Isolation of >1 nm Diameter Single-Wall Carbon Nanotube Species Using Aqueous Two-Phase Extraction

Surfactant Micelle-Driven High-Efficiency and High-Resolution Length Separation of Carbon Nanotubes for Electronic Applications

High-efficiency extraction of long single-wall carbon nanotubes (SWCNTs) with excellent optoelectronic properties from SWCNT solution is critical for enabling their application in high-performance optoelectronic devices. Here, a straightforward and high-efficiency method is reported for length separation of SWCNTs by modulating the concentrations of binary surfactants. The results demonstrate that long SWCNTs can spontaneously precipitate for binary-surfactant but not for single-surfactant systems. This effect is attributed to the formation of compound micelles by binary surfactants that squeeze the free space of long SWCNTs due to their large excluded volumes. With this technique, it can readily separate near-pure long (≥500 nm in length, 99% in content) and short (≤500 nm in length, 98% in content) SWCNTs with separation efficiencies of 26% and 64%, respectively, exhibiting markedly greater length resolution and separation efficiency than those of previously reported methods. Thin-film transistors fabricated from extracted semiconducting SWCNTs with lengths >500 nm exhibit significantly improved electrical properties, including a 10.5-fold on-state current and 14.7-fold mobility, compared with those with lengths <500 nm. The present length separation technique is perfectly compatible with various surfactant-based methods for structure separations of SWCNTs and is significant for fabrication of high-performance electronic and optoelectronic devices.

Cavity-enhanced photon indistinguishability at room temperature and telecom wavelengths

Indistinguishable single photons in the telecom-bandwidth of optical fibers are indispensable for long-distance quantum communication. Solid-state single photon emitters have achieved excellent performance in key benchmarks, however, the demonstration of indistinguishability at room-temperature remains a major challenge. Here, we report room-temperature photon indistinguishability at telecom wavelengths from individual nanotube defects in a fiber-based microcavity operated in the regime of incoherent good cavity-coupling. The efficiency of the coupled system outperforms spectral or temporal filtering, and the photon indistinguishability is increased by more than two orders of magnitude compared to the free-space limit. Our results highlight a promising strategy to attain optimized non-classical light sources.

Iterative Strategy for Sorting Single-Chirality Single-Walled Carbon Nanotubes from Aqueous to Organic Systems

Significant advancements in electronic devices and integrated circuits have been facilitated by semiconducting single-walled carbon nanotubes (SWCNTs) sorted by conjugated polymers (CPs). However, the variety of CPs with single-chirality selectivity is limited, and the sorting results are strongly dependent on the chiral distribution of the starting materials. To address this, we develop an iterative strategy to achieve single-chirality SWCNT separation from aqueous to organic systems, based on a multistep tandem extraction technique that allows a gentle and nondestructive separation of surfactants from SWCNTs, ensuring an efficient system transfer. In parallel, we refined the iterative sorting process between CPs. Employing two starting materials with narrow diameter distributions, using three CPs, we successfully sorted out five single-chirality SWCNTs of the (9,5), (8,6), (10,5), (8,7), and (11,3) species in organic systems. This strategy bridges the gap between aqueous and organic separation systems, achieving efficient complementarity between them.

Surfactant-Aided Stabilization of Individual Carbon Nanotubes in Water around the Critical Micelle Concentration

Surfactants are widely used to disperse single-walled carbon nanotubes (SWCNTs) and other nanomaterials for liquid-phase processing and characterization. Traditional techniques, however, demand high surfactant concentrations, often in the range of 1-2 wt/v% of the solution. Here, we show that optimal dispersion efficiency can be attained at substantially lower surfactant concentrations of approximately 0.08 wt/v%, near the critical micelle concentration. This unexpected observation is achieved by introducing "bare" nanotubes into water containing the anionic surfactant sodium deoxycholate (DOC) through a superacid-surfactant exchange process that eliminates the need for ultrasonication. Among the diverse ionic surfactants and charged biopolymers explored, DOC exhibits the highest dispersion efficiency, outperforming sodium cholate, a structurally similar bile salt surfactant containing just one additional oxygen atom compared to DOC. Employing all-atomistic molecular dynamics simulations, we unravel that the greater stabilization by DOC arises from its higher binding affinity to nanotubes and a substantially larger free energy barrier that resists nanotube rebundling. Further, we find that this barrier is nonelectrostatic in nature and does not obey the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability, underscoring the important role of nonelectrostatic dispersion and hydration interactions at the nanoscale, even in the case of ionic surfactants like DOC. These molecular insights advance our understanding of surfactant chemistry at the bare nanotube limit and suggest low-energy, surfactant-efficient solution processing of SWCNTs and potentially other nanomaterials.

Molecular Doping Modulation and Applications of Structure-Sorted Single-Walled Carbon Nanotubes: A Review

Single-walled carbon nanotubes (SWCNTs) that have a reproducible distribution of chiralities or single chirality are among the most competitive materials for realizing post-silicon electronics. Molecular doping, with its non-destructive and fine-tunable characteristics, is emerging as the primary doping approach for the structure-controlled SWCNTs, enabling their eventual use in various functional devices. This review provides an overview of important advances in the area of molecular doping of structure-controlled SWCNTs and their applications. The first part introduces the underlying physical process of molecular doping, followed by a comprehensive survey of the commonly used dopants for SWCNTs to date. Then, it highlights how the convergence of molecular doping and structure-sorting strategies leads to significantly improved functionality of SWCNT-based field-effect transistor arrays, transparent electrodes in optoelectronics, thermoelectrics, and many emerging devices. At last, several challenges and opportunities in this field are discussed, with the hope of shedding light on promoting the practical application of SWCNTs in future electronics.

Understanding the partitioning behavior of single-walled carbon nanotubes using an aqueous two-phase extraction system composed of non-ionic surfactants and polymers

In this work, a Pluronic/Dextran system was developed to discover the mechanism of the aqueous two-phase extraction (ATPE) technique, which is widely employed for the sorting of single-walled carbon nanotubes (SWCNTs) and other types of nanomaterials. The role of the phase-forming components and partitioning modulators was comprehensively investigated to gain greater insights into the differentiation process. The obtained results revealed that sodium dodecyl sulfate and sodium dodecylbenzene sulfonate operated as excellent partitioning modulators, enabling the diameter-based sorting of SWCNTs. Additionally, the data strongly suggested that different densities of various SWCNT species drove the movement of SWCNTs in the ATPE system. Consequently, the largest diameter SWCNTs were first influenced by surfactants and, thus, the nanotubes migrated towards a lower density top phase in the following order (7,5) > (8,3) > (6,5) > (6,4). Based on the in-depth analysis of the partitioning system, a mechanism was proposed that described the method in which the popular ATPE separation technique operates.

Highly-Selective Harvesting of (6,4) SWCNTs Using the Aqueous Two-Phase Extraction Method and Nonionic Surfactants

Monochiral single-walled carbon nanotubes (SWCNTs) are indispensable for advancing the technology readiness level of nanocarbon-based concepts. In recent times, many separation techniques have been developed to obtain specific SWCNTs from raw unsorted materials to catalyze the development in this area. This work presents how the aqueous two-phase extraction (ATPE) method can be enhanced for the straightforward isolation of (6,4) SWCNTs in one step. Introducing nonionic surfactant into the typically employed mixture of anionic surfactants, which drive the partitioning, is essential to increasing the ATPE system's resolution. A thorough analysis of the parameter space by experiments and modeling reveals the underlying interactions between SWCNTs, surfactants, and phase-forming agents, which drive the partitioning. Based on new insight gained on this front, a separation mechanism is proposed. Notably, the developed method is highly robust, which is proven by isolating (6,4) SWCNTs from several raw SWCNT materials, including SWCNT waste generated over the years in the laboratory.

Scalable, Highly Pure, and Diameter-Sorted Boron Nitride Nanotube by Aqueous Polymer Two-Phase Extraction

Boron nitride nanotube (BNNT) has attracted recent attention owing to its exceptional material properties; yet, practical implementation in real-life applications has been elusive, mainly due to the purity issues associated with its large-scale synthesis. Although different purification methods have been discussed so far, there lacks a scalable solution method in the community. In this work, a simple, high-throughput, and scalable purification of BNNT is reported via modification of an established sorting technique, aqueous polymer two-phase extraction. A complete partition mapping of the boron nitride species is established, which enables the segregation of the highly pure BNNT with a major impurity removal efficiency of > 98%. A successful scaling up of the process is illustrated and provides solid evidence of its diameter sorting behavior. Last, towards its macroscopic assemblies, a liquid crystal of the purified BNNT is demonstrated. The effort toward large-scale solution purification of BNNT is believed to contribute significantly to the macroscopic realization of its exceptional properties in the near future.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

A Short Review on Nanostructured Carbon Containing Biopolymer Derived Composites for Tissue Engineering Applications

The development of new scaffolds and materials for tissue engineering is a wide and open realm of material science. Among solutions, the use of biopolymers represents a particularly interesting area of study due to their great chemical complexity that enables creation of specific molecular architectures. However, biopolymers do not exhibit the properties required for direct application in tissue repair-such as mechanical and electrical properties-but they do show very attractive chemical functionalities which are difficult to produce through in vitro synthesis. The combination of biopolymers with nanostructured carbon fillers could represent a robust solution to enhance composite properties, producing composites with new and unique features, particularly relating to electronic conduction. In this paper, we provide a review of the field of carbonaceous nanostructure-containing biopolymer composites, limiting our investigation to tissue-engineering applications, and providing a complete overview of the recent and most outstanding achievements.

Spectroscopic Insights into the Influence of Filling Carbon Nanotubes with Atomic Nanowires for Photophysical and Photochemical Applications

Studying the optical performance of carbon nanotubes (CNTs) filled with guest materials can reveal the fundamental photochemical nature of ultrathin one-dimensional (1D) nanosystems, which are attractive for applications including photocatalysis. Here, we report comprehensive spectroscopic studies of how infiltrated HgTe nanowires (NWs) alter the optical properties of small-diameter (d _t < 1 nm) single-walled carbon nanotubes (SWCNTs) in different environments: isolated in solution, suspended in a gelatin matrix, and heavily bundled in network-like thin films. Temperature-dependent Raman and photoluminescence measurements revealed that the HgTe NW filling can alter the stiffness of SWCNTs and therefore modify their vibrational and optical modes. Results from optical absorption and X-ray photoelectron spectroscopy demonstrated that the semiconducting HgTe NWs did not provide substantial charge transfer to or from the SWCNTs. Transient absorption spectroscopy further highlighted that the filling-induced nanotube distortion can alter the temporal evolution of excitons and their transient spectra. In contrast to previous studies on functionalized CNTs, where electronic or chemical doping often drove changes to the optical spectra, we highlight structural distortion as playing an important role.

Up/Down Tuning of Poly(ionic liquid)s in Aqueous Two-Phase Systems

Thermally induced reversible up/down migration of poly(ionic liquid)s (PILs) in aqueous two-phase systems (ATPSs) was achieved for the first time in this study. Novel ATPSs were fabricated using azobenzene (Azo)- and benzyl (Bn)-modified PILs, and their upper and lower phases could be easily tuned using the grafting degree (GD) of the Azo and Bn groups. Bn-PIL with higher GD_Bn could go up into the upper phase and Azo-PIL come down to the lower phase when the temperature increased (>65 °C); this behavior was reversed at lower temperatures. Moreover, a reversible two-phase/single-phase transition was realized under UV irradiation. Experimental and simulation results revealed that the difference in the hydration capacity between Bn-PIL and Azo-PIL accounted for their unique phase-separation behavior. A versatile platform for fabricating ATPSs with tunable stimuli-responsive behavior can be realized based on our findings, which can broaden their applications in the fields of smart separation systems and functional material development.

van der Waals SWCNT@BN Heterostructures Synthesized from Solution-Processed Chirality-Pure Single-Wall Carbon Nanotubes

Single-wall carbon nanotubes in boron nitride (SWCNT@BN) are one-dimensional van der Waals heterostructures that exhibit intriguing physical and chemical properties. As with their carbon nanotube counterparts, these heterostructures can form from different combinations of chiralities, providing rich structures but also posing a significant synthetic challenge to controlling their structure. Enabled by advances in nanotube chirality sorting, clean removal of the surfactant used for solution processing, and a simple method to fabricate free-standing submonolayer films of chirality pure SWCNTs as templates for the BN growth, we show it is possible to directly grow BN on chirality enriched SWCNTs from solution processing to form van der Waals heterostructures. We further report factors affecting the heterostructure formation, including an accelerated growth rate in the presence of H₂, and significantly improved crystallization of the grown BN, with the BN thickness controlled down to one single BN layer, through the presence of a Cu foil in the reactor. Transmission electron microscopy and electron energy-loss spectroscopic mapping confirm the synthesis of SWCNT@BN from the solution purified nanotubes. The photoluminescence peaks of both (7,5)- and (8,4)-SWCNT@BN heterostructures are found to redshift (by ∼10 nm) relative to the bare SWCNTs. Raman scattering suggests that the grown BN shells pose a confinement effect on the SWCNT core.

Variations in bile salt surfactant structure allow tuning of the sorting of single-wall carbon nanotubes by aqueous two-phase extraction

Being some of the most efficient agents to individually solubilize single-wall carbon nanotubes (SWCNTs), bile salt surfactants (BSS) represent the foundation for the surfactant-based structure sorting and spectroscopic characterization of SWCNTs. In this work, we investigate three BSS in their ability to separate different SWCNT chiral structures by aqueous two-phase extraction (ATPE): sodium deoxycholate (DOC), sodium cholate (SC) and sodium chenodeoxycholate (CDOC). The small difference in their chemical structure (just one hydroxyl group) leads to significant differences in their stacking behavior on SWCNT walls with different diameter and chiral structure that, in turn, has direct consequences for the chiral sorting of SWCNTs using these BSS. By performing several series of systematic ATPE experiments, we reveal that, in general, the stacking of DOC and CDOC is more enantioselective than the stacking of SC on the SWCNT walls, while SC has a clear diameter preference for efficiently solubilizing the SWCNTs in comparison to DOC and CDOC. Moreover, combining sodium dodecylsulfate with SC allows for resolving the ATPE sorting transitions of empty and water-filled SWCNTs for a number of SWCNT chiralities. We also show that addition of SC to combinations of DOC and sodium dodecylbenzenesulfonate can enhance separations of particular chiralities.

Ion-Incorporative, Degradable Nanocellulose Crystal Substrate for Sustainable Carbon-Based Electronics

Electronic wastes from transient electronics accumulate biologically harmful materials with global concern. Recycling these wastes could prevent the deposition of hazardous chemicals and toxic materials to the environment while saving scarce natural compounds and valuable resources. Here, we report a sustainable electronic device, taking advantage of carbon resources and a biodegradable cellulose composite. The device consists of an ambient-stable carbon nanotube as a semiconductor, graphene as electrodes, and a free-standing cellulose filter paper/nanocellulose composite as a dielectric layer. The dual-functional cellulose composite acting simultaneously as a robust substrate and a dielectric is demonstrated, which is compatible with solution device fabrication processes. An optimized channel dimension of 5 mm × 3 mm with the addition of ions that facilitates a charge transport realized a device with an on-current per width of 9.6 μA mm^-1, an on/off ratio >10², a field-effect mobility of 2.03 cm² V^-1 s^-1, and long-term stability over 30 days under ambient conditions. Successful separation of the carbonaceous components via an eco-friendly solution sorting protocol allowed the recycled device to display excellent electronic performance, with a recapture efficiency of 90%. This effort demonstrates a processable, low-cost, and sustainable electronic system that can be applied in the current realm of the semiconducting and sensing industry.

The Effects of Lengths of Flavin Surfactant N-10-Alkyl Side Chains on Promoting Dispersion of a High-Purity and Diameter-Selective Single-Walled Nanotube

Flavin with defined helical self-assembly helps to understand chemical designs for obtaining high-purity semiconducting (s)-single-walled carbon nanotubes (SWNT) in a diameter (d_t)-selective manner for high-end applications. In this study, flavins containing 8, 12, 16, and 20 n-alkyl chains were synthesized, and their single/tandem effects on d_t-selective s-SWNT dispersibility were investigated at isomolarity. Flavins with n-dodecyl and longer chain lengths (FC12, FC16, and FC20) act as good surfactants for stable SWNT dispersions whereas n-octyl flavin (FC8) exhibits poor dispersibility owing to the lack of SWNT buoyancy. When used with small-d_t SWNT, FC8 displays chirality-selective SWNT dispersion. This behavior, along with various flavin helical motifs, prompts the development of criteria for 'side chain length (l_S)' required for stable and d_t-selective SWNT dispersion, which also explains l_S-dependent d_t-enrichment behavior. Moreover, SWNT dispersions with flavins with dodecyl and longer l_S exhibit increased metallic (m)-SWNT, background absorption-contributing carbonaceous impurities (CIs) and preferential selectivity of s-SWNT with slightly larger d_t. The increased CIs that affect the SWNT quantum yield were attributed to a solubility parameter. Furthermore, the effects of flavin l_S, sonication bath temperature, centrifugal speed, and surfactant concentration on SWNT purity and s-/m-SWNT ratio were investigated. A tandem FC8/FC12 provides fine-tuning of d_t-selective SWNT dispersion, wherein the FC8 ratio governs the tendency towards smaller d_t. Kinetic and thermodynamic assemblies of tandem flavins result in different sorting behaviors in which wide d_t-tunability was demonstrated using kinetic assembly. This study highlights the importance of appropriate side chain length and other extrinsic parameters to obtain d_t-selective or high-purity s-SWNT.

Polyoxometalate steric hindrance driven chirality-selective separation of subnanometer carbon nanotubes

Subnanometer single-chirality single-walled carbon nanotubes (SWCNTs) are of particular interest in multiple applications. Inspired by the interdisciplinary combination of redox active polyoxometalates and SWCNTs, here we report a cluster steric hindrance strategy by assembling polyoxometalates on the outer surface of subnanometer SWCNTs via electron transfer and demonstrate the selective separation of monochiral (6,5) SWCNTs with a diameter of 0.75 nm by a commercially available conjugated polymer. The combined use of DFT calculations, TEM, and XPS unveils the mechanism that selective separation is associated with tube diameter-dependent interactions between the tube and clusters. Sonication drives the preferential detachment of polyoxometalate clusters from small-diameter (6,5) SWCNTs, attributable to weak tube–cluster interactions, which enables the polymer wrapping and separation of the released SWCNTs, while strong binding clusters with large-diameter SWCNTs provide steric hindrance and block the polymer wrapping. The polyoxometalate-assisted modulation, which can be rationally customized, provides a universal and robust pathway for the separation of SWCNTs.

We develop a cluster steric hindrance strategy by assembling polyoxometalates on subnanometer single-walled carbon nanotubes (SWCNTs) and demonstrate the selective separation of single-chirality (6,5) SWCNTs via polymer extraction.

Recent Advances in Structure Separation of Single-Wall Carbon Nanotubes and Their Application in Optics, Electronics, and Optoelectronics

Structural control of single-wall carbon nanotubes (SWCNTs) with uniform properties is critical not only for their property modulation and functional design but also for applications in electronics, optics, and optoelectronics. To achieve this goal, various separation techniques have been developed in the past 20 years through which separation of high-purity semiconducting/metallic SWCNTs, single-chirality species, and even their enantiomers have been achieved. This progress has promoted the property modulation of SWCNTs and the development of SWCNT-based optoelectronic devices. Here, the recent advances in the structure separation of SWCNTs are reviewed, from metallic/semiconducting SWCNTs, to single-chirality species, and to enantiomers by several typical separation techniques and the application of the corresponding sorted SWCNTs. Based on the separation procedure, efficiency, and scalability, as well as, the separable SWCNT species, purity, and quantity, the advantages and disadvantages of various separation techniques are compared. Combined with the requirements of SWCNT application, the challenges, prospects, and development direction of structure separation are further discussed.

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.

Reconfiguring Organic Color Centers on the sp2 Carbon Lattice of Single-Walled Carbon Nanotubes

Organic color centers (OCCs) are atomic defects that can be synthetically created in single-walled carbon nanotube hosts to enable the emission of shortwave infrared single photons at room temperature. However, all known chemistries developed thus far to generate these quantum defects produce a variety of bonding configurations, posing a formidable challenge to the synthesis of identical, uniformly emitting color centers. Herein, we show that laser irradiation of the nanotube host can locally reconfigure the chemical bonding of aryl OCCs on (6,5) nanotubes to significantly reduce their spectral inhomogeneity. After irradiation the defect emission narrows in distribution by ∼26% to yield a single photoluminescence peak. We use hyperspectral photoluminescence imaging to follow this structural transformation on the single nanotube level. Density functional theory calculations corroborate our experimental observations, suggesting that the OCCs convert from kinetic structures to the more thermodynamically stable configuration. This approach may enable localized tuning and creation of identical OCCs for emerging applications in bioimaging, molecular sensing, and quantum information sciences.

Enrichment of high-purity large-diameter semiconducting single-walled carbon nanotubes

Semiconducting single-walled carbon nanotubes SWCNTs (s-SWCNTs) are considered one of the most promising alternatives to traditional silicon-based semiconductors. In particular, large-diameter s-SWCNTs (>1.2 nm) exhibit more advantages over small-diameter ones in high-performance electronic applications because of their higher charge carrier mobility and reduced Schottky barrier height. Great efforts have been made to enriching large-diameter s-SWCNTs from mass-produced raw CNTs that contain both metallic SWCNTs and s-SWCNTs. Among separation technologies, the effective and scalable ones are conjugated polymer wrapping (CPW), gel permeation chromatography (GC), aqueous two-phase extraction (ATPE), and density gradient ultracentrifugation (DGU). In this review, we survey recent progress on enriching large-diameter s-SWCNTs using those methods and outline the strategies and challenges in the separation according to the electronic type and chirality of SWCNTs. Finally, we highlight some applications of the enriched large-diameter s-SWCNTs and outlook for the future of SWCNT-based electronic devices.

Enzyme-Responsive Aqueous Two-Phase Systems in a Cationic-Anionic Surfactant Mixture

Enzyme-instructed self-assembly is an increasingly attractive topic owing to its broad applications in biomaterials and biomedicine. In this work, we report an approach to construct enzyme-responsive aqueous surfactant two-phase (ASTP) systems serving as enzyme substrates by using a cationic surfactant (myristoylcholine chloride) and a series of anionic surfactants. Driven by the hydrophobic interaction and electrostatic attraction, self-assemblies of cationic-anionic surfactant mixtures result in biphasic systems containing condensed lamellar structures and coexisting dilute solutions, which turn into homogeneous aqueous phases in the presence of hydrolase (cholinesterase). The enzyme-sensitive ASTP systems reported in this work highlight potential applications in the active control of biomolecular enrichment/release and visual detection of cholinesterase.

Unconventional Thermoelectric Materials for Energy Harvesting and Sensing Applications

Heat is an abundant but often wasted source of energy. Thus, harvesting just a portion of this tremendous amount of energy holds significant promise for a more sustainable society. While traditional solid-state inorganic semiconductors have dominated the research stage on thermal-to-electrical energy conversion, carbon-based semiconductors have recently attracted a great deal of attention as potential thermoelectric materials for low-temperature energy harvesting, primarily driven by the high abundance of their atomic elements, ease of processing/manufacturing, and intrinsically low thermal conductivity. This quest for new materials has resulted in the discovery of several new kinds of thermoelectric materials and concepts capable of converting a heat flux into an electrical current by means of various types of particles transporting the electric charge: (i) electrons, (ii) ions, and (iii) redox molecules. This has contributed to expanding the applications envisaged for thermoelectric materials far beyond simple conversion of heat into electricity. This is the motivation behind this review. This work is divided in three sections. In the first section, we present the basic principle of the thermoelectric effects when the particles transporting the electric charge are electrons, ions, and redox molecules and describe the conceptual differences between the three thermodiffusion phenomena. In the second section, we review the efforts made on developing devices exploiting these three effects and give a thorough understanding of what limits their performance. In the third section, we review the state-of-the-art thermoelectric materials investigated so far and provide a comprehensive understanding of what limits charge and energy transport in each of these classes of materials.

Versatile Types of Inorganic/Organic NIR-IIa/IIb Fluorophores: From Strategic Design toward Molecular Imaging and Theranostics

In vivo imaging in the second near-infrared window (NIR-II, 1000-1700 nm), which enables us to look deeply into living subjects, is producing marvelous opportunities for biomedical research and clinical applications. Very recently, there has been an upsurge of interdisciplinary studies focusing on developing versatile types of inorganic/organic fluorophores that can be used for noninvasive NIR-IIa/IIb imaging (NIR-IIa, 1300-1400 nm; NIR-IIb, 1500-1700 nm) with near-zero tissue autofluorescence and deeper tissue penetration. This review provides an overview of the reports published to date on the design, properties, molecular imaging, and theranostics of inorganic/organic NIR-IIa/IIb fluorophores. First, we summarize the design concepts of the up-to-date functional NIR-IIa/IIb biomaterials, in the order of single-walled carbon nanotubes (SWCNTs), quantum dots (QDs), rare-earth-doped nanoparticles (RENPs), and organic fluorophores (OFs). Then, these novel imaging modalities and versatile biomedical applications brought by these superior fluorescent properties are reviewed. Finally, challenges and perspectives for future clinical translation, aiming at boosting the clinical application progress of NIR-IIa and NIR-IIb imaging technology are highlighted.

Single-Chirality Near-Infrared Carbon Nanotube Sub-Cellular Imaging and FRET Probes

Applications of single-walled carbon nanotubes (SWCNTs) in bioimaging and biosensing have been limited by difficulties with isolating single-chirality nanotube preparations with desired functionalities. Unique optical properties, such as multiple narrow near-infrared bands and several modes of signal transduction, including solvatochromism and FRET, are ideal for live cell/organism imaging and sensing applications. However, internanotube FRET has not been investigated in biological contexts. We developed single-chirality subcellular SWCNT imaging probes and investigated their internanotube FRET capabilities in live cells. To functionalize SWCNTs, we replaced the surfactant coating of aqueous two-phase extraction-sorted single-chirality nanotubes with helical polycarbodiimide polymers containing different functionalities. We achieved single-chirality SWCNT targeting of different subcellular structures, including the nucleus, to enable multiplexed imaging. We also targeted purified (6,5) and (7,6) chiralities to the same structures and observed internanotube FRET within these organelles. This work portends the use of single-chirality carbon nanotube optical probes for applications in biomedical research.

A Spin-Coated Hydrogel Platform Enables Accurate Investigation of Immobilized Individual Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) have been used in a variety of sensing and imaging applications over the past few years due to their unique optical properties. In the solution phase, SWCNTs are employed as near-infrared (NIR) fluorescence-based sensors of target analytes via modulations in emission intensity and/or wavelength. In an effort to lower the limit of detection, research has been conducted into isolating SWCNTs adhered to surfaces for potential single molecule analyte detection. However, it is known that SWCNT fluorescence is adversely affected by the inherently rough surfaces that are conventionally used for their observation (e.g., glass coverslip), potentially interfering with fluorescence-based analyte detection. Here, using a spin-coating method with thin films of alginate and SWCNTs, we demonstrate that a novel hydrogel platform can be created to investigate immobilized individual SWCNTs without significantly perturbing their optical properties as compared to solution-phase values. In contrast to the glass coverslip, which red-shifted DNA-functionalized (6,5)-SWCNTs by an average of 3.4 nm, the hydrogel platform reported emission wavelengths that statistically matched the solution-phase values. Additionally, the heterogeneity in the wavelength measurements, as determined from the width of created histograms, was reduced nearly by a factor of 3 for the SWCNTs in the hydrogel platform when compared to glass coverslips. Using long SWCNTs, i.e., those with an average length above the diffraction limit of our microscope, we show that a glass coverslip can induce optical heterogeneity along the length of a single SWCNT regardless of its surface functionalization. This is again significantly mitigated when examining the long SWCNTs in the hydrogel platform. Finally, we show that upon the addition of a model analyte (calcium chloride), the optical response can be spatially resolved along the length of a single SWCNT, enabling localized analyte detection on the surface of a single nanoscale sensor.

DNA-Directed Assembly of Carbon Nanotube-Protein Hybrids

Here, we report the controlled assembly of SWCNT–GFP hybrids employing DNA as a linker. Two distinct, enriched SWCNTs chiralities, (6,5), (7,6), and an unsorted SWCNT solution, were selectively functionalized with DNA and hybridized to a complementary GFP^DNA conjugate. Atomic force microscopy images confirmed that GFP attachment occurred predominantly at the terminal ends of the nanotubes, as designed. The electronic coupling of the proteins to the nanotubes was confirmed via in-solution fluorescence spectroscopy, that revealed an increase in the emission intensity of GFP when linked to the CNTs.

Selective Chemistry-Based Separation of Semiconducting Single-Walled Carbon Nanotubes and Alignment of the Nanotube Array Network under Electric Field for Field-Effect Transistor Applications

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are considered as a replacement for silicon in field-effect transistors (FETs), solar cells, logic circuits, and so forth, because of their outstanding electronic, optical, and mechanical properties. Herein, we have studied the reaction of pristine SWCNTs dispersed in a pluronic F-68 (PF-68) polymer solution with para-amino diphenylamine diazonium sulfate (PADDS) to separate nanotubes based on their metallicity. The preferential selectivity of the reactions was monitored by changes in the semiconducting (S22 and S33) and metallic (M11) bands by ultraviolet-visible-near infrared spectroscopy. Metallic selectivity depended on the concentrations of PADDS, reaction time, and the solution pH. Furthermore, separation of pure s-SWCNTs was confirmed by Raman spectroscopy and Fourier-transform infrared spectroscopy. After the removal of metallic SWCNTs, direct current electric field was applied to the pure s-SWCNT solution, which effectively directed the nanotubes to align in one direction as nanotube arrays with a longer length and high density. After that, electrically aligned s-SWCNT solution was cast on a silicon substrate, and the length of the nanotube arrays was measured as ∼2 to ∼14 μm with an areal density of ∼2 to ∼20 tubes/μm of s-SWCNTs. Next, electrically aligned s-SWCNT arrays were deposited on the channel of the FET device by drop-casting. Field-emission scanning electron microscopy and electrical measurements have been carried out to test the performance of the aligned s-SWCNTs/FETs. The fabricated FETs with a channel length of 10 μm showed stable electrical properties with a field-effect mobility of 30.4 cm2/Vs and a log10 (I on/I off) current ratio of 3.96. We envisage that this new chemical-based separation method and electric field-assisted alignment could be useful to obtain a high-purity and aligned s-SWCNT array network for the fabrication of high-performance FETs to use in digital and analog electronics.

Thermoelectric Properties of Thin Films from Sorted Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) remain one of the most promising materials of our times. One of the goals is to implement semiconducting and metallic SWCNTs in photonics and microelectronics, respectively. In this work, we demonstrated how such materials could be obtained from the parent material by using the aqueous two-phase extraction method (ATPE) at a large scale. We also developed a dedicated process on how to harvest the SWCNTs from the polymer matrices used to form the biphasic system. The technique is beneficial as it isolates SWCNTs with high purity while simultaneously maintaining their surface intact. To validate the utility of the metallic and semiconducting SWCNTs obtained this way, we transformed them into thin free-standing films and characterized their thermoelectric properties.

Supramolecular Chemistry-Based One-Pot High-Efficiency Separation of Solubilizer-Free Pure Semiconducting Single-Walled Carbon Nanotubes: Molecular Strategy and Mechanism

Single-walled carbon nanotubes (SWCNTs) have the potential to revolutionize nanoscale electronics and power sources; however, their low purity and high separation cost limit their use in practical applications. Here we present a supramolecular chemistry-based one-pot, less expensive, scalable, and highly efficient separation of a solubilizer/adsorbent-free pure semiconducting SWCNT (sc-SWCNT) using flavin/isoalloxazine analogues with different substituents. On the basis of both experimental and computational simulations (DFT study), we have revealed the molecular requirements of the solubilizers as well as provided a possible mechanism for such a highly efficient selective sc-SWCNT separation. The present sorting method is very simple (one-pot) and gives a promising sc-SWCNT separation methodology. Thus, the study provides insight for the molecular design of an sc-SWCNT solubilizer with a high (n,m)-chiral selectivity, which benefits many areas including semiconducting nanoelectronics, thermoelectric, bio and energy materials, and devices using solubilizer-free very pure sc-SWCNTs.

Chirality Pure Carbon Nanotubes: Growth, Sorting, and Characterization

Single-walled carbon nanotubes (SWCNTs) have been attracting tremendous attention owing to their structure (chirality) dependent outstanding properties, which endow them with great potential in a wide range of applications. The preparation of chirality-pure SWCNTs is not only a great scientific challenge but also a crucial requirement for many high-end applications. As such, research activities in this area over the last two decades have been very extensive. In this review, we summarize recent achievements and accumulated knowledge thus far and discuss future developments and remaining challenges from three aspects: controlled growth, postsynthesis sorting, and characterization techniques. In the growth part, we focus on the mechanism of chirality-controlled growth and catalyst design. In the sorting part, we organize and analyze existing literature based on sorting targets rather than methods. Since chirality assignment and quantification is essential in the study of selective preparation, we also include in the last part a comprehensive description and discussion of characterization techniques for SWCNTs. It is our view that even though progress made in this area is impressive, more efforts are still needed to develop both methodologies for preparing ultrapure (e.g., >99.99%) SWCNTs in large quantity and nondestructive fast characterization techniques with high spatial resolution for various nanotube samples.

Separation of Specific Single-Enantiomer Single-Wall Carbon Nanotubes in the Large-Diameter Regime

The enantiomer-level isolation of single-walled carbon nanotubes (SWCNTs) in high concentration and with high purity for nanotubes greater than 1.1 nm in diameter is demonstrated using a two-stage aqueous two-phase extraction (ATPE) technique. In total, five different nanotube species of ∼1.41 nm diameter are isolated, including both metallics and semiconductors. We characterize these populations by absorbance spectroscopy, circular dichroism spectroscopy, resonance Raman spectroscopy, and photoluminescence mapping, revealing and substantiating mod-dependent optical dependencies. Using knowledge of the competitive adsorption of surfactants to the SWCNTs that controls partitioning within the ATPE separation, we describe an advanced acid addition methodology that enables the fine control of the separation of these select nanotubes. Furthermore, we show that endohedral filling is a previously unrecognized but important factor to ensure a homogeneous starting material and further enhance the separation yield, with the best results for alkane-filled SWCNTs, followed by empty SWCNTs, with the intrinsic inhomogeneity of water-filled SWCNTs causing them to be worse for separations. Lastly, we demonstrate the potential use of these nanotubes in field-effect transistors.

Toward Complete Resolution of DNA/Carbon Nanotube Hybrids by Aqueous Two-Phase Systems

Sequence-dependent interactions between DNA and single-wall carbon nanotubes (SWCNTs) are shown to provide resolution for the atomic-structure-based sorting of DNA-wrapped SWCNTs. Previous studies have demonstrated that aqueous two-phase (ATP) systems are very effective for sorting DNA-wrapped SWCNTs (DNA-SWCNTs). However, most separations have been carried out with a polyethylene glycol (PEG)/polyacrylamide (PAM) ATP system, which shows severe interfacial trapping for many DNA-SWCNT dispersions, resulting in significant material loss and limiting multistage extraction. Here, we report a study of several new ATP systems for sorting DNA-SWCNTs. We have developed a convenient method to explore these systems without knowledge of the corresponding phase diagram. We further show that the molecular weight of the polymer strongly affects the partition behavior and separation results for DNA-SWCNTs in PEG/dextran (DX) ATP systems. This leads to the identification of the PEG1.5kDa/DX250kDa ATP system as an effective vehicle for the chirality separation of DNA-SWCNTs. Additionally, this ATP system exhibits greatly reduced interfacial trapping, enabling for the first time continuous multistep sorting of four species of SWCNTs from a single dispersion. Enhanced stability of DNA-SWCNTs in the PEG1.5kDa/DX250kDa ATP system also allows us to investigate pH dependent sorting of SWCNTs wrapped by C-rich sequences. Our observations suggest that hydrogen bonding may form between the DNA bases at lower pH, enabling a more ordered wrapping structure on the SWCNTs and improvement in sorting (11,0). Together, these findings reveal that the new ATP system is suitable for searching DNA sequences leading toward more complete resolution of DNA-SWCNTs. A new concept of "resolving sequences", evolved from the old notion of "recognition sequences", is proposed to describe a broader range of behaviors of DNA/SWCNT interactions and sorting.

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.

Multi-Drug/Gene NASH Therapy Delivery and Selective Hyperspectral NIR Imaging Using Chirality-Sorted Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) can serve as drug delivery/biological imaging agents, as they exhibit intrinsic fluorescence in the near-infrared, allowing for deeper tissue imaging while providing therapeutic transport. In this work, CoMoCAT (Cobalt Molybdenum Catalyst) SWCNTs, chirality-sorted by aqueous two-phase extraction, are utilized for the first time to deliver a drug/gene combination therapy and image each therapeutic component separately via chirality-specific SWCNT fluorescence. Each of (7,5) and (7,6) sorted SWCNTs were non-covalently loaded with their specific payload: the PI3 kinase inhibitor targeting liver fibrosis or CCR5 siRNA targeting inflammatory pathways with the goal of addressing these processes in nonalcoholic steatohepatitis (NASH), ultimately to prevent its progression to hepatocellular carcinoma. PX-866-(7,5) SWCNTs and siRNA-(7,6) SWCNTs were each imaged via characteristic SWCNT emission at 1024/1120 nm in HepG2 and HeLa cells by hyperspectral fluorescence microscopy. Wavelength-resolved imaging verified the intracellular transport of each SWCNT chirality and drug release. The therapeutic efficacy of each formulation was further demonstrated by the dose-dependent cytotoxicity of SWCNT-bound PX-866 and >90% knockdown of CCR5 expression with SWCNT/siRNA transfection. This study verifies the feasibility of utilizing chirality-sorted SWCNTs for the delivery and component-specific imaging of combination therapies, also suggesting a novel nanotherapeutic approach for addressing the progressions of NASH to hepatocellular carcinoma.

Self-Sorting of 10-µm-Long Single-Walled Carbon Nanotubes in Aqueous Solution

Single-walled carbon nanotubes (SWCNTs) are a class of 1D nanomaterials that exhibit extraordinary electrical and optical properties. However, many of their fundamental studies and practical applications are stymied by sample polydispersity. SWCNTs are synthesized in bulk with broad structural (chirality) and geometrical (length and diameter) distributions; problematically, all known post-synthetic sorting methods rely on ultrasonication, which cuts SWCNTs into short segments (typically <1 µm). It is demonstrated that ultralong (>10 µm) SWCNTs can be efficiently separated from shorter ones through a solution-phase "self-sorting". It is shown that thin-film transistors fabricated from long semiconducting SWCNTs exhibit a carrier mobility as high as ≈90 cm² V^-1 s^-1 , which is ≈10 times higher than those which use shorter counterparts and well exceeds other known materials such as organic semiconducting polymers (<1 cm² V^-1 s^-1 ), amorphous silicon (≈1 cm² V^-1 s^-1 ), and nanocrystalline silicon (≈50 cm² V^-1 s^-1 ). Mechanistic studies suggest that this self-sorting is driven by the length-dependent solution phase behavior of rigid rods. This length sorting technique shows a path to attain long-sought ultralong, electronically pure carbon nanotube materials through scalable solution processing.

Photosensitization of Single-Crystal Oxide Substrates with Quantum Confined Semiconductors

Dye-sensitized solar cells have been studied for many years as a potential inexpensive and scalable alternative to silicon solar cells. They have recently expanded their list of photosensitizers to include quantum dots. In recent years, there has been substantial progress in the field of quantum dot solar cells, with certified efficiencies now reaching 13.4%. Fundamental studies on nanomaterial/semiconductor electrode coupling have led to a deeper understanding of photoinduced electron-transfer processes that are important for both of these devices. This Feature Article will highlight the use of a model system, nanomaterials sensitizing single-crystal oxide substrates, that is useful for investigating how changes in nanomaterial shape, dimensionality, size, and local environment affect the photoinduced charge separation efficiency.

Endowing a Light-Inert Aqueous Surfactant Two-Phase System with Photoresponsiveness by Introducing a Trojan Horse

The ability to modulate the phase behavior of an aqueous surfactant two-phase (ASTP) system reversibly with light is of great importance in both fundamental and applied science. Thus far, most of the light-responsive ASTP systems are based on covalent modification of the component molecules. In this article, we, for the first time, achieve photoresponsiveness in a light-inert ASTP system by physically introducing a phototrigger with the aid of a Trojan horse. The ASTP system formed from sodium laurate (SL) and dodecyltributylammonium bromide (DBAB) does not show light responsiveness by physically mixing a light-responsive azobenzene compound, 2-(4-(phenyldiazenyl)phenoxy)acetate sodium (Azo). However, in the presence of the host-guest complex SL@β-CD formed from β-CD and sodium laurate (SL), the ASTP turns quickly into a homogeneous suspension under visible light, which recovers to the original ASTP state again under 365 nm UV irradiation. Because the SL@β-CD complex exists harmonically with the ASTP system, it can be viewed as a "Trojan horse" that becomes fatal only when the encapsulated SL is triggered to release. In the presence of the Trojan horse, the photoresponsiveness of the ASTP system can be manipulated reversibly by alternatively exerting UV and visible light. Using this strategy, we are able to collect trace amounts of oily components from water. The current strategy points out that it is possible to achieve light responsiveness in light-inert systems with a physical method, which may have profound impact on both the fundamental and applied science.

Manufacture of Networks from Large Diameter Single-Walled Carbon Nanotubes of Particular Electrical Character

We have demonstrated that the aqueous two-phase extraction (ATPE) can differentiate between large diameter single-walled carbon nanotubes (CNTs) by electrical character. Introduction of "hydration modulators" to the ATPE machinery has enabled us to isolate metallic and semiconducting CNTs with ease. We have also shown that often there is a trade-off between the purity of the obtained fractions and the ability to separate both metallic and semiconducting CNTs at the same time. To isolate the separated CNTs from the matrices, we have proposed a method based on precipitation and hydrolysis, which can eliminate the need to use lengthy dialysis routines. In the final step, we prepared thin free-standing films from the sorted material and probed how electrical charge is transported through such macroscopic ensembles.

Solution-Processing of High-Purity Semiconducting Single-Walled Carbon Nanotubes for Electronics Devices

High-purity semiconducting single-walled carbon nanotubes (s-SWCNTs) are of paramount significance for the construction of next-generation electronics. Until now, a number of elaborate sorting and purification techniques for s-SWCNTs have been developed, among which solution-based sorting methods show unique merits in the scale production, high purity, and large-area film formation. Here, the recent progress in the solution processing of s-SWCNTs and their application in electronic devices is systematically reviewed. First, the solution-based sorting and purification of s-SWCNTs are described, and particular attention is paid to the recent advance in the conjugated polymer-based sorting strategy. Subsequently, the solution-based deposition and morphology control of a s-SWCNT thin film on a surface are introduced, which focus on the strategies for network formation and alignment of SWCNTs. Then, the recent advances in electronic devices based on s-SWCNTs are reviewed with emphasis on nanoscale s-SWCNTs' high-performance integrated circuits and s-SWCNT-based thin-film transistors (TFT) array and circuits. Lastly, the existing challenges and development trends for the s-SWCNTs and electronic devices are briefly discussed. The aim is to provide some useful information and inspiration for the sorting and purification of s-SWCNTs, as well as the construction of electronic devices with s-SWCNTs.

Separation of Small-Diameter Single-Walled Carbon Nanotubes in One to Three Steps with Aqueous Two-Phase Extraction

An aqueous two-phase extraction (ATPE) technique capable of separating small-diameter single-walled carbon nanotubes in one, two, or at the most three steps is presented. Separation is performed in the well-studied two-phase system containing polyethylene glycol and dextran, but it is achieved without changing the global concentration or ratio of cosurfactants. Instead, the technique is reliant upon the different surfactant shell around each nanotube diameter at a fixed surfactant concentration. The methodology to obtain a single set of surfactant conditions is provided, and strategies to optimize these for other diameter regimes are discussed. In total, 11 different chiralities in the diameter range 0.69-0.91 nm are separated. These include semiconducting and both armchair and nonarmchair metallic nanotube species. Titration of cosurfactant suspensions reveal separation to be driven by the pH of the suspension with each ( n, m) species partitioning at a fixed pH. This allows for an ( n, m) separation approach to be presented that is as simple as pipetting known volumes of acid into the ATPE system.

Single-step isolation of carbon nanotubes with narrow-band light emission characteristics

Lack of necessary degree of control over carbon nanotube (CNT) structure has remained a major impediment factor for making significant advances using this material since it was discovered. Recently, a wide range of promising sorting methods emerged as an antidote to this problem, all of which unfortunately have a multistep nature. Here we report that desired type of CNTs can be targeted and isolated in a single step using modified aqueous two-phase extraction. We achieve this by introducing hydration modulating agents, which are able to tune the arrangement of surfactants on their surface, and hence make selected CNTs highly hydrophobic or hydrophilic. This allows for separation of minor chiral species from the CNT mixture with up to 99.7 ± 0.02% selectivity without the need to carry out any unnecessary iterations. Interestingly, our strategy is also able to enrich the optical emission from CNTs under selected conditions.

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.

Probing the mechanics of aqueous two-phase extraction using large diameter single-walled carbon nanotubes

We have demonstrated that large diameter (1.8 ± 0.4 nm) carbon nanotubes (CNTs) can be separated by means of the aqueous two-phase extraction (ATPE). This rapid and convenient tool has enabled us to isolate fractions of particular CNT diameter distribution. We have shown how a range of parameters can be used to fine tune the characteristics of the isolated material. Interestingly, by varying the pH of the medium, we have suppressed the extraction of low diameter CNTs and only large diameter CNTs were obtained. A number of other factors such as selected surfactant concentration steps, temperature or amount of starting CNT material have been found to have a significant effect on the end result of the CNT differentiation. The findings have provided us with more insight regarding the underlying mechanics of ATPE for processing polydisperse CNT mixtures.

Diameter-Dependent Optical Absorption and Excitation Energy Transfer from Encapsulated Dye Molecules toward Single-Walled Carbon Nanotubes

The hollow cores and well-defined diameters of single-walled carbon nanotubes (SWCNTs) allow for creation of one-dimensional hybrid structures by encapsulation of various molecules. Absorption and near-infrared photoluminescence-excitation (PLE) spectroscopy reveal that the absorption spectrum of encapsulated 1,3-bis[4-(dimethylamino)phenyl]-squaraine dye molecules inside SWCNTs is modulated by the SWCNT diameter, as observed through excitation energy transfer (EET) from the encapsulated molecules to the SWCNTs, implying a strongly diameter-dependent stacking of the molecules inside the SWCNTs. Transient absorption spectroscopy, simultaneously probing the encapsulated dyes and the host SWCNTs, demonstrates this EET, which can be used as a route to diameter-dependent photosensitization, to be fast (sub-picosecond). A wide series of SWCNT samples is systematically characterized by absorption, PLE, and resonant Raman scattering (RRS), also identifying the critical diameter for squaraine filling. In addition, we find that SWCNT filling does not limit the selectivity of subsequent separation protocols (including polyfluorene polymers for isolating only semiconducting SWCNTs and aqueous two-phase separation for enrichment of specific SWCNT chiralities). The design of these functional hybrid systems, with tunable dye absorption, fast and efficient EET, and the ability to remove all metallic SWCNTs by subsequent separation, demonstrates potential for implementation in photoconversion devices.