Scientists at the Washington University in St Louis have recently developed a new type of delivery vehicle for very small molecules, in the form of an amazingly small, smart polymer nanobox. The structure, a few nanometers in size, is able to open up and release its contents when exposed to light, and also to reseal perfectly when the light source is turned off. This amazing ability would have made historical figures, prone to poisoning their opponents with powders hidden in rings, jealous.

However, the developer of the smart capsule, Younan Xia, PhD, whose laboratory hosted the research as well, is more of a healer than a poisoner. The expert is the James M. McKelvey professor in the WU Department of Biomedical Engineering. He reveals that the nanocages are only visible when they number in the billions and are placed inside a test tube. The only sign that they are there is the fact that the water color changes to a slight ruby-red hue. Details of the structures and the production process appear in the advanced online issue of the respected scientific journal Nature Materials.

“But the really cool part, and the cool part of nanotechnology generally, is that the tiny gold cages have very different properties than bulk gold,” Xia says of the nanostructures, adding that their behavior to light is the thing that changes the most. Michael Faraday, the famed physicist, was the first scientist to propose that gold turned reddish in a test tube because the solution contained very small particles. “His original sample of a gold colloid is still in the Faraday Museum in London. Isn't that amazing? It's over 150 years later and it's still there,” the WU expert adds.

The basic operating principle of the cage is very ingenious. The polymer covering the structure responds to what is known as a critical temperature. When the environment is at a lower temperature than the threshold, the polymer chains stand out like tiny bristles, facing away from the tiny box, and thus sealing the pores that prevent the contents from dripping. Conversely, when the limit temperature is exceeded, the polymer chains collapse, breaking the seal and releasing their contents. The heat, in this case, comes from the amount of light the polymers absorb.

“It's a bit counter-intuitive. Typically when you go to higher temperature, a molecule will expand, but this one does the opposite,” Xia concludes.