Sum-frequency vibrational spectroscopy of a Langmuir film: Study of molecular orientation of a two-dimensional system

Size- and Shape-Dependent Interactions of Lipid-Coated Silver Nanoparticles: An Improved Mechanistic Understanding through Model Cell Membranes and In Vivo Toxicity

The widespread use of silver nanoparticles (AgNPs) in various applications and industries has brought to light the need for understanding the complex relationship between the physicochemical properties (shape, size, charge, and surface chemistry) of AgNPs that affect their ability to enter cells and cause toxicity. To evaluate their toxicological outcomes, this study systematically analyzed a series of homogeneous hybrid lipid-coated AgNPs spanning sizes from 5 to 100 nm with diverse shapes (spheres, triangles, and cubes). The hybrid lipid membrane comprises hydrogenated phosphatidylcholine (HPC), sodium oleate (SOA), and hexanethiol (HT), which shield the AgNP surface from surface oxidation and toxic Ag+ ion release to minimize its contribution to toxicity. To reduce any significant effects by surface chemistry, the HPC, SOA, and HT membrane composition ratio was kept constant, and the AgNPs were assessed using embryonic zebrafish (Danio rerio). While a direct comparison cannot be drawn due to the lack of complementary sizes below 40 nm for triangular plates and cubes due to synthetic challenges, significant mortality was observed for spherical AgNPs (AgNSs) of 5, 20, 40, and 60 nm at 120 h postfertilization at concentrations ≥6 mg Ag/L. In contrast, the 10, 80, and 100 nm AgNSs, 40, 70, and 100 nm triangular plate AgNPs (AgNPLs), and 55, 75, and 100 nm cubic AgNPs (AgNCs) showed no significant mortality at 5 days postfertilization following exposure to AgNPs at concentrations up to 12 mg Ag/L. With constant surface chemistry on the AgNPs, size is the dominant factor driving toxicological responses, with smaller nanoparticles (5 to 60 nm) being the most toxic. Larger AgNSs, AgNCs, and AgNPLs from 75 to 100 nm do not show any evidence of toxicity. However, when closely examining sizes between 40 and 60 nm for AgNSs, AgNCs, and AgNPLs, there is evidence that discriminates shape as a driver of toxicity since sublethal responses generally were observed to follow a pattern, suggesting toxicity is most significant for AgNSs followed by AgNPLs and then AgNCs, which is the least toxic. Sum frequency generation vibrational spectroscopy showed that irrespective of size or shape, all hybrid lipid-coated AgNPs interact with membrane surfaces and "snorkel" between phases into the lipid monolayer with minimal energetic cost. These findings decisively demonstrate that not only smaller AgNPs but also the shape of the AgNPs influences their biological compatibility.

Sixty years of electrochemical optical spectroscopy: a retrospective

Sixty years ago, Reddy, Devanatan, and Bockris performed the first in situ electrochemical ellipsometry experiment, which ushered in a new era in the study of electrochemistry, using optical spectroscopy. After six decades of development, electrochemical optical spectroscopy, particularly electrochemical vibrational spectroscopy, has advanced from a phase of immaturity with few methods and limited applications to a phase of maturity with excellent substrate generality and significantly improved resolutions. Here, we divide the development of electrochemical optical spectroscopy into four phases, focusing on the proof-of-concept of different electrochemical optical spectroscopy studies, the emergence of plasmonic enhancement-based electrochemical optical spectroscopic (in particular vibrational spectroscopic) methods, the realization of electrochemical vibrational spectroscopy on well-defined surfaces, and the efforts to achieve operando spectroelectrochemical applications. Finally, we discuss the future development trend of electrochemical optical spectroscopy, as well as examples of new methodology and research paradigms for operando spectroelectrochemistry.

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications

Interfacial engineering has long been a vital means of improving thin-film device performance, especially for organic electronics, perovskites, and hybrid devices. It greatly facilitates the fabrication and performance of solution-processed thin-film devices, including organic field effect transistors (OFETs), organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light-emitting diodes (OLEDs). However, due to the limitation of traditional interfacial materials, further progress of these thin-film devices is hampered particularly in terms of stability, flexibility, and sensitivity. The deadlock has gradually been broken through the development of self-assembled monolayers (SAMs), which possess distinct benefits in transparency, diversity, stability, sensitivity, selectivity, and surface passivation ability. In this review, we first showed the evolution of SAMs, elucidating their working mechanisms and structure-property relationships by assessing a wide range of SAM materials reported to date. A comprehensive comparison of various SAM growth, fabrication, and characterization methods was presented to help readers interested in applying SAM to their works. Moreover, the recent progress of the SAM design and applications in mainstream thin-film electronic devices, including OFETs, OSCs, PVSCs and OLEDs, was summarized. Finally, an outlook and prospects section summarizes the major challenges for the further development of SAMs used in thin-film devices.

The dynamic nature of natural and fatty acid modified calcite surfaces

Calcium carbonate, particularly in the form of calcite, is an abundant mineral widely used in both human-made products and biological systems. The calcite surface possesses a high surface energy, making it susceptible to the adsorption of organic contaminants. Moreover, the surface is also reactive towards a range of chemicals, including water. Consequently, studying and maintaining a clean and stable calcite surface is only possible under ultrahigh vacuum conditions and for limited amounts of time. When exposed to air or solution, the calcite surface undergoes rapid transformations, demanding a comprehensive understanding of the properties of calcite surfaces in different environments. Similarly, attention must also be directed towards the kinetics of changes, whether induced by fluctuating environments or at constant condition. All these aspects are encompassed in the expression "dynamic nature", and are of crucial importance in the context of the diverse applications of calcite. In many instances, the calcite surface is modified by adsorption of fatty acids to impart a desired nonpolar character. Although the binding between carboxylic acid groups and calcite surfaces is strong, the fatty acid layer used for surface modification undergoes significant alterations when exposed to water vapour and liquid water droplets. Therefore, it is also crucial to understand the dynamic nature of the adsorbed layer. This review article provides a comprehensive overview of the current understanding of both the dynamics of the calcite surface as well as when modified by fatty acid surface treatments.

Liposomes and Lipid Droplets Display a Reversal of Charge-Induced Hydration Asymmetry

The unique properties of water are critical for life. Water molecules have been reported to hydrate cations and anions asymmetrically in bulk water, being a key element in the balance of biochemical interactions. We show here that this behavior extends to charged lipid nanoscale interfaces. Charge hydration asymmetry was investigated by using nonlinear light scattering methods on lipid nanodroplets and liposomes. Nanodroplets covered with negatively charged lipids induce strong water ordering, while droplets covered with positively charged lipids induce negligible water ordering. Surprisingly, this charge-induced hydration asymmetry is reversed around liposomes. This opposite behavior in charge hydration asymmetry is caused by a delicate balance of electrostatic and hydrogen-bonding interactions. These findings highlight the importance of not only the charge state but also the specific distribution of neutral and charged lipids in cellular membranes.

How Does pH Affect the Adsorption of Human Serum Protein in the Presence of Hydrophobic and Hydrophilic Nanoparticles at Air-Water and Lipid-Water Interfaces?

This study investigates interaction between hydrophilic (11-mercaptoundecanoic acid (MUA)) and hydrophobic (1-undecanethiol (UDT)) gold nanoparticles (GNPs) with human serum albumin (HSA) protein on air-water and lipid-water interfaces at pH 3 and 7. Vibrational sum frequency generation (VSFG) spectroscopy is used to analyze changes in the intensity of interfacial water molecules and the C-H group of the protein. At the air-water interface, the hydrophobic interaction between the HSA protein and hydrophobic GNPs at pH 3 leads to their accumulation at the interface, resulting in an increased C-H intensity of the protein with a slight decrease in water intensity. Whereas, at pH 7, where the negative charge of the protein results in the reduced surface activity of the HSA compared to pH 3, the interaction between alkyl chain of the hydrophobic GNPs and alkyl group of the protein results in the adsorption of the protein-capped GNPs at the interface. This leads to an increased intensity of the C-H group of protein and water molecules. However, negatively charged hydrophilic GNPs do not induce significant changes in the interfacial water structure or the C-H group of the protein due to the electrostatic force of repulsion with the negatively charged HSA at pH 7. In contrast, at the lipid-water interface, both hydrophobic and hydrophilic GNPs interact with HSA protein, causing disordering of interfacial water molecules at pH 3 and ordering at pH 7. Interestingly, similar behavior of the protein with both types of GNPs results in comparable ordering/disordering at the interface depending on the pH of solution. Furthermore, the VSFG results obtained with the deuterated lipid suggest that changes in ordering and disorder occur due to increased protein adsorption in the presence of GNPs, causing alterations in the membrane structure. These findings give a better understanding of the mechanisms that govern protein-nanoparticle interaction and their consequential effects on the structure, function, and behavior of molecules at the biological membrane interface, which is crucial for developing safe and effective nanoparticle-based therapeutics.

Calcite Surfaces Modified with Carboxylic Acids (C2 to C18): Layer Organization, Wettability, Stability, and Molecular Structural Properties

A fundamental understanding of the interactions between mineral surfaces and amphiphilic surface modification agents is needed for better control over the production and uses of mineral fillers. Here, we controlled the carboxylic acid layer formation conditions on calcite surfaces with high precision via vapor deposition. The properties of the resulting carboxylic acid layers were analyzed using surface-sensitive techniques, such as atomic force microscopy (AFM), contact angle measurements, angle resolved X-ray photoelectron spectroscopy (XPS), and vibrational sum-frequency spectroscopy. A low wettability was achieved with long hydrocarbon chain carboxylic acids such as stearic acid. The stearic acid layer formed by vapor deposition is initially patchy, but with increasing vapor exposure time, the patches grow and condense into a homogeneous layer with a thickness close to that expected for a monolayer as evaluated by AFM and XPS. The build-up process of the layer occurs more rapidly at higher temperatures due to the higher vapor pressure. The stability of the deposited fatty acid layer in the presence of a water droplet increases with the chain length and packing density in the adsorbed layer. Vibrational sum frequency spectroscopy data demonstrate that the stearic acid monolayers on calcite have their alkyl chains in an all-trans conformation and are anisotropically distributed on the plane of the surface, forming epitaxial monolayers. Vibrational spectra also show that the stearic acid molecules interact with the calcite surface through the carboxylic acid headgroup in both its protonated and deprotonated forms. The results presented provide new molecular insights into the properties of adsorbed carboxylic acid layers on calcite.

"Nonlinear" pursuit of understanding pollutant accumulation and chemistry at environmental and biological interfaces

Over the past few decades, the public recognition of the prevalence of certain classes of pollutants, such as perfluoroalkyl substances and nanoplastics, within the environment, has sparked growing concerns over their potential impact on environmental and human health. Within both environmental and biological systems, the adsorption and structural organization of pollutants at aqueous interfaces can greatly impact the chemical reactivity and transformation. Experimentally probing chemical behavior at interfaces can often pose a problem due to bulk solvated molecules convoluting molecular signatures from interfacial molecules. To solve this problem, there exist interface-specific nonlinear spectroscopy techniques that can directly probe both macroscopic planar interfaces and nanoplastic interfaces in aqueous environments. These techniques can provide essential information such as chemical adsorption, structure, and reactivity at interfaces. In this perspective, these techniques are presented with obvious advantages for studying the chemical properties of pollutants adsorbed to environmental and biological interfaces.

Interfacial Inversion, Interference, and IR Absorption in Vibrational Sum Frequency Scattering Experiments

Molecular interfacial structure greatly determines the properties of nano- and microscale systems. Vibrational sum frequency scattering (SFS) spectroscopy is a unique interface-selective tool to measure the interfacial vibrational spectrum of sub-micron to micron-scale objects dispersed in liquid and solid media. The interfacial structure is extracted from the interfacial susceptibility, a physical property derived from the intensity. Here, we describe the effect of infrared absorption that occurs in a bulk medium that is spectroscopically complex and use the results to investigate the effects of interfacial inversion, interfacial interference, and interfacial interference combined with absorption. We use the same three chemicals to do so, hexadecane oil, water, and a neutral Span80 surfactant. For all cases, the effective surface susceptibility can be retrieved from the intensity. We further find that inverting the phases results in different interfacial structures, even though they are composed of the same three chemicals, and explain this in terms of the different interactions that are necessary to stabilize the drops: steric stabilization for water drops in oil vs. charge stabilization for oil drops in water. Interfacial interference can be used to estimate the surface density of different compounds.

Disentangling Origins of Adhesive Bonding at Interfaces between Epoxy/Amine Adhesive and Aluminum

Joining metals by adhesive bonding is essential in widespread fields such as mobility, dentistry, and electronics. Although adhesive technology has grown since the 1920s, the roles of interfacial phenomena in adhesive bonding are still elusive, which hampers the on-demand selection of surface treatment and adhesive types. In the present study, we clarified how chemical interactions and mechanical interlocking governed adhesive bonding by evaluating adhesion properties at the interfaces between epoxy/amine adhesive and two kinds of Al adherends: a flat aluminum hydroxide (Al_xO_yH_z) and technical Al plate with roughness. Spectroscopic and microscopical data demonstrate that the protonation of the amino groups in an amine hardener converts Al(OH)₃ on the Al_xO_yH_z surface to AlO(OH). The interfacial protonation results in an interfacial dipole layer with positive charges on the adhesive side, whose electrostatic interaction increases the interfacial fracture energy. The double cantilever beam tests for the flat Al_xO_yH_z and technical Al substrates clarify that the mechanical interlocking originating from the surface roughness further increases the fracture energy. This study disentangles the roles of the chemical interactions and mechanical interlocking occurring at the epoxy adhesive/Al interface in the adhesion mechanism.

Selective Action of Antimalarial Hydroxychloroquine on the Packing of Phospholipids and Interfacial Water Associated with Lysosomal Model Membranes: A Vibrational Sum Frequency Generation Study

Understanding the structural change of lysosomal membranes induced by hydroxychloroquine (HCQ) drug is essential as it has been considered as one of the probable mechanisms of its antimalarial action. In this context, vibrational sum frequency generation (VSFG) spectra of the O-H region of water and C-H of the hydrocarbon chain of negatively charged and zwitterionic phospholipids associated with the lysosomal membrane in the absence and presence of different concentrations of HCQ have been measured at the air/water interface. The interfacial water at the negatively charged and zwitterionic lipids gets restructured in the presence of HCQ; however, the mechanism of restructuring is different due to the charge of the head groups of lipids. Interestingly, the presence of HCQ leads to a disorder in the negatively charged lipids, irrespective of their chemical nature, mainly by creating the gauche defect in the hydrocarbon chain of the lipid. In contrast, the ordering of the zwitterionic lipid does not show any appreciable change with the addition of HCQ. The finding on the selectivity of HCQ in affecting the ordering of the lipid depending on its head group charge and restructuring of interfacial water may be useful in understanding the molecular level mechanism of the antimalarial action of HCQ.

Orientation of the Dysferlin C2A Domain is Responsive to the Composition of Lipid Membranes

Dysferlin is a 230 kD protein that plays a critical function in the active resealing of micron-sized injuries to the muscle sarcolemma by recruiting vesicles to patch the injured site via vesicle fusion. Muscular dystrophy is observed in humans when mutations disrupt this repair process or dysferlin is absent. While lipid binding by dysferlin's C2A domain (dysC2A) is considered fundamental to the membrane resealing process, the molecular mechanism of this interaction is not fully understood. By applying nonlinear surface-specific vibrational spectroscopy, we have successfully demonstrated that dysferlin's N-terminal C2A domain (dysC2A) alters its binding orientation in response to a membrane's lipid composition. These experiments reveal that dysC2A utilizes a generic electrostatic binding interaction to bind to most anionic lipid surfaces, inserting its calcium binding loops into the lipid surface while orienting its β-sheets 30-40° from surface normal. However, at lipid surfaces, where PI(4,5)P2 is present, dysC2A tilts its β-sheets more than 60° from surface normal to expose a polybasic face, while it binds to the PI(4,5)P2 surface. Both lipid binding mechanisms are shown to occur alongside dysC2A-induced lipid clustering. These different binding mechanisms suggest that dysC2A could provide a molecular cue to the larger dysferlin protein as to signal whether it is bound to the sarcolemma or another lipid surface.

Elucidating Interfacial Chain Conformation of Superhydrophilic Polymer Brushes by Vibrational Sum Frequency Generation Spectroscopy

Surface-tethered macromolecules (polymer brushes) are a potent means to modify surfaces with stimuli-responsive properties while avoiding delamination problems. This vibrational sum frequency generation spectroscopy study describes how the conformation of hydrophilic polymer brushes changes in response to environmental conditions, that is, changes in humidity (in air) and upon exposure to liquid water. Three hydrophilic brushes were prepared on silicon oxide surfaces by surface-initiated reversible deactivation radical polymerization of cationic (quaternary ammonium), anionic (sulfonate), and zwitterionic (containing both) monomers. The average tilt angle of methyl groups was analyzed and used to deduce the chain conformations of the polymer brushes. In air, the brush films absorb water and swell with increasing humidity. This is accompanied by the rotation of interfacial polymer chains. The degree of water uptake and chain conformation vary with the nature of the charged hydrophilic moieties. The hydrophilic polymer brush surfaces appear to remain relatively dry except in near-condensation conditions. In water, the quaternary ammonium groups of cationic and zwitterionic brushes are aligned nearly parallel to the surface. The anionic brush chains appear to assume nearly random conformations in water.

Interaction mechanism of chitosan oligomers in pure water with cell membrane models studied by SFG vibrational spectroscopy

Chitosan is a versatile and biocompatible cationic antimicrobial polymer obtained from sustainable sources that is effective against a wide range of microorganisms. Although it is soluble only at low pH, chitosan oligomers (ChitO) are soluble in pure water and thus more appropriate for antibacterial applications. Although there is a vast literature on chitosan's antimicrobial activity, the molecular details of its interaction with biomembranes remain unclear. Here we investigate these molecular interactions by resorting to phospholipid Langmuir films (zwitterionic DPPC and anionic DPPG) as simplified membrane models (for mammalian and bacterial membranes, respectively), and using SFG vibrational spectroscopy to probe lipid tail conformation, headgroup dynamics and interfacial water orientation. For comparison, we also investigate the interactions of another simple cationic antimicrobial polyelectrolyte, poly(allylamine) hydrochloride - PAH. By forming the lipid films over the polyelectrolyte solutions, we found that both have only a very small interaction with DPPC, but PAH adsorption is able to invert the interfacial water orientation (membrane potential). This might explain why ChitO is compatible with mammalian cells, while PAH is toxic. In contrast, their interaction with DPPG films is much stronger, even more so for ChitO, with both insertion within the lipid film and interaction with the oppositely charged headgroups. Again, PAH adsorption inverts the membrane potential, while ChitO does not. Finally, ChitO interaction with DPPG is weaker if the antimicrobial is injected underneath a pre-assembled Langmuir film, and its interaction mode depends on the time interval between end of film compression and ChitO injection. These differences between ChitO and PAH effects on the model membranes highlight the importance of molecular structure and intermolecular interactions for their bioactivity, and therefore this study may provide insights for the rational design of more effective antimicrobial molecules.

Mechanism by Which Cholesterol Induces Sphingomyelin Conformational Changes at an Air/Water Interface

This work investigates the interactions in cholesterol and sphingomyelin monolayers at the molecular level by high-resolution broadband sum frequency generation vibrational spectroscopy (HR-BB-SFG-VS). The SFG spectra of natural egg sphingomyelin (ESM) as a function of cholesterol concentration are obtained at an air/water interface under different polarization combinations. The analysis of the spectra shows that cholesterol can induce sphingomyelin conformational changes at an air/water interface. The mechanism is proposed. When cholesterol is inserted into the ESM monolayer, the inherent intramolecular hydrogen bonds between the phosphate moiety and 3OH in the sphingosine backbones are destroyed. During this process, the sphingosine backbones become more ordered, while the conformation of the N-linked long acid chain remains unaltered. The OH of the cholesterol head group can bind to the -PO^-2 of the ESM molecule, and the orientation of the -PO^-2 in the head groups changes to be more parallel to the interface.

Chiral Sum Frequency Generation Spectroscopy Detects Double-Helix DNA at Interfaces

Many DNA-based technologies involve the immobilization of DNA and therefore require a fundamental understanding of the DNA structure-function relationship at interfaces. We present three immobilization methods compatible with chiral sum frequency generation (SFG) spectroscopy at interfaces. They are the "anchor" method for covalently attaching DNA on a glass surface, the "island" method for dropcasting DNA on solid substrates, and the "buoy" method using a hydrocarbon moiety for localizing DNA at the air-water interface. Although SFG was previously used to probe DNA, the chiral and achiral SFG responses of single-stranded and double-stranded DNA have not been compared systemically. Using the three immobilization methods, we obtain the achiral and chiral C-H stretching spectra. The results introduce four potential applications of chiral SFG. First, chiral SFG gives null response from single-stranded DNA but prominent signals from double-stranded DNA, providing a simple binary readout for label-free detection of DNA hybridization. Second, with heterodyne detection, chiral SFG gives an opposite-signed spectral response useful for distinguishing native (D-) right-handed double helix from non-native (L-) left-handed double helix. Third, chiral SFG captures the aromatic C-H stretching modes of nucleobases that emerge upon hybridization, revealing the power of chiral SFG to probe highly localized molecular structures within DNA. Finally, chiral SFG is sensitive to macroscopic chirality but not local chiral centers and thus can detect not only canonical antiparallel double helix but also other DNA secondary structures, such as a poly-adenine parallel double helix. Our work benchmarks the SFG responses of DNA immobilized by the three distinct methods, building a basis for new chiral SFG applications to solve fundamental and biotechnological problems.

Choose your own adventure: Picosecond or broadband vibrational sum-frequency generation spectroscopy

Vibrational sum-frequency generation (VSFG) spectroscopy is a method capable of measuring chemical structure and dynamics within the interfacial region between two bulk phases. At the core of every experimental system is a laser source that influences the experimental capabilities of the VSFG spectrometer. In this article, we discuss the differences between VSFG spectrometers built with picosecond and broadband laser sources as it will impact everything from material costs, experimental build time, experimental capabilities, and more. A focus is placed on the accessibility of the two different SFG systems to newcomers in the SFG field and provides a resource for laboratories considering incorporating VSFG spectroscopy into their research programs. This Tutorial provides a model decision tree to aid newcomers when determining whether the picosecond or femtosecond laser system is sufficient for their research program and navigates through it for a few specific scenarios.

Laser pulses into bullets: tabletop shock experiments

This article discusses tabletop high-throughput laser experiments on shock waves in solids and liquids, where the more usual laser pump pulse is replaced by a 0.5 mm diameter laser-launched bullet, a thin metal disk called a flyer plate. The hypervelocity flyer (up to 6 km s^-1 or Mach 18) can have kinetic energy (∼1 J) to briefly produce extreme conditions of temperature and pressure, thousands of K and tens of GPa (1 GPa = 10 000 bar) in a small volume with a rise time <2 ns. The experiments are performed using a "shock compression microscope", a microscope fitted with the laser flyer launcher plus an optical velocimeter, a high-speed laser interferometer that measures the motion of the flyer plate or the sample material after impact. This makes it possible to generate extreme conditions at the push of a button in an intrinsically safe environment, and probe with any of the diagnostics used in microscope experiments, such as high-speed video, optical emission, nonlinear coherent spectroscopies and so on. The barrier to entering this field is relatively low since many laser laboratories already possess much of the needed instrumentation. A brief introduction to shock waves and instrumentation is presented. Then several examples of recent applications are described, including shocked water, the photophysics of fluorescent molecules under extreme conditions, shocked protein solutions, shocked metal-organic frameworks (MOFs), shocked explosives, chemical catalysis in a shocked liquid, and molecules at shocked interfaces. Since one can shoot a bullet at practically anything, there are many emerging opportunities in chemistry, biophysics, materials science, physics and hypervelocity aerodynamics.

Revealing the Molecular Physics of Lattice Self-Assembly by Vibrational Hyperspectral Imaging

Lattice self-assemblies (LSAs), which mimic protein assemblies, were studied using a new nonlinear vibrational imaging technique called vibrational sum-frequency generation (VSFG) microscopy. This technique successfully mapped out the mesoscopic morphology, microscopic geometry, symmetry, and ultrafast dynamics of an LSA formed by β-cyclodextrin (β-CD) and sodium dodecyl sulfate (SDS). The spatial imaging also revealed correlations between these different physical properties. Such knowledge shed light on the functions and mechanical properties of LSAs. In this Feature Article, we briefly introduce the fundamental principles of the VSFG microscope and then discuss the in-depth molecular physics of the LSAs revealed by this imaging technique. The application of the VSFG microscope to the artificial LSAs also paved the way for an alternative approach to studying the structure-dynamic-function relationships of protein assemblies, which were essential for life and difficult to study because of their various and complicated interactions. We expect that the hyperspectral VSFG microscope could be broadly applied to many noncentrosymmetric soft materials.

The Past and the Future of Langmuir and Langmuir-Blodgett Films

The Langmuir-Blodgett (LB) technique, through which monolayers are transferred from the air/water interface onto a solid substrate, was the first method to allow for the controlled assembly of organic molecules. With its almost 100 year history, it has been the inspiration for most methods to functionalize surfaces and produce nanocoatings, in addition to serving to explore concepts in molecular electronics and nanoarchitectonics. This paper provides an overview of the history of Langmuir monolayers and LB films, including the potential use in devices and a discussion on why LB films are seldom considered for practical applications today. Emphasis is then given to two areas where these films offer unique opportunities, namely, in mimicking cell membrane models and exploiting nanoarchitectonics concepts to produce sensors, investigate molecular recognitions, and assemble molecular machines. The most promising topics for the short- and long-term prospects of the LB technique are also highlighted.

Contact Ion Pair Formation Is Not Necessarily Stronger than Solvent Shared Ion Pairing

Vibrational sum frequency spectroscopy (VSFS) and pressure-area Langmuir trough measurements were used to investigate the binding of alkali metal cations to eicosyl sulfate (ESO₄) surfactants in monolayers at the air/water interface. The number density of sulfate groups could be tuned by mixing the anionic surfactant with eicosanol. The equilibrium dissociation constant for K⁺ to the fatty sulfate interface showed 10 times greater affinity than for Li⁺ and approximately 3 times greater than for Na⁺. All three cations formed solvent shared ion pairs when the mole fraction of ESO₄ was 0.33 or lower. Above this threshold charge density, Li⁺ formed contact ion pairs with the sulfate headgroups, presumably via bridging structures. By contrast, K⁺ only bound to the sulfate moieties in solvent shared ion pairing configurations. The behavior for Na⁺ was intermediate. These results demonstrate that there is not necessarily a correlation between contact ion pair formation and stronger binding affinity.

Headgroup-Specific Interaction of Biological Lipid Monolayer/Water Interface with Perfluorinated Persistent Organic Pollutant (f-POP): As Observed with Interface-Selective Vibrational Spectroscopy

Perfluoro compounds are widely used in various manufacturing processes, which leads to their bioaccumulation and subsequent adverse effects on human health. Using interface-selective vibrational spectroscopy (heterodyne-detected vibrational sum frequency generation (HD-VSFG)), we have elucidated the molecular mechanism of the perturbation of lipid monolayers on the water surface using a prototype perfluorinated persistent organic pollutant, perfluoroheptanoic acid (PFHA). PFHA disrupts the well-ordered all-trans conformation of a cationic lipid (1,2-dipalmitoyl-3-trimethylammonium propane (DPTAP)) monolayer and reduces the interfacial electric field at the lipid/water interface. In contrast, the hydrophobic packing of an anionic lipid (1,2-dipalmitoyl-sn-glycero-3-phospoglycerol (DPPG)) monolayer remains largely unaffected in the presence of PFHA, though the interfacial electric field is reduced. For a zwitterionic lipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC))/water interface, both alkyl chain ordering and interfacial electric field are fairly perturbed by PFHA. Lipid headgroup-specific interaction of PFHA and the repulsive interaction of oleophobic fluoroalkyl chain with the lipid alkyl chains govern these distinct perturbations of the lipid monolayers on the water surface.

Water-Induced Restructuring of the Surface of a Deep Eutectic Solvent

We study the molecular-scale structure of the surface of Reline, a DES made from urea and choline chloride, using heterodyne-detected vibrational sum frequency generation (HD-VSFG). Reline absorbs water when exposed to the ambient atmosphere, and following structure-specific changes at the Reline/air interface is crucial and difficult. For Reline (dry, 0 wt %, w/w, water) we observe vibrational signatures of both urea and choline ions at the surface. Upon increase of the water content, there is a gradual depletion of urea from the surface, an enhanced alignment, and an enrichment of the surface with choline cations, indicating surface speciation of ChCl. Above 40% w/w water content, choline cations abruptly deplete from the surface, as evidenced by the decrease of the vibrational signal of the -CH2- groups of choline and the rapid rise of a water signal. Above 60% w/w water content, the surface spectrum of aqueous Reline becomes indistinguishable from that of neat water.

Recovery of Fatty Acid Monolayers by Salts Investigated by Sum-Frequency Generation Spectroscopy

Langmuir monolayers consisting of fatty acids with relatively short alkyl chains (C₁₄H₂₉COOH (pentadecanoic acid), C₁₅H₃₁COOH (palmitic acid), and C₁₆H₃₃COOH (heptadecanoic acid)) are stable at a neutral pH (pH ≈ 6) but become unstable at a high pH (pH ≈ 11). Further addition of a small amount of divalent salt in subphase water was found to recover the monolayer at a high pH because binding of the divalent cations to the carboxylic headgroups renders the molecule more stable against dissolution in subphase water. This revival of the monolayer was observed via a pressure-area isotherm measurement and sum-frequency generation spectrum in the CH_x and OH ranges. Fatty acids with longer alkyl chains needed less amount of MgCl₂ to recover the monolayer at a high pH. A much lower concentration of Mg²⁺ as compared to Ca²⁺ is required to revive fatty acid molecules to the surface. Monovalent and trivalent salts were compared with the above divalent salts on the ability to recover the fatty acid monolayers.

Conformational Order of Alkyl Side Chain of Poly(3-alkylthiophene) Promotes Hole-Extraction Ability in Perovskite/Poly(3-alkylthiophene) Heterojunction

Molecular structures of hole transport materials (HTMs) have significant impact on the optoelectronic properties of perovskite/HTM heterojunction. But the structure-property relationship in the heterojunction remains poorly understood. By using poly(3-alkylthiophene) (P3AT) as the HTM model, here we apply sum frequency generation vibrational spectroscopy to establish correlations among conformations of P3ATs, the hole extraction ability of P3ATs from the perovskite layer, and the charge mobility of P3ATs. It is revealed that with similar energy-level alignment, the conformational order of alkyl side chains in regioregular P3ATs can effectively regulate the hole extraction ability of P3ATs from perovskite layer by tuning reorganization energy. By contrast, the charge mobility of P3ATs strongly depends on the P3AT backbone's coplanarity. Our findings decouple the roles of the long-hidden conformational order of alkyl side chain and the polythiophene backbone's coplanarity on the performance of perovskite/HTM heterojunction, offering useful guidelines for boosting the performance of optoelectronic devices.

Sum frequency generation spectroscopy of the attachment disc of a spider

The pyriform silk of the attachment disc of a spider was studied using infrared-visible vibrational sum frequency generation (SFG) spectroscopy. The spider can attach dragline and radial lines to many kinds of substrates in nature (concrete, alloy, metal, glass, plant branches, leaves, etc.) with the attachment disc. The adhesion can bear the spider's own weight, and resist the wind on its orb web. From our SFG spectroscopy study, the NH group of arginine side chain and/or NH₂ group of arginine and glutamine side chain in the amino acid sequence of the attachment silk proteins are suggested to be oriented in the disc. It was inferred from the observed doublet SFG peaks at around 3300 cm^-1 that the oriented peptide contains two kinds of structures.

Dynamic Duo: Vibrational Sum Frequency Scattering Investigation of pH-Switchable Carboxylic Acid/Carboxylate Surfactants on Nanodroplet Surfaces

Surfactants containing pH-switchable, carboxylic acid moieties are utilized in a variety of environmental, industrial, and biological applications that require controlled stability of hydrophobic droplets in water. For nanoemulsions, kinetically stable oil droplets in water, surface adsorption of the anionic form of the carboxylic acid surfactant stabilizes the droplet, whereas a dominant surface presence of the neutral form leads to destabilization. Through the use of dynamic light scattering, ζ-potential, and vibrational sum frequency scattering spectroscopy (VSFSS), we investigate this mechanism and the relative surface population of the neutral and charged species as pH is adjusted. We find that the relative population of the two surfactant species at the droplet surface is distinctly different than their bulk equilibrium concentrations. The ζ-potential measurements show that the surface concentration of the charged surfactant stays nearly constant throughout the stabilizing pH range. In contrast, VSFSS shows that the neutral carboxylic acid form increasingly adsorbs to the surface with increased acidity. The spectral features of the headgroup vibrational modes confirm this behavior and go further to reveal additional molecular details of their adsorption. A significant hydrogen-bonding interaction occurs between the headgroups that, along with hydrophobic chain-chain interactions, assists in drawing more carboxylic acid surfactant to the interface. The charged surfactant provides the stabilizing force for these droplets, while the neutral surfactant introduces complexity to the interfacial structure as the pH is lowered. The results are significantly different than what has been found for the planar oil/water studies where stabilization of the interface is not a factor.

Vibrational Sum-Frequency Generation Hyperspectral Microscopy for Molecular Self-Assembled Systems

In this review, we discuss the recent developments and applications of vibrational sum-frequency generation (VSFG) microscopy. This hyperspectral imaging technique can resolve systems without inversion symmetry, such as surfaces, interfaces and noncentrosymmetric self-assembled materials, in the spatial, temporal, and spectral domains. We discuss two common VSFG microscopy geometries: wide-field and confocal point-scanning. We then introduce the principle of VSFG and the relationships between hyperspectral imaging with traditional spectroscopy, microscopy, and time-resolved measurements. We further highlight crucial applications of VSFG microscopy in self-assembled monolayers, cellulose in plants, collagen fibers, and lattice self-assembled biomimetic materials. In these systems, VSFG microscopy reveals relationships between physical properties that would otherwise be hidden without being spectrally, spatially, and temporally resolved. Lastly, we discuss the recent development of ultrafast transient VSFG microscopy, which can spatially measure the ultrafast vibrational dynamics of self-assembled materials. The review ends with an outlook on the technical challenges of and scientific potential for VSFG microscopy.

Revealing molecular conformation-induced stress at embedded interfaces of organic optoelectronic devices by sum frequency generation spectroscopy

Interface stresses are pervasive and critical in conventional optoelectronic devices and generally lead to many failures and reliability problems. However, detection of the interface stress embedded in organic optoelectronic devices is a long-standing problem, which causes the unknown relationship between interface stress and organic device stability (one key and unsettled issue for practical applications). In this study, a kind of previously unknown molecular conformation-induced stress is revealed at the organic embedded interface through sum frequency generation (SFG) spectroscopy technique. This stress can be greater than 10 kcal/mol per nm² and is sufficient to induce molecular disorder in the organic semiconductor layer (with energy below 8 kcal/mol per nm²), finally causing instability of the organic transistor. This study not only reveals interface stress in organic devices but also correlates instability of organic devices with the interface stress for the first time, offering an effective solution for improving device stability.

Restructuring of Membrane Water and Phospholipids in Direct Interaction of Neurotransmitters with Model Membranes Associated with Synaptic Signaling: Interface-Selective Vibrational Sum Frequency Generation Study

Comprehensive molecular-level understanding of the role of interfacial water and phospholipids associated with synaptic membranes during their direct interaction with neurotransmitters is essential because of their involvement in synaptic signaling. Herein, the interfacial regions of the synaptic membranes mimicking anionic and zwitterionic phospholipids are probed in the presence of dopamine and serotonin neurotransmitters using surface-specific vibrational sum frequency generation technique. Neurotransmitters intrude into the headgroup region of both zwitterionic and anionic lipids by restructuring the interfacial water associated with the phospholipids, although the restructuring mechanism is different for both lipids. Neurotransmitters also decrease the overall ordering of both the phospholipids probably by creating gauche defects. Neurotransmitters restructure the surface water, conformation, and the ordering of the hydrocarbon chains of the zwitterionic and anionic phospholipids associated with synaptic membranes, which could be potentially an important step for synaptic signaling.

Assessing the Impact of Solvent Selection on Vibrational Sum-Frequency Scattering Spectroscopy Experiments

The development of vibrational sum-frequency scattering (S-VSF) spectroscopy has opened the door to directly probing nanoparticle surfaces with an interfacial and chemical specificity that was previously reserved for planar interfacial systems. Despite its potential, challenges remain in the application of S-VSF spectroscopy beyond simplified chemical systems. One such challenge includes infrared absorption by an absorptive continuous phase, which will alter the spectral lineshapes within S-VSF spectra. In this study, we investigate how solvent vibrational modes manifest in S-VSF spectra of surfactant stabilized nanoemulsions and demonstrate how corrections for infrared absorption can recover the spectral features of interfacial solvent molecules. We also investigate infrared absorption for systems with the absorptive phase dispersed in a nonabsorptive continuous phase to show that infrared absorption, while reduced, will still impact the S-VSF spectra. These studies are then used to provide practical recommendations for anyone wishing to use S-VSF to study nanoparticle surfaces where absorptive solvents are present.

A Unified Mathematical Formalism for First to Third Order Dielectric Response of Matter: Application to Surface-Specific Two-Colour Vibrational Optical Spectroscopy

To take advantage of the singular properties of matter, as well as to characterize it, we need to interact with it. The role of optical spectroscopies is to enable us to demonstrate the existence of physical objects by observing their response to light excitation. The ability of spectroscopy to reveal the structure and properties of matter then relies on mathematical functions called optical (or dielectric) response functions. Technically, these are tensor Green’s functions, and not scalar functions. The complexity of this tensor formalism sometimes leads to confusion within some articles and books. Here, we do clarify this formalism by introducing the physical foundations of linear and non-linear spectroscopies as simple and rigorous as possible. We dwell on both the mathematical and experimental aspects, examining extinction, infrared, Raman and sum-frequency generation spectroscopies. In this review, we thus give a personal presentation with the aim of offering the reader a coherent vision of linear and non-linear optics, and to remove the ambiguities that we have encountered in reference books and articles.

Evident phase separation and surface segregation of hydrophobic moieties at the copolymer surface using atomic force microscopy and SFG spectroscopy

HYPOTHESIS

Copolymers are developed to enhance the overall physical and chemical properties of polymers. The surface nature of a copolymer is relevant to creating efficient materials to improve adhesion and biocompatibility. We hypothesize that the improved adhesion, as a surface property, is due to phase separation, surface segregation, and the overall molecular organization of different polymer components at the copolymer surface.

EXPERIMENTS

The surface structure of a copolymer composed of 2-hydroxyethyl methacrylate (HEMA) monomer and 2-phenoxyethyl methacrylate (PhEMA) monomer was analyzed in comparison to the polyHEMA and polyPhEMA homopolymers using atomic force microscopy (AFM) and sum frequency generation (SFG) spectroscopy.

FINDINGS

The contrast in the phase images was due to the variance in the hydrophobic level provided by the hydroxyl and phenoxy modified monomers in the copolymer. The distribution of the adhesion values, supporting the presence of hydrophobic moieties, across the polymer surface defined the surface segregation of these two components. SFG spectra of the copolymer thin film showed combined spectral features of both polyHEMA and polyPhEMA thin films at the polymer surface. The tilt angles of the alpha-methyl group of homopolymers using the polarization intensity ratio analysis and the polarization mapping method were estimated to be in the range from 48° to 66°.

Conformational disorder of organic cations tunes the charge carrier mobility in two-dimensional organic-inorganic perovskites

The chemical nature of the organic cations governs the optoelectronic properties of two-dimensional organic-inorganic perovskites. But its mechanism is not fully understood. Here, we apply femtosecond broadband sum frequency generation vibrational spectroscopy to investigate the molecular conformation of spacer organic cations in two-dimensional organic-inorganic perovskite films and establish a correlation among the conformation of the organic cations, the charge carrier mobility, and broadband emission. Our study indicates that both the mobility and broadband emission show strong dependence on the molecular conformational order of organic cations. The gauche defect and local chain distortion of organic cations are the structural origin of the in-plane mobility reduction and broad emission in two-dimensional organic-inorganic perovskites. Both of the interlayer distance and the conformational order of the organic cations affect the out-of-plane mobility. This work provides molecular-level understanding of the conformation of organic cations in optimizing the optoelectronic properties of two-dimensional organic-inorganic perovskites.

Effect of Oxidation Level on the Interfacial Water at the Graphene Oxide-Water Interface: From Spectroscopic Signatures to Hydrogen-Bonding Environment

The interfacial region of the graphene oxide (GO)-water system is nonhomogenous due to the presence of two distinct domains: an oxygen-rich surface and a graphene-like region. The experimental vibrational sum-frequency generation (vSFG) spectra are distinctly different for the fully oxidized GO-water interface as compared to the reduced GO-water case. Computational investigations using ab initio molecular dynamics were performed to determine the molecular origins of the different spectroscopic features. The simulations were first validated by comparing the simulated vSFG spectra to those from the experiment, and the contributions to the spectra from different hydrogen bonding environments and interfacial water orientations were determined as a function of the oxidation level of the GO sheet. The ab initio simulations also revealed the reactive nature of the GO-water interface.

Adsorption of Iodine Species (I3-, I-, and IO3-) at the Nuclear Paint Monolayer-Water Interface and Its Relevance to a Nuclear Accident Scenario

Following a nuclear accident, radioactive iodine causes great concern to public health and safety. Organic iodide, because of its ability to escape reactor containment building and high environmental mobility, constitutes a predominant fraction of airborne radioiodine at places far away from the accident site. As the iodine released from a reactor core is inorganic iodine, it is vital to understand the mechanism of organic iodide formation inside reactor containment. In this context, we investigated the surface prevalence and adsorption of various inorganic iodines, I^-, I₃^-, and IO₃^-, at a nuclear paint (used in nuclear installations) monolayer-water interface, mimicking the painted inner walls of an accident-affected containment building that are exposed to the iodine-containing condensed water layer. Vibrational sum frequency generation (VSFG) measurements in the OH and CH stretch regions reveal that the paint-water interface changes its charge characteristics with the pH of the water that affects the degree of interaction with the iodine species. At the acidic condition (bulk pH < 7), the paint becomes positively charged and strongly adsorbs the negatively charged iodine species dissolved in the aqueous phase, whereas at the alkaline condition (bulk pH > 9.5), the paint becomes net neutral and weakly interacts with the iodine species. These interactions change the conformation of the paint such that its hydrophobic alkyl groups orient increasingly away from the aqueous phase. The order of adsorption increases as IO₃^- < I^- < I₃^- for the different iodine species studied.

Electric-Field-Induced Interface Behavior of Dodecyl Sulfate with Large Organic Counterions: A Molecular Dynamics Study

Dodecyl sulfate with tetramethylammonium counterions has been employed to systematically investigate the influence of different static electric fields on molecular structural properties, surface tension, by adopting molecular dynamics (MD) simulations with IR and sum frequency generation (SFG) spectrum calculations. The results indicated that dodecyl sulfate (DS^-) and large organic TMA⁺ counterions can form a mixed adsorption layer in which one head group of DS^- is surrounded by two tetramethylammonium (TMA⁺) and one water molecule. Additionally, it was observed that the surface tension significantly decreases with the increasing static electric field strength since the surfactant stands straighter at the interface as the electric field increases. The result can be instructively adopted in the manufacturing field to control surface tension. Moreover, it was found that the SFG stretch intensities of methylene decrease and the stretch intensities of the methyl group increase with increasing static electric fields. The result indicated that the static electric fields can make DS^- more orderly and upright at the interface.

Investigating the interactions of corona-free SWCNTs and cell membrane models using sum-frequency generation

The understanding of the interactions between biomolecules and nanomaterials is of great importance in many areas of nanomedicine and bioapplications. Numerous studies in this area have been performed. However, toxicological aspects involving the interaction between phospholipids and carbon nanotubes (CNTs) remain undefined, especially for those cases in which a protein corona is not formed around the nanomaterial (corona-free nanomaterials). This study focuses on the interaction of Langmuir films of dipalmitoylphosphatidylglycerol (DPPG) and dipalmitoylphosphatidylcholine (DPPC) with corona-free, single-walled CNTs. Surface pressure-area isotherms and sum-frequency generation (SFG) vibrational spectroscopy, a non-linear optical technique used to study surfaces and interfaces, were used to investigate the lipid tail orientation and conformation, aiming to understand the interactions between phospholipids and single walled carbon nanotubes functionalized by carboxylic acid (SWCNTs-COOH) at the air-water interface under low ionic strength conditions. Data from isotherms and SFG spectra revealed that the SWCNT adsorption at the air-water interface is induced by the presence of both lipids, although at a lesser extent for DPPG due to its anionic head group, which could result in repulsion of SWCNTs-COOH that also bear a negative charge. Furthermore, lipid monolayers remained conformationally ordered, indicating insertion of SWCNTs into the lipid monolayer. Our results corroborate previous works and simulations in the literature, but made it possible to perform an in-depth investigation of the interaction of these nanomaterials with components of phospholipid membranes.

On the Behavior of Perfluorinated Persistent Organic Pollutants (POPs) at Environmentally Relevant Aqueous Interfaces: An Interplay of Hydrophobicity and Hydrogen Bonding

The behavior of perfluorinated persistent organic pollutants (POPs), especially perfluoroalkyl carboxylic and sulfonic acids, at aqueous interfaces is crucial for their transport and speciation in the environment and subsequent immunotoxicity. Here, we investigate the surface prevalence and interfacial interaction of a prototype perfluorinated-POP, perfluoroheptanoic acid (PFHA), with environmentally relevant amphiphiles of varying hydrophobicity and head groups (C_nH_2n+1-X; n: 8 vs 16; -X: -OH vs -COOH) using interface-selective vibrational sum frequency generation (VSFG) spectroscopy. SFG intensity spectra in the CH- and OH-stretch regions reveal that PFHA prevails at aqueous interfaces that contain amphiphiles of intermediate chain length such as 1-octanol (n = 8) and heptanoic acid (n = 6). PFHA partially expels as well as increases the alkyl chain order of octanol on the water surface. Whereas heptanoic acid, though less hydrophobic than octanol, is retained at the water surface through hydrogen-bonding with the PFHA head group (^(PFHA)COO^-···HOOC^{(heptanoic-acid)}). Long chain amphiphiles (n = 16) such as hexadecanol and palmitic acid expel PFHA from the water surface regardless of the difference in their head groups. Interestingly, the dangling OH (3710 cm^-1) which is diminished at the hydrogenated-amphiphile-water interface is preserved at the perfluorinated-POP-water interface.

Vibrational Energy Redistribution between CH Stretching Modes in Alkyl Chain Monolayers Revealed by Time-Resolved Two-Color Pump-Probe Sum Frequency Spectroscopy

The vibrational dynamics of the various CH stretching modes in a fatty acid Langmuir-Blodgett film was studied using a resonant narrowband infrared (IR) laser pulse for pumping and a broadband femtosecond IR visible pulse pair for detection in a sum frequency spectroscopy setup. The resulting two-dimensional spectra indicate that pumping either the antisymmetric methyl or methylene stretch results in the transfer of energy to the other modes on a time scale faster than 2 ps. This rapid process is followed by energy redistribution to other modes, presumably the bending and internal rotational modes, with a time constant of approximately 85 ps. The formation of gauche defects is not observed within the first 250 ps. The whole spectrum recovers on a time scale of several nanoseconds, indicating dissipation of the excitation energy into the substrate.

The Prevailing Role of Hotspots in Plasmon-Enhanced Sum-Frequency Generation Spectroscopy

The plasmonic amplification of nonlinear vibrational sum frequency spectroscopy (SFG) at the surfaces of gold nanoparticles is systematically investigated by tuning the incident visible wavelength. The SFG spectra of dodecanethiol-coated gold nanoparticles chemically deposited on silicon are recorded for 20 visible wavelengths. The vibrational intensities of thiol methyl stretches extracted from the experimental measurements vary with the visible color of the SFG process and show amplification by coupling to plasmon excitation. Because the enhancement is maximal in the orange-red region rather than in the green, as expected from the dipolar model for surface plasmon resonances, it is attributed mostly to hotspots created in particle multimers, in spite of their low surface densities. A simple model accounting for the longitudinal surface plasmons of multimers allows the recovery of the experimental spectral dispersion.

Takes Two to Tango: Choreography of the Coadsorption of CTAB and Hexanol at the Oil-Water Interface

Mixed surfactant systems at the oil-water interface play a vital role in applications ranging widely from drug delivery to oil-spill remediation. Synergistic mixtures are superior emulsifiers and more effective at modifying surface tension than either component alone. Mixtures of surfactants with dissimilar polar head groups are of particular interest because of the additional degree of control they offer. The interplay of hydrophobic and electrostatic effects in these systems is not well understood, in part because of the difficulty in examining their behavior at the buried oil-water interface where they reside. Here, surface-specific vibrational sum frequency spectroscopy is utilized in combination with surface tensiometry and computational methods to probe the cooperative molecular interactions between a cationic surfactant cetyltrimethylammonium bromide (CTAB) and a nonionic alcohol (1-hexanol) that induce the two initially reluctant surfactants to coadsorb synergistically at the interface. A careful deuteration study of CTAB reveals that hexanol cooperates with CTAB such that both molecules preferentially orient at the interface for sufficiently large enough concentrations of hexanol. This work's methodology is unique and serves as a guide for future explorations of macroscopic properties in these complex systems. Results from this work also provide valuable insights into how interfacial ordering impacts surface tensiometry measurements for nonionic surfactants.

Chiral Inversion of Amino Acids in Antiparallel β-Sheets at Interfaces Probed by Vibrational Sum Frequency Generation Spectroscopy

A parallel study of protein variants with all (l-), all (d-), or mixed (l-)/(d-) amino acids can be used to assess how backbone architecture versus side chain identity determines protein structure. Here, we investigate the secondary structure and side chain orientation dynamics of the antiparallel β-sheet peptide LK7β (Ac-Leu-Lys-Leu-Lys-Leu-Lys-Leu-NH2) composed of all (l-), all (d-), or alternating (l-Leu)/(d-Lys) amino acids. Using interface-selective vibrational sum frequency generation spectroscopy (VSFG), we observe that the alternating (l-)/(d-) peptide lacks a resonant C-H stretching mode compared to the (l-) and (d-) variants and does not form antiparallel β-sheets. We rationalize our observations on the basis of density functional theory calculations and molecular dynamics (MD) simulations of LK7β at the air-water interface. Irrespective of the handedness of the amino acids, leucine side chains prefer to orient toward the hydrophobic air phase while lysine side chains prefer the hydrophilic water phase. These preferences dictate the backbone configuration of LK7β and thereby the folding of the peptide. Our MD simulations show that the preferred side chain orientations can force the backbone of a single strand of (l-) LK7β at the air-water interface to adopt β-sheet Ramachandran angles. However, denaturation of the β-sheets at pH = 2 results in a negligible chiral VSFG amide I response. The combined computational and experimental results lend critical support to the theory that a chiral VSFG response requires macroscopic chirality, such as in β-sheets. Our results can guide expectations about the VSFG optical responses of proteins and should improve understanding of how amino acid chirality modulates the structure and function of natural and de novo proteins at biological interfaces.

Hydration mediated interfacial transitions on mixed hydrophobic/hydrophilic nanodroplet interfaces

Interfacial phase transitions are of fundamental importance for climate, industry, and biological processes. In this work, we observe a hydration mediated surface transition in supercooled oil nanodroplets in aqueous solutions using second harmonic and sum frequency scattering techniques. Hexadecane nanodroplets dispersed in water freeze at a temperature of ∼15 °C below the melting point of the bulk alkane liquid. Addition of a trimethylammonium bromide (C_XTA⁺) type surfactant with chain length equal to or longer than that of the alkane causes the bulk oil droplet freezing transition to be preceded by a structural interfacial transition that involves water, oil, and the surfactant. Upon cooling, the water loses some of its orientational order with respect to the surface normal, presumably by reorienting more parallel to the oil interface. This is followed by the surface oil and surfactant alkyl chains losing some of their flexibility, and this chain stretching induces alkyl chain ordering in the bulk of the alkane phase, which is then followed by the bulk transition occurring at a 3 °C lower temperature. This behavior is reminiscent of surface freezing observed in planar tertiary alkane/surfactant/water systems but differs distinctively in that it appears to be induced by the interfacial water and requires only a very small amount of surfactant.