Nanoﬁber templates were prepared according to our previous
study, with slight modiﬁcation . A 10 w/v% suspension of
poly(caprolactone) (PCL, Mw=80,000) in dichromethane/ethanol
solvent which containing magnetite nanoparticles (20 wt.% relative
to PCL) was prepared. The mixture suspension was electrospun into
a random nanoﬁber mesh at a ﬁeld strength of 15 kV/10 cm. The
nanoﬁber surface was activated in 2 M NaOH solution for 4 h, after
which they were washed and alternatively soaked in 150 mM CaCl2
and Na2HPO4 solution. This process was followed by an apatite miner-
alization within 1.5 times simulated body ﬂuid (1.5× SBF) at 37 °C for
seven days. The specimen was then heat-treated at 500 °C for 5 h to
remove out the polymer phase and to form hollow nanotubes.
The crystal phase and chemical bond status of the samples were
characterized by X-ray diffraction (XRD, Rigaku) and Fourier trans-
form infrared (FT-IR, Varian 640-IR) spectroscopy, respectively. The
surface electrical potential of the nanotubes was examined using a
zeta potential measurement (Malvern Zetasizer Nano) at pH=7.0
and 25 °C. The instrument determines the electrophoretic mobility
of the particles automatically and converts it to the zeta (ζ) potential
using Smoluchowski's equation. The morphology of the samples was
characterized by scanning electron microscopy (SEM, Hitachi
S-3000H) and transmission electron microscope (TEM, JEOL-7100).
Based on the TEM images, the size of the magnetic nanoparticles
(MNPs) and the apatite nanotubes was measured, and the existence
of MNPs within the nanotubes was also conﬁrmed. The magnetic
properties of the samples were studied using a superconducting
quantum interference device (SQUID, Quantum Design MPMS-XL7)
in an applied magnetic ﬁeld of ±20 kOe at room temperature. The
magnetic properties of the particles were evaluated in terms of satu-
ration magnetization and area.
3. Results and discussion
The crystalline phases of the MHAnt were investigated with XRD
as shown in Fig. 1. For references, the MNPs and their composite
nanoﬁber with PCL before and after the mineralization were also
characterized. As expected, typical magnetite peaks (‘M’) were
Fig. 1. XRD patterns of the samples: (a) MNPs, (b) PCL-MNPs composite nanoﬁber,
(c) mineralized composite nanoﬁber and (d) MHAnt after heat-treatment at 500 °C. M:
MNPs; P: PCL; A: apatite. Dashed lines in (d) are HA reference peaks (JCPDS card No. 24–33).
Fig. 2. (a) TEM image of MNPs. (b) SEM image of composite nanoﬁber and inset TEM image showing the presence of MNPs on the surface region. SEM images of the mineralized
nanoﬁbers (c) before and (d) after heat-treatment. Inset in (d) shows a hollow tubular structure.
R.K. Singh et al. / Materials Letters 75 (2012) 130–133
shown in the as-prepared iron oxide nanoparticles (Fig. 1(a)) and ad-
ditional PCL peaks (‘P’) were also shown in the composite nanoﬁber
(Fig. 1(b)). After mineralization, the poorly crystallized apatite
peaks at 2θ≈26° and 32° (‘A’) were well developed (Fig. 1(c)).
After heat-treatment at 500 °C, the apatite peaks became more pro-
nounced (Fig. 1(d)), and the magnetite peaks were also preserved.
There were no other peaks observed except those of apatite and
The morphologies of the produced samples were examined by SEM
and TEM, as shown in Fig. 2. The particle size of the magnetite nano-
particles was found to be 12±1.34 nm, as deduced from the TEM
image (Fig. 2(a)). The nanoparticles were observed to be easily dis-
persed in the dichloromethane/ethanol solvent where the PCL could
also be dissolved to prepare the composite suspension for electrospin-
ning. A nanoﬁber mesh with sizes of hundreds of nanometers was well
generated without any bead formation from SEM image (Fig. 2(b)). A
high magniﬁcation TEM image shown in the inset revealed that the
black dotted MNPs were primarily positioned at the surface region of
the nanoﬁber. This was likely responsible for the inorganic particles
being segregated at the border region of the ﬁber during the electro-
spinning process. Previously, we observed a similar phenomenon dur-
ing the electrospinning of a tricalcium phosphate sol and a polymer
mixture, where the inorganic sol was distributed outer surface region
while the polymer was inner . This distribution of MNPs in large
quantity at the surface region is highly beneﬁcial to obtain MHAnt.
The surface of the composite nanoﬁbers was then functionalized
with apatite mineral as shown in the SEM image (Fig. 2(c)). Surface
activation with alkaline solution and further solution-mediated nucle-
ation and crystallization processes led to the successful deposition of
an apatite mineral phase on the nanoﬁber surface. Further heat-
treatment of the mineralized nanoﬁber at 500 °C revealed a hollow tu-
bular structure (Fig. 2(d)).
The high resolution image of the MHAnt was revealed by TEM
(Fig. 3). Hollow-structured long tubes were well developed and con-
sisted of a number of highly elongated mineral nanocrystallites with
a morphology similar to bone mineral and that generally observed in
apatite obtained by biomimetic processes. The width of the inner hol-
low part of the tube was measured to be 650 nm (±80 nm) and the
wall thickness was 137 nm (±24 nm). Although magnetite nanopar-
ticles were not clearly discerned from the apatite nanocrystallites,
some particulate forms (representative regions are indicated by ar-
rows) in dark-colored area are considered to indicate their presence.
The nanocrystallite morphology of the apatite embedded with the
MNPs was actually slightly different from the pure apatite without
the nanoparticles, as previously reported; a more dotted structure
was observed MHAnt than in pure HA nanotubes which have a more
plate-like structure. Therefore, we examined the atomic composition
of the areas with EDS. In addition to the Ca and P peaks, a high
level of Fe peaks was found, conﬁrming the existence of iron oxide
The magnetic properties of the MHAnt were investigated using
SQUID magnetometry. The magnetization curve as a function of an
applied magnetic ﬁeld (Fig. 4) shows a ferromagnetic property
(i.e. the narrow hysteresis loop and low coercivity) and a saturation
magnetization (Ms) of 27.20 emu/g. In fact, the Ms of pure magnetite
nanoparticles was measured to be around 71 emu/g, while that of
bulk magnetite is known to be 92 emu/g . When compared to the
pure magnetite materials, the currently developed magnetic HA had a
much lower Ms value, which is easily understandable when considering
the decrease in the relative mass fraction of the magnetite. Although the
Ms value of the MHAnt did not reach that of pure magnetite, the prop-
erties revealed in this study including the ferromagnetic behavior and
fairly high Ms should indicate that the MHAnt can be used as biocom-
patible magnetic materials. Another important parameter for the mag-
netic characterization is the magnetic loss/cycle or the area of the
hysteresis loop. The integrated hysteresis loop area calculated for a
maximum applied ﬁeld of 20 kOe was found to be 57.22×103
Since the area under the loop is proportional to the energy loss and
hence the heat generated by a sample under a magnetic ﬁeld it is sug-
gested the nanotubes ﬁnd possible uses in hyperthermia treatment.
Taken together, this study provides useful information of the applicabil-
ity of the developed HA hollow magnetic nanotubes in biomedicine, ei-
ther used directly as a delivery system of therapeutic molecules or as a
secondary phase in artiﬁcial bone matrices, to treat the bone disease
and trauma via hyperthermia treatment or other therapeutic efﬁcacies
potentiated under magnetic ﬁelds.
Magnetic hydroxyapatite nanotubes (MHAnt) were produced
using a template made of magnetite nanoparticles/PCL polymer and
Fig. 3. TEM nanostructure image of the MHAnt and the EDS proﬁle in the inset showing
the existence of Fe and thus MNPs. Arrows pointing to dark dotted areas indicate the
possible presence of MNPs.
Fig. 4. Magnetic properties of the magnetic HA nanotubes as determined by SQUID
magnetometry. The magnetization versus magnetic ﬁeld curve shows that the nano-
tubes exhibit a ferromagnetic property with a saturation magnetization of
27.20 emu/g. The inset shows an enlargement of the curve in the initial range.
132 R.K. Singh et al. / Materials Letters 75 (2012) 130–133
its subsequent surface mineralization and thermal treatment. The
MHAnt showed a hollow tubular structure with an inner shell size
of about 650 nm and a shell thickness of about 137 nm. The MHAnt
exhibited a ferromagnetic property with a saturation magnetization
of 27.20 emu/g. The MHAnt developed herein may ﬁnd usefulness
in many biomedical applications, including the treatment of bone re-
pair and related diseases.
This work was supported by the Priority Research Centers Pro-
gram (No. 2009–0093829) and WCU program (R31-10069) through
the National Research Foundation of Korea (NRF) funded by the Min-
istry of Education, Science and Technology. Authors highly thank the
help of IBST, Dankook University.
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