Nano News: 09/01/15

Manipulating the rotational direction of artificial molecular motors using supramolecules

A research group consisting of Takashi Uchihashi, MANA Scientist, Jonathan Hill, MANA Scientist, Tomonobu Nakayama, Unit Director, and Christian Joachim, MANA Principal Investigator (also a group leader at the CEMES-CNRS, France), at the NIMS International Center for Materials Nanoarchitectonics (MANA), along with a research team led by Professor Teruo Ono at the Institute for Chemical Research of Kyoto University, jointly fabricated molecular motors on a metal substrate using supramolecules, and successfully reversed rotation of molecular motors by rearranging bonding between molecules that constitute a supramolecule. A molecular motor is a kind of nanomachine vital in sustaining everyday activities of living organisms. It is a dream of nanotechnology researchers to fabricate a mechanical system driven by nanomachines in the same manner that biological systems develop molecular motors in a self-organizing fashion. While molecular motors already have been created on substrate surfaces using organic molecules, they had a major issue in that they were incapable of switching their rotational direction. This issue is caused by their structural rigidity associated with strong bonding among the molecules that comprise a motor.

Conceptual diagram showing a molecular motor in action. A porphyrin dimer rotates in the direction indicated by the solid arrow through injection of electric current into the dimer from the probe of a scanning tunneling microscope. In this study ("Current-Driven Supramolecular Motor with In Situ Surface Chiral Directionality Switching"), the joint research team fabricated structurally flexible molecular motors using a supramolecule, and succeeded for the first time in manipulating the rotational direction of the motors. A supramolecule has a complex structure, consisting of several molecules that are loosely connected to each other by hydrogen bonds and/or other kinds of weaker bonds relative to covalent bonds. A motor made of a supramolecule rotates in one direction when electric current is injected into the molecule. In addition, the team succeeded in reversing the motor’s rotational direction by rearranging motor parts via application of electric current under certain conditions. The team accomplished this because supramolecule-comprising molecules were bonded by moderate strength, which is neither too strong nor too weak. Moreover, since the team applied the principle of self-organization in biological systems to the fabrication of molecular motors, they believe that mass production of the products is feasible. Building on these positive outcomes, the team will aim to create nanomachines with superior functionality at a larger scale. Also, studies on the behavior of artificial molecular motors may help understand the detailed mechanism of how naturally-occurring molecular motors in biological systems work. This study was conducted as a part of a Grant-in-Aid for Challenging Exploratory Research program titled “Creation of a new molecular motor based on the Einstein-de Haas effect” (Takashi Uchihashi, principal investigator).

Reversible writing with light

The medium is the message. Dr. Rafal Klajn of the Weizmann Institute’s Organic Chemistry Department and his group have given new meaning to this maxim: An innovative method they have now demonstrated for getting nanoparticles to self-assemble focuses on the medium in which the particles are suspended; these assemblies can be used, among other things, for reversibly writing information. This approach is an elegant alternative to present methods that require nanoparticles to be coated with light-sensitive molecules; these then switch the particles’ state when light is shined on them. According to the group’s research, which recently appeared in ("Light-controlled self-assembly of non-photoresponsive nanoparticles"), putting regular, uncoated nanoparticles into a light-sensitive medium would be simpler, and the resulting system more efficient and durable than existing ones. The possible applications range from rewritable paper, to water decontamination, to the controlled delivery of drugs or other substances.

Nanoparticles in a light-sensitive medium scatter in the light (left) and aggregate in the dark (right) This method could be the basis of future "rewritable paper" The medium, in this case, is made up of small “photo-switchable” (or “photoresponsive”) molecules called spiropyrans. In the version of the photoresponsive molecule employed by Klajn and his group, absorbing light switches the molecule to a form that is more acidic. The nanoparticles then react to the change in acidity in their environment: It is this reaction that causes the particles to aggregate in the dark and disperse in the light. This means that any nanoparticles that respond to acid – a much larger group than those that respond to light – can now potentially be manipulated into self-assembly. By using light – a favored means of generating nanoparticle self-assembly – to control the reaction, one can precisely govern when and where the nanoparticles will aggregate. And since nanoparticles tend to have different properties if they are floating freely or clustered together, the possibilities for creating new applications are nearly limitless. Klajn points out that these molecules have a long history at the Weizmann Institute: “Two Institute scientists, Ernst Fischer and Yehuda Hirshberg, were the first to demonstrate the light-responsive behavior of spiropyrans in 1952. Later on, in the 1980s, Prof. Valeri Krongauz used these molecules to develop a variety of materials including photosensitive coatings for lenses. Now, 63 years after the first demonstration of its light-responsive properties, we are using the same simple molecule for another use, entirely,” he says. The advantages of the medium-based approach are clear. For one, the particles do not seem to degrade over time – a problem that plagues the coated nanoparticles. “We ran one hundred cycles of writing and rewriting with the nanoparticles in a gel-like medium – what we call reversible information storage – and there was no deterioration in the system. So you could use the same system over and over again,” says Klajn. “And, although we used gold nanoparticles for our experiments, theoretically one could even use sand, as long as it was sensitive to changes in acidity.” In addition to durable “rewritable paper,” Klajn suggests that future applications of this method might include removing pollutants from water – certain nanoparticles can aggregate around contaminants and release them later on demand – as well as the controlled delivery of tiny amounts of substances, for example, drugs, that could be released with light.

Hot electrons point the way to perfect light absorption

Light-absorbing films can be found in many everyday applications such as solar cells or sensors. They are used to convert light into electrical current or heat. The films literally trap the light. Although such absorber films are applied widely, scientists still do not know which mechanism permits the most efficient absorption of light. A team of physicists at Bielefeld University, the University of Kaiserslautern, and the University of Würzburg have now proved that the very efficient scattering of light in ultrathin rough films traps light until it is absorbed completely. The researchers are publishing their findings in the journal ("Perfect absorption in nanotextured thin films via Anderson-localized photon modes"). This research can help to make thin absorber films even more efficient and thereby save energy. The experiments applied ultrashort light pulses. When such pulses penetrate smooth ultrathin films, they emerge on the other side practically unchanged and scarcely weakened. In rough films, in contrast, irregularities prevent the light pulse from spreading through the material. When there are many irregularities leading to light scattering, the pulse proceeds along a closed path and remains trapped until the light is absorbed. Two effects enabled the physicists to confirm this light trapping mechanism. First, a small part of the trapped light is scattered out of the absorber layer. By tracking this scattered light intensity over time, it is possible to see directly how long light is trapped in the film. A second effect delivers information on the spatial localization of the trapped light and the local absorption of energy. The absorption of an ultrashort light pulse excites electrons in the absorber material, heating them up briefly to temperatures of several thousand degrees Celsius – comparable to the temperature on the surface of the sun. At these temperatures, electrons are emitted from the material, as can be confirmed by high-resolution electron microscopy. Measurements show that the light is trapped in small areas with a diameter of less than one micrometre, and this is where it is also absorbed. The underlying effect of this so-called Anderson localization was already described more than 60 years ago, and it has been observed several times since then. What is new is that the mechanism also functions for thin absorber layers. ‘This opens up new ways to develop highly efficient absorbers and can therefore contribute to developing improved thin-film solar cells or sensors,’ says Professor Dr. Walter Pfeiffer from Bielefeld University. The idea behind the research is to make thin-film absorbers more efficient so that they can be used in everyday applications. In future, the researchers aim to study what structure films should have in order to trap light perfectly and to use this to develop a universal concept of efficient light absorption via Anderson localization.