Stochastic on-off conductivity switching observed in phenylene-ethynylene oligomers has been explained in terms of changes in ring conformations, or electron localization, or both. We report the ...observation of stochastic on-off switching in the simplest of wired molecules: octanedithiol, decanedithiol, and dodecanedithiol bonded on an Au(111) surface. Stochastic switching was observed even when a top gold contact was pressed on by a conducting atomic force microscope tip at constant force. The rate of switching increased substantially at 60°C, a temperature at which these films are commonly annealed. Because such switching in alkanethiols is unlikely to be caused by internal molecular electronic changes and cannot be fully accounted for by breaking of the top contact, we argue that the cause is the well-known mobility of molecules tethered to gold via a thiol linkage.
We have studied electron transport properties of unsubstituted oligo(phenylene ethynylene) (OPE) (1) and nitro-substituted OPE (2) covalently bound to two gold electrodes. The conductance values of ...single 1 and 2 are ∼13 and ∼6 nS, respectively. In addition to a decrease in the conductance, the presence of the nitro moiety leads to asymmetric I−V characteristics and a negative differential resistance-like (NDR-like) behavior. We have altered the nitro-substituted OPE by electrochemically reducing the nitro group and by varying the pH of the electrolyte. The conductance decreases linearly with the electron-withdrawing capability (i.e., Hammett substituent values) of the corresponding reduced species. In contrast, the conductance of 1 is independent of the pH and the electrode potential.
The electrical properties of self-assembled monolayers (SAMs) on metal surfaces have been explored for a series of molecules to address the relation between the behavior of a molecule and its ...structure. We probed interfacial electron transfer processes, particularly those involving unoccupied states, of SAMs of thiolates or arylates on Au by using shear force-based scanning probe microscopy (SPM) combined with current−voltage (i − V) and current−distance (i − d) measurements. The i − V curves of hexadecanethiol in the low bias regime were symmetric around 0 V and the current increased exponentially with V at high bias voltage. Different than hexadecanethiol, reversible peak-shaped i − V characteristics were obtained for most of the nitro-based oligo(phenylene ethynylene) SAMs studied here, indicating that part of the conduction mechanism of these junctions involved resonance tunneling. These reversible peaked i − V curves, often described as a negative differential resistance (NDR) effect of the junction, can be used to define a threshold tip bias, V TH, for resonant conduction. We also found that for all of the SAMs studied here, the current decreased with increasing distance, d, between tip and substrate. The attenuation factor β of hexadecanethiol was high, ranging from 1.3 to 1.4 Å-1, and was nearly independent of the tip bias. The β-values for nitro-based molecules were low and depended strongly on the tip bias, ranging from 0.15 Å-1 for tetranitro oligo(phenylene ethynylene) thiol, VII, to 0.50 Å-1 for dinitro oligo(phenylene) thiol, VI, at a −3.0 V tip bias. Both the V TH and β values of these nitro-based SAMs were also strongly dependent on the structures of the molecules, e.g. the number of electroactive substituent groups on the central benzene, the molecular wire backbone, the anchoring linkage, and the headgroup. We also observed charge storage on nitro-based molecules. For a SAM of the dintro compound, V, ∼25% of charge collected in the negative scan is stored in the molecules and can be collected at positive voltages. A possible mechanism involving lateral electron hopping is proposed to explain this phenomenon.
The surface modification of synthetic tissue engineering scaffolds is essential to improving their hydrophilicity and cellular compatibility. Plasma treatment is an effective way to increase the ...hydrophilicity of a surface, but the incorporation of biomolecules is also important to control cellular adhesion and differentiation, among many other outcomes. In this work, oriented polycaprolactone (PCL) electrospun fibers were modified by air-plasma treatment, followed by the covalent attachment of laminin. The amount of protein incorporated onto the fiber surface was controlled by varying the reaction time and the protein solution concentration. The protein concentration and coverage were quantified using X-ray photoelectron spectroscopy (XPS), solid-state ultraviolet–visible spectroscopy (UV–vis) and two fluorescence-based assays. XPS results showed a nearly linear increase in protein coverage with increasing protein soaking solution concentration until a monolayer was formed. Results from XPS and the NanoOrange fluorescence assay revealed multilayer protein coverage at protein solution concentrations between 25 and 50 μg/mL, whereas the UV–vis assay demonstrated multilayer coverage at lower protein solution concentrations. The effect of protein concentration on the neurite outgrowth of neuron-like PC12 cells was evaluated, and outgrowth rates were found to be positively correlated to increasing protein concentration.
Poly(methyl methacrylate) (PMMA)–polyacrylonitrile (PAN) fibers were prepared using a conventional single-nozzle electrospinning technique. The as-spun fibers exhibited core–shell morphology as ...verified by transmission electron microscopy (TEM) and atomic force microscopy (AFM). AFM-phase and modulus mapping images of the fiber cross-section and X-ray photoelectron spectroscopy (XPS) analysis indicated that PAN formed the shell and PMMA formed the core material. XPS, thermogravimetric analysis (TGA), and elemental analysis were used to determine fiber compositional information. Soaking the fibers in solvent demonstrated removal of the core material, generating hollow PAN fibers.
The impact of mat porosity of polycaprolactone (PCL) electrospun fibers on the infiltration of neuron-like PC12 cells was evaluated using two different approaches. In the first method, bi-component ...aligned fiber mats were fabricated via the co-electrospinning of PCL with polyethylene oxide (PEO). Variation of the PEO flow rate, followed by selective removal of PEO from the PCL/PEO mesh, allowed for control of the porosity of the resulting scaffold. In the second method, aligned fiber mats were fabricated from various concentrations of PCL solutions to generate fibers with diameters between 0.13 ± 0.06 and 9.10 ± 4.1 μm. Of the approaches examined, the variation of PCL fiber diameter was found to be the better method for increasing the infiltration of PC12 cells, with the optimal infiltration into the ca. 1.5-mm-thick meshes observed for the mats with the largest fiber diameters, and hence largest pore sizes.
Presented here are several convergent synthetic routes to conjugated oligo(phenylene ethynylene)s. Some of these oligomers are free of functional groups, while others possess donor groups, acceptor ...groups, porphyrin interiors, and other heterocyclic interiors for various potential transmission and digital device applications. The syntheses of oligo(phenylene ethynylene)s with a variety of end groups for attachment to numerous metal probes and surfaces are presented. Some of the functionalized molecular systems showed linear, wirelike, current versus voltage (I(V)) responses, while others exhibited nonlinear I(V) curves for negative differential resistance (NDR) and molecular random access memory effects. Finally, the syntheses of functionalized oligomers are described that can form self‐assembled monolayers on metallic electrodes that reduce the Schottky barriers. Information from the Schottky barrier studies can provide useful insight into molecular alligator clip optimizations for molecular electronics.
Functionalized conjugated oligo(phenylene ethylene)s with donor groups, acceptor groups, porphyrin interiors, and heterocycle interiors (see scheme) have been synthesized by several convergent synthetic routes. These compounds have potential for wire and digital device applications: some showed linear, wirelike, current versus voltage (I(V)) responses, while others exhibited nonlinear I(V) curves for negative differential resistance (NDR) and molecular random access memory effects.
Biomaterial bridges constructed from electrospun fibers offer a promising alternative to traditional nerve tissue regeneration substrates. Aligned and unaligned polycaprolactone (PCL) electrospun ...fibers were prepared and functionalized with the extracellular matrix proteins collagen and laminin using covalent and physical adsorption attachment chemistries. The effect of the protein modified and native PCL nanofiber scaffolds on cell proliferation, neurite outgrowth rate, and orientation was examined with neuronlike PC12 cells. All protein modified scaffolds showed enhanced cellular adhesion and neurite outgrowth compared to unmodified PCL scaffolds. Neurite orientation was found to be in near perfect alignment with the fiber axis for cells grown on aligned fibers, with difference angles of less than 7° from the fiber axis, regardless of the surface chemistry. The bioavailability of PCL fibers with covalently attached laminin was found to be identical to that of PCL fibers with physically adsorbed laminin, indicating that the covalent chemistry did not change the protein conformation into a less active form and the covalent attachment of protein is a suitable method for enhancing the biocompatibility of tissue engineering scaffolds.
Phthalocyanines, naphthalocyanins, and their derivatives are frequently used as light modulating materials. These compounds, with their stable planar square structure and highly delocalized ...π-electron system, are being used in numerous technological applications, such as pigments in chemical sensors, and more recently as photosensitizers for photodynamic therapy. The nonlinear optical properties (NLO) of these compounds are of particular importance. Using density functional method (DFT), we calculated the optical properties of phthalocyanine and naphthalocyanine complexes with Si as a central atom. We examined the effect of hydrophilic axial substituents and the size of polycyclic aromatic hydrocarbons surrounding the porphyrazine-Si kernel on the optical properties of title molecules. Both UV−vis and RSA spectra are calculated and are compared with available experimental results. The time-dependent DFT (TDDFT) with the B3LYP functional predicts that the characteristic UV−vis absorption maxima are blue-shifted; however, the relative error is almost constant for phthalocyanine and naphthalocyanine compounds. The TDDFT triplet−triplet absorption spectra of Si-phthalocyanine and Si-naphthalocyanine complexes reproduce experimental data well.
Poly(methyl methacrylate) (PMMA) was electrospun in the presence of a low molecular weight, hyperbranched poly(ethylene imine) additive partially functionalized with perfluorinated and aliphatic ...end-groups (M n ∼ 1600 g/mol). The additive exhibited surface segregation with an insignificant influence on the rheological behavior of PMMA solutions. A morphological transition from beaded electrospun fibers to uniform fibers was observed upon introduction of additive at low PMMA concentrations. XPS revealed a surface enrichment of fluorine and nitrogen, which are both present in the hyperbranched additive. Surface fluorine content depended primarily on the amount of additive in solution, and a dependency on the PMMA/additive weight ratio was not observed.
PDF
