Orientation Of Monolayers Research Articles

Characterization of Porphyrin Surface Orientation in Monolayers on Au(111) and Si(100) Using Spectroscopically Labeled Molecules

The synthesis and surface IR characterization are reported for a series of porphyrins bearing vibrational spectroscopic labels that afford distinction of the two in-plane axes of the porphyrin ring. The labeled porphyrins include three different types of tethers for surface attachment, those containing either methylthio (−CH2S−) or benzylthio (−BzS−) for attachment to either Au or Si and a tripodal alkenyl group for attachment to Si. The spectroscopic labels are placed so as to enable distinction between the methyl group of a p-tolyl substituent that lies along the molecular axis distal to the tether versus the p-tolyl substituents that lie along the orthogonal in-plane axis lateral to the tether. The porphyrins include isotopically labeled species containing CD3 and species wherein a CF3 replaces CH3. The spectroscopically labeled molecules allow evaluation of both the tilt angle (θ) with respect to the surface normal and the rotation angle (φ) about the molecular axis. These two angles cannot be uniquely determined for typical porphyrins because the in-plane modes of the porphyrin are (nearly) degenerate, and unique vibrational signatures cannot be identified that define the two orthogonal in-plane axes. The surface IR studies suggest that all of the porphyrins on both Au and Si exhibit a distribution of tilt and rotation angles. The distribution of φ angles is (nearly) random about the molecular axis; the distribution of tilt angles is less broad, owing to steric interactions between the porphyrin substituents and the surface. The surface coverage affects the distribution of both the tilt and rotation angles. At lower surface coverage, the molecules exhibit larger tilt angles and rotation angles, that is, the porphyrin is more coplanar with the surface. The fact that all of the porphyrins, which bear structurally different types of tethers, on both Au and Si exhibit qualitatively similar surface orientation characteristics suggests that the adsorption geometry is primarily controlled by properties intrinsic to the porphyrin macrocycle rather than by properties of the tether and/or the surface.

Controlled Adsorption Orientation for Double-Decker Complexes

Control of the molecular orientation of double-decker complexes comprised of rare-earth metals sandwiched with naphthalocyanine and porphyrin/phthalocyanine, at the interface of a graphite surface and 1-phenyloctane solution, has been demonstrated for the future application of molecular rotor assembly. Scanning tunneling microscope images clarified the internal structures of the top ligands. The observed images indicate that naphthalocyanine ligands were selectively adsorbed onto the graphite surface and the counterpart ligands of porphyrin/phthalocyanine were oriented off the surface. Control of the orientation of monolayers of mixed double-decker complexes has also been demonstrated.

Formation and structure of organized molecular films for fluorinated comb copolymers

AbstractThe molecular orientation in monolayer and multilayer films of fluorinated comb copolymers with various monomer ratios can be characterized by in‐plane and out‐of plane X‐ray diffraction (XRD), near‐edge X‐ray absorption fine structures (NEXAFS) spectroscopy at the C K‐edges, and atomic force microscopic observation. And further, the fine structure in the solid state and phase transition behavior of newly synthesized comb copolymers having fluorocarbon and hydrocarbon side‐chains were investigated by temperature controlled wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). From the WAXD profiles, two kinds of short spacing peaks based on the formation of the sub‐cell for fluorinated and hydrogenated side‐chains were confirmed at 5.0 and 4.1 Å, respectively. Furthermore, two kinds of endothermic peaks, which corresponded to melting peaks of both side‐chain crystals, appeared in the heating process of the DSC thermograms. From these experimental findings, the phase separation structure having the independently packed immiscible side‐chain crystalline formed in the whole polymer crystal. In addition, it was found that these comb polymers formed highly ordered (double) layer structures estimated using WAXD and small‐angle X‐ray scattering (SAXS). These fluorinated comb copolymers form the monolayer on the water surface and their transferred film with phase separated structure at nanometer size on the solid. There were hydrogenated domains at 10–20 nm diameter scales in these phase separated surface structures of the monolayers. From these experimental results, these copolymer monolayers are expected to be used as in new molecular devices such as nano‐lithography based on the surface patterning of polymer nano‐materials. Copyright © 2006 John Wiley & Sons, Ltd.

Flow-induced molecular orientation of amphiphile monolayers: Incorporation of hexatic elasticity into Ericksen-Leslie theory

By introducing the local lattice elasticity into the pure Ericksen-Leslie (EL) theory which only considers the deformation of the director, we developed a more general theory for understanding the behaviors of hexatic liquid crystal under flow. In the cases of amphiphilic monolayers in tilted phases (L2 and L'2), the exact solutions of the new EL equation in two types of flow, pure extension and simple shear, explain well most of the features of flow-induced tilt azimuth orientation observed by Fuller's group [Science, 274, 233 (1996)] and Schwartz's group [Nature (London) 410, 348 (2001)]. In particular, the "shear band" domain generated by flow discovered by the former is proved theoretically as the result of two-dimensional Wulf construction in L2 and L'2 phases.

Functional Monolayers for Improved Resistance to Protein Adsorption: Oligo(ethylene glycol)-Modified Silicon and Diamond Surfaces

The interaction of proteins with semiconductors such as silicon and diamond is of great interest for applications such as electronic biosensing. We have investigated the use of covalently bound oligo(ethylene glycol), EG, monolayers on diamond and silicon to minimize nonspecific protein adsorption. Protein adsorption was monitored by fluorescence scanning as a function of the length of the ethylene glycol chain (EG3 through EG6) and the terminal functional group (methyl- versus hydroxyl-terminated EG3 monolayer). More quantitative measurements were made by eluting adsorbed avidin from the surface and measuring the intensity of fluorescence in the solution. The attachment chemistry of the tri(ethylene glycol) molecules and monolayer orientation was studied by X-ray photoelectron spectroscopy. Improvement in the selectivity of surfaces modified with EG functionality was demonstrated in two model biosensing assays. We find that high-quality EG monolayers are formed on silicon and diamond and that these EG3 monolayers are as effective as EG3 self-assembled monolayers on gold at resisting nonspecific avidin adsorption. These results show promise for use of silicon and diamond materials in many potential applications such as biosensing and medical implants.

Molecular Orientation of Self-assembled Monolayer of Octadecanethiol on Platinum Surface Studied by Femtosecond Broad-bandwidth Sum Frequency Generation Spectroscopy

Abstract The molecular orientation in self-assembled monolayer (SAM) of octadecanethiol (ODT) on platinum surface prepared in ethanol solution was examined by broad-bandwidth sum frequency generation (BB-SFG) spectroscopy. The bands attributed to the CH3 symmetric and asymmetric stretching vibrational modes were observed in the SFG spectra. From the analysis of SFG intensity ratio of these two modes, the alkyl chain on platinum was found to be more perpendicular to the substrate than those on gold surface.

Detection of flexoelectric effect from 4-heptyloxy-4′-cyanobiphenyl monolayers at an air-water interface by Maxwell displacement current and optical second harmonic generation

The flexoelectric effect of 4-heptyloxy-4'-cyanobiphenyl (7OCB) monolayers at the air-water interface is studied by Maxwell displacement current (MDC) and optical second harmonic generation measurements. Though MDC was expected to increase during the compression of 7OCB monolayers in L2L2' phase from the MDC theory developed previously, decrease of MDC was detected in these phases. This abnormalous phenomenon is found to be due to the quench of flexoelectric effect by the flow orientation of monolayers.

Leucine Fastener Formation Mechanism between Peptide β-Sheets in a Monolayer Studied by Infrared Multiple-Angle Incidence Resolution Spectroscopy

A leucine zipper and a leucine fastener formed between peptide molecules have been hypothetical models of a molecular association via interdigitation. As a molecular interaction mechanism, a "leucine zipper" with the aid of an alpha-helix backbone in biological peptides is believed to play an important role in the molecular association, but no experimental evidence to prove the zipping has been reported thus far. In the same fashion, a "leucine fastener" built on the structure of peptide beta-sheets has never been experimentally captured either. In the present study, very fine changes of molecular stress and orientation in monolayers of a synthesized tetraleucine-containing amphiphile before and after the molecular interdigitation have readily been detected by infrared multiple-angle incidence resolution spectroscopy, which was recently developed for the analysis of structural anisotropy in thin materials. It has been suggested that the conventional molecular orientation model of the leucine fastener should be modified, and the backbone structure (parallel beta-sheet in the present study) plays a necessary role for the interlock of the leucine fastener.

Symmetry Control of Polymer Colloidal Monolayers and Crystals by Electrophoretic Deposition on Patterned Surfaces

Colloidal crystals with body-centered cubic packing (see Figure) can be fabricated by electrophoretic deposition of charged latex particles onto patterned surfaces. Laser-interference lithography produces SiO2 layers patterned with controlled symmetry that can then be used to control the orientation and symmetry of colloidal monolayers and crystals deposited onto the electrodes.

Monolayer–multilayer transitions in a lung surfactant model: IR reflection–absorption spectroscopy and atomic force microscopy

A hydrophobic pulmonary surfactant protein, SP-C, has been implicated in surface-associated activities thought to facilitate the work of breathing. Model surfactant films composed of lipids and SP-C display a reversible transition from a monolayer to surface-associated multilayers upon compression and expansion at the air/water (A/W) interface. The molecular-level mechanics of this process are not yet fully understood. The current work uses atomic force microscopy on Langmuir-Blodgett films to verify the formation of multilayers in a dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylglycerol, cholesterol, and SP-C model system. Isotherms of SP-C-containing films are consistent with exclusion and essentially complete respreading during compression and expansion, respectively. Multilayer formation was not detected in the absence of SP-C. Most notable are the results from IR reflection-absorption spectroscopy (IRRAS) conducted at the A/W interface, where the position and intensity of the Amide I band of SP-C reveal that the predominantly helical structure changes its orientation in monolayers versus multilayers. IRRAS measurements indicate that the helix tilt angle changed from approximately 80 degrees in monolayers to a transmembrane orientation in multilayers. The results constitute the first quantitative measure of helix orientation in mixed monolayer/multilamellar domains at the A/W interface and provide insight into the molecular mechanism for SP-C-facilitated respreading of surfactant.

Molecule orientation in self-assembled monolayers determined by infrared-visible sum-frequency generation spectroscopy

The molecular orientation in self-assembled monolayers of thiophenol (TP) on Ag(1 1 1) and of para-nitroanilino-dodecane-thiol (NAT) on Au was investigated by infrared-visible sum-frequency generation between 500 and 1500 cm−1. Ab initio calculations were performed to determine the interface nonlinear response. The combined experimental and theoretical results reveal that the S–C bond of TP is tilted 37∘ from the Ag(1 1 1) surface normal, with the aromatic ring plane perpendicular to the surface. On Au, the tilt angle of the NAT molecule stabilizes below 60∘ at the end of film growth.

Characterization of the Molecular Orientation of Self-Assembled Monolayers of Alkanethiols on Obliquely Deposited Gold Films by Using Infrared−Visible Sum-Frequency Spectroscopy

This paper reports the use of infrared (IR)−visible sum-frequency spectroscopy to characterize the surface of gold films deposited by physical vapor deposition at an oblique angle of incidence and the orientation of the methyl end groups of self-assembled monolayers (SAMs) of n-alkanethiols formed on these gold films. The oblique deposition of the gold breaks the macroscopic azimuthal degeneracy that is characteristic of gold films grown without a preferred direction of deposition. By tuning the frequency of the incident IR light to either the symmetric or the asymmetric methyl stretch resonance and by recording the azimuthal dependence of the sum-frequency signal, the orientation of the methyl groups of the n-alkanethiols on the obliquely deposited gold films were shown to be distributed around a preferred in-plane direction coincident with the direction of the gold deposition. The angles between the normal of the sample and the methyl groups were determined to be 85 ± 2° and 44 ± 10° for the SAMs formed from pentadecanethiol and hexadecanethiol, respectively. In contrast, when using gold films that were not obliquely deposited, no preferred in-plane orientation of the methyl groups could be observed.

Quantitative Functional Group Orientation in Langmuir Films by Infrared Reflection−Absorption Spectroscopy: CO Groups in Behenic Acid Methyl Ester and sn2-13C-DSPC

IR reflection−absorption spectroscopy (IRRAS) is used to determine the orientation of carbonyl groups in behenic acid methyl ester (BME) and in sn2-13C 1,2 distearoylphosphatidylcholine (sn2-13C-DSPC) in aqueous monolayer films. The CO stretching contour of BME consists of three overlapped peaks arising from unhydrated, monohydrated, and doubly hydrated forms at 1737, 1721, and 1702 cm-1, respectively. Dichroic ratios constructed from the p- and s-polarized IRRAS intensities of the resolved components of the contour were simulated with the optical theory of V. L. Kuzmin.1,2 On the basis of valence bond and molecular modeling considerations, the carbonyl group is essentially perpendicular to the chain axes. The transition moment is determined by IRRAS to indeed lie along the CO bond for the unhydrated groups (thus providing good support for the theory) but changes direction for hydrated groups. A satisfactory fit between simulation and experiment could not be obtained by assuming a uniaxial distribution of the zigzag all-trans methylene chain to which the carbonyl group is rigidly attached. A slight preferential orientation of the chain planes in the direction perpendicular to that of the trough barrier movement is found for both BME and sn2-13C DSPC. In addition, spectral quality in the 1000−1300 cm-1 region for BME was sufficient to determine that the orientation of the single-bond C−O stretch of the ester moiety was within 20° of the vertical. For sn2-13C-DSPC, each CO bond stretching contour was split into a doublet arising from hydrated and unhydrated forms. The best fit to the IRRAS data for the unhydrated CO mode in each chain led to a suggested chain tilt of 30°. This value, in good accord with X-ray diffraction measurements,3 was confirmed through analysis of the CH2 stretching vibrations. The transition moments of the hydrated CO bonds in sn2-13C-DSPC were again found to not lie along the bond direction. The current analysis provides a new method for determining functional group orientation in monolayers. The approach, when coupled with X-ray and neutron reflectivity measurements, will provide unique information about the structures and interactions of film constituents.

Orientation of o-, m-, and p-Methylbenzylmercaptans Adsorbed on Au(111) Probed by Broad-Bandwidth Sum Frequency Generation Spectroscopy

The molecular orientation in self-assembled monolayers of ortho, meta, and para isomers of methylbenzylmercaptans (OMBM, MMBM, and PMBM, respectively) on Au(111) has been studied using broad-bandwidth sum frequency generation spectroscopy and cyclic voltammetry. Cyclic voltammograms for the reductive desorption suggest that all methylbenzylmercaptans form closely packed monolayers. Sum frequency vibrational spectra for OMBM, MMBM, and PMBM show the CH3 symmetric stretching vibration mode. The vibrational intensities are greatest in PMBM and smallest in OMBM, implying the difference in the orientation of the methyl group. The vibrational intensities are compared with the hyperpolarizabilities of methylbenzylmercaptans from a calculation based on the density functional theory. OMBM molecules adsorb on Au(111) tilting their benzyl moieties so that the direction of the methyl group is parallel with the surface, whereas MMBM and PMBM molecules adsorb on Au(111) with their benzyl moieties being perpendicular to the surface.

Orientation modulation of a synthetic polypeptide in self-assembled monolayers: a TOF-SIMS study.

Structure and orientation of molecules are key properties of functionalized surfaces. Using time-of-flight secondary ion mass spectrometry (TOF-SIMS), here we investigate how to modulate these parameters upon the immobilization process varying the conditions of self-assembly. The molecule of interest, a template-assembled synthetic protein (TASP), consists of a central peptide ring with orthogonally arranged residues. Thioalkane chains allow the directed self-assembly of the molecule on a gold surface; four serine residues on the opposite side of the ring can be used as anchoring sites for various functional sensing molecules. The TASP conformation and its orientation in self-assembled monolayers (SAMs) play a central role for the accessibility of these serine residues. To study the influence of the self-assembly conditions, two series of samples were prepared. Pure TASP monolayers of different surface densities are compared to mixed TASP/alkanethiol monolayers prepared by sequential adsorption varying sequence and particular incubation times as well as by coadsorption modifying incubation times and TASP/alkanethiol mass ratios. Switching the TASP orientation from a state where the molecules are lying flat on the surface to an upright orientation turned out to be possible by inserting the TASP into a preformed alkanethiol monolayer of an appropriate surface density. This study demonstrates that TOF-SIMS is an excellent tool not only to investigate the surface composition, but also the molecular structure of functionalized surfaces.

Relation between rheological properties and structural changes in monolayers of model lung surfactant under compression

rSP-C surfactant monolayers spread on a native physiological model substrate show two plateau regions in the π/ A-isotherm. The first corresponds to the main phase transition in the monolayer from a LE to a LC phase. Its course is non-horizontal because of the complex composition of the lung surfactant. The second plateau, which is much more pronounced, cannot be attributed to a change of the phase state. Brewster angle microscopy images taken in this region show a sharp apparent decrease of the aggregation degree from the LE to the LC state. This process can be considered as a change in the monolayer orientation relative to the direction of the propagated light. Such a change can be the result of monolayer folding and formation of a thicker layer, which is supported by results of rheological measurements. The dilatation elasticity obtained from oscillating barrier and longitudinal wave measurements reveals a pure elastic behaviour with a steep increase in the second plateau region. Because of the insolubility of the pure lipid components, a possible explanation is squeezing protein components of rSP-C or its complexes with lipids out of the monolayer into the bulk.

Compression-shear-induced tilt azimuthal orientation of amphiphilic monolayers at the air-water interface: a C(infinity)-->C2v transition in the flow of a two-dimensional hexatic structure.

Compression-shear-induced tilt azimuthal orientation of amphiphilic monolayer in tilting phases (L2 and L'2) at the air-water interface is analyzed as dynamical equilibrium of the elastic distortion of orthogonally hexagonal structure of the molecules under compression-induced shear flow. It is shown theoretically that the compression can induce molecular tilts lying along and/or against the flow direction. All these tilts makeup the initial random tilt azimuth of the molecular tails along a uniform direction. At a threshold compression speed, it causes a C(infinity)-->C(2v)-symmetry transition at the air-water interface. With Maxwell displacement current and optical second-harmonic generation measurements, the above theoretical results are verified experimentally in a monolayer of 4-heptyloxy-4(')-cyanobiphenyl.

Vibrational Sum-Frequency Studies of a Series of Phospholipid Monolayers and the Associated Water Structure at the Vapor/Water Interface

The orientation and conformation of phospholipid monolayers and the structure of water associated with these monolayers adsorbed at the vapor/water interface have been investigated using vibrational sum-frequency spectroscopy. The phospholipids studied are saturated diacyl phosphatidylcholines, phosphatidylethanolamines, phosphatidylglycerols, and phosphatidylserines with chain lengths of 14, 16, and 18 carbons. Spectra have been acquired under different polarization schemes for monolayers at the vapor/D2O interface to examine chain ordering and at the vapor/H2O interface to examine water structure. Comparisons are made between the monolayers based on chain length and headgroup differences. These studies show significant differences between the aqueous environments associated with the zwitterionic lipids and the charged lipids.

Bonding and Orientation in Self-Assembled Monolayers of Oligophenyldithiols on Au Substrates

The structure and orientation within prototype self-assembled monolayers (SAMs) containing aromatic subunits absorbed on Au substrates has been investigated using infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). Several oligophenyl(di)thiols, namely biphenylthiol (HS−C6H4−C6H5, BPT), biphenyldithiol (HS−C6H4−C6H4−SH, BPDT), biphenyldimethyldithiol (HS−CH2−C6H4−C6H4−CH2−SH, BPDMT), and terphenyldimethyldithiol (HS−CH2−C6H4−C6H4−C6H4−CH2−SH, TPDMT) have been studied. At least one of these molecules, BPDT, has recently been discussed in connection with the fabrication of a simple electronic device using the aromatic oligophenylthiolate SAM as an active component. Our results reveal that organodithiols with a short oligophenyl backbone, that is, BPDT and BPDMT, do not form well oriented layers but demonstrate that longer backbones, for example, a terphenyl unit, do indeed lead to the formation of SAMs with a high degree of molecular orientation.

Order and Orientation in Self-Assembled Long Chain Alkanephosphate Monolayers Adsorbed on Metal Oxide Surfaces

Octadecylphosphoric acid ester was employed to establish monolayers on various flat oxide substrates. The films were prepared via self-assembly from a heptane/propan-2-ol solution. The alkanephosphate was found to produce oriented, well-ordered films on titanium oxide, niobium oxide, and aluminum oxide surfaces. Contact-angle measurements and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) indicate that these layers closely resemble those formed by a similar thiol−gold system, with respect to packing density, inclination, and molecular order. The preparation scheme appears to be rather universally applicable to a variety of metal oxide substrates. These systems show promise as an approach to functionalizing different oxide surfaces with well-ordered organic monolayers, with potential applications in the fields of corrosion protection, adhesion promotion, biochemical analysis, and sensors.