Transport Dynamics of Water Molecules Confined between Lipid Membranes

Water molecules confined between biological membranes exhibit a distinctive non-Gaussian displacement distribution, far different from that of bulk water. Here, we introduce a new transport equation for water molecules in the intermembrane space, quantitatively explaining molecular dynamics simulation results. We find that the unique transport dynamics of water molecules stems from the lateral diffusion coefficient fluctuation caused by their longitudinal motion in the direction perpendicular to the membranes. We also identify an interfacial region where water possesses distinct physical properties, which is unaffected by changes in the intermembrane separation.

Water-Ion Interaction Determines the Mobility of Ions in Highly Concentrated Aqueous Electrolytes

Solvation engineering plays a critical role in tailoring the performance of batteries, particularly through the use of highly concentrated electrolytes, which offer heterogeneous solvation structures of mobile ions with distinct electrochemical properties. In this study, we employed spectroscopic techniques and molecular dynamics simulations to investigate mixed-cation (Li+/K+) acetate aqueous electrolytes. Our research unravels the pivotal role of water in facilitating ion transport within a highly viscous medium. Notably, Li+ cations primarily form ion aggregates, predominantly interacting with acetate anions, while K+ cations emerge as the principal charge carriers, which is attributed to their strong interaction with water molecules. Intriguingly, even at a concentration as high as 40 m, a substantial amount of water molecules persistently engages in hydrogen bonding with one another, creating mobile regions rich in K+ ions. Our observations of a redshift of the OH stretching band of water suggest that the strength of the hydrogen bond alone cannot account for the expansion of the electrochemical stability window. These findings offer valuable insights into the cation transfer mechanism, shedding light on the contribution of water-bound cations to both the ion conductivity and the electrochemical stability window of aqueous electrolytes for rechargeable batteries. Our comprehensive molecular-level understanding of the interplay between cations and water provides a foundation for future advances in solvation engineering, leading to the development of high-performance batteries with improved energy storage and safety profiles.

Dual phase-detected infrared photothermal microscopy

Infrared photothermal microscopy (IPM) has recently gained considerable attention as a versatile analytical platform capable of providing spatially resolved molecular insights across diverse research fields. This technique has led to numerous breakthroughs in the study of compositional variations in functional materials and cellular dynamics in living cells. However, its application to investigate multiple components of temporally dynamic systems, such as living cells and operational devices, has been hampered by the limited information content of the IP signal, which only covers a narrow spectral window (< 1 cm-1). Here, we present a straightforward approach for measuring two distinct IPM images utilizing the orthogonality between the in-phase and quadrature outputs of a lock-in amplifier, called dual-phase IR photothermal (DP-IP) detection. We demonstrate the feasibility of DP-IP detection for IPM in distinguishing two different micro-sized polymer beads.

Molecular dynamics simulation study of water structure and dynamics on the gold electrode surface with adsorbed 4-mercaptobenzonitrile

Understanding water dynamics at charged interfaces is of great importance in various fields, such as catalysis, biomedical processes, and solar cell materials. In this study, we implemented molecular dynamics simulations of a system of pure water interfaced with Au electrodes, on one side of which 4-mercaptobenzonitrile (4-MBN) molecules are adsorbed. We calculated time correlation functions of various dynamic quantities, such as the hydrogen bond status of the N atom of the adsorbed 4-MBN molecules, the rotational motion of the water OH bond, hydrogen bonds between 4-MBN and water, and hydrogen bonds between water molecules in the interface region. Using the Luzar-Chandler model, we analyzed the hydrogen bond dynamics between a 4-MBN and a water molecule. The dynamic quantities we calculated can be divided into two categories: those related to the collective behavior of interfacial water molecules and the H-bond interaction between a water molecule and the CN group of 4-MBN. We found that these two categories of dynamic quantities exhibit opposite trends in response to applied potentials on the Au electrode. We anticipate that the present work will help improve our understanding of the interfacial dynamics of water in various electrolyte systems.

Efficient and Inexpensive Synthesis of 15N-Labeled 2-Azido-1,3-dimethylimidazolinium Salts Using Na15NO2 Instead of Na15NNN

15N-Labeled azides are important probes for infrared and magnetic resonance spectroscopy and imaging. They can be synthesized by reaction of primary amines with a 15N-labeled diazo-transfer reagent. We present the synthesis of 15N-labeled 2-azido-1,3-dimethylimidazolinium salts 1 as a 15N-labeled diazo-transfer reagent. Nitrosation of 1,3-dimethylimidazolinium-2-yl hydrazine (2) with Na15NO2 under acidic conditions gave 1 as a 1:1 mixture of α- and γ-15N-labeled azides, α- and γ-1, rather than γ-1 alone. The isotopomeric mixture thus obtained was then subjected to the diazo-transfer reaction with primary amines 3 to afford azides 4 as a 1:1 mixture of β-15N-labeled azides β-4 and unlabeled ones 4'. The efficient and inexpensive synthesis of 1 as a 1:1 mixture of α- and γ-1 using Na15NO2 instead of Na15NNN facilitates their wide use as a 15N-labeled diazo-transfer reagent for preparing 15N-labeled azides as molecular probes.

Monitoring the synthesis of neutral lipids in lipid droplets of living human cancer cells using two-color infrared photothermal microscopy

There has been growing interest in the functions of lipid droplets (LDs) due to recent discoveries regarding their diverse roles. These functions encompass lipid metabolism, regulation of lipotoxicity, and signaling pathways that extend beyond their traditional role in energy storage. Consequently, there is a need to examine the molecular dynamics of LDs at the subcellular level. Two-color infrared photothermal microscopy (2C-IPM) has proven to be a valuable tool for elucidating the molecular dynamics occurring in LDs with sub-micrometer spatial resolution and molecular specificity. In this study, we employed the 2C-IPM to investigate the molecular dynamics of LDs in both fixed and living human cancer cells (U2OS cells) using the isotope labeling method. We investigated the synthesis of neutral lipids occurring in individual LDs over time after exposing the cells to excess saturated fatty acids while simultaneously comparing inherent lipid contents in LDs. We anticipate that these research findings will reveal new opportunities for studying lesser-known biological processes within LDs and other subcellular organelles.

Dielectric Response and Linear Absorption Spectroscopy of Ionic Systems

Time-dependent electric fields applied to ionic systems can induce both a dielectric and a conductive response, leading to the generation of macroscopic polarization and current, respectively. It has long been recognized that it is not possible to determine the two types of responses separately. However, this aspect is often not adequately accounted for in dielectric and absorption spectroscopies of ionic systems. To clarify this, we theoretically investigate the dielectric and conductive responses of ionic systems containing polyatomic ions based on linear response theory. We derive general expressions for the frequency-dependent dielectric functions, conductivity, and absorption coefficient, including those measured experimentally. Furthermore, we show that the dielectric and conductive responses cannot be uniquely distinguished even at the theoretical level and, therefore, cannot represent experimentally measured quantities. Instead, dielectric and absorption spectra of ionic systems should be expressed in terms of the generalized dielectric function that encompasses both dielectric and conductive responses. We propose a computational method to calculate this generalized dielectric function reliably. Model calculations on concentrated aqueous solutions of NaCl, a monatomic salt, and LiTFSI, a polyatomic salt, show that the dielectric and linear absorption spectra of the two systems based on the generalized dielectric function are significantly different from purely dielectric counterparts in the far-IR, terahertz, and lower-frequency regions. Moreover, the spectra are mainly determined by the autocorrelations of total dipole and total current, but dipole-current cross-correlation can also significantly contribute to the spectra of the LiTFSI solution. The present theoretical approach could be extended to nonlinear spectroscopy of ionic liquids and electrolyte solutions.

The hydrogen-bonding dynamics of water to a nitrile-functionalized electrode is modulated by voltage according to ultrafast 2D IR spectroscopy

We report the hydrogen-bonding dynamics of water to a nitrile-functionalized and plasmonic electrode surface as a function of applied voltage. The surface-enhanced two-dimensional infrared spectra exhibit hydrogen-bonded and non-hydrogen-bonded nitrile features in similar proportions, plus cross peaks between the two. Isotopic dilution experiments show that the cross peaks arise predominantly from chemical exchange between hydrogen-bonded and non-hydrogen-bonded nitriles. The chemical exchange rate depends upon voltage, with the hydrogen bond of the water to the nitriles breaking 2 to 3 times slower (>63 vs. 25 ps) under a positive as compared to a negative potential. Spectral diffusion created by hydrogen-bond fluctuations occurs on a ~1 ps timescale and is moderately potential-dependent. Timescales from molecular dynamics simulations agree qualitatively with the experiment and show that a negative voltage causes a small net displacement of water away from the surface. These results show that the voltage applied to an electrode can alter the timescales of solvent motion at its interface, which has implications for electrochemically driven reactions.

Long-term cargo tracking reveals intricate trafficking through active cytoskeletal networks in the crowded cellular environment

A eukaryotic cell is a microscopic world within which efficient material transport is essential. Yet, how a cell manages to deliver cellular cargos efficiently in a crowded environment remains poorly understood. Here, we used interferometric scattering microscopy to track unlabeled cargos in directional motion in a massively parallel fashion. Our label-free, cargo-tracing method revealed not only the dynamics of cargo transportation but also the fine architecture of the actively used cytoskeletal highways and the long-term evolution of the associated traffic at sub-diffraction resolution. Cargos frequently run into a blocked road or experience a traffic jam. Still, they have effective strategies to circumvent those problems: opting for an alternative mode of transport and moving together in tandem or migrating collectively. All taken together, a cell is an incredibly complex and busy space where the principle and practice of transportation intriguingly parallel those of our macroscopic world.

Revisiting Ultrafast Dynamics in Carbonate-Based Electrolytes for Li-Ion Batteries: Clarifying 2D-IR Cross-Peak Interpretation

Understanding chemical exchange in carbonate-based electrolytes employed in Li-ion batteries (LIBs) is crucial for elucidating ion transport mechanisms. Ultrafast two-dimensional (2D) IR spectroscopy has been widely used to investigate the solvation structure and dynamics of Li-ions in organic carbonate-based electrolytes. However, the interpretation of cross-peaks observed in picosecond carbonyl stretch 2D-IR spectra has remained contentious. These cross-peaks could arise from various phenomena, including vibrational couplings between neighboring carbonyl groups in the first solvation shell around Li-ions, vibrational excitation transfers between carbonyl groups in distinct solvation environments, and local heating effects. Therefore, it is imperative to resolve the interpretation of 2D-IR cross-peaks to avoid misinterpretations regarding ultrafast dynamics found in LIB carbonate-based electrolytes. In this study, we have taken a comprehensive investigation of carbonate-based electrolytes utilizing 2D-IR spectroscopy and molecular dynamics (MD) simulations. Through meticulous analyses and interpretations, we have identified that the cross-peaks observed in the picosecond 2D-IR spectra of LIB electrolytes predominantly arise from intermolecular vibrational excitation transfer processes between the carbonyl groups of Li-bound and free carbonate molecules. We further discuss the limitations of employing a picosecond 2D-IR spectroscopic technique to study chemical exchange and intermolecular vibrational excitation transfer processes, particularly when the effects of the molecular photothermal process cannot be ignored. Our findings shed light on the dynamics of LIB electrolytes and resolve the controversy related to 2D-IR cross-peaks. By discerning the origin of these features, we could provide valuable insights for the design and optimization of next-generation Li-ion batteries.

Construction of N-Rich Aminal-Linked Porous Organic Polymers for Outstanding Precombustion CO2 Capture and H2 Purification: A Combined Experimental and Theoretical Study

A large number of scientific investigations are needed for developing a sustainable solid sorbent material for precombustion CO2 capture in the integrated gasification combined cycle (IGCC) that is accountable for the industrial coproduction of hydrogen and electricity. Keeping in mind the industrially relevant conditions (high pressure, high temperature, and humidity) as well as good CO2/H2 selectivity, we explored a series of sorbent materials. An all-rounder player in this game is the porous organic polymers (POPs) that are thermally and chemically stable, easily scalable, and precisely tunable. In the present investigation, we successfully synthesized two nitrogen-rich POPs by extended Schiff-base condensation reactions. Among these two porous polymers, TBAL-POP-2 exhibits high CO2 uptake capacity at 30 bar pressure (57.2, 18.7, and 15.9 mmol g-1 at 273, 298, and 313 K temperatures, respectively). CO2/H2 selectivities of TBAL-POP-1 and 2 at 25 °C are 434.35 and 477.93, respectively. On the other hand, at 313 K the CO2/H2 selectivities of TBAL-POP-1 and 2 are 296.92 and 421.58, respectively. Another important feature to win the race in the search of good sorbents is CO2 capture capacity at room temperature, which is very high for TBAL-POP-2 (15.61 mmol g-1 at 298 K for 30 to 1 bar pressure swing). High BET surface area and good mesopore volume along with a large nitrogen content in the framework make TBAL-POP-2 an excellent sorbent material for precombustion CO2 capture and H2 purification.

Evaluation of Metastatic Potential in Esophageal and Gastroesophageal Junction (GEJ) Cancer with Adherence to Elective Nodal Volume Guidelines: A Retrospective Analysis of Elective Nodal Irradiation

PURPOSE/OBJECTIVE(S)

In 2015, expert guidelines on esophageal/GEJ cancer contouring for intensity-modulated radiation therapy (IMRT) were published in IJROBP, which delineate recommended elective nodal basins (celiac, para-aortic, gastrohepatic ligament, supraclavicular) to be irradiated depending on the primary tumor location. We hypothesize that incomplete coverage of these areas increases the risk of the development of distant failures.

MATERIALS/METHODS

Patients treated for non-metastatic esophageal or GEJ cancer with chemoradiotherapy pre-operatively or definitively from 2012 to 2021 were retrospectively identified from a single institution database. Radiation plans of eligible patients were then analyzed by tumor location. Plans were deemed guideline-compliant if radiation dose coverage, between 41.4 to 45 Gy, encompassed nodal basins recommended by the 2015 guidelines. The primary endpoint of this study was the overall rate of distant disease. Other endpoints included locoregional failures, defined as failures within the radiation field but outside of the primary tumor, and local failures within the gross tumor volume. Summary and descriptive statistics were used to define collected variables. Differences were measured using chi-square and Fisher's exact test for categorical variables and two-sided t-tests for continuous measures. Assessment of distant, locoregional, and local failures were assessed using univariate logistic regression with statistical significance at p < 0.05.

RESULTS

With a median follow-up of 25.0 months, 37 patients, with a median age of 66, were included in the study. Most patients were male (94.6%) with cT3 (54.1%), cN0 (43.2%), moderately differentiated (47.1%) adenocarcinoma (75.7%) located at the GEJ (56.8%). The median radiation dose used was 50.4 Gy, with the majority of patients receiving concurrent carboplatin and paclitaxel (83.8%). Four patients received induction chemotherapy and 20 (55.6%) underwent esophagectomy. When examining guideline compliance, 17 (46.0%) radiation plans demonstrated adequate ENI. The most common improperly covered nodal basin was para-aortic (65.0%), followed by gastrohepatic (30.0%). No patients with sufficient ENI coverage (0/17) developed distant failure compared to 45.0% (9/20) with insufficient coverage (p = 0.001). There were inappreciable differences in locoregional or local failure rates between those with and without complete ENI. Patients with complete ENI were more likely to be of larger craniocaudal length (p = 0.007) or have N2 disease (p = 0.003). When examining other tumor characteristics (histologic subtype, location, HER2 status, esophagectomy rate) of patients with and without complete ENI, no further differences were noted.

CONCLUSION

These results suggest that proper coverage of nodal basins, when indicated by expert guidelines, could improve distant metastasis. ENI analysis of previous prospective CRT studies for esophageal cancer could validate these findings.

Molecular Mechanism of Selective Al2O3 Atomic Layer Deposition on Self-Assembled Monolayers

Area-selective atomic layer deposition (AS-ALD) of insulating metallic oxide layers could be a useful nanopatterning technique for making increasingly complex semiconductor circuits. Although the alkanethiol self-assembled monolayer (SAM) has been considered promising as an ALD inhibitor, the low inhibition efficiency of the SAM during ALD processes makes its wide application difficult. We investigated the deposition mechanism of Al2O3 on alkanethiol-SAMs using temperature-dependent vibrational sum-frequency-generation spectroscopy. We found that the thermally induced formation of gauche defects in the SAMs is the main causative factor deteriorating the inhibition efficiency. Here, we demonstrate that a discontinuously temperature-controlled ALD technique involving self-healing and dissipation of thermally induced stress on the structure of SAM substantially enhances the SAM's inhibition efficiency and enables us to achieve 60 ALD cycles (6.6 nm). We anticipate that the present experimental results on the ALD mechanism on the SAM surface and the proposed ALD method will provide clues to improve the efficiency of AS-ALD, a promising nanoscale patterning and manufacturing technique.

Two-color infrared photothermal microscopy

Infrared photothermal microscopy is an infrared (IR) imaging technique that enables non-invasive, non-destructive, and label-free investigations at the sub-micrometer scale. It has been applied in various research areas targeting pharmaceutical and photovoltaic materials as well as biomolecules in living systems. Despite its potency in observing biomolecules in living organisms, its practical application for cytological research has been restricted by the deficiency of molecular information from the IR photothermal signal, due to the narrow spectral width of a quantum cascade laser that is one of the most preferred IR excitation light sources for current IR photothermal imaging (IPI) techniques. Here, we address this issue by bringing modulation-frequency multiplexing into IR photothermal microscopy for developing a two-color IR photothermal microscopy technique. We demonstrate that the two-color IPI technique can be used to obtain the IR microscopic images of two discrete IR absorption bands and to distinguish two different chemical species in live cells with a sub-micrometer spatial resolution. We anticipate that the more general multi-color IPI technique and its use for metabolic studies of live cells could be realized by extending the present modulation-frequency multiplexing method.

Discriminating Congested Vibrational Peaks of Condensed Organic Materials with Time- and Frequency-Resolved Coherent Anti-Stokes Raman Scattering Spectroscopy

The spectral congestion of highly overlapping vibrational peaks of molecules in condensed phases is a persistent challenge in conventional linear vibrational spectroscopy, making it difficult to accurately determine the spectroscopic parameters. This study demonstrates the utility of time- and frequency-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy with a time-delayed picosecond probe pulse in resolving congested C-H stretching vibrational peaks of condensed organic matters. The results show that the overlapping vibrational peaks of polymeric films and oily liquids, which are not easily distinguishable in spontaneous Raman spectroscopy, can be separated in the time-resolved CARS (tr-CARS) spectra. To understand the physical basis of the enhanced spectral resolution, we examine the time series of CARS spectra obtained by varying the delay time between the pump and probe pulses. The global fit analysis indicates that the effective suppression of faster Raman free-induction-decay components and instantaneous nonresonant background signals contributes to improved spectral resolution. Additionally, the present study reveals that the CARS spectra at a sufficient probe delay time are highly sensitive to the incident and detection polarizations, further improving vibrational peak distinguishability through polarization-controlled tr-CARS.

Axial profiling of interferometric scattering enables an accurate determination of nanoparticle size

Interferometric scattering (iSCAT) microscopy has undergone significant development in recent years. It is a promising technique for imaging and tracking nanoscopic label-free objects with nanometer localization precision. The current iSCAT-based photometry technique allows quantitative estimation for the size of a nanoparticle by measuring iSCAT contrast and has been successfully applied to nano-objects smaller than the Rayleigh scattering limit. Here we provide an alternative method that overcomes such size limitations. We take into account the axial variation of iSCAT contrast and utilize a vectorial point spread function model to uncover the position of a scattering dipole and, consequently, the size of the scatterer, which is not limited to the Rayleigh limit. We found that our technique accurately measures the size of spherical dielectric nanoparticles in a purely optical and non-contact way. We also tested fluorescent nanodiamonds (fND) and obtained a reasonable estimate for the size of fND particles. Together with fluorescence measurement from fND, we observed a correlation between the fluorescent signal and the size of fND. Our results showed that the axial pattern of iSCAT contrast provides sufficient information for the size of spherical particles. Our method enables us to measure the size of nanoparticles from tens of nanometers and beyond the Rayleigh limit with nanometer precision, making a versatile all-optical nanometric technique.

Thermoresponse of Odd-Even Effect in n-Alkanethiolate Self-Assembled Monolayers on Gold Substrates

This study examines thermoresponse of odd-even effect in self-assembled monolayers (SAMs) of n-alkanethiolates (SC_n , n=3-18) formed on template-stripped gold (Au^TS ) using macro- and microscopic analytical techniques, contact angle goniometry (CAG) and vibrational sum frequency generation (VSFG) spectroscopy, respectively. Both CAG and VSFG analyses showed that the odd-even effect in liquid-like SAMs (n=3-9) disappeared upon heating at 50-70 °C, indicating that the heating led to increased structural disorder regardless of odd and even carbon numbers. In contrast, the opposite thermoresponse was observed for odd and even SC_n molecules in wax- and solid-like SAMs (n=10-18). Namely, temperature-dependent orientational change of terminal CH₃ relative to the surface normal was opposite for the odd and even molecules, thereby leading to mitigated odd-even effect. Our work offers important insights into thermoresponse of supramolecular structure in condensed organic matter.

Micro-Raman spectroscopic analysis of liquid-liquid phase separation

Liquid-liquid phase separation (LLPS) plays a significant role in various biological processes, including the formation of membraneless organelles and pathological protein aggregation. Although many studies have found various factors that modulate the LLPS process or the liquid-to-solid phase transition (LSPT) using microscopy or fluorescence-based methods, the molecular mechanistic details underlying LLPS and protein aggregation within liquid droplets remain uncharacterized. Therefore, structural information on proteins inside liquid droplets is required to understand the mechanistic link to amyloid formation. In the present study, we monitored droplet formation related to protein fibrillation using micro-Raman spectroscopy in combination with differential interference contrast (DIC) microscopy to study the conformational change in proteins and the hydrogen-bonding (H-bonding) structure of water during LLPS. Interestingly, we found that the O-D stretching band for water (HOD in H₂O) inside the droplets exhibited a distinct Raman spectrum from that of the bulk water, suggesting that the time-dependent change in the hydration environment in the protein droplets during the process of LLPS can be studied. These results demonstrate that the superior spatial resolution of micro-Raman spectroscopy offers significant advantages in investigating the molecular mechanisms of LLPS and following LSPT processes.

Molecular Photothermal Effects on Time-Resolved IR Spectroscopy: Solute-Solvent Intermolecular Energy Transfer

Time-resolved IR pump-probe (IR-PP) and two-dimensional IR (2D-IR) spectroscopy are valuable tools for studying ultrafast chemical and biological processes in solutions. However, the corresponding signals at long times are obscured by the molecular photothermal effects resulting from the heat dissipation of vibrationally photoexcited molecules to the surroundings. Recently, a phenomenology model was used to describe molecular photothermal effects on IR-PP signals and the diagonal and cross-peaks of 2D-IR spectra at long pump-probe delay times. Here, we consider the thermal diffusion equation with a time-dependent heat source term to describe the solute-solvent energy transfer process. An approximate solution to the nonhomogeneous differential equation shows that the molecular photothermal effect is determined by the mean intermolecular distance between IR-absorbing molecules. We show that the time profile of heat dissipation from a vibrationally excited molecule to the surroundings, which provides information about the mechanisms involved in the solute-solvent intermolecular energy transfer process in solutions, can be directly measured by analyzing the molecular photothermal IR-PP and 2D-IR signals. We anticipate that the present work can be used to interpret local heating-induced time-resolved IR spectroscopic signals and understand the rate of and the mechanisms involved in the conversion from high-frequency molecular vibrational energy to solvent kinetic energy in condensed phases.

Molecular photothermal effects on time-resolved IR spectroscopy

Time-resolved IR pump-probe (IR-PP) and two-dimensional IR (2D-IR) spectroscopy are valuable techniques for studying various ultrafast chemical and biological processes in solutions. The time-dependent changes of nonlinear IR signals reflecting fast molecular processes such as vibrational energy transfer and chemical exchange provide invaluable information on the rates and mechanisms of solvation dynamics and structural transitions of multi-species vibrationally interacting molecular systems. However, due to the intrinsic difficulties in distinguishing the contributions of molecule-specific processes to the time-resolved IR signals from those resulting from local heating, it becomes challenging to interpret time-resolved IR-PP and 2D-IR spectra exhibiting transient growing-in spectral components and cross-peaks unambiguously. Here, theoretical considerations of various effects of vibrational coupling, energy transfer, chemical exchange, the generation of hot ground states, molecular photothermal process, and their combinations on the lineshapes and time-dependent intensities of IR-PP spectra and 2D-IR diagonal and cross-peaks are presented. We anticipate that the present work will help researchers using IR pump-probe and 2D-IR techniques to distinguish local heating-induced photothermal signals from genuine nonlinear IR signals.

Lithium-Ion Solvation Structure in Organic Carbonate Electrolytes at Low Temperatures

Lithium-ion batteries face insufficient capacity at low temperatures. The lithium-ion desolvation process in the vicinity of a solid electrolyte interphase (SEI) layer is considered the major problem. Thus, an accurate determination of lithium-ion solvation structures is a prerequisite for understanding this process. Here, using a cryostat combined with an FTIR spectrometer, we found that as the temperature decreased, the number of coordinating carbonates in the first solvation shell of the lithium ion increased with a decreased population of the contact ion pair (CIP). More specifically, we found that two or more carbonate molecules replace a single PF₆^- anion upon CIP dissociation. This experimental finding shows that the prevailing notion that four carbonate molecules coordinate each lithium ion to form a tetrahedral structure is invalid for describing lithium-ion solvation structures. We anticipate that the present work will elucidate one of the molecular origins behind the low performance of lithium-ion batteries at low temperatures.

Silver Nanoclusters Serve as Fluorescent Rivets Linking Hoogsteen Triplex DNA and Hairpin-Loop DNA Structures

Greater understanding of the mutual influence between DNA and the associated nanomaterial on the properties of each other can provide alternative strategies for designing and developing DNA nanomachines. DNA secondary structures are essential for encapsulating highly emissive silver nanoclusters (DNA/AgNCs). Likewise, AgNCs stabilize secondary DNA structures, such as hairpin DNA, duplex DNA, and parallel-motif DNA triplex. In this study, we found that the fluorescence of AgNCs encapsulated within a Hoogsteen triplex DNA structure can be turned on and off in response to pH changes. We also show that AgNCs can act as nanoscale rivets, linking two functionally distinctive DNA nanostructures. For instance, we found that a Hoogsteen triplex DNA structure with a seven-cytosine loop encapsulates red fluorescent AgNCs. The red fluorescence faded under alkaline conditions, whereas the fluorescence was restored in a near-neutral environment. Hairpin DNA and random DNA structures did not exhibit this pH-dependent AgNCs fluorescence. A fluorescence lifetime measurement and a small-angle X-ray scattering analysis showed that the triplex DNA-encapsulated AgNCs were photophysically convertible between bright and dark states. An in-gel electrophoresis analysis indicated that bright and dark convertibility depended on the AgNCs-riveted dimerization of the triplex DNAs. Moreover, we found that AgNCs rivet the triplex DNA and hairpin DNA to form a heterodimer, emitting orange fluorescence. Our findings suggest that AgNCs between two cytosine-rich loops can be used as nanorivets in designing noncanonical DNA origami beyond Watson-Crick base pairing.

Label-Free Live-Cell Imaging of Internalized Microplastics and Cytoplasmic Organelles with Multicolor CARS Microscopy

As the bioaccumulation of microplastics (MPs) is considered as a potential health risk, many efforts have been made to understand the cellular dynamics and cytotoxicity of MPs. Here, we demonstrate that label-free multicolor coherent anti-Stokes Raman scattering (CARS) microscopy enables separate vibrational imaging of internalized MPs and lipid droplets (LDs) with indistinguishable shapes and sizes in live cells. By simultaneously obtaining polystyrene (PS)- and lipid-specific CARS images at two very different frequencies, 1000 and 2850 cm^-1, respectively, we successfully identify the local distribution of ingested PS beads and native LDs in Caenorhabditis elegans. We further show that the movements of PS beads and LDs in live cells can be separately tracked in real time, which allows us to characterize their individual intracellular dynamics. We thus anticipate that our multicolor CARS imaging method could be of great use to investigate the cellular transport and cytotoxicity of MPs without additional efforts for pre-labeling to MPs.

Midwavelength Infrared Colloidal Nanowire Laser

Realizing bright colloidal infrared emitters in the midwavelength infrared (or mid-IR), which can be used for low-power IR light-emitting diodes (LEDs), sensors, and deep-tissue imaging, has been a challenge for the last few decades. Here, we present colloidal tellurium nanowires with strong emission intensity at room temperature and even lasing at 3.6 μm (ω) under cryotemperature. Furthermore, the second-harmonic field at 1.8 μm (2ω) and the third-harmonic field at 1.2 μm (3ω) are successfully generated thanks to the intrinsic property of the tellurium nanowire. These unique optical features have never been reported for colloidal tellurium nanocrystals. With the colloidal midwavelength infrared (MWIR) Te nanowire laser, we demonstrate its potential in biomedical applications. MWIR lasing has been clearly observed from nanowires embedded in a human neuroblastoma cell, which could further realize deep-tissue imaging and thermotherapy in the near future.

Vibrational Modes Promoting Exciton Relaxation in the B850 Band of LH2

Exciton relaxation dynamics in multichromophore systems are often modeled using Redfield theory, where bath fluctuations mediate the relaxation among the exciton eigenstates. Identifying the vibrational or phonon modes that are implicated in exciton relaxation allows more detailed understanding of exciton dynamics. Here we focus on a well-studied light-harvesting II complex (LH2) isolated from the photosynthetic purple bacterium Rhodoblastus acidophilus strain 10050. Using two synchronized mode-locked lasers, we carried out a polarization-dependent two-dimensional electronic spectroscopy (2DES) study of an ultrafast exciton relaxation in the B850 band of LH2. 2DES data with different polarization configurations enable us to investigate the exciton relaxation between the k = ±1 exciton states. Then, we identify vibrational modes coupled to the exciton relaxation by analyzing the coherent wavepackets in the 2DES signals. Focusing on the coherent vibrational wavepackets, the data suggest that certain symmetry-breaking modes of monomeric units play a key role in exciton relaxation.

TfNN15N: A γ-15N-Labeled Diazo-Transfer Reagent for the Synthesis of β-15N-Labeled Azides

Azides are infrared (IR) probes that are important for structure and dynamics studies of proteins. However, they often display complex IR spectra owing to Fermi resonances and multiple conformers. Isotopic substitution of azides weakens the Fermi resonance, allowing more accurate IR spectral analysis. Site-specifically ¹⁵N-labeled aromatic azides, but not aliphatic azides, are synthesized through nitrosation. Both ¹⁵N-labeled aromatic and aliphatic azides are synthesized through nucleophilic substitution or diazo-transfer reaction but as an isotopomeric mixture. We present the synthesis of TfNN¹⁵N, a γ-¹⁵N-labeled diazo-transfer reagent, and its use to prepare β-¹⁵N-labeled aliphatic as well as aromatic azides.

Direct observation of protein structural transitions through entire amyloid aggregation processes in water using 2D-IR spectroscopy

Amyloid proteins that undergo self-assembly to form insoluble fibrillar aggregates have attracted much attention due to their role in biological and pathological significance in amyloidosis. This study aims to understand the amyloid aggregation dynamics of insulin (INS) in H₂O using two-dimensional infrared (2D-IR) spectroscopy. Conventional IR studies have been performed in D₂O to avoid spectral congestion despite distinct H–D isotope effects. We observed a slowdown of the INS fibrillation process in D₂O compared to that in H₂O. The 2D-IR results reveal that different quaternary structures of INS at the onset of the nucleation phase caused the distinct fibrillation pathways of INS in H₂O and D₂O. A few different biophysical analysis, including solution-phase small-angle X-ray scattering combined with molecular dynamics simulations and other spectroscopic techniques, support our 2D-IR investigation results, providing insight into mechanistic details of distinct structural transition dynamics of INS in water. We found the delayed structural transition in D₂O is due to the kinetic isotope effect at an early stage of fibrillation of INS in D₂O, i.e., enhanced dimer formation of INS in D₂O. Our 2D-IR and biophysical analysis provide insight into mechanistic details of structural transition dynamics of INS in water. This study demonstrates an innovative 2D-IR approach for studying protein dynamics in H₂O, which will open the way for observing protein dynamics under biological conditions without IR spectroscopic interference by water vibrations.

This study aims to understand the structural transition dynamics of INS during amyloid aggregation in H₂O using 2D-IR spectroscopy. The results show that distinct fibrillations in D₂O and H₂O originated from different quaternary structures of INS.

Coherent Nonlinear Spectroscopy with Multiple Mode-Locked Lasers

Coherent multidimensional spectroscopy has been widely used to study the structure and dynamics of chemical and biological systems. Each ultrashort pulse from a single mode-locked laser is split into multiple pulses by beam splitters. Their arrival times at a given molecular sample are controlled with mechanical time-delay generators for time-resolved measurements of molecular responses. Such nonlinear vibrational, electronic, or vibrational-electronic spectroscopy can now be carried out with multiple mode-locked lasers with highly stabilized repetition and sometimes carrier-envelope-offset frequencies. By precisely controlling the repetition frequencies of multiple mode-locked lasers, one can achieve automatic delay time scanning, known as asynchronous optical sampling, to investigate various relaxation processes associated with photochemical or photobiological phenomena at one sweep in time. In this Perspective, the current developments and applications of multiple mode-locked laser-based techniques to time-resolved nonlinear spectroscopy of chromophores in condensed phases are discussed. The author's perspective on this approach is also presented.

Time-Variable Chiroptical Vibrational Sum-Frequency Generation Spectroscopy of Chiral Chemical Solution

Vibrational sum-frequency generation (VSFG) spectroscopy, a surface-specific technique, was shown to be useful even for characterizing the vibrational optical activity of chiral molecules in isotropic bulk liquids. However, accurately determining the spectroscopic parameters is still challenging because of the spectral congestion of chiroptical VSFG peaks with different amplitudes and phases. Here, we show that a time-variable infrared-visible chiroptical three-wave-mixing technique can be used to determine the spectroscopic parameters of second-order vibrational response signals from chiral chemical liquids. For varying the delay time between infrared and temporally asymmetric visible laser pulses, we measure the chiral VSFG, achiral VSFG, and their interference spectra of bulk R-(+)-limonene liquid and perform a global fitting analysis for those time-variable spectra to determine their spectroscopic parameters accurately. We anticipate that this time-variable VSFG approach will be useful for developing nearly background-free chiroptical characterization techniques with enhanced spectral resolution.

Machine Learning Approach for Describing Water OH Stretch Vibrations

A machine learning approach employing neural networks is developed to calculate the vibrational frequency shifts and transition dipole moments of the symmetric and antisymmetric OH stretch vibrations of a water molecule surrounded by water molecules. We employed the atom-centered symmetry functions (ACSFs), polynomial functions, and Gaussian-type orbital-based density vectors as descriptor functions and compared their performances in predicting vibrational frequency shifts using the trained neural networks. The ACSFs perform best in modeling the frequency shifts of the OH stretch vibration of water among the types of descriptor functions considered in this paper. However, the differences in performance among these three descriptors are not significant. We also tried a feature selection method called CUR matrix decomposition to assess the importance and leverage of the individual functions in the set of selected descriptor functions. We found that a significant number of those functions included in the set of descriptor functions give redundant information in describing the configuration of the water system. We here show that the predicted vibrational frequency shifts by trained neural networks successfully describe the solvent-solute interaction-induced fluctuations of OH stretch frequencies.

Adsorbed Water Structure on Acrylate-Based Biocompatible Polymer Surface

The role of water in the excellent biocompatibility of the acrylate-based polymers widely used for antibiofouling coating material has been realized previously. Here, we report femtosecond mid-infrared pump-probe spectroscopy of the OD stretch band of HOD molecule adsorbed on highly biocompatible poly(2-methoxyethyl) acrylate [PMEA] and poorly biocompatible poly(2-phenoxyethyl) acrylate [PPEA], both of which reveal that there are two water species with significantly different vibrational lifetime. PMEA interacts more strongly with water than PPEA through the H-bonding interaction between carbonyl (C═O) and water. The vibrational lifetime of the OD stretch in PPEA is notably longer by factors of 3 and 7 than those in PMEA and bulk water, respectively. The IR-pump visible-probe photothermal imaging further unravels substantial spatial overlap between polymer CO group and water for hydrated PMEA and a significant difference in surface morphology than those in PPEA, which exhibits the underlying relationships among polymer-water interaction, surface morphology, and biocompatibility.

Solvation Structure around Li+ Ions in Organic Carbonate Electrolytes: Spacer-Free Thin Cell IR Spectroscopy

Organic carbonate electrolytes are widely used materials for lithium-ion batteries. However, detailed solvation structures and solvent coordination numbers (CNs) of lithium cations in such solutions have not been accurately described nor determined yet. Because transmission-type IR spectroscopy is not of use for measuring the carbonyl stretch modes of electrolytes due to their absorption saturation problem, we here show that simple spacer-free thin cell IR spectroscopy can provide quantitative information on the number of solvating carbonate molecules around each lithium ion. We could estimate the solvent (carbonate) CNs of lithium ions in dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propylene carbonate, and butylene carbonate over a wide range of lithium salt concentrations accurately, and they are compared with the previous results obtained with attenuated total reflection IR spectroscopy technique. We anticipate that our spacer-free thin cell approach will potentially be used to investigate the solvation dynamics, chemical exchange process, and vibrational energy transfers between solvating carbonate molecules in lithium salt electrolytes when combined with time-resolved IR spectroscopy.

Quantitative complementarity of wave-particle duality

To test the principle of complementarity and wave-particle duality quantitatively, we need a quantum composite system that can be controlled by experimental parameters. Here, we demonstrate that a double-path interferometer consisting of two parametric downconversion crystals seeded by coherent idler fields, where the generated coherent signal photons are used for quantum interference and the conjugate idler fields are used for which-path detectors with controllable fidelity, is useful for elucidating the quantitative complementarity. We show that the quanton source purity μ _s is tightly bounded by the entanglement E between the quantons and the remaining degrees of freedom by the relation [Formula: see text], which is experimentally confirmed. We further prove that the experimental scheme using two stimulated parametric downconversion processes is an ideal tool for investigating and understanding wave-particle duality and Bohr's complementarity quantitatively.

Substituent Effects on the Vibrational Properties of the CN Stretch Mode of Aromatic Nitriles: IR Probes Useful for Time-resolved IR Spectroscopy

Developing ideal IR probes is essential to understand the structure and dynamics of biomolecules with time-resolved IR spectroscopies and imaging techniques. Especially, nitrile (CN) group has recently been proposed to serve as IR probes of the local environment of proteins. Herein, we investigated the effect of a substituent on the vibrational properties of the benzonitrile. The electron-donating and withdrawing character of p-substituent on benzonitrile are expected to modulate the vibrational frequency, molar extinction coefficient, and vibrational lifetime of CN probe. FT-IR revealed the positive correlation between electron-donating character and the molar extinction coefficient of CN stretch mode. Infrared pump-probe (IR-PP) measurements showed that the vibrational lifetime of CN stretch mode exhibits a relatively weak correlation with the electron-donating strength. Among the investigated samples, 4-dimethylamino benzonitrile with the strongest electron-donating strength shows enhanced absorption and extended vibrational lifetime. Utilizing substituent effects will be a practical strategy to improve the performance of the IR probe.

Low-Frequency Vibronic Mixing Modulates the Excitation Energy Flow in Bacterial Light-Harvesting Complex II

Oscillatory features observed in two-dimensional electronic spectroscopy (2DES) manifest coherent vibrational and electronic dynamics and even the interplay of them. Recently, we developed a 2DES technique utilizing a pair of synchronized repetition-frequency-stabilized lasers, which enables the wide dynamic range measurements of 2DES signals rapidly. Here, we apply this dual-laser 2DES technique to investigate the electronic energy transfer (EET) process in bacterial light-harvesting complex II consisting of B800 and B850 circular aggregates at ambient temperature, and the coherent vibrational wavepakcet associated with the EET between the two aggregates are measured. Examining the principal component analysis of the time-resolved 2DES signal, we found that the EET from B800 to low-lying B850 states is modulated by a low-frequency (156 cm^-1) vibrational mode of the exciton donor (B800). This observation suggests that the donor transition density is modulated by this vibration, which, in turn, modulates the energy transfer dynamics.

Real-Time Reaction Monitoring with In Operando Flow NMR and FTIR Spectroscopy: Reaction Mechanism of Benzoxazole Synthesis

In operando observation of reaction intermediates is crucial for unraveling reaction mechanisms. To address the sensitivity limitations of commercial ReactIR, a flow cell was integrated with a Fourier transform infrared (FTIR) spectrometer yielding a "flow FTIR" device coupled with an NMR spectrometer for the elucidation of reaction mechanisms. The former device detects the low-intensity IR peaks of reaction intermediates by adjusting the path length of the FTIR sample cell, whereas the flow NMR allows the quantitative analysis of reaction species, thus offsetting the limitations of IR spectroscopy resulting from different absorption coefficients of the normal modes. Using the flow NMR and FTIR device, the controversial mechanism of benzoxazole synthesis was conclusively determined by spectroscopic evaluation of the reaction intermediates. This system enabled the accurate acquisition of previously elusive kinetic data, such as the reaction time and rate-determining step. The implementation of reaction flow cells into NMR and FTIR systems could be widely applied to study various reaction mechanisms, including dangerous and harsh reactions, thus avoiding contact with potentially harmful reaction intermediates.

Broadband Infrared Spectroscopy of Molecules in Solutions with Two Intrapulse Difference-Frequency-Generated Mid-Infrared Frequency Combs

Mid-infrared (mid-IR) spectroscopy is an incisive tool for studying structures and dynamics of complicated molecules in condensed phases. Developing a compact and broadband mid-IR spectrometer has thus been a long-standing challenge. Here, we show that a highly coherent and broadband mid-IR frequency comb can be generated by using an intrapulse difference-frequency-generation with a train of pulses from a few-cycle pulse Ti:sapphire oscillator. By tightly focusing the oscillator output beam into a single-pass, fan-out-type periodically poled lithium niobate crystal and tilting the orientation of the crystal, we show that a mid-IR frequency comb with more than an octave spectral bandwidth from 1550 cm^-1 (46 THz) to 3650 cm^-1 (110 THz) and vanishing carrier-envelope-offset phase can be generated. Using two coherent mid-IR frequency combs with different repetition frequencies, we demonstrate that a broadband mid-IR dual-frequency comb spectroscopy of aromatic compounds or amino acids in solutions is feasible. We thus anticipate that researchers will find our mid-IR frequency combs useful for developing ultrafast and broadband linear and nonlinear IR spectroscopy of chemically reactive or biologically important molecules in condensed phases.

Two-dimensional IR spectroscopy reveals a hidden Fermi resonance band in the azido stretch spectrum of β-azidoalanine

Azido stretch modes in a variety of azido-derivatized nonnatural amino acids and nucleotides have been used as a site-specific infrared (IR) probe for monitoring changes in their conformations and local electrostatic environments. The vibrational bands of azide probes are often accompanied by complex line shapes with shoulder peaks, which may arise either from incomplete background subtraction, Fermi resonance, or multiple conformers. The isotope substitution in the infrared probe has thus been introduced to remove Fermi resonances without causing a significant perturbation to the structure. Here, we synthesized and labeled the mid-N atoms of aliphatic azide derivatives with 15N to study the effects of isotope labelling on their vibrational properties. The FT-IR spectra of the aliphatic azide with asymmetric lineshape became a single symmetric band upon isotope substitution, which might be an indication of the removal of the hidden Fermi resonance from the system. We also noticed that the 2D-IR spectrum of unlabeled aliphatic azide has cross-peaks, even though it is not apparently identifiable. The 1D slice spectra obtained from the 2D-IR spectra reveal the existence of a hidden Fermi resonance peak. Furthermore, we show that this weak Fermi resonance does not produce discernible oscillatory beating patterns in the IR pump-probe spectrum, which has been used as evidence of the Fermi resonance. Therefore, we confirm that isotope labelling combined with 2D-IR spectroscopy is the most efficient and incisive way to identify the origin of small shoulder peaks in the linear and nonlinear vibrational spectra of various IR probe molecules.

Fluorescence-Combined Interferometric Scattering Imaging Reveals Nanoscale Dynamic Events of Single Nascent Adhesions in Living Cells

Focal adhesions (FAs) are dynamic protein nanostructures that form mechanical links between cytoskeletal actin fibers and the extracellular matrix. Here, we demonstrate that interferometric scattering (iSCAT) microscopy, a high-speed and time-unlimited imaging technique, can uncover the real-time dynamics of nanoscopic nascent adhesions (NAs). The high sensitivity and stability of the iSCAT signal enabled us to trace the whole life span of each NA spontaneously nucleated under a lamellipodium. Such high-throughput and long-term image data provide a unique opportunity for statistical analysis of adhesion dynamics. Moreover, we directly revealed that FAs play critical roles in both the extrusion of filopodia as nucleation sites on the leading edge and the one-dimensional transport of cargos along cytoskeletal fibers as fiber docking sites. These experimental results show that iSCAT is a sensitive tool for tracking real-time dynamics of nanoscopic objects involved in endogenous and exogenous biological processes in living cells.