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.

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.

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.

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.

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.

Cold-induced Conversion of Connective Tissue Skeleton in Brown Adipose Tissues.

Thermogenesis via fatty acid-induced uncoupled mitochondrial respiration is the primary function of brown adipose tissue (BAT). In response to changes in ambient temperatures, the weight and specific gravity of BAT change, depending on the quantity of lipid droplets stored in brown adipocytes (BA). Such conditions should result in the reconstruction of connective tissue skeletons, especially of collagen fiber networks, although the mechanisms have not been clarified. This study showed that, within 4 hr of exposing mice to a cold environment, collagen fibers in the extracellular matrix (ECM) of BAT became discontinuous, twisted, emancipated, and curtailed. Surprisingly, the structure of collagen fibers returned to normal after the mice were kept at room temperature for 19 hr, indicating that the alterations in collagen fiber structures are physiological processes association with adaptation to cold environments. These dynamic changes in connective tissue skeletons were not observed in white adipose tissues, suggesting that they are unique to BAT. Interestingly, the vascular permeability of BAT was also augmented by exposure to cold. Collectively, these findings indicate that dynamic changes in ECM collagen fibers provide high flexibility to BAT, enabling the adjustment of tissue structures and the regulation of vascular permeability, resulting in adaptation to changes in ambient temperatures.

Submilligram-scale separation of near-zigzag single-chirality carbon nanotubes by temperature controlling a binary surfactant system.

Mass production of zigzag and near-zigzag single-wall carbon nanotubes (SWCNTs), whether by growth or separation, remains a challenge, which hinders the disclosure of their previously unknown property and practical applications. Here, we report a method to separate SWCNTs by chiral angle through temperature control of a binary surfactant system of sodium cholate (SC) and SDS in gel chromatography. Eleven types of single-chirality SWCNT species with chiral angle less than 20° were efficiently separated including multiple zigzag and near-zigzag species. Among them, (7, 3), (8, 3), (8, 4), (9, 1), (9, 2), (10, 2), and (11, 1), were produced on the submilligram scale. The spectral detection results indicate that lowering the temperature induced selective adsorption and reorganization of the SC/SDS cosurfactants on SWCNTs with different chiral angles, amplifying their interaction difference with gel. We believe that this work is an important step toward industrial separation of single-chirality zigzag and near-zigzag SWCNTs.

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.

Quantitative analysis of the effect of reabsorption on the Raman spectroscopy of distinct (n, m) carbon nanotubes.

We quantitatively analyze the effect of reabsorption on the Raman spectroscopy of (10, 3) and (8, 3) single-chirality single-wall carbon nanotube (SWCNT) solutions by varying the detection depth in confocal micro-Raman measurements and SWCNT concentration the in sample solution. The increase of the detection depth and concentration of SWCNTs enhances the reabsorption effect and decreases the intensities of the Raman features. More importantly, reabsorption exhibits different effects on different Raman features such as the radial breathing mode (RBM) and G+ band, strongly depending on the resonance degree of the scattered light energy and the interband transition of SWCNTs. When (10, 3) SWCNTs are excited with a 633 nm laser, the scattered light from RBM has stronger resonance with the interband transition of the SWCNTs than that from the G+ band, leading to a faster reduction in the RBM intensity and a lower intensity ratio of RBM to the G+ band. In contrast, when (8, 3) SWCNTs are excited with a 633 nm laser, reabsorption has the same effect on the RBM and G+ band intensities and thus maintains a constant intensity ratio of RBM to the G+ band. Furthermore, we precisely establish a quantitative relationship of the intensities of the Raman features such as RBM, the G+ band and their intensity ratio as a function of the focal depth and SWCNT concentration by theoretical calculations and numerical simulation, which reproduces the experimental results well. These results are very useful in the precise analysis of the Raman spectroscopy of SWCNTs and thus their applications in molecular detection and imaging.

Cascade Reaction-Based Chemiresistive Array for Ethylene Sensing.

Chemiresistive sensors, which are based on semiconducting materials, offer real-time monitoring of environment. However, detection of nonpolar chemical substances is often challenging because of the weakness of the doping effect. Herein, we report a concept of combining a cascade reaction (CR) and a chemiresistive sensor array for sensitive and selective detection of a target analyte (herein, ethylene in air). Ethylene was converted to acetaldehyde through a Pd-catalyzed heterogeneous Wacker reaction at 40 °C, followed by condensation with hydroxylamine hydrochloride to emit HCl vapor. HCl works as a strong dopant for single-walled carbon nanotubes (SWCNTs), enabling the main sensor to detect ethylene with excellent sensitivity (10.9% ppm-1) and limit of detection (0.2 ppm) in 5 min. False responses that occur in the main sensor are easily discriminated by reference sensors that partially employ CR. Moreover, though the sensor monitors the variation of normalized electric resistance (ΔR/R0) in the SWCNT network, temporary deactivation of CR yields a sensor system that does not require analyte-free air for a baseline correction (i.e., estimation of R0) and recovery of response. The concept presented here is generally applicable and offers a solution for several issues that are inherently present in chemiresistive sensing systems.

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.

Brighter near-IR emission of single-walled carbon nanotubes modified with a cross-linked polymer coating.

Photoluminescence (PL) in the near-infrared (NIR) region is an attractive feature of single-walled carbon nanotubes (SWNTs). In this study, we investigated the effect of the chemical structure of the cross-linked polymer coating of polymer-coated SWNTs on the NIR PL emission intensity. We found that brighter NIR emission can be achieved using a more hydrophobic polymer coating.

Photoluminescence Intensity Fluctuations and Temperature-Dependent Decay Dynamics of Individual Carbon Nanotube sp3 Defects.

Recent demonstration of room temperature, telecommunication wavelength single photon generation by sp3 defects of single wall carbon nanotubes established these defects as a new class of quantum materials. However, their practical utilization in development of quantum light sources calls for a significant improvement in their imperfect quantum yield (QY∼10-30%). PL intensity fluctuations observed with some defects also need to be eliminated. Aiming toward attaining fundamental understanding necessary for addressing these critical issues, we investigate PL intensity fluctuation and PL decay dynamics of aryl sp3 defects of (6,5), (7,5), and (10,3) single wall carbon nanotubes (SWCNTs) at temperatures ranging from 300 to 4 K. By correlating defect-state PL intensity fluctuations with change (or lack of change) in PL decay dynamics, we identified random variations in the trapping efficiency of E11 band-edge excitons (likely resulting from the existence of a fluctuating potential barrier in the vicinity of the defect) as the mechanism mainly responsible for the defect PL intensity fluctuations. Furthermore, by analyzing the temperature dependence of PL intensity and decay dynamics of individual defects based on a kinetic model involving the trapping and detrapping of excitons by optically allowed and forbidden (bright and dark) defect states, we estimate the height of the potential barrier to be in the 3-22 meV range. Our analysis also provides further confirmation of recent DFT simulation results that the emissive sp3 defect state is accompanied by an energetically higher-lying optically forbidden (dark) exciton state.

Oxidative Stress of Carbon Nanotubes on Proteins Is Mediated by Metals Originating from the Catalyst Remains.

Nanomaterials introduced into biological systems are immediately coated by proteins in vivo. They induce oxidative stress on adsorbed proteins and hence alter the protein structures, which determines the fate pathways and biological impacts of nanomaterials. Carbon nanotubes (CNTs) have been suggested to cause protein oxidation. In this work, we discovered that CNTs induce oxidative stress on proteins in cooperation with coexisting metals originating from catalyst remains. Protein sulfhydryl groups were readily oxidized by the coexistence of CNTs and metals. Numerical simulations of the reaction demonstrated that the metals effectively mediate electron transfer between the CNTs and protein sulfhydryl groups. Thus, the coexistence of CNTs and metals, even in low concentrations, generates oxidative stress on proteins with high reaction rates. Metal catalysts used for CNT growth, in turn, catalyze the oxidation reaction of proteins. The proposed protein oxidation mechanism will advance the fundamental understanding of the biological safety and toxicity of nanomaterials synthesized using metal catalysts.

Fasting-dependent Vascular Permeability Enhancement in Brown Adipose Tissues Evidenced by Using Carbon Nanotubes as Fluorescent Probes.

Brown adipose tissue (BAT), which is composed of thermogenic brown adipocytes (BA) and non-parenchymal components including vasculatures and extracellular matrix, contribute to the maintenance of body temperature. BAT distribution is detected by positron emission tomography-computed tomography (PET/CT) using 18F-fluorodeoxy glucose (18F-FDG) or single-photon-emission computed tomography-computed tomography (SPECT/CT) using [123/125I]-beta-methyl-p-iodophenyl-pentadecanoic acid. Although sympathetic nerve activity and thermogenic capacity of BA is downregulated under fasting conditions in mice, fasting-dependent structural changes and fluid kinetics of BAT remain unknown. Here we show that the fasting induces fine and reversible structural changes in the non-parenchymal region in murine BAT with widened intercellular spaces and deformed collagen bands as revealed by electron microscopy. Interestingly, a newly introduced near infrared fluorescent probe of single-walled carbon nanotubes (CNTs) coated with phospholipid polyethylene glycol (PLPEG) easily demonstrated enhanced vascular permeability in BAT by the fasting. PLPEG-CNTs extravasated and remained in intercellular spaces or further redistributed in parenchymal cells in fasted mice, which is a previously unknown phenomenon. Thus, PLPEG-CNTs provide a powerful tool to trace fluid kinetics in sub-tissue levels.

Narrow-band single-photon emission through selective aryl functionalization of zigzag carbon nanotubes.

The introduction of sp3 defects into single-walled carbon nanotubes through covalent functionalization can generate new light-emitting states and thus dramatically expand their optical functionality. This may open up routes to enhanced imaging, photon upconversion, and room-temperature single-photon emission at telecom wavelengths. However, a significant challenge in harnessing this potential is that the nominally simple reaction chemistry of nanotube functionalization introduces a broad diversity of emitting states. Precisely defining a narrow band of emission energies necessitates constraining these states, which requires extreme selectivity in molecular binding configuration on the nanotube surface. We show here that such selectivity can be obtained through aryl functionalization of so-called 'zigzag' nanotube structures to achieve a threefold narrowing in emission bandwidth. Accompanying density functional theory modelling reveals that, because of the associated structural symmetry, the defect states become degenerate, thus limiting emission energies to a single narrow band. We show that this behaviour can only result from a predominant selectivity for ortho binding configurations of the aryl groups on the nanotube lattice.

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.

Extended-conjugation π-electron systems in carbon nanotubes.

Extending π-electron systems are among the most important topics in physics, chemistry and materials science because they can result in functional materials with applications in electronics and optics. Conventional processes for π-electron extension, however, can generate products exhibiting chemical instability, poor solubility or disordered structures. Herein, we report a novel strategy for the synthesis of π-conjugated polymers within the interiors of carbon nanotubes (CNTs). In this process, thiophene-based oligomers are encapsulated within CNTs as precursors and are subsequently polymerized by thermal annealing. This polymerization increases the effective conjugation length of the thiophenes, as confirmed by transmission electron microscopy and absorption peak red shifts. This work also demonstrates that these polythiophenes can serve as effective markers for individual CNTs during Raman imaging with single-wavelength laser excitation due to their strong absorbance. In addition, stable carrier injection into the encapsulated polythiophenes is found to be possible via electrochemical doping. Such doping has the potential to produce π-electron-based one-dimensional conductive wires and highly stable electrochromic devices.

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.

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.