Carboxylic Acid-terminated Self-assembled Monolayers Research Articles

Probing the Structure and Orientation of Carboxylic Acid-Terminated Self-Assembled Monolayers.

Self-assembled monolayers (SAMs), such as alkanethiols (AT), are widely used as functional coatings or interfaces between different materials. There is an assumption that the arrangement and alignment of the hydrocarbon chains in films made from carboxyl-terminated alkanethiols are similar to those made from alkanethiols. Here, the structure of the outermost layer and near-surface region of SAMs formed from carboxyl-terminated alkanethiols of various lengths has been analyzed. The chemical composition of the samples was measured using X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS (AR-XPS), allowing the film thickness. Metastable induced photoelectron spectroscopy (MIES) as a surface analytical tool sensitive only for the outermost layer in conjunction with density functional theory (DFT) calculations provided insights into the composition of the topmost layer, showing that it consists mainly of the backbone of the SAM-forming molecules. Through combining AR-XPS concentration depth profiles and the measurement of the composition of the outermost layer, it can be shown that SAMs tend to favor a gauche orientation, enabling interactions between the functional groups.

Selective Positioning of Nanosized Metal–Organic Framework Particles at Patterned Substrate Surfaces

Herein, we describe the selective positioning of metal-organic framework (MOF) nanoparticles UiO-66 (Universitet i Oslo; Zr6O4(OH)4(bdc)6; bdc2- = benzene-1,4-dicarboxylate) and MIL-101 (Material Institut Lavoisier, Cr3O(OH) (H2O)2(bdc)3) at defined positions on a patterned substrate. For this purpose, patterned alkyne- and carboxylic acid-terminated self-assembled organic monolayer (SAM)-modified silicon surfaces were prepared by liquid immersion and microcontact printing (μCP). Preformed UiO-66 and MIL-101 nanometer-sized MOFs (NMOFs) were synthesized by solvothermal synthesis, and the nanocrystallite particles' exterior surface was functionalized in order to generate reactive sites (such as azides and amines) at the NMOFs. Copper-catalyzed alkyne azide cycloaddition and N-hydroxysuccinimide-mediated amide formation were used to selectively position the NMOFs at the surface of pre-patterned substrates. The resulting surfaces were thoroughly investigated by scanning electron microscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy, confirming the validity of the presented approach. We hope that our research paves the way for microsystem integration of NMOFs, for example, in microfluidic devices/reactors, and further investigation of their enhanced catalytic activity.

Reversible Self-Assembled Monolayers (rSAMs) as Robust and Fluidic Lipid Bilayer Mimics

Lipid bilayers, forming the outer barrier of cells, display a wide array of proteins and carbohydrates for modulating interfacial biological interactions. Formed by the spontaneous self-assembly of lipid molecules, these bilayers feature liquid crystalline order, while retaining a high degree of lateral mobility. Studies of these dynamic phenomena have been hampered by the fragility and instability of corresponding biomimetic cell membrane models. Here, we present the construct of a series of oligoethylene glycol-terminated reversible self-assembled monolayers (rSAMs) featuring lipid-bilayer-like fluidity, while retaining air and protein stability and resistance. These robust and ordered layers were prepared by simply immersing a carboxylic acid-terminated self-assembled monolayer into 5-50 μM aqueous ω-(4-ethylene glycol-phenoxy)-α-(4-amidinophenoxy)decane solutions. It is anticipated that this new class of robust and fluidic two-dimensional biomimetic surfaces will impact the design of rugged cell surface mimics and high-performance biosensors.

Interfacial Charge Contributions to Chemical Sensing by Electrolyte-Gated Transistors with Floating Gates.

The floating gate, electrolyte-gated transistor (FGT) is a chemical sensing device utilizing a floating gate electrode to physically separate and electronically couple the active sensing area with the transistor. The FGT platform has yielded promising results for the detection of DNA and proteins, but questions remain regarding its fundamental operating mechanism. Using carboxylic acid-terminated self-assembled monolayers (SAMs) exposed to solutions of different pH, we create a charged surface and hence characterize the role that interfacial charge concentration plays relative to capacitance changes. The results agree with theoretical predictions from conventional double-layer theory, rationalizing nonlinear responses obtained at high analyte concentrations in previous work using the FGT architecture. Our study elucidates an important effect in the sensing mechanism of FGTs, expanding opportunities for the rational optimization of these devices for chemical and biochemical detection.

Adsorption of hyaluronic acid on solid supports: Role of pH and surface chemistry in thin film self-assembly

Owing to its biocompatibility, resistance to biofouling, and desirable physicochemical and biological properties, hyaluronic acid (HA) has been widely used to modify the surface of various materials. The role of various physicochemical factors in HA adsorption remains, however, to be clarified. Herein, we employed quartz crystal microbalance with dissipation (QCM-D) in order to investigate HA adsorption at different pH conditions onto three substrates—silicon oxide, amine-terminated self-assembled monolayer (SAM) on gold, and carboxylic acid-terminated SAM on gold. The QCM-D experiments indicated specific pH conditions where either strong or weak HA adsorption occurs. The morphology of the adsorbed HA layers was investigated by atomic force microscopy (AFM), and we identified that strong HA adsorption produced a complete, homogenous and smooth HA layer, while weak HA adsorption resulted in rough and inhomogeneous HA layers. The observed specifics of the kinetics of HA adsorption, including a short initial linear phase and subsequent long non-linear phase, were described by using a mean-field kinetic model taking HA diffusion limitations and reconfiguration in the adsorbed state into account. The findings extend the physicochemical background of design strategies for improving the use of passive HA adsorption for surface modification applications.

High-Contrast Imaging of Biomolecular Interactions Using Liquid Crystals Supported on Roller Printed Protein Surfaces

In this study, we report a new method for the high contrast imaging of biomolecular interactions at roller printed protein surfaces using thermotropic liquid crystals (LCs). Avidin was roller printed and covalently immobilized onto the obliquely deposited gold surface that was decorated with carboxylic acid-terminated self-assembled monolayers (SAMs). The optical response of LCs on the roller printed film of avidin contrasted sharply with that on the obliquely deposited gold surface. The binding of biotin-peroxidase to the roller printed avidin was then investigated on the obliquely deposited gold substrate. LCs exhibited a non-uniform and random orientation on the roller printed area decorated with the complex of avidin and biotin-peroxidase, while LCs displayed a uniform and planar orientation on the area without roller printed proteins. The orientational transition of LCs from uniform to non-uniform state was triggered by the erasion of nanometer-scale topographies on the roller printed surface after the binding of biotin-peroxidase to the surface-immobilized avidin. The specific binding events of protein-receptor interactions were also confirmed by atomic force microscopy and ellipsometry. These results demonstrate that the roller printing of proteins on obliquely deposited gold substrates could provide a high contrast signal for imaging biomolecular interactions using LC-based sensors.

Measuring ligand–receptor binding events on polymeric surfaces with periodic wave patterns using liquid crystals

In this study, we measured ligand–receptor binding events on polymeric substrates with periodic nanostructures using liquid crystals (LCs). Periodic sinusoidal wave patterns were generated through the buckling of the poly-(dimethylsiloxane) (PDMS) substrate on a cylindrical surface followed by replicating the associated relief structures on a poly-(urethane acrylate) (PUA) surface, where a film of gold was deposited. Avidin was then covalently immobilized onto a gold surface decorated with carboxylic acid-terminated self-assembled monolayer via NHS/EDC chemistry. The optical response of the device showed that the orientation of nematic 4-cyano-4′-pentylbiphenyl (5CB) was parallel to the plane of the avidin surface. However, the formation of the avidin–biotin complexes disturbed the sinusoidal topographies of the surface, and induced a random orientation of LCs, which produced a distinctive change in the optical response. We also confirmed the specific ligand–receptor interactions using ellipsometry and atomic force microscopy (AFM). These results suggest that polymeric surfaces with continuous wavy features could be used to develop novel LC-based sensors for the detection of ligand–receptor binding events.

Structure of Carboxyl-Acid-Terminated Self-Assembled Monolayers from Molecular Dynamics Simulations and Hybrid Quantum Mechanics–Molecular Mechanics Vibrational Normal-Mode Analysis

Self-assembled monolayers (SAMs) are excellent models for studying interfacial reactions and various properties of thin films. Carboxylic acid-terminated SAMs (CATSAMs) are of special interest because their surface properties are highly pH-dependent. To elucidate the complicated pH-dependent structural properties of CATSAMs, we combine force-field molecular dynamics simulations with hybrid quantum mechanics-molecular mechanics (QM/MM) calculations of vibrational frequencies. These studies show that at low pH the carboxylic acids behave similarly to bulk carboxylic acids, hydrogen bonding mainly to the aqueous phase. With increasing pH, intralayer hydrogen bonds and monolayer disorder increase. The computed QM/MM vibrational data show that the observed range of carbonyl frequencies is due to inherent condensed phase properties of the CATSAMs such as fluctuating hydrogen bond environment and steric interactions. The results reported herein underscore the role H-bonding and steric congestion play in pH-depen...

Visual observation of avidin–biotin affinity by fluorescent G4.5 poly(amidoamine) dendrimer

An air-treated G4.5 poly(amidoamine) (PAMAM) dendrimer displayed the enhanced fluorescence enough to be utilized as a fluorescence marker to visualize avidin–biotin affinity: On a fluorescence microscopic image, the avidin labeled by a fluorescent G4.5 PAMAM dendrimer was observed to be selectively bound on the biotin pattern that was prepared by amide-bonding of biotin on a carboxylic acid-terminated self-assembled monolayer and in turn by UV-irradiation with a photomask on the monolayer.

Micropatterned Dynamically Adhesive Substrates for Cell Migration

We present a novel approach to examine cell migration using dynamically adhesive substrates consisting of three spatially and functionally distinct regions: the first is permanently nonadhesive to cells, the second is permanently adhesive, and the final region is electrochemically switched from nonadhesive to adhesive. We applied a double microcontact printing approach to pattern gold surfaces with carboxylic acid-terminated self-assembled monolayers (SAMs) that permit initial cell adhesion, with methyl-terminated SAMs that permit adsorption of a nonadhesive, and with tri(ethylene glycol)-terminated SAMs that can be electrochemically "switched" to permit cell migration from a prespecified pattern onto a new pattern. Using these substrates, we investigated the migration of epithelial cells from monolayers onto narrow, branching tracks of extracellular matrix in order to characterize how lead cells influence the direction of movement of followers. Time-lapse imaging revealed that, on average, five cells consistently chose one branch before other cells entered the second branch, providing evidence to suggest that intercellular communication plays an important role in guiding the cohesive movement of epithelial sheets. This platform may be of use in furthering our understanding of the mechanisms underlying cellular decision-making during migration in both individual and multicellular contexts.

XPS and ToF-SIMS Investigation of α-Helical and β-Strand Peptide Adsorption onto SAMs

14-mer alpha-helix and 15-mer beta-strand oligopeptides composed of leucine (L) and lysine (K) were used to investigate peptide adsorption and orientation onto well-defined methyl and carboxylic acid-terminated self-assembled monolayer (SAM) surfaces with X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS showed that both peptides reached monolayer thickness on both SAMs, but significantly higher solution concentrations were required to reach this coverage on the methyl SAMs. This shows that the peptides adsorb more strongly onto the carboxyl-terminated SAMs. The excess oxygen detected by XPS and the H(3)O(+) signal detected by ToF-SIMS for the SAMs with adsorbed peptides indicated that water molecules are associated with the adsorbed peptides, even under ultrahigh-vacuum conditions. Changes in the number of L and K fragments detected by ToF-SIMS indicate that the beta-strand oriented differently on the two SAMs. The L side chains were preferentially associated with the methyl-terminated SAM, and the K side chains were preferentially associated with the carboxyl SAM. In contrast, little change in the ToF-SIMS K/L ratio was observed for the alpha-helix peptide absorbed on the two SAMs, indicating that ToF-SIMS was not as sensitive to the orientation of the alpha-helix peptide.

Atomic Layer Deposition of Aluminum Oxide on Carboxylic Acid-Terminated Self-Assembled Monolayers

In situ infrared absorption spectroscopy is used to monitor atomic layer deposition (ALD) of aluminum oxide (Al2O3) on carboxylic acid-terminated self-assembled monolayers (SAMs), Si(111)-(CH2)10-COOH (or COOH-SAMs), directly grafted on silicon (111) at approximately 100 degrees C. The quality of resulting Al2O3 films is comparable to Al2O3 on SiO2. Both the SAM film and the Si/SAM interface remain chemically stable during growth and upon post annealing to 400 degrees C, suggesting that the tight packing of the alkyl chains and COOH-SAM head groups presents a diffusion barrier and promotes ordered nucleation for ALD.

Covalent Nanoparticle Assembly onto Random Copolymer Films

We present a novel class of nanoparticle-decorated surfaces: amine-functionalized silica nanoparticles covalently attached to poly(styrene-random-acrylic acid) films and carboxylic acid-terminated self-assembled monolayers (SAMs). The dependence of the particle attachment kinetics on the concentration of particles in solution and acrylic acid moieties in the polymer backbone was investigated and was compared to the observed kinetics with SAM substrates. The kinetics on the polymer films included three distinct stages, which were governed by the acrylic acid concentration-dependent morphological changes of the films under the reaction conditions. The first stage was an induction period with little change in the particle coverage with time, followed by a rapid rise in the coverage, and finally a plateau. The maximum coverage achieved for the polymer films, 70%, was nearly twice that of the SAM substrates, which followed diffusion-limited coverage kinetics prior to reaching saturation. This enhanced coverage is attributed to the swelling of the acrylic acid groups at the film surface in the reaction solvent, which increases the surface area and roughness of the substrate. This approach is a reproducible way of preparing nanoparticle-decorated, chemically robust surfaces with controlled coverages and have potential applications for controlling surface wettability, optical properties, and cellular adhesion.

Effect of Solution Concentration, Surface Bias and Protonation on the Dynamic Response of Amplitude-Modulated Atomic Force Microscopy in Water

The dynamic response of amplitude-modulated atomic force microscopy (AM-AFM) is studied at the solid/water interface with respect to changes in ionic concentration, applied surface potential, and surface protonation. Each affects the electric double layer in the solution, charge on the tip and the sample surface, and thus the forces affecting the dynamic response. A theoretical model is developed to relate the effective stiffness and hydrodynamic damping of the AFM cantilever that is due to the tip/surface interaction with the phase and amplitude signals measured in the AM-AFM experiments. The phase and amplitude of an oscillating cantilever are measured as a function of tip-sample distance in three experiments: mica surface in potassium nitrate solutions with different concentrations, biased gold surface in potassium nitrate solution, and carboxylic acid-terminated self-assembled monolayers (SAMs) on gold in potassium nitrate pH buffers. Results show that, over the range where the higher harmonic modes of the oscillation are negligible, the effective stiffness of the AFM cantilever increases to a maximum as the tip approaches the surface before declining again as a result of the repulsive electrical double layer interaction. For attractive electrical double-layer interactions, the effective stiffness declines monotonically as the tip approaches the surface. Similarly, the hydrodynamic damping of the tip increases and then decreases as the tip approaches the solid/water interface, with the magnitude depending on the species present in the solution.

Characterization of Carboxylic Acid-Terminated Self-Assembled Monolayers by Electrochemical Impedance Spectroscopy and Scanning Electrochemical Microscopy

Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are used to monitor changes in the ionization of monolayers of 11-mercaptoundecanoic acid. When using an anionic redox probe, Fe(CN)6(-4), the charge-transfer resistance of the 11-mercaptoundecanoic acid monolayer-modified interface increases in a sigmoidal fashion as the solution is made basic. The opposite effect is observed when using a cationic redox probe. The inflection points of these two titration curves, however, differ when using the different redox probes. This result is taken as being characteristic of the influence that applied potential has on the ionization of the monolayer. The role of substrate potential on the ionization of the monolayer is further investigated by SECM. The SECM measurement monitors the concentration of Ru(NH3)6(+3) as the potential of the substrate is varied about the potential of zero charge. For monolayers of 11-mercaptoundecanoic acid in solutions buffered near the pKa of the terminal carboxylic acid, potential excursions positive of the PZC cause an increase in the concentration of Ru(NH3)6(+3) local to the interface, and potential excursions negative of the PZC cause a decrease in the local concentration of Ru(NH3)6(+3). Similar experiments conducted with an interface modified with 11-undecanethiol had no impact on the local concentration of Ru(NH3)6(+3). These results are interpreted in terms of the influence that applied potential has on the pH of the solution local to the interface and the impact that this has on the ionization of the monolayer.

Calcium Carbonate Storage in Amorphous Form and Its Template-Induced Crystallization

Calcium carbonate crystallization in organisms often occurs through the transformation from the amorphous precursor. It is believed that the amorphous phase could be temporarily stabilized and stored, until its templated transition to the crystalline form is induced. Here we develop a bioinspired crystallization strategy that is based on the above mechanism. Amorphous calcium carbonate (ACC) spherulitic particles are induced to form on a self-assembled monolayer (SAM) of hydroxyl-terminated alkanethiols on a gold surface. The ACC can then be stored in a dry atmosphere as a reservoir for ions and be induced to crystallize on command by the introduction of water and a secondary surface that is functionalized with carboxylic acid-terminated SAM. This secondary surface acts as a template for oriented and patterned nucleation. Various oriented crystalline arrays and micropatterned films are formed. We also show that the ACC phase can be doped with foreign ions (e.g., magnesium) and organic molecules (e.g., dyes) and that these dopants later function as growth modifiers of calcite crystals and become incorporated into the crystals during the transformation process of ACC to calcite. We believe that our strategy opens the way to using a stabilized amorphous phase as a versatile reservoir system that can be converted in a highly controlled fashion to a crystalline form upon contacting a specially designed nucleating template in water.

Functionalization of Poly(oligo(ethylene glycol) methacrylate) Films on Gold and Si/SiO2 for Immobilization of Proteins and Cells: SPR and QCM Studies

Thin films of a biocompatible and nonbiofouling poly(oligo(ethylene glycol) methacrylate) ( pOEGMA) with various thicknesses were formed on gold and Si/SiO 2 substrates by a combination of the formation of self-assembled monolayers (SAMs) terminating in bromoester-an initiator of atom transfer radical polymerization (ATRP)-and surface-initiated ATRP. After the formation of the pOEGMA films, terminal hydroxyl groups of side chains divergent from the methacrylate backbones were activated with N, N'-disuccinimidyl carbonate (DSC), and the DSC-activated pOEGMA films were reacted with (+)-biotinyl-3,6,9-trioxaundecanediamine (Biotin-NH 2) to form biotinylated pOEGMA films. By surface plasmon resonance experiments with the target protein (streptavidin) and model proteins (fibrinogen and lysozyme), we verified that the resulting films showed the enhanced signal-to-noise ratio ( approximately 10-fold enhancement) for the biospecific binding of streptavidin compared with the biotinylated substrate prepared from carboxylic acid-terminated SAMs. Quartz crystal microbalance measurements were also carried out to obtain the surface coverage of streptavidin and fibrinogen adsorbed onto the biotinylated pOEGMA films with various thicknesses and to investigate the effect of film thicknesses on the biospecific binding of streptavidin. Both the binding capacity of streptavidin and the signal-to-noise ratio of streptavidin/fibrinogen were found to be saturated at the 20 nm thick pOEGMA film. In addition, to demonstrate a wide applicability of the pOEGMA films, we constructed micropatterns of streptavidin and cells by microcontact-printing biotin-NH 2 and poly- l-lysine onto the DSC-activated pOEGMA films, respectively.

Detection of organophosphorous nerve agents using liquid crystals supported on chemically functionalized surfaces

Abstract We report use of thin films of the nematic liquid crystal E7 supported on chemically functionalized surfaces to indicate the presence of vapors of the organophosphorous nerve agents sarin (GB), soman (GD), tabun (GA) and VX. The surfaces were prepared by the deposition of metal perchlorate salts onto carboxylic acid-terminated self-assembled monolayers. When using surfaces prepared from aluminum perchlorate salts, the nematic film of E7 underwent a transition from a perpendicular orientation to a tilted or planar orientation upon exposure to vapors of GB, GD, GA or VX generated from a small drop of agent placed on a piece of filter paper. The orientational transition of the liquid crystal was readily apparent as an optical signal that was visible to the naked eye. The agent VX, which has the lowest vapor pressure of these organophosphorous nerve agents (∼140 ppb), was reported by the liquid crystal within 60 s. The agents GB, GD, and GA, which have higher vapor pressures, triggered responses in the liquid crystal that were evident within 15 s, 15 s, and 60 s, respectively. By preparing surfaces from perchlorate salts of aluminum(III), zinc(II) and iron(III) it was possible to distinguish between GB, VX and either GD or GA, thus demonstrating the feasibility of designing surfaces patterned with metal salts to detect and positively identify chemical warfare agents.

Reorganization of Immobilized Horse and Yeast Cytochrome c Induced by pH Changes or Nitric Oxide Binding

The redox properties of horse and yeast cytochrome c electrostatically immobilized on carboxylic acid-terminated self-assembled monolayers (SAMs) have been determined over a broad pH range (pH 3.5-8) in the absence and presence of nitric oxide. Below pH 6, both proteins exhibit comparable midpoint potentials, coverages, and electron-transfer rate constants, which suggests that they are adsorbed on the SAM in a similar fashion. Above pH 6, a sharp decrease in electron-transfer rate constants is observed for immobilized yeast cytochrome c, which is indicative of a change in the electron tunneling pathway between the heme and the electrode and hence suggests that the protein reorients on the surface. Such a decrease is not observed for horse cytochrome c and therefore must be related to the specific charge distribution on yeast cytochrome c. Apart from the charge distribution on the protein, the reorientation also seems to be related to the charge on the SAM surface. The presence of nitric oxide causes a decrease in electron-transfer rate constants of both yeast and horse cytochrome c at low pH. This is probably due to the fact that nitric oxide induces a conformational change of the protein and also changes the reorganization energy for electron transfer.

Fluoro-N,N,N‘,N‘-Tetramethylformamidinium Hexafluorophosphate: A Reagent for Formation of Interchain Carboxylic Anhydrides on Self-Assembled Monolayers

In this paper, we report the reactivity of fluoro-N,N,N',N'-tetramethylformamidinium hexafluorophosphate (TFFH), a reagent for transformation of carboxylic acids into acid fluorides in solution, toward self-assembled monolayers (SAMs) of 16-mercaptohexadecanoic acid on gold. Contrary to the solution-based reactions, we found that only interchain carboxylic anhydrides (ICAs), not acid fluorides (AFs), were obtained at surfaces by the facile interchain reaction under most reaction conditions studied. AFs were found to be formed only when tetrabutylammonium fluoride, a reagent inducing fast decomposition of ICAs, was added to the reaction mixture. The reactivity of TFFH toward carboxylic acid-terminated SAMs was different from that of cyanuric fluoride, which has been reported previously (Langmuir 2005, 21, 11765-11772). This study provides more insight into the role of the proximity effect in SAM-based reactions as well as another approach to the formation of ICAs from carboxylic acid-terminated SAMs.