The synthesis of highly crystalline and monodisperse gamma-Fe(2)O(3) nanocrystallites is reported. High-temperature (300 degrees C) aging of iron-oleic acid metal complex, which was prepared by the thermal decomposition of iron pentacarbonyl in the presence of oleic acid at 100 degrees C, was found to generate monodisperse iron nanoparticles. The resulting iron nanoparticles were transformed to monodisperse gamma-Fe(2)O(3) nanocrystallites by controlled oxidation by using trimethylamine oxide as a mild oxidant. Particle size can be varied from 4 to 16 nm by controlling the experimental parameters. Transmission electron microscopic images of the particles showed 2-dimensional and 3-dimensional assembly of particles, demonstrating the uniformity of these nanoparticles. Electron diffraction, X-ray diffraction, and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the gamma-Fe(2)O(3) structures. Monodisperse gamma-Fe(2)O(3) nanocrystallites with a particle size of 13 nm also can be generated from the direct oxidation of iron pentacarbonyl in the presence of oleic acid with trimethylamine oxide as an oxidant.

Quantum dots (QDs) and magnetic nanoparticles (MPs) are of interest for biological imaging, drug targeting, and bioconjugation because of their unique optoelectronic and magnetic properties, respectively. To provide for water solubility and biocompatibility, QDs and MPs were encapsulated within a silica shell using a reverse microemulsion synthesis. The resulting SiO2/MP-QD nanocomposite particles present a unique combination of magnetic and optical properties. Their nonporous silica shell allows them to be surface modified for bioconjugation in various biomedical applications.

Reverse microemulsion techniques combined with templating strategies have led to the synthesis of four types of nanoparticles. First, homogeneous SiO2-coated Fe2O3 (SiO2/Fe2O3) nanoparticles with controlled SiO2 shell thickness (1.8−30 nm) were synthesized by reverse microemulsion. These nanocomposite particles were used as templates for the deposition of a mesoporous silica shell. The iron oxide core in SiO2/Fe2O3 could be partially and completely etched to produce rattle-type SiO2/Fe2O3 nanoballs and hollow SiO2 nanoballs, respectively. These facile synthetic methods led to the formation of different nanoparticle architectures with tailored silica shell thickness and porosity.

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Polymer-or SiO 2 -coated magnetic nanoparticles have been widely investigated due to their superparamagnetic properties and biocompatibility. 1-4 For many applications, these magnetic nanoparticles would benefit from having SiO 2 or polymer shells to impart ease of functionalization and biocompatibility. Poly(ethylene glycol) conjugation onto the SiO 2 layer has increased cell uptake efficiency in silicacoated magnetic nanoparticles and made them feasible for cell separation applications. 1 Self-assembled block copolypeptides on magnetic nanoparticles could be used as a smart drug delivery system controlled by magnetic field. 2 Magnetic nanoparticles can also be processed with quantum dots within a silica matrix to yield magnetic and fluorescent nanocomposites. 3 Layer-by-layer synthesis, 5 charged particle interaction, 6 and mercaptosilane functionalization 7 have been proposed to form various architectures of transition metal nanoclusters with silica nanospheres. These strategies can be employed to derive a novel heterogeneous catalyst system, whereby the high surface-to-volume ratio of transition metal nanoclusters can be used to achieve high reactivity. However, such ultrafine particles do not facilitate large-scale separations by conventional methods in liquid-phase processes. To allow for the easy removal of catalysts from reaction mixtures, supports containing magnetic nanoparticles have been developed. [8][9][10][11] However, silica-coated magnetic nanoparticles have not been studied as a catalyst support. (1) Yoon, T. J.; Kim, J. S.; Kim, B. G.; Yu, K. N.; Cho, M. H.; Lee, J. K. Angew. Chem., Int. Ed. 2005, 44, 2. (2) Euliss, L. E.; Grancharov, S. G.; O'Brien, S.; Deming, T. J.; Stucky, G. D.; Murray, C. B.; Held, G. A. Nano Lett. 2003, 3, 1489. (3) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.; Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4991. (4) (a) Phillipse, A. P.; Bruggen, M. P. B.; Pathmamanoharan, C. Langmuir 1994, 10, 92. (b) Kobayashi, Y.; Horie, M.; Konno, M.; RodriguezGonzález, B.; Liz-Marzan, L. M.Figure 1. TEM micrographs of as-prepared SiO2/Fe2O3 (left inset), Pd/ HS-SiO2/Fe2O3 (left), and Pd/H2N-SiO2/Fe2O3 (right).

A reverse microemulsion method is reported for preparing monodispersed silica-coated gold (or silver) nanoparticles without the use of a silane coupling agent or polymer as the surface primer. This method enables a fine control of the silica shell thickness with nanometer precision. As compared to the Stöber method reported for direct silica coating, which can only coat large gold particles ( approximately 50 nm in diameter) at low concentrations (<1.5 x 10(10) particles/mL), this new approach is capable of coating gold particles of a wide range of sizes (from 10 to 50 nm) at a much higher concentration ( approximately 1.5 x 10(13) particles/mL). Moreover, it enables straightforward surface functionalization via co-condensation between tetraethyl orthosilicate and another silane with the desired functional groups. The functional groups introduced by this method are readily accessible and thus useful for various applications.

Highly crystalline and monodisperse cobalt ferrite nanocrystals were fabricated by the high-temperature aging of a metal-surfactant complex followed by mild oxidation. Particle sizes were varied from 4 to 9 nm by changing the experimental conditions. Transmission electron microscopic (TEM) images of the particles showed two-and three-dimensional assembly of the particles, demonstrating the uniformity of the nanocrystals. Electron diffraction, X-ray diffraction, and high-resolution transmission electron microscopic images of the nanocrystals confirmed the highly crystalline nature of the cobalt ferrite structure. The elemental analysis confirmed the stoichiometry of cobalt ferrite, despite some variations in the relative atomic composition of nanocrystals. The nanocrystals were found to have typical behaviors of magnetic nanocrystals and the narrow energy barrier distributions of magnetic anisotropy, implying that the nanocrystals obtained are very uniform.The development of uniform magnetic nanocrystals has been intensively pursued because of their applications in magnetic data storage, ferrofluids, medical imaging, drug targeting, and catalysis. 1,2 Recently, several metallic magnetic nanocrystals of uniform particle size have been fabricated. 3 Magnetic oxide nanocrystals having uniform particle size have been synthesized. 4,5 However, a very difficult size-selection process was required to obtain magnetic nanocrystals having uniform particle size distribution. Recently, we developed a new procedure for producing highly crystalline and monodisperse γ-Fe 2 O 3 nanocrystals without a size-selection process. 6 These iron oxide nanocrystals were synthesized from the controlled oxidation of uniform iron nanoparticles generated from the thermal decomposition of an iron-surfactant complex. We have extended the synthetic method to fabricate bimetallic oxide nanocrystals and report upon the synthesis of monodisperse and highly crystalline cobalt ferrite nanocrystals.In the synthetic procedure, uniform iron-cobalt alloy nanoparticles were first generated from the thermal decomposition of a metal-oleate complex and further oxidized to yield cobalt ferrite nanocrystals. The precursor, (η 5 -C 5 H 5 )CoFe 2 (CO) 9 , was prepared using the previously reported synthetic procedure. 7 A typical synthesis of cobalt ferrite nanocrystals with a particle diameter of 6 nm is as follows. A total of 0.2 g of (η 5 -C 5 H 5 )-CoFe 2 (CO) 9 (1.23 mmol; total metal atoms) was added to a mixture containing 5 mL of dioctyl ether and 1.04 g of oleic acid (3.69 mmol) at room temperature under an argon atmosphere. The mixture was heated to reflux (∼300°C) and kept at that temperature for 1 h. The color of the reaction mixture changed to clear blue at 230°C and to black at the reflux temperature. The blue color formed at 230°C seemed to be due to a metal oleate complex. The resulting black solution was cooled to room temperature, and 0.28 g of dehydrated (CH 3 ) 3 -NO (3.69 mmol) was added. The mixture was then heated to 130°C under an argon...

Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains.Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here we demonstrate that δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the δM plots of γ-Fe 2 O 3 nanoparticles isolated by thick SiO 2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the δM plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions, as well as offer new perspectives in the use of Henkel-and δM-plots to quantify the rather elusive inhomogeneous magnetizations states in nanoparticles.Additional information on the δM and the in-field Mössbauer techniques, table with the complete results of the Mössbauer spectra fits, details of the Monte Carlo simulations, FC and ZFC magnetization curves of the VST series (Fig. S1a), Langevin scaling of M(H;T) data measured in VST45 (Fig. S1b), details on the estimate of the "magnetic size" from Langevin fits, δM plots of all the VST series and graphical analysis of the intraparticle and interparticle contributions to the dip (Fig. S2), example of hysteresis loops measured after ZFC and FC (for sample VST17, Fig. S3); X-ray diffraction patterns and lattice parameter across of the maghemite cores of different size (Fig. S4); complete results from Monte Carlo simulations showing the dependence of δM on core anisotropy (Fig. S5), surface anisotropy ( Fig. S6), exchange coupling constant ( Fig. S7) and disordered surface thickness (Fig. S8).