Magnetic Nanomaterials: Chemical Design, Synthesis, and Potential

Feb 7, 2018 - We hope that this Account will clarify the progress on the controllable preparation of MNMs with specific compositions, sizes, and struc...
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Cite This: Acc. Chem. Res. 2018, 51, 404−413

Magnetic Nanomaterials: Chemical Design, Synthesis, and Potential Applications Kai Zhu,†,‡ Yanmin Ju,†,§ Junjie Xu,† Ziyu Yang,† Song Gao,∥ and Yanglong Hou*,†,‡ †

Beijing Key Laboratory for Magnetoelectric Materials and Devices (BKLMMD), BIC-EAST, Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China ‡ Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China § College of Life Science, Peking University, Beijing 100871, China ∥ College of Chemical and Molecular Engineering, Peking University, Beijing 100871, China CONSPECTUS: Magnetic nanomaterials (MNMs) have attracted significant interest in the past few decades because of their unique properties such as superparamagnetism, which results from the influence of thermal energy on a ferromagnetic nanoparticle. In the superparamagnetic size regime, the moments of nanoparticles fluctuate as a result of thermal energy. To understand the fundamental behavior of superparamagnetism and develop relevant potential applications, various preparation routes have been explored to produce MNMs with desired properties and structures. However, some challenges remain for the preparation of well-defined magnetic nanostructures, including exchange-coupled nanomagnets, which are considered as the next generation of advanced magnets. In such a case, effective synthetic methods are required to achieve control over the chemical composition, size, and structure of MNMs. For instance, liquid-phase chemical syntheses, a set of emerging approaches to prepare various magnetic nanostructures, facilitate precise control over the nucleation and specific growth processes of nanomaterials with diverse structures. Among them, the high-temperature organic-phase method is an indispensable one in which the microstructures and physical/chemical properties of MNMs can be tuned by controlling the reaction conditions such as precursor, surfactant, or solvent amounts, reaction temperature or time, reaction atmosphere, etc. In this Account, we present an overview of our progress on the chemical synthesis of various MNMs, including monocomponent nanostructures (e.g., metals, metal alloys, metal oxides/carbides) and multicomponent nanostructures (heterostructures and exchange-coupled nanomagnets). We emphasize the high-temperature organic-phase synthetic method, on which we have been focused over the past decade. Notably, multicomponent nanostructures, obtained by growing or incorporating different functional components together, not only retain the functionalities of each single component but also possess synergic properties that emerge from interfacial coupling, with improved magnetic, optical, or catalytic features. Herein, potential applications of MNMs are covered in three representative areas: biomedicine, catalysis, and environmental purification. Regarding biomedicine, MNMs can detect or target biological entities after being modified with specific biomolecules, and they can be applied to magnetic resonance imaging, imaging-guided drug delivery, and photothermal therapy. Apart from their magnetic features, the catalytic performance of some MNMs resulting from their highly specific chemical components and surface structure will be briefly introduced, highlighting its impact in the methanol oxidation reaction, the oxygen reduction reaction, the oxygen and hydrogen evolution reactions, and the Fischer−Tropsch synthesis. Finally, environmental purification, primarily for water remediation, will be highlighted with two main aspects: the effective capture of bacteria and the removal of adverse ions in wastewater. We hope that this Account will clarify the progress on the controllable preparation of MNMs with specific compositions, sizes, and structures and generate broad interest in the realms of biomedicine and catalysis as well as in environmental issues and other potential applications.

1. INTRODUCTION Magnetic nanomaterials (MNMs) have recently undergone intensive research because of their suitable properties for a diverse set of potential applications in biomedicine, catalysis, etc.1−9 At the nanoscale, magnetic materials display novel physical effects that distinguish them from their bulk counter© 2018 American Chemical Society

parts. This phenomenon is known as nanomagnetism, and it endows MNMs with unique properties such as superparamagnetism. It should be noted that well-defined nanostructures in Received: August 20, 2017 Published: February 7, 2018 404

DOI: 10.1021/acs.accounts.7b00407 Acc. Chem. Res. 2018, 51, 404−413

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In this section, we will provide some typical synthetic strategies to produce representative monocomponent and multicomponent magnetic nanostructures.

MNMs are critical to achieve these unique properties. However, great challenges remain in obtaining monodisperse magnetic nanostructures because of the dipolar interactions and surface effects of MNMs at the nanoscale, as well as in addressing the control of grains, antioxidation, etc. Fortunately, chemical synthesis offers innovative approaches for precise control over the nucleation and growth processes of MNMs at the atomic level. Therefore, the size and morphology of MNMs can be tuned through a specific growth strategy by adjusting the surfactants, solvents, and reaction parameters. Our group is focusing on developing effective methods for the synthesis of high-quality MNMs and adopting various techniques to investigate their special properties and potential applications. This Account mainly summarizes our contributions in developing MNMs with various nanostructures to achieve fundamental magnetism phenomena and for diverse applications. First, we will briefly review a general synthetic strategy for the controllable synthesis of MNMs with an emphasis on the high-temperature organic-phase method, and then we will address their potential applications in diverse fields including biomedicine, catalysis, and environmental purification.

2.2. Monocomponent MNMs

2.2.1. Metal NPs: Fe, Ni, Co. Iron NPs are one of the most representative kinds of nanostructures. The pioneering synthesis of monodisperse Fe NPs involved the decomposition of Fe(CO)5 in the presence of oleic acid (OA), a process that has many remaining challenges in the light of the low stability of Fe NPs under air.10 Regarding the sensitivity of Fe NPs toward oxygen, Hou and Gao11 developed a facile aqueous synthesis process to produce 65 nm body-centered cubic (bcc) iron NPs (Figure 1A), during which poly(N-vinylpyrrolidone) (PVP) plays a crucial role in passivation of the metal surface for antioxidation.

2. CHEMICAL SYNTHESIS OF MNMS 2.1. General Strategy

Liquid-phase chemical syntheses offer effective routes to produce magnetic nanostructures because of their precise control over the size, composition, and structure of the resulting nanomaterials. Among them, the high-temperature organic-phase method requires high reaction temperatures (as high as the boiling point of the reactants involved) and a more inert environment, which results in better control over and higher precision in the features the products. According to the LaMer model of monodisperse nanoparticles (NPs),7 the concentration of monomers is enriched with increasing reactant amounts in the initial stage of reaction. Burst nucleation occurs once the concentration of the reactants reaches a critical point, leading to a decrease in the amount of monomers. The growth process takes place after the concentration drops to the “nucleation level”, which is driven by adjusting the free energy of the system. Thus, the final sizes and structures of the resulting NPs can be determined by controlling the relationship between the processes of nucleation and growth.3 Specially, when surfactants are employed to promote the oriented growth process, shaped NPs or one/two-dimensional nanostructures are obtained. In nature, the surface energy of the formed nuclei controls the growth process, and therefore, it affects the targeted products, which may be tuned precisely by varying the solvent and surfactant amounts. Surfactants are also critical to produce monodisperse NPs by optimizing the binding reactions between the NPs and their official groups.3 The mechanisms involved can be described as follows: (a) forming protective layers to avoid oxidation; (b) reducing the surface energy of the NPs to avoid aggregation; (c) controlling the sizes of the NPs by tuning the nucleation and growth processes; and (d) promoting oriented growth of the nucleus to form shaped MNMs. Moreover, the reaction parameters also make a great difference in the final structure. According to nanomagnetism, the magnetic properties of MNMs are highly size-dependent. When the grain size decreases to a critical value, a single-domain grain forms, in which the coercivity is size-dependent and reaches a maximum. Thus, controlling the grain size of MNMs becomes a key point for highperformance nanomagnets.

Figure 1. (A) TEM image of 65 nm Fe NPs. Reprinted with permission from ref 11. Copyright 2003 Elsevier B.V. (B) TEM image of 3.7 nm Ni NPs. Reprinted with permission from ref 12. Copyright 2003 Royal Society of Chemistry. (C, D) TEM images of (C) 6 nm and (D) 9 nm Co NPs. Reprinted with permission from ref 13. Rights managed by AIP Publishing LLC.

To obtain smaller and monodisperse NPs, a high-temperature organic-phase method was developed after the critical role of surfactants in the reaction was understood. Monodisperse Ni NPs with sizes of about 3.7 nm were synthesized by the reduction of Ni(acac)3 in hexadecylamine (HDA) without further sizeselective processes (Figure 1B).12 In the case of Co NPs, a bulky trialkylphosphine limited their growth, and thus, smaller particles with sizes of 2−6 nm were produced (Figure 1C), while a less bulky trialkylphosphine led to larger particles with sizes of 7−11 nm (Figure 1D), which may be attributed to the reverse coordination of the surfactants with neutral metal surface sites.13 2.2.2. Metal Alloy Nanostructures. As metal alloy nanostructures, FePd and FePt are particularly representative because of their chemical stability and high magnetocrystalline anisotropy. Hou et al.14 developed a facile high-temperature organic-phase thermal decomposition process to prepare monodisperse FePd NPs, with adamantanecarboxylic acid and tributylphosphine as stabilizers. They found that tuning the molar ratio of surfactants to offer suitable surface energy is a prerequisite for the preparation of monodisperse NPs. FePd NPs with tunable size from 11 to 16 nm (Figure 2A,B) exhibited superparamagnetic properties at room temperature. 405

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obvious size change and aggregation of the 8 nm fct-FePd NPs was observed (Figure 2C,D). Anisotropic nanostructures such as nanowires (NWs) might possess properties distinct from those of NPs. Hou and coworkers reported facile solvothermal approaches to prepare controllable FePt NWs (Figure 2E,F).17 Ethylenediamine could remarkably influence the morphology of the FePt NWs, acting as a bidentate ligand to form a relatively stable complex ion. Subsequently, a high-temperature organic-phase method was developed for uniform FePt nanorods/nanowires (NRs/NWs). The one-dimensional structure was attributed to the oriented growth of FePt nuclei formed within a reverse-micelle-like structure. Diverse packing densities of oleylamine on FePt nuclei caused the anisotropic growth, leading to the one-dimensional shape of NWs (Figure 2G), while dilution of oleylamine with octadecene induced the formation of NRs with lengths of about 20 nm (Figure 2H).18 In addition, permanent rare-metal permanent nanomagnets with a large magnetic anisotropy can be similarly prepared by the method previously described with a further annealing step. Hou et al.19 developed a synthetic process for 32 nm SmCo5 NPs based on the synthesis of Co@Sm2O3 core@shell nanostructures followed by reductive annealing aided by Ca granules. The coercivity of the resulting SmCo5 nanomagnets could reach up to 8 kOe at room temperature. 2.2.3. Metallic Oxides. MFe2O4 NPs (M = Fe, Co, Mn, etc.) are attracting wide interest because of their magnetic properties and chemical stability. Among them, Fe3O4, which possesses a cubic-closest-packed inverse spinel structure and semimetallic performance, has shown great potential in the fields of magnetic separation and biomedicine. Fe3O4 NPs could be prepared by a facile route based on Fe complexes, usually under alkaline conditions.20 To achieve more precise control of their size and

Figure 2. (A, B) TEM images of (A) 11 nm and (B) 16 nm FePd NPs. Reprinted from ref 14. Copyright 2004 American Chemical Society. (C, D) TEM images of (C) fcc-FePt@Fe3O4 and (D) fct-FePt NPs. Reprinted with permission from ref 16. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (E) TEM image of FePt NWs and (F) HRTEM image after annealing. Reprinted with permission from ref 17. Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (G, H) TEM images of (G) FePt NWs and (H) FePt NRs. Reprinted with permission from ref 18. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Monodisperse face-centered cubic (fcc) FePt NPs were first synthesized by a wet-chemical route based on the reduction of Pt(acac)2 and decomposition of Fe(CO)5.15 However, when changed to a face-centered tetragonal (fct or L10) phase by annealing, the fcc structures tended to aggregate. Hou and coworkers prepared fcc-FePt-Fe3O4@MgO NPs that were converted into fct-FePt/MgO NPs by thermal annealing in an Ar/H2 atmosphere.16 In this process, hydrogen reduced Fe3O4 to Fe, which further diffused into a FePt matrix to form fct-FePt. A layer of MgO protected the NPs against aggregation and was subsequently etched away to produce clean fct-FePt NPs. No

Figure 3. (A) TEM image of 16 nm Fe3O4 NPs. Reprinted from ref 22. Copyright 2002 American Chemical Society. (B−D) TEM images of (B) 7 ± 0.5 nm, (C) 8 ± 0.4 nm, and (D) 10 ± 0.8 nm Fe3O4 NPs. The scale bars are 20 nm. Reprinted from ref 21. Copyright 2009 American Chemical Society. (E) TEM image of octahedral Fe3O4 NPs. Reprinted with permission from ref 24. Copyright 2009 Royal Society of Chemistry. (F) TEM image of Fe3O4 nanoprisms. Reprinted with permission from ref 23. Copyright 2010 Royal Society of Chemistry. (G−I) TEM images of (G) 14 nm spherical and (H) 32 nm and (I) 53 nm truncated octahedral FeO NPs and (J) SEM image of truncated octahedral FeO NPs. Reprinted with permission from ref 25. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (K, L) TEM images of (K) Co(OH)2 and (L) CoO NPLs. Reprinted from ref 26. Copyright 2005 American Chemical Society. 406

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phase transformation by tuning the bonding between Fe and C atoms.28 As expected, all of the above-mentioned iron carbides possess weak ferromagnetic properties (Figure 5).

distribution, investigations involving organic-phase methods were conducted, indicating that the particle size could be tuned from 4 to 20 nm by controlling the solvent or surfactant (Figure 3A−D).21,22 OA was expected to contact with Fe3+ to form Fe3+−OOC−, and oleylamine acted as a capping agent during the particle growth. In regard to iron oxide shapes, octahedral Fe3O4 NPs and Fe3O4 nanoprisms (Figure 3E,F) can also be obtained analogously by using oleylamine to tune the energy of each facet and promote oriented growth in specific directions.23,24 Similar reactions of Fe(acac)3 are suitable for the formation of FeO NPs. The surfactant ratio can regulate the competitive growth in specific directions to form truncated octahedral NPs.25 Furthermore, the FeO NPs can be converted into other iron oxides, such as Fe3O4, after annealing. Figure 3G−J shows various morphologies and sizes of FeO NPs that were obtained with various heating parameters. In addition, cobalt oxides have also been synthesized using similar routes. A hydrothermal process was carried out with Co(NO3)3 in the presence of PVP to form Co(OH)2 nanoplatelets (NPLs), which could be converted into CoO NPLs (Figure 3K,L).26 2.2.4. Metal Carbides. Iron carbides, such as Fe5C2, Fe3C, and Fe2C, hold promise as suitable platforms for biomedicine because of their stability and high magnetic saturation compared with iron oxides. However, they have rarely been studied, mostly because of the great challenges associated with synthetic strategies, especially those targeting control over the phase, size, and morphology. Recently, Hou and co-workers reported a facile chemical route to synthesize Fe5C 2 NPs by the decomposition of Fe(CO)5 in octadecylamine.27 During the synthesis, the highly crystalline Fe NPs were carbonized to form Fe5C2 NPs. It is noteworthy that bromide, which was added to tune the surface energy, was critical for the formation of iron carbide NPs, but the mechanism involved is not yet fully understood. As shown in Figure 4A,B, the Fe5C2 NPs produced

Figure 5. Magnetic properties of iron carbides at (A) 300 K and (B) 2 K. Reprinted with permission from ref 28. Copyright 2017 Royal Society of Chemistry.

Clearly, diverse types of metal carbides could be obtained by controlling the synthetic process, especially by introducing halide ions, whose selective absorption may have affected the carbon penetration and content. Furthermore, the specific formation of different phases is achieved by tuning the atomic bonding during the synthesis. 2.3. Multicomponent MNMs

2.3.1. Heterostructures. Magnetic heterostructures possess unique properties due to their diverse chemical composition, which includes a magnetic part merged with additional functional components to produce multifunctional composites. The integration of different components and their intimate contact at the interface provide newfangled properties. A seed-mediated process is frequently carried out to prepare multicomponent MNMs, in which presynthesized monocomponent MNMs usually serve as seeds for either heteroepitaxial growth or chemical deposition of other components. Representatively, core@shell heterostructures are under intensive investigation because of their special properties such as protection or modification by the shell and the synergistic effects of each component. Xu et al.29 developed an organicphase method to prepare Fe3O4@Au@Ag NPs with controllable magnetic properties, as illustrated in Figure 6A,B. Fe3O4 NPs (i) were first synthesized as mentioned above, and HAuCl4 was reduced in their presence to synthesize the Fe3O4@Au core@ shell structure (ii). Then (ii) was transferred to the aqueous phase (iii). Finally, a thicker Au or Au@Ag layer was grown on the hydrophilic structure (iii), resulting in tunable plasmonic properties. FePt-based heterostructures such as FePt−Au have also been widely investigated. FePt can provide outstanding magnetic properties, whereas Au is well-known for its applications in catalysis or biomedicine. A facile synthesis process for FePt−Au heterostructured nanowires (HNWs) was reported, in which the heteroepitaxial growth of Au NPs onto FePt NRs was realized (Figure 6C).30 With precise control of Au complexes, FePt−Au with tunable structures can be obtained (Figure 6D−F). By a similar methodology, the synthesis of FePt−Au heterostructured NPs was developed (Figure 6H−J) where an inducing reductive atmosphere led to the formation of FePt−Au structures from heterodimers to FePt−Aun NPs.31

Figure 4. (A) TEM and (B) HRTEM images of Fe5C2. Reprinted from ref 27. Copyright 2012 American Chemical Society. (C−F) TEM images of (C) hexagonal Fe2C, (D) monoclinic Fe2C, (E) monoclinic Fe5C2, and (F) orthorhombic Fe3C NPs. Reprinted with permission from ref 28. Copyright 2017 Royal Society of Chemistry.

were about 20 nm in size with an amorphous shell. Furthermore, they presented a versatile solution chemistry route for producing various iron carbide NPs, including Fe2C with hexagonal and monoclinic syngony, Fe5C2 with monoclinic syngony, and Fe3C with orthorhombic syngony, based on Fe@Fe3O4 nanostructures in one system (Figure 4C−F), in which halide ions control the 407

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both the high coercivity of the hard phase and the high magnetization of the soft phase into one system (Figure 7).

Figure 7. Exchange-coupling effect between hard and soft magnetic phases. Reprinted with permission from ref 2. Copyright 2014 Royal Society of Chemistry.

Theoretical calculations indicate that only when the size of the soft magnetic phase is smaller than twice the thickness of the domain wall of the hard magnetic phase can the effective exchange-coupling effect work, which demands very precise control in the preparation process. Chemical synthesis supplies a reliable strategy for this and opens up corridors for a deep understanding of the interactions between magnetic moments at the nanoscale.2,32 Coating hard magnetic cores with soft magnetic shells is an effective route to obtain the exchange-coupling effect. For example, fct-phase FePt NPs have coercivities of up to 33 kOe, while the saturation magnetization is only 33 emu/g. However, when they are coated with a soft magnetic shell, the magnetization increases to 133 emu/g.16 During this process, a reductive reaction of Co(acac)2 (or Ni(acac)3 or Fe(acac)2) was carried out on the surface of as-synthesized fct-FePt NPs to form fct-FePt@Co (or Ni, Fe2C) NPs (Figure 8A−C). Analogously, fct-FePd/α-Fe exchange-coupled nanocomposites were developed as shown in Figure 8D−G.33 The seed-mediated synthesis provides an effective bottom-up approach for obtaining exchange-coupled nanomagnets, which may act as building blocks for high-performance nanostructured materials in the future. In addition, some efforts have been made to synthesize rareearth permanent MNMs. Nd2Fe14B-based exchange-coupled nanocomposites are under exploration to improve (BH)max. For instance, Nd2Fe14B/α-Fe nanocomposites with optimized magnetic properties have been prepared by annealing of a Nd−Fe−B−O mixture, providing a high (BH)max above 10 MG· Oe.34 Apart from the Nd−Fe−B series, SmCo5-based nanocomposites have been synthesized similarly.35 To obtain singledomain nanomagnets, Sm[Co(CN)6]·4H2O@GO (GO = graphene oxide) was initially prepared, and subsequent reductive annealing with Ca granules formed the 200 nm core@shellstructured SmCo5@Co and SmCo5@Sm2Co17 NPs with larger coercivity and high saturation magnetization (Figure 8H−J).36 Substantial homogeneous mixing of the metal source was sufficient to ensure the coreduction of each component during annealing, avoiding aggregation of Fe and the formation of α-Fe. As described above, chemical synthesis offers an effective approach to produce mono- and multicomponent MNMs. The controlled synthesis of MNMs depends on the design of chemical reactions. We should consider various parameters for obtaining specific MNMs, including the intrinsic crystalline structure, the surface energy, the nucleation and growth processes, and the reaction thermodynamics and kinetics. Overall, chemical synthesis paves a profound way to understand

Figure 6. (A, B) Schematic illustrations of the syntheses of (A) Fe3O4@ Au [(i) Fe3O4; (ii) Fe3O4@Au; (iii) hydrophilic Fe3O4@Au] and (B) Fe3O4@Au(@Ag. Reprinted from ref 29. Copyright 2007 American Chemical Society. (C) Schematic illustration of FePt−Au HNWs. FePt NWs were first prepared at high temperature, and then Au nanocrystals were grown onto them to form FePt−Au HNWs. By control over the amount of H2, different morphologies were obtained, such as (D) tadpole-like, (E) dumbbell-like, and (F) bead-like HNWs. Reprinted with permission from ref 30. Copyright 2011 Tsinghua University Press and Springer-Verlag Berlin Heidelberg. (G) Schematic illustration of FePt−Au heterostructured NPs and (H−J) TEM images of (H) FePt concave nanocubes and (I) FePt−Au1 and (J) FePt−Aun heterostructured NPs. The scale bars are 50 nm. Reprinted with permission from ref 31. Copyright 2013 Royal Society of Chemistry.

2.3.2. Exchange-Coupled MNMs. To meet the requirements for an even higher maximum-energy [(BH)max] product in industry, the exchange-coupling effect between hard and soft magnetic phases is considered as a core principle. Generally, when the size of the soft parts is small enough, the hard and soft phases will switch coherently just like a single phase, combining 408

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Figure 8. (A−C) TEM images of 8/4 nm (A) fct-FePt@Co, (B) fct-FePt@Ni, and (C) fct-FePt@Fe2C NPs. Reprinted with permission from ref 16. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (D−F) Schematic diagram of the synthesis of Pd@Fe3O4 NPs and TEM images of (E) 6.8 nm and (F) 9.4 nm Pd@Fe3O4 NPs; (G) HRTEM image and (K) hysteresis loops of FePd/Fe exchange-coupled nanocomposites. Reprinted with permission from ref 33. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (H) TEM image of Sm[Co(CN)6]·4H2O@GO, (I) TEM and (J) HRTEM images of single-domain SmCo5@Co, (L) hysteresis loop of single-domain SmCo5@Co, and (M) demagnetization curves of SmCo5-based nanomagnets with various compositions. Reprinted with permission from ref 36. Rights managed by Nature Publishing Group.

the synthetic mechanism and interactions of each part and finally produces high-quality MNMs.

hydrophilic macromolecules/biomolecules, have been developed to assess the biocompatibility and stability of MNMs before their application in the clinic. In addition, MNMs detect or target biological entities after modification, and on this basis, magnetic resonance imaging (MRI) and imaging-guided photothermal therapy and drug delivery have been developed in research laboratories. The biomedical applications of MNMs have become one of the leading research fields because of their irreplaceable advantages. In this section, we provide an overview

3. POTENTIAL APPLICATIONS 3.1. Biomedicine

MNMs have shown great promise in biomedicine because of their unique behaviors under an applied magnetic field, which has no limitation of penetration in real-time monitoring or drug/ gene delivery. Several modification routes, such as linking to 409

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Figure 9. (A) T2-weighted MR images of tumor-bearing mice before and after intravenous injection of Fe5C2−ZHER2:342 and Fe5C2−PEG for different time points. (B) Tumor growth curves different groups. Reprinted with permission from ref 41. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 10. (A) Cyclic voltammograms for methanol oxidation. Reprinted with permission from ref 43. Copyright 2012 Royal Society of Chemistry. (B, C) Linear sweep voltammetry curves for (B) the OER and (C) the HER catalyzed by Co3O4@GCN. Reprinted with permission from ref 44. Copyright 2015 Tsinghua University Press and Springer-Verlag Berlin Heidelberg. (D, E) Temperature-programmed surface reaction diagrams of the FTS catalyzed by (D) Fe5C2/SiO2 and (E) Fe2O3/SiO2 and (F) product selectivities of Fe5C2/SiO2 and Fe2O3/SiO2. Reprinted from ref 27. Copyright 2012 American Chemical Society.

possibilities for multimodal imaging as well as magnetic therapeutic agents. In order to provide advanced options in the clinic to replace suboptimal contrast agents for MRI probes, researchers have made efforts to fabricate MNMs with even higher magnetization. As a result of the breakthrough of Fe5C2 NPs with tunable size and suitable colloidal stability, they have proved to be excellent MRI contrast agents and display high r2 relaxivity.38 3.1.2. Imaging-Guided Drug Delivery. A magnetic-NPbased drug delivery system can transport drug to specific sites and subsequently control drug/gene release remotely under realtime monitoring. Hou and co-workers encapsulated doxorubicin within hollow-pore-structured manganese phosphate that showed a pH-dependent drug release and effective uptake by multidrug-resistant cells.39 Thus, the fabricated hollow manganese phosphate NPs can simultaneously act as T1-weighed MRI contrast agents and drug delivery vehicles. When exposed to

of several novel MRI contrast agents that exhibited better performance compared with those available for clinical use. In addition, these agents can serve as nanoplatforms to kill tumor cells by chemotherapy and photothermal therapy (mainly on iron carbides), which may open new routes for cancer treatment. 3.1.1. Magnetic Resonance Imaging. MRI, which is based on the response of the nuclear spin in hydrogen to an applied magnetic field, can provide high-resolution imaging for monitoring of tissue morphology and anatomical details. To enhance the contrast and amplify signals from the background tissue, contrast agents, generally in the form of T1 positivecontrast agents and T2 negative-contrast agents, are usually required. Hou and co-workers developed 12 nm magnetite nanocrystals modified by protocatechuic acid (PA) via a rapid ligand exchange process.37 These nanocrystals could act as dualmode contrast agents with a high T1 relaxivity of 17.8 mM−1 s−1 and T2 relaxivity of 220 mM−1 s−1 in MRI, supplying new 410

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reaction responses (Figure 10D,E). Further characterizations indicated the product selectivity of Fe5C2 NPs, through which the FTS yielded more longer-chain hydrocarbons (Figure 10F).

late endosome/lysosome-mimetic environments (pH 5.4), the NPs showed a higher T1 enhancement than under physiological conditions (pH 7.4) due to the release of Mn2+ at the lower pH. In addition, considering their outstanding magnetic properties, Fe5C2 NPs were developed into a multistimuli-controlled drug carrier by coating them with bovine serum albumin.40 The nanoplatform provided a burst drug release when stimulated by acidic conditions or near-infrared light, and it enabled targeted drug delivery under the guidance of a magnetic field, which has great potential for theranostics. 3.1.3. Imaging-Guided Photothermal Therapy. Efficient heat generation under illumination with laser radiation has been recently developed for cancer treatment. Through light-activated heating of NPs conveyed into tumors, photothermal therapy (PTT) achieves a high temperature in the tumor site, killing tumor cells without damaging the surrounding healthy tissue. Hou and co-workers developed Fe5C2 NPs with a carbon shell that possess a unique photothermal property for efficient tumor ablation and excellent contrast for MRI.41 T2-weighted MR images were taken before and after the intravenous injection of active-targeting Fe5C2−ZHER2:342 or negative-targeting Fe5C2− PEG NPs on tumor-bearing mice. The signal dropping of the former group was clearer than the latter one in Figure 9A, which demonstrated that more active targeting could be achieved in tumor sites. Figure 9B shows an obvious tumor ablation efficiency of Fe5C2−ZHER2:342 under group irradiation. Since the synthesis of iron carbide NPs has been further optimized to produce NPs with more uniform and smaller sizes, Au as the quintessential optical component was incorporated into iron carbide to form Janus NPs (JNPs) for imaging-guided PTT. As Au−Fe2C JNPs can simultaneously present optical and magnetic properties, they have been successfully evaluated as a multifunctional nanoplatform that can be used for MRI/multispectral photoacoustic tomography (MSOT)/computed tomography (CT) triple-mode imaging-guided PTT in tumor ablation.42

3.3. Environmental Purification

Water pollution is regarded as a serious environmental problem with many types of contaminants, among which microbial contamination cannot be neglected. Thousands of kinds of microalgae and bacteria as well as toxic ions exist in the wastewater discharged from industry and agriculture, which limits its further use. MNMs have been applied in the purification of wastewater, especially in removing organic species, killing bacteria, degrading dyes, and providing more accepted approaches for the capture and separation of microorganisms in wastewater. Several attempts have been made to achieve the magnetic separation of contaminants in wastewater via well-defined MNMs such as modified Fe3O4@amino acid.45 Arginine, lysine, and poly-L-lysine are all suitable for both Gram-positive Bacillus subtilis and Gram-negative Escherichia coli. Through adsorption, final capture efficiencies could reach over 97%. As for the removal of adverse ions, nanocomposites of various titanates and Fe3O4 were synthesized by self-assembly and showed a Pb2+ adsorption capacity of up to 382.3 mg/g that may be attributed to the reaction between Pb2+ and octahedral FeO6 on the active surface of the nanocomposites.46

4. CONCLUSION AND OUTLOOK MNMs have opened up novel approaches to create magnetic materials with unique properties, especially considering their nanoscale effects. With precise control over their chemical synthesis, newfangled properties of MNMs have been achieved, making their potential applications possible. The surface energy of formed nuclei and their subsequent growth can be accurately tuned by using appropriate amounts of solvents and surfactants, providing representative MNMs with desired sizes, compositions, and structures, such as metals, metal alloys, and metal oxides/carbides. In particular, by incorporation of different monocomponent parts through designed processes, including heteroepitaxial growth and chemical deposition in seed-mediated syntheses, multicomponent MNMs have been obtained, offering synergic properties through interfacial coupling and promising novel multifunctional MNMs. With further optimization work such as surface modification, MNMs can attain diverse properties and be applied in fields such as biomedicine, catalysis, and environmental purification, which are of great significance given currently emerging demands. We focus on seeking environmentally friendly, cheaper, and more effective chemical routes to prepare MNMs with advanced properties in terms of efficiency and stability. We hope to discover more multifunctional MNMs suitable for diverse situations that can help solve specific problems in areas related to health, energy, catalysis, and so forth.

3.2. Catalysis

Besides their magnetic features, MNMs can also be regarded as catalysts, which is attributed to the specific chemical components and the great extent of their exposed facets. Moreover, because of the high surface-to-volume ratio and quick response to an applied magnetic field, MNMs usually act as catalyst supports. Based foremost on electrocatalysts like Pt−C NPs, MNMs such as FePt have been synthesized and shown efficient performance in electrochemical processes with low cost. Moreover, transition metal oxides and carbides are all under investigation. Methanol oxidation is extensively applied for the fabrication of methanal using noble-metal species such as Pt-based catalysts. Recently, Hou and co-workers synthesized a type of concave nanocubes (CNCs) based on FePt that exhibited effective catalytic performance, showing clearly superior catalytic activity for methanol oxidation (Figure 10A).43 Furthermore, the oxygen and hydrogen evolution reactions (OER and HER) were also effectively catalyzed with hybrids of Co3O4 and graphitic carbon nitride (GCN) nanocomposites, making their use suitable for renewable energy applications.44 Figure 10B,C shows linear sweep voltammetry curves for the OER and HER, indicating effective catalytic performance. The Fischer−Tropsch synthesis (FTS) is considered as an efficacious route to fabricate liquid fuels through gas-phase reactions in the presence of Fe-based catalysts, which provide an alternative to petroleum as a motor fuel.27 The above-mentioned Fe5C2 NPs have been applied to catalyze the FTS with immediate

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yanglong Hou: 0000-0003-0579-4594 Notes

The authors declare no competing financial interest. 411

DOI: 10.1021/acs.accounts.7b00407 Acc. Chem. Res. 2018, 51, 404−413

Article

Accounts of Chemical Research Biographies

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Kai Zhu received his B.Sc. degree from Shandong University in China. He is currently a Ph.D. candidate at Peking University in China, working on the design and controllable synthesis of magnetic nanomaterials. Yanmin Ju received her B.Sc. degree from Nanjing Agricultural University in China. She is currently a Ph.D. candidate at Peking University, working on the biomedical applications of magnetic nanomaterials. Junjie Xu received his B.Sc. degree from Sichuan University in China. He is currently a Ph.D. candidate at Peking University, working on the controlled synthesis of magnetic nanomaterials. Ziyu Yang received his Ph.D. degree from Peking University. He is currently a postdoctoral researcher at the University of Electronic Science and Technology of China, working on self-assembly of nanomaterials. Song Gao is a professor at Peking University. His research interest is focused on molecular nanomagnets. Yanglong Hou is a professor at Peking University. His current research interest is focused on the synthesis, functionalization, and applications of magnetic nanomaterials.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Key R&D Program of China (2017YFA0206301 and 2016YFA0200102) and the National Natural Science Foundation of China (21621061, 51672010, 51631001, 81421004, and 51590882).

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