Nanoscale machines can be made by mechanically locking molecules together, say researchers in the US.

Fraser Stoddart and colleagues at the University of California Los Angeles used rotaxanes and pseudorotaxanes to create such devices.

Rotaxanes consist of a molecular ring mounted on a spindle. The ring is prevented from leaving by bulky groups, or stoppers, at either end of the spindle. Pseudorotaxanes are similar to rotaxanes, but they instead rely on non-covalent interactions to hold the ring in place. 

Long rotaxanes can have more than one stable site on the spindle where the ring can rest. As the ring moves between these sites, a switch is created. This can be achieved by reduction and oxidation, photochemistry, or by temperature control. Pseudorotaxane switches are operated by the threading and dethreading of the ring.

The group built both kinds of molecular switch onto nanopores to create chemically-operated valves, attached by the rings to a gold cantilever to make actuators. They also made a nanoscale photoelectrochemical cell that can power the dethreading of a pseudorotaxane.

Thorough studies of how these switchable molecules self-assemble, pack, align, and function on surfaces are essential for device design, say the researchers.

Colin R Batchelor