Light vortex: Circularly polarized luminescence from a stirred and gelled solution of dye

If you hold one end of a rope and swing it up and down with your arm while the other end is tied to a fence, the rope forms a wave. The amplitude oscillates vertically. If you swing the rope left and right instead, the it oscillates horizontally. If the rope runs through a narrow gap between two trees, only the vertical wave can pass through to the end of the rope. Light can also be viewed as a wave.

The oscillation of ordinary light from a light bulb has no preferred direction. It varies in all directions perpendicular to the direction of propagation of the light. As the two trees do with the rope, special glasses, known as polarizing filters, allow only those light waves which oscillate in a specific plane to pass through. The light that passes through is known as linearly polarized light. Another variation is also possible: circularly polarized light. In this case, the light wave oscillates in a helical pattern because the amplitude describes a circle around the axis of propagation. The amplitude can rotate around to the left or the right.

The shape and orientation of molecules can influence the polarization plane of light when it passes through a given substance. It is thus not surprising that some molecules that emit light (luminesce) can give off polarized light. This luminescence can be circularly polarized if the emitting molecules (luminophores) are arranged helically.

The Japanese researchers from the Tokyo University of Science and the Nara Institute of Science and Technology have now found a new twist for emitting circularly polarized light: simply stir. Why does this work? Stirring causes spiral vortexes to form in liquids, which can induce the luminophores to adopt a helical arrangement.

The researchers were even able to preserve the forcibly twisted directionality of the luminescence by causing the solution containing the luminophore molecules, a green rhodamine dye, to gel while being stirred. A gel is formed like the gelatine glaze on a cake. Below a certain temperature the molecules of a gelling agent form a loose network with cavities that contain the other components of the liquid. If the dye solution with a suitable gelling agent is cooled under stirring, the stir-induced spiral arrangement of the luminophores is maintained in the gel. Depending on the direction of stirring, the gel emits left- or right-polarized luminescence. Without stirring, the light emitted is not polarized.

More information: Kunihiko Okano, Circularly Polarized Luminescence of Rhodamine B in a Supramolecular Chiral Medium Formed by a Vortex Flow, Angewandte Chemie International Edition, Permalink to the article: http://dx.doi.org/ … ie.201104708

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What makes tires grip the road on a rainy day?

A team of scientists from Italy and Germany has recently developed a model to predict the friction occurring when a rough surface in wet conditions (such as a road on a rainy day) is in sliding contact with a rubber material (such as a car tire tread block) in an article to be published shortly in the Springer journal EPJE.

In their study, B.N.J. Persson from the Jülich Research Center in Germany and M. Scaraggi from the Polytechnic of Bari in Italy examined the flow of liquid at the contact between randomly rough surfaces. The contact interface looks like a labyrinth with vertically narrow void channels intersecting randomly. This causes channels to be either filled with water or not when in wet conditions.

For the first time, the authors applied a statistical analytical method to determine the average fluid flow at the interface of rough surfaces. Understanding this flow is important because it is inherently linked to the phenomenon of friction at the contact between the two surfaces.

Previous attempts to understand friction in such conditions used numerical approaches that required large computing power. They were based on calculating real roughness contacts by singling out each individual portion of the overall rough surface under study. Often, heavy approximations in the description of the simulated surface were applied to decrease the computational time.

The model presented in this paper provides theoretical predictions of friction as a function of the surface sliding velocity. It confirms previous experimental friction measurements made with a smooth steel ball sliding on a rough rubbery surface patterned with parallel grooves. The authors' model confirmed the experimental observation of a changing friction level related to a change in the angle between the direction of movement of the ball and the parallel to the grooves.

Potential applications would require that such a model be used to help create surfaces, such as microstructured tyres, which do not lower their grip when it rains.

Story Source:

The above story is reprinted from materials provided by Springer Science+Business Media.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

B. N. J. Persson, M. Scaraggi. Lubricated sliding dynamics: Flow factors and Stribeck curve. The European Physical Journal E, 2011; 34 (10) DOI: 10.1140/epje/i2011-11113-9

Quantum dots cast light on biomedical processes

The light emitted by quantum dots is both more intense and longer lasting than that produced by the fluorescent markers commonly used in medical and biological applications. Yet these nano-scale light sources still suffer from one major drawback: they do not dissolve in water. Researchers at the University of Twente's MESA+ Institute for Nanotechnology and at the A*STAR agency in Singapore have found a way to remedy this. They have developed a coating which allows quantum dots to be used inside the human body, even inside living cells. The researchers published details of their coating recipe in the October issue of Nature Protocols.

The new coating enables quantum dots, which are semiconductor nanocrystals, to literally cast light on biological processes. These dots are "nuggets," consisting of several hundred to several thousand atoms, that emit visible light when they are exposed to invisible UV radiation, for example. They range from a few nanometres to several tens of nanometres in size. The coating's benefits are not limited to improved solubility in water alone. Other molecules can "lock on" to its surface. This could make coated quantum dots sensitive to certain substances, for example, or allow them to bind to specific types of cells, such as tumour cells.

Better option

Scientists studying biological processes often use fluorescent tags that bind to biomolecules. This makes it relatively easy to track such molecules, even inside living cells. Quantum dots are a better option. They emit long-lasting, bright light, the colour of which depends on the size of the quantum dots used. For a number of reasons, including their toxicity, they were previously unsuitable for use in living organisms.

The researchers therefore developed an amphiphilic coating, i.e. one with both hydrophobic and hydrophilic properties. The "water hating" side of the polymer material attaches to the surface of the quantum dot. Its exposed hydrophilic side then makes the quantum dot/coating combination soluble in water. The coating builds up on the surface of the quantum dot through a process of self-assembly. The coating polymer has the added benefit that other molecules can be bound to it. Another important plus is that it does not adversely affect the quantum dot's light-emitting properties.

The study is a collaborative venture between the University of Twente's MESA+ Institute for Nanotechnology and the A*STAR agency's Institute of Materials Research and Engineering, in Singapore. It is headed by Professor Julius Vancso, Professor of Materials Science and Technology of Polymers at the University of Twente, who is also a visiting scientist at the Singapore institute.

Story Source:

The above story is reprinted from materials provided by University of Twente.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Dominik Jańczewski, Nikodem Tomczak, Ming-Yong Han, G Julius Vancso. Synthesis of functionalized amphiphilic polymers for coating quantum dots. Nature Protocols, 2011; 6 (10): 1546 DOI: 10.1038/nprot.2011.381

Shaken, not stirred: Scientists spy molecular maneuvers

 Stir this clear liquid in a glass vial and nothing happens. Shake this liquid, and free-floating sheets of protein-like structures emerge, ready to detect molecules or catalyze a reaction. This isn't the latest gadget from James Bond's arsenal -- rather, the latest research from the U. S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) scientists unveiling how slim sheets of protein-like structures self-assemble. This "shaken, not stirred" mechanism provides a way to scale up production of these two-dimensional nanosheets for a wide range of applications, such as platforms for sensing, filtration and templating growth of other nanostructures.

"Our findings tell us how to engineer two-dimensional, biomimetic materials with atomic precision in water," said Ron Zuckermann, Director of the Biological Nanostructures Facility at the Molecular Foundry, a DOE nanoscience user facility at Berkeley Lab. "What's more, we can produce these materials for specific applications, such as a platform for sensing molecules or a membrane for filtration."

Zuckermann, who is also a senior scientist at Berkeley Lab, is a pioneer in the development of peptoids, synthetic polymers that behave like naturally occurring proteins without degrading. His group previously discovered peptoids capable of self-assembling into nanoscale ropes, sheets and jaws, accelerating mineral growth and serving as a platform for detecting misfolded proteins.

In this latest study, the team employed a Langmuir-Blodgett trough -- a bath of water with Teflon-coated paddles at either end -- to study how peptoid nanosheets assemble at the surface of the bath, called the air-water interface. By compressing a single layer of peptoid molecules on the surface of water with these paddles, said Babak Sanii, a post-doctoral researcher working with Zuckermann, "we can squeeze this layer to a critical pressure and watch it collapse into a sheet."

"Knowing the mechanism of sheet formation gives us a set of design rules for making these nanomaterials on a much larger scale," added Sanii.

To study how shaking affected sheet formation, the team developed a new device called the SheetRocker to gently rock a vial of peptoids from upright to horizontal and back again. This carefully controlled motion allowed the team to precisely control the process of compression on the air-water interface.

"During shaking, the monolayer of peptoids essentially compresses, pushing chains of peptoids together and squeezing them out into a nanosheet. The air-water interface essentially acts as a catalyst for producing nanosheets in 95% yield," added Zuckermann. "What's more, this process may be general for a wide variety of two-dimensional nanomaterials."

This research is reported in a paper titled, "Shaken, not stirred: Collapsing a peptoid monolayer to produce free-floating, stable nanosheets," appearing in the Journal of the American Chemical Society (JACS) and available in JACS online. Co-authoring the paper with Zuckermann and Sanii were Romas Kudirka, Andrew Cho, Neeraja Venkateswaran, Gloria Olivier, Alexander Olson, Helen Tran, Marika Harada and Li Tan.

This work at the Molecular Foundry was supported by DOE's Office of Science and the Defense Threat Reduction Agency.

Story Source:

The above story is reprinted from materials provided by DOE/Lawrence Berkeley National Laboratory.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Babak Sanii, Romas Kudirka, Andrew Cho, Neeraja Venkateswaran, Gloria K. Olivier, Alexander M. Olson, Helen Tran, R. Marika Harada, Li Tan, Ronald N. Zuckermann. Shaken, Not Stirred: Collapsing a Peptoid Monolayer To Produce Free-Floating, Stable Nanosheets. Journal of the American Chemical Society, 2011; : 111012114427004 DOI: 10.1021/ja206199d