Gold nanoclusters self-assemble to form novel 2D and 3D materials

Self-assembly is one of the fundamental principles of nature, directing the growth of larger ordered and functional systems from smaller building blocks. Self-assembly can be observed at all length scales, from molecules to galaxies.

Now researchers at the Nanoscience Centre of the University of Jyväskylä and the HYBER Centre of Excellence of Aalto University, both in Finland, report a new type of self-assembly, in which tiny gold nanoclusters just a couple of nanometres in size form two- and three-dimensional materials. Each nanocluster comprises 102 gold atoms and a surface layer of 44 thiol molecules. The study, conducted with funding from the Academy of Finland and the European Research Council, is reported in a paper in Angewandte Chemie International Edition.

The atomic structure of the 102-atom gold nanocluster was first resolved by Roger Kornberg’s group at Stanford University in 2007. Since then, further studies of the nanocluster’s properties have been conducted in the Jyväskylä Nanoscience Centre. In this latest study, the Finnish researchers have shown that the nanocluster’s thiol surface possesses a large number of acidic groups able to form directed hydrogen bonds with neighboring nanoclusters, initiating directed self-assembly.

This self-assembly took place in a water-methanol mixture and produced two distinctly different superstructures, which were imaged by a high-resolution electron microscope at Aalto University. In one of the structures, two-dimensional, hexagonally-ordered layers of gold nanoclusters were stacked together, each layer being just one nanocluster thick. Under different synthesis conditions, the nanoclusters would instead self-assemble into three-dimensional spherical, hollow capsid structures, where the thickness of the capsid wall corresponds again to just one nanocluster.

While the details of the formation mechanisms for the superstructures warrant further investigation, these initial observations suggest a new route to synthetically-made, self-assembling nanomaterials.

“Today, we know of several tens of different types of atomistically-precise gold nanoclusters, and I believe they can exhibit a wide variety of self-assembling growth patterns that could produce a range of new meta-materials,” said Hannu Häkkinen, who coordinated the research at the Nanoscience Centre. “In biology, typical examples of self-assembling functional systems are viruses and vesicles. Biological self-assembled structures can also be de-assembled by gentle changes in the surrounding biochemical conditions. It’ll be of great interest to see whether these gold-based materials can be de-assembled and then re-assembled to different structures by changing something in the chemistry of the surrounding solvent.”

“The free-standing two-dimensional nanosheets will bring opportunities towards new-generation functional materials, and the hollow capsids will pave the way for highly lightweight colloidal framework materials,” predicted postdoctoral researcher Nonappa from Aalto University.

“In a broader framework, it has remained as a grand challenge to master the self-assemblies through all length scales to tune the functional properties of materials in a rational way,” said Olli Ikkala from Aalto University. “So far, it has been commonly considered sufficient to achieve sufficiently narrow size distributions of the constituent nanoscale structural units to achieve well-defined structures. The present findings suggest a paradigm change to pursue strictly defined nanoscale units for self-assemblies.”

This story is adapted from material from the Academy of Finland, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.