Terahertz Absorption Properties of Electrolyte Solutions Based on Microfluidic Chip
WU Ya-xiong, SU Bo*, HE Jing-suo, ZHANG Cun-lin
Key Laboratory of Terahertz Optoelectronics, Ministry of Education; Beijing Key Laboratory for Terahertz Spectroscopy and Imaging; Beijing Advanced Innovation Center for Imaging Technology, Department of Physics, Capital Normal University, Beijing 100048, China
Abstract：As the vibration and rotational energy levels of many biological molecules fall in the terahertz band, which has the characteristics of low electron energy (about 4meV) and not destroying the samples, terahertz wave can be used to detect biological samples. Many biological molecules need to keep their bioactivity in the liquid environment, to explore the influences of acid and alkali environment in the salt solution, and to study their biological characteristics in the salt buffer solution. However, water as a polar liquid has strong absorption of terahertz wave, so it is necessary to explore how to reduce the absorption of terahertz wave by water, which is mainly caused by hydrogen bonds between water molecules. At present, the most common method is to reduce the distance between water and the terahertz wave and to destroy the hydrogen bonds between the water molecules. In this paper, by the aid of the terahertz time-domain spectroscopy system, the influences of different kinds and different concentrations of electrolyte on the hydrogen bond between water molecules are investigated by observing the change of spectral intensity in the range of 0.1～10 THz by using a sandwich microfluidic chip, which can not only reduce the distance between water and the terahertz wave, but also explore the influences of electrolyte on hydrogen bonds between water molecules. The specific performances are that some electrolytes can promote the association of hydrogen bonds, while the other destroy the formation of hydrogen bonds between water molecules, and this can be shown by the intensity of the spectra. If the association of hydrogen bonds between water molecules is promoted, terahertz absorption will increase and spectral intensity will decrease, and if the association is destroyed, terahertz absorption will decrease and spectral intensity will increase. The research results show that the strength of the terahertz spectrum increases when potassium chloride and potassium bromide are injected into the water, which indicates that they can destroy the hydrogen bonds and make the spectral intensity become larger. However, when magnesium chloride and calcium chloride are injected, the intensity of the terahertz spectrum is weakened. This indicates that they have an associative effect on hydrogen bonds, which makes the spectral intensity smaller. By using terahertz technology in the range of 0.1 to 1.0 THz to study the characteristics of potassium chloride, potassium bromide, magnesium chloride and calcium chlorideelectrolyte solutions with different concentration, it is found that these electrolytes can only affect the spectral intensity, and do not introduce new characteristic absorption peaks and cause interference to the sample. This has some practical value for the study of biological molecules such as Escherichia coli, Bacillus subtilis etc., which have characteristic absorption spectra in the range of 0.1～1.0 THz. With the help of microfluidic chip, injecting the required electrolyte in the solution can not only identify the samples to be measured, study the spectral information of the samples, and explore its biological characteristics, but also provide a prerequisite for further promoting the application of the terahertz technology in biochemistry.
Key words：Terahertz; Microfluidics; Chip; Electrolyte; Hydrogen bond
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