Infrared, Raman, near-infrared, far-infrared, and terahertz spectroscopies will be referred to here as the five sisters of vibrational spectroscopy. Molecular vibrations sensitively reflect the molecular structure and the strength of chemical bonds, making vibrational spectroscopy extremely useful for molecular identification and the study of molecular structures. It is also excellent for studying interactions between molecules and their surrounding environments (such as hydrogen bonds). Each of the five sisters of vibrational spectroscopy has its own advantages. For example, infrared and Raman spectroscopies are both very powerful for molecular identification and the study of molecular structures, while near-infrared spectroscopy is strong in non-destructive, in situ analysis as a whole. Far-infrared, terahertz, and low-frequency Raman spectroscopies provide bands due to lattice vibrations and skeletal vibrations, giving useful information for the study of intermolecular interactions. This review discusses the history, principles, characteristics, and applications including recent application examples of these five sisters.

Microparticles immobilized on a glass substrate can be dissociated by applying a combined ultrasonic–gravitational field. The threshold voltage required for particle dissociation is governed according to the density of the microparticles and the intermolecular binding force between the particle and the substrate. In this study, we have demonstrated a novel sensing strategy that utilizes changes in these parameters—induced by surface reactions—to achieve high-sensitivity detection and to quantify molecular association constants. For example, the binding of gold nanoparticles (AuNPs) to the microparticle surface leads to an increase in particle density, resulting in a measurable shift in the dissociation voltage. By analyzing this voltage shift, the amount of AuNP binding can be quantitatively determined. Additionally, by altering the molecular interactions at the particle–substrate interface, we can probe changes in intermolecular binding forces. These binding forces are closely related to the equilibrium association constants of the molecular complexes formed at the interface. Thus, the dissociation behavior provides a quantitative readout of molecular binding events. The methodology described herein offers a unique platform for evaluating both physical properties (such as particle density) and chemical parameters (such as binding affinities) at the solid–liquid interface, without the need for complex optical or target-molecule labeling techniques. This paper provides an overview of our recent research on this method, focusing on the principles of particle detachment, sensitivity of detection, and the application of this approach to determine equilibrium constants of molecular complex.

Surface-enhanced Raman spectroscopy (SERS) represents an advanced extension of Raman spectroscopy—an analytical methodology capable of simultaneously detecting multiple species— with sensitivity reaching the single-molecule level. However, its widespread application has been hindered by inherent challenges, particularly in achieving signal uniformity, reproducibility, and durability. To address these limitations, ongoing research focuses on the development of SERS substrates exploiting chemical enhancement mechanisms based on novel materials, rather than the conventional electromagnetic mechanism afforded by noble metal nanostructures. Herein, we present the development of the SERS substrates based on the chemical mechanisms fabricated from metal-free materials that exhibit exceptional uniformity, reproducibility, and durability, thereby overcoming the intrinsic shortcomings of traditional metal-based SERS platforms. Furthermore, their measurement sensitivity has been markedly improved, effectively addressing a major limitation of chemically enhanced systems. The subsequent presentation will elucidate the distinctive characteristics of these advanced substrates and highlight their potential applications across diverse analytical domains.

A new Raman imaging system using a pair of tunable band pass filters was designed and developed. A tunable filter system composed of two bandpass filters was incorporated into a microscopic Raman setup to transmit light at specified wavenumbers and bandwidths. By scanning the transmission range, high-resolution Raman images were acquired in a short time. It is demonstrated that the developed system is capable of acquiring chemical images that distinguish two types of particles on a metal surface.

Quantitative ¹H Nuclear Magnetic Resonance (qHNMR) is a powerful tool for the high-precision analysis of polyphenolic compounds. Conventionally, signals from carbon-bonded protons (C-H) are used for quantification, while signals from phenolic hydroxyl groups (Ar-OH)—a key structural feature of polyphenols—have been excluded due to line broadening caused by proton exchange. In this study, we investigated the effect of metal salt addition on sharpening Ar-OH signals to establish them as reliable quantitative indicators. Using isoferulic acid, gallic acid, and rutin as model compounds, we employed DMSO-d₆ saturated with MgCl₂ as the solvent. This approach resulted in a notable sharpening of all Ar-OH signals. For rutin, the results suggested the possibility of stable observation for both the multiple Ar-OH signals and the hydroxyl groups of the sugar moiety. The quantitative values obtained from each Ar-OH signal were in excellent agreement with those derived from C-H signals, demonstrating high reproducibility. These results indicate that the saturation of the sample solution with MgCl₂ effectively suppresses proton exchange, converting Ar-OH signals into reliable quantitative markers. This method represents a valuable technique for the accurate quantification and purity assessment of polyphenolic compounds based on their specific structural characteristics.

The desorption behavior of perfluoroalkyl carboxylic acids (PFCAs) adsorbed on porous carbon materials (PCMs) by superheated water was investigated using superheated water chromatography (SWC). The thermal effect on the desorption efficiency of perfluorooctanoic acid (PFOA) from an activated carbon fiber (ACF) was examined. The results showed that the desorption efficiency of PFOA increased with rising temperature and reached nearly 100 % at 150°C and 3 MPa without degradation of PFOA. In addition, the influence of raw material and structure of PCM on the desorption efficiencies of PFOA and trifluoroacetic acid (TFA) was studied across three ACFs, two coconut shell-derived activated carbons (CAC), and one coal-based activated carbon (CAC). The results indicated that PCMs with more complex pore structures exhibited lower desorption efficiency of PFOA, whereas the desorption efficiency of TFA was largely independent of PCM structural parameters likely due to its low hydrophobicity. Furthermore, the desorption efficiencies of TFA and PFOA from ACF with SWC were compared with those of conventional desorption methods. Lower desorption efficiencies for both compounds were observed with alkaline aqueous desorption at 70°C (TFA: 93 % and PFOA: 64 %) and vacuum vaporization at 150°C (TFA: 82 % and PFOA: 45 %) compared to SWC (TFA: 100 % and PFOA: 105 %). These results suggest that superheated water is a safe and environmentally friendly desorption solvent for the reactivation of PCMs that have adsorbed PFCAs.

In this study, the vaporization behavior of an ethyl alcohol-water system at room temperature was examined based on the vaporization amount and liquid temperature change. A solution of ethyl alcohol, with a concentration of approximately 5 cm³, was injected into a dish with a diameter of 28 mm. The vaporization amount was measured using an electronic balance, and the temperature change was measured using a digital thermometer. The findings indicated that the weight ratio of the evaporation rates of ethyl alcohol and water ranged from 2.56 to 30 wt%, which corresponds to the molecular weight ratio of ethyl alcohol and water being 1:1. Additionally, an investigation was conducted into the relationship between the decrease in liquid temperature (ΔT) and the ratio of ethyl alcohol to water. It was determined that the magnitude of the liquid temperature decrease was substantial when the value exceeded 1.5. Moreover, the temperature increase resulting from dissolution heat exhibited a maximum at approximately 30 wt% ethyl alcohol solution. In light of the findings, it is postulated that the observed sequence of vaporization behaviors within the present study can be elucidated by taking into account the vaporization heat and the dissociation heat resulting from the rupture of hydrogen bonds.