Modulation Infrared Reflection Absorption Spectroscopy Research Articles

CO oxidation over Ru(0 0 0 1) at near-atmospheric pressures: From chemisorbed oxygen to RuO 2

RuO 2(1 1 0) was formed on Ru(0 0 0 1) under oxygen-rich reaction conditions at 550 K and high pressures. This phase was also synthesized using pure O 2 and high reaction temperatures. Subsequently the RuO 2 was subjected to CO oxidation reaction at stoichiometric and net reducing conditions at near-atmospheric pressures. Both in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and post-reaction Auger electron spectroscopy (AES) measurements indicate that RuO 2 gradually converts to a surface oxide and then to a chemisorbed oxygen phase. Reaction kinetics shows that the chemisorbed oxygen phase has the highest reactivity due to a smaller CO binding energy to this surface. These results also show that a chemisorbed oxygen phase is the thermodynamically stable phase under stoichiometric and reducing reaction conditions. Under net oxidizing conditions, RuO 2 displays high reactivity at relatively low temperatures (⩽450 K). We propose that this high reactivity involves a very reactive surface oxygen species, possibly a weakly bound, atomic oxygen or an active molecular O 2 species. RuO 2 deactivates gradually under oxidizing reaction conditions. Post-reaction AES measurements reveal that this deactivation is caused by a surface carbonaceous species, most likely carbonate, that dissociates above 500 K.

Optimizing the PMIRRAS signal from a multilayer system and application to self-assembled monolayers in contact with liquids

Polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) has become a powerful technique for the study of thin films on metal surfaces. We present a simple tool which allows the calculation and optimization of the PMIRRAS signal in multilayer systems. In order to illustrate the technique, an in situ experimental study of self-assembled monolayers on gold in contact with a μ m thick water film is presented and the results compared with the calculation. A systematic series of measurements was performed on such a system and the signal intensity was recorded as a function of incident angle of the infrared beam. A strong, but broad peak in the signal-to-noise is observed at about 70 ° for an absorption peak at 1115 cm −1, but its position strongly depends on the thickness of the water layer. This and other examples illustrate the importance of the calculations and simulations for the optimization of experimental parameters.

Characterizing Changes In The Structure And Orientation Of Supported Model Membranes Upon Binding Of Cholera Toxin B

Cholera is caused by a protein toxin secreted by the bacterium Vibrio cholerae. This toxin is a member of the AB5 superfamily of toxins composed of an active (A) and a binding (B) pentameric unit. The five binding subunits of the pentamer bind specifically to the GM1, which is present in the outer leaflet of the host's plasma membrane; however, the mechanism for the translocation of the toxin is not well understood. Model membranes consisting of DMPC+cholesterol+GM1 were supported on gold electrode surfaces. The changes in the structure and orientation of the model membrane upon binding of the cholera toxin B unit were explored using differential capacitance, chronocoulometry and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Changes in the structure and orientation of the toxin protein with respect to a changing electric field were similarly investigated. The IR data suggests that the binding of the toxin to the membrane causes a decrease in the number of gauche conformers, which is caused by the constriction of the acyl chains due to interactions between the toxin and the GM1 glycolipids. The bound toxin induces some minor defects in the membrane; however, these defects are not significant enough to cause measurable changes in the average orientation of the membrane lipids. The major change in the bilayer upon binding of the cholera B unit was a remarkable decrease in the relative hydration of the membrane. This decrease in hydration is mostly like due to the separation of the bilayer from the aqueous electrolyte by the presence of the bound protein layer on the membrane surface. This work is part of ongoing study to understand the mechanism for translocation of the cholera toxin across the plasma membrane.

Electric Field Driven Conformational Changes of Gramicidin D in a Model Membrane Supported on a Au(111) Electrode Surface

In this work, we show molecular resolution scanning tunnelling microscopy (STM) images of gramicidin, a model antibacterial peptide, inserted into a phospholipid matrix supported at a gold electrode surface. The resolution of the images is superior to that obtained in previous attempts to image gramicidin in a lipid environment using atomic force microscopy (AFM). This breakthrough has allowed visualization of individual peptide molecules surrounded by individual lipid molecules. We have observed several important features: the peptide molecules do not aggregate, the peptide molecules adopt a single conformation corresponding to a specific ion channel form, and the lipid molecules adjacent to the peptide molecules are systematically longer than those in the lipid matrix. These results constitute a new approach to obtain structural characteristics of antibiotic peptides in lipid assemblies that is necessary for the understanding of their biological activity.We then applied the polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) to investigate the effect of the electric field on the conformation and orientation of gramicidin molecules in a bilayer supported at the gold electrode surface. We observed potential controlled changes in the orientation and conformation of the gramicidin molecules in the supported bilayer. Careful analysis of the IR data indicated that the potential applied to the electrode affects the bilayer structure and these changes cause reorientation and conformational transformations of gramicidin molecules.

Selective Dissociation of O3 and Adsorption of CO on Various Au Single Crystal Surfaces

The O3 dissociation and CO adsorption on various gold single crystal surfaces were investigated using X-ray photoelectron spectroscopy, temperature-programmed desorption, and polarization modulation infrared reflection absorption spectroscopy (PM-IRAS). Dissociation of O3 proceeded on the Au(111) surface at 323 K, whereas no O3 dissociation occurs on the Au(100) surface, indicating that the O3 dissociation on gold strongly depends on the structure of the gold surface. PM-IRA peaks of CO adsorbed on Au(111) and Au(100) were observed around 2,080 cm−1 at CO pressures above 0.5 Torr at 273 K. On the other hand, the peak of CO adsorbed on step sites was observed at 2,117 cm−1 above 0.01 Torr for Au(311). Thus, CO adsorbed preferentially onto the step sites of the gold surface.

XPS, time-of-flight-SIMS and polarization modulation IRRAS study of Cr 2O 3 thin film materials as anode for lithium ion battery

Ultra-thin Cr 2O 3 films (12.0, 17.3 and 29.6 nm thick) were produced on Cr metal by thermal oxidation, and their electrochemical properties in 1 M LiClO 4 in propylene carbonate (PC) were investigated by cyclic voltammetry and chronopotentiometry. The reductive electrolyte decomposition and the conversion/deconversion process were observed and analyzed by X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The initial irreversible capacity due to the reduction of electrolyte and the incomplete deconversion process during the first cycle is 70% of the first discharge capacity. A stable charge/discharge capacity of 460 mAh g −1 was obtained in the 3rd to 10th cycles. XPS and PM-IRRAS evidenced the growth of a solid electrolyte interphase (SEI) layer that is constituted of Li 2CO 3 formed by reductive decomposition of the electrolyte. The SEI layer thickness and/or density is modified by the conversion/deconversion reaction. ToF-SIMS evidenced the volume expansion/shrink resulting from the conversion/deconversion reaction. ToF-SIMS also revealed an incomplete conversion process limited by mass transport, which partitions the oxide into a converted outer part assigned to Li 2O containing Cr traces and an unconverted inner part ascribed to Cr 2O 3 or lower Cr oxide containing Li. It was found that the deconversion re-homogenizes the oxide film in a single layer but with lithium trapped in it. The present study provides a detailed understanding of the interfacial reaction on the oxide anode undergoing a conversion/deconversion reaction.

SAMs on Gold Derived from the Direct Adsorption of Alkanethioacetates Are Inferior to Those Derived from the Direct Adsorption of Alkanethiols

Self-assembled monolayers (SAMs) on gold derived from the direct adsorption of thioacetic acid S-decyl ester (C10SAc) and thioacetic acid S-octadecyl ester (C18SAc) were compared to the corresponding SAMs derived from the analogous adsorption n-decanethiol (C10SH) and n-octadecanethiol (C18SH). All SAMs were characterized using ellipsometry, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). The comparison revealed that the SAMs generated from the thioacetates are not as densely packed and well ordered as the SAMs generated from the thiols. Furthermore, studies of the kinetics of adsorption found that the thioacetates adsorb more slowly than the corresponding thiols.

The positive influence of electrochemical cyclic potentiodynamic passivation (CPP) of a SS316LS surface on its response to fibronectin and pre-osteoblasts

The influence of an electrochemical surface passivation technique (cyclic potentiodynamic polarization, CPP) on the physico-chemical surface properties of SS316LS and its subsequent response to fibronectin (Fn) and pre-osteoblasts were investigated. Contact angle and zeta-potential measurements showed that the CPP-modified surface is more hydrophilic and more positively charged than the unmodified surface. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was used to investigate the interaction of Fn with both surfaces. The saturated surface concentration of adsorbed Fn was higher on the CPP-modified surface. As well, significant changes were identified in the secondary structure of Fn adsorbed on both surfaces, in comparison to its native state. This data also indicated a higher degree of Fn unfolding on the CPP-modified surface. Cell studies indicated that the attachment, proliferation and morphology of pre-osteoblasts were significantly improved on the CPP-modified surface, which was attributed to the more open conformation of Fn on the CPP-modified surface. Thus, the CPP surface passivation method was demonstrated to yield a SS316LS surface of enhanced biocompatibility.

Surface reaction and transport kinetics of hydrogen through palladium-based membranes under gas co-existing with hydrogen atmospheres

In order to understand how gas co-existing with hydrogen affects the hydrogen permeability of a silver 23wt%-palladium membrane, the surface reaction of hydrogen was evaluated from the continuity of the surface reaction rate and the bulk diffusion flux of hydrogen based on the results of hydrogen-permeation measurements. The interference effect of the co-existing species was quantified as a function of temperature, partial pressure and the components of the co-existing gas. In order to investigate the behaviour of the co-existing species on the membrane surface, in situ observation using the Polarisation Modulated Infrared Reflection Absorption Spectroscopy (PM-IRRAS) was carried out. An infrared absorption peak due to the adsorption of carbon monoxide on the membrane was observed under the atmosphere of 7.8% carbon monoxide – 2.3% water vapour – 89.9% hydrogen. From the dependence of the infrared absorption peak on temperature, co-existing carbon monoxide was found to adsorb onto the membrane surface irreversibly.

CO Oxidation on Pt-Group Metals from Ultrahigh Vacuum to Near Atmospheric Pressures. 1. Rhodium

The CO oxidation reaction on Rh(111) was studied both at low pressures (≤2 × 10−4 Torr) under steady-state conditions and at high pressures (0.01−88 Torr) in a batch reactor at various gaseous reactant compositions. Surface CO and O coverages were determined using polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and X-ray photoelectron spectroscopy (XPS). CO titration experiments were also carried out on surfaces with known oxygen coverages. Both CO and O inhibition were evident at low pressures so that only within a relatively narrow temperature range were the reaction conditions optimized such that the CO conversion reached ∼20% of the CO flux to the surface. For high pressures and with stoichiometric or slightly oxidizing reactant ratios (O2/CO ≤ 2), the reaction fell into three regimes: (i) a CO-inhibited low temperature regime where the reaction rate was determined by CO desorption; (ii) a mass transfer limited regime at high temperatures; and (iii) a transient, high-rate regime lying between regimes (i) and (ii) where the reaction was not completely controlled by mass transfer limitation. For all reaction conditions investigated (when O2/CO ≤ 2), the surface oxygen coverage did not exceed ∼0.5 monolayer. With very oxidizing reactants (O2/CO ≥ 5), the reactivity of the Rh surface decreased dramatically at high temperatures due to oxidation. Furthermore, the so-called “superior oxide reactivity” for CO oxidation that has been proposed in several recent studies is not evident in this investigation.

Solid‐supported amphiphilic triblock copolymer membranes grafted from gold surface

AbstractGold‐supported amphiphilic triblock copolymer brushes composed of two hydrophilic poly(2‐hydroxyethyl methacrylate) (PHEMA) blocks and a hydrophobic poly(n‐butyl methacrylate) (PBMA) middle part were synthesized using a surface‐initiated ATRP. Attenuated total reflectance Fourier transform infrared spectroscopy, polarization modulation infrared reflection absorption spectroscopy (PM‐IRRAS), ellipsometry, contact angle measurements, and atomic force microscopy were used for the characterization of PHEMA‐co‐PBMA‐co‐PHEMA brushes. The PM‐IRRAS analysis revealed an increase of the chain tilt toward the gold surface during growth of the individual blocks. We suggest that the orientation of the amphiphilic polymer brushes is influenced by both the chain length and the interchain interactions. Additionally, a detachment of the polymer membranes from the solid support and subsequent gel permeation chromatography analyses allowed us to establish their compositions. We applied block‐selective solvents (water and hexane) as well as a good solvent for the whole polymer chain (ethanol) to study the morphology and solvent responsive behavior of the amphiphilic brushes. The presented results could serve as a good starting point for the fabrication of functional solid‐supported membranes for biosensing applications. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1–13, 2009

Electrochemical and PM-IRRAS a Glycolipid-Containing Biomimetic Membrane Prepared Using Langmuir−Blodgett/Langmuir−Schaefer Deposition

Differential capacitance, chronocoulometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to characterize the structure and orientation of a DMPC + cholesterol + GM 1 (60:30:10 mol %) bilayer supported at a Au(111) electrode surface prepared using combined Langmuir-Blodgett/Langmuir-Schaefer (LB/LS) deposition. The electrochemical measurements indicate that the incorporation of ganglioside GM 1 into the membrane significantly improves the quality of the bilayer, reflected in the very low capacitance value of approximately 0.8 microF cm (-2). An analysis of the infrared data suggests that the incorporation of the glycolipid into the membrane changes both the orientation of the lipid acyl chains in the membrane and the hydration of the membrane, particularly with respect to the interfacial region of the lipids.

Reversible Exchange of Self-Assembled Monolayers of Semifluorinated n-Alkanethiols and n-Alkanethiols on Au/Mica Surfaces

Scanning tunnelling microscopy (STM) and photoelastic modulation infrared reflection absorption spectroscopy (PM-IRRAS) have been used to investigate the exchange of self-assembled monolayers (SAMs) of n-octanethiolate by semifluorinated n-octanethiolate from ethanolic solution at 340 K. The process occurs via displacement of the thiols from domain boundaries. By contrast, the reverse exchange process occurs more homogeneously via the formation of mixed thiolate domains. The ability of semifluorinated octanethiolate to adopt a range of adsorption sites with approximately similar adsorption energies may lead to this contrasting behavior.

Brewster Angle Microscopy and PMIRRAS Study of DNA Interactions with BGTC, a Cationic Lipid Used for Gene Transfer

The lipid bis(guanidinium)-tris(2-aminoethyl)amine-cholesterol (BGTC) is a cationic cholesterol derivative bearing guanidinium polar headgroups which displays high transfection efficiency in vitro and in vivo when used alone or formulated as liposomes with the neutral colipid 1,2-di-[ cis-9-octadecenoyl]- sn-glycero-3-phosphoethanolamine (DOPE). Since transfection may be related to the structural and physicochemical properties of the self-assembled supramolecular lipid-DNA complexes, we used the Langmuir monolayer technique coupled with Brewster angle microscopy (BAM) and polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) to investigate DNA-BGTC and DNA-BGTC/DOPE interactions at the air/water interface. We herein show that BGTC forms stable monolayers at the air/water interface. When DNA is injected into the subphase, it adsorbs to BGTC at 20 mN/m. Whatever the (+/-) charge ratio of the complexes used, defined as the ratio of positive charges of BGTC in the monolayer versus negative charges of DNA injected in the subphase, the DNA interacts with the cationic lipid and forms either an incomplete (no constituent in excess) or a complete (DNA in excess) monolayer of oriented double strands parallel to the lipid monolayer plan. We also show that, under a homogeneous BGTC/DOPE (3/2) monolayer at 20 mN/m, DNA adsorbs homogeneously to form an organized but incomplete layer whatever the charge ratio used (DNA in default or in excess). Compression beyond the collapse of these mixed DNA-BGTC/DOPE systems leads to the formation of dense DNA monolayers under an asymmetric lipid bilayer with a bottom layer of BGTC in contact with DNA and a top layer mainly constituted of DOPE. These results allow a better understanding of the mechanisms underlying the formation of the supramolecular BGTC-DNA complexes efficient for gene transfection.

In situ detection and characterization of potable water biofilms on materials by microscopic, spectroscopic and electrochemistry methods

We studied biofilm formation on stainless steel occurring in a drinking water distribution system which operated in parallel at 20 and 37 °C, in order to focus on the effect of temperature rather than on other operational and water quality parameters. A surface conditioning step was followed as a function of time on this material until adhesion of bacterial colonies by using microscopic methods: scanning electron microscopy (SEM) and atomic force microscopy (AFM); a spectroscopic method: polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and an electrochemical method: rotating disk electrode (RDE). Correlations between surface analysis, the duration of immersion of the sample and the influence of temperature have been identified clearly before bacterial adhesion. In cold water, these results showed an initial conditioning step of surface occurring during the first 8 days, with detection of superficial acidic functions grafted on surface, until adsorption of proteins. After 12 days, formation of independent bacteria microcolonies, reaching 2–3 μm in length was observed. In tepid water, the first step was reduced to 2 days during which carbonates, acidic functions, and proteins were detected. After 90 days, the biofilm entered in a stable population state, which appeared as a bacteria rich film, including possibly Legionella. The spatial variation of the biofilm was limited as deduced from the thickness determination (about 4 μm for 3-month period), using a RDE. The combination of these different techniques confirms successive steps for biofilm formation on stainless steel in a tap water. Then, we scrutinized the external near environment of bacteria including extracellular polymeric substances (EPS) and then further characterize the morphology of dominant bacteria (shape, size, flagellum) on gold substrate by AFM in air.

Formate stability and carbonate hydrogenation on strained Cu overlayers on Pt(1 1 1)

Formate (HCOO) synthesis, decomposition and the hydrogenation of carbonate (CO 3) on Cu overlayers deposited on a Pt(1 1 1) single crystal are investigated to examine the reactivity of a Cu surface under tensile strain with defects present. Formate is synthesized from a 0.5 bar mixture of 70% CO 2 and 30% H 2 at varying temperatures, and the evolution is followed with polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Subsequent TPD reveals decomposition of the formate species into CO 2 and H 2 at 420 ± 5 K for strained Cu at sub-monolayer to monolayer coverages. This is a significantly lower decomposition temperature than obtained earlier on pristine Cu(1 1 1) (∼460 K), as well as for thicker Cu layers where we assign an observed decomposition peak at 440 ± 5 K to relaxed, but defect-rich Cu(1 1 1). However, the thermal stability of formate on strained and defect-rich Cu is similar to previous results obtained for supported, and lattice-strained, Cu nanoparticles. The hydrogenation of carbonate produced by 0.3 bar CO 2 exposure at room temperature was monitored with XPS and TPD showing a significant loss of carbonate when subjected to 0.2 bar H 2 at room temperature. However, the presence of formate on the surface, or any other hydrogenation product, could not be established during or after H 2 exposure by PM-IRRAS, EELS or TPD. Even so, the results suggest that carbonate and its hydrogenation may constitute a relevant pathway to methanol production.

A High Pressure PM-IRRAS Study of CO and O2 Coadsorption and Reactivity on PtSn Alloy Surfaces

The adsorption of CO on the two terminations, p(2 × 2) and (√3 × √3) R30°, of the bulk Pt3Sn(111) alloy is studied by high pressure polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The influence of O2 is also investigated. While no differences are observed when CO is chemisorbed alone, the behavior of the two terminations clearly differs in presence of O2. The presence of oxygen induces modifications on the (√3 × √3)R30° surface. Finally we study how the interaction between O2 and the surface modifies the reactivity. Hence we show that the p(2 × 2) termination is more efficient than the (√3 × √3) R30° for CO oxidation. However this termination remains more efficient than pure platinum.

Characterisation of morphology of self-assembled PEG monolayers: a comparison of mixed and pure coatings optimised for biosensor applications

For detection of low concentrations of analytes in complex biological matrices using optical biosensors, a high surface loading with capture molecules and a low nonspecific binding of nonrelevant matrix molecules are essential. To tailor biosensor surfaces in such a manner, poly(ethylene glycols) (PEG) in varying lengths were immobilised covalently onto glass-type surfaces in different mixing ratios and concentrations, and were subsequently modified with three different kinds of receptors. The nonspecific binding of a model protein (ovalbumin, OVA) and the maximum loading of the respective analytes to these prepared surfaces were monitored using label-free and time-resolved reflectometric interference spectroscopy (RIfS). The three different analytes used varied in size: 150 kDa for the anti-atrazine antibody, 60 kDa for streptavidin and 5 kDa for the 15-bp oligonucleotide. We investigated if the mixing of PEG in different lengths could increase the surface loadings of analyte mimicking a three-dimensional matrix as was found using dextrans as sensor coatings. In addition, the effect on the surface loading was investigated with regard to the size of the analyte molecule using such mixed PEGs on the sensor surface. For further characterisation of the surface coatings, polarisation modulation infrared reflection absorption spectroscopy, atomic force microscopy, and ellipsometry were applied.

Polarization Modulation Infrared Reflection Absorption Spectroscopy Investigations of Thin Silica Films Deposited on Gold. 2. Structural Analysis of a 1,2-Dimyristoyl-sn-glycero-3-phosphocholine Bilayer

In this paper we report on the structural analysis of bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) using polarization modulation infrared reflection absorption spectroscopy (PM IRRAS). The lipid bilayers were formed on SiO2|Au and Au surfaces using the Langmuir-Blodgett and Langmuir-Schaeffer techniques. As we showed in part 1 (Zawisza, I.; Wittstock, G.; Boukherroub, R.; Szunertis, S. Langmuir 2007, 23, 9303-9309), SiO2 layers of 7 nm thickness, synthesized by plasma-enhanced chemical vapor deposition on 200 nm thick gold covered glass slides, allow PM IRRAS investigations. Only minor changes in the order and structure of the lipid bilayer are observed when deposited on SiO2|Au and Au surfaces. The choline moiety in the leaflet directed toward the SiO2 surface exists in trans conformation and shows a tilt of 28 degrees with the surface normal of the CN bond. On the silica surface in the second leaflet directed toward air and in two layers deposited on the Au surface, trans and gauche isomers of the choline moiety are present and the tilt of the CN bond increases to 55 degrees with respect to the surface normal. The order and molecular orientation in the DMPC bilayers on SiO2 and Au surfaces are not affected by time. The analysis of the phosphate stretching mode on the Au surface shows slight dehydration of this group and reorientation of the phosphate moiety.

Secondary structure of a truncated form of lecithin retinol acyltransferase in solution and evidence for its binding and hydrolytic action in monolayers

Lecithin retinol acyltransferase (LRAT) is a 230 amino acids membrane-associated protein which catalyzes the esterification of all- trans-retinol into all- trans-retinyl ester. The enzymatic activity of a truncated form of LRAT (tLRAT) which contains the residues required for catalysis but which is lacking N- and C-terminal hydrophobic segments has been shown to depend on the detergent used for its solubilization. Moreover, it is unknown whether tLRAT can bind membranes in the absence of these hydrophobic segments. The present study has allowed to measure the membrane binding and hydrolytic action of tLRAT in lipid monolayers by use of polarization modulation infrared reflection absorption spectroscopy and Brewster angle microscopy. Moreover, the proportion of the secondary structure components of tLRAT was determined in three different detergents by infrared absorption spectroscopy, vibrational circular dichroism and electronic circular dichroism which allowed to explain its detergent dependent activity. In addition, the secondary structure of tLRAT in the absence of detergent was very similar to that in Triton X-100 thus suggesting that, compared to the other detergents assayed, the secondary structure of this protein is very little perturbed by this detergent.