19 research outputs found
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 DNAāSWCNT 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
nanotubeās diameter and metallicity. Thermodynamic analysis
suggests that this chirality-dependent partition is determined by
nanotubeās 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 nanotubeās 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 Ā°C for 1 h, at a heating rate of 50 Ā°CĀ·minā»Ā¹ under different pyrolysis atmospheric conditions (Nā, COā, and steam). The flow rate of Nā and COā was 700 ml/min, while that of steam was 31 ml/min. Also, biochar was produced under Nā and COā 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 Nā + 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 Nā + 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 (GŹ¹) and the loss tangent (tan Ī“). 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 DNAāCarbon Nanotube Hybrids in Monovalent Salts
DNAācarbon 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 forceādistance 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 E<sub>11</sub> 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<sub>11</sub> 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