Nanomachines have been developed to swim like bacteria
Magnetotactic bacteria drive their swimming motion with flagella, like other microbes, but they can also orient themselves with respect to a magnetic field, using nanoscopic ferromagnetic crystals which they keep in specialised compartments, the magnetosomes. Researchers in the US have now created an artificial nanoscopic device whose swimming is similarly guided but not driven by external magnetic fields.
Ayusman Sen’s group at Pennsylvania State University has recently reported that nanorods made of platinum at one end and gold at the other can propel themselves around in a solution of dilute hydrogen peroxide (H2O2), driven by the decomposition of H2O2 catalysed by platinum. Surprisingly, the rods move in the direction of the platinum end, suggesting that the oxygen produced by the reaction moves the rod by reducing the interfacial tension (rather than acting as a rocket exhaust). That motion, while non-Brownian, still went off in random directions, much like a bacterium’s motion in the absence of stimuli. The speed of two to ten body lengths per second was also comparable to bacterial swimming.1
Now the researchers have reined in their nanoscopic speed boats with a guidance system resembling that of magnetotactic bacteria. Two small (100nm wide) segments of ferromagnetic nickel, incorporated between the segments of precious metal (Pt 550nm, Au 200nm, Au 350nm), allow the rod to respond to guidance by magnetic fields. Using detailed statistical analysis,2 the researchers show that the field provides only orientation, while the driving force continues to arise from the catalysed decomposition of H2O2.
Tethering the rods to other components might ’result in new classes of nano/micromachines’, say the authors.
1 W F Paxton et al, J. Am. Chem. Soc., 2004, 126, 13424
2 T R Kline et al, Angew. Chem. Int. Ed. 2005, 44, 744
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