This review focuses on the synthesis, protection, functionalization, and application of magnetic nanoparticles, as well as the magnetic properties of nanostructured systems. Substantial progress in the size and shape control of magnetic nanoparticles has been made by developing methods such as co-precipitation, thermal decomposition and/or reduction, micelle synthesis, and hydrothermal synthesis. A major challenge still is protection against corrosion, and therefore suitable protection strategies will be emphasized, for example, surfactant/polymer coating, silica coating and carbon coating of magnetic nanoparticles or embedding them in a matrix/support. Properly protected magnetic nanoparticles can be used as building blocks for the fabrication of various functional systems, and their application in catalysis and biotechnology will be briefly reviewed. Finally, some future trends and perspectives in these research areas will be outlined.

Magnetic nanoparticles: Synthesis, protection, functionalization, and application

Poly(N-isopropylacrylamide): experiment, theory and application

Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

This feature article highlights work from the authors' laboratories on the synthesis, assembly, reactivity, and optical applications of metallic nanoparticles of nonspherical shape, especially nanorods. The synthesis is a seed-mediated growth procedure, in which metal salts are reduced initially with a strong reducing agent, in water, to produce ∼4 nm seed particles. Subsequent reduction of more metal salt with a weak reducing agent, in the presence of structure-directing additives, leads to the controlled formation of nanorods of specified aspect ratio and can also yield other shapes of nanoparticles (stars, tetrapods, blocks, cubes, etc.). Variations in reaction conditions and crystallographic analysis of gold nanorods have led to insight into the growth mechanism of these materials. Assembly of nanorods can be driven by simple evaporation from solution or by rational design with molecular-scale connectors. Short nanorods appear to be more chemically reactive than long nanorods. Finally, optical applica...

Anisotropic metal nanoparticles: Synthesis, assembly, and optical applications

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

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Present and Future of Surface-Enhanced Raman Scattering

Etude par DSC de la transition de type thermoretrait de gels aqueux de N-isopropylamide (NIPA) de solutions de poly(NIPA) et de PVME. Comparaison des temperatures de transition des gels et des solutions obtenues par DSC avec les valeurs obtenues par mesure du gonflement des gels et par mesure des points de trouble des solutions. Interpretation des resultats a l'aide du concept d'interaction hydrophobe. Influence d'additifs de faible masse sur les temperatures de transition

Thermal analysis of the volume phase transition with N-isopropylacrylamide gels

Silver nanoparticles prepared through a borohydride-reduction method were directly coated with silica by means of a seeded polymerization technique based on the Stober method. Various amine catalysts were used for initialization of a sol-gel reaction of TEOS with no need for a prior surface modification. Use of dimethylamine (DMA) as a catalyst was found to be necessary to obtain a proper coating. The silica shell thickness was varied from 28 to 76 nm for TEOS concentrations of 1-15 mM at 11.1 M water and 0.8 M DMA. The optical spectra of the core-shell silver-silica composite particles show a qualitative agreement with predictions by Mie theory.

Silica coating of silver nanoparticles using a modified Stober method.

The synthesis of monodispersed, amorphous cobalt nanoparticles coated with silica in aqueous/ethanolic solution is described. Both the core size and the silica shell thickness can be controlled through the synthetic conditions. Furthermore, the transformation of the cobalt cores into crystalline, metallic cobalt upon annealing in air is demonstrated through X-ray diffraction data. Both the initial, amorphous nanoparticles and their crystalline counterpart are magnetic, which promises important applications for ferrofluid preparation and for magnetic recording media.

Preparation and Properties of Silica-Coated Cobalt Nanoparticles†

Gold nanoparticles prepared through a conventional citrate-reduction method were directly coated with silica by means of a seeded polymerization technique based on the Stober method. The method required no surface modification. The addition of tetraethylorthosilicate and water prior to ammonia was found to be critical to obtain a proper coating. The silica shell thickness was varied from 30 to 90 nm for TEOS concentrations of 0.0005–0.02 M at 10.9 M of water and 0.4 M of ammonia. The optical spectra of the core–shell gold–silica composite particles agreed with predictions of Mie theory.

Direct coating of gold nanoparticles with silica by a seeded polymerization technique.

To prepare high dielectric thin film of polymer-based materials, nanometer sized barium titanate (BaTiO3) particles, which should have high dielectric coefficients and low energy dissipation factors due to nano-size effects, were dispersed in polyvinylidene fluoride (PVDF) or siloxane-modified polyamideimide (SPAI). The BaTiO3 particles with crystal sizes of 10.5–34.6 nm were synthesized with a complex alkoxide method. Polymer/N-methyl-2-pyrrodinone solution suspending the BaTiO3 particles was spin-coated on ITO glass substrates to prepare polymer-based composite films with thickness of submicron meters. The BaTiO3 particles were dispersed more homogeneously in the PVDF film than in the SPAI film. The good dispersion of the particles in the PVDF film brought about a smooth surface of the film that had a root mean square roughness less than 20 nm at a particle volume fraction of 30%. The roughness was less than one-tenth of the roughness of the SPAI composite film. An increase in the BaTiO3 crystal size from 10.5 to 34.6 nm in the PVDF film at a particle volume fraction of 30% increased the dielectric constant of the film from 20.1 to 31.8. The BaTiO3–PVDF composite film attained high dielectric constant that had more than twice the dielectric constant of the BaTiO3–SPAI composite film. The dissipation factor of the PVDF composite film was as low as 0.05 at 104 Hz.

Mikio Konno

Papers

Thermal analysis of the volume phase transition with N-isopropylacrylamide gels

Silica coating of silver nanoparticles using a modified Stober method.

Preparation and Properties of Silica-Coated Cobalt Nanoparticles†

Direct coating of gold nanoparticles with silica by a seeded polymerization technique.

Fabrication and dielectric properties of the BaTiO3–polymer nano-composite thin films