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Original stir bar made of ferromagnetic metal with PTFE or Teflon coating. Chemists have discovered that the inert rods commonly used for mixing chemicals can cause unwanted results during the Billups-Birch reaction with carbon and other nanotubes. (Image source: Brandon Martin)

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According to a new study, using the wrong stirrer in a chemical laboratory can cause errors.

Scientists have discovered that a stir bar made of PTFE (commonly called Teflon) can introduce errors in standard laboratory reactions used to manipulate the properties of carbon or boron nitride nanotubes.

The stirring rod is a spherical ferromagnetic metal rod covered with polytetrafluoroethylene and located at the bottom of the beaker. The rotating magnetic field rotates the stirring rod. They allow researchers to mix solutions in closed flasks without manual stirring.

A new paper on ACS Omega outlines what happens when scientists use a PTFE stir bar to reduce functionalized nanotubes through Billups-Birch, a long-used reaction that can release electrons to bond with other atoms.

Scientists often use reduction to make nanotubes more suitable for functionalization, that is, the process of adding molecules such as proteins to customize them for use in applications.

This can be as simple as dispersing the nanotubes in a chemical bath containing the molecules you want to add. The researchers said that Billups-Birch, a one-step method for functionalizing nanotubes with various molecules, is just such a method. Edward Billups, Emeritus Professor of Chemistry at Rice University, helped develop the method.

When they used it to modify boron nitride nanotubes, the researchers were surprised to find that their tube turned gray, and the PTFE stirring rod turned black. Standard thermogravimetric analysis is usually sufficient to see evidence of functionalization, but no problems are found-but the researchers did.

"Other than that, we can't get consistent results," said Angel Marty, associate professor of chemistry, bioengineering, materials science, and nanoengineering.

"Sometimes we get very high functionalization-or obvious functionalization-sometimes we don't. That's really weird."

They found that the lithium in the amino solvent used in the Billups-Burch reaction reacted with the white polytetrafluoroethylene in the rods, turning them black.

"Because carbon nanotubes are black, it is easy to believe that the nanotubes are deposited on the rods throughout the reaction," Martí said. "But this is not the case. We found that under Billups-Birch conditions, PTFE reacts.

"Teflon usually doesn't react with anything," he said. "This is why it is used for stirring rods and cookware. This is why it is easy to overlook what we see in the laboratory."

Marti said that a review of the literature did not find that Billups-Burch avoided the use of PTFE. "That's also very strange. Maybe everyone else knows-but just in case we decide to explore this problem. That's why we decided to write a paper."

Researchers suspect that an accidental reaction with Teflon will produce free radicals, which will reduce the efficiency of the reaction and may attack boron nitride or carbon nanotubes. At the same time, their quick solution to the problem may be the simplest.

"Now we use glass-coated stir bars," Martí said. "Glass is completely inert. This provides us with repeatability and good functionality."

The National Science Foundation, the Air Force Office of Scientific Research, and the Welch Foundation supported this research.

Original research DOI: 10.1021/acsomega.8b03677

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