Angewandte
Chemie
Remarkably,the solution of 2 exhibits a thermoreversible
phase transition at 608C (see the Supporting Information,
Figure S8). Upon heating to 658C,cryo-TEM of the solution
revealed a hollow spherical structure with diameters ranging
from several hundreds nanometers to a few micrometers with
a layer thickness of about 16 nm (Figure 3b). However,the
image showed that the lateral pores in the shell are
completely closed,demonstrating that the porous capsules
transform spontaneously into closed ones upon heating
without any noticeable changes in spherical shape. After
12 hours of annealing at room temperature,the shells started
to form small openings (Figure 3c). With further increases of
annealing time,the number of the openings gradually
increases and the pore sizes become more uniform (Fig-
ure 3d,e). Complete recovery to the original porous capsules
were observed over a period of approximately 7 days resting
at room temperature (Figure 3 f,g), indicating that the struc-
tural transformation between open and closed states is
accompanied with considerable hysteresis. This hysteretic
behavior in open/closed motion of the pores seems to arise
from the kinetic effect related to slow break-up of strong p–p
stacking interactions between the rod segments when opening
the pores. These results demonstrate that the lateral pores in
the hollow sphere are reversibly gated in response to temper-
ature without affecting the overall spherical shape (Fig-
ure 3 h).
Figure 4. a) 1H NMR spectra of ethylene oxide regions of 2. b) Time
course of calcein release from closed capsules of 2. Fluorescence
microscopy images of HeLa cells c) after intracellular delivery of the
capsules and d) rhodamine-labeled DNA oligomer. Images of the
capsules after DNA encapsulation showing that the blue fluorescence
from the capsules (inset in c) overlaps with the red fluorescence from
the DNA (inset in d). Scale bars in the insets: 4 mm.
This gating behavior of the pores can be explained by the
fact that the oligo(ethylene oxide) dendritic exterior exhibit a
lower critical solution temperature (LCST) behavior in
aqueous media.[14] Above the LCST,the ethylene oxide
segments are dehydrated to collapse into molecular globules,
which leads to a decrease in the effective hydrophilic volume
as the hydrodynamic volume of the polyether dendrons
decreases. As a result,the porous structure with a highly
curved local interface transforms into a closed structure with
flat interface to reduce interfacial energy associated with
unfavorable segmental contacts. This dehydration was con-
firmed by 1H NMR experiments. Upon heating above the
LCST,the resonances associated with the ethylene oxide
chains are noticeably broadened together with a decrease in
intensity (Figure 4a),demonstrating the loss of hydrogen
bonding interactions between ether oxygen atoms and water
molecules.[15]
period of 5 days,indicating that,when cooled down to room
temperature and then annealed,the capsules undergo a
transition from closed to open states,releasing the entrapped
guests from the internal cavity.
To address the potential utility of the porous capsules as a
virus-like delivery vehicle,intracellular delivery experiments
were performed in mammalian cells. Toward this direction,
fluorescently labeled DNA oligomer (TAMRA-DNA) as a
cargo was encapsulated within 2 as described above. The
intracellular fluorescence distribution after the treatment of
DNA-encapsulated capsules showed that nearly all of the
cells were stained,showing blue (the capsules) and red
fluorescence (TAMRA-DNA) simultaneously (Figure 4c,d).
This result indicates that the capsules can encapsulate the
relatively large DNA molecules (molecular weight ca.
6700 Da),deliver the encapsulated cargo into the inside of
the cell.
A reversible open/closed gating motion of the pores in the
shell,triggered by external stimuli,suggests that the porous
capsules may selectively encapsulate guest molecules and
then release them in a controlled manner. To substantiate
thermoresponsive gating behavior of the lateral pores,
encapsulation experiments were performed with hydrophilic
guest molecules. Calcein (100 mm),as a guest,was added to
the pre-equilibrated solution of 2 at room temperature where
the capsule exists in its open form. The resulting solution was
then heated to 658C,at which temperature the pores in the
shell close. Release of encapsulated calcein was accompanied
by an increase in fluorescence emission as the free calcein in
solution was dequenched.[16] As shown in Figure 4b,essen-
tially no leakage of the entrapped calcein was observed over a
period of 12 hours at room temperature. After 12 hours,
however,the entrapped calcein was released gradually over a
The notable feature of the dumbbell-shaped rod amphi-
philes investigated herein is their ability to self-assemble into
a capsule structure with gated nanopores in the shell. These
lateral nanopores undergo a transition from the open state to
the closed state upon heating,which is capable of blocking
cargo transport. Accordingly,the responsive nanopores
endow the spherical objects with a reversible encapsulation
capability of cargos under a controlled manner,with preser-
vation of their hollow spherical structure. Considering that
the capsules with reversibly gated lateral pores are internal-
ized in their closed form to deliver the entrapped cargos into
the inside of cells,our capsules can be considered as synthetic
analogues to viral capsids.[7] Such a fascinating function of the
pores may provide a new strategy for the design of synthetic
systems with virus-like functions. Furthermore,the capability
of the capsules to encapsulate large molecules suggests that
Angew. Chem. Int. Ed. 2008, 47, 4662 –4666
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Kim, Jung-Keun
