A synthetic route to size-controlled fcc and fct FePt nanoparticles

We report here a new synthetic route to FePt nanoparticles using a stoichiometric mixture of Na2Fe(CO)4 and Pt(acac)2. The structure of FePt nanoparticles, their size, chemical composition, and magnetic property can be controlled by various synthetic parameters, such as the solvent type, nature, and molar ratio of surfactants and stabilizers, synthesis temperature, and purification process. Partially ordered fct (L10) nanoparticles with room temperature magnetic coercivity can be synthesized directly in tetracosane solution at 389 degrees C. The fcc FePt synthesized in nonadecane can be transformed into the magnetically important fct phase at 430 degrees C without significant particle sintering.     

Polymer mediated self-assembly of magnetic nanoparticles

We present a simple polymer-mediated process of assembling magnetic FePt nanoparticles on a solid substrate. Alternatively absorbing the PEI molecule and FePt nanoparticles on a HO-terminated solid surface leads to a smooth FePt nanoparticle assembly with controlled assembly thickness and dimension. Magnetic measurements show that the thermally annealed FePt nanoparticle assembly as thin as three nanoparticle layers is ferromagnetic. The magnetization direction of this thin FePt nanoparticle assembly is readily controlled with the laser-assisted magnetic writing. The reported process can be applied to various substrates, nanoparticles, and functional macromolecules and will be useful for future magnetic nanodevice fabrication.     

Water-based ferrofluids from FexPt1-x nanoparticles synthesized in organic media

This paper describes an effective method to transfer oleic acid/oleylamine-capped colloidal FePt nanoparticles dispersed in hexane into water, using tetramethylammonium hydroxide (TMAOH) as a phase transfer agent. FexPt1-x nanoparticles with different compositions (x = 0.32, 0.40, 0.48, 0.60, 0.66, 0.69) in the size range of 2-4 nm were synthesized by a high-temperature organometallic route with oleic acid and oleylamine as stabilizers. The surface of such nanoparticles was modified through removal of the organic, hydrophobic layer and adsorption of TMAOH, which provides the nanoparticles with sufficient surface charge so that an electrostatic double layer builds up, and the FePt nanoparticles can be fully redispersed in aqueous solution, even with high concentrations. The water-dispersible FePt nanoparticles were characterized by transmission electron microscopy, electrophoretic mobility, X-ray diffraction, and Fourier transform infrared spectroscopy.     

Formation of FePt nanoparticles having high coercivity

Ultrasonication of toluene solutions of the heteropolynuclear cluster complex, Pt3Fe3(CO)15, in the presence of oleic acid and oleylamine affords surface-capped fcc FePt nanoparticles having an average diameter of ca. 2 nm. Self-assembled arrays of these nanoparticles on oxidized Si wafers undergo a fcc-to-fct phase transition at 775 degrees C to form ferromagnetic FePt nanocrystals ca. 5.8 nm in diameter well dispersed on the Si wafer surface. Room-temperature coercivity measurements of these annealed FePt nanoparticles confirm a high coercivity of ca. 22.3 kOe. Such high coercivity for fct FePt nanoparticles might result from use of a heterpolynuclear complex as a single-source precursor of Fe and Pt neutral atoms or from use of ultrasonication to form fcc FePt nanoparticles under conditions of exceptionally rapid heating. Experiments to determine the critical experimental conditions required to achieve such high room-temperature coercivities in ferromagnetic nanoparticles are underway.     

Magnetic nanoparticle assembly on surfaces using click chemistry

Controlled assembly of ferromagnetic nanoparticles on surfaces is of crucial importance for a range of spintronic and data storage applications. Here, we present a novel method for assembling monolayers of ferromagnetic FePt nanoparticles on silicon oxide substrates using "click chemistry". Reaction of alkyne-functionalized FePt nanoparticles with azide-terminated self-assembled monolayers (SAMs), on silicon oxide, leads to the irreversible attachment of magnetic nanoparticles to the surface via triazole linkers. Based on this covalent interaction, well-packed monolayers of FePt nanoparticles were prepared and nanoparticle patterns are generated on surfaces via microcontact printing (μCP).     

Synthesis of FePt nanocubes and their oriented self-assembly

Monodisperse FePt nanocubes are synthesized at 205 degrees C by controlling decomposition of Fe(CO)5 and reduction of Pt(acac)2 and addition sequence of oleic acid and oleylamine. Different from the assembly of the sphere-like FePt nanoparticles, which shows 3D random structure orientation, self-assembly of the FePt nanocubes leads to a superlattice array with each FePt cube exhibiting (100) texture. Thermal annealing converts the chemically disordered fcc FePt to chemically ordered fct FePt, and the annealed assembly shows a strong (001) texture in the directions both parallel and perpendicular to the substrate. This shape-controlled synthesis and self-assembly offers a promising approach to fabrication of magnetically aligned FePt nanocrystal arrays for high density information storage and high performance permanent magnet applications.     

Building Nanocomposite Magnets by Coating a Hard Magnetic Core with a Soft Magnetic Shell

Controlling exchange coupling between hard magnetic and soft magnetic phases is the key to the fabrication of advanced magnets with tunable magnetism and high energy density. Using FePt as an example, control over the magnetism in exchange‐coupled nanocomposites of hard magnetic face‐centered tetragonal (fct) FePt and soft magnetic Co (or Ni, Fe₂C) is shown. The dispersible hard magnetic fct‐FePt nanoparticles are first prepared with their coercivity (H_c) reaching 33 kOe. Then core/shell fct‐FePt/Co (or Ni, Fe₂C) nanoparticles are synthesized by reductive thermal decomposition of the proper metal precursors in the presence of fct‐FePt nanoparticles. These core/shell nanoparticles are strongly coupled by exchange interactions and their magnetic properties can be rationally tuned by the shell thickness of the soft phase. This work provides an ideal model system for the study of exchange coupling at the nanoscale, which will be essential for building superstrong magnets for various permanent magnet applications in the future.     

Direct synthesis of fct-structured FePt nanoparticles at low temperature with assistance of poly(N-vinyl-2-pyrrolidone)

Direct synthesis of fct-structured FePt nanoparticles was successfully achieved by using poly(N-vinyl-2-pyrrolidone) as a protective reagent at lower temperature than the case using low molecular weight ligands as a protective reagent. Experimental data suggest that a transformation of FePt nanoparticles from face-centered cubic to face-centered tetragonal (fct) structure takes place at reaction temperature of 261 degrees C. The results of XRD and the magnetic properties exhibit that the FePt nanoparticles synthesized at 261 degrees C have partially ordered fct-structure and a ferromagnetic behavior at room temperature.     

Enhanced magnetic properties of self-assembled FePt nanoparticles with MnO shell

Self-assembled FePt/MnO nanoparticles with different morphology and size were synthesized with a polyol process. With the MnO coating, FePt nanoparticles exhibit a high blocking temperature and magnetic moment. The low-temperature hysteresis loop of FePt nanoparticles can be shifted through the AFM pinning of the MnO shell. The aggregation of FePt nanoparticles during the L10 phase transformation can be significantly decreased by coating with the MnO shell.     

Versatile routes toward functional, water-soluble nanoparticles via trifluoroethylester-PEG-thiol ligands

This paper reports the synthesis of a trifluoroethylester-PEG-thiol ligand (TFEE-PEG-SH) and its use to create water-soluble, chemically functional Au metal and FePt magnetic nanoparticles. The trifluoroethylester terminus facilitates attachment of any primary-amine-containing molecule via amide bond formation at room temperature without the use of coupling agents. Three possible routes of nanoparticle functionalization are demonstrated: synthesis of Au nanoparticles in the presence of functionalized R-PEG-SH; ligand-exchange of R-PEG-SH onto both Au and FePt nanoparticles; and exchange of TFEE-PEG-SH onto Au nanoparticles followed by subsequent amide condensation. A series of primary-amine-containing molecules, including biotin and fluorescamine, are easily attached to the water-soluble particles, and the resulting materials are characterized by NMR, UV-visible absorption, and emission spectroscopies.     

Accelerated color change of gold nanoparticles assembled by DNAzymes for simple and fast colorimetric Pb2+ detection

The combination of high metal selectivity of DNAzymes with the strong distance-dependent optical properties of metallic nanoparticles has presented considerable opportunities for designing colorimetric sensors for metal ions. We previously communicated a design for a colorimetric lead sensor based on the assembly of gold nanoparticles by a Pb(2+)-dependent DNAzyme. However, heating to 50 degrees C followed by a cooling process of approximately 2 h was required to observe the color change. Herein we report a new improved design that allows fast (<10 min) detection of Pb(2+) at ambient temperature. This improvement of sensor performance is a result of detailed studies of the DNAzyme and nanoparticles, which identified "tail-to-tail" nanoparticle alignment, and large (42 nm diameter) nanoparticle size as the major determining factors in allowing fast color changes. The optimal conditions for other factors such as temperature (35 degrees C) and concentrations of the DNAzyme (2 microM), its substrate (3 nM), and NaCl (300 mM) have also been determined. These results demonstrate that fundamental understanding of the DNAzyme biochemistry and nanoparticle science can lead to dramatically improved colorimetric sensors.     

单分散铁氧体纳米颗粒的生长机制与成分偏聚的透射电子显微研究

理解纳米晶的生长机制对单分散纳米晶的可控合成至关重要。本文以热分解法制备的双金属铁氧体(钴铁氧和锰铁氧)纳米颗粒为例,利用透射电子显微镜(TEM)系统研究了铁氧体纳米晶的生长机制,揭示了由此造成的成分偏聚现象。对不同时间阶段的反应产物的分析结果表明,两步加热法(即先后在相对低的温度和相对高的温度下加热反应)是制备高质量的单分散铁氧体纳米晶的关键;通过控制低温反应阶段的时间可实现纳米晶的形核阶段和生长阶段的有效分离,从而有利于单分散纳米晶的合成。利用扫描透射电子显微镜(STEM)及电子能量损失谱(EELS)谱学成像技术分析,我们进一步发现了双金属铁氧体纳米晶中的成分偏聚现象,表明双金属铁氧体纳米晶在形核阶段主要形成富Fe的核芯,而在生长阶段则形成更富Co/Mn的双金属铁氧体壳层。这些结果对制备高质量的单分散铁氧体纳米晶具有重要的指导意义,同时也有助于正确理解热分解法制备的铁氧体纳米晶的表面成分和相关表面物理化学性质。     

Size Control of FePt Nanoparticles Produced by Seed Mediated Growth Process

Monodisperse FePt nanoparticles with average size of 2.4?nm were successfully synthesized via chemical co-reduction of iron acetylacetonate, Fe(acac)₃, and platinum acetylacetonate, Pt(acac)₂, by 1,2-hexadecanediol as a reducing agent and oleic acid and oleyl amine as surfactant. Then using the seed mediated growth process smaller sized FePt nanoparticles are used as seeds for the growth of larger sized FePt particles and there is no specific limitation to achieve upper size range by this method. In this work, we could synthesize FePt nanoparticles up to 4.0?nm. Monodispersity with relatively narrow size distribution and having the same elemental composition with the atomic percentage of Fe _x Pt_100?x (x?=?63) are the main advantages of this method. As-made FePt nanoparticles have the chemical disordered face centered cubic structure with superparamagnetic behavior at room temperature. After annealing these particles become ferromagnetic with high magnetocrystalline anisotropy and their coercivity increases with increasing particle sizes and reaches a maximum value of 5,200?Oe for size of 46.5?nm     

Understanding mercapto ligand exchange on the surface of FePt nanoparticles

Tailoring the surface of nanoparticles is essential for biological applications of magnetic nanoparticles. FePt nanoparticles are interesting candidates owing to their high magnetic moment. Established procedures to make FePt nanoparticles use oleic acid and oleylamine as the surfactants, which make them dispersed in nonpolar solvents such as hexane. As a model study to demonstrate the modification of the surface chemistry, stable aqueous dispersions of FePt nanoparticles were synthesized after ligand exchange with mercaptoalkanoic acids. This report focuses on understanding the surface chemistry of FePt upon ligand exchange with mercapto compounds by conducting X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) studies. It was found that the mercapto end displaces oleylamine on the Pt atoms and the carboxylic acid end displaces the oleic acid on the Fe atoms, thus exposing carboxylate and thiolate groups on the surface that provide the necessary electrostatic repulsion to form stable aqueous dispersions of FePt nanoparticles.     

Engineered biocompatible nanoparticles for in vivo imaging applications

Iron-platinum alloy nanoparticles (FePt NPs) are extremely promising candidates for the next generation of contrast agents for magnetic resonance (MR) diagnostic imaging and MR-guided interventions, including hyperthermic ablation of solid cancers. FePt has high Curie temperature, saturation magnetic moment, magneto-crystalline anisotropy, and chemical stability. We describe the synthesis and characterization of a family of biocompatible FePt NPs suitable for biomedical applications, showing and discussing that FePt NPs can exhibit low cytotoxicity. The importance of engineering the interface of strongly magnetic NPs using a coating allowing free aqueous permeation is demonstrated to be an essential parameter in the design of new generations of diagnostic and therapeutic MRI contrast agents. We report effective cell internalization of FePt NPs and demonstrate that they can be used for cellular imaging and in vivo MRI applications. This opens the way for several future applications of FePt NPs, including regenerative medicine and stem cell therapy in addition to enhanced MR diagnostic imaging.     

Study of nucleation and growth in the organometallic synthesis of magnetic alloy nanocrystals: the role of nucleation rate in size control of CoPt3 nanocrystals

High quality CoPt(3) nanocrystals were synthesized via simultaneous reduction of platinum acetylacetonate and thermodecomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic acid and hexadecylamine as stabilizing agents. The high flexibility and reproducibility of the synthesis allows us to consider CoPt(3) nanocrystals as a model system for the hot organometallic synthesis of metal nanoparticles. Different experimental conditions (reaction temperature, concentration of stabilizing agents, ratio between cobalt and platinum precursors, etc.) have been investigated to reveal the processes governing the formation of the metal alloy nanocrystals. It was found that CoPt(3) nanocrystals nucleate and grow up to their final size at an early stage of the synthesis with no Ostwald ripening observed upon further heating. In this case, the nanocrystal size can be controlled only via proper balance between the rates for nucleation and for growth from the molecular precursors. Thus, the size of CoPt(3) nanocrystals can be precisely tuned from approximately 3 nm up to approximately 18 nm in a predictable and reproducible way. The mechanism of homogeneous nucleation, evolution of the nanocrystal ensemble in the absence of Ostwald ripening, nanocrystal faceting, and size-dependent magnetic properties are investigated and discussed on the example of CoPt(3) magnetic alloy nanocrystals. The developed approach was found to be applicable to other systems, e.g., FePt and CoPd(2) magnetic alloy nanocrystals.     

Silica encapsulation and magnetic properties of FePt nanoparticles

Core-shell nanoparticles have emerged as an important class of functional nanostructures with potential applications in many diverse fields, especially in health sciences. We have used a modified aqueous sol-gel route for the synthesis of size-selective FePt@SiO2 core-shell nanoparticles. In this approach, oleic acid and olyel amine stabilized FePt nanoparticles are first encapsulated through an aminopropoxysilane (APS) monolayer and then subsequent condensation of triethoxysilane (TEOS) on FePt particle surface. These well-defined FePt@SiO2 core-shell nanoparticles with narrow size distribution become colloidal in aqueous media, and can thus be used as carrier fluid for biomolecular complexes. In comparison, the scarce hydrophilic nature of oleic acid monolayers on FePt particle surface yields an edgy partial coating of silica when only TEOS is applied for the surface modification. The synthesized core-shell nanoparticles were characterized by direct techniques of high resolution transmission electron microscopy (HRTEM), EDS and indirectly via UV-vis absorption and FTIR studies. The FePt@SiO2 nanoparticles exhibit essential characteristics of superparamagnetic behavior, as investigated by SQUID magnetometry. The blocking temperatures (T(B)) of FePt and FePt@SiO2 (135 and 80 K) were studied using zero field cooled (ZFC)/field cooled (FC) curves.     

A fluorescent magnetic nanoalloy--Lanthanon-doped FePt:RE

Employing dibenzo-24-crown-8-ether (DB24C8) as a phase-transfer catalyst, monodispersed fluorescent lanthanon-doped magnetic FePt:RE (RE=Eu, Dy, and Ce) nanoparticles about 3 nm in size were synthesized through the reduction of H2PtCl6.6H2O, Fe2(C2O4)3.5H2O, and RE(NO3)3 (RE=Eu, Dy, and Ce) by propylene glycol using oleic acid as the stabilizer in the solvent-thermal system. The conversion of the as-synthesized chemically disordered fcc FePt:RE nanoparticles to a chemically ordered L1 0 structure occurred after annealing treatment at 873 K, and was simultaneously accompanied by a coercivity increase. It is interesting that the amorphous formation trend is strengthened in an europium-doped FePt:Eu alloy accompanied by enhancement of the coercive force. Its thermal stability indicated that the addition of europium can inhibit the phase transformation. Moreover, the optical measurement results proved that FePt:Dy alloy nanoparticles have fluorescent properties.     

Synthesis and characterization of FePt nanoparticles and FePt nanoparticle/SiO2-matrix composite films

Superparamagnetic face-centered cubic (fcc) FePt nanoparticles were synthesized using a polyol process. The effect of reaction temperature and molar ratio of Fe(CO)₅ to Pt(acac)₂ on the structure, composition and morphology of nanoparticles has been investigated. The optimum processing condition has been obtained for producing well-monodisperse fcc-phase FePt nanoparticles with the 2:1?molar ratio of Fe-Pt at 220?°C. In order to circumvent the problem of FePt particle coalescence during high temperature annealing for the L1₀ ordering, FePt nanoparticle/SiO₂-matrix composite films have been fabricated by sol?Cgel method. The experimental results confirm that the amorphous SiO₂ matrix effectively inhibits the grain growth and particle aggregation during 700?°C annealing for 1?h. Well-monodisperse face-centered tetragonal (fct) FePt particles embedded in the SiO₂ matrix can be obtained with the long-range chemical order parameter S of ~0.74, indicating partially ordered L1₀ phase transition in FePt/SiO₂ composite films. The FePt/SiO₂ system exhibits a hysteretic behavior with smaller coercive field of 1,450 Oe. The incomplete phase transition from cubic deredat height maxsium (A ₁-disordered phase to tetragonal L1₀-ordered phase) might be responsible for it.     

Homogeneous Hydrogenation Catalysis with Monodisperse,Dendrimer‐Encapsulated Pd and Pt Nanoparticles

Extraordinarily stable , monodisperse noble metal nanoparticles can be prepared by using dendrimers as both templates and stabilizers. Dendrimer‐encapsulated Pd nanoparticles (see the schematic representation) exhibit high catalytic activity for the hydrogenation of alkenes in water. The catalytic activity and selectivity of these materials can be controlled by adjusting the dendrimer generation.