BioEd Online

Electric currents are being used to move bacteria around silicon chips and trap them at specific locations. The technique could help to assemble nanomachines from miniature parts, and to create a new generation of biological sensors.

Nanodevices are typically built by connecting tiny components. But such a delicate task is not easy. So, many researchers are exploring ways to fix components in place using the binding properties of biological molecules, notably DNA.

Robert Hamers and his colleagues from University of Wisconsin-Madison propose using entire microbes instead. The cells have surface proteins that attach to certain biological molecules. Once the cells are placed at specific sites on a silicon wafer, nanoparticles tagged with these molecules can bind to the cells in those locations. This is easier than dragging the nanoparticles themselves to the right spot, because their high density makes them harder to move through fluid media than the less dense living cells.

The technique gives one a way to fix components such as quantum dots or carbon nanowires at very precise locations, explains Paul Cremer, a bioanalytical chemist at Texas A&M University in College Station. "That's potentially very exciting," he says.

Golden rods

The researchers use Bacillus mycoides, rod-shaped bacteria that are about 5 micrometres long. They pass a solution containing the cells over a silicon wafer with gold electrodes on its surface. The charge on the electrodes captures the bacteria, which flow along the electrodes' edges like luggage on a conveyor belt.

The electrodes have tiny gaps between them. When a bacterium reaches a gap, it is trapped there by the electric field. It can be released by reducing the field between the electrodes, or permanently immobilized by increasing the voltage enough to break its cell wall.

Cells have been manipulated using electric currents before but it is typically done using larger cells, which are moved around as they are observed under a microscope. Hamers' work is unique because the locations of the bacteria are detected electrically.

When a cell bridges the gap between two electrodes, it acts like a wire and increases the current, signalling the bacterium's presence. Hamers presented the work on 17 March at a meeting of the American Chemical Society in San Diego.

"I think of it like catch-and-release fishing," he says. "You can collect the cell, measure it and then if you want you can release the field and let it go again."

He believes that electrical detection will allow the method to be used on organisms that are too small to be seen with an optical microscope. It should also help the automation of nanoscale assembly. "You don't want to have to visually inspect every electrode to see what's happening," he says. "You could have a computer detect it electrically."

As well as providing the glue for miniature devices, the system could also be used to detect harmful biological agents such as anthrax spores or certain strains of Escherichia coli bacteria. The electrodes on the chip could be coated with biomolecules designed to bind to particular pathogens and hold them in place, and other pathogens would flow away when the electrode voltage was reduced below a certain threshold.