ARTICLE
pubs.acs.org/JPCC
pH-Triggered Doxorubicin Delivery Based on Hollow Nanoporous
Silica Nanoparticles with Free-Standing Superparamagnetic
Fe3O4 Cores
Xuefeng Zhang,†,‡ Liviu Clime,† Helene Roberge,† Francois Normandin,† L'H. Yahia,§ E. Sacher,§ and
Teodor Veres*,†,‡
†
Industrial Materials Institute, National Research Council of Canada, 75 Boulevard de Mortagne, Boucherville, Quebec, Canada J4B 6Y4
nergie, Materiaux et Telecommunications, 1650 Boulevard Lionel Boulet, Varennes, Quebec, Canada J3X 1S2
INRS E
§
Ecole Polytechnique de Montreal, Case Postale 6079, succursale Centre-Ville, Montreal, Quebec, Canada H3C 3A7
‡
ABSTRACT: Superparamagnetic core/shell nanoparticles, composed
of a free-standing Fe3O4 magnetic core inside a hollow nanoporous
silica shell, have been synthesized, and their structures and magnetic
and drug release behavior were systematically investigated. The hollow/nanoporous silica shells, containing secondary amine groups, were
functionalized with 1, 2-cyclohexanedicarboxylic anhydride as click
linkers, producing pH-sensitive amides and terminal carboxylic groups
for the conjugation and release of the anticancer drug doxorubicin. The
results show that the effective release occurred at pH 5 and was 3 times
higher than that at pH 7.4. The kinetics of release was assumed as
a Fickian diffusion process and fitted by the Higuchi model to uncover
an intrinsic relationship of the released amount with time, temperature,
and pH.
’ INTRODUCTION
Nanoporous silica-based materials have attracted much attention for their potential applications in drug delivery due to their
excellent biocompatibility, very high surface areas available for
the loading of drug molecules, and diversity in surface functionalization.1-7 Among all the potential candidates, the controllable
drug release systems that can be triggered by various external
stimuli, such as pH,8-11 enzyme,12,13 ultrasound,14 and light,15,16
attracted the most attention because of the possibility for targeted
release, thereby minimizing the systemic side effect of the drugs
delivered. While nanoporous silica-based delivery systems have
been recently proposed,17-22 their use for localized and targeted
drug delivery is limited by the lack of a magnetic component
which allows both magnetic localization and MRI imaging. Core/
shell architectures in which magnetic nanoparticle cores are surrounded by nanoporous silica shells represent an appealing solution, providing the possibility for external magnetically localized
release and imaging capability using magnetic resonance.23-27
Doxorubicin (DOX) is one of the most widely used anticancer
drugs due to its promising potential against solid tumors. The
therapy, however, is limited by dose-dependent toxic side effects
which can potentially lead to heart failure due to cardiotoxicity.28
To overcome this problem, targeted drug delivery can provide
therapeutically effective drug release directly at the tumor site to
improve the treatment of cancers. In this paper, we report a novel
pH-triggered drug delivery nanocarrier based on superparamagnetic
r 2010 American Chemical Society
Fe3O4/nanoporous silica hollow nanoparticles [Fe3O4/silica(H)], in which DOX molecules are conjugated with aminofunctionalized silica shells. As a proof of concept, a pH-triggered
amide linker is created in the proximity of carboxylic acid groups
by Fe3O4/silica(H) nanoparticles functionalized with 1,2-cyclohexanedicarboxylic anhydride. The amide bonds were found to
be relatively stable at neutral pH 7.4, but can be rapidly hydrolyzed at a pH of less than 6.0. The pH-responsive amide bonds
could potentially allow the majority of DOX to be magnetically
delivered and released only in the cancerous tissues (pH 4-6)
with less loss in normal tissue or biological fluids (pH ≈ 7). The
multifunctional nanoparticles reported herein, combining the
nanoporous hollow architecture, magnetic manipulation, and the
pH-triggered release of the drug molecules, thus represent
interesting candidates for targeted drug delivery.
’ EXPERIMENTAL SECTION
Materials. All reagents used in this study are commercially
available. Oleic acid (OA; 90%), 1-hexanol anhydrous (99%),
octyl ether (98%), ammonia solution (NH4OH; 28-30 wt %
in water), Triton X-100, hexane (95%), cylcohexane (99.5%),
dimethyl sulfoxide (DMSO; 99%), 1,2-cis-cyclohexanedicarboxylic
Received: August 10, 2010
Revised:
November 30, 2010
Published: December 16, 2010
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anhydride (98%), triethylamine (98%), tetraethoxysilane (TEOS;
99.999%), and doxorubicin hydrochloride (98%) were purchased
from Sigma-Aldrich Inc. Iron pentacarbonyl (99.5%) was purchased from Strem Chemicals, Inc. and N-[(trimethoxysilyl)propyl]poly(ethylenimine) (PS076; 50%) from UCT Specialties, LLC.
Synthesis of Fe3O4 Nanoparticles. The oleic acid-coated
Fe3O4 (Fe3O4/OA) nanoparticles were synthesized on the basis
of a well-known process.29 Under a nitrogen flow, a mixture of
20 mL of octyl ether and 1.92 mL of oleylamine was mixed
at room temperature for ∼10 min. This solution was subsequently heated to 100 °C in 20 min, remaining nearly colorless.
At 100 °C, 0.4 mL of iron pentacarbonyl was quickly injected into
the solution under a fast argon flow, and the temperature was
raised to 290 °C at a rate of 2 °C/min. The solution was refluxed
at 290 °C for 2 h and cooled to room temperature by removing
the heating source. During the reflux process, the solution
experienced a color change from light yellow to colorless to
black. The resultant product of 15 nm Fe3O4/OA nanoparticles
was precipitated by adding excess anhydrous ethanol and separated by centrifugation (9000 rpm). The product purified at least
three times was dried under vacuum and then kept in vacuum for
long-term storage.
Synthesis of Core/Shell Fe3O4/Silica Nanoparticles. The
core/shell Fe3O4/silica nanoparticles were synthesized by hydrolyzing TEOS in a water-in-oil (W/O) microemulsion that
contained the Fe3O4/OA nanoparticles as seeds. The purified
Fe3O4/OA nanoparticles were first dispersed in cyclohexane
with a concentration of 1 mg/mL, and then 0.5 mL of Fe3O4containing cyclohexane solution was rapidly injected into a
mixture of 1.77 g of Triton X-100, 1.6 mL of 1-hexanol anhydrous,
and 7 mL of cyclohexane under a strong vortex for about 1 h.
Subsequently, 0.5 mL of ∼6% ammonia solution was added to
the above solution and the resulting mixture shaken for another
1 h. Finally, 25 μL of TEOS was added, and the mixture was
allowed to react for 24 h. The product was precipitated by adding excess anhydrous ethanol and separated by centrifugation
(9000 rpm). This process was repeated at least three times to
completely remove the unreacted TEOS, and then the precipitate was washed by deionized water three times. The resultant
product was stored in deionized water or dried under vacuum for
long-term storage.
Synthesis of Core/Shell Fe3O4/Silica(H) Nanoparticles.
The purified Fe3O4/OA nanoparticles were first dispersed in
cyclohexane with a concentration of 1 mg/mL, and then 0.5 mL
of Fe3O4-containing cyclohexane solution was rapidly injected
into a mixture of 1.77 g of Triton X-100, 1.6 mL of 1-hexanol
anhydrous, and 7 mL of cyclohexane under a strong vortex for
about 1 h. Subsequently, 0.5 mL of ∼6% ammonia solution was
added to the above solution and the resulting mixture shaken for
another 1 h. A 25 μL volume of TEOS was then added, and the
mixture was allowed to react for 24 h. Finally, 25 μL of PS076 was
added and the reaction carried out for another 24 h under the
same conditions. The product was separated and washed with
hexane and ethanol several times. The resultant product was
dried under vacuum or directly dispersed in deionized water
for use.
Loading and Release of DOX. A 2 mg portion of Fe3O4/
silica(H) nanoparticles was dispersed in 20 mL of DMSO,
followed by sonication for 30 min. 1,2-cis-Cyclohexanedicarboxylic anhydride was subsequently added and the resulting
mixture magnetically stirred for 2 h. The grafted nanoparticles
ARTICLE
Figure 1. Fe3O4/OA nanoparticles: (a) self-assembly TEM image
(the inset in (a) is the corresponding size distribution); (b) highresolution TEM image (the inset in (b) is the corresponding fast Fourier
transform pattern marked in (b)). The figure obviously reveals that the
nanoparticle is highly crystalline to extend to the outer edges, and the
lattice distance is equal to 0.42 nm, corresponding to the (200) plane of
the Fe3O4 phase. Moreover, it can be seen that the FFT pattern is a
symmetrical lattice, indicating the monocrystalline nature.
were separated by centrifugation at 9000 rpm and mildly washed
by DMSO three times. The grafted nanoparticles and doxorubicin hydrochloride salt (1 mg) were dispersed in 20 mL of DMSO
solution and 100 μL of triethylamine and the resulting dispersion
magnetically stirred for various amounts of time. To remove the
free DOX molecules, the DOX-coupled nanoparticles were
separated by centrifugation and mildly washed by pH 7.4 phosphoric acidic buffer solution three times. The release of DOX
from coupled Fe3O4/silica(H) nanoparticles was carried out at
37 °C and room temperature and in pH 7.4, 6.0, and 5.0 phosphoric acidic buffer solutions, respectively. The separated supernatant solution was monitored by UV-vis spectroscopy.
Characterization. The size and morphology of the nanoparticles
were analyzed using a modified transmission electron microscopy
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Figure 3. XRD patterns of (a) the standard card of Fe3O4 powders
(JCPDS 880315) and (b) Fe3O4/OA, (c) Fe3O4/silica, and (d) Fe3O4/
silica(H) nanoparticles.
Figure 2. TEM images of (a) Fe3O4/silica and (b) Fe3O4/silica(H)
nanoparticles. (c) HAADF-TEM image of Fe3O4/silica(H) nanoparticles. (d) EDS analysis along the diameter of the nanoparticle. (e) Highresolution HAADF-TEM image indicating the nanoporous structure of
the silica shell. (f) High-resolution TEM image of the core region.
(TEM) instrument operated at a voltage of 30 kV. The microstructure and composition of the samples were obtained by using
high-resolution TEM (HRTEM), selected area electron diffraction
(SAED), electron microscopy, and electron energy loss spectroscopy
(EELS) on a JEOL 2010 (200 kV). TEM samples were prepared
by dropping 25 μL of particle dispersion in hexane on amorphous
carbon-coated copper grids and drying under vacuum overnight.
Fourier transmission infrared (FTIR) spectra were collected with a
Nicolet Fourier spectrophotometer at wave numbers between 600
and 4000 cm-1. An X-ray diffractometer with Cu KR (λ = 0.154 nm)
radiation at a voltage of 30 kV and current of 30 mA was used to
study the phase structure of the nanoparticles. Magnetic measurements were performed with a Quantum Design PPMS model 6000
magnetometer. A Thermo Scientific NanoDrop 3300 fluorospectrometer was employed to detect the concentration variation of
fluorescein molecules.
’ RESULTS AND DISCUSSION
The oleic acid-coated Fe3O4 (Fe3O4/OA) nanoparticles were
as shown in Figure 1. It is found that the nanoparticles are
essentially spherical in shape and form a self-assembled monolayer superlattice. On the basis of the statistic of ∼200 particles,
the diameter of the nanoparticles is estimated to be 15.1 (
1.26 nm. The high-resolution TEM image and its corresponding
Figure 4. FRIR spectra of (a) Fe3O4/silica(H), (b) Fe3O4/OA, and (c)
Fe3O4/silica nanoparticles.
fast Fourier transform (FFT) pattern in Figure 1b indicate the
nanoparticles are single crystalline, that is, extended to the
surface.
Parts a and b of Figure 2 show TEM images of Fe3O4/silica
nanoparticles and Fe3O4/silica(H) nanoparticles, respectively.
For Fe3O4/silica nanoparticles, Fe3O4 nanoparticles were completely encapsulated into a silica shell with a mean shell thickness
of about 18 nm and exhibited a high uniformity of the core/shell
structure and a good monodispersersibility in water. It is noteworthy that hollow nanostructures were obtained after the
amino-functionalized silica coating process by in situ hydrolyzing
PS076 molecules (Figure 2b). The cores of the nanoparticles
were not located in the centers of the hollow shells, but had a
tendency to stick to the internal walls of the shells. Besides the
Fe3O4/silica(H) nanoparticles, some hollow silica shells without
Fe3O4 cores were found in the final product. These hollow silica
shells should derive from the pure silica nanoparticles formed by
TEOS. We estimate this fraction at less than 10%. Figure 2c
shows a high-angle annular dark-field (HAADF) image of Fe3O4/
silica(H) nanoparticles, providing a clear contrast of the hollow
structure. In the image it can be seen that a small number of silica
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Figure 5. Schematic diagram for the formation mechanism of Fe3O4/silica(H) nanoparticles.
Figure 6. Hysteresis loops at 50 and 300 K of (a) Fe3O4/OA and (b) Fe3O4/silica and hollow Fe3O4/silica(H) nanoparticles. (c) and (d) are the
corresponding magnifications of (a) and (b) near the origin. The inset in (d) shows the ZFC-FC magnetization curve of Fe3O4/silica(H) nanoparticles
under an applied magnetic field of 50 Oe.
shells have collapsed, which suggests that the first silica shell
may play a role of sacrificial template in the reaction process. By a
combination measurement from bright- and dark-field images,
it was estimated that the thickness of the amino-functionalized
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ARTICLE
Figure 7. Conjugation and release scheme of DOX molecules with secondary amine groups of silica shells using 1,2-cyclohexanedicarboxylic anhydride
as a linker.
silica shells and the diameter of the Fe3O4/silica(H) nanoparticles were approximately 7.8 and 64.0 nm, respectively. Parts d-f
of Figure 2 show the energy-dispersive X-ray spectroscopy (EDS)
and high-resolution TEM images of an individual Fe3O4/silica(H) nanoparticle. The elemental analyses revealed the core/shell
structure, with the core rich in Fe and O and the shell rich in Si
and O, and confirmed the hollow region between the core and
shell. The nanoporous structures of the silica shells were further
confirmed by HAADF-TEM as shown in Figure 2e. The HRTEM
analysis of the core region (Figure 2f) indicated a very well formed
crystalline structure with a lattice spacing of 4.17 Å which corresponded to the (200) planes of face-centered cubic (fcc) Fe3O4.
All the diffraction peaks in XRD patterns (Figure 3) of Fe3O4/
OA, Fe3O4/silica, and Fe3O4/silica(H) nanoparticles are in good
agreement with that of standard Fe3O4 powders (JCPDS 880315)
without the presence of other iron oxides. The peak around 20° is
the characteristic peak of silica. FTIR spectra of Fe3O4/silica(H)
nanoparticles (Figure 4) indicated a characteristic peak (νas) for
formation of tSi;O;Sit bonds at 1041 cm-1 and a stretching vibration (ν) of tSi;OH bonds at about 977 cm-1, which
confirmed the incomplete condensation of PS076 molecules.
Moreover, the characteristic peaks of secondary amine groups
appeared at 650 [νout-of-plane(NH)], 1305 [νas(CN)], 1558
[νin-of-plane(NH)], 2948 [νas(dCH2)], and 2840 [νas(-CH)
in -CH2-NH-CH2-] cm-1.30
Although the complete mechanism for the formation of a
nanoporous/hollow silica shell is still not completely understood, we propose a possible formation process as shown in
Figure 5 within a typical W/O microemulsion system consisting
of oil, water, and surfactant. Triton X-100 molecules were used as
the surfactant to form a monolayer at the W/O interface, with
the hydrophobic tails toward the oil phase and the hydrophilic
poly(ethylene oxide) (PO) heads in the aqueous phase. When
Fe3O4/OA nanoparticles were added, the PO heads could be
strongly anchored on the surface of nanoparticles by replacing
OA molecules, thus generating nanoparticles in the aqueous
reaction cell for the subsequent formation of initial silica shells
and amine-functionalized silica shells. Additionally, the presence
of the Triton X-100 molecules anchored on the nanoparticle
surface limited the condensation of the TEOS molecules and the
formation of a compact silica shell. As a consequence, the initial
silica shell was a “hybrid” state composed of hydrolyzed silica
Figure 8. Normalized UV-vis absorption spectra of DOX molecules
for separated supernatant solutions after the various loading times for
(a) Fe3O4/silica(H) nanoparticles and (b) Fe3O4/silica nanoparticles
without pH linkers. The inset in (a) is the loading profile of DOX
molecules with time.
debris and Triton X-100 molecules. Subsequently, PS076 molecules with a long-chain backbone structure were added to the
reaction, leading to their hydrolysis and direct reaction with the
silica debris. After removal of the Triton X-100 molecules and
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Figure 9. Release profiles of DOX molecules from Fe3O4/silica(H) nanoparticles based on UV absorption as a function of time in buffer solutions of pH
5.0, 6.0, and 7.4: (a, b) room temperature, (c, d) 37 °C.
silica debris by ethanol washing and high-velocity centrifugation,
the cavities of Fe3O4/silica(H) were formed and finally retained
due to the steric hindrance of the backbones of hydrolyzed PS076.
This process was similar to the thermal annealling used to remove
the surfactants, in which similar porous silica shells were obtained.31
Magnetic measurements were performed to confirm the magnetic nature of the cores as well as to determine the magnetization of the phase created. Figure 6 shows the hysteresis loops,
measured at temperatures of 300 and 50 K and the corresponding magnification at the origin of Fe3O4/OA, Fe3O4/silica, and
Fe3O4/silica(H) nanoparticles. At low temperatures (50 K) the
Fe3O4/OA nanoparticles show a ferromagnetic behavior with a
small coercivity (14 Oe) and magnetization at saturation of 73.40
emu/g. A superparamagnetic behavior with zero remanence was
observed at 300 K. As expected, due to the nonmagnetic silica
shells, the magnetizations of Fe3O4/silica and Fe3O4/silica(H)
samples were reduced to 2.96 and 5.01 emu/g, respectively. It
should be noted that the magnetization at saturation measured
for the hollow Fe3O4/silica(H) sample had a magnetic moment
40% larger than that of the Fe3O4/silica sample, implying that the
removed silica debris was removed from the resultant product by
magnetic separations. The temperature-dependent zero-field-cooling
(ZFC) and field-cooling (FC) magnetization curve of Fe3O4/
silica(H) nanoparticles was measured at an applied magnetic field
of 50 Oe and showed a broad maximum at ∼111 K corresponding to the signature of the superparamagnetic behavior of Fe3O4
nanoparticle cores.
The hollow architecture of the Fe3O4/silica(H) nanoparticles
makes them particularly suitable as drug carriers. The conjugation
scheme of DOX with secondary amides, via 1,2-cyclohexanedicarboxylic anhydride as a linker, is shown in Figure 7. The
nanoporous silica shells of the Fe3O4/silica(H) nanoparticles
was first treated with 1,2-cyclohexanedicarboxylic anhydride to
form the amide linkers and carboxylic-terminal surface. The carboxylic groups then reacted with amine groups of DOX molecules. The formed amide linkers serve as an effective pHtriggered switch due to the effect of neighboring carboxylic
acid groups. They are chemically stable at neutral pH, while at a
low pH they are negatively charged to regenerate the amine
groups.8,32 Therefore, this synthetic approach has the potential
to allow an easy control of DOX molecule delivery by decreasing
the pH of the solution in contact with the nanoparticle surface.
The UV-vis absorbance of the supernatant solution collected
from the Fe3O4/silica(H) nanoparticle sample was measured
before and after loading of DOX molecules (Figure 8 a). The
mass of the DOX molecules loaded into the Fe3O4/silica(H)
nanoparticles was estimated to be 15.3 mg/100 mg, which was
normalized to 100%. For comparison, we also measured the
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Figure 10. Correlation between the release rate constant (kH) and pH
for the release of DOX molecules from Fe3O4/silica(H) nanoparticles at
room temperature and 37 °C.
coupling capacity of DOX molecules in the Fe3O4/silica nanoparticles with hydroxyl groups on the surface. As shown in
Figure 8 b, the result shows that the maximum payload after
24 h is only 0.64 mg for 100 mg of nanoparticles, far less than that
of Fe3O4/silica(H) nanoparticles. This indicates that DOX
molecules are only loaded by physical adsorption rather than
chemical bonding because the hydroxyl groups cannot react with
the amine group of the DOX molecules. Figure 9 shows the
release kinetics of DOX molecules as a function of the pH and
temperature. It was found that, at room temperature and at a pH
of 7.4, only 3.2% of the initial DOX molecules' payload was
released within 10 h. This low loss of the initial charge indicates
that the amide bonds were stable enough to retain the DOX
molecules. As the pH decreased, a sharp increase was measured
in the concentration of the DOX molecules released in the supernatant solution. The concentration measured within 1 h after the
pH was reduced to 6 and 5 was estimated at 6.7% and 14.0%,
respectively. Assuming that the release is dominated by a simple
diffusion kinetic process from the inner cavities and nanopores of
the silica shells, the values measured for the concentration of the
released DOX molecules could be explained by the Higuchi
model, Qt = kHt1/2,33,34 where Qt is the release amount of guest
molecules (DOX), t is time, and kH is the release rate constant.
Porous silica systems for the noncontrolled release of drugs have
been satisfactorily explained by the Higuchi model.10,35 As expected, the released amount at room temperature and at 37 °C
can be fitted linearly as the square root of time (Figure 9b). From
Figure 9b, two regimes of release are indicated with distinct and
very different release kinetics exhibiting linear behaviors over
time. In the first 1 h, a sharp slope indicates a faster release rate,
followed by a slower release process that is still linear up to 10 h
measured in our experiments. The later release rate constant
corresponds to about 1.99 at pH 5, which is 2 times larger than
that at pH 7.4 (0.94). In accordance with the Higuchi model, the
release kinetics of the DOX molecules suggests that they can
be loaded into the nanopores of the silica shells, and even within
the inner cavities of the nanoparticles, which can be ascribed
to the nanoporous architectures of the silica shells. Therefore,
the rapid release in the initial stage is likely due to the DOX
molecules coupled to the surface of the nanoparticles, which
diffuse more easily into the buffer through hydrolyzing amides in
comparison with those that are loaded within the cavities.
Figure 11. (a, b) Suspension of DOX-coupled Fe3O4/silica(H) nanoparticles in PBS 7.4 buffer at room temperature. (c, d) Magnetic
separation process by an external magnet.
To establish the DOX release kinetics closer to the physiological conditions, the release behaviors were studied at 37 °C in
PBS buffers. From Figure 9c,d a rapid release of DOX molecules
within 1 h is observed, similar to that at room temperature. The
amount of DOX content released after 1 h at pH 5, 6, and 7.4 was
estimated to be 34.7%, 24.5%, and 7.2%, respectively. From 1 to
10 h, the amount of released DOX increases steadily up to 73.2%
of the total payload at pH 5. The data in Figure 9d, plotted as a
function of the square root of time for the second release regime
(1-10 h), are in good agreement with the linear relationship of
the Higuchi model with the corresponding release rate constants
of 17.8, 11.9, and 6.7 at pH 5, 6, and 7.4, respectively.
In the Higuchi model, the release rate constant (kH) can be
expressed as kH = 2CoD1/2/π1/2, where Co is the initial concentration of drug in the matrix and D is the diffusion coefficient. The
diffusion coefficient (D) is related to the temperature and the
structure of the matrix, which were constant in our case, leading
to the conclusion that the changes in the release rate constant
(kH) at various pH values were mainly caused by the initial
concentration of drug in the matrix (Co). It should be noted that
the release rate constants of DOX molecules are inversely dependent on the pH, suggesting that the pH plays a critical role in controlling the release. The coupling and hydrolysis of the amides are
pH-dependent. At pH 7.4, a small amount of DOX molecules can
be released, because most of them are bound by amides, resulting
in a low concentration of free DOX molecules, i.e., an “effective
initial concentration”. Conversely, at pH 5 and 6, more hydrolyzed amides contribute to the higher effective initial concentrations. Figure 10 shows the correlation between the release rate
constant (kH) and pH at room temperature and 37 °C. In light of
this function, a linear relationship was established as a theoretical
guidance to the drug release kinetics dependent on the pH,
temperature, and time.
Water solubility, good dispersibility, and rapid magnetic
response to an external magnetic field are critical for biomedical
applications of the nanoparticles. After being coupled with DOX
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molecules, the Fe3O4/silica(H) nanoparticles still reserve these
merits. Figure 11 shows the suspension of DOX-coupled Fe3O4/
silica(H) nanoparticles in PBS 7.4 buffer at room temperature
and the magnetic separation process. One can observe that the
suspension is stable and after 12 h some precipitation appears.
The precipitation, however, can be redispersed to the initial state
by slight hand shaking. Furthermore, the nanoparticles in the
suspension can also be separated by an external magnet, indicating their manipulation ability.
’ CONCLUSIONS
In conclusion, we have reported a very effective and reproducible route for the synthesis of amino-functionalized silicacoated superparamagnetic Fe3O4 nanoparticles with a hollow
structure and nanoporous shell. The highly uniform core/shell
topology leads to weakened magnetic interactions, consequently
providing an excellent aqueous dispersibility. By combining the
magnetic properties of the Fe3O4 cores and the hollow/nanoporous silica shells, we propose a new hollow-magnetic carrier
with high molecular binding capacity, and as a proof-of-concept,
we demonstrated their use for the pH-triggered control and release
of the anticancer drug molecule DOX. The release kinetics
clearly indicated that the DOX molecules can be loaded not
only on the outside surface of the silica shells, but also in the inner
cavities and nanopores ascribed to the porous architecture of the
shells. Additionally, we have also demonstrated a pH-triggered
release of DOX molecules which is dependent on the pHtriggered hydrolysis kinetics of the amides. The amount of conjugated DOX was approximately 15.3 mg/100 mg of Fe3O4/
silica(H) nanoparticles. After 10 h, 73.2% of the DOX content
was effectively released at pH 5, while only 21.8% was released at
pH 7.4. We expect that the use of Fe3O4/silica(H) nanoparticles
can be easily extended to other guest molecules such as genes,
protein, and DNA, with potential application in various fields
of targeted delivery.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Fax: (450) 641-5105.
’ ACKNOWLEDGMENT
This work was jointly supported by the NSERC-CRD grant,
the Canadian Institutes of Health Research, and the Industrial
Materials Institute, National Research Council of Canada (IMINRC). We thank Dr. Gianluigi Botton from the McMaster
University Center for Electron Microscopy for his help with
the TEM characterization. We are grateful to NMD Inc. for
financial support and Dr. Blaise Gilbert and Dr. Omar Quraishi
for insightful advice regarding the use of the magnetic carriers for
biomedical applications. Finally, we thank Dr. Lidjia Malic for a
very helpful suggestion on the manuscript.
ARTICLE
(4) Hetrick, E. M.; Shin, J. H.; Stasko, N. A.; Johnson, C. B.; Wespe,
D. A.; Holmuhamedov, E.; Schoenfisch, M. H. ACS Nano 2008, 2, 235.
(5) Nguyen, T. Q.; Wu, J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H.
Science 2000, 288, 652.
(6) Zhou, W.; Thomas, J. M.; Shephard, D. S.; Johnson, B. F. G.;
Ozkaya, D.; Maschmeyer, T.; Bell, R. G.; Ge, Q. Science 1998, 280, 705.
(7) Chi, F.; Guo, Y. N.; Liu, J.; Liu, Y.; Huo, Q. J. Phys. Chem. C 2010,
114, 2519.
(8) Xu, P.; Kirk, E.A. V.; Zhan, Y.; Murdoch, W. J.; Radosz, M.; Shen,
Y. Angew. Chem., Int. Ed. 2007, 46, 4999.
(9) Wu, W.; Zhou, T.; Berliner, A.; Banerjee, P.; Zhou, S. Chem.
Mater. 2010, 22, 1966.
(10) Aznar, E.; Marcos, M. D.; Martínez-Ma
nez, R.; Sancenon, F.;
Soto, J.; Amoros, P.; Guillem, C. J. Am. Chem. Soc. 2009, 131, 6833.
(11) Yang, Y. J.; Tao, X.; Hou, Q.; Ma, Y.; Chen, X. L.; Chen, J. F.
Acta Biomater. 2010, 6, 3092.
(12) Patel, K.; Angelos, S.; Dichtel, W. R.; Coskun, A.; Yang, Y. W.;
Zink, J. I.; Stoddart, J. F. J. Am. Chem. Soc. 2008, 130, 2382.
(13) Thornton, P. D.; Heise, A. J. Am. Chem. Soc. 2010, 132, 2024.
(14) Kapoor, S.; Bhattacharyya, A. J. J. Phys. Chem. C 2009, 113,
7155.
(15) Nguyen, T. D.; Leung, K. C. F.; Liong, M.; Liu, Y.; Stoddart,
J. F.; Zink, J. I. Adv. Funct. Mater. 2007, 17, 2101.
(16) Mal, N. K.; Fujiwara, M.; Tanaka, Y. Nature 2003, 421, 350.
(17) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.;
Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4990.
(18) Zhang, X. F.; Dong, X. L.; Huang, H.; Lv, B.; Zhu, X. G.; Lei,
J. P.; Ma, S.; Liu, W.; Zhang, Z. D. Acta Mater. 2007, 55, 3727.
(19) Yang, Q.; Wang, S.; Fan, P.; Wang, L.; Di, Y.; Lin, K.; Xiao, F. S.
Chem. Mater. 2005, 17, 5999.
(20) Guerrero-Martínez, A.; Perez-Juste, J.; Liz-Marzan, L. M. Adv.
Mater. 2010, 22, 1182.
(21) Yoon, T. J.; Kim, J. S.; Kim, B. G.; Yu, K. N.; Cho, M. H.; Lee,
J. K. Angew. Chem., Int. Ed. 2005, 44, 1068.
(22) Chattopadhyay, P.; Gupta, R. B. Ind. Eng. Chem. Res. 2002, 41,
6049.
(23) Cao, S. W.; Zhu, Y. J.; Ma, M. Y.; Li, L.; Zhang, L. J. Phys. Chem.
C 2008, 112, 1851.
(24) Urbina, M. C.; Zinoveva, S.; Miller, T.; Sabliov, C. M.; Monroe,
W. T.; Kumar, C. S. S. R. J. Phys. Chem. C 2008, 112, 11102.
(25) Zhang, L.; Qiao, S.; Jin, Y.; Chen, Z.; Gu, H.; Lu, G. Q. Adv.
Mater. 2008, 20, 805.
(26) Zhang, L.; Qiao, S.; Jin, Y.; Yang, H.; Budihartono, S.; Stahr, F.;
Yan, Z.; Wang, X.; Hao, Z.; Lu, G. Q. Adv. Funct. Mater. 2008, 18, 3203.
(27) Zhang, L.; Qiao, S.; Cheng, L.; Yan, Z.; Lu, G. Q. Nanotechnology
2008, 19, 435608.
(28) Crowe, D. L. Recent Res. Dev. Cancer 2002, 4, 65.
(29) Woo, K.; Hong, J.; Choi, S.; Lee, H. W.; Ahn, J. P.; Kim, C. S.;
Lee, S. W. Chem. Mater. 2004, 16, 2814.
(30) The Handbook of Infrared Spectra; Simons, W. W., Ed.; Sadtler
Research Laboratories Press: Philadelphia, PA, 1978.
(31) Zhao, W.; Gu, J.; Zhang, L.; Chen, H.; Shi, J. J. Am. Chem. Soc.
2005, 127, 8916.
(32) Morris, J. J.; Page, M. I. J. Chem. Soc., Chem. Commun. 1978, 591.
(33) Higuchi, T. J. J. Pharm. Sci. 1963, 52, 1145.
(34) Higuchi, W. I. J. Pharm. Sci. 1962, 51, 802.
(35) Vallet-Regí, M.; Balas, F.; Arcos, D. Angew. Chem., Int. Ed. 2007,
46, 7548.
’ REFERENCES
(1) Torney, F.; Trewyn, B. G.; Lin, V. S.Y.; Wang, K. Nat. Nanotechnol. 2007, 2, 295.
(2) Slowing, I. I.; Trewyn, B. G.; Giri, S.; Lin, V. S. Y. Adv. Funct.
Mater. 2007, 17, 1225.
(3) Shin, J. H.; Metzger, S. K.; Schoenfisch, M. H. J. Am. Chem. Soc.
2007, 129, 4612.
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