DNA-controlled partition of carbon nanotubes in polymer aqueous two-phase systems.

Sorting single-wall carbon nanotubes (SWCNTs) of different chiralities is both scientifically interesting and technologically important. Recent studies have shown that polymer aqueous two-phase extraction is a very effective way to achieve nanotube sorting. However, works published to date have demonstrated only separation of surfactant-dispersed SWCNTs, and the mechanism of chirality-dependent SWCNT partition is not well understood. Here we report a systematic study of spontaneous partition of DNA-wrapped SWCNTs in several polymer aqueous two-phase systems. We show that partition of DNA-SWCNT hybrids in a given polymer two-phase system is strongly sequence-dependent and can be further modulated by salt and polymer additives. With the proper combination of DNA sequence, polymer two-phase system, and partition modulators, as many as 15 single-chirality nanotube species have been effectively purified from a synthetic mixture. As an attempt to provide a unified partition mechanism of SWCNTs dispersed by surfactants and by DNA, we present a qualitative analysis of solvation energy for SWCNT colloids in a polymer-modified aqueous phase. Our observation and analysis highlight the sensitive dependence of the hydration energy on the spatial distribution of hydrophilic functionalities.

Single-step total fractionation of single-wall carbon nanotubes by countercurrent chromatography.

Development of simple processes to fractionate synthetic mixtures of single-wall carbon nanotubes (SWCNTs) into individual species is crucial to many applications. Existing methods for single-chirality SWCNT purification are cumbersome, often requiring multiple steps and different conditions for different species. Here, we report a method to achieve total fractionation of a synthetic SWCNT mixture by countercurrent chromatography, resulting in purification of many single-chirality SWCNT species in a single run. This method is based on a tunable partition of sodium deoxycholate dispersed SWCNTs in a polyethylene glycol/dextran aqueous two-phase system. By running the mobile phase with 0.02% of sodium deoxycholate and a gradient of sodium dodecyl sulfate from 0.1% to 0.7% (w/w), we observe clear diameter-dependent elution, with ∼ 90% total recovery. Among all the fractions collected, a number of them are enriched in single-chirality (9,4), (7,5), (7,6), (8,3), (6,5) species, while most of the remaining ones contain no more than 2-3 major species. We also observe strong (n,m)-dependent elution peak width due to the enantiomer-resolved partition. These results demonstrate countercurrent chromatography (CCC) as an effective way to obtain high purity (n, m) species, and suggest the potential of CCC as an analytical tool for chirality distribution mapping of synthetic SWCNT mixtures.

Rod hydrodynamics and length distributions of single-wall carbon nanotubes using analytical ultracentrifugation.

Because of their repetitive chemical structure, extreme rigidity, and the separability of populations with varying aspect ratio, SWCNTs are excellent candidates for use as model rodlike colloids. In this contribution, the sedimentation velocities of length and density sorted single-wall carbon nanotubes (SWCNTs) are compared to predictions from rod hydrodynamic theories of increasing complexity over a range of aspect ratios from <50 to >400. Independently measuring all contributions to the sedimentation velocity besides the shape factor, excellent agreement is found between the experimental findings and theoretical predictions for numerically calculated hydrodynamic radius values and for multiterm analytical expansion approximations; values for the hydrodynamic radii in these cases are additionally found to be consistent with the apparent hydrated particle radius determined independently by buoyancy measurements. Lastly, we utilize this equivalency to calculate the apparent distribution of nanotube lengths in each population from their sedimentation coefficient distribution without adjustable parameters, achieving excellent agreement with distributions from atomic force microscopy. The method developed herein provides an alternative for the ensemble measurement of SWCNT length distributions and others rodlike particles.

Spontaneous partition of carbon nanotubes in polymer-modified aqueous phases.

The distribution of nanoparticles in different aqueous environments is a fundamental problem underlying a number of processes, ranging from biomedical applications of nanoparticles to their effects on the environment, health, and safety. Here, we study distribution of carbon nanotubes (CNTs) in two immiscible aqueous phases formed by the addition of polyethylene glycol (PEG) and dextran. This well-defined model system exhibits a strikingly robust phenomenon: CNTs spontaneously partition between the PEG- and the dextran-rich phases according to nanotube's diameter and metallicity. Thermodynamic analysis suggests that this chirality-dependent partition is determined by nanotube's intrinsic hydrophobicity and reveals two distinct regimes in hydrophobicity-chirality relation: a small diameter (<1 nm) regime, where curvature effect makes larger diameter tubes more hydrophobic than small diameter ones, and a large diameter (>1.2 nm) regime, where nanotube's polarizability renders semiconducting tubes more hydrophobic than metallic ones. These findings reveal a general rule governing CNT behaviors in aqueous phase and provide an extremely simple way to achieve spatial separation of CNTs by their electronic structures.

High-resolution length fractionation of surfactant-dispersed carbon nanotubes.

Length fractionation of colloidal single-wall carbon nanotube (SWCNT) dispersions is required for many studies. Size-exclusion chromatography (SEC) has been developed as a reliable method for high-resolution length fractionation of DNA-dispersed SWCNTs but has not been applied to surfactant-dispersed SWCNTs due to their lower dispersion stability and tendency to adsorb onto SEC stationary phases. Here, we report that SEC length fractionation can be achieved for bile salt dispersed SWCNTs by using porous silica-based beads as the stationary phase and bile salt solution as the mobile phase. We demonstrate that the SEC length sorting method can be combined with existing ultracentrifugation SWCNT sorting methods to produce "orthogonally sorted" samples, including length sorted semiconducting SWCNTs, which are important for electronics applications as well as length sorted empty-core SWCNTs. Importantly, we show that unlike simple length fractionation by SEC or any other method, orthogonal sorting produces samples of consistent quality for different length fractions, with similar UV-vis-nearIR absorption and Raman spectral features.

Electrostatically driven interactions between hybrid DNA-carbon nanotubes.

Single-stranded DNA is able to wrap around single-wall carbon nanotubes (CNT) and form stable DNA-CNT hybrids that are highly soluble in solution. Here we report quantitative measurements and analysis of the interactions between DNA-CNT hybrids at low salts. Condensation of DNA-CNT hybrids by neutral osmolytes leads to liquid crystalline phases, and varying the osmotic pressure modulates the interhybrid distance that is determined by x-ray diffraction. Thus obtained force-distance dependencies of DNA-CNT hybrids show a remarkable resemblance to that of double-stranded DNA with differences that can be largely accounted for by their different diameters. This establishes their common physical nature of electrostatically driven interactions. Quantitative modeling further reveals the roles of hydration in mediating the interhybrid forces within the last nanometer of surface separation. This study also suggests the utility of osmotic pressure to control DNA-CNT assemblies at subnanometer precision.

Concentration measurement of length-fractionated colloidal single-wall carbon nanotubes.

The determination of the carbon concentration of single-wall carbon nanotubes (SWCNTs) in a given dispersion is a basic requirement for many studies. The commonly used optical absorption-based concentration measurement is complicated by the spectral change due to variations in nanotube chirality and length. In particular, the origin of the observed length-dependent spectral change and its effect on concentration determination has been the subject of considerable debate. Here, we use length-fractionated DNA-wrapped SWCNTs to establish the relationship between SWCNT carbon concentration and optical absorption spectra by directly quantifying the amount of wrapping DNA and, independently, the DNA/carbon nanotube mass ratio. We find that SWCNT carbon concentrations derived from either the E(11) peak or spectral baseline deviate significantly from the SWCNT carbon concentrations derived from the DNA measurement method. Instead, SWCNT carbon concentrations derived from the spectral integration of the E(11) optical transition region match most closely with the DNA-derived SWCNT carbon concentrations. We also observe that shorter SWCNT fractions contain more curved carbon nanotubes, and propose that these defective nanotubes are largely responsible for the observed spectral variation with nanotube length.

Evolution of DNA sequences toward recognition of metallic armchair carbon nanotubes.

The armchair carbon nanotube is an ideal system to study fundamental physics in one-dimensional metals and potentially a superb material for applications such as electrical power transmission. Synthesis and purification efforts to date have failed to produce a homogeneous population of such a material. Here we report evolutionary strategies to find DNA sequences for the recognition and subsequent purification of (6,6) and (7,7) armchair species from synthetic mixtures. The new sequences were derived by single-point scanning mutation and sequence motif variation of previously identified ones for semiconducting tubes. Optical absorption spectroscopy of the purified armchair tubes revealed well-resolved first- and second-order electronic transitions accompanied by prominent sideband features that have neither been predicted nor observed previously. Resonance Raman spectroscopy showed a single Lorentzian peak for the in-plane carbon-carbon stretching mode (G band) of the armchair tubes, repudiating the common practice of using such a line shape to infer the absence of metallic species. Our work demonstrates the exquisite sensitivity of DNA to nanotube metallicity and makes the long-anticipated pure armchair tubes available as seeds for their mass amplification.

A facile and low-cost length sorting of single-wall carbon nanotubes by precipitation and applications for thin-film transistors.

Semiconducting single-wall carbon nanotubes (SWCNTs) with long lengths are highly desirable for many applications such as thin-film transistors and circuits. Previously reported length sorting techniques usually require sophisticated instrumentation and are hard to scale up. In this paper, we report for the first time a general phenomenon of a length-dependent precipitation of surfactant-dispersed carbon nanotubes by polymers, salts, and their combinations. Polyelectrolytes such as polymethacrylate (PMAA) and polystyrene sulfonate (PSS) are found to be especially effective on cholate and deoxycholate dispersed SWCNTs. By adding PMAA to these nanotube dispersions in a stepwise fashion, we have achieved nanotube precipitation in a length-dependent order: first nanotubes with an average length of 650 nm, and then successively of 450 nm, 350 nm, and 250 nm. A similar effect of nanotube length sorting has also been observed for PSS. To demonstrate the utility of the length fractionation, the 650 nm-long nanotube fraction was subjected to an aqueous two-phase separation to obtain semiconducting enriched nanotubes. Thin-film transistors fabricated with the resulting semiconducting SWCNTs showed a carrier mobility up to 18 cm(2) (V s)(-1) and an on/off ratio up to 10(7). Our result sheds new light on the phase behavior of aqueous nanotube dispersions under high concentrations of polymers and salts, and offers a facile, low-cost, and scalable method to produce length sorted semiconducting nanotubes for macroelectronics applications.

Isolation of specific small-diameter single-wall carbon nanotube species via aqueous two-phase extraction.

Aqueous two-phase extraction is demonstrated to enable isolation of single semiconducting and metallic single-wall carbon nanotube species from a synthetic mixture. The separation is rapid and robust, with remarkable tunability via modification of the surfactant environment set for the separation.

Analyzing surfactant structures on length and chirality resolved (6,5) single-wall carbon nanotubes by analytical ultracentrifugation.

The structure and density of the bound interfacial surfactant layer and associated hydration shell were investigated using analytical ultracentrifugation for length and chirality purified (6,5) single-wall carbon nanotubes (SWCNTs) in three different bile salt surfactant solutions. The differences in the chemical structures of the surfactants significantly affect the size and density of the bound surfactant layers. As probed by exchange of a common parent nanotube population into sodium deoxycholate, sodium cholate, or sodium taurodeoxycholate solutions, the anhydrous density of the nanotubes was least for the sodium taurodeoxycholate surfactant, and the absolute sedimentation velocities greatest for the sodium cholate and sodium taurodeoxycholate surfactants. These results suggest that the thickest interfacial layer is formed by the deoxycholate, and that the taurodeoxycholate packs more densely than either sodium cholate or deoxycholate. These structural differences correlate well to an observed 25% increase in fluorescence intensity relative to the cholate surfactant for deoxycholate and taurodeoxycholate dispersed SWCNTs displaying equivalent absorbance spectra. Separate sedimentation velocity experiments including the density modifying agent iodixanol were used to establish the buoyant density of the (6,5) SWCNT in each of the bile salt surfactants; from the difference in the buoyant and anhydrous densities, the largest hydrated diameter is observed for sodium deoxycholate. Understanding the effects of dispersant choice and the methodology for measurement of the interfacial density and hydrated diameter is critical for rationally advancing separation strategies and applications of nanotubes.

Molecular-crowding-induced clustering of DNA-wrapped carbon nanotubes for facile length fractionation.

Emerging applications require single-wall carbon nanotubes (SWCNTs) of well-defined length. Yet the use of length-defined SWCNTs is limited, in part due to the lack of an easily accessible materials preparation method. Here, we present a new strategy for SWCNT length fractionation based on molecular crowding induced cluster formation. We show that the addition of polyethylene glycol (PEG) as a crowding agent into DNA-wrapped SWCNT dispersion leads to the formation of reversible, nematic, and rodlike microclusters, which can be collected by gentle centrifugation. Since shorter SWCNTs form clusters at higher polyethylene glycol concentration, gradual increase in PEG concentration results in length fractionated SWCNTs. Using atomic force microscopy (AFM) we show that fractions with average lengths of 60-500 nm and standard deviations of 30-40% can be obtained. The concept of molecular-crowding-based fractionation should be applicable to other nanoparticle dispersions.

Protein-directed assembly of arbitrary three-dimensional nanoporous silica architectures.

Through precise control of nanoscale building blocks, such as proteins and polyamines, silica condensing microorganisms are able to create intricate mineral structures displaying hierarchical features from nano- to millimeter-length scales. The creation of artificial structures of similar characteristics is facilitated through biomimetic approaches, for instance, by first creating a bioscaffold comprised of silica condensing moieties which, in turn, govern silica deposition into three-dimensional (3D) structures. In this work, we demonstrate a protein-directed approach to template silica into true arbitrary 3D architectures by employing cross-linked protein hydrogels to controllably direct silica condensation. Protein hydrogels are fabricated using multiphoton lithography, which enables user-defined control over template features in three dimensions. Silica deposition, under acidic conditions, proceeds throughout protein hydrogel templates via flocculation of silica nanoparticles by protein molecules, as indicated by dynamic light scattering (DLS) and time-dependent measurements of elastic modulus. Following silica deposition, the protein template can be removed using mild thermal processing yielding high surface area (625 m(2)/g) porous silica replicas that do not undergo significant volume change compared to the starting template. We demonstrate the capabilities of this approach to create bioinspired silica microstructures displaying hierarchical features over broad length scales and the infiltration/functionalization capabilities of the nanoporous silica matrix by laser printing a 3D gold image within a 3D silica matrix. This work provides a foundation to potentially understand and mimic biogenic silica condensation under the constraints of user-defined biotemplates and further should enable a wide range of complex inorganic architectures to be explored using silica transformational chemistries, for instance silica to silicon, as demonstrated herein.

Tricontinuous Cubic Nanostructure and Pore Size Patterning in Mesostructured Silica Films Templated with Glycerol Monooleate.

The fabrication of nanostructured films possessing tricontinuous minimal surface mesophases with well-defined framework and pore connectivity remains a difficult task. As a new route to these structures, we introduce glycerol monooleate (GMO) as a template for evaporation-induced self-assembly. As deposited, a nanostructured double gyroid phase is formed, as indicated by analysis of grazing-incidence small-angle x-ray scattering data. Removal of GMO by UV/O(3) treatment or acid extraction induces a phase change to a nanoporous body-centered structure which we tentatively identify as based on the IW-P surface. To improve film quality, we add a co-surfactant to the GMO in a mass ratio of 1:10; when this co-surfactant is cetyltrimethylammonium bromide, we find an unusually large pore size (8-12 nm) in acid extracted films, while UV/O(3) treated films yield pores of only ca. 4 nm. Using this pore size dependence on film processing procedure, we create a simple method for patterning pore size in nanoporous films, demonstrating spatially-defined size-selective molecular adsorption.

Cell-directed integration into three-dimensional lipid-silica nanostructured matrices.

We report a unique approach in which living cells direct their integration into 3D solid-state nanostructures. Yeast cells deposited on a weakly condensed lipid/silica thin film mesophase actively reconstruct the surface to create a fully 3D bio/nano interface, composed of localized lipid bilayers enveloped by a lipid/silica mesophase, through a self-catalyzed silica condensation process. Remarkably, this integration process selects exclusively for living cells over the corresponding apoptotic cells (those undergoing programmed cell death), via the development of a pH gradient, which catalyzes silica deposition and the formation of a coherent interface between the cell and surrounding silica matrix. Added long-chain lipids or auxiliary nanocomponents are localized within the pH gradient, allowing the development of complex active and accessible bio/nano interfaces not achievable by other synthetic methods. Overall, this approach provides the first demonstration of active cell-directed integration into a nominally solid-state three-dimensional architecture. It promises a new means to integrate "bio" with "nano" into platforms useful to study and manipulate cellular behavior at the individual cell level and to interface living organisms with electronics, photonics, and fluidics.