Computer-generated rendering of a USN. (Image courtesy of Loïc Samuel and Rice University.)
Produced through a 20-step chemical synthesis, USNs consist of 244 atoms. Their “rotors” operate like a bacterium’s flagellum, a whip-like appendage used primarily for locomotion.
When exposed to ultraviolet light, the double bond which holds the USN’s rotor to its body becomes a single bond, allowing it to rotate a quarter step. As the rotor seeks to return to a lower energy state, it jumps adjacent atoms for another quarter turn. The process repeats as long as the light is on.
The USNs are propelled forward at 18nm per revolution, with their motors running at over one million RPM. “These are the fastest-moving molecules ever seen in solution,” said Chemist James Tour, who runs the laboratory at Rice University where they were constructed.
In a paper published in the journal Nano Letters, Tour and his colleagues report that the USNs showed an enhancement in diffusion of 26 percent.
The researchers also equipped the USNs with “pontoons” that fluoresce red when excited by laser in order to keep track of them. The same team previously built molecular cars with four wheels, axels and independent suspensions, but the USNs could not be observed using the same methods.
“We had used scanning tunneling microscopy and fluorescence microscopy to watch our cars drive, but that wouldn't work for the submersibles," Tour explained. "They would drift out of focus pretty quickly.”
To solve this problem, the researchers sandwiched a drop of diluted acetonitrile liquid containing USNs between two slides. They then used a custom confocal fluorescence microscope to hit the slides from opposite sides with ultraviolet light (for the motors) and a red laser (for the pontoons).
Rice graduate student Victor García-López holds a vial containing millions of USNs. (Image courtesy of Jeff Fitlow and Rice University.)
Although the USNs cannot be steered, they demonstrate that molecular motors can be powerful enough to drive them through solutions of similarly-sized molecules. “This is akin to a person walking across a basketball court with 1,000 people throwing basketballs at him,” Tour said.
The long-term goal is to produce USNs capable of carrying cargo for medical and other purposes.
For example, it’s not difficult to imagine swarms of USNs being used to deliver precise chemical payloads to specifically targeted sites of cancer or infection. They could even be used to augment our immune systems. Could USNs play a crucial role in the next generation of medicine?
For more information, read the abstract online or visit the Wiess School of Natural Sciences website.