In 2005, Bartels and colleagues made the first molecule that can move in a straight line – an already significant result because normal molecules tend to move around randomly in all directions. The new breakthrough now shows that this nano-walker can carry a load – in this case, two carbon dioxide molecules – and transport them along a straight path.

The researchers made their molecule carrier out of anthraquinone, which consists of three fused benzene rings with one oxygen atom on each side. They then placed the molecule on the surface of a clean and polished copper surface. Next, using a needle valve, the researchers introduced carbon dioxide molecules into the vacuum chamber where the experiment was being performed so that they deposited onto the surface of the sample.

Using scanning tunnelling microscopy, the California team observed that the anthraquinone is able to pick up up to two carbon dioxide molecules and carry them in a specific direction across the copper surface. Loaded down with its "cargo", the anthraquinone moves more slowly than without a load. A carrier with one carbon dioxide molecule requires double the energy and one carrying two molecules needs triple the energy. "This is not unlike a human being carrying heavy loads in one or both hands," explains Bartels.

"[Our work] is an unprecedented step forward towards the realization of molecular-scale machinery," adds Bartels. "Our experiments show a means to transport molecules reliably. This will become as important to the molecular machinery of the future as trucks and conveyor belts are for factories today." The result could also help scientists understand how to attach therapeutic drugs to molecules so that they can be transported to specific targets through the bloodstream.

The researchers would now like to make their molecule carrier travel around corners, rotate its cargo or send out photons to tell scientists where it is located. They would also like to find molecules that can carry cargos other than carbon dioxide.

The team reported its work in Sciencexpress.