Fabrication of Thermoresponsive Plasmonic Core-Satellite Nanoassemblies with a Tunable Stoichiometry via Surface-Initiated Reversible Addition-Fragmentation Chain Transfer Polymerization from Silica Nanoparticles

This work presents a fabrication of thermoresponsive plasmonic core-satellite nanoassemblies. The structure has a silica nanoparticle core surrounded by gold nanoparticle satellites using thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) chains as scaffolds. The thiol-terminated PNIPAM shell is densely grafted on the silica core via surface-initiated reversible addition-fragmentation chain transfer polymerization and used to anchor numerous gold nanoparticle satellites with a tunable stoichiometry. Below and above lower critical solution temperature, the chain conformation of PNIPAM reversibly changes between swollen and shrunken state. The reversible change of the polymer size varies the refractive index of the local medium surrounding the satellites and the distance between them. The two effects together lead to the thermoresponsive plasmonic properties of the nanoassemblies. Under different satellite densities, two distinctive plasmonic features appear

Synthesis of hybrid silica nanoparticles densely grafted with thermo and pH dual-responsive brushes via surface-initiated ATRP

This work introduces a synthesis of well-defined thermo and pH dual-responsive poly(N-isopropylacrylamide)-b-poly(4-vinylpyridine)-grafted silica nanoparticles (SNPs-g-PNIPAM-b-P4VP) via surface-initiated atom transfer radical polymerization (ATRP). ATRP initiators were attached onto the surfaces of silica nanoparticles followed by ATRP of N-isopropylacrylamide (NIPAM). During the surface-initiated ATRP, free sacrificial initiator and halogen exchange were utilized to render the polymerization in a controlled manner. Because of the retention of the polymer chain’s end-group functionality, chain extension with 4-vinylpyridine (4VP) from the obtained PNIPAM-grafted SNPs (SNPs-g-PNIPAM) was successfully conducted. Kinetics of the chain extension was studied in detail, showing the livingness of this reinitiation process. Subsequent quaternization of the outer P4VP block with methyl iodide led to the synthesized hybrid nanoparticles being well dispersed in aqueous solution. This well dispersibility affords the possibility to study the pH- and thermoresponsive behavior of the diblock copolymer chains on the nanoparticle surface

Surface-initiated controlled radical polymerizations from silica nanoparticles, gold nanocrystals, and bionanoparticles

In recent years, core/shell nanohybrids containing a nanoparticle core and a distinct surrounding shell of polymer brushes have received extensive attention in nanoelectronics, nanophotonics, catalysis, nanopatterning, drug delivery, biosensing, and many others. From the large variety of existing polymerization methods on the one hand and strategies for grafting onto nanoparticle surfaces on the other hand, the combination of grafting-from with controlled radical polymerization (CRP) techniques has turned out to be the best suited for synthesizing these well-defined core/shell nanohybrids and is known as surface-initiated CRP. Most common among these are surface-initiated atom transfer radical polymerization (ATRP), surface-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization, and surface-initiated nitroxide-mediated polymerization (NMP). This review highlights the state of the art of growing polymers from nanoparticles using surface-initiated CRP techniques. We focus on mechanistic aspects, synthetic procedures, and the formation of complex architectures as well as novel properties. From the vast number of examples of nanoparticle/polymer hybrids formed by surface-initiated CRP techniques, we present nanohybrid formation from the particularly important and most studied silica nanoparticles, gold nanocrystals, and proteins which can be regarded as bionanoparticles

Magnetic field-induced assembly of superparamagnetic cobalt nanoparticles on substrates and at liquid-air interface

Superparamagnetic cobalt nanoparticles (Co NPs) are an interesting material for self-assembly processes because of their magnetic properties. We investigated the magnetic field-induced assembly of superparamagnetic cobalt nanoparticles and compared three different approaches, namely, the assembly on solid substrates, at water–air, and ethylene glycol–air interfaces. Oleic acid- and trioctylphosphine oxide-coated Co NPs were synthesized via a thermolysis of cobalt carbonyl and dispersed into either hexane or toluene. The Co NP dispersion was dropped onto different substrates (e.g., transmission electron microscopy (TEM) grid, silicon wafer) and onto liquid surfaces. Transmission electron microscopy (TEM), scanning force microscopy, optical microscopy, as well as scanning electron microscopy showed that superparamagnetic Co NPs assembled into one-dimensional chains in an external magnetic field. By varying the concentration of the Co NP dispersion (1–5 mg/mL) and the strength of the magnetic field (4–54 mT), the morphology of the chains changed. Short, thin, and flexible chain structures were obtained at low NP concentration and low strength of magnetic field, whereas they became long, thick and straight when the NP concentration and the magnetic field strength increased. In comparison, the assembly of Co NPs from hexane dispersion at ethylene glycol–air interface showed the most regular and homogeneous alignment, since a more efficient spreading could be achieved on ethylene glycol than on water and solid substrates