Intelligent breath-responsive Cu-BTC / chitosan / polyvinyl alcohol film, and preparation method and application thereof

Abstract

This invention discloses a smart, respirable Cu-BTC / chitosan / polyvinyl alcohol membrane, its preparation method, and its applications. The invention employs a one-pot solution-phase process. First, a copper salt precursor, polyvinyl alcohol, and a plasticizer are added to a chitosan solution and mixed. Then, an organic ligand solution is added for a hydrothermal reaction to synthesize the Cu-BTC / chitosan / polyvinyl alcohol membrane solution. After cooling, cinnamaldehyde is loaded, and the membrane is obtained through freeze-thaw physical crosslinking and drying. The prepared composite membrane exhibits a typical octahedral morphology, with uniform distribution and tight bonding to the matrix, solving the problems of uneven crystal distribution and base membrane defects in traditional in-situ growth methods. The resulting composite membrane possesses dual respirable response characteristics (acidity / humidity), intelligently releasing cinnamaldehyde according to the acidic and high-humidity microenvironment caused by fruit and vegetable spoilage, effectively inhibiting Staphylococcus aureus and significantly reducing the spoilage rate of blueberries during post-harvest storage.

Description

Technical Field

[0001] This invention belongs to the field of food packaging materials technology, specifically relating to a smart breathable responsive Cu-BTC / chitosan / polyvinyl alcohol film, its preparation method, and its application. Background Technology

[0002] In recent years, with increasing public awareness of food safety and environmental protection, the development of green, safe, and intelligent preservation-functional active packaging materials has become a research hotspot in the field of food science. The waxy layer of blueberries is easily detached due to physical friction during post-harvest handling or transportation, leading to accelerated moisture loss, increased risk of pathogen invasion, and ultimately shortened shelf life. While traditional petroleum-based plastic packaging such as polyethylene has good barrier properties, it lacks active antibacterial function and faces contamination issues.

[0003] Chitosan (CS) and polyvinyl alcohol (PVA), as excellent biodegradable polymers, have attracted much attention due to their outstanding biocompatibility, excellent film-forming properties, and environmentally friendly characteristics of being completely decomposed into water and carbon dioxide by microorganisms. Chitosan is readily degraded in natural soil and compost environments, possessing not only natural antibacterial properties but also pH sensitivity due to its amino protonation characteristics. However, its mechanical properties and moisture barrier properties are poor when used alone. Polyvinyl alcohol also has excellent environmental degradation capabilities and superior mechanical strength and gas barrier properties, which can compensate for the physical defects of chitosan, but it lacks biological activity. Therefore, mixing the two to construct a biodegradable composite matrix is ​​an ideal environmentally friendly preservation packaging solution. Volatile essential oils are mostly natural plant-derived antibacterial agents with broad-spectrum antibacterial properties. They mainly achieve antibacterial effects by disrupting microbial cell membranes, denaturing proteins, and interfering with metabolism. However, they are volatile and easily oxidized. Direct blending into the membrane matrix often leads to a burst release of active ingredients in the early stages of storage, while the concentration is insufficient in the later stages, making it difficult to achieve long-term preservation.

[0004] Metal-organic frameworks (MOFs) are a new type of porous materials with high specific surface area and tunable pore size. Among them, Cu-BTC not only has an excellent pore structure that can be used to load active molecules, but its Cu-O coordination bonds also have unique humidity sensitivity. Furthermore, the synergistic effect of combining MOFs with polymers can improve the mechanical properties, antibacterial activity, and smart responsiveness of the materials.

[0005] In summary, although there have been studies on active packaging films, there are few reports on composite films that can achieve intelligent controlled release in response to both acidity and humidity changes in the postharvest microenvironment of fruits and vegetables, and possess excellent mechanical properties. Among them, there are no reports on methods for preparing respiratory-responsive controlled-release films by combining Cu-BTC and chitosan / polyvinyl alcohol films.

[0006] Based on this, this application is hereby submitted. Summary of the Invention

[0007] To address the problems of existing fruit and vegetable preservation packaging materials lacking intelligent responsiveness, uncontrollable release of active ingredients, and poor mechanical properties, this invention provides an intelligent respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol film with acidity and humidity respiratory response characteristics, prepared by a one-pot solution phase method, along with its preparation method and applications.

[0008] The objective of this invention is achieved through the following technical solution:

[0009] A method for preparing a smart respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol membrane, characterized by comprising the following steps:

[0010] Step A: Prepare chitosan solution and polyvinyl alcohol solution separately;

[0011] Step B: Add the copper salt precursor, polyvinyl alcohol solution and plasticizer to the chitosan solution and mix evenly to obtain a copper-containing matrix mixture;

[0012] Step C: Prepare an organic ligand solution and add it to the copper-containing matrix mixture obtained in Step B. Perform a hydrothermal reaction under closed conditions to synthesize Cu-BTC / CS / PVA membrane solution in a one-pot solution phase.

[0013] Step D: After the membrane solution obtained in step C is cooled, cinnamaldehyde is added, stirred evenly, and after molding, physical cross-linking and drying, the Cu-BTC / chitosan / polyvinyl alcohol membrane is obtained.

[0014] Preferably, in step A, the concentration of the chitosan solution is 1.5%-2.5% (w / v), and the solvent is an acetic acid solution of 1%-3% (v / v); the concentration of the polyvinyl alcohol solution is 10%-14% (w / w); in the mixing system of step B, the mass ratio of chitosan to polyvinyl alcohol is 1:2 to 1:8.

[0015] Preferably, in the mixing system of step B, the mass ratio of chitosan to polyvinyl alcohol is 1:4.

[0016] Preferably, in step B, the copper salt precursor is copper nitrate trihydrate; the plasticizer is glycerol; the ratio of copper nitrate trihydrate: polyvinyl alcohol solution: glycerol is (0.4~0.9) g: 40 mL: 3 mL; and the mixing conditions are stirring in a water bath at 60±5℃ for 30-90 min.

[0017] Preferably, in step C, the organic ligand is pyromellitic acid, and the solvent is anhydrous ethanol; the mass ratio of pyromellitic acid to copper nitrate trihydrate is (0.7~1.5):(0.4~0.9).

[0018] The hydrothermal reaction is carried out at a temperature of 80-90℃ for 20-28 hours.

[0019] Preferably, in step D, the temperature when adding cinnamaldehyde is controlled below 40°C, and the amount added is 3.0%-7.0% of the dry weight of the membrane matrix; the physical crosslinking is to pour the membrane liquid into a mold, freeze it at -20°C±2°C for 12-24 hours, thaw it at room temperature for 20-40 minutes, and repeat the cycle 3 times.

[0020] Preferably, in step D, the drying method is vacuum drying, the temperature is 35-45℃, and the drying time is 20-28 hours.

[0021] The intelligent respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol membrane prepared by the method described in this invention.

[0022] The application of the intelligent respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol film described in this invention in fruit and vegetable preservation.

[0023] Preferably, the film is used for preserving blueberries and has an antibacterial effect against Staphylococcus aureus.

[0024] The principle of this invention: This invention utilizes a one-pot solution-phase method to synthesize a Cu-BTC metal-organic framework in a chitosan / polyvinyl alcohol polymer matrix. Cu-BTC possesses a rich porous structure, enabling efficient loading of the antibacterial agent cinnamaldehyde. The chitosan molecular chain contains numerous amino groups; under the acidic environment produced by fruit and vegetable spoilage, these amino groups undergo protonation, leading to electrostatic repulsion and swelling of the polymer network. Simultaneously, the acidic environment causes protonation of the carboxylic acid groups in Cu-BTC, weakening the metal-ligand coordination and synergistically promoting cinnamaldehyde release. The Cu-O coordination bonds in Cu-BTC are sensitive to water molecules; under the transpiration of fruits and vegetables, competitive coordination of water molecules leads to adjustment or partial collapse of the pore structure, causing the hydrophilic matrix to absorb water and swell, collectively triggering the release of cinnamaldehyde.

[0025] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0026] 1. This invention employs a one-pot solution-phase preparation process. Compared to in-situ growth, this method produces Cu-BTC crystals with higher density, more uniform distribution, and a more complete crystal structure, and exhibits stronger adhesion to the substrate film. Furthermore, the reaction conditions are mild, demonstrating good process repeatability and operational feasibility.

[0027] 2. The composite membrane prepared by this invention exhibits acidity and humidity-responsive breathing characteristics. Under simulated fruit and vegetable storage conditions, the release of cinnamaldehyde is significantly higher than in neutral or low-humidity environments, enabling the release of antibacterial agents as needed based on the degree of fruit and vegetable spoilage.

[0028] 3. The composite film prepared by this invention has significant effects in the preservation of blueberries. Experiments have shown that the film can effectively inhibit Staphylococcus aureus, significantly reduce the spoilage rate and weight loss rate of blueberries, delay fruit softening, and maintain nutrients such as vitamin C. Attached Figure Description

[0029] Figure 1 This is a schematic diagram illustrating the preparation process and intelligent breathing response mechanism of the Cu-BTC / CS / PVA-Cin composite thin film in Examples 4-6 of the present invention.

[0030] Figure 2 The images show the appearance of the Cu-BTC / CS / PVA-Cin composite films prepared in Examples 4-6 and the control examples of the present invention.

[0031] Figure 3 Scanning electron microscope (SEM) images of Cu-BTC / CS / PVA-Cin composite films prepared in Example 4 (one-pot solution phase method), Comparative Example 1 (in-situ synthesis of membrane substrate method 1), and Comparative Example 2 (in-situ synthesis of membrane substrate method 2) of the present invention, and transmission electron microscope (TEM) image of Example 4.

[0032] Figure 4 is a comparison diagram of the mechanical properties of the composite films prepared in the embodiments and control examples of the present invention.

[0033] Figure 5 is a comparison of the water vapor barrier properties of the composite films prepared in the embodiments and control examples of the present invention.

[0034] Figure 6 Cinnamaldehyde release curves of MOF-Cin3.2, MOF-Cin4.55 and MOF-Cin6.3 films prepared in Examples 4-6 of this invention under different pH and humidity conditions.

[0035] Figure 7 The graphs show the inhibitory effects of MOF-Cin3.2, MOF-Cin4.55, and MOF-Cin6.3 films prepared in Examples 4-6 of this invention on Staphylococcus aureus under different pH and humidity conditions.

[0036] Figure 8 Comparative graphs showing the appearance changes of blueberries during storage after packaging with MOF-Cin3.2, MOF-Cin4.55, and MOF-Cin6.3 films prepared in Examples 4-6 of this invention.

[0037] Figure 9 The graph shows the effect of MOF-Cin3.2, MOF-Cin4.55 and MOF-Cin6.3 film packaging prepared in Examples 4-6 of this invention on the spoilage rate of blueberries during storage.

[0038] Figure 10The graph shows the effect of MOF-Cin3.2, MOF-Cin4.55 and MOF-Cin6.3 film packaging prepared in Examples 4-6 of this invention on the changes in flavonoid, total phenol and anthocyanin content of blueberries during storage.

[0039] Figure 11 The graph shows the effect of MOF-Cin3.2, MOF-Cin4.55 and MOF-Cin6.3 film packaging prepared in Examples 4-6 of this invention on the changes in malondialdehyde content during blueberry storage.

[0040] Figure 12 The graph shows the effect of MOF-Cin3.2, MOF-Cin4.55 and MOF-Cin6.3 film packaging prepared in Examples 4-6 of this invention on the change of total bacterial count during blueberry storage.

[0041] Figure 13 The figure shows the effect of MOF-Cin3.2, MOF-Cin4.55 and MOF-Cin6.3 film packaging prepared in Examples 4-6 of this invention on the moisture migration of blueberries during storage. Detailed Implementation

[0042] The present invention will now be described in further detail with reference to the embodiments and accompanying drawings. Although detailed embodiments of the present invention are provided herein, the reader should understand that these embodiments are merely illustrative examples of the present invention, and the present invention can be implemented and embodied in various other ways. Therefore, the specific functional details disclosed herein should not be construed as limiting, but rather as supporting descriptions of the claims.

[0043] Chitosan, polyvinyl alcohol, copper nitrate trihydrate, 1,3,5-benzenetricarboxylic acid, and cinnamaldehyde used in the following embodiments and comparative examples of this invention were all purchased from Aladdin Biochemical Technology Co., Ltd., glycerin was purchased from Guangzhou Zhufeng Import & Export Trading Co., Ltd., and Chilean blueberries were imported under customs declaration number 516620251660009036. The goods were stored by Guangzhou Nansha International Cold Chain Co., Ltd.

[0044] Example 1

[0045] Preparation of CS / PVA membrane:

[0046] 1.2 g of chitosan was dissolved in 60 mL of 2% (v / v) acetic acid solution and stirred until completely dissolved to obtain a 2% CS solution. 4.8 g of polyvinyl alcohol was dissolved in 40 mL of cold water and stirred at 85℃ and 800 r / min for 30 min until completely dissolved to obtain a 12% PVA solution.

[0047] Prepare a 100 mL mixture by mixing CS and PVA solutions at a mass ratio of 1:4, and add 3 mL of glycerol. After mixing, heat the mixture in a 60°C water bath at 1000 rpm for 1 hour. Pour an appropriate amount of this mixture into a petri dish to prepare a circular membrane, then freeze at -20°C for 18 hours and cool at room temperature (30 minutes), repeating three cycles. Finally, vacuum dry at 40°C for 24 hours to obtain the film, labeled CS / PVA-1:4.

[0048] Example 2

[0049] The difference from Example 1 is that the CS / PVA mass ratio is 1:2, which is labeled as CS / PVA-1:2.

[0050] Example 3

[0051] Unlike Example 1, the CS / PVA mass ratio is 1:8, and it is labeled as CS / PVA-1:8.

[0052] Example 4

[0053] A one-pot solution-phase method for preparing a Cu-BTC / chitosan / polyvinyl alcohol membrane with controlled release of cinnamaldehyde in response to respiration, the specific steps of which are as follows:

[0054] 1. Dissolve 1.2 g of chitosan in 60 mL of 2% (v / v) acetic acid solution and stir until completely dissolved to obtain a 2% CS solution. Dissolve 4.8 g of polyvinyl alcohol in 40 mL of cold water and stir at 85℃ and 800 r / min for 30 min until completely dissolved to obtain a 12% PVA solution.

[0055] 2. Add 0.6 g of copper nitrate trihydrate, 40 mL of the above PVA solution and 3 mL of glycerol to 60 mL of the above CS solution in sequence, and heat in a 60°C water bath at 1000 rpm for 1 hour to obtain a homogeneous copper-containing matrix solution A.

[0056] 3. Dissolve 1.0 g of trimesic acid in 16 mL of anhydrous ethanol, stir until completely dissolved, and then add it to solution A from step 2. Pour the mixture into a hydrothermal reactor and react at 85°C for 24 hours to synthesize Cu-BTC.

[0057] 4. After the reaction solution has cooled to below 40°C, add 0.6 g of cinnamaldehyde (4.55% of the dry weight of the membrane matrix) and stir magnetically for 10 minutes to ensure uniform dispersion. Pour the membrane solution into a glass mold and freeze at -20°C for 18 hours, then thaw at room temperature for 30 minutes. Repeat this cycle three times for physical crosslinking. Finally, vacuum dry at 40°C for 24 hours and equilibrate in a constant temperature and humidity chamber (25°C, 53% RH) for 48 hours to obtain the Cu-BTC / CS / PVA-Cin composite film, labeled MOF-Cin4.55.

[0058] Example 5

[0059] 1. Dissolve 1.2 g of chitosan in 60 mL of 2% (v / v) acetic acid solution and stir until completely dissolved to obtain a 2% CS solution. Dissolve 4.8 g of polyvinyl alcohol in 40 mL of cold water and stir at 85℃ and 800 r / min for 30 min until completely dissolved to obtain a 12% PVA solution.

[0060] 2. Add 0.4 g of copper nitrate trihydrate, 40 mL of the above PVA solution and 3 mL of glycerol to 60 mL of the above CS solution in sequence, and heat in a 60°C water bath at 1000 rpm for 1 hour to obtain a homogeneous copper-containing matrix solution A.

[0061] 3. Dissolve 0.7 g of trimesic acid in 16 mL of anhydrous ethanol, stir until completely dissolved, and then add it to solution A from step 2. Pour the mixture into a hydrothermal reactor and react at 85°C for 24 hours to synthesize Cu-BTC.

[0062] 4. After the reaction solution has cooled to below 40°C, add 0.4 g of cinnamaldehyde (3.2% of the dry weight of the membrane matrix) and stir magnetically for 10 minutes to ensure uniform dispersion. Pour the membrane solution into a glass mold and freeze at -20°C for 18 hours, then thaw at room temperature for 30 minutes. Repeat this cycle three times for physical crosslinking. Finally, vacuum dry at 40°C for 24 hours and equilibrate in a constant temperature and humidity chamber (25°C, 53% RH) for 48 hours to obtain the Cu-BTC / CS / PVA-Cin composite film, labeled MOF-Cin3.2.

[0063] Example 6

[0064] 1. Dissolve 1.2 g of chitosan in 60 mL of 2% (v / v) acetic acid solution and stir until completely dissolved to obtain a 2% CS solution. Dissolve 4.8 g of polyvinyl alcohol in 40 mL of cold water and stir at 85℃ and 800 r / min for 30 min until completely dissolved to obtain a 12% PVA solution.

[0065] 2. Add 0.9 g of copper nitrate trihydrate, 40 mL of the above PVA solution, and 3 mL of glycerol sequentially to 60 mL of the above CS solution. Heat in a 60°C water bath at 1000 rpm for 1 hour with stirring to obtain a homogeneous copper-containing matrix solution A.

[0066] 3. Dissolve 1.5 g of trimesic acid in 16 mL of anhydrous ethanol, stir until completely dissolved, and then add it to solution A from step 2. Pour the mixture into a hydrothermal reactor and react at 85°C for 24 hours to synthesize Cu-BTC.

[0067] 4. After the reaction solution has cooled to below 40°C, add 0.9 g of cinnamaldehyde (6.3% of the dry weight of the membrane matrix) and stir magnetically for 10 minutes to ensure uniform dispersion. Pour the membrane solution into a glass mold and freeze at -20°C for 18 hours, then thaw at room temperature for 30 minutes. Repeat this cycle three times for physical crosslinking. Finally, vacuum dry at 40°C for 24 hours and equilibrate in a constant temperature and humidity chamber (25°C, 53% RH) for 48 hours to obtain the Cu-BTC / CS / PVA-Cin composite film, labeled MOF-Cin6.3.

[0068] Comparative Example 1

[0069] The steps for preparing Cu-BTC / chitosan / polyvinyl alcohol membranes using a membrane-based in-situ synthesis method (Method 1) are as follows:

[0070] 1. Dissolve 1.2 g of chitosan in 60 mL of 2% (v / v) acetic acid solution and stir until completely dissolved to obtain a 2% CS solution. Dissolve 4.8 g of polyvinyl alcohol in 40 mL of cold water and stir at 85℃ and 800 r / min for 30 min until completely dissolved to obtain a 12% PVA solution.

[0071] 2. Add 40 mL of PVA solution and 3 mL of glycerol to 60 mL of CS solution. After mixing, heat the mixture in a 60°C water bath at 1000 rpm for 1 hour. Pour an appropriate amount of the mixture into a petri dish for film preparation, then freeze at -20°C for 18 hours and cool at room temperature for 30 minutes, repeating three cycles. Finally, vacuum dry at 40°C for 24 hours to obtain the film.

[0072] 3. The prepared base film was placed in a 60 mg / mL copper nitrate trihydrate solution and shaken at 25°C for 24 hours to obtain CS / PVA-Cu. 2+ membrane.

[0073] 4. The prepared CS / PVA-Cu 2+The membrane was placed in a 60 mg / mL BTC ethanol solution in a hydrothermal reactor and reacted at 80°C for 24 hours. Finally, the synthesized product was removed and washed three times with ethanol.

[0074] 5. Dissolve 10 g of cinnamaldehyde in 100 mL of anhydrous ethanol to prepare a 0.1 g / mL cinnamaldehyde ethanol solution. Add 10 g of Cu-BTC@CS / PVA to this solution, seal, and stir at 25°C for 24 hours. After the reaction is complete, remove the sample and allow it to air dry for 24 hours. Finally, place all membrane samples in a constant temperature and humidity chamber at 25°C and 53% relative humidity for 48 hours for subsequent testing. Label as In-situ_1.

[0075] Comparative Example 2

[0076] The steps for preparing Cu-BTC / chitosan / polyvinyl alcohol membranes using a membrane-based in-situ synthesis method (Method 2) are as follows:

[0077] 1. Dissolve 1.2 g of chitosan in 60 mL of 2% (v / v) acetic acid solution and stir until completely dissolved to obtain a 2% CS solution. Dissolve 4.8 g of polyvinyl alcohol in 40 mL of cold water and stir at 85℃ and 800 r / min for 30 min until completely dissolved to obtain a 12% PVA solution.

[0078] 2. Weigh 0.6 g of copper nitrate (II) trihydrate, 40 mL of PVA solution, and 3 mL of glycerol and add them to 60 mL of CS solution. After mixing, heat the mixture in a 60°C water bath at 1000 rpm for 1 hour. Pour an appropriate amount of the mixture into a petri dish for film preparation, then freeze at -20°C for 18 hours and cool at room temperature for 30 minutes, repeating three cycles. Finally, vacuum dry at 40°C for 24 hours to obtain CS / PVA-Cu. 2+ film.

[0079] 3. The prepared CS / PVA-Cu 2+ The mixed membrane was placed in a 60 mg / mL BTC ethanol solution, poured into a hydrothermal reactor, and reacted at 80°C for 24 hours. The synthesized product was removed, washed three times with ethanol, and vacuum dried at 40°C for 24 hours to obtain a Cu-BTC@CS / PVA film.

[0080] 4. Dissolve 10 g of cinnamaldehyde in 100 mL of anhydrous ethanol to prepare a 0.1 g / mL cinnamaldehyde ethanol solution. Add 10 g of Cu-BTC@CS / PVA to this solution, seal, and stir at 25°C for 24 hours. After the reaction is complete, remove the sample and allow it to air dry for 24 hours. Finally, place all membrane samples in a constant temperature and humidity chamber at 25°C and 53% relative humidity for 48 hours for subsequent testing. Label as In-situ_2.

[0081] Comparative Example 3

[0082] Commercially available PE film.

[0083] Performance Analysis

[0084] Figure 2 (AC) are images of the composite films prepared by the one-pot solution-phase method, the first membrane-based in-situ synthesis method, and the second membrane-based in-situ synthesis method, respectively. Observation shows that all three methods can produce translucent blue films. The film obtained by the one-pot solution-phase method has a smoother and more even surface, while the films prepared by the two membrane-based in-situ synthesis methods exhibit varying degrees of wrinkling. Figure 2 (DE) Images show the appearance of composite films with different MOF-Cin loadings prepared by a one-pot solution-phase method. All three films exhibit a uniform, continuous, and intact structure without obvious pores. MOF-Cin is well dispersed in the polymer matrix without significant agglomeration. As the MOF-Cin loading increases, the film color changes from light yellow-green to bright green, and finally to lake blue.

[0085] Figure 3 Scanning electron microscope (SEM) images of Cu-BTC / CS / PVA-Cin composite films prepared using the methods described in Comparative Examples 1, 2, and 4 are shown, along with a transmission electron microscope (TEM) image of Example 4. As can be seen from the figures, the MOF crystal density synthesized by the solution-phase one-pot method is higher than that of the two in-situ synthesis methods. The crystals are densely distributed, both inside and on the surface of the film. Furthermore, the MOF structure formed by the one-pot method has clear boundaries, a smooth surface, and no residue around it. Figure 3(As indicated by the arrows), indicating a stable crystalline structure and good crystallization effect. The MOF synthesized by in-situ synthesis method one exhibits surface collapse and wrinkling, blurred boundaries, and residue adhesion. Its CS / PVA base film displays a cracked, interwoven fiber structure, indicating that the MOF structure synthesized by this method is unstable and the film structure is relatively rigid. The MOF structure prepared by in-situ synthesis method two is more stable than that prepared by in-situ synthesis method one, with less edge and surface collapse, clearer boundaries, and less surface adhesion, but its overall quality is still inferior to the one-pot method. These results indicate that the solution-phase one-pot method can synthesize more, denser, and better-crystallized Cu-BTC crystals. Transmission electron microscopy results further confirm the successful synthesis and good crystallization of Cu-BTC, showing clear lattice fringes in the images, presumably due to cinnamaldehyde being uniformly distributed on the MOF surface and in the pores of the base film.

[0086] The film sample (15 mm × 80 mm) was placed in a desiccator (25 ° C, 53 % RH) for equilibration for 16 hours, and its tensile strength and elongation at break were determined using an electronic universal testing machine. Figure 4 The figures show a comparison of the mechanical properties of the films in the embodiments and comparative examples of this invention. As can be seen from the figures, the tensile strength of CS / PVA_1:8 is significantly higher than that of CS / PVA_1:2 and CS / PVA_1:4 films. This may indicate that the increased content of hydroxyl-rich polymeric PVA helps to form dense crystalline regions of hydroxyl groups, thus improving film strength. CS / PVA_1:4 exhibits the highest elongation at break (158.4%), indicating that it has the best ductility among all tested films. CS / PVA_1:4, with its high ductility and moderate toughness, demonstrates superior performance for food preservation films compared to other films. The tensile strength of the film prepared by in-situ synthesis method two is significantly higher than that of other film groups. This may be because after forming the Cu@CS / PVA film, when it is placed in an ethanol solution containing Cu, the ethanol rapidly replaces the water molecules in the film, disrupting the original hydrogen bond network and promoting the rearrangement of polymer chains. The film prepared by in-situ synthesis method one, due to immersion in a Cu-containing solution… 2+ In solution, the CS / PVA molecules absorb water and swell, disrupting the original hydrogen bond network and increasing the interchain spacing, resulting in a loose film structure and thus lower tensile strength. Films prepared by the solution-phase one-pot method, however, exhibit both high ductility and moderate toughness, demonstrating potential for application in composite film preparation. The MOF-Cin4.55 group of films showed the highest tensile strength, indicating that this film maintains a certain degree of ductility while possessing superior tensile properties. This may be because the addition of an appropriate amount of cinnamaldehyde promotes cross-linking between the CS / PVA networks, while excessive MOF particles (such as in the MOF-Cin6.3 group) may lead to uneven MOF growth and aggregation, which in turn reduces tensile strength.

[0087] Cut the membrane into circular pieces slightly larger than 5 cm in diameter, and add an appropriate amount of desiccant (CaCl2) inside the membrane. Maintain the environment at a temperature of 25°C and a humidity of 50% RH, weighing until the change in mass of the permeation cup is no more than 0.005 g. Calculate the membrane's water vapor permeation rate and water vapor permeability coefficient. Figure 5 It is known that increasing chitosan and decreasing polyvinyl alcohol enhances water vapor permeation, and the content of both in the membrane is closely related to water vapor permeation. The WVT of the composite membrane decreases with increasing Cu-BTC and cinnamaldehyde filler content, possibly because the increased essential oil content reduces the hydrophilicity of the membrane, and the micropores of Cu-BTC crystals retain water molecules, slowing down water molecule diffusion.

[0088] A 0.20 g sample membrane of uniform thickness was immersed in 50 mL of buffer solutions with different pH values ​​(2.8, 5.6, 6.86, 8.0) and placed in a shaker at 25℃ and 250 rpm for reaction. Samples were taken at predetermined time points to determine the release rate. Figure 6 As shown in Figure A, after soaking the membranes in different pH buffers for 1 and 3 days, the cinnamaldehyde release concentration of all three membranes increased with decreasing pH. After 1 day of soaking, the cinnamaldehyde release concentration of the MOF-Cin6.3 group in pH 5.6 buffer (441.5 µg / mL) was significantly higher than that in the pH 6.86 environment. After 3 days of soaking, the release concentrations of the MOF-Cin6.3 and MOF-Cin4.55 groups reached 889.1 µg / mL and 417.9 µg / mL, respectively, under pH 5.6 conditions, which were also significantly higher than those of the pH 6.86 group. These results indicate that an acidic environment can promote the release of cinnamaldehyde from the composite membranes.

[0089] A 0.20 g sample membrane of uniform thickness was immersed in 50 mL of a water-ethanol buffer solution with different water contents (0%, 30%, 60%, 90%). The mixture was shaken at 25°C and 250 rpm, and samples were taken at predetermined time points to determine the release rate. Figure 6 As shown in Figure B, the cinnamaldehyde release concentration of all three membranes increased significantly with increasing humidity. After one day of soaking, under 60% humidity conditions, the cinnamaldehyde concentrations in the MOF-Cin6.3 and 4.55 groups increased by 755.7 µg / mL and 880.7 µg / mL, respectively, significantly higher than that in the MOF-Cin3.2 group. The release trend after three days of soaking was basically the same as that on day one.

[0090] Depend on Figure 7As shown in Figure A, under acidic conditions, MOF-Cin4.55 exhibited a weak inhibitory effect on the standard Staphylococcus aureus strain (SA1). However, the membrane solution soaked in buffer at pH 5 showed a greater inhibitory effect on the viable count of Staphylococcus aureus than the treatment group in buffer at pH 6. Furthermore, MOF-Cin4.55 demonstrated significant antibacterial activity against both clinical Staphylococcus aureus strains (SA2) and (SA3) (p < 0.05). Figure 7 As shown in section B, all films exhibit humidity-dependent antibacterial effects, but their response thresholds and intensities differ significantly. For the MOF-Cin3.2 film, complete eradication of SA1 is achieved at 60% humidity, while MOF-Cin4.55 and MOF-Cin6.3 films show antibacterial activity against SA1 at 30% humidity. The inhibitory effect of MOF-Cin6.3 on SA1 shows a non-linear change with increasing loading, suggesting that the humidity-responsive inhibition of the MOF-Cin system may have a critical loading threshold.

[0091] Application Examples

[0092] Blueberries of similar maturity, uniform size, and without mechanical damage were selected and packaged using the films prepared in Examples 4-6. Commercially available PE cling film was used as a control group, and unpackaged blueberries served as a blank control. All treatment groups were stored in a constant temperature and humidity environment of 20°C and 50% relative humidity. The appearance, flavonoids, total phenols, anthocyanins, and malondialdehyde content of the blueberries were monitored and recorded periodically during storage.

[0093] Performance Analysis

[0094] Depend on Figure 8 It is evident that contaminating bacteria were observed in both the blank control group and the PE group as early as day 15. During storage, blueberries continuously lost moisture and carbon dioxide through stomata, leading to a gradual decline in quality and freshness. The blank control group showed the earliest signs of wrinkling and the lowest fruit plumpness, while blueberries packaged with the Cu-BTC / CS / PVA-Cin composite film did not show contaminating bacteria until day 25, with the MOF-Cin 6.3 and MOF-Cin 4.55 groups showing better results, indicating that this composite film can delay fruit senescence and achieve effective preservation. With prolonged storage, the spoilage rate of blueberries under different storage conditions showed a continuous upward trend. Figure 9It is evident that after 10 days of storage, the blank control group, PE group, and MOF-Cin3.2 group were the first to show signs of spoilage. By day 25, the spoilage rates of the blank control group and PE group had reached as high as 46.7% and 37.2%, respectively, while the spoilage rates of the MOF-Cin4.55 group and MOF-Cin6.3 group were only 6.1% and 7.7%, respectively, significantly lower than the other groups. This indicates that the MOF-Cin4.55 film was the most effective in reducing the spoilage rate of blueberries.

[0095] Weigh 25 g of blueberry sample, add 250 mL of 1% hydrochloric acid-methanol solution, grind thoroughly with a pestle under ice bath conditions, and then extract by shaking in a 4℃ dark environment for 20 minutes. Subsequently, centrifuge at 4000 r / min for 10 minutes, and collect the supernatant as the extract to be tested. Using a UV spectrophotometer, with 1% hydrochloric acid-methanol solution as a blank reference, measure the absorbance at wavelengths of 280 nm, 325 nm, 530 nm, and 600 nm, respectively, and calculate the relative contents of total phenols, flavonoids, and anthocyanins. Figure 10 It was found that the MOF-Cin6.3 group had the highest flavonoid content (bar chart) during the entire storage period, peaking on day 20. By day 25, the flavonoid content in the blank control group, PE group, and MOF-Cin6.3 group decreased, while the MOF-Cin3.2 and MOF-Cin4.55 groups continued to increase. The total phenolic content (solid line graph) of all treatment groups showed a trend of first increasing and then decreasing, with the MOF-Cin4.55 group showing the slowest increase in total phenolic and anthocyanin content (dashed line graph) except for the PE group. The results indicate that the MOF-Cin4.55 group was the most effective in maintaining the flavonoid, total phenolic, and anthocyanin content in blueberries.

[0096] Malondialdehyde (MDA) is a product of lipid peroxidation, and its content changes can reflect the degree of cell membrane damage. It is determined using the thiobarbituric acid-UV spectrophotometric method. Figure 11 It was found that the malondialdehyde (MDA) content in the blank control group increased significantly during storage, from an initial 0.006 µmol / g FW to 0.022 µmol / g FW, indicating that cell membrane damage intensified with storage time. In contrast, the MDA content in the other film-packaged blueberries showed little change throughout storage and was significantly lower than that in the blank control group. This suggests that the dense hydrogen bond network structure of the composite film and its loaded cinnamaldehyde can inhibit membrane lipid peroxidation and reduce cell damage, thereby mitigating oxidative stress during blueberry storage and helping to maintain fruit quality.

[0097] Figure 12The study shows the trend of total bacterial colony count in blueberries under different treatment groups during storage. The total bacterial count increased rapidly in the blank control group and the PE group, indicating that fruits without preservation treatment and those packaged only with PE film were more susceptible to bacterial contamination. Blueberries packaged with composite films MOF-Cin3.2, MOF-Cin4.55, and MOF-Cin6.3 showed a slower bacterial growth rate. The MOF-Cin4.55 group exhibited the slowest bacterial growth throughout storage, indicating that this composite film loading had the best inhibitory effect on bacterial proliferation.

[0098] Low-field nuclear magnetic resonance (NMR) is a non-destructive testing technique suitable for characterizing the distribution and migration of water in fruits. Figure 13 It was found that after 20 days of storage, the weight loss rate of blueberries in each composite film group was similar. However, with the increase of cinnamaldehyde loading, the content of non-free water in blueberries showed an increasing trend, indicating that water evaporation was intensified, which was conducive to the formation of a local high-humidity environment, thereby triggering the humidity response mechanism of the film and promoting the release of active substances. The proportion of non-free water in the MOF-Cin6.3 group was increased, which may be because the excessive film loading caused damage to the blueberry cell membrane and leakage of contents, causing some free water to combine with damaged macromolecules and then be converted into non-free water. In contrast, the non-free water content of the MOF-Cin4.55 group was 0.55%, which maintained appropriate water evaporation and better maintained the integrity of cell structure, indicating that the composite film at this loading level is more suitable for preservation.

[0099] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a smart respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol membrane, characterized in that, Includes the following steps: Step A: Prepare chitosan solution and polyvinyl alcohol solution separately; Step B: Add the copper salt precursor, polyvinyl alcohol solution and plasticizer to the chitosan solution and mix evenly to obtain a copper-containing matrix mixture; Step C: Prepare an organic ligand solution and add it to the copper-containing matrix mixture obtained in Step B. Perform a hydrothermal reaction under closed conditions to synthesize Cu-BTC / CS / PVA membrane solution in a one-pot solution phase. Step D: After the membrane solution obtained in step C is cooled, cinnamaldehyde is added, stirred evenly, and after molding, physical cross-linking and drying, the Cu-BTC / chitosan / polyvinyl alcohol membrane is obtained.

2. The method for preparing a smart respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol membrane according to claim 1, characterized in that, In step A, the concentration of the chitosan solution is 1.5%-2.5% (w / v), and the solvent is an acetic acid solution of 1%-3% (v / v); the concentration of the polyvinyl alcohol solution is 10%-14% (w / w); in the mixing system of step B, the mass ratio of chitosan to polyvinyl alcohol is 1:2 to 1:

8.

3. The method for preparing a smart respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol membrane according to claim 2, characterized in that, In the mixing system of step B, the mass ratio of chitosan to polyvinyl alcohol is 1:

4.

4. The method for preparing a smart respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol membrane according to claim 3, characterized in that, In step B, the copper salt precursor is copper nitrate trihydrate; the plasticizer is glycerol; the ratio of copper nitrate trihydrate: polyvinyl alcohol solution: glycerol is (0.4~0.9) g: 40 mL: 3 mL; the mixing conditions are stirring for 30-90 min in a water bath at 60±5℃.

5. The method for preparing a smart respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol membrane according to claim 4, characterized in that, In step C, the organic ligand is pyromellitic acid, and the solvent is anhydrous ethanol; the mass ratio of pyromellitic acid to copper nitrate trihydrate is (0.7~1.5):(0.4~0.9). The hydrothermal reaction is carried out at a temperature of 80-90℃ for 20-28 hours.

6. The method for preparing a smart respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol membrane according to claim 1, characterized in that, In step D, the temperature when adding cinnamaldehyde is controlled below 40°C, and the amount added is 3.0%-7.0% of the dry weight of the membrane matrix; the physical crosslinking is to pour the membrane solution into a mold, freeze it at -20°C±2°C for 12-24 hours, thaw it at room temperature for 20-40 minutes, and repeat the cycle 3 times.

7. The method for preparing a smart respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol membrane according to claim 1, characterized in that, In step D, the drying method is vacuum drying, the temperature is 35-45℃, and the drying time is 20-28 hours.

8. A smart respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol membrane prepared by the method according to any one of claims 1-7.

9. The application of the intelligent respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol film according to claim 8 in fruit and vegetable preservation.

10. The application of the intelligent respiratory-responsive Cu-BTC / chitosan / polyvinyl alcohol film according to claim 9 in fruit and vegetable preservation, characterized in that, Used for blueberry preservation, the film has an antibacterial effect against Staphylococcus aureus.

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