Silicon From Sea Life

The ornate glass cages that diatoms call home are getting a high-tech makeover, courtesy of chemistry. Materials scientists have developed a way to convert the unicellular algae's intricate silica exoskeletons into silicon without destroying their architectural finery (Nature 2007, 446, 172).

The resulting rococo microstructures could find use in sensing, optical, and electronic applications, according to Georgia Institute of Technology's Kenneth H. Sandhage, who led the research effort.

To strip the oxygen atoms out of the silica structures, Sandhage's group seals the exoskeletons in a steel ampoule along with some magnesium granules and then heats up the apparatus to 650 ^oC. That's hot enough to generate magnesium gas, which reduces the silica to elemental silicon. It's not so hot, however, that the silicon becomes volatile, so the glassy cage's original shape remains intact. A bath in hydrochloric acid removes any oxidized magnesium from the structure, leaving only silicon behind.

David J. Norris, a materials science professor at the University of Minnesota, calls the technique "a powerful new tool for modifying biologically derived or inspired materials." In a commentary that accompanies the report, Norris writes: "Silicon is arguably the 'gold standard' among electronic materials, and this approach is akin to the magic touch of a modern Midas. It should allow a variety of intricate glass structures, both natural and artificial, to be transformed into silicon."

The magnesium-mediated reduction also boosts the structures' surface area by introducing a multitude of nanoscale pores. High-surface-area silicon has potential for use in sensing applications, and Sandhage and colleagues demonstrate that the reduced diatom shell works well as a nitric oxide microsensor.

Sandhage says his work with diatoms is "something of a lucky coincidence." In 1991, he went to Germany on an Alexander von Humboldt Foundation fellowship and spent some time touring the country with the other Humboldt fellows by bus. After a while, Sandhage says, he grew tired of looking at castles and started to chat with bus mate and marine biologist Monica Schoenwaelder about her work with diatoms.

"I had been making ceramics out of macroscopic silica, but I never thought about doing it with microscopic structures until then," Sandhage recalls.