Credit: © 2007 Nature

The silica microshells of marine algae (diatoms) are intricately patterned with nanopores and protrusions. To make replicas of these structures out of silicon — a more technologically useful material — the silica has to be chemically reduced. However, this usually requires very high temperatures that would destroy the original morphology of the diatom shells.

Now, Kenneth Sandhage and co-workers1 at the Georgia Institute of Technology in Atlanta in the USA have made silicon replicas of diatom microshells by reacting them with magnesium gas at a cool (relatively speaking) 650 °C. Initially, a mixed solid of interconnected silicon and magnesia was obtained. Coarser grains of silicon were reoxidized to silica and then — along with the magnesia — were dissolved in acid, leaving behind a continuous and highly porous nanocrystalline silicon structure with a high surface area.

It is suggested that because its production can be readily scaled up, this new material could be used for a range of electronic, optical and sensing applications. The potential to make such devices was demonstrated by using a silicon replica of a diatom shell to sense nitric oxide gas. When one of these replicas was connected to electrodes and a voltage put across it, changes in impedance were observed on exposure to nitric oxide. Although further optimization is required, the devices are highly sensitive and have faster response times than previously reported porous silicon sensors.