Measuring dipolar width across liquid-liquid interfaces with 'molecular rulers'

Time-resolved heterodyne-detected electronic sum frequency generation (TR-HD-ESFG) spectroscopy: A new approach to explore interfacial dynamics

Aqueous interfaces containing organic/inorganic molecules are important in various biological, industrial, and atmospheric processes. So far, the study on the dynamics of interfacial molecules has been carried out with time-resolved vibrational sum-frequency generation (TR-VSFG) and time-resolved electronic sum-frequency generation (TR-ESFG) techniques. Although the ESFG probe is powerful for investigating interfacial photochemical dynamics of solute molecules by monitoring the electronic transition of transients or photoproducts at the interface, heterodyne detection is highly desirable for obtaining straightforward information, particularly in time-resolved measurements. So far, heterodyne detection has been realized only for TR-VSFG measurements but not for TR-ESFG measurements. In this paper, we report on femtosecond time-resolved heterodyne-detected ESFG (TR-HD-ESFG) spectroscopy for the first time. With TR-HD-ESFG developed, we measured the time-resolved electronic ΔImχ(2) spectra (pump-induced changes in the imaginary part of the second-order susceptibility) of a prototype dye, malachite green (MG), at the air/water interface. The obtained ΔImχ(2) spectra clearly show not only the ground-state bleach but also the excited-state band of MG at the air/water interface, demonstrating the high potential of TR-HD-ESFG as a new powerful tool to investigate ultrafast reaction dynamics at the interface.

Reactivity of Monoethanolamine at the Air-Water Interface and Implications for CO2 Capture

The development of CO2-capture technologies is key to mitigating climate change due to anthropogenic greenhouse gas emissions. These cover a number of technologies designed to reduce the level of CO2 emitted into the atmosphere or to eliminate CO2 from ambient air. In this context, amine-based sorbents in aqueous solutions are broadly used in most advanced separation techniques currently implemented in industrial applications. It has been reported that the gas/liquid interface plays an important role in the early stages of the capture process, but how the interface influences the chemistry is still a matter of debate. With the help of first-principles molecular dynamics simulations, we show that monoethanolamine (MEA), a prototypical sorbent molecule, has a weak affinity for the air-water interface, where in addition it exhibits a lower nucleophilicity compared to bulk solution. The change in reactivity is due to the combination of structural and electronic factors, namely, the shift of the conformational equilibrium and the stabilization of the N-atom lone pair. Based on these results, strategies for improving the efficiency of alkanolamine sorbents are proposed.

In Situ Probing of the Surface Properties of Droplets in the Air

Surface properties of nanodroplets and microdroplets are intertwined with their immense applicability in biology, medicine, production, catalysis, the environment, and the atmosphere. However, many means for analyzing droplets and their surfaces are destructive, non-interface-specific, not conducted under ambient conditions, require sample substrates, conducted ex situ, or a combination thereof. For these reasons, a technique for surface-selective in situ analyses under any condition is necessary. This feature article presents recent developments in second-order nonlinear optical scattering techniques for the in situ interfacial analysis of aerosol droplets in the air. First, we describe the abundant utilization of such droplets across industries and how their unique surface properties lead to their ubiquitous usage. Then, we describe the fundamental properties of droplets and their surfaces followed by common methods for their study. We next describe the fundamental principles of sum-frequency generation (SFG) spectroscopy, the Langmuir adsorption model, and how they are used together to describe adsorption processes at planar liquid and droplet surfaces. We also discuss the history of developments of second-order scattering from droplets suspended in dispersive media and introduce second-harmonic scattering (SHS) and sum-frequency scattering (SFS) spectroscopies. We then go on to outline the developments of SHS, electronic sum-frequency scattering (ESFS), and vibrational sum-frequency scattering (VSFS) from droplets in the air and discuss the fundamental insights about droplet surfaces that the techniques have provided. Finally, we describe some of the areas of nonlinear scattering from airborne droplets which need improvement as well as potential future directions and utilizations of SHS, ESFS, and VSFS throughout environmental systems, interfacial chemistry, and fundamental physics. The goal of this feature article is to spread knowledge about droplets and their unique surface properties as well as introduce second-order nonlinear scattering to a broad audience who may be unaware of recent progress and advancements in their applicability.

Electrostatics and Chemical Reactivity at the Air-Water Interface

It has been recently discovered that chemical reactions at aqueous interfaces can be orders of magnitude faster compared to conventional bulk phase reactions, but despite its wide-ranging implications, which extend from atmospheric to synthetic chemistry or technological applications, the phenomenon is still incompletely understood. The role of strong electric fields due to space asymmetry and the accumulation of ions at the interface has been claimed as a possible cause from some experiments, but the reorganization of the solvent around the reactive system should provide even greater additional electrostatic contributions that have not yet been analyzed. In this study, with the help of first-principles molecular dynamics simulations, we go deeper into this issue by a careful assessment of solvation electrostatics at the air-water interface. Our simulations confirm that electrostatic forces can indeed be a key factor in rate acceleration compared to bulk solution. Remarkably, the study reveals that the effect cannot simply be attributed to the magnitude of the local electric field and that the fluctuations of the full electrostatic potential resulting from unique dynamical behavior of the solvation shells at the interface must be accounted for. This finding paves the way for future applications of the phenomenon in organic synthesis, especially for charge transfer or redox reactions in thin films and microdroplets.

Recent progress in simulating microscopic ion transport mechanisms at liquid-liquid interfaces

Transport of ions through liquid-liquid interfaces is of fundamental importance to a wide variety of applications. However, since it is quite challenging for experimentalists to directly and selectively observe molecules at the interfaces, microscopic mechanisms of ion transport have been largely presumed from kinetic information. This Perspective illustrates recent examples that molecular dynamics simulations with proper free energy surfaces clarified mechanistic pictures of ion transport. The key is a proper choice of coordinates and defining/calculating free energy surfaces in multidimensional space. Once the free energy surfaces for realistic systems are available, they naturally provide new insight into the ion transport in unprecedented details, including water finger, transient ion pairing, and electron transfer.

Development of interface-/surface-specific two-dimensional electronic spectroscopy

Structures, kinetics, and chemical reactivities at interfaces and surfaces are key to understanding many of the fundamental scientific problems related to chemical, material, biological, and physical systems. These steady-state and dynamical properties at interfaces and surfaces require even-order techniques with time-resolution and spectral-resolution. Here, we develop fourth-order interface-/surface-specific two-dimensional electronic spectroscopy, including both two-dimensional electronic sum frequency generation (2D-ESFG) spectroscopy and two-dimensional electronic second harmonic generation (2D-ESHG) spectroscopy, for structural and dynamics studies of interfaces and surfaces. The 2D-ESFG and 2D-ESHG techniques were based on a unique laser source of broadband short-wave IR from 1200 nm to 2200 nm from a home-built optical parametric amplifier. With the broadband short-wave IR source, surface spectra cover most of the visible light region from 480 nm to 760 nm. A translating wedge-based identical pulses encoding system (TWINs) was introduced to generate a phase-locked pulse pair for coherent excitation in the 2D-ESFG and 2D-ESHG. As an example, we demonstrated surface dark states and their interactions of the surface states at p-type GaAs (001) surfaces with the 2D-ESFG and 2D-ESHG techniques. These newly developed time-resolved and interface-/surface-specific 2D spectroscopies would bring new information for structure and dynamics at interfaces and surfaces in the fields of the environment, materials, catalysis, and biology.

An octanol hinge opens the door to water transport

Despite their prevalent use as a surrogate for partitioning of pharmacologically active solutes across lipid membranes, the mechanism of transport across water/octanol phase boundaries has remained unexplored. Using molecular dynamics, graph theoretical, cluster analysis, and Langevin dynamics, we reveal an elegant mechanism for the simplest solute, water. Self-assembled octanol at the interface reversibly binds water and swings like the hinge of a door to bring water into a semi-organized second interfacial layer (a “bilayer island”). This mechanism is distinct from well-known lipid flipping and water transport processes in protein-free membranes, highlighting important limitations in the water/octanol proxy. Interestingly, the collective and reversible behavior is well-described by a double well potential energy function, with the two stable states being the water bound to the hinge on either side of the interface. The function of the hinge for transport, coupled with the underlying double well energy landscape, is akin to a molecular switch or shuttle that functions under equilibrium and is driven by the differential free energies of solvation of H₂O across the interface. This example successfully operates within the dynamic motion of instantaneous surface fluctuations, a feature that expands upon traditional approaches toward controlled solute transport that act to avoid or circumvent the dynamic nature of the interface.

Despite their pharmacological relevance, the mechanism of transport across water/octanol phase boundaries has remained unexplored. Octanol molecular assemblies are demonstrated to reversibly bind water and swing like the hinge of a door.

In Situ Spectroscopic Probing of Polarity and Molecular Configuration at Aerosol Particle Surfaces

The growth of aerosol particles in the atmosphere is related to chemical reactions in the gas and particle phases and at aerosol particle surfaces. While research regarding the gas and particle phases of aerosols is well-documented, physical properties and chemical reactivities at aerosol particle surfaces have not been studied extensively but have long been recognized. In particular, in situ measurements of aerosol particle surfaces are just emerging. The main reason is a lack of suitable surface-specific analytical techniques for direct measurements of aerosol particles under ambient conditions. Here we develop in situ surface-specific electronic sum frequency scattering (ESFS) to directly identify spectroscopic behaviors of molecules at aerosol particle surfaces. As an example, we applied an ESFS probe, malachite green (MG). We examined electronic spectra of MG at aerosol particle surfaces and found that the polarity of the surfaces is less polar than that in bulk. Our quantitative orientational analysis shows that MG is orientated with a polar angle of 25°-35° at the spherical particle surfaces of aerosols. The adsorption free energy of MG at the aerosol surfaces was found to be -20.75 ± 0.32 kJ/mol, which is much lower than that at the air/water interface. These results provide new insights into aerosol particle surfaces for further understanding the formation of secondary organic aerosols in the atmosphere.

Molecular reactions at aqueous interfaces

This Review aims to critically analyse the emerging field of chemical reactivity at aqueous interfaces. The subject has evolved rapidly since the discovery of the so-called 'on-water catalysis', alluding to the dramatic acceleration of reactions at the surface of water or at its interface with hydrophobic media. We review critical experimental studies in the fields of atmospheric and synthetic organic chemistry, as well as related research exploring the origins of life, to showcase the importance of this phenomenon. The physico-chemical aspects of these processes, such as the structure, dynamics and thermodynamics of adsorption and solvation processes at aqueous interfaces, are also discussed. We also present the basic theories intended to explain interface catalysis, followed by the results of advanced ab initio molecular-dynamics simulations. Although some topics addressed here have already been the focus of previous reviews, we aim at highlighting their interconnection across diverse disciplines, providing a common perspective that would help us to identify the most fundamental issues still incompletely understood in this fast-moving field.

Expanded Poly(tetrafluoroethylene) Blood Vessel Grafts with Embedded Reactive Oxygen Species (ROS)-Responsive Antithrombogenic Drug for Elimination of Thrombosis

Treatment of cardiovascular diseases suffers from the lack of transplantable small-diameter blood vessel (SDBV) grafts that can prohibit/eliminate thrombosis. Although expanded poly(tetrafluoroethylene) (ePTFE) has the potential to be used for SDBV grafts, recurrence of thrombus remains the biggest challenge. In this study, a reactive oxygen species (ROS)-responsive antithrombogenic drug synthesis and a bulk coating process were employed to fabricate functional ePTFE grafts capable of prohibiting/eliminating blood clots. The synthesized drug that would release antiplatelet ethyl salicylate (ESA), in responding to ROS, was dissolved in a polycaprolactone (PCL) solution, followed by a bulk coating of the as-fabricated ePTFE grafts with the PCL/drug solution. Nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) were employed to investigate and confirm the synthesis and presence of the ROS-responsive drug in the ePTFE grafts. The ESA release functions were demonstrated via the drug-release profile and dynamic anticoagulation tests. The biocompatibility of the ROS-responsive ePTFE grafts was demonstrated via lactate dehydrogenase (LDH) cytotoxicity assays, live and dead cell assays, cell morphology, and cell-graft interactions. The ROS-responsive, antithrombogenic ePTFE grafts provide a feasible way for maintaining long-term patency, potentially solving a critical challenge in SDBV applications.

Interface-Specific Two-Dimensional Electronic Sum Frequency Generation Spectroscopy

High even-order surface/interface specific spectroscopy has the potential to provide more structural and dynamical information about surfaces and interfaces. In this work, we developed a novel fourth-order interface-specific two-dimensional electronic sum frequency generation (2D-ESFG) for structures and dynamics at surfaces and interfaces. A translating wedge-based identical pulses encoding system (TWINs) was introduced to generate phase-locked pulse pairs for coherent pump beams in 2D-ESFG. As a proof-of-principle experiment, fourth-order 2D-ESFG spectroscopy was used to demonstrate couplings of surface states for both n-type and p-type GaAs (100). We found surface dark state within the bandgap of the GaAs in 2D-ESFG spectra, which could not be observed in one-dimensional ESFG spectra. To our best knowledge, this is a first demonstration of interface-specific two-dimensional electronic spectroscopy. The development of the 2D-ESFG spectroscopy will provide new structural probes of spectral diffusion, conformational dynamics, energy transfer, and charge transfer for surfaces and interfaces.

Liquids relax and unify strain in graphene

Solid substrates often induce non-uniform strain and doping in graphene monolayer, therefore altering the intrinsic properties of graphene, reducing its charge carrier mobilities and, consequently, the overall electrical performance. Here, we exploit confocal Raman spectroscopy to study graphene directly free-floating on the surface of water, and show that liquid supports relief the preexisting strain, have negligible doping effect and restore the uniformity of the properties throughout the graphene sheet. Such an effect originates from the structural adaptability and flexibility, lesser contamination and weaker intermolecular bonding of liquids compared to solid supports, independently of the chemical nature of the liquid. Moreover, we demonstrate that water provides a platform to study and distinguish chemical defects from substrate-induced defects, in the particular case of hydrogenated graphene. Liquid supports, thus, are advantageous over solid supports for a range of applications, particularly for monitoring changes in the graphene structure upon chemical modification.

Here, the authors report water as a superior platform to suspend graphene compared to solid substrates that induce non-uniformity and do not provide structural flexibility. They utilize confocal Raman spectroscopy to study graphene floating freely on the surface of water to show that a liquid support relieves the pre-existing strain.

Second Harmonic Generation Spectroscopy of Membrane Probe Dynamics in Gram-Positive Bacteria

Bacterial membranes are complex mixtures with dispersity that is dynamic over scales of both space and time. To capture adsorption onto and transport within these mixtures, we conduct simultaneous second harmonic generation (SHG) and two-photon fluorescence measurements on two different gram-positive bacterial species as the cells uptake membrane-specific probe molecules. Our results show that SHG not only can monitor the movement of small molecules across membrane leaflets but also is sensitive to higher-level ordering of the molecules within the membrane. Further, we show that the membranes of Staphylococcus aureus remain more dynamic after longer times at room temperature in comparison to Enterococcus faecalis. Our findings provide insight into the variability of activities seen between structurally similar molecules in gram-positive bacteria while also demonstrating the power of SHG to examine these dynamics.

Two dimensional diffusion-controlled triplet-triplet annihilation kinetics

Diffusion controlled chemical reactions are usually observed in three dimensional media. In contrast, planar bimolecular reactions taking place between reagents adsorbed at a soft interface are two-dimensional and therefore cannot be studied within the same formalism. Indeed, soft interfaces allow the adsorbed species to freely diffuse in a liquid-like manner. Here, we present the first experimental observation of a diffusion-controlled reaction in an environment that is planar at the ångström scale. By means of time-resolved surface second harmonic generation, an inherently surface sensitive technique, we observed that the kinetics of the diffusion of the reagents in the plane decreases as the surface concentration of adsorbed species increases. This is of course not the case for bulk reactions where the rates always increase with the reactant concentration. Such changes in the kinetics regime were rationalised as the evolution from a regular 2D free diffusion regime to a geometry-controlled scheme.

Hindered Isomerization at the Silica/Aqueous Interface: Surface Polarity or Restricted Solvation?

Time-resolved fluorescence measurements performed in a total internal reflection (TIR) geometry examined the photophysical behavior of coumarin 152 (C152) adsorbed to a silica/aqueous interface. Results imply that interfacial C152 has a remarkably different photoisomerization rate compared to its bulk solution value. C152's fluorescence in bulk water is dominated by a short, sub-nanosecond emission lifetime as the solute readily forms a nonemissive, twisted, intramolecular charge transfer (TICT) state. Time-resolved-TIR data from the silica/aqueous interface show that C152 emission contains a contribution from a longer-lived state (τ = 3.5 ns) that matches C152's fluorescence lifetime in nonpolar solvents where a photoexcited TICT state does not form. This long-lived excited state is assigned to C152 solvated in the interfacial region, where strong substrate-solvent hydrogen bonding prevents the aqueous solvent from stabilizing C152's TICT isomer. Similar results are observed for C152 in frozen water, emphasizing the silica surface's ability to restrict solvent mobility and change the interfacial solvation and reactivity from bulk solution limits.

Metal-dielectric hybrid nanoantennas for efficient frequency conversion at the anapole mode

Background: Dielectric nanoantennas have recently emerged as an alternative solution to plasmonics for nonlinear light manipulation at the nanoscale, thanks to the magnetic and electric resonances, the strong nonlinearities, and the low ohmic losses characterizing high refractive-index materials in the visible/near-infrared (NIR) region of the spectrum. In this frame, AlGaAs nanoantennas demonstrated to be extremely efficient sources of second harmonic radiation. In particular, the nonlinear polarization of an optical system pumped at the anapole mode can be potentially boosted, due to both the strong dip in the scattering spectrum and the near-field enhancement, which are characteristic of this mode. Plasmonic nanostructures, on the other hand, remain the most promising solution to achieve strong local field confinement, especially in the NIR, where metals such as gold display relatively low losses. Results: We present a nonlinear hybrid antenna based on an AlGaAs nanopillar surrounded by a gold ring, which merges in a single platform the strong field confinement typically produced by plasmonic antennas with the high nonlinearity and low loss characteristics of dielectric nanoantennas. This platform allows enhancing the coupling of light to the nanopillar at coincidence with the anapole mode, hence boosting both second- and third-harmonic generation conversion efficiencies. More than one order of magnitude enhancement factors are measured for both processes with respect to the isolated structure. Conclusion: The present results reveal the possibility to achieve tuneable metamixers and higher resolution in nonlinear sensing and spectroscopy, by means of improved both pump coupling and emission efficiency due to the excitation of the anapole mode enhanced by the plasmonic nanoantenna.

Exploring Fluorescent Dyes at Biomimetic Interfaces with Second Harmonic Generation and Molecular Dynamics

The adsorption of a DNA fluorescent probe belonging to the thiazole orange family at the dodecane/water and dodecane/phospholipid/water interfaces has been investigated using a combination of surface second harmonic generation (SSHG) and all-atomistic molecular dynamics (MD) simulations. Both approaches point to a high affinity of the cationic dye for the dodecane/water interface with a Gibbs free energy of adsorption on the order of -45 kJ/mol. Similar affinity was observed with a monolayer of negatively charged DPPG (1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol)) lipids. On the other hand, no significant adsorption could be found with the zwitterionic DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) lipids. This was rationalized in terms of Coulombic interactions between the monolayer surface and the cationic dye. The similar affinity for the interface with and without DPPG, despite the favorable Coulombic attraction in the latter case, could be explained after investigating the interfacial orientation of the dye. In the absence of a monolayer, the dye adsorbs with its molecular plane almost flat at the interface, whereas in the presence of DPPG it has to intercalate into the monolayer and adopt a significantly different orientation to benefit from the electrostatic stabilization.

New insight into probe-location dependent polarity and hydration at lipid/water interfaces: comparison between gel- and fluid-phases of lipid bilayers

Location dependent polarity and hydration probed by a new series of 4-aminophthalimide-based fluorescent molecules (4AP-Cn;n= 2–10, 12) show different behaviour at gel- and fluid-phase lipid/water interfaces.

Lipopolysaccharide Density and Structure Govern the Extent and Distance of Nanoparticle Interaction with Actual and Model Bacterial Outer Membranes

Design of nanomedicines and nanoparticle-based antimicrobial and antifouling formulations and assessment of the potential implications of nanoparticle release into the environment requires understanding nanoparticle interaction with bacterial surfaces. Here we demonstrate the electrostatically driven association of functionalized nanoparticles with lipopolysaccharides of Gram-negative bacterial outer membranes and find that lipopolysaccharide structure influences the extent and location of binding relative to the outer leaflet-solution interface. By manipulating the lipopolysaccharide content in Shewanella oneidensis outer membranes, we observed the electrostatically driven interaction of cationic gold nanoparticles with the lipopolysaccharide-containing leaflet. We probed this interaction by quartz crystal microbalance with dissipation monitoring (QCM-D) and second harmonic generation (SHG) using solid-supported lipopolysaccharide-containing bilayers. The association of cationic nanoparticles increased with lipopolysaccharide content, while no association of anionic nanoparticles was observed. The harmonic-dependence of QCM-D measurements suggested that a population of the cationic nanoparticles was held at a distance from the outer leaflet-solution interface of bilayers containing smooth lipopolysaccharides (those bearing a long O-polysaccharide). Additionally, smooth lipopolysaccharides held the bulk of the associated cationic particles outside of the interfacial zone probed by SHG. Our results demonstrate that positively charged nanoparticles are more likely to interact with Gram-negative bacteria than are negatively charged particles, and this interaction occurs primarily through lipopolysaccharides.

Reaction dynamics at liquid interfaces

The liquid interface is a narrow, highly anisotropic region, characterized by rapidly varying density, polarity, and molecular structure. I review several aspects of interfacial solvation and show how these affect reactivity at liquid/liquid interfaces. I specifically consider ion transfer, electron transfer, and SN2 reactions, showing that solvent effects on these reactions can be understood by examining the unique structure and dynamics of the liquid interface region.

Complementary surface second harmonic generation and molecular dynamics investigation of the orientation of organic dyes at a liquid/liquid interface

The second-order nonlinear response of two dyes adsorbed at the dodecane/water interface was investigated by surface second harmonic generation (SSHG). These dyes consist of the same chromophoric unit, 2-pyridinyl-5-phenyloxazole, with an alkyl chain located at the two opposite ends. The analysis of the polarization dependence of the SSHG intensity as usually performed points to similar tilt angles of the two dyes with respect to the interface but does not give information on the absolute direction. Molecular dynamics (MD) simulations reveal that both dyes lie almost flat at the interface but have opposite orientations. A refined SSHG data analysis with the width of the orientational distribution yields tilt angles that are in very satisfactory agreement with the MD simulations.

Fabrication of metal nanoelectrodes by interfacial reactions

Despite great improvements in the past decades, the controllable fabrication of metal nanoelectrodes still remains very challenging. In this work, a simple and general way to fabricate metal nanoelectrodes (Ag, Au, and Pt) is developed. On the basis of interfacial reactions at nano-liquid/liquid interfaces supported at nanopipettes, the nanoparticles can be formed in situ and have been used to block the orifices of pipettes to make nanoelectrodes. The effect of the driving force for interfacial reaction at the liquid/liquid interface, the ratio of redox species in organic and aqueous phases, and the surface charge of the inner wall of a pipette have been studied. The fabricated nanoelectrodes have been characterized by scanning electron microscopy (SEM) and electrochemical techniques. A silver electrode with about 10 nm in radius has been employed as the scanning electrochemical microscopy (SECM) probe to explore the thickness of a water/nitrobenzene (W/NB) interface, and this value is equal to 0.8 ± 0.1 nm (n = 5). This method of fabrication of nanoelectrodes can be extended to other metal or semiconductor electrodes.

Edge nonlinear optics on a MoS₂ atomic monolayer

The translational symmetry breaking of a crystal at its surface may form two-dimensional (2D) electronic states. We observed one-dimensional nonlinear optical edge states of a single atomic membrane of molybdenum disulfide (MoS2), a transition metal dichalcogenide. The electronic structure changes at the edges of the 2D crystal result in strong resonant nonlinear optical susceptibilities, allowing direct optical imaging of the atomic edges and boundaries of a 2D material. Using the symmetry of the nonlinear optical responses, we developed a nonlinear optical imaging technique that allows rapid and all-optical determination of the crystal orientations of the 2D material at a large scale. Our technique provides a route toward understanding and making use of the emerging 2D materials and devices.

Excited-state dynamics of charged dyes at alkane/water interfaces in the presence of salts and ionic surfactants

The excited-state dynamics of the cationic dye malachite green (MG) and of the dianionic dye eosin B at the dodecane/water interface has been investigated using femtosecond time-resolved surface second harmonic generation (TR-SSHG). By using different probe wavelengths, the contributions of monomeric and aggregated MG to the signal could be spectroscopically distinguished. The effect of the addition of a small amount of surfactants was found to strongly depend on the relative charges of surfactant and dye. For surfactant/dye pairs with opposite charges, the TR-SSHG signal is dominated by the contribution from aggregates, whereas for pairs with the same charges, the signal intensity becomes vanishingly small. These effects are explained in terms of electrostatic interactions between surfactants and dyes that favor either attraction of the dye toward the interface or its repulsion toward the bulk. As a very similar behavior is observed with MG upon addition of NaSCN, we conclude that, in this case, this effect reflects the affinity of SCN¯ for the interface. On the other hand, the guanidinium cation was found to have a different effect than that of a positively charged surfactant on the SSHG signal of MG, indicating this cation does not accumulate in the interfacial region.

LSPR properties of metal nanoparticles adsorbed at a liquid-liquid interface

Unlike the solid-air and solid-liquid interfaces, the optical properties of metal nanoparticles adsorbed at the liquid-liquid interface have not been theoretically exploited to date. In this work, the three dimensional finite difference time domain (3D-FDTD) method is employed to clarify the localized surface plasmon resonance (LSPR) based optical properties of gold nanoparticles (NPs) adsorbed at the water-oil interface, including near field distribution, far field absorption and their relevance. The LSPR spectra of NPs located at a liquid-liquid interface are shown to differ significantly from those in a uniform liquid environment or at the other interfaces. The absorption spectra exhibit two distinct LSPR peaks, the positions and relative strengths of which are sensitive to the dielectric properties of each liquid and the exact positions of the NPs with respect to the interface. Precise control of the particles' position and selection of the appropriate wavelength of the excitation laser facilitates the rational design and selective excitation of localized plasmon modes for interfacial NPs, a necessary advance for the exploration of liquid-liquid interfaces via surface enhanced Raman spectroscopy (SERS). According to our calculations, the SERS enhancement factor for Au nanosphere dimers at the water-oil interface can be as high as 10(7)-10(9), implying significant promise for future investigations of interfacial structure and applications of liquid-liquid interfaces towards chemical analysis.

Microgel particles at the fluid-fluid interfaces

Microgels are colloidal gel particles that consist of chemically cross-linked three-dimensional polymer networks. They are able to dramatically swell or shrink in response to a variety of external stimuli such as temperature, pH, ionic strength, electric field, and enzyme activities. Very recently, microgel particles were employed as stabilizers for emulsions. Being soft, porous and stimuli sensitivity, it has been shown that emulsions stabilized by these microgel particles can offer an unparalleled degree of control on emulsions' stability, well beyond what can be achieved by using small molecular surfactants or conventional solid colloidal particles. In this feature article, we review recent studies where microgel particles were employed as emulsion stabilizers, focusing on the behavior of microgel particles at the liquid-liquid interfaces. We also highlight that emulsions stabilized by soft microgel particles can serve as a template for the fabrication of novel functional materials which will have a great potential to be applied in a variety of applications.

Excited-state dynamics of organic dyes at liquid/liquid interfaces

Liquid/liquid interfaces play a crucial role in numerous areas of science. However, direct spectroscopic access to this thin (~1 nm) region is not possible with conventional optical methods. After a brief review of the most used techniques to perform interfacial optical spectroscopy, we will focus on time-resolved surface second harmonic generation, which allows the measurement of the excited-state dynamics of probe molecules at interfaces. By comparing these dynamics with those measured in bulk solutions, precious information on the properties of the interfacial region can be obtained. To illustrate this, several studies performed in our group will be presented.