Materials scientists have always had a strong fascination for -- and a desire to imitate -- biological systems, with their amazing variety of complicated, yet highly ordered, structures. But now it is no longer just a
matter of copying -- researchers are trying to incorporate
microorganisms such as
bacteria,
viruses,
and
fungi into the selective
synthesis of novel materials. American researchers at
Northwestern University in Evanston have now employed living
fungi as "templates" in the
synthesis of highly ordered structures of
nanoparticles.
The principle behind the method used by
Chad A. Mirkin and his team is as simple as it is astonishing. Nanoscopic
gold particles, coupled to short
DNA strands, are dispersed into a nutrient medium. The medium is then inoculated with fungal
spores. As the
fungus starts to grow, it forms a fibrous web, called the hyphae
or mycellia. The
gold particles then selectively attach to the surface of the hyphae to form a very dense coating. The resulting tubular structures can be conserved and examined after being slowly dried and embedded in epoxy resin. If they are dried quickly and pressed into a film, they form a fibrous golden material. Since the hyphae grow with a constant diameter characteristic of each particular
fungus, the resulting
tubes are very regular.
It gets even better: The DNA strands attached to the gold particles can be used to couple to a second layer of gold particles, as long as these are first attached to the appropriate complementary strand of DNA. This method allows for the construction of a complex secondary structure. Moreover, the fungi can do
even more; they survive the "gold-plating" and the hyphae continue to grow unimpeded if they are fed the proper sugar
nutrients. Therefore, if the medium is changed and gold particles of a different size are added, the resulting
tubes have different coatings from one section to the next.
"In this way, we can use
microorganisms as living templates or biological slaves to generate macroscopic architectures with strict control over the microscopic and nanoscopic dimensions of the resulting materials", explains Mirkin. Initial electrical transport studies suggest that although these materials look like
metals based upon their lustrous gold color, they actually behave as new forms of
semiconductors. "We would like to use this technique to produce materials with novel made-to-order opto-electronic, magnetic, or catalytic properties,"
says Mirkin.