i n my s h o e s
The road less traveled to nanowire sensors One researcher fabricated his nanowires in an unusual way, and that has made all the difference.
W
hen Reg Penner began to think about nanowires, the University of California (UC), Irvine, researcher asked himself what any good electrochemist would: What can I detect with them? The fundamentals of sensing with carbon nanotubes were reported in Science in 2000 by two groups that described changes in the conductivity of nanowires upon exposure to various gases (2000, 287, 622–625; 1801–1804). But relatively few papers about nanowire sensors were published subsequently (e.g., Science 2001, 293, 1289–1292; 2227–2231), indicating that some serious obstacles lay in the path to practical implementation. “I think one of the [factors] that’s neglected in this whole nanosensing business, whether it’s particles or wires or films, is the importance of the stability of the material,” says Penner. Because nanowires are made with such small amounts of material, they are “incredibly fragile” mechanically and chemically, he explains. “I really do think that progress has been impeded by this stability problem.” Penner suspected that the choice of materials for nanowires—typically, semiconductors—could be part of the problem, so he decided to work with noble metals. “[I]n a lot of ways, they would be superior to using semiconductors,” he says. Thin metal films will register increases in resistance if analytes adsorb on the surface. Although the change is small (1–5%), the S/N is good, he explains. And gold, silver, and platinum resist oxidation in aqueous solutions and over a broad range of pH. So, Penner decided to make nanowires from silver. The “only” issue his group had to tackle was fabrication.
Why noble metals matter The researchers found noble-metal nanowires quite difficult to grow, so they developed an electrochemical meth2890
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od for nucleating wires on the edges of a graphite substrate, which is cut like a flight of steps with long treads (Nano Lett. 2004, 4, 665–670). The researchers oxidized the substrate gently and applied a negative nucleation pulse (–1 V) for 100 ms. Silver nuclei, 5–20 nm in diameter, formed on the graphite edges, (a)
(c) 130 nm
(b)
(d)
197 nm
Scanning electron micrographs of Ag wires grown for (a, c) 30 s or (b, d) 1500 s. (Adapted from Anal. Chem. 2005, 77, 5205–5214.)
and the researchers let them grow for ~100 s to ~1 h until they coalesced. The mesowires—at 150–950 nm in diameter, they were larger than true nanowires—looked like densely packed beads on a string. The researchers exposed them to ammonia and other amine vapors and measured the resulting resistance changes. Gold, copper, platinum, and silver wires showed resistance increases as high as several hundred percent. But for the silver wires, the change was reversible: When the ammonia was removed, the resistance dropped to its original value within seconds (Anal. Chem. 2005, 77, 5205– 5214). Although the mesowires looked identical to one another, some registered increases of
