Gold nanoparticles are encapsulatedwithin thermoresponsive pNIPAMmigrogels through an easy two‐stepprotocol. The core/shell structure ofthe composite is characterized by TEM,AFM, PCS, and UV‐vis spectroscopy. The restricted environment and thehigh porosity of the microgel shell arestudied through the overgrowth of thegold core.

Small-angle neutron scattering and dynamic light scattering have been used to study the thermodynamics of swelling and the associated structure modifications of highly cross-linked temperature-sensitive poly (N-isopropylacrylamide) [poly(NIPAM)] microgels in D2O. A particle core-shell model is proposed, with the core containing most of the cross-linker molecules. The Flory-Rehner theory, with the inclusion of a concentration dependent Flory solvency parameter, successfully describes the experimental swelling, despite the inhomogeneous character of the particles. Interestingly, the shell evolution with temperature controls the whole particle swelling, exerting an external pressure over the core, which in turn influences its size during the swelling process. Scaling laws for the correlation lengths were found with respect to temperature and polymer concentration. Finally, it has been encountered that for the collapsed microgel states, the particle surface seems to have a fractal character.

The swelling of cationic microgel particles has been studied experimentally, in the weak screening regime. The solution pH was selected as the external variable triggering the swelling, which was followed by dynamic light scattering. The particle charge was determined by conductometric and potentiometric titrations, leading to a good correlation between the charge of the microgel network and its size. This leads to the conclusion that the swelling is mainly charge controlled. The Flory-Huggins thermodynamic theory for gels, including a term accounting for the counterion distribution within the microgel, has been used to interpret the experimental data. The osmotic term associated with the counterions explains fairly well the observed behavior, and additional contributions due to the internal microgel microscopic structure are not necessary, as Pincus et al. have suggested.

Thermoresponsive nanocomposites comprising a gold nanoparticle core and a poly(N‐isopropylacrylamide) (pNIPAM) shell are synthesized by grafting the gold nanoparticle surface with polystyrene, which allows the coating of an inorganic core with an organic shell. Through careful control of the experimental conditions, the pNIPAM shell cross‐linking density can be varied, and in turn its porosity and stiffness, as well as shell thickness from a few to a few hundred nanometers is tuned. The characterization of these core–shell systems is carried out by photon‐correlation spectroscopy, transmission electron microscopy, and atomic force microscopy. Additionally, the porous pNIPAM shells are found to modulate the catalytic activity, which is demonstrated through the seeded growth of gold cores, either retaining the initial spherical shape or developing a branched morphology. The nanocomposites also present thermally modulated optical properties because of temperature‐induced local changes of the refractive index surrounding the gold cores.

The evolution of granular shear flow is investigated as a function of height in a split-bottom Couette cell. Using particle tracking, magnetic-resonance imaging, and large-scale simulations we find a transition in the nature of the shear as a characteristic height H * is exceeded. Below H * there is a central stationary core; above H * we observe the onset of additional axial shear associated with torsional failure. Radial and axial shear profiles are qualitatively different: the radial extent is wide and increases with height while the axial width remains narrow and fixed.PACS numbers: 45.70. Mg, 83.50.Ax Shear bands in dense granular materials are localized regions of large velocity gradients; they are the antithesis of the broad uniform flows seen in slowly-sheared Newtonian fluids [1,2,3,4,5,6]. Until recently it was generally assumed that all granular shear bands were narrow. However, in 2003 Fenistein et al. [7] discovered that in modified Couette cells granular shear bands can be made arbitrarily broad. In this geometry, the bottom of a cylindrical container is split at radius r = R s and shear is produced by rotating both the outer ring and the cylindrical boundary of the container while keeping the central disk (r < R s ) stationary. For very shallow packs, the shear band measured at the top surface is narrow and located at r = R s so that the inner region directly above the central disk is stationary while the remaining part rotates as a solid. As the filling height of the material, H, increases, the shear band increases in radial width and moves toward the cylinder axis. For sufficiently large H, the shear band overlaps the axis at r = 0 and one might expect qualitatively new behavior. Indeed, Unger et al. [8] predicted that the shape of the boundary between moving and stationary material would undergo a first-order transition as H is increased past a threshold value H * : the shearing region which for H < H * is open at the top and intersects the free surface abruptly collapses to a closed cupola completely buried inside the bulk.Previous experiments focused primarily on the surface flows in shallow containers and left unexplored many questions about the shape and evolution of the shear profiles for large H. Here, we combine magnetic resonance imaging (MRI) and high-speed video observations with large-scale simulations to explore shear flow both for shallow and tall packs. In addition to monitoring the evolution of the flow profiles in the radial direction, we also examine shear in the vertical direction. Instead of a first order collapse of the shear zone as proposed by Unger et al.[8], we find that above H * ≃ 0.6R s , the inner core of immobile material disappears gradually as shear along the central axis of the cylinder sets in. Our setup is similar to that of Fenistein et al. [7] except that we rotate the inner disk instead of the outer ring and cylinder (Fig.1b inset). In the absence of inertial effects, this makes no difference to the results. For surface observations with high-speed video ...

The synthesis, characterization, and assembly of different types of nanoparticles, which was established as a necessary prerequisite for the application of nanotechnology, have dramatically advanced over the last 20 years. [1,2] However, it has recently been realized that the incorporation of multiple functionalities within nanoscale systems would become much more useful for most of the foreseen applications. Thus, the fabrication of multifunctional nanoparticles has become a major challenge. Among these systems, the incorporation of active (optically, catalytically, magnetically…) nanoparticles within so-called "smart" thermosensitive microgels has received significant attention over the last few years. [3,4] The incorporation of nanoparticles can be accomplished either by in situ formation, by post-synthesis assembly or by direct polymerization on the nanoparticles surface. [5,6] We have recently reported the growth of thermosensitive poly(N-isopropylacrylamide) (pNIPAM) microgels on the surface of gold nanoparticles, involving several steps, including the formation of a first polystyrene thin layer, followed by pNIPAM polymerization after the required purification process. [7,8] Although gold nanoparticle growth could be achieved within the microgel shell, this synthesis was restricted to spherical nanoparticle seeds, whereas, for example, nanorods (which display a much more interesting optical response) were not properly incorporated. Thus, there was a need to both simplify the coating process and make it more widely applicable.We report here a novel procedure where butenoic acid is used for the synthesis of gold nanoparticles in aqueous surfactant solutions, in the presence of preformed Au seeds. Apart from the interesting observation that butenoic acid can be used as a reducing agent, this is particularly interesting because it provides the particles with a vinyl functionality, which should be useful for the direct pNIPAM polymerization on the nanoparticles surface and their subsequent encapsulation, while avoiding complicated surface functionalization steps. Although we have only optimized the reduction process for nanosphere growth, we also demonstrate that butenoic acid can replace cetyltrimethylammonium bromide (CTAB) molecules from Au nanoparticle surfaces, including Au nanorods, and adsorb on the metal surface, thereby facilitating the polymerization of pNIPAM on the metal core. The improved stability of the nanocomposites and the porosity of the pNIPAM shell allows subsequent reduction of metal atoms on the metal core, which was exploited for the overgrowth of pNIPAM encapsulated Au spheres and rods with both Au and Ag under mild conditions. In a previous publication [9] we demonstrated the ability of these composite colloids to mechanically trap non-common surface-enhanced Raman scattering (SERS) analytes. However, the use of 60 nm gold spheres as colloidal cores and the impossibility of forming hot spots due to the physical barrier imposed by the pNIPAM shell severely limited the enhancing capabili...

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