Synthesis of colloidal Janus nanoparticles by asymmetric capping of mesoporous silica with phenylsilsesquioxane

Article abstract of DOI:10.1039/c4cc10064f

Colloidal mesoporous silica nanoparticles asymmetrically capped with non-porous phenylsilsesquioxane have been prepared by adding phenyltriethoxysilane to an aqueous dispersion of mesostructured silica-surfactant composite nanoparticles. The integration of colloidal stability, mesoporosity and the Janus structure is quite promising for materials design applicable in various fields, including catalysis, biomedicine and coatings.

Full text of DOI:10.1039/c4cc10064f

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Colloidal Janus Nanoparticles by Asymmetric Capping
of Mesoporous Silica with Phenylsilsesquioxane
Cite this: DOI: 10.1039/x0xx00000x
a
a
a, b
Hiroto Ujiie, Atsushi Shimojima* and Kazuyuki Kuroda*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Colloidal mesoporous silica nanoparticles asymmetrically 50 nm in diameter) possess an asymmetric structure, where a
mesoporous silica nanosphere is capped with
hemispherical phenylsilsesquioxane shell (see Fig. 1). Although
post-modification of mesoporous silica nanoparticles with
a nonporous,
capped with non-porous phenylsilsesquioxane have been
prepared by adding phenyltriethoxysilane to an aqueous
dispersion of mesostructured silica–surfactant composite
nanoparticles. The integration of colloidal stability,
organosilane
functionalisation,
is
a
well-established
this report is the first on asymmetric
method
for
their
14-17
mesoporosity and the Janus structure is quite promising for modification to produce Janus nanoparticles. In addition to the
materials design applicable in various fields, including differences in the hydrophobicity, reactivity and thermal/mechanical
properties of silica and phenylsilsesquioxane, the combination of
porous and nonporous components will be useful for designing
unique nanocarriers, nanocoatings and complex assemblies.
catalysis, biomedicine and coatings.
Janus particles with asymmetric structures or surface properties have
attracted great interest because these particles exhibit unusual
1
-3
behaviour that cannot be observed for isotropic particles. A typical
example is Janus particles with hydrophilic and hydrophobic faces,
which are assembled at oil–water interfaces to form water repellent
4
5
membranes and Pickering emulsions. The Janus-type structure is
also important for maximising the functions of individual
components without interfering with each other. Precise design
based on the type, size and shape of each component promises a
wide range of applications in displays, sensors and drug delivery
systems, although the easy and large-scale fabrication of Janus
particles remains a major challenge.
Mesoporous silica nanoparticles (<100 nm in diameter) possess
high surface areas, large pore volumes, tunable pore sizes and
modifiable surfaces, which make these particles potentially useful as
6
7,8
9
Fig. 1. Synthetic procedure for the Janus nanoparticles. The green parts
catalysts, drug-delivery carriers, and coatings. Recently, Janus-
type mesoporous silica particles with magnetite and metal
counterparts have been prepared by the anisotropic growth of
represent phenylsilsesquioxane (PhSiO1.5).
1
0
mesostructured silica on the core particles and by sputtering metals
CMSS with an average particle diameter of 30 nm (Fig. S1 in
1
1,12
18
on a monolayer of mesoporous silica particles,
respectively. ESI†) were prepared according to our previous report. The molar
Zhao and co-workers reported the synthesis of Janus particles by the composition of the starting mixture was 1 tetraethoxysilane (TEOS):
1
3
growth of mesoporous organosilica on mesoporous silica particles.
Nonetheless, the types of mesoporous silica-based Janus particles triethanolamine (TEA): 1200 H
0.50
cetyltrimethylammonium
bromide
(CTAB):
0.25
2
O. To the dispersion of CMSS (pH
reported so far have been very limited and their particle sizes are 8.2), PTES was added, and the mixture was stirred at 80 °C for 12 h.
generally greater than 100 nm. Further development of a synthetic The dispersion was then dialysed with a mixture of 2M acetic acid
approach is therefore significant from both scientific and practical and ethanol (7:3 v/v) to remove the surfactant (see ESI for details).
points of view.
The samples before and after dialysis are denoted as PTES_R-as and
Herein, we report a new class of mesoporous Janus nanoparticles PTES_R-dia, respectively, where R is the molar ratio of PTES to
prepared by adding phenyltriethoxysilane (PTES) to a dispersion of TEOS used for the preparation.
colloidal mesostructured silica–surfactant composite nanoparticles
Figure 2 shows transmission electron microscopy (TEM) and field-
(
hereafter denoted as CMSS). The obtained Janus nanoparticles (< emission scanning electron microscopy (FE-SEM) images of the
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nanoparticles prepared with varying amounts of PTES (PTES_R-dia).
The core particles before growth (R = 0) have worm-like mesopores Fig. 3. Evolution of the morphology of the nanoparticles (R = 0.5) during the
reaction observed by TEM. The time lapsed after the additionVioewf PATrtiEclSe Otonltihnee
CMSS dispersion is shown in each image. The su Dr fOa cI :t a1 0n t. 1s 0 w3 9e r/ eC 4r eCmC o1 v0 e0 d6 4b Fy
dialysis before the observations. Scale bars: 20 nm.
with open pores on their surfaces. When R ≥ 0.5, it is clearly shown
that the core particles are capped with a hemispherical shell with
smooth surfaces.
The dispersions of CMSS before and after dialysis (PTES_0-as
and PTES_0-dia, respectively) were almost transparent, while those
of PTES_0.5-as and PTES_0.5-dia became translucent (Fig. 4A).
Although the Rayleigh scattering becomes stronger after the shell
formation, the dynamic light scattering (DLS) measurements of all
these samples reveal single peaks at approximately 50 nm and the
absence of any peaks over a few hundred nanometres, indicating
good dispersibility even after the shell formation and subsequent
surfactant removal (Fig. 4B). The particle diameters measured by
DLS do not precisely match the real particle size observed by TEM
but are at least overestimated due to the hydration layer around the
particles.
Fig. 2. TEM and FE-SEM (insets) images of PTES_R-dia: a) R = 0, b) R =
0
.1, c) R = 0.33, d) R = 0.5, e) R = 0.75 and f) R = 1.
The size of the non-porous shell increases with an increase in the
amount of added PTES. An increase of the C contents was also
confirmed (Fig. S2 in ESI†). When R = 0.5, the C/Si ratio evaluated
by the CHN analysis and thermogravimetry was 2.4, which is
relatively higher than that corresponding to the ratio of phenyl
groups to Si atoms in the dispersion (C/Si = 2.0). The difference in
the C/Si ratios is possibly because of the adsorption of
cetyltrimethylammonium (denoted as C16TMA) cations and/or to the
higher solubility of silica compared with that of organosiloxane in an
1
9-21
aqueous solution at near neutral to basic conditions.
As a control
experiment, the reaction of PTES in an aqueous solution of CTAB
and TEA without CMSS was observed to yield dispersed
nanoparticles of approximately 10 nm in diameter (data not shown).
In the PTES_R-dia samples, the coexistence of these nanoparticles
was not confirmed by TEM, suggesting that PTES was mostly
consumed for the capping of CMSS.
Fig. 4. (A) Appearances of PTES_0-as, PTES_0-dia, PTES_0.5-as and
PTES_0.5-dia. (B) Particle diameter distributions of the nanoparticles
measured by dynamic light scattering (DLS) (inset: ζ potentials of each
sample).
The ζ potential values of PTES_0.5-as and -dia were almost the
The growth of hemispherical shells during the reaction was same (+51 mV and +52 mV, respectively), while those of PTES_0-
monitored by TEM (Fig. 3). At early stages of the reaction (0–5 min), as and -dia were largely different (+47 mV and +3 mV, respectively).
asymmetric structures were not clearly observed. After 10 min, non- The positive potentials can be ascribed to the C16TMA cations
1
8
porous shells were clearly observed (lower right part of the particle), adsorbed on the surface of the particles, leading to the high
and the shells subsequently became thicker (~1 h). Further growth of dispersibility of the particles. For PTES_0.5-dia, the amount of
residual C16TMA ions was relatively large compared with PTES_0-
dia. CHN analysis revealed that ~6% of C16TMA cations remained
in PTES_0.5-dia, whereas the amount of residual C16TMA ions in
PTES_0-dia was under the detection limit. This difference is
possibly because of hydrophobic interactions between the alkyl
chains of C16TMA ions and the phenyl groups on the nanoparticles.
the shell was not notably observed during 1–6 h, which indicates the
completion of hydrolysis and condensation of PTES. This trend in
the growth of the shell was also confirmed by CHN analysis; the C
content gradually increased and became constant after 1 h (Table S1
in ESI†).
2
9
The Si CP/MAS NMR spectrum of PTES_0.5-dia (Fig. S3 in
2
3
x
ESI†) consist of the signals assigned to T , T (T represents
3
4
y
x y
PhSi(OH)3–x(OSi) ), Q and Q (Q represents Si(OH)4–y(OSi) ) units
at −69.0 ppm, −78.1 ppm, −100.0 ppm and −110.1 ppm, respectively,
confirming that phenylsilsesquioxane was formed and that the silica
networks originating from the core nanoparticles were retained. In
the IR spectrum of PTES_0.5-dia (Fig. S4 in ESI†), new peaks
assigned to phenyl groups are clearly observed. Importantly, the
4
3
4
degree of condensation of the Q units (Q /(Q + Q )) is apparently
higher than that before the reaction with PTES, suggesting that
2
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phenylsiloxane units are covalently attached to the surface of the continuous growth of the particles initially formed on CMSS
mesoporous silica nanoparticles by Si–O–Si bonds.
without additional nucleation. It is also possible that CMSS are
The nitrogen adsorption/desorption isotherms and pore size located at the interface between water and PTES oil droplets as an
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DOI: 10.1039/C4CC10064F
distributions of PTES_0-dia and PTES_0.5-dia are shown in Fig. 5. emulsifier; hence, nucleation and growth essentially occur on one
Both samples exhibit a Type-IV curve, suggesting the presence of side of CMSS. To verify the importance of slow hydrolysis rate, we
mesopores. The pore size distributions reveal two peaks used pre-hydrolysed PTES (Fig. S6 in ESI†). Hydrolysis of PTES
corresponding to intraparticle mesopores (< 5 nm) and interparticle under a weakly acidic condition was performed according to a
2
5
voids (>10 nm). The Brunauer–Emmett–Teller (BET) surface area previous report. After the addition of the pre-hydrolysed solution
2
−1
and pore volume of PTES_0.5-dia were calculated to be 430 m g
to the dispersion of CMSS, a homogeneous dispersion without PTES
3
−1
and 0.75 cm g , respectively. These values are smaller than those droplets was obtained immediately. The TEM image of the resulting
2
3
of PTES_0-dia (BET surface area: 780 m /g, pore volume: 1.6 cm
sample demonstrated that asymmetric structures were not formed
−
1
g ), suggesting that the shell is essentially non-porous. The smaller (Fig. S7 in ESI†). A phenylsilsesquioxane shell was formed
pore size of PTES_0.5-dia (ca. 3.2 nm) compared with that for uniformly around the core nanoparticles. Similarly, the Janus
PTES_0-dia (ca. 4.3 nm) is not attributed to residual surfactants structure was not observed when methyltriethoxysilane (MTES) was
because a similar pore size resulted when the surfactant was almost used instead of PTES, which can be explained by the relatively
completely removed under more severe conditions. It is plausible higher hydrolysis rate of MTES. Thus, the proper selection of
that the phenylsilsesquioxane units exist not only in the shell, but organoalkoxysilane is the key to the asymmetric shell formation.
also on the surface of the inner mesopores. When the amount of
PTES was varied in the range of R = 0.1–1.0, the pore size was selectively etched with aq. Na
almost constant (ca. 3.0 nm) independent of the R value (Fig. S5 in silica compared with organosiloxane.
Notably, the mesoporous silica parts in our Janus particles can be
2
CO based on the higher solubility of
3
2
0,21
The selective silica etching
ESI†). This fact indicates that a relatively small portion of PTES is of PTES_0.5-dia produced cap-like nanoparticles (Fig. 6),
involved in the surface modification of the mesopores, and the demonstrating the hemispherical shape of the shell. This finding is in
remaining portion is consumed for the growth of the shell.
contrast to the generation of hollow nanoparticles (Fig. S7 in ESI†)
by etching of the aforementioned isotropically grown particles
prepared using pre-hydrolysed PTES.
Fig. 6. FE-SEM (left) and scanning TEM (right) images of PTES_0.5-dia
after selective etching of the mesoporous silica component.
In conclusion, colloidal mesoporous Janus nanoparticles have been
successfully prepared by the addition of phenyltriethoxysilane to an
aqueous dispersion of mesostructured silica nanoparticles. The high
dispersibility, mesoporosity and structural asymmetry of the Janus
ꢀ
Fig. 5. (Left) Nitrogen adsorption/desorption isotherms and (right) pore size nanoparticles are potentially useful, especially for applications as a
distributions (calculated by NLDFT method) of PTES_0-dia and PTES_0.5-
dia.
phase-selective catalyst or
a
drug-delivery carrier. These
nanoparticles are also attractive as building blocks to create higher
order structures.
Growth of CMSS by the addition of tetraalkoxysilane has recently
2
2
been reported. In this case, isotropic growth of the mesostructured
shell occurs, while the pore size of the core particles is unchanged.
The decrease of the pore size (~1 nm) observed for PTES_0.5-dia
can be explained by the amphiphilic nature of hydrolysed PTES
molecules to facilitate their penetration into the internal surfactant
assemblies in CMSS. A substantial amount of free surfactants is also
present in the liquid phase of the dispersion of CMSS. The formation
of the non-porous phenylsilsesquioxane shell despite the presence of
surfactant is consistent with the general findings that mesostructures
Acknowledgements
The authors thank Mr. K. Onishi and E. Yamamoto (Waseda
Univ.) for discussion. They also thank Mr. M. Yoshikawa (Waseda
Univ.) for his help on NMR measurement. This work was supported
in part by a Grant-in-Aid for Challenging Exploratory Research (No.
2
5620191) and by a Grant-in-Aid for Scientific Research on
Innovative Areas “New Polymeric Materials Based on Element-
Blocks (Area No. 2401)” from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Japan.
2
3
cannot be formed from organotrialkoxysilane alone.
The formation mechanism of the hemispherical shell is not clearly
understood but appears to be related to the hydrolysis rate of PTES.
Notes and references
Department of Applied Chemistry, Faculty of Science and Engineering,
a
Generally, hydrolysis of alkoxysilane under basic conditions
−
proceeds via OH attack against centred Si. Compared with TEOS, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
PTES is hydrolysed much more slowly because of the steric and
Fax:
(+81)
3-5286-3199,
E-mail:
shimojima@waseda.jp,
2
4
inductive effects of phenyl groups. In our system, therefore, the
added PTES exists as an oil phase for a relatively long period. A
slow supply of hydrolysed PTES to the water phase should favour
kuroda@waseda.jp.
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b
Kagami Memorial Research Institute for Materials Science and
Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku,
Tokyo 169-0051, Japan
View Article Online
DOI: 10.1039/C4CC10064F
†
Electronic Supplementary Information (ESI) available: Experimental
details, Table S1 and Figs. S1–S7. See DOI: 10.1039/c000000x/
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