Scientists create technology to exploit electrical nanowires hidden in soil and oceans

How do the nanowires work?

Essentially, all living things need oxygen to expel excess electrons during the process of converting nutrients into energy. But what happens if there’s no access to oxygen, for example, in the soil or deep at the bottom of an ocean? It turns out some bacteria adapted to this quandary by essentially "breathing minerals" through very small protein filaments or “nanowires”.

What’s more, the scientists found a particular bacteria that can be manipulated to produce these nanowires on an industrial scale. As Professor Malvankar stated, they discovered that a “bacteria called Geobacter have evolved over billions of years how to interface with outside world electronically” using nanowires comprised of chains of “heme” molecules just like the ones carrying oxygen in our blood. But instead, the Geobacter’s hemes transfer electricity. The micrometer-long heme chains provide a continuous path for electrons.

When light is added to the bacteria, “you actually get almost 100 times more electricity,” revealed Malvankar.

The secret ingredient

How does the Geobacter do it? As Geobacter is a very common bacteria, found in the ground under our feet, Malvankar believes it provides the solution to what he considers to be the main bottleneck problem for them to solve — “how can we connect living systems with electronics efficiently at large scale?”

The key lies in their finding of a metal-containing protein known as cytochrome OmcS, which acts as a natural photoconductor. This was their main discovery, according to Malvankar. The metal is what gives the bacterial nanowires high-electron conductivity.

“They look like a real wire. The heme molecule is like a wire and the protein is like insulation around the wire,” explained the scientist, comparing it to how our phone chargers have insulation.

Why do some bacteria need nanowires?

The answer, according to Malvankar, lies in the quote “Life is nothing but an electron looking for a place to rest“ by Albert Szent-Györgyi, a biochemist, who discovered the oxygen metabolism cycle and won 1937 Nobel prize in “Physiology or Medicine.”

Growing nanowires allows the bacteria to survive without the presence of oxygen. The bacteria use the wires to send electrons, allowing them to access a much larger area. Light accelerates respiration by bacteria due to rapid electron transfer between nanowires.

“What excites me about this work is it extends the range of how far electrons can go in a living system,” shared Malvankar, adding that “typically, in humans, an electron can only go over a nanometer, or a billionth of a meter, because proteins are not very good at moving electrons very long. Whereas here, the electrons can go hundreds of micrometers.” That makes it almost several million times longer, all thanks to the network of nanowires.

How the nanowires are produced

Malvankar shared that they can now produce these multifunctional “living electronics” at a much faster rate and at a lower cost.

A big plus of the material in the wires is that it combines useful properties — it is self-repairing, flexible, biodegradable, and non-toxic. Malvankar compared it to silicon, which works well under certain conditions, while the nanowires are even more flexible. They can work in a number of different and extreme environments — at high and extremely low temperatures, or in situations of low acidity —while remaining very stable and robust.

As far as production, the nanowires can now be grown in the laboratory and will retain their properties even though they are not alive.

The scientists can use the bacteria to grow the wires, then put them “in a blender”, get rid of the bacteria and leave just the “purified filaments — the wires which are then stable enough to be utilized.”

What are potential applications for the nanowires?

Malvankar sees a number of exciting applications possible for the biomaterials they are developing, which would be cheaper, faster, and more precise than other current technologies of this kind.

The nanowires could be very useful in addressing greenhouse gas emissions, which cause rising temperatures and warming of the planet. As they develop the nascent field of electrogenetics (which uses electrostimulation to control biological processes), the researchers aim to figure out how to activate the electric grid on the ocean floor to stop methane from being released into the atmosphere.

As Malvankar elaborated, they hope to put “electrodes in the ocean or soil, and those electrodes will be used to induce these electrical connections between bacteria. And that could be one way we can locally stop release of methane to the environment, or locally, you know, clean up the environment of toxic contaminants. So, if there's an oil spill, this bacteria can eat up oil. So we can accelerate that process locally by inserting these electrodes.”

The nanowires could also be a revolutionary tool for use in DNA sequencing, computing, light-harvesting, or in health monitoring through the creation of a new generation of self-powered body sensors that measure glucose or oxygen levels.

Another application of these wires stems from the fact that “they show properties no other protein showed before”, according to the scientist. The wires his lab is working on are very adaptable, performing particularly well at very low temperatures. When scientists cooled them, the electron transfer actually sped up about 300-fold. This could work for low-temperature sensors or other electronics that need to perform in extreme conditions.

One such extreme condition could be outer space — the exploration of Mars, in particular. It may be possible, Malvankar speculates, to use their methods to induce the bacterial nanowires to produce specific chemicals, biofuels, or nutrients necessary for Mars colonization. It also may turn out that Mars, whose soil is particularly rich in iron, already has such bacteria present on its surface. By using their techniques to activate it, the scientists might be able to “basically mimic how we evolved life on Earth,” proposed Malvankar.

Study Abstract:

Light-induced microbial electron transfer has potential for efficient production of value-added chemicals, biofuels and biodegradable materials owing to diversified metabolic pathways. However, most microbes lack photoactive proteins and require synthetic photosensitizers that suffer from photocorrosion, photodegradation, cytotoxicity, and generation of photoexcited radicals that are harmful to cells, thus severely limiting the catalytic performance. Therefore, there is a pressing need for biocompatible photoconductive materials for efficient electronic interface between microbes and electrodes. Here we show that living biofilms of Geobacter sulfurreducens use nanowires of cytochrome OmcS as intrinsic photoconductors. Photoconductive atomic force microscopy shows up to 100-fold increase in photocurrent in purified individual nanowires. Photocurrents respond rapidly (<100 ms) to the excitation and persist reversibly for hours. Femtosecond transient absorption spectroscopy and quantum dynamics simulations reveal ultrafast (~200 fs) electron transfer between nanowire hemes upon photoexcitation, enhancing carrier density and mobility. Our work reveals a new class of natural photoconductors for whole-cell catalysis.