A method for preparing a foldable paper-based microfluidic chip for LIBS heavy metal detection

Abstract

The application belongs to the field of laser spectrum chemical analysis technology and heavy metal detection, and particularly relates to a preparation method of a foldable paper-based microfluidic chip for LIBS heavy metal detection. The method comprises paper-based microfluidic chip structure design and substrate construction, COF enrichment functional zone construction, nanoparticle signal enhancement functional zone construction, and integration and stabilization treatment of the foldable paper-based microfluidic chip. The application integrates the COF enrichment functional zone and the nanoparticle signal enhancement functional zone on the paper-based microfluidic chip, realizes spatial alignment and contact of the two by using the foldable structure, and completes heavy metal enrichment and signal enhancement in the same platform. The integrated design effectively reduces sample transfer and loss, reduces matrix interference, and significantly improves the detection sensitivity of LIBS to trace heavy metals. The method has the advantages of simple operation, short detection period, small sample consumption, etc., is suitable for on-site rapid detection, improves the practical applicability, and has a wide market application prospect.

Description

Technical Field

[0001] This invention belongs to the field of laser spectrochemical analysis technology and heavy metal detection, specifically relating to a method for preparing a foldable paper-based microfluidic chip for LIBS heavy metal detection. Background Technology

[0002] With the acceleration of industrialization and the intensification of agricultural activities, large amounts of wastewater containing heavy metals are being discharged into the environment, leading to increasingly serious heavy metal pollution problems. Heavy metals such as lead, cadmium, and chromium are highly toxic, persistent, and bioaccumulative, easily accumulating in environmental media and the food chain, posing a serious threat to ecosystems and human health. Therefore, achieving rapid, sensitive, and portable detection of trace heavy metals in water bodies is of great significance.

[0003] Laser-induced breakdown spectroscopy (LIBS), a novel atomic emission spectroscopy technique, utilizes high-energy lasers to generate plasma on the sample surface, enabling qualitative and quantitative elemental detection by analyzing the plasma spectrum. LIBS offers advantages such as requiring no complex pretreatment, simultaneous detection of multiple elements, fast response speed, and suitability for on-site detection in complex environments, demonstrating promising application potential in fields like environmental monitoring, food safety, and geological analysis. However, in trace heavy metal detection, LIBS faces technical bottlenecks including relatively high detection limits, significant matrix effects, and difficulties in direct analysis of water samples.

[0004] Enrichment pretreatment to increase the local concentration of target elements has become an important approach to improve the sensitivity of LIBS detection. Covalent organic frameworks (COFs), due to their advantages such as large specific surface area, tunable pore size, and designable chemical functional groups, can achieve the detection of Pb. 2+ Cr 3+ Cd 2+ Highly efficient and selective adsorption of metal ions is achieved. Simultaneously, metal nanoparticles (such as AuNPs and AgNPs) can significantly enhance the LIBS signal through surface plasmon resonance. However, existing techniques often separate enrichment and enhancement, resulting in numerous and time-consuming steps, and nanoparticles are prone to aggregation in the liquid phase. Furthermore, most structures cannot achieve spatial isolation and temporal control of enrichment and enhancement within the chip.

[0005] Therefore, developing a method for fabricating a foldable paper-based microfluidic chip for LIBS heavy metal detection, in order to realize an integrated "enrichment-enhancement-LIBS" system for trace heavy metal detection, has significant technical value. Summary of the Invention

[0006] To address the problems of high detection limit, low sensitivity, significant matrix effect, and inability to integrate enrichment and enhancement steps in existing LIBS trace heavy metal detection methods, this invention provides a method for fabricating a foldable paper-based microfluidic chip for LIBS heavy metal detection.

[0007] To achieve the above technical objectives, the present invention adopts the following solution: A method for fabricating a foldable paper-based microfluidic chip for LIBS heavy metal detection, the specific steps of which are as follows: Step one, paper-based microfluidic chip structure design and substrate construction, includes the following processes: Step 1: Overall Chip Structure Design Design an integrated foldable paper-based microfluidic chip: The paper-based microfluidic chip is divided into an enrichment functional region and a signal enhancement functional region that are interconnected along the fold line; the enrichment functional region and the signal enhancement functional region can correspond to each other in spatial position after folding. Step 2: Construction of microfluidic channel and functional area patterns: After the paper-based substrate is operated according to the design of Process 1, an enrichment functional area and a signal enhancement functional area are formed; the signal enhancement functional area is the nanoparticle signal enhancement functional area (4); the enrichment functional area is the COF enrichment functional area (3), and microfluidic channels are provided on both sides of the COF enrichment functional area (3), which are connected to the sample inlet (1) and the sample outlet (5) respectively. Step 3: Construction of hydrophobic barriers and fold lines: A hydrophobic barrier is constructed on the paper substrate surface using wax printing, laser ablation, or hydrophobic coating. The hydrophobic barrier is constructed in the area outside the nanoparticle signal enhancement functional area (4), COF enrichment functional area (3), microfluidic channel, inlet (1), and outlet (5). At the same time, a folding line is set between the nanoparticle signal enhancement functional area (4) and the COF enrichment functional area (3) to ensure the flexibility and structural stability of the paper substrate microfluidic chip during the folding process.

[0008] Step two, the construction of the COF enrichment functional region, includes the following process: Step 1: Selection and preparation of COF materials: Select the target heavy metal ion (such as Pb) 2+ A covalent organic framework material with high selective adsorption capacity is used as the enrichment material, wherein the covalent organic framework material is DAA-TFPT-COF; The DAA-TFPT-COF is a covalent organic framework material containing an imine bond structure, which is prepared by a condensation reaction of an aldehyde-containing polyaldehyde monomer and an amino-containing aromatic diamine monomer.

[0009] Step 2, Construction of the COF fixed bed: The DAA-TFPT-COF material from process one is dispersed in a mixture of water and polymer binder to form a COF suspension; then the COF suspension is loaded into the COF enrichment functional region (3) to form a COF enrichment bed, thus obtaining a paper base loaded with COF.

[0010] Step 3: Drying and Stabilization Treatment A paper substrate loaded with COF was subjected to low-temperature drying to obtain a paper-based microfluidic chip with a COF-enriched bed.

[0011] Step 3, the construction of the signal enhancement functional region of nanoparticles, includes the following processes: Step 1: Selection of Nanoreinforcing Materials Metal nanoparticles with plasma enhancement effects are selected as LIBS signal enhancement materials; the metal nanoparticles include gold nanoparticles, silver nanoparticles or gold core-silver shell structured nanoparticles, with Au@Ag core-shell structured nanoparticles being preferred.

[0012] Process 2: Loading of nanoparticles on paper substrate: The metal nanoparticles in process one are dispersed in a mixed solution of water, dispersion stabilizer and viscosity modifier to form a nanoparticle suspension; then the nanoparticle suspension is loaded into the nanoparticle signal enhancement functional area (4), and after standing for 1~5 min, a paper base loaded with nanoparticles is obtained.

[0013] Step 3: Drying and Curing The paper substrate loaded with nanoparticles was dried to obtain a paper-based microfluidic chip loaded with nanoparticles.

[0014] Step four, the integration and stabilization of the foldable paper-based microfluidic chip, includes the following processes: Step 1: Alignment and folding matching confirmation of functional areas: After completing the construction of the COF enrichment functional region (3) and the nanoparticle signal enhancement functional region (4), a paper-based microfluidic chip loaded with functional materials is obtained. First, the paper-based microfluidic chip loaded with functional materials is inspected in its unfolded state to confirm the relative positional relationship between the two functional regions on both sides of the fold line. Through folding test, it is verified that after the paper-based microfluidic chip is folded along the preset fold line, the COF enrichment functional region (3) and the nanoparticle signal enhancement functional region (4) can accurately correspond in spatial position and overlap and contact after folding.

[0015] Step 2: Overall chip drying and functional area stabilization treatment: Secondly, the paper-based microfluidic chip loaded with functional materials undergoes a uniform low-temperature drying process to obtain the final foldable paper-based microfluidic chip for LIBS heavy metal detection.

[0016] Preferably, the fold lines in step one of process one are formed by mechanical indentation.

[0017] Preferably, the paper-based substrate in step one of process two is quantitative filter paper or chromatography filter paper.

[0018] Preferably, the DAA-TFPT-COF material described in step two of process one is prepared by reacting 2,4,6-tris(4-formylphenyl)-1,3,5-triazine with 2,5-diaminobenzoic acid.

[0019] Preferably, the polymeric binder in step two of process two includes polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), or chitosan; the mass ratio of the DAA-TFPT-COF material, water, and polymeric binder is 1:(100~500):(1~10).

[0020] Preferably, the loading method in step two of process two includes dripping or impregnation, wherein when dripping, 10~60 μL is added per square centimeter based on the area of ​​the COF enrichment functional region (3).

[0021] Preferably, the conditions for the low-temperature drying process in step two, process three, are: drying at 30~60 ℃ for 10~30 min.

[0022] Preferably, the dispersion stabilizer in step three, process two, includes polyvinylpyrrolidone (PVP) or polyvinyl alcohol (PVA); the viscosity modifier includes glycerol, ethylene glycol or propylene glycol; the mass ratio of the metal nanoparticles, water, dispersion stabilizer and viscosity modifier is 1:(1000~10000):(1~20):(50~500); the loading method includes dripping, spraying or impregnation, wherein when dripping, 10~60 μL is added per square centimeter according to the area of ​​the signal enhancement functional region (4) of the nanoparticles.

[0023] Preferably, the drying process in step three includes slow drying in an environment with a relative humidity of 70% followed by low-temperature heat treatment at 40°C for curing, with a drying time of 10~30 min.

[0024] Preferably, the conditions for low-temperature drying in step four, process two, are: drying at 30~60 ℃ for 30~60 min.

[0025] Preferably, in step four, the foldable paper-based microfluidic chip in process two can be partially encapsulated or reinforced in the non-functional area of ​​the foldable paper-based microfluidic chip according to actual use needs; the partial encapsulation or reinforcement includes covering with a transparent protective film or support layer; the partial encapsulation is only set at the edge of the paper-based microfluidic chip or in the non-microfluidic area, and the encapsulation material used is a transparent polymer film.

[0026] The application of foldable paper-based microfluidic chips for LIBS heavy metal detection includes the following processes: S1. Liquid sample loading and heavy metal enrichment: First, a standard solution containing metal ions is prepared. Then, the standard solution is added to the inlet (1) of the paper-based microfluidic chip. Driven by capillary action, the sample solution flows through the COF enrichment functional region (3) along the pre-constructed microfluidic channel (2) to obtain a paper-based microfluidic chip enriched with heavy metal ions. S2. Construction of the contact and signal enhancement system for the folding trigger functional area: The paper-based microfluidic chip enriched with heavy metal ions obtained in step S1 is folded along a preset folding line so that the COF enrichment functional region (3) and the nanoparticle signal enhancement functional region (4) are aligned and overlapped in space.

[0027] S3. Drying and LIBS testing: The paper-based microfluidic chip folded in step S2 was subjected to low-temperature drying. After drying, the paper-based microfluidic chip was unfolded along the original fold line so that the COF enrichment functional region (3) enriched with the target heavy metal was exposed on the surface of the paper-based microfluidic chip. LIBS was used to collect signals from the target region to obtain the collected LIBS spectral signal. The signal was then analyzed by a univariate model with the concentration of heavy metal ions. The linear relationship between the intensity of the sensitive spectral line of heavy metal and the concentration of heavy metal ions was compared to construct a standard curve.

[0028] S4. Sample detection: Replace the standard solution with the sample to be tested, and follow the steps from S1 to S3 to obtain the LIBS spectral signal. Substitute the signal into the standard curve constructed in S3 to achieve the detection of metal ions in the unknown sample.

[0029] Preferably, the metal ions in S1 include Pb. 2+ Cd 2+ and Cr 3+ The concentration range of the standard solution is 0.05~10 mg / L.

[0030] Preferably, the low-temperature drying process in S3 uses a temperature of 40°C and a drying time of 5-10 min; the parameters for signal acquisition are: laser wavelength of 1064 nm or 532 nm, laser energy of 20-60 mJ, delay time of 2 μs, integration time of 10 μs, and laser pulse repetition frequency of 1 Hz.

[0031] The beneficial effects of this invention are: Compared with existing technologies, the method for fabricating a foldable paper-based microfluidic chip for LIBS heavy metal detection provided by this invention has the following advantages: (1) Compared with the problems of separation of enrichment and signal enhancement processes and cumbersome operation steps in traditional LIBS detection, this invention integrates the COF enrichment functional region and the nanoparticle signal enhancement functional region on the same paper-based microfluidic chip, and uses a foldable structure to achieve spatial alignment and contact between the two, so that heavy metal enrichment and signal enhancement can be completed on the same platform. This integrated design effectively reduces sample transfer and loss, reduces matrix interference, and significantly improves the detection sensitivity of LIBS for trace heavy metals.

[0032] (2) This invention uses a covalent organic framework material with electron-rich coordination sites as an enrichment medium, which can selectively adsorb target heavy metal ions, thereby achieving effective separation of complex sample matrices in a paper-based microfluidic system. Compared with direct LIBS detection or conventional adsorption materials, this invention has significant advantages in reducing matrix effects and improving detection accuracy.

[0033] (3) This invention introduces metal nanoparticles with plasma enhancement effect into the signal enhancement functional region, so that the enriched heavy metal and nanoparticles form a tight contact interface in the folded state, thereby enhancing the plasma generation and radiation emission intensity during laser-induced breakdown. This structural design effectively improves the problems of weak signal and large fluctuation in traditional LIBS, and improves the stability of the detection signal and the quantitative analysis capability.

[0034] (4) This invention uses a paper-based microfluidic chip as a carrier, relying on capillary force to drive liquid flow, without the need for an external pump or complex fluid control device; the enrichment zone and enhancement zone can be combined through folding operation, and the operation steps are simple and intuitive. Compared with traditional laboratory pretreatment and detection methods, this invention has the advantages of simple operation, short detection cycle, and small sample consumption, and is suitable for rapid on-site detection. Attached Figure Description

[0035] Figure 1This is a schematic diagram of the foldable paper-based microfluidic chip of the present invention in its unfolded state; in the figure, 1-sample inlet; 2-microfluidic channel; 3-COF enrichment functional area; 4-nanoparticle signal enhancement functional area; 5-sample outlet.

[0036] Figure 2 This is a schematic diagram of the foldable paper-based microfluidic chip of the present invention after folding, illustrating the spatial alignment and overlapping contact state of the COF enrichment functional region and the nanoparticle signal enhancement functional region.

[0037] Figure 3 This is a flowchart illustrating the workflow of the foldable paper-based microfluidic chip described in this invention when used for LIBS detection. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail with the help of the following embodiments.

[0039] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art.

[0040] While only preferred methods and materials have been described in this invention, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe the methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0041] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0042] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0043] Example 1: Step one, paper-based microfluidic chip structure design and substrate construction, includes the following processes: Step 1: Overall Chip Structure Design Design an integrated foldable paper-based microfluidic chip: The paper-based microfluidic chip is divided into an enrichment functional region and a signal enhancement functional region along a preset fold line. The enrichment functional region is used to load COF enrichment material, and the signal enhancement functional region is used to load metal nanoparticle signal enhancement material. After the chip is folded along the fold line, the enrichment functional region and the signal enhancement functional region can be precisely aligned and contact each other in space to achieve in-situ contact and synergistic effect between the enrichment material and the signal enhancement material.

[0044] Step 2: Construction of microfluidic channel and functional area patterns: After the paper-based substrate is operated according to the design of Process 1, an enrichment functional region and a signal enhancement functional region are formed; the signal enhancement functional region is the nanoparticle signal enhancement functional region 4; the enrichment functional region is the COF enrichment functional region 3, and microfluidic channels are provided on both sides of the COF enrichment functional region 3, which are connected to the sample inlet 1 and the sample outlet 5 respectively. Step 3: Construction of hydrophobic barriers and fold lines: Slow-speed quantitative filter paper was selected as the substrate of the paper-based microfluidic chip. A hydrophobic barrier was constructed in the area outside the nanoparticle signal enhancement functional region 4, COF enrichment functional region 3, microfluidic channel, inlet 1 and outlet 5 by wax printing to limit the flow of liquid sample within the predetermined microfluidic channel 2. At the same time, fold lines were set between the nanoparticle signal enhancement functional region 4 and the COF enrichment functional region 3 by mechanical indentation to ensure the flexibility and structural stability of the paper-based microfluidic chip during the folding process.

[0045] Step two, the construction of the COF enrichment functional region, includes the following process: Step 1: Selection and Adsorption Performance Evaluation of COF Materials A covalent organic framework (COF) material with high selective adsorption capacity for Pb²⁺ was selected as the enrichment material. This COF material contains electron-rich coordination sites, enabling efficient and selective enrichment of heavy metal ions through coordination, chelation, or electrostatic adsorption. In this embodiment, DAA-TFPT-COF is preferably used as an example. DAA-TFPT-COF material is synthesized from 2,4,6-tris(4-formylphenyl)-1,3,5-triazine and 2,5-diaminobenzoic acid via a Schiff base condensation reaction under solvothermal conditions. The specific synthesis method is as follows: 0.0264 g of 2,4,6-tris(4-formylphenyl)-1,3,5-triazine and 0.0152 g of 2,5-diaminobenzoic acid were dissolved in a mixed solvent of 1,3,5-trimethylbenzene and 1,4-dioxane. The mixture was reacted in an ampoule at 120 °C for 72 h. After the reaction was completed, the material was washed and vacuum dried to obtain the DAA-TFPT-COF material. The carboxyl group and nitrogen-containing aromatic heterocyclic structure introduced into the structure of the DAA-TFPT-COF material are Pb. 2+The material provides the main coordination and electrostatic adsorption sites, while the covalent organic framework constructed by imine bonds endows it with good structural stability, regular pore structure, and high specific surface area, fully exposing the aforementioned active sites and synergistically promoting the enrichment of Pb²⁺ during adsorption, thereby achieving the desired Pb adsorption. 2+ Highly efficient enrichment.

[0046] To ensure the applicability and enrichment efficiency of the selected DAA-TFPT-COF material in paper-based microfluidic chips, the adsorption performance of DAA-TFPT-COF was evaluated. Specifically, at room temperature, 10 mg of DAA-TFPT-COF material was added to 20 mL of Pb at initial concentrations of 0–10 mg / L (0, 0.05, 0.1, 0.5, 1, 10 mg / L). 2+ In the standard solution, the pH was adjusted to 6.0 using 0.01 mol / L HNO3 or 0.01 mol / L NaOH solution, and adsorption experiments were conducted under constant temperature shaking conditions at 25 ℃ and 150 rpm. Samples were taken at different adsorption times (1, 5, 10, 15, 20, 25, and 30 min) to detect the Pb content in the solution. 2+ The remaining concentration of DAA-TFPT-COF was used to evaluate the effect of DAA-TFPT-COF on Pb. 2+ The adsorption performance of Pb was measured experimentally under different initial concentrations. 2+ The residual concentration of the solution after different adsorption times is shown in Table 1.

[0047] Table 1

[0048] Table 1 shows that under different initial concentrations, the concentration of Pb in the solution... 2+ The concentrations of Pb gradually decreased with increasing adsorption time, indicating that the DAA-TFPT-COF material has a high affinity for Pb. 2+ It exhibits significant adsorption. In the initial adsorption phase (1-5 min), Pb in the solution... 2+ The concentration decrease was quite significant. Under the condition of an initial concentration of 1.00 mg / L, the concentration of Pb in the solution decreased significantly after 5 minutes of adsorption. 2+ The concentration decreased from 1.00 mg / L to 0.61 mg / L, with a removal rate of approximately 39%. At an initial concentration of 10.00 mg / L, after 5 minutes of adsorption, the solution concentration decreased from 10.00 mg / L to approximately 5.20 mg / L, with a removal rate of approximately 48%. With further extension of adsorption time, Pb... 2+ The concentration continued to decrease, but the rate of decrease gradually slowed down and gradually stabilized in the 20-30 minute range.

[0049] DAA-TFPT-COF can target Pb levels in the mg / L range within a short time (5 min). 2+ The significant adsorption demonstrates that this material possesses excellent adsorption kinetics and high affinity, enabling it to meet the requirements of subsequent paper-based microfluidic chips for Pb adsorption at even lower concentrations. 2+ To meet the enrichment and detection requirements, the adsorption time was uniformly set to 5 minutes to improve detection efficiency.

[0050] Step 2, Construction of the COF fixed bed: 50 mg of DAA-TFPT-COF material was dispersed in 10 mL of deionized water, and 0.25 g of polyvinylpyrrolidone was added as a binder to prepare a uniform and stable COF suspension. The suspension was loaded into the COF enrichment functional region 3 defined by the hydrophobic barrier, and 10 μL was added dropwise each time. After natural permeation, the addition continued until a total of 60 μL was added. Under the capillary action and the binder-assisted fixation, the COF material was uniformly embedded in the paper-based pore structure to form a stable COF enrichment bed.

[0051] Step 3: Drying and Stabilization Treatment The paper-based microfluidic chip loaded with COF was dried at 40°C for 10 min to remove the solvent and further enhance the bonding stability between the COF material and the paper-based fiber.

[0052] Step 3, the construction of the signal enhancement functional region of nanoparticles, includes the following processes: Step 1: Selection and preparation of nano-reinforcing materials: Metal nanoparticles with good localized surface plasmon resonance (SPR) effect were selected as LIBS signal enhancement materials. In this embodiment, Au@Ag core-shell structured nanoparticles were preferably used. These nanoparticles were obtained by first preparing an Au nanoparticle core and then growing an Ag shell on its surface. The specific method is as follows: 100 μL of 25 mM tetrachloroauric acid solution was added to 10 mL of deionized water and heated to boiling. Then, 50 μL of 75 mM sodium citrate solution was rapidly added under continuous stirring, and the reaction was continued to boil for 2 min until the solution turned orange-red, resulting in an Au nanoparticle suspension with a particle size of approximately 40-50 nm. This suspension was then cooled to room temperature for later use. 500 μL of the above Au nanoparticle suspension was added to 8 mL of deionized water and dispersed evenly under magnetic stirring. Then, 50 μL of 75 mM sodium citrate solution was added. Subsequently, 45 μL of 10 mM silver nitrate solution was added, followed immediately by 90 μL of 10 mM ascorbic acid solution. The reaction was continued under magnetic stirring at room temperature for 2 min. In this process, Ag is reduced and deposited on the surface of the Au core to form a shell structure, resulting in Au@Ag core-shell nanoparticles with an overall particle size of approximately 50–60 nm. After the reaction, the resulting nanoparticle suspension is purified by centrifugation to remove unreacted ions and byproducts. These nanoparticles can effectively enhance the local electromagnetic field intensity and plasma formation process under laser irradiation, thereby improving the signal intensity and detection sensitivity of the characteristic emission lines of the target heavy metal elements.

[0053] Process 2: Loading of nanoparticles on paper substrate: 5 mg of Au@Ag core-shell nanoparticles were dispersed in 5 mL of deionized water, and 0.05 g of PVP and 0.25 mL of glycerol were added. The mixture was sonicated for 10 min under ice bath conditions to prepare a uniform and stable nanoparticle suspension. This suspension was then added dropwise to a pre-defined nanoparticle signal enhancement region 4 in a paper-based microfluidic chip, 10 μL at a time, allowing for natural permeation before continuing to add until a total of 60 μL was added. This ensured that the nanoparticles were uniformly embedded in the pores of the paper fibers and formed a stable load. After standing for 5 min, the paper substrate loaded with nanoparticles was obtained.

[0054] Step 3: Drying and Curing The paper substrate loaded with nanoparticles was slowly dried for 10 minutes in an environment with a relative humidity of approximately 70% to suppress capillary flow during solvent evaporation and promote uniform deposition of nanoparticles into the pores of the paper substrate. Subsequently, the paper substrate loaded with nanoparticles was dried in a drying oven at 40°C for 30 minutes to allow the stabilizer molecular chains to rearrange and enhance the bonding force between the nanoparticles and the paper substrate fibers, thereby obtaining a signal enhancement functional region that is uniformly distributed, firmly attached, and structurally stable.

[0055] Step four, the integration and stabilization of the foldable paper-based microfluidic chip, includes the following processes: Step 1: Alignment and folding matching confirmation of functional areas: After completing the construction of COF enrichment functional region 3 and nanoparticle signal enhancement functional region 4, the paper-based microfluidic chip was structurally inspected in its unfolded state. The folding test was used to verify that after the paper-based microfluidic chip was folded along the preset folding line, COF enrichment functional region 3 and nanoparticle signal enhancement functional region 4 could be accurately aligned and fully contacted in space.

[0056] Step 2: Overall chip drying and functional area stabilization treatment: The paper-based microfluidic chip loaded with functional materials was placed in a forced-air drying oven and dried at 40 °C for 60 min to further enhance the stability of the functional materials on the paper substrate.

[0057] Step 5: Construction of the foldable paper-based microfluidic chip and detection method for LIBS heavy metal detection, including the following processes: Process 1: Liquid sample loading and Pb 2+ Enrichment: A 200 μL water sample is added to the inlet 1 of the paper-based microfluidic chip. Driven by capillary action, the water sample flows along the pre-constructed microfluidic channel through the COF enrichment functional region 3. In this embodiment, the Pb in the water sample is... 2+ The initial concentrations were set at 0.05 mg / L, 0.1 mg / L, 0.5 mg / L, 1.0 mg / L, and 10 mg / L, respectively. The Pb concentration in the water samples... 2+ When flowing through COF enrichment functional zone 3, Pb is selectively adsorbed and enriched in the enrichment functional zone mainly through coordination and electrostatic interactions with electron-rich coordination sites in the COF material, thereby reducing Pb concentration in the water sample. 2+ In-situ enrichment.

[0058] Comparative example: Follow the procedure in step 5, except that the paper-based microfluidic chip is replaced with ordinary filter paper and the water sample to be tested is dropped onto the surface of the filter paper; the rest of the steps are the same as in step 5, procedure 1.

[0059] Step 2: Construction of the contact and signal enhancement system for the folding trigger functional area: In Pb 2+ After enrichment (adsorption for 5 min), the paper-based microfluidic chip is folded along the preset folding line so that the COF enrichment functional region 3 and the nanoparticle signal enhancement functional region 4 are aligned and in contact in space to form a stable contact interface.

[0060] Step 3, Drying and LIBS Testing: The folded paper-based microfluidic chip was dried at 40 °C for 5 min to allow the liquid to evaporate and form a stable solid-phase detection region. The chip was then unfolded along a pre-defined fold line, exposing the COF-enriched functional region with enhanced signal on the chip surface. Laser-induced breakdown spectroscopy (LIBS) was used to detect this region at a wavelength of 1064 nm, an energy of 50 mJ, a delay of 2 μs, an integration time of 10 μs, and a pulse repetition frequency of 1 Hz. LIBS spectral signals from the stable solid-phase detection region were acquired, with a focus on analyzing the signal intensities of the sensitive characteristic emission lines of Pb: PbI 283.31 nm, PbI 368.35 nm, and PbI 405.78 nm. A univariate computational model was constructed using these three emission lines.

[0061] The modeling effect of LIBS signals collected by directly dripping heavy metal solutions onto the surface of ordinary filter paper was compared with that of LIBS signals collected by direct dripping of heavy metal solutions onto the surface of ordinary filter paper. The results are shown in Table 2.

[0062] Table 2

[0063] All three lead spectral lines showed good model correction coefficients. R 2 All values ​​were greater than 0.98, with Pb I at 405.78 nm showing the best effect. The formula for Pb content detection was constructed as y = 1.91 × 10⁻⁶. -6 x + 1.2 × 10 -5 , R 2 =0.9963, where x is the Pb spectral line signal intensity (in au) and y is the Pb detection content (in mg / L).

[0064] Example 2: Using Pb 2+ For example, the application of paper-based microfluidic chips in Pb analysis of actual water samples. 2+ Applications in detection To verify the feasibility of applying the foldable paper-based microfluidic chip described in this invention to actual water sample testing, tap water samples and river water samples were selected as testing objects, and a spiked recovery experiment was used to evaluate the detection accuracy.

[0065] Tap water and river water samples were collected and filtered through a 0.45 μm microporous membrane. Pb was then added to the samples at concentrations of 0.10 mg / L, 0.50 mg / L, and 1.00 mg / L, respectively. 2+ The standard solution is spiked.

[0066] The spiked water sample was tested according to the method described in step five of Example 1. Specifically, 200 μL of the water sample was added through the inlet 1 of the paper-based microfluidic chip. Driven by capillary action, the water sample flowed along the microfluidic channel through the COF enrichment functional region 3, where Pb... 2+ The COF material selectively adsorbs and enriches the COF in this region. After enrichment for 5 min, the paper-based microfluidic chip is folded along a preset folding line, aligning and contacting the COF enrichment functional region 3 and the nanoparticle signal enhancement functional region 4 in spatial position, thus forming a signal enhancement interface. The folded paper-based microfluidic chip is then dried at 40 °C for 5 min to allow solvent evaporation and form a stable solid-phase detection region. The chip is then unfolded again along the folding line, and LIBS is used to detect the COF enrichment functional region 3. The detection parameters are consistent with those in Example 1. Pb is calculated using the signal intensities of three characteristic spectral lines: PbI 283.31 nm, PbI 368.35 nm, and PbI 405.78 nm. 2+ Concentration, and calculate the recovery rate based on the spiked concentration. Pb in actual water samples 2+ The results of the spiked recovery experiment are shown in Table 3.

[0067] Table 3

[0068] The data in the table show that the recoveries of samples spiked at concentrations of 0.10, 0.50, and 1.00 mg / L were 94.0%–103.7%, with coefficients of variation ranging from 3.3% to 5.5%. This indicates that the method of the present invention has good stability and accuracy and can be used to detect Pb in actual water samples. 2+ The detection.

[0069] Note: The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. Therefore, although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention. All technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for fabricating a foldable paper-based microfluidic chip for LIBS heavy metal detection, characterized in that, Includes the following steps: Step one, paper-based microfluidic chip structure design and substrate construction, includes the following processes: Step 1: Overall Chip Structure Design Design an integrated foldable paper-based microfluidic chip: The paper-based microfluidic chip is divided into an enrichment functional region and a signal enhancement functional region that are interconnected along the fold line; the enrichment functional region and the signal enhancement functional region can correspond to each other in spatial position after folding. Step 2: Construction of microfluidic channel and functional area patterns: After the paper-based substrate is operated according to the design of Process 1, an enrichment functional area and a signal enhancement functional area are formed; the signal enhancement functional area is the nanoparticle signal enhancement functional area (4); the enrichment functional area is the COF enrichment functional area (3), and microfluidic channels are provided on both sides of the COF enrichment functional area (3), which are connected to the sample inlet (1) and the sample outlet (5) respectively. Step 3: Construction of hydrophobic barriers and fold lines: A hydrophobic barrier is constructed on the paper substrate surface using wax printing, laser ablation, or hydrophobic coating. The hydrophobic barrier is constructed in the area outside the nanoparticle signal enhancement functional area (4), COF enrichment functional area (3), microfluidic channel, inlet (1), and outlet (5). At the same time, a folding line is set between the nanoparticle signal enhancement functional area (4) and the COF enrichment functional area (3) to ensure the flexibility and structural stability of the paper substrate microfluidic chip during the folding process. Step two, the construction of the COF enrichment functional region, includes the following process: Step 1: Selection and preparation of COF materials: A covalent organic framework material with high selective adsorption capacity for target heavy metal ions was selected as the enrichment material, wherein the covalent organic framework material is DAA-TFPT-COF; The DAA-TFPT-COF is a covalent organic framework material containing an imine bond structure, which is prepared by a condensation reaction of an aldehyde-containing polyaldehyde monomer and an amino-containing aromatic diamine monomer. Step 2, Construction of the COF fixed bed: The DAA-TFPT-COF material from process one is dispersed in a mixture of water and polymer binder to form a COF suspension; then the COF suspension is loaded into the COF enrichment functional region (3) to form a COF enrichment bed, thus obtaining a paper base loaded with COF. Step 3: Drying and Stabilization Treatment A paper-based microfluidic chip with a COF-loaded bed was obtained by low-temperature drying of the paper substrate. Step 3, the construction of the signal enhancement functional region of nanoparticles, includes the following processes: Step 1: Selection of Nanoreinforcing Materials Metal nanoparticles with plasma-enhancing effects are selected as LIBS signal enhancement materials. The metal nanoparticles include gold nanoparticles, silver nanoparticles, or gold core-silver shell structured nanoparticles; Au@Ag core-shell structured nanoparticles are preferred. Process 2: Loading of nanoparticles on paper substrate: The metal nanoparticles in process one are dispersed in a mixed solution of water, dispersion stabilizer and viscosity modifier to form a nanoparticle suspension; then the nanoparticle suspension is loaded into the nanoparticle signal enhancement functional area (4), and after standing for 1~5 min, a paper base loaded with nanoparticles is obtained. Step 3: Drying and Curing The paper substrate loaded with nanoparticles was dried to obtain a paper-based microfluidic chip loaded with nanoparticles. Step four, the integration and stabilization of the foldable paper-based microfluidic chip, includes the following processes: Step 1: Alignment and folding matching confirmation of functional areas: After completing the construction of the COF enrichment functional region (3) and the nanoparticle signal enhancement functional region (4), a paper-based microfluidic chip loaded with functional materials is obtained. First, the paper-based microfluidic chip loaded with functional materials is inspected in the unfolded state to confirm the relative positional relationship of the two functional regions on both sides of the fold line. Through folding tests, it was verified that after the paper-based microfluidic chip was folded along the preset folding line, the COF enrichment functional area (3) and the nanoparticle signal enhancement functional area (4) could accurately correspond in spatial position and overlap and contact after folding. Step 2: Overall chip drying and functional area stabilization treatment: Secondly, the paper-based microfluidic chip loaded with functional materials undergoes a uniform low-temperature drying process to obtain the final foldable paper-based microfluidic chip for LIBS heavy metal detection.

2. The method according to claim 1, characterized in that, In step one, the fold lines are formed by mechanical indentation; in step two, the paper base material is quantitative filter paper or chromatography filter paper.

3. The method according to claim 1, characterized in that, In step two, the DAA-TFPT-COF material described in process one is prepared by reacting 2,4,6-tris(4-formylphenyl)-1,3,5-triazine with 2,5-diaminobenzoic acid; the polymeric binder described in step two includes polyvinylpyrrolidone, polyvinyl alcohol, or chitosan; the mass ratio of the DAA-TFPT-COF material, water, and polymeric binder is 1:(100~500):(1~10).

4. The method according to claim 1, characterized in that, In step two, the loading methods include dripping or impregnation. When dripping, 10-60 μL is added per square centimeter based on the area of ​​the COF enrichment functional region (3).

5. The method according to claim 1, characterized in that, The conditions for the low-temperature drying process described in step two, process three, are: drying at 30~60 ℃ for 10~30 min.

6. The method according to claim 1, characterized in that, The dispersion stabilizer mentioned in step three of process two includes polyvinylpyrrolidone (PVP) or polyvinyl alcohol (PVA); the viscosity modifier includes glycerol, ethylene glycol or propylene glycol; the mass ratio of the metal nanoparticles, water, dispersion stabilizer and viscosity modifier is 1:(1000~10000):(1~20):(50~500); the loading method includes dripping, spraying or impregnation, wherein when dripping, 10~60 μL is added per square centimeter according to the area of ​​the signal enhancement functional region (4) of the nanoparticles.

7. The method according to claim 1, characterized in that, The drying process described in step three includes slow drying in an environment with a relative humidity of 70% followed by low-temperature heat treatment at 40°C for curing, with a drying time of 10~30 min.

8. The method according to claim 1, characterized in that, The low-temperature drying conditions in step four, process two, are: drying at 30~60 ℃ for 30~60 min; the foldable paper-based microfluidic chip in process two can be locally encapsulated or reinforced in the non-functional area of ​​the foldable paper-based microfluidic chip according to actual use needs; the local encapsulation or reinforcement includes covering with a transparent protective film or support layer; the local encapsulation is only set at the edge of the paper-based microfluidic chip or in the non-microfluidic area, and the encapsulation material used is a transparent polymer film.

9. The use of the foldable paper-based microfluidic chip prepared according to any one of claims 1-8 for LIBS heavy metal detection, characterized in that, Includes the following steps: S1. Liquid sample loading and heavy metal enrichment: First, a standard solution containing metal ions is prepared. Then, the standard solution is added to the inlet (1) of the paper-based microfluidic chip. Driven by capillary action, the sample solution flows through the COF enrichment functional region (3) along the pre-constructed microfluidic channel (2) to obtain a paper-based microfluidic chip enriched with heavy metal ions. S2. Construction of the contact and signal enhancement system for the folding trigger functional area: The paper-based microfluidic chip enriched with heavy metal ions obtained in step S1 is folded along a preset folding line so that the COF enrichment functional region (3) and the nanoparticle signal enhancement functional region (4) are aligned and overlapped in space. S3. Drying and LIBS testing: The paper-based microfluidic chip folded in step S2 is subjected to low-temperature drying. After drying, the paper-based microfluidic chip is unfolded along the original fold line so that the COF enrichment functional region (3) enriched with the target heavy metal is exposed on the surface of the paper-based microfluidic chip. LIBS is used to collect signals from the target region to obtain the collected LIBS spectral signal. The signal is then analyzed by a single variable model with the concentration of heavy metal ions. The linear relationship between the intensity of the sensitive spectral line of heavy metal and the concentration of heavy metal ions is compared to construct a standard curve. S4. Sample detection: Replace the standard solution with the sample to be tested, and follow the steps from S1 to S3 to obtain the LIBS spectral signal. Substitute the signal into the standard curve constructed in S3 to achieve the detection of metal ions in the unknown sample.

10. The use according to claim 9, characterized in that, Metal ions in S1 include Pb 2+ Cd 2+ and Cr 3+ The concentration range of the standard solution is 0.05~10 mg / L; the temperature used for the low-temperature drying treatment in S3 is 40°C, and the drying time is 5~10 min; the parameters for signal acquisition are: laser wavelength of 1064 nm or 532 nm, laser energy of 20~60 mJ, delay time of 2 μs, integration time of 10 μs, and laser pulse repetition frequency of 1 Hz.

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