Terahertz Transmission Characteristics of Electrolyte Solution
QIAN Kun, BAI Zhi-chen, WU Rui, WANG Jia-hui, SU Bo*, WEN Yi-wei, ZHANG Cun-lin
Beijing Key Laboratory for Terahertz Spectroscopy and Imaging; Key Laboatory of Terahertz Optoelectronics, Ministry of Education; Beijing Advanced Innovation Center for Imaging Theory and Technology, Department of Physics, Capital Normal University, Beijing 100048, China
Abstract：The vibration and rotational energy levels of many biomacromolecules are in the terahertz band, and the terahertz wave has low photon energy and high peak power. Therefore, detection with terahertz technology can ensure the biological molecules are not destroyed to a large extent. However, most of the biomolecules can maintain their biological activity only in an aqueous solution, and water is a polar molecule, which has strong absorption of terahertz wave, so it is not easy to use conventional terahertz technology to detect the characteristics of biological samples in an aqueous solution. In this paper, a terahertz microfluidic chip with a sandwich structure is designed, including substrate, cover and microchannel layers. The substrate and cover are made of COC and PMMA. COC material has high transparency to terahertz wave and transparent to visible light. It is ideal for making a terahertz microfluidic chips, but it is expensive and hard to obtain. To reduce the amount of COC, the COC is embedded in the PMMA of the substrate and the cover to ensure that the terahertz wave can pass through the COC. The diameter of COC is 5 mm, the thickness is the same as that of PMMA material, both of which are 2 mm, aligned with the center of the microchannel. A strong adhesive double-sided adhesive with a thickness of 50 μm is selected as the microchannel layer, and the center of the double-sided adhesive is hollowed out as the microchannel, with a length of 3 cm and a width of 4 mm. The THz microfluidic chip is composed of substrate, cover and microchannel. The THz detection area is 4 mm in diameter. The combination of microfluidic technology and terahertz technology reduces the consumption of samples, shortens the distance between terahertz wave and samples, and provides the possibility of detecting liquid samples. It is found that the strong absorption of THz wave by water is mainly due to the hydrogen bond in water, while the electrolyte solution will affect the hydrogen bond in water. In this paper, the electrolyte solutions were prepared with different potassium chloride concentrations, potassium sulfate, copper chloride and copper sulfate solutions, and their terahertz transmission spectra were studied by terahertz microfluidic technology. The results show that THz’s transmission intensity of THz of the four electrolyte solutions is lower than that of pure deionized water, but the experimental phenomena are different. The transmission intensity of THz of copper chloride solution increases with the increase of concentration, while potassium chloride, potassium sulfate and copper sulfate solution decreases with the increase of concentration.
