A Microfluidic Chip-based Absorbance Detection System Utilizing Liquid-core/Liquid-cladding Waveguides

With the development of science and technology, the analytical instruments have been more and more miniaturized, automated, and portable. miniaturized total analysis systems were just proposed for these purposes. Microfluidic-based absorbance detections excel at instrument miniaturization. However, the sensitivity of the detector is required to be higher for samples in much less volume. Liquid-core waveguides (LCWs) have the function of confining the light propagation along a special path based on the total internal reflection. Since much less light loss and longer effective absorbance path have been achieved in the LCWs, the LCWs have been used to improve the detection sensitivity. In this paper, two kind of liquids with different refractive indices were used to form liquid-liquid laminar flow, thus forming a liquid-core/liquid-cladding waveguide, and a simple and sensitive microfluidic-based absorbance detection system was established utilizing liquid-core/liquid-cladding waveguides.In chapter 1, optical detection systems based on LCW techniques were reviewed, including LCW-based absorption spectrophotometric detection, fluorescence detection, chemiluminescence detection and Raman spectroscopy detection. The applications of liquid-liquid laminar flow techniques in microfluidic systems were introduced. Finally, the purpose and design of this paper were proposed.In chapter 2, a liquid-core/liquid-cladding waveguide system was formed by liquid-liquid laminar flow, and a microfluidic-based absorbance detection system using liquid-liquid waveguides was established. Liquid-liquid laminar flow was formed by an organic phase with high refractive index and an aqueous phase with low refractive index in the micro-channel, wherein the organic phase and the aqueous phase were used as core and cladding, respectively. Thereby the liquid-core/liquid-cladding waveguide system was formed. The total internal reflection occurred at the core/cladding interface when the angle of the incident light was above the critical angle, thus longer effective absorbance path has been achieved and the detection sensitivity was improved. The analytical performance of the system was demonstrated using methyl red in hexanol solution as the inner core and water as the cladding. Length and depth of the micro-channel, and width of the liquid core were optimized. The linear range was 0.01～1.00 mmol/L, and the regression equation for the standard curve was y=0.0343+0.8219x (x:mmol/L, R2=0.9973). The LOD was 9.7×10-6 mol/L (3a), and a precision of 6.1% RSD (n=11) was obtained. This system was used for the detection of Sudan Ⅱ, the linear range was 0.005～0.500 mmol/L, and the regression equation for the standard curve was y=0.1191+4.3923x (x:mmol/L, R2=0.9966). The LOD was 1.8×10-6 mol/L (3σa), and a precision of 6.4% RSD (n=11) was obtained. This system was used for the detection of Sudan Ⅱ in eggs, the recovery was 93.3% ～ 107.5%.In chapter 3, the microfluidic-based absorbance detection system with liquid-liquid waveguides was summarized.