Fabrication of Thermoresponsive Cross-Linked Poly(N-isopropylacrylamide) Nanocapsules and Silver Nanoparticle-Embedded Hybrid Capsules with Controlled Shell Thickness

We report on the fabrication of thermoresponsive cross-linked hollow poly(N-isopropylacrylamide) (PNIPAM) nanocapsules and silver nanoparticle-embedded hybrid PNIPAM nanocapsules with controlled shell thickness via the combination of surface-initiated atom transfer radical polymerization (ATRP) and "click" cross-linking. Starting from initiator- functionalized silica nanoparticles, the surface-initiated ATRP of N-isopropylacrylamide (NIPAM) and 3-azidopropylacrylamide (AzPAM) afforded hybrid silica nanoparticles surface coated with P(NIPAM-co-AzPAM) brushes. Hybrid PNIPAM nanocapsules were then fabricated by the "click" cross-linking of PNIPAM shell layer with a trifunctional molecule, 1,1,1-tris(4-(2-propynyloxy)phenyl)ethane, followed by the subsequent removal of silica cores via HF etching. Shell cross-linked hybrid silica nanoparticles can further serve as templates for the in situ preparation of silver nanoparticles within the cross-linked PNIPAM layer. After HF etching, silver nanoparticle-embedded hybrid PNIPAM nanocapsules were obtained. Due to the thermoresponsiveness of PNIPAM, cross-linked PNIPAM nanocapsules and silver nanoparticle-embedded hybrid PNIPAM nanocapsules exhibit thermo-induced collapse/swelling transitions. In the latter case, the spatial distribution of Ag nanoparticles within the hybrid PNIPAM nanocapsules can be facilely modulated by temperature variations, as revealed by the thermo-induced red shift of surface plasmon absorption band. Dynamic laser light scattering (LLS) measurements revealed that PNIPAM nanocapsules and Ag nanoparticle- embedded hybrid PNIPAM nanocapsules exhibit more prominent thermo-induced dimensional changes, as compared to shell cross-linked hybrid silica/PNIPAM nanoparticles loaded with or without Ag nanoparticles, respectively. Due to that the surface-initiated ATRP can be conducted in a controlled manner, the current strategy employed for the fabrication of structurally stable cross-linked PNIPAM nanocapsules and Ag nanoparticle-embedded hybrid PNIPAM nanocapsules can be further applied to the preparation of other functional hollow hybrid nanostructures with controlled dimensions.