Magnetophoresis behaviour at low gradient magnetic field and size control of nickel single core nanobeads

Article abstract of DOI:10.1016/j.jmmm.2011.02.027

Magnetic separation of organic compounds, proteins, nucleic acids and other biomolecules, and cells from complex reaction mixtures is becoming the most suitable solution for large production in bioindustrial purification and extraction processes. Optimal magnetic properties can be achieved by the use of metals. However, they are extremely sensitive to oxidation and degradation under atmospheric conditions. In this work Ni nanoparticles are synthesised by conventional solution reduction process with the addition of a non-ionic surfactant as a surface agent. The nanoparticles were surfacted in citric acid and then coated with silica to form single core Ni nanobeads. A magnetophoresis study at different magnetic field gradients and at the different steps of synthesis route was performed using Horizontal Low Gradient Magnetic Field (HLGMF) systems. The reversible aggregation times are reduced to a few seconds, allowing a very fast separation process.

Full text of DOI:10.1016/j.jmmm.2011.02.027

Journal of Magnetism and Magnetic Materials 323 (2011) 1945–1949
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Magnetophoresis behaviour at low gradient magnetic field and size control
of nickel single core nanobeads
M. Benelmekki a,n, A. Montras b, A.J. Martins a, P.J.G. Coutinho a, Ll.M. Martinez c
a Centro de F´ısica, Universidade do Minho, Braga, Portugal
b
ꢀ
ꢀ
Sepmag Tecnologies, Parc Tecnologic del Valles, Barcelona, Spain
c Sepmag Technologies, Atlanta, GA, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Magnetic separation of organic compounds, proteins, nucleic acids and other biomolecules, and cells
from complex reaction mixtures is becoming the most suitable solution for large production in
bioindustrial purification and extraction processes. Optimal magnetic properties can be achieved by the
use of metals. However, they are extremely sensitive to oxidation and degradation under atmospheric
conditions. In this work Ni nanoparticles are synthesised by conventional solution reduction process
with the addition of a non-ionic surfactant as a surface agent. The nanoparticles were surfacted in citric
acid and then coated with silica to form single core Ni nanobeads. A magnetophoresis study at different
magnetic field gradients and at the different steps of synthesis route was performed using Horizontal
Low Gradient Magnetic Field (HLGMF) systems. The reversible aggregation times are reduced to a few
seconds, allowing a very fast separation process.
Received 19 January 2011
Received in revised form
15 February 2011
Available online 26 February 2011
Keywords:
Single core nanobeads
Surfactant effects
Magnetophoresis behaviour
& 2011 Elsevier B.V. All rights reserved.
1. Introduction
magnetic micro- and nanobeads have low efficiency for large
volume production and potential industrial application. These
In recent years, substantial progress has been made in devel-
oping technologies in the field of magnetic microspheres, mag-
netic nanospheres and ferrofluids. Nanospheres and microspheres
containing a magnetic core in a nonmagnetic matrix are used in
numerous biological applications [1]. They are used, for example,
as carriers, which can be targeted to a particular site using an
external magnetic field.
Magnetic separation of organic compounds, proteins, nucleic
acids and other biomolecules, and cells from complex reaction
mixtures is becoming the most suitable solution for large scale
production in bioindustrial purification and extraction processes.
For in-vivo applications, it is important that well-defined bio-
compatible coatings surround the magnetic particles to prevent
any aggregation, and also enable efficient protection of the body
from toxicity. However, for in-vitro applications, biocompatible
coatings are not indispensable; particles could be coated with
non-toxic materials inert to chemical and biological mediums.
The current commercial nanoparticles for in-vitro application are
coated with polystyrene, different copolymers or silica [2–4].
Due to the low surface/volume ratio of the microspheres, and
the unclear magnetic behaviour of the particles, commercial
limitations make their use as efficient and low cost carriers for
bioindustrial purification and extraction processes difficult [5].
Scientific bibliography shows the important labours for the
improvement and understanding of the magnetic behaviour of
magnetic nanoparticles and magnetic micro- and nanospheres.
Most of the scientific studies converge on liquid-phase synthesis
of the magnetite particles. Despite their lower magnetic suscept-
ibility, magnetite and maghemite are particularly desirable to
be synthesised by liquid phase due to their great potential as
oxidatively stable nanomagnetic particles with diverse applica-
tions [6]. The liquid-phase process of magnetic nanoparticles
synthesis is very laborious and comprises different stages, nor-
mally developed under atmospheric conditions: precipitation,
stabilisation, coating and suspension.
The essential steps in the synthesis and different applications
are the manipulation, recovery and removal of the magnetic
particles using external magnetic fields (magnetophoresis) [7].
In the simplest form, magnetic separation can be achieved by
placing a permanent magnet in the proximity of the suspension
containing the magnetic particles conjugated with the captured
targets. This approach is limited to separate micron scale mag-
netic beads. More sophisticated magnetic separation devices were
developed to capture magnetic particles during continuous flow.
These flow-trough devices generally fall into columnar flow
devices, planar flow devices or devices that combine columnar
and planar flow systems [8]. Columnar flow systems consist
n Corresponding author. Tel.: +351253604060; fax: +351253604061.
E-mail addresses: benelmekki@fisica.uminho.pt,
maria.benelmekki@gmail.com (M. Benelmekki).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2011.02.027

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M. Benelmekki et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1945–1949
of a large flow column ranging from millimetres to tens of
centimeters in size, accompanied with an assembly of large external
permanent magnet or electromagnets, which generate a non-uni-
form field to capture the magnetic particles [9]. These devices suffer
from low field gradients and require conjugate beads to be highly
magnetic. Planar flow devices consist of flow channels with dimen-
v/v) containing 3% (v/v) TEOS. The pH value of the solution was
maintained at 11 for 90 min.
2.2. Characterisation methods
A JEOL JEM-1210 electron microscope operating at 200 keV
was used for transmission electron microscopy characterisation.
The samples were prepared by deposition of a droplet of particles
solution on a copper grid coated with carbon and allowed to dry.
SEM imaging was performed with a FEI Nova 200 (FEG/SEM).
sions on the order of 10 mm. Magnetic material or electromagnets
are patterned directly next to or inside the flow channel. These
devices offer high magnetic gradient, but the small cross-sectional
area and the limited number of flow channels restrict the
throughput [10].
Dynamic light scattering (DLS) and
z potential were deter-
In this work, we report the synthesis and characterisation of
single core Ni nanobeads. Ni nanoparticles with diameter ranging
from 80 to 120 nm were synthesised by conventional solution
reduction process. With the addition of a non-ionic surfactant as a
surface agent, the diameter was reduced and controlled to 50–
55 nm. To avoid magnetic aggregation, and assure monodispersed
nickel nanospheres, the Ni nanoparticles were surfacted in citric
acid. At the end of the process the surfacted nanoparticles were
coated with silica to form single-core Ni nanobeads. HRTEM and
SQUID characterisation was performed on the samples to define
their structural and magnetic properties.
mined with a Malvern Zetasizer, NANO ZS (Malvern Instruments
Limited, UK), using a He–Ne laser (wavelength of 633 nm) and
a detector angle of 1731. Nanoparticles dispersion was analysed in
a polystyrene cell or in a folded capillary cell, for size distribution
or zeta potential measurements, respectively. The zeta poten-
tial values were calculated using the Smoluchowski equation.
Hysteresis loops were measured with a superconducting quan-
tum interference device (SQUID) magnetometer (Quantum Design
MPMS5XL). The experimental results were corrected for the
holder contribution.
An extensive magnetophoresis study was performed using
Horizontal Low Gradient Magnetic Field (HLGMF) [11]. These
systems provide high magnetic forces, implying fast separation
and low magnetic nanoparticle losses and the developed pro-
cesses are easily scalable for industrial application. As reported by
De la Cuevas et al. [12], the magnetophoresis process at HLGMF is
driven by a cooperative phenomenon consisting of reversible
aggregation of the particles, followed by moving of the aggregates
to the walls of the bottle. Different reversible aggregation times
were observed at different HLGMFs, at each step of the synthesis
route. The obtained results prove that in the case of Ni–silica
core–shell aqueous suspensions on applying a low gradient
magnetic field, magnetic interaction overcomes the electrostatic
repulsion, allowing very fast separation processes.
2.3. Magnetophoresis experiments
The magnetophoresis setups employed in our experiment are
the SEPMAG LAB 1 ꢂ 25 mL 1042 and 2042 systems [11]. The
system consists of a cylindrical cavity containing a high perma-
nent magnetic field with a uniform horizontal gradient pointing
toward the walls of the cylindrical vessel. The magnetophoresis
experiments are performed in low gradient magnetic field (LGMF)
(o100 T/m), by placing a bottle of radius 1.5 cm containing
25 mL of nanoparticles aqueous solution inside the SEPMAG
cylindrical cavity. The initial grey-black dispersion becomes
transparent progressively, reaching a transparent final state with
all particles close to the walls of the bottle. Opacity measure-
ments were performed using the external light source SEPMAG
CBL Q250 mL [11]. Due to the magnetic field gradient values in
the three sepmag systems used in this study, 15, 30 and 60 T/m,
the absolute values of the magnetic field vary radially from 0 to
0.225, 0 to 0.45, and 0 to 0.9 T, respectively, being zero at the
centre and maximal at the cylindrical walls. At the instant zero
(when the bottle is inserted on the device) only the nanoparticles
nearer the axis are not saturated. Once the process starts, the
beads move radially and in a few seconds all the beads are in the
region where the magnetic field is high enough to saturate them.
The amount of the volume not saturated at the starting time is
below 20% for the 15 T/m system, less than 5% for 30 T/m and less
than 2% for 60 T/m. Typical magnetophoresis curves consist of a
plateau corresponding to the reversible aggregation time followed
by a progressive decay of the suspension opacity until 100%
transparency is reached.
2. Materials and methods
2.1. Synthesis of single core nickel nanobeads
Single core nickel nanobeads were prepared by mixing hydra-
zine monohydrate (N2H4–H2O) 2 M, nickel (II) chloride hexahy-
drate (NiCl2 ꢀ 6 H2O) 0.2 M and sodium hydroxide (NaOH) 8 M in
the molar proportion 5:1:32. The pH value was about 11. The
resulting solution was kept at 60 1C for 1 h until black powder
precipitated completely. Finally, the product was washed with
distilled water and ethanol several times, and then dried in a
vacuum drying oven at room temperature. In order to control the
size distribution of the nickel nanoparticles, the non-ionic surfac-
tant Triton Tx-100, was introduced in the reaction mixture at a
molar concentration below its critical micelles concentration
(CMC) and at a [Ni]/[Tx-100] molar ratio of 5 [13].
3. Results and discussion
Citric acid (CA) was used to stabilise the magnetic nanoparti-
cles, according to the method proposed by Campelj et al. [14].
Approximately 3 g of nanoparticles were mixed with 65 mL of
aqueous solution of 2% CA (0.02 mg mLꢁ1). The pH value of the
mixture was adjusted to pH¼5 and then raised to pH¼10 using
concentrated ammonia. The adsorption step of the CA was
maintained at 80 1C and rigorously stirred for 60 min. The
prepared suspension was washed with distilled water and mag-
netically separated several times to remove the excess of CA.
The CA-modified nanoparticles were coated with silica using
tetraethyl orthosilicate (TEOS). 10 mL of CA-modified nanoparti-
cles aqueous solution was mixed in a 20 mL ethanol solution (35%
3.1. Influence of synthesis parameters on the size of nickel
nanoparticles
Following the protocol described in Section 2.1 monodisperse
nickel nanoparticles of a size below 120 nm were obtained. As
reported by Liu et al. [15], these monodispersed nickel nano-
spheres are characterised by magnetic single domain structure.
Other work refers to Ni single domain structure when the
diameter of the nanoparticles is below 100 nm, reaching the
superparamagnetic properties when the diameter is below
30 nm [16].

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The results of the room temperature hysteresis loop measured by
SQUID are depicted in Fig. 1. Typical ferromagnetic properties of the
Ni nanospheres are observed. The Ni nanoparticles reach the regime
of saturation at about 1 kOe (0.1 T). This result demonstrates that the
Ni nanoparticles are working at saturation regime inside the sepmag
system.
The Ni particles were coated with silica to obtain core–shell
structure, chemically inert toward air and acids and suitable for the
formation of covalent bonds and the binding of functional groups [16].
The CA-modified nanoparticles were coated under the conditions
described in Section 2.1. The size and morphology of the silica coated
nanoparticles were examined by using high resolution transmission
electron microscopy (HRTEM). As shown in Fig. 2, the nanoparticles
were successfully coated and monodispersed single core Ni nano-
beads were obtained. The thickness of the silica coating is about
2 nm.
In the classical model of LaMer and Dinegar, the process of
synthesising monodispersed particles occurs in two steps: (1) nuclea-
tion and (2) growth [3]. These two steps depend on the reactants and
the parameters controlling the reaction. When nickel is added to
hydrazine a complex is formed. This complex is irreversibly reduced
to metal by the addition of sodium hydroxide, which catalyses the
reduction action of hydrazine [13].
The reaction mechanism includes the formation of nickel hydro-
xide nanoparticles, which upon dissolution, reduction, nucleation and
growth result in nickel nanoparticles. The size of the formed
nanoparticles depends strongly on collision and coalescence rates of
nickel nucleus during nucleation step. To achieve a better control of
the size of the nanoparticles, the non-ionic surfactant Tx-100 (Triton)
was chosen to prepare Ni nanoparticles. The polar heads of X-100
(Triton) adsorbed on Ni nanoparticles can form a protective layer
(Fig. 3) and act as an inhibitor, which is in favour of the formation of
the spherical nanoparticles with homogeneous size.
As shown in Fig. 3, the size of Ni was successfully controlled and a
homogeneous distribution of about 55 nm diameter size of the
nanoparticles was obtained, allowing single core nanobeads of about
57 nm (55 nm core and 2 nm silica shell). In Fig. 4, we can observe
that the hysteresis loop at 20 K is asymmetric around H¼0
(DH¼75 Oe). This behaviour could be explained by the exchange
bias phenomena, associated with the exchange anisotropy created at
the interface between the NiO and Ni [15,17]. The HRTEM images of
these nanoparticles (not shown in this report) demonstrate the
formation of a discontinuous NiO layer on the surface of the Ni
nanoparticles. A detailed study of the surface oxidation of these
nanoparticles has been reported in another work [18]. Hysteresis loop
at 300 K shows that the nanoparticles reach the saturation regime at
about 1 kOe (0.1 T), confirming that inside the sepmag systems the
nanoparticles are working in the regime of saturation.
3.2. Magnetophoresis behaviour of the obtained nanoparticles and
core–shell structure
Magnetophoresis measurements were performed on the
obtained Ni nanobeads at different magnetic field gradients: 60,
30 and 15 T/m. The concentration of the suspensions was fixed to
5 g/L. Fig. 5 depicts the magnetophoresis behaviour of nanobeads
obtained without using Tx-100 (Triton) as surface inhibitor. As
previously mentioned, a typical magnetophoresis curve presents a
plateau corresponding to the reversible aggregation time, fol-
lowed by a progressive decrease of the suspension opacity [12].
The curves depicted in Fig. 5 indicate that the separation time is
Fig. 1. SEM images of citric acid modified Ni nanoparticles and hysteresis loop of Ni nanoparticles at room temperature.

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M. Benelmekki et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1945–1949
Fig. 2. HRTEM images of the obtained Ni–silica core–shell structures showing silica coating of about 2 nm.
Fig. 4. Hysteresis loop measured at 20 and 300 K. Inset shows the asymmetry
around H¼0 at 20 K.
The results obtained when Ni nanoparticles were precipitated in
the presence of Tx-100 (Triton) surfactant are shown in Fig. 6. By
applying 15 T/m magnetic field gradient, at pHꢃ10.5, the aggregation
time increases from less than 2 s for non-modified Ni nanoparticles to
4 s for CA-modified nanoparticles. The CA-modified nanoparticles
were prepared as described in Section 2.1. CA has three dissociable
protons associated with its three carboxylate groups. It has been
claimed that CA acts as an adsorbate tribasic acid on metal oxides
(adsorbent) [19]. The deprotonation of CA depends on the pH of the
suspension. At pHꢃ5.5 CA appears mainly with two deprotonated
species (AH2ꢁ), and is then adsorbed onto the nanoparticles surface
positively charged [20]. At pHꢃ10, CA appears at their fully depro-
tonated state (A3ꢁ). The CA may be adsorbed on the surface of the
nanoparticles by coordinating via one or two of the carboxylate
functionalities depending on the steric necessity and the curvature of
the surface. When the CA affinity limit is reached, the CA adsorption
increases further and a rearrangement of CA binding towards a
tighter packed surface structure can occur. Only one carboxylate
Fig. 3. Description of synthesis route of the controlled size Ni–silica core–shell
structures. Ni nanoparticles havean homogeneous size of about 55 nm.
significantly reduced by increasing the magnetic field gradient. At
60 T/m (not shown), the separation process is completed within a few
seconds, making it difficult to monitor the process and distinguish
the different expected steps of the magnetophoresis process. On
applying 30 T/m, no plateau corresponding to reversible aggregation
time was observed and complete separation was achieved within
10 s. At 15 T/m. the separation process is significantly slower;
however, the curve still shows a very short plateau corresponding
to the reversible aggregation time of the Ni–silica core–shell
structure. These results indicate that, as expected, the obtained
metallic single core nanobeads allow a very fast separation process.

M. Benelmekki et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1945–1949
1949
nanobeads, as well as on the concentration of the suspen-
sion [12,22,23]. The measured potentials of the non-modified Ni
z
nanoparticles and the CA-modified Ni nanoparticles are ꢁ16 and
ꢁ43 mV, respectively. Considering this important differences
between the surface charge of the non-modified and CA-modified
Ni nanoparticles, and the contribution of the high electrostatic
stabilisation to obtain a stable suspension, we expected a larger
reversible aggregation time of the CA-modified nanoparticles with
respect to the non-modified nanoparticles [23]. However a difference
of 2 s was observed. This result suggests that in the case of metallic
single core nanobeads, the attractive magnetic interactions overcome
the electrostatic repulsion, allowing a very short separation time.
In the case of Ni–silica core–shell structure, the aggregation
time takes about 3 s (Fig. 6) and the measured
z potential is
ꢁ30 mV.
4. Conclusions
Fig. 5. Magnetophoresis curves of synthesised Ni–silica core–shell structures at
15 and 30 T/m magnetic field gradient. Nucleation of Ni nanoparticles was
performed without Tx-100 (Triton) as surface inhibitor.
Monodispersed single core Ni–silica core–shell structures were
successfully prepared using a wet chemical solution method. The
control of the Ni nanoparticles size through controlling the nickel
nucleation step was persued using a non-ionic surfactant. Ni core was
coated with a thin film of silica to improve its chemical resistance.
Magnetophoresis behaviour at different magnetic field gradients and
at different steps of synthesis route was monitored. The results
demonstrate that in the case of single core Ni–silica core–shell
structures, the attractive magnetic interactions overcome the electro-
static repulsion, allowing a very short separation time. These results
may have a very high impact in bioindustrial purification and
extraction processes in terms of efficiency and quality separation.
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group of CA will be bound to the nanoparticles surface. In these
conditions, there will be at least one carboxylic acid group exposed to
the solvent that should be responsible for the surface charge.
Furthermore, the presence of a terminal carboxylic group provides a
possible way of extending the bond formation [21]. The chemisorp-
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The reversible aggregation strongly depends on the total interac-
tion potential between the particles, magnetisation and size of the