Measuring approximately 4 x 2 nm, Empa’s molecular car forges ahead on a copper surface with its four electrically driven wheels. |
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 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 synthesized 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” says Empa researcher Karl-Heinz Ernst.
Range per tank of fuel: still room for improvement
The
downside: the small car, which measures approximately 4 x 2 nm—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 6 nm forward—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
carbon-carbon 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”. 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
ultraviolet lasers.
Electrically driven directional motion of a four-wheeled molecule on a metal surface
