Ethylenediaminetetraacetic acid as capping ligands for highly water-dispersible iron oxide particles
- Yunfeng Yi†1, 3,
- Ying Zhang†2,
- Yixiao Wang3,
- Lihua Shen3,
- Mengmeng Jia3,
- Yu Huang3,
- Zhenqing Hou3Email author and
- Guohong Zhuang4, 5Email author
© Yi et al.; licensee Springer. 2014
Received: 18 December 2013
Accepted: 8 January 2014
Published: 14 January 2014
Monodispersed magnetite (Fe3O4) particles were synthesized using a high-temperature hydrolysis reaction with the assistance of ethylenediaminetetraacetic acid (EDTA) as capping ligands. These particles were composed of small primary nanocrystals and their sizes could be tuned from about 400 to about 800 nm by simply changing the EDTA or precursor concentration. Surface-tethered EDTA made the particles highly water-dispersible. The as-prepared magnetite particles also showed superparamagnetic behavior at room temperature, and their magnetic properties were size dependent. In addition, the particles had a strong response to external magnetic field due to their high magnetization saturation values. These properties were very important for some potential biomedical applications, such as magnetic separation and magnetic-targeted substrate delivery.
Over the past decade, magnetic nanocrystals (e.g., Fe3O4, γ-Fe2O3) have attracted much attention due to their unique magnetic properties and important applications such as targeted drug delivery [1, 2], biomolecular separations [3, 4], treatment of hyperthermia in cancer [5, 6], and as contrast agents in magnetic resonance imaging (MRI) [7, 8]. Up to now, many methods have been developed to prepare Fe3O4 nanocrystals with small sizes on the nanometer scale, which include hydrothermal synthesis [9, 10], chemical coprecipitation [11–13], and thermal decomposition and/or reduction [14, 15]. Besides these nanosized particles, the secondary structural superparamagnetic Fe3O4 particles have also attracted increasing attention due to their practical applications in magnetic separation and magnetic-targeted substrate delivery [16, 17]. Generally, these secondary structural Fe3O4 particles consist of small Fe3O4 nanocrystals. As-prepared Fe3O4 particles are stable in solution and reveal rapid magnetic response to the externally applied magnetic field. Over the past decade, these secondary structural Fe3O4 particles are prepared by a common two-step process, including cooperative assembly , microemulsion templating , and spontaneous assembly . Compared to the two-step process of assembling the pre-synthesized Fe3O4 nanocrystals into uniform secondary structures, the direct one-step growth route to synthesize the secondary structural Fe3O4 particles seems to be a simpler way, which is also economical for large-scale production.
Herein we reported a general approach for the fabrication of monodispersed, highly water-dispersible, and superparamagnetic Fe3O4 particles by a one-step hydrothermal procedure using an ethylenediaminetetraacetic acid (EDTA)-assisted route. Biocompatible EDTA was chosen because it can act as a crystal grain growth inhibitor for the synthesis of variously sized Fe3O4 particles, and the carboxylate groups of EDTA have a strong coordination affinity to the iron cations on the Fe3O4 surface, which might favor the attachment of hydrophilic groups on the surface of the Fe3O4 particles. Herein, the Fe3O4 particles synthesized with the assistance of EDTA were also intrinsically stabilized with a layer of hydrophilic ligand in situ, which was essential for their long-term stability in aqueous media without any surface modification.
Synthesis of Fe3O4 particles
In a typical synthesis of 725 nm Fe3O4 particles, 1.3 g of anhydrous FeCl3 was first vigorously mixed with 40 mL of ethylene glycol (EG) to form a clear solution. Then, 0.47 g of EDTA was added and the mixture was heated at 110°C, followed by dissolving of anhydrous sodium acetate (NaOAc) (2.4 g), Then the mixture was transferred into a 100-mL Teflon-lined stainless-steel autoclave and sealed in air. The autoclave was kept at 200°C for 10 h. The black products were collected by a magnet and washed with ethanol three times, and the products were dried at 60°C for further use.
The x-ray diffraction (XRD) patterns were collected between 20° and 80° (2θ) on an x-ray diffraction system (X’Pert Pro, PANalytical Co., Almelo, The Netherlands) with a graphite monochromator and Cu Kα radiation (λ = 0.15406 nm). Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were obtained (JEOL JEM-2100; JEOL, Tokyo, Japan) operated at an accelerating voltage of 200 kV. The samples for TEM and high-resolution transmission electron microscope (HR-TEM) analyses were prepared by spreading a drop of as-prepared magnetite nanoparticle-diluted dispersion on copper grids coated with a carbon film followed by evaporation under ambient conditions. Atom force microscope (AFM) characterization was carried out using Scan Asyst-Air (Bruker Multimode 8, Bruker Corporation, Billerica, MA, USA). Measurements were carried out in air, and imaging was performed in tapping mode. The height, amplitude, and phase images were recorded. The scanning electron microscopy (SEM) images were obtained using LEO 1530 microscope (LEO, Munich, Germany).
Results and discussion
In summary, a modified solvothermal approach was used to synthesize monodispersed Fe3O4 particles with the assistance of EDTA, which are composed of numerous primary Fe3O4 nanocrystals with sizes of 7 to 15 nm. Their sizes could be easily tuned over a wide range of 400 to 800 nm by simply varying the concentration of FeCl3 or EDTA. More importantly, owing to the presence of the carboxylate groups attached on the surface, the Fe3O4 particles have excellent water dispersibility and dispersing stability. In addition, the growth mechanism of the secondary structural Fe3O4 particles is discussed. The magnetite particles are also superparamagnetic at room temperature and have a high magnetization, which enhance their response to external magnetic field and therefore should greatly facilitate the manipulation of the particles in practical uses.
This work was supported by the Natural Science Foundation of China (grant nos. 31271071 and 81072472) and the Natural Science Foundation of Fujian Province (grant no. 2012 J01416) and The Medical Science and Technology Innovation Project of Nanjing Military Command (10MA078, 2010).
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