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Complementary electrochemical and spectroscopic techniques were used to characterize the behavior and composition of the passive layer formed at the gold surface in a thiosulfate electrolyte in the presence of cupric ions. Raman studies of three different cationic (calcium, ammonium, and sodium) thiosulfate leaching solutions revealed that the concentrations of thiosulfate, trithionate, sulfate, and tetrathionate remained constant in the bulk solution over a 3-h time period. The initial leaching current densities of these three systems were identical; however, significant differences in the open circuit potentials of these systems were observed. To provide additional information about the nature of the passive layer, gold nanorod array electrodes were fabricated and employed as substrates for studying the species present at the gold–thiosulfate interface using surface-enhanced Raman spectroscopy (SERS). The composition and behavior of the passive layer at the gold–thiosulfate interface greatly differed from those of the bulk solutions. The passive layer consisted primarily of elemental sulfur and sulfide-like species, with thiosulfate and its oxidation products, such as tri- and tetrathionates, as minority components. The nature of the cation (calcium, sodium, or ammonium) of the thiosulfate salt showed no significant effect on the composition of the passive layer at leaching times longer than 100 min. In addition, the presence of cupric ions also had no significant effect on the formation of the passive layer. However, copper is a much better oxidant than oxygen in gold–thiosulfate leaching reactions.

High-resolution atomic force microscopy (AFM) was used to image the real-time in situ degradation of crystalline by three types of T. reesei cellulolytic enzymes TrCel6A, TrCel7A, and TrCel7Band their mixtures. TrCel6A and TrCel7A are exo-acting cellobiohydrolases processing cellulose fibers from the nonreducing and reducing ends, respectively. TrCel7B is an endoglucanase that hydro- lyzes amorphous cellulose within fibers. When acting alone on native cellulose fibers, each of the three enzymes is incapable of significant degradation. However, mixtures of two enzymes exhibited synergistic effects. The degradation effects of this synergism depended on the order in which the enzymes were added. Faster hydrolysis rates were observed when TrCel7A (exo) was added to fibers pretreated first with TrCel7B (endo) than when adding the enzymes in the opposite order. Endo-acting TrCel7B removed amorphous cellulose, softened and swelled the fibers, and exposed single microfibrils, facilitating the attack by the exo-acting enzymes. AFM images revealed that exo-acting enzymes processed the TrCel7B-pretreated fibers preferentially from one specific end (reducing or nonreducing). The most efficient (almost 100%) hydrolysis was observed with the mixture of the three enzymes. In this mixture, TrCel7B softened the fiber and TrCel6A and TrCel7A were directly observed to process it from the two opposing ends. This study provides high-resolution direct visualization of the nature of the synergistic relation between T. reesei exo- and endo-acting enzymes digesting native crystalline cellulose.

Changes at the gold–solution interface in an alkaline thiosulfate solution under leaching conditions were investigated using surface enhanced Raman spectroscopy (SERS). Oxidation product of thiosulfate, such as tetrathionate, adsorbed thiosulfate and products of thiosulfate decomposition such as adsorbed sulfide or elemental sulfur were identified as the main contributor to the film that forms on the gold surface as a result of direct contact with the thiosulfate solution. The roles of thiourea and ammonia additives on the enhanced leaching kinetics of thiosulfate-based electrolyte solutions were also investigated. The SERS spectra indicate that these two additives form surface complexes with Au(I) and their presence inhibits either the formation or adsorption of the oxygen-containing sulfur species responsible for passivation of the gold surface. Leaching currents were determined from the analysis of current–potential curves and mixed potential theory. The initial leaching rates were higher in the presence of the additives, suggesting that they inhibit the passivating species from interfering with the Au leaching reaction and assist this reaction by forming complexes with the released Au(I) ion.

Natural Sciences and Engineering Research Council of Canada; National Natural Science Foundation of China [20873116]

Polarized attenuated total reflection (ATR-IR) spectroscopy and fluorescence microscopy techniques were used to characterize a 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) membrane supported on porous, cationic hydrogel beads. Fluorescence microscopy images showed that the DPhPC coated the external surface of the hydrogel scaffold. In addition, a fluorescence assay of the emission intensity of the Tb3+/dipicolinic acid complex demonstrated that the DPhPC coating acted as a barrier to Tb3+ efflux from the scaffolded vesicle and successfully sealed the porous hydrogel bead. Fluorescence quenching and ATR-IR spectroscopic measurements revealed that the lipid coating has a bilayer structure. The phytanoyl chains were found to exhibit significant trans-gauche isomerization, characteristic of the fluid liquid phase. However, no lipid lateral mobility was observed by fluorescence recovery after photobleaching (FRAP) measurements. The phosphocholine headgroup was found to be well hydrated and oriented such that the cationic choline group tucked in behind the anionic phosphate group, consistent with an electrostatic attraction between the cationic scaffold and zwitterionic lipid. The absence of lipid lateral mobility may be due to the strength of this attraction. the Natural Science and Engineering Research Council of Canada (NSERC)

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Langmuir, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.langmuir.6b00635 Quantitative subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) was used to determine the molecular orientation and identify the metal–molecular interactions responsible for the adsorption of adenine from the bulk electrolyte solution onto the surface of the Au(111) electrode. The recorded p-polarized IR spectra of the adsorbed species were subtracted from the collected s-polarized IR spectra to remove the IR contributions of the vibrational bands of the desorbed molecules that are located within the thin layer cavity of the spectroelectrochemical cell. The intense IR band around 1640 cm–1, which is assigned to the pyrimidine ring stretching vibrations of the C5–C6 and C6–N10 bonds, and the IR band at 1380 cm–1, which results from a combination of the ring stretching vibration of the C5–C7 bond and the in-plane CH bending vibration, were selected for the quantitative analysis measurements. The transition dipoles of these bands were evaluated by DFT calculations. Their orientations differed by 85 ± 5°. The tilt angles of adsorbed adenine molecules were calculated from the intensity of these two vibrations at different potentials. The results indicate that the molecular plane is tilted at an angle of 40° with respect to the surface normal of the electrode and rotates by 16° around its normal axis with increasing electrode potential. This orientation results from the chemical interaction between the N10 and gold atoms coupled with the π–π parallel stacking interactions between the adjacent adsorbed molecules. Furthermore, the changes in the molecular plane rotation with the electric field suggests that the N1 atom of adenine must also participate in the interaction between the molecule and metal. Spanish Ministry of Science and Technology (CTQ2010- 19823), Spanish Ministry of Economy and Competitiveness (CTQ2014-57515-C2-1-R), Junta de Andalucia (PAI FQM202) and Discovery Grant from Natural Sciences and Engineering Council of Canada

The immersion method was used to measure the potential of zero free charge (E pzfc) of a bare Au(111) electrode and the electrode modified by self-assembled monolayers (SAMs) of 1-thio-β-D-glucose (β-Tg) and 2,3-di-O-phytanyl-sn-glycerol-1-tetraethylene glycol-D,L-α-lipoic acid ester (DPTL). The measurements were performed in three electrolyte solutions: 0.1 M NaH2PO4, 0.1 MKClO4 and 0.1 M NaF. The E pzc of the Au(111) electrode has different values in these electrolyte solutions due to the specific adsorption of phosphate and fluoride anions on the Au(111) surface. In contrast, when the gold surface is covered by a SAM of β-Tg, the adsorption of anions is suppressed and similar values of E pzfc were measured in the three electrolytes. Additional chronocoulometric experiments were performed to determine the potentials of the zero total charge (E pztc) for the thiol covered electrode. Significant differences between numerical values of potentials of the zero free charge E pzfc and potentials of the zero total charge E pztc were observed.

The biodegradation of cellulose involves the enzymatic action of cellulases (endoglucanases), cellobiohydrolases (exoglucanases), and β-glucosidases that act synergistically. The rate and efficiency of enzymatic hydrolysis of crystalline cellulose in vitro decline markedly with time, limiting the large-scale, cost-effective production of cellulosic biofuels. Several factors have been suggested to contribute to this phenomenon, but there is considerable disagreement regarding the relative importance of each. These earlier investigations were hampered by the inability to observe the disruption of crystalline cellulose and its subsequent hydrolysis directly. Here, we show the application of high-resolution atomic force microscopy to observe the swelling of a single crystalline cellulose fiber and its-hydrolysis in real time directly as catalyzed by a single cellulase, the industrially important cellulase 7B from Trichoderma reesei. Volume changes, the root-mean-square roughness, and rates of hydrolysis of the surfaces of single fibers were determined directly from the images acquired over time. Hydrolysis dominated the early stage of the experiment, and swelling dominated the later stage. The high-resolution images revealed that the combined action of initial hydrolysis followed by swelling exposed individual microfibrils and bundles of microfibrils, resulting in the loosening of the fiber structure and the exposure of microfibrils at the fiber surface. Both the hydrolysis and swelling were catalyzed by the native cellulase; under the same conditions, its isolated carbohydrate-binding module did not cause changes to crystalline cellulose. We anticipate that the application of our AFM-based analysis on other cellulolytic enzymes, alone and in combination, will provide significant insight into the process of cellulose biodegradation and greatly facilitate its application for the efficient and economical production of cellulosic ethanol.

Quantitative subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIF- TIRS) was used to determine the conformation and orientation of sodium dodecyl sulfate (SDS) molecules adsorbed at the single crystal Au(111) surface. The SDS molecules form a hemimicellar/hemicylindrical (phase I) structure for the range of potentials between −200 ≤ E < 450 mV and condensed (phase II) film for electrode potentials ≥500 mV vs Ag/AgCl. The SNIFTIRS measurements indicate that the alkyl chains within the two adsorbed states of SDS film are in the liquid-crystalline state rather than the gel state. However, the sulfate headgroup is in an oriented state in phase I and is disordered in phase II. The newly acquired SNIFTIR spectroscopy measurements were coupled with previous electrochemical, atomic force microscopy, and neutron reflectivity data to improve the current existing models of the SDS film adsorbed on the Au(111) surface. The IR data support the existence of a hemicylindrical film for SDS molecules adsorbed at the Au(111) surface in phase I and suggest that the structure of the condensed film in phase II can be more accurately modeled by a disordered bilayer.

Rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) were employed to measure leaching current of gold in an alkaline thiosulfate solution. First, the leaching current was determined with the help of RDE by measuring currents around the open circuit potential (ocp) and applying the mixed potential theory. The leaching current measured by this method may have contribution from another parasitic reaction such as oxidation of the leaching reagent. Therefore, a second method that employs RRDE was applied to verify that the leaching current measured at the RDE represents the gold dissolution reaction. In this approach gold was used for the disk and the ring was made of a glassy carbon. The gold ions produced at the disk during the leaching reaction were reduced at the ring. The collection efficiency allowed then to calculate the rate of gold ions production at the disk from the measured ring current. The leaching currents measured using this procedure were in excellent agreement with currents measured using RDE and the mixed potential theory confirming that the leaching currents measured by the first method were free of parasitic reactions and could be used as a measure of gold dissolution rate.

In this study, a Au(111) electrode is functionalized with a monolayer of 1-thio-β-D-glucose (β-Tg), producing a hydrophilic surface. A monolayer of β-Tg was formed on a Au(111) surface by either (1) potential-assisted deposition with the thiol in a supporting electrolyte or (2) passive incubation of a gold substrate in a thiol-containing solution. For each method, the properties of the β-Tg monolayer were investigated using cyclic voltammetry (CV), differential capacitance (DC), and chronocoulometry. In addition, electrochemical scanning tunneling microscopy (EC-STM) was used to obtain images of the self-assembled monolayer with molecular resolution. Potential-assisted assembly of β-Tg onto a Au(111) electrode surface was found to be complicated by oxidation of β-Tg molecules. The EC-STM images revealed formation of a passive layer containing honeycomb-like domains characteristic of a formation of S(8) rings, indicating the S-C bond may have been cleaved. In contrast, passive self-assembly of thioglucose from a methanol solution was found to produce a stable, disordered monolayer of β-Tg. Since the passive assembly method was not complicated by the presence of a faradaic process, it is the method of choice for modifying the gold surface with a hydrophilic monolayer.

The surface of a gold electrode was functionalized with a hydrophilic monolayer of 1-thio-β-D-glucose formed by spontaneous self-assembly. The Langmuir-Blodgett/Langmuir-Schaefer (LB/LS) method was then used to assemble a bilayer onto the modified Au(111) surface. The bilayer lipid membrane (BLM) was separated from the Au(111) electrode surface by incorporating the monosialoganglioside GM1 into the inner leaflet of a bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol. To make the inner leaflet, monolayers of GM1/DMPC/cholesterol with mole ratios of 1:6:3, 2:5:3, and 3:4:3 were used. The outer leaflet was composed of a 7:3 mole ratio of DMPC/cholesterol. Because of the amphiphilic properties of GM1, the hydrophobic acyl chains were incorporated into the BLM, whereas the large hydrophilic carbohydrate headgroups were physically adsorbed to the Au(111) electrode surface, creating a "floating" BLM (fBLM). This model contained a water-rich reservoir between the BLM and the gold surface. In addition, because of the bilayer being physically adsorbed onto the support, the fluidity of the BLM was maintained. The compression isotherms were measured at the air/water interface to determine the phase behavior and optimal transfer conditions. The images acquired using atomic force microscopy (AFM) and the force-distance measurements showed that the structure of the fBLM evolved with increasing GM1 content from 10 to 30 mol %, undergoing a transition from a corrugated to a homogeneous phase. This change was associated with a significant increase in bilayer thickness (from ∼5.3 to 7.3 nm). The highest-quality fBLM was produced with 30 mol % GM1.

The first version of this article was published in January issue of the Canadian Chemical News in 2000. This issue featured Electrochemistry and coincided with the 200th anniversary of the famous. Volta pile experiment. It also coincided with the 50th anniversary of the International Society of Electrochemistry. In that article I reflected on the past and I pondered on the future of electrochemistry. It was written for general community of Canadian chemists. The present version has been updated. However most of my original thoughts have been retained. I have also retained the original language of an article written to a non-specialized readership.

Highly ordered microporous films of silver-containing regular arrays of spherical pores of differing diameters were prepared by electrochemical deposition into the interstitial spaces of a template formed by nanosphere lithography. These nanostructured electrodes in conjunction with a Raman microprobe spectrometer were used to obtain surface-enhanced Raman spectra (SERS) of beta-thioglucose (beta-TG) under potential control. The SERS results were compared with SERS of beta-TG on an electrochemically roughened silver electrode surface. The bands in the experimental spectra were assigned to particular vibrations with the help of ab initio predictions of the spectra. The results of this study show that beta-TG self-assembled at a silver electrode forms a hydrophilic film that may be used in biomimetic research to enhance interactions between the electrode and the hydrophilic portion of a model membrane, and to prevent interactions of proteins inserted into this membrane with the metal surface. However, in contact with the aqueous electrolyte an anomerization reaction takes place and the beta-TG film is a mixture of the alpha- and beta-anomers of thioglucose. A partial oxidation of the self-assembled molecules was also observed. In addition, the orientation of the adsorbed molecules changes as a function of the applied potential.

Atomic force microscopy (AFM) was used to study native cellulose films prepared from a bacterial cellulose source, Acetobacter xylinum, using a novel application of the Langmuir-Blodgett technique. These films allowed high-resolution AFM images of single fibers and their microfibril structure to be obtained. Two types of experiments were performed. First, the fibers were characterized using samples that were dried after LB deposition. Next, novel protocols that allowed us to image single fibers of cellulose in films that were never dried were developed. This procedure allowed us to perform in situ AFM imaging studies of the enzymatic hydrolysis of single cellulose fibers in solution using cellulolytic enzymes. The in situ degradation of cellulose fibers was monitored over a 9 h period using AFM. These studies provided the first direct, real-time images of the enzymatic degradation of a single cellulose fiber. We have demonstrated the tremendous potential of AFM to study the mechanism of the enzymatic digestion of cellulose and to identify the most effective enzymes for the digestion of various cellulose structures or isomorphs.

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) has been applied to determine the conformation, orientation, and hydration of a monolayer of 2,3-di-O-phytanyl-sn-glycerol-1-tetraethylene glycol-dl-alpha-lipoic acid ester (DPTL) self-assembled at a gold electrode surface. This Archaea analogue thiolipid has been recently employed to build tethered lipid bilayers. By synthesizing DPT(d16)L, a DPTL molecule with a deuterium substituted tetraethylene glycol spacer, it was possible to differentiate the C-H stretch vibrations of the phytanyl chains from the tetraethylene glycol spacer and acquire the characteristic IR spectra for the chains, spacer, and lipoic acid headgroup separately. Our results show that the structure of the monolayer displays remarkable stability in a broad range of electrode potentials and that the phytanyl chains remain in a liquid crystalline state. The tetraethylene glycol chains are coiled, and the IR spectrum for this region shows that it is in the disordered state. The most significant result of this study is the information that in contrast to expectations the spacer region is poorly hydrated. Our results have implications for the design of a tethered lipid membrane based on this thiolipid.

By describing studies of three prototypical surfactants with similar hydrophobic tails but differently charged headgroups, this review provides a summary of the rich phase behavior of soluble surfactant molecules at electrified interfaces. With the use of electrochemical, scanning probe microscopy, and neutron scattering techniques we have been able to fully explore the adsorption and surface aggregation of these molecular systems. Furthermore, we have been able to provide compelling evidence of electric field-driven phase transitions in these surfactant films and their aggregated structures. Cumulatively, our results demonstrate that the electrical state of a surface (namely surface charge or applied potential) plays an integral role in determining the morphology of surfactants at solid interfaces. Unlike other aggregate shape determining factors such as the surfactant packing parameter, the electrical parameter can readily be adjusted in situ, providing a tunable means to control films of soft condensed matter.

In this work, we show molecular resolution scanning tunneling microscopy (STM) images of gramicidin, a model antibacterial peptide, inserted into a phospholipid matrix. 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 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.

Atomic force microscopy (AFM) was used to characterize a phospholipid bilayer composed of 70 mol % 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 30 mol % cholesterol, at a Au(111) electrode surface. Results indicate that addition of cholesterol relaxes membrane elastic stress, increases membrane thickness, and reduces defect density. The thickness and thermotropic properties of the mixed DMPC-cholesterol bilayer supported at the gold electrode surface are quite similar to the properties of the mixed membrane in unilamellar vesicles. The stability of the supported membrane at potentials negative to the potential of zero charge E(pzc) was investigated. This study demonstrates that the bilayer supported at the gold electrode surface is stable provided the applied potential (E - E(pzc)) is less than -0.3 V. At larger polarizations, swelling of the membrane is observed. Polarizations larger than -1 V cause electrodewetting of the bilayer from the gold surface. At these negative potentials, the bilayer remains in close proximity to the metal surface, separated from it by a approximately 2 nm thick layer of electrolyte.

Differential capacitance, chronocoulometry and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to characterize the structure and orientation of DMPC + cholesterol (70:30 mol%) phospholipid bilayers prepared using combined Langmuir–Blodgett/Langmuir–Schaefer (LB/LS) deposition. The electrochemical measurements indicate that incorporation of cholesterol into the membrane improves the quality of the bilayer if both leaflets contain cholesterol. Phospholipid bilayers containing DMPC and cholesterol prepared using LB/LS deposition were found to have smaller tilt angles (∼27°) than bilayers prepared by vesicle spreading (∼52°), indicating that LB/LS deposition is a superior method for preparing supported phospholipid bilayers at a gold electrode surface.

A combination of the Langmuir-Blodgett and Langmuir-Schaefer techniques has been used to build a 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) bilayer at a Au(111) electrode surface with hydrogen-substituted acyl chains in the top leaflet (solution side) and deuterium-substituted acyl chains in the bottom leaflet (gold side). Polarization modulation infrared reflection-absorption spectroscopy was used to determine changes in the conformation and orientation of the acyl chains of DMPC caused by the incorporation of two selected perfluorinated compounds, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), into the top leaflet of the bilayer. The incorporation of perfluorinated compounds into the DMPC bilayer caused a broadening of the methylene peaks and a shift in the methylene band positions toward higher frequencies. In addition, the tilt angle of the acyl chains decreased in comparison to the tilt angle of a pure DMPC bilayer. The reported tilt angles were smaller upon insertion of PFOS ( approximately 24 degrees ) than in the presence of PFOA ( approximately 30 degrees ). Overall, the results show that the incorporation of the perfluorinated acids has an effect on the bilayer similar to that of cholesterol by increasing the membrane fluidity and thickness due to a decrease in the tilt angle of the acyl chains.

The behavior, secondary structure, and orientation of a recently discovered bacteriocin-like peptide BacSp222 in a lipid model system supported at a gold electrode was investigated by chronocoulometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and attenuated total reflectance infrared (ATR-IR) spectroscopy. The IR spectra show that the secondary structure of BacSp222 is predominantly α-helical. Analysis of the spectra in the amide I region shows that the α-helical fragment of the peptide is inserted into bilayer at the potential range at which the bilayer is stable and attached to the Au(111) surface, i.e., from -0.5 to 0.3 V vs Ag/AgCl. Insertion of BacSp222 to the membrane significantly changes the conformation of the acyl chains of lipid molecules, from all-trans to partially melted; however, the chains become less tilted. Based on these results, we propose that BacSp222 interacts with the DMPC bilayer through the barrel-stave pore formation. In this model, α-helix of BacSp222 inserts into the membrane with an angle between the α-helix axis and membrane normal equal to ∼18°. The changes in orientation of the α-helical fragment of the peptide indicate that the orientation of BacSp222 with respect to the bilayer surface is potential-dependent. The peptide is inserted into the membrane driven by the electrostatic field generated by negative charge at the metal surface. It is not inserted at negative potentials where the membrane is detached from the metal and no longer exposed to the electrostatic field of the metal.

Ni electrodeposition experiments at a constant current density 220 A/m2 were performed to determine the optimum concentrations of chloride and an organic additive used for an industrial Ni electrowinning. White light interference microscopy was used to acquire digital images of the morphology of the electrodeposited nickel. The scaling analysis was employed to parameterize the morphological information encoded in the images. The standard deviation of the surface height, δ, the critical scaling length, L c, and the optical roughness, 4δ/L c, were determined as a function of the chloride concentration and the amount of organic additives. These parameters were plotted as a function of the two compositional variables. These three-dimensional plots allowed us to find conditions corresponding to the minimum of 4δ/L c, at which the deposited nickel is well leveled.

Electrochemical scanning tunneling microscopy (EC-STM) has been employed to study the structure of a film formed by cationic surfactant N-decyl-N,N,N-trimethylammonium triflate (DeTATf) adsorbed on the Au(111) electrode surface. The film is disordered at potentials corresponding to either large negative charge densities or to positive charge densities. At small negative charge densities, an ordered adlayer of flat-lying DeTATf molecules is formed. High-resolution images of this adlayer reveal that the triflate anion is coadsorbed with the N-decyl-N,N,N-trimethylammonium cation, effectively forming an ion pair at the electrode surface. This is a significant result because it explains why this surfactant behaves like a zwitterionic surfactant at the metal/solution interface.

We have applied a recently developed method (Langmuir 2006, 22, 5509-5519) to determine charge numbers per adsorbed molecule and packing densities in self-assembled monolayers (SAMs) of octadecanethiol (C18SH), a representative long-chain thiol. Our method yields values of area per molecule that are physically reasonable, in contrast to the popular reductive desorption method, which gives molecular areas that are smaller than those determined by the van der Waals radii. In a nonadsorbing electrolyte, we were able to model the dependence of the charge number per adsorbed molecule on the electrode potential, taking into account that the desorption process is a substitution reaction between the solvent and the adsorbate. We have also shown that the charge number per adsorbed thiol is affected by the specific adsorption of the anion of the electrolyte. In the latter case, the thiol competes for adsorption sites at the surface not only with water but also with the anion of the electrolyte, and this competition has an effect on the measured charge number.

Combined Langmuir-Blodgett vertical withdrawing and Langmuir-Schaefer horizontal touch (LB-LS) methods were employed to transfer DMPC bilayers onto a Au(111) electrode surface. Charge density measurements and photon polarization modulation infrared reflection absorption spectroscopy were employed to investigate electric field induced changes in the structure of the bilayer. The results show that the physical state and the molecular arrangement found in the monolayer at the air-water interface is to a large extent preserved in the bilayer formed by the LB-LS method. This approach provides an opportunity to produce supported bilayers with a well-designed architecture. The properties of the bilayer formed by the LB-LS method were compared to the properties of the bilayer produced by spontaneous fusion of unilamellar vesicles investigated in an earlier study (Bin, X.; Zawisza, I.; Lipkowski, J. Langmuir 2005, 21, 330-347). The tilt angles of the acyl chains are much smaller in the bilayer formed by the LB-LS method and are closer to the angles observed for vesicles and stacked hydrated bilayers. The tilt angles of the phosphate and choline groups are also smaller and are characteristic of an orientation in which the area per DMPC molecule is small. The electric field induced changes of these angles are also less pronounced in the bilayer formed by the LB-LS method. We have shown that these differences are a result of the higher packing density of the phospholipid molecules in the bilayer formed by the LB-LS method.

Reduction of fermentation-derived lactic acid (LA) offers a renewables-based pathway to propylene glycol (PG), a large-scale commodity chemical, currently manufactured by the oxidation of petroleum-derived propene. Complementing our previously described catalytic hydrogenation of LA to PG, we now report electrocatalytic hydrogenation (ECH) of LA in an aqueous electrolyte using constant current electrolysis. A reticulated vitreous carbon (RVC) electrode serves to agglomerate, support, and supply current to a 5% Ru/C powder catalyst, the same catalyst used in the classical hydrogenations. The ECH conditions are mild (ambient pressure, 70 °C vs 1500 psi H2, 150 °C) relative to the chemical hydrogenation. More surprisingly, the major electrohydrogenation product is lactaldehyde (LAL), with small quantities of PG also formed. Variable current studies in the range of 10–100 mA show an increase in product yields and a shift in selectivity toward PG with increasing current. Experiments carried out with different acids as electrolytes reveal a distinct effect of the anion on the yields of the two products. In situ ATR-FTIR studies of the ECH of LA point to a chelating bidentate carboxylate adsorption mode for lactate on the Ru surface and offer insight into the effects of electrolyte anions on surface adsorption and reactivity.

Electrochemical methods have been used to provide a thermodynamic description of the adsorption of citric acid (H 3 A) and dihydrogen citrate (H 2 A − ) on a Au(1 1 1) surface. We compare the adsorption of citric acid species from an electrolyte (pH 1) corresponding to the fully-protonated tri-acid species and an electrolyte (pH 3) corresponding to an approximately equimolar mixture of H 3 A and H 2 A − . Our results indicate a stronger adsorption of the monovalent ion compared to the neutral citric acid although both sets of experiments reveal similar charge numbers per adsorbed ion species. The latter values are dependent on the electrical variable (electrode potential or charge density) and approach a value of −3 at the most positive potentials (charge densities) studied for both sets of data (pH 1 and pH 3). This indicates that the adsorbed species is the fully de-protonated citrate ion, A −3 . From our chronocoulometry data we determine the outer Helmholtz potential for the citrate covered gold surface which is significantly larger for the system at pH 3 compared to pH 1. The results of this study on a two-dimensional gold surface can be extended to three-dimensional citrate-stabilized gold colloids, and rationalize the pH induced coagulation of colloidal sols.

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Langmuir, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.langmuir.5b02813. The scaffolded vesicle has been employed as an alternative means of developing natural model membranes and envisioned as a potential nutraceutical transporter. Furthering the research of the scaffolded vesicle system, a nucleophilic substitution reaction was implemented to form an ester linkage between palmitate and terminal hydroxyl groups of dextran in order to hydrophobically modify the hydrogel scaffold. An average tilt angle of 38° of the hydrophobic palmitate modifying layer on the surface of the hydrogel was determined from dichroic ratios obtained from infrared spectra collected in the attenuated total reflection (ATR) configuration. ATR-IR studies of the DMPC coated acylated hydrogel demonstrated that the hydrocarbon chains of the DMPC coating was similar to those of DMPC bilayers and that the underlying palmitate layer had a negligible effect on the average tilt angle (26°) of the DMPC coating. The 2 permeability of this acylated hydrogel was investigated with fluorescence spectroscopy and the terbium/ dipicolinic acid assay. The hydrophobic modification on the surface of the hydrogel bead allowed for an efficient deposition of a DMPC layer that served as an impermeable barrier to terbium efflux. About 72% of DMPC coated acylated hydrogel beads showed ideal barrier properties. The remaining 28% were leaking but the half-life of terbium efflux of the DMPC coated acylated hydrogel was increasing and the total amount of leaked terbium was decreasing with the incubation time. The half-life time and the retention were considered a marked improvement relative to past scaffolded vesicle preparations. The process of acylating hydrogel beads for efficient DMPC deposition has been identified as another viable method for controlling the permeability of the scaffolded vesicle. Discovery Grant from Natural Sciences and Engineering Council of Canada

Shell-isolated gold nanoparticles (SHINs) were employed to record shell-isolated nanoparticle-enhanced Raman spectra (SHINERS) of a passive layer formed at a gold surface during gold leaching from thiosulfate solutions. The (3-aminopropyl)triethoxysilane (APTES) and a sodium silicate solution were used to coat gold nanoparticles with a protective silica layer. This protective silica layer prevented interactions between the thiosulfate electrolyte and the gold core of the SHINs when the SHINs-modified gold electrode was immersed into the thiosulfate lixiviant. The SHINERS spectra of the passive layer, formed from thiosulfate decomposition, contained bands indicative of hydrolyzed APTES. We have demonstrated how to exploit the presence of these APTES bands as an internal standard to compensate for fluctuations of the surface enhancement of the electric field of the photon. We have also developed a procedure that allows for removal of the interfering APTES bands from the SHINERS spectra. These methodological advancements have enabled us to identify the species forming the passive layer and to determine that the formation of elemental sulfur, cyclo-S8, and polymeric sulfur chains is responsible for inhibition of gold dissolution in oxygen rich thiosulfate solutions.

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was employed to investigate the interaction of cholesterol with the headgroups of dimyristoylphosphatidycholine (DMPC) molecules under a static electric field. DMPC/cholesterol (7:3 molar ratio) mixtures form a bilayer on a Au(111) electrode surface by fusion and spreading of small unilamellar vesicles. PM-IRRAS experiments provided detailed information concerning the conformation and hydration of headgroups of DMPC bilayers in the presence and absence of 30% cholesterol. The presence of 30% cholesterol increases the space between the headgroups of DMPC molecules and hence increases the hydration of the DMPC/cholesterol mixed bilayer. The conformational state of the headgroups of DMPC molecules in the mixed bilayer is also significantly changed. The phosphate group is closer to the surface compared with the pure DMPC bilayer. The conformation of the -O-C-C-N moiety changes from gauche to trans in the presence of cholesterol.

The paper deals with phosphoric acid organic esters such as commercially available diphenylphosphate and custom made cyclic phosphoric ester of oligo(propylene oxide). The N,N-dimethylformamide and propylene carbonate solutions of these compounds were tested for potential application as electrolytes in a protonic electrochromic cell with tungsten trioxide as an active compound. As a reference, a solution of phosphoric acid of identical concentration was used. Cyclic chronovoltamperometry studies were used to test both the stability window of the electrolytes on a platinum electrode and the stability of indium tin oxide and tungsten trioxide electrodes against the electrolyte solution. The proton intercalation process was also studied. The experiments were performed both for dry solutions (vacuum drying of compounds) and for ones, which were on purpose contaminated with water. Additionally, the dc conductivity of the studied systems was measured for the whole possible operational temperature range. Generally, it was observed that the ester solutions give a better photochromic response of the WO3 electrode but are more aggressive to its surface. This observation is valid especially for the water-contaminated systems.

The thermodynamics of the so-called perfectly polarizable electrode was employed to analyze the voltammograms of a Pt(1 1 1) electrode in KBr solutions with an excess of a supporting electrolyte (0.1 M HClO4 and 0.1 M KClO4 + 10?3 M HClO4 + xM KBr where x varied between 5 × 10?4 and 1 × 10?2). The surface Gibbs excess, the Gibbs energy of adsorption and the charge number at a constant electrode potential and a constant chemical potential have been determined. The effect of pH on the magnitude of these parameters has been evaluated. The thermodynamic parameters for bromide have been compared to the parameters determined from previous thermodynamic studies of (bi)sulfate, chloride and OH adsorption at the Pt(1 1 1) electrode surface. As expected, bromide adsorption is stronger than for the other anions and halide adsorption seems to be limited by close packing of the adlayer. The calculated charge number values suggest that Br adsorption involves a full charge transfer, as in the case of chloride

For a monolayer of 2,3-di-phytanyl-sn-glycerol-1-tetraethylene glycol-D,L-a-lipoic acid ester lipid (DPTL) self-assembled (SAM) at a gold electrode surface we propose a new method to determine the charge number per adsorbed molecule and the packing density (area per molecule) in the monolayer. The method relies on chronocoulometry to measure the charge density at the SAM covered gold electrode surface. Two series of measurements have to be performed. In the first series, charge densities are measured for a monolayer transferred from the air-solution to the metal-solution interface using the Langmuir-Blodgett (LB) technique. This series of measurements allows one to determine charge numbers per adsorbed DPTL molecule. The second series is performed using a gold electrode covered with a self-assembled monolayer. The charge densities obtained in this series are then used to calculate the packing density with the help of charge numbers per adsorbed DPTL determined in the first series. The area per adsorbed molecule determined by the new method was compared to the area per molecule determined by the popular reductive desorption method. The molecular area determined with the new method is about 20% larger than the area calculated from the van der Waals model, which is a physically reasonable result. In contrast, the popular reductive desorption method gives an area per molecule 20% lower than the minimum estimated based on a van der Waals model. This is a physically unreasonable result. It is also shown that the charge numbers per adsorbed molecule depend on the electrode potential and may assume values smaller than the number of electrons participating in the reductive desorption step. An explanation of the origin of the "partial charge numbers" is provided. We recommend the new method be used in future studies of thiol adsorption at metal surfaces.

A thermodynamic method based on the work done by Frumkin and Petrii [A.N. Frumkin, O.A. Petrii, Electrochim. Acta 20 (1975) 347], to calculate the so-called double layer capacity for a Pt(1 1 1) electrode is proposed. The analysis requires careful measurement of the total charge density versus potential curves for a series of solutions with composition (0.1 − x) M KClO4 + x M HClO4. A method in which the total charge densities are determined by integration of cyclic voltammograms recorded in solutions with or without chloride is described. Following this procedure the double layer capacity curves were calculated. The double layer capacity curves displayed three peaks that were tentatively assigned to the solvent reorientation, onset of OH adsorption and completion of the OH adlayer. In the hydrogen adsorption region, the double layer capacity values were 14 ± 5 μF/cm2, in good agreement with previous estimates reported in the literature by using other approaches.

Atomic force microscopy (AFM) was used to study the morphology of electrodeposited Cu at current densities from 183 to 253 A m −2 . Digital image analysis was employed to parameterize the morphological information encoded in AFM images and to extract information concerning the mechanism of the electrodeposition reaction. It has been shown how the limiting roughness, δ, the critical scaling length, Lc and the aspect ratio, 4δ/Lc, vary as a function of the deposition time and electrodeposition conditions, such as temperature, current density and the amount of organic additives. It has been demonstrated how laboratory experiments of short duration and the scaling analysis of AFM images can be used to predict roughness of the metal sample after 2 weeks of industrial electrorefining. © 2005 Elsevier Ltd. All rights reserved.

The description of the monolayer formed at Au(111) by 2-mercaptobenzimidazole (MBI) under potential control has been based on electrochemical data (charge measurements) and spectroscopic information from the subtractively normalized interfacial Fourier transform infrared spectroscopy method (SNIFTIRS). From the quantitative analysis of the SNIFTIR spectra, a surface coverage Γ/Γ max was extracted for each sample potential. The evolution of the coverage with potential was in full agreement with the charge density curve. The shift of the pzc in the presence of MBI indicates that the adsorbed molecules have a nonzero component of the permanent dipole moment in the direction perpendicular to the electrode surface. Thanks to the high quality of the spectra, it was possible to determine the orientation of MBI molecules at the surface in the monolayer and submonolayer range. The angle between the C 2 -axis of the molecule and the direction normal to the surface is close to 64 ± 4° and its small change (

The thermodynamics of the so-called perfectly polarizable electrode was employed to analyze the total charge densities for a Pt(1 1 1) electrode in a series of solutions (0.1 ? x) M HClO4 + x M KClO4 + 2.5 × 10?3 M NaCl with a constant ionic strength and variable pH. The total charge densities were calculated by integration of cyclic voltammetry curves. The adsorption of chloride present in the electrolyte blocked hydrogen and OH adsorption at potentials 0.3 < E < 0.7 V (SHE). Consequently, the differential capacity, charge density and surface energy become pH independent in that potential range providing pH independent integration constants for the integration of CVs. For E < 0.3 V (SHE), a complete thermodynamic analysis using charge and potential as independent variables has been performed. The Gibbs excesses of adsorbed hydrogen were determined without a need to introduce any arbitrary correction for the so-called charging of the double layer and further assumptions about the charge number per adsorbed species. Using the thermodynamic method, the charge numbers at a constant potential (electrosorption valency) and at a constant chemical potential (reciprocal of the Esin–Markov coefficient) for adsorbed hydrogen were calculated using the Gibbs excess data. The charge numbers are equal to ?1, indicating that the adsorbed species is a totally discharged hydrogen atom. For 0.1 M HClO4 + 2.5 × 10?3 M NaCl solution Gibbs excesses of both hydrogen and chloride were determined. In a narrow potential range 0.2 < E < 0.3 V (SHE) chloride and hydrogen atoms are simultaneously adsorbed at the electrode surface. We have demonstrated that their adsorption has a competitive character

A model biological membrane was formed by fusion of mixed cholesterol and DMPC (dimyristoylphosphatidylcholine) phospholipid vesicles onto a gold-coated quartz support. The gold surface was charged and the influence of the charge at the solid support on the structure and integrity of the phospholipid bilayer was investigated using the specular reflection of neutrons and electrochemical measurements. When the surface charge density is close to zero, the lipid vesicles fuse directly on the surface to form a bilayer with a small number of defects and hence low water content. When the support's surface is negatively charged the film swells and incorporates water due to the field driven poration of the membrane. When the charge density is more negative then -8 microC cm(-2) the bilayer is detached from the metal surface. However, it remains in close proximity to the metal electrode, suspended on a thin cushion of water. The film thicknesses, calculated from neutron reflectivity, have allowed us to determine the tilt angle of the lipid molecules as a function of the support's charge density. The lipid molecules are tilted 55 degrees from the surface normal at zero charge density but become significantly more perpendicular (30 degrees tilt angle) at charge densities more negative than -8 microC cm(-2). The tilt angle measurements are in very good agreement with previous IR studies. This paper describes the highlights of a more in-depth study which is fully described in [1].

The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a nearly defect-free Pt(1 1 1) electrode in NaCl solutions with an excess of an inert electrolyte (0.1 M HClO 4 ). A complete thermodynamic analysis using the electrode potential and the charge as independent variables has been performed. The Gibbs excess, Gibbs energy of adsorption, charge numbers at constant electrode potential and constant chemical potential for chloride adsorption at the Pt(1 1 1) surface have been determined. Our results show that the polarity of the chemisorption bond is very small. The highest packing density of chloride determined by the thermodynamic method corresponds to 0.5 ML and is only 10% smaller than the coverage expected for a close packed monolayer of chlorine atoms. We demonstrated that at negative potentials, where Cl and hydrogen adsorption overlaps, the adsorption has a competitive character.

We have synthesized colloidal Pt nanoparticles with a mean particle size of 2.6±0.4 nm by reducing PtCl2 dissolved in N,N-dimethylacetamide with t-BuMe2SiH. The latter compound acted both as a reducing agent and a stabilizer of the Pt nanoparticles. Pt nanoparticles were deposited onto the Au substrate and IR reflectance absorption spectroscopy (IRRAS) was applied to investigate CO adsorption and oxidation at the surface of the catalyst. The reactivity of the catalyst covered with Nafion was compared with the reactivity of the catalyst without Nafion. In addition, the reactivity of the colloidal Pt was compared with the reactivity of bulk polycrystalline Pt. We found that CO oxidation proceeds at lower over-potentials at nanoparticles than at polycrystalline Pt. The IRRAS data indicate that the difference in the reactivity may be explained by a different mechanism of the oxidation reaction; Langmuir–Hinshelwood at Pt nanoparticles and island formation and growth at polycrystalline Pt. We have also observed that a film of Nafion slows down the CO oxidation reaction. The IRRAS spectra for CO adsorbed at Pt nanoparticles covered by Nafion were significantly different from the spectra recorded for the nanoparticles in the absence of Nafion. The spectroscopic features suggest that in the presence of Nafion the nanoparticles experience regions of lower and higher proton concentration.

Differential capacity, charge density measurements, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) were employed to study the fusion of small unilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) on a Au(111) electrode surface. The differential capacity and charge density data showed that the vesicles fuse onto the gold surface at charge densities between -10 microC/cm(2) < sigma(M) < 10 microC/cm(2) to form a bilayer. When sigma(M) < -10 microC/cm(2), the film is detached from the surface but it remains in close proximity to the surface. PM-IRRAS experiments provided IR spectra for the bilayer in the adsorbed and the desorbed state. Ab initio normal coordinate calculations were performed to assist interpretation of the IR spectra. The IR bands were analyzed quantitatively, and this analysis provided information concerning the conformation and orientation of the acyl chains and the polar head region of the DMPC molecule. The orientation of the chains, hydration, and conformation of the headgroup of the DMPC molecule strongly depend on the electrode potential.

The horizontal touch and Langmuir–Blodgett techniques have been used to transfer a monolayer of n-octadecanol from the gas–solution interface of a Langmuir trough onto the metal–solution interface of a Au(1 1 1) electrode. Chronocoulometry has been used to determine the charge density at the electrode surface covered by the film of n-octadecanol. The surface pressure of this film was calculated from the charge density data and was found to be controlled by the electrode potential. We have demonstrated that by dialing the potential applied to the electrode via a potentiostat the monolayer adsorbed on the surface can be compressed or decompressed. Two states of the monolayer were observed. The transition between these states took place at a film pressure ∼12 mN m −1 . Neutron reflectometry and polarization modulation Fourier transform infrared reflection absorption spectroscopy have been employed to determine the nature of the two states. The results show that octadecanol molecules form a two-dimensional solid film at all film pressures. At film pressures larger than 12 mN m −1 , the film has low compressibility and n-octadecanol molecules assume a small tilt angle with respect to the surface normal. At film pressures lower than 12 mN m −1 a compressive film is formed in which the tilt angle progressively increases with decreasing surface pressure. We have demonstrated that the properties of a monolayer of n-octadecanol at the metal–solution interface display many similarities to the properties

A mixed bilayer of cholesterol and dimyristoylphosphatidylcholine has been formed on a gold-coated block of quartz by fusion of small unilamellar vesicles. The formation of this bilayer lipid membrane on a conductive surface allowed us to study the influence of the support's surface charge on the structure and hydration of the bilayer lipid membrane. We have employed electrochemical measurements and the specular reflection of neutrons to measure the thickness and water content in the bilayer lipid membrane as a function of the charge on the support's surface. When the surface charge density is close to zero, the lipid vesicles fuse directly on the surface to form a bilayer with a small number of defects and hence small water content. When the support's surface is negatively charged the film swells and incorporates water. When the charge density is more negative than -8 micro C cm(-2), the bilayer starts to detach from the metal surface. However, it remains in a close proximity to the metal electrode, being suspended on a thin cushion of the electrolyte. The field-driven transformations of the bilayer lead to significant changes in the film thicknesses. At charge densities more negative than -20 micro C cm(-2), the bilayer is approximately 37 A thick and this number is comparable to the thickness determined for hydrated multilayers of dimyristoylphosphatidylcholine from x-ray diffraction experiments. The thickness of the bilayer decreases at smaller charge densities to become equal to approximately 26 A at zero charge. This result indicates that the tilt of the acyl chains with respect to the bilayer normal changes from approximately 35 degrees to 59 degrees by moving from high negative charges (and potentials) to zero charge on the metal.

The behaviour of a self-assembled monolayer of 2-mercaptobenzimidazole (MBI) at the Au(1 1 1) electrode has been examined using cyclic voltammetry and in situ FTIR spectroscopy. The charge associated with the reductive desorption is pH independent while the oxidative partial redeposition charge increases when the pH is lowered. This is due to differences between the nature and the solubility of the MBI desorption product. In alkaline and neutral media MBI desorbs as the thiolate. In contrast, in acidic solutions the thiol is the desorbed product. Subtractively normalized interfacial reflection Fourier transform absorption spectroscopy (SNIFTIRS) has been applied to investigate the MBI monolayer in contact with aqueous solutions of different pH. The SNIFTIRS data are in agreement with the electrochemical results. Moreover, quantitative analysis of the IR data provided evidence that adsorbed MBI molecules assume a tilted orientation with an angle of 60 ±5° between the C2 axis of the molecule and the direction normal to the gold surface.

The adsorption of citrate on Au(1 1 1) from citric acid containing electrolytes has been quantified using in situ subtractively normalised interfacial Fourier transform infrared spectroscopy (SNIFTIRS). These spectra, together with spectral simulations, provide evidence that citric acid, which contains 3 carboxylic acid groups, deprotonates fully upon adsorption. Spectral simulations are consistent with a limiting coverage of about 3 × 10 −10 mol cm −2 and the surface bound carboxylate groups adopting a tilted orientation. A model for the molecular adsorption is proposed.