Size-tunable supramolecular nanoparticles mediated by ternary cucurbit[8]uril host-guest interactions

Article abstract of DOI:10.1039/c3cc43045f

The formation of size-tunable, supramolecular nanoparticles (SNPs), employing cucurbit[8]uril-assisted naphthol-viologen charge-transfer complexes, is strongly time and temperature dependent. Yet, the ternary complex formation is fast at all temperatures

Full text of DOI:10.1039/c3cc43045f

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Size-tunable supramolecular nanoparticles mediated
by ternary cucurbit[8]uril host–guest interactions†
Cite this: Chem. Commun., 2013,
49, 6740
Carmen Stoffelen and Jurriaan Huskens*
Received 24th April 2013,
Accepted 8th June 2013
DOI: 10.1039/c3cc43045f
www.rsc.org/chemcomm
The formation of size-tunable, supramolecular nanoparticles (SNPs),
Although multivalency plays an important role in the dynamic
employing cucurbit[8]uril-assisted naphthol–viologen charge-transfer molecular assembly and disassembly processes targeting the
complexes, is strongly time and temperature dependent. Yet, the formation of stable materials,14–16 the mechanism of the forma-
ternary complex formation is fast at all temperatures employed. This tion and size control of the SNPs remains elusive. Beyond the Ad–
indicates that SNP formation requires dynamic disassembly and CD binding motif, a variety of molecular recognition units are
reassembly of the constituting ternary complex units.
available in supramolecular chemistry, however, none of these
has been used for the preparation of size-tuneable SNPs. Thus, a
Supramolecular chemistry promises the assembly of molecular basic question arises whether SNPs can be formed using different
units into well-defined architectures, via specific non-covalent interaction motifs. Additionally, the expected changes in inter-
interactions.1–3 Nanoparticles (NPs) possess unique size-dependent action strength and exchange rate may have a profound influence
properties that are not observable in bulk materials.4 The combi- on the assembly and disassembly processes of the SNPs.
nation of both fields is attractive, since stable, yet reversible hybrid
Cucurbit[n]urils (CB[n]s) constitute a class of macrocyclic molec-
assemblies hold great promise for biomedical applications.5–7 ules that form inclusion complexes through hydrogen bonds and
External triggers can induce the selective disassembly of supra- hydrophobic and ion–dipole interactions,17,18 e.g. when using
molecular systems, which leads to responsive drug delivery positively charged guests such as methyl viologen (MV). Moreover,
formulations.8,9
MV and naphthol (Np) can form a charge-transfer (CT) complex
Supramolecular nanoparticles (SNPs) are particles in which which is included in the cavity of CB[8] to give a ternary complex
multiple copies of different building blocks are brought with micromolar affinity.19 Utilizing this or similar ternary supra-
together by specific noncovalent interactions, resulting in molecular motifs, different nanoparticulates20,21 and microstruc-
assemblies that are typically larger than the building blocks tures22 have been reported by the groups of Scherman and Abell. In
themselves. The most well studied SNP, a siRNA delivery the NP studies,20,21 however, the CBs have been used merely to
system, has been developed by the group of Davis.10,11 DNA crosslink intramolecularly connected guest moieties attached to a
delivery vectors were formed owing to electrostatic interactions single polymer chain, thus smaller NPs are provided due to the
between a b-cyclodextrin (CD)-containing polycation in the collapse of the starting guest-modified polymer. The strong bind-
presence of negatively charged DNA. Stabilization of these ing affinity of this interaction motif, but with a radically different
relatively weakly bound particles was achieved by supramolec- kinetics compared to CD complexes, makes it an interesting
ular host–guest interactions of the CD-containing polycation candidate to explore its potential in the formation of SNPs and
with monovalent adamantyl (Ad)-grafted poly(ethylene glycol).12 to study their assembly kinetics.
In contrast, NP formation exclusively based on supramolecular
Here, we report a novel strategy for the preparation of size-
CD–Ad interactions has been demonstrated by the group of tunable SNPs in water using CB[8] as an intermolecular host for
Tseng, whereby the size of the formed NPs could be altered by MV and Np-containing building blocks, with the formation of a
varying the ratio of the monovalent and multivalent Ad host–guest-assisted CT complex. As shown in Fig. 1, the host–
derivatives.13
guest recognition between MV-PEI, Np8-PAMAM and Np-PEG
(MW B 5000 g molÀ1) in the presence of CB[8] leads to SNPs
composed of four building blocks and held together by multiple
ternary complexes. Characterization of the multivalent Np8-
PAMAM and the monovalent Np-PEG (Fig. 1b) showed that the
reactive binding sites of the parent amino-terminated dendrimer
and PEG were fully functionalized with Np guest moieties.
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; Fax: +31-534894645
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc43045f
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To evaluate whether the SNP formation is based on the
selective host–guest interaction of the different supramolecular
building blocks, the formation of the CT complex was studied.
An increase in UV/Vis absorbance (Fig. S5, ESI†) as well as a
decrease in fluorescence intensity (Fig. S6, ESI†) indicate that the
SNPs are assembled in aqueous solution as a result of formation
of a ternary supramolecular complex. Congruently, no SNPs were
formed in the absence of MV-PEI or CB[8], or upon addition of
CB[7] instead of CB[8] (Fig. S7, ESI†). The dicationic MV moiety is
required to enable inclusion of the neutral Np inside the cavity of
the CB[8] host, hence no CT complex is formed in the absence of
MV. Additionally, intermolecular crosslinking between the MV
and Np derivatives cannot occur in the presence of CB[7]; its
cavity is too small to host both guest moieties.
Remarkably, a decrease in fluorescence was observed
directly after mixing the different supramolecular building
blocks at room temperature, whereas consistent SNP formation
is only detectable after 48 h. This leads to the presumption
that the supramolecular host–guest interactions, forming the
CB-assisted ternary complexes, are established immediately
after mixing the components. Initially, undefined, kinetically
trapped structures are formed which slowly reassemble into
stable SNPs by exchange of the guest moieties via (slow)
dissociation and (fast) reassociation. In order to monitor the
assembly behaviour of the supramolecular building blocks in
more detail, time-dependent DLS measurements were carried
out at different temperatures. As seen in Fig. 3, the observed
sizes using SNP formulations containing 25% Np derived from
DLS data varies with time, and the extent of variations in the
SNP size depends on the temperature. At RT the observed
amplitude of the SNP size variation was very high and the
measurements were not consistent. In contrast, SNP assembly
occurred faster at elevated temperatures. Samples kept at 40 1C
showed distinct particle formation and consistent DLS mea-
surements already 2 h after sample preparation, and a stable
SNP size (96 Æ 14 nm) was evident after 10 h.
Fig. 1 (a) Supramolecular nanoparticle formation by ternary complex formation
between cucurbit[8]uril (CB[8]), methyl viologen (MV) and naphthol (Np)
moieties. (b) Supramolecular building blocks involved in particle formation: methyl
viologen-poly(ethylene imine) (MV-PEI), naphthol-poly(ethylene glycol) (Np-PEG),
cucurbit[8] uril and naphthol8-poly(amidoamine) (Np8-PAMAM). (c) Ternary
complex formation by inclusion of MV in CB[8], followed by inclusion of Np.
The degree of functionalization of the multivalent MV-PEI poly-
mer (B10 kDa) was evaluated using UV/Vis spectroscopy, micro-
calorimetry and 1H-NMR, and all techniques showed each polymer
chain to be functionalized with, on average, four MV guest
moieties.
Upon addition of aqueous MV-PEI to a previously prepared
aqueous solution of CB[8], Np-PEG and Np8-PAMAM, SNPs were
formed using a 0.67 mM concentration of each of the three
molecular recognition groups (CB[8], MV and Np). The concen-
trations of Np-PEG and Np8-PAMAM were varied in different
ratios, while keeping the total Np concentration constant, in
order to study the influence of the multivalent–monovalent
competition on the SNP size. Thereby, the resulting intrinsic
ratio of the three functional host–guest recognition units CB[8]/
MV/Np was kept constant at 1 : 1 : 1 in accordance with the
stoichiometry of the basic ternary complex motif. After 2 days,
particle formation was confirmed using DLS and SEM. As
shown in Fig. 2 (right panel), utilizing 20% Np derived from
Np8-PAMAM, SNPs with an average hydrodynamic diameter (dh)
of 85 Æ 15 nm were obtained as observed using DLS. In good
agreement, SEM (Fig. 2, left panel) showed particles with an
average diameter of 74 Æ 13 nm.
Fig. 2 Characterization of CB-mediated SNPs prepared using Np : MV : CB[8] =
1 : 1 : 1, with 20% Np derived from Np8-PAMAM : DLS data (left panel); SEM
image (right panel).
Fig. 3 (a) Hydrodynamic SNP diameters averaged over four measurements and
(b) corresponding standard deviations as obtained by time-dependent DLS
measurements as a function of temperature.
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The temperature-dependent particle formation is ascribed crosslinking of Np building blocks is established with time, as
to the dynamic character of the supramolecular host–guest indicated by the time-dependent measurements. Stable particle
binding. After initial rapid ternary complex formation, disas- formation requires termination of the multivalent interactions
sembly and reassembly of the four building blocks is required that are established between Np8-PAMAM and MV-PEI in the
for well-defined particle formation. At elevated temperatures presence of CB[8]. The results obtained for the size dependence
the required rearrangement is enhanced which leads to faster show that more than 50% monovalent Np-PEG is required to
SNP formation. Such a time-dependent particle formation has stabilize the NPs. Distinct supramolecular NP size control is
not been reported so far for SNPs based on CD–Ad host–guest achieved by fine-tuning the balance of multivalent vs. mono-
interactions. In all cases reported, SNP formation was observed valent interactions. As expected, increasing the concentration of
directly after mixing the supramolecular components. We the multivalent dendrimer leads to the formation of larger SNPs.
attribute the difference in dynamics to the stronger and more
In conclusion, we have developed a novel, versatile strategy
slowly exchanging complexes of CB vs. CD. An important for the preparation of SNPs, in which CB[8] acts as an inter-
advantage of the use of supramolecular interactions for the molecular connector between MV-PEI, Np8-PAMAM and Np-PEG
formation of SNPs is their responsive character. The triggered upon formation of a ternary supramolecular CT-complex. Of
disassembly of the SNPs reported here has been induced by the particular importance are the observed assembly kinetics and
reductant Na2S2O4 (Fig. S8, ESI†). After addition of the reducing the size-focusing in time, which indicates progression towards
agent, initially observed NPs could not be detected anymore thermodynamic equilibrium. The distinct size tunability as well
using DLS and SEM. Reduction of the dicationic MV species as the stimulus-responsive particle disassembly make such NPs
leads to the formation of MV + radical cations which undergo a potential candidate for biomedical applications in drug delivery,
stable homo-ternary complex formation involving two MV peptide therapeutics as well as in vivo sensing.
radical cations in one CB[8] host.23 Thereby, a different supra-
molecular complex is formed, and the Np entities are released Sciences of the Netherlands Organization for Scientific
This work was supported by the Council for Chemical
from the CB[8] cavities and, consequently, the SNPs dissolve.
SNP size control was achieved by varying the ratio of mono-
valent Np-PEG to multivalent Np8-PAMAM, while keeping the
overall Np concentration constant and maintaining an equimo-
lar CB[8]–MV–Np stoichiometry. SNPs were observed for all
samples as shown by DLS (Fig. S9, ESI†) and SEM (Fig. S10(a)–
(e), ESI†) and the size of the observed SNPs strongly depends on
the origin of the Np derivative. In particular, by increasing the
amount of Np derived from Np8-PAMAM from 10% to 35%, an
increase in particle size from 57 Æ 11 nm to 115 Æ 13 nm and
from 51 Æ 13 nm to 137 Æ 16 nm was observed using SEM and
DLS, respectively. As seen in Fig. 4, the observed SNP size
exhibits an apparent linear relationship with the relative amount
of Np derived from Np8-PAMAM. For samples containing 50% or
more of Np derived from Np8-PAMAM, DLS showed dh beyond
1000 nm (Fig. S9, ESI†), as well as precipitation. The formation of
SNPs is established by multivalent interactions in the core
and monovalent interactions of Np-PEG at the outer surface of
the particles. Thereby an equilibrium between capping and
Research (NWO-CW; Vici grant 700.58.443 to J.H.).
Notes and references
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6742 Chem. Commun., 2013, 49, 6740--6742
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This journal is The Royal Society of Chemistry 2013