Dodecanethiol Self-assembled Monolayers Research Articles

A rotating gold ring–gold disk electrode study on electrochemical reductive desorption and oxidative readsorption of a self-assembled monolayer of dodecanethiol

The electrochemical reductive desorption and oxidative readsorption of a self-assembled monolayer (SAM) of dodecanethiol (C 12SH) in an alkaline solution were investigated using a rotating gold ring–gold disk electrode. Dodecanethiolate (C 12S −), which was reductively desorbed from the C 12SH SAM modified gold disk electrode, was detected by the gold ring electrode which was kept at a potential where the oxidative readsorption took place. As the rotation rate was increased, the cathodic peak potential for the reduction of the SAM at the disk electrode became more positive and the full width at half maximum of the anodic peak for the oxidative readsorption at the ring electrode decreased. These results suggest that reductively produced C 12S − tends to remain on the electrode surface and might form a molecular assembly such as a micelle on the disk electrode surface, thus preventing the residual C 12SH SAM from desorbing.

자기 집합 단분자막 개질 금 전극을 이용한 수용액 중 폴리피를 성장에 관한 In-situ EQCM 연구

Self-assembled monolayer(SAM)로 변형된 금 전극 위로 폴리피롤의 전기화학적 석출 과정을 수용액 상태에서 in-situ EQCM (Electrochemical Quartz Crystal Microbalance)과 ex-situ AFM (Atomic Force Microscopy)을 이용하여 조사하였다. 금 전극에서 cyclic voltammetry로 살펴본 폴리머의 석출은 산화 제한 전위 (anodic limiting potential) 값에 매우 의존적이었으며 주사 횟수에는 의존하지 않았다. 제한 산화 전위가 0.8V (vs Ag | ArCl) 이상일 때 폴리머의 석출은 크게 증가하였다. 그리고 주사 횟수가 증가하면서 질량의 비이상적 변화가 관찰되었는데 이것은 폴리피롤 필름의 rearrangement가 원인이라고 생각된다. 1-dodecanethiol SAM 전극과 thiophene SAM전극에서는 폴리머가 3차원적으로 성장하며 필름의 rearrangement를 수반하였지만 BPUS <TEX>$(Bis(\omega(N-pyrrolyl)-n-undecyl)disulfide)$</TEX> SAM 전극에서는 2차원적인 layer-by-layer 성장을 하고 필름의 rearrangement는 관찰되지 않았다. 폴리머가 급격하게 전극 면으로 석출되면 사슬 모양과 도너츠 모양의 폴리머를 만들며, 정류 상태에 이르면서 주름잡힌 폴리머 필름이 생성되는 것이 원자 힘 현미경 (Atomic Force Microscopy) 이미지로 관찰되었다. The growth of Polypyrrole film has been investigated during electropolymerization in an aqueous solution on bare and SAM modified gold electrodes by in-situ EQCM and ex-situ AFM. According to the result of cyclic voltammetry measurements, in the case of a bare gold electrode, the electrochemical deposition of polypyrrole were dependent on the limiting oxidative potential, but not on scan numbers. When the limiting potential higher than 0.8 V was applied on the electrode, the amount of polypyrrole deposited on a gold electrode was rapidly increased and the abnormal mass change attributed to the rearrangement of polypyrrole films was observed as the scan number increased. The polypyrrole film Prepared on electrodes modified with 1-dodecanethiol SAM or thiophene SAM grew 3-dimensionally with the rearrangement of film. However, in the case of BPUS SAM, 2-dimensional layer-by-layer growth of film was observed without the rearrangement of film. AFM images showed films with chain-shaped and/ or donut-shaped polymers when grown rapidly and a wrinkled film at the steady state condition.

Voltammetric Properties of the Reductive Desorption of Alkanethiol Self-Assembled Monolayers from a Metal Surface

The current peak that appears on a linear-scan voltammogram for the reductive desorption of alkanethiol self-assembled monolayers (SAMs) from a gold surface in an aqueous alkaline solution exhibits intriguing features: the narrow full width at half-maximum (fwhm) of the peak, e.g., 20 mV for dodecanethiol SAMs, the saturation of fwhm in the SAM composed of long-chain alkanethiols, an asymmetric shape, the shift of the peak potential with increasing the alkyl chain length, and the peak area that is greater than what is expected from the (√3 × √3)R30° structure of adsorbed alkanethiols on Au(111). A Pade approximant expression for the adsorption isotherm proposed by Blum and Huckaby based on the two-dimensional Ising model, in combination with the semi-infinite linear diffusion of desorbed species, well explains these salient features of the reductive desorption behavior. The double-layer charging current can amount to one-third of the charge calculated from the area of the peak of the reductive desorption...

Construction and Characterization of a Nanowell Electrode Array

Gold nanowell electrode arrays, with a nanowell depth of approximately 600 nm, have been prepared using commercially available anopore membranes as templates. Electrodes were prepared by forming a copper film on anopore, followed by electrodeposition of gold throughout the pore and finally removal of the copper to expose the nanowells. The electrodes were characterized by atomic force microscopy, scanning electron microscopy, and electrochemical methods. Dodecanethiol self-assembled monolayers were found to passivate the electrode. C. R. Martin coined the term “template synthesis” to describe a method in which the pores of a nanoporous membrane act as templates for the synthesis of nanostructures of a desired material. 1 As the pores within these membranes are nearly cylindrical and of uniform diameter, monodisperse nanocylinders of a desired material can be obtained. This method has been applied to the formation of nanowires, 2 metal nanostructures 3 and hollow metal tubules, 4 conductive 5 and insulating polymers, 6 and semiconductors. 7 (For a review of general template-based methods for the preparation of nanomaterials see ref 1.) The formation of gold metal nanostructures by Martin’s group using anopore disks as a template 3 was accomplished by sputtering a thin layer of Ag onto one side of the membrane in order to form an electrode, followed by the galvanostatic deposition of Ag to seal the pores and potentiostatic deposition of Au into the pores on top of the Ag microcylinders. Nitric acid was then used to dissolve away the Ag, leaving Au nanostructures. In this report we discuss a modification of this method which allows us to form an array of Au nanoelectrodes, 200 nm in diameter and which have a controlled depth, embedded within an anopore template. The nanoelectrode array was characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), and electrochemical methods. Passivation of the electrodes with alkanethiol molecules in a self-assembled monolayer (SAM) was also investigated. Anodisc membranes obtained from Whatman were used as the template material for gold nanoelectrode fabrication.

Effect of temperature on friction observed between a Si 3N 4 tip and a dodecanethiol self-assembled monolayer on Au(1 1 1)

Recent computational studies (Phys. Rev. Lett. 70 (1993) 1960; Phys. Rev. B 62 (2000) 17055) predicted that friction of ordered organic monolayer had characteristic dependence on temperature, where the maximum friction was observed around rotator transition point of the monolayer. This remained to be confirmed experimentally. Using a friction force microscope (FFM) combined with a temperature regulation module, we attempted to investigate such dependence on temperature (130 K–room temperature) on a self-assembled monolayer of dodecanethiol prepared on Au(1 1 1). The observed friction showed strong dependence on temperature and good agreement with the computational prediction.

Influence of the metal electronic properties on the sum-frequency generation spectra of dodecanethiol self-assembled monolayers on Pt (111), Ag (111) and Au (111) single crystals

Infrared-visible sum-frequency generation (SFG) spectroscopy is performed at different visible wavelengths between 450 and 650 nm in order to investigate the interaction between metallic substrates (Pt, Ag and Au) and 1-dodecanethiol self-assembled monolayers. We show that such measurements provide a means to study the electronic properties of metals as well as the interference phenomena between the SFG signals from the adsorbate and the substrate. The common features of the three interfaces are the vibration modes of the terminal methyl groups. However, these resonances appear as peaks for Pt (111) and as dips in the case of Ag (111). Their shape is not modified when the visible wavelength is tuned between 450 and 650 nm. Moreover, the metal sum-frequency generation contribution is slightly modified in that spectral range. For Au (111) samples, the sum-frequency generation spectrum drastically evolves with the visible wavelength: the peak resonant sum-frequency generation signal at 450 nm becomes rather dip-shaped at 650 nm. The non-linear response of gold is also enhanced when the wavelength associated with the sum-frequency process is close to 480 nm. These results are interpreted on the basis of the metal electronic properties that are responsible for the non-resonant contribution to the SFG signal.

Direct comparison of the electronic coupling efficiency of sulfur and selenium anchoring groups for molecules adsorbed onto gold electrodes

We performed air and ultra-high vacuum scanning tunneling microscopy experiments in order to compare the electronic coupling provided by S and by Se used as alligator clips for bisthiol- and biselenol-terthiophene molecules adsorbed onto gold. The molecules were inserted in a dodecanethiol self-assembled monolayer. Their apparent height above the dodecanethiol matrix was used as a measure of the electronic coupling strength corresponding to S and Se, respectively. We show that the insertion behaviors of the two molecules are qualitatively the same, and that Se provides systematically a better coupling link than S whatever the tunneling conditions.

Characterization of self-assembled alkanethiol monolayers using a low-current scanning tunneling microscope

A low-current scanning tunneling microscope (STM) was used to image ordered alkanethiol self-assembled monolayers (SAMs) on Au (111) surface. The effects of the tunneling current on the STM images of both decanethiol and dodecanethiol SAMs were studied. At low current, with the tip above the sample surface, domain boundaries and ordered alkanethiol molecules can be seen clearly. When the tunneling current was increased, destructive STM images of decanethiol and dodecanethiol SAMs were observed. However, when the tunneling current was reduced back to the original low values, the ordered structures of alkanethiol molecules reappeared. This reversible process suggests that there is no rearrangement in the sulfur head groups under different tunneling current conditions. In this article, we present a model which supports the important role of the alkyl chain in the STM imaging of SAMs. In our model, the medium conductance is a key factor.

Controlling the formation of Au nanoparticles using functionalized molecular buffer layers

Nanoparticles of gold are produced by evaporation of gold atoms onto functionalized self-assembled monolayers (SAMs). SAMs of dodecanethiol (DT) and mercaptoundecanoic acid (MUA) on Au(1 1 1) are used as substrates and the gold particles are imaged with a scanning tunneling microscope. Gold atoms evaporated on DT monolayers diffuse rapidly to the interface between the molecular layer and the gold substrate where they form two-dimensional islands of 1 ML in height. In the case of monolayers of MUA, the strong interaction between evaporated gold and the acid group of the molecule gives rise to effective pinning of the gold atoms on top of the molecular layer and leads to the growth of three-dimensional gold nanocrystals.

Hydrophobic Organization of Monolayer-Protected Au Clusters on Thiol-Functionalized Au(111) Surfaces

The present study explains a novel hydrophobic organization of 4.8 ± 0.5 nm core diameter Au clusters in contrast to the organization using covalent and electrostatic interactions where specific bifunctional molecules are used. This unique method of organization is demonstrated using quartz crystal microbalance (QCM), UV−vis spectroscopy, cyclic voltammetry, X-ray photoelectron spectroscopy, X-ray diffraction, and I−V measurements. QCM results show a slow attainment of saturation coverage (1010 clusters/cm2) of Au nanoclusters on the self-assembled monolayer (SAM) functionalized substrate, and the equilibrium constant (Keq) is three times less compared to that for the monolayer formation using dodecanethiol. The electronic and optical properties (e.g., surface plasmon band ∼525 nm) of these films show that the Au colloids maintain their individual character without fusion to larger units, and the current−voltage behavior shows nonlinearity. X-ray photoelectron spectra of the functionalized gold surface treated with monolayer-protected Au clusters (MPCs) reveal that S 2p shows a 0.2 eV shift compared to that of a dodecanethiol SAM. Cyclic voltammetric studies confirm the redox accessibility of these MPCs with an E° value of 0.65 V (ΔE ≈ 60 mV, Ipa/Ipc ≈ 1) and a surface coverage of 2.15 × 10-9 mol/cm2 on the SAM-functionalized surface. The hydrophobic organization of MPCs on the functionalized gold substrate forms an ideal platform for examining the existing theoretical models associated with the adsorption of colloids and proteins, as well as cellular attachment and adhesion at solid surfaces.

Measurements of the potential dependence of electric field magnitudes at an electrode using fluorescent probes in a self-assembled monolayer

Abstract This report details the measurement of the dependence of the electric field magnitude in the electrical double layer on applied potential for a metal/organic thin film/electrolyte system. The self-assembled monolayers (SAMs) used in these experiments were formed from n-alkyl thiols on roughened Ag and roughened or smooth Au electrodes. The electric field magnitudes were calculated from the Stark shifts of a cationic fluorescent probe, an (aminostyryl)pyridinium derivative, that was immobilized in the SAM. Attention is focused on two gold-supported SAMs, one in which the probe was embedded within the SAM, and another in which the probe was external to the SAM. The composition of the monolayers and the orientation of the probe and n-alkyl thiol molecules within them were characterized using ex situ reflection-absorption Fourier transform infrared spectroscopy (RA-FTIR). For the case in which the probe was embedded within a dodecane thiol SAM, the change in electric field magnitude was determined to be 3×105 V cm−1 per volt change in the applied potential, while for the case of the probe outside of a propyl thiol SAM a value of 4×104 V cm−1 per volt change in the applied potential was obtained. These values are considerably smaller than expectations based on existing models for the potential distributions within such structures, and possible reasons for this behavior are discussed. The fluorescence intensity from the probe was observed to depend on potential, an effect that is also discussed. Measurements of the Stark shift versus applied potential at different ionic strengths provide a value of the potential of zero charge for these SAMs of −0.4 V versus SCE, in good agreement with previously measured values. The conditions under which fluorescent probes can be used effectively at metal electrodes are also discussed.

Manipulation and immobilization of alkane-coated gold nanocrystals using scanning tunneling microscopy

Colloidal nanoparticles, comprised of gold nanocrystals, of mean diameter 2.8 nm, coated with an insulating chemically stable self-assembled monolayer of dodecanethiol, have been prepared. Monolayers of nanoparticles have been physisorbed on highly oriented pyrolitic graphite, first by self-assembly, and second by assembly as Langmuir films and subsequent deposition. Nanoparticles have been self-assembled on gold, and immobilized by chemisorption, using decanedithiol during assembly as a linking molecule. Scanning tunneling microscope images of the monolayers are obtained. At high substrate–tip voltages, &amp;gt;0.6 V, the tip is able to climb above the nanoparticles. The tunneling is then a two-step event, tunneling from the substrate to the gold nanocrystal, and subsequently from the gold nanocrystal to the tip. At low voltage, 0.25 V, the Coulomb blockade prevents one extra electron occupying the gold nanocrystal. The tip cannot then climb above the nanoparticles. The theoretical threshold of the blockade is estimated from the nanoparticle size, and shown to be consistent with the observations. At low substrate–tip voltages, rastering of the tip sweeps the nanoparticles from the raster area (but not at high tip voltage). This result has not been described previously, and it is envisaged that it could be used to separate nanoparticles of differing size. However, immobilized isolated nanoparticles are not removed, but only pushed temporarily aside by the scanning tip.

Current Images of CdSe Colloidal Nanodots Observed by Conductive-tip Atomic Force Microscopy

AbstractWe have fabricated submonolayer-thick films of CdSe colloidal nanodots in order to investigate electronic properties of individual nanodots by conductive-tip atomic force microscopy (AFM). Topographic and current images of isolated single CdSe colloidal dots on single crystalline Au (111) surface which was covered with dodecanethiol self-assembled monolayer were obtained by AFM operating in contact mode with a conductive tip under appropriate bias voltages. In the current image, it is found that the dot regions have higher electric resistances due to tunneling resistance through the CdSe dots. We also found 10 nm-scale electric inhomogeneity around the dots, which may corresponds to the previously reported etch-pits of Au (111) surfaces formed during the deposition of the dodecanethiol molecules.

Oriented Nanocomposites of Ultrahigh-Molecular-Weight Polyethylene and Gold

Polymer nanocomposites were prepared by mixing ultrahigh-molecular-weight polyethylene and gold colloids coated with a self-assembled monolayer of dode-canethiol. Subsequently, these materials were oriented by solid state drawing which induced the formation of uniaxially oriented arrays of gold particles. As a result of the orientation of the gold arrays, the VIS-NIR spectra of the drawn composites strongly depended in polarized light on the angle between polarization direction and the orientation axis of the particle arrays, with shifts in the absorption maxima up to ca. 100 nm. It is assumed that these color shifts originated from the small dimensions of the dispersed metal phase in combination with their uniaxial orientation.

Covalent surface chemical modification of electrodes for cardiac pacing applications.

We report the covalent surface modification of active-fixation pacemaker electrodes with butanethiol or dodecanethiol self-assembled monolayers (SAMs) using a platinum or gold metal-thiolate bond (i.e., changing the chemical identity of the exposed metal electrode from metal to organic) in such fashion that (a) the surface is organic in functionality with lipophilic physicochemical characteristics, (b) a possible degradation product is gold (I)-alkanethiolate with putative anti-inflammatory actions, and (c) current density/electric field strength is increased. Superior acute and chronic pacing performance with dodecanethiol-modified, gold-coated, platinum-iridium alloy pacemaker electrodes was observed with inferential evidence of reduced inflammation and scar. This approach may have applicability in other areas of bioelectrodes with practical applications in clinical cardiology, surgery, neuroscience, and subcutaneous sensors.

Placement of conjugated oligomers in an alkanethiol matrix by scanned probe microscope lithography

We report the in situ replacement of conjugated molecules in an insulating matrix by scanned probe microscope lithography. High yield, programmable patterning of a self-assembled monolayer of dodecanethiol was performed by applying voltage pulses from a scanning tunneling microscope. Conjugated oligomers were observed to be subsequently chemisorbed onto the patterned sites.

Dynamic characteristics of adsorbed monolayers of 1-dodecanethiol on gold (111) terraces from in-situ scanning tunneling microscopy imaging

The dynamics of 1-dodecanethiol self-assembled monolayers (SAMs) on Au(111) produced by contacting pure thiol was followed by in situ STM at 298 K. Initially, these SAMs constituted a heterogeneous surface consisting of disordered and ordered adsorbate domains at terraces, step edges and pits, forming a p(6×1) superlattice which later changed to the ( 3 × 3 )R30° structure. Subsequently, the c(4×2) superlattice was also observed. Surface dynamics involves the coalescence and Ostwald ripening phenomena at pits occurring simultaneously with adsorbate ordering at different surface domains. The difference in surface mobility between adsorbate-free and 1-dodecanethiol-covered Au(111) can be explained taking into account both gold lattice relaxation due to adsorption and alkyl chain interaction at SAM. Data analysis in terms of clustering theory allowed us to conclude that a single mechanism is likely involved in the overall surface phenomena in which the mass transport is either a nonsteady state surface diffusion or an interface transfer along step edges as rate-determining step.

Determination of Thickness of a Self-Assembled Monolayer of Dodecanethiol on Au(111) by Angle-Resolved X-ray Photoelectron Spectroscopy

Determination of Thickness of a Self-Assembled Monolayer of Dodecanethiol on Au(111) by Angle-Resolved X-ray Photoelectron Spectroscopy

Structure and Thermodynamics of Self-Assembled Monolayers on Gold Nanocrystallites

Equilibrium structures and thermodynamic properties of dodecanethiol self-assembled monolayers on small (Au140) and larger (Au1289) gold nanocrystallites were investigated with the use of molecular dynamics simulations. Compact passivating monolayers are formed on the (111) and (100) facets of the nanocrystallites, with adsorption site geometries differing from those found on extended flat Au(111) and Au(100) surfaces, as well as with higher packing densities. At lower temperatures the passivating molecules organize into preferentially oriented molecular bundles with the molecules in the bundles aligned approximately parallel to each other. Thermal disordering starts at T ≳200 K, initiating at the boundaries of the bundles and involving generation of intramolecular conformational (gauche) defects which occur first at bonds near the chains' outer terminus and propagate inward toward the underlying gold nanocrystalline surface as the temperature is increased. The disordering process culminates in melting of...

Three-Dimensional Hexagonal Close-Packed Superlattices of Passivated Ag Nanocrystals

Abstract Silver nanocrystals passivated by dodecanethiol self-assembled monolayers were produced using an aerosol technique described in detail elsewhere [1]. Self-assembling passivated nanocrystal-superlattices (NCS's) involve self-organization into monolayers, thin films, and superlattices of size-selected nanoclusters encapsulated in a protective compact coating [2,3,4,5,6,7]. We report the preparation and structure characterization of three-dimensional (3-D) hexagonal close-packed Ag nanocrystal supercrystals from Ag nanocrystals of ˜4.5 nm in diameters. The crystallography of the superlattice and atomic core lattices were determined using transmission electron microscopy (TEM) and high-resolution TEM. SEM was used to image the nanocrystal superlattices formed on an amorphous carbon film of an TEM specimen grid (fig. la). The superlattice films show well shaped, sharply faceted, triangular shaped sheets. Figure lb depicts numerous Ag nanocrystal aggregates uniformly distributed over the imaging region. Inset in this figure is an enlargement of the boxed region at the edge of a supercrystal typifying the ordered nanocrystal packing.