March 15, 2007 – According to research reported by the Georgia Institute of Technology, the 3D shells of tiny ocean creatures known as diatoms could provide the foundation for novel electronic devices, including gas sensors able to detect pollution faster and more efficiently than conventional devices.
Using a chemical process that converts the shells’ original silica (silicon dioxide, SiO2) into the semiconductor material silicon, researchers have created a new class of gas sensors based on the unique and intricate shells produced by diatoms. The converted shells, which retain the 3D shape and nanoscale detail of the originals, could also be useful as battery electrodes, chemical purifiers, and in other applications requiring complex shapes that nature can produce better than humans.
“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,” says 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 Nature. The research was sponsored by the U.S. Air Force Office of Scientific Research and the U.S. Office of Naval Research.
According to the journal paper, scientists estimate that roughly 100,000 species of diatoms exist in nature, and each forms a microshell with a unique and often complex 3D 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, the team says it 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, professor Nils Kroger (School of Chemistry and Biochemistry at Georgia Tech) and Mark Hildebrand, PhD, (Scripps Institution of Oceanography) are currently conducting research that could allow for genetic engineering of diatom microshell shapes.
Silicon is normally produced from silica at temperatures well above the silicon melting point (1414 degrees Celsius), so that solid silicon replicas cannot be directly produced from silica structures with conventional processing. The Georgia Tech researchers used a reaction based on magnesium (Mg) 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 — making 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 microns in length, could also be used to immobilize enzymes for purifying drugs in high-performance liquid chromatography and as improved electrodes in lithium-ion batteries.
Tests showed that the silicon the researchers produced was photoluminescent — which also shows the fabrication process produced a nanoporous, nanocrystalline structure, and may have interesting photonic applications in addition to electronics.
Though Sandhage and his colleagues have demonstrated the potential of their technique, they must address significant challenges: 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, before they can produce useful devices.
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 Kroger and Nicole Poulson, to grow the diatoms.
Sandhage, a ceramist by training, says he would like to work directly with electronics engineers and others who have specific interests in silicon-based devices.
In addition to Sandhage, other researchers who contributed to the paper included Zhihao Bao, Michael R. Weatherspoon, Samual Shian, Ye Cia, Phillip D. Graham, Shawn M. Allan, Gul Ahmad, Matthew B. Dickerson, Benjamin C. Church, Zhitao Kang, Harry W. Abernathy III, Christopher J. Summers, and Meilin Liu.