A PVA@AL@CDs hydrogel and a preparation method thereof

Abstract

The application discloses a PVA@AL@CDs hydrogel and a preparation method thereof, and particularly relates to the field of food preservation monitoring technology, and comprises a polyvinyl alcohol matrix, alizarin, carbon dots with unique fluorescence characteristics and water dispersed in the polyvinyl alcohol matrix. The application further provides a preparation method of the PVA@AL@CDs hydrogel, which comprises the following steps: completely dissolving citric acid and urea in deionized water to obtain a mixture; microwave heating reaction of the mixture in a microwave oven, followed by cooling to room temperature, centrifugal washing, and filtering the supernatant through a microporous filter membrane to obtain CDs powder; dissolving PVA in water, stirring, adding AL, and finally adding the CDs powder, and then mixing to obtain a PVA@AL@CDs gel solution; pouring the obtained PVA@AL@CDs gel solution into a mold, freezing, and then thawing at room temperature to obtain the PVA / AL / CDs hydrogel. The PVA@AL@CDs hydrogel has a colorimetric and fluorescent dual-mode response, and simultaneously has the functions of preservation and indicating the freshness degree of food.

Description

Technical Field

[0001] This application relates to the field of food preservation monitoring technology, and in particular to a PVA@AL@CDs hydrogel and its preparation method. Background Technology

[0002] Smart active packaging is a novel type of food packaging material with preservation properties that extend the shelf life of food. It also provides great convenience to consumers by monitoring changes in the atmosphere inside the packaging in real time and tracking dynamic changes in food quality. Currently, biodegradable films based on natural polymers are widely used in the preparation of smart active packaging materials due to their excellent biodegradability and low toxicity.

[0003] Smart active packaging materials integrate pH or ammonia-sensitive indicators with the packaging material to achieve real-time monitoring of food freshness. During microbial and chemical degradation, food produces various compounds, especially total volatile basic nitrogen (TVB-N), which can cause changes in the environmental pH value.

[0004] However, the poor flexibility and low water content of these films severely limit their preservation and indication effects. Furthermore, most smart active packaging materials rely on a single response signal, making them susceptible to environmental interference. Therefore, there is an urgent need to develop a smart active packaging material that combines dual-mode indication, indication, and preservation functions. Summary of the Invention

[0005] This invention provides a PVA@AL@CDs hydrogel and its preparation method. The PVA@AL@CDs hydrogel has both colorimetric and fluorescence dual-mode responses, and also functions as a preservative and indicator of food freshness.

[0006] The objective of this invention is achieved through the following technical solution: A PVA@AL@CDs hydrogel comprises a polyvinyl alcohol matrix, and alizarin, carbon dots with unique fluorescent properties, and water dispersed in the polyvinyl alcohol matrix; wherein the mass percentages of polyvinyl alcohol, alizarin, carbon dots, and water are 9%-10%, 0.004%, 0.04%, and 90%-91%, respectively.

[0007] Citric acid carbon dots (CDs) are widely used in food-related fields due to their high quantum yield, flexible synthesis process, excellent biocompatibility, and antibacterial and antioxidant properties. Under alkaline conditions, the deprotonation of citric acid carbon dots regulates fluorescence intensity by inducing molecular conformational changes. The combination with pH dyes can further amplify this fluorescence response by generating a fluorescence intrinsic rate effect, thereby significantly improving sensing performance.

[0008] Meanwhile, alizarin (AL), a natural dye extracted from madder root, not only has unique pH sensitivity, antibacterial and antioxidant activities, but also has extremely high stability due to its naphthoquinone structure. Therefore, it is widely used in food packaging to monitor food freshness.

[0009] Preferably, the carbon dots are obtained by reacting citric acid and urea by microwave heating, with a mass ratio of citric acid to urea of ​​1:2.

[0010] This invention also provides a method for preparing PVA@AL@CDs hydrogel, the method comprising the following steps: S1. Dissolve citric acid and urea completely in deionized water to obtain a mixture; S2. The mixture is placed in a microwave oven and heated to react. After cooling to room temperature, it is centrifuged and washed. The supernatant is filtered through a microporous membrane to obtain CDs powder. S3. Dissolve PVA in water, stir, add AL, and finally add CDs powder. After mixing, you can get PVA@AL@CDs gel solution. S4. Pour the obtained PVA@AL@CDs gel solution into a mold, freeze it, and then thaw it at room temperature to obtain PVA / AL / CDs hydrogel.

[0011] Preferably, in step S2, the microwave heating reaction time is 5 minutes.

[0012] Preferably, in step S2, the centrifugal washing conditions are: 10,000 rpm, 10 min.

[0013] Preferably, in step S3, the stirring temperature when PVA is dissolved in water is 95°C and the stirring time is 2 hours.

[0014] Preferably, in step S3, the temperature at which AL is dissolved in the PVA solution is 50°C and the mixing reaction time is 1 hour.

[0015] Preferably, in step S3, after adding CDs powder, the mixing reaction time is 1 hour.

[0016] Preferably, in step S4, the freezing temperature is -20℃, the freezing time is 12h, and the thawing time is 1h.

[0017] This invention also provides applications of PVA@AL@CDs hydrogels, specifically including (1), (2), (3) and (4): (1) Colorimetric signal response based on pH sensitivity of AL; (2) Fluorescence signal response based on the pH-dependent fluorescence characteristics of CDs and the internal filtration effect of AL on CDs; (3) Reagents used to prepare for testing the freshness grade of shrimp; (4) Preservatives used in the preparation of shrimp.

[0018] Compared with the prior art, the advantages or beneficial effects of the technical solution of this application include: 1. The material provided in this application can simultaneously utilize the colorimetric response of AL and the fluorescence response of CDs. The two signals can be mutually corrected, effectively avoiding interference from environmental factors on a single signal, thus improving the accuracy and reliability of detection. The fluorescence mode has higher sensitivity.

[0019] 2. Hydrogels have both freshness indication and preservation functions, which can actively extend the shelf life of food and monitor quality changes at the same time, simplifying the packaging process.

[0020] 3. The synergistic effect of hydrogen bonds between AL and CDs and PVA significantly enhances the mechanical properties, water resistance, and color stability of the hydrogel. The raw materials are safe, biocompatible, and contain no toxic cross-linking agents.

[0021] 4. It is prepared by freeze-thaw method, which is simple and the raw materials are readily available, making it suitable for industrial production.

[0022] 5. With the help of smartphones and self-developed mini-programs, it can realize on-site, real-time, and intuitive quantitative assessment of freshness, providing great convenience for consumers. Attached Figure Description

[0023] Figure 1 This is a schematic diagram illustrating the synthesis of dual-mode intelligent active packaging materials and their application in shrimp freshness monitoring and preservation.

[0024] Figure 2 (A) Fourier transform infrared spectra of CDs and citric acid; (B) Particle size distribution of CDs; (C) Emission and UV-Vis absorption spectra of CDs, with insets showing photographs of CDs solutions under fluorescent and 365 nm UV irradiation; (D) FT-IR spectrum of smart active hydrogel.

[0025] Figure 3 (A) T2 inversion spectra of different hydrogels; (B) T2 relaxation time-area ratio of different hydrogels; (C) water content and water solubility of hydrogels; (D) swelling ratio of hydrogels.

[0026] Figure 4(A) Fluorescence emission spectra and images of CDs at different pH values ​​(365 nm excitation); (B) Fluorescence emission spectra of CDs solution (365 nm excitation) and UV absorption spectra of alizarin (pH=12); (C) Colorimetric and fluorescence changes of hydrogels in buffer solutions at different pH values; (D) Colorimetric and fluorescence changes of hydrogels exposed to 500 ppm TMA for different times; (E) Colorimetric ED value changes of hydrogels exposed to 500 ppm TMA for different times; (F) Fluorescence ED value changes of hydrogels exposed to 500 ppm TMA for different times; (G) Colorimetric and fluorescence changes of hydrogels at 4℃; (H) Colorimetric ED value changes of hydrogels at 4℃; (I) Fluorescence ED value changes of hydrogels at 4℃.

[0027] Figure 5 (A) DPPH radical scavenging activity of different hydrogels; (B) DPPH radical scavenging activity of different CDs concentrations; (C) ABTS radical scavenging activity of different hydrogels; (D) ABTS radical scavenging activity of different CDs concentrations; (E) F / F0 ratio of fluorescence intensity of CDs after mixing with seven single metal ions; (F) Addition of other metal ions and Fe 3+ The relative fluorescence intensity of CDs after (G)Fe 3+ Changes in the emission spectrum of CDs with increasing concentration (0-25.6 mM); (H)CDs and CDs+Fe 3+ FT-IR spectra; (I) CDs and CDs+Fe 3+ (J) Zeta potential; (J) Minimum inhibitory concentration (MIC) of CDs against Vibrio parahaemolyticus and images of bacterial colonies formed after incubation with different hydrogels and Vibrio parahaemolyticus; (K) Alizarin release from PVA / AL / CDs hydrogel in solution; (L) CDs release from PVA / AL / CDs hydrogel in solution; (M) Cell activity of different hydrogels.

[0028] Figure 6(A) Photographs of PVA / AL / CDs hydrogels under visible and ultraviolet light; (B) Diurnal variation of ED value of PVA / AL / CDs hydrogels and TVB-N value of shrimp under visible light; (C) Diurnal variation of ED value of PVA / AL / CDs hydrogels and TVB-N value of shrimp under ultraviolet light; (D) Nonlinear correlation between ED value and TVBN value of PVA / AL / CDs hydrogels under visible light; (E) Nonlinear correlation between ED value and TVBN value of PVA / AL / CDs hydrogels under ultraviolet light; (F) Quantitative assessment of shrimp freshness using a self-designed WeChat mini-program and portable device; (G) Correlation assessment of parallel measurements of shrimp using smartphones and Kjeldahl nitrogen determination method under visible light; (H) Correlation assessment of parallel measurements of shrimp using smartphones and Kjeldahl nitrogen determination method under ultraviolet light.

[0029] Figure 7 Application of intelligent active hydrogels in shrimp preservation. Changes in (A) TVC; (B) TVB-N; (C) pH value during shrimp storage at 4℃ for 9 days. Detailed Implementation

[0030] The following detailed description of the embodiments of this application, in conjunction with the accompanying drawings, will provide a thorough understanding of how this application uses technical means to solve technical problems and achieve corresponding technical effects, enabling its implementation. The embodiments of this application and the various features within them can be combined with each other without conflict, and all resulting technical solutions are within the protection scope of this application.

[0031] It should be clearly stated that the embodiments described below are merely some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0032] Example 1: Preparation of PVA@AL@CDs hydrogel The main reagents used in this experiment are shown in Table 1.

[0033] Table 1. Reagents and materials used in the experiment

[0034] The main instruments and equipment used in this study are shown in Table 2.

[0035] Table 2 Instruments and Equipment

[0036] The experimental method for synthesizing PVA@AL@CDs hydrogels is as follows: 1. Synthesis of CDs First, 1 g of citric acid and 2 g of urea were completely dissolved in 20 ml of deionized water. The mixture was reacted in a microwave oven for 5 min, and then cooled to room temperature. The mixture was centrifuged and washed twice (10,000 rpm, 10 min), and the supernatant was filtered through a 0.22 μm microporous membrane and freeze-dried to obtain a brownish-yellow CDs powder.

[0037] 2. Dissolve 5 g of PVA1799 in 50 mL of water and stir in a 95°C magnetically stirred water bath for 2 h. Next, adjust the water bath temperature to 50°C and mix 2 mg of AL with the PVA gel solution thoroughly for 1 h. Finally, add 20 mg of CDs and continue mixing for 1 h to obtain the PVA / AL / CDs gel solution.

[0038] 3. Pour the obtained gel solution into a mold, freeze it at -20℃ for 12 h, and then thaw it at room temperature for 1 h to obtain the PVA / AL / CDs hydrogel. In addition, hydrogels with other different components were prepared according to similar steps, and the amounts used are shown in Table 3.

[0039] Table 3. Abbreviations and composition of the hydrogels prepared in this study

[0040] Example 2: Characterization of CDs and PVA / AL / CDs hydrogels The chemical structures of CDs and citric acid were determined using an FT-IR-4700 spectrometer in the range of 500–4000 cm⁻¹, with a resolution of 4 cm⁻¹ and 32 scans. The particle size of CDs was determined using a particle size analyzer. The fluorescence emission spectra (400 nm–800 nm) and ultraviolet absorption spectra (200 nm–800 nm) of CDs were recorded using a multi-mode microplate reader. The structures of different hydrogels were analyzed using ATR-FT-IR, performed in transmission mode with a resolution of 4 cm⁻¹, 200 scans, and a wavenumber range of 500–4000 cm⁻¹.

[0041] Experimental results are as follows Figure 2 As shown, in this study, we successfully synthesized CDs using citric acid as the main raw material by microwave heating for 5 min. The functional groups of the CDs were analyzed using FT-IR spectroscopy. Figure 2 As shown in section A, the absorption peak of CDs appears at 3344 cm⁻¹. −1 1459 cm −1 and 1192 cm −1The dispersion of these hydrophilic groups in water is attributed to the stretching vibrations of OH, C=O, and CN, respectively. The presence of these hydrophilic groups ensures their good dispersion in water. Furthermore, at 1610 cm⁻¹... -1 The presence of an absorption peak at the C=C bond indicates the formation of a carbon structure within the graphite core. The particle size of the CDs was determined using a particle size analyzer. Figure 2 Part B shows that the average particle size of the CDs is approximately 5.3 ± 1.8 μm, indicating the formation of nanoscale CDs with a uniform overall particle size distribution. The optical properties of the carbon dots were characterized by fluorescence emission and UV-Vis absorption spectroscopy. Figure 2 The C portion of the graphite core shows that CDs have a typical UV absorption peak at 260 nm, which can be attributed to the π-π* transition of C=C, further demonstrating the formation of carbon structures in the graphite core. The optimal emission peak of CDs is located at 520 nm, consistent with previous literature.

[0042] in addition, Figure 2 In Part C of the diagram, the illustration in the upper left corner shows optical images of CDs under sunlight and ultraviolet light, which exhibit pale yellow and green fluorescence, respectively.

[0043] After synthesis, CDs were introduced into a PVA matrix along with AL, and a bimodal smart active hydrogel was prepared using a simple freeze-thaw method. The structural interactions of the PVA / AL / CDs hydrogel were characterized by FT-IR. Figure 2 As shown in section D, PVA hydrogels without added AL or Zn-MOF reach approximately 3299 cm⁻¹. −1 2925 cm −1 1321 cm −1 and 1091 cm −1 The bands at these points represent typical absorption bands, corresponding to the OH stretching vibration, CH2 stretching vibration, CH-CH2 bending vibration, and CO stretching vibration, respectively.

[121] When AL was introduced into the PVA hydrogel, the OH absorption peak showed a significant blue shift (from 3299 cm⁻¹ to 3297 cm⁻¹), indicating a hydrogen bond interaction between the hydroxyl groups in PVA and the oxygen-containing groups (OH, C=O) in AL. When CDs were added to the PVA hydrogel, the OH absorption peak shifted from 3299 cm⁻¹ to 3297 cm⁻¹. −1 Blue shifted to 3293 cm −1 This may be attributed to the hydrogen bonding interactions between oxygen-containing groups (OH, COOH) in PVA and CDs. Additionally, in PVA / CDs hydrogels at 1600 cm⁻¹... -1The presence of C=C bonds at the point confirms the successful doping of CDs. Furthermore, the addition of AL and CDs to the PVA hydrogel resulted in a more significant blue shift of the OH absorption peak, indicating a stronger hydrogen bond interaction among PVA, AL, and CDs.

[0044] Example 3: Mechanical property testing of PVA / AL / CDs hydrogel 1. Gel strength The gel strength of the smart active hydrogel was determined using a texture analyzer (Stable Micro Systems, Surrey, UK). Cylindrical hydrogel samples were prepared with a diameter of 2.2 cm and a height of 2 cm. Experimental parameters were set as follows: cylindrical probe (P / 0.5R), pre-experiment speed (1 mm / s), testing speed (1 mm / s), post-experiment speed (1 mm / s), and penetration distance (4 mm).

[0045] 2. Texture properties The textural properties of the smart hydrogel were determined using a texture analyzer (Stable Micro Systems, Surrey, UK). Cylindrical hydrogel samples were prepared with a diameter of 2.2 cm and a height of 2 cm. Experimental parameters were set as follows: cylindrical probe (P / 36 R), pre-test speed (1 mm / s), test speed (1 mm / s), post-test speed (1 mm / s), and compression ratio (40%).

[0046] 3. Moisture distribution The water distribution and state of the hydrogel were determined using an NMI20 low-field NMR instrument (NIUMAG, China) according to previously reported methods. Briefly, 3.5 g of hydrogel was placed in a 25 mm diameter NMR glass tube, which was then inserted into the NMR probe. T2 values ​​were determined using a Carr-Purcell-Meiboom-Gill sequence. Each sample was tested three times.

[0047] 4. Moisture content, water solubility, and swelling rate The water content of the hydrogel was determined by drying a 2 g hydrogel sample at 105 °C for 24 h. The formula (1) for calculating the water content (MS) is as follows: (1).

[0048] Where m1 and m2 are the masses of the hydrogel before and after drying, respectively.

[0049] Next, 5 mL of distilled water was added to each centrifuge tube containing the dried hydrogel, and the mixture was allowed to stand for 24 h. Then, the insoluble hydrogel samples were dried at 105 °C for 24 h and weighed (m3). Water solubility (WS) was calculated using formula (2): (2).

[0050] First, we tested the initial mass of the hydrogel (m4). Then, we immersed the hydrogel in 5 mL of water and recorded its mass again (m5). Before weighing, the surface moisture of the hydrogel was wiped off. The swelling ratio was calculated using the following formula (3): (3).

[0051] Experimental results are as follows Figure 3 As shown in Table 4, excellent mechanical properties are crucial for evaluating the preservation indication effect of the dual-mode smart active hydrogel. To assess the hydrogel's resistance to deformation, we measured its gel strength, and the results are shown in Table 4.

[0052] Table 4. Gel strength and textural properties of hydrogels

[0053] After the addition of AL, the gel strength of the PVA hydrogel increased from 83.98 g to 93.25 g. This improvement is attributed to the hydrogen bonding interaction between the oxygen-containing groups of PVA and AL, which makes the hydrogel network more uniform and dense, thereby enhancing the gel strength. When CDs are added to the PVA hydrogel, the gel strength increases to 107.44 g. This is because the nano-sized CDs, acting as fillers, are dispersed in the PVA hydrogel, helping to resist and transfer stress, thus improving the gel strength. When AL and CDs are present in the PVA hydrogel simultaneously, the gel strength increases to 116.05 g. This phenomenon is attributed to the strong interaction among PVA, AL, and CDs.

[0054] To further demonstrate that AL and CDs enhance the mechanical properties of PVA hydrogels, we measured their textural properties. As shown in Table 3, the textural properties and gel strength of the hydrogels exhibit the same trend. Notably, the PVA / AL / CDs hydrogel is significantly superior to the PVA hydrogel in both hardness and cohesion, indicating that this hydrogel possesses stronger internal binding forces and resistance to external deformation. Furthermore, the PVA / AL / CDs hydrogel also demonstrates the best performance in elasticity and resilience, showing a significant difference compared to the PVA hydrogel. This indicates that the hydrogel can better recover its original shape after repeated compression, thanks to the reversible hydrogen bonds in its structure that can continuously break and recombine under external force, effectively dissipating energy and thus endowing the hydrogel with excellent elasticity and resilience.

[0055] NMR analysis is used to study the water distribution of hydrogels and its effect on mechanical properties. For example... Figure 3 As shown in Part A, the relaxation times for bound water, non-flowing water, and free water in the hydrogel range from 0.1 to 10 ms (T0). 21 ), 10-100 ms (T 22 100-1000 ms (T) 23 Relative water content is calculated by measuring peak area P. 21 P 22 and P 23 ( Figure 3 (As shown in Part B of the figure) was measured, where P 23 Excessive phosphorus content (PL) is generally detrimental to hydrogel formation and leads to poor mechanical properties. The introduction of AL shifts the relaxation time of PVA hydrogels to the left, indicating a more uniform and dense network structure, thereby enhancing mechanical properties. In PVA / AL hydrogels, PL... 21 and P 22 Increase, P 23 The decrease indicates that the groups in AL can bind with free water and convert it into bound water or non-flowing water. The addition of CDs also leads to a leftward shift in relaxation time, further improving network density and mechanical properties. P in PVA / CDs hydrogels... 21 and P 22 Increase, P 23 The decrease indicates that the surface groups of CDs interact with free water, converting it into bound water or non-flowing water. When AL and CDs are simultaneously introduced into the PVA hydrogel, the relaxation time continuously shifts to the left, and P... 21 and P 22 Further increase, P 23 The further reduction indicates that the hydrogel's network structure is more uniform and compact. This phenomenon is attributed to the synergistic effect between PVA, AL, and CDs, which forms a strong interaction force and significantly improves the mechanical properties of the hydrogel.

[0056] Water resistance is an important characteristic of dual-mode smart active hydrogels. For example... Figure 3 As shown in section C, the addition of AL reduces the water content of the PVA hydrogel. This is because the enhanced hydrogen bonding interaction between AL and PVA makes the network structure more compact, thereby reducing the free water content. Similarly, the addition of CDs also reduces the water content of the hydrogel. This is because CDs make the PVA hydrogel network structure more compact, thus reducing the water content. When AL and CDs are introduced into the hydrogel simultaneously, the water content decreases further, indicating that the synergistic effect of the two makes the network structure more compact. In addition, the trend of water solubility changes of different hydrogels is consistent with the water content (…). Figure 3As shown in section C), the PVA / AL / CDs hydrogel has the lowest water solubility, which is also attributed to the more uniform and compact cross-linked network of the hydrogel, indicating its excellent water resistance.

[0057] In addition, the swelling ratio of the hydrogel was evaluated. For example... Figure 3 As shown in section D, the addition of AL reduces the swelling rate of the PVA hydrogel. This is because the high-density cross-linked gel network prevents further water molecule penetration, thus limiting the swelling of the hydrogel. Similarly, the addition of CDs further enhances the cross-linking density, leading to a decrease in the swelling rate. When AL and CDs are simultaneously incorporated into the hydrogel, the swelling rate drops to 11.29%, the lowest level, indicating that the cross-linking structure of the PVA / AL / CDs composite hydrogel is the most dense, limiting water penetration and maintaining strong structural stability. These results demonstrate that the PVA / AL / CDs hydrogel can not only absorb some of the exudate from shrimp tissue, helping to reduce the accumulation of free water and inhibit microbial growth, but also maintain its structural integrity to the greatest extent, thereby improving its stability and practicality in food preservation applications.

[0058] Example 4: Colorimetric performance test of PVA / AL / CDs hydrogel 1. pH sensitivity CDs solutions (0.1 mg / mL) with different pH values ​​(4-12) were prepared, and UV and fluorescence measurements were performed using a Spectra Max i3 multi-microplate reader (Molecular Devices, USA), with scanning wavelengths of 400-800 nm and excitation wavelengths of 365 nm.

[0059] The color responses of different hydrogels were recorded by taking photos with a smartphone. Different hydrogel samples were immersed in different pH buffer solutions (4-12) for 1 h. After the hydrogels were removed, they were placed in a small darkroom and photographed with a smartphone (under visible light and 365 nm excitation light). Then, the color values ​​(R, G, B values) of the hydrogels were extracted using Adobe Photoshop (PS) software, and the color difference (ED) was calculated using formula (3.4).

[0060] 2. Trimethylamine sensitivity Different hydrogels were placed in culture dishes, and 500 ppm of trimethylamine (TMA) was added. Images of the hydrogels were captured using a smartphone at 0, 0.25, 0.5, 0.75, 1, 2, 3, 6, and 12 h (under visible light and 365 nm excitation light). The RGB values ​​of the captured images were extracted, and the color response (ED) of different hydrogels to volatile TMA was calculated using formula (2.1).

[0061] 3. Stability Different hydrogels were packaged in plastic petri dishes (90 mm) and sealed with Parafilm M (Bemis, USA) to avoid the influence of relative humidity, and then stored in a refrigerator at 4°C for 14 days. Images of the hydrogels (under visible light and 365 nm excitation light) were taken using a smartphone at 0, 2, 4, 6, 8, 10, 12, and 14 days. Based on the recorded color values ​​(R, G, B values), the formula was used... Calculate the color stability (ED) of different hydrogels.

[0062] Experimental results are as follows Figure 4 As shown, the pH responsiveness of CDs is crucial for their potential applications in smart sensing. Therefore, the relevant fluorescence response behavior was evaluated in solutions with different pH values ​​(pH = 4-12). Figure 4 As shown in Part A, the fluorescence intensity of CDs continuously decreases with increasing pH, and the emission spectrum exhibits a significant blue shift (520 nm - 470 nm) in alkaline environments. This is mainly due to the conversion of OH and COOH functional groups to O in alkaline environments. − and COO − The resulting deprotonation causes the color in the illustration to gradually change from green to blue. Furthermore, the fluorescence emission peak of CDs overlaps with the UV absorption peak of AL (e.g., Figure 4 (As shown in Part B of the diagram), this demonstrates that AL can quench the fluorescence of CDs through an internal filtration effect. Next, the pH-responsive behavior of the AL- and CD-incorporated hydrogel was further evaluated. Figure 4 Part C of the diagram shows the color changes of different hydrogels under visible and ultraviolet light within a pH range of 4-12. Due to the lack of a pH indicator, the pure PVA hydrogel showed essentially no color change under both visible and ultraviolet light. In contrast, the PVA / AL hydrogel exhibited a color change from light green to purple under visible light with increasing pH, but remained unchanged under ultraviolet light. The PVA / CDs hydrogel showed essentially no color change under visible light with increasing pH, but exhibited a color change from green to blue under ultraviolet light, consistent with the changes of CDs in solutions with different pH values. Interestingly, the addition of CDs significantly enhanced the color change of the PVA / AL hydrogel under visible light, possibly due to increased gel strength, which maintained the structural integrity of the AL molecules, thus improving its significant response to pH changes. Furthermore, under ultraviolet light, the PVA / AL / CDs hydrogel gradually changed from green to deep blue, mainly due to the internal filtration effect triggered by the addition of AL, leading to a more significant fluorescence change.

[0063] To further verify the application potential of hydrogels in freshness monitoring, TMA was selected as a typical volatile deterioration marker for testing. First, the colorimetric and fluorescence changes corresponding to the response of hydrogels to TMA were recorded (e.g., Figure 4 As shown in section D of the diagram, the trend of color change is similar to that observed in pure pH solution. For further quantitative analysis, color changes were recorded using a smartphone, converted to RGB values, and finally to ED values ​​(e.g., [image of ED values]). Figure 4 (As shown in sections E and F). The ED value of pure PVA hydrogel remained almost unchanged under visible and ultraviolet light. Adding alkaline (AL) significantly increased the ED value of the PVA / AL hydrogel under visible light, indicating that the addition of AL enables reliable TMA detection via pH response characteristics. Under ultraviolet light, the ED value also increased slightly, due to the quenching of the hydrogel's inherent blue fluorescence by alizarin. Adding CDs to the PVA hydrogel resulted in no change in the ED value of the PVA / CDs hydrogel under visible light, but a significant change under ultraviolet light, indicating that the addition of CDs enables reliable TMA detection via pH response characteristics. Surprisingly, the ED value of the PVA / AL / CDs hydrogel increased more rapidly and reached a higher equilibrium level under both visible and ultraviolet light. This is because CDs have a higher specific surface area, thus providing more surface active sites, increasing the opportunity for the hydrogel to contact TMA, thereby improving adsorption efficiency and quantity. These results fully demonstrate the superior performance of PVA / AL / CDs hydrogel in TMA detection.

[0064] The color stability of PVA / AL / CDs hydrogels directly affects the accuracy of monitoring; therefore, it must be evaluated to ensure reliability in practical applications. For example... Figure 4 As shown in section G, the colorimetric and fluorescence images of pure PVA hydrogels and PVA / CDs hydrogels remained unchanged during 14 days of storage, while the colors of the remaining hydrogels changed slightly, mainly due to the oxidation of AL. Correspondingly, as... Figure 4 As shown in sections H and I, the ED values ​​of PVA hydrogel and PVA / CDs hydrogel remained below 2 under visible and ultraviolet light, respectively. Conversely, the ED values ​​of PVA / AL hydrogel increased to 11.1 and 6.5 for colorimetric and fluorescence measurements, respectively, after 14 days of storage. This indicates that the color change of the hydrogel is visible to the naked eye, which is not conducive to detection (ED values ​​<5 are invisible to the naked eye, and >5 are visible to the naked eye). However, after the addition of CDs, the ED values ​​of both the colorimetric and fluorescence measurements of PVA / AL / CDs hydrogel remained below 5, indicating that its color change was not significant. This demonstrates that the addition of CDs helps to form a more uniform and dense hydrogel, thereby maintaining the stability of AL.

[0065] Example 5: Bioactivity test of PVA / AL / CDs hydrogel 1. Antioxidant activity DPPH free radical scavenging ability determination: Extracts of different hydrogel samples were added to DPPH ethanol solution (0.2 mM), and after standing in the dark for a period of time, the absorbance of the mixed solution at 527 nm was measured. Distilled water was used instead of DPPH working solution in the control group, and distilled water was used instead of sample solution in the blank group. The DPPH free radical scavenging ability was calculated by the following formula (5): (5).

[0066] Note: A X A represents the absorbance value of the experimental group. X0 A1 represents the absorbance value of the control group; A2 represents the absorbance value of the blank group.

[0067] ABTS free radical scavenging capacity determination: Extracts of different hydrogel samples were added to ABTS solution, and the absorbance of the mixed solution at 734 nm was measured after standing in the dark for a period of time. Distilled water was used instead of ABTS working solution in the control group, and distilled water was used instead of sample solution in the blank group. The ABTS free radical scavenging rate was calculated according to formula (6): (6).

[0068] Note: A X A represents the absorbance value of the experimental group. X0 A1 represents the absorbance value of the control group; A2 represents the absorbance value of the blank group.

[0069] Different hydrogel sample extracts, 150 µL FeSO4 (8 mM), 125 µL H2O2 (20 mM), and 500 µL salicylic acid (3 mM, dissolved in ethanol) were mixed and heated in a 37°C water bath for 20 min. After centrifugation at 2000 rpm for 5 min, the absorbance of the supernatant was measured at 510 nm. The scavenging rate of hydroxyl radicals was calculated by the following formula (7): (7).

[0070] Note: A X A represents the absorbance value of the experimental group. X0 A1 represents the absorbance value of the control group; A2 represents the absorbance value of the blank group.

[0071] 2. Antibacterial activity (1) The ability of CDs to chelate Fe3+ CDs chelate Fe 3+ Proficiency assay: CDs (0.5 mg / mL) were reacted with Na + K + Ca 2+ Mg 2+Cu 2+ Al 3+ and Fe 3+ The mixture contained metal ions of 6 mM concentration and identical volume. Other metal ions were mixed with Fe... 3+ After mixing, an equal volume of CDs was added. The fluorescence intensity of the solution was recorded at an excitation wavelength of 365 nm. The fluorescence quenching rate F / F0 was used to assess the degree of fluorescence quenching of CDs by metal ions, where F and F0 represent the fluorescence intensity of each solution (with and without metal ions).

[0072] (2) Antibacterial properties Antimicrobial activity assay of Zn-MOF and different hydrogels: Escherichia coli (O157:H7, ATCC 25922) and Staphylococcus aureus (S. aureus, ATCC 25923) were used as representative strains of Gram-negative and Gram-positive bacteria, respectively. The antimicrobial activity of Zn-MOF and different hydrogels was evaluated by plate counting method.

[0073] To assess the antibacterial properties of different hydrogels, 300 mg of each sterilized hydrogel was separately mixed with 1 mL of bacterial suspension (1 × 10⁻⁶). 5 The bacterial suspension (cfu / mL) was incubated at 37°C and 150 rpm for 12 h. 200 μL of the bacterial suspension was then spread onto LB agar plates and incubated at 37°C for 24 h.

[0074] To further evaluate the antibacterial properties of different hydrogels, bacterial suspensions incubated with hydrogels were thoroughly mixed with NucGreen and EthD-III dyes and incubated at room temperature in the dark for 15 min. Bacterial activity was then observed using a fluorescence upright microscope (DM 3000, Germany).

[0075] 3. Biocompatibility To assess the biocompatibility of the hydrogel, cell viability was tested using a CCK-8 kit and Caco-2 cells. After sterilization under UV light for 1 h, the hydrogel was immersed in complete culture medium at a concentration of 0.1 μg / mL for 24 h to prepare an extract, which was then filtered using a sterile filter (PES membrane, 0.22 μm). Caco-2 cells (100 μL, 10⁵ cells / mL) were seeded in 96-well plates and cultured at 37°C and 5% CO₂ for 48 h. After removing the complete culture medium, 100 μL of the extract was added, and the cells were cultured for another 48 h. Then, 10 μL of CCK-8 solution was added to each well, and after incubation for 1 h, the absorbance was measured at 450 nm. Caco-2 cells cultured in complete culture medium without the hydrogel were used as a control group. Cell viability was calculated using formula (8): (8).

[0076] Wherein, ODtest and ODcontrol are the absorbance of the experimental group and the control group, respectively.

[0077] Experimental results are as follows Figure 5 As shown, the antioxidant activity of different hydrogels was evaluated using DPPH and ABTS free radical scavenging experiments. Figure 5 As shown in Part A, the DPPH radical scavenging activities of both PVA / AL and PVA / CDs hydrogels were significantly higher than those of pure PVA hydrogels. The addition of AL and CDs increased the DPPH radical scavenging rate by more than 6-fold and 8-fold, respectively. When AL and CDs were added to the PVA hydrogel simultaneously, the DPPH radical scavenging rate exceeded 15-fold, indicating a synergistic effect between AL and CDs. This may be because AL and CDs may react with free radicals through different mechanisms, and their effects may complement each other, thereby enhancing the overall free radical scavenging capacity. Furthermore, the DPPH radical scavenging rate of CDs showed a concentration-dependent effect, which may be attributed to the antioxidant properties of the carboxyl groups in CDs.

[139] (like Figure 5 (As shown in Part B of the diagram). To further evaluate the antioxidant activity of different hydrogels, we conducted ABTS radical scavenging experiments (e.g., [reference needed]). Figure 5 As shown in section C), a trend similar to that observed in the DPPH scavenging experiment was obtained. The scavenging rates of PVA / AL and PVA / CDs hydrogels were 3 and 6 times that of PVA hydrogels, respectively, while the scavenging rate increased by more than 10 times after the simultaneous addition of AL and CDs. In addition, the scavenging rate of CDs for ABTS radicals also showed a concentration-dependent effect (e.g., Figure 5 (as shown in section D of the diagram). These results demonstrate that the antioxidant capacity of PVA / AL / CDs hydrogels is significantly enhanced by the addition of AL and CDs, showing their strong potential for food storage applications.

[0078] In food preservation applications, the antibacterial ability of hydrogels is a key indicator, helping to effectively prevent food spoilage caused by bacteria. CDs can deprive bacteria of the iron (Fe) they need for growth. 3+ This leads to bacterial inhibition or death due to iron deficiency. To verify the effect of CDs on Fe... 3+ To assess the chelating ability of CDs, fluorescence quenching experiments were conducted by adding CDs solution to a 6 mM solution of seven metal ions (e.g., ...). Figure 5 (As shown in part E). We found Fe 3+ The fluorescence quenching effect on CDs was the strongest, which proves that CDs can chelate Fe. 3+ .like Figure 5As shown in the F portion, CDs retain their resistance to Fe even in the presence of other metal ions. 3+ The fluorescence quenching behavior of CDs. Therefore, CDs affect Fe. 3+ It has specific selectivity. Figure 5 G shows the CDs solution with Fe 3+ Emission spectra of solutions (0-25.6 mM) after mixing. Fluorescence intensity of CDs increases with Fe... 3+ The decrease in concentration with increasing concentration confirms the presence of abundant Fe on the surface of CDs. 3+ Binding sites, and 25.6 mM Fe 3+ This caused CDs to completely lose their fluorescent properties. To verify and investigate the chelation of Fe by CDs... 3+ The mechanism was determined by FT-IR measurement of the addition of Fe to CDs. 3+ Changes in post-functional groups (such as) Figure 5 (As shown in the H part). The results show that, with the addition of Fe... 3+ Later, 1718 cm −1 The C=O at that location shifted to 1709 cm. −1 1417 cm −1 The OH blue shifted to 1401 cm. −1 This indicates that the oxygen-containing groups in CDs are related to Fe. 3+ Coordinate bonds are formed. Specifically, the O atoms on the CDs surface interact with Fe through lone pairs of electrons. 3+ Coordination occurs, thereby forming Fe-O coordinate bonds.

[142] .like Figure 5 As shown in section I, the negative zeta potential (-3.68 mV) indicates that the CDs surface is rich in hydroxyl and carboxylic acid groups. With Fe... 3+ With the addition of Fe, the zeta potential of CDs changed from -3.68 mV to 12.40 mV, which proves that the hydroxyl and carbonyl groups on the CD surface react with Fe. 3+ The interaction is consistent with the results of FT-IR spectroscopy.

[0079] In this study, Vibrio parahaemolyticus (Vibrio parahaemolyticus) was selected. V. parahaemolyticus (ATCC 17802) was used as a model bacterium to evaluate the antibacterial effects of different hydrogels. For example... Figure 5As shown in Figure J, pure PVA hydrogel did not exhibit any inhibitory effect on Vibrio parahaemolyticus, indicating that it lacks antibacterial ability. PVA / AL hydrogel showed some inhibitory effect on Vibrio parahaemolyticus, likely due to the inherent antibacterial activity of AL. In contrast, the addition of CDs inhibited the growth of most Vibrio parahaemolyticus, demonstrating the strong antibacterial ability of CDs. More interestingly, the simultaneous addition of AL and CDs to the hydrogel resulted in a 100% inhibition rate against Vibrio parahaemolyticus, indicating a synergistic effect between AL and CDs. These two substances may play a role at different stages of bacterial growth and reproduction, thereby enhancing the antibacterial effect. Furthermore, we confirmed the minimum inhibitory concentration (MIC) of CDs using a plate coating experiment; the MIC of CDs against Vibrio parahaemolyticus was 0.32 mg / mL. These results demonstrate that CDs and AL significantly enhance the antibacterial properties of the hydrogel, making it a promising candidate material for food preservation applications.

[0080] The excellent antibacterial and antioxidant properties of the dual-mode smart active hydrogel may be attributed to the release of AL and CDs. To investigate the release efficiency, the release curves of AL and CDs showed rapid release within the first 4 hours, followed by a gradual decrease in the release rate (e.g., ...). Figure 5 (As shown in the KL section). The initial rapid release is due to the accumulation of AL and CDs on the hydrogel surface, followed by a slower release from within the hydrogel. The continuous release of AL and CDs from the hydrogel suggests that the PVA / AL / CDs hydrogel has the potential for long-term preservation of shrimp.

[0081] The biocompatibility of the hydrogel samples was assessed using the CCK-8 assay (e.g., Figure 5 (As shown in section M). Cell viability results showed that, compared with the control group, the activity of all groups of hydrogels after incubation with Caco-2 cells was above 90%. ISO 10993-5 standard stipulates that materials are considered cytotoxic when cell viability is below 70%, and the above results indicate that the PVA / AL / CDs hydrogel has good biocompatibility.

[0082] Example 6: Application of PVA / AL / CDs hydrogel in monitoring shrimp freshness Shrimp were encased in two PVA / AL / Zn-MOF-0.3 hydrogels (8.5 cm × 8.5 cm × 0.5 cm) and then placed in a food-grade transparent square box (10.8 cm × 10.8 cm × 5.9 cm) and stored at 4°C for 10 days. Images of the PVA / AL / Zn-MOF-0.3 hydrogels were captured using a smartphone and a darkroom. Next, the RGB values ​​of 16 points on the hydrogels were extracted using a WeChat mini-program to calculate the average ED value. The relationship between TVB-N and the average ED value was derived using a nonlinear fitting formula and integrated into the mini-program to assess shrimp freshness. This program was finally tested on 30 unknown shrimp samples. Simultaneously, the TVB-N of the shrimp was determined according to the method of GB 5009.288-2016.

[0083] Results Analysis: The freshness of shrimp was visually monitored by observing changes in the color of the prepared PVA / AL / CDs hydrogel. Figure 6 Part A shows daily images of PVA / AL / CDs hydrogels containing wrapped shrimp stored at 4°C for 10 days under visible and ultraviolet light, captured using a smartphone. Under visible light, the hydrogel changes from yellow to deep purple. Under ultraviolet light, the hydrogel changes from bright green to blue-green. To achieve more accurate analysis, we independently developed a WeChat mini-program called "FreshnessMonitor," which automatically converts the captured data into ED values ​​and freshness grades. This mini-program supports two signal detection modes: fluorescence and colorimetric readings. First, 16 uniformly spaced points are selected on the hydrogel; the mini-program automatically extracts the RGB values ​​of these points and calculates the average ED value. (See image for details.) Figure 6 As shown in section BC, the changes in colorimetric and fluorescence ED values ​​over 10 days are consistent with the TVB-N values ​​of shrimp, and their relationship can be calculated using logistic nonlinear regression. (R) 2 =0.9966) (e.g. Figure 6 (as shown in section D), used for colorimetric readings. (R) 2 =0.9978) (e.g. Figure 6 The E part (shown in the diagram) is used for fluorescence readings. These two formulas are then integrated into a mini-program for more accurate freshness assessment. Therefore, after capturing an image, the mini-program allows for automatic capture of RGB values ​​via fluorescence and colorimetric reading modes, calculation of the ED value, and ultimately conversion into a freshness grade, which is then displayed on the smartphone screen (e.g., as shown in the diagram). Figure 6(As shown in part F of the diagram). To further verify the reliability of this dual-mode detection platform, we used 30 blind samples to compare its detection results with the traditional Kjeldahl method. The results show that there is a high correlation between the smartphone-based detection method and the Kjeldahl method (Pearson correlation coefficients r = 0.9785 and 0.9884 for colorimetric and fluorescence modes, respectively). Figure 6 (As shown in the GH section). This high correlation indicates that both detection modes produce highly consistent results. However, the fluorescence mode exhibits higher sensitivity, making it more reliable in detecting food freshness. Therefore, by integrating a self-correcting function for dual-mode readout, this smartphone-based colorimetric-fluorescence sensing platform can be used for real-time monitoring of shrimp freshness.

[0084] Example 7: Application of PVA / AL / CDs hydrogel in shrimp preservation To evaluate the effectiveness of PVA hydrogel and PVA / AL / Zn-MOF-0.3 hydrogel in extending the shelf life of shrimp, changes in TVB-N, total viable count (TVC), and pH were tested during storage. Fresh shrimp samples were randomly assigned to a control group (unpackaged), a PVA hydrogel-packaged group, and a PVA / AL / Zn-MOF-0.3 hydrogel-packaged group. Each shrimp sample was directly wrapped with either PVA hydrogel or PVA / AL / Zn-MOF-0.3 hydrogel and stored at 4°C for 10 days. TVB-N and pH were measured daily, and TVC was measured every two days.

[0085] TVB-N in shrimp samples was determined using the automated Kjeldahl method as described in GB 5009.228-2016; pH value was determined according to GB 5009.237-2016; and TVC value was determined according to GB 4789.2-2022.

[0086] Results Analysis: To evaluate the preservation ability of the smart active hydrogel, this study measured the key spoilage indicators (TVC, TVB-N and pH value) of shrimp samples treated with the hydrogel at different storage times (4℃).

[0087] TVC (Total Volume Currency) is a key indicator of shrimp freshness, as the proliferation and growth of microorganisms are the main cause of shrimp spoilage. Typically, when TVC falls below 5.0 log... 10 CFU g -1 At that time, the shrimp was considered fresh, but when the TVC reached 6.0 log 10 CFU g -1 At this point, it indicates that the shrimp has spoiled. For example... Figure 7 As shown in Part A, the TVC of fresh shrimp is 4.2 log. 10CFU g -1 During storage, untreated shrimp reached 6.3 logs on day 6. 10 CFU g -1 The levels exceeded the spoilage threshold. In contrast, shrimp coated with PVA hydrogel showed only 5.9 log50 on day 6. 10 CFU g -1 This is because PVA hydrogel can absorb some of the exudate, thereby inhibiting microbial growth and enhancing the preservation effect of shrimp. Surprisingly, the TVC growth of shrimp coated with PVA / AL / CDs hydrogel was slow, reaching only 5.3 log on day 6. 10 CFU g -1 It was in the second stage of freshness, and only reached the spoilage threshold (6.2 log) on ​​the 9th day. 10 CFU g -1 This indicates that the hydrogel has a strong antibacterial effect under the synergistic effect of AL and CDs.

[0088] TVB-N values ​​are used to quantify the degree of protein breakdown in shrimp. A TVB-N value below 20 mg / 100 g indicates freshness, while a value above 30 mg / 100 g indicates spoilage. Figure 7 As shown in Part B, the TVB-N value of untreated shrimp dropped to a secondary freshness level on day 3 and reached the spoilage threshold on day 5, while shrimp coated with PVA hydrogel spoiled on day 6 due to reduced protein decomposition. Surprisingly, shrimp coated with PVA / AL / CDs hydrogel reached a secondary freshness level on day 4 and reached the spoilage threshold on day 8. This was attributed to the antibacterial and antioxidant properties of AL and CDs, which reduced the amount of bacterial proteases and protected proteins from oxidative damage by capturing free radicals, ultimately reducing protein decomposition and lowering the TVB-N value. To further confirm that PVA / AL / CDs hydrogel reduces protein decomposition in shrimp, we measured the pH value, which reflects the acidity or alkalinity of the shrimp meat. A pH value exceeding 7.8 indicates shrimp spoilage.

[125] .like Figure 7 As shown in section C, the shelf life of shrimp encapsulated in PVA / AL / CDs hydrogel was extended to 8 days, consistent with the TVB-N results. These results indicate that the PVA / AL / CDs hydrogel extended the shelf life of shrimp by at least 3 days compared to the control group, demonstrating its potential as an effective smart active hydrogel.

[0089] Overall Conclusion: Four types of hydrogels (PVA, PVA / AL, PVA / CDs, and PVA / AL / CDs) were prepared using PVA, AL, and CDs as raw materials via a freeze-thaw method. The effects of AL and CDs on the physical and functional properties of PVA hydrogels were systematically studied, and their freshness indication and preservation performance in shrimp preservation were evaluated. The main conclusions are as follows: (1) Experiments on the structure and properties of the hydrogel showed that the addition of AL and CDs enhanced the hydrogen bonding interactions of the PVA hydrogel, improved the gel strength and textural properties, and reduced the water solubility and free water ratio, thereby enhancing the mechanical properties. In addition, due to the inherent pH fluorescence response characteristics of CDs and the fluorescence intrinsic rate effect of AL, the PVA / AL / CDs hydrogel exhibited colorimetric / fluorescence dual-mode responses in different pH solutions and TMA atmospheres. Furthermore, after 14 days of storage, the colorimetric and fluorescence ED values ​​of the PVA / AL / CDs hydrogel were both less than 5, indicating its excellent stability. In addition, the sustained-release properties of AL and CDs endowed the hydrogel with significant antioxidant and antibacterial capabilities, while also exhibiting good biocompatibility.

[0090] (2) Application studies of the intelligent active hydrogel show that the synergistic effect of AL and CDs effectively inhibits the increase of TVC, TVB-N and pH in shrimp samples, extending the shelf life by 3 days. Furthermore, during shrimp spoilage, the PVA / AL / CDs hydrogel exhibits a color change from yellow to dark purple under visible light and from bright green to blue-green under ultraviolet light. Both color changes show a non-linear correlation with TVB-N values ​​(Pearson correlation coefficients r = 0.9785 and 0.9884 for colorimetric and fluorescence modes), achieving sensitive detection with self-reference correction for both signal modes. Based on a smartphone detection system, a portable real-time freshness monitoring device was successfully constructed, enabling dynamic assessment of shrimp quality.

Claims

1. A PVA@AL@CDs hydrogel, characterized in that, It contains a polyvinyl alcohol matrix, as well as alizarin, carbon dots with unique fluorescent properties, and water dispersed in the polyvinyl alcohol matrix; wherein the mass percentages of polyvinyl alcohol, alizarin, carbon dots, and water are 9%-10%, 0.004%, 0.04%, and 90%-91%, respectively.

2. The PVA@AL@CDs hydrogel according to claim 1, characterized in that, Carbon dots are obtained by reacting citric acid and urea with microwave heating, with a mass ratio of citric acid to urea of ​​1:

2.

3. A method for preparing a PVA@AL@CDs hydrogel, characterized in that, Includes the following steps: S1. Dissolve citric acid and urea completely in deionized water to obtain a mixture; S2. The mixture is placed in a microwave oven and heated to react. After cooling to room temperature, it is centrifuged and washed. The supernatant is filtered through a microporous membrane to obtain CDs powder. S3. Dissolve PVA in water, stir, add AL, and finally add CDs powder. After mixing, you can get PVA@AL@CDs gel solution. S4. Pour the obtained PVA@AL@CDs gel solution into a mold, freeze it, and then thaw it at room temperature to obtain PVA / AL / CDs hydrogel.

4. The preparation method according to claim 3, characterized in that, In step S2, the microwave heating reaction time is 5 minutes.

5. The preparation method according to claim 3, characterized in that, In step S2, the centrifugal washing conditions are: 10000 rpm, 10 min.

6. The preparation method according to claim 3, characterized in that, In step S3, the stirring temperature for dissolving PVA in water is 95℃, and the stirring time is 2 hours.

7. The preparation method according to claim 3, characterized in that, In step S3, the temperature at which AL is dissolved in the PVA solution is 50°C, and the mixing reaction time is 1 hour.

8. The preparation method according to claim 7, characterized in that, In step S3, after adding CDs powder, the mixing reaction time is 1 hour.

9. The preparation method according to claim 3, characterized in that, In step S4, the freezing temperature is -20℃, the freezing time is 12h, and the thawing time is 1h.

10. The application of the PVA@AL@CDs hydrogel according to any one of claims 1-2 and 3-9, characterized in that, Including (1), (2), (3) and (4): (1) Colorimetric signal response based on pH sensitivity of AL; (2) Fluorescence signal response based on the pH-dependent fluorescence characteristics of CDs and the internal filtration effect of AL on CDs; (3) Reagents used to prepare for testing the freshness grade of shrimp; (4) Preservatives used in the preparation of shrimp.

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