New technique controls dimensions of gold nanorods while manufacturing on a large scale

"This technique should facilitate the economical manufacture of large volumes of gold nanorods," says Dr. Joseph Tracy, an associate professor of materials science and engineering at NC State and senior author of a paper on the work. "And that should be good news for both the science community and the biomedical research and development community."

The NC State team started with an existing technique, in which gold nanorods are formed by mixing two chemical solutions together. However, that technique only converts 30 percent of the gold into nanorods -- the rest remains dissolved in solution.

To convert the remaining 70 percent of the gold into nanorods, the researchers added a continuous stream of ascorbic acid (better known as vitamin C) to the solution, while constantly stirring the mixture. The ascorbic acid essentially pulls the gold out of the solution and deposits it on the existing nanorods.

But the researchers also found that the slower they added the ascorbic acid, the stubbier the nanorods became. This is important because the optical properties of gold nanorods depend on their "aspect ratio" -- their relative height and width. For example, long, thin gold nanorods absorb light at wavelengths greater than 800 nanometers (in the near infrared spectrum), while shorter, wider gold nanorods absorb light at wavelengths below 700 nanometers (red or dark red).

"The ability to fine-tune these optical properties will likely be useful for the development of new biomedical applications," Tracy says.