Magnetic nanoparticles separation based on nanostructures

Article abstract of DOI:10.1016/j.jmmm.2006.10.1104

This study describes a magnetic array, which consists of depositing Fe nanowires on a porous alumina membrane. Such a device can be used as a planar magnetic separator. Its performance for the collection of Fe₃O₄ nanoparticles is experimentally shown. For magnetization of such iron nanowires in the vertical direction, we propose equations to calculate the theoretical absorption ratio.

Full text of DOI:10.1016/j.jmmm.2006.10.1104

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 312 (2007) 354–358
www.elsevier.com/locate/jmmm
Magnetic nanoparticles separation based on nanostructures$
Ã
Jianfei Suna, Rui Xub,1, Yu Zhanga, Ming Maa, Ning Gua,
aState key Laboratory of BioElectronics, School of Biological Science and Medical Engineering, SouthEast University, Nanjing 210096, China
bDepartment of Physics, SouthEast University, Nanjing 210002, China
Received 19 July 2006; received in revised form 25 September 2006
Available online 27 November 2006
Abstract
This study describes a magnetic array, which consists of depositing Fe nanowires on a porous alumina membrane. Such a device can be
used as a planar magnetic separator. Its performance for the collection of Fe3O4 nanoparticles is experimentally shown. For
magnetization of such iron nanowires in the vertical direction, we propose equations to calculate the theoretical absorption ratio.
r 2006 Elsevier B.V. All rights reserved.
Keywords: Magnetic separation; Magnetic nanorod array; Planar separator
1. Introduction
advantages of compactness, design flexibility and easy
integration over conventional separators that are generally
composed of specifically placed permanent magnets or
magnetizable wire matrices combined with a uniform
magnetic field [8,9]. However, high-field gradients require
high geometric curvature, which seems incompatible with
the design of devices. For example, membranes of magnetic
metals, prepared by electroplating or sputtering, are often
exploited as separators with magnetization by a uniform
magnetic field. However, due to the shape anisotropy, the
produced gradient is not satisfying.
Actually, more and more nanostructures with unique
and exotic properties are found due to their size and shape.
This could lead to novel applications in industry [10,11]. In
this paper, we propose a new type of separator with
vertically arranged Fe nanowire arrays. Compared with the
existing planar separators, our device offers advantages of
high-field gradients, large-scale treatment, low-cost fabri-
cation and no heat generation. The latter feature is very
important for practical applications because the heat
produced by separators may be potentially harmful to
biomolecules [12].
Magnetic separation techniques, playing an important
role in molecular biology, have been paid more and more
attention due to the increasing utilization of reagents
containing magnetic labels [1–3]. The most attractive
advantage of using magnetic separation is the simplicity
of manipulation. Once the target biological cells or
molecules are immobilized on magnetic nanoparticles,
these target biomolecules can be separated from a sample
solution, flexibly manipulated in various reagents, and
easily transported to a desired location by controlling
magnetic fields produced by a permanent magnet or an
electromagnet [4,5]. However, as biochemical experiments
are now done at molecular levels with the emergence of
micro total analytical system (mTAS), it is a challenging
task to avoid inconsistencies between relatively large
separators and increasingly small volumes of samples [6].
Recently, Jin-Woo Choi et al. [7] proposed a planar
separator prototype using planar inductance with a
permalloy as the ferrocore. The planar separator has the
$This project was financially supported by the NFSC
(grant No. 60171005, No. 60371027, No. 90406023).
In this article, the polydispersed magnetite nanoparticles
(about 40 nm in diameter) are chosen as objects to be
separated from colloidal suspensions. For nanoscaled
particles, the Brownian motion will no more be negligible,
which requires strong enough magnetic fields, and field
gradients to overcome the influence of Brownian motion.
Ã
Corresponding author. Tel.: +86 025 83794960.
E-mail addresses: sunzaghi@seu.edu.cn (J. Sun),
xurui04@mails.gscas.ac.cn (R. Xu), guning@seu.edu.cn (N. Gu).
1He is now with the Institute of High-Energy Physics, Chinese Academy
of Science, 100039 Beijing, China.
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.10.1104

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J. Sun et al. / Journal of Magnetism and Magnetic Materials 312 (2007) 354–358
355
Therefore, the planar separator based on magnetic
nanowire arrays, seems particularly useful to capture such
small particles due to the nanometric diameters of
nanowires.
Refs. [15,16], the flux produced by the magnetic film is just
like that of a bar magnet along the length direction. The
field distribution of nanowires array film has an advantage
of good anisotropy in z-direction over that of magnetically
metallic membranes, prepared by electroplating.
For nanoscaled colloidal suspensions, magnetic nano-
particles separation behaves as a statistical mechanics
process due to thermal fluctuations. Supposing that one
nanowire is one absorptive centers at the film surface and
the energy of one captured particle is –e0, the equilibrium
absorptive ratio is
2. Theoretic background
A magnetic force exerted on a nanoparticle in an
inhomogeneous magnetic field, can be expressed by
Fm
¼
12 Vmðwp ꢀ wf Þ=m0rB2, where Vm is the volume of a
magnetic nanoparticle, wp and wf are the susceptibility of
particles and fluidic medium, respectively, B is the magnetic
flux density and m0 is the permeability of free space [13]. As
far as separator equipment is concerned, the generation of
high-field gradients is a key issue since the field intensity is
achieved by an externally exerted uniform field. The
magnetic nanowire array is fabricated via electrochemical
deposition of Fe into channels of alumina films, which
exhibit parallel alignment normal to the substrate plane.
Therefore, along axis of the magnetized nanowires, high-
field gradients can be generated due to such large aspect
ratios. The configuration of porous alumina films is shown
in Fig. 1. Macroscopically, it behaves as a compact film,
having the thickness of Al substrate. In practical applica-
tions, the uniform magnetic field is applied vertically to the
film, as it is interesting for planar films to generate high
magnetization fields in the normal direction.
N
r ¼
,
(1)
N0
where N is the captured particle number in equilibrium and
N0 is the total number of absorptive centers. Regarding the
suspension as thermal source and particle source, the
captured particles can be considered as a system ex-
changing particle and energy with source, which complies
with grand canonical distribution. When N particles are
captured, the energy of system is ꢀNe0. Considering that N
particles in N0 absorptive positions have N0!=N!ðN ꢀ N0!Þ
permutations, the systematic grand canonical partition
function and the average captured particle number are
expressed in Eqs. (2) and (3), respectively:
N0
X
N0!
0
0
0
X ¼
ebðmþꢀ ÞN
¼ ½1 þ ebðmþꢀ ÞꢁN
(2)
N!ðN0 ꢀ NÞ!
N¼0
A single magnetized nanowire can be regarded as an
annular current it is often macroscopically called magne-
tization current. According to Ampere’s molecular current
viewpoint, the magnetic field generated by a magnetized
rod is identical to a electrified solenoid. Hence, in a plane,
the nanowire can be regarded as an annular. Generally, the
pore density of porous alumina film is so high
(109–1011 cmꢀ2 [14]) that the annular currents can be
considered as continuous distributions. Then due to the
effect resulting from mutual neutralization of annular
currents with reverse directions, all annular currents will
be reduced to one annular current, i.e. to a single nanowire
with high magnetization, somewhat similar to the
mechanism of magnetization in a magnet. According to
q
qa
q
qm
N0
N ¼ ꢀ ln X ¼ kT ln X ¼
,
(3)
1 þ eꢀbðꢀ þmÞ
0
where a ¼ ꢀbm, b is the Lagrange’s unknown factor, which
is determined by (kT)ꢀ1. Here, k is Boltzmann’s constant
and T is the absolute temperature. m is the chemical
potential of the system. At equilibrium, it is identical to
that of the source, namely the suspension. Using Eq. (3),
we obtain the absorptive ratio
N
1
r ¼
¼
.
(4)
N0 1 þ eꢀbðꢀ þmÞ
0
Eq. (4) reveals that the trapping process will consequen-
tially reach equilibrium and the trapping efficiency depends
on temperature, magnetization, and the chemical potential
of suspensions.
3. Experimental section
The porous alumina template was prepared by electro-
chemical anodization of a 0.1 mm thick Al disk as the
anode, using a pure graphite plate as a cathode [14]. The set
up is shown in Fig. 2. The voltage was set to 20 V DC using
0.3 M sulfuric acid as electrolyte. Typically, the experiment
was carried out for 10 h. After the formation of an alumina
film on the surface of the Al substrate, the Al disk was
maintained but the electrolyte was removed and replaced
by a 5 wt% phosphoric acid solution, applied during one
more hour to form larger pores. When it finished, the
Fig. 1. (a) Is a morphological sketch of a porous alumina film, and (b) is a
morphological sketch of metal nanowires deposited in the channels of an
alumina film. The black rods mean Fe nanowires.

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356
time, the supernate was drawn out into a cuvette as sample
copy by a syringe. Fe concentrations of all the samples
were measured by atomic absorption spectroscopy (polar-
ized zeeman atomic absorption spectrophotometer). We
think that Fe signals are due to Fe3O4 nanoparticles,
therefore the remnant Fe in supernate could characterize
the separation efficiency.
Simultaneously, after the separation experiment, the
porous alumina film was observed by SEM and the
elemental analysis for a selected area was performed by
energy-dispersive X-ray spectrograph (EDX) to investigate
Fe content absorbed on the film surface. The porous
alumina film with Fe deposition before separation was
characterized for comparison.
Fig. 2. The set up of fabricating alumina film and magnetic array. The
aluminum disk was placed in the bottom of the tank and connected with
the anode of power supply (red column). The cathode of power supply
(black column) was connected with the graphite plate.
4. Results and discussion
phosphoric acid solution was poured out and the film was
also fixed in the tank. The film, and the reaction tank was
washed with distilled water for several times.
The morphological SEM image of alumina template
surface is shown in Fig. 3 which reveals the high density of
channels of porous alumina film. TEM photos of Fe
nanowires after the removal of template and substrate are
shown in Fig. 4, where the diameter of nanowires is about
15 nm and the length is about several micrometers,
respectively. Based on these data, the aspect ratio can be
calculated to be above 100, exhibiting the strong geome-
trically magnetic anisotropy. So it may well be inferred that
the film will be easily magnetized along the direction
normal to its surface while an ordinary ferruginous slice is
often hard to be magnetized in this direction due to the
large demagnetization factor on disk-like shape. The
magnetic characterization of magnetic nanowire arrays is
shown in Fig. 5, from which the ferromagnetism of
Fe–Al2O3–Al, corresponding to the ternary system consist-
ing of nanowire, template and substrate is documented.
The Fe concentration–time curve is shown in Fig. 6.
There is a sharp decline after 5 min, predicating that
equilibrium is reached after maximum 5 min. The absorp-
tive ratio, which is up to 90%, is calculated using the final
concentration in supernatant divided by the original
The preparation of Fe nanowires was based on the same
set-up, but the DC voltage was replaced with AC voltage
and the sulfuric acid electrolyte was replaced with the
mixed solution consisted with 120 g Lꢀ1 Fe2SO4 ꢂ 7H2O,
45 g Lꢀ1 boracic acid and 1.0 g Lꢀ1 vitamin C. Using the
graphite plate as counter-electrode and setting the voltage
on 20 V, 50 Hz AC, the Fe2+ were reduced into Fe
and confined to the wire-like shape by the channels of
alumina template. The reaction time was correlative to the
length of formed nanowires, and in our experiments, it was
about 10 min. The Fe nanowires array was characterized
by scanned electron microscopy (SEM) and the removed
Fe nanowires were characterized by transmission electron
microscopy (TEM) . The removal of the Al substrate
and alumina template was achieved by treating the disk
in a saturated KOH solution at 60 1C. After Al and
alumina were both dissolved, the solution was separated
by centrifugation in 3000 r/min and washed with distilled
water. Such treatments were repeated for several
times until the pH value of solution was approximately
equal to 7.
The magnetic separation experiments were done in a
glass tank (40 ꢃ 30 ꢃ 6 mm3). The porous film was fixed at
the bottom by cyanoacrylate adhesive. The objects to be
separated were glutamic acid capped magnetite nanopar-
ticles with mean diameter of 40–50 nm, which were
prepared in our lab and dispersed in water [17]. A
peristaltic pump was employed to drive the highly diluted
aqueous suspension to flow into the glass tank. A 0.25 T
homogeneous magnetic field generated by an electro-
magnet was externally applied to the tank with the
magnetic flux perpendicular to the magnetic nanowire film.
Due to the field gradient from the magnetized nanowires
array, the magnetic nanoparticles were captured by the
film. This could be observed by naked eye, as the
suspension turns limpid. The separation experiment was
carried out at different separation times. In each separation
Fig. 3. SEM image of porous alumina film without Fe deposition.

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357
Fig. 4. (a) Is the TEM image of a single wire and (b) is the TEM image of aligned wires.
concentration in the suspension. From Eq. (4), it is seen
that 100% separation of magnetic nanoparticles is im-
possible and once the equilibrium arrives, the absorptive
ratio is independent of time. Thermal fluctuations con-
tribute to irregularities in the concentration curve (Fig. 6).
These are consistent with our experimental results. From
the experimental point of view, these phenomena can be
explained by the theory proposed by Takayasu [18], where
the size of nanoparticles plays an important role in the
capturing process. In our experiments, the pure iron disk
was also experimentalized for comparison with magnetic
array film. Also, the colloidal suspension was experimen-
talized under a uniform magnetostatic field in the absence
of magnetic array film or other ferromagnetic components.
The results show no significant particulate separation is
observed under both cases. This is because that the
adequate field gradient fails to be generated. For the
former, the shape anisotropy diminishes the gradient too
weakly to exert forces on nanoparticles. For the latter,
there exists only uniform magnetic field, under which the
magnetic forces on the nanoparticles are zero.
Fig. 5. Magnetic hysteresis of Fe–Al2O3–Al ternary system. The
magnetization is perpendicular to the surface.
Another interesting result comes from EDX experi-
ments. As illustrated in Fig. 7, before adsorption, no Fe
peak is found while 2 weak Fe peaks are found after
adsorption, indicating that Fe is not fully deposited into
the channels of the used alumina film while the Fe peaks
result from the nanoparticles captured into the surface of
film. Because of the larger size of nanoparticles than that of
the channels, there remain vast nanoparticles without
entering the channels. A typical SEM photo is shown in
Fig. 8. These phenomena suggest applications in biosen-
sors, that the difference of Fe content in surface can be
exploited to characterize the amount of adsorbed particles,
offering a novel approach for the biological detection and
measurement of biological affinity.
Fig. 6. Fe concentration vs. time curve (under room temperature). The
parameter in y-axis represents the Fe concentration in different samples
measured by atomic absorption spectroscopy and the parameter in x-axis
is separation time.

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358
Fig. 7. (a) Is EDX spectrum after adsorption. Two weak Fe peaks are visible. No Fe peaks are revealed in the reference EDX spectrum (b).
Acknowledgments
The authors thank Prof. Rolf Steiger in ETH, Switzer-
land for a critical reading of an early version of the
manuscript and for help revision. We would also like to
thank Miss Yin Jun, Mr. Zhang Yi, Mr. Liu Zhan-hui,
Mrs. Shao Li and Mr. Xu Ai-qun for helpful discussion
and technical assistance.
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