Who needs headlights? These nanocars glow!

BY THOMAS BURNETT
Special to the Rice News

How do you make a vehicle visible if it is as small as a nanocar? Rice University professor James Tour has been making single-molecule nanocars in his lab for years. They’re too small to see with a regular microscope, so he decided to mark them with a bright, glow-in-the-dark tag that would make them easy to spot.

The addition of a fluorescent molecule allows nanocars to emit light for easier tracking.

At first Tour, graduate student Jason Guerrero and postdoctoral research associate Guillaume Vives added a fluorescent tag to their vehicles, but these marked cars had difficulty moving. The tag did not always attach properly, and when it did, it appeared to interfere with the motion of the nanoscopic cars.

The breakthrough came when Tour’s group began using a fluorescent molecule called BODIPY to construct the axles of the car itself, which eliminated the need for any external additions. In the presence of green laser light, the axles of the car emit yellow light and allow for easier tracking. The findings appeared in the March 8 issue of Organic Letters.

Tour said using BODIPY molecules in the construction of nanocars is attractive for several reasons. They are highly stable and soluble in most organic solvents, which makes them easier to assemble. And in contrast to previous ”Z-shaped” nanocars, these molecules enable the scientists to construct a chassis that’s perpendicular to the axles, which enhances the straight-line motion of the cars and reduces the chance that they’ll stick to the surface on which they are rolling. These BODIPY-based nanocars are considered “highly fluorescent” because they emit light about three-quarters of the time.

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“These vehicles are a versatile building block to prepare even more complex nanocars by varying the inner portion,” Tour said.

The main advantage of a fluorescent nanocar is that it is easier to observe. With earlier models, scanning tunneling microscopy (STM) made it possible to view the cars, but STM limited studies of rolling cars to metal surfaces. With single-molecule fluorescence microscopy (SMFM), nanocars are visible on common surfaces like glass.

The Rice team constructed three prototypes of these fluorescent cars: two that roll in straight lines and one that rolls in circles. Though they don’t have a steering mechanism, their speed and direction are predictable. And because they are so incredibly small, Tour has found that “they can rely exclusively on the thermal energy from the surface to roll about four car-lengths per second, or nine nanomiles per hour.”

Another fascinating feature about these cars is that they need different kinds of wheels to roll on different surfaces. When rolling on gold, buckyball wheels work well because their electrical charge transfer gives them good traction. Wheels made of carborane are smaller than buckyballs. They lose their grip on gold surfaces, but they perform well on glass, due to their hydrogen-bonding properties. Climbing hills is still a formidable challenge for designers; Tour said today’s fleet of nanocars rolls only on flat surfaces. Now that these nanocars glow, they are much easier to track, and Tour is busy studying the motion of these unique vehicles.

Tour’s nanocar research is funded by the National Science Foundation.

— Thomas Burnett ’00 is an aspiring science writer. He lives in the Washington, D.C., area.