Controlling the Size of Gold Nanoparticles

Gold nanoparticles have gotten a great deal of airplay in the last few years as a potentially suitable technology for medical device applications--including as drug-delivery vehicles for treating such diseases as cancer. Now, researchers at North Carolina State University (NC State; Raleigh) have demonstrated that the bulkiness of molecules used to create gold nanoparticles dictates their size.

Scientists often use organic molecules known as ligands to create gold nanoparticles. These ligands effectively bring gold atoms together in a solution, from which the gold nanoparticles form. The ligands essentially line up side by side and surround the nanoparticles in all three dimensions. The scientists have shown that larger ligands result in smaller nanoparticles.

The researchers assessed three types of thiol ligands--a family of ligands commonly used to synthesize gold nanoparticles. In order of bulkiness from smallest to largest, the ligands included linear hexanethiolate (-SC6), cyclohexanethiolate (-SCy), and 1-adamantanethiolate (-SAd). Because fewer -SAd and -SCy ligands can line up next to each other in three dimensions, fewer gold atoms are brought together in the core. Therefore, the nanoparticles are smaller. -SC6, the least bulky of the thiolates, can create the largest nanoparticles.

"While we've shown that this is an effective means of controlling size in gold nanoparticles, we think it may have implications for other materials as well," remarks Peter Krommenhoek, a PhD student at NC State and lead author of a paper on this research published in ACS Nano.

The researchers discovered that when particularly small nanoparticles form, they tend to form at very specific sizes, called discrete sizes. For example, some types of nanoparticles may consist of 25 or 28 atoms, but never 26 or 27 atoms. The research team also found that the bulkiness of the ligands also changed the discrete sizes of the nanoparticles. "This is interesting, in part, because each discrete size represents a different number of gold atoms and ligands, which could influence the nanoparticle's chemical behavior," Tracy notes. "That question has yet to be addressed."

"This work advances our understanding of nanoparticle formation, and gives us a new tool for controlling the size and characteristics of gold nanoparticles," explains Joseph Tracy, an assistant professor of materials science and engineering at NC State and coauthor of the paper.