Researchers Shinjiro Takano, Yuya Hamasaki, and Tatsuya Tsukuda of the University of Tokyo have successfully visualized the geometric structure of growing gold nanoclusters in their earliest stages. During this process, they also successfully "grew" a novel structure of elongated nanoclusters, which they named "gold quantum needles." Thanks to their responsiveness to light in the near-infrared range, these "needles" could enable much higher-resolution biomedical imaging and more efficient light-energy conversion. The findings were published in the Journal of the American Chemical Society.
Although gold might conjure up images of pomp and luxury, it is also an essential element of modern nanotechnology due to its unique structures and properties at the nanoscale. Gold nanoclusters composed of fewer than 100 atoms are typically synthesized by reducing, that is, adding electrons, gold precursor ions in the presence of protective ligands. However, synthesizing gold nanoclusters of the desired size, shape, and composition is still a challenge.
"Over the past years," says Tsukuda, the principal investigator, "much effort has been devoted to understanding the correlation between the structure and physicochemical properties of the nanoclusters. However, the formation process is regarded as a black box. We initiated this project with the belief that understanding the initial stages of cluster formation will lead to the development of new, targeted synthesis methods for desired structures."
The researchers thus set out to determine the geometric structures of gold nanoclusters at the initial stages of their formation. They used slightly unusual synthesis conditions to trap the nanoclusters in the very first stages of growth. Single-crystal X-ray diffraction analysis, an X-ray for chemical compounds, if you will, revealed that gold nanoclusters grew anisotropically, at a different rate in different directions. Moreover, the analysis revealed an entirely new structure: pencil-shaped nanoclusters composed of triangular trimers and tetrahedral tetramers. The researchers named them "gold quantum needles" because the electrons confined in these nanoclusters demonstrated quantized behavior, a quantum phenomenon in which electrons can take only specific potential energies.
"We could retroactively explain the formation processes of a series of small gold nanoclusters under our unusual synthetic conditions," Tsukuda explains. "However, the formation of needles with a base of a triangle of three gold atoms instead of a nearly spherical cluster is a serendipitous finding that was far beyond our imagination."
The structural snapshots the researchers acquired of the stepwise growth of gold nanoclusters greatly contribute to our understanding of the formation mechanism. However, Tsukuda is already thinking about the next steps.
"We would like to explore synthesizing other, unique nanoclusters by refining the synthesis conditions further. We would also like to collaborate with other experts to promote the application of gold quantum needles, leveraging their exceptional optical properties."
