Nanocapsules bubble up

Japanese researchers have developed a technique for the production of silicon dioxide nanocapsules using tiny bubbles of carbon dioxide in a silicon copolymer.

Nanocapsules have very thin shells and diameters in the nanometre range. They hold a tiny volume of liquid and protect their cargo while transporting it through a foreign medium, such as in a human blood vessel, without any loss. Applications for nanocapsules include the delivery of medication and the encapsulation of scents, printer ink, and adhesives. Once at their destinations, the payloads are released by pressure or friction.

Lei Li and Hideaki Yokoyama, from Japan’s National Institute of Advanced Industrial Science and Technology, developed the new technique. They coated silicon wafers, which act as a support, with thin films of a plastic that consists of molecules with segments of different types of polymers. These are known as block copolymers, in this case made of polystyrene and silicone.

The researchers made their copolymer films so that nanoscopic droplets of silicone “float” in a matrix of polystyrene. Supercritical carbon dioxide (CO₂) is then forced into this film under high pressure at 60°C. The CO₂ lodges within the droplets of silicone in the block copolymer and forms bubbles, but it cannot force its way into the polystyrene matrix.

The scientists then cool the film down to 0°C in order to freeze the polystyrene matrix and then slowly reduce the pressure back to atmospheric levels. The CO₂ returns to the gas phase, expands, and escapes from the bubbles without collapsing them.

Finally, the researchers expose the polymer film to ozone and UV light. Under these conditions, the polystyrene matrix is completely destroyed; the silicone surrounding the bubbles is oxidised to silicon dioxide (SiO₂). This results in a thin film of tightly packed, tiny cavities with a thin shell of silicon dioxide. These nanocapsules have diameters of less than 40 nanometres and walls that are about two nanometres wide.

The particular advantage of this method is that the resulting nanocapsules are organised into a two-dimensional structure that can be controlled by varying the segments of the block copolymer.