Without vacuum, nanocars become ‘speed bumps’

Driving nanocars exposed to open air—rather than in a vacuum—gets sticky, as researchers found out. The hydrophobic single-molecule cars get stuck to the “road” and create what amount to large speed bumps.

The work comes as competitors prepare for first NanoCar Race in Toulouse, France, in October. Researchers from Rice University, where the first nanocars were developed, are members of one of five international teams that plan to enter the competition.

Though the race will be run in an ultra-cold vacuum, the researchers thought it wise to study how their latest model of nanocars would fare in a more natural setting.

“Our long-term goal is to make nanomachines that operate in ambient environments,” says James Tour, chair in chemistry and a professor of computer science and of materials science and nanoengineering at Rice.

“That’s when they will show potential to become useful tools for medicine and bottom-up manufacturing.”

The newest generation of Rice nanocars features adamantane wheels that are slightly hydrophobic (water-repellent). Tour says some hydrophobicity is important to help keep the nanocars attached to a surface, but if the tires are too hydrophobic, the cars could become permanently immobilized. That is because hydrophobic things tend to stick together to minimize the amount of surface area that is in contact with water. Things that are hydrophilic, or water-liking, are more amenable to floating freely in water, Tour says.

In the latest tests with the new tires, the nanocars were placed on surfaces that were either clean glass or glass coated with the polymer polyethylene glycol (PEG). Glass is the most frequently used substrate in nanocar research. Tour says the PEG-coated glass slides were used for their anti-fouling—nonsticky—properties, while the clean glass slides were treated with hydrogen peroxide so the hydrophobic wheels wouldn’t stick.

He says the cars weren’t so much being driven as undergoing “directed diffusion” in the tests. The point, he says, was to establish the kinetics of nanocar movement and understand the potential energy surface interaction between the car and surface over time.

“We want to know what makes a nanocar ‘hit the brakes’ and how much external energy we need to apply to start it moving again,” he says.

The researchers let their cars run freely on a solid surface exposed to the air and tracked their movements by exciting embedded fluorescent tags.

The cars that moved via Brownian diffusion slowed down during the 24 hours that the slides were under observation. Tour says slides absorbed molecules from the air; as more and more of these molecules stuck to the surface, the slides become progressively more “dirty” throughout the experiment.

Each nanocar is a single, complex molecule that contains just a few hundred atoms, so any other molecules they encounter on the roadway are huge obstacles that act like sticky foam. Each collision with one of these obstructions makes the nanocar slow down, and eventually the cars become permanently stuck.

Gufeng Wang, a collaborator on the tests and assistant professor of analytical chemistry at North Carolina State University, says that from an energy perspective—that is, the energetic relationship between the molecular cars and those that make up the road—molecules adsorbed from air generate many potential energy wells, just like puddles on the potential energy surface. These puddles can slow or permanently trap the nanocars.

Tests showed that nearly twice as many of the cars appeared to move on the nonsticking PEG slides, and all moved a little faster than those on the bare glass.

The researchers note that they could not view the new models with scanning tunneling microscopes because those only work in a vacuum and they emit energy that could influence movement of the cars. For this reason, the researchers tagged each nanocar with a fluorescent marker and used confocal microscopes to track the cars’ movements.

The National Science Foundation and North Carolina State University supported the research, which appears in the Journal of Physical Chemistry C.