Paper-based electrochemical device and method for detecting heavy metals Pb and Cd in milk

Abstract

The invention discloses a paper-based electrochemical device and method for detecting heavy metals Pb and Cd in milk. The method comprises the following steps: firstly, manufacturing a patterned paper base, cutting off Whatman No.1 filter paper according to a designed PVC sticker pattern, manufacturing a paper base hydrophobic wall by utilizing a wax printing method, and modifying with ammonium sulfate; the method comprises the following steps: constructing a paper-based electrochemical detection device, putting a paper base into a paper-based clamping groove, inserting a twistable iron wire into a concave point of the clamping groove, connecting to the upper end of the device, adjusting the twistable iron wire to a proper angle, enabling a sample to slowly flow to a filtering area, and dripping filtrate into a 2mL centrifugal tube; inserting the miniature three-electrode system into a 2mL centrifugal tube, connecting the miniature three-electrode system with an electrochemical workstation, enriching Pb and Cd heavy metal ions in the sample by adopting a chronoamperometry, transferring the miniature three-electrode system into an electrolyte filled with an acetic acid buffer solution, and detecting the Pb and Cd heavy metal ions by using a differential pulse voltammetry to obtain a potential-current curve. The prepared paper-based electrochemical device is simple in pretreatment and low in cost, and has certain feasibility.

Description

Technical Field

[0001] The patent of this invention relates to the field of food testing, and in particular to a paper-based electrochemical device and method for detecting heavy metals such as Pb and Cd in milk. Background Art

[0002] The main heavy metals in milk include Pb, Cd, and Hg. These elements can harm health if ingested, making testing for heavy metals in milk essential. Given the high risk of heavy metals accumulating in milk through the food chain, a flexible, simple, and instant detection method is urgently needed to accurately, sensitively, and selectively detect multiple heavy metals in milk.

[0003] Currently, traditional methods for detecting lead and cadmium ions in milk include graphite furnace atomic absorption spectrometry, inductively coupled plasma mass spectrometry, and flame atomic absorption spectrometry. These methods are time-consuming and require specialized techniques, failing to meet the requirements for rapid, accurate, portable, sensitive, and low-cost on-site testing. These traditional methods for detecting heavy metal ions in food often suffer from the use of numerous chemical reagents, which can easily lead to secondary contamination during wastewater treatment; large, complex instruments and cumbersome operation; significant matrix effects, making them unsuitable for detecting complex analytes; and prolonged sample pretreatment times and significant background signal interference. Electrochemical detection offers advantages such as simple equipment, low cost, portability, and rapid heavy metal ion analysis. However, electrochemical determination of heavy metal ions in milk is difficult due to the complexation of lead and cadmium with proteins in milk. Therefore, developing new methods for electrochemical detection of heavy metal ions in milk is of great practical value.

[0004] After searching, a Chinese invention patent with publication number CN114113249A discloses a method for extracting and detecting heavy metals in infant formula. The method comprises the following steps: using an analytical balance to weigh the milk powder into a centrifuge tube, adding deionized water, and shaking to obtain a uniform solution; adding hydrogen peroxide to the uniform solution and ultrasonically treating it; adding hydrochloric acid and acetic acid and ultrasonically treating it; collecting the supernatant after centrifugation, filtering it through a cellulose membrane, adding sodium hydroxide to adjust the pH, obtaining a test solution, and transferring it to an electrochemical cell; using a nanoporous carbon-modified screen-printed electrode to detect heavy metal ions in the test solution in the electrochemical cell, and detecting its current response by square wave voltammetry to obtain the heavy metal content in the milk powder.

[0005] The present application differs from the aforementioned reference documents in the following ways:

[0006] 1. The present application has fewer pretreatment steps for milk. The method for separating heavy metals in milk involves constructing a paper-based platform and using the salting-out principle combined with ammonium sulfate modification to separate protein from Pb and Cd heavy metals in milk. The above-mentioned comparative document extracts heavy metals from milk powder solutions through multiple pretreatment steps such as acidification, ultrasound, and centrifugation, and uses nanoporous carbon-modified screen-printed electrodes to detect their current responses. There are essential differences between the two in their technical implementation solutions.

[0007] 2. This application emphasizes low cost, simple operation, and simple pretreatment, and is suitable for instant detection in non-laboratory environments. The above-mentioned comparative document is suitable for complex matrix analysis of solid or powder samples, focusing on high sensitivity and anti-interference ability. There are essential differences between the two in their application scenarios. After searching, the Chinese invention patent with publication number CN109580730A discloses a method for detecting the concentration of heavy metal ions in water, which comprises preparing a detection solution containing halogen ions, and measuring the standard current of the detection solution containing halogen ions by an electrochemical measuring device; taking a sample solution containing heavy metal ions, and adding the sample solution to the detection solution obtained in step 1, so that the heavy metal ions react with the halogen ions; after the complexation reaction is completed, measuring the detection current of the reaction solution by an electrochemical measuring device; calculating the difference between the detection current and the standard current, and using this difference, inferring the content of heavy metal ions in the sample solution, thereby determining the concentration of heavy metal ions.

[0008] The present application differs from the aforementioned reference documents in the following ways:

[0009] 1. This application belongs to the field of food testing, targeting heavy metals such as Pb and Cd in milk, and is suitable for solving the problem of protein interference in milk testing, emphasizing portability and instant detection; while the above-mentioned comparative document belongs to the field of chemical testing technology, suitable for water quality testing (such as industrial wastewater and natural water bodies), and the detection objects are various heavy metal ions in water; there are essential differences between the two in technical fields and application scenarios.

[0010] 2. This application combines chronoamperometry enrichment and differential pulse voltammetry stripping to achieve high-sensitivity detection through an electrochemical workstation. At the same time, acidification pretreatment and ammonium sulfate-modified paper-based salting-out separation steps are introduced to separate heavy metal ions from proteins in the milk matrix, thereby improving selectivity and anti-interference capabilities. The above-mentioned comparative document uses the current difference method to change the conductivity of the solution through the complexation reaction of halogen ions and heavy metals, and uses the difference between the standard current and the detection current to infer the heavy metal concentration. The method is simple but does not involve enrichment or dissolution steps, and the sensitivity may be low. There are essential differences between the two in detection methods and technical principles. Summary of the Invention

[0011] In response to the above defects or improvement needs of the prior art, the purpose of the embodiments of the present invention is to provide a paper-based electrochemical device and detection method for detecting Pb and Cd heavy metals in milk. The paper-based electrochemical device is a simple and easy-to-operate device for detecting Pb and Cd heavy metal ions in milk.

[0012] In the first aspect, the present invention provides a paper-based electrochemical detection device for detecting heavy metals such as Pb and Cd in milk. The device comprises a 3D-printed irregular cube window model with grooves, a paper-based card slot, a twistable iron wire, and a detachable micro three-electrode system. The upper right corner of the 3D-printed irregular cube window model with grooves is a circular groove into which a thin rod is inserted, and a removable connecting mold is provided on the thin rod. There are two grooves, one for placing a 2mL centrifuge tube and the other for holding an acetic acid buffer solution. The paper-based card slot is connected to the twistable iron wire and the removable connecting mold, and the angle can be adjusted arbitrarily. The paper-based card slot has a circular opening corresponding to the paper-based filtration area.

[0013] In the second aspect, the present invention provides a paper-based electrochemical detection method for detecting heavy metals such as Pb and Cd in milk, comprising the following steps:

[0014] Step 1. Use Sketchup 2025 software to design a dumbbell-shaped patterned PVC sticker model as a wax printing template;

[0015] Step 2: Using Whatman No. 1 filter paper as a paper-based detection platform, a hydrophobic wall was prepared by wax printing to form a dumbbell-shaped hydrophilic channel, which was then modified with ammonium sulfate;

[0016] Step 3, acidifying and pretreating the sample;

[0017] Step 4: Assemble a paper-based electrochemical device, which includes a 3D-printed irregular cube window model with grooves, a paper-based card slot, a twistable iron wire, and a detachable micro three-electrode system. The upper right corner of the 3D-printed irregular cube window model with grooves is a circular groove, into which a thin rod is inserted. The thin rod is covered with a removable connecting mold. There are two grooves, one for accommodating a 2mL centrifuge tube and the other for holding an acetic acid buffer solution. The paper-based card slot is connected to the twistable iron wire and the removable connecting mold, and the angle can be adjusted at will. The paper-based card slot has a circular opening corresponding to the paper-based filtration area. Adjust the angle of the paper-based card slot and complete the sample filtration.

[0018] Step 5: Insert the detachable micro three-electrode system into the sample;

[0019] Step 6: Connect the device to an electrochemical workstation and use chronoamperometry (it) to concentrate Pb(II) and Cd(II) on the glassy carbon electrode.

[0020] Step 7: Insert the detachable micro three-electrode system into the acetate buffer solution, connect it to the electrochemical workstation, and use differential pulse voltammetry (DPV) scanning to dissolve Pb and Cd ions and detect the Pb and Cd ion concentrations.

[0021] Further preferably, in step 1, the dumbbell-shaped patterned PVC sticker is a rectangle with a length of 50 mm and a width of 25 mm, with a dumbbell-shaped hydrophilic region in the center of the rectangle, an ammonium sulfate modified region and a leached region as a circle with a radius of 6 mm, and a rectangular transition region with a length of 20 mm and a width of 6 mm in the middle. The surrounding area is a hydrophobic region.

[0022] Further preferably, in step 1, the wax printing method is to use a hot plate to heat paraffin until it melts into a liquid state, use a 1 mL pipette to absorb a small amount of paraffin droplets, and smooth them on the peripheral part of the dumbbell-shaped sticker. After the paraffin is completely infiltrated and solidified into the inside of the filter paper, tear off the dumbbell-shaped sticker in the middle part.

[0023] Further preferably, in step 2, the concentration of ammonium sulfate is 4.2 g / mL and the amount used is 10 μL.

[0024] Further preferably, in step 3, the reagent used for acidification pretreatment is hydrochloric acid with a concentration of 50 mM.

[0025] Further preferably, in step 4, the detachable micro three-electrode system is: working electrode: GRE glassy carbon electrode, size 30mm×4mm; auxiliary electrode: platinum wire electrode, size 30mm×2mm; reference electrode: saturated calomel electrode, size 30mm×6mm.

[0026] Further preferably, in step 4, the chronoamperometry deposition time is 400s and the sensitivity is 10 -5 A.

[0027] Further preferably, in step 5, the pH of the acetate buffer solution is 4.0, and the concentration is 0.1M.

[0028] Further preferably, in step 6, differential pulse voltammetry is used to detect Pb 2+ When the potential range is -0.7~0V; when detecting Cd 2+ When the potential range is -1.2~-0.6V, the sensitivity is 10 -6 A.

[0029] The principle of the patented invention is as follows:

[0030] The fibrous structure of the paper substrate allows milk proteins to be retained at the upper end of the dumbbell-shaped hydrophilic region. Ammonium sulfate-modified paper substrates can salt out acidified milk samples, separating macromolecules such as protein from the Pb and Cd heavy metal ions. Combining these two properties, milk proteins are effectively precipitated and retained at the upper end of the paper substrate, achieving excellent separation results. Chronoamperometry was used to deposit trace Pb and Cd heavy metal ions from the sample onto the surface of a glassy carbon electrode, and differential pulse voltammetry was used to strip the Pb and Cd heavy metal ions from the electrode surface. This yielded differential pulse voltammograms of the Pb and Cd heavy metal ions.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. The present invention is low-cost, easy to operate, and requires simple pretreatment, making it suitable for immediate detection in non-laboratory environments;

[0033] 2. The method involved in separating heavy metals from milk involves constructing a paper-based platform and utilizing the salting-out principle combined with ammonium sulfate modification to separate protein from heavy metals such as Pb and Cd in milk, requiring minimal pretreatment steps for the milk.

[0034] 3. Combining chronoamperometry enrichment with differential pulse voltammetry stripping, high-sensitivity detection is achieved using an electrochemical workstation. Acidification pretreatment and ammonium sulfate-modified paper-based salting-out separation steps are also introduced to separate heavy metal ions from proteins in the milk matrix, achieving excellent selectivity and anti-interference capabilities. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of the paper-based electrochemical sensor structure (a) and an enlarged view of the paper-based card slot (b) of the patented invention;

[0036] Figure 2 The back side (a) and front side (b) of the paper base of the present invention are shown;

[0037] Figure 3 This is a schematic diagram of the paper base size of the patented invention;

[0038] Figure 4 This is a schematic diagram of the wax printing method of the present invention;

[0039] Figure 5 This is a feasibility diagram of the paper-based electrochemical sensor device patented in this invention for detecting Pb(Ⅱ)(b) and Cd(Ⅱ)(a) heavy metal ions. DETAILED DESCRIPTION

[0040] The following describes a specific embodiment of a paper-based electrochemical device for detecting heavy metals such as Pb and Cd in milk with the aid of accompanying drawings. The following examples are provided to illustrate the present invention and are not intended to limit the scope of the present invention.

[0041] like Figure 1 (a) As described above, the device for performing electrochemical detection on paper-based substrates of the present invention comprises a paper-based card slot 1 (including a circular hole 10, a connecting groove 11, Figure 1 (b) is an enlarged view of the paper-based card slot), a twistable iron wire 4, a thin rod 2, a detachable connecting groove 3, a cube model with irregular grooves and windows 7 (including grooves 5 and 9), and a detachable micro-three-electrode system (including a glassy carbon electrode 7, a saturated calomel electrode 6, a platinum wire electrode 8, and a bottom plate with a circular hole 13). Groove 5 holds 0.1M, pH 4.0 acetic acid buffer solution, and groove 9 is used to hold a 2mL centrifuge tube. The detachable micro-three-electrode system is connected to the electrochemical workstation via an electrode clamp for electrochemical testing. It is placed in groove 9 for chronoamperometry (ITEM) testing and in groove 5 for differential pulse voltammetry (DPV) testing, producing a differential pulse voltammetry curve.

[0042] like Figure 4 As shown in the figure, the wax printing method is used to make the paper-based hydrophobic wall. The wax ball is placed in a 250mL beaker and melted into a liquid at a constant temperature of 85℃. Figure 3 1 filter paper, cut it with scissors, tear off the area around the dumbbell shape, use a 1mL pipette to take an appropriate amount of wax liquid and add it around the dumbbell-shaped sticker. During the wax dripping process, try to make the pipette tip and the paper base smooth. Wait until the wax liquid completely penetrates into the paper base and solidifies, tear off the dumbbell-shaped part of the sticker to obtain a paper base with a hydrophobic wall, and stick a PVC waterproof sticker on the back of the paper base to prevent leakage, as shown. Figure 3 As shown in (a) and (b), 100 μL of (NH4)2SO4 was added to one end of the dumbbell-shaped paper substrate and allowed to dry naturally.

[0043] The present invention is further illustrated by examples. It should be understood that the examples are intended to illustrate the feasibility of the present invention and are not intended to limit the present invention. Simple improvements to the present invention based on the essence of the present invention fall within the scope of protection claimed by the present invention.

[0044] To verify the feasibility of detection using the paper-based electrochemical sensor device, two different experimental groups for Pb and Cd were set up: Pb blank group, Pb+EM (experimental milk) group, Pb+EM+HCl (hydrochloric acid) group, and Pb+EM+HCl+(NH4)2SO4 (ammonium sulfate) group. The EM components were whey protein, 5% casein, and NaCl in a ratio of 15:10:21. The Cd group was the same as the Pb group, with 0.1M acetate buffer solution, pH=4.0, as the electrolyte solution. The purpose of such setting is to use the Pb and Cd blank groups as controls to determine the peak positions of Pb and Cd ions; to compare the Pb+EM and Cd+EM groups with the Pb and Cd groups to compare the degree of interference after adding EM; to compare the Pb+EM+HCl group and the Cd+EM+HCl group with the Pb+EM and Cd+EM groups, and to compare the magnitude of the response current of the two groups to determine whether the acidification and complexation effect of hydrochloric acid is effective; to compare the magnitude of the response current of the Pb+EM+HCl+(NH4)2SO4 group and the Cd+EM+HCl+(NH4)2SO4 group with the first three groups to determine whether the acidification and salting-out steps are effective. Through analysis and comparison, the feasibility of the patent of this invention is verified. Taking the Pb group as an example, the Cd group is set up in the same way as the Pb group, and the specific ratios and treatment methods are as follows:

[0045] Pb blank group: 1 mL of 0.01 M (CH3COO)2Pb solution was diluted to 64 mL, filtered through a paper-based electrochemical device modified with 100 μL (NH4)2SO4 to 1 mL in a 2 mL centrifuge tube.

[0046] Pb+EM group: 1 mL of 0.01 M (CH3COO)2Pb solution and 3 mL of EM solution were mixed and diluted to 64 mL, and filtered through a paper-based electrochemical device modified with 100 μL of (NH4)2SO4 to 1 mL in a 2 mL centrifuge tube.

[0047] Pb+EM+HCl group: 1 mL of 0.01 M (CH3COO)2Pb solution, 3 mL of EM solution, and 4 mL of 50 mM HCl solution were mixed and diluted to 64 mL. The solution was filtered through a paper-based electrochemical device modified with 100 μL of (NH4)2SO4 to 1 mL in a 2 mL centrifuge tube.

[0048] Pb+EM+HCl+(NH4)2SO4 group: 1 mL of 0.01 M (CH3COO)2Pb solution, 3 mL of EM solution, 4 mL of 50 mM HCl solution, and 2 mL of 4.2 g / L (NH4)2SO4 solution were mixed and diluted to 64 mL. The solution was filtered through a paper-based electrochemical device modified with 100 μL of (NH4)2SO4 to 1 mL in a 2 mL centrifuge tube.

[0049] Example 1

[0050] Polish the glassy carbon electrode using metallographic sandpaper. Then, apply 1.0μm, 0.3μm, and 0.05μm alumina powder to suede, adding a few drops of deionized water, and polish the glassy carbon electrode for 1 minute, 1 minute, and 3 minutes, respectively. After polishing, place the electrode surface in pure water, ethanol, and pure water, and ultrasonically clean it once for 30 seconds each time. Cyclic voltammetry (CV) was performed using an electrochemical workstation over a potential range of -0.1 to 0.5 V. The electrolyte consisted of 5 mM potassium ferrocyanide and 0.1 M potassium chloride. Polishing was considered successful when the redox peak ΔE was less than 100 mV. After testing, rinse with pure water and dry. Assemble the paper-based electrochemical device. Place the 2mL centrifuge tube into the groove 9, connect the connecting groove 11 of the paper base card slot with an arbitrarily twisted iron wire 4 with a diameter of 1mm (the length can be adjusted according to the position of the card slot and the groove 9), and connect the other end of the iron wire to the middle connecting groove 3. Place the paper base into the card slot and twist the iron wire to a suitable angle so that the filtrate can drip into the 2mL centrifuge tube when the paper base filters the sample. Use an electrochemical workstation for detection, which is connected to a micro three-electrode system for chronoamperometry-differential pulse voltammetry detection. The specific method is to insert the 2mL centrifuge tube into the card slot. Figure 1 At the middle groove 9, wait for the paper-based filter sample to reach 1 mL of the centrifuge tube, insert the micro three-electrode system into the 2 mL centrifuge tube, and enrich the Pb in the solution by chronoamperometry. 2+ , the enrichment time is 400s. 10mL, 0.1M, pH=4.0 acetate buffer solution is placed in groove 5. After completion, the micro three-electrode system is inserted into groove 5 and differential pulse voltammetry is performed. Detection of Pb 2+ When the potential range is adjusted to -0.65 to -0.40 V, the sensitivity is adjusted to 10 -6 A, after testing the Pb group in sequence, we get Figure 5 (b).

[0051] Example 2

[0052] Polish the glassy carbon electrode using metallographic sandpaper. Then, apply 1.0μm, 0.3μm, and 0.05μm alumina powder to suede, adding a few drops of deionized water, and polish the glassy carbon electrode for 1 minute, 1 minute, and 3 minutes, respectively. After polishing, place the electrode surface in pure water, ethanol, and pure water, and ultrasonically clean it once for 30 seconds each time. Cyclic voltammetry (CV) was performed using an electrochemical workstation over a potential range of -0.1 to 0.5 V. The electrolyte consisted of 5 mM potassium ferrocyanide and 0.1 M potassium chloride. Polishing was considered successful when the redox peak ΔE was less than 100 mV. After testing, rinse with pure water and dry. Assemble the paper-based electrochemical device. Place the 2mL centrifuge tube into the groove 9, connect the connecting groove 11 of the paper base card slot with an arbitrarily twisted iron wire 4 with a diameter of 1mm (the length can be adjusted according to the position of the card slot and the groove 9), and connect the other end of the iron wire to the middle connecting groove 3. Place the paper base into the card slot and twist the iron wire to a suitable angle so that the filtrate can drip into the 2mL centrifuge tube when the paper base filters the sample. Use an electrochemical workstation for detection, which is connected to a micro three-electrode system for chronoamperometry-differential pulse voltammetry detection. The specific method is to insert the 2mL centrifuge tube into the card slot. Figure 1 At the middle groove 9, wait for the paper-based filter sample to reach 1 mL of the centrifuge tube, insert the micro three-electrode system into the 2 mL centrifuge tube, and enrich the Pb in the solution by chronoamperometry. 2+ , the enrichment time is 400s. 10mL, 0.1M, pH=4.0 acetate buffer solution is placed in groove 5. After the completion, the micro three-electrode system is inserted into groove 5 and differential pulse voltammetry detection is performed. Detection of Cd 2+ When the potential range is adjusted to -1.20 to -0.60 V, the sensitivity is adjusted to 10 -6 A, after testing the Cd group in sequence, we get Figure 5 (a).

[0053] Depend on Figure 5 As can be seen, the current responses of the Pb+EM+HCl+(NH4)2SO4 and Cd+EM+HCl+(NH4)2SO4 groups at the characteristic peaks were significantly higher than those of the other groups, indicating that ammonium sulfate modification significantly enhances the detection signals of Pb and Cd, improving detection sensitivity. Compared with the groups with EM alone or EM and HCl added simultaneously, the ammonium sulfate-modified sensors exhibited more stable background signals and less fluctuation when detecting Pb and Cd, demonstrating that ammonium sulfate can cause salting out, effectively reducing the impact of interfering substances in the matrix on the detection signal. The curves of the Pb+EM+HCl+(NH4)2SO4 and Cd+EM+HCl+(NH4)2SO4 groups were relatively smooth within the visible potential range, with good baseline stability, demonstrating the accuracy and reliability of the detection results.

[0054] The above embodiments are intended only to illustrate the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the embodiments, it should be understood by those skilled in the art that various combinations, modifications, or equivalent substitutions of the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention and should be encompassed by the claims of the present invention.

Claims

1. A paper-based electrochemical device for detecting heavy metals Pb and Cd in milk, characterized by: The device comprises a 3D-printed irregular cube window model with grooves, a paper-based card slot, a twistable iron wire, and a detachable micro three-electrode system. The 3D-printed irregular cube window model has a circular groove in the upper right corner, into which a thin rod is inserted, and a removable connecting mold is mounted on the thin rod. The grooves are two, one for accommodating a 2mL centrifuge tube and the other for containing an acetic acid buffer solution. The paper base card slot is connected to the twistable iron wire and the movable connection mold, and the angle can be adjusted arbitrarily. The paper base card slot has a circular opening corresponding to the paper base filtering area.

2. A paper-based electrochemical detection method for detecting heavy metals Pb and Cd in milk, characterized by: The following steps are involved: Step 1: Design a dumbbell-shaped patterned PVC sticker as a wax printing template; Step 2: Using Whatman No. 1 filter paper as a paper-based detection platform, a hydrophobic wall was prepared by wax printing to form a dumbbell-shaped hydrophilic channel, which was then modified with ammonium sulfate; Step 3, acidifying and pretreating the sample; Step 4: Assemble a paper-based electrochemical device, which includes a 3D-printed irregular cube window model with grooves, a paper-based card slot, a twistable iron wire, and a detachable micro three-electrode system; the 3D-printed irregular cube window model with grooves has a circular groove in the upper right corner, into which a thin rod is inserted, and the thin rod is covered with a removable connecting mold; the number of the grooves is two, one for accommodating a 2mL centrifuge tube and the other for holding an acetic acid buffer solution; the paper-based card slot is connected to the twistable iron wire and the removable connecting mold, and the angle can be adjusted at will. The paper-based card slot has a circular opening corresponding to the paper-based filtration area; Adjust the angle of the paper-based card slot and complete sample filtration; Step 5: Insert the detachable micro three-electrode system into the sample, connect it to the electrochemical workstation, and use chronoamperometry to enrich Pb and Cd ions at the glassy carbon electrode; Step 6: Insert the detachable micro three-electrode system into the acetate buffer solution, connect it to the electrochemical workstation, and use differential pulse voltammetry to scan and dissolve Pb and Cd ions to detect the Pb and Cd ion concentrations.

3. A paper-based electrochemical detection method for detecting heavy metals Pb and Cd in milk according to claim 2, characterized in that: The dumbbell-shaped patterned PVC sticker in step 1 is: a rectangle with a length of 50 mm and a width of 25 mm, with a dumbbell-shaped hydrophilic area in the middle of the rectangle, an ammonium sulfate modified area and a leached area as a circle with a radius of 6 mm, a rectangular transition area with a length of 20 mm and a width of 6 mm in the middle, and a hydrophobic area around it.

4. The paper-based electrochemical detection method for detecting heavy metals Pb and Cd in milk according to claim 2, characterized in that: The wax printing method described in step 1 is as follows: use a hot plate to heat paraffin until it melts into a liquid state, use a 1 mL pipette to absorb a small amount of paraffin droplet, and smooth it on the periphery of the dumbbell-shaped sticker. After the paraffin is completely infiltrated and solidified into the inside of the filter paper, tear off the dumbbell-shaped sticker in the middle.

5. The paper-based electrochemical detection method for detecting heavy metals Pb and Cd in milk according to claim 2, characterized in that: The concentration of ammonium sulfate in step 2 is 4.2 g / mL, and the amount used is 10 μL.

6. The paper-based electrochemical detection method for detecting heavy metals Pb and Cd in milk according to claim 2, characterized in that: The reagent used for the acidification pretreatment in step 3 is hydrochloric acid with a concentration of 50 mM.

7. The paper-based electrochemical detection method for detecting heavy metals Pb and Cd in milk according to claim 2, characterized in that: The detachable micro three-electrode system in step 4 is as follows: working electrode: glassy carbon electrode, size 30mm×4mm; auxiliary electrode: platinum wire electrode, size 30mm×2mm; Reference electrode: saturated calomel electrode, size 30mm×6mm.

8. The paper-based electrochemical detection method for detecting heavy metals Pb and Cd in milk according to claim 2, characterized in that: The chronoamperometric deposition time in step 4 was 400 s and the sensitivity was 10 -5 A.

9. The paper-based electrochemical detection method for detecting heavy metals Pb and Cd in milk according to claim 2, characterized in that: The pH of the acetate buffer solution in step 5 is 4.0, and the concentration is 0.1M.

10. The paper-based electrochemical detection method for detecting heavy metals Pb and Cd in milk according to claim 2, characterized in that: Differential pulse voltammetry detection of Pb in step 6 2+ When the voltage range is -0.65~-0.40V; detect Cd 2+ When the voltage range is -1.20~-0.60V, the sensitivity is 10 -6 A.

Images