Covalent functionalisation

Covalent functionalisation and the surface chemistry of CNTs have been envisaged as very important factors for the processing and applications of nanotubes. Recently, many efforts on polymer composite reinforcement have been focused on an integration of chemically modified nanotubes containing different functional groups into the polymer matrix. Covalent functionalisation of CNTs can be achieved by either direct addition reactions of reagents to the sidewalls of nanotubes or the modification of appropriate surface-bound functional e.g. carboxylic acid) groups on to the nanotubes.  

Oxidation by ozonolysis is one of generic set of techniques which is utilised to functionalise CNTs. For example, in one recent work MWNTs oxidised by ozonolysis were then modified with long-alkyl chains via an esterification process. These MWNTS were used to produce PVC composites, which demonstrated improved solubility in organic solvents.  

A completely different strategy for the surface functionalisation of CNTs with nitrogen-containing groups is the treatment of CNTs under atomic nitrogen flow obtained by molecular nitrogen dissociation in an Ar -t N2 microwave plasma. X-ray photoelectron spectroscopy of the nanotube surface demonstrated the presence of amides, oximes and mainly amine and nitrile groups.  

Finally, an approach which is based on plasma polymerisation treatment enables the coating of CNTs with a very thin ( 3 nm) polymer layer. Polymer 

Main Initial immobilization of initiators Attaching of already preformed 

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The covalent functionalisation of CNTs is the alternative and extremely promising approach for applications in fields such as that of functional and composite materials and that of biology. According to the location of the functional groups, two main strategies are used to covalently functionalise CNTs with biomolecules (i) defect functionalisation, and (ii) sidewall functionalisation. 

McDevitt et al. (2007) reported that the tumour-targeting CNT constructs were synthesised via sidewall covalent functionalisation using 1,3-dipolar cycloaddition to attach the antibody IgG, radiometal-ion chelates and fluorescent probes. 

Studies from our laboratories by Pantarotto et al. (2004a, b), Wu et al. (2005) and Kostarelos et al. (2007) using covalently functionalised CNTs (1,3-dipolar cycloaddition reaction chemistry) have reproducibly described that CNTs were uptaken by cells via pathways other than endocytosis. This work has experimentally observed that CNTs were able to interact with plasma membranes and cross into the cytoplasm without the apparent need of engulfment into a cellular compartment... 

There are only a few examples of a follow-on regioselective and covalent functionalisation of the interior scaffold of a dendrimer [47]. 

Non-covalent functionalised carbon nanotubes as highly specific electronic biosensors... 

Using sol-gel procedures, scandium triflate, a known water-tolerant Lewis acid, has been immobilised on a silica gel which had previously covalently functionalised with a sulphonic acid linker. The formation of the silica-gel supported species 41 and 42 is shown in Figure 43. 

Although SWCNTs show attractive features, the development of selective gas sensors based on SWCNTs remains a great challenge. Several methods exposed in Section 10.4 have been explored to overcome this problem. The most mature routes to reach gas selectivity are focused on the chemical covalent or non-covalent functionalisation of SWCNTs, the decoration of SWCNTs with metallic nanoparticles and the diversification of the metallic electrodes. 

As has been pointed out by Cantalini et al. (2003), the cross-sensitivity of SWCNT gas sensors can lead to false alarms in a complex atmosphere. To exploit CNT-based devices, it is crucial to find ways to give them selectivity. To modify the chemical activity of SWCNTs and enhance their sensitivity to a specific gas, the common approach is to functionalise the nanotube side-wall. There are two types of functionalisation depending on the binding of the functional group onto the nanotube sidewall covalent functionafisation and non-covalent functionalisation. Covalent functionalisation of SWCNTs is often obtained from the esterification of carboxylic add groups formed on... 

There are two major approaches to nanotube functionalisation non-covalent supramolecular modifications and covalent functionalisation. ... 

The non-covalent functionalisation of nanotubes normally involves van der Waals, 71-71, CH-ti or electrostatic interactions between polymer molecules and the CNT surface. 

The advantage of non-covalent functionalisation is that it does not alter the structure of the nanotubes and therefore both the initial electrical and mechanical properties should also remain unchanged. However, the efficiency of the load transfer might decrease as the forces between the wrapping molecules and the nanotube surface may be relatively weak. 

Figure 4.2 Synthesis of the compatibilizer (P2) and schematic illustration of non-covalent functionalisation of MWNTs by P2. Reproduced with permission from Mandal and Nandi." EBIBT- 3-[l-ethyl-2(2-bromoisobutyrate)] thiophene DMAEMA- 2-(dimethylamino ethyl methacrylate) HMTETA- 1,1,4,7,10,10-hexamethyltriethylenetetra-mine ATRP- atom transfer radical polymerization.

Martinez and co-workers reported an interesting approach which is based on non-covalent functionalisation of SWNTs with amphiphihc block copolymers (BCs) containing polyethylenoxide (Figure 4.3). They have shown... 

Figure 4.4 Schematic presentation of covalent functionalisation of CNTs via 1,3-dipolar cyclo-addition for enhancing the ability to process CNTs and facilitating the preparation of hybrid composites, which is achieved solely by mixing. EtOH, ethanol. Reproduced with permission from Georgakilas et alf ...

The presence of appropriate active functional groups such as carboxylic acids or amines on the CNT surface allows for further covalent functionalisation with polymer molecules (polymer grafting). Two main approaches for the covalent functionalisation of CNTs with polymers have been reported grafting from and grafting The main differences, advantages... 

Thus, the above reports demonstrated that both non-covalent and covalent functionalisation of nanotubes can serve as effective tools for improving both the nanotube dispersion and nanotube-matrix interfacial interaction, enabling the fabrication of reinforced polymer composites. 

Non-covalent functionalisation approaches also enable the preparation of polymer composites with significantly improved mechanical properties. For example, in the recent work of Yuan et al three different polymers such as polyimide without side-chain (PI), polyimide-graft-glyceryl 4-nonylphenyl... 

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