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New study reveals a graphene sheet behaves ‘like a mirror’ for water molecules

Posted By Graphene Council, Saturday, July 18, 2020
A recently published study led by Virginia Commonwealth University researchers sheds new light on how water interacts with the nanomaterial graphene, a single, thin layer of carbon atoms arranged in a hexagonal honeycomb lattice.

The researchers’ findings could hold implications for a variety of applications, including sensors, fuel cell membranes, water filtration, and graphene-based electrode materials in high-performance supercapacitors.

The study, “Solvent–Solvent Correlations across Graphene: The Effect of Image Charges,” was published in the American Chemical Society journal ACS Nano and was led by Neda Ojaghlou, Ph.D., who conducted the research as a doctoral student in the Department of Chemistry in the College of Humanities and Sciences.

The project addressed an important area of study for medicine, industry and science: Understanding how liquids — mainly water —interact with surfaces. These interactions are measured in several ways, but particularly by monitoring “wetting,” inferred from the shape of a drop on a surface. If a droplet is flat, the surface is considered “hydrophilic,” like a wet glass. If the droplet resembles a sphere, it is “hydrophobic,” like a droplet on a hot pan.

“An extremely important surface to study the wetting is a graphene sheet. Graphene is one of the most prominent nanomaterials,” Ojaghlou said. “Its chemical, electrical and mechanical properties underlie a wide range of applications from cellphones to tennis racquet production, and from electronic devices to car manufacturing. Graphene wetting is also important in biological surfaces and designing supercapacitors.”

In this study, the researchers investigated the enhanced graphene’s propensity to wet if there is water on the other side of the sheet. They used advanced computer simulations to study this effect at the molecular level.

“By improving the graphene model, we have shown for the first time how graphene’s conductivity leads to wetting transparency. Conductivity means the displacement of electric charges of carbon atoms to respond to the presence of water electric dipole moments. These electric fluctuations on carbon atoms, which are extremely hard to simulate, modulate the interaction of water molecules on the two sides of the sheet,” Ojaghlou said. “In short, we have taken the graphene conductivity into account, and that provides a much better explanation of the wetting of graphene when there is water on the other side.”

Dusan Bratko, Ph.D., professor in the Department of Chemistry and an author of the paper, said the findings are an important discovery.

"When in contact with water, graphene interferes with hydrogen bonds among the water molecules, replacing them with weaker dispersion attraction to carbon atoms. Nonetheless, neat graphene is found to be weakly hydrophilic. This is partly explained by the graphene’s conductivity, which adds an interesting attractive mechanism between aqueous dipoles and transient charges induced on carbon atoms,” Bratko said.

“A previously unknown feature unveiled by the team’s computational approach is the synergy of the induction effects when water is present on both sides of a graphene sheet,” he said. “In this new picture, graphene plays an active role in communicating between the opposing hydration layers. As a result, graphene is considerably easier to wet from both sides than from one side alone. This is important as the former scenario occurs in many practical applications. The two distinct behaviors have been indicated in experiments in water and can be expected with other dipolar and ionic liquids or solutions.”

Mahdi Shafiei, Ph.D., also a former doctoral student at VCU and author of the paper, said the team’s findings could be explained as showing how a graphene sheet “behaves like a mirror for water molecules.”

“In our work, we explain the image charges on conducting graphene for the first time,” Shafiei said. “Our work has at least two significant impacts: It sheds light on the behavior of water droplets on graphene supported by water, and we expand the theoretical knowledge about conducting graphene and image charges on them."

The research was made possible thanks to support from the Department of Energy and the National Science Foundation.

In addition to Ojaghlou, Bratko and Shafiei, the paper was also authored by Mathieu Salanne, Ph.D., a professor at Sorbonne Université, and the late Alenka Luzar, Ph.D., a VCU professor of chemistry who passed away in 2019.

Tags:  ACS Nano  College of Humanities and Sciences  Dusan Bratko  fuel cells  Graphene  Mahdi Shafiei  Medicine  Neda Ojaghlou  Sensors  Virginia Commonwealth University 

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Tour scores prestigious Centenary Prize

Posted By Graphene Council, Friday, June 26, 2020
Rice University chemist James Tour has won a Royal Society of Chemistry Centenary Prize. The award, given annually to up to three scientists from outside Great Britain, recognizes researchers for their contributions to the chemical sciences industry or education and for successful collaborations. Tour was named for innovations in materials chemistry with applications in medicine and nanotechnology.

The prestigious award, established in 1947, comes with a 5,000-pound (about $6,260) cash prize and a medal. Winners are invited to undertake a lecture tour of the United Kingdom, but the COVID-19 pandemic has delayed that until 2021.

Additional winners this year are Teri Odom, the chair and Charles E. and Emma H. Morrison Professor of Chemistry at Northwestern University, and Eric Anslyn, the Welch Regents Chair and University Distinguished Teaching Professor at the University of Texas at Austin. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice.

“Receiving the Royal Society of Chemistry 2020 Centenary Prize is an enormous honor,” Tour said. “The award recognizes the accomplishments of my research group over a period of 32 years. I am greatly indebted to a host of students, postdocs and collaborators that have carried the weight of this research endeavor.

“We have sought to use chemistry to extend the boundaries of new materials development for use in medicine, electronic devices, nano-enhanced structures and renewable energy platforms,” he said. “It is a joy to realize the work done by this array of people in and with my laboratory has afforded such advances that are being recognized by this Centenary Prize.”

Work by Tour and his group in recent years includes the development of versatile laser-induced graphene, flash graphene from waste material, light-activated nanodrills that destroy cancer cells and “superbug” bacteria, silicon-oxide memory circuits that have flown on the International Space Station, the development of graphene quantum dots from coal, asphalt-based materials to capture carbon dioxide from gas wells, and the use of nanoparticles to quench damaging superoxides after an injury or stroke.

“We live in an era of tremendous global challenges, with the need for science recognized now more so than ever — so it is important to recognize those behind the scenes who are making significant contributions towards improving the world we live in,” said acting Royal Society of Chemistry chief executive Helen Pain. “In recognizing the work of Professor Tour, we are also recognizing the important contribution this incredible network of scientists makes to improve our lives every day.”

Tags:  Graphene  Healthcare  Helen Pain  James Tour  Medicine  nanotechnology  Rice University  Royal Society of Chemistry 

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Risk analyses for nanoparticles Nanosafety research without animal experiments

Posted By Graphene Council, Thursday, June 18, 2020
They are already in use in, say, cosmetics and the textile industry: Nanoparticles in sun blockers protect us from sunburn, and clothing with silver nanoparticles slows down bacterial growth. But the use of these tiny ingredients is also linked to the responsibility of being able to exclude negative effects for health and the environment. Nanoparticles belong to the still poorly characterized class of nanomaterials, which are between one and 100 nanometers in size and have a wide range of applications, for example in exhaust gas catalytic converters, wall paints, plastics and in nanomedicine. As new and unusual as nanomaterials are, it is still not clear whether or not they pose any risks to humans or the environment.

This is where risk analyses and life cycle assessments (LCA) come into play, which used to rely strongly on animal experiments when it came to determining the harmful effects of a new substance, including toxicity. Today, research is required to reduce and replace animal experiments wherever possible. Over the past 30 years, this approach has led to a substantial drop in animal testing, particularly in toxicological tests. The experience gained with conventional chemicals cannot simply be transferred to novel substances such as nanoparticles, however. Empa scientists are now developing new approaches, which should allow another substantial reduction in animal testing while at the same time enabling the safe use of nanomaterials.

"We are currently developing a new, integrative approach to analyze the risks of nanoparticles and to perform life cycle assessments," says Beatrice Salieri from Empa's Technology and Society lab in St. Gallen. One new feature, and one which differs from conventional analyses, is that, in addition to the mode of action of the substance under investigation, further data is included, such as the exposure and fate of a particle in the human body, so that a more holistic view is incorporated into the risk assessment.

These risk analyses are based on the nanoparticles' biochemical properties in order to develop suitable laboratory experiments, for example with cell cultures. To make sure the results from the test tube ("in vitro") also apply to the conditions in the human body ("in vivo"), the researchers use mathematical models ("in silico"), which, for instance, rely on the harmfulness of a reference substance. "If two substances, such as silver nanoparticles and silver ions, act in the very same way, the potential hazard of the nanoparticles can be calculated from that," says Salieri. 

But for laboratory studies on nanoparticles to be conclusive, a suitable model system must first be developed for each type of nanoparticle. "Substances that are inhaled are examined in experiments with human lung cells," explains Empa researcher Peter Wick who is heading the "Particles-Biology Interactions" lab in St. Gallen. On the other hand, intestinal or liver cells are used to simulate digestion in the body.

This not only determines the damaging dose of a nanoparticle in cell culture experiments, but also includes all biochemical properties in the risk analysis, such as shape, size, transport patterns and the binding – if any – to other molecules. For example, free silver nanoparticles in a cell culture medium are about 100 times more toxic than silver nanoparticles bound to proteins. Such comprehensive laboratory analyses are incorporated into so-called kinetic models, which, instead of a snapshot of a situation in the test tube, can depict the complete process of particle action.

Finally, with the aid of complex algorithms, the expected biological phenomena can be calculated from these data. "Instead of 'mixing in' an animal experiment every now and then, we can determine the potential risks of nanoparticles on the basis of parallelisms with well-known substances, new data from lab analyses and mathematical models," says Empa researcher Mathias Rösslein. In future, this might also enable us to realistically represent the interactions between different nanoparticles in the human body as well as the characteristics of certain patient groups, such as elderly people or patients with several diseases, the scientist adds.

As a result of these novel risk analyses for nanoparticles, the researchers also hope to accelerate the development and market approval of new nanomaterials. They are already being applied in the "Safegraph" project, one of the projects in the EU's "Graphene Flagship" initiative, in which Empa is involved as a partner. Risk analyses and LCA for the new "wonder material" graphene are still scarce. Empa researchers have recently been able to demonstrate initial safety analyses of graphene and graphene related materials in fundamental in vitro studies. In this way, projects such as Safegraph can now better identify potential health risks and environmental consequences of graphene, while at the same time reducing the number of animal experiments.

Tags:  Beatrice Salieri  Empa  Graphene  Medicine  nanomaterials  nanoparticles 

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