Electrical phenomena are important in biology, as Galvani discovered long ago when he learnt to make dead frogs twitch. But in biology electrical currents are generally carried by currents of ions rather than electrons. The transport of electrons is important in processes like photosynthesis, but the distances over which the electrons are transported are very small – the nanometer or two that defines the thickness of a lipid membrane. So the discovery of what look like electrically conducting nanowires in a soil bacterium is rather surprising. The discovery, from a group at UMASS Amherst (press release here), was reported in Nature (subscription required for full article) a few weeks ago.
The bacteria in question are soil bacteria that make their living by metabolising iron; to do this they seem to have evolved electrically conducting filaments called pili that allow them to do electrochemistry at a distance on a particle of iron oxide. Pili are common in many types of bacteria; they’re used by pathogenic bacteria to inject toxins into host cells, and for transfer of DNA between bacteria. They’re composed of protein molecules which self-assemble into long filaments, which are anchored into the bacterial cell wall by a large protein complex.
This report still leaves some unanswered questions in my mind. The conductivity of the pili was measured using atomic force microscope based conductance mapping of a graphite surface decorated with pili that had been broken off bacterial surfaces; it would be more convincing (though much more difficult) to quantify the conductivity along the length of the filament, rather than across the thickness. More importantly, perhaps, it doesn’t yet seem to be clear what is the structural feature of the pilus-making protein in this particular bacteria that leads to its electrical conductivity (as opposed to pili from other types of bacteria, which are shown in the paper to be non-conductive). It’s still a remarkable and suggestive result, though.
Thanks to Jim Moore for a comment drawing my attention to this press release.
http://www.manchester.ac.uk/press/title,36799,en.htm
The link above is more interesting nano news from your neck of the woods, Richard.
‚ÄúOne-atom-thick materials promise a ‘new industrial revolution‚Äù. Aparently, these 2 D crystals have some really interesting properties and are not that difficut to make.
But back to the conductive pili, Do you think that they might be helpful in making solar cells? I was thinking that if you coupled the nanocrystals ( that absorb light) with the conductive pili you would increase the effeciency in transfereing the electrons to the circuit.
Thanks (again) for the tip. I’ll check out the paper on monday.
It’s definitely potentially useful to have nanowires for solar cells. Actually, Wilhelm Huck at Cambridge has shown already that if you grow a forest of nanowires (in his case, his wires are individual semiconducting polymer molecules) from one electrode of your cell that does boost the efficiency in just the way you suggest. The issue with the pili is just what the mechanism is they conduct electricity by. And of course from the practical point of view one has to ask whether the design is going to be economically viable for large area production, which will probably be an issue for a protein-based system.
From the article
“The materials have been created by extracting¬†individual atomic planes from conventional bulk crystals by using a technique called ‘micromechanical cleavage’. Depending on a parent crystal, their one-atom-thick counterparts can be metals, semiconductors, insulators, magnets, etc. Previously, it was thought that such thin materials could not exist in principle, but the research team have, for the first time, demonstrated that they are not only possible but fairly easy to make.
They found that the atomically thin sheets they extracted were not only stable under ambient conditions but also exhibited extremely high crystal quality, which is what gives them their unique properties.
Dr Kostya Novoselov, a key investigator in this research, added: “Probably the most important part is that our discovery is not limited to just one or two new materials. It is a whole class of new materials, thousands of them. And they have a variety of properties, allowing one to choose a material most appropriate for a particular application.”