10.1002/anie.201907063
Angewandte Chemie International Edition
COMMUNICATION
almost intact vesicle bodies upon treatment with CO2 for 40 min
(Figure 3f). A handful of small pores on each vesicle surface (as
indicated by arrows, Figure 3f) reflected the imperfect connection
during the course of membrane enclosing. As the surface pores
coalesced and healed, defect-free vesicles were ultimately gained
(60 min CO2, Figure S10). It is worth noting when all of FLP
monomers were linked by CO2, prolonging gas supply brought no
change on vesicular sizes and shapes (Figure S11). Moreover,
once formed, these CO2-constructed nanocapsules can be stable
in solution for more than two months. Since the dynamic feature
of CO2 bridged bonding to FLP monomers, the obtained
o
nanocapsules can be dissociation upon heating over 60 C and
reversibly rebuilt with a new round of gas purging (Figure S12).
On the basis of the above phenomenon, a possible mechanism
was proposed to explicate this CO2-driven vesicular formation
process (Figure 3g): 1) At the early stage of the reaction, because
TB3 and TP3 are disk-shaped rigid molecules that contain lateral
trefoil-like FLP groups, thus CO2 can fast polymerize with these
monomers through in-plane multiple cross-linking, leading to CO2-
bridged planar networks (Figure 3g, i-ii); 2) With the reaction
proceeding, the small sheets epitaxially grow along in 2D direction
to produce large lamellar membranes. To reduce total free energy,
part of the membranes start to concave inward and bud out from
the membranes (Figure 3g, iii); 3) The separated membranes can
spontaneously change into kippah-like vesicles (Figure 3g, iv). To
decrease the total interfacial area exposing to solution, the further
CO2 linking reaction causes that the kippah vesicles highly curve,
yielding bowl-shaped capsules (Figure 3g, v). 4) Excess CO2 will
lead to a higher cross-linking effect and encourage the curvature
increase among the intermediates, which turns the semi-closed
nanobowls into full-closed vesicles (Figure 3g, vi). 5) Because the
bonding between CO2 and FLP is dynamic covalent,[11] proof-
reading and error-correcting capabilities of dynamic chemistry
can heal small defects produced during membrane enclosing to
obtain the well-structured vesicles (Figure 3g, vii). Similar sheet-
to-sphere phase transition have been found in other reaction-
induced nanocapsule formation by use of columnar macrocyclic
hosts and disk-shaped molecules.[16] Particularly, our vesicle
system features favourable gas tunability, that is, the level of CO2
can regulate the membrane curvature.
To universalize this gas-induced vesicle-forming approach, we
highly desired that it could be suitable to other gas substances. If
possible, a series of worthless industrial waste gases or pollutant
gases are hopeful to be transformed into valuable nanostructural
materials. To this end, besides CO2 (greenhouse gas), we chose
three representative polyatomic gases, CO (pollutant gas), N2O
(vehicle exhaust) and ethylene (C2H4, petrochemical exhaust), to
validate the universality. Adopting these three kinds of gases is
because they have potential to be activated by FLP chemistry.[17]
When adding different gas molecules (1.0 mM) into TB3 and TP3
mixture (1:1, 0.25 mM), interestingly, vesicles with distinct sizes
and morphologies were yielded (Figure 4). TEM images revealed
that the addition of CO can enlarge the final vesicular size to 540
± 80 nm, 80% larger than that of CO2-formed ones (Figure 4a). In
contrast, C2H4 and N2O gave rise to opposite effects: C2H4 led to
formation of smaller vesicles, only half the size of CO2-vesicles
(160 ± 30 nm, Figure 4d), whereas N2O resulted in the smallest
vesicles, whose diameter is one-fourth of CO2-vesicles (80 ± 15
nm, Figure 4g). To further confirm that the different morphological
Figure 3. (a-f) TEM tracking the vesicular formation process upon addition of
CO2: (a) 10 min, (b) 12 min, (c,d) 15-20 min, (e) 25 min, (f) 40 min (scale bar:
500 nm). (g) Proposed membrane-enclosed mechanism: i) Complementary FLP
monomers polymerize with CO2 into 2D planar networks; ii) small nanosheets
grow into large lamellar membrane via 2D epitaxial growth; iii) part membranes
start to concave and separate from the membranes; iv) separated membranes
self-bend into kippah-like vesicles; v) kippah vesicles further self-curve into high
curvature of bowl-shaped vesicles; vi) membrane enclosing with small porous
defects; vii) dynamic proof-reading and error-correcting of FLP and cross-linkers
to obtain intact vesicles.
molecular simulation, if two kinds of FLP monomers and CO2 can
polymerize into a perfect 2D framework, considering the height of
aromatic side groups, the single-layered molecular thickness is
evaluated to be 1.28 nm, which is near half of the actual value of
membrane thickness (Figure S8). This result suggest that the 2D
membranes possibly adopt bilayer structure like other amphiphilic
vesicles. The major difference is that covalent bonds are bridged
between the building units in the lateral directions in the present
system, whereas in conventional vesicles the laterally molecular
association depends on noncovalent bonds. Subsequently, these
2D membranes began to appear random depression and concave,
and seemed to separate from the bulk membrane (Figure 3b). In
the presence of CO2 for 15‒20 min, these separated membranes
produced spontaneous bending and formed a kind of kippah-like
vesicular objects, while the initial 2D structures totally vanished
(Figure 3c-d). If the aeration time was extended to 25 min, the
kippah vesicle membrane can further self-curve into bowl-shaped
structure with a higher curvature (Figure 3e). Afterwards, the open
mouths in the bowl-shaped vesicles gradually closed and formed
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