A dual-channel intelligent indicator label for monitoring the freshness of high-protein foods and its preparation method

Abstract

This invention discloses a dual-channel intelligent indicator label for monitoring the freshness of high-protein foods. It uses cassava starch and polyvinyl alcohol as a composite matrix, and combines solvents and / or film-forming aids to load polyvinylpyrrolidone-alizarin nanoparticles and carbon quantum dots onto this composite matrix to form a thin film. This film serves as an intelligent indicator label to monitor biogenic amines produced during spoilage of high-protein foods, thereby monitoring their freshness. The indicator label of this invention enables accurate, real-time, and visual monitoring of the freshness of high-protein foods under sunlight and ultraviolet light. Through self-correction and elimination of interference from external factors, it improves sensitivity and reliability.

Description

Technical Field

[0001] This invention belongs to the field of smart label preparation, specifically relating to a dual-channel smart indicator label for monitoring the freshness of high-protein foods and its preparation method. Background Technology

[0002] With the improvement of living standards, high-protein foods are increasingly popular due to their rich content of vitamins, proteins, and various natural minerals. However, these foods are prone to microbial growth during transportation, leading to spoilage and the production of biogenic amines. Therefore, the presence of biogenic amines can serve as a direct indicator of food spoilage. Traditional methods for detecting biogenic amines mainly include high-performance liquid chromatography (HPLC), gas chromatography (GC), and mass spectrometry (MS). However, most of these methods rely on specialized laboratory equipment. While novel electronic nose technology and electrochemical sensing offer advantages such as convenience and efficiency, they require specialized technicians to operate, which cannot meet the needs of ordinary customers. Therefore, developing an intelligent, visual label for monitoring food freshness is urgently needed.

[0003] Currently, pH-sensitive smart label detection systems are a type of visual label that reflects the freshness of food within packaging and expresses it intuitively through color changes. Smart labels based on natural dyes, such as anthocyanins and beetroot red, are used to indicate food freshness. These dyes have advantages such as low cost and non-toxicity, but they are unstable. Furthermore, most current pH colorimetric labels, as a single detection mode, are easily affected by environmental factors, resulting in poor accuracy. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a dual-channel intelligent indicator label for monitoring the freshness of high-protein foods and its preparation method, which addresses the shortcomings of the prior art. It can realize the intuitive observation and differentiation of the freshness of high-protein foods by visually observing color changes under sunlight and ultraviolet light.

[0005] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows:

[0006] A dual-channel intelligent indicator label for monitoring the freshness of high-protein foods comprises a thin film formed by loading polyvinylpyrrolidone-alizarin nanoparticles and carbon quantum dots onto a composite matrix of cassava starch and polyvinyl alcohol, combined with solvents and / or film-forming aids. The polyvinylpyrrolidone-alizarin nanoparticles (PVP-AZ NPs) have a size of 450–700 nm and are prepared by directly forming ester bonds between polyvinylpyrrolidone (PVP) and alizarin (AZ), which can improve the water solubility of alizarin. The carbon quantum dots have a size of 1.5–2.5 nm and exhibit aggregation-induced emission.

[0007] According to the above scheme, the mass ratio of cassava starch to polyvinyl alcohol in the composite matrix is ​​(1.5-2.5):1; wherein the degree of alcoholysis of polyvinyl alcohol (PVA) is not less than 98 mol%.

[0008] According to the above scheme, the loading of polyvinylpyrrolidone-alizarin nanoparticles (PVP-AZ NPs) is 0.003 to 0.006% of the total mass of the composite matrix, and the loading of carbon quantum dots (RQD) is 0.01 to 0.025% of the total mass of the composite matrix.

[0009] According to the above scheme, the solvent is ethanol and water, wherein the volume ratio of ethanol to water is preferably (50-100):1; the film-forming aid is glycerol.

[0010] The preparation method of the above-mentioned dual-channel intelligent monitoring indicator label for the freshness of high-protein foods includes the following steps:

[0011] (1) Add cassava starch and polyvinyl alcohol to ultrapure water, heat at 80-100℃ for 0.5-1.5h, and then cool to 30-50℃ to obtain a composite matrix solution;

[0012] (2) Add an aqueous solution of polyvinylpyrrolidone-alizarin nanoparticles (PVP-AZNPs), an ethanol solution of carbon quantum dots (RQDs), and glycerol to the composite matrix solution obtained in step (1), stir until uniform, and obtain a film-forming solution.

[0013] (3) Let the film-forming solution obtained in step (2) stand, defoam, pour it into a container and spread it evenly. Then dry it at 30-80℃ for 12-24 hours to obtain a colorimetric / fluorescent dual-channel film, which is an indicator label for dual-channel intelligent monitoring of the freshness of high-protein food. Store it in a constant temperature and humidity environment.

[0014] According to the above scheme, in step (1), the mass ratio of cassava starch to polyvinyl alcohol (PVA) is (1.5~2.5):1; the content of cassava starch in ultrapure water is 30~40mg / mL, and the content of polyvinyl alcohol (PVA) in ultrapure water is 10~20mg / mL.

[0015] According to the above scheme, in step (2), the amount of polyvinylpyrrolidone-alizarin nanoparticles is 0.003-0.006% of the total mass of cassava starch and polyvinyl alcohol, the amount of carbon quantum dots is 0.01-0.025% of the total mass of cassava starch and polyvinyl alcohol, and the amount of glycerol is 8-12% of the total mass of cassava starch and polyvinyl alcohol; the polyvinylpyrrolidone-alizarin nanoparticles are dissolved in water to prepare an aqueous solution with a concentration of 0.2-0.3 mg / mL (labeled as PVP-AZ NPs aqueous solution); the carbon quantum dots are dispersed in ethanol to prepare an ethanol solution with a concentration of 0.5-1.5 mg / mL (labeled as RQDs ethanol solution).

[0016] According to the above scheme, in step (2), the stirring time is 1 to 2 hours.

[0017] According to the above scheme, in step (3), the thickness of the flat layer is 0.04 to 0.06 mm.

[0018] According to the above scheme, in step (3), the constant temperature and humidity environment conditions are: 20~30℃, 40~75% RH.

[0019] According to the above scheme, the preparation method of polyvinylpyrrolidone-alizarin nanoparticles (PVP-AZ NPs) is as follows: Under magnetic stirring, polyvinylpyrrolidone, 4-dimethylaminopyridine, triethylamine and alizarin (AZ) are sequentially added to dimethyl sulfoxide. After stirring and reacting in the dark, the mixture is dialyzed in a dimethyl sulfoxide aqueous solution to remove unreacted polyvinylpyrrolidone, alizarin and other impurities. The mixture is then dialyzed again in ultrapure water and freeze-dried to obtain PVP-AZ NPs. The mass ratio of polyvinylpyrrolidone, 4-dimethylaminopyridine, triethylamine and alizarin is (2-4):1:(1-4):(0.05-0.15), and the concentration of alizarin in dimethyl sulfoxide is 0.01-0.04 g / mL. The stirring reaction time is 1-2 days. The volume concentration of dimethyl sulfoxide in the dimethyl sulfoxide aqueous solution used for the first dialysis is 30-80%.

[0020] According to the above scheme, the preparation method of carbon quantum dots (RQDs) is as follows: melamine and 2,2'-dithiocarboxylic acid are dissolved and dispersed in acetic acid by ultrasonication, then transferred to a reactor and heated at 150-200℃ for 4-8 hours. After cooling to room temperature, the resulting reaction solution is dropped into boiling water to form a suspension, then filtered, washed, and vacuum dried to obtain RQDs. The mass ratio of melamine to 2,2'-dithiocarboxylic acid to acetic acid is 1:(2-4):(100-300), and the concentration of acetic acid is not less than 80%.

[0021] The dual-channel smart indicator tag described in this invention can be used as a smart indicator tag to monitor the biogenic amines produced by the spoilage of high-protein foods, thereby monitoring the freshness of high-protein foods.

[0022] The method for monitoring the freshness of high-protein foods using the indicator label of this invention is as follows: The indicator label is affixed to the inner wall of a sealed container holding the high-protein food, ensuring that the indicator label does not come into contact with the food. A camera is then used to record the color of the indicator label and its color changes to determine the freshness of the high-protein food. Specifically, under sunlight, the color change from orange-yellow to purple indicates the process of the high-protein food changing from fresh to spoiled; under ultraviolet light, the color change from red fluorescence to blue fluorescence indicates the process of the high-protein food changing from fresh to spoiled. When the high-protein food spoils and gradually produces biogenic amine molecules, the indicator label gradually changes from orange-yellow to purple under sunlight, and from red fluorescence to blue fluorescence under ultraviolet light, achieving a visually identifiable colorimetric / fluorescence dual-channel response. This is because, in the presence of biogenic amine molecules, the polyvinylpyrrolidone-alizarin nanoparticles in the indicator tag change the color of the indicator tag from orange-yellow to purple under sunlight due to deprotonation. At the same time, in the presence of biogenic amine molecules, the carbon quantum dots change from an aggregated state to a dispersed state, causing the color of the indicator tag to change from red to bright blue under ultraviolet light. Furthermore, the deprotonated polyvinylpyrrolidone-alizarin nanoparticles (PVP-AZ NPs) and the dispersed carbon quantum dots (RQDs) further reduce the blue fluorescence intensity of the indicator tag through internal filtration.

[0023] According to the above plan, the high-protein food proteins include milk, eggs, meat and beans, with typical examples being fresh shrimp, tuna, eggs and chicken breast.

[0024] According to the above scheme, after the indicator label is affixed to the sealed container holding high-protein food, the response time is generally 10 to 20 minutes; the temperature inside the sealed container is generally -20 to 25°C.

[0025] Biogenic amines are produced by the degradation of amino acids due to external microbial activity during food spoilage or endogenous tissue metabolism. Therefore, biogenic amines can serve as important biomarkers for monitoring food quality and aiding in disease diagnosis. In experiments, ammonia is typically used as a mimic to examine the responsiveness of the prepared indicator labels to biogenic amines. The indicator labels prepared in this invention exhibit a visually identifiable colorimetric / fluorescent dual-channel response in an ammonia environment ranging from 25 to 25,000 ppm.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] The dual-channel intelligent indicator label of this invention can intuitively display the freshness of high-protein foods, and the freshness of high-protein foods can be intuitively judged or compared by the color of the indicator label. Moreover, the indicator label of this invention uses dual channels to quickly detect the freshness of high-protein foods, and displays specific color responses under both sunlight and ultraviolet light. Through self-correction and elimination of interference from external factors, the detection results of the two channels can be mutually verified, improving the sensitivity and reliability of detection, and providing convenient, fast, accurate, and real-time visual monitoring of the freshness and spoilage of high-protein foods.

[0028] In summary, the dual-channel intelligent monitoring label for the freshness of high-protein foods described in this invention has advantages such as high sensitivity, visualization, accuracy, and stability, providing a new real-time visual monitoring method for the freshness monitoring of high-protein foods. Attached Figure Description

[0029] Figure 1 This is a schematic diagram illustrating the principle of the dual-channel intelligent monitoring indicator label for detecting the freshness of high-protein foods, as described in this invention, for detecting the freshness of shrimp.

[0030] Figure 2 In the image, A is the TEM image of PVP-AZ NPs, and B is the SEM image of RQDs.

[0031] Figure 3 To investigate the optical properties of quantum dots (RQDs) in different aqueous ethanol solutions, RQDs were dissolved in ethanol solutions with varying water contents, ranging from 0% to 90% by volume. In the figures, A represents the UV absorption spectra of RQDs in ethanol solutions with different water contents, B represents photographs of RQDs in ethanol solutions with different water contents under sunlight and UV light, and C represents the fluorescence emission spectra of RQDs in ethanol solutions with different water contents.

[0032] Figure 4 In the table, A shows the image of the indicator tag prepared in Example 1 under visible light and its fluorescence under ultraviolet light; B shows the ammonia water photograph and ultraviolet-visible spectrum curve of PVP-AZ NPs under different pH conditions; C shows the structural change diagram of PVP-AZ NPs; D shows the ammonia water photograph and fluorescence spectrum of RQDs at pH = 7-12; E shows the ultraviolet-visible spectrum curve of PVP-AZ NPs under alkaline conditions and the fluorescence excitation and fluorescence emission spectra of RQDs; and F shows the fluorescence emission spectrum of quantum dot solution (pH = 10) with different concentrations of alizarin nanoparticles (PVP-AZ NPs).

[0033] Figure 5In the image, A shows the color response of the PA-R membrane to ammonia at concentrations of 0 mg / mL, 0.125 mg / mL, 0.25 mg / mL, and 0.375 mg / mL during the PA-R membrane preparation process; B shows the color response of the PA-R membrane to ammonia at different response times; and C shows the color response of the PA-R membrane to ammonia at different concentrations.

[0034] Figure 6 In the diagram, A shows the visible light image and fluorescence of the indicator label prepared in Example 1 and the shrimp stored at three different temperatures; B shows the TVB-N content of the shrimp stored at different temperatures for different days; and C is a schematic diagram of the indicator label monitoring the freshness of the shrimp. Detailed Implementation

[0035] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the present invention is not limited to the following embodiments.

[0036] In the following examples, alizarin (AZ), polyvinylpyrrolidone (PVP), 4-dimethylaminopyridine, melamine, and dithiosalicylic acid were purchased from Maclean Biochemical Technology Co., Ltd. (Shanghai, China); cassava starch and polyvinyl alcohol were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China); and glycerol and ammonia were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

[0037] Example 1

[0038] I. Preparation of polyvinylpyrrolidone-alizarin nanoparticles (PVP-AZ NPs) and carbon quantum dots (RQDs):

[0039] (1) Preparation of PVP-AZ NPs: Under magnetic stirring, polyvinylpyrrolidone (PVP, 0.9 g, molecular weight 5800), dimethylaminopyridine (0.3 g), triethylamine (0.6 g) and AZ (30 mg) were added sequentially to a beaker containing 30 mL DMSO. After stirring in the dark for 24 h, the mixture was dialyzed in DMSO aqueous solution (DMSO to aqueous solution volume ratio of 3:1, 3500 Da) for 1 day to remove unreacted polyvinylpyrrolidone, AZ and other impurities. The mixture was then dialyzed in ultrapure water for 2 days. Finally, the mixture was freeze-dried and PVP-AZ NPs were collected.

[0040] (2) Preparation of RQDs: Melamine (201.6 mg) and dithiosalicylic acid (544 mg) were dissolved and dispersed in 40 mL of 99% acetic acid by ultrasonication. Subsequently, the resulting mixed solution was transferred to a reactor and heated at 180 °C for 10 h. After cooling to room temperature, the reaction solution was added dropwise to 2 L of boiling water. The suspension was filtered, washed, and vacuum dried at 60 °C for 6 h.

[0041] Figure 2 In the image, A is a TEM image of PVP-AZ NPs dispersed in water, showing that PVP-AZ NPs exhibit a regular spherical structure with an average lateral size of approximately 572 nm and a size generally in the range of 450–700 nm; B is an HR-TEM image of RQDs dispersed in ethanol, showing that RQDs exhibit a regular dispersed spherical structure with an average particle size of 2.0 nm and a size generally in the range of 1.5–2.5 nm.

[0042] To verify the fluorescence properties of RQDs in ethanol solutions with different water contents, RQDs were dispersed in ethanol solutions with varying water contents (0%, 10%, 30%, 50%, 70%, and 90%). Color photographs of the RQDs dispersed in these ethanol solutions under sunlight and ultraviolet light were recorded using a camera, as shown below. Figure 3 As shown. By Figure 3 It can be seen that RQDs ethanol solutions with different water contents all exhibit a significant absorption peak at 310 nm, with the peak intensity gradually decreasing as the water content increases. Correspondingly, RQDs ethanol solutions with different water contents change from clear and transparent to turbid under sunlight. Under 365 nm ultraviolet light, clear RQDs ethanol solutions with low water content emit blue fluorescence, while turbid RQDs ethanol solutions with high water content emit red fluorescence. With increasing water content, the intensity of the blue fluorescence peak at 443 nm gradually decreases; when the water content exceeds 50%, the fluorescence peak at 443 nm almost disappears, while a new red fluorescence peak appears at 603 nm. In subsequent experiments, unless the water content is specifically mentioned, ethanol will be assumed to be the dispersing solvent for the RQDs.

[0043] II. Preparation of Indicator Labels for Dual-Channel Intelligent Monitoring of Freshness in High-Protein Foods

[0044] Add 3.4g of tapioca starch and 1.7g of polyvinyl alcohol to 100mL of ultrapure water, then heat at 100℃ for 1h. After cooling to 40℃, add PVP-AZ NPs aqueous solution (0.25mg / mL, 2mL), RQDs ethanol solution (1mg / mL, 2mL), and 1g of glycerol, and continue stirring for 1h to obtain the film-forming solution. Let the film-forming solution stand to defoam, then pour it into a petri dish (3mL volume, 0.6cm² surface area). 2The sample was dried in a vacuum drying oven at 55℃ for 12 hours to obtain a colorimetric / fluorescence dual-channel film, named PA-R film, which is a dual-channel intelligent indicator label for monitoring the freshness of high-protein foods. It was stored in a constant temperature and humidity incubator (25℃, 50% RH). In this indicator label, the composite matrix is ​​composed of cassava starch and polyvinyl alcohol in a mass ratio of approximately 2:1; the loading of PVP-AZ NPs is 0.0047% of the total mass of the composite matrix, and the loading of RQD is 0.019% of the total mass of the composite matrix.

[0045] In addition, by replacing the PVP-AZ NPs aqueous solution in the above preparation method with ultrapure water, the corresponding prepared film is named RQDs membrane; by replacing the RQDs ethanol solution in the above preparation method with ethanol solvent, the corresponding prepared film is named PVP-AZ NPs membrane; by replacing the PVP-AZ NPs solution in the above preparation method with ultrapure water and replacing the RQDs ethanol solution in the above preparation method with ethanol solvent, the corresponding prepared film is named Blank membrane.

[0046] The PA-R membrane, RQDs membrane, AZ NPs membrane, and Blank membrane were all cut into 2×2cm squares and placed on top of a 6-well plate containing ammonia water (2500ppm). The response time was timed for 15 minutes from the start of fumigation, and the color of the PA-R membrane was recorded before and after 15 minutes of fumigation using a camera. Figure 4 Figure A shows visible light images and ultraviolet fluorescence images of the indicator tags prepared in Example 1 under visible light. It can be seen that neither the Blank film nor the RQDs film showed significant color changes before and after responding to ammonia under sunlight. However, after responding to ammonia, the color of the PVP-AZ NPs film changed from pale yellow to purple, and the color of the PA-R film changed from orange-yellow to purple. Therefore, under sunlight, the color response of the film to ammonia is closely related to the PVP-AZ NPs. Under ultraviolet irradiation, neither the Blank film nor the PVP-AZ NPs film emitted fluorescence signals before and after responding to ammonia. However, after responding to ammonia, the fluorescence of the RQDs film changed from red to blue, indicating that the RQDs may serve as a fluorescent indicator for ammonia. The PA-R film also exhibited a similar fluorescence color change. In particular, the blue fluorescence intensity of the PA-R film was significantly lower than that of the RQDs film. This may be due to the conformational change of the PVP-AZ NPs, causing the fluorescence of the RQDs to be quenched through an internal filtration effect. The above results show that the prepared PA-R membrane (i.e., the dual-channel intelligent indicator label for monitoring the freshness of high-protein foods) exhibits significant color changes in response to ammonia under both sunlight and ultraviolet light, and is expected to become an ideal candidate material for dual-channel intelligent labels for detecting BAs.

[0047] Figure 4Figure B shows photographs and UV-Vis spectra of PVP-AZ NPs in aqueous solutions under different pH conditions, verifying the pH responsiveness of PVP-AZ NPs. Figure 4 As can be seen from B: at pH=2, the PVP-AZ NPs solution shows a maximum absorption peak at 425nm, and the peak intensity gradually increases; at pH=7, a new peak appears at 537nm, and the solution color changes from pale yellow to pale purple; as the pH continues to increase, the absorption peak intensity of the solution gradually increases, and the color transitions from light purple to dark purple. Figure 4 The C value represents the conformational changes of AZ under acidic and alkaline conditions. The above data indicate that PVP-AZ NPs can serve as a response signal to ammonia under sunlight.

[0048] Figure 4 The image shows photographs and fluorescence spectra of RQDs in ammonia water at pH 7-10, verifying the dispersibility and fluorescence transition of RQDs in solutions with different pH values. The concentration of RQDs in the ammonia water was 0.1 mg / mL. Figure 4 As shown in D, under alkaline conditions, the fluorescence emission spectrum of the RQDs solution at an excitation wavelength of 365 nm shows that with increasing pH, the intensity of the fluorescence peak at 603 nm decreases, while the intensity of the blue fluorescence peak at 443 nm gradually increases; correspondingly, the solution changes from turbid to clear, and the fluorescence changes from red to blue. These results indicate that RQDs are dispersible in polar solutions.

[0049] Figure 4 D and F represent the UV-Vis spectra of PVP-AZ NPs (2.5 mg / mL) and the fluorescence excitation and emission spectra of RQDs (0.1 m / mL) under alkaline conditions (using ammonia water at pH = 10). Under alkaline conditions, the conformational change of PVP-AZ NPs can produce an internal filtration effect on the dispersed RQDs, causing the film to change from bright blue to blue. Figure 4 D indicates that the RQDs solution exhibits a significant fluorescence emission peak at 443 nm under an excitation wavelength of 365 nm. Figure 4 F, representing the PVP-AZ NPs solution, exhibits a maximum absorption peak at 537 nm due to deprotonation. The intensity of this absorption peak gradually decreases with increasing PVP-AZ NPs concentration. This indicates that the deprotonated PVP-AZ NPs quench the fluorescence of RQDs via IFE. These results indirectly confirm that the prepared PA-R film achieves a color response to ammonia under sunlight through the deprotonation of PVP-AZ NPs, and a fluorescence response under ultraviolet light through the dispersed state of RQDs and IFE.

[0050] III. Optimization of Conditions in the Preparation Process of the Indicator Label (i.e., PA-R Membrane) for the Dual-Channel Intelligent Detection of Freshness of High-Protein Foods

[0051] Figure 5 Figure A shows that when the PVP-AZ NPs concentrations were 0, 0.125, 0.25, and 0.375 mg / mL, the indicator labels (the indicator labels were prepared using PVP-AZ NPs concentrations of 0, 0.125, 0.25, and 0.375 mg / mL as in Example 1, and the rest were the same as in Example 1) were all cut into 2×2 cm squares and covered on a 6-well plate containing ammonia water (ammonia water concentration of 2500 ppm). The response time was timed for 15 min from the start of fumigation, and the color photos of the PA-R membrane before and after 15 min of fumigation were recorded using a camera. Figure 5 Figure A shows that with increasing PVP-AZ NPs concentration, the color change of the PA-R membrane before and after responding to ammonia under sunlight becomes more pronounced. At a concentration of 0.25 mg / mL, the color change is easily visible to the naked eye. Under ultraviolet light, the blue fluorescence of the PA-R membrane after responding to ammonia gradually decreases with increasing PVP-AZ NPs concentration. When the PVP-AZ NPs concentration exceeds 0.25 mg / mL, the fluorescence intensity does not decrease, indicating that deprotonated PVP-AZ NPs achieve maximum fluorescence quenching of RQDs through IFE. Therefore, 0.25 mg / mL of PVP-AZ NPs was selected as the optimal concentration for preparing the PA-R membrane.

[0052] PA-R membranes prepared with the optimal concentration of PVP-AZ NPs (0.25 mg / mL) were cut into 2×2 cm squares and placed on top of a 6-well plate containing ammonia (2500 ppm). The response time was timed for 15 minutes from the start of fumigation, and color photographs of the PA-R membranes were recorded using a camera. The effect of response time on the ammonia response behavior of the PA-R membrane was further investigated. Figure 5 As shown in section B, with the extension of the response time, the color of the PA-R membrane under sunlight changes from orange-yellow to deep yellow, and then to purple; correspondingly, the fluorescence of the PA-R membrane changes from red to blue under ultraviolet light, and the blue fluorescence gradually quenches; after the response time reaches 16 minutes, the color change inside the membrane tends to stabilize under sunlight and ultraviolet light irradiation. Therefore, 16 minutes is selected as the optimal ammonia response time for the PA-R membrane.

[0053] PA-R membranes prepared with the optimal concentration of PVP-AZ NPs (0.25 mg / mL) were cut into 2×2 cm squares and placed on top of 6-well plates containing ammonia water of different concentrations (ammonia water concentration in the range of 0-25000 ppm). The response time was timed for 16 min from the start of fumigation, and color photos of the PA-R membranes were recorded using a camera. The effect of the PA-R membranes on the response behavior of different concentrations of ammonia water under the optimal detection conditions of sunlight and 365 nm ultraviolet light was further investigated. Figure 5 The C-value shows that under sunlight, as the ammonia concentration increases, the color of the PA-R film changes from an initial orange-yellow to a dark yellow and then to purple; under ultraviolet light, the fluorescence of the film changes from red to pale blue. This unique dual-channel color change is easily discernible to the naked eye, indicating that the PA-R film can achieve visually sensitive detection of ammonia.

[0054] IV. Indicator labels are used to monitor the freshness of shrimp.

[0055] Detection method: A 1×3cm PA-R film (i.e., the indicator label prepared in Example 1) was affixed to the petri dish lid as a freshness indicator label. Uniformly sized fresh shrimp were placed in the sterile petri dish, ensuring they did not touch each other, and then sealed for storage. Storage was carried out at 25℃, 4℃, and -20℃ for 1–5 days, and the color change of the label was recorded using a camera. A damp wipe was used instead of a shrimp sample for the same experiment to eliminate humidity interference. The total volatile basic nitrogen (TVB-N) content of the shrimp was determined using a Kjeldahl nitrogen analyzer.

[0056] Figure 6 Figure A shows that after shrimp were stored at 25°C for 1 day or at 4°C for 3 days, the indicator label changed from orange to purple under sunlight. Under ultraviolet light, the red fluorescence of the label gradually turned into blue fluorescence, indicating that the shrimp had spoiled. After being stored at -20°C for 5 days, the color and fluorescence of the shrimp hardly changed, indicating that the shrimp were still fresh.

[0057] Further verification was conducted on the TVB-N content in shrimp samples to validate the accuracy of the PA-R film as an indicator label for monitoring shrimp freshness. Shrimp with a TVB-N value <12 mg / 100g were considered fresh, 12 mg / 100g–20 mg / 100g were considered stale but still edible, and >20 mg / 100g were considered spoiled. After storage at 25℃ for 1 day and at 4℃ for 3 days, the TVB-N content in the shrimp was 63±17 mg / 100g and 37±5.5 mg / 100g, respectively. Both exceeded the TVB-N limit for edible shrimp (20 mg / 100g), indicating spoilage. This data was consistent with the color response of the indicator label. The change in TVB-N content after storage at -20℃ for 5 days showed that the TVB-N content was below 10 mg / 100g, confirming that the shrimp remained fresh. These results demonstrate that the indicator label of the present invention can intuitively and easily monitor the freshness of fresh shrimp during the storage process, enabling consumers to understand the quality of shrimp in real time and ensuring food safety.

[0058] The above description is merely a preferred embodiment for explaining the present invention and is not intended to limit the present invention in any way. Therefore, any modifications or changes made to the present invention under the same inventive spirit should still be included within the scope of protection intended by the present invention.

Claims

1. A dual-channel intelligent indicator label for monitoring the freshness of high-protein foods, characterized in that, It is a thin film formed by loading polyvinylpyrrolidone-alizarin nanoparticles and carbon quantum dots onto the composite matrix using cassava starch and polyvinyl alcohol as a composite matrix, combined with solvents and / or film-forming aids. The carbon quantum dots have a size of 1.5–2.5 nm. The polyvinylpyrrolidone-alizarin nanoparticles are prepared as follows: under magnetic stirring, polyvinylpyrrolidone, 4-dimethylaminopyridine, triethylamine and alizarin are added sequentially to dimethyl sulfoxide. After stirring and reacting in the dark, the mixture is dialyzed sequentially in dimethyl sulfoxide aqueous solution and ultrapure water, and then freeze-dried to obtain polyvinylpyrrolidone-alizarin nanoparticles with a size of 450–700 nm.

2. The dual-channel intelligent indicator label for monitoring the freshness of high-protein foods according to claim 1, characterized in that, The mass ratio of cassava starch to polyvinyl alcohol in the composite matrix is ​​(1.5-2.5):1; the loading of polyvinylpyrrolidone-alizarin nanoparticles is 0.003-0.006% of the total mass of the composite matrix, and the loading of carbon quantum dots is 0.01-0.025% of the total mass of the composite matrix.

3. The dual-channel intelligent indicator label for monitoring the freshness of high-protein foods according to claim 1, characterized in that, The solvent is ethanol and water; the film-forming aid is glycerol.

4. The method for preparing the dual-channel intelligent monitoring indicator label for the freshness of high-protein foods according to claim 1, characterized in that, In the preparation method of polyvinylpyrrolidone-alizarin nanoparticles, the ratio of polyvinylpyrrolidone, 4-dimethylaminopyridine, triethylamine and alizarin is (2-4):1:(1-4):(0.05-0.15), the concentration of alizarin in dimethyl sulfoxide is 0.01-0.04 g / mL, and the stirring reaction time is 1-2 days.

5. The method for preparing the dual-channel intelligent monitoring indicator label for the freshness of high-protein foods according to claim 1, characterized in that, Includes the following steps: (1) Add cassava starch and polyvinyl alcohol to ultrapure water, heat at 80-100℃ for 0.5-1.5 h, and then cool to 30-50℃ to obtain a composite matrix solution; (2) Add an aqueous solution of polyvinylpyrrolidone-alizarin nanoparticles, an ethanol solution of carbon quantum dots, and glycerol to the composite matrix solution obtained in step (1), stir evenly, and obtain a film-forming solution; (3) After the film-forming solution obtained in step (2) is allowed to stand to defoam, it is poured into a container and spread evenly. Then, it is dried at 30-80℃ for 12-24 hours to obtain a colorimetric / fluorescent dual-channel film, which is an indicator label for dual-channel intelligent monitoring of the freshness of high-protein food.

6. The method for preparing the dual-channel intelligent monitoring indicator label for the freshness of high-protein foods according to claim 5, characterized in that, In step (1), the mass ratio of cassava starch to polyvinyl alcohol is (1.5-2.5):1; the concentration of cassava starch in ultrapure water is 30-40 mg / mL, and the concentration of polyvinyl alcohol in ultrapure water is 15-30 mg / mL; in step (3), the thickness of the spread is 0.04-0.06 mm; the indicator label is stored in a constant temperature and humidity environment, and the constant temperature and humidity environment conditions are: 20-30℃, 40-75% RH.

7. The method for preparing the dual-channel intelligent monitoring indicator label for the freshness of high-protein foods according to claim 5, characterized in that, In step (2), the amount of polyvinylpyrrolidone-alizarin nanoparticles is 0.003-0.006% of the total mass of cassava starch and polyvinyl alcohol, the amount of carbon quantum dots is 0.01-0.025% of the total mass of cassava starch and polyvinyl alcohol, and the amount of glycerol is 8-12% of the total mass of cassava starch and polyvinyl alcohol; the polyvinylpyrrolidone-alizarin nanoparticles are dissolved in water to prepare an aqueous solution with a concentration of 0.2-0.3 mg / mL; the carbon quantum dots are dispersed in ethanol to prepare an ethanol solution with a concentration of 0.5-1.5 mg / mL.

8. The method for preparing the dual-channel intelligent monitoring indicator label for the freshness of high-protein foods according to claim 5, characterized in that, The preparation method of carbon quantum dots is as follows: melamine and 2,2'-dithiocarboxylic acid are dissolved and dispersed in acetic acid by ultrasonication, and then transferred to a reactor. The mixture is heated at 150-200℃ for 4-8 hours. After cooling to room temperature, the resulting reaction solution is dropped into boiling water to form a suspension. The suspension is then filtered, washed, and vacuum dried to obtain carbon quantum dots. The mass ratio of melamine to 2,2'-dithiocarboxylic acid to acetic acid is 1:(2-4):(100-300), and the concentration of acetic acid is not less than 80%.

9. The application of the indicator label of claim 1 in monitoring the freshness of high-protein foods, characterized in that, The application method is as follows: the indicator label is affixed to the inner wall of a sealed container holding high-protein food, ensuring that the indicator label does not come into contact with the high-protein food. Then, a camera is used to record the color of the indicator label and its color changes to determine the freshness of the high-protein food. Specifically, under sunlight, the color change of the indicator label from orange to purple indicates the change from fresh to spoiled high-protein food; under purple light, the color change of the indicator label from red fluorescence to blue fluorescence indicates the change from fresh to spoiled high-protein food.

10. The application according to claim 9, characterized in that, After the indicator label is affixed to a sealed container holding high-protein food, the response time is 10 to 20 minutes, and the temperature inside the sealed container is -20 to 25°C.

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