NANO REVIEW Magnetic Iron Oxide Nanoparticles: Synthesis and Surface Functionalization Strategies Wei Wu �� Quanguo He �� Changzhong Jiang Received: 8 July 2008 / Accepted: 11 September 2008 / Published online: 2 October 2008 �� to the authors 2008 Abstract Surface functionalized magnetic iron oxide nanoparticles (NPs) are a kind of novel functional materi- als, which have been widely used in the biotechnology and catalysis. This review focuses on the recent development and various strategies in preparation, structure, and mag- netic properties of naked and surface functionalized iron oxide NPs and their corresponding application briefly. In order to implement the practical application, the particles must have combined properties of high magnetic satura- tion, stability, biocompatibility, and interactive functions at the surface. Moreover, the surface of iron oxide NPs could be modified by organic materials or inorganic materials, such as polymers, biomolecules, silica, metals, etc. The problems and major challenges, along with the directions for the synthesis and surface functionalization of iron oxide NPs, are considered. Finally, some future trends and pro- spective in these research areas are also discussed. Keywords Magnetic iron oxide NPs Surface functionalization Preparation Application Introduction Nanoparticle are submicron moieties (diameters ranging from 1 to 100 nm according to the used term, although there are examples of NPs several hundreds of nanometers in size) made of inorganic or organic materials, which have many novel properties compared with the bulk materials [1]. On this basis, magnetic NPs have many unique magnetic properties such as superparamagnetic, high coercivity, low Curie temperature, high magnetic susceptibility, etc. Magnetic NPs are of great interest for researchers from a broad range of disciplines, including magnetic fluids, data storage, catalysis, and bioapplica- tions [2���6]. Especially, magnetic ferrofluids and data storage are the applied researches that have led to the intergration of magnetic NPs in a myriad of commercial applications. Currently, magnetic NPs are also used in important bioapplications, including magnetic biosepara- tion and detection of biological entities (cell, protein, nucleic acids, enzyme, bacterials, virus, etc.), clinic diagnosis and therapy (such as MRI (magnetic resonance image) and MFH (magnetic fluid hyperthermia)), targeted drug delivery and biological labels. However, it is crucial to choose the materials for the construction of nano- structure materials and devises with adjustable physical and chemical properties. To this end, magnetic iron oxide NPs became the strong candidates, and the application of small iron oxide NPs in in vitro diagnostics has been practiced for nearly half a century [7]. In the last decade, increased investigations with several types of iron oxides have been carried out in the field of magnetic NPs (mostly includes the Fe3O4 magnetite, FeIIFeIII2O4, ferrimagnetic, superparamagnetic when the size is less than 15 nm), a-Fe2O3 (hematite, weakly ferromagnetic or antiferro- magnetic), c-Fe2O3 (maghemite, ferrimagnetic), FeO W. Wu C. Jiang (&) Department of Physics, Wuhan University, Wuhan 430072, People���s Republic of China e-mail: czjiang@whu.edu.cn W. Wu e-mail: w.wu@163.com Q. He Key Laboratory of Green Packaging and Bio-Nanotechnology Applications (Hunan Province), Hunan University of Technology, Zhuzhou 412008, People���s Republic of China e-mail: hequanguo@163.com 123 Nanoscale Res Lett (2008) 3:397���415 DOI 10.1007/s11671-008-9174-9

(wustite, �� antiferromagnetic), e-Fe2O3 and b-Fe2O3) [8], among which magnetite and maghemite is the very promising and popular candidates since its biocompati- bility have already proven. However, it is a technological challenge to control size, shape, stability, and dispersibility of NPs in desired sol- vents. Magnetic iron oxide NPs have a large surface-to- volume ratio and therefore possess high surface energies. Consequently, they tend to aggregate so as to minimize the surface energies. Moreover, the naked iron oxide NPs have high chemical activity, and are easily oxidized in air (especially magnetite), generally resulting in loss of mag- netism and dispersibility. Therefore, providing proper surface coating and developing some effective protection strategies to keep the stability of magnetic iron oxide NPs is very important. These strategies comprise grafting of or coating with organic molecules, including small organic molecules or surfactants, polymers, and biomolecules, or coating with an inorganic layer, such as silica, metal or nonmetal elementary substance, metal oxide or metal sul- fide. Practically, it is worthy that in many cases the protecting shells not only stabilize the magnetic iron oxide NPs, but can also be used for further functionalization. In the following, we focus mainly on recent develop- ment and various strategies in the preparation, structure and magnetic properties of various surface functionalized strategies of magnetic iron oxide NPs and their corre- sponding applications, as well as the research advances on functionalizations of magnetic iron oxide NPs worldwide. Further the problems and major challenges still should be solved are pointed out, and the directions in these resear- ches are also discussed. Synthesis of Iron Oxide NPs In the last decades, much research has been developed to the synthesis of iron oxide NPs, and many reports have descri- bed efficient synthesis approaches to produce the shape- controlled, stable, biocompatible, and monodispersed iron oxide NPs. The most common methods including co-pre- cipitation, thermal decomposition, hydrothermal synthesis, microemulsion, sonochemical synthesis, and sonochemical synthetic route can all be directed to the synthesis of high quality of iron oxide NPs. In addition, these NPs can also be prepared by the other methods such as electrochemical synthesis [9, 10], laser pyrolysis techniques [11], microor- ganism or bacterial synthesis (especially the Magnetotactic bacteria and iron reducing bacteria) [12, 13], etc. As follows, we try to present typical and recent examples for the dis- cussions of each synthetic pathway and the corresponding formation mechanism. Co-Precipitation The most conventional method for obtaining Fe3O4 or c-Fe2O3 is by co-precipitation. This method consists of mixing ferric and ferrous ions in a 1:2 molar ratio in highly basic solutions at room temperature or at elevated temper- ature. The size and shape of the iron oxide NPs depends on the type of salt used (such as chlorides, sulfates, nitrates, perchlorates, etc.), the ferric and ferrous ions ratio, the reaction temperature, the PH value, ionic strength of the media, and the other reaction parameters (e.g. stirring rate, dropping speed of basic solution). Recently we have repor- ted the co-precipitation synthesis of Fe3O4 NPs and their corresponding morphology, structure, and magnetic prop- erties at different reaction temperature was investigated [14]. This method would critically affect the physical and chemical properties of the nanosized iron oxide particles. Generally, the saturation magnetization (MS) values found in nanostructured materials are usually smaller than the corresponding bulk phase, provided that no change in ionic configurations occurs [15]. Accordingly, experimental value for MS in magnetic iron oxide NPs have been reported to span the 30���80 emu g-1 range, lower than the bulk magnetic value 100 emu g-1. Inaddition, Fe3O4 NPsare notverystable under ambient conditions and are easily oxidised to Fe2O3 or dissolved in an acidic medium. In order to avoid the possible oxidation in the air, the synthesis of Fe3O4 NPs must be done inananaerobicconditions. Based onthispoint, Fe3O4 NPs can also be utilized to prepare the Fe2O3 NPs by oxidation or anneal treatment under oxygen atmosphere. And oxidation is not the important influence factor for Fe2O3 NPs due to its own chemical stability in alkaline or acidic environment. However, this method generates particles with a wide particle size distribution, which requires secondary size selection sometimes. A wide particle size distribution will result in a wide range of blocking temperatures (TB) due to TB depends on particle size and, therefore non-ideal magnetic behavior for many applications. Kang et al. [16] reported a synthesis of monodispersed, uniform, and narrow size distributional Fe3O4 NPs (the diameter of NPs was 8.5 �� 1.3 nm) by co-precipitation without surfactants, the reaction in an aqueous solution with a molar ratio of FeII/ FeIII = 0.5 and a pH = 11���12, and the colloidal suspensions of the magnetite can be then directly oxidized by aeration to form colloidal suspensions of c-Fe2O3. In contrast, many recent publications have described efficient routes to obtain the monodispersed NPs, surfactants such as dextran or polyvinyl alcohol (PVA) can be added in the reaction media, or the particles can be coated in a subsequent step [17, 18]. Surfactants act as protecting agent for controlling particle size and stabilizing the colloidal dispersions. Additionally, the disadvantage of these aqueous solution syntheses is that the high pH value of the reaction mixture 398 Nanoscale Res Lett (2008) 3:397���415 123

has to be adjusted in both the synthesis and purification steps, and the process toward uniformed and monodi- spersed NPs has only very limited success. On the other hand, wastewaters with very basic pH values are also generated in the experiment, which require subsequent treatments for protecting the environment. Thermal Decomposition An organic solution phase decomposition route has been widely used in iron oxide NPs synthesis, and decomposi- tion of Fe(cup)3 (cup = N-nitrosophenylhydroxylamine), Fe(acac)3 (acac = acetylacetonate), or Fe(CO)5 followed by oxidation can lead to high-quality monodispersed iron oxide NPs, which usually requires relatively higher tem- peratures and a complicated operation. Sun and Zeng [19] have reported a general decomposition approach for the synthesis of size-controlled monodispersed magnetite NPs based on high temperature (265 ��C) reaction of Fe(acac)3 in phenyl ether in the presence of alcohol, oleic acid, and oleylamine. With the smaller magnetite NPs as seeds, larger monodispersed magnetite NPs of up to 20 nm in diameter can be synthesized and dispersed into nonpolar solvent by seed-mediated growth method. The process does not require a size-selection procedure and is readily scaled up for mass production. The as-synthesized Fe3O4 nanoparticle assemblies can be transformed easily into c-Fe2O3 NPs by annealing at high temperature (250 ��C) and oxygen for 2 h. Generally, direct decomposition of Fe(Cup)3 single precursor can lead to monodispersed c-Fe2O3 NPs [20]. The thermal decomposition of Fe(CO)5 produces iron NPs and the following oxidation by a chemical reagent can also lead to monodispersed c-Fe2O3 NPs [21]. For instance, Hyeon et al. [22] reported a synthesis of highly crystalline and monodispersed iron NPs without size-selection process by the thermal decomposition of iron pentacarbonyl in the presence of oleic acid at 100 ��C. The resulting iron NPs were transformed to monodispersed c-Fe2O3 nanocrystal- lites by controlled oxidation using trimethylamine oxide as a mild oxidant. Particle size can be varied from 4 to 16 nm by controlling the experimental parameters. Although the thermal decomposition method has many advantages for producing highly monodispersed particles with a narrow size distribution, it has the big disadvantage that the resulting NPs are generally only dissolved in nonpolar solvents. Microemulsion Microemulsion is a thermodynamically stable isotropic dispersion of two immiscible phases (water and oil) under the surfactant present, the surfactant molecules may form a monolayer at the interface between the oil and water, with the hydrophobic tails of the surfactant molecules dissolved in the oil phase and the hydrophilic head groups in the aqueous phase. As in the binary systems (water/surfactant or oil/surfactant), self-assembled structures of different types can be formed, ranging, for example, from (inverted) spherical and cylindrical micelles to lamellar phases and bicontinuous microemulsions, which may coexist with predominantly oil or aqueous phases [23]. In this sense, microemulsion and inverse micelles route can be employed for obtaining the shape- and size-controlled iron oxide NPs. Particularly, water-in-oil (w/o) microemulsions are formed by well-defined nanodroplets of the aqueous phase, dispersed by the assembly of surfactant molecules in a continuous oil phase. Vidal-Vidal et al. [24] have reported the synthesis of monodisperse maghemite NPs by the one- pot microemulsion method. The spherical shaped particles, capped with a monolayer coating of oleylamine (or oleic acid), show a narrow size distribution of 3.5 �� 0.6 nm, are well crystallized and have high saturation magnetization values (76.3 Am2/kg for uncoated NPs, 35.2 Am2/kg for oleic acid coated NPs, and 33.2 Am2/kg for oleylamine coated NPs). Moreover, the results show that oleylamine act as precipitating and capping agent. However, cyclo- hexylamine acts only as precipitating agent and does not avoid particle aggregation. Chin and Yaacob reported the synthesis of magnetic iron oxide NPs (less than 10 nm) via w/o microemulsion, furthermore, in comparison to the particles produced by Massart���s procedure [25], particles produced by microemulsion technique were smaller in size and were higher in saturation magnetization [26]. However, despite the presence of surfactants, the aggregation of the produced NPs usually needs several washing processes and further stabilization treatments. Hydrothermal Synthesis Iron oxide NPs with controlled size and shape are techno- logically important due to strong correlation between these parameters and magnetic properties. The microemulsion and thermal decomposition methods usually lead to com- plicated process or require relatively high temperatures. As an alternative, hydrothermal synthesis includes various wet- chemical technologies of crystallizing substance in a sealed container from the high temperature aqueous solution (generally in the range from 130 to 250 ��C) at high vapor pressure (generally in the range from 0.3 to 4 MPa). This technique has also been used to grow dislocation-free single crystal particles, and grains formed in this process could have a better crystallinity than those from other processes, so hydrothermal synthesis is prone to obtain the highly crystalline iron oxide NPs. Several authors have reported the synthesis of iron oxide NPs by hydrothermal method [27���30]. There are two major Nanoscale Res Lett (2008) 3:397���415 399 123