Wrapped |
DNA,
a molecule famous for storing the genetic blueprints for all living
things, can do other things as well. In a new paper, researchers at the
National Institute of Standards and Technology (NIST) describe how
tailored single strands of DNA can be used to purify the highly desired
“armchair” form of carbon nanotubes.
Armchair-form
single wall carbon nanotubes are needed to make “quantum wires” for
low-loss, long distance electricity transmission and wiring. Single-wall
carbon nanotubes are usually about a nanometer in diameter, but they
can be millions of nanometers in length. It’s as if you took a
one-atom-thick sheet of carbon atoms, arranged in a hexagonal pattern,
and curled it into a cylinder, like rolling up a piece of chicken wire.
If you’ve tried the latter, you know that there are many possibilities,
depending on how carefully you match up the edges, from neat, perfectly
matched rows of hexagons ringing the cylinder, to rows that wrap in
spirals at various angles—“chiralities” in chemist-speak.
Chirality
plays an important role in nanotube properties. Most behave like
semiconductors, but a few are metals. One special chiral form—the
so-called “armchair carbon nanotube”—behaves like a pure metal and is
the ideal quantum wire, according to NIST researcher Xiaomin Tu.
Armchair
carbon nanotubes could revolutionize electric power systems, large and
small, Tu says. Wires made from them are predicted to conduct
electricity 10 times better than copper, with far less loss, at a sixth
the weight. But researchers face two obstacles: producing totally pure
starting samples of armchair nanotubes, and “cloning” them for mass
production. The first challenge, as the authors note, has been “an
elusive goal.”
Separating
one particular chirality of nanotube from all others starts with
coating them to get them to disperse in solution, as, left to
themselves, they’ll clump together in a dark mass. A variety of
materials have been used as dispersants, including polymers, proteins
and DNA. The NIST trick is to select a DNA strand that has a particular
affinity for the desired type of nanotube. In earlier work, team leader
Ming Zheng and colleagues demonstrated DNA strands that could select for
one of the semiconductor forms of carbon nanotubes, an easier target.
In this new paper, the group describes how they methodically stepped
through simple mutations of the semiconductor-friendly DNA to “evolve” a
pattern that preferred the metallic armchair nanotubes instead.
“We
believe that what happens is that, with the right nanotube, the DNA
wraps helically around the tube,” explains Constantine Khripin, “and the
DNA nucleotide bases can connect with each other in a way similar to
how they bond in double-stranded DNA.”
According
to Zheng, “The DNA forms this tight barrel around the nanotube. I love
this idea because it’s kind of a lock and key. The armchair nanotube is a
key that fits inside this DNA structure—you have this kind of molecular
recognition.”
Once
the target nanotubes are enveloped with the DNA, standard chemistry
techniques such as chromatography can be used to separate them from the
mix with high efficiency.
“Now
that we have these pure nanotube samples,” says team member Angela
Hight Walker, “we can probe the underlying physics of these materials to
further understand their unique properties. As an example, some optical
features once thought to be indicative of metallic carbon nanotubes are
not present in these armchair samples.”
Evolution of DNA sequences towards recognition of metallic armchair carbon nanotubes
DNA sequence motifs for structure-specific recognition and separation of carbon nanotubes
