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.

We have developed a novel method to separate metallic and semiconducting single-wall carbon nanotubes (SWCNTs) with high purities using agarose gel. When an SWCNTs/sodium dodecyl sulfate (SDS) dispersion was applied to a column containing agarose gel beads, semiconducting SWCNTs were trapped by the beads, while metallic SWCNTs passed through the column. After the semiconducting SWCNTs adsorbed to the beads were eluted with sodium deoxycholate solution, the column could be used for repeated separation. Because this continuous, repeatable separation method is applicable to a low-cost, large-scale process, it should enable the industrial production of metallic and semiconducting SWCNTs.

The separation of semiconducting and metallic single-wall carbon nanotubes (SWCNTs) by agarose gel chromatography was investigated. SWCNTs dispersed in sodium dodecyl sulfate (SDS) solution were applied to the top of the gel column. Metallic SWCNTs were eluted with SDS solution. Subsequently, the semiconducting SWCNTs that remained in the gel column were collected with sodium deoxycholate (DOC) solution as the eluant. By the successive addition of DOC solutions with concentrations ranging from 0.05 to 2 wt % and fractional collection at each concentration, we found that smaller-diameter enriched S-SWCNTs were eluted first with the lower concentration DOC solutions and then larger-diameter enriched S-SWCNTs were eluted with the higher DOC concentration solutions. Thus, diameter-selective enrichment of semiconducting SWCNTs was achieved. Diameter-selective enrichment of metallic SWCNTs was also demonstrated by adding a series of SDS solutions with different concentrations. These results demonstrate that agarose gel can be used to simultaneously separate metallic and semiconducting SWCNTs and to perform diameter separation of these SWCNTs.

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.

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.

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.

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.

Metallic and semiconducting single-wall carbon nanotubes (SWCNTs) were separated by selective adsorption and desorption using agarose gel beads and SWCNTs/sodium dodecyl sulfate dispersion. Comparison to the batch method, the purities of metallic and semiconducting SWCNTs obtained by a column method were improved to 90 and 95%, respectively. When linear-gradient elution was applied to the column method, semiconducting SWCNTs and presumed mixed bundles could be separated into early-and late-eluted fractions, respectively. In the semiconducting fractions, chirality separation was also detected.