Self-assembly of amphiphilic molecules in droplet compartments: An approach toward discrete submicrometer-sized one-dimensional structures

Article abstract of DOI:10.1002/anie.201106632

Self-assembly in a droplet: A supramolecular system that governs self-assembly events over hierarchies from the nano- to the micrometer range has been developed by combining a bottom-up strategy based on molecular programming with top-down droplet microsc

Full text of DOI:10.1002/anie.201106632

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DOI: 10.1002/anie.201106632
Supramolecular Chemistry
Self-Assembly of Amphiphilic Molecules in Droplet Compartments:
An Approach Toward Discrete Submicrometer-Sized One-
Dimensional Structures**
Munenori Numata,* Daiki Kinoshita, Nobuko Taniguchi, Hitoshi Tamiaki, and Akio Ohta
Great progress has been made in the construction of well-
defined nanoarchitectures through the self-assembly of pro-
grammed molecules with intermolecular interactions con-
might require a combination of top-down and bottom-up
strategies.
Oil-in-water (O/W) emulsions (i.e., oil droplets) have a
broad range of applications in, for example, drug delivery,
[
1]
trolled through a sophisticated molecular design. In partic-
ular, the unique self-assembling ability of amphiphilic mole-
cules finds application in the creation of diverse nanostruc-
tures for use as nanomaterials or as building units for further
[
4]
high-throughput analysis, and microreactors. Each droplet
offers a spherical interface between two immiscible liquids;
this interface can act as a scaffold of submicrometer
dimensions for the self-assembly of amphiphilic molecules.
In addition, because the droplet size is controllable in a top-
down manner (e.g., through sonication or the use of micro-
fluidic devices), a controlled number of amphiphilic mole-
cules can be compartmentalized into the droplet by changing
its size and/or the concentration of the dissolved molecules. If
the trapped amphiphilic molecules could also form 1D
assembled structures on the surfaces of the isolated droplets,
these 1D structures would be monodisperse—a potentially
expeditious route towards discrete submicrometer-sized 1D
structures. To test this intriguing hypothesis, we designed
amphiphilic molecules having the potential to undergo 1D
self-assembly after entrapping on the droplet surface.
Remarkably, we found that moderate heating of an O/W
emulsion (i.e., dynamic shrinking of droplets) containing
appropriately designed amphiphilic molecules induced a
dramatic structural change from spherical droplets to discrete
tubular structures having dimensions in the submicrometer
regime.
[
2,3]
hierarchical nanoarchitectures.
Despite rapid advances in
this area, no effective strategies have been developed to date
to control the sizes of one-dimensional (1D) self-assembled
structures (e.g., rods and tubes) in the submicrometer regime
because the component amphiphilic molecules tend to
aggregate in a noncontrollable manner, resulting in the
formation of dispersive structures. In contrast, spherically
closed structures (e.g., micelles and vesicles) feature a limited
number of amphiphilic molecules packed in a restricted space.
An interesting challenge in supramolecular chemistry is the
construction of versatile self-assembling systems in which
more than a thousand component molecules interact simul-
taneously beyond molecular programs to form uniform 1D
architectures in the submicrometer region. One solution
would be to use noncovalent bonding to self-organize a
limited number of molecules onto a 1D template possessing a
well-regulated length (e.g., oligomeric molecular templates
[3g]
synthesized in a bottom-up manner). To construct further
comprehensive systems that govern self-assembly events
hierarchically from the nano- to the micrometer range
We prepared Zn–chlorophyll-based amphiphilic mole-
cules featuring dendritic tetra(ethylene glycol) (TEG) units at
position 17 and isonicotinic acid (Chl-4Py, Figure 1a) and
nicotinic acid (Chl-3Py, see Figure S1 in the Supporting
[
*] Prof. M. Numata, D. Kinoshita, N. Taniguchi
Department of Biomolecular Chemistry
Graduate School of Life and Environmental Sciences
Kyoto Prefectural University
[5]
Information) moieties at position 3 of the chlorophyll ring.
These amphiphilic molecules, which feature a perpendicular
orientation between the TEG units and the coordination
moieties, are suitable for 1D self-assembly on the surfaces of
dynamically shrinking droplets, stabilized mainly through Zn–
pyridine coordination (Figures 1b and c). In addition, we also
prepared Chl-OMe and Chl-OH, amphiphilic molecules that
can form stiffer 1D assembled structures through p–p
stacking in addition to Zn–oxygen coordination (see Fig-
ure S1 in the Supporting Information).
First, we investigated the fundamental self-assembly of
Chl-4Py in a homogeneous water/THF mixture. We mixed
THF solutions containing 2.0 mm Chl-4Py with water at
various water/THF compositions. The Soret and Q bands of
Chl-4Py gradually shifted from 424 to 447 nm and from 651 to
668 nm, respectively, upon increasing the composition of
water in THF (Figure 2a); accordingly, the circular dichroism
(CD) intensity also increased dramatically (Figure 2b). In
Shimogamo, Sakyo-ku, Kyoto 606-8522 (Japan)
E-mail: numata@kpu.ac.jp
Prof. H. Tamiaki
Department of Bioscience and Biotechnology
Ritsumeikan University, Kusatsu, Shiga 525-8577 (Japan)
Prof. A. Ohta
School of Chemistry, College of Science
and Engineering, Kanazawa University
Kakuma, Kanazawa, Ishikawa 920-1192 (Japan)
[
**] We thank the “Kyoto-Advanced Nanotechnology Network” from
MEXT (Japan) for TEM observations. We also thank Mr. K. Kiguchi
and Prof. A. Ikeda for kindly supporting our TEM observations. This
study was partially supported by Grants-in-Aid for Scientific
Research (B) (grant number 22310069) and Exploratory Research
(
grant number 23651118).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106632.
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Figure 3. a) UV/Vis and b) CD spectra (0.5 cm cell, RT) of the emul-
sion prepared from 2.5 mm Chl-4Py in DCE (blue lines: O/W emulsion;
red lines: after heating of the emulsion) and the corresponding
homogeneous water/THF (99:1, v/v) solution (green lines) and the
DCE solution (black dotted lines). c) Size distributions of the droplets
formed from 2.5 mm (red line), 7.5 mm (blue line), and 12.5 mm
(
orange line) solutions of Chl-4Py in DCE; black bold line: size
Figure 1. a) Structure of Chl-4Py. b) 1D self-assembly of Chl-4Py at the
DCE–water interface. c) Chl-4Py compartmentalized on a DCE droplet
and manifesting its inherent 1D assembling ability at the water–DCE
interface through a dramatic change in the shape of the DCE droplet
from spherical to tubular upon evaporating the DCE.
distribution of rodlike structures obtained from homogeneous water/
THF (99:1, v/v) solution. d) Transmittance at 800 nm plotted with
respect to temperature. Inset: Photographs of the emulsion sample
(left) and clear solution obtained after heating (right).
Dynamic light scattering (DLS) provided fur-
ther quantitative data regarding the size dis-
tribution (Figure 3c).
Based on these results, we focused our
attention on the self-assembling behavior of
Chl-4Py on 1,2-dichloroethane (DCE)/water
emulsion (droplet) surfaces. We prepared
DCE solutions of Chl-4Py at three different
concentrations (2.5, 7.5, and 12.5 mm) and used
a probe-type sonicator to disperse each of
them into an excess of distilled water. DLS of
the obtained emulsion samples revealed that
the average diameter of the DCE droplets was
Figure 2. a) UV/Vis and b) CD spectra (1.0 cm cell, RT) of Chl-4Py solutions at various
water/THF compositions. c) Cryo-TEM image of the obtained rodlike structures.
[6]
contrast, we observed no such spectral changes in either the
UV/Vis or CD spectra of Chl-3Py, presumably because of an
unfavorable Zn–pyridine orientation (see Figures S2 and S3
in the Supporting Information). These results suggest that
Chl-4Py has a 1D self-assembling ability mediated through
effective Zn–pyridine coordination. Cryogenic transmission
electron microscopy (cryo-TEM) of the obtained aqueous
solution [water/THF, 99:1 (v/v)] confirmed that Chl-4Py self-
assembled to form rodlike structures (Figure 2c). The diam-
eter of each rodlike structure was quite consistent at
approximately 30 nm, whereas the length extended to several
hundreds of nanometers with a rather wide distribution.
approximately 250 nm with narrow distributions (Fig-
[7]
ure 3c). TEM analysis of the emulsion sample revealed
vesiclelike inner structures of the DCE droplets (Figures 4a
and b), suggesting that the amphiphilic Chl-4Py was entrap-
ped at the DCE–water interface. UV/Vis and CD spectra
supported this hypothesis. Figure 3a presents the UV/Vis
spectra of the emulsion sample prepared from 2.5 mm Chl-
4Py in DCE and the rodlike structure obtained from a
homogeneous water/THF (99:1, v/v) solution. The Soret and
Q bands appear at 446 and 668 nm, respectively, in each
spectrum; these features are almost identical, but they are
red-shifted relative to those of monomeric Chl-4Py, indicating
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droplet surface. When we heated the obtained emulsion
sample at 658C for 4 min, the turbid emulsion transformed
into a clear green solution (Figure 3d, inset), suggesting that
most of the DCE molecules gradually evaporated from the
droplets. This energy-consuming process induced the shrink-
ing of the droplets and enhanced the concentration of Chl-4Py
on the droplet surfaces, further facilitating their self-assem-
[
8]
bly. As expected, the CD intensity of the resulting solution
was greater than that of the initial emulsion (Figure 3b),
suggesting tighter molecular packing of Chl-4Py units leading
to stiffer self-assembled structures formed on the droplets’
surfaces.
The cryo-TEM images of the clear aqueous solutions
showed that the DCE droplets underwent dramatic morpho-
logical changes, from spherical to tubular structures having
dimensions in the submicrometer regime. The TEM images in
Figure 4c–f show that the lengths and diameters of the tubular
structures were almost uniform, in contrast to the rodlike
structures formed in the homogeneous solution. The narrow
size distributions imply that the number of assembling Chl-
4
Py units was controlled by the compartment effect of each
DCE droplet. We confirmed this effect by changing the
concentration of Chl-4Py and the pipetted volume of the
DCE solutions, both of which should directly affect the
number of Chl-4Py molecules compartmentalized within each
droplet. In the case of the DCE droplets prepared from 50 mL
of the 2.5 mm Chl-4Py solution, the average length and outer
diameter of the tubular structure were approximately 45 and
2
0 nm, respectively (Figure 4c). The hydrophobic chlorophyll
units appeared as a dark layer having a thickness of
approximately 1.0 nm, consistent with the calculated molec-
ular length of a chlorophyll moiety; in contrast, the solvated
TEG units were not directly visible in the cryo-TEM image. In
the case of the DCE droplets prepared from 50 mL of the
7.5 mm Chl-4Py solution, the average length and outer
diameter both increased to approximately 60 and 25 nm,
respectively. The wall thickness also increased to 4–10 nm
Figure 4. a) TEM image of DCE droplets prepared from a 12.5 mm
DCE solution and b) magnified image. c–f) Cryo-TEM images of the
tubular structures obtained from DCE solutions of Chl-4Py at
c) 2.5 mm (inset: magnified image), d) 7.5 mm, and e) 12.5 mm.
f) Magnified image of (e). For other TEM images, see Figure S9–11 in
the Supporting Information.
(Figure 4d). Furthermore, in the case of the DCE droplets
prepared from 50 mL of the 12.5 mm Chl-4Py solution, the
length and outer diameter of the tubular structures increased
further to approximately 200 and 45 nm, respectively. Nota-
bly, in contrast to the structures formed from the lower-
concentration (i.e., 2.5 and 7.5 mm) samples, the tubular
structures obtained from the 12.5 mm Chl-4Py solution
featured multiple layers (Figures 4e and f), with a wall
thickness of approximately 15 nm with very regular intervals.
We estimated the interval between the dark layers to be
approximately 4 nm, in good agreement with the calculated
molecular length of Chl-4Py. Similarly, when we decreased
the pipetted volume of 12.5 mm Chl-4Py to 10 mL, the droplet
size decreased to approximately 180 nm, as shown by DLS
analysis (see Figure S8 in the Supporting Information). This
result implies that the number of molecules entrapped in each
droplet also decreased. Upon heating the emulsion sample,
the tubule size decreased (see Figure S8 in the Supporting
Information) and we could not observe any of the multilayer
structures inside the tubes as we had seen in Figure 4e.
Consequently, a small droplet resulted in small tubes. These
results clearly indicate that a controlled number of amphi-
that Chl-4Py units were present at the DCE–water interface,
where they exerted their inherent 1D self-assembling ability.
In contrast, the CD spectrum of the emulsion sample was
different from that of the water/THF solution, suggesting that
the Chl-4Py units self-assembled on the droplet surface in a
molecular arrangement unlike that in the rodlike structure
(
Figure 3b). These spectral discrepancies are characteristic
and common with those of other samples containing different
concentrations of Chl-4Py (see Figures S6 and S7 in the
Supporting Information). Overall, as supported by TEM and
UV/Vis and CD spectra, we conclude that the amphiphilic
Chl-4Py aligns at the DCE–water interface, with its hydro-
philic TEG units solvated by water.
The aqueous solutions containing Chl-4Py showed rever-
sible transformations from transparent to translucent states at
temperatures above 308C, which is the lower critical solution
temperature (LCST) originating from dehydration of the
TEG units (Figure 3d). Therefore, we heated the emulsion
sample moderately above the LCST, expecting that the self-
assembly of Chl-4Py would be further accelerated on the
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philic molecules can be compartmentalized into each droplet
by changing its size and/or the concentration of the dissolved
molecules and that the self-assembly of Chl-4Py occurred
within each droplet. We also confirmed that heating above the
LCSTwas necessary to create the tubular structures; when we
evaporated DCE under reduced pressure without heating, in
a reference experiment, the droplet merely shrank to form
spherical assembled structures similar to micelles (see Fig-
measurements suggested that Chl-OH and Chl-OMe had
potential to form 1D assemblies on the droplets’ surfaces,
cryo-TEM did not show any such tubular structures after
heating their droplets (see Figures S4 and S5 in the Support-
ing Information).
Unlike the conventional tubular structures created from
various amphiphilic molecules in homogeneous solvents, we
expected the tubular structures obtained from the DCE
droplets to contain DCE within the tubes. Indeed, the cryo-
TEM images often provided evidence for the presence of
traces of volatilized DCE within the tubes, appearing as local
heating under electron beam irradiation (see Figure S11 in
the Supporting Information). The unique features of the
tubular DCE droplets stimulated us to investigate whether
hydrophobic functional molecules could be extracted into the
DCE phase from the uncapped edges. To test this hypothesis,
we mixed a hexane solution of cyanine dye (1,1-didodecyl-
3,3,3’,3’-tetramethylindocarbocyanine perchlorate, 0.2 mm,
400 mL) with the same volume of aqueous solution containing
the tubular structure prepared from 2.5 mm Chl-4Py in DCE
(50 mL). After stirring the two immiscible layers for 30 min
with a magnetic stirrer, the pink color of the cyanine dye
disappeared from the hexane layer (Figure 5a). The UV/Vis
spectrum of the resulting aqueous solution showed that the
extraction process did not affect the tubular structure. In
addition, a new signal representing the characteristic absorp-
tion band of the cyanine dye appeared near 520 nm, bridging
the green gap (see Figure S16 in the Supporting Information).
Because the cyanine dye itself cannot be extracted into the
aqueous phase, this result implies that it had been extracted
from hexane into the DCE layer directly. Fluorescence
spectra of the resulting aqueous solution provided further
evidence for the extracted cyanine dye existing within the
DCE phase. Upon excitation at 520 nm, a strong emission
peak appeared at 572 nm, consistent with that of a cyanine
[
9]
ure S15 in the Supporting Information). The increase in
hydrophobicity of the TEG units upon heating above the
LCST encouraged the Chl-4Py units to aggregate on the
droplet surface to avoid unfavorable contact with the water
phase. Furthermore, when the entire surface of the droplet
was covered with Chl-4Py units prior to shrinking, the
dehydration of the TEG units decreased the effective
volume of each Chl-4Py moiety, leading to their aggregation
in DCE layers and the creation of multilayer structures.
Next, we correlated the number of compartmentalized
molecules with the surface area of the prepared DCE
droplets. When we employed 50 mL of the 2.5 mm Chl-4Py
solution in DCE, we estimated the number of compartmen-
talized molecules and the surface area to be 6300 and 1.26 ꢀ
5
2
1
0 nm , respectively. Assuming that the area occupied by a
2
single Chl-4Py molecule is approximately 3.7 nm (from a
CPK model), we calculated the total area occupied by all the
2
2
Chl-4Py molecules to be 2.33 ꢀ 10 nm . As a result, the ratio
of the surface area of the droplet to the total occupied area
was 5.4, indicating that sufficient space was available on the
droplet surface for the self-assembly of the Chl-4Py units. In
the case where we added of 50 mL of the 7.5 mm Chl-4Py
solution, we estimated this ratio to be 1.6. This value
decreased further to 0.8 in the case of the 12.5 mm Chl-4Py
solution (50 mL), implying that not all of the Chl-4Py
molecules could be organized on the droplet surface, leading
to the creation of multilayered tubular structures upon
shrinking of the droplet.
Small-angle X-ray scattering (SAXS) analysis of the
multilayered tubular structure obtained from the 12.5 mm
Chl-4Py solution (50 mL) showed a broad reflection peak
assignable to a d spacing of 4.4 nm (see Figure S12 in the
Supporting Information), consistent with the extended molec-
ular length calculated for Chl-4Py (4.2 nm). This result
suggests that Chl-4Py self-assembled into single-layer struc-
tures, which further aggregated to form multilayered struc-
tures. The calculated molecular model for the 1D self-
assembled structure displayed (see Figure S13 in the Support-
ing Information) that the Chl-4Py units aligned one-dimen-
sionally merely through Zn–pyridine coordination, without
direct p–p stacking between adjacent chlorophyll units. The
periodical alignment of chlorophyll units bestows the created
1
D structures with regular intervals for interdigitation
through p–p stacking, leading to the formation of robust
assembled structures through 2D assembly on each droplet’s
surface and, subsequently, to the spontaneous shape transi-
tion from spherical to tubular structures. Notably, among our
tested amphiphilic chlorophyll derivatives, including Chl-OH
and Chl-OMe, the creation of this unique 1D structure with
molecular clefts was possible only for Chl-4Py (see Figure S14
in the Supporting Information). Although, spectroscopic
Figure 5. a) Photographs of two immiscible solutions. Left: an aque-
ous solution containing Chl-4Py and a hexane solution containing the
cyanine dye. Right: the mixture of (a) after shaking. b) Fluorescence
spectra of the cyanine dye after extraction into the tubular structure
(blue line) and the sample containing the same concentration of the
cyanine dye dissolved in DCE/hexane (99:1, v/v; red line). All samples
were excited at 520 nm (ET=energy transfer).
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dye dissolved in DCE/hexane (99:1, v/v; Figure 5b; lem
=
[
[
1] a) J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995;
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5
68 nm). Notably, the fluorescence intensity of the cyanine
dye decreased relative to that prior to entrapping within the
tubes, and a new peak, which we ascribe to the emission from
the chlorophyll unit, appeared at 668 nm. Upon excitation at
1
1
401 – 1443; b) N. L. Rosi, C. A. Mirkin, Chem. Rev. 2005, 105,
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absorption band in the green region (green gap). These results
show that energy transfer occurred from the cyanine dye to
the chlorophyll unit, and that the cyanine dye units were
located within the tubes. Together, these findings suggest that
the DCE solvent, which compartmentalized and pre-organ-
ized the Chl-4Py units in the initial stage, remained within the
tubular structure. Because chlorophyll is very efficient in
harvesting blue and red light, the present system, incorporat-
ing a functional dye bridging the green gap, would favor the
efficient utilization of solar energy.
In conclusion, we have developed a novel supramolecular
system for the formation of discrete 1D self-assembled
architectures having dimensions in the submicrometer
regime. This system combines a conventional supramolecular
strategy with a dynamic liquid–liquid interface provided by
shrinking droplets. In this system, the organic droplets act not
only as dynamic templates but also as compartmentalizing
solutions for amphiphilic molecules, allowing several thou-
sands of molecules to spontaneously self-assemble on the
isolated droplets’ surfaces to form discrete 1D architectures
beyond molecular programs. In other words, the amphiphilic
molecules program the submicrometer-sized droplets. The
size-regulated nanospace filled with organic solvent inside
each tube might also be applicable as a container for a limited
number of polymer strands or nanoparticles, or as a reaction
nanovessel for preparing these materials in a bottom-up
fashion. Recent developments in microfabrication techniques
should enable us to further control the droplets—in a top-
down manner—in terms of their size, concentration, and
stability, allowing the number of self-assembling components
to be regulated more precisely. We believe that this present
system will open up new opportunities for supramolecular
chemistry combining nano- and microscience.
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6] In a control experiment, we confirmed that once Chl-4Py was
solubilized in DCE, it could not be distributed into the aqueous
phase upon vigorously shaking. All of the Chl-4Py molecules
were, therefore, trapped in the DCE droplet after forming the
emulsion.
7] It was difficult to estimate the precise diameter of the droplets
obtained immediately after sonication, because DCE gradually
evaporated during the DLS measurements, even at room temper-
ature.
8] DLS analysis confirmed that fusion of DCE droplets did not occur
during the heating process.
Experimental Section
[
[
The emulsion samples were prepared according to the following
general procedure: In a vial (internal diameter: 1.5 cm), a DCE
solution (50 mL) containing Chl-4Py (2.5, 7.5, or 12.5 mm) was
dispersed into distilled water (2950 mL) using a probe-type sonicator
for 2 min, with the vial immersed in an ice water bath. Immediately
after stopping the sonication, the obtained turbid O/W emulsion was
heated moderately (658C, 4 min) in a water bath to give a clear green
solution. The resultant aqueous solutions were stable for several
months without forming any precipitates.
[
[
Received: September 19, 2011
Revised: December 9, 2011
Published online: January 11, 2012
9] To investigate the necessity of the O/W emulsion, we heated a
DCE solution containing 12.5 mm Chl-4Py at 608C without
forming an emulsion. Cryo-TEM analysis did not show any
identifiable structures, indicating that self-assembly of the Chl-
4Py units on the DCE–water interface was a prerequisite for the
creation of tubular structures.
Keywords: droplets · nanotubes · non-equilibrium conditions ·
.
self-assembly · supramolecular chemistry
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Angew. Chem. Int. Ed. 2012, 51, 1844 –1848