Microfluidic chip based on gradient electric field and its application in micro-nano plastic enrichment

Abstract

This invention discloses a microfluidic chip based on a gradient electric field and its application in the enrichment of micro- and nano-plastics, belonging to the field of environmental pollutant detection and treatment. The chip includes a sample channel and an enrichment channel arranged vertically and parallel to each other. Multiple electrodes are uniformly distributed along the length of the channel on the upper surface of the sample channel and the lower surface of the enrichment channel, with a one-to-one correspondence between the electrodes on the sample channel and the electrodes on the enrichment channel. The electrodes can be independently voltaged, forming a positive gradient electric field that increases along the sample flow direction, causing negatively charged micro- and nano-plastics in the sample channel to migrate to the enrichment channel under the action of dielectric force. This chip is suitable for automated, high-throughput enrichment of low-concentration micro- and nano-plastics in aquatic environments, exhibiting good versatility and scalability, providing an effective tool for the monitoring and control of microplastics.

Description

Technical Field

[0001] This invention belongs to the field of environmental pollutant detection and treatment, specifically involving microfluidic chips based on gradient electric fields and their application in micro-nano plastic enrichment. Background Technology

[0002] Microplastics (particle size <5 mm) and nanoplastics (particle size <1 μm) are emerging global environmental pollutants that have been widely detected in marine, freshwater, soil, and even atmospheric media. They exhibit strong environmental persistence, readily adsorb and transport other toxic pollutants, accumulate through the food chain, and ultimately threaten human health. Therefore, developing efficient and sensitive micro / nanoplastics enrichment and detection technologies is crucial for environmental pollution assessment and health risk management. Currently, the enrichment and removal of micro / nanoplastics in water bodies mainly relies on traditional physicochemical methods, such as membrane filtration, adsorption, and flocculation. However, these methods have significant limitations in application: membrane filtration technology faces problems such as easy pore clogging and rapid flux decay when treating nanoscale particles; adsorption methods based on materials such as activated carbon have poor selectivity and difficulty in solid-liquid separation; chemical flocculation is inefficient for plastic particles with weak surface charges and may introduce secondary pollution. These methods generally suffer from cumbersome processing procedures and insufficient capture capacity for low concentrations and nanoscale particles, making it difficult to meet the demands for real-time, online, and high-throughput enrichment and removal.

[0003] In recent years, microfluidic technology has provided a new approach for the efficient manipulation of micro and nano-plastics due to its advantages such as precise fluid control, low sample consumption, and ease of integration and automation. Researchers have developed microfluidic chips based on principles such as optical tweezers, acoustic waves, and inertial forces for the capture, sorting, or detection of plastic particles. However, existing technologies still suffer from drawbacks such as reliance on expensive and complex external field generation devices, which limits their on-site applicability, or their applicability only to particles within a specific size range, resulting in poor adaptability to a wide size spectrum.

[0004] Therefore, there is an urgent need to develop a microfluidic enrichment technology that is clear in principle, simple in structure, highly efficient in enrichment, and applicable to a wide range of micro and nano plastics. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide a microfluidic chip based on gradient electric field and its application in micro-nano plastic enrichment, thus solving the problems in the prior art.

[0006] The objective of this invention can be achieved through the following technical solutions: The microfluidic chip based on gradient electric field includes: sample channels and enrichment channels that are distributed vertically, parallel to each other, and aligned vertically. Multiple electrodes are uniformly distributed along the length of the channels on the upper surface of the sample channels and the lower surface of the enrichment channels, and the electrodes on the sample channels correspond one-to-one with the electrodes on the enrichment channels. The electrodes can be independently voltaged to form a positive gradient electric field that increases along the sample flow direction, causing the negatively charged micro-nanoplastics in the sample channels to migrate to the enrichment channels under the action of dielectric force.

[0007] Furthermore, the sample channel and the enrichment channel are located on the surfaces of two thin glass slides, and the two thin glass slides are attached to each other, so that the sample channel and the enrichment channel are aligned vertically and connected.

[0008] Furthermore, the electrodes are made of gold and there are 5 of them; each electrode is 3 mm long and 0.9 mm wide, and the distance between adjacent electrodes is 3 mm.

[0009] Furthermore, the voltage of the electrode is set sequentially along the flow direction as follows: 1 V, 5 V, 10 V, 15 V, 20 V.

[0010] Furthermore, the sample channel has a cross-sectional dimension of 33 mm in length, 0.3 mm in height, and 0.6 mm in width; the enrichment channel has a cross-sectional dimension of 33 mm in length, 0.15 mm in height, and 0.6 mm in width.

[0011] The above-mentioned microfluidic chip is used in the enrichment of micro-nanoplastics in water samples.

[0012] A method for enriching micro / nanoplastics in a water sample, using the aforementioned microfluidic chip, includes: A water sample containing micro- and nano-plastics is injected into the sample channel; a positive gradient voltage is applied to the electrode, causing the negatively charged micro- and nano-plastics in the sample channel to migrate to the enrichment channel under the action of dielectric force; then deionized water is injected into the enrichment channel to enrich the micro- and nano-plastics, and finally the solution at the outlet of the enrichment channel is collected.

[0013] Furthermore, the flow rate of the water sample injection channel containing micro-nanoplastics is 200 μL / h ~ 1500 μL / h.

[0014] Furthermore, the particle size of the micro-nanoplastics is 20 nm to 2 μm.

[0015] The above-mentioned method for fabricating microfluidic chips includes the following steps: S1. Take two glass slides and two thin glass slides, clean and dry them for later use; S2, the upper electrode and the lower electrode are formed on two glass slides by photolithography, development and magnetron sputtering of conductive gold layers respectively; S3, design the pattern of sample channel and enrichment channel, then use a laser beam to scan the surface of two thin glass slides along the preset pattern path, and then activate the ultraviolet laser to form sample channel and enrichment channel on the surface of the two thin glass slides respectively. S4, Align and bond the glass slide covered with electrodes and the thin glass slide with channels so that the sample channel and the enrichment channel are aligned and connected vertically, forming a microfluidic chip including upper and lower electrodes, sample channels and enrichment channels. S5. Two PDMS blocks are respectively bonded to both ends of the microfluidic chip. Two through holes are opened on the PDMS, which are respectively opposite to the sample channel and the enrichment channel. A needle is installed in the through hole for connecting the syringe.

[0016] The beneficial effects of this invention are: 1. This invention constructs a spatially programmed gradient electric field with progressively increasing intensity inside a microfluidic chip by sequentially arranging segmented electrodes along the sample flow direction and applying incrementally increasing voltages. Under the action of this electric field, the dielectric force experienced by negatively charged micro / nano plastic particles flowing through the upper sample channel gradually increases. Thus, in the dynamic balance between fluid drag and electric field force, the particles migrate smoothly and controllably from the inlet to the outlet to the lower enrichment channel. Finally, the particles are efficiently captured and spatially concentrated in the near-static enrichment channel, completing a high-throughput, wide-size-range integrated "flow-separation-enrichment" process from a continuous sample flow.

[0017] 2. This invention achieves the "gradual" migration of micro / nano plastic particles by designing a gradient electric field, avoiding rapid particle accumulation and blockage at the inlet due to excessively strong electric fields. This method exhibits highly efficient enrichment capabilities for negatively charged micro / nano plastics in a wide size range from 20 nm to 2 μm, and is particularly adept at capturing low-concentration nanoscale particles that are difficult to handle using traditional methods.

[0018] 3. The chip of this invention adopts a classic layered flow channel and planar electrode design, which can be driven by only a DC power supply and an injection pump. The equipment is simple and the operation process is convenient, which is conducive to realizing on-site and rapid sample pretreatment and detection.

[0019] 4. This invention establishes a complete technical closed loop from "COMSOL simulation modeling → parameter optimization → chip fabrication → experimental verification". Simulation results can effectively predict actual enrichment performance, greatly reducing experimental trial-and-error costs, shortening the chip R&D and optimization cycle, and improving the reliability and repeatability of the process. 5. The chip of this invention realizes the spatial separation of sample flow and particle enrichment. The concentrated solution after enrichment can be directly used for downstream detection and is easy to be used with analytical equipment such as microscopes and Raman spectrometers, laying the foundation for building a fully automated microplastic detection platform. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the overall structure of the microfluidic chip; Figure 2 This is a schematic diagram illustrating the working principle of microfluidic chips enriching microplastics. Figure 3 The image shows a comparison of fluorescence microscopy images of 2 μm polystyrene microspheres with orange fluorescence before and after enrichment at different flow rates using a designed dielectrophoresis microfluidic chip. Figure 4 The image shows a comparison of fluorescence microscopy images of 500 nm polystyrene microspheres with green fluorescence before and after enrichment at different flow rates using a designed dielectrophoresis microfluidic chip. Figure 5 This is a comparison of scanning microscope images of 20 nm polystyrene microspheres before and after enrichment at different flow rates using a designed dielectrophoresis microfluidic chip. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] like Figure 1 As shown, the microfluidic chip based on gradient electric field includes: sample channels and enrichment channels that are distributed vertically and parallel to each other. Multiple electrodes are uniformly distributed along the length of the channel on the upper surface of the sample channel and the lower surface of the enrichment channel. The upper and lower electrodes correspond one to one. The electrodes can be independently applied with voltage and form a positive gradient electric field that increases along the sample flow direction. This allows the negatively charged micro-nanoplastics in the sample channel to migrate to the enrichment channel under the action of dielectric force.

[0024] The sample channel and enrichment channel are located on the surfaces of two thin glass slides, and the two thin glass slides are attached to each other, so that the sample channel and enrichment channel are aligned vertically and connected.

[0025] In this invention, preventing the sample liquid in the upper channel (sample channel) from flowing into the lower channel (enrichment channel) due to gravity can be explained using the concept of the Reynolds coefficient: In the designed microfluidic chip, a fluid dynamics model based on the laminar flow assumption was constructed to accurately simulate the flow behavior of the sample liquid within the microchannels. The applicability of this model is based on the low Reynolds number (Re) condition satisfied by the liquid flow in the microscale chip channels. The Reynolds number is defined by the following equation: Re=ρvd / μ Where ρ is the liquid density (unit: kg / m³) 3 v is the fluid velocity (unit: m / s), d is the characteristic length of the channel (unit: m), and μ is the fluid viscosity (unit: Pa·s). At the various flow rates (200 μL / h ~ 1500 μL / h) set in this invention, the calculated Re values ​​are all much less than 1, clearly indicating that viscous force is the dominant force, and the inertial effect can be ignored, which meets the laminar flow conditions.

[0026] In low Reynolds number environments, fluid viscosity is extremely strong. In parallel channels, even if two liquid layers are in close contact and have different flow velocities, they will only flow in parallel and in layers, and will not automatically mix due to the flow itself.

[0027] like Figure 2 As shown, the method for enriching micro / nanoplastics with microfluidic chips is as follows: A sample solution containing micro- and nano-plastics is injected into the sample channel at a set flow rate. A positive gradient voltage is applied to the electrode, causing the negatively charged micro- and nano-plastics in the sample channel to migrate to the enrichment channel under the action of dielectric force. Then, deionized water is injected into the enrichment channel to enrich the micro- and nano-plastics. Finally, the solution at the outlet of the enrichment channel is collected for detection or subsequent processing.

[0028] exist Figure 2 In the process, the surface electrode of the sample channel is connected to the negative terminal of the power supply, and the surface electrode of the enrichment channel is connected to the positive terminal of the power supply. DC voltages of 1 V, 5 V, 10 V, 15 V and 20 V are applied sequentially to the five electrodes from the sample channel inlet to the sample channel outlet.

[0029] Example 1 This embodiment describes the specific fabrication and application process of a microfluidic chip; Step (1): In the fabrication process of the microfluidic chip, a three-dimensional model of the chip is first established using COMSOL Multiphysics software to simulate the electric field distribution and particle motion trajectory. The optimized structure is then fabricated into a microfluidic chip through processes such as photolithography, etching, and bonding. In this embodiment, the fabrication method of the microfluidic chip includes the following steps: The microfluidic chip used in this study consists of four functional layers: the first layer is the upper electrode layer, the second layer is the sample channel layer, the third layer is the enrichment channel layer, and the fourth layer is the lower electrode layer.

[0030] S1, Fabrication of the chip substrate. Two glass slides and two thin glass slides were ultrasonically cleaned with acetone, ethanol, and deionized water, respectively. This cleaning process was repeated three times. Then, nitrogen gas was used to blow onto the surfaces of the glass slides and thin glass slides to dry them and ensure that no impurities were attached. S2, fabrication of the first layer (upper electrode layer) and the fourth layer (lower electrode layer). The dried glass slide from the first step is fixed on the chuck of the spin coater. Positive photoresist (RZJ-304) is dropped onto the glass slide. First, it is rotated at low speed (600 rpm) for 10 seconds to evenly spread the photoresist, then rotated at high speed (3000 rpm) for 40 seconds. The coated glass slide is then placed on a hot plate and baked at 100°C for 1 minute, then allowed to cool naturally to room temperature. A photomask with electrode patterns is aligned with the glass slide, and then the glass slide is placed in a UV lithography machine for exposure. The exposed glass slide is immersed in developer (RZX-3038) for 1 minute, then rinsed with deionized water and dried. The patterned glass slide is placed in a magnetron sputtering system to deposit a chromium layer, which forms a thin adhesion layer of approximately 20 nm on the surface of the glass slide, and then a 200 nm thick conductive gold layer is sputtered. Finally, the sputtered slides were immersed in acetone and sonicated for 10 minutes to accelerate the peeling process. S3, Preparation of the second layer (sample channel layer) and the third layer (enrichment channel layer). First, a two-dimensional graphic with precise sample channel dimensions and shape, as well as enrichment channels, is drawn using the software provided with the laser processing system. Then, a thin glass sheet is mounted onto the laser processing platform. The laser beam is controlled to scan the glass surface along a preset pattern path. Finally, the ultraviolet laser is activated, causing the glass material to immediately form microchannels in the irradiated area.

[0031] S4, Bonding of electrode and channel layers. A glass slide covered with electrodes is aligned and bonded to a thin glass slide with microchannels using a vacuum bonding machine to form a chip structure including upper and lower channel layers and electrode layers; S5, PDMS Preparation. Mix the PDMS base and curing agent at a weight ratio of 10:1 until homogeneous. Then, remove air bubbles from the mixture using a vacuum system. Pour the mixture into a mold and cure in an oven at 100 °C for 1 h. After natural cooling, use a punching tool to drill two holes, 1 mm in diameter, corresponding to the upper and lower channels. This allows subsequent samples to be introduced and removed from the chip through the holes in the PDMS. S6, Bonding PDMS to the microfluidic chip. The perforated PDMS block and microfluidic chip are placed in a plasma cleaner. The plasma is turned on, and a 1-minute timer begins once a purple-red glow appears. The PDMS block and microfluidic chip are then removed and aligned using tweezers. They are transferred to a 100°C oven and baked for 20 minutes to achieve adhesion. Finally, the bonded microfluidic chip is removed, secured with clips, and a syringe (for connecting a syringe) is inserted into the hole in the PDMS.

[0032] In this embodiment, the electrodes are gold electrodes, and there are five of them. Each electrode is 3 mm long and 0.9 mm wide, with a spacing of 3 mm between adjacent electrodes. The gradient voltage along the flow direction is 1 V, 5 V, 10 V, 15 V, and 20 V, and can be adjusted according to the particle concentration and size. The sample channel has a cross-sectional dimension of 33 mm long, 0.3 mm high, and 0.6 mm wide; the enrichment channel has a cross-sectional dimension of 33 mm long, 0.15 mm high, and 0.6 mm wide.

[0033] Step (2): After preparation, the first syringe pump draws in the sample solution and connects it to the microplastic particle inlet of the chip through a PTFE tube. The second syringe pump draws in deionized water and connects it to the enrichment channel inlet of the chip. The positive terminal of the DC power supply is connected to the five electrodes in sequence, and the negative terminal is grounded. The sample channel outlet is connected to the waste liquid tube through a PTFE tube, and the enrichment channel outlet is connected to the collection tube through a PTFE tube. After connection, 2 μm polystyrene microspheres with orange fluorescence are added to the syringe of the first syringe pump, and deionized water is added to the second syringe pump. After addition, DC voltages of 1 V, 5 V, 10 V, 15 V, and 20 V are applied to the five electrodes in sequence. Working principle Figure 2 As shown; Step (3): Subsequently, a simulated water sample containing orange fluorescent polystyrene microspheres was injected into the inlet of the microplastic particles at a flow rate of 200 μL / h. After enrichment, the enrichment channel was eluted with deionized water at a flow rate of 100 μL / h. After all the sample was injected, the DC power was turned off and the concentrate was collected. Finally, the concentrate was observed using a fluorescence microscope to compare the microplastic enrichment before and after the microfluidic chip was applied. Before enrichment, the polystyrene was as follows: Figure 3 As shown in (a), the enriched polystyrene is as follows: Figure 3 As shown in (b), it is clear that the microplastics are more numerous under a fluorescence microscope after enrichment, which proves that the designed microfluidic chip can effectively enrich polystyrene microplastics.

[0034] Example 2 The only difference between Example 2 and Example 1 is that in step (3), a simulated water sample containing polystyrene microspheres is injected into the inlet of the microplastic particles at a flow rate of 600 μL / h; enriched polystyrene, such as Figure 3 As shown in (c), it is clear that the microplastics are more numerous under a fluorescence microscope after enrichment, which also proves that the designed microfluidic chip can effectively enrich polystyrene microplastics.

[0035] Example 3 The only difference between Example 3 and Example 1 is that in step (3), a simulated water sample containing polystyrene microspheres is injected into the inlet of the microplastic particles at a flow rate of 1000 μL / h. enriched polystyrene, such as Figure 3 As shown in (d), it is clear that the microplastics are more numerous under a fluorescence microscope after enrichment, which also proves that the designed microfluidic chip can effectively enrich polystyrene microplastics.

[0036] Example 4 The only difference between Example 4 and Example 1 is that in step (3), a simulated water sample containing polystyrene microspheres is injected into the inlet of the microplastic particles at a flow rate of 1500 μL / h. enriched polystyrene, such as Figure 3 As shown in (e), it is clear that the microplastics are more numerous under a fluorescence microscope after enrichment, which also proves that the designed microfluidic chip can effectively enrich polystyrene microplastics.

[0037] Example 5 The only difference between Example 5 and Example 1 is that: in step (2), 500 nm polystyrene microspheres with green fluorescence are selected for enrichment, and in step (3), simulated water samples of polystyrene microspheres are injected into the inlet of microplastic particles at flow rates of 200 μL / h, 600 μL / h, 1000 μL / h, and 1500 μL / h, respectively.

[0038] enriched pre-polystyrene, such as Figure 4 As shown in (a), the enriched polystyrene is as follows: Figure 4 As shown in (b)-(e), Figure 4 (b), (c), (d), and (e) in the figure correspond to the effect of microfluidic chip enrichment of 500 nm polystyrene microspheres with green fluorescence at sample flow rates of 200 μL / h, 600 μL / h, 1000 μL / h, and 1500 μL / h, respectively. It can be clearly seen that the microplastics are more numerous under the fluorescence microscope after enrichment, which also proves that the designed microfluidic chip can effectively enrich polystyrene microplastics.

[0039] Example 6 The only difference between Example 6 and Example 1 is that: in step (2), 20 nm polystyrene microspheres are selected for enrichment, and in step (3), simulated water samples of polystyrene microspheres are injected into the inlet of microplastic particles at flow rates of 200 μL / h, 600 μL / h, 1000 μL / h, and 1500 μL / h, respectively.

[0040] enriched pre-polystyrene, such as Figure 5 As shown in (a), the enriched polystyrene is as follows: Figure 5 As shown in (b)-(e), Figure 5 Figures (b), (c), (d), and (e) show the effects of microfluidic chip enrichment of 20 nm polystyrene microspheres at sample flow rates of 200 μL / h, 600 μL / h, 1000 μL / h, and 1500 μL / h, respectively. It can be clearly seen that the number of microplastics under the scanning microscope is greater after enrichment, which also proves that the designed microfluidic chip can effectively enrich polystyrene microplastics.

[0041] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0042] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A microfluidic chip based on a gradient electric field, characterized in that, include: The sample channel and enrichment channel are arranged vertically, parallel to each other, and aligned vertically. Multiple electrodes are uniformly distributed along the length of the channel on the upper surface of the sample channel and on the lower surface of the enrichment channel, and the electrodes on the sample channel correspond one-to-one with the electrodes on the enrichment channel. The electrodes can be independently voltaged and form a positive gradient electric field that increases along the sample flow direction, causing the negatively charged micro-nanoplastics in the sample channel to migrate to the enrichment channel under the action of dielectric force.

2. The microfluidic chip based on gradient electric field according to claim 1, characterized in that, The sample channel and enrichment channel are located on the surfaces of two thin glass slides, and the two thin glass slides are attached to each other, so that the sample channel and enrichment channel are aligned vertically and connected.

3. The microfluidic chip based on gradient electric field according to claim 1, characterized in that, The electrodes are made of gold and there are 5 of them; each electrode is 3 mm long and 0.9 mm wide, and the distance between adjacent electrodes is 3 mm.

4. The microfluidic chip based on gradient electric field according to claim 3, characterized in that, The voltage of the electrodes is set sequentially along the flow direction as follows: 1 V, 5 V, 10 V, 15 V, 20 V.

5. The microfluidic chip based on gradient electric field according to claim 1, characterized in that, The sample channel has a cross-sectional dimension of 33 mm in length, 0.3 mm in height, and 0.6 mm in width; the enrichment channel has a cross-sectional dimension of 33 mm in length, 0.15 mm in height, and 0.6 mm in width.

6. The application of the microfluidic chip according to any one of claims 1-5 in the enrichment of micro-nanoplastics in water samples.

7. A method for enriching micro / nanoplastics in a water sample, using the microfluidic chip according to any one of claims 1-5, characterized in that, include: Inject water samples containing micro-nanoplastics into the sample channel; A positive gradient voltage is applied to the electrode, causing the negatively charged micro- and nano-plastics in the sample channel to migrate to the enrichment channel under the action of dielectric force; Then, deionized water was injected into the enrichment channel to enrich the micro- and nano-plastics, and finally the solution at the outlet of the enrichment channel was collected.

8. The method for enriching micro- and nano-plastics in a water sample according to claim 7, characterized in that, The flow rate of the water sample injection channel containing micro-nanoplastics is 200 μL / h ~ 1500 μL / h.

9. The method for enriching micro- and nano-plastics in a water sample according to claim 7, characterized in that, The particle size of the micro-nanoplastics is 20 nm to 2 μm.

10. A method for fabricating a microfluidic chip according to any one of claims 1-5, characterized in that, Includes the following steps: S1. Take two glass slides and two thin glass slides, clean and dry them for later use; S2, the upper electrode and the lower electrode are formed on two glass slides by photolithography, development and magnetron sputtering of conductive gold layers respectively; S3, design the pattern of sample channel and enrichment channel, then use a laser beam to scan the surface of two thin glass slides along the preset pattern path, and then activate the ultraviolet laser to form sample channel and enrichment channel on the surface of the two thin glass slides respectively. S4, Align and bond the glass slide covered with electrodes and the thin glass slide with channels so that the sample channel and the enrichment channel are aligned and connected vertically, forming a microfluidic chip including upper and lower electrodes, sample channels and enrichment channels. S5. Two PDMS blocks are respectively bonded to both ends of the microfluidic chip. Two through holes are opened on the PDMS, which are respectively opposite to the sample channel and the enrichment channel. A needle is installed in the through hole for connecting the syringe.

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