How Membrane Geometry Regulates Protein Sorting Independently of Mean Curvature.

Biological membranes have distinct geometries that confer specific functions. However, the molecular mechanisms underlying the phenomenological geometry/function correlations remain elusive. We studied the effect of membrane geometry on the localization of membrane-bound proteins.

Membrane curvature enables N-Ras lipid anchor sorting to liquid-ordered membrane phases.

Trafficking and sorting of membrane-anchored Ras GTPases are regulated by partitioning between distinct membrane domains. Here, in vitro experiments and microscopic molecular theory reveal membrane curvature as a new modulator of N-Ras lipid anchor and palmitoyl chain partitioning. Membrane curvature was essential for enrichment in raft-like liquid-ordered phases; enrichment was driven by relief of lateral pressure upon anchor insertion and most likely affects the localization of lipidated proteins in general.

Triazatriangulene as binding group for molecular electronics.

The triazatriangulene (TATA) ring system was investigated as a binding group for tunnel junctions of molecular wires on gold surfaces. Self-assembled monolayers (SAMs) of TATA platforms with three different lengths of phenylene wires were fabricated, and their electrical conductance was recorded by both conducting probe-atomic force microscopy (CP-AFM) and scanning tunneling microscopy (STM). Similar measurements were performed for phenylene SAMs with thiol anchoring groups as references.

Wet chemical synthesis of soluble gold nanogaps.

A central challenge in molecular electronics is to create electrode pairs separated by only a few nanometers that can accommodate a single molecule of interest to be optically or electrically characterized while residing in the gap. Current techniques for nanogap fabrication are largely based on top-down approaches and often rely on subsequent deposition of molecules into the nanogap. In such an approach, the molecule may bridge the gap differently with each experiment due to variations at the metal-molecule interface.

Deterministic assembly of linear gold nanorod chains as a platform for nanoscale applications.

We demonstrate a method to assemble gold nanorods highly deterministically into a chain formation by means of directed capillary assembly. This way we achieved straight chains consisting of end-to-end aligned gold nanorods assembled in one specific direction with well-controlled gaps of ∼6 nm between the individual constituents. We determined the conditions for optimum quality and yield of nanorod chain assembly by investigating the influence of template dimensions and assembly temperature.

Ultrathin reduced graphene oxide films as transparent top-contacts for light switchable solid-state molecular junctions.

A new type of solid-state molecular junction is introduced, which employs reduced graphene oxide as a transparent top contact that permits a self-assembled molecular monolayer to be photoswitched in situ, while simultaneously enabling charge-transport measurements across the molecules. The electrical switching behavior of a less-studied molecular switch, dihydroazulene/vinylheptafulvene, is described, which is used as a test case.

Quantum interference effects at room temperature in OPV-based single-molecule junctions.

Interference effects on charge transport through an individual molecule can lead to a notable modulation and suppression on its conductance. In this letter, we report the observation of quantum interference effects occurring at room temperature in single-molecule junctions based on oligo(3)-phenylenevinylene (OPV3) derivatives, in which the central benzene ring is coupled to either para- or meta-positions. Using the break-junction technique, we find that the conductance for a single meta-OPV3 molecule wired between gold electrodes is one order of magnitude smaller than that of a para-OPV3 molecule.

Bonding and electronic transport properties of fullerene and fullerene derivatives in break-junction geometries.

Fullerenes are considered anchoring groups for molecular electronics due to a large contact area and their affinity for noble metals. The conductances of fullerene-terminated molecules, however, are found to be even lower than for thiol termination. The effects of weak molecule-metal coupling and symmetry breaking are studied by transport measurements of C(60) and functionalized C(60).

Electrical annealing and temperature dependent transversal conduction in multilayer reduced graphene oxide films for solid-state molecular devices.

The transversal conductance through thin multi-layered films of reduced graphene oxide was studied as a function of temperature in a solid-state device setup designed for molecular electronic measurements. Upon cooling to cryogenic temperatures, the resistivity of the films increased by about three orders of magnitude compared to the value at room temperature, and this temperature dependence was described by a variable range hopping model. Above a certain threshold voltage the films could be annealed electrically at low temperatures.

A triptycene-based approach to solubilising carbon nanotubes and C60.

We describe herein the synthesis of a triptycene-based surfactant designed with the ability to solubilise single-walled carbon nanotubes (SWNTs) and C(60) in water through non-covalent interactions. Furthermore, an amphiphilic naphthalene-based surfactant with the same ability to solubilise SWNTs and C(60) has also been prepared. The compounds synthesised were designed with either two ionic or non-ionic tails to ensure a large number of supramolecular interactions with the solvent, thereby promoting strong solubilisation.

Aligned growth of gold nanorods in PMMA channels: parallel preparation of nanogaps.

We demonstrate alignment and positional control of gold nanorods grown in situ on substrates using a seed-mediated synthetic approach. Alignment control is obtained by directing the growth of spherical nanoparticle seeds into nanorods in well-defined poly(methyl methacrylate) nanochannels. Substrates with prepatterned metallic electrodes provide an additional handle for the position of the gold nanorods and yield nanometer-sized gaps between the electrode and nanorod.

End-to-end assembly of gold nanorods via oligopeptide linking and surfactant control.

We report two novel approaches for fabricating self-assembled chains of end-to-end linked Au nanorods separated by a nanogap. In one approach, bi-functional cysteine end-capped oligopeptides of different lengths are used as the linking agent. The widths of the produced nanogaps scale with the length and tertiary structure of the peptide linker.

Solution-processed ultrathin chemically derived graphene films as soft top contacts for solid-state molecular electronic junctions.

A novel method using solution-processed ultrathin chemically derived graphene films as soft top contacts for the non-destructive fabrication of molecular junctions is demonstrated. We believe this protocol will greatly enrich the solid-state test beds for molecular electronics due to its low-cost, easy-processing and flexible nature.

Molecular junctions based on SAMs of cruciform oligo(phenylene ethynylene)s.

Cruciform oligo(phenylene ethynylene)s (OPEs) with an extended tetrathiafulvalene (TTF) donor moiety (OPE5-TTF and OPE3-TTF) and their simple analogues (OPE5-S and OPE3) without conjugated substituents were used to form high-quality self-assembled monolayers (SAMs) on ultraflat gold substrates. Molecular junctions based on these SAMs were investigated using conducting-probe atomic force microscopy (CP-AFM). The TTF substituent changes the molecular orbital energy levels and decreases the HOMO-LUMO energy gap, resulting in a 9-fold increase in conductance for both TTF cruciform OPEs compared to the unsubstituted analogues.

Influence of the preparation route on the supramolecular organization of lipids in a vesicular system.

A confocal fluorescence microscopy-based assay was used for studying the influence of the preparation route on the supramolecular organization of lipids in a vesicular system. In this work, vesicles composed of cholesterol and CTAB (1/1 mol %) or cholesterol and DOPC (2/8 mol %) and incorporating two membrane dyes were prepared by either a compressed fluid (CF)-based method (DELOS-susp) or a conventional film hydration procedure. They were subsequently immobilized and imaged individually using a confocal fluorescence microscope.

Aligning nanodiscs at the air-water interface, a neutron reflectivity study.

Nanodiscs are self-assembled nanostructures composed of a belt protein and a small patch of lipid bilayer, which can solubilize membrane proteins in a lipid bilayer environment. We present a method for the alignment of a well-defined two-dimensional layer of nanodiscs at the air-water interface by careful design of an insoluble surfactant monolayer at the surface. We used neutron reflectivity to demonstrate the feasibility of this approach and to elucidate the structure of the nanodisc layer.

Savinase action on bovine serum albumin (BSA) monolayers demonstrated with measurements at the air-water interface and liquid Atomic Force Microscopy (AFM) imaging.

We studied the enzymatic action of Savinase on bovine serum albumin (BSA) organized in a monolayer spread at the air/water interface or adsorbed at the mica surface. We carried out two types of experiments. In the first one we followed the degradation of the protein monolayer by measuring the surface pressure and surface area decrease versus time.

Voltammetry and in situ scanning tunnelling spectroscopy of osmium, iron, and ruthenium complexes of 2,2':6',2''-terpyridine covalently linked to Au(111)-electrodes.

We have studied self-assembled molecular monolayers (SAMs) of complexes between Os(II)/(III), Fe(II)/(III), and Ru(II)/(III) and a 2,2',6',2''-terpyridine (terpy) derivative linked to Au(111)-electrode surfaces via a 6-acetylthiohexyloxy substituent at the 4'-position of terpy. The complexes were prepared in situ by first linking the terpy ligand to the surface via the S-atom, followed by addition of suitable metal compounds. The metal-terpy SAMs were studied by cyclic voltammetry (CV), and in situ scanning tunnelling microscopy with full electrochemical potential control of substrate and tip (in situ STM).

A statistical approach to inelastic electron tunneling spectroscopy on fullerene-terminated molecules.

We report on the vibrational fingerprint of single C(60) terminated molecules in a mechanically controlled break junction (MCBJ) setup using a novel statistical approach manipulating the junction mechanically to address different molecular configurations and to monitor the corresponding vibrational modes. In the IETS spectra, the vibrations of the anchoring C(60) dominate the spectra; thus information on the unit anchored with C(60) to the electrodes is masked by the modes arising from the anchoring groups. However, we have identified the additional modes from the fluorene backbone optically.

Monitoring the aggregation of single casein micelles using fluorescence microscopy.

The aggregation of casein micelles (CMs) induced by milk-clotting enzymes is a process of fundamental importance in the dairy industry for cheese production; however, it is not well characterized on the nanoscale. Here we enabled the monitoring of the kinetics of aggregation between single CMs (30-600 nm in diameter) by immobilizing them on a glass substrate at low densities and subsequently imaging them with fluorescence microscopy. We validated the new method by a quantitative comparison to ensemble measurements of aggregation.

Free-standing biomimetic polymer membrane imaged with atomic force microscopy.

Fluid polymeric biomimetic membranes are probed with atomic force microscopy (AFM) using probes with both normal tetrahedrally shaped tips and nanoneedle-shaped Ag(2)Ga rods. When using nanoneedle probes, the collected force volume data show three distinct membrane regions which match the expected membrane structure when spanning an aperture in a hydrophobic scaffold. The method used provides a general method for mapping attractive fluid surfaces.

From nanofabrication to self-fabrication--tailored chemistry for control of single molecule electronic devices.

Single molecule electronics is a field of research focused on the use of single molecules as electronics components. During the past 15 years the field has concentrated on development of test beds for measurements on single molecules. Bottom-up approaches to single molecule devices are emerging as alternatives to the dominant top-down nanofabrication techniques.

Fulleropyrrolidine end-capped molecular wires for molecular electronics--synthesis, spectroscopic, electrochemical, and theoretical characterization.

In continuation of previous studies showing promising metal-molecule contact properties a variety of C(60) end-capped "molecular wires" for molecular electronics were prepared by variants of the Prato 1,3-dipolar cycloaddition reaction. Either benzene or fluorene was chosen as the central wire, and synthetic protocols for derivatives terminated with one or two fullero[c]pyrrolidine "electrode anchoring" groups were developed. An aryl-substituted aziridine could in some cases be employed directly as the azomethine ylide precursor for the Prato reaction without the need of having an electron-withdrawing ester group present.