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Date: 17 April 2014
LLNL Researchers Detected a Signature for Water inside Single-Walled Carbon Nanotubes  


Topic Name: LLNL Researchers Detected a Signature for Water inside Single-Walled Carbon Nanotubes
Category: Nanobiotechnology
    
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Research persons: Jason Holt, Julie Herberg

Location: Lawrence Livermore National Laboratory, United States

Details

LLNL Researchers Detected a Signature for Water inside Single-Walled Carbon Nanotubes

Researchers have identified a signature for water inside single-walled carbon
nanotubes, helping them understand how water is structured and how it moves
within these tiny channels.

This is the first time researchers were able to get a snapshot of the water
inside the carbon nanotubes.

Single-walled carbon nanotubes (SWCNTs) offer the potential to act as a unique
nanofiltration system. While experiments have demonstrated extremely fast flow
in these channels, it is still unclear why, and few studies have experimentally
probed the detailed structure and movement of the water within nanotubes.

That’s where Lawrence Livermore scientists Jason Holt, Julie Herberg, and

University of North Carolina
’s, Yue Wu and colleagues come in.

As described in an article appearing in the July edition of Nanoletters, they
used a technique called Nuclear Magnetic Resonance (NMR) to get a glimpse of the
water confined inside one-nanometer diameter SWCNTs.

The nanotubes, special molecules made of carbon atoms in a unique arrangement,
are hollow and more than 50,000 times thinner than a human hair. The confined
water exhibited very different properties from that of bulk water, and this
allowed it to be distinguished in the NMR spectrum.

Carbon nanotubes have long been touted for their superior thermal, mechanical
and electrical properties, but recent work suggests they can be used as
nanoscale filters.

Earlier Livermore studies have suggested that carbon nanotubes may be used for
desalination and demineralization because of their small pore size and enhanced
flow properties. Conventional desalination membranes are typically much less
permeable and require large pressures, entailing high energy costs. However,
these more permeable nanotube membranes could reduce the energy costs of
desalination significantly.

While the technology offers great promise, there still are important unanswered
scientific questions.

“There have been many predictions about how water behaves within carbon
nanotubes,” said Holt, the principal investigator of the project, which is
funded through LLNL’s Laboratory Directed Research and Development (LDRD). “With
experiments like these, we can directly probe that water and determine how close
those predictions were.”

Founded in 1952, Lawrence
Livermore National Laboratory
is a national security laboratory, with a
mission to ensure national security and apply science and technology to the
important issues of our time. Lawrence Livermore National Laboratory is managed
by Lawrence Livermore National Security, LLC for the
U.S. Department of Energy's
National Nuclear Security Administration.
About Single-Walled Nanotube
Most single-walled nanotubes (SWNT) have a diameter of close to 1 nanometer,
with a tube length that can be many thousands of times longer. The structure of
a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite
called graphene into a seamless cylinder. The way the graphene sheet is wrapped
is represented by a pair of indices (n,m) called the chiral vector. The integers
n and m denote the number of unit vectors along two directions in the honeycomb
crystal lattice of graphene. If m=0, the nanotubes are called "zigzag". If n=m,
the nanotubes are called "armchair". Otherwise, they are called "chiral".

Single-walled nanotubes are a very important variety of carbon nanotube because
they exhibit important electric properties that are not shared by the
multi-walled carbon nanotube (MWNT) variants. Single-walled nanotubes are the
most likely candidate for miniaturizing electronics beyond the micro
electromechanical scale that is currently the basis of modern electronics. The
most basic building block of these systems is the electric wire, and SWNTs can
be excellent conductors. One useful application of SWNTs is in the development
of the first intramolecular field effect transistors (FETs). The production of
the first intramolecular logic gate using SWNT FETs has recently become possible
as well. To create a logic gate you must have both a p-FET and an n-FET. Because
SWNTs are p-FETs when exposed to oxygen and n-FETs when unexposed to oxygen, it
is possible to protect half of a SWNT from oxygen exposure, while exposing the
other half to oxygen. This results in a single SWNT that acts as a NOT logic
gate with both p and n-type FETs within the same molecule.

Single-walled nanotubes are still very expensive to produce, around $1500 per
gram as of 2000, and the development of more affordable synthesis techniques is
vital to the future of carbon nanotechnology. If cheaper means of synthesis
cannot be discovered, it would make it financially impossible to apply this
technology to commercial-scale applications. Several suppliers offer as-produced
arc discharge SWNTs for ~$50–100 per gram as of 2007.
About Nuclear Magnetic Resonance
Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the quantum
mechanical magnetic properties of an atom's nucleus. NMR also commonly refers to
a family of scientific methods that exploit nuclear magnetic resonance to study
molecules.

All nuclei that contain odd numbers of protons or neutrons have an intrinsic
magnetic moment and angular momentum. The most commonly measured nuclei are
hydrogen-1 (the most receptive isotope at natural abundance) and carbon-13,
although nuclei from isotopes of many other elements (e.g.113Cd, 15N, 14N 19F,
31P, 17O, 29Si, 10B, 11B, 23Na, 35Cl, 195Pt) can also be observed.

NMR resonant frequencies for a particular substance are directly proportional to
the strength of the applied magnetic field, in accordance with the equation for
the Larmor precession frequency.

NMR studies magnetic nuclei by aligning them with an applied constant magnetic
field and perturbing this alignment using an alternating magnetic field, those
fields being orthogonal. The resulting response to the perturbing magnetic field
is the phenomenon that is exploited in NMR spectroscopy and magnetic resonance
imaging, which use very powerful applied magnetic fields in order to achieve
high spectral resolution, details of which are described by the chemical shift
and the Zeeman effect.
In figure, An NMR spectrum showing features associated with water external and internal to the carbon nanotube


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