Efficiently self-healing boronic ester crystals.

The perception of organic crystals being rigid static entities is quickly eroding, and molecular crystals are now matching a number of properties previously thought to be unique to soft materials. Here, we present crystals of a boronate ester that encompass many of the elastic and plastic mechanical properties of polymers such as bending, twisting, coiling and highly efficient self-healing of up to 67%, while they maintain their long-range structural order. The approach utilizes the concept of dynamic covalent chemistry and proves it can be applied towards ordered materials. This work expands our current understanding of the properties of crystalline molecular materials, and it could have implications towards the development of mechanically robust organic crystals that are capable of self-repair for durable all-organic electronics and soft robotics.

Turning on Solid-State Fluorescence with Light.

A bioinspired fluorophore that is analogous to the substrate in the bioluminescence of fireflies was prepared and reacts when exposed to weak blue LED light. Upon excitation, this material is photodecarboxylated with a nearly 81-fold enhancement of the solid-state emission, the fluorescence quantum yield of the product in solution is approximately 90 %, and violent disintegrative effects occur as a result of the release of carbon dioxide. Crystallographic and computational results, together with global spectral analysis of the kinetics, confirmed that most of the emission observed in the decay-associated spectra is intrinsic to the product molecule, with only a minor contribution from an excimer through π-π stacking of the molecules in the crystal.

Mechanical and Crystallographic Analysis of Cholesterol Crystals Puncturing Biological Membranes.

Ischemic heart disease often leads to myocardial infarction and remains the most common cause for death in humans. Although the exact impetus for the infarction remains elusive, a mechanism has been proposed that relates the disease to the observed high cholesterol levels in the body. The mechanism claims that cholesterol crystallizes inside the arterial plaque into needle-shaped crystals. The crystals puncture the fibrous cap of the plaque, whereby the necrotic contents of the plaque are spilled, subsequently clotting the blood vessels. This hypothesis has not been given sufficient attention partly due to the purported softness of the organic crystals and the common platy habit of the known crystal forms of cholesterol. In this work it is shown that, from hydrophobic solutions that attempt to emulate the plaque contents, a new solid form of cholesterol crystallizes as prisms with mucronate tips, and they are sufficiently strong to puncture a lamb pericardium, which mimics the plaque cap. The properties of the crystals were assessed by mechanical, structural, and crystallographic analyses. The results support the hypothesis that the cholesterol crystals can be considered, at least within the framework of the proposed mechanism, a possible cause of myocardial infarction.

Interference of citrate-stabilized gold nanoparticles with ß2-microglobulin oligomeric association.

Protein fibrillation is involved in many serious diseases, and protein oligomers are proved to be precursors of amyloid fibrils. NMR and QCMD experiments allowed us to establish that the interaction between citrate-stabilized gold nanoparticles and a paradigmatic amyloidogenic protein, ß2-microglobulin, is able to interfere with protein association into oligomers.

Encapsulation of Plant Viral Particles in Calcite Crystals.

The concept of biomineralization and encapsulation of organic molecules into inorganic matrices to alter and enhance their physical properties has been evolved and perfected in natural systems. Being inspired by the natural biomineralization of foreign components into calcite, here the inclusion of a plant virus, cowpea mosaic virus (CPMV) of 5.4% by mass into crystals of calcite is reported. The viral particles are labeled with a fluorescent tag (Alexa Fluor 532), and are observed within the calcite matrix using confocal fluorescence microscopy. Upon encapsulation, the calcite crystals exhibit an irregular and aggregated morphology, as visualized with atomic force and electron microscopy. The viral particles protected inside the calcite crystals are able to resist harsh chemical agents. While spherical viral particles such as CPMV can be easily included in calcite, viruses such as the tobacco mosaic virus are not compatible with the host, presumably due to their high aspect ratio. The results provide a simple and scalable method to incorporate viral particles into inorganic matrix, and could prove useful in thermal stabilization of sensitive viral biological agents such as vaccines in the future.

Direct Observation of Asphaltene Nanoparticles on Model Mineral Substrates.

The propensity for adherence to solid surfaces of asphaltenes, a complex solubility class of heteropolycyclic aromatic compounds from the heavy fraction of crude oil, has long been the root cause of scale deposition and remains an intractable problem in the petroleum industry. Although the adhesion is essential to understanding the process of asphaltene deposition, the relationship between the conformation of asphaltene molecules on mineral substrates and its impact on adhesion and mechanical properties of the deposits is not completely understood. To rationalize the primary processes in the process of organic scale deposition, here we use atomic force microscopy (AFM) to visualize the morphology of petroleum asphaltenes deposited on model mineral substrates. High imaging contrast was achieved by the differential adhesion of the tip between asphaltenes and the mineral substrate. While asphaltenes form smooth continuous films on all substrates at higher concentrations, they deposit as individual nanoparticles at lower concentrations. The size, shape, and spatial distribution of the nanoaggregates are strongly affected by the nature of the substrate; while uniformly distributed spherical particles are formed on highly polar and hydrophilic substrates (mica), irregular islands and thicker patches are observed with substrates of lower polarity (silica and calcite). Asphaltene nanoparticles flatten when adsorbed on highly oriented pyrolytic graphite due to π-π interactions with the polycyclic core. Force-distance profiles provide direct evidence of the conformational changes of asphaltene molecules on hydrophilic/hydrophobic substrates that result in dramatic changes in adhesion and mechanical properties of asphaltene deposits. Such an understanding of the nature of adhesion and mechanical properties tuned by surface properties, on the level of asphaltene nanoaggregates, would contribute to the design of efficient asphaltene inhibitors for preventing asphaltene fouling on targeted surfaces. Unlike flat surfaces, the AFM phase contrast images of defected calcite surfaces show that asphaltenes form continuous deposits to fill the recesses, and this process could trigger the onset for asphaltene deposition.

In situ quartz crystal microbalance monitoring of the adsorption of polyoxometalate on a polyampholyte polymer matrix.

Hybridization of polyoxometalates (POMs) via an organic-inorganic association constitutes a new route to develop heterogeneous POM catalysts with tunable supramolecular architecture. As the structural stability of POMs is strongly influenced by the pH conditions, a quantitative understanding of the POMs-polymer association is important in practical applications. Herein, we use Quartz Crystal Microbalance (QCM) to systematically investigate the interactions of Keggin phosphotungstic acid POM with a polyampholyte polymer-coated QCM sensor as a function of pH. The mass of adsorbed POMs increases when pH decreases from 5.6 to 2, indicating that electrostatic forces play a major role in the formation of POM-polymer hybrids. This finding is complemented by AFM images that show an increase in the size of the hybrid entities from 5 to 12 nm as the pH decreases from 5.6 to 2. The POM adsorbed amount at a particular pH value reaches an equilibrium level with time. The hybrids further gain in adsorbed mass only when lowering the pH value of the POM solution. The hybrid structure formed above pH 2 shows resistance to leaching as indicated by the steady level of the adsorbed mass during a rinsing step with water. However, at pH 2, the rinsing step causes desorption of some weakly adsorbed POMs. It is shown that leached POMs can be re-adsorbed back into the polymer matrix during a second contact with a POM solution at pH 2. This adsorption-desorption cycles of POMs were successfully repeated. Our experiments shed light into the coexistence of tightly as well as loosely bound POMs in hybrid catalyst formed at pH 2. The loosely bound POMs can potentially act as homogeneous catalysts when desorbed. However, these leached POMs can be re-adsorbed back into the matrix, preserving the heterogeneous state of the catalyst. Our results show that QCM is a powerful technique to study in situ the dynamics of the adsorption of POMs on a polymer matrix under different pH conditions.

Controlling the dispersion of supported polyoxometalate heterogeneous catalysts: impact of hybridization and the role of hydrophilicity-hydrophobicity balance and supramolecularity.

The hybridization of polyoxometalates (POMs) through an organic-inorganic association offers several processing advantages in the design of heterogeneous catalysts. A clear understanding of the organization of these hybrid materials on solid surfaces is necessary to optimise their properties. Herein, we report for the first time the organization of Keggin phosphotungstic [PW12O40](3-) and Wells-Dawson (WD) phosphomolybdic [P2Mo18O62](6-) anions deposited on mica (hydrophilic), and highly oriented pyrolytic graphite (HOPG) (hydrophobic) surfaces. Next, the supramolecular organization of the organic-inorganic hybrid materials formed from the association of POM anions and dimethyldioctadecylammonium bromide (DODA) is investigated as a function of the hydrophilic or hydrophobic nature of the surfaces. The height of the Keggin-POM anions, measured with tapping mode (TM-AFM) is always in good agreement with the molecular dimension of symmetric Keggin-POM anions (ca. 1 nm). However, the asymmetric WD-POM anions form monolayer assemblies on the surfaces with the orientation of their long molecular axis (ca. 1.6 nm) depending on the hydrophilic or hydrophobic properties of the substrate. Namely, the long axis is parallel on mica, and perpendicular on HOPG. When hybridized with DODA, the organization of the hybrid material is dictated by the interaction of the alkyl side chains of DODA with the substrate surface. On HOPG, the DODA-POM hybrid forms small domains of epitaxially arranged straight nanorod structures with their orientation parallel to each other. Conversely, randomly distributed nanospheres are formed when the hybrid material is deposited on freshly cleaved mica. Finally, a UV-ozone treatment of the hybrid material allows one to obtain highly dispersed isolated POM entities on both hydrophilic and hydrophobic surfaces. The hybridization strategy to prevent the clustering of POMs on various supports would enable to develop highly dispersed POM-based heterogeneous catalysts with enhanced functionalities.

Study of mesoporous CdS-quantum-dot-sensitized TiO2 films by using X-ray photoelectron spectroscopy and AFM.

CdS quantum dots were grown on mesoporous TiO2 films by successive ionic layer adsorption and reaction processes in order to obtain CdS particles of various sizes. AFM analysis shows that the growth of the CdS particles is a two-step process. The first step is the formation of new crystallites at each deposition cycle. In the next step the pre-deposited crystallites grow to form larger aggregates. Special attention is paid to the estimation of the CdS particle size by X-ray photoelectron spectroscopy (XPS). Among the classical methods of characterization the XPS model is described in detail. In order to make an attempt to validate the XPS model, the results are compared to those obtained from AFM analysis and to the evolution of the band gap energy of the CdS nanoparticles as obtained by UV-vis spectroscopy. The results showed that XPS technique is a powerful tool in the estimation of the CdS particle size. In conjunction with these results, a very good correlation has been found between the number of deposition cycles and the particle size.

Supramolecular organization in organic-inorganic heterogeneous hybrid catalysts formed from polyoxometalate and poly(ampholyte) polymer.

Hybridization of polyoxometalates (POMs) via the formation of an organic-inorganic association constitutes a new route to develop a heterogeneous POM catalyst with tunable functionality imparted through supramolecular assembly. Herein, we report on strategies to obtain tunable well-defined supramolecular architectures of an organic-inorganic heterogeneous hybrid catalyst formed by the association of a hydrophobically substituted polyampholyte copolymer (poly N, N-diallyl-N-hexylamine-alt-maleic acid) and phosphotungstic acid (H3PW12O40) POMs. The self-assembling property of the initial polyampholyte copolymer matrix is modulated by controlling the pH of the hybridization solution. When deposited on a mica surface, isolated, long and extended polymer chains are formed under basic conditions (pH 7.9), while globular or coiled structures are formed under acidic conditions (pH 2). The supramolecular assembly of the POM-polymer hybrid is found to be directed by the type and quantities of charges present on the polyampholyte copolymer, which themselves depend on the pH conditions. The hypothesis is that the Keggin type [PW12O40](3-) anions, which have a size of ~1 nm, electrostatically bind to the positive charge sites of the polymer backbone. The hybrid material stabilized at pH 5.3 consists of POM-decorated polymer chains. Statistical analysis of distances between pairs of POM entities show narrow density distributions, suggesting that POM entities are attached to the polymer chains with a high level of order. Conversely, under acidic conditions (pH 2), the hybrid shows the formation of a core-shell type of structure. The strategies reported here, to tune the supramolecular assembly of organic-inorganic hybrid materials, are highly valuable for the design and a more rational utilization of POM heterogeneous catalysts in several chemical transformations.