Solubilization of Carbon Nanobelts in Aqueous Solutions: Optical and Colloidal Properties.

Carbon nanobelts (CNBs) correspond to carbon nanotube (CNT) segments and are insoluble in most common aqueous solutions, posing challenges across diverse applications. In this study, [12] CNB, which corresponds to a (6,6) CNT segment, was solubilized by aliphatic surfactant micelles through host-guest complexation, which was confirmed by comprehensive analyses involving spectrophotometry, mass spectrometry, and molecular dynamics simulations. Through this solubilization, zero-Stokes shift emission of the CNB could occur, which could be ascribed to the symmetry-allowed transition. In contrast, CNB was insoluble in non-aliphatic surfactant solutions. The mechanism by which CNB is solubilized using aliphatic surfactants is completely distinct from that of the CNT dispersion mechanism. The present finding provides knowledge of the effectiveness of aliphatic compounds in solubilizing CNBs and their derivatives (carbon nanohoops), which show significant potential for various applications in aqueous systems, including biological applications.

Toward Confined Carbyne with Tailored Properties.

Confining carbyne to a space that allows for stability and controlled reactivity is a very appealing approach to have access to materials with tunable optical and electronic properties without rival. Here, we show how controlling the diameter of single-walled carbon nanotubes opens the possibility to grow a confined carbyne with a defined and tunable band gap. The metallicity of the tubes has a minimal influence on the formation of the carbyne, whereas the diameter plays a major role in the growth. It has been found that the properties of confined carbyne can be tailored independently from its length and how these are mostly determined by its interaction with the carbon nanotube. Molecular dynamics simulations have been performed to interpret these findings. Furthermore, the choice of a single-walled carbon nanotube host has been proven crucial even to synthesize an enriched carbyne with the smallest energy gap currently reported and with remarkable homogeneity.

Photoluminescence Quantum Yield of Single-Wall Carbon Nanotubes Corrected for the Photon Reabsorption Effect.

Photoluminescence (PL) from single-wall carbon nanotubes (SWCNTs) enables structural identification, but to derive the content rate of the specific chirality species it is necessary to know the quantum yield of each chirality. However, in the PL of SWCNTs, because the Stokes shift is small, the photon reabsorption effect is dominant and the apparent PL spectral shape and emission intensity are greatly modified depending on the concentration. This problem makes quantitative identification of SWCNTs by PL difficult. In this study, the concentration dependence of the PL of SWCNTs separated into a few chiralities was analyzed in detail, including the effect of reabsorption. It is clear that all changes in the PL spectrum occurring in the high concentration range can be explained simply by the reabsorption effect, and additional effects such as Coulomb interactions between SWCNTs can be negligible. Furthermore, a reliable quantum yield was derived from the emission intensity corrected for the reabsorption effect. The PL quantum yield varied with SWCNT chirality and exhibited a clear "family pattern". This is consistent with the theoretical report showing that the chirality-dependent PL quantum yield is dominated mainly by relaxation by optical phonons from E22 to E11.

Direct Proof of a Defect-Modulated Gap Transition in Semiconducting Nanotubes.

Measurements of optical properties at a nanometer level are of central importance for the characterization of optoelectronic devices. It is, however, difficult to use conventional light-probe measurements to determine the local optical properties from a single quantum object with nanometrical inhomogeneity. Here, we successfully measured the optical gap transitions of an individual semiconducting carbon nanotube with defects by using a monochromated electron source as a probe. The optical conductivity extracted from an electron energy-loss spectrum for a certain type of defect presents a characteristic modification near the lowest excitation peak ( E11), where excitons and nonradiative transitions, as well as phonon-coupled excitations, are strongly involved. Detailed line-shape analysis of the E11 peak clearly shows different degrees of exciton lifetime shortening and electronic state modification according to the defect type.

Determination of Enantiomeric Purity of Single-Wall Carbon Nanotubes Using Flavin Mononucleotide.

Although enantiomeric separation of single-wall carbon nanotubes is possible, their enantiomeric purity (EP) remains an issue due to a lack of effective evaluation methods. In this work, we report the EP of (6,5) carbon nanotube enantiomers using flavin mononucleotide (FMN) as an enantiomer-sensitive dispersant. The enantiomers (6,5) and (11,-5) were separated by a gel column chromatography method and dispersed in a FMN aqueous solution. In these solutions, (6,5) and (11,-5) showed E11 optical transitions at different wavelengths due to handedness-dependent interactions with the FMN molecule, which enabled us to estimate each concentration, namely, the EP. We prepared six intermediate-purity enantiomer samples by mixing the (6,5) and (11,-5) enantiomers and measured their circular dichroism (CD) spectra. The CD signal was confirmed to change linearly with the EP. Using this relationship, we can estimate the EP of any mixture of (6,5) and (11,-5) from its CD intensity.

A 104-week pulmonary toxicity assessment of long and short single-wall carbon nanotubes after a single intratracheal instillation in rats.

We compared long-term pulmonary toxicities after a single intratracheal instillation of two types of dispersed single-wall carbon nanotubes (SWCNTs), namely, those with relatively long or short linear shapes with average lengths of 8.6 and 0.55 µm, respectively. Both types of SWCNTs were instilled intratracheally in male F344 rats at 0.2 or 1.0 mg/kg (long SWCNTs) or 1.0 mg/kg (short SWCNTs). Pulmonary responses were characterized at 26, 52 and 104 weeks after a single instillation. Inflammatory changes, test substance deposition, test substance engulfment by macrophages, and alveolar wall fibrosis were observed in the lungs of almost all test rats at 52 and 104 weeks after short nanotube instillation. The incidences of these changes were much lower in the long nanotube-treated groups. In almost all rats of the long nanotube-treated groups, fibrosis and epithelium loss in the terminal bronchiole with test substance deposition were observed. These bronchiolar changes were not observed after administering short nanotubes. Both bronchiolo-alveolar adenoma and carcinoma were found in the negative-control group, the high-dose long-nanotube group, and the short-nanotube group at 104 weeks post-instillation, although the incidences were not statistically different. The genotoxicity of the SWCNTs was also evaluated by performing in vivo comet assays with lung cells obtained 26 weeks post-instillation. No significant changes in the percent tail deoxyribonucleic acid were found in any group. These findings suggested that most long SWCNTs were deposited at the terminal bronchioles and that a considerable amount of short SWCNTs reached the alveolus, resulting in chronic inflammatory responses, but no genotoxicity in the lungs.

Characteristics and improvement of wideband wavelength-tunable narrow-linewidth source by spectral compression in quasi-dispersion-increasing comb-profile fiber.

We investigated the characteristics and behavior of spectral compression in a quasi-dispersion-increasing comb-profile fiber (CPF). A periodical breathing behavior and sidelobe emission process in the CPF were observed in numerical analysis. Then, taking account of the numerical results, we developed an improved CPF in which the sidelobe suppression was dramatically improved to -24.2 dB while keeping a narrow spectral width of ~0.6 nm. As a seed pulse source, we developed a high-repetition-rate Er-doped ultrashort-pulse fiber laser with single-wall carbon nanotubes and used the improved CPF to realize a high-power, narrow-linewidth source with wide wavelength tunability in the 1.62-1.90 µm band.

Simultaneous chirality and enantiomer separation of metallic single-wall carbon nanotubes by gel column chromatography.

We report the chirality and enantiomer separation of metallic single-wall carbon nanotubes (SWCNTs) using gel chromatography, which has been the last remaining issue in SWCNT separation that has yet to be achieved. The key to the separation is summarized as the following three points: (i) the use of a preseparated metallic SWCNT mixture to eliminate the semiconducting SWCNTs that are more interactive with the gel; (ii) the reduction of the concentration of dispersant to increase the interaction between the metallic SWCNTs and the gel; and (iii) the use of a long column to increase the number of interaction sites that enhance the slight differences between metallic SWCNT species. Using these three separation conditions, we obtained chirality-sorted metallic SWCNTs, especially (10,4) metallic SWCNTs were highly enriched. Circular dichroism spectra demonstrated the enantiomer separation of metallic SWCNTs. The discrimination of the enantiomers is derived from the dextran in the gel, which is the only enantiomeric moiety in this system. This is the first report on the enantiomer separation of metallic SWCNTs and will contribute to progress in the fundamental physics and applications of SWCNTs.

Optical isomer separation of single-chirality carbon nanotubes using gel column chromatography.

We report a gel column chromatography method for easily separating the optical isomers (i.e., left- and right-handed structures) of single-chirality carbon nanotubes. This method uses the difference in the interactions of the two isomers of a chiral single-wall carbon nanotube (SWCNT) with an allyl dextran-based gel, which result from the selective interaction of the chiral moieties of the gel with the isomers. Using this technique, we sorted optical isomers of nine distinct (n, m) single-chirality species from HiPco SWCNTs, which is the maximum number of isolable species of SWCNTs reported to date. Because of its advantages of technical simplicity, low cost, and high efficiency, gel column chromatography allows researchers to prepare macroscopic ensembles of single-structure SWCNTs and enables the complete discovery of intrinsic properties of SWCNTs and advances their application.

Ultrafast generation of fundamental and multiple-order phonon excitations in highly enriched (6,5) single-wall carbon nanotubes.

Using a macroscopic ensemble of highly enriched (6,5) single-wall carbon nanotubes, combined with high signal-to-noise ratio and time-dependent differential transmission spectroscopy, we have generated vibrational modes in an ultrawide spectral range (10-3000 cm(-1)). A total of 14 modes were clearly resolved and identified, including fundamental modes of A, E1, and E2 symmetries and their combinational modes involving two and three phonons. Through comparison with continuous wave Raman spectra as well as calculations based on an extended tight-binding model, we were able to identify all the observed peaks and determine the frequencies of the individual and combined modes. We provide a full summary of phonon frequencies for (6,5) nanotubes that can serve as a basic reference with which to refine our understanding of nanotube phonon spectra as well as a testbed for new theoretical models.

Conformational analysis of single perfluoroalkyl chains by single-molecule real-time transmission electron microscopic imaging.

Whereas a statistical average of molecular ensembles has been the conventional source of information on molecular structures, atomic resolution movies of single organic molecules obtained by single-molecule real-time transmission electron microscopy have recently emerged as a new tool to study the time evolution of the structures of individual molecules. The present work describes a proof-of-principle study of the determination of the conformation of each C-C bond in single perfluoroalkyl fullerene molecules encapsulated in a single-walled carbon nanotube (CNT) as well as those attached to the outer surface of a carbon nanohorn (CNH). Analysis of 82 individual molecules in CNTs under a 120 kV electron beam indicated that 6% of the CF2-CF2 bonds and about 20% of the CH2-CH2 bonds in the corresponding hydrocarbon analogue are in the gauche conformation. This comparison qualitatively matches the known conformational data based on time- and molecular-average as determined for ensembles. The transmission electron microscopy images also showed that the molecules entered the CNTs predominantly in one orientation. The molecules attached on a CNH surface moved more freely and exhibited more diverse conformation than those in a CNT, suggesting the potential applicability of this method for the determination of the dynamic shape of flexible molecules and of detailed conformations. We observed little sign of any decomposition of the specimen molecules, at least up to 10(7) e·nm(-2) (electrons/nm(2)) at 120 kV acceleration voltage. Decomposition of CNHs under irradiation with a 300 kV electron beam was suppressed by cooling to 77 K, suggesting that the decomposition is a chemical process. Several lines of evidence suggest that the graphitic substrate and the attached molecules are very cold.

Arginine side chains as a dispersant for individual single-wall carbon nanotubes.

Charged peptides and proteins disperse single-wall carbon nanotubes (SWCNTs) in aqueous solutions. However, little is known about the role of their side chains in their interactions with SWCNTs. Homopolypeptide-SWCNT systems are ideal for investigating the mechanisms of such interactions. In this study, we demonstrate that SWCNTs are individually dispersed by poly-L-arginine (PLA). The debundled SWCNTs exhibited a distinct fluorescence. The dispersibility of SWCNTs with PLA was greater than that of SWCNTs with poly-L-lysine (PLL). Molecular dynamics simulations suggest that the side chains of PLA have stronger interactions with the sidewalls of SWCNTs compared with those of PLL. The guanidinium group at the end of the side chain of an arginine residue plays an important role in the interaction with SWCNTs, likely through hydrophobic, van der Waals, and π-π interactions. PLA can be useful as a tool for the dispersion of SWCNTs and can be used to non-covalently anchor materials to SWCNTs with strong binding.

High-efficiency single-chirality separation of carbon nanotubes using temperature-controlled gel chromatography.

We report the use of temperature-controlled gel chromatography for the high-efficiency single-chirality separation of single-wall carbon nanotubes (SWCNTs). This new method uses temperature to selectively control the interaction between the sodium dodecyl sulfate (SDS)-wrapped SWCNTs and an allyl dextran-based gel. Temperature control enhances the differences in the interactions of various (n, m) SWCNTs with the gel, enabling the separation of high-purity (n, m) single-species in a single-step process. With this technique, we successfully sorted seven (n, m) single-species including (6, 4), (6, 5), (7, 5), (8, 3), (8, 4), (7, 6), and (8, 6) from raw HiPco-SWCNTs at a series of temperatures. Our technique offers the advantages of technical simplicity, low cost, and high yield, representing an important step toward the industrial-scale separation of single-chirality SWCNTs.

Coenzyme corona formation on carbon nanotubes leads to disruption of the redox balance in metabolic reactions.

Carbon nanotubes (CNTs) have adverse impacts on metabolism in biological systems. The impacts should be associated with interactions of the CNTs with coenzymes, such as nicotinamide adenine dinucleotide (NAD), because most metabolic processes are governed by coenzyme-dependent reactions. This study demonstrates that NAD molecules adsorb onto the CNT surface, leading to the formation of interfacial NAD layers-in other words, a coenzyme corona (coenzyme-based biomolecular corona). Coenzyme corona formation is accompanied by the oxidation of NAD at biological concentrations through electron transfer. Similar phenomena are observed for NAD derivatives. Molecular dynamics simulations indicate that the adsorption of NAD onto CNTs is driven by interactions between the aromaphilic groups of NAD and the CNT surfaces, leading to coenzyme corona formation. Generally, in living biological systems, the balance of NAD redox (NADH/NAD+ redox) is maintained to sustain metabolism. The present results suggest that CNTs affect coenzyme-dependent metabolic reactions by disrupting the redox balance through coenzyme corona formation and subsequent coenzyme oxidation. The proposed molecular mechanism not only advances the fundamental understanding of the biological impact of CNTs in terms of metabolism but also contributes to biological CNT applications.

Near-Infrared Photoluminescence of Carbon Nanotubes Powered by Biochemical Reactions of Luciferin/Luciferase.

We report the near-infrared (NIR) photoluminescence of single-wall carbon nanotubes (SWCNTs) generated by chemical energy derived from enzymatic reactions. NIR photoluminescence from SWCNTs has attracted much attention for medical applications, such as bioimaging and biosensors, because of its high transparency and low scattering in biological tissues; however, visible excitation light cannot reach deep tissues. We developed a novel method in which the NIR luminescence of SWCNTs is powered by the biochemical reaction of luciferin/luciferase from fireflies. The luminescence could be detected by a highly sensitive measurement system using an infrared camera, and the optimal conditions for luminescence were investigated. Spectroscopic analysis of the NIR luminescence using chirality-sorted SWCNTs confirmed that the luminescence was derived from SWCNTs. This is the first report achieving NIR photoluminescence of SWCNTs using chemical energy, which does not require external energies, e.g., excitation light or electronic power, and will be applicable to biological imaging and sensing.

Chirality-dependent electrical transport properties of carbon nanotubes obtained by experimental measurement.

Establishing the relationship between the electrical transport properties of single-wall carbon nanotubes (SWCNTs) and their structures is critical for the design of high-performance SWCNT-based electronic and optoelectronic devices. Here, we systematically investigated the effect of the chiral structures of SWCNTs on their electrical transport properties by measuring the performance of thin-film transistors constructed by eleven distinct (n, m) single-chirality SWCNT films. The results show that, even for SWCNTs with the same diameters but different chiral angles, the difference in the on-state current or carrier mobility could reach an order of magnitude. Further analysis indicates that the electrical transport properties of SWCNTs have strong type and family dependence. With increasing chiral angle for the same-family SWCNTs, Type I SWCNTs exhibit increasing on-state current and mobility, while Type II SWCNTs show the reverse trend. The differences in the electrical properties of the same-family SWCNTs with different chiralities can be attributed to their different electronic band structures, which determine the contact barrier between electrodes and SWCNTs, intrinsic resistance and intertube contact resistance. Our present findings provide an important physical basis for performance optimization and application expansion of SWCNT-based devices.

Selective Detection of Toxic C1 Chemicals Using a Hydroxylamine-Based Chemiresistive Sensor Array.

Formaldehyde (FA) is a deleterious C1 pollutant commonly found in the interiors of modern buildings. C1 chemicals are generally more toxic than the corresponding C2 chemicals, but the selective discrimination of C1 and C2 chemicals using simple sensory systems is usually challenging. Here, we report the selective detection of FA vapor using a chemiresistive sensor array composed of modified hydroxylamine salts (MHAs, ArCH2ONH2·HCl) and single-walled carbon nanotubes (SWCNT). By screening 32 types of MHAs, we have identified an ideal sensor array that exhibits a characteristic response pattern for FA. Thus, trace FA (0.02-0.05 ppm in air) can be clearly discriminated from the corresponding C2 chemical, acetaldehyde (AA). This system has been extended to discriminate methanol (C1) from ethanol (C2) in combination with the catalytic conversion of these alcohols to their corresponding aldehydes. Our system offers portable and reliable chemical sensors that discriminate the subtle differences between C1 and C2 chemicals, enabling advanced environmental monitoring and healthcare applications.

Single chirality extraction of single-wall carbon nanotubes for the encapsulation of organic molecules.

The hollow inner spaces of single-wall carbon nanotubes (SWCNTs) can confine various types of molecules. Many remarkable phenomena have been observed inside SWCNTs while encapsulating organic molecules (peapods). However, a mixed electronic structure state of the surrounding SWCNTs has impeded a detailed understanding of the physical/chemical properties of peapods and their device applications. We present a single-chirality purification method for SWCNTs that can encapsulate organic molecules. A single-chiral state of (11,10) SWCNTs with a diameter of 1.44 nm, which is large enough for molecular encapsulation, was obtained after a two-step purification method: metal-semiconductor sorting and cesium-chloride sorting. The encapsulation of C(60) to the (11,10) SWCNTs was also succeeded, promising a route toward single-chirality peapod devices.

Discovery of surfactants for metal/semiconductor separation of single-wall carbon nanotubes via high-throughput screening.

We report novel surfactants that can be used for the separation of metallic (M) and semiconducting (S) single-wall carbon nanotubes (SWCNTs). Among the M/S separation methods using surfactants in an aqueous solution, sodium dodecyl sulfate plays a key role in density gradient ultracentrifugation (DGU) and agarose gel separations. In this study, we screened 100 surfactants for M/S separation using a high-throughput screening system. We identified five surfactants, which could be used for both DGU and agarose gel separations, suggesting that the basic principle of these separations is common. These surfactants have relatively low dispersibilities, which is likely due to their common structural features, i.e., straight alkyl tails and charged head groups, and appeared to enable M- and S-SWCNTs to be distinguished and separated. These surfactants should stimulate research in this field and extend the application of electrically homogeneous SWCNTs not only for electronics but also for biology and medicine.

Confined water inside single-walled carbon nanotubes: global phase diagram and effect of finite length.

Studies on confined water are important not only from the viewpoint of scientific interest but also for the development of new nanoscale devices. In this work, we aimed to clarify the properties of confined water in the cylindrical pores of single-walled carbon nanotubes (SWCNTs) that had diameters in the range of 1.46 to 2.40 nm. A combination of x-ray diffraction (XRD), nuclear magnetic resonance, and electrical resistance measurements revealed that water inside SWCNTs with diameters between 1.68 and 2.40 nm undergoes a wet-dry type transition with the lowering of temperature; below the transition temperature T(wd), water was ejected from the SWCNTs. T(wd) increased with increasing SWCNT diameter D. For the SWCNTs with D = 1.68, 2.00, 2.18, and 2.40 nm, T(wd) obtained by the XRD measurements were 218, 225, 236, and 237 K, respectively. We performed a systematic study on finite length SWCNT systems using classical molecular dynamics calculations to clarify the effect of open ends of the SWCNTs and water content on the water structure. It was found that ice structures that were formed at low temperatures were strongly affected by the bore diameter, a = D - σ(OC), where σ(OC) is gap distance between the SWCNT and oxygen atom in water, and the number of water molecules in the system. In small pores (a < 1.02 nm), tubule ices or the so-called ice nanotubes (ice NTs) were formed irrespective of the water content. On the other hand, in larger pores (a > 1.10 nm) with small water content, filled water clusters were formed leaving some empty space in the SWCNT pore, which grew to fill the pore with increasing water content. For pores with sizes in between these two regimes (1.02 < a < 1.10 nm), tubule ice also appeared with small water content and grew with increasing water content. However, once the tubule ice filled the entire SWCNT pore, further increase in the water content resulted in encapsulation of the additional water molecules inside the tubule ice. Corresponding XRD measurements on SWCNTs with a mean diameter of 1.46 nm strongly suggested the presence of such a filled structure.