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Technology: Bacterial image detector spots anything that moves

作者:胥妥鹛    发布时间:2019-03-03 01:07:04    

By DANIEL CLERY A humble marine bacterium may be the key to robot vision systems which have hitherto existed only in the realms of science fiction. Using a protein derived from the purple pigment of Halobacterium halobium, Japanese researchers have taken the first steps by making a basic image detector with characteristics similar to those of a real retina (Science, 17 January, p 255). The protein, called bacteriorhodopsin, is similar to the protein rhodopsin, which detects light in a real retina. In the middle of the bacteriorhodopsin molecule is a nonprotein component called retinal. The energy of a photon causes the retinal to change shape, from straight to bent. Some atoms in the retinal carry a charge, and when it changes shape the charges move with respect to each other. This movement creates a potential difference in the retinal, which can be used to generate an electric current. To build their detector, Tsutomu Miyasaka and colleagues from the Fuji film company in Kanagawa, Japan, created a film of bacteriorhodopsin just one molecule thick on the surface of water, and transferred it onto a plate of glass coated with a transparent layer of tin oxide. The oxide is an electrical conductor, so acts as an electrode. Laying down several more films on top of the first creates a multilayer film of bacteriorhodopsin only a few tens of nanometres thick. All the molecules in the films have the same orientation, so when light strikes them they all generate a potential difference in the same direction. The researchers then covered the multilayer film with a water-based electrolyte gel and finally covered that with a gold electrode. This left the protein and electrolyte sandwiched in a photocell between two electrodes, one of which was transparent. Light passing through the tin oxide electrode is absorbed by the bacteriorhodopsin. This generates a potential difference that causes a current to flow from the electrolyte, through the electrodes, and into a circuit. Bacteriorhodopsin does not react to steady incoming light but to changes in intensity. The Japanese photocell therefore produces a pulse of current when first illuminated which drops to nothing if the light stays constant. When illumination is turned off, the protein produces a pulse of current in the opposite direction. This feature of the photocell is what makes it most like a real eye. Rhod-opsin in a retina behaves the same way, and this helps animals and humans to spot moving objects against an unchanging background. Conventional electronic photodetectors like those in a video camera do not have this ability. They simply detect the intensity of light and process it electronically. Researchers at the California Institute of Technology in Pasadena have constructed a retina out of silicon which closely mimics the function of a real eye, but it is cumbersome compared to the bacteriorhodopsin photocell. The Japanese researchers have gone on to make another photocell where the tin oxide electrode has been replaced with a square array of 64 electrodes made of indium-tin oxide printed on the glass. Each square in the array is 2.5 millimetres across and is connected to a light-emitting diode in a matching array of 64 LEDs. Each time the bacteriorhodopsin sends a current pulse through one of the electrodes, its corresponding LED lights up. The researchers shined images of letters at the photocell array. Nothing appears on the LEDs when the letters are shone steadily at the screen. However, when the letters move or are flashed on and off very quickly,

 

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