Dynamics of HCl collisions with hydroxyl- and methyl-terminated self-assembled monolayers

Molecular beam scattering techniques are used to explore the energy exchange and thermal accommodation efficiencies of HCl in collisions with long-chain OH- and CH(3)-terminated self-assembled monolayers (SAMs) on gold. Upon colliding with the nonpolar methyl-terminated SAM, HCl (E(i) = 85 kJ/mol) is found to transfer the majority, 83%, of its translational energy to the surface. The extensive energy loss for HCl helps to bring the molecules into thermal equilibrium with the monolayer. Specifically, 72% of the HCl approaches thermal equilibrium prior to desorption. For the molecules that do not thermally accommodate, but scatter after an impulsive collision with the surface, the final translational energy is observed to be directly proportional to the surface temperature as the thermal surface energy and gas translational energy exchange during the collision. For the OH-terminated SAM, the impulsively scattered HCl escapes from the surface with slightly more average energy. The rigid nature of the OH-terminated SAM is due to the extended intra-monolayer hydrogen-bonding network that restricts some of the low-energy modes of the surface. However, despite the rigid nature of this system, the extent of thermal accommodation for HCl on these two surfaces is remarkably similar. It appears that the potential energy well between the impinging HCl and the polar surface groups is sufficient enough to trap HCl molecules that would otherwise scatter impulsively from this rigid SAM.     

Oxidation states and CO ligand exchange kinetics in a self-assembled monolayer of a triruthenium cluster studied by in situ infrared spectroscopy

Oxidation states and CO ligand exchange kinetics in a self-assembled monolayer (SAM) of an oxo-centered triruthenium cluster [Ru(3)(mu3-O)(mu-CH3COO)6(CO)(L1)(L2)] (L1 = [(NC5H4)CH2NHC(O)(CH2)10S-]2, L2 = 4-methylpyridine) have been extensively investigated on the surface of a gold electrode in aqueous and nonaqueous solutions. The SAM exhibits three consecutive one-electron transfers and four oxidation states, which have been characterized by electrochemistry, in situ infrared spectroscopy, and in situ sum frequency generation (SFG) vibrational spectroscopy measurements. The original electron-localized state of the Ru cluster center was changed to electron delocalization states by oxidation or reduction of the central Ru ions. These changes are revealed by the IR absorptions of the CO ligand and the bridging acetate ligands of the triruthenium cluster in the SAM. The IR absorptions of the two kinds of ligands are strongly dependent on the oxidation state of the Ru cluster center. One-electron oxidation of the central Ru ion in the SAM triggers a CO ligand liberation process. Solvent molecules may then occupy the CO site to result in a CO-free SAM. One-electron reduction of this CO-free SAM in a CO-saturated solution leads to re-coordination of the CO ligand into the SAM. Both processes can be precisely controlled by tuning the electrode potential. The kinetics of the CO exchange cycle in the SAM, including liberation and coordination, has been investigated by in situ IR and SFG measurements for the first time. The CO exchange cycle is significantly dependent on the temperature. The reaction rate greatly decreases with decreasing solution temperature, which is an important factor in the CO ligand exchange process. The activation energies of both CO liberation and coordination have been evaluated from the reaction rate constants obtained at various temperatures.     

Direct application strategy to immobilise a thioctic acid self-assembled monolayer on a gold electrode

Immobilisation of a self-assembled monolayer (SAM) onto an electrode surface is often achieved by immersing it in a solution for over 24 h. A biological or biologically derived recognition component can then be linked to the SAM in fabricating a biosensor. This time consuming immobilisation step can be a drawback in biosensor development, especially when repeated preparations of the biosensor are required. In this work, an alternative immobilisation strategy involving the direct application of a known quantity of the ethanolic solution of the alkanethiol, thioctic acid, on a gold electrode surface was studied. The solution was left to dry at room temperature for approximately 20 min. Comparable results including the relative percentage decrease in double layer capacitance, the surface coverage and the percentage of binding to the bacterial protein, Protein A, were obtained relative to those obtained with SAM formed by the immersion method. Shewhart’s statistical analysis technique was applied to examine the stability in terms of the relative percentage decrease in double layer capacitance. In these tests, within 99.7% confidence control limits, only a 1% deterioration was observed over a 3-month period. Therefore, all these results have demonstrated that the direct application method yields a stable thioctic acid SAM on a gold electrode surface with characteristics similar to those obtained with an immersion method. However, formation of a SAM using direct application can be achieved within a significantly shorter period of time compared to immersion method.     

Magnetic and Electrochemical Properties of a TEMPO‐Substituted Disulfide Diradical in Solution,in the Crystal,and on a Surface

A study of the magnetic and electrochemical properties of a TEMPO‐substituted disulfide diradical in three different environments was carried out: in solution, in the crystal, and as a self‐assembled monolayer (SAM) on an Au(111) substrate, and the relationship between them was explored. In solution, this flexible diradical shows a strong spin‐exchange interaction between the two nitroxide functions that depends on the temperature and solvent. Structural, dynamic, and thermodynamic information has been extracted from the EPR spectra of this dinitroxide. The magnetic interactions in the crystal include intra‐ and intermolecular contributions, which have been studied separately and shown to be antiferromagnetic in both cases. Finally, we demonstrate that both the magnetic and electrochemical properties are preserved upon chemisorption of the diradical on a gold surface. The resulting SAM displayed anisotropic magnetic properties, and angle‐resolved EPR spectra of the monocrystal allowed a rough determination of the orientation of the molecules in the SAM.     

Interface chemistry and molecular interactions of phosphonic acid self-assembled monolayers on oxyhydroxide-covered aluminum in humid environments

Barrier properties of self-assembled octadecylphosphonic acid (ODPA) monolayers on plasma-modified oxyhydroxide-covered aluminum surfaces were analyzed by means of in situ photoelastic modulated infrared reflection absorption spectroscopy (PM-IRRAS). The surface hydroxyl density prior to ODPA adsorption was increased by means of a low-temperature H(2)O-plasma treatment. Adsorption isotherms of H(2)O on ODPA self-assembled monolayer (SAM) modified surfaces in comparison to bare oxide covered aluminum surfaces showed that the ODPA SAM leads to a strongly reduced amount of adsorbed water based on the inability of water to form hydrogen bonds to the low-energy aliphatic surface. However, the ODPA SAM covered surfaces did not show a significant inhibition of the H(2)O/D(2)O isotope exchange reaction between the D(2)O gas phase and the hydroxyl groups of the aluminum oxyhydroxide film, as the interfacial layer between the ODPA SAM and the metal substrate, while the interfacial phosphonate group as well as the orientation of the SAM is not affected by the adsorption of water. It can be followed that the strong adhesion promoting and high corrosion resistances of organophosphonate monolayers on oxyhydroxide-covered aluminum is a result of the strong acid-base interaction of the phosphonate headgroup with the Al ions in the oxyhydroxide film, even in the presence of high interfacial water activity and the molecular interactions of the aliphatic chains. However, the barrier effect of such monolayers on the transport of water is negligible.     

Reaction Mechanisms in Heterogeneous Catalysis; C?H Activation as a Case Study

Heterogeneous catalysis is changing from an empirical art to an exact science. The various methods for the analysis of solids and surfaces, constantly refined by materials science and surface science, seem to be almost unlimited. The increasing availability of atomic resolution microscopy as well as synchrotron radiation allows the characterization of catalyst particles, surface structures, surface processes and surface intermediates. We have learned to determine the surface structure sensitivity of catalytic reactions. Thermodynamic and kinetic data of catalytic reactions are now determined routinely. Isotopic exchange and labeling experiments provide information about reactant-catalyst interactions. How much have we learned through these techniques about the nature or mechanism of heterogeneously catalyzed reactions? The following article attempts to summarize the progress and the problems encountered in mechanistic studies of C? H bond formation and activation in a hydrogen atmosphere as an example for the present state of the understanding of reaction mechanisms in heterogeneous catalysis.     

Self-assembled monolayer of organic iodine on a Au surface for attachment of redox-active metal clusters

The attachment of a bifunctional iodo-organo-phosphinate compound to gold (Au) surfaces via chemisorption of the iodine atom is described and used to chelate a redox-active metal cluster via the phosphinate group. XPS, AFM, and electrochemical measurements show that (4-iodo-phenyl)phenyl phosphinic acid (IPPA) forms a tightly bound self-assembled monolayer (SAM) on Au surfaces. The surface coverage of an IPPA monolayer on Au was quantified by an electrochemical method and found to be 0.40 +/- 0.03 nmol/cm2, roughly corresponding to 0.4 monolayers. We show that the Au/IPPA SAM, but not the underivatized Au, adsorbs Mn4O4(Ph2PO2)6 from solution by a phosphinate exchange reaction to yield Au/IPPA/Mn4O4(Ph2PO2)5 SAM. The resulting SAM is firmly bound and not removed by sonication, as confirmed by manganese XPS (Mn 2p1/2) and by AFM. Electrochemistry confirms that Mn4O4(Ph2PO2)6 is anchored on the Au/IPPA surface and that redox chemistry can be mediated between the electrode and the surface-attached complex. Mn4O4(Ph2PO2)6 contains the reactive Mn4O46+ cubane core, a redox-active bioinspired catalyst.     

Gas‐Phase Thermodynamics as a Validation of Computational Catalysis on Surfaces: A Case Study of Fischer–Tropsch Synthesis

Density functional theory has become a valuable tool to study surface catalysis. However, due to the scarcity of clean and reliable experimental data on surfaces, the theoretical methods employed to explore heterogeneous catalytic mechanisms are usually less well validated than those for gas‐phase reactions. We argue herein that gas‐phase reactions and the corresponding surface reactions are related through the Born–Haber cycle and computational catalysis on surfaces will be less meaningful if gas‐phase behavior cannot first be suitably determined. In this contribution, we have constructed a set of gas‐phase reactions relevant to the Fischer–Tropsch synthesis as a case study. With this set, we have tested the validity of the widely used PBE and B3LYP functionals and found that neither of them are capable of describing all kinds of gas‐phase reactions properly, such that some surface reactions may be biased falsely against the others. Significantly, XYG3, which is a double‐hybrid functional that includes Hartree–Fock‐like exchange and many‐body perturbation correlation effects, presents a significant improvement for all of the gas‐phase reactions, holding promise for further development for surface catalysis.     

Surface modification and functionalization through the self-assembled monolayer and graft polymerization

The modification of a surface at the molecular level with precise control of the building blocks generates an integrated molecular system. This field has progressed rapidly in recent years through the use of self-assembled monolayer (SAM) interfaces. Recent developments on surface-initiated chemical reactions, functionalization, and graft polymerization on SAM interfaces are emphasized in the present review. A number of surface modifications by grafting are reviewed. The grafting of polyaniline on a glass surface, previously modified with a silane self-assembled monolayer (SAM), is examined in detail for both planar and 3-D systems, such as fibers, nanoparticles, and even polymer patterned surfaces. We also discuss the graft polymerization of water-soluble polymers on the surface of silicon nanoparticles, which generate stable aqueous colloidal solutions and have numerous applications. Finally, we compare and review some surface-modification techniques on the surfaces of polymers, such as two-solvent entrapment, polymer blending, and chemical grafting, which improve their biocompatibility.     

Preferential adsorption of amino-terminated silane in a binary mixed self-assembled monolayer

The composition and structure of a binary mixed self-assembled monolayer (SAM) of 3-aminopropyltriethoxysilane (APS, NH(2)(CH(2))(3)Si(OCH(2)CH(3))(3)) and octadecyltrimethoxysilane (ODS, CH(3)(CH(2))(17)Si(OCH(3))(3)) on a silicon oxide surface have been characterized by water contact-angle measurements, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy. XPS demonstrated that APS in the mixed SAM is significantly enriched in comparison to that in solution, indicating the preferential adsorption of APS during the SAM formation. AFM observations showed that the mixed SAM becomes rougher. SFG revealed that the coadsorption of APS induced a conformation disordering in the ODS molecules present in the mixed SAM. The surface enrichment of APS has been explained in terms of differences in the surface adsorption rates of the two components as well as in the self-congregation states of APS molecules in the bulk solution. Furthermore, the structure of the water molecules on the mixed SAM surface in contact with the aqueous solutions at different pH's has also been studied. The results indicate that the mixed-SAM modified surface is positively charged at pH < 5 and negatively charged at pH > 7.     

A simple method for controlled immobilization of proteins on modified SAMs

This paper describes a method for modifying self-assembled monolayers (SAMs) with the nitrilotriacetic acid (NTA) group for subsequent immobilization of hexahistidine tagged proteins. The method has two important improvements over previous ones; firstly it avoids the need to carry out a complex synthesis of the chelator alkanethiols prior to deposition because the reactions are performed in situ on a preassembled SAM. This in situ approach also avoids phase segregation of alkanethiols with different functional groups, especially bulky ones such as NTA and tri(ethylene glycol), since a simple SAM is employed as the starting material. The approach reported here uses mercaptohexadecanoic acid to form a well-ordered homogeneous carboxyl-terminated SAM on a gold surface. The carboxyl group was then condensed with an NTA derivative containing an amino group to form a peptide bond. The product is a surface that, after chelating Ni(2+) ions, binds histidine tagged proteins. The loading of NTA groups can be controlled by choice of reaction conditions thereby removing the need for a second alkanethiol to dilute the surface density of chelator groups and prevent molecular crowding. Both factors allow rapid attainment of optimal protein loading. Fluorescence imaging demonstrated that (His)(6) enhanced green fluorescent protein was reversibly immobilized and importantly, was functional on the surface. Furthermore, data from surface plasmon resonance, cyclic voltammetry and fluorescence spectrometry provided additional information on the specific and reversible immobilization of (His)(6) proteins on the NTA-modified SAM surface.     

A new data management concept for AES and SAM

The problem of interchangeability of surface analytical data is gaining increasing importance in multi-method surface analysis. There are various surface analytical instruments in different laboratories on several automation levels. For these instruments, which are controlled by computer systems working with company-specific software under various operating systems, a standardised data format is necessary to allow an exchange of data. Therefore, we are developing a software package for the transfer, archiving and editing of surface analytical data called MAXMIND (management and exchange of method independent data), which is based on public domain software products. It allows the storage of all available and necessary information on experimental conditions and all parameters specific for a number of analytical techniques such as AES, SAM, XPS, SIMS, STM and AFM. Additionally, all data concerning the sample-preparation and the state of the instrument are included in order to allow a well-founded evaluation of the data and improved reproducibility of the experiment. This concept is demonstrated in detail for the two Auger techniques AES (Auger Electron Spectroscopy) and SAM (Scanning Auger Microscopy).Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Orthogonally reactive SAMs as a general platform for bifunctional silica surfaces

We report the synthesis and self-assembly of azide and amine trimethoxysilanes that result in mixed monolayers on silica. The amine and azide functional groups can be independently reacted with acid chlorides and terminal alkynes, respectively. Consequently, these orthogonally reactive monolayers represent a general starting point for making bifunctional surfaces. Using X-ray photoelectron spectroscopy, we determined the azide/amine surface ratio as well as the reactivity of the azide and amine functional groups in the mixed self-assembled monolayer (SAM). Significantly, the surface azide/amine ratio was much lower than the azide/amine ratio in the self-assembly mixture. After determining the self-assembly mixture composition that would afford 1:1 azide-amine mixed monolayers, we demonstrated their subsequent functionalization. The resulting bifunctional surface has a similar functional group ratio to the azide/amine precursor SAM demonstrating the generality of this approach.     

Thermal Effect on Positive Patterned Self-Assembled Monolayer Grown from a Droplet of Alkanethiol

Atomic force microscope technique is widely used for the spatial narrow deposition of molecules inside the bare space of preexisting self-assembled monolayer (SAM) matrix. Using molecular dynamics simulation, we studied the formation of positively patterned SAM from a globule of 1-octadecanethiol (ODT) on predesigned SAM matrix of 1-dodecanethiol (DDT) and effect of temperature on it. The alkyl chains of ODT SAM were densely packed and ordered by means of chemisorption through sulfur atoms. The circular SAM of ODT contained defects due to the molecules those were standing upside down or trapped inside ODT SAM. We found that with the increase of temperature, these defects moved out by flipping of inverted ODT molecules or building spaces to be adsorbed on Au surface. The ODT molecules on the top of the pile of stable circular SAM or those are upside down and trapped disperse in a unique fashion namely serial pushing through which molecules firstly make a free space to enter inside the adsorbed thiol molecules and then push neighboring molecules to get enough space to be adsorbed on the gold surface. The stability of ODT SAM was confirmed by analyzing different structural properties such as tilt angle, tilt orientation. and backbone orientation. We also calculated the diffusion coefficient of the ODT molecules which were on the top of SAM island. © 2019 Wiley Periodicals, Inc.     

Chemical stability of nonwetting, low adhesion self-assembled monolayer films formed by perfluoroalkylsilanization of copper

A self-assembled monolayer (SAM) has been produced by reaction of 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS) with an oxidized copper (Cu) substrate and investigated by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, and contact angle measurement. FFM showed a significant reduction in the adhesive force and friction coefficient of PFMS modified Cu (PFMS/Cu) compared to unmodified Cu. The perfluoroalkyl SAM on Cu is found to be extremely hydrophobic, yielding sessile drop static contact angles of more than 130 degrees for pure water and a "surface energy" (which is proportional to the Zisman critical surface tension for a Cu surface with 0 rms roughness) of 14.5 mJm2(nMm). Treatment by exposure to harsh conditions showed that PFMS/Cu SAM can withstand boiling nitric acid (pH=1.8), boiling water, and warm sodium hydroxide (pH=12, 60 degrees C) solutions for at least 30 min. Furthermore, no SAM degradation was observed when PFMS/Cu was exposed to warm nitric acid solution for up to 70 min at 60 degrees C or 50 min at 80 degrees C. Extremely hydrophobic (low surface energy) and stable PFMS/Cu SAMs could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for antiwetting, low adhesion surfaces or dropwise condensation on heat exchange surfaces.     

Microscopic wetting of self-assembled monolayers with different surfaces: a combined molecular dynamics and quantum mechanics study

Molecular dynamics simulations are used to study the micronature of the organization of water molecules on the flat surface of well-ordered self-assembled monolayers (SAMs) of 18-carbon alkanethiolate chains bound to a silicon (111) substrate. Six different headgroups (-CH(3), -C═C, -OCH(3), -CN, -NH(2), -COOH) are used to tune the character of the surface from hydrophobic to hydrophilic, while the level of hydration is consistent on all six SAM surfaces. Quantum mechanics calculations are employed to optimize each alkyl chain of the different SAMs with one water molecule and to investigate changes in the configuration of each headgroup under hydration. We report the changes of the structure of the six SAMs with different surfaces in the presence of water, and the area of the wetted surface of each SAM, depending on the terminal group. Our results suggest that a corrugated and hydrophobic surface will be formed if the headgroups of SAM surface are not able to form H-bonds either with water molecules or between adjacent groups. In contrast, the formation of hydrogen bonds not only among polar heads but also between polar heads and water may enhance the SAM surface hydrophilicity and corrugation. We explicitly discuss the micromechanisms for the hydration of three hydrophilic SAM (CN-, NH(2)- and COOH-terminated) surfaces, which is helpful to superhydrophilic surface design of SAM in biomimetic materials.     

Soft-UV photolithography using self-assembled monolayers

We report thiol-on-gold self-assembled monolayers (SAMs) that can be photodeprotected using soft UV irradiation (lambda = 365 nm) to yield CO(2)H functionalized surfaces complementing those reported previously, which yielded NH(2) functionalized surfaces. The photolysis of these SAMs were monitored using a combination of surface sensitive techniques. In the SAM environment the photodeprotection yields are lower than those obtained for equivalent reactions in dilute solution. The protected carboxylic acids SAMs are shown to have a low yield approximately 50% due to competing photoreduction reactions of the nitro group. The results from infrared studies show that, as the photolysis progresses, the long chain protected residues reorganize and shield the functional COOH groups, thereby reducing the hydrophilic character of the surface.     

Olefin cross-metathesis of a vinyl-terminated self-assembled monolayer (SAM) on Au(111): electrochemical study using a ferrocenyl redox center

Self-assembled thiol monolayers bound to single-crystal Au(111) surfaces containing a terminal olefin have been prepared and used to monitor electrochemically the cross-metathesis (CM) between the surface and an olefin-terminated ferrocenyl (Fc) derivative from solution over time. Mixed SAM surfaces were prepared by first adsorbing a diluent for 2 days followed by the olefinic alkanethiol for known adsorption time intervals; three diluents of varying length were used. The oxidation peak areas from the voltammetry show the CM reaction yields a maximum amount of product at 100-150 min. Beyond this time, thiol desorption is apparent and the Fc oxidation peaks diminished. A kinetic simulation of the interfacial reactions involving CM and desorption reactions are described and aided in the interpretation of the voltammetric responses. The length of the diluent and the coverage of surface olefins were important factors in limiting undesirable self-CM reactions on the surface, and a model of the relationship between the diluent and surface concentration of olefin is described. This study shows that attention to monolayer formation and reaction conditions are important parameters when maximizing CM yields on surfaces.     

Photochemical patterning of a self-assembled monolayer of 7-diazomethylcarbonyl-2,4,9-trithiaadmantane on gold films via Wolff rearrangement

Photolithographic attachment of functional organic molecules via ester or amide linkages to self-assembled monolayers (SAMs) on gold thin films was achieved by employing a novel photoreactive surface anchor, 7-diazomethylcarbonyl-2,4,9-trithiaadmantane. The photoreactive SAM was prepared by the spontaneous physical adsorption of the photoreactive surface anchor onto gold surfaces. The alpha-diazo ketone moiety of the SAM was found to display the classical Wolff rearrangement reactivity to produce a ketene intermediate on the exposed area. Organic molecules such as alcohols and amines can thus be attached to the gold surfaces selectively by the facile in situ formation of ester or amide linkages. The structure and reactivity of the photoreactive surface anchor were characterized by real-time FT-IR, fluorescence, and polarization modulation infrared reflectance absorption spectroscopy (PM-IRRAS). The Wolff rearrangement reactivity of the SAM suggested that a "surface-isolated" carbonylcarbene may be generated when the SAM was exposed to 255-nm irradiation.