Solvothermal Synthesis of Size-Controlled Monodispersed Superparamagnetic Iron Oxide Nanoparticles

: Superparamagnetic iron oxide nanoparticles are of great interest in magnetic targeted drug delivery due to their unique properties. In this paper, size-controlled superparamagnetic iron oxide nanoparticles were synthesized in an ethylene glycol / diethylene glycol (EG / DEG) binary solvent system via a facile solvothermal method. X-ray di ﬀ raction (XRD), a scanning electron microscope (SEM), and a vibrating sample magnetometer (VSM) were used to conﬁrm that the prepared samples were superparamagnetic Fe 3 O 4 nanospheres. When the V EG / V DEG was varied from 100 / 0 to 80 / 20, 60 / 40, and 40 / 60, the average diameters of the resulting Fe 3 O 4 nanospheres were approximately 700, 500, 300, and 100 nm, respectively. In addition, the saturation magnetization ( M s ) of Fe 3 O 4 nanoparticles with a size of 100, 300, 500, and 700 nm was 72.14, 75.94, 80.28, and 85.41 emu / g, and the corresponding remanent magnetization ( M r ) was 3.34, 3.97, 3.26, and 4.28 emu / g, respectively. The relevant formation mechanisms of Fe 3 O 4 nanoparticles are proposed at the end. These superparamagnetic Fe 3 O 4 nanoparticles with high saturation magnetization may have use as targeted drug carriers.


Introduction
Delivering a drug accurately and safely to its target site remains the yardstick for the design of drug delivery systems. Targeted drug delivery has attracted a significant amount of attention due to the unique advantage of precise control over a drug's release [1]. In particular, magnetic drug targeting is currently recognized as one of the most promising approaches to drug delivery [2][3][4]. A pharmaceutical drug is loaded onto the surface of the magnetic nanoparticles, which are released at the target site with an external localized magnetic field gradient, thereby achieving an accurate and targeted dose [5].
During the process of delivering magnetic drug-loaded nanoparticles, the magnetic force that is exerted on the particles plays a crucial role and is strongly dependent on the size of the particle [6]. Furthermore, the magnetic force is proportional to d 3 , where d is the diameter of the particle [7]. In addition, to minimize the aggregation that is caused by the magnetic attractive forces, magnetic nanoparticles must have dependable superparamagnetic properties. As a result, superparamagnetic nanoparticles become magnetic in the presence of the external magnetic field, but revert to a nonmagnetic state that allows them to be excreted when the external magnetic field is removed [8]. Therefore, the size and superparamagnetism of magnetic nanoparticles are predominantly important to controlling the transport of a targeted drug. 2 of 8 Superparamagnetic iron oxide (Fe 3 O 4 ) nanoparticles are frequently used as the drug carrier because of their low toxicity [9]. Among the reported synthetic methods, including ball-milling [10], co-precipitation [11,12], thermal decomposition [13,14], and microemulsion templating [15], the solvothermal method has been proven to be feasible for the synthesis of magnetic nanoparticles with a controllable morphology and microstructure [16][17][18][19][20]. A considerable amount of effort has been devoted to the size-controlled synthesis of magnetic nanoparticles, mainly focusing on changing the reaction time, temperature, surfactants, and the initial concentration of reagents [20][21][22][23][24][25]. However, despite these advances, the synthesis of size-controllable magnetic nanoparticles based on changing the solvent deserves further research.
In this study, size-tunable superparamagnetic Fe 3 O 4 nanoparticles were synthesized via a facile solvothermal method based on a binary solvent system with ethylene glycol (EG) and diethylene glycol (DEG). To obtain superparamagnetic nanoparticles with a high saturation magnetization, polyvinyl pyrrolidone (PVP) and sodium citrate (Na 3 Cit) were used together as structure guide agents. The Fe 3 O 4 nanoparticles have nearly monodispersed sizes that are tunable in the range of 60-800 nm by varying the volume ratio of EG/DEG. The reaction mechanism of the solvothermal system is discussed. These superparamagnetic ferrite nanoparticles may be candidate targeted drug carriers.

Synthesis of Nanoparticles
The monodispersed superparamagnetic iron oxide nanoparticles were synthesized by the solvothermal method. In a typical synthesis, 13 g FeCl 3 ·6H 2 O was dissolved in 350 mL of ethylene glycol and diethylene glycol. Subsequently, 2 g NaAc, 2 g polyvinyl pyrrolidone (PVP), and sodium citrate (Na 3 Cit) were added to the solution's ultrasonic processing. After an hour, the solution was sealed in a 400 mL Teflon-lined stainless-steel autoclave. The autoclave was heated to 210 • C for 12 h, and then cooled to room temperature naturally. The black products were collected by magnetic decantation and centrifugation, followed by repeated washing with deionized water and ethanol. The final products were dried in a vacuum oven at 50 • C for 12 h.

Characterization
A scanning electron microscope (SEM, JSM-7500F, Japan) and a transmission electron microscope (TEM, JEM 1200EX, Japan) were used to examine the structure of the samples. Phase analyses of the products were performed by X-ray diffraction (XRD, Bruker AXSD8 Advance, Germany) with Cu target radiation. The magnetic properties of the particles were measured at room temperature (298 K) using a vibrating sample magnetometer (VSM, Lake Shore7410, USA).  Table 1 shows the average crystal sizes of the Fe 3 O 4 nanoparticles calculated using the Scherrer equation, and the measurements of the FWHM of the (311) peaks. It is known that the superparamagnetic limit for magnetite is~20 nm [26][27][28]. Hence, the as-prepared Fe 3    To confirm the morphology of the samples, we recorded SEM images of the Fe 3 O 4 monodispersed nanoparticles, which are shown in Figure 2. The mean diameter of the Fe 3 O 4 nanoparticles increased with the increasing ratio of V EG /V DEG during the solvothermal reaction. When the V EG /V DEG was varied from 100/0 to 80/20, 60/40, and 40/60, the average diameters of the resulting Fe 3 O 4 nanospheres were approximately 700, 500, 300, and 100 nm, respectively. Additionally, it can be confirmed from the TEM image of Fe 3 O 4 nanoparticles shown in Figure 3 that the as-prepared Fe 3 O 4 nanospheres are clusters of numerous superfine particles.  To confirm the morphology of the samples, we recorded SEM images of the Fe 3 O 4 monodispersed nanoparticles, which are shown in Figure 2. The mean diameter of the Fe 3 O 4 nanoparticles increased with the increasing ratio of V EG /V DEG during the solvothermal reaction. When the V EG /V DEG was varied from 100/0 to 80/20, 60/40, and 40/60, the average diameters of the resulting Fe 3 O 4 nanospheres were approximately 700, 500, 300, and 100 nm, respectively. Additionally, it can be confirmed from the TEM image of Fe 3 O 4 nanoparticles shown in Figure 3 that the as-prepared Fe 3 O 4 nanospheres are clusters of numerous superfine particles. monodispersed nanoparticles, which are shown in Figure 2. The mean diameter of the Fe 3 O 4 nanoparticles increased with the increasing ratio of V EG /V DEG during the solvothermal reaction. When the V EG /V DEG was varied from 100/0 to 80/20, 60/40, and 40/60, the average diameters of the resulting Fe 3 O 4 nanospheres were approximately 700, 500, 300, and 100 nm, respectively. Additionally, it can be confirmed from the TEM image of Fe 3 O 4 nanoparticles shown in Figure 3 that the as-prepared Fe 3 O 4 nanospheres are clusters of numerous superfine particles.

Magnetic Properties
The room-temperature hysteresis loop of the Fe 3 O 4 nanoparticles was measured using a vibrating sample magnetometer (VSM). The magnetization curves of the samples are shown in Figure 4, and the magnetic parameters are listed in Table 2. All Fe 3 O 4 nanoparticle samples show excellent superparamagnetic properties with really narrow hysteresis loops. From Table 2, the remanent magnetization (M r ) of Fe 3 O 4 nanoparticles with a size of 100, 300, 500, and 700 nm was 3.34, 3.97, 3.26, and 4.28 emu/g, respectively. The saturation magnetization ( M s ) of Fe 3 O 4 nanoparticles with a size of 100, 300, 500, and 700 nm was 72.14, 75.94, 80.28, and 85.41 emu/g, respectively. In this work, it was further found that the saturation magnetization (M s ) and the remanent magnetization (M r ) can be mildly affected by the size of Fe 3 O 4 nanoparticles.

Magnetic Properties
The room-temperature hysteresis loop of the Fe 3 O 4 nanoparticles was measured using a vibrating sample magnetometer (VSM). The magnetization curves of the samples are shown in Figure 4, and the magnetic parameters are listed in Table 2. All Fe 3 O 4 nanoparticle samples show excellent superparamagnetic properties with really narrow hysteresis loops. From Table 2, the remanent magnetization (M r ) of Fe 3 O 4 nanoparticles with a size of 100, 300, 500, and 700 nm was 3.34, 3.97, 3.26, and 4.28 emu/g, respectively. The saturation magnetization (M s ) of Fe 3 O 4 nanoparticles with a size of 100, 300, 500, and 700 nm was 72.14, 75.94, 80.28, and 85.41 emu/g, respectively. In this work, it was further found that the saturation magnetization (M s ) and the remanent magnetization (M r ) can be mildly affected by the size of Fe 3 O 4 nanoparticles.

Magnetic Properties
The room-temperature hysteresis loop of the Fe 3 O 4 nanoparticles was measured using a vibrating sample magnetometer (VSM). The magnetization curves of the samples are shown in Figure 4, and the magnetic parameters are listed in Table 2. All Fe 3 O 4 nanoparticle samples show excellent superparamagnetic properties with really narrow hysteresis loops. From Table 2, the remanent magnetization (M r ) of Fe 3 O 4 nanoparticles with a size of 100, 300, 500, and 700 nm was 3.34, 3.97, 3.26, and 4.28 emu/g, respectively. The saturation magnetization ( M s ) of Fe 3 O 4 nanoparticles with a size of 100, 300, 500, and 700 nm was 72.14, 75.94, 80.28, and 85.41 emu/g, respectively. In this work, it was further found that the saturation magnetization (M s ) and the remanent magnetization (M r ) can be mildly affected by the size of Fe 3 O 4 nanoparticles.

Formation Mechanism of Fe 3 O 4 Nanoparticles with Tunable Sizes
As a typical solvothermal reaction, the formation process of magnetic nanoparticles includes two main steps: nucleation and ripening growth [29]. In this case, the ethylene glycol and diethylene glycol act as the solvent and reductant, respectively, the iron (ш) chloride as the iron source, and the sodium acetate (NaAC) provides the hydroxyl-ion and maintains the acid-base equilibrium of the reaction system. The PVP and Na 3 Cit are structure guide agents.
As we previously reported [30], during the reaction process, parts of the hydrolyzed Fe 3+ ions reduce to Fe(OH) 2 , and combine with the remaining Fe(OH) 3 to finally form Fe 3 O 4 nanoparticles. As is shown in Figure 5a, the small-molecule citrate groups that have adhered on the magnetic nanocrystals act as both resistance and bridging agents, which prevent the further growth of nanocrystals and agglomerate the neighboring nanocrystals into nanoclusters with low remanent magnetization. As for the PVP long-chain molecules shown in Figure 5b, in the initial process, the macromolecule stabilizers mostly act as bridging agents to guide the neighboring nanocrystals to self-assemble into larger nanoparticles with high saturation magnetization. After the larger nanoparticles are fully covered by the PVP long-chain molecules, the PVP long-chain molecules act as space blocks and prevent the further growth of the nanoparticles. In order to obtain superparamagnetic nanoparticles with high saturation magnetization, the binary Na 3 Cit/PVP is added into the reaction system as a structure guide agent.

Formation Mechanism of Fe 3 O 4 Nanoparticles with Tunable Sizes
As a typical solvothermal reaction, the formation process of magnetic nanoparticles includes two main steps: nucleation and ripening growth [29]. In this case, the ethylene glycol and diethylene glycol act as the solvent and reductant, respectively, the iron (ш) chloride as the iron source, and the sodium acetate (NaAC) provides the hydroxyl-ion and maintains the acid-base equilibrium of the reaction system. The PVP and Na 3 Cit are structure guide agents.
As we previously reported [30], during the reaction process, parts of the hydrolyzed Fe 3+ ions reduce to Fe OH) 2 , and combine with the remaining Fe OH) 3 to finally form Fe 3 O 4 nanoparticles. As is shown in Figure 5a, the small-molecule citrate groups that have adhered on the magnetic nanocrystals act as both resistance and bridging agents, which prevent the further growth of nanocrystals and agglomerate the neighboring nanocrystals into nanoclusters with low remanent magnetization. As for the PVP long-chain molecules shown in Figure 5b, in the initial process, the macromolecule stabilizers mostly act as bridging agents to guide the neighboring nanocrystals to self-assemble into larger nanoparticles with high saturation magnetization. After the larger nanoparticles are fully covered by the PVP long-chain molecules, the PVP long-chain molecules act as space blocks and prevent the further growth of the nanoparticles. In order to obtain superparamagnetic nanoparticles with high saturation magnetization, the binary Na 3 Cit/PVP is added into the reaction system as a structure guide agent. In addition, the EG/DEG binary solvent system also plays a key role in controlling the size of the Fe 3 O 4 nanoparticles [31]. From the above-mentioned analysis, the formation of magnetic nanoparticles is closely related to the aggregation process. When pure EG is used, the precursor iron alkoxides transform into crystal nuclei and further agglomerate rapidly into large nanoparticles due to their high surface energy. When DEG is added to form a binary solvent with EG, the size of Fe 3 O 4 nanoparticles decreases as the V EG /V DEG ratio decreases. DEG forms a more stable coordination iron alkoxide with Fe ions than EG, so the growth rate of Fe 3 O 4 grains will slow down, consequently decreasing the size. Furthermore, when EG/DEG coordinates with FeCl 3 to form an iron alkoxide, HCl is a byproduct from the reaction. The accumulation of HCl will inhibit further iron alkoxide formation. When NaAc is introduced, it can react with HCl to reduce the concentration of H + , and allow the coordination reaction to be completed [32,33]. In addition, the EG/DEG binary solvent system also plays a key role in controlling the size of the Fe 3 O 4 nanoparticles [31]. From the above-mentioned analysis, the formation of magnetic nanoparticles is closely related to the aggregation process. When pure EG is used, the precursor iron alkoxides transform into crystal nuclei and further agglomerate rapidly into large nanoparticles due to their high surface energy. When DEG is added to form a binary solvent with EG, the size of Fe 3 O 4 nanoparticles decreases as the V EG /V DEG ratio decreases. DEG forms a more stable coordination iron alkoxide with Fe ions than EG, so the growth rate of Fe 3 O 4 grains will slow down, consequently decreasing the size. Furthermore, when EG/DEG coordinates with FeCl 3 to form an iron alkoxide, HCl is a byproduct from the reaction. The accumulation of HCl will inhibit further iron alkoxide formation. When NaAc is introduced, it can react with HCl to reduce the concentration of H + , and allow the coordination reaction to be completed [32,33].

Conclusions
In summary, we have demonstrated a facile synthesis of size-controllable superparamagnetic Fe 3 O 4 nanoparticles via a solvothermal method based on an ethylene glycol/diethylene glycol (EG/DEG) binary solvent system. When the V EG /V DEG was varied from 100/0 to 80/20, 60/40, and 40/60, the average diameters of the resulting Fe 3 O 4 nanospheres were approximately 700, 500, 300, and 100 nm, respectively. In addition, the saturation magnetization (M s ) of Fe 3 O 4 nanoparticles with a size of 100, 300, 500, and 700 nm was 72.14, 75.94, 80.28, and 85.41 emu/g, and the corresponding remanent magnetization (M r ) was 3.34, 3.97, 3.26, and 4.28 emu/g, respectively. These superparamagnetic Fe 3 O 4 nanoparticles with high saturation magnetization show outstanding potential as targeted drug carriers.