Chitin-based fluorescent film for Zr < 4 + > detection and preparation method thereof

Abstract

The invention discloses a chitin-based fluorescent film for Zr < 4 + > detection and a preparation method thereof. The material takes a chitin film as a substrate and is chemically modified in situ by a one-pot method. The method comprises the following steps: firstly, dissolving chitin in an alkali / urea system, preparing a chitin film through a tape casting method, and washing the film with deionized water until the pH value is neutral; then, a multi-component Mannich reaction is adopted, the chitin thin film obtained after hot pressing reacts with quercetin molecules and aldehyde compounds, and the modified covalent grafted chitin-based fluorescent thin film is obtained. Through introduction of quercetin molecules, the chitin film is endowed with fluorescence performance, and meanwhile, carbonyl and polyhydroxy structures of quercetin have the capability of easily forming strong complexes with metal ions, so that the modified chitin-based fluorescent film has the characteristic of quantitatively detecting the metal ions. The preparation method disclosed by the invention accords with green chemistry, is simple to operate and stable in performance, and can be used for rapidly and highly selectively detecting zirconium (IV) ions.

Description

Technical Field

[0001] This invention belongs to the field of chitin-based fluorescent thin film detection technology, specifically relating to a method for detecting Zr. 4+ Chitosan-based fluorescent thin films detected and their preparation method. Background Technology

[0002] Transition metals are indispensable in materials science, nuclear energy industry, and environmental monitoring, but their concentrations can pose ecological risks if they exceed safety thresholds. As a representative transition metal of group IVB, zirconium and its compounds are widely used in nuclear reactor structural materials, high-temperature ceramics (such as experimental crucibles), and biomedical devices (such as dental implants) due to their excellent corrosion resistance, mechanical stability, and neutron absorption properties. However, the application of zirconium also carries potential hazards: inhalation of dust or particulate matter generated during processing can damage the respiratory system; zirconium ions (Zr) in the environmental medium... 4+ The chemical migration of Zr (typically, the corrosion and dissolution of nuclear fuel cladding under high temperature and high pressure hydrochemistry) not only severely impairs the service performance of materials but may also lead to the risk of radionuclide leakage; furthermore, Zr 4+ Bioaccumulation of zirconium ions can lead to ecotoxicity, and clinical evidence shows that exposure is significantly associated with pulmonary granulomas, tissue inflammation, skin irritation, and even genotoxicity. Unfortunately, research on fluorescent probes that can specifically recognize zirconium ions remains very limited. Therefore, there is an urgent need to develop accurate and sensitive fluorescent probes for zirconium ions. 4+ Detection technologies to address the increasingly serious environmental safety and health challenges.

[0003] Currently Zr 4+ The detection of metal ions mainly relies on traditional analytical methods such as complexometric titration, volumetric analysis, electrochemical voltammetry, and inductively coupled plasma mass spectrometry (ICP-MS). However, these techniques are often limited by complex sample pretreatment, time-consuming operations, high costs, and strong instrument dependence, which restricts their practical application. In contrast, fluorescence detection analysis methods have become an effective platform for metal ion detection due to their advantages of simple operation, rapid response, and high sensitivity. Flavonols are considered ideal fluorescent probe groups due to their excellent photophysical properties. Among them, quercetin, as a representative of natural flavonols, is widely found in fruits and vegetables. Notably, it is currently the only known naturally derived fluorophore with both ESIPT and AIE bifunctional properties, showing significant potential in metal ion detection.

[0004] Meanwhile, most of the reported Zr 4+Detection sensors are based on small-molecule organic compounds. Although some sensors have low detection limits, they often suffer from problems such as cumbersome synthesis and potential environmental hazards associated with organic reagents. For example, in 2025, Singh et al. synthesized a naphthol-derived 1,2,3-triazole (NPTZ) via click chemistry for the selective detection of Zr. 4+ However, the synthesis of small molecule probes is complex, and the detection methods are not convenient enough. (G. Singh, Komal, A. Radha, H. Kaur, B. Rani, B. S. Gill, D. Baliyan, A. Kaur, B. Mohan, Formulation of naphthol-derived 1,2,3-triazole: A “turn-on” chemosensor for Zr(IV) with anticancer and antibacterial activities, Journal of Inorganic Biochemistry 270 (2025)). In contrast, unlike traditional small molecule probes, this invention employs a biopolymer-based sensing platform derived from chitin grafted with quercetin. This macromolecular structure not only retains the molecular recognition ability of the fluorophore but also integrates the biocompatibility and environmental friendliness of the polysaccharide matrix. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention aims to provide a chemical modification method for Zr. 4+ Chitosan-based fluorescent thin films detected and their preparation method.

[0006] This invention utilizes the Mannich reaction chemical modification method to firmly anchor quercetin molecules, which possess dual functions of chelating metal ions and fluorescence, onto a chitin film substrate, successfully constructing a modified film rich in regularly distributed metal ion recognition sites. This modification endows the film with highly sensitive and visual recognition capabilities based on changes in fluorescence intensity, and enables precise quantitative detection of specific metal ion concentrations in the environment based on fluorescence signals.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] To achieve the objective of this invention, in a first aspect, chitin powder is completely dissolved, and the chitin solution is cross-linked using a cross-linking agent. After centrifugation of the cross-linked solution, a chitin gel film is prepared by casting, and finally obtained by hot pressing. The prepared chitin film is then subjected to a Mannich multicomponent reaction with quercetin molecules and aldehyde compounds under suitable conditions to obtain the material used for detecting Zr. 4+Chitosan-based fluorescent functional thin films.

[0009] A type of Zr 4+ The chitin-based fluorescent thin film was detected. The chemical structural formula of the chitin-based fluorescent thin film is as follows:

[0010]

[0011] A type of Zr 4+ The method for preparing the chitin-based fluorescent thin film includes the following steps:

[0012] (1) Chitosan powder is completely dissolved in an alkali / urea solvent system and chitosan solution is obtained by freeze-thaw cycle treatment. The chitosan solution is cross-linked with a cross-linking agent. The cross-linked solution is centrifuged and poured into a mold. It is gelled at room temperature, washed with deionized water until neutral pH, and finally replaced with ethanol solvent and hot-pressed to obtain chitosan film.

[0013] (2) The prepared chitin film was subjected to a Mannich multicomponent reaction with quercetin molecules and aldehyde compounds under suitable conditions to obtain a solution for detecting Zr. 4+ Chitosan-based fluorescent functional thin films.

[0014] Preferably, in step (1), the alkali / urea solvent system is one of NaOH / urea aqueous solution, KOH / urea aqueous solution or LiOH / urea aqueous solution; the crosslinking agent is one of epichlorohydrin, glutaraldehyde or glyoxal.

[0015] More preferably, in step (1), the weight percentage of alkali and urea solvent is 11:4; and the ratio of chitin solution to crosslinking agent is 100g:3mL.

[0016] More preferably, in step (1),

[0017] In step (1), the freeze-thaw cycle is performed 2 to 5 times;

[0018] The crosslinking temperature is 0.5–10°C, and the crosslinking time is 0.5–2 hours.

[0019] The centrifugation temperature is 0.5–10°C, the centrifugation speed is 6000–8000 rpm, and the centrifugation time is 8–20 min;

[0020] The specific steps of the hot pressing are drying at 80-120°C and 0.5-1 bar for 5-15 minutes.

[0021] Preferably, the specific steps of step (2) are as follows: the prepared chitin film is immersed in a solution containing quercetin and aldehyde compounds, reacted at 25-80°C for 1-24 hours, and then washed and dried to obtain the material for detecting Zr. 4+ Chitosan-based fluorescent thin films.

[0022] More preferably, the aldehyde compound is one of formaldehyde, benzaldehyde, or vanillin.

[0023] More preferably, in the solution containing quercetin and aldehyde compounds, the concentration of quercetin is 0.0037–0.0444 g / mL, and the concentration of aldehyde compounds is 0.0685–0.0548 mg / mL.

[0024] More preferably, the solvent for the solution containing quercetin and aldehyde compounds is ethanol, methanol, an ethanol / water mixture, or a methanol / water mixture.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (I) This invention employs a simple one-step reaction to covalently bond quercetin and chitin films. The raw materials are relatively inexpensive, the reaction conditions are mild, and the preparation process is simple. Chitin, a natural biopolymer, is used as the basic framework, and a fluorophore from the same natural quercetin source is grafted onto it. This utilizes the biodegradability and environmental friendliness of chitin itself. This strategy avoids complex small-molecule organic synthesis routes, making the preparation process simpler and greener, and reducing potential negative environmental impacts.

[0027] (II) This invention utilizes the inherent advantages of fluorescence detection, such as ease of operation and rapid response. The solid form based on biopolymers may further simplify the operation (e.g., compared to small molecule probes that require solution preparation, it may be easier to prepare test strips, membranes, or simple devices). Furthermore, this fluorescent film exhibits strong anti-interference capabilities, specific recognition, and high sensitivity, effectively distinguishing zirconium ions from other common metal ions. This design enhances the convenience and practicality of detection, as well as its potential for application in less complex laboratory environments.

[0028] (III) The chitin-based fluorescent film of this invention has a wide range of applications, exhibiting good detection performance under a wide pH range (2-8). It emits light at a wavelength of 540 nm, displaying a strong yellow-green fluorescence visible to the naked eye. It has a fast response speed, enabling real-time monitoring. Furthermore, it does not generate secondary pollution during use, which is beneficial for Zr in water bodies and the environment. 4+ Trace analysis and detection of this material has good application prospects and promotion value.

[0029] (iv) In this invention, the quercetin molecules grafted into the chitosan-based fluorescent film specifically recognize and chelate zirconium ions. This binding suppresses the excited-state intramolecular proton transfer (ESIPT) of quercetin itself, and through the chelation-enhanced fluorescence (CHEF) effect and the dominant ligand-metal charge transfer (LMCT) process, the film produces a significant "on" yellow-green fluorescence signal at a wavelength of 540 nm, achieving selective fluorescence sensing of zirconium ions. Attached Figure Description

[0030] Other features, objects, and advantages of this application will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0031] Figure 1 Detection of Zr according to an embodiment of the present invention 4+ The reaction equations involved in the Mannich reaction in the preparation of chitin-based fluorescent thin films;

[0032] Figure 2 Detection of Zr in another embodiment of the present invention 4+ Infrared spectral verification of successful preparation of chitin-based fluorescent thin films;

[0033] Figure 3 Detection of Zr in another embodiment of the present invention 4+ Fluorescence changes of chitosan-based fluorescent films in response to zirconium ions;

[0034] Figure 4 Detection of Zr in another embodiment of the present invention 4+ Fluorescence emission spectra of chitosan-based fluorescent films for different concentrations of zirconium ions;

[0035] Figure 5 Detection of Zr in another embodiment of the present invention 4+ The relationship between the fluorescence enhancement of chitosan-based fluorescent films and zirconium ion concentration;

[0036] Figure 6 Detection of Zr in another embodiment of the present invention 4+ Detection of the anti-interference ability of chitosan-based fluorescent thin films in the presence of different metal ions;

[0037] Figure 7 Detection of Zr in another embodiment of the present invention 4+ Graph showing the fluorescence intensity variation of chitosan-based fluorescent films under different pH conditions;

[0038] Figure 8 Detection of Zr in another embodiment of the present invention 4+The fluorescence intensity variation of chitosan-based fluorescent thin film needles in the presence of different concentrations of zirconium ion aqueous solutions during response time. Detailed Implementation

[0039] The present application will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, only the parts relevant to the invention are shown in the drawings. It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of this application can be combined with each other. The present application will now be described in detail with reference to the accompanying drawings and embodiments.

[0040] Example 1

[0041] A type of Zr 4+ The method for preparing the chitin-based fluorescent thin film for detection includes the following steps:

[0042] 1. Weigh 2g of chitin powder and dissolve it in 100g of an 11wt% NaOH / 4wt% urea aqueous solution system. After three freeze-thaw cycles, a 2wt% chitin solution is obtained. Add 3mL of epichlorohydrin solution to 100g of the 2wt% chitin solution and crosslink it in an ice bath at 0℃ for 1h.

[0043] 2. After the cross-linking is completed, the chitin solution is centrifuged at 8000 rpm at 0℃ for 10 min. Then, the film is cast by casting. The gel film is washed with deionized water until the pH is neutral. After ethanol replacement, it is hot-pressed and dried at 90℃ and 0.8 bar for 10 min to obtain the chitin film.

[0044] 3. Immerse the 0.5g chitin film prepared in step 2 in 50mL of an ethanol solution containing 0.74g quercetin and 0.034mL formaldehyde, and react at 40℃ for 4h.

[0045] 4. After the reaction is complete, remove the chitosan membrane, clean it with ethanol, and dry it to obtain the material for detecting Zr. 4+ Chitosan-based fluorescent thin films.

[0046] The chemical equation for the Mannich reaction is shown in the appendix. Figure 1 middle.

[0047] Through infrared spectrum (attached) Figure 2 This invention clearly demonstrates the detection method for Zr. 4+ Successful preparation of chitin-based fluorescent thin films.

[0048] Example 2

[0049] A type of Zr 4+ The chitin-based fluorescent film detected against Zr 4+ Selective detection of fluorescence response:

[0050] Take 18 10mL sample tubes, add 0.5g of chitin-based fluorescent film to each sample tube, and then add 3mL of Ni-containing solution with a concentration of 0.01mol / L to each sample tube. + Cs + Li + K + Zn 2+ Mg 2+ 、Tb 3+ Ca 2+ Mn 2+ Al 3+ Fe 3+ Ag + Co 2+ Fe 2+ Cu 2+ Hg 2+ Cr 3+ and Zr 4+ The above samples were subjected to aqueous solution treatment, allowed to stand for 10 minutes, then removed and dried. The fluorescence spectral changes were then measured. The excitation wavelength was set to 360 nm, and the maximum emission wavelength to 540 nm. This was used for Zr. 4+ The fluorescence response of the chitin-based fluorescent film to zirconium ions is shown in the attached figure. Figure 3 As shown in the figure. The results show that the chitosan-based fluorescent film exhibits significant fluorescence enhancement at 540 nm when bound to zirconium ions. The results indicate that this invention is suitable for Zr... 4+ The chitin-based fluorescent film exhibits a highly sensitive fluorescence enhancement effect for zirconium ions.

[0051] Example 3

[0052] A type of Zr 4+ The chitin-based fluorescent film detected against Zr 4+ Quantitative fluorescence detection: Take 12 10mL sample tubes and add 0eq–8eq of 0.01mol / L zirconium ion aqueous solution (0.1–0.9mL) to each sample tube. Then add 0.5g of chitin-based fluorescent film to each sample tube. Let the samples stand for 10min, then remove and dry them, and measure their fluorescence spectrum changes. This method is used for Zr... 4+ The fluorescence emission spectra of the chitin-based fluorescent films for different concentrations of zirconium ions are shown in the attached figure. Figure 4 As shown in the figure. A standard curve was obtained by fitting the fluorescence intensity at 540 nm of the fluorescence emission spectrum to the corresponding zirconium ion equivalent ratio (see attached figure). Figure 5As shown in the figure, the results show a good linear relationship, indicating that the method for Zr involved in this invention has good linearity. 4+ The chitin-based fluorescent thin film used in the test can quantitatively detect zirconium ion concentration.

[0053] Example 4

[0054] A type of Zr 4+ Interference resistance testing of chitosan-based fluorescent thin films:

[0055] Take 18 sample tubes (10 mL each) and add 3 mL of a 0.01 mol / L Ni-containing solution to each. + Cs + Li + K + Zn 2 + Mg 2+ 、Tb 3+ Ca 2+ Mn 2+ Al 3+ Fe 3+ Ag + Co 2+ Fe 2+ Cu 2+ Hg 2+ Cr 3+ and Zr 4+ An aqueous solution was prepared, and then 3 mL of 0.01 mol / L Zr was added to each of the 18 sample tubes. 4+ After thoroughly mixing the aqueous solution, add 0.5 g of chitin-based fluorescent film to each sample tube. Let the samples stand for 10 min, then remove and dry them. Measure the fluorescence spectrum changes of each sample. The excitation wavelength was set to 360 nm, and the maximum emission wavelength to 540 nm. (For Zr...) 4+ The test results of the anti-interference ability of the chitosan-based fluorescent thin film are shown in the attached figure. Figure 6 As shown in the figure. The results show that even in the presence of other metal cations, the fluorescence intensity of the chitin-based fluorescent film is still significantly enhanced. This result indicates that the chitin-based fluorescent film prepared in this invention is suitable for Zr. 4+ The chitin-based fluorescent thin film tested showed strong anti-interference ability in the presence of different metal ions.

[0056] Example 5

[0057] pH for Zr 4+ The chitin-based fluorescent thin film detected identifies Zr 4+ Impact:

[0058] Eleven groups of aqueous solutions with different pH values ​​ranging from 2 to 12 were prepared as pH adjusters. Twenty-two 10 mL sample tubes were divided into two groups of eleven each. In the first group, 0.5 g of chitin-based fluorescent film and 3 mL of pH adjuster (pH 2–12) were added to each sample tube. In the second group, 3 mL of 0.01 mol / L Zr solution was added to each sample tube. 4+ An aqueous solution was prepared, along with 3 mL of a pH adjuster (pH 2–12), and 0.5 g of a chitin-based fluorescent film was added. The sample solution was allowed to stand for 10 min, then removed and dried. The fluorescence spectral changes were then measured. This solution was used for Zr. 4+ The fluorescence intensity changes of the chitin-based fluorescent thin film under different pH conditions are shown in the attached figure. Figure 7 As shown in the figure. The results show that the chitosan-based fluorescent film itself exhibits weak fluorescence in the pH range of 2–12, and the fluorescence change is relatively stable in the pH range of 2–12. When the chitosan-based fluorescent film is reacted with Zr... 4+ After binding, a significant enhancement in fluorescence was observed in the pH range of 2–8. These results indicate that the Zr prepared according to this invention… 4+ The chitin-based fluorescent thin film used for detection is applicable to Zr under different pH conditions (2–8). 4+ Testing.

[0059] Example 6

[0060] A type of Zr 4+ Response time determination of the chitin-based fluorescent thin film:

[0061] Take 11 10 mL sample tubes and number them 1 to 11. Add 3 mL of 0.01 mol / L Zr to each sample tube. 4+ An aqueous solution was prepared, and 0.5 g of a chitin-based fluorescent film was added. The solution was immediately placed in a fluorescence spectrometer for detection after a response time of 5 s to 5 min, yielding a time-varying fluorescence spectrum. This was used for Zr. 4+ The fluorescence intensity change of the chitin-based fluorescent film in aqueous zirconium ion solution of the same concentration during the detection is as follows: Figure 8 As shown. The results show that the Zr prepared in this invention 4+ The chitin-based fluorescent film tested showed a rapid response, with the fluorescence value reaching its peak at 60 seconds.

[0062] Comparative Example 1

[0063] A type of Zr 4+ The method for preparing the chitin-based fluorescent thin film for detection includes the following steps:

[0064] 1. Weigh 2g of chitin powder and dissolve it in 100g of an 11wt% NaOH / 4wt% urea aqueous solution system. After three freeze-thaw cycles, a 2wt% chitin solution is obtained. Add 3mL of epichlorohydrin solution to 100g of the 2wt% chitin solution and crosslink it in an ice bath at 0℃ for 1h.

[0065] 2. After the cross-linking is completed, the chitin solution is centrifuged at 8000 rpm at 0℃ for 10 min. Then, the film is cast by casting. The gel film is washed with deionized water until the pH is neutral. After ethanol replacement, it is hot-pressed and dried at 90℃ and 0.8 bar for 10 min to obtain the chitin film.

[0066] 3. Immerse the 0.5g chitin film prepared in step 2 in 50mL of ethanol solution containing 0.74g quercetin and 0.102mL formaldehyde, and react at 60℃ for 4h.

[0067] 4. After the reaction is complete, remove the chitosan membrane, clean it with ethanol, and dry it to obtain the material for detecting Zr. 4+ Chitosan-based fluorescent thin films.

[0068] Comparative Example 2

[0069] A type of Zr 4+ The method for preparing the chitin-based fluorescent thin film for detection includes the following steps:

[0070] 1. Weigh 2g of chitin powder and dissolve it in 100g of an 11wt% NaOH / 4wt% urea aqueous solution system. After three freeze-thaw cycles, a 2wt% chitin solution is obtained. Add 3mL of epichlorohydrin solution to 100g of the 2wt% chitin solution and crosslink it in an ice bath at 0℃ for 1h.

[0071] 2. After the cross-linking is completed, the chitin solution is centrifuged at 8000 rpm at 0℃ for 10 min. Then, the film is cast by casting. The gel film is washed with deionized water until the pH is neutral. After ethanol replacement, it is hot-pressed and dried at 90℃ and 0.8 bar for 10 min to obtain the chitin film.

[0072] 3. Immerse the 0.5g chitin film prepared in step 2 in 50mL of an ethanol solution containing 1.48g quercetin and 0.068mL formaldehyde, and react at 100℃ for 4h.

[0073] 4. After the reaction is complete, remove the chitosan membrane, clean it with ethanol, and dry it to obtain the material for detecting Zr. 4+ Chitosan-based fluorescent thin films.

[0074] Comparative Example 3

[0075] A type of Zr 4+ The method for preparing the chitin-based fluorescent thin film for detection includes the following steps:

[0076] 1. Weigh 2g of chitin powder and dissolve it in 100g of an 11wt% NaOH / 4wt% urea aqueous solution system. After three freeze-thaw cycles, a 2wt% chitin solution is obtained. Add 3mL of epichlorohydrin solution to 100g of the 2wt% chitin solution and crosslink it in an ice bath at 0℃ for 1h.

[0077] 2. After the cross-linking is completed, the chitin solution is centrifuged at 8000 rpm at 0℃ for 10 min. Then, the film is cast by casting. The gel film is washed with deionized water until the pH is neutral. After ethanol replacement, it is hot-pressed and dried at 90℃ and 0.8 bar for 10 min to obtain the chitin film.

[0078] 3. Immerse the 0.5g chitin film prepared in step 2 in 50mL of an ethanol solution containing 0.074g quercetin and 0.034mL formaldehyde, and react at 60℃ for 4h.

[0079] 4. After the reaction is complete, remove the chitosan membrane, clean it with ethanol, and dry it to obtain the material for detecting Zr. 4+ Chitosan-based fluorescent thin films.

[0080] The chitin-based fluorescent film of this invention for Zr 4+ The detection sensitivity mainly depends on the grafting degree of the quercetin molecule, which is determined by the efficiency of the Mannich reaction in step (2) of claim 2. Precise control of the degree of substitution of the quercetin molecule can be achieved by systematically adjusting key parameters such as the molar ratio of reactants and the reaction temperature.

[0081] Experimental data (Table 1) show that: Comparative Example 1 indicates that the degree of substitution significantly decreases when formaldehyde is in excess. This is because excess formaldehyde easily undergoes self-polymerization, leading to a decrease in the concentration of free formaldehyde in the system, thereby inhibiting the forward progress of the Mannich reaction; Comparative Example 2 confirms that excessively high reaction temperatures lead to a decrease in grafting efficiency, possibly due to the exacerbation of side reactions under high-temperature conditions; Comparative Example 3 shows that when the amount of quercetin added is insufficient, the reaction cannot proceed fully, which also affects the final grafting efficiency. The above research results provide an optimized range of reaction condition parameters for this invention, paving the way for obtaining Zr with ideal properties. 4+ This has laid an important foundation for fluorescent thin film materials with high performance.

[0082] Table 1. Mannich reaction conditions and degree of substitution results in Example 1 and Comparative Examples 1-3

[0083]

[0084]

[0085] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features with similar functions disclosed in this application.

Claims

1. A method for Zr 4+ The chitin-based fluorescent thin film detected is characterized by: The chemical structural formula of the chitosan-based fluorescent thin film is:

2. The method for Zr as described in claim 1 4+ The method for preparing the chitin-based fluorescent thin film for detection is characterized by: Includes the following steps: (1) Chitosan powder is dispersed in an alkali / urea solvent system and chitosan solution is obtained by freeze-thaw cycle treatment. The chitosan solution is cross-linked with a cross-linking agent. The cross-linked solution is centrifuged and poured into a mold. It is gelled at room temperature, washed with deionized water until neutral pH, and finally replaced with ethanol solvent and hot-pressed to obtain chitosan film. (2) The prepared chitin film was subjected to a Mannich multicomponent reaction with quercetin molecules and aldehyde compounds under suitable conditions to obtain a solution for detecting Zr. 4+ Chitosan-based fluorescent functional thin films.

3. The preparation method according to claim 2, characterized in that: The alkali / urea solvent system mentioned in step (1) is one of NaOH / urea aqueous solution, KOH / urea aqueous solution or LiOH / urea aqueous solution; The crosslinking agent is one of epichlorohydrin, glutaraldehyde, or glyoxal.

4. The preparation method according to claim 3, characterized in that: The weight percentage of the alkali / urea solvent is 11:4; the ratio of chitin solution to crosslinking agent is 100g:3mL.

5. The preparation method according to claim 2, characterized in that: In step (1), the freeze-thaw cycle is performed 2 to 5 times; The crosslinking temperature is 0.5–10°C, and the crosslinking time is 0.5–2 hours. The centrifugation temperature is 0.5–10°C, the centrifugation speed is 6000–8000 rpm, and the centrifugation time is 8–20 min; The specific steps of the hot pressing are drying at 80-120°C and 0.5-1 bar for 5-15 minutes.

6. The preparation method according to claim 2, characterized in that: The specific steps of step (2) are as follows: The prepared chitin film was immersed in a solution containing quercetin and aldehyde compounds and reacted at 25–80°C for 1–24 hours. After washing and drying, the film was used to detect Zr. 4+ Chitosan-based fluorescent thin films.

7. The preparation method according to claim 6, characterized in that: The aldehyde compound is one of formaldehyde, benzaldehyde, or vanillin.

8. The preparation method according to claim 6, characterized in that: In the solution containing quercetin and aldehydes, the concentration of quercetin is 0.0037–0.0444 g / mL, and the concentration of aldehydes is 0.0685–0.0548 mg / mL.

9. The preparation method according to claim 8, characterized in that: The solvent for solutions containing quercetin and aldehydes is ethanol, methanol, an ethanol / water mixture, or a methanol / water mixture.

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