Preparation method of paper-based sensor for detecting ammonia gas and application thereof

Abstract

The application belongs to the field of nanoscience and fluorescent sensing technology, and particularly relates to a preparation method of a paper-based sensor for detecting ammonia and application thereof. First, Zn(PA)(BPE) is prepared, and the surface porous structure of Zn(PA)(BPE) is used to adsorb ammonia so as to improve the reaction sensitivity to ammonia. Then, Zn(PA)(BPE) is further embedded with CNQDs to form a Zn(PA)@CNQDs ratio type fluorescent substance, and finally, the fluorescent substance is dissolved in ultrapure water to obtain a solution. The solution is added dropwise on filter paper, and is loaded on the filter paper through nitrogen blowing, so that a ratio type fluorescent paper-based sensor is obtained. The application uses Zn(PA)(BPE) and CNQDs to form a ratio type fluorescent paper-based sensor, which not only makes CNQDs more stable, but also improves the background anti-interference ability of ammonia detection, has high selectivity and sensitivity, and has a good application prospect in the detection of the spoilage process of meat products.

Description

Technical Field

[0001] This invention belongs to the field of nanoscience and fluorescence sensing technology, specifically relating to a method for preparing a paper-based sensor for detecting ammonia and its application. Background Technology

[0002] Ammonia is a gas less dense than air, with an irritating odor and toxicity. It is produced by the decarboxylation of amino acids in decaying animal tissue and is commonly found in chicken and other high-protein meats. Currently, commonly used methods for detecting ammonia and fatty amines include gas chromatography-mass spectrometry, electrochemical methods, colorimetry, high-performance liquid chromatography, and fluorescence detection. However, these methods are generally time-consuming, labor-intensive, and require complex pretreatment, and are not suitable for detection in complex storage environments. Among these methods, fluorescence detection of ammonia and fatty amines is simpler and more convenient.

[0003] Coordination polymers are complexes with different properties obtained by combining different metal ions with different organic ligands. These complexes possess stable, porous, three-dimensional structures. The porous cavity structure provides multiple reaction sites for interaction with the target substance, while also exhibiting good material stability. Different metal ions combined with organic ligands form coordination polymers with varying properties, making these complexes a widely used method for detecting ammonia and amine gases. Furthermore, compared to enhanced fluorescence probes for ammonia detection, ratiometric fluorescent probes feature dual-wavelength emission. The wavelength ratio varies independently of probe concentration and light source intensity, significantly reducing interference from other detection conditions.

[0004] In existing technologies, a document discloses "A sensing material for ammonia detection and its preparation method and application" (CN115452894A), specifically involving a carbon dot fluorescent probe with low detection limit and high sensitivity; however, the stability of this probe is insufficient. Patent "An organohalogenated cuprous material for high-efficiency ammonia fluorescence detection and its preparation and application" (CN 115028188A) proposes a fluorescent detection probe with high sensitivity and a simple synthesis method; however, the material used in this probe is based on organohalogenated cuprous materials, which exhibit certain biotoxicity in food spoilage detection.

[0005] Currently, most materials used for ammonia sensing are organic conductive gas-sensitive materials. While these materials offer the advantage of high sensitivity, they also suffer from high cost, poor selectivity, and poor stability. Therefore, developing a simple, rapid, accurate, portable, and easy-to-use biosensor by combining fluorescent probes with a paper substrate is of great significance. Summary of the Invention

[0006] To address the shortcomings of existing fluorescent molecular probes for ammonia, this invention proposes a ratiometric fluorescent probe with high sensitivity and strong stability. On the one hand, it utilizes the porous structure of the Zn(PA)(BPE) surface to adsorb ammonia, thereby increasing the sensitivity to ammonia. On the other hand, the ratiometric fluorescent probe is formed by embedding Zn(PA)(BPE) with CNQDs, which not only makes CNQDs more stable but also improves the background interference resistance for detecting ammonia.

[0007] To achieve the above objectives, the present invention provides a method for preparing a paper-based sensor for detecting ammonia, comprising the following steps:

[0008] (1) Preparation of Zn(PA)(BPE): Zn(NO3)2·6H2O was mixed with DMF to obtain a mixed solution, which was denoted as solution A; dihydroxynaphthyl acid, 1,2-bis(4-pyridyl)ethane and ultrapure water were mixed to obtain a mixed solution, and the pH of the mixed solution was adjusted to alkaline with KOH, which was denoted as solution B; then solution A and solution B were mixed, and after sonication, they were placed in a high-pressure reactor and heated for a period of time to obtain a solid substance. After washing with ultrapure water and vacuum freeze-drying, a solid powder was obtained, which is Zn(PA)(BPE);

[0009] (2) Preparation of Zn(PA)@CNQDs ratiometric fluorescent material: A certain amount of sodium citrate, ammonium chloride and water were mixed and then the Zn(PA)(BPE) powder obtained in step (1) was added. After mixing evenly, the mixture was transferred to a high-pressure reactor and placed in an oven for reaction. After the reaction, the mixture was naturally cooled to room temperature to obtain a mixture. The mixture was then placed in a dialysis membrane and placed in the dialysis solution for a period of time for purification to remove unreacted precursor substances. After the purified mixture was naturally deposited, the precipitate was collected and freeze-dried to obtain a solid powder, which is the Zn(PA)@CNQDs ratiometric fluorescent material.

[0010] (3) Prepare a ratiometric fluorescent paper-based sensor by loading a fluorescent probe onto filter paper; dissolve the Zn(PA)@CNQDs ratiometric fluorescent material prepared in step (2) in ultrapure water to obtain a Zn(PA)@CNQDs solution; then drop the Zn(PA)@CNQDs solution onto filter paper, dry it by nitrogen blowing and load it onto the filter paper to obtain a ratiometric fluorescent paper-based sensor, which is a paper-based sensor for detecting ammonia.

[0011] Preferably, in step (1), the ratio of Zn(NO3)2·6H2O to DMF in solution A is 15g:200mL; and the ratio of dihydroxynaphthyl acid, 1,2-bis(4-pyridyl)ethane and ultrapure water in solution B is 1.6g:1.5g:400mL.

[0012] Preferably, the volume ratio of liquid A and liquid B when mixing in step (1) is 1:5.

[0013] Preferably, the concentration of KOH in step (1) is 1M, and the pH is adjusted to an alkaline pH of 8; the ultrasonic time is 5 min, and the reaction is carried out in an oven at 120°C for 72 h.

[0014] Preferably, the ratio of Zn(PA)(BPE), sodium citrate, ammonium chloride and water in step (2) is 20g:10g:53g:500mL.

[0015] Preferably, the reaction conditions in the oven in step (2) are 180°C for 4 hours; the molecular weight of the dialysis membrane is 1000 Da; the dialysis solution is ultrapure water, distilled water or deionized water; and the storage period is 24 hours.

[0016] Preferably, the ratio of Zn(PA)@CNQDs ratiometric fluorescent material to pure aqueous solution in step (3) is 1.5 mg: 1-128 mL; more preferably, the ratio is 1.5 mg: 16 mL.

[0017] Preferably, the Zn(PA)@CNQDs solution described in step (3) is added dropwise onto filter paper, with the dosage ratio being approximately 1 cm. 2 Add 15-20 μl of Zn(PA)@CNQDs solution to filter paper.

[0018] The ratiometric fluorescent paper-based sensor prepared by this invention is used for ammonia detection. The specific steps are as follows:

[0019] (1) Construction of standard curve: Ammonia solutions of different concentrations were placed in the same closed environment with ratiometric fluorescent paper-based sensors. One concentration of ammonia solution corresponds to one ratiometric fluorescent paper-based sensor. After a period of reaction, the concentration of ammonia in the gas phase was calculated according to Henry's Law, and the G / B ratio in the RGB value of the corresponding paper-based sensor was calculated. Then, a standard curve was established based on the ammonia concentration and the G / B ratio.

[0020] (2) Detection of unknown samples: Place the sample liquid to be tested and the ratio-type fluorescent paper-based sensor in the same sealed environment. After reacting for a period of time, calculate the G / B ratio in the RGB value of the paper-based sensor and then substitute it into the standard curve in step (1) to realize the detection of ammonia concentration.

[0021] Preferably, the concentration of the ammonia solution in step (1) is 0.1 to 0.6 M; the temperature of the environmental conditions in steps (1) and (2) is 25 °C, and the reaction time is 24 h; the ratiometric fluorescent paper-based sensor is placed adjacent to the ammonia solution or the sample solution to be tested.

[0022] Beneficial effects:

[0023] (1) The Zn(PA)(BPE) prepared in this invention is a complex with a porous and stable three-dimensional structure. Due to the high thermal stability of this complex, this invention mixes and reacts it with the precursor material for synthesizing carbon dots. Under the condition that the structure remains unchanged, the carbon dots are synthesized in the porous cavity of Zn(PA)(BPE) to obtain a stable composite product Zn(PA)@CNQDs. This product has two emission wavelengths under 365nm wavelength excitation. Compared with single emission fluorescent probes, the ratio value of this complex ratiometric fluorescent probe is independent of probe concentration and light source intensity, which can greatly reduce the interference of other detection conditions.

[0024] (2) The Zn(PA)(BPE) in the complex has multiple binding sites, and its reaction with ammonia increases the transition of electrons to energy level orbitals, thereby leading to fluorescence changes. Studies have shown that the detection limit of this complex for ammonia can reach 288 nM, and it does not react with volatile organic reagents such as acetone, methanol, ethanol, and ethane. Therefore, this invention provides a novel ratiometric fluorescent paper-based sensor with high selectivity and sensitivity, which can be packaged and stored with meat products and has good application prospects in detecting meat spoilage. Attached Figure Description

[0025] Figure 1 The fluorescence emission spectra of Zn(PA)(BPE), CNQDs, and Zn(PA)@CNQDs ratiometric fluorescent materials are shown.

[0026] Figure 2 The images show the fluorescence spectra of Zn(PA)@CNQDs solutions of different concentrations after reaction with 20 μl of 0.1 M ammonia solution. In Figure a, the concentration of Zn(PA)@CNQDs solution is 1.5 mg / mL; in Figure b, the concentration is 1.5 mg / 2 mL; in Figure c, the concentration is 1.5 mg / 4 mL; in Figure d, the concentration is 1.5 mg / 8 mL; in Figure e, the concentration is 1.5 mg / 16 mL; in Figure f, the concentration is 1.5 mg / 32 mL; in Figure g, the concentration is 1.5 mg / 64 mL; and in Figure h, the concentration is 1.5 mg / 128 mL.

[0027] Figure 3 A standard curve was established to show the relationship between G / B and ammonia concentration based on fluorescence images from a paper-based sensor.

[0028] Figure 4The graph shows the fluorescence color changes of the paper-based sensor after reacting with different volatile organic gases.

[0029] Figure 5 The fluorescence color change of the paper-based sensor after 1 hour at different temperatures. Detailed Implementation

[0030] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0031] Example 1:

[0032] (1) Synthesis of Zn(PA)(BPE):

[0033] Solution A was obtained by mixing 0.8 mmol of Zn(NO3)2·6H2O with 8 mL of DMF and stirring. Solution B was obtained by mixing 0.4 mmol of dihydroxynaphthyl acid with 0.8 mmol of 1,2-bis(4-pyridyl)ethane with 40 mL of ultrapure water. Solution B was obtained by adjusting the pH of the mixture to 8 with 1 M KOH. Solution A and solution B were mixed at a volume ratio of 1:5, and then sonicated for 5 min before being placed in a high-pressure reactor and heated at 120 °C for 72 h to obtain a solid substance. The solid substance was washed three times with ultrapure water and then freeze-dried under vacuum for 2 days to obtain a solid powder, which is Zn(PA)(BPE).

[0034] (2) Preparation of Zn(PA)@CNQDs ratiometric fluorescent material:

[0035] 0.1g of sodium citrate, 0.53g of ammonium chloride and 5mL of water were mixed and then 0.2g of Zn(PA)(BPE) powder obtained in step (1) was added. The mixture was then transferred to a high-pressure reactor and placed in an oven at 180℃ for reaction. After 4 hours of reaction, the mixture was naturally cooled to room temperature to obtain a mixture. The mixture was then placed in a dialysis membrane with a molecular weight of 1000Da and purified in ultrapure water for 24 hours to remove unreacted precursors to obtain a purified mixture. After natural sedimentation, the precipitate was removed and washed three times by centrifugation with ultrapure water. Then, it was freeze-dried under vacuum to obtain the product, which is Zn(PA)@CNQDs ratiometric fluorescent material.

[0036] (3) A ratiometric fluorescent paper-based sensor was prepared by loading a fluorescent probe onto filter paper;

[0037] Add 1.5 mg of the Zn(PA)@CNQDs ratiometric fluorescent material obtained in step (2) to 16 mL of pure aqueous solution to obtain a Zn(PA)@CNQDs solution. Take 20 μl of the Zn(PA)@CNQDs solution and drop it onto a 1 cm... 2 The filter paper is dried by blowing nitrogen for 5 minutes, and then loaded onto the filter paper to obtain a ratiometric fluorescence sensor.

[0038] Performance testing:

[0039] I. Optical properties of the Zn(PA)@CNQDs ratiometric fluorescent material prepared in Example 1 after reaction with ammonia at different concentrations were tested.

[0040] (1) Prepare aqueous solutions of Zn(PA)@CNQDs with different concentrations of 1.5 mg / mL, 1.5 mg / 2 mL, 1.5 mg / 4 mL, 1.5 mg / 8 mL, 1.5 mg / 16 mL, 1.5 mg / 32 mL, 1.5 mg / 64 mL, and 1.5 mg / 128 mL.

[0041] (2) Take 200 μl of 0.1 M ammonia solution and add it to 2 mL of Zn(PA)@CNQDs solution of different concentrations to obtain a mixed solution; one concentration of Zn(PA)@CNQDs aqueous solution corresponds to one part of ammonia solution, and the two have a one-to-one correspondence; after standing for 5 min, detect the fluorescence intensity of the mixed solution. From Figure 2 It can be observed that the fluorescence enhancement varies after reacting the complex with the same concentration of ammonia; therefore, it can be inferred that when the concentration of Zn(PA)@CNQDs is too high, the fluorescence is quenched due to the fluorescence internal filtration effect after reacting with ammonia. In order for the ratiometric fluorescent material Zn(PA)@CNQDs to react better with ammonia, this invention selects a concentration of 1.5 mg / 16 mL of Zn(PA)@CNQDs.

[0042] II. Method for Detecting Ammonia Gas Using the Zn(PA)@CNQDs Paper-Based Sensor Prepared in Example 1

[0043] (1) The concentration of ammonia gas was detected using the ratiometric fluorescent sensor prepared in Example 1, and the concentration of the paper-based sensor was calculated using the following formula.

[0044]

[0045] c(Zn(PA)@CNQDs) is the concentration of the Zn(PA)@CNQDs solution;

[0046] v(Zn(PA)@CNQDs) is the volume of the Zn(PA)@CNQDs solution;

[0047] A(paper) represents the area of ​​the filter paper;

[0048] (2) Ammonia solutions of different concentrations were placed in the same sealed environment as the ratiometric fluorescent paper-based sensor. In this embodiment, the sealed environment was a gas generator. The dried ratiometric fluorescent sensor was fixed in the gas generator, and ammonia solutions of different concentrations (0, 0.1M, 0.2M, 0.3M, 0.4M, 0.5M, 0.6M) of the same volume were added to the orifice of the gas generator. After the addition was completed, the gas generator was sealed. The two were placed in the same sealed environment. The gas generator was placed at 25°C, and the concentration of ammonia in the gas phase of the gas generator was calculated using the following formula.

[0049]

[0050] R - molar gas constant; T - reaction environment temperature; n(NH3,g) - molar amount of ammonia in the gas phase; V(NH3,g) - container volume; K B - Henry's constant; n0(NH3·H2O) - molar amount of ammonia; V(H2O) - total volume of solution; ρ(H2O) - density of water; M(H2O) - relative molecular mass of water;

[0051] (3) After 24 hours of reaction, the fluorescence color of the ratiometric fluorescence sensor was recorded under a 365nm UV lamp using a smartphone. The paper substrate was replaced after each recording of the fluorescence image change at a concentration (i.e., one Zn(PA)@CNQDs paper substrate sensor corresponds to one concentration). Each concentration was repeated three times. The fluorescence color of the paper substrate sensor gradually changed from blue-green to bright green. The higher the ammonia concentration, the brighter the corresponding green fluorescence of the sensor.

[0052] Ammonia gas is quantitatively analyzed by extracting the RGB values ​​of the image and calculating the ratio of the G to B values. A standard curve is then established based on the relationship between the G / B ratio and ammonia concentration. Figure 3 The standard curve equation is y = 0.281x + 0.5901(R). 2 =0.993), and the detection limit is 288 nM. The results show that the standard curve has good linearity and a low detection limit, indicating good performance in detecting ammonia.

[0053] (4) The ratiometric fluorescence sensor was placed in the same environment as a volatile organic solution of the same concentration (0.1M). After standing for a period of time, the fluorescence color of the ratiometric fluorescence sensor under a 365nm ultraviolet lamp was recorded using a smartphone. Except for the ratiometric fluorescence sensor under 0.1M ammonia concentration, which turned bright green, the fluorescence color of the ratiometric fluorescence sensor under other volatile gases remained almost unchanged, still being blue-green. Results Figure 4 As shown, this demonstrates that the sensor has good selectivity for ammonia.

[0054] (5) The paper-based sensor was placed at different temperatures (25℃, 35℃, 45℃, 55℃, 65℃, 75℃) for 1 hour. The fluorescence color of the paper-based sensor under a 365nm ultraviolet lamp was recorded using a smartphone, and the RGB values ​​of the images were extracted. The results are shown in Table 1. Figure 5 As shown, the sensor's G and B values ​​did not change significantly between 25℃ and 65℃, while the G value remained largely unchanged at 75℃, although the B value decreased slightly. This indicates that the ratiometric fluorescence sensor has good stability.

[0055] Table 1 shows the G and B values ​​extracted from the paper-based sensor after 1 hour at different temperatures.

[0056]

[0057] (6) The corresponding ammonia concentration can be obtained by using the G / B value of the fluorescence color of the paper-based sensor through the standard curve.

[0058] Ammonia water samples with concentrations of 0.25M, 0.45M, and 0.55M were prepared respectively. 1 mL of each of these samples was placed in the same environment (25℃) with the paper-based sensor for reaction. The corresponding ammonia concentrations were calculated to be 0.53899μM, 0.97017μM, and 1.18576μM according to the formula in step (2). After 24 hours of reaction, the RGB values ​​of the corresponding paper-based sensor were obtained. The corresponding ammonia concentration was compared with the actual ammonia concentration by substituting the obtained G / B value into the standard curve in step (3). The results are shown in Table 2.

[0059] Table 2 shows the measurement results and recovery rate of ammonia samples.

[0060]

[0061] Table 2 shows that the recovery rates are 87%, 108%, and 90%, respectively. The recovery rates are good and the results are accurate, indicating that within a certain range of ammonia concentration, the corresponding ammonia concentration can be obtained by the RGB value of the fluorescence color of the paper-based sensor.

[0062] Note: The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. Therefore, although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention. All technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a paper-based sensor for detecting ammonia, characterized in that, Includes the following steps: (1) Mix Zn(NO3)2·6H2O with DMF to obtain a mixed solution, which is denoted as solution A; mix dihydroxynaphthyl acid, 1,2-bis(4-pyridyl)ethane and ultrapure water to obtain a mixed solution, and adjust the pH of the mixed solution to alkaline with KOH, which is denoted as solution B; then mix solution A and solution B, sonicate and heat in a high-pressure reactor for a period of time to obtain a solid substance, and then wash with ultrapure water and freeze dry under vacuum to obtain a solid powder, which is Zn(PA)(BPE); In step (1), the ratio of Zn(NO3)2·6H2O to DMF in solution A is 15g:200mL; the ratio of dihydroxynaphthyl acid, 1,2-bis(4-pyridyl)ethane, and ultrapure water in solution B is 1.6g:1.5g:400mL; and the volume ratio of solution A to solution B when mixed is 1:

5. The concentration of KOH is 1M, adjusted to an alkaline pH of 8; the ultrasonication time is 5 minutes, and the heating conditions are 120℃ for 72 hours. (2) After mixing a certain amount of sodium citrate, ammonium chloride and water, add Zn(PA)(BPE) obtained in step (1), mix evenly and transfer to a high-pressure reactor. Place the high-pressure reactor in an oven for reaction. After the reaction, cool naturally to room temperature to obtain a mixture. Put the obtained mixture into a dialysis membrane and place it in the dialysis solution for a period of time for purification to remove unreacted precursor substances. After the purified mixture is naturally deposited, collect the precipitate, freeze-dry the precipitate to obtain a solid powder, which is Zn(PA)@CNQDs ratiometric fluorescent material. The ratio of Zn(PA)(BPE), sodium citrate, ammonium chloride and water used in step (2) is 20g:10g:53g:500mL; (3) Dissolve the Zn(PA)@CNQDs ratiometric fluorescent material prepared in step (2) in ultrapure water to obtain Zn(PA)@CNQDs solution; then take Zn(PA)@CNQDs solution and drop it onto filter paper, dry it by nitrogen blowing and load it onto filter paper to obtain ratiometric fluorescent paper-based sensor, which is a paper-based sensor for detecting ammonia. The ratio of Zn(PA)@CNQDs ratiometric fluorescent material to ultrapure water in step (3) is 1.5 mg: 1-128 mL.

2. The method for preparing a paper-based sensor for detecting ammonia according to claim 1, characterized in that, The reaction conditions in the oven in step (2) are 180°C for 4 hours; the molecular weight of the dialysis membrane is 1000 Da; the dialysis solution is ultrapure water, distilled water or deionized water; and the storage period is 24 hours.

3. The method for preparing a paper-based sensor for detecting ammonia according to claim 1, characterized in that, The ratio of Zn(PA)@CNQDs ratiometric fluorescent material to ultrapure water in step (3) is 1.5 mg: 16 mL.

4. The method for preparing a paper-based sensor for detecting ammonia according to claim 1, characterized in that, The Zn(PA)@CNQDs solution described in step (3) is added dropwise onto filter paper, with the dosage ratio being per cm. 2 Add 15-20 μl of Zn(PA)@CNQDs solution to filter paper.

5. The application of the paper-based sensor for detecting ammonia prepared according to any one of claims 1-4 in ammonia detection, characterized in that, The steps are as follows: (1) Construction of standard curve: Ammonia solutions of different concentrations were placed in the same closed environment with ratiometric fluorescent paper-based sensors. One concentration of ammonia solution corresponds to one ratiometric fluorescent paper-based sensor. After a period of reaction, the concentration of ammonia in the gas phase was calculated according to Henry's law, and the G / B ratio in the corresponding RGB value of the paper-based sensor was calculated. Then, a standard curve was established based on the ammonia concentration and the G / B ratio. (2) Detection of unknown samples: Place the sample liquid to be tested and the ratio-type fluorescent paper-based sensor in the same sealed environment. After reacting for a period of time, calculate the G / B ratio in the RGB value of the paper-based sensor and then substitute it into the standard curve in step (1) to realize the detection of ammonia concentration.

6. The application according to claim 5, characterized in that, In step (1), the concentration of the ammonia solution is 0.1~0.6M; the temperature of the environmental conditions described in steps (1) and (2) is 25℃, and the reaction time is 24h. The ratiometric fluorescent paper-based sensor is placed adjacent to the ammonia solution or the sample solution to be tested.

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