Preparation method of MOFs-loaded alizarin high-stability freshness indicator label based on 3D printing technology

Abstract

The present invention belongs to the field of intelligent indicator labels, and specifically relates to a method for preparing a highly stable freshness indicator label of MOFs-loaded alizarin based on 3D printing technology; the steps are: first preparing highly stable ZIF‑8@alizarin nanoparticles; then selecting hydrophilic colloid and polyvinyl alcohol as substrates to prepare a 3D printing base solution; then adding ZIF‑8@alizarin nanoparticles to the 3D printing base solution to obtain a mixed solution, adjusting the pH to 2 to 4, and obtaining a 3D printing stock solution; finally, adding the 3D printing stock solution to a printing chamber, drawing a self-selected model, selecting a nozzle, setting the printing temperature and speed, and then printing. The printed product is cooled and formed to obtain a freshness indicator label. The present invention utilizes the high specific surface area and porosity of MOFs materials to increase the stability of the indicator label while also ensuring the sensitivity of the indicator label. At the same time, it can also meet the personalized needs of the food packaging industry, and has broad application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of smart indicator labels, and in particular relates to a method for preparing a highly stable freshness indicator label of MOFs loaded with alizarin based on 3D printing technology. Background Art

[0002] Food safety has always been a hot topic of public concern, and food freshness is a crucial component of quality and safety. With advances in science and technology and the continuous development of new testing methods, food freshness testing has rapidly developed. Traditional food freshness testing methods primarily include sensory evaluation, physical and chemical analysis, and microbiological testing. However, most of these methods require specialized laboratory equipment, are time-consuming and labor-intensive, and cannot provide real-time, online testing of samples. Therefore, the development of intelligent indicator labels for food freshness monitoring is urgently needed. Freshness indicator labels are a new packaging technology that allows for real-time and efficient visualization of food freshness. Freshness indicator labels typically consist of two components: a solid substrate and an indicator loaded within the substrate. Their principle is based on the color change caused by the interaction of the indicator with components produced by food spoilage during storage. Freshness indicator labels do not require the sample to be destroyed; consumers can easily obtain food freshness information by observing the color change on the label. Their convenience, speed, and non-destructive nature have attracted widespread attention.

[0003] Among existing freshness indicator label technologies, the patent "Process for Preparing an Anthocyanin-Active Intelligent Packaging Film" (CN201810667474.X) discloses an anthocyanin freshness indicator film that can determine the freshness of meat by analyzing the color changes of the anthocyanin. However, this technology presents two major issues: the pigment indicator is unstable and easily affected by light and temperature, interfering with the accuracy of the test results; and the substrate is hydrophilic, which can easily swell during use as an indicator label, causing leakage of the indicator. To overcome these shortcomings, the patent "Method for Preparing an Indicator Label for Real-Time Meat Freshness Detection" (CN114062358A) discloses a microencapsulated structural material enclosed in hydrophobic PVDF filter paper, which effectively improves the stability of the freshness indicator label. However, this also presents another significant issue: the microencapsulated structure reduces the contact points between the indicator and spoilage components, thereby reducing the indicator's sensitivity. Therefore, balancing the stability and sensitivity of the freshness indicator becomes a conflicting issue.

[0004] Metal-organic frameworks (MOFs) are porous framework compounds composed of metal ions or metal oxide clusters connected to organic ligands via strong coordination bonds. They combine the advantages of structural tunability, high porosity, high surface area, and good thermal and chemical stability. The open windows and pores of MOFs allow for interaction between the diffusion matrix and the indicator small molecules, forming a relatively stable host-guest structure. 3D printing, also known as "additive manufacturing," is an intelligent molding technology that digitizes complex machining processes through computer programs to create high-precision geometric models. This technology is considered a key technological tool for driving the transformation of Industry 4.0. Combining these two approaches for use in food freshness indicator labels is currently a rare topic. This present invention aims to improve the stability of indicator labels by leveraging the high surface area and porosity of MOFs to effectively increase the stability of the indicator while ensuring its sensitivity. Furthermore, 3D printing technology can be used to meet the personalized needs of different food products and consumer preferences. Summary of the Invention

[0005] To address the problem that existing indicators have difficulty in balancing their sensitivity and stability, the present invention provides a high-specific-surface-area MOFs loaded with alizarin as a highly stable indicator carrier, and combines it with 3D printing technology to prepare a new type of freshness indicator label.

[0006] In order to achieve the above objectives, the specific implementation steps of the present invention are as follows:

[0007] Step 1: Preparation of highly stable ZIF-8@Alizarin nanoparticles:

[0008] A certain amount of zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 2-methylimidazole (MI), and alizarin were dissolved in a certain volume of methanol to obtain zinc nitrate hexahydrate solution, 2-methylimidazole solution, and alizarin solution. The three solutions were then mixed in a certain proportion to obtain a mixed solution, stirred for a certain period of time, centrifuged, and washed three times with methanol. The precipitate obtained after centrifugation and washing was collected and dried to obtain the product, which is the ZIF-8@alizarin nanoparticles.

[0009] Step 2: Preparation of 3D printing substrate solution;

[0010] A gel material that is easy to form 3D printing and its mixed system are selected as the base material for 3D printing. The present invention mainly selects hydrophilic colloid and polyvinyl alcohol as the base material, wherein the main preparation process is as follows:

[0011] S1. Add a certain amount of polyvinyl alcohol to a glycerol aqueous solution, heat and stir until fully dissolved to obtain a polyvinyl alcohol solution;

[0012] S2. After the hydrophilic colloid is added to the distilled water, heated and stirred until fully dissolved to obtain a uniform hydrophilic colloid solution;

[0013] S3. The polyvinyl alcohol solution obtained in S1 and the hydrophilic colloid solution obtained in S2 are mixed to obtain a 3D printing base solution;

[0014] Step 3: Add the ZIF-8@alizarin nanoparticles prepared in Step 1 to the 3D printing base solution prepared in Step 2, stir for a period of time under a certain temperature condition to obtain a mixed solution, and adjust the pH of the solution to 2-4 with citric acid to obtain a 3D printing stock solution; the 3D printing base solution loaded with alizarin will produce a wider color response change under acidic conditions, thereby effectively expanding the application range of the indicator label;

[0015] Step 4: Add the 3D printing reserve solution in step 3 into the printing chamber of the 3D printer. After drawing the selected model through computer software, select a nozzle with a certain inner diameter, set the printing temperature and speed, and then perform 3D printing. After printing, the product is quickly cooled down on a low-temperature molding platform and then fixed into shape, thus obtaining a highly stable freshness indicator label of MOFs loaded with alizarin based on 3D printing technology.

[0016] Preferably, in step 1, the dosage relationship of the hexahydrate, zinc nitrate and methanol is 1 g:30-50 mL; the dosage relationship of the 2-methylimidazole (MI) and methanol is 1-2 g:30-50 mL; the dosage relationship of the alizarin and methanol is 0.02-0.05 g:30-50 mL; the volume ratio of the zinc nitrate hexahydrate solution, the 2-methylimidazole solution and the alizarin solution when mixed is 1:1:1; the stirring time is 4-24 h; and the drying temperature is 40-60° C.

[0017] Preferably, the volume fraction of the glycerol solution in step 2 S1 is 2% to 4%; the heating temperature is 90 to 100° C.; and the mass concentration of polyvinyl alcohol in the polyvinyl alcohol solution is 1% to 3%.

[0018] Preferably, in step 2 S2, the hydrophilic colloid is one of agar, gellan gum, and carrageenan, the mass concentration of the hydrophilic colloid solution is 2% to 4%, and the heating temperature is 90 to 100°C.

[0019] Preferably, the volume ratio of the polyvinyl alcohol solution to the hydrophilic colloid solution in step 2 S3 is 1:1-10.

[0020] Preferably, the volume ratio of the ZIF-8@Alizarin nanoparticles to the 3D printing substrate solution in step 3 is 1 mg:4-6 mL.

[0021] Preferably, the certain temperature condition in step 3 is 55-70° C., and the stirring time is 30-60 min.

[0022] Preferably, the mass concentration of the citric acid solution in step three is 0.05% to 0.1%.

[0023] Preferably, the inner diameter of the nozzle in step 4 is 0.5-3 mm; the printing temperature is set to 50-70° C., the speed is 2-10 mm / s, and the temperature of the low-temperature molding platform is set to 10-30° C.

[0024] Beneficial effects of the present invention:

[0025] The present invention prepares a highly stable freshness indicator label of MOFs loaded with alizarin based on 3D printing technology. The high specific surface area and porosity of the MOFs material can increase the stability of the indicator label while ensuring the sensitivity of the indicator label.

[0026] The ZIF-8@alizarin nanomaterial prepared by the present invention effectively increases the thermal stability of alizarin, can overcome the temperature disadvantages of alizarin as an indicator material, and expand the application range of indicator labels.

[0027] By combining 3D printing technology as a construction carrier for food freshness indicator labels, the personalized needs of the food packaging industry can be effectively met. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A is a scanning electron micrograph of ZIF-8@alizarin nanoparticles prepared in Example 1; B is a cross-sectional electron micrograph of the indicator label prepared in Example 1; and C is an electron micrograph of the indicator label loaded with alizarin alone in Comparative Example 1.

[0029] Figure 2 Thermogravimetric analysis of ZIF-8@Alizarin nanoparticles prepared in Example 1;

[0030] Figure 3 This is a graph showing the storage stability of the indicator labels prepared in Example 1 and Comparative Example 1;

[0031] Figure 4 This is a graph showing the sensitivity response of the indicator labels prepared in Example 1 and Comparative Example 1 to ammonia. DETAILED DESCRIPTION

[0032] To address the problem that existing indicators have difficulty in balancing their sensitivity and stability, the present invention provides a high-specific-surface-area MOFs loaded with alizarin as a highly stable indicator carrier, and combines it with 3D printing technology to prepare a new type of freshness indicator label.

[0033] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0034] Example 1:

[0035] Step 1: Dissolve 1 g of zinc nitrate hexahydrate, 2 g of 2-methylimidazole (MI), and 20 mg of alizarin in 50 mL of methanol solution and stir until dissolved. Mix the three solutions in equal proportions and stir for 4 h. Centrifuge and wash the mixture three times with methanol. Collect the resulting solid precipitate and dry it in a vacuum drying oven at 40°C to obtain ZIF-8@alizarin nanoparticles.

[0036] Step 2: Select agar and polyvinyl alcohol as the base materials for 3D printing:

[0037] Take 1g of polyvinyl alcohol and add it to 100mL of aqueous solution containing 2mL of glycerol, heat and stir at 90℃ until fully dissolved to obtain polyvinyl alcohol solution;

[0038] Take 2g of agar and add it to 100mL of aqueous solution, then heat and stir at 90℃ to obtain an agar solution with a mass concentration of 2%;

[0039] The polyvinyl alcohol solution and the agar solution were mixed at a volume ratio of 1:1 at 90° C. to obtain a 3D printing base mixed solution;

[0040] Step 3: 50 mg of ZIF-8@Alizarin nanoparticles were added to 200 mL of the 3D printing substrate mixed solution, magnetically stirred at 55°C for 60 min, and then the pH of the solution was adjusted to 2 with 0.05% citric acid to obtain a 3D printing stock solution.

[0041] Step 4: Add the mixed 3D printing stock solution to the print chamber of a 3D printer. Using computer software, draw a cube and import it into the 3D printing program. Set the nozzle diameter to 3mm, the material printing temperature to 50°C, the printing speed to 10mm / s, and the build platform temperature to 10°C. After printing, the product is rapidly cooled on a low-temperature build platform and then fixed in place. This results in the highly stable MOFs-loaded alizarin freshness indicator label based on 3D printing technology.

[0042] Example 2:

[0043] Step 1: Take 1g of zinc nitrate hexahydrate, 1g of 2-methylimidazole (MI) and 50mg of alizarin and dissolve them in 30mL of methanol solution respectively and stir until dissolved; after the three solutions are mixed and stirred in equal proportions for 24h, centrifuge and wash the collected solid precipitate three times with methanol, and dry it in a vacuum drying oven at 60℃ to obtain ZIF-8@alizarin nanoparticles.

[0044] Step 2: Use agar and polyvinyl alcohol as the base materials for 3D printing: Take 3g of polyvinyl alcohol and add it to 100mL of aqueous solution containing 4mL of glycerol. Heat and stir at 100°C until fully dissolved to obtain a polyvinyl alcohol solution.

[0045] 4 g of agar was added to 100 mL of aqueous solution and heated at 100°C to obtain a 4% agar solution. The polyvinyl alcohol solution and the agar solution were mixed at 100°C in a volume ratio of 1:10 to obtain a 3D printing substrate mixed solution.

[0046] Step 3: 20 mg of ZIF-8@Alizarin nanoparticles were added to 120 mL of a 3D printing substrate mixed solution and magnetically stirred at 70°C for 30 min. The pH of the solution was adjusted to 4 with 0.1% citric acid to obtain a 3D printing stock solution.

[0047] Step 4: Add the mixed 3D printing stock solution into the printing chamber of the 3D printer, draw it into a cylinder through computer software, and then import it into the 3D printing program. The nozzle diameter is 1 mm, the printing temperature of the material is 70°C, the printing speed is 2 mm / s, and the molding platform temperature is set to 30°C. After printing, the product is quickly cooled using a low-temperature molding platform and then fixed into shape to obtain the MOF-loaded alizarin high-stable freshness indicator label based on 3D printing technology.

[0048] Example 3:

[0049] Step 1: Take 1g of zinc nitrate hexahydrate, 1.5g of 2-methylimidazole (MI) and 30mg of alizarin and dissolve them in 50mL of methanol solution respectively and stir until dissolved. After mixing and stirring the three solutions for 8h, centrifuge and wash them three times with methanol to collect the solid precipitate, and dry it in a vacuum drying oven at 50℃ to obtain ZIF-8@alizarin nanoparticles.

[0050] Step 2: Use agar and polyvinyl alcohol as the base materials for 3D printing: add 2g of polyvinyl alcohol to 100mL of an aqueous solution containing 3mL of glycerol; heat and stir at 95°C until fully dissolved to obtain a polyvinyl alcohol solution;

[0051] Take 3g of agar and add it to 100mL of aqueous solution, then heat it at 95℃ while stirring to obtain an agar solution with a mass concentration of 3%;

[0052] The polyvinyl alcohol solution and the agar solution were mixed at a volume ratio of 1:5 at 95° C. to obtain a 3D printing base mixed solution;

[0053] Step 3: 30 mg of ZIF-8@Alizarin nanoparticles were added to 120 mL of a 3D printing substrate mixed solution. After magnetic stirring at 60°C for 45 minutes, the pH of the solution was adjusted to 3 with 0.2% citric acid to obtain a 3D printing stock solution.

[0054] Step 4: Add the mixed 3D printing stock solution into the printing chamber of the 3D printer, draw it into a simple cube through computer software, and then import it into the 3D printing program. The nozzle diameter is 1 mm, the printing temperature of the W material is 60°C, the printing speed is 15 mm / s, and the molding platform temperature is set to 20°C. After printing, the product is quickly cooled on the low-temperature molding platform and then fixed into shape to obtain the MOF-loaded alizarin high-stable freshness indicator label based on 3D printing technology.

[0055] Comparative Example 1:

[0056] In order to further verify the technical effect of the invention, a comparative example is now set. The comparative example is opposite to Example 1, except that the load process is deleted. The specific implementation steps are as follows:

[0057] Step 1: Select agar and polyvinyl alcohol as the base materials for 3D printing:

[0058] Take 1g of polyvinyl alcohol and add it to 100mL of aqueous solution containing 2mL of glycerol, heat and stir at 90℃ until fully dissolved to obtain polyvinyl alcohol solution;

[0059] Take 2g of agar and add it to 100mL of aqueous solution, then heat and stir at 90℃ to obtain an agar solution with a mass concentration of 2%;

[0060] The polyvinyl alcohol solution and the agar solution were mixed at a volume ratio of 1:1 at 90° C. to obtain a 3D printing base mixed solution;

[0061] Step 2: 5 mg of ZIF-8@Alizarin nanoparticles were added to 200 mL of a 3D printing substrate mixed solution, magnetically stirred at 55°C for 60 min, and then the pH of the solution was adjusted to 2 with 0.05% citric acid to obtain a 3D printing stock solution.

[0062] Step 3: Add the mixed 3D printing stock solution to the print chamber of a 3D printer. A simple cube is drawn using computer software and then imported into the 3D printing program. The nozzle diameter is 3 mm, the material is printed at a temperature of 50°C, the printing speed is 10 mm / s, and the build platform temperature is set at 10°C. After printing, the product is rapidly cooled on a low-temperature build platform and then fixed in place. This results in the highly stable MOF-loaded alizarin freshness indicator label based on 3D printing technology.

[0063] In order to further illustrate the technical effect of the present invention, the indicator labels prepared in Example 1 and Comparative Example 1 were subjected to a series of characterizations, and the specific results are as follows:

[0064] (1) Scanning electron microscopy analysis

[0065] As attached Figure 1 As shown in Figure A, the ZIF-8@Alizarin nanoparticles are regular dodecahedral nanostructures with uniform particle size. As shown in Figures B and C, the cross-section of the indicator tag loaded with ZIF-8@Alizarin nanoparticles is rougher than that of the indicator tag loaded with alizarin alone. This is due to the low dispersibility of ZIF-8 in the substrate solution. However, the rough cross-sectional structure can increase the porosity of the indicator tag, thereby increasing the sensitivity of the tag, thereby compensating for the disadvantage of reduced exposure sites after being loaded with MOFs.

[0066] (2) Thermogravimetric analysis

[0067] In order to characterize the thermal stability of ZIF-8@Alizarin nanoparticles before and after encapsulation, the materials were subjected to thermogravimetric analysis. Figure 2 The results clearly show that alizarin begins to decompose at 180°C, with a weight loss of approximately 10%. At 300°C, the weight has decomposed by more than half, a 55% decrease. Meanwhile, at 450°C, the weight of ZIF-8@alizarin nanoparticles only decreases by 10%, consistent with the thermal stability of ZIF8 itself. This demonstrates that ZIF-8@alizarin nanoparticles effectively improve the thermal stability of alizarin.

[0068] (3) Color stability determination of indicator labels

[0069] The indicator label reflects the quality information of the food through color change, so the color stability of the indicator label must be guaranteed. The common storage temperatures of 4°C (refrigerated) and 25°C (room temperature) were selected for food. The 3D printed indicator label was placed in a PET fresh-keeping box for 14 days. The color signal was collected every two days, and the stability of the indicator label was determined by calculating the color difference value (ΔE). The results are as follows Figure 3 As shown in the results, at 4°C, the color difference after 14 days for the indicator tag loaded with alizarin alone was 3.19%, while that for the tag loaded with ZIF8@alizarin was 2.49%. At 25°C, the color difference after 14 days for the indicator tag loaded with alizarin alone was 5.46%, while that for the tag loaded with alizarin was only 3.16%. These results indicate that the stability of the tag after loading is significantly improved, especially under adverse conditions such as high temperatures, where the increased stability is even more pronounced.

[0070] (4) Sensitivity measurement of indicator labels

[0071] In order to better verify the sensitivity of the indicator tag loaded with ZIF-8@Alizarin nanoparticles, the indicator film was made to respond to ammonia. Figure 4As shown, the sensitivity of the loaded alizarin alone was 44.15% at 60 minutes of contact with ammonia, and the sensitivity of the loaded ZIF8@alizarin was 39.7% at 60 minutes. There was a small difference between the two, indicating that the sensitivity did not decrease significantly.

[0072] In summary, the present invention designs a MOF-loaded alizarin high-stability freshness indicator label based on 3D printing technology, which can not only increase the stability of the indicator label, but also ensure the sensitivity of the indicator label, and well balance the relationship between the stability and sensitivity of the indicator label, and can realize the visual detection of food freshness.

[0073] 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 a highly stable freshness indicator label of MOFs loaded with alizarin based on 3D printing technology, characterized in that: The following steps are involved: Step 1, respectively taking a certain amount of zinc nitrate hexahydrate, 2-methylimidazole and alizarin and dissolving them in a certain volume of methanol to obtain a zinc nitrate hexahydrate solution, a 2-methylimidazole solution and an alizarin solution; then mixing the three solutions in a certain proportion to obtain a mixed solution, stirring for a certain time, centrifuging and washing the solution three times with methanol; collecting the precipitate obtained after centrifugal washing and drying the obtained product, which is ZIF-8@alizarin nanoparticles; the dosage relationship of the zinc nitrate hexahydrate and methanol is 1g:30-50mL; the dosage relationship of the 2-methylimidazole and methanol is 1-2g:30-50mL; the dosage relationship of the alizarin and methanol is 0.02-0.05g:30-50mL; the volume ratio of the zinc nitrate hexahydrate solution, the 2-methylimidazole solution and the alizarin solution when mixed is 1:1:1; The ZIF-8@Alizarin nanoparticles are regular dodecahedral nanostructures with uniform particle size. Step 2: Preparation of 3D printing substrate solution; S1. Add a certain amount of polyvinyl alcohol to a glycerol aqueous solution, heat and stir until fully dissolved to obtain a polyvinyl alcohol solution; S2. After adding the hydrophilic colloid to distilled water, heating and stirring until fully dissolved to obtain a uniform hydrophilic colloid solution; S3. The polyvinyl alcohol solution obtained in S1 and the hydrophilic colloid solution obtained in S2 are mixed to obtain a 3D printing base solution; Step 3: Add the ZIF-8@Alizarin nanoparticles prepared in step 1 to the 3D printing base solution prepared in step 2, stir for a period of time under certain temperature conditions to obtain a mixed solution, and adjust the pH of the solution to 2-4 with citric acid to obtain a 3D printing stock solution; the volume ratio of the ZIF-8@Alizarin nanoparticles to the 3D printing base solution is 1 mg:4-6 mL; Step 4: Add the 3D printing reserve solution in step 3 into the printing chamber of the 3D printer. After drawing the selected model through computer software, select a nozzle with a certain inner diameter, set the printing temperature and speed, and then perform 3D printing. After printing, the product is quickly cooled down on a low-temperature molding platform and then fixed into shape, thus obtaining a highly stable freshness indicator label of MOFs loaded with alizarin based on 3D printing technology.

2. The method for preparing a MOFs-loaded alizarin high-stability freshness indicator label based on 3D printing technology according to claim 1, characterized in that: The stirring time in step 1 is 4 to 24 hours; the drying temperature is 40 to 60°C.

3. The method for preparing a MOFs-loaded alizarin high-stability freshness indicator label based on 3D printing technology according to claim 1, characterized in that: In step 2 S1, the volume fraction of the glycerol solution is 2% to 4%; the heating temperature is 90 to 100° C.; and the mass concentration of polyvinyl alcohol in the polyvinyl alcohol solution is 1% to 3%.

4. The method for preparing a MOFs-loaded alizarin high-stability freshness indicator label based on 3D printing technology according to claim 1, characterized in that: In step 2 S2, the hydrophilic colloid is one of agar, gellan gum, and carrageenan, and the mass concentration of the hydrophilic colloid solution is 2% to 4%; the heating temperature is 90 to 100°C.

5. The method for preparing a MOFs-loaded alizarin high-stability freshness indicator label based on 3D printing technology according to claim 1, characterized in that: The volume ratio of the polyvinyl alcohol solution to the hydrophilic colloid solution in step 2 S3 is 1:1-10.

6. The method for preparing a MOFs-loaded alizarin high-stability freshness indicator label based on 3D printing technology according to claim 1, characterized in that: The certain temperature condition in step 3 is 55-70° C., and the stirring time is 30-60 min.

7. The method for preparing a MOFs-loaded alizarin high-stability freshness indicator label based on 3D printing technology according to claim 1, characterized in that: The mass concentration of the citric acid solution in step 3 is 0.05% to 0.1%.

8. The method for preparing a MOFs-loaded alizarin high-stability freshness indicator label based on 3D printing technology according to claim 1, characterized in that: The inner diameter of the nozzle in step 4 is 0.5-3 mm; the printing temperature is set to 50-70° C., the speed is 2-10 mm / s, and the temperature of the low-temperature molding platform is set to 10-30° C.

9. Use of the MOFs-loaded alizarin high-stability freshness indicator label prepared according to the method of any one of claims 1 to 8 in indicating the freshness of food.

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