A new technique that produces nanoscale gold rods in large quantities while also controlling their optical properties and dimensions could facilitate their high-volume manufacture for applications in biomedicine. Because the optical properties of the gold nanorods are desirable for applications such as bioimaging and cancer treatment, the technique developed at North Carolina State University could make it more economical to manufacture them in large quantities. "And that should be good news for both the science community and the biomedical research and development community," said Dr. Joseph B. Tracy, an associate professor of materials science and engineering at NC State and senior author of a paper on the work. NC State University researchers found they could control the dimensions of the nanorods by varying how quickly they added ascorbic acid. Courtesy of Joseph Tracy, North Carolina State University. The NC State team began with an existing technique, in which gold nanorods are formed by mixing two chemical solutions together. However, that process converts only 30 percent of the gold into nanorods — the rest remains dissolved in solution. To convert the remaining 70 percent, 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 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 are dependent on their aspect ratio: long, thin gold nanorods absorb light at wavelengths greater than 800 nm, while shorter, wider gold nanorods absorb light at wavelengths below 700 nm. "The ability to fine-tune these optical properties will likely be useful for the development of new biomedical applications," Tracy said. The paper, "Large-Scale Synthesis of Gold Nanorods through Continuous Secondary Growth," is published online in Chemistry of Materials. Lead author is Krystian A. Kozek, a former NC State undergraduate. Co-authors include Klaudia M. Kozek, an NC State undergraduate who worked on the project while in high school; and Wei-Chen Wu and Sumeet R. Mishra, both NC State Ph.D. students. For more information, visit: www.ncsu.edu