When
the gold nanocages are irradiated with a laser, the strongly
absorbed light is converted into heat through the photothermal
effect. When the temperature rises above a certain
threshold (the low critical solution temperature, LCST), the
pNIPAAm coating undergoes a conformational change. When
the polymer is collapsed, the pores of the nanocage are
exposed, allowing for effectors pre-loaded in the interior to
be released (though some release will also occur from effectors
trapped in the polymer layer). The loading process is simply
the reverse—pNIPAAM-coated gold nanocages are mixed
with a solution of effector at a temperature above the LCST,
then quickly cooled below the LCST to trap the contents
inside. Conveniently, the LCST of pNIPAAM can be tuned
from 32–50 1C through the incorporation of different amounts
of acrylic acid (AAm) during the polymerization. The studies
presented here used a temperature of 39 1C as this temperature
is above body temperature (37 1C), but below the threshold
where thermal damage can occur in biological systems (42 1C).
Fig. 8C and D, shows the release profiles of a PEGconjugated
alizarin dye as the laser irradiation time and laser
power were increased, respectively.21 By adjusting these
parameters, it is possible to controllably release the loaded
effectors both in solution and in vitro, as the amount of dye
release increases with both time and laser power. This system
is versatile, and has also been demonstrated to release both
chemotherapeutic drugs and enzymes, which retained B80%
of their bioactivity after the release process.