Preparation method and application of anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film

Abstract

The present application belongs to the field of food detection, and specifically relates to a method for preparing an anthocyanin @ UiO‑66‑NH2 nano gas-sensitive pigment dual-signal indicator film and its use; the steps are: step one, preparation of UiO‑66‑NH2 nanomaterials; step two, encapsulation of anthocyanins in a metal-organic framework; step three, preparation of an indicator film; step four, establishment of a freshness model and non-destructive real-time evaluation. The present invention is based on the characteristics of UiO‑66‑NH2 nanomaterials with strong adsorption, high porosity, and multiple functionalized sites, and constructs an anthocyanin @ UiO‑66‑NH2 nano gas-sensitive pigment dye, which can effectively solve the problem of poor stability of anthocyanins; at the same time, the fluorescent substance luminol is added, and the freshness indicator film prepared based on this improvement will show visible light color and fluorescent color changes during the application stage, which can realize non-destructive visual detection of fresh cold meat, effectively improving the sensitivity and stability of traditional gas-sensitive pigment indicator films.

Description

Technical Field

[0001] The present invention belongs to the technical field of food freshness detection, and specifically relates to a preparation method and application of an anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film. Background Art

[0002] Fresh meat is widely sought after for its delicious taste and nutritional value. However, improper handling during production, transportation, and storage can lead to high spoilage rates and losses due to environmentally induced microbial growth. These microorganisms produce amines through amino acid decarboxylation and amination of carbon-based organic matter, causing food spoilage and producing toxic substances that are harmful to human health. Therefore, the design of intelligent packaging with monitoring, detection, recording, tracking, and communication capabilities is of great significance. In recent years, freshness-indicating films have attracted considerable attention in food research due to their ability to provide intuitive information on food quality and safety. In the food industry, anthocyanins are considered promising natural pigments that can replace synthetic pigments. They are widely available and respond to varying pH values. pH-sensitive color changes offer a simple and effective method for monitoring food quality, as the production of amines is always accompanied by pH changes during food spoilage. These pH fluctuations alter the pigment structure, resulting in visible color changes.

[0003] Anthocyanins, a class of natural gas-sensing materials, react with spoilage gases to produce a distinct color change and are widely available. Numerous studies have demonstrated that anthocyanins from various sources can be used as bio-based pH-sensitive dyes for monitoring food freshness. However, anthocyanins undergo structural degradation under light or high temperature, resulting in significant deviations in the results of food freshness indicator films. This significantly hinders their application in food testing and is a major challenge in the field of food freshness indication. Therefore, improving the stability of anthocyanins is crucial for the practical application of food freshness indicator films. Furthermore, most current freshness indicator films can only determine the freshness of chilled meat by color changes under visible light. However, this single signal can lead to low sensitivity and high detection limits. Therefore, it is necessary to develop a non-destructive, effective, and rapid detection method for evaluating freshness. Summary of the Invention

[0004] In order to solve the technical problems in the above background, the present application provides a method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film, which is used for evaluating the freshness of fresh chilled meat, solving the technical problems of poor stability of natural gas-sensitive membranes, strong subjectivity of response results and low sensitivity in the existing technology.

[0005] This paper first designs a photo- and thermo-stable phytochemical dye using different plant anthocyanins and the metal-organic framework UiO-66-NH2, significantly improving the photostability and thermal stability of anthocyanins. Furthermore, considering that a single signal may be insufficient for determining freshness, a fluorescence signal is introduced as an auxiliary, enabling dual-signal freshness detection, resulting in more accurate and reliable results. The film's structure is also characterized, and its pH sensitivity and storage stability are examined. The result is an excellent indicator film that has achieved significant technical results in meat freshness detection applications.

[0006] This application achieves the above-mentioned purpose through the following technical means.

[0007] A method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film comprises the following steps:

[0008] S1. Anthocyanin extraction: Anthocyanin raw material is washed with distilled water and then air-dried to obtain an air-dried raw material. Anhydrous ethanol and water are mixed to obtain an ethanol-water extraction solution. The air-dried raw material is then added to the ethanol-water extraction solution, stirred uniformly at room temperature, and the supernatant is collected by vacuum filtration. The resulting supernatant is subjected to rotary evaporation to obtain concentrated anthocyanins. Finally, the solid powder is obtained by freeze-drying, which is the anthocyanin extract. The anthocyanin raw materials include, but are not limited to, grape skin, mulberry, roselle, and black wolfberry.

[0009] S2. Synthesis of UiO-66-NH2 Nanomaterials:

[0010] ZrCl4 and 2-aminoterephthalic acid powders are respectively dissolved in N,N-dimethylformamide and ultrasonically treated until completely dissolved to obtain a ZrCl4 solution and a 2-aminoterephthalic acid solution; the 2-aminoterephthalic acid solution is placed in an oil bath, and acetic acid is added dropwise to the 2-aminoterephthalic acid solution, followed by the ZrCl4 solution, and stirred evenly; then, stirring is stopped and the oil bath is maintained at a constant temperature overnight to obtain a reaction solution; after cooling the reaction solution to room temperature, anhydrous ethanol is added, and a precipitate is collected by centrifugation at room temperature. Finally, the precipitate is washed, centrifuged, and vacuum-dried to obtain a product, namely, UiO-66-NH2 nanomaterial;

[0011] S3. Encapsulation of anthocyanins in a metal-organic framework: The UiO-66-NH2 nanoparticles obtained in step S3 and the anthocyanin extract obtained in step S1 were added to anhydrous ethanol and ultrasonically mixed to obtain a mixed solution; the mixed solution was centrifuged, the precipitate was collected, and the precipitate was dispersed in distilled water for washing. After washing, the precipitate was collected and vacuum-dried in the dark. The product obtained after drying was anthocyanin-loaded UiO-66-NH2, denoted as anthocyanin@UiO-66-NH2;

[0012] S4. Preparation of dual-signal freshness indicator membrane:

[0013] Add corn starch and polyvinyl alcohol to distilled water, heat and stir to form a mixed solution; then cool to room temperature, add glycerol to the mixed solution and stir to obtain a starch / polyvinyl alcohol polymer solution;

[0014] The anthocyanin @UiO-66-NH2 and luminol obtained in step S3 are added to the starch / polyvinyl alcohol polymer under ultrasonic treatment. After ultrasonic treatment, the mixture is poured into a mold and dried to form a film, thereby obtaining a dual-signal indicator film.

[0015] Preferably, in step S1, the volume ratio of anhydrous ethanol to water is 3:1; the ratio of the air-dried raw material to the ethanol-water extract solution is 1 g:10 mL; the stirring is magnetic stirring, and the time is 2 hours; the temperature of rotary evaporation is 50°C, and the time is 40 minutes; the temperature of freeze-drying is -40°C, and the freeze-drying time is 2 days.

[0016] Preferably, the dosage of ZrCl4, 2-aminoterephthalic acid, N,N-dimethylformamide, acetic acid and anhydrous ethanol in step S2 is 300-350 mg: 200-300 mg: 1-2 mL: 20 mL: 10 mL.

[0017] Preferably, the temperature of the constant temperature oil bath in step S2 is 80°C; the rate of adding acetic acid is 3s / drop; the speed of the magnetic stirring is 400r / min, and the stirring time is 30min; washing is performed using anhydrous ethanol; the centrifugal speed is 8000-10000r / min, and the centrifugal time is 20-25min; the temperature of the vacuum drying is 75°C, and the time is 10-12h.

[0018] Preferably, in step S3, the ratio of UiO-66-NH2, anthocyanin extract and anhydrous ethanol added is 45-50 mg: 20-30 mg: 10 mL; the ultrasonic mixing time is 20 min; the number of distilled water washings is 3-5 times; the centrifugation conditions are: speed 10000 r / min, time 10-15 min; the vacuum drying temperature is 40 ° C, and the drying time is 12 h.

[0019] Preferably, the amount of polyvinyl alcohol, corn starch, luminol and distilled water in step S4 is 1-1.7 g: 3-4 g: 10-20 mg: 100 mL; the amount of glycerol is 1 wt% of the amount of corn starch; the amount of anthocyanin @UiO-66-NH2 is 1% to 5% of the total mass of corn starch and polyvinyl alcohol; the conditions of the ultrasonic treatment are: ultrasonic power of 100 W, ultrasonic time of 20 min, and ultrasonic temperature of 30 ° C; the mold includes a disposable culture dish and a polytetrafluoroethylene plate; the drying temperature is 40 ° C, and the drying time is 12 h.

[0020] The anthocyanin@UiO-66-NH2 nanometer gas-sensitive pigment dual-signal indicator film of the present invention is used for detecting the freshness of meat products; the meat products include fresh meat, cooked meat products and semi-finished products;

[0021] Based on the use of dual-signal indicator membrane in meat product freshness detection, the operation steps are as follows:

[0022] (1) Cut meat samples into equal weights, place them in a reaction container and store them at low temperature. Take out the samples at regular intervals and determine the TVB-N content using the Kjeldahl method based on the different volatile amine content of the meat products at different freshness levels. The meat products are then divided into three freshness levels: fresh: TVB-N content <15 mg / 100 g; sub-fresh: TVB-N content 15-25 mg / 100 g; and spoiled: TVB-N content >25 mg / 100 g. The low temperature condition is 4°C and the storage time is 0-480 h. Different freshness levels correspond to different storage days.

[0023] (2) Obtain the color difference of the dual-signal indicator membrane before the reaction, then store the dual-signal indicator membrane and meat products of different grades in the same closed reaction container, obtain the color difference of the indicator membrane again after the reaction, and calculate the color difference value based on the color difference before and after the reaction, recorded as ΔE, and then establish a correlation curve between ΔE and the corresponding TVB-N content, thereby making quantitative judgments and realizing the freshness detection of meat products; at the same time, meat products at different grades will cause the dual-signal indicator membrane to change color, so that the freshness of the meat products can be judged according to the color change of the dual-signal indicator membrane, realizing visual detection.

[0024] Preferably, the reaction time in step (2) is 1 to 16 minutes; the dual-signal indicator membrane is fixed on the top of the reaction container, and the distance from the meat product is 10-50 mm.

[0025] Preferably, in step (2), the color difference extraction step is as follows: using a colorimeter to extract the color difference of the indicator film before and after the reaction, that is, the L, a, and b values; the values ​​before the reaction are recorded as L0, a0, and b0; the values ​​after the reaction are recorded as L1, a1, and b1; the values ​​of L, a, and b before and after the reaction are subtracted, that is, △L=L1-L0, △a=a1-a0, △b=b1-b0; and according to the formula Calculate the color difference value.

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

[0027] 1. The smart indicator zone in this application uses anthocyanins embedded in the metal-organic framework UiO-66-NH2 as a highly stable pH indicator. UiO-66-NH2 has high porosity, a large surface area, and diverse functional sites, allowing specific guest molecules to be encapsulated within its pores. The UiO-66-NH2 selected in this application is safer than other MOFs nanomaterials, and the resulting complex has significantly improved light and heat stability compared to natural anthocyanins.

[0028] 2. This application discloses a method for preparing a dual-signal indicator membrane for the freshness of chilled meat based on anthocyanin @UiO-66-NH2. The membrane comprises a metal-organic framework material UiO-66-NH2 loaded with anthocyanins, a fluorescent substance luminol, and a film-forming matrix solution. The membrane is then cast and dried to form a film after ultrasonically eliminating bubbles. The polyvinyl alcohol and starch matrix blocks the transmittance of ultraviolet light, thereby minimizing the degradation of anthocyanins by light. The matrix is ​​non-toxic, low-cost, highly stable, and has good film-forming properties.

[0029] 3. The colorimetric / fluorescent dual-signal indicator film developed in this application can react with volatile components produced during the spoilage process of fresh meat, resulting in changes in visible light and fluorescent color. These two color changes can effectively monitor the freshness of fresh meat products. Compared with a single visible light indicator, this indicator also has fluorescent indication properties. Its fluorescent signal changes sensitively, enabling accurate detection of the freshness of fresh meat, and has a low detection limit, ensuring detection accuracy.

[0030] 4. The method for visually detecting meat freshness established in this application can effectively determine the freshness of chilled meat during its processing, transportation, and storage based on the color changes of the indicator film under visible and ultraviolet light, enabling real-time and rapid monitoring. The combination of natural anthocyanins, traditional color indicators, and fluorescent nanomaterials enriches the color signal of the indicator label and increases the accuracy of measurement results. This method allows for easy visual evaluation of meat freshness without the need for expensive testing equipment, and is both low-cost and easy to operate. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the preparation process of anthocyanin@UiO-66-NH2 and dual-signal indicator membrane.

[0032] Figure 2 The color change and UV absorption spectrum of anthocyanin @UiO-66-NH2 at different pH values ​​in this application.

[0033] Figure 3 Deprotonation of luminol in an alkaline environment (A) and luminescence principle after addition of hydrogen peroxide (B) in Example 1.

[0034] Figure 4 This is the Fourier infrared image of the synthesized material UiO-66-NH2.

[0035] Figure 5 Schematic diagram of the color change of the smart membrane in Example 1 and the ammonia reaction time.

[0036] Figure 6 The indicator film color and TVB-N content (C) of pork stored at 4°C for different storage days under daylight (A) and ultraviolet light (B). DETAILED DESCRIPTION

[0037] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0038] It should be understood that the terms described in the present invention are only for describing particular embodiments and are not intended to limit the present invention. Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art in the art. Although the present invention has only described preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the content of this specification shall prevail.

[0039] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be illustrative only.

[0040] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to. Figure 1This is the preparation process of anthocyanin@UiO-66-NH2 and dual-signal indicator membrane. The specific operation is detailed in the examples.

[0041] Example 1:

[0042] A method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film comprises the following steps:

[0043] S1. Anthocyanin extraction (anthocyanin raw materials include grape skin, mulberry, roselle, and black wolfberry);

[0044] The anthocyanin extract is prepared by the following steps: fresh grape skin, mulberry, roselle, and black wolfberry fruit are washed with distilled water and then naturally dried to obtain the corresponding dried raw materials; at the same time, anhydrous ethanol and water are mixed in a volume ratio of 3:1 to obtain an ethanol-water extraction solution; the dried raw materials are then added to the ethanol-water extraction solution, and the ratio of the dried raw materials to the ethanol-water extraction solution is 1g:10mL; magnetic stirring is performed at room temperature for 2 hours, and after stirring evenly, the supernatant is vacuum filtered to obtain an anthocyanin extract; the anthocyanin extract is subjected to rotary evaporation at 50°C for 40 minutes to concentrate the anthocyanins and remove ethanol to obtain concentrated anthocyanins, which is then freeze-dried at -40°C for 2 days to obtain a solid powder, namely the anthocyanin extract;

[0045] In this embodiment, four anthocyanin extracts were obtained, namely, anthocyanin extract from grape skin, anthocyanin extract from mulberry, anthocyanin extract from roselle, and anthocyanin extract from black wolfberry;

[0046] S2. Synthesis of UiO-66-NH2 Nanomaterials:

[0047] 320 mg of ZrCl4 and 300 mg of 2-aminoterephthalic acid powder were dissolved in 20 mL of N,N-dimethylformamide, and ultrasonicated until completely dissolved to obtain a ZrCl4 solution and a 2-aminoterephthalic acid solution;

[0048] The obtained 2-aminoterephthalic acid solution was added to a round-bottom flask and immersed in silicone oil for an oil bath (the temperature of the oil bath was 80° C.), 20 mL of acetic acid was added dropwise to the 2-aminoterephthalic acid solution at a rate of 3 s / drop, and then ZrCl4 solution was added dropwise at a rate of 3 s / drop, and magnetic stirring was performed at a speed of 400 r / min for 30 min. After stirring evenly, stirring was stopped, and the constant temperature oil bath at 80° C. was maintained overnight to obtain a reaction solution; after the reaction solution was cooled to room temperature, 10 mL of anhydrous ethanol was added, and the mixture was centrifuged at room temperature (8000 r / min, 20 min) to obtain a precipitate, which was collected and added with 10 mL of anhydrous ethanol for centrifugal washing twice (centrifugal conditions: 8000 r / min, 20 min). Finally, the precipitate was taken and vacuum dried overnight to obtain the product, namely UiO-66-NH2 nanoparticles; wherein Figure 4 This is the Fourier infrared image of the synthesized material UiO-66-NH2.

[0049] S3. Encapsulation of anthocyanins in metal-organic frameworks;

[0050] 50 mg of UiO-66-NH2 nanoparticles and 30 mg of anthocyanin extract were dissolved in 10 mL of anhydrous ethanol and ultrasonically dissolved for 20 minutes to obtain a mixed solution; the mixed solution was then centrifuged at 10,000 r / min for 15 minutes, the precipitate was collected, and washed three times with distilled water to remove unencapsulated anthocyanins; finally, the mixture was vacuum-dried at 40°C in the dark overnight to obtain anthocyanin-loaded UiO-66-NH2, which was recorded as anthocyanin@UiO-66-NH2; four anthocyanin extracts were obtained in this example, namely, anthocyanin extract from grape skin, anthocyanin extract from mulberry, anthocyanin extract from roselle, and anthocyanin extract from black wolfberry;

[0051] S4. Preparation of dual-signal freshness indicator film;

[0052] (1) 1.7 g of polyvinyl alcohol and 3.4 g of corn starch were mixed and added to 100 mL of distilled water. The mixture was magnetically stirred at 400 rpm for 1 h at 100° C. After the mixture was cooled to room temperature, 1 wt% of glycerol based on the mass of corn starch was added to enhance the stability of the indicator solution. The mixture was stirred at room temperature for 20 min to obtain a starch / polyvinyl alcohol polymer solution.

[0053] (2) The anthocyanin@UiO-66-NH2 prepared in step S3 and 20 mg of luminol were added to 15 mL of corn starch / polyvinyl alcohol polymer solution under ultrasonic treatment to obtain a freshness indicator layer solution after dissolution; wherein the amount of anthocyanin@UiO-66-NH2 was 5% of the total mass of corn starch and polyvinyl alcohol; then, the freshness indicator layer solution was cast into a disposable culture medium with a size of 90×90 mm; and dried at 40°C for 12 hours to form a dual-signal indicator membrane. Among them, the dual-signal indicator membranes prepared based on the four raw materials of grape skin, mulberry, roselle and black wolfberry were marked as A1@UiO-66-NH2, A2@UiO-66-NH2, A3@UiO-66-NH2, and A4@UiO-66-NH2.

[0054] Comparative Example 1: The operation is the same as that of Example 1, except that in step (2) of step S4, anthocyanin @UiO-66-NH2 is replaced by the anthocyanin extract in step S1; corresponding indicator films are obtained, wherein the indicator films prepared based on the four raw materials of grape skin, mulberry, roselle and black wolfberry are marked as A1, A2, A3 and A4 respectively.

[0055] In order to understand the possible response mechanism of the functional factors anthocyanin and luminol in the membrane matrix, the UV-visible absorption spectra of anthocyanin@UiO-66-NH2 under different pH (2-12) environments were studied in this example. Figure 2 As shown, (A) shows anthocyanin extract from grape skin, (B) shows anthocyanin extract from mulberry, (C) shows anthocyanin extract from roselle, and (D) shows anthocyanin extract from black wolfberry. The UV spectrum shows that the maximum UV absorption peak of anthocyanin @UiO-66-NH2 is at 535nm. As the pH value increases, the maximum absorption peaks of the four anthocyanin @UiO-66-NH2 gradually decrease and exhibit a red shift. The color of the corresponding solutions also gradually changes, indicating a structural transformation of the anthocyanins. This demonstrates that anthocyanins are natural dyes with a good color response to pH, making them ideal freshness indicators in smart packaging.

[0056] Figure 3 This diagram shows the principle of luminol luminescence. The presence of amino groups in the luminol molecule makes it more sensitive to protonation and deprotonation, thus giving it pH-responsive properties. Blue fluorescence gradually decreases with increasing pH, and fluorescence quenching occurs at pH ≥ 11. Under alkaline conditions and in the presence of hydrogen peroxide, luminol generates an excited 3-aminophthalic acid dianion (3APA*). This excited 3-APA* returns to its ground state, producing blue fluorescence.

[0057] S5. Optimization of reaction conditions when applying the dual-signal indicator membrane;

[0058] A dual-signal indicator film (20 mm × 20 mm) was suspended 10 mm above the liquid surface and incubated with 1% ammonia water. The color change of the indicator film was recorded using a smartphone every 2 min for a total of 16 min. Figure 5 Figure (A) shows the color response of four different anthocyanin-based films in an ammonia atmosphere (8 mmol / L) under sunlight. When exposed to an ammonia atmosphere, anthocyanins from different sources reacted with ammonia in sunlight, producing a significant color development. Based on the color difference, the optimal reaction was achieved by exposing the films to an ammonia atmosphere for 15 minutes.

[0059] The color difference response of different anthocyanin-based films was further investigated at a reaction time of 15 minutes. Figure 5 Figures (B) and (C) show that different anthocyanins have different responses to ammonia, which is related to their anthocyanin content. Among them, the membrane added with Roselle anthocyanin@UiO-66-NH2 has the best response to the simulated volatile biogenic amine model.

[0060] The dual-signal indicator membrane is used to monitor the freshness of chilled pork. Taking A3@UiO-66-NH2 as an example, the steps are as follows:

[0061] (1) Cut fresh pork into 20 ± 0.5 g pieces and place them in a sterile packaging box. Stick a dual-signal indicator film (A3@UiO-66-NH2, size 20 mm × 20 mm) on the top of the packaging box without contacting the meat. Store in a refrigerator at 4 ± 1 °C.

[0062] (2) Take out the dual-signal indicator film every 48 hours, take pictures of the dual-signal indicator film under fluorescent light and ultraviolet light with a camera, and record the color changes of the dual-signal indicator film.

[0063] (3) The freshness grade of pork was determined using the automatic Kjeldahl method in the national standard. 10 g of meat was mixed with 75 mL of distilled water and homogenized for 20 minutes. Standard hydrochloric acid titration solution (0.100 mol / L) was used. An automatic Kjeldahl nitrogen analyzer was used. The reagent addition and waste discharge functions were turned off. The volume of alkali and water was set to 0 mL, and the volume of boric acid receiving solution was set to 25 mL. The distillation time was 3 minutes. The nitrogen analyzer with automatic potentiometric titration was used to determine the endpoint. The titration endpoint pH was set to 4.65. 1 g of magnesium oxide was added to the distillation tube containing the treated sample, immediately connected to the distiller, and the determination was started according to the above requirements. The TVB-N content in pork was measured every 48 hours.

[0064] The results are as attached Figure 6The figure shows the change in TVB-N content over storage days, along with the corresponding color changes of the indicator film under sunlight and UV light. Pork freshness is categorized into three levels based on the change in TVB-N content: fresh meat from 0 to 72 hours, sub-fresh meat from 96 to 144 hours, and spoiled meat from 168 to 240 hours. The color of the label changes significantly with increasing refrigeration time, making it easier to identify with the naked eye.

[0065] Under sunlight, the color changes from reddish-pink (0-72 hours) to bluish-pink (96-144 hours) to green (168-240 hours). Furthermore, under ultraviolet light, the color response changes from reddish-pink (0-72 hours) to orange-pink (96-144 hours) to yellow-green (168-240 hours). The color changes are distinct and demonstrate excellent sensitivity. The color changes of the indicator film correspond to the changes in the TVB-N content in the pork, indicating the freshness of the pork. This demonstrates that the prepared dual-signal indicator film can intuitively and effectively monitor the freshness of fresh pork in real time.

[0066] Example 2:

[0067] A method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film comprises the following steps:

[0068] S1. Anthocyanin extraction;

[0069] The anthocyanin extract is prepared by the following steps: fresh grape skin, mulberry, roselle, and black wolfberry fruit are washed with distilled water and then dried naturally to obtain the corresponding dried raw materials; at the same time, anhydrous ethanol and water are mixed in a volume ratio of 3:1 to obtain an ethanol-water extract solution; the dried raw materials are then added to the ethanol-water extract solution, and the ratio of the dried raw materials to the ethanol-water extract solution is 1:10 (g / mL); after magnetic stirring at room temperature for 2 hours, the supernatant is vacuum filtered to obtain an anthocyanin extract; the anthocyanin extract is subjected to rotary evaporation at 50°C for 40 minutes to concentrate the anthocyanins and remove ethanol to obtain concentrated anthocyanins, which is then freeze-dried at -40°C for 2 days to obtain a solid powder, which is anthocyanin extract; in this embodiment, four anthocyanin extracts are obtained, namely, anthocyanin extract from grape skin, anthocyanin extract from mulberry, anthocyanin extract from roselle, and anthocyanin extract from black wolfberry.

[0070] S2. Synthesis of UiO-66-NH2 nanomaterials;

[0071] 320 mg of ZrCl4 and 300 mg of 2-aminoterephthalic acid powder were dissolved in 20 mL of N,N-dimethylformamide, and ultrasonicated until completely dissolved to obtain a ZrCl4 solution and a 2-aminoterephthalic acid solution;

[0072] The obtained 2-aminoterephthalic acid solution was added to a round-bottom flask and immersed in silicone oil for an oil bath. 20 mL of acetic acid was added dropwise to the 2-aminoterephthalic acid solution at a rate of 3 s / drop, and then ZrCl4 solution was added dropwise at a rate of 3 s / drop. The mixture was magnetically stirred at a speed of 400 r / min for 30 min and stirred evenly. Then, stirring was stopped and the oil bath was maintained at a constant temperature of 80°C overnight to obtain a reaction solution. After the reaction solution was cooled to room temperature, 10 mL of anhydrous ethanol was added and centrifuged at room temperature (8000 r / min, 20 min) to obtain a precipitate A. The precipitate A was washed with 10 mL of anhydrous ethanol and centrifuged again twice (8000 r / min, 20 min). The precipitate was taken and vacuum dried overnight to obtain the product, which was UiO-66-NH2 nanoparticles.

[0073] S3. Encapsulation of anthocyanins in metal-organic frameworks;

[0074] 45 mg of UiO-66-NH2 nanoparticles and 20 mg of anthocyanin were dissolved in 10 mL of anhydrous ethanol and sonicated for 20 minutes to obtain a mixed solution. The mixed solution was then centrifuged at 10,000 rpm for 15 minutes, and the precipitate was collected and washed three times with distilled water to remove unencapsulated anthocyanin. Finally, the anthocyanin-loaded UiO-66-NH2 was obtained by vacuum drying at 40°C in the dark overnight, referred to as anthocyanin@UiO-66-NH2.

[0075] S4. Preparation of dual-signal freshness indicator film;

[0076] Step 1: Add corn starch and polyvinyl alcohol to 100 mL of distilled water and heat in a water bath at 100°C to obtain a film-forming matrix solution;

[0077] Step 2: Add glycerol, ethanol solution of anthocyanin@UiO-66-NH2 material and luminol to the film-forming matrix solution obtained in step 1, and mix them evenly by ultrasonication to obtain a film-forming solution;

[0078] Step 3: The film-forming liquid obtained in step 2 is allowed to stand to remove bubbles hidden in the film-forming liquid, and then cast into a film; after drying, an anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film is obtained.

[0079] In the film-forming matrix solution, the content of corn starch is 0.04 g / mL, the content of polyvinyl alcohol is 0.01 g / mL, and the content of glycerol is 0.01 g / mL.

[0080] In the film-forming solution: the amount of anthocyanin @UiO-66-NH2 is 5% of the total mass of corn starch and polyvinyl alcohol; the amount of luminol added is 20 mg.

[0081] A preferred embodiment is that the anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film is stored in a constant temperature and humidity environment of 25°C and 50% RH.

[0082] A preferred embodiment is: the preparation of anthocyanin@UiO-66-NH2 material is based on Roselle anthocyanin powder as raw material, and the steps include: adding Roselle anthocyanin powder and UiO-66-NH2 nanoparticles to ethanol in sequence, ultrasonically mixing in a dark environment, centrifugally washing to remove impurities, and then drying in the dark.

[0083] Among them: the ultrasonic power is 100W, the ultrasonic time is 20min, and the ultrasonic temperature is 30℃.

[0084] Note: The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. Therefore, although this specification has described the present invention in detail with reference to the above embodiments, it should be understood by those skilled in the art that the present invention may still be modified or replaced by equivalents. All technical solutions and improvements that do not depart from the spirit and scope of the present invention should be included within the scope of the claims of the present invention.

Claims

1. A method for preparing anthocyanin@UiO-66-NH2 nano-gas-sensitive pigment dual-signal indicator film, comprising the following steps: S1. Anthocyanin extraction: Anthocyanin raw materials are washed with distilled water and then air-dried to obtain an air-dried raw material; anhydrous ethanol and water are mixed to obtain an ethanol-water extraction solution; the air-dried raw material is then added to the ethanol-water extraction solution, stirred uniformly at room temperature, and the supernatant is collected by vacuum filtration. The obtained supernatant is subjected to rotary evaporation to obtain concentrated anthocyanins; and finally, the solid powder is obtained by freeze-drying, which is the anthocyanin extract. The anthocyanin raw materials include but are not limited to grape skin, mulberry, roselle, and black wolfberry. S2. Synthesis of UiO-66-NH2 Nanomaterials: ZrCl4 and 2-aminoterephthalic acid powders are respectively dissolved in N,N-dimethylformamide and ultrasonically treated until completely dissolved to obtain a ZrCl4 solution and a 2-aminoterephthalic acid solution; the 2-aminoterephthalic acid solution is placed in an oil bath, and acetic acid is added dropwise to the 2-aminoterephthalic acid solution, followed by the ZrCl4 solution, and stirred evenly; then, stirring is stopped and the oil bath is maintained at a constant temperature overnight to obtain a reaction solution; after cooling the reaction solution to room temperature, anhydrous ethanol is added, and a precipitate is collected by centrifugation at room temperature. Finally, the precipitate is washed, centrifuged, and vacuum-dried to obtain a product, namely, UiO-66-NH2 nanomaterial; S3. Encapsulation of anthocyanins in a metal-organic framework: The UiO-66-NH2 nanoparticles obtained in step S3 and the anthocyanin extract obtained in step S1 were added to anhydrous ethanol and ultrasonically mixed to obtain a mixed solution; the mixed solution was centrifuged, the precipitate was collected, and the precipitate was dispersed in distilled water for washing. After washing, the precipitate was collected and vacuum-dried in the dark. The product obtained after drying was anthocyanin-loaded UiO-66-NH2, denoted as anthocyanin@UiO-66-NH2; S4. Preparation of dual-signal freshness indicator membrane: Add corn starch and polyvinyl alcohol to distilled water, heat and stir to form a mixed solution; then cool to room temperature, add glycerol to the mixed solution and stir to obtain a starch / polyvinyl alcohol polymer solution; The anthocyanin @UiO-66-NH2 and luminol obtained in step S3 are added to the starch / polyvinyl alcohol polymer under ultrasonic treatment. After ultrasonic treatment, the mixture is poured into a mold and dried to form a film, thereby obtaining a dual-signal indicator film.

2. The method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 1, characterized in that: In step S1, the volume ratio of anhydrous ethanol to water is 3:1; the ratio of the air-dried raw material to the ethanol-water extract solution is 1 g:10 mL; the stirring is magnetic stirring, and the stirring time is 2 hours; the temperature of the rotary evaporation is 50° C., and the time is 40 minutes; the temperature of the freeze-drying is -40° C., and the freeze-drying time is 2 days.

3. The method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 1, characterized in that: The dosage of ZrCl4, 2-aminoterephthalic acid, N,N-dimethylformamide, acetic acid and anhydrous ethanol in step S2 is 300-350 mg: 200-300 mg: 1-2 mL: 20 mL: 10 mL.

4. The method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 1, characterized in that: In step S2, the temperature of the constant temperature oil bath is 80° C.; the rate of adding acetic acid is 3 seconds per drop; the speed of the magnetic stirring is 400 r / min, and the stirring time is 30 minutes; washing is performed using anhydrous ethanol; The centrifugal speed is 8000-10000 r / min, and the centrifugal time is 20-25 min; the vacuum drying temperature is 75° C., and the time is 10-12 h.

5. The method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 1, characterized in that: In step S3, the ratio of UiO-66-NH2, anthocyanin extract and anhydrous ethanol added is 45-50 mg: 20-30 mg: 10 mL; the ultrasonic mixing time is 20 min; the number of distilled water washings is 3-5 times; the centrifugation conditions are: speed 10000 r / min, time 10-15 min; the vacuum drying temperature is 40 ° C, and the drying time is 12 h.

6. The method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 1, characterized in that: The amount of polyvinyl alcohol, corn starch, luminol and distilled water in step S4 is 1-1.7 g: 3-4 g: 10-20 mg: 100 mL; the amount of glycerol is 1 wt% of the amount of corn starch; the amount of anthocyanin @UiO-66-NH2 is 1% to 5% of the total mass of corn starch and polyvinyl alcohol; the conditions of the ultrasonic treatment are: ultrasonic power of 100 W, ultrasonic time of 20 min, and ultrasonic temperature of 30 ° C; the mold includes a disposable culture dish and a polytetrafluoroethylene plate; the drying temperature is 40 ° C, and the drying time is 12 h.

7. Anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film prepared according to the method of any one of claims 1 to 6.

8. Use of the anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 7 in detecting the freshness of meat products.

9. The use according to claim 8, characterized in that Here are the steps: (1) Cut meat samples into equal weights, place them in a reaction container and store them at low temperature. Take out the samples at regular intervals and determine the TVB-N content using the Kjeldahl method based on the different volatile amine content of the meat products at different freshness levels. The meat products are then divided into three freshness levels: fresh: TVB-N content <15 mg / 100 g; sub-fresh: TVB-N content 15-25 mg / 100 g; and spoiled: TVB-N content >25 mg / 100 g. The low temperature condition is 4°C and the storage time is 0-480 h. Different freshness levels correspond to different storage days. (2) Obtain the color difference of the dual-signal indicator membrane before the reaction, then store the dual-signal indicator membrane and meat products of different grades in the same closed reaction container, obtain the color difference of the indicator membrane again after the reaction, and calculate the color difference value based on the color difference before and after the reaction, recorded as ΔE, and then establish a correlation curve between ΔE and the corresponding TVB-N content, thereby making quantitative judgments and realizing the freshness detection of meat products; at the same time, meat products at different grades will cause the dual-signal indicator membrane to change color, so that the freshness of the meat products can be judged according to the color change of the dual-signal indicator membrane, realizing visual detection.

10. The use according to claim 9, characterized in that The reaction time in step (2) is 1 to 16 minutes; the dual-signal indicator membrane is fixed on the top of the reaction container, and the distance from the meat product is 10-50 mm; The steps of color difference extraction are as follows: use a colorimeter to extract the color difference of the indicator film before and after the reaction, that is, the L, a, and b values; the values ​​before the reaction are recorded as L0, a0, and b0; the values ​​after the reaction are recorded as L1, a1, and b1; the difference between the values ​​of L, a, and b before and after the reaction is calculated, that is, △L=L1-L0, △a=a1-a0, △b=b1-b0; and according to the formula Calculate the color difference value.

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