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Tuesday, January 20, 2009

Nanosensors Made Easy

Nanosensors in a row: A scanning-electron microscope image (top) shows single nanowire detectors lined up on a silicon chip. An array of detectors (center), in which each row contains nanowires coated with a different DNA, lights up (bottom) when different fluorescent-tagged target molecules attach to the nanowires.
Credit: Penn State

Treated nanowires could serve as very sensitive toxin or pathogen detectors. But while nanowire sensors have been made in the lab, they have been difficult to mass-produce, mainly because there is no quick and easy way to place the tiny wires at precise locations on a surface.

Now researchers at Penn State University have come up with a way to guide single nanowires into place on a silicon chip using an electric field. Once the nanowires are in place, the researchers deposit electrodes on top to make arrays of sensing devices. This is a step toward affordable, sensitive handheld sensors that could quickly screen for hundreds of pathogens and toxic chemicals or catch the first signs of disease.

The new technique is simple, fast, and compatible with conventional silicon-chip fabrication. Others have already made single nanowire sensors, but "you need to take the next step and integrate them on a large scale using approaches that are manufacturable," says Theresa Mayer, an electrical-engineering professor at Penn State and one of the lead researchers on the new work, which appeared in Science. "What we're really interested in doing is adding new function to silicon integrated circuits."

Making nanowire detectors involves coating them with molecules that bind to certain target molecules, such as viruses or proteins. When a single target molecule attaches to the coating, the nanowire's conductivity changes. Detecting this electrical signal leads to sensors that are smaller, cheaper, and more sensitive than using current diagnostic chips, which rely on large microscopes to detect fluorescent molecules attached to the target molecule. "We would like to do this in a tiny chip all electrically," Mayer says. "So it would be potentially low cost, ultraportable, low power, and compatible for diagnostics at the point of care."

The difficulty has been finding an easy way to integrate the nanowires with electronics. Typically, researchers have deposited nanowires randomly on a surface, hunted them down using a microscope, and then made devices, says Ali Javey, an electrical-engineering and computer-science professor at the University of California, Berkeley. Harvard chemist Charles Lieber has devised a technique to line up nanowires using polymer bubbles, but it needs extra equipment and might be a challenge to automate.

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Using an electric field might be simpler. The Penn State researchers coat eight-micrometer-long and 300-nanometer-wide silica-coated nanowires with three different types of DNA. They etch three rows of shallow rectangular wells on a silicon chip; each well is designed to hold a single nanowire. Then they apply an electric field to the first row--the field's strength is designed to vary along the row--and deposit a solution of the first type of DNA-coated nanowire. "There's a very strong force that pulls the wires in the direction of the highest field strength," says Penn State chemistry professor Christine Keating, who led the work with Mayer.

One by one, the nanowires fall into the wells, going from the well exposed to the strongest field to that under the lowest field. The other two rows of wells are filled in the same way. "Assembling single wires has been very tricky," Javey says. "This is probably the best result I've seen in terms of assembling individual wires at discrete locations."

This is just a first step for the Penn State researchers, though. They deposit electrodes on their nanowire arrays, showing that an electrical connection to the wires is possible. But right now, they detect the DNA-coated wires optically using fluorescent-tagged target molecules. Keating says that to make a practical device, they need to assemble silicon nanowires and connect the wires to transistors on the silicon chip.

Mark Reed, a professor of electrical engineering and applied physics at Yale, has come up with a different way to make arrays of silicon-nanowire detectors, using an etching process similar to that employed to make integrated-circuit chips. Reed says that nearly all of the sensors etched on the chips work. The new electric-field technique, meanwhile, has a lower yield. About 70 percent of the sensors in the arrays worked; the other wells remained empty or got filled with multiple nanowires.

The new technique could also face competition from the nanosensing technology developed at Lieber's lab, which startup Vista Therapeutics is commercializing. Nevertheless, University of Pittsburgh chemistry professor Alexander Star says that the new method "is a really elegant and sophisticated way to assemble functional nanostructures."

Javey says that the process would need to work on smaller nanowires. Narrower nanowires are much more sensitive, but they're also more difficult to work with because they are flexible and break easily. "This is beautiful work," he says, "but [if it] can be scaled down to 10-nanometer wires . . . then that would make it even more exciting."