Ion scattering and X-ray photoelectron spectroscopy of copper overlayers vacuum deposited onto mercaptohexadecanoic acid self-assembled monolayers

Metal overlayers deposited in vacuum onto self-assembled monolayer (SAM) systems serve as models for more complex metalized polymers. Metals (M) deposited onto SAMs with different organic functional end groups exhibit a wide range of behavior, ranging from strong chemical interactions with the end group to complete penetration of the metal through the SAM. In this work, we have characterized the interactions of Cu with the ---COOH of mercaptohexadecanoic acid (MHA, HOOC(CH₂)₁₅SH) SAMs self assembled on gold films by using X-ray photoelectron spectroscopy (XPS) to examine the chemical interactions, and a combination of XPS and ion scattering spectroscopy (ISS) to deduce the growth mode and penetration rate of the deposited Cu. We found that submonolayer amounts of Cu react with HOOC, whereas the rest of the Cu remains metallic and penetrates beneath the SAM surface to the SAM  Au interface. Considerable amounts of Cu (5 nm or more) will penetrate beneath the SAM layer, which is ca. 2.5 nm thick, despite the submonolayer presence of Cu at the SAM surface. The penetration rate depends strongly on the Cu deposition rate. Depositing copper onto MHA at 220 K or less, or using faster Cu deposition rates, results in slower or even completely suppressed penetration of the Cu through the SAM layer, whereas exposure to X-rays greatly enhances the penetration rate of large amounts of Cu through the SAM layer. The reacted copper is, based on the XPS 2p and LMM peaks, in the +2 oxidation state, but cannot be identified with a simple, stoichiometric oxide such as Cu₂O, CuO, or Cu (OH)₂.     

Spectroscopic fingerprints of amine and imide functional groups in self-assembled monolayers.

The electronic structures of naphthalene tetracarboxylic diimide (NTCDI) and 1,4-bis(4,6-diamino-1,3,5-triazin-2-yl)benzene (BDG) monolayer assemblies grown on Au(111) are investigated by photoemission spectroscopy, X-ray absorption, and density functional theory. The different spectroscopic features in the absorption and core-level photoemission spectra are understood in terms of contributions from different core and molecular levels at N- and O-atom sites. This study provides clear spectroscopic fingerprints for amine and imide functional end groups, which drive the self-organization process in a number of planar, pi-conjugated molecular structures.     

Dipole-dipole interactions and the structure of self-assembled monolayers

The structure of self-assembled monolayers (SAMs) on the gold (111) surface is still a matter of debate despite a considerable experimental and theoretical effort. We address the problem from a new perspective, studying the influence of electrostatic interactions on the degree of disorder in COOH-terminated SAMs. We show that the HS(CH2)(n-1)COOH molecules carry two dipole moments associated with their head- and tail-groups. Depending on the coupling of these dipole moments along the molecules, the structure of the COOH-SAMs either resembles the structure of the corresponding alkanethiol monolayers (short molecules, strong dipole coupling) or shows a more complex behavior (long molecules, weak dipole coupling). In the latter case, the monolayer exhibits a crystalline-like order with respect to the hydrocarbon chains and a high degree of disorder with respect to the carboxylic head-groups. These results resolve the controversy of experimental data on the degree of order in COOH-monolayers with near-edge X-ray absorption fine structure spectroscopy (Himmel, H.-J.; Weiss, K.; J?ger, B.; Dannenberger, O.; Grunze, M.; W?ll, Ch. Langmuir 1997, 13, 4943), probing the tail-groups, showing that the monolayer is largely disordered, and the infrared data (Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558) on the C-H stretching modes suggesting a crystalline-like order of the hydrocarbon chains.     

Electrochemically deposited palladium as a substrate for self-assembled monolayers

The vast majority of reports of self-assembled monolayers (SAMs) on metals focus on the use of gold. However, other metals, such as palladium, platinum, and silver offer advantages over gold as a substrate. In this work, palladium is electrochemically deposited from PdCl2 solutions on glassy carbon electrodes to form a substrate for alkanethiol SAMs. The conditions for deposition are optimized with respect to the electrolyte, pH, and electrochemical parameters. The palladium surfaces have been characterized by scanning electron microscopy (SEM) and the surface roughness has been estimated by chronocoulometry. SAMs of alkane thiols have been formed on the palladium surfaces, and their ability to suppress a Faradaic process is used as an indication for palladium coverage on the glassy carbon. The morphology of the Pd deposit as characterized by SEM and the blocking behavior of the SAM formed on deposited Pd delivers a consistent picture of the Pd surface. It has been clearly demonstrated that, via selection of experimental conditions for the electrochemical deposition, the morphology of the palladium surface and its ability to support SAMs can be controlled. The work will be applied to create a mixed monolayer of metals, which can subsequently be used to create a mixed SAM of a biocomponent and an alkanethiol for biosensing applications.     

Immobilization of acetylcholinesterase in lipid membranes deposited on self-assembled monolayers

Human red blood cell acetylcholinesterase was incorporated into planar lipid membranes deposited on alkanethiol self-assembled monolayers (SAMs) on gold substrates. Activity of the protein in the membrane was detected with a standard photometric assay and was determined to be similar to the protein in detergent solution or incorporated in lipid vesicles. Monolayer and bilayer lipid membranes were generated by fusing liposomes to hydrophobic and hydrophilic SAMs, respectively. Liposomes were formed by the injection method using the lipid dimyristoylphosphatidylcholine (DMPC). The formation of alkanethiol SAMs and lipid monolayers on SAMs was confirmed by sessile drop goniometry, ellipsometry, and electrochemical impedance spectroscopy. In this work, we report acetylcholinesterase immobilization in lipid membranes deposited on SAMs formed on the gold surface and compare its activity to enzyme in solution.     

Intermolecular interactions in self-assembled monolayers of tetrathiafulvalene derivatives

The elaboration of mixed self-assembled monolayers (SAMs) of tetrathiafulvalene derivatives allows the modulation of intermolecular interactions and provides evidence of segregated distribution of redox centers.     

Triboelectrification between smooth metal surfaces coated with self-assembled monolayers (SAMs)

Using a modified surface forces apparatus, we have simultaneously measured the friction and triboelectrification between both similar and dissimilar molecularly smooth hexadecanethiol-coated metal surfaces on mica substrates. On shearing dissimilar surfaces, the tribocurrent increases dramatically as the load or pressure is increased, with large fluctuations about the mean. Neither charge transfer nor fluctuations are observed when the symmetric surfaces are sheared against each other. We also find that the type of friction, i.e., stick-slip or smooth sliding, the load and friction force, the sliding distance, and recent previous history have additional fine influences on the triboelectrification. Our results suggest that frictional dissipation induces electron-hole formation and charge transfer between two shearing surfaces due to molecular-level roughness and defects and local dielectric constant changes, giving rise to the observed tribocurrents.     

Electroactive mixed self-assembled monolayers: Lateral interactions model updated to interactions between redox and non-redox species

The lateral interactions model, dedicated to random and non-random distributed electroactive species on redox responsive self-assembled monolayers (SAM), was extended to interactions between redox and non-redox species. This approach supports an unusual result achieved in the field of electrochemical transduction without covalent links between redox and complexant units in mixed SAM.     

Affinity interactions between phenylboronic acid-carrying self-assembled monolayers and flavin adenine dinucleotide or horseradish peroxidase

A method is provided for the recognition of glycated molecules based on their binding affinities to boronate-carrying monolayers. The affinity interaction of flavin adenine dinucleotide (FAD) and horseradish peroxidase (HRP) with phenylboronic acid monolayers on gold was investigated by using voltammetric and microgravimetric methods. Conjugates of 3-aminophenylboronic acid and 3,3'-dithiodipropionic acid di(N-hydroxysuccinimide ester) or 11-mercaptoundecanoic acid were prepared and self-assembled on gold surfaces to generate monolayers. FAD is bound to this modified surface and recognized by a pair of redox peaks with a formal potential of -0.433 V in a 0.1 M phosphate buffer solution, pH 6.5. Upon addition of a sugar to the buffer, the bound FAD could be replaced, indicating that the binding is reversible. Voltammetric, mass measurements, and photometric activity assays show that the HRP can also be bound to the interface. This binding is reversible, and HRP can be replaced by sorbitol or removed in acidic solution. The effects of pH, incubation time, and concentration of H(2)O(2) were studied by comparing the catalytic reduction of H(2)O(2) in the presence of the electron-donor thionine. The catalytic current of the HRP-loaded electrode was proportional to HRP concentrations in the incubation solution in the range between 5 microg mL(-1) and 0.1 mg mL(-1) with a linear slope of 3.34 microA mL mg(-1) and a correlation coefficient of 0.9945.     

Host-guest interactions at self-assembled monolayers of cyclodextrins on gold

We have developed synthesis routes for the introduction of short and long dialkylsulfides onto the primary side of alpha-, beta-, and gamma-cyclodextrins. Monolayers of these cyclodextrin adsorbates were characterized by electrochemistry, wettability studies, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and atomic force microscopy (AFM). The differences in thickness and polarity of the outerface of the monolayers were measured by electro-chemistry and wettability studies. On average about 70% of the sulfide moieties were used for binding to the gold, as measured by XPS. Tof-SIMS measurements showed that the cyclodextrin adsorbates adsorb without any bond breakage. AFM measurements revealed for beta-cyclodextrin monolayers a quasi-hexagonal lattice with a lattice constant of 20.6 A, which matches the geometrical size of the adsorbate. The alpha-cyclodextrin and gamma-cyclodextrin monolayers are less ordered. Interactions of the anionic guests 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS) and 2-(p-toluidinyl)naphthalene-6-sulfonic acid (2,6-TNS) and the highly ordered monolayers of heptapodant beta-cyclodextrin adsorbates were studied by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy. The SPR measurements clearly showed interactions between a beta-cyclodextrin monolayer and 1,8-ANS. Electrochemical impedance spectroscopy measurements gave high responses even at low guest concentrations (< or = 5 microM). The association constant for the binding of 1,8-ANS (K = 289,000 +/- 13,000M-1) is considerably higher than the corresponding value in solution. (Partial) methylation of the secondary side of the beta-cyclodextrin strongly decreases the binding.     

Investigation of the interactions between alkanethiol self-assembled monolayers and a liquid overlayer using impedance spectroscopy

We report on changes in the complex impedance response of a quartz crystal microbalance (QCM) that result from the growth of an alkanethiol monolayer on the electrodes of the device. The purpose of this work is to understand the evolution of the interactions between alkanethiol-gold monolayers and a liquid overlayer as a function of time after initial deposition by evaluating the position, shape, and linewidth of the impedance spectra associated with the monolayer formation. We relate the complex impedance response of the QCM to the mass and viscosity of the monolayer through an established equivalent circuit model. The data show the organization of alkanethiol SAMs occurs at approximately the same rate for aliphatic chain lengths in the range of C(9)-C(16), as long as the thiol is readily soluble in the solvent system used. Our data are consistent with SAM annealing being mediated by the sulfur-gold adsorption and desorption equilibrium. Additionally, we have found that examination of a C(18) SAM is limited by the deposition conditions and thiol solubility. Bulk deposition was confirmed visually and related to the evolution of the peak position and shape changes with deposition time.     

Structure and desorption energetics of ultrathin D2O ice overlayers on serine- and serinephosphate-terminated self-assembled monolayers

This paper reports on the structure and desorption dynamics of thin D2O ice overlayers (0.2-10 monolayers) deposited on serine- and serinephosphate- (with H+, Na+, Ca2+ counterions) terminated self-assembled monolayers (SAMs). The D2O ice overlayers are deposited on the SAMs at approximately 85 K in ultrahigh vacuum and characterized with infrared reflection absorption spectroscopy (IRAS). Reflection absorption (RA) spectra obtained at sub-monolayer D2O coverage reveal that surface modes, e.g. free dangling OD stretch, dominate on the serine SAM surface, whereas vibrational modes characteristic for bulk ice are more prominent on the serinephosphate SAMs. Temperature programmed desorption mass spectrometry (TPD-MS) and TPD-IRAS are subsequently used to investigate the energetics and the structural transitions occurring in the ice overlayer during temperature ramping. D2O ice (approximately 2.5 monolayers) on the serine SAMs undergoes a gradual change from an amorphous- to a crystalline-like phase upon increasing the substrate temperature. This transition is not as pronounced on the serine phosphate SAM most likely because of reduced mobility due to strong pinning to the surface. We show also that the energy of desorption for a sub-monolayer of D2O ice on serinephosphate SAM surfaces with a Na+ and Ca2+ counterions is equally high or even exceeds previously reported values for analogous high-energy SAMs.     

The relationship between threshold voltage and dipolar character of self-assembled monolayers in organic thin-film transistors

We report a quantitative study that describes and correlates the threshold voltage of low-voltage organic field-effect transistors with the molecular structure of self-assembled monolayer dielectrics. We have observed that the component of the dipole moment of such self-assembled molecules perpendicular to the surface correlates linearly with the threshold voltage shift in devices. The model was validated using three different organic semiconductors (pentacene, α,α'-dihexylsexithiophene, and fullerene-C(60)) on six different self-assembled monolayers. The correlation found can help optimize future devices, by tuning the dipole moments of the molecules that constitute the self-assembled monolayer.     

Plasma surface modification of organic materials: comparison between polyethylene films and octadecyltrichlorosilane self-assembled monolayers

Low-pressure low-frequency NH3 plasmas have been used for the surface modification of bulk polyethylene films and of octadecyltrichlorosilane (OTS) self-assembled monolayers deposited on oxidized silicon wafers. The incorporation of nitrogen-containing groups by the plasma treatment has been followed by contact angle measurements and by X-ray photoelectron spectroscopy. The surface degradation of the OTS monolayers due to plasma etching has been measured separately by optical ellipsometry with subnanometric accuracy. Our data show clear evidence for the existence of an optimum treatment time, yielding a high density of NH2 functional groups without significant variation of the structural features of the organic material. Self-assembled monolayers appear as excellent model systems to characterize the effects of plasma discharges on polyolefins. In particular, they allow testing the influence of molecular orientation, packing density, and crystallinity on the final results.     

pH-Dependent rectification in self-assembled monolayers based on electrostatic interactions

Asymmetric electrostatic interactions dependent on pH between the redox molecules and the terminal group on the top of the self-assembled monolayer (SAM) afford control of the electron transfer property of the SAM having the imidazole terminal group.     

Organization-induced charge redistribution in self-assembled organic monolayers on gold

The charge redistribution that occurs within dipolar molecules as they self-assemble into organized organic monolayer films has been studied. The extent of charge transfer is probed by work function measurements, using low-energy photoelectron spectroscopy (LEPS), contact potential difference (CPD), and X-ray photoelectron spectroscopy (XPS), with the latter providing fine details about the internal charge distribution along the molecule. In addition, two-photon photoelectron spectroscopy is applied to investigate the electronic structure of the adsorbed layers. We show that charge transfer acts to reduce the dipole-dipole interaction between the molecules but may either decrease or increase the molecule-to-surface dipole moment.     

Water in nanoconfinement between hydrophilic self-assembled monolayers

Molecular dynamics (MD) simulations of water confined to subnanometer thicknesses between carboxyl-terminated alkanethiol self-assembled monolayers (SAMs) on gold were performed to address conflicts in the literature on the structure and response of water in confinement. The amount of water was varied to yield submonolayer to bilayer structures. The orientation of the water is affected by the confinement, especially in the submonolayer case. We find that the diffusion coefficient decreases as the film becomes thinner and at higher pressures. However, in all cases studied, liquid diffusion is always found. At maximal suppression, the diffusion constant is 2 orders of magnitude smaller than the bulk value.     

Electrochemical stability of self-assembled alkylphosphate monolayers on conducting metal oxides

Alkylphosphate self-assembled monolayers (SAMs) were prepared on Nb-doped SrTiO(3) (Nb-STO) conducting metal oxide substrates. Unlike thiols on gold, the alkylphosphate SAMs on Nb-STO exhibited an electrochemical stability over a wide voltage range from -2 to 2 V. Cyclic voltammetry showed that the SAM modification inhibited the electrochemical activity of the underlying conducting substrate with an efficiency dependent on the chain length. Impedance spectroscopy showed that SAM-modified Nb-STO substrates have a significantly higher resistance than bare substrates.     

Directed electroless growth of metal nanostructures on patterned self-assembled monolayers

The directed placement of Cu nanostructures on surfaces has been studied using a combination of scanning probe lithography and electroless metal deposition onto nanopatterned SAMs of 16-mercaptohexadecanoic acid (16-MHA) on Au. In situ studies using nanoscale molecular gradients reveal how controlling the areal density of the 16-MHA molecules dictates the nucleation and growth of the metal nanostructures. The influence of controlling pattern line spacing and tip path on pattern feature fidelity is also discussed.     

Mimicking enzymes: cooperation between organic functional groups and metal ions in the cleavage of phosphate diesters

A series of ligands derived from the bis-2-pyridinylmethylamine structure, which bear either additional hydroxyl or aromatic amino groups, were prepared and their Zn(II) complexes were studied as catalysts for the cleavage of bis-p-nitrophenyl phosphate (BNP) and 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP) diesters. A comparative kinetic study indicated that the insertion of organic groups, capable of acting as nucleophiles or as hydrogen-bond donors, substantially increases the hydrolytic activity of the metal complex. Dissection of the effects of the individual groups revealed that the increase in reactivity can reach up to three orders of magnitude. The improved efficiency of the systems studied, combined with the benefits resulting from the low pK(a) value of the active nucleophile, result in an acceleration of the BNP cleavage at pH 7 of six orders of magnitude. The pH-dependent reactivity profiles follow a bell-shaped curve and the maximum reactivity is observed at pH 9. The mechanism of the reactions and the structure of the complexes were investigated in detail by means of kinetic analysis, NMR spectroscopy experiments, and theoretical calculations. The reactivity of the complexes that cleave HPNP closely resembles the reactivity observed for BNP, but the accelerations achieved are lower as a result of different reaction mechanisms.