A Carbon Dioxide Bubble-Induced Vortex Triggers Co-Assembly of Nanotubes with Controlled Chirality

Article abstract of DOI:10.1002/anie.201701749

It is challenging to prepare co-organized nanotube systems with controlled nanoscale chirality in an aqueous liquid flow field. Such systems are responsive to a bubbled external gas. A liquid vortex induced by bubbling carbon dioxide (CO₂) gas was used to stimulate the formation of nanotubes with controlled chirality; two kinds of achiral cationic building blocks were co-assembled in aqueous solution. CO₂-triggered nanotube formation occurs by formation of metastable intermediate structures (short helical ribbons and short tubules) and by transition from short tubules to long tubules in response to chirality matching self-assembly. Interestingly, the chirality sign of these assemblies can be selected for by the circulation direction of the CO₂ bubble-induced vortex during the co-assembly process.

Full text of DOI:10.1002/anie.201701749

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Accepted Article
Title: CO2-Triggered Co-Assembled Nanotubes with Controlled
Chirality Through a CO2-Bubble-Induced Vortex
Authors: Ling Zhang, Laicheng Zhou, Na Xu, and Zhenjie Ouyang
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201701749
Angew. Chem. 10.1002/ange.201701749
Link to VoR: http://dx.doi.org/10.1002/anie.201701749
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10.1002/anie.201701749
CO
CO
2
2
-Triggered Co-Assembled Nanotubes with Controlled Chirality Through a
-Bubble-Induced Vortex
Ling Zhang,* Laicheng Zhou, Na Xu and Zhenjie Ouyang
Abstract: The design of co-organized nanotube systems with controlled nanoscale chirality that are responsive to
the liquid flow field induced by bubbling external gas in aqueous media remains a significant challenge. Here, we
report success in utilizing the liquid vortexes induced by bubbling carbon dioxide (CO
formation of nanotubes with controlled chirality based on the co-assembly of two kinds of achiral cationic building
blocks in aqueous solution. The results reveal that CO -triggered nanotube formation occurs via the formation of
2
) gas to stimulate the
2
short helical ribbons and short tubules as metastable intermediate structures, and via transition from short tubules
to long tubules based on chirality match self-assembly. Interestingly, the chirality sign of these assemblies can be
2
selected by the circulation direction of the CO -bubble-induced vortex during the co-assembling process.
Tubular and helical nanostructures with controllable chirality from the co-assembly of exclusively
achiral molecules in aqueous solution are attracting attention owing to their crucial roles in the
fields of chiral recognition, sensing, catalysis, chiroptical switches, medicine and biological
[
1, 2]
applications.
Tubular and helical structures seem to be rather common in nature. A key feature
of nature’s complex multifunctional co-assembly is that it can respond to external stimuli with
[3]
controllable chirality in an aqueous environment. Thus it is very important to have an overall
understanding of the co-assembly process of external-stimuli-triggered tubular and helical
nanostructures with controllable chirality in aqueous solution for materials science and biology.
[
*] Dr. L. Zhang, L. C. Zhou, N. Xu, Z. J. Ouyang, PCFM Lab, and GDHPRC Lab, School of Chemistry, Sun Yat-Sen University,
Guangzhou 510275, China. E-mail: ceszl@mail.sysu.edu.cn
**] This research was supported by the program of National Natural Science Foundation of China (Grant 21374137), and Natural
[
Science Foundation of Guangdong province (Grant 2014A030313194). The authors gratefully acknowledge Prof. Yongming Chen for
helpful discussion and support, and Jinmei Huang for her help in the drawing.
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Angewandte Chemie International Edition
10.1002/anie.201701749
Although there are still several methods available for controlling handedness based on
spontaneous symmetry breaking, such as the direction of vortex stirring via rotational or magnetic
[4]
[5]
[6]
forces, irradiation with circularly polarized light (CPL) and air/water interfacial assembly,
the liquid vortex in gas-bubble-induced single-circulation-cell type of flow for controlled chirality
remains a significant challenge.
Gas-liquid bubble-flow and flow regimes can be studied in a flat bubble column. A bubble
column is a piece of multiphase equipment that is frequently used in the chemical, biological, and
[
7]
petrochemical industries. The flow structure in a flat bubble column is determined by the gas
flow rate, the aeration mode and the column aspect ratio. In the case of the ratio of height to width
(
H/D) equaling 1 for a gas flow rate of 1 l/min which corresponds to a superficial gas velocity of
.4125 mm/s, the flow has an essentially stationary character in which one circulation cell
depicted with an arrow) spreads over the full width of the bubble column with a bubble jet
0
(
[
8]
located laterally. Depending on the lateral location of the stationary bubble jet, a stationary
vortex in the bubble column develops with either a clockwise (CW) or counterclockwise (CCW)
[
8, 9]
circulation of steady-state flow.
The circulation direction of liquid vortexes induced by
bubbling gas for controlled chirality (right-handed, P; left-handed, M) of co-assembled nanotubes
has not yet been explored.
Herein we report CO
controlled by the circulation direction of a CO
Different from other external trigger modes such as pH, temperature, light, reducing or oxidizing
agents, and even voltage, CO gas, as a pivotal endogenous metabolite, possesses a nontoxic
2
–triggered co-assembled tubular and helical nanostructures with chirality
2
-bubble-induced vortex in water (Figure 1).
2
[10]
nature and good biocompatibility and is close to physiological conditions. To date, although
there are several reports on the regulation of vesicle structures based on molecular self-assembly
[
11]
through CO
2
,
this is the first report that the control of supramolecular chirality for
CO
CO
2
-triggered co-assembled nanotube systems is realized by the circulation direction of a
-bubble-induced vortex for an aqueous solution of achiral building blocks.
2
In this paper, we have selected 1-ethyl-3-(3-dimethylaminoisopropyl) carbodiimide (EDC) for
introducing N,N-dimethylamino species by the activation of carboxylate groups to form
carbodiimide adducts to the azobenzene molecule (Figure 1a; see Figure S1 for molecular
structure of compound 1). For molecular coassembly, we also designed compound 2 which is
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Angewandte Chemie International Edition
10.1002/anie.201701749
made up of a hydrophobic tail containing alkyl and azobenzene functionalities attached through an
ethylene glycol hydrophilic linker to N,N-dimethylamino head groups (Figure 1a; see Figure S2).
To investigate CO -triggered co-assembled tubular and helical nanostructures with controlled
chirality, we chose a flat bubble column with gas flow rate of 1 l/min and a liquid height of 2 cm
2
with the location of the bubble jet to obtain a CO
two-circulation-cell type of flow.
2
-bubble-induced single-circulation-cell or
a)
N
N
H
HCO
3
N
N
HCO3
H
O
O
N
H
N
H
N
O
N
O
O
O
O
O
CO2
O
O
+
+
N
N
N
N
N
O
N
O
N
O
N
O
C H
C H
C H
10 21
1
0
21
C H
0
21
10 21
1
1
2
Cation 1
Cation 2
b)
2
Figure 1. a) CO -switchable structural change of molecules 1 and 2. b) Schematic representation of the formation
of CO
2
-triggered co-organized nanotubes of cations 1 and 2 with controllable chirality by the circulation direction
of CO
2
-bubble-induced vortex.
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Compounds 1 and 2 could be dissolved in CH
were suspended in water. When CO gas was bubbled through the aqueous solution of compounds
and 2 with a molar ratio of 1:1, they could be dissolved in water to form a viscous,
yellow-orange solution. By introducing CO into the solution, the N,N-dimethylamino species
were gradually protonated by the acidic gaseous medium, leading to the formation of cations 1 and
(Figure 1a), and a successive variation of the hydrophilic–hydrophobic balance and a better
2 2
Cl at room temperature, but the two compounds
2
1
2
2
dissolution in water.
2 2
Experimental evidence supported the above CO -switchable mechanism. First, CO gas was
passed through the solution for 20 min, the zeta potential increased rapidly from +4.2 to +25 mV,
accompanied by a decrease of the pH value from approximately 7.2 to 5.68, suggesting the
protonation of N,N-dimethylamino headgroups generates extra positive charge. However, because
CO
2
is a weak acid, even though the duration of CO
2
addition was prolonged, the protonation of
saturated
N,N-dimethylamino species could stop at the solution pH corresponding to CO
2
[11c]
concentration (pH 5.6).
Interestingly, CO could activate compounds 1 and 2 (non-protonated monomers) to begin an
2
unexpected formation of co-assembled tubular structures of cations 1 and 2 (protonated monomers)
in aqueous solution. A representative cryogenic transmission electron microscopy (cryo-TEM)
image showed the CO
The nanotubes were straight and discrete and had a uniform diameter of ~106 nm (Figure 3a). For
a control experiment with the presence of only cation 1 or 2 under a CO -bubble-induced liquid
2
-triggered co-assembly of high aspect ratio nanotubes in aqueous solution.
2
flow field, no tubular and helical nanostructures could be detected in the cryo-TEM observations,
indicating that molecular interactions between cations 1 and 2 may play very important roles for
2
CO -triggered co-assembled nanotubes. The mixing ratio of compound 1 to compound 2 did not
influence the formation of the nanotube or the helicity of the nanotube, but influenced the number
of nanotubes. A molar ratio of 1:1 of compounds 1 and 2 was found to be the optimal ratio for
co-assembly.
Another observation was to monitor the formation of CO
cations 1 and 2 based on the change in electronic absorption spectra of the mixture of cations 1
and 2 in water over CO aeration time (Figure 2). To eliminate the influences of other external
2
-triggered co-assembled nanotubes of
2
stimuli (such as stirring and shaking) on co-assembly, a UV/Vis absorption spectrum was recorded
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Angewandte Chemie International Edition
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immediately when the mixture of compounds 1 and 2 was added to water (with no stirring or no
shaking), and a weak signal was observed before CO treatment. Upon bubbling CO , the
absorption increased in intensity, and a further increase of intensity was observed upon prolonged
CO aeration time. For a comparison of the spectra of the aggregates, the UV/Vis spectra of the
2
2
2
protonated monomer (cation 1 or 2) and the non-protonated monomer (compound 1 or 2) in water
as control experiments are also displayed in Figure S3. Compound 1 or 2 was treated in the same
way as the mixture (with no stirring or no shaking), and a weak signal of compound 1 or 2 before
CO
2 2
treatment and the increase of absorption band intensity upon CO stimulation were also
observed, being consistent with those of the aggregates in Figure 2.
0.8
2
1
1
0 min
5 min
0 min
0.6
5
1
0
min
min
min
0.4
0.2
0.0
200
250
300
350
400
450
Wavelength (nm)
2
Figure 2. Monitoring the formation of CO -triggered co-assembled nanotubes of cations 1 and 2 based on the
change in electronic absorption spectra of cations 1 and 2 in water over CO
2
aeration time at room temperature.
Figure 3. a) Cryo-TEM images of co-assembled micrometre-long nanotubes of cations 1 and 2 generated by
exposure to CO under aqueous conditions. Scale bar: 100 nm. b) Summed cryo-TEM images of a cross-section of
the CO -triggered co-assembled nanotubes. Dark regions are bilayer walls. White bar indicates an approximate
thickness of 5 nm.
2
2
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Angewandte Chemie International Edition
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[
12]
Theoretical discussions have suggested that the formation of tubular lipid assemblies requires
2D bilayer crystalline membranes. The wall thickness was measured on slightly defocused
cryo-electron microscopy images to obtain maximal phase contrast. The highest density was
[
13]
obtained from the electron-dense overcrowded alkene part of the amphiphile. Considering the
molecular structure of cations 1 and 2, the wall thickness of the nanotube (~ 5 nm, indicated by the
white bar in Figure 3b) suggests a bilayer structure.
It is generally accepted that molecular dipole interaction plays an important role in the
[
14]
alignment of molecules and in promoting the formation of a well-organized bilayer. Exciton
coupling of azobenzene-based chromophores results from the interaction of their transition dipole
moments in space. In the case of the non-protonated monomer (compound 1 or 2) and the
protonated monomer (cation 1 or 2) and the mixture of cations 1 and 2 in DMSO solution, an
absorption maxima appeared at ~360 nm, characteristic of π-π* transition of trans-azobenzene
[
15]
derivatives (see Figure S4, Figure S5). For the co-assembled bilayer structure of cations 1 and 2
formed upon CO stimulation in water, the absorption maximum of trans azobenzene blue shifted
to ~320 nm (see Figure S5) relative to the mixture of cations 1 and 2 in DMSO solution (λmax
60 nm). This hypsochromic shift is indicative of an increase in the π-π stacking of the
2
=
3
[
16]
azobenzenes (H-aggregated) within the tubules in water. It was also found that no absorption
peaks appear at ~450 nm which is ascribable to the n-π* transition of the cis isomer, suggesting
the absence of cis-azobenzene during the co-assembly. In addition, infrared spectroscopy of the
–
1
nanotube on a KBr plate (see Figure S6) showed CH
2
stretching vibrations at 2918 cm (νanti) and
–
1
2
849 cm (νsym), indicating that the alkyl chains are closely packed to form a crystalline domain.
Furthermore, photoisomerization to the cis configuration was not observed for cations 1 and 2
when co-assembled in water, suggesting that they are tightly packed within the nanotube structure.
2
We attribute the high stability of the CO -triggered co-assembled nanotubular structures to the
crystalline domain formation of the hydrophobic moieties of cations 1 and 2 in water, to
π-stacking of the azobenzene moieties, to intermolecular hydrogen bonds (N-H···O=C) between
-
3
cations 1 and 2 and to electrostatic screening by condensed HCO
counterions.
Cryo-TEM measurements showed helical nanoribbons during the course of the nanotube
formation, which indicated supramolecular chirality at the nanoscale. Furthermore, the influence
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c)
1
5
2.0
.5
.0
.5
0.0
CW
CCW
10
1
5
0
1
0
-5
-10
250
300
350
400
450
250
300
350
400
450
Wavelength (nm)
Wavelength (nm)
d)
2
Figure 4. Chiral co-assembly of cations 1 and 2 generated from CO -bubble-induced liquid circulation based on
cryo-TEM measurement, a) CCW; b) CW, Scale bar: 100 nm, and their possible packing mode within the
co-assembly for M helicity and P helicity, including the H-bond interaction between cations 1 and 2. c) CD spectra
and UV/Vis absorption spectrum. d) The relationship between the observed chirality and the direction of
2
CO -bubble-induced liquid circulation.
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2
of the liquid vortex in the CO -bubble-induced single-circulation-cell type of flow could be
controlled to obtain either clockwise (CW) or counterclockwise (CCW) circulation by changing
the location of the lateral bubble jet from left to right. As shown in Figure 4a and 4b,
2
CO -bubble-induced CCW and CW circulation could predominantly produce the metastable M
helicity and P helicity by cryo-TEM observations, respectively. The results indicate that the
supramolecular helicity of the nanostructures could be controlled by varying the circulation
direction in the CO
The supramolecular chirality for the co-assemblies of cations
CO -bubble-induced single-circulation-cell type of flow could be detected from the CD signals of
the azobenzene chromophores. The CD spectra showed the optical activity and their CD signal
inverted with circulation sense in CO bubble-induced single-circulation-cell type of flow: a
2
-bubble-induced single-circulation-cell type of flow.
1
and 2 under
2
2
negative Cotton effect was observed at 320 nm with counterclockwise (CCW) liquid circulation,
conversely, a positive Cotton effect was observed at 320 nm with clockwise (CW) liquid
[
17]
circulation (Figure 4c). The CD data of the void cuvette as a control experiment could exclude
the occurrence of artifactual CD signals due to nanotube deposition on the walls of the CD cuvette
(
see Figure S7). The results are in agreement with the cryo-TEM observations, indicating that CO
2
bubble-induced single-circulation-cell type of flow in co-assembled solutions could promote
asymmetric environments for symmetry breaking, which provide the supramolecular chirality to
the co-assemblies of achiral building blocks (Figure 4d).
[
4k, 5a, 18]
According to the stereoregular packing of azobenzene chromophores
supramolecular chirality could be due to the helical stacking of the azobenzene units of cations 1
and 2 on a molecular level under a CO -bubble-induced liquid flow field. For the CO -triggered
, the emergence of
2
2
co-assembled nanotubes, the π-π interactions between the aromatic rings of azobenzene and
intermolecular hydrogen bonding interactions (N-H···O=C) between cations 1 and 2 play very
important roles for forming chiral helical assemblies, resulting in the emergence and amplification
of supramolecular chirality. Their possible packing mode within the co-assembly of cations 1 and
2
for M helicity and P helicity, including the H-bond interaction between cations 1 and 2, are
displayed in Figure 4. The CO bubbling co-assembled solutions show good correlation between
the chirality sign of their CD spectra and the circulation direction of the CO -bubble-induced
2
2
single-circulation-cell type of flow, indicating alternation of the stereoregular packing of
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azobenzene chromophores with the circulation direction. When a stationary bubble jet moves
centrally, the liquid carried up by the central bubble jet returns down along the side walls of the
[
19]
bubble column, forming two symmetrical liquid circulations (see Figure S8). In the case of the
symmetrical, two-circulation-cells type of flow, no optical activity could be detected in the CD
spectra, indicating that this system simultaneously formed both left- and right-handed helices in
equal amounts which canceled any chiroptical signal. Although supramolecular chirality
originating from assemblies of achiral molecular building blocks upon vortex stirring has been
generally acknowledged, the determination of the optically active state by the circulation sense in
a single-circulation-cell or a two-circulation-cell type of flow is here first reported.
Figure 5. Illustration of pathway of the formation of CO
2
-triggered co-assembled nanotubes of cations 1 and 2.
Scale bar: 100 nm. a), b) Transition from ribbons to short helical ribbons and then to short tubules under CW and
CCW circulation, respectively. c), d) Transition from short tubules to long tubules under CW and CCW circulation,
respectively. a), c) for right-handed, P. b), d) for left-handed, M.
A transition from ribbons to short helical ribbons and then to short tubules was observed by
Cryo-TEM measurements under both CW and CCW circulation (Figure 5a and 5b), and
2 2
CO -bubble-induced CCW and CW circulation in the CO -bubble-induced single-circulation-cell
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Angewandte Chemie International Edition
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type of flow could predominantly produce the left- and right-handed short tubules, respectively.
The results further demonstrated that the sense of the helicity could be controlled by switching the
circulation direction from clockwise (CW) to counterclockwise (CCW) in CO -bubble-induced
2
single-circulation-cell type of flow. Figure 5c showed that right-handed short tubules
predominantly obtained under CW circulation could further self-assemble to obtain right-handed
long tubules based on chirality match self-assembly, the same as for the formation of left-handed
long tubules (Figure 5d). This indicated the pathway of the formation of CO
co-assembled long nanotubes and chirality controlled by the circulation direction of
CO -bubble-induced single-circulation-cell type of flow.
In conclusion, we have demonstrated a novel CO -triggered co-assembled nanotube system
with controlled chirality that is realized by the circulation direction of CO -bubble-induced
2
-triggered
2
2
2
single-circulation-cell type of flow. The uniqueness of the co-assembled nanotubes reported in this
study is that 1) controlled chirality is possible by switching the circulation direction of
CO
CO
2
-bubble-induced single-circulation-cell type of flow and 2) the pathway to the formation of
-triggered co-assembled long nanotubes from short nanotubes is based on chirality match
2
self-assembly. This is the first example of how the sense of a macroscopic circulation induced by
gas bubbling in single-circulation-cell type of flow can determine the optically active state of a
gas-triggered co-assembled nanotube system, and such studies could provide new strategies to
regulate the chirality of nanostructures based on the study of fluid hydrodynamics.
2
Keywords: Co-assembled tubular structures · Controlled chirality · CO -bubble-induced liquid
circulation direction · Achiral azobenzene amphiphiles
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17] As a precautionary note, we have ruled out the possibility that CO
of aligned aggregates and have excluded the occurrence of instrumental artifacts such as linear dichroism
LD). And such a study of the mechano-chiral effect requires a high number of repetitive experiments, whose
reproducibility depends on the careful control of the experimental parameters.
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