Boroxine nanotubes: Moisture-sensitive morphological transformation and guest release

Article abstract of DOI:10.1002/adfm.201302005

Boroxines, (R-BO)₃, which can be easily synthesized via a dehydration reaction of boronic acids, R-B(OH)₂, selectively self-assemble in toluene into nanofibers, nanorods, nanotapes, and nanotubes, depending on the aromatic substituent (R). Spectroscopic measurements show that the nanotube consists of a J-aggregate of the boroxine. Humidification converts the morphology from the nanotube to a sheet as a result of the hydrolysis of the boroxine components and subsequent molecular-packing rearrangement from the J-aggregate to an H-aggregate. Such a transformation leads to the compulsive release of guest molecules encapsulated in the hollow cylinder of the nanotube. The hydrolysis and the molecular-packing rearrangement described above are suppressed by coordination of pyridine to the boron atom, with the resulting moiety acting as a Lewis acid of the boroxine component. The pyridine-coordinated nanotube is transformed into a helical coil by humidification. Guest release during the nanotube-to-helical-coil transformation is much slower than during the nanotube-to-sheet transformation, but faster than from a nanotube that did not undergo morphological transformation. The storage and release of guest molecules from the boroxine nanotubes can be precisely controlled by adjusting the moisture level and the concentration of Lewis bases, such as amines. Self-assembly of boroxines in toluene produce various nanostructures, including nanotubes, depending on the boroxines' aromatic substituents. Humidification-induced morphological transformations from nanotubes to sheets or helical coils are observed, accompanied by the hydrolysis of the boroxine components and a molecular-packing rearrangement from J- to H-aggregates. These morphological changes enable the precisely controlled release of guest molecules encapsulated in the nanotubes' hollow cylinders.

Full text of DOI:10.1002/adfm.201302005

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Boroxine Nanotubes: Moisture-Sensitive Morphological
Transformation and Guest Release
Kazuyuki Ishikawa, Naohiro Kameta,* Mitsutoshi Masuda, Masumi Asakawa,
and Toshimi Shimizu
nanostructures, supramolecular nano-
Boroxines, (R-BO) , which can be easily synthesized via a dehydration reac-
3
structures are advantageous because
their morphology, size, and dimensions
can be finely and quickly re-tuned by
use of external stimuli to induce struc-
tion of boronic acids, R–B(OH) , selectively self-assemble in toluene into
2
nanofibers, nanorods, nanotapes, and nanotubes, depending on the aro-
matic substituent (R). Spectroscopic measurements show that the nano-
tube consists of a J-aggregate of the boroxine. Humidification converts the
morphology from the nanotube to a sheet as a result of the hydrolysis of
the boroxine components and subsequent molecular-packing rearrange-
ment from the J-aggregate to an H-aggregate. Such a transformation leads
to the compulsive release of guest molecules encapsulated in the hollow
cylinder of the nanotube. The hydrolysis and the molecular-packing rear-
rangement described above are suppressed by coordination of pyridine to the
boron atom, with the resulting moiety acting as a Lewis acid of the boroxine
component. The pyridine-coordinated nanotube is transformed into a helical
coil by humidification. Guest release during the nanotube-to-helical-coil
transformation is much slower than during the nanotube-to-sheet transfor-
mation, but faster than from a nanotube that did not undergo morphological
transformation. The storage and release of guest molecules from the boroxine
nanotubes can be precisely controlled by adjusting the moisture level and the
concentration of Lewis bases, such as amines.
[1]
tural changes. As a result, we are able
to control and switch not only the chem-
ical properties of the supramolecular
nanostructures but also their physical
properties.
Supramolecular nanotubes, which are
formed by self-assembly of well-designed
amphiphilic molecules in solvents, can act
as open-ended nanocapsules for encap-
sulation of molecules (e.g., drugs and
dyes), macromolecules (e.g., DNA and
[
2]
proteins), and nanoparticles. However,
controlling the release of such encapsu-
lated guest molecules from the hollow
cylinder of the nanotube to bulk media is
challenging. Morphological transforma-
tion of the nanotubes via external stimuli
should strongly influence guest release.
Nanotube-to-nanosphere and nanotube-
to-nanofiber transitions, as well as tran-
sitions of nanotubes into nanotubes
with different inner diameters, have already been achieved by
environmental changes such as changes in pH, salt concen-
1
. Introduction
[
3]
[
3]
[4]
[5]
[6]
Well-designed low-molecular-weight compounds self-assemble
in solvents to form supramolecular nanostructures, which
also become soft materials such as gels and liquid crystals
based on the hierarchization of the supramolecular nanostruc-
tures. Compared to covalently bonded polymers and inorganic
tration, temperature, dilution, solvation, and complexa-
tion, or by specific reactions of components in the nanotubes
[7]
[8]
with additives such as metals, cyclodextrin, poly(propylene
[9]
[10]
glycol), and enzymes. Furthermore, we recently developed
photoresponsive nanotubes that are able to instantly release
guests as the tubular structures shrink and unfold in response
[11]
to ultraviolet-visible (UV/vis) irradiation.
Dr. N. Kameta, Dr. M. Masuda, Dr. T. Shimizu
Nanosystem Research Institute (NRI)
National Institute of Advanced Industrial
Science and Technology (AIST)
Tsukuba Central 5, 1–1–1 Higashi
Tsukuba, Ibaraki, 305–8565, Japan
E-mail: n-kameta@aist.go.jp
Herein we investigate the self-assembly behavior of various
boroxines (Scheme 1) to form constructed nanotubes, whose
morphology can be tuned by hydrolyzing the boroxine molecules
to boronic acid. We describe the controlled release of guest mol-
ecules through moisture-stimulated morphological transforma-
tions from nanotube to sheet, and from nanotube to helical coil.
Dr. K. Ishikawa, Dr. N. Kameta, Dr. M. Masuda,
Dr. M. Asakawa, Dr. T. Shimizu
Nanotube Research Center (NTRC)
National Institute of Advanced Industrial
Science and Technology (AIST)
Tsukuba Central 5, 1–1–1 Higashi
Tsukuba, Ibaraki, 305–8565, Japan
2
. Self-Assembly Behavior/Morphological
Transformation of Nanotubes
Boroxines ((R–BO) ), which are formed by dehydration of
3
DOI: 10.1002/adfm.201302005
boronic acids (R–B(OH) , R = alkyl, aryl), have an exclusively
2
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system. Boroxines have received attention owing to their
promise as advanced functional materials for two-dimensional
covalent organic frameworks (COFs), which are formed by
dehydration of R–B(OH) (R = C H Br, C H B(OH) , and so
2
6
4
6
4
2
[
13]
on). COFs are of great interest for gas storage, chemical reac-
tors, nanopatterning, and opto-electronics. On the other hand,
systematic studies of the self-assembled structures of borox-
ines (R = aromatic ring without substituted groups, as shown
in Scheme 1), which never form COFs, have scarcely been per-
formed until now.
We carried out self-assembly experiments by refluxing the
boroxines (1 mg) in toluene (1 mL) for 1 h. The hot solutions
were gradually cooled to room temperature. Scanning electron
microscopy (SEM) observations revealed that the boroxines
independently formed nanofibers 10−20 nm wide and several
tens of micrometers in length (R = 2− and 3-thienyl), nanorods
5
3
0−400 nm wide and with sub-micrometer lengths (R = phenyl,
-biphenyl, and 4-biphenyl), and nanotapes 50−100 nm
wide and several tens of micrometers in length (R = 1− and
-naphtyl, 2- and 9-anthracenyl) (Figure 1). All the self-assem-
2
blies shown in Figure 2 had no hollow structures, whereas a
self-assembly formed from boroxine (R = pyrenyl, abbrevi-
ated as (Py–BO) hereafter) had well-defined open ends in its
3
Scheme 1. Chemical structures of boroxines ((R-BO) ) and their corre-
sponding boronic acids (R-B(OH)2).
3
fibers (Figure 2a). Transmission electron microscopy (TEM)
observation clearly displays a hollow cylinder, indicating that
the self-assembled morphology was tubular (Figure 2b). The
nanotubes had uniform inner diameters of ∼8 nm and a mem-
brane thickness of ∼4 nm.
six-membered ring, regardless of the nature of the substituent
[
12]
(
R). The persistence of the six-membered ring is attributable
to the resonance stabilization of the conjugated six π-electron
Figure 1. SEM images of the self-assemblies formed from boroxines ((R–BO) , R = various aromatic rings) in toluene: (a,b) nanofibers, (c–e) nanorods,
3
and (f–i) nanotapes.
6
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by hydrolysis of (Py–BO) in the nanotube,
3
showed
a hypsochromic shift compared
with that (λmax = 347 nm) of Py-B(OH) dis-
2
solved in toluene (Figure 4b). These results
indicated that the boronic acid forms H-type
aggregates in the sheet (Figure 5a). Thus,
the morphological transformation from
nanotube to sheet was caused not only by
the structural change from boroxine to the
boronic acid, but also by the rearrangement
of the molecular packing from J-type to
H-type aggregation.
3
. Moisture-Responsive Guest
Release
We expected that the humidity-induced mor-
phological transformation from nanotube to
sheet would strongly influence the release
behavior of pre-encapsulated guests in the
nanotubes’ hollow cylinders. Figure 6 shows
the release profile of cis-jasmone (a flavor
Figure 2. (a) SEM image of nanotubes self-assembled from (Py–BO) . (b) TEM image of nano-
3
tubes self-assembled from (Py–BO) (c) SEM image of sheets formed by the humidity-induced
3
morphological transformation of nanotubes (relative humidity 55%). (d) SEM image of helical
coils formed by humidity-induced morphological transformation of pyridine-coordinated nano- component of jasmine essential oil) as a
tubes (relative humidity 55%).
guest molecule. Under low relative humidity
6%), which strictly inhibits the hydrolysis
(
of the boroxine as shown in Figure 3b, slow
release of the encapsulated cis-jasmone into the bulk media was
observed. Such slow release can be ascribed to the nanotubes’
Boroxines easily evolve into their corresponding boronic
acids via hydrolysis reactions (Scheme 1). We expected such
hydrolysis reactions to alter the morphologies of the boroxine
self-assemblies. We stored the nanotubes self-assembled from
[
14]
high axial ratio. Further humidification promoted the release
of the cis-jasmone (Figure 6 and Supporting Information,
Figure S2), as a result of the morphological transformation
from nanotube to sheet. The rapid release rate observed here
was found to correspond to the hydrolysis rate of the boroxine
in the nanotube (Figure 3b). The complete release of the cis-
jasmone was clearly related to the time required for the com-
plete hydrolysis of the boroxine (compare the blue data points
(
Py–BO) in a desiccator, in which the humidity was precisely
3
controlled by saturated aqueous solutions of salts. This humid
environment induced a morphological transformation from
nanotube to sheet structures (Figure 2c), through an unzip-
ping process[
6b]
of the nanotube (Supporting Information,
Figure S1). Infrared spectroscopy for the sheet showed the
−1
O–H stretching vibration band at 3235 cm ,
corresponding to 1-pyrenylboronic acid (Py–
B(OH) ) (Figure 3a), suggesting that the
2
hydrolysis of the boroxine is closely related to
this morphological transformation. The time
dependence for the absorbance of the O–H
stretching vibration band provided informa-
tion about the rate of boroxine hydrolysis,
which depended strongly on the humidity
(Figure 3b).
UV–vis spectroscopic measurements were
carried out to analyze the molecular packing
in the nanotube and sheet. The absorp-
tion band (λmax = 429 nm) of the nanotube
self-assembled from (Py–BO)3 showed a
bathochromic shift compared with that (λmax
=
357 nm) of (Py–BO) that dissolved in tol-
3
uene (Figure 4a), indicating that (Py–BO)3
forms J-type aggregates in the nanotube
Figure 3. (a) IR spectral change induced by the hydrolysis of (Py–BO) in nanotubes under
3
−
1
5
5% humidity for 2 days. (b) Changes in the IR absorbance at 3235 cm upon the hydrolysis
(
Figure 5b). On the other hand, the absorp-
of (Py–BO) in nanotubes under different humidity conditions. (c) Changes in the IR absorb-
3
tion band (λmax = 302 nm) of the sheet com-
posed of Py–B(OH) , which was formed different humidity conditions.
−1
ance at 3235 cm upon the hydrolysis of (Py–BO) in pyridine-coordinated nanotubes under
3
2
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Figure 5. (a) Schematic image of the H-aggregate of Py–B(OH)2 in a
sheet. (b) Schematic image of the J-aggregate of (Py–BO) in nanotubes
3
and helical coils.
anions; such binding is accompanied by a molecular orbital
2
3[15]
change from sp to sp
and increases the boronate esters’
and boroxines’ stability against hydrolysis. We investigated the
effects of bases, pyridine and ammonia, on the hydrolysis of the
boroxine nanotubes, their morphological transformation, and
their release of guest molecules.
Figure 4. (a) Absorption spectra of (Py–BO) in nanotubes dispersed in
3
toluene at 25 °C and of (Py–BO) dissolved in toluene without aggrega-
3
tion at 100 °C. (b) Absorption spectra of Py–B(OH) in sheets dispersed
2
in toluene at 25 °C and of Py–B(OH) dissolved in toluene without aggre-
2
gation at 100 °C. (c) Absorption spectra of nanotubes and helical coils
dispersed in toluene at 25 °C. Absorbance of each spectrum appears to
be normalized.
in Figure 6 with the blue data points in Figure 3b). The sheet
may have no cavity for storing cis-jasmone, as we expected from
the observed fast release of the guest molecules (Figure 7).
4
. Amine- and Moisture-Responsive Guest
Release
Boron in boronic acids, boronate esters, and boroxines has
been known to act as a hard Lewis acid and can bind bases and
Figure 6. Release profile of cis-jasmone encapsulated in the hollow cyl-
inders of nanotubes with and without morphological transformations.
6
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Figure 6) was slower than that associated with the morpholog-
ical transformation from nanotube to sheet (circles in Figure 6);
suggests that the helical coil had a hollow cylinder to store cis-
jasmone. On the other hand, the faster release from the hel-
ical coil (squares in Figure 6) compared with the release from
the nanotubes (triangles in Figure 6) suggests that the release
from the helical coil occurs not only from both open ends, as
is observed for the nanotubes, but also at the interstices of the
helical coils (Figure 7). In lieu of coordination with pyridine,
ammonia, which is volatized from 28% NH aqueous solution,
3
also allowed unmodified nanotubes to transform into helical
coils and actively released cis-jasmone (Supporting Informa-
tion, Figures S3 and S4).
5
. Conclusions
We have constructed nanotubes by self-assembly of (Py–BO)3
in toluene without specific chemical modifications. Humidifica-
tion of the nanotubes and coordination with amines induced
nanotube-to-sheet and nanotube-to-helical coil transitions,
respectively. Those morphological transformations enabled
precise control of guest molecule release from the nanotubes.
These unique properties of the boroxine nanotubes have
never been reported for other boron-based materials.[
13,17]
The
results of the present study could facilitate the development of
advanced supramolecular nanocapsules for analytical, biolog-
ical, medical, cosmetic, and other practical applications.
6
. Experimental Section
Figure 7. (a) Schematic illustration of guest release from a sheet, a nano-
tube, and a helical coil.
Synthesis of Boroxines: Boronic acids were refluxed in toluene in
the presence of molecular sieves (4Å) for 1 h. After the solvent was
evaporated, the obtained residues were recrystallized from chloroform
for several times. The products, formed as powders, were stored in a
vacuum desiccator.
Pyridine (3.5 mg, 44.2 μmol) was dropped onto dried nano-
tubes (10.0 mg, 14.6 μmol), and then the nanotubes were
washed with cyclohexane to remove any unbound pyridine. The
unbound pyridine in cyclohexane was determined by UV–vis
spectroscopy. The number of pyridine molecules coordinated to
each boron atom in the boroxine was estimated to be 0.6−0.9,
+
Boroxine (R = 2-thienyl). MALDI-TOF mass (m/z): [M + H] calculated
for C H B O S : 331.01; found: 331.87. Elemental analysis: Calcd. (%):
C, 43.70; H, 2.75. Found: C, 43.72; H, 2.73.
1
2
10
3
3 3
+
Boroxine (R = 3-thienyl). MALDI-TOF mass (m/z): [M + H] calculated
for C H B O S : 331.01; found: 331.84. Elemental analysis: Calcd. (%):
C, 43.70; H, 2.75. Found: C, 43.75; H, 2.79.
1
2
10
3
3 3
which is reflective of a 1:1 complex formed between the
+
boroxine (R = phenyl) and pyridine.[16] Although the J-aggregate
Boroxine (R = phenyl). MALDI-TOF mass (m/z): [M + H] calculated
for C H B O : 313.14; found: 313.86. Elemental analysis: Calcd. (%): C,
1
8
15
3
3
of the nanotubes was slightly changed upon coordination with
pyridine, as observed from the slight UV–vis spectral change
6
9.35; H, 4.85. Found: C, 69.30; H, 4.81.
Boroxine (R = 3-biphenyl). MALDI-TOF mass (m/z): [M + H]+
(Figure 4c), the nanotubes never exhibited any morphological
calculated for C H B O : 541.03; found: 541.22. Elemental analysis:
36 28 3 3
changes. However, humidification of the pyridine-coordinated
nanotubes induced a morphological transformation from nano-
tubes to helical coils with 10 nm width (Figure 2d). IR also indi-
cated that the pyridine-coordinated boroxine had undergone
hydrolysis; however, the degree of hydrolysis, i.e., the enhance-
ment of the O–H stretching vibration band, was remarkably
smaller than that observed for the above system without pyri-
dine (Figures 3b and 3c). Since the J-aggregate structure was
clearly maintained after the morphological transformation
from nanotube to helical coil (Figure 4c), we concluded that
the coordinated pyridine contributed to suppressing boroxine
hydrolysis and boronic acid formation.
Calcd. (%): C, 80.07; H, 5.04. Found: C, 80.15; H, 5.12.
+
Boroxine (R = 4-biphenyl). MALDI-TOF mass (m/z): [M + H]
calculated for C H B O : 541.03; found: 541.25. Elemental analysis:
36 28
3
3
Calcd. (%): C, 80.07; H, 5.04. Found: C, 80.19; H, 5.15.
+
Boroxine (R = 1-naphthyl). MALDI-TOF mass (m/z): [M + H]
calculated for C H B O : 463.19; found: 463.36. Elemental analysis:
30
22
3
3
Calcd. (%): C, 78.01; H, 4.58. Found: C, 77.98; H, 4.50.
+
Boroxine (R = 2-naphthyl). MALDI-TOF mass (m/z): [M + H]
calculated for C H B O : 463.19; found: 463.54. Elemental analysis:
30
22
3
3
Calcd. (%): C, 78.01; H, 4.58. Found: C, 77.99; H, 4.51.
+
Boroxine (R = 2-anthracenyl). MALDI-TOF mass (m/z): [M + H]
calculated for C H B O : 613.23; found: 612.23. Elemental analysis:
42
28
3
3
Calcd. (%): C, 82.41; H, 4.45. Found: C, 82.47; H, 4.49.
+
Boroxine (R = 9-anthracenyl). MALDI-TOF mass (m/z): [M + H]
The release of cis-jasmone associated with the morpholog-
ical transformation from nanotube to helical coil (squares in
calculated for C H B O : 613.23; found: 612.25. Elemental analysis:
42 28 3 3
Calcd. (%): C, 82.41; H, 4.45. Found: C, 82.49; H, 4.50.
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+
Boroxine (R = pyrenyl). MALDI-TOF mass (m/z): [M + H] calculated
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SEM and TEM Observations: The self-assemblies were dropped onto
carbon grids and dried under vacuum. The SEM (Hitachi S-4800) and
TEM (Hitachi H-7000) instruments were operated at 30 and 75 keV,
respectively.
Fourier Transform-IR and UV–vis Spectroscopic Measurements: IR
spectra of the self-assemblies were measured with a Fourier transform
IR spectrometer (JASCO FT-620) operated at 4 cm resolution with an
unpolarized beam and an attenuated total reflection accessory system
Diamond MIRacle, horizontal ATR accessory with a diamond crystal
prism, Pike Technologies, USA) and a mercury cadmium telluride
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Preparation of Nanotubes Encapsulating cis-Jasmone:
A toluene
solution (5 mL) of cis-jasmone (24.0 mg, 146 μmol) was dropped onto
dried boroxine nanotubes (10.0 mg, 14.6 μmol). Capillary action enabled
the nanotubes to encapsulate cis-jasmone. After aging overnight
in a refrigerator, the solution was filtered through a polycarbonate
membrane with 0.2 μm pore size. The residual nanotubes were washed
several times with toluene to remove any unencapsulated cis-jasmone.
The amount of encapsulated cis-jasmone was determined as follows:
the nanotubes were transformed to sheet in the presence of water,
thereby the encapsulated cis-jasmone was compulsively released. The
released cis-jasmone was selectively extracted into cyclohexane, and
the UV–vis spectrum of the solution was measured. The amount of
encapsulated cis-jasmone in the nanotubes (5.0 mg) was calculated to
be 0.30−0.35 mg.
Release Experiments: Nanotubes encapsulating cis-jasmone were
placed into the desiccator, in which the humidity was precisely controlled.
Relative humidities of 6%, 32%, 55%, and 86% were achieved by
keeping saturated aqueous solutions of NaOH, CaCl /6H O, Ca(NO ) ,
and KCl, respectively, in the desiccator. After certain time, the amount
of residual cis-jasmone in the nanotubes was determined by UV/vis
spectroscopic measurements as described above. The release ratio was
calculated from the concentration of the residual cis-jasmone and the
initial concentration of the encapsulated cis-jasmone.
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Supporting Information
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Acknowledgements
This work was partly supported by a Grant-in-Aid for Young Scientists
(B) (no. 24750143) from the Ministry of Education, Culture, Sports,
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