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

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

Protein-based biochars as potential renewable fillers in styrene-butadiene rubber composites

In this study, chicken feather meal (CFM) and canola protein (CP) were converted into biochar and their suitability as reinforcing fillers in styrene-butadiene rubber (SBR) composites was evaluated. The protein-based feedstocks were pyrolyzed at 700 °C for 1 h, at a heating rate of 50 °C·min⁻¹ under different pyrolysis atmospheric conditions (N₂, CO₂, and steam). The flow rate of N₂ and CO₂ was 700 ml/min, while that of steam was 31 ml/min. Also, biochar was produced under N₂ and CO₂ gas flow and subsequently activated using steam. The physicochemical properties of the resulting CFM and CP biochars were characterized based on elemental and proximate analyses, surface area, Fourier-transform infrared spectroscopy (FTIR), and thermal field emission scanning electron microscopy. Results showed that activated CP, pyrolyzed under nitrogen and subsequently steam cooled (CP N₂ + SC), had enhanced physicochemical properties such as lower ash content, higher fixed carbon content, and reduced polar surface functional groups compared to the other studied biochars in this study. Results also showed that CFM and CP pyrolyzed under nitrogen and subsequently steam cooled displayed higher final moduli and better filler dispersion in rubber than the other biochar samples produced in this study. Given that results showed favorable physicochemical properties and higher final moduli for CP N₂ + SC, this sample was further tested using dynamic mechanical analysis (DMA) and results showed slight differences with carbon black regarding the temperature dependence of the shear storage modulus (Gʹ) and the loss tangent (tan δ). These differences were attributed to increased filler-filler interactions, reduced filler-rubber interactions, and a lower dispersion in rubber than carbon black. The observed differences were not large enough to explain the inability of the biochar to adequately reinforce the rubber. However, the biochar filled rubber sample had a comparable viscoelastic behavior to carbon black over the studied temperature range

Attractive Interactions between DNA–Carbon Nanotube Hybrids in Monovalent Salts

DNA–carbon nanotube (DNA-CNT) hybrids are nanometer-sized, highly charged, rodlike molecules with complex surface chemistry, and their behaviors in aqueous solutions are governed by multifactorial interactions with both solvent and cosolutes. We have previously measured the force between DNA-CNTs as a function of their interaxial distance in low monovalent salts where interhybrid electrostatic repulsion dominates. The characteristics of DNA-CNT forces were further shown to closely resemble that of double-stranded DNA (dsDNA) in low salts. However, contrasting behaviors emerge at elevated monovalent salts: DNA-CNT condenses spontaneously, whereas dsDNA remains soluble. Here we report force–distance dependencies of DNA-CNTs across wide-ranging monovalent salt concentrations. DNA-CNT force curves are observed to deviate from dsDNA curves above 300 mmol/L NaCl, and the deviation grows with increasing salts. Most notably, DNA-CNT forces become net attractive above 1 mol/L NaCl, whereas dsDNA forces are repulsive at all salt concentrations. We further discuss possible physical origins for the observed DNA-CNT attraction in monovalent salts, in consideration of the complex surface chemistry and unique polyelectrolyte properties of DNA-CNT hybrids

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 E₁₁ 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 E₁₁ 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

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

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