And with the motors running at more than a million RPM, that translates into speed. Though the sub’s top speed amounts to less than 1 inch per second, Tour said that’s a breakneck pace on the molecular scale.
“These are the fastest-moving molecules ever seen in solution,” he said.
Expressed in a different way, the researchers reported this month in the American Chemical Society journal Nano Letters that their light-driven nanosubmersibles show an “enhancement in diffusion” of 26 percent. That means the subs diffuse, or spread out, much faster than they already do due to Brownian motion, the random way particles spread in a solution.
A chemical schematic shows the design of single-molecule nanosubmersibles created at Rice University. The sub's fluorescent pontoons are blue; the motor is red. (Illustration by Victor García-López
Nanoletters - Unimolecular Submersible Nanomachines. Synthesis, Actuation, and Monitoring
While they can’t be steered yet, the study proves molecular motors are powerful enough to drive the sub-10-nanometer subs through solutions of moving molecules of about the same size.
“This is akin to a person walking across a basketball court with 1,000 people throwing basketballs at him,” Tour said.
Tour’s group has extensive experience with molecular machines. A decade ago, his lab introduced the world to nanocars, single-molecule cars with four wheels, axles and independent suspensions that could be “driven” across a surface.
Nanomotors, which operate more like a bacteria’s flagellum than a propeller, complete each revolution in four steps. When excited by light, the double bond that holds the rotor to the body becomes a single bond, allowing it to rotate a quarter step. As the motor 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.
For comparison tests, the lab also made submersibles with no motors, slow motors and motors that paddle back and forth. All versions of the submersibles have pontoons that fluoresce red when excited by a laser, according to the researchers.
“One of the challenges was arming the motors with the appropriate fluorophores for tracking without altering the fast rotation,” García-López said.
Once built, the team turned to Gufeng Wang at North Carolina State University to measure how well the nanosubs moved.
“We had used scanning tunneling microscopy and fluorescence microscopy to watch our cars drive, but that wouldn’t work for the submersibles,” Tour said. “They would drift out of focus pretty quickly.”
The North Carolina team sandwiched a drop of diluted acetonitrile liquid containing a few nanosubs between two slides and used a custom confocal fluorescence microscope to hit it from opposite sides with both ultraviolet light (for the motor) and a red laser (for the pontoons).
The microscope’s laser defined a column of light in the solution within which tracking occurred, García-López said. “That way, the NC State team could guarantee it was analyzing only one molecule at a time,” he said.
Rice’s researchers hope future nanosubs will be able to carry cargoes for medical and other purposes. “There’s a path forward,” García-López said. “This is the first step, and we’ve proven the concept. Now we need to explore opportunities and potential applications.”
Co-authors of the paper are Rice alumnus Pinn-Tsong Chiang and postdoctoral researcher Gedeng Ruan; North Carolina State graduate student Fang Chen; Angel Martí, an associate professor of chemistry, of bioengineering and of materials science and nanoengineering, and Anatoly Kolomeisky, a professor of chemistry and of chemical and biomolecular engineering, both at Rice.
Wang is an assistant professor of analytical chemistry at North Carolina State. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering.
Unimolecular submersible nanomachines (USNs) bearing light-driven motors and fluorophores are synthesized. NMR experiments demonstrate that the rotation of the motor is not quenched by the fluorophore and that the motor behaves in the same manner as the corresponding motor without attached fluorophores. No photo or thermal decomposition is observed. Through careful design of control molecules with no motor and with a slow motor, we found using single molecule fluorescence correlation spectroscopy that only the molecules with fast rotating speed (MHz range) show an enhancement in diffusion by 26% when the motor is fully activated by UV light. This suggests that the USN molecules give ∼9 nm steps upon each motor actuation. A non-unidirectional rotating motor also results in a smaller, 10%, increase in diffusion. This study gives new insight into the light actuation of motorized molecules in solution.
SOURCES - Rice University, Nanoletters