Bionic bacteria: gold nanoparticles make gadgets of living microbes.

Blurring the boundaries between biology and the realm of electromechanical machinery, technologists have already used components of the natural world such as DNA to make robots. They've also coaxed living nerve cells to grow on a microchip, allowing neuroscientists to eavesdrop on cell-to-cell signaling (SN: 10/6/01, p. 216; 6/12/04, p. 382).

Now, two chemical engineers have created an electromechanical device out of living microbes. Vikas Berry and Ravi F. Saraf of the University of Nebraska in Lincoln have converted bacteria into humidity sensors by studding the cells' surfaces with gold nanoparticles.

The bacteria-as-sensors are exceptionally responsive in dry environments. One potential use would be to precisely measure humidity in dry places such as Mars, says Saraf.

The sensors function even after the bacteria die. But the fact that living microbes can function this way suggests more-sophisticated cell-based devices, such as biological transistors and tiny, microbe-powered batteries that could energize components on microchips, Saraf says.

"If you give the microorganism food, it will drive a device. That's where we're going," he says. "There are a lot of advantages to using these kinds of biological systems in electrical devices," comments chemist Robert J. Hamers of the University of Wisconsin-Madison. However, having to keep the organisms alive could be a drawback.

In the Oct. 21 Angewandte Chemie, the Nebraska researchers describe how they fabricated the new humidity sensors by first growing bridges of Bacillus cereus across electrodes on a silicon wafer. A thin layer of the amino acid lysine, used to put a positive charge on the wafer's surface, also fed the bacteria.

Next, Berry and Saraf dipped the bacteria-studded silicon into a solution of gold nanoparticles, each of which was also coated with lysine. Because the bacterial cells were negatively charged, and the wafer and nanoparticles had positive charges, the particles glommed on to the bacteria and stayed there when the wafer dried.

These microbes could read the humidity in the air because they lose water and shrink in dry conditions. So, as the humidity around them dropped, the cells' golden beads moved closer together. With a voltage applied across electrodes spanned by the bacteria, the current rose sharply through a quantum mechanical process known as tunneling. The researchers report that decreasing the humidity from 20 percent to zero caused a whopping 40-fold current increase.

The Nebraska team got into its microbial line of work by accident. While studying electron flow through assemblies of gold nanoparticles on silicon, Berry and Saraf discovered an unexpected colony of B. cereus. By using lysine to control electric charge in their experiments, the researchers had unwittingly set out a picnic for the bugs.