Bacteria naturally found in the soil could be the source of electrically conductive nanowires that could be used in compact electronic devices, sensors, and even in devices that could generate alternative fuels from natural sources. The wires, which are produced by genetic modification, are smaller even than those that can be made by industry using current methods. Moreover, they do not require harsh chemical processes or pollutants in the manufacture.
The research was carried out by microbiologists at the University of Massachusetts Amherst, sponsored by the US Office of Naval Research, and led by Dr Derek Lovley. The team started with a microbe called Geobacter sulferreducens, which naturally produces conducting fibres made from proteins, that connect the bacterium to particles of iron oxide in the ground that support its growth. The current that it carries is too small to be useful for technology, but it is still measurable.
The conductivity of fibres is believed to stem from their structure, where the amino acids that make up the proteins are arranged so that their unsaturated regions with free electrons line up. To improve the conductivity, Lovley’s team genetically modified the bacteria to replace to the natural amino acids with tryptophan, an amino acid commonly found in the proteins that make up muscle fibres. Tryptophan is highly efficient at transporting electrons. "We arranged the amino acids to produce a synthetic nanowire that we thought might be more conductive," said Lovley. "We hope that Geobacter would still produce nanowires and it might double their conductivity."
To their surprise, the nano wires produced by the modified bacteria were 2,000 times as conductive as the unmodified nanowires. With a diameter of 1.5nm, they were 1000 times smaller than the best materials that can currently be made using industrial nanotechnology; these tend to be about one billionth of a metre in diameter. "This is an exciting time to be on the cutting edge of creating new types of electronics materials," said Lovley. "The fact that we can do this with sustainable, renewable materials makes it even more rewarding."
The nanowires are also sensitive to changes in pH. This means, Lovely said, that they could be used in medical sensors that could be implanted, for example, in the heart or kidneys; in the heart, pH is an indicator of heart rate, while in the kidneys it indicates how well the organ is functioning. There are also applications in electronics as component size continues to shrink, while the researchers military sponsors are interested in the possibility of using such wires to feed current to modified microbes that can produce butanol as a fuel for military vehicles. Another defence application might be in sensors for unmanned vehicles to detect pollutants, toxic materials or traces of explosives. The researchers discuss their work in Small.