Tag Archives: Au

Golden nanoglue

This starts out as a graphene story before taking an abrupt turn. From a June 5, 2018 news item on Nanowerk,

Graphene has undoubtedly been the most popular research subject of nanotechnology during recent years. Made of pure carbon, this material is in principle easy to manufacture: take ordinary graphite and peel one layer off with Scotch tape. The material thus obtained is two-dimensional, yielding unique properties, different from those in three-dimensional materials.

Graphene, however, lacks one important property, semiconductivity, which complicates its usage in electronics applications. Scientists have therefore started the quest of other two-dimensional materials with this desired property.

Molybdenum disulfide, MoS2 is among the most promising candidates. Like graphene, MoS2 consists of layers, interacting weakly with one another. In addition to being a semiconductor, the semiconducting properties of MoS2 change depending on the number of atomic layers.

A June 5, 2018 University of Oulu press release, which originated the news item,  gives more detail about the work,

For the one or few layer MoS2 to be useful in applications, one must be able to join it to other components. What is thus needed is such a metallic conductor that electric current can easily flow between the conductor and the semiconductor. In the case of MoS2, a promising conductor is provided by nickel, which also has other desired properties from the applications point of view.

However, an international collaboration, led by the Nano and molecular systems research unit at the University of Oulu has recently discovered that nanoparticles made of nickel do not attach to MoS2. One needs gold, which ‘glues’ the conductor and the component together. Says docent Wei Cao of NANOMO: “The synthesis is performed through a sonochemical method.” Sonochemistry is a method where chemical reactions are established using ultrasound. NANOMO scientist Xinying Shi adds: “The semiconductor and metal can be bridged either by the crystallized gold nanoparticles, or by the newly formed MoS2-Au-Ni ternary alloy.”

The nanojunction so established has a very small electrical resistivity. It also preserves the semiconducting and magnetic properties of MoS2. In addition, the new material has desirable properties beyond those of the original constituents. For example, it acts as a photocatalyst, which works much more efficiently than pure MoS2. Manufacturing the golden nanojunction is easy and cheap, which makes the new material attractive from the applications point of view.

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

Metallic Contact between MoS2 and Ni via Au Nanoglue by Xinying Shi, Sergei Posysaev, Marko Huttula, Vladimir Pankratov, Joanna Hoszowska, Jean‐Claude Dousse, Faisal Zeeshan, Yuran Niu, Alexei Zakharov, Taohai Li. Small Volume 14, Issue22 May 29, 2018 1704526 First published online: 24 April 2018 https://doi.org/10.1002/smll.201704526

This paper is behind a paywall.

There is a pretty illustration of the ‘golden nanojunctions’,

Golden nanoglue (Courtesy of the University of Oulu)

Create gold nanoparticles and nanowires with water droplets.

For some reason it took a lot longer than usual to find this research paper despite having the journal (Nature Communications), the title (Spontaneous formation …), and the authors’ names. Thankfully, success was wrested from the jaws of defeat (I don’t care if that is trite; it’s how I felt) and links, etc. follow at the end as usual.

An April 19, 2018 Stanford University news release (also on EurekAlert) spins fascinating tale,

An experiment that, by design, was not supposed to turn up anything of note instead produced a “bewildering” surprise, according to the Stanford scientists who made the discovery: a new way of creating gold nanoparticles and nanowires using water droplets.

The technique, detailed April 19 [2018] in the journal Nature Communications, is the latest discovery in the new field of on-droplet chemistry and could lead to more environmentally friendly ways to produce nanoparticles of gold and other metals, said study leader Richard Zare, a chemist in the School of Humanities and Sciences and a co-founder of Stanford Bio-X.

“Being able to do reactions in water means you don’t have to worry about contamination. It’s green chemistry,” said Zare, who is the Marguerite Blake Wilbur Professor in Natural Science at Stanford.

Noble metal

Gold is known as a noble metal because it is relatively unreactive. Unlike base metals such as nickel and copper, gold is resistant to corrosion and oxidation, which is one reason it is such a popular metal for jewelry.

Around the mid-1980s, however, scientists discovered that gold’s chemical aloofness only manifests at large, or macroscopic, scales. At the nanometer scale, gold particles are very chemically reactive and make excellent catalysts. Today, gold nanostructures have found a role in a wide variety of applications, including bio-imaging, drug delivery, toxic gas detection and biosensors.

Until now, however, the only reliable way to make gold nanoparticles was to combine the gold precursor chloroauric acid with a reducing agent such as sodium borohydride.

The reaction transfers electrons from the reducing agent to the chloroauric acid, liberating gold atoms in the process. Depending on how the gold atoms then clump together, they can form nano-size beads, wires, rods, prisms and more.

A spritz of gold

Recently, Zare and his colleagues wondered whether this gold-producing reaction would proceed any differently with tiny, micron-size droplets of chloroauric acid and sodium borohydide. How large is a microdroplet? “It is like squeezing a perfume bottle and out spritzes a mist of microdroplets,” Zare said.

From previous experiments, the scientists knew that some chemical reactions proceed much faster in microdroplets than in larger solution volumes.

Indeed, the team observed that gold nanoparticle grew over 100,000 times faster in microdroplets. However, the most striking observation came while running a control experiment in which they replaced the reducing agent – which ordinarily releases the gold particles – with microdroplets of water.

“Much to our bewilderment, we found that gold nanostructures could be made without any added reducing agents,” said study first author Jae Kyoo Lee, a research associate.

Viewed under an electron microscope, the gold nanoparticles and nanowires appear fused together like berry clusters on a branch.

The surprise finding means that pure water microdroplets can serve as microreactors for the production of gold nanostructures. “This is yet more evidence that reactions in water droplets can be fundamentally different from those in bulk water,” said study coauthor Devleena Samanta, a former graduate student in Zare’s lab and co-author on the paper.

If the process can be scaled up, it could eliminate the need for potentially toxic reducing agents that have harmful health side effects or that can pollute waterways, Zare said.

It’s still unclear why water microdroplets are able to replace a reducing agent in this reaction. One possibility is that transforming the water into microdroplets greatly increases its surface area, creating the opportunity for a strong electric field to form at the air-water interface, which may promote the formation of gold nanoparticles and nanowires.

“The surface area atop a one-liter beaker of water is less than one square meter. But if you turn the water in that beaker into microdroplets, you will get about 3,000 square meters of surface area – about the size of half a football field,” Zare said.

The team is exploring ways to utilize the nanostructures for various catalytic and biomedical applications and to refine their technique to create gold films.

“We observed a network of nanowires that may allow the formation of a thin layer of nanowires,” Samanta said.

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

Spontaneous formation of gold nanostructures in aqueous microdroplets by Jae Kyoo Lee, Devleena Samanta, Hong Gil Nam, & Richard N. Zare. Nature Communicationsvolume 9, Article number: 1562 (2018) doi:10.1038/s41467-018-04023-z Published online: 19 April 2018

Not unsurprisingly given Zare’s history as recounted in the news release, this paper is open access.

Surgery on nanoparticles?

Chemists performed “surgery” on a 23-gold-atom nanoparticle according to a June 12, 2017 news item on Nanowerk (Note: A link has been removed),

A team of chemists led by Carnegie Mellon University’s [CMU] Rongchao Jin has for the first time conducted site-specific surgery on a nanoparticle. The procedure, which allows for the precise tailoring of nanoparticles, stands to advance the field of nanochemistry.

The surgical technique developed by Qi Li, the study’s lead author and a 3rd year graduate student in the Jin group, will allow researchers to enhance nanoparticles’ functional properties, such as catalytic activity and photoluminescence, increasing their usefulness in a wide variety of fields including health care, electronics and manufacturing. The findings were published in Science Advances (“Molecular “surgery” on a 23-gold-atom nanoparticle”).

Here’s an image the researchers have provided,

Caption: Carnegie Mellon chemists used a two-step metal exchange method to remove two S-Au-S staples from the surface of a nanoparticle. Credit: Carnegie Mellon University

A June 12, 2017 CMU press release (also on EurekAlert), which originated the news item, provides more details about the research,

“Nanochemistry is a relatively new field, it’s only about 20 years old. We’ve been racing to catch up to fields like organic chemistry that are more than 100 years old,” said Jin, a chemistry professor in the Mellon College of Science. “Organic chemists have been able to tailor the functional groups of molecules for quite some time, like tailoring penicillin for better medical functions, for example. We dreamed that we could do something similar in nanoscience. Developing atomically precise nanoparticles has allowed us to make this dream come true.”

In order to make this “nano-surgery” a reality, researchers needed to begin with atomically precise nanoparticles that could be reliably produced time after time. Jin’s lab has been at the forefront of this research. Working with gold nanoparticles, he and his team have developed methods to precisely control the number of atoms in each nanoparticle, resulting in uniformly-sized nanoparticles with every batch. With reliably precise particles, Jin and colleagues were able to identify the particles’ structures, and begin to tease out how that structure impacted the particles’ properties and functionality.

With these well-defined nanoparticles in hand, Jin’s next step was to find a way to surgically tailor the particles in order to learn more about­ – and hopefully enhance – their functionality.

In their recent study, Jin and colleagues performed nano-surgery on a gold nanoparticle made up of 23 gold atoms surrounded by a protective surface of ligands in staple-like motifs. Using a two-step metal exchange method, they removed two S-Au-S staples from the particle’s surface. In doing this they revealed the structural factors that determine the particle’s optical properties and established the role that the surface plays in photoluminescence. Significantly, the surgery increased the particle’s photoluminescence by about 10-fold. Photoluminescence plays a critical role in biological imaging, cancer diagnosis and LED technology, among other applications.

Jin and coworkers are now trying to generalize this site-specific surgery method to other nanoparticles.

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

Molecular “surgery” on a 23-gold-atom nanoparticle by Qi Li, Tian-Yi Luo, Michael G. Taylor, Shuxin Wang, Xiaofan Zhu, Yongbo Song, Giannis Mpourmpakis, Nathaniel L. Rosi, and Rongchao Jin. Science Advances 19 May 2017: Vol. 3, no. 5, e1603193 DOI: 10.1126/sciadv.1603193

This paper is open access.

Stabilizing a carbon-gold complex (gold carbene shines green)

If you find carbon bonds and catalysis interesting, this is the posting for you. A July 8, 2014 news item on Nanowerk highlights research at Heidelberg University (Germany; Note: A link has been removed),

With a chemical “trick”, scientists at Heidelberg University have succeeded in isolating a stable gold carbene complex. Chemist Prof. Dr. Bernd F. Straub and his team are the first to have created the basis for directly examining the otherwise unstable gold-carbon double bond. Prof. Straub explains that highly reactive gold carbene molecules play an important role in landmark catalysing processes taking place at high speed. The research findings have been published in the German and the international edition of Angewandte Chemie (“Isolation of a Non-Heteroatom-Stabilized Gold–Carbene Complex”).

A July 8, 2014 Heidelberg University press release, which originated the news item, describes the catalytic process in general and the specific complex created by the researchers,

Chemical reactions can be accelerated with the aid of catalysts; consequently materials and pharmaceuticals can be manufactured from the raw materials of nature. The study of gold compounds in catalytic processes has proved particularly intensive and successful, according to Prof. Straub. “In numerous scientific studies in the last ten years, experts have been proposing gold carbenes as essential short-lived intermediates in catalytic reactions,” the Heidelberg researcher explains. However, with their high reactivity they escape detailed study: hardly has a gold carbene fragment consisting of the elements gold and carbon emerged – Au for aurum and C for carbon – when it continues to react.

In order to first create a stable complex and isolate a gold carbene structure for research, the two elements were “lured into a cage like a hungry tiger with a bait,” says Matthias Hussong, who is working on his doctoral dissertation in Prof. Straub’s team. The researchers first shielded the gold and carbon from its environment by surrounding them with low-reactive, space-filling chemical groups. Then the two elements were bonded in a carefully planned step – and so the Au=C fragment was “caught” in the gold carbene complex.

The chemists were able to impart “an amazing stability” to the gold carbene, says Prof. Straub – and at the same time to make it literally visible. “Almost all gold complexes are colourless, while the ‘stable’ gold carbene is emerald green,” states the scientist, who heads a research group at Heidelberg University‘s Institute of Organic Chemistry. Further Heidelberg studies showed that gold in its compounds is more than a “soft proton”, as the chemical behaviour of gold had been described to date.

If the gold fragment is replaced by a “real” proton, e.g. the nucleus of hydrogen, the lightest element, this analogous protonated carbene displays a reddish purple colour. “The gold in the gold carbene complex behaves differently from a proton – that is very clear to the eye,” states Prof. Straub. He and his team are now continuing to explore the understanding of gold catalysis, with the aim of using these findings to make catalytic processes more efficient.

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

Isolation of a Non-Heteroatom-Stabilized Gold–Carbene Complex by  M. Sc. Matthias W. Hussong, Dr. Frank Rominger, Petra Krämer, and Prof. Dr. Bernd F. Straub. Angewandte Chemie DOI: 10.1002/anie.201404032 Article first published online: 20 JUN 2014

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

This paper is behind a paywall.

All the talk of emerald green reminded me of Angelina Jolie,

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The earrings can be found here.