Directing the assembly of charged organic molecules by a hydrophilic-hydrophobic nanostructured monolayer at electrified interfaces

Nanostructured monolayers of water-insoluble amphiphilic 5-alkoxy-isophthalic acids direct the reversible self-assembly of water-soluble positively and negatively charged molecules under electrochemical control. The surface potential is in control of the monolayer composition, structure, and guest dynamics.     

Supramolecular polymers at work

Sophisticated polymeric materials with ‘responsive’ properties are beginning to reach the market. The use of reversible, noncovalent interactions is a recurring design principle for responsive materials. Now, recently developed hydrogen-bonding units allow this design principle to be taken to its extreme. Supramolecular polymers, where hydrogen bonds are the only force keeping the monomers together, form materials whose (mechanical) properties respond strongly to a change in temperature or solvent. In this review, we describe the developments that have led to hydrogen-bonded supramolecular polymers and discuss the use of these materials in advanced applications.     

Cell biology at interfaces

A brief review is presented of the significant developments in the understanding of the processes involved in cell adhesion both to other cells and to substrates. The relationship between general cellular behaviour and cell adhesion is a result of the importance of the cytoplasmic cytoskeleton to most cellular processes. Interaction between a substrate and the cell is mediated through intramembranous proteins, such as the integrins. The intramembranous proteins, in turn, influence the assembly of the microfilamentous structures in the cytoplasm. Changes in the state of the microfilaments are accompanied by modifications in the behaviour of both microtubules and intermediate filaments. The expression of different types of cytoskeletal configuration result from differing types of cell-cell or cell-substratum encounters. This leads to significant changes in resultant cellular behaviour. It is argued that an understanding of changes that result from cell-biomaterial interactions, at the ultrastructural level, is necessary in order to assess the biocompatability of implant materials.     

Superhydrophobicity and superhydrophilicity of regular nanopatterns

The hydrophilicity, hydrophobicity, and sliding behavior of water droplets on nanoasperities of controlled dimensions were investigated experimentally. We show that the "hemi-wicking" theory for hydrophilic SiO(2) samples successfully predicts the experimental advancing angles and that the same patterns, after silanization, become superhydrophobic in agreement with the Cassie-Baxter and Wenzel theories. Our model topographies have the same dimensional scale of some naturally occurring structures that exhibit similar wetting properties. Our results confirm that a forest of hydrophilic/hydrophobic slender pillars is the most effective superwettable/water-repellent configuration. It is shown that the shape and curvature of the edges of the asperities play an important role in determining the advancing angles.     

Intergranular films at Au-sapphire interfaces

The existence of nanometer-thick amorphous equilibrium films at metal-ceramic interfaces has been experimentally verified for the Au–Al₂O₃ system. The films were formed using a novel experimental approach, in which thin sputtered films of Au were dewetted on a sapphire substrate which was previously partially wetted with drops of anorthite glass (CaO–2SiO₂–Al₂O₃). High-resolution transmission electron microscopy and qualitative analytical transmission electron microscopy were used to confirm the existence of the amorphous films. In addition, positive and relatively large Hamaker constants were calculated for the Au-film-Al₂O₃ interface, which indicates the existence of an attractive van der Waals force which stabilizes the film, similar to equilibrium films at grain boundaries in ceramics. A ∼ 1 nm thick surface film was also detected on the (0001) surface of sapphire substrates partially wetted by anorthite glass. The refractive index required to stabilize the surface films, via a positive Hamaker constant, is explored.     

Emergent phenomena at oxide interfaces

Recent technical advances in the atomic-scale synthesis of oxide heterostructures have provided a fertile new ground for creating novel states at their interfaces. Different symmetry constraints can be used to design structures exhibiting phenomena not found in the bulk constituents. A characteristic feature is the reconstruction of the charge, spin and orbital states at interfaces on the nanometre scale. Examples such as interface superconductivity, magneto-electric coupling, and the quantum Hall effect in oxide heterostructures are representative of the scientific and technological opportunities in this rapidly emerging field.     

Protein nanopatterns for improved immunodetection sensitivity

In this work, we clearly demonstrate the capability of protein nanopatterns of improving the quality factors of immunosensing devices, such as lowering of the limit of detection and increase of sensitivity. This beneficial effect is obtained by the formation on the sensor's surface of bioadhesive domains of nanometric dimensions in a nonadhesive matrix by means of colloidal lithography.     

Protein-protein interactions in reversibly assembled nanopatterns

We describe herein a platform to study protein-protein interactions and to form functional protein complexes in nanoscopic surface domains. For this purpose, we employed multivalent chelator (MCh) templates, which were fabricated in a stepwise procedure combining dip-pen nanolithography (DPN) and molecular recognition-directed assembly. First, we demonstrated that an atomic force microscope (AFM) tip inked with an oligo(ethylene glycol) (OEG) disulfide compound bearing terminal biotin groups can be used to generate biotin patterns on gold achieving line widths below 100 nm, a generic platform for fabrication of functional nanostructures via the highly specific biotin-streptavidin recognition. Subsequently, we converted such biotin/streptavidin patterns into functional MCh patterns for reversible assembly of histidine-tagged (His-tagged) proteins via the attachment of a tris-nitriloacetic acid (trisNTA) biotin derivative. Fluorescence microscopy confirmed reversible immobilization of the receptor subunit ifnar2-His10 and its interaction with interferon-alpha2 labeled with fluorescent quantum dots in a 7 x 7 dot array consisting of trisNTA spots with a diameter of approximately 230 nm. Moreover, we carried out characterization of the specificity, stability, and reversibility as well as quantitative real-time analysis of protein-protein interactions at the fabricated nanopatterns by imaging surface plasmon resonance. Our work offers a route for construction and analysis of functional protein-based nanoarchitectures.     

Metallic conduction at organic charge-transfer interfaces

The electronic properties of interfaces between two different solids can differ strikingly from those of the constituent materials. For instance, metallic conductivity-and even superconductivity-have recently been discovered at interfaces formed by insulating transition-metal oxides. Here, we investigate interfaces between crystals of conjugated organic molecules, which are large-gap undoped semiconductors, that is, essentially insulators. We find that highly conducting interfaces can be realized with resistivity ranging from 1 to 30 kohms per square, and that, for the best samples, the temperature dependence of the conductivity is metallic. The observed electrical conduction originates from a large transfer of charge between the two crystals that takes place at the interface, on a molecular scale. As the interface assembly process is simple and can be applied to crystals of virtually any conjugated molecule, the conducting interfaces described here represent the first examples of a new class of electronic systems.     

Lattice compatibility at interfaces after recrystallisation

Abstract

The nature, distribution, and implications of local, i.e. intragrain, lattice rotations have been explored in recrystallised nickel using electron backscattered diffraction. The data processing methodology for such methods is discussed in some detail. The results reveal that orientation perturbations with a periodicity of ～1 μm occur frequently within grains, and that there is a strong tendency for two grains which feature either both large perturbations or both small perturbations to be neighbours. The results are interpreted in terms of lattice compatibility, potential sites for instigation of secondary recrystallisation, and the meaningful specification of grain boundary misorientation.     

Vortex dimples at charged helium interfaces

We have investigated theoretically the profile of electrically charged helium interfaces in the presence of a superfluid vortex line perpendicular to the interface. The free surface of superfluid³He-B as well as the interface between phase-separated³He and⁴He mixtures have been considered. The superflow-induced dimple profile around the vortex line was calculated within the linear theory assuming a weak curvature of the interface. Our results show that the depth of the dimple increases strongly with an external electric field as the critical value of the electrohydrodynamic surface instability is approached. The chances of observing these charge-enhanced dimples are also discussed.     

End-tethered chain molecules at liquid interfaces

Polymeric chains attached by one end to an interfce and immersed in a liquid environment provide a powerful tool for the modification of interfacial properties. Recent developments have advanced our understanding and control over interfacial polymer layers at the molecular level, with clear implications to tribological, biosensing and multicomponent systems design considerations.     

Adhesion mechanisms at soft polymer interfaces

Based on several significant examples, we analyse the adhesion mechanisms at soft polymer interfaces with a special emphasis first on the role of connector molecules, that is, polymer chains bound to the interface and which transmit stress through a stretching and extraction mechanism, and second on the necessary relay that must be taken by additional dissipation mechanisms acting at larger scales if one wants to reach typical fracture toughnesses in the range of a few 10 J m(-2). Examples of such bulk dissipation mechanisms will be discussed for interfaces between polymer melts and for pressure-sensitive adhesives in contact with a solid surface. We shall particularly point out the fact that the level of adhesion results from a competition between adhesive failure usually driven by both the interactions and the friction properties of the interface and bulk strong deformations which take place in the bulk of the adhesive layer. Controlling the friction properties of the interface then becomes a tool to finely tune adhesive properties.     

Silicide characterization at alumina–niobium interfaces

Alumina–niobium interfaces formed by liquid-film-assisted joining with copper/niobium/copper interlayers exhibited microstructures that depend on the nature of the alumina components. Characterization of these interfaces in the transmission electron microscope provided insight on the relationship between interfacial microstructure and fracture performance. Interfaces between sapphire and niobium and those between high-purity (99.9%) polycrystalline alumina and niobium were free of secondary phases. However, niobium silicides were found at interfaces between lower-purity (99.5%) alumina and niobium, identified by electron diffraction analysis as the body-centered tetragonal α-Nb₅Si₃ phase. Spatially resolved compositional analysis was conducted on silicide particles at and away from the interface.     

Impurity-induced degradation at zirconia/sapphire interfaces

Sapphire fibre surface damage caused by a polycrystalline zirconia coating has been analysed using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). “Pitting” of the sapphire surface was partially attributed to formation of a transient liquid phase (interphase) capable of local dissolution of alumina at zirconia grain contacts. Chemical etching was used to verify that the interphase material was silicate-based and resided at triple points between zirconia grains and the fibre surface. An additional crystalline calcium hexaluminate phase (hibonite) was found on some fibres. Origin of these impurities and resulting consequences of their presence are rationalized in view of observed surface modification. This revised version was published online in November 2006 with corrections to the Cover Date.