Dumbbells lend weight to molecular machines

An international team of chemists has devised a method that could allow them to organise tiny molecular machines on a surface to build devices that pack in thousands of times as many switching units than is possible with a conventional silicon chip.

Fraser Stoddart from the University of California Los Angeles and his colleagues have designed and made numerous molecules based on hanging ring-shaped molecules on other chain-like molecules and loops. By incorporating functional chemical groups along the length of the chain or around these loops, they have shown that it is possible to make the molecular beads switch between these various functional groups using heat, light, or electricity. The ultimate aim of creating such molecular-scale devices is to use them as switching units or logic gates in a future computer based on molecules instead of silicon chips.

Before that will be possible, however, the nanoscientists must find a way to organise arrays of these molecules on a surface so that input and output connections can be made between the molecules and the outside world.

Stoddart and his colleagues said that in order to exploit their molecular machines they will need to find a way to organise them at interfaces, deposit them on functional surfaces, or immobilise them into membranes or porous materials. This will allow the molecular machines to work together and to be "addressed" on the nanometre scale. The researchers believe that modifying the surface of an electrode to incorporate an organised layer of molecular machines could be the key to success.

The team has succeeded in applying a layer just a few molecules thick of a particular molecular machine to a glass surface. The molecular machine in question is a switchable rotaxane, a ring-shaped molecule held on a short chain by two blocking groups, making it resemble a dumbbell with a collar around the handle.

A technique was used to make thin layers of this dumbbell-shaped component in solution on a glass slide coated with ITO (indium tin oxide). By using two solutions, one containing the dumbbells and the other a soapy surfactant compound, the researchers were able to force the molecules to organise themselves using electrostatic repulsion and attraction between the surfactant, the molecules, and the surface. Ultimately they became attached with the same orientation to the ITO layer on the glass slide.

The researchers then tested their thin layers of dumbbell molecules to see whether they would work as planned. They found that the thin film exhibited a reversible switching behaviour when exposed cyclically to an acid and then a base. This, they said, demonstrates that the thin film is capable of transducing a chemical input signal, the acid-base, into an electrical output signal. This could lead to interfacing other molecular machines in a similar way.

Ultimately, control using a light source or electricity will be required before such layers will be useful in the development of molecular computers, but this first small step to organising molecular machines could lead to the required breakthrough.