Laser-assisted synthesis of superparamagnetic Fe@Au core-shell nanoparticles

Article abstract of DOI:10.1021/jp0560967

A novel method combining wet chemistry for synthesis of an Fe core, 532 nm laser irradiation of Fe nanoparticles and Au powder in liquid medium for deposition of an Au shell, and sequential magnetic extraction/ acid washing for purification has been developed to fabricate oxidation-resistant Fe@Au magnetic core-shell nanoparticles. The nanoparticles have been extensively characterized at various stages during and up to several months after completion of the synthesis by a suite of electron microscopy techniques (HRTEM, HAADF STEM, EDX), X-ray diffraction (XRD), UV-vis spectroscopy, inductively coupled plasma atomic emission spectroscopy, and magnetometry. The surface plasmon resonance of the Fe@Au nanoparticles is red shifted and much broadened as compared with that of pure colloidal nano-gold, which is explained to be predominantly a shell-thickness effect. The Au shell consists of partially fused ～3-nm-diameter fee Au nanoparticles (lattice interplanar distance, d = 2.36 A). The 18-nm-diameter magnetic core is bcc Fe single domain (d = 2.03 A). The nanoparticles are superparamagnetic at room temperature (300 K) with a blocking temperature, T_b, of ≈170 K. After 4 months of shelf storage in normal laboratory conditions, their mass magnetization per Fe content was measured to be 210 emu/g, ～96% of the Fe bulk value.

Full text of DOI:10.1021/jp0560967

7122
J. Phys. Chem. B 2006, 110, 7122-7128
Laser-Assisted Synthesis of Superparamagnetic Fe@Au Core-Shell Nanoparticles
Jin Zhang,†,‡ Michael Post,† Teodor Veres,§ Zygmunt J. Jakubek,*,‡ Jingwen Guan,‡
Dashan Wang,† Francois Normandin,§ Yves Deslandes,† and Benoit Simard‡
Institute for Chemical Process and EnVironmental Technology, National Research Council of Canada,
Ottawa, Ontario, Canada; Steacie Institute for Molecular Sciences, National Research Council of Canada,
Ottawa, Ontario, Canada; and Industrial Materials Institute, National Research Council of Canada,
BoucherVille, Que´bec, Canada
ReceiVed: October 24, 2005; In Final Form: February 14, 2006
A novel method combining wet chemistry for synthesis of an Fe core, 532 nm laser irradiation of Fe
nanoparticles and Au powder in liquid medium for deposition of an Au shell, and sequential magnetic extraction/
acid washing for purification has been developed to fabricate oxidation-resistant Fe@Au magnetic core-
shell nanoparticles. The nanoparticles have been extensively characterized at various stages during and up to
several months after completion of the synthesis by a suite of electron microscopy techniques (HRTEM,
HAADF STEM, EDX), X-ray diffraction (XRD), UV-vis spectroscopy, inductively coupled plasma atomic
emission spectroscopy, and magnetometry. The surface plasmon resonance of the Fe@Au nanoparticles is
red shifted and much broadened as compared with that of pure colloidal nano-gold, which is explained to be
predominantly a shell-thickness effect. The Au shell consists of partially fused ∼3-nm-diameter fcc Au
nanoparticles (lattice interplanar distance, d ) 2.36 Å). The 18-nm-diameter magnetic core is bcc Fe single
domain (d ) 2.03 Å). The nanoparticles are superparamagnetic at room temperature (300 K) with a blocking
temperature, Tb, of ≈170 K. After 4 months of shelf storage in normal laboratory conditions, their mass
magnetization per Fe content was measured to be 210 emu/g, ∼96% of the Fe bulk value.
Introduction
The main objective of the present project was to produce
oxidation-resistant Fe@Au nanoparticles employing a laser
ablation method. Laser ablation of solvent-suspended metal
powders has previously been used in our group for synthesis
of Au-Ag alloy nanoparticles of predetermined composition.16
Two general routes for synthesis of the Fe@Au nanoparticles
have been explored. The first one consists of making both the
Fe core and the Au shell using the laser ablation method. The
second one, which is reported here, consists of preparing the
Fe core by wet chemistry and subsequently coating the core
with Au by laser ablation of Au powder. Magnetic extraction
and acid washing of nanoparticles constitute the third step in
both routes. The main advantages of the second route are a
higher overall yield and better control of the size of the magnetic
core.
The recent surge in research on core-shell nanoparticles is
driven by their multiple potential applications as well as interest
in their fundamental properties. Shell addition can enhance
stability and dispersibility of naked core particles and modify
or enhance their properties.1,2 To date, gold has been the
preferred coating material because of its well-known optical
properties and chemical functionability.3,4
Magnetic-core gold-shell nanoparticles are of particular
importance because of their potential biodiagnostic applications,
such as rapid magnetic separation and concentration of bioma-
terials.5 Recently, gold-coated Fe3O4 magnetic core nanoparticles
(MNP) have been synthesized through wet chemical methods.6
However, pure Fe core nanoparticles are expected to be superior
for such applications because of the high saturation magnetiza-
tion of iron, Ms ) 218 emu/g at 300 K, which is a factor of >2
larger than that of Fe3O4 (93 emu/g). Unprotected Fe nanopar-
ticles are highly reactive in aqueous environment; thus, for
biomedical applications they have to be coated with a protective
layer such as a Au shell. Unfortunately, successful preparation
of Fe@Au nanoparticles has been an enduring scientific
challenge. There have been several reports of the synthesis of
gold/iron/gold and iron/gold (Fe@Au) MNPs.7-9 However,
detailed characterization of such nanoparticles has indicated
rather fast oxidation of the Fe core.10-14 Recently, Ban et al.
reported15 the synthesis of Fe@Au nanoparticles by partial
replacement reaction. Although the authors measured high mass
magnetization per pure Fe content of their nanoparticles (170
emu/g), long-term oxidation resistance of the Fe core was not
addressed.
Experimental Section
Homogeneous Nucleation Synthesis of Fe Nanoparticles.
Monodispersed Fe nanoparticles were prepared under nitrogen
atmosphere via thermal decomposition of iron pentacarbonyl,
Fe(CO)5.17 In a typical homogeneous nucleation synthesis of
∼8-nm-diameter particles, 2.28 g of oleic acid (OA) was stirred
in 20 mL of octyl ether, the solution was heated to 40 °C, and
0.3 mL of Fe(CO)5 was injected in a 1:3 molar ratio to the OA.
Following the injection, the reaction mixture became orange-
yellow. After 30 min of refluxing, the mixture became black.
The mixture was stirred for an additional 2 h under reflux and
then cooled to room temperature. The particles were precipitated
by addition of a large amount of ethanol and subsequent
centrifugation. The precipitate was washed with ethanol or
methanol. The solid residue was then extracted with 45 mL of
hexane to form a black homogeneous solution.
† Institute for Chemical Process and Environmental Technology.
‡ Steacie Institute for Molecular Sciences.
§ Industrial Materials Institute.
Au Coating Process. In the coating process, 0.067 g of Fe
nanoparticles in 25 mL of hexane was added to a three-neck
10.1021/jp0560967 CCC: $33.50 Published 2006 by the American Chemical Society
Published on Web 03/23/2006

Superparamagnetic Fe@Au Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 14, 2006 7123
Figure 1. Schematic diagram of the experimental process for fabrication of Fe@Au magnetic core-shell nanoparticles.
coating and purification steps of the procedure are illustrated
round-bottom flask containing 0.30 g of fine Au powder
(Nanophase, 80-nm nominal diameter), 25 mL of water, and
15 mL of octane. Cationic surfactant cetyltrimethylammonium
bromide (CTAB) was used as a stabilizing agent in aqueous
phase (0.02 M). As a cosurfactant, 10 mL of 1-butanol was
used. Nitrogen gas was flowed over the mixture to provide an
inert nonreactive atmosphere. The mixture was vigorously stirred
and irradiated with a pulsed 532-nm laser beam (Coherent,
Infinity; 20 Hz, 65 mJ/pulse). The laser beam was unfocused
and had a diameter of ∼3.5 mm at the surface of the mixture.
Gold nanoparticles, with surface plasmon resonance (SPR)
located in the visible, absorbed 532-nm radiation more ef-
ficiently than the Fe NPs,18 which do not have SPR in the
visible. During a single laser pulse, gold particles consecutively
absorbed several photons, which led to their intense heating and
subsequent decomposition.19,20 Because the melting temperature
of a Au NP decreases with decreasing size, the decomposition
process intensified during the irradiation. Nanoparticle charging
followed by Coulomb disintegration might also have played a
role.21 Au atoms, clusters, and nanoparticles condensed on Fe
NPs, forming shells. Further interaction with the laser beam
resulted in fusion of the shell particles and formation of a
complete Au shell protecting the Fe core from oxidation. During
the coating process the Fe nanoparticles were protected from
oxidation by free radicals created by photodecomposition of a
solvent and a cosurfactant (e.g., propanol or 1-butanol).22 After
2 h of laser irradiation, the mixture was centrifuged for 10-15
min to separate the oil phase from the aqueous solution.
in Figure 1.
Characterization. The NPs were thoroughly characterized
at various stages of the project, during and up to several months
after completion of the synthesis, by a suite of transmission
electron microscopy (TEM) techniques (HRTEM, HAADF
STEM, EDX), X-ray diffractometry (XRD), UV-vis spectro-
scopy, inductively coupled plasma atomic emission spectroscopy
(ICP-AES), and magnetometry. Bright field TEM (Philips,
CM20) was employed to determine the size distribution of the
nanoparticles. To avoid the nonmonotonic contrast, such as that
generated by diffraction or Fresnel fringes, Z-contrast imaging,
generated by high-angle annular dark field (HAADF) scanning
TEM (STEM), was used for investigation of structure and
morphology of the Fe@Au core-shell NPs. Details of the core-
shell structure were also studied by high-resolution TEM
(HRTEM). XRD of the Fe@Au core-shell NPs dried in a
vacuum was also performed. A Bruker D8 Advance X-ray
diffractometer using Cu KR radiation (λ ) 1.54056 Å) was
scanned in the 25° e 2θ e 100° range with a step size of 0.02°.
Atomic composition of nanoparticle samples was analyzed by
global and single-particle EDX spectroscopy and ICP-AES. The
optical response of NP solutions was monitored by UV-vis
spectroscopy (Varian, Cary 3). Functionalization of the Au shell
was investigated by FT-Raman spectroscopy. Finally, magne-
tization of the nanoparticles was measured with a Quantum
Design Physical Property Measurement System (PPMS) using
the extraction method with sensitivities of 2 × 10-5 and 1 ×
10-8 emu for DC and AC magnetization, respectively. For the
Fe@Au core-shell nanoparticles, DC magnetization was mea-
sured at 10 and 300 K; also, ZFC and FC curves were recorded
at 50 Oe in the 2-300 K temperature range. In the measure-
ments, a sample of dried Fe@Au nanoparticles (6.08 mg) was
immobilized in a plastic capsule with zero magnetic contribution.
Extraction and Purification of Fe@Au Nanoparticles.
Following the laser irradiation, the mixture contained pure Au
NPs, uncoated or partially Au-coated Fe NPs, and fully Au-
coated Fe NPs (Fe@Au core-shell NPs). The Fe-containing
nanoparticles were extracted with a rare earth magnet, redis-
persed in water, magnetically extracted again, and washed with
a methanol/chloroform solution (1:1 volume ratio) to remove
CTAB surfactant. Next, the nanoparticles were acid washed
(HCl, 11.9%) to remove the uncoated and incompletely Au-
coated Fe nanoparticles. The magnetic extraction/acid washing
procedure was repeated until the acid wash contained no iron.
Finally, the acid-resistant Fe@Au NPs were magnetically
extracted and redispersed in water/CTAB solution. The Au
Results and Discussion
Thorough characterization of the nanoparticles synthesized
in this project has shown that upon completion of the three-
step procedure presented above, oxidation-resistant Fe@Au
core-shell magnetic nanoparticles have been successfully
synthesized.

7124 J. Phys. Chem. B, Vol. 110, No. 14, 2006
Zhang et al.
Figure 2. TEM micrograph and corresponding diameter distribution
histogram of Fe nanoparticles produced by thermodecomposition.
The magnetic Fe core of the Fe@Au nanoparticles is a single
domain; its average size is significantly larger (see below) than
that of the original Fe nanoparticles produced through thermal
decomposition of Fe(CO)5, which was determined by electron
microscopy to be equal to 7.8 nm (Figure 2). Panels a and b of
Figure 3 show TEM micrographs and corresponding particle
diameter histograms for the magnetically extracted Fe-containing
nanoparticles before and after acid treatment, respectively.
Because Fe has a lower electron density than Au, the dark
contrast in the magnified TEM image in Figure 3a (inset)
corresponds to gold, whereas the bright one to iron. The
magnified image in Figure 3a (inset) shows that the gold shell
consists of incompletely fused nanoparticles. The average
particle size is about 10.9 nm before (Figure 3a) and 22.6 nm
after acid washing (Figure 3b), which indicates that nanoparticles
with the Au shell thicker than 3-4 nm are more likely to survive
the acid treatment.
Figure 3. Bright field TEM micrographs and corresponding diameter
distribution histograms of Fe-containing nanoparticles (a) before and
(b) after acid treatment.
The structure of the Fe@Au nanoparticles was further studied
by HRTEM (Figure 4). The image in Figure 4a was recorded
with the electron beam focused at the surface of a particle.
Multidomain structure with a regular lattice interplanar distance
(d ) 2.36 Å), which corresponds to fcc Au (111), was observed.
The size of Au crystallites ranged from ∼2 to 4 nm. With the
electron beam focused inside the particle, a ∼18-nm-diameter
crystallite with the lattice interplanar distance of d ) 2.03 Å
corresponding to bcc Fe (100) was observed (Figure 4b). By
focusing the e-beam at the bottom of the nanoparticle, multiple
domains with a size and a lattice interplanar distance similar to
those at the top surface were observed, which indicated that
the bottom part of the core-shell NPs was also made of gold
crystallites. Analyzing HRTEM micrographs, we estimated the
average Fe core diameter and the Au shell thickness to be equal
to ∼18 and ∼4 nm, respectively. More precise evaluation of
the size distributions was done using HAADF STEM micro-
graphs (Figure 5). Electrons collected with an HAADF detector
were not Bragg scattered; hence, HAADF images showed little
or no diffraction effects. The size of the shell-building Au
particles and the average total diameter of the core-shell
nanoparticles were estimated to be ∼3-4 and 22.6 nm,
respectively, in good agreement with the HRTEM estimates.
The average diameter of the Fe core in Fe@Au, equal to ∼18
nm as estimated by HRTEM or ∼18.6 nm as estimated by
HAADF, is significantly larger than the initial average size of
the Fe nanoparticles (7.8 nm). A mechanism of the Fe core
growth during the laser irradiation is likely similar to that of
the Au shell formation outlined above. Despite weak absorption
of Fe nanoparticles in the visible, prolonged 532-nm laser
irradiation leads to their heating and, consequently, decomposi-
tion. The Fe atoms and small clusters condense on a surface of
larger particles. Coalescence of stabilizing agent decorated metal
nanoparticles has been demonstrated for thiolate-stabilized nano-
gold.23 The photodecomposition is more efficient for the small
particles, because of a melting temperature depression with
decreasing nanoparticle diameter; this leads to a shift of
nanoparticle size distribution toward larger diameters. The
melting temperature of an 8-nm-diameter Fe particle was
predicted to be as low as 570 °C, down from 1538 °C for bulk
iron.24
The presence of a highly crystalline Au shell in our Fe@Au
nanoparticles was confirmed by observation of a well-developed
fcc Au XRD pattern (Figure 6). It should be noted that X-ray
diffraction is inconclusive with respect to determination of the
structure and chemical composition of an Fe-containing core
coated by a heavy atom (Au) shell, because the shell precludes
observation of diffraction peaks corresponding to either pure
iron or iron oxide core.25
Pure colloidal nano-Fe in aqueous media exhibits only a weak
continuous absorption spectrum in the visible range,18 whereas
Au NPs show a size-dependent absorption due to strong SPR.
Thus, the color of the colloidal solutions studied in this project

Superparamagnetic Fe@Au Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 14, 2006 7125
Figure 6. XRD pattern of Fe@Au nanoparticles.
Figure 4. HRTEM micrographs of Fe@Au core-shell nanoparticles
recorded with the focal plane of the objective lens set inside (a) the
shell and (b) the core of the nanoparticle.
Figure 7. Colloidal solutions and their corresponding UV-vis
absorption spectra at various stages of the Fe@Au production process:
(a) after laser-assisted Au coating of Fe nanoparticles; (b) residual
solution after magnetic confinement of Fe-containing nanoparticles,
which are visible as a dark cluster near the bottom of the sample bottle;
(c) solution of redispersed Fe-containing nanoparticles prior to acid
washing; (d) magnetically extracted Fe@Au nanoparticles redispersed
in H2O with CTAB.
Figure 5. HAADF micrograph of Fe@Au nanoparticles.
was predominantly determined by optical response of the gold
fraction. Following the laser irradiation and prior to the magnetic
extraction of the Fe-containing nanoparticles, the solution
appeared ruby red and the SPR absorption peak was observed
at 528 nm (Figure 7a). The residual solution after magnetic
confinement of nanoparticles displayed red color. The optical
response of that solution with the SPR maximum observed at
516 nm was largely determined by pure Au nanoparticles (Figure
7b). The solution containing magnetically extracted and redis-
persed nanoparticles prior to acid treatment was dark red; the
SPR band was significantly broader, as compared with the two
solutions discussed above, and red shifted to ∼546 nm (Figure
7c). Finally, the solution containing the Fe@Au nanoparticles,
magnetically extracted following acid washing and redispersed
in H2O with 0.02 M CTAB, showed purple color with the SPR
band even broader and shifted further to the red (Figure 7d) as
compared with the solution shown in Figure 7c.
The spectrum of the Fe@Au colloid has been modeled using
complex dielectric functions of bulk Fe26 and Au27 and the
geometric parameters of the nanoparticles determined by
electron microscopy. Details of the method have previously been
described by Mulvaney.28
The core and the shell dielectric constant functions were
approximated from the bulk functions by scaling the bulk
damping frequency, Γ, by Rb/Re factor, where Rb and Re are
the bulk and effective mean free paths, respectively. This

7126 J. Phys. Chem. B, Vol. 110, No. 14, 2006
Zhang et al.
Figure 10. Hysteresis loops of Fe@Au core-shell nanoparticles at
10 K (dotted lines, O) and 300 K (solid line, b) measured 4 months
after synthesis of the nanoparticles.
Figure 8. UV-vis absorption spectrum of Fe@Au core-shell nano-
particles in water with CTAB observed (solid line) and calculated using
bulk dielectric constant functions of Fe and Au and geometric
parameters determined by TEM (dotted line).
Boltzman constant. Using the value of the anisotropy constant
determined at liquid-He temperatures for pure Fe, K ) 5.4 ×
104 J m-3, the average diameter of 12.6 nm is obtained, which
is significantly smaller than the HRTEM estimate of 18 nm.
However, it should be pointed out that at 170 K the Fe
anisotropy constant, assuming a temperature dependence similar
to that of magnetite,30 could be as much as 50% smaller, leading,
in turn, to the noticeably larger core diameter estimate of 15.2
nm.
Figure 10 shows the 10 and 300 K hysteresis loops recorded
for the Fe@Au nanoparticles. Prior to the magnetic measure-
ments the sample was shelf stored in normal laboratory
conditions for 4 months. The room temperature (300 K)
saturation magnetization, Ms, was estimated by extrapolation
to be 0.3 emu. With the Fe mass fraction in the Fe@Au sample
measured by ICP-AES to be 23.5% and the total mass of the
sample to be 6.08 mg, the Fe@Au saturation magnetization per
pure Fe is calculated as 210 emu/g, which is only slightly smaller
than the bulk Fe value of 218 emu/g. The Fe mass fraction was
alternatively estimated at 29.4% using the average sizes of the
Fe core and the Fe@Au nanoparticle equal to 18 and 22.6 nm,
as discussed above, and bulk densities of Au and Fe equal to
19300 and 7874 kg/m3, respectively, yielding the saturation
magnetization per Fe content equal to 179 emu/g. Given the
approximate nature of the second calculation (average particle
sizes used, residual content of CTAB in the dry sample
neglected, etc.), the agreement of both estimates with the bulk
Fe value is very good. The magnetization measurements support
our claim of the core being highly crystalline Fe. The second
saturation magnetization estimate, the smaller and less precise
of the two, is still almost twice as large as the saturation
magnetization of Fe3O4. Cho et al.14 determined, using the
average values of the core and the shell sizes, the saturation
magnetization per Fe contents of their sample to be 152 emu/g.
The somewhat smaller value as compared with our estimates
may result form the partial Fe core oxidation, which is evident
from the temperature dependence of resistance and Mo¨ssbauer
spectra of their sample. More recently, Cho et al.12 reported a
saturation magnetization per Fe content of Fe@Au nanoparticles
equal to 162 emu/g with the Fe fraction determined by ICP-
AES. Their sample had a poor oxidation resistance, which
resulted in the saturation magnetization decreasing to 50% of
the initial value after just 5 days of storage. Most recently, Ban
et al.15 reported the synthesis of Fe@Au nanoparticles with
saturation magnetization per Fe content equal to 170 emu/g,
which suggests a high content of pure Fe in the core. The authors
Figure 9. ZFC and FC curves of Fe@Au core-shell nanoparticles
measured 4 months after synthesis of the nanoparticles.
approach takes into account nanoparticle size effects, but
neglects all other effects.
The spectrum was initially simulated in the 400-700 nm
range for typical acid-treated Fe@Au nanoparticles (see Figures
5b and 7) with the shell diameter Rs ) 22 nm and the core
diameter Rc ) 18 nm immersed in water (ꢀm ) 1.77) using the
scaled complex dielectric constant functions and taking Rb )
50 nm for the bulk electron mean free path for both Fe and Au.
Taking into account the fact that the observed spectrum was
heterogeneously broadened due to the distribution of nanopar-
ticle core and shell sizes in the sample, the agreement between
the observed and the simulated spectrum was remarkably good.
With just slightly adjusted (effective) values of the simulation
parameters, Rs ) 22.8 nm, Rc ) 18.4 nm, and Rb ) 52 nm,
well within the acceptable experimental range, the simulated
spectrum was brought into near perfect agreement with the
observed one (see Figure 8).
Zero-field-cooled (ZFC) and field-cooled (FC) magnetization
curves of the Fe@Au nanoparticles are shown in Figure 9. The
field-dependent blocking temperature, Tb ) 170 K, was
estimated as the average value of the temperatures corresponding
to the onset of irreversibility between the FC and ZFC curves
(225 K) and the maximum of the ZFC curve (115 K). Assuming
that the energy barriers are correctly described by the anisotropy
energy KV, where K is the total anisotropy constant of the
particles and V is the volume of the magnetic core, the average
diameter (D) of the magnetic core can be estimated29 from the
KV = 25kBTb relationship with V ) πD3/6, where kB is the

Superparamagnetic Fe@Au Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 14, 2006 7127
Figure 11. FT-Raman spectrum of Fe@Au-S(CH2)11OH in CH2Cl2
(red trace), HS(CH2)11OH in CH2Cl2 (blue trace), and pure CH2Cl2
(black trace). Downward-pointing arrows indicate SH stretch peaks.
Figure 13. EDX spectra of a single Fe@Au nanoparticle with the
electron beam traversing the particle (a) at the center through both the
shell and the core and (b) away from the center through the shell only.
spectrum of Fe@Au nanoparticles (a). Figure 13 shows EDX
spectra of an isolated Fe@Au nanoparticle recorded with the
electron beam traversing the particle (a) through the center, and
thus through both the shell and the core, and (b) away from the
center through the shell only. In addition to the C and Cu peaks
originating from the sample support, trace b shows only Au
peaks, whereas trace a shows both Au and Fe peaks, as expected
for a core/shell nanoparticle. The reader should note that no
oxygen peak is present in either trace.
The Fe@Au nanoparticles synthesized in this project are
stabilized with the cationic surfactant CTAB. Various applica-
tions of the Fe@Au magnetic nanoparticles may require use of
different surfactants or surface functionalization. Due to the
strong chemical affinity of Au and -SH, CTAB can be readily
displaced by dodecanethiol, N-mercaptoundecanoic acid (MUA),
and similar derivatives. To test the Au shell functionability, the
Fe@Au NPs were redispersed in dichloromethane (CH2Cl2) and
capped with 11-mercapto-1-undecanol (HSC11H22OH). It has
been reported that 11-mercaptoundecanate immobilized on the
surface of Au nanoparticles was used as a template for peptide
elongation.31 The S-H and C-S stretches were observed by
FT-Raman spectroscopy for pure 11-mercapto-1-undecanol at
1380 and 2700 cm-1, respectively (Figure 11, middle trace).
Both stretches were, however, absent from the spectra when
11-mercapto-1-undecanol was added to the solution of Fe@Au
in CH2Cl2, indicating attachment of the 11-mercapto-1-unde-
canol to the Au surface (Figure 11, top trace).
Figure 12. EDX spectra of (a) Fe@Au and (b) Fe nanoparticles stored
in normal laboratory conditions for 2 weeks.
claimed that no oxygen was present in the core, but some of
their arguments were unconvincing. As mentioned above, XRD
is inconclusive with respect to the core structure and chemical
composition when a heavy atom (Au) shell is present. Also, in
the EDX spectrum they omitted the energies below 1 keV, where
the oxygen peak would appear if oxygen was present in the
core. It should also be pointed out that they did not discuss
long-term oxidation resistance of their nanoparticles.
At room temperature (300 K) our Fe@Au nanoparticles are
superparamagnetic. The coercivity, Hc, is approximately 12 Oe,
and the remanent magnetization, Mr, is 0.0019 emu. As expected
for the superparamagnetic material, the Mr/Ms ratio is very small,
0.006. At 10 K, coercivity equals 200 Oe. For comparison, Cho
et al.14 measured for their Fe@Au nanoparticles coercivity equal
to 240 Oe at 5 K. A close examination of the 10 K curve reveals
a slope change with the field approaching zero value, which
can be attributed to the presence of a small fraction of
agglomerated and interacting nanoparticles in the sample.
Excellent stability of our Fe@Au nanoparticles against core
oxidation was verified by EDX spectroscopy. Figure 12 shows
EDX spectra of (a) Fe@Au and (b) unprotected Fe nanoparticles
after 2 weeks of shelf storage in normal laboratory conditions.
Although a strong oxygen peak is present in the spectrum of
unprotected Fe nanoparticles (b), no oxygen is observed in the
Conclusions
A novel method for the synthesis of oxidation-resistant
Fe@Au core-shell magnetic nanoparticles has been developed.
It combines wet chemistry for the Fe core synthesis with laser
irradiation for the Au shell formation and magnetic extraction/
acid washing for sample purification. The effect of irradiation
is complex, but it most likely involves the fragmentation of Au
particles into small clusters, which through repeated heating and
melting tightly coat the iron core. Magnetic extraction separates
from the laser-irradiated mixture Fe-containing nanoparticles,
whereas acid washing removes those nanoparticles that are
uncoated or incompletely Au-coated. The purified product
contains Fe@Au core-shell nanoparticles; the nanoparticles are
superparamagnetic at room temperature (300 K) and exhibit an

7128 J. Phys. Chem. B, Vol. 110, No. 14, 2006
Zhang et al.
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excellent long-term oxidation resistance. After 4 months of shelf
storage in normal laboratory conditions, they showed high
saturation magnetization per Fe content, 210 emu/g, which is
comparable (96%) to that of pure bulk iron. The high saturation
magnetization, as compared with iron oxide and other magnetic
material containing nanoparticles, makes the Fe@Au structure
very attractive for theranostic applications where, in addition
to surface functionability, efficient and rapid magnetic manipu-
lation is required. The Fe@Au nanoparticles can be prepared
in either aqueous or organic medium, and well-known gold
chemistry can be applied readily.
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Acknowledgment. We thank Prof. G. A. Botton and N.
Braidy from McMaster University for HAADF STEM measure-
ments. J.Z. thanks the National Research Council and the Natural
Sciences and Engineering Research Council (NSERC) for the
support of a NSERC visiting fellowship.
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