Self-assembled thy@zif-8 nanoparticles, self-reinforced multifunctional fresh-keeping composite film and preparation method and application thereof

Abstract

This invention belongs to the field of functional films, and relates to self-assembled Thy@ZIF-8 nanoparticles, a self-reinforced multifunctional preservation composite film, its preparation method, and its application. The self-assembled Thy@ZIF-8 nanoparticles are nanoparticles formed by encapsulating thymol (Thy) in the metal-organic framework material ZIF-8. This invention prepares and optimizes self-assembled Thy@ZIF-8 nanoparticles and, for the first time, uses them as a material component in the preparation of post-harvest fruit preservation films. Utilizing the porous structure and high specific surface area of ​​ZIF-8 nanoparticles, the active substance Thy is encapsulated to prepare synthetic Thy@ZIF-8 nanoparticles. Through changes in the pH of the microenvironment during post-harvest storage of fruit, the structure of the Thy@ZIF-8 nanoparticles is disrupted, releasing Thy, achieving the purpose of slow release and prolonged antibacterial effect.

Description

Technical Field

[0001] This invention belongs to the field of functional films, specifically relating to a self-assembled Thy@ZIF-8 nanoparticle, a self-reinforced multifunctional food preservation composite film, its preparation method, and its application. Background Technology

[0002] Fresh fruit is essential for a healthy diet and is one of the world's most popular foods. Unfortunately, more than a third of fruit spoils before it can be consumed each year, resulting in significant food waste and economic losses. Therefore, extending the shelf life of fruit is crucial. However, fruit freshness is affected by a variety of factors, including oxygen, respiration, and microorganisms. On the one hand, oxygen and respiration accelerate oxidative spoilage, water loss, and skin shrinkage in fruit. On the other hand, rich nutrients promote the growth and reproduction of microorganisms, thus exacerbating mold and decay. Among numerous preservation technologies, food packaging materials play a vital role in preventing food spoilage and extending shelf life. Currently, the most commonly used food preservation packaging materials on the market are polymer plastic wrap, typically made from petroleum-based polymers such as polyvinylidene chloride and polyethylene. While cost-effective and convenient to use, they present challenges in recycling and degradation, leading to serious environmental pollution. In recent years, with the development of biotechnology, the shift from traditional preservation packaging materials to biodegradable and environmentally friendly packaging materials has become an inevitable trend in modern food packaging development. Existing biodegradable green food packaging materials can be mainly classified into biopolymers such as starch, protein, polysaccharides, and lipids, which possess excellent properties such as renewability, biodegradability, good biocompatibility, and environmental friendliness. Compared with films made from lipids or proteins, polysaccharide-based films are the most attractive due to their superior gas barrier properties and mechanical properties. Among them, chitosan (CS) is a relatively common natural polymer that has attracted much attention for many years due to its excellent performance. However, due to its linear structure and the presence of numerous hydrogen bonds, CS films have low antioxidant activity and can only dissolve in acidic environments, further limiting their applications.

[0003] This invention aims to develop a self-reinforced, multifunctional, and preservative-active packaging material with excellent antioxidant and antibacterial properties by incorporating active natural compounds into CS films. This material not only inhibits the growth of pathogenic bacteria and extends the shelf life of fresh food, but also achieves biodegradability and protects environmental health. Among numerous active natural compounds, proanthocyanidins (PA), also known as condensed tannins, are natural antioxidants. They are natural bioflavonoids formed by the condensation of flavan-3-ols and are widely found in fruits, vegetables, seeds, and flowers. Grafting PA into CS films can further enhance its solubility and antioxidant properties. In addition, thymol (Thy) is also favored for its significant antibacterial and antioxidant properties and good biocompatibility. However, Thy's poor thermal stability, volatility, and water solubility limit its bioactivity to some extent, and its unique flavor can also affect the original sensory characteristics of packaged foods. Encapsulating Thy into nanocarriers is one effective strategy to address the aforementioned problems. Compared to directly encapsulating Thy into a biopolymer matrix, the addition of nanocarriers allows for better control of the release of active substances, thus providing more durable antibacterial and antioxidant properties to the composite film. Regarding nanocarriers, metal-organic frameworks (MOFs), represented by ZIF-8, have been widely used in biomedical drug delivery and sustained release due to their porous structure, high specific surface area, and structural diversity. Furthermore, ZIF-8 releases unsaturated Zn under acidic conditions. 2+ By disrupting the bacterial structure, active materials can more easily enter bacterial cells, achieving synergistic antibacterial effects. Based on this, the present invention prepares Thy@ZIF-8 nanoparticles by encapsulating Thy into ZIF-8 nanoparticles, and simultaneously introduces them into a CS-PA matrix to prepare a self-reinforced multifunctional preservation CS-PA / Thy@ZIF-8 composite film, which exhibits excellent antioxidant and sustained antibacterial effects in postharvest fruit preservation. Summary of the Invention

[0004] To address the problems of existing traditional food packaging materials, such as difficulty in degradation, environmental pollution, and poor preservation effect, this invention aims to provide a biodegradable preservation film and its preparation method. The film uses chitosan, a natural polymer, as a base, proanthocyanidins and thymol, natural active compounds, as encapsulating agents, and ZIF-8, a metal-organic framework material, as a nanocarrier. The resulting self-reinforced multifunctional preservation composite film can be used for post-harvest preservation of fruits.

[0005] The first objective of this invention is to provide ZIF-8-loaded Thy nanoparticles and their preparation method, in order to solve the problems of poor thermal stability, volatility, and water solubility of Thy in practical applications, as well as the impact of its unique flavor on the original sensory characteristics of packaged foods. Utilizing the advantages of ZIF-8's high porosity and large specific surface area, Thy@ZIF-8 nanoparticles are prepared by encapsulating Thy with ZIF-8 as a carrier.

[0006] Specifically, the first aspect of the present invention provides a self-assembled Thy@ZIF-8 nanoparticle, wherein the self-assembled Thy@ZIF-8 nanoparticle is a nanoparticle formed by encapsulating thymol Thy with the metal-organic framework material ZIF-8.

[0007] A second aspect of the present invention provides a method for preparing the above-mentioned self-assembled Thy@ZIF-8 nanoparticles, comprising the following steps:

[0008] 2-Methylimidazole and thymol were dissolved in a first organic solvent, and after ultrasonic treatment, they were combined with Zn. 2+ The precursor solution was mixed evenly, stirred and reacted, and then centrifuged to obtain Thy@ZIF-8 nanoparticles.

[0009] The second objective of this invention is to provide a self-reinforced multifunctional preservative CS-PA / Thy@ZIF-8 composite film based on Thy@ZIF-8 nanoparticles and its preparation method. This is achieved through hydrogen bonds formed between the polysaccharide matrix and the various components of the nanoparticles, as well as their interaction with Zn. 2+ The coordination bonds formed result in excellent compatibility between Thy@ZIF-8 nanoparticles and the CS-PA polysaccharide matrix, thereby further enhancing the physical and antibacterial properties of the CS polysaccharide matrix film.

[0010] Specifically, a third aspect of the present invention provides a self-reinforced multifunctional preservation composite film, wherein the film is based on chitosan and at least a portion of the chitosan is grafted with proanthocyanidins, and the film contains the aforementioned self-assembled Thy@ZIF-8 nanoparticles.

[0011] A fourth aspect of the present invention provides a method for preparing the above-described self-reinforced multifunctional food preservation composite film, comprising the following steps:

[0012] Chitosan-proanthocyanidin conjugates CS-PA and Thy@ZIF-8 nanoparticles were dissolved in distilled water and stirred rapidly at room temperature to prepare CS-PA dispersions and Thy@ZIF-8 dispersions. The CS-PA dispersions and Thy@ZIF-8 dispersions were added to a chitosan film-forming solution for blending reaction. The resulting blended solution was then used to form a film and dried to obtain a CS-PA / Thy@ZIF-8 composite film.

[0013] The third objective of this invention is to apply the self-reinforced multifunctional preservation CS-PA / Thy@ZIF-8 composite film to post-harvest preservation of fruits and vegetables (such as strawberries and cherry tomatoes). Through changes in pH within the fruit's microenvironment during storage, the Thy@ZIF-8 nanoparticles in the composite film release Thy and unsaturated Zn. 2+ In addition, the PA released from the CS-PA polysaccharide matrix achieves synergistic antibacterial and antioxidant effects, thereby extending the shelf life of fruits.

[0014] Specifically, the fifth aspect of the present invention provides the application of the above-mentioned self-assembled Thy@ZIF-8 nanoparticles or the above-mentioned self-reinforced multifunctional preservation composite film in postharvest storage of fruits and vegetables.

[0015] The technical effects of this invention are as follows:

[0016] First, this invention prepares and optimizes self-assembled Thy@ZIF-8 nanoparticles and, for the first time, uses them as a material component in the preparation of post-harvest preservation films for fruits. The active substance Thy is encapsulated using the porous structure and high specific surface area of ​​ZIF-8 nanoparticles to prepare synthetic Thy@ZIF-8 nanoparticles. Through changes in the pH of the microenvironment during post-harvest storage of fruits, the structure of the Thy@ZIF-8 nanoparticles is disrupted, releasing Thy, achieving a slow release and prolonged antibacterial effect. Furthermore, ZIF-8 itself releases unsaturated Zn under acidic conditions. 2+ It can disrupt the bacterial structure, making it easier for active molecules to enter the bacterial cell and achieve synergistic antibacterial effects.

[0017] Secondly, this invention synthesizes a self-reinforced multifunctional preservative CS-PA / Thy@ZIF-8 composite film by filling the prepared Thy@ZIF-8 nanoparticles into a CS-PA polysaccharide matrix. Characterization results show that the hydrogen bonds formed between the polysaccharide matrix and the various components of the nanoparticles, as well as the hydrogen bonds formed between Thy@ZIF-8 and Zn, contribute to the preservation of the CS-PA / Thy@ZIF-8 composite film. 2+ The formed coordination bonds significantly improve the physical properties of the composite film, thus achieving self-reinforcement. Furthermore, the combination of Thy@ZIF-8 nanoparticles and active substances in the CS-PA polysaccharide matrix also achieves synergistic antioxidant and antibacterial effects.

[0018] Third, the self-reinforced multifunctional preservation CS-PA / Thy@ZIF-8 composite film of this invention was tested for its preservation effect on postharvest strawberries and cherry tomatoes under actual storage conditions. Experimental results showed that the composite film significantly extended the shelf life of postharvest fruits, with significant differences in nutritional indicators and sensory evaluation compared to the control group. Furthermore, no other auxiliary materials, large-scale facilities, or ultraviolet / irradiation light sources were required during the experiment; the preservation effect was visually perceptible. The prepared composite film has advantages such as low cost, good biocompatibility, and biodegradability.

[0019] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0020] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings.

[0021] Figure 1 This is a schematic diagram illustrating the principle of composite film preparation and preservation in this invention.

[0022] Figure 2 The encapsulation efficiency of nanoparticles with different amounts of Thy is shown.

[0023] Figure 3 The PA content of conjugates with different PA addition amounts is shown.

[0024] Figure 4 Characterization results of ZIF-8 and Thy@ZIF-8 nanoparticles are shown. (a) and (b) are SEM images of ZIF-8 and Thy@ZIF-8, respectively; (c) and (d) are particle size analyses of ZIF-8 and Thy@ZIF-8, respectively; (e) Zeta potential analysis of Thy, ZIF-8 and Thy@ZIF-8; (f) XRD patterns of ZIF-8 and Thy@ZIF-8; (g) FT-IR patterns of Thy, ZIF-8 and Thy@ZIF-8; (h) TG curves of Thy, ZIF-8 and Thy@ZIF-8.

[0025] Figure 5 The characterization results of the CS-PA / Thy@ZIF-8 composite film are shown. (a) SEM image, (b) AFM image, (c) XRD pattern, (d) FT-IR pattern, and (e) XPS pattern of the CS film, CS-PA film, CS-PA / Thy film, and CS-PA / Thy@ZIF-8 composite film.

[0026] Figure 6The performance test results of the CS-PA / Thy@ZIF-8 composite film are shown. (a) Stress-strain curves; (b) transmittance; (c) water contact angle; (d) cumulative release of Thy under different pH conditions; (e) antioxidant properties of the CS film, CS-PA film, CS-PA / Thy film, and CS-PA / Thy@ZIF-8 composite film.

[0027] Figure 7 The antibacterial activity of the CS-PA / Thy@ZIF-8 composite film is shown. (a) and (b) are the growth curves of PE plastic wrap, CS film, CS-PA film, CS-PA / Thy film, and CS-PA / Thy@ZIF-8 composite film co-cultured with Staphylococcus aureus and Escherichia coli, respectively; (c) bacterial dead / live staining images after co-culturing different films with Staphylococcus aureus and Escherichia coli; (d) bacterial colony images and SEM images after co-culturing different films with Staphylococcus aureus and Escherichia coli.

[0028] Figure 8 The biocompatibility of the CS-PA / Thy@ZIF-8 composite film is shown. (a) Hemolysis rate of sheep erythrocytes by CS film, CS-PA film, CS-PA / Thy film, and CS-PA / Thy@ZIF-8 composite film; (b) Cell viability of L929 cells after co-culturing with different films; (c) Cell viability of L929 cells co-cultured with CS-PA / Thy@ZIF-8 composite film at different concentrations; (d) Degradability of the composite film; (e) Peanut sprout growth experiment to evaluate the safety of the composite film.

[0029] Figure 9 The following data are presented: (a) appearance; (b) weight; (c) firmness; (d) vitamin C content; (e) TSS content; (f) MDA content; (g) TA content; (h) CAT activity; (i) sensory evaluation of strawberries in different storage groups (day 2 of storage); and (j) sensory evaluation of strawberries in different storage groups (day 6 of storage).

[0030] Figure 10 The appearance of cherry tomatoes after storage is shown using PE preservation film, CS film, CS-PA film, CS-PA / Thy film, and CS-PA / Thy@ZIF-8 composite film.

[0031] Figure 11The following data are presented: (a) weight loss; (b) firmness; (c) vitamin C content; (d) TSS content; (e) TA content; (f) MDA content; (g) total phenol content; (h) flavonoid content; and (i) CAT activity of cherry tomatoes after storage in PE preservation film, CS film, CS-PA film, CS-PA / Thy film, and CS-PA / Thy@ZIF-8 composite film. Detailed Implementation

[0032] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein.

[0033] This invention provides a self-assembled Thy@ZIF-8 nanoparticle, wherein the self-assembled Thy@ZIF-8 nanoparticle is a nanoparticle formed by encapsulating thymol Thy with the metal-organic framework material ZIF-8.

[0034] According to a preferred embodiment of the present invention, the ZIF-8 content in the self-assembled Thy@ZIF-8 nanoparticles is 70-85% by weight, and the Thy content is 15-30% by weight.

[0035] This invention also provides a method for preparing the above-mentioned self-assembled Thy@ZIF-8 nanoparticles, comprising the following steps:

[0036] 2-Methylimidazole and thymol were dissolved in a first organic solvent, and after ultrasonic treatment, they were combined with Zn. 2+ The precursor solution was mixed evenly, stirred and reacted, and then centrifuged to obtain Thy@ZIF-8 nanoparticles.

[0037] According to a preferred embodiment of the present invention, the first organic solvent is anhydrous methanol; the Zn 2+ The precursor is Zn(NO3)2·6H2O; 2-methylimidazole and Zn 2+ The molar ratio of the precursor is 2–12:1; the mass ratio of 2-methylimidazole to thymol is 200–300:1.

[0038] According to a preferred embodiment of the present invention, the conditions for the stirred reaction include: a temperature of 10–30°C and a time of 16–36 h. The stirred reaction is followed by washing and vacuum drying steps.

[0039] Furthermore, the present invention provides a self-reinforced multifunctional preservation composite film, wherein the film is based on chitosan, and at least a portion of the chitosan is grafted with proanthocyanidins, and the film contains the aforementioned self-assembled Thy@ZIF-8 nanoparticles.

[0040] According to a preferred embodiment of the present invention, the content of self-assembled Thy@ZIF-8 nanoparticles in the self-reinforced multifunctional preservation composite film is 25-35% by weight; the content of proanthocyanidins is 15-25% by weight.

[0041] The present invention also provides a method for preparing the above-mentioned self-reinforced multifunctional food preservation composite film, comprising the following steps:

[0042] Chitosan-proanthocyanidin conjugates CS-PA and Thy@ZIF-8 nanoparticles were dissolved in distilled water and stirred rapidly at room temperature to prepare CS-PA dispersions and Thy@ZIF-8 dispersions. The CS-PA dispersions and Thy@ZIF-8 dispersions were added to a chitosan film-forming solution for blending reaction. The resulting blended solution was then used to form a film and dried to obtain a CS-PA / Thy@ZIF-8 composite film.

[0043] According to a preferred embodiment of the present invention, the preparation method of chitosan-proanthocyanidin conjugate CS-PA includes: adding a hydrogen peroxide solution containing ascorbic acid to an acetic acid solution of chitosan, reacting for a period of time, adding proanthocyanidins, reacting at room temperature, adjusting the reaction solution to neutral after the reaction is completed, centrifuging to collect the precipitate, washing, and freeze-drying to obtain the chitosan-proanthocyanidin conjugate CS-PA.

[0044] The present invention does not impose any particular limitation on the reaction conditions for the preparation of composite films. The reaction can be carried out at room temperature. The reaction time is sufficient as long as the solution is stirred evenly and free of particles. The same effect can be achieved by appropriately changing the stirring time and speed.

[0045] According to a preferred embodiment of the present invention, the method for preparing the chitosan film-forming solution includes: dissolving chitosan in glacial acetic acid to prepare a chitosan solution, adding glycerol as a plasticizer, and obtaining the chitosan film-forming solution.

[0046] The method for forming the film is as follows: the blend solution is poured onto a plastic plate.

[0047] The self-assembled Thy@ZIF-8 nanoparticles or self-reinforced multifunctional preservation composite film of the present invention can be used for postharvest storage of fruits and vegetables. As an example, the present invention tested the preservation effect of the enhanced multifunctional preservation composite film on strawberries and cherry tomatoes, but its application is obviously not limited to these two fruits.

[0048] The present invention will be further illustrated by the following embodiments.

[0049] Unless otherwise specified in the examples, all procedures were performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0050] Preparation Example 1

[0051] (1) Synthesis of ZIF-8 self-assembled material: 2.4 g zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 6.9 g 2-methylimidazole were dissolved in 30 mL of anhydrous methanol. The mixture was gently stirred at room temperature for 2 h. The zinc nitrate methanol solution was slowly added to the 2-methylimidazole methanol solution while stirring. After reacting for 24 h, a white suspension was obtained. The white precipitate was collected by centrifugation at 9000 rpm / min for 5 min. The precipitate was washed 3-5 times with an appropriate amount of anhydrous methanol and dried at 80 °C overnight.

[0052] (2) Synthesis and optimization of Thy@ZIF-8 nanoparticles: 2.4 g zinc nitrate hexahydrate was dissolved in 30 mL anhydrous methanol as solution A with 10 mg, 20 mg, 30 mg, 40 mg and 50 mg thymol (Thy), respectively. 6.9 g 2-methylimidazole was dissolved in 30 mL anhydrous methanol as solution B. The mixture was stirred gently at room temperature for 2 h. Solution A was added to solution B while stirring and mixed evenly. After reacting for 24 h, a white suspension was obtained. The white precipitate was collected by centrifugation at 9000 rpm / min for 5 min. The precipitate was washed 3-5 times with an appropriate amount of anhydrous methanol and dried at 80 °C overnight.

[0053] The encapsulation efficiency of nanoparticles with different amounts of Thy was tested, and the results are as follows: Figure 2 As shown, Thy@ZIF-8 nanoparticles with a high encapsulation rate containing 30mg Thy were subsequently selected, with a ZIF-8 nanoparticle content of 100mg.

[0054] Preparation Example 2

[0055] Synthesis and optimization of chitosan-proanthocyanidin (CS-PA) conjugates: CS was completely dissolved in 100 mL of acetic acid solution (2%, v / v) to a final concentration of 1% (w / v). Then, 2 mL of hydrogen peroxide solution (1.0 mol / L) containing 0.108 g ascorbic acid was added to the CS solution. After reacting for 30 min, PA was added to the CS solution according to the molar ratios of PA:CS = 0.01:1, 0.02:1, 0.03:1, 0.04:1, 0.05:1, 0.06:1, 0.07:1, 0.08:1, 0.09:1, and 0.10:1. The reaction was carried out at room temperature for 24 h. The pH was adjusted to 7.0 with NaOH solution (1 mol / L). The precipitate was collected by centrifugation at 8000 rpm / min for 5 min, washed three times with distilled water, and freeze-dried to obtain the product.

[0056] The PA content of conjugates with different PA addition amounts was determined, such as... Figure 3As shown, the PA:CS = 0.04:1 conjugate has the highest PA content, and further increasing the amount of PA will not increase the PA content in the conjugate. Therefore, the PA addition amount was determined to be PA:CS = 0.04:1.

[0057] Comparative preparation example

[0058] (1) Preparation of CS membrane: Dissolve CS in 50 mL of 1% glacial acetic acid and stir rapidly at room temperature for 30 min to prepare CS solution. After stirring, add glycerol as plasticizer to obtain CS film-forming solution. Pour the solution into a plastic plate (14 cm × 9 cm × 2.5 cm) and dry to successfully prepare CS membrane.

[0059] (2) Preparation of CS-PA membrane:

[0060] CS-PA was dissolved in 50 mL of distilled water and stirred rapidly at room temperature for 30 min to prepare a CS-PA solution. After stirring, glycerol was added as a plasticizer to obtain a CS-PA film-forming solution. The solution was poured into a plastic plate (14 cm × 9 cm × 2.5 cm) and dried to successfully prepare a CS-PA film.

[0061] (3) Preparation of CS-PA / Thy membrane

[0062] CS-PA was dissolved in 50 mL of distilled water and stirred rapidly at room temperature for 30 min to prepare a CS-PA solution. Thy was dissolved in 50 mL of distilled water and stirred rapidly at room temperature until homogeneous. The CS-PA solution and Thy solution were then mixed and stirred. After stirring, glycerol was added as a plasticizer to obtain a CS-PA / Thy film-forming solution. The solution was poured into a plastic plate (14 cm × 9 cm × 2.5 cm) and dried to successfully prepare a CS-PA / Thy film.

[0063] Example 1

[0064] This embodiment illustrates the preparation method of the self-assembled Thy@ZIF-8 nanoparticles of the present invention.

[0065] 1) Preparation of Thy@ZIF-8 nanoparticles: 6.9 g of 2-methylimidazole and 30 mg of thymol were weighed and dissolved in 30 mL of anhydrous methanol. After sonication, the mixture was dissolved in 30 mL of methanol solution with 2.4 g of Zn(NO3)2·6H2O and mixed thoroughly. The mixture was then magnetically stirred at room temperature for 24 h and centrifuged at 9000 rpm / min for 5 min to obtain a white precipitate. The precipitate was washed 3-5 times with an appropriate amount of anhydrous methanol and then vacuum dried overnight at 80 °C to obtain Thy@ZIF-8 nanoparticles.

[0066] 2) Characterization of Thy@ZIF-8 nanoparticles: such as Figure 4 As shown, the ZIF-8 nanoparticles exhibit a classic rhombic dodecahedral structure with a particle size of approximately 116.67 nm. Thy@ZIF-8 nanoparticles show a similar morphology to ZIF-8, but with a slightly increased average diameter of approximately 141.77 nm. Figure 4 (a)-(d)). Additionally, the Zeta potential of the encapsulated Thy@ZIF-8 nanoparticles changed ( Figure 4 (e)). XRD and FT-IR spectra further confirmed the successful synthesis of Thy@ZIF-8 nanoparticles. Figure 4 (f)-(g)).

[0067] 3) Performance determination of Thy@ZIF-8 nanoparticles: such as Figure 4 As shown in (h), the TG curves reveal that Thy completely evaporates at approximately 160.1 °C, attributed to its unique thermal instability and volatility. The initial degradation of ZIF-8 occurs at 210 °C, with a weight loss of 2.06%, which can be attributed to the low water content in the ZIF-8 nanoparticles. Furthermore, the ZIF-8 framework degrades at 590 °C, with a weight loss of approximately 12.52%. Notably, Thy@ZIF-8 exhibits two degradation stages, with maximum temperatures of 195.6 °C and 614.8 °C, respectively. The first stage is likely due to the degradation of the tightly bound Thy within Thy@ZIF-8, while the second stage is due to the degradation of the ZIF-8 framework. Thermogravimetric analysis further confirms that Thy is successfully encapsulated within ZIF-8, and that Thy@ZIF-8 exhibits excellent thermal stability due to the protective effect of the ZIF-8 framework.

[0068] Example 2

[0069] This embodiment illustrates the preparation method of the self-reinforced multifunctional food preservation CS-PA / Thy@ZIF-8 composite film based on Thy@ZIF-8 nanoparticles according to the present invention.

[0070] 1) Preparation of the composite film: CS was dissolved in 50 mL of 1% glacial acetic acid and rapidly stirred at room temperature for 30 min to prepare a CS solution. After stirring, glycerol was added as a plasticizer to obtain a CS film-forming solution. Further, CS-PA and Thy@ZIF-8 dispersions were prepared by dissolving them in 30 mL and 20 mL of distilled water, respectively. The dispersions were rapidly stirred at room temperature for 30 min, and then added to the CS solution for blending. After reacting for 3 h, 100 mL of the blend solution was poured into a plastic plate (14 cm × 9 cm × 2.5 cm), dried, and a CS-PA / Thy@ZIF-8 composite film was successfully prepared. The content of self-assembled Thy@ZIF-8 nanoparticles was 30% by weight; the content of proanthocyanidins was 20% by weight.

[0071] 2) Characterization of composite thin films: such as Figure 5 As shown in (a), the SEM images of each group of films reveal that the CS film surface is smooth and uniform, without cracks or pores, indicating the formation of a dense structure. After adding PA and Thy to CS, the unevenness of the film surface gradually becomes apparent, with irregularities and small pores appearing. This is due to phase separation caused by the insolubility of Thy in water, and the film roughness increases significantly. However, encapsulating Thy in ZIF-8 significantly improves the roughness. Furthermore, the SEM images of the CS-PA / Thy@ZIF-8 composite film show that the Thy@ZIF-8 nanoparticles are uniformly dispersed in the CS-PA matrix without phase separation or aggregation, indicating good compatibility between Thy@ZIF-8 and the film-forming matrix.

[0072] XRD and FT-IR spectra show that the crystal structure of ZIF-8 nanoparticles remained unchanged. Hydrogen bonds and electrostatic interactions may have formed between ZIF-8 and Thy nanoparticles in Thy@ZIF-8 nanoparticles. Figure 5 (c) and (d)).

[0073] AFM image ( Figure 5 (b) shows that the CS film is smooth and uniform, without cracks or pores, indicating its dense structure. However, after adding PA and Thy, protrusions and pores appeared on the surface, which may be due to phase separation caused by the insolubility of Thy, resulting in a significant increase in surface roughness (Ra increased from 1.03 to 25.4). Encapsulating Thy in ZIF-8 improved the surface roughness, reducing Ra to 2.79.

[0074] XPS analysis further validated the above findings, such as Figure 5The shifts in the (c) C1s and Zn 2p spectra indicate weak hydrogen and coordination bonds between CS, PA, Thy, and ZIF-8. Specifically, when Zn 2p is combined with CS-PA, the Zn 2p peaks at 1043.86 eV and 1020.77 eV shift, indicating interactions between Zn and the reactive groups in CS and PA. The N1s peak at 401.18 eV indicates a Zn-N coordination bond between Thy@ZIF-8 and CS. The excellent interfacial compatibility resulting from hydrogen bonding and coordination interactions promises to improve the gas barrier properties and mechanical properties of the composite film, making it an ideal candidate material for food packaging applications.

[0075] Example 3

[0076] This embodiment is used to illustrate the performance determination of the self-reinforced multifunctional preservation CS-PA / Thy@ZIF-8 composite film based on the present invention and the preservation effect test of post-harvest fruit under actual storage conditions.

[0077] (1) Physical properties of the composite film: The physical properties of the CS-PA / Thy@ZIF-8 composite film were analyzed using tensile strength and light transmittance. For example... Figure 6 As shown in (a), the stress-strain curves indicate that the CS film has low tensile strength. With the addition of Thy@ZIF-8, the tensile strength of the CS-PA / Thy@ZIF-8 film improves, which is attributed to the self-reinforcing effect of Thy@ZIF-8 and the interfacial interaction between Thy@ZIF-8 and the CS-PA matrix, thereby improving the mechanical properties of the composite film. Figure 6 As shown in (b), the transmission spectra of different films reveal that the pure CS film is transparent with high ultraviolet transmittance, while the transparency of the composite film gradually decreases and its ultraviolet blocking ability improves. When PA is added to the CS matrix, the ultraviolet transmittance of the CS-PA film decreases significantly because PA itself is red. Furthermore, doping CS-PA with Thy@ZIF-8 nanoparticles significantly reduces the transmittance in the ultraviolet region, possibly due to the low transmittance and scattering effect of the Thy@ZIF-8 nanoparticles. Figure 6 As shown in (c), the CS-PA / Thy@ZIF-8 composite film exhibits a higher water contact angle, indicating good surface hydrophobicity of this material. Furthermore, the controlled-release behavior of free CS-PA / Thy membranes and CS-PA / Thy@ZIF-8 membranes under acidic and neutral conditions was evaluated using pH 3, 5, and PBS solutions. Figure 6(d) It can be observed that the release of Thy from all membranes follows a similar two-step biphasic process. In the first 10 hours, the CS-PA / Thy membrane experienced a significant Thy burst release, primarily caused by the swelling of the surface-exposed Thy and the CS membrane matrix. The real-time release rate reached over 65% of the cumulative release rate at 10 hours; benefiting from the inherent binding effect of the CS membrane, the second stage transitioned to a slow release until isochoric release was achieved. Compared to the CS-PA / Thy membrane, the CS-PA / Thy@ZIF-8 membrane exhibited a significantly slower Thy release process throughout the testing interval, with a real-time release rate as low as 60% at 140 hours.

[0078] (2) Antioxidant activity of composite films: The antioxidant activity of CS films, CS-PA films, CS-PA / Thy films, and CS-PA / Thy@ZIF-8 composite films was determined using DPPH and ABTS solutions. Different groups of films were added to 95% ethanol solutions containing 0.35 mM DPPH and incubated for 1 h in the dark. The absorbance at 517 nm was then read using a UV-Vis spectrophotometer. ABTS was measured by co-incubating with the different groups of films in the dark for 20 min, followed by reading the absorbance at 734 nm. The results showed ( Figure 6 (e) The CS film exhibits poor antioxidant capacity, with its scavenging rates of DPPH radicals and ABTS+ remaining consistently low. Conversely, the addition of different components to the composite film enhances its antioxidant activity to varying degrees. Among them, the CS-PA / Thy film and the CS-PA / Thy@ZIF-8 composite film show the highest scavenging rates of DPPH radicals and ABTS+, indicating that the enhanced antioxidant capacity of the composite film is directly related to the addition of PA and Thy. In particular, the hydrogen atom donor of the phenolic hydroxyl group in Thy is crucial for the scavenging of DPPH radicals, while the single-electron transfer between Thy and ABTS+ is the main reason for the scavenging of ABTS+.

[0079] (3) Antibacterial properties of composite films: Staphylococcus aureus and Escherichia coli are the most common Gram-positive and Gram-negative foodborne pathogens in food hygiene. The antibacterial activities of PE preservation film, CS film, CS-PA film, CS-PA / Thy film and CS-PA / Thy@ZIF-8 composite film were tested using these two bacteria as representatives. Figure 7The antibacterial activity of the CS-PA / Thy@ZIF-8 composite film is shown. (a) and (b) are the growth curves of PE plastic wrap, CS film, CS-PA film, CS-PA / Thy film, and CS-PA / Thy@ZIF-8 composite film co-cultured with Staphylococcus aureus and Escherichia coli, respectively; (c) are the bacterial live / dead staining images after co-culturing different films with Staphylococcus aureus and Escherichia coli, where green fluorescence represents live bacterial cells and red fluorescence represents dead bacterial cells. It can be seen that most of the bacteria in the control group are alive, while the CS, CS-PA, CS-PA / Thy, and CS-PA / Thy@ZIF-8 experimental groups all caused bacterial death, and the composite film showed the most red fluorescence, representing the most dead bacterial cells. The bacterial solution (1×10⁻⁶) cultured overnight was used to incubate the bacteria. 6 (CFU / mL) were co-cultured with different films for 30 min, then centrifuged at 8000 rpm for 5 min. 100 μL of the supernatant was then evenly spread onto TSB medium solid plates. For example... Figure 7 As shown in (d), the results showed that, compared with the control group of PE preservation film, the number of bacteria on the plate decreased with the increase of film incorporation. Among them, there were almost no bacterial colonies on the CS-PA / Thy@ZIF-8 composite film plate, indicating that almost all bacteria in this group were killed. This was attributed to the synergistic antibacterial effect of CS-PA matrix and Thy@ZIF-8 nanoparticles.

[0080] (4) Biocompatibility of the composite films: The cytotoxicity of PE preservation film, CS film, CS-PA film, CS-PA / Thy film, and CS-PA / Thy@ZIF-8 composite film was evaluated using sheep erythrocyte hemolysis assay and mouse fibroblast cell line L-929 cell proliferation assay. Erythrocytes isolated from fresh sheep whole blood were incubated with different film solutions at 37℃ for 30 min, centrifuged at 3000 rpm for 5 min, and the supernatant was collected. The absorbance at 540 nm was read using a UV-Vis spectrophotometer. Cell proliferation assays were performed by measuring the absorbance at 450 nm of L-929 cells after co-incubation with different film solutions and different concentrations of CS-PA / Thy@ZIF-8 composite film for 24 h using a CCK-8 assay kit. Figure 8 As shown in (a), (b), and (c), the results show that L-929 cells maintained a viability of over 90% when co-incubated with high-concentration membrane solutions and different concentrations of CS-PA / Thy@ZIF-8 composite membranes, and no significant hemolysis was observed in sheep erythrocytes, reflecting that the CS-PA / Thy@ZIF-8 composite membrane has good biocompatibility.

[0081] (5) Degradability of the composite film:

[0082] The composite film was cut into 9cm × 9cm circles, with a PE film used as a control group. Both were buried 8-10cm deep in soil, and their morphology was photographed at fixed times each day. The results are as follows: Figure 8 As shown in (d), the CS-PA / Thy@ZIF-8 composite film exhibits good degradation performance.

[0083] (6) Biosafety of composite films:

[0084] Peanut seeds were soaked in water at room temperature to germinate. After germination, the peanut seedlings were transferred to a solution containing CS-PA / Thy@ZIF-8 film (concentration 1 mg / mL). -1 The seedlings were cultured in petri dishes at room temperature. As a control, another group of peanut seedlings were cultured in petri dishes containing only tap water. Under the same experimental conditions, images of the peanut seedlings' growth and appearance were taken at fixed time points on days 3, 6, 10, 15, and 18. The results are as follows: Figure 8 As shown in (e), it can be seen that the CS-PA / Thy@ZIF-8 composite film did not affect the growth of peanut sprouts, demonstrating good biocompatibility.

[0085] (7) Postharvest Fruit Preservation Effect Test: Using freshly picked strawberries and cherry tomatoes of similar quality as experimental materials, the preservation effects of PE preservation film, CS film, CS-PA film, CS-PA / Thy film, and CS-PA / Thy@ZIF-8 composite film were studied. Strawberries were stored at room temperature, and cherry tomatoes at 4℃. The selected fruits were divided into 6 groups and stored under PE preservation film, CS film, CS-PA film, CS-PA / Thy film, and CS-PA / Thy@ZIF-8 composite film, respectively. The Control group was the control group without any preservation treatment. During storage, physiological indicators such as decay index, weight loss rate, firmness, pH, and soluble solids were monitored and recorded daily at the same time. Sensory evaluation was conducted based on five indicators: appearance, smell, color, liking, and purchase desire. Each indicator was scored out of 10 points, for a total of 50 points. A total of 50 volunteers participated.

[0086] The test results for strawberries are as follows Figure 9 As shown, specifically, as Figure 9 As shown in (a)-(h), the results indicate that the CS-PA / Thy@ZIF-8 composite film maintained better levels of weight loss, TSS, TA, and Vc in strawberries stored under conditions other than those of other films. This suggests that the composite film has better O2, CO2, and water vapor barrier properties than single-component films, further confirming the effects of hydrogen bonding and Zn. 2+The interaction of coordinate bonds and the synergistic antibacterial and antioxidant effects of PA and Thy enable the composite film to achieve self-enhanced preservation performance. For example... Figure 9 As shown in (i) and (j), it can be seen that on the second and sixth days of storage, the CS-PA / Thy@ZIF-8 composite film group had the highest scores in all sensory evaluations. In particular, on the sixth day, all indicators exceeded those of other experimental groups, indicating that the preservation effect of the CS-PA / Thy@ZIF-8 composite film group is more prominent as the storage time increases.

[0087] Test results for preserving cherry tomatoes are as follows: Figure 10 As shown, since the cherry tomatoes were stored at 4℃, none of the groups showed obvious signs of rotting. However, it can still be seen that the cherry tomatoes in the CS-PA / Thy@ZIF-8 composite film group were the freshest, indicating that the CS-PA / Thy@ZIF-8 composite film has the best preservation effect.

[0088] Figure 11 The results show various indicators of cherry tomatoes stored for different durations using different film groups. It can be seen that the CS-PA / Thy@ZIF-8 composite film maintained better levels of weight loss, firmness, vitamin C content, TSS content, TA, MDA content, flavonoid content, total phenolic content, and CAT activity compared to other films.

[0089] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A self-reinforcing multifunctional food preservation composite film, characterized in that, The film is based on chitosan, and at least a portion of the chitosan is grafted with proanthocyanidins. The film contains self-assembled Thy@ZIF-8 nanoparticles. The self-assembled Thy@ZIF-8 nanoparticles are nanoparticles formed by encapsulating thymol Thy with the metal-organic framework material ZIF-8. The film is prepared by a method comprising the following steps: dissolving chitosan-proanthocyanidin conjugate CS-PA and Thy@ZIF-8 nanoparticles in distilled water and stirring rapidly at room temperature to obtain CS-PA dispersion and Thy@ZIF-8 dispersion; adding CS-PA dispersion and Thy@ZIF-8 dispersion to chitosan film-forming solution for blending reaction; forming a film from the blended solution after reaction and drying it to obtain CS-PA / Thy@ZIF-8 composite film. The preparation method of the self-assembled Thy@ZIF-8 nanoparticles includes the following steps: dissolving 2-methylimidazol and thymol in a first organic solvent, ultrasonicating, and then reacting with Zn. 2+ The precursor solution was mixed evenly, stirred and reacted, and then centrifuged to obtain Thy@ZIF-8 nanoparticles.

2. The self-reinforced multifunctional food preservation composite film according to claim 1, characterized in that, The self-assembled Thy@ZIF-8 nanoparticles contain 70-85% ZIF-8 by weight and 15-30% Thy by weight.

3. The self-reinforced multifunctional food preservation composite film according to claim 1, characterized in that, The first organic solvent is anhydrous methanol; The Zn 2+ The precursor is Zn(NO3)2·6H2O; 2-Methylimidazole and Zn 2+ The molar ratio of the precursor is 2~12:1; The mass ratio of 2-methylimidazole to thymol is 200~300:

1.

4. The self-reinforced multifunctional food preservation composite film according to claim 1, characterized in that, The conditions for the stirring reaction include: a temperature of 10~30℃ and a time of 16~36 h; The reaction process also includes washing and vacuum drying.

5. The self-reinforced multifunctional food preservation composite film according to claim 1, characterized in that, The self-reinforced multifunctional preservation composite film contains 25-35% by weight of self-assembled Thy@ZIF-8 nanoparticles and 15-25% by weight of proanthocyanidins.

6. The method for preparing the self-reinforced multifunctional food preservation composite film according to claim 1, comprising the following steps: Chitosan-proanthocyanidin conjugates CS-PA and Thy@ZIF-8 nanoparticles were dissolved in distilled water and stirred rapidly at room temperature to prepare CS-PA dispersions and Thy@ZIF-8 dispersions. The CS-PA dispersions and Thy@ZIF-8 dispersions were added to a chitosan film-forming solution for blending reaction. The resulting blended solution was then used to form a film and dried to obtain a CS-PA / Thy@ZIF-8 composite film.

7. The preparation method according to claim 6, wherein, Stir quickly until the solution is homogeneous and free of granules; The preparation method of chitosan-proanthocyanidin conjugate CS-PA includes: adding hydrogen peroxide solution containing ascorbic acid to acetic acid solution of chitosan, reacting for a period of time, adding proanthocyanidins, reacting at room temperature, adjusting the reaction solution to neutral after the reaction is completed, centrifuging to collect the precipitate, washing, and freeze-drying to obtain the chitosan-proanthocyanidin conjugate CS-PA. The method for preparing the chitosan film-forming solution includes: dissolving chitosan in glacial acetic acid to prepare a chitosan solution, adding glycerol as a plasticizer, and obtaining the chitosan film-forming solution. The method for forming the film is as follows: the blend solution is poured onto a plastic plate.

8. The application of the self-reinforcing multifunctional preservation composite film according to any one of claims 1-5 in postharvest storage of fruits and vegetables.

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