Temperature-controlled uptake and release in PNIPAM-modified porous silica nanoparticles

Temperature-dependent uptake and release of small molecules within porous silica nanoparticles has been achieved by treatment of preformed, thiol-functionalized micro-to-mesoporous silica nanoparticles (MSN) with pyridyl disulfide-terminated poly(N-isopropylacrylamide) (PNIPAM-S-S-Py). The resulting nanoparticle-polymer composites show uptake and release of fluorescein at room temperature (below the lower critical solution temperature, LCST, of the polymer) and a low level of leakage at 38 degrees C (above LCST, < 2% after 2 h). The data are consistent with a mode of action in which fluorescein diffusion occurs readily when the polymer is in the random coil conformation but is significantly retarded when the polymer adopts the globule conformation. This mode of action is opposite to that observed for systems in which the PNIPAM is grown from the porous silica surface or co-condensed with silica and is accompanied by a greater than 10-fold improvement in fluorescein retention in the pore-closed conformation.