Electrically transmissive alkyne-anchored monolayers on gold

Well-ordered, tightly-packed (surface coverage 0.97 × 10−9 mol cm−2) monolayer films of 1,4-bis((4-ethynylphenyl)ethynyl)benzene (1) on gold are prepared via a simple self-assembly process, taking advantage of the ready formation of alkynyl C–Au σ-bonds. Electrochemical measurements using [Ru(NH3)6]3+, [Fe(CN)6]3−, and ferrocenylmethanol [Fe(η5-C5H4CH2OH)(η5-C5H5)] redox probes indicate that the alkynyl C–Au contacted monolayer of 1 presents a relatively low barrier for electron transfer. This contrasts with monolayer films on gold of other oligo(phenylene ethynylene) derivatives of comparable length and surface coverage, but with different contacting groups. Additionally, a low voltage transition (Vtrans = 0.51 V) from direct tunneling (rectangular barrier) to field emission (triangular barrier) is observed. This low transition voltage points to a low tunneling barrier, which is consistent with the facile electron transport observed through the C–Au contacted self-assembled monolayer of 1.P. C. and J. L. S. are grateful for financial assistance from Ministerio de Economia y Competitividad from Spain and fondos FEDER in the framework of projects MAT2016-78257-R and CTQ2015-70174-P, respectively. J. L. S. also acknowledges the funded project Hierarchical Self Assembly of Polymeric Soft Systems, “SASSYPOL”, from the 7th Framework Programme (CEE, Ref-607602). L. H., S. M., J. L. S, and P. C. acknowledge support from DGA/Fondos FEDER (construyendo Europa desde Aragón) for funding PLATON (E31_17R) and CLIP (E47_17R) research groups. R. J. N. thanks EPSRC for funding (EP/M029522/1, EP/K007785/1, EP/M014169/1 and EP/M005046/1), and P. J. L. also gratefully acknowledges support from the Australian Research Council (FT120100073; DP140100855).Peer reviewe

Influence of surface coverage on the formation of 4,4 '- bipyridinium (viologen) single molecular junctions

Single-molecule conductance experiments using the STM-based I(s) method and samples of N,N’-di(4-(trimethylsilylethynyl)benzyl)-4,4’-bipyridinium bis(tetrafluoroborate) ([1](BF4)2) prepared on gold substrates with low-surface coverage of [1](BF4)2 (Γ = 1.25·10⁻¹¹ mol·cm⁻²) give rise to molecular junctions with two distinct conductance values. From the associated break-off distances and comparison experiments with related compounds the higher conductance junctions are attributed to molecular contacts between the molecule and the electrodes via the N,N’-dibenzyl-4,4´-bipyridinium (viologen) moiety and one trimethylsilylethynyl (TMSE) group (G = (5.4 ± 0.95)×10⁻⁵ G0, break-off distance (1.56 ± 0.09) nm). The second, lower conductance junction (G = (0.84 ± 0.09)×10⁻⁵ G0) is consistent with an extended molecular conformation between the substrate and tip contacted through the two TMSE groups giving rise to a break-off distance (1.95 ± 0.12) nm that compares well with the Si...Si distance (2.0 nm) in the extended molecule. Langmuir monolayers of [1](BF4)2 formed at the air-water interface can be transferred onto a gold-on-glass substrate by the Langmuir-Blodgett (LB) technique to give well-ordered, compact films with surface coverage Γ = 2.0·10⁻¹⁰ mol·cm⁻². Single-molecule conductance experiments using the STM-based I(s) method reveal only the higher conductance junctions (G = (5.4 ± 0.95)×10⁻⁵ G0, break-off distance (1.56 ± 0.09) nm) due to the restricted range of molecular conformations in the tightly packed, well-ordered LB films.S.M. and P.C. are grateful for financial assistance from Ministerio de Economía y Competitividad from Spain and fondos FEDER in the framework of projects MAT2016-78257-R. S.M. and P.C. also acknowledge DGA/fondos FEDER (construyendo Europa desde Aragón) for funding the research group Platón (E-54). S.M. acknowledges funding from the University of Zaragoza (grant number JIUZ02016-CIE-04). R.J.N, S.J.G and D.C.M are grateful for financial assistance from the EPSRC (grant EP/M029522/1). P.J.L. and J.B.G.G. gratefully acknowledge support from the Australian Research Council (FT120100073; DP140100855).Peer reviewe

Melanin films on Au(1 1 1): Adsorption and molecular conductance

We have studied the adsorption and electronic properties of thin melanin films on Au(1 1 1) by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional (DF) calculations. We have found that the minimum melanin unit detected under different adsorption conditions is consistent with the structural model for eumelanin protomolecules based on tetramer macrocycles formed by four monomer units (hydroquinone, indolequinone and its tautomers) with an inner porphyrin ring. DF calculations reveal that the entire π structure of the tetramers is implied in the chemisorption process through its frontier orbitals (HOMO and LUMO), a fact that is reflected in the change of intramolecular bonds. Also van der Waals interactions give an important contribution to the adsorption energy (≈0.02 eV/Å 2). Dried thin melanin films (1 monolayer in thickness) exhibit good electronic conductance due to the presence of localized states near the Fermi level while dried thicker films exhibit a semiconductor-like behavior. Illumination of the thicker films with white light results in significant photo-induced tunneling currents when the melanin-covered Au is made negative with respect to the tip.Fil: Orive, Alejandro Gonzalez. Universidad de La Laguna; EspañaFil: Hernández Creus, Alberto. Universidad de La Laguna; EspañaFil: Carro, Pilar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad de La Laguna; EspañaFil: Salvarezza, Roberto Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentin

Dependance of Poly(acrylic acid) Interfacial Adhesion on the Nanostructure of Electrodeposited ZnO Films

Understanding the impact of the intricate morphology and surface chemistry of ZnO nanorod arrays on their interactions with polyelectrolyte polymers is crucial for the development of nascent ZnO-based adhesion-promoting materials. AFM-based single molecule force spectroscopy was applied for the analysis of the adsorption of poly(acrylic acid) (PAA) on zinc oxide (ZnO) film covered stainless steel substrates in aqueous electrolytes at pH 7. Based on the electrodeposition process, the morphology of zinc oxide films could be varied ranging from platelet-like crystals to nanorods. This approach allowed for the morphology dependent analysis of macromolecular adsorption processes on complex ZnO nanostructures which have diverse applications in the field of adhesion-promoting thin films. The surface chemical composition, as determined by X- ray photoelectron spectroscopy, could be correlated to the AFM-based desorption studies. Only equilibrium desorption events (plateaus), centered at 42 pN, were observed on mirror polished, preconditioned stainless steel. However, for platelet-like ZnO films, the poly(acrylic acid) desorption showed a mixture of rupture events (mean rupture forces of about 350 pN) and equilibrium desorption, while ZnO nanorod structures showed solely rupture events with mean rupture forces of about 1300 pN. These results indicate that simultaneous multiple ruptures of carboxylate-zinc bonds occur due to the macromolecular coordination of poly(acrylic acid) to the ZnO nanorods. The analysis of the interfacial adhesion processes is further supported by the dwell time dependence of desorption processes

Structure and electronic and charge-transfer properties of mercaptobenzoic acid and mercaptobenzoic acid-undecanethiol mixed monolayers on AU(111)

The surface structure, molecular conductance, and charge-transfer properties of pure mercaptobenzoic acid (MBA) and mixed MBA?undecanethiol (UDT) self-assembled monolayers (SAMs) on Au(111) are studied by scanning tunneling microscopy, scanning tunneling spectroscopy, electrochemical techniques, and density functional theory calculations. MBA forms ordered diluted lattices without evidence of Au adatom complexes at terraces (absence of vacancy islands) at low MBA concentrations, whereas disordered lattices with possible formation of these complexes are found at high concentrations (presence of vacancy islands). In addition, some other interesting facts have been established. The stability of the MBA SAMs is improved in mixed MBA-UDT SAMs, revealing aromatic-aliphatic attractive interactions. The mixed SAMs at equivalent amounts of thiols hinder the vacancy island formation typical of the pure UDT SAMs. The MBA SAMs exhibit good molecular conductance with filled states from S and aromatic ring contributions near the Fermi level. Finally, it has been proven that MBA molecules efficiently wire the electron transfer through UDT SAMs.Fil: Rodriguez Gonzalez, Miriam C.. Universidad de la Laguna; EspañaFil: Gonzalez Orive, Alejandro. Universidad de la Laguna; EspañaFil: Carro, Pilar. Universidad de la Laguna; EspañaFil: Salvarezza, Roberto Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; ArgentinaFil: Hernandez Creus, Alberto. Universidad de la Laguna; Españ

Effect of Terminal Modifications on the Adsorption and Assembly of hIAPP(20–29)

The assembly of peptides and proteins into nanoscale amyloid fibrils via formation of intermolecular β-sheets not only plays an important role in the development of degenerative diseases but also represents a promising approach for the synthesis of functional nanomaterials. In many biological and technological settings, peptide assembly occurs in the presence of organic and inorganic interfaces with different physicochemical properties. In an attempt to dissect the relative contributions of the different molecular interactions governing amyloid assembly at interfaces, we here present a systematic study of the effects of terminal modifications on the adsorption and assembly of the human islet amyloid polypeptide fragment hIAPP(20–29) at organic self-assembled monolayers (SAMs) presenting different functional groups (cationic, anionic, polar, or hydrophobic). Using a selection of complementary in situ and ex situ analytical techniques, we find that even this well-defined and comparatively simple model system is governed by a rather complex interplay of electrostatic interactions, hydrophobic interactions, and hydrogen bonding, resulting in a plethora of observations and dependencies, some of which are rather counterintuitive. In particular, our results demonstrate that terminal modifications can have tremendous effects on peptide adsorption and assembly dynamics, as well as aggregate morphology and molecular structure. The effects exerted by the terminal modifications can furthermore be modulated in nontrivial ways by the physicochemical properties of the SAM surface. Therefore, terminal modifications are an important factor to consider when conducting and comparing peptide adsorption and aggregation studies and may represent an additional parameter for guiding the assembly of peptide-based nanomaterials

Adsorption and Fibrillization of Islet Amyloid Polypeptide at Self-Assembled Monolayers Studied by QCM-D, AFM, and PM-IRRAS

Aggregation and fibrillization of human islet amyloid polypeptide (hIAPP) plays an important role in the development of type 2 diabetes mellitus. Understanding the interaction of hIAPP with interfaces such as cell membranes at a molecular level therefore represents an important step toward new therapies. Here, we investigate the fibrillization of hIAPP at different self-assembled alkanethiol monolayers (SAMs) by quartz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM), and polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). We find that hydrophobic interactions with the CH₃-terminated SAM tend to retard hIAPP fibrillization compared to the carboxylic acid-terminated SAM where attractive electrostatic interactions lead to the formation of a three-dimensional network of interwoven fibrils. At the hydroxyl- and amino-terminated SAMs, fibrillization appears to be governed by hydrogen bonding between the peptide and the terminating groups which may even overcome electrostatic repulsion. These results thus provide fundamental insights into the molecular mechanisms governing amyloid assembly at interfaces

New Findings for the Composition and Structure of Ni Nanoparticles Protected with Organomercaptan Molecules

Here we explore the synthesis of alkanethiolcoated Ni NPs following the one-phase reaction method by Brust et al.1 The reduction of NiCl2 with NaBH4 in the presence of dodecanethiol (C12SH) yields a complex product that is difficult to identify as illustrated in the figure of merit. We synthesized Ni(II) dodecanethiolate (C12S) (without the addition of NaBH4) for comparison and performed an exhaustive characterization with TEM, HR-TEM, AFM, MFM, XPS, XRD, UV−vis, magnetism, and FT-IR. It is found that the organic coating is not quite a well-organized self-assembled monolayer (SAM) surrounding the Ni cluster as previously reported.2,3 XPS and XRD data show slight differences between both syntheses; however, Ni(II) thiolate appears to be more stable than reduced Ni when exposed to ambient air, indicating the propensity of metallic Ni to oxidize. It has been shown that irradiating with TEM electrons over various metal thiolates leads to nanoparticle formation.4 We irradiated over Ni(II) thiolate and observed no evidence of NP formation whereas irradiating a reduced Ni sample exhibited an ∼3.0 nm nanoparticle diameter. Magnetism studies showed a difference between both samples, indicating ferromagnetic character for the reduced Ni sample. According to our results, the product of the synthesis is comprised of ultrasmall metallic clusters embedded in some form of Ni(II) C12S. In this work, we open a discussion of the chemical nature of the core and the shell in the synthesis of Ni NPs protected with organomercaptan molecules.Fil: Calderón Segovia, Matías Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Zelaya, Maria Eugenia. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); ArgentinaFil: Benitez, Guillermo Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Schilardi, Patricia Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Hernandez Creus, Alberto. Universidad de La Laguna, Instituto de Materiales y Nanotecnología, Departamento de Química Física; EspañaFil: Gonzalez Orive, Alejandro. Universidad de La Laguna, Instituto de Materiales y Nanotecnología, Departamento de Química Física; EspañaFil: Salvarezza, Roberto Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Ibañez, Francisco Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentin

Design and Synthesis of Aviram-Ratner-Type Dyads and Rectification Studies in Langmuir-Blodgett (LB) Films

The design and synthesis of Aviram-Ratner-type molecular rectifiers, featuring an anilino-substituted extended tetracyanoquinodimethane (exTCNQ) acceptor, covalently linked by the sigma-spacer bicyclo[2.2.2]octane (BCO) to a tetrathiafulvalene (TTF) donor moiety, are described. The rigid BCO spacer keeps the TTF donor and exTCNQ acceptor moieties apart, as demonstrated by X-ray analysis. The photophysical properties of the TTF-BCO-exTCNQ dyads were investigated by UV/Vis and EPR spectroscopy, electrochemical studies, and theoretical calculations. Langmuir-Blodgett films were prepared and used in the fabrication and electrical studies of junction devices. One dyad showed the asymmetric current-voltage (I-V) curve characteristic for rectification, unlike control compounds containing the TTF unit but not the exTCNQ moiety or comprising the exTCNQ acceptor moiety but lacking the donor TTF part, which both gave symmetric I-V curves. The direction of the observed rectification indicated that the preferred electron current flows from the exTCNQ acceptor to the TTF donor