Ultralow detection limit MZI iron ion sensor based on three-dimensional network structure hydrogel coating and detection system

Abstract

The invention discloses a poly (acrylamide-acrylic acid) / sodium alginate (P (AAm-co-AAc) / SA) three-dimensional network structure hydrogel coated MZI iron ion sensor with ultralow detection limit and a detection system. The MZI iron ion sensor is composed of a broadband light source, a single-mode optical fiber, a flow cell, an MZI interferometer and a spectrograph. A broadband light source emits light, the light is input into a hydrogel-coated MZI interferometer based on dislocation welding, and an output interference spectrum is monitored and recorded in real time by a spectrometer. After the hydrogel adsorbs the iron ions, the refractive index and the volume of the hydrogel can be changed, so that the intensity or the wavelength of an interference spectrum is changed, and rapid, accurate and trace detection of the iron ions in the water environment is finally realized by establishing a relationship between different iron ion concentrations and interference spectrum drifting. The sensor has the remarkable advantages of low cost, ultralow detection limit, wide detection range, fast response and repeatability in detection, and has a wide application prospect in the fields of future environmental water quality monitoring, industrial production, high-end manufacturing and the like.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic biochemical sensing, specifically relating to an iron ion sensor and detection system based on an ultra-low detection limit Mach-Zehnder interferometer (MZI) coated with a three-dimensional network hydrogel of poly(acrylamide-acrylic acid) / sodium alginate (P(AAm-co-AAc) / SA). Background Technology

[0002] Iron ions are one of the key trace elements for maintaining ecosystem balance and normal physiological activities of organisms. In environmental monitoring, the concentration of iron ions in aqueous solutions is a core indicator for assessing water quality safety and ecological stability, and is of great significance for drinking water purification, water environment management, and aquaculture. In industrial production, the detection of iron ions in aqueous solutions involves multiple stages, including raw material quality control, process monitoring, and wastewater discharge; its concentration directly affects product quality, equipment lifespan, and environmental safety. In high-end manufacturing, iron ions are considered a critical harmful impurity in aqueous solutions. As a conductive metal ion, even trace amounts of iron ions can have a fatal impact on the performance and defect rate of precision components.

[0003] Currently, several mature methods for detecting iron ions exist, including spectrophotometry, fluorescence probe methods, inductively coupled plasma mass spectrometry (ICP-MS), and atomic absorption spectrometry. Spectrophotometry achieves quantitative analysis by measuring the absorbance of colored complexes formed by iron ions and chromogenic reagents; however, this method has a high detection limit and is susceptible to interference from coexisting ions. Fluorescence probe methods detect iron ions by detecting changes in fluorescence intensity caused by the specific binding of a fluorescent probe to the iron ion; however, the probe is easily affected by external environmental factors. ICP-MS ionizes the sample and achieves qualitative and quantitative detection of the element based on the mass-to-charge ratio of the target isotope; however, this method is expensive, complex to operate, and susceptible to mass spectrometry interference. Atomic absorption spectrometry quantitatively analyzes iron ions by measuring the absorbance at a specific wavelength; however, this method is complex, costly, and typically only feasible in specialized laboratories.

[0004] Fiber optic sensors are widely used in biochemical sensing and detection fields due to their advantages such as simple structure, high sensitivity, resistance to electromagnetic interference, ease of multiplexing, and real-time remote monitoring. Their working principle is to sensitively sense and respond to changes in the external environment by monitoring changes in light wave parameters (intensity, wavelength, etc.). The Mach-Zehnder interferometer is a refractive index-based fiber optic sensor with high sensitivity and fast response speed. It easily and sensitively captures changes in the refractive index of the environment surrounding the detection fiber, making it highly suitable for the detection of trace iron ions.

[0005] Hydrogels are polymeric gel network systems with a three-dimensional network structure and water as the dispersion medium. Hydrogels exhibit excellent swelling properties, superior biocompatibility, and good mass transport and permeability. The porous network structure and unique swelling characteristics of three-dimensional hydrogels make it easier for iron ions to be adsorbed by the active functional groups in the hydrogel network, thereby increasing the ion adsorption rate. These properties give them many unique advantages in detecting iron ions in aqueous solutions.

[0006] Researchers have conducted extensive and in-depth studies on iron ion sensors and detection systems: [ZL Bian, Q. Xu, FH Chu, et al. Fe] 3+ Sensing Based on HydrogelOptical Fiber Doped with Nitrogen Carbon Dots[J]. Journal of Electronic Materials, 2024, 53(2): 1094-1104.】This method is based on the fluorescence quenching effect. By doping a fluorescent indicator into a hydrogel, the concentration of iron ions is quantitatively detected by utilizing changes in fluorescence intensity. However, this sensor has problems such as a high detection limit (0.802 μM), a narrow detection range (0-60 μM), and insufficient validation with actual samples. Comparison document: Technology 2 [YQ Zhou, XY Huang, BY Xia, et al. An ultrasensitive optical fiber SPRsensor enhanced by functionalized carbon quantum dots for Fe 3+ [Measurement[J].Journal of Alloys and Compounds, 2025: 180784.] This method proposes a fiber optic surface plasmon resonance (SPR) sensor for the detection of iron ions. The fiber structure consists of three segments: a multimode fiber, a coreless fiber, and another multimode fiber. A 60nm gold film is deposited on the fiber surface to excite the SPR effect. Organically functionalized carbon quantum dots are coated on the gold film surface to capture iron ions, causing a change in the effective refractive index of the fiber surface. Quantitative detection of iron ions is achieved by monitoring the wavelength shift of the SPR resonance peak. However, this sensor requires gold film deposition and organically functionalized carbon quantum dot coating, making the process relatively complex. Furthermore, the sensor has a high detection limit (0.288 pM) and a long response time for iron ion detection (approximately 30 minutes), limiting its application in rapid detection scenarios such as environmental water quality monitoring, industrial production, and high-end manufacturing. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention proposes an ultra-low detection limit MZI iron ion sensor and detection system based on a three-dimensional network hydrogel coating of poly(acrylamide-acrylic acid) / sodium alginate (P(AAm-co-AAc) / SA). The mechanism by which the P(AAm-co-AAc) / SA hydrogel enables rapid and accurate detection of iron ions is as follows: the sodium alginate (SA) molecular chain is rich in hydroxyl (-OH) and carboxyl (-COOH) groups, and iron ions, as trivalent cations, can form stable chelates with multiple oxygen atoms; the carboxyl groups contained in acrylic acid (AAc) can provide more iron ion binding sites, accelerating the response speed; although the amide groups (-CONH2) of acrylamide (AAm) do not directly chelate iron ions, they can stabilize the "carboxyl / hydroxyl-iron ion" complex through hydrogen bonding, further enhancing the binding ability. Acrylamide (AAm) and acrylic acid (AAc) form a loose three-dimensional network through copolymerization. The introduction of sodium alginate (SA) further increases the hydrophilicity of the three-dimensional network structure. The formed "water channels" allow iron ions to rapidly diffuse into the interior of the hydrogel and bind to deep coordination sites, rather than reacting only on the surface. This invention provides an iron ion sensor and detection system with an ultra-low detection limit by combining a P(AAm-co-AAc) / SA three-dimensional network hydrogel with a misaligned MZI fiber optic sensor, featuring low cost, ultra-low detection limit (0.1 fM), and wide detection range (10). -8 mol / L-10 -16 The significant advantages of mol / L and fast response (10 min).

[0008] This sensor is implemented through the following technical solution:

[0009] S1. Fabrication of a three-segment misaligned MZI interferometer (4);

[0010] S2. Preparation of P(AAm-co-AAc) / SA three-dimensional network structure hydrogel (42) and sensor functionalization process;

[0011] S3. Construction of ultra-low detection limit MZI iron ion sensor and detection system;

[0012] S4. Actual performance testing of iron ion sensors (gradient concentration, time response, repeatability, specificity, and real sample testing).

[0013] In step S1, the misaligned MZI is formed by vertically misaligned splicing of single-mode fiber (41), single-mode fiber (43), and single-mode fiber (44) in sequence. The single-mode fiber (43) serves as the middle misaligned splicing part with a length of 3cm and a relative vertical displacement of 4~5μm.

[0014] In step S2, the P(AAm-co-AAc) / SA three-dimensional network hydrogel (42) is obtained by covalent crosslinking of poly(acrylamide-acrylic acid) / sodium alginate, and is prepared by evaporation-induced self-assembly and coated on the surface of the MZI interferometer (4).

[0015] The experimental platform setup in step S3 mainly includes the connection of a broadband light source (1), a single-mode fiber (2), an MZI interferometer (4), and a spectrometer (5).

[0016] Fe in step S4 3+ The concentration detected was 10. -8 mol / L-10 -16 mol / L.

[0017] Fe in step S4 3+ The detection time is 30 minutes.

[0018] In step S4, the repeatability test is selected to be repeated 3 times.

[0019] Fe in step S4 3+ The eluent was ethylenediaminetetraacetic acid (EDTA) solution.

[0020] The heavy metal ions included in the specific detection in step S4 are: Fe 3+ Na + K + Li + Cu 2+ Co 2+ Sn 2 + 、Tb 3+ .

[0021] In step S4, all test solutions were kept at pH 6.0 using phosphate-PBS buffer.

[0022] The working principle of the misaligned MZI iron ion concentration detection sensor and detection system based on poly(acrylamide-acrylic acid) / sodium alginate (P(AAm-co-AAc) / SA) three-dimensional network structure hydrogel is as follows: When the incident light from the broadband light source (1) is transmitted through a single-mode optical fiber (2) and then incident on the three misaligned MZI structure (4). When incident light from single-mode fiber (41) is incident on single-mode fiber (43), the core mismatch occurs due to the vertical position shift at the connection point. Part of the incident light in single-mode fiber (41) is coupled into the cladding of single-mode fiber (43) and continues to propagate, exciting the cladding mode optical field in single-mode fiber (43). Simultaneously, another part of the light that does not enter the cladding continues to propagate forward into the core of single-mode fiber (43). When the incident light finishes propagating in the cladding and core of single-mode fiber (43), it is recoupled into the core of single-mode fiber (44) at the second misalignment point for propagation. Therefore, a misaligned MZI interferometer is realized. The propagation conditions of the incident light in the cladding and core of single-mode fiber (43) are different. The optical field energy propagated in the cladding is easily affected by the external hydrogel environment, thus the phase difference between the core mode and the cladding mode changes accordingly, which can be expressed as:

[0023] (1)

[0024] In the formula, and These represent the effective refractive indices of the core and cladding, respectively. Let L be the wavelength of the input light and L be the effective length of the single-mode fiber (43). The interference light intensity after re-coupled into the single-mode fiber (44) can be expressed as:

[0025] (2)

[0026] In the formula, and These represent the light field intensities propagating along the fiber core and cladding, respectively. The phase difference between the two beams of light is denoted as α. The three-dimensional network structure hydrogel (42) has swelling properties. The hydroxyl and carboxyl groups rich in sodium alginate molecular chains form a stable chelate structure with iron ions, thereby changing the refractive index and volume of the hydrogel, which in turn causes a change in the phase of the cladding propagation light. As can be seen from equations (1) and (2), the interference spectrum will also change accordingly. By monitoring the changes in wavelength or intensity in the optical fiber interference spectrum, the detection of iron ions can be finally achieved.

[0027] This invention fully utilizes the three-dimensional network structure of poly(acrylamide-acrylic acid) / sodium alginate (P(AAm-co-AAc) / SA) hydrogel to inhibit Fe 3+With excellent adsorption capacity and high sensitivity based on low-cost misaligned MZI, the sensor ultimately passed testing and demonstrated an ultra-low detection limit (0.1 fM) and a wide detection range (10). -8 mol / L-10 -16 (mol / L), with a response time of 10 min, and was ultimately applied to Fe in a real aquatic environment. 3+ The sensor exhibits good recovery rate and accurate and rapid response to iron ions, a wide detection range and low detection limit, as well as good repeatability and specificity, making it promising for a wide range of applications. Attached Figure Description

[0028] Figure 1 An ultra-low detection limit MZI iron ion sensor and detection system based on poly(acrylamide-acrylic acid) / sodium alginate (P(AAm-co-AAc) / SA) three-dimensional network hydrogel coating.

[0029] Figure 2 Sensor at 10 -8 mol / L-10 -16 Interference spectra and fitting curves of interference peak intensity changes within the mol / L iron ion concentration range.

[0030] Figure 3 Time response graph of iron ion sensor.

[0031] Figure 4 Figures from three repeatability tests of the iron ion sensor.

[0032] Figure 5 Experimental diagram illustrating the specificity of the iron ion sensor.

[0033] Figure 6 Figures showing the detection and recovery of iron ions in pure water and natural water samples by the sensor. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to specific embodiments:

[0035] Example 1: Fabrication of a misaligned MZI interferometer (4)

[0036] In this embodiment, an MZI interferometer (4) is fabricated using a three-segment single-mode fiber misaligned fusion splicing method. The specific fabrication process is as follows: Take out single-mode fiber (41) and single-mode fiber (44), peel off the coating layer of about 2cm, then wipe the peeled area with anhydrous ethanol and cut the end face. Take out single-mode fiber (43), peel off the coating layer of about 5cm and wipe it, and after cutting both ends, 3cm remains. Use the manual mode of the fiber fusion splicer to manually adjust the relative vertical displacement between the fibers to complete the fabrication of the misaligned structure. Place them in the groove of the fusion splicer in sequence and manually misalign them for fusion splicing. The three segments of single-mode fiber form a misaligned structure. The vertical relative displacement of single-mode fiber (41) and single-mode fiber (44) relative to single-mode fiber (43) is 4~5μm. At the same time, positioning tape needs to be pasted during the fusion splicing process.

[0037] Example 2: Preparation of P(AAm-co-AAc) / SA three-dimensional network hydrogel (42) and sensor functionalization process

[0038] In this embodiment, the preparation of the functionalized P(AAm-co-AAc) / SA three-dimensional network hydrogel (42) and the specific process of sensor functionalization are as follows: 0.5g acrylamide (AAm), 0.05g acrylic acid (AAc), 10mg methylenebisacrylamide (MBA), and 25mg potassium persulfate (KPS) solid powder were added to a beaker containing 10ml of deionized water. After stirring and dissolving thoroughly with a magnetic stirrer at 30°C for 1 hour, the solution was treated with ultrasonic oscillation to remove air bubbles, resulting in a uniform P(AAm-co-AAc) composite solution. The monomers acrylamide (AAm) and acrylic acid (AAc), together with the crosslinking agent methylenebisacrylamide (MBA), were used to prepare a ternary copolymerized P(AAm-co-AAc) network through free radical polymerization under the action of the initiator potassium persulfate (KPS). The misaligned MZI interferometer (4) was placed in a piranha solution for 30 minutes to remove all impurities from the surface. Then it was cleaned with deionized water and anhydrous ethanol and dried under a nitrogen atmosphere. The dried misaligned MZI interferometer (4) was immersed in a P(AAm-co-AAc) solution and left to stand for 5 minutes to allow it to adhere to the surface of the sensing area. After being removed, it was dried in a drying oven at 60°C for 1 hour to form a film. Then, 0.2g of sodium alginate (SA) was added to a beaker containing 10ml of deionized water and stirred thoroughly with a magnetic stirrer at 45°C for 1 hour to dissolve the film. After that, the air bubbles in the solution were removed by ultrasonic oscillation to obtain a uniform sodium alginate (SA) solution with a mass fraction of 2%. The coated sensing area was immersed in the sodium alginate (SA) solution and left to stand for 10 minutes. The modified misaligned MZI interferometer (4) was rinsed with deionized water and dried under a nitrogen atmosphere.

[0039] Example 3: Construction of Ultra-Low Detection Limit MZI Iron Ion Sensor and Detection System

[0040] In this embodiment, the sensor and detection system are built as follows: A broadband light source (1) with a wavelength of 1500nm-1620nm is selected and connected to one end of a misaligned MZI interferometer (4) via a single-mode fiber (2). The misaligned MZI interferometer (4) is fixed in a flow cell (3) and its other end is connected to a spectrometer (5). The broadband light source (1) provides a light source for the experiment. The flow cell (3) is used to add the test solution and eluent. The spectrometer (5) is used to monitor and record the real-time changes in the spectrum. After the functionalized hydrogel (42) modified on the surface of the MZI interferometer (4) adsorbs iron ions, the refractive index and volume change, which in turn affects the wavelength or intensity in the fiber interference spectrum. The spectrometer (5) monitors, records and analyzes the changes in real time to realize the detection of the concentration of iron ions in the water environment.

[0041] Example 4: Gradient concentration, time response, repeatability, specificity, and real sample detection

[0042] Conduct Fe 3+ Solution gradient concentration, time response, repeatability, specificity, and real sample detection were performed. All solutions were maintained at pH 6.0 using phosphate-PBS buffer. After each experiment, ethylenediaminetetraacetic acid (EDTA) was added to the flow cell (3), and the MZI interferometer (4) was immersed in the EDTA solution for 10 minutes for elution, followed by multiple rinses with phosphate-PBS buffer. The specific steps were as follows:

[0043] (a)Fe 3+ Concentration detection: Different concentrations of Fe were prepared using phosphate-buffered saline (PBS) buffer solution at pH 6.0. 3+ Solution, prepared to a concentration of 10 -16 mol / L, 10 -15 mol / L, 10 -14 mol / L, 10 -13 mol / L, 10 -12 mol / L, 10 -11 mol / L, 10 - 10 mol / L, 10 -9 mol / L, 10 -8 mol / L. The modified functionalized P(AAm-co-AAc) / SA three-dimensional network hydrogel (42) was rinsed multiple times with deionized water and anhydrous ethanol and then placed in a flow cell (3). Phosphate-PBS buffer was added to the flow cell (3), and the initial spectrum was recorded using a spectrometer (5). Different concentrations of Fe 3+ The solution was added to the flow cell (3), and the interference spectrum was then recorded using a spectrometer (5). Different concentrations of Fe were added sequentially in ascending order of concentration. 3+Solution, repeat the above steps to obtain Fe solutions of different concentrations. 3+ The interference spectrum of the solution is used to plot a fitted curve of the intensity change. For example... Figure 2 As shown, this iron ion sensor and detection system has an ultra-low detection limit (the lowest actual detection concentration is 0.1 fM) and a wide detection range (10). -8 mol / L-10 -16 mol / L).

[0044] (b) Time response test: Phosphate-PBS buffer was added to the flow cell (3), and the initial spectrum was recorded using a spectrometer (5). Subsequently, 10 ppm of PBS buffer was added to the flow cell (3). -12 Fe concentration of mol / L 3+ The solution was tested and the spectral response was recorded for 30 minutes using a spectrometer (5). The interference spectrum was recorded once per minute for the first 10 minutes, and then at 15, 20, 25 and 30 minutes. The time response of the sensor was evaluated by the drift of the interference spectrum. Figure 3 As shown, the time response of the iron ion sensor and detection system is 10 minutes.

[0045] (c) Repeatability test: Phosphate-PBS buffer was added to the flow cell (3), and the initial spectrum was recorded using a spectrometer (5). Subsequently, 10 ppm of PBS buffer was added to the flow cell (3). -12 Fe concentration of mol / L 3+ The solution was prepared, and the spectral response was detected and recorded for 30 minutes using a spectrometer (5). EDTA was added to the flow cell (3), and the optical fiber was immersed in the EDTA solution for 10 minutes for elution. After elution, the eluted spectrum was recorded using a spectrometer (5). Then, the MZI interferometer (4) was rinsed multiple times with phosphate-PBS buffer and the spectrum was recorded. The above steps were repeated to obtain three sets of interference spectral intensity response changes over time after repeated addition and elution. Figure 4 As shown, the iron ion sensor and detection system have good repeatability.

[0046] (d) Specificity test: 10 μL of phosphate-buffered saline (PBS) buffer solution at pH 6.0 was used to prepare 10 μL of PBS buffer solution. -12 Fe concentration of mol / L 3+ Na + K + Li + Cu 2+ Co 2+ Sn 2+ 、Tb 3+Solution. Phosphate-PBS buffer was added to the flow cell (3), and the initial spectrum was recorded using a spectrometer (5). The test solutions of different ions were added to the flow cell (3), and the interference spectra were then recorded using a spectrometer (5) to obtain the specific selectivity intensity changes of the MZI interferometer (4) for different ions. For example... Figure 5 As shown, the iron ion sensor and detection system have good specificity.

[0047] (e) Real Sample Testing: The phosphate-PBS buffer solution with pH 6.0 in the above experiment was prepared using pure water and natural water, and the solutions were prepared with concentrations of 10... -14 mol / L, 10 -12 mol / L, 10 -10 mol / L Fe 3+ Solution. First, the initial spectrum was recorded using a spectrometer (5), and then 10 solutions were added sequentially to the flow cell (3) in order of increasing concentration. -14 mol / L, 10 -12 mol / L, 10 -10 mol / L Fe 3+ Solutions were obtained to obtain different concentrations of Fe. 3+ Interference spectra of solutions, plotting Fe in pure water and tap water 3+ A comparison graph showing the measured concentration versus the actual added concentration. (Example) Figure 6 As shown, the iron ion sensor and detection system have high recovery rates in both pure water and natural water samples.

[0048] The embodiments described above provide a detailed explanation of the technical solution of the present invention, but the scope of protection of this disclosure is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of this disclosure, and all such changes and modifications will fall within the protection scope of the present invention.

Claims

1. An ultra-low detection limit MZI iron ion sensor and detection system based on a three-dimensional network hydrogel coating of poly(acrylamide-acrylic acid) / sodium alginate (P(AAm-co-AAc) / SA), comprising a broadband light source (1), a single-mode optical fiber (2), a flow cell (3), an MZI interferometer (4), and a spectrometer (5), characterized in that: A broadband light source (1) is connected to one end of an MZI interferometer (4) via a single-mode fiber (2). The MZI interferometer (4) is fixed in a flow cell (3), and its other end is connected to a spectrometer (5). The working wavelength of the broadband light source (1) is 1500nm-1620nm, which is used to provide light. The spectrometer (5) is used to monitor and record the spectral changes of the MZI interferometer. The MZI interferometer (4) is composed of single-mode fibers (41), (43), and (44) spliced ​​in a staggered manner. The length of the single-mode fiber (43) as the middle part of the staggered splice is 3cm, and the vertical displacement is 4~5μm. The surface of the MZI interferometer (4) is coated with poly(acrylamide-acrylic acid) / sodium alginate (P(AAm-co)). -AAc) / SA) hydrogel (42) is used to bind ferric ions in the test solution; the MZI interferometer (4) is fixed in the flow cell (3), and by adding different concentrations of the test iron ion solution, the light emitted by the broadband light source (1) passes through the MZI interferometer (4) and is detected by the spectrometer (5), and multiple characteristic wavelength interference peaks are shown on the transmission spectrum curve; when the hydrogel (42) modified on the surface of the MZI interferometer (4) binds iron ions, the volume and refractive index of the hydrogel on the fiber surface will change, which will then be reflected in the spectrometer (5) as a change in the intensity or wavelength of the interference spectrum. By establishing the relationship between different iron ion concentrations and the intensity or wavelength shift of the interference spectrum, the concentration of iron ions in the water environment can be detected.

2. The preparation process of the misaligned MZI interferometer (4) according to claim 1 is as follows: Take out the single-mode fiber (41) and the single-mode fiber (44), peel off the coating layer with a length of 2cm, then wipe the peeled area with anhydrous ethanol and cut the end face; then take out the single-mode fiber (43), peel off the coating layer with a length of 5cm and wipe it, and cut the ends to leave 3cm; use the manual mode of the fusion splicer to manually adjust the relative vertical position between the fibers to complete the preparation of the misaligned structure; put them into the groove of the fusion splicer in sequence and manually misalign them for fusion splicing, and the three single-mode fibers form a misaligned structure. The vertical relative displacement of the single-mode fiber (41) and the single-mode fiber (44) relative to the single-mode fiber (43) is 4~5μm. At the same time, positioning tape needs to be pasted during the fusion splicing process.

3. The preparation and sensor functionalization process of the P(AAm-co-AAc) / SA three-dimensional network structure hydrogel (42) according to claim 1 is as follows: 0.5g acrylamide (AAm), 0.05g acrylic acid (AAc), 10mg methylenebisacrylamide (MBA), and 25mg potassium persulfate (KPS) solid powder are added to a beaker containing 10ml deionized water. After stirring and dissolving thoroughly with a magnetic stirrer at 30°C for 1 hour, ultrasonic oscillation is used to remove bubbles in the solution, resulting in a uniform P(AAm-co-AAc) composite solution. The monomers acrylamide (AAm) and acrylic acid (AAc), together with the crosslinking agent methylenebisacrylamide (MBA), are used to prepare the ternary copolymer P(AAm-co-AAc) through free radical polymerization under the action of the initiator potassium persulfate (KPS). Network; The misaligned MZI interferometer (4) was placed in the piranha solution for 30 minutes to remove all impurities on the surface, then cleaned with deionized water and anhydrous ethanol, and dried under a nitrogen atmosphere; the dried misaligned MZI interferometer (4) was immersed in P(AAm-co-AAc) solution and left to stand for 5 minutes to adhere to the surface of the sensing area, and then dried in a drying oven at 60°C for 1 hour to form a film; 0.2g of sodium alginate (SA) was added to a beaker containing 10ml of deionized water, and stirred and dissolved thoroughly with a magnetic stirrer at 45°C for 1 hour, and then ultrasonic oscillation was used to remove bubbles in the solution to obtain a uniform sodium alginate (SA) solution with a mass fraction of 2%, the coated sensing area was immersed in the sodium alginate (SA) solution and left to stand for 10 minutes, the modified misaligned MZI interferometer (4) was rinsed with deionized water and dried under a nitrogen atmosphere.

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