Bottom-up approach used to fabricate nanowire (NW) resonator arrays. In step a) the probe molecules attach to the NW. In b) the nanowires are suspended in holes of photoresist. In c) add'l photoresist is applied sandwich the NW's in. In d) the resist is etched and melted is added to mount the NW. Finally in e) The detection is tested.  (Source: Penn State and Nature Nanotechnology)

An array of nanowires. Note the small grey 20 micron bar on the lower left. The nanowires have a diameter of approximately 300 nm and are visible as the small extensions from the white blobs (which are the metal mounts to the chip).  (Source: Penn State)
Chalk up another promising application of nanotechnology

Nanowires, nanoscale wires of organic or semiconductors material, are a hot new field of research due to their unique electrical and thermal properties.  Now researchers are looking to put the tiny wires to a new use -- in medical sensing.

A team at Penn State University has come up with a new bottom-up approach to help make attaching nanowires to a chip easier.  Such nanowires, with attached sensor molecules could create detectors such as "diving board resonators" which would monitor minute changes in resonance based on molecules bound to the sensor molecules at their tips.  Such chemical sensors could be used to create integrated detectors of cancer or other diseases, which emit chemical markers.

Rustom B. Bhiladvala, research assistant professor, electrical engineering who helped conduct the research describe the process stating, "Diagnostic chips can be made more useful by assembling, at predetermined locations on the chip, large numbers of nanowires pretreated off chip.  Using this new bottom-up method, our group has demonstrated that thousands of single wires can be successfully aligned and anchored to form tiny diving board resonator arrays."

The traditional top-down approach to designing resonators is more uniform than the new approach but also more limited.  In this approach nanoresonators are carved from base silicon.  Changes in the materials and the addition of chemical sensors must occur after these nanoresonators have been carved.  The varieties of molecules available for chemical sensors are also limited by the base materials.

The bottom-up approach involves attaching nanowires to a premade chip, through the use of photoresist and electrical current.  Such an approach takes advantage of self-assembly similar to that used to assemble a CMOS chip, previously detailed at DailyTech.  Each bottom up chip is unique, but by controlling the nanowire solution, material used in the nanowires, and the current applied, similar chips can be made.

Advantages of the nanowires method are that the wires can be attached to any location on the chip desired.  The wires can also feature a broader assortment of organic or inorganic materials and feature more types of probe molecules, which could be leveraged to make new, cutting-edge sensors.

Theresa S. Mayer, professor of electrical engineering, explains these advantages and disadvantages, stating, "We can achieve high device integration yields, but the devices are not as uniform as top-down manufactured devices.  However, we can access materials that are not easy to integrate into the devices with top-down methods. We can also integrate wires treated off-chip with entirely different probe molecules that are attached to the wires using condition optimized for that molecule."

The team used the new bottom-up approach to fabricate a proof-of-concept chip using a resonator array of single-crystal silicon or polycrystalline rhodium nanowires -- rhodium is a metal similar to platinum, also used in quantum computing.  The chips could detect molecules based on vibrations created when the molecules bound to the probe-tips of the nanowires.

The measurements required a partial vacuum to negate the effects of air dampening.  However the sensor was able to operate at pressures of up to a thousandth of an atmosphere, which are achievable by small inexpensive pumps.  This easy solution raises the technology's promise to the medical industry.

Meyer acknowledges that the approach is very new, but sees the approach as eventually being adopted for commercial medical sensors.  She states, "Bottom-up fabrication is an entirely new nanomanufacturing approach and we need to create devices that have properties that match what we can now make using top-down fabrication.  Our vision is to make large arrays of nano size devices with multiple probes for multiple targets by placing different groups of functionalized nanowires sequentially on chips."

The group's research appears in the current issue of Nature Nanotechnology (PDF).

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