NANOCOMPUTING:
Yet Another Role for DNA?
Dennis Normile and Robert F. Service
As they struggle to join nanotubes and nanowires into simple X shapes, molecular electronics researchers dream of making much more complex circuitry. "Everybody is trying to make larger arrays" of devices, says Tom Mallouk, a chemist at Pennsylvania State University, University Park. "What we're seeing now is just the beginning." To move from the simple to the complex, though, scientists will need to develop a much defter touch.
Some think the key to that dexterity lies in that consummate molecular sleight-of-hand artist, DNA. By taking advantage of DNA's ability to recognize molecules and self-assemble--not to mention the huge toolkit of enzymes and techniques biologists have developed for working with the molecule--they hope to use DNA as a template for crafting metallic wiring, or even to wire circuits with strands of DNA itself.
Mallouk's group, also led by chemist Christine Keating and electrical engineers Tom Jackson and Theresa Mayer, starts by growing metal nanowires in the tiny pores of commercially available filtration membranes. Because the researchers can vary the composition of the metals laid down in the pores, they make nanowires with one type of metal, such as platinum, on the ends, and another metal, such as gold, in the middle. By attaching gold-linking thiol groups to single-stranded DNA, they can bind the DNA to the gold midsections of the nanowire. To coax the nanowires to assemble into different shapes, they simply attach complementary DNA strands to the gold segments of other nanowires. The complementary strands then bind to each other, welding pairs of wires together.
In initial experiments, the team has used the technique to make simple shapes such as crosses and triangles. And they are currently using it in an attempt to assemble more complex circuitry, Keating says: "You can envision using this to carry out the deterministic assembly of a circuit." That hasn't happened yet, in part because the DNA on some nanowires tends to bind indiscriminately to other noncomplementary DNA rather than its partner strand. But because biochemists have learned to solve this problem with applications such as DNA chips, Keating is confident that DNA will soon become a type of addressable glue for a wide variety of molecular electronics components.
Matchmaker. A piece of single-stranded DNA links corresponding sequences on nanowires to forge a cross.
ILLUSTRATION: J. MOGLIA
Erez Braun and his group at the Technion-Israel Institute of Technology in Haifa take a different approach. Instead of using DNA to join wires together, they make wires by silver-plating DNA itself (Science, 20 March 1998, p. 1967). The researchers start with a pair of gold electrodes 1200 nanometers apart on a sheet of glass. First they attach snippets of DNA 12 oligonucleotides long to each electrode. Then they immerse the electrodes in a solution containing short lengths of viral DNA. The viral DNA attaches itself to the snippets, creating a DNA bridge between the electrodes. Next, by soaking the bridge in a solution containing silver ions, Braun and colleagues coat it with silver. The result is a nanometer-scale metallic wire between the electrodes, with properties that can be varied by fiddling with the developing conditions.
Braun says they have extended the approach and are now close to completing a three-terminal switching device that would function much like a transistor. They are also studying how they might scale up these processes to create more complex networks.
More exotically, it's even possible that wires might be made of DNA itself. First, though, researchers will need a much better understanding of DNA's basic electrical properties. Since the first report, in 1993, that DNA can carry current, measurements of its conductivity have ranged from zero, a perfect insulator, to superconductivity when the electrodes are spaced very closely together. Christian Schönenberger, a physicist at the Swiss Nanoscience Center in Basel, says most researchers now think that DNA is a semiconductor whose conductivity depends on how it is "doped" with foreign molecules. The wide range of conductivity is good news, Schönenberger says. "It means that we can, in principle, tailor the doping and control the conductivity." To make electronic devices, though, scientists must sort out precisely which parts of DNA's complex chemistry do the doping--and that may be no simple task.
Volume 293,
Number 5531,
Issue of 3 Aug 2001,
p. 783.
Copyright © 2001 by The American Association for the Advancement of Science.
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