Nanosensors Made Easy

A trick to assemble nanowires on silicon could lead to cheap, tiny sensing devices.

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.

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

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."

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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.