Tag Archives: Chakra P. Joshi

Putting a gold atom in a silver nanocluster changes things

Considering that the King Abdullah University of Science and Technology (KAUST) opened on Sept. 23, 2009 (mentioned in my Sept. 24, 2009 post; scroll down about 50% of the way), the university has done a remarkable job of establishing itself within the research community. Here’s some of the latest news from KAUST in a July 15, 2016 news item on Nanowerk,

The appearance of metals, such as their shiny surface or their electrical conductivity, is determined by the ensemble of atoms that comprise the metal. The situation differs on the molecular scale, and KAUST researchers have shown that replacing a single atom in a cluster of 25 silver atoms with one gold atom fundamentally changes its properties …

Composing a silver nanocrystal: the center silver atom (a) surrounded by a cage of 12 other silver atoms (b) embedded by further atoms (c) and stabilized by further ligands (d). Reproduced with permission from ref 1.© 2016 John Wiley and Sons.

Composing a silver nanocrystal: the center silver atom (a) surrounded by a cage of 12 other silver atoms (b) embedded by further atoms (c) and stabilized by further ligands (d). Reproduced with permission from ref 1.© 2016 John Wiley and Sons.

A July (??), 2016 KAUST news release, which originated the news item, provides more detail,

Metal atom nanoclusters are made from a core of a few metal atoms surrounded by a protective shell of stabilizing ligands. Nanoclusters come in different sizes, but each stable variation of nanoclusters has exactly the same number of metal atoms. This leads to very controllable properties, noted Osman Bakr, KAUST associate professor of material science and engineering and leader of the research team.

“Nanoclusters have unique arrangements of atoms and size-dependent absorption, fluorescence, electronic and catalytic properties,” he said.

A popular metal nanocluster is [Ag25(SR)18], which consists of of 25 silver atoms. This nanocluster is unique as it corresponds to a gold nanocluster that has exactly the same number of atoms. Both clusters have different properties due to the different metal used. To understand how exactly the atomic composition affects these properties, the researchers replaced a single silver atom with gold.

Replacing a single atom in a nanocluster is a difficult task. Direct chemical methods can be used, but these give little control over how many atoms are replaced, making it difficult to ascribe particular properties to the nanocluster structure.

Instead, the researchers used a galvanic replacement process that relies on difference in the electrochemical potential between the incoming and outgoing atoms to induce atomic replacements. To their surprise, the process produced a reliable and precise atomic exchange in which only the center silver atom is replaced by gold.

The replacement yielded dramatic changes in the nanocluster. A solution of the silver nanoclusters appears orange, whereas after the replacement of the central atom the color turns dark green.

This indicates more fundamental changes in properties, Bakr said. “The ambient stability and fluorescence of the nanocluster were enhanced by a factor of 25 as a result of this single atom replacement. Furthermore, we are now able to demonstrate the importance of a single atom impurity on nanoparticles and modulate the properties at the single atom level,” he noted.

The reliable replacement of only a single gold atom opens the door to a more systematic investigation of metal nanoclusters, which can help to uncover the mechanisms of the chemical and physical changes arising from the replacement.

Here’s a link to and a citation for the paper,

Templated Atom-Precise Galvanic Synthesis and Structure Elucidation of a [Ag24Au(SR)18] Nanocluster by Dr. Megalamane S. Bootharaju, Chakra P. Joshi, Dr. Manas R. Parida, Prof. Omar F. Mohammed and Prof. Osman M. Bakr. Angewandte Chemie International Edition DOI: 10.1002/anie.201509381 Version of Record online: 27 NOV 2015

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

They’ve certainly waited a while to tout this research. Ah well. This paper is behind a paywall.

Nano-alchemy: silver nanoparticles that look like and act like gold

This work on ‘nano-alchemy’ comes out of the King Abduhllah University of Science and Technology (KAUST) according to a Sept. 22, 2015 article by Lisa Zynga for phys.org (Note: A link has been removed),

In an act of “nano-alchemy,” scientists have synthesized a silver (Ag) nanocluster that is virtually identical to a gold (Au) nanocluster. On the outside, the silver nanocluster has a golden yellow color, and on the inside, its chemical structure and properties also closely mimic those of its gold counterpart. The work shows that it may be possible to create silver nanoparticles that look and behave like gold despite underlying differences between the two elements, and could lead to creating similar analogues between other pairs of elements.

“In some aspects, this is very similar to alchemy, but we call it ‘nano-alchemy,'” Bakr [Osman Bakr, Associate Professor of Materials Science and Engineering at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia] told Phys.org. “When we first encountered the optical spectrum of the silver nanocluster, we thought that we may have inadvertently switched the chemical reagents for silver with gold, and ended up with gold nanoparticles instead. But repeated synthesis and measurements proved that the clusters were indeed silver and yet show properties akin to gold. It was really surprising to us as scientists to find not only similarities in the color and optical properties, but also the X-ray structure.”

In their study, the researchers performed tests demonstrating that the silver and gold nanoclusters have very similar optical properties. Typically, silver nanoclusters are brown or red in color, but this one looks just like gold because it emits light at almost the same wavelength (around 675 nm) as gold. The golden color can be explained by the fact that both nanoclusters have virtually identical crystal structures.

The question naturally arises: why are these silver and gold nanoclusters so similar, when individual atoms of silver and gold are very different, in terms of their optical and structural properties? As Bakr explained, the answer may have to do with the fact that, although larger in size, the nanoclusters behave like “superatoms” in the sense that their electrons orbit the entire nanocluster as if it were a single giant atom. These superatomic orbitals in the silver and gold nanoclusters are very similar, and, in general, an atom’s electron configuration contributes significantly to its properties.

Here’s one of the images used to illustrate Zynga’s article and the paper published by the American Chemical Society,

(Left) Optical properties of the silver and gold nanoclusters, with the inset showing photographs of the actual color of the synthesized nanoclusters. The graph shows the absorption (solid lines) and normalized emission (dotted lines) spectra. (Right) Various representations of the X-ray structure of the silver nanocluster. Credit: Joshi, et al. ©2015 American Chemical Society

(Left) Optical properties of the silver and gold nanoclusters, with the inset showing photographs of the actual color of the synthesized nanoclusters. The graph shows the absorption (solid lines) and normalized emission (dotted lines) spectra. (Right) Various representations of the X-ray structure of the silver nanocluster. Credit: Joshi, et al. ©2015 American Chemical Society

I encourage you to read Zynga’s article in its entirety. For the more technically inclined, here’s a link to and a citation for the researchers’ paper,

[Ag25(SR)18]: The “Golden” Silver Nanoparticle by Chakra P. Joshi, Megalamane S. Bootharaju, Mohammad J. Alhilaly, and Osman M. Bakr.J. Am. Chem. Soc., 2015, 137 (36), pp 11578–11581 DOI: 10.1021/jacs.5b07088 Publication Date (Web): August 31, 2015

Copyright © 2015 American Chemical Society

This paper is behind a paywall.