Alkanethiol Self-assembled Monolayers Research Articles

Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes

Summary Surface and interfacial engineering of heterogeneous metal catalysts is effective and critical for optimizing selective hydrogenation for fine chemicals. By using thiol-treated ultrathin Pd nanosheets as a model catalyst, we demonstrate the development of stable, efficient, and selective Pd catalysts for semihydrogenation of internal alkynes. In the hydrogenation of 1-phenyl-1-propyne, the thiol-treated Pd nanosheets exhibited excellent catalytic selectivity (>97%) toward the semihydrogenation product (1-phenyl-1-propene). The catalyst was highly stable and showed no obvious decay in either activity or selectivity for over ten cycles. Systematic studies demonstrated that a unique Pd-sulfide/thiolate interface created by the thiol treatment was crucial to the semihydrogenation. The high catalytic selectivity and activity benefited from the combined steric and electronic effects that inhibited the deeper hydrogenation of C=C bonds. More importantly, this thiol treatment strategy is applicable to creating highly active and selective practical catalysts from commercial Pd/C catalysts for semihydrogenation of internal alkynes.

Toward understanding weak matrix effects in TOF SIMS

Chemical imaging methods, including imaging mass spectrometry, are increasingly used for the analysis of samples ranging from biological tissues to electronic devices. A barrier to wider adoption of imaging mass spectrometry is the presence of matrix effects which complicate quantitative analysis. Interactions between an analyte molecule and its surroundings (the “matrix”) can substantially alter both the yield and type of ions observed. Furthermore, such “intrinsic” effects can be confused with nonlinear response due to detector saturation and other instrument-related complications. As a result, quantitative analyses of time-of-flight secondary ion mass spectrometry (TOF SIMS) data that attempt to account for matrix effects are rare. The authors discuss analysis of such data using maximum a posteriori reconstruction based on physically motivated models, and present progress toward the quantitative extraction of chemical concentration profiles and component spectra in the presence of matrix effects, using mixed self-assembled alkanethiolate monolayers as a test system. The authors demonstrate that the incorporation of matrix effects to lowest order using a series-expansion approach is an effective strategy and that doing so provides improved quantitative performance in measuring surface compositions and can also yield information about interactions between species during the SIMS process.

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

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

Direct Mapping of Heterogeneous Surface Coverage in DNA-Functionalized Gold Surfaces with Correlated Electron and Fluorescence Microscopy

The characterization of biofunctionalized surfaces such as alkanethiol self-assembled monolayers (SAMs) on gold modified with DNA or other biomolecules is a challenging analytical problem, and access to a routine method is desirable. Despite substantial investigation from structural and mechanistic perspectives, robust and high-throughput metrology tools for SAMs remain elusive but essential for the continued development of these devices. We demonstrate that scanning electron microscopy (SEM) can provide image contrast of the molecular interface during SAM functionalization. The high-speed, large magnification range, and ease of use make this widely available technique a powerful platform for measuring the structure and composition of SAM surfaces. This increased throughput allows for a better understanding of the nonideal spatial heterogeneity characteristic of SAMs utilized in real-world conditions. SEM image contrast is characterized through the use of fluorescently labeled DNA, which enables correlative SEM and fluorescence microscopy. This allows identification of the DNA-modified regions at resolutions that approach the size of the biomolecule. The effect of electron beam irradiation dose is explored, which leads to straightforward lithographic patterning of DNA SAMs with nanometer resolution and with control over the surface coverage of specifically adsorbed DNA.

THE KINETICS OF THE AS GROWN AND ANNEALED SELF-ASSEMBLED MONOLAYER STUDIED BY FORCE SPECTROSCOPY

The growth of biological systems like DNA, peptides and proteins are accredited to the self-assembly processes from the molecular level to the nanoscale. The flawless immobilization of DNA on any surface is quite an important step to the development of DNA-based biosensors. The present paper reports the use of atomic force microscopy to determine the mechanical properties of the as grown and annealed self-assembled monolayer (SAM) as well as the mutated DNA immobilized on the SAM. The SAM of alkane thiol (16-mercapto-1-hexadecanol) was developed on Au surface, which was then annealed and analyzed for its structural and mechanical properties. The surface coverage, height and monolayer’s order was studied as a function of incubation time and annealing time. Excessive annealing led to the defragmentation and desorption of SAM structures due to breaking of hydrocarbon bonds. AFM was employed to determine the detach separation, pull-off and work of adhesion of the as grown and annealed SAM.

Wafer-scale bioactive substrate patterning by chemical lift-off lithography.

The creation of bioactive substrates requires an appropriate interface molecular environment control and adequate biological species recognition with minimum nonspecific attachment. Herein, a straightforward approach utilizing chemical lift-off lithography to create a diluted self-assembled monolayer matrix for anchoring diverse biological probes is introduced. The strategy encompasses convenient operation, well-tunable pattern feature and size, large-area fabrication, high resolution and fidelity control, and the ability to functionalize versatile bioarrays. With the interface-contact-induced reaction, a preformed alkanethiol self-assembled monolayer on a Au surface is ruptured and a unique defect-rich diluted matrix is created. This post lift-off region is found to be suitable for insertion of a variety of biological probes, which allows for the creation of different types of bioactive substrates. Depending on the modifications to the experimental conditions, the processes of direct probe insertion, molecular structure change-required recognition, and bulky biological species binding are all accomplished with minimum nonspecific adhesion. Furthermore, multiplexed arrays via the integration of microfluidics are also achieved, which enables diverse applications of as-prepared substrates. By embracing the properties of well-tunable pattern feature dimension and geometry, great local molecular environment control, and wafer-scale fabrication characteristics, this chemical lift-off process has advanced conventional bioactive substrate fabrication into a more convenient route.

Strong Hydrogen Bonds at the Interface between Proton-Donating and -Accepting Self-Assembled Monolayers on Au(111)

Hydrogen-bonding heterogeneous bilayers on substrates have been studied as a base for new functions of molecular adlayers by means of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), and density functional theory (DFT) calculations. Here, we report the formation of the catechol-fused bis(methylthio)tetrathiafulvalene (H2Cat-BMT-TTF) adlayer hydrogen bonding with an imidazole-terminated alkanethiolate self-assembled monolayer (Im-SAM) on Au(111). The heterogeneous bilayer is realized by sequential two-step immersions in solutions for the individual Im-SAM and H2Cat-BMT-TTF adlayer formations. In the measurements by AFM, a grained H2Cat-BMT-TTF adlayer on Im-SAM is revealed. The coverage and the chemical states of H2Cat-BMT-TTF on Im-SAM are specified by XPS. On the vibrational spectrum measured by IRAS, the strong hydrogen bonds between H2Cat-BMT-TTF and Im-SAM are characterized by the remarkably red-shifted OH stretching mode at 3140 cm-1, which is much lower than that for hydrogen-bonding water (typically ∼3300 cm-1). The OH stretching mode frequency and the adsorption strength for the H2Cat-BMT-TTF molecule hydrogen bonding with imidazole groups are quantitatively examined on the basis of DFT calculations.

On-Demand Modulation of Bacterial Cell Fates on Multifunctional Dynamic Substrates.

This paper reports unprecedented dynamic surfaces based on zwitterionic low-density self-assembled monolayers (LDSAMs) of alkanethiolates on gold, which integrate three interconvertible states-bacteria-adherable, bactericidal, and nonfouling states-through electrical modulations. The conformations of alkanethiolates were electrically modulated to generate zwitterionic, anionic, and cationic surfaces, which responded differently to bacteria and determined the fate of bacteria. Furthermore, the reversible switching of multifunctions of the surface was realized for killing bacteria and subsequently releasing dead bacteria from the surface. For practical application of our strategy, we examined the selective antibacterial effect of our surface for eradication of mycoplasma contaminants in contaminated mammalian cell cultures.

Alkanethiol self-assembled monolayer on copper polycrystalline thin films: Influence on resistivity

Thin polycrystalline copper films of low surface roughness and low electrical resistivity were deposited by physical vapor deposition onto mica at room temperature and in thicknesses ranging from 10 to 80 nm. The crystalline orientation of the films was mainly in the [111] direction, with a surface roughness of under 8.0 nm for thicker films. The copper films were coated with dodecanethiol (DDT), CH3(CH2)10HS by direct immersion into a millimolar solution of the molecules in ethanol for 20 h. Self-Assembled monolayers (SAMs) formation was observed based on XPS studies. The results showed that DDT-SAMs were an effective barrier against the oxidation of the metallic surface, and the resistivity increased between 15% and 70% over the bulk value, depending on the film thickness.

Integration of organic electrochemical transistors and immuno-affinity membranes for label-free detection of interleukin-6 in the physiological concentration range through antibody-antigen recognition.

We demonstrate the label-free and selective detection of interleukin-6 (IL-6), a key cell-signaling molecule in biology and medicine, by integrating an OECT with an immuno-affinity regenerated cellulose membrane. The objective of the membrane is to increase the local concentration of IL-6 at the sensing electrode and, thereby, enhance the device response for concentrations falling within the physiological concentration range of cytokines. The OECT gate electrode is functionalized with an oligo(ethylene glycol)-terminated self-assembled alkanethiolate monolayer (SAM) for both the immobilization of anti IL-6 antibodies and the inhibition of non-specific biomolecule binding. The OECT gate/electrolyte interface is exploited for the selective detection of IL-6 through the monitoring of antigen-antibody binding events occurring at the gate electrode.

Energy Level Alignment of Organic Molecules with Chemically Modified Alkanethiolate Self-Assembled Monolayers

We have employed two-photon photoemission spectroscopy to nondestructively resolve the unoccupied energy levels of fullerene C60 molecules deposited on alkanethiolate self-assembled monolayers (SAMs). By fluorine substitution of the hydrogen atoms in the alkyl chain, the work function (WF) increased from 4.3 eV for the alkanethiolate-SAM (H-SAM) to 5.7 eV for the fluorine-substituted SAM (F-SAM), owing to the formation of surface dipole layers. When C60 is deposited on the H-SAM and F-SAM, the energy positions of the unoccupied/occupied levels of C60 are pinned to the vacuum level (Fermi level (EF) + WF). As a result of the energy level alignment, on the F-SAM, the relative energy from EF of the highest occupied molecular orbital of C60 almost equals that of the lowest unoccupied molecular orbital, implying that the C60 film on the F-SAM exhibits both p- and n-type (ambipolar) charge transport properties, while C60 is known as a typical n-type semiconductor. The energetics are preserved even with multilay...

Comparison of Biomolecular Force Fields for Alkanethiol Self-Assembled Monolayer Simulations

Reasonable all-atom or united-atom biomolecular force fields have been developed to represent the properties of proteins and lipid membranes in molecular dynamics simulations. However, since they have not been parametrized for self-assembled monolayers (SAMs), their utility in simulating SAMs and protein–SAM systems has not been confirmed. Here, we compared six popular biomolecular force fields, Lipid14, GAFF, L-OPLS, CHARMM36, Slipids, and GROMOS54a7, to simulate alkanethiol SAMs of short to long chains (C10–C18). Our results show that none of these force fields reproduce the chain length dependence of the tilt angle, and twist angle. Although the droplet contact angles on SAMs are well represented by all force fields, only GAFF and Lipid14 yield phase transition temperatures that are reasonably close to the experimental values. Overall, our comprehensive comparison suggests that GAFF and Lipid14 are better choices for SAM simulations; further improvements in the force field parameters for SAMs are requi...

Delocalized versus localized excitations in the photoisomerization of azobenzene-functionalized alkanethiolate SAMs

Self-assembled monolayers of azobenzene-functionalized alkanethiolates form molecular ensembles with preferential orientation and significant excitonic coupling among the azobenzene chromophores. We have studied their optical switching with differential reflectance and two-photon-photoemission spectroscopy tuning the excitation wavelength through the excitonically broadened S2 absorption band. While the effective isomerization cross-section increases towards shorter wavelengths, the fraction of cis molecules in the photostationary state decreases. We attribute this observation to the absorption of the cis isomer in the SAM. The photoisomerization in the SAM thereby follows the behavior of non-interacting chromophores in solution, despite the formation of H-aggregates. Our study thus reveals that photoswitching occurs via localized excitations while strongly excitonically coupled, delocalized states do not contribute significantly.

Formation Kinetics of Mixed Self-Assembled Monolayers of Alkanethiols on GaAs(100).

We report on the formation kinetics of mixed self-assembled monolayers (SAMs) comprising 16-mercaptohexadecanoic acid (MHDA) and 11-mercapto-1-undecanol (MUDO) thiols on GaAs(100) substrates. These compounds were selected for their potential in constructing highly selective and efficient architectures for biosensing applications. The molecular composition and quality of one-compound and mixed SAMs were determined by the Fourier transform infrared absorption spectroscopy measurements. The formation of enhanced-quality mixed SAMs was investigated as a function of the molecular composition of the thiol mixture and the proportion of ethanol/water solvent used during their arrangement. Furthermore, the formation of mixed SAMs has been carried out by successive immersion of MHDA SAMs in MUDO thiol solutions and MUDO SAMs in MHDA thiol solution through the process involving thiol-thiol substitution. Our results, in addition to confirming that water-ethanol-based solvents improve the packing density of single thiol monolayers, demonstrate the attractive role of water-ethanol solvents in forming superior quality mixed SAMs.

Self-assembled monolayers of sulfonate-terminated alkanethiols investigated by frequency modulation atomic force microscopy in liquid

A molecular-scale understanding of self-assembled monolayers (SAMs) of sulfonate-terminated alkanethiols is crucial for interfacial studies of functionalized SAMs and their various applications. However, such an understanding has been difficult to achieve because of the lack of direct information on these molecular-scale structures in real space. In this study, we investigated the structures of sulfonate SAMs of sodium 11-mercapto-1-undecanesulfonate (MUS) by frequency modulation atomic force microscopy (FM-AFM) in liquid. The subnanometer-resolution FM-AFM images showed that the single-component MUS SAM prepared in pure water had random surface structures. In contrast, the MUS SAM prepared in a water–ethanol mixed solvent showed periodic striped structures with a flat-lying conformation. The results suggest a significant solvent effect on molecular-scale structures of long-chain sulfonate SAMs. In addition, we investigated the molecular-scale structures of mixed SAMs of MUS and 11-mercapto-1-undecanol (MUO) with alkane chains of the same length. The FM-AFM images of the mixed SAMs showed clear phase separation between MUS SAM and MUO SAM domains. In the MUO SAM domains, the incorporated MUS molecules appeared as protrusions. The results obtained in this study provide direct structural information on long-chain sulfonate and mixed SAMs.

Effect of the Chain Length and Temperature on the Adhesive Properties of Alkanethiol Self-Assembled Monolayers.

Stable and hydrophobic self-assembled monolayers of alkanethiols are promising materials for use as lubricants in microdevices and nanodevices. We applied high-rate dynamic force spectroscopy measurements to study in detail the influence of the chain length and temperature on the adhesion between methyl-terminated thiol monolayers and a silicon nitride tip. We used the Johnson-Kendall-Roberts model to calculate the number of molecules in adhesive contact and then the Dudko-Hummer-Szabo model to extract the information about the position and the height of the activation barrier per single molecule. Both parameters were determined and analyzed in the temperature range from 25 to 65 °C for three thiols: 1-decanethiol (measured previously), 1-tetradecanethiol, and 1-hexadecanethiol. We associate the increase of the activation barrier parameters versus the chain length with lower stiffness of longer molecules and higher effectiveness of adhesive bond formation. However, we relate the thermal changes of the parameters rather to rearrangements of molecules than to the direct influence of temperature on the adhesive bonds.

Time and pressure dependent deformation of microcontact printed channels fabricated using self-assembled monolayers of alkanethiol on gold

Abstract In this work, the replication-based microcontact printing method has been presented to study the deformation effect of different printing times and printing pressures on the microcontact printed structures. Cost-effective microcontact printing channels of self-assembled monolayers of alkanethiol have been prepared on gold surface. The alkanethiol inking the polydimethylsiloxanes stamp effectively forms the self-assembled monolayers on the noble gold surface that protects the metal against etchant solution and thereby forms channel-like structures. To address the deformation issue, variations in the printing time in the range of 30 s–60 min and the printing pressure ranging from 840 to 4200 Pa have been studied. The estimation of differing the channel width and channel space with varying printing time and pressure shows the best resolution structures printed under minimal printing time at atmospheric pressure.

Effects of Embedded Dipole Layers on Electrostatic Properties of Alkanethiolate Self-Assembled Monolayers

Alkanethiolates (ATs) forming self-assembled monolayers (SAMs) on coinage metal and semiconductor substrates have been used successfully for decades for tailoring the properties of these surfaces. Here, we provide a detailed analysis of a highly promising class of AT-based systems, which are modified by one or more dipolar carboxylic acid ester groups embedded into the alkyl backbone. To obtain comprehensive insight, we study nine different embedded-dipole monolayers and five reference nonsubstituted SAMs. We systematically varied lengths of the alkyl segments, ester group orientations, and number of ester groups contained in the chain. To understand the structural and electronic properties of the SAMs, we employ a variety of complementary experimental techniques, namely, infrared reflection absorption spectroscopy (IRS), high-resolution X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM), and Kelvin probe (KP) AFM. These experiments are comp...

Potential of Zero Charge and Its Temperature Derivative for Au(111) Electrode|Alkanethiol SAM|1.0 M Aqueous Electrolyte Solution Interfaces: Impact of Electrolyte Solution Ionic Strength and Its Effect on the Structure of the Modified Electrode|Electrolyte Solution Interface

We demonstrate how small and rapid temperature perturbations (produced by the indirect laser-induced temperature jump (ILIT) technique) of solid metal electrode|electrolyte solution interfaces may be used to determine the potential of zero (total) charge (Epzc) and its temperature derivative (dEpzcdT) of Au(111) electrode surfaces modified by alkanethiol self-assembled monolayers in contact with high ionic strength (i.e., 1.0 M) aqueous electrolyte solutions. The Epzc’s measured for two different types of SAMs (made from either HS(CH2)n−1CH3 (5 ≤ n ≤ 12, Epzc = −(0.99 ± 0.12) V vs SSCE) or HS(CH2)nOH (3 ≤ n ≤ 16, Epzc = (0.46 ± 0.22) V vs SSCE)) are considerably different than those measured previously at much lower electrolyte solution ionic strengths. For mixed monolayers made from both HS(CH2)n−1CH3 and HS(CH2)nFc (where Fc refers to ferrocene), the difference in Epzc decreases as a function of the surface concentration of the Fc moiety (i.e., [Fc]), and it completely disappears at a surprisingly small...

Inhibiting Metal Oxide Atomic Layer Deposition: Beyond Zinc Oxide.

Atomic layer deposition (ALD) of several metal oxides is selectivity inhibited on alkanethiol self-assembled monolayers (SAMs) on Au, and the eventual nucleation mechanism is investigated. The inhibition ability of the SAM is significantly improved by the in situ H2-plasma pretreatment of the Au substrate prior to the gas-phase deposition of a long-chain alkanethiol, 1-dodecanethiol (DDT). This more rigorous surface preparation inhibits even aggressive oxide ALD precursors, including trimethylaluminum and water, for at least 20 cycles. We study the effect that the ALD precursor purge times, growth temperature, alkanethiol chain length, alkanethiol deposition time, and plasma treatment time have on Al2O3 ALD inhibition. This is the first example of Al2O3 ALD inhibition from a vapor-deposited SAM. The inhibitions of Al2O3, ZnO, and MnO ALD processes are compared, revealing the versatility of this selective surface treatment. Atomic force microscopy and grazing-incidence X-ray fluorescence further reveal insight into the mechanism by which the well-defined surface chemistry of ALD may eventually be circumvented to allow metal oxide nucleation and growth on SAM-modified surfaces.