacial bond (10,27-29). However, although chemical functionalization can lead... 

Chemical functionalization of CNT surfaces could improve their dispersion in the polymer matrix and enhance the nanotube-polymer interfacial interaction and the mechanical load transfer. The effects of nanotube functionalization on the properties of CNT-TPU composites have been investigated in details. Xia and Song have synthesized polycaprolactone polyurethane (PU)-grafted SWNTs (PU-g-SWNTs) and corresponding PU-g-SWNT-PU composites by in-situ polymerization. The results show that PU-g-SWNTs improve the dispersion of SWNTs in the PU matrix and strengthen the interfacial interaction between the PU and SWNTs. Compared with neat PU and pristine SWNT PU composites, PU-g-SWNT-PU composites demonstrate remarkable enhancement on Young s modulus. The Young s modulus of a 0.7 wt /o PU-g-SWNT-PU composite increases by 178% over the blank PU and 88% over the 0.7 wt% pristine SWNT-PU composite, respectively. 

To address these issues, several strategies for the preparation of such composites have been reported. Here we report some of these strategies involving physical mixing in solution, infiltration of monomers in the presence of nanotube sheets and chemical functionalization of CNTs by plasma treatment. 

Covalent functionalization of nanotubes can improve nanotube dispersion in solvents and polymers significantly. Various strategies for chemical functionalization of CNTs have been... 

A key advantage of CNTs and CNFs is the possibility to functionalize or dope their surface with various elements to improve the dispersion of the inorganic active phase or introduce new organic active sites for performing subsequent catalytic reactions. Chemical functionalization of CNTs and/or CNFs is critically important for developing new, highly efficient, carbon-based materials for downstream catalytic applications. It is expected that doped CNTs could represent an important class of new metal-free catalysts with better catalytic performance and improved resistance toward deactivation compared with traditional supported transition metals and oxides. 

Chemical modification of CNTs is an essential step towards the fabrication of CNT-based electrochemical sensors. Raman spectroscopy provides an effective way to monitor the modification process and to characterize the functionalized CNTs. 

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). 

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... 

A good understanding of the chemical properties of CNTs is mandatory for enhancing the efficiency of practical devices and also for comprehending related fundamental processes such as their electrochemistry. In the following, we will address the chemical reactivity of CNTs and in Section 3.3.3, the different (bio) chemical functionalization procedures that can be performed for applications, will be discussed in more detail. 

The chemical modification of CNTs can be endohedral (inside the cavity of the tube) or exohedral [42]. There are some examples in the literature that have demonstrated the filling of CNTs with fullerenes, biomolecules (proteins, DNA), metals and oxides that have been driven inside by capillary pressure [39, 42, 72-78]. However, in this section we will focus on exohedral functionalization, taking place just at the external walls of the tubes. Both covalent (chemical-bond formation) and noncovalent (physiadsorption) functionlizations can be carried out. In the following... 

Attaching chemical functionalities to CNTs can improve their solubility and allow for their manipulation and processability [24]. The chemical functionalization can tailor the interactions of nanotubes with solvents, polymers and biopolymer matrices. Modified tubes may have physical or mechanical properties different from those of the original nanotubes and thus allow tuning of the chemistry and physics of carbon nanotubes. Chemical functionalization can be performed selectively, the metallic SWCNTs reacting faster than semiconducting tubes [25]. 

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).

Double-walled carbon nanotubes (DWNTs), first observed in 1996, constitute a unique family of carbon nanotubes (CNTs). -2 DWNTs occupy a position between the single-walled carbon nanotubes (SWNTs) and the multiwalled carbon nanotubes (MWNTs), as they consist of two concentric cylinders of rolled graphene. DWNTs possess useful electrical and mechanical properties with potential applications. Thus, DWNTs and SWNTs have similar threshold voltages in field electron emission, but the DWNTs exhibit longer lifetimes.3 Unlike SWNTs, which get modified structurally and electronically upon functionalization, chemical functionalization of DWNTs surfaces would lead to novel carbon nanotube materials where the inner tubes are intact. The stability of DWNTs is controlled by the spacing of the inner and outer layers but not by the chirality of the tubes 4 therefore, one obtains a mixture of DWNTs with varying diameters and chirality indices of the inner and outer tubes. DWNTs have been prepared by several techniques, such as arc discharge5 and chemical vapor depo-... 

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). 

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