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Posted: January 31, 2008

Programming advanced materials

(Nanowerk News) 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.
Source: MIT Technology Review (Peter Fairley)
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