Reflection Absorption Spectroscopy Research Articles

Vibrational properties of CO2 adsorbed on the Fe3O4 (111) surface: Insights gained from DFT.

By virtue of density functional theory calculations, this work discusses several carbonate, carboxylate, and bicarbonate species on two thermodynamically relevant metal terminations of the (111) surface of magnetite, Fe3O4. We present adsorption energies and vibrational wavenumbers and conclude in assigning the observed infrared reflection-absorption spectroscopy bands. CO2 prefers to adsorb molecularly on the Fetet1 terminated Fe3O4(111) surface, a finding consistent with observation. Calculations compared with the experiment lead to interpreting results in favor of the Fetet1 (single metal) terminated Fe3O4(111) surface as the regular surface termination. Formation of carbonate and bicarbonate requires metal impurities on that surface. Such impurities exist, for instance, on the Feoct2 (double metal) termination, which can thus be used as a model for "metal-rich terminations" of more complex surfaces.

Phase Diagram for a Lysyl-Phosphatidylglycerol Analogue in Biomimetic Mixed Monolayers with Phosphatidylglycerol: Insights into the Tunable Properties of Bacterial Membranes.

Ion pairing between the major phospholipids of the Staphylococcus aureus plasma membrane (phosphatidylglycerol – PG and lysyl‐phosphatidylglycerol – LPG) confers resistance to antimicrobial peptides and other antibiotics. We developed 3adLPG, a stable synthetic analogue which can substitute for the highy‐labile native LPG, in biophysical experiments examining the membrane‐protecting role of lipid ion pairing, in S. aureus and other important bacteria. Here we examine the surface charge and lipid packing characteristics of synthetic biomimetic mixtures of DPPG and DP3adLPG in Langmuir monolayers, using a combination of complementary surface‐probing techniques such as infrared reflection‐absorption spectroscopy and grazing‐incidence x‐ray diffraction. The resultant phase diagram for the ion paired lipids sheds light on the mixing behavior of lipids in monolayer models of resistant phenotype bacterial membranes, and provides a platform for future biophysical studies.

Doping-Induced Electron Transfer at Organic/Oxide Interfaces: Direct Evidence from Infrared Spectroscopy.

Charge transfer at organic/inorganic interfaces critically influences the properties of molecular adlayers. Although for metals such charge transfers are well documented by experimental and theoretical results, in the case of semiconductors, clear and direct evidence for a transfer of electrons or holes from oxides with their typically high ionization energy is missing. Here, we present data from infrared reflection–absorption spectroscopy demonstrating that despite a high ionization energy, electrons are transferred from ZnO into a prototype strong molecular electron acceptor, hexafluoro-tetracyano-naphthoquinodimethane (F6-TCNNQ). Because there are no previous studies of this type, the interpretation of the pronounced vibrational red shifts observed in the experiment was aided by a thorough theoretical analysis using density functional theory. The calculations reveal that two mechanisms govern the pronounced vibrational band shifts of the adsorbed molecules: electron transfer into unoccupied molecular levels of the organic acceptor and also the bonding between the surface Zn atoms and the peripheral cyano groups. These combined experimental data and the theoretical analysis provide the so-far missing evidence of interfacial electron transfer from high ionization energy inorganic semiconductors to molecular acceptors and indicates that n-doping of ZnO plays a crucial role.

Tracking the prelude of the electroreduction of carbon monoxide via its interaction with Cu(100): Studies by operando scanning tunneling microscopy and infrared spectroscopy

• Seriatim STM-PMIRS tracks operando structures of the Cu surface and adsorbed CO. • CO can be chemisorbed on Cu at ambient temperatures under applied potentials. • CO is adsorbed prior to its electrochemical reduction on Cu(100) in 0.1 M KOH. • Changes in the alkali cation do not lead to observable CO vibrational-energy shifts. • CO adsorption yields a Cu(100)- c (2 × 2)-CO, or Cu(100)-(√2×√2)R45°-CO, adlattice. The first isolable intermediate in the electrochemical reduction of carbon dioxide is carbon monoxide. This species, or its hydrated form, formic acid, is also the primary end product from all but a handful of metallic electrodes; with the latter, hydrogen gas is generated, but it emanates from the reduction of water and not from CO 2 . Only one electrode material, zerovalent copper, can spawn, in greater-than-trace quantities, a variety of species that are more highly reduced than CO. Hence, if the aim is to pursue a reaction trail of the reduction of CO 2 to products other than CO, it would be both logical and expedient to track the electrocatalytic reaction of CO itself. Heterogeneous electrocatalysis is a surface phenomenon; it transpires only when the reactant, CO in this case, chemisorbs on, or chemically interacts with, the Cu electrode surface. There is no electrocatalytic reaction if there is no CO adsorption. In ultrahigh vacuum, no CO resides on the Cu(100) surface at temperatures higher than 200 K. However, under electrochemical conditions, CO is chemisorbed on Cu at ambient temperatures at a given potential. We thus paired, in seriatim fashion, scanning tunneling microscopy (STM) and polarization-modulation IR reflection-absorption spectroscopy (PMIRS) to document the influence of applied potential on the coverage, the molecular orientation, and the adlattice structure of CO adsorbed on Cu(100) in alkaline solutions; the results are described in this paper.

Unusual Behavior of the Bipolar Molecule 25-Hydroxycholesterol at the Air/Water Interface-Langmuir Monolayer Approach Complemented with Theoretical Calculations.

In this study, 25-hydroxycholesterol (25-OH), a biamphiphilic compound with a wide range of biological activities, has been investigated at the air/water interface. We were interested in how two hydroxyl groups attached at distal positions of the 25-OH molecule (namely, at C(3) in the sterane system and at C(25) in the side chain) influence its surface behavior. Apart from traditional Langmuir monolayers, other complementary surface-sensitive techniques, such as electric surface potential measurements, Brewster angle microscopy (BAM, enabling texture visualization and film thickness measurements), and polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS), were applied. Experimental data have been interpreted with the aid of theoretical study. Our results show that 25-OH molecules in the monomolecular layer are anchored to the water surface alternatively with C(3) or C(25) hydroxyl groups. Theoretical calculations revealed that the populations of these alternative orientations were not equal and molecules anchored with C(3) hydroxyl groups were found to be in excess. As a consequence of such an arrangement, surface films of 25-OH are of lower stability as compared to cholesterol (considered as a non-oxidized analogue of 25-OH). Moreover, it was found that, upon compression, the transition from mono- to bilayer occurred. The molecular mechanism and interactions stabilizing bilayer structure were proposed. The explanation of the observed unusual surface behavior of 25-OH may contribute to an understanding of differences in biological activity between chain- and ring-oxidized sterols.

Interaction of nitrofurantoin with lipid langmuir monolayers as cellular membrane models distinguished with tensiometry and infrared spectroscopy

Knowing how a drug interacts with cell membranes is important to understand and predict its effects at the molecular level. Therefore, we aimed to study the interaction of nitrofurantoin (NFT), a compound with potential antibiotic and antitumor properties, with lipidic biological interfaces using Langmuir monolayers. We employed the phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phospho-l-serine (DPPS), which were spread on the surface of water to form Langmuir films, to investigate the membrane-drug interactions. The interaction of the drug with the lipid monolayers was evaluated by using surface pressure-area isotherms, surface pressure-time kinetic curves, Brewster angle microscopy (BAM), and polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). Nitrofurantoin shifted the isotherms to lower DPPC molecular areas, indicating monolayer condensation, and to higher DPPS molecular areas, indicating monolayer expansion. Meanwhile, BAM images showed the appearance of interfacial aggregates for DPPS, but not for DPPC, in the presence of NFT. PM-IRRAS spectra showed that bands related to methylene stretches changed their relative intensities and maximum position related to their asymmetric and symmetric modes for both lipids. This suggested an alteration of the monolayer packing degree and the conformational order of the lipid alkyl chains, which were related to an increase in configurational order for DPPS, but disorder for DPPC. In conclusion, NFT caused distinctive changes in the thermodynamic, morphological, and structural properties of DPPC and DPPS monolayers, which may be associated with its bioactivity in cellular membranes and other lipidic interfaces of pharmaceutical interest.

Investigation of Au SAMs Photoclick Derivatization by PM-IRRAS.

In this work, we present a clean one-step process for modifying headgroups of self-assembled monolayers (SAMs) on gold using photo-enabled click chemistry. A thiolated, cyclopropenone-caged strained alkyne precursor was first functionalized onto a flat gold substrate through self-assembly. Exposure of the cyclopropenone SAM to UVA light initiated the efficient photochemical decarbonylation of the cyclopropenone moiety, revealing the strained alkyne capable of undergoing the interfacial strain-promoted alkyne-azide cycloaddition (SPAAC). Irradiated SAMs were derivatized with a series of model azides with varied hydrophobicity to demonstrate the generality of this chemical system for the modification and fine-tuning of the surface chemistry on gold substrates. SAMs were characterized at each step with polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) to confirm the successful functionalization and reactivity. Furthermore, to showcase the compatibility of this approach with biochemical applications, cyclopropenone SAMs were irradiated and modified with azide-bearing cell adhesion peptides to promote human fibroblast cell adhesion, and then imaged by live-cell fluorescence microscopy. Thus, the "photoclick" methodology reported here represents an improved, versatile, catalyst-free protocol that allows for a high degree of control over the modification of material surfaces, with applicability in materials science as well as biochemistry.

New look for an old molecule – Solid/solid phase transition in cholesterol monolayers

Surface pressure (π) - molecular area (A) isotherms of cholesterol were precisely measured to get insight into the orientation of molecules in Langmuir monolayers, which allowed to obtain detailed information on their phase behaviour. This was possible from the detailed analysis of the interfacial compressibility modulus versus surface pressure (Cs-1- π) plots (obtained from the experimental surface pressure, π - area, A isotherms) and films thickness measurements (applying Brewster angle microscope, BAM) complemented with polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). At first glance, the isotherm for cholesterol is characterized by the major slope change of surface pressure versus area per molecule. However, a more detailed analysis showed the presence of a discontinuity and slope change both upon the compression and expansion of the monolayer. This discontinuity is more accurately reflected in the Cs-1- π plot as a pseudo-plateau visible at π values between approximately 5 and 10 mN/m. This plateau was found to be temperature-dependent. Also, film thickness versus area plot (th-A) exhibits a pseudo-plateau in this region of surface pressures, in which the monolayer thickness increased gradually from 1.15 nm to 1.5 nm. Interestingly, although cholesterol has been intensively investigated in Langmuir monolayers, the existence of such a plateau have been overlooked previously. By linking experimental thickness values with theoretical molecular conformations, we have identified the presence of this plateau to the solid-solid (S-S') second-order transition. Using 2D analog of Clausius-Clapeyron equation, the thermodynamic functions (ΔH and ΔS) for this transition have been calculated. Based on monolayer experiments, the orientation of molecules in both solid phases was assumed to differ in the orientation of short alkyl chain attached to C17, which has additionally been confirmed with PM-IRRAS analysis.

Amplification of Elementary Surface Reaction Steps on Transition Metal Surfaces Using Liquid Crystals: Dissociative Adsorption and Dehydrogenation.

Elementary reaction steps, including adsorption and dissociation, of a range of molecular adsorbates on transition metal surfaces have been elucidated in the context of chemical catalysis. Here we leverage this knowledge to design liquid crystals (LCs) supported on ultrathin polycrystalline gold films (predominant crystallographic face is (111)) that are triggered to undergo orientational transitions by dissociative adsorption and dehydrogenation reactions involving chlorine and carboxylic acids, respectively, thus amplifying these atomic-scale surface processes in situ into macroscopic optical signals. We use electronic structure calculations to predict that 4'-n-pentyl-4-biphenylcarbonitrile (5CB), a room temperature nematic LC, does not bind to Au(111) in an orientation that changes upon dissociative adsorption of molecular chlorine, a result validated by experiments. In contrast, 4-cyano-4-biphenylcarboxylic acid (CBCA) is calculated to bind strongly to Au(111) in a perpendicular orientation via dehydrogenation of the carboxylic acid group, which we confirmed using polarization-modulation infrared reflection-absorption spectroscopy. A maximum coverage of 0.07 monolayer of CBCA on the gold surface is sufficient to cause a perpendicular orientation of the LC. Dissociative adsorption of Cl2 gas on the gold surface, resulting in 0.5 monolayer coverage of Cl, displaces CBCA from Au(111) and thus triggers a strikingly visible change in orientation of the LC. Infrared spectroscopy established the orientation of adsorbed CBCA to be parallel to the Cl covered surface, with the COOH plane perpendicular to the surface, as predicted by first-principles calculations. These results demonstrate the use of first-principles calculations and transition metal surfaces to design LCs that report in situ targeted atomic-scale surface processes.

Controlling polaron formation at hematite surfaces by molecular functionalization probed by XUV reflection-absorption spectroscopy.

Small polaron formation is known to limit the photocatalytic charge transport efficiency of hematite via ultrafast carrier self-trapping. While small polaron formation is known to occur in bulk hematite, a complete description of surface polaron formation in this material is not fully understood. Theoretical predictions indicate that the kinetics and thermodynamics of surface polaron formation are different than those in bulk. However, to test these predictions requires the ability to experimentally differentiate polaron formation dynamics at the surface. Near grazing angle extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy is surface sensitive and provides element and oxidation state specific information on a femtosecond time scale. Using XUV-RA, we provide a systematic comparison between surface and bulk polaron formation kinetics and energetics in photoexcited hematite. We find that the rate of surface polaron formation (250 ± 40 fs) is about three times slower than bulk polaron formation (90 ± 5 fs) in photoexcited hematite. Additionally, we show that the surface polaron formation rate can be systematically tuned by surface molecular functionalization. Within the framework of a Marcus type model, the kinetics and energetics of polaron formation are discussed. The slower polaron formation rate observed at the surface is found to result from a greater lattice reorganization relative to bulk hematite, while surface functionalization is shown to tune both the lattice reorganization as well as the polaron stabilization energies. The ability to tune the kinetics and energetics of polaron formation and hopping by molecular functionalization provides the opportunity to synthetically control electron transport in hematite.

Structural effects on water molecules on the low index planes of Pt modified with alkyl amines and the correlation with the activity of the oxygen reduction reaction

Abstract Our previous study shows that the activity of the cathodic reaction (oxygen reduction reaction: ORR) of fuel cells is increased markedly by the adsorption of alkyl amines on a well-defined Pt(111) electrode. On Pt(100) and Pt(110) electrodes, however, the adsorption of alkyl amines deactivates the ORR significantly. We have studied the possible factors of the activation and deactivation by infrared (IR) reflection absorption spectroscopy. An IR band of icelike water is found on all the electrodes after the modification with alkyl amines. The band of the icelike water on Pt(111) is located at a higher wavenumber than those on Pt(100) and Pt(110). These findings indicate that the cluster size of icelike water on Pt(111) is smaller than those on Pt(100) and Pt(110). The cluster size of the ice correlates with the ORR activity on Pt electrodes.

Iron(III) Speciation Observed at Aqueous and Glycerol Surfaces: Vibrational Sum Frequency and X-ray.

Aqueous solutions of FeCl3 have been widely studied to shed light on a number of processes from dissolution, mineralization, biology, electrocatalysis, corrosion, to microbial biomineralization. Yet there are little to no molecular level studies of the air-liquid FeCl3 interface. Here, both aqueous and glycerol FeCl3 solution surfaces are investigated with polarized vibrational sum frequency generation (SFG) spectroscopy. We also present the first ever extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy measurements of solvated ions and complexes at a solution interface, and observe with both X-ray photoelectron spectroscopy (XPS) and XUV-RA the existence of Fe(III) at the surface and in the near surface regions of glycerol FeCl3 solutions, where glycerol is used as a high vacuum compatible proxy for water. XPS showed Cl- and Fe(III) species with significant Fe(III) interfacial enrichment. In aqueous solutions, an electrical double layer (EDL) of Cl- and Fe(III) species at 0.5 m FeCl3 concentration is observed as evidenced from an enhancement of molecular ordering of water dipoles, consistent with the observed behavior at the glycerol surface. At higher concentrations in water, the EDL appears to be substantially repressed, indicative of further Fe(III) complex enrichment and dominance of a centrosymmetric Fe(III) species that is surface active. In addition, a significant vibrational red-shift of the dangling OH from the water molecules that straddle the air-water interface reveals that the second solvation shell of the surface active Fe(III) complex permeates the topmost layer of the aqueous interface.

Tunable Emission in Heteroepitaxial Ln‐SURMOFs

AbstractBy using a layer‐by‐layer (LbL) approach, lanthanide‐based, monolithic metal–organic framework (MOF) thin films are fabricated for optical applications. In particular, the LbL approach allows manufacturing of heteroepitaxial Tb(III)‐Eu(III)(BTC) coatings with precise thickness control. Adjusting the Tb(III)‐to‐Eu(III) ratio allows tuning of the emission color. The hetero‐multilayer architecture makes it possible to suppress the direct Tb(III)‐to‐Eu(III) energy transfer, an unwanted phenomenon present in the corresponding mixed‐metal bulk MOF structures. The resulting Ln‐MOF thin films, or Ln‐surface‐anchored MOFs (SURMOFs), are characterized by X‐ray diffraction, infra‐red reflection absorption spectroscopy, UV–vis, and photoluminescence measurements. The results demonstrate that the heteroepitaxial SURMOF architectures carry huge potential for fabricating optical coatings for a wide range of applications.

Surfactant Charge Modulates Structure and Stability of Lipase-Embedded Monolayers at Marine-Relevant Aerosol Surfaces.

Lipases, as well as other enzymes, are present and active within the sea surface microlayer (SSML). Upon bubble bursting, lipases partition into sea spray aerosol (SSA) along with surface-active molecules such as lipids. Lipases are likely to be embedded in the lipid monolayer at the SSA surface and thus have the potential to influence SSA interfacial structure and chemistry. Elucidating the structure of the lipid monolayer at SSA interfaces and how this structure is altered upon interaction with a protein system like lipase is of interest, given the importance of how aerosols interact with sunlight, influence cloud formation, and provide surfaces for chemical reactions. Herein, we report an integrated experimental and computational study of Burkholderia cepacia lipase (BCL) embedded in a lipid monolayer and highlight the important role of electrostatic, rather than hydrophobic, interactions as a driver for monolayer stability. Specifically, we combine Langmuir film experiments and molecular dynamics (MD) simulations to examine the detailed interactions between the zwitterionic dipalmitoylphosphatidylcholine (DPPC) monolayer and BCL. Upon insertion of BCL from the underlying subphase into the lipid monolayer, it is shown that BCL permeates and largely disorders the monolayer while strongly interacting with zwitterionic DPPC molecules, as experimentally observed by Langmuir adsorption curves and infrared reflectance absorbance spectroscopy. Explicitly solvated, all-atom MD is then used to provide insights into inter- and intramolecular interactions that drive these observations, with specific attention to the formation of salt bridges or ionic-bonding interactions. We show that after insertion into the DPPC monolayer, lipase is maintained at high surface pressures and in large BCL concentrations by forming a salt-bridge-stabilized lipase-DPPC complex. In comparison, when embedded in an anionic monolayer at low surface pressures, BCL preferentially forms intramolecular salt bridges, reducing its total favorable interactions with the surfactant and partitioning out of the monolayer shortly after injection. Overall, this study shows that the structure and dynamics of lipase-embedded SSA surfaces vary based on surface charge and pressure and that these variations have the potential to differentially modulate the properties of marine aerosols.

Orthogonal chemical functionalization of patterned Au/TiW substrate for selective immobilization of nanoparticles

The evolution of nanobiosensors stresses the need for multi-material nanopatterned surfaces to enhance sensing performances. Titanium tungsten (TiW) has been mastered and routinely implemented in nanoelectronic devices, in a reproducible way and at industrial production scales. Such a material may be envisioned for use in (bio)chemical nanoelectronic sensors, but the surface functionalization of such material has yet to be studied. In the present article, the orthogonal chemical functionalization of patterned Au on TiW substrates has been explored for the first time. Surface functionalizations were assessed by x-ray photoelectron spectroscopy, polarization modulation infrared reflection-absorption spectroscopy and time-of-flight secondary ion mass spectrometry imaging. Au/TiW patterned substrates were functionalized with mercapto-undecamine. Thanks to the orthogonality of thiol/Au versus phosphonic acid/TiW reactions, only the Au features were modified leading to the amine derivatized surface. This allowed for the localizing of carboxy-functionalized nanoparticles by electrostatic interaction on Au with a selectivity above 10 when compared to TiW.

Modification of Gibbs monolayers by chromium (III) compounds

The present paper reveals the effects produced by a chromium (III) sulfate colloidal solution on Gibbs monolayers of (i) sodium dodecyl sulfate (SDS) and (ii) turkey red oil (TRO) formed at the air/water interface. Their intermolecular interactions and the modifications of the interphase properties play a significant role in the stabilization of concentrated reversed emulsions which have high relevance for the leather tanning technology. The changes produced in the amphiphilic monolayers were monitored employing highly sensitive surface characterization techniques such as Brewster angle microscopy (BAM), infrared reflection-absorption spectroscopy (IRRAS) and a Langmuir balance. The experimental data leads to the conclusion that amphiphile–chromium (III) complexes are formed. These complexes are characterized by an increased surface activity. In the case of the SDS containing system, the packing density increases and the hydrophobic interactions promote the formation of a stiff and brittle interface layer. Yet, only a loose packing is observed in the TRO monolayers, probably due to steric hindrance of the bulky hydrophobic chains.

Molecular beam/infrared reflection-absorption spectroscopy apparatus for probing heterogeneously catalyzed reactions on functionalized and nanostructured model surfaces.

A new custom-designed ultrahigh vacuum (UHV) apparatus combining molecular beam techniques and in situ surface spectroscopy for reactivity measurements on complex nanostructured model surfaces is described. It has been specifically designed to study the mechanisms, kinetics, and dynamics of heterogeneously catalyzed reactions over well-defined model catalysts consisting of metal nanoparticles supported on thin oxide films epitaxially grown on metal single crystals. The reactivity studies can be performed in a broad pressure range starting from UHV up to the ambient pressure conditions. The UHV system includes (i) a preparation chamber providing the experimental techniques required for the preparation and structural characterization of single-crystal based model catalysts such as oxide supported metal particles or ordered oxide surfaces and (ii) the reaction chamber containing three molecular beams-two effusive and one supersonic, which are crossed at the same point on the sample surface, infrared reflection-absorption spectroscopy for the detection of surface-adsorbed species, and quadrupole mass spectrometry for gas phase analysis. The supersonic beam is generated in a pulsed supersonic expansion and can be modulated via a variable duty-cycle chopper. The effusive beams are produced by newly developed compact differentially pumped sources based on multichannel glass capillary arrays. Both effusive sources can be modulated by a vacuum-motor driven chopper and are capable of providing high flux and high purity beams. The apparatus contains an ambient pressure cell, which is connected to the preparation chamber via an in situ sample transfer system and provides an experimental possibility to study the reactivity of well-defined nanostructured model catalysts in a broad range of pressure conditions-up to ambient pressure-with the gas phase analysis based on gas chromatography. Additionally, a dedicated deposition chamber is connected to the preparation chamber, which is employed for the in situ functionalization of model surfaces with large organic molecules serving as promoters or modifiers of chemical reactions. We present a general overview of the apparatus as well as a description of the individual components and their interplay. The results of the test measurements involving the most important components are presented and discussed.

Interfacial Supramolecular Structures of Amphiphilic Receptors Drive Aqueous Phosphate Recognition.

Phosphate remediation is important for preventing eutrophication in fresh waters and maintaining water quality. One approach for phosphate removal involves the utilization of molecular receptors. However, our understanding of anion recognition in aqueous solution and at aqueous interfaces is underdeveloped, and the rational design of surface-immobilized receptors is still largely unexplored. Herein, we evaluated the driving forces controlling phosphate binding to elementary amphiphilic receptors anchored at air-water interfaces. We designed biologically inspired receptors with neutral thiourea, positively charged guanidinium, and thiouronium units that all formed Langmuir monolayers. Phosphate binding was quantitatively examined using surface pressure-area isotherms and infrared reflection-absorption spectroscopy (IRRAS). The receptors within this homologous series differ in functional group, charge, and number of alkyl chains to help distinguish the fundamental components influencing anion recognition at aqueous interfaces. The two charged receptors bearing two alkyl chains each displayed strong phosphate affinities and 103- and 101-fold anti-Hofmeister selectivity over chloride, respectively. Neutral thiourea and the single-chain guanidinium receptor did not bind phosphate, revealing the importance of electrostatic interactions and supramolecular organization. Consistently, charge screening at high ionic strength weakens binding. Spectroscopic results confirmed phosphate binding to the double alkyl chain guanidinium receptor, whereas surface pressure isotherm results alone showed a minimal change, thus emphasizing the importance of interfacial spectroscopy. We found that the binding site identity, charged interface created by the electrical double layer, and supramolecular superstructure all affect interfacial binding. These detailed insights into phosphate recognition at aqueous interfaces provide a foundation to develop efficient receptors for phosphate capture.

Probing the Molecular Structure and Orientation of the Leaf Surface of Brassica oleracea L. by Polarization Modulation-Infrared Reflection-Absorption Spectroscopy.

The surface of most aerial plant organs is covered with the cuticle, a membrane consisting of a variety of organic compounds, including waxes, cutin (a polyester) and polysaccharides. The cuticle serves as the multifunctional interface between the plant and the environment, and plays a major role in protecting plants against various environmental stress factors. Characterization of the molecular arrangements in the intact cuticle is critical for the fundamental understanding of its physicochemical properties; however, this analysis remains technically challenging. Here, we describe the nondestructive characterization of the intact cuticle of Brassica oleracea L. leaves using polarization modulation-infrared (IR) reflection-absorption spectroscopy (PM-IRRAS). PM-IRRAS has a probing depth of less than several hundreds of nanometers, and reveals the crystalline structure of the wax covering the cuticle surface (epicuticular wax) and the nonhydrogen-bonding character of cutin. Combined analysis using attenuated total reflection-IR spectra suggested that hemicelluloses xylan and xyloglucan are present in the outer cuticle region close to the epicuticular wax, whereas pectins are dominant in the inner cuticle region (depth of ≤2 μm). PM-IRRAS can also determine the average orientation of the cuticular molecules, as indicated by the positive and negative spectral peaks. This unique advantage reveals the orientational order in the intact cuticle; the hydrocarbon chains of the epicuticular wax and cutin and the backbones of hemicelluloses are oriented perpendicular to the leaf surface. PM-IRRAS is a versatile, informative and easy-to-use technique for studying plant cuticles because it is nondestructive and does not require sample pretreatment and background measurements.

The effect of protonation in a family of peptide based gemini amphiphiles on the interaction in Langmuir films

Abstract The behavior and properties of four new gemini amphiphilic pseudopeptides (GAPs) were studied in monomolecular films formed at the air-water interface using surface pressure measurements, polarization-modulation infrared reflection-absorption spectroscopy as well as computational modeling. The molecules used differed in the structure of the polar heads containing amino and amide groups and in the number of apolar hydrocarbon chains. To better understand the role of the polar heads in the intra- and intermolecular interaction, the experiments were performed at different pH values of the water subphase. The experimental and theoretical results show that the interaction between the molecules forming monolayers change in function of pH; these changes depend on the pseudopeptide used. We propose that the effects observed are due to the differences in the protonation state of the polar heads in the four molecules. This outcome may give a handle on preparation of pH-dependent supramolecular structures based on tailor-made GAPs.