Controlled synthesis and stability of Co@SiO2 aqueous colloids

Article abstract of DOI:10.1111/j.1551-2916.2007.01509.x

Magnetic nanoparticles have emerged as an important class of functional nanostructures with potential applications of magnetic resonance imaging, drug targeting, and bio-conjugation. We have developed a modified sol-gel approach to synthesize stable and well-dispersed magnetic Co@SiO₂ nanoparticles with improved control over shell thickness and larger core diameters. These well-defined Co@SiO₂ core-shell nanoparticles exhibit useful magnetic properties, and the protective silica shell allows them to be surface modified for bioconjugation for various biomedical applications. The core-shell nanoparticles were characterized by transmission electron microscopy, energy-dispersive spectroscopy, elemental mapping, and the line compositional analyses to demonstrate that uniform individually isolated coreshell nanoparticles are obtained through the improved synthetic route.

Full text of DOI:10.1111/j.1551-2916.2007.01509.x

J. Am. Ceram. Soc., 90 [3] 950–956 (2007)
DOI: 10.1111/j.1551-2916.2007.01509.x
r 2007 The American Ceramic Society
ournal
J
Controlled Synthesis and Stability of Co@SiO Aqueous Colloids
2
M. Aslam, S. Li, and V. P. Dravidw
Department of Materials Science and Engineering, International Institute for Nanotechnology, Northwestern
University, Evanston, Illinois 60208
Magnetic nanoparticles have emerged as an important class of
functional nanostructures with potential applications of magnet-
ic resonance imaging, drug targeting, and bio-conjugation. We
have developed a modified sol–gel approach to synthesize stable
with photonic properties different from the one based on con-
ventional particles.
1
0
Various sol–gel-based precursor strategies have been used for
generating core–shell structures of iron oxide; however, reports
on core–shell structure formation for metallic particles like Co,
and well-dispersed magnetic Co@SiO nanoparticles with im-
2
6
,11,12
proved control over shell thickness and larger core diameters.
These well-defined Co@SiO core–shell nanoparticles exhibit
Fe, and Ni are scarce.
3 4
In the case of Fe O nanoparticles, a
dense and homogeneous silica layer could be directly grown
from tetraethoxysilane (TEOS)-based high-pH gelation, while in
other cases like Ag, Au, and Co, the surfaces of the particles are
first modified with a primer silane-based monolayer that acts as
a coupling agent for silica growth on the metal particle sur-
2
useful magnetic properties, and the protective silica shell allows
them to be surface modified for bioconjugation for various bio-
medical applications. The core–shell nanoparticles were charac-
terized by transmission electron microscopy, energy-dispersive
spectroscopy, elemental mapping, and the line compositional
analyses to demonstrate that uniform individually isolated core–
shell nanoparticles are obtained through the improved synthetic
route.
8
,13,14
face.
Both these methods result in a dense coating of ce-
ramic material on top of the core nanoparticles. This simple and
attractive approach has so far been applied to smaller particles.
In this study, we report a homogeneous coating of silica as
thick as 100 nm on core nanoparticles exceeding 70 nm in size.
We have also investigated whether a larger size offers con-
formal coverage, heterogeneous nucleation, and formation of
naked silica particles, which have been reported for micron size
I. Introduction
AGNETIC nanomaterials in their various forms offer simple
and versatile strategies for promising applications in bio-
1
5,16
particles.
M
1
–5
Herein, we report the successful synthesis of a series of strik-
ing core–shell nanoparticles with a control over core size as well
as shell thickness. We have used a citrate-based approach to
stabilize the Co particles and then applied the monolayer func-
tionalization chemistry of aminopropoxysilane (APS) mole-
cules, which reveal good linkage with Co layers, for further
growth of TEOS through slow hydrolysis. We report their mor-
phological and chemical identity using transmission electron
microscopy (TEM), energy-dispersive spectroscopy (EDS) and
magnetic properties as a function of various parameters. The
magnetic properties are directly correlated to the morphological
evolution at different temperatures. Moreover, the magnetic
properties show an increment in saturation magnetization coer-
civity, reaching a maximum value at 4001C and then decreasing
drastically during the annealing process.
medical applications.
In particular, core–shell magnetic na-
nostructures offer the interesting possibility of making use of the
much-improved properties of an arbitrary magnetic core, and a
protective coating that is amenable to surface modification for
numerous diagnostic and therapeutic purposes. The surface
modification of the shell not only provides a route for func-
tionalization but also enhances the stability and biocompatibil-
ity of the core particles. For example, bare metal/alloy
nanoparticles (e.g., Co, CoSm, FePt, etc.), which have poor sta-
bility under ambient conditions but have better magnetic prop-
erties compared with oxide magnetic particles, could be ideal
candidates in core–shell form for possible in vivo bio-applica-
tions. However, most of these high-quality metallic particles
(
narrow and uniform particle size distribution with standard
deviation r5%) are prepared under nonaqueous conditions,
and surface functionalization strategies to disperse them in
water are very critical, which invariably leads to a poor yield
3
,6,7
II. Experimental Procedure
and a limited time stability under ambient conditions.
In
principle, the properties could be tailored in a controlled fashion
by independently designing the composition, structure, and di-
mension of a biocompatible shell on top of the magnetic core. In
addition, the shell can function as a unique platform for many
bio-conjugation strategies and thus provide a multifunctional
colloidal delivery system. Apart from biomedical applications
of magnetic core–shell structures, the other core–shell structures
have also been explored as building blocks to build supracrystals
(
1) Materials
Cobalt(II) chloride hydrate (99.999%), sodium borohydride
99%), sodium citrate dehydrate (99%), 3-aminopropyltrieth-
(
oxysilane (99%), and TEOS (99%) were obtained from Sigma-
Aldrich (St. Louis, MO), and used as received. Ethyl alcohol
8
,9
(
200 proof) was obtained from Pharmco (Brookfield, CT), and
used as received. Deionized water was distilled by a Milli-Q
water purification system (Millipore, Billerica, MA). All glass-
ware and Teflon-coated magnetic stir bars were cleaned with
aqua regia, followed by copious rinsing with distilled water
before drying in an oven.
C. Randall—contributing editor
Manuscript No. 21947. Received June 27, 2006; approved November 13, 2006.
This work was supported by Grant No. U54CA119341 from the NIH National Cancer
Institute (NCI) under Center for Cancer Nanotechnology Excellence (CCNE) program at
Northwestern and partially supported by NSF-NSEC and NU-IBNAM. This work was
performed at the EPIC/NIFTI facility of the NUANCE center (supported by NSF-NSEC,
NSF-MRSEC, Keck Foundation, the State of Illinois, and Northwestern University) at
Northwestern University. The contents are solely the resoponsibility of the authors and do
not necessarily represent the official views of the NIH.
(2) Synthesis Procedure
(A) Co Nanoparticle Synthesis: Citrate-stabilized Co na-
noparticles were first prepared from the conventional NaBH4
reduction of CoCl . In a typical procedure, 02.6 mg of Co(II)
2
chloride hydrate and 6 mg of sodium citrate were added to 100
mL of deoxygenated water. Deoxygenation was achieved by
bubbling nitrogen through the solution in a vigorously stirred
w
Author to whom correspondence should be addressed. e-mail: v-dravid@northwestern.
edu
9
50

March 2007
Synthesis and Stability of Co@SiO Aqueous Colloids
2
951
1000-mL beaker. The resulting Co and citrate concentrations
were 0.2 mM, resulting in the required 1:1 ratio for [citrate]/[Co].
Nitrogen bubbling was continued for almost 1 h, and then a
(a)
4
multifold amount of NaBH was slowly added to the vigorously
stirred solution. The mixture slowly turned black, indicating the
formation of cobalt.
(
B) SiO Shell Formation on Co Nanoparticles: Follow-
x
ing the in situ synthesis of Co nanoparticle solution, a 7-mL APS
in 200-mL ethanol was added slowly to the black-colored solu-
tion after 5 min. The solution was then stirred for 30 min, and
42-mL TEOS in 200-mL ethanol was added to ensure complete
coating of the exposed nanoparticle surfaces. The slow hydrol-
ysis of TEOS aids the growth of the oxide layer on APS-
modified Co particles. The reaction was allowed to proceed
for 3 h before centrifuging to isolate the silica-coated particles.
The magnetic portion was then separated in 50–100 mL of
ethanol dispersion, magnetically decanted, and resuspended in
2
µm
1
1
0 mL of Milli-Q water. The APS:TEOS ratio was varied from
:2 to 1:7, with the [citrate]/[Co] ratio from 0.1 to 1.
(b)
(
C) Acid and Thermal Treatment: To check the stability
and composition of these core–shell structures under acute con-
ditions, the particles underwent thermal and chemical treat-
ments. Various molarity HCl acid baths were used to check the
stability of these particles at low pH. The Co@SiO nanopar-
2
ticles (1–5 mg/mL of H O) were dispersed in hydrochloric acidic
2
solutions at room temperature for 1 h. Products were centri-
fuged and rinsed with deionized water.
The stability of the cobalt core was also tested through a
sintering route at 2001–8001C. During the procedure, the Co@
2
SiO nanoparticles were placed in an alumina boat, which was
set in the center hot zone of an alumina tube furnace. The con-
tent was heated at 2001C for 4 h in air ambient under ambient
pressure. Then, the furnace was naturally air cooled to room
temperature under the protection of argon. The sample for
TEM observation was scraped from the alumina boat and dis-
persed in alcohol via the assistance of ultrasonication. A similar
calcination procedure was repeated at 4001, 6001, and 8001C,
respectively.
1
µm
Fig. 1. (a) Large-area scanning electron micrograph of Co@SiO core–
2
shell nanoparticles prepared with [citrate]/[Co] B0.2. The bright center
indicates metallic cobalt and the gray shell is semiconducting silica.
(b) Low-magnification transmission electron spectroscopy image of
Co@SiO particles deposited on a carbon-coated copper grid by sim-
ply drying a drop of solution.
2
(3) Characterization of the Samples
The size, shape, and structure of core–shell particles were meas-
ured by using a Hitachi HF2000 and Hitachi 8100 transmission
electron microscope (Pleasanton, CA) operated at 200 kV.
Chemical analyses were performed by an Oxford EDS system
particles was characterized by TEM. Figure 1 shows low-reso-
lution representative (a) SEM and (b) TEM images of Co@SiO
nanoparticles. The SEM image of the Co@SiO core–shell na-
2
(
Eynsham, U.K.) attached to the HF2000 machine, while se-
2
lected-area electron diffraction (SAED) patterns were taken
from the Hitachi 8100 instrument. TEM samples were prepared
by evaporating a dilute suspension on a carbon-coated grid.
noparticle reveals a bright metal cobalt core at the center and
gray silica shells around it. Nanoparticles in water were drop
casted on a substrate and slowly dried in air. Nearly 90%–95%
particles were individually isolated in a single silica shell, and the
spherical shape was maintained after silica coating. Low-mag-
Magnetic measurements of Co@SiO nanoparticles were per-
2
formed using a SQUID analyzer (Quantum Design, MPMS,
San Diego, CA). Magnetic hysteresis was measured at room
temperature for the samples annealed at temperatures of 2001–
2
nification TEM images of core–shell Co@SiO nanoparticles at
several spots were taken so as to demonstrate good-quality na-
noparticles revealing individually isolated cobalt cores with very
few dimers and trimers (two or three particles in a single shell).
Trapping of multiple particles in a single silica shell is due to
agglomeration of cobalt nanoparticles before or during the coat-
ing process. Typical distributions for nuclei per shell were on the
order of 90% monomers, 5% dimers, 4% trimers, and 1%
greater than trimers. A perfect monomer formation is generally
favored through decreased concentration of cobalt nuclei and
good long-time dispersion after citrate stabilization to ensure
that the core particles are well separated before coating begins.
The TEM analysis of the silica-coated initial particles (Figs.
8
001C. Apart from TEM and SEM, dynamic light scattering
(
DLS) particle size determination was performed on 3-mL sam-
ples removed from the solutions prepared similar to those de-
scribed in the experimental section. The samples were placed in
standard vials (B27.4 mm diameter) as the sample cells. Meas-
urements were conducted on a DLS system N4Plus, Beckman-
Coulter Corp (Fullerton, CA). DLS measurements gave an av-
erage size of 220 nm, confirming that Co@SiO nanoparticles
2
are well dispersed in water.
III. Results and Discussion
2(a)–(e)) suggests that the Co core can be tuned from 25 to 100
All nanoparticles were synthesized in the presence of sodium
citrate and reducing agent NaBH was added dropwise until we
4
nm by varying the citrate:Co ratio from 1 to 0.1. A prominent
effect of the APS:TEOS ratio is the better control of shell for-
mation on individual particles. For example, when the AP-
S:TEOS ratio is 1:3 with [citrate]/[Co]ꢀ0.2, we note a core
size of 70 nm and a shell of B60 nm size (Fig. 2(a)). In com-
parison, when the APS:TEOS ratio is 1:9, the shell thickness
increases to 110 nm (Fig. 2(c)) with a core size of B25 nm. In-
observed a grayish color appearance. The nanoparticles were
first stabilized by citrate after the reduction, and were found to
be stable for several hours. Our immediate goal was to explore a
wider range of control on particle size and silica shell thickness
using this synthesis approach. The core–shell structure of the

9
52
a)
Journal of the American Ceramic Society—Aslam et al.
Vol. 90, No. 3
citrate salt as a stabilizer provides a wider range of control on
particle sizes and better stability. The variation in APS:TEOS
makes stable colloids possible with thinner shells when the cit-
rate concentration is low, keeping the total surface area of the
Co particles minimum. This optimization is important for
(
(b)
Co@SiO core–shell structures. The time at which the shell is
2
deposited is crucial to control the stability of the final colloid, as
the Co cores can aggregate and even oxidize if the timing is not
correct. This can be explained by kinetically controlled growth
of silica on cobalt nanoparticles. To be precise, the formation of
a ‘‘seed’’ APS layer occurs first and this layer then catalyzes the
formation of silica. The size of the Co core determined by TEM
remains constant (for a particular citrate/Co ratio) with a stand-
ard deviation o20%, while the thickness of the silica shell in-
creases with increasing APS:TEOS content in the solution.
Thus, the major role has been played by the APS layer to carry
out an exchange in the first step with citrate ions.
(c)
(d)
Figure 3 shows the results of energy-filtered elemental map-
ping using the Co, Si, and O edges, respectively. The bright
contrast in the images represents Co (a), Si (b), and O (c), re-
spectively. A typical result is a Co core and SiO shell structure
x
with complete individual SiO shells as can be seen in Fig. 3(b).
x
The O element-specific brightness of the same particle is marked
by circles in image (c). Further, to characterize as-prepared and
(e)
6
001C annealed sample, which shows a distinct growth of CoO
shells after heating, we have recorded and analyzed EDS com-
positional line profiles (Fig. 4). The dashed line in Fig. 4(a) in-
dicates the location of the scan line of the spatial-resolution
EDS, and the corresponding chemical concentration profile is
shown in Fig. 4(b).
(f)
Overall, the EDS data show distinct differences in the spatial
profiles of as-prepared and annealed sample (Fig. 4(b) versus
Fig. 4(d)). Co and Si profiles obtained from the as-prepared
2
Co@SiO structure (Fig. 4(a)) reveal excellent fits with a nano-
particle core diameter of 42 nm, and a shell thickness of B110
nm. Importantly, we find that the TEM images recorded on the
annealed samples (Fig. 4(c)) exhibit the appearance of a thin
Fig. 2. (a)–(e) Transmission electron spectroscopy image at low mag-
nification of a monolayer of Co@SiO particles deposited on a carbon-
2
CoO layer between the Co core and the SiO shell (indicated by
2
arrow). Interestingly, the growth of an oxide layer does not in-
hibit the crystallization of the inner Co core by thermal treat-
ment under an ambient condition; however, the oxidation of Co
is slowed down by the protective oxide layer. The line scan en-
ters from the shell of a nanoparticle, crosses it through the core,
and then again enters the shell. The increase of the counts at the
center of the nanoparticle is likely due to the increase of the
sample thickness; therefore, we only focused on the relative in-
tensities of the three detected elements (Co, Si, and O). It is
found that the Co species are dominant at the core of the na-
noparticles. A Co peak occurs at the center of the particle, sug-
gesting the presence of Co at the center of the nanoparticle. As
the scan leaves the center and enters the shell, the Co signal
coated copper grid by simply drying a drop of solution. The core–shell
structure is visible at this magnification. (f) Superimposed graphs indi-
cating the variation of Co core size with respect to [citrate]/[Co]
concentration and silica shell thickness change with aminopropoxysi-
lane:tetraethoxysilane (APS:TEOS) amount.
terestingly, when the citrate/Co concentration decreases further
to 0.1, we observe an increase in Co core size to 100 nm
(Fig. 2(e)), whereas the APS:TEOS ratio gives rise to a shell
thickness of 15 nm.
Although our approach has some similarity to the earlier re-
an important variation to note is that the selection of
10,13
ports,
(a) Co
(b) Si
(c) O
Fig. 3. EELS element mapping of the Co component (Co L2,3 edge) (a), the Si component (Si edge) (b), and the O edge (c) in the core–shell particles
revealing a Co@SiO structure. Scale bar of the image one is the same as in Fig. 1(a).
2

March 2007
Synthesis and Stability of Co@SiO Aqueous Colloids
2
953
(a)
(
c)
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
1
1
1
400
200
000
(
b)
(d)
Si
8
6
4
2
00
00
00
00
0
O
Co
−10
0
20
40
Distance (nm)
nanoparticles (uncalcined) and (b) energy-dispersive spectroscopy (EDS) line
scans for the same nanoparticles shown above, with Co, Si, and O data plotted as black dots, red dots, and blue dots, respectively. (c) TEM image of a
Co@SiO nanoparticle calcined at 6001C and the respective (d) EDS line scans. An additional intermediate shell appears due to the oxidation of Co to
60
80 100 120 140
0
50
100
Distance (nm)
150
200
Fig. 4. (a) Transmission electron spectroscopy (TEM) image of Co@SiO
2
2
cobalt oxide.
disappears within the detection limit. The Si signal gradually
increases and as the beam scans through the Co/Si interface, it
shows a slight decrease in the signal due to less Si surface inter-
action.
One of the objectives of this study was to understand the
stability of such interesting magnetic structures where homoge-
neous silica shells protect the magnetic core. Figure 6 shows
2
TEM images of Co@SiO structures annealed at various tem-
Elemental analysis determined by EDS shows Na concentra-
tions of B2 at.% (not shown for brevity). Sodium is present to
aid in the formation of the citrate-passivating layer. While the
presence of stoichiometric Co, Si, and O is apparent after the
sodium borohydride reduction of Co salt, we find evidence to
suggest the presence of a citrate layer, the small amount of Na-
citrate layer present most likely located on the surface of Co
even after the silica growth. This is not entirely surprising as
citrate is a good stabilizer on the Co surface and prevents ag-
gregation and oxidation. The presence of sodium was observed
in EDS, which is probably due to the presence of some sodium
citrate primer on top of the Cobalt surface. SAED (Fig. 5(b))
indicates the amorphous nature of cobalt in the as-synthesized
sample. The sharp diffraction maxima in the diffraction pattern
of the heat-treated sample were more well defined than the pre-
heat-treated one, indicating that an improvement in crystallinity
peratures and simultaneously illustrates the change in magnetic
properties with respect to annealing temperature. TEM obser-
vation of the particles upon annealing reveals a remarkable
change. For example, in TEM micrographs of particles from the
sample that have been annealed in air at 6001C, although the
core is still clearly differentiated from the shell, a slightly darker
area is visible between the Co core and silica shell, which sup-
ports the idea of cobalt oxide or probably some Co/Si com-
pounds being present. At a higher temperature (Fig. 6(d)), the
surface of the particles appears deformed, probably due to the
sintering. This sample also shows the disappearance of the core
and a further progression of loss to cobalt toward an oxide or
silicide compound (indicated by arrows). However, the core–
shell morphology is intact after annealing, as this may be im-
portant for applications where nanostructures are prepared at
room temperature and annealing is subsequently performed.
The retention of magnetic properties after annealing is an
important parameter in the context of protective insulation pro-
vided by the silica shells. As compared with the as-prepared
sample, the 2001C annealed sample demonstrates a considerable
enhancement in the saturation magnetization, while the maxi-
mum value of saturation magnetization was observed for the
sample annealed at 4001C (Fig. 7(c)). These results are in good
agreement with the earlier reports where the strongest XRD
14
(
Fig. 5(d)) had taken place during the heating procedure, and
this is also supported by the high-resolution transmission elec-
tron microscopy (HRTEM) images revealing the lattice fringes
of crystalline cobalt (Figs. 5(e) and (f)). The amorphous nature
of silica is still maintained after annealing at 6001C. The results
are corroborated by X-ray diffraction (XRD) patterns indicat-
ing the presence of crystalline cobalt attained at higher tem-
peratures.

9
54
Journal of the American Ceramic Society—Aslam et al.
Vol. 90, No. 3
(b)
(
a)
3
2
1
0
0
0
0
(
A)
(
c)
(
b)
d)
(
(
a)
2
0 nm
(e)
(
c)
(d)
−
−
−
10
20
30
−
20000
0
20000
Field (Oe)
(
B)
(
e)
(f)
1
0
0
−
10
Fig. 5. (a) Transmission electron spectroscopy (TEM) image of a
Co@SiO nanoparticle (as synthesized) and (b) selected-area electron
diffraction (SAED) for the same nanoparticles. (c) TEM image of a
Co@SiO nanoparticle calcined at 6001C and (d) the respective SAED
2
−500
0
500
Field (Oe)
2
2
Fig. 7. (A) Hysteresis loops for (a) as-prepared Co@SiO samples, an-
pattern. (e) and (f) show the high-resolution transmission electron mi-
nealed at (b) 2001C, (c) 4001C, (d) 6001C, and (e) 8001C as under an
ambient condition. The dotted line indicates the shift in saturation
magnetization with respect to the normal. (B) Illustrative magnified
view for the first three samples.
croscopy images of the crystalline particle annealed at 6001C.
100 nm
1
00 nm
(b) 400°C
(c) 600°C
(d) 800°C
(
a) 200°C
−20000
0
20000 −20000
0
20000 −20000
0
20000 −20000
0
20000
Field (Oe)
Field (Oe)
Field (Oe)
Field (Oe)
Fig. 6. Transmission electron spectroscopy images of Co@SiO
respective hysteresis curves taken at room temperature.
2
nanoparticles sintered at (a) 2001C, (b) 4001C, (c) 6001C, and (d) 8001C and their

March 2007
Synthesis and Stability of Co@SiO Aqueous Colloids
2
955
Fig. 8. (a) Transmission electron spectroscopy photograph of a silica nanoshell after etching of the Co core in 1-M HCl and (b) intensity line scan across
a hollow shell particle.
peaks of metallic cobalt were observed for the 4001C annealed
IV. Conclusions
1
3
sample. It can be noted in Fig. 7 that as-prepared samples can
be saturated at a relatively lower magnetic field, while after sin-
tering (2001 and 4001C), the saturation magnetization threshold
shifts toward a higher magnetic field. Further annealing at
In summary, we report a simple modified approach to fabricate
stable, well-dispersed magnetic Co@SiO nanoparticles with
2
improved control over shell thickness and larger core diameters.
Citrate-stabilized Co nanoparticles are first formed and then the
citrate layer is place exchanged with a covalently linking ami-
nosilane layer, which further facilitates silica growth by the slow
hydrolysis of TEOS. TEM, EDS, elemental mapping, and line
compositional chemical analyses were used to demonstrate that
uniform isolated core–shell nanoparticles are obtained through
the improved synthetic route, and the individual nanoparticles
are coated with self-assembled silica via chemical bonding.
These particles exhibit viable conjugation with fluorescent–
avidin tags to enable subcellular targeting and monitoring.
6001C (Fig. 7(d)) causes a considerable decrease in saturation
magnetization and a marginal magnetization for the 8001C sin-
tered sample (Fig. 7(e)). This trend indicates that as the particle
size increases, the slope of the magnetization curve decreases,
which indicates slow saturation, usually related to interactions
1
7
between the magnetic dipoles present in the cobalt domains.
Also, the maximum saturation magnetization obtained is com-
1
8
parable to those for bulk metallic cobalt (162 emu/g). The
most interesting case is when, after annealing at a high tem-
1
9
perature (above 6001C), the porous silica likely cracks, allow-
ing diffusion of oxygen. This ultimately converts most of the
magnetic cobalt into cobalt oxide, with another possible reac-
tion between cobalt oxide and silica leading to a very tiny
amount of protected cobalt. This effect is clearly reflected in
the magnetic properties, which show a significant decrease in
magnetization. Similar results have been found for CdS quan-
tum dots that undergo photodegradation in air and yet remain
unchanged after silica protection. In general, it can be stated
that grain size increases with temperature, indicating slow sat-
uration, but the loss of cobalt into cobalt oxide causes a decrease
in magnetization, indicating disordered magnetic moments at
very few sites.
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4
5
To investigate the fluidic stability of these core–shell struc-
tures, the particles were first tested with different acidic solu-
tions. Figure 8 shows a TEM image of Co@SiO particles
2
‘
(
6
obtained by selectively etching the Co core in 1-M aqueous
HCl solution. It took almost 12 h to dissolve the Co core com-
pletely and was evident by the color change from dark gray to
pinkish due to chloride formation of cobalt. The size and shape
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orated from their interiors during TEM sample preparation.
Figure 8(b) shows an intensity line scan of elemental Si across an
individual particle using the Gatan elemental mapping tech-
7
8
9
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nique. This confirms the hollow structure with high silica in-
tensity seen at the edge and lower in the center. One of the main
objectives of developing the metal-silica core–shell colloids is
for specific adsorption of proteins, cytotoxicity with cells, and
selective new magnetic resonance imaging (MRI) probe. Our
ongoing studies are focused toward tagging bio-molecules and
fluorescent tags to these highly stable refractory shells, which
provide a convenient platform for many bio-functionalization
phenomena.
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(
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&