and TRI/Princeton, 601 Prospect AV enue, P. O. Box 625, Princeton, New Jersey 08542
Received February 15, 2005; Revised Manuscript Received March 14, 2005
ABSTRACT
We describe a simple and versatile technique to produce magnetic tubes by filling carbon nanotubes (CNTs) with paramagnetic iron oxide
particles (∼10 nm diameter). Commercial ferrofluids were used to fill CNTs with an average outer diameter of 300 nm made via chemical vapor
deposition into alumina membranes. Transmission electron microscopy study shows a high density of particles inside the CNT. Experimentsusing external magnetic fields demonstrate that almost 100% of the nanotubes become magnetic and can be easily manipulated in magnetic
field. These one-dimensional magnetic nanostructures can find numerous applications in nanotechnology, memory devices, optical transducers
for wearable electronics, and in medicine.
1. Introduction. The unique physicochemical properties of
carbon nanotubes1 have stimulated the search for possible
applications in different areas of engineering.2-6 Especially,
electrical properties of CNTs are very attractive, but all
attempts to make CNTs magnetic7,8 have had limited success.
The nanotubes containing magnetic particles did not show
useful magnetic properties because the amount and location
of magnetic material inside the tube was difficult to control.Other techniques, which have been used to produce magnetic
needles, are expensive and time-consuming,9 leading to a
low yield. While increasing the magnetization, it is also
important to prevent nanoneedles from agglomerating when
a magnetic field is not applied. Encapsulating paramagnetic
particles into CNTs makes paramagnetic needles and allows
for control of their movement.
The phenomenon of spontaneous penetration of fluids into
wettable capillaries is taken as a guiding idea to load the
nanotubes with magnetic nanoparticles. As known from
everyday experience, when a capillary is set in contact with
a wetting fluid, the fluid spontaneously penetrates inside.
This method of filling nanotubes with molten metals was
suggested long ago.10-12 The drawback of using the melts
in the way suggested in refs 10-12 is that it is time-
consuming, includes the quite tedious and unpredictable step
of nanotube opening, and, at the end, the filling efficiency
is too low to consider this method for large scale production.
Ferrous metals, which are of interest for magnetic applica-
tions, have high melting points (1535 °C for Fe, 1453 °C
for Ni, and 1495 °C for Co),13 and their melts react with
carbon. To avoid these shortcomings, we suggest a three-
step protocol which consists of (1) synthesis of CNTs by
the method of noncatalytyic chemical vapor deposition
(CVD) into the pores of an alumina template (this methodprovides tubes with open ends and they will be free of
ferromagnetic catalyst particles); (2) filling of nanotubes with
suspensions of functional nanoparticles; (3) separation of
nanotubes from alumina membrane. Step three can be made
before step two. Then the individual nanotubes are filled with
a suspension in a similar way.
It has been previously demonstrated that CVD nanotubes
produced by template synthesis can be filled with water,3,14
glycerin, ethylene glycol,15 hydrocarbons, and other liquids.
However, filling nanotube channels with particle-loaded
fluids or colloidal solutions has not been previously reported,
although, a recent demonstration using a Coulter counter16
shows that particulate flow inside nanotubes is possible. The
dragging of nanoparticles into the nanotubes is mostly
affected by the process of enforcement of the fluid body as
a whole continuum.17
The objectives of this work are to use commercially
available ferrofluids to fill the CNTs, evaporate the carrying
fluid from the tubes, and produce magnetic nanotubes.
2. Materials and Experimental Details. In all experi-
ments we used carbon nanotubes produced in our laboratory
by the CVD technique. The nanotubes were formed in the
† Chemistry Department, Drexel University.‡Department of Materials Science and Engineering, Drexel University.§ Department of Electrical and Computer Engineering, Drexel University.| TRI/Princeton.
straight cylindrical pores connecting both faces of the
alumina membrane. The alumina template Anodisc of 13 mm
in diameter was purchased from Whatman. The pore diameter
and thickness of the membrane determine the dimensions
of the nanotubes. In our experiments, the pore size was 300nm in average, and the membrane thickness was 60 µm. The
synthesis of CNT by CVD is described in detail in refs 18
and 33. The resulting CNTs have open ends from one or
both sides, and their walls are highly disordered and
hydrophilic to allow water to invade the tubes.3,14,19 This
makes it possible to fill the nanotubes with both organic-
and water-based ferrofluids. To fill the CNTs with magnetic
particles, we used the following commercially available
ferrofluids: water based (EMG 508) and organic based
(EMG 911) (Ferrotec Corporation) which carry magnetite
(Fe3O4) particles with a characteristic diameter of 10 nm.
Samples were characterized by transmission electron
microscopy (TEM) using a JEOL JEM-2010F (operated at200 kV) with a point-to-point resolution of 0.23 nm. The
TEM samples were prepared by dispersing the nanotubes in
2-propanol and then placing them onto a copper grid coated
with a lacy carbon film. Images of nanotubes in experiments
with external magnetic field of strength µ0 H ) 0.011-0.012
T ( µ0 is the permeability of vacuum) were taken on a Leica
DM LFS microscope with a Leica HCX APO 63×/0.90
U-V-I water immersion lens and a MagnaFire SP model
S99805 camera.
3. Results and Discussion. 3.1. Filling of CNT with
Magnetic Particles. The procedure of nanotube filling with
ferrofluid is shown schematically in Figure 1. Two different
methods have been used. The first method assumes filling
nanotubes in the alumina membrane, while the second
method deals with filling nanotubes released from the
membrane.
Filling tubes in alumina membranes. Upon deposition of
a droplet of ferrofluid onto the membrane, the fluid invaded
the pores. To control the magnetic anisotropy of prepared
magnetic CNTs, we also applied a magnetic field. A
permanent magnet ( µ0 H ∼ 0.4 T) was mounted underneath
the membrane. Even without a field, the penetration occurs
almost instantaneously. An applied magnetic field only
increases the rate of penetration, because it creates an
additional force to direct magnetic nanoparticles toward the
magnet, i.e., into the tubes. The effect of field-induced
acceleration, however, is insignificant at the fields used in
our experiments. After evaporation of the carrying liquid at
room temperature, the membrane was broken into tiny pieces
and dispersed in 2-propanol for the TEM examination. TEM
micrographs show the typical distribution of magnetic grains
inside the nanotubes (Figure 2). During the processing, the
Figure 1. Filling of carbon nanotubes. First method. Alumina membrane with carbon nanotubes produced by CVD (a) is brought incontact with ferrofluid (b). Ferrofluid invades pores (c). Carrying fluid is dried to leave only magnetic particles in CNTs (d). Aluminamembrane is dissolved in NaOH to produce magnetic CNTs (e). Second method. Alumina membrane with carbon nanotubes produced byCVD (f) is dissolved in NaOH to produce individual CNTs (g). Droplet of ferrofluid is deposited atop nanotube layer (h). Ferrofluid fillsnanotubes, then carrying fluid is evaporated and nanotubes are left with magnetic particles inside.
Figure 2. TEM images of CNTs filled with organic-basedferrofluid EMG 911. (a) CNTs filled with ferrofluid in magneticfield. Nanotubes are embedded in alumina, which can be seen inthe lower left corner. (b) High-resolution TEM image of a fragmentof the nanotube, filled with magnetite particles without appliedmagnetic field. The particles are agglomerated inside the nanotube.
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magnetic particles deposited outside the nanotubes were
washed out, while the particles deposited inside were held
by adhesion forces. Statistical analysis of a large number of
tubes showed that nanoparticles fill the nanotubes equally
with and without magnetic field. This suggests that filling
is primarily driven by capillary forces.
Filling released nanotubes. In a second technique, the
alumina template after preparation of the CNT by the CVDmethod was dissolved in 4.0 M NaOH. After sonication, the
solution was vacuum filtered through the polyester nucle-
opore membrane (Osmonic Corp.) with the pore size of 0.2
µm. Then the filtrate was dispersed in toluene. In the next
step, a few milliliters of solution containing CNTs dispersed
in toluene was filtered again by using a similar polyester
membrane. After filtration, the residue was rinsed with
alcohol and distilled water, and then dried. Typically, a gray
area of concentrated nanotubes appeared on the substrate after
drying. A drop of ferrofluid was deposited on that gray spot.
Again, after complete evaporation of the solvent, the substrate
was rinsed with alcohol, immersed into a small vial and 5
mL of alcohol was added. The vial was put into a sonicatorand the deposit was removed from the membrane. This
dispersion was analyzed by TEM (Figures 3 and 4).
As shown, both approaches give similar results: the
nanotubes are filled with magnetic nanoparticles. The density
of magnetic grains is so high that it can be probed by a
variety of macroscopic techniques, including optical micros-
copy, SEM and TEM. Suspended in liquids, magnetic
nanotubes follow the change of the direction of applied
magnetic field. Figure 5 shows the typical behavior of a
suspension of magnetic nanotubes in water placed in a
magnetic field of µ0 H ) 0.01 T. Magnetic nanotubes can be
oriented in the plane of a silicon wafer with gold electrodes
(Figure 5 a,b) or can be forced to stand normally to the wafer
surface (Figure 5c). In the field, the nanotubes form long
chains with lengths greater than the widths of the golden
islands, 25 µm. As seen from that panel, all nanotubes are
sensitive to the application of magnetic field, independent
of whether they are on an Au or Si surface. In particular, all
nanotubes seen in the camera align perpendicularly to the
wafer (Figure 5c). This proves that we have an almost 100%
yield of magnetic nanotubes after filling. Single nanotubes
could also be manipulated, e.g., positioned between two gold
electrodes using dielectrophoresis as described in ref 20.
3.2. Nanotube Magnetization. Average magnetic properties
of the nanotubes filled with the magnetic particles were
measured using an alternating gradient magnetometer (AGM
by Princeton Measurements Inc.). Membranes with the filled
nanotubes were rinsed with toluene to remove ferrofluid
particles from the surface of the membrane. Pieces of the
membrane were cut out and their areas were measured under
the microscope. An example of the magnetization curve for
a sample of area 5.9177 × 10-6 m2 is given in Figure 6.
Using the total magnetization value of this sample of M )
9.138351 × 10-8 Am2 for the applied field of 0.007 T, the
Figure 3. TEM image of a released tube filled without magnetic field. The closed tip of the nanotube shown is revealed after dissolutionof the aluminum template in NaOH followed by sonicating, filtering, and rinsing in the alcohol. This image shows high density of particlesinside the nanotube even after several hours processing.
Figure 4. Filling of released nanotubes. TEM image of (a) part of the branched CNT, and (b) CNTs with open ends, filled withmagnetic particles from water-based ferrofluid EMG 508. Thisimage shows that the nanoparticles are collected inside the CNTand form quite dense structure. Attached to the walls of the CNTby adhesion forces, the particles stay intact after processing.
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cies. We experimented with a water-based suspension of
CNTs in a field of 0.007 T. The critical frequency was found
to vary between f cr ) 6-10 Hz. Taking these values, the
magnetization is estimated through the formula
where L ) 10-15 µm, d e ) 300 nm, and µ0 H ) 0.007 T.
The rotational experiments give m ) 5 × 10-15- 2 × 10-14
Am2, which is slightly higher, but is otherwise in good
agreement with the magnetic moment obtained from the
membrane magnetization measurements by AGM. The
rotational field experiments also demonstrate that the pro-
duced magnetic nanotubes are highly responsive to the
applied field typical in MEMS applications.
It is worthwhile to compare the energy of dipole-dipole
interactions with respect to the energy of thermal excitations.
The minimum energy of two needle-like magnets corre-
sponding to their parallel orientation is estimated to be E dd
) µ0m2/ L2d e ∼ 10-18- 10-17 J, which is much greater than
the energy of thermal excitations, E t )
T ∼
10
-21
J. Thisestimate may explain why the CNT needles are prone to
forming chains as seen in Figures 5a,b.
4. Potential Applications. The proposed approach allows
magnetic nanotubes to inherit paramagnetic nature of en-
capsulated magnetic particles, yet become much more
responsive to an applied field. Even without extra treatment,
our magnetic nanoneedles stay separated in suspension. In
contrast, ferromagnetic needles, e.g., γ-Fe2O3, of the same
length (but with a much smaller aspect ratio) and same
concentration require some extra treatment to prepare stable
suspensions. Given the high magnetization that we can
achieve and the compatibility of CNTs with many polymeric
materials, the magnetic nanotubes are a highly attractivemultifunctional material. They can be added to polymers to
produce composites with aligned paramagnetic needles,
arranged in a variety of patterns on a surface or incorporated
into polymer fibers. The list of applications of magnetic
nanotubes can be extended to include materials for wearable
electronics,23-25 cantilever tips in magnetic force micro-
scopes,26 magnetic stirrers in microfluidic devices, or mag-
netic valves in nanofluidic devices.27 Use of these tubes as
connecting pipes in nanofluidic devices has already been
demonstrated.28 Experiments shown in Figure 5c suggest the
use of these magnetic nanotubes instead of nanoposts in
fluidic chips for DNA separation.29,30 That is, one can apply
magnetic field to freeze nanotubes and to vary the intertubespacing in order to unravel DNA coils and separate them as
described in refs 29-32. Thus, the versatile technique
suggested in this paper offer an opportunity to make a step
toward the nanoengineering of complex multifunctional
nanosystems.
One of the most attractive applications is using the
magnetic CNTs as capsules or nanosubmarines for magneti-
cally guided drug delivery to desired locations in the body,
as well as for diagnostics without surgical interference. Other
particulate fluids, e.g., solutions of quantum dots or polymer
particles can be used as well. In our previous studies, we
demonstrated that not only water,3,14 but a large number of
organic fluids including glycerin, alcohols, benzene, and
cyclohexane can be encapsulated into the same kind of
nanotubes. These liquids can be used as carriers for some
other particulate and emulsion systems or biopolymer solu-
tions for nanotube filling. We have also demonstrated selec-
tive sealing of tube tips with polypyrrole by using bipolar
electrochemistry,33 making closed capsules for liquid deliv-
ery.5. Conclusions. We have demonstrated a relatively simple,
inexpensive, reproducible, scalable, and fast method of filling
the carbon nanotubes with functional nanoparticles, in
particular, with magnetic nanograins. This technique opens
an opportunity for engineering magnetic nanotubes based on
the phenomenon of spontaneous penetration of wetting fluids
into capillaries. The magnetization of magnetic CNTs is
controlled by the number of encapsulated nanograins and
thus can be made very magnetic. In our experiments, for
example, the number of magnetic grains in the tubes varies
between N ∼ 104-105. The filled nanotubes follow the
applied magnetic field, thus manifesting their magnetic
nature. Thus we showed that the yield of magnetic nanotubesafter wet filling was close to 100%. Controllable manipula-
tion of magnetic nanotubes with micromagnets points to a
straightforward way for utilization of these nanoneedles in
different nanofluidic and electronic devices.
Generally, the paper provides a procedure of making
nanotubes functional. Other particulate fluids, emulsions, and
polymer solutions can be used to fill nanotubes and transform
them into multifunctional nanostructures.
Acknowledgment. The authors thank Dr. P. G. Ndungu,
Dr. S. Babu, Dr. B. M. Kim, and D. Mattia for help in carbon
nanotube preparation and study. The appreciation of DrexelUniversity group is extended to NTI and NSF NIRT for
financial support.
Supporting Information Available: Windows Media
Player movie showing the motion of CNT filled with
magnetic nanoparticles in rotating magnetic field of µ0 H )
0.007 T, and frequency of f ) 1 Hz. This material is available
free of charge via the Internet at http://pubs.acs.org.
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