Chemists Solve an 84-Year-Old Theory On How Molecules Move Energy After Light Absorption

Chemists Solve an 84-Year-Old Theory On How Molecules Move Energy After Light Absorption

  10:22:00 AM    Science Lover

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The same principle that causes figure skaters to spin faster as they draw their arms into their bodies has now been used by Michigan State University researchers to understand how molecules move energy around following the absorption of light.

MSU chemist Jim McCusker and postdoctoral researcher Dong Guo proved an 84-year-old theory. (Credit: Photo courtesy of MSU.)

Conservation of angular momentum is a fundamental property of nature, one that astronomers use to detect the presence of satellites circling distant planets. In 1927, it was proposed that this principle should apply to chemical reactions, but a clear demonstration has never been achieved.

In the current issue of Science, MSU chemist Jim McCusker demonstrates for the first time the effect is real and also suggests how scientists could use it to control and predict chemical reaction pathways in general.

"The idea has floated around for decades and has been implicitly invoked in a variety of contexts, but no one had ever come up with a chemical system that could demonstrate whether or not the underlying concept was valid," McCusker said. "Our result not only validates the idea, but it really allows us to start thinking about chemical reactions from an entirely different perspective."

The experiment involved the preparation of two closely related molecules that were specifically designed to undergo a chemical reaction known as fluorescence resonance energy transfer, or FRET. Upon absorption of light, the system is predisposed to transfer that energy from one part of the molecule to another.

McCusker's team changed the identity of one of the atoms in the molecule from chromium to cobalt. This altered the molecule's properties and shut down the reaction. The absence of any detectable energy transfer in the cobalt-containing compound confirmed the hypothesis.

"What we have successfully conducted is a proof-of-principle experiment," McCusker said. "One can easily imagine employing these ideas to other chemical processes, and we're actually exploring some of these avenues in my group right now."

The researchers believe their results could impact a variety of fields including molecular electronics, biology and energy science through the development of new types of chemical reactions.

Dong Guo, a postdoctoral researcher, and Troy Knight, former graduate student and now research scientist at Dow Chemical, were part of McCusker's team. Funding was provided by the National Science Foundation.

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Scientists Observe Single Ions Moving Through Tiny Carbon-Nanotube Channel

Scientists Observe Single Ions Moving Through Tiny Carbon-Nanotube Channel

  1:22:00 PM    Science Lover

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For the first time, a team of MIT chemical engineers has observed single ions marching through a tiny carbon-nanotube channel. Such channels could be used as extremely sensitive detectors or as part of a new water-desalination system. They could also allow scientists to study chemical reactions at the single-molecule level.

MIT chemical engineers built tiny channels out of carbon 
nanotubes -- hollow tubes whose walls aremade of lattices 
of carbon atoms. Small moleculessuch as sodium ions and 
protons can flow through the channels. 
(Credit: Graphic byPatrick Gillooly)

Carbon nanotubes -- tiny, hollow cylinders whose walls are lattices of carbon atoms -- are about 10,000 times thinner than a human hair. Since their discovery nearly 20 years ago, researchers have experimented with them as batteries, transistors, sensors and solar cells, among other applications.

In the Sept. 10 issue of Science, MIT researchers report that charged molecules, such as the sodium and chloride ions that form when salt is dissolved in water, can not only flow rapidly through carbon nanotubes, but also can, under some conditions, do so one at a time, like people taking turns crossing a bridge. The research was led by associate professor Michael Strano.

The new system allows passage of much smaller molecules, over greater distances (up to half a millimeter), than any existing nanochannel. Currently, the most commonly studied nanochannel is a silicon nanopore, made by drilling a hole through a silicon membrane. However, these channels are much shorter than the new nanotube channels (the nanotubes are about 20,000 times longer), so they only permit passage of large molecules such as DNA or polymers -- anything smaller would move too quickly to be detected.

Strano and his co-authors -- recent PhD recipient Chang Young Lee, graduate student Wonjoon Choi and postdoctoral associate Jae-Hee Han -- built their new nanochannel by growing a nanotube across a one-centimeter-by-one-centimeter plate, connecting two water reservoirs. Each reservoir contains an electrode, one positive and one negative. Because electricity can flow only if protons -- positively charged hydrogen ions, which make up the electric current -- can travel from one electrode to the other, the researchers can easily determine whether ions are traveling through the nanotube.

They found that protons do flow steadily across the nanotube, carrying an electric current. Protons flow easily through the nanochannel because they are so small, but the researchers observed that other positively charged ions, such as sodium, can also get through but only if enough electric field is applied. Sodium ions are much larger than protons, so they take longer to cross once they enter. While they travel across the channel, they block protons from flowing, leading to a brief disruption in current known as the Coulter effect.

Strano believes that the channels allow only positively charged ions to flow through them because the ends of the tubes contain negative charges, which attract positive ions. However, he plans to build channels that attract negative ions by adding positive charges to the tube.

Once the researchers have these two types of channels, they hope to embed them in a membrane that could also be used for water desalination. More than 97 percent of Earth's water is in the oceans, but that vast reservoir is undrinkable unless the salt is removed. The current desalination methods, distillation and reverse osmosis, are expensive and require lots of energy. So a nanotube membrane that allows both sodium and chloride ions (which are negatively charged) to flow out of seawater could become a cheaper way to desalinate water.

This study marks the first time that individual ions dissolved in water have been observed at room temperature. This means the nanochannels could also detect impurities, such as arsenic or mercury, in drinking water. (Ions can be identified by how long it takes them to cross the channel, which depends on their size). "If a single arsenic ion is floating in solution, you could detect it," says Strano.

Story Source:
The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Massachusetts Institute of Technology. The original article was written by Anne Trafton, MIT News Office.

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Nanotubes Weigh A Single Atom

Nanotubes Weigh A Single Atom

  5:49:00 PM    Science Lover

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How can you weigh a single atom? European researchers have built an exquisite new device that can do just that. It may ultimately allow scientists to study the progress of chemical reactions, molecule by molecule.

A diagram (above) and real-life image (inset) of a carbon nanotube.
(Credit: CARDEQ Project (www.cardeq.eu)

Carbon nanotubes are ultra-thin fibres of carbon and a nanotechnologist’s dream.

They are made from thin sheets of carbon only one atom thick – known as graphene – rolled into a tube only a few nanometres across. Even the thickest is more than a thousand times thinner than a human hair.

Interest in carbon nanotubes blossomed in the 1990s when they were found to possess impressive characteristics that make them very attractive raw materials for nanotechnology of all kinds.

“They have unique properties,” explains Professor Pertti Hakonen of Helsinki University of Technology. “They are about 1000 times stronger than steel and very good thermal conductors and good electrical conductors.”

Hakonen is coordinator of the EU-funded CARDEQ project (http://www.cardeq.eu/) which is exploiting these intriguing materials to build a device sensitive enough to measure the masses of atoms and molecules.

Vibrating strings

A carbon nanotube is essentially an extremely thin, but stiff, piece of string and, like other strings, it can vibrate. As all guitar players know, heavy strings vibrate more slowly than lighter strings, so if a suspended carbon nanotube is allowed to vibrate at its natural frequency, that frequency will fall if atoms or molecules become attached to it.

It sounds simple and the idea is not new. What is new is the delicate sensing system needed to detect the vibration and measure its frequency. Some nanotubes turn out to be semiconductors, depending on how the graphene sheet is wound, and it is these that offer the solution that CARDEQ has developed.

Members of the consortium have taken the approach of building a semiconducting nanotube into a transistor so that the vibration modulates the current passing through it. “The suspended nanotube is, at the same time, the vibrating element and the readout element of the transistor,” Hakonen explains.

“The idea was to run three different detector plans in parallel and then select the best one,” he says. “Now we are down to two. So we have the single electron transfer concept, which is more sensitive, and the field effect transistor concept, which is faster.”

Single atoms

Last November, CARDEQ partners in Barcelona reported that they had sensed the mass of single chromium atoms deposited on a nanotube. But Hakonen says that even smaller atoms, of argon, can now be detected, though the device is not yet stable enough for such sensitivity to be routine. “When the device is operating well, we can see a single argon atom on short time scales. But then if you measure too long the noise becomes large.”

CARDEQ is not alone in employing carbon nanotubes as mass sensors. Similar work is going on at two centres in California – Berkeley and Caltech – though each has adopted a different method to measuring the mass.

All three groups have announced they can perform mass detection on the atomic level using nanotubes, but CARDEQ researchers provided the most convincing data with a clear shift in the resonance frequency.

But a single atom is nowhere near the limit of what is possible. Hakonen is confident they can push the technology to detect the mass of a single nucleon – a proton or neutron.

“It’s a big difference,” he admits, “but typically the improvements in these devices are jump-like. It’s not like developing some well-known device where we have only small improvements from time to time. This is really front-line work and breakthroughs do occur occasionally.”

Biological molecules

If the resolution can be pared down to a single nucleon, then researchers can look forward to accurately weighing different types of molecules and atoms in real time.

It may then become possible to observe the radioactive decay of a single nucleus and to study other types of quantum mechanical phenomena.

But the real excitement would be in tracking chemical and biological reactions involving individual atoms and molecules reacting right there on the vibrating nanotube. That could have applications in molecular biology, allowing scientists to study the basic processes of life in unprecedented detail. Such practical applications are probably ten years away, Hakonen estimates.

“It will depend very much on how the technology for processing carbon nanotubes develops. I cannot predict what will happen, but I think chemical reactions in various systems, such as proteins and so on, will be the main applications in the future.”

The CARDEQ project received funding from the FET-Open strand of the EU’s Sixth Framework Programme for ICT research.

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