Octadecanethiol Monolayer Research Articles

Electrochemical and Raman studies of the biointeraction between Escherichia coli and mannose in polydiacetylene derivative supported on the self-assembled monolayers of octadecanethiol on a gold electrode.

Here, we describe a new method to study the biointeraction between Escherichia coli and mannose by using supramolecular assemblies composed of polydiacetylene supported on the self-assembled monolayer of octadecanethiol on a gold electrode. These prepared bilayer materials simply are an excellent protosystem to study a range of important sensor-related issues. The experimental results from UV-vis spectroscopy, resonance Raman spectroscopy, and electrochemistry confirm that the specific interactions between E. coli and mannose can cause conformational changes of the polydiacetylene backbone rather than simple nonspecific adsorption. Moreover, the direct electrochemical detection by polydiacetylene supramolecular assemblies not only opens a new path for the use of these membranes in the area of biosensor development but also offers new possibilities for diagnostic applications and screening for binding ligands.

Using Nanografting to Achieve Directed Assembly of de novo Designed Metalloproteins on Gold

Parallel three-helix bundle metalloproteins incorporating C-terminal thiol groups have been designed to orient vertically on a gold(111) surface. A small area of pre-assembled octadecanethiol was exchanged with the proteins under the action of an AFM tip. The resulting “nanografted” metalloprotein was imaged at low applied force. The measured height of the octadecanethiol monolayer was 2.2(1) nm and the mean height above this of the grafted metalloprotein layer was 3.1(4) nm. Thus the total height of the grafted metalloprotein is 5.3(4) nm. The predicted height of the vertically oriented metalloprotein is 5.2 nm. We take this as good evidence for spatial control of monolayer assembly of three-helix bundles with predictable orientation.

Cyclic Voltammetric and Scanning Electrochemical Microscopic Study of Menadione Permeability through a Self-Assembled Monolayer on a Gold Electrode

Menadione (2-methyl-1,4-naphthoquinone) reduction and menadiol oxidation at octadecanethiol (C18SH) monolayer modified gold electrodes were investigated by cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM). The modified electrode acts as a better barrier toward the ferrocyanide transport than toward the menadione species. This difference is attributed to permeation of the organic substrates into the hydrophobic monolayer. A simple model is proposed and applied to extract the rate constant of the kinetically limiting permeation step from the cyclic voltammograms. However, a better estimate of the transport properties is obtained by SECM. The same trends are observed with CV and SECM, and a similar pH dependence shows the loss of an intermediate formed during menadione reduction from the monolayer with increasing pH. This loss can probably be assigned to the rapid expulsion of the more hydrophilic reduced species from the monolayer.

Vapor Deposition of Oxotitanium(IV) Phthalocyanine on Surface-Modified Substrates: Effects of Organic Surfaces on Molecular Alignment

Surface modifications of gold-on-glass substrates with organic thin layers exerted remarkable effects on the molecular alignment in the ordered solid films of oxotitanium(IV) phthalocyanine (OTiPc) grown by conventional vapor deposition (VD). While VD onto a gold-on-glass substrate heated at 150 °C gave a highly ordered phase I (β-form) crystalline film with parallel molecular orientations, ordered phase II (α-form) crystalline films with an obliquely standing molecular arrangement were selectively grown on the surfaces of the substrate modified with 9-cycle Langmuir−Blodgett layers of OTiPc, a self-assembled monolayer of octadecanethiol, and a spin-coated thin film of polyimide. Moreover, VD of OTiPc onto a rubbed polyimide film gave an in-plane ordered phase II film in which obliquely standing OTiPc molecules are aligned with the molecular plane parallel to the rubbing direction. This film revealed high dichroic behavior in the p-polarized visible and IR absorption spectra. The substrate-surface effects...

A multiplex infrared-visible sum-frequency spectrometer with wavelength tunability of the visible probe

We present a multiplex infrared-visible sum-frequency (SF) spectrometer with wavelength tunability of the visible probe. The spectrometer is based on a two-channel optical parametric amplifier producing a broad-bandwidth femtosecond infrared pulse and a narrow-bandwidth picosecond visible pulse (470–800 and 401 nm). A SF spectrum over a 400 cm−1 spectral region was obtainable by a single measurement. SF spectra of an octadecanethiol monolayer on a gold substrate were measured with visible probes of different wavelengths from 401 to 633 nm. The spectral shape showed remarkable dependence on the visible probe wavelength. The dependence was ascribed to resonance to a surface electronic state of the substrate existing in the visible region.

Scanning Tunneling Spectroscopy of Insulating Self-Assembled Monolayers on Au(111)

Self-assembled monolayers (SAMs) of dodecanethiol (DDT), octadecanethiol (ODT), and resorcinarene C10 tetrasulfide (RC10TS) on Au(111) were characterized by scanning tunneling spectroscopy (STS) under ultrahigh vacuum conditions and evaluated as ultrathin resists. A voltage division factor η was used to parametrize the equilibrium Fermi energy level Ef in terms of an applied voltage bias. Nominally symmetric tunneling (η ∼ 0.5) conditions were established by acquiring conductance spectra over a range of set point voltages. The electrical conductivity of the monolayers was observed to be dependent on both the monolayer thickness and the nature of the Au/S bonding. Leakage current densities across DDT and RC10TS SAMs on Au were estimated, with the latter comparing favorably with a 1.5-nm layer of SiO2 on Si.

Biotinylation of colloidal gold particles using interdigitated bilayers: a UV–visible spectroscopy and TEM study of the biotin–avidin molecular recognition process

Abstract The surface modification of colloidal gold particles with biotin groups using a technique based on interdigitated bilayers is described. The biotinylation of the colloidal particles is accomplished by a two-step process. The first step involves the self-assembly of a close-packed primary monolayer of octadecanethiol (ODT) molecules on the colloidal particle surface. Thereafter, a secondary monolayer of alkylbiotin molecules is self-assembled on the ODT-capped gold colloidal particles by interdigitation of the alkyl groups of the biotin molecules with the hydrocarbon chains of the primary monolayer. The resulting biotinylation of the colloidal particles is checked by the high-affinity biotin–avidin interaction, which leads to cross-linking of the colloidal particles and an easily measurable change in the optical properties of the sol. This approach may be important for the incorporation of molecules with functional groups that become inactive due to modification of the molecule with colloidal particle surface-anchoring groups such as thiol groups and thus shows scope for development.

Development of a Multiplex Infrared-visible Sum-frequency Generation Spectrometer with Wavelength Tunability of the Visible Probe.

We present a newly developed multiplex sum-frequency generation (SFG) spectrometer that has wavelength tunability of the visible probe. The tunable wavelength range of the visible probe was 470∼800, and 401 nm. The frequency width of the visible probe were 7∼10 cm−1. An SFG spectrum over a 400 cm−1 spectral region was obtainable by a single measurement. We measured SFG spectra of a self-assembled monolayer of octadecanethiol on a gold substrate with the visible probes laser of various wavelengths (401, 477, 535, 633 nm). The observed spectra were drastically changed with the wavelength of the visible probe. We ascribed this spectral change to the change in the phase and magnitude of the contribution from the gold substrate because the interband transition threshold of gold is located in the wavelength range of the visible probe.

Vibrationally resonant sum-frequency generation spectral shape dependent on the interval between picosecond-visible and femtosecond-infrared laser pulses

We present the pulse interval dependent vibrationally resonant sum-frequency generation (SFG) spectra generated by picosecond-visible and femtosecond-infrared (IR) laser pulses on a self-assembled monolayer of octadecanethiol (ODT) on a Au substrate. The bandwidths of vibrational bands became broader when the visible pulse arrived at a sample earlier than the infrared pulse. This phenomenon was interpreted as a transient coherent optical effect of the SFG process emerging in the frequency domain.

Atomic force microscopy on bare and thiol monolayer covered gallium arsenide

In this work, we study the functionalized gallium arsenide (GaAs) surface with self-assembled monolayer (SAM) by means of atomic force microscopy (AFM). First, we study the flatness of the dummy GaAs surface through AFM in electrolyte. Second, with the same technique, we study the effect of SAMs of octadecanethiol (C 18H 37SH) and alcohol-thiol (HOCH 2(CH 2) 10SH) on GaAs surface. Finally, for GaAs treated with octadecanethiol monolayer, we observe a hysteresis—demonstrating that the surface is hydrophobic and that the Van der Waals interaction between the tip and the sample is very high. GaAs treated with alcohol-thiol shows hysteresis, which shows that the surface is hydrophilic and uncharged.

Total and free charge densities on mercury coated with self-assembled phosphatidylcholine and octadecanethiol monolayers and octadecanethiol/phosphatidylcholine bilayers

The thermodynamic ‘total’ charge density is the charge to be supplied to the electrode to keep the applied potential constant when the electrode surface is increased by unity, while the extrathermodynamic ‘free’ charge density is the charge actually experienced by the diffuse layer ions. The total charge density at dioleoylphosphatidylcholine (DOPC) and octadecanethiol (ODT) monolayers and mixed ODT/DOPC bilayers self-assembled on mercury from aqueous solutions was determined from chronocoulometric single potential steps to a final potential negative enough to cause complete desorption of the film. The effect of different alkali metal ions and of tetramethylammonium on DOPC desorption was examined. The total charge for ODT monolayers and ODT/DOPC bilayers, +56±3 μC cm−2, agrees with the value obtained by integration of the current under the reductive desorption voltammetric peaks, only provided the scan rate is higher than 100 mV s−1. An approximate model of the interface of the ODT-coated electrode, which accounts for partial charge transfer from sulfur to mercury and for the degree of dissociation of the sulfhydryl group upon self-assembly, was employed to estimate the free charge density.

Characterization of defects in the formation process of self-assembled thiol monolayers by electrochemical impedance spectroscopy

Abstract Self-assembled monolayers of octadecanethiol (ODT) on gold have been studied by electrochemical impedance spectroscopy (EIS). The fractional coverage, pinhole size and separation have been examined as a function of immersion time of Au in the ODT deposition solution. The fractional coverage of ODT monolayer increases sharply from zero to more than 99% of its maximum within the first minute. However, it takes hours or a day for the fractional coverage to approach its final value. The pinhole separation increases from ca. 1.8 μm to 53 μm as the assembling time increases from 5 s to 24 h, indicating that the number of pinholes decreases. The pinhole radius increases slowly from ca. 0.3 μm to 1.6 μm in the same time interval. The slight trend of increasing pinhole size with adsorption time is attributed to the increasing influence of collapsed sites in the ODT monolayer.

Studies of Adsorption Kinetics and Defects of Self‐Assembled Thiol Monolayers on Gold by Capacitance Plane Plot

AbstractThe capacitive property of an electrode/electrolyte interface can be described by complex capacitance. The capacitance plane plots (CPPs) of ideal polarized and kinetic controlled electrodes are derived based on the concept of complex capacitance. By using CPPs, the capacitance of electrode/electrolyte interface can be conveniently determined. In this work, CPPs obtained in ac impedance experiments are employed for the first time in studying the kinetics of adsorption process of the thiol monolayer. The coverage of octadecanethiol (ODT) monolayer on gold is examined as a function of adsorption time. The adsorption process of ODT molecules on gold exhibits two distinct phases: an initial rapid step followed by a slow one. The simple Langmuir model best explains our experimental data in the initial adsorption stage. CPPs and cyclic voltammetry (CV) indicate that, in the initial adsorption step, the ODT monolayer contains defects whose number decreases with the increasing of adsorption time.

Structural characterization of GaAs/thiol/electrolyte interface

The present work is motivated by attempts in our laboratory to develop the physical basis for the design of biosensors (suited for aqueous medium) based on biofunctional supported membranes. We studied the electrical properties of n-doped gallium arsenide (GaAs) in contact with electrolyte and the high insulating properties of self-assembled monolayers (SAMs) of octadecanethiol (C 18H 37SH) by impedance spectroscopy and cyclic voltammetry. The average thickness of the thiol monolayer was measured by ellipsometry and confirmed by impedance. The analysis of the impedance in terms of equivalent circuits of the GaAs/electrolyte and GaAs/insulator/electrolyte interface is discussed. This analysis allowed measurements of the defect area fraction of the thiol monolayer.

High-Resolution Electron Energy Loss Spectroscopy Investigation of the Vibrational Structure and Order of an Octadecanethiol Monolayer on Gold Substrates

The vibrational spectra induced by electron beam from octadecanethiol monolayers adsorbed on gold are reported as a contribution to study in detail the structure of the layers, through an analysis of the interaction mechanisms between the electrons and these organic ultrathin films. To show the different spatial distribution of scattered electrons from dipole and impact interactions with the monolayer, angular distributions of elastic and inelastic scattered electrons were measured for various primary energies. The absolute electron scattering loss probabilities from both dipole and impact interactions were estimated for different vibrational modes. The study of these different mechanisms was first hampered by the poor order of the layers adsorbed on evaporated gold. It became therefore highly desirable to perform the experiments on a single-crystal substrate. Using Au(111) and Au(100) single crystals as substrates, it was possible to separate the spectral response from impact and dipole interacting elect...

Can N-ethyl- N′-octadecyl viologen dimerize on octadecanethiol-coated gold electrode ?

Electrochemical and ex situ FTIR spectral techniques have been used to study the insertion of asymmetric alkyl viologen, N-ethyl- N′-octadecyl viologen (C 18EV 2+) into the self-assembled monolayers (SAMs) of octadecanethiol (ODT) on Au electrode surface. In the IR reflection absorption (IRRA) spectroscopic measurements, the relative magnitude of the CH 2 and CH 3 stretching modes of ODT and their peak positions indicated that ODT forms a densely packed crystalline-like assembly on polycrystalline Au electrode. Furthermore, a considerable increase in the CH 2 stretching mode intensity relative to that of the CH 3 stretching mode in the C 18EV 2+-inserted ODT monolayer suggested a greater disorder in the alkyl chains of the C 18EV 2+-intercalated ODT monolayer than in the pure ODT monolayer. In the cyclic voltammetry studies, C 18EV 2+ showed a neat broad redox wave at low surface coverage of C 18EV 2+ on the ODT-coated electrode and a sharp reduction and multiple oxidation peaks at high surface coverage. The potential difference subtractively normalized interfacial FTIR (SNIFTIR) spectral studies showed that at high surface coverage of C 18EV 2+ inserted on the ODT-coated electrode, the electrogenerated radical cations of C 18EV 2+ are close and in an orientation proper enough to form the dimer; the spectral features characteristic of the dimer were observed. On the other hand, at similar surface coverage of C 18EV 2+, such spectral features were not observed for the SAMs of C 18EV 2+ on bare Au electrode because of the less ordered orientation of C 18EV 2+.

Crystalline Solids of Alloy Clusters

AuAg alloy clusters protected with octadecanethiol (ODT) monolayers form crystalline solids whose powder X-ray diffractograms can be indexed to single cubic unit cells. These solids can also be regarded as superlattices of alloy clusters. The bulk Au:Ag ratio is significantly different from the metal salt feed ratio used for the synthesis of the alloy clusters. The surface plasmon resonance band of the alloy clusters shift monotonically with change in composition and only one distinct band is seen in the composition range, indicating that the single metal core containing alloy clusters are not forming. The shifts observed can be linearly related to the compositions. Atomic emission spectroscopy gives similar compositions as determined from absorption spectroscopy. The monolayer coverage determined from thermogravimetry increases with an increase in silver concentration. Thermal stability of the monolayers is comparable to pure Au and Ag clusters and no change in desorption temperature is observed with composition. Both monolayer melting and superstructure melting transitions are seen in the differential scanning calorimetry of the solids. Infrared spectroscopy suggests that the monolayers are essentially solidlike and the conformations are similar to the monolayers on pure Ag clusters. Some of the bands exhibit factor group splitting in the Au-rich composition, which is observed in other compositions upon annealing.

Electrochemical Cleaning of Surface-Confined Carbon Contamination in Self-Assembled Monolayers on Polycrystalline Ag and Au

A protocol for electrochemical cleaning of carbon-contaminated alkanethiol SAMs at mechanically polished (MP) Ag surfaces is characterized by surface Raman spectroscopy and electrochemistry. Vibrational information in the ν(C−S), ν(C−C), ν(C−H), and δ(C−H) regions is particularly useful in elucidating the degree of order and amount of contamination in propanethiol, dodecanethiol, and octadecanethiol monolayers before and after negative potential exposure in several aqueous electrolytes. Specifically, Raman spectra indicate that electrochemical cleaning of alkanethiol SAMs at potentials negative of the thiolate reduction removes carbonaceous species and greatly increases the film order near the sulfur headgroup.

Recognition of vitamin K1 with a molecularly imprinted self-assembled monolayer film.

In this paper, we present a novel way to design a self-assembled monolayer, based on molecular imprinting protocols, for the selective recognition of hydrophobic molecules.A monolayer of octadecanethiol, templated with n-hexadecane, was fabricated on a gold substrate, and its voltammetric behavior was examined with vitamin K1 and K3.The monolayer film showed a voltammetric response only to hydrophobic vitamin K1 molecules, while a film without the hexadecane template showed neither a peak assigned to vitamin K1 nor K3.The results demonstrate that the present technique can be used as a simple and highly selective surface-designing tool for the fabrication of a self-assembled monolayer based electrode.

Fractional coverage of defects in self-assembled thiol monolayers on gold

Au electrodes are alkylated by self-assembled organic monolayers of octadecanethiol from alcohol solution. The electron tunnelling resistance of a monolayer-coated gold electrode has been investigated by ac impedance. The relation between the fractional coverage of different defects and the corresponding film thickness at these ‘collapsed’ sites has been deduced from electron tunnelling theory. By using the concepts of average film thickness at defect (da) and average fractional coverage of defect (θa), we have obtained the θa∼da plot. The influence of the apparent standard rate constant on the shape of the θa∼da plot has been discussed. In our experiments, Fe(CN)63−/4− is used as a redox probe to study the θa∼da plot of an octadecanethiol monolayer. The θa versus da plot indicates that the defects with da<6 methylene groups and θa<0.1 can increase the apparent standard rate constant from 1.9×10−10 cm s−1, which is the theoretical value calculated from electron tunnelling theory, to 2.9×10−7 cm s−1. The average thickness of the whole monolayer (ATWM), which is obtained from the θa versus da plot and which can indicate the blocking property of the monolayer, is 11 methylene groups.