Stir Bars Made of Teflon can Introduce Errors into Standard Lab Reaction

The stirrers that are used to stir cream and coffee possibly do not make much difference to the drink. However, in a chemistry lab, using the wrong stirrer can end up skewing the science.

Researchers at the Rice University have found out that stir bars composed of PTFE, more widely known as Teflon, can bring in errors into a standard lab reaction used to manipulate the properties of boron-nitride or carbon nanotubes.

Stir bars are pellet-like rods of ferromagnetic metal enclosed in PTFE that rest in the bottom of a beaker and are turned by a rotating magnetic field. They enable a solution to be blended in a closed flask without manual stirring.

A paper was published in the American Chemical Society journal ACS Omega by the Rice lab of Angel Martí, summarizing what happens upon using PTFE stir bars to functionalize nanotubes through Billups-Birch reduction, a long-used reaction partly created by Rice Professor Emeritus of Chemistry Edward Billups that lets electrons to bind with other atoms.

Usually, the reduction process is used to make nanotubes more amenable to functionalization, which is the process of tailoring them for applications by incorporating molecules like proteins. That can be as easy as spreading nanotubes in a chemical bath filled with the molecules needed to be added. One such method is Billups-Birch, which is a single-step process used to functionalize nanotubes with a range of molecules, state the scientists.

When they used it to change nanotubes of boron-nitride, the scientists were stunned to observe their tubes become gray, while the PTFE stir bars became black. Standard thermogravimetric analysis—often enough to observe evidence of functionalization—didn’t observe anything unusual, but the scientists noticed it.

Aside from that, we couldn’t get consistent results. Sometimes we would get very high functionalization—or apparent functionalization—and sometimes we wouldn’t. That was really strange.

Angel Martí, Rice lab, Rice University.

They identified that the lithium in the ammonia-based solvent used in the Billups-Birch reaction was reacting with the white PTFE from the bars, changing them to black color.

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

“Teflon doesn’t generally react with anything,” he said. “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.”

Martí stated that the search of the literature gave nothing about eliminating PTFE in Billups-Birch.

“That was odd, too,” he said. “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 suppose the surprising reaction with Teflon is producing radicals that decrease the efficiency of the reaction and that can impact the carbon or boron-nitride nanotubes. Meanwhile, their rapid solution to the issue is probably the simplest.

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

Angel Martí, Rice lab, Rice University

The lead author of the paper is Rice graduate student Carlos de los Reyes. Co-authors are Rice graduate student Ashleigh Smith McWilliams, research assistant Kendahl Walz-Mitra, undergraduate students Katharyn Hernandez and Selin Ergülin, and Matteo Pasquali, the A.J. Hartsook Professor of Chemical and Biomolecular Engineering and a professor of materials science and nanoengineering and of chemistry. Martí is an associate professor of chemistry, bioengineering, and materials science and nanoengineering.

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

(Video credit: Rice University)