Mater. Res. Soc. Symp. Proc. Vol. 1416 © 2012 Materials Research Society DOI: 10.1557/opl.2012.736
Microemulsion Synthesis of Iron Core/Iron Oxide Shell Magnetic Nanoparticles and Their Physicochemical Properties Katsiaryna Kekalo, Katherine Koo, Evan Zeitchick and Ian Baker Thayer School of Engineering at Dartmouth College, Hanover, New Hampshire, USA ABSTRACT Iron magnetic nanoparticles were synthesized under an inert atmosphere via the reaction between FeCl3 and NaBH4 in droplets of water in a microemulsion consisting of octane with cetyl trimethylammonium bromide and butanol as surfactants. A thin Fe3O4 layer was produced on the iron nanoparticles using slow, controlled oxidation at room temperature. A silica shell was deposited on the Fe3O4 using 3-aminopropyltrimethoxysilane following the method of Zhang et al. [Mater. Sci. Eng. C 30 (2010) 92–97]. The structure and chemistry of the resulting nanoparticles were studied using variety of methods and their magnetic properties were determined. The diameter of the iron core was typically 8-16 nm, while the thickness of the Fe3O4 shell was 2-3 nm. The presence of the silica layer was confirmed using Fourier transform infra-red spectroscopy and the number of NH2-groups on each nanoparticle was determined based on colorimetric tests using ortho-phthalaldehyde. Keywords: microemulsion synthesis, magnetic nanoparticles, Fe/Fe oxide, magnetic properties, size distribution. INTRODUCTION Iron core/iron oxide shell magnetic nanoparticles coated with SiO2 and with NH2 groups attached are being developed since they are able to transform alternating magnetic field (AMF) energy into heat, a feature useful for localized hyperthermia treatment of tumors [1,2]. The small size of the nanoparticles enables their uptake by cells and, upon the application of an external magnetic field, cause damage to the cells by heating to 42-45 °C. Fe3O4 and γ-Fe2O3 nanoparticles are most commonly used for this purpose , but Fe core/ Fe oxide shell magnetic nanoparticles can cause higher magnetic energy to heat transfer in an applied AMF due to the higher saturation magnetization, Ms, of iron . Reverse micelles (surfactant-stabilized water-in-oil microemulsions) have been successfully used as nanoreactors for the synthesis of metal nanoparticles including Co , Cu , Ag , Pt , Au [8-10], and Fe [1, 2, 9-11]. The same approach is used here to produce Fe nanoparticles. In addition to their use for hyperthermia, Fe core/ Fe oxide shell magnetic nanoparticles have shown superior properties as contrast agents for magnetic resonance imaging when compared to commercial superparamagnetic iron oxide nanoparticles . Nanoparticles for use in biomedical applications have a number of requirements including biocompatibility, non-toxicity, monodispersivity, resistance to agglomeration and, importantly, long-term stability. This paper describes work on nanoparticles with silica and amine groups and the determination of the stability of Fe/Fe oxide nanoparticles in a saline solution.
EXPERIMENTAL DETAILS Two water-in-oil microemulsions, one containing a FeCl3 aqueous solution, the other containing a NaBH4 aqueous solution were created using octane with cetyltrimethylammonium bromide (CTAB) and butanol as surfactants. The individual microemulsions were continuously stirred under flowing argon (Ar) for 30 min. The NaBH4 emulsion was quickly and carefully transferred to the FeCl3 microemulsion in droplets over 5 minutes, limiting air exposure as much as possible. Dispersed water droplets from both emulsions collide and exchange contents allowing the reduction of FeCl3 to iron nanoparticles. Nanoparticles were stirred for 15 minutes then washed three times with water, and then twice more with methanol using a strong permanent magnet to concentrate the particles between washings. This allowed the removal of excess CTAB, octane and n-butanol. The oil:water volume ratio used was 2.5:1; the oil:surfactant:co-surfactant molar ratio was 7.5:2:1, with equal volumes of 0.20 M solution of FeCl3 and 0.85 M solution of NaBH4 being used. A 40 % excess of NaBH4 was used because NaBH4 is subject to hydrolysis on contact with water. Careful oxidation of the surface particles was performed by adding a solution of trimethylamine n-oxide (1.9 wt.%) in methanol to the nanoparticles in a vial filled with argon. The vial was flooded with argon, sealed, and placed in an ultrasound bath for 1 hr. The weight ratio magnetic nanoparticles:oxidizer was 20:1. After one hour the particles were washed with methanol and dried under flowing argon. The next step in nanoparticle processing was the removal of the CTAB coating. CTABcoated nanoparticles were dispersed with hexane in a glass bottle containing 25% w/w solution of tetramethylammonium hydroxide (TMAH). The bottle was then flooded with argon, sealed, shaken and sonicated until the particles were dispersed in the water phase and not in the hexane (see visually). Afterwards, a strong magnet was used to concentrate the particles while the water and hexane were decanted. The particles were then washed twice with methanol and dried under flowing argon. Next, the nanoparticles were coated with an aminosilane molecule using the following procedure. Post CTAB removal, the nanoparticles were placed in a bottle filled with 40 ml hexane and 6 ml (3-aminopropyl) trimethoxysilane (APTMES). The bottle was placed in a bath sonicator with argon flowing through the top of the bottle. After 10 minutes of sonication, 200 µl of glacial acetic acid was introduced into the mixture, the bottle was capped to ensure an inert atmosphere, and sonicated for up to six days. A magnet was used to immobilize the particles in the bottle while the hexane solution was decanted. The particles were rinsed with hexane twice, then dispersed in methanol and transferred to a flat-bottomed boiling flask, where the methanol medium was replaced with toluene. The flat bottom boiling flask was attached to a Dean-Stark extraction apparatus, with an input for argon, and a reflux condenser. The toluene was boiled for six hours under argon to remove trace water and promote silane condensation on the particle surface. In order to produce multiple coatings, this procedure was repeated. The particles were then carefully dried under argon and stored in a vacuum desiccator. Transmission electron micrographs of the nanoparticles were taken using a FEI Technai F20ST field emission gun transmission electron microscope (TEM) operated at 200 kV. X-ray diffraction (XRD) measurements were performed using a Rigaku D/MAX diffractometer with Cu–Kα radiation (range 2 theta from 20 to 140 degrees, step size 0.02 degrees, scan speed 1 second per step, 40kV and 300mA). The quasi-static magnetic properties of the nanoparticles
were determined (Ms, remanence magnetization, Mr, and coercivity, Hc) from hysteresis loop measurements using a Lakeshore model 7300 vibrating sample magnetometer (VSM). FT-IR spectra were recorded on FT/IR-6200 type A machine with an ATR PRO450-S accessory and ZnSe ATR crystal, area 599.994 – 4000.12 cm-1, standard light source, a TGS detector at a resolution of 1 cm-1 with an aperture of 3.5 mm and a scanning speed of 2 mm/sec with a 10000 Hz filter. The concentration of amines on the nanoparticles was quantified using a fluorescent microplate assay. The method was based upon the reaction of known amounts of orthophthalaldehyde (OPA) with amine surface nanoparticles, separation of the nanoparticles and determination of the residual OPA in the supernatant by reaction with an excess of soluble amine and subsequent measurement of the fluorescent product. The method was adapted from the assays described in Hoinard et al  and Janolino and Swaisgood . Fluorescence was measured in a Spectramax 190 microplate using 340 nm excitation and 455 nm emissions. Since the Fe/Fe oxide nanoparticles could be used both in in vitro and in vivo studies, their long-term stability was examined. In addition, data about the stability in air enabled determinations of storage and shelf life of this material and data about the stability of nanoparticles and their properties in water and saline solution  will help to understand the limits of their usage in biomedical applications. RESULTS AND DISCUSSION The nanoparticle procedures described above enabled the production of Fe/Fe oxide nanoparticles with yields up to 98% of the theoretical value. The magnetic nanoparticles produced via the microemulsion technique are spherical and have 8-16 nm core and 2-3 nm shell (Figure 1). X-ray diffraction pattern of the nanoparticles (Figure 2) showed a peak at 2θ = 44°, indicating the presence of iron (the blue lines). The next largest peak is directly to the left of the strong iron peak, at 2θ = 30° is iron oxide (red lines indicate iron oxide). Fe3O4 and γ-Fe2O3 have very similar set of diffraction peaks and it is hard to determine whether the peaks in Figure 2 correspond to Fe3O4 or γ-Fe2O3
Figure 1. Bright field TEM image of Fe/Fe oxide nanoparticles. Figure 2. XRD pattern of Fe/Fe oxide nanoparticles: blue lines – Fe, red lines – Fe3O4 or γ-Fe2O3.
The Ms, Mr and Hc were measured to be 81 emu/g, 10 emu/g and 130 Oe, respectively (Figure 3) – the Ms of the nanoparticles was determined by extrapolation of the magnetization curve to be 10 kOe. The thickness of the Fe oxide shell played a significant role in the stability of nanoparticles: if the oxide layer is too thin it cannot prevent the nanoparticles from burning in air and/or during mechanical crushing of aggregates. It was shown that Fe core/ Fe oxide shell nanoparticles retain their magnetic and physicochemical properties for long periods if they are stored in vacuum and even in air. However, water and, especially saline Figure 3. Magnetization curve of Fe core/ Fe solution, causes rapid oxidation after storage oxide shell nanoparticles after production. for three days (Figure 4), whereas nanoparticles stored in an inert atmosphere for three days still have a core/shell structure (Figure 4a). In contrast, after three days in water they are destroyed and small iron oxide particles (Figure 4b) are visible, while in saline solution the nanoparticles, probably, transform to iron hydroxide clusters (Figure 4c). Refrigeration of the samples at 4 °C decreases both the rates of oxidation and hydrolysis, but cannot prevent oxidation and hydrolysis even for three days. The degradation in the magnetic properties of the nanoparticle after storage for three days in air, in water or in saline solution is shown in Figure 5. This degradation indicates the transformation of the Fe/Fe oxide nanoparticles into less magnetic materials (perhaps iron oxides and/or hydroxides).
Figure 4. Bright field TEM images of Fe/Fe oxide nanoparticles after storage for 3 days in (a) inert atmosphere, (b) water, and (c) saline solution.
The APTMES-coated nanoparticles were analyzed using FT-IR spectroscopy (Figure 6). Changes in the spectra due to the creation of mono- or multi-coatings indicate the presence of APTMES on nanoparticle surface. The peaks observed around 1000 cm-1 are similar to those observed in pure APTMES. UV-VIS spectrometry was used to determine the concentration of the NH2- groups on the nanoparticles’ surface. The silica coating may further increase the stability of the Fe/Fe oxide MNP as barrier for oxygen penetration. It was found that nanoparticles contained up to 4.2 x 10-4 mole of NH2- groups per gram of material.
Figure 5. Magnetization curves of Fe/Fe oxide nanoparticles after 3 days of treatment
Figure 6. FT-IR spectra of Fe/Fe [email protected]
CONCLUSIONS It was shown that Fe core/ Fe oxide shell nanoparticles retain their magnetic and physicochemical properties for at least 3 days if they are stored in an inert atmosphere and even in air. However, storage in water and, especially in a saline solution causes rapid oxidation and
hydrolysis, as well as a corresponding degradation in the magnetic properties. A silica layer and NH2-groups were incorporated onto the surface of the nanoparticles. Further work will be performing to find out influence of silica coating on the MNP stability. ACKNOWLEDGEMENTS This research was supported by National Institute of Health (NIH) grant 1U54CA151662-01. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing official policies, either expressed or implied of the NIH or the U.S. Government.
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