THE DEVICE: Butyl methyl sulphide is kind of like a lop-sided pinwheel, with a sulphur atom as the pin, and two arms—one a larger butyl group and the other a smaller methyl group—that spin around. This nanometer-size motor is bound to a metal surface at the sulphur atom, and powered by electrons from a scanning tunneling microscope (STM).
“We chose this molecule because we wanted it to be relatively easy to understand,” said Charles Sykes, an associate professor of chemistry at Tufts University and the senior researcher on the project.
The microscope, which can observe single molecules, has a sharp metal tip that the researchers used to deliver electrons to the motor from above. At the same time, the scope is watching the molecule spin. By measuring changes in current, it can determine how fast the motor is spinning and in what direction.
WHAT'S NEW: Sykes's motor isn't the first single-molecule motor, but it is the first to be powered electrically. Light-driven and chemically-fueled motors preceded the current model, but the electronic approach has some key advantages.
“Light is too stochastic,” said Herre van der Zant, a professor at Delft University of Technology in the Netherlands, who was not involved in this study, making it hard to precisely power the motor. Light can also become too intense, and actually burn the motor. “An electrical impulse is much faster and gives more direct control,” Van der Zant said.
Chemical fuel mandates that a motor be in a fluid, and can also be less precise, as the chemicals will diffuse throughout the liquid and not stay focused on the motor, Sykes told The Scientist.
Daniel Dundas, a researcher at Queen's University of Belfast, said the researchers were able to develop what others had only designed in theory because they took advantage of the microscope's ability to fuel the motor and observe its rotation simultaneously. “The main novelty is the use of STM techniques to carry out the measurements,” Dundas told The Scientist.
IMPORTANCE: Though motors have use in living systems, Columbia University professor Henry Hess said the most promising application of the new machine could be in technology, using the rotation of the motor to switch something on or off by making or breaking a connection.
Tiny motors could also be used to steer fluids in a particular direction, process signals, sense viscosity, or act as miniature microwave transmitters or nanoscale radio wave antennas. But Sykes said these ideas represent a 50-year outlook.
For now, proof of principal is the most important thing, and not necessarily the potential applications of a tiny electric motor, said Hess, who did not participate in this research. Simply demonstrating that such a motor can work “is a world record, in a sense,” he told The Scientist.
NEEDS IMPROVEMENT: An important part of developing a motor is directing which way it spins so that its motion does the work desired consistently. Sykes and his team actually developed two motors with opposite orientations of its side chain components—one with a right-handed structure, and one with a left. The left-handed form of the motor preferred to spin clockwise, doing so 5 percent more frequently than it spun in the opposite direction.
“It would be very important to increase that number,” and induce the motor to spin exclusively in one direction, Van der Zant told The Scientist. “But it's not clear at all whether you can achieve that with this set up...and make this truly unidirectional.” Van der Zant is working on a molecular motor that uses electrical fields as opposed to electrons alone as a driving force, and said that it could allow for more control over speed and direction.
Sykes said he was pleased to see any preference in direction at all. “Right now, we're very happy to have proof of principal, because there are no real guidelines in terms of what to expect.”
Van der Zant agreed. “I think the main thing is to understand this motion on a nanometer scale, and this will help us understand how motors in general work.”
H.L. Tierney, et al., “Experimental demonstration of a single-molecule electric motor,” Nature Nanotechnology, doi:10.1038/nnano.2011.142, 2011.