Thermal Plasma Synthesis of Superparamagnetic Iron Oxide Nanoparticles
for Biomedical Applications
Pingyan Lei and Steven L. Girshick
Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN, USA
Abstract: Superparamagnetic iron oxide nanoparticles are of interest as contrast agents for magnetic resonance
imaging, and can be heated by an alternating magnetic field, facilitating tumor destruction by hyperthermia. We
report synthesis of superparamagnetic iron oxide nanoparticles using a DC thermal plasma. Ferrocene vapor and
oxygen were injected into an argon/helium plasma that was then expanded through a subsonic nozzle. Particles
were collected on glass fiber filters located in the reactor exhaust. In-situ measurements of particle size distributions were made using an aerosol sampling probe interfaced to a scanning mobility particle sizer (SMPS). Collected powder was characterized by transmission electron microscopy (TEM), X-ray diffraction, and vibrating
sample magnetometry (VSM). Synthesized particles consisted of the superparamagnetic maghemite (γ-Fe2O3) or
magnetite (Fe3O4) phases and hematite (α-Fe2O3) impurities. TEM images show primary particle diameters of 5-8
nm, while SMPS measurements indicate that the aerosol at the reactor exhaust consisted of small agglomerates,
with mobility diameters mostly in the 10-20 nm range. VSM measurements confirmed that the powder was superparamagnetic, with saturation magnetizations in the 15-30 emu/g range, depending on the oxygen flow rate.
Keywords: Thermal plasma, iron oxide, nanoparticles, superparamagnetism, biomedical applications
1. Introduction
Superparamagnetic iron oxide nanoparticles
(SPIONs) have potential biomedical applications
due to their nanoscale dimensions and magnetic
properties [1, 2]. Iron oxide nanoparticles of two
crystalline phases, magnetite (Fe3O4) and maghemite
(γ-Fe2O3) exhibit superparamagnetism when the
crystallite size is smaller than about 20 nm. SPIONs
provide enhanced contrast for magnetic resonance
imaging (MRI) [3], and can be heated by both Néelian and Brownian relaxation under an alternating
magnetic field, producing a localized temperature
increase, which can be used for hyperthermia therapy in cancer treatment [4].
Several previous studies reported synthesis of
iron oxide nanoparticles using various types of
plasmas [5-9]. However, in these studies either the
magnetic properties were not characterized [5,8], or
the particle size and/or phase composition were not
conducive to superparamagnetism [6,7,9]. In the
present work, a DC thermal plasma was used to synthesize iron oxide nanoparticles. Measurements
were made of particle size distributions, morphology,
phase composition and magnetic properties. The
results demonstrate the first (to our knowledge)
plasma synthesis of superparamagnetic iron oxide
nanoparticles.
2. Experimental Setup
The experimental setup used to synthesize iron
oxide nanoparticles is shown in Fig. 1. The DC
plasma was operated at a current of 250 amperes,
using as plasma gas a 30/5 slm mixture of argon and
helium. Ferrocene (Fe(CH)10) vapor was used as the
iron precursor. Ferrocene is a stable powder at room
temperature, and sublimes upon heating. Ferrocene
was sublimated in a heated bed at 120°C, controlled
by a heating mantle. The ferrocene vapor was entrained in argon, which flowed through the heated
bed at 0.5 slm. Oxygen was introduced downstream
of the heated bed, and the mixture was injected into
the plasma at the upstream end of a converging nozzle made of boron nitride. The flow exiting the nozzle expanded into a 250-mm-diameter chamber,
maintained at a pressure in the range 40-53 kPa. A
ceramic tube (32-mm OD, 25-mm ID, 356-mm
length) was positioned 51 mm above the nozzle exit
to provide a longer high temperature region for inflight particle annealing [10]. Cold argon quench
gas was injected into the ceramic tube. The flow
rates of quench gas and oxygen were treated as variable parameters.
3. Results and Discussion
Figure 1 Experimental apparatus for synthesizing
superparamagnetic iron oxide nanoparticles.
The size distribution of plasma-synthesized iron
oxide nanoparticles was measured on-line with
SMPS, consisting of a differential mobility analyzer
and a condensation particle counter, by extracting
aerosol from the reactor exhaust using an ejector
driven by high-pressure nitrogen. A sample collector located in the exhaust line was used to collect
product nanoparticles on a glass fiber filter for offline characterization to study chemical composition
and magnetic properties of the synthesized powder.
Nanoparticle phase composition was investigated
using X-ray diffraction (XRD), which was performed on a Siemens D-500 diffraction meter using
a 2.2-kW sealed cobalt source. Magnetic measurements were conducted on a Princeton micro vibrating sample magnetometer using a maximum applied
field of one Tesla at room temperature. Nanoparticle morphology was characterized by TEM, which
was conducted on a Tecnai G² F30 electron microscope. Samples for TEM were collected on lacey
carbon grids by an electrostatic precipitator with an
applied voltage of 3 kV, downstream of a bipolar
charger (Po radiation source) located in the sampling
line.
The magnetic properties of plasma-synthesized
iron oxide nanoparticles were characterized by VSM.
Fig. 2 shows hysteresis loops measured at room
temperature with various oxygen flow rates, with a
constant ferrocene feeding rate of ~7.9 sccm. Magnetic moments were normalized by sample mass
measured on the glass fiber filter. The oxygen flow
rate is seen to have a strong effect on the saturation
magnetization. The highest saturation magnetization,
28.4 emu/g, was achieved with an oxygen flow rate
of 200 sccm. The lack of evident hysteresis (to
within the line thickness) in the curves is a signature
of superparamagnetism. For the case of 200-sccm
oxygen flow rate, the coercivity and remanence
equalled 22.1 Oe and 1.87 emu/g, respectively. The
measured saturation magnetization is lower than reported bulk values for the maghemite and magnetite
phases [11]. As discussed below, we hypothesize
that the lower magnetization may result from the
presence of phase impurities, particularly of the
hematite phase, which is antiferromagnetic.
Figure 2 VSM measurements of hysteresis loops of
iron oxide nanoparticles at various oxygen flow rates. TEM images of nanoparticles produced at 200
sccm of oxygen, without argon quenching flow, are
shown in Fig. 3. The TEM image in Fig. 3(a) shows
a large agglomerate (believed to have formed during
deposition on the TEM grid) of iron oxide nanoparticles, with the diameter of primary particles in the
range 5-8 nm. The high-resolution TEM image in
Fig. 3(b) clearly shows the lattice fringes of a single
iron oxide nanoparticle. The d-spacing measured in
the image equals ~2.65 Å, corresponding to the maghemite (310) plane.
(a)
(b)
Figure 4 XRD patterns of plasma-synthesized iron
oxide nanoparticles compared to magnetite and maghemite standards (purchased from Sigma-Aldrich). Figure 3 TEM images of plasma-synthesized iron
cxide nanoparticles. Fig. 4 compares the XRD pattern of plasmasynthesized iron oxide nanoparticles, produced at an
oxygen flow rate of 200 sccm and zero argon quench
flow, with standard magnetite and maghemite nanopowder purchased from Sigma-Aldrich. The
standard maghemite nanopowder shows small peaks
from hematite impurities, which may be caused by
exposure to air. Magnetite and maghemite have the
same cubic structure and similar crystal lattice parameters (magnetite a=0.8396 nm, maghemite
a=0.83474 nm) [11]. Their XRD patterns are virtually identical. As seen in Fig. 4, the plasmasynthesized magnetic nanoparticles contain the maghemite (or magnetite) phase as well as the antiferromagnetic hematite phase. Hematite contamination decreases the average saturation magnetization
of the product. The formation of the hematite phase
may be caused by the high-temperature plasma process. Another possible cause of hematite formation
is that the superparamagnetic iron oxide nanoparticles are not stable due to their small sizes. A thin
layer of the hematite phase may form quickly on the
surfaces of such small iron oxide nanoparticles when
they are exposed to air. Measured size distributions of plasmasynthesized iron oxide particles with various argon
quench gas flow rates are shown in Fig. 5. These
particles were produced with ~7.9 sccm ferrocene
vapor and 200 sccm oxygen. SMPS measures the
mobility diameter of particles, which may consist of
agglomerates that contain several primary particles.
At the same precursor feeding rate and oxygen flow
rate, the average particle size is seen to decrease as
the quench gas flow rate increases. The mode of the
size distributions in Fig. 5 decreases from ~18 nm to
~12 nm as the quench gas flow rate increases from 0
to 30 slm.
Figure 5 SMPS size distribution measurements
for various flow rates of argon quench gas. 4. Summary
A DC thermal plasma system was used to synthesize superparamagnetic iron oxide nanoparticles
using ferrocene as iron precursor. Plasmasynthesized magnetic iron oxide nanoparticles had
primary particle sizes of 5-8 nm. The powders produced are predominantly the superparamagnetic ma-
ghemite (or magnetite) phase, and also show evidence of hematite impurities. To our knowledge this
is the first report of plasma synthesis (by any type of
plasma) of superparamagnetic iron oxide nanoparticles. The oxygen flow rate has a strong effect on the
magnetic properties of the product powder, likely by
affecting the iron oxide stoichiometry and phase
composition. Introducing argon quench into the reaction chamber is an effective means to reduce the
agglomerate size of plasma-synthesized magnetic
iron oxide nanoparticles. At the highest quench gas
flow rate tested the mean agglomerate size equalled
~16 nm, indicating that each agglomerate contains
several primary particles.
5. Acknowledgments
This research was primarily supported by the
U.S. National Science Foundation under Award
Number CBET-0730184, and partially supported by
the Minnesota Futures Grant Program. Parts of the
characterization work were carried out in the College of Science and Engineering Characterization
Facility, and the Institute for Rock Magnetism, University of Minnesota.
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