Self-assembly of soft hybrid materials directed by light and a magnetic field

Article abstract of DOI:10.1002/adma.201304689

Dual-responsive soft matter: A soft hybrid material composed of superparamagnetic nanoparticles and cyclodextrin vesicles self-assembles in microscale linear aggregates in water in response to magnetic field as well as light.

Full text of DOI:10.1002/adma.201304689

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Self-Assembly of Soft Hybrid Materials Directed by Light
and a Magnetic Field
Jan Hendrik Schenkel, Avik Samanta, and Bart Jan Ravoo*
Self-assembly is a most efficient way to prepare functional
nanomaterials.[ If nanoscale building blocks are equipped
with molecular recognition units, the self-assembly of a nano-
material can be directed by noncovalent interactions. As a
consequence, such self-assembled materials are inherently
dynamic, self-healing and adaptive and – depending on the type
of noncovalent interaction – can also be stimulus-responsive. In
this respect, self-assembled materials that respond to external
stimuli such as light or magnetic field are particularly attrac-
tive, since these stimuli can be delivered with high spatial and
temporal resolution so that superstructures may arise that are
not accessible in the absence of the stimulus. Nanomaterials
In this study, we report a hybrid material that is composed
of iron oxide (magnetite) nanoparticles and vesicles composed
of amphiphilic cyclodextrins (CDs). Cyclodextrin vesicles (CDV)
with an average diameter of 150-200 nm are obtained by dis-
1]
[12]
persing amphiphilic β-CD in aqueous solution.
A crucial
advantage of CDV in comparison with conventional liposomes
is the fact that CDV can selectively form inclusion complex
with hydrophobic guest molecules (such as adamantanes,
t-butylbenzenes, and azobenzenes) and hence can be decorated
with functional guest molecules such as carbohydrates, pep-
tides and proteins simply by mixing the host vesicles with the
[13]
desired mixture of guest-appended biomolecules.
Azoben-
that respond to either light[ or magnetic field are by now
rather well-established. However, nanomaterials that respond
to light and magnetic field are still very rare.[
2]
[3]
zenes constitute a well-known class of photoresponsive guest
molecules for CDs since they can reversibly isomerize from
trans to cis by irradiation at 350 nm and from cis to trans by
irradiation by 455 nm. trans-Azobenzene is apolar and rod-
like and forms stable inclusion complex with α-CD and β-CD,
whereas cis-azobenzene is polar and bent and does not fit into
either CD. In recent years, the photoreversible molecular rec-
ognition of CDs with azobenzenes has been used to develop
4]
In this article we report a soft hybrid material composed of
magnetic nanoparticles and vesicles which self-assemble in
microscale linear aggregates under the influence of a magnetic
field. The linear aggregates can be stabilized by a noncovalent
and photoresponsive cross-linker, which can be photoisomer-
ized between an adhesive and a nonadhesive configuration.
Thus, the hybrid material responds to light and to magnetic
field, and a stable self-assembled structure can only be obtained
in a magnetic field in the presence of the photoresponsive
cross-linker. The self-assembled structure is metastable in the
absence of the cross-linker and it cannot form if the magnetic
field is off or if the cross-linker is photo-inactivated.
[
14]
[15]
light-responsive materials such as hydrogels, vesicles, and
[
16]
mesoporous nanoparticles. Our group has reported a pho-
toresponsive ternary system which can selectively capture and
release biomolecules (such as DNA or proteins) on the basis of
supramolecular interaction of CDV and azobenzene modified
guest molecules. We have also shown that a divalent azoben-
zene cross-linker acts as a photoreversible supramolecular glue
[
17]
[18]
Nanoscale magnetic materials have recently attracted con-
siderable attention because of their innumerable applications
in biomedicine. Amongst others, magnetic nanoparticles are
in the adhesion of CDV.
Here we demonstrate that “magnetic” CDV can be prepared
by incorporating oleic acid stabilized magnetic iron oxide
nanoparticles (MNP) in the bilayer of the CDV. These hybrid
materials composed of magnetic nanoparticles absorbed in
the membrane of cyclodextrin vesicles (MNP-CDV) align in
an external magnetic field due to the dipolar interaction of
the superparamagnetic MNP. Moreover, these magnetic vesi-
cles can form photoactive inclusion complexes with a divalent
azobenzene cross-linker. Thus, the self-assembly of MNP-CDV
is controlled by magnetic field as well as light.
used in drug and gene delivery,[ magnetic hyperthermia, and
contrast agents in magnetic resonance imaging.[ Significant
progress has also been made in the preparation of magnetic
hybrid materials containing both inorganic nanoparticles and
5]
[6]
7]
[
3b] [8]
organic compounds, such as magnetic micelles,
,
magnetic
[
9]
polymer spheres and dendrimer-functionalized magnetic
nanoparticles.[ The use of vesicles or liposomes to design
magnetic hybrid materials is attractive because they are water
soluble, biocompatible and biodegradable. Several groups also
reported liposomes with magnetic nanoparticles embedded in
the bilayer membrane.[
10]
Amphiphilic β-CD (see Figure 1) was synthesized as described
[12]
in the literature.
This macrocyclic amphiphile forms uni-
11]
lamellar bilayer vesicles in aqueous solution at physiological
pH. Multilamellar CDV are readily obtained by dispersing a
film of amphiphilic CD in aqueous solution by gentle stirring
[
12]
at room temperature. The diameter of these multilamellar
CDV is around 400 nm according to dynamic light scattering
(DLS). The CDV form inclusion complexes with guest mol-
ecules since many CD cavities are exposed outside on the sur-
face of vesicles. Photoresponsive cross-linker G1 (see Figure 1)
was synthesized as reported in the Supporting Information.
The analytical and spectroscopic data for G1 are consistent
J. H. Schenkel, A. Samanta, Prof. B. J. Ravoo
Organic Chemistry Institute and
Graduate School of Chemistry
Westfälische Wilhelms-Universität Münster
Corrensstrasse 40, 48149, Münster, Germany
E-mail: b.j.ravoo@uni-muenster.de
DOI: 10.1002/adma.201304689
1
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diameter of around 500 nm. With AFM, it was observed that the
MNP-CDV are rather soft and flexible particles that adsorb and
spread on the mica substrate (see Figure S2). These findings
were confirmed by DLS (see Figure 2). In aqueous medium the
amphiphilic β-CD self-organize in a molecular bilayer so that
the hydrophobic alkyl chains minimize their exposure to water
while the CD and oligo(ethylene glycol) moieties cover the sur-
face the membrane. These bilayers can obviously accommodate
hydrophobic MNP in the hydrophobic membrane interior, even
though the average diameter of the particles (9 nm) is sub-
[
12]
stantially larger than the thickness of the bilayer (5 nm). We
note that our observations are consistent with several literature
reports describing the absorption of hydrophobic nanoparticles
[
11]
in liposomes.
The MNP-CVD were fluorescently labeled by including 10%
[20]
sulforhodamine B labeled amphiphilic β-cyclodextrin.
The
sulphorhodamine B labeled MNP-CDV hybrids can be observed
in fluorescence microscopic images (see Figure 3). It is evident
from Figure 3A that the hybrid vesicles are spherical and the
diameter is around 500 nm (consistent with TEM and DLS).
However, the MNP-CDV hybrids align in the direction of an
applied external magnetic field and form linear aggregates with
a length of around 10 μm within a few seconds (see Figure 3B).
We assume that the MNP-CDV align due to the attractive
dipole-dipole interactions of the superparamagnetic MNP in the
membrane of the CDV. Indeed, the aggregates spontaneously
dissociate when the magnetic field was switched off within a
few seconds (see Figure 3C). It can be concluded that the mag-
netic field assisted self-assembly of the MNP-CDV into linear
aggregates is completely reversible. A real time video of the for-
mation and disassembly of the linear aggregates is available as
Supporting Information with this article. In the video, it can be
seen that formation and disassembly of the aggregates occurs
within seconds upon application or removal of the magnetic
field. Also, it can be seen that the formation and disassembly
can be repeated multiple times, indicating the reversible nature
of the self-assembly process induced by the magnetic field. Fur-
thermore, it was observed that the linear aggregates can reor-
ient with a changing direction of the applied magnetic field,
thus acting as “magnetic stirring bars” that can be manipulated
Figure 1. (A) Schematic representation of magnetic nanoparticle-cyclo-
dextrin vesicle hybrids (MNP-CDV). (B) Photoresponsive azobenzene
cross-linker (G1).
with its molecular structure. Oleic acid coated iron oxide nano-
particles were synthesized as described.[19] The average size
of the MNP is around 9 nm according to TEM and DLS (see
Figure 2). These MNP are highly superparamagnetic (e.g. they
can be easily precipitated with a permanent magnet).
Superparamagnetic cyclodextrin vesicles (MNP-CDV) were
obtained by absorbing hydrophobic MNP into the hydrophobic
bilayer domain during preparation of the vesicles. To this end, a
mixture of the oleic acid coated MNP and amphiphilic β-CD was
dissolved in chloroform and a thin film was made by rotary evap-
oration of the solvent. The film was hydrated by 20 mM phos-
phate buffer (pH 7.2) and the suspension was stirred for 12 h.
The resulting MNP-CDV were examined by TEM, AFM and
DLS. In TEM, it could be observed that the MNP are confined
to the CDV (see Figure 2 and additional images in Figure S1
in Supporting Information) and that the MNP-CDV have a
[
3c]
by the external field.
Figure 2. (A) TEM image of iron oxide MNP. (B) TEM image of MNP-CDV hybrid. (C) DLS size distribution of MNP and MNP-CDV hybrids. Condi-
−
1
−1
tions: [MNP] = 0.67 mg mL , [MNP-CDV] = 0.5 mg mL MNP in 100 μM of CDV in aqueous solution.
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Figure 3. Fluorescence microscopy images of sulforhodamine B labeled magnetic vesicles. (A) MNP-CDV in the absence of an external magnetic
field. (B) Formation of linear aggregates of MNP-CDV in a magnetic field (H). (C) Spontaneous disassembly of linear aggregates of MNP-CDV after
switching off the magnetic field. (D) Linear aggregates of MNP-CDV formed in an external magnetic field and stabilized by the cross-linker trans-G1.
The aggregates deform but persist after switching off the external field. (E) Disaggregation of MNP-CDV and cis-G1 after UV irradiation for 30 min in
the absence of an external magnetic field. (F) Re-aggregation of MNP-CDV and trans-G1 in a magnetic field after visible light irradiation for 30 min.
−
1
Conditions: [MNP-CDV] = 0.4 mg mL MNP in 50 μM of CDV (90% of amphiphilic β-CD and 10% sulforhodamine B labeled amphiphilic β-CD) in 20
mM phosphate buffer at pH 7.2, [trans-G1] = 20 μM. The scale bars denote 10 μm.
The microscale aggregates of MNP-CDV are not stable in
the absence of a magnetic field since the MNP are superpara-
magnetic. We hypothesized that the aggregates of MNP-CDV
can be stabilized using divalent guest G1 that cross-links the
CDV.[ The formation of an inclusion complex of G1 and CDV
is light-responsive since the trans isomer of azobenzene is a
Figure 4B), provided the irradiation time is sufficient (30 min at
both 350 and 455 nm).
Having established that MNP-CDV can be reversibly cross-
linked by photoresponsive G1, their interaction in an external
magnetic field was investigated under the fluorescence micro-
scope (see Figure 3). It was observed that if MNP-CDV were
first exposed to a magnetic field and then cross-linked with
G1, ca. 100 μm long threads of compactly clustered MNP-
CDV were formed (see Figure 3D). These aggregates could
also be observed by AFM, although they are difficult to charac-
terize due to their soft nature (see Figure S2 in the Supporting
Information). Importantly, these aggregates persist for sev-
eral hours also in the absence of the external magnetic field.
We assume that these large aggregates result from the forma-
tion of small linear aggregates of MNP-CDV in the magnetic
field (as shown before in Figure 3B) which are subsequently
stabilized and interconnected by numerous intervesicular
non-covalent cross-links. The aggregates reorient according
to the direction of the external magnetic field and gradually
deform due to Brownian motion, but they do not disaggre-
gate by switching off the magnetic field (see video provided
as Supporting Information). However, UV irradiation of the
cross-linked aggregates of MNP-CDV and trans-G1 at 350 nm
for 30 min leads to the dispersion of the thread like aggregates
and the MNP-CDV recover their initial size and spherical shape
(see Figure 3E). This observation is fully consistent with the
photoisomerization of trans-G1 to cis-G1, which eliminates the
cross-links in the aggregates. The resulting MNP-CDV self-
organize into linear aggregates again as soon as the external
18]
−1
suitable guest for β-CD (K = 6000 M ), but the cis isomer is
a
−
1
not (K ∼1 M ). The azobenzene moiety can be reversibly pho-
a
toisomerised from trans to cis by irradiation at 350 nm and
from cis to trans by irradiation at 455 nm. We have previously
shown that a divalent guest very similar to G1 can induced the
photoresponsive adhesion of CDV into amorphous microscale
clusters.[ Similarly, addition of trans-G1 to a solution of MNP-
CDV induces a rapid aggregation and adhesion of MNP-CDV
18]
(see Figure 4). Optical density measurements at 600 nm were
performed with MNP-CDV after extrusion through a polycar-
bonate membrane, which reduced the initial average particle
size to around 150 nm, i.e. similar to CDV that are not loaded
with MNP. It can be taken from Figure 4A that the OD600 of a
solution of 50 μM solution of MNP-CDV is around 0.05. When
1
00 μM trans-G1 was added (after 2 min) to the MNP-CDV,
the OD600 gradually increased to a maximum of 0.25. These
measurements confirm the spontaneous aggregation and adhe-
sion of MNP-CDV in presence of trans-G1, consistent with our
reported findings for CDV not loaded with MNP.[ UV irradia-
tion of the mixture of MNP-CDV and trans-G1 induces the dis-
persion of the aggregates leading to a decrease of OD600. The
reversibility of the light-induced formation and dispersion of
MNP-CDV aggregates is almost quantitative over 2 cycles (see
18]
1
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the linear aggregates are stable under visible light irradia-
tion (and in the absence of any irradiation) but disintegrate
under UV irradiation. Thus, we have made a soft material that
responds to magnetic field as well as to light, and the com-
bination of both leads to superstructures that are not acces-
sible when only one external stimulus is applied. We envisage
that these magnetic hybrids may be of potential use as a drug
delivery system, contrast agent in MRI and also in hypo-
thermia therapy. The biocompatibility of the hybrid material
can be significantly enhanced by using host-guest chemistry
to functionalize the outer surface with carbohydrates, peptides
and/or proteins.
Experimental Section
Materials: All chemicals used in this study were purchased from Acros
Organics (Schwerte, Germany) or Sigma–Aldrich Chemie (Taufkirchen,
Germany) and were used without further purification. β-Cyclodextrin was
kindly donated by Wacker Chemie (Burghausen, Germany). All solvents
were dried according to conventional methods before use.
Synthesis: The synthesis of divalent azobenzene (G1) was carried
out in four steps. Commercially available tetraethyleneglycol was
monotosylated, and further treated with 4-phenylazophenol in the
presence of K CO3 to give (E)-2-(2-(2-(2-(4-(phenyldiazenyl)phenoxy)
2
ethoxy)ethoxy)ethoxy)ethanol. The second hydroxyl group was
mesylated, and further treated with piperazine in the presence of Et N
3
to give rise to the target G1. Details of the synthesis are reported in the
Supporting Information. The spectroscopic and analytical data for G1
are consistent with its molecular structure. The synthesis of CDV was
performed as described in the literature.[
12b]
Chemical reactions were
carried out in oven-dried glassware under an inert gas atmosphere.
Analytical TLC was performed on Merck silica gel 60 F254 plates.
All compounds were visualized by dipping in alkaline permanganate
solution. Column chromatography was carried out by using Kieselgel 60
1
13
(
230–400 mesh). H and C NMR spectroscopic measurements were
carried out by using Bruker ARX 300 MHz or Varian 500 MHz INOVA
spectrometers. Chemical shifts were referenced to internal standards
1
13
CDCl (δ = 7.26 ppm for H and 77.0 ppm for C) or TMS (δ = 0.00
3
1
13
ppm for H and C). HRMS was performed by using a Bruker MicroTof
instrument.
Figure 4. Photoresponsive aggregation of MNP-CDV and trans-G1. (A)
Time-dependent optical density measurements at λ = 600 nm (G1 added
after 2 min) (B) Aggregation and dispersion of MNP-CDV by G1 under
alternate irradiation with UV light (350 nm) and visible light (455 nm).
Conditions: [MNP-CDV] = 0.4 mg mL MNP in 50 μM of CDV in 20 mM
phosphate buffer at pH 7.2.
Preparation of MNP: Oleic acid coated MNP were synthesized
by co-precipitation of ferric and ferrous salts in alkaline medium
as described in the literature.[ To this end, 2 g of ferrous sulfate
19]
−
1
heptahydrate (FeSO4 7H O > 99%) and 3.49 g of ferric chloride
2
hexahydrate (FeCl3 6H O > 99%) were dissolved in 100 mL Milli Q
2
water in a clean flask. Next, 25 mL of 25% (w/w) NH H O was added
3
2
to the mixture and the solution was kept stirring at room temperature.
The solution turned black, leading to a black precipitate. Then ∼1 mL
of oleic acid was added dropwise into the dispersion at 80 °C under
argon atmosphere. The resulting MNP were well dispersed into water
and protected by a double layer of oleic acid. The water dispersion of
nanoparticles was extracted with toluene. By addition of NaCl, MNP
coated with a monolayer of oleic acid were transferred into the toluene
phase. Finally the toluene dispersed MNP were refluxed under argon
atmosphere to remove the water.
Preparation of MNP-CDV: Amphiphilic β-CD and oleic acid coated
MNP were taken in a 25 mL flask. The mixture was dissolve in 2–3 mL of
chloroform and dried by slow rotary evaporation to yield a thin film at the
bottom of the flask. Residual solvent was removed under high vacuum.
magnetic field is reapplied. Upon subsequent irradiation of vis-
ible light at 455 nm for 30 min (to obtain trans-G1 from cis-
G1) the 100 μm thread-like aggregates slowly form again (see
Figure 3F). These experiments show that a metastable linear
assembly of MNP-CDV can be stabilized by photoresponsive
noncovalent cross-linking.
In conclusion, we have developed a magnetic soft material
based on iron oxide nanoparticles absorbed in amphiphilic
cyclodextrin vesicles. MNP-CDV form 10 μm linear aggre-
gates in aqueous solution if exposed to an external magnetic
field. The assembly of MNP-CDV in a magnetic field results
in a metastable state that is only accessible as long as the field
is present. However, the metastable state can be stabilized
using noncovalent cross-linking by a divalent azobenzene
guest. Due to the photoresponsive nature of the cross-links,
1
0 mL of aqueous buffer (100 mM phosphate buffer, pH 7.2) was added
and the suspension was stirred overnight. Fluorescent MNP-CDV
hybrids were prepared by incorporating 10 mol% sulforhodamine
[20]
labeled amphiphilic β-CD with amphiphilic β-CD.
Light microscopy and fluorescence microscopy: Microscopic images
were recorded with an Olympus inverted research microscope CKX41.
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The microscope was equipped with a DX 20 L-FW camera (Kappa
opto-electronics GmbH), which was controlled by the program Kappa
CameraControl (version 2.7.5.7032). A mercury burner U-RFL-T was used
for the generation of the excitation light. 10 μL of MNP-CDV solution
was taken on a microscopy slide. A magnetic field was introduced by
bringing a hand-held rectangular permanent magnet in close proximity
of the microscopy slide.
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UV/Vis Spectroscopy: Optical density measurements were carried out at
in 1.5 mL disposable cuvettes with dimensions 12.5 × 12.5 × 45 mm and
1
0 mm path length using a Uvikon 923 double-beam spectrophotometer.
The optical density was measured at λ = 600 nm (OD600), which is far
from absorption of the azobenzene chromophore. Measurements were
performed for 30 min to 40 min, unless otherwise noted, with data
points collected every 12 s. The freshly prepared solution of MNP-CDV
was extruded through a polycarbonate filter resulting in an average
particle size of around 150 nm at the starting point of each experiment.
Irradiation Experiments: Two different light sources were utilized for
light irradiation experiments. One source was a Rayonet photochemical
reactor (The Southern New England Ultraviolet Company) equipped
with 16 RPR- 3500 lamps used to generate UV light (350 nm) to photo-
isomerize azobenzene moieties from trans to cis. The other source was a
Philips Lumileds royal blue LUXEON K2 emitter (LXK2-PR14-Q00) used
to generate visible light (455 nm) to isomerize azobenzene moieties
from cis to trans. The MNP-CDV were irradiated for 30 min in disposable
plastic cuvettes.
Transmission Electron Microscopy (TEM): MNP were deposited on a
covered holey copper grid, by applying a drop of NPs dissolved in toluene
on the surface. TEM analyses were performed by using a Zeiss 200 FE
electron microscope with schottky emitter and energy Ω filter operating
at 200 kV. The microscope was equipped with the CCD camera Gatan
USC 4000. The electron microscope was constructed by CARL ZEISS AG,
Oberkochen and the camera by GATAN GMBH, München. The size of the
nanoparticles was determined with ImageJ version 1.39u, Java 1.6.0_02
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Acknowledgment
[
TEM was performed by Dr. Martin Peterlechner at the Institute of
Materials Physics of the Westfälische Wilhelms-Universität Münster. A.
S. acknowledges a fellowship of the Graduate School of Chemisty. This
work was supported by the Deutsche Forschungsgemeinschaft (grant Ra
2
796; b) D. A. Uhlenheuer, D. Wasserberg, C. Haase, H. D. Nguyen,
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