Thermodynamic Determination of the Metal/Semiconductor Separation of Carbon Nanotubes Using Hydrogels

The metal/semiconductor separation of single-wall carbon nanotubes (SWCNTs) using hydrogels, such as agarose gel and Sephacryl, together with sodium dodecyl sulfate is one of the most successful techniques necessary for industrial applications. Despite recent improvements in the technique, little is known about the separation mechanism. Here, we show that SWCNTs are reversibly adsorbed onto hydrogels in the presence of sodium dodecyl sulfate. The results enabled us to examine the thermodynamics of the adsorption reaction and thereby elucidate the separation mechanism. The adsorbability of SWCNTs onto the hydrogels was described by the standard Gibbs free energy for the adsorption, as well as the area of the hydrogels allowing the adsorption. We demonstrated, for the first time, that the free energy of adsorption for semiconducting SWCNTs was 0–12 kJ/mol lower than that for metallic SWCNTs in the temperature range of 290–320 K (<i>e.g.</i>, <i>ca</i>. −4 kJ/mol for the agarose gel and <i>ca</i>. −9 kJ/mol for Sephacryl at 300 K), which permits metal/semiconductor separation. Importantly, the difference in the free energy was attributed to the difference in the enthalpy of adsorption: the enthalpy of adsorption of metallic SWCNTs was <i>ca</i>. 70 kJ/mol higher than that of semiconducting SWCNTs. Thus, the enthalpy of adsorption was found to be an important parameter in the metal/semiconductor separation of SWCNTs using hydrogels. In addition, the thermodynamic parameters depended on the hydrogel type and the surfactant concentration, which is most likely why under certain conditions hydrogels and surfactants produce different separations, <i>e.g.,</i> chirality-selective or diameter-selective separation

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

pH- and Solute-Dependent Adsorption of Single-Wall Carbon Nanotubes onto Hydrogels: Mechanistic Insights into the Metal/Semiconductor Separation

The gel separation of single-wall carbon nanotubes (SWCNTs) suspended in sodium dodecyl sulfate (SDS) is expected to be one of the most successful methods of large-scale and high-purity separation. Understanding the mechanism of the gel separation helps improve the quality and quantity of separation and reveals the colloidal behaviors of SWCNTs, which reflects their band structures. In this study, we characterize the pH- and solute-dependent adsorption of SWCNTs onto agarose and Sephacryl hydrogels and provide a mechanistic model of the metal/semiconductor separation. The adsorbability of SWCNTs is substantially reduced under acidic pH conditions. Importantly, the pH dependence differs between metallic and semiconducting species; therefore, the adsorbability is related to the band-structure-dependent oxidation of the SWCNTs. Oxidation confers positive charges on SWCNTs, and these charges enhance the electrostatic interactions of the SWCNTs with SDS, thereby leading to the condensation of SDS on the SWCNTs. This increase in SDS density reduces the interactions between the SWCNTs and hydrogels. Under highly basic conditions, such as pH ∼12.5, or in the presence of salts, the adsorption is dissociative because of the condensation of SDS on the SWCNTs through electrostatic screening by counterions. Desorption of the SWCNTs from the hydrogels due to the addition of urea implies a hydrophobic interface between SDS-dispersed SWCNTs and the hydrogels. These results suggest that the metal/semiconductor separation can be explained by the alteration of the interaction between SDS-dispersed SWCNTs and the hydrogels through changes in the conformation of SDS on the SWCNTs depending on the SWCNTs’ band structures

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

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 (<i>n, m</i>) SWCNTs with the gel, enabling the separation of high-purity (<i>n, m</i>) single-species in a single-step process. With this technique, we successfully sorted seven (<i>n, m</i>) 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

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

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 (<i>n, m</i>) SWCNTs with the gel, enabling the separation of high-purity (<i>n, m</i>) single-species in a single-step process. With this technique, we successfully sorted seven (<i>n, m</i>) 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

Purification of Single-Wall Carbon Nanotubes by Controlling the Adsorbability onto Agarose Gels Using Deoxycholate

One of the key challenges to the industrialization of single-wall carbon nanotubes (SWCNTs) is the commercial-scale production of highly purified SWCNTs separated into metallic and semiconducting species. In the present study, the purification of SWCNTs, i.e., the removal of amorphous carbon or bundled SWCNTs, was performed by quantifying and controlling their adsorbability onto agarose gel. The quantification of the adsorbability was achieved by assuming the Langmuir isotherm, and control over the adsorbability was exerted using 0.05–1% sodium deoxycholate (DOC). The results show that the adsorbability depends on the concentration of DOC. At a low DOC concentration (approximately 0.05%), impurities such as amorphous carbon or bundled SWCNTs were preferentially adsorbed onto the gels, whereas, at an intermediate DOC concentration (ca. 0.25%), high-purity SWCNTs were mainly adsorbed onto the gels. Thus, the impurities, which are difficult to remove by conventional methods, could be separated from unpurified SWCNTs by controlling the adsorbability, leading to the extraction of high-purity SWCNTs. In the purification, diameter-selective separation of SWCNTs was also observed. The purification method using a gel column can be conducted simply and continuously, so that it can be applied for the high-throughput production of high-purity SWCNTs

Intramolecular S_N′-Type Aromatic Substitution of Benzylic Carbonates at their Para-Position

The benzylic carbonates, which connect with an active methine through an <i>o</i>-phenylene tether at their meta-position, are cyclized by Pd(η³-C₃H₅)Cp–S-Phos catalyst, yielding 3-methyl-9,10-dihydrophenanthrenes. In the catalytic cyclization, the internal nucleophile attacks not the ortho-carbon but the para-carbon of the benzylic ester. The [3 + 2] cycloaddition of <i>m</i>-(silylmethyl)benzyl carbonates with alkylidene malonates was developed from the palladium-catalyzed intramolecular S_N′-type aromatic substitution

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