1. Multifunctional iron oxide–carbon hybrid
microrods†
Lu Zhu,*a
Weijie Huang,b
Zachary S. Rinehart,a
Jason Tamc
and Yiping Zhaob
In this work, iron oxide microrods (MRs) with different crystal phases were successfully fabricated by a facile
solvothermal method and sequential annealing processes. It was found that the carbon content remained in
the structure when annealing at low temperature (150
C). The carbon in the MRs contributed to the higher
dye adsorption and drug loading capabilities of the MRs. The Fe3O4–C sample showed superior adsorption
for both a cationic dye (methylene blue) and an anionic dye (methyl orange) with an equilibrium adsorption
capability of 11.7 mg gÀ1
and 20.8 mg gÀ1
, respectively. When applied as a drug carrier for a tissue
plasminogen activator, the mass loading ratio of the MRs was as high as 12.9% for chemical loading and
7.8% for physical loading. With the high dye adsorption/drug loading ratio, such magnetic structures
show promise for use in water treatment and advanced medical applications.
1. Introduction
Due to their unique physical and chemical properties, iron
oxide (FexOy) based nanomaterials have attracted tremendous
interest in many different elds, such as energy storage, catal-
ysis, drug delivery, and bio-imaging.1–4
The applications of
FexOy nanostructures are strongly dependent on their crystal
phases, morphologies, and compositions. Among the different
phases of iron oxides, the a-phase has the a-Al2O3 structure and
it is the most stable structure.5
However, it barely responds to
a magnetic eld at room temperature. By applying a reducing-
oxidation process, a-Fe2O3 can be converted to Fe3O4 or g-
Fe2O3. With the changing of crystal structures, the magnetic
properties of these iron oxides also change. In addition, because
of its high theoretical capacity ($924 mA h gÀ1
),6
low cost, and
non-toxic properties, Fe3O4 is considered as a promising
candidate for lithium ion battery anodes. The superior lithium
storage properties of hierarchical Fe3O4 hollow spheres that are
composed of ultrathin porous nanosheets have been demon-
strated and reported.1,7
From the biomedical point of view,
magnetite (Fe3O4) and maghemite (g-Fe2O3) nanoparticles
(NPs) have become major focuses for use as tumor-targeting
drug carriers8
and cancer cell separators,9
because of their
unique magnetic properties. In these applications, the
magnetic force would not directly interact with cells, which
could help minimize the potential side effects.9
Superparamagnetic FexOy based NPs are also widely investi-
gated as imaging contrast agents for magnetic resonance
imaging (MRI).10–14
The magnetism of the materials helps
improve the spatial resolution and the tissue penetration
length. As for the environmental applications, hematite (a-
Fe2O3) is considered a promising photocatalyst material that
has potential to utilize visible light to degrade pollutants and
detoxify pathogens in waste water because of its narrow
bandgap ($2.2 eV).15–17
Other than a-Fe2O3, the nanostructured
Fe3O4 and g-Fe2O3 were used to remove pollutants18
or served as
catalysts.19
Composite FexOy nanostructures have also been
synthesized to improve the performances and match different
application requirements. For example, Fe3O4–carbon
composites have been investigated as advanced anode materials
for batteries.20–22
Also, it has been reported that the dumbbell-
like Pt–Fe3O4 NPs showed excellent catalytic properties for
oxygen reduction reactions.23
Compared to NPs, nanorod and nanowire based nano-
materials have shown more interesting and improved chemical
and physical properties,21,24–26
and have attracted extensive
attentions for potential applications in magnetism, electronics,
optics, catalysis, sensors and biomedical applications.6,26–30
It
was reported that the magnetic Fe3O4 nanorod fabricated by
a physical vapor deposition technique helped to greatly improve
the thrombolytic efficiency under a rotating magnetic eld.31
Effective photocatalytic a-Fe2O3 nanorods (NRs) were reported
by Pradip Basnet, et al.17
The a-Fe2O3 NRs showed high meth-
ylene blue (MB) degradation rate and bacterial inactivation
activity under visible light illumination.17
Sun et al.32
also re-
ported that with the coating of Ag/AgCl, the a-Fe2O3 nano-
spindles showed improved photocatalytic degradation of
Rhodamine B (RhB) under simulated sunlight illumination
compared to bare a-Fe2O3 materials. And spindle shaped
a
College of Engineering, University of Georgia, Athens, GA 30602, USA. E-mail: zhulu@
uga.edu
b
Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA
c
Department of Materials Science and Engineering, University of Toronto, Toronto, ON
M5S 3E4, Canada
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra19489c
Cite this: RSC Adv., 2016, 6, 98845
Received 2nd August 2016
Accepted 29th September 2016
DOI: 10.1039/c6ra19489c
www.rsc.org/advances
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2. Fe3O4–C NRs were reported as a promising anode material for
lithium ion batteries by Zhang et al.21
To meet the requirements of different applications, many
efforts have been devoted to fabricate FexOy nanostructures
with different morphology, shape, crystal phase, and composi-
tions. The co-precipitation method is widely used to fabricate
Fe3O4 NPs.9,13,33
However, the size and morphology of the
resulting products of this method are not uniform. Thermal
decomposition of organic precursors was reported by Jongnam
Park et al. in 2004, then widely adopted later by the commu-
nity.14,34,35
This method is not only a large scale synthesis
method, but also provides uniform, monodisperse and size-
tunable FexOy products. Even though this method produces
ideal NPs, the products are hydrophobic. In order to apply these
NPs into a biological system, a subsequent surface modication
step is necessary and important to turn them hydrophilic and
thus form a stable suspension in water.34
Zhou et al.36
fabricated
rod-like FeC2O4$2H2O by microemulsion method under room
temperature and then used them as the precursor material to
fabricate a-Fe2O3, g-Fe2O3, and Fe3O4 rods by annealing them
under different conditions. Hydrothermal/solvothermal
synthesis is another popular method to synthesize FexOy
nanostructures with preferred morphologies nowadays. Both
Fe3O4 and Fe2O3 micro/nano-rods could be prepared through
these facile synthesis methods.20,22,37
Other than the wet-
chemical methods, physical vapor deposition (PVD) is also an
effective way to fabricate FexOy materials with different
nanostructures.17
Here we report a systematic study of a series of iron oxide
microrods (MRs), including their fabrication methods, physical
and chemical properties, and applications in different elds.
Starting from a single precursor material Fe–glycolate, four
different iron oxides were obtained by changing the oxidation/
reduction processes. It is found that the obtained magnetic
Fe3O4–C and g-Fe2O3–C hybrid MRs have the potential to be
used in water treatments to remove pollutants and their high
drug loading ability and fast clot lysis speed (as fast as 16 mm
minÀ1
) have also been demonstrated, which indicates that they
are also promising candidates to be used in stroke treatment.
2. Experimental section
2.1. Materials and Fe–glycolate synthesis
Ferric nitride (Fe(NO3)3$9H2O, Alfa Aesar), glucose (Sigma) and
ethylene glycol (EG, Amresco) were used without further puri-
cation. In a typical synthesis of Fe3O4 MR precursors, 0.7575 g
Fe(NO3)3$9H2O and 0.5 g glucose were thoroughly dissolved
into 75 ml EG. Then the homogeneous mixture was transferred
into a 100 ml Teon-lined stainless steel autoclave and main-
tained at a temperature of 220
C for 12 h. The green product,
Fe–glycolate was then collected by centrifugation, washed twice
with absolute ethanol (EtOH) and dried in an oven at 65
C
overnight. The EG mediated synthesis promote the growth of
1D micro-rod structures. EG rst coordinated with Fe3+
to form
iron alkoxide, which became the nuclei for the owing structure
growth.38
Because the –OH groups in glucose molecules can be
protonated by a strong acid to form H2O, the addition of glucose
helped to slow the accumulation of H+
during solvothermal
synthesis. H+
was generated during the formation of metal
alkoxide and its accumulation would inhibit further metal
alkoxide formation.38–40
According to the study by Fei-Xiang Ma
et al.,22
changing the glucose amount would vary the particle
size of Fe–glycolate. The addition of glucose could also intro-
duce a small amount of carbon in to the Fe–glycolate struc-
tures.22
A control synthesis was performed without glucose. As
shown in Fig. S1 of ESI,† without glucose, only bead-like
structures with a mean diameter of 350 Æ 50 nm formed aer
12 h solvothermal synthesis, which indicates that the glucose
works not only as a source of carbon, but also serves potentially
as a rod-shape directing agent.
2.2. Reduction and oxidation processes
The Fe–glycolate powders were annealed in different atmo-
spheres to obtain different iron oxides samples. As shown in
Scheme 1, the Fe3O4–C samples were obtained by annealing the
Fe–glycolate MRs in N2 (3 h) or with a N2 ow carrying EtOH (50
SCCM N2 ow directly bubbled through 100 ml EtOH in a ask)
for 1 h, 2 h, and 3 h at 350
C. The Fe3O4–C powders were further
annealed in air at 150
C for 2 h to achieve g-Fe2O3–C. To
prepare the a-Fe2O3 samples, the Fe3O4–C powders were
annealed in air at 600
C for 2 h. In addition, reducing the a-
Fe2O3 MRs, in the N2 ow carrying EtOH at 350
C for 1 h, pure
Fe3O4 MRs were obtained.
2.3. Characterizations
Morphologies of the samples were investigated by a eld-
emission scanning electron microscope (FESEM) equipped
with an energy dispersive X-ray spectroscopy (FEI Inspect F).
Transmission electron microscopy (TEM)/scanning trans-
mission electron microscope (STEM) analysis and correspond-
ing selected area electron diffraction (SAED) were carried out
using a Hitachi HF-3300 TEM/STEM at 300 kV to further
investigate the morphologies and atomic structures of Fe3O4–C,
g-Fe2O3–C, a-Fe2O3 and Fe3O4 samples. The crystal structures of
all the as-prepared samples were characterized by an X-ray
diffractometer (XRD; PANalytical X'Pert PRO MRD) with a Cu
Ka source (l ¼ 1.5405980 ˚A) at 45 kV and 40 mA. The Raman
Scheme 1 Annealing processes of iron oxide MR samples.
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3. spectra of a-Fe2O3, g-Fe2O3–C, and Fe3O4–C samples were taken
by a Renishaw inVia Raman microscope equipped with a 514
nm laser source. Thermogravimetric analysis (TGA, Mettler
Toledo TGA/SDTA851e) was performed from 50
C to 800
C.
Magnetic properties were measured at room temperature by
a vibrating sample magnetometer (VSM, Model EZ7; Micro-
Sense, LLC, Lowell, MA, USA) with a 2.15 T electromagnet. The
magnetization of the sample was measured over a range of
applied elds from À1.5 to +1.5 kOe. The measurements were
conducted in step eld mode at a step size of 250 Oe sÀ1
. Zeta
potential of different MRs samples were measured by a Malvern
Zetasizer Nano ZS system at 25
C.
The dye adsorption properties of some samples were
measured as follows: 2 mg as-prepared iron oxide MR samples
were added into 10 ml methyl orange (MO) or MB solution (30
mM) at room temperature. Then the mixtures were shaken at
300 rpm for 30 min. At 5 min, 20 min and 30 min time interval,
an aliquot sample was taken out and centrifuged at 12 000 rpm
to remove the iron oxide MRs and the concentration change of
MO and MB in the remaining solution were investigated by UV-
Vis spectroscopy (JASCO V-570) through their representative
concentration – extinction calibration curves (Fig. S2 of the
ESI†).
Drug loading capability of the FexOy samples were charac-
terized by tissue plasminogen activator (tPA) loading experi-
ments. Since drugs could potentially chemically or physically
adsorb on the materials, and their release effect would be
different, we investigated both drug loading effects. For chem-
ical tPA loading, which is dened as the loading between the
carboxyl group (–COOH) modied MRs and tPA molecules, iron
oxide MRs were rst dispersed in EtOH/water mixture with
a volume ratio of 4 : 1. Then 3-aminopropyltriethoxysilane
(APTES) and dimethylformamide (DMF) were added into the
mixture and shaken for 2 hours at room temperature to func-
tionalize the MR surfaces with amine groups (–NH2). The –NH2
modied MRs were then separated from the suspension by
a strong permanent magnet and washed 3 times and re-
dispersed in phosphate-buffered saline (PBS). Next, 25%
glutaraldehyde (GA) was added to obtain a GA concentration of
0.5% and shaken for 30 min at 30
C to modify the MRs with
–COOH groups. Aer washing, the –COOH modied MRs were
added into a tPA solution (500 mg mlÀ1
) for 12 hours at 4
C to
immobilize tPA onto the MRs' surfaces. The physical tPA
loading, which is the linking of MRs and tPA molecules by van
der Waals force, was achieved by directly mixing MRs with tPA
solution (500 mg mlÀ1
). The mixture was gently shaken and then
stored at 4
C for 24 h. During this process, certain amount of
tPA could be either physically or chemically absorbed by the
porous MRs. The amount of immobilized tPA was determined
by using a Pierce™ BCA protein assay kit from Thermo Scien-
tic (Rockford, IL) to measure the concentration of unbound
tPA in the supernatant before and aer mixing with the –COOH
modied MRs. The in vitro blood clot lysis experiments (exper-
imental set-up is shown in Fig. S3 of the ESI†) were performed
in polydimethylsiloxane (PDMS) channels as reported
previously.41
3. Results and discussion
Fig. 1 shows the representative morphologies of different as-
prepared samples. The uniform precursor Fe–glycolate MRs
(Fig. 1A) were assembled from plate like nanostructures, and
these MRs have an average length of L ¼ 1.4 Æ 0.3 mm, with an
average diameter of D ¼ 0.7 Æ 0.1 mm. By annealing in N2 at 350
C for 3 h (Fig. 1B), Fe3O4–C composite MRs were achieved. The
rod shape was retained aer annealing, however, the surface of
the MRs became coarse aer decomposition of Fe–glycolate. By
annealing the Fe–glycolate in a reducing environment, N2
carrying EtOH, the surfaces of the MRs became smoother
(Fig. 1C) as compared to those in Fig. 1B. Aer further anneal-
ing in air at 150
C for 2 h (Fig. 1D), the structure of the MRs is
well retained. When the annealing temperature increased to
600
C, the carbon content in the structures was oxidized by O2
in the air, and the Fe3O4 was also oxidized into Fe2O3. As shown
in the zoomed-in image of Fig. 1E, the a-Fe2O3 MRs changed to
nano-porous structures. These porous a-Fe2O3 rods can then be
reduced in a N2 and EtOH environment at 350
C, leading to
a transformation from a porous a-Fe2O3 to a porous Fe3O4 MR
structure without carbon. During these annealing processes,
the size of the MRs changes slightly. As summarized in Table 1,
the Fe–glycolate MRs have an average length of 1.4 Æ 0.3 mm,
the Fe3O4–C is about 1.2 Æ 0.3 mm, the a-Fe2O3 is about 1.0 Æ 0.2
mm and the Fe3O4 is about 0.9 Æ 0.3 mm. With the sequential
annealing processes, the size of MRs gradually shrank, which
might have contributed to the loss of carbon content in the
structure and the surface melting of iron oxide nanostructures
under high temperature. However, all the samples remained as
microrod structures.
The porous nature of the samples (Fe3O4–C, g-Fe2O3–C, a-
Fe2O3 and Fe3O4 MRs) was further investigated by TEM, and the
images (A1–D1) and (A2–D2) in Fig. 2 clearly demonstrate that
the four MR samples were all composed of small crystal grains
with voids inside the rods. The lattice fringes in HRTEM images
(A2–D2 in Fig. 2) show some representative atomic planes in
each sample. As shown in (A2) of Fig. 2, the measured lattice
spacing of 0.253 nm matches the (311) plane of cubic Fe3O4 in
Fe3O4–C MRs. The ring structures in the SAED pattern in (A3) of
Fig. 2 match well with the (220), (311), (400), (511) and (440)
planes of cubic Fe3O4 and reveal that the Fe3O4–C MR is poly-
crystalline. For the g-Fe2O3–C MRs, the measured lattice
spacing of 0.252 nm in (B2) of Fig. 2 agrees well with the (311)
plane of cubic g-Fe2O3, and the diffraction rings shown in (B3)
of Fig. 2 t the (220), (311), (400), (511) and (440) planes of cubic
g-Fe2O3. However, both the HRTEM image (C2) and the corre-
sponding SAED pattern (C3) shown in Fig. 2 indicate that by
annealing at 600
C, the Fe3O4–C sample is oxidized into a-
Fe2O3 and the resulting MRs are very close to a single-crystal-
like structure with distinguishable diffraction spots. The
lattice spacing of 0.145 nm in (C2) matches the (300) plane of
rhombohedral a-Fe2O3. For the Fe3O4 MR sample, the lattice
spacing of 0.162 nm in (D2) matches the (511) plane of cubic
Fe3O4, and the diffraction spots arranged in the rings of (D3)
correspond to different crystal planes of cubic Fe3O4.
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4. The X-ray powder diffraction (XRD) patterns in Fig. 3 reveal
the crystalline structures of different FexOy samples. For Fe–
glycolate MRs shown in Fig. 3A, only one peak is located in the
low diffraction angle region, $11
, demonstrating the crystal
structure of a metal alkoxide Fe–glycolate.22,38,40
Fig. 3B conrms
the transformation of different iron oxide phases by varying the
annealing conditions. By annealing Fe–glycolate in N2, weak
peaks of cubic Fe3O4 (PDF reference code: 01-089-0688) appear,
which demonstrate that the Fe–glycolate MRs were decomposed
to Fe3O4 MRs. However, annealing the Fe–glycolate sample in
a reducing environment (N2 + EtOH) while keeping other
conditions the same helped to improve the crystallinity. The XRD
peaks of this cubic Fe3O4 became very prominent. The further
annealing of the sample in air at 150
C for 2 h changes the color
of products from black to dark red, which indicates a change in
the oxidation state of iron oxide from Fe3O4 to g-Fe2O3,42
even
though the XRD patterns of magnetite Fe3O4 and maghemite g-
Fe2O3 are very similar. By annealing Fe3O4–C MRs in air at 600
C, both the change of sample color from black to red and the
sharpening in XRD pattern peaks (Fig. 3B and D) of hematite a-
Fe2O3 demonstrate that the sample transformed to its most
stable iron oxide phase. By applying the Scherrer equation on the
XRD pattern, the crystal size of the MRs is estimated to be 47.1
nm from the a-Fe2O3 (110) XRD peak. The XRD pattern in
(Fig. 3B and E) shows that reducing the a-Fe2O3 MRs in N2 +
EtOH at 350
C for 1 h forms Fe3O4 MRs with high crystallinity.
The crystal size was estimated to be approximately 47.1 nm by
using the Fe3O4 (311) peak and the Scherrer equation.
The carbon content of the g-Fe2O3–C and Fe3O4–C samples,
which were annealed under different conditions, was quanti-
tatively determined by thermogravimetric analysis (TGA). Fig. 4
shows that when the temperature is lower than 150
C, the mass
change is not obvious for both the g-Fe2O3–C and the Fe3O4–C
samples. However, it can be observed that for the Fe3O4–C
samples, the total mass slightly increased. With the increasing
of temperature, the mass of the Fe3O4–C sample, which is ob-
tained from annealing Fe–glycolate in N2, rst increased 1.8%,
then decreased 9.8%. For the Fe3O4–C sample that had been
annealed in the present of EtOH, the mass rst increased 2.3%,
then decreased 13.4%. For these two samples, the mass
increases at lower temperature (below 230
C) was attributed to
the oxidation of Fe3O4 to Fe2O3. As shown in Fig. 4, the small
changes in g-Fe2O3–C samples below 230
C also demonstrate
that Fe3O4 were gradually transformed to Fe2O3 while the
carbon remained in the structure in the low temperature range.
It was observed that upon heating to 600
C, all four samples
showed large amount of mass loss. Upon heating to 600
C, the
mass loss of Fe3O4–C (EtOH + N2) is 13.4%. It should be noted
that by heating in air, the transformation of Fe3O4 to Fe2O3 can
cause a mass increase because of the addition of 1
2O into Fe3O4.
Its theoretical mass increase is 3.45%. Assuming that the
amount of Fe3O4 is totally converted to Fe2O3, the 13.4% mass
loss is in fact corresponding to a 16.9% carbon content in the
structures. For g-Fe2O3–C (EtOH + N2), the carbon content is
14.6% and it is all attributed from carbon content in the
structure because of the pre-annealing process in air at 150
C
had already oxidized the Fe3O4. For Fe3O4–C (N2), the mass loss
is 9.8%, corresponding to 13.3% carbon content. And for g-
Fe2O3–C (N2), the mass loss is 11.0% (contributed by the carbon
Fig. 1 SEM images of (A) Fe–glycolate; (B) Fe3O4–C (annealed in N2); (C) Fe3O4–C (annealed in EtOH + N2); (D) g-Fe2O3–C (E) a-Fe2O3 and
(F) Fe3O4 MRs. The scale bars are 4 mm (500 nm for scale bars in the insert images).
Table 1 Shape information of different FexOy MRs
Sample Length L (mm) Width D (mm)
Fe–glycolate 1.4 Æ 0.3 0.7 Æ 0.1
Fe3O4–C (N2) 1.2 Æ 0.2 0.5 Æ 0.1
Fe3O4–C (N2 + EtOH) 1.1 Æ 0.2 0.5 Æ 0.1
g-Fe2O3–C (N2) 1.1 Æ 0.2 0.5 Æ 0.1
g-Fe2O3–C (N2 + EtOH) 1.3 Æ 0.3 0.4 Æ 0.1
a-Fe2O3 1.0 Æ 0.2 0.4 Æ 0.1
Fe3O4 0.9 Æ 0.3 0.4 Æ 0.1
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5. content). It should be noted that there is a 2% difference in
carbon content between Fe3O4–C and g-Fe2O3–C samples,
which means that only a small amount of carbon ($2%) was
oxidized during the annealing of the Fe3O4–C samples in air at
150
C in order to obtain the g-Fe2O3–C samples. It should also
be noted that the annealing environment would affect the
carbon content in these MRs. The TGA results show that by
annealing in the N2 and EtOH mixture, the carbon content in
the product is higher than in the samples prepared in pure N2,
which might be caused by the EtOH depositing extra carbon
onto the MRs during annealing. During annealing under high
temperature (350
C), decomposition of Fe–glycolate occurs.22
Even though the detailed mechanism of this reaction is still
unclear, during the experiments, it was found that aer
complete decomposition in N2 for 3 h at 350
C (ref. 22) then
cooled down to room temperature, a very small amount of
Fig. 2 The TEM images and corresponding SAED of Fe3O4–C (A1–A3), g-Fe2O3–C (B1–B3), a-Fe2O3 (C1–C3) and Fe3O4 (D1–D3).
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6. Fe3O4–C product at the top of the sample changed to a red color
when exposed to air, which indicates the oxidization of Fe3O4 to
Fe2O3. It might result from some decomposition by-products
accumulated at the product surface that react with O2 in air
once contacted, then oxidized the surface Fe3O4 to Fe2O3 and
might also react with part of the carbon content. Thus,
annealing the Fe–glycolate MRs in a slightly reducing environ-
ment would not only help improve the crystallinity of Fe3O4 but
also preserve the carbon content in the structure.
Raman measurements were carried out to further conrm
the existence and removal of carbon content in different
samples. The Raman spectra in Fig. 5 conrm that both the D
band located at $1374 cmÀ1
and G band located $1592 cmÀ1
for carbon are present in Fe3O4–C and g-Fe2O3–C MRs. This
indicates that even annealing in air at 150
C for 2 h, most of the
carbon content would still remain in the structure. However, the
peak intensity ratios of D band and G band (ID/IG) of these two
samples are different. For the Fe3O4–C sample, ID/IG is esti-
mated as 0.73, which indicates the carbon has low crystallinity,
but for g-Fe2O3–C, ID/IG is 0.56, which indicates better carbon
crystallinity.43
Clearly, such structural and compositional transitions of
the MRs could result in some unique properties, which may
be used for different potential applications. First, the
magnetic properties of these iron oxide based MRs (except for
a-Fe2O3) can be tuned systematically via different annealing
conditions. Fig. 6 shows typical magnetic hysteresis loops for
some representative samples. For samples annealed under
N2 and EtOH ow, all show ferromagnetic properties under
room temperature and the results are summarized in Table 2.
The pure Fe3O4 MRs have the highest saturated magnetiza-
tion, Ms ¼ 87 emu gÀ1
. However, the change in annealing
time could modify the magnetic properties of these rods.
When annealed under a reducing environment, both Fe3O4
and Fe3O4–C samples can achieve high saturated magneti-
zation. The highest saturated magnetization, Ms, of the Fe3O4
MR samples is 87 emu gÀ1
when annealed for 1 h, and 40 emu
gÀ1
for the Fe3O4–C MR samples when annealed for 2 h,
which is close to the saturated magnetization of bulk Fe3O4
material (92–100 emu gÀ1
).44
For Fe3O4–C (EtOH + N2, 1 h), Ms
is 39 emu gÀ1
, and for Fe3O4–C (N2, 3 h) the value is reduced
to 20 emu gÀ1
. The decreasing of the Ms might be caused by
Fig. 3 (A) XRD patterns of Fe–glycolate; (B) (a) Fe3O4–C (annealed in N2), (b) Fe3O4–C (annealed in EtOH + N2), (c) g-Fe2O3–C, (d) a-Fe2O3 and
(e) Fe3O4 rods.
Fig. 4 TGA curves of Fe3O4–C and g-Fe2O3–C samples under air
flow.
Fig. 5 Raman spectra of a-Fe2O3, g-Fe2O3–C, and Fe3O4–C rods with
an excitation wavelength of 514 nm.
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7. the poor crystallinity of Fe3O4 and the availability of carbon
content in the structures. It is noted that the Fe3O4–C (N2),
g-Fe2O3–C (N2) and g-Fe2O3–C (EtOH + N2) samples have very
small remnant magnetization, Mr, values (0.1, 0.01
and 0.6 emu gÀ1
, respectively), which indicates that
they might be superparamagnetic materials. For super-
paramagnetic materials, the coefficient of squareness, Kp
Kp ¼
Mr
Ms
, of the hysteresis loop should be or very close to
0.45,46
The Kp values of Fe3O4–C (N2), g-Fe2O3–C (N2) and g-
Fe2O3–C (EtOH + N2) samples are 0.005, 0.000625 and 0.02
respectively. These low values suggest the
superparamagnetic-like behaviors of these MRs. Due to these
unique magnetic properties, these MR samples can be easily
collected and separated from solutions for future
applications, for example, collecting and recycling catalysts
aer water treatments.
In addition, as shown in Fig. 1, some iron oxide MRs may
have large surface area due to the surface roughness and pores
in the structure. In fact, aer removing the C from MRs, some
MRs become signicantly more porous, increasing their surface
area which increases the dye adsorption ability for environ-
mental applications. The dye adsorption capabilities of ve
different samples were investigated by using both MB and MO
solutions. The discoloration of MB and MO solutions was
observed aer the MRs were added and suspended into the
solutions. The equilibrium adsorption capabilities qe were
estimated from the equilibrium MB or MO concentration by the
following equation,
qe
À
mol gÀ1
Á
¼
ðC0 À CeÞ ðmol LÀ1
ÞV ðLÞ
m ðgÞ
; (1)
where m and V are the mass of iron oxide MRs and the volume of
dye solutions, respectively. The C0 (initial dye concentration)
and Ce (equilibrium dye concentration aer adsorption) was
estimated by UV-Vis measurements calibrated through the
Beer's law (Fig. S4 of the ESI†). The results are summarized in
Table 3. MRs that were annealed in N2 showed better dye
adsorption abilities than samples that were annealed with
EtOH, because the annealing with EtOH made the samples
slightly hydrophobic. For example, the adsorption capacities of
Fe3O4–C MRs annealed in N2 is 11.7 mg gÀ1
for MB and 20.8 mg
gÀ1
for MO, while the values of Fe3O4–C MRs annealed with
EtOH are 2.2 mg gÀ1
and 4.3 mg gÀ1
for MB and MO, respec-
tively. Due to the amorphous carbon in its structure, the Fe3O4–
C (N2) sample showed good dye adsorption capability for both
MB and MO. Zeta-potential of these samples were measured in
order to further understand the adsorption of dyes (Table 3).
The zeta potential values help to explain the adsorption differ-
ence for different dyes. The zeta potential of g-Fe2O3–C (N2)
sample is À17.5 mV, which makes the adsorption capability of
cationic dye MB (19.2 mg gÀ1
) higher than that of the anionic
dye MO (4.9 mg gÀ1
). Considering their magnetic properties,
these MRs can be potentially used for water treatments to
remove pollutants: the high adsorption ability would help to
collect and carry the pollutants and then they can be easily
separated from liquid by simply applying magnetic elds.
Different Fe3O4 based materials have been studied for dye
absorbance. For example, Fe3O4@C NPs with 44.38 mg gÀ1
MB
and 11.22 mg gÀ1
cresol red (CR) adsorption capabilities were
reported by Zhang et al.47
The graphene nanosheet (GNS)/
Fig. 6 Vibrating sample magnetometer curve of different FexOy rods.
Table 2 Magnetic properties of different FexOy MRs
Sample
Annealing
time
Ms
(emu gÀ1
)
Mr
(emu gÀ1
)
Coercivity
Hc (Oe)
Fe3O4–C (N2 + EtOH) 1 h 39 3 63
Fe3O4–C (N2 + EtOH) 2 h 40 6 101
Fe3O4–C (N2 + EtOH) 3 h 35 5 106
Fe3O4–C (N2) 3 h 20 0.1 8
Fe3O4 1 h 87 26 268
Fe3O4 2 h 78 25 245
Fe3O4 3 h 76 20 270
g-Fe2O3–C (N2) 2 h 16 0.01 1
g-Fe2O3–C (N2 + EtOH) 2 h 30 0.6 10
Table 3 Properties and multi-functional applications of different FexOy MRs
Sample
Zeta potential
(mV)
MB qe
(mg gÀ1
)
MO qe
(mg gÀ1
)
tPA loading
Chemical loading % Physical loading %
Fe3O4–C (N2) À18.8 11.7 20.8 11.7 6.6
Fe3O4–C (N2 + EtOH) À19.5 2.2 4.3 12.9 7.8
Fe3O4 13.8 1.4 2 9.3 4.4
g-Fe2O3–C (N2) À17.5 19.2 4.9 12.4 6.2
g-Fe2O3–C (N2 + EtOH) 17.3 3.1 3.3 12.1 7.3
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8. magnetite (Fe3O4) composite reported by Ai et al.48
showed a MB
adsorption capability of 35 mg gÀ1
with an initial MB concen-
tration of 25 mg lÀ1
at 25
C. The MB adsorption capability in
our work is 11.7 mg gÀ1
and MO adsorption capability is 20.8
mg gÀ1
of Fe3O4–C MRs. However, it is hard to compare these
values to our results directly since the reported structures (core–
shell NPs and NPs-sheets composites) are very different from
ours (MRs), and the tested conditions are different, such as pH
value, initial dye concentration, and temperature. Meanwhile,
due to the differences of synthesis methods, the carbon content
in these structures also varied.
From a bio-medical point of view, the drug loading capa-
bilities of these magnetic MRs are also very promising. Stroke
is one of the leading causes of death globally. Localized
treatment of the blood clot is required because the dosage of
tissue plasminogen activator (tPA) used to clear blood clots
oen leads to internal bleeding. According to the work of
Jiangnan Hu et al.,31
Fe3O4 NRs fabricated by physical vapor
deposition method can load 6% tPA and have $30 min tPA
release time for thrombolysis. It is expected that the porous
MRs synthesized by our current method would load more tPA
molecules. Chemically and physically tPA loading experiments
of MRs were carried out to determine their potentials for
stroke treatments. The results are summarized in Table 3. The
mass loading ratio is dened as loaded tPA mass over the MRs'
mass. The results show that with the existence of carbon, these
MRs can carry a greater tPA dose both chemically and physi-
cally. For example, Fe3O4–C annealed in EtOH + N2 showed
12.9% tPA chemical loading ratio while Fe3O4 MRs showed
9.3% loading ratio. The high drug loading ratios of these
samples indicate that they can be candidates for stroke treat-
ment in the future. And our preliminary result shows that
when cultured with neutral stem cells, the Fe3O4–C MRs do not
show apparent cytotoxicity. By considering their magnetic
properties, samples that are superparamagnetic can reduce
the cluster-forming possibilities during experiments. To
demonstrate the clot dissolving ability of these tPA-loaded
MRs, clot lysis experiments with Fe3O4–C (N2), and g-Fe2O3–
C (EtOH + N2) MRs were carried out in PDMS channels to
mimic the thrombolysis in blood vessels. The tPA molecules
were specically carried to the target clot by the magnetic MRs.
Then under a rotating magnetic eld, the mechanical rotation
of the MRs helped enhance the uid transportation into the
clot structure, which directly increased the amount of tPA
delivered into the clot. Our results show that these two MRs
can achieve a clot lysis speed of 11 mm minÀ1
and 16 mm
minÀ1
, respectively. These fast clot lysis speeds again indicate
their future use in the stroke treatments.
4. Conclusions
In summary, rod-shaped iron alkoxide (Fe–glycolate) has been
successfully synthesized via a low cost and facile solvothermal
method and used as a template to obtain iron oxide-based MRs.
From the same precursor, by changing the annealing condi-
tions, Fe3O4, g-Fe2O3 and a-Fe2O3 can be obtained with slight
morphology changes. Magnetic properties of these FexOy MRs
can be also tuned by annealing under different conditions. By
annealing under ow of N2 carrying EtOH aer removing the
carbon content, the Fe3O4 MRs obtained the largest saturated
magnetization. The as-prepared MRs showed excellent dye
adsorption and drug loading capabilities. By applying
a magnetic eld, tPA loaded Fe3O4–C and Fe3O4 MRs were able
to dissolve blood clots in PDMS channels. These MRs are ex-
pected to be used in water treatments and also advanced drug
delivery in the future.
Acknowledgements
This work was supported by National Science Foundation under
the contract ECCS-1303134 and National Institutes of Health
under the contract R21 NS084148-01A1. The authors would like
to thank Mr Steven R. Larson and Mr Layne H. Bradley for
proofreading the manuscript.
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