Synthetic molecular machines hold promise for nanorobotics, improved drug delivery systems, and chemical synthesis. Now, researchers have taken a step toward creating these nano-machines by building a four-wheeled molecule - dubbed a nanocar - that can convert electrical energy into forward motion.
The nanocar is a synthetic organic molecule made of four molecular motors. When placed on a conductive substrate and electrically stimulated. the so-called wheels rotate and propel the molecule forward.
This is not the first example of synthetic molecular movement.
"There have been previous examples of molecules that could move on surfaces," says lead researcher Tibor Kudernac, a postdoctoral student at the Univ. of Twente in the Netherlands. "But although controlled movement of single molecules along a surface has been reported, the molecules in those examples act as passive elements that either diffuse along a preferential direction with equal probability for forward and backward movement or are dragged by an STM [scanning tunneling microscope] tip," Kudernac says.
The nanocar transforms electrical energy into mechanical motion and therefore could be used to perform tasks at the nanoscale, such as carrying atoms or molecules from one location to another.
The proof-of-principle research demonstrates that a synthetic molecule can be designed and made to perform mechanical tasks, Kudernac says. "This particular molecule will probably never find any application, but the demonstration that we can do something like that probably will lead to further designs and other molecules that can do something more complex, more application-related."
The nanocar makes use of molecular wheels - chiral units that undergo geometric changes and rotate in one direction as a result of electrical or vibrational energy input.
For the car to move forward, all four wheels must move in the same direction. The researchers met this requirement by identifying and then designing the molecule to have a specific configuration: the meso-(R, S-R, S) isomer of the molecule. They placed the meso-isomer on a copper surface and used an STM to both fuel the molecule and observe its movement. The STM tip applied a voltage pulse (> 500 mV), and after 1 0 pulses, the nanocar moved 6 nm across the copper surface.
Kudernac and his colleagues will now focus their research on the next critical challenges, one of which is to perform the same results at ambient conditions; the current findings were obtained at very low temperatures (7 K) and in ultrahigh vacuum (less than 1O-10 mbar). Because STM requires a conductive substrate, they will also experiment with using light to provide energy to the nanocar, which will make their work applicable to a variety of substrates.
* The 3D geometry of the four-wheeler is essential to its forward motion. Image courtesy of Randy Wind and Martin Roelfs.
* The nanocar consists of four chiral units that act like molecular motors. For the molecule to move forward, all four wheels must move in the same direction. Of all the possible isomers that the molecule could assume, this requirement is met only by the meso-(R, S-R, S) isomer. The direction of the motors is represented by the red arrows. Image courtesy of Nature.