Liquid/Liquid Extraction And Electrochemical Detection Based On Microfluidic Droplet Systems

The miniaturization, integration and portability of chemical-analysis equipments are one of the most important trends in the development of modern analytical apparatus and analytical science. Microfluidic chip based systems are such microsystems keeping up with the current trend. On a microfluidic chip, operations in traditional laboratory such as sample injection, mixing, reaction, separation and detection can be performed. With the miniaturization of the size of the microchannel, microfluidic chip possesses the advantages of low sample and reagent consumption, fast mass and heat transfer rate, short analysis time and so on, which are very suitable for expensive and small-volume sample testing and for manufacturing portable apparatus for analysis in various situations. Recently, microfluidic droplet has emerged as a novel technique in the field of microfluidics. Microfluidic droplet systems have higher surface to volume ratios, faster heat and mass transfer rate and shorter reaction times than those of continuous flow based microfluidic systems. The microfluidic droplet can be used as individual microreactor units for parallel ultramicroanalysis and high throughput experimentation. Currently, the study on microfluidic droplet systems has attracted the attention of many researchers. Based on the background, novel methods for liquid/liquid extraction and bipolar electrochemical detection based on microfluidic droplet systems had been developed in this thesis. This thesis had been divided into three Parts.In Part1, Chapter1, the characteristics and applications of microfluidic systems and microfluidic droplet systems had been briefly introduced. Recent development of liquid/liquid extraction and electrochemical detection based on microfluidic systems had been reviewed. In Part2, Chapter2, a microfluidic droplet liquid/liquid extraction system modulated by the interfacial Galvani potential difference had been developed. The system was based on a home-built sampling platform, which was consisting of a sampling probe at the nanoliter scale and a slotted-vial array. The interfacial Galvani potential difference that used for modulating the analyte extraction was chemically controlled by the distribution of different salts. The performance of the extraction system had been demonstrated using methyl orange anion as a model analyte and1,2-dichloroethane as an extractant. When the interfacial potential difference at equilibrium was below-0.2V, the methyl orange anion could transfer from water phase with the volume of4.0nL into1,2-dichloroethane droplet phase with the same volume within5s. And the extraction efficiency was higher than80%. Moreover, the extraction efficiency could be modulated by adjusting the interfacial potential difference at equilibrium via changing the ionic component and its concentration in both phases. And the correlation between the extraction and the interfacial potential difference at equilibrium followed the classical Nernst equation. Compared with the extraction modulated by the interfacial potential difference via external electrodes, the complex process for electrode fabrication in microchannel was omitted in the proposed system but with a higher extraction efficiency and faster response.In Part3, including Chapter3and Chapter4, a novel microfluidic droplet sensor based on bipolar electrochemistry had been developed. Chapter3, the proposed microfluidic droplet sensor based on bipolar electrochemistry had been optimized and then it was further used for the detection of hydrogen peroxide. This novel approach was performed on a indium tin oxide glass-poly(dimethylsioxane)(ITO glass-PDMS) microchip. ITO glass was selectively etched for patterning the driving electrodes and bipolar electrodes, and several microwells were punched on the PDMS. The anodic and cathodic end of each bipolar electrode was buried in the microwell that located at both sides of the electrode, respectively. Firstly, tris(2,2’-bipyridyl) ruthenium(II)/2-(dibutylamino)ethanol (Ru(bpy)3+/DBAE) system contained in microliter-sized microfluidic droplets and the analyte droplet were added into the microwell at the anodic and cathodic side of each bipolar electrode, respectively. Then an appropriate external voltage was imposed on the driving electrodes to induce the oxidation of Ru(bpy)32+/DBAE and the reduction of the analyte, respectively. During the oxidation process of Ru(bpy)3+/DBAE system, the electrochemiluminescence singals were emitted from anodic end of each bipolar electrode, which could be recorded for visual analyte detection. Ru(bpy)3+/DBAE system and a certain concentration of potassium ferricyanide dissolved in potassium chloride solution were used for optimizing the layout and dimension of electrodes and the component and concentration of the electrolyte at the anodic and cathodic side of bipolar electrode. The results indicated that a wide driving electrode, a narrow and short bipolar electrode and a short gap between driving electrode and bipolar electrode were benefit to obtain the bipolar electrochemiluminescence at a low external voltage. And the best composition of electrochemiluminescence detection solution was1mM Ru(bpy)32+/0.1mM DBAE. Moreover, the anodic electrochemiluminescence singal was closely related to the type and concentration of the redox substance at the cathodic side of the bipolar electrode. Therefore, it was possible to detect the redox substance at the cathodic side of bipolar electrode by its anodic electrochemiluminescence singal. Furthermore, the cathode end of the bipolar electrode was modified with the composites of gold nanoparticles and horseradish peroxidase, and the modified microfluidic droplet sensor based on bipolar electrochemistry was used for biosensing of hydrogen peroxide. With an appropriate external voltage, hydrogen peroxide was catalyzed reduction by horseradish peroxidase, which could be indirectly detected by the electrochemiluminescence singal of Ru(bpy)32+/DBAE emitted from anodic end of the bipolar electrode. During recording the electrochemiluminescence singal, the current passed through the bipolar electrode could also be collected for the detection of hydrogen peroxide. A good linear Log-Log relationship between the electrochemiluminescence singal and hydrogen peroxide concentration was obtained in the concentration range of10-5to10-2M. And a good linear Log-Log relationship between the current singal and hydrogen peroxide concentration was obtained in the concentration range of5×10-5to10-1M. The results obtained by amperometric detection were in good aggrement with that obtained by electrochemiluminescence imaging (Correlation coefficient0.9952). The proposed sensor had the advantages of simple in structure, highly sensitive, fast response, wide dynamic response, avoiding cross-contamination between sensing analyte and reporting reagent, etc, which might be widely used for analyte detection.In Chapter4, the microfluidic droplet array sensor based on bipolar electrochemiluminescence imaging developed in Chapter3had been used for detection of organic compounds. Four types of quinones, such as p-benzoquinone (BQ),2,6-dichloro-1,4-benzoquinone (DCBQ),2,3,5,6-tetrachloro-1,4-benzoquine (TCBO) and7,7’,8,8’-tetracyanoquinodimethane (TCNQ) dissolved in dimethylformamide phase were selected as the model analytes to demonstrate the sensor performance. The electrochemiluminescence imaging of Ru(bpy)32+/DBAE system in water phase emitted from the anodic side of the bipolar electrode was used for detection of these types of quinones. A good linear Log-Log relationship between the electrochemiluminescence gray value and the concentration of each type of quinones was obtained. The limit of detection of BQ, DCBQ, TCBQ and TCNQ was220μM,70μM,250μM and200μM, respectively. This novel approach expanded the applications of electrochemiluminescence detection.