DNA in nanotubes sorts molecules
Technology Research News
are especially selective gatekeepers, allowing the right chemicals through
the cell membrane at the right time. The ability to allow some molecules
through the membrane while blocking others is useful in biotechnology and
nanotechnology applications that require that specific molecules be selected
Researchers from the University of Florida have made a synthetic
membrane that recognizes certain biochemical molecules and allows them to
The method could be used to make biological sensors like those needed
for genetics research, and to sort biological molecules, said Charles Martin,
a professor of chemistry at the University of Florida.
The synthetic membrane is made up of tiny gates and molecular gatekeepers.
The gates are gold nanotubes and DNA strands attached to the nanotubes determine
which molecules pass through, said Martin.
To form a working membrane, the researchers had to match the size
of the gate to the size of the gatekeeper. "A gate-keeping man can block
a gate the size of a window, but this man could not block a gate the size
of a garage door," said Martin. "So we need to make nanotubes with diameters
comparable to the size of the DNA molecules -- in the 10 nanometer range."
A nanometer is one millionth of a millimeter, or the span of 10 hydrogen
The researchers used template synthesis to make nanotubes of the
correct size. They electrically deposited gold atoms inside the 30-nanometer
diameter pores of a polycarbonate template membrane to make gold nanotubes
with inner diameters ranging from 8 to 12 nanometers.
The researchers also had to determine what kind of DNA strands would
be the best gatekeepers. They used DNA hairpin molecules to select target
DNA strands and allow them to pass through the nanotubes.
The DNA gatekeeper molecule is made from the same building blocks
as biological DNA -- the bases adenine, cytosine, guanine and thymine attached
to a sugar-phosphate backbone. Single-stranded DNA can combine with other
strands that contain complementary bases -- adenine across from thymine,
and cytosine across from guanine.
Hairpin DNA molecules contain complementary bases on a single strand,
and when these bases join, the single-strand forms a hairpin-like shape.
The researchers' hairpins strands contain a loop before the joined portion.
The researchers affixed hairpin DNA to the insides of the gold nanotubes
in the membrane so that the loops floated freely. DNA present in the environment
that contained a stretch of bases complementary to the exposed loops combined
with the gatekeeper DNA, then passed through when other complementary strands
The flow of DNA molecules that contain exactly matching segments
is increased because when there is an excess of matching segments, the segments
connected to the loops quickly disconnect, according to Martin. This rapid
displacement of connected segments accelerates the transport of matching
segments through the membrane.
The researchers found that the nanotube membrane transported five
times as many DNA segments that contained exactly complementary sequences
to the DNA hairpin loop as segments that contain the complementary sequence
with a single base mismatch
The researchers are working on making the membranes more selective
and quicker, said Martin. The method also needs to be tested with more realistic
samples, like DNA extracted from cells.
The method could be ready for practical use in in five to ten years,
Ultimately, the research is aimed at tapping the vast experience
of nature, according to Martin."Living systems use bio and nano devices
routinely -- ion channels, ribosomes, protein complexes, molecular motors
et cetera," he said. "I'm trying to make bio and nano devices that mimic
the functions of the natural ones."
Martin's research colleagues were Punit Kohli, C. Chad Harrell,
Zehui Cao, Rahela Gasparac, and Weihong Tan. The work appeared in the August
13, 2004 issue of Science. The research was funded by the National
Science Foundation (NSF).
Timeline: 5-10 years
TRN Categories: Biotechnology; Nanotechnology
Story Type: News
Related Elements: Technical paper, "DNA-Functionalized Nanotube
Membranes with Single-Base Mismatch Selectivity," Science, August 13, 2004
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