Chapter 9: Terahertz spectroscopy of liquids and biomolecules

The terahertz regime has particular value for liquid and biomolecular spectroscopy. In the case of liquids, terahertz is sensitive to relaxational and collective motions in liquids [1-13]. Applications include determination of sugar, alcohol, and water content. While there are no narrow band identification features for liquids in the terahertz range, the ability of THz to transmit through packaging materials and high sensitivity of relative water content is considered highly appealing for its use as a method to rapidly verify labeled contents. The determination of the water, sucrose, alcohol, liquid fuel, and petroleum content using terahertz have been demonstrated [1, 10]. The fundamental findings from terahertz measurements of liquids include the hydration number associated with solutes [14, 15], the extent of the perturbation of the liquid structure by the solute [16, 17], and the role of interactions in binary liquids [13, 18] . New collective mode vibrations have been identified for alcohols [19, 20], and the changes in the relaxational dynamics due to mixing, and the role of collective vibrations in ionic liquids [21-24]. In order to achieve these many findings, sensitive measurement techniques and data analysis have been developed. In parallel, great strides in modeling have been made to effectively model the picosecond dielectric response for these highly complex systems. Biological applications of terahertz have been explored from spectroscopy of biologically relevant molecules as small as sucrose up to organisms such as bacterial spores. Significant progress has been made in fundamental characterization of small biomolecules with accurate modeling of both intramolecular modes, and the intermolecular modes for crystalline material. Initial measurements of small proteins have been explored; however, theoretical understanding is not as well developed. While a variety of groups have demonstrated sensitivity in the THz dielectric response to protein and nucleic acid functional state, the origin of this sensitivity is still some-what controversial. In this chapter, we will discuss measurement methods, modeling of the terahertz response for these systems, and major results. We will conclude with a discussion on future directions for the applications of terahertz for liquid and biomolecular characterization.