Mechanical Properties of a Metal-Organic Framework formed by Covalent Cross-Linking of Metal-Organic Polyhedra.

Overcoming the brittleness of metal-organic frameworks (MOFs) is a challenge for industrial applications. To increase the mechanical strength, MOFs have been blended with polymers to form composites. However, this also brings challenges, such as integration and integrity of MOF in the composite, which can hamper the selectivity of gas separations. In this report, an "all MOF" material with mechanical flexibility has been prepared by covalent cross-linking of metal-organic polyhedra (MOPs). The ubiquitous Cu24 isophthalate MOP has been decorated with a long alkyl chain having terminal alkene functionalities so that MOPs can be cross-linked via olefin metathesis using Grubbs second generation catalyst. Different degrees of cross-linked MOP materials have been obtained by varying the amount of catalyst in the reaction. Rheology of these structures with varying number of cross-links was performed to assess the cross-link density and its homogeneity throughout the sample. The mechanical properties were further investigated by the nanoindentation method, which showed increasing hardness with higher cross-link density. Thus, this strategy of cross-linking MOPs with covalent flexible units allows us to create MOFs of increasing mechanical strength while retaining the MOP cavities.

Role of interparticle interactions on microstructural and rheological properties of cellulose nanocrystal stabilized emulsions.

HYPOTHESIS: Microstructural and rheological properties of particle-stabilized emulsions are highly influenced by the nanoparticle properties such as size and surface charge. Surface charge of colloidal particles not only influences the interfacial adsorption but also the interparticle network formed by the non-adsorbed particles in the continuous phase. EXPERIMENTS: We have studied oil-in-water emulsions stabilized by cellulose nanocrystals (CNCs) with two different degrees of surface charge. Surface charge was varied by means of acidic or basic desulfation. Confocal microscopy coupled with rheology as well as cryogenic scanning electron microscopy were employed to establish a precise link between the microstructure and rheological behavior of the emulsions. FINDINGS: CNCs desulfated with hydrochloric acid (a-CNCs) were highly aggregated in water and shown to adsorb faster to the oil-water interface, yielding emulsions with smaller droplet sizes and a thicker CNC interfacial layer. CNCs desulfated using sodium hydroxide (b-CNCs) stabilized larger emulsion droplets and had a higher amount of non-adsorbed CNCs in the water phase. Rheological measurements showed that emulsions stabilized by a-CNCs formed a stronger network than for b-CNC stabilized emulsions due to increased van der Waals and H-bonding interactions that were not impeded by electrostatic repulsion.

Analysis of network formation and long-term stability in silica nanoparticle stabilized emulsions.

Emulsions are widely used in industrial applications, including in food sciences, cosmetics, and enhanced oil recovery. For these industries, an in depth understanding of the stability and rheological properties of emulsions under both static and dynamic conditions is vital to their successful application. Presented here is a thorough assessment of a model nanoparticle (NP) stabilized dodecane-in-water emulsion as a route to improved understanding of the relationship between NP properties, microstructure and droplet-droplet interactions on the stability and rheological properties of emulsions. Emulsions are obtained here with low NP loadings without the need for added electrolyte through the use of an optimized silica NP (SNP) surface modification procedure. The prepared emulsions were characterized via optical microscopy, cryo-scanning electron microscopy (cryo-SEM), zeta potential analysis and laser scanning confocal microscopy (LSCM), enabling quantification of the emulsion droplet size, SNP interfacial coverage/morphology and surface charge. The correlation of these properties with the rheology of the emulsions is investigated through small amplitude oscillatory shear experiments which provide significant insight into the origins of the emulsions' rheological behavior and their stability. In addition, long-term stability, droplet-droplet network formation and microstructural evolution are found to be readily detectable shortly after preparation through measured progression of the emulsion's rheological properties.

A stimulus-responsive, in situ-forming, nanoparticle-laden hydrogel for ocular drug delivery.

Most medications targeting optic neuropathies are administered as eye drops. However, their corneal penetration efficiencies are typically < 5%. There is a clear, unmet need for novel transcorneal drug delivery vehicles. To this end, we have developed a stimulus-responsive, in situ-forming, nanoparticle-laden hydrogel for controlled release of poorly bioavailable drugs into the aqueous humor of the eye. The hydrogel is formulated as a composite of hyaluronic acid (HA) and methylcellulose (MC). The amphiphilic nanoparticles are composed of poly(ethylene oxide) (PEO) and poly(lactic acid) (PLA). Experimental design aided the identification of hydrogel composition and nanoparticle content in the formulation, and the formulation reliably switched between thixotropy and temperature-dependent rheopexy when it was tested in a rheometer under conditions that simulate the ocular surface, including blinking. These properties should ensure that the formulation coats the cornea through blinking of the eyelid and facilitate application of the medication as an eye drop immediately prior to the patient's bedtime. We subsequently tested the efficacy of our formulation in whole-eye experiments by loading the nanoparticles with cannabigerolic acid (CBGA). Our formulation exhibits over a 300% increase in transcorneal penetration over control formulations. This work paves the way for the introduction of novel products targeting ocular diseases to the market.

Fluorescent polycatecholamine nanostructures as a versatile probe for multiphase systems.

Shape and size controlled nanostructures are critical for nanotechnology and have versatile applications in understanding interfacial phenomena of various multi-phase systems. Facile synthesis of fluorescent nanostructures remains a challenge from conventional precursors. In this study, bio-inspired catecholamines, dopamine (DA), epinephrine (EP) and levodopa (LDA), were used as precursors and fluorescent nanostructures were synthesized via a simple one pot method in a water-alcohol mixture under alkaline conditions. DA and EP formed fluorescent spheres and petal shaped structures respectively over a broad spectrum excitation wavelength, whereas LDA did not form any particular structure. However, the polyepinephrine (PEP) micropetals were formed by weaker interactions as compared to covalently linked polydopamine (PDA) nanospheres, as revealed by NMR studies. Application of these fluorescent structures was illustrated by their adsorption behavior at the oil/water interface using laser scanning confocal microscopy. Interestingly, PDA nanospheres showed complete coverage of the oil/water interface despite its hydrophilic nature, as compared to hydrophobic PEP micropetals which showed a transient coverage of the oil/water interface but mainly self-aggregated in the water phase. The reported unique fluorescent organic structures will play a key role in understanding various multi-phase systems used in aerospace, biomedical, electronics and energy applications.

Hydrothermal Gelation of Aqueous Cellulose Nanocrystal Suspensions.

We report the facile preparation of gels from the hydrothermal treatment of suspensions of cellulose nanocrystals (CNCs). The properties of the hydrogels have been investigated by rheology, electron microscopy, and spectroscopy with respect to variation in the temperature, time, and CNC concentration used in preparation. Desulfation of the CNCs at high temperature appears to be responsible for the gelation of the CNCs, giving highly porous networks. The viscosity and storage modulus of the gels was shown to increase when samples were prepared at higher treatment temperature. Considering the wide natural abundance and biocompatibility of CNCs, this simple, green approach to CNC-based hydrogels is attractive for producing materials that can be used in drug delivery, insulation, and as tissue scaffolds.