Nanotechnology is all about making
machines and materials molecule-by-molecule. Such precision promises to
enable microscopic machines, faster electronics, and materials that harbor
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.
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 protein molecules.
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
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.
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
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.
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 said.
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
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 Foundation.
Government; Private; University
Nanotechnology; Integrated Circuits
Related Elements: Technical paper, "DNA-Templated
Carbon Nanotube Field-Effect Transistor," Science, November 21, 2003