Surface-specific Sum-frequency Generation Research Articles

Spontaneous Appearance of Triiodide Covering the Topmost Layer of the Iodide Solution Interface Without Photo-Oxidation.

Ions containing iodine atoms at the vapor-aqueous solution interfaces critically affect aerosol growth and atmospheric chemistry due to their complex chemical nature and multivalency. While the surface propensity of iodide ions has been intensely discussed in the context of the Hofmeister series, the stability of various ions containing iodine atoms at the vapor-water interface has been debated. Here, we combine surface-specific sum-frequency generation (SFG) vibrational spectroscopy with ab initio molecular dynamics simulations to examine the extent to which iodide ions cover the aqueous surface. The SFG probe of the free O-D stretch mode of heavy water indicates that the free O-D group density decreases drastically at the interface when the bulk NaI concentration exceeds ∼2 M. The decrease in the free O-D group density is attributed to the spontaneous appearance of triiodide that covers the topmost interface rather than to the surface adsorption of iodide. This finding demonstrates that iodide is not surface-active, yet the highly surface-active triiodide is generated spontaneously at the water-air interface, even under dark and oxygen-free conditions. Our study provides an important first step toward clarifying iodine chemistry and pathways for aerosol formation.

Ion-Specific Water–Macromolecule Interactions at the Air/Aqueous Interface: An Insight into Hofmeister Effect

The specificity of ions in inducing conformational changes in macromolecules is introduced as the Hofmeister series; however, the detailed underlying mechanism is not comprehensible yet. We utilized surface-specific sum frequency generation (SFG) vibrational spectroscopy to explore the Hofmeister effect at the air/polyvinylpyrrolidone (PVP)/water interface. The spectral signature observed from the ssp polarization scheme reveals ion-specific ordering of water molecules following the Hofmeister series attributed to the ion-macromolecule interactions. Along with this, the presence of ions does not reflect any significant influence on the structure of the PVP macromolecule. However, the ppp-SFG spectra in the CH-stretch region reveal the impact of ions on the orientation angle of vinyl chain CH2-groups, which follows the Hofmeister series: SO42- > Cl- > NO3- > Br- > ClO4- > SCN-. The minimal orientation angle of CH2-groups indicates significant reordering in PVP vinyl chains in the presence of chaotropic anions ClO4-, and SCN-. The observation is attributed to the ion-specific water-macromolecule interactions at the air/aqueous interface. It is compelling to observe the signature of spectral blue shifts in the OH-stretch region in the ppp configuration in the presence of chaotropic anions. The origin of spectral blue shifts has been ascribed to the existence of weaker interactions between the interfacial water molecules and the backbone CH- and CH2-moieties of the PVP macromolecules. The ion-specific modulation in water-macromolecule interactions is endorsed by the relative propensity of anion's adsorption toward the air/aqueous interface. The experimental findings highlight the existence and cooperative participation of ion-specific water-macromolecule interactions in the mechanism of the Hofmeister effect, along with the illustrious ion-water and ion-macromolecule interactions.

Investigation of Water Evaporation Process at Air/Water Interface using Hofmeister Ions.

Evaporation is an interfacial phenomenon in which a water molecule breaks the intermolecular hydrogen (H-) bonds and enters the vapor phase. However, a detailed demonstration of the role of interfacial water structure in the evaporation process is still lacking. Here, we purposefully perturb the H-bonding environment at the air/water interface by introducing kosmotropic (HPO4-2, SO4-2, and CO3-2) and chaotropic ions (NO3- and I-) to determine their influence on the evaporation process. Using time-resolved interferometry on aqueous salt droplets, we found that kosmotropes reduce evaporation, whereas chaotropes accelerate the evaporation process, following the Hofmeister series: HPO4-2 < SO4-2 < CO3-2 < Cl- < NO3- < I-. To extract deeper molecular-level insights into the observed Hofmeister trend in the evaporation rates, we investigated the air/water interface in the presence of ions using surface-specific sum frequency generation (SFG) vibrational spectroscopy. The SFG vibrational spectra reveal the significant impact of ions on the strength of the H-bonding environment and the orientation of free OH oscillators from ∼36.2 to 48.4° at the air/water interface, where both the effects follow the Hofmeister series. It is established that the slow evaporating water molecules experience a strong H-bonding environment with free OH oscillators tilted away from the surface normal in the presence of kosmotropes. In contrast, the fast evaporating water molecules experience a weak H-bonding environment with free OH oscillators tilted toward the surface normal in the presence of chaotropes at the air/water interface. Our experimental outcomes showcase the complex bonding environment of interfacial water molecules and their decisive role in the evaporation process.

The dielectric function profile across the water interface through surface-specific vibrational spectroscopy and simulations

The dielectric properties of interfacial water on subnanometer length scales govern chemical reactions, carrier transfer, and ion transport at interfaces. Yet, the nature of the interfacial dielectric function has remained under debate as it is challenging to access the interfacial dielectric with subnanometer resolution. Here we use the vibrational response of interfacial water molecules probed using surface-specific sum-frequency generation (SFG) spectra to obtain exquisite depth resolution. Different responses originate from water molecules at different depths and report back on the local interfacial dielectric environment via their spectral amplitudes. From experimental and simulated SFG spectra at the air/water interface, we find that the interfacial dielectric constant changes drastically across an ∼1 Å thin interfacial water region. The strong gradient of the interfacial dielectric constant leads, at charged planar interfaces, to the formation of an electric triple layer that goes beyond the standard double-layer model.

Polarization-Dependent Sum-Frequency Generation Spectroscopy for Ångstrom-Scale Depth Profiling of Molecules at Interfaces.

The three-dimensional spatial distribution of molecules at soft matter interfaces is crucial for processes ranging from membrane biophysics to atmospheric chemistry. While several techniques can access surface composition, obtaining information on the depth distribution is challenging. We develop a noninvasive, polarization-resolved, surface-specific sum-frequency generation spectroscopy providing quantitative depth information. We demonstrate the technique on formic acid molecules at the air-water interface. With increasing molar fraction from 2.5% to 10%, the formic acid molecules shift, on average, ∼0.9 Å into the bulk. The consistency with the simulation data manifests that the technique allows for probing the Ångstrom-scale depth profile.

Direct Observation of Adsorption Morphologies of Cationic Surfactants at the Gold Metal-Liquid Interface.

Understanding interfacial phenomena is important in processes like corrosion, catalysis, and electrochemical reactions. Specifically, in corrosion inhibition, the assembly of adsorbed surfactants at metal-water interfaces in well-packed, ordered layers is desired. We provide direct evidence of the role of alkyl tails of surfactants in the formation of ordered adsorbed layers at metal-water interfaces. We have employed surface-specific sum frequency generation (SFG) spectroscopy to probe the in situ adsorption and self-assembly of cationic surfactants, alkyldimethylbenzyl ammonium bromides of tail lengths n = 4 (C4) and 12 (C12), without any applied potential or stimulus, at the gold-water interface. Our SFG measurements show that C12 Quat adsorbs as an ordered monolayer, whereas C4 Quat adsorbs in a disordered monolayer. All-atom molecular dynamics (MD) simulations of these surfactants corroborate with SFG results. These findings provide new insights on how hydrophobic interactions between alkyl tails of surfactants affect their self-assembly at metal-water interfaces.

Surface Dipoles Induce Uniform Orientation in Contacting Polar Liquids

Molecular interactions at the solid–liquid interface have a profound impact on the macroscale wetting properties of surfaces. Surface-specific sum frequency generation (SFG) spectroscopy, one of a limited number of techniques having the capability to probe such interfaces, generates a surface vibrational spectrum that is sensitive to molecular structure and is used to determine the orientation of molecules at the interface. This study utilized SFG spectroscopy to study the interactions between the polar liquid acetonitrile and selectively fluorinated self-assembled monolayer (FSAM) interfaces generated by the adsorption of CF3(CH2)17SH and CD3(CF2)6(CH2)10SH on gold; these SAMs exhibit strong oriented molecular dipoles at their chain termini. We used SFG spectroscopy to examine the C–N and C–H stretching regions, 2000–2300 and 2800–3050 cm–1, respectively, to probe the solid–liquid interface of the FSAMs in contact with acetonitrile. The appearance of positive- or negative-going peaks in the SFG spectrum of acetonitrile as a function of the variation in the terminal group of the FSAMs could be attributed, to a first-order approximation, to the orientation (CH3 up or down) of the acetonitrile molecules with respect to changes in the direction of the FSAM dipole. Coherent molecular spectroscopy was used as a direct probe of this effect by examining the differences in the SFG spectra to determine the relative orientation of acetonitrile with respect to the underlying SAM and, therefore, provided direct evidence for the dipole-influenced wettability behavior of these unique model surfaces.

Electrostatic Interactions Control the Functionality of Bacterial Ice Nucleators.

Bacterial ice-nucleating proteins (INPs) promote heterogeneous ice nucleation more efficiently than any other material. The details of their working mechanism remain elusive, but their high activity has been shown to involve the formation of functional INP aggregates. Here we reveal the importance of electrostatic interactions for the activity of INPs from the bacterium Pseudomonas syringae by combining a high-throughput ice nucleation assay with surface-specific sum-frequency generation spectroscopy. We determined the charge state of nonviable P. syringae as a function of pH by monitoring the degree of alignment of the interfacial water molecules and the corresponding ice nucleation activity. The net charge correlates with the ice nucleation activity of the INP aggregates, which is minimal at the isoelectric point. In contrast, the activity of INP monomers is less affected by pH changes. We conclude that electrostatic interactions play an essential role in the formation of the highly efficient functionally aligned INP aggregates, providing a mechanism for promoting aggregation under conditions of stress that prompt the bacteria to nucleate ice.

Peptide-Controlled Assembly of Macroscopic Calcium Oxalate Nanosheets.

The fabrication of two-dimensional (2D) biomineral nanosheets is of high interest owing to their promise for applications in electronics, filtration, catalysis, and chemical sensing. Using a facile approach inspired by biomineralization in nature, we fabricate laterally macroscopic calcium oxalate nanosheets using β-folded peptides. The template peptides are composed of repetitive glutamic acid and leucine amino acids, self-organized at the air–water interface. Surface-specific sum frequency generation spectroscopy and molecular dynamics simulations reveal that the formation of oxalate nanosheets relies on the peptide–Ca2+ ion interaction at the interface, which not only restructures the peptides but also templates Ca2+ ions into a calcium oxalate dihydrate lattice. Combined, this enables the formation of a critical structural intermediate in the assembly pathway toward the oxalate sheet formation. These insights into peptide–ion interfacial interaction are important for designing novel inorganic 2D materials.

Topologically disordered mesophase at the topmost surface layer of crystalline ice between 120 and 200 K

The structure and dynamics of an ice surface upon melting are of paramount importance in a variety of phenomena on earth and in the universe because undercoordinated water molecules at an ice surface have peculiar physical and chemical properties distinct from fully coordinated molecules in bulk. Using surface-specific sum-frequency generation spectroscopy and molecular dynamics simulations, we demonstrate that a topologically disordered hydrogen-bond network emerges at the topmost surface layer of crystalline ice Ih(0001) at $\ensuremath{\sim}120\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ that is much lower than the conventionally believed premelting temperature of $\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The ice surface undergoes a cascade of structural transitions from a low-temperature solid phase to high-temperature quasiliquid phase via the topologically disordered mesophase between 120 and 200 K. Because the lower limit of temperature of the earth's atmosphere is $\ensuremath{\sim}120\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ around the mesopause, the topmost surface layers of crystalline ice on earth are unlikely to be the perfectly ordered solid.

Counteracting Interfacial Energetics for Wetting of Hydrophobic Surfaces in the Presence of Surfactants.

Surface active agents (surfactants) are commonly used to improve the wetting of aqueous solutions on hydrophobic surfaces. The improved wettability is usually quantified as a decrease of the contact angle θ of a droplet on the surface, where the contact angle θ is given by the three surface tensions involved. Surfactants are known to lower the liquid–vapor surface tension, but what they do to the two other surface tensions is less clear. We propose an improved Zisman method for quantifying the wetting behavior of surfactants at the solid surface. This allows us to show that a number of very common surfactants do not change the wettability of the solid: they give the same contact angle as a simple liquid with the same liquid–vapor surface tension. Surface-specific sum-frequency generation spectroscopy shows that nonetheless surfactants are present at the solid surface. The surfactants therefore change the solid–liquid and solid–vapor surface tensions by the same amount, leading to an unchanged contact angle.

Molecular Structure of Hydrophobins Studied with Site-Directed Mutagenesis and Vibrational Sum-Frequency Generation Spectroscopy.

Hydrophobins are surface-active fungal proteins that adsorb to the water–air interface and self-assemble into amphiphilic, water-repelling films that have a surface elasticity that is an order of magnitude higher than other molecular films. Here we use surface-specific sum-frequency generation spectroscopy (VSFG) and site-directed mutagenesis to study the properties of class I hydrophobin (HFBI) films from Trichoderma reesei at the molecular level. We identify protein specific HFBI signals in the frequency region 1200–1700 cm–1 that have not been observed in previous VSFG studies on proteins. We find evidence that the aspartic acid residue (D30) next to the hydrophobic patch is involved in lateral intermolecular protein interactions, while the two aspartic acid residues (D40, D43) opposite to the hydrophobic patch are primarily interacting with the water solvent.

Nitrated Fatty Acids Modulate the Physical Properties of Model Membranes and the Structure of Transmembrane Proteins.

Nitrated fatty acids (NO2 -FAs) act as anti-inflammatory signal mediators, albeit the molecular mechanisms behind NO2 -FAs' influence on diverse metabolic and signaling pathways in inflamed tissues are essentially elusive. Here, we combine fluorescence measurements with surface-specific sum frequency generation vibrational spectroscopy and coarse-grained computer simulations to demonstrate that NO2 -FAs alter lipid organization by accumulation at the membrane-water interface. As the function of membrane proteins strongly depends on both, protein structure as well as membrane properties, we consecutively follow the structural dynamics of an integral membrane protein in presence of NO2 -FAs. Based on our results, we suggest a molecular mechanism of the NO2 -FA in vivo activity: Driven by the NO2 -FA-induced lipid layer reorganization, the structure and function of membrane-associated (signaling) proteins is indirectly affected.

Mechanistic Insights into Electrochemical Oxidation of Small Alcohol Molecules

Small alcohol molecules such as methanol and ethanol represent promising fuels to power fuel cells for mobile electronic devices, transportation and stationary applications. Ethanol is of particular interest due to its renewability by generation from biomass feedstocks, large energy density and compatibility with existing infrastructures for fuel storage and delivery. Although substantial progress has been made in the recent years for the development of direct ethanol fuel cell (DEFC) technology, obstacles are still present for the practical implication, largely in the lack of efficient and selective electrocatalysts for complete oxidation of ethanol to CO2. Platinum (Pt) is commonly used as electrocatalyst in fuel cells, including DEFCs. However, ethanol oxidation on Pt is found to be incomplete and produces undesired partial oxidation products such as acetaldehyde and acetic acid. It is thereby considered that Pt itself cannot cleave the C-C bond in ethanol molecules and a second transition metal such as Rh and Ir is needed to facilitate this rate- and selectivity-limiting step. However, evidences are also present in the literature for the formation of strongly binding adsorbates, such as -CO and -CH x , at low potentials (<0.5 V vs. RHE), from both in situ molecular spectroscopy (e.g., Infrared (IR) and surface enhanced Raman spectroscopy (SERS)) and differential electrochemical mass spectroscopy (DEMS) studies. These results suggest that oxidative removal of these C1 species, instead of C-C bond cleavage, is the sole rate-limiting factor on Pt catalysts. In attempt to resolve this controversy, we have performed systematic studies of methanol, ethanol and ethylene glycol oxidation on polycrystalline Pt electrodes. In particular, surface-specific sum frequency generation (SFG) spectroscopy is combined with electrochemistry to determine the reaction intermediates and evaluate their dependence on electrode potentials. It is found that C1 adsorbates form at low potentials, and the coverages increase as the electrode potential is raised from 0.1 V to ~0.4 V. While the -CO feature drops above 0.4 V for methanol and ethylene glycol oxidation, it persists up to 0.5 V and then drops for ethanol oxidation. Our results confirm the presence of active Pt sites for C-C bond cleavage and underline the role of -CH x species generated from the β-carbon in governing the kinetics of alcohol oxidation reactions. It is proposed that the slow transition from -CH x to -CO gives rise to the higher overpotentials for ethanol oxidation compared to methanol and ethylene glycol oxidation.

Plasmonic Field Enhancement for Vibration Spectroscopy at Metal/Water Interfaces

Electrochemical (EC) reactions play vital roles in many disciplines, and its molecular-level understanding is highly desired, in particular under reactions. The vibration spectroscopy is a powerful in situ technique for chemical analysis, yet its application to EC reactions is hindered by the strong attenuation of infrared (IR) light in both electrodes and electrolytes. Here we demonstrate that by incorporating appropriate sub-wavelength plasmonic structures at the metal electrode, the IR field at the EC interface can be greatly enhanced via the excitation of surface plasmon. This scheme facilitates in situ vibrational spectroscopic studies, especially using the surface-specific sum-frequency generation technique.

Deconstructing the Heterogeneity of Surface-Bound Catalysts: Rutile Surface Structure Affects Molecular Properties

Catalytic interfaces involving surface-bound molecular catalysts often exhibit a large structural heterogeneity from uncontrolled variation in surface morphology. Conventional spectroscopic techniques typically average over these different structural motifs within the sample, making it difficult to link the underlying surface morphology to the properties of the immobilized catalyst. Here we present the first direct comparison of the vibrational dynamics of a CO2 reduction catalyst bound to two different single-crystalline TiO2 surfaces, rutile (001) and (110), probed with transient surface-specific sum-frequency generation spectroscopy. We find that the change in surface structure between crystallographic faces alters both the vibrational frequency and relaxation time of the symmetric carbonyl stretching mode of the catalyst, with (001) displaying a lower frequency and longer relaxation time. This results from a change in the catalyst electronic structure and indicates that the molecular properties of the catalyst, likely including the catalytic properties, depend on the specific TiO2 surface to which it is bound. The comparison of the molecular properties on these two single crystal surfaces is an essential step toward understanding how semiconductor surface structure influences catalyst behavior and identifying optimal surface structures for improved catalytic performance.

Observation of ice-like water layers at an aqueous protein surface.

We study the properties of water at the surface of an antifreeze protein with femtosecond surface sum frequency generation spectroscopy. We find clear evidence for the presence of ice-like water layers at the ice-binding site of the protein in aqueous solution at temperatures above the freezing point. Decreasing the temperature to the biological working temperature of the protein (0 °C to -2 °C) increases the amount of ice-like water, while a single point mutation in the ice-binding site is observed to completely disrupt the ice-like character and to eliminate antifreeze activity. Our observations indicate that not the protein itself but ordered ice-like water layers are responsible for the recognition and binding to ice.

Structural Properties of gp41 Fusion Peptide at a Model Membrane Interface

We explore the structure and orientation of N-terminal (23 amino acids) of HIV gp41 envelop protein at the interface of a phospholipid monolayer. Using surface specific sum frequency generation, we probe the response of the Amide I vibrational states of the protein, and compare the experimental results to the modeled response of several secondary structures that have previously been reported in literature. To obtain the modeled response, we derive the line-shape expressions under cumulant expansion within the Brownian oscillator model, and express the intensities of the theoretical response according to the components of the Raman tensors and the infrared transition dipole moments of the relevant Amide I modes, under different orientation angles. This approach enables us to identify one plausible secondary structure among those considered, and allows us to determine the orientation angle, under which the protein inserts into the monolayer. We discuss the relevance of our findings toward understanding the functionality of this polypeptide at the membrane interface.

Nuclear Quantum Effects Affect Bond Orientation of Water at the Water-Vapor Interface

Using combined theoretical and experimental approaches, we demonstrate that the bond orientation of water at the water-vapor interface depends markedly on the water isotope (H-D) composition. While the interfacial water structures of H(2)O and D(2)O are indistinguishable, the intramolecular symmetry breaking in HDO is directly reflected at the surface: the OD bonds preferably orient down towards the bulk water, whereas the OH bond tends to orient up into the vapor phase. Path integral molecular dynamics simulations show good agreement with surface-specific sum-frequency generation (SFG) spectroscopy results, revealing that the distinct interfacial bond orientations originate from nuclear quantum effects. The enhanced localization of the heavier D atom leads to stronger hydrogen bonds, giving rise to OD bonds pointing down into the bulk.

The Polyphenol EGCG Inhibits Amyloid Formation Less Efficiently at Phospholipid Interfaces than in Bulk Solution

Age-related diseases, like Alzheimer's disease and type 2 diabetes mellitus, are characterized by protein misfolding and the subsequent pathological deposition of fibrillized protein, also called amyloid. Several classes of amyloid-inhibitors have recently been tested, traditionally under bulk conditions. However, it has become apparent that amyloid fibrils and oligomers assemble and exert their cytotoxic effect at cellular membranes, rather than in bulk solution. Knowledge is therefore required of inhibitor activity specifically at the phospholipid membrane interface. Here we show, using surface-specific sum-frequency generation (SFG) spectroscopy and atomic force microscopy (AFM), that the commonly used (-)-epigallocatechin gallate (EGCG) is a much less efficient amyloid inhibitor at a phospholipid interface than in bulk solution. Moreover, EGCG is not able to disaggregate existing amyloid fibrils at a phospholipid interface, in contrast to its behavior in bulk. Our results show that interfaces significantly affect the efficiency of inhibition by EGCG inhibitors and should therefore be considered during the design and testing of amyloid inhibitors.