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Volume 87 Issue 48 | pp. 30-31
Issue Date: November 30, 2009

From Thesis To Business

Flexible, high-aspect-ratio nanowires turn researcher into entrepreneur
Department: Science & Technology
Keywords: Nanowire, Silver-gallium, Conductivity
Scanning electron microscopy video shows what happens when a silver-coated atomic force microscopy tip is dipped into a drop of liquid gallium at room temperature: Gallium readily adheres to the silver-coated tip, and as the tip is retracted, a nanowire of Ag2Ga alloy forms and eventually detaches from the gallium drop (J. Appl. Phys. 2005, 98, 073510-1).
Credit: NaugaNeedles

At NaugaNeedles' facilities in Louisville, a technician checks a large flat television screen connected to a microscope stage and a complex system of optical lenses as he prepares to begin the day's work. Using a joystick, he dips the silver-coated tip of an atomic force microscope (AFM) cantilever into a droplet of liquid gallium. As he retracts the tip, a wire of silver-gallium alloy forms at the silver-coated tip from the point of contact. The wire eventually separates from the droplet, just as a thin stream of honey lifted by a honey dipper eventually breaks. By the end of the day, the technician will have made about 40 probes with long, thin, conductive metallic tips.

The metallic wires have uniform diameters that can vary from 25 to 1,000 nm; their lengths can vary from 1 to 100 µm.

Attached to an AFM tip, the wires impart a highly desirable quality: a high aspect ratio; in other words, being long and thin. A standard AFM tip is tapered but broadens quickly. "If you push it to any great distance, it would rip a cell open," explains Robert W. Cohn, a professor of electrical and computer engineering at the University of Louisville. On the other hand, an AFM probe with a high aspect ratio can reach small cavities that a standard probe can't enter, such as the deep trenches of an integrated circuit. In addition, like the old whip antennas on cars, the wires are rigid but flexible, bending generously before they break.

The silver-gallium nanowires are the fruits of Mehdi M. Yazdanpanah's Ph.D. work at the University of Louisville under Cohn's direction. Already they are enabling experiments that had been difficult to do consistently or with sufficient resolution before. And NaugaNeedles, which Yazdanpanah founded to commercialize the invention, may be on the way to becoming a viable small business.

Yuguang Cai, an assistant professor of chemistry at the University of Kentucky, has used NaugaNeedles' high-aspect-ratio nanowires to characterize biomolecule-decorated patterned surfaces that his research group designs for biosensing or bioanalysis applications. A recent example is nucleic acids preferentially bound to the edge of a protein pattern on a silane film. To characterize the binding, Cai and coworker Pei Gao applied a method called Kelvin probe force microscopy to measure surface potentials. A generic pyramid-shaped tip could not resolve the nucleic acid molecules bound to the edge of the protein pattern, Cai and Gao write in Analytical & Bioanalytical Chemistry (2009, 304, 207). A high-aspect-ratio silver-gallium tip, however, afforded a spatial resolution of 30 nm, and they could clearly see the molecules at the edge.

The high-aspect-ratio tips reduce the nonlocal interactions between a probe and the sample. That means the probe detects only what is directly beneath its small-diameter tip. This quality is not available in other commercial probes, say the researchers who have used them. "The closest product on the market as a high-aspect-ratio probe is probably carbon nanotube-modified tips," says Andy Wain, a research scientist with the Electrochemistry & Corrosion Group at the National Physical Laboratory, the U.K.'s national measurement institute.

Another unique feature is the high conductivity throughout the length of the metallic wire, says Minhua Zhao, a research associate at the National Institute of Standards & Technology. "Conductive coatings on other commercial probes may be worn out during scanning," he says. Because of the high aspect ratio and high conductivity, the wires "are particularly suitable for applications in electric force microscopy," he says. "For instance, I could clearly distinguish different polystyrene-block-polyethylene oxide crystals from surface potential measurements using these high-aspect-ratio nanoneedles," when conventional probes revealed no differences.

One of the early users of NaugaNeedles' nanowires is Babak Sanii, a materials science researcher at Lawrence Berkeley National Laboratory. "I made a couple of nanowires," using the reported method (J. Appl. Phys. 2005, 98, 073510), he tells C&EN. But "there were clearly tricks to the trade to have more control over the size of the nanowires," he says. So he contacted NaugaNeedles in December 2008, and "we've bought dozens of their nanowires since," he says.

In conventional AFM systems, interactions at the AFM tip cause the cantilever to deflect. The deflection is a measure of local conditions at the tip. Because the nanowires are flexible, in some ultrasensitive detection schemes, their motions alone can measure those local conditions.

Taking advantage of this property of the nanowires, Sanii uses them as the cantilevers themselves and detects their deflections with what he calls a "home-brewed" detection scheme. The nanowires' small size "gives us orders-of-magnitude improvement in how gently we can probe in water, which we intend to demonstrate by imaging the membranes of living cells," he tells C&EN. An example of an experiment Sanii now hopes to do when his detection scheme is fully operational is finding inhomogeneous lipid domains on the surface of living cells. These domains, also called lipid rafts, are thought to participate in protein clustering and cell signaling, but evidence of their existence has been hard to obtain (C&EN, Feb. 9, page 31).

[+]Enlarge
QUALITY CONTROL
NaugaNeedles employee David Mudd prepares to review images of nanowires before the products are shipped to a customer.
Credit: Nauganeedles
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QUALITY CONTROL
NaugaNeedles employee David Mudd prepares to review images of nanowires before the products are shipped to a customer.
Credit: Nauganeedles

Back in the U.K., Wain has yet to put the nanowires to work. He's waiting for custom-coated needles for his particular application. He wants to use them as nanoscale electrodes to map electrochemical activity with resolution better than that achieved by scanning electrochemical microscopy, a more conventional technique for such local measurements. The technique, Wain explains, involves scanning an ultramicroelectrode over a substrate immersed in an electrolyte solution and measuring the current due to electrochemistry occurring locally at the probe's tip. Resolution is limited by the size of the probe—the smaller, the better. "The important criterion here is that electrochemistry only takes place at the very tip of the probe; the rest of the probe must be insulated from the solution. NaugaNeedles is developing a coating procedure for their needles for this purpose," Wain tells C&EN.

The silver-gallium nanowires have come a long way since mid-2003, when Yazdanpanah first observed the spontaneous growth of rods upon dipping a silver-coated AFM tip into liquid gallium. "It's similar to growing rock candy or pulling crystal from a melt," says Cohn, who is also director of the University of Louisville's ElectroOptics Research Institute & Nanotechnology Center. He recalls that Yazdanpanah had been growing needlelike crystals from other combinations of metals, but the silver-gallium combo "has been the easiest to do so far."

By the end of 2004, Yazdanpanah believed he could build a business based on the silver-gallium nanowires serving as tips of AFM probes. With encouragement from Cohn, who heads NaugaNeedles' board of advisers but has no financial stake in the company, Yazdanpanah learned the ropes of business plans, market research, and other tools of entrepreneurship and then launched the company on July 7, 2007.

Early financial assistance came from the University of Louisville. When NaugaNeedles licensed the nanowire technology from the university in 2008, the university awarded Cohn and Yazdanpanah a $75,000 grant to help with market research. And in early October, Yazdanpanah began a stint as one of the 13 inaugural entrepreneur postdoctoral fellows of the Ewing Marion Kauffman Foundation, a Kansas City, Mo.-based philanthropy devoted to entrepreneurship. In addition to salary, the yearlong fellowship offers fellows mentoring and internships to hone their business skills.

Seed money for the company itself was $45,000 from two private investors. Until now, NaugaNeedles has raised $618,000 in funding from private, state, and federal sources. The big break in funding, Yazdanpanah says, is $120,000 the company won in March for presenting the best business plan among the competitors for the Vogt Award, a private program that aims to cultivate entrepreneurship in Louisville.

The current staff of one full-time employee—Yazdanpanah himself—and three part-time employees will grow to three full-time and three or four part-time employees in January, Yazdanpanah tells C&EN. Sales for the first six months of 2009 are $25,000, the company says, and it projects 2010 sales of several hundred thousand dollars. It also expects production to ramp up to 100 needles per day by the end of 2009.

"Mehdi is very personable, enthusiastic, and with lots of drive to make things happen," Cohn says of Yazdanpanah. "He worked hard on the business plan and market research, he looked good on paper, and he interviewed well," he says of Yazdanpanah's preparation for and performance at the Vogt Award and at the Kauffman postdoctoral fellows competitions. "I think he's on his way to a self-sustaining business."

 
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