Terahertz time-domain spectroscopy (THz-TDS) is a detection method of biological molecules with label-free, non-ionizing, non-intrusive, no pollution and real-time monitoring. But owing to the strong THz absorption by water, it is mainly used in the solid state detection of biological molecules. In this paper, we present a microfluidic chip technique for detecting biological liquid samples using the transmission type of THz-TDS system. The microfluidic channel of the microfluidic chip is fabricated in the quartz glass using Micro-Electro-Mechanical System (MEMS) technology and sealed with polydimethylsiloxane (PDMS) diaphragm. The length, width and depth of the microfluidic channel are 25mm, 100μm and 50μm, respectively. The diameter of THz detection zone in the microfluidic channel is 4mm. The thicknesses of quartz glass and PDMS diaphragm are 1mm and 250μm, individually. Another one of the same quartz glass is used to bond with the PDMS for the rigidity and air tightness of the microfluidic chip. In order to realize the automation of sampling and improve the control precise of fluid, a micropump, which comprises PDMS diaphragm, pump chamber, diffuser and nozzle and flat vibration motor, is integrated on the microfluidic chip. The diffuser and nozzle are fabricated on both sides of the pump chamber, which is covered with PDMS diaphragm. The flat vibration motor is stuck on the PDMS diaphragm as the actuator. We study the terahertz absorption spectroscopy characteristics of glycerol with the concentration of 98% in the microfluidic chip by the aid of the THz-TDS system, and the feasibility of the microfluidic chip for the detection of liquid samples is proved.
A terahertz microfluidic chip which was fabricated out of quartz and polydimethylsiloxane (PDMS) was designed. The quartz acted as a substrate, on which a microfluidic channel with a height of 50 micrometer was etched by lithography technology. PDMS is a kind of easy fabricated and cheap polymer to cover the quartz substrate. It’s widely used in microfluidic applications for its numerous characteristics such as good insulation, colorlessness and transparency, high-voltage endurance and excellent optical quality. The transmittance of the chip was measured first. Increasing the frequency from 0.2 THz to 1.4 THz decreases the transmittance of microfluidic chip rapidly, where reaches as high as 80% when the frequency is 0.2 THz. The chip shows a high transmittance of above 40% from the range from 0.2 THz to 0.8 THz. Then THz spectra of glycerol, water and their mixture in the microfluidic chip were measured, respectively. Not only the time domain spectra but frequency-dependent amplitude spectra present significant differences of different kinds of liquids and different concentrations of the same glycerol. The absorption spectra result shows a monotonic increase in the absorption coefficient of the glycerol, water and the mixture with increasing frequency, respectively. It matches the fact that different molecules or the same molecules of different concentrations present different absorption coefficients in the terahertz band. All of the above indicate that this microfluidic chip is available in measurement of the samples in liquid and it’s able to realize real-time and label-free measurement for biochemistry samples in terahertz time-domain-spectroscopy(THz-TDS). Since the depth of the micro-channel is 50 μm，the detectable liquid in the chip only has an value of less than 10 μL, microfluidic chips offer a new platform for performing THz spectroscopy of small quantities of biomolecules using low-power THz sources.