1 August 2005

New Type Of Molecular Motor Suggested

by Kate Melville

Compared to the specialized chemistry that powers the muscles and cellular motors of the human body, the micro-machines built by scientists today look pretty crude. But that may be about to change thanks to a new theory outlining how molecular machines can be driven individually (using only one molecule) by applying an electric current that creates an internal energy source. While the work is only theoretical at this stage, the concept, published in Physical Review Letters, could speed the development of molecular sized machinery.

The researchers, two Northwestern University chemists, said the downside of conventional molecular machines are that they are driven as an ensemble, usually by external light or chemistry, and are made up of many molecules. The complexity and size factors make these machines difficult to control. "People envision using molecular machines for computing techniques, sensors, bioengineering and solar cells," said Tamar Seideman, who led the research effort. "Molecular machines have unique functions and properties that are different from macroscopic machines, not only and not primarily because they are of the nanoscale. Rather, they use truly molecular features such as their energy level structure, their dynamics and their response to external stimuli."

Seideman and co-researcher Chao-Cheng Kaun propose a motor based around a single buckyball (fullerene) molecule. The buckyball is placed between two gold electrodes, forming what is known as a molecular junction. When an electric current is run through the electrodes, the electrons transfer energy to the molecule, causing the molecule to vibrate and creating an internal energy source. The buckyball oscillates between the electrodes, as if on an invisible spring. Importantly, because the single molecule can be driven individually, the resulting motion can be controlled precisely.

Interestingly, because the conductivity of the molecular junction depends on the location of the buckyball between the electrodes, the current oscillates with time at the frequency of the buckyball oscillations. The oscillating current translates into an electromagnetic field, making the tiny motor a nanoscale radiation field generator- something the researchers believe had not been envisaged before.

Seideman said a practical demonstration of such a molecular motor should not be too far away. "We are encouraged by the rapid progress of experimental methods of making little molecular junctions of this type," he concluded.

Source: Media release - North Western University