Solid-state contact layer of all-solid-state ion-selective electrode and preparation method and application of solid-state contact layer

Abstract

The invention relates to the technical field of sensing of aquatic water ion detection, and discloses a solid-state contact layer of an all-solid-state ion selective electrode, FeCoNi-coated ZIF-8 doped with three transition metals of iron, cobalt and nickel is synthesized through a one-pot method, nano porous carbon based on ZIF-8 is generated through pyrolysis, and finally a transition metal doped ZIF-8-derived carbon nanomaterial is obtained. And mixing the carbon nanomaterial with a hydrophobic material and an ionic liquid to construct an ion-electron transduction layer of the all-solid-state ion selective electrode with excellent hydrophobic performance. The solid-state contact layer prepared by the invention not only can effectively inhibit the formation of a water layer between an ion sensitive film and a conductive substrate and solve the problem of potential drift of an ion selective electrode in measurement, but also has the advantages that the electric double-layer capacitance effect of the carbon nanomaterial and the synergistic pseudocapacitance effect among various transition metals act together; and the ion-electron transduction capability of the electrode is improved.

Description

Technical Field

[0001] This invention relates to the field of all-solid-state ion-selective electrodes, specifically to a solid contact layer of an all-solid-state ion-selective electrode, its preparation method, and its application. Background Technology

[0002] Ion-selective electrodes (ISEs) enable rapid and direct in-situ analysis, eliminating errors caused by sample preservation or storage and changes in environmental conditions. They offer advantages such as easy miniaturization, low power consumption, rapid detection, and low manufacturing costs, making them the most attractive sensing platform for environmental water analysis. Solid-state contact ion-selective electrodes (SC-ISEs) directly deposit ISMs onto the surface of a solid electrode, adding a solid contact layer (SC) between the ISM and the conductive substrate as an ion-electron transduction layer. This replaces the internal filling liquid, directly depositing the ISM onto the solid electrode surface and eliminating the cumbersome liquid-phase contact of liquid-state contact ion-selective electrodes (LC-ISEs), thus improving the electrode's stability and reliability.

[0003] In the use of solid-state ion-selective electrodes (ISMEs), a thin aqueous phase, known as a "water layer," forms at the interface between the solid contact layer and the ion-selective membrane. This water layer causes potential drift and delayed response. Since all ISMs absorb moisture to some extent, a hydrophobic solid contact layer is crucial in preventing water layer formation and accumulation. Graphene, with its single-layer hexagonal honeycomb structure, possesses excellent electrical conductivity, a large specific surface area, and hydrophobicity. Graphene, used as a solid contact layer, exhibits double-layer capacitance, and its significant role in suppressing water layers and improving electrode performance has been proven. Ultrathin, defect-free graphene sheets with high conductivity and uniform porosity can be produced using fluid dynamics-assisted layering technology, increasing the effective surface area available for forming the double layer. When drop-coated onto the electrode substrate surface as a solid contact layer, its enhanced double-layer capacitance effectively reduces potential drift under continuous operation and external interference. Its excellent chemical inertness and extremely low reactivity under environmental conditions minimize interference from oxygen, carbon dioxide, and light, further stabilizing the electrode potential. Besides direct drop coating, graphene can also be deposited directly onto the electrode surface via chemical reduction to obtain graphene oxide (CRGO or ERGO), as well as through inkjet printing and laser induction. Furthermore, graphene needs to be combined with other materials such as metal oxides, metal nanoparticles, and polymers to improve sensing performance. These compounds increase the cost, complexity, and manufacturing time of the sensor, and may reduce its flexibility and biocompatibility. Additionally, graphene has a limited surface area and is prone to stacking, and the sensitivity of the resulting ion-selective electrodes has not yet reached ideal levels. Other carbon nanomaterials can complement graphene, such as carbon nanotubes, carbon black, and 3DOM carbon. These methods have enabled sensors to exhibit low detection limits and excellent stability in diverse and challenging real-world environments. However, most carbon material surfaces react with water or oxygen, potentially introducing additional functional groups during electrode fabrication, leading to additional drift and affecting stability and interference resistance. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a solid contact layer for an all-solid-state ion-selective electrode, its preparation method, and its application.

[0005] The first aspect of this invention provides a solid contact layer for an all-solid-state ion-selective electrode. In the one-pot preparation of ZIF-8, transition metals Fe, Co, and Ni are incorporated and grown at 0°C to 5°C to obtain FeCoNi@ZIF-8. The dispersion obtained by pyrolyzing FeCoNi@ZIF-8 and mixing it with a hydrophobic material and an ionic liquid is then coated into a film to obtain the final product. The molar ratio of Fe, Co, Ni to the ligand of ZIF-8 is 0.006~0.007:0.006~0.007:0.015~0.02:1; the mass ratio of FeCoNi@ZIF-8, hydrophobic material to ionic liquid is 6~8:1.7~2.2:0.7~1.2.

[0006] The transition metals Fe, Co, and Ni selected in this invention not only possess excellent electronic conductivity but also exhibit multiple oxidation states, enabling them to efficiently participate in redox processes through electron gain and loss, thus being the primary source of redox capacitance (pseudocapacitance). Among transition metals, iron, cobalt, and nickel are low in cost and possess excellent electrochemical activity. Furthermore, due to the synergistic effect of transition metals, they can significantly enhance the redox activity of materials, thereby improving the pseudocapacitive effect.

[0007] In another preferred embodiment, the hydrophobic material is polyvinylidene fluoride-hexafluoropropylene or polyvinylidene fluoride; the fluorine-containing functional groups in these two materials can play a hydrophobic role; The ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)ammonium salt; this ionic liquid is the most hydrophobic ionic liquid: the weak polarity of its cation makes it difficult for it to bind with water molecules through hydrogen bonds, and it also hinders the contact between water molecules and nitrogen atoms on the ring through steric hindrance; the anionic structure contains two trifluoromethyl groups (-CF3), which have strong hydrophobic properties; the combined effect of the anionic and cation groups greatly reduces the interaction ability with water molecules.

[0008] In another preferred embodiment, the Fe is derived from FeSO4·7H2O or Fe(NO3)2·6H2O; The Co comes from Co(NO3)2·6H2O; The Ni is derived from Ni(NO3)2·6H2O.

[0009] A second aspect of the present invention provides a method for preparing the solid contact layer of the all-solid-state ion-selective electrode, comprising the following steps: FeSO4·7H2O or Fe(NO3)2·6H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O, and zinc salt are dissolved in an alcoholic reagent to obtain mixture A; 2-methylimidazole is used as an organic ligand and dissolved in methanol to obtain mixture B; The two solutions were stored at 0–5°C to reach this temperature, and then reacted in an ice-water bath at 0–5°C to obtain FeCoNi@ZIF-8. Specifically, when rapidly mixing the two solutions, the metal salt solution should be poured into the organic ligand solution. Preferably, the mixing time of the two solutions is within 30 seconds. After mixing, stirring was continued while maintaining the system temperature. Preferably, the stir bar speed was set to 500 rpm for the first 10 minutes, then the speed was reduced to 250 rpm and stirring was continued for 6 hours, maintaining the system temperature for aging for 24 hours. The solution separated into layers, and the upper half of the solution was removed. The lower half of FeCoNi@ZIF-8 was centrifuged at 8000 rpm for 10 minutes. Then, FeCoNi@ZIF-8 was washed three times with anhydrous methanol using a centrifuge at 8000 rpm for 10 minutes each time.

[0010] In this invention, mixtures A and B are first heated to 0-5°C before reaction. Polyvinylidene fluoride-hexafluoropropylene is first reacted with acetone, followed by the addition of FeCoNi@ZIF-8-NPC. The purpose of this is that polyvinylidene fluoride acts as a mesh to support FeCoNi@ZIF-8-NPC. The FeCoNi@ZIF-8-NPC is first sonicated to ensure uniform dispersion of its carbon nanoparticles. Then, the ionic liquid is added, and sonication is used to uniformly disperse the ionic liquid within the voids of the carbon nanoparticles, providing a rapid transport channel for the target ions. This allows the ions to quickly reach the ion electron transduction sites in the electrode contact layer, reducing the response time of the ion-selective electrode and enabling rapid detection of ammonium ions.

[0011] Further, FeCoNi@ZIF-8 was dried in an oven at 70°C for 12 hours, or a freeze dryer could be used for 12 hours. The dried FeCoNi@ZIF-8 was then ground in an agate mortar for 10 minutes to obtain uniformly dispersed FeCoNi@ZIF-8 with a particle size of approximately 300 nm. The powdered sample was then stored in a dark, dry environment.

[0012] FeCoNi@ZIF-8 was pyrolyzed under a protective atmosphere to obtain FeCoNi@ZIF-8-NPC; FeCoNi@ZIF-8-NPC, polyvinylidene fluoride-hexafluoropropylene, and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt were dissolved in acetone and sonicated to obtain a dispersion. Specifically, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) powder was first dissolved in acetone and stirred at 55°C for 2 hours. FeCoNi@ZIF-8-NPC was then dissolved in the polyvinylidene fluoride-hexafluoropropylene copolymer solution, and grinding beads were added. The mixture was sonicated in a water bath for 1 hour to ensure uniform dispersion of FeCoNi@ZIF-8-NPC. Finally, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt ionic liquid was added and sonicated for 1 hour. The dispersion should be stored at low temperature and protected from light, and sonicated for 10-30 minutes before each use.

[0013] A solid contact layer is obtained by drop-coating the dispersion onto the substrate using a drop-coating method.

[0014] In another preferred embodiment, the pyrolysis temperature is 950℃~1050℃ and the time is 60min~80min.

[0015] In another preferred embodiment, the alcohol reagent is methanol.

[0016] In another preferred embodiment, the protective atmosphere is nitrogen.

[0017] The third aspect of the present invention provides the application of the solid contact layer in the preparation of an all-solid-state ion-selective electrode, wherein the all-solid-state ion-selective electrode is composed of a conductive substrate, a solid contact layer, and a sensitive film; The conductive substrate is any one of glassy carbon, screen-printed electrode rigid conductive substrate, and flexible conductive substrate. The solid contact layer is located between the ion-sensitive film and the conductive substrate; The sensitive membrane is prepared from an ion carrier, an ion exchanger, a polymer substrate material, and a plasticizer; wherein, the ion carrier is any one of potassium ion, sodium ion, calcium ion, magnesium ion, ammonium ion, nitrate ion, and chloride ion; the ion exchanger is a tetraphenylborane derivative or a tetraalkyl quaternary ammonium salt; the polymer substrate material is polyvinyl chloride; the plasticizer is one of di-n-octyl sebacate (DOS), o-nitrophenyl octyl ether (o-NPOE), di-n-octyl phthalate (DNOP), and butyl phthalate (DBP), or it can be a polymer ion-sensitive membrane such as a polyaniline pH-sensitive membrane.

[0018] The fourth aspect of this invention provides the application of the all-solid-state ion-selective electrode in detecting ion concentrations in an aquatic environment.

[0019] In another preferred embodiment, the aquatic environment is an aquaculture water body.

[0020] Compared with the prior art, the present invention has the following beneficial effects: In this invention, the pyrolysis of FeCoNi@ZIF-8 preserves the original MOF framework, and the transition metal nanoparticles formed during this process are uniformly embedded in the carbon nanopores. Their large specific surface area increases the active sites and enhances ion-electron transfer at the interface, thereby improving the electrode's response time and detection limit. The fluorine-containing functional groups of polyvinylidene fluoride-hexafluoropropylene and the hydrophobic anionic group [TFSI-] of the ionic liquid provide excellent hydrophobicity. Based on the solid-state transconducting layer obtained after the pyrolysis of FeCoNi@ZIF-8, the prepared all-solid-state ion-selective electrode exhibits excellent stability, anti-interference ability, sensitivity, and repeatability. The constructed all-solid-state ammonium ion-selective electrode has a high sensitivity to 10... -1 ~10 -6 M of NH 4+ It exhibits linear performance and a Stirling response, with a sensitivity of 56.73 mV / dec and a detection limit of 10. -6.4 M.

[0021] The solid contact layer in this invention is a MOF-derived carbon material, which has high chemical inertness and structural stability, avoiding the degradation problem of conductive polymers and resulting in extremely low potential drift (<300 μV / h) of the electrode. The huge specific surface area and abundant porous structure endow the contact layer with extremely high capacitance, and the introduction of trace transition metals into redox capacitance further enhances the capacitance performance of the contact layer without generating interference signals. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the manufacturing process of FeCoNi@ZIF-8-NPC of the present invention.

[0023] Figure 2 Transmission electron microscopy (TEM) images of FeCoNi@ZIF-8 and FeCoNi@ZIF-8-NPC are shown; where A is the TEM image of FeCoNi@ZIF-8 and B is the TEM image of FeCoNi@ZIF-8-NPC.

[0024] Figure 3 The image shows the X-ray diffraction pattern of FeCoNi@ZIF-8-NPC, where B represents the metal peak analysis results of A.

[0025] Figure 4 This is a test diagram of the contact angle of the solid contact layer.

[0026] Figure 5 The graph shows the sensitivity test results of the ammonium ion selective sensor in the example.

[0027] Figure 6 This is a water layer test diagram of the ammonium ion selective sensor in the embodiment.

[0028] Figure 7 The figure shows the long-term stability test results of the ammonium ion selective sensor in the example.

[0029] Figure 8 Comparison of short-term stability tests of two ammonium ion selective electrodes before and after modification of the contact layer.

[0030] Figure 9 The results of constant current charge-discharge experiments for an ammonium ion selective sensor are shown. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0032] Unless otherwise specified, the experimental methods described in the following embodiments are conventional methods; unless otherwise specified, the reagents and materials are commercially available.

[0033] The following example uses an all-solid-state ammonium ion electrode, combined with... Figure 1 The fabrication process of an all-solid-state ion-selective electrode with FeCoNi@ZIF-8-NPC as the solid-state transduction layer is described, using all AR-grade reagents. The specific process is as follows:

[0034] The glassy carbon electrode surface is treated, including polishing and cleaning. Glassy carbon electrodes are used here to prepare the all-solid-state ion-selective electrode. Alternatively, rigid or flexible electrodes such as screen-printed electrodes or flexible inkjet-printed electrodes can be used. Other appropriate cleaning and activation steps are then performed on the electrode before modifying it with a FeCoNi@ZIF-8-NPC solid-state contact layer. The specific treatment process is as follows:

[0035] Using 1500-grit, 2000-grit, and 3000-grit sandpaper, the electrode was placed vertically on the sandpaper and polished for 30 seconds using the "rainbow method." After polishing with each type of sandpaper, the electrode was rinsed with deionized water for 30-60 seconds and then ultrasonically cleaned in deionized water and anhydrous ethanol for 30 seconds, followed by drying with nitrogen. After polishing, the electrode was placed vertically on chamois leather and polished with alumina abrasive powder with particle sizes of 0.3µm and 0.05µm, polishing the electrode in a figure-eight pattern. After ultrasonic cleaning in deionized water and anhydrous ethanol for 30 seconds, the electrode was dried with nitrogen for later use.

[0036] FeCoNi@ZIF-8 was synthesized by preparing a polymetallic salt solution and a 2-methylimidazole ligand solution.

[0037] High-purity nitrogen gas was bubbled into anhydrous methanol for 5-10 minutes to purge air. 2.9749 g of Zn(NO3)2·6H2O, 0.0834 g of FeSO4·7H2O, 0.0873 g of Co(NO3)2·6H2O, and 0.2617 g of Ni(NO3)2·6H2O were dissolved in 100 mL of anhydrous methanol to prepare a polymetallic salt solution, named Mixed Solution A. FeSO4·7H2O can be replaced with Fe(NO3)2·6H2O. 3.94 g of 2-methylimidazole was weighed and dissolved in 100 mL of anhydrous methanol to prepare Mixed Solution B. A magnetic stirrer was set to 300 rpm, and Mixed Solutions A and B were stirred thoroughly for 30 minutes to ensure complete dissolution. Mixed Solutions A and B were then sealed and stored in a refrigerator until the solution temperature dropped to 0-5°C. Mixed solution B was placed in an ice-water bath, with its temperature controlled at 0-5℃. A magnetic stirrer was set to a speed of 500 r / min. Mixed solution A was rapidly poured into mixed solution B and stirred for 30 seconds. After 30 minutes, the stirring speed was reduced to 250 r / min, and stirring continued for 12 hours to allow FeCoNi@ZIF-8 to grow fully. The solution was then placed in a refrigerator for 24 hours to achieve the optimal morphology. After aging, the solution separated into layers, as shown in the scanning electron microscope (SEM) image of the NPC prepared by the pyrolysis of FeCoNi@ZIF-8. The dopant morphology of ZIF-8 was not affected by the addition of trace transition metals, and the MOF could crystallize and grow normally. After deheating, FeCoNi@ZIF-8-NPC still retained the dodecahedral structure of ZIF-8, indicating that the NPC retained the large specific surface area of ​​ZIF-8 and had a large number of active sites on its surface. The pale purple substance appearing in the lower layer is the doped trace transition metal, such as... Figure 1 As shown, the overall MOF still maintains the FeCoNi@ZIF-8 structure of ZIF-8.

[0038] Next, FeCoNi@ZIF-8 was washed and dried. The supernatant was removed from the separated solution, and the FeCoNi@ZIF-8 in the lower half of the mixture was separated by centrifugation at 8000 rpm for 8 min. After centrifugation, the supernatant was removed, and FeCoNi@ZIF-8 was reconstituted with anhydrous methanol to ensure thorough dispersion. The FeCoNi@ZIF-8 was then washed using a centrifuge at 8000 rpm for 10 min. This washing process was repeated three times, with reconstitution with anhydrous methanol each time. The resulting purple precipitate was the washed FeCoNi@ZIF-8.

[0039] FeCoNi@ZIF-8 was dried. An oven was used with the temperature set at 70℃ for 12 hours to obtain fully dried FeCoNi@ZIF-8. Alternatively, a freeze dryer can be used for 12 hours to obtain fully dried FeCoNi@ZIF-8.

[0040] FeCoNi@ZIF-8-derived nanoporous carbon, FeCoNi@ZIF-8-NPC, was obtained by high-temperature pyrolysis in nitrogen. The pyrolysis process can be performed using a muffle furnace or a tube furnace; a muffle furnace is used as an example here. An alumina crucible was selected, and after washing the crucible and protective cover, it was dried at 100°C for 2 hours. The thoroughly dried FeCoNi@ZIF-8 was then ground using an agate mortar for 10 minutes, yielding dried FeCoNi@ZIF-8 powder. The FeCoNi@ZIF-8 powder was placed in the alumina crucible, ideally spreading it evenly across the bottom, and then sealed with the protective cover. The alumina crucible was placed in a muffle furnace. After sealing the furnace, a vacuum pump was used to evacuate the gas inside until the internal pressure reached -0.1 MPa. Nitrogen gas was slowly introduced into the furnace until atmospheric pressure was reached. The process was repeated three times, with the final step involving introducing nitrogen gas until the pressure reached -0.8 MPa. This ensured the furnace was filled with nitrogen and free of oxygen, preventing the MOF from reacting with oxygen during high-temperature pyrolysis. The pyrolysis temperature was set to 950–1050 °C, with a heating rate of 5 °C / min and a pyrolysis time of 60–80 min. The furnace was then allowed to cool naturally to room temperature. The vent valve was opened to restore the internal pressure to atmospheric pressure. The furnace could then be opened normally. Upon opening the crucible, the pale purple MOF powder turned black, yielding FeCoNi@ZIF-8-NPC.

[0041] FeCoNi@ZIF-8-NPC powder was ground in an agate mortar for 10-20 min. The powder was then washed with anhydrous methanol and centrifuged to remove impurities. The centrifuge speed was set to 9000 r / min and the centrifugation time to 10 min. The powder was reconstituted with methanol before each centrifugation, repeated three times. The NPC powder was then dried using a freeze dryer or oven. The resulting FeCoNi@ZIF-8-NPC powder was observed under a transmission electron microscope (TEM) as shown below. Figure 2 As shown, after high-temperature pyrolysis, the MOF surface changes from smooth and flat to full of pores. The XRD results are as follows... Figure 3 As shown in the figure, the peak positions correspond to iron(II,III) oxide, cobalt, and nickel, which are the incorporated transition metals. This indicates that the trace amounts of incorporated transition metals are retained during the high-temperature pyrolysis process. Meanwhile, the XRD results show that NPC does not contain zinc. This is because during the pyrolysis process at 1000℃, Zn evaporates directly and does not remain in the FeCoNi@ZIF-8-NPC structure.

[0042] The contact layer dispersion was prepared using FeCoNi@ZIF-8-NPC, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)ammonium salt ionic liquid (EMImTFSI), and acetone. 20 mg of PVDF-HFP powder was weighed and dissolved in 20 mL of acetone. The mixture was stirred for 2 hours at 500 r / min and 50 °C to ensure complete dissolution of the PVDF-HFP powder, resulting in a PVDF-HFP acetone dispersion. Further, 8–12 mg of FeCoNi@ZIF-8-NPC was weighed and dissolved in 1 mL of the acetone dispersion. The mixture was sonicated for 1 hour to ensure complete dispersion of PVDF-HFP. Then, 1–3 μL of EMImTFSI was added, and the mixture was sonicated for another 1 hour to obtain a fully dispersed FeCoNi@ZIF-8-NPC contact layer dispersion.

[0043] Using a drop-coating method, 10-15 μL of contact layer dispersion was drop-coated onto the surface of a 4 mm diameter glassy carbon electrode to modify the contact layer. The drop-coating amount could be adjusted according to the area of ​​different sensitive regions of the electrode to ensure complete coverage of the sensitive region by the dispersion. The electrode was then dried in a 40°C oven for 12 hours to allow the organic solvent in the dispersion to fully evaporate, thus completing the contact layer modification on the electrode surface. The hydrophobicity of the NPC solid contact layer was measured using contact angle. Figure 4 As shown, deionized water remains in a spherical shape on the contact layer surface with a contact angle of 122.687°, indicating that the prepared contact layer dispersion can form a hydrophobic surface after being modified and drop-coated on the electrode surface.

[0044] Preparation of NH4 + A sensitive membrane solution was prepared by drop-coating an ion-sensitive membrane onto the electrode surface and then drying. Further, a membrane was coated onto the electrode surface using the drop-coating method and then dried. The ion-selective electrode sensitive membrane consisted of an ion carrier, an ion exchanger, a polymer substrate material, and a plasticizer. The ion carrier could be a commercially available ion carrier containing potassium, sodium, calcium, magnesium, ammonium, nitrate, chloride, or hydrogen ions. The ion exchanger was a tetraphenylborane derivative or a tetraalkyl quaternary ammonium salt. The polymer substrate material was polyvinyl chloride. The plasticizer was one of di-n-octyl sebacate (DOS), o-nitrophenyl octyl ether (o-NPOE), di-n-octyl phthalate (DOP), and butyl phthalate (DBP), or it could be a polymer ion-sensitive membrane such as a polyaniline pH-sensitive membrane. In this example, NH4 was selected as the non-viable bacterium. + The ion carrier is di-n-octyl sebacate (DOS), the plasticizer is sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaTFPB), and the polymer matrix material is polyvinyl chloride (PVC). NH4 is prepared using these methods. +Select the membrane. Weigh 2 mg of viable bacterium, 0.6 mg of NaTFPB, 65.8 mg of PVC, and 144 μL of DOS. Add them to a 2 mL sample vial, then add 1 mL of tetrahydrofuran (THF) to dissolve them. Add the stir bar, tighten the cap, and set the magnetic stirrer speed to 500 r / min. Stir for 1 h to obtain a homogeneous sensitive membrane solution.

[0045] The sensitive film solution was drop-coated onto the electrode contact layer surface using a drop-coating method. Further, an appropriate amount of sensitive film solution was drop-coated according to the size of the electrode's sensitive area; in this case, 15 μL was drop-coated onto an electrode surface with a diameter of 4 mm. The electrode with the drop-coated sensitive film was then placed in a constant temperature drying oven at 40°C for 12 hours to dry.

[0046] The electrode after modifying the contact layer and sensitive film was placed at 10... -3 The electrode was activated in M ​​NH4Cl solution for 24 h, and then its electrochemical performance, such as sensitivity, stability, and detection limit, was tested using an electrochemical workstation.

[0047] Each of the GC / ISM and GC / NPC / ISM electrodes was tested with three electrodes. Figure 5 As shown. The results show that the interelectrode reproducibility of the electrodes is significantly improved after modifying the NPC contact layer. The three electrodes of GC / NPC / ISM can exhibit consistent potential responses in the same NH4Cl standard solution, while the three electrodes of GC / ISM without the solid contact layer have different potential responses in the standard solution, and the interelectrode reproducibility is worse than that of GC / NPC / ISM with the modified contact layer; at a low concentration of 10 -6 ~10 -8 In the M standard solution, GC / ISM does not exhibit a proper potential response, with a sensitivity of -51.88 mV / dec and a detection limit of 10. -6 M, GC / NPC / ISM still exhibits a correct potential response in low-concentration standard solutions, with a sensitivity of -56.73 mV / dec and a detection limit of 10. - 6.4 M.

[0048] Figure 6The results of water layer tests are presented for a GC / ISE ammonium ion selective electrode without a contact layer and a GC / NPC / ISE ammonium ion selective electrode with a modified contact layer. Both electrodes were connected to the same reference electrode, and water layer tests were conducted sequentially: the electrodes were first immersed in 0.1M NH4Cl solution for 1 hour, then placed in 0.1M NaCl solution for 1 hour, and finally returned to 0.1M NH4Cl solution. The potential changes of the electrodes were collected. It can be seen that compared with the electrode without a contact layer, the electrode modified with the NPC contact layer exhibited less potential fluctuation in NaCl solution, and the potential recovered immediately after being returned to NH4Cl solution and remained constant thereafter. In contrast, the electrode without a contact layer showed a continuous decrease in potential when first immersed in 0.1M NH4Cl solution, significant and continuous potential fluctuations in NaCl solution, and the potential not only failed to recover immediately after being returned to NH4Cl solution but also continued to drift.

[0049] Figure 7 For the long-term stability test of the electrode, during 10 hours of continuous monitoring, the electrode potential modified with the NPC contact layer remained stable, with an electrode potential drift of only 0.26 mV / h. Figure 8 For short-term stability testing of the electrodes, the electrodes were placed in a 0.1M NH4Cl solution and tested with a constant current of ±1nA for 60 seconds. The sensor was first subjected to a +1nA current for 60 seconds, and then the current direction was reversed within the same time period (-1nA, 60 seconds), and the potential response was recorded. The short-term stability of the unmodified contact layer electrode was significantly worse than that of the modified contact layer electrode. Furthermore, comparing the modification amount of NPC dispersion, it was found that a modification amount of 10~15µL was optimal, resulting in better short-term electrode stability.

[0050] Figure 9 The constant-current charge-discharge (GCD) test results using FeCoNi@ZIF-8-NPC and ZIF-8-NPC as solid contact layers (NPC) clearly show that the capacitance performance of FeCoNi@ZIF-8-NPC is significantly better than that of ZIF-8-NPC. This indicates that the pseudocapacitive effect introduced by the transition metal can improve the capacitance performance of the electrode.

[0051] The sensor is applied to the detection of ammonia nitrogen in aquaculture water as follows: An ion-selective electrode and an Ag / AgCl glass reference electrode are used to form an electrode pair. The reference electrode is connected to the ground terminal of the potential detection system, and the ion-selective electrode is connected to the potential signal input terminal of the potential detection system. During detection, the electrode sensitive membrane and the reference electrode core are immersed in the water sample to be tested. The ion concentration in the water sample is obtained by reading the potential from the detection system.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. 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 specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A solid contact layer for an all-solid-state ion-selective electrode, characterized in that, In the one-pot preparation of ZIF-8, transition metals Fe, Co, and Ni are incorporated and grown at 0℃~5℃ to obtain FeCoNi@ZIF-8; after pyrolysis of FeCoNi@ZIF-8, a dispersion is obtained by mixing it with hydrophobic materials and ionic liquids, and the dispersion is then coated into a film. The molar ratio of Fe, Co, Ni to the ligand of ZIF-8 is 0.006~0.007:0.006~0.007:0.015~0.02:1; the mass ratio of FeCoNi@ZIF-8, hydrophobic material to ionic liquid is 6~8:1.7~2.2:0.7~1.

2.

2. The solid contact layer of the all-solid-state ion-selective electrode according to claim 1, characterized in that, The hydrophobic material is polyvinylidene fluoride-hexafluoropropylene or polyvinylidene fluoride; The ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)ammonium salt.

3. The solid contact layer of the all-solid-state ion-selective electrode according to claim 2, characterized in that, The Fe comes from FeSO4·7H2O or Fe(NO3)2·6H2O; The Co comes from Co(NO3)2·6H2O; The Ni is derived from Ni(NO3)2·6H2O.

4. A method for preparing the solid contact layer of the all-solid-state ion-selective electrode according to claim 3, characterized in that, Includes the following steps: FeSO4·7H2O or Fe(NO3)2·6H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O and zinc salt are dissolved in an alcoholic reagent to obtain mixed solution A; 2-methylimidazole is used as an organic ligand and dissolved in methanol to obtain mixed solution B; Mixed solution A and mixed solution B were reacted at 0℃~5℃ to obtain FeCoNi@ZIF-8; FeCoNi@ZIF-8 was pyrolyzed under a protective atmosphere to obtain FeCoNi@ZIF-8-NPC; Polyvinylidene fluoride-hexafluoropropylene is first dissolved in acetone, then FeCoNi@ZIF-8-NPC is added and sonicated, followed by the addition of an ionic liquid and sonication to obtain a dispersion. A solid contact layer is obtained by drop-coating a dispersion onto a conductive electrode substrate.

5. The preparation method according to claim 4, characterized in that, The pyrolysis temperature is 950℃~1050℃, and the time is 60min~80min.

6. The preparation method according to claim 4, characterized in that, The alcohol reagent is methanol.

7. The preparation method according to claim 4, characterized in that, The protective atmosphere is nitrogen.

8. The application of the solid contact layer according to claim 3 in the preparation of an all-solid-state ion-selective electrode, characterized in that, The all-solid-state ion-selective electrode consists of a conductive substrate, a solid contact layer, and a sensitive film; The conductive substrate is a rigid conductive substrate or a flexible conductive substrate; The solid contact layer is located between the ion-sensitive film and the conductive substrate; The sensitive membrane is prepared from an ion carrier, an ion exchanger, a polymer substrate material, and a plasticizer.

9. The application of the all-solid-state ion-selective electrode of claim 8 in detecting ion concentration in an aquatic environment.

10. The application according to claim 9, characterized in that, The suitable aquatic environment is aquaculture water.

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