Naotoshi Nakashima

Polybenzimidazole (PBI) is shown to individually dissolve/disperse single‐walled carbon nanotubes (SWNTs) in N,N‐dimethylacetamide (DMAc), which is demonstrated by vis‐near IR absorption and photoluminescence spectroscopy and atomic force microscopy observations. By casting these dispersions, SWNTs/PBI composite films were successfully fabricated on substrates without any sign of macroscopic aggregation. The thermal stability and mechanical properties of the composite films were investigated using thermogravimetric analysis (TGA) and tensile tests, respectively, and it was found that, first, the addition of SWNTs to PBI does not deteriorate the thermal stability of the matrix film, and second, the mechanical properties of the PBI film were reinforced by ca. 50% with only 0.06 wt % addition of the SWNTs to the film without reducing the thermal stability of the PBI. Raman spectroscopy of the composite films revealed the existence of an interaction between the PBI and the SWNTs. The individual dissolution of the SWNTs and efficient reinforcement of the PBI are due to the π–π interaction between the PBI and the sidewalls of the SWNTs.

We designed and synthesized 4‐dodecyloxybenzenediazonium tetrafluoroborate (1), which preferentially reacts with metallic single‐walled carbon nanotubes (SWNTs) by kinetic control. We first determined the suitable experimental conditions for the preferential reaction of 1 with individually dissolved SWNTs by monitoring the decrease in absorbance for the metallic SWNT in the range of 400–650 nm in the absorption spectrum of the SWNTs. The reacted SWNTs were thoroughly rinsed with THF to obtain THF‐insoluble SWNTs. The Raman spectrum of the THF‐insoluble SWNTs showed a strong peak near 180 cm⁻¹, which corresponds to a semiconducting breathing band. The metallic breathing bands (≈220 cm⁻¹) and Breit–Wingner–Fano (BWF) modes (1520 cm⁻¹) corresponding to the metallic SWNTs were much weaker than those of the pristine SWNTs. We also confirmed that metallic peaks in the range of 400–650 nm in the absorption spectrum of THF‐insoluble SWNTs that were individually dissolved in an aqueous micelle of sodium cholate were almost nondetectable. All the results indicate that the THF‐insoluble SWNTs are semiconducting.

The individual solubilization of single‐walled carbon nanotubes (SWNTs), achieved by using ten different anionic‐, zwitterionic‐, and nonionic‐steroid biosurfactants and three different sugar biosurfactants, was examined. Aqueous micelles of anionic cholate analogues, such as sodium cholate (SC), sodium deoxycholate (SDC), sodium taurocholate (STC), sodium taurodeoxycholate (STDC), sodium glycocholate (SGC), as well as N,N‐bis(3‐D‐gluconamidopropyl)cholamide (BIGCHAP) and N,N‐bis(3‐D‐gluconamidopropyl)deoxycholamide (deoxy‐BIGCHAP), exhibited good abilities to dissolve the SWNTs individually. Aqueous micelles of nonionic biosurfactants, such as sucrose monocholate (SMC), n‐octyl‐β‐D‐glucoside (OG), n‐decyl‐β‐D‐maltoside (DM), and n‐decanoyl‐N‐methylglucamide (MEGA‐10), could dissolve the SWNTs, however, the solubilization abilities were weaker than those of the anionic cholate analogues. In sharp contrast, the solubilization abilities of the zwitterionic micelles of 3‐[(3‐cholamidopropyl)dimethylammonio]propanesulfonic acid (CHAPS) and 3‐[(3‐cholamidopropyl)dimethylammonio]‐2‐hydroxypropanesulfonic acid (CHAPSO) were very low, and almost zero for OG. It is evident that the chemical structures, in particular the substituent groups of the surfactants, play an important role in the solubilization of SWNTs. The near‐IR photoluminescence behaviors of the SWNTs dissolved in aqueous micelles and in 1 mM biosurfactants were investigated. The chirality indices of the SWNTs dissolved in these solutions depend on the chemical structures of the biosurfactants. The Raman spectra of the SWNTs dissolved in a 1 mM solution of SC suggest the selective extraction of the metallic SWNTs. Finally, a possible solubilization mechanism using steroid surfactants is described. The SWNTs dissolved individually in water‐containing biocompounds are useful in many areas of nano‐ and materials chemistry.

We describe the design of polycyclic aromatic compounds with high performance that dissolve single‐walled carbon nanotubes (SWNTs). Synthetic amphiphiles trimethyl‐(2‐oxo‐2‐phenylethyl)‐ammonium bromide (1) and trimethyl‐(2‐naphthalen‐2‐yl‐2‐oxo‐ethyl)‐ammonium bromide (2) carrying a phenyl or a naphtyl moiety were not able to dissolve/disperse SWNTs in water. By contrast, trimethyl‐(2‐oxo‐2‐phenanthren‐9‐yl‐ethyl)‐ammonium bromide (3) solubilized SWNTs, although the solubilization ability was lower than that of trimethyl‐(2‐oxo‐2‐pyrene‐1‐yl‐ethyl)‐ammonium bromide (4) (solubilization behavior observed by using 4 was described briefly in reference 4a). Transmission electron microscopy (TEM), as well as visible/near‐IR, fluorescence, and near‐IR photoluminescence spectroscopies were employed to reveal the solubilization properties of 4 in water, and to compare these results with those obtained by using sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium bromide (HTAB) as solubilizers. Compound 4 solubilized both the as‐produced SWNTs (raw‐SWNTs) and purified SWNTs under mild experimental conditions, and the solubilization ability was better than that of SDS and HTAB. Near‐IR photoluminescence measurements revealed that the chiral indices of the SWNTs dissolved in an aqueous solution of 4 were quite different from those obtained by using micelles of SDS and HTAB; for a SWNTs/4 solution, the intensity of the (7,6), (9,5), and (12,1) indices were strong and the chirality distribution was narrower than those of the micellar solutions. This indicates that the aqueous solution of 4 has a tendency to dissolve semiconducting SWNTs with diameters in the range of 0.89–1.0 nm, which are larger than those SWNTs (0.76–0.97 nm) dissolved in the aqueous micelles of SDS and HTAB.

Carbon nanotubes (CNTs) have been in the forefront of nanoscience and nanotechnology because of their remarkable electronic, mechanical and thermal properties and specific functions. CNTs have high potentials for possible applications in the fields of energy, electronics, IT and materials. However, because of the insolubility of the nanotubes in solvents, chemical, biochemical and biological (medical) approaches using these materials have been rather limited. Soluble CNTs in aqueous and organic systems are of interests since they may open the door in such fields. In this review article, our concept to obtain individually dissolved single-walled CNTs using synthetic and biological compounds carrying a condensed aromatic and their fundamental properties is summarized.

Aqueous dispersions of fullerene C₇₀-filled carbon nanotubes (C₇₀@SWNTs or peapods) and empty single-walled carbon nanotubes (empty SWNTs) were prepared with the aid of trimethyl-(2-oxo-2-pyrene-1-yl-ethyl)-ammonium bromide (1), which is a carbon nanotube solubilizer. This is the first report describing the preparation and characterization of the transparent dispersion/dissolution of the peapods. The UV−vis−near-IR spectra of C₇₀@SWNTs−1 and empty SWNTs−1 were almost identical. We found by means of transmission electron microscopy and atomic force microscopy that the empty SWNTs and C₇₀-peapods form helical nanostructures in the shapes of rings, irregular rings, lassos, handcuffs, catenanes, pseudorotaxanes, and figure-eight structures. The mechanism of the superstructure formation has been discussed in relation to the unique characteristics of stiff polymer chains with the aid of an off-lattice Monte Carlo simulation.

Fullerene–porphyrin conjugates coordinated with Fe and Zn ions (1·Fe and 1·Zn) were designed and synthesized. Compounds 1·Fe and 1·Zn were reconstituted into apomyoglobin successfully to produce C₆₀-modified myoglobins, 1·Fe-Mb and 1·Zn-Mb, respectively. The axial-ligand exchange reaction revealed that 1·Fe-Mb maintains the intrinsic properties of native Mb, except for the autooxidation rate constant, suggesting similar microenvironment of the porphyrin moieties in the proteins. Cyclic voltammogram (CV) of a graphite electrode modified with a film of 1·Fe-Mb–didodecyldimethylammonium bromide (DDAB) showed two reversible redox couples with E⁰′ = −206 and −1048 mV which are attributable to Fe^2+/3+ and the reduction of the porphyrin ring, respectively. Differential pulse voltammogram of an electrode modified with a 1·Fe-Mb–tridodecylmethylammonium bromide (TDAB) film in water containing 0.5 M tetraethylammonium chloride and 10 mM 2,2′,2″-nitrilotriethanol showed three cathodic peaks at E_½,1 = −372, E_½,2 = −555, and E_½,3 = −1028 mV which are attributable to Fe^2+/3+, C₆₀ ^0/1−, and the reduction of the porphyrin ring, respectively. The electrodes modified with 1·Fe-Mb and 1·Zn-Mb gave anodic photocurrent coupled with on–off light irradiation. The action spectrum of photocurrent for a 1·Zn-Mb–DDAB film was in accord with the UV–vis absorption spectrum of 1·Zn-Mb. Transient absorption spectra of 1·Zn in benzonitrile and 1·Zn-Mb in a 50 mM phosphate buffer at 100 ns after the ns-laser light pulse irradiation at 532 nm showed three absorption maxima at 700, 830, and 1000 nm which are assignable to the triplet excited state of C₆₀ (³C₆₀*), the triplet excited state of the zinc porphyrin (³ZnP*), and C₆₀ radical anion (C₆₀˙⁻), respectively. Existence of the apparent C₆₀˙⁻ indicates the generation of charge-separation state, ZnP˙⁺–C₆₀˙⁻. The rate constants for the generation of the charge separation states in 1·Zn and 1·Zn-Mb calculated from the fluorescence data were 8.2 × 10⁸ s⁻¹ and 6.4 × 10⁸ s⁻¹, respectively, and the corresponding quantum yields were 0.62 and 0.59.