18512
J. Phys. Chem. C 2007, 111, 18512-18519
Structural and Magnetic Properties of Gold and Silica Doubly Coated γ-Fe2O3
Nanoparticles
Keeseong Park,† Gan Liang,*,‡ Xiaojun Ji,§ Zhi-Ping Luo,| Chun Li,§ Mark C. Croft,⊥,# and
John T. Markert†
Department of Physics, UniVersity of Texas at Austin, Austin, Texas 78712, Department of Physics,
Sam Houston State UniVersity, HuntsVille, Texas 77341, Department of Experimental Diagnostic Imaging,
M. D. Anderson Cancer Center, UniVersity of Texas, Houston, Texas 77030, Microscopy and Imaging Center,
Texas A&M UniVersity, College Station, Texas 77843-2257, Department of Physics and Astronomy,
Rutgers UniVersity, Piscataway, New Jersey 08854, and National Synchrotron Light Source,
BrookhaVen National Laboratory, Upton, New York 11973
ReceiVed: July 22, 2007; In Final Form: September 25, 2007
Extensive structural and magnetic characterization measurements were carried out on gold and silica doubly
coated γ-Fe2O3 nanoparticles, which were recently demonstrated to have an efficient photothermal effect and
high transverse relaxivities for MRI applications. Powder X-ray diffraction and X-ray absorption spectroscopy
show the phase of the uncoated and coated nanoparticles to be that of the γ-Fe2O3 structure. The sizes, structure,
and chemical compositions of the nanoparticles were determined by transmission electron microscopy. The
magnetization results indicate that coating of the iron oxide nanoparticles by gold/silica decreases the blocking
temperature from 160 to 80 K. Such a decrease can be well-explained by spin disorder, causing reduction of
the effective volume of the γ-Fe2O3 core. Moreover, it was found that in the temperature (T) range between
100 K and room temperature, the gold/silica coating can cause a slight magnetic change in the γ-Fe2O3 cores
from superparamagnetic to almost superparamagnetic. Finally, it was found that the coercivity for both the
uncoated and the coated nanoparticles decreases almost linearly with T1/2 with the former decreasing faster
than the latter, and this coercivity result confirms that the blocking temperature is decreased by gold/silica
coating. These results are valuable for evaluating the future applications of this class of multifunctional,
hybrid magnetic nanoparticles in biomedicine.
1. Introduction
Superparamagnetic nanoparticles (SPNPs) have great potential
for magnetic resonance imaging (MRI), due to their high
transverse relaxivity,1,2 and for a wide range of other biomedical
applications such as drug delivery.3-6 Indeed, SPNP-enhanced
MRI techniques have been under development in recent years
for imaging of macrophage activity, atherosclerotic lesions,
detection of lymph node metastasis, staging of liver tumors,
and monitoring the in vivo distribution of cellular trafficking.1,2,7-11
For in vivo applications, the SPNPs must be coated with a
biocompatible and diamagnetic material (such as organic
polymers or silica matrixes) to prevent the formation of large
aggregates of SPNPs and to facilitate functions such as the
attachment of drugs. For example, the dispersion of uncoated
γ-Fe2O3 NPs in aqueous solution is stable only in highly acidic
or basic media. In recent years, much effort has been devoted
to the syntheses and characterization of silica coated superparamagnetic iron oxide (SPIO) nanoparticles (NPs) including
γ-Fe2O3 NPs.12-17
It has been proven that silica coated SPIO NPs have an
excellent biocompatibility and can homogeneously disperse in
* Corresponding author. Tel.: (936) 294-1608; fax: (936) 294-1585;
e-mail: [email protected].
† University of Texas at Austin.
‡ Sam Houston State University.
§ M. D. Anderson Cancer Center, University of Texas.
| Texas A&M University.
⊥ Rutgers University.
# Brookhaven National Laboratory.
aqueous solutions with a wide range of pH values.12-17 Thus,
they can be used for the MRI diagnosis of cells or tissues altered
by diseases such as cancer. However, to kill the cancer cells or
cure the affected tissues by photothermal therapy, the silica
coated SPIO NPs need to be coated with particles that have an
excellent heat generating ability under controllable conditions.
In recent years, nanostructures with highly controlled optical
properties have attracted great interest due to their potential
diagnostic and therapeutic applications in biological and biomedical systems.18 Among such nanostructures, the gold derived
NPs are especially attractive due to their ease of preparation,
good biocompatibility, ready bioconjugation, and unique optical
properties.19-23 Gold nanoshells have a strong absorbance for
the wavelength tunable in the near-infrared (NIR) region of the
electromagnetic spectrum. They are, therefore, very promising
candidates for localized photothermal therapy because they can
mediate strong plasmon-induced surface heat flux upon absorption of the NIR radiation.23-25 Thus, if the Au nanoparticles
can be coated on the surface or dispersed into the surface layer
of the silica matrix of the silica coated γ-Fe2O3 NPs, then such
Au/SiO2 doubly coated γ-Fe2O3 NPs could be made bifunctional
(i.e., diagnosis and cure of the cancer cells).
Recently, we successfully synthesized Au/SiO2 doubly coated
γ-Fe2O3 NPs and found that they can form a stable solution in
water and have high transverse relaxivities for MRI applications.26 Furthermore, the broad absorbance between 100 and
900 nm in the UV-vis spectra indicates that these doubly coated
particles are suitable for photothermal therapy with light in the
10.1021/jp0757457 CCC: $37.00 © 2007 American Chemical Society
Published on Web 11/29/2007
Structural/Magnetic Properties of SPIO Au Nanoshells
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18513
NIR region.26 However, in that work, a detailed structural and
magnetic characterization of the uncoated and Au/SiO2 doubly
coated γ-Fe2O3 NPs was absent. A better understanding of the
structural and magnetic properties of this class of doubly coated
(bifunctional) NPs is urgently needed for their applications in
biomedicine. In this paper, we present detailed results of
characterization by temperature- and field-dependent magnetization, transmission electron microscopy (TEM), X-ray diffraction
(XRD), and X-ray absorption spectroscopy (XAS) measurements
on these Au/SiO2 doubly coated γ-Fe2O3 NPs.
2. Experimental Procedures
The synthesis of gold and silica doubly coated γ-Fe2O3 NPs
has been described in detail elsewhere.26 The first step is the
synthesis of the silica coated γ-Fe2O3 NPs using the well-known
Stöber process.27 An aqueous ammonia solution (30 wt %, 7
mL) and 0.5 mL of tetraethylorthosilicate (TEOS) were added
to the γ-Fe2O3 (or SPIO) solution at room temperature to form
silica NPs. The functional groups at the surface of the silica
nanoparticles are primarily silanol (Si-OH) or ethoxy (Si-OEt)
groups.28 These silica nanoparticles were then treated with
0.04 mL of 3-minopropyl trimethoxysilane (APTMS) for 6 h
to introduce the amino-terminated silica surface. The reaction
mixture was refluxed for 30 min to complete the reaction.28 In
the second step of the synthesis, 1 mL of γ-Fe2O3-embedded
silica solution was added to 5 mL of undiluted tetrakis(hydroxymethyl) phosphonium chloride (THPC) gold solution,
causing the THPC gold nanocrystals to attach onto the silica
surface. The THPC gold solution was prepared by reduction of
chloroauric acid (HAuCl4) with THPC, which produces small
gold particles with a net negative interfacial charge.29 Finally,
the gold nanoshell was prepared by reduction of the K-Gold
solution with formaldehyde (37%). UV-vis absorption spectra
of the nanoshells were measured with a Beckman Coutler DU800 UV-vis spectrometer after the reaction to verify the
formation of the nanoshell. The spectra are identical to those
reported in our previous article.26 The broad absorbance between
100 and 900 nm in the UV-vis spectra indicates that these Au/
SiO2 doubly coated particles are suitable for photothermal
therapy with light in the NIR region.26 The water-based Fe2O3
NPs (MEG 304) were purchased from Ferrotech (Nashua, NH).
TEOS, APTMS, ammonia solution, THPC, HAuCl4, and
formaldehyde were purchased from Sigma-Aldrich (St. Louis,
MO).
X-ray diffraction (XRD) measurements were performed at
room temperature using a Rigaku 2005 X-ray diffractometer
with Cu KR radiation. The TEM measurements were carried
out using a JEOL 2010 TEM system at a working voltage of
200 kV. All imaging magnifications were calibrated using the
standards of SiC lattice fringes30 for high magnifications and
commercial cross-line grating replica for low magnifications.
To prepare the TEM samples, a small drop of the sample
solution was transferred to the top surface of a carbon film
supported Cu grid that was previously glow discharged to
achieve better dispersion and then dried in air. The average size
of the particles was estimated based on a sufficient number of
sampling, typically over 50. The EDS measurements were
performed using an Oxford Instruments EDS detector with an
INCA energy platform.
The magnetic properties of the NPs were studied using a
Quantum Design MPMS SQUID magnetometer at temperatures
ranging from 5 to 300 K and in magnetic fields (H) ranging
from 0 to up to 50 kOe. In the magnetization measurement,
both the uncoated and the Au/SiO2 coated Fe2O3 NPs were in
Figure 1. X-ray diffraction patterns for (a) uncoated γ-Fe2O3 NPs,
(b) silica coated γ-Fe2O3, and (c) Au and silica doubly coated γ-Fe2O3
NPs.
Figure 2. X-ray absorption spectra at Fe K-edge for γ-Fe2O3 NPs (12
nm) and other reference compounds.
the form of dried powders. Gelatin capsules (from Capsuline.com) and water-resistive polycarbonate capsules (from Unipec Inc., Rockville, MD) were used as the sample containers
for the uncoated and Au/SiO2 coated Fe2O3 NPs, respectively.
Since both the amount of the Au/SiO2 coated Fe2O3 particles
(0.6 mg) and the volume fraction of the magnetic Fe2O3 cores
in the sample were very small, the magnetic moment of the
polycarbonate capsule was measured for background subtraction.
The Fe K-edge XAS data were taken in fluorescence mode
at room temperature at beamline X-19A at the National
Synchrotron Light Source (NSLS) at Brookhaven National Lab.
A double-crystal Si (111) monochromator was used for energy
selection, which was detuned by reducing the incident photon
flux 20% from its maximum value to suppress contamination
from harmonics. The energy resolution (∆E/E) of the X-19A
beam line was 2 × 10-4, corresponding to about 1.4 eV for the
edge energy of the Fe K-edge. The energy calibration of the
spectra was made by simultaneously measuring the spectrum
of a FeO slide as a reference. The XAS spectra presented in
this paper were background subtracted and normalized to unity
in the continuum region.
18514 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Park et al.
Figure 3. (a) TEM image of the uncoated γ-Fe2O3 NPs. (b) EDS spectrum that confirms that the composition is Fe and O without any other
impurities. The signal of Cu is from the Cu grid, and C is from the support film on the Cu grid. (c) Enlargement of the framed area in panel a.
3. Results and Discussion
(A) XRD and XAS Results. Figure 1 shows the powder XRD
patterns for (a) commercial IONPs, (b) SiO2 coated IONPs, and
(c) Au/SiO2 doubly coated IONPs, with each pattern normalized
to its maximum intensity. All of the peaks in the patterns of
the commercial IONPs can be indexed with the cubic structure
corresponding to either γ-Fe2O3 or Fe3O4 phase. Since magnetite
(Fe3O4) has a cubic spinel structure and lattice constant similar
to maghemite (γ-Fe2O3), it is quite difficult to distinguish
γ-Fe2O3 from Fe3O4 using only XRD data. Thus, the existence
of Fe3O4 in the commercial IONPs cannot be excluded based
only on the XRD data. On the other hand, the X-ray absorption
near-edge structure (XANES) of XAS offers a powerful means
to distinguish the iron oxide species. Fe lattice sites with
different formal valences and local coordination can manifest
different chemical shifts and near-edge spectral features in their
Fe K-edge spectra. In Figure 2, we compare the Fe K-edge
spectra (labeled by γ-Fe2O3, 12 nm) of the commercial IONPs
to the powder spectra of four reference compounds: micrometersized γ-Fe2O3, R-Fe2O3, Fe3O4, and FeO. It can be seen that
both the shape and the edge energy (defined as the energy at
absorption coefficient µ ) 0.5) for the spectrum of the
commercial IONPs are very close to those for the micrometer-
sized γ-Fe2O3 but very different from the rest of the spectra in
Figure 2. This clearly demonstrates that the commercial IONPs
used in this study are in the γ-Fe2O3 phase.
Figure 1b shows that the XRD pattern of the SiO2 (silica)
coated γ-Fe2O3 NPs is very similar to that (Figure 1a) of the
uncoated γ-Fe2O3 NPs. This indicates that the coated silica is
in an amorphous form before the step of mixing the γ-Fe2O3embedded silica solution with the THPC gold solution during
the synthesis process. For the Au and silica doubly coated
γ-Fe2O3 particles, the XRD pattern shown in Figure 1c displays
three wide peaks that can be identified as the (111), (200), and
(220) reflection lines of the Au fcc-cubic phase, indicating that
the gold particles are crystallized. In addition, there are four
narrow peaks in the pattern that can be attributed to the
crystallized SiO2 phase (PDF 31-1233), indicating that a certain
portion of the silica is crystallized during the steps of coating
Au on the surface of the γ-Fe2O3-embedded silica NPs. At
present, while the mechanism of the crystallization of silica due
to the coating of the Au particles is unclear, some recent research
indicates that Au nanoparticles on silica spheres can induce
crystallization of silica at low temperatures.31 The background
of this pattern is very similar to the XRD pattern of amorphous
SiO2, which decreases rapidly with the increase of 2θ from 23
Structural/Magnetic Properties of SPIO Au Nanoshells
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18515
Figure 5. Images of BF (a) and central dark-field image (b) for Au/
SiO2 doubly coated γ-Fe2O3 NPs. In panel b, only the crystalline Fe2O3
and Au particles are highlighted. Larger particles indicated by arrowheads are Fe2O3. Mapping Si and O are shown in panels c and d,
respectively. EDS spectrum is shown in panel e.
Figure 4. (a) TEM image in lower magnification for Au/SiO2 doubly
coated γ-Fe2O3 NPs; large spherical particles, as marked by the black
dashed circles for two of them, are the silica spheres, and the small
black dots represent Au particles. (b) Enlarged image of several Au
particles that clearly show the lattice fringes. (c) Enlarged image from
the framed area (enclosed by the dashed loop) in panel a.
to 40° and then slowly beyond 40°. Considering the fact that
most of these coated particles are spherical (see TEM results)
and the observation that usually spherical SiO2 particles are in
the amorphous form, we believe that only a small fraction of
the silica is crystallized as a minor phase and that most of the
silica nanoshells are in the amorphous phase.
Interestingly, the Fe2O3 peaks (Figure 1a,b) appearing in the
patterns of the SiO2 coated and uncoated Fe2O3 particles are
almost unnoticeable in the pattern of the Au/SiO2 coated
particles shown in Figure 1c. This very small intensity of the
Fe2O3 peaks can be explained by the strong absorption of and
scattering from the coated Au particles. Indeed, such a washingout effect was also observed in XRD patterns for some Au
passivated Fe NPs.32
(B) TEM and EDS Results. Figure 3a shows a TEM image
of the pure or uncoated Fe2O3 particles at low magnification.
The average size of the particles is measured to be DFe2O3 )
12.4 nm with a standard deviation of 4.5 nm, based on the
measurements of 412 particles. This value of average size is
slightly larger than (but within one standard deviation of) the
nominal size of 10 nm given by the vendor (Ferrotech). The
EDS data in Figure 3b confirm that the particle composition is
Fe and O without any other impurities. The signal of Cu is from
the Cu grid, and C is from the support film on the Cu grid. The
framed area (enclosed by the dashed rectangular loop) in Figure
3a is enlarged in Figure 3c, where lattice fringes of the
crystalline Fe2O3 crystals are visible.
A typical TEM image of the Au and silica doubly coated
Fe2O3 nanoparticles is shown in Figure 4a. The large spherical
particles, two of which are marked by the dashed circles at the
left side of the image, are identified as the silica spheres.
The average size of these particles is measured to be Dsilica )
81.5 nm with a standard deviation of 17.0 nm, based on the
measurements of 60 particles. The smaller dark particles
distributed on the surfaces of these silica spheres are Au
nanoparticles. The average diameter of these Au particles is
measured to be DAu ) 6.0 nm with a standard deviation of
1.6 nm, based on the measurements of 365 particles. The coated
Au only forms dispersed discrete nanoparticles on the surface
of silica spheres, rather than a continuous Au layer. An enlarged
lattice image of a gold nanoparticle is shown in Figure 4b, where
the Au lattice fringes are clearly visible. Figure 4c is an
enlargement of a rectangular area (enclosed by the white,
rectangular dashed loop) of a silica/Au coated nanoparticle
shown in Figure 4a. In the core region of this coated nanoparticle, lattice fringes of the Fe2O3 particle are visible (in phase
contrast), as shown by the white dash loop in Figure 4c. The
Fe2O3 particles are larger than the Au nanoparticles, with an
average size of around 12 nm. These Fe2O3 particles are
embedded inside the silica spheres. The silica, as confirmed by
either electron diffraction or the imaging as shown in Figure
4c, remains in an amorphous phase. The Au particles exhibit
the darkest contrast because of their highest scattering absorption
to electrons as compared to other elements. It should be pointed
out that since the Au particles (located on the surface of the
SiO2 sphere) and the Fe2O3 core are at different heights along
the electron beam, they cannot be focused on at the same time
to visualize the lattice fringes. However, the fringes for Au and
Fe2O3 can be visualized at different defocus values, as taken
for Figure 4b,c, respectively.
Another view of the Au and silica doubly coated Fe2O3 NPs
is presented as a bright field (BF) image in Figure 5a. Figure
5b is a central dark-field (CDF) image from the same area but
taken by tilting the incident electron beam to highlight the
crystallites inside. As mentioned before, because the mass of
Fe2O3 is much lighter than Au, the image contrast (scatteringabsorption contrast) of Fe2O3 is very weak, making it hard to
identify in the presence of the strong Au contrast in the BF
image taken at low scattering angles. Fe2O3 is only visible in
18516 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Park et al.
Figure 6. Temperature-dependent magnetization curves, in both FC and ZFC nodes, measured at (a) 10 Oe and (b) 500 Oe for uncoated γ-Fe2O3
NPs, respectively. Panel c shows the magnetization curves measured at 500 Oe for the SiO2 coated γ-Fe2O3 NPs without Au, and panel d shows
the magnetization curves measured at 500 Oe for Au/SiO2 doubly coated γ-Fe2O3 NPs.
the previous phase contrast image in Figure 4b. However, in
the CDF image in Figure 5b, as the contrast is formed by
diffracted beams at high scattering angles (diffraction contrast),
both crystalline Fe2O3 and Au particles exhibit high contrast in
the CDF image formed from their common diffracted beam
positions. It should be pointed out that only those particles that
have their diffracted beams along this tilted incident beam
direction are imaged (i.e., only selective fractions of the Fe2O3
and Au particles show up in the CDF image in Figure 5b).
Importantly, it can be recognized that larger Fe2O3 particles are
at the center of the silica spheres, as pointed out by some
arrowheads in the Figure 5. The maps of Si (Figure 5c) and O
(Figure 5d) are similar, along the silica spheres in the BF image
in Figure 5a. The EDS spectrum taken over this entire area (with
the background signals of Cu and C removed) is shown in Figure
5e, and it clearly shows the evidence of O, Si, Fe, and Au. A
quantitative analysis yields a composition of 62.6% O, 26.4%
Si, 1.7% Fe, and 9.3% Au (all in atom percent). The very low
concentration of Fe confirms that only small Fe2O3 particles
are present inside the silica.
(C) Magnetic Properties. Figure 6 presents the field cooled
(FC) and zero-field cooled (ZFC) magnetization M(T) curves
for the uncoated and Au/SiO2 doubly coated γ-Fe2O3 NPs. The
M(T) curves were measured in a temperature range between 5
and 300 K and at two applied fields: 10 and 500 Oe for the
uncoated particles and 500 Oe for the coated ones. Figure 6b
shows that the 500 Oe ZFC and FC curves for the γ-Fe2O3 NPs
(with DFe2O3 ) 12.4 nm) are irreversible below the irreversible
temperature Tirr ≈ 300 K, and the blocking temperature TB,
defined as the temperature at the maximum of the ZFC
curve,33-35 is TB ≈ 160 K. Such observed irreversibility between
ZFC and FC curves and a maximum in the ZFC curve are typical
for an assembly of SPNP. We note that the values of Tirr and
TB observed here for our 12 nm γ-Fe2O3 NPs are higher than
Figure 7. Magnetic hysteresis loops for (a) uncoated γ-Fe2O3 NPs
and (b) Au/SiO2 coated γ-Fe2O3 NPs, measured at different temperatures
in fields between -50 and +50 kOe.
the values Tirr ) 175 K and TB ≈ 120 K measured at the same
field (500 Oe) by Jeong et al.35 for their γ-Fe2O3 particles with
Structural/Magnetic Properties of SPIO Au Nanoshells
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18517
Figure 8. Magnetic hysteresis loops of Figure 8 plotted in range of applied fields between -2 and 2 kOe.
a smaller particle size of 5-8 nm. This particle size caused a
change in TB and can be explained by the theoretical relation34,35
TB )
KV
25kB
(1)
where V [(1/6)πD3av] is the average volume of the NPs, K is the
uniaxial anisotropy constant, and kB the Boltzmann constant.
Indeed, such an increase of TB due to the increase of particle
size has also been observed previously for γ-Fe2O3 NPs with
an average diameter in the range between 5 and 13 nm.35-37
For example, Mukadam et al.36 found that the TB of their
γ-Fe2O3 NPs increased from 80 to 160 K with increasing particle
size from 8 to 13 nm, when measured at 100 Oe. The 40 K
difference in TB observed between our ∼12 nm NPs and the
∼8 nm NPs of Jeong et al.35 appears to be comparable but
slightly smaller than the ∆TB/∆Dav rate observed by Mukadam
et al.36
A comparison between the curves shown in Figure 6a,b
indicates that with the decrease of the applied field H from 500
Oe to a much lower value of 10 Oe, Tirr and TB both increase
to above 300 K. Similar high Tirr and TB values (measured at
the same field H ) 10 Oe) were also observed by another
group34 for γ-Fe2O3 with a similar particle size Dav ) 11.1 nm.
The observed increase of TB with decreasing applied field was
also previously observed for γ-Fe2O3 NPs of other particle
sizes,36,38 and it can be explained by the theoretical relation39,40
(
TB ) TB0 1 -
)
CH2
TB0
(2)
where TB0 is the blocking temperature at zero-field and C is a
field-independent parameter.
By comparing the the ZFC curve in Figure 6c with that in
Figure 6b, it can be seen that the blocking temperature TB
decreases from 160 K to about 80 K with the coating of Au/
18518 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Park et al.
Figure 9. Plots of coercivity (HC) against T1/2 for uncoated γ-Fe2O3
NPs (solid circles) and Au/SiO2 doubly coated γ-Fe2O3 NPs (open
circles). Solid lines are the linear fit of the HC data against T1/2 according
to eq 6.
SiO2 on the γ-Fe2O3 NPs. Comparison between the magnetization curves in Figure 6c,d shows that further coating of Au
particles on the surface of the SiO2 spheres does not change
TB. We can attribute the 80 K decrease of TB mainly to the
reduction of the average effective volume of the γ-Fe2O3 cores,
which is caused by the interfacial interaction between the silica
and the outer layer of the iron oxide core. Our TEM results
have shown that the SiO2 nanoshells (about 35 nm thick) are
coated on γ-Fe2O3 cores and that the Au particles (Dav ≈ 6
nm) are only dispersed in a thin layer near the outer surface of
the SiO2 nanoshells. This result means that the Fe ions located
near the surface of the γ-Fe2O3 cores can only interact with
SiO2 (silica) near the γ-Fe2O3/SiO2 interface, not with the Au
particles. Such an interaction between Fe ions and SiO2 could
produce a thin layer of misaligned or disordered Fe spins near
the surface of the spherical γ-Fe2O3 cores. The spins in this
magnetically disordered layer should have a negligible contribution to the total magnetization M for the sample and thus can
be excluded from the particle volume V in eq 1. Thus, if the
average thickness of the spin disordered layers is t, we can define
an average effective volume, Veff,41 for the γ-Fe2O3 cores in
the Au/SiO2 coated NPs
Veff )
π(Dav - 2t)3
6
(3)
Then, eq 1 should be replaced by
TB )
KVeff
25kB
(4)
Combining eqs 3 and 4, we have
(
TB(t) ) TB(0) 1 -
2t
Dav
)
3
(5)
where TB(0) is the blocking temperature at t ) 0.
Recently, Rosa et al.41 measured the value of t for γ-Fe2O3
NPs embedded in an amorphous SiO2 matrix using a Faraday
rotation technique. They found that t was 1.25 ( 0.07 nm and
was almost unchanged for all γ-Fe2O3 NPs with the average
diameter in the range of 6.2 nm e Dav e 21.8 nm. For our
γ-Fe2O3 NPs, Dav (12.4 nm) falls into this range, and thus, t )
1.25 nm can be used for estimating TB. Using eq 5 and the
values TB(0) ) 160 K, Dav ) 12.4 nm, and t ) 1.25 nm, TB for
the Au/SiO2 coated γ-Fe2O3 NPs is estimated to be 81 K, which
is in excellent agreement with the experimental value of 80 K.
Figure 7 shows the magnetic hysteresis M(H) loops of both
the uncoated and the coated γ-Fe2O3 NPs, measured in fields
up to 50 kOe and at temperatures from 5 to 300 K. In Figure 8,
we show these hysteresis loops in the zoomed region between
H ) -2 and 2 kOe to demonstrate more clearly the irreversibility in this region. For fields greater than 2 kOe, all the M(H)
hysteresis loops for both the uncoated and the coated Fe2O3
particles are reversible at all temperatures. For fields less than
2 kOe, the hysteresis loops of the uncoated particles are
completely reversible or superparamagnetic only in the temperature range T g 100 K. For the coated particles, the
irreversibility is seen in Figure 8 at all temperatures. Even at
300 K, there is a very small but noticeable irreversibility within
(50 Oe. Thus, it seems that the Au/SiO2 coating extends the
irreversibility to higher temperatures. Since the irreversibility
of the hysteresis loop for the coated particle is extremely small
in the temperature range 100 K e T e 300 K, we can say that
the Au/SiO2 coated γ-Fe2O3 NPs are almost superparamagnetic
in this temperature range. Another effect of Au/SiO2 coating
on the M(H) loops is the decrease of Hirr, which is defined as
the field at which the irreversibility occurs. For example, from
Figure 8, we see that at 5 K, Hirr is about 1 kOe for the Au/
SiO2 coated γ-Fe2O3 NPs but about 2 kOe for the uncoated
γ-Fe2O3.
From Figure 7, we see that the saturation moment per gram,
Msat, is about 73 emu/g at 5 K and 50 kOe for the uncoated
γ-Fe2O3 NPs, corresponding to 2.07 µB per formula unit (f.u.)
or 1.03 µB/Fe3+. This value is about 83% of the resultant
moment (1.25 µB/Fe3+) of the bulk ferrimagnetic maghemite
(γ-Fe2O3).42 With the increase of temperature from 5 to 300 K,
Msat decreases monotonically from 73 to 61 emu/g. This range
in the values of Msat is comparable to that reported by some
other groups for their uncoated γ-Fe2O3 NPs of similar
sizes.36,38,43 For Au/SiO2 coated γ-Fe2O3 NPs, the Msat value at
300 K and 50 kOe is about 5.6 emu/g, which is about 9.2% of
the Msat value (61 emu/g) for the uncoated γ-Fe2O3 NPs. This
decrease in Msat is due to the increase of the mass per γ-Fe2O3
NP by coating Au/SiO2 on the γ-Fe2O3 NPs. Even though the
Msat (measured in emu/g) value is decreased substantially by
Au/SiO2 coating, the low amount of the maghemite phase in
the gold and silica doubly coated sample is still sufficient for
the T2-weighted MR imaging application, as demonstrated by
our previous research.26 A simple calculation using the values
of the mass density and the measured Dav for the γ-Fe2O3 core,
Au particles, and SiO2 matrix can confirm that such a decrease
of Msat requires an average mass density of 0.2 g/cm3 for the
Au/SiO2 nanoshells, which corresponds to a 99.53% volume
fraction of SiO2 and 0.47% volume fraction of Au with
0.11 g/cm3 for SiO2 and 19.3 g/cm3 for Au.
It is well-known that the coercivity, HC, for superparamagnetic systems varies with temperature according to the wellknown expression12,33
()
HC
T
)1HC0
TB
1/2
(6)
where HC0 is the coercivity at 0 K. Thus, in Figure 9, we show
the HC versus T1/2 plots below the blocking temperature TB for
both the coated and the uncoated γ-Fe2O3 NPs, with the values
Structural/Magnetic Properties of SPIO Au Nanoshells
of HC obtained from the hysteresis loops shown in Figure 8.
The data in Figure 9 show that the T1/ 2 dependence of HC is
slightly deviated from linearity; such a deviation from linearity
has been previously observed for SiO2 coated γ-Fe2O3 NPs.12
A least-squares fit (shown by the straight lines in Figure 8) to
the data by eq 6 yields HC0 ) 239 Oe and TB ) 139.3 K for
the uncoated γ-Fe2O3 NPs and HC0 ) 263 Oe and TB )
113.4 K for the Au/SiO2 coated γ-Fe2O3 NPs. This fitting result
shows that the Au/SiO2 coating increases HC0 but decreases TB.
The decrease of TB by coating is consistent with the TB results
obtained from the ZFC M(T) curves in Figure 6. However, the
data points in Figure 9 indicate that HC is increased by coating
only at low temperatures (i.e., below 50 K). Above 50 K, the
values of HC for the coated NPs are actually slightly smaller
than that for the uncoated NPs.
4. Conclusion
In summary, we have carried out the first extensive structural
and magnetic characterization of uncoated and Au/SiO2 doubly
coated γ-Fe2O3 NPs by XRD, TEM, EDS, XAS, and magnetization measurements. The XRD and XAS results confirm that
the iron oxide NPs before and after coating have the phase of
γ-Fe2O3. However, it appears that the coating of Au particles
on silica could induce small crystallization of silica even at room
temperature. The TEM measurement shows that the gold
nannoparticles are dispersed in the outer layer of the silica
spheres and that the average diameters of the γ-Fe2O3 cores,
Au/SiO2 doubly coated Fe2O3 NPs, and Au NPs are about 12,
82, and 6 nm, respectively. The magnetization results indicate
that coating of γ-Fe2O3 by Au/SiO2 decreases the blocking
temperature TB from 160 to 80 K and that such a decrease of
TB can be well-explained by the spin disorder and reduction of
the effective volume of the γ-Fe2O3 core. Thus, this study
provides new evidence that silica coating can induce spin
disorder in the outer layer of SPIO particles. The hysteresis loops
indicate that for T g 100 K, the uncoated γ-Fe2O3 NPs are
superparamagnetic and the Au/SiO2 coated γ-Fe2O3 NPs are
almost superparamagnetic. The coercivity HC for both the
uncoated and the coated nanoparticles decreases almost linearly
with T1/2 with the former decreasing faster than the latter. The
linear fitting of the HC versus T1/2 data points confirms that TB
is decreased by Au/SiO2 coating. All of these new results
provide us with a better understanding of the structural and
magnetic properties for gold and silica doubly coated SPIO NPs
and thus are very valuable for evaluating the future applications
of this class of multifunctional NPs in biomedicine.
Acknowledgment. The work at Sam Houston State University was supported by the National Science Foundation under
Grant CHE-0718482, a grant from the SHSU EGR program,
and an award from the Research Corporation. The work at
University of Texas at Austin was supported by National Science
Foundation under Grant DMR-0605828 and the Welch Foundation under Grant F-1191. The work at the U.T.M.D. Anderson
Cancer Center was supported by the National Cancer Institute
under Grant R01 CA119387 and by the John S. Dunn Foundation.
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