Researchers determine structure of gold nanoparticles
Nature’s beautiful patterns can be found anywhere you look: in the shape of a snowflake, the symmetry of a honeycomb, the Fibonacci spiral of a nautilus shell, and even in the structure of a tiny gold nanoparticle. Rongchao Jin, associate professor of chemistry at Carnegie Mellon, and chemistry graduate students Chenjie Zeng and Yuxiang Chen have successfully crystallized a Au133 nanoparticle and determined its entire structure, which consists of a series of beautiful patterns.
Nanoparticles are clusters of atoms that are between one and 100 nanometers in size and consist of a core of atoms surrounded by surface-protecting molecules. In the case of Au133, the core is made up of 133 gold atoms and the surface molecules are known as thiolate ligands. These thiolate ligands are extremely important for the nanoparticle because they consist of sulfur atoms that bond tightly with the gold core, holding each nanoparticle together and preventing them from aggregating with one another.
“The most important part is the surface, because it stabilizes the nanoparticle,” Zeng said. “But we know the least about it.” Although nanoparticles can be imaged through electron microscopy, these images only show the cores of nanoparticles. This occurs because heavy atoms like gold can scatter electrons very well, while the thiolate ligands on the surface cannot. Because of this, the researchers must turn to a technique known as X-ray diffraction (XRD), a method in which the way a crystal diffracts X-rays reveals the crystal’s structure.
This is where the team faced its greatest obstacle, because to do XRD, a single crystal of nanoparticles is needed. Jin explained this idea using the analogy of ice cubes, which are single crystals of water. “So that’s easy, you put your solution in the freezer and the next morning you will have single crystals and if you take a small piece and take it to the XRD, you will be able to see the oxygen and the hydrogen,” Jin said. “So we are doing the same thing, but instead of water molecules, we are trying to crystallize nanoparticles — and obviously it is much more difficult.”
Although the chemists crystallized smaller nanoclusters in the past, crystallizing larger nanoparticles such as Au133 was much more daunting. After tackling the problem for two years, the recipe they perfected had two main factors. The first is that a very high-purity batch of Au133 nanoparticles is needed. In other words, a solution containing only Au133 nanoparticles is more likely to be crystallized successfully than a solution contaminated with nanoclusters of other sizes.
The second key factor is the choice of surface protecting molecules. The group used a molecule called 4- tert -butylbenzenethiol, better known as TBBT, which consists of a rigid benzene ring — a ring of six carbon atoms with a hydrogen atom attached to each carbon. The rigidness of these surface molecules allows the Au133 nanoparticles to pack together very snugly, with each nanoparticle in the same orientation. It is this consistent orientation that allows the nanoparticles to be crystallized. “We can only crystallize the particles if they are packed in a periodical way,” Zeng said.
Having successfully crystallized Au133 nanoparticles, the researchers were able to analyze its structure. They found that several of the gold shells were constructed in an octahedron structure. The most beautiful part, though, was the surface of the nanoparticle: the surface molecules attached to the core atoms in sulfur-gold-sulfur links that resembled little staples on the outside of the nanoparticle. “We think the most amazing thing is how these staples protect the gold core,” Zeng said. “We found that the staples aren’t random. They actually form these very beautiful helical-like patterns that we call stripes.”
Besides these stripes, the chemists found that there was yet another pattern. The carbon tails of the surface molecules formed elegant swirls around the nanoparticle. “You can see a lot of these same patterns in nature,” Jin emphasized. “For example, the octahedron structure is the same shape as a mosaic virus, the helical structures look just like DNA, and the swirls are just like our galaxy. We see all the same patterns in the tiny world of a nanoparticle.” Despite how vast our galaxy is and how minuscule a nanoparticle is, both share the same patterns.
Besides being a beautiful attestation of nature’s universality, these Au133 nanoparticles have applications in catalysis, electronics, and healthcare. For example, Jin’s group has been using them to catalyze — speed up — the reaction that converts carbon monoxide, a toxic gas, to carbon dioxide.
Zeng explained that the Au133 nanoparticles are extremely stable, as they can exist in air at room temperature. “We think that their stability is related to the very symmetric surface patterns,” she said. “We think it’s nature’s strategy for fabricating robust nanoparticles.”