杏吧原创

WHEN Peidong Yang first took living organisms and connected them to electrified silicon wires, no one thought any good could come of it. 鈥淲hen I proposed the idea, people didn鈥檛 believe it would work,鈥� says Yang.

The microbes weren鈥檛 the only ones that got a shock. Yang鈥檚 experiments at the University of California, Berkley, and those of a few others, are showing that some organisms can not only survive an encounter with raw electrons pumped through the silicon, but live for weeks this way. In the process, they have opened up a new path towards sustainable energy. The hope is that this fusion of biology and electricity can solve one of chemistry鈥檚 biggest problems: how to take the freely available power of sunlight and convert it into a cheap, green energy source for everyone.

And not just that. By making microbes that pair some of our best light-harvesting technology with nature鈥檚 way of using the sun鈥檚 energy 鈥� photosynthesis 鈥� we might be able to create tiny, green factories that pump out any useful chemicals we desire.

鈥淣ature knows how to do chemistry and humans know how to make electricity,鈥� says Thomas Moore, who studies solar energy capture at Arizona State University. 鈥淚t makes a lot of sense to put the two things together.鈥�

Our desire to harness the sun鈥檚 power has roots going back a long time. in Colorado likes to highlight the foresight of an Italian chemist named Giacomo Ciamician. Writing in the journal Science in 1912, Ciamician wondered whether harnessing the sun鈥檚 energy the way plants do would be possible. 鈥�,鈥� he wrote. 鈥淲ould it not be advantageous to make better use of radiant energy?鈥�

鈥�Meshing plants with technology would turbocharge photosynthesis鈥�

Fast forward a century and you might think we have achieved his vision. The average consumer solar panel made from rigid silicon crystals now converts between 15 and 20 per cent of the sunlight that hits it into electricity. And other types of solar cell that are flexible and cheap are rapidly getting more efficient too. Plants actually look rather forlorn in comparison: the maximum theoretical amount of sunlight they can convert to biomass is 4.5 per cent. In real conditions, most achieve only about 1 per cent.

But that comparison ignores a problem that has solar energy tangled up in the weeds. Sunshine is not a constant. It only beams down on us during the day and even then it often hides behind clouds, making the stream of electricity from photovoltaic cells intermittent. That would be fine if we could easily store the electricity to use when we need it. But although our technology is getting better at matching electricity supply and demand, we still rely heavily on batteries, which are expensive, bulky and degrade a little with every charge cycle.

Plants have evolved a nifty antidote for the sun鈥檚 disappearances. They store their energy not on the basis of charged particles, but in chemical bonds. In other words, they make fuel.

鈥淔uels have intrinsically more storage capacity than a battery,鈥� says , one of the pioneers of artificial photosynthesis. 鈥淭here wouldn鈥檛 be enough room for nature to store energy as a battery does.鈥�

But whipping up artificial systems that do the same thing is onerous. Plants work their biochemical magic by taking in water and using the sun鈥檚 power to split it into oxygen, electrons and charged hydrogen ions, otherwise known as protons. Then those protons and electrons are combined with carbon dioxide to form sugars (see diagram). The whole delicate dance is chaperoned by biomolecules with elaborate chemical architectures that are tough to equal.

Delicate dance

It is Nocera who has probably come furthest along the path to replicating this feat. In 2011, he unveiled what is among the . Its design is simple and its components inexpensive. It looks more like a shiny grey postage stamp than a leaf (see below). It鈥檚 actually a silicon wafer impregnated with catalysts. But it can certainly make like a leaf. Pop it into water in the sunshine and bubbles of oxygen and hydrogen begin to form. That hydrogen is the key. It is a fuel that can be transferred into pressurised storage canisters or fuel cells, which can convert it back to electricity at will.

Nocera鈥檚 efforts are impressive, but they aren鈥檛 enough to drive an energy revolution. Hydrogen may be a fuel, but dreams of a hydrogen economy have been around for years and progress has stalled. That is partly because the fuel cells needed to convert hydrogen into electricity depend on catalysts made from expensive and scarce metals such as platinum. Plus, society has a massive infrastructure for carbon-based liquid fuels. Hydrogen gas doesn鈥檛 fit the mould.

Plants don鈥檛 have this trouble, since they store the energy they harvest as sugar, a fuel that they can metabolise. We would like artificial leaves to do something similar 鈥� spit out a fuel that suits our infrastructure. This biofuel would still produce carbon dioxide when burned, but because the artificial leaf sucks it in first, the net emissions would be next to zero.

Yet we have never mastered that final step. 鈥淲e know how to do solar power to electricity well with photovoltaics,鈥� says Moore. 鈥淏ut we don鈥檛 know how to do solar power to carbon-based fuel.鈥�

So a new idea began to emerge. Plants are the masters of orchestrating the biochemistry of fuel synthesis. But human technology surpasses them in terms of generating electrons. Would it be possible to mesh the two together and create a turbocharged cyborg version of photosynthesis?

Nocera was one of the first to investigate. Working with bioengineer , he began by taking the water-splitting, hydrogen-producing synthetic leaf from 2011 and pairing it with a soil bacterium called Ralstonia eutropha. The bacteria fed on the hydrogen, blended it with carbon dioxide and spat out a biofuel. Silver鈥檚 lab tinkered with the bacterium鈥檚 genome to get it to produce .

It worked, but only just. The hydrogen-producing catalyst that Nocera had developed also produced highly reactive oxygen atoms. They were so reactive, in fact, that they disrupted the bacteria鈥檚 biochemical machinery, killing them off within hours. But in work published last year, Nocera鈥檚 team revealed a fresh catalyst that could play nice with the microbes. The than the previous one and wildly efficient. It converted a whopping 10 per cent of the energy in sunlight into fuel (Science, vol 352, p 1210).

It was neat, but not in a league of its own. Yang had already begun his experiments with microbes and electricity. And he wanted to go further than Nocera. He wanted to feed microbes not hydrogen, but raw electrons.

鈥淭he scientific community didn鈥檛 recognise that this sort of thing would be possible 10 years ago,鈥� says Moore. But we discovered recently that some types of bacteria naturally survive on pure electricity, by directly ingesting electrons. We also now know that Geobacter microbes can take in electrons and use them in chemical reactions. But what Yang wanted to do was something else entirely.

Starting in 2013, his research group showed that certain types of non-photosynthetic bacteria could silicon nanowires. Two years later, the team discovered that the nanowires could . The microbes seemed perfectly happy with the arrangement, ingesting the electrons and using carbon dioxide and water to create liquid fuels such as acetate from hydrogen, carbon and oxygen.

Then came what might be a game-changer. Yang and his colleagues took another non-photosynthetic bacteria, Moorella thermoacetica, which naturally generates acetate, and added a mix of chemicals, including cadmium ions and the amino acid cysteine. They saw that light-absorbing particles made from cadmium sulphide had appeared on the surface of the bacteria. It seemed that the microbes had from the chemicals (Science, vol 351, p 74). 鈥淚t takes a lot of effort for us to make the semiconductor nanostructures,鈥� Yang says. 鈥淗ere, the bacterial cells created this semiconductor surface themselves.鈥� It鈥檚 a sun-powered fuel factory that replicates itself.

鈥淵ang鈥檚 work is very exciting 鈥� it鈥檚 completely new,鈥� says , who is also developing bionic photosynthesis systems.

Questions remain, however, over durability. So far, the electron-consuming bacteria can survive in Yang鈥檚 apparatus for only several weeks. Yang is still focused on understanding their biochemistry. He hopes doing so will help him improve the efficiency of what he calls his 鈥減hotosynthetic cyborg system鈥�, which is currently 2.5 per cent. 鈥淭his is all new and we need to understand the details,鈥� he says. 鈥淥therwise it is black magic.鈥�

And you can forget the sleek silicon wafer style fully artificial leaf; these systems are still prototypes. It鈥檚 unclear to what extent they can be scaled up. Nocera and Silver are working on a pilot reactor in India, which should provide some answers.

Nocera is at pains not to oversell bionic leaves. 鈥淚 don鈥檛 have any false pretences that next year I will solve the global energy problem,鈥� he says. The cost of producing fuel from bionic leaves will probably remain higher than extracting oil for the foreseeable future. Nocera says market interventions like carbon pricing will be necessary before devices like his make economic sense. 鈥淲hen we could see this type of technology applied has little to do with discovery and a lot more to do with global markets,鈥� he says.

In fact, what鈥檚 causing the most excitement right now is all the other things a bionic leaf might produce. 鈥淚t鈥檚 not just about making fuel,鈥� says Moore. 鈥淏iology also makes other fantastic chemicals that are valuable for our society.鈥�

The first fruits are already being picked. Take ammonia, a molecule made of nitrogen and hydrogen atoms that is a crucial part of fertiliser. We used about 166 million tonnes of it in 2016. Yet we still make it using the energy-intensive and 100-year-old Haber process, which creates lots of carbon dioxide. But a new way of producing ammonia along the same lines as a bionic leaf is on the cards.

King has recently extracted the biochemical machinery that certain types of bacteria use to convert nitrogen in the air to ammonia. Put this into a solution, bubble through nitrogen and add a cadmium sulphide semiconductor and you have a with a twist: it now effectively produces ammonia from sunlight. 鈥淲e鈥檝e removed the living cell and its complexity and simply worked with an enzyme,鈥� says King.

Cyborgs evolve

Purifying the nitrogenase enzyme from bacteria isn鈥檛 likely to scale up because it鈥檚 so time consuming. Instead Kings is hoping to show how nitrogenase works and so help synthetic chemists mimic it with easy to handle artificial analogues.

Yang sees a different way forward; not deconstructing cells, but making them more elaborate. At the moment his 鈥渓eaves鈥� are simple cells, a package of enzymes and biological machinery encapsulated in a membrane. But 鈥渆volve鈥� them into more complex cells, with internal units each equipped to do specialised chemical transformations, and you could end up with cells that work as processing lines for complex and interesting chemicals.

鈥淲e can start thinking about this as a general renewable chemical synthesis platform,鈥� says Nocera. Because the bacteria can be genetically manipulated, it is possible to have them make plastics, pharmaceutical drugs or compounds whose synthesis would otherwise require a lot of fuel. It鈥檚 this sort of application that Nocera thinks will be the first to be economically viable. 鈥淎s these processes become cheaper, the next important step would be fuel production.鈥�

A neat stamp of approval for such ideas recently came from NASA. Yang has received a funding package from the agency鈥檚 new Center for the Utilization of Biological Engineering in Space. This outfit plans to use living organisms to produce some essentials for astronauts, including food, fuel and oxygen.

The plan will be to get Yang鈥檚 bionic leaves to pull off King鈥檚 trick of taking nitrogen and carbon dioxide and producing ammonia as fertiliser for food crops in space and oxygen to breathe. 鈥淥n Earth, fuel is it and oxygen is of no value,鈥� says Reisner. In space, of course, oxygen is crucial.

Yang is even imagining building a system that combines different types of bionic cells with various functions. These might work more like an organism, with sensing cells checking when oxygen supplies get low, for example, and getting the leaf cells to dial up their photosynthesis.

We have certainly come a long way since Yang first tried connecting his bacteria to electricity. 鈥淚t鈥檚 getting closer to the movie The Martian,鈥� says Nocera. Perhaps one day bionic leaves will eat their electrons on another planet.

This article appeared in print under the headline 鈥淢ake like a leaf鈥�