Some of the long gold nanowires grown from nanorods in Eugene Zubarev’s lab at Rice University. Image: Zubarev Research Group/Rice University.
Some of the long gold nanowires grown from nanorods in Eugene Zubarev’s lab at Rice University. Image: Zubarev Research Group/Rice University.

A boost of vitamin C helped scientists at Rice University turn small gold nanorods into fine gold nanowires.

Common, mild ascorbic acid is the not-so-secret sauce that helped the Rice lab of chemist Eugene Zubarev grow pure batches of nanowires from stumpy nanorods without the drawbacks of previous techniques.

"There's no novelty per se in using vitamin C to make gold nanostructures because there are many previous examples," Zubarev said. "But the slow and controlled reduction achieved by vitamin C is surprisingly suitable for this type of chemistry in producing extra-long nanowires." The scientists report their work in a paper in ACS Nano.

The Rice lab's nanorods are about 25nm thick at the start of the process – and remain that way while they lengthen to become long nanowires. Above 1000nm in length, the objects are considered nanowires, and that matters. The wires' aspect ratio – length over width – dictates how they absorb and emit light, and how they conduct electrons. Combined with gold's inherent metallic properties, that could enhance the value of the nanowires for sensing, diagnostic, imaging and therapeutic applications.

Zubarev and lead author Bishnu Khanal, a Rice chemistry alumnus, succeeded in making their particles go far beyond the normal transition from nanorod to nanowire, theoretically to unlimited length. The researchers also showed that the process is fully controllable and reversible. That makes it possible to produce nanowires of any desired length, and thus any desired configuration for electronic or light-manipulating applications, especially those that involve plasmons, the light-triggered oscillation of electrons on a metal's surface. The nanowires' plasmonic response can be tuned to emit light from visible to infrared and theoretically far beyond, depending on their aspect ratios.

The lengthening process is slow, taking hours to grow a micron-long nanowire. "In this paper, we only reported structures up to 4–5µm in length," Zubarev said. "But we're working to make much longer nanowires."

The growth process only appeared to work with pentahedrally twinned gold nanorods, which contain five linked crystals. These five-sided rods – "Think of a pencil, but with five sides instead of six," Zubarev said – are stable along their long, flat surfaces, but not at the tips.

"The tips also have five faces, but they have a different arrangement of atoms," he explained. "The energy of those atoms is slightly lower, and when new atoms are deposited there, they don't migrate anywhere else." That keeps the growing wires from gaining girth. Every added atom increases the wire's length, and thus its aspect ratio.

The nanorods' reactive tips get help from a surfactant called CTAB, which covers the flat surfaces of the nanorods. "The surfactant forms a very dense, tight bilayer on the sides, but it cannot cover the tips effectively," Zubarev said.

That leaves the tips open to an oxidation or reduction reaction, with the ascorbic acid providing electrons that combine with gold ions so that they settle at the tips in the form of gold atoms. And unlike carbon nanotubes in a solution that easily aggregate, the nanowires keep their distance from one another.

"The most valuable feature is that it is truly one-dimensional elongation of nanorods to nanowires," Zubarev said. "It does not change the diameter, so in principal we can take small rods with an aspect ratio of maybe two or three and elongate them to 100 times the length." He said the process should also apply to other metal nanorods, including silver.

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.