Crystals Made to Sprout Tiny Tubes

Micrometers-wide tubes that grow spontaneously could be used to make tiny chemical devices.

Researchers have discovered a way to grow tiny micrometer-scale tubes from materials that act as catalysts and gas sensors. By making networks of these tubes, the researchers say that they could create compact lab-on-a-chip devices in which the channels themselves are made of the catalyst or sensing material. "You could throw chemicals through a very high surface area tube and potentially do very efficient catalysis," says Lee Cronin, a professor of chemistry at the University of Glasgow, U.K., who led the work.

Magic tubes: Inorganic crystals placed in a solution of fluorescent molecules start to grow tubes automatically. The tubes are robust enough to be injected with liquid (top right). Researchers can also fuse two tubes together (middle) and create designs with the tubes by directing growth with electric fields (bottom right). Credit: Lee Cronin, University of Glasgow

In a paper published in the journal Nature Chemistry, Cronin and his colleagues report that they can control the diameters of the tubes and the speed with which they grow. What is more, by using simple tricks, they can control the tubes' direction of growth and can merge two tubes together to make different structures.

Growing microfluidic devices this way could be simpler than using current lithography techniques, says Cronin. "We're able to grow tubes in the same way you control lines on an Etch A Sketch," he says. "You just grow very quickly in a few seconds the device you want."

The inorganic crystals that the researchers use belong to a class of chemicals known as polyoxometalates. These negatively charged clusters of metal and oxygen atoms are excellent catalysts for many different reactions in the chemical industry. They are also good at sensing and adsorbing gases, and are used to remove toxic compounds like nitrogen oxides and sulphur dioxide from flue-gas streams. By using different metal atoms, researchers can create polyoxometalates with various chemical properties. "Polyoxometalates have large structural diversity and versatility, as well as a lot of options to modify physical and chemical behavior," says Paul Kogerler, a professor of chemistry at RWTH Aachen University, in Germany.

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To create their microtubes, the Glasgow researchers use crystals containing tungsten. When they put these negatively charged metal-oxide crystals in water and add positively charged fluorescent molecules, the crystals start to sprout tubes in just a few seconds.

Cronin explains that the positive and negative molecules join up to form a membrane on the crystal's surface. The pressure inside this membrane builds up until it ruptures and the metal-oxide material inside pours out in a jet. As it streams out, it automatically starts to form a hollow tube through which more and more material can flow out. The tube grows until all that's left of the crystal is the hollow membrane shell.