Programming Advanced Materials

[Kristina Kirby]

Thursday, January 31, 2008

Technology Review

Programming Advanced Materials
Researchers create three-dimensional structures using DNA-directed
assembly.
By Peter Fairley

http://www.technologyreview.com/Nanotech/20137/?nlid=845
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In 1996, scientists at IBM and Northwestern University used single-
stranded DNA as if it were molecular Velcro to program the self-
assembly of nanoparticles into simple structures. The work helped
launch the then-nascent nanotechnology field by suggesting the
possibility of building novel materials from the bottom up. Twelve
years later, researchers from Northwestern and Brookhaven National
Laboratory report separately in the journal Nature that they have
finally delivered on that promise, using DNA linkers to transform
nanoparticles into perfect crystals containing up to one million
particles.

"The crystal structures are deliberately designed," says
Northwestern's Chad Mirkin, one of the materials scientists who
pioneered DNA linking in the 1990s and a coauthor of one of today's
reports. "This is a new way of making things."

Ohio State University physicist David Stroud calls the work "quite
valuable." He predicts that the breakthrough will enable the assembly
of new materials with novel optical, electronic, or magnetic
properties that have, until now, existed only in the minds and models
of materials scientists. "Even now I'm surprised they could do it,"
says Stroud.

To date, efforts at programmed nanoparticle self-assembly in three
dimensions have produced mostly disordered clumps. These clumps can
have value; indeed, Mirkin's startup company NanoSphere has used the
technology to develop medical diagnostics that have gained approval
from the Food and Drug Administration.

But more complex and exotic materials imagined by Stroud and others
require ordered structures. The hang-up, says Stroud, is that
nanoparticles are immense relative to the atoms that form most
crystals. As a result, the nanoparticles move relatively slowly,
especially with DNA strands attached. When cooled to allow the
complementary strands of DNA to link up, the nanoparticles tend to get
frozen into a disordered arrangement before they can find their way to
the orderly lattice of a crystal.

The authors of the new reports--a team at Northwestern led by Mirkin
and chemist George Schatz, and physicist Oleg Gang's team in
Brookhaven National Laboratory's functional materials center, in
Upton, NY--overcame the particles' sluggishness by using longer DNA
strands that give the particles more flexibility during crystal
formation. "Typically, we think that crystallinity requires very rigid
structures, so one could imagine it's necessary to have a very rigid
DNA shell on the particles to have good crystals," says Gang. "In
reality, it's the opposite."

While the details of the Northwestern and Brookhaven systems differ,
both pad out their DNA strands with sequences that act as spacers and
flexors, in addition to complementary sequences on the DNA ends that
bind particles together. The groups start by binding one of two types
of DNA to gold nanoparticles. The DNA types are complementary to each
other. These two pools of modified particles are then mixed and
cooled. DNA strands with complementary DNA form a double helix, tying
together their respective nanoparticles, while identical DNA strands
act like springs to repel their respective particles. The spacers on
each DNA strand, meanwhile, allow bound particles to twist and bend so
each particle in the mix can bind the largest number of complementary
particles.

The result is exactly what theory predicts: a crystal lattice in which
each particle of one type is surrounded by eight of the others marking
the corners of a cube. Mirkin's group further demonstrated that
tweaking the temperature and DNA sequences could nudge the same mix of
particles to form a distinct crystal structure in which each particle
has 12 neighbors.

Mirkin says that he and his team are just getting started. "To me,
it's really only the start rather than the ending," he says. Over the
past three years, Mirkin's group has been demonstrating methods to
place different DNA linkers on different faces of nonspherical
particles, such as triangle-faced prisms and virus particles. That, he
says, should enable programming of more complex materials with
repeating patterns of three or more components. "The really intriguing
possibility here is the ability to program the formation of any
structure you want," says Mirkin.

Stroud says that the structures already produced will be useful as the
DNA-programmed assembly is extended to particles other than gold.
Applications could include photonic crystals, in which the precise
periodicity of particles can tune the overall materials to manipulate
specific wavelengths of light, and photovoltaics that capture a
broader range of the solar spectrum.

The structures are highly porous--10 percent particles and DNA and 90
percent water. That could hinder applications in which water is
undesirable. Drain out the water, and the crystals collapse. Gang says
that one could stabilize the crystals by filling the lattice with a
polymer, but he is also exploring alternate stabilization schemes that
would preserve the lattice's open space.
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http://www.technologyreview.com/Nanotech/20137/?nlid=845

Copyright Technology Review 2008.