High-sensitivity optical fiber MZI ammonia gas sensor based on Sn-MOF modification and preparation method of high-sensitivity optical fiber MZI ammonia gas sensor

Abstract

The invention discloses a Sn-MOF modification-based high-sensitivity optical fiber MZI ammonia gas sensor and a preparation method thereof, and belongs to the field of optical fiber sensors, the Sn-MOF modification-based high-sensitivity optical fiber MZI ammonia gas sensor comprises an optical fiber sensing unit based on Mach-Zehnder interference and a Sn-MOF gas-sensitive coating coated on the optical fiber sensing unit; the optical fiber sensing unit is of an SMF-NCF-PCF-NCF-SMF structure formed by sequentially welding a single-mode optical fiber, a coreless optical fiber and a photonic crystal fiber, and the PCF and the NCF on the two sides are welded in a staggered mode in the x direction to form an asymmetric staggered welding structure; the Sn-MOF gas-sensitive coating is uniformly coated on a sensing area formed by the PCF and the NCF on the two sides through a dip-coating method, and the Sn-MOF is prepared by adopting a solvothermal method. According to the invention, the Sn-MOF material is combined with the asymmetric staggered structure of the MZI optical fiber, so that high-sensitivity, high-specificity and rapid detection of NH3 is realized.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensors, and in particular to a high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification and its preparation method. Background Technology

[0002] NH3 is an important component of exhaled human breath, and its metabolic pathway is closely related to kidney function. Proteins are broken down into amino acids in the gastrointestinal tract, and the undigested portion is converted into ammonia by intestinal flora. A small amount of ammonia is exhaled directly through the lungs, while most enters the bloodstream and is converted into urea in the liver, then excreted by the kidneys. In addition, some urea diffuses into the intestines and is re-decomposed into ammonia by bacterial urease. When kidney function is impaired, the kidneys' ability to excrete urea decreases, leading to an increase in blood urea concentration, which in turn promotes increased ammonia production in the intestines. Simultaneously, the kidneys' ability to excrete ammonia weakens, ultimately leading to ammonia accumulation in the blood, which is then expelled through respiration, increasing the ammonia concentration in exhaled breath. Therefore, NH3 can serve as a non-invasive diagnostic biomarker for chronic kidney disease.

[0003] Given the significant value of NH3 in the non-invasive diagnosis of kidney disease, the rapid and accurate detection of NH3 concentration in exhaled breath has become a key research focus. NH3 sensors based on the principle of optical interference have become a research hotspot in this field due to their advantages of resistance to electromagnetic interference and high sensitivity. Common interference types include Mach-Zehnder interferometry (MZI), Fabry-Perot interferometry (FPI), Michelson interferometry, and Sagnac interferometry. In recent years, researchers have continuously improved the detection performance of sensors by combining interference principles with functional materials. In 2018, Yi Zhu et al. coated ZnO nanoflowers and microspheres onto a refractive index sensor of single-mode fiber (SMF)-multimode fiber (MMF)-single-mode fiber, enabling the sensor to be selective for NH3. Experimental results showed that the NH3 sensitivity of the nanoflower sensor was 2.6 times that of the microsphere sensor. In 2023, Lijun Li et al. achieved a highly sensitive and rapid response to NH3 through a symmetrical structure of SMF-e-TCF (etched thin-core fiber)-SMF and by coating the TCF surface with a graphene / polyaniline composite film. Experimental results showed a sensitivity of -27.8 pm / ppm for NH3. In 2024, Zetian Liu et al. fused a capillary tube and a coreless optical fiber (NCF) between two SMF segments as the sensing region, and coated the NCF surface with a ZnO thin film, achieving specific detection of NH3. Experimental results showed a sensitivity of -35.52 pm / ppm for NH3, with response and recovery times of 105 s and 85 s, respectively.

[0004] Although sensors based on interferometric optics and functional materials exhibit high sensitivity and tunability in refractive index sensing, they still face challenges in the complex environment of human exhaled gases—relying solely on refractive index changes is insufficient for specific identification of NH3. Metal-organic frameworks (MOFs), with their abundant specific adsorption sites, offer an effective approach to solving the problem of specific NH3 detection, becoming a key breakthrough in improving sensor performance. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a high-sensitivity fiber MZI ammonia sensor based on Sn-MOF modification and its preparation method. In view of the problems of insufficient specificity, slow response speed and high detection limit of existing NH3 sensors in non-invasive diagnosis of chronic kidney disease, the present invention achieves high sensitivity, high specificity and rapid detection of NH3 by combining Sn-MOF material with the asymmetric misalignment structure of MZI fiber.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A high-sensitivity fiber MZI ammonia gas sensor based on Sn-MOF modification includes a fiber sensing unit based on Mach-Zehnder interferometry and a Sn-MOF gas-sensitive coating coated on the fiber sensing unit. The fiber optic sensing unit is formed by sequentially fusing single-mode fiber, coreless fiber, and photonic crystal fiber to form an SMF-NCF-PCF-NCF-SMF structure, wherein the PCF and the two NCFs on both sides are staggered in the x-direction to form an asymmetric staggered fusion structure. The Sn-MOF gas-sensitive coating is uniformly coated on the sensing area composed of the PCF and the NCF on both sides by dip-coating method. The Sn-MOF is prepared by solvothermal method, and the raw materials are tin tetrachloride pentahydrate and trimesic acid. The Sn-MOF has a porous structure that matches the diameter of NH3 molecules and abundant Sn metal active sites, which are used for the specific adsorption of NH3 molecules.

[0007] A further improvement of the technical solution of the present invention is that the PCF and the left NCF are staggered and fused in the x direction with a center distance of 45 μm, and the PCF and the right NCF are staggered and fused in the x direction with a center distance of 87 μm.

[0008] A further improvement of the technical solution of the present invention is that: in the asymmetric misaligned fusion spliced ​​fiber sensing unit, the length of NCF is 1 mm and the length of PCF is 2934.95 μm.

[0009] A further improvement of the technical solution of the present invention is that the PCF is a double-layer grapefruit-shaped structure with the following parameters: the diameter of the germanium-doped core is 2.8±1.0 μm, the diameter of the cladding is 125±1.0 μm, the diameter of the inner layer macropore is 7.3±1.0 μm, the spacing between the inner layer micropores is 8.2±1.0 μm, the diameter of the outer layer micropore is 3.7±1.0 μm, and the spacing between the outer layer micropores is 5.0±1.0 μm.

[0010] A method for fabricating a high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification includes the following steps: S1. Preparation of Sn-MOF by solvothermal method; S2. Apply a Sn-MOF gas-sensitive coating to the outside of the fiber optic sensing unit by dip-coating method; S3, Asymmetric misaligned fusion spliced ​​fiber optic sensing unit.

[0011] A further improvement to the technical solution of this invention lies in the following: the solvothermal preparation process of Sn-MOF specifically includes: Dissolve 1.052 g of tin tetrachloride pentahydrate in 20 mL of methanol and stir until completely dissolved to form solution A; dissolve 1 g of trimesic acid in 20 mL of methanol and stir until completely dissolved to form solution B; Pour solution A into solution B, mix, and then stir continuously on a mixer for 6 hours to obtain mixed solution C; The mixed solution C was transferred to a 50 mL polytetrafluoroethylene-lined high-pressure reactor and subjected to a hydrothermal reaction at 180 °C for 24 h. After the reaction was completed and cooled to room temperature, the reaction product was washed with ethanol and then centrifuged at 8000 rpm for 10 min. The washing-centrifugation operation was repeated 3 times, and the white solid collected was Sn-MOF.

[0012] A further improvement to the technical solution of this invention lies in the following: the specific process parameters for the immersion lifting method are as follows: The pulling speed is 0.1-10 mm / s. After coating, the fiber sensing unit is placed in a 60 ℃ oven to dry for 1 h to enhance the adhesion between the Sn-MOF gas-sensitive coating and the fiber surface.

[0013] A further improvement to the technical solution of this invention lies in: an asymmetric misaligned fusion spliced ​​fiber optic sensing unit, comprising the following steps: Pre-fusion splicing: Using a fusion splicer, the SMF and NCF are precisely connected after end-face cleaning. The cleaning discharge power is set to 13.2 mA, the cleaning discharge time to 60 ms, the fiber pre-fusion power to 10.8 mA, the fiber pre-fusion time to 10 ms, the main discharge power to 18 mA, the main discharge time to 100 ms, the re-discharge power to 12.7 mA, and the re-discharge time to 700 ms, to achieve low-loss fusion splicing of SMF-NCF. Real-time monitoring of welding: Connect the pre-welded SMF-NCF to the PCF, turn on the attenuation welding, control the misalignment distance through the micron-level displacement platform of the welding machine, and at the same time turn on the CCD imaging system to monitor the welding interface. The main discharge power is changed to 14.5 mA and the main discharge time is 1250 ms to prevent the air hole of the PCF from collapsing. Precise positioning and welding: Open the manual welding mode and connect the SMF-NCF-PCF to another SMF-NCF. Connect the spectrometer to monitor the transmission loss, interference fringe contrast and free spectral range in the 1550 nm band in real time. When the x-direction offset is 87 μm, the y-direction offset is 0 μm and the angle θ is aligned, the FSR is stable at 1.84 nm and the interference fringe contrast is 25 dB, and the welding is completed.

[0014] The technological advancements achieved by this invention due to the adoption of the above technical solutions are as follows: 1. This invention has a sensitivity of 0.02 nm / ppm for low concentration NH3 (2.5-20 ppm) and 0.008 nm / ppm for high concentration NH3 (approximately 20-50 ppm), with detection limits as low as 0.02 ppm (low concentration range) and 0.05 ppm (high concentration range), enabling the detection of low concentrations of exhaled ammonia in the early stages of chronic kidney disease; the response time is 9 s and the recovery time is 12 s, meeting the requirements for real-time detection.

[0015] 2. The Sn active site and pyromellitic acid ligand in the Sn-MOF proposed in this invention can specifically adsorb NH3. The response trough shift of interfering gases such as 16% ethanol and 99.9% carbon dioxide is much lower than that of NH3, showing outstanding specificity.

[0016] 3. The double-layer grapefruit-shaped PCF used in this invention is resistant to bending and avoids strain interference; the humidity range of 37%-80% has little impact on detection (wavelength humidity sensitivity -0.0015 nm / %), which is suitable for the high humidity exhalation environment of the human body; controlling the room temperature for detection can eliminate temperature interference.

[0017] 4. The NH3 sensor prepared by this invention has a compact and miniaturized structure, requires no power supply, is easy to integrate, and is suitable for home monitoring and clinical application; the preparation process is mature, the cost is controllable, and it has the potential for large-scale application. Attached Figure Description

[0018] Figure 1 In the image, (a), (b), and (c) show the upward movement of the right NCF in the y-direction after fusing the NCF and PCF on the left; (d) and (e) show the movement of the PCF in the x-direction; (f) shows the rotation of the PCF at the θ angle; (g) and (h) are schematic diagrams of SMF-NCF-PCF-NCF-SMF and actual images under SEM, respectively, where the arrows indicate the light transmission path; and (i) is a cross-sectional view of the PCF under SEM. Figure 2 In the figure, (a) is the interference spectrum, (b) is the interference spectrum with a refractive index range of 1.3330-1.33314, (c) is the linear fit, (d) is the response time of the sensor to the refractive index solution, (e) is the interference spectrum with a temperature range of 24-78 ℃, and (f) is the linear fit graph. Figure 3 This is a flowchart of the Sn-MOF preparation process; Figure 4 In the image, (a) is the SEM image of Sn-MOF at 5000 magnification, and (b) is the SEM image of Sn-MOF at 23000 magnification. Figure 5 In the image, (a) shows the interference spectrum before and after coating, and (b) shows the interference spectrum from 0 to 10. 4 The interference spectrum of s, (c) is the Allen deviation of the three valleys, and (d) is the power spectral density of the three valleys; Figure 6 Diagram of the experimental setup; Figure 7 In the figure, (a) is the wavelength shift for 2.5-50 ppm NH3 concentration, (b) is the linear fit, (c) is the response and recovery time for 2.5 ppm NH3 at room temperature, (d) is the specificity, (e) is the transmission spectrum for 78-24 ℃, (f) is the linear fit, (g) is the transmission spectrum for 37-80%, and (h) is the linear fit.

[0019] Among them, 1. Broadband light source, 2. Beaker, 3. Spectrometer, 4. Glass tube, 5. SMF-NCF-PCF-NCF-SMF, 6. Dropper, 7. Flask, 8. Heating stage. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments: A high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification, comprising: The asymmetric misaligned fusion splicing fiber sensing unit based on Mach-Zehnder interferometry (MZI) consists of a single-mode fiber (SMF), a coreless fiber (NCF), and a photonic crystal fiber (PCF) sequentially fused to form an SMF-NCF-PCF-NCF-SMF structure. The PCF and the left NCF are misaligned in the x-direction by a distance of 45 μm, and the PCF and the right NCF are misaligned in the x-direction by a distance of 87 μm, thus forming an asymmetric misaligned fusion splicing structure. The Sn-MOF gas-sensitive coating is uniformly coated on the sensing area composed of the PCF and the NCF on both sides by dip-coating method. The Sn-MOF is prepared by solvothermal method, and the raw materials are tin tetrachloride pentahydrate (SnCl4·5H2O) and trimesic acid (H3BTC). The Sn-MOF has a porous structure that matches the diameter of NH3 molecules (about 0.3 nm) and abundant Sn metal active sites, which are used for the specific adsorption of NH3 molecules.

[0021] The parameters of the double-layer grapefruit-shaped PCF are as follows: the diameter of the germanium-doped core is 2.8±1.0 μm, the diameter of the cladding is 125±1.0 μm, the diameter of the inner macropore is 7.3±1.0 μm, the spacing of the inner micropore is 8.2±1.0 μm, the diameter of the outer micropore is 3.7±1.0 μm, and the spacing of the outer micropore is 5.0±1.0 μm.

[0022] In the asymmetric misaligned fusion spliced ​​fiber sensing unit, the length of the NCF is 1 mm and the length of the PCF is 2934.95 μm.

[0023] A method for fabricating a high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification includes the following steps: S1. Preparation of Sn-MOF by solvothermal method; The solvothermal preparation process of Sn-MOF specifically includes: Dissolve 1.052 g of tin tetrachloride pentahydrate in 20 mL of methanol and stir until completely dissolved to form solution A; dissolve 1 g of trimesic acid in 20 mL of methanol and stir until completely dissolved to form solution B; Solution A was quickly poured into solution B, and the mixture was stirred continuously on a mixer for 6 hours to obtain mixed solution C. The mixed solution C was transferred to a 50 mL polytetrafluoroethylene-lined high-pressure reactor and subjected to a hydrothermal reaction at 180 °C for 24 h. After the reaction was completed and cooled to room temperature, the reaction product was washed with ethanol and then centrifuged at 8000 rpm for 10 min. The washing-centrifugation operation was repeated 3 times, and the white solid collected was Sn-MOF.

[0024] S2. Apply a Sn-MOF gas-sensitive coating to the outside of the fiber optic sensing unit by dip-coating method; The specific process parameters of the dip-coating method are as follows: the coating speed is 0.1-10 mm / s, and after coating, the optical fiber sensing unit is placed in a 60 ℃ oven to dry for 1 h to enhance the adhesion between the Sn-MOF gas-sensitive coating and the optical fiber surface.

[0025] S3, Asymmetric misaligned fusion spliced ​​fiber optic sensing unit.

[0026] The asymmetric misaligned fusion spliced ​​fiber optic sensing unit is fabricated using a three-step process: pre-fusion splicing, real-time monitoring, and precise positioning. Specifically, it includes: Pre-fusion splicing stage: Using a Fujikura FSM-100P+ fusion splicer, the SMF and NCF are precisely connected after end-face cleaning. The cleaning discharge power is set to 13.2 mA, the cleaning discharge time to 60 ms, the fiber pre-fusion power to 10.8 mA, the fiber pre-fusion time to 10 ms, the main discharge power to 18 mA, the main discharge time to 100 ms, the re-discharge power to 12.7 mA, and the re-discharge time to 700 ms, to achieve low-loss fusion splicing of SMF-NCF. Real-time monitoring of the welding stage: Connect the pre-welded SMF-NCF to the PCF, turn on the attenuation welding, control the misalignment distance through the micron-level displacement platform of the welding machine, and at the same time turn on the CCD imaging system to monitor the welding interface. The main discharge power is changed to 14.5mA and the main discharge time is 1250 ms to prevent the air hole of the PCF from collapsing. Precise positioning and splicing stage: Open the manual splicing mode and connect the SMF-NCF-PCF to another SMF-NCF. Connect the Yokogawa spectrometer (AQ6370E) to monitor the transmission loss, interference fringe contrast and free spectral range (FSR) in the 1550 nm band in real time. When the x-direction offset is 87 μm, the y-direction offset is 0 μm and the angle θ is aligned, the FSR is stable at 1.84 nm and the interference fringe contrast is 25 dB, and the splicing is completed.

[0027] The specific adsorption mechanism of Sn-MOF for NH3 is as follows: the active site of Sn metal acts as a Lewis acid site, which coordinates with the lone pair electrons in the NH3 molecule; at the same time, the carboxyl group in the Sn-MOF organic ligand enhances the physical adsorption of NH3 through hydrogen bonding or van der Waals forces. The dual action achieves the specific recognition of NH3, and there is no obvious adsorption response to interfering gases such as ethanol and carbon dioxide.

[0028] The fiber optic sensing unit exhibits the following performance characteristics for NH3 detection: a sensitivity of 0.02 nm / ppm and a detection limit (LOD) of 0.02 ppm in the low concentration range (2.5-20 ppm), a sensitivity of 0.008 nm / ppm and a detection limit (LOD) of 0.05 ppm in the high concentration range (20-50 ppm), and response and recovery times of 9 s and 12 s, respectively.

[0029] The temperature control range of the heating stage is 24 ℃ to 80 ℃. By adjusting the temperature of the heating stage, the temperature sensitivity of the sensor before and after coating is 0.013 nm / ℃ and 0.023 nm / ℃, respectively. The temperature sensitivity after coating Sn-MOF film is increased compared with that before coating because Sn-MOF will expand as the temperature increases.

[0030] The asymmetric misaligned fusion splice structure amplifies the effective refractive index difference between the core mode and the cladding mode, enhances the wavelength shift signal of the interference spectrum induced by NH3 adsorption, and improves the sensing sensitivity; the bending resistance of the double-layer grapefruit-shaped PCF can avoid the interference of fiber strain on the NH3 detection results.

[0031] The sealed gas chamber is also equipped with a hygrometer. The left side of the SMF-NCF-PCF-NCF-SMF fiber optic asymmetric misalignment structure within the chamber is connected to a broadband light source and a beaker for NH3 exhaust gas treatment, while the right side is connected to a spectrometer and a flask on a heating stage for adding ammonia. Because monitoring the relative humidity within the chamber is required, the hygrometer probe is placed on the left side of the glass tube before sealing the chamber. Within a relative humidity range of 37%-80%, the wavelength humidity sensitivity of the fiber optic sensing unit is -0.0015nm / %, enabling stable detection of NH3 concentration.

[0032] like Figure 1 As shown, the optical fiber structure of the sensor SMF-NCF-PCF-NCF-SMF and the cross-sectional view of the PCF are illustrated. Figure 1 In the image, (a), (b), and (c) show the upward movement of the right NCF in the y-direction after fusing the NCF and PCF on the left; (d) and (e) show the movement of the PCF in the x-direction; (f) shows the rotation of the PCF at the θ angle; (g) and (h) are schematic diagrams of SMF-NCF-PCF-NCF-SMF and actual images under SEM, respectively, where the arrows indicate the light transmission path; and (i) is a cross-sectional view of the PCF under SEM. Among them, SMF (Single-Mode Fiber), NCF (No-Core Fiber), and PCF (Photonic Crystal Fiber) are all types of optical fibers.

[0033] The fiber structure fabrication process involves cutting two 1 mm NCF segments and one 2934.95 μm PCF segment; pre-fusion splicing the SMF with the first NCF segment; controlling the PCF to be misaligned by 45 μm on the left side of the NCF segment and fusion splicing; and, combined with OSA (Optical Spectrum Analysis) monitoring, controlling the PCF to be misaligned by 87 μm on the right side of the second NCF segment and fusion splicing, ensuring an FSR of 1.84 nm and a fringe contrast of 25 dB.

[0034] To verify the response of the fabricated sensor to changes in external environmental parameters, refractive index sensing and temperature sensing experiments were conducted. First, the two sides of the MZI-based SMF-NCF-PCF-NCF-SMF misalignment structure were connected to BBS and OSA, respectively. Figure 2 In the figure, (a) shows the interference spectrum of the sensor with an FSR of 1.84 nm. Then, glucose solutions with different refractive indices were added to the sensing region, and the spectral shifts were recorded. The results are as follows: Figure 2 As shown in (b) and (c), the interference spectrum exhibits a blue shift as the refractive index increases from 1.3330 to 1.33314. After three repeated experiments, linear fitting yielded a refractive index sensitivity of -23278.44 nm / RIU and a linearity (R0) of [missing value]. 2 The accuracy rate is 99.69%, indicating that the sensor's sensitivity is ideal. Figure 2 In the diagram, (d) shows the sensor's response time to solutions with different refractive indices, indicating that the spectrum stabilizes in the refractive index solution after 81 s, which is a relatively fast stabilization time. Finally, a temperature sensing experiment was conducted. Figure 2 In the middle, (e) and (f) results show that the interference spectrum redshifts as the temperature increases, and the temperature sensitivity of the sensor is found to be 0.013 nm / ℃ through fitting.

[0035] Among them, MZI (Mach-Zehnder Interferometry) is a Mach-Zehnder interferometer, BBS (Broadband Light Source) is a broadband light source, and OSA (Optical Spectrum Analyzer) is a spectrometer.

[0036] Figure 3 The preparation process of Sn-MOF was demonstrated: 1.052 g of tin tetrachloride pentahydrate was dissolved in 20 mL of methanol (solution A), and 1 g of trimesic acid was dissolved in 20 mL of methanol (solution B); solutions A and B were mixed (solution C) and stirred for 6 h, then transferred to a polytetrafluoroethylene high-pressure reactor and hydrothermally heated at 180 ℃ for 24 h; after cooling, the mixture was washed with ethanol and centrifuged at 8000 rpm for 10 min, repeated 3 times to obtain Sn-MOF. The Sn-MOF was then coated onto the optical fiber sensing area to form an NH3 sensor.

[0037] Figure 4In the image, (a) shows a SEM image of Sn-MOF at 5000 magnification; (b) shows a SEM image of Sn-MOF at 23000 magnification. At 5000 magnification, Sn-MOF exhibits a large number of spherical particles with varying sizes and a relatively dispersed distribution. The overall morphology is quite regular, with spherical shape as the primary characteristic. At 23000 magnification, the details of individual spherical particles are more clearly revealed. The particle surfaces are relatively smooth, and the spherical morphology is very obvious, further demonstrating that Sn-MOF has a good spherical morphology, and the particle size is within the micrometer range.

[0038] Figure 5 The stability of Sn-MOF thin films deposited in the fiber optic sensing region was demonstrated. Figure 5 In the diagram, (c) the Allen bias of the interference peak on the ordinate is a statistical tool for quantifying the long-term stability of a signal. By analyzing the fluctuation characteristics at different time scales, it distinguishes noise types and evaluates system performance. The lowest point of the Allen bias curve corresponds to the optimal averaging time; the smaller the value, the smaller the fluctuation in the intensity or phase of the interference peak during long-term monitoring, and the better the stability. Figure 5 In the image, (a) shows that as the EtOH (ethanol) organic solvent evaporates during the coating process, the interference spectrum FSR and fringe contrast gradually decrease, and the final stable interference spectrum is consistent with the FSR before coating. Then, Figure 5 In the image, (b) shows the interference spectrum with three valleys in the wavelength range of 1552-1559 nm, measured at 0-10 nm. 4 Stability test, Figure 5 In the diagram (c), it can be seen that dip 2 and dip 3 have smaller and more stable Allen deviation values, indicating that the sensor resolution is 0.0004 nm. Figure 5 From (d), it can be seen that dip 2 has the lowest power spectral density. Therefore, considering all factors, dip 2 is the best choice.

[0039] Figure 6The diagram shows the experimental setup. Light from a broadband light source 1 with a wavelength range of 300 to 1100 nm is transmitted to the sensor via a standard SMF. The interference spectrum of the sensor is observed using a Yokogawa OSA (AQ6370E) spectrometer with a minimum wavelength resolution of 0.02 nm. In the gas detection experiment, the sensor is fixed in a specially designed groove with UV-curable adhesive, and a glass tube 4 is placed over it. One end of the glass tube 4 is connected to the gas outlet at the top of beaker 2. Ammonia solutions of different concentrations are added to flask 7 via a dropper 6, causing the ammonia to evaporate and enter the gas chamber. The other end of the glass tube 4 is connected to a tail gas treatment device via a pipeline. For humidity detection, a hygrometer probe is inserted into the glass tube 4 to monitor the ambient humidity in real time. For temperature detection, the sensor is placed on a heating stage 8, with its two sides connected to the broadband light source 1 and the spectrometer 3, respectively. In the diagram, the dashed arrows indicate the direction of light transmission; the solid arrows indicate the direction of NH3 flow.

[0040] Example 1 To evaluate the potential of the prepared gas sensor for detecting ammonia, the present invention tested the dynamic response of the prepared NH3 sensor to different NH3 concentrations at room temperature. Figure 7 In the figures, (a) and (b) show the wavelength shift and linear fitting of the Sn-MOF-coated sensor under different NH3 concentrations. The results show that the trough wavelength of the transmission spectrum redshifts with increasing NH3 concentration. The shift is approximately 0.12 nm for NH3 concentrations as low as 2.5 ppm and approximately 0.61 nm for NH3 concentrations as high as 50 ppm. To improve the fitting effect, a piecewise fitting method was used. The NH3 sensitivity of the first piecewise fitting was 0.02 nm / ppm, and R0 was [not specified]. 2 The NH3 sensitivity of the second segment was 98.12%; the sensitivity of the second segment was 0.008 nm / ppm, R 2 The detection limit (LOD) of NH3 was 97.72%. Calculations showed that the LODs of NH3 were 0.02 ppm and 0.05 ppm, respectively. This is because NH3 molecules adsorb onto the Sn-MOF film in the fiber optic sensing region, resulting in charge transfer and a change in refractive index, which in turn shifts the wavelength of the transmission spectrum's trough. Simultaneously, as the NH3 concentration increases, the number of metal active sites in the Sn-MOF decreases, leading to a decrease in sensitivity. Compared to most existing NH3 sensors, this invention not only improves sensitivity but also enables the detection of low concentrations of NH3. The detection limit (LOD) of NH3 can be expressed as: LOD=R / S In the formula: R is the resolution of the sensor; S is the sensitivity for detecting NH3.

[0041] In this embodiment, the sensing of NH3 gas at room temperature is realized.

[0042] Example 2 The response time and recovery time of a gas sensor directly determine whether the sensor can meet the requirements of "rapid gas identification, timely alarm cancellation, and repeated accurate detection" in practical applications. The response time is defined as the time required for the sensor to reach 90% of its maximum value after being exposed to the target gas, while the recovery time is defined as the time required for the sensor signal to recover to 90% of its baseline after the gas is removed. Figure 7 In Figure (c), the response and recovery times of the sensor at room temperature with 2.5 ppm NH3 are shown. Injecting 2.5 ppm NH3 into the gas cell and then blowing it out after spectral stabilization yielded response and recovery times of 9 s and 12 s, respectively. The excellent response and recovery times are primarily attributed to the loose porous structure and stacked three-dimensional network structure of the Sn-MOF, which generates a sufficiently large specific surface area to carry a large number of NH3 molecules. Simultaneously, the abundant Sn metal active sites increase the potential for ammonia adsorption.

[0043] Example 3 The most important performance characteristic of high-performance NH3 sensors is specificity. Figure 7 In Figure (d), the specificity of the sensor for NH3 at room temperature is demonstrated. The sensor's specificity was evaluated by exposing it to different interfering gases, including 16% ethanol (C2H6O), 99.9% carbon dioxide (CO2), 99.9% oxygen (O2), 99.9% nitrogen (N2), and 13% acetone (C3H6O). The average trough offsets were approximately -0.24 nm, -0.28 nm, 0.07 nm, 0.02 nm, and 0.17 nm, respectively, while the trough offset for 0.005% NH3 reached 0.61 nm. The trough offset was higher than that for the interfering gases despite the much lower NH3 concentration, indicating good specificity of the sensor for NH3. This is mainly attributed to the abundant Sn metal active sites and functionalized ligand (trimethylammonium benzoate) in the prepared Sn-MOF material, which can specifically adsorb NH3 while remaining insensitive to other gases.

[0044] Example 4 The impact of changes in the external environment on sensors cannot be ignored, as it determines the effectiveness of sensors in practical applications. Therefore, this invention further investigates the effects of temperature and humidity on sensor performance. To ensure the accuracy of the experimental data, each experiment was repeated three times, and the average value was taken. Figure 7 In the figure, (e) and (f) show the transmission spectra of 2.5 ppm NH3 at different temperatures and the linear fitting. It can be seen that as the ambient temperature increases from 24 ℃ to 78 ℃, the trough shifts to longer wavelengths by approximately 1.2 nm. The temperature sensitivity is obtained as 0.023 nm / ℃ through linear fitting, and R0... 2The accuracy was 99.17%. The results indicate that temperature has a certain impact on the sensor; therefore, the experiment should be conducted at the same temperature (room temperature) to eliminate temperature interference. Furthermore, the temperature sensitivity increased after coating the Sn-MOF film compared to before coating, because Sn-MOF expands with increasing temperature. Figure 7 In the table, (g) and (h) represent the transmission spectra of 2.5 ppm NH3 at different relative humidities at room temperature and the linear fit. It can be seen that as the relative humidity increases from 37% to 80%, the trough wavelength blue shifts by approximately 0.06 nm and the trough intensity increases by approximately 1 dB. Linear fitting yields humidity sensitivities of -0.0015 nm / % and 0.023 dB / % for the trough wavelength and intensity, respectively; R 2 The values ​​were 94.87% and 99.72%, respectively. The experimental results indicate that humidity has a relatively small impact on the sensor's response to NH3 detection, thus the sensor can be used to detect NH3 concentration in environments with high relative humidity (90%). The reason high humidity has a certain negative impact on the sensor's adsorption capacity is that water molecules competitively occupy the metal active sites on the sensor's thin film surface, thereby reducing its ability to adsorb NH3. It is worth noting that water molecules are weaker electron donors compared to NH3 molecules, therefore the influence of water molecules is negligible.

[0045] In summary, this invention innovatively combines Sn-MOF with an MZI-based fiber misalignment structure, resulting in a sensor with high sensitivity, fast response, and high specificity for NH3, which can be used for non-invasive screening of chronic kidney diseases. Furthermore, this invention is of great significance for the development of high-performance, cost-effective NH3 sensors.

[0046] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification, characterized in that: Including a fiber optic sensing unit based on Mach-Zehnder interferometry and a Sn-MOF gas-sensitive coating applied to the fiber optic sensing unit; The fiber optic sensing unit is formed by sequentially fusing single-mode fiber, coreless fiber, and photonic crystal fiber to form an SMF-NCF-PCF-NCF-SMF structure, wherein the PCF and the two NCFs on both sides are staggered in the x-direction to form an asymmetric staggered fusion structure. The Sn-MOF gas-sensitive coating is uniformly coated on the sensing area composed of the PCF and the NCF on both sides by dip-coating method. The Sn-MOF is prepared by solvothermal method, and the raw materials are tin tetrachloride pentahydrate and trimesic acid. The Sn-MOF has a porous structure that matches the diameter of NH3 molecules and abundant Sn metal active sites, which are used for the specific adsorption of NH3 molecules.

2. The high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification according to claim 1, characterized in that: The PCF and the left NCF are staggered and fused together in the x-direction with a center distance of 45 μm, and the PCF and the right NCF are staggered and fused together in the x-direction with a center distance of 87 μm.

3. The high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification according to claim 2, characterized in that: In the asymmetric misaligned fusion spliced ​​fiber sensing unit, the length of the NCF is 1 mm and the length of the PCF is 2934.95 μm.

4. The high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification according to claim 2, characterized in that: The PCF is a double-layer grapefruit-shaped structure with the following parameters: germanium-doped core diameter is 2.8±1.0 μm, cladding diameter is 125±1.0 μm, inner layer macropore diameter is 7.3±1.0 μm, inner layer micropore spacing is 8.2±1.0 μm, outer layer micropore diameter is 3.7±1.0 μm, and outer layer micropore spacing is 5.0±1.0 μm.

5. A method for fabricating a high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification, characterized in that: Includes the following steps: S1. Preparation of Sn-MOF by solvothermal method; S2. Apply a Sn-MOF gas-sensitive coating to the outside of the fiber optic sensing unit by dip-coating method; S3, Asymmetric misaligned fusion spliced ​​fiber optic sensing unit.

6. The method for fabricating a high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification according to claim 5, characterized in that: The solvothermal preparation process of Sn-MOF specifically includes: Dissolve 1.052 g of tin tetrachloride pentahydrate in 20 mL of methanol and stir until completely dissolved to form solution A; dissolve 1 g of trimesic acid in 20 mL of methanol and stir until completely dissolved to form solution B; Pour solution A into solution B, mix, and then stir continuously on a mixer for 6 hours to obtain mixed solution C; The mixed solution C was transferred to a 50 mL polytetrafluoroethylene-lined high-pressure reactor and subjected to a hydrothermal reaction at 180 °C for 24 h. After the reaction was completed and cooled to room temperature, the reaction product was washed with ethanol and then centrifuged at 8000 rpm for 10 min. The washing-centrifugation operation was repeated 3 times, and the white solid collected was Sn-MOF.

7. The method for fabricating a high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification according to claim 5, characterized in that: The specific process parameters for the dip-coating method are as follows: The pulling speed is 0.1-10 mm / s. After coating, the fiber sensing unit is placed in a 60 ℃ oven to dry for 1 h to enhance the adhesion between the Sn-MOF gas-sensitive coating and the fiber surface.

8. The method for fabricating a high-sensitivity fiber optic MZI ammonia sensor based on Sn-MOF modification according to claim 5, characterized in that: The asymmetric misaligned fusion spliced ​​fiber optic sensing unit includes the following steps: Pre-fusion splicing: Using a fusion splicer, the SMF and NCF are precisely connected after end-face cleaning. The cleaning discharge power is set to 13.2mA, the cleaning discharge time to 60ms, the fiber pre-fusion power to 10.8mA, the fiber pre-fusion time to 10ms, the main discharge power to 18mA, the main discharge time to 100ms, the re-discharge power to 12.7mA, and the re-discharge time to 700ms, to achieve low-loss fusion splicing of SMF-NCF. Real-time monitoring of welding: Connect the pre-welded SMF-NCF to the PCF, turn on the attenuation welding, control the misalignment distance through the micron-level displacement platform of the welding machine, and at the same time turn on the CCD imaging system to monitor the welding interface. The main discharge power is changed to 14.5 mA and the main discharge time is 1250 ms to prevent the air hole of the PCF from collapsing. Precise positioning and welding: Open the manual welding mode and connect the SMF-NCF-PCF to another SMF-NCF. Connect the spectrometer to monitor the transmission loss, interference fringe contrast and free spectral range in the 1550 nm band in real time. When the x-direction offset is 87 μm, the y-direction offset is 0 μm and the angle θ is aligned, the FSR is stable at 1.84 nm and the interference fringe contrast is 25 dB, and the welding is completed.

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