In the preparation and storage of ferrofluids, the stability is of utmost importance. Organic compounds are often employed to passivate the surface of the iron oxide NPs during or after the preparing procedure to avoid agglomeration. The main reasons for magnetic iron oxide NPs have hydrophobic surfaces with a large surface area-to-volume ratio if in the absence of any proper surface coating, and the hydrophobic interactions between the NPs will cause them to aggregate and form large clusters, resulting in increased particle size. Additionally, in order to expand the biological application scope of the iron oxide NPs, some biomolecules are also employed to enhance its biocompatibility.
In recent years, considerable efforts have been devoted to the design and controlled fabrication of nanostructured materials displaying specific functional properties. Organic compounds coated on iron oxide NPs offer a high potential application in several areas. The structure of organic compounds functionalized magnetic iron oxide NPs consists of two major parts: preserved the magnetic property of magnetic iron oxides and preserved the other properties of organic molecules. Generally, if iron oxides were always assumed as the core, its structure can roughly be divided into three types: core-shell, matrix, and shella-core-shellb. As plotted in Fig. , coating an ensemble of iron oxide NPs by organic material yields core-shell nanostructure. In these structures, the cores may be any kind of iron oxide particles, such as magnetite or maghemite. Likewise, the shells may consist of any sort of materials including organic ones. Matrix structure includes two typical structures: mosaic and shell-core. Comparatively speaking, the shell-core structure consists of an organic compound NPs core and iron oxide particle shells. Iron oxide NPs may connect with the organic core by the interaction of chemical bonds. The mosaic structure comprising the shell layer was made of organic molecules coated to a lot of uniformly iron oxide magnetic NPs. Among the different matrixes that can be used to embed the NPs, polymers are of particular interest because of their wide range of properties. Furthermore, a shell layer made of organic molecules coated to a shell-core structurally functionalized iron oxide NPs that will form the shella-core-shellbstructure, which occurs in many current reports and generally obtained bylayer-by-layertechnology. The shellamay be the polymer or biomolecules, likewise, the shellbcan be the same or different functional materials. Moreover, multi-component conducting organic material systems with iron oxide NPs can be tailored to obtain desired mechanical properties besides novel electrical, magnetic, and optical properties.
The representative structure of organic materials functionalized magnetic iron oxide NPs (if iron oxide NPs were always assumed as the core)
Organic compounds functionalized iron oxide NPs provide not only the basic magnetism characteristics of magnetic NPs, but also possess good biocompatibility and biodegradability of the functional organic materials. Moreover, organic moleculars can provide the ensemble functional reactive group, e.g. aldehyde groups, hydroxyl groups, carboxyl groups, amino groups, etc. It is crucial that their groups can link to the active biosubstance such as antibody, protein, DNA, enzyme, etc., for the further application. In the following section, the organic small molecules, macromolecules or polymer and biological molecules functionalized iron oxide NPs and the corresponding properties will be discussed.
Small Molecules and Surfactants
A suitable surface functionalization and choice of solvent are crucial to achieving sufficient repulsive interactions to prevent agglomeration so as to obtain a stable colloidal solution and further expand the scope of application. According to the surface characteristics of the functionalized NPs, small molecules or surfactants functionalized iron oxide NPs can be simply divided into three types: oil-soluble, water-soluble, and amphiphilic. Oil-soluble type refers to the surface of functionalized iron oxide NPs containing the molecular which have a weak attraction for the solvent environment, it is generally hydrophobic group, such as the fatty acid, alkyl phenol (n = 6–10, linear or branched). Conversely, water-soluble type refers to the surface of functionalized iron oxide NPs containing the chemical groups which have a strong attraction for the solvent environment, it is generally hydrophilic groups, such as the ammonium salt, polyol, lycine. Amphiphilic type refers to the surface of functionalized iron oxide NPs containing both hydrophilic and hydrophobic chemical groups, the main chain of these functionalized small molecules or surfactants showed concurrence of hydrophobic and hydrophilic structural regions, which also will make the functionalized NPs possess both oil-solubility and water-solubility, such as sulfuric lycine.
Oil-soluble type functionalization employed in order to prevent or decrease the agglomeration of iron oxide NPs and increase the stability give rise to the monodispersity, for instance, iron oxide NPs frequently dispersed in long-chain substance of hexadecane, the classic example being oleic acid (CH3
H), which has a C18 tail with a cis-double-bond in the middle, forming a kink. Such kinks have been postulated as necessary for effective stabilization, and indeed stearic acid (CH3
H) with no double-bond in its C18 tail, cannot stabilize the iron oxide NPs [7
]. Additionally, Oleic acid is widely used in ferrite nanoparticle synthesis because it can form a dense protective monolayer, thereby producing highly uniform and monodisperse particles. However, relative to the naked iron oxide NPs, the average diameter of functionalized NPs will increase in the range of 0–5 nm, and its saturation magnetization (MS
) almost unchanged.
Several authors have reported the magnetic iron oxide NPs coated or dispersed in the oil-soluble organic molecules. Shaoo et al. [43
] have reported the surface-coated magnetite NPs with a 6–8 nm average diameter by oleic acid, lauric acid, dodecyl phosphonate, hexadecyl phosphonate, and dihexadecyl phosphate etc., they found that alkyl phosphonates and phosphates could be used for obtaining thermodynamically stable dispersions of magnetite NPs. The organic molecules’ ligands seem to form a quasi-bilayer structure with the primary layer strongly bonded to the surface of the NPs. This study demonstrates that alkyl phosphonates and phosphates bind efficiently to iron oxide particle surfaces, the good biocompatibility of phosphonate and phosphate ligands may advance the utilization of encapsulated magnetic NPs in medical applications, such as MRI and other biophysical purposes. Bourlions et al. [44
] have reported the synthesis of capped ultrafine γ-Fe2
NPs under anaerobic conditions by the thermal treatment of a hydrated iron(III) hydroxide caprylate gel in boiling tetraline and by retention of the organophilic mantle around the parent particle throughout the final solid. The capped particles, which possessed a spherical morphology and an average size of 4 nm, were easily solvated in organic solvents to provide stable and homogeneous magnetic organosols.
However, the oil-soluble type functionalized iron oxide NPs are relatively easy to prepare and control. Currently the mainly research focus are the synthesis of water-soluble type functionalized iron oxide NPs with water solubility (good disperse in aqueous environment, can not produce agglomeration), biological compatibility (good for protein, antibody, biotin, DNA etc., low cytotoxicity, low effect their activity), and biodegradability. Water-soluble functionalized iron oxide NPs can be widely utilized in bioseparation and biodetection.
Several routes were employed for obtaining the water-soluble functionalized iron oxide NPs. One method is to directly add the biocompatible small organic molecules such as amino acid [45
], citric acid [43
], vitamin [47
], cyclodextrin [49
], etc. during the synthesis procedure. Recently Xia et al. [52
] reported a facile synthetic approach for preparing water-soluble Fe3
NPs with a surrounded layer by use of polyethylene glycol nonylphenyl ether (NP5) and cyclodextrin (CD) in aqueous medium. Moreover, the author had also built nanostructured spherical aggregates (mosaic type structure) from individual functionalized NPs (core-shell type structure) and which can be tailored by the concentration of NP5 and CD. These mesoporous aggregates of magnetite NPs can be used in magnetic carrier technology. Though this direct method has many advantages, owing to small organic molecules do not have good stability especially in an alkaline or acidic environment, it often decomposes and results in the functionalized iron oxide NPs agglomeration. Furthermore, the morphology of functionalized iron oxide NPs often presents as an aggregate clusters.
Another method is to transform the oil-soluble type into water-soluble type functionalized iron oxide NPs, and the ligand-exchange reaction is the major approach to realize this aim [53
]. Ligand exchange is a well-known method for tuning the surface properties of NPs. It involves adding an excess of ligand to the nanoparticle solution, which results in the displacement of the original ligand on the NPs’ surface. Especially, ligand-exchange reactions on noble metal NPs, via the self-assembly of thiols, have been used for many years already [54
]. However, more recently, several groups have reported the use of ligand exchange to alter the surface properties of iron oxide NPs. Sun et al. [56
] have prepared a monodispersed magnetite NPs with a tunable diameter range from 3 to 20 nm by the high temperature solution phase reaction of iron(III) acetylacetonate, Fe(acac)3
, with 1,2-hexadecanediol in the presence of oleic acid and oleylamine. And the hydrophobic NPs can be transformed into hydrophilic ones by mixing with bipolar surfactants such as tetramethylammonium 11-aminoundecanoate, allowing preparation of aqueous nanoparticle dispersions. Subsequently, as illustrated in Fig. , Lattuada and Hatton [57
] reported that the oleic groups initially present on the above nanoparticle surfaces were replaced via ligand-exchange reaction with various capping agents bearing reactive hydroxyl moieties. They also found that particle sizes in the range of 6 to 11 nm can be tuned without using the seeded-growth process by simply varying the heating rate of the solution containing the nucleating particles. This route was proposed a flexible methodology for the preparation of various types of monodisperse, water-soluble magnetic NPs coated by different polymer brushes. A similar approach was used by Fan et al. [58
] to prepare a water-dispersible γ-Fe2
NPs with narrow size distributions (13 ± 2 nm) via site-exchange reaction, and these NPs were superparamagnetic and stable without aggregation in a wide range of pH 3–9. Additionally, attachment of the biotin functionality onto the surfaces of our NPs enabled the magnetic affinity isolation of avidin–FITC from its incubation buffer with 96% efficiency. However, ligand-exchange reaction often contributes to the complicated operation and the difficulty of control exchange rate, these problems are urgent to be solved.
Figure 2 Scheme for the magnetic NPs functionalization procedure described in this work. Steps 1A and 1B: ligand-exchange reactions. Step 2: acylation of hydroxyl groups to prepare ATRP surface initiators. Step 3A: surface-initiated ring opening polymerization (more ...)
Moreover, the silane agent is often considered as a candidate for modifying on the surface of iron oxide NPs directly, for the advantages of the biocompatibility as well as high density of surface functional endgroups, and allowing for connecting to other metal, polymer or biomolecules [59
]. In general, silane-coated iron oxide NPs still maintains the physical characteristics of naked NPs with high saturation magnetization, the decrease value of saturation magnetization often less than 10 emu g−1
. The 3-aminopropyltriethyloxysilane (APTES), p
-aminophenyl trimethoxysilane (APTS), and mercaptopropyltriethoxysilane (MPTES) agents were mostly employed for providing the amino and sulfhydryl group, respectively. The physicochemical mechanism of the silane agent modifying on the surface of iron oxide NPs according to Arkles is depicted in Fig. [61
]. The hydroxyl groups on the iron oxide NPs surface reacted with the methoxy groups of the silane molecules leading to the formation of Si–O bonds and leaving the terminal functional groups available for immobilization the other substance.
Physicochemical mechanism for modifying the silane agents on the surface of iron oxide NPs
We have prepared magnetite NPs with average diameter 25 ± 5 nm by modified chemical co-precipitation, and silane bridged surface tailoring are exploited to obtain amino-coated and thiolated magnetite NPs via corresponding APTES and MPTES modification, respectively. The result reveals that the surface functionalized magnetic particles have a slight dimensional increase in average diameter while retain almost original saturation magnetization; APTES is prone to maintain the morphology of the magnetite NPs, but MPTES has an outstanding hypochromic effect though it caused a slight decrease in the saturation magnetization. The finding is of practical significance for magnetic NPs applications associated with bioconjugation, target carriers, and biosensing [62
]. In addition, silanes were found to render the iron oxide NPs highly stable and water-dispersible, and it was also found to form a protective layer against mild acid and alkaline environments. Moreover, silane ligand-exchange reaction can make the hydrophobic iron oxide NPs into water-dispersible [63
Although most studies have focused on the development of small organic molecules and surfactants coating up to now, recently polymers functionalized iron oxide NPs are receiving more and more attention, owing to the advantages of polymers coating will increase repulsive forces to balance the magnetic and thevan der Waalsattractive forces acting on the NPs. In addition, polymers coating on the surface of iron oxide NPs offer a high potential in the application of several fields. Moreover, polymer functionalized iron oxide NPs have been extensively investigated due to interest in their unique physical or chemical properties. To make use of these materials for fundamental or applied research, access to well-defined NPs samples whose properties can be “tuned” through chemical modification is necessary. In a number of cases it has now been shown that, through careful choice of the passivating and activating polymers and/or reaction conditions, can produce NPs with tailored and desired properties.
Polymer coating materials can be classified into synthetic and natural (see Table ). Table also provides a list of synthetic and natural materials that are used to functionalize the iron oxide NPs along with their advantages, respectively. The saturation magnetization value of iron oxide NPs will decrease after polymers functionalization.
Organic macromolecules and their advantages of functionalized iron oxide NPs
Currently, there are two major developing directions to form polymers functionalized iron oxide NPs, as follows: One is for the purpose of expanding application range by introducing the functional polymers. For instance, Gupta et al. [91
] have reported a microemulsion polymerization process to prepare a poly(ethyleneglycol) (PEG)-modified superparamagnetic iron oxide NPs with magnetic core and hydrophilic polymeric shell. Highly monodispersed iron oxide NPs were synthesized by using the aqueous core of aerosol-OT (AOT)/n
-Hexane reverse micelles (without microemulsions) in N2
atmosphere. The average size of the PEG-modified NPs was found to be around 40–50 nm with narrow size distribution. It is important that the cytotoxicity profile of the NPs on human dermal fibroblasts as measured by standard 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay showed that the particles are nontoxic and may be useful for various in vivo and in vitro biomedical applications. Another is for the purpose of manufacturing a monodisperse NPs with well-defined shape and a controlled composition. Monomer polymerization method or in-suit synthesis in the microporous of polymeric microsphere was employed for obtaining the uniform magnetic composite NPs with high content of iron oxide NPs [92
]. As Zhang et al. [93
] reported, an approach employs polymer microgels as templates for the synthesis of iron oxide NPs. At first, precursor cations were incorporated into the poly(N
-isopropyl acrylamide-acrylic acid-2-hydroxyethyl acrylate) microgel particles crosslinked with NN
’-methylene bisacrylamide (BIS), then iron oxide NPs were in-suit synthesized in the microgel by co-precipitation. They show that NPs with predetermined dimensions and size-dependent properties can be synthesized by using a very delicate balance between the reaction conditions, the composition and the structure of microgel templates, and the concentration of NPs in the microgel. The synthesis of superparamagnetic Fe3
NPs in the microgel templates was extremely efficient: for NPs concentration as high as 0.724 g per gram polymer, the NPs had an average size of ca. 8.1 nm, low polydispersity, and showed excellent magnetic properties, while the microspheres carrying these NPs remained stimuli-responsive. Additionally, atom transfer radical polymerization (ATRP) method was utilized for obtaining monodisperse NPs with well-defined core-shell shape. ATRP can offer polymeric shells with low polydispersity and this method is easy to control the molecular weight, thereby, the thickness of the polymeric shell. Recently several polymer-coated iron oxide NPs through surface-initiated ATRP in polar solvents were successfully developed [94
]. ATRP can be used in preparing polymers with narrow molecular weight distribution, block copolymers, graft polymers, random copolymers and finally to realize controlling the thickness of the polymeric shell and the size of polymer functionalized iron oxide NPs. As illustrated in Fig. , Sun et al. [98
] have reported the synthesis of the iron oxide/polystyrene (PS) core/shell NPs via surface initiated ATRP. At first a method was employed to covalently bond initiators onto the surface of iron oxide NPs, which was the combination of ligands exchange reaction and condensation of triethoxysilane having an ATRP initiating site, 2-bromo-2-methyl-N-(3-(triethoxysilyl)propyl) propanamide. Then the polystyrene shell was grafted from the initiating sites on the surface of iron oxide NPs through ATRP. The covalently bonded polymeric shell could prevent the undesired site exchange of the MNPs surface functionalities. From the as-synthesized macroinitiators, the polymer functionalized iron oxide NPs with a well-defined, covalently bonded PS shell have been successfully prepared through the surface initiated ATRP, which exhibits a characteristic of a controlled/”living” polymerization.
Figure 4 Illustration of the synthesis route of polystyrene coated magnetic NPs with core/shell structure. From 
Moreover, polymer are often employed to make up the deficiency of small organic molecules and surfactant for the purpose of obtaining the surface functionalized iron oxide NPs. Therefore, besides the above research focus, how to prepare the functionalized iron oxide NPs with ideal chemical stability and good biocompatibility remains the great challenges. The right choice of organic materials can provide a structure suitable for obtaining the well-controlled functionalized NPs. For example, Frankamp et al. [99
] developed an approach for direct controlling the magnetic interaction between iron oxide NPs through dendrimer-mediated self-assembly. The resulting assemblies featured systematically increasing average interparticle spacing over a 2.4 nm range with increasing dendrimer generation. This increase in spacing modulated the collective magnetic behavior by effective lowering of the dipolar coupling between the particles. The dependence of blocking temperature on interparticle spacing was found to point toward a much more dramatic interdependence at close interparticle spacing, and a weaker correlation at larger spacing. Overall, the development of other methods for constructing the surface engineering of iron oxide NPs is of great important.
Various biological molecules such as protein [101
], polypeptide [103
], antibody [104
], biotin and avidin [106
], etc., may also be bound to the surface of iron oxide NPs directly or indirectly by chemically coupling via some functional endgroups to make the NPs target specific. The biological molecules functionalized iron oxide NPs will greatly improve the particles’ biocompatibility. Such magnetic NPs can be very useful to assist an effective separation of proteins, DNA, cells, biochemical products, etc.
With appropriate surface chemistry, biomolecules can immobilize on iron oxide NPs. Zhang et al. [107
] have reported a microemulsion approach to prepare a human serum albumin (HAS)-coated Fe3
magnetic NPs as a radioisotope carrier labeled with 188
Re and explored the optimal labeling conditions with 188
Re. The procedure for preparing HAS-coated Fe3
magnetic NPs as follows: With cotton oil as oil phase, a mixture of HSA and magnetite solution as water phase and Span-83 as emulsion agent. The mixture of the above two solutions was ultrasonicated for 15 min, then added rapidly dropwise into cotton oil at 130 °C under stirring condition. The diameter of HSA-coated magnetic NPs was about 200 nm. The particles can be labeled with 188
Re for the purpose of in vivo regional target therapy.
In contrast, the major strategy for surface functionalization by biological molecules includes two steps, first synthesizing the small molecules or polymers functionalized NPs, and then coupled to the biomolecules by chemical bond or physical adsorption. Recently, Lee et al. [108
] developed a route for conjugating the γ-Fe2
NPs with single strand oligonucleotides. The water-soluble magnetic NPs with carboxyl groups on their surfaces were prepared at first, and then by using 1-ethyl-3-(3-dimrthylaminopropyl)carbodiimide hydrochloride (EDC) as a liker reagent, they successfully modified a protein, streptavidin, on the surface of γ-Fe2
NPs. Streptavidin functionalized Fe2
can catch a biotin-labeled single strand oligonucleotides through the strong affinity between streptavidin and biotin.
Additionally, superparamagnetic iron oxide core of individual NPs becomes more efficient at dephasing the spins of surrounding water protons, enhancing spin−spin relaxation times (T2
relaxation times) so that the NPs act as magnetic relaxation switches (MRS) [109
]. Based on this phenomenon, Perez and colleagues [110
] have developed biocompatible magnetic nanosensors that act as MRS to detect molecular interactions in the reversible self-assembly of disperse magnetic particles into stable nanoassemblies recently. MRS technology can be used to detect different types of molecular interactions (DNA-DNA, protein-protein, protein-small molecule, and enzyme reactions) with high efficiency and sensitivity using magnetic relaxation measurements MRI. Furthermore, the magnetic changes are detectable in turbid media and in whole-cell lysates without protein purification. The developed magnetic nanosensors can be used in a variety of biological applications such as in homogenous assays, as reagents in miniaturized microfluidic systems, as affinity ligands for rapid and high-throughput magnetic readouts of arrays, as probes for magnetic force microscopy, and potentially for in vivo imaging. For instance, oligonucleotides functionalized iron oxide NPs aggregate in the presence of target oligonucleotides (20 pM limit), resulting in a measurable increase (30 ms) in the T2
relaxation times of the surrounding water. It was further discovered that base pair insertions in the target strand resulted in only 2–5 ms increases in the relaxation times, while single base pair mismatches resulted in 1–21 ms increases in T2
, indicating that these systems could potentially be used to selectively detect DNA mutations.