Preparation of silica nanosprings using cationic gelators as template

Article abstract of DOI:10.1246/bcsj.78.2069

Cationic cyclo(dipeptide) gelators 10mimClO₄, 11mimClO ₄, and 11pyClO₄ were synthesized as templates for the sol-gel transcription reaction of alkoxysilanes. They were able to cause physical opaque gels in alcohols such as

Full text of DOI:10.1246/bcsj.78.2069

ꢀ 2005 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 78, 2069–2074 (2005) 2069
Preparation of Silica Nanosprings Using Cationic Gelators as Template
Ã
Yonggang Yang, Hiroaki Fukui, Masahiro Suzuki, Hirofusa Shirai, and Kenji Hanabusa
Department of Functional Polymer Science, Faculty of Textile Science and Technology,
Shinshu University, Ueda 386-8567
Received April 28, 2005; E-mail: hanaken@giptc.shinshu-u.ac.jp
Cationic cyclo(dipeptide) gelators 10mimClO4, 11mimClO4, and 11pyClO4 were synthesized as templates for the
sol–gel transcription reaction of alkoxysilanes. They were able to cause physical opaque gels in alcohols such as meth-
anol, ethanol, and 1-propanol. Three-dimensional fibrous networks responsible for physical gelation were found in SEM
images of gels. Sol–gel transcription reactions using the self-assemblies of three gelators as the templates were carried
out in the presence of propylamine as a catalyst. When tetraethoxysilane (TEOS) was selected as a precursor, three kinds
of morphologies, such as inner-helical nanotubes, right-handed nanobundles, and right-handed helical nanosprings were
observed by electron microscopy. The helical nanosprings were preferentially formed as compared with nanotubes and
nanobundles. On the other hand, the sol–gel polymerization of bis(triethoxysilyl)methane, 1,2-bis(triethoxysilyl)ethane,
and 1,4-bis(triethoxysilyl)benzene as precursors gave nanoballs instead of tubular structures. When tetrabutylammonium
fluoride (TBAF) was used as a catalyst, a helical tubular structure coated by silica nanoballs was identified.
Nanostructures, especially one-dimensional nanostructures
such as wires, rods, belts, and tubes, could be used in many
Results and Discussion
fields due to their electronic, optical, magnetic, and thermo-
electric properties.1 Both chemical and physical methods were
designed to synthesize these nanostructures.2 The helicity and
periodicity of helical nanosprings, which belong to a new
group of one-dimensional nanostructure, are of special interest
for nano-engineering. Up to now, only a few papers have been
published for the synthesis of nanosprings.3 For example, the
chemical vapor deposition (CVD) technique has been adopted
to prepare nanosprings. Unfortunately, the yield of nano-
springs was still low and the nanosprings were combined with
other nanostructures; namely, the morphologies were not uni-
form.3 It is of interest to create new methods and to find new
ways for preparing nanosprings.
Helical structures can often be found in nature; for instance,
ꢀ-helical polypeptides, double helical nucleic acids, spirulina,
and spiral shells. With respect to artificial nanomaterials, hel-
ical nanowires, helical nanotubes,4 inner helical nanotubes,5
and hybrid helical bundles6 have been prepared. Helical nano-
materials can be used as asymmetric reaction catalysts,7 helical
sensor,8 and optical materials.9 Generally, surfactants,10 li-
pids,11 and gelators3,4 were selected as the templates for the
preparation of these helical nanomaterials. It is thought that
the templates act as structure-directing agents. Gelators are
one of the candidates to act as templates, because they can
show many kinds of morphologies; for example, rod, single
helical strand, double helix, multiple helix, and twist ribbon.12
And the sol–gel transcription method has been a promising
method for the preparation of inner helical inorganic nano-
tubes.13 In particular, chiral gelators have the potential to
transfer their helicity and periodicity to the inorganic nanoma-
terials by using the sol–gel transcription method. In this paper,
we report the preparation of silica nanosprings using cationic
gelators as template.
Gelation Properties of Gelators 11pyClO4, 10mimClO4,
and 11mimClO4. Gelators 11pyClO4, 10mimClO4, and
11mimClO4, used as the templates, were synthesized by the
pathway in Fig. 1. The compounds 11pyClO4, 10mimClO4,
and 11mimClO4 showed nearly the same gelation properties,
being capable of gelling acetone, THF, and alcohols such as
methanol, ethanol, 1-propanol, and t-butyl alcohol. Another
notable property is that they can form transparent gels in water
with less than 0.5 wt %. Because these gelators cause opaque
gels in alcohols, CD spectra were taken using water as the
solvent in which the transparent gels were formed. The CD
O
OH
O
O
O
O
Br
HO
Br
n
H
H
HN
n
HN
1) pyridine or
imidazole
NH
NH
diisopropylcarbodiimide/
4-dimethylaminopyridine
O
O
H
H
2) NaClO4
N
+ ClO4
O
O
N
O
O
O
N
-
-
+ ClO4
n
11
H
H
O
HN
HN
NH
NH
O
O
H
H
11pyClO4
n = 10; 10mimClO4
n = 11; 11mimClO4
Fig. 1. Synthesis of gelators 11pyClO4, 10mimClO4, and
11mimClO4.
Published on the web October 31, 2005; DOI 10.1246/bcsj.78.2069

2070 Bull. Chem. Soc. Jpn., 78, No. 11 (2005)
Preparation of Silica Nanosprings
20
15
11mimClO
4
10
5
0
11pyClO
4
Fig. 4. FE-SEM image of silica obtained by sol–gel tran-
scription method using gelator 10mimClO4.
-5
200
250
300
350
400
450
Wavelength/nm
Fig. 2. CD spectra of the hydrogels of 11pyClO4 and
11mimClO4 (concentration: 5 mg/1.0 mL).
Fig. 5. FE-SEM image of silica obtained by sol–gel tran-
scription method using gelator 10mimClO4.
Fig. 3. FE-SEM image of xerogel prepared from t-butyl
alcohol gels of 11pyClO4 (concentration: 50 mg/0.5 mL).
spectra were strongly active as shown in Fig. 2. The source of
the CD activity is unclear at this time, but it is assumed that the
strong induced CD band originates from a chiral structure of
gel aggregate. Furthermore, they are readily soluble in DMF,
but insoluble in toluene and decane. To study visually the
supramolecular self-assemblies of gelators, we used FE-SEM.
Almost resembling morphologies were observed for the three
gelators. Figure 3 shows the SEM image of the xerogel ob-
tained from t-butyl alcohol gel of 11pyClO4 by the freeze-
and-pump method. We can detect two kinds of nanofibers;
namely, one is huge nanofibers which are uniform in diameter
(600–400 nm) and the other is fine nanofibers whose diameter
is around 50 nm. Unfortunately, it was difficult to decide the
helicity of their fibers from the SEM image of Fig. 3, although
they were built up by chiral compounds.
Formation of Silica Nanosprings Using Propylamine as
Catalyst. Figures 4 and 5 show the FE-SEM images of the
silica obtained by the sol–gel polymerization of TEOS by
the transcription method using gelator 10mimClO4. Figure 6
shows SEM images of the silica that were obtained using
the self-assemblies of 11mimClO4 and 11pyClO4 as tem-
plates. There are almost the same morphologies in the three
silica obtained by replication of 10mimClO4, 11mimClO4,
and 11pyClO4. Namely, we can recognize three kinds of
Fig. 6. FE-SEM images of silica obtained by sol–gel tran-
scription method using gelators 11mimClO4 (a) and
11pyClO4 (b).
morphologies building up a hierarchical structure. The first
morphology is helical nanotubes with an inner diameter
around 10–40 nm. These nanotubes possess an inner helical
structure (Fig. 5 and Fig. 7b). The second morphology is
right-handed helical nanobundles which are constructed by
gathering nanotubes (Fig. 5). The diameters of helical nano-
bundles are around 100 nm. The third morphology is the most
interesting nanosprings. Many right-handed helical nano-
springs with a diameter around 300 nm were constructed by
right-handed helical nanobundles in these silica (Fig. 4,
Fig. 5, and Fig. 6). The inner diameters of nanosprings were
around 100 nm. The TEM image of nanosprings looks compli-
cated, but we confirmed they possess helical and tubular struc-
tures (Fig. 7a). These morphologies, from nanotube to right-
handed nanospring via right-handed helical nanobundle, can

Y. Yang et al.
Bull. Chem. Soc. Jpn., 78, No. 11 (2005) 2071
Fig. 7. TEM images of the silica obtained by sol–gel transcription method using gelators (a) 11mimClO4 and (b) 10mimClO4.
Fig. 8. The formation of inner-helical silica tube, helical silica bundle, and silica nanospring.
help us not only to understand the hierarchical structure of
self-assembly of gelators during the sol–gel transcription proc-
ess, but also to understand that the morphologies of the self-
assembly formed by gelators were sensitively influenced by
surroundings such as solvents, catalyst, and TEOS. From the
comparison of Fig. 3 and Fig. 4, it is thought that the helicity
and periodicity of the self-assemblies of gelators are amplified
during the sol–gel transcription process.
The postulated mechanism of the formation of nanotubes,
helical nanobundles, and nanosprings is shown in Fig. 8. First
of all, helical gel fibers are constructed through the self-assem-
bly of gelators. Secondly, silica oligomers are adsorbed on the
surfaces of the helical gel fibers. At the same time the helical
gel bundles are constructed by these fibers and spring-like gel
aggregates are formed by the self-assembly and self-coiling of
the helical gel bundles. The sol–gel polymerization of TEOS
oligomers proceeds on the surface of the helical gel fibers,
helical gel bundles, and spring-like gel aggregates side by side.
After the polymerization is completed, gelators are removed
by washing with hot methanol. Finally, helical nanotubes,
helical nanobundles, and nanosprings of silica are constructed.
Although the silica nanosprings were combined with nano-
tubes and helical nanobundles, the nanosprings were preferen-
tially formed (Fig. 4). If we carefully select appropriate gela-
tors and reaction conditions, we can probably produce pure
nanosprings. Considering that the gelators show a variety of
morphologies, the sol–gel polymerization method using them
as templates should be a powerful method for the synthesis
of nano materials.
Formation of Hybrid Silica Nanoparticles. The research
of hybrid silica has been developing rapidly from the periodic
mesoporous materials with organic segments in the walls14 to
hybrid nanofibers.6 The organic segments in hybrid silica can
improve the mechanical properties. We studied the sol–gel
transcription of bis(triethoxysilyl)methane, 1,2-bis(triethoxy-
silyl)ethane, and 1,4-bis(triethoxysilyl)benzene as the precur-
sors. In the case of the sol–gel transcription of bis(triethoxy-
silyl)methane using the gelator 11pyClO4 as a template, short

2072 Bull. Chem. Soc. Jpn., 78, No. 11 (2005)
Preparation of Silica Nanosprings
Fig. 9. FE-SEM images of hybrid silica obtained by sol–gel
polymerization of bis(triethoxysilyl)methane (a) and 1,2-
bis(triethoxysilyl)ethane (b) using gelator 11pyClO4.
Fig. 11. FE-SEM (a) and TEM (b) images of silica ob-
tained by sol–gel polymerization of TEOS in the presence
of TBAF using gelator 11mimClO4.
kinds of morphologies were observed in the silica obtained
from TEOS. They were helical nanotubes, right-handed
nanobundles, and right-handed helical nanosprings. The
helical nanosprings were preferentially formed as compared
with others. Sol–gel polymerization with bis(triethoxysilyl)-
methane, 1,2-bis(triethoxysilyl)ethane, and 1,4-bis(triethoxy-
silyl)benzene as precursors gave nanoballs instead of tubular
structures. When TBAF instead of propylamine was used as
a catalyst, a helical tubular structure coated by silica nanoballs
was identified.
Experimental
Fig. 10. TEM image of hybrid silica obtained by sol–gel
polymerization of bis(triethoxysilyl)methane using gelator
11pyClO4.
Chemicals. The perchlorate salts, 11pyClO4, 10mimClO4,
and 11mimClO4, were obtained by ionic exchange16 between
sodium perchlorate and the corresponding bromide salts which
were synthesized according to the literature.17 Cyclo(L-aspara-
ginyl-L-phenylalanyl) was prepared by cyclization of L-aspara-
ginyl-L-phenylalanine methyl ester.18
Cyclo(L-ꢀ-11-bromoundecylasparaginyl-L-phenylalanyl).
Cyclo(L-asparaginyl-L-phenylalanyl) (10.23 g, 0.039 mol) was
dissolved in 250 mL of DMF, and then 4-dimethylaminopyridine
p-toluenesulfonate (17.22 g, 0.059 mol), 11-bromo-1-undecanol
(10.00 g, 0.039 mol), and diisopropylcarbodiimide (5.83 g, 0.046
mol) were added. The solution was stirred at room temperature
overnight. After evaporating DMF under reduced pressure, the
residue was washed with water. The crude product was recrystal-
lized from a mixture of methanol and diethyl ether. Cyclo(L-ꢁ-11-
bromoundecylasparaginyl-L-phenylalanyl) was obtained in a yield
of 13.9 g (72%).
N-Methylimidazolium Bromide Salt of Cyclo(L-aspara-
ginyl-L-phenylalanyl) Derivative (11mimBr). A mixture of
cyclo(L-ꢁ-11-bromoundecylasparaginyl-L-phenylalanyl) (5.95 g,
12 mmol) and N-methylimidazole (5 mL, 62.5 mmol) in DMF
(30 mL) was heated at 60 ꢀC for 24 h under a nitrogen atmo-
sphere. After the solvent was removed in vacuum, the residue
was dissolved in 50 mL of methanol. Diethyl ether was added
dropwise to the solution until the cloudy endpoint was reached.
The solution was cooled at 0 ꢀC, and then the precipitate was
filtered and dried. Yield: 6.10 g (88%).
N-Methylimidazolium Perchlorate Salt of Cyclo(L-aspara-
ginyl-L-phenylalanyl) Derivative (11mimClO4). To 50 mL of
saturated sodium perchlorate aqueous solution, the aqueous solu-
tion of 11mimBr (2.00 g, 3.46 mmol) was added under stirring.
After the mixture was stirred for 12 h, the precipitate was filtered,
washed with distilled water and dried. The crude product was
recrystallized from a mixture of methanol and diethyl ether. Yield
clusters tying in a row were obtained (Fig. 9a), however, the
clusters possessed no inner structure (Fig. 10). With respect
to 1,2-bis(triethoxysilyl)ethane, only nanoballs tying in a row
were found (Fig. 9b). These hybrid nanoballs are several tens
of nanometers in diameter. The sol–gel transcription of 1,4-
bis(triethoxysilyl)benzene gave the cotton-like hybrid silica,
however, it shrank into nonporous transparent small particles
during the air-drying procedure. Although we do not succeed
in preparing tubular structures with the sol–gel transcription
of silsesquioxanes as precursors as yet, it is interesting to note
that the particles could be controlled in nano-size (less than 50
nm),15 which can increase the surface area of hybrid silica.
Formation of Tubular Structures Using TBAF as
Catalyst. We attempted to prepare silica with tubular struc-
ture by the sol–gel polymerization of TEOS using TBAF as the
catalyst instead of propylamine. Figure 11 shows FE-SEM and
TEM images of silica obtained by sol–gel polymerization of
TEOS in the presence of TBAF using gelator 11mimClO4.
Right-handed helical silica nanotubes, whose surface was coat-
ed with nanoparticles, were identified. The diameters of nano-
particles were less than 10 nm. The inner diameters of the
nanotubes are around 30 nm. Because when TBAF was used
as catalyst, the polymerization speed of TEOS was very fast,
so many nanoparticles were identified. Furthermore, when
the concentration of TBAF was increased, only silica balls
were obtained.
Conclusion
Three cationic gelators, synthesized as templates for sol–gel
polymerization, formed huge nanofibers and fine nanofibers
whose diameters are ca. 500 nm and ca. 50 nm in gels. Three
1
1.96 g (95%). H NMR (400 MHz, DMSO): ꢂ 1.26–2.0 (18H, m,

Y. Yang et al.
Bull. Chem. Soc. Jpn., 78, No. 11 (2005) 2073
CH2), 2.92 (2H, d, CH2Ph), 3.10 (2H, d, CH2COO), 3.84 (3H, s,
CH3), 3.92 (2H, t, CH2O), 4.03 (1H, t, CH(PhCH2)), 4.14 (2H, t,
CH2-methylimidazole), 4.22 (1H, t, CH(CH2COO)), 7.17–7.28
(5H, m, Ar), 7.69 (1H, s, NCH), 7.75 (1H, s, NCH), 7.97 (1H,
s, NH), 8.17 (1H, s, NH), 9.08 (1H, s, NCHN). Elemental analysis
calcd (%) for C28H41ClN4O8 (596.26): C, 56.32; H, 6.92; N,
9.38%. Found: C, 56.03; H, 6.73; N, 9.35%.
N-Methylimidazolium Perchlorate Salt of Cyclo(L-aspara-
ginyl-L-phenylalanyl) Derivative (10mimClO4). 1H NMR (400
MHz, DMSO): ꢂ 1.26–2.0 (16H, m, CH2), 2.92 (2H, d, CH2Ph),
3.10 (2H, d, CH2COO), 3.84 (3H, s, CH3), 3.92 (2H, t, CH2O),
4.03 (1H, t, CH(PhCH2)), 4.14 (2H, t, CH2-methylimidazole),
4.22 (1H, t, CH(CH2COO)), 7.17–7.28 (5H, m, Ar), 7.69 (1H,
s, NCH), 7.75 (1H, s, NCH), 7.97 (1H, s, NH), 8.17 (1H, s,
NH), 9.08 (1H, s, NCHN). Elemental analysis calcd (%) for
Characterization. Transmission electron microscope (TEM)
images were obtained using a JEOL JEM-2010. Field emission
scanning electron microscopy (FE-SEM) was taken on a Hitachi
S-5000. Circular dichroism (CD) spectra were measured on a
JASCO J650 spectrophotometer (cell diameter 0.1 mm). 1H NMR
spectra were recorded with a Bruker AVANCE 400 spectrometer
using TMS as an internal standard. Elemental analyses were per-
formed on a Perkin-Elmer series II CHNS/O analyzer 2400.
This work was supported by Grant-in-Aid for 21st Century
COE Program and a grant (No. 15350132) by the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
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Á
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The typical procedure for the sol–gel polycondensation
of TEOS using TBAF as the catalyst is as follows. Gelator
11mimClO4 (30 mg), 10 mL of TBAF aq (0.05 M), and 42 mL
of TEOS were dissolved in 0.30 mL of t-butanol to form a trans-
parent solution. After being cooled down with an ice-water bath to
form an opaque gel, the reaction mixture was kept at room tem-
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11mimClO4 was removed by washing with methanol and then
dried in air overnight.
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