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© 2010 American Chemical Society
Interfacially Controlled Synthesis of Hollow Mesoporous Silica Spheres with
Radially Oriented Pore Structures
Juan Li,†,‡ Jun Liu,*,† Donghai Wang,† Ruisong Guo,*,‡ Xiaolin Li,† and Wen Qi†
†
Pacific Northwest National Laboratory, Richland, Washington 99352, and ‡School of Material Science and
Engineering, Tianjin University, Tianjin 300072, P. R. China
Received March 28, 2010. Revised Manuscript Received May 13, 2010
This paper reports an alternative process to prepare hollow mesoporous silica spheres (HMS) using a single cationic
surfactant with a tunable wall thickness and radially oriented pore structures. Using N,N-dimethylformide (DMF) as
the intermediate solvent bridging the organic and aqueous phase, hollow mesoporous silica spheres were synthesized
with interfacial hydrolysis reactions at the surface of liquid droplets. These spheres have an ordered pore structure
aligned along the radial direction, and the wall thickness and sphere sizes can be tuned by adjusting the experimental
conditions. Transmission electron microscopy and nitrogen absorption techniques were used to characterize HMS and
its formation procedure. A hypothetic formation mechanism was proposed on the basis of a morphology transformation with the correct amount of DMF and a careful observation of the early hydrolysis stages. Au and magnetic Fe3O4
nanoparticles have been encapsulated in the HMS hollow core for potential applications.
1. Introduction
Since the pioneering work by the scientists at the Mobil
Corporation in 1992, great effort has been devoted to synthesizing
mesoporous silica nanoparticles with controlled particle sizes,
morphologies, and tunable pore structures.1 In these materials,
the surface area, pore sizes, and pore structures can be precisely
controlled with different surfactants/polymer molecules or by
carefully tuning the synthetic conditions. Recently, hollow porous
silica particles and spheres attracted wide attention for drug
delivery, optical and magnetic imaging, absorption, encapsulation, and catalysis applications because the large cavities in these
particles or spheres can be used as high-capacity reservoirs to
store chemicals, biological agents, or other nanoparticles. Furthermore, the mesoporosity provides easy pathways for molecular
diffusion.2 Hollow mesoporous particles with three-dimensional
pore channels3-6 and core-shell micellar silica nanoparticles7
have been investigated for high-capacity loading and controlled
release of biomolecules and drugs. High drug loading and
sustained, or even switchable, release7 have been reported. Dyeloaded hollow silica particles can be used for direct optical imaging of cancer cells.7 Hollow mesoporous silica spheres (HMS)
loaded with magnetic particles have the potential for magnetic
imaging and magnetic separation.8 HMS have also been prepared
with mixed magnetic and semiconductor nanoparticles.9 It is
interesting that enzymes trapped in multilayered silica vesicles
could catalyze the hydrolysis of tributyrin with a high activity,10
showing the potential of such materials for biomolecule encapsulation and catalytic applications.
There are several general approaches to prepare hollow
mesoporous materials. The core templated approach is based
on surface deposition on inorganic or organic materials or
polymers.10-13 This method is simple to use, but it is difficult to
scale up. The templates need to be removed after the synthesis,
and it is difficult to load the materials in the cavities. The second
approach uses water/oil emulsion as the template and forms
hollow mesoporous structures at the water-oil interfaces.4,14
The most widely studied method is to use surfactant vesicles as
the template. The vesicle template approach was first studied by
Pinnavaia et al.15,16 using neutral bolaamphiphile surfactant that
contains two polar head groups linked by a hydrophobic alkyl
chain. Silicate precursors were condensed in interlayered regions
of multilamellar vesicles, and they formed micro- and mesoporous multilayer lamella silica with a high surface area and a high
thermal stability. Also, many other symmetric and asymmetric
diamine surfactants as well as different silicate precursors have
been used to prepare vesicle silicate particles with a wide range of
particle sizes, morphologies, and pore structures.17-19 Vesicle-like
*Corresponding authors. E-mail: [email protected] (J.L.), [email protected]
(R.G.).
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Langmuir 2010, 26(14), 12267–12272
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Published on Web 06/24/2010
DOI: 10.1021/la101225j
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Li et al.
silica particles can be obtained with nonionic surfactants,20-22
cationic surfactants,23,24 and fluorocarbon surfactants.25 Another
widely studied method is to use mixed systems of cationicanionic surfactants26 and fluorocarbon-hydrocarbon surfactants.27 In the cationic and anionic system, the positive and
negative charges are neutralized, and the two surfactants strongly
bind to one another, favoring the formation of the lamella sheets
and vesicles that function as the template for mesoporous silica
structures. In the fluorocarbon surfactant system, the fluorocarbon surfactant forms the template for the vesicle template, and the
hydrocarbon surfactant forms the template for the silicate mesostructures. Both cationic and nonionic surfactant28,29 can be used
with the fluorocarbon surfactants, and the morphologies and
structures can be controlled by adjusting the surfactant ratio and
experimental conditions.30 Periodic mesoporous organosilica
hollow spheres with tunable wall thickness have also been prepared with fluorocarbon and cationic double templates.31
Many scientific and practical issues still need to be addressed to
allow an efficient synthesis of HMS. For example, many widely
studied approaches involve expensive or special surfactants. In
the vesicle template synthesis, the hollow silica spheres are based
on surfactant bilayers and lamella structures. The silica particles
have onionlike structures and mono- or multilayered shell structures. The wall or the shell structures range from microporous,
mesoporous, spongelike, and, in a few cases, ordered three-dimensional mesoporosity. For many applications, such as controlled
release and catalysis, ideally the pore channels should be oriented
perpendicularly with respect to the walls. Although oriented
mesoporous particles have been reported with a hard template
approach,11 there is a need to develop a simple, one-step method
that not only produces hollow mesoporous particles and spheres
with controlled pore orientation using commonly available commercial surfactant but also could integrate the encapsulation of
functional materials inside the HMS during the synthesis.
In this paper, we report a simple and one-step route to
synthesize HMS particles with radially oriented mesoporous wall
structures and tunable wall thickness at room temperature using
one commonly available cationic surfactant. These mesoporous
silica spheres are formed through an interfacial reaction at the
surface of liquid droplets and are different from the surfactant
bilayer vesicle structures. These materials can be used to load
multiple gold and Fe3O4 nanoparticles.
2. Experimental Methods
Materials. All chemicals were used directly without further purification, including N,N-dimethylformamide (DMF,
(20) Wang, H. N.; Wang, Y. H.; Zhou, X.; Zhou, L.; Tang, J.; Lei, J.; Yu, C.
Adv. Funct. Mater. 2007, 17, 613.
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C. Z. Chem. Mater. 2008, 20, 6238.
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19, 4279.
(25) Tan, B.; Lehmler, H. J.; Vyas, S. M.; Knutson, B. L.; Rankin, S. E. Adv.
Mater. 2005, 17, 2368.
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(27) Lin, C. X.; Yuan, P.; Yu, C. Z.; Qiao, S. Z.; Lu, G. Q. Microporous
Mesoporous Mater. 2009, 126, 253.
(28) Yuan, P.; Yang, S.; Wang, H. N.; Yu, M. H.; Zhou, X. F.; Lu, G. Q.; Zou,
J.; Yu, C. Z. Langmuir 2008, 24, 5038.
(29) Gu, X.; Li, C. L.; Liu, X. H.; Ren, J. W.; Wang, Y. Q.; Guo, Y. L.; Guo, Y.;
Lu, G. Z. J. Phys. Chem. C 2009, 113, 6472.
(30) Yang, S.; Zhou, X. F.; Yuan, P.; Yu, M. H.; Xie, S. G.; Zou, J.; Lu, G. Q.;
Yu, C. Z. Angew. Chem., Int. Ed. 2007, 46, 8579.
(31) Djojoputro, H.; Zhou, X. F.; Qiao, S. Z.; Wang, L. Z.; Yu, C. Z.; Lu, G. Q.
J. Am. Chem. Soc. 2006, 128, 6320.
12268 DOI: 10.1021/la101225j
anhydrous, 99.8%, Sigma-Aldrich), cetyltrimethylammonium
chloride solution (CTAC, 25 wt % solution in water, Aldrich),
tetraethyl orthosilicate (TEOS, Fluka), triethanolamine (TEA,
minimum 98%, Sigma), ethanol (anhydrous, Sigma-Aldrich),
and deionized (DI) H2O. Hydrochloric acid (HCl, 37%, ACS
Reagent, Sigma-Aldrich) and ethanol were used for template
extraction.
Synthesis of HMS Spheres. Our methods for making HMS
spheres were derived from a method used to prepare regular
mesoporous nanoparticles.32-34 Typically, 6.4 mL of DI H2O,
1.05 mL of ethanol, and 1.04 mL of 25 wt % CTAC were mixed
and stirred at room temperature for 30 min. A total of 1.85 g of
TEA was subsequently added and further stirred for 30 min until
dissolved to control the pH. The mixtures were stirred at room
temperature, to which the mixture of 0.692 mL of TEOS and an
appropriate amount of DMF was added under stirring. The
whole mixtures were further stirred for an appropriate time at
room temperature.
After the reaction, we stopped the stirring and added 5 mL of
ethanol into the translucent, colloidal aqueous suspension to
precipitate the mesoporous materials. The mixture was centrifuged to collect the aggregate HMS at the bottom. The resulting
precipitates were washed by hydrochloric acid in ethanol solvent
(3 mL of concrntrated HCl in 240 mL of ethanol) with sonication.
This procedure was repeated three times. Subsequent washing was
performed by centrifugation at 9000 rpm for 6 min and redispersed in DI H2O under sonication. This washing procedure was
repeated three times. All products were stocked in DI H2O.
To study the effect of stirring and sonication, similar procedures were followed, except that the mixed solution was sonicated
in a water bath (Branson 1510) during part or the entire period of
reaction. The same procedure was used to extract the surfactant.
Preparation of Au Colloids. Colloidal Au nanoparticles were
prepared according to the citrate method reported in the
literature.35 A total of 500 mg of HAuCl4 was dissolved in 250 g
of DI H2O to form 2 mg/mL of HAuCl4 aqueous solution. Then
4.31 mL of this HAuCl4 aqueous solution was added into 200 g of
DI H2O. This solution was loosely covered with Al foil and heated
to boil. Then 20 mL of 1% sodium citrate aqueous solution was
added. The mixture was boiled for 5 min to form the wine red gold
colloidal solution with gold nanoparticles about 15 nm in
diameter.35 This colloidal solution was used as the seed solution
to grow the particles to a larger size. A total of 100 mL of the asprepared colloidal solution was heated to boil. Then 25 mL of
HAuCl4 (2 mg/mL) aqueous solution was added dropwise during
boiling. After boiling for 5 min, the mixture was cooled down to
room temperature. The resultant solution was centrifuged at 3500
rpm for 10 min to collect the large gold particles at the bottom
section of the centrifuge tube.
Preparation of Magnetic Fe3O4 Nanoparticles. Fe3O4 nanoparticles were synthesized according to a method reported in the
literature.36 A total of 1 g of KOH and 0.5 g of KNO3 was dissolved
in 75 mL of DI H2O, and the solution was heated to boil. Subsequently, 2.25 g of FeSO4 dissolved in 15 mL of DI H2O was
added. After boiling for about 5 min, the granular black precipitate
was separated by a magnet. Then 200 mL of hot DI H2O was
added. After dispersion, a magnet was brought into contact with
the beaker for a short period of time (5 min), and the large particles
attracted to the magnet were removed. The small Fe3O4 particles
left in the solution were washed by hot DI H2O three times and
dispersed in 5 mL of DI H2O. The crystalline phase of Fe3O4
(magnetite) was confirmed by X-ray diffractometry analysis.
Synthesis of Mesoporous Silica Spheres with Au and
Fe3O4 Nanoparticles. Mesoporous silica nanoparticles with
(32)
(33)
(34)
(35)
(36)
Moller, K.; Kobler, J.; Bein, T. Adv. Funct. Mater. 2007, 17, 605.
Kobler, J.; Moller, K.; Bein, T. ACS Nano 2008, 2, 791.
Moller, K.; Kobler, J.; Bein, T. J. Mater. Chem. 2007, 17, 624.
Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 55.
David, I.; Welch, A. J. E. Trans. Faraday Soc. 1956, 52, 1642.
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Article
Au cores were prepared with the following method: 6.4 mL of DI
H2O, 1.05 mL of ethanol, and 1.04 mL of 25 wt % CTAC were
stirred at room temperature for 30 min and then put under a bath
sonication while the rest of the reactants were added. A total of
1.85 g of TEA was subsequently added into the mixture. After
that, 0.2 mL of colloidal Au solution was added. Then a mixture
of 0.692 mL of TEOS and 2 mL of DMF was added. The entire
mixtures were further sonicated for 5 min followed by stirring at
room temperature for 1 h. After the reaction, all of the products
were washed by hydrochloric acid in ethanol solvent (3 mL
of concentrated HCl in 240 mL of ethanol) with sonication. This
procedure was repeated twice. Subsequent washing was performed by repeated centrifugation at 9000 rpm for 6 min and
redispersed in DI H2O under sonication. All products were
stocked in DI H2O.
Mesoporous spheres with Fe3O4 nanoparticles were prepared as
follows: 6.4 mL of DI H2O, 1.05 mL of ethanol, and 1.04 mL of
25 wt % CTAC were stirred at room temperature for 30 min and
then put under a bath sonication while the rest of the reactants were
added. A total amount of 1.85 g of TEA was subsequently added
into the mixture. After that, 0.1 mL of Fe3O4 aqueous suspension
was added. A mixture of 0.692 mL of TEOS and 2 mL of DMF was
then added. The mixture was further sonicated for 1 h.
Characterization. Transmission electron microscopy (TEM)
was performed on a JEOL JSM-2010 TEM microscope operated
at 200 kV. Scanning electron microscopy (SEM) was carried out
on an FEI Helios Nanolab dual-beam focused ion beam (FIB)/
SEM. The surface area was determined using Quantachrome
Autosorb Automated Gas Sorption Systems. The result of the
Brunauer-Emmett-Teller (BET) surface area was obtained with
the single-point adsorption method.
Figure 1. TEM and SEM images of particle morphologies with
different amounts of DMF. (a) No DMF. (b) 0.2 mL of DMF.
(c) 0.976 mL of DMF. (d) 2 mL of DMF. (e) 4 mL of DMF. (f) A
single sphere with almost no cavity obtained by using 4 mL of
DMF. (g, h) SEM images of hollow spheres synthesized with 2 mL
of DMF. Broken spheres are marked out with arrows.
3. Results and Discussion
DMF was found to be important in morphology control. We
obtained very small mesoporous nanoparticles to HMS by tuning
the amount of DMF. As shown in the TEM images of Figure 1,
without DMF, nanoparticles of about 30-40 nm in diameter with
disordered mesopore structures were produced (Figure 1a). While
a small amount of DMF (200 μL) was added, similar mesoporous
nanoparticles were obtained (Figure 1b). However, when the
amount of DMF was increased to 0.976 mL, some hollow spheres
with radially oriented mesopore features were observed in addition to the small nanoparticles (Figure 1c). These hollow spheres
are about 200 nm in diameter, and the mesopore features are
about 3 nm wide, consistent with what is expected from the
cationic synthesis approach.23,24 The wall thickness of the hollow
spheres is about 60 nm. Further addition of DMF (more than
2 mL of DMF) caused the formation of mostly HMS spheres
(Figure 1d-f). The radially oriented mesopore can be clearly
observed at a high magnification from Figure 1d. Figure 1e is a
lower magnification TEM image with 4 mL of DMF. A few
regular, “dense” mesoporous nanoparticles are stilled observed,
but the exact amount is difficult to quantify from the TEM
images. Figure 1f shows one single mesoporous particle with a
very small cavity in the center. The hollow structure of the HMS
obtained with 2 mL of DMF was further confirmed by SEM
images (Figure 1g,h), from which spheres with broken shells were
clearly observed.
To prove the existence of the mesoporosity, the materials made
of hollow silica particles were precipitated with ethanol, dried,
and calcined at 500 °C for BET measurement. The nitrogen
absorption isotherm of the sample synthesized with 4 mL of DMF
was obtained and is shown in Figure 2. The material has a surface
area of 800 m2/g (Figure 2a). The Barrett-Joyner-Halenda
(BJH) method gives an average pore size of 3 nm (Figure 2b)
from the adsorption branch.
Langmuir 2010, 26(14), 12267–12272
Figure 2. Brunauer-Emmett-Teller (BET) result of the hollow
mesoporous spheres synthesized using 4 mL of DMF. (a) Nitrogen
absorption-desorption isotherm curves. (b) Pore size distribution
from the absorption curve using the Barrett-Joyner-Halenda
(BJH) method.
A series of experiments were carried out to study the growth of
the hollow spheres. It is critical to understand how the particles
formed. Therefore, we collected the samples synthesized with
2 mL of DMF of different reaction times. Figure 3 shows the
particle morphologies at different reaction times. Before 30 min,
only disordered, amorphous precipitates were observed. However,
at 30 min, hollow spheres already formed with well-defined and
radially oriented features (Figure 3a). The wall thickness was less
than 40 nm. After 1 h, similar morphologies were observed, and
the wall thickness increased slightly (Figure 3b). After 3 h, the wall
thickness further increased to more than 60 nm, and the wall
structure seemed to become denser (Figure 3c). This series of TEM
results suggests that the particles were produced in the early stages
of the reaction, and further reaction resulted in increasing the wall
thickness of the particles. Another noticeable feature is that the
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Figure 5. Grain boundary structures in fused silica spheres with 2
mL of DMF. (a) Fused particles with disordered boundary in the
interior and ordered pore structure on the edge of the particles.
(b) Two overlapped HMS spheres with ordered boundary and
sharp edge.
Figure 3. Hollow silica spheres formed at different reaction times
with the addition of 2 mL of DMF. (a) After 30 min of reaction.
(b) After 60 min of reaction. (c) After 3 h of reaction. (d) Small
particles and spheres with 1 h of sonication after the ingredients are
mixed.
Figure 4. High-magnification TEM images of the wall structures
in HMS spheres with 4 mL of DMF. (a) The oriented pore
structures on the outer surfaces. (b) The disordered structures in
the inner walls. White arrows marked the ordered pore structures.
Inset is a zoom-in image showing the ordered porous wall of the
HMS.
formation of the hollow particles may be kinetically controlled. If
the mixed solution containing all the ingredients is sonicated at the
beginning of the reaction, the large hollow particles will be broken
down into smaller hollow particles, and mostly filled nanoparticles
will be formed, as shown by Figure 3d for a sample that was
sonicated for 1 h at the beginning of the reaction.
Figure 4 shows high-magnification TEM images of some
hollow spheres with 4 mL of DMF and further reveals how the
interfacial reactions take place. Figure 4a is a sphere with a small
cavity at the center. The mesostructure near the outer surface of
the walls seems to be more ordered than the structures close to the
center of the sphere. Figure 4b also shows that the inner walls of
the spheres remained disordered for these thick-walled particles.
These results suggest that the self-assembly of the mesopore
structure most likely started at the outer surface of the spheres.
Furthermore, when compared with the morphologies of the
spheres formed at an early stage of reaction (inset in Figure 4b),
the mesostructure also became denser and more uniform.
12270 DOI: 10.1021/la101225j
Sometimes, fused hollow mesoporous particles/spheres are
observed connecting two or more spheres together. The grain
boundary structures of these fused particles are interesting. Figure
5 shows the grain boundaries of the fused spheres with different
degrees of overlapping. In Figure 5a, the boundary in the interior
of the particles is totally fused together and becomes quite
disordered because of the high degree of overlap, but on the edge,
the pore structures on both spheres remain intact until they meet
at the grain boundary. There is a sharp grain boundary and not a
gradual change of the mesostructure in the gain boundary region.
Similarly, in Figure 5b, the mesostructures in both spheres are
well maintained, and there is again a sharp grain boundary
dividing the two spheres. This observation again suggests that
the mesostructures are formed in the very early stage of reaction
and are locked in. There is almost no change in the porous
structure even if the particles are fused together.
On the basis of these TEM results and our observation of the
hydrolysis process, it is believed that the formation of the hollow
mesoporous spheres may be explained by an interfacial reaction
mechanism (Figure 6). When pure TEOS is mixed with the
surfactant solution, initially the solubility of TEOS is very poor
until it is hydrolyzed by TEA. Without stirring, the TEOS rapidly
phase separates from the aqueous surfactant phase and forms its
own liquid phase above the aqueous phase because of the density
difference, and no reaction will be observed for many days.
Reactions take place through slow hydrolysis of TEOS that favor
the formation of regular mesoporous nanoparticles (Figure 1a).
DMF is soluble in both the TEOS and the aqueous surfactant
solution. DMF added in the solution probably plays two roles.
First, as a bridging solvent, it helps the dispersion of TEOS in the
aqueous solution to form small droplets of TEOS and DMF
(Figure 6a). The inside of the droplet is silicate-rich, and the
outside aqueous phase is surfactant-rich. Another effect for DMF
is that it speeds up the hydrolysis at the interface. When the
surfactant and the TEOS meet at the water-TEOS interface,
hydrolysis and condensation of the silicate occur and form
ordered mesostructures. The surfactant and water diffuse toward
the centers of the spheres, and the silicate species and DMF
diffuse toward the aqueous phase, forming the radially aligned
mesoporous walls (Figure 6b). Since the formation of the droplets
is not an equilibrium process, the sizes of the droplets depend on
the stirring and rate of mixing. Sonicating the solution in the early
stages of the reaction can have a large effect and breaks up the
droplets, and thus, the sizes of the spheres are reduced. Figure 6c
shows a schematic of the grain boundary structure.
The above interfacial reaction mechanism is supported by controlled experiments. First, when TEOS is added to the aqueous
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Figure 6. Schematics of the formation of hollow mesoporous spheres and the wall structures. (a) TEOS droplets in aqueous solution.
(b) Formation of oriented mesostructures at the interface. (c) Grain boundary structures. (d) Immiscible interface formed between TEOS and
the aqueous phase without stirring. (e) Interface formed between the aqueous phase and the TEOS-DMF solution without stirring, showing
the reacted interfacial region. (f) An almost clear solution of mixed TEOS and aqueous solution after 5 min of stirring, without adding DMF.
(g) A mixed solution of TEOS and aqueous solution with DMF, after 5 min of stirring, showing the reaction products in the solution. Images
were taken right after mixing of the solutions.
Figure 7. Au and Fe3O4 nanoparticles trapped in the hollow mesoporous spheres. Usually several particles are encapsulated inside of one
HMS. (a-c) Typical TEM images of multiple gold nanoparticles in hollow silica spheres. (d-f) TEM images of multiple magnetic
nanoparticles in hollow silica spheres. The images were taken with the materials obtained in one representative synthesis of Au@HMS and
Fe3O4@HMS, respectively.
surfactant solution without stirring, the TEOS phase separates
from the surfactant solutions and forms a stable interface without
any reaction for many days (Figure 6d). However, when the
TEOS-DMF is slowly added to the surfactant solution without
stirring, the TEOS-DMF phase separates to form the interface,
Langmuir 2010, 26(14), 12267–12272
but an interfacial reaction zone that grows thicker with time is
immediately observed, which supports the interfacial reaction
mechanism (Figure 6e). Figure 6f,g compares the solutions with
and without DMF after 5 min of stirring. The solution without
DMF remains clear without much reaction, and the solution with
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DMF becomes translucent due to particle formation. This observation confirms that DMF indeed accelerates the hydrolysis
reactions.
For controlled release, catalysis, and sensing, it is important to
demonstrate that the cavities in the spheres can be filled with other
materials or chemicals. Figure 7a-c shows multiple gold nanoparticles trapped inside the mesoporous spheres. The gold nanoparticles were prepared according to the well-known citrate
reduction methods, and the particle sizes were increased through
multiple growth. The nanoparticles were then dispersed in the
surfactant solutions. Since the gold nanoparticles were negatively
charged in the solution at the pH conditions (pH 10), most likely,
the cationic surfactants were attracted to the particle surfaces
through the positively charged headgroup through ion pairing,
leaving the tails exposed.37 When mixed with the TEOS and
DMF, the particle core with the surfactant tails on the surface was
easily displaced in the more hydrophobic TEOS droplets. Similar
results were reported in other systems that showed single nanoparticles trapped inside radially aligned micellar structures.38
Similarly, magnetic Fe3O4 particles were prepared according to
a method reported in the literature, and we have shown that
multiple magnetic particles can be loaded in the hollow mesoporous spheres (Figure 7d-f). The Fe3O4 loaded silica spheres are
magnetic and can be easily separated from the solution with a
magnet. In Figure 7, the mesoporous walls in the particles are not
as well ordered. This may be attributed to the effect of other
(37) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley & Son:
New York, 1978; p 34.
(38) Gorelikov, I.; Matsuura, N. Nano Lett. 2008, 8, 369.
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Li et al.
impurities that coexist with the nanoparticles from the preparation method, such as citrate ions in the gold nanoparticle solution.
4. Summary and Conclusions
In summary, we have developed a simple and fast method to
synthesize HMS spheres through interfacial hydrolysis reactions
at the surface of liquid droplets of TEOS-DMF. By tuning the
amount of DMF and experimental conditions, we can control the
pore structure well aligned along the radial direction, the wall
thickness, and sphere sizes. We proposed a hypothetic mechanism
to explain the formation process of HMS spheres on the basis of
morphology transformation with the amount of DMF and the
careful observation of the hydrolysis. Our as-synthesized HMS
spheres have been used to encapsulate multiple Au and magnetic Fe3O4 nanoparticles and can be extended to encapsulate
other materials like drugs and dyes as potential stably dispersed
catalysts at high temperatures, magnetic imaging agents, drug
cargos, and optical imaging agents.
Acknowledgment. This research is supported by the U.S.
Department of Energy, Office of Basic Energy Sciences, Division
of Materials Sciences and Engineering, under Award KC020105FWP12152. Pacific Northwest National Laboratory (PNNL) is a
multiprogram national laboratory operated for DOE by Battelle
under Contract DE-AC05-76RL01830. Juan Li thanks the partially financial support from the China Scholarship Council. Jun
Liu thanks Wayne Cosby at PNNL for his help in the preparation
of this manuscript.
Langmuir 2010, 26(14), 12267–12272