2004/9/25 朝日新聞　　　　　　　　　　　　Science News誌　　　ＭＩＴ release

ホウレンソウで発電します　ＭＩＴ、初の光合成電池開発
http://www.asahi.com/science/update/0925/001.html

　米マサチューセッツ工科大（ＭＩＴ）は、ホウレンソウを使った「太陽電池」を開発したと発表した。米化学会機関誌の最新号に掲載された。発電量はまだごくわずかだが、「光合成を利用した固体電池は世界初」としている。

　植物の光合成は、光のエネルギーを利用して有機物をつくる。この働きを発電に応用するアイデアは以前からあったが、光合成を担うたんぱく質を分離して使うには水分が必要で、固体電池には不向きとされてきた。研究チームは、ある種の物質を混ぜると、たんぱく質が乾燥状態でも長持ちすることを発見、入手の容易さなどからホウレンソウを材料に選んだ。

　完成した電池は、ホウレンソウから抽出したたんぱく質をガラスと特殊な半導体で挟んだサンドイッチ構造。ガラス面に光を当てると、弱い電流が生じた。「電力変換効率は１２％くらい。多層構造にするなどの工夫で、実用に耐えうる２０％を突破したい」と研究チームは説明している。

Science News 　　Week of June 5, 2004

Protein Power: Solar cell produces electricity from spinach and bacterial proteins
http://www.sciencenews.org/articles/20040605/fob2.asp

Inspired by the efficiency with which plants convert sunlight into sugar, researchers have fabricated a solar cell that uses photosynthetic proteins to convert light into electricity. Although the prototype device can't yet rival commercial solar cells made of silicon, it demonstrates a new strategy for making longer-lasting photovoltaic cells.

	：	To make the solar cell, a team of biologists and engineers led by Marc Baldo of the Massachusetts Institute of Technology (MIT) harvested photosynthetic proteins from spinach and the bacterium Rhodobacter sphaeroides and deposited the proteins onto a glass support. Because the proteins naturally reside in an aqueous environment inside a cell membrane, it took some creative chemistry to keep the approximately 2 billion isolated proteins functional on a solid surface. Consider the new material that MIT molecular biologist Shuguang Zhang developed to stabilize the proteins. It consists of synthetic peptides that self-assemble into structures resembling cell membranes. When embedded in the synthetic membranes, the photosynthetic proteins retain their function. The MIT group placed a thin layer of this membrane complex on a glass surface coated with indium tin oxide, which served as a transparent electrode. The researchers then added a soft layer of an organic semiconductor and topped it all with a silver electrode.
GREEN MACHINE. In this prototype solar cell, photosynthetic proteins (spheres embedded in yellow peptides) absorb light and pump electrons (e-) into a silver electrode.

When the researchers shone light of certain wavelengths onto the device, the photosynthetic proteins absorbed the photons and shunted excited electrons through the semiconductor layer and into the silver electrode, creating a current. Baldo and his colleagues describe the working device in the June issue of Nano Letters.

"This is very exciting work," says Peter Peumans of Stanford University, noting that the new strategy opens many possibilities for making not just solar cells but also other protein-based electronic devices. However, he says, to make a useful solar cell, the MIT team will have to dramatically increase the device's efficiency.

To boost the solar cell's power output, Baldo and his colleagues are exploring ways of packing more photosynthetic proteins into their 1-millimeter-by-1-millimeter device. One potential way of achieving that goal is to roughen the glass to increase the amount of surface area that can hold the proteins.

Even if Baldo and his colleagues can't boost their new solar cell's efficiency to match that of commercial photovoltaic devices, there could be other advantages to a protein-based design.

For example, many solar cell materials degrade over time, but a protein-based solar cell could be self-repairing, says Baldo. Just as living plants replenish their photosynthetic proteins by swapping out the old copies for new ones, it might become possible to flush a solution of fresh proteins through a solar cell to replace the photosynthetic molecules as they degrade, Baldo explains.

Stephen Forrest of Princeton University says that experiments such as Baldo's could also give researchers a greater understanding of the mechanisms underlying photosynthesis. "Nature has taken a very long time to optimize solar energy collection and conversion," he says, "and it has many strategies for doing that."

MIT　September 15, 2004

Green, leafy spinach may soon power more than Popeye's biceps
http://web.mit.edu/newsoffice/2004/spinach-0915.html

For the first time, MIT researchers have incorporated a plant's ability to convert sunlight to energy into a solid-state electronic “spinach sandwich” device that may one day power laptops and cell phones.

At the heart of the device is a protein complex dubbed Photosystem I (PSI). Derived from spinach chloroplasts, PSI is 10 to 20 nanometers wide. Around 100,000 of them would fit on the head of a pin. “They are the smallest electronic circuits I know of,” said researcher Marc A. Baldo, assistant professor of electronic engineering and computer science at MIT.

Baldo and other researchers from MIT, the University of Tennessee and the U.S. Naval Research Laboratory, including electrical and biomedical engineers, nanotechnology experts and biologists, collaborated on the world's first solid-state photosynthetic solar cell. The work was reported in NanoLetters, a publication of the American Chemical Society.

“We have crossed the first hurdle of successfully integrating a photosynthetic protein molecular complex with a solid-state electronic device,” Baldo said.

Plants’ ability to generate energy has been optimized by evolution, so a spinach plant is extremely efficient, churning out a lot of energy relative to its size and weight. But combining biological and non-biological materials in one device has stymied researchers in the past. Biological materials need water and salt to survive?both are deadly for electronics.

From wet to dry
A new twist in the current work is a membrane of peptide surfactants - similar to the main ingredient in soap - that helped the photosynthetic complexes self-assemble and stabilize while the circuit was fabricated.

So far, scientists and engineers’ efforts to harness the photosynthetic properties of green plants have been most successful with naturally soft organic materials in liquid solutions. But if organic solar cells are to be practical for commercial devices, they need to be integrated with solid-state electronics.

The researchers ground up ordinary spinach and purified it with a centrifuge to isolate a protein deep within the cell.

The resulting dark green pellets that smell like cut grass were purified still further and coaxed into a water-soluble state. One of the challenges was to keep the proteins in the same configuration as they appear naturally in the organism.

Here's where peptides come in. The 80,000-plus kinds of proteins in our body, when in fragments called peptides, transform themselves like tiny LEGOs? into millions of substances. Shuguang Zhang, associate director of MIT's Center for Biomedical Engineering, discovered that these same peptides can be tweaked into forming completely new natural materials that perform useful functions. One of his designer nanomaterials, which acts like the main ingredient in soaps and detergents, turns out to be ideal for keeping protein complexes functional on a cold, hard surface.

The spinach-sandwich device has no water. Proteins usually need water to survive, but using Zhang's detergent peptide, the researchers were able to stabilize the protein complexes in a dry environment for at least three weeks. “Detergent peptide turned out to be a wonderful material to keep proteins intact on the surface with electronics,” Zhang said. He speculates that the detergent material has some water trapped within it, similar to the way plant seeds hoard oils that maintain the seeds’ integrity in dry conditions.

Building the sandwich
The bottom layer of the molecular electronic device is transparent glass coated with a conductive material. A thin layer of gold helps the chemical reaction that assembles the spinach chlorophyll Photosystem I complexes. The researchers then evaporate a soft organic semiconductor that prevents electrical shorts and protects the protein complexes from the layer of metal that completes the sandwich.

The researchers shone laser light on the device to create optical excitation, then measured the resulting current. “An important caveat is that we got very little current out, mostly because we had just a thin layer of the complexes in our devices,” Baldo said. “Most of the optical excitation passed straight through without being absorbed. Of the light that was absorbed, we estimate that we converted around 12 percent to charge.”

The researchers hope to achieve a power conversion efficiency of 20 percent or more (which would provide an extremely efficient power source) by creating multiple layers of PSI or assembling them on rough surfaces or 3-D surfaces, like skyscrapers that concentrate a huge amount of surface area within a relatively small space.

Patrick J. Kiley (S.B. 2003) of MIT also worked on this research, which is funded by the Defense Advanced Research Projects Agency, the Air Force Office of Scientific Research, and the National Science Foundation.