Plants, which grow through exposure to sunlight, have the ability to efficiently convert particles of light into electrons. By using this function as a component and fine laboratory-developed molecular wires as connections, an artificial eye harnessing the power of living organisms has been created. The hybrid sensor formed from this combination of organic and artificial materials can detect even extremely minute amounts of light, enabling light to be utilized to the utmost limits.
Galatea Transformed
Pygmalion and Galatea
© Erich Lessing/PPS
In Greek mythology, one story tells of a statue transformed into a living woman. Pygmalion, the king of Cyprus, fell in love with Galatea, the statue of a woman that he had carved himself, and wished for her to become a living being. Aphrodite, the goddess of love, granted his wish and transformed Galatea into a real woman.
However, despite all the advances of modern technology, we humans cannot artificially create even a single cell from nothing—the phenomenon of life is that complex and delicate. This situation has naturally led to the idea of using components from living organisms to create previously unattainable artificial devices. University of Tokyo professor and chemist Hiroshi Nishihara has long pondered the next step beyond nanotechnology research that is currently the mainstream. One notion of his is to combine living organisms and artificial devices to create a completely new material. Research into the process of extracting parts of living organisms began in the 1960s. However, the idea of using these parts as components, as chemical materials, is an entirely new undertaking.
One day we too may be able to create life from inanimate matter, just as Galatea was changed into a living person. However, this ability is certain to present both technological and ethical difficulties. In the near future, on the other hand, we will be able to extract material from living organisms to create new materials. This is the beginning of an age in which life and inanimate matter will be fused.
Using plant functions as components
Julius von Sachs, the German plant physiologist who discovered that plants use sunlight to create starch
© SPL/PPS
Four billion years have passed since life first appeared upon Earth. We are completely unable to imitate this phenomenon of life, brought to its present state by the process of evolution, using current technology. The plants that flourish in verdant splendor upon the Earth are bathed in the light of the sun, which they convert into electrons to create energy. This process is called photosynthesis.
What causes plants to grow was long a mystery. It was the Flemish physician Jan van Helmont who performed the first experiments leading to our current understanding of photosynthesis. Van Helmont perceived that a willow tree could grow even when given only water. Numerous other scholars followed in his path, but it was not until 1862 that the German plant physiologist Julius von Sachs discovered that plants live by using the power of sunlight to transform carbon dioxide into starch.
There are several steps involved from the absorption of sunlight to the creation of starch. The first of these steps is the conversion of sunlight to electrons within the plant. In plants, this conversion of light to electrons is performed with almost 100 percent efficiency, leaving no waste whatsoever.
“In future light-harnessing technologies, the ability to convert light to electrons with 100 percent efficiency will be extremely important,” notes Professor Nishihara. The ability to reliably detect weak light at room temperature will be indispensable for future technologies. However, this ability has proved difficult to realize, with artificially created materials having light/electron conversion rates in the lower double digits, at best. The reason for this is that with the increasing miniaturization of modern devices, the slightest irregularity in the crystal structure of their inorganic materials becomes a decisive flaw.
This has led to the first ever attempt to actually extract from plants the ability to convert light into electrons and use it as a component in a device. Professor Nishihara and his team were aware that fine metallic particles of two nanometers or less in diameter were well suited to detecting individual electrons, and they possessed the technology to synthesize such particles. If it were possible to convert light into electrons, this ability could be combined with fine metallic particles to develop the ultimate photovoltaic cell, photon detection system, or single-electron phototransistor.
Thus it was that Professor Nishihara and his colleagues, including two friends, botanist and professor at Tokyo University of Science Yasunori Inoue, and applied physicist and head researcher at the National Institute of Advanced Industrial Science and Technology Takashi Hiraga, came together to engage in a brainstorming session, each contributing from his own specialty, in order to give birth to the notion of extracting the components responsible for converting light into electrons from a type of algae called cyanobacteria, and connecting them with gold nanoparticles and molecular wires to a raw material. It would be the world's first hybrid organic/artificial system, designed to make maximum use of light.
Building a photo sensor from algae found in hot springs
Spinach was often used in previous research attempts to harness the power of plant photosynthesis. However, the component extracted from spinach, a protein complex called photosystem I (PSI), is susceptible to heat and breaks down as the temperature rises.For it to be of use as a material, such a component needs to be stable even at temperatures slightly higher than room temperature.
Thus attention was focused upon cyanobacteria algae, which flourishes in hot springs at temperatures of between 50°C and 60°C (approx. 108°F to 118°F). Besides being heat-stable, it is widely available and inexpensive, and is readily susceptible to the gene manipulation that is necessary later in the process. Moreover, it was known that whereas the PSI extracted from spinach would only last for about two days, the algae-derived PSI could be stably used for 80 to 90 days.
The component extracted from the algae converts light into electrons. The electro-conductive portion of the component (i.e. the vitamin K1) is removed and replaced with artificial molecular wires, and the electrons are extracted.
In order to extract PSI from the algae and turn it into a photo sensor capable of detecting weak light, Nishihara's team joined up with researchers from other fields* to push the project forward. The PSI extracted from the algae would form the component that converts light into electrons, but in order to artificially extract the converted electrons, new strategies would be necessary. Molecular wires were employed to this end.
- *Professor Inoue, Head Researcher Hiraga, professor at Tokyo Institute of Technology and specialist in spectroscopic chemistry Masaaki Fujii, professor at Shizuoka University and electronic engineering specialist Makoto Minakata, and professor at Nagoya University and electronic engineering specialist Kazuo Nakazato.
In the algae's PSI, the extracted electrons pass through chains of vitamin K1 on their way to being used as energy for the plant. Thus, the members of the team in charge of building the molecular wires hit on the idea of removing the vitamin K1 from the PSI complex, then replacing it with molecular wires formed from chains of gold nanoparticles. For the purposes of this project, the wires needed to have a structure similar to that of vitamin K1, good electrical conductivity, a certain degree of length, and elasticity to make them easier to work with later. As a result, such molecular wires were also designed as part of the process of producing the first hybrid photo sensor combining organic and artificial materials.
Creating an artificial eye from living organisms and artificial substances
Image produced by a hybrid biological-artificial photo sensor using components derived from algae. The power of plants was harnessed to successfully create an artificial photo sensor.
This was the process by which the world's first hybrid photo sensor, or artificial eye, combining organic and artificial materials, was developed. The ends of the molecular wires are attached to electrodes in order to reliably pick up electrical signals after the light is converted to electrons. Did this artificial eye work? In actuality, the team has already succeeded in converting light absorbed by the PSI to electrons and removing them in the form of a current. Furthermore, when a transistor (FET) was substituted for the electrodes and a projector used to divide up and project a 60x80 pixel image of the woodblock print Thirty-Six Views of Mt. Fuji in a time series, a distinct output image was successfully obtained.
While only a single FET was used in this test, recent preliminary image sensing experiments involving arrays of multiple FETs have also been successful. While the size of the gate electrode is still 17 square micrometers, if it were possible to reduce the electrode area to the nanoscale, then one could both reduce the interface capacitance of the electrode as well as limit the number of molecular wires on the electrode, making it possible to detect even minute numbers of light particles, or photons, at room temperature.
This utilization of a component possessed by living organisms not only yields a highly efficient energy conversion rate, but also has the benefits of low production costs and environmental load. It shows promise for the development of artificial retinas and other biocompatible photo sensors, ultra low-level photon detecting systems and phototransistor cells. In the meantime, solutions to problems unique to biological systems, such as durability, will require further study.
Editorial contributor / Date of article posted
Hiroshi Nishihara, Professor of Chemistry, School of Science, Graduate Faculty, University of Tokyo / November 2010