A nano car with molecular 4-wheel drive

November 10, 2011

To carry out mechanical work, one usually turns to engines, which transform chemical, thermal or electrical energy into kinetic energy in order to, say, transport goods from A to B. Nature does the same thing; in cells, so-called motor proteins - such as kinesin and the muscle protein actin - carry out this task. Usually they glide along other proteins, similar to a train on rails, and in the process "burn" ATP (adenosine triphosphate), the chemical fuel, so to speak, of the living world.

A number of chemists aim to use similar principles and concepts to design molecular transport machines, which could then carry out specific tasks on the nano scale. According to an article in the latest edition of science magazine Nature, scientists at the University of Groningen and at Empa have successfully taken "a decisive step on the road to artificial nano-scale transport systems". They have synthesised a molecule from four rotating motor units, i.e. wheels, which can travel straight ahead in a controlled manner. "To do this, our car needs neither rails nor petrol; it runs on electricity. It must be the smallest electric car in the world - and it even comes with 4-wheel drive" comments Empa researcher Karl-Heinz Ernst.

Range per tank of fuel: still room for improvement

The downside: the small car, which measures approximately 4x2 nanometres - about one billion times smaller than a VW Golf - needs to be refuelled with electricity after every half revolution of the wheels - via the tip of a scanning tunnelling microscope (STM). Furthermore, due to their molecular design, the wheels can only turn in one direction. "In other words: there's no reverse gear", says Ernst, who is also a professor at the University of Zurich, laconically.

According to its "construction plan" the drive of the complex organic molecule functions as follows: after sublimating it onto a copper surface and positioning an STM tip over it leaving a reasonable gap, Ernst's colleague, Manfred Parschau, applied a voltage of at least 500 mV. Now electrons should "tunnel" through the molecule, thereby triggering reversible structural changes in each of the four motor units. It begins with a cis-trans isomerisation taking place at a double bond, a kind of rearrangement - in an extremely unfavourable position in spatial terms, though, in which large side groups fight for space. As a result, the two side groups tilt to get past each other and end up back in their energetically more favourable original position - the wheel has completed a half turn. If all four wheels turn at the same time, the car should travel forwards. At least, according to theory based on the molecular structure.

To drive or not to drive - a simple question of orientation

And this is what Ernst and Parschau observed: after ten STM stimulations, the molecule had moved six nanometres forwards - in a more or less straight line. "The deviations from the predicted trajectory result from the fact that it is not at all a trivial matter to stimulate all four motor units at the same time", explains "test driver" Ernst.

Another experiment showed that the molecule really does behave as predicted. A part of the molecule can rotate freely around the central axis, a C-C single bond - the chassis of the car, so to speak. It can therefore "land" on the copper surface in two different orientations: in the right one, in which all four wheels turn in the same direction, and in the wrong one, in which the rear axle wheels turn forwards but the front ones turn backwards - upon excitation the car remains at a standstill. Ernst und Parschau were able to observe this, too, with the STM.

Therefore, the researchers have achieved their first objective, a "proof of concept", i.e. they have been able to demonstrate that individual molecules can absorb external electrical energy and transform it into targeted motion. The next step envisioned by Ernst and his colleagues is to develop molecules that can be driven by light, perhaps in the form of UV lasers
-end-
Literature

Electrically driven directional motion of a four-wheeled molecule on a metal surface, T. Kudernac, N. Ruangsupapichat, M. Parschau, B. Macia, N. Katsonis, S.R. Harutyunyan, K.-H. Ernst, B.L. Feringa, Nature 479 (2011), doi: 10.1038/nature10587

Swiss Federal Laboratories for Materials Science and Technology (EMPA)

Related Nature Articles from Brightsurf:

Future climate changes in nature reserves
The Earth's nature reserves are set to be affected by future climate change in very different ways.

Tailored light inspired by nature
An international research team with Prof. Cornelia Denz from the Institute of Applied Physics at the University of M√ľnster (Germany) develop for the first time light fields using caustics that do not change during propagation.

Accounting for nature in economies
Gross Domestic Product, the standard metric for measuring national economies, doesn't account for the valuable services provided by nature.

Nature unveiling herself before science
21st century societal challenges such as demographic developments and an ageing population demand for new functional materials, such as for bone prostheses.

Energized by enzymes -- nature's catalysts
Scientists at Pacific Northwest National Laboratory are using a custom virtual reality app to design an artificial enzyme that converts carbon dioxide to formate, a kind of fuel.

Mother nature and child development
A world first review of the importance of nature play could transform children's play spaces, supporting investment in city and urban parks, while also delivering important opportunities for children's physical, social and emotional development.

How nature tells us its formulas
A team from TU Wien and the University of Heidelberg has developed methods with which these models can be directly obtained from experimental measurements.

Reconnecting with nature key for sustainability
People who live in more built up areas and spend less free-time in nature are also less likely to take actions that benefit the environment, such as recycling, buying eco-friendly products, and environmental volunteering.

Limiting the loss of nature
With only about half of Earth's terrestrial surface remaining as natural vegetation, a University of Queensland-led team has proposed an international goal to halt its continued loss.

Combatting air pollution with nature
Air pollution is composed of particles and gases that can have negative impacts on both the environment and human health.

Read More: Nature News and Nature Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.