Controlling adhesion force by means of nanoscale surface roughness

Control of adhesion is a crucial aspect in the design of microelectromechanical and nanoelectromechanical devices. To understand the dependence of adhesion on nanometer-scale surface roughness, a roughness gradient has been employed. Monomodal roughness gradients were fabricated by means of silica nanoparticles (diameter ～12 nm) to produce substrates with varying nanoparticle density. Pull-off force measurements on the gradients were performed using (polyethylene) colloidal-probe microscopy under perfluorodecalin, in order to restrict interactions to van der Waals forces. The influence of normal load on pull-off forces was studied and the measured forces compared with existing Hamaker-approximation-based models. We observe that adhesion force reaches a minimum value at an optimum particle density on the gradient sample, where the mean particle spacing becomes comparable with the diameter of the contact area with the polyethylene sphere. We also observe that the effect on adhesion of increasing the normal load depends on the roughness of the surface.     

Versatile wettability gradients prepared by chemical modification of polymer brushes on polymer foils

A method to create a wettability gradient by variation of the chemical functionality in a polymer brush is presented. A poly(N-methyl-vinylpyridinium) (QP4VP) brush was created on a poly(ethylene-alt-tetrafluoroethylene) (ETFE) foil by the grafting of 4-vinylpyridine and subsequent quaternization. The instability of QP4VP, a strong polyelectrolyte, in alkaline media was exploited to transform it to the neutral poly(vinyl(N-methyl-2-pyridone)) (PVMP), as confirmed with ATR-IR spectroscopy. The slow transformation resulted in a substantial, time-dependent decrease in wettability. A nearly linear gradient in water contact angle (CA) was created by immersion of a QP4VP brush modified sample into a sodium hydroxide solution, resulting in CAs ranging from 10° to 60°. The concurrent decrease in the number of charged functional groups along the gradient was characterized by loading an anionic dye into the polymer brush and measuring the UV transmittance of the sample. The versatility of the wettability gradient was demonstrated by exchanging the counterions of the N-methyl-vinylpyridinium groups, whereby a reversal of gradient direction was reproducibly achieved.     

Enhanced chaos synchronization in unidirectionally coupled vertical-cavity surface-emitting semiconductor lasers with polarization-preserved injection

We experimentally study chaos synchronization in unidirectionally coupled vertical-cavity surface-emitting semiconductor lasers (VCSELs) with polarization-preserved and polarization-selected optical injection. The measurements show, in agreement with theoretical predictions, that the maximum cross coefficient of 0.884 obtained with polarization-preserved optical injection is significantly higher than the maximum cross coefficient of 0.724 obtained with polarization-selected optical injection.     

Evidence that protons can be the active catalysts in Lewis acid mediated hetero-Michael addition reactions

The mechanism of Lewis acid catalysed hetero‐Michael addition reactions of weakly basic nucleophiles to α,β‐unsaturated ketones was investigated. Protons, rather than metal ions, were identified as the active catalysts. Other mechanisms have been ruled out by analyses of side products and of stoichiometric enone–catalyst mixtures and by the use of radical inhibitors. No evidence for the involvement of π‐olefin–metal complexes or for carbonyl–metal‐ion interactions was obtained. The reactions did not proceed in the presence of the non‐coordinating base 2,6‐di‐tert‐butylpyridine. An excellent correlation of catalytic activities with cation hydrolysis constants was obtained. Different reactivities of mono‐ and dicarbonyl substrates have been rationalised. A ¹H NMR probe for the assessment of proton generation was established and Lewis acids have been classified according to their propensity to hydrolyse in organic solvents. Brønsted acid‐catalysed conjugate addition reactions of nitrogen, oxygen, sulfur and carbon nucleophiles are developed and implications for asymmetric Lewis acid catalysis are discussed.     

2D Crystal Engineering of Nanosheets Assembled from Helical Peptide Building Blocks

The successful integration of 2D nanomaterials into functional devices hinges on developing fabrication methods that afford hierarchical control across length scales of the entire assembly. We demonstrate structural control over a class of crystalline 2D nanosheets assembled from collagen triple helices. By lengthening the triple helix unit through sequential additions of Pro‐Hyp‐Gly triads, we achieved sub‐angstrom tuning over the 2D lattice. These subtle changes influence the overall nanosheet size, which can be adjusted across the mesoscale size regime. The internal structure was observed by cryo‐TEM with direct electron detection, which provides real‐space high‐resolution images, in which individual triple helices comprising the lattice can be clearly discerned. These results establish a general strategy for tuning the structural hierarchy of 2D nanomaterials that employ rigid, cylindrical structural units.     

De novo design of a stable N-terminal helical foldamer

A peptide NTH-18 was synthesized in which a N-terminal helix is stabilised by two crossed disulfide bonds to a C-terminal extension. The design was inspired by the structure of the neurotoxic peptide apamin, which has previously been used to stabilise helices in miniature enzymes. CD- and NMR-spectroscopy indicated that NTH-18 adopted a fold similar to that found in apamin. However, the arrangement of the elements of secondary structures was inverted relative to apamin; a N-terminal alpha-helix was connected by a reverse turn to a C-terminal extension of non-canonical secondary structure. NTH-18 displayed significant stability to heat and changes of pH. The high definition of the N-terminal end of the alpha-helix of NTH-18 should make this peptide a useful vehicle to stabilise alpha-helices in proteins with applications in protein engineering and molecular recognition.     

Gradients of topographical structure in thin polymer films

We report the fabrication of centimeter-long gradients of topography. Structured polymer thin films were made by spin-coating an immiscible poly(methyl methacrylate)/poly(2-vinylpyridine) (PMMA/P2VP) blend from a common solvent on a substrate presenting a surface-energy gradient. Due to the interplay between polymer-wetting of the substrate and polymer phase-separation, different morphologies were observed along the sample. Atomic force microscopy (AFM) revealed a gradual transition from a flat to a structured morphology as the surface energy of the substrate was reduced. The transition occurred for a well-defined range of surface energies. A selective solvent was used to remove one of the polymer phases, increasing the topographical contrast and providing insight into the different stages of the transition. Image analysis allowed a quantitative characterization of the structure gradient.     

Convenient preparation of trans-arylalkenes via palladium(II)-catalyzed isomerization of cis-arylalkenes

A convenient method for the isomerization of cis-arylalkenes to their trans isomers using a palladium(II) catalyst is described. The reaction conditions are mild and general across a range of arylalkenes. The synthesis of a trans-resveratrol derivative from a mixture of alkene isomers was also completed.