Like their normally sized analogues, nanoscale machines and
robots need
motors
to function. Some time ago, a team at
Pennsylvania State University developed a
clever engine to "drive" nanoscopic metal rods; however, until now, these tiny
"submarines" roamed at random through the
solution. Now the research team headed
by Ayusman Sen has made more progress: their "sequel" can be steered by remote
control.
How do these steerable nanoscopic submarines work? First, it is important that
the
nanorods be made of "stripes" of different
metals: one end of
gold, followed
by a very narrow band of
nickel, a band of
gold, another very narrow band of
nickel, and the other end of
platinum. The
platinum end is responsible for the
propulsion because it is here that the "fuel" is converted. The fuel in question
is
hydrogen peroxide that is present in the
solution. It is catalytically
converted by the platinum, producing
oxygen, which also dissolves in the
solution. Having a high
oxygen concentration, the solution surrounding the
platinum end is less polar than the solution surrounding the other end of the
nanorod. The
surface tension between the solution and the metal surface is thus
no longer equal at both ends of the rods and the rod is pulled inescapably in
the direction of the oxygen-containing region of the solution. Because
oxygen is
constantly being formed, the gradient is maintained and the rod moves through
the solution with its platinum end in front. The orientation of the
nanorods in
the solution is random; the overall motion is thus undirected.
This is where the "
remote control"-an external magnetic field-comes in. The
"receiving
antennas" are the previously magnetized nickel bands in the nanorods.
The crucial trick here is that the width of the bands must be smaller than their
diameter, so that the nanorods can be magnetized crosswise, rather than along
their long axis. When the magnetic field is switched on, the nanorods line up at
right angles to its field lines and maintain this orientation as they zip around
under
hydrogen peroxide power. By changing the direction of the magnetic field,
the researchers can vary the orientation of the nanorods-and thus the direction
of their movement-at will. The nanorods are steerable.
"In principle, we should also be able to couple our nanorod motors to other
nano-objects in order to drive them," says Sen. "This opens up new possibilities
for a whole new class of micro- and
nanomachines." The dimensions and magnetic
properties of the nanorods are comparable to
magnetotactic bacteria, which are
oriented and steered by the magnetic field of the earth. Sen: "Thus, our
nanorods are functional models for such organisms."