“We obtained promising results that indicated that this fungus can hydrolyze metal complexes to nano-oxides at room temperature,” Murali Sastry told nanotechweb.org. “Since these fungi grow on the roots of plants, we thought they may also be capable of leaching out silicate complexes from silica present in the soil to complete the cycle. While this was a rather long shot, it worked.”
To date, biological methods have produced both crystalline and amorphous bulk silica. Sastry and colleagues believe this is the first time anyone has tried to develop a bioleaching process for synthesizing silica in nanoparticle form.
On exposure to sand, the fungus leached out silica nanoparticles within about a day. The nanoparticles were crystalline and between 2 and 5 nm in size. They tended to cluster together.
Spectroscopy showed that the silica nanoparticles contained proteins. The researchers removed these proteins by carrying out calcination of the powders at 400 °C for two hours. This produced hollow, quasi-spherical structures.
“This is commercially exciting,” said Sastry. “It is green chemistry: no toxic chemicals are employed and the microbes aren't pathogenic to humans. Furthermore, nano-oxides are derived from a cheap source - sand - at room temperature.”
The team believes that the fungus reacted with the sand to form silicic acid. Fungal proteins then formed a complex with the acid. In turn, this complex was hydrolyzed by specific hydrolyzing proteins or enzymes secreted by the fungus to form silica nanoparticles.
“The biggest problem with this approach is that we have no rational strategy - given [that] some fungi can hydrolyze metal complexes when they should not really be doing so, it is almost impossible to predict which fungus/bacterium will be responsive to a given metal ion stress,” said Sastry. “Once we figure out which genes are responsible and look at their similarity with oxide-producing micro-organisms such as diatoms, we may be able to work out a rational approach. Right now, it is pretty much hit or miss as to which fungus will work best for a particular reaction.”
The researchers are now aiming to commercialize their work, tackling issues such as scale-up. “We would also be looking to understand the biochemistry much better - protein sequencing, gene identification etc - to make it a truly viable option for chemical synthesis of nano-oxides,” said Sastry.
The scientists reported their work in Advanced Materials.