Dual stimuli-responsive self-assembled supramolecular nanoparticles

Article abstract of DOI:10.1002/anie.201310829

Supramolecular nanoparticles (SNPs) encompass multiple copies of different building blocks brought together by specific noncovalent interactions. The inherently multivalent nature of these systems allows control of their size as well as their assembly and disassembly, thus promising potential as biomedical delivery vehicles. Here, dual responsive SNPs have been based on the ternary host-guest complexation between cucurbit[8]uril (CB[8]), a methyl viologen (MV) polymer, and mono- and multivalent azobenzene (Azo) functionalized molecules. UV switching of the Azo groups led to fast disruption of the ternary complexes, but to a relatively slow disintegration of the SNPs. Alternating UV and Vis photoisomerization of the Azo groups led to fully reversible SNP disassembly and reassembly. SNPs were only formed with the Azo moieties in the trans and the MV units in the oxidized states, respectively, thus constituting a supramolecular AND logic gate. A guiding light: UV/Vis irradiation and chemical reduction have been used for the reversible disassembly/reassembly and irreversible disintegration, respectively, of responsive supramolecular nanoparticles stabilized by ternary host-guest complexes (see picture). The disassembly proceeds through an initial aggregation phase that is attributed to different response times of the multivalent core and the monovalent shell upon photoswitching.

Full text of DOI:10.1002/anie.201310829

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Communications
DOI: 10.1002/anie.201310829
Self-Assembled Systems
Dual Stimuli-Responsive Self-Assembled Supramolecular
Nanoparticles**
Carmen Stoffelen, Jens Voskuhl, Pascal Jonkheijm, and Jurriaan Huskens*
Abstract: Supramolecular nanoparticles (SNPs) encompass
multiple copies of different building blocks brought together by
specific noncovalent interactions. The inherently multivalent
nature of these systems allows control of their size as well as
their assembly and disassembly, thus promising potential as
biomedical delivery vehicles. Here, dual responsive SNPs have
been based on the ternary host–guest complexation between
cucurbit[8]uril (CB[8]), a methyl viologen (MV) polymer, and
mono- and multivalent azobenzene (Azo) functionalized
molecules. UV switching of the Azo groups led to fast
disruption of the ternary complexes, but to a relatively slow
disintegration of the SNPs. Alternating UV and Vis photo-
isomerization of the Azo groups led to fully reversible SNP
disassembly and reassembly. SNPs were only formed with the
Azo moieties in the trans and the MV units in the oxidized
states, respectively, thus constituting a supramolecular AND
logic gate.
Although a variety of external controls have been used for
the triggered release of proteins and cells from surfaces,[6]
RNA from micelles,[7] and DNA from hydrogels[8] and
electrostatic delivery vectors,[9] only chemical reduction[10]
and a magnetic field[11] have been used as specific triggers
for the release of drugs encapsulated in SNPs.
Azobenzene (Azo) is known to bind to a- and b-CD in the
stable trans form. Upon irradiation with light at a wavelength
shorter than 360 nm, it undergoes a conversion into the
unstable, bulkier cis form, which is released from the CD
cavity. This phenomenon has been used to build photo-
responsive supramolecular vesicles,[12] mesoporous silica
nanoparticles,[13] and hydrogels,[14] which can bind and release
DNA, drugs, and proteins, respectively. In contrast, thermal
trans–cis isomerization has been used to fabricate an amine-
displacement assay driven by the complexation of Azo by
cucurbit[7]uril.[15] Recently, the formation of a ternary light-
responsive host–guest complex between Azo, methyl viol-
ogen (MV), and cucurbit[8]uril CB[8] was reported.[16] Upon
irradiation of the system with UV light, the cis-Azo was
formed, thus disrupting the ternary complex formed between
trans-Azo, MV, and CB[8]. By using this photoinduced
mechanism, changes in the surface wettability[16a] as well as
the polymerization of supramolecular building blocks[16b]
could be triggered using light. This system holds the, so far
uninvestigated, promise to function as a reversible supra-
molecular glue for the controlled assembly and disassembly of
supramolecular nanoparticles.
T
he self-assembly of molecules into higher ordered supra-
molecular structures constitutes a powerful approach for the
design of novel materials such as nanoparticles,[1] hydrogels,[2]
and polymers.[3] These aggregates are held together by
noncovalent weak interactions such as van der Waals, hydro-
phobic, and electrostatic interactions. More robust host–guest
interactions, for example, the inclusion of hydrophobic guests
in cyclodextrins (CDs)[4] or the inclusion of amines and
hydrophobic guest molecules into cucurbit[n]urils (CB[n]),[5]
have been studied extensively in the last decade.
Among these systems, supramolecular nanoparticles
(SNPs) based on host–guest interactions currently receive
high interest.[1] The self-assembly of small building blocks
offers size control and colloidal stability through an interplay
of multivalent and monovalent interactions. Their drug-
encapsulation properties and the easy implementation of
targeting ligands make these aggregates promising candidates
as site-selective biomedical delivery vectors.[1a,c]
Recently, we showed by employing a ternary charge-
transfer complex between CB[8], MV, and naphthol-func-
tionalized components that SNPs could be formed with
controllable particle sizes.[17] Here we report a fully reversible
and dual-responsive SNP system with Azo building blocks, in
which SNP assembly and disassembly can be switched
reversibly in multiple cycles by the photoswitching of Azo,
and irreversibly by chemical reduction of the MV units. SNPs
based on the ternary host–guest generated from CB[8], MV-
polymer, and mono- and multivalent Azo-functionalized
guest molecules have been formed with controllable particle
sizes. As shown schematically in Figure 1, disintegration of
the formed SNPs is observed after photochemical conversion
from trans- to cis-Azo on irradiation with UV light (l <
400 nm). Isomerization back to trans-Azo is induced by
visible light, which leads to full restoration of the SNPs.
Additionally, the SNPs have been fully disassembled by
chemical reduction.
[*] C. Stoffelen, Dr. J. Voskuhl, Prof. Dr. P. Jonkheijm,
Prof. Dr. J. Huskens
Molecular Nanofabrication group
MESA+Institute for Nanotechnology, University of Twente
P.O. Box 217, 7500 AE, Enschede (The Netherlands)
E-mail: j.huskens@utwente.nl
Homepage: http://www.utwente.nl/tnw/mnf
[**] This work was supported by the Council for Chemical Sciences of
the Netherlands Organization for Scientific Research (NWO-CW;
Vici grant 700.58.443 to J.H.). J.V. and P.J. thank the European
Research Council for funding through grant 310105.
To this end, Azo-terminated poly(amidoamine) dendri-
mer (PAMAM) generation 1 (Azo-PAMAM8) was synthe-
sized (see Figure S1 in the Supporting information) to act as
the multivalent photoresponsive supramolecular cross-linker
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310829.
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Figure 1. A) Schematic presentation of the supramolecular nanoparticle (SNP) self-assembly and triggered disassembly mediated by formation
and disruption of the ternary complex formed between cucurbit[8]uril (CB[8]), methyl viologen (MV), and azobenzene (Azo) moieties. B) The
supramolecular host (CB[8]) and responsive guest molecules involved in SNP formation: azobenzene-functionalized poly(ethylene glycol) (Azo-
PEG), methyl viologen functionalized poly(ethylene imine) (MV-PEI), and Azo8-poly(amidoamine) (Azo8-PAMAM).
in the core of the particles. In the presence of the previously
reported MV-substituted poly(ethyleneimine) (MV-PEI,
degree of substitution: 4.5 MV units per polymer chain)[17]
and CB[8], this Azo-substituted dendrimer forms multiple
ternary complexes in aqueous solution. Monovalent, Azo-
functionalized poly(ethylene glycol) (Azo-PEG, MW =
5000 gmolÀ1) was also prepared (see Figure S4 in the
Supporting Information) to provide colloidal stability and
size control of the formed SNPs. As described by us[17] for
a similar, although nonswitchable, ternary complex motif, the
monovalent Azo-PEG competes with the multivalent Azo8-
PAMAM for binding to CB[8], thereby leading to termina-
tion of the aggregates and resulting in the size-controlled
formation of SNPs. The Azo-PEG also facilitates the water
solubility of the assembled SNPs as the PEG chains are
exposed at the outside of the SNPs.
Size-controlled self-assembly of the particles was
observed by mixing in various ratios the four supramolecular
blocks, that is, mono- and multivalent Azo, multivalent MV-
PEI, and CB[8], involved in the formation of the ternary
complex. A solution of MV-PEI was added to a mixture of
Figure 2. Size determinations of SNPs prepared with different formula-
tions: A)–C) SEM images of the resulting SNPs as a function of the
Azo fraction derived from Azo8-PAMAM dendrimers (A: 10% B: 20%
C: 30%) upon formation of the assembly using Azo/MV/CB[8]=1:1:1.
Azo8-PAMAM dissolved in DMSO, aqueous CB[8], and
aqueous Azo-PEG. Successful size tuning by varying the ratio
of the two Azo compounds, keeping the overall stoichiometry
of Azo/MV/CB[8] = 1:1:1 (Figure 2), was observed on ana-
lyzing the mixtures after two days by scanning electron
microscopy (SEM) and dynamic light scattering (DLS). As
shown in Figure 2, the observed SNP size depends strongly on
the fraction of Azo8-PAMAM used during particle self-
D) SNP diameters as measured by DLS (gray bars) and SEM (black
bars).
assembly. By increasing the amount of Azo derived from
Azo8-PAMAM from 10% to 30%, while decreasing the
amount of Azo-PEG correspondingly, the recorded particle
size was found to increase from 55 nm to 110 nm by DLS and
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from 35 nm to 70 nm by SEM. The commonly observed
discrepancy between the results from DLS and SEM can be
attributed to DLS reporting hydrodynamic diameters and to
the loss of water during SEM sample preparation. Overall, the
size increased monotonously and was approximately linearly
dependent on the amount of Azo8-PAMAM up to 30%. DLS
analysis further revealed that addition beyond 35% Azo8-
PAMAM led to uncontrolled particle growth, which yielded
large aggregates with a high polydispersity (see Figure S7 in
the Supporting Information). This observation can be
ascribed to the supramolecular competition between the
multivalent dendrimers that reside in the core of the SNPs,
while the monovalent Azo-PEG assembles at the exterior of
the SNPs. An increasing amount of the multivalent compo-
nent allows an extended growth of the SNPs, although
a minimum amount of monovalent supramolecular stopper
is apparently required for the formation of stable SNPs. To
prove the supramolecular specificity of the SNP self-assem-
bly, CB[7] instead of CB[8] was used during the assembly of
the SNPs; for steric reasons the smaller CB[7] does not allow
the formation of the ternary complexes. In the presence of
MV-PEI, Azo-PEG, and Azo8-PAMAM (20% Azo from
Azo8-PAMAM), DLS analysis revealed, as expected, no
distinct particle formation when CB[7] was used (see Fig-
ure S9 in the Supporting Information). This observation
shows that CB[8] is essential for SNP formation through the
generation of multiple ternary complexes.
ments showed that the absorption band at 345 nm was
reinstalled within 20 min after exposing the sample to
ambient light (Figure 3), which is indicative of cis–trans
isomerization and re-formation of the ternary complex.
Subsequently we irradiated previously prepared solutions
of the SNPs with UV light (l < 400 nm) and followed the
change in the SNP size by DLS (Figure 4A). After an initial
UV exposure time of 1 h, aggregated particles with a high
polydispersity were observed (Figure 4A). In contrast, full
As a next step, the light-induced disassembly and reas-
sembly of the particles was investigated by making use of the
photoresponsive ternary complex by conversion of trans-Azo
into cis-Azo. UV/Vis spectroscopy of the complex formed
between CB[8], methyl viologen, and Azo-PEG as well as
1H NMR spectroscopy of Azo-PEG in D2O showed that 80–
90% of the trans-Azo-PEG was converted into cis-Azo-PEG
within 1 minute after irradiating the aqueous solution with
UV light (l < 400 nm; Figure 3 and see Figure S12 in the
Supporting Information). Time-dependent UV/Vis measure-
Figure 4. A) DLS size determination of as-prepared SNPs, and subse-
quently after 1 h UV light, after 14 h UV light, after 8 h ambient light,
and after chemical reduction of MV by Na2S2O4. B) UV/Vis light-
induced switching to trigger SNP assembly and disassembly for four
consecutive cycles.
disintegration of the SNPs was observed after prolonging the
irradiation time to 14 h. This particle disassembly process is
relatively slow in comparison to the 1 min switching time of
Azo in the individual ternary complex as described above.
This difference can be explained by 1) an incomplete trans-to-
cis isomerization of Azo and 2) a slow rearrangement of
multivalently interacting building blocks. It has been reported
that the photostationary state of the UV irradiation of Azo
contains up to 20% trans-Azo.[12a] Presumably, upon UV
illumination, the cis-Azo-PEG is immediately expelled from
the SNP shell, which leads to exposure of the core. The
photostationary state ensures at the same time that the
multivalent Azo8-PAMAM will initially remain interacting
through a few moieties inside the core, thus leading to
significant clustering of the nanoparticles. Since the assembly
of SNPs is slow, as has been observed before,[17] because of
rearrangement of the multivalently interacting components,
the disintegration of the remaining building blocks only takes
place over the course of hours (Figure 4A).
Figure 3. UV/Vis absorbance of Azo-PEG/methyl viologen/CB[8]
(1:1:0.95) before and after 1 min irradiation with UV light as well as
the time-dependent interconversion of cis- to trans-Azo-PEG in ambient
light.
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To verify the reversibility of the cis-to-trans switching that
leads to the reassembly of the individual components to
nanoparticles, the disassembled SNP samples were kept in
ambient light to induce the reformation of trans-Azo, thus
enabling the re-formation of the ternary complex with MV
and CB[8]. Complete SNP reassembly was observed after
keeping the samples for 8 h in visible light (Figure 4A). In
contrast, no distinct particle reformation was observed for
SNP samples kept in the dark for 6–14 h after UV irradiation
(see Figure S14 in the Supporting Information). The SNP
assembly/disassembly process was conducted several times by
switching between UV and visible light. Figure 4B shows the
alternate switching for four complete cycles, and in all states
a complete re-formation (ca. 116 nm Æ 16 nm) or rupture
(ca. 2–4 nm) of the SNPs was observed. These results confirm
the successful development of a reversible, light-responsive
supramolecular nanoparticle system.
The constitutive ternary supramolecular complex con-
taining MV also results in the SNPs being susceptible to
changes induced by chemical reduction of the MV moieties.
Chemically triggered particle degradation was observed after
the addition of the reducing agent Na2S2O4. DLS analysis
demonstrates the disappearance of particles with a size of
about 110 nm and formation of small aggregates of about 2–
4 nm (Figure 4A) after 12 h. This observation is in agreement
with the supramolecular mechanism reported before. The
addition of a chemical reductant leads to formation of the
MV+ radical cation, which is known to from a stable homo-
ternary complex with CB[8],[18] thus causing the release of the
Azo guest unit (Figure 1) and concomitant disassembly of the
SNPs.[16a]
Figure 5. Molecular AND logic operator in dual-responsive SNPs:
SNPs are only formed with trans-Azo and the MV dication; with cis-Azo
(by photoirradiation) and/or MV in the radical cation form (by
chemical reduction), only small building blocks are observed.
disassembly and reassembly can be induced by illumination
by UV and visible light, respectively, whereas irreversible
disassembly occurs following chemical reduction. The dual
responsive character and its reversibility make this SNP
system a promising candidate for advanced material engi-
neering.
Received: December 13, 2013
Revised: January 14, 2014
To prove that particle reassembly requires both the trans-
Azo as well as the dicationic MV species, the two particle
disassembly processes were carried out consecutively. The
formed SNPs were disassembled first by chemical reduction
with Na2S2O4, followed by UV irradiation. DLS studies (see
Figure S17 in the Supporting Information) show that the
initially formed SNPs were neither observed after chemical
reduction nor after UV light irradiation (Figure 5). Further-
more, SNP reassembly was not observed after subsequent
treatment with visible light, which can be ascribed to the still-
present ternary complexes formed between MV+ radical
cations and CB[8].
In logic terms, the SNP system described here functions as
an AND operator. Only when the Azo moiety is in the trans
form and the MV moiety in the dicationic form are SNPs
formed (Figure 5). Switching either of them leads to inhib-
ition of the particle formation process. Molecular systems
representing logic operators have been shown and reviewed
before.[19] To our knowledge, this is the first example in which
the result of the operation is visualized by the formation of
supramolecular particles.
Published online: February 24, 2014
Keywords: host–guest systems · nanoparticles ·
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photochemistry · self-assembly · supramolecular chemistry
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