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A pristine stir bar made of ferromagnetic metal with a PTFE, or Teflon, coating. chemists discovered the inert bars, which are commonly used to mix chemicals, cause unwanted results during Billups-Birch reactions with carbon and other nanotubes. (Credit: Brandon Martin)

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Using the wrong stir bar in a chemistry lab can introduce errors, according to a new study.

Scientists have discovered that stir bars made of PTFE, more commonly known as Teflon, can introduce errors into a standard lab reaction used to manipulate the properties of carbon or boron-nitride nanotubes.

Stir bars are pellet-like rods of ferromagnetic metal covered in PTFE that sit in the bottom of a beaker. A rotating magnetic field turns the stir bars. They allow researchers to mix a solution in a closed flask without manual stirring.

A new paper in ACS Omega outlines what happens when scientists use PTFE stir bars to functionalize nanotubes through Billups-Birch reduction, a long-used reaction that frees electrons to bind with other atoms.

Scientists often use reduction to make nanotubes more amenable to functionalization, the process of customizing them for applications by adding molecules like proteins.

That can be as simple as dispersing nanotubes in a chemical bath laden with the molecules you want to add. Billups-Birch, a one-step process used to functionalize nanotubes with a variety of molecules, is one such method, researchers say. Edward Billups, professor emeritus of chemistry at Rice University, helped develop the method.

When they used it to modify nanotubes of boron-nitride, the researchers were surpised to see their tubes turn gray while the PTFE stir bars turned black. Standard thermogravimetric analysis, usually adequate to see evidence of functionalization, didn’t see anything wrong—but the researchers did.

“Aside from that, we couldn’t get consistent results,” says Angel Martí, an associate professor of chemistry, of bioengineering, and of materials science and nanoengineering.

“Sometimes we would get very high functionalization—or apparent functionalization—and sometimes we wouldn’t. That was really strange.”

They found the lithium in the ammonia-based solvent used in the Billups-Birch reaction was reacting with the white PTFE from the bars, turning them black.

“Because carbon nanotubes are black, it would be easy to believe that nanotubes were depositing on the bars throughout the reaction,” Martí says. “But that’s not what happens. We found that in Billups-Birch conditions, the PTFE reacts.

“Teflon doesn’t generally react with anything,” he says. “That’s why it’s used in stir bars, and in cookware. That’s why it’s also easy to overlook what we saw happening in the lab.”

A search of the literature turned up nothing about avoiding PTFE in Billups-Birch, Marti says. “That was odd, too. Maybe everybody else knows—but just in case we decided to explore the problem. That’s why we decided to write a paper.”

The researchers suspect the unexpected reaction with Teflon is creating radicals that reduce the efficiency of the reaction and that can attack the boron-nitride or carbon nanotubes. In the meantime, their quick solution to the problem is perhaps the simplest.

“Now we use glass-coated stir bars,” Martí says. “Glass is completely inert. That gives us reproducibility and good functionalization.”

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

Original Study DOI: 10.1021/acsomega.8b03677

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