Synthesis And Applications Of Thermoresponsive Polymer/Inorganic Hybrid Nanoparticles

Organic/inorganic hybrid nanoparticles have attracted ever increasing attention in the past decade due to their fascinating optical, electronic, magnetic, and catalytic properties. Recent progress in this area involves the preparation of hybrid nanoparticles coated with stimuli-responsive polymer brushes, which are attractive building blocks for the design and fabrication of smart nanostructured devices. In this dissertation, The fabrication of poly(N-isopropylacrylamide) (PNIPAM) grafted hybrid silica nanoparticles via surface initiated living polymerization and the thermal phase transition behavior of PNIPAM brushes at the silica surface were investigated in detail. On basis of the thermoresponsive PNIPAM grafted silica nanoparticles, we prepared thermoresponsive/photo-switchable fluorescent hybrid silica nanoparticles and thermoresponsive nanocapsules. The thermoresponsive PNIPAM grafted silica nanoparticles can also serve as templates for the self-assembly of Au nanoparticles.1. This section reports on the fabrication of hybrid silica nanoparticles densely grafted with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brushes and their thermal phase transition behavior. Surface-initiated atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAM) was conducted in 2-propanol at ambient temperature using CuCl/CuCl2/Me6TREN as the catalytic system, starting from the surface of silica nanoparticles derivatized with ATRP initiators. The surface-initiated ATRP can be conducted in a well-controlled manner, as revealed by the linear kinetic plot, linear evolution of number-average molecular weights (Mn) versus monomer conversions, and the relatively narrow molecular weight distributions of the grafted PNIPAM chains. Laser light scattering (LLS) and optical transmittance were then employed to study the thermal phase transitions of PNIPAM brushes at the surface of silica nanoparticles. Both the intensity-average hydrodynamic radius, <Rh>, and average radius of gyration, <Rg>, exhibit a two-stage decrease upon heating over the broad temperature range of 20–37 oC. The first phase transition takes place in the temperature range of 20–30 oC, which can be tentatively ascribed to the n-cluster-induced collapse of the inner region of PNIPAM brushes close to the silica core; the second phase transition occurs above 30 oC, which can be ascribed to the outer region of PNIPAM brushes, possessing much lower chain density compared to that of the inner part.2. This section reports on the fabrication of hybrid silica nanoparticles densely grafted with thermoresponsive PNIPAM brushes with inner and outer layers selectively labeled with fluorescence resonance energy transfer (FRET) donors, 4-(2-acryloyloxyethylamino)-7-nitro-2,1,3-benzoxadiazole (NBDAE), and photo-switchable acceptors, 1’-(2-methacryloxyethyl)-3’,3’-dimethyl-6-nitro- spiro (2H-1-benzopyran-2,2’-indoline) (SPMA), respectively, via surface-initiated sequential ATRP. P(NIPAM-co-NBDAE)-b-P(NIPAM-co- SPMA) brushes at the surface of silica core exhibit collapse in the broad temperature range of 20-37 oC. UV irradiation of the aqueous dispersion of hybrid silica nanoparticles induces the transformation of SPMA moieties in the outer layer of polymer brushes from non-fluorescent spiropyran (SP) form to fluorescent merocyanine (MC) form, leading to occurrence of FRET process between NBDAE and SPMA residues. Most importantly, the FRET efficiency can be facilely tuned via thermo-induced collapse/swelling of P(NIPAM-co-NBDAE)-b-P(NIPAM-co-SPMA) brushes by changing the relative distance between donor and acceptor species located within the inner and outer layer of polymer brushes, respectively. Thus, hybrid silica nanoparticles coated with P(NIPAM-co-NBDAE)-b-P(NIPAM-co-SPMA) brushes can serve as a sensitive ratiometric fluorescent thermometer. On the other hand, when the hybrid nanoparticle dispersion was irradiated with visible light again after UV irradiation, the MC form of SPMA moieties reverts back to the non-fluorescent SP form, leading to the turn-off of FRET process. Overall, aqueous dispersion of this novel type of hybrid silica nanoparticles is capable of emitting multicolor fluorescence, which can be facilely tuned by UV irradiation, visible light, and temperatures, or a proper combination of them.3. This section report on the fabrication of thermoresponsive cross-linked hollow PNIPAM nanocapsules and silver nanoparticle-embedded hybrid PNIPAM nanocapsules with controlled shell thickness via the combination of surface-initiated 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.4. This section reports on the fabrication of thermoresponsive hybrid silica nanospheres anchored with gold nanoparticles of tunable spatial distribution. Starting from initiator-functionalized silica nanoparticles, surface-initiated ATRP of NIPAM afforded hybrid silica nanoparticles coated with PNIPAM brushes. The halogen end groups of grafted PNIPAM chains were substituted by azido groups and hybrid silica nanoparticles coated with 1,2-dithiolane end-capped PNIPAM brushes were then obtained via“click”reaction of azido groups with 1,2-dithiolane-3-pentanoic acid-N-propargylamide. The obtained 1,2-dithiolane functionalized hybrid silica nanoparticles were employed as templates for the self-assembly of citrate-capped gold nanopartilces. Laser light scattering (LLS) measurements revealed that PNIPAM brushes at the surface of silica exhibit reversibly thermo-induced collapse/swelling transitions and this phase transition behavior was not affected after the attachment of gold nanoparticles. Due to the thermoresponsiveness of PNIPAM brushes, the spatial distances between gold nanoparticles attached at the surface of PNIAPM brushes could be facilely modulated by temperature variations, as revealed by the thermo-induced red shift of surface plasmon absorption band.