Direct Measurement of Charge Reversal on Lipid Bilayers Using Heterodyne-Detected Second Harmonic Generation Spectroscopy

Temperature Dependence of Proton Coverage and the Total Potential at Fused Silica:Water Interfaces from Phase-Resolved Nonlinear Optics

We study the temperature and ionic strength dependence of interfacial amphoterism via phase- and amplitude-resolved SHG measurements at the fused silica:water interface, finding that the total interfacial potential becomes increasingly negative when raising the temperature from 20 to 60 °C. The interfacial structure, reported by calibrated measurements of the second-order nonlinear susceptibility of the interface, remains largely temperature invariant. Our approach is presented in form of a blueprint for second harmonic generation (SHG) amplitude and phase measurements at buried aqueous interfaces using an affordable Y-crystal-based oscillator. A Galilean beam expander addresses the signal vs local oscillator mismatch, while a beam block eliminates otherwise interfering front reflections from the flat optical windows used here. The results show nearly linear voltage increases with temperature that rise faster at low vs high ionic strength and are consistent with temperature-dependent equilibrium constants governing the amphoteric silica:water interface. Proton surface coverages at pH 2.5, 6, and 11 increase with temperature as the surface becomes more negatively charged, reaching up to 1013 protons cm-2 at 60 °C. These findings aid interfacial model development, benchmark atomistic simulations, explore temperature-dependent Hofmeister effects, and enhance understanding of interfacial electrocatalysis.

Water flipping and the oxygen evolution reaction on Fe2O3 nanolayers

Hematite photoanodes are promising for the oxygen evolution reaction, however, their high overpotential (0.5-0.6 V) for water oxidation and limited photocurrent make them economically unviable at present. The work needed to orient dipoles at an electrode surface may be an overlooked contribution to the overpotential, especially regarding dipoles of water, the electron source in the oxygen evolution reaction (OER). Here, we employ second harmonic amplitude and phase measurements to quantify the number of net-aligned Stern layer water molecules and the work associated with water flipping, on hematite, an earth abundant OER semiconductor associated with a high overpotential. At zero applied bias, the pH-dependent potentials for Stern layer water molecule flipping exhibit Nernstian behavior. At positive applied potentials and pH 13, approximately one to two monolayers of water molecules points the oxygen atoms towards the electrode, favorable for the OER. The work associated with water flipping matches the cohesive energy of liquid water (44 kJ mol-1) and the OER current density is highest. This current is negligible at pH 5, where the work approaches 100 kJ mol-1. Our findings suggest a causal relationship between the need for Stern layer water flipping and the OER overpotential, which may lead to developing strategies for decreasing the latter.

Second harmonic generation null angle polarization analysis for determining interfacial potential at charged interfaces

Methods of quantifying the electrostatics of charged interfaces are important in a range of research areas. The surface-selective nonlinear optical technique second harmonic generation (SHG) is a sensitive probe of interfacial electrostatics. Recent work has shown that detection of the SHG phase in addition to its amplitude enables direct quantification of the interfacial potential. However, the experimental challenge of directly detecting the phase interferometrically with sufficient precision and stability has led to the proposal and development of alternative techniques to recover the same information, notably through wavelength scanning or angle scanning, each of which has their own associated experimental challenges. Here, we propose a new polarization-based approach to recover the required phase information, building upon the previously established nonlinear optical null ellipsometry (NONE) technique. Although NONE directly returns only relative phase information between different tensor elements of the second-order susceptibility, it is shown that a symmetry relation that connects the tensor elements of the potential-dependent third-order susceptibility can be used to generate the absolute phase reference required to calculate the interfacial potential. The sensitivity of the technique to potential at varying surface charge densities and ionic strengths is explored by means of simulated data of the silica:water interface. The error associated with the use of the linearized Poisson-Boltzmann approximation is discussed and compared to the error associated with the precision of the measured NONE null angles. Overall, the results suggest that NONE is a promising approach for performing phase-resolved SHG based quantification of interfacial potentials that experimentally requires only the addition of standard polarization optics to the basic single-wavelength, fixed-angle SHG apparatus.

NaCl, MgCl2, and AlCl3 Surface Coverages on Fused Silica and Adsorption Free Energies at pH 4 from Nonlinear Optics

We employ amplitude- and phase-resolved second harmonic generation experiments to probe interactions of fused silica:aqueous interfaces with Al3+, Mg2+, and Na+ cations at pH 4 and as a function of metal cation concentration. We quantify the second-order nonlinear susceptibility and the total interfacial potential in the presence and absence of a 10 mM screening electrolyte to understand the influence of charge screening on cation adsorption. Strong cation:surface interactions are observed in the absence of the screening electrolyte. The total potential is then employed to estimate the total number of absorbed cations cm-2. The contributions to the total potential from the bound and mobile charges were separated using Gouy-Chapman-Stern model estimates. All three cations bind fully reversibly, indicating physisorption as the mode of interaction. Of the isotherm models tested, the Kd adsorption model fits the data with binding constants of 3-30 and ∼300 mol-1 for the low (<0.1 mM) and high (0.1-3 mM) concentration regimes, corresponding to adsorption free energies of -13 to -18 and -24 kJ mol-1 at room temperature, respectively. The maximum surface coverages are around 1013 cations cm-2, matching the number of deprotonated silanol groups on silica at pH 4. Clear signs of decoupled Stern and diffuse layer nonlinear optical responses are observed and found to be cation-specific.

In Situ Heterodyne-Detected Second-Harmonic Generation Study of the Influence of Cholesterol on Dye Molecule Adsorption on Lipid Membrane

Cholesterol plays an essential role in regulating the functionality of biomembranes. This study employed in situ second-harmonic generation (SHG) to investigate the adsorption behavior of the dye molecule 4-(4-(diethylamino)styryl)-N-methyl-pyridinium iodide (D289) on a biomimic membrane composed of 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (sodium salt) (DPPG) and cholesterol. The time-dependent polarization SHG intensity exhibited an initial rapid increase, followed by a subsequent decline. The initial increased SHG intensity is responsible for the electrostatic interaction-driven adsorption of D289 onto the membrane, while the decrease in the SHG signal results from the broadening of the orientation distribution within the membrane. Heterodyne-detected SHG (HD-SHG) measurements demonstrated that the adsorption of dye molecules influenced the phase of the induced electric field. The interfacial potential Φ(0) as a function of time was measured, and we found that even after reaching a stable Stern layer state, the diffusion layer continued to exhibit a dynamic change. This study offers a comprehensive understanding of the influence of cholesterol on adsorption, reorientation dynamics, and dynamic changes in the reorientation of water in the diffusion layer.

Theoretical Study of the Two-Dimensional Vibrational Sum Frequency Generation Spectroscopy of the Air-Water Interface at Varying Temperature and Its Connections to the Interfacial Structure and Dynamics

We performed a theoretical study of the temperature variation of two-dimensional vibrational sum frequency generation (2D-VSFG) spectra of the OH stretch modes at air-water interfaces in the mid-IR region. The calculations are performed at four different temperatures from 250 to 325 K by using a combination of techniques involving response function formalism of nonlinear spectroscopy, electronic structure calculations, and molecular dynamics simulations. Also, the calculations are performed for isotopically dilute solutions so that the intra- and intermolecular coupling between the vibrational modes of interest can be ignored. We have established the connections of temperature variation of various frequency- and time-dependent features of the calculated spectra to the changes in the underlying structure and dynamics of the interfaces. The results reveal that interfacial water is dynamically more heterogeneous than bulk water, with three dominant dynamical processes exhibiting their corresponding time-dependent features in the 2D-VSFG spectrum. These are the spectral diffusion of hydrogen-bonded OH groups at the interface, conversion of an initially hydrogen-bonded OH group to a dangling OH which is a stable state for surface water, unlike the bulk water, and the third one, which involves the conversion of an initially free or dangling OH group to its hydrogen-bonded state at the interface. The temporal appearance of the cross peaks corresponding to interconversion of the hydrogen-bonded state to the dangling state or vice versa of an interfacial OH group is found to take place at a slower rate than the dynamics of spectral diffusion of hydrogen-bonded molecules at the interface, which, in turn, is slower than the corresponding spectral diffusion of bulk water molecules. The temperature variation of these dynamic processes can be linked to the decay of appropriate hydrogen-bond and non-hydrogen-bond time correlation functions of interfacial water molecules for the different air-water systems studied in this work.

Quantification of Stern Layer Water Molecules, Total Potentials, and Energy Densities at Fused Silica:Water Interfaces for Adsorbed Alkali Chlorides, CTAB, PFOA, and PFAS

We have employed amplitude- and phase-resolved second-harmonic generation spectroscopy to investigate ion-specific effects of monovalent cations at the fused silica:water interface maintained under acidic, neutral, and alkaline conditions. We find a negligible dependence of the total potential (as negative as -400 mV at pH 14), the second-order nonlinear susceptibility (as large as 1.5 × 10-21 m2 V-1 at pH 14), the number of Stern layer water molecules (1 × 1015 cm-2 at pH 5.8), and the energy associated with water alignment upon going from neutral to high pH (ca. -24 kJ mol-1 to -48 kJ mol-1 at pH 13 and 14, close to the cohesive energy of liquid water but smaller than that of ice) on chlorides of the alkali series (M+ = Li+, Na+, K+, Rb+, and Cs+). Attempts are presented to provide estimates for the molecular hyperpolarizability of the cations and anions in the Stern layer at high pH, which arrive at ca. 20-fold larger values for αtotal ions(2) = αM+(2) + αOH-(2) + αCl-(2) when compared to water's molecular hyperpolarizability estimate from theory and point to a sizable contribution of deprotonated silanol groups at high pH. In contrast to the alkali series, a pronounced dependence of the total potential and the second-order nonlinear susceptibility on monovalent cationic (cetrimonium bromide, CTAB) and anionic (perfluorooctanoic and perfluorooctanesulfonic acid, PFOA and PFOS) surfactants was quantifiable. Our findings are consistent with a low surface coverage of the alkali cations and a high surface coverage of the surfactants. Moreover, they underscore the important contribution of Stern layer water molecules to the total potential and second-order nonlinear susceptibility. Finally, they demonstrate the applicability of heterodyne-detected second-harmonic generation spectroscopy for identifying perfluorinated acids at mineral:water interfaces.

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.

Elucidating Trivalent Ion Adsorption at Floating Carboxylic Acid Monolayers: Charge Reversal or Water Reorganization?

We study the adsorption of trivalent neodymium on floating arachidic acid films at the air-water interface by two complementary surface specific probes, sum frequency generation spectroscopy and X-ray fluorescence near total reflection. In the absence of background ions, neodymium ions compensate for the surface charge of the arachidic acid film at a bulk concentration of 50 μM without any charge reversal. Increasing the bulk concentration to 1 mM does not change the neodymium surface coverage but affects the interfacial water structure significantly. In the presence of a high concentration of NaCl, there is overcharging at 1 mM Nd³⁺, i.e., 30% more Nd³⁺ than needed to compensate for the surface charge. These results show that the total coverage of neodymium ions is not enough to describe the complete picture at the interface, and interfacial water and ion coverage needs to be considered together to understand more complex ion adsorption and transport processes.

Optical method for quantifying the potential of zero charge at the platinum-water electrochemical interface

When an electrode contacts an electrolyte, an interfacial electric field forms. This interfacial field can polarize the electrode's surface and nearby molecules, but its effect can be countered by an applied potential. Quantifying the value of this countering potential ('potential of zero charge' (pzc)) is, however, not straightforward. Here we present an optical method for determining the pzc at an electrochemical interface. Our approach uses phase-sensitive second-harmonic generation to determine the electrochemical potential where the interfacial electric field vanishes at an electrode-electrolyte interface with Pt-water as a model experiment. Our method reveals that the pzc of the Pt-water interface is 0.23 ± 0.08 V versus standard hydrogen electrode (SHE) and is pH independent from pH 1 to pH 13. First-principles calculations with a hybrid explicit-implicit solvent model predict the pzc of the Pt(111)-water interface to be 0.23 V versus SHE and reveal how the interfacial water structure rearranges as the electrode potential is moved above and below the pzc. We further show that pzc is sensitive to surface modification; deposition of Ni on Pt shifts the interfacial pzc in the cathodic direction by ~360 mV. Our work demonstrates a materials-agnostic approach for quantifying the interfacial electrical field and water orientation at an electrochemical interface without requiring probe molecules and confirms the long-held view that the interfacial electric field is more intense during hydrogen electrocatalysis in alkaline than in acid.

Monitoring membranes: The exploration of biological bilayers with second harmonic generation

Nature's seemingly controlled chaos in heterogeneous two-dimensional cell membranes stands in stark contrast to the precise, often homogeneous, environment in an experimentalist's flask or carefully designed material system. Yet cell membranes can play a direct role, or serve as inspiration, in all fields of biology, chemistry, physics, and engineering. Our understanding of these ubiquitous structures continues to evolve despite over a century of study largely driven by the application of new technologies. Here, we review the insight afforded by second harmonic generation (SHG), a nonlinear optical technique. From potential measurements to adsorption and diffusion on both model and living systems, SHG complements existing techniques while presenting a large exploratory space for new discoveries.

Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities

The electric double layer governs the processes of all charged surfaces in aqueous solutions; however, elucidating the structure of the water molecules is challenging for even the most advanced spectroscopic techniques. Here, we present the individual Stern layer and diffuse layer OH stretching spectra at the silica/water interface in the presence of NaCl over a wide pH range using a combination of vibrational sum frequency generation spectroscopy, heterodyned second harmonic generation, and streaming potential measurements. We find that the Stern layer water molecules and diffuse layer water molecules respond differently to pH changes: unlike the diffuse layer, whose water molecules remain net-oriented in one direction, water molecules in the Stern layer flip their net orientation as the solution pH is reduced from basic to acidic. We obtain an experimental estimate of the non-Gouy-Chapman (Stern) potential contribution to the total potential drop across the insulator/electrolyte interface and discuss it in the context of dipolar, quadrupolar, and higher order potential contributions that vary with the observed changes in the net orientation of water in the Stern layer. Our findings show that a purely Gouy-Chapman (Stern) view is insufficient to accurately describe the electrical double layer of aqueous interfaces.

Label-Free Imaging of Membrane Potentials by Intramembrane Field Modulation, Assessed by Second Harmonic Generation Microscopy

Optical interrogation of cellular electrical activity has proven itself essential for understanding cellular function and communication in complex networks. Voltage-sensitive dyes are important tools for assessing excitability but these highly lipophilic sensors may affect cellular function. Label-free techniques offer a major advantage as they eliminate the need for these external probes. In this work, it is shown that endogenous second-harmonic generation (SHG) from live cells is highly sensitive to changes in transmembrane potential (TMP). Simultaneous electrophysiological control of a living human embryonic kidney (HEK293T) cell, through a whole-cell voltage-clamp reveals a linear relation between the SHG intensity and membrane voltage. The results suggest that due to the high ionic strengths and fast optical response of biofluids, membrane hydration is not the main contributor to the observed field sensitivity. A conceptual framework is further provided that indicates that the SHG voltage sensitivity reflects the electric field within the biological asymmetric lipid bilayer owing to a nonzero χeff(2) tensor. Changing the TMP without surface modifications such as electrolyte screening offers high optical sensitivity to membrane voltage (≈40% per 100 mV), indicating the power of SHG for label-free read-out. These results hold promise for the design of a non-invasive label-free read-out tool for electrogenic cells.

First Hyperpolarizability of Water in Bulk Liquid Phase: Long-Range Electrostatic Effects Included via the Second Hyperpolarizability

The molecular first hyperpolarizability β contributes to second-order optical non-linear signals collected from molecular liquids. For the Second Harmonic Generation (SHG) response, the first hyperpolarizability β (2ω,ω,ω) often depends on...

Divalent Ion Specific Outcomes on Stern Layer Structure and Total Surface Potential at the Silica:Water Interface

The second-order nonlinear susceptibility, χ⁽²⁾, in the Stern layer and the total interfacial potential drop, Φ(0)_tot, across the oxide:water interface are estimated from SHG amplitude and phase measurements for divalent cations (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) at the silica:water interface at pH 5.8 and various ionic strengths. We find that interfacial structure and total potential depend strongly on ion valency. We observe statistically significant differences between the experimentally determined χ⁽²⁾ value for NaCl and that of the alkali earth series but smaller differences between ions of the same valency in that series. These differences are particularly pronounced at intermediate salt concentrations, which we attribute to the influence of hydration structure in the Stern layer. Furthermore, we corroborate the differences by examining the effects of anion substitution (SO₄^2- for Cl^-). Finally, we identify that hysteresis in measuring the reversibility of ion adsorption and desorption at fused silica in forward and reverse titrations manifests itself both in Stern layer structure and in total interfacial potential for some of the salts, most notably for CaCl₂ and MgSO₄ but less so for BaCl₂ and NaCl.

Angstrom-Resolved Interfacial Structure in Buried Organic-Inorganic Junctions

Charge transport processes at interfaces play a crucial role in many processes. Here, the first soft x-ray second harmonic generation (SXR SHG) interfacial spectrum of a buried interface (boron-Parylene N) is reported. SXR SHG shows distinct spectral features that are not observed in x-ray absorption spectra, demonstrating its extraordinary interfacial sensitivity. Comparison to electronic structure calculations indicates a boron-organic separation distance of 1.9 Å, with changes of less than 1 Å resulting in easily detectable SXR SHG spectral shifts (ca. hundreds of milli-electron volts).

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.

Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability

Isolating the hydrogen-bonding structure of water immediately at the surface is challenging, even with surface-specific techniques like sum-frequency generation (SFG), because of the presence of aligned water further away in the diffuse layer. Here, we combine zeta potential and SFG intensity measurements with the maximum entropy method referenced to reported phase-sensitive SFG and second-harmonic generation results to deconvolute the SFG spectral contributions of the surface waters from those in the diffuse layer. Deconvolution reveals that at very low ionic strength, the surface water structure is similar to that of a neutral silica surface near the point-of-zero-charge with waters in different hydrogen-bonding environments oriented in opposite directions. This similarity suggests that the known metastability of silica colloids against aggregation under both conditions could arise from this distinct surface water structure. Upon the addition of salt, significant restructuring of water is observed, leading to a net decrease in order at the surface.

Sensitivity of sum frequency generation experimental conditions to thin film interference effects

Sum-frequency generation (SFG) spectroscopy has furthered our understanding of the chemical interfaces that guide key processes in biology, catalysis, environmental science, and energy conversion. However, interpreting SFG spectra of systems containing several internal interfaces, such as thin film electronics, electrochemical cells, and biofilms, is challenging as different interfaces within these structures can produce interfering SFG signals. One potential way to address this issue is to carefully select experimental conditions that amplify the SFG signal of an interface of interest over all others. In this report, we investigate a model two-interface system to assess our ability to isolate the SFG signal from each interface. For SFG experiments performed in a reflective geometry, we find that there are few experimental conditions under which the SFG signal originating from either interface can be amplified and isolated from the other. However, by performing several measurements under conditions that alter their interference, we find that we can reconstruct each signal even in cases where the SFG signal from one interface is more than an order of magnitude smaller than its counterpart. The number of spectra needed for this reconstruction varies depending on the signal-to-noise level of the SFG dataset and the degree to which different experiments in a dataset vary in their sensitivity to each interface. Taken together, our work provides general guidelines for designing experimental protocols that can isolate SFG signals stemming from a particular region of interest within complex samples.

Debye-Hückel Free Energy of an Electric Double Layer with Discrete Charges Located at a Dielectric Interface

Poisson-Boltzmann theory provides an established framework to calculate properties and free energies of an electric double layer, especially for simple geometries and interfaces that carry continuous charge densities. At sufficiently small length scales, however, the discreteness of the surface charges cannot be neglected. We consider a planar dielectric interface that separates a salt-containing aqueous phase from a medium of low dielectric constant and carries discrete surface charges of fixed density. Within the linear Debye-Hückel limit of Poisson-Boltzmann theory, we calculate the surface potential inside a Wigner-Seitz cell that is produced by all surface charges outside the cell using a Fourier-Bessel series and a Hankel transformation. From the surface potential, we obtain the Debye-Hückel free energy of the electric double layer, which we compare with the corresponding expression in the continuum limit. Differences arise for sufficiently small charge densities, where we show that the dominating interaction is dipolar, arising from the dipoles formed by the surface charges and associated counterions. This interaction propagates through the medium of a low dielectric constant and alters the continuum power of two dependence of the free energy on the surface charge density to a power of 2.5 law.

Phase-Sensitive Second-Harmonic Generation of Electrochemical Interfaces

The interaction between molecular species and charged interfaces plays an indispensable role in a multitude of electrochemical devices. Yet, very little is understood about the nature of this interaction, in particular, the interfacial electric field. Second-order nonlinear spectroscopy such as second-harmonic generation (SHG) can provide chemical information on these interfacial interactions; however, the phase information has received limited attention in electrochemical SHG studies. Here, we demonstrate that the phase of the SHG is essential to the measurement of the electric field at the electrode-electrolyte interface. Our in situ SHG experiment provides strong evidence in support of the parabolic model with complex nonlinear susceptibilities. We conclude that if the absolute phase of the total SHG signal with both χ⁽²⁾ and χ⁽³⁾ contributions can be obtained, it would be possible to measure the potential of zero charge of any electrochemical material.