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A silicon replica made from
diatom microshells glows. The conversion process from the original
silica retains the intricate structure of the microshells.
Image shows a sensor created from
a microporous silicon structure converted from the shell (frustule)
of a single diatom.
Images courtesy Ken Sandhage |
"When we conducted measurements for the
detection of nitric oxide, a common pollutant, our single
diatom-derived silicon sensor possessed a combination of speed,
sensitivity, and low voltage operation that exceeded conventional
sensors," said Kenneth H. Sandhage, a professor in the School of
Materials Science and Engineering at the Georgia Institute of
Technology. "The unique diatom-derived shape, high surface area and
nanoporous, nanocrystalline silicon material all contributed towards
such attractive gas sensing characteristics."
The unique devices, part of a broader long-term
research program by Sandhage and his research team, were described in
the March 8 issue of the journal Nature. The research was sponsored by
the U.S. Air Force Office of Scientific Research and the U.S. Office
of Naval Research.
Scientists estimate that roughly 100,000 species of
diatoms exist in nature, and each forms a microshell with a unique and
often complex 3-D shape that includes cylinders, wheels, fans, donuts,
circles and stars. Sandhage and his research team have worked for
several years to take advantage of those complex shapes by converting
the original silica into materials that are more useful.
Ultimately, they would like to conduct such
conversion reactions on genetically-modified diatoms that generate
microshells with tailored shapes. However, to precisely alter and
control the structures produced, further research is needed to learn
how to manipulate the genome of the diatom. Since scientists already
know how to culture diatoms in large volumes, harnessing the diatom
genetic code could allow mass-production of complex and tailored
microscopic structures. Sandhage’s colleagues, Prof. Nils Kröger
(Georgia Tech School of Chemistry and Biochemistry at Georgia Tech)
and Dr. Mark Hildebrand (Scripps Institution of Oceanography) are
currently conducting research that could ultimately allow for genetic
engineering of diatom microshell shapes.
"Diatoms are fabulous for making very precise
shapes, and making the same shape over and over again by a
reproduction process that, under the proper growth conditions, yields
microshells at a geometrically-increasing rate," Sandhage noted. "Diatoms
can produce three-dimensional structures that are not easy to produce
using conventional silicon-based processes. The potential here is for
making enormous numbers of complicated 3-D shapes and tailoring the
shapes genetically, followed by chemical modification as we have
conducted to convert the shells into functional materials such as
silicon."
Silicon is normally produced from silica at
temperatures well above the silicon melting point (1,414 degrees
Celsius), so that solid silicon replicas can not be directly produced
from silica structures with such conventional processing. So the
Georgia Tech researchers used a reaction based on magnesium gas that
converted the silica of the shells into a composite containing silicon
(Si) and magnesium oxide (MgO). The conversion took place at only 650
degrees Celsius, which allowed preservation of the complex channels
and hollow cylindrical shape of the diatom.
The magnesium oxide, which makes up about
two-thirds of the composite, was then dissolved out by a hydrochloric
acid solution, which left a highly porous silicon structure that
retained the original shape. The structure was then treated with
hydrofluoric acid (HF) to remove traces of silica created by reaction
with the water in the hydrochloric acid solution.
The researchers then connected individual
diatom-derived silicon structures to electrodes, applied current and
used them to detect nitric oxide. The highly porous silicon shells,
which are about 10 micrometers in length, could also be used to
immobilize enzymes for purifying drugs in high-performance liquid
chromatography (HPLC) and as improved electrodes in lithium-ion
batteries.
"Silicon can form compounds that have a high
lithium content," Sandhage said. "Because diatom-derived silicon
structures have a high surface area and are thin walled and highly
porous, the rate at which you can get lithium ions into and out of
such silicon structures can be high. For a given battery size, you
could store more power, use it more rapidly or recharge the battery
faster by using such structures as electrodes."
In testing, the researchers showed that the silicon
they produced was photoluminescent – meaning it glows when illuminated
by certain wavelengths of light. That shows the fabrication process
produced a nanoporous, nanocrystalline structure – and may have
interesting photonic applications in addition to the electronic ones.
Though Sandhage and his collaborators have
demonstrated the potential of their technique, significant challenges
must be overcome before they can produce useful sensors, battery
electrodes and other structures. The sensors will have to be packaged
into useful devices, for example, connected into arrays of devices
able to detect different gases and scaled up for volume manufacture.
The Aulacoseira diatoms used in the research
reported by Nature were millions of years old, obtained from samples
mined and distributed as diatomaceous earth. To provide samples with
other geometries, Sandhage’s group has set up a cell culturing lab,
with the assistance of Georgia Tech colleagues Nils Kröger and Nicole
Poulson, to grow the brownish-colored diatoms.
Sandhage, who is a ceramist by training, would now
like to work directly with electronics engineers and others who have
specific interests in silicon-based devices.
"We can target diatoms of a certain shape, generate
the right chemistry, and then work with applications engineers to get
these unique structures into practice," he said. "We are now at the
point where we have a good idea of the chemical palette that is
accessible with the conversion approaches we have taken. The next step
is really to start making packaged devices." |
