Infrared Reflection Absorption Spectra Research Articles

Infrared Reflection Absorption Spectroscopy (IRRAS) of Water-Soluble Surfactants: Is it Surface-Specific?

Infrared reflection absorption spectroscopy (IRRAS) is commonly used to study the structure and chemistry of molecules residing at surfaces, including water surfaces, which has far-reaching applications including atmospheric chemistry and food science. However, there is some debate regarding the surface-specificity of IRRAS when examining soluble surfactants on aqueous solutions, and there is some evidence that the surface-specificity may differ between IRRAS of ionic surfactants and soluble organic acids. This paper presents infrared reflection absorption (IRRA) spectra of soluble organic acids underneath monolayers of insoluble surfactants, where the contributions from the insoluble surfactants are subtracted from the spectra to capture "subsurface effects". These "subsurface" spectra demonstrate that IRRA spectra of soluble organic acids are surface specific, and this observation is supported by a simplified model for reflections from "subsurface" layers. Finally, the observations presented here are compared to literature observations regarding the surface-specificity of IRRAS when studying ionic surfactants. Overall, this work demonstrates the utility of IRRAS for studying the structures and chemistry of soluble organic acids at aqueous surfaces.

Structural relaxation of water during rapid cooling from ambient temperatures.

Experiments investigating the properties of deeply supercooled liquid water are needed to develop a comprehensive understanding of water's anomalous properties. One approach involves transiently heating nanoscale water films into the supercooled region for several nanoseconds at a time and then interrogating the water films after they have quenched to cryogenic temperatures. To relate the results obtained with this approach to other experiments and simulations on supercooled water, it is important to understand how closely the quenched structure tracks the (metastable) equilibrium structure of water as a function of the transient heating temperature. A key step involves quantifying the extent to which water that is transiently heated to ambient temperatures [hyperquenched water (HQW)] subsequently relaxes toward the structure of low-density amorphous (LDA) ice as it cools. We analyzed the infrared reflection-absorption spectra of LDA, HQW, and crystalline ice films to determine their complex indices of refraction. With this information, we estimate that HQW retains ∼50%-60% of a structural motif characteristic of water at high temperatures with the balance comprised of a low-temperature motif. This result, along with results from x-ray diffraction experiments on water and amorphous ices, allows one to quantify the fraction of the high-temperature motif at approximately zero pressure as a function of temperature from 150 to 350K.

The influence of solvents on the appearance of the absorption bands of the polystyrene films deposited from solutions on metal mirrors

Infrared reflection-absorption spectra (IRRAS) at the near-normal incidence of polystyrene films from benzene, toluene, and chloroform solutions were analyzed in this paper. The appearance of the spectrum can be affected so that false conclusions can be drawn about the positions and the shape of the absorption bands. The knowledge of these influences of residual solvents in the polymer film is important for the correct interpretation of the reflection-absorption spectra. Unlike other approaches, a single reflection at a 20° incidence angle was used. A new drop-carting technique was used for the deposition of polymer film solutions on metal mirrors. Reflection-absorption spectra at a near-normal incidence angle were obtained using a dispersive infrared spectrometer.

Adsorption and Oxidation of CO on Co3O4/Ir(100) Thin Films

The combination of noble metals and reducible metal oxides often displays superior catalytic properties in low-temperature CO oxidation reactions. In the present study, we investigated the adsorption and activation of CO on submonolayer and monolayer CoOx films deposited on Ir(100) at low temperature under ultra-high vacuum (UHV) conditions. Using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED), we found that the CoOx films have a (3 × 1) structure and consist of a Co–O bilayer and an O–Co–O trilayer associated with Co2+ and Co3+ states, respectively. A combination of XPS, temperature-programmed desorption (TPD), and infrared reflection absorption spectrum (IRAS) reveals that the presence of the interface of the submonolayer CoOx film and Ir significantly enhances the catalytic activity of cobalt oxide for CO oxidation. In contrast, a monolayer film of CoOx without exposing CoOx–Ir interface sites is hardly active for CO oxidation. CO adsorbs on the bridge sites of Ir atoms on the submonolayer CoOx/Ir(100) surface at 100 K. Upon heating, CO partially reacts with oxygen on the surface to produce CO2, which desorbs at 215 K, while unreacted CO molecules adsorbed on Ir bridge sites migrate to top sites. Increasing the temperature leads to CO2 desorption at 295 and 325 K, arising from the reaction of CO with chemisorbed oxygen on the Ir surface and oxygen of cobalt oxide, respectively. These results reveal the importance of the interface between Ir and cobalt oxide in the oxidation of CO.

Luminescence of Macrocyclic Mononuclear DyIII Complexes and Their Immobilization on Functionalized Silicon-Based Surfaces.

Two mononuclear DyIII complexes, [Dy(L1)(NCS)3] (Dy-EDA) and [Dy(L2)(NCS)3] (Dy-DAP), where Ln (n = 1-2) corresponds to a macrocyclic ligand derived from 2,6-pyridinedicarboxaldehyde and ethylenediamine (L1) and 1,3-diaminepropane (L2) were immobilized on functionalized silicon-based surfaces. This was achieved by the microcontact printing (μCP) technique, generating patterns on a functionalized surface via covalent bond formation through the auxiliary -NCS ligands present in the macrocyclic complex species. With this strategy, it was possible to control the position of the immobilized molecules on the surface. Water contact angle measurements, X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectra (IRRAS), and atomic force microscopy (AFM) confirmed that the surfaces were successfully functionalized. Furthermore, the optical properties in a broad temperature range were investigated for the as-prepared compounds. At room temperature, Dy-EDA was shown to emit in the deep blue region (Commission Internationald'Eclairage (CIE): (0.175, 0.128)), while Dy-DAP in the white region (CIE: (0.252, 0.312)). The different CIE values were due to the contribution of the strong emission of the ligand in the case of Dy-EDA. Besides, surface photoluminescence measurements showed that the immobilized complexes retained their bulk emissive properties.

Spread Layers of Lysozyme Microgel at Liquid Surface.

The spread layers of lysozyme (LYS) microgel particles were studied by surface dilational rheology, infrared reflection–absorption spectra, Brewster angle microscopy, atomic force microscopy, and scanning electron microscopy. It is shown that the properties of LYS microgel layers differ significantly from those of ß-lactoglobulin (BLG) microgel layers. In the latter case, the spread protein layer is mainly a monolayer, and the interactions between particles lead to the increase in the dynamic surface elasticity by up to 140 mN/m. In contrast, the dynamic elasticity of the LYS microgel layer does not exceed the values for pure protein layers. The compression isotherms also do not exhibit specific features of the layer collapse that are characteristic for the layers of BLG aggregates. LYS aggregates form trough three-dimensional clusters directly during the spreading process, and protein spherulites do not spread further along the interface. As a result, the liquid surface contains large, almost empty regions and some patches of high local concentration of the microgel particles.

Hydrogenation of Formate Species Using Atomic Hydrogen on a Cu(111) Model Catalyst.

The reaction mechanism of the CH3OH synthesis by the hydrogenation of CO2 on Cu catalysts is unclear because of the challenge in experimentally detecting reaction intermediates formed by the hydrogenation of adsorbed formate (HCOOa). Thus, the objective of this study is to clarify the reaction mechanism of the CH3OH synthesis by establishing the kinetic natures of intermediates formed by the hydrogenation of adsorbed HCOOa on Cu(111). We exposed HCOOa on Cu(111) to atomic hydrogen at low temperatures of 200-250 K and observed the species using infrared reflection absorption (IRA) spectroscopy and temperature-programmed desorption (TPD) studies. In the IRA spectra, a new peak was observed upon the exposure of HCOOa on Cu(111) to atomic hydrogen at 200 K and was assigned to the adsorbed dioxymethylene (H2COOa) species. The intensity of the new peak gradually decreased with heating from 200 to 290 K, whereas the IR peaks representing HCOOa species increased correspondingly. In addition, small amounts of formaldehyde (HCHO), which were formed by the exposure of HCOOa species to atomic hydrogen, were detected in the TPD studies. Therefore, H2COOa is formed via hydrogenation by atomic hydrogen, which thermally decomposes at ∼250 K on Cu(111). We propose a potential diagram of the CH3OH synthesis via H2COOa from CO2 on Cu surfaces, with the aid of density functional theory calculations and literature data, in which the hydrogenation of bidentate HCOOa to H2COOa is potentially the rate-determining step and accounts for the apparent activation energy of the methanol synthesis from CO2 on Cu surfaces.

Changing the optical properties of poly(methyl methacrylate) superficial films during UV irradiation

In this work, the simple spectroscopic method is presented to study the change of the optical constants n and k of the poly(methyl methacrylate), UV-irradiated. The infrared reflection-absorption spectra were recorded at a single incidence angle. The reflection-absorption spectra of the poly(methyl methacrylate) thin films were processed using classical dispersion analysis and a fitting program to obtain optical constants n and k. The shift, the variation of intensity, or broadening of the absorption bands during the UV degradation, were watched in the extinction coefficient k spectrum. The changes in the extinction spectrum of UV-irradiated polymer films are consistent with the photodegradation mechanisms described in the literature.

Atmospheric chemical processes of microcystin-LR at the interface of sea spray aerosol

Microcystins are the most toxic toxins released by cyanobacteria and they have adverse effects on aquatic ecosystems and even human health. Although the removal and detoxification of microcystins in various water bodies have been extensively studied, the interaction mechanism and reaction process of microcystins once they enter the atmosphere are largely unknown, especially at the organic-enriched sea spray aerosol (SSA) interface. Herein, using the surface technique of Langmuir trough coupled in-situ infrared reflection-absorption spectra, we studied the interfacial behavior of microcystin-LR (MC-LR) in artificial seawater containing humic acid and typical surfactants in the presence or absence of UV-irradiation. Zwitterionic 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and anionic stearic acid (SA) were chosen as typical film-forming species, results obtained from the surface pressure-area isotherms showed that MC-LR caused greater expansion of the DSPC monolayer. The comparable results of MC-LR in DSPC/SA-containing systems indicated that the interaction ability was closely related to the monolayer molecular structure and was regulated by electrostatic interaction. Furthermore, the presence of humic acid (HA) could enhance the interaction between microcystin and monolayer molecules. UV-irradiation experiments showed that the photosensitized reaction greatly promoted the removal of microcystin embedded in the SSA surface compared with the direct photolysis effect in the absence of HA. These findings highlight that the toxic effects of microcystins after entering the atmosphere may be weakened by photochemical reactions.

Diagnostics of SARS-CoV-2 infection using electrical impedance spectroscopy with an immunosensor to detect the spike protein

Mass testing for the diagnostics of COVID-19 has been hampered in many countries owing to the high cost of the methodologies to detect genetic material of SARS-CoV-2. In this paper, we report on a low-cost immunosensor capable of detecting the spike protein of SARS-CoV-2, including in samples of inactivated virus. Detection is performed with electrical impedance spectroscopy using an immunosensor that contains a monolayer film of carboxymethyl chitosan as matrix, coated with an active layer of antibodies specific to the spike protein. In addition to a low limit of detection of 0.179 fg/mL within an almost linear behavior from 10−20 g/mL to 10−14 g/mL, the immunosensor was highly selective. For the samples with the spike protein could be distinguished in multidimensional projection plots from samples with other biomarkers and analytes that could be interfering species for healthy and infected patients. The excellent analytical performance of the immunosensors was validated with the distinction between control samples and those containing inactivated SARS-CoV-2 at different concentrations. The mechanism behind the immunosensor performance is the specific antibody-protein interaction, as confirmed with the changes induced in C–H stretching and protein bands in polarization-modulated infrared reflection absorption spectra (PM-IRRAS). Because impedance spectroscopy measurements can be made with low-cost portable instruments, the immunosensor proposed here can be applied in point-of-care diagnostics for mass testing even in places with limited resources.

Role of low-energy electrons in the solubility switch of Zn-based oxocluster photoresist for extreme ultraviolet lithography.

The electron-induced chemistry of a resist material for extreme ultraviolet lithography (EUVL) consisting of Zn oxoclusters with methacrylate (MA) and trifluoroacetate (TFA) ligands (Zn(MA)(TFA)) has been studied. Electron energies of 80 eV and 20 eV mimic the effect of photoelectrons released by the absorption of EUV photons and low-energy secondary electrons (LESEs) produced by those photoelectrons. The chemical conversion of the resist is studied by mass spectrometry to monitor the volatile species that desorb during electron irradiation, combined with reflection absorption infrared spectra (RAIRS) measured before and after irradiation. The observed reactions are closely related to those initiated upon EUV absorption. Also, the conversion of the Zn(MA)(TFA) resist layer that is required in EUVL is achieved by a similar energy input upon electron irradiation. The dominant component of the desorbing gas is CO2, but CO detection also suggests Zn oxide formation during electron irradiation. In contrast, species deriving from the ligand side chains predominantly remain within the resist layer. RAIRS gives direct evidence that, during electron irradiation, C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds of the MA ligands are more rapidly consumed than the carboxylate groups. This supports that chain reactions occur and contribute to the solubility switch in the resist in EUVL. Remarkably, 20 eV electrons still evolve roughly 50% of the amount of the gas that is observed at 80 eV for the same electron dose. The present results thus provide complementary and new insight to the EUV-induced chemistry in the Zn(MA)(TFA) resist and point towards the important contribution of low-energy electrons therein.

Tailoring electric field standing waves in reflection–absorption infrared spectroscopy to enhance absorbance from adsorbates on ice surfaces

The spectroscopic detection of molecules adsorbed onto ice surfaces at coverages similar to those encountered under typical environmental conditions requires high surface selectivity and sensitivity that few techniques can afford. An experimental methodology allowing a significant enhancement in the absorbance from adsorbed molecules is demonstrated herein. It exploits Electric Field Standing Wave (EFSW) effects intrinsic to grazing incidence Reflection-Absorption Infrared (RAIR) spectroscopy, where film thickness dependent optical interferences occur between the multiple reflections of the IR beam at the film-vacuum and the substrate-film interfaces. In this case study, CH4 is used as a probe molecule and is deposited on a 20 ML coverage dense amorphous solid water film adsorbed onto solid Ar underlayers of various thicknesses. We observe that, at thicknesses where destructive interferences coincide with the absorption features from the CH stretching and HCH bending vibrational modes of methane, their intensity increases by a factor ranging from 10 to 25. Simulations of the RAIR spectra of the composite stratified films using a classical optics model reproduce the Ar underlayer coverage dependent enhancements of the absorbance features from CH4 adsorbed onto the ice surface. They also reveal that the enhancements occur when the square modulus of the total electric field at the film's surface reaches its minimum value. Exploiting the EFSW effect allows the limit of detection to be reduced to a coverage of (0.2 ± 0.2) ML CH4, which opens up interesting perspectives for spectroscopic studies of heterogeneous atmospheric chemistry at coverages that are more representative of those found in the natural environment.

Cyanate and Cyanurate Adsorption at Silver Electrodes in Neutral Solutions: In Situ ATR-SEIRAS and DFT Studies

The adsorption and reactivity of cyanate at silver electrodes is studied spectroelectrochemically in sodium perchlorate solutions and compared to those of cyanuric acid (1,3,5-triazine-2,4,6-trione, C3H3N3O3). The surface enhanced infrared reflection absorption spectra obtained under attenuated total reflection conditions (ATR-SEIRAS) for low cyanate concentrations show that some amount of adsorbed CO is formed at the silver electrode surface at low electrode potentials. Adsorbed cyanate species are formed at higher potentials and predominate at the electrode surface. However, for cyanate concentrations above 1 mM, the ATR-SEIRA spectra show potential-dependent absorption features between 1800 and 1300 cm-1, which are similar to those obtained after dosing cyanuric acid to the sodium perchlorate solution. This observation suggests that adsorbed cyanuric acid related species are formed from cyanate anions at the silver surface sites as the result of an electroless trimerization reaction. The infrared spectra collected in the cyanuric acid containing solutions also show a higher amount of adsorbed CO at low potential when compared to that observed in cyanate-containing solutions, thus indicating certain instability of cyanuric acid at the silver surfaces. Density functional theory (DFT) calculations have allowed the study of the effect of crystallographic orientation and bonding geometry on the adsorption energy and vibrational frequencies of adsorbates. Also, the effect of adsorbate coverage on the vibrational frequencies has been investigated. In the case of cyanate, DFT calculations show that the most favorable adspecies is N-bonded cyanate adsorbed in 4-fold hollow (100) sites. The experimental spectra in cyanuric acid solutions agree with the formation of adlayers of specifically adsorbed triketo-monocyanurate species that adsorb, in a tridentate configuration, perpendicular to the electrode surface. The polyoriented nature of the silver surface used in the spectroelectrochemical experiments, together with the eventual existence of cyanate in domains with different local coverages, can explain the broadness of the band for the asymmetric OCN stretching. For adsorbed triketo-monocyanurate, high local coverages are at the origin of the band observed experimentally at ca. 1790 cm-1 that can be associated with the in-phase collective CO stretching.

The Adsorption and Film Forming Behavior of Vinyltriethoxysilane(VS) on Low Carbon Steel Surfaces

The adsorption and film forming behavior of vinyltriethoxysilane (VS) on Q235 low carbon steel surfaces, as well as its effect on the properties of film layer were systematically investigated by reflection absorption infrared spectrum (RA-IR), electrochemical impedance spectroscopy (EIS) and contact angle measurement technology. The experimental results indicate that the oscillatory phenomenon exists in the adsorption of VS on low carbon steel surfaces, which directly determines the adsorption morphology and the arrangement pattern of VS on metal surfaces, generating VS films with different protective properties. The desorption occurring in the initial stage was accompanied with film restructuring, which was helpful to the readsorption of VS and the film formation with dense structure and excellent protective performance.

Evidence of the Berreman effect in TiO2:Co thin layers deposited by DC magnetron Co-Sputtering: A correlation between optical properties and LO-TO splitting modes

In this work TiO2:Co thin films were deposited by DC magnetron Co-sputtering method, varying the substrate temperature between 300 K and 523 K. The anatase and rutile phases were identified from XRD patterns, according to the variation of the synthesis parameters and the Ti foils-like substrate. The random distribution of the Co into the TiO2 semiconductor matrix was determined by AES spectroscopy. Infrared reflection-absorption spectra of nanostructured TiO2:Co thin films grown on Ti foils were obtained in order to characterize the surface and interface of the samples on metallic substrates. Characteristics bands with changes in the reflection minima position and line shapes were observed between 600 and 1000 cm−1 wavenumber range. LO and TO modes of TiO2:Co identified in the samples, showed a connection with the so-called Berreman effect. Structural, optical and reflection-absorption properties were studied from XRD, Uv–Vis and FT-IR spectroscopy measurements, respectively.

Direct 12-Electron Oxidation of Ethanol on a Ternary Au(core)-PtIr(Shell) Electrocatalyst.

Understanding the roles of metals and atomic structures in activating various elementary steps of electrocatalytic reactions can help rational design of binary or ternary catalysts for promoting activity toward desirable products via favorable pathways. Here we report on a newly developed ternary Au@PtIr core-shell catalyst for ethanol oxidation reaction (EOR) in alkaline solutions, which exhibits an activity enhancement of 6 orders of magnitude compared to AuPtIr alloy catalysts. Analysis of in situ infrared reflection absorption spectra for Au@PtIr and its bimetallic subsets, Au@Pt and PtIr alloy, found that monatomic steps and Au-induced tensile strain on PtIr facilitate C-C bond splitting via ethanol dissociative adsorption and Ir promotes dehydrogenation at low potentials. As evidenced by the CO band being observed only for the PtIr alloy that is rather inactive for ethanol dissociative adsorption, we propose that splitting the C-C bond at the earliest stage of EOR activates a direct 12-electron full oxidation pathway because hydrogen-rich fragments can be fully oxidized without CO as a poisoning intermediate. The resulting synergy of complementary effects of Au core and surface Ir leads to an outstanding performance of Au@PtIr for EOR as characterized by a low onset potential of 0.3 V and 8.3 A mg-1all-metals peak current with 57% currents generated via full ethanol oxidation.

(Invited) Challenges and Opportunities in Developing Anode Catalysts for Direct Ethanol and Ammonia Fuel Cells

Direct liquid fuel cells (DLFC) are attractive alternatives of batteries and hydrogen fuel cells for applications where high energy density and easy to refill are critical. For example, in powering a 200W-1kWh drone, a DLFC system can weigh only 1/10 of a battery and refill is simpler than hydrogen and faster than battery recharging. Among liquid fuels, ethanol stands out for its high energy density and ammonia is carbon free. A drawback is the lack of anode catalysts for complete oxidation of ethanol and ammonia at low potentials with sufficient activity per mass of platinum group metals (PGM). Dr. Shimshon Gottesfeld is currently leading an ARPA-E project on “Direct ammonia hydroxide exchange membrane fuel cells”, and I am in the team to develop high performance catalysts for ammonia oxidation reaction (AOR) with low PGM contents. While I learned from reading his papers on a wide range of topics and talking with him at conferences in many years, working under his leadership made me further appreciate his contribution in advancing fuel cell technologies. Inspired by his sharp and open mind in choosing and solving technical issues and in pursuing deep fundamental understandings, I’d like to discuss the challenges in developing anode catalysts for ethanol and ammonia oxidation and to illustrate promising directions and opportunities based on new insights in reaction mechanisms and material/structure/performance correlations. The performances of ethanol oxidation reaction (EOR) and AOR in alkaline media on the best catalysts that we recently developed will be presented and compared. The results show that facile EOR is possible at ambient temperature when a direct 12-electron EOR pathway is activated. Analyses of in situ infrared reflection absorption spectra indicate that the C-C bond splits via ethanol dissociation and full oxidation completes without CO as poisoning intermediate. However, carbonate formation in alkaline media lowers pH and reduces EOR activity. Carbon-free ammonia circumvents this issue, but AOR activity needs enhancement from elevated temperature due to the high activation barrier for N-N bond formation. Most studies so far focused on Pt-based catalysts and the polarization curves were measured at ambient temperature. Our study of temperature-dependent AOR kinetics found that activity enhancement from 25 to 60 oC is in the order of Ir (5.5) > IrPt (4.2) > Pt (2.0).1 For both EOR and AOR, Ir is effective in promoting dehydrogenation, and thus, in lowering the onset potentials. Thus, Ir-based catalysts are more promising than Pt-based catalysts for AOR. I’ll also show that the C-C bond cleavage in EOR and the N-N bond formation in AOR prefer distinctly different atomic structures. The new insights point out opportunities and directions for future studies. Acknowledgements The author thanks Zhixiu Liang, Liang Song, Jingyi Chen, Shiqing Deng, and Eli Stavitski for their collaborations. The work was funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000805 and by the Division of Chemical Sciences, Geosciences and Biosciences Division, US Department of Energy under contract DE-SC0012704. References Song, L. et al. Temperature-Dependent Kinetics and Reaction Mechanism of Ammonia Oxidation on Pt, Ir, and PtIr Alloy Catalysts. J. Electrochem. Soc. 165, J3095–J3100 (2018).

Platinum-Iridium Modified Gold Nanoparticles Catalysts for Electrooxidation of Ethanol in Alkaline Media

Ethanol is one of the most promising alternative fuels for fuel cells (FC). It is a renewable resource which can be produced on a large scale from biomass originating from agriculture, forestry and urban residues. The energy density of ethanol is as high as 8.01 kWh/kg, which is comparable to that of gasoline. Ethanol is easy to store, transfer and refuel owing to its non-toxicity and high boiling point in comparison with methanol and ammonia. [1] However, the sluggish kinetics and low energy efficiency for ethanol oxidation reaction (EOR) hinder the application of direct ethanol fuel cells (DEFCs). To address this issue, great effort has been made focusing on catalysts discovery as well as reaction mechanism understanding. [2] Here we report a systematic study on Pt-Ir modified Au/C electrocatalysts for efficient ethanol oxidation in alkaline media. In-situ electrochemical Infrared reflection absorption spectra (EC-IRRAS) reveals a high selectivity toward full oxidization to carbonate at low potentials, resulting in a high energy conversion efficiency. EC-IRRAS also indicate that the C-C bond splits upon ethanol interaction with the surface of PtIrAu/C ternary catalyst. More detailed discussion on this topic will be presented at the meeting. Acknowledgements This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-SC0012704 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. [1] Liang An, Tianshou Zhao, Yinshi Li, Carbon-neutral sustainable energy technology: direct ethanol fuel cells. Renewable and Sustainable Energy Reviews 2015, 50: 1462- 1468 [2] Alexey Serov, Iryna V. Zenyuk, Christopher G Arges, Marian Chatenet. Hot topics in alkaline exchange membrane fuel cells. Journal of Power Sources 2018, 375: 149- 157

Evaluation of EGCG Loading Capacity in DMPC Membranes.

Catechins are molecules with potential use in different pathologies such as diabetes and cancer, but their pharmaceutical applications are often hindered by their instability in the bloodstream. This issue can be circumvented using liposomes as their nanocarriers for in vivo delivery. In this work, we studied the molecular details of (-)-epigallocatechin-3-gallate (EGCG) interacting with 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) monolayer/bilayer systems to understand the catechin loading ability and liposome stability, using experimental and computational techniques. The molecular dynamics simulations show the EGCG molecules deep inside the lipid bilayer, positioned below the lipid ester groups, generating a concentration-dependent lipid condensation. This effect was also inferred from the surface pressure isotherms of DMPC monolayers. In the polarization-modulated infrared reflection absorption spectra assays, the predominant effect at higher concentrations of EGCG (e.g., 20 mol %) was an increase in lipid tail disorder. The steady-state fluorescence data confirmed this disordered state, indicating that the catechin-induced liposome aggregation outweighs the condensation effects. Therefore, by adding more than 10 mol % EGCG to the liposomes, a destabilization of the vesicles occurs with the ensuing release of entrapped catechins. The loading capacity for DMPC seems to be limited by its disordered lipid arrangements, typical of a fluid phase. To further increase the clinical usefulness of liposomes, lipid bilayers with more stable and organized assemblies should be employed to avoid aggregation at large concentrations of catechin.

The cyclic peptide labaditin does not alter the outer membrane integrity of Salmonella enterica serovar Typhimurium

Antimicrobial peptides are a promising class of new antibiotics with the ability to kill bacteria by disrupting their cell membrane, which is especially difficult for Gram-negative bacteria whose cell wall contains an outer layer of lipopolysaccharides (LPS). Here we show that the cyclic decapeptide Labaditin (Lo), with proven activity against the Gram-positive Staphylococcus aureus and Streptococcus mutans, is not able to kill the Gram-negative Salmonella enterica serovar Typhimurium (S.e.s. Typhimurium). We found that Lo induced significant changes in the surface pressure isotherms of Langmuir monolayers representing the Salmonella enterica serovar Typhimurium inner membrane (S.e.s. Typhimurium IM), and caused leakage in large unilamellar vesicles made with this IM lipid composition. On the basis of these results one should expect bactericidal activity against S.e.s. Typhimurium. However, Lo could not interact with a monolayer of LPS, causing no significant changes in either the surface pressure isotherms or in the polarization-modulated infrared reflection absorption spectra (PM-IRRAS). Therefore, the failure of Lo to kill S.e.s. Typhimurium is associated with the lack of interaction with LPS from the outer bacteria membrane. Our approach with distinct monolayer compositions and combined techniques to investigate molecular-level interactions is useful for drug design to fight antibiotic-resistant bacteria.