It's actually the absorption component that the scientists exploit to open and close the nanocages. When the light is absorbed it is converted to heat, and the nanocages are covered with a special polymer that responds to heat in an interesting way.
The polymer, poly(N-isopropylacrylamide), and its derivatives has what's called a critical temperature. When it reaches this temperature it undergoes 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," says Xia. "Typically when you go to higher temperature, a molecule will expand, but this one does the opposite."
Like everything else about this system, the polymer is tunable. The scientists can control its critical temperature by altering its composition. For medical applications, they tune the critical temperature to one right above body temperature (37 degrees Celsius) but well below 42 degrees Celsius (107 degree Fahrenheit), the temperature at which heat would begin to kill cells.
Next comes the fun part. Once they had made their smart capsules, the scientists tested them by loading them with a bright red dye called alazarin crimson, or rose madder. The dye made it easy to detect and measure any release with a spectrometer.
The cages were loaded by shaking them in a solution of the dye at a temperature above the critical temperature of the smart polymer. Next, they we
|Contact: Diana Lutz|
Washington University in St. Louis