water spins gold into wire
Technology Research News
One approach to building microscopic devices
is getting tiny particles of material to simply fall into place.
Researchers from North Carolina State University have found a way to coax
microscopic gold particles to assemble into wires that are five times
thinner than the diameter of a red blood cell.
The researchers put a pair of electrodes into water suffused with gold
particles, pumped an alternating current of electrons through, and found
that the free particles aggregated at the ends of the electrodes and eventually
grew into a wire connecting them. An alternating current constantly switches
the direction electrons are traveling along a wire.
"The particles are brought to a high concentration in the end of the wire
because that is where the electric field is strongest, and once they are
present there in high concentrations they aggregate," increasing the length
of the wire, said Eric Kaler, a professor of chemical engineering at North
The effect had not been predicted. "The whole process is surprising. Theory
does not predict this phenomenon, and it has not been seen before," Kaler
The nature of the process makes the wires self-repairing. When the researchers
increased the current through the microwire to the point where the wire
snapped, the electric field at the break attracted new particles to aggregate
near the gap and restore the connection, according to Kaler.
By changing the strength and location of the electric fields, the researchers
were able to make the wires branch in a way similar to frost forming on
a window. The researchers also used the method to coax microparticles
of latex to aggregate along with the gold in order to grow gold wires
surrounded by an insulator, according to Kaler.
One of the most useful things about the process is it happens in water.
"These wires can connect circuits underwater, so that provides a means
to connect... aqueous structures like cells to electronic devices," said
Kaler. There is still work to be done to achieve this, he added. The challenge
is preserving the cells in the electrical environment needed to build
the wires. "But there is a good chance it could work," he said.
The wires also have potential as chemical sensors. They can be coated
with single-molecule layers of substances that bind to, or physically
connect with, certain chemicals, said Kaler. The electrical resistance
of the wire changes when the chemicals are bound to this outer layer,
causing electrons to travel through it at different speeds that can be
correlated to the concentration of the chemical.
In the researchers' experiments, wires one micron in diameter grew at
speeds ranging from 50 microns to 500 microns a second. The faster speed
is quick enough to bridge a one-centimeter gap between electrodes in less
than half a minute. A micron is one thousandth of a millimeter; a red
blood cell is five microns in diameter, and an E. coli bacterium is one
micron in diameter.
One advantage of the method is it does not require a physical template
to map out where the wires are going to grow. Instead, the wire assembly
"is driven by an external field," said Kaler. Because the strength and
location of the electric field guides both the rate and location of the
growth, the method is less tedious and expensive than current template
methods, he said. The method could be used commercially at any time, he
The researchers are looking to build more organized structures using the
method, said Kaler. "Ultimately we would like a toolbox of approaches
to build nanostructures" in place, he said.
Kaler's research colleagues were Kevin D. Hermanson, Simon O. Lumsdon,
Jacob P. Williams, and Orlin D. Velev from North Carolina State University.
They published the research in the November 2, 2001 issue of Science.
The research was funded by the National Science Foundation.
TRN Categories: Nanotechnology; Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "Diclectrophoretic Assembly
of Electrically Functional Microwires from Nanoparticle Suspensions,"
Science, November 2, 2001.
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