Single-Molecule Electric Motor May Impact Microfluidics, Nanoscale Electronics

Kate Dixon

Testing the boundaries of nanotechnology and miniaturization, a team of chemists at Tufts University (Medford, MA) has developed what they claim is the world’s first single-molecule electric motor. Beating out the previous record of 200-nm across, the Tufts team’s minuscule molecular motor could open the door to a new class of medical devices.

While working on self-assembled monolayers (SAMs) in 2006, the researchers found that they were able to image individual molecules outside of a SAM structure at low concentrations. Featuring a hexagonal shape, the molecules were free to rotate around the sulfur metal bond. “At high concentrations, however, the molecules block each other from rotating, and we could image the stationary molecule,” says Colin Murphy, a lead author of the published research and a doctoral candidate in chemistry at Tufts. “It is well established that in order to get directed motion in molecular systems, you need a chiral or asymmetrical element.”

With this in mind, the team employed a butyl methyl sulfide molecule with carbon and hydrogen atoms radiating off of it. Forming two asymmetrical arms, the structure featured four carbons on one side and one on the other. One molecule was placed on a conductive copper substrate while the carbon chains were left free to rotate around the sulfur-copper bond.

The researchers then attempted to control the resulting molecular motor with an electrical charge. This task was achieved using the metal tip on a low-temperature scanning tunneling microscope (LT-STM), which uses electrons instead of light to visualize molecules. By controlling the temperature of the molecule, the team was able to directly affect the rate of the molecule’s rotation. The researchers found that a temperature of approximately –450ºF was optimal for tracking the motion of the motor.

“It may be possible that the motor still works at higher temperatures. But, at these temperatures, it is rotating too rapidly for the team to measure with current instruments,” Murphy explains. “We are also looking into different ways we could adjust the surface or molecule in order for the system to remain stable at higher temperatures. Further studies of different molecules and fashioning specific electrodes will lead to enhanced control at higher temperatures.”

Tracking the rotational motion of the molecule over time proved difficult as well. Because the scanning rate of the instrument was much lower than the rotation rate, the researchers relied on spectroscopy to monitor the rotation and to correlate the data with the motion. “Eventually, we were able to develop a counting method that could tell us when the molecule was in what position relative to the electrode and thus what direction it was going over a set amount of time,” Murphy says.

The single-molecule motor could potentially be used to facilitate fluid movement through the microscale channels of microfluidic devices, according to Murphy. For example, the incorporation of these small motors along the channel walls could help to overcome friction created by fluid moving against the pipe walls. The motor molecules could also be coupled to electromagnetic fields in order to yield minimally invasive sensing arrays or may be used in nanoscale electronics.