U-shaped micro-nano optical fiber chemical sensor based on super-mode interference and preparation method of U-shaped micro-nano optical fiber chemical sensor

Abstract

The invention discloses a U-shaped micro-nano optical fiber chemical sensor based on super-mode interference and a preparation method, and belongs to the technical field of optical fiber sensing, the U-shaped micro-nano optical fiber chemical sensor comprises a single-mode optical fiber I, a tapering four-core optical fiber and a single-mode optical fiber II which are sequentially and coaxially welded, the surface of the tapering four-core optical fiber is uniformly coated with a Ni < 2 + > chitosan polymer ion imprinting film, and the surface of the tapering four-core optical fiber is coated with a Ni < 2 + > chitosan polymer ion imprinting film. The single-mode fiber-tapering four-core fiber-single-mode fiber structure is formed by bending a tapering four-core fiber part into a U shape and then packaging the tapering four-core fiber part. The preparation method comprises the following steps: preparing a micro-nano optical fiber with a single-mode optical fiber-four-core optical fiber-single-mode optical fiber structure by using an optical fiber fusion splicer; performing oxyhydrogen flame heating fused tapering on the four-core optical fiber part in the SFS structure to form a taper area, and forming a single-mode optical fiber-tapering four-core optical fiber-single-mode optical fiber structure; a tapering four-core optical fiber area is uniformly coated with a layer of specific Ni < 2 + > chitosan polymer ion imprinting film at a low speed by utilizing an electric displacement platform, and low-concentration Ni < 2 + > in water can be selectively adsorbed.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber sensing, and particularly relates to a U-shaped micro-nano optical fiber chemical sensor based on supermode interference and a preparation method thereof. Background Art

[0002] With the continuous development of China's economy and the acceleration of the industrialization process, the discharge of heavy metal-containing wastewater has increased significantly, posing a serious threat to water quality safety. Heavy metals refer to metallic elements with a density greater than 5 g / cm³ and having a relatively high atomic number (usually greater than 20), mainly including biologically toxic elements such as nickel, chromium, mercury, lead, and the metalloid arsenic. Heavy metal ions have characteristics such as high toxicity, difficult degradation, and persistence, causing serious harm to the environment and human health, making the task of solving the current heavy metal pollution problem a top priority. Therefore, it is of great significance to detect heavy metal ions in real time and accurately. The World Health Organization has set a strict allowable exposure limit (1.19 μM) for nickel ions (Ni 2+ )

[0003] Currently, people use various technologies to detect the nickel concentration in the environment, including electrochemical detection, colorimetric detection, fluorescence probes, high-performance liquid chromatography (HPLC), and atomic absorption spectrometry (AAS). However, these methods usually require sample pretreatment, which is both expensive and complex, and lacks sufficient sensitivity for detecting low concentrations of Ni 2+ ions. Therefore, real-time and in-situ detection of trace metal ions in complex environments remains a major challenge.

[0004] Ion imprinting technology provides a promising solution by creating polymers with specific binding sites for Ni 2+ ions, thereby improving the selectivity and sensitivity of the sensor. This technology involves synthesizing polymers that mimic the size, shape, and function of the target ion, ensuring high affinity and selectivity. Optical fiber sensors have become an excellent choice due to their small size, low cost, easy integration, and resistance to harsh environments. Micro-nano optical fiber sensors based on ion imprinting technology not only improve the sensitivity of optical fiber sensors but also enhance their detection selectivity for Ni²⁺ by coating cross-linked chitosan polymer ion-imprinted films onto the optical fibers.

[0005] Micro-nano optical fiber sensors have become a research hotspot due to their miniaturized size, cost-effectiveness, environmental robustness, and compatibility with functional coatings. The combination of optical fibers and ion imprinting technology has brought new opportunities for the detection of heavy metal ions.

[0006] In 2022, researchers such as Zhen Yi from Nanchang Hangkong University proposed to use a single-mode fiber-coreless fiber-single-mode fiber (STNCS) structure to achieve highly sensitive and selective detection of Ni²⁺. However, they did not adopt a four-core fiber (STFS) and U-shaped design, utilize the supermode interference effect and enhanced evanescent field to enhance sensitivity, and did not precisely control the film thickness and uniformity and optimize the chitosan concentration to balance the film thickness and binding sites. For details, please refer to the literature "Research on Trace Detection Applications of Heavy Metal Ions Based on Micro-Nano Fiber Sensing". Summary of the Invention

[0007] The purpose of the present invention is to provide a U-shaped micro-nano fiber chemical sensor based on supermode interference and its preparation method, which can selectively detect the content of Ni in water 2+ and improve the detection sensitivity of Ni at low concentrations. 2+

[0008] To achieve the above purpose, the present invention provides a U-shaped micro-nano fiber chemical sensor based on supermode interference, which includes two coaxial single-mode fibers and a tapered four-core fiber in the center. The tapered four-core fiber includes a tapered region in the middle of the four-core fiber. The tapered region is formed by heating and melting the four-core fiber into a four-core micro-nano fiber. The chitosan polymer ion-imprinted film (Ni-II-CS) based on Ni ions is uniformly coated on the surface of the tapered four-core fiber. The coated STFS structure is bent into a U shape, and the U-shaped fiber sensor is encapsulated. 2+ 2+

[0009] Further, the diameter of the four-core fiber before tapering is 124.58 μm, containing 4 cores, each core having a diameter of 8.54 μm. The central core is located at the center of the fiber, and the outer layer contains 3 cores arranged in an equilateral triangle. The distance between the outer cores is 55.48 μm; the distance between the central core and the outer cores is 33.44 μm.

[0010] Further, the tapered four-core fiber is provided with 4 cores, each core having a diameter of 0.29 μm. The central core is located at the center of the tapered four-core fiber, and 3 cores are evenly arranged in its outer layer in an equilateral triangle. The distance between the outer cores is 1.89 μm, and the distance between the central core and the outer cores is 1.14 μm.

[0011] Further, the outer diameter of the single-mode fiber is 124.58 μm, and the core diameter is 8.54 μm.

[0012] Further, the diameter of the tapered region of the tapered four-core fiber is 4.25 μm.

[0013] Further, the bending diameter of the U-shaped micro-nano fiber sensor is 3 mm.​​​

[0014] The present invention also provides a preparation method of a U-shaped micro-nano fiber chemical sensor based on supermode interference, comprising the following steps: Step 1: First, use an optical fiber cutter to cut two sections of single-mode optical fiber and one section of four-core optical fiber respectively, and then use a fusion splicer to fuse and splice one section of single-mode optical fiber at both ends of the four-core optical fiber to form a single-mode fiber-four-core fiber-single-mode fiber (SFS) structure; Step 2: Use an optical fiber taper machine to melt and taper the four-core optical fiber until the diameter of the tapered four-core optical fiber is 4.25 μm, forming a single-mode fiber-tapered four-core fiber-single-mode fiber (STFS) structure; Step 3: Coat the surface of the tapered four-core optical fiber with a Ni 2+ chitosan polymer ion-imprinted film (Ni 2+ -II-CS); Step 4: Bend the micro-nano fiber sensor coated in Step 3 into a U shape, and then perform encapsulation, with a bending diameter of 3 mm.

[0015] Further, according to the Ni 2+ chitosan polymer ion-imprinted film (Ni 2+ -II-CS) described in Step 3, it is coated on the surface of the tapered four-core optical fiber according to the following steps: Step 3.1: Dissolve 400 mg of chitosan (CS) in 20 mL of 2% acetic acid solution (C2H4O2), and use a centrifugal stirrer to stir at a speed of 1500 rpm for 6 hours until completely dissolved to obtain a chitosan sol with a concentration of 20 mg / mL; Step 3.2: Add 36 mg of nickel chloride (NiCl2) to the chitosan sol and stir for 2 hours to form non-covalent bonds between Ni 2+ ions and the amino (-NH2) and hydroxyl (-OH) groups of chitosan molecules, thereby introducing template ions; Step 3.3: Add 2 mL of epichlorohydrin (ECH) and stir for 4 hours to promote the cross-linking reaction, forming a cross-linked Ni 2+ -CS solution of the template ions; Step 3.4: Immerse the cleaned STFS-structured micro-nano fiber sensor in the Ni 2+ -CS solution, and then use an electric displacement platform to move the solution at a speed of 0.25 mm / s to ensure the formation of a uniform cross-linked Ni 2+ -CS film on the surface of the tapered four-core optical fiber; Step 3.5: Air-dry the micro-nano fiber sensor coated with the cross-linked Ni 2+ -CS film at room temperature for 5 minutes, and then dry it in a constant-temperature oven at 65°C for 12 h to ensure complete curing of the film; Step 3.6: Immerse the micro-nano fiber sensor coated with Ni 2+ -CS film into 1 mM hydrochloric acid solution (HCl) to remove Ni 2+ in the -CS film 2+ , generating a Ni 2+ chitosan polymer ion-imprinted film (Ni 2+ -II-CS) to obtain an optical fiber sensor with an STFS structure.

[0016] Compared with the prior art, the beneficial technical effects of the present invention are as follows: The U-shaped micro-nano fiber chemical sensor based on supermode interference provided by the present invention adopts the low-speed uniform brushing method (0.25 mm / s) of an electric displacement platform to form a uniform and appropriate-thickness coating of cross-linked Ni²⁺-CS film on the surface of the tapered four-core optical fiber. Compared with the in-situ polymerization method of impregnation that is difficult to control the thickness and uniformity, the brushing method can better meet the expected requirements.

[0017] The U-shaped micro-nano fiber chemical sensor based on supermode interference provided by the present invention elutes Ni²⁺ in a 1 mM hydrochloric acid solution with a pH of about 3.5 to effectively remove metal ions in the polymer and form a Ni 2+ chitosan polymer ion-imprinted film with specific binding sites. This method fully considers the solubility change of chitosan under acidic conditions and ensures the performance stability of the sensor.

[0018] A U-shaped micro-nano fiber chemical sensor based on supermode interference and a preparation method provided by the present application use an optical fiber fusion mechanism to obtain a micro-nano fiber with a single-mode fiber-four-core fiber-single-mode fiber (SFS) structure; for the four-core fiber part in the SFS structure, oxyhydrogen flame heating and melting are carried out to form a tapered region, forming a single-mode fiber-tapered four-core fiber-single-mode fiber (STFS) structure; an electric displacement platform is used to uniformly coat a layer of specific Ni 2+ chitosan polymer ion-imprinted film (Ni 2+ -II-CS) on the tapered four-core fiber region, which can selectively adsorb low-concentration Ni 2+ in water. By introducing the ion-imprinting technology, the present invention combines the Ni 2+ -II-CS film with the micro-nano fiber sensor. Using the Ni 2+ -II-CS film with a stable cavity that matches the size, charge, and spatial structure of Ni 2+ , high-sensitivity detection and selective adsorption of Ni 2+ in water are achieved. The sensor has the characteristics of high sensitivity, simple preparation, and the ability to selectively adsorb Ni 2+ in water.

[0019] By using the ion imprinting technique, with the target heavy metal as the template, non-covalent bonds are formed through amino and hydroxyl groups, and cross-linking polymerization is carried out with ECH. Subsequently, Ni²⁺ is removed with hydrochloric acid to effectively remove Ni in the cross-linked Ni²⁺-CS film 2+ ions, forming a chitosan polymer ion imprinting (Ni 2+ -II-CS) film with specific binding sites for Ni²⁺ ions. In this way, specific recognition sites are formed in the chitosan cross-linked material, which can effectively adsorb Ni²⁺ in water, achieving precise removal of heavy metals, and having the advantages of strong specificity and high sensitivity. Description of the Drawings

[0020] Figure 1 The structure diagram of the STFS micro-nano fiber sensing system coated with the Ni 2+ chitosan polymer ion imprinting film provided by the present invention; Figure 2 The measurement diagram of the fiber waist diameter of the fiber bundle under the scanning electron microscope (SEM) after the multi-mode fiber is tapered provided by the present invention; Figure 3 The end face diagram of the untapered four-core fiber; Figure 4 The mode field distribution of 4 supermodes in the tapered four-core fiber; Figure 5 The packaged sensor; Figure 6 The spectrum shift curve of the U-shaped micro-nano fiber chemical sensor coated with the Ni 2+ chitosan polymer ion imprinting film provided by the present invention at 10 nM Ni 2+ ; Figure 7 The spectrum shift amount of the U-shaped micro-nano fiber chemical sensor for different concentrations of Ni 2+ and the corresponding linear fitting results; Figure 8 The curve of the wavelength drift of the interference spectrum corresponding to different concentrations of Ni 2+ changing with time; Among them, 1. Single-mode fiber 1, 2. Tapered four-core fiber, 3. Ni 2+ chitosan polymer ion imprinting film, 4. Single-mode fiber 2. Detailed Embodiment

[0021] The concept, specific structure and technical effects achieved by the present invention will be elaborated in detail below in conjunction with the drawings, so as to more deeply understand the objectives, features and advantages of the present invention.

[0022] Obviously, the embodiments described in the present invention are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention without creative efforts fall within the scope of protection of the present invention.

[0023] The present invention discloses a U-shaped micro-nano fiber chemical sensor based on supermode interference. As Figure 1 shown, it includes a single-mode fiber 1 - a tapered four-core fiber - a single-mode fiber 2 that are coaxially fused in sequence. The tapered four-core fiber 2 is formed by heating and melting and drawing. The tapered four-core fiber 2 can flexibly adjust the coupling strength between the cores, further adjust the interference effect of the supermode, and can also enhance the evanescent field, thereby achieving higher sensitivity. When Ni 2+ ions are captured by the functionalized fiber surface, this will cause a change in the interference beat length, resulting in a spectral shift.

[0024] The Ni 2+ ion-imprinted film of chitosan polymer Ni 2+ -II-CS is uniformly coated on the surface of the tapered four-core fiber 2. As Figure 5 shown, the coated micro-nano fiber sensor is bent into a U shape and then encapsulated. By bending the tapered four-core fiber into a U shape, the sensitivity can be further improved, and the bending diameter is 3 mm.

[0025] Preferably, as Figure 3 shown, the diameter of the four-core fiber before tapering is 124.58 μm, containing 4 cores, each core diameter is 8.54 μm. The central core is located at the center of the fiber, and the outer layer contains 3 cores arranged in an equilateral triangle. The distance between the outer cores is 55.48 μm, and the distance between the central core and the outer cores is 33.44 μm.

[0026] Preferably, the outer diameter of the single-mode fiber is 125 μm, and the core diameter is 9 μm; Preferably, the bending diameter of the U-shaped micro-nano fiber sensor is 3 mm; The present invention also provides a preparation method for a U-shaped micro-nano fiber chemical sensor based on supermode interference, which is as follows: Step 1: First, use an optical fiber cutter to precisely and smoothly cut the end faces of the single-mode fiber 1 and the single-mode fiber 4 respectively, then dock them with the left end face and the right end face of a section of four-core fiber with both ends ensured to be flat, and then use an optical fiber fusion splicer to fuse the two optical fiber end faces docked at both ends of the four-core fiber to form a single-mode fiber - four-core fiber - single-mode fiber SFS structure; Step 2: Heat and melt the four-core optical fiber of the SFS structure with the hydrogen gas generated by electrolyzing water to draw it into a tapered region until the diameter of the drawn four-core optical fiber is measured to be approximately 4.25 μm under a scanning electron microscope; as Figure 2 shown, it is the measurement diagram of the fiber waist diameter of the drawn four-core optical fiber under an electron microscope provided by the present invention; this fiber diameter can not only ensure that the optical field has sufficient evanescent fields to interact with the surrounding medium to change the interference between supermodes, but also ensure that the sensing optical fiber has a certain mechanical strength and will not break easily.

[0027] Step 3: Use an electric displacement platform to coat the surface of the drawn four-core optical fiber 2 with a chitosan polymer ion-imprinted film (Ni 2+ -II-CS) of Ni ions by the brushing method; 2+ -II-CS); Step 4: Bend the micro-nano optical fiber sensor coated in Step 3 into a U shape, and then perform encapsulation, with a bending diameter of 3 mm; Further, the chitosan polymer ion-imprinted film (Ni 2+ -II-CS) of Ni ions described in Step 3 is coated on the surface of the drawn four-core optical fiber 2 according to the following steps: 2+ -II-CS) is coated on the surface of the drawn four-core optical fiber 2 according to the following steps: Step 3.1: Dissolve 400 mg of chitosan CS in 20 mL of 2% acetic acid solution C2H4O2, and use a centrifugal stirrer to stir at a speed of 1500 rpm for 6 hours until completely dissolved to obtain a chitosan sol with a concentration of 20 mg / mL; Step 3.2: Add 36 mg of nickel chloride NiCl2 to the chitosan sol and stir for 2 hours to form non-covalent bonds between Ni 2+ ions and the amino -NH2 and hydroxyl -OH groups of chitosan molecules, and introduce the template ion Ni 2+ ; Step 3.3: Add 2 mL of epichlorohydrin ECH and stir for 4 hours to promote the cross-linking reaction and form a three-dimensional network structure polymer of the template ion; Step 3.4: Immerse the cleaned STFS structure drawn four-core optical fiber 2 into the cross-linked Ni 2+ -CS, and then use the electric displacement platform to move the optical fiber at a speed of 0.25 mm / s to ensure the formation of a uniform cross-linked Ni 2+ -CS film on the surface of the drawn four-core optical fiber 2; Place an electric displacement platform under the cleaned STFS structure drawn four-core optical fiber 2, and fix a container containing Ni 2+ -CS solution on the platform. The electric displacement platform moves from one end of the drawn four-core optical fiber to the other end at a speed of 0.25 mm / s. This coating process is only done once to ensure the formation of a uniform cross-linked Ni on the surface of the drawn four-core optical fiber.2+ -CS film; Step 3.5: Coating the cross-linked Ni 2+ -CS micro-nano fiber optic sensor was air-dried at room temperature for 5 minutes and then dried in a constant temperature oven at 65°C for 12 hours to ensure that the film was completely cured; Step 3.6: Coating with cross-linked Ni 2+ -CS film optical fiber sensor was immersed in hydrochloric acid solution HCl with a pH of about 3.5 to remove Ni 2+ Ni in CS films 2+ ions, generating Ni with specific binding sites 2+ Chitosan polymer ion imprinted film (Ni 2+ -II-CS).

[0028] The present invention provides two Ni 2+ The test method of ions is as follows: Method 1: Immerse the fiber optic sensor in 10nM Ni 2+ The changes in the spectrum with increasing immersion time were recorded in the solution.

[0029] like Figure 4 As shown in the figure, when the immersion time of Ni²⁺ solution is prolonged, the interference spectrum shows a red shift phenomenon. This phenomenon is due to the interaction between Ni²⁺ and Ni 2+ Chitosan polymer ion imprinted film (Ni 2+ The chelation of Ni²⁺-II-CS binding sites causes a change in the effective refractive index. Initially, within 30 seconds, the abundant binding sites on the film surface significantly enhance Ni²⁺ binding efficiency, leading to a dramatic refractive index change and a significant red shift. As the binding sites gradually become occupied (for >30 seconds), the effective binding probability decreases, the rate of refractive index change slows, and the magnitude of the red shift decreases. When the immersion time reaches 10 minutes, the spectrum stabilizes, indicating that the film's selective adsorption of Ni²⁺ has reached a dynamic equilibrium.

[0030] Method 2: By diluting high concentration Ni 2+ Solutions were prepared with concentrations of 10 nM, 30 nM, 50 nM, 70 nM, and 100 nM Ni 2+ The optical fiber sensor is immersed in the prepared Ni solution with concentrations from low to high. 2+ solution and record the spectral changes.

[0031] like Figure 7 As shown, the wavelength drift of the interference spectrum of the sensor is related to Ni 2+ The concentration showed a linear response in the range of 10 ~ 100 nM, and the corresponding Ni 2+ The detection sensitivity is 0.04488 nm / nM, and the linear correlation R2 is 98.06%.

[0032] As Figure 8 shown, the curve of the interference spectrum offset in Ni²⁺ solutions with different concentrations changing with time shows that when the micro-nano fiber sensor is immersed in deionized water for 20 minutes and recorded every 5 minutes, the wavelength offset deviation is extremely small, indicating its excellent stability. After drying, it is successively immersed in Ni²⁺ solutions with concentrations of 10 nM, 30 nM, 50 nM, 70 nM, and 100 nM for testing. The results show that in the first 5 minutes, the offset linearly increases in the ion binding dominant stage. Subsequently, the growth rate slows down in the next 25 - 30 minutes as the binding sites tend to be saturated, and the offset reaches an equilibrium state after 30 minutes. This phenomenon reveals the dynamic adsorption equilibrium process of Ni²⁺ with the binding sites on the film.

[0033] The above embodiments are only for illustrating the present application and are not intended to limit the present application. Those of ordinary skill in the relevant technical fields can also make various changes and modifications without departing from the scope of the present application. Therefore, all equivalent technical solutions should also fall within the scope of the disclosure of the present application.

Claims

1. A U-shaped micro-nano fiber chemical sensor based on supermode interference, characterized in that: It includes a structure of a single-mode optical fiber 1 - a tapered four-core optical fiber 2 - a single-mode optical fiber 4 that are coaxially welded in sequence. The surface of the tapered four-core optical fiber 2 is uniformly coated with Ni 2+ a chitosan polymer ion-imprinted film 3. The structure of the single-mode optical fiber 1 - the tapered four-core optical fiber 2 - the single-mode optical fiber 4 is encapsulated after the tapered four-core optical fiber 2 is partially bent into a U shape.

2. The U-shaped micro-nano fiber chemical sensor based on supermode interference according to claim 1, characterized in that: The tapered four-core optical fiber (2) is formed by heating and melting a four-core optical fiber, and the surface of the tapered four-core optical fiber (2) is coated with a chitosan polymer ion-imprinted film based on Ni 2+ ions by an ion imprinting method.

3. The U-shaped micro-nano fiber chemical sensor based on supermode interference according to claim 1, characterized in that: The diameters of single-mode optical fiber one (1) and single-mode optical fiber two (4) connected to both ends of the tapered four-core optical fiber (2) are 125 μm, and the core diameter is 9 μm.

4. The U-shaped micro-nano fiber chemical sensor based on supermode interference according to claim 2, characterized in that: There are 4 cores arranged in the tapered four-core optical fiber (2), each core has a diameter of 0.29 μm. The central core is located at the center of the tapered four-core optical fiber (2), and 3 cores are evenly arranged on its outer layer in an equilateral triangle layout. The distance between the outer cores is 1.89 μm, and the distance between the central core and the outer cores is 1.14 μm.

5. The U-shaped micro-nano optical fiber chemical sensor based on supermode interference according to claim 4, characterized in that: The diameter of the tapered four-core optical fiber (2) before tapering is 124.58 μm, and the diameter after tapering is 4.25 μm.

6. A preparation method of a U-shaped micro-nano optical fiber sensor based on supermode interference, which is characterized by including the following steps: Step 1: Splice a section of single-mode optical fiber at both ends of the four-core optical fiber to form a single-mode optical fiber - four-core optical fiber - single-mode optical fiber SFS structure; Step 2: Melt and taper the four-core optical fiber to form an STFS structure of single-mode optical fiber one (1) - tapered four-core optical fiber (2) - single-mode optical fiber two (4). The diameter of the tapered four-core optical fiber after tapering is 4.25 μm; Step 3: Coat Ni on the surface of the tapered four-core optical fiber (2) 2+ Chitosan polymer ion-imprinted film Step 4: Bend the STFS structure into a U shape from the tapered four-core optical fiber (2) part and then encapsulate it. The bending diameter is 3 mm.

7. The preparation method of a U-shaped micro-nano fiber optic sensor based on supermode interference according to claim 6, characterized in that: The chitosan polymer ion-imprinted film Ni 2+ -II-CS based on Ni ions is obtained by the following treatment: 2+ ions is obtained by the following treatment: Step 3.1: Prepare chitosan sol: Dissolve chitosan in 2% acetic acid solution C2H4O2, and stir until completely dissolved to obtain chitosan sol; Step 3.2: Add nickel chloride to the above chitosan sol and stir; Step 3.3: Add epichlorohydrin and stir to form a cross-linked chitosan solution Ni 2+ -CS; Step 3.4: Immerse the tapered four-core optical fiber (2) with an STFS structure that has been cleaned thoroughly into crosslinked Ni 2+ -CS to form a uniform crosslinked Ni 2+ -CS thin film on the surface of the tapered four-core optical fiber; Step 3.5, Air-dry and dry the micro-nano fiber chemical sensor with the coated cross-linked Ni 2+ -CS film at room temperature to ensure complete curing of the film; Step 3.

6. Immerse the optical fiber sensor with crosslinked Ni 2+ -CS membrane into dilute hydrochloric acid solution to remove Ni 2+ ions in the polymer, generating a Ni 2+ chitosan polymer ion-imprinted thin film (3).

8. The preparation method of a U-shaped micro-nano optical fiber sensor based on supermode interference according to claim 7, characterized in that: In step 3.4, the surface of the tapered four-core optical fiber 2 was coated with a chitosan polymer ion-imprinted film containing Ni ions by the brushing method using an electric displacement platform, and the electric displacement platform moved at a speed of 0.25 mm / s. 2+ ​ 9. The preparation method of a U-shaped micro-nano optical fiber sensor based on supermode interference according to claim 7, characterized in that: The concentration of chitosan prepared in Step 3.1 is 20 mg / mL, and it is stirred at a speed of 1500 rpm for 6 hours using a centrifugal stirrer. It is stirred for 2 hours in Step 3.2, 4 hours in Step 3.3, and the dilute hydrochloric acid in Step 3.6 is 1 mM hydrochloric acid with a pH of 3.

5.

10. The preparation method of a U-shaped micro-nano fiber optic sensor based on supermode interference according to claim 7, characterized in that: In Step 3.2, the mass ratio of nickel chloride to chitosan is 9:100, and in Step 3.3, the volume-mass ratio of epichlorohydrin to chitosan is 1:200 mL / g.

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