Microfluidic Enrichment Method And Experimental Study Based On Electroosmotic Induced Pressure Flow

With the rapid development of microfluidic technology,microfluidic detection platforms have been widely used in various detection systems,including disease diagnosis,drug analysis,food safety,environment monitoring,and many others.However,the microfluidic detection platform normally suffers the limited sensitivity due to its own restriction,such as small sample volume,small detection space,etc.In order to improve the sensitivity of the detection devices based on the microfluidic platform,microfluidic enrichment technology has been studied.Microfluidic enrichment is to achieve spatial aggregation of target objects by using various methods in the microfluidic platform and increase their local concentration.In this way,the detection threshold of targets can be lowered and the sensitivity of the detection system can be improved,which is especially important for the detection of dilute sample analytes.Many strategies or techniques have been explored for concentrating low-abundance analytes in microfluidic platforms.Among these techniques,electric enrichment methods have a great competitive advantage due to the simplicity of its implementation and the need for few peripheral elements.In this dissertation,the electroosmotic flow combining with pressure flow,which is called the electroosmotic induced pressure flow has been studied,based on the electroosmotic flow theory in microchannels.And the microfluidic electrokinetic enrichment method and experimental research based on the electroosmotic induced pressure flow has been carried out.Using this method,fluorescent ions and fluorescent microspheres have been concentrated efficiently in double T-shaped micochannel,and the fluorescence detection of stress hormone cortisol at a low concentration has also been realized.In the dissertation,the following work has been completed.Firstly,through the theoretical analysis of the electroosmotic flow in the microchannel,combined with the multiphysics coupling analysis software COMSOL,the study of the electroosmotic induced pressure flow in the straight microchannel and the electrokinetic enrichment effect of anions under its action has been carried out.And the limitation of sample driving and substances concentrating in the straight microchannel by the action of electric field has been found.Therefore,the study of electrokinetic driving and concentrating effect of anions in the T-shaped microchannel has been then carried out.According to the influence of the channel structure on the flow field distribution and enrichment performance,a double T-shaped microchannel with good driving and enrichment performance has been obtained.Subsequently,the influence of the parameters,including the channel dimensions,electrode size and the applied electric field strength,on the enrichment performance of anions in the double T-shaped microchannel has been investigated.And a three-dimensional numerical simulation model with the optimized structure size has been established,to analyze the electrokinrtic enrichment performance of the negatively charged substances in the double T-shaped channel under different voltages,which provided a theoretical foundation for subsequent experiment research.Secondly,the research on the integrated manufacturing process of the chip with double T-shaped microchannel and microelectrode has been carried out.In this dissertation,the soft lithography method was choosed to realize the production of the chip,and the liquid metal was choosed as the microelectrode material.The fabrication process of the liquid metal electrodes in micro-scale space has been emphatically studied.An integrated microchannel structure with liquid metal electrode channel and sample solution channel was designed.To obtain the liquid metal electrode with good performance,different microchannel configurations were modeled and fabricated,and the fluid distribution of the liquid metal after being pressed into the microchannel with different configurations or under different pressing conditions was repeated researched by numerical simulation and experimental test.As a result,the manufacturing process of liquid metal microelectrode with excellent shape and accurately coupled with the sample channel has been obtained,and the microchip with double T-shaped fluidic channel and microelectrode has been successfully fabricated.Thirdly,the experimental study on the electrokinetic enrichment of fluorescent anions in the double T-shaped channel has been carried out.At first,an experimental platform with fluorescence display for substances electrokinetic enrichment was established and the enrichment process of fluorescent sodium ions under different voltages was studied.The results showed that when the enrichment time was 300s,the higher the voltage,the faster the concentrating process and the better the enrichment effect.The enrichment rate could achieve 83 by applying the voltage of 90 V for 300 s,when the concentration of fluorescent anions in the sample solution was 7.8×10^-8 mol/L.And the experimental results were compared with numerical simulation results and a good agreement was found under the same boundary conditions,which verified the feasibility of numerical simulation analysis.Then the electrokinetic enrichment performance for fluorescent sodium ions in two double T-shaped microchannels with different distances between inflow branch channels was researched by experiment.It was found that the increase of the inflow branch channels spacing was beneficial for the enrichment of fluorescent anions under certain time conditions.The conclusion was also consistent with numerical simuation analysis of the two-dimensional structure,which demonsrates the reliability of the numerical simulation analysis again.Fourthly,the study on the electrokinetic enrichment performance of fluorescent microspheres in the double T-shaped channel has been carried out.The electrokinetic enrichment experiments of fluorescent microspheres with different diameters under different experimental conditions were completed.The results showed that the enriched area of fluorescent microspheres displayed some dispersion compared with that of fluorescent ions,and the dipersion was affected by the particle size and the electric field strength.In order to explain the enrichment characteristics of fluorescent microspheres,a numerical analysis model of particle trajectories was established through the force analysis of suspended particles in the fluid in the microchannel.And the trajectory analysis of particles in the fluid under different boundary conditions was carried out.The numerical simulation results show that the surface charge of the particles and the electric field strength were the main factors affecting the enrichment characteristics of the particles.Finally,the application experiments on the detection of stress hormone cortisol based on the electrokinetic enrichment effect in the double T-shaped channel has been carried out.The detection principle of stress hormone cortisol based on the fluorescence immunoassay mechanism was studied at first.As a result,the electrokinetic enrichment experiments of cortisol test samples in the double T-shaped microchannels were carried out,and a new enrichment effect was discovered.That was,through the enrichment experiment of the first sample,cortisol aptamers,it was found that the sample could achieve a good enrichment performance,and the enrichment area was stable and far away from the microelectrode,which would be of benefit to both the sample and the electrode.Then,the new enrichment effect was applied to achieve effective detection of stress hormone cortisol at the concentration of 0.1μg/m L.And through the enrichment of the detection samples,the strength of the original fluorescent detection signal was increased by 100 times,and the strength of the recovered fluorescent detection signal was increased by 40 times.The microfluidic enrichment method based on the electroosmotic induced pressure flow proposed in this dissertation has no requirments of complicated manufacturing process of the channel.It only needs a single DC voltage loading module to achive sample driving and concentrating functions,which is easy to realize the integration in a small space.The fabrication method of the liquid metal microelectrodestudied in this dissertation is easy to operate,and the electrode structure and the sample solution channel are integrally formed without special alignment equipment and alignment process.The application of this enrichment method has achieved effective enrichment of various types of negatively charged substances,while many microorganisms in nature carry negative charges.In conclusion,this method has application potential in the portable microfluidic detection devices for detection of diversified low-concentration biochemical substance.