Hierarchic template approach for synthesis of silica nanocapsules with tuned shell thickness

Article abstract of DOI:10.1246/cl.2011.840

Nanoporous silica capsules with mesosized diameter (ca. 60 nm) were prepared by using polystyrene (PS) beads and polyethyleneimine (PEI) as the hierarchic core and shell templates, respectively. The coverage of PEI on the PS beads enhanced the positive charge density on the template surface and favored the formation of well-separated hollow spheres with diameters smaller than 100 nm. By tuning the PEI shell template layer on the solid PS bead templates at different pH values, the shell thickness of the hollow silica sphere could also be controlled at the nanometer level in the range 312 nm.

Full text of DOI:10.1246/cl.2011.840

doi:10.1246/cl.2011.840
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Published on the web July 16, 2011
Hierarchic Template Approach for Synthesis of Silica Nanocapsules
with Tuned Shell Thickness
Masahiro Mashimo,1 Qingmin Ji,* Shinsuke Ishihara, Hideki Sakai,
,2
2
2
1
1
2,3
2,3
Masahiko Abe, Jonathan P. Hill, and Katsuhiko Ariga*
1
Department of Pure and Applied Chemistry, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510
2
World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA),
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044
3
JST, CREST, 1-1 Namiki, Tsukuba, Ibaraki 305-0044
(
Received May 27, 2011; CL-110449; E-mail: JI.Qingmin@nims.go.jp)
Nanoporous silica capsules with mesosized diameter (ca.
0 nm) were prepared by using polystyrene (PS) beads and
template surface. We have found that sole usage of amine-
functionalized PS beads with diameters less than 100 nm as
template does not yield a silica layer on the bead surfaces, which
may be due to the reduction in surface positive charges upon
decreasing the bead size. We thus considered preparing a shell
layer possessing high cationic charge density on the PS bead to
guarantee the formation of silica of sufficient thickness. PEI is a
cationic polymer containing amine groups. Besides being widely
used in therapeutic applications for enhancing the attachment of
weakly anchoring cells, it has also been used as template for the
6
polyethyleneimine (PEI) as the hierarchic core and shell
templates, respectively. The coverage of PEI on the PS beads
enhanced the positive charge density on the template surface and
favored the formation of well-separated hollow spheres with
diameters smaller than 100 nm. By tuning the PEI shell template
layer on the solid PS bead templates at different pH values, the
shell thickness of the hollow silica sphere could also be
controlled at the nanometer level in the range 3­12 nm.
2
3
synthesis of silica nanocomposites. The PS­PEI hierarchic
core and shell template strengthen the adsorption of the SiO2
precursors. In addition, by tuning the PEI layer on the PS bead at
Silica (SiO2) capsules are promising materials with potential
1
­3
4,5
applications in drug delivery,
as novel microreactors,
different pH values, the shell thickness of the SiO capsules can
2
6
,7
8
9,10
catalysts, sensors, optical or electrochemical architectures,
be easily controlled at the nanometer level.
etc. Their expansive hollow interiors make encapsulation of
a variety of substances possible, and their permeable shells
promote the facile transport of guest materials. Many templates
have been used to prepare silica capsules (hollow spheres), such
as emulsion droplets, nanoparticles, vesicles, and polymer
The synthesis of SiO2 capsules through the hierarchic
template approach is illustrated in Figure 1. Carboxylate
(­COOH)-functionalized PS beads (PS­COOH) of 60-nm
diameter (2.5 wt % solution, 50 ¯L) and an aqueous solution
¹
1
of PEI (10 g L , 200 ¯L) were mixed in 1 mL of H O. The
2
11­16
spheres.
Polystyrene (PS) beads are attractive templates
resulting mixture was adjusted to the appropriate pH using
HNO3(aq). Hydrolyzed tetramethylorthosilicate (TMOS) solu-
tion (40 ¯L), which had been obtained by vigorously mixing
TMOS (148 ¯L) with HCl(aq) (0.01 M, 1 mL) for 20 min, was
added to the template suspension and stirred overnight. The
due to their low cost and availability in various monodisperse
diameters. Caruso et al. used layer-by-layer (LbL) self-assembly
of prefabricated SiO2 nanoparticles and polymers on PS cores to
fabricate hollow SiO2 spheres.11 There are also reports of the use
of PS beads with amine- or zwitterion-functionalized surfaces
for growth of silica shells.1
7,18
However, most of the resulting
SiO2 capsules are larger than 100 nm in diameter and are not of
monodisperse dimensions. Large sizes of the shells usually
reduces the effectiveness of SiO2 capsules in optical or
biomedical applications because of low transparency and limited
1
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cell permeability.
On the other hand, dispersibility of SiO2
capsules is also important for application although silica tends to
aggregate during the sol­gel formation and calcination process.
Despite the availability of many syntheses of SiO2 capsules,
facile methods for controlling the shell thickness, which is an
important property for regulating guest release, are scarce and
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rarely effective.
For instance, shell thickness control due to
variation of reaction solvent or quantity of silica precursors
usually results in changes of the hollow dimensions or
aggregated state of the spheres.
Here, we report the synthesis of mesoscopic SiO2 capsules
of monodisperse 60-nm diameter by using a hierarchic core and
shell template method with PS beads and polyethyleneimine
Figure 1. Schematic illustration for the formation of the silica
capsules through a hierarchic core and shell template approach.
(i) Mixing under different pH values; (ii) add silica precursors;
(iii) stirring overnight; (iv) calcination at 600 °C.
(PEI). The resulting SiO2 capsules possess a homogenous size
distribution, and this method also allows a tunable shell
thickness from 3 to 12 nm depending on the PEI layer on the
Chem. Lett. 2011, 40, 840­842
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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distribution profiles of the PS­PEI beads prepared at pH 7 and 8
show clearly two scattering peaks. At pH 7, the two peaks are
located at 81 and 223 nm, while at pH 8 they are found at 96 and
2
56 nm (Figures 3c and 3d). The particle distribution in the
small diameter range should be due to isolated PS­PEI beads
with those of larger size indicating 2 or 3 aggregated PS­PEI
beads. At pH greater than 7, ­COOH is completely dissociated
¹
to ­COO ions in the PS­COOH beads, enabling enhanced
adsorption of positive PEI molecules at the surface. The pKa of
the primary amine groups in PEI is 8.6. Although the amine
groups in PEI will be positively charged in the pH range 7­8, the
charge density will gradually decrease with increasing pH value.
These charge changes affect the adsorbed amount of PEI on PS­
COOH surface under weakly alkaline conditions. The DLS
results indicate that a thicker PEI layer was adsorbed on the PS­
COOH surface at pH 8 than at pH 7. It should be noted that the
PS­PEI beads become less well-dispersed and begin to form
agglomerates at pH >8. The decreasing positive charge density
of PEI molecules leads to a reduction of the repulsion forces
between neighboring beads.
Figure 2. TEM images of (A) PS­PEI template prepared by
mixing of PS­COOH bead and PEI solution at pH 7, (B) thin
silica layer formed on PS­PEI template which prepared by
mixing of PS­COOH bead and PEI solution at pH 4.
Of the PS­PEI beads prepared at various pH values, those
formed at pH 7­8 could be used as the core­shell templates to
form hollow SiO capsules. TMOS was hydrolyzed in hydro-
2
chloric acid yielding silicic acid as the SiO2 precursor.
Negatively charged silicic acid is adsorbed on the positively
charged PEI layer then undegoes condensation to form the
polymeric SiO layer. Therefore, the coverage of PEI directly
2
affects the completeness of formation of SiO2 layer on the
template surface. PS­PEI beads prepared at low pH values
(pH 4­6), where there is incomplete coverage by the PEI layer,
were poorly formed due to the partial formation of a very thin
SiO2 layer on the template surface (Figure 2B). These thin SiO2
layers are too structurally weak to maintain spherical morphol-
ogy during the calcination process yielding a collapsed or
deformed yet still hollow morphology (Figure 4A). When using
PS­PEI beads at pH 7­8 as the templates, we successfully
obtained SiO2 capsules with inner diameters of 60 « 8 nm
(Figures 4B and 4C). In particular, SiO2 capsules obtained using
PS­PEI templates prepared at pH 7.0, 7.2, 7.4, 7.6, 7.8, and
8.0 gave average shell thicknesses of 3.3 « 0.5, 3.4 « 0.3,
5.8 « 0.7, 7.8 « 0.3, 9 « 0.7, and 12.2 « 0.8 nm, respectively
(Figure 5), clearly indicating the shell thickness controllability
Figure 3. DLS size distribution profiles of (a) PS­COOH
beads, PS­PEI beads prepared by mixing of PS­COOH beads
and PEI solution at (b) pH 5, (c) pH 7, and (d) pH 8.
reaction solution was then purified several times by centrifuga-
tion followed by water redispersion and finally freeze-dried
under vacuum. The template was removed by calcination at
600 °C for 4 h.
Figure 2A shows the core­shell morphology of a particle
of PS­COOH beads which was coated with PEI at pH 7. The
PS­PEI core­shell template has a diameter around 60 nm and
possesses a smooth surface. It is difficult to distinguish the PS­
COOH beads from the PS­PEI beads by transmission electron
microscopy (TEM) observation. However, we could clearly
observe the changes due to PEI adsorption on the PS­COOH
beads using dynamic light scattering (DLS). As shown in
Figure 3a, the size distribution profile of PS­COOH bead
dispersion indicates a well-dispersed state with an average
particle size around 69 nm. The slight difference in size (60 nm)
from that observed by TEM is probably due to a layer of
hydrogen-bonded water molecules on the PS­COOH surface
and slight swelling of hydrophilic chains in PS­COOH beads.
The scattering profile for PS­PEI beads prepared at pH 5 shows
a main peak centered at 129 nm with a small shoulder at
using this method. These dry SiO capsules look similar to loose
2
cotton and are easily resuspended in water. The BET surface area
of the hollow SiO2 capsules with 60-nm diameter and 12-nm
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shell thickness was 264 m g . The BJH pore size distribution is
around 2.4 nm. Although the condensation of SiO2 precursors is
affected by the pH values of their preparation, the condensation
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4
rate becomes saturated in the pH range of 7­8. Considering the
trend in the thickness of the PEI layer on the PS­COOH surface
within this pH region, it is likely that variation of the PEI layer
thickness is responsible for the SiO2 formation on the template
surface. The denser amine scaffold provided by the thicker PEI
layer facilitates the condensation of SiO2 precursor acting like a
weak surface catalyst, thus promoting the formation of a thicker
SiO2 layer. This feature enables modulation of the shell
thickness of the SiO2 capsule between 3 and 12 nm.
59 nm (Figure 3b). The main peak at 129 nm is likely due to
aggregation of 2 or 3 PEI­PS beads. The shoulder peak, which
indicates a similar size to PS­COOH bead, suggests incomplete
coverage of PEI on the PS­COOH surface under these
conditions. At pH 5, fewer ­COOH groups of the PS­COOH
¹
beads are dissociated as ­COO ions. The weakened surface
In conclusion, we present a new strategy for preparation of
uniform SiO2 capsules (<100 nm) by using PEI-coated PS­
COOH beads as the core­shell template. The adsorption of PEI
charges may induce partial aggregation of the PS­COOH beads
and the poorer adsorption of PEI on the surface. The size
Chem. Lett. 2011, 40, 840­842
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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on the template facilitates regulation of the formation of SiO2 in
the PEI layer and allows control to be exerted over the shell
thickness of the hollow SiO2 capsules. The SiO2 capsules
possess homogeneous, well dispersible, and less than 100 nm
diameter characteristics, which are promising to improve their
application as novel drug carriers, catalyst supports antireflection
coating materials, etc. This synthesis at near neutral pH
conditions would also make an interesting model for the
formation of silica biomaterials.
This work was supported by a Japan Society for the
Promotion of Science (JSPS) Fellowship, World Premier
International Research Center (WPI) Initiative on Materials
Nanoarchitectonics, MEXT, Japan, and JST, CREST.
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Chem. Lett. 2011, 40, 840­842
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/