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).

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.

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.

The researchers can change the tubes’ diameters and the rate at which they grow by changing the concentration of the fluorescent molecules. The tubes range from 1 to 120 micrometers wide. By applying a voltage, they can make the tubes grow in specific directions. They can make branched tubes in two different ways. One is to let two tubes collide, which makes a single tube emerge at the collision point. The other is to puncture a tube with a micromanipulator needle so that the material flows out and grows another branch. To show that the tubes are hollow and can carry liquids, the researchers inject fluorescent dye through them.

Kogerler says that the work is promising because the tubes maintain their structure and do not decompose. Also, they have a relatively high surface-to-volume ratio, which is beneficial for catalysis and sensing applications. But it is not yet evident that they will be ideal for these applications. That is because the surface of the tubes is not just made of interlinked polyoxometalate molecules: it also contains positively charged fluorescent molecules. “The question is, would you gain any kind of reactivity from that?” Kogerler asks.

Kogerler says that it would be really interesting if the researchers could find a way to grow similar tubes in the nanometer range. Metal-oxide compounds are known to form structures on this scale, and the approach would yield even larger surfaces.

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