19 research outputs found
Cucurbit[8]uril Mediated DonorâAcceptor Ternary Complexes: A Model System for Studying Charge-Transfer Interactions
A supramolecular self-assembly approach is described
which allows
for the convenient preparation of a wide range of charge-transfer
(CT) donorâacceptor complexes in aqueous solutions. When one
equiv of the macrocyclic host cucurbit[8]Âuril (CB[8]) is added to
an aqueous donor and acceptor solution, a heteroternary complex forms
inside the hostâs cavity with a well-defined face-to-face
ÏâÏ-stacking geometry of the donor and acceptor.
This heteroternary, CB[8]-mediated complex offers the opportunity
to study the CT phenomena at low concentrations and free from complications
arising from any donorâdonor and acceptorâacceptor interactions
as a result of the large binding affinities and the very high selectivity
over the formation of these homoternary complexes. Thus, this supramolocular
self-assembly strategy is a practical donorâacceptor mix-and-match
approach with synthetic advantages over much more cumbersome tethering
schemes. While the characteristic UV/vis features of a few CB[8] ternary
systems had been described as a CT band, we present for the first
time systematic evidence for the existence of CT interactions between
several donorâacceptor pairs that are mediated by the host
CB[8]. Correlation of the experimentally obtained CT λ<sub>max</sub> to computed HOMOâLUMO energies demonstrated that the CT process
in the hostâs cavity can be described by the Mulliken model.
Furthermore, the literature claim of a âCT driving forceâ
for the formation of CB[8] ternary complexes was scrutinized and evaluated
by calorimetric (ITC) and ESI-MS measurements. The findings indicated
that neither in the aqueous medium nor in the âgas-phaseâ
is CT of energetic relevance to the Gibbs free binding energy. In
contrast, electrostatic considerations combined with solvation effects
are much better suited to rationalize the observed trends in binding
affinities. Additionally, the CT λ<sub>max</sub> was found to
be much more red-shifted (â„75 nm) inside the CB[8] cavity than
in any polar organic solvents or water, indicating a significant stabilization
of the CT excited state within the host cavity, further demonstrating
the unique electrostatic, polar properties of the host cavity
A selective supramolecular photochemical sensor for dopamine
<div><p>Dopamine (DA) is an important biomarker for diseases and biological disorders. Existing techniques for DA detection suffer from drawbacks including low sensitivity and selectivity as well as interfering signals from non-target molecules. A simple and selective photochemical sensor for the determination of DA in a supramolecular manner is presented. This approach utilises the complexation properties of a highly fluorescent water-soluble complex of perylene bis(diimide) dye with the macrocyclic host cucurbit[8]uril. The method can be used for the determination of DA in aqueous media, with detection limits below 2 Ă 10<sup>â 5</sup> M, even in the presence of known interferents including ascorbic acid and the catecholamines epinephrine and norepinepherine.</p></div
Facile Method for Preparing Surface-Mounted Cucurbit[8]uril-Based Rotaxanes
Surface-immobilized rotaxanes are
of practical interest for myriad
applications including molecular rotors and analytical sensing. Herein,
we present a facile method for the preparation of cucurbit[8]Âuril
(CB[8])-based rotaxanes on gold (Au) surfaces threaded onto a viologen
(MV<sup>2+</sup>) axle. The surface-bound CB[8] rotaxanes were characterized
by contact angle measurements and optical microscopy. Direct imaging
of the rotaxanes was accomplished by attaching either azobenzene-functionalized
silica (Si-azo) colloids or fluorescein-labeled dopamine that were
bound to the Au surface through a supramolecular heteroternary (1:1:1)
complex with CB[8]. The surface density of CB[8] rotaxanes was examined
based on their detection of dopamine. The calculated surface density
is 4.8 Ă 10<sup>13</sup> molecules·cm<sup>â2</sup>, which is only slightly lower than the theoretical value of 5.0
Ă 10<sup>13</sup> molecules·cm<sup>â2</sup>. Surface-functionalized
rotaxanes can be reversibly switched using external stimuli to bind
electron-rich second guests for CB[8], including both small molecules
such as dopamine and appropriately-functionalized colloidal particles.
Such controlled reversibility gives rise to potential applications
including selective sensing or reusable templates for preparing well-defined
colloidal arrays. The formation of the surface-bound rotaxane structure
is critical for successfully anchoring CB[8] host molecules onto Au
substrates, yielding an interlocked architecture and preventing the
dissociation of binary hostâguest complex MV<sup>2+</sup>âCBÂ[8].
The MV<sup>2+</sup>âCBÂ[8] rotaxane structure thus effectively
maintains the material density on the Au surface and dramatically
enhances the stability of the functional surface
Quantitative SERS Using the Sequestration of Small Molecules Inside Precise Plasmonic Nanoconstructs
We show how the macrocyclic host, cucurbit[8]Âuril (CB[8]),
creates
precise subnanometer junctions between gold nanoparticles while its
cavity simultaneously traps small molecules; this enables their reproducible
surface-enhanced Raman spectroscopy (SERS) detection. Explicit shifts
in the SERS frequencies of CB[8] on complexation with guest molecules
provides a direct strategy for absolute quantification of a range
of molecules down to 10<sup>â11</sup> M levels. This provides
a new analytical paradigm for quantitative SERS of small molecules
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Formation of Cucurbit[8]uril-Based Supramolecular Hydrogel Beads Using Droplet-Based Microfluidics
Herein
we describe the use of microdroplets as templates for the
fabrication of uniform-sized supramolecular hydrogel beads, assembled
by supramolecular cross-linking of functional biopolymers with the
macrocyclic host molecule, cucurbit[8]Âuril (CB[8]). The microdroplets
were formed containing diluted hydrogel precursors in solution, including
the functional polymers and CB[8], in a microfluidic device. Subsequent
evaporation of water from collected microdroplets concentrated the
contents, driving the formation of the CB[8]-mediated hostâguest
ternary complex interactions and leading to the assembly of condensed
three-dimensional polymeric scaffolds. Rehydration of the dried particles
gave monodisperse hydrogel beads. Their equilibrium size was shown
to be dependent on both the quantity of material loaded and the dimensions
of the microfluidic flow focus. Fluorescein-labeled dextran was used
to evaluate the efficacy of the hydrogel beads as a vector for controlled
cargo release. Both passive, sustained release (hours) and triggered,
fast release (minutes) of the FITC-dextran was observed, with the
rate of sustained release dependent on the formulation. The kinetics
of release was fitted to the Ritger-Peppas controlled release equation
and shown to follow an anomalous (non-Fickian) transport mechanism
Release of High-Energy Water as an Essential Driving Force for the High-Affinity Binding of Cucurbit[<i>n</i>]urils
Molecular dynamics simulations and isothermal titration
calorimetry
(ITC) experiments with neutral guests illustrate that the release
of high-energy water from the cavity of cucurbitÂ[<i>n</i>]Âuril (CB<i>n</i>) macrocycles is a major determinant for
guest binding in aqueous solutions. The energy of the individual encapsulated
water molecules decreases with increasing cavity size, because larger
cavities allow for the formation of more stable H-bonded networks.
Conversely, the total energy of internal water increases with the
cavity size because the absolute number of water molecules increases.
For CB7, which has emerged as an ultrahigh affinity binder, these
counteracting effects result in a maximum energy gain through a complete
removal of water molecules from the cavity. A new design criterion
for aqueous synthetic receptors has therefore emerged, which is the
optimization of the size of cavities and binding pockets with respect
to the energy and number of residing water molecules
Photocontrol over Cucurbit[8]uril Complexes: Stoichiometry and Supramolecular Polymers
Herein we report the photocontrol
of cucurbit[8]Âuril (CB[8])-mediated
supramolecular polymerization of azobenzene-containing monomers. The
CB[8] polymers were characterized both in solution and in the solid
state. These hostâguest complexes can be reversibly switched
between highly thermostable photostationary states. Moreover, a remarkable
stabilization of <i>Z</i>-azobenzene was achieved by CB[8]
complexation, allowing for structural characterization in the solid
state
Ultrahigh-Water-Content Supramolecular Hydrogels Exhibiting Multistimuli Responsiveness
Hydrogels are three-dimensional networked materials that
are similar
to soft biological tissues and have highly variable mechanical properties,
making them increasingly important in a variety of biomedical and
industrial applications. Herein we report the preparation of extremely
high water content hydrogels (up to 99.7% water by weight) driven
by strong hostâguest complexation with cucurbit[8]Âuril (CB[8]).
Cellulosic derivatives and commodity polymers such as polyÂ(vinyl alcohol)
were modified with strongly binding guests for CB[8] ternary complex
formation (<i>K</i><sub>eq</sub> = 10<sup>12</sup> M<sup>â2</sup>). When these polymers were mixed in the presence
of CB[8], whereby the overall solid content was 90% cellulosic, a
lightly colored, transparent hydrogel was formed instantaneously.
The supramolecular nature of these hydrogels affords them with highly
tunable mechanical properties, and the dynamics of the CB[8] ternary
complex cross-links allows for rapid self-healing of the materials
after damage caused by deformation. Moreover, these hydrogels display
responsivity to a multitude of external stimuli, including temperature,
chemical potential, and competing guests. These materials are easily
processed, and the simplicity of their preparation, their availability
from inexpensive renewable resources, and the tunability of their
properties are distinguishing features for many important water-based
applications
Photocontrol over Cucurbit[8]uril Complexes: Stoichiometry and Supramolecular Polymers
Herein we report the photocontrol
of cucurbit[8]Âuril (CB[8])-mediated
supramolecular polymerization of azobenzene-containing monomers. The
CB[8] polymers were characterized both in solution and in the solid
state. These hostâguest complexes can be reversibly switched
between highly thermostable photostationary states. Moreover, a remarkable
stabilization of <i>Z</i>-azobenzene was achieved by CB[8]
complexation, allowing for structural characterization in the solid
state
Photocontrol over Cucurbit[8]uril Complexes: Stoichiometry and Supramolecular Polymers
Herein we report the photocontrol
of cucurbit[8]Âuril (CB[8])-mediated
supramolecular polymerization of azobenzene-containing monomers. The
CB[8] polymers were characterized both in solution and in the solid
state. These hostâguest complexes can be reversibly switched
between highly thermostable photostationary states. Moreover, a remarkable
stabilization of <i>Z</i>-azobenzene was achieved by CB[8]
complexation, allowing for structural characterization in the solid
state