Phenomenology of Intermediate Molecular Dynamics at Metal-Oxide Interfaces.

Reaction intermediates buried within a solid-liquid interface are difficult targets for physiochemical measurements. They are inherently molecular and locally dynamic, while their surroundings are extended by a periodic lattice on one side and the solvent dielectric on the other. Challenges compound on a metal-oxide surface of varied sites and especially so at its aqueous interface of many prominent reactions. Recently, phenomenological theory coupled with optical spectroscopy has become a more prominent tool for isolating the intermediates and their molecular dynamics. The following article reviews three examples of the SrTiO3-aqueous interface subject to the oxygen evolution from water: reaction-dependent component analyses of time-resolved intermediates, a Fano resonance of a mode at the metal-oxide-water interface, and reaction isotherms of metastable intermediates. The phenomenology uses parameters to encase what is unknown at a microscopic level to then circumscribe the clear and macroscopically tuned trends seen in the spectroscopic data.

In vitro investigation of protein assembly by combined microscopy and infrared spectroscopy at the nanometer scale.

The nanoscale structure and dynamics of proteins on surfaces has been extensively studied using various imaging techniques, such as transmission electron microscopy and atomic force microscopy (AFM) in liquid environments. These powerful imaging techniques, however, can potentially damage or perturb delicate biological material and do not provide chemical information, which prevents a fundamental understanding of the dynamic processes underlying their evolution under physiological conditions. Here, we use a platform developed in our laboratory that enables acquisition of infrared (IR) spectroscopy and AFM images of biological material in physiological liquids with nanometer resolution in a cell closed by atomically thin graphene membranes transparent to IR photons. In this work, we studied the self-assembly process of S-layer proteins at the graphene-aqueous solution interface. The graphene acts also as the membrane separating the solution containing the proteins and Ca2+ ions from the AFM tip, thus eliminating sample damage and contamination effects. The formation of S-layer protein lattices and their structural evolution was monitored by AFM and by recording the amide I and II IR absorption bands, which reveal the noncovalent interaction between proteins and their response to the environment, including ionic strength and solvation. Our measurement platform opens unique opportunities to study biological material and soft materials in general.

Exploring the Molecular-Scale Structures at Solid/Liquid Interfaces of Li-Ion Battery Materials: A Force Spectroscopy Analysis with Sparse Modeling.

In this study, we clarify the liquid structure formed at the interface between LiCoO2 (LCO), the cathode material of Li-ion batteries, and propylene carbonate (PC), which is used as a solvent in the electrolyte, on a molecular scale. We apply sparse modeling-based modal analysis to force spectroscopy data measured by frequency modulation atomic force microscopy (FM-AFM) and show that each component in the FM-AFM force curve, such as oscillatory solvation force, background, and noise, can be automatically decomposed. Moreover, by combining detailed force curve analysis with solid/liquid interface simulations based on first-principles calculation, we have identified that there are distinct damped vibrational modes in the force curves at the LCO/PC interface with a period of about 0.57 nm and those with shorter periods, which likely correspond to the solvation forces associated with bulk-state PC molecules and those with PC molecules in "lying down" orientations.

A Fusion-Growth Protocell Model Based on Vesicle Interactions with Pyrite Particles.

Protocell models play a pivotal role in the exploration of the origin of life. Vesicles are one type of protocell model that have attracted much attention. Simple single-chain amphiphiles (SACs) and organic small molecules (OSMs) possess primitive relevance and were most likely the building blocks of protocells on the early Earth. OSM@SAC vesicles have been considered to be plausible protocell models. Pyrite (FeS2), a mineral with primitive relevance, is ubiquitous in nature and plays a crucial role in the exploration of the origin of life in the mineral-water interface scenario. "How do protocell models based on OSM@SAC vesicles interact with a mineral-water interface scenario that simulates a primitive Earth environment" remains an unresolved question. Hence, we select primitive relevant sodium monododecyl phosphate (SDP), isopentenol (IPN) and pyrite (FeS2) mineral particles to build a protocell model. The model investigates the basic physical and chemical properties of FeS2 particles and reveals the effects of the size, content and duration of interaction of FeS2 particles on IPN@SDP vesicles. This deepens the understanding of protocell growth mechanisms in scenarios of mineral-water interfaces in primitive Earth environments and provides new information for the exploration of the origin of life.

Molecular Insight into the Processes and Mechanisms of N2 Adsorption and Accumulation at the Hydrophobic Solid/Liquid Interface.

In this study, molecular dynamics (MD) simulations were employed to elucidate the processes and underlying mechanisms that govern the adsorption and accumulation of gas (represented by N2) at the hydrophobic solid-liquid interface, using the GROMACS program with an AMBER force field. Our findings indicate that, regardless of surface roughness, the presence of water molecules is a prerequisite for the adsorption and aggregation of N2 molecules on solid surfaces. N2 molecules dissolved in water can cluster even without a solid substrate. In the gas-solid-liquid system, the exclusion of water molecules at the hydrophobic solid-liquid interface and the adsorption of N2 molecules do not occur simultaneously. A loosely arranged layer of water molecules is initially formed on the hydrophobic solid surface. The two-stage process of N2 molecule adsorption and accumulation at the hydrophobic solid/liquid interface involves initial adsorption to the solid surface, displacing water molecules, followed by N2 accumulation via self-interaction after saturating the substrate's surface. The process and underlying mechanisms of gas adsorption and accumulation at hydrophobic solid/liquid interfaces elucidated in this study offer a molecular-level understanding of nano-gas layer formation.

Strong Metal-Support Interaction Triggered by Molten Salts.

Strong metal-support interaction (SMSI) plays a vital role in tuning the geometric and electronic structures of metal species. Generally, a high-temperature treatment (>500 °C) in reducing atmosphere is required for constructing SMSI, which may induce the sintering of metal species. Herein, we use molten salts as the reaction media to trigger the formation of high-intensity SMSI at reduced temperatures. The strong ionic polarization of the molten salt promotes the breakage of Ti-O bonds in the TiO2 support, and hence decreases the energy barrier for the formation of interfacial bonds. Consequently, a high-intensity SMSI state is achieved in TiO2 supported Ir nanoclusters, evidenced by a large number of Ir-Ti bonds at the interface, at a low temperature of 350 °C. Moreover, this method is applicable for triggering SMSI in various supported metal catalysts with different oxide supports including CeO2 and SnO2. This newly developed SMSI construction methodology opens a new avenue and holds significant potential for engineering advanced supported metal catalysts toward a broad range of applications.

Solid-Liquid Interfacial Engineered Large-Area Two-Dimensional Covalent Organic Framework Films.

Constructing uniform covalent organic framework (COF) film on substrates for electronic devices is highly desirable. Here, a simple and mild strategy is developed to prepare them by polymerization on a solid-liquid interface. The universality of the method is confirmed by the successful preparation of five COF films with different microstructures. These films have large lateral size, controllable thickness, and high crystalline quality. And COF patterns can also be directly achieved on substrates via hydrophilic and hydrophobic interface engineering, which is in favor of preparing device array. For application studies, the PyTTA-TPA (PyTTA: 4,4',4'',4'''-(1,3,6,8-Tetrakis(4-aminophenyl)pyrene and TPA: terephthalaldehyde) COF film has a high photoresponsivity of 59.79 µA W-1 at 420 nm for photoelectrochemical (PEC) detection. When employed as an active material for optoelectronic synaptic devices for the first attempt, it shows excellent light-stimulated synaptic plasticity properties such as short-term plasticity (STP), long-term plasticity (LTP), and the conversion of STP to LTP, which can be used to simulate biological synaptic functions.

Molecular-Resolution Imaging of Interfacial Solvation of Electrolytes for Lithium-Ion Batteries by Frequency Modulation Atomic Force Microscopy.

Solvation structures formed by ions and solvent molecules at solid/electrolyte interfaces affect the energy storage performance of electrochemical devices, such as lithium-ion batteries. In this study, the molecular-scale solvation structures of an electrolyte, a solution of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in propylene carbonate (PC) at the electrolyte-mica interface, were measured using frequency-modulation atomic force microscopy (FM-AFM). The spacing of the characteristic force oscillation in the force versus distance curves increased with increasing ion concentration, suggesting an increase in the effective size of molecules at the interface. Molecular dynamics simulations showed that the effective size of molecular assemblies, namely, solvated ions formed at the interface, increased with increasing ion concentrations, which was consistent with the experimental results. Knowledge of molecular-scale structures of solid/electrolyte interfaces obtained by a combination of FM-AFM and molecular dynamics simulations is important in the design of electrolytes for future energy devices and in improving their properties.

Fluoropolymer: A Review on Its Emulsion Preparation and Wettability to Solid-Liquid Interface.

In the preparation of a superamphiphobic surface, the most basic method is to reduce the surface free energy of the interface. The C-F bond has a very low surface free energy, which can significantly change the wettability of the solid-liquid interface and make it a hydrophobic or oleophobic, or even superamphiphobic surface. Based on the analysis of a large number of research articles, the preparation and application progress in fluoropolymer emulsion were summarized. After that, some corresponding thoughts were put forward combined with our professional characteristics. According to recent research, the status of the fluoropolymer emulsion preparation system was analyzed. In addition, all related aspects of fluoropolymer emulsion were systematically classified in varying degrees. Furthermore, the interaction between fluoropolymer structure and properties, especially the interaction with nanomaterials, was also explored. The aim of this review is to try to attract more scholars' attention to fluorocarbon interfacial materials. It is expected that it will make a certain theoretical and practical significance in the preparation and application of fluoropolymer.

Influence of Selected Factors on the Adsorption Layer Structure of Polyamino Acids and Their Block Copolymers at the Solid-Aqueous Solution Interface.

The adsorption mechanism of different polymers containing ionic polyamino acids monomers in the chain structure at the solid-liquid interface was investigated. Initially, the influence of molecular weight and solution pH on simple polyamino acids (poly(L-aspartic acid) and poly(L-lysine) binding was determined. Considering the obtained dependencies, the polymer adsorption layer conformation was proposed in the systems containing block copolymers (both diblock and symmetrical triblock) consisting of polypeptide as well as poly(ethylene glycol) fragments. The presented studies focused on the application of two experimental methods. The polymer adsorption was carried out using the batch method and the adsorbate concentration was determined spectrophotometrically. Then, the turbidimetric measurements were taken. The analysis of the obtained results showed that the adsorption process of block copolymers depends on two factors. Firstly, the solution pH determines both the nature of the interactions of the copolymer structural units with the solid surface and the conformation of the polypeptide chains. The second parameter influencing the adsorption layer structure is the ratio of the lengths of both blocks. Introducing a short PEG fragment into the polymer main chain may improve the polymer adsorption properties by increasing the number of interactions with the adsorbent surface.

pH-Responsive Cobalt(II)-Coordinated Assembly Containing Quercetin for Antimicrobial Applications.

The development of novel drug delivery systems (DDSs) with promising antibacterial properties is essential for facing the emergency of increasing resistance to antimicrobial agents. The antibacterial features of quercetin and its metal complexes have been broadly investigated. However, several drawbacks affect their activity and effectiveness. In this work, we propose a DDS based on a pH-responsive cobalt(II)-coordinated assembly containing quercetin and polyacrylic acid. This system is suggested to trigger the release of the model drug in a pH-dependent mode by exploiting the localized acidic environment at the bacterial infection sites under anaerobic conditions. The delivery system has been designed by accurately examining the species and the multiple equilibria occurring in solution among the assembly components. The formation of cobalt(II) complexes with quercetin in the absence or presence of the pH-responsive polyacrylic acid was investigated in buffered aqueous solution at pH 7.4 using spectrophotometric (UV-Vis) and calorimetric (ITC) techniques. The determined binding affinities and thermodynamic parameters that resulted are essential for the development of a DDS with improved binding and release capabilities. Furthermore, the affinity of the polymer-cobalt(II) complex toward the model antimicrobial flavonoid was explored at the solid-liquid interface by quartz crystal microbalance (QCM-D) experiments, which provided marked evidence for drug loading and release under pH control.

Accelerated Surface Reconstruction through Regulating the Solid-Liquid Interface by Oxyanions in Perovskite Electrocatalysts for Enhanced Oxygen Evolution.

A comprehensive understanding of surface reconstruction was critical to developing high performance lattice oxygen oxidation mechanism (LOM) based perovskite electrocatalysts. Traditionally, the primary determining factor of the surface reconstruction process was believed to be the oxygen vacancy formation energy. Hence, most previous studies focused on optimizing composition to reduce the oxygen vacancy formation energy, which in turn facilitated the surface reconstruction process. Here, for the first time, we found that adding oxyanions (SO4 2- , CO3 2- , NO3 - ) into the electrolyte could effectively regulate the solid-liquid interface, significantly accelerating the surface reconstruction process and enhancing oxygen evolution reaction (OER) activities. Further studies indicated that the added oxyanions would adsorb onto the solid-liquid interface layer, disrupting the dynamic equilibrium between the adsorbed OH- ions and the OH- ions generated during surface reconstruction process. As such, the OH- ions generated during surface reconstruction process could be more readily released into the electrolyte, thereby leading to an acceleration of the surface reconstruction. Thus, it was expected that our finding would provide a new layer of understanding to the surface reconstruction process in LOM-based perovskite electrocatalysts.

Passive Gate-Tunable Kinetic Photovoltage along Semiconductor-Water Interfaces.

Moving boundaries of electric double layer at solid-liquid interface enables unprecedented persistent energy conversion and provokes a kinetic photovoltaic effect by moving an illumination region along the semiconductor-water interface. Here, we report a transistor-inspired gate modulation of kinetic photovoltage by applying a bias at the semiconductor-water interface. The kinetic photovoltage of both p-type and n-type silicon samples can be facilely switched on/off, stemming from the electrical-field-modulated surface band bending. In contrast to the function of solid-state transistors relying on external sources, passive gate modulation of the kinetic photovoltage is achieved simply by introducing a counter electrode with materials of desired electrochemical potential. This architecture provides the ability to modulate the kinetic photovoltage over three orders of magnitude and opens up a new way for self-powered optoelectronic logic devices.

Protein Adsorption Loss─The Bottleneck of Single-Cell Proteomics.

Single-cell proteomics is a promising field to provide direct yet comprehensive molecular insights into cellular functions without averaging effects. Here, we address a grand technical challenge impeding the maturation of single-cell proteomics─protein adsorption loss (PAL). Even though widely known, there is currently no quantitation on how profoundly and selectively PAL has affected single-cell proteomics. Therefore, the mitigations to this challenge have been generic, and their efficacy was only evaluated by the size of the resolved proteome with no specificity on individual proteins. We use the existing knowledge of PAL, protein expression, and the typical surface area used in single-cell proteomics to discuss the severity of protein loss. We also summarize the current solutions to this challenge and briefly review the available methods to characterize the physical and chemical properties of protein surface adsorption. By citing successful strategies in single-cell genomics for measurement errors in individual transcripts, we pinpoint the urgency to benchmark PAL at the proteome scale with individual protein resolution. Finally, orthogonal single-cell proteomic techniques that have the potential to cross validate PAL are proposed. We hope these efforts can promote the fruition of single-cell proteomics in the near future.

The relationship between solid-liquid interface interaction and gelling capacity of h-BN 2D material: a rheological study.

Diurea modified h-BN nanosheet is a novel kind of 2D gelator that could gel the lubricating oils under the stimulus of ultrasound. Morphological analyzations in previous study confirmed that the ultrasound induced layer-by-layer (LBL) structure of BN gelator is critical for the gelation. However, the elastic response in LBL structure, which is crucial for the formation of a stable gel system, has not been explicitly illustrated yet. The challenge is that the LBL gelator structure is based on 2D material and thus lacks vertical linkage between gelator layers, which is significantly different from the traditional gel systems that generally possess highly crosslinked gelator network. In this work, by investigating the viscoelastic behavior of the BN-based gel via rheometer, it is found the solid-liquid interface interaction, which is regulated by the diurea molecular structure in the BN gelator, is the key factor for triggering the stable elastic response in the LBL structure, and the elasticity mainly originates from the interface interaction induced bending deformation of h-BN 2D material. The findings further elucidate the gelling mechanism of BN gelators and enlighten the structure design of ultrasound-responsive gelator based on 2D materials.

Energy conversion via metal nanolayers.

Current approaches for electric power generation from nanoscale conducting or semiconducting layers in contact with moving aqueous droplets are promising as they show efficiencies of around 30%, yet even the most successful ones pose challenges regarding fabrication and scaling. Here, we report stable, all-inorganic single-element structures synthesized in a single step that generate electrical current when alternating salinity gradients flow along its surface in a liquid flow cell. Nanolayers of iron, vanadium, or nickel, 10 to 30 nm thin, produce open-circuit potentials of several tens of millivolt and current densities of several microA cm-2 at aqueous flow velocities of just a few cm s-1 The principle of operation is strongly sensitive to charge-carrier motion in the thermal oxide nanooverlayer that forms spontaneously in air and then self-terminates. Indeed, experiments suggest a role for intraoxide electron transfer for Fe, V, and Ni nanolayers, as their thermal oxides contain several metal-oxidation states, whereas controls using Al or Cr nanolayers, which self-terminate with oxides that are redox inactive under the experimental conditions, exhibit dramatically diminished performance. The nanolayers are shown to generate electrical current in various modes of application with moving liquids, including sliding liquid droplets, salinity gradients in a flowing liquid, and in the oscillatory motion of a liquid without a salinity gradient.

Unexpectedly High Capacitance of the Metal Nanoparticle/Water Interface: Molecular-Level Insights into the Electrical Double Layer.

The electrical double-layer plays a key role in important interfacial electrochemical processes from catalysis to energy storage and corrosion. Therefore, understanding its structure is crucial for the progress of sustainable technologies. We extract new physico-chemical information on the capacitance and structure of the electrical double-layer of platinum and gold nanoparticles at the molecular level, employing single nanoparticle electrochemistry. The charge storage ability of the solid/liquid interface is larger by one order-of-magnitude than predicted by the traditional mean-field models of the double-layer such as the Gouy-Chapman-Stern model. Performing molecular dynamics simulations, we investigate the possible relationship between the measured high capacitance and adsorption strength of the water adlayer formed at the metal surface. These insights may launch the active tuning of solid-solvent and solvent-solvent interactions as an innovative design strategy to transform energy technologies towards superior performance and sustainability.

Reducing Kapitza resistance between graphene/water interface via interfacial superlattice structure.

The control of thermal transport across solid/liquid interface has attracted great interests for efficient thermal management in the integrated devices. Based on molecular dynamics simulations, we study the effect of interfacial superlattice structure on the Kapitza resistance between graphene/water interface. Compared to the original interface, introducing interfacial superlattice structure can result in an obvious reduction of Kapitza resistance by as large as 40%, exhibiting a decreasing trend of Kapitza resistance with the decrease of superlattice period. Surprisingly, by analyzing the structure of water block and atomic vibration characteristics on both sides of the interface, we find the interfacial superlattice structure has a minor effect on the water structure and overlap in the vibrational spectrum, suggesting that the improved interfacial heat transfer is not mainly originated from the liquid block. Instead, the spectral energy density analysis reveals that phonon scattering rate in the interfacial graphene layer is significantly enhanced after superlattice decoration, giving rise to the increased thermal resistance between the interfacial graphene layer and its nearest neighboring layer. As this thermal resistance is coupled to the Kapitza resistance due to the local nature of interfacial superlattice decoration, the enhanced thermal resistance in the solid segment indirectly reduces the Kapitza resistance between graphene/water interface, which is supported by the enhancement of the spectral interfacial thermal conductance upon superlattce decoration at microscopic level. Our study uncovers the physical mechanism for controlling heat transfer across solid/liquid interface via interfacial superlattice structure, which might provide valuable insights for designing efficient thermal interfaces.

Kinetics of solid-liquid interface motion in molecular dynamics and phase-field models: crystallization of chromium and silicon.

The results of molecular dynamics (MD) simulations of the crystallization process in one-component materials and solid solution alloys reveal a complex temperature dependence of the velocity of the crystal-liquid interface featuring an increase up to a maximum at 10-30% undercooling below the equilibrium melting temperature followed by a gradual decrease of the velocity at deeper levels of undercooling. At the qualitative level, such non-monotonous behaviour of the crystallization front velocity is consistent with the diffusion-controlled crystallization process described by the Wilson-Frenkel model, where the almost linear increase of the interface velocity in the vicinity of melting temperature is defined by the growth of the thermodynamic driving force for the phase transformation, while the decrease in atomic mobility with further increase of the undercooling drives the velocity through the maximum and into a gradual decrease at lower temperatures. At the quantitative level, however, the diffusional model fails to describe the results of MD simulations in the whole range of temperatures with a single set of parameters for some of the model materials. The limited ability of the existing theoretical models to adequately describe the MD results is illustrated in the present work for two materials, chromium and silicon. It is also demonstrated that the MD results can be well described by the solution following from the hodograph equation, previously found from the kinetic phase-field model (kinetic PFM) in the sharp interface limit. The ability of the hodograph equation to describe the predictions of MD simulation in the whole range of temperatures is related to the introduction of slow (phase field) and fast (gradient flow) variables into the original kinetic PFM from which the hodograph equation is obtained. The slow phase-field variable is responsible for the description of data at small undercoolings and the fast gradient flow variable accounts for local non-equilibrium effects at high undercoolings. The introduction of these two types of variables makes the solution of the hodograph equation sufficiently flexible for a reliable description of all nonlinearities of the kinetic curves predicted in MD simulations of Cr and Si. This article is part of the theme issue 'Transport phenomena in complex systems (part 1)'.

Surfaces Affect Screening Reliability in Formulation Development of Biologics.

PURPOSE: The ability to predict an antibody's propensity for aggregation is particularly important during product development to ensure the quality and safety of therapeutic antibodies. We demonstrate the role of container surfaces on the aggregation process of three mAbs under elevated temperature and long-term storage conditions in the absence of mechanical stress. METHODS: A systematic study of aggregation is performed for different proteins, vial material, storage temperature, and presence of surfactant. We use size exclusion chromatography and micro-flow imaging to determine the bulk concentration of aggregates, which we combine with optical and atomic force microscopy of vial surfaces to determine the effect of solid-liquid interfaces on the bulk aggregate concentration under different conditions. RESULTS: We show that protein particles under elevated temperature conditions adhere to the vial surfaces, causing a substantial underestimation of aggregation propensity as determined by common methods used in development of biologics. Under actual long-term storage conditions at 5°C, aggregate particles do not adhere to the surface, causing an increase in bulk concentration of particles, which cannot be predicted from elevated temperature screening tests by common methods alone. We also identify specific protein - surface interactions which promote oligomer formation in the nanometre range. CONCLUSIONS: Special care should be taken when interpreting size exclusion and particle count data from stability studies if different temperatures and vial types are involved. We propose a novel combination of methods to characterise vial surfaces and bulk solution for a full understanding of protein aggregation processes in a sample.