Terahertz bio-sensing techniques

This chapter reviews research involving the sensing and analysis of biological materials using terahertz (THz) waves. Because the characteristic energy of biological materials, generated by molecular motions such as rotation and vibration, lies in the THz frequency range, THz spectroscopy is the best tool for the direct sensing of biological molecules and the associated techniques. This review begins with water, outlining its bulk dielectric property and the temperature dependence thereof, and its solvation dynamics in the presence of various solutes including proteins. THz techniques have also been used to study the dynamic behaviour of proteins, as well as to sense ligand binding to proteins. Through the measurement of dielectric functions in the THz range, the binding-state dependence of molecules such as DNA and triple-strand β (beta)-glucans can be characterized. For molecules, such as DNA, RNA, nucleobases, nucleosides and polypeptides, resonant features arising from their characteristic motions are reviewed along with the identification of peaks using density functional theory. The temperature-dependent dielectric property of water has led to the invention of a THz molecular imaging technique using nanoparticles, the principle of which is briefly described with an outline of its potential applications, such as cancer diagnosis and drug delivery monitoring.