Astronomy Domine

Scientists first coated a silicon chip with a layer of live Bacillus cereus bacteria. Some of the long, rod-shaped microbes lodged between two etched electrodes on the chip’s surface, forming a bridge. The chip was then washed in a solution containing tiny gold particles, each one about 30 nanometers across The gold nanoparticles attached to long hair-like proteins on the surface of the bacteria, transforming them into gold-plated bridges that completed an electronic circuit. The hair-like proteins are called teichoic acid molecules. They are negatively charged and provide a surface for the positively-charged gold nanoparticles to attach to. Without them, the gold nanoparticles would repel one another due to their like-charges and no bridge between the two electrodes could ever form. By wrapping themselves around the gold nanoparticles, the teichoic acid molecules therefore act as metal insulators, creating what engineers call a “dielectric barrier.” “To any electronic person, that’s a field day,” said Ravi Saraf, a University of Nebraska chemical engineer who led the discovery. “You can go nuts with it.” The cellborg sensor is extremely sensitive: a drop from 20 percent to zero humidity results in a 40-fold decrease in current flow. In humidity sensors that are solely electronic, the decrease is only 10-fold. Once assimilated, the gilded bacteria can survive for only about two days, but even when dead, their bodies still swell and contract in response to changes in humidity. They can go on working this way for months, Saraf said. .