Optical imaging of surface chemistry and dynamics in confinement

Azo-Enhanced Raman Scattering Probing Proton Transfer between Water and Nanoscale Zero-valent Iron

The interaction between a solid and water at their interface, especially proton transfer, impacts molecular-scale catalysis, macroscopic environmental science, and geoscience. Although being highly desired, directly probing proton transfer between a solid and water is a great challenge, given the subnanometer to nanometer scale of the interface. The fundamental challenge lies in the lack of a measurement tool to sensitively observe local proton concentration without introducing an exogenous electrode or nanoparticle with a minimum size of tens of nanometers. Here, we demonstrate an azo-enhanced Raman scattering strategy to design a 2 nm long small-molecule pH probe with a chelating group anchoring to the solid surface. Empowered by the intramolecular Raman enhancing sensitivity, the probe directly observes proton transfer between water and nanoscale zero-valent iron (nZVI), a famous environmental material for pollution control. This molecular-scale interfacial probing methodology offers a powerful tool to pave the way for advanced environmental and geochemical discernment and management.

Unconventional structural evolution of an oxide surface in water unveiled by in situ sum-frequency spectroscopy

Oxide-water interfaces host a wide range of important reactions in nature and modern industrial applications; however, accurate knowledge about these interfaces is still lacking at the molecular level owing to difficulties in accessing buried oxide surfaces. Here we report an experimental scheme enabling in situ sum-frequency vibrational spectroscopy of oxide surfaces in liquid water. Application to the silica-water interface revealed the emergence of unexpected surface reaction pathways with water. With ab initio molecular dynamics and metadynamics simulations, we uncovered a surface reconstruction, triggered by deprotonation of surface hydroxylated groups, that led to unconventional five-coordinated silicon species. The results help demystify the multimodal chemistry of aqueous silica discovered decades ago, bringing in fresh information that modifies the current understanding. Our study will provide new opportunities for future in-depth physical and chemical characterizations of other oxide-water interfaces.

Dynamic Second Harmonic Imaging of Proton Translocation Through Water Needles in Lipid Membranes

Proton translocation through lipid membranes is a fundamental process in the field of biology. Several theoretical models have been developed and presented over the years to explain the phenomenon, yet the exact mechanism is still not well understood. Here, we show that proton translocation is directly related to membrane potential fluctuations. Using high-throughput wide-field second harmonic (SH) microscopy, we report apparently universal transmembrane potential fluctuations in lipid membrane systems. Molecular simulations and free energy calculations suggest that H+ permeation proceeds predominantly across a thin, membrane-spanning water needle and that the transient transmembrane potential drives H+ ions across the water needle. This mechanism differs from the transport of other cations that require completely open pores for transport and follows naturally from the well-known Grotthuss mechanism for proton transport in bulk water. Furthermore, SH imaging and conductivity measurements reveal that the rate of proton transport depends on the structure of the hydrophobic core of bilayer membranes.

Bridging molecular-scale interfacial science with continuum-scale models

Solid-water interfaces are crucial for clean water, conventional and renewable energy, and effective nuclear waste management. However, reflecting the complexity of reactive interfaces in continuum-scale models is a challenge, leading to oversimplified representations that often fail to predict real-world behavior. This is because these models use fixed parameters derived by averaging across a wide physicochemical range observed at the molecular scale. Recent studies have revealed the stochastic nature of molecular-level surface sites that define a variety of reaction mechanisms, rates, and products even across a single surface. To bridge the molecular knowledge and predictive continuum-scale models, we propose to represent surface properties with probability distributions rather than with discrete constant values derived by averaging across a heterogeneous surface. This conceptual shift in continuum-scale modeling requires exponentially rising computational power. By incorporating our molecular-scale understanding of solid-water interfaces into continuum-scale models we can pave the way for next generation critical technologies and novel environmental solutions.

A charge-dependent long-ranged force drives tailored assembly of matter in solution

The interaction between charged objects in solution is generally expected to recapitulate two central principles of electromagnetics: (1) like-charged objects repel, and (2) they do so regardless of the sign of their electrical charge. Here we demonstrate experimentally that the solvent plays a hitherto unforeseen but crucial role in interparticle interactions, and importantly, that interactions in the fluid phase can break charge-reversal symmetry. We show that in aqueous solution, negatively charged particles can attract at long range while positively charged particles repel. In solvents that exhibit an inversion of the net molecular dipole at an interface, such as alcohols, we find that the converse can be true: positively charged particles may attract whereas negatives repel. The observations hold across a wide variety of surface chemistries: from inorganic silica and polymeric particles to polyelectrolyte- and polypeptide-coated surfaces in aqueous solution. A theory of interparticle interactions that invokes solvent structuring at an interface captures the observations. Our study establishes a nanoscopic interfacial mechanism by which solvent molecules may give rise to a strong and long-ranged force in solution, with immediate ramifications for a range of particulate and molecular processes across length scales such as self-assembly, gelation and crystallization, biomolecular condensation, coacervation, and phase segregation.

Monitoring the response of a model protocell to dye and surfactant molecules through second harmonic generation and fluorescence imaging

Probing the interaction between molecules and protocells is crucial for understanding the passive transport of functional molecules in and out of artificial and real cells. Second-harmonic generation (SHG) has been proven to be a powerful method for analyzing the adsorption and cross-membrane transport of molecules on lipid bilayers. In this study, we used SHG and two-photon fluorescence (TPF) imaging to study the interaction of charged dye molecules (D289) with a lipid vesicle. Unexpectedly, it was observed that the transport of D289 at a relatively high concentration is not as efficient as that at a lower dye concentration. Periodic shrinking of the model protocell and discharging of D289 out from the vesicle were revealed by combined analyses of SHG and TPF images. The response of the vesicle to a surfactant was also analyzed with D289 as a probe. This work demonstrates that the combined SHG and TPF imaging method is a unique approach that can provide detailed information on the interaction of molecules and lipids (both morphology and molecular kinetics). Determining these subtle interfacial kinetics in molecules is important for understanding the mechanism of many biophysical processes occurring on lipids.

Interfacial chemical reactivity enhancement

Interfacial enhancements of chemical reaction equilibria and rates in liquid droplets are predicted using a combined theoretical and experimental analysis strategy. Self-consistent solutions of reaction and adsorption equilibria indicate that interfacial reactivity enhancement is driven primarily by the adsorption free energy of the product (or activated complex). Reactant surface activity has a smaller indirect influence on reactivity due to compensating reactant interfacial concentration and adsorption free energy changes, as well as adsorption-induced depletion of the droplet core. Experimental air-water interfacial adsorption free energies and critical micelle concentration correlations provide quantitative surface activity estimates as a function of molecular structure, predicting an increase in interfacial reactivity with increasing product size and decreasing product polarity, aromaticity, and charge (but less so for anions than cations). Reactions with small, neutral, or charged products are predicted to have little reactivity enhancement at an air-water interface unless the product is rendered sufficiently surface active by, for example, interactions with interfacial water dangling OH groups, charge transfer, or voltage fluctuations.

Compact oblique-incidence nonlinear widefield microscopy with paired-pixel balanced imaging

Nonlinear (vibrational) microscopy has emerged as a successful tool for the investigation of molecular systems as it combines label-free chemical characterization with spatial resolution on the sub-micron scale. In addition to the molecular recognition, the physics of the nonlinear interactions allows in principle to obtain structural information on the molecular level such as molecular orientations. Due to technical limitations such as the relatively complex imaging geometry with the required oblique sample irradiation and insufficient sensitivity of the instrument this detailed molecular information is typically not accessible using widefield imaging. Here, we present, what we believe to be, a new microscope design that addresses both challenges. We introduce a simplified imaging geometry that enables the measurement of distortion-free widefield images with free space oblique sample irradiation achieving high spatial resolution (∼1 µm). Furthermore, we present a method based on a paired-pixel balanced detection system for sensitivity improvement. With this technique, we demonstrate a substantial enhancement of the signal-to-noise ratio of up to a factor of 10. While both experimental concepts presented in this work are very general and can, in principle, be applied to various microscopy techniques, we demonstrate their performance for the specific case of heterodyned, sum frequency generation (SFG) microscopy.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

New insights on the catalytic center of proteins from peptidylprolyl isomerase group based on the FOD-M model

Generating the structure of the hydrophobic core is based on the orientation of hydrophobic residues towards the central part of the protein molecule with the simultaneous exposure of polar residues. Such a course of the protein folding process takes place with the active participation of the polar water environment. While the self-assembly process leading to the formation of micelles concerns freely moving bi-polar molecules, bipolar amino acids in polypeptide chain have limited mobility due to the covalent bonds. Therefore, proteins form a more or less perfect micelle-like structure. The criterion is the hydrophobicity distribution, which to a greater or lesser extent reproduces the distribution expressed by the 3D Gaussian function on the protein body. The vast majority of proteins must ensure solubility, so a certain part of it-as it is expected-should reproduce the structuring of micelles. The biological activity of proteins is encoded in the part that does not reproduce the micelle-like system. The location and quantitative assessment of the contribution of orderliness to disorder is of critical importance for the determination of biological activity. The form of maladjustment to the 3D Gauss function may be varied-hence the obtained high diversity of specific interactions with strictly defined molecules: ligands or substrates. The correctness of this interpretation was verified on the basis of the group of enzymes Peptidylprolyl isomerase-E.C.5.2.1.8. In proteins representing this class of enzymes, zones responsible for solubility-micelle-like hydrophobicity system-the location and specificity of the incompatible part in which the specific activity of the enzyme is located and coded were identified. The present study showed that the enzymes of the discussed group show two different schemes of the structure of catalytic center (taking into account the status as defined by the fuzzy oil drop model).

High throughput wide field second harmonic imaging of giant unilamellar vesicles

Cell-sized giant unilamellar vesicles (GUVs) are an ideal tool for understanding lipid membrane structure and properties. Label-free spatiotemporal images of their membrane potential and structure would greatly aid the quantitative understanding of membrane properties. In principle, second harmonic imaging is a great tool to do so, but the low degree of spatial anisotropy that arises from a single membrane limits its application. Here, we advance the use of wide-field high throughput SH imaging by SH imaging with the use of ultrashort laser pulses. We achieve a throughput improvement of 78% of the maximum theoretical value and demonstrate subsecond image acquisition times. We show how the interfacial water intensity can be converted into a quantitative membrane potential map. Finally, for GUV imaging, we compare this type of nonresonant SH imaging to resonant SH imaging and two photon imaging using fluorophores.

Passive transport of Ca2+ ions through lipid bilayers imaged by widefield second harmonic microscopy

In biology, release of Ca2+ ions in the cytosol is essential to trigger or control many cell functions. Calcium signaling acutely depends on lipid membrane permeability to Ca2+. For proper understanding of membrane permeability to Ca2+, both membrane hydration and the structure of the hydrophobic core must be taken into account. Here, we vary the hydrophobic core of bilayer membranes and observe different types of behavior in high-throughput wide-field second harmonic imaging. Ca2+ translocation is observed through mono-unsaturated (DOPC:DOPA) membranes, reduced upon the addition of cholesterol, and completely inhibited for branched (DPhPC:DPhPA) and poly-unsaturated (SLPC:SLPA) lipid membranes. We propose, using molecular dynamics simulations, that ion transport occurs through ion-induced transient pores, which requires nonequilibrium membrane restructuring. This results in different rates at different locations and suggests that the hydrophobic structure of lipids plays a much more sophisticated regulating role than previously thought.

Ion-Induced Transient Potential Fluctuations Facilitate Pore Formation and Cation Transport through Lipid Membranes

Unassisted ion transport through lipid membranes plays a crucial role in many cell functions without which life would not be possible, yet the precise mechanism behind the process remains unknown due to its molecular complexity. Here, we demonstrate a direct link between membrane potential fluctuations and divalent ion transport. High-throughput wide-field non-resonant second harmonic (SH) microscopy of membrane water shows that membrane potential fluctuations are universally found in lipid bilayer systems. Molecular dynamics simulations reveal that such variations in membrane potential reduce the free energy cost of transient pore formation and increase the ion flux across an open pore. These transient pores can act as conduits for ion transport, which we SH image for a series of divalent cations (Cu2+, Ca2+, Ba2+, Mg2+) passing through giant unilamellar vesicle (GUV) membranes. Combining the experimental and computational results, we show that permeation through pores formed via an ion-induced electrostatic field is a viable mechanism for unassisted ion transport.

Single-Molecule Trapping and Measurement in a Nanostructured Lipid Bilayer System

The repulsive electrostatic force between a biomolecule and a like-charged surface can be geometrically tailored to create spatial traps for charged molecules in solution. Using a parallel-plate system composed of silicon dioxide surfaces, we recently demonstrated single-molecule trapping and high precision molecular charge measurements in a nanostructured free energy landscape. Here we show that surfaces coated with charged lipid bilayers provide a system with tunable surface properties for molecular electrometry experiments. Working with molecular species whose effective charge and geometry are well-defined, we demonstrate the ability to quantitatively probe the electrical charge density of a supported lipid bilayer. Our findings indicate that the fraction of charged lipids in nanoslit lipid bilayers can be significantly different from that in the precursor lipid mixtures used to generate them. We also explore the temporal stability of bilayer properties in nanofluidic systems. Beyond their relevance in molecular measurement, such experimental systems offer the opportunity to examine lipid bilayer formation and wetting dynamics on nanostructured surfaces.

Water Orientation at the Anatase TiO2 Nanoparticle Interface: A Probe of Surface pKa Values

The acid-base properties of surfaces significantly influence catalytic and (photo)electrochemical processes. Estimation of acid dissociation constants (pKa values) for colloids is commonly performed through electroanalytical techniques or spectroscopic methods employing label molecules. Here, we show that polarimetric angle-resolved second harmonic scattering (AR-SHS) can be used as an all-optical, label-free probe of colloid surface pKa values. We apply AR-SHS to dispersions of 100 nm anatase TiO2 particles to extract surface potential and surface susceptibility, a measure of interfacial water orientation, as a function of pH. The surface potential follows changes in surface charge density, while the interfacial water orientation inverts at pH ∼4.8, ∼6, and ∼7.6. As the variation in bulk pH modifies the populations of Ti-OH2+, Ti-OH, and Ti-O- interfacial groups, a change in water orientation reports on the ratio of protonated/deprotonated species. Such observation allows for pKa evaluation from plots of surface susceptibility versus pH. A Nerstian trend in the surface potential is additionally demonstrated.

The Possible Mechanism of Amyloid Transformation Based on the Geometrical Parameters of Early-Stage Intermediate in Silico Model for Protein Folding

The specificity of the available experimentally determined structures of amyloid forms is expressed primarily by the two- and not three-dimensional forms of a single polypeptide chain. Such a flat structure is possible due to the β structure, which occurs predominantly. The stabilization of the fibril in this structure is achieved due to the presence of the numerous hydrogen bonds between the adjacent chains. Together with the different forms of twists created by the single R- or L-handed α-helices, they form the hydrogen bond network. The specificity of the arrangement of these hydrogen bonds lies in their joint orientation in a system perpendicular to the plane formed by the chain and parallel to the fibril axis. The present work proposes the possible mechanism for obtaining such a structure based on the geometric characterization of the polypeptide chain constituting the basis of our early intermediate model for protein folding introduced formerly. This model, being the conformational subspace of Ramachandran plot (the ellipse path), was developed on the basis of the backbone conformation, with the side-chain interactions excluded. Our proposal is also based on the results from molecular dynamics available in the literature leading to the unfolding of α-helical sections, resulting in the β-structural forms. Both techniques used provide a similar suggestion in a search for a mechanism of conformational changes leading to a formation of the amyloid form. The potential mechanism of amyloid transformation is presented here using the fragment of the transthyretin as well as amyloid Aβ.

Why local and non-local terms are essential for second harmonic generation simulation?

Second Harmonic Generation (SHG) today represents one of the most powerful techniques to selectively probe all types of interfaces. However, the origin of the SHG signal at a molecular level is still debated since the local dipole contribution, which is strongly correlated to the molecular orientation can be counterbalanced by non-local quadrupole contributions. Here, we propose a method to simulate the SHG signal of a model water/air interface from the molecular response of each contribution. This method includes both local and non-local terms, which are represented, respectively, by the dependency of the polarisability and hyperpolarisability upon the chemical environment of the molecule and by the bulk quadrupole response. The importance of both terms for the sound simulation of the SHG signals and their interpretation is assessed. We demonstrate that the sole dipole term is unable to simulate a SHG signal, even if the dependency of the hyperpolarisability on the local environment is considered. The inclusion of the bulk quadrupole contribution, which largely dominates the dipole contribution, is essential to predict the SHG response, although the accuracy of the prediction is increased when the dependency upon the local environment is considered.

Modeling charge separation in charged nanochannels for single-molecule electrometry

We model the transport of electrically charged solute molecules by a laminar flow within a nanoslit microfluidic channel with electrostatic surface potential. We derive the governing convection–diffusion equation, solve it numerically, and compare it with a Taylor–Aris-like approximation, which gives excellent results for small Péclet numbers. We discuss our results in light of designing an assay that can measure simultaneously the hydrodynamic size and electric charge of single molecules by tracking their motion in such nanoslit channels with electrostatic surface potential.

Nonmonotonic Pair Potentials in the Interaction of Like-Charged Objects in Solution

We consider the long-standing like-charge attraction problem, wherein under certain conditions, similarly charged spheres suspended in aqueous electrolyte have been observed to display a minimum in their interaction potential, contrary to the intuitively expected monotonically varying repulsion. Recently, we described an interfacial mechanism invoking the molecular nature of the solvent that explains this anomalous experimental observation. In our model for the interaction of negatively charged particles in water, the minimum in the pair potential results from the superposition of competing contributions to the total free energy. One of these contributions is the canonical repulsive electrostatic term, whereas the other is a solvation-induced attractive contribution. We find that whereas both contributions grow approximately exponentially with decreasing interparticle separation, the occurrence of a stable, long-ranged minimum in the pair potential arises from differences in the precise interparticle separation dependence of the two terms. Specifically, the interfacial solvation term exhibits a more gradual decay with distance than the electrostatic repulsion, permitting the attractive contribution to dominate the interaction at large distances. Importantly, these disparities become evident in quantities calculated from exact numerical solutions to the governing nonlinear Poisson-Boltzmann (PB) equation for the spatial electrical potential distribution in the system. In marked contrast, we find that the linearized PB equation, applicable in the regime of low surface electrical potentials, does not support nonmonotonic trends in the total interaction free energy within the present model. Our results point to the importance of exact descriptions of electrostatic interactions in real systems that most often do not subscribe to particular mathematical limits where analytical approximations may provide a sufficiently accurate description of the problem.

Evaluating the cross-membrane dynamics of a charged molecule on lipid films with different surface curvature

Does the curvature of a phospholipid membrane influence the permeability of the lipid bilayers? This is a question of great importance yet hard to answer. In this work the permeability of a positively charged rod like probing molecule (D289 dye) on the bilayers of DOPG lipid vesicles was investigated using angle resolved second harmonic generation method. It was revealed that the permeability of D289 on the surface of small vesicles with ∼ 100 nm diameter was notably lower than that on giant vesicles with ∼ 1000 nm diameter. With the increasing of temperature or the introducing of dimethyl sulfoxide (DMSO) in the solutions, the D289 permeability of the lipid bilayers was notably enhanced as expected, on both the small and the giant vesicles. Still, the D289 permeability of the lipid film with more curvature is lower than the relatively flat film in all these cases. This work demonstrated a general protocol for the investigating of surface permeability of lipid films with various curvature.

Deciphering second harmonic generation signals

Second harmonic generation (SHG) has emerged as one of the most powerful techniques used to selectively monitor surface dynamics and reactions for all types of interfaces as well as for imaging non-centrosymmetric structures, although the molecular origin of the SHG signal is still poorly understood. Here, we present a breakthrough approach to predict and interpret the SHG signal at the atomic level, which is freed from the hyperpolarisability concept and self-consistently considers the non-locality and the coupling with the environment. The direct ab initio method developed here shows that a bulk quadrupole contribution significantly overwhelms the interface dipole term in the purely interfacial induced second-order polarisation for water/air interfaces. The obtained simulated SHG responses are in unprecedented agreement with the experimental signal. This work not only paves the road for the prediction of SHG response from more complex interfaces of all types, but also suggests new insights in the interpretation of the SHG signal at a molecular level. In particular, it highlights the modest influence of the molecular orientation and the high significance of the bulk quadrupole contribution, which does not depend on the interface, in the total experimental response.

Second harmonic generation is one of the most powerful techniques used to selectively probe interfaces of all types. The direct ab initio method developed here allows predicting the signal and highlights the importance of local and non-local effects.

Structure and dynamics of nanoconfined water and aqueous solutions

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed.

Imaging Cu2+ binding to charged phospholipid membranes by high-throughput second harmonic wide-field microscopy

The interaction of divalent copper ions (Cu²⁺) with cell membranes is crucial for a variety of physiological processes of cells, such as hormone synthesis and cellular energy production. These interactions would not be possible without membrane hydration. However, the role of water has not received a lot of attention in membrane studies. Here, we use high-throughput wide-field second harmonic (SH) microscopy to study the interaction between Cu²⁺ and hydrated freestanding Montal-Müller lipid membranes. The symmetric lipid membranes are composed of 1,2-diphytanoyl-sn-glycero-3-phosphocholine and either 1,2-diphytanoyl-sn-glycero-3-phosphate or 1,2-diphytanoyl-sn-glycero-3-phospho L-serine and are brought into contact with divalent Cu²⁺, which are added to one leaflet while maintaining the ionic strength balance. We observe transient domains of high SH intensity. In these areas, Cu²⁺ ions bind to the charged head groups, leading to charge neutralization on one side of the membrane. This exposes the ordered water at the non-interacting side of the membrane interface, which can be used to compute the interfacial membrane potential difference. We find that the domains of lipids with phosphatidic acid head groups display a higher interfacial membrane potential than those with phosphatidylserine head groups, which converts into higher dynamic electrostatic free energies and binding constants.

Unveiling the Molecular Dynamics in a Living Cell to the Subcellular Organelle Level Using Second-Harmonic Generation Spectroscopy and Microscopy

Second-harmonic generation (SHG) microscopy has been proved to be a powerful method for investigating the structures of biomaterials. SHG spectra were also generally used to probe the adsorption and cross-membrane transport of molecules on lipid bilayers in situ and in real time. In this work, we applied SHG and two-photon fluorescence (TPF) spectra to investigate the dynamics of an amphiphilic ion with an SHG and TPF chromophore, D289 (4-(4-diethylaminostyry)-1-methyl-pyridinium iodide), on the surface of human chronic myelogenous leukemia (K562) cells and the subcellular structures inside the cells. The adsorption and cross-membrane transport of D289 into the cells and then into the organelles such as mitochondria were revealed. SHG images were also recorded and used to demonstrate their capability of probing molecular dynamics in organelles in K562 cells. This work demonstrated the first SHG investigation of the cross-membrane transport dynamics on the surface of subcellular organelles. It may also shed light on the differentiation of different types of subcellular structures in cells.

Osmotic Transport at the Aqueous Graphene and hBN Interfaces: Scaling Laws from a Unified, First-Principles Description

Osmotic transport in nanoconfined aqueous electrolytes provides alternative venues for water desalination and "blue energy" harvesting. The osmotic response of nanofluidic systems is controlled by the interfacial structure of water and electrolyte solutions in the so-called electrical double layer (EDL), but a molecular-level picture of the EDL is to a large extent still lacking. Particularly, the role of the electronic structure has not been considered in the description of electrolyte/surface interactions. Here, we report enhanced sampling simulations based on ab initio molecular dynamics, aiming at unravelling the free energy of prototypical ions adsorbed at the aqueous graphene and hBN interfaces, and its consequences on nanofluidic osmotic transport. Specifically, we predicted the zeta potential, the diffusio-osmotic mobility, and the diffusio-osmotic conductivity for a wide range of salt concentrations from the ab initio water and ion spatial distributions through an analytical framework based on Stokes equation and a modified Poisson-Boltzmann equation. We observed concentration-dependent scaling laws, together with dramatic differences in osmotic transport between the two interfaces, including diffusio-osmotic flow and current reversal on hBN but not on graphene. We could rationalize the results for the three osmotic responses with a simple model based on characteristic length scales for ion and water adsorption at the surface, which are quite different on graphene and on hBN. Our work provides fundamental insights into the structure and osmotic transport of aqueous electrolytes on 2D materials and explores alternative pathways for efficient water desalination and osmotic energy conversion.

Water at charged interfaces

The ubiquity of aqueous solutions in contact with charged surfaces and the realization that the molecular-level details of water-surface interactions often determine interfacial functions and properties relevant in many natural processes have led to intensive research. Even so, many open questions remain regarding the molecular picture of the interfacial organization and preferential alignment of water molecules, as well as the structure of water molecules and ion distributions at different charged interfaces. While water, solutes and charge are present in each of these systems, the substrate can range from living tissues to metals. This diversity in substrates has led to different communities considering each of these types of aqueous interface. In this Review, by considering water in contact with metals, oxides and biomembranes, we show the essential similarity of these disparate systems. While in each case the classical mean-field theories can explain many macroscopic and mesoscopic observations, it soon becomes apparent that such theories fail to explain phenomena for which molecular properties are relevant, such as interfacial chemical conversion. We highlight the current knowledge and limitations in our understanding and end with a view towards future opportunities in the field.

A New Imaginary Term in the Second-Order Nonlinear Susceptibility from Charged Interfaces

Nonresonant second harmonic generation (SHG) phase and amplitude measurements obtained from the silica-water interface at varying pH values and an ionic strength of 0.5 M point to the existence of a nonlinear susceptibility term, which we call χ_X⁽³⁾, that is associated with a 90° phase shift. Including this contribution in a model for the total effective second-order nonlinear susceptibility produces reasonable point estimates for interfacial potentials and second-order nonlinear susceptibilities when χ_X⁽³⁾ ≈ 1.5χ_water⁽³⁾. A model without this term and containing only traditional χ⁽²⁾ and χ⁽³⁾ terms cannot recapitulate the experimental data. The new model also provides a demonstrated utility for distinguishing apparent differences in the second-order nonlinear susceptibility when the electrolyte is NaCl versus MgSO₄, pointing to the possibility of using heterodyne-detected SHG to investigate ion specificity in interfacial processes.

Polymeric Delivery of Therapeutic Nucleic Acids

The advent of genome editing has transformed the therapeutic landscape for several debilitating diseases, and the clinical outlook for gene therapeutics has never been more promising. The therapeutic potential of nucleic acids has been limited by a reliance on engineered viral vectors for delivery. Chemically defined polymers can remediate technological, regulatory, and clinical challenges associated with viral modes of gene delivery. Because of their scalability, versatility, and exquisite tunability, polymers are ideal biomaterial platforms for delivering nucleic acid payloads efficiently while minimizing immune response and cellular toxicity. While polymeric gene delivery has progressed significantly in the past four decades, clinical translation of polymeric vehicles faces several formidable challenges. The aim of our Account is to illustrate diverse concepts in designing polymeric vectors towards meeting therapeutic goals of in vivo and ex vivo gene therapy. Here, we highlight several classes of polymers employed in gene delivery and summarize the recent work on understanding the contributions of chemical and architectural design parameters. We touch upon characterization methods used to visualize and understand events transpiring at the interfaces between polymer, nucleic acids, and the physiological environment. We conclude that interdisciplinary approaches and methodologies motivated by fundamental questions are key to designing high-performing polymeric vehicles for gene therapy.

Leaving the Limits of Linearity for Light Microscopy

Nonlinear microscopy has enabled additional modalities for chemical contrast, deep penetration into biological tissues, and the ability to collect dynamics on ultrafast timescales across heterogenous samples. The additional light fields introduced to a sample offer seemingly endless possibilities for variation to optimize and customize experimentation and the extraction of physical insight. This perspective highlights three areas of growth in this diverse field: the collection of information across multiple timescales, the selective imaging of interfacial chemistry, and the exploitation of quantum behavior for future imaging directions. Future innovations will leverage the work of the studies reviewed here as well as address the current challenges presented.

Mapping Electrochemical Heterogeneity at Gold Surfaces: A Second Harmonic Imaging Study

Designing efficient catalysts requires correlating surface structure and local chemical composition with reactivity on length scales from nanometers to tens of microns. While much work has been done on this structure/function correlation on single crystals, comparatively little has been done for catalysts of relevance in applications. Such materials are typically highly heterogeneous and thus require methods that allow mapping of the structure/function relationship during electrochemical conversion. Here, we use optical second harmonic imaging combined with cyclic voltammetry to map the surface of gold nanocrystalline and polycrystalline electrodes during electrooxidation and to quantify the spatial extent of surface reconstruction during potential cycling. The wide-field configuration of our microscope allows for real-time imaging of an area ∼100 μm in diameter with submicron resolution. By analyzing the voltage dependence of each pixel, we uncover the heterogeneity of the second harmonic signal and quantify the fraction of domains where it follows a positive quadratic dependence with increasing bias. There, the second harmonic intensity is mainly ascribed to electronic polarization contributions at the metal/electrolyte interface. Additionally, we locate areas where the second harmonic signal follows a negative quadratic dependence with increasing bias, which also show the largest changes during successive cyclic voltammetry sweeps as determined by an additional correlation coefficient analysis. We assign these areas to domains of higher roughness that are prone to potential-induced surface restructuring and where anion adsorption occurs at lower potentials than expected based on the cyclic voltammetry.

Surface Chemistry Can Unlock Drivers of Surface Stability of SARS-CoV-2 in a Variety of Environmental Conditions

The surface stability and resulting transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), specifically in indoor environments, have been identified as a potential pandemic challenge requiring investigation. This novel virus can be found on various surfaces in contaminated sites such as clinical places; however, the behavior and molecular interactions of the virus with respect to the surfaces are poorly understood. Regarding this, the virus adsorption onto solid surfaces can play a critical role in transmission and survival in various environments. In this article, we first give an overview of existing knowledge concerning viral spread, molecular structure of SARS-CoV-2, and the virus surface stability is presented. Then, we highlight potential drivers of the SARS-CoV-2 surface adsorption and stability in various environmental conditions. This theoretical analysis shows that different surface and environmental conditions including temperature, humidity, and pH are crucial considerations in building fundamental understanding of the virus transmission and thereby improving safety practices.

Computing Surface Acidity Constants of Proton Hopping Groups from Density Functional Theory-Based Molecular Dynamics: Application to the SnO2(110)/H2O Interface

Proton transfer at metal oxide/water interfaces plays an important role in electrochemistry, geochemistry, and environmental science. The key thermodynamic quantity to characterize this process is the surface acidity constant. An ab initio method that combines density functional theory-based molecular dynamics (DFTMD) and free energy perturbation theory has been established for computing surface acidity constants. However, it involves a reversible proton insertion procedure in which frequent proton hopping, e.g., for strong bases and some oxide surfaces (e.g., SnO₂), can cause instability issues in electronic structure calculation. In the original implementation, harmonic restraining potentials are imposed on all O-H bonds (denoted by the V_rH scheme) to prevent proton hopping and thus may not be applicable for systems involving spontaneous proton hopping. In this work, we introduce an improved restraining scheme with a repulsive potential V_rep to compute the surface acidities of systems in which proton hopping is spontaneous and fast. In this V_rep scheme, a Buckingham-type repulsive potential V_rep is applied between the deprotonation site and all other protons in DFTMD simulations. We first verify the V_rep scheme by calculating the pK_a values of H₂O and aqueous HS^- solution (i.e., strong conjugate bases) and then apply it to the SnO₂(110)/H₂O interface. It is found that the V_rep scheme leads to a prediction of the point of zero charge (PZC) of 4.6, which agrees well with experiment. The intrinsic individual pK_a values of the terminal five-coordinated Sn site (Sn_5cOH₂) and bridge oxygen site (Sn₂O_brH⁺) are 4.4 and 4.7, respectively, both being almost the same as the PZC. The similarity of the two pK_a values indicates that dissociation of terminal water has almost zero free energy at this proton hopping interface (i.e., partial water dissociation), as expected from the acid-base equilibrium on SnO₂.

Wavelength-scanning second-harmonic generation for determining absolute charge density at aqueous interfaces

We develop a new, to the best of our knowledge, optical scheme based on second-harmonic generation (SHG) at multiple wavelengths for unequivocal separation of the second-order and the electric-field-induced third-order nonlinear optical contributions from aqueous interfaces. The third-order SHG originating from the field-induced reorientation order of water molecules in the electrical double layer offers an optical label-free and inherent probe to the surface charge density and surface potential in the absolute scales. We verify this wavelength-scanning SHG scheme both theoretically and experimentally, and show that the approach is applicable to water interfaces with bulk ionic strength below 500 µM and can achieve a detection sensitivity for a surface charge density of ∼10^-4C/m².

Imaging the Heterogeneity of the Oxygen Evolution Reaction on Gold Electrodes Operando: Activity is Highly Local

Understanding the mechanism of the oxygen evolution reaction (OER), the oxidative half of electrolytic water splitting, has proven challenging. Perhaps the largest hurdle has been gaining experimental insight into the active site of the electrocatalyst used to facilitate this chemistry. Decades of study have clarified that a range of transition-metal oxides have particularly high catalytic activity for the OER. Unfortunately, for virtually all of these materials, metal oxidation and the OER occur at similar potentials. As a result, catalyst surface topography and electronic structure are expected to continuously evolve under reactive conditions. Gaining experimental insight into the OER mechanism on such materials thus requires a tool that allows spatially resolved characterization of the OER activity. In this study, we overcome this formidable experimental challenge using second harmonic microscopy and electrochemical methods to characterize the spatial heterogeneity of OER activity on polycrystalline Au working electrodes. At moderately anodic potentials, we find that the OER activity of the electrode is dominated by <1% of the surface area and that there are two types of active sites. The first is observed at potentials positive of the OER onset and is stable under potential cycling (and thus presumably extends multiple layers into the bulk gold electrode). The second occurs at potentials negative of the OER onset and is removed by potential cycling (suggesting that it involves a structural motif only 1–2 Au layers deep). This type of active site is most easily understood as the catalytically active species (hydrous oxide) in the so-called incipient hydrous oxide/adatom mediator model of electrocatalysis. Combining the ability we demonstrate here to characterize the spatial heterogeneity of OER activity with a systematic program of electrode surface structural modification offers the possibility of creating a generation of OER electrocatalysts with unusually high activity.

Interfacial solvation can explain attraction between like-charged objects in aqueous solution

Over the past few decades, the experimental literature has consistently reported observations of attraction between like-charged colloidal particles and macromolecules in aqueous solution. Examples include nucleic acids and colloidal particles in the bulk solution and under confinement, and biological liquid-liquid phase separation. This observation is at odds with the intuitive expectation of an interparticle repulsion that decays monotonically with distance. Although attraction between like-charged particles can be rationalized theoretically in the strong-coupling regime, e.g., in the presence of multivalent counterions, recurring accounts of long-range attraction in aqueous solution containing monovalent ions at low ionic strength have posed an open conundrum. Here, we show that the behavior of molecular water at an interface-traditionally disregarded in the continuum electrostatics picture-provides a mechanism to explain the attraction between like-charged objects in a broad spectrum of experiments. This basic principle will have important ramifications in the ongoing quest to better understand intermolecular interactions in solution.

Coherent Vibrational Spectroscopy of Electrochemical Interfaces with Plasmonic Nanogratings

The fundamental understanding of electrochemistry urges accurate knowledge of all interfacial properties at the molecular level, but the retrieval of such information is a real challenge. Optical spectroscopies facilitated by surface plasmon enhancement can shed light on this field, yet past studies relied on either highly inhomogeneous "hot spots" or planar plasmon modes with limited enhancement. Here we report an in situ sum-frequency vibrational spectroscopy scheme using plasmonic nanogratings, which enable strong, coherent surface plasmon excitation even on planar electrodes. With two classical reactions, the gold oxidation and pyridine adsorption in water, we demonstrate the realization of coherent vibrational spectroscopy in the strong absorption region, revealing the polar orientation and ordering of interfacial species that are crucial toward the mechanistic understanding of electrochemical phenomena.

Second Harmonic Generation for Moisture Monitoring in Dimethoxyethane at a Gold-Solvent Interface Using Plasmonic Structures

Second harmonic generation (SHG) is forbidden from most bulk metals because metals are characterized by centrosymmetric symmetry. Adsorption or desorption of molecules at the metal interface can break the symmetry and lead to SHG responses. Yet, the response is relatively low, and minute changes occurring at the interface, especially at solid/liquid interfaces, like in battery electrodes are difficult to assess. Herein, we use a plasmonic structure milled in a gold electrode to increase the overall SHG signal from the interface and gain information about small changes occurring at the interface. Using a specific homebuilt cell, we monitor changes at the liquid/electrode interface. Specifically, traces of water in dimethoxyethane (DME) have been detected following changes in the SHG responses from the plasmonic structures. We propose that by plasmonic structures this technique can be used for assessing minute changes occurring at solid/liquid interfaces such as battery electrodes.

Reactivity mapping of nanoscale defect chemistry under electrochemical reaction conditions

Electrocatalysts often show increased conversion at nanoscale chemical or topographic surface inhomogeneities, resulting in spatially heterogeneous reactivity. Identifying reacting species locally with nanometer precision during chemical conversion is one of the biggest quests in electrochemical surface science to advance (electro)catalysis and related fields. Here, we demonstrate that electrochemical tip-enhanced Raman spectroscopy can be used for combined topography and reactivity imaging of electro-active surface sites under reaction conditions. We map the electrochemical oxidation of Au nanodefects, a showcase energy conversion and corrosion reaction, with a chemical spatial sensitivity of about 10 nm. The results indicate the reversible, concurrent formation of spatially separated Au₂O₃ and Au₂O species at defect-terrace and protrusion sites on the defect, respectively. Active-site chemical nano-imaging under realistic working conditions is expected to be pivotal in a broad range of disciplines where quasi-atomistic reactivity understanding could enable strategic engineering of active sites to rationally tune (electro)chemical device properties.

Identifying reacting species locally with nanometer precision is a major challenge in electrochemical surface science. Using operando Raman nanoscopy, authors image the reversible, concurrent formation of nanometer-spatially separated Au₂O₃ and Au₂O species during Au nanodefect oxidation.

Molecule-Membrane Interactions in Biological Cells Studied with Second Harmonic Light Scattering

The nonlinear optical phenomenon second harmonic light scattering (SHS) can be used for detecting molecules at the membrane surfaces of living biological cells. Over the last decade, SHS has been developed for quantitatively monitoring the adsorption and transport of small and medium size molecules (both neutral and ionic) across membranes in living cells. SHS can be operated with both time and spatial resolution and is even capable of isolating molecule-membrane interactions at specific membrane surfaces in multi-membrane cells, such as bacteria. In this review, we discuss select examples from our lab employing time-resolved SHS to study real-time molecular interactions at the plasma membranes of biological cells. We first demonstrate the utility of this method for determining the transport rates at each membrane/interface in a Gram-negative bacterial cell. Next, we show how SHS can be used to characterize the molecular mechanism of the century old Gram stain protocol for classifying bacteria. Additionally, we examine how membrane structures and molecular charge and polarity affect adsorption and transport, as well as how antimicrobial compounds alter bacteria membrane permeability. Finally, we discuss adaptation of SHS as an imaging modality to quantify molecular adsorption and transport in sub-cellular regions of individual living cells.

Label-free Optical Imaging of Membrane Potential

Offering high temporal resolution, voltage imaging is an important and essential technique in neuroscience. Among different optical imaging approaches, the label-free approach remains attractive due to its unique value coming from free of exogenous chromophores. The intrinsic voltage-indicating signals arising from membrane deformation, membrane spectral change, phase shift, light scattering, and membrane hydration haven been reported. First demonstrated 70 years ago, label-free optical imaging of membrane potential is still at an early stage and the field is challenged by the relatively small signals generated by the intrinsic optical properties. We review major contrast mechanisms used for label-free voltage imaging and discuss several recent exciting advances that could potentially enable membrane potential imaging in mammalian neurons at high speed and high sensitivity.

Spatiotemporal Imaging of Water in Operating Voltage-Gated Ion Channels Reveals the Slow Motion of Interfacial Ions

Ion channels are responsible for numerous physiological functions ranging from transport to chemical and electrical signaling. Although static ion channel structure has been studied following a structural biology approach, spatiotemporal investigation of the dynamic molecular mechanisms of operational ion channels has not been achieved experimentally. In particular, the role of water remains elusive. Here, we perform label-free spatiotemporal second harmonic (SH) imaging and capacitance measurements of operational voltage-gated alamethicin ion channels in freestanding lipid membranes surrounded by aqueous solution on either side. We observe changes in SH intensity upon channel activation that are traced back to changes in the orientational distribution of water molecules that reorient along the field lines of transported ions. Of the transported ions, a fraction of 10^-4 arrives at the hydrated membrane interface, leading to interfacial electrostatic changes on the time scale of a second. The time scale of these interfacial changes is influenced by the density of ion channels and is subject to a crowding mechanism. Ion transport along cell membranes is often associated with the propagation of electrical signals in neurons. As our study shows that this process is taking place over seconds, a more complex mechanism is likely responsible for the propagation of neuronal electrical signals than just the millisecond movement of ions.

Ricocheting Droplets Moving on Super-Repellent Surfaces

Droplet bouncing on repellent solid surfaces (e.g., the lotus leaf effect) is a common phenomenon that has aroused interest in various fields. However, the scenario of a droplet bouncing off another droplet (either identical or distinct chemical composition) while moving on a solid material (i.e., ricocheting droplets, droplet billiards) is scarcely investigated, despite it having fundamental implications in applications including self-cleaning, fluid transport, and heat and mass transfer. Here, the dynamics of bouncing collisions between liquid droplets are investigated using a friction-free platform that ensures ultrahigh locomotion for a wide range of probing liquids. A general prediction on bouncing droplet-droplet contact time is elucidated and bouncing droplet-droplet collision is demonstrated to be an extreme case of droplet bouncing on surfaces. Moreover, the maximum deformation and contact time are highly dependent on the position where the collision occurs (i.e., head-on or off-center collisions), which can now be predicted using parameters (i.e., effective velocity, effective diameter) through the concept of an effective interaction region. The results have potential applications in fields ranging from microfluidics to repellent coatings.

Demonstration of flat-top beam illumination in widefield multiphoton microscopy

Multiphoton microscopy provides a suitable technique for imaging biological tissues with submicrometer resolution. Usually a Gaussian beam (GB) is used for illumination, leading to a reduced power efficiency in the multiphoton response and vignetting for a square-shaped imaging area. A flat-top beam (FTB) provides a uniform spatial intensity distribution that equalizes the probability of a multiphoton effect across the imaging area. We employ a customized widefield multiphoton microscope to compare the performance of a square-shaped FTB illumination with that based on using a GB, for both two-photon fluorescence (TPF) and second-harmonic generation (SHG) imaging. The variation in signal-to-noise ratio across TPF images of fluorescent dyes spans ∼5.6  dB for the GB and ∼1.2  dB for the FTB illumination, respectively. For the GB modality, TPF images of mouse colon and Convallaria root, and SHG images of chicken tendon and human breast biopsy tissue showcase ∼20  %   area that are not imaged due to either insufficient or lack of illumination. For quantitative analysis that depends on the illuminated area, this effect can potentially lead to inaccuracies. This work emphasizes the applicability of FTB illumination to multiphoton applications.

Mapping of real-time morphological changes in the neuronal cytoskeleton with label-free wide-field second-harmonic imaging: a case study of nocodazole

We demonstrate the use of wide-field high-throughput second-harmonic (SH) microscopy for investigating cytoskeletal morphological changes on the single-cell level. The method allows for real-time, in vitro, label-free measurements of cytoskeletal changes that can, under certain conditions, be quantified in terms of orientational distribution or in terms of changes in the number of microtubules. As SH generation is intrinsically sensitive to noncentrosymmetrically structured microtubules, but not to isotropic or centrosymmetric materials, we use it to probe the microtubule structure in the cytoskeleton when it undergoes dynamic changes induced by the application of nocodazole, a well-known microtubule-destabilizing drug that reversibly depolymerizes microtubules. In addition, the orientational directionality of microtubules in neurites and cell bodies is determined label-free using SH polarimetry measurements. Finally, we use spatiotemporal SH imaging to show label-free, real-time nocodazole-induced morphological changes in neurons of different age and in a single axon.

Direct Quantification of Water Surface Charge by Phase-Sensitive Second Harmonic Spectroscopy

We developed and verified a phase-sensitive second harmonic generation spectroscopic scheme that allows for direct determination of the absolute surface charge density and surface potential of a water interface without the need for prior interfacial information. The method relies on selective probing of surface-field-induced reorientation order of water molecules in the electrical double layer and is, hence, independent of the interfacial molecular bonding structure. Application of this technique to a mixed surfactant monolayer on water suggests the manifest effect of the chain-chain interactions among the monolayer on adsorption of soluble ionic surfactants. We also deduce the third-order nonlinear susceptibility of bulk water and prove its applicability to analysis of charges of various water interfaces. In addition, we show that Debye-Hückle theory should be avoided in the spectroscopic analysis for its potential significant error, as evidenced experimentally and theoretically.