A Stirring Advance

Researchers who want to combine tiny volumes of liquid have few simple options when it comes to mixing for lab-on-a-chip applications or microliter bioassays. Passive diffusion is slow, and violent stirring can break droplets apart instead of mixing them together. Thanks to a team in Singapore, researchers can now reach for the world’s smallest magnetic stir bars. At just 17 μm long and 75 nm to 1.4 μm thick, these super small stir bars can mix as little as 4 pL of liquid (Angew. Chem. Int. Ed. 2013, DOI: 10.1002/anie.201303249). A team at Nanyang Technological University led by Hongyu Chen created the stir bars from oleic acid-stabilized Fe₃O₄ nanoparticles that are 40 nm in diameter. They modified the nanoparticles with citric acid to render them water-soluble and dissolved them. The team then used a magnet to align the nanoparticles and gave them a silica coating to ensure they stayed straight and rigid. They centrifuged the mixture to purify the stir bars, which they dispersed in solutions of various concentrations for stirring. While stirring, the nanosized stir bars remain suspended in solution indefinitely. And removing them is as simple as placing a stationary magnet beneath the droplet and letting the stir bars settle out of solution over the course of about five minutes.