Resonant photoconductance of molecular junctions formed in gold nanoparticle arrays

We investigate the photoconductance properties of oligo(phenylene vinylene) (OPV) molecules in metal-molecule-metal junctions. The molecules are electrically contacted in a two-dimensional array of gold nanoparticles. The nanoparticles in such an array are separated by only few nanometers. This allows to bridge the distance between the nanoparticles with molecules considered as molecular wires such as OPV. We report on the photoconductance of electrically contacted OPV upon resonant optical excitation of the molecules. This resonant photoconductance is sublinear in laser intensity, which suggests that trap state dynamics of the optically excited charge carriers dominate the optoelectronic response.     

Binding at molecule/gold transport interfaces. V. Comparison of different metals and molecular bridges

The geometric and electronic structural properties of symmetric and asymmetric metal cluster-molecule-cluster' complexes have been explored. The metals include Au, Ag, Pd, and Al, and both benzenedithiol and the three isometric forms of dicyanobenzene are included as bridging molecules. Calculated properties such as cluster-molecule interface geometry, electronic state, degree of metal --> molecule charge transfer, metal-molecule mixing in the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy region, the HOMO-LUMO gap, cluster --> cluster' charge transfer as a function of external field strength and direction, and the form of the potential profile across such complexes have been examined. Attempts are made to correlate charge transport with the characteristics of the cluster-complex systems. Indications of rectification in complexes that are asymmetric in the molecule, clusters, and molecule-cluster interfaces are discussed. The results obtained here are only suggestive because of the limitations of the cluster-complex model as it relates to charge transport.     

Fabrication and characterization of metal-molecule-metal junctions by conducting probe atomic force microscopy

Metal-molecule-metal junctions were fabricated by contacting Au-supported alkyl or benzyl thiol self-assembled monolayers (SAMs) with an Au-coated atomic force microscope (AFM) tip. The tip-SAM microcontact is approximately 15 nm(2), meaning the junction contains approximately 75 molecules. Current-voltage (I-V) characteristics of these junctions were probed as a function of SAM thickness and load applied to the microcontact. The measurements showed: (1) the I-V traces were linear over +/-0.3 V, (2) the junction resistance increased exponentially with alkyl chain length, (3) the junction resistance decreased with increasing load and showed two distinct power law scaling regimes, (4) resistances were a factor of 10 lower for junctions based on benzyl thiol SAMs compared to hexyl thiol SAMs having the same thickness, and (5) the junctions sustained fields up to 2 x 10(7) V/cm before breakdown. I-V characteristics determined for bilayer junctions involving alkane thiol-coated tips in contact with alkane thiol SAMs on Au also showed linear I-Vs over +/-0.3 V and the same exponential dependence on thickness. The I-V behavior and the exponential dependence of resistance on alkyl chain length are consistent with coherent, nonresonant electron tunneling across the SAM. The calculated conductance decay constant (beta) is 1.2 per methylene unit ( approximately 1.1 A(-)(1)) for both monolayer and bilayer junctions, in keeping with previous scanning tunneling microscope and electrochemical measurements of electron transfer through SAMs. These measurements show that conducting probe-AFM is a reliable method for fundamental studies of electron transfer through small numbers of molecules. The ability to vary the load on the microcontact is a unique characteristic of these junctions and opens opportunities for exploring electron transfer as a function of molecular deformation.     

Strong effects of molecular structure on electron transport in carbon/molecule/copper electronic junctions

Carbon/molecule/copper molecular electronic junctions were fabricated by metal deposition of copper onto films of various thicknesses of fluorene (FL), biphenyl (BP), and nitrobiphenyl (NBP) covalently bonded to flat, graphitic carbon. A "crossed-wire" junction configuration provided high device yield and good junction reproducibility. Current/voltage characteristics were investigated for 69 junctions with various molecular structures and thicknesses and at several temperatures. The current/voltage curves for all cases studied were nearly symmetric, scan rate independent, repeatable at least thousands of cycles and exhibited negligible hysteresis. Junction conductance was strongly dependent on the dihedral angle between phenyl rings and on the nature of the molecule/copper "contact". Junctions made with NBP showed a decrease in conductivity of a factor of 1300 when the molecular layer thickness increased from 1.6 to 4.5 nm. The slope of ln(i) vs layer thickness for both BP and NBP was weakly dependent on applied voltage and ranged from 0.16 to 0.24 A(-1). These attenuation factors are similar to those observed for similar molecular layers on modified electrodes used to study electrochemical kinetics. All junctions studied showed weak temperature dependence in the range of approximately 325 to 214 K, implying activation barriers in the range of 0.06 to 0.15 eV. The carbon/molecule/copper junction structure provides a robust, reproducible platform for investigations of the dependence of electron transport in molecular junctions on both molecular structure and temperature. Furthermore, the results indicate that junction conductance is a strong function of molecular structure, rather than some artifact resulting from junction fabrication.     

Redox-driven conductance switching via filament formation and dissolution in carbon/molecule/TiO2/Ag molecular electronic junctions

Carbon/molecule/TiO2/Au molecular electronic junctions show robust conductance switching, in which a metastable high conductance state may be induced by a voltage pulse which results in redox reactions in the molecular and TiO2 layers. When Ag is substituted for Au as the "top contact", dramatically different current/voltage curves and switching behavior result. When the carbon substrate is biased negative, an apparent breakdown occurs, leading to a high conductance state which is stable for at least several hours. Upon scanning to positive bias, the conductance returns to a low state, and the cycle may be repeated hundreds of times. Similar effects are observed when Cu is substituted for Au and for three different molecular layers as well as "control" junctions of the type carbon/TiO2/Ag/Au. The polarity of the "switching" is reversed when the Ag layer is between the carbon and molecular layers, and the conductance change is suppressed at low temperature. Pulse experiments show very erratic transitions between high and low conductivity states, particularly near the switching threshold. The results are consistent with a switching mechanism based on Ag or Cu oxidation, transport of their ions through the TiO2, and reduction at the carbon to form a metal filament.     

"Contact voltage" in nanoparticle/molecule connections

This work presents conclusive evidence that connecting Pt and Co nanoparticles stabilized by an aluminum-organic shell with molecular spacers interacting with this shell can induce notable changes in the electronic structure of the metal. X-ray absorption spectroscopy measurements at the Al K-, the Pt L(III)-, and the Co K-edge provide consistent evidence for this effect. The changes induced by cross-linking with an acidic spacer are discussed in detail as an example to elucidate the mechanism of this effect. It turns out that a reconfiguration of the protection shell that occurs upon networking is responsible for the observed changes.     

Simultaneous detection of intracellular tumor mRNA with bi-color imaging based on a gold nanoparticle/molecular beacon

The successful treatment of most cancers depends on early detection. Tumor mRNA as a specific marker provides new avenues to monitor tumor progression in the early stages and assesses response to treatment. However, single tumor mRNA testing usually yields "false positive" results because cancer is associated with multiple tumor mRNA. It is indispensable to develop simple and effective approaches for the detection of multiple tumor mRNA. In this study, we used a combination of tumor-specific mRNA markers to avoid the inherent limitations associated with the single-marker technique. A gold nanoparticle (AuNP) was assembled with a bi-molecular beacon (bi-MB), and termed AuNP/bi-MB, which simultaneously targeted to two types of tumor mRNA in breast cancer cells. This imaging agent could prevent effectively false positive results and provide comprehensive and dependable information for the early detection of cancer. It would be beneficial to identify the stage of tumor progression and assess treatment decisions with the real-time detection of the relative expression levels of tumor mRNA in cancer cells. This strategy would offer an appealing approach toward the early detection of cancer by using multianalysis of tumor mRNA.     

Conductivity enhancement in raw polypyrrole and polypyrrole nanoparticle dispersions

For certain commercial applications of polypyrrole (PPy), an enhancement of the electrical conductivity of the material is the key to industrial success. In this paper it will be shown that raw PPy with high conductivity (>150 S/cm) can be obtained in very good yield (>90%) by appropriate selection of both bi‐functional additives and reaction conditions which increase the oxidative polymerization rate of pyrrole. The presence of new active centers for the polymerization is demonstrated by UV measurements. In addition, raw PPy synthesized according to the improved method of synthesis shows good stability of the conductivity upon aging at high temperature (150°C) in air. Finally, new PPy dispersions are reported with an average particle size of 67 nm obtained by sonochemical synthesis that can be incorporated into conventional plastic paints for direct metallization of plastics. The conductivity of the new metallization paint developed was 0.4 S/cm with a PPy content of 10 wt%. Copyright © 2006 John Wiley & Sons, Ltd.     

Low-frequency relaxation modes in ferroelectric liquid crystal/gold nanoparticle dispersion: impact of nanoparticle shape

Low-frequency (1 mHz–100 Hz) dielectric relaxation modes were experimentally studied in ferroelectric liquid crystal (FLC)/gold nanoparticles (nanospheres and nanorods) dispersion. It was demonstrated that the dielectric spectra of nanodispersion are strongly influenced by the shape of nanoparticles. Using different formalisms of the impedance spectroscopy, three possible low-frequency relaxation processes were found in the dispersions and the pure FLC. Due to the electrical double layers (EDLs) near nanoparticles and the alignment layers, one can observe the relaxation of the EDL polarisation around the nanoparticles (Schwarz’s relaxation) and near the driving indium tin oxide (ITO) electrodes (electrode polarisation). The other possible relaxation process is interfacial polarisation (Maxwell–Wagner mode) in which the frequency is unaffected by the nanoparticles. It was shown that Schwarz’s relaxation frequency strongly depended on the shape and size of the nanoparticles. Moreover, dispersion of nanoparticles significantly reduced direct current conductivity of the FLC mixture.     

Rigid, specific, and discrete gold nanoparticle/antibody conjugates

A general method of rigid, specific labeling of proteins with gold clusters has been devised. The method relies on the conjugation of a glutathione monolayer-protected gold cluster (MPC) with a single chain Fv antibody fragment (scFv), mutated to present an exposed cysteine residue. Efficient formation of a gold-thiolate bond between the MPC and scFv depends on activation of the gold cluster by chemical oxidation. Once formed, the MPC-scFv conjugate is treated with a reductant to quench cluster reactivity. The procedure has been performed with an MPC with an average Au(71) core and an scFv directed against a tetrameric protein, the influenza neuraminidase. A complex of the MPC-scFv conjugate with the neuraminidase was isolated, and the presence of four gold clusters was verified by cryoelectron microscopy.     

Magneto-optical enhancement by plasmon excitations in nanoparticle/metal structures

Coupling magnetic materials to plasmonic structures provides a pathway to dramatically increase the magneto-optical response of the resulting composite architecture. Although such optical enhancement has been demonstrated in a variety of systems, some basic aspects are scarcely known. In particular, reflectance/transmission modulations and electromagnetic field intensification, both triggered by plasmon excitations, can contribute to the magneto-optical enhancement. However, a quantitative evaluation of the impact of both factors on the magneto-optical response is lacking. To address this issue, we have measured magneto-optical Kerr spectra on corrugated gold/dielectric interfaces with magnetic (nickel and iron oxide) nanoparticles. We find that the magneto-optical activity is enhanced by up to an order of magnitude for wavelengths that are correlated to the excitation of propagating or localized surface plasmons. Our work sheds light on the fundamental principles for the observed optical response and demonstrates that the outstanding magneto-optical performance is originated by the increase of the polarization conversion efficiency, whereas the contribution of reflectance modulations is negligible.     

SERS in PAH-Os and gold nanoparticle self-assembled multilayers

We present a detailed structural and surface-enhanced Raman scattering (SERS) study of poly(allylamine) modified with Os(byp)2ClPyCHO (PAH-Os) and gold nanoparticles self-assembled multilayers [PAH-Os+(Au-nanoparticlesPAH-Os)n, n=1 and 5]. Atomic force microscopy and variable-angle spectroscopic ellipsometry measurements indicate that the first nanoparticle layer grows homogenously by partially covering the substrate without clustering. Analyzing the sample thickness and roughness we infer that the growth process advances thereafter by filling with nanoparticles the interstitial spaces between the previously adsorbed nanoparticles. After five immersion steps the multilayers reach a more compact structure. The interaction between plasmons of near-gold nanoparticles provides a new optical absorption around 650 nm which, in addition, allows a more effective SERS process in that spectral region than at the single-plasmon resonance (approximately 530 nm). We compare the electronic resonance Raman and SERS amplification mechanisms in these self-assembled multilayers analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe large changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities which we relate to "hot spots." Numerical discrete dipole approximation calculations including the interaction between gold nanoparticles are presented, providing a qualitative model for the coupled-plasmon absorption and redshifted Raman hot spots in these samples.     

Signalling probe displacement electrochemical aptasensor for malignant cell surface nucleolin as a breast cancer biomarker based on gold nanoparticle decorated hydroxyapatite nanorods and silver nanoparticle labels

Nucleolin is a multifunctional protein that is markedly overexpressed on the surface of most cancer cells. By taking advantage of the high affinity and specificity of the AS1411 aptamer for nucleolin, a signalling probe displacement electrochemical aptasensor was developed. The thiolated AS1411 aptamer was conjugated to hydroxyapatite nanorods (HApNRs) decorated with gold nanoparticles (AuNPs). To further increase the electrical conductivity of the interface, the ionic liquid 1-ethyl-3-methylimidazolium alanine with its high ion conductivity was placed on the electrode surface. Then, the aptamer was immobilized on the modified electrode and conjugated to signalling c-DNA tagged with AgNPs (c-DNA@AgNPs). In the presence of the MCF7 target cells, the signalling probe is displaced and released from the electrode surface. This leads to a decrease in the current that is proportional to the concentration of cancer cells in the range from 10 to 10⁶ cells mL^?1, with a detection limit as low as 8?±?2 cells mL^?1 (n?=?3) (based as 3σ/m, where σ is the standard deviation of the blank and m is the slope of the calibration plot). This method presents a promising tool for highly sensitive and selective detection of surface nucleolin on MCF7 cancer cells.

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Graphical abstract HApNR-AuNP-AS1411 aptamer nanocomposite as an electrochemical sensing interface was immobilized on the gold electrode surface and conjugated to signaling c-DNA tagged with AgNPs for determination of surface nucleolin on MCF7 cancer cells.

     

Enhanced electrochemiluminescence of luminol at the gold nanoparticle/carbon nanotube/electropolymerised molecular imprinting composite membrane interface for selective recognition of triazophos

This paper reports a surface molecular self-assembly strategy for imprinting triazophos in the electropolymerised poly(aminthiophenol) (PATP) membranes at the surface of gold nanoparticle (AuNP)/carbon nanotube (CNT) composites modified glassy (GC) electrode for electrochemiluminescent (ECL) detection of pesticide triazophos. The electrochemical and ECL behaviours of luminol at the imprinted PATP/AuNP/CNT/GC electrode were investigated before and after the rebinding of triazophos. It was also found that the ECL intensity was strikingly enhanced by the adsorbed triazophos molecules in the imprinted PATP/AuNP/CNT composite membranes, which was about 5.2-fold as compared with the blank ECL intensity. On this basis, the molecularly imprinted polymer (MIP)-ECL sensor is established for high sensitive and selective detection of triazophos residues in vegetable samples. The resulting MIP-ECL sensor shows wide linear ranges from 3.1 × 10^?8 to 3.1 × 10^?5 g L^?1 with lower detection limit of 3.1 × 10^?9 g L^?1 for triazophos. Moreover, the MIP-ECL sensor has the advantages of high sensitivity, speed, specificity, stability and can become a promising technique for organophosphate pesticide detection.     

Nematodynamics and structures in junctions of cylindrical micropores

ABSTRACT

We explore equilibrium structures and flow-driven deformations of nematic liquid crystals confined to 3D junctions of cylindrical micropores with homeotropic surface anchoring. The topological state of the nematic ordering field in such basic unit of porous networks is controlled by nematic orientation profiles in individual pores, anchoring frustration along the edges of joining pores and coupling to the material flow field. We numerically investigate formation of the flow-aligned configurations in single cylindrical pores and pore junctions. Depending on the arrangement of inlet and outlet flows in the junction, we demonstrate existence of numerous stationary nematic configurations, characterised by specific bulk defects and surface disclinations along joining edges. Observed bulk defects are nonsingular escaped structures, disclinations in the form of loops or disclination lines pinned to the joining edges of the pores. Furthermore, we show examples of defect dynamics during the flow-induced topological transformations.     

A modular molecular photovoltaic system based on phospholipid/alkanethiol hybrid bilayers: photocurrent generation and modulation

Monolayer quantities of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), incorporated with either fullerenes or ruthenium tris(bipyridyl) (Ru(bpy)(3)(2+)) complexes, were formed on ferrocene-terminated C11-alkanethiol self-assembled monolayers (SAMs) through lipid fusion. Thus formed hybrid structures are characterized by quartz crystal microbalance, UV-vis spectroscopy, cyclic voltammetry and impedance analysis. In comparison to lipid monolayers deposited on C12-alkanethiol SAMs, photocurrent generation from these ferrocene-based structures is significantly modulated, displaying attenuated anodic photocurrents and enhanced cathodic photocurrents. While a similar trend was observed for the two photoagents studied, the degree of such modulations was always found to be greater in fullerene-incorporated bilayers. These findings are evaluated in the context of the film structure, energetics of the involved photo(electrochemical) species and cross-membrane electron-transfer processes.     

Controlled gold nanoparticle diffusion in nanotubes: Platfom of partial functionalization and gold capping

Multifunctionality of nanotubes (NTs) is essential in biomedical and biotechnological applications, such as drug/gene delivery, bioseparation, and single-molecule detection. Each functionality should be located at optimal positions, depending on their roles such as targeting, tracking, and transporting. This enables avoidance of possible malfunctions or interference caused by having randomly distributed multiple groups (e.g., hydrophobic and hydrophilic) in the same space. In the aspect of multifunctionality, however, a general selective partial functionalization method of NT inner surfaces still remains a challenge. For this reason, we investigated a selective partial functionalization method of NTs using controlled gold nanoparticle (Au NP) diffusion in nanotubes and the preparation method of Au-capped silica nanotubes. Silica nanotubes (SNTs) were prepared using template sol-gel synthesis, and the inside of SNT was selectively modified with (3-trimethoxysilylpropyl) diethylenetriamine (DETA-silane). Au NPs of 2-nm size were then incubated with SNTs with DETA layer inside. Spontaneous diffusion of negatively charged Au NPs from bulk into the positively charged nanochannels of SNTs led trapped Au NPs onto the inner surface of SNTs. The degree of functionalization was controlled by the channel diameter, Au NP concentration, and solvent type. These SNTs partially modified with Au NPs were then used for localized selective chemical functionalization of SNTs. This was accomplished by the reaction between thionylated Au NPs trapped on the inside of SNTs and Alexa555-maleimide. Au-capped SNTs were prepared from SNTs with Au NPs inside by seed-mediated gold growth.     

A thermodynamic investigation into the binding properties of DNA functionalized gold nanoparticle probes and molecular fluorophore probes

We report the first quantitative analysis of the oligonucleotide binding thermodynamics for DNA functionalized gold nanoparticle probes and compare our findings to molecular fluorophore probes on a sequence-for-sequence basis. With proper design, nanoparticle probes show significantly increased binding over molecular fluorophore probes under identical conditions. This is significant because probe binding strength directly influences detection sensitivity limits.     

Highly specific colorimetric recognition and sensing of sulfide with glutathione-modified gold nanoparticle probe based on an anion-for-molecule ligand exchange reaction

Colorimetric recognition and sensing of sulfide with high selectivity by virtue of glutathione-modified gold nanoparticle (GSH-AuNP) is presented. The assay is developed based on a specific anion-for-molecule ligand exchange reaction, through which sulfide replaces glutathione self-assembled on the AuNP. The unique feature of the GSH-AuNP probe makes it a novel colorimetric sensor for sulfide.