Tuesday, 30 July 2013

Tetrapod nanocrystals light the way to stronger polymers

Tetrapod nanocrystals light the
way to stronger polymers
Fluorescent tetrapod quantum dots
or tQDs (brown) serve as stress
probes that allow precise
measurement of polymer fiber
tensile strength with minimal impact
on mechanical properties. Inserts
show relaxed tQDs (upper) and
stressed tQDs (lower). Credit:
Alivisatos group
Fluorescent tetrapod nanocrystals
could light the way to the future
design of stronger polymer
nanocomposites. A team of
researchers with the U.S. Department
of Energy (DOE)'s Lawrence Berkeley
National Laboratory (Berkeley Lab)
has developed an advanced opto-
mechanical sensing technique based
on tetrapod quantum dots that
allows precise measurement of the
tensile strength of polymer fibers
with minimal impact on the fiber's
mechanical properties.
In a study led by Paul Alivisatos,
Berkeley Lab director and the Larry
and Diane Bock Professor of
Nanotechnology at the University of
California (UC) Berkeley, the research
team incorporated into polymer
fibers a population of tetrapod
quantum dots (tQDs) consisting of a
cadmium-selenide (CdSe) core and
four cadmium sulfide (CdS) arms. The
tQDs were incorporated into the
polymer fibers via electrospinning,
among today's leading techniques for
processing polymers, in which a large
electric field is applied to droplets of
polymer solution to create micro-
and nano-sized fibers. This is the
first known application of
electrospinning to tQDs.
"The electrospinning process allowed
us to put an enormous amount of
tQDs, up to 20-percent by weight,
into the fibers with minimal effects
on the polymer's bulk mechanical
properties," Alivisatos says. "The
tQDs are capable of fluorescently
monitoring not only simple uniaxial
stress, but stress relaxation and
behavior under cyclic varying loads.
Furthermore, the tQDs are elastic
and recoverable, and undergo no
permanent change in sensing ability
even upon many cycles of loading to
failure."
Alivisatos is the corresponding author
of a paper describing this research in
the journal Nano Letters titled
"Tetrapod Nanocrystals as
Fluorescent Stress Probes of
Electrospun Nanocomposites."
Coauthors were Shilpa Raja, Andrew
Olson, Kari Thorkelsson, Andrew
Luong, Lillian Hsueh, Guoqing Chang,
Bernd Gludovatz, Liwei Lin, Ting Xu
and Robert Ritchie.
Polymer nanocomposites are
polymers that contain fillers of
nanoparticles dispersed throughout
the polymer matrix. Exhibiting a wide
range of enhanced mechanical
properties, these materials have
great potential for a broad range of
biomedical and material applications.
However, rational design has been
hampered by a lack of detailed
understanding of how they respond
to stress at the micro- and
nanoscale.
"Understanding the interface between
the polymer and the nanofiller and
how stresses are transferred across
that barrier are critical in
reproducibly synthesizing
composites," Alivisatos says. "All of
the established techniques for
providing this information have
drawbacks, including altering the
molecular-level composition and
structure of the polymer and
potentially weakening mechanical
properties such as toughness. It has
therefore been of considerable
interest to develop optical
luminescent stress-sensing
nanoparticles and find a way to
embed them inside polymer fibers
with minimal impact on the
mechanical properties that are being
sensed."
From left, Andrew Olson, Shilpa Raja
and Andrew Luong are members of
Paul Alivisatos's research group who
used electrospinning to incorporate
tetrapod quantum dot stress probes
into polymer fibers. Credit: Roy
Kaltschmidt, Berkeley Lab
The Berkeley Lab researchers met
this challenge by combining
semiconductor tQDs of CdSe/CdS,
which were developed in an earlier
study by Alivisatos and his research
group, with electrospinning. The
CdSe/CdS tQDs are exceptionally
well-suited as nanoscale stress
sensors because an applied stress
will bend the arms of the tetrapods,
causing a shift in the color of their
fluorescence. The large electric field
used in electrospinning results in a
uniform dispersal of tQD aggregates
throughout the polymer matrix,
thereby minimizing the formation of
stress concentrations that would act
to degrade the mechanical properties
of the polymer. Electrospinning also
provided a much stronger bond
between the polymer fibers and the
tQDs than a previous diffusion-based
technique for using tQDs as stress
probes that was reported two years
ago by Alivisatos and his group. Much
higher concentrations of tQDs could
also be a achieved with
electrospinning rather than diffusion.
When stress was applied to the
polymer nanocomposites, elastic and
plastic regions of deformation were
easily observed as a shift in the
fluorescence of the tQDs even at low
particle concentrations. As particle
concentrations were increased, a
greater fluorescence shift per unit
strain was observed. The tQDs acted
as non-perturbing probes that tests
proved were not adversely affecting
the mechanical properties of the
polymer fibers in any significant way.
"We performed mechanical tests
using a traditional tensile testing
machine with all of our types of
polymer fibers," says Shilpa Raja, a
lead author of the Nano Letters
paper along with Andrew Olson, both
members of Alivisatos' research
group. "While the tQDs undoubtedly
change the composition of the fiber -
it is no longer pure polylactic acid
but instead a composite – we found
that the mechanical properties of the
composite and crystallinity of the
polymer phase show minimal
change."
The research team believes their tQD
probes should prove valuable for a
variety of biological, imaging and
materials engineering applications.
"A big advantage in the development
of new polymer nanocomposites
would be to use tQDs to monitor
stress build-ups prior to material
failure to see how the material was
failing before it actually broke apart,"
says co-lead author Olson. "The tQDs
could also help in the development
of new smart materials by providing
insight into why a composite either
never exhibited a desired
nanoparticle property or stopped
exhibiting it during deformation from
normal usage."
For biological applications, the tQD is
responsive to forces on the
nanoNewton scale, which is the
amount of force exerted by living
cells as they move around within the
body. A prime example of this is
metastasizing cancer cells that move
through the surrounding extracellular
matrix. Other cells that exert force
include the fibroblasts that help
repair wounds, and cardiomyocytes,
the muscle cells in the heart that
beat.
"All of these types of cells are known
to exert nanoNewton forces, but it is
very difficult to measure them," Raja
says. "We've done preliminary
studies in which we have shown that
cardiomyocytes on top of a layer of
tQDs can be induced to beat and the
tQD layer will show fluorescent shifts
in places where the cells are beating.
This could be extended to a more
biologically-relevant environment in
order to study the effects of
chemicals and drugs on the
metastasis of cancer cells."
Another exciting potential application
is the use of tQDs to make smart
polymer nanocomposites that can
sense when they have cracks or are
about to fracture and can strengthen
themselves in response.
"With our technique we are
combining two fields that are usually
separate and have never been
combined on the nanoscale, optical
sensing and polymer nanocomposite
mechanical tunability," Raja says. "As
the tetrapods are incredibly strong,
orders of magnitude stronger than
typical polymers, ultimately they can
make for stronger interfaces that can
self-report impending fracture."
Provided by Lawrence Berkeley
National Laboratory

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