A team from Rice University has developed single-molecule nanosubmarines that could eventually be used to deliver medicines directly inside the body. Back in 2006, this same group developed nanocars, a single-molecule vehicle with four wheels, axles, and independent suspensions that could be “driven” across a terrestrial surface. Unfortunately, many of the nano-scale devices developed since then either required or emitted toxic chemicals. Several years ago, however, a new motor developed by a group in the Netherlands caught the attention of Rice chemist James Tour, who applied it to his new nanosubmarine.
The new Unimolecular Submersible Nanomachines (USNs) consist of 244 atoms that, when bound together in a particular formation, comprise a single-molecule device that’s powered by ultraviolet light. With each full revolution of the sub’s tail-like propeller, the device moves forward 18 nanometers. The motors run at more than a million RPMs, allowing for a top speed of slightly less than one inch per second. As noted in a Rice release, “These are the fastest-moving molecules ever seen in a solution. The motors are more like a bacterium’s flagellum than a propeller. After the molecule is excited by light, the double bond that holds the rotor to the body transforms into a single bond. This allows it to rotate a quarter step. Then, as the motor naturally tries to return to its resting state, it jumps yet again for another quarter turn. This process is repeated so long as the light remains on.
Technically speaking, the devices display an “enhancement in diffusion” of 26%, which means the nanosubs spread out much faster than they would from Brownian motion alone, the random way that tiny objects diffuse in a solution. Unfortunately, the subs cannot be steered, but the research shows that the molecular motors are capable of traveling through solutions of moving molecules of roughly the same size. Tour says this is akin to a person being able to walk across a basketball court while 1,000 people throwing basketballs at them. Looking ahead, the team hopes more sophisticated versions of the nanosub will carry cargoes for medical and other purposes. “There’s a path forward,” noted study co-author Victor García-López. “This is the first step, and we’ve proven the concept. Now we need to explore opportunities and potential applications.”
Tour says many scientists have created microscopic machines with motors over the years, but most have either used or generated toxic chemicals. He says a motor that was conceived in the last decade by a group in the Netherlands proved suitable for Rice’s submersibles, which were produced in a 20-step chemical synthesis. “These motors are well-known and used for different things,” says Victor García-López, lead author and Rice graduate student. “But we were the first ones to propose they can be used to propel nanocars and now submersibles.” The motors, 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,” says García-López.
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,” says Tour. “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 says. “That way, the NC State team could guarantee it was analyzing only one molecule at a time.” The team hopes future nanosubs will be able to carry cargoes for medical and other purposes. “There’s a path forward,” says García-López. “This is the first step, and we’ve proven the concept. Now we need to explore opportunities and potential applications.”
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