Progress in phase-sensitive sum frequency generation spectroscopy

Sum rule comparison of narrowband and broadband sum frequency generation spectra and comparison with theory

Earlier sum frequency generation (SFG) experiments involve one infrared and one visible laser, and a measurement of the intensity of the response, yielding data on the surface sensitive properties of the sample. Recently, both the real and imaginary components of the susceptibility were measured in two different sets of experiments. In one set, a broadband infrared laser was used, permitting observations at very short times, while in another set the infrared laser was narrowband, permitting higher spectral resolution. The differences in the spectrum obtained by the two will be most evident in studying narrow absorption bands, e.g., the band due to dangling OH bonds at a water interface. The direct comparisons in the integrated amplitude (sum rule) of the imaginary part of the dangling OH bond region differ by a factor of 3. Due to variations in experimental setup and data processing, corrections were made for the quartz reference, Fresnel factors, and the incident visible laser wavelength. After the corrections, the agreement differs now by the factors of 1.1 within broadband and narrowband groups and the two groups now differ by a factor of 1.5. The 1.5 factor may arise from the extra heating of the more powerful broadband laser system on the water surface. The convolution from the narrowband SFG spectrum to the broadband SFG spectrum is also investigated and it does not affect the sum rule. Theory and narrowband experiments are compared using the sum rule and agree to a factor of 1.3 with no adjustable parameters.

Appraisal of TIP4P-type models at water surface

In view of the current situation in which non-polarizable rigid water models have been scarcely examined against surface-specific properties, we appraise TIP4P-type models at the liquid water surface on the basis of heterodyne-detected sum frequency generation (HD-SFG) spectroscopy. We find in the HD-SFG spectrum of the water surface that the peak frequency of the hydrogen-bonded OH band, the half width at half maximum of the hydrogen-bonded OH band, and the full width at half maximum of the free OH band are best reproduced by TIP4P, TIP4P/Ew, and TIP4P/Ice, respectively, whereas it is already well known that TIP4P/2005 best reproduces the surface tension. These TIP4P-type models perform better at the water surface in terms of the present appraisal items than some polarizable models in the literature.

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

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

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.

Ions Speciation at the Water-Air Interface

In typical aqueous systems, including naturally occurring sweet and salt water and tap water, multiple ion species are co-solvated. At the water-air interface, these ions are known to affect the chemical reactivity, aerosol formation, climate, and water odor. Yet, the composition of ions at the water interface has remained enigmatic. Here, using surface-specific heterodyne-detected sum-frequency generation spectroscopy, we quantify the relative surface activity of two co-solvated ions in solution. We find that more hydrophobic ions are speciated to the interface due to the hydrophilic ions. Quantitative analysis shows that the interfacial hydrophobic ion population increases with decreasing interfacial hydrophilic ion population at the interface. Simulations show that the solvation energy difference between the ions and the intrinsic surface propensity of ions determine the extent of an ion's speciation by other ions. This mechanism provides a unified view of the speciation of monatomic and polyatomic ions at electrolyte solution interfaces.

Enhanced Sum Frequency Generation for Monolayers on Au Relative to Silica: Local Field Factors and SPR Effect

Using metals as signal magnified substrates, surface plasmon-enhanced sum frequency generation (SFG) vibrational spectroscopy is a promising technique to probe weak molecular-level signals at surfaces and interfaces. In this study, the vibrational signals of the n-alkane monolayer on the gold (Au) and silica substrates are investigated using the broadband femtosecond SFG. The enhancement factors are discovered to be up to ∼1076 and ∼31 for the methyl symmetric and asymmetric stretching (ss and as) modes of the monolayer, respectively. By systematically analyzing the second-order nonlinear susceptibility tensor components (χ_ijks), the Fresnel coefficients (F_ijks), and the surface plasmon resonance (SPR) effect, we find that the interplay between F_ijk and χ_ijk terms and the SPR effect dominate the SFG signal enhancement. Our study reveals that the relative contributions of different influencing factors (i.e., Fresnel coefficients and SPR) to the SFG signal enhancement provide an approach to interpreting enhanced SFG vibrational signals detected from probe molecules on distinct substrates and may finally guide the design of the experimental methodology to improve the detection sensitivity and signal-to-noise ratio.

Interfacial Liquid Water on Graphite, Graphene, and 2D Materials

The optical, electronic, and mechanical properties of graphite, few-layer, and two-dimensional (2D) materials have prompted a considerable number of applications. Biosensing, energy storage, and water desalination illustrate applications that require a molecular-scale understanding of the interfacial water structure on 2D materials. This review introduces the most recent experimental and theoretical advances on the structure of interfacial liquid water on graphite-like and 2D materials surfaces. On pristine conditions, atomic-scale resolution experiments revealed the existence of 1-3 hydration layers. Those layers were separated by ∼0.3 nm. The experimental data were supported by molecular dynamics simulations. However, under standard working conditions, atomic-scale resolution experiments revealed the presence of 2-3 hydrocarbon layers. Those layers were separated by ∼0.5 nm. Linear alkanes were the dominant molecular specie within the hydrocarbon layers. Paradoxically, the interface of an aged 2D material surface immersed in water does not have water molecules on its vicinity. Free-energy considerations favored the replacement of water by alkanes.

Experimental and Theoretical Heterodyne-Detected Sum Frequency Generation Spectroscopy of Isotopically Pure and Diluted Water Surfaces

The χ⁽²⁾ (second-order nonlinear optical susceptibility) spectrum of the water surface has been a matter of debate for a few decades. Here, we report that we experimentally measured the isotopic dilution dependence of the χ⁽²⁾ spectrum and theoretically reproduced it by employing the quantum/classical mixed approach with a new idea to subtract an artifact. The present theoretical framework allows for clarifying the effects of the intramolecular, intermolecular, and Fermi resonance couplings on the OH-stretch vibrational spectra of water at the surface as well as in the bulk.

Accurate phase detection in time-domain heterodyne SFG spectroscopy

Heterodyne detection is a ubiquitous tool in spectroscopy for the simultaneous detection of intensity and phase of light. However, the need for phase stability hinders the application of heterodyne detection to electronic spectroscopy. We present an interferometric design for a phase-sensitive electronic sum frequency generation (e-SFG) spectrometer in the time domain with lock-in detection. Our method of continuous phase modulation of one arm of the interferometer affords direct measurement of the phase between SFG and local oscillator fields. Errors in the path length difference caused by drifts in the optics are corrected, offering unprecedented stability. This spectrometer has the added advantage of collinear fundamental beams. The capabilities of the spectrometer are demonstrated with proof-of-principle experiments with GaAs e-SFG spectra, where we see significantly improved signal to noise ratio, spectral accuracy, and lineshapes.

Flotation surface chemistry of water-soluble salt minerals: from experimental results to new perspectives

The flotation separation of water-soluble salt minerals has to be conducted under the condition of saturation in brines which represents a challenging but exciting topic of colloid and surface chemistry. Despite several proposals on explaining the success of this industrial application for many decades, our understanding of the flotation separation is still far from complete yet, owing to the complexity of the highly selective collection of salt crystals by air bubbles in brines. Here, we thoroughly review the experimental results for halogen, oxyanion, and double salts and match them with the proposed theories on the flotation of soluble salts to identify the agreed and disagreed cases. The experimental results show that the flotation of these salts varies from collectors (surfactants applied to control the crystal hydrophobicity) to collectors and is strongly affected by the brine ion composition and pH conditions. We find some exceptional flotation results that cannot be simply explained by the crystal surface charge and wettability. Furthermore, we outline several disputes and discrepancies between the experiments and the theories when different collectors are applied. Apart from the extensive consideration of surface hydration, the presence of external ion species exhibits ubiquitous effects on the surface properties of salt crystals and the colloidal properties of collectors. We conclude that the interactions between salt ions, water molecules, collectors, and salt crystals must be considered more thoroughly, and the activity of collectors at the air-liquid interface should also be the focus. Advanced techniques such as molecular dynamics simulation, atomic force microscopy, X-ray photoelectron spectroscopy, and sum-frequency generation spectroscopy are expected to be promising research tools for future studies.

Proton Configuration in Water Chain on Pt(533)

In this paper, a wetting behavior of Pt(533) is studied by using heterodyne-detected vibrational sum-frequency generation spectroscopy under an ultrahigh-vacuum condition at 145 K. The imaginary parts of the surface nonlinear susceptibility (Imχ⁽²⁾) of the H-bonded OH stretching region are successfully obtained for submonolayer water coverage that show negative bands indicating H-down (proton pointing to the substrate) configurations both for the water at the step and at the terrace. The growth manner of the Imχ⁽²⁾ signal with coverage and the results of an isotopic dilution are consistent with a model in which a one-dimensional (1D) chain at the step forms a "zigzag" structure that contains H-down orientations. This finding resolves the previous controversy in the literature concerning the proton configuration in the 1D water chain at the step.

Tutorials in vibrational sum frequency generation spectroscopy. III. Collecting, processing, and analyzing vibrational sum frequency generation spectra

In this Tutorial series, we aim to provide an accessible introduction to vibrational sum frequency generation (VSFG) spectroscopy, targeted toward people entering the VSFG world without a rigorous formal background in optical physics or nonlinear spectroscopy. In this article, we discuss in detail the processes of collecting and processing VSFG data, and user-friendly processing software (sfgtools) is provided for use by people new to the field. Some discussion of analyzing VSFG spectra is also given, specifically with a discussion of fitting homodyne VSFG spectra, and a discussion of what can be learned (both qualitatively and quantitatively) from VSFG spectra.

Polarization-Dependent Heterodyne-Detected Sum-Frequency Generation Spectroscopy as a Tool to Explore Surface Molecular Orientation and Ångström-Scale Depth Profiling

Sum-frequency generation (SFG) spectroscopy provides a unique optical probe for interfacial molecules with interface-specificity and molecular specificity. SFG measurements can be further carried out at different polarization combinations, but the target of the polarization-dependent SFG is conventionally limited to investigating the molecular orientation. Here, we explore the possibility of polarization-dependent SFG (PD-SFG) measurements with heterodyne detection (HD-PD-SFG). We stress that HD-PD-SFG enables accurate determination of the peak amplitude, a key factor of the PD-SFG data. Subsequently, we outline that HD-PD-SFG can be used not only for estimating the molecular orientation but also for investigating the interfacial dielectric profile and studying the depth profile of molecules. We further illustrate the variety of combined simulation and PD-SFG studies.

Thiocyanate Ions Form Antiparallel Populations at the Concentrated Electrolyte/Charged Surfactant Interface

Anions play significant roles in the separation of lanthanides and actinides. The molecular-scale details of how these anions behave at aqueous interfaces are not well understood, especially at high ionic strengths. Here, we describe the interfacial structure of thiocyanate anions at a soft charged interface up to 5 M bulk concentration with combined classical and phase-sensitive vibrational sum frequency generation (PS-VSFG) spectroscopy and molecular dynamics (MD) simulations. At low concentrations thiocyanate ions are mostly oriented with their sulfur end pointing toward the charged surfactants. The VSFG signal reaches a plateau at around 100 mM bulk concentration, followed by significant changes above 1 M. At high concentrations a new thiocyanate population emerges with their sulfur end pointing toward the bulk liquid. The -CN stretch frequency is different for up and down oriented SCN^- ions, indicating different coordination environments. These results provide key molecular-level insights for the interfacial behavior of complex anions in highly concentrated solutions.

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

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

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

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

Accurate molecular orientation at interfaces determined by multimode polarization-dependent heterodyne-detected sum-frequency generation spectroscopy via multidimensional orientational distribution function

Many essential processes occur at soft interfaces, from chemical reactions on aqueous aerosols in the atmosphere to biochemical recognition and binding at the surface of cell membranes. The spatial arrangement of molecules specifically at these interfaces is crucial for many of such processes. The accurate determination of the interfacial molecular orientation has been challenging due to the low number of molecules at interfaces and the ambiguity of their orientational distribution. Here, we combine phase- and polarization-resolved sum-frequency generation (SFG) spectroscopy to obtain the molecular orientation at the interface. We extend an exponentially decaying orientational distribution to multiple dimensions, which, in conjunction with multiple SFG datasets obtained from the different vibrational modes, allows us to determine the molecular orientation. We apply this new approach to formic acid molecules at the air–water interface. The inferred orientation of formic acid agrees very well with ab initio molecular dynamics data. The phase-resolved SFG multimode analysis scheme using the multidimensional orientational distribution thus provides a universal approach for obtaining the interfacial molecular orientation.

Salting-Up of Surfactants at the Surface of Saline Water as Detected by Tensiometry and SFG and Supported by Molecular Dynamics Simulation

Surfactant adsorption at the air-water interface is critical to many industrial processes but its dependence on salt ions is still poorly understood. Here, we investigate the adsorption of sodium dodecanoate onto the air-water interface using model saline waters of Li⁺ or Cs⁺ at pH values 8 and 11. Both cations enhance the surfactant adsorption, as expected, but their largest effects on the adsorption also depend on pH. Specifically, surface tension measurements, sum-frequency generation spectroscopy, and microelectrophoresis show that small (hard) Li⁺ enhances the surfactant adsorption more than large (soft) Cs⁺ at pH 11. This effect is fully reversed at pH 8. We argue that this salting-up (increasing adsorption) reversal is attributable to the conversion of the neutralized carboxylic (-COOH) headgroup at pH 8 into the charged carboxylate (-COO^-) headgroup at pH 11, which, respectively, interact with Cs⁺ and Li⁺ favorably. Molecular dynamics simulation shows that the affinity of Cs⁺ to the interface is decreased and eventually overtaken by Li⁺ as the carboxylic groups are deprotonated. This study highlights the importance of the charge and size of salt ions in selecting surfactants and electrolytes for industrial applications.

Time-resolved electronic sum-frequency generation spectroscopy with fluorescence suppression using optical Kerr gating

Excited state dynamics of molecules at interfaces can be studied using second-order non-linear spectroscopic methods such as time-resolved electronic sum-frequency generation (SFG). However, as such measurements inherently generate very small signals, they are often overwhelmed by signals originating from fluorescence. Here, this limitation is overcome by optical Kerr gating of the SFG signal to discriminate against fluorescence. The new approach is demonstrated on the excited state dynamics of malachite green at the water/air interface, in the presence of a highly fluorescent coumarin dye, and on the photo-oxidation of the phenolate anion at the water/air interface. The generality of the use of optical Kerr gating to SFG measurements is discussed.

Probing Orientations and Conformations of Peptides and Proteins at Buried Interfaces

Molecular structures of peptides/proteins at interfaces determine their interfacial properties, which play important roles in many applications. It is difficult to probe interfacial peptide/protein structures because of the lack of appropriate tools. Sum frequency generation (SFG) vibrational spectroscopy has been developed into a powerful technique to elucidate molecular structures of peptides/proteins at buried solid/liquid and liquid/liquid interfaces. SFG has been successfully applied to study molecular interactions between model cell membranes and antimicrobial peptides/membrane proteins, surface-immobilized peptides/enzymes, and physically adsorbed peptides/proteins on polymers and 2D materials. A variety of other analytical techniques and computational simulations provide supporting information to SFG studies, leading to more complete understanding of structure-function relationships of interfacial peptides/proteins. With the advance of SFG techniques and data analysis methods, along with newly developed supplemental tools and simulation methodology, SFG research on interfacial peptides/proteins will further impact research in fields like chemistry, biology, biophysics, engineering, and beyond.