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January 18, 2017

Microbial Nanowires Make for “Green” Electronics

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    An artist's rendition of Geobacter expressing electrically conductive nanowires. Microbiologists at UMass Amherst have discovered a new type of natural wire produced by bacteria that could greatly accelerate the development of sustainable "green" conducting materials for the electronics industry. [UMass Amherst]

    The inner workings of that new cell phone or tablet could be made from bacteria in the not so distant future, as investigators from the University of Massachusetts Amherst just reported about a new type of natural wire produced by bacteria that could greatly accelerate the development of sustainable "green" conducting materials for the electronics industry. In the new study, the researchers studied microbial nanowires—protein filaments that bacteria use naturally to make electrical connections with other microbes or minerals.

    “Microbial nanowires are a revolutionary electronic material with substantial advantages over man-made materials,” explained senior study investigator Derek Lovley, Ph.D., professor of microbiology at UMass Amherst. “Chemically synthesizing nanowires in the lab requires toxic chemicals, high temperatures, and/or expensive metals. The energy requirements are enormous. By contrast, natural microbial nanowires can be mass-produced at room temperature from inexpensive renewable feedstocks in bioreactors with much lower energy inputs. And the final product is free of toxic components."

    The findings from this study were published recently in mBio in an article entitled “Expressing the Geobacter metallireducens PilA in Geobacter sulfurreducens Yields Pili with Exceptional Conductivity.”

    The advantages of developing biologically derived electronic material are seemingly limitless, and the findings in this new study enable researchers to take a big leap forward toward reaching that goal. 

    "Microbial nanowires offer an unprecedented potential for developing novel materials, electronic devices, and sensors for diverse applications with a new environmentally friendly technology," Dr. Lovley added. "This is an important advance in microbial nanowire technology. The approach we outline in this paper demonstrates a rapid method for prospecting in nature to find better electronic materials."

    The conductive property of these bacteria can be found in their pili—hair-like projections on the surface of many bacteria that are often involved in the exchange of genetic material between microbes. However, for some bacteria, the pili produce filaments that can conduct electrical signals, often referred to as e-pili. 

    Until now Dr. Lovely's lab had been working with the nanowires of just one bacterium, Geobacter sulfurreducens. When his lab began looking at the protein filaments of other Geobacter species, they were surprised to find a wide range in conductivities.

    "Our early studies focused on the one Geobacter species because we were just trying to understand why a microbe would make tiny wires," Dr. Lovley noted. "Now we are most interested in the nanowires as an electronic material and would like to better understand the full scope of what nature may have to offer for these practical applications."

    For instance, one species recovered from uranium-contaminated soil produced poorly conductive filaments. However, another species, Geobacter metallireducens—coincidentally the first Geobacter species ever isolated—produced nanowires 5000 times more conductive than the G. sulfurreducens wires. "I isolated G. metallireducens from mud in the Potomac River 30 years ago, and every couple of years it gives us a new surprise," Dr. Lovely recalled.

    Yet, for this current study, the Amherst team did not study the G. metallireducens strain directly. Instead, they took the gene for the protein that assembles into microbial nanowires from it and inserted this into G. sulfurreducens. The result was a genetically modified G. sulfurreducens that expresses the G. metallireducens protein, making nanowires much more conductive than G. sulfurreducens would naturally produce.

    “We have found that G. sulfurreducens will express filament genes from many different types of bacteria,” Dr. Lovely stated. “This makes it simple to produce a diversity of filaments in the same microorganism and to study their properties under similar conditions."

    Dr. Lovely added that “with this approach, we are prospecting through the microbial world to see what is out there in terms of useful conductive materials. There is a vast reservoir of filament genes in the microbial world, and now we can study the filaments produced from those genes even if the gene comes from a microbe that has never been cultured."

    The researchers suggested that the G. metallireducens nanowires' extraordinarily high conductivity was due to its greater abundance of aromatic amino acids. Closely packed aromatic rings appear to be a key component of microbial nanowire conductivity, and more aromatic rings potentially mean better connections for electron transfer along the protein filaments. The high conductivity of the nanowires makes them an attractive material for the construction of conductive materials, electronic devices, and sensors for medical or environmental applications.

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