Controlling the size of magnetic nanoparticles using pluronic block copolymer surfactants

Article abstract of DOI:10.1021/jp0457702

We have successfully controlled the size of magnetic nanoparticles by adjusting the surfactant/solvent ratio. γ-Fe₂O₃ nanoparticles of 5.6 and 12.7, and Fe⁰ nanoparticles of 22.3 nm in diameter were prepared, all having spherical shape and uniform size as confirmed by TEM. Mo?ssbauer spectra confirmed Fe³⁺ for the 5.6 and 12.7 nm particles and Fe³⁺ and Fe⁰ for 22.3 nm particles, in good agreement with synchrotron XRD patterns. Both room temperature and 5 K H-M measurements show that 22.3 nm particles have much higher magnetization than their oxide counterparts, in agreement with their being Fe⁰. T-M measurements show superparamagnetism for 5.6 and 12.7 nm particles and ferromagnetism for 22.3 nm particles.

Full text of DOI:10.1021/jp0457702

15
2005, 109, 15-18
Published on Web 12/13/2004
Controlling the Size of Magnetic Nanoparticles Using Pluronic Block Copolymer
Surfactants
Jr-iuan Lai,†,| Kurikka V. P. M. Shafi,†,| Abraham Ulman,*,†,|, Katja Loos,†,|,# Yongjae Lee,‡
Thomas Vogt,‡ Wei-Li Lee,§ and N. P. Ong§
Department of Biological and Chemical Sciences and Engineering, Polytechnic UniVersity,
6 Metrotech Center, Brooklyn, New York 11201, Physics Department, BrookhaVen National Laboratory,
P.O. Box 5000, Upton, New York 11973-5000, Department of Physics, Princeton UniVersity,
Princeton, New Jersey 08544, and The NSF MRSEC for Polymers at Engineered Interfaces
ReceiVed: September 17, 2004; In Final Form: NoVember 5, 2004
We have successfully controlled the size of magnetic nanoparticles by adjusting the surfactant/solvent ratio.
γ-Fe2O3 nanoparticles of 5.6 and 12.7, and Fe0 nanoparticles of 22.3 nm in diameter were prepared, all having
spherical shape and uniform size as confirmed by TEM. Mo¨ssbauer spectra confirmed Fe3+ for the 5.6 and
12.7 nm particles and Fe3+ and Fe0 for 22.3 nm particles, in good agreement with synchrotron XRD patterns.
Both room temperature and 5 K H-M measurements show that 22.3 nm particles have much higher
magnetization than their oxide counterparts, in agreement with their being Fe0. T-M measurements show
superparamagnetism for 5.6 and 12.7 nm particles and ferromagnetism for 22.3 nm particles.
Advances in nanoparticle synthesis stem from their funda-
mental and technological importance,1 as nanoparticles exhibit
electrical, optical and magnetic properties that are different from
their bulk counterparts. However, these materials can be toxic
and hence coating their surface with a thin biocompatible
polymer shell might be advantageous for biomedical applica-
tions. Pluronic triblock copolymerss(EO)x(PO)y(EO)x, poly-
(ethylene glycol)-block-poly(propylene glycol)-block-poly-
(ethylene glycol)sare biocompatible and have been used as
surfactants in the synthesis of different nanoparticles.2 Here we
report on using Pluronic 127 for the thermal synthesis of Fe
and Fe2O3 nanoparticles. The particles are produced with the
polymer coated on their surface.
In a typical synthesis, 0.23 mL Fe(CO)5 was added to 250
mL of degassed mesitylene (preheated to 70 °C) and stirred for
5 min. Subsequently, 226 mg of the polymer surfactant Pluronic
F127 was added into the solution. Once the polymer dissolved,
the temperature was raised to the boiling point and the solution
was refluxed for 24 h. To produce Fe nanoparticles, the
experiment was carried out in a reducing environment (5% H2;
95% Ar). After this, the solution was cooled to room temperature
and the resulting precipitate was collected by centrifugation,
washed with toluene to remove the excess of polymer, and dried
in a vacuum overnight. The molar ratio between Fe(CO)5 and
mesitylene was 1:400. To obtain different sizes of Fe nanopar-
ticles, the ratio surfactant (polymer)/solvent (Su/So) needed to
be adjusted.3 For small, medium, and large Fe nanoparticles,
we found that optimized ratios of surfactant/solvent were 1:4000,
1:40 000, and 1:400 000, respectively. We anticipated that
increasing the amount of surfactant for the same amount of
carbonyl will result in smaller particles, due to the availability
of a stabilizing agent for larger surface area. Notice, however,
that since smaller particles have larger surface area, they should
be more reactive and prone to oxidation.4
Figure 1 shows the TEM images and resulting histograms
for nanoparticles prepared with three different Su/So ratios.
Since the Pluronic surfactant is not stained, it cannot be
observed, and the TEM images show only the particle cores.
The histograms reveal particle size distributions as 5.6 ( 0.73
(1:4000), 12.7 ( 3.24 (1:40 000), and 22.3 ( 3.08 nm
(1:400 000), respectively. The images clearly show spherical
particles with narrow size distribution. It also becomes obvious
that particle size increases with the decreasing Su/So ratio, and
hence can be controlled by adjusting the Su/So ratio. A TEM
image of 22.3 nm particles (Figure 1c) shows chain-like
aggregates, which was not observed in images of either 5.6
(Figure 1a) or 12.7 (Figure 1b) nm particles. We shall return to
this phenomenon later.
Figure 2 shows synchrotron powder XRD patterns for the
three nanoparticle sizes. The patterns for 5.6 (Figure 2a) and
12.7 (Figure 2b) nm nanoparticles match the cubic γ-Fe2O3
crystal structure, which is a spinel, where oxygen atoms are
organized in an fcc structure in which iron ions occupy both
octahedral and the tetrahedral sites. On the other hand, the
synchrotron powder XRD pattern of 22.3 nm nanoparticles
(Figure 2c) matches the Fe crystal structure, which is bcc. Since
all particles are synthesized under a reducing environment,
* Corresponding author. E-mail: aulman@duke.poly.edu.
† Polytechnic University.
‡ Brookhaven National Laboratory.
§ Princeton University.
| NSF MRSEC for Polymers at Engineered Interfaces.
Present address: Department of Chemistry, Bar-Ilan University, Ramat-
Gan 52900, Israel.
# Present address: Faculteit der Wiskunde en Natuurwetenschappen,
Polymer Chemistry, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG
Groningen, The Netherlands.
10.1021/jp0457702 CCC: $30.25 © 2005 American Chemical Society

16 J. Phys. Chem. B, Vol. 109, No. 1, 2005
Letters
Figure 1. (a) TEM bright field images of (a) sample with 1:4000 Su/So ratio, (b) sample with 1:40 000 Su/So ratio, and (c) sample with 1:400 000
Su/So ratio. Corresponding histograms are attached below the TEM images.
Figure 2. Synchrotron powder XRD patterns for (a) 5.6 nm; (b) 12.7 nm; and (c) 22.3 nm particles.
one should assume that they were all formed as iron nanopar-
ticles and that the smaller nanoparticles, due to their large
surface area, were oxidized upon exposure to ambient into the
corresponding γ-Fe2O3 nanoparticles. However, only the 22.3
nm particles exhibit Fe crystal structure. Since the magnetic
moment of Fe is much larger than the magnetic moment of
γ-Fe2O3, it will have a dipolar field extending a further distance
than the γ-Fe2O3. This is in good agreement with the observa-
tion of chain aggregation, where the larger nanoparticles
organize with their poles aligned by the dipolar field. This is
equivalent to the experiments of Faraday showing the lines
of force of a magnet by iron fillings,5 and much later Francis
Bitter used it to demonstrate the domain structures in bulk
magnets.6
The magnetic properties of these nanoparticles were studied
by using SQUID at field range of (5 T (H-M) and a
temperature range from 2 to 300 K (T-M). H-M SQUID
measurements were used to ascertain the induced magnetization
from the nanoparticles at the applied field. Due to their small
Figure 3. H-M measurements at 5 K.
size, we expected superparamagnetic behavior. According to
the TGA results, the polymer contents are 85%, 60%, and 40%
for the 5.6, 12.7, and 22.3 nm particles, respectively. Therefore,
the magnetization reported here (in emu/g) is after subtracting
the polymer content. The isothermal magnetization of the three
samples at room temperature displays hardly any hysteresis. The
magnetization at an applied field 5 T was 10, 13, and 92 emu/g
for particles with mean diameter of 5.6, 12.7, and 22.3 nm,
respectively. When the samples were cooled to 5 K (Figure 3),
the magnetization at an applied field of 5 T increased to 14,
17, and 106 emu/g for particles with mean diameter of 5.6, 12.7,
and 22.3 nm, respectively, and hysteresis loops appear (Figure

Letters
J. Phys. Chem. B, Vol. 109, No. 1, 2005 17
Figure 4. T-M measurements for (a) 5.6, (b) 12.7, and (c) 22.3 nm particles.
Figure 5. Mo¨ssbauer spectra for (a) 5.6, (b) 12.7, and (c) 22.3 nm particles.
TABLE 1: Summary of H-M Measurement Results
images suggest that 5.6 nm particles have stronger particle-
particle interaction than 12.7 nm particles (Figure 1). In T-M
measurements, ZFC and FC curves split above Tb for the 5.6
nm sample, compared to 12.7 nm particles, where ZFC and FC
curves overlap almost perfectly up to Tb, then split from that
point. These measurements confirm that 5.6 nm particles have
stronger particle-particle interaction than their 12.7 nm coun-
terparts. This stronger interaction might lead to the higher
observed Tb. Furthermore, 12.7 nm particles behave like single
domain superparamagnetic particles.
Magnetization (emu/g)
particle
size (nm)
coercivity
(Oe)
300 K
5 K
5.6
12.7
22.3
10
13
92
14
17
106
1300
1200
1200
3 inset), with coercivity of 1300, 1200, and 1200 Oe, respec-
tively. The results of H-M measurements are summarized in
Table 1.
The Mo¨ssbauer spectra of the three samples are shown in
Figure 5. The Mo¨ssbauer spectra for γ-Fe2O3 samples are
characterized by a doublet at room temperature and by a sextet
at 4.2 K. The change is due to the fact that the moments in the
particles are random at room temperature and therefore para-
magnetic, while below the blocking temperature, there is an
internal magnetic field generating the Zeeman splitting of the
levels. The spectra of the two samples with small particles are
as expected for γ-Fe2O3 with hyperfine field value of 500 KG.
The Mo¨ssbauer spectra for the Fe particles display a superposi-
tion of a doublet and a sextet. Deconvolution of the spectrum
suggests the presence of two γ-Fe2O3 components with isomer
shifts of 0.5 and 0.3, confirming that the Fe is in a trivalent
state. In addition, a sextet pertaining to metallic Fe with isomer
shift of 0 and hyperfine field of 330 KG can be observed.
In conclusion, by adjusting the Su/So ratio we have success-
fully controlled the size of magnetic nanoparticles. Three
different nanoparticle sizes were prepared, 5.6, 12.7, and 22.3
nm, all shown by TEM to have spherical shape and uniform
size. The TEM image also revealed a chain-like pattern for 22.3
nm Fe particles. Synchrotron powder XRD patterns showed 5.6
and 12.7 nm particles to be γ-Fe2O3 and 22.3 nm particles to
be Fe. Mo¨ssbauer spectra confirmed Fe3+ for the 5.6 and 12.7
nm particles and Fe3+ and Fe0 for 22.3 nm particles, in good
According to the XRD patterns, the 22.3 nm particles are
Fe, supporting the observed magnetization data. The saturation
magnetization is 76 emu/g for γ-Fe2O3 and 221 emu/g for Fe;7
due to the small size, magnetic order is easier to be randomized
and the observed saturation magnetization is smaller. The
different coercivity field observed at low temperature is due to
the different content ratio of Fe and γ-Fe2O3 in the samples. In
general, the γ-Fe2O3 phase has a larger coercive field than Fe
metal.
Figure 4 shows the T-M measurements for particles with
mean diameter 5.6, 12.7, and 22.3 nm, respectively. Both
γ-Fe2O3 samples have maximums in ZFC measurement and
constant moments below Tb for FC measurement. These are the
typical T-M measurements for superparamagnetic materials.8
That ZFC and FC curves almost overlap above Tb indicates
narrow particle size distribution, which is in good agreement
with histograms derived from TEM measurements (Figure 1).
The larger Fe nanoparticles, with their much stronger magnetic
moments, lead to T-M curves indicating ferromagnetic behav-
ior.
The blocking temperatures are surprisingly not systematic.
This may be a consequence of the different content of these
composites and the critical size for maximum coercivity, which
is different for Fe and γ-Fe2O3 nanoparticle samples. TEM

18 J. Phys. Chem. B, Vol. 109, No. 1, 2005
Letters
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agreement with XRD patterns. Both room temperature and 5 K
H-M measurements show that 22.3 nm particles have much
higher magnetization then their oxide counterparts, in agreement
with their being Fe. T-M measurements show superparamag-
netism for 5.6 and 12.7 nm particles and ferromagnetism for
22.3 nm. Experiments underway in our laboratory to extend
this synthesis to mixed oxide nanoparticles.
(6) Bitter, F. Phys. ReV. 1930, 36, 978.
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