In campy old movies, Lucretia Borgia swans around emptying powder from her ring into wine glasses carelessly left unattended. The poison ring is usually a confection of gold filigree holding a cabochon or faceted gemstone that can be broken to empty the ring's contents. It is invariably enormous so large it is rather odd nobody seems to notice it.
Lucretia would have given her eyeteeth for the "smart capsule" devised in Younan Xia's laboratory at Washington University in St. Louis. A tiny cage of gold covered with a smart polymer, it responds to light, opening to empty its contents, and resealing when the light is turned off. Infinitely more cunning and discreet than Lucretia's ring, the nanocage is too small to be seen except indirectly: billions change the color of liquid in a test tube.
No Lucretia, Xia is a healer rather than a poisoner. The smart nanocage is designed to be filled with a medicinal substance, such as a chemotherapy drug or bactericide. Releasing carefully titrated amounts of a drug only near the tissue that is the drug's intended target, this delivery system will maximize the drug's beneficial effects while minimizing its side effects.
The method for making the capsules and tests of their performance appeared online on Nov. 1, 2009, as part of the advance online publications program of the journal Nature Materials.
The first step in making a smart capsule is to mix up a batch of silver nanocubes. Tiny single-crystal cubes of silver can be made by adding silver nitrate (AgNO3) to a solution that donates electrons to the silver ions, allowing them to precipitate as solid silver. The addition of another chemical encourages the silver atoms to deposit on some parts of a seed crystal rather than others, coaxing the seeds to form sharp-edged cubes rather than misshapen lumps.
A second step clips all eight corners off the cubes.
The clipped silver cubes then serve as "
|Contact: Diana Lutz|
Washington University in St. Louis