"In a standard silicon-based chip, information processing is limited by the distance between units that store and share information," said Kiehl. "With these DNA crystals, we can lay out devices closely so that the interconnects are very short. If nanoparticles are spaced even 20 angstroms apart on such a DNA crystal scaffolding, a chip could hold 10 trillion bits per square centimeter--that's 100 times as much information as in the 64 Gigabit D-RAM memory projected for 2010. By showing how to assemble nanoscale components in periodic arrangements, we've taken the first step toward this goal."
Eventually, a chip made from DNA crystals and nanoparticles could be valuable in such applications as real-time image processing, Kiehl said. Nanocomponents could be clustered in pixel-like "cells" that would process information internally and also by "talking" to other cells. The result could be improved noise filtering and detection of edges or motion. Someday, the technology may even help computers identify images with something approaching the speed of the human eye and brain, said Kiehl.
The team devised a DNA scaffolding for arrays of nanoparticles of gold, but the scaffolding could also hold arrays of carbon nanotubes or other molecules. Information could be stored as an electrical charge on certain nanoparticles; the presence or absence of charge would constitute one bit of information. Alternatively, nanoparticles could be magnetic, and the magnetic states would be read as information. Because DNA strands contain four chemical bases spaced every 3.4 angstroms, information might be stored on that small a scale, Kiehl said.
To manufacture the scaffolding, the researchers took advantage of the fact that each base spontaneously pairs up with, or "sticks to," one of the other bases to form the famous DNA double helix. The team synthesized four different two-dimensional "tiles" of DNA, each tile having an extension that sticks to the extension on another tile. Like self-assembling jigsaw pieces, the tiles joined themselves into a flat crystal with a repeating pattern. One tile had a stretch of DNA that extended above the plane of the tile; to this the researchers anchored a spherical, 55-atom nanoparticle of gold. Under an electron microscope, the gold nanoparticles appeared as regular lines of bright spots. A regular pattern of nanoparticles is important in arranging them to process or store information.
"Gold is a metal, and a matrix between metals and organic molecules like DNA is very hard to make," Kiehl said. "If we can make DNA scaffolding for gold, we think we can do it for carbon nanotubes and other organic molecules. The technique is well suited to laying out locally interconnected circuitry, which is of great interest for circumventing the interconnect bottleneck-- the well-known problem where wires, rather than devices (transistors), limit computing speed."
Other scientists have used DNA as nanoparticle "glue," but such arrangements are prone to structural flaws, which limits their usefulness, said Kiehl. In contrast, the virtually perfect arrangement of molecules within a DNA crystal allows precise control over the arrangement of the particles.
Among the next steps for the researchers is to demonstrate that nanoparticles bound to the DNA crystals can function electrically.
"We're working on instrumentation to do electrical characterization of gold nanoparticles and other nanocomponents on DNA," said Kiehl. "We hope to show, for example, that DNA doesn't interfere with the electrical functioning of the nanocomponents."
The work was supported by the National Science Foundation.
Richard Kiehl, (612) 625-8073
Deane Morrison, University News Service, (612) 624-2346, firstname.lastname@example.org