Fermi Resonance of the N-D Stretching Mode Probing the Local Hydrogen-Bonding Environment in Proteins.

Local H-bonding interactions are crucial for proteins to undergo various structural transitions and form different secondary structures. However, identifying slight distinctions in the local H-bonding of proteins is rather challenging. Here, we demonstrate that the Fermi resonance of the N-D stretching mode can provide an effective probe for the localized H-bonding environment of proteins both at the surface/interface and in the bulk. Using sum frequency generation vibrational spectroscopy and infrared spectroscopy, we established a correlation between the Fermi resonance of the N-D mode and protein secondary structures. The H-bond of N-D···C═O splits the N-D modes into two peaks (∼2410 and ∼2470 cm-1). The relative strength ratio (R) between the ∼2410 cm-1 peak and the ∼2470 cm-1 peak is very sensitive to H-bond strength and protein secondary structure. R is less than 1 for α-helical peptides, while R is greater than 1 for ß-sheet peptides. For R < 2.5, both α-helical/loop structures and ß-sheet structures exhibit almost identical Fermi coupling strengths (W = 28 cm-1). For R > 2.5, W decreases from 28 to 14 cm-1 and depends on the aggregation degree of the ß-sheet oligomers or fibrils. The initial local H-bonding status impacts the misfolding dynamics of proteins at the lipid bilayer interface.

Probing the Local Near-Field Intensity of Plasmonic Nanoparticles in the Mid-infrared Spectral Region.

The enhanced local field of gold nanoparticles (AuNPs) in mid-infrared spectral regions is essential for improving the detection sensitivity of vibrational spectroscopy and mediating photochemical reactions. However, it is still challenging to measure its intensity at subnanometer scales. Here, using the NO2 symmetric stretching mode (νNO2) of self-assembled 4-nitrothiophenol (4-NTP) monolayers on AuNPs as a model, we demonstrated that the percentage of excited νNO2 mode, determined by femtosecond time-resolved sum-frequency generation vibrational spectroscopy, allows us to directly detect the local field intensity of the AuNP surface in subnanometer ranges. The local-field intensity is tuned by AuNP diameters. An approximate 17-fold enhancement was observed for the local field on 80 nm AuNPs compared to the Au film. Additionally, the local field can regulate the anharmonicity of the νNO2 mode by synergistic effect with molecular orientation. This work offers a promising approach to probe the local field intensity distribution around plasmonic NP surfaces at subnanometer scales.

Two-Dimensional Conjugated Metal-Organic Frameworks with a Ring-in-Ring Topology and High Electrical Conductance.

Electrically conducting two-dimensional (2D) metal-organic frameworks (MOFs) have garnered significant interest due to their remarkable structural tunability and outstanding electrical properties. However, the design and synthesis of high-performance materials face challenges due to the limited availability of specific ligands and pore structures. In this study, we have employed a novel highly branched D3h symmetrical planar conjugated ligand, dodechydroxylhexabenzotrinaphthylene (DHHBTN) to fabricate a series of 2D conductive MOFs, named M-DHHBTN (M=Co, Ni, and Cu). This new family of MOFs offers two distinct types of pores, elevating the structural complexity of 2D conductive MOFs to a more advanced level. The intricate tessellation patterns of the M-DHHBTN are elucidated through comprehensive analyses involving powder X-ray diffraction, theoretical simulations, and high-resolution transmission electron microscope. Optical-pump terahertz-probe spectroscopic measurements unveiled carrier mobility in DHHBTN-based 2D MOFs spanning from 0.69 to 3.10 cm2 V-1 s-1. Among M-DHHBTN famility, Cu-DHHBTN displayed high electrical conductivity reaching 0.21 S cm-1 at 298 K with thermal activation behavior. This work leverages the "branched conjugation" of the ligand to encode heteroporosity into highly conductive 2D MOFs, underscoring the significant potential of heterogeneous double-pore structures for future applications.

Intermolecular Protein-Water Coupling Impedes the Coupling Between the Amide A and Amide I Mode in Interfacial Proteins.

The coupling between different vibrational modes in proteins is essential for chemical dynamics and biological functions and is linked to the propagation of conformational changes and pathways of allosteric communication. However, little is known about the influence of intermolecular protein-H2O coupling on the vibrational coupling between amide A (NH) and amide I (C═O) bands. Here, we investigate the NH/CO coupling strength in various peptides with different secondary structures at the lipid cell membrane/H2O interface using femtosecond time-resolved sum frequency generation vibrational spectroscopy (SFG-VS) in which a femtosecond infrared pump is used to excite the amide A band, and SFG-VS is used to probe transient spectral evolution in the amide A and amide I bands. Our results reveal that the NH/CO coupling strength strongly depends on the bandwidth of the amide I mode and the coupling of proteins with water molecules. A large extent of protein-water coupling significantly reduces the delocalization of the amide I mode along the peptide chain and impedes the NH/CO coupling strength. A large NH/CO coupling strength is found to show a strong correlation with the high energy transfer rate found in the light-harvesting proteins of green sulfur bacteria, which may understand the mechanism of energy transfer through a molecular system and assist in controlling vibrational energy transfer by engineering the molecular structures to achieve high energy transfer efficiency.

Probing the Molecular Structure and Dynamics of Membrane-Bound Proteins during Misfolding Processes by Sum-Frequency Generation Vibrational Spectroscopy.

Protein misfolding and amyloid formation are implicated in the protein dysfunction, but the underlying mechanism remains to be clarified due to the lack of effective tools for detecting the transient intermediates. Sum frequency generation vibrational spectroscopy (SFG-VS) has emerged as a powerful tool for identifying the structure and dynamics of proteins at the interfaces. In this review, we summarize recent SFG-VS studies on the structure and dynamics of membrane-bound proteins during misfolding processes. This paper first introduces the methods for determining the secondary structure of interfacial proteins: combining chiral and achiral spectra of amide A and amide I bands and combining amide I, amide II, and amide III spectral features. To demonstrate the ability of SFG-VS in investigating the interfacial protein misfolding and amyloid formation, studies on the interactions between different peptides/proteins (islet amyloid polypeptide, amyloid ß, prion protein, fused in sarcoma protein, hen egg-white lysozyme, fusing fusion peptide, class I hydrophobin SC3 and class II hydrophobin HFBI) and surfaces such as lipid membranes are discussed. These molecular-level studies revealed that SFG-VS can provide a unique understanding of the mechanism of interfacial protein misfolding and amyloid formation in real time, in situ and without any exogenous labeling.

Orientation of Thiocyanate Ions Tuning the Electron-Phonon Interactions in Pseudohalide Perovskites.

Although the importance of electron-phonon interactions on the optoelectronic properties of perovskites has been well documented, the structural origin of electron-phonon interactions remains largely unexplored. In this study, using pseudohalide perovskites Cs2Pb(SCN)2I2(1-x)Br2x as a model, we have revealed how the orientation of SCN- anions tunes the electron-phonon interactions and the effective charge-carrier mobility by utilizing femtosecond sum frequency generation vibrational spectroscopy, supplemented by photoluminescence spectroscopy and femtosecond optical-pump terahertz-probe spectroscopy. The coupling between neighboring SCN- anions decreases as the Br content (x) increases but does not have a significant effect on the electron-phonon interactions. In contrast, the orientation angle of SCN- anions has a strong correlation with the electron-phonon interaction and effective charge-carrier mobility, that is, a more parallel orientation of SCN- anions leads to a higher electron-phonon interaction and lower effective charge-carrier mobility. This finding provides a molecule-level understanding of the inorganic lattice structure in tuning electron-phonon interactions and may offer valuable guidance for optimizing the optoelectronic properties of perovskites.

Visualizing Water Monomers and Chiral OH-(H2O) Complexes Infiltrated in a Macroscopic Hydrophobic Teflon Matrix.

Insights into the interaction of fluoroalkyl groups with water are crucial to understanding the polar hydrophobicity of fluorinated compounds, such as Teflon. While an ordered hydrophobic-like 2D water layer has been demonstrated to be present on the surface of macroscopically hydrophobic fluorinated polymers, little is known about how the water infiltrates into the Teflon and what is the molecular structure of the water infiltrated into the Teflon. Using highly sensitive femtosecond sum frequency generation vibrational spectroscopy (SFG-VS), we observe for the first time that monomeric H2O and chiral OH-(H2O) complexes are present in macroscopically hydrophobic Teflon. The species are inhomogeneously distributed inside the Teflon matrix and at the Teflon surface. No water clusters or single-file water "wires" are observed in the matrix. SFG free induction decay (SFG-FID) experiments demonstrate that the OH oscillators of physically absorbed molecular water at the surface dephase on the time scale of <230 fs, whereas the water monomers and hydrated hydroxide ions infiltrated in the Teflon matrix dephase much more slowly (680-830 fs), indicating that the embedded monomeric H2O and OH-(H2O) complexes are decoupled from the outer environment. Our findings can well interpret ultrafast water permeation through fluorous nanochannels and the charging mechanism of Teflon, which may tailor the desired applications of organofluorines.

Determination of the Thickness of Interfacial Water by Time-Resolved Sum-Frequency Generation Vibrational Spectroscopy.

The physics and chemistry of a charged interface are governed by the structure of the electrical double layer (EDL). Determination of the interfacial water thickness (diw) of the charged interface is crucial to quantitatively describe the EDL structure, but it can be utilized with very scarce experimental methods. Here, we propose and verify that the vibrational relaxation time (T1) of the OH stretching mode at 3200 cm-1, obtained by time-resolved sum frequency generation vibrational spectroscopy with ssp polarizations, provides an effective tool to determine diw. By investigating the T1 values at the SiO2/NaCl solution interface, we established a time-space (T1-diw) relationship. We find that water has a T1 lifetime of ≥0.5 ps for diw ≤ 3 Å, while it displays bulk-like dynamics with T1 ≤ 0.2 ps for diw ≥ 9 Å. T1 decreases as diw increases from ∼3 Å to 9 Å. The hydration water at the DPPG lipid bilayer and LK15ß protein interfaces has a thickness of ≥9 Å and shows a bulk-like feature. The time-space relationship will provide a novel tool to pattern the interfacial topography and heterogeneity in Ångstrom-depth resolution by imaging the T1 values.

Stacking Arrangement and Orientation of Aromatic Cations Tune Bandgap and Charge Transport of 2D Organic-Inorganic Hybrid Perovskites.

Chemical modifications on aromatic spacers of 2D perovskites have been demonstrated to be an effective strategy to simultaneously improve optoelectronic properties and stability. However, its underlying mechanism is poorly understood. By using 2D phenyl-based perovskites ([C6 H5 (CH2 )m NH3 ]2 PbI4 ) as models, the authors have revealed how the chemical nature of aromatic cations tunes the bandgap and charge transport of 2D perovskites by utilizing sum-frequency generation vibrational spectroscopy to determine the stacking arrangement and orientation of aromatic cations. It is found that the antiparallel slip-stack arrangement of phenyl rings between adjacent layers induces an indirect band gap, resulting in anomalous carrier dynamics. Incorporation of the CH2 moiety causes stacking rearrangement of the phenyl ring and thus promotes an indirect to direct bandgap transition. In direct-bandgap perovskites, higher carrier mobility correlates with a larger orientation angle of the phenyl ring. Further optimizing the orientation angle by introducing a para-substituted element in a phenyl ring, higher carrier mobility is obtained. This work highlights the importance of leveraging stacking arrangement and orientation of the aromatic cations to tune the photophysical properties, which opens up an avenue for advancing high-performance 2D perovskites optoelectronics via molecular engineering.

Transcriptome analysis of Chinese sucker (Myxocyprinus asiaticus) head kidney and discovery of key immune-related genes to cold stress after swimming fatigue.

For Chinese sucker (Myxocyprinus asiaticus), passing through a dam with fast flow and cold water are always unavoidable, and this process can cause stress, disease or even death. In this study, comparative transcriptome analysis was conducted to investigate the potential immune mechanism in head kidney of M. asiaticus with swimming fatigue stress and cold stress after fatigue. In general, a total of 181,781 unigenes were generated, and 38,545 differentially expressed genes (DEGs) were identified. In these DEGs, 22,593, 7286 and 8666 DEGs were identified among groups of fatigue vs. cold, control vs. cold, and control vs. fatigue, respectively. Enrichment analysis revealed these DEGs were involved in coagulation cascades and complement, natural killer cell mediated cytotoxicity, antigen processing and presentation, Toll-like receptor signaling pathways, and chemokine signaling pathway. Notably, immune genes including heat shock protein 4a (HSP4a), HSP70 and HSP90α genes were significantly up-regulated in fishes with cold stress after fatigue. Differently, more immune genes in control vs. cold compared with that in control vs. fatigue were significantly down-regulated expression, such as claudin-15-like, Toll-like receptor 13, antimicrobial peptide (hepcidin), immunoglobulin, CXCR4 chemokine receptor, T-cell receptor, complement factor B/C2-A3, and interleukin 8. In this study, the number of DEGs in the head kidney was less than that our previous study in the spleen, which we speculated was more sensitive to changes in water temperature than the head kidney. In summary, lots of immune-related genes in the head kidney were down-regulated under cold stress after fatigue, suggesting that M. asiaticus might have experienced severe immunosuppression in the process of passing through the dam.

Lactiplantibacillus plantarum attenuates Coxsackievirus B3-induced pancreatitis through the BAX/BCL2/CASP3 signaling pathway.

Lactiplantibacillus plantarum is a lactic acid bacterium widely used in food production. Coxsackievirus B3 (CVB3) is an important human pathogen associated with acute pancreatitis development, and no antiviral therapeutics or vaccines are approved to treat or prevent its infection. However, whether L. plantarum could inhibit CVB3 infection remains unclear. Here, L. plantarum FLPL05 showed antiviral activity against CVB3 infection in vivo and in vitro. Pretreatment with L. plantarum FLPL05 reduced serum amylase levels, CVB3 viral load in the pancreas, serum pro-inflammatory cytokine levels, and macrophage infiltration in CVB3-infected mice. In mice, L. plantarum FLPL05 inhibited CVB3-induced pancreas apoptosis via the B cell leukemia/lymphoma 2 (BCL2)/BCL2-associated X protein (BAX)/caspase-3 (CASP3) signaling pathway. Furthermore, L. plantarum FLPL05 reduced CVB3 replication, protected cells from the cytopathic effect of CVB3 infection, and inhibited cell apoptosis. Moreover, L. plantarum FLPL05's exopolysaccharide (EPS) had activity against CVB3 in vitro, reducing the CVB3 titer and improving cell activity. Therefore, L. plantarum FLPL05 pretreatment improved CVB3-induced pancreatitis by partially reversing pancreatitis, which might be associated with EPS. Consequently, L. plantarum FLPL05 could be a potential probiotic with antiviral activity against CVB3.

QCR7 affects the virulence of Candida albicans and the uptake of multiple carbon sources present in different host niches.

Background: Candida albicans is a commensal yeast that may cause life-threatening infections. Studies have shown that the cytochrome b-c1 complex subunit 7 gene (QCR7) of C. albicans encodes a protein that forms a component of the mitochondrial electron transport chain complex III, making it an important target for studying the virulence of this yeast. However, to the best of our knowledge, the functions of QCR7 have not yet been characterized. Methods: A QCR7 knockout strain was constructed using SN152, and BALb/c mice were used as model animals to determine the role of QCR7 in the virulence of C. albicans. Subsequently, the effects of QCR7 on mitochondrial functions and use of carbon sources were investigated. Next, its mutant biofilm formation and hyphal growth maintenance were compared with those of the wild type. Furthermore, the transcriptome of the qcr7Δ/Δ mutant was compared with that of the WT strain to explore pathogenic mechanisms. Results: Defective QCR7 reduced recruitment of inflammatory cells and attenuated the virulence of C. albicans infection in vivo. Furthermore, the mutant influenced the use of multiple alternative carbon sources that exist in several host niches (GlcNAc, lactic acid, and amino acid, etc.). Moreover, it led to mitochondrial dysfunction. Furthermore, the QCR7 knockout strain showed defects in biofilm formation or the maintenance of filamentous growth. The overexpression of cell-surface-associated genes (HWP1, YWP1, XOG1, and SAP6) can restore defective virulence phenotypes and the carbon-source utilization of qcr7Δ/Δ. Conclusion: This study provides new insights into the mitochondria-based metabolism of C. albicans, accounting for its virulence and the use of variable carbon sources that promote C. albicans to colonize host niches.

Observing Nonpreferential Absorption of Linear and Cyclic Carbonate on the Silicon Electrode.

Silicon is reported to be a promising anode material due to its high storage capacity and excellent energy conversion rate. Molecular-level insight into the interaction between silicon electrodes and electrolyte solutions is essential for understanding the formation of a stable solid electrolyte interphase (SEI), but it is yet to be explored. In this study, we apply femtosecond sum frequency generation vibrational spectroscopy to investigate the initial adsorption of various pure and mixed electrolyte molecules on the silicon anode surface by monitoring the SFG signals from the carbonyl group of electrolyte molecules. When the silicon comes in contact with a pure carbonate solution, the linear carbonates of diethyl carbonate and ethyl methyl carbonate adopt two conformations with opposite C═O orientations on the silicon interface while the cyclic carbonates of ethylene carbonate and propylene carbonate almost adopt one conformation with C═O bonds pointing toward the silicon electrode. When the silicon comes in contact with the mixed linear and cyclic carbonate solutions, the total SFG intensity from the mixed solutions is approximately 2∼5 times weaker than those of pure cyclic carbonates. The C═O bonds of cyclic carbonates point toward the silicon electrode, while the C═O bonds of linear carbonates face toward the bulk solution at the silicon/mixed solution interface. No preferential absorption behaviors of the linear and cyclic carbonate electrolytes on the silicon electrode are observed. Such findings may help to understand the mechanism by which the SEI formed on the silicon anode is unstable.

Boosting Charge Transport in a 2D/3D Perovskite Heterostructure by Selecting an Ordered 2D Perovskite as the Passivator.

We demonstrate that an ordered 2D perovskite can significantly boost the photoelectric performance of 2D/3D perovskite heterostructures. Using selective fluorination of phenyl-ethyl ammonium (PEA) lead iodide to passivate 3D FA0.8 Cs0.2 PbI3 , we find that the 2D/3D perovskite heterostructures passivated by a higher ordered 2D perovskite have lower Urbach energy, yielding a remarkable increase in photoluminescence (PL) intensity, PL lifetime, charge-carrier mobilities (ϕµ), and carrier diffusion length (LD ) for a certain 2D perovskite content. High performance with an ultralong PL lifetime of ≈1.3 µs, high ϕµ of ≈18.56 cm2  V-1 s-1 , and long LD of ≈7.85 µm is achieved in the 2D/3D films when passivated by 16.67 % para-fluoro-PEA2 PbI4 . This carrier diffusion length is comparable to that of some perovskite single crystals (>5 µm). These findings provide key missing information on how the organic cations of 2D perovskites influence the performance of 2D/3D perovskite heterostructures.

Changes of bacterial and fungal communities and relationship between keystone taxon and physicochemical factors during dairy manure ectopic fermentation.

BACKGROUND: Due to interactions with variety of environmental and physicochemical factors, the composition and diversity of bacteria and fungi in manure ectopic fermentation are constantly changing. The purpose of this study was to investigated bacterial and fungal changes in dairy manure ectopic fermentation, as well as the relationships between keystone species and physicochemical characteristics. METHODS: Ectopic fermentation was carried out for 93 days using mattress materials, which was combined with rice husk and rice chaff (6:4, v/v), and dairy waste mixed with manure and sewage. Physicochemical characteristics (moisture content, pH, NH4+-N (NN), total organic carbon (TO), total nitrogen (TN) and the C/N ratio) of ectopic fermentation samples were measured, as well as enzymatic activity (cellulose, urease, dehydrogenase and alkaline phosphatase). Furthermore, the bacterial and fungal communities were studied using 16S rRNA and 18S rRNA gene sequencing, as well as network properties and keystone species were analyzed. RESULTS: During the ectopic fermentation, the main pathogenic bacteria reduced while fecal coliform increased. The C/N ratio gradually decreased, whereas cellulase and dehydrogenase remained at lower levels beyond day 65, indicating fermentation maturity and stability. During fermentation, the dominant phyla were Chloroflexi, Firmicutes, Proteobacteria, Bacteroidetes, and Actinobacteria of bacteria, and Ascomycota of fungi, while bacterial and fungal community diversity changed dramatically and inversely. The association between physicochemical characteristics and community keystone taxon was examined, and C/N ratio was negative associated to keystone genus. CONCLUSION: These data indicated that microbial composition and diversity interacted with fermentation environment and parameters, while regulation of keystone species management of physicochemical factors might lead to improved maturation rate and quality during dairy manure ectopic fermentation. These findings provide a reference to enhance the quality and efficiency of waste management on dairy farm.

Surface Structure Engineering of PdAg Alloys with Boosted CO2 Electrochemical Reduction Performance.

Converting carbon dioxide into high-value-added formic acid as a basic raw material for the chemical industry via an electrochemical process under ambient conditions not only alleviates greenhouse gas effects but also contributes to effective carbon cycles. Unfortunately, the most commonly used Pd-based catalysts can be easily poisoned by the in situ formed minor byproduct CO during the carbon dioxide reduction reaction (CRR) process. Herein, we report a facile method to synthesize highly uniformed PdAg alloys with tunable morphologies and electrocatalytic performance via a simple liquid synthesis approach. By tuning the molar ratio of the Ag+ and Pd2+ precursors, the morphologies, composition, and electrocatalytic activities of the obtained materials were well-regulated, which was characterized by TEM, XPS, XRD, as well as electrocatalytic measurements. The CRR results showed that the as-obtained Pd3Ag exhibited the highest performance among the five samples, with a faradic efficient (FE) of 96% for formic acid at -0.2 V (vs. reference hydrogen electrode (RHE)) and superior stability without current density decrease. The enhanced ability to adsorb and activate CO2 molecules, higher resistance to CO, and a faster electronic transfer speed resulting from the alloyed PdAg nanostructure worked together to make great contributions to the improvement of the CRR performance. These findings may provide a new feasible route toward the rational design and synthesis of alloy catalysts with high stability and selectivity for clean energy storage and conversion in the future.

Homogeneous interfacial water structure favors realizing a low-friction coefficient state.

HYPOTHESIS: The use of water to reduce friction has always played a significant role in a wide range of areas ranging from biology to engineering. Many efforts have been made to extensively investigate the water behavior between two contacted surfaces, but its role in water-based friction remains incompletely understood. EXPERIMENTS: Herein, we utilize the sum-frequency generation (SFG) spectroscopy to identify interfacial water structures upon adjusting the wettability of titanium dioxide (TiO2) and silicon surfaces. And the corresponding wettability-tunable underwater friction is measured by atomic force microscopy (AFM). FINDINGS: It demonstrates that enhanced wettability induces higher friction on the TiO2 surface but lower friction on the silicon surface. Although the tribological properties of TiO2 show independence of surface forces in contrast to the case of silicon, both TiO2 and silicon surfaces covered with homogeneous water molecules correspond to a lower friction coefficient. This observation indicates that a homogeneous interfacial water structure, dominating over surface forces, is of the utmost importance for achieving low friction. Our results shed new light on the origins of friction in the presence of water and reveal the ubiquitous role of interfacial water structures on friction.

Analysis of Movements and Behavior of Bighead Carps (Hypophthalmichthys nobilis) Considering Fish Passage Energetics in an Experimental Vertical Slot Fishway.

An understanding of fish movement behavior in response to flow field variables is important for exploring the hydrodynamic strategies of fish in fish passages. In this paper, bighead carps were taken as an example. The fish movement behavior response to water flow field information by means of estimating the energetic expenditure using an IBM approach in an experimental fishway was investigated. Fish swimming velocity, drag force, and energy expenditure were analyzed in varied flow conditions related to hydraulic variables, including velocity (V), turbulent kinetic energy (TKE), and strain rate (SR). The result indicated that the fish will require more energy in high TKE zones. This study provides a reference for optimizing the design of fish passages and fisheries management. This method can be applied to assess the efficiency of fish bypass structures and conduct fish survival studies.

Evidence for a Local Field Effect in Surface Plasmon-Enhanced Sum Frequency Generation Vibrational Spectra.

Surface plasmon-enhanced vibrational spectroscopy has been demonstrated to be an important highly sensitive diagnostic technique, but its enhanced mechanism is yet to be explored. In this study, we couple femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) with surface plasmon generated by the excitation of localized gold nanorods/nanoparticles and investigate the plasmonically enhanced factors (EFs) of SFG signals from poly(methyl methacrylate) films. Through monitoring the SFG intensity of carbonyl and ester methyl groups, we have established a correlation between EFs and the coupling of localized surface plasmon resonance with SFG and visible beams. It is found that the total enhanced factor is approximately proportional to the square of an enhanced factor of the SFG electromagnetic field and the fourth power of the enhanced factor of the visible electromagnetic field. The local field effect is roughly expressed to be the square of an enhanced factor of the visible electromagnetic field. This finding will help to guide the experimental design of plasmon-enhanced SFG to drastically improve the detection sensitivity and thus provide greater insight into the ultrafast dynamics near plasmonic surfaces.

Observing Two-Dimensional Spontaneous Reaction between a Silicon Electrode and a LiPF6-Based Electrolyte In Situ and in Real Time.

Two-dimensional spontaneous reactions between an electrode and an electrolyte are very important for the formation of a solid electrolyte interphase (SEI) but difficult to study because studying such reactions requires surface/interface sensitive techniques with sufficiently structural and temporal resolutions. In this study, we have applied femtosecond broadband sum-frequency generation vibrational spectroscopy (SFG-VS) to investigate the interaction between a silicon electrode and a LiPF6-based diethyl carbonate electrolyte solution in situ and in real time. We found that two kinds of diethyl carbonate species are present on the silicon surface and their C═O stretching aligns in opposite directions. Intrinsically spontaneous chemical reactions between silicon electrodes and a LiPF6 electrolyte solution are observed. The reactions generate silicon hydride and cause corrosion of the silicon electrodes. Coating of the silicon surface with a poly(vinyl alcohol) layer can effectively retard and attenuate these reactions. This work demonstrates that SFG-VS can provide a unique and powerful state-of-the-art tool for elucidating the molecular mechanisms of SEI formation.