The little motor that might

Hank Hogan, hank@hankhogan.com

Researchers have built a molecule-size motor powered by light that could be used for such tasks as moving a minuscule object a few nanometers. The scientists, however, have bigger plans: They hope to someday use this or other similar motors to move objects large enough to see, with the energy to do so coming from the sun.

Pulling off that feat will present several challenges. The main trick will be getting vast numbers of these so-called “nano-motors” to work together, according to University of Florida chemistry and physiology professor Weihong Tan. “The key,” he said, “is to accumulate molecular-level forces.”

A molecular “nanomotor” converts visible light into motion. The expansion can be reversed, and the molecule will contract into a smaller size upon exposure to ultraviolet light. Courtesy of Weihong Tan, University of Florida.

Tan, who headed the project, noted that other light-driven nanomotors have been built but that this version is unique and offers several benefits. The new motor involves only a single molecule, which makes the motion independent of chemical concentration. Schemes that depend upon the interaction of two molecules, as was the case for earlier motors, don’t provide that independence.

The new nanomotor also is highly efficient and is easy to prepare and regulate. The final advantage of the new motor, Tan said, is the potential for solar energy utilization.

The device, which Tan, graduate students Huaizhi Kang and Yan Chen and the rest of the group reported on in Nano Letters, consists of a single strand of DNA. The researchers inserted azobenzene molecules at each end of the strand. In one conformation, the azobenzene forced the molecule into a closed hairpin loop measuring about 2.2 nm across. When exposed to visible light (450 nm), the azobenzene caused the DNA bases to dehybridize, thereby releasing the loop. As a result, the molecule uncoiled, reaching a length of about 10.2 nm.

Subsequent exposure to ultraviolet light (350 nm) changed the azobenzene’s characteristics and allowed the DNA to hybridize again, thereby recreating the hairpin curve. Tests showed that the nano-motor could be used repeatedly, with the molecule opening and closing many times without any apparent degradation.

The investigators measured the molecular movement by attaching fluorescein to one end of the molecule and a quencher at the other. When the molecule was closed, the two were near each other, and the fluorescence was low. When the molecule was uncoiled, the fluorescence was high.

The force exerted by a given nanomotor was a few piconewtons, similar in size to the force applied by kinesin molecular motor proteins transporting vesicles inside a cell. The efficiency of the conversion of light energy for the nanomotor was considerably better than that of traditional solar cells. The open-close efficiency was 40 to 50 percent, with the high figure being due in part to this being a single-molecule event.

“Intramolecular motion loses little energy to the environment,” Tan said.

Tan reported that the group has two long-term goals. One is to generate other new molecule-based nanostructures, assemble them into nanomotors and then study their properties. The other is to use the nanomotors to convert sunlight into power and mechanical motion.