Thursday, January 31, 2008

Programming Advanced Materials

Researchers create three-dimensional structures using DNA-directed assembly.

By Peter Fairley

Ordered nano order: Sequences of DNA attached to gold nanoparticles (upper image) program the particles’ self-assembly into novel crystals (lower image). X-ray diffraction confirms the crystals--partly squashed by the electron microscopy that produced these images--to be perfect lattices of tens of thousands of particles. Credit: Oleg Gang

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.