Typical example of magnetic stir bars in a chemistry lab: surface changes are clearly observableCREDIT: Evgeniy O. Pentsak

A surprisingly high level of surface contaminations of magnetic stir bars escapes regular cleaning and brings highly reactive traces of metal species from previous experiments to the next ones.

With electron microscopy experiments and DFT calculations, it was shown that the plastic surface of magnetic stir bars can form reactive centers that absorb metal atoms from solution followed by growth of metal nanoparticles. The process readily takes place on the surface of PTFE-coated magnetic stir bars, ubiquitously used in modern chemistry and biology labs.

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The regular in-use magnetic stir bars carry bunches of metal nanoparticles on their surface. It was demonstrated that the presence of a previously used magnetic stir bar in a reaction medium is sufficient for initiating a full-scale catalytic reaction (promoted by leaching of metal species from the PTFE surface).

Magnetic stir bars are commonly regarded as reusable consumables, and in many labs they last for months and years. This study shows that in a regular catalysis lab, almost all magnetic stir bars become permanently contaminated with metal nanoparticles after about a week of use. Regular routine cleaning procedures do not remove such contamination completely. Indeed, subsequent release of metal traces in the next reactions is unacceptable even in small quantitates, as it may add critical bias to many experimental settings.

In the study, the authors examined stir bars from different laboratories, and only one bar out of 60 was found uncontaminated. They further investigated the origins of contamination, performed online ESI-MS monitoring of the contamination process, and demonstrated its impact on catalysis.

Metal contamination is a critical issue, which has paramount importance for the development of high-performance catalytic and synthetic systems. Although several issues dealing with metal contamination have been already discussed in literature, the chemical reactivity of PTFE remains underexplored, as it was believed to be an inert material.