Tailoring the structure of nanocrystal superlattices is an important step toward controlled design of novel nanostructured materials and devices. We demonstrate how the long-range order and macroscopic dimensions of magnetic nanoparticle arrays can be controlled by the use of a modulated magnetic field. Inducing a dipolar attraction during the initial stage of the drying-mediated self-assembly process was sufficient to assemble the superparamagnetic oleate-capped maghemite nanocubes into large and defect-free superstructures with both translational and orientational order. The characteristic dimensions of the superlattice are controlled by the particle concentration as well as the duration of the applied magnetic field. The superparamagnetic maghemite nanocubes assemble into large and highly oriented thin arrays by applying the magnetic field perpendicular to the substrate surface only during the initial phase of drying-mediated self-assembly. Micrometer-sized and thick three-dimensional mesocrystals are obtained when the drying dispersion is subjected to an external magnetic field of moderate strength for the entire duration of the assembly process. The discovery of how translational and orientation order of nanocrystal superlattices can be induced by a temporal modulation of an anisotropic interparticle force offers new insight on the importance of the initial nucleation stage in the self-assembly process and suggests new routes for controlled self-assembly of dipolar nanocrystals.he assembly of nanoparticles into ordered superstructures is a generic phenomenon observed in materials of biological, geological, and man-made origin. Important examples include biomineralization (1) and ancient paints (2) and precipitates origination from slowly hydrolyzing aqueous solutions (3). The possibility to self-organize nanoparticles into two-and threedimensional superlattices with a high degree of translational order has attracted much interest since the early observations of iron oxide ''super crystals'' (4). Indeed, understanding and optimizing the structures, at all length scales, of nanocrystal superlattices is an important step toward controlled design of novel nanostructured materials and devices. Arranging nanocrystals into ordered arrays and macroscopic structures with a specific packing arrangement has been used to engineer the band-gap of semiconductor quantum dot arrays (5) and highdensity storage media (6, 7).Mixing monodisperse spheres of different sizes and interactions can yield binary nanoparticle superlattices with a wide range of superstructures and tunable properties (8, 9). Although long-range translational order is essential, it is also of importance to attain a high degree of orientational order (10-13) to optimize, e.g., the magnetic properties (14). Indeed, oriented organization and attachment of nanocrystals is an important path for biomineralization processes (15, 16) to create materials with highly anisotropic mechanical properties.Fundamentally, self-assembly into well defined structures requir...

This review describes recent efforts on the synthesis, dispersion and surface functionalization of the three dominating oxide nanoparticles used for photocatalytic, UV-blocking and sunscreen applications: titania, zinc oxide, and ceria. The gas phase and liquid phase synthesis is described briefly and examples are given of how weakly aggregated photocatalytic or UV-absorbing oxide nanoparticles with different composition, morphology and size can be generated. The principles of deagglomeration are reviewed and the specific challenges for nanoparticles highlighted. The stabilization of oxide nanoparticles in both aqueous and non-aqueous media requires a good understanding of the magnitude of the interparticle forces and the surface chemistry of the materials. Quantitative estimates of the Hamaker constants in various media and measurements of the isoelectric points for the different oxide nanoparticles are presented together with an overview of different additives used to prepare stable dispersions. The structural and chemical requirements and the various routes to produce transparent photocatalytic and nanoparticle-based UV-protecting coatings, and UV-blocking sunscreens are described and discussed.

Aluminum salts, developed almost a century ago, remain the most commonly used adjuvant for licensed human vaccines. Compared to more recently developed vaccine adjuvants, aluminum adjuvants such as Alhydrogel are heterogeneous in nature, consisting of 1–10 micrometer-sized aggregates of nanoparticle aluminum oxyhydroxide fibers. To determine whether the particle size and aggregated state of aluminum oxyhydroxide affects its adjuvant activity, we developed a scalable, top-down process to produce stable nanoparticles (nanoalum) from the clinical adjuvant Alhydrogel by including poly(acrylic acid) (PAA) polymer as a stabilizing agent. Surprisingly, the PAA:nanoalum adjuvant elicited a robust TH1 immune response characterized by antigen-specific CD4+ T cells expressing IFN-γ and TNF, as well as high IgG2 titers, whereas the parent Alhydrogel and PAA elicited modest TH2 immunity characterized by IgG1 antibodies. ASC, NLRP3 and the IL-18R were all essential for TH1 induction, indicating an essential role of the inflammasome in this adjuvant’s activity. Compared to microparticle Alhydrogel this nanoalum adjuvant provided superior immunogenicity and increased protective efficacy against lethal influenza challenge. Therefore PAA:nanoalum represents a new class of alum adjuvant that preferentially enhances TH1 immunity to vaccine antigens. This adjuvant may be widely beneficial to vaccines for which TH1 immunity is important, including tuberculosis, pertussis, and malaria.

Here we demonstrate how monodisperse iron oxide nanocubes and nanospheres with average sizes between 5 and 27 nm can be synthesized by thermal decomposition. The relative importance of the purity of the reactants, the ratio of oleic acid and sodium oleate, the maximum temperature, and the rate of temperature increase, on robust and reproducible size and shape-selective iron oxide nanoparticle synthesis are identified and discussed. The synthesis conditions that generate highly monodisperse iron oxide nanocubes suitable for producing large ordered arrays, or mesocrystals are described in detail.

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Fine tuning of the axial ratio of ceria nanorods by a time-dependent, face-specific growth and dissolution process has been demonstrated for the first time. Nearly monodisperse ceria nanorods can be synthesized by thermal decomposition of a ceria complex in the presence of only one type of surfactant (oleic acid). Strikingly, we find that the ceria nanorods are slowly and selectively shortened with time without any change of the rod diameter. The growth and subsequent dissolution along the [100] direction has been related to the high surface energy and the strong tendency for the {100} surface to reconstruct to reduce the dipolar moment. The addition of a second surfactant, octadecylamine, to the reaction mixture promotes the growth rate in other crystallographic directions and thus yields ceria nanodumbbells and nanospheres.

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