Sum frequency generation vibrational spectroscopy (SFG-VS) has been proven to be a uniquely effective spectroscopic technique in the investigation of molecular structure and conformations, as well as the dynamics of molecular interfaces. However, the ability to apply SFG-VS to complex molecular interfaces has been limited by the ability to abstract quantitative information from SFG-VS experiments. In this review, we try to make assessments of the limitations, issues and techniques as well as methodologies in quantitative orientational and spectral analysis with SFG-VS. Based on these assessments, we also try to summarize recent developments in methodologies on quantitative orientational and spectral analysis in SFG-VS, and their applications to detailed analysis of SFG-VS data of various vapour/neat liquid interfaces. A rigorous formulation of the polarization null angle (PNA) method is given for accurate determination of the orientational parameter D = ⟨cos θ ⟩/⟨cos3 θ⟩, and comparison between the PNA me...

Quantitative spectral and orientational analysis in surface sum frequency generation vibrational spectroscopy (SFG-VS)

Understanding the structure and dynamics of water's constituent ions, proton and hydroxide, has been a subject of numerous experimental and theoretical studies over the last century. Besides their obvious importance in acid-base chemistry, these ions play an important role in numerous applications ranging from enzyme catalysis to environmental chemistry. Despite a long history of research, many fundamental issues regarding their properties continue to be an active area of research. Here, we provide a review of the experimental and theoretical advances made in the last several decades in understanding the structure, dynamics, and transport of the proton and hydroxide ions in different aqueous environments, ranging from water clusters to the bulk liquid and its interfaces with hydrophobic surfaces. The propensity of these ions to accumulate at hydrophobic surfaces has been a subject of intense debate, and we highlight the open issues and challenges in this area. Biological applications reviewed include proton transport along the hydration layer of various membranes and through channel proteins, problems that are at the core of cellular bioenergetics.

Protons and Hydroxide Ions in Aqueous Systems

Quantitative spectral and orientational analysis in surface sum frequency generation vibrational spectroscopy (SFG-VS). Int Rev Phys Chem

Lithium-ion batteries, simply known as lithium batteries, are distinct among high energy density charge-storage devices. The power delivery of batteries depends upon the electrochemical performances and the stability of the electrode, electrolytes and their interface. Interfacial phenomena of the electrode/electrolyte involve lithium dendrite formation, electrolyte degradation and gas evolution, and a semi-solid protective layer formation at the electrode–electrolyte interface, also known as the solid–electrolyte interface (SEI). The SEI protects electrodes from further exfoliation or corrosion and suppresses lithium dendrite formation, which are crucial needs for enhancing the cell performance. This review covers the compositional, structural and morphological aspects of SEI, both artificially and naturally formed, and metallic dendrites using in situ/in operando cells and various in situ analytical tools. Critical challenges and the historical legacy in the development of in situ/in operando electrochemical cells with some reports on state-of-the-art progress are particularly highlighted. The present compilation pinpoints the emerging research opportunities in advancing this field and concludes on the future directions and strategies for in situ/in operando analysis.

In situ analytical techniques for battery interface analysis

Demand for accessible and affordable healthcare for infectious and chronic diseases present significant challenges for providing high-value and effective healthcare. Traditional approaches are expanding to include point-of-care (POC) diagnostics, bedside testing, and community-based approaches to respond to these challenges.1 Innovative solutions utilizing recent advances in mobile technologies, nanotechnology, imaging systems, and microfluidic technologies are envisioned to assist this transformation.

Infectious diseases have considerable economic and societal impact on developing settings. For instance, malaria is observed more commonly in sub-Saharan Africa and India.2 The societal impact of acquired immune deficiency syndrome (AIDS) and tuberculosis is high, through targeting adults in villages and leaving behind declining populations.3 In resource-constrained settings, it is estimated that about 32% of the disease burden is from communicable diseases such as respiratory infections, AIDS, and malaria, while 43% of the burden is from noncommunicable diseases, such as cardiovascular diseases, neuropsychiatric conditions, and cancer.4 Developing diagnostic platforms that are affordable, robust, and rapid-targeting infectious diseases is one of the top priorities for improving healthcare delivery in the developing world.5 The early detection and monitoring of infectious diseases and cancer through affordable and accessible healthcare will significantly reduce the disease burden and help preserve the social fabric of these communities. Further, improved diagnostics and disease monitoring technologies have potential to enhance foreign investment, trade, and mobility in the developing countries.6

Highly sensitive and specific lab assays such as cell culture methods, polymerase chain reaction (PCR), and enzyme-linked immunosorbent assay (ELISA) are available for diagnosis of infectious diseases in the developed world. They require sample transportation, manual preparation steps, and skilled and well-trained technicians. These clinical conventional methods provide results in several hours to days, precluding rapid detection and response at the primary care settings. Another diagnostic challenge is identifying multiple pathogens. Since common symptoms like sore throat and fever can be caused by multiple infectious agents (e.g., bacteria and viruses), it is important to accurately identify the responsible agent for targeted treatment. Therefore, high-throughput sensors for multiplexed identification would help improve patient care.7

Medical instruments in centrally located institutions in the developed world rely on uninterrupted electricity and running water and require controlled environmental conditions. It may not be viable to satisfy some of these criteria in some POC settings, where well-trained healthcare personnel are not available and clean water access is unreliable.7,8 Further, in remote settings without infrastructure, rain and dust can act as contaminants.7 Diagnostic devices for POC testing in these settings are identified by the World Health Organization to be affordable, sensitive, user-friendly, specific to biological agents, and providing rapid response to small sample volumes.9 Optical biosensor devices are emerging as powerful biologic agent detection platforms satisfying these considerations.10

Optical sensing platforms employ various methods, including refractive index change monitoring, absorption, and spectroscopic-based measurements.11 Optical sensors that are based on refractive index monitoring cover a range of technologies, including photonic crystal fibers, nano/microring resonator structures, interferometric devices, plasmonic nano/micro arrays, and surface plasmon resonance (SPR)-based platforms.11,12 The latter two are plasmonic-based technologies. Plasmonics is an enabling optical technology with applications in disease monitoring, diagnostics, homeland security, food safety, and biological imaging applications. The plasmonic-based biosensor platforms along with the underlying technologies are illustrated in the Figure ​Figure1.1. Here, we reviewed SPR, localized surface plasmon resonance (LSPR), and large-scale plasmonic arrays (e.g., nanohole arrays).



Figure 1

Plasmonic-based technologies for versatile biosensor applications. SPR stands for surface plasmon resonance, LSPR for localized surface plasmon resonance, SPRi for surface plasmon resonance imaging, and SERS for surface-enhanced Raman scattering.

https://canarycenter.stanford.edu/content/dam/sm/canarycenter/documents/research/bamm-lab/ChemicalReview.pdf

Advances in Plasmonic Technologies for Point of Care Applications

The interfacial regions between bulk media, although often comprising only a fraction of the material present, are frequently the site of reactions and phenomena that dominate the macroscopic properties of the entire system. Spectroscopic investigations of such interfaces are often hampered by the lack of surface specificity of most available techniques. Sum frequency generation vibrational spectroscopy (SFS) is a non‐linear optical technique which provides vibrational spectra of molecules solely at interfaces. The spectra may be analysed to provide the polar orientation, molecular conformation, and average tilt angle of the adsorbate to the surface normal. This article is aimed at newcomers to the field of SFS, and via a tutorial approach will present and develop the general sum frequency equations and then demonstrate how the fundamental theory elucidates the important experimental properties of SFS.

Implementing the Theory of Sum Frequency Generation Vibrational Spectroscopy: A Tutorial Review

A Protocol for the Reproducible Silanization of Mica Validated by Sum Frequency Spectroscopy and Atomic Force Microscopy

A composite dielectric/metal substrate, consisting of a thin mica sheet backed with a gold film, has been employed to enhance the intensity of sum frequency vibrational spectroscopy (SFS) signals arising from model monolayers of octadecylsiloxane (ODS) adsorbed at the mica/air interface. In addition to enhanced intensities in comparison to SF spectra of ODS on mica sheets without gold backing, resonant line profiles were found to vary as a function of the thickness of the mica component of the composite substrate. This experimental result concurs with a recent theoretical study of the composite substrate which showed that interference between SF signals arising from the ODS monolayer and from the displaced gold surface results in variations in the resonant phase of the SF vibrational mode. The interference phase/thickness effect has been investigated for several composite samples, each containing a series of steps of well-defined mica thicknesses. A periodic relationship between the mica thickness and the...

Enhanced Sum Frequency Generation from a Monolayer Adsorbed on a Composite Dielectric/Metal Substrate

A theoretical model has been developed to describe the generation of Sum Frequency (SF) light from a monolayer film adsorbed to the surface of a dielectric and metal composite substrate. This type of substrate provides enhanced SF signals from the monolayer without modifying the intrinsic (hydrophilic) properties of the dielectric surface. The fundamental equations of both resonant and nonresonant SF generation have been extended from a single interface to encompass the two displaced sources, the air/dielectric interface at which the monolayer film is located, and the dielectric/metal interface. The model describes the coherent addition of three separate SF signals which coherently interfere with each other, thereby affecting the line-shape observed in the net SF spectrum. Representative calculations have been made for a highly ordered monolayer of an aliphatic hydrocarbon molecule, adsorbed on a dielectric with the optical properties of mica, which is itself in contact with a metal modeled to resemble go...

Interference Effects in Sum Frequency Spectra from Monolayers on Composite Dielectric/Metal Substrates

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Papers

Implementing the Theory of Sum Frequency Generation Vibrational Spectroscopy: A Tutorial Review

A Protocol for the Reproducible Silanization of Mica Validated by Sum Frequency Spectroscopy and Atomic Force Microscopy

Enhanced Sum Frequency Generation from a Monolayer Adsorbed on a Composite Dielectric/Metal Substrate

Interference Effects in Sum Frequency Spectra from Monolayers on Composite Dielectric/Metal Substrates