Division of Applied Physics, Faculty of Pure and Applied Sciences,University of Tsukuba
Nano-Micro Conference, 2017, 1, 01006
Published Online: 04 October 2017 (Abstract)
Use of terahertz waves for sensing various type of materials has been an important subject of research. The target materials include biological tissues, various kind of liquids, industrial products, etc. In most of cases, water is the most important absorber of terahertz waves. It is crucial, therefore, to understand the terahertz absorption properties of water in various situations.
It is known that the terahertz absorption of water is strongly affected by the interaction with other molecules in solutions, gels, crystals, or other types of phases. In aqueous solutions, for example, the solute is stabilized by the interaction with surrounding water molecules, and the water molecules incorporated in this interaction are very often strongly bound to the solute, leading to reduced mobility and reduced terahertz absorption.
We have constructed a high-precision terahertz time-domain spectroscopy apparatus for hydration study. Results of hydration study of protein (hen egg white lysozyme) aqueous solution with several ammonium salts [1,2] is briefly described below. Terahertz absorption coefficient of the protein-salt aqueous solutions and salt aqueous solutions were obtained as a function of the salt concentration. The difference between the results of protein-salt aqueous solutions and those of salt aqueous solutions exhibits the nature of the hydration water of the protein and also the effect of salt on them. It was found that the hydration layer of the protein is affected by ions (mostly anions) added to the solution, and that so-called kosmotropic ions (structure makers), such as sulfate ion, reduce the amount of immobile hydration water, whereas chaotropic ions (structure breakers), such as thiocyanate ions, increase it. It is shown that the dynamical properties of hydration water molecules is correlated with the stability of the macromolecules, whereas their density with the solubility.
Measurements on other types of water-rich systems have shown that the mobility of water sometimes increases after hydration, or that the hydration number changes drastically upon a small change in the concentration. Study of hydration using terahertz waves is expected to produce fruitful results in various fields including solution chemistry, sol-gel science, biology, and medicine.
 Katsuyoshi Aoki; Kentaro Shiraki; Toshiaki Hattori, Observation of salt effects on hydration water of lysozyme in aqueous solution using terahertz time-domain spectroscopy. Applied Physics Letters. 103, 173704 (2013). doi:10.1063/1.4826699
 Katsuyoshi Aoki; Kentaro Shiraki; Toshiaki Hattori, Salt effects on the picosecond dynamics of lysozyme hydration water investigated by terahertz time-domain spectroscopy and an insight into the Hofmeister series for protein stability and solubility. Physical Chemistry Chemical Physics. 18, 15060 (2016). doi:10.1039/C5CP06324H