Selective desorption of high-purity (6,5) SWCNTs from hydrogels through surfactant modulation

An Automated Gradient Titration Fluorescence Methodology for High-Resolution Identification of Aqueous Two-Polymer Phase Extraction Conditions for Single-Wall Carbon Nanotubes

A significant advance in rate and precision of identifying the co-surfactant concentrations leading to differential extraction of specific single-wall carbon nanotube (SWCNT) species in aqueous two-polymer phase extraction experiments is reported. These gains are achieved through continuous titration of co-surfactant and other solution components during automated fluorescence measurements on SWCNT dispersions. The resulting fluorescence versus concentration curves display intensity and wavelength shift transitions traceable to the nature of the adsorbed surfactant layer on specific SWCNT structures at the (n,m) species and enantiomer level at high resolution. The increased precision and speed of the titration method resolve previously invisible complexity in the SWCNT fluorescence during the transition from one surfactant dominating the SWCNT interface to the other, offering insight into the fine details of the competitive exchange process. For the first time, we additionally demonstrate that the competitive process of the surfactant switch is direction independent (reversible) and hysteresis-free; the latter data effectively specifies an upper bound for the time scale of the exchange process. Titration curves are compared to literature results and initial advanced parameter variation is conducted for previously unreasonable to investigate solution conditions.

Surfactant Chemistry and Polymer Choice Affect Single-Wall Carbon Nanotube Extraction Conditions in Aqueous Two-Polymer Phase Extraction

Quantitative determination of the effects of surfactant chemistry and polymer chain length on the concentration conditions necessary to yield extraction of specific single-wall carbon nanotube (SWNCT) species in an aqueous two-polymer phase extraction (ATPE) separation are reported. In particular, the effects of polyethylene glycol (PEG) chain length, surfactant ratios, and systematic structural variations of alkyl surfactants and bile salts on the surfactant ratios necessary for extraction were investigated using a recently reported fluorescence-based method. Alkyl surfactant tail length was observed to strongly affect the amount of surfactant necessary to cause PEG-phase extraction of nanotube species in ATPE, while variation in the anionic sulfate/sulfonate head group chemistry has less impact on the concentration necessary for extraction. Substitution of different bile salts results in different surfactant packings on the SWCNTs, with substitution greatly affecting the alkyl surfactant concentrations required for (n,m) extraction. Finally, distinct alkyl-to-bile surfactant ratios were found to extract specific (n,m) SWCNTs across the whole effective window of absolute concentrations, supporting the hypothesized competitive adsorption mechanism model of SWCNT sorting. Altogether, these results provide valuable insights into the underlying mechanisms behind ATPE-based SWCNT separations, towards further development and optimization of the ATPE method for SWCNT chirality and handedness sorting.

Gel Chromatography for Separation of Single-Walled Carbon Nanotubes

Carbon nanotubes (CNTs), having either metallic or semiconducting properties depending on their chirality, are advanced materials that can be used for different devices and materials (e.g., fuel cells, transistors, solar cells, reinforced materials, and medical materials) due to their excellent electrical conductivity, mechanical strength, and thermal conductivity. Single-walled CNTs (SWNTs) have received special attention due to their outstanding electrical and optical properties; however, the inability to selectively synthesize specific types of CNTs has been a major obstacle for their commercialization. Therefore, researchers have studied different methods for the separation of SWNTs based on their electrical and optical properties. Gel chromatography methods enable the large-scale separation of metallic/semiconducting (m/s) SWNTs and single-chirality SWNTs with specific bandgaps. The core principle of gel chromatography-based SWNT separation is the interaction between the SWNTs and gels, which depends on the unique electrical properties of the former. Controlled pore glass, silica gel, agarose-based gel, and allyl dextran-based gel have been exploited as mediums for gel chromatography. In this paper, the interaction between SWNTs and gels and the different gel chromatography-based SWNT separation technologies are introduced. This paper can serve as a reference for researchers who plan to separate SWNTs with gel chromatography.

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.

Aqueous two-polymer phase extraction of single-wall carbon nanotubes using surfactants

This review details the current state of the art in aqueous two-phase extraction (ATPE) based separations of surfactant dispersed single-wall carbon nanotubes by their chemical species, i.e., (n,m) structure, semiconducting or metallic nature, and enantiomeric handedness. Discussions of the factors affecting each separation, including workflow effects, variations of different surfactant and nanotube materials, and the underlying physical mechanism are presented. Lastly an outlook on the applications of ATPE at bench scale and implementation to larger scales is discussed, along with identification of research directions that could further support ATPE development.

Three-dimensional porous graphene oxide-maize amylopectin composites with controllable pore-sizes and good adsorption-desorption properties: Facile fabrication and reutilization, and the adsorption mechanism

Three-dimensional (3D) porous graphene oxide-maize amylopectin (GO-MA) composites with controllable pore-sizes composites in the range of 6-40 nm were prepared by facile hydrothermal-assisted assembly approaches. The morphologies, pore sizes, specific surface area (SSA) and compositions of GO-MA_x:y composites with and different GO-to-MA mass ratios (x:y) were characterized by scanning electron microscopy (SEM), N₂ adsorption-desorption isotherms, Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). To reveal the adsorption-desorption mechanism, effects of contact time, temperature, initial adsorbate concentration, pH value of the solution on the adsorption process were studied in detail. The adsorption capacities of 3D GO-MA_20:1 composite for organic contaminants including tert-butyl hydroquinone (TBHQ), p-aminophenol (PAP), p-nitrophenol (PNP), o-nitrophenol (MNP), hydroquinone (HQ), alizarin red S (ARS) and neutral red (NR) were 22.17, 116.4, 44.78, 36.96, 16.10, 39.92 and 24.23 mg g^-1, respectively. The adsorption capacities of GO-MA_30:1 composite for inorganic substances including Pb²⁺, Mn²⁺, Cr₂O₇^2-, Cd²⁺, Cu²⁺, Nd³⁺, La³⁺, Y³⁺, Yb³⁺ and Er³⁺ were 84.76, 7.92, 13.6, 17.64, 30.56, 25.52, 12.48, 16.96, 23.32 and 30.32 mg g^-1, respectively. In addition, GO-MA_x:y composites also exhibited high mechanical properties and good reusability. Consequently, GO-MA_x:y composites could be used as reusable adsorbents for removal/enrichment inorganic/organic substances in aqueous solutions.

Radio frequency heating of metallic and semiconducting single-walled carbon nanotubes

Here we report the effect of metallic (m-) and semiconducting (s-) properties of single-walled carbon nanotubes (SWCNTs) on the response of SWCNT films to radio frequency (RF) heating. We separated high-purity m- and s-SWCNTs from an initial SWCNTs mixture and prepared thin films using vacuum filtration method. The areal density of the films is 9.6 μg cm-2, and the DC conductivities are in the range of 7800-49 000 S m-1. We show rapid and non-contact Joule heating of films using a fringing-field RF applicator, and we observe maximum heating rates in the frequency range of 60-70 MHz. We determine that the more conductive m-SWCNT films reflect RF fields and heat at a maximum rate of 1.51 °C s-1 compared to maximum heating rate of 25.6 °C s-1 for s-SWCNT films. However, m-SWCNTs heat up faster than s-SWCNTs when dispersed in a dielectric medium. Our results confirm the non-monotonic relationship between RF heating rate and conductivity for CNT-based materials such that conductivity is required for heating but high values are correlated with reflections. Our findings also suggest that RF heating could be a possible metric for evaluating film purity because impurities in the films affect the conductivity and thus RF heating rate. We anticipate that RF heating may occur in SWCNT-based electronics and affect their performance.

Synthesis, purification, properties and characterization of sorted single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) have attracted significant attention due to their outstanding mechanical, chemical and optoelectronic properties, which makes them promising candidates for use in a wide range of applications. However, as-produced SWCNTs have a wide distribution of various chiral species with different properties (i.e. electronic structures). In order to take full advantage of SWCNT properties, highly purified and well-separated SWCNTs are of great importance. Recent advances have focused on developing new strategies to effectively separate nanotubes into single-chirality and/or semiconducting/metallic species and integrating them into different applications. This review highlights recent progress in this cutting-edge research area alongside the enormous development of their identification and structural characterization techniques. A comprehensive review of advances in both controlled synthesis and post-synthesis separation methods of SWCNTs are presented. The relationship between the unique structure of SWCNTs and their intrinsic properties is also discussed. Finally, important future directions for the development of sorting and purification protocols for SWCNTs are provided.

Release of Retained Single-Walled Carbon Nanotubes in Gels

The separation of single-chirality, even-enantiomeric single-walled carbon nanotubes (SWCNTs) has been well established using gel permeation chromatography. Successful SWCNTs separation has been considered to be the selective adsorption and desorption of specific SWCNTs on the porous sites of Sephacryl gels. This work reports two nonspecific releases of SWCNTs retained on Sephacryl gels: (1) a considerable number of SWCNTs were eluted using a low-concentration SDS condition solution (0.5 wt %) from the gels exclusively eluted with a high-concentration SDS eluting solution (5 wt %) after being stocked overnight and (2) the retained SWCNTs in Sephacryl gels can be eluted using a low-concentration SDS condition solution (0.5 wt %) after being stocked overnight without any treatments. Inspired by extracellular matrix systems, these releases are attributed to the strain-induced gel relaxation. The roles of surfactants, especially SDS, in the retention and release of SWCNTs on Sephacryl gels were discussed on the basis of spectral dilution and titration experiments using single-chirality (6,5) SWCNT as the probe.

Strongly Bound Sodium Dodecyl Sulfate Surrounding Single-Wall Carbon Nanotubes

NMR techniques have been widely used to infer molecular structure, including surfactant aggregation. A combination of optical spectroscopy, proton NMR spectroscopy, and pulsed field gradient NMR (PFG NMR) is used to study the adsorption number for sodium dodecyl sulfate (SDS) with single-wall carbon nanotubes (SWCNTs). Distinct transitions in the NMR chemical shift of SDS are observed in the presence of SWCNTs. These transitions demonstrate that micelle formation is delayed by SWCNTs due to the adsorption of SDS on the nanotube surface. Once the nanotube surface is saturated, the free SDS concentration increases until micelle formation is observed. Therefore, the adsorption number of SDS on SWCNTs can be determined by the changes to the apparent critical micelle concentration (CMC). PFG NMR found that SDS remains strongly bound onto the nanotube. Quantitative analysis of the diffusivity of SDS allowed calculation of the adsorption number of strongly bound SDS on SWCNTs. The adsorption numbers from these techniques give the same values within experimental error, indicating that a significant fraction of the SDS interacting with nanotubes remains strongly bound for as long as 0.5 s, which is the maximum diffusion time used in the PFG NMR measurements.