Won k Seo

Separation of nanoparticles in a density gradient: FeCo@C and gold nanocrystals.

Size and geometric control of nanomaterials are important to the discovery of intrinsic size/shape dependent properties and bottom up approaches for the fabrication of functional nanodevices. Two general strategies have been employed to create size-uniform nanocrystals. One method is direct particle size control during synthesis by adjusting growth parameters; 7–9] the other is post-synthesis separation. Much capacity exists to improve size separation efficacy in the latter case. Differential centrifugation can remove large and unstable particles from colloidal systems, but lacks precise control over particle size. Addition of adjustable amounts of “anti-solvent” (including CO2) [12] into colloidal systems may make precipitation processes more controllable. Other methods include filtration (including diafiltration), electrophoresis, and chromatographic methods that can produce particle fractions with narrow shape and size distributions. To maintain or improve the quality of nanoparticle (NP) separation, whilst addressing the issues of adhesion and clogging in liquid–solid phase separation processes, a completely liquid phase separation method is highly appealing. Isopycnic centrifugation, which is often used for biomacromolecule separation, relies upon a density gradient and ultracentrifugation to separate components according to subtle density differences, and has been applied for diameter and electronic-dependent separation of single-walled carbon nanotubes (SWNT). However, the isopycnic densitygradient centrifugation method reaches a limitation when it is extended to the separation of metal nanoparticles. Such a method requires that the components for separation have densities within a gradient range. Aqueous density gradient media usually have densities less than 1.4 gcm , which is much less than the density of metal nanoparticles. Size or shape separation of such heavy nanocrystals remains an issue, both in their preparation and utility for various applications. In contrast to isopycnic separation, ultracentrifugal rate separation can utilize density gradients to separate nanocrystals with higher densities than the gradient media itself. We have previously applied such a method to achieve length separation of suspended SWNTs and pegylated graphene oxide. In this report, the method was extended to metallic NP size separation. Nanoparticles of various size, suspension chemistry, and composition, including FeCo@C and gold nanoparticles (Au NPs), were separated using the method. FeCo nanocrystals coated in graphitic shells have superior magnetic properties, and have shown promise for applications in biolabeling and magnetic resonance imaging (MRI). However, the chemical deposition method used for their preparation produces nanocrystals with wide size distribution. They are thus ideal candidates for post-synthesis separation. FeCo@C NPs with average diameters of about 4 nm were separated first by our density gradient rate (DGR) separation method by using a 10+ 20+ 30+ 40% gradient and centrifugation for 3.5 h. TEM results of typical fractions (Figure 1A) indicate that fraction 8 (labeled as “f8” in Figure 1B) contained circa 1.5 nm NPs. The average particle diameter of subsequent fractions (f11, 15, 19, 24, and 27) gradually increased from 2.5 to 5.6 nm. By varying the step gradient densities and centrifuge exposure time, this method could be used for separation of nanoparticles of a larger size range, which was demonstrated by (on average) 7 nm FeCo@C NP separation (see TEM images of initial 7 nm FeCo@C NPs in the Supporting Information). A gradient of higher density steps (20+ 30+ 40+ 60%) was used. The use of higher density gradient media helps to control the sedimentation rate by reducing the density difference between the NPs and the environmental medium, and increasing the medium viscosity. After centrifugation for 2.5 h, several bands formed in the centrifuge vessel (Figure 2A), just as in the 4 nm NP case. Sampling fractions along the centrifuge vessel yielded nanoparticles of increasing size (with increasing density), as revealed by TEM (Figure 2B). From f5 to f16, the average particle size increased from 2 to 6.5 nm. It is noteworthy that the Fe/Co atomic ratios of f8 and f16 were measured by energy dispersive spectra (EDS) and found to be different (Figure 2B, bottom right). For f8, the ratio Fe/Co= 48:52; for f16, Fe/Co= 40:60. Previously such stoichiometry was analyzed by calcination/burning of the graphitic shells at 500 8C, dissolving the metal species in an HCl solution, and measuring the iron and cobalt concentrations on the basis of the ultraviolet[*] Prof. Dr. X. M. Sun, L. Bai State Key Laboratory of Chemical Resource Engineering Beijing University of Chemical Technology Beijing 100029 (China)

High‐contrast in vivo visualization of microvessels using novel FeCo/GC magnetic nanocrystals

FeCo‐graphitic carbon shell nanocrystals are a novel MRI contrast agent with unprecedented high per‐metal‐atom‐basis relaxivity (r1 = 97 mM‐1 sec‐1, r2 = 400 mM‐1 sec‐1) and multifunctional capabilities. While the conventional gadolinium‐based contrast‐enhanced angiographic magnetic MRI has proven useful for diagnosis of vascular diseases, its short circulation time and relatively low sensitivity render high‐resolution MRI of morphologically small vascular structures such as those involved in collateral, arteriogenic, and angiogenic vessel formation challenging. Here, by combining FeCo‐graphitic carbon shell nanocrystals with high‐resolution MRI technique, we demonstrate that such microvessels down to ∼100 μm can be monitored in high contrast and noninvasively using a conventional 1.5‐T clinical MRI system, achieving a diagnostic imaging standard approximating that of the more invasive X‐ray angiography. Preliminary in vitro and in vivo toxicity study results also show no sign of toxicity. Magn Reson Med, 2009.

Synthesis of Ultrasmall Ferromagnetic Face-Centered Tetragonal FePt–Graphite Core–Shell Nanocrystals

Chemically L10-ordered face-centered tetragonal (fct) FePt nanocrystals are of great interest because of their large magnetocrystalline anisotropy,[1,2] which makes them excellent candidates for applications in ultrahigh-density recording media and for use in biological separation.[3–5] There have been considerable efforts to organize FePt nanocrystals for such applications by nanocrystal self-assembly[3] and polymer templating.[6] It has been predicted that fct FePt particles as small as 2.8 nm exhibit sufficiently high ferromagnetic stability against superparamagnetism at room temperature.[7] These ultrasmall ferromagnetic particles could be useful for various high-density applications.

Synthesis and structure of ansa-cyclopentadienyl pyrrolyl titanium complexes: [(η5-C5H4)CH2(2-C4H3N)]Ti(NMe2)2 and [1,3-{CH2(2-C4H3N)}2(η5-C5H3)]Ti(NMe2)

Abstract Reaction of ansa-cyclopentadienyl pyrrolyl ligand (C5H5)CH2(2-C4H3NH) (2) with Ti(NMe2)4 affords bis(dimethylamido)titanium complex [(η5-C5H4)CH2(2-C4H3N)]Ti(NMe2)2 (3) via amine elimination. A cyclopentadiene ligand with two pendant pyrrolyl arms, a mixture of 1,3- and 1,4-{CH2(2-C4H3NH)}2C5H4 (4), undergoes an analogous reaction with Ti(NMe2)4 to give [1,3-{CH2(2-C4H3N)}2(η5-C5H3)]Ti(NMe2) (5). Molecular structures of 3 and 5 have been determined by single crystal X-ray diffraction studies.

The Role of Water for the Phase-Selective Preparation of Hexagonal and Cubic Cobalt Oxide Nanoparticles

A selective preparation and the formation mechanism of hexagonal and cubic CoO nanoparticles from the reaction of [Co(acac)(2)] (acac=acetylacetonate) and amine have been investigated. CoO nanoparticles with a hexagonal pyramidal shape were yielded under decomposition conditions with amine. Importantly, the addition of water altered the final phase to cubic and comprehensively changed the reaction mechanism. The average sizes of the hexagonal and cubic CoO nanoparticles could be controlled either by changing the amine concentration or by using different reaction temperatures. Detailed formation mechanisms are proposed on the basis of gas chromatography-mass spectrometry data and color changes of the reaction mixture. The hexagonal CoO phase is obtained through two distinct pathways: solvolysis with C-C bond cleavage and direct condensation by amine. On the other hand, the cubic CoO nanoparticles were synthesized by strong nucleophilic attack of hydroxide ions from water and subsequent C-C bond breaking. The resulting caboxylate ligand can stabilize a cobalt hydroxide intermediate, leading to the generation of a thermodynamically stable CoO phase.

Propargylic substitution reactions with various nucleophilic compounds using efficient and recyclable mesoporous silica spheres embedded with FeCo/graphitic shell nanocrystals

Phosphomolybdic acid (PMA, H3PMo12O40) functioned as a catalyst for reactions of secondary propargylic alcohols and nucleophiles. Highly stable and magnetically recyclable mesoporous silica spheres (MMS) embedded with FeCo-graphitic carbon shell nanocrystals (FeCo/GC@MSS) were fabricated by a modified Stöber process and chemical vapor deposition (CVD) method. The FeCo/GC@MSS were loaded with phosphomolybdic acid (PMA@FeCo/GC@MSS), and their catalytic activity was investigated. Propargylic reactions of 1,3-diphenyl-2-propyn-1-ol with a wide range of nucleophiles bearing activating substituents were catalyzed under mild conditions. It was found that the MMS possess mesoporosities and have enough inner space to load FeCo and phosphomolybdic acid. The FeCo/GC@MSS were found to be chemically stable against acid etching and oxidation. This suggests that the nanocrystals can be used as a support for an acid catalyst. Moreover, the magnetic property of the nanocrystals enabled the facile separation of catalysts from the products.

Ultra‐small, Uniform, and Single bcc‐Phased FexCo1‐x/Graphitic Shell Nanocrystals for T1 Magnetic Resonance Imaging Contrast Agents

We have synthesized ultra-small and uniform Fe(x)Co(1-x)/graphitic carbon shell (Fe(x)Co(1-x)/GC) nanocrystals (x=0.13, 0.36, 0.42, 0.50, 0.56, and 0.62, respectively) with average diameters of <4 nm by thermal decomposition of metal precursors in approximately 60 nm MCM-41 and methane CVD. The composition of the Fe(x)Co(1-x)/GC nanocrystals can be tuned by changing the Fe:Co ratios of the metal precursors. The Fe(x)Co(1-x)/GC nanocrystals show superparamagnetic properties at room temperature. The Fe(0.50)Co(0.50)/GC, Fe(0.56)Co(0.44)/GC, and Fe(0.62)Co(0.38)/GC nanocrystals have a single bcc FeCo structure, whereas the Fe(0.13)Co(0.87)/GC, Fe(0.36)Co(0.64)/GC, and Fe(0.42)Co(0.58)/GC nanocrystals have a mixed structure of bcc FeCo and fcc Co. The single bcc-phased Fe(x)Co(1-x)/GC nanocrystals functionalized with phospholipid-poly(ethylene glycol) (PL-PEG) in phosphate buffered saline (PBS) are demonstrated to be excellent T(1) MRI contrast agents.

Air-stable CuInSe2 nanoparticles formed through partial cation exchange in methanol at room temperature

Spherical CuInSe2 nanoparticles were synthesized through a partial cation exchange reaction of Cu2−xSe nanoparticles in methanol at room temperature. An additive, tributylphosphine, rapidly reduced copper and selenium ions, and facilitated the dissolution of Cu ions. Poly(vinylpyrrolidone) retained the spherical morphology of the nanoparticles by anchoring on the surface. The fraction of In3+ ions increased as the reaction progressed, which led to a decrease of the optical bandgap from 2.06 eV of Cu2−xSe to 1.03 eV of CuInSe2. The resulting CuInSe2 particles exhibited high air and thermal stability, and can be applied as colloidal inks for thin film fabrication in optoelectronic devices.

A Highly Stable and Magnetically Recyclable Nanocatalyst System: Mesoporous Silica Spheres Embedded with FeCo/Graphitic Shell Magnetic Nanoparticles and Pt Nanocatalysts.

We have developed a highly stable and magnetically recyclable nanocatalyst system for alkene hydrogenation. The materials are composed of mesoporous silica spheres (MSS) embedded with FeCo/graphitic shell (FeCo/GC) magnetic nanoparticles and Pt nanocatalysts (Pt-FeCo/GC@MSS). The Pt-FeCo/GC@MSS have superparamagnetism at room temperature and show type IV isotherm typical for mesoporous silica, thereby ensuring a large enough inner space (surface area of 235.3 m(2)  g(-1), pore volume of 0.165 cm(3)  g(-1), and pore diameter of 2.8 nm) to undergo catalytic reactions. We have shown that the Pt-FeCo/GC@MSS system readily converts cyclohexene into cyclohexane, which is the only product isolated and Pt-FeCo/GC@MSS can be seperated very quickly by an external magnetic field after the catalytic reaction is finished. We have demonstrated that the recycled Pt-FeCo/GC@MSS can be reused further for the same hydrogenation reaction at least four times without loss in the initial catalytic activity.

Feasibility study of the IE-SASW method for nondestructive evaluation of containment building structures in nuclear power plants

Abstract The IE-SASW method, a combination of impact-echo (IE) acoustics with spectral analysis of surface waves (SASW), is proposed as a newly developed nondestructive testing method in concrete structures. This feasibility study examines the IE technique and uses elastic P-wave velocity data as measured from the SASW method on concrete members in nuclear power plant containment structures. It was shown that both the thickness of the concrete specimens used in this study and the depth of the introduced defects (i.e. voids) could be identified by the IE-SASW method. In contrast, the reinforced steel bar itself could not be identified by the IE-SASW method. Additionally, GPR (ground penetrating radar) techniques were used to examine the same specimens in order to establish some level of performance and reliability to compare with the performance of the IE-SASW method. The GPR method provides an objective and reliable image corresponding to the reinforced steel bars. The experimental studies show that it is more feasible to use the IE-SASW method rather than GPR to detect voids that were positioned beneath the steel reinforcing bars in the concrete specimens.