Molecularly designed nanoparticles by dispersion of self-assembled organosiloxane-based mesophases

Article abstract of DOI:10.1002/anie.201404515

The design of siloxane-based nanoparticles is important for many applications. Here we show a novel approach to form core-shell silica nanoparticles of a few nanometers in size through the principle of dispersion of ordered mesostructures into single nanocomponents . Self-assembled siloxane-organic hybrids derived from amphiphilic alkyl-oligosiloxanes were postsynthetically dispersed in organic solvent to yield uniform nanoparticles consisting of dense lipophilic shells and hydrophilic siloxane cores. Insitu encapsulation of fluorescent dyes into the nanoparticles demonstrated their ability to function as nanocarriers. Self-assembled hybrid nanoparticles: A new type of oligosiloxane precursor self-assembles into reverse-micellar mesostructures, which can be transformed to nanoparticles with a siloxane core and an organic shell by dispersion in nonpolar organic solvents.

Full text of DOI:10.1002/anie.201404515

Angewandte
Chemie
DOI: 10.1002/anie.201404515
Hybrid Nanoparticles
Molecularly Designed Nanoparticles by Dispersion of Self-Assembled
Organosiloxane-Based Mesophases**
Shigeru Sakamoto, Yasuhiro Tamura, Hideo Hata, Yasuhiro Sakamoto, Atsushi Shimojima,* and
Kazuyuki Kuroda*
Abstract: The design of siloxane-based nanoparticles is
important for many applications. Here we show a novel
approach to form core–shell silica nanoparticles of a few
nanometers in size through the principle of “dispersion of
ordered mesostructures into single nanocomponents”. Self-
assembled siloxane–organic hybrids derived from amphiphilic
alkyl-oligosiloxanes were postsynthetically dispersed in
organic solvent to yield uniform nanoparticles consisting of
dense lipophilic shells and hydrophilic siloxane cores. In situ
encapsulation of fluorescent dyes into the nanoparticles
demonstrated their ability to function as nanocarriers.
surface properties as well as with the ability to accommodate
various guest species thus addresses an important need in
these fields.
The self-assembly of amphiphilic organosilanes has
attracted growing interest as a direct approach to silica–
organic hybrid nanomaterials.[11–14] Micellar and vesicular
particles have been prepared from lipids or block copolymers
bearing Si(OEt)3 groups;[13,14] however, difficulty remains in
precisely controlling their dimensions, particularly in the 1–
10 nm range. In this context, we focused on the formation of
a periodic and extended reverse-type mesophase where
siloxane spheres surrounded by an organic shell are arranged
periodically to establish a straightforward dispersion process
yielding uniform and discrete nanoparticles of a few nano-
meters in size.
Herein, we report a novel approach to form core–shell
silica nanoparticles of a few nanometers in size through the
principle of “dispersion of ordered mesostructures into single
nanocomponents” (Scheme 1). Self-assembled siloxane–
organic hybrids derived from amphiphilic trialkyl-substituted
F
unctionalized nanoparticles have found many applications
including in biomedicine,[1–5] optics,[6,7] electronics,[8,9] and
catalysis.[10] As host or coating matrices, silica is widely
utilized because of its transparency, biocompatibility, insulat-
ing property, and controllable porosity. Fine control of the
particle sizes and surface properties while retaining a high
dispersibility is crucial for various new applications. For
example, the optimal nanoparticle size for drug delivery
systems is limited to the range of 3–7 nm to avoid an
inflammatory immune response.[5] Control of these factors is
also crucial for the alignment and dispersion of the nano-
particles to provide unique functions for electronic and
optical devices.[10] The methodology for the preparation of
uniform and stable silica nanoparticles with tailored sizes and
[*] S. Sakamoto, Y. Tamura, Dr. H. Hata, Prof. A. Shimojima,
Prof. K. Kuroda
Department of Applied Chemistry, Waseda University
Ohkubo-3, Shinjuku-ku, Tokyo 169-8555 (Japan)
E-mail: shimojima@waseda.jp
kuroda@waseda.jp
Prof. K. Kuroda
Scheme 1. Self-assembly/dispersion approach to organically modified
silica nanoparticles: (i) an evaporation-induced self-assembly process
involving hydrolysis and polycondensation of 1(Sin); (ii) dispersion of
a hybrid mesophase (1H(Sin)) in a nonpolar solvent to form single
nanoparticles. Hydrophilic guest species can be loaded into the
nanoparticles during the self-assembly process.
Kagami Memorial Research Institute for Materials Science &
Technology, Waseda University
Nishiwaseda-2, Shinjuku-ku, Tokyo 165-0032 (Japan)
Dr. Y. Sakamoto
Nanoscience and Nanotechnology Research Center
Osaka Prefecture University, Sakai 599-8570 (Japan)
[**] This work was supported in part by a Grant-in-Aid for Scientific
Research (A) from the Japan Society for the Promotion of Science
(JSPS) and by a Grant-in-Aid for Scientific Research on the
Innovative Areas: ‘Fusion Materials’ (Area No. 2206) from the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT) (Japan). We are grateful to Prof. O. Terasaki (Stockholm
Univ.), Dr. H. Miyata (Canon Inc.), Dr. Y. Hagiwara and R. Hoshi
(Waseda Univ.), and Dr. J. Du (Queens Univ., Canada) for
experimental help and suggestions.
oligosiloxanes were postsynthetically dispersed in organic
solvent to yield uniform nanoparticles consisting of dense
lipophilic shells and hydrophilic siloxane cores. To explore
their ability to function as nanocapsules, we demonstrate the
in situ encapsulation of hydrophilic dyes into the nanoparti-
cles. The synthetic development of such a molecularly
designed nanoparticle is a promising step toward various
advanced applications.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404515.
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This concept has been realized by the design of linear and
alkoxylated oligosiloxane precursors, in which three alkyl
chains are attached to the terminal Si atom (1(Sin) in
Scheme 1). The larger volume of the hydrophobic trialkylsilyl
group relative to the linear oligosiloxane chain should favor
the formation of reverse micelles and their assemblies. The
self-assembly and siloxane network formation are induced by
the evaporation of solvent from the solution containing the
hydrolyzed precursor, which allows the synthesis of mono-
disperse silica nanoparticles stabilized against irreversible
aggregation by the organic outer shell. This strategy is
fundamentally different from the conventional approaches
that use reverse micelles or block copolymer assemblies as
templates for the confined growth of inorganic nanoparti-
cles.[15,16]
The precursors 1(Sin) were synthesized by the reaction of
trihexyl(hydroxy)silane with alkoxylated oligosiloxanes (see
the Supporting Information for details) and were hydrolyzed
in tetrahydrofuran (THF) under acidic conditions. After
À
complete hydrolysis of the constituent Si OMe groups, the
solutions were cast onto a glass substrate to evaporate the
volatile components so that self-assembly of the hydrolyzed
molecules and the concomitant polysiloxane formation could
proceed. The resulting film was solid, but exhibited some
plasticity. The resulting hybrid material 1H(Si4) from the
1(Si4) precursor was characterized in detail by electron
microscopy, X-ray diffraction (XRD), and solid-state NMR
spectroscopy.
The scanning electron microscopy (SEM) imaging of the
surface of the cast film revealed hexagonally arranged
spherical particles with a uniform diameter of ca. 3.3 nm
(Figure 1a). The particle size roughly corresponds to twice
the extended molecular length of 1(Si4) (ca. 1.7 nm). This film
exhibits an XRD peak corresponding to a d spacing of 2.7 nm
(Figure 1b). Assuming that this peak is assigned to the (111)
plane of a face-centered cubic structure, the calculated
particle size (3.3 nm) is in good agreement with that
determined by SEM. The solid-state 29Si magic-angle spinning
(MAS) NMR spectrum of 1H(Si4) shows the Q3 and Q4
signals along with the M1 and Q2 signals (Qm: Si(OSi)m-
(OH)4Àm, M1: C3Si(OSi); Figure 1c), confirming the formation
of crosslinked siloxane networks with a certain degree of
uncondensed silanols. The presence of the alkyl chains as well
as the elimination of alkoxy groups were confirmed by 13C
cross-polarization magic-angle spinning (CP-MAS) NMR
spectroscopy (see the Supporting Information, Figure S1).
The hybrid material 1H(Si4) was readily dispersed in
hexane to give a colloidal solution. When the clear solution is
allowed to dry, island-shaped aggregates (Figure 2a) and
discrete nanoparticles (Figure 2b) are clearly observed by
SEM and the relative proportion of these two forms depends
on the dilution ratio. The discrete particles are also observed
by transmission electron microscopy (TEM) (inset of Fig-
ure 2b). The affinity of these nanoparticles to the nonpolar
solvent strongly suggests that they have a reverse micellar
structure with the alkyl chains facing outward and the
hydrophilic silanol groups inward. Furthermore, these nano-
particles can be re-assembled into various macroscopic
morphologies, such as spherical particles (ca. 200 nm in
Figure 1. a) Typical SEM micrograph and its Fourier transform image
of the hybrid mesostructure 1H(Si4). The sample was imaged without
prior deposition of a conductive metal top-coat. b) XRD pattern and
c) solid-state 29Si MAS NMR spectrum of 1H(Si4).
diameter) through the addition of ethanol to the hexane
solution (Figure S2), and as thin films by coating the hexane
solution onto substrates (data not shown). These secondary
structures can again be re-dispersed in hexane. The ability of
these nanoparticles to undergo reversible assembly/disper-
sion illustrates their chemical stability.
Thus, core–shell hybrid nanoparticles as small as 3.3 nm in
diameter have been obtained. The maximum thickness of the
organic shell is ca. 0.7 nm when the C6 chains are in an
extended state, and the diameter of the siloxane core is then
calculated to be ca. 1.9 nm. A recent report on the templated
synthesis of silica nanospheres in metal complex capsules
confirmed that a nanosphere of 2.9 nm in diameter is
composed of roughly 170 SiO2 units.[17] Considering that our
nanoparticles have a similar degree of condensation (i.e. Q3
unit is predominant), as confirmed by 29Si MAS NMR
spectroscopy, the number of SiO2 units in our case is
estimated to be 48. Therefore, each nanoparticle of 1H(Si4)
is presumed to be composed of at least 12 molecules of 1(Si4).
Our previous studies have shown that the structural
periodicity of the alkylsiloxane mesophase is dependent on
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reactions, and radical polymerizations. For biomedical appli-
cations, grafting of hydrophilic PEG chains to the terminal
=
C C bonds is of great significance. Our direct-assembly
method thus provides a versatile route for the production of
organically modified nanoparticles. This is superior to the
conventional method based on the postsynthetic modification
of bare silica nanoparticles,[18,19] which is often complicated by
self-condensation of the silylating agents and/or irreversible
aggregation of the nanoparticles during modification.
To demonstrate the ability of our silica nanoparticles to
serve as nanocapsules, we investigated the encapsulation of
a hydrophilic fluorescent dye, 7-hydroxycoumarin, into the
siloxane core by dissolving the dye in the solution of
hydrolyzed 1(Si4) prior to evaporation-induced self-assembly.
The obtained particles can be dispersed in hexane, and the
resulting clear solution exhibits absorption at lmax = 327 nm
(Figure 3), suggesting that the dyes are incorporated in the
nanoparticles. The fluorescent peak at 390 nm suggests that
Figure 2. Direct observation of the nanoparticles deposited from
a hexane solution. SEM images of a) island-shaped aggregates and
b) discrete nanoparticles. The samples were prepared by applying
drops of the solutions on copper TEM grids followed by drying in air.
The inset of (b) shows the TEM image of the discrete nanoparticles.
the alkyl chain length of the precursors.[11] In the present case,
precise size control of the nanoparticles can be achieved by
varying the length of the oligosiloxane chains in the presence
of trihexyl groups. Homologous precursors with a disiloxane
unit, 1(Si2), as well as a linear trisiloxane unit, 1(Si3), form
mesophases with smaller d spacings (Figure S3), which
indicates a decrease of the resultant particle size. These
mesophases were fully dispersible in hexane. Notably, 1H-
(Si2) was a viscous liquid mainly consisting of the trimer of
hydrolyzed 1(Si2) having a cyclic trisiloxane core, as con-
firmed by liquid-state 29Si NMR spectroscopy and matrix-
assisted laser desorption/ionization time-of-flight mass spec-
trometry (MALDI-TOF-MS, Figure S4). The smaller number
of crosslinkable units available with this disiloxane precursor
should result in a decreased number of the molecular
components in a particle. Hence, the direct observation of
individual nanoparticles has not been achieved mainly
because of their extremely small numbers of SiO2 units.
Varying the chemical composition of the outer organic
layer plays a key role in tuning the surface and therefore the
nanoparticlesꢀ properties with regard to interparticle inter-
actions, solubility, and functionality.[19,20] As an example, we
Figure 3. UV/Vis spectrum (black line) and fluorescence spectrum (red
line, excitation wavelength: 327 nm) of the hexane dispersion of 7-
hydroxycoumarin-containing 1H(Si4). The inset shows the photograph
of the dispersion under UV irradiation (365 nm).
the incorporated dye molecules are not aggregated and in
their neutral state,[20] implying that the dyes are fixed to the
siloxane core through a weak hydrophilic interaction and/or
hydrogen bonding. This represents a new method to “dis-
solve” hydrophilic dyes in nonpolar solvents and such an
encapsulation of dyes in silica matrices is expected to enhance
their stability against degradation.[21] Similar results were
observed for different types of hydrophilic dyes (Figure S7),
confirming that this encapsulation capability is of general
versatility.
In conclusion, we have developed a novel route to
organically modified silica nanoparticles with diameters of
a few nanometers from well-defined single-molecule precur-
sors. The small silica core is covered with a dense layer of
alkyl chains, which allows for the dispersion of the bulk
mesophase into its individual nanoparticle building blocks
without the occurrence of irreversible nanoparticle aggrega-
tion. Furthermore, we succeeded in encapsulating a fluores-
cent dye into the siloxane cores. This study paves the way to
new possibilities for designing functionalized hybrid nano-
=
performed the reaction of the precursor bearing C C bonds
at the terminus of the hexyl groups forming a periodic
structure that exhibits an XRD peak at d = 2.3 nm (Fig-
ure S5). The slightly shorter periodicity compared to that of
1H(Si4) aggregates may be attributed to the different packing
=
=
due to the presence of C C bonds. The C C bonds should
allow further reactions, such as hydrosilylation, coupling
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was stirred for 30 min to completely dissolve the dye, and then cast on
glass substrates and air-dried.
particles through the targeted tailoring of the chemical and
physical properties of the core and shell components.
Dispersion of hybrid materials (1H(Si4)) into nanoparticles: The
hybrid materials derived from 1(Si4) can be well dispersed in hexane
(regardless of whether they contain dyes). A tiny amount of
undispersed polymeric species was removed by filtration to give
clear and colorless solutions.
Experimental Section
Synthesis of 1(Si4): All reactions were performed under nitrogen
atmosphere using standard Schlenk techniques. The alkoxytrisiloxane
was synthesized by adding H2O (17.5 mL) and 6n HCl (12.5 mL) to
a mixture of tetramethoxysilane (TMOS, 500 g) and tetrahydrofuran
(THF, 300 mL), followed by heating at reflux for 3 h. After removal of
the solvent in vacuo, octamethoxytrisiloxane [(CH3O)3SiOSi-
(OCH3)2OSi(OCH3)3] was isolated by vacuum distillation. Partial
Characterization: Liquid-state 29Si, 13C, and 1H NMR spectra
were recorded on a JEOL Lambda-500 spectrometer at resonance
frequencies of 99.3, 125.7, and 500 MHz, respectively. The solution
was put in a 5 mm glass tube, where a small amount of [D8]THF was
added for obtaining lock signals, and a small amount of chromium-
(III) acetylacetonate was added for the relaxation of 29Si nuclei. Solid-
state 29Si MAS NMR and 13C CP/MAS NMR spectra were recorded
on a JEOL JNM-CMX-400 spectrometer at resonance frequencies of
79.42 and 100.5 MHz with recycle delays of 100 s and 12 s, respec-
tively. The qÀ2q XRD patterns of the mesostructured hybrid
materials were recorded on a Mac Science M03XHF22 diffractometer
with Mn-filtered Fe Ka radiation or on a Rigaku RINT 2000
diffractometer with Ni-filtered Cu Ka radiation. TEM studies were
carried out on a JEOL JEM-2010 electron microscope operated at
200 kV. SEM observation was carried out on a HITACHI S-5500
microscope operated at 30 kV. For TEM and SEM observation,
mesostructures were directly formed by applying drops of the
hydrolyzed solution on a carbon-supported grid. In the case of the
particle observation, the hexane solutions of particles were applied
dropwise onto the same grid and air-dried. UV/Vis absorption spectra
were obtained with a Shimadzu UV-3100PC instrument. Fluores-
cence spectra were recorded on a Hitachi F-4500 spectrofluoropho-
tometer at the excitation wavelength of 327 nm. For spectroscopic
data, transparent hexane solutions were measured, after filtration of
the solution with a 0.2 mm filter. MALDI-TOF mass analysis was
carried out with a Shimadzu AXIMACFR instrument. The matrix of
1,8,9-anthracenetriol and the sample in hexane were spotted on the
MALDI-TOF MS source target and allowed to dry in air prior to
analysis.
À
À
replacement of OMe groups with Cl was then performed by
stirring a mixture of octamethoxytrisiloxane (25 g), tetrachlorosilane
(SiCl4; 1.3 mL), and AlCl3 (0.32 g) at RT for 1 day. Removal of AlCl3
followed by vacuum distillation gave a mixture of (CH3O)3SiOSi-
(OCH3)2OSi(OCH3)3 and (CH3O)2ClSiOSi(OCH3)2OSi(OCH3)3
(approximate molar ratio of 1:1, as shown in Figure S8). Trihexylsi-
lanol was synthesized by stirring a mixture of trihexylsilane (5.0 g),
THF (60 mL), H2O (2.0 mL), and Pearlmanꢀs catalyst (22 mg) for
30 min in an ice bath. The resulting solution was filtered to remove
the catalyst and the solvent was evaporated. The resulting solid
(trihexylsilanol) was dissolved in hexane (50 mL) and added to
a stirred mixture of alkoxytrisiloxane (20 g), pyridine (5.2 mL), and
THF (100 mL), and the mixture was stirred at RT. To eliminate the
À
residual Cl groups, methanol (1.3 mL) was added to the mixture.
After removal of pyridine hydrochloride by filtration, the residue was
distilled under vacuum to yield 1(Si4) as a clear, colorless liquid
(8.5 g). Spectroscopic data for 1(Si4): 1H NMR (500 MHz, [D8]THF):
d = 0.61–0.64 (m, 2H), 0.88–0.89 (t, 3H; CH3), 1.31–1.39 (m, 8H),
3.51–3.55 ppm (s, 21H; OCH3); 13C NMR (125.7 MHz,[D8]THF): d =
14.42, 15.88, 23.42, 23.73, 32.43, 34.18, 50.96, 51.08, 51.10 ppm; 29Si
NMR (99.3 MHz, [D8]THF): d = À93.97 (Q2), À92.76 (Q2), À86.17
(Q1), 10.21 ppm (M1); MS (MALDI-TOF): [M+K+] = 672,
[M+Na+] = 656.
Synthesis of 1(Si2) and 1(Si3): Compounds 1(Si2) and 1(Si3) were
synthesized by the reaction of trihexylsilanol with monochlorinated
alkoxysilane and alkoxydisiloxane, respectively. The silylating agent
used for the synthesis of 1(Si2) was the mixture of SiCl(OCH3)3
(> 60%) and Si(OCH3)4 obtained by adding methanol dropwise to
SiCl4 in a N2 flow. The silylating agent for 1(Si3) was the siloxane
Received: April 21, 2014
Published online: && &&, &&&&
Keywords: amphiphiles · inorganic–organic hybrids ·
.
dimer with a Si Cl group.[12] Other procedures were the same as those
nanoparticles · organosiloxanes · self-assembly
À
for the synthesis of 1(Si4). Spectroscopic data for 1(Si2): 1H NMR
(500 MHz, [D8]THF): d = 0.60–0.63 (m, 2H), 0.88–0.90 (t, 3H; CH3),
1.30–1.39 (m, 8H), 3.48 ppm (s, 9H; OCH3); 13C NMR (125.7 MHz,
[D8]THF): d = 14.40, 15.90, 23.36, 23.70, 32.39, 34.10, 50.95 ppm; 29Si
NMR (99.3 MHz,[D8]THF): d = À84.84 (Q1), 9.93 ppm (M1). Spectro-
scopic data for 1(Si3): 1H NMR (500 MHz, [D8]THF): d = 0.61 (m,
2H), 0.89 (t, 3H; CH3), 1.30 (m, 8H), 3.48–3.52 ppm (s, 15H; OCH3);
13C NMR (125.7 MHz, [D8]THF): d = 14.35, 15.83, 23.25, 23.58, 32.29,
34.01, 50.82, 51.85, 50.94 ppm; 29Si NMR (99.3 MHz, [D8]THF): d =
À92.58 (Q2), À86.15 (Q1), 10.14 ppm (M1); MS (MALDI-TOF):
[M+K+] = 566, [M+Na+] = 550.
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Synthesis of hybrid material 1H(Sin): Hydrolysis and polycon-
densation of 1(Sin) were performed in a mixture with the molar ratio
of 1(Sin)/THF/H2O/HCl = 1:30:(2nÀ1) ꢁ 2:0.01. The mixture was
stirred at room temperature until complete hydrolysis was confirmed
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compositions of 1(Si2)/THF/H2O/HCl = 1:30:12:0.01 and 1(Si4)/
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cast on glass substrates and air-dried at RT for 1 day. In the case of
1H(Si2), the film was aged for 1.5 h at 808C to form the structure with
a better arrangement.
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ratio of 1(Si4)/THF/H2O/HCl/dye = 1:30:15:0.002:0.2. The mixture
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Communications
Hybrid Nanoparticles
S. Sakamoto, Y. Tamura, H. Hata,
Y. Sakamoto, A. Shimojima,*
K. Kuroda*
&&&& — &&&&
Molecularly Designed Nanoparticles by
Dispersion of Self-Assembled
Organosiloxane-Based Mesophases
Self-assembled hybrid nanoparticles: A
new type of oligosiloxane precursor self-
assembles into reverse-micellar meso-
structures, which can be transformed to
nanoparticles with a siloxane core and an
organic shell by dispersion in nonpolar
organic solvents.
6
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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