From Tunneling to Hopping: A Comprehensive Investigation of Charge Transport Mechanism in Molecular Junctions Based on Oligo(p-phenylene ethynylene)s

The charge transport mechanism of oligo(p-phenylene ethynylene)s with lengths ranging from 0.98 to 5.11 nm was investigated using modified scanning tunneling microscopy break junction and conducting probe atomic force microscopy methods. The methods were based on observing the length dependence of molecular resistance at single molecule level and the current-voltage characteristics in a wide length distribution. An intrinsic transition from tunneling to hopping charge transport mechanism was observed near 2.75 nm. A new transitional zone was observed in the long length molecular wires compared to short ones. This was not a simple transition between direct tunneling and field emission, which may provide new insights into transport mechanism investigations. Theoretical calculations provided an essential explanation for these phenomena in terms of molecular electronic structures.

Surface-enhanced Raman scattering of 4-aminothiophenol self-assembled monolayers in sandwich structure with nanoparticle shape dependence: Off-surface plasmon resonance condition

A universal metal-molecule-metal sandwich architecture by the self-assembly of Ag nanoparticles (NPs) and Au NPs of various shapes interconnected with 4-aminothiophenol (4-ATP) molecules was presented. These Ag NPs/4-ATP/Au NPs sandwich structures were characterized by surface enhanced Raman scattering (SERS) using an off-surface plasmon resonance condition. Enhancement factors (EF) on the order of 10(8) for 9b(b(2)) vibration mode were observed for the 4-ATP self-assembled monolayers (SAMs) in such sandwich structures. The factors are 2 orders of magnitude larger than that on the monolayer of Au NPs of various shapes under similar condition. More importantly, remarkable increase in the intensity of b(2) vibrational modes, which is characteristic of the charge transfer (CT) behavior between metal NPs and 4-ATP molecules, was observed in these sandwich structures under 1064 nm excitation. The obtained EF on these sandwich structure for 9b(b(2)) is larger than that for 7a vibration mode by a factor of similar to 10(2), demonstrating the importance of the contribution of the CT mechanism and the CT behavior of metal contacts, which play a significant role in metal-molecule-metal nanosystems.

Synthesis, self-assembly in water, and cytotoxicity of MPEG-block-PLLA/DX conjugates

Docetaxel (DX) is one of the most effective antineoplastic drugs. Its current clinical administration is limited because of its hydrophobicity and Serious side effects. A polymer/DX conjugate is designed and successfully prepared to solve these problems. It is monomethoxy-poly(ethylene glycol)-block-poly(L-lactide)/DX (MPEG-PLLA/DX) It was synthesized by reacting DX with carboxyl-terminated copolymer MPEG-PLLA, which was prepared by reacting succinic anhydride with hydroxyl-terminated copolymer monomethoxy-poly(ethylene glycol)-block-poly (L-lactide) (MPEG-PLLA). Its structure and molecular weight was confirmed by H-1 NMR and GPC. The MPEG-PLLA/DX micelles in aqueous solution were prepared Using a SO]vent displacement method and characterized by dynamic light scattering for size and size distribution, and by transmission electron microscopy for surface morphology. Its antitumor activity against HeLa cancer cells evaluated by MTT assay showed that it had a similar antitumor activity to Pure D at the same drug content.

Type I collagen-mediated synthesis and assembly of UV-photoreduced gold nanoparticles and their application in surface-enhanced Raman scattering

We reported a simple method to synthesize gold nanoparticles (NPs) by photoreducing HAuCl4 in acetic acid solution in the presence of type I collagen. It was found that the collagen takes an important role in the formation of gold NPs. The introduction of collagen made the shape of the synthesized gold nanocrystals change from triangular and hexangular gold nanoplates to size-uniform NPs. On the other hand, thanks to the special characters of collagen molecules, such as its linear nanostructure, are positively charged when the pH < 7, and the excellent self-assembly ability, photoreduced gold NPs were assembled onto the collagen chains and formed gold NPs films and networks. A typical probe molecule, 4-aminothiophenol, was used to test the surface-enhanced Raman scattering activity of these gold NPs films and networks and the results indicated good Raman activity on these substrates.

Luminescent supramolecular microstructures containing Ru(bpy)(3)(2+): Solution-based self-assembly preparation and solid-state electrochemiluminescence detection application

In this correspondence, we report on the first preparation of novel, robust Ru(bpy)(3)(2+)-containing supramolecular microstructures via a solution-based self-assembly strategy, carried out by directly mixing H2PtCl6 and Ru(bpy)(3)Cl-2 aqueous solutions at room temperature. It reveals that both the molar ratio and concentration of reactants have a heavy influence on the morphologies of such microstructures. The electrochemical behavior of the Ru(bpy)(3)(2+) components contained in the solid film of the microstructures formed on the electrode surface is also studied and found to exhibit a diffusion-controlled voltammetric feature. Most importantly, such microstructures exhibit excellent electrochemiluminescence (ECL) behaviors and therefore hold great promise as new luminescent materials for solid-state ECL detection in capillary electrophoresis (CE) or CE microchip.

Silver nanoparticles coated with adenine: preparation, self-assembly and application in surface-enhanced Raman scattering

We describe herein the preparation of silver nanoparticles (AgNPs) using nucleobase adenine as protecting agent through the in situ chemical reduction of AgNO3 with NaBH4 in an aqueous medium at room temperature. As-prepared AgNPs were characterized by UV-visible spectra, transmission electron microscopy and x-ray photoelectron spectroscopy. All these data confirmed the formation of AgNPs. On the basis of electrostatic interactions between as-prepared AgNPs and anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS), we successfully fabricated (PSS/AgNP)n (n = 0-9) multilayers on a 3-mercaptopropyltrimethoxysilane/AgNP functionalized indium tin oxide (ITO) substrate via the layer-by-layer self-assembly technique and characterized as-formed multilayers with UV-visible spectra. Furthermore, these ITO substrates coated with multilayers of different thickness were investigated as surface-enhanced Raman scattering (SERS)-active substrates using p-aminothiophenol as a probe molecule, implying that these multilayers substrates may be promising for a new type of SERS-active substrate.

Electrochemiluminescence-based DNA detection using guanine oxidation at electrostatic self-assembly of Ru(bpy)(3)(2+)-doped silica nanoparticles on indium tin oxide electrode

A Ru(bpy)(3)(2+)-doped silica nanoparticle-[Ru@Silica] modified indium tin oxide electrode was prepared by simple electrostatic self-assembly technique, and one-electron catalytic oxidation of guanine bases in double-strand and denatured DNA was realized using the electrochemiluminescence detection means.

Phase Behavior and dewetting for polymer blend films studied by in situ AFM and XPS: From thin to ultrathin films

In this work, the film thickness (l(0)) effect on the phase and dewetting behaviors of the blend film of poly(methyl methacrylate)/poly (styrene-ran-acrylonitrile) (PMMA/SAN) has been studied by in situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The thinner film shows the more compatibility of the blend, and the phase separation of the film occurs at l(0) > 5R(g) (radius of gyration). An initially time-independent q*, the characteristic wavenumber of the phase image, which is in good agreement of Cahn's linearized theory for the early stage of spinodal decomposition, has been obtained in real space and discussed in detail. For 5R(g) > l(0) > 3R(g), a "pseudo-dewetting/(phase separation + wetting)" behavior occurs, where the pseudo-wetting is driven by the concentration fluctuation mechanism. For 10 < 3R(g), a "real dewetting/(phase separation + wetting)" behavior occurs.

Monte Carlo simulation on symmetric ABA/AB copolymer blends in confined thin films

The effects of blend composition on morphology, order-disorder transition (ODT), and chain conformation of symmetric ABA/AB copolymer blends confined between two neutral hard walls have been investigated by lattice Monte Carlo simulation. Only lamellar structure is observed in all the simulation morphologies under thermodynamic equilibrium state, which is supported by theoretical prediction. When the composition of AB diblock copolymer (phi) increases, both lamellar spacing and the corresponding ODT temperature increase, which can be attributed to the variation of conformation distribution of the diblock and the triblock copolymer chains. In addition, both diblock and triblock copolymer, chains with bridge conformation extend dramatically in the direction parallel to the surface when the system is in ordered state. Finally, the copolymer chain conformation depends strongly on both the blend composition and the incompatibility parameter chi N.

Bimolecular quadruplexes and their transitions to higher-order molecular structures detected by ESI-FTICR-MS

Four individual quadruplexes, which are self-assembled in ammonium acetate solution from telomeric sequences of closely related DNA strands - d(G(4)T(4)G(4)), d(G(3)T(4)G(4)), d(G(3)T(4)G(3)), and d(G(4)T(4)G(3)) - have been detected in the gas phase using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). The bimolecular quadruplexes associate with the same number of NH4+ in the gas phase as NMR shows that they do in solution. The quadruplex structures formed in solution are maintained in the gas phase. Furthermore, the mass spectra show that the bimolecular quadruplexes generated by the strands d(G(3)T(4)G(3)) and d(G(4)T(4)G(3)) are unstable, being converted into trimolecular and tetramolecular structures with increasing concentrations of NH4+ in the solution. Circular dichroism (CD) spectra reveal structural changes during the process of strand stoichiometric transitions, in which the relative orientation of strands in the quadruplexes changes from an antiparallel to a parallel arrangement. Such changes were observed for the strand d(G(4)T(4)G(3)), but not for the strand d(G(3)T(4)G(3)). The present work provides a significant insight into the formation of various DNA quadruplexes, especially the higher-order species.

Self-assembly of ionic liquids-stabilized Pt nanoparticles into two-dimensional patterned nanostructures at the air-water interface

In this paper, we demonstrate the self-assembly of ionic liquids (ILs)-stabilized Pt nanoparticles into two-dimensional (2D) patterned nanostructures at the air-water interface under ambient conditions. Here, ILs are not used as solvents but as mediators by virtue of their pronounced self-organization ability in synthesis of self-assembled, highly organized hybrid Pt nanostructures. It is also found that the morphologies of the 2D patterned nanostructures are directly connected with the quantities of ILs. Due to the special structures of ILs-stabilized Pt nanoparticles, 2D patterned Pt nanostructures can be formed through the pi-pi stack interactions and hydrogen bonds. The resulting 2D patterned Pt nanostructures exhibit good electrocatalytic activity toward oxygen reduction.

Thermal annealing of Au nanorod self-assembled nanostructured materials: Morphology and optical properties

Three-dimensional Au nanorod and An nanoparticle nanostructured materials were prepared by layer-by-layer self-assembly. The plasmonic properties of the An nanorod and An nanoparticle self-assembled nanostructured materials (abbreviated as AuNR and AuNP SANMs) are tunable by the controlled self-assenibly process. The effect of thermal annealing at 180 and 500 degrees C to the morphologies, plasmonic properties and surface-enhanced Raman scattering (SERS) responses of these SANMs were investigated. According to the experimental results, these properties correlate with the structure of the SANMs.

Self-assembled silver nanoparticle monolayer on glassy carbon: an approach to SERS substrate

In this paper, the fabrication of an active surf ace-enhanced Raman scattering (SERS) substrate by self-assembled silver nanoparticles on a monolayer of 4-aminophenyl-group-modified glassy carbon (GC) is reported. Silver nanoparticles are attached to the substrate through the electrostatic force between the negatively charged silver nanoparticles and the positively charged 4-aminophenyl groups on GC. The active SERS substrate has been characterized by means of tapping-mode atomic force microscopy (AFM), indicating that large quantities of silver nanoparticles are uniformly coated on the substrate. Rhodamine 6G (R6G) and p-aminothiophenol (p-ATP) are used as the probe molecules for SERS, resulting in high sensitivity to the SERS response, with the detection limit reaching as low as 10(-9) m. This approach is easily controlled and reproducible, and more importantly, can extend the range of usable substrates to carbon-based materials for SERS with high sensitivity.

Weak epitaxy growth affording high-mobility thin films of disk-like organic semiconductors

Thin films of phthalocyanine compounds show weak epitaxial growth on a monodomain film of a rod-like molecule (see figure). The resulting organic electronic devices exhibit high charge carrier mobilities close to those of the single-crystal devices.

Self-assembly of diblock copolymer mixtures in confined states: A Monte Carlo study

The self-assembly of diblock copolymer mixtures (A-b-B/A-b-C or A-b-B/B-b-C mixtures) subjected to cylindrical confinement (two-dimensional confinement) was investigated using a Monte Carlo method. In this study, the boundary surfaces were configured to attract blocks A but repel blocks B and C. Relative to the structures of the individual components, the self-assembled structures of mixtures of the diblock copolymers were more complex and interesting. Under cylindrical confinement, with varying cylinder diameters and interaction energies between the boundary surfaces and the blocks, we observed a variety of interesting morphologies. Upon decreasing the cylinder's diameter, the self-assembled structures of the A(15)B(15)/A(15)C(15) mixtures changed from double-helix/cylinder structures (blocks B and C formed double helices, whereas blocks A formed the outer barrel and inner core) to stacked disk/cylinder structures (blocks B and C formed the stacked disk core, blocks A formed the outer cylindrical barrel), whereas the self-assembled structures of the A(15)B(7)/B7C15 mixtures changed from concentric cylindrical barrel structures to screw/cylinder structures (blocks C formed an inside core winding with helical stripes, whereas blocks A and B formed the outer cylindrical barrels) and then finally to the stacked disk/cylinder structures.