by Andre on 6 November 2007
Chemotaxis in biological cells is a relatively complex process that requires a sensing apparatus that talks to a locomotion apparatus. This most famous of movies shows a neutrophil streaming after a bacterium and as you can imagine, there must be some pretty intricate chemistry behind this kind of behaviour:
That’s one of the reasons that scientists haven’t replicated this in an artificial system. Recently though, as you can read in my inaugural physicsworld.com news story, the Ayusman and Velegol groups from Penn State have come up with an alternative:
The team made a large number of tiny metal rods that were 2 µm long. Each rod was gold along one half of its length and platinum along the other. The rods were placed in a dish containing pure water and a piece of gel that contained hydrogen peroxide. The hydrogen peroxide slowly leached from the gel into the water, creating a concentration gradient in the surrounding water.
After about 110 hours, the team noticed that more than 70% of the rods had accumulated next to the gel. According to the researchers, this movement occurred because hydrogen peroxide undergoes different chemical reactions at the gold and platinum ends of the rods. This they say, drives fluid along the rod causing it to move. The particles’ speed increases with the local concentration of hydrogen peroxide and so on average the rods are “attracted” to the gel – a simple realization of chemotaxis.
A movie of the rods swarming towards the gel is available on the Ayusman lab website here.
In a previous paper in JACS, the Ayusman group showed (I think quite convincingly) that the force driving the locomotion is due to the electrochemical decomposition of H2O2 on the platinum surface and the reduction of H2O2 on the gold half resulting in an ion flux that drives fluid along the rods and causes them to move. That’s not the only possibility though. The Golestanian and Jones groups at the University of Sheffield have shown that non-conductive particles half-coated with platinum are also motile in H2O2 suggesting that a simpler mechanism is also possible. For these simpler particles, it seems that the motility is caused by the osmotic pressure that is built up when the H2O2 is broken down (resulting in more numerous products than there were reactants) on one side of the particle only. But you can get that story straight from one of the authors because Richard Jones has his own blog!
A professor of mine once told us that Purcell had shown that nanoswimmers were impossible because “life at low Reynolds number" is so different. It is very different, but he misread Purcell and if swimming bacteria weren’t convincing enough, surely swimming rods are.