A nitrogen-doped graphene modified paper-based electrode, a preparation method thereof and application thereof

Abstract

The application relates to the technical field of heavy metal ion detection, in particular to a nitrogen-doped graphene modified paper-based electrode and a preparation method and application thereof. The preparation method provided by the application comprises the following steps: providing filter paper, the filter paper has a hydrophobic region and a non-hydrophobic region, and the surface of the hydrophobic region is provided with a hydrophobic barrier; printing a three-electrode system and a conductive pad on the surface of the filter paper to obtain a paper-based electrode, the three-electrode system comprises a working electrode, a counter electrode and a reference electrode; mixing an ammonium chloride and ammonia water mixed solution and an oxidized graphene solution to obtain an electrolyte; and dropping the electrolyte on the paper-based electrode to perform electrochemical deposition, thereby obtaining a nitrogen-doped graphene modified paper-based electrode. The preparation method is simple in operation and mild in conditions, and the nitrogen-doped graphene electrode prepared by the method has strong anti-interference performance, low detection limit and high sensitivity when simultaneously detecting Cd 2+ , Pb 2+ and Hg 2+ three ions in a water environment.

Description

Technical Field

[0001] This invention relates to the field of heavy metal ion detection technology, and in particular to a nitrogen-doped graphene-modified paper-based electrode, its preparation method, and its application. Background Technology

[0002] Graphene, due to its stable crystal structure and band structure, possesses excellent electrical and thermal conductivity, high mechanical strength, strong resistance to degradation, large specific surface area, and numerous active sites. However, its practical applications are limited, primarily because its zero-bandgap electronic structure cannot be directly used in the semiconductor field. To address this issue, introducing a bandgap between the conduction and valence bands of graphene by doping with other elements is a powerful means of widening its bandgap and endowing it with new properties. Nitrogen doping is an important method for modifying graphene, offering low cost, high controllability, and broad application prospects. As an adjacent element to carbon, nitrogen has a higher electronegativity than carbon and a similar radius to carbon atoms. It can influence the spin density and charge distribution of carbon atoms, inducing more positive charges onto carbon atoms and generating more active sites.

[0003] Currently, the main methods for preparing nitrogen-doped graphene include chemical vapor deposition, solvothermal methods, and plasma treatment methods. Traditional chemical vapor deposition and plasma methods have drawbacks such as harsh reaction conditions and the need for special instruments, while solvothermal methods have long reaction times and low controllability. Summary of the Invention

[0004] The purpose of this invention is to provide a nitrogen-doped graphene-modified paper-based electrode, its preparation method, and its application. The preparation method is simple to operate and operates under mild conditions. The nitrogen-doped graphene obtained can simultaneously detect Cd in an aqueous environment. 2+ Pb 2 + and Hg 2+ When using three types of ions, it exhibits strong anti-interference capabilities, low detection limits, and high sensitivity.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides a method for preparing a nitrogen-doped graphene-modified paper-based electrode, comprising the following steps:

[0007] A filter paper is provided, wherein the hydrophobic region and the non-hydrophobic region of the filter paper have a hydrophobic barrier on the surface of the hydrophobic region;

[0008] A three-electrode system and conductive pads are printed on the surface of the filter paper to obtain a paper-based electrode. The three-electrode system includes a working electrode, a counter electrode, and a reference electrode.

[0009] An electrolyte is obtained by mixing a mixed solution of ammonium chloride and ammonia with a graphene oxide solution.

[0010] The electrolyte is dropped onto the paper-based electrode and electrochemically deposited to obtain a nitrogen-doped graphene-modified paper-based electrode.

[0011] Preferably, the concentration of graphene oxide in the electrolyte is 0.5–2 mg / mL.

[0012] Preferably, the total concentration of ammonium chloride and ammonia in the electrolyte is 0.01–0.25 mol / L.

[0013] Preferably, the molar ratio of ammonium chloride to ammonia in the mixed solution of ammonium chloride and ammonia is 1:1.

[0014] Preferably, the electrochemical deposition is performed at a constant potential of -1.3 to -1.9 V for 200 to 600 s, with a sampling interval of 0.1 s and a sensitivity of 1*10⁻⁶. -3 A / V.

[0015] Preferably, the method for preparing the hydrophobic barrier is wax printing.

[0016] Preferably, the working electrode is a circular working electrode, and the raw material for preparation is hydrophilic carbon paste;

[0017] The counter electrode is an arc-shaped counter electrode, and the raw material for preparation is hydrophilic carbon paste; the distance between the working electrode and the counter electrode is 0.67 mm;

[0018] The reference electrode is prepared from conductive silver paste.

[0019] The circular working electrode, the arc-shaped counter electrode, and part of the reference electrode are located in the non-hydrophobic region of the filter paper.

[0020] Preferably, both the working electrode and the counter electrode are connected to conductive pads, and the conductive pads are prepared from conductive silver paste; the conductive silver paste extends into the hydrophobic region.

[0021] The remaining portion of the reference electrode extends into the hydrophobic region.

[0022] The present invention also provides a nitrogen-doped graphene-modified paper-based electrode prepared by the preparation method described in the above technical solution.

[0023] This invention also provides the application of the nitrogen-doped graphene-modified paper-based electrode described above in the detection of heavy metal ions, wherein the heavy metal ions include Cd. 2+ Pb 2+ and Hg 2+ One or more of them.

[0024] This invention provides a method for preparing a nitrogen-doped graphene-modified paper-based electrode, comprising the following steps: providing filter paper, wherein the filter paper has hydrophobic and non-hydrophobic regions, and the surface of the hydrophobic regions has a hydrophobic barrier; printing a three-electrode system and conductive pads on the surface of the filter paper to obtain a paper-based electrode, wherein the three-electrode system includes a working electrode, a counter electrode, and a reference electrode; mixing a mixed solution of ammonium chloride and ammonia with a graphene oxide solution to obtain an electrolyte; and dripping the electrolyte onto the paper-based electrode to perform electrochemical deposition to obtain the nitrogen-doped graphene-modified paper-based electrode. The preparation method of this invention overcomes the shortcomings of chemical vapor deposition, solvothermal methods, and other methods for preparing graphene-doped electrodes, and has the advantages of simple operation, mild conditions, and direct and simultaneous modification onto the surface of the paper-based electrode after doping; simultaneously, the paper-based electrode prepared by this method can detect Cd in an aqueous environment. 2+ Pb 2+ and Hg 2+ When three ions are used, the anti-interference ability is strong, the detection limit is low and the sensitivity is high. The specific reasons include: (1) The doping of nitrogen element (including pyridine nitrogen and pyrrole nitrogen) increases the electronegativity of the electrode. Pyridine nitrogen can coordinate with heavy metal cations to enhance the catalytic activity of the electrode. The active sites of pyrrole nitrogen can interact with heavy metal ions to fix them on the electrode, induce the enrichment of heavy metal ions and their deposition on the electrode surface, thereby generating a larger current response during the release of heavy metal ions; (2) The nitrogen-doped graphene film causes greater defects in graphene, providing more conductive channels, effectively increasing the specific surface area of ​​the electrode and the active sites for electron transfer, which can promote the penetration of electrolyte and is beneficial for the detection of heavy metal ions. Attached Figure Description

[0025] Figure 1 The diagram shows the electrode pattern, a complete paper-based electrode, and a schematic diagram of the three-dimensional structure of the paper-based electrode.

[0026] Figure 2 XPS plots and N-fitting peaks were performed on the nitrogen-doped graphene in the nitrogen-doped graphene-modified paper electrode described in Example 8.

[0027] Figure 3 The C1s spectra of graphene oxide, reduced graphene oxide, and nitrogen-doped graphene are shown.

[0028] Figure 4 SEM images of graphene oxide, reduced graphene oxide, and nitrogen-doped graphene from Example 7;

[0029] Figure 5 The image shows the AFM pattern of the nitrogen-doped graphene-modified paper electrode described in Example 7.

[0030] Figure 6 The CV curve of the paper-based electrode described in Example 8;

[0031] Figure 7 The CV curve of the nitrogen-doped graphene-modified paper electrode described in Example 8;

[0032] Figure 8 The current response of the nitrogen-doped graphene-modified paper-based electrodes described in Examples 1-5 to cadmium ions is shown.

[0033] Figure 9 The current response of the nitrogen-doped graphene-modified paper-based electrodes described in Examples 2 and 6-8 to cadmium ions is shown.

[0034] Figure 10 The current response of the nitrogen-doped graphene-modified paper-based electrodes described in Examples 7 and 9-12 to cadmium ions is shown.

[0035] Figure 11 This is a schematic diagram of the fabrication process of the paper-based microfluidic chip in the application example;

[0036] Figure 12 This is a schematic diagram of the paper-based microfluidic chip in the application example;

[0037] Figure 13 For example, the paper-based microfluidic chip is used to measure Cd. 2+ Pb 2+ and Hg 2+ Peak current versus concentration curves for the three ions;

[0038] Figure 14 For example, the paper-based microfluidic chip is used to measure Cd. 2+ Pb 2+ and Hg 2+ Current deviation curves before and after the addition of interfering ions for three types of ions;

[0039] Figure 15 The dissolution curves of heavy metal ions were measured for three different batches of paper-based microfluidic chips prepared according to the method described in the application example. Detailed Implementation

[0040] This invention provides a method for preparing a nitrogen-doped graphene-modified paper-based electrode, characterized by comprising the following steps:

[0041] A filter paper is provided, wherein the hydrophobic region and the non-hydrophobic region of the filter paper have a hydrophobic barrier on the surface of the hydrophobic region;

[0042] A three-electrode system and conductive pads are printed on the surface of the filter paper to obtain a paper-based electrode. The three-electrode system includes a working electrode, a counter electrode, and a reference electrode.

[0043] An electrolyte is obtained by mixing a mixed solution of ammonium chloride and ammonia with a graphene oxide solution.

[0044] The electrolyte is dropped onto the paper-based electrode and electrochemically deposited to obtain a nitrogen-doped graphene-modified paper-based electrode.

[0045] In this invention, unless otherwise specified, all raw materials used in the preparation are commercially available products well known to those skilled in the art.

[0046] The present invention provides filter paper, wherein the hydrophobic region and the non-hydrophobic region of the filter paper have a hydrophobic barrier on the surface of the hydrophobic region.

[0047] The present invention does not impose any special limitations on the pattern of the hydrophobic region; it can be set according to actual needs.

[0048] In this invention, the preferred method for preparing the hydrophobic barrier is wax printing. The specific preparation process preferably includes: cleaning the surface of filter paper without a hydrophobic barrier using nitrogen gas, and then using a wax printer to print a pre-designed hydrophobic barrier pattern onto the cleaned filter paper. This invention does not impose any special limitations on the printing process; any process well-known to those skilled in the art can be used. In this invention, the filter paper is preferably Whatman No. 1 filter paper.

[0049] A working electrode, a counter electrode, and a reference electrode are printed on the surface of the filter paper to obtain a paper-based electrode (a schematic diagram of the paper-based electrode is shown in Figure 1). Figure 1 As shown in Figure E, the working electrode includes a circular working electrode and a conductive pad.

[0050] In this invention, the process for preparing the working electrode, the counter electrode, and the reference electrode preferably includes:

[0051] like Figure 1 As shown, the working electrode, counter electrode, and reference electrode were designed using the drawing software Corel DRAW 2020.

[0052] The filter paper is cleaned by blowing with nitrogen gas, placed under a screen, and hydrophilic carbon paste is sequentially applied to the surface of the filter paper for the first curing. Then, conductive silver paste is applied to the surface of the filter paper for the second curing.

[0053] In this invention, the working electrode is preferably a circular working electrode, and the raw material for preparation is preferably hydrophilic carbon paste; the working electrode is preferably connected to conductive pads, and the raw material for preparation is preferably conductive silver paste; the counter electrode is preferably an arc-shaped counter electrode, and the raw material for preparation is preferably hydrophilic carbon paste; the counter electrode is preferably connected to conductive pads, and the raw material for preparation is preferably conductive silver paste; the raw material for preparation of the reference electrode is preferably conductive silver paste. In this invention, the hydrophilic carbon paste is preferably CI-2067; the conductive silver paste is preferably silver / silver chloride ink, and the silver / silver chloride ink is preferably CIE-4250. In this invention, the circular working electrode, the arc-shaped counter electrode, and part of the reference electrode are preferably located in the non-hydrophobic region of the filter paper; the conductive pads connecting the working electrode and the counter electrode preferably extend into the hydrophobic region; the remaining part of the reference electrode preferably extends into the hydrophobic region.

[0054] In this invention, the squeegee used for the squeegee printing is preferably a water-oil dual-purpose pointed squeegee with a hardness of 75; the length of the squeegee customized according to the screen is 5cm; this invention does not impose any special limitations on the squeegee printing process, and adopts a process known to those skilled in the art to ensure that the screen or filter paper is not slipped during the printing process, so as to prevent the pattern from being unclear.

[0055] In this invention, the first curing preferably includes sequential standing and drying, wherein the standing is preferably placed in an open space for half an hour; the drying temperature is preferably 100°C and the time is preferably 10 minutes. The second curing method is preferably drying, wherein the second curing temperature is preferably 120°C and the time is preferably 30 minutes.

[0056] The preparation method of the present invention further includes mixing a mixed solution of ammonium chloride and ammonia water with a graphene oxide solution to obtain an electrolyte.

[0057] In this invention, the concentration of graphene oxide in the electrolyte is preferably 0.5-2 mg / mL, more preferably 0.8-1.6 mg / mL, and most preferably 1.4-1.5 mg / mL.

[0058] In this invention, the total concentration of ammonium chloride and ammonia in the electrolyte is preferably 0.01-0.25 mol / L, more preferably 0.05-0.20 mol / L, and most preferably 0.05-0.10 mol / L; the molar ratio of ammonium chloride to ammonia is preferably 1:1.

[0059] The present invention does not impose any special limitations on the mixing process; any process known to those skilled in the art can be used.

[0060] After obtaining the electrolyte and the paper-based electrode, the present invention further includes placing the paper-based electrode in the electrolyte and performing electrochemical deposition to prepare nitrogen-doped graphene on the surface of the paper-based electrode.

[0061] In this invention, placing the paper-based electrode in the electrolyte is preferably done by using a pipette to draw up the electrolyte and drop it onto the paper-based electrode, ensuring that the electrolyte just covers the paper-based electrode.

[0062] Before performing the electrochemical deposition, the working electrode, reference electrode, and counter electrode are connected to an electrochemical workstation using conductive clamps.

[0063] In this invention, the constant potential for electrochemical deposition is preferably -1.3 to -1.9 V, more preferably -1.4 to -1.8 V, and most preferably -1.5 to -1.7 V; the time is preferably 200 to 600 s, more preferably 300 to 500 s, and most preferably 350 to 450 s; the sampling interval is preferably 0.1 s, and the sensitivity is preferably 1*10 -3 .

[0064] In this invention, the nitrogen doping content in the nitrogen-doped graphene is preferably 3.06%; the nitrogen doping form preferably includes pyridine nitrogen and pyrrole nitrogen.

[0065] In this invention, the mass percentage of carbon atoms in the nitrogen-doped graphene is preferably 78.71%, and the mass percentage of oxygen atoms in the nitrogen-doped graphene is preferably 12.77%.

[0066] The present invention also provides a nitrogen-doped graphene-modified paper-based electrode prepared by the preparation method described in the above technical solution.

[0067] This invention also provides the application of the nitrogen-doped graphene-modified paper-based electrode described above in the detection of heavy metal ions, wherein the heavy metal ions include Cd. 2+ Pb 2+ and Hg 2+ One or more of them.

[0068] In this invention, the preferred application is to prepare a paper-based microfluidic chip (denoted as NG / pSPE-μPAD sensor) according to the above-described process for preparing nitrogen-doped graphene-modified paper-based electrodes.

[0069] In this invention, the paper-based microfluidic chip includes an upper paper-based electrode and a lower paper-based electrode. The upper paper-based electrode is similar to the paper-based electrode described in the above-mentioned technical solution, except that it does not include a circular working electrode and a conductive pad connecting the circular working electrode, and the circular working electrode has a through-hole structure. The lower paper-based electrode is similar to the paper-based electrode described in the above-mentioned technical solution, except that it also has a sample injection area, which is a non-hydrophobic region bridging the working electrode. The distance between the sample injection area and the detection area is preferably 7.5 mm, and the width of the bridging channel is preferably 6 mm. The upper and lower paper-based electrodes share a working electrode at the through-hole structure, and nitrogen-doped graphene is applied to the working electrode of the lower paper-based electrode (the modification process uses the upper counter electrode and reference electrode, and the lower working electrode; the electrochemical deposition conditions are the same as those described in the above-mentioned technical solution). The circular working electrode of the un-drilled paper-based electrode corresponds to the drilling position of the upper layer, and the conductive pads of the upper and lower layers are at a 90-degree angle; the junction between the upper and lower paper-based electrodes is connected with double-sided adhesive.

[0070] In this invention, the above-mentioned structure of the paper-based microfluidic chip can effectively prevent contamination of the reference electrode and counter electrode during the modification process, thereby affecting the subsequent detection of heavy metal ions. That is, when nitrogen doping graphene, a three-electrode system is also required. After the modification is completed, the three electrodes should be cleaned. This operation may damage the reference electrode and counter electrode, affecting the subsequent detection and analysis of heavy metal ions.

[0071] The following detailed description, in conjunction with embodiments, illustrates the nitrogen-doped graphene-modified paper-based electrode, its preparation method, and its applications provided by the present invention. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0072] Examples 1-5

[0073] Electrode patterns were designed using CorelDRAW 2020 graphics software, including a circular working electrode, an arc-shaped counter electrode, and a reference electrode; the diameter of the circular working electrode was 5mm (e.g., ...). Figure 1 As shown in Figure A), the width of the counter electrode, reference electrode, and conductive pad is 1.4 mm (as shown in Figure A). Figure 1 Figure B shows a schematic diagram of the three electrodes and conductive pads, Figure 1C shows a schematic diagram of the complete paper-based electrode, and Figure 1E shows a schematic diagram of the three-dimensional layering of the paper-based electrode.

[0074] Customized templates: Customized screen printing stencils based on the layered electrode pattern, with each layer using 200 mesh.

[0075] Wax printing: Before wax printing, the dust and other impurities on the surface of the filter paper need to be blown away with nitrogen. Then, the pre-designed hydrophobic barrier pattern is printed onto the filter paper using a wax printer.

[0076] Printing hydrophilic carbon paste: Before screen printing, the filter paper needs to be purged with nitrogen again. The printing table must be level and clean. Use a water-oil dual-purpose squeegee with a hardness of 75, customized to a length of 5cm according to the screen. Select Whatman No. 1 filter paper as the printing substrate. Place the filter paper under the screen, apply an appropriate amount of hydrophilic carbon paste (CI-2067) to the screen, and hold the squeegee to smear the working electrode and counter electrode. Avoid slipping the screen or filter paper during printing to prevent unclear patterns.

[0077] Drying and curing: Before the hydrophilic carbon paste is completely dry, place the screen-printed filter paper in an open space for half an hour, and then put it in an oven for 10 minutes to cure it at 100℃.

[0078] Printing conductive silver paste: Remove the cured filter paper and, using the same method as in step four, print conductive silver paste (silver / silver chloride ink CIE-4250) to prepare the reference electrode and conductive pads. Ensure precise alignment of the two printing positions to form a complete current path. After printing, place in an oven for 30 minutes at 120℃ for curing. After cutting, place the prepared electrodes in a sealed bag and store at room temperature for later use.

[0079] An electrolyte was prepared by mixing a mixture of ammonium chloride and ammonia (molar ratio of ammonium chloride and ammonia was 1:1) with a graphene oxide solution (the concentration of graphene oxide in the electrolyte was 1 mg / mL, and the total concentration of ammonium chloride and ammonia was shown in Table 1).

[0080] The electrolyte was added dropwise at a uniform rate using a pipette, ensuring it just covered the paper-based electrode. The wires were correctly connected to form a three-electrode system. The electrochemical workstation was then turned on for electrochemical deposition. The electrochemical deposition was performed at a constant potential of -1.3V for 400 seconds, with a sampling interval of 0.1 seconds and a sensitivity of 1*10⁻⁶. -3 After electrochemical deposition, excess liquid is removed and the electrode is dried to form a nitrogen-doped graphene film on the surface of the circular working electrode, thus obtaining a paper-based electrode modified with nitrogen-doped graphene.

[0081] Table 1. Total concentrations (mol / L) of ammonium chloride and ammonia in the electrolytes of Examples 1-6

[0082] Example Example 1 Example 2 Example 3 Example 4 Example 5 Total concentration 0.01 0.02 0.05 0.125 0.25

[0083] The current response of the nitrogen-doped graphene-modified paper-based electrodes described in Examples 1-5 to cadmium ions was measured using DPV. To avoid random errors due to large fluctuations, three sets of experiments were performed for each concentration, and the average value was taken. The DPV parameters were set as follows: starting voltage -1.1V, ending voltage -0.7V, potential increase 0.004V, settling time 2s, and sensitivity 1*10. 4 A / V, the test results are as follows Figure 8 As shown, by Figure 8 It can be seen that as the concentration of NH4Cl / NH3·H2O increases from 0.01 mol / L to 0.05 mol / L, the nitrogen content gradually increases, and the current response intensity also gradually increases. However, as the concentration of NH4Cl / NH3·H2O increases from 0.05 mol / L to 0.25 mol / L, the current response intensity decreases instead of increasing. This may be because as the concentration increases, the amount of nitrogen deposited gradually reaches saturation. Continuing to increase the nitrogen source will result in excessive accumulation of nitrogen, which will hinder electron transfer and significantly reduce the current response intensity.

[0084] Examples 6-8

[0085] Electrode patterns were designed using CorelDRAW 2020 graphics software, including a circular working electrode, an arc-shaped counter electrode, and a reference electrode; the diameter of the circular working electrode was 5mm (e.g., ...). Figure 1 As shown in Figure A), the width of the counter electrode, reference electrode, and conductive pad is 1.4 mm (as shown in Figure A). Figure 1 Figure B shows a schematic diagram of the three electrodes and conductive pads, Figure C shows a schematic diagram of the complete paper-based electrode, and Figure E shows a schematic diagram of the three-dimensional layering of the paper-based electrode.

[0086] Customized templates: Customized screen printing stencils based on the layered electrode pattern, with each layer using 200 mesh.

[0087] Wax printing: Before wax printing, the dust and other impurities on the surface of the filter paper need to be blown away with nitrogen. Then, the pre-designed hydrophobic barrier pattern is printed onto the filter paper using a wax printer.

[0088] Printing hydrophilic carbon paste: Before screen printing, the filter paper needs to be purged with nitrogen again. The printing table must be level and clean. Use a water-oil dual-purpose squeegee with a hardness of 75, customized to a length of 5cm according to the screen. Select Whatman No. 1 filter paper as the printing substrate. Place the filter paper under the screen, apply an appropriate amount of hydrophilic carbon paste (CI-2067) to the screen, and hold the squeegee to smear the working electrode and counter electrode. Avoid slipping the screen or filter paper during printing to prevent unclear patterns.

[0089] Drying and curing: Before the hydrophilic carbon paste is completely dry, place the screen-printed filter paper in an open space for half an hour, and then put it in an oven for 10 minutes to cure it at 100℃.

[0090] Printing conductive silver paste: Remove the cured filter paper and, using the same method as in step four, print conductive silver paste (silver / silver chloride ink CIE-4250) to prepare the reference electrode and conductive pads. Ensure precise alignment of the two printing positions to form a complete current path. After printing, place in an oven for 30 minutes at 120℃ for curing. After cutting, place the prepared electrodes in a sealed bag and store at room temperature for later use.

[0091] An electrolyte was prepared by mixing a mixture of ammonium chloride and ammonia (molar ratio of ammonium chloride and ammonia was 1:1) with a graphene oxide solution (the concentration of graphene oxide in the electrolyte is shown in Table 2, and the total concentration of ammonium chloride and ammonia was 0.05 mol / L).

[0092] The electrolyte was added dropwise at a uniform rate using a pipette, ensuring it just covered the paper-based electrode. The wires were correctly connected to form a three-electrode system. The electrochemical workstation was then turned on for electrochemical deposition. The electrochemical deposition was performed at a constant potential of -1.3V for 400 seconds, with a sampling interval of 0.1 seconds and a sensitivity of 1*10⁻⁶. -3 After electrochemical deposition, excess liquid is removed and the electrode is dried to form a nitrogen-doped graphene film on the surface of the circular working electrode, thus obtaining a paper-based electrode modified with nitrogen-doped graphene.

[0093] Table 2. Concentration of graphene oxide in the electrolytes of Examples 6-8 (mg / mL)

[0094] Example Example 6 Example 7 Example 8 Total concentration 0.5 1.5 2

[0095] The current response of the nitrogen-doped graphene-modified paper-based electrodes described in Examples 2 and 6-8 to cadmium ions was measured using DPV. To avoid random errors due to large fluctuations, three sets of experiments were performed for each concentration, and the average value was taken. The DPV parameters were set as follows: starting voltage -1.1V, ending voltage -0.7V, potential increase 0.004V, settling time 2s, and sensitivity 1*10. 4 A / V, the test results are as follows Figure 9 As shown, by Figure 9It can be seen that as the concentration of graphene oxide (GO) increases from 0.5 mg / mL to 1.5 mg / mL, the current response increases accordingly. This is because as the GO concentration increases, more rGO is reduced and grown on the electrode surface, enhancing conductivity. When the concentration is 2 mg / mL, the current response no longer increases, and the experiment found that the measurement at this concentration is extremely unstable. The reason for this is that excessive rGO agglomerates due to interlayer π-π interactions, resulting in increased resistance. Agglomerated particles are likely to detach from the electrode surface due to fluid forces, leading to signal instability.

[0096] Examples 9-12

[0097] Referring to Example 7, the difference lies in the electrochemical deposition time. The electrochemical deposition times described in Examples 9-12 are shown in Table 3.

[0098] Table 3 shows the electrochemical deposition times (s) for Examples 9-12.

[0099] Example Example 9 Example 10 Example 11 Example 12 time 200 300 500 600

[0100] The current response of the nitrogen-doped graphene-modified paper-based electrodes described in Examples 7 and 9-12 to cadmium ions was measured using DPV. To avoid random errors due to large fluctuations, three sets of experiments were performed for each concentration, and the average value was taken. The DPV parameters were set as follows: starting voltage -1.1V, ending voltage -0.7V, potential increase 0.004V, settling time 2s, and sensitivity 1*10. 4 A / V, the test results are as follows Figure 10 As stated, by Figure 10 It can be seen that the current response continuously increases and reaches its maximum value during the process of increasing from 200s to 400s. However, with the increase of deposition time, the current response decreases instead. This may be because the deposition potential of -1.5V is a relatively high potential. At 400s, the nitrogen-doped graphene has already been deposited. Depositing too much nitrogen would severely damage the six-membered ring structure of graphene and block the porous structure of the paper substrate, significantly reducing the specific surface area of ​​the electrode and affecting the electron transport rate. Therefore, a deposition time of 400s was selected for subsequent experiments.

[0101] Test case

[0102] The nitrogen-doped graphene in the nitrogen-doped graphene-modified paper electrode described in Example 7 was characterized by XPS, and the test results are as follows. Figure 2As shown, A is the XPS spectrum of nitrogen-doped graphene, and B is the fitted peak of N in the nitrogen-doped graphene. A shows that the XPS spectrum reveals the presence of carbon, oxygen, and nitrogen atoms in the nitrogen-doped graphene, with carbon content of 78.71%, oxygen content of 12.77%, and nitrogen content of 3.06%, indicating successful nitrogen doping. B shows that peak N1s can be fitted into two distinct small peaks: pyrrole nitrogen at 398.48 eV and pyridine nitrogen at 400.56 eV. In the oxygen reduction catalytic reaction, pyridine nitrogen exhibits high reactivity, while pyrrole nitrogen demonstrates excellent electrochemical performance. The presence of these two nitrogen types is beneficial for the electrochemical detection of heavy metal ions.

[0103] The C1s peaks of the graphene oxide, reduced graphene oxide, and nitrogen-doped graphene described in Example 7 were fitted, and the test results are as follows: Figure 3 As shown (where A is the C1s spectrum of graphene oxide, B is the C1s spectrum of reduced graphene oxide, and C is the C1s spectrum of nitrogen-doped graphene), by Figure 3 It can be seen that the peaks at 284.78, 284.16, and 286.57 eV in the XPS spectrum of graphene oxide correspond to C-C, CO, and C=O bonds; the peaks at 284.77, 285.36, 286.70, and 288.42 eV in the XPS spectrum of reduced graphene oxide correspond to C-C, C≡C, CO, and C=O bonds; and the peaks at 284.79, 286.84, and 288.53 eV in the XPS spectrum of nitrogen-doped graphene correspond to C-C, CO, and C=O. Based on the XPS data, the C, O content, and C / O ratio in the samples are summarized in the table. It can be observed that from GO and rGO to NG, the C atom content gradually increases, while the O atom content gradually decreases, and the ratio between the two gradually increases (the atomic percentage of C in graphene oxide is 71.37%, the atomic percentage of O is 21.72%, and the C / O ratio is 3.2286; the atomic percentage of C in reduced graphene oxide is 76.33%, the atomic percentage of O is 16.05%, and the C / O ratio is 4.755; the atomic percentage of C in nitrogen-doped graphene is 78.71%, the atomic percentage of O is 122.77%, and the C / O ratio is 6.12). This indicates that during the reduction of graphene oxide, some CO and C=O bonds break, forming other chemical bonds, such as C-C bonds, resulting in successful graphene oxide reduction; similarly, during nitrogen doping, some CO and C=O bonds also break, and nitrogen forms CN bonds with a small number of newly formed carbon active sites, resulting in successful nitrogen doping.

[0104] The graphene oxide, reduced graphene oxide, and nitrogen-doped graphene described in Example 7 were subjected to SEM testing, and the test results are as follows. Figure 4As shown (where AB is the SEM image of graphene oxide, CD is the SEM image of reduced graphene oxide, and EH is the SEM image of nitrogen-doped graphene), by Figure 4 It is known that nitrogen-doped graphene has many flocculent wrinkles on its surface with varying pore sizes. Compared with reduced graphene oxide, the wrinkles are more obvious and have a more three-dimensional shape. These wrinkles may originate from defects caused by the doping of nitrogen atoms into the graphene sheets and the lack of oxygen-containing groups. The wrinkled structure gives nitrogen-doped graphene a larger specific surface area and more active sites, which can promote the permeation of electrolytes and provide transport channels for ion diffusion.

[0105] The nitrogen-doped graphene-modified paper electrode described in Example 7 was characterized by AFM, and the characterization results are as follows: Figure 5 As shown (where A is a two-dimensional image of a paper-based electrode modified with nitrogen-doped graphene, B is a three-dimensional image of a paper-based electrode modified with nitrogen-doped graphene, and C is a magnified three-dimensional image of a paper-based electrode modified with nitrogen-doped graphene), by... Figure 5 It can be seen that, at a scanning range of 5*5μm, the hill-like cluster structure of nitrogen-doped graphene can be clearly observed. The clusters are neatly arranged and uniformly cover the surface, with a vertical height of 0–151.9 nm. The results further indicate the successful doping of nitrogen, and the characterization results are consistent with the SEM images.

[0106] The paper-based electrode described in Example 7 and the nitrogen-doped graphene-modified paper-based electrode were subjected to cyclic voltammetry scans in a mixed solution of potassium ferricyanide and potassium chloride (potassium ferricyanide concentration of 5 mmol / L and potassium chloride concentration of 0.1 mol / L) to test their electrochemical performance; the test results are as follows. Figures 6-7 As shown, where Figure 6 The CV curve of the paper-based electrode described in Example 7 is shown below. Figure 7 The CV curve of the nitrogen-doped graphene-modified paper-based electrode described in Example 7; by Figures 6-7 It can be seen that the nitrogen-doped graphene electrode has a higher peak current response compared with the bare electrode (paper-based electrode): the current response of the latter at a scan rate of 50 mV / s is the same as that of the former at 75 mV / s. When the scan rate is 100 mV / s, the peak current of the latter is 32.89% higher than that of the former. This fully demonstrates that the nitrogen-doped graphene modified electrode exhibits superior conductivity. The reason may be that after nitrogen deposition, the high specific surface area of ​​nitrogen-doped graphene effectively reduces the diffusion path of ions to the electrode surface, provides more active sites, and enhances the electron transfer rate in electrochemical reactions.

[0107] Application examples

[0108] according to Figure 11The fabrication process described above prepares a paper-based microfluidic chip (denoted as NG / pSPE-μPAD sensor). The front structure of the upper layer of the paper-based microfluidic chip is shown below. Figure 12 As shown in Figure A, the back structure is as follows: Figure 12 As shown in B; the lower layer of the paper-based microfluidic chip is as follows: Figure 12 (As shown in C): Two paper-based electrodes are prepared according to the process of preparing the paper-based electrodes in Examples 1-13. A hole is removed from the circular working electrode of one of the paper-based electrodes and this hole is used as the upper layer. The un-drilled paper-based electrode is used as the lower layer (wherein, the upper layer does not include the circular working electrode and the conductive pads connecting the circular working electrode; the lower layer also has a sample injection area, which is a blank area that has not passed through a hydrophobic printed hydrophobic barrier and is bridged with the working electrode; the distance between the blank area and the detection area is 7.5 mm, and the width of the bridging channel is 6 mm). The circular working electrode of the un-drilled paper-based electrode corresponds to the drilling position of the upper layer. The conductive pads of the upper and lower layers are at a 90-degree angle. The upper and lower layers are overlapped according to the above requirements, and double-sided tape is used to connect the junction of the upper and lower layers; (this makes the upper layer form three...) The electrode system uses the lower working electrode, achieving the goal of sharing a single working electrode between the upper and lower layers while using different reference and counter electrodes. After overlapping, an electrolyte (graphene concentration of 1.5 mg / mL, total concentration of ammonium chloride and ammonia of 0.05 mol / L) is added to the upper layer. Conductive clips are used to clamp the upper reference and counter electrodes and the lower working electrode for electrochemical deposition (deposition potential of -1.5 V, time of 400 s) to achieve nitrogen doping. Then, the overlapped bilayer paper-based chip is unfolded, at which point the lower working electrode has completed the nitrogen doping modification step. A heavy metal solution is dropped into the sample injection area, where it flows along the microfluidic channel formed by the hydrophobic barrier towards the detection area and forms the negative electrode under the influence of the negative potential. Then, differential pulse stripping voltammetry is used to complete electrochemical detection.

[0109] The differential pulse stripping voltammetry test conditions were: buffer pH 4.85, enrichment potential -1.1V, and enrichment time 150s. The paper-based microfluidic chip was used for the simultaneous quantitative determination of Cd concentrations in the range of 5-100 μg / L. 2+ Pb 2+ and Hg 2+ The electrochemical behavior of three ions was studied, and dissolution peak curves were obtained. A curve showing the relationship between peak current and concentration was also established. Figure 13 As shown, by Figure 13 It can be seen that the dissolution curve has a very obvious peak shape, the peak current value increases with increasing concentration, and the peak current of the three ions all show a good linear relationship with concentration: Cd 2+ Pb 2+ and Hg2+ The linear equations were I = -0.3791c - 8.034, I = -0.5393c - 7.606, and I = -0.8443c - 4.325, respectively, with correlation coefficients of 0.9912, 0.9938, and 0.9952, and sensitivities of 0.3791, 0.5393, and 0.8443 μA / μg / L, respectively. The limit of detection (LOD) was calculated based on LOD = 3σ / k, where σ is the standard deviation of multiple measurements of the blank sample, and k is the slope of the standard curve. The calculated Cd... 2+ Pb 2+ and Hg 2+ The theoretical detection limits were 0.5661, 0.3979, and 0.2541 μg / L, respectively, which meet the requirements of GB5749-2022 Standard for Drinking Water Quality, which stipulates that the limits for cadmium are no more than 5 μg / L, the limits for lead are no more than 10 μg / L, and the limits for mercury are no more than 1 μg / L.

[0110] Selectivity experiment (by adding interfering ions, comparing Cd before and after addition) 2+ Pb 2+ and Hg 2+ An experiment evaluating the selectivity of a sensor was conducted by analyzing the changes in the peak values ​​of the dissolution currents of three types of ions.

[0111] To a concentration of 50 μg / L of Cd 2+ Pb 2+ and Hg 2+ The three ion buffer solutions were each supplemented with an interfering ion Ni at a concentration of 1000 μg / L. 2+ Cu 2+ Mn 2+ Zn 2+ and Ca 2+ Differential pulse stripping voltammetry was performed to observe the changes in the peak current; where I represents the current before the addition of Cd. 2+ Pb 2+ and Hg 2+ The current responses of the three ions are shown, where I0 is the current response after adding a certain interfering ion. Therefore, the deviation is the percentage of I0 / I. The test results are as follows: Figure 14 As shown, by Figure 14 It can be seen that in the presence of interfering ions Ni 2+ Cu 2+ Mn 2+ Zn 2+ and Ca 2+ In the case of Cd 2+ The current deviations before and after were 96.6%, 91.4%, 97.3%, 95.8%, and 96.1%, respectively, for Pb. 2+The current deviations before and after were 95.3%, 93.1%, 96.8%, 97.1%, and 95.9%, respectively. 2+ The current deviations before and after were 95.7%, 95.6%, 97.7%, 96.2%, and 97.2%, respectively. It can be seen that after adding a large number of interfering ions, the deviation of the dissolution peak current was less than 10%, with Cu... 2+ The existence of Cd 2+ and Pb 2+ The interference is the greatest (the reason may be that, apart from Hg) 2+ In addition, Cu 2+ Its reducing power is higher than that of Cd. 2+ and Pb 2+ (It is easier to deposit on the electrode surface), but the current response is reduced by only 8.6% and 6.9%. Except for Cu 2+ All other deviations were within 5%, indicating that the NG / pSPE-μPAD sensor can selectively detect Cd while resisting the influence of interfering ions. 2+ Pb 2+ and Hg 2+ Three types of ions;

[0112] Reproducibility testing (referring to the testing of different batches of electrodes prepared in parallel using the same method under the same conditions for Cd) 2+ Pb 2 + and Hg 2+ An experiment was conducted to detect three ions using differential pulse stripping voltammetry, and the reproducibility of the sensor was evaluated by comparing changes in the peak stripping current.

[0113] Figure 15 To measure the dissolution curves of heavy metal ions in three different batches of paper-based microfluidic chips prepared according to the method described in the application example. Figure 15 The peak values ​​and calculated relative standard deviations are shown in Table 4. Figure 15 As shown in Table 4, the RSD of the three heavy metal ions measured by the three different batches of paper-based microfluidic chips was all within 6%, indicating that the NG / pSPE-μPAD sensor prepared in this chapter has good reproducibility.

[0114] Table 4. Peak current and relative standard deviation of three different batches of paper-based microfluidic chips.

[0115]

[0116] Real water sample testing:

[0117] Beijing tap water was collected as an experimental sample for a spiked recovery experiment. Before testing, the pH of the sample was adjusted to 4.85 using sodium acetate-acetate buffer, and then different concentrations of Cd were added. 2+ Pb2+ and Hg 2+ Standard solution, making Cd 2+ Pb 2+ and Hg 2+ The concentrations were 5, 25, and 50 μg / L, respectively, and electrochemical analysis was performed under optimized experimental conditions. The test results are shown in Table 5: the concentration results obtained from the actual samples were compared with the values ​​of the added concentrations. The recovery rate was in the range of 96.4%-106.2%, the recovery rate deviation did not exceed 7%, and the relative standard deviation was also within the acceptable range. This demonstrates that the NG / pSPE-μPAD sensor has high accuracy and reliability and has great potential for practical water body applications.

[0118] Table 5 shows the detection results of the paper-based microfluidic chip described in the application examples.

[0119]

[0120] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for fabricating a paper-based microfluidic chip, characterized in that, Includes the following steps: An upper paper-based electrode and a lower paper-based electrode are provided. The upper paper-based electrode is prepared by providing filter paper, which has hydrophobic and non-hydrophobic regions, and the surface of the hydrophobic regions has a hydrophobic barrier. A counter electrode, a reference electrode, and conductive pads of a three-electrode system are printed on the surface of the filter paper, and holes are drilled at the working electrode position to obtain the upper paper-based electrode. The lower paper-based electrode is prepared by providing filter paper, which has hydrophobic and non-hydrophobic regions, and the surface of the hydrophobic regions has a hydrophobic barrier. A three-electrode system and conductive pads are printed on the surface of the filter paper to obtain the lower paper-based electrode. The three-electrode system includes a working electrode, a counter electrode, and a reference electrode. The lower paper-based electrode also has a sample injection area, which is a blank area that has not passed through the hydrophobic printed hydrophobic barrier and is bridged with the working electrode. The distance between the blank area and the detection area is 7.5 mm, and the width of the bridging channel is 6 mm. An electrolyte is obtained by mixing a mixed solution of ammonium chloride and ammonia with a graphene oxide solution; the concentration of graphene oxide in the electrolyte is 1.5 mg / mL, and the total concentration of ammonium chloride and ammonia is 0.05 mol / L. The circular working electrode of the lower paper-based electrode is aligned with the punched hole of the upper paper-based electrode, and the conductive pads of the upper and lower paper-based electrodes are overlapped at a 90-degree angle. Double-sided adhesive is used to connect the junction of the upper and lower layers. After overlapping, electrolyte is dropped onto the upper layer for electrochemical deposition. The deposition potential is -1.5V, and the time is 400s. The overlapping double-layer paper-based chips are unfolded to obtain the paper-based microfluidic chip.

2. The preparation method according to claim 1, characterized in that, The molar ratio of ammonium chloride to ammonia in the mixed solution of ammonium chloride and ammonia is 1:

1.

3. The preparation method according to claim 1, characterized in that, The sampling interval for the electrochemical deposition was 0.1 s, and the sensitivity was 1. 10 -3 A / V.

4. The preparation method according to claim 1, characterized in that, The method for preparing the hydrophobic barrier is the wax printing method.

5. The preparation method according to claim 1, characterized in that, The working electrode is a circular working electrode, and the raw material for preparation is hydrophilic carbon paste; The counter electrode is an arc-shaped counter electrode, and the raw material for preparation is hydrophilic carbon paste; the distance between the working electrode and the counter electrode is 0.67 mm; The reference electrode is prepared from conductive silver paste. The circular working electrode, the arc-shaped counter electrode, and part of the reference electrode are located in the non-hydrophobic region of the filter paper.

6. The preparation method according to claim 1, characterized in that, Both the working electrode and the counter electrode are connected to conductive pads, and the conductive pads are made of conductive silver paste; the conductive silver paste extends into the hydrophobic region. The remaining portion of the reference electrode extends into the hydrophobic region.

7. The paper-based microfluidic chip prepared by the preparation method according to any one of claims 1 to 6.

8. The application of the paper-based microfluidic chip according to claim 7 in the detection of heavy metal ions, characterized in that, The heavy metal ions include Cd. 2+ Pb 2+ and Hg 2+ One or more of them.

Images