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Glotzer and Nicholas Kotov, associate
professor of chemical engineering, and their graduate students and
post doctoral researchers have co-authored a paper scheduled to appear
Oct. 13 in the journal Science.
"The importance of this work is in making a key connection between the
world of proteins and the world of nanotechnology" Kotov said. "Once
we know how to manipulate the forces between the nanoparticles and
their ability to self-organize, it will help us in a variety of
practical applications from light-harvesting nanoparticle devices to
new drugs which can act like proteins, but are actually nanoparticles."
The sheets, which can appear colored under UV illumination from bright
green to dark red depending on the nanoparticle size, are made from
cadmium telluride crystals, a material used in solar cells. The sheets
are about 2 microns in width, about 1/5 the thickness of a human hair.
Scientists have long known how to coax nanoparticles into forming
sheets, Glotzer said. But those sheets have only been achieved when
the particles were on a surface or at an interface between two fluids,
never while suspended in a single fluid.
The work started in Kotov's lab three years ago, when he and his team
observed the sheets in experiments. Though they created them, they
weren't sure how.
"We were aware of certain proteins in living organisms that
self-assemble into layers, called S-layers," Kotov said. S-layer
proteins comprise the outermost cell envelope of a wide variety of
bacteria and other single-celled, prokaryotic organisms called archaea,
and they are able to form 2-d sheets with square, hexagonal, and other
packings at surfaces and interfaces, as well as suspended in fluid.
The group sought to make the connection between the forces governing
S-layer protein assembly and the forces governing the nanoparticle
assembly. That's when Glotzer's group, whose expertise is in computer
modeling and simulation, became involved.
"It's likely that the forces between S-layer proteins are highly
anisotropic, and we suspected this was also a feature of the
nanoparticles," Glotzer said. "Computer simulations allowed us to
further develop and test this hypothesis."
Post doctoral researcher Zhenli Zhang of Glotzer's group tried various
combinations of forces based on information gleaned from experiments
performed by post doctoral Zhiyong Tang of Kotov's group. The team
discovered that the unique shape of the CdTe nanocrystals gave rise to
a combination of forces that conspired to produce the unusual
two-dimensional packing. Subsequent experiments by Kotov's group
showed that if any of the forces were missing, the sheets would not
form, confirming the simulation predictions.
"Self-assembly is nature's basic building principle for producing
organized arrays of biomolecules with controlled geometrical and
physicochemical surface properties," Glotzer said. "In the fabrication
of functional nanoscale materials and devices, self-assembly offers
substantial advantages over traditional manufacturing approaches, if
we can design the building blocks appropriately. This is what we're
trying to do."
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