Solubilized SWNTs

Figure 5. (a) Raman spectrum of SWNTs solubilized in perfluorohexane. The inset in part a shows the photograph of the solution of SWNTs in perfluorohexane. TEM images of the solubilized SWNTs are shown in parts b and c. 

Initial attempts to functionalize these compounds were limited to oxidation reactions [85] that resulted in shortened nanotubes with carboxylic acid groups on the open edges. Haddon and coworkers first reported use of these acid groups to attach long alkyl chains by means of amide linkages [86]. Later, Sun and coworkers showed that esterification can also be used to functionalize SWNTs [87]. These procedures afforded solubilized SWNTs, which enabled characterization and further solution-based investigations. 

A nonplanar donor-acceptor interaction also serves to underscore the binding between porphyrins and SWNTs. The basic interaction was first reported in 2003 by Murakami et al. who found that porphyrin molecules (zinc protoporphyrin IX) could solubilize SWNTs in organic solvents. These researchers also reported a decrease in fluorescence intensity for the proposed ZnPP-SWNT ensembles in solution as compared to solutions of ZnPP alone. Because there is great potential for nanotubes in optoelectronic and photovoltaic applications, this discovery engendered tremendous interest in the study of the noncovalent interactions between porphyrins and SWNTs. Here, a key motivation is that the noncovalent functionalization of SWNTs via the use of porphyrins is expected to be benign... 

Soon after Guldi et al. published on their system, Stod-dart and coworkers published a report wherein dynamic chemistry was used to form self-assembled oligomers that were found to be capable of binding and, possibly even, sorting SWNTs by size. Their system is composed of 5,15-fran.y-pyridyl metalloporphyrin, 13, which can act as a linear divalent ligand and cw-protected Pd or Pt complexes (Figure 13). While neither the Pd/Pt ligand nor the porphyrin displayed an ability to solubilize SWNTs on its own, they were found to completely solubilize SWNTs in aqueous acetonitrile solutions when used in conjunction with one another. 

A report in 2004 by the Sun group showed that 5,10,15,20-tetrakis(hexadecyloxyphenyl)-21//, 23//-porph-ine (THPP) could selectively solubilize SWNTs. Thus, this readily prepared molecule allowed for efficient separation of mixtures of metallic and semiconducting SWNTs, which have different electrical conductivities among other properties. The mechanism was only speculated upon, but it is thought that the porphyrin is better able to interact with the semiconducting surface of the semiconducting SWNT than with the metallic versions. It was noted that... 

Supramolecular interaction involving porphyrins also provides a means for solubilizing SWNTs in water. The first such example was reported in 2005 by Chen et al.,... 

Gable-type porphyrin dimers 46 were found to have the ability not only to solubilize SWNTs, but also discriminate their structures [56-58[. These porphyrins consisting of a rigid spacer and the two porphyrin units were prepared via the Suzuki-Miyaura coupling reaction as shown in Scheme 2. In addition, we introduced the asynunetrical moieties at the periphery of the porphyrin units. The chiral porphyrins 46 were found to extract CoMo-... 

SCHEME 28. Solubilization of SWNTs (279) with artificial (277) or natural (278) schizophy]]ans 251.252... 

Unlike the smaller, spheroidal fullerenes discussed previously, carbon nanotubes are not easily solubilized, even in organic solution. The reality is that all SWNTs and MWNTs are insoluble in all solvent systems. They also have a strong tendency to bind together and aggregate due to van der Waals attractive forces along the length of the nanotube. Since the length-to-diameter... 

Some of the better solvents for pure SWNTs are the amide-containing ones, like DMF or N-methylpyrrolidone, but they still do not permit full dissolution, just dispersion (Boul et al., 1999 Liu et al., 1999). The addition of surfactants to carbon nanotube suspensions can aid in their solubilization, and even permit their complete dispersion in aqueous solution. The hydro-phobic tails of surfactant molecules adsorb onto the surface of the carbon nanotube, while the hydrophilic parts permit interaction with the surrounding polar solvent medium. 

Detergents have been used for simple solubilization of SWNTs in aqueous solution. Ionic detergents such as SDS will coat the nanotube surface and expose the negatively charged sulfonate groups to the surrounding aqueous environment, thus allowing SWNT dispersion in aqueous... 

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). 

A representative example of indirect imaging with SWNTs labeled with fluorescent molecules was illuminated by Dai and coworkers.102 They solubilized as-produced SWNTs via sonication with fluorescein-modified poly (ethylene glycol) (Fluor-PEG, 114 PEG units). The absorption maximum of the nanotube-attached Fluor-PEG was red shifted 3 nm from that of free fluorescein, and the fluorescence intensity was quenched about 67% in phosphate buffer solution (pH 7.4). In the bioevaluation with BT474 breast cancer cells, substantial intracellular fluorescence was observed, suggesting cellular uptake of the Fluor-PEG-functionalized SWNTs.102... 

We could solubilize purified single-walled carbon nanotubes (SWNTs)11 in the fluorous medium by reacting them with heptadecafluoroundecylamine. For this purpose, we first prepared SWNTs by the arc discharge method and purified them by HNO3 and FL treatment.11 To solubilize the nanotubes, 1.5 mg of SWNTs, 22 uL of the heptadecafluoroundecylamine (SWNTs/amine = 2 1), and 3 mL of perfluorohexanne were sealed in a 7 mL Teflon-lined stainless steel autoclave and heated at 130 °C for 48 h. This process produced a clear solution of SWNTs in perfluorohexane, as can be seen from Figure 5a. 

The solubilization of SWNTs appears to occur by the interaction of the -NH2 groups of the amine with the — COOH groups present in the surface of the SWNTs, as proposed hy Chen et al.12 The Raman spectrum of the solution gives the characteristic... 

We have extended the method of solubilization of SWNTs to oxide nanorods. We prepared ZnO nanorods by the solvothermal decomposition of 250 mg (0.911 mmol) of zinc acetate dihydrate in the presence of 6 mL (0.102 mmol) of ethylene-... 

Electrochemical polymerization of pyrrole on an SWNT electrode using an aqueous HCl 0.5 M solution as electrolyte, resulted in deposition of a PPy film onto the SWNT layer leading to a composite with a bilayer structure, as demonstrated by Raman spectroscopy [112]. Anew method was developed by S.Cosner eta/, in 2008 [111] SWNTs were functionalized by electropolymerizable pyrrole groups following covalent and noncova-lent strategies. The covalent pyrrole grafting was carried out by ester formation between pyrrole alcohol and chemically oxidized SWNTs. The strong Ti-interactions between pyrene and SWNTs were exploited for the noncovalent adsorption of a new pyrene-pyrrole derivative on the pristine CNT surface. The pyrrole-ester-SWNTs were solubilized in THE and electropolymerized by controlled potential electrolysis at 0.95 V. The PPy/SWNT... 

Metallic and semiconducting SWNTs are apparently wrapped differently by DNA or more generally in the same concept by surfactant molecules, as discussed above for various approaches to exploit such differences for postproduction separation purposes. The nanotubes also have different interactions with selected functionalization or solubilization agents. Such selective interactions, sometimes considered as non-covalent functionalizations, have been found to allow relatively facile post-production separation at significant quantities. ... 

Sun and co-workers exploited the selectivity in the non-covalent functionalization of SWNTs with planar aromatic molecules, such as derivatized porphyrin or pyrene (Figure 6.3), for the post-production separation. The separation method simply splits the starting nanotube mixture by selectively solubilizing semiconducting SWNTs and leaving their metallic counterparts behind, and consequently is capable of handling significant sample quantities. Experimentally, as-produced samples of... 

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