Nanowire Platforms Are Diving Boards for Diagnostics

R&D DIGEST

Detection techniques for cancer and other diseases are increasingly moving to chips assembled from nano­wires that act as resonator arrays. Such devices are used to target molecules as they bind to probes on the nanowires. The change in vibration of the nanowire announces the target molecule's presence.

What is distinctive about the work performed by Pennsylvania State University researchers is the fabrication method the university team is using to create these arrays.

The typical process uses a top-down approach. In this method, nanoresonators are carved from silicon to produce nearly identical devices. Although the repeatability is a benefit, it also limits the changes that can be designed into the chip. “It is less well suited for applications such as on-chip sensing that benefit from integration of devices comprised of many different types of materials,” says Theresa Mayer. She is a professor of electrical engineering and coauthor of the study.

Adding chemical probes or making other changes must be done after the devices are fabricated on the chip.

Alternatively, the researchers are experimenting with fabrication from the bottom up. “Diagnostic chips can be made more useful by assembling, at predetermined locations on the chip, large numbers of nanowires [that have been] pretreated off-chip,” says Rustom Bhiladvala, research assistant and professor of electrical engineering. “Using this new bottom-up method, our group has demonstrated that thousands of single wires can be aligned and anchored to form tiny diving board resonator arrays.”

To demonstrate the principle, the team fabricated chips with nanowires of single-crystal silicon or polycrystalline rhodium attached at one end and suspended over a depression.

Controlling where the wires go is a significant challenge, the researchers say. Using a photo­resist material that can be etched when exposed to light, the researchers make tiny rectangular wells on the chip. The wells are aligned above an insulated electrode on the chip's surface. A solution of nano­wires, with probe molecules attached, is flowed over the surface while the electrodes are activated.

The electric field pulls the nanowires to the surface and aligns them perpendicular to the chip. When the wires reach a well, they drop down, allowing only one wire per well. The number of wires in the solution is controlled so that only a few remain outside the wells.

For the final step, the team places a different layer of photoresist material on top of the chip. They then remove only a cube on the tip of the wire. An electrodeposit metal is flowed into those cubes, providing an anchor for the nanowire. Then the rest of the photoresist material and any nanowires left on the surface are dissolved.

This method does not produce uniform devices, the team notes. But it does enable flexibility so that the team can manufacture nanowires off chip, using any inorganic or organic material that will produce nanowires. In addition, probe molecules can be attached using a variety of chemicals, and each group can be attached to the chip in various numbers and locations. “Ultimately, we envision ultrasmall integrated sensor chips where the sensing elements [nanowires] are integrated directly with the on-chip processing in a single low-power, low-cost package,” Mayer says.

The National Institutes of Health and the National Science Foundation supported for the research. Members of the research team have filed a provisional patent on the assembly process.

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