Self-assembled Monolayer Surface Research Articles

Investigating the substitution of intermolecular hydrogen bonds on the surface of self-assembled monolayer by scanning electrochemical microscopy

In this work, the substitution of intermolecular hydrogen bonds on the surface of 3-mercaptopropionic acid self-assembled monolayer (MPA SAM) was investigated using scanning electrochemical microscopy (SECM). • The substitution of intermolecular hydrogen bonds on the surface of MPA SAM was investigated by scanning electrochemical microscopy. • The formation of charge-assisted hydrogen bonds was more likely to induce intermolecular hydrogen bonds substitution. • All phenomena were properly characterized by the changes in current of substrate and tip electrodes. • The substitution of intermolecular hydrogen bonds is conductive to understanding the molecular selective recognition process on biofilm surface. Investigating the substitution of intermolecular hydrogen bonds on the simulated biofilm surface at molecular level is beneficial to understand the mechanism of molecular selective recognition occurring on biofilm surface. In this paper, the substitution of intermolecular hydrogen bonds on the surface of simulated biofilm, self-assembled monolayer of 3-mercaptopropionic acid (MPA SAM), is studied using scanning electrochemical microscopy (SECM). When acetic acid exists on MPA SAM surface, the intermolecular hydrogen bonds between acetic acid and MPA SAM are able to replace those between the redox mediator ferrocenemethanol (FcCH 2 OH) and MPA SAM, because the stronger charge-assisted hydrogen bonds (CAHB) formed between acetic acid and MPA SAM have the ability of substitution. The effect of the substitution of intermolecular hydrogen bonds is more significant with increasing the concentration of acetic acid within certain limits. This work may provide ideas for deeper understanding of molecular selective recognition on biofilm surface.

Potential-Dependent Adhesion Forces between dsDNA and Electroactive Surfaces

A promising approach to regulating the interactions between polyelectrolytes and materials is the use of electroactive surfaces that can change their charge state. However, common electroactive groups are too unstable to be practical for this purpose. Here we have performed a single molecule force spectroscopy study of the interactions between dsDNA and an 1,1'-biferrocenylene (BFD = bis(fulvalene)diiron)-terminated self-assembled monolayer surface that allows us to reversibly change the charge state. We found that the interaction force between DNA and the surface is correlated to the oxidation state of the BFD groups, which is conveniently controlled by the electrochemical potentials. We discovered that the electroactive SAM produces much stronger interaction forces than its nonelectroactive counterpart. A model based on the Grahame equation was able to quantitatively reproduce the experimentally observed relation between the applied potentials and adhesion forces. Our electroactive surface provides a model system for quantitative studies of the interactions between polyelectrolyte and charged surfaces in liquid. These insights may enable new opportunities for actively manipulating the binding, orientations, and conformations of polyelectrolytes for biosensing, nanomotors, and other applications.

Electron Transfer in Binary Hemin-Modified Alkanethiol Self-Assembled Monolayers on Gold: Hemin's Lateral and Interfacial Interactions.

Orientated coupling of redox enzymes to electrodes by their reconstitution onto redox cofactors, such as hemin conjugated to self-assembled monolayers (SAMs) formed on the electrodes, poses the requirements for a SAM design enabling reconstitution. We show that the kinetics of electron transfer (ET) in binary SAMs of alkanethiols on gold composed of in situ hemin-conjugated 11-amino-1-undecanethiol (AUT) and diluting OH-terminated alkanethiols with 11, 6, and 2 methylene groups (MC11OH, MC6OH, and MC2OH) depends on both the SAM composition and surface density of hemin, Γheme. In AUT/MC11OH SAMs composed of equal linker/diluent lengths, the heterogeneous ET rate constant ks decreased with the Γheme and varied between 70 and 500 s-1. For shorter diluents, the ks of 245-330 s-1 (C6) and 300-340 s-1 (C2) showed a little (if any) Γheme dependence. In AUT/MC11OH SAMs, the increasing Γheme resulted in the steric crowding of hemin species and their neighboring lateral interactions in the plane of hemin localization, affecting the potential distribution at the SAM/electrode interface and inducing local electrostatic effects interfering with hemin oxidation. In AUT/MC6OH and AUT/MC2OH SAMs, hemin discharged at the plane of the closest approach to the gold surface, equal to the diluent length and permeable to electrolyte ions, which lessened those effects. All studied binary SAMs provided steric hindrance for protein reconstitution on the hemin cofactor conjugated to the extended AUT linker. Further use of SAM-modified electrodes with the covalently attached hemin as interfaces for heme proteins' reconstitution should consider SAMs with loosely dispersed redox centers terminating more rigid molecular wires. Such wires place hemin at fixed distances from the electrode surface and thus ensure the interfacial properties required for the effective on-surface reconstitution of proteins and enzymes.

A mechanistic study of Pt particle electrodeposition and growth on a self-assembled monolayer as an active template

• Application of monolayers on gold as templates for deposition of Pt particles. • Evaluation of particle growth using cyclic voltammetry and chronoamperometry. • Evaluation of growth mechanism with SH model using chronoamperometric deposition. • Evaluation of growth mechanism using time dependant image J analysis of the SEM. • Comparing mechanism of growth evaluated using SH model with SEM analysis. Platinum is an essential component in many commercial catalytic processes. However, due to its cost and availability considerable effort is made to control the amount, location, and accessibility of the platinum species on an electrode surface. The current study investigates a low-cost, and versatile method for the deposition of Pt particles using a self-assembled monolayer (SAM) as an active assisted template on a gold electrode. Alkane (HSCH 2 (CH 2 ) 8 CH 3 ), alcohol (HSCH 2 (CH 2 ) 9 CH 2 OH) and carboxylate (HSCH 2 (CH 2 ) 8 CH 2 COOH) terminated monolayers were used to probe the impact of hydrophobicity on particle size, population density and growth mechanism. Cyclic voltammetry (CV) and chronoamperometry (CA) were employed to induce particle growth. From the CA data a dimensionless theoretical plot was calculated based on the Scharifker and Hills model to establish the mechanism as either progressive or instantaneous. The time-dependent study revealed a progressive nucleation mechanism for Pt particles synthesized on all three electrode surfaces, whereas microscopy and image analysis revealed an instantaneous nucleation mechanism for the alkane template only and progressive nucleation pathways for the alcohol and carboxylate SAM surfaces. The results indicated that SAM templating could be used to effectively control particle growth in a predictable fashion, however there is a limitation in application of the existing model due to a combination of factors including the migration of the SAM monomers on the electrode surface, the hydrophobicity of the terminal moieties and the occurrence of secondary and subsequent nucleation and growth phases. In addition, post-particle deposition CV was used to determine the impact of the SAM template on the active area of the Pt particles available for catalysis and showed that the hydrophobic SAM coatings gave significantly higher activity.

Redox-Induced Lipid Vesicle Fusion Onto Electroactive Self-Assembled Monolayers

Supported lipid bilayers (SLBs) are popular model systems to study cell membrane functionalities and various biomolecular interaction forces. A key advantage of SLBs on metallic surfaces, is the application of spectroscopic, electrochemical, and surface plasmon resonance techniques for biomolecular recognition studies. The most common method for forming SLBs is vesicle fusion, which involves the adsorption, deformation, and rupture of small unilamellar vesicles (SUVs) of lipids from aqueous suspension to the substrate surface.1 The formation of continuous single bilayers by vesicle fusion can be problematic due to the many experimental parameters influencing vesicle rupture and bilayer spreading (i.e., lipid and vesicle properties, physicochemical characteristics of the surface, temperature, and solvent environment)1. Vesicle fusion typically works only with smooth hydrophilic surfaces, such as glass, silica, and mica. Although approaches have been developed to form bilayers on technologically relevant surfaces such gold (e.g., surface functionalization with hydrophilic organic films,2 solvent-assisted lipid bilayer formation3 or the addition of a vesicle-destabilizing agent4), there remains a need for active strategies that are fast, versatile, and scalable.We present a redox-induced approach for the formation of single bilayers on gold functionalized with electroactive self-assembled monolayers (SAMs) of ferrocenylalkanethiolates. The electrochemical oxidation of the SAM-bound ferrocene (Fc) to ferrocenium (Fc+) involves coupled electron transfer and ion pairing reactions. Counteranions from solution pair with the ferroceniums to stabilize the oxidized cations and neutralize the excess positive charge at the SAM/aqueous interface. The surface-confined redox reaction triggers the formation of single bilayer membranes from SUVs of anionic or zwitterionic phospholipids onto gold surfaces modified with the electroactive SAM. The ion pairing association of the charged lipid head groups with the electrogenerated ferroceniums drives the assembly of the phospholipids on the SAM surface to produce solid-supported bilayers of high surface coverage (≳ 90%) from gel- or fluid-phase forming phospholipids within minutes at room temperature. The redox-mediated strategy reported in this work is a conceptual advance in the preparation of solid-supported lipid bilayers. It is fast, insensitive to the phase state of the phospholipid in the vesicle precursor, and not limited to hydrophilic surfaces. References Richter, R. P.; Bérat, R.; Brisson, A. R., Formation of Solid-Supported Lipid Bilayers: An Integrated View. Langmuir 2006, 22 (8), 3497-3505.Silin, V. I.; Wider, H.; Woodward, J. T.; Valincius, G.; Offenhausser, A.; Plant, A. L., The Role of Surface Energy on the Formation of Hybrid Bilayer Membranes. J. Am. Chem. Soc. 2002, 124, 14676-14683.Ferhan, A. R.; Yoon, B. K.; Park, S.; Sut, T. N.; Chin, H.; Park, J. H.; Jackman, J. A.; Cho, N.-J., Solvent-Assisted Preparation of Supported Lipid Bilayers. Nat. Protoc. 2019, 14 (7), 2091-2118.Cho, N.-J.; Cho, S.-J.; Cheong, K. H.; Glenn, J. S.; Frank, C. W., Employing an Amphipathic Viral Peptide to Create a Lipid Bilayer on Au and TiO2. J. Am. Chem. Soc. 2007, 129 (33), 10050-10051. Figure 1

Probing Ca2+-induced electron transfer on the surface of self-assembled monolayer using SECM

The electron transfer process of protein on the surface of biomembrane is closely related to various physiological phenomena occurring on the biomembrane. Calcium cations (Ca2+), as a ubiquitous messenger, induced electron transfer of negatively charged protein on the surface of negatively charged biomembrane is a problem worth exploring. In this work, the 3-Mercaptopropionic acid self-assembled monolayer (MPA SAM) with terminal carboxyl groups was constructed on gold surface as a simplified model for simulated surface of negatively charged biomembrane. The electron transfer of negatively charged redox ion induced by Ca2+ on the surface of MPA SAM was investigated using scanning electrochemical microscopy. It was found that Ca2+ can induce electron transfer and enhance the electron transfer of negatively charged redox ions. The result is attributed to the reduction of the energy barrier of the electron transfer of negatively charged redox ion on the surface of MPA SAM because Ca2+ neutralizes the electrostatic repulsion interaction between negatively charged redox ion and negatively charged carboxylate surface of MPA SAM. The conditions of lower pH also benefit the above-mentioned electron transfer, which further confirms the mechanism of Ca2+-induced electron transfer. This work may provide a reference for studying the electron transfer process on the surface of simulated biomembrane.

Self-assembled monolayer enabling improved buried interfaces in blade-coated perovskite solar cells for high efficiency and stability

Despite the rapidly increased power conversion efficiency (PCE) of perovskite solar cells (PVSCs), it is still quite challenging to bring such promising photovoltaic technology to commercialization. One of the challenges is the upscaling from small-sized lab devices to large-scale modules or panels for production. Currently, most of the efficient inverted PVSCs are fabricated on top of poly[bis(4-phenyl)(2, 4, 6-trimethylphenyl)amine] (PTAA), which is a commonly used hole-transporting material, using spin-coating method to be incompatible with large-scale film deposition. Therefore, it is important to develop proper coating methods such as blade-coating or slot-die coating that can be compatible for producing large-area, high-quality perovskite thin films. It is found that due to the poor wettability of PTAA, the blade-coated perovskite films on PTAA surface are often inhomogeneous with large number of voids at the buried interface of the perovskite layer. To solve this problem, self-assembled monolayer (SAM)-based hole-extraction layer (HEL) with tunable headgroups on top of the SAM can be modified to provide better wettability and facilitate better interactions with the perovskite coated on top to passivate the interfacial defects. The more hydrophilic SAM surface can also facilitate the nucleation and growth of perovskite films fabricated by blade-coating methods, forming a compact and uniform buried interface. In addition, the SAM molecules can also be modified so their highest occupied molecular orbital (HOMO) levels can have a better energy alignment with the valence band maxima (VBM) of perovskite. Benefitted by the high-quality buried interface of perovskite on SAM-based substrate, the champion device shows a PCE of 18.47% and 14.64% for the devices with active areas of 0.105 cm² and 1.008 cm², respectively. In addition, the SAM-based device exhibits decent stability, which can maintain 90% of its initial efficiency after continuous operation for over 500 h at 40 ℃ in inert atmosphere. Moreover, the SAM-based perovskite mini-module exhibits a PCE of 14.13% with an aperture area of 18.0 cm². This work demonstrates the great potential of using SAMs as efficient HELs for upscaling PVSCs and producing high-quality buried interface for large-area perovskite films.

Achieving High-Performance Molecular Rectification through Fast Screening Alkanethiol Carboxylate-Metal Complexes Electroactive Units

Achieving High-Performance Molecular Rectification through Fast Screening Alkanethiol Carboxylate-Metal Complexes Electroactive Units

Electrochemical and X-ray Photoelectron Spectroscopy Surface Characterization of Interchain-Driven Self-Assembled Monolayer (SAM) Reorganization.

Herein, we report a combined strategy encompassing electrochemical and X-ray photoelectron spectroscopy (XPS) experiments to investigate self-assembled monolayer (SAM) conformational reorganization onto an electrode surface due to the application of an electrical field. In particular, 3-mercaptopriopionic acid SAM (3MPA SAM) modified gold electrodes are activated with a 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysulfosuccinimide (NHSS) (EDC-NHSS) mixture by shortening the activation time, from 2 h to 15/20 min, labelled as Protocol-A, -B and -C, respectively. This step, later followed by a deactivation process with ethanolamine (EA), plays a key role in the reaction yields (formation of N-(2-hydroxyethyl)-3-mercaptopropanamide, NMPA) but also in the conformational rearrangement observed during the application of the electrical field. This study aims at explaining the high performance (i.e., single-molecule detection at a large electrode interface) of bioelectronic devices, where the 3MPA-based SAM structure is pivotal in achieving extremely high sensing performance levels due to its interchain interaction. Cyclic voltammetry (CV) experiments performed in K4Fe(CN)6:K3Fe(CN)6 for 3MPA SAMs that are activated/deactivated show similar trends of anodic peak current (IA) over time, mainly related to the presence of interchain hydrogen bonds, driving the conformational rearrangements (tightening of SAMs structure) while applying an electrical field. In addition, XPS analysis allows correlation of the deactivation yield with electrochemical data (conformational rearrangements), identifying the best protocol in terms of high reaction yield, mainly related to the shorter reaction time, and not triggering any side reactions. Finally, Protocol-C’s SAM surface coverage, determined by CV in H2SO4 and differential pulse voltammetry (DPV) in NaOH, was 1.29 * 1013 molecules cm−2, being similar to the bioreceptor surface coverage in single-molecule detection at a large electrode interface.

Binding of Sulfate-Terminated Surfactants with Different Alkyl Chain Lengths to Viologen Sites Covalently Embedded in the Interior of a Self-Assembled Monolayer on a Au Electrode

We investigated the binding of anionic surfactants of lower concentrations than their critical micelle concentrations (cmcs) to the cationic redox-active viologen site in the interior of a self-assembled monolayer (SAM) on a polycrystalline Au electrode. We embedded the viologen site in the midway of the alkyl chain to facilitate the ion-pairing binding, which depends on the oxidation state of the viologen. We found that the binding of anionic surfactants and inorganic anions causes a negative shift of the formal potential of the redox couple of the viologen radical cation/viologen dication in line with the binding equilibrium. In contrast, the anion binding was weak and trivial when viologens are located at the SAM surface, indicative of the enhancement of the binding by the electrostatic interaction in the microenvironment with the low dielectric constant. The negative shift of the formal potential of viologen in the interior was greater for the surfactants with longer alkyl chain lengths, indicative of the efficacy of the alkyl chain-chain interaction. The chain-length-dependent potential shift followed the linear Traube rule but with a smaller slope than that in the original rule. We also demonstrated that the conjugated layer of the viologen SAM with dodecyl sulfate at a lower concentration than the cmc completely blocks the direct electron transfer (ET) from the Au electrode to solution-phase Fe(CN)63- but allows mediated ET around the formal potential of the viologen.

Dynamic in vitro hemocompatibility of oligoproline self-assembled monolayer surfaces.

The blood compatibility of self-assembled monolayers (SAMs) of oligoproline, a nonionic antifouling peptide, was investigated using the cone-and-plate assay imitating arterial blood flow conditions. End-capped oligoprolines composed of 6 and 9 proline residues (Pro6 and Pro9) and a Cys residue were synthesized for preparing SAMs (Pro-SAMs) on Au-sputtered glass. The surface of Pro-SAMs indicated hydrophilic property with a smooth topology. The adsorption of blood components and the adhesion of blood cells, including leukocytes and platelets, were strongly suppressed on Pro-SAMs. Moreover, Pro9-SAM did not trigger the activation of platelets (i.e., the conformational change of GPIIb/IIIa and P-selectin (CD62P) expression on platelets and the formation of aggregates). Our results demonstrate that Pro9-SAM completely inhibited acute thrombogenic responses and the activation of platelets under dynamic conditions.

Metal‐Supported Phospholipid Bilayers Formed by Redox Command

AbstractA surface‐confined redox reaction triggers the formation of single bilayer membranes from small unilamellar vesicles of anionic or zwitterionic phospholipids onto gold surfaces modified with a ferrocene‐terminated self‐assembled monolayer (SAM). The ion pairing association of the charged lipid headgroups with the electrogenerated cationic ferroceniums drives the assembly of the phospholipids on the SAM surface to produce solid‐supported bilayers of high surface coverage (≳90%) from gel‐ or fluid‐phase forming phospholipids within minutes at room temperature. The redox‐mediated strategy reported in this work is a conceptual advance in the preparation of solid‐supported lipid bilayers. It is fast, compatible with negatively‐charged and zwitterionic phospholipids, insensitive to the phase state of the phospholipid in the vesicle precursor, and not limited to hydrophilic surfaces.

Enhanced Self-Assembled Monolayer Surface Coverage by ALD NiO in p-i-n Perovskite Solar Cells.

Metal halide perovskites have attracted tremendous attention due to their excellent electronic properties. Recent advancements in device performance and stability of perovskite solar cells (PSCs) have been achieved with the application of self-assembled monolayers (SAMs), serving as stand-alone hole transport layers in the p-i-n architecture. Specifically, phosphonic acid SAMs, directly functionalizing indium–tin oxide (ITO), are presently adopted for highly efficient devices. Despite their successes, so far, little is known about the surface coverage of SAMs on ITO used in PSCs application, which can affect the device performance, as non-covered areas can result in shunting or low open-circuit voltage. In this study, we investigate the surface coverage of SAMs on ITO and observe that the SAM of MeO-2PACz ([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid) inhomogeneously covers the ITO substrate. Instead, when adopting an intermediate layer of NiO between ITO and the SAM, the homogeneity, and hence the surface coverage of the SAM, improve. In this work, NiO is processed by plasma-assisted atomic layer deposition (ALD) with Ni(MeCp)2 as the precursor and O2 plasma as the co-reactant. Specifically, the presence of ALD NiO leads to a homogeneous distribution of SAM molecules on the metal oxide area, accompanied by a high shunt resistance in the devices with respect to those with SAM directly processed on ITO. At the same time, the SAM is key to the improvement of the open-circuit voltage of NiO + MeO-2PACz devices compared to those with NiO alone. Thus, the combination of NiO and SAM results in a narrower distribution of device performance reaching a more than 20% efficient champion device. The enhancement of SAM coverage in the presence of NiO is corroborated by several characterization techniques including advanced imaging by transmission electron microscopy (TEM), elemental composition quantification by Rutherford backscattering spectrometry (RBS), and conductive atomic force microscopy (c-AFM) mapping. We believe this finding will further promote the usage of phosphonic acid based SAM molecules in perovskite PV.

Atomistic insights into dynamic growth of pentacene thin films on metal surfaces functionalized with self-assembled monolayers

The fabrication of high-quality organic semiconductor thin films has crucial impact on the electrical properties of organic electronic devices. Here, we provide atomistic insights into dynamic growth of pentacene thin films on different Au surfaces functionalized with self-assembled monolayers (SAMs). First, disordered crystal structures are formed on both bare Au and smooth SAM functionalized Au surfaces, while the upright crystalline islands emerge on rough SAM surfaces. During the growth process on rough SAM surfaces, organic semiconductor molecules undergo the nucleation and reorientation, and eventually form thin films with terrace morphologies. In addition, the interaction energies among pentacene molecules and between pentacene and SAM molecules will affect the formation and reorientation of the seed nucleus. Furthermore, the effect of substrate temperature on the pentacene growth is also evaluated. The high substrate temperature delays the onset of nucleus reorientation but accelerate the reorientation process. Finally, free energy calculations demonstrate that the surface roughness plays an important role in the formation and orientation of the seed nucleus. Meanwhile, the energy barrier at the step edge of the terrace morphology can prevent molecules from crossing the step edge.

Impacts of terminal molecules on metal-atom diffusion into alkane self-assembled-monolayer films: first-principles study

The effects of terminal molecules on metal-atom diffusion into alkane self-assembled-monolayer (SAM) films were studied using first-principles calculations. It was shown that the diffusion of Al and Au atoms easily occurs when the alkane molecules have a CH3 termination, while the diffusion is suppressed by COOK terminal molecules for Au atom and by COOH and COOK molecules for Al atom. This is in good agreement with experiments. We showed that such suppression is caused by the orbital-hybridization-induced interaction between a metal atom and terminal molecules: the ionic bonding between negatively charged Au and surrounding positively charged K atoms, and the covalent-like bonding between Al and neighboring O atoms. We also showed that, once the metal atoms form films on the SAM surface, the metal atoms bond to each other and stabilize, thus also suppressing the diffusion into the SAM substrate.

Visualization of Thermal Transport Properties of Self-Assembled Monolayers on Au(111) by Contact and Noncontact Scanning Thermal Microscopy.

Thermal transport properties of patterned binary self-assembled monolayers (SAMs) on Au(111) were examined using scanning thermal microscopy (SThM) with both contact and noncontact methods. We fabricated two-dimensional (2D) patterns with two separate domains of n-hexadecanethiol/benzenethiol, benzenethiol/n-butanethiol, or n-hexadecanethiol/n-butanethiol. In the experimental setup, the efficiency of thermal transport from a SThM tip to the SAM surface can be evaluated in terms of the temperature change at the SThM tip. In the contact regime, where a SThM tip physically contacts the SAM surface, direct thermal transport through the SAM and radiation-based thermal transport through the space where SAMs exist may contribute to a drop in temperature at the tip. In the noncontact regime, thermal transport relies on radiation-based heat dissipation from the heated tip to the SAMs. 2D mapping of the spatial temperature distribution on SAMs reflects the difference in thermal transport properties of the two SAM domains. We found that the contact method is effective for visualizing the temperature contrast, which reflects the thermal transport properties of the constituent molecules when the domains of the SAMs have a similar height, while the noncontact method allows visualization of the temperature distribution, which is related to the height of each domain of the SAMs, rather than the chemical structures of the constituent molecules. Combination of contact and noncontact SThM enables 2D imaging of thermal transport properties and topographic imaging simultaneously and represents a new technique for investigating the thermal properties of materials surfaces, which is essential for nanoscale thermal management.

Ordering of Small Molecules on Hydrophobic Self-Assembled n-Alkanethiols: Delicate Balance of Interfacial and Intermolecular Interactions

Growth of molecular thin films with desired orders and orientations has become technologically relevant as the electronic industries seek new opportunities and applications. However, the delicate balance of interfacial and intermolecular forces and their complex influence on thin-film growths still require more understanding. Here, the effects of a hydrophobic self-assembled monolayer (SAM) surface on the crystallization of four common solvents—acetonitrile, ethanol, methanol, and water—are investigated. Despite the absence of significant substrate–molecule forces, unexpected oriented growth is observed for these molecules except water. Acetonitrile and ethanol form a sustaining vertical assembly order with long-range crystalline structures. Coincident epitaxy with small lattice mismatches is found to be essential to these orderings, which are energetically favored but without a dominant azimuthal orientation. In contrast, a preferred in-plane registry of methanol overlayers is observed for an ultrathin nominal thickness and becomes lost in slightly thicker films. Such thickness-dependent ordering of methanol assemblies can be explained with semi-commensurate epitaxy with a tensile strain of ∼6.6% along hydrogen-bonded chains, whose quick relief results in the loss of order and even the phase. These rich observations suggest that SAM surfaces offer good opportunities for selective crystallization of molecular films worthy of further investigations.

Model Asphaltenes Adsorbed onto Methyl- and COOH-Terminated SAMs on Gold.

Petroleum asphaltenes are surface-active compounds found in crude oils, and their interactions with surfaces and interfaces have huge implications for many facets of reservoir exploitation, including production, transportation, and oil-water separation. The asphaltene fraction in oil, found in the highest boiling-point range, is composed of many different molecules that vary in size, functionality, and polarity. Studies done on asphaltene fractions have suggested that they interact via polyaromatic and heteroaromatic ring structures and functional groups containing nitrogen, sulfur, and oxygen. However, isolating a single pure chemical structure of asphaltene in abundance is challenging and often not possible, which impairs the molecular-level study of asphaltenes of various architectures on surfaces. Thus, to further the molecular fundamental understanding, we chose to use functionalized model asphaltenes (AcChol-Th, AcChol-Ph, and 1,6-DiEtPy[Bu-Carb]) and model self-assembled monolayer (SAM) surfaces with precisely known chemical structures, whereby the hydrophobicity of the model surface is controlled. We applied solutions of asphaltenes to these SAM surfaces and then analyzed them with surface-sensitive techniques of near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). We observe no adsorption of asphaltenes to the hydrophobic surface. On the hydrophilic surface, AcChol-Ph penetrates into the SAM with a preferential orientation parallel to the surface; AcChol-Th adsorbs in a similar manner, and 1,6-DiEtPy[Bu-Carb] binds the surface with a bent binding geometry. Overall, this study demonstrates the need for studying pure and fractionated asphaltenes at the molecular level, as even within a family of asphaltene congeners, very different surface interactions can occur.

Exploring a molecular switch for dopamine oxidation induced by charge reversal using scanning electrochemical microscopy

Oxidation of dopamine inside synaptic vesicles has been considered as one of inducements for Parkinson's disease. Dopamine molecules are often oxidized to quinones, making damages to neurotransmission process. Thus, exploring a controllable molecular switch for dopamine oxidation would make great significance in physiology. In this work, a binary homocysteine-dodecanethiol self-assembled monolayers was constructed on gold surface as a specific molecular switch. Scanning electrochemical microscopy was adopted to investigate the switch behavior for dopamine oxidation. The results showed that the molecular switch was at the ON state in a weak alkaline solution and dopamine oxidation would occur. While, the molecular switch was at the OFF state in a weak acidic solution and dopamine oxidation would be cut off. This controllable ON-OFF phenomenon was attributed to charge reversal on the surface of self-assembled monolayer. Then, Density Functional Theory calculation was adopted to calculate the HOMO-LUMO band gap of dopamine-homocysteine cluster. The theoretical calculation exhibited that charge reversal of homocysteine could adjust the energy barrier of dopamine oxidation. This work would offer some contributions to studies of dopamine oxidation in cellular physiology.

Bridging Gaussian Density Fluctuations from Microscopic to Macroscopic Volumes: Applications to Non-Polar Solute Hydration Thermodynamics.

The hydration of hydrophobic solutes is intimately related to the spontaneous formation of cavities in water through ambient density fluctuations. Information theory-based modeling and simulations have shown that water density fluctuations in small volumes are approximately Gaussian. For limiting cases of microscopic and macroscopic volumes, water density fluctuations are known exactly and are rigorously related to the density and isothermal compressibility of water. Here, we develop a theory—interpolated gaussian fluctuation theory (IGFT)—that builds an analytical bridge to describe water density fluctuations from microscopic to molecular scales. This theory requires no detailed information about the water structure beyond the effective size of a water molecule and quantities that are readily obtained from water’s equation-of-state—namely, the density and compressibility. Using simulations, we show that IGFT provides a good description of density fluctuations near the mean, that is, it characterizes the variance of occupancy fluctuations over all solute sizes. Moreover, when combined with the information theory, IGFT reproduces the well-known signatures of hydrophobic hydration, such as entropy convergence and solubility minima, for atomic-scale solutes smaller than the crossover length scale beyond which the Gaussian assumption breaks down. We further show that near hydrophobic and hydrophilic self-assembled monolayer surfaces in contact with water, the normalized solvent density fluctuations within observation volumes depend similarly on size as observed in the bulk, suggesting the feasibility of a modified version of IGFT for interfacial systems. Our work highlights the utility of a density fluctuation-based approach toward understanding and quantifying the solvation of non-polar solutes in water and the forces that drive them toward surfaces with different hydrophobicities.