Paper-based nanochannel sensor based on nanoporous adsorbent modification and its small molecule separation, enrichment and detection method

Abstract

The application discloses a paper-based nanochannel sensor based on nanoporous adsorbent modification and a small molecule separation, enrichment and detection method thereof. The sensor comprises a porous paper-based material layer and a conductive layer which are modified with nanoporous adsorbents. The method comprises the following steps: adding an electrochemical probe on the sensor to detect an electrochemical signal; adding a small molecule target standard solution to enrich the small molecule target and detect the electrochemical signal; detecting the concentration of the small molecule target according to the signal change rate and establishing a standard curve; when applied, filtering a solution to be detected to separate and enrich the small molecule target; obtaining the signal change rate of the electrochemical signal of the target after filtration, and comparing the signal change rate with the standard curve to obtain the concentration result and complete the detection. The application provides a low-cost, rapid preparation, portable and disposable paper-based nanochannel sensor and method, can realize the separation and enrichment of the target small molecule, can convert the weak change caused by the enrichment of the target into a measurable electrochemical signal, and realizes the concentration detection.

Description

TECHNICAL FIELD

[0001] The present application relates to a small molecule separation and enrichment detection method, and particularly relates to a paper-based nanochannel sensor based on a nanoporous adsorbent modified sensor and a small molecule separation and enrichment detection method thereof. BACKGROUND

[0002] In order to improve agricultural productivity and product quality, a large number of chemicals, including fertilizers, pesticides, veterinary drugs and food additives, are used in the production, storage, transportation and sale of agricultural products. However, the excessive or unreasonable use of these chemicals will have adverse effects on the ecological environment and food safety. For example, antibiotics are mostly difficult to be degraded by organisms, but are discharged into the water environment in the original form or metabolite form, causing pollution of surface water and groundwater. Therefore, it is necessary to develop a low-cost technology for detecting and removing small molecule pollutants in agricultural wastewater.

[0003] With their large surface area, high porosity and good structural stability, various nanoporous adsorbents have been applied to the enrichment and detection of pollutants in agricultural wastewater. The types of nanoporous materials include, but are not limited to, nanoporous carbon, metal-organic frameworks, covalent organic frameworks, zeolites, mesoporous silica, etc. However, nanoporous materials tend to aggregate in aqueous solution, hindering the occurrence of adsorption behavior. Therefore, it is necessary to use suitable support materials to load nanoporous adsorbents. Among them, porous paper-based materials have the advantages of low cost, recyclability and easy mass production, and have been used as flexible carriers to load nanoporous adsorbents and prepare sensor devices. In addition, thanks to their customizable pore size and high porosity, porous paper-based materials are also used to build filtration / separation membranes and are widely used in the field of agricultural wastewater treatment. However, due to their relatively large pore size and lack of small molecule recognition sites, the conventional paper-based materials still lack the ability to capture and separate small molecules. Therefore, it is possible to consider loading nanoporous adsorbents with small molecule recognition and adsorption capacity into paper-based materials to improve their filtration and separation performance.

[0004] On the other hand, with the development of microfluidic technology, sensors prepared with porous paper-based materials as substrates have become a powerful tool in the field of analytical sensing. Paper-based sensors are simple to prepare, low in cost, easy to mass-produce, and are conducive to portable and disposable real-time detection on site, and have been widely used in laboratory research and practical detection. However, existing paper-based sensors are usually constructed based on immunological strategies, which require biological macromolecule labeling, increasing the complexity of operation and instability of detection, and the detection objects are mainly macromolecules such as proteins and bacteria, which are difficult to use for the detection of trace small molecule pollutants. Therefore, it is urgent to develop a paper-based sensor with small molecule pollutant separation and enrichment function and detection function to improve the efficiency of wastewater treatment. SUMMARY

[0005] To solve the problems in the background art, the application provides a paper-based nanochannel sensor modified by a nanoporous adsorbent and a small molecule separation, enrichment and detection method thereof.

[0006] The technical scheme adopted by the application is:

[0007] One, a paper-based nanochannel sensor modified by a nanoporous adsorbent:

[0008] The paper-based nanochannel sensor comprises a porous paper-based material layer modified by a nanoporous adsorbent NPA and a conductive layer deposited on the two sides of the porous paper-based material layer, and a conductive circuit is led out on the two conductive layers to connect an external electrochemical workstation.

[0009] The porous paper-based material layer is specifically a paper material layer made of inorganic or organic fibers, and the paper material layer is specifically natural plant fiber paper, polyether sulfone PES filter paper, cellulose acetate CA filter paper, polyamide PA filter paper or glass fiber filter paper, etc. The inside of the porous paper-based material layer presents irregular micrometer and nanometer pore structures, and the thickness is in the order of micrometers, and the porous paper-based material layer has the characteristics of high flexibility, high strength and renewability. The nanoporous adsorbent NPA is specifically metal organic framework, covalent organic framework, zeolite or mesoporous silica, etc. The nanoporous adsorbent NPA has a three-dimensional porous structure (including "microporous" and "mesoporous"), a high specific surface area and a high adsorption capacity. The conductive layer is specifically a coating prepared from a metal conductive material, and the metal conductive material includes gold, silver, copper alloy and aluminum alloy, etc. The conductive circuit is specifically prepared by printing conductive ink, and the conductive ink is specifically conductive silver ink and conductive carbon paste, etc.

[0010] Two, a preparation method of a paper-based nanochannel sensor:

[0011] The method comprises the following steps:

[0012] 1) A conductive layer is deposited on the two sides of the porous paper-based material layer, a hydrophobic barrier is constructed at the edges of the porous paper-based material layer and the two conductive layers, then a conductive circuit is printed on the two conductive layers and sealed, only the symmetrical regions of the two conductive layers in the preset area exposed to the solution in the middle are used as the sensing interface, and the integrated flexible paper-based electrode IFPE is prepared after the sensing interface is washed with deionized water to completely remove the pollution on the surface of the sensing interface.

[0013] 2) The sensing interface of the integrated flexible paper-based electrode (IFPE) is pretreated and then modified using a nanoporous adsorbent (NPA) to obtain a paper-based nanopore sensor (NPA-IFPE).

[0014] The modification is performed on the outer surface and the interior of the sensing interface. The outer surface refers to the surface of the porous paper-based material layer in contact with the outside world. The interior refers to the cross-sectional part of the porous paper-based material layer, which, when enlarged, presents a micro- and nano-scale irregular pore structure, allowing the outside solution to pass through the pores. After modification, the pore diameter of the porous paper-based material layer is significantly reduced to 1-100 nm, forming a nanopore with small molecule specific recognition sites.

[0015] In step 1), a conductive layer is deposited on both sides of the porous paper-based material layer, a hydrophobic barrier is constructed on the two conductive layers, and then a conductive circuit is printed on the two conductive layers. Specifically, two mask paper masks are overlaid on both sides of the porous paper-based material layer, and then an ion sputtering deposition method is used to deposit a conductive layer with the same thickness on both sides. The two conductive layers serve as the working electrode and the counter electrode, respectively. Then, a wax printing method assisted by heating is used to wax print a hydrophobic ring layer on the deposited porous paper-based material layer and the two conductive layers to construct a hydrophobic barrier. Finally, conductive ink is used to manually print a conductive circuit on the surface of the two conductive layers as a lead. The porous paper-based material layer is connected to the external electrochemical workstation through the two conductive layers and the two conductive circuits.

[0016] The ion sputtering deposition method specifically involves using an ion sputtering instrument to deposit for 300-600 s at a spraying current of 30 mA. The thickness of the deposited conductive layer is 80-120 nm.

[0017] The heating-assisted wax printing method specifically involves first attaching two filter paper pieces pre-soaked in paraffin to the surfaces of the two conductive layers. Then, a metal stamp is preheated at 120℃ and pressed onto the two filter paper pieces for 5 s for wax printing, causing the paraffin to be transferred to the surface of the conductive layer to form a hydrophobic ring layer and construct a hydrophobic barrier. The paraffin modification makes the part outside the conductive layer hydrophobic, and the solution flow is restricted to the sensing area to avoid lateral permeation of the solution.

[0018] The step 2) is to pretreat the sensing interface of the integrated flexible paper-based electrode IFPE, and then modify the sensing interface using the nanoporous adsorbent NPA. Specifically, the sensing interface is pretreated by acid-base hydrolysis or metal ion anchoring to increase the surface active sites, and then two reaction precursor solutions of the nanoporous adsorbent NPA are prepared by dissolving two kinds of reaction precursors of the nanoporous adsorbent NPA in deionized water respectively. The integrated flexible paper-based electrode IFPE is clamped between two electrolytic cells, and a through hole with the same shape as the sensing interface is formed on the side of each electrolytic cell close to the integrated flexible paper-based electrode IFPE. The two reaction precursor solutions are poured into the two electrolytic cells respectively and immerse the sensing interface. Then, the sensing interface of the integrated flexible paper-based electrode IFPE is modified by using the reverse diffusion-interface growth method to make the two reaction precursor solutions transmit in the pores of the sensing interface. The nanoporous adsorbent NPA is uniformly modified on the outer surface and internal channels of the porous paper-based material at room temperature after sealing modification. The modified integrated flexible paper-based electrode IFPE is taken out, cleaned with methanol and deionized water in sequence, and then dried in an oven to prepare a paper-based nanochannel sensor NPA-IFPE.

[0019] The sensor uses the nanoporous adsorbent NPA to modify the porous paper-based material to construct a paper-based nanochannel film with a nanoscale pore size. The method gives the conventional paper-based material a large number of small molecule specific recognition and adsorption sites, and realizes the construction of a small molecule sensitive nanometer limited space.

[0020] The two reaction precursor solutions of the nanoporous adsorbent NPA are prepared by dissolving two kinds of reaction precursors of the nanoporous adsorbent NPA in deionized water or an organic solvent respectively. Specifically, a first reaction precursor solution is prepared by dissolving a polymer monomer in deionized water or an organic solvent, and then a second reaction precursor solution is prepared by dissolving a bridging agent in deionized water or an organic solvent. The polymer monomer and the bridging agent are Zn(NO3)2·6H2O and 2-methylimidazole, ZrCl4 and phthalic acid, or 1,3,5-triformylbenzene and p-phenylenediamine, etc., which can all be organic ligands.

[0021] The sealing modification at room temperature is performed for 4-16 hours, and the drying in the oven is performed at 60°C.

[0022] Three, a small molecule separation and enrichment detection method of a paper-based nanochannel sensor:

[0023] The method comprises the following steps:

[0024] 1) Electrolyte solution was added on the counter electrode side of the paper-based nanochannel sensor NPA-IFPE as an electrochemical probe, and the initial electrochemical signal generated by the electrochemical probe on the working electrode side of the nanochannel was detected using an electrochemical workstation.

[0025] 2) The paper-based nanochannel sensor NPA-IFPE was clamped in a vacuum-assisted filtration device, with the counter electrode side of the paper-based nanochannel sensor NPA-IFPE facing upwards as the feed side and the working electrode as the permeation side. A series of pre-set concentration gradient standard solutions of small molecule target objects were added dropwise on the feed side, and the respective electrochemical signals generated by the electrochemical probe were detected using an electrochemical workstation. The concentration of the small molecule target object was quantitatively detected according to the signal change rate of the initial electrochemical signal and the detected electrochemical signal obtained in steps 1) and 2), and a standard curve of the concentration of the small molecule target object and the signal change rate was established.

[0026] 3) In actual detection, the same operation as in steps 1)-2) was used for the solution to be detected, achieving enrichment of the small molecule target object in the solution to be detected and separation of interfering substances in the solution to be detected. The signal change rate of the electrochemical signal generated by the electrochemical probe in actual detection was detected by the electrochemical workstation, and the concentration of the small molecule target object in the solution to be detected was obtained by comparing with the standard curve of the concentration of the small molecule target object and the signal change rate, completing the detection.

[0027] The small molecule target object is specifically a common small molecule pollutant such as an antibiotic, a pesticide, and a dye commonly found in agricultural wastewater. In detection, the specific binding of the nano-porous adsorbent NPA is used to capture the small molecule target object. Other small molecule interferents are not interfered with by the nanochannel and will pass through the nanochannel with the solution and exist in the filtrate, thereby achieving separation of different small molecules. After the paper-based nanochannel sensor NPA-IFPE enriches the small molecule target object, the surface properties of the outer surface and the inner surface change, causing a change in the diffusion flux of the electrochemical probe in the nanochannel, which is reflected as a change in the electrochemical signal generated by the electrochemical probe on the working electrode, thereby achieving quantitative detection of the concentration of the small molecule target object.

[0028] In step 1), the electrolyte solution added on the counter electrode side is specifically 5 μL of 1 mM K3Fe(CN)6 solution as an electrochemical probe.

[0029] The step 2) is to drop the small molecule target standard solution with a series of preset concentration gradient into the feeding side, and use the electrochemical workstation to detect the respective electrochemical signals generated by the electrochemical probe, specifically, the small molecule target standard solution with each preset concentration is dropped into the feeding side, the vacuum assisted filtration method is used to make the small molecule target standard solution completely filter through the penetration side of the sensing interface under the pressure driving to realize the dynamic enrichment of the small molecule target in the small molecule target standard solution, and then the double electrode-electrochemical method is used to detect the electrochemical signals generated by the electrochemical probe before and after the enrichment of the small molecule target by using the electrochemical workstation.

[0030] The beneficial effects of the present application are:

[0031] 1. The present application combines the nanoporous adsorbent and the porous paper-based material, constructs the nano-limited space with small molecule recognition ability and signal transmission ability in the paper-based material, and realizes the specific binding and detection of the target small molecule without biological macromolecule labeling.

[0032] 2. The present application combines the specific adsorption of the nanoporous adsorbent and the selective action of the nanochannel, and has the small molecule separation and enrichment function.

[0033] 3. The present application significantly reduces the pore diameter of the porous paper-based material to 1-100 nanometers, and the pore diameter is adjustable, and a simple method for preparing paper-based nanochannel films with different pore diameters is provided.

[0034] 4. The present application prepares an integrated flexible paper-based electrode based on common porous paper-based materials, can output the electrochemical signal on the porous paper-based material without an external electrode, provides a low-cost, rapid preparation method of a portable and disposable paper-based sensor, can realize the separation and enrichment of the target small molecule, can convert the weak change caused by the enrichment of the target into a measurable electrochemical signal, and realizes the concentration detection of the target small molecule. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 A is a scanning electron microscope image of the outer surface of the PES-Au film;

[0036] Figure 1 B is a scanning electron microscope image of the inside of the PES-Au film;

[0037] Figure 1 C is a schematic diagram of the element distribution of the PES-Au film;

[0038] Figure 2 is a scanning electron microscope image of the wax sealing area of the PES-Au film after wax sealing;

[0039] Figure 3A is a scanning electron microscope image of the outer surface of the ZIF-8@P-IFPE paper-based nanochannel sensor of the application;

[0040] Figure 3 B is a scanning electron microscope image of the inside of the ZIF-8@P-IFPE paper-based nanochannel sensor of the application;

[0041] Figure 4 A is a UV absorbance spectrum of a tetracycline solution before and after filtration by the ZIF-8@P-IFPE of the application;

[0042] Figure 4 B is a UV absorbance spectrum of an indigo solution before and after filtration by the ZIF-8@P-IFPE of the application;

[0043] Figure 4 C is a UV absorbance spectrum of a mixed solution of tetracycline and indigo before and after filtration by the ZIF-8@P-IFPE of the application;

[0044] Figure 5 A is an electrochemical signal response graph of the ZIF-8@P-IFPE paper-based nanochannel sensor for detection of tetracycline at different concentrations;

[0045] Figure 5 B is a fitted standard curve graph of the ZIF-8@P-IFPE paper-based nanochannel sensor for detection of tetracycline at different concentrations;

[0046] Figure 6 A is a schematic diagram of the ZIF-8@P-IFPE paper-based nanochannel sensor for separation and enrichment of tetracycline;

[0047] Figure 6 B is a schematic diagram of the ZIF-8@P-IFPE paper-based nanochannel sensor for detection of tetracycline;

[0048] Figure 7 A is an internal scanning electron microscope image of a paper-based nanochannel film with a pore size of 1.2 μm prepared by the application;

[0049] Figure 7 B is an internal scanning electron microscope image of a paper-based nanochannel film with a pore size of 500 nm prepared by the application;

[0050] Figure 7 C is an internal scanning electron microscope image of a paper-based nanochannel film with a pore size of 3 nm prepared by the application. DETAILED DESCRIPTION

[0051] To enable those skilled in the art to better understand the technical solutions of the application, the method provided by the application is described in detail below with reference to the accompanying drawings and examples. However, it should be pointed out that the implementation of the application is not limited to the following embodiments.

[0052] The specific embodiments of the present application are as follows:

[0053] Example 1:

[0054] Preparation of polyether sulfone PES paper-based nanochannel sensor ZIF-8@P-IFPE modified by zeolitic imidazolate framework ZIF-8:

[0055] A commercially available polyether sulfone PES filter paper was selected as the porous paper-based material carrier. First, two mask plates made of label paper were covered on both sides of the surface of the polyether sulfone PES filter paper, and then a layer of gold was deposited on the two sides of the polyether sulfone PES filter paper by ion sputtering deposition method, specifically, the ion sputtering deposition method was deposited for 400 s under a spraying current of 30 mA, and the thickness of the gold layer after deposition was 100 nm, and a PES-Au film was prepared. The two layers of gold layer were used as the working electrode and the counter electrode, respectively; then a hydrophobic ring layer was printed on the PES-Au film by a heating-assisted wax printing method to construct a hydrophobic barrier, so that the part outside the gold layer had hydrophobicity to avoid horizontal penetration of the solution. The heating-assisted wax printing method was as follows: first, two filter paper pieces pre-soaked with paraffin were attached to the surface of the two layers of gold layer, respectively, then the metal stamp was preheated at 120°C and pressed on the two filter paper pieces for 5 s for wax printing, so that the paraffin was transferred to the PES-Au film by heat to form a hydrophobic ring layer, and the hydrophobic barrier was constructed; finally, silver conductive ink was used to manually print silver conductive lines on the surface of the two layers of gold layer as lead wires, so that the silver conductive lines were connected with the two layers of gold layer on both sides of the PES-Au film to form a double electrode system. The polyether sulfone PES filter paper was connected to the external electrochemical workstation through the two layers of gold layer and the two conductive circuits; the seal was performed by using adhesive tape, and only the two sides of the gold layer of the preset area of the middle part in contact with the solution were exposed as the sensing interface, which was a circular PES-Au conductive area with a diameter of 5 mm. After the sensing interface was washed with deionized water to completely remove the pollution on the surface of the sensing interface, an integrated flexible paper-based electrode IFPE (P-IFPE) was prepared.

[0056] The sensing interface was pretreated by using a metal ion anchoring method to functionalize the P-IFPE. Specifically, the P-IFPE was first washed with water, then immersed in a Zn 2+ -phytic acid mixed solution (0.1 g of Zn(NO3)2·6H2O and 400 μL of phytic acid were dissolved in 30 mL of deionized water) for 20 min, and continuously stirred using a magnetic rotor. Then, the electrode was taken out and washed with water to remove the surface residual substances. Finally, the electrode was further immersed in an aqueous solution of Zn(NO3)2·6H2O (Zn 2+ content of 4 g·L -1 ) for 30 min to promote the coordination of Zn 2+ .

[0057] The sensing interface of the integrated flexible paper-based electrode IFPE was modified using a reverse diffusion-interface growth method. Specifically, 0.274 g of Zn(NO3)2·6H2O crystals were weighed and dissolved in 50 mL of deionized water to prepare a first reaction precursor solution. 5.665 g of 2-methylimidazole was weighed and dissolved in 50 mL of deionized water to prepare a second reaction precursor solution. The integrated flexible paper-based electrode IFPE was sandwiched between two self-made 3D-printed electrolytic cells. Each of the two electrolytic cells had a through-hole with the same shape as the sensing interface and directly facing the sensing interface on the side closest to the integrated flexible paper-based electrode IFPE. The sensing interface and two through holes were precisely aligned. 2 mL of freshly prepared Zn(NO3)2·6H2O solution was added to one side of the electrolytic cell, and 2 mL of 2-methylimidazole solution was added to the other side of the electrolytic cell to immerse the sensing interface. The zeolite imidazole framework ZIF-8 modification process was carried out. The sensing interface of the integrated flexible paper-based electrode IFPE was modified by the reverse diffusion-interface growth method, so that the two reaction precursor solutions could be transported in the pores of the sensing interface. After sealing and modifying at room temperature for 12 h, the nanoporous adsorbent NPA was uniformly modified on the outer surface and internal channels of the porous paper-based material. The modified integrated flexible paper-based electrode IFPE was taken out, washed three times with a large amount of methanol, then washed with deionized water, and then dried in an oven at 60 °C to prepare the paper-based nanochannel sensor NPA-IFPE (ZIF-8@P-IFPE).

[0058] The paper-based nanochannel sensor NPA-IFPE (ZIF-8@P-IFPE) was characterized as follows:

[0059] The morphology of the P-IFPE integrated flexible paper-based electrode was characterized using scanning electron microscopy (SEM). After depositing a gold layer on the PES film, the surface pores remained open, allowing only a small number of gold nanoparticles to enter the channels, thus slightly reducing the pore size to 100-300 nm. Figure 1 As shown in Figure A. Cross-sectional SEM reveals micron-sized channels (0.1-1.5 μm) formed by interwoven fibers, as... Figure 1 As shown in B. Further characterization was performed using an energy dispersive spectroscopy (EDS) instrument, as shown in Figure B. Figure 1 As shown in Figure C, oxygen (O), sulfur (S), and gold (Au) signals were observed, and the gold nanoparticles were uniformly distributed. These results indicate that a conductive gold layer was successfully deposited on the PES filter paper without affecting its porous structure. After wax modification, a portion of the PES-Au membrane was covered by wax, and the wax-sealed area was tight and pore-free, preventing solution penetration into non-sensing areas, such as... Figure 2 As shown, this demonstrates the successful construction of a hydrophobic barrier in a P-IFPE integrated flexible paper-based electrode.

[0060] After ZIF-8 modification, the morphology, structure, and composition of the ZIF-8@P-IFPE paper-based nanochannel sensor were characterized using SEM and EDS. The characterization results showed that ZIF-8 crystals grew on polymer fibers both on the electrode surface and inside, such as... Figure 3 A and Figure 3 As shown in Figure B, the ZIF-8 crystals exhibit a complete morphology and their size can vary with the pore size, flexibly adapting to the growth environment within the membrane. Modification with ZIF-8 significantly reduces the pore size of the channels within the PES membrane, constructing nanochannels with pore sizes at the nanoscale (20-80 nm). This demonstrates that the method successfully establishes a nano-confined space suitable for small molecule sensing, which is beneficial for the subsequent separation, enrichment, and detection of target small molecules.

[0061] The enrichment and separation experiments of tetracycline using the paper-based nanochannel sensor NPA-IFPE (ZIF-8@P-IFPE) are detailed below:

[0062] Using tetracycline as the small molecule target and indigo as the interfering agent, 10 mg of tetracycline standard and indigo standard were weighed and dissolved separately in 1 mL of deionized water to obtain 10 μg·L⁻¹. -1 A mixed solution was prepared by mixing tetracycline standard solution and indigo standard solution. 5 mL of each of the tetracycline standard solution, indigo standard solution, and mixed solution were added dropwise to the feed side. Three separate filtration experiments were conducted.

[0063] Vacuum-assisted filtration was used to completely filter tetracycline standard solution, indigo standard solution, and mixed solution through the permeation side of the sensing interface under pressure. This allowed for the dynamic enrichment of small molecule target substances in the small molecule target standard solution and the separation of interfering substances in the interfering substance standard solution. The UV absorbance spectra of each solution before and after filtration were measured, and the absorbance changes of tetracycline and indigo at characteristic peaks were recorded and calculated. The enrichment rate was then calculated based on the absorbance changes. Figure 4 As shown in A, for 10 μg·L -1 The absorbance of the tetracycline solution at 357 nm decreased significantly after filtration. Based on the change in absorbance at 357 nm, the tetracycline enrichment rate was calculated to be as high as 90.88%, indicating that the ZIF-8 modified paper-based nanochannel electrode has good tetracycline recognition and enrichment capabilities, which is beneficial for subsequent tetracycline detection. Similarly, 10 μg·L⁻¹ tetracycline solution was filtered using ZIF-8@P-IFPE. -1 The absorbance at 300 nm in the indigo solution showed no significant change, decreasing by only 0.68%. Figure 4As shown in Figure B, it is evident that almost all indigo is filtered out without being adsorbed or retained by the composite membrane. ZIF-8@P-IFPE exhibits distinctly different enrichment capabilities for tetracycline and indigo, demonstrating that the constructed ZIF-8-modified nanoconfined space possesses different enrichment capabilities for small molecules with different sizes, structures, and surface charge distributions.

[0064] ZIF-8@P-IFPE was further applied to the separation of tetracycline and indigo. 10 μg·L⁻¹ -1 Equal volumes of tetracycline and indigo solutions were mixed and filtered. The ultraviolet absorption spectra of the solutions before and after filtration were measured. Figure 4 As shown in Figure C, the original mixed solution contained characteristic absorption peaks of tetracycline and indigo. After filtration through ZIF-8@P-IFPE, the filtrate only showed a characteristic absorption peak of indigo at 300 nm. The tetracycline enrichment rate was calculated to be as high as 85.15% based on the absorbance change, while the indigo concentration decreased by only 19.76%. As a control, the separation and enrichment effect of the original PES filter paper was investigated. After filtration with PES filter paper, the absorbance of tetracycline and indigo in the mixed solution decreased by 14.72% and 28.18%, respectively, and its enrichment rate for tetracycline and separation rate of the mixture were much lower than those of ZIF-8@P-IFPE. Therefore, based on the specific adsorption of ZIF-8 and the charge and size selectivity of the constructed nano-confined space, the prepared ZIF-8@P-IFPE not only exhibits high adsorption capacity for target antibiotics but also achieves selective separation of different small molecule pollutants. This will be beneficial for the separation and detection of small antibiotic molecules in agricultural wastewater.

[0065] The paper-based nanochannel sensor NPA-IFPE (ZIF-8@P-IFPE) detects tetracycline, as detailed below:

[0066] Before detection, the silver wires on both sides of the ZIF-8@P-IFPE paper-based nanochannel sensor were connected to the wires of the electrochemical workstation. 5 μL of 1 mM K3Fe(CN)6 solution was added to the counter electrode side of the ZIF-8@P-IFPE as an electrochemical probe. Differential pulse voltammetry was used to detect the initial electrochemical signal generated on the working electrode side after the electrochemical probe passed through the nanochannel using the electrochemical workstation. The magnitude I0 of the reduction current generated by K3Fe(CN)6 at the characteristic peak was recorded.

[0067] The paper-based nanochannel sensor NPA-IFPE was clamped in a vacuum-assisted filtration device, with the counter electrode side of the NPA-IFPE facing upwards as the feed side and the working electrode as the permeation side. It was diluted with deionized water at a concentration of 10 μg·L⁻¹. -1The tetracycline standard solution is used to obtain a series of preset concentration gradient tetracycline solutions, and 5 mL of tetracycline solution is added dropwise on the feed side, and the dynamic enrichment of tetracycline in the tetracycline solution is carried out by using a vacuum-assisted filtration method to make the tetracycline solution completely filter through the permeation side of the sensing interface under the pressure driving. Then, the electrochemical probe generates various electrochemical signals after the enrichment of tetracycline is detected by using a two-electrode-electrochemical method through an electrochemical workstation, and the size I of the reduction current generated by K3Fe(CN)6 at the characteristic peak is recorded. Finally, the ZIF-8@P-IFPE is taken out, washed with deionized water and dried. As shown in A of Figure 5 , the electrochemical signal response of ZIF-8@P-IFPE to tetracycline of different concentrations is shown. With the increase of the concentration, the reduction current generated by K3Fe(CN)6 at the characteristic peak gradually decreases. The current change rate (I0-I) / I0 before and after filtration is calculated as the quantitative response signal of the tetracycline concentration, and the standard curve between the tetracycline concentration and the current change rate is established, as shown in B of Figure 5 . The current change rate and the logarithm of the tetracycline concentration satisfy the linear relationship Y=16.738Ln(X)+10.399, and the correlation coefficient R 2 =0.9955. The linear detection range is 1-20000ng mL -1 , and the detection limit is 0.22ng mL -1 (S / N=3).

[0068] In actual detection, the detection solution is detected, the enrichment of the tetracycline target in the detection solution and the separation of the interfering substances in the detection solution are realized, the signal change rate of the electrochemical signal generated by K3Fe(CN)6 in actual detection is detected by an electrochemical workstation, the tetracycline concentration result in the detection solution is obtained by comparing the standard curve of the tetracycline concentration and the signal change rate, and the detection is completed.

[0069] The method of the present application does not need biological macromolecular markers, and still shows good detection performance, which benefits from the modification of ZIF-8 on the inside and outside of the PES filter paper, which gives the sensor a large number of tetracycline specific recognition sites; the nano-limited space constructed in the PES plays a signal amplification role. In addition, the raw materials of the sensor used in the method are easy to obtain, the preparation process is simple, and large-scale production is easy. The method has low cost, and the cost of a single ZIF-8@P-IFPE paper-based nanochannel sensor is about 1.5 yuan, which can meet the one-time rapid detection needs of agricultural wastewater in the laboratory and on site.

[0070] The separation and enrichment and detection mechanism of the small molecule target:

[0071] The separation and enrichment mechanism of tetracycline is as shown in Figure 6As shown in Figure A, the ZIF-8 modified on the internal and external surfaces of ZIF-8@P-IFPE exhibits strong and specific binding to tetracycline through π-π interactions and electrostatic interactions. Furthermore, tetracycline molecules are negatively charged in solution and often exist as aggregates of multiple molecules, which are trapped by the composite membrane. In contrast, indigo molecules are much smaller than the pore size of the nanochannels and are not affected by the steric hindrance of the nanochannels. Moreover, indigo molecules are electrically neutral and lack the groups that specifically bind to ZIF-8. Therefore, thanks to the specific adsorption of ZIF-8 and the charge and size selectivity of the constructed nanoconfined space, the prepared ZIF-8@P-IFPE can be used for the separation and enrichment of small tetracycline molecules.

[0072] The detection mechanism of tetracycline is as follows: Figure 6 As shown in B, ZIF-8 provides a large number of tetracycline recognition sites; the growth of ZIF-8 inside polyethersulfone reduces the micron-scale (0.1-1.5 μm) pore size to the nanoscale (20-80 nm), constructing a nanoscale confined space conducive to small molecule sensing. The inner and outer surfaces of ZIF-8@P-IFPE are positively charged due to the presence of ZIF-8; after adsorbing negatively charged tetracycline molecules, the surface charge of ZIF-8@P-IFPE changes from positive to negative, generating a negatively charged double layer, which is beneficial for electrolyte anions (here, [Fe(CN)6)). 3- The diffusion of [Fe(CN)6] generates repulsion. When the ionic strength of the electrolyte solution is 1 mM, the double layer thickness is approximately 10 nm. Considering that tetracycline has a three-dimensional structure with a length of 1.41 nm, a width of 0.46 nm, and a height of 0.86 nm, and that it aggregates in solution, the adsorption of tetracycline will increase the steric hindrance within the nanochannel, hindering the diffusion of [Fe(CN)6]. 3- Diffusion. In summary, after adsorbing tetracycline, ZIF-8@P-IFPE will affect [Fe(CN)6]. 3- The diffusion of [Fe(CN)6] is hindered, preventing it from reaching the working electrode through the nanochannels. 3- Decrease. Therefore, [Fe(CN)6] 3- The reduction current generated on the working electrode side is reduced. ZIF-8@P-IFPE for [Fe(CN)6] 3- The diffusion hindrance is related to the concentration of tetracycline. Therefore, the rate of change of reduction current can be used as a quantitative signal for tetracycline detection.

[0073] Example 2:

[0074] Preparation of paper-based nanochannel films with different pore sizes:

[0075] By adjusting the modification time of the nanoporous adsorbent, paper-based nanochannel films with different pore sizes can be prepared. Their internal morphology and pore size variations are as follows:Figure 7 As shown. After 3 hours of modification, uniformly distributed ZIF-8 seeds were observed inside the PES film. At this time, the pore size of the composite film was approximately 1.2 μm. Figure 7 As shown in Figure A; the modification time was 8 hours. The ZIF-8 crystal continued to grow and increase in size in the confined space, but some areas still retained large pores with a diameter of approximately 500 nm. Figure 7 As shown in Figure B; with increasing time, the reaction precursor continues to diffuse within the pores. After 12 hours, the ZIF-8 crystals stack up, significantly reducing the pore size of the composite film to approximately 50 nm, as shown in Figure B. Figure 3 As shown in B; when the modification time is extended to 16 hours, most of the pores are blocked, forming a relatively dense ZIF-8 layer with a pore size of approximately 3 nm, as shown in Figure B. Figure 7 As shown in C. The above results indicate that the method of modifying nanoporous adsorbents can easily adjust the pore size of porous paper-based materials, which is beneficial for preparing paper-based nanochannel films with different pore sizes.

Claims

1. A method for preparing a paper-based nanochannel sensor modified with a nanoporous adsorbent, wherein the paper-based nanochannel sensor modified with a nanoporous adsorbent includes a porous paper-based material layer modified with the nanoporous adsorbent NPA and conductive layers deposited on both sides of the porous paper-based material layer, with conductive circuits led out from the two conductive layers to connect to an external electrochemical workstation, characterized in that: The method includes the following steps: 1) A conductive layer is deposited on both sides of the porous paper-based material layer. A hydrophobic barrier is constructed at the edge of the porous paper-based material layer and the two conductive layers. Then, conductive circuits are printed on the two conductive layers and sealed. Only the symmetrical areas on both sides of the conductive layer with a predetermined area in the middle are exposed as the sensing interface. After rinsing the sensing interface with deionized water, an integrated flexible paper-based electrode IFPE is prepared. 2) The sensing interface of the integrated flexible paper-based electrode IFPE was pretreated and then modified with nanoporous adsorbent NPA to prepare the paper-based nanochannel sensor NPA-IFPE. In step 2), the sensing interface of the integrated flexible paper-based electrode IFPE is pretreated and then modified with nanoporous adsorbent NPA. Specifically, the sensing interface is first pretreated with acid-base hydrolysis or metal ion anchoring. Then, two reaction precursor standards of the nanoporous adsorbent NPA are dissolved in deionized water to prepare two reaction precursor solutions. The integrated flexible paper-based electrode IFPE is sandwiched between two electrolytic cells. Each of the two electrolytic cells has a through-hole with the same shape as the sensing interface and facing the sensing interface on the side closest to the integrated flexible paper-based electrode IFPE. The two reaction precursor solutions are then placed between the two electrodes. The precursor solutions were poured into two electrolytic cells and immersed in the sensing interface. Then, the sensing interface of the integrated flexible paper-based electrode IFPE was modified using a reverse diffusion-interface growth method, which allowed the two reaction precursor solutions to be transported in the pores of the sensing interface. After sealing and modification at room temperature, the modified integrated flexible paper-based electrode IFPE was removed and then cleaned with methanol and deionized water in sequence. Finally, it was dried in an oven to prepare the paper-based nanochannel sensor NPA-IFPE. By adjusting the modification time of the nanoporous adsorbent NPA, paper-based nanochannel sensors NPA-IFPE with different pore sizes were prepared.

2. The method for preparing a paper-based nanochannel sensor modified with a nanoporous adsorbent according to claim 1, characterized in that: In step 1), a conductive layer is deposited on each of the two surfaces of the porous paper-based material layer, and a hydrophobic barrier is constructed on the two conductive layers. Then, conductive circuits are printed on the two conductive layers. Specifically, two masks are placed on the two surfaces of the porous paper-based material layer, and a conductive layer of the same thickness is deposited using ion sputtering deposition. The two conductive layers serve as the working electrode and the counter electrode, respectively. Then, a hydrophobic ring is printed on the deposited porous paper-based material layer and the two conductive layers using a heat-assisted wax printing method to construct a hydrophobic barrier. Finally, conductive circuits are printed on the surface of the two conductive layers using conductive ink as leads. The porous paper-based material layer is connected to an external electrochemical workstation in sequence through the two conductive layers and the two conductive circuits.

3. The method for preparing a paper-based nanochannel sensor modified with a nanoporous adsorbent according to claim 2, characterized in that: The ion sputtering deposition method specifically involves using an ion sputtering instrument to deposit a conductive layer for 300-600 s at a sputtering current of 30 mA, resulting in a thickness of 80-120 nm after deposition. The heat-assisted wax printing method involves first attaching two pre-impregnated paraffin filter papers to the surfaces of two conductive layers, then preheating the metal mold at 120°C and pressing it onto the two filter papers for 5 seconds to perform wax printing. This allows the paraffin heat to transfer to the surface of the conductive layer, forming a hydrophobic ring and completing the hydrophobic barrier.

4. The method for preparing a paper-based nanochannel sensor modified with a nanoporous adsorbent according to claim 1, characterized in that: The method described above involves dissolving two reaction precursor standards of the nanoporous adsorbent NPA in deionized water or an organic solvent to prepare two reaction precursor solutions. Specifically, the polymer monomer is dissolved in deionized water or an organic solvent to prepare a first reaction precursor solution, and then the bridging agent is dissolved in deionized water or an organic solvent to prepare a second reaction precursor solution. The polymer monomer and the bridging agent are Zn(NO3)2·6H2O and 2-methylimidazolium, ZrCl4 and phthalic acid, or 1,3,5-tricarboxyphenyl and p-phenylenediamine, respectively. The sealing and modification at room temperature specifically refers to sealing and modifying at room temperature for 4-16 hours; the drying in an oven specifically refers to drying in an oven at 60 ℃.

5. A method for small molecule separation, enrichment, and detection using a paper-based nanochannel sensor modified with a nanoporous adsorbent, prepared according to any one of claims 1-4, characterized in that: The method includes the following steps: 1) An electrolyte solution was dropped onto the counter electrode side of the paper-based nanochannel sensor NPA-IFPE as an electrochemical probe, and the initial electrochemical signal generated by the electrochemical probe passing through the nanochannel on the working electrode side was detected using an electrochemical workstation; 2) The paper-based nanochannel sensor NPA-IFPE is clamped in a vacuum-assisted filtration device, with the counter electrode side of the NPA-IFPE facing upwards as the feed side and the working electrode as the permeation side. A series of small molecule target standard solutions with preset concentration gradients are added dropwise to the feed side, and the electrochemical signals generated by the electrochemical probe are detected by an electrochemical workstation. The concentration of the small molecule target is quantitatively detected based on the initial electrochemical signals obtained in steps 1) and 2) and the signal change rate of the detected electrochemical signals, and a standard curve of small molecule target concentration and signal change rate is established. 3) In actual testing, the same operation as the small molecule target standard solution in steps 1)-2) is used to enrich the small molecule target in the test solution and separate the interfering substances in the test solution. The signal change rate of the electrochemical signal generated by the electrochemical probe during actual testing is detected by an electrochemical workstation. The concentration result of the small molecule target in the test solution is obtained by comparing with the standard curve of small molecule target concentration and signal change rate, and the detection is completed.

6. The method for small molecule enrichment and detection based on a paper-based nanochannel sensor modified with nanoporous adsorbents according to claim 5, characterized in that: In step 1), adding an electrolyte solution to the counter electrode side specifically involves adding 5 μL of a 1 mM K3Fe(CN)6 solution as an electrochemical probe to the counter electrode side.

7. The method for small molecule enrichment and detection based on a paper-based nanochannel sensor modified with a nanoporous adsorbent according to claim 5, characterized in that: In step 2), a series of small molecule target standard solutions with preset concentration gradients are added dropwise to the feed side. An electrochemical workstation is used to detect the electrochemical signals generated by the electrochemical probe. Specifically, each preset concentration of small molecule target standard solution is added dropwise to the feed side. A vacuum-assisted filtration method is used to ensure that the small molecule target standard solution is completely filtered through the permeation side of the sensing interface to achieve dynamic enrichment of the small molecule target in the small molecule target standard solution. Then, a dual-electrode electrochemical method is used to detect the electrochemical signals generated by the electrochemical probe before and after the enrichment of the small molecule target by the electrochemical workstation.

Images