Carbon nanotubes functionalization groups

C. H. Andersson, H. Grennberg, Reproducibility and efficiency of carbon nanotube end-group generation and functionalization, European Journal of Organic Chemistry, pp. 4421-4428, 2009. 

Biomedical applications of carbon nanotubes functionalized by heteryl groups 05CC571. 

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. 

Potentiodynamic and potentiostatic methods can also be used to activate nanotubes. Similar to chemical oxidation, electrochemical pretreatment can effectively remove impurities and cause the creation of carbon-oxygen functional groups at the exposed edge plane and defect sites. The same authors reported on an electrochemical pretreatment that involved potentiostating the electrode at +1.7 V vs. Ag/AgCl in pH 7 phosphate buffer for 3 min followed by 3 min at —1.5 V (84). Both the chemical and electrochemical oxidations improved the electrode response (smaller voltammetric A p and larger values) for Fe(CN)g " , serotonin, and caffeic acid. 

Carbon nanotubes mixed with ruthenium oxide powder, and immersed in a liquid electrolyte, have been shown by a Chinese research group to function as supercapacilors with much larger capacitance per unit volume than is normally accessible (Ma et al. 2000). 

Kostarelos, K. et al. (2007) Cellular uptake of functionalized carbon nanotubes is independent of functional group and cell type. Nature Nanotechnology, 2 (2), 108-113. 

Due to their moderate specific surface area, carbon nanotubes alone demonstrate small capacitance values. However, the presence of heteroatoms can be a source of pseudocapacitance effects. It has been already proven that oxygenated functional groups can significantly enhance the capacitance values through redox reactions [11]. Lately, it was discovered that nitrogen, which is present in carbon affects also the capacitance properties [12]. 

An additional and very attractive aspect of molecular qubits is the fact that they are stable in solution, and that the ligand shell can be functionalized with specific chemical groups. In recent years, this has enabled depositing molecular clusters onto different substrates and grafting them to nanostructures or devices, such as carbon nanotube single electron transistors or point contacts [112]. These devices... 

An important route to solubilization of carbon nanotubes is to functionalize their surface to form groups that are more soluble in the desired solvent environment. It has been shown that acid treatment of nanotube bundles, particularly with HC1 or HNO3 at elevated temperatures, opens up the aggregate structure, reduces nanotube length, and facilitates dispersion (An et al., 2004 Kordas et al., 2006). Nitric acid treatment oxidizes the nanotubes at the defect sites of the outer graphene sheet, especially at the open ends (Hirsch, 2002 Alvaro et al., 2004), and creates carbonyl, carboxyl, and hydroxyl groups, which aid in their solubility in polar solvents. 

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. 

In another example [56] SWNT was modified with peroxytrifluoroacetic add (PTFAA). Raman spectrum of the carbon nanotubes after the FIFA A treatment shows a D-line substantially increased indicating the formation of defect sites with sp3-hybridized carbon atoms on the sidewalls due to the addition of the functional groups. The RBM bands in the region of 170-270cm-1 decreased and shifted to higher... 

With the aid of a bi-functionalized reagent (terminated with pyrenyl unit at one end and thiol group at the other end), gold nanoparticles were self-assembled onto the surface of solubilized carbon nanotubes [147], Raman spectrum of the gold nanoparticle bearing CNTs is enhanced possibly due to charge transfer interactions between nanotubes and gold nanoparticles. 

The same group reported the simultaneous radiolabeling (with DOTA-anchored 4Cu) and fluorescence studies, coupled with biodistribution in vivo and in vitro (92). It is believed that appropriately functionalized SWNTs can efficiently reach tumor tissues in mice with no apparent toxicity (159). Furthermore, water-solubilised carbon nanotubes are nontoxic when taken up by cells even in high concentration (92). These studies have been complemented by the recent PET imaging of water-soluble 86Y labelled carbon nanotubes in vivo (mice) (160,161), to explore the potential usefulness of carbon nanocarriers as scaffolds for drug delivery. The tissue biodistribution and pharmacokinetics of model DOTA functionalized nanotubes have been explored in vivo (mouse model). MicroPET images indicated accumulation of activity mainly in the kidney, liver, spleen, and to a much less... 

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 becomes essential for multiple reasons. Firstly, chemical modification can allow debundling and therefore solubilization of the tubes, which is an important feature for their processability. Secondly, insertion of functional groups enables attachment of more complex moieties that find applications in several fields. 

---