Research And Application On Microfluidic Chip System Based On Conductive Particle-PDMS 3D Composite Microelectrode

Microfluidic chip can complete operations such as sample preparation,biochemical reaction,separation and detection on the region of square centimeter in biological,chemical,medical and other fields.It has the advantages of small size,low cost,less sample consumption,rapid detection,high detection resolution and high sensitivity,which make it widely applicated in many fields such as drug synthesis screening,environmental detection and protection,health quarantine,judicial authentication and biochemical analysis.Many technical means have been used in the construction of microfluidic platform.Electrical technology has become excellent means in microfluidic field due to its short response time,convenient installation,easy integration and good compatibility.Microelectrode plays an important role in microfluidic chip.It can not only be used as an actuator to transmit external electrical signals into the chip to realize electrophoresis,dielectrophoresis,electroporation and electrofusion,but also act as sensors to react on its surface for electrochemical detection and impedance analysis.According to the thickness,microelectrodes are often divided into two-dimensional(2D)electrodes at nanometer level and three-dimensional(3D)electrodes at micron level.The 3D electrode has attracted the attention of researchers at home and abroad because of its good uniformity of spatial electric field,large effective electrode surface area,high current intensity and good robustness.According to the different materials,microelectrodes can be divided into ITO electrode,metal electrode,carbon electrode and composite electrode.The fabrication of composite electrode is less dependent on equipment and easy to operate,which is beneficial to the large-scale production and industrial application of microfluidic chips.Polydimethylsiloxane(PDMS)is the most common chip material in the field of microfluidics.It has the characteristics of low surface energy and poor metal binding ability.It is difficult to fabricate and integrate 3D microelectrodes on PDMS-based microfluidic chip,which is easy to fall off and difficult to process.In order to find the processing materials and technology of 3D microelectrode with simple operation and high reliability,and to solve the technical problems of processing and integrating 3D microelectrodes in PDMS microfluidic chips,This paper proposes a conductive particle-PDMS with excellent electrical and mechanical properties as a composite electrode material.Based on it,a set of high-reliability,easy-to-operate,3D microelectrodes of different shapes and sizes can be processed on PDMS microfluidic chips,and related application experiments are also designed.The main research work includes the following aspects:(1)The conductive theory and conductive network model of conductive particles-PDMS composite electrode materials had been studied.Based on the formation mechanism of conductive network and the movement mode of free electrons in composite electrode materials,and referring to the crosslinking characteristics of PDMS,a micro conductivity model with metallic silver powder and carbon graphite powder as conductive particles was established.It was considered that there was a double network composed of PDMS cross-linking network and conductive particle contact network in the composite electrode material.They cross-link and influence each other,and are reflected in the relevant parameters of electrical properties.The equivalent resistance of composite electrode material is calculated by contribution degree method.It is considered that the resistance of composite electrode is composed of the resistance of conductive particles and the direct contact resistance between particles.The tunneling effect caused by the movement of electrons through PDMS is regarded as part of the contact resistance,and the two contact resistances are the main factors that affect the total resistance of the material.The conductive mechanism of composite electrode materials is complex,and there are many influencing factors,including the type,concentration and size of conductive particles,the proportion of PDMS,curing temperature,mixing process,etc.This study lays a foundation for obtaining composite electrode materials with excellent electrical properties in a wide range of particles,and has great significance for the preparation of conductive particles PDMS composite electrode materials.(2)The composite electrode materials with excellent electrical and mechanical properties had been manufactureed.Firstly,the dispersion effect of direct dispersion and indirect dispersion on silver and graphite powder was studied.Ethyl acetate(EAC)was used as auxiliary solvent and Tween 80 was used as surfactant in the indirect dispersion method.The performance tests showed that the indirect dispersion method was more effective for the dispersion of conductive particles.The effects of the type,size and content of conductive particles on the electrical and mechanical properties of the composite electrode materials were investigated.The effects of the ratio of prepolymer to crosslinking agent and crosslinking temperature on the electrical and mechanical properties of the composite electrode materials were studied.There is a balance problem in the electrical and mechanical properties of Ag PDMS and graphite PDMS composite electrode materials.As the content of conductive particles increases,the conductive network is gradually formed,and the conductivity of the electrode is constantly enhanced,but the hardness of the materials is also increasing,gradually losing the softness that PDMS should have,and finally losing its workability and stability.After considering the balance of electrical and mechanical properties,75%10nm silver PDMS and 36%10nm graphite PDMS were selected as composite electrodes with excellent performance.The microstructure of the composite electrodes was characterized.It was verified that there was a double network model in the composite electrodes:the conductive network formed by the contact of conductive particles and the crosslinked network formed by the solidification of PDMS.The balanced composite electrode materials provide an important support for the fabrication of 3D microelectrodes.(3)The technology of miniaturization and integration for 3D composite electrode had been studied.After the preparation of composite electrode materials with balanced performance,the key to make microelectrode is to shape the composite electrode materials into a desired 3D shape.In this paper,the molding method is used as the miniaturization method of composite electrode materials.The soft lithography technology is used to make the concave mold for electrode miniaturization.The length,width and height of the cavity can be adjusted at will.The uncured paste composite electrode material was filled into the concave mold,and then crosslinked to obtain the microelectrode with excellent electrical and mechanical properties.Finally,the vacuum-filled PDMS is poured on the mold,and the PDMS is seamlessly connected with the PDMS in the composite microelectrode during thermal curing.When the PDMS is stripped from the mold,3D composite microelectrode can be easily glued out of the cavity and integrated into the microfluidic chip.Using this method,3D composite microelectrode with the minimum width of 50?M can be fabricated.Theoretically,Theoretically,the electrode depth-to-width ratio can be up to 3:1.The microelectrode has its own electrode lead,which reduces the contact resistance caused by the interface contact of various materials.The process has high reliability and good repeatability,which has a guiding role in the processing of 3D composite microelectrodes,and has great significance for the large-scale production and industrial application of microfluidic chips.(4)A dielectrophoretic separation chip based on Ag-PDMS 3D microelectrode has been designed and fabricated.In the design stage of the chip,the dielectric electrophoretic theory and the laminar flow theory related to the fluid mechanics were deduced and analyzed,and the dielectric electrophoretic force and flow force of the polystyrene microspheres in the separation microchannel of the microfluidic chip were discussed.COMSOL was used to model and simulate the electric field and flow field involved in the separation process,and to predict the separation trajectories of 5,10 and20?m polystyrene microspheres.A 10nm Ag-PDMS composite electrode material with a mass fraction of 75%was used as the composite electrode material.The mold and chip were made by soft lithography and molding method respectively.The chip was encapsulated with the alignment bonding platform to obtain a microfluidic chip with two inlets and three outlets and a 3D composite microelectrode with sidewall.Three different sizes of polystyrene microspheres were experimentally separated,and the influence of the peak-to-peak value of the excitation signal voltage and the change of the flow rate on the separation result is analyzed.Under the conditions of a peak-to-peak value of the excitation signal is 23V,the frequency is 1 MHZ,the sample flow rate is v₁=3?L/min,v₂=1?L/min,three kinds of polystyrene microspheres were continuously separated for more than 5 minutes,then counted in the collection tanks at three outlets.The results proved that the separation efficiency can reach 150 pcs/min,and the separation accuracy rate can reach more than 90%.This has laid a solid foundation for the research of separating different kinds of particles in later stage.(5)An electrochemical detection chip based on a miniature three-electrode system has been designed and fabricated for ascorbic acid.The Ag/Ag Cl-PDMS reference electrode with stable electrochemical performance is prepared by the electrolytic chlorination method.A three-electrode system is constructed with micro GP-PDMS as the working electrode,Ag/Ag Cl-PDMS as the reference electrode,and Ag-PDMS as the auxiliary electrode.Integrating the 3D microelectrode,sample reservoir and flow paths onto a microfluidic chip to form a chip electrochemical sensor.Through surface plasma treatment,the working electrode of GP-PDMS has hydroxyl on the surface,which was highly attractive to two protons in L-ascorbic Acid(AA),and played a guiding and accelerating role in the oxidation process of AA.Chip electrochemical sensor can be more conveniently and automatically detect AA concentration.It can get rid of the tedious preparation and complicated operation of traditional electrochemical detection.The preparation of electrochemical sensor for detecting AA on a chip has good catalytic performance.The detection method(CV method and DPV method)can be flexibly selected according to the concentration range of AA,the sensitivity is up to0.206A/(M·cm2),the minimum detection limit is 175n M,the linear range is DPV method:1-250?M,CV method:50-1200?M,it has good selectivity and repeatability.Finally,the electrochemical sensor was used to detect two actual samples with different concentrations,and the results were consistent with those obtained by ultraviolet spectroscopy.This method provides a new solution for the sample inspection in the fields of drug analysis and food safety.In summary,the technical route of this study starts from the study of the conductive mechanism of the composite electrode,to the manufacture of the composite electrode with excellent conductive performance and the coupling of the 3D composite microelectrode with the microfluidic chip,and finally to the two kinds of application research of this type of microfluidic device.In this paper,the electrical and mechanical properties and preparation methods of conductive particle-PDMS composite electrode materials have been deeply researched,which provides technical support for the application of microfluidic chip in new fields.