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Stir bar stashes sensitive solids

Magnetic capsule springs open at high spin speeds to release its payload

by Bethany Halford
September 1, 2021 | A version of this story appeared in Volume 99, Issue 32


Credit: Stéphane Rosset
When it spins at high speed, a magnetic stirring capsule opens to release a blue solid.

A capsule that releases its contents under just the right conditions seems like something a spy might use. But unlike James Bond’s martinis, this new tool for chemists is stirred, not shaken. Magnetic stirring capsules, reported by chemists at the University of Geneva, release solids when they spin at high speed.

A hollow cylinder made of polytetrafluoroethylene with magnet at its base and a cap for the cylinder that also houses a magnet.
Credit: Clément Mazet
This magnetic stirring capsule is 17 mm in diameter and has a volume of 1.5 cm3.

Clément Mazet, Sylvain Taillemaud, and Stéphane Rosset developed the magnetic stirring capsules to help chemists prevent air- and moisture-sensitive reagents from reacting with their surroundings. Chemists typically use glove boxes full of inert gas to work with such reagents, but the bulky gloves can be awkward. A few years ago, chemists at the Massachusetts Institute of Technology reported paraffin capsules that melt and release sensitive reagents. The downsides were that the paraffin had to be removed from the reaction products, and the reactions had to be run at elevated temperatures so that the capsules would melt.

The new magnetic stirring capsules don’t eliminate the need for a glove box—that’s where the capsules are loaded. But they can be used at low temperature and in both polar and nonpolar solvents. Because the capsules are made from inert polytetrafluoroethylene, they can be cleaned and reused, and they leave no residue. The chemists have made them in two sizes: 1.5 cm3 and 5.0 cm3 (Helv. Chim. Acta 2021, DOI: 10.1002/hlca.202100110).

Neil Garg, an organic chemist at the University of California, Los Angeles, says in an email that the capsules “have the potential to be a game changer, since they avoid the use of chemical additives, like wax, that have been used previously but can sometimes impact reactivity and necessitate multistep purifications.”

Garg says he’d be interested in using the capsules for research and teaching. But he’ll have to wait. Although Mazet and colleagues have patented the magnetic stirring capsules, they’re not yet for sale.



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