Engineering New Uses for Gold

MIT researchers are working on ways to modify these gold nanorods so they could be used as drug delivery or anti-tumor devices. Image Courtesy of Andy Wijaya

Gold nanorods have shown promise in a number of medical applications. For instance, researchers have determined that the tiny gold particles could be used to destroy tumor cells, deliver drugs, and as a contrast agent for in vivo and in vitro imaging. But before gold nanorods can live up to their potential, scientists need to figure out how to fine tune their surface chemistry, according to Kimberly Hamad-Schifferli, MIT assistant professor of biological and mechanical engineering. “For all of these nifty applications to work, someone has got to sit down and do the dirty work of understanding the surface,” she says. Hamad-Schifferli and her colleagues published two papers in August describing ways to manipulate the nanorods’ surface, which could allow researchers to design nanorods with specific functions.

Nanorods differ from traditional, spherical gold nanoparticles in one important respect: they can absorb infrared light, which means they could be activated by an infrared laser without damaging surrounding cells. Before that can happen, scientists must figure out how to deal with an organic molecule known as cetyl trimethylammonium bromide (CTAB) that coats the outer surface of gold nanorods and tends to detach from and reattach itself to the surface. The molecule, a byproduct of the synthesis reaction that produces the nanorods, makes it difficult to attach other molecules for drug delivery as drugs or DNA.

The team’s two recent papers describe how the CTAB influences heat dissipation and how to remove the CTAB and replace it with another organic molecule. In the first paper, published in the Journal of Physical Chemistry, they found that a low concentration of the CTAB in the surrounding solution accelerates heat dissipation after the nanorod is hit with infrared light. When the concentration of CTAB is high, heat is dissipated more slowly.

That information could help scientists design nanorods that fight cancer agents by burning away tumor cells when activated with infrared light. In the second paper, published in the journal Langmuir, the team demonstrated how to replace CTAB with a more useful molecule — a sulfur-containing group known as a thiol. This molecule binds more strongly to the nanorod, so it doesn’t detach and reattach like CTAB. In addition, other molecules, such as DNA, can be easily attached to the end of the thiol.
These surface chemistry studies are critical to lay the groundwork for development of gold nanorods, according to Hamad-Schifferli. “People have dreamed up all of these cool applications for nanorods, but one of the biggest bottlenecks to making this a reality is this interface,” she says. In the future, Hamad-Schifferli and her colleagues hope to build gold nanorods that carry DNA designed for a specific function in the target cell.

Andy Wijaya, an MIT graduate student in chemical engineering, prepares a gold nanorod solution for pump-probe spectroscopy with (standing, left to right) postdoc Aaron Schmidt and professors Gang Chen and Kimberly Hamad-Schifferli observing. Image Courtesy of Christopher Huang

Tags: gold nanorods, MIT