Cucurbit[8]uril-Regulated Colloidal Dispersions Exhibiting Photocontrolled Rheological Behavior.

In situ photocontrol over shear-thickening of condensed colloidal dispersions is of paramount importance in a wide range of applications including process technology and photorheological fluids. Its development and practicability, however, are hampered by the lack of well-designed photoresponsive systems. Here, a colloidal suspension whose rheological behavior is readily switchable between shear-thinning and shear-thickening using an external light stimulus is reported. This smart colloidal solution contains hybrid raspberry-like colloids prepared by employing cucurbit[8]uril as a supramolecular linker to assemble functional Fe3 O4 nanoparticles onto a silica core. The formed raspberry colloids are photoresponsive and can be reversibly disassembled under UV irradiation.

Property Self-Optimization During Wear of MoS2.

Knowledge of their bulk physical properties often guides selection of appropriate tribological coating materials. However, these properties as well as the microstructure evolve dramatically under the extreme conditions imposed during mechanical wear. The dynamic response ultimately governs the material's wear performance; thus, understanding the dynamic evolution of the system is critical. This work characterizes the change in mechanical properties and microstructure as a function of wear cycles in model MoS2 films using a combination of nanowear testing, transmission electron microscopy, and site-specific nanopillar compression. Notably, mechanical wear enhances the mechanical properties of the MoS2 while simultaneously evolving a microstructure that reduces the coefficient of friction and wear rate. We hypothesize that this self-optimizing behavior underpins the exceptional lubricity and antiwear performance of MoS2.

Hollow mesoporous raspberry-like colloids with removable caps as photoresponsive nanocontainers.

The fabrication, characterisation and controlled cargo release of hollow mesoporous raspberry-like colloids (HMRCs), which are assembled by utilising host-guest complexation of cucurbit[8]uril (CB[8]) are described. CB[8] is employed as a supramolecular linker to 'stick' the viologen functionalised paramagnetic iron oxide nanoparticles onto an azobenzene functionalised hollow mesoporous silica core. The formed HMRCs are photoresponsive and can be reversibly disassembled upon light irradiation, endowing them with an ability to release loaded cargo under photocontrol. While the assembled HMRCs retain cargo inside their cavity, disassembled particles with their iron oxide nanoparticle 'caps' removed will release the loaded cargo through the mesoporous shell of the hollow silica colloids. A model system using a boronic acid derivative as the cargo in the HMRCs and Alizarin Red salt as a sensor for the released boronic acid is demonstrated.

Cucurbit[8]uril-Regulated Nanopatterning of Binary Polymer Brushes via Colloidal Templating.

Sub-micrometer/nanoscale patterned polymer brushes are prepared by employing cucurbit[8]uril (CB[8]) as a supramolecular recognition motif to assemble functional silica colloids onto Au surfaces as a sacrificial nanopatterning template. By employing CB[8]-mediated host-guest interactions at the interface, it is possible to readily generate nanopatterned materials in aqueous media under ambient conditions.

Mechanical Properties of Molybdenum Disulfide and the Effect of Doping: An in Situ TEM Study.

Direct observations on nanopillars composed of molybdenum disulfide (MoS2) and chromium-doped MoS2 and their response to compressive stress have been made. Time-resolved transmission electron microscopy (TEM) during compression of the submicrometer diameter pillars of MoS2- and Cr-doped MoS2 (Cr: 0, 10, and 50 at %) allow the deformation process of the material to be observed and can be directly correlated with mechanical response to applied load. The addition of chromium to the MoS2 changed the failure mode from plastic deformation to catastrophic brittle fracture, an effect that was more pronounced as chromium content increased.

Aromatic donor-acceptor interactions in non-polar environments.

We have evaluated the strength of aromatic donor-acceptor interactions between dialkyl naphthalenediimide and dialkoxynaphthalene in non-polar environments. (1)H NMR, UV-vis spectroscopy and isothermal titration calorimetry were used to characterise this interaction. We concluded that the strength of donor-acceptor interactions in heptane is sufficient to drive supramolecular assemblies in this and other aliphatic solvents.

Cucurbit[8]uril directed stimuli-responsive supramolecular polymer brushes for dynamic surface engineering.

In situ modification of surfaces with thin layers of polymers is of growing interest as adjustment of surface properties can be made on demand. We present herein a supramolecular 'grafting to' polymer brush via the recognition of surface-bound cucurbit[8]uril (CB[8]) rotaxanes towards end-functionalised polyethylene glycol (PEG). This dynamic supramolecular method represents advantages over traditional approaches, which employ covalent bond formation in the 'grafting to' process. Brush properties can be easily modified post-preparation by exchanging the polymers with small molecules in a controlled, reversible manner. Including both redox- and light-responsive guests in a single rotaxane entity, the CB[8]-mediated preparation of the polymer brush offers unique opportunities to switch the brush composition efficiently. While the PEG brushes are well hydrated in a good solvent (water) and stretch away from the surface, they collapse in a poor solvent (toluene), leading to the formation of a dense layer on the surface. This collapsed conformation protects the heteroternary complexes of CB[8]-rotaxane from dissociation and maintains the attachment of polymers on the surface.

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]uril (CB[8])-based rotaxanes on gold (Au) surfaces threaded onto a viologen (MV(2+)) 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 × 10(13) molecules·cm(-2), which is only slightly lower than the theoretical value of 5.0 × 10(13) molecules·cm(-2). 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 host-guest complex MV(2+)⊂CB[8]. The MV(2+)⊂CB[8] rotaxane structure thus effectively maintains the material density on the Au surface and dramatically enhances the stability of the functional surface.

A synthetic receptor for nicotine from a dynamic combinatorial library.

Designing synthetic receptors that bind biologically relevant guests in an aqueous solution remains a considerable challenge. We now report a new synthetic receptor for nicotine, selected from a dynamic combinatorial library, that binds this guest in water at neutral pH through a combination of hydrophobic and π-π interactions.

Dynamic combinatorial discovery of a [2]-catenane and its guest-induced conversion into a molecular square host.

A simple water-soluble naphthalenedithiol building block is converted quantitatively into a series of octameric [2]-catenanes, composed of two interlocked molecular squares. When this mixture is re-equilibrated in the presence of an adamantyl ammonium guest, the catenanes disassemble into their macrocyclic components that bind the guest with nanomolar affinity in water.

Dynamic combinatorial libraries of hydrazone-linked pseudo-peptides: dependence of diversity on building block structure and chirality.

Expanding on our earlier building block architecture [(MeO)(2)CH-Linker-Pro-X-NHNH(2) where X = Phe, Cha], we have produced a series of new pseudo-dipeptides [(MeO)(2)CH-Linker-Pro-X-NHNH(2) where X = Val, Leu, Ile, Ala] for use in hydrazone-based dynamic combinatorial libraries (DCLs); reverse order analogues [Phe-Pro and Val-Pro] and two enantio-analogues [Pro-Phe and Pro-Val] were also prepared. The behaviours of these building blocks in DCLs, as single components and in mixtures, were studied systematically using HPLC and mass spectrometry in order to gain insight into the relationship between building block structure and good library diversity. Subtle changes in building block structure lead to significant changes in library distribution and in the ability to produce diverse libraries in mixtures.

Dynamic combinatorial libraries of disulfide cages in water.

[structure: see text] Dynamic combinatorial libraries (DCLs) containing water-soluble disulfide-linked cages (alongside macrocyclic structures) have been generated and characterized. Unlike most other strategies for generating molecular cages, the structures are held together by covalent bonds, which are formed under thermodynamic control. The diversity of the cages generated opens new possibilities for a generalized combinatorial strategy toward molecular encapsulation.

Reversible covalent chemistry in drug delivery.

The targeting of drugs specifically to their sites of action is an important strategy for increasing drug efficacy. Chemists have come up with many elegant schemes that aim to convert drugs into magic bullets. This review focuses on the chemistry that underlies these schemes, with particular emphasis on two types of cleavable covalent bonds that are frequently used to link drugs to their various carriers: disulfide bonds and hydrazone bonds. These linkages have been used to release drugs under specific conditions; in the case of disulfides, cleavage is triggered by the mildly reducing environment found in intracellular fluids, and in the case of hydrazones, the acidic conditions that prevail in endosomes cause release of the drug. The applications of these chemistries in drug delivery are reviewed.