Organic Phase Syntheses of Magnetic Nanoparticles and Their

Review pubs.acs.org/CR

Organic Phase Syntheses of Magnetic Nanoparticles and Their Applications Liheng Wu, Adriana Mendoza-Garcia, Qing Li, and Shouheng Sun* Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States ABSTRACT: In the past two decades, the synthetic development of magnetic nanoparticles (NPs) has been intensively explored for both fundamental scientific research and technological applications. Different from the bulk magnet, magnetic NPs exhibit unique magnetism, which enables the tuning of their magnetism by systematic nanoscale engineering. In this review, we first briefly discuss the fundamental features of magnetic NPs. We then summarize the synthesis of various magnetic NPs, including magnetic metal, metallic alloy, metal oxide, and multifunctional NPs. We focus on the organic phase syntheses of magnetic NPs with precise control over their sizes, shapes, compositions, and structures. Finally we discuss the applications of various magnetic NPs in sensitive diagnostics and therapeutics, high-density magnetic data recording and energy storage, as well as in highly efficient catalysis.

CONTENTS 1. Introduction 2. Fundamental Features of Magnetic Nanoparticles 2.1. Size Effect 2.2. Structure and Shape Effect 2.3. Composition Effect 3. Synthesis of Magnetic Nanoparticles 3.1. Formation of Monodisperse Nanoparticles 3.2. Magnetic Metal Nanoparticles 3.2.1. Fe Nanoparticles 3.2.2. Co Nanoparticles 3.2.3. Ni Nanoparticles 3.2.4. FeCo Nanoparticles 3.2.5. MPt (M = Fe, Co) Nanoparticles 3.2.6. FePd Nanoparticles 3.2.7. SmCo5 Nanoparticles 3.3. Magnetic Oxide Nanoparticles 3.4. Multifunctional Magnetic Nanoparticles 3.4.1. Core/Shell Nanoparticles 3.4.2. Dumbbell Nanoparticles 4. Magnetic Nanoparticles for Biomedical Applications 4.1. Surface Modification of Magnetic Nanoparticles 4.1.1. Ligand Exchange 4.1.2. Ligand Addition 4.1.3. Silica Coating 4.2. Magnetic Nanoparticles as MRI Contrast Agents 4.2.1. T1 MRI Contrast Agents 4.2.2. T2 MRI Contrast Agents 4.2.3. Multimodal MRI Contrast Agents 4.3. Magnetic Nanoparticles for MFH 4.3.1. Basic Principal of MFH © 2016 American Chemical Society

4.3.2. Magnetic Nanoparticles as MFH Agents 5. Magnetic Nanoparticles for Data and Energy Storage Applications 5.1. Magnetic Nanoparticles for Data Storage 5.1.1. Fundamental of Magnetic Recording 5.1.2. Magnetic Nanoparticles as Recording Media 5.2. Magnetic Nanoparticles as Building Blocks of Permanent Magnet 6. Magnetic Nanoparticles for Catalytic Applications 6.1. Magnetic Nanoparticles as Catalyst Supports 6.2. Magnetic Nanoparticles as Catalysts in Electrochemical Reactions 6.3. Magnetic Nanoparticles as Catalysts in Thermochemical Reactions 7. Conclusion Remarks Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

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1. INTRODUCTION Recent advances in synthetic methodologies have allowed the preparations of many different kinds of functional nanoparticles (NPs) with standard deviations in diameter less than 10%. Such controls achieved in NP size monodispersity have enabled more

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Special Issue: Nanoparticle Chemistry Received: November 24, 2015 Published: June 29, 2016 10473

DOI: 10.1021/acs.chemrev.5b00687 Chem. Rev. 2016, 116, 10473−10512

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size, spins of free electrons within the NP are aligned by ferromagnetic coupling into one direction and the NP acts as a single-domain magnet. The critical diameter (Dc) for a spherical magnetic NP to reach the single domain limit is

fundamental understanding of NP properties for nanotechnological applications.1−7 Magnetic NPs represent an important class of functional NPs and have been investigated extensively for their interesting nanoscale magnetism and potential applications. Back in 1930, Frenkel and Dorfman first predicted that a particle of ferromagnetic materials with the size below a critical limit would possess a single magnetic domain within which magnetic moments of free electrons are aligned parallel, resulting in a large net magnetization.8 Since then, tremendous research efforts have been devoted to the study of magnetism at the nanoscale. It has been found that reducing the sizes of ferromagnetic materials can indeed change the materials from multidomain to single domain behaviors along with the increase in coercivity (Hc).9−12 Over the years, significant progress has been made to precisely control the size of magnetic NPs via chemical synthesis. The synthesis has even led to the development of more exotic magnetic NPs with control over their shapes, compositions, structures, and multifunctionalities. Such studies have not only advanced our understanding of their unique nanoscale magnetic properties but also demonstrated great potential of these NPs in technological applications, including medical diagnostics and therapeutics,13−18 magnetic data storage,19−21 nanocomposite permanent magnet,22 and catalysis.23−27 Magnetic NPs discussed in this review are Fe-, Co-, and Nibased ferromagnetic elemental, alloy, oxide, or composite structures. They can be prepared by many different chemical methods, among which the organic phase synthesis has become a popular choice nowadays due to its ease of controlling the NP formation and of protecting these NPs against uncontrolled oxidation during the synthetic process. These NPs are typically smaller than 20 nanometers (nm) and are often in single magnetic domain, below which their surface atom percentage increases exponentially, causing drastic changes in magnetism and other properties. In this review, we first briefly introduce the fundamental features of magnetic NPs. We then focus on the chemical syntheses, specifically the organic phase syntheses, of a series of magnetic NPs with controlled sizes, shapes, compositions, and structures. Finally we discuss various applications of these magnetic NPs. In particular, we review recent developments on using monodisperse magnetic NPs for sensitive diagnostics and therapeutics, for high-density magnetic data recording and energy storage, as well as for highly efficient catalytic reactions that relate to energy conversions. As this review focuses mainly on the organic phase syntheses of magnetic NPs and their applications, readers who are interested in knowing other synthetic methods are recommended to read some representative review articles published previously.16,17,28

estimated to be Dc ≈

36 AK , μ0 Ms2

where A is the exchange constant,

K is the effective anisotropy constant that measures the energy per unit volume required to flip magnetization direction, μ0 is the vacuum permeability, and Ms is the saturation magnetization.32 Dc for most magnetic materials lies in tens to hundreds of nanometers, but for the one with an extremely large K, its single domain size can be close to the micrometer regime. The Ms, K, and Dc values of some typical magnetic materials are listed in Table 1.19,29,31,33 Table 1. Magnetic Properties of Some Typical Magnetic Materials materials

Ms (emu cm−3)

K (106 J m−3)

Dc (nm)

Fe Co Ni Fe3O4 L10-FePt SmCo5

1745 1400 490 460 1140 910

0.048 0.45 −0.005 −0.011 7 20

15 70 55 128 60 750

Different from a multidomain NP in which the alignment of magnetization directions within each domain is controlled by both anisotropy energy KV (V is the domain volume) and domain wall motion, in a single domain NP, there exists no domain wall motion and magnetization reversal is mostly dependent on KV. As a result, the single domain NPs can have higher Hc than the multidomain counterparts. The sizedependent Hc of ferromagnetic NPs is shown in Figure 1.29

Figure 1. Schematic illustration of size-dependent H c of a ferromagnetic particle. Reprinted from ref 29. Copyright 1996 American Chemical Society.

2. FUNDAMENTAL FEATURES OF MAGNETIC NANOPARTICLES One of the most prominent features of magnetic NPs is their size-dependent magnetic properties. In addition to the size, the fine control of magnetic NPs on the shape, composition, and structure enables further tuning of their magnetic properties. Here, we discuss briefly the fundamental features of magnetic NPs. More detailed descriptions on magnetism at the nanoscale can be found in the related textbooks, as well as in some representative review articles.29−31

As the NP size continues to decrease, however, thermal energy kBT (kB is Boltzmann constant and T is temperature) starts to compete with KV (V is the volume of single-domain NP),34 causing the reduction of Hc. When the NP size is further reduced to a critical value at Ds, kBT overtakes KV, leading to a spontaneous magnetization reversal. At this state, these NPs become superparamagnetic; they show no coercivity but can still have high Ms that is close to the value of their larger ferromagnetic counterparts. The size-dependent Hc changes have been well-demonstrated in many magnetic NP systems.35−37 Figure 2A gives just one example of the study on Co0.6Fe2.4O4 NPs, showing that the reduction of the NP size from 20 to 10 nm leads to the Hc

2.1. Size Effect

Magnetic NPs display unique size-dependent magnetic properties. When the size of a magnetic NP is below a certain critical 10474


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Figure 2. (A) Transmission electron microscopy (TEM) images of Co0.6Fe2.4O4 NPs with different sizes and their corresponding size-dependent Hc. Reprinted from ref 37. Copyright 2014 American Chemical Society. (B) TEM images of Fe3O4 NPs with different sizes and their corresponding sizedependent Ms. Reprinted from ref 39. Copyright 2005 American Chemical Society.

decrease from 810 to 0 Oe.37 The size reduction also affects the saturation magnetization (Ms) of the magnetic NPs. In general, smaller magnetic NPs often have a larger degree of spin-canting effect.38 Figure 2B shows the size-dependent Ms of monodisperse Fe3O4 NPs.39 As the size of the Fe3O4 NPs increases from 4 to 12 nm, the Ms increases correspondingly from 25 emu g−1Fe to 101 emu g−1Fe. Such spin-canting effect may aggravate further by NP’s surface binding with a surfactant, as demonstrated in surfactant-induced surface spin quenching on the Ni NPs.40,41 The smaller the NPs, the more noticeable the spin canting/quenching effect. The magnetization reversal behaviors of magnetic NPs are temperature-sensitive. The time scale of the thermal fluctuation of the NP magnetization direction is measured by Néel

( ), where τ

relaxation time: τN = τ0 exp −9

42

KV kBT

0

erally, the larger the anisotropy constant, the larger the coercivity. This is demonstrated in the FePt alloy system (Figure 3).44 The equiatomic FePt alloy can adopt two different

is a constant at

10 s. For a specific measurement time (τm, about 100 s for a typical laboratory measurement condition), if the relaxation time τN of superparamagnetic NPs is shorter than τm, the NPs should show superparamagnetic properties. In contrast, if τN is longer than τm, the NP magnetization direction cannot complete the reversal process during the measurement period and the measured magnetization value is not zero. In this case, the superparamagnetic NPs are in the so-called “blocked” state. The temperature at which τm = τN is called the blocking KV temperature (TB), which is given by the equation: TB = k ln τm . B

Figure 3. (A and B) Schematics of the local structures of (A) A1-FePt and (B) L10-FePt. Reprinted with permission from ref 44. Copyright 2006 Wiley-VCH. (C and D) Magnetic hysteresis loops of (C) superparamagnetic and (D) ferromagnetic NPs.

τ0

The size-dependent TB change is demonstrated in hexagonalclose-packed (hcp) Co NPs of 3, 6, 8, and 11 nm, which show the TB of 15, 50, 120, and 260 K, respectively.43 The τN and TB are important parameters used to select superparamagnetic NPs to achieve the desired magnetic heating efficiency under an alternating magnetic field, which will be discussed in section 4.

crystal structures. One is the solid solution type face-centered cubic (fcc) structure (commonly referred to as A1 structure, Figure 3A), within which the Fe and Pt atoms occupy randomly the fcc lattice points. Such a cubic structure is magnetically isotropic and soft. The other is the chemically ordered L10 structure (Figure 3B), with Fe and Pt forming the alternate stacking along the [001] direction.45,46 Crystallographically, the L10-FePt has a primitive tetragonal lattice (space group P4/ mmm). In literature, however, the L10-FePt structure is also called the face-centered-tetragonal (fct) or the body-centered-

2.2. Structure and Shape Effect

Magnetic properties of NPs are strongly dependent on their structure. Crystal structure affects intrinsic spin−orbital interaction, which is often expressed as magnetocrystalline anisotropy. The magnitude of magnetocrystalline anisotropy is determined by magnetocrystalline anisotropy constant. Gen10475


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tetragonal (bct) structure to highlight the local ordering of Fe and Pt. The bct-structure has a smaller unit containing only one Fe and one Pt. But from the magnetism point of view, the fctstructure highlights more clearly the anisotropic interaction between Fe and Pt along one crystallographic direction. To avoid these definition confusions and to align our definition with most publications, in this review article, we use L10-FePt to represent the chemically ordered FePt structure. Due to the strong 3d (Fe) and 5d (Pt) coupling, the L10-FePt has a large anisotropy constant K (up to 7 × 106 J/m3). When prepared in sub-10 nm, the A1-FePt NPs are superparamagnetic (Figure 3C), while the L10-FePt NPs are strongly ferromagnetic (Figure 3D); they belong to a class of the strongest nanomagnets ever developed for important magnetic applications.47−50 Similar to the FePt, metallic Co NPs also show the structure-dependent magnetic properties with the K value increased from fcc-Co to hcp-Co.51 Magnetic NPs also show shape-dependent magnetic properties. Spherical NPs have no shape anisotropy due to their isotropic structure. However, when magnetic NPs are made in one-dimensional shape, their magnetic easy axis can be aligned longitudinally. Theoretical calculation indicates that an increase in the aspect ratio of Fe NPs from 1.1 to 1.5 and 2.0 enhances the coercivity by 4 and 7 times, respectively.52 Shape-anisotropy enhanced magnetic coercivity has been experimentally demonstrated in Co nanorods with various aspect ratios.53−55 Compared to superparamagnetic Co NPs, the Co nanorods with the diameter of 15 nm and aspect ratio of 10 have an Hc of 4500 Oe at room temperature.55 The shape anisotropy effect is also essential for the FeCo alloy nanorods to serve as magnetic media in high density magnetic tape recording.56

Figure 4. (A) TEM images of 15 nm (ZnxMn1−x)Fe2O4 NPs. (B) TEM images of 15 nm (ZnxFe1−x)Fe2O4 NPs. (C) Schematic illustration of magnetic spin alignment in a (ZnxFe1−x)Fe2O4 NP with different Zn2+ doping under an applied magnetic field. (D) Zn2+ doping dependent Ms changes in (ZnxM1−x)Fe2O4 (M = Fe, Mn) NPs. Reprinted with permission from ref 58. Copyright 2009 Wiley-VCH.

magnetization values is critical for developing sensitive magnetic probes for biomedical applications. The composition control in magnetic NPs can be used to tune not only the magnetization values but also the coercivity. For example, in the FePt alloy NPs, their magnetic properties are strongly affected by the Fe/Pt atomic ratio. Studies on the composition-dependent magnetic properties of the FexPt1−x NPs with x from 0.3 to 0.8 indicate that the maximum coercivity can be obtained at x = 0.55.59,60 Similar theoretical and experimental investigations on bulk CoxFe3−xO4 also reveal that the coercivity of the cobalt ferrite increases with x from 0 to 0.7 but decreases when x is beyond 0.7, which is validated in the CoxFe3−xO4 NPs.61,62

2.3. Composition Effect

In multicomponent magnetic NP systems, magnetic properties of the NPs can be tuned by their compositions. This is seen in the magnetic ferrite MFe2O4 (M = Fe, Co, Ni, andMn) with an inverse spinel structure.57 In the Fe3O4 structure, O anions form close-packed fcc structure, Fe ions locate in parts of the interstitial octahedral (O) and tetrahedral (T) sites. The Fe3O4 structure can be better written as [Fe3+]T[Fe2+Fe3+]OO4. In this structure, the Fe−O−Fe bonding for the Fe3+ in T- and O-sites results in antiferromagnetic coupling and cancellation of magnetic moments of Fe3+. As a result, the total magnetic moment of the Fe3O4 structure comes from the net magnetic moments of Fe2+. By substituting Fe2+ (d6) with Mn2+ (d5), Co2+ (d7), or Ni2+ (d8), the net magnetizations of the MFe2O4 can be engineered from 4 μB to 5 μB, 3 μB, and 2 μB, respectively (μB, Bohr magneton, a physical constant and the natural unit for the magnetic moment of an electron). Experimentally, the same trend has been validated in a series of monodisperse MFe2O4 NPs with the same size of 12 nm.57 The saturation moments are 110, 101, 99, to 85 emu g−1metal for the MnFe2O4, Fe3O4, CoFe2O4, and NiFe2O4 NPs, respectively. The magnetization values of the MFe2O4 NPs can be further enhanced by doping Zn2+ into the structure.58 Once Zn2+ is doped in the T-site of the unit, the antiparallel exchange coupling between Fe3+ can be partially alleviated. The Ms of 15 nm ZnxFe3−xO4 NPs increases from 114 to 161 emu g−1metal for x = 0 to x = 0.4, and further decreases to 115 emu g−1metal for x = 0.8 due to the dominant antiferromagnetic coupling between Fe3+ ions at higher Zn2+ doping (Figure 4).58 The magnetization values can be further enhanced by substituting part of Fe2+ with Mn2+ ions. This composition engineering of NP

3. SYNTHESIS OF MAGNETIC NANOPARTICLES As the properties of magnetic NPs are strongly dependent on their dimensions, compositions, and structures, extensive efforts have been made to synthesize these NPs with fine controls over the size, shape, composition, and structure. Magnetic NPs can be prepared in hydrolytic (e.g., aqueous) solutions via the sol− gel process,63−67 the coprecipitation method,68−70 the microwave-assisted synthesis,71−75 and the hydrothermal reaction,76−81 which have been well-summarized.16,17,28 Magnetic NPs can also be produced from reactions in organic solutions. Recent studies have shown that the organic phase synthesis of magnetic NPs possess some distinct advantages over the conventional hydrolytic one: it provides a chemically inert reaction environment for active metal atoms to nucleate and grow; it allows the reaction to proceed at a temperature up to the boiling point of the organic solvent used (often in the range between 180 and 350 °C) at atmospheric pressure for much tighter control on NP uniformity and structure; and it allows the preparation of magnetic NPs with multifunctionalities. As thus, the organic phase synthesis has been applied to prepare magnetic NPs of Fe-, Co-, and Ni-based elemental, alloy, oxide, core/shell, and dumbbell structures. 10476


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Table 2. Common Reactants Used to Prepare Some Representative Magnetic NPsa NPs Fe

Co

Ni

FeCo

FePt

CoPt3 Fe3O4

MnFe2O4 CoFe2O4 Zn0.4Fe2.6O4 FeO a

precursors/reagents

surfactants

solvents

ref

Fe(CO)5 Fe(CO)5 Fe[N(SiMe3)2]2 Co2(CO)8 Co2(CO)8 Co2(CO)8 CoCl2/Superhydride Ni(OOCCH3)2 Ni(acac)2 Ni(acac)2 Fe(CO)5, Co2(CO)8 Fe(CO)5, Co(N(SiMe3)2)2 Fe(acac)3, Co(acac)2 Fe(CO)5, Pt(acac)2/HD Fe(CO)5, Pt(acac)2 FeCl2, Pt(acac)2/superhydride Fe(CO)5, Pt(acac)2 Co2(CO)8, Pt(acac)2 Fe(acac)3/HD Fe(acac)3 Fe(III)-oleate Fe(acac)3 Mn(acac)2, Fe(acac)3/HD MnCl2, Fe(acac)3 Co(acac)2, Fe(acac)3/HD CoCl2, Fe(acac)3 ZnCl2, Fe(acac)3 Fe(acac)3

OAm OAm, HDA•HCl OAc, HDA OAc, TBP OAc, DOA OAc, TOPO OAc, TBP OAc, TBP, TBA OAm, TOP OAm, TOP OAc, TOP OAc, HDA OAc, OAm OAc, OAm OAc, OAm OAc, OAm OAm ACA, HDA OAc, OAm OAm − OAc OAc, OAm OAc, OAm OAc, OAm OAc, OAm OAc, OAm OAc, OAm

1-octadecene 1-octadecene mesitylene diphenyl ether tetralin 1,2-dichlorobenzene dioctyl ether diphenyl ether TOP benzyl ether 1,2-dichlorobenzene toluene OAm dioctyl ether benzyl ether diphenyl ether 1-octadecene diphenyl ether diphenyl ether benzyl ether 1-octadecene benzyl ether benzyl ether dioctyl ether benzyl ether dioctyl ether dioctyl ether OAm

87 88 113 117 118 91 123 43 92 128 139 140 141 59 170 153 174 158 93 206 95 35 94 57 94 57 58 237

See the abbreviations at the end of this review.

LaMer and Dinegar, the synthesis of monodisperse NPs involves several consecutive stages, as illustrated in Figure 5.84

The organic phase reaction is often run in a high boiling point organic solvent, such as 1-octadecene, diphenyl ether, benzyl ether, 1,2-dichlorobenzene, 1,2,3,4-tetrahydronaphthalene (tetralin), or even liquid alkylamine. In the reaction system, appropriate surfactant(s) should also be present to stabilize the formed NPs. These surfactants are common longchain aliphatic acids (e.g., oleic acid), amines (e.g., oleylamine), phosphines [e.g., trialkylphosphine (TOP)], or phosphine oxides [e.g., trialkylphosphine oxide (TOPO)]. The important factor in choosing a proper surfactant is the binding chemistry between the functional group of the surfactant and the surface of the NPs. Although there are no specific rules to follow, the chemical binding concept based on “Pearson acid base” or more commonly “hard and soft acid and base” can be used as a general guideline for choosing the right surfactant(s) for the synthetic purpose. The size, shape, and composition of the NPs are controlled by one or more reaction parameters such as reactant concentration, solvent polarity, and reaction temperature/time. Organic phase syntheses of inorganic NPs have been nicely reviewed.1,6,82,83 Herein, we highlight the syntheses of monodisperse magnetic NPs (standard deviation in diameter σ < 10 %). The metal precursors, surfactants, and solvents used to prepare some representative magnetic NPs are summarized in Table 2.

Figure 5. LaMer model of the nucleation and growth process to monodisperse NPs. Reprinted from ref 84. Copyright 1950 American Chemical Society.

In the synthetic solution, when the concentration of “monomers” increases to the nucleation threshold (stage I), nuclei start to form.85 At stage II, the “burst nucleation” takes place, resulting in the formation of nuclei and at the same time the rapid decrease of the monomer concentration. When the concentration drops to the level below the nucleation threshold in stage III, no new nuclei form. The reaction is then at the growth stage, in which the system free energy drives the attachment of monomer to the preformed nuclei. The growth process lasts until the monomer concentration drops below the

3.1. Formation of Monodisperse Nanoparticles

The formation mechanism of monodisperse NPs, especially magnetic NPs, has not been fully understood due to the complex reaction kinetics involved in the organic phase reduction or decomposition chemistry. On the basis of the classical model of burst nucleation and growth proposed by 10477


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and crystalline phase of the as-synthesized NPs can be obtained.101 For example, the nucleation and growth of Au NPs were studied by in situ X-ray scattering.102 By quantifying the X-ray scattering data, the number of NPs and their size distribution were determined on an absolute scale and the effects of different surfactants on the nucleation rate were evaluated. The method was also used to study the growth mechanism of the CdSe NPs.103 From the NP size distribution and NP concentration data obtained, the growth of the NPs was thought to be limited by the thermal activation of the Se precursor. Using the in situ X-ray scattering, the fast nucleation and growth kinetics of Ag NPs were studied.104 It is believed that these in situ techniques will help to better understand the formation mechanism of NPs and can certainly be extended to study the growth of magnetic NPs.

saturation level. In order to synthesize monodisperse NPs, it is critical to shorten the nucleation process and to maintain the uniform rate of growth around each nucleus. To achieve the efficient “burst nucleation” in the organic phase synthesis, two methods have been developed. One method relies on “hot-injection” that was introduced in the synthesis of monodisperse CdE (E = S, Se, and Te) NPs.86 The rapid injection of reaction precursors at a preset high temperature can result in the burst nucleation of CdE, which is followed by controlled growth to form monodisperse CdE NPs, as illustrated in Figure 6.83 This “hot-injection” method

3.2. Magnetic Metal Nanoparticles

3.2.1. Fe Nanoparticles. Fe is an important ferromagnetic material with high mass magnetization value at ∼220 emu g−1. Fe NPs have long been known and studied for many applications, including the one as the recording media for the early stage magnetic tape recording.105 Many methods have been utilized to synthesize Fe NPs, including the reduction of an iron salt by a reducing agent (e.g., sodium borohydride) in an aqueous solution.106,107 But the chemical reactivity of Fe makes it difficult to achieve the size and phase purity control in the synthesis. Industrially, metallic Fe NPs are mass-produced via the dehydration of iron hydroxide to iron oxide, followed by thermal reduction in H2.105 But this large-scale synthesis cannot provide Fe NPs with the desired uniformity. Alternatively, Fe NPs can be synthesized by thermal decomposition of iron pentacarbonyl, Fe(CO)5. In the early stage of the study, the decomposition was run in a polymer matrix to prevent the formed NPs from aggregation.108 The need of monodisperse Fe NPs motivated the studies of the synthesis via thermal decomposition of Fe(CO)5 in organic solution phase, as demonstrated in the synthesis of monodisperse Fe NPs with controlled size from 5 to 20 nm via the decomposition of Fe(CO)5 in dioctylether with oleic acid and oleylamine as surfactants.109 However, the Fe NPs prepared from this method were not well crystallized and were readily oxidized into iron oxides when the NPs were exposed to air. To improve the oxidation resistance of Fe NPs, the as-synthesized Fe NPs (Figure 7A) were oxidized into the core/shell Fe/Fe3O4 NPs by a controlled amount of trimethylamine N-oxide (Me3NO) at the NP synthesis temperature.87 These core/shell Fe/Fe3O4 NPs with a layer of crystalline Fe3O4 (Figure 7B) showed much robust stability against air oxidation in the ambient environment. To further improve the chemical stability of the Fe NPs, it is better to have these Fe NPs well-crystallized. The crystalline body-centered-cubic (bcc) Fe NPs were synthesized via the decomposition of Fe(CO)5 in the presence of hexadecylamonium chloride (HDA·HCl) (Figure 7C).88 The presence of Cl− in HDA·HCl was believed to favor the thermodynamic growth of crystalline Fe NPs. These bcc-Fe NPs were much more stable than the amorphous Fe NPs prepared in the absence of HDA·HCl, showing higher magnetic moment (50% improvement) and more robust chemical stability (only 20% mass magnetization value loss after 1 month of air exposure). Further studies indicated that the chloride ion (Cl−) present in the reaction solution helped to mediate the thermal decomposition of Fe(CO)5 and growth of bcc-Fe NPs due to the “strong” Cl−

Figure 6. “Hot-injection” synthesis of monodisperse NPs. Reprinted with permission from ref 83. Copyright 2007 Wiley-VCH.

has been applied to synthesize many magnetic NPs, including monodisperse Fe,87,88 Co,89−91 and Ni NPs.92 The second method utilizes the “heating-up” approach, in which the precursors, surfactants, and solvent are mixed together at room temperature and the mixture is then heated to a desired temperature to initiate the nucleation process and to control the NP growth. Compared to the “hot injection” method, the “heating-up” one is more convenient for the large-scale synthesis as it does not require the sudden introduction of a large volume of reactant solution into the reaction system.93−95 The “heating-up” process suits even better the synthesis of more complex NPs, such as core/shell and dumbbell NPs, as the formation of such composite structures normally needs heterogeneous nucleation and growth of the second component on the preformed seeding monodisperse NPs.96,97 Understanding the fundamentals of nucleation and growth is critical for the synthesis of NPs in a more controlled and predictive way. However, it has been proven to be difficult to study the nucleation and growth process due to the lack of experimental techniques to capture and to analyze the desired intermediates. In the past few years, the fast development of in situ imaging techniques offers a great opportunity to probe realtime NP formation in a liquid solution. This was demonstrated in the in situ TEM observation of the dynamic growth of Pt NPs.98 The study revealed that the growth of Pt NPs was via both the attachment of monomeric species following the classical growth model and the direct NP growth from particle−particle coalescence. The size distribution of the final NP product was narrowed through the combination of these two growth processes. This in situ technique was also applied to study the growth kinetics of one-dimensional Pt3Fe nanorods, which were formed through the growth of winding polycrystalline NP chains by the shape-directed NP attachment, followed by straightening and orientation.99 Further development in in situ TEM using a graphene liquid cell even allowed atomic-level resolution imaging for visualizing the critical steps involved in the formation of NPs.100 Another emerging method developed to analyze the NP growth in real time is the in situ synchrotron X-ray scattering technique, from which the information related to the number, size, shape, size distribution, 10478


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from 13 to 30 nm were synthesized at different HDA/palmitic acid ratios.114,115 3.2.2. Co Nanoparticles. The bulk Co usually adopts either an fcc or a hcp structure.116 In a thermodynamic equilibrium condition, the fcc structure is preferred above 425 °C while the hcp one is favored at a lower temperature. But within a Co NP, both phases can coexist at room temperature. Monodisperse Co NPs with multitwinned fcc structures were synthesized by thermal decomposition of Co2(CO)8 in diphenylether with oleic acid and tributylphosphine (TBP) as surfactants.117 In this synthesis, TBP was used to control Co nucleation and growth and oleic acid was used for Co NP stabilization. When TBP was replaced by dioctylamine, the similar decomposition reaction at 208 °C in tetralin led to the formation of monodisperse polycrystalline Co NPs.118 Here the weaker amine surfactant was used to promote the decomposition of Co2(CO)8 and to facilitate the coating of the Co NPs with a layer of iron oxide. The Co NP sizes were controlled to be in the range of 8−12 nm by the amount of oleic acid added in the reaction system. Alternatively, polycrystalline Co NPs could be produced in a reverse micelle by reducing cobalt(II) bis(2-ethylhexyl) sulfosuccinate [Co(AOT)2] with NaBH4.119 Interestingly, these polycrystalline Co NPs (Figure 8A) could be converted to hcp-Co NPs (Figure

Figure 7. TEM images of different Fe NPs prepared from organic phase reactions. (A) Amorphous Fe NPs showing a thin layer of amorphous Fe3O4 coating due to the uncontrolled surface oxidation by air. (B) 5 nm/5 nm Fe/Fe3O4 NPs prepared by controlled surface oxidation of the amorphous Fe NPs into the crystalline Fe3O4. (C) 10.5/2.5 nm bcc-Fe/Fe3O4 NPs. (D) An assembly of 7 nm cubic Fe NPs. (A and B) Reprinted from ref 87. Copyright 2006 American Chemical Society. (C) Reprinted from ref 88. Copyright 2011 American Chemical Society. (D) Reprinted with permission from ref 113. Copyright 2004 American Association for the Advancement of Science.

binding to Fe. The halide effect on the formation of bcc-Fe NPs was further supported by the preparation of the bcc-Fe NPs from the decomposition of Fe(CO)5 in the presence of cetyltrimethylammonium bromide,110 NH4Cl, or NH4Br.111 Recent studies indicate that Fe(CO)5 as a reaction precursor has another benefit in the formation of iron carbide. Serving as both the Fe and C sources, Fe(CO)5 was used to prepare core/ shell Fe/FexC NPs via the reaction between the preformed Fe NPs and Fe(CO)5 at 150 °C under an Ar atmosphere.112 This FexC shell acts as an effective protection layer over the metallic Fe core, and the Fe/FexC NPs have even higher mass magnetization values than the Fe/Fe3O4 NPs. Similarly, reacting Fe NPs with octadecylamine leads to Fe5C2 NPs, indicating that Fe NPs can react with a proper C source to form iron carbide NPs for magnetic and catalytic applications.110 Despite the success of using Fe(CO)5 as a precursor to prepare Fe NPs, Fe(CO)5 does have its limitation: it is a wellknown toxic carbonyl complex and its thermal decomposition chemistry can be very complicated due to the unequal Fe-CO bonding. To avoid this complication, a new Fe-complex Fe[N(SiMe3)2]2 was designed and prepared as the precursor for the synthesis of Fe NPs.113 The reductive decomposition of Fe[N(SiMe3)2]2 under a H2 atmosphere at 150 °C in the presence of HDA and oleic acid yielded high quality 7 nm bccFe NPs with cubic shape (Figure 7D). Self-assembly of these nanocubes gave three-dimensional (3D) superlattices with the cubes arranged face-to-face due to the dominated hydrophobic interactions among the cubic faces. With the use of palmitic acid instead of oleic acid, uniform Fe NPs with controlled sizes

Figure 8. TEM images of different Co NPs prepared from organic phase reactions. (A) 7 nm polycrystalline Co. (B) 7 nm hcp-Co NPs obtained from the solution phase annealing of the polycrystalline Co. (C) 9 nm ε-Co NPs prepared from superhydride reduction of CoCl2. Inset: High-resolution TEM (HR-TEM) image of a single NP. (D) 10 nm ε-Co NPs prepared from the thermal decomposition of Co2(CO)8. (A and B) Reprinted from ref 121. Copyright 2012 American Chemical Society. (C) Reprinted with permission from ref 123. Copyright 1999 American Institute of Physics. (D) Reprinted with permission from ref 91. Copyright 2001 American Association for the Advancement of Science.

8B) via the solution-phase annealing in dioctylether.120,121 The hcp-Co NPs could also be synthesized directly from the high temperature reduction of Co(CH3COO)2·4H2O by 1,2dodecanediol in diphenylether (or dioctylether) in the presence of oleic acid and TOP.43,117 In this synthetic condition, increasing the amount of surfactants led to smaller hcp-Co NPs 10479


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synthesis monodisperse Ni NPs include NiAc2 (Ac = acetate), Ni(acac)2 (acac = acetylacetonate), Ni(oleate)2, or even Ni(COD)2 (COD = cycloocta-1,5-diene). NiAc2 was often a choice for the synthesis of Ni NPs via polyol reduction in the surfactant mixture of oleic acid, TBP, and tributylamine (TBA) in diphenylether at 250 °C.43 The size of the Ni NPs can be controlled by the bulkiness of the surfactants. Ni(acac)2 was used to prepare ultrasmall (3 nm) Ni NPs by borane tributylamine (BTB) reduction.127 In this synthesis, oleylamine was found to be good as both solvent and surfactant. Monodisperse Ni NPs were also prepared by “hot-injection” of oleylamine solution Ni(acac)2 into the phosphine solvent at 215 °C and could be assembled easily into a 2D or 3D superlattice (Figure 9A).92 Alternate to the “hot injection”

(3−6 nm), while substituting TOP with TBP resulted in larger hcp-Co NPs (10 to 13 nm). Here the bulkiness of the phosphine cosurfactant was attributed to the size control, the bulkier the surfactant, the smaller the NPs. In addition to the fcc and hcp structures, Co NPs were found to adopt a metastable ε-structure.122 This structure is isomorphic with the β phase of Mn, containing 20 Co atoms in its unit cell and being magnetically softer than the hcp-Co.51 Monodisperse ε-Co NPs were synthesized by reduction of CoCl2 with superhydride (LiBEt3H, Et = ethyl) at 200 °C in dioctylether in the presence of oleic acid and trialkylphosphine (Figure 8C).123 In the synthesis, superhydride was transferred from tetrahydrofuran (THF) into dioctylether before it was injected into the reaction solution. The Co NP size was controlled from 2 to 11 nm by the steric effect of TOP (for smaller NPs) or TBP (for larger NPs). The ε-Co NPs synthesized from this reaction were finally stabilized by oleic acid and were uniform in both size and shape, allowing their spontaneous self-assembly into the 2D or 3D magnetic superlattice,123 which served as an excellent model system for studying magneto-transport.90 The metastable ε-Co NPs could be converted into more stable hcp-Co NPs at 300 °C and fccCo NPs at 500 °C along with their magnetic property changes from magnetically soft ε-Co to hard hcp-Co and then to soft fcc-Co.123 Monodisperse ε-Co NPs could also be prepared from the thermal decomposition of Co2(CO)8 in 1,2-dichlorobenzene with oleic acid and TOPO as the surfactants.91 The interesting observation from this synthesis was that ∼10 s after the injection of the carbonyl precursor, hcp-Co nanodisks were formed in the reaction solution. However, these nanodisks were not stable in the reaction/growth condition and were converted into spherical ε-Co NPs (Figure 8D). By changing the precursor/surfactant ratio and reaction time, the NP size was controlled from 3 to 17 nm.124 Further study indicated that TOPO played an important role in the kinetic growth of the Co into the hcp-Co nanodisks and further to ε-Co NPs likely due to the weak TOPO binding to Co.89 On the contrary, due to the strong binding between oleic acid and Co, the amount of oleic acid significantly affect the formation of Co NPs. If the amount of oleic acid was much more than that of Co-carbonyl precursor used for the reaction, then Co NPs could not be formed, rather the growth stopped at the cluster stage.125 Monodisperse Co NPs could also be prepared by a simple heating-up process in which all reagents were dissolved in the solvent followed by rapidly heating the reaction mixture to the desired temperature.126 More interesting development in the synthesis of Co NPs was the preparation of Co nanorods via a new reductive decomposition of [Co(η3-C8H13)(η4-C8H12)] under a H2 atmosphere in anisole at 150 °C using HDA and an alkyl acid as surfactants.53,54 The aspect ratios of the Co nanorods were controlled by the length of the alkyl chain of the alkyl acid. These nanorods provided ideal building blocks for studying shape-induced nanorod assembly and anisotropic magnetic properties. 3.2.3. Ni Nanoparticles. Unlike in the syntheses of Fe and Co NPs where both metal carbonyl and metal salt can be selected as a proper metal precursor, the Ni-precursors that are suited for the synthesis of Ni NPs is very limited. Ni(CO)4 is a well-known carbonyl complex and is thermally unstable. But its extremely high toxicity prevents its routine use as the Niprecursor for the NP synthesis. A proper Ni-salt is often selected as the precursor. The common salts used for the

Figure 9. TEM images of (A) 5 nm Ni NPs, (B) 8 nm Ni NPs, and (C) 12 nm Ni nanocubes. (A) Reprinted with permission from ref 92. Copyright 2005 Wiley-VCH. (B) Reprinted with permission from ref 128. Copyright 2013 American Association for the Advancement of Science. (C) Reprinted from ref 129. Copyright 2012 American Chemical Society.

approach, mixing Ni(acac)2 with oleylamine and TOP in benzyl ether and controlling the heating from 100 to 230 °C at a heating rate of 40 °C/min could also lead to monodisperse Ni NPs (Figure 9B).128 When Ni(COD)2 was used as a precursor and HDA as a surfactant, Ni nanorods were synthesized.41 The surfactant HDA was believed to be able to direct the anisotropic growth of Ni in the reaction solution. In the presence HDA and TOP, the reduction of Ni(acac)2 under the 1 bar H2 atmosphere produced 12 nm Ni cubes (Figure 9C).129 The amount of TOP was found to be critical for the cube formation as too much TOP could modify the Ni NP growth into spherical 5 nm NPs. Ni normally adopts the fcc structure, but a unconventional hcp-Ni NPs have been prepared by thermal decomposition of Ni(oleate)2 in 1-octadecene at 300 °C.130 Further study showed that the hcp-No NP quality (size, shape, and phase purity) could be further improved by reducing Ni(acac)2 at 140 °C in the mesitylene solution of HDA and TOP under the 10 bar H2 atmosphere.131,132 It is worth mentioning that although TOP has been widely used in the synthesis of monodisperse Ni NPs, the strong binding between Ni and P could lead to the contamination of P in the Ni NP.133 10480


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uniform alloy MPt NPs. This synthetic challenge was finally overcome by a carefully designed high temperature organic phase reaction between Fe(CO)5 and Pt(acac) 2 in a dioctylether solvent. In the synthesis, 59 Pt(acac) 2 was first mixed with dioctylether, 1,2-hexadecanediol. Then the mixture was stirred and heated to 100 °C under a gentle flow of N2 or Ar to remove air and moisture from the reaction system. About 30 min later, oleylamine and oleic acid were added and the reaction solution was protected under a blanket of N2 or Ar. A premeasured volume of Fe(CO)5 was added by a syringe though a septa rubber, and the stirred solution should then be heated up to the boiling point (close to 300 °C) and kept for about 30 min for the reaction to proceed. In this synthesis, the Fe(CO)5 decomposition chemistry and Pt(acac)2 polyol reduction chemistry were combined to form FePt alloy NPs. Oleic acid and oleylamine were introduced to protect the FePt NPs. In general, the FePt compositions were controlled by the molar ratios of Fe(CO)5/Pt(acac)2. An excess of Fe(CO)5 was required to obtain FePt with Fe close to or surpassing 50% due to the Fe(CO)5 vapor separation from the reaction system and the extra Fe consumption by oleic acid. The synthesis produced monodisperse 4 nm FePt NPs which could be further enlarged by the seed-mediated growth. As a result, a series of FePt NPs with controlled sizes and Fe/Pt composites were prepared. Figure 10A shows a representative TEM image of the 6 nm FePt NPs. Further improvement of the synthesis indicated that the larger FePt NPs (up to 10 nm) could be produced more easily via a one-step reaction without the presence of the alkanediol reducing agent.144 More detailed studies indicated that the final FePt alloy NPs were formed via heat-induced

In an extreme case, the TOP could induce the total conversion of Ni NPs to Ni2P NPs.134 3.2.4. FeCo Nanoparticles. A FeCo alloy is known to have the highest magnetic moment among the well-known magnetic elements. Its Ms can reach 245 emu g−1, making the NPs of the FeCo alloy extremely promising for high moment magnetic applications.56,135 Synthetically, it is more challenging to control the size and composition of these FeCo alloy NPs than the single component Fe or Co NPs due to their different nucleation and growth processes.136−138 But the right control of an organic phase reaction can still yield good quality FeCo NPs. The obvious choice of metal precursors for the synthesis of FeCo NPs is Fe- and Co-carbonyls. This has been demonstrated in the synthesis of monodisperse FeCo NPs via the simultaneous thermal decomposition of Fe(CO)5 and Co2(CO)8 in 1,2-dichlorobenzene.139 However, the slower decomposition rate of Fe(CO)5 made it difficult to control the composition in the final FeCo NPs. To overcome this problem, a new Co-precursor Co(η3-C8H13)(η4-C8H12) or Co(N(SiMe3)2)2 was developed to react with Fe(CO)5 under H2 in the presence of HDA and oleic acid to form FeCo NPs.140 In this case, the Fe/Co ratios were much better controlled, and Fe-rich FeCo alloy NPs could be obtained. Interestingly, when these FeCo NPs were annealed under Ar at 500 °C for 30 min, a thin carbon shell was coated around each FeCo NP, efficiently protecting the FeCo NPs against fast oxidation. The FeCo/C NPs showed a stable Ms of 220 emu g−1 even after their exposure to air for 2 weeks. In addition to the thermal decomposition approach, reduction of metal salt can also be utilized to synthesize monodisperse FeCo NPs. For example, uniform FeCo NPs were synthesized via reductive decomposition of Fe(acac)3 and Co(acac)2 in the mixture of surfactants and reducing agent 1,2-hexadecanediol under Ar + 7% H2 gas.141 The Fe/Co ratio was controlled by changing the initial ratio of metal precursors. The surfactant mixture of oleic acid and oleylamine was used to obtain 20 nm FeCo NPs, while a combination of oleic acid and TOP gave 10 nm FeCo NPs. Monodisperse FeCo NPs could also be prepared via a controlled metal diffusion from the core/shell Co/Fe NPs.142 The core/shell Co/Fe NPs were synthesized by coating the Co NPs with a layer of Fe. By controlling the thickness of the Fe coating, the Fe/Co compositions of the final FeCo NPs were easily controlled. 3.2.5. MPt (M = Fe, Co) Nanoparticles. MPt (M = Fe, Co) alloy NPs have been extensively studied as ferromagnetic NPs for magnetic recording,21 permanent magnet,143 and other magnetic applications due to their unique structure-dependent magnetic properties. Compared to the synthesis of elemental or even FeCo alloy NPs, the preparation of alloy MPt NPs is even more challenging due to the obvious difference in chemical reactivity between the noble metal Pt and the first-row transition metal M. For example, when the decomposition chemistry is sought after for the synthesis, Pt(0)-complexes are often stabilized by an alkylphosphine (R3P) ligand, as Pt(PR3)4, not by a common CO ligand used to stabilize Fe or Co in Fe(CO)5 or Co2(CO)8. While Fe(CO)5 or Co2(CO)8 can be decomposed easily to give Fe or Co, the similar decomposition condition does not work for Pt(PR)4. As a result, earlier attempts to prepare MPt NPs via the decomposition chemistry often yielded the mixture of M and Pt NPs. On the other hand, when the metal salt reduction chemistry is used to prepare MPt NPs, the easy reduction and growth of Pt metal tends to produce similar results as in the decomposition chemistry, not

Figure 10. TEM images of monodisperse MPt (M = Fe, Co) NPs with different sizes and shapes: (A) 6 nm FePt, (B) 9 nm FePt, (C) 4.8 nm CoPt3, and (D) cubic FePt. (A) Reprinted with permission from ref 59. Copyright 2000 American Association for the Advancement of Science. (B) Reprinted from ref 144. Copyright 2004 American Chemical Society. (C) Reprinted from ref 158. Copyright 2002 American Chemical Society. (D) Reprinted from ref 170. Copyright 2006 American Chemical Society. 10481


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controlled doping of the third metal into the preformed FePt NPs, as demonstrated in the synthesis of monodisperse FePtCu NPs via the reduction of Cu(acac)2 by oleylamine in the presence of the FePt NPs.168 The compositions and sizes of the FePtCu NPs were controlled by the sizes of the seeding FePt NPs and the amount of Cu precursor added in the reaction mixture. Similarly, trimetallic FePtRu NPs were obtained by doping Ru inside the FePt NPs.169 Shape-controlled synthesis of MPt alloy NPs is often realized in a specifically controlled reaction environment. For example, cubic FePt NPs were synthesized by controlling the Fe/Pt precursor ratio and addition sequence of oleic acid and oleylamine (Figure 10D).170 It was believed that the preferred growth on (111) was achieved due to the stronger oleate binding on the (100) surface. These FePt nanocubes could easily self-assemble into well-defined NP arrays with (100) texture. By adjusting the ratio of surfactants/precursors, the concentration of 1,2-hexadecanediol, and the reaction time, FePt NPs with octapod, cuboctahedron, truncated cube, and cube shapes were successfully obtained,171 indicating that the growth rates on (111) and (100) could indeed be controlled to achieve the desired growth into a specific shape. Even more precise shape control could be realized by adding W(CO)6 during the synthesis of MPt3 nanocubes (M = Fe, Co, and Ni) through coreduction of MCl2 and Pt(acac)2.172 When reacting Fe(CO)5 and Pt(acac)2 in the benzyl ether solution of oleic acid and oleylamine without stirring or the mixture of 1octadecene and oleylamine, uniform FePt nanorods/nanowires were formed with their length controlled simply by reaction duration173 or by oleylamine/1-octadecene volume ratios.174 The nanowire formation was attributed to the controlled anisotropic growth of FePt in the cylindrical reverse micelle preformed by the self-assembly of oleylamine in the reaction medium. Figure 11 shows the TEM images of several representative FePt nanowires (∼2 nm diameter) prepared in

diffusion of Fe and Pt atoms in the NPs. Figure 10B is the TEM image of the 9 nm FePt NPs prepared from this one-pot reaction. Even larger FePt NPs could be obtained via reductive diffusion of preformed core/shell NPs, as demonstrated in the synthesis of 17 nm FePt NPs by reductive annealing of Pt/ Fe2O3 NPs at 550 °C.145,146 In addition to Fe(CO)5, other carbonyls, such as Na2Fe(CO)4, were also used to react with Pt(acac)2 to prepare FePt NPs with an easier Fe/Pt composition control.147 In the synthesis, Na2Fe(CO)4 served as a strong reducing agent. Upon the reduction of Pt(acac)2, the Fe2− in the Na2Fe(CO)4 was oxidized to Fe that could alloy directly with Pt in the reaction condition. The high temperature (389 °C) reaction in tetracosane even resulted in ferromagnetic FePt NPs with their room temperature Hc of 1300 Oe. Furthermore, a new heteropolynuclear cluster complex, Pt3Fe3(CO)15,148 or FePt(CO)4(dppm)Br 2 (dppm = CH2 (PPH 2) 2 ),149 was also synthesized and used as a single-source of metal precursor for the synthesis of FePt NPs. From the related decomposition reaction, FePt NPs with 1:1 Fe:Pt ratio were easily separated. An alternative approach to FePt NPs is via the coreduction of metal salts in a nonhydrolytic solvent. Different from the coreduction of metal salts in a polyalcohol solution, which generates polydisperse FePt NPs,150−152 the coreduction of metal salts by a stronger reducing agent is a much better route to the synthesis of FePt NPs with the desired composition control. For example, simultaneous reduction of FeCl2 and Pt(acac)2 in diphenyl ether by superhydride in the presence of oleic acid, oleylamine produced monodisperse 4 nm FePt NPs.153 The initial molar ratio of the metal precursors was retained in the final FePt NPs. In this coreduction approach, FeCl2 could be replaced by Fe(acac)2, Fe(acac)3, or even Fe(stearate)3 without affecting the reaction output.154−157 The strategy developed for the synthesis of FePt NPs can be readily extended to the synthesis of CoPt NPs. Thermal decomposition of Co2(CO)8 and reduction of Pt(acac)2 in diphenyl ether produced monodisperse CoPt3 NPs with sizes tunable from 1.5 to 7.2 nm.158 Figure 10C is the representative TEM image of the 4.8 nm CoPt3 NPs prepared from this method. In a similar reaction condition, Co2(CO)8 could be replaced by Co(CO)3(NO) with simpler decomposition chemistry.159 CoPt NPs could also be synthesized via a redox transmetalation between Co2(CO)8 and Pt(hfac)2 (hfac = hexafluoroacetylacetone).160 In this synthesis, the alloy formation was driven by redox transmetalation reactions between Co(0) from decomposition and Pt(II) precursors without the need of an additional reducing agent. The composition of the CoPt NPs was controlled from CoPt to CoPt3 by metal precursor ratios. This redox transmetalation method has been utilized to synthesize Co/Pt core/shell NPs through the reaction between Co NPs and Pt(hfac)2.161 Slight modification of the recipe used for the synthesis of the binary MPt NPs can lead to the formation of multimetallic alloy NPs. For example, similar to what was used to prepare FePt NPs,59 by adding a third metal salt (e.g., metal acetate and metal acetylacetonate), a series of uniform trimetallic FePtM (M = Co, Au, or Ag) NPs can be prepared.162−166 The NP quality could be further improved by modifying the reaction condition, as demonstrated in the synthesis of monodisperse 4 nm FePtAu NPs with controlled Fe/Pt/Au compositions by using HAuCl4·xH2O as the Au precursor and tetradecylphosphonic acid as an oleic acid alternative.167 Alternative to the one-pot reaction, the trimetallic NPs could be prepared by

Figure 11. TEM images of (A) 200 nm, (B) 50 nm, (C) 20 nm FePt nanowires, and (D) HR-TEM image of two single 50 nm long FePt nanowires showing the growth along the [111] direction. Reprinted with permission from ref 174. Copyright 2007 Wiley-VCH. 10482


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the mixture of oleylamine and 1-octadecene.174 The nanowire quality and yield could be further improved by adding sodium oleate in the reaction mixture. The presence of sodium oleate in the reaction mixture seemed to favor the formation of reverse micelle structure in the solution and thus increased the yield of nanowires.175 In the similar reaction condition, substituting Fe(CO)5 with Co2(CO)8 could lead to the formation of CoPt nanowires.175 The method could be easily extended to synthesize trimetallic FePtM nanowires (M = Au, Pd, Ni, and Cu).176−178 The thicker nanowires179,180 or more complicated aggregates of one-dimensional nanostructures181 could also be obtained. Monodisperse MPt (M = Fe, Co) NPs prepared from the organic phase reactions often adopt the chemically disordered A1 structure and are superparamagnetic at room temperature. To obtain ferromagnetic L10-MPt NPs, high temperature annealing (usually higher than 550 °C) is required for the structure transformation.182 However, the NPs protected with oleic acid/oleylamine cannot survive this high temperature annealing condition and tend to aggregate/sinter. One way to solve this aggregation/sintering problem was to embed (by grounding) the as-synthesized FePt NPs in a large amount of NaCl.183−185 With the NaCl salt serving as a physical barrier, the FePt NPs were better stabilized in the high-temperature annealing. Once NaCl was dissolved in water, the L10-FePt NPs were separated. However, this process was only partially successful due to the limited control of NaCl coating over every FePt NP in the physical mixture of FePt and NaCl. To ensure that every FePt NP was protected to eliminate any possible sintering in the high-temperature annealing condition, the A1FePt NPs were coated with a layer of SiO2. Thermal annealing of the A1-FePt/SiO2 NPs, followed by subsequent removal of SiO2 in the NaOH solution, gave L10-FePt NPs.186−188 Alternatively, mechanically robust MgO was coated over the A1-FePt NPs. The MgO coating was realized by thermal decomposition of Mg(acac)2 in benzyl ether in the presence of the A1-FePt NPs.189 The MgO shell effectively protected the FePt NPs against sintering at annealing temperatures up to 800 °C. After annealing, the MgO shell was removed by washing the product with a diluted acid, resulting in the clean L10-FePt NPs. The ferromagnetic L10-FePt NPs could be recaptured by oleate/alkylthiol and redispersed in hexane. This process is illustrated in Figure 12A, and the related FePt NPs are shown in Figure 12 (panels B−D). The room temperature coercivity of these L10-FePt NPs reached only 1 T, indicating an incomplete crystal ordering within the NPs. To improve Fe/Pt diffusion and to achieve higher degree of crystal ordering within each FePt NP, FePt/Fe3O4 core/shell NPs190 or dumbbell-like FePt-Fe3O4 NPs191,192 were prepared first from the organic phase reactions. Annealing in the Ar + 5% H2 atmosphere reduced Fe3O4 to metallic Fe, created the necessary defects in the Fe matrix and favored the interfacial Fe−Pt diffusion into the ordered L10-FePt. By controlling the molar ratio of Fe and Pt in the core/shell or dumbbell-like NPs, strongly ferromagnetic L10-FePt NPs with room temperature Hc up to 3.3 T were obtained. These L10-FePt NPs are not only magnetically hard but also chemically robust for magnetic and catalytic applications.168,192 3.2.6. FePd Nanoparticles. FePd is structurally similar to FePt, which can adopt both chemically disordered A1 structure and ordered L10 structure. Thermal decomposition of Fe(CO)5 and reduction of Pd(acac)2 in the presence 1-adamantanecarboxylic acid and trialkylphosphine was used to prepare 16 nm

Figure 12. (A) Schematic illustration of the conversion of A1-FePt NPs to L10-FePt NPs. (B) TEM image of the as-synthesized A1-FePt NPs. (C) TEM image of the L10-FePt/MgO NPs. (D) TEM image of the L10-FePt NPs recaptured by oleic acid/hexadecanethiol. (D) Hysteresis loops of the L10-FePt NPs at 300, 100, and 5 K. Reprinted with permission from ref 189. Copyright 2009 Wiley-VCH.

A1-FePd NPs.193 When the reaction was run in the presence of oleic acid and oleylamine, sea-urchin-like FePd-Fe3O4 nanocomposites were obtained and Fe/Pd molar ratios were controlled by the amount of Fe(CO)5 added in the reaction mixture and by the reaction temperature.194 Upon reductive annealing at 500 °C, the A1-FePd-Fe3O4 nanocomposites were converted into exchange-coupled L10-FePd-Fe, providing an interesting system for evaluating exchange coupling at the nanoscale. The partially ordered L10-FePd NPs could also be prepared by reductive annealing of Pd/Fe3O4 NPs at 600 °C.195 These ferromagnetic L10-FePd NPs are also an interesting NP system for both magnetic and catalytic studies. 3.2.7. SmCo5 Nanoparticles. With the largest magnetic anisotropy (K = ∼2 × 107 J m−3) and high Currie temperature (Tc = ∼730 °C), the SmCo5 alloy is an important magnetic material for high-temperature permanent magnet applications. The success achieved in the organic phase syntheses of many magnetic NPs has motivated the search for the similar approach to monodisperse SmCo5 NPs with strong ferromagnetism. An early attempt to synthesize SmCo5 NPs in dioctyl ether by the decomposition of Co2(CO)8 and reduction of Sm(acac)3 gave low Sm-content NPs with Hc at only 50 Oe.196 An improved synthesis using polyol as a reducing agent resulted in the SmCo NPs with sizes below 10 nm and Hc only at 2.2 kOe at 5 K.197 The weak magnetism of the as-synthesized SmCo NPs was thought to be caused by the facile oxidation of Sm. Consequently, numerous efforts were devoted to the SmCo NP protection,198−201 but none of them was successful due to the extreme chemical instability of Sm. An alternative strategy applied to make SmCo NPs was to reduce SmCo-oxide NPs by 10483


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a strong reducing agent, as demonstrated in a two-step approach to nanocrystalline SmCo5 by high-temperature (900 °C) reductive annealing of the core/shell Co/Sm2O3 NPs in the presence of Ca.202 By properly tuning the thickness of the Sm2O3 shell, different nanostructured SmCo were obtained after the reductive annealing with their Hc reaching up to 8 kOe at room temperature. Similarly, nanostructured SmCo5 (or Sm2Co17) particles were prepared by the high-temperature reduction of Sm2O3 and Co NPs.203 These SmCo NPs were strongly ferromagnetic with room temperature Hc reaching up to 12 kOe and Ms at 70 emu g−1. It is important to note that in this high-temperature annealing condition, it is difficult to produce well-dispersed SmCo NPs without aggregation/ sintering. A test on the Ca reduction of the SmCo-O NPs in the CaO matrix showed the promising sign of success that ferromagnetic SmCo NPs could indeed be synthesized.204 In this test, the 7 nm SmCo-O NPs were presynthesized and then embedded in a CaO matrix formed by base precipitation of CaCl2. After reductive annealing at 900 °C and water washing to remove excess Ca and CaO, 5 nm SmCo5 NPs were obtained. These NPs showed a room-temperature coercivity of 7.2 kOe. However, the CaO coating could not be controlled uniformly and only a small fraction of SmCo-O NPs were reduced to SmCo5 NPs by Ca under the reductive annealing condition. Further optimization of the reaction process is still needed to achieve high-yield synthesis.

Figure 13. TEM images of different MFe2O4 NPs: (A) 6 nm Fe3O4, (B) 12 nm Fe3O4, (C) 14 nm CoFe2O4, and (D) 14 nm MnFe2O4. Reprinted from ref 94. Copyright 2004 American Chemical Society.

3.3. Magnetic Oxide Nanoparticles

Fe3O4 NPs was studied using X-ray diffraction (XRD) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, indicating that the obtained Fe3O4 NPs could be oxidized to γ-Fe2O3 NPs at 250 °C in the O2 atmosphere and further converted to α-Fe2O3 NPs at 500 °C in the Ar atmosphere. As a comparison, direct annealing of the Fe3O4 NPs in the Ar atmosphere at 500 °C showed no structural change. This thermal decomposition method could be easily extended to the preparation of MFe2O4 by simply adding M(acac)2 in the reaction mixture (Figure 13, panels C and D)94 with the size controlled by the precursor concentration,205 or by using just oleylamine as both the reducing agent and surfactant.206 In a modified synthesis, the M(acac)2 could be replaced by MCl2 (M = Fe, Co, Ni, Mn, and Zn). The reaction between MCl2 and Fe(acac)3 in dioctylether solution of oleic acid and oleylamine led to the formation of monodisperse MFe2O4 NPs, which allowed the detailed studies of M-doping effect on MFe2O4 magnetism.57,58 Iron oxide NPs could also be made directly by thermolysis of Fe(CO)5 under aerobic conditions.207 A more interesting development in the synthesis of iron oxide NPs was using Fe-oleate complex as the starting precursor. This Fe-oleate complex was synthesized by reacting metal carbonyl with oleic acid, which could be thermally decomposed into monodisperse iron oxide NPs in the presence of trimethylamine N-oxide.208−210 With the use of the seedmediated growth, iron oxide NPs with one-nanometer-level diameter control from 4 to 13 nm were achieved.211 Further studies shows that the Fe-complex could be prepared more conveniently by reacting FeCl2 with Na-oleate (Figure 14),95 FeO(OH) with oleic acid,212 or metal-fatty acid salts.213 Thermal decomposition of the Fe-oleate complex at different reflux temperatures using different organic solvents was found to reliably produce uniform iron oxide NPs with the sizes tunable from 5 to 22 nm. The metal-oleate decomposition method was also applied to the synthesis of MFe2O4 NPs (M =

NPs of magnetic iron oxides, especially cubic and hexagonal ferrites, have shown many important applications due to their acceptable magnetic performance and their robust chemical stability. The common cubic ferrites are Fe3O4, Fe2O3, and MFe2O4 (M = Co, Mn, Ni, Zn, etc.), and hexagonal ferrites are MO·6Fe2O3 or MFe12O19 (M = Ba, Sr, etc.). The cubic ferrites are structurally isotropic and magnetically softer (smaller K and Hc), while the hexagonal ferrites are magnetically harder with a unique magnetic easy axis along the crystallographic c direction. Due to their chemical stability, numerous methods have been developed to synthesize these oxide NPs, among which the aqueous phase synthesis has been the most popular approach for both lab-scale fabrication and mass production. However, the need for deep understanding of NP magnetism and surface chemistry require an even tighter control on the NP size, shape, composition, and structure, which has posed quite a synthetic challenge to the conventional aqueous phase synthesis method. The organic phase reaction provides a promising alternative to the production of monodisperse iron oxide NPs that are suitable for both basic research and practical applications. The reductive thermal decomposition of Fe(acac)3 in phenyl ether at 265 °C was used to prepare monodisperse 4 nm Fe3O4 NPs.93 In this synthesis, oleic acid and oleylamine served as the surfactants and 1,2-hexadecandiol as a reducing agent to reduce Fe(III) to Fe(II) for the formation of Fe3O4. Changing the solvent to benzyl ether and reacting at 300 °C resulted in 6 nm Fe3O4 NPs (Figure 13A) with good crystallinity and magnetic control.94 Oleic acid was believed to react with Fe(III) to form Fe(III)−OOC− and oleylamine bound to Fe(III), providing a more efficient stabilization for the Fe3O4 NPs despite that their surface was surrounded by Fe(III). The sizes of the Fe3O4 NPs could be further increased up to 20 nm via the seed-mediated growth method by using the 4 or 6 nm NPs as seeds. Figure 13B shows the 12 nm Fe3O4 NPs prepared from the seedmediated growth method. The thermal stability of the obtained 10484


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Compared with the cubic structured ferrites, the hexagonal ferrites show generally much higher Hc and are considered as the ideal recording medium in the next-generation magnetic tape recording.56,228,229 A representative example is barium ferrite (BaO·6Fe2O3 or BaFe12O19, abbreviated as BaFe) NPs. Typically, BaFe is synthesized through solid-state reactions between Ba- and Fe-precursors at very high temperature (>1000 °C).230−233 It is very difficult to control the size and morphology of the BaFe NPs through conventional synthetic methods. Recently, high-temperature organic phase reaction was also tested to prepare BaFe NPs.234 The synthesis applied the decomposition of Fe(acac)3 and Ba(stearate)2 in 1octadecene with Ba/Fe compositions controlled and optimized by the metal precursor ratios. Once annealed in O2 at 700 °C for 1 h, nanostructured hexagonal BaFe were obtained with room temperature Hc surpassing 4000 Oe. The method was readily extended to the synthesis of hexagonal strontium ferrite NPs with similar magnetic properties. The organic phase synthesis method was also applied to prepare antiferromagnetic MO NPs (M = Fe, Co, Ni, Mn, etc.). In an antiferromagnetic MO oxide, magnetic spins in M are aligned antiparallel due to the superexchange coupling across M-O-M. Coupled with ferromagnetic material through exchange bias, antiferromagnetic material plays an important role in giant magnetoresistance and spintronics devices.235 Antiferromagnetic oxide NPs, including FeO NPs,236−238 CoO NPs,239−243 NiO NPs,244,245 and MnO NPs,246−249 were synthesized by the thermal decomposition of a proper metal precursor in a nonhydrolytic solvent. These monodisperse MO NPs provide some unique models for studying nanoscale antiferromagnetism and exchange bias.

Figure 14. Schematic illustration of a simple approach to Fe-oleate complex that allows the large-scale synthesis of monodisperse iron oxide NPs and a representative TEM image of 12 nm Fe3O4 NPs prepared from the decomposition of Fe-oleate complex. Reprinted with permission from ref 95. Copyright 2004 Nature Publishing Group.

Co, Ni, and Mn).214,215 Noticeably, the Fe-oleate complex allowed the ultralarge-scale synthesis of monodisperse iron oxide NPs.95 It is noteworthy that γ-Fe2O3 and Fe3O4 are structurally similar exhibiting only ∼1% difference in lattice constant, which makes it difficult to determine what kind of iron oxide NPs are obtained as the final product. Previous studies showed that X-ray absorption and X-ray magnetic circular dichroism spectroscopy could be used to identify the crystal phase of iron oxide NPs.95 Studying the product from the thermal decomposition of Fe-oleate indicated that the 5 nm NPs were dominated by the γ-Fe2O3 phase, while the 22 nm NPs were pure Fe3O4 phase. Some other characterization techniques such as 57Fe Mössbauer spectroscopy,216−218 Raman spectroscopy,219,220 and X-ray photoemission spectroscopy221,222 have also been utilized to distinguish the right phase of iron oxide. The kinetics of iron oxide NP formation from the Fe-oleate decomposition was studied by thermogravimetric-mass spectrometric analysis and in situ magnetic measurements.223 The decomposition at above 300 °C induced a sudden increase in the concentration of NPs, followed by a rapid ripening process that narrowed the NP size distribution. Further studies showed that Fe-oleate was thermally more stable when excess oleic acid was present.224 The bubbles generated from the boiling solvent could favor the nucleation/growth process by absorbing the heat released from the exothermic decomposition reaction and by promoting the mass transfer.225 The solvent boiled under the high temperature might also decompose, forming reactive intermediates that affected the crystallization of the iron oxide NPs.226 More understanding on the decomposition chemistry of metal salts and formation of oxide NP in the presence of surfactants also facilitates the preparation of iron oxide NPs with the desired shapes. This was demonstrated in the shapecontrolled synthesis of MnFe2O4 NPs.205 Polyhedral NPs were synthesized when the surfactant/metal precursor ratio was smaller than 3:1. Increasing the ratio to 3:1 gave rise to cubic NPs. When CoFe2O4 NPs were prepared via seed-mediated growth of 5 nm NPs, a high growth rate led to the polyhedral NPs while a slow one resulted in the cubic NPs.36 Similarly, controlling the NP growth conditions has led to the formation of Fe3O4 nanocubes with the tunable sizes from 20 to 160 nm,35 and Zn0.4Fe2.6O4 nanocubes from 18 to 140 nm,227 as well as CoxFe3−xO4 nanocubes from 10 to 60 nm with x tuned by the initial ratio of metal precursors for compositiondependent magnetic studies.37

3.4. Multifunctional Magnetic Nanoparticles

The search for more advanced magnetic nanomaterials for important technological applications has called for the preparation of multifunctional magnetic NPs that combine additional functionality with the magnetic NP systems. The progress on the design and synthesis of multifunctional NPs has been summarized in several recent comprehensive review articles.250−254 Here, we highlight the organic phase synthesis of multifunctional magnetic core/shell and dumbbell-like NPs. They are normally synthesized via the process that is similar to the seed-mediated growth, in which the presynthesized NPs are used as seeds for the nucleation and growth of other inorganic components. 3.4.1. Core/Shell Nanoparticles. With a unique heterostructure, organic-phase synthesized core/shell NPs have been extensively studied in semiconductor NPs to modify their optical and electronic properties.255−258 The organic-phase synthesis of core/shell NPs has been successfully deployed to magnetic core/shell NPs. For example, using metallic magnetic NPs as seeds, various magnetic core/shell NPs including Fe/ Fe3O4,87,88 Ni/NiO,259 Co/CoSe,260 and Co/Co2P261 were synthesized by reacting the magnetic metal NP seeds with O2, selenium, or TOP. When excess amount of O2, selenium, or TOP was present, all the magnetic metal NPs would react, forming the corresponding hollow oxide, selenide, or phosphide NPs through Kirkendall effect. Magnetic metal NPs could also be used to synthesize core/shell NPs through redox transmetalation, as demonstrated in the synthesis of Co/ Au, Co/Pd, Co/Pt, and Co/Cu NPs.161 Besides these controlled redox approaches, a more general method to synthesize magnetic core/shell NPs is to grow a 10485


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thicker the shell, the more red shift of the absorption peak. Alternatively, controlled isotropic nucleation and growth of the Fe3O4 shell on the Au core led to the formation of Au/Fe3O4 core/shell NPs with intimate contact between the core and shell (Figure 15D).273 Having noble metal coated over a magnetic core will make the core/shell NPs more interesting. However, it is a more challenging synthetic target due to the easy self-nucleation and growth of noble metal in the common reaction condition. The direct seed-mediated growth of Au over the existing Fe3O4 NPs was not easily controlled, as demonstrated in the reduction of Au(OOCCH3)3 by 1,2-hexadecandiol at 180 °C in the presence of Fe3O4 NPs.274 To prevent the self-nucleation of Au in the synthesis, a different room-temperature seed-mediated growth method was used to prepare Fe3O4/Au NPs via the reduction of HAuCl4 on Fe3O4 NPs using oleylamine as both reducing agent and the surfactant (Figure 16).275 The Fe3O4/Au NPs

uniform shell on the core NPs through the seed-mediated synthesis. For example, monodisperse FePt/Fe3O4 NPs were synthesized via thermal decomposition of Fe(acac)3 in the presence of 4 nm FePt NP seeds (Figure 15A).96 The shell

Figure 15. TEM images of some representative core/shell NPs: (A) FePt/Fe3O4, (B) FePt/PbS, (C) Au/Fe3O4 (with the hollow shell), and (D) Au/Fe3O4. (A) Reprinted from ref 96. Copyright 2004 American Chemical Society. (B) Reprinted from ref 269. Copyright 2010 American Chemical Society. (C) Reprinted with permission from ref 271. Copyright 2008 Wiley-VCH. (D) Reprinted from ref 273. Copyright 2006 American Chemical Society.

Figure 16. (A) Schematic illustration of the formation of Fe3O4/Au seeding NPs and the subsequent surface modification. (B) Schematic illustration of the growth of thicker Au shell or multicomponent Au/ Ag shell and the associated plasmonic absorption peak shift. Reprinted from ref 275. Copyright 2007 American Chemical Society.

thickness was tunable from 0.5 to 3 nm by controlling the amount of FePt NPs and Fe(acac)3. This seed-mediated growth was also applied to coat other ferrite shell262 or soft magnetic shell (e.g., Co, Ni, and Fe2C)191 on FePt core NPs, as well as many other bimagnetic core/shell NPs including CoFe2O4/ MnFe2O4,263,264 Zn0.4Fe2.6O4/CoFe2O4,227 and Fe/MFe2O4 (M = Fe, Co, and Mn)265 NPs. Due to the exchange coupling between the magnetically hard and soft components, magnetic properties of these core/shell NPs were effectively tuned. For example, the 60 nm Zn0.4Fe2.6O4 NPs showed an Hc of 140 Oe, while the core/shell Zn0.4Fe2.6O4/CoFe2O4 NPs exhibited 14 times enhancement in Hc, resulting in a significant increase of energy conversion efficiency of magnetic energy to thermal energy.227 The seed-mediated growth method was further extended to prepare core/shell NPs of FePt/ZnO,266 Fe3O4/ ZnO,267 Co/CdSe,268 FePt/PbS (Figure 15B),269 and Fe3O4/ Cu2‑xS NPs.270 The morphology of the core/shell NPs could be further modified as demonstrated in the preparation of Pt/CoO NPs with the “yolk−shell” structure,260 which was obtained by controlled oxidation of the Pt/Co NPs. Similarly, the oxidation of Au/Fe NPs in air produced Au/Fe3O4 core/hollow-shell NPs (Figure 15C),271 and sulfurization of FePt/Co (by reacting with S) gave FePt/CoS2 core/hollow-shell NPs.272 The Au/ Fe3O4 NPs exhibited a superparamagnetic property from the iron oxide shell and plasmonic property from the Au core.271 The plasmonic resonance of the core/shell NPs was found to be dependent on the thickness of the iron oxide shell, the

were then transferred from the organic phase to the aqueous phase via the transfer agent CTAB/sodium citrate. These hydrophilic NPs acted as the seeds for growing a thicker Au shell or even a multicomponent Au/Ag shell. The hydrophilic Fe3O4/Au NPs could also be used as seeds for the anisotropic growth of Au into a star-shaped shell.276 In this kind of core/ shell NPs, the plasmonic properties were tuned by the thickness and the shape of the Au shell or by the component of the Au/ Ag shells. Alternatively, the core/shell NPs might be prepared by direct coating of the Au shell over the magnetic NPs in aqueous solution. In this case, magnetic NPs should first be transferred into aqueous medium and then functionalized with the −NH2 group to facilitate the Au nucleation and growth into a shell.277,278 More conveniently, the hydrophobic iron oxide NPs were modified directly with NH2-terminated organosilane to direct the formation of the Au shell.279 These core/shell NPs are especially interesting as multifunctional probes for biomedical applications. 3.4.2. Dumbbell Nanoparticles. Different from core/shell NPs in which the shells are uniformly formed on the seeding NPs, dumbbell NPs are formed by anisotropic nucleation and growth of one or even more discrete components on the surface of seeding NPs. A successful synthesis of the heterogeneous dumbbell NPs relies on many parameters, including lattice matching between different components, as well as synthetic conditions (e.g., solvent polarity, seed-toprecursor ratio, surfactants, growth rate, and temperature). 10486


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affect the formation of the dumbbell structure. For example, treating the FePt NPs with the excess oleylamine facilitated the nucleation of PbS around the FePt NPs, leading to the formation of FePt/PbS core/shell NPs. In contrast, removing the oleylamine surfactant and recapping the FePt NPs with oleic acid resulted in FePt-PbS dumbbell NPs.269 The magnetic dumbbell NPs can also be prepared by nucleation and growth of noble metal NPs on the preformed magnetic NPs. For example, in the presence of magnetic ferrite MFe2O4 NPs, the reduction of silver acetate or nitrate by oleylamine under mild reaction temperature generated AgMFe2O4 dumbbell NPs.287,288 By controlling the reaction temperature, the Ag NP sizes were easily controlled. Different from the crystalline MFe2O4 NP seeds, when amorphous Fe/ FexOy core/shell NPs were used as seeds, the nucleation and growth of Ag (via the reduction of AgNO3 by oleylamine in 1octadecene at 180 °C) was found to occur on multiple sites around each Fe/FexOy NP (Figure 18, panels A and B).289 As

The most studied dumbbell NPs are the noble metalmagnetic oxide NPs. The Au-Fe3O4 dumbbell NPs were prepared by the thermal decomposition of Fe(CO)5 on the surface of the preformed Au NPs, followed by air oxidation (Figure 17, panels A and B).97 The size of the Fe3O4 NPs was

Figure 17. (A) Schematic illustration of the synthesis of Au−Fe3O4 dumbbell NPs. (B) Representative TEM image and (C) HR-TEM image of the obtained Au−Fe3O4 dumbbell NPs. Reprinted from ref 97. Copyright 2005 American Chemical Society.

controlled from 4 to 20 nm by adjusting the ratio of Fe(CO)5 and Au seeds. HR-TEM image of a representative Au-Fe3O4 dumbbell NP confirmed the single crystallinity of both Au and Fe3O4 NPs with only ∼3% lattice mismatch between the Au (111) and Fe3O4 (111) junctions (Figure 17C). With the use of the same methodology, various metal−iron oxide dumbbell NPs have been prepared, including Ag-Fe3O4,280 AuAg− Fe3O4,280 Pt−Fe3O4,281 FePt−Fe3O4,192 and PtPd−Fe3O4 NPs.282 Noble metal-magnetic oxide dumbbell-like NPs could also be synthesized by the thermal decomposition of Fe(acac)3 in dioctylether273 or Fe-oleate in 1-octadecene283 in the presence of the noble metal NPs. Due to the interfacial interaction between two different components, the physical and chemical properties of the dumbbell NPs can be significantly different from the single component. For example, compared to the pure Au NPs with strong plasmonic resonance at 520 nm, the Au−Fe3O4 NPs showed a clear red-shift of the plasmonic absorption at 538 nm.97 With superparamagnetic Fe3O4 copresent in the dumbbell structure, the Au−Fe3O4 NPs were tested as a promising dual optical/magnetic probe for diagnostic and therapeutic applications.284,285 The interfacial effect also rendered the dumbbell NPs more efficient in catalyzing CO oxidation280 and H2O2 reduction.282,286 The anisotropic growth of the dumbbell NPs is strongly dependent on the polarity of solvents. Taking the formation of Au−Fe3O4 dumbbell NPs as an example, the synthesis was believed to start by the nucleation of Fe, generated from the decomposition of Fe(CO)5, on the Au NPs after which Au became more negatively charged, prohibiting further Fe nucleation/growth on the other sites of the Au NPs.97 As a result, a single site nucleation was achieved for the formation of dumbbell NPs in a nonpolar solvent.97,273 However, when a more polar solvent, such as benzyl ether or phenyl ether, was used as the reaction medium, the “extra” negative charge was better dissipated, allowing multiple-sites nucleation/growth of Fe on the Au NP surface.97,273 The type of surfactant could also

Figure 18. (A) Schematic illustration of the formation of Ag-hollow FexOy dumbbell NPs via the multiple site nucleation of Ag on the amorphous Fe/FexOy core/shell NPs, followed by further oxidation of Fe and subsequent ripening of the Ag domains. (B) TEM image of the hybrid NPs containing several Ag domains on each Fe/FexOy NP. (C) TEM image of the final Ag-hollow FexOy dumbbell NPs. Reprinted with permission from ref 289. Copyright 2011 Wiley-VCH.

the reaction proceeded, the internal Fe was further oxidized and small Ag NPs on the FexOy surface were subject to further ripening to larger NPs, producing an interesting dumbbell Ag− FexOy NPs with a hollow FexOy component (Figure 18C). The dumbbell NPs may be formed in a controlled microemulsion environment, as demonstrated in the synthesis of Ag−Fe3O4, Ag−FePt, and Ag−Au NPs.290 They may also be prepared by surface dewetting of core/shell NPs due to the large lattice mismatch between the core and the shell, as seen in the heating-assisted conversion from core/shell FePt/CdS NPs to 10487


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dumbbell FePt-CdS NPs,291 or from core/shell γ-Fe2O3/MS (M = Zn, Cd, and Hg) to dumbbell γ-Fe2O3-MS.292 When the surface of the NPs is catalytically active, it may offer the desired sites for an anisotropic growth of another component, forming the dumbbell structure. This was demonstrated in the syntheses of FePt−Au NPs293 and Au−Ni NPs.294 With the use of the preformed two-component dumbbell NPs as seeds, more complex dumbbell NPs can be synthesized. This was shown in the synthesis of Fe 3 O4 −Au−Fe 3 O 4 dumbbell NPs through the fusion of the Au sides from two Au−Fe3O4 NPs in the presence of S (Figure 19, panels A and

Au NPs, the same growth condition generated only larger Au NPs, not Au2−Au1 dumbbell NPs. These studies clearly indicated the influence of the Fe3O4 NPs on the anisotropic growth of Au2 on Au1 in the Au1−Fe3O4 NPs. More systematic studies on the addition of metal M (M = Au, Ag, Ni, and Pd) to the Pt−Fe3O4 dumbbell NPs (Figure 19D)297 showed that similar to the formation of Au2−Au1−Fe3O4 dumbbell NPs,296 the reactions gave rise to the ternary dumbbell-like M−Pt− Fe3O4 NPs, within which Au, Ag, Ni, and Pd all grew from the Pt surface.

4. MAGNETIC NANOPARTICLES FOR BIOMEDICAL APPLICATIONS Magnetic NPs, especially superparamagnetic NPs, have been extensively studied for their biomedical applications.13−18,298,299 Due to the superparamagnetic behavior and high magnetic moments in an external magnetic field, these NPs stabilized in biological environments can respond quickly to an external magnetic field, generating a secondary field around each NP and interfering with the proton nuclear spin relaxation. Such NPs can serve as sensitive magnetic probes (contrast agents) in magnetic resonance imaging (MRI). Moreover, when exposed to an alternating magnetic field (AMF), they can fast-switch their magnetization directions, transferring magnetic energy into thermal energy, which is an especially promising approach as magnetic fluid hyperthermia (MFH) to eliminate heatsensitive cancer cells. Magnetic NPs have also been used for efficient separation of biomolecules (e.g., bacteria, proteins, and cells)300−303 and as a magnetic platform for targeted drug delivery.304−307 In this section, we highlight the surface modification of magnetic NPs synthesized from organic phase reactions, followed by their applications as MRI contrast agents for sensitive diagnosis and as hyperthermia agents for MFH. 4.1. Surface Modification of Magnetic Nanoparticles

Figure 19. (A) TEM image of Fe3O4−Au−Fe3O4 dumbbell NPs. (B) HR-TEM image of a representative Fe3O4−Au−Fe3O4 NP. (C) Schematic illustration of the formation of the Au2−Au1−Fe3O4 dumbbell NPs and their corresponding TEM image. (D) Schematic illustration of the formation of the Ag−Pt−Fe3O4 dumbbell NPs and their corresponding TEM image. (A and B) Reprinted from ref 273. Copyright 2006 American Chemical Society. (C) Reprinted from ref 296. Copyright 2009 American Chemical Society. (D) Reprinted with permission from 297. Copyright 2011 Nature Publishing Group.

Magnetic NPs prepared from organic phase reactions are coated with a long-chain hydrocarbon layer that provides steric repulsion for NP stabilization in nonpolar solvents. To meet the requirements for biomedical applications, the surface of these magnetic NPs needs to be hydrophilic and biocompatible. The most common methods used for surface modification of these magnetic NPs are ligand exchange, ligand addition, and hydrophilic SiO2 coating (Figure 20). 4.1.1. Ligand Exchange. The ligand exchange is the direct replacement of hydrophobic surfactants on magnetic NPs with hydrophilic ligands (Figure 20A). Normally, the hydrophilic ligands consist of two functional groups, within which one group can strongly bind to the NP surface and the other is hydrophilic so that the NPs can be dispersed in an aqueous solution or be further functionalized. One earlier example of the ligand exchange was the dopamine modification of the iron oxide NP surface due to the strong binding of dopamine with surface Fe(III) via the diol bidentate bonding.302 The dopamine could further be functionalized with nitrilotriacetic acid for protein separation. Coupling the dopamine with a biocompatible polymeric ligand (e.g., polyethylene glycol (PEG)) provides an even stronger steric repulsion and improves the colloidal stability in an aqueous solution. Moreover, the PEG coating on the magnetic NPs can reduce their interaction with proteins, making them “stealth” to the innate immune system,308 as demonstrated in the stabilization of monodisperse Fe3O4 NPs with dopamine-terminated PEG.309,310 The PEG-coated Fe3O4 NPs were stable in

B).273 Without S, the preformed Au−Fe3O4 NPs were stable. But in the presence of S, Au−S strong bond formation destabilized the Au NPs, aided the coalescence of two Au NPs from two Au−Fe3O4 NPs, and led to the formation of Fe3O4− Au−Fe3O4 NPs. Complex dumbbell NPs can be synthesized by seed-mediated growth of a third component on the seeding dumbbell NPs. A typical example is the growth of PdX (X = S, Se) on Au−Fe3O4 NPs.273,295 Probably due to the interaction between X and Au, the PdX nucleated only on the Au surface leaving the Fe3O4 surface intact. At a low PdX/Au−Fe3O4 ratio, PdX−Au−Fe3O4 NPs were obtained. But with increased precursor concentration of PdX, the PdX could grow into nanorods or even branched nanorods. Reducing HAuCl4 with oleylamine at 80 °C in the presence of the Au−Fe3O4 NPs resulted in Au2−Au1−Fe3O4 dumbbell NPs with Au2 nucleate and grow only on the Au1 side (Figure 19C).296 In this Au2− Au1−Fe3O4 structure, Au2 could not overgrow Au1, otherwise the Au2−Au1 would separate from the Fe3O4. It is interesting to know that when the Au−Fe3O4 NPs were replaced by the pure 10488


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doped ferrite NPs,58 and Fe/MnFe2O4 core/shell NPs,265 for diagnostic and therapeutic applications. 4.1.2. Ligand Addition. Ligand addition is another important strategy developed to transfer hydrophobic NPs into hydrophilic media by encapsulating these NPs with an amphiphilic ligand (Figure 20B). The ligand addition is achieved through the hydrophobic interaction between the lipophilic region of amphiphilic ligands and the hydrophobic section of the surfactant attached on the NP surface. The hydrophilic region of the amphiphilic ligand is responsible for NP dispersion in an aqueous solution. PEG-phospholipid copolymer, which is a typical amphiphilic ligand used for surface modification of quantum dots,322−324 has been successfully utilized to modify hydrophobic magnetic NPs.135,325 By simply mixing the as-synthesized magnetic NPs with an amphiphilic ligand in a proper solvent (e.g., chloroform), effective ligand addition could be achieved.325−328 Many other amphiphilic polymers have also been developed, such as poly(maleic anhydride-alt-1-octadecene),317 poly(maleic anhydride alt-1-tetradecene),329 poly(styrene-blockacrylic acid) copolymers,330 and PEG-block-polylactide.331 The carboxylic and amino groups present in these polymers are the common sites for further bioconjugation. 4.1.3. Silica Coating. Silica (SiO2) coating was widely used to modify the magnetic NPs with much more robust stability and biological compatibility (Figure 20C).15,332,333 Typically, amorphous SiO2 shell was formed via the sol−gel reaction (Stöber approach) of the hydrolysis of tetraethyl orthosilicate (TEOS) and subsequent condensation.334−336 The SiO2 shell thickness was controlled by the concentration of TEOS, the amount of seeding NPs or by the reaction time. Before the SiO2 coating, the hydrophobic magnetic NPs need to be transferred from an organic phase to an aqueous phase for the sol−gel reaction. Hydrophilic ligands such as polyvinylpyrrolidone 3 3 6 − 3 3 8 and cetyltrimethylammonium bromide (CTAB)339,340 were often used as the phase transfer agents. The CTAB even acted as an organic template for the formation of a mesoporous SiO2 shell around each magnetic NP core (Figure 21A).340 Hydrophobic Fe3O4 NPs could also be coated

Figure 20. Schematic illustration of surface modification of hydrophobic magnetic NPs via (A) ligand exchange, (B) ligand addition, and (C) silica coating.

physiological environment without agglomeration. More importantly, these NPs showed much reduced nonspecific uptake by macrophage cells, indicating enhanced circulation time by PEG coating. If the tail group of the dopamineterminated PEG is −COOH or −NH2 then the PEG-modified magnetic NPs can be further functionalized with various biomolecules via the common coupling chemistry for drug delivery.309−311 Similarly, some other polymers, such as the phosphine oxide-terminated PEG,312−314 poly(vinylpyrrolidone),315,316 polyethylenimine,317 and poly(vinyl alcohol),318 have also been used to make the magnetic NPs water-dispersible and biocompatible. Although the PEG-based polymer makes magnetic NPs biocompatible, it also causes an increase of the hydrodynamic size of the NPs. For magnetic NPs to be magnetically responsive, their surface coating should be thinner and their hydrodynamic sizes should be smaller. The thin coating was achieved by replacing the oleic acid/oleylamine around the iron oxide NPs with a compact zwitterionic dopamine sulfonate ligand, in which the dopamine provides the strong binding to the oxide surface and zwitterion offers the desired NP stability in the biologically relevant solutions.319 Recently, 3,4dihydroxyhydrocinnamic acid was also demonstrated to stabilize Fe3O4 NPs in aqueous solution.320 The modified NPs showed an excellent water solubility and stability over a wide pH range (pH = 3−12), making it easy to couple with dyes, oligonucleotides, and enzymes for more specific bioapplications. A third form of the small ligand, 2,3dimercaptosuccinic acid (DMSA), attracted even more attention for bioapplications. In the demonstrated surface modification, DMSA attached to Fe3O4 via a COO-chelate bonding.39,321 The ligand shell was further coupled with other functional molecules via thiol-based linker chemistry. This DMSA was used to modify many different magnetic NPs, including ferrite MFe2O4 (M = Mn, Fe, Co, and Ni) NPs,57 Zn-

Figure 21. Schematic illustration of coating SiO2 on hydrophobic magnetic NPs via (A) sol−gel approach and (B) reverse microemulsion approach. (A) Reprinted with permission from ref 340. Copyright 2008 Wiley-VCH. (B) Reprinted from ref 342. Copyright 2006 American Chemical Society. 10489


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Figure 22. (A) TEM image of 3 nm iron oxide NPs. (B) Magnetization curves (M−H) at 300 K of the iron oxide NPs of different sizes. (C) Schematic illustration of surface spin-canting effects of NPs of different sizes. (D) T1-weighted MR images of the 3 nm iron oxide NPs at different concentrations. (E) Plot of 1/T1 over Fe concentration of the iron oxide NPs with different sizes. (F) T1 weighted-MR images of MCF-7 cell pellets after 24 h incubation with 3 and 12 nm iron oxide NPs on a 1.5 T MR scanner. Reprinted from ref 314. Copyright 2011 American Chemical Society.

magnetization due to the random spin−spin interaction/ dephasing; this process is also called spin−spin relaxation. The T2 relaxation time is determined by the time it takes for the transverse magnetization to decay to 37% of its original value. The efficiency of a contrast agent is usually expressed by its relaxivity ri (i = 1, 2), which is defined by the change in relaxation rate Δ(1/Ti) after the introduction of a contrast agent. Its unit is mM−1 s−1. The larger the ri, the better the MRI sensitivity. 4.2.1. T1 MRI Contrast Agents. MRI contrast agents are used to enhance the proton relaxation rates to improve the MRI sensitivity. The contrast agents that facilitate the T1 relaxation are called T1 contrast agents. Paramagnetic complexes based on Gd3+ with 7 unpaired electrons are the common T1 enhancement agents.349 In the presence of the paramagnetic complexes, the strong interaction between protons and Gd3+ induces faster T1 relaxation and thus results in brighter MR images. However, the Gd-complexes are generally short-lived in biological systems, and free Gd3+ ions can leach out from the Gd-complexes, inducing undesired toxicities.350 To overcome the drawbacks of the Gd-based T1 contrast agents, monodisperse MnO NPs (Mn2+ has 5 d-electrons) were developed as a new T1 contrast agent.325 Water-dispersible and biocompatible MnO NPs were prepared by the PEGphospholipid encapsulation. But these MnO NPs showed a r1 value of only ∼0.4 mM−1 s−1, lower than the Gd-based contrast agents (∼4 mM−1 s−1).351 Superparamagnetic iron oxide NPs were also studied as a T1 contrast agent. But their high magnetization values limited their use in T1 weighted imaging due to their excessively high r2 value that is good for T2 contrast. To improve their T1 contrast enhancement, iron oxide NPs were made smaller so that surface-spins from these small

with SiO2 directly in the reverse microemulsion formed by mixing poly(5)oxyethylene-4-nonylphenyl-ether with cyclohexane (Figure 21B).333,341,342 In the process, ammonium hydroxide was often added to catalyze the hydrolysis of TEOS and to facilitate the NP transfer of NPs from hydrophobic zone to the hydrophilic one.343,344 Using the reverse microemulsion method, other magnetic NPs including FePt345 and metaldoped ferrite NPs346−348 could also be coated with SiO2. These SiO2-coated NPs were easily modified with PEG or 3(aminopropyl) triethoxysilane for next-step surface functionalization. 4.2. Magnetic Nanoparticles as MRI Contrast Agents

MRI is an imaging technique that measures the difference of proton nuclear magnetic resonance of water molecules around solid tissue and its surrounding biological solution. In a strong permanent magnetic field, the proton nuclear spins can be split into low-energy (along the magnetic field) and high-energy (against the magnetic field) states. When a secondary electromagnetic radio frequency pulse is applied at the resonance frequency (also called Larmor frequency), the lowenergy spins can absorb the radio frequency energy, and as a result, the spin direction can be switched from the low-energy state to the higher energy state. Once the pulse is turned off, the excited spins tend to relax to their original low-energy state, generating two simultaneous and independent relaxation processes: longitudinal (T1) relaxation process and transverse (T2) relaxation process. The T1 process allows the spins to “realign” along the permanent magnetic field direction, releasing the absorbed RF energy into the surrounding; the process is also called spin−lattice relaxation. The T1 relaxation time is referred to as the time required for the longitudinal magnetization value to recover approximately 63% of its original value. The T2 process causes the decay of transverse 10490


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Figure 23. (A) TEM images of various ferrite NPs with the size of 12 nm. (B) Schematics of spin alignments of magnetic ions in the inverse spinel structure of MFe2O4 with different transition metal M (M = Mn, Fe, Co, and Ni) and their corresponding spin moments. (C) T2-weighted MR images and the color maps of a series of ferrite NPs at 1.5 T. (D) The r2 relaxivity of different ferrite NPs. (E) Color maps of in vivo T2-weighted MR images of a mouse implanted with the cancer cell line NIH3T6.7 after injection of MnFe2O4−Herceptin (a−c) or CLIO-Herceptin conjugates (d− f). Here, CLIO refers to as cross-linked iron oxide. Reprinted with permission from ref 57. Copyright 2007 Nature Publishing Group.

NPs could dominate their magnetic response.314,352−354 Therefore, a series of iron oxide NPs with the sizes of 1.5, 2.2, 3, and 12 nm were studied (Figure 22).314 The NPs with the size below 3 nm exhibited small magnetization value due to the strong surface spin-canting effect. The 3 nm iron oxide NPs showed a high r1 of 4.77 mM−1 s−1, which was higher than that of the 12 nm iron oxide NPs (2.37 mM−1 s−1). In addition, the 3 nm NPs showed a much smaller r2/r1 ratio at 6.12 than the 12 nm NPs with a r2/r1 ratio of 24.5, demonstrating that the 3 nm iron oxide NPs were a superior T1 contrast agent. In vitro T1weighted MR images of MCF-7 cells with the 3 nm iron oxide NPs confirmed the significant signal enhancement with brighter images compared with the 12 nm NPs (Figure 22F). The 3 nm NPs also helped to enhance the MRI detection sensitivity down to 0.2 mm. A T1 contrast agent based on PEG-coated 4 nm iron oxide NPs was demonstrated to have an even higher r1 relaxivity of 7.3 mM−1 s−1, which is about two times higher than that of a standard Gd-based T1 contrast agent Magnevist.353 Another efficient T1 contrast agent based on 5.5 nm iron oxide NPs was also reported to have a high r1 of 9.5 mM−1 s−1 and a low r2/r1 ratio of 2.97 at 3 T.354 The high relaxivity allows the significant enhancement of T1-weighted imaging of blood vessels and vascular organs with high spatial resolution. 4.2.2. T2 MRI Contrast Agents. Superparamagnetic NPs with high magnetization values can significantly shorten the T2 relaxation time of the nearby protons, reducing the proton signal intensity and darkening the T2-weighted MR images. These superparamagnetic NPs are known as T2 contrast agents. As the T2 contrast effect is strongly dependent on the magnetization of NPs, it can be further improved by enhancing the Ms of superparamagnetic NPs. Superparamagnetic NPs with different sizes and compositions have been investigated for enhancing their T2 contrast effects. Monodisperse Fe3O4 NPs served as an excellent model system for studying the NP size effect on the T2 contrast.39 A series of biocompatible Fe3O4 NPs with the core size from 4 to 12 nm and magnetization values from 25 to 102 emu g−1Fe were

prepared. Consistent with the magnetization enhancement, the 12 nm NPs showed the strongest MR contrast effect with a r2 value of 218 mM−1 s−1. Recently, an extremely high r2 relaxivity at 761 mM−1 s−1 was achieved by using the water-dispersible 22 nm cubic Fe3O4 NPs with their Ms of 106 emu g−1Fe,327 which yielded a superior in vivo MR image of tumors. Due to their tunable magnetization values, the MFe2O4 NPs provide another model system for studying NP-composition dependent MRI contrast effects. A systematic study of the 12 nm MFe2O4 (M = Ni, Co, Fe, and Mn) NPs indicated that the NPs with higher magnetization values showed better T2 contrasts (Figure 23).57 With the inverse spinel-type structure, these ferrite NPs showed the M-dependent (M = Ni, Co, Fe, and Mn) saturation magnetizations (Figure 23B) of 85, 99, 101, and 110 emu g−1(Fe+M), respectively. Consistent with this trend, their corresponding r2 relaxivities were measured to be 152, 172, 218, and 358 mM−1 s−1 (Figure 23D). With the highest r2 value, the MnFe2O4 NPs showed the highest contrast in the T2weighted MR images. When conjugated with the antibody Herceptin, these MnFe2O4 NPs became the best contrast agent for highly sensitive in vivo MR detection of small tumors (Figure 23E). The magnetization value of the 15 nm Zn0.4Fe2.6O4 NPs could be further increased to 161 emu g−1 (Fe+Zn) due to the Zn(II)-doping induced alleviation of the antiferromagnetic coupling between Fe3+ ions, resulting in a much higher r2 value of 687 mM−1 s−1.58 Metallic magnetic NPs have also been investigated as efficient T2 contrast agents due to their high magnetization values. The 15 nm bcc-Fe/Fe3O4 NPs with the oxide shell thickness of 2.5 nm were synthesized and modified by an amphiphilic oleylamine-PEG ligand.88 With a stabilized saturation magnetization value at of 164 emu g−1Fe, the modified bcc-Fe/Fe3O4 NPs exhibited a r2 of 220 mM−1 s−1, which is 2 times higher than the typical iron oxide contrast agent Feridex (110 mM−1 s−1). When the 9/3.2 nm bcc-Fe/ Fe3O4 NPs were coated with DMSA, they had a Ms of 150 emu g−1Fe and r2 value of 324 mM−1 s−1 that was much higher than that from the comparable iron oxide NPs (145 mM−1 s−1).355 10491


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scanning confocal microscope at 594 nm, showing typical morphology of epithelial cells that were targeted specifically by the Au−Fe3O4 NPs. Moreover, the optical signal was very stable without any observable degradation even after 3 days of imaging tests. Alternatively, core/shell Fe3O4/Au NPs277 and fluorescent organic dyes coupled magnetic NPs340,359 were also developed for multimodal imaging. Recent advances in the design of multimodal probes based on magnetic NPs allow other imaging techniques, such as computed tomography (CT) and positron emission tomography (PET), to be combined with MRI to achieve even higher imaging sensitivity. For example, core/shell Fe3O4/TaOx NPs were demonstrated as dual contrast agents for both MRI and CT detections, enabling the clear imaging of newly formed blood vessels around tumors by CT and vascular regions of tumors by MRI.360 Positron-emitting radionuclides (e.g., 64Cu and 124I) labeled on the surface of magnetic NPs through proper surface modification/conjugation were tested as MRIPET dual-modal imaging agents to provide both high sensitivity and high spatial resolution for in vivo MRI and PET.361,362

Similarly, Fe/MFe2O4 (M = Fe, Co, and Mn) core/shell NPs with controlled core and shell sizes were also tested, among which the Fe/MnFe2O4 NPs showed a r2 of 356 mM−1 s−1.265 Greater T2 contrast effect was achieved by using 7 nm FeCo/ graphite core/shell NPs with even higher magnetization of 215 emu g−1metal.135 Modified with PEG-phospholipid, the watersoluble FeCo/graphite NPs showed a r2 of 644 mM−1 s−1. 4.2.3. Multimodal MRI Contrast Agents. In order to improve MRI sensitivity and accuracy, multimodal imaging probes using multifunctional magnetic NPs have been actively pursued.356 For example, in the efforts of developing a T1−T2 dual-mode contrast agent, 15 nm MnFe2O4 NPs were coated with a uniform layer of SiO2 on which 1.5 nm thick Gd2O(CO3)2 layer was deposited.346 In this dual-modal contrast agent, MnFe2O4 NPs were used as the T2 contrast agent and Gd2O(CO3)2 as the T1 contrast agent. The thickness of the SiO2 layer was tuned to optimize the magnetic coupling between the superparamagnetic NP core and paramagnetic shell. With the 16 nm SiO2 coating, the dual-modal NP contrast agent had a r1 of 33.1 mM−1 s−1 and a r2 of 274 mM−1 s−1 with both T1 and T2 signals enhanced. This concept was also extended to core/shell nanocomposites with Mn-containing metal organic framework357 and Gd2O3-embedded iron oxide NPs.358 Combining an optical imaging modality with a MRI one is another approach to achieve the desired imaging sensitivity. The dumbbell-like Au−Fe3O4 NPs, which contain optically active Au and magnetically active Fe3O4, were demonstrated to be a promising dual imaging probe (Figure 24).284 The oleic

4.3. Magnetic Nanoparticles for MFH

In an AMF, magnetic NPs can be used to convert the magnetic energy into thermal energy. As cancer cells are more sensitive to heat than the normal ones, the heat generated from the NPs around the cancerous area can inhibit the cancer growth or even eliminate these cancer cells.363,364 This physical approach for cancer therapy is referred to as MFH. Recent developments in the controlled synthesis of monodisperse magnetic NPs and their surface functionalization have made significant progress in MFH.298,365−367 4.3.1. Basic Principal of MFH. In general, heat can be generated from magnetic NPs by any of the following three magnetic processes: hysteresis losses, Néel relaxation, and Brownian relaxation.368 For the single-domain magnetic NPs, heat is produced by Néel and Brownian relaxation of the NPs. In Néel relaxation, the magnetization direction flips within the NP without changing the NP’s physical orientation. As discussed in section 2.1, the Néel relaxation time (τN) is

( ). KV kBT

given by the equation: τN = τ0 exp

In the case of

Brownian relaxation, the NPs rotate within the medium. The Brownian relaxation time (τB) of a NP is given by the equation: 3ηV τB = k TH , where η is the viscosity of the medium and VH is the

Figure 24. (A) Schematic illustration of the surface modification of the dumbbell Au−Fe3O4 NPs. (B) T2-weighted MR images of (i) 20 nm Fe3O4, (ii) 3−20 nm Au−Fe3O4, (iii) 8−20 nm Au−Fe3O4 NPs, and (iv) A431 cells labeled with 8−20 nm Au−Fe3O4 NPs. (C) Reflection image of the A431 cells labeled with 8−20 nm Au−Fe3O4 NPs. Reprinted with permission from ref 284. Copyright 2008 Wiley-VCH.

B

hydrodynamic volume of the NP. As the two relaxation modes take place in an AMF, an effective relaxation time (τeff) should τ τ be defined by the equation: τeff = τ N+ Bτ .368 The τeff is strongly N

B

dependent on the size (or volume) and magnetic anisotropy of the NPs. The heating efficiency is determined by the specific loss power (SLP), which is referred to as the thermal power dissipation divided by the mass of magnetic NPs and is CV dT expressed as SLP = m s dt , where C is the volumetric specific heat capacity of the sample, Vs the sample volume, m the mass of magnetic NPs in the sample, and dT/dt the initial linear rise in the temperature versus time dependence. On the basis of Rosensweig’s theory,369 the SLP for monodisperse NPs is

acid/oleylamine coated Au−Fe3O4 NPs were modified with HS-PEG on the Au side through the Au−S bonding and dopamine-PEG on the Fe3O4 side through the covalent bonding between dopamine and Fe3O4. These NPs were tested as the magnetic-optical dual probe for imaging A431 human epithelial carcinoma cells that overexpress the epidermal growth factor receptor. The surface of the NPs was functionalized with the epidermal growth factor receptor antibody to target specifically to the A431 human epithelial carcinoma cells. The A431 cells labeled with the 8−20 nm Au−Fe3O4 NPs showed an enhanced T2-weighted MR images with the r2 of 80.4 mM−1 s−1. These A431 cells were also visualized with a

expressed as SLP =

μ0 MsH

(2πf )2 τ

2ρ

1 + (2πfτ )2

L(ξ), where μ0 is the

vacuum permeability, H the strength of the AMF, f the frequency of the AMF, Ms the saturation magnetization of the NPs, ρ the density of the magnetic NPs, τ the relaxation time, 10492


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Figure 25. (A) Schematic illustration of the exchange-coupled CoFe2O4/MnFe2O4 NPs and their magnetization curves at 300 and 5 K. (B) The SLP values of CoFe2O4, MnFe2O4, and CoFe2O4/MnFe2O4 NPs. (C) Schematics of the in vivo hyperthermia treatment of a mouse. (D) Nude mice xenografted with cancer cells (U87MG) before treatment (upper row, dotted circle) and 18 days after different treatments. (E) Plot of tumor volume versus days after different treatments. Reprinted with permission from ref 263. Copyright 2011 Nature Publishing Group.

The SLP is strongly dependent on the Ms of the magnetic NPs. The higher the Ms, the higher the SLP. Thus, magnetic NPs with high Ms have been pursued to enhance the SLP. For example, the stable bcc-Fe/Fe3O4 NPs were demonstrated as a superior agent for MFH compared to the amorphous Fe/Fe3O4 NPs at the similar size.88 The heating efficiency of these NPs was evaluated in an AMF of H = 26 kA m−1 and f = 177 kHz. Due to their much higher Ms over the amorphous Fe/Fe3O4 NPs (164 vs 104 emu g−1Fe), the bcc-Fe/Fe3O4 NPs showed a SLP of 140 W g−1, which was significantly higher than that from the amorphous Fe/Fe3O4 NPs (10 W g−1).88 Zn(II)-doped ferrite NPs with high Ms could also be employed for MFH.58 For the 15 nm Zn0.4Mn0.6Fe2O4 NPs with a Ms of 175 emu g−1metal (higher than that of the 15 nm Fe3O4 at 117 emu g−1metal), their SLP value reached 432 W g−1 (vs 333 W g−1 from the Fe3O4 NPs) (H = 37.3 kA m−1 and f = 500 kHz),263 which was about 4 times higher than the 115 W g−1 value from the Feridex under the same AMF condition. When the Zn0.4Mn0.6Fe2O4 NPs was tested in vitro to treat HeLa cells, it was found that 10 min of AMF exposure elimiated 84.4% of the cancer cells, while with Feridex, only 13.5% of the cancer cells died. Magnetic anisotropy plays a critical role in SLP enhancement as well. Taking the well-known ferrite NPs as an example, the K can be easily tuned by different transition metal doping, as demonstrated in the CoxFe3−xO4 system. Both experimental and theoretical results showed that the K value of the CoFe2O4

and L(ξ) the Langevin function. It is clear that the SLP increases as the strength and frequency of the AMF increases.370 However, for safe clinical hyperthermia therapy under electromagnetic irradiation, the applied strength and frequency of the AMF need to be carefully considered.371,372 4.3.2. Magnetic Nanoparticles as MFH Agents. Magnetic NPs, including iron oxide NPs coated with polymers and encapsulated into cationic liposomes, have been used in magnetic hyperthermia to eliminate cancer cells.373,374 Clinical trials of MFH are underway.365,375 For successful applications of this hyperthermia therapy, sufficient SLP from the magnetic NPs is required. Therefore, important parameters of magnetic NPs must be carefully controlled.376 The magnetic heating rate of the 5 to 14 nm Fe3O4 NPs was studied, showing an increased SLP from 180 to 447 W g−1 under H = 24.5 kA m−1 and f = 400 kHz.377 However, the SLP did not always increase with increased sizes, and the optimal size should be near the transition from superparamagnetic to ferri/ferromagnetic.378−381 In the case of uniform cubic Fe3O4 NPs with controlled sizes from 12 to 38 nm, the optimal SLP of 540 W g−1 was obtained for the 19 nm Fe3O4 NPs (H = 14 kA m−1 and f = 320 kHz), which was over 10 times and 2 times higher than that from the 12 and 24 nm NPs, respectively.381 In vitro experiments using the 19 nm NPs demonstrated an efficient hyperthermia treatment on KB cancer cells with 50% cell mortality at an equilibrium temperature of 43 °C after 1 h of the AMF exposure. 10493


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(∼2 × 105 J m−1) was much larger than that of the Fe3O4 (∼1.6 × 104 J m−1).61,62,382 As a result, at the size of 9 nm, monodisperse CoFe2O4 NPs exhibited 3 times higher SLP (443 W g−1) than Fe3O4 NPs (152 W g−1).263 Theoretical modeling suggested that the SLP of superparamagnetic NPs could be optimized when the K was in the range from 0.5 × 104 to 4.0 × 104 J m−1,263 but the Ms factor should also be considered to maximize the SLP. To obtain the right K/Ms combination, nanocomposites containing exchange coupled hard and soft magnetic phases were designed. A representative example was the exchange-coupled core/shell NPs composed of magnetically hard CoFe2O4 (K = 2 × 105 J m−3) core and magnetically soft MnFe2O4 shell (K = 3 × 103 J m−1).263 The exchangecoupled 15 nm CoFe2O4/MnFe2O4 NPs maintained the superparamagnetism at room temperature and had a K of 1.5 × 104 J m−3, which is in the optimal K range (Figure 25A). The core/shell NPs had an appropriately 5 times higher SLP value of 2280 W g−1 compared to single component NPs (443 W g−1 for the CoFe2O4 NPs and 411 W g−1 for the MnFe2O4 NPs) in an AMF of H = 37.3 kA m−1 and f = 500 kHz (Figure 25B). In vivo hyperthermia therapy was investigated by injecting 75 μg of the CoFe2O4/MnFe2O4 NPs into the U87MG human brain cancer cells in mice and subsequently treating with the AMF for 10 min (Figure 25C). The tumor was eliminated after the hyperthermia treatment with the core/shell NPs for 18 days. As a comparison, the tumors did not shrink by using Feridex, or chemotherapeutic doxorubicin under the similar treatments, but actually increased with the growth behavior similar to untreated control (Figure 25, panels D and E).

Figure 26. Schematics of (A) perpendicular recording hard-disk drive and (B) heat-assisted magnetic recording system. A laser is used to heat the medium locally in order to lower the magnetic recording field. Reprinted with permission from ref 383. Copyright 2009 Nature Publishing Group.

field changes the magnetization direction of a small area of the recording medium under the write-head, reserving what is recorded on the film as one bit. The read-head is made of a giant magnetoresistance sensor and can translate the recorded magnetic bit information back to digitized electric signals. Currently, the magnetic recording medium in a HDD is based on the thin film assembly of ∼8.5 nm nanomagnets of CoCrPt made from the sputtering method, which can support recording densities over 750 Gb in−2.21 To increase the recording density in the HDD, the size of the recording media needs to be further reduced. However, lack of control over the size and size distribution of nanomagnets made from the sputtering method limits the increase in recording density. Moreover, due to the superparamagnetic limit, this kind of recording medium cannot support stable ferromagnetism when their sizes are further reduced. To retain a much higher recording density at room temperature, the recording medium should have high magnetic anisotropy K to withstand thermal fluctuation. On the other hand, if the NPs with much larger K value are used, a new recording magnetic field that is much stronger than the one used in the conventional HDD must be generated first to reverse the magnetization direction of these hard magnetic NPs. As the current technology has its limit in producing this strong magnetic field for the recording purposes, an alternative approach to use heat to lower the NP magnetization reversal barrier is applied. This is called “heatassisted magnetic recording (HAMR)” in which heat is generated from laser (Figure 26B).383 With the use of the HAMR, small NPs of 3−5 nm with high K can be used to support magnetic recording at an areal density over 1.5 Tb in−2.21,384 Magnetic tape recording is another important recording technique. The recording basic is the same as the HDD

5. MAGNETIC NANOPARTICLES FOR DATA AND ENERGY STORAGE APPLICATIONS Different from superparamagnetic NPs, ferromagnetic NPs, once magnetized, can retain remnant magnetization (Mr) even after the external magnetic field is removed. Due to this property, an array of ferromagnetic NPs can be utilized as a recording medium for magnetic data storage or as a permanent magnet component to store magnetic energy. 5.1. Magnetic Nanoparticles for Data Storage

The synthesis of monodisperse magnetic NPs has enabled the detailed understanding of nanomagentism, which makes it possible to design new magnetic NPs to increase the bit density for magnetic recording. As discussed in section 2, fine control of ferromagnetic NPs over a specific size, structure, and composition is critical for optimizing the NP ferromagnetism for magnetic recording. From a simple geometry viewpoint, the smaller the ferromagnetic NPs, the higher the recording areal density. But considering the superparamagnetic limit of the ferromagnetic NPs, there should have a balance between the NP size and the thermal stability of the NP-like recording media. For a commercially viable recording media, the energy barrier required to reverse the magnetization from one direction to another should be at least 60 times higher than the thermal energy (KV/kBT ≥ 60).21 5.1.1. Fundamental of Magnetic Recording. There are two important schemes for magnetic recording: hard disk drive (HDD) recording and magnetic tape recording. Figure 26A is the schematic of the modern HDD perpendicular recording.383 It consists of three components: an assembly of ferromagnetic NPs, a write-head, and a read-head. In the writing process, the write-head is responsible for converting the flux current that carries the digital information into a flux external field. The flux 10494


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recording but with different device configuration and recording media. Magnetically softer NPs, such as γ-Fe2O3 and FeCobased NPs, are selected as the common recording media. But because of their small K values, the NPs have to be made in a needle shape (to obtain large shape anisotropy) with much larger dimensions than those used in HDD. This leads to a much smaller areal density in the magnetic tape media than that in HDD. But magnetic tape has a much larger surface area. Therefore, the overall storage capacity of a magnetic tape device is comparable to the HDD. With its much lower cost, the magnetic tape recording is playing an important role in big data storage.20 5.1.2. Magnetic Nanoparticles as Recording Media. The L10-FePt NP system is the most promising candidate for HAMR due to its very large K (7 × 106 J m−3), Tc (750 K), and robust chemical stability. A fully ordered L10-FePt enables a thermally stable grain with very small size down to 3 nm at KV/ kBT = 60 (T = 350 K). To be utilized as the recording medium, the L10-FePt NPs must be prepared with uniform sizes and assembled in an array with the preferred magnetic alignment. An earlier study showed that the organic phase synthesis could be used to prepare monodisperse FePt NPs, and their assembly could support magnetization reversals.59 In this study, the assynthesized 4 nm A1-FePt NPs were superparamagnetic. Once assembled into a thin NP film and annealed in an inert atmosphere at 560 °C for 30 min, the A1-FePt NPs were converted into ferromagnetic L10-FePt NPs without any obvious sign of NP aggregation (Figure 27A). Preliminary recording experiments performed on an assembly with the Hc

of 1800 Oe showed that these 4 nm L10-FePt NPs could support magnetization reversals at linear densities of 500, 1040, 2140, and 5000 flux changes per millimeter, and the recorded information could be read back nondestructively (Figure 27B). High-density recording requires uniform and small FePt NPs with strong ferromagnetism. However, earlier studies indicated that it was challenging to convert the A1-FePt NPs into fully ordered L10-FePt NPs while still maintaining the NP uniformity. In a following-up investigation, monodisperse FePt (3.5 nm) and MnO (11 nm) NPs were coassembled and annealed at 650 °C to obtain the L10-FePt NP assembly.385 Because of the protection offered by the thermally stable MnO NPs, the L10-FePt NPs had no aggregation in the annealed binary assembly. A more convenient approach to protect the FePt NPs against high temperature aggregation/sintering was to coat the A1-FePt NPs with a robust shell such as SiO2187,188 or MgO.189,192 The FePt NPs might also be doped with a third element, such as Au,164 Ag,165 or Cu,386 to lower the phase transformation temperature. Magnetic FeCo-based NPs have been used in magnetic tape recording. In the commercial Linear Tape Open generation-5 tape drives, a density of 1.2 Gb in−2 and a cartridge capacity of 1.5 TB have been realized by using the rod-shape FeCo NPs.56 The demand for even higher recording density requires the magnets to be smaller, which is difficult to achieve with the current FeCo NPs due to their small K. Chemical synthesis of monodisperse magnetic NPs allows the precise control of the NP size, shape, and magnetic properties, providing a new NP system as recording media for high density data storage. Recently, monodisperse 18 nm CoFe2O4 NPs with a Hc of 1000 Oe were synthesized and assembled in a thin polymer matrix for magnetic recording demonstration.387 In this study, the CoFe2O4 NPs were coated with a diblock copolymer and then spin-coated into a thin film with the 3 layers of NPs assembled inside. An external field was applied to magnetically align these NPs. Magnetic read/write experiments indicated that digital information could be stored in the assembled CoFe2O4 NP arrays and read back nondestructively. Further studies indicated that the CoxFe3−xO4 NPs could offer an even larger K than the CoFe2O4 NPs when x = ∼0.6.61,382 Therefore, the monodisperse 20 nm Co0.6Fe2.4O4 NPs with a room temperature Hc of 1930 Oe were synthesized and assembled at the water−air interface into a large area monolayer assembly (Figure 28A) as the recording media.37 The magnetic field image of the readback amplitude (Figure 28B) demonstrated a successful magnetic recording in the monolayer assembly. 5.2. Magnetic Nanoparticles as Building Blocks of Permanent Magnet

Another important application of ferromagnetic NPs is as building blocks of a permanent magnet for magnetic energy storage applications. A figure-of-merit for evaluating the performance of a permanent magnet is the maximum energy product (BH)max, which corresponds to the area of the largest rectangle inside the second quadrant of the B−H hysteresis loop. The higher the (BH)max, the smaller the magnet needed for a given energy storage/output in magnetic devices such as direct-current motors and wind turbines. In order to obtain a permanent magnet with high (BH)max, both high Hc and Mr are required. In 1991, a hard−soft composite material was proposed to show the desirable properties of both phases when the two phases were properly coupled via exchange interaction across

Figure 27. (A) Scanning electron microscopy (SEM) image of an array of 4 nm FePt NPs obtained after annealed at 560 °C. (B) Magneto-resistive read-back signals from written bit transitions. The individual line scans reveal magnetization reversal transitions at linear densities of (a) 500, (b) 1040, (c) 2140, and (d) 5000 flux changes per millimeter. Reprinted with permission from ref 59. Copyright 2000 American Association for the Advancement of Science. 10495


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Figure 29. TEM images of (A) randomly occupied 4 nm/4 nm FePt/ Fe3O4 NP arrays and (B) phase-segregated 4 nm/12 nm FePt/Fe3O4 NP arrays. Corresponding hysteresis loops of (C) the nanocomposites from the array (A) and (D) the nanocomposites from the array (B) after annealing at 650 °C for 1 h under Ar + 5% H2. Reprinted with permission from ref 143. Copyright 2002 Nature Publishing Group.

and to promote partial interdiffusion between Fe and FePt to a new soft phase Fe3Pt. For the assembly of the 4 nm FePt and 4 nm Fe3O4 NPs, the optimal mass ratio was found to be FePt:Fe3O4 = 10:1, and the FePt-Fe3Pt nanocomposite showed a smooth hysteresis loop (Figure 29C), implying a coherent switching and thus an efficient coupling of the FePt and Fe3Pt phases. Structural characterizations revealed that the soft phase Fe3Pt was about 5 nm and was uniformly dispersed into the hard phase FePt matrix. Such an exchange-coupled nanocomposite yielded an enhanced energy product (BH)max of 20.1 MGOe which 50% higher than 13 MGOe expected from the nonexchange-coupled isotropic FePt. As a control, the annealed nanocomposite from the phase-segregated assembly of 4 nm FePt NPs and 12 nm Fe3O4 NPs showed a kinked loop (Figure 29D), indicating that there existed an uncoupled soft Fe phase in the composite. To control the hard−soft distribution within the nanocomposite structure, bimagnetic core/shell FePt/Fe3O4 NPs were synthesized with the core size controlled at 4 nm and shell thickness tuned from 0.5−3 nm.96 Upon reductive annealing, the 4 nm/1 nm FePt/Fe3O4 NPs were converted into a hard magnetic composite with smooth hysteresis loop and much increased magnetic properties (Hc = 13.5 kOe and Ms = 1040 emu cc−1, (BH)max = 18 MGOe). In a recent design, an exchange-coupled system was directly prepared from core/shell L10-FePt/Co NPs without thermal annealing.191 In this case, the L10-FePt NPs were first prepared by thermal annealing of the A1-FePt-Fe3O4 dumbbell NPs coated with MgO. Once annealed to obtain the L10-FePt/MgO NPs, the MgO was removed by an acid wash, and the obtained L10-FePt NPs were redispersed and used as seeds for the growth of soft magnetic Co shell. The L10-FePt NPs were ferromagnetic at room temperature with Hc = 33 kOe and Ms = 33 emu g−1. In the exchange-coupled L10-FePt/Co NPs, the magnetic properties were tuned by the thickness of the Co shell with the Hc controlled from 33 to 0.6 kOe and Ms from 33 to 133 emu g−1. Self-assembly of these core/shell NPs should provide a promising approach to FePt-based nanocomposites for under-

Figure 28. (A) SEM image of the monolayer assembly of Co0.6Fe2.4O4 NPs. (B) Magnetic field image of the read-back amplitude corresponds to the write signals at linear densities ranging from 254 (left) to 31 (right) kilo flux changes per inch. Reprinted from ref 37. Copyright 2014 American Chemical Society.

the interface.388 In an ideal exchange-coupled system, the magnetic hard and soft phases are situated alternately and in direct contact with each other. The size of the soft phase should be smaller than twice the domain wall width of the hard magnetic phase.389 The domain wall width of the common hard-phase magnet falls in the range of 3−15 nm. Thus, the soft-phase magnet in the exchange-coupled composite should have a critical dimension of 6−30 nm. A successful exchangecoupled composite is expected to show a single phase behavior and to have both a large Hc and an increased Mr. The common physical techniques, such as mechanical milling and melt spinning, have been used to fabricate exchange-coupled nanocomposites,390−393 but they have shown difficulty in controlling the hard and soft phases at the nanoscale and in achieving the uniform phase distribution for the effective exchange coupling. The solution phase synthesis discussed in this review, followed by a rational assembly, may offer a promising solution. The FePt NPs prepared from the organic phase synthesis were first selected for the model system of exchange coupling due to their strong ferromagnetism and their chemical stability.143 In the demonstration, both A1-FePt and Fe3O4 NPs were prepared and then self-assembled in 3D binary arrays by mixing their dispersions in hexane followed by slow evaporation of the hexane. Figure 29 (panels A and B) shows the TEM images of two binary assemblies of 4 nm FePt with 4 and 12 nm Fe3O4 NPs, indicating clearly the size effect on the binary assembly. The binary assembly was then annealed under Ar + 5% H2 at 650 °C for 1 h to remove surfactants, to reduce Fe3O4 into metallic Fe, to transform A1-FePt into L10-FePt, 10496


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Figure 30. (A) Schematic of the synthesis of Fe2O3/SiO2/Pd nanocomposites. (B) TEM images of as-prepared SiO2/Fe2O3 (left inset) and Fe2O3/ SiO2−SH/Pd nanocomposite. (C) Conversion of hydrogenation of nitrobenzene over (●) Fe2O3/SiO2−NH2/Pd, (■) Fe2O3/SiO2−SH/Pd, and (▲) commercial Pd/C. (Inset) Magnetic separation of Fe2O3/SiO2−NH2/Pd from the reaction medium. Reprinted from ref 406. Copyright 2006 American Chemical Society.

6. MAGNETIC NANOPARTICLES FOR CATALYTIC APPLICATIONS The advances in synthetic methodology toward the magnetic NPs make it possible to extend the application fronts of these NPs not only in magnetism but also in catalysis. There have been tremendous efforts in exploring the dispersible magnetic NPs as supports to anchor homogeneous catalysts, to stabilize and to recycle them for long-term use. More importantly, these transition-metal based magnetic NPs themselves can serve as efficient catalysts for many chemical reactions. In this section, we highlight a few examples on using magnetic NPs as catalyst supports and as catalysts for chemical reactions.

standing the exchange-coupling at the nanoscale and for building high performance nanocomposite magnets. Similar to L10-FePt, L10-FePd also exhibits a large anisotropy value (K = 1.0 × 106 J m−3). Exchange-coupled nanocomposite of L10-FePd-Fe was reported from the sea urchin-like FePdFe3O4 nanostructures.194 Annealing at 500 °C under a reducing atmosphere (Ar + 5% H2) allowed the transformation of chemically disordered A1-FePd (Fe/Pd = 45:55) into ordered L10-FePd and the reduction of Fe3O4 into Fe. The composite showed a single phase hysteresis loop (Hc = 2.6 kOe, Ms = 90 emu g−1), indicating the exchange-coupling between Fe and L10-FePd was established. The SmCo5 alloy has an even higher magnetic anisotropy (K = ∼2 × 107 J m−3) and is an important practical material for permanent magnet applications. But its low magnetization value limits its magnet performance, which may be improved by fabricating the nanocomposites of SmCo5-Fe with SmCo5 and Fe being effectively exchange-coupled. The early experiments showed that the exchange-coupled SmCo5-Fe could be fabricated via simultaneous calcium reduction of Sm-Co-oxide and Fe3O4 NPs in a KCl matrix.394 Due to the protection of the KCl matrix, the final grain sizes of the SmCo5 phase and the Fe phase remained in the nanoscale with average sizes of 29 and 8 nm, respectively. The largest Hc of 11.6 kOe was achieved for the SmCo5-Fe0.23 while the largest Mr of 56 emu g−1 was obtained for the SmCo5-Fe1.7. Alternatively, the nanocomposite could be made by reducing a mixture of Sm2O3, Co, and Fe NPs.203 The energy products (BH)max were about 6.0, 7.1, and 7.8 MGOe for the SmCo5, SmCo5.5-Fe0.6, and SmCo6.5-Fe1.2, respectively. Currently, the synthetic challenge is still lying in the preparation of monodisperse SmCo5 NPs and protection of them from sintering/oxidation during the fabrication process.

6.1. Magnetic Nanoparticles as Catalyst Supports

Due to the strong response to an external magnetic field, magnetic NPs can function as dispersible magnets in a reaction system, providing an ideal platform on which a catalyst can be anchored by proper linker chemistry and recycled by magnetic separation. The most commonly used catalyst supports are iron oxide NPs. In the studies, these magnetic NPs are often coated with a layer of robust shell, such as silica or a polymer shell, to protect the iron oxide core from an unexpected corrosion and to facilitate the catalyst coupling.25 So far, a large variety of catalysts, including metal NPs,395−397 semiconductors,398 organic molecules,399−401 metal−organic complexes,402,403 and enzymes,404 have been grafted onto the magnetic NP supports for various organic reactions. More details of the studies can be found in several representative topical review papers.24−27,405 A typical example of using magnetic NPs as the catalyst support involves depositing Pd nanoclusters on the silicacoated Fe2O3 NPs for the hydrogenation of nitrobenzene.406 In this study, the Fe2O3/SiO2 core/shell NPs were prepared from the reverse microemulsion method, followed by further functionalization with (3-mercaptopropyl)-trimethoxysilane or 10497


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N-(2-aminoethyl)-3-aminopropyltrimethoxysilane as an affinity ligand to capture Pd nanoclusters (

Organic Phase Syntheses of Magnetic Nanoparticles and Their

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