Kwangjin An

Large-Scale Synthesis of Uniform and Extremely Small-Sized Iron Oxide Nanoparticles for High-Resolution T1 Magnetic Resonance Imaging Contrast Agents

Uniform and extremely small-sized iron oxide nanoparticles (ESIONs) of < 4 nm were synthesized via the thermal decomposition of iron-oleate complex in the presence of oleyl alcohol. Oleyl alcohol lowered the reaction temperature by reducing iron-oleate complex, resulting in the production of small-sized nanoparticles. XRD pattern of 3 nm-sized nanoparticles revealed maghemite crystal structure. These nanoparticles exhibited very low magnetization derived from the spin-canting effect. The hydrophobic nanoparticles can be easily transformed to water-dispersible and biocompatible nanoparticles by capping with the poly(ethylene glycol)-derivatized phosphine oxide (PO-PEG) ligands. Toxic response was not observed with Fe concentration up to 100 μg/mL in MTT cell proliferation assay of POPEG-capped 3 nm-sized iron oxide nanoparticles. The 3 nm-sized nanoparticles exhibited a high r(1) relaxivity of 4.78 mM(-1) s(-1) and low r(2)/r(1) ratio of 6.12, demonstrating that ESIONs can be efficient T(1) contrast agents. The high r(1) relaxivities of ESIONs can be attributed to the large number of surface Fe(3+) ions with 5 unpaired valence electrons. In the in vivo T(1)-weighted magnetic resonance imaging (MRI), ESIONs showed longer circulation time than the clinically used gadolinium complex-based contrast agent, enabling high-resolution imaging. High-resolution blood pool MR imaging using ESIONs enabled clear observation of various blood vessels with sizes down to 0.2 mm. These results demonstrate the potential of ESIONs as T(1) MRI contrast agents in clinical settings.

Synthesis of Uniform Ferrimagnetic Magnetite Nanocubes

We synthesized uniform ferrimagnetic magnetite nanocubes in the size range from 20 to 160 nm. The magnetic property of the nanocubes was characterized, and magnetic separation of the histidine-tagged protein was demonstrated.

Size and Shape Control of Metal Nanoparticles for Reaction Selectivity in Catalysis

A nanoparticle with well‐defined surfaces, prepared through colloidal chemistry, enables it to be studied as a model heterogeneous catalyst. The colloidal synthetic approach provides versatile tools to control the size and shape of nanoparticles. Traditional nucleation and growth mechanisms have been utilized to understand how nanoparticles can be uniformly synthesized and unprecedented shapes can be controlled. Now, the size of metal particles can be controlled to cluster regimes by using dendrimers. By using seeds and foreign atoms, specific synthetic environments such as seeded growth and crystal overgrowth can be induced to generate various shaped mono‐ or bi‐metallic, core/shell, or branched nanostructures. For green chemistry, catalysis in 21st century is aiming for 100 % selectivity to produce only one desired product at high turnover rates. Recent studies on nanoparticle catalysts clearly demonstrate size and shape dependent selectivity in many catalytic reactions. By combining in situ surface characterization techniques, real‐time monitoring of nanoparticles can be performed under reaction environments, thus identifying several molecular factors affecting catalytic activity and selectivity.

Large-scale soft colloidal template synthesis of 1.4 nm thick CdSe nanosheets.

Two-dimensional (2D) nanocrystals have attracted tremendous attention from many researchers in various disciplines because of their unique properties. Since ways of making graphene were devised, there have been significant research efforts to synthesize free-standing 2D nanocrystals of various materials, including metals, oxides, and chalcogenides. Many of these 2D nanocrystals have been generated from exfoliation of materials with layered structures, and tiny amounts of products are generally produced. CdSe nanocrystals are among the most intensively studied nanostructured materials, owing to their many size-dependent optical and electrical characteristics and resulting exciting applications. Herein, we report on the large-scale synthesis of single-layered and lamellar-structured 2D CdSe nanocrystals with wurtzite crystal structure as thin as 1.4 nm by a soft colloidal template method. These free-standing 2D nanocrystals with insulating organic layers at the interface could find many interesting electronic and optoelectronic applications, including in quantum cascade lasers and resonant tunneling diodes utilizing their multiple quantum well structures. Compared to materials with layered crystal structures such as graphite, the synthesis of free-standing 2D nanocrystals of nonlayered materials such as CdSe is extremely challenging, because selective growth along one specific facet among several with similar energies is required. For example, in CdSe with a hexagonal wurtzite crystal structure, a (0001) facet has significantly higher surface energy than other facets, which leads to the formation of many one-dimensional nanostructures. Although there is a slight difference in the surface energies of (1120) and (1100) facets, quantum-confined thin CdSe 2D nanocrystals could not be synthesized using a conventional colloidal chemical route that employs thermal decomposition of precursors at high temperature, because the small difference in the surface energies of these two facets is negated by the high reaction temperature. Consequently, there have been only a few reports on the successful chemical synthesis of 2D CdSe nanocrystals. For example, CdSe inorganic–organic hybrid lamellar structures and CdSe nanoplatelets with zinc-blende structure were synthesized using colloidal chemical routes. However, their 2D growth mechanism has not been clearly elucidated. Furthermore, nanostructural control to form single-layered or multiple-layered nanosheets has not been demonstrated. In the current approach to creating 2D CdSe nanocrystals, we employed a soft template method, and we were able to synthesize not only free-standing single-layered CdSe nanosheets but also lamellar-structured nanosheets by controlling the interaction between organic layers in 2D templates of cadmium chloride alkyl amine complexes. It has been reported that the complex of cadmium halide and diamine can form a cadmium halide /diamine alternating layered structure through diamine bridging and hydrogen bonding between hydrogen atoms of the amine and halogen atoms. Likewise, a [CdCl2(RNH2)2] lamellar complex, which is used herein as a soft template, is expected to form lamellar structures composed of 2D arrays of CdCl2 and alkyl amine by van der Waals attraction between hydrocarbon sidechains of the alkyl amine. The small-angle X-ray scattering (SAXS) patterns of [CdCl2(RNH2)2] complexes with butylamine (BA), octylamine (OA), and dodecylamine (DA) show 00l orders of reflection, which confirms that the complexes formed typical lamellar structures (Supporting Information, Figure S1). A [CdCl2(OA)2] lamellar complex was chosen as the soft template for the synthesis of lamellarstructured CdSe nanosheets because of its optimum reactivity. [*] J. S. Son, Dr. J. Joo, K. Park, Dr. J. H. Kim, Dr. K. An, J. H. Yu, S. G. Kwon, Dr. S.-H. Choi, Prof. T. Hyeon National Creative Research Initiative Center for Oxide Nanocrystalline Materials and School of Chemical and Biological Engineering Seoul National University Seoul 151-744 (Korea) Fax: (+82)2-886-8457 E-mail: thyeon@snu.ac.kr

Synthesis of Uniform Hollow Oxide Nanoparticles through Nanoscale Acid Etching

We synthesized various hollow oxide nanoparticles from as-prepared MnO and iron oxide nanocrystals. Heating metal oxide nanocrystals dispersed in technical grade trioctylphosphine oxide (TOPO) at 300 degrees C for hours yielded hollow nanoparticles retaining the size and shape uniformity of the original nanocrystals. The method was highly reproducible and could be generalized to synthesize hollow oxide nanoparticles of various sizes, shapes, and compositions. Control experiments revealed that the impurities in technical grade TOPO, especially alkylphosphonic acid, were responsible for the etching of metal oxide nanocrystals to the hollow structures. Elemental mapping analysis revealed that the inward diffusion of phosphorus and the outward diffusion of metal took place in the intermediate stages during the etching process. The elemental analysis using XPS, EELS, and EDX showed that the hollow nanoparticles were amorphous metal oxides containing significant amount of phosphorus. The hollow nanoparticles synthesized from MnO and iron oxide nanocrystals were paramagnetic at room temperature and when dispersed in water showed spin relaxation enhancement effect for magnetic resonance imaging (MRI). Because of their morphology and magnetic property, the hollow nanoparticles would be utilized for multifunctional biomedical applications such as the drug delivery vehicles and the MRI contrast agents.

MR Tracking of Transplanted Cells With “Positive Contrast” Using Manganese Oxide Nanoparticles

Rat glioma cells were labeled using electroporation with either manganese oxide (MnO) or superparamagnetic iron oxide (SPIO) nanoparticles. The viability and proliferation of SPIO‐labeled cells (1.9 mg Fe/ml) or cells electroporated with a low dose of MnO (100 μg Mn/ml) was not significantly different from unlabeled cells; a higher MnO dose (785 μg Mn/ml) was found to be toxic. The cellular ion content was 0.1–0.3 pg Mn/cell and 4.4 pg Fe/cell, respectively, with cellular relaxivities of 2.5–4.8 s−1 (R1) and 45–84 s−1 (R2) for MnO‐labeled cells. Labeled cells (SPIO and low‐dose MnO) were each transplanted in contralateral brain hemispheres of rats and imaged in vivo at 9.4T. While SPIO‐labeled cells produced a strong “negative contrast” due to the increase in R2, MnO‐labeled cells produced “positive contrast” with an increased R1. Simultaneous imaging of both transplants with opposite contrast offers a method for MR “double labeling” of different cell populations. Magn Reson Med 60:1–7, 2008.

Simple and Generalized Synthesis of Oxide−Metal Heterostructured Nanoparticles and their Applications in Multimodal Biomedical Probes

Heterostructured nanoparticles composed of metals and Fe3O4 or MnO were synthesized by thermal decomposition of mixtures of metal-oleate complexes (for the oxide component) and metal-oleylamine complexes (for the metal component). The products included flowerlike-shaped nanoparticles of Pt-Fe3O4 and Ni-Fe3O4 and snowmanlike-shaped nanoparticles of Ag-MnO and Au-MnO. Powder X-ray diffraction patterns showed that these nanoparticles were composed of face-centered cubic (fcc)-structured Fe3O4 or MnO and fcc-structured metals. The relaxivity values of the Au-MnO and Au-Fe3O4 nanoparticles were similar to those of the MnO and Fe3O4 nanoparticles, respectively. Au-Fe3O4 heterostructured nanoparticles conjugated with two kinds of 12-base oligonucleotide sequences were able to sense a complementary 24-mer sequence, causing nanoparticle aggregation. This hybridization-mediated aggregation was detected by the overall size increase indicated by dynamic light scattering data, the red shift of the surface plasmon band of the Au component, and the enhancement of the signal intensity of the Fe3O4 component in T2-weighted magnetic resonance imaging.

Evidence of highly active cobalt oxide catalyst for the Fischer-Tropsch synthesis and CO2 hydrogenation.

Hydrogenations of CO or CO2 are important catalytic reactions as they are interesting alternatives to produce fine chemical feedstock hence avoiding the use of fossil sources. Using monodisperse nanoparticle (NP) catalysts, we have studied the CO/H2 (i.e., Fischer-Tropsch synthesis) and CO2/H2 reactions. Exploiting synchrotron based in situ characterization techniques such as XANES and XPS, we were able to demonstrate that 10 nm Co NPs cannot be reduced at 250 °C while supported on TiO2 or SiO2 and that the complete reduction of cobalt can only be achieved at 450 °C. Interestingly, cobalt oxide performs better than fully reduced cobalt when supported on TiO2. In fact, the catalytic results indicate an enhancement of 10-fold for the CO2/H2 reaction rate and 2-fold for the CO/H2 reaction rate for the Co/TiO2 treated at 250 °C in H2 versus Co/TiO2 treated at 450 °C. Inversely, the activity of cobalt supported on SiO2 has a higher turnover frequency when cobalt is metallic. The product distributions could be tuned depending on the support and the oxidation state of cobalt. For oxidized cobalt on TiO2, we observed an increase of methane production for the CO2/H2 reaction whereas it is more selective to unsaturated products for the CO/H2 reaction. In situ investigation of the catalysts indicated wetting of the TiO2 support by CoO(x) and partial encapsulation of metallic Co by TiO(2-x).

High Structure Sensitivity of Vapor-Phase Furfural Decarbonylation/Hydrogenation Reaction Network as a Function of Size and Shape of Pt Nanoparticles

Vapor-phase transformations of furfural in H(2) over a series of Pt nanoparticles (NPs) with various particle sizes (1.5-7.1 nm size range) and shapes (rounded, cubes, octahedra) encapsulated in poly(vinylpyrrolidone) (PVP) and dispersed on MCF-17 mesoporous silica were investigated at ambient pressure in the 443-513 K temperature range. Furan and furfuryl alcohol (FFA) were two primary products as a result of furfural decarbonylation and hydrogenation reactions, respectively. Under conditions of the study both reactions exhibited structure sensitivity evidenced by changes in product selectivities, turnover rates (TORs), and apparent activation energies (E(A)s) with Pt particle size and shape. For instance, upon an increase in Pt particle size from 1.5 to 7.1 nm, the selectivity toward FFA increases from 1% to 66%, the TOR of FFA production increases from 1 × 10(-3) s(-1) to 7.6 × 10(-2) s(-1), and E(A) decreases from 104 kJ mol(-1) to 15 kJ mol(-1) (9.3 kPa furfural, 93 kPa H(2), 473 K). Conversely, under the same experimental conditions the decarbonylation reaction path is enhanced over smaller nanoparticles. The smallest NPs (1.5 nm) produced the highest selectivity (96%) and highest TOR values (8.8 × 10(-2) s(-1)) toward furan formation. The E(A) values for decarbonylation (∼62 kJ mol(-1)) was Pt particle size independent. Furan was further converted to propylene via a decarbonylation reaction, but also to dihydrofuran, tetrahydrofuran, and n-butanol in secondary reactions. Furfuryl alcohol was converted to mostly to 2-methylfuran.

Colloid chemistry of nanocatalysts: A molecular view

Recent advances of a colloidal chemistry can offer great opportunities to fabricate and design nanocatalysts. Comprehensive understanding of a basic concept and theory of the colloidal synthetic chemistry facilitates to engineer elaborate nano-architectures such as bi- or multi-metallic, heterodimers, and core/shell. This colloidal solution technique not only enables to synthesize high surface mesoporous materials, but also provides a versatile tool to incorporate nanoparticles into mesoporous materials or onto substrates. For green chemistry, catalysis research has been pursued to design and fabricate a catalyst system that produces only one desired product (100% selectivity) at high turnover rates to reduce the production of undesirable wastes. Recent studies have shown that several molecular factors such as the surface structures, composition, and oxidation states affect the turnover frequency and reaction selectivity depending on the size, morphology, and composition of metal nanoparticles. Multipath reactions have been utilized to study the reaction selectivity as a function of size and shape of platinum nanoparticles. In the past, catalysts were evaluated and compared with characterizations before and after catalytic reaction. Much progress on in situ surface characterization techniques has permitted real-time monitoring of working catalysts under various conditions and provides molecular information during the reaction.