A gold nanocage devised in the laboratory of Younan Xia, Ph.D, the James M. McKelvey Professor in the School of Engineering & Applied Science, responds to light, opening to empty its contents and then resealing when the light is turned off.
The discovery could make a big difference in treating diseases. The smart nanocage could be filled with medicine, such as a chemotherapy drug or bactericide. It would then release carefully titrated amounts of a drug only near the tissue that is the drug’s intended target, maximizing the drug’s beneficial effects while minimizing its side effects.
The method for making the capsules and tests of their performance appeared online Nov. 1 as part of the advance online publications program of the journal Nature Materials.
The nanocages are engineered to absorb light at near-infrared frequencies, to which human tissue is particularly transparent.
When near-infrared light shines on the cages, they heat slightly, just enough to trigger a change in a polymer attached to their surfaces.
The polymer, poly(N-isopropylacrylamide), and its derivatives have what is called a critical temperature. When they reach this temperature, they undergo a transformation called a phase change.
If the temperature is lower than the critical temperature, the polymer chains are water-loving and stand out from the cage like brushes. The brushes seal the cage’s pores and prevent its cargo from leaking out.
If the temperature is above the critical temperature, on the other hand, the polymer chains shun water, shrink together and collapse. As they shrink, the pores of the cage open, and its contents flood out.
“It’s a bit counter-intuitive,” Xia said. “Typically when you go to higher temperature, a molecule will expand, but this one does the opposite.”
In tests, the cages were filled with a bright dye that made it easy to detect any leakage.
The cages were loaded by shaking them in a solution of the dye at a temperature above the polymer’s critical temperature.
Next, they were dunked in an ice bath to trigger the polymer to trap the dye inside the cages. The cages were then opened again by bathing them in the light of a near-infrared laser.
Absorbed light warmed the gold cages and provoked the polymer’s phase change. The polymer collapsed, the cages’ pores were exposed, and dye spilled out.
For more information and to see a graphic explaining how the nanocages were made, visit news-info.wustl.edu/news/page/normal/14996.html.