DNA assembles nanotube transistor
Technology Research News
Nanotechnology is all about making machines
and materials molecule-by-molecule. Such precision promises to enable
microscopic machines, faster electronics, and materials that harbor new
Because it is difficult and tedious to manually put atoms and
molecules in place, researchers are looking for ways to cause materials
to self-assemble. Self-assembly is an especially attractive concept because
it has the potential to be quick, relatively easy, and very inexpensive.
One way to make things assemble automatically is to coax nature's
self-assembly molecule -- DNA -- to assemble into templates that can in
turn cause other molecules to line up in all the right places.
Researchers from the Technion-Israel Institute of Technology have
brought the idea a large step forward by demonstrating a DNA-template
self-assembly process that makes transistors in a test tube using an assortment
of raw ingredients: carbon nanotubes, silver, gold, and four types of
The process could eventually be used to make many types of materials,
molecular machines and electronics, and even entire computers.
DNA is made up of four bases -- adenine, cytosine, guanine and
thymine -- attached to a sugar-phosphate backbone. In cells, two strands
of DNA zip together into the familiar double helix when their bases line
up -- adenine connects to thymine and cytosine to guanine -- and sequences
of bases act as templates to build proteins. Nanotubes are rolled-up sheets
of carbon atoms that form naturally in soot and can be smaller than one
nanometer in diameter, or 75,000 times narrower than a human hair.
Researchers have been able to make artificial DNA molecules that
have tailor-made sequences of bases for some time. The key to using this
type of DNA as a template for tiny components and new materials is finding
ways to connect nonbiological materials like metal and carbon nanotubes
to specific sequences of DNA bases. "Combining DNA, proteins, metal particles
and carbon nanotubes and a test tube is not easy since these materials
are alien to each other," said Erez Braun, a professor of physics at the
Technicon-Israel Institute of Technology.
The researchers accomplished this by co-opting the natural antibody
process. Antibodies connect to specific proteins that make up the outside
cell walls of pathogens like bacteria in order to capture and dispose
of the bacteria.
The researchers' process self-assembles a transistor in several
steps. First, the researchers coax a long double strand of DNA and a short
single strand to position the nanotube.
The short single strand is coated with a protein from an E. coli
bacteria that connects to a target span of 500 bases on the double strand.
The span measures about 250 nanometers, or 250 millionths of a millimeter.
An antibody to the bacteria protein then binds to the protein, followed
by a second antibody that binds to the first one. Finally, a carbon nanotube
that has been coated with a second type of protein binds to the second
antibody, connecting the nanotube along the target sequence of the double
strand of DNA.
The DNA-nanotube assembly is then stretched out on a silicon wafer,
where the E. coli protein carries out a second job as a resist, or shield.
When a solution of silver is mixed with the DNA, silver molecules
attach only to those segments of DNA that are unprotected by the protein.
This sets up the second step of the wire-building process. When the researchers
add suspended gold particles and electrify the solution, gold deposits
around the silver clusters to form gold wires on both sides of the nanotube.
These gold wires are the source and drain electrodes of a transistor.
The nanotube forms the transistor's semiconducting channel, and the silicon
surface acts as a gate electrode, which controls the flow of current running
through the device to turn it on or off.
"We harnessed a basic biological process... responsible for mixing
genes in cells... to create sequence-specific DNA junctions and networks,
to coat DNA with metal in a sequence-specific manner and to [position]
molecular objects on [a specific] address in a DNA molecule," said Braun.
The demonstration "is a very significant [advance] in developing
the technology for assembling carbon nanotube-based devices," said Deepak
Srivastava, a senior scientist and technical lead in computational nanotechnology
at the NASA Ames Research Center. "People have always talked about using
wet chemistry for assembling molecular electronic components into precise
locations," he said. "This is a first proof of the principal."
The research is novel because it uses biological molecular recognition
techniques to assemble synthetic building blocks, said Srivastava. The
technique could eventually be used in a next generation of electronics
and in other applications that require nanoscale molecular components
to assemble into complex system-level architectures -- like embedded sensors,
molecular machines and nano-manufacturing applications, he said.
The researchers' next step is to construct a device on a DNA junction,
said Braun. This would involve getting rid of the silicon substrate that
acts as a gate for the current prototype transistor. Once this is possible,
"the road is open for self-assembling more complex logic circuits," he
Today's computer chips are largely made up of transistors arranged
into circuits that carry out the basic logic of computing. Researchers
are working to make transistors smaller in order to speed computing; smaller
components are faster because electrical signals have less distance to
travel. Self-assembly processes could eventually prove less expensive
than today's silicon manufacturing techniques.
It is not clear how long it will take before the self-assembly
process can be used to manufacture components, said Braun. "It's hard
to predict applications," he said. "A lot needs to be done before it becomes
technology, but it's a good step forward since self-assembly of carbon
nanotube devices opens many possibilities for electronics and diagnostics."
Braun's research colleagues were Kinneret Keren, Rotem S. Berman,
Evgeny Buchstab, and Uri Savon. The work appeared in the November 21,
2003 issue of Science. The research was funded by the Israeli Science
Foundation, the Techinion-Israel Institute of Technology, and the Clore
Funding: Government; Private; University
TRN Categories: Nanotechnology; Integrated Circuits
Story Type: News
Related Elements: Technical paper, "DNA-Templated Carbon
Nanotube Field-Effect Transistor," Science, November 21, 2003
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