Metallic cobalt nanoparticles for heating applications

Article abstract of DOI:10.1016/j.jmmm.2006.11.178

We investigated the properties of metallic cobalt particles which were prepared by metal organic synthesis. By X-ray diffraction we identified the FCC Co phase and obtained a particle size of 6 nm. VSM measurements revealed a specific magnetization of 77.5 Am²/kg which is 46% of the bulk value. From the analysis of the magnetization curve the parameters of the particle size distribution were estimated. In order to assess the suitability of the material for heating applications AC susceptometry as well as calorimetrical measurements of the specific loss power at 400 kHz and 13-25 kA/m were performed. We obtained values from 500 to 1300 W/g.

Full text of DOI:10.1016/j.jmmm.2006.11.178

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 311 (2007) 224–227
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Metallic cobalt nanoparticles for heating applications
a,
Matthias Zeisberger , Silvio Dutz , Robert Mu
Ã
a
a
ller , Rudolf Hergt ,
a
¨
nnemann
b
Nina Matoussevitch , Helmut Bo
b
¨
a
Institut f u¨ r Physikalische Hochtechnologie, A.-Einstein-Str. 9, D-07745 Jena, Germany
Forschungszentrum Karlsruhe, ITC-CPV, Post Box 3640, D-76021 Karlsruhe, Germany
b
Available online 8 December 2006
Abstract
We investigated the properties of metallic cobalt particles which were prepared by metal organic synthesis. By X-ray diffraction we
2
identified the FCC Co phase and obtained a particle size of 6 nm. VSM measurements revealed a specific magnetization of 77.5 Am /kg
which is 46% of the bulk value. From the analysis of the magnetization curve the parameters of the particle size distribution were
estimated. In order to assess the suitability of the material for heating applications AC susceptometry as well as calorimetrical
measurements of the specific loss power at 400 kHz and 13–25 kA/m were performed. We obtained values from 500 to 1300 W/g.
r 2006 Elsevier B.V. All rights reserved.
Keywords: Co-nanoparticle; Specific heating power; Particle size distribution; AC susceptometry; Hyperthermia
1
. Introduction
The limitations concerning the particle concentration
and the field parameters as well as the effect of the heat
conductivity of the tissue [5] result in the demand of a very
high specific heating power (SHP). According to our
estimations a SHP in the order of 1000 W/g or higher
should be a goal of the material development.
Magnetic nanoparticles are a very promising tool for
several biomedical applications. In particular, the heat
generation of these particles in a magnetic AC field can be
used for local hyperthermia in the therapy of tumors [1,2].
The simplest method is the direct injection of aqueous
suspensions of these particles into the tumor before
applying the field. More sophisticated techniques to load
the tumor with the material are magnetic targeting or
antibody targeting. The heat generated by the particles
increases with increasing field amplitude and frequency.
However, there are limitations for these parameters
because of medical [3] and technical reasons. According
to our previous investigations, a frequency of 400 kHz and
a field amplitude of 10 kA/m are reasonable values for
hyperthermia.
Up to now, the research in materials for hyperthermia
has focussed on superparamagnetic iron oxide (Fe O ,
3
4
g-Fe O ) particles [2]. However, this type of particles
2
3
commonly shows SHP values less than 100 W/g (at 400 kHz
and 10 kA/m).
One possible way to obtain a higher SHP is the use of
magnetic materials with higher specific magnetization, e.g.
metallic cobalt or iron. As a first step in this direction we
investigated the properties of metallic Co particles.
For in vivo applications the concentration of magnetic
material in the tissue should be limited in order to avoid
side effects. This problem was investigated in the context of
MRI contrast agents where maximum Fe concentrations of
2. Sample preparation
The particles were prepared by controlled thermolysis of
Co (CO) in toluene in the presence of Al alkyls [6]. The
2
8
0
.1 mmol/kg are recommended [4].
reaction results in metallic Co particles with a size up to
0 nm. After the particle synthesis a controlled oxidation is
1
used to produce particles which are stable against further
oxidation. The Co particles were transferred from toluene
into kerosene by using the surfactants Korantin SH and
Ã
Corresponding author. Tel.: +49 3641 206107; fax: +49 3641 206199.
E-mail address: zeisberger@ipht-jena.de (M. Zeisberger).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.11.178

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225
LP4. These surfactants allow for a stable suspension of the
particles.
Three samples were used for the measurements. The first
3
0
3
2
1
0
5
was a powder sample. Another one was a suspension of the
particles in kerosene and the third type of sample was
prepared by drying about 20 ml of the fluid on a small piece
of paper. The paper was rolled and inserted into the
container used for the VSM measurements. In this kind of
sample the particles are immobilized but in contrast to the
powder the concentration is much lower resulting in much
lower demagnetization effects which simplifies the analysis
of the data.
2
1
0
9
10
5
0
-
-
5
-3
0
3
-
-
-
10
15
20
1
8
200 300 400
3
. VSM and XRD investigations
-
1500
-1000
-500
0
500
1000
1500
In order to obtain the specific magnetization of the
particles the powder sample was investigated by VSM. The
data are shown in Fig. 1. The specific saturation
2
magnetization was 77.5 A m /kg (46% of the bulk value
field [kA/m]
Fig. 2. Magnetization data of the immobilized fluid sample and fits of the
low-and high-field range.
of Co) which is slightly higer than the value reported in
Ref. [7] for Co particles with an oxide shell. This difference
to the bulk properties is due to the oxide shell which is
created during the preparation process. Another VSM
measurement was performed with the immobilized sample.
The data which are presented in Fig. 2 correspond to a
modified Langevin function which can be analyzed
according to the method of Chantrell [8] in order to obtain
the parameters of the particle size distribution which is
assumed as lognormal:
X-ray diffraction. The Yꢀ2Y-scan shows a peak at 44.341
corresponding to the cubic (FCC) phase of Co. From the
full-width at half-maximum of this peak which is 2.271 we
obtained a mean particle size of 6 nm.
4. Characterization of the AC behavior
The characterization of the behavior in AC magnetic
fields was performed by two methods: AC susceptometry
(ACS) and calorimetric measurement of the SHP. The
arrangement used for ACS is shown in Fig. 3. The field coil
(+14 mm ꢁ 75 mm, 75 turns) which is connected to a
function generator provides an AC field with an amplitude
up to 60 A/m and a frequency in the range from 20 Hz to
ꢀ
ꢁ
2
1
ln ðd=DÞ
f ðdÞ ¼ pffiffiffiffiffi exp ꢀ
.
(1)
2
2
psd
2s
From separate fits in the low (m ¼ cVH for |m|5m ) and
s
high-field range (m ¼ m (1ꢀH /H) for |m|Em ) we
s
0
s
1
MHz. The sample is in a cylindric container (inner size
obtained D ¼ 7.0 nm and s ¼ 0:34.
+
3 mm ꢁ 7 mm) which is placed inside the pick-up coil (25
In order to obtain information about the phase and
the particle diameter we investigated the powder sample by
turns). A compensation coil is used to cancel out the
background signal. The output signal is detected by a lock-
in amplifier providing the in-phase and out-of-phase
8
6
4
2
0
0
0
0
0
0
signals that correspond to the real (w ) and imaginary part
0
0
(w ) of the susceptibility. Fig. 4 shows the result of the
measurements which were performed for the fluid as well as
for the immobilized sample. The data show that the fluid
0
00
sample shows much higher values of w and w and a more
pronounced frequency dependence than the immobilized
sample which is due to the Brownian rotation of the
particles in the liquid. In the fluid sample w shows a
distinct maximum at 25 kHz which corresponds to a
0
0
-
-
-
-
20
40
60
80
ꢀ
6
Brownian relaxation time of t ¼ 6.4 ꢁ 10 s. From the
B
relation t ¼ pZDh3/k T and a dynamic viscosity of
B
B
Z ¼ 1 mPas for kerosene we estimated a hydrodynamic
diameter D of the particles of 20 nm.
h
The calorimetric measurement of the SHP was per-
formed using an arrangement as shown in Fig. 5. A water
cooled copper coil which is connected to a 5 kW RF
generator provides an AC field of 400 kHz and amplitudes
in the range from 12 to 24 kA/m. The fluid sample is placed
-1000
-500
0
500
1000
field [kA/m]
Fig. 1. Specific magnetization of the Co powder.

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M. Zeisberger et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 224–227
226
temperature
measurement
temperature
sensor
Ref-In
In
sample
Lock-In
Amplifier
pick-up
coil
RF generator
f = 400 kHz
P = 5 kW
RF
Cu coil
(water cooled)
generator
Fig. 5. Experimental set-up of the SLP measurement.
compensation
coil
1
1
1
400
200
000
calorimetric
from acs data
field coil
Fig. 3. Experimental set-up for ACS measurements.
8
6
4
00
1
1
1
1
60
40
20
00
00
00
1
2
14
16
18
20
22
24
26
real part (fluid)
imaginary part (fluid)
real part (immob.)
field [kA/m]
8
6
4
2
0
0
0
0
0
imaginary part (immob.)
Fig. 6. SLP data of the Co fluid measured calorimetrically (dots) for
different field amplitudes and field dependence according to Eq. (3).
are significantly higher than those we found with super-
paramagnetic iron oxide particles in previous investiga-
tions [9].
For materials with linear magnetic properties the specific
0
0
heating power can be calculated from the w data [10]
according to the relation
1
0
100
1k
10k
100k
1M
frequency [Hz]
0
0
2
SHP ¼ m pf w H =r,
(3)
0
Fig. 4. ACS spectra of the Co fluid and the immobilized sample.
where f is the frequency and H the amplitude of the field
and r the density of the particles.
0
0
into the center of the coil and the temperature T(t) is
monitored using a thermocouple. From these data the SHP
is calculated by
With this equation and w (400 kHz) ¼ 13.9 as measured
for the fluid we obtain the curve in Fig. 6 which is in good
0
0
accordance with the calorimetrical data. The w data of the
immobilized sample are nearly independent on the
frequency within the range we investigated. The absolute
value is about 4. As this value corresponds to pure Ne
relaxation, whereas the above-mentioned value of the
liquid includes Brown and Neel relaxation we can estimate
the contribution of the Brown and Neel mechanism to
the total SHP to 70% or 30%, respectively.
cfluidmfluid dT
SHP ¼
,
(2)
mparticle dt
´
el
where cfluid is the specific heat and mfluid or mparticle are
the masses of the fluid or the particles, respectively. The
symbols in Fig. 6 show the SHP data obtained from
the fluid for different field amplitudes. The absolute values
´
´

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227
5
. Conclusions
References
[
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[
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Our results demonstrate that metallic cobalt nanoparti-
5
cles show a SHP which is significantly higher than the data
measured with superparamagnetic iron oxide particles in
previous investigations. The comparison with the results
from ACS shows that the behaviour of the fluid is based on
¨
pf, H. Hofmann, et al., J. Magn. Magn. Mater
2
[4] M. Laniado, A. Chachuat, Radiologe 35 (1995) 266.
[
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, C.G. d’ Ambly, R. Hergt, et al., J. Magn. Magn. Mater
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´
1
measured SHP (70%) results from the Brown mechanism.
Our results demonstrate the potential of these metallic
particles for heating applications. For in vivo applications
the problem of toxicity has to be solved by covering the
particles with a suitable shell (e.g., silica [2] or gold [11]).
¨
nnemann, N. Matousevitch, et al., Z. Phys. Chem.
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7] M.L. Vadala, M.A. Zalich, D.B. Fulks, et al., J. Magn. Magn. Mater
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¨ ¨
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Acknowledgments
[
[
10] L.D. Landau, E.M. Lifshitz, Electrodynamics of Continuous Media,
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The work was supported by the DFG priority program
‘Colloidal Magnetic Liquids’’ under contract HE 2878/9-2.
‘
The authors thank Ch. Schmidt for performing the X-ray
investigations.