Chitosan-based membrane as well as preparation method and application thereof

Abstract

The invention discloses a chitosan-based membrane as well as a preparation method and application thereof, and belongs to the technical field of packaging materials. The chitosan-based film has a double-layer structure consisting of an inner layer and an outer layer, the outer layer is a water-blocking antibacterial layer formed by a silver complex, and the inner layer is an antibacterial antioxidant layer formed by polyphenol; the silver complex is a complex which is formed by the reaction of pyrrole-2-formaldehyde-g-chitosan and silver acetate; the polyphenol is formed by grafting 3, 4-dihydroxy methyl phenylacetate on the chitosan. The silver complex water-blocking antibacterial layer is constructed on the outer layer, and the polyphenol antibacterial anti-oxidation layer is constructed on the inner layer, so that the double fresh-keeping mechanism of external protection and internal maintenance is realized, and the comprehensive performance and application potential of the material are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of packaging materials technology, and more specifically relates to a chitosan-based film, its preparation method, and its application. Background Technology

[0002] Fresh fruits are rich in vitamins and other nutrients, playing a vital role in maintaining human health and preventing disease. However, harvested fresh fruits are highly susceptible to microbial contamination, oxidation, and moisture loss, leading to spoilage, flavor loss, and significant resource waste and economic losses. Nearly one-third of fruits rot and spoil each year due to microbial invasion and improper storage. To extend the shelf life of fruits, methods such as freezing, steam heat treatment, irradiation sterilization, and the addition of preservatives are commonly used to reduce losses during transportation. However, these technologies suffer from high energy consumption and potential health hazards associated with synthetic preservatives, which are inconsistent with current green development principles.

[0003] Traditional petroleum-based fruit preservation films only physically isolate fruit from the external environment, failing to effectively inhibit microbial growth or resist quality deterioration caused by ultraviolet radiation. Furthermore, they are difficult to degrade, leading to serious environmental problems after use. Green packaging materials developed from functional biopolymers, on the other hand, possess superior antibacterial, antioxidant, mechanical, and optical properties, along with the advantage of biodegradability, aligning with the principles of green chemistry and sustainable development.

[0004] Chitosan, a biopolymer, possesses excellent biocompatibility, biodegradability, non-toxicity, and film-forming properties. However, pure chitosan materials suffer from poor mechanical properties and limited antibacterial capabilities, restricting their practical applications. Chemical modification, typically employing active groups, can significantly enhance the specific functions of chitosan molecules. However, currently used chemical modification methods suffer from harsh reaction conditions, the use of toxic cross-linking agents, limited functionality, or poor stability, making it difficult to achieve a synergistic integration of multiple functions such as antibacterial, antioxidant, and water-blocking properties. Furthermore, the complex processes hinder large-scale production and application.

[0005] Based on this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a chitosan-based membrane, its preparation method, and its applications, in order to solve the problems existing in the prior art. This invention proposes a chitosan-based membrane with a double-layer structure. By constructing a silver complex water-blocking and antibacterial layer on the outer layer and a polyphenol antibacterial and antioxidant layer on the inner layer, a dual preservation mechanism of "external protection + internal maintenance" is achieved, significantly improving the overall performance and application potential of the material.

[0007] To achieve the above objectives, the present invention provides the following solution: One of the technical solutions of the present invention is to provide a chitosan-based membrane, wherein the chitosan-based membrane has a double-layer structure consisting of an inner layer and an outer layer, the outer layer being a water-blocking and antibacterial layer formed by silver complexes, and the inner layer being an antibacterial and antioxidant layer formed by polyphenols; The silver complex is a complex formed by the reaction of pyrrole-2-carboxaldehyde-g-chitosan and silver acetate; pyrrole-2-carboxaldehyde-g-chitosan is prepared by reacting chitosan and pyrrole-2-carboxaldehyde. The polyphenols are formed by grafting methyl 3,4-dihydroxyphenylacetic acid onto chitosan.

[0008] The second technical solution of the present invention provides a method for preparing the above-mentioned chitosan-based membrane, comprising the following steps: Chitosan was dissolved in an aqueous acetic acid solution, and pyrrole-2-carboxaldehyde was added to react with it. After alkaline precipitation, washing, and drying, pyrrole-2-carboxaldehyde-g-chitosan was obtained. Pyrrole-2-carboxaldehyde-g-chitosan was dissolved in an aqueous acetic acid solution, and silver acetate was added to react with it. Glycerin was then added and stirred evenly. After degassing, the outer membrane solution was obtained. Chitosan was dissolved in an aqueous acetic acid solution, methyl 3,4-dihydroxyphenylacetic acid was added and reacted, then glycerol was added and stirred until homogeneous. The inner membrane solution was obtained by degassing. The outer membrane is formed by liquid injection and curing, and then the inner membrane liquid is injected and dried to obtain the chitosan-based membrane.

[0009] The reaction formulas involved in the preparation of the inner membrane solution are as follows: Figure 6 As shown, specifically, chitosan is reacted with methyl 3,4-dihydroxyphenylacetate, and methyl 3,4-dihydroxyphenylacetate is grafted onto chitosan to prepare phenolic chitosan. The proton NMR spectra of chitosan and phenolic chitosan are shown below. Figure 7 As shown. In Figure 7 In the diagram, blue lines represent phenolic chitosan, and black lines represent chitosan. (The remaining text appears to be a fragment and doesn't translate directly.) Figure 7 It can be seen that chitosan 1 The 1H NMR spectrum signal peaks appeared at 2.04, 3.18, 3.72–3.93, and 4.95 ppm, corresponding to various hydrogen protons on the chitosan chain. Compared with chitosan, phenolic chitosan... 1 A new peak appeared in the 1H NMR spectrum near the chemical shift of 3.5 ppm, corresponding to the methylene proton attached to the benzene ring; new multiple absorption peaks appeared in the range of 6.72–6.85 ppm, corresponding to various hydrogen protons on the benzene ring. This indicates that methyl 3,4-dihydroxyphenylacetate has successfully undergone ammonolysis with chitosan to form polyphenol-grafted chitosan.

[0010] Silver acetate is chosen over other silver salts in the preparation of silver ion coordination compounds for three reasons: First, silver acetate has good water solubility and shares the same acetate anion with the coordination reaction system, avoiding the introduction of new anions and improving the coordination efficiency of silver ions; second, the coordination of pyrrole-2-carboxaldehyde-g-chitosan with silver ions requires a specific pH range, and the acetate ions in silver acetate react with acetic acid to form acetate-acetic acid (CH3COO-). - The -CH3COOH) buffer system helps to regulate the reaction pH; thirdly, the acetate anion in silver acetate avoids the occurrence of side reactions such as oxidation-reduction and precipitation, which affect the properties of the material. For example, if silver nitrate is used, its nitrate ions have a certain oxidizing property under acidic conditions, which reduces the antioxidant performance of the material and affects the preservation effect.

[0011] Choosing acetic acid aqueous solution as the solvent for chitosan has the following advantages: First, as a weak organic acid, acetic acid provides a suitable acidic environment, effectively protonating the amino groups on the chitosan chains, causing them to dissolve due to charge repulsion. This avoids the degradation of chitosan molecular chains that may be caused by strong acids such as hydrochloric acid, thus optimizing the material's performance. Second, acetic acid is volatile and can be effectively removed through evaporation during subsequent drying processes, minimizing the likelihood of impurities remaining in the final product and meeting the safety requirements for biomaterials. Third, acetic acid exhibits good solution stability; the chitosan solution dissolved in acetic acid maintains good stability at room temperature, is less prone to gelation or precipitation, and facilitates subsequent operations. Fourth, acetic acid itself is non-toxic, low-corrosive, inexpensive, and readily available, making it safe and convenient to handle and suitable for industrial-scale production.

[0012] Compared with other plasticizers (such as sorbitol, polyethylene glycol, and mannitol), the core advantages of choosing glycerol as a plasticizer are: (1) Glycerol has excellent plasticizing efficiency. As a small polyhydroxy molecule, it can effectively insert into the chitosan molecular chain and destroy the strong interaction of chitosan itself through hydrogen bonds, thereby greatly reducing the brittleness of the film and improving its extensibility. (2) Glycerol and chitosan acetic acid solution are completely miscible and do not easily separate into phases; more importantly, it does not compete with silver ions for coordination or reaction, and can completely maintain the coordination structure between pyrrole-2-carboxaldehyde-g-chitosan and silver ions, thus not affecting the antibacterial efficacy of the film. (3) Glycerol is non-toxic, biodegradable, inexpensive, and readily available.

[0013] Preferably, the mass fraction of acetic acid in the acetic acid aqueous solution is 0.5-2.5%; and the mass fraction of chitosan solution obtained by dissolving chitosan in the acetic acid aqueous solution is 1-3%.

[0014] Preferably, the chitosan has a molecular weight of 50-300 kDa and a degree of deacetylation of ≥90%.

[0015] Preferably, when preparing the outer membrane solution, the mass ratio of chitosan, pyrrole-2-carboxaldehyde, silver acetate and glycerol is (10~30):(4~8):(3~7):(4~8).

[0016] Preferably, the reaction time for adding pyrrole-2-carboxaldehyde is 6-8 hours.

[0017] Furthermore, the alkaline precipitation step involves adding 1 / 2 volume of NaOH solution to the reaction system to precipitate the precipitate; the mass fraction of the NaOH solution is 3-5%.

[0018] Preferably, the reaction conditions for adding silver acetate are: reaction in the dark for 3-5 hours.

[0019] Preferably, when preparing the inner membrane solution, the mass ratio of chitosan, methyl 3,4-dihydroxyphenylacetate and glycerol is (6~20):(2~6):(2~5).

[0020] Preferably, the reaction conditions for adding methyl 3,4-dihydroxyphenylacetic acid are: reaction in the dark for 5-7 hours.

[0021] The third technical solution of the present invention is to provide the application of the above-mentioned chitosan-based film in fruit preservation or food packaging.

[0022] The present invention discloses the following technical effects: (1) Highly efficient and long-lasting antibacterial properties. The outer layer of the chitosan-based membrane forms a stable antibacterial layer through silver complexes, which can effectively inhibit planktonic microorganisms around food. The inner layer introduces polyphenol structures to exert a synergistic effect of antioxidation and antibacterial activity. The combination of the inner and outer layers significantly enhances the antibacterial duration and broad spectrum.

[0023] (2) Excellent preservation and barrier functions. The outer layer of the chitosan-based membrane is coordinated with silver ions to reduce the spacing between chitosan molecular chains and weaken the hydrophilicity of the molecular chains, thereby improving the membrane density and blocking the water molecule permeation channels, effectively preventing food from losing water and nutrients; the inner layer of polyphenols further maintains the freshness of food by scavenging free radicals and delaying cell aging; achieving dual protection of "external protection + internal maintenance" and significantly improving the preservation performance.

[0024] (3) Good safety and environmental protection. Chitosan is used as the matrix, and the material is naturally biodegradable; and the double-layer structure design ensures high safety and meets the requirements of green packaging.

[0025] (4) The preparation process is simple and has broad application prospects. The preparation process is based on solution casting and layer-by-layer solidification, which is simple to operate and easy to scale up, and has good potential for promotion in fruit and various food packaging. In summary, the chitosan-based film of the present invention achieves an organic combination of antibacterial, antioxidant, water-blocking and preservation functions through a dual-layer synergistic design, and has both practicality and environmental friendliness. Attached Figure Description

[0026] Figure 1 This is a photograph of the chitosan-based membrane obtained in Example 1; Figure 2 This is a scanning electron microscope image of the chitosan-based membrane obtained in Example 1; Figure 3 The test sample's activity against DPPH free radical scavenging; Figure 4 This is a diagram showing the changes in the appearance of strawberries during storage; Figure 5 This is a graph showing the weight loss of strawberries during storage. Figure 6 The reaction formulas involved in the preparation process of the inner membrane solution; Figure 7 The images show the hydrogen spectroscopy spectra of chitosan and phenolic chitosan. Detailed Implementation

[0027] The first aspect of the present invention provides a chitosan-based membrane having a double-layer structure consisting of an inner layer and an outer layer, wherein the outer layer is a water-blocking and antibacterial layer formed by a silver complex, and the inner layer is an antibacterial and antioxidant layer formed by polyphenols.

[0028] A second aspect of the present invention provides a method for preparing a chitosan-based membrane, comprising the following steps: Step 1: Dissolve chitosan in an aqueous acetic acid solution to obtain a chitosan solution; add pyrrole-2-carboxaldehyde to the chitosan solution for reaction, add 1 / 2 volume of NaOH solution to precipitate the precipitate, filter, wash, and vacuum dry to obtain pyrrole-2-carboxaldehyde-g-chitosan; dissolve the prepared pyrrole-2-carboxaldehyde-g-chitosan in an aqueous acetic acid solution to obtain a pyrrole-2-carboxaldehyde-g-chitosan solution, add silver acetate for reaction, then add glycerol and stir evenly, and vacuum degas to obtain the outer membrane solution; Step 2: Dissolve chitosan in an aqueous acetic acid solution to obtain a chitosan solution; add methyl 3,4-dihydroxyphenylacetate to the chitosan solution for reaction, add glycerol and stir until homogeneous, then perform vacuum degassing to obtain the inner membrane solution. Step 3: Inject the outer layer membrane liquid into the mold and let it stand under a forced air to solidify into an outer layer membrane; then inject the inner layer membrane liquid into the mold with the prepared outer layer membrane and let it stand under a forced air to dry, thus obtaining the chitosan-based membrane.

[0029] Step 1 and Step 2 are not in any particular order.

[0030] In a preferred embodiment of the present invention, in steps 1 and 2, the mass fraction of acetic acid in the acetic acid aqueous solution is 0.5-2.5%; the mass fraction of chitosan in the chitosan solution is 1-3%, the molecular weight of chitosan is 50-300 kDa, and the degree of deacetylation of chitosan is ≥90%.

[0031] In a preferred embodiment of the present invention, in step 1, the reaction conditions for adding pyrrole-2-carboxaldehyde are: stirring at room temperature for 6-8 hours; and the mass fraction of the NaOH solution is 3-5%.

[0032] In this invention, pyrrole-2-carboxaldehyde-grafted chitosan exhibits the following superior effects: First, the introduced nitrogen-containing heterocyclic structure enhances the positive charge of the chitosan molecule, thereby significantly improving the antibacterial activity of the material; Second, the imine group (-C=N-) formed at room temperature, together with the nitrogen atom with lone pair electrons in the pyrrole ring, constructs an NN-bident coordination structure, a feature that significantly strengthens the antimicrobial activity against silver ions (Ag). + The chelating ability of chitosan allows it to form stable complexes through efficient coordination. These complexes, together with the grafted and modified chitosan, produce a synergistic antibacterial effect, giving the material a stronger and more durable antibacterial effect.

[0033] In a preferred embodiment of the present invention, in step 1, the mass fraction of the pyrrole-2-carboxaldehyde-g-chitosan solution is 1-3%; the reaction conditions for adding silver acetate are: stirring at room temperature in the dark for 3-5 hours.

[0034] In a preferred embodiment of the present invention, in step 1, the mass ratio of chitosan, pyrrole-2-carboxaldehyde, silver acetate and glycerol is (10~30):(4~8):(3~7):(4~8).

[0035] In a preferred embodiment of the present invention, in step 2, the reaction conditions for adding methyl 3,4-dihydroxyphenylacetic acid are: stirring at room temperature in the dark for 5-7 hours.

[0036] In this invention, the addition of methyl 3,4-dihydroxyphenylacetate has the following excellent effects: First, due to the strong electrophilic activity of the carbonyl group in its ester structure, it can directly undergo aminolysis with the primary amino group in the chitosan molecule, thereby grafting it into the chitosan molecule, avoiding the use of coupling agents and simplifying the reaction process. Second, by introducing catechol structural units into the chitosan matrix material, it can be endowed with multiple excellent functions. The catechol group can form strong hydrogen bonds, π-π stacking, and metal coordination with various material surfaces (including food), thus giving the material universal and strong wet adhesion, allowing it to directly and tightly adhere to the surfaces of various foods. At the same time, the catechol structure also has significant antibacterial activity, inhibiting bacterial growth by interfering with microbial membrane function or inducing oxidative stress; in addition, the strong antioxidant capacity of the catechol structure can effectively scavenge free radicals and block oxidation chain reactions, thereby synergistically extending the shelf life of food.

[0037] In a preferred embodiment of the present invention, in step 2, the mass ratio of chitosan, methyl 3,4-dihydroxyphenylacetate and glycerol is (6~20):(2~6):(2~5).

[0038] In this invention, the purpose of using a water-blocking and antibacterial layer formed by silver complexes as the outer layer and an antibacterial and antioxidant layer formed by polyphenols as the inner layer is as follows: The outer layer utilizes the silver complexes to form a dense physical barrier, whose main function is to block the intrusion of external moisture and oxygen, and continuously release silver ions to kill planktonic microorganisms in the environment, thus establishing the first line of protection for food. The polyphenol structure of the inner layer adheres tightly to the food surface due to its strong adhesion, directly removing free radicals generated by the food's own metabolism, achieving antioxidant preservation, and inhibiting the growth of microorganisms on the food surface through its inherent antibacterial properties. The positions of the two cannot be interchanged, because if the polyphenol layer is placed on the outside, its antioxidant components will be rapidly consumed and ineffective due to direct exposure to light and air; while placing the silver layer on the inside may increase the risk of metal ions migrating into the food, and it will not be able to effectively intercept external microorganisms.

[0039] The third aspect of this invention provides the application of the chitosan-based film described above in fruit preservation or food packaging.

[0040] The chitosan-based membrane obtained by this invention has superior antibacterial, antioxidant, water vapor barrier properties, and safety, and has good application prospects in fruit preservation or food packaging.

[0041] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0042] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0043] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0044] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This specification and embodiments are merely exemplary.

[0045] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0046] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0047] Unless otherwise specified, all raw materials used in the following embodiments, comparative examples, and performance tests of this invention are commercially available products, and the source of these commercially available products does not affect the technical effect of this invention.

[0048] Unless otherwise specified, "room temperature" or "normal temperature" as used in this invention refers to 25±3℃.

[0049] Example 1 A method for preparing a chitosan-based membrane: (1) 2g of chitosan (molecular weight of 100kDa, degree of deacetylation of 95%) was added to a 1% acetic acid aqueous solution and stirred to dissolve, thus preparing a 2% chitosan solution (I); 0.6g of pyrrole-2-carboxaldehyde was added to the chitosan solution (I) and stirred at room temperature for 7h; 1 / 2 volume of 4% NaOH solution was added in a constant pressure dropping funnel while stirring, and the precipitate was precipitated. The precipitate was filtered under reduced pressure and washed once with distilled water and once with 95% ethanol. After vacuum drying, pyrrole-2-carboxaldehyde-g-chitosan was obtained.

[0050] The prepared pyrrole-2-carboxaldehyde-g-chitosan was dissolved in a 1% (w / w) aqueous acetic acid solution to prepare a 2% (w / w) pyrrole-2-carboxaldehyde-g-chitosan solution. 0.5g of silver acetate was added, and the mixture was stirred at room temperature in the dark for 4 hours. Then, 0.6g of glycerol was added and stirred until homogeneous. The mixture was then degassed under vacuum to obtain the outer membrane solution.

[0051] (2) 1.3g of chitosan (molecular weight of 100kDa and degree of deacetylation of 95%) was added to a 1% acetic acid aqueous solution and stirred to dissolve, thus preparing a 2% chitosan solution (II); 0.4g of methyl 3,4-dihydroxyphenylacetate was added to the chitosan solution (II) and stirred at room temperature in the dark for 6 hours; then 0.35g of glycerol was added, stirred evenly, and vacuum degassing was performed to obtain the inner membrane solution.

[0052] (3) Inject the outer membrane liquid into the mold, let it stand under the blower and solidify into an outer membrane; then inject the inner membrane liquid into the mold with the prepared outer membrane, let it stand under the blower and dry to obtain a chitosan-based membrane.

[0053] Figure 1 This is a photograph of the chitosan-based membrane obtained in Example 1. Figure 2 This is a scanning electron microscope image of the chitosan-based membrane obtained in Example 1.

[0054] like Figure 1 As shown, the film is light beige, resulting from the oxidation of a small amount of phenol residues, confirming the presence of a catechol structure; its surface is smooth with a soft luster, and its texture is thin and transparent, showcasing the film's lightweight and flexible characteristics as a preservation material. From a microscopic perspective, it is visible that... Figure 2 The film has a smooth and uniform surface without obvious pores, protrusions or impurities, exhibiting a fine and dense microstructure. This uniform and dense surface morphology helps to improve its barrier and protective performance in fruit and food preservation applications.

[0055] Example 2 A method for preparing a chitosan-based membrane: (1) 1 g of chitosan (molecular weight of 300 kDa and degree of deacetylation of 90%) was added to a 0.5% acetic acid aqueous solution and stirred to dissolve, thus preparing a 1% chitosan solution (I). 0.4 g of pyrrole-2-carboxaldehyde was added to the chitosan solution (I) and stirred at room temperature for 6 h. 1 / 2 volume of 3% NaOH solution was added to the solution under constant pressure in the stirring state, and the precipitate was precipitated. The solution was filtered under reduced pressure and washed once with distilled water and once with 95% ethanol. After vacuum drying, pyrrole-2-carboxaldehyde-g-chitosan was obtained.

[0056] The prepared pyrrole-2-carboxaldehyde-g-chitosan was dissolved in a 0.5% acetic acid aqueous solution to prepare a 1% pyrrole-2-carboxaldehyde-g-chitosan solution. 0.3g of silver acetate was added, and the mixture was stirred at room temperature in the dark for 3 hours. Then, 0.4g of glycerol was added and stirred until homogeneous. The mixture was then degassed under vacuum to obtain the outer membrane solution.

[0057] (2) Add 0.6g of chitosan (molecular weight of 300kDa, degree of deacetylation of 90%) to a 0.5% acetic acid aqueous solution and stir to dissolve to prepare a 1% chitosan solution (II); add 0.2g of methyl 3,4-dihydroxyphenylacetate to the chitosan solution (II) and stir at room temperature in the dark for 5h; then add 0.2g of glycerol, stir evenly, and degas under vacuum to obtain the inner membrane solution.

[0058] (3) Inject the outer membrane liquid into the mold, let it stand under the blower and solidify into an outer membrane; then inject the inner membrane liquid into the mold with the prepared outer membrane, let it stand under the blower and dry to obtain a chitosan-based membrane.

[0059] Example 3 A method for preparing a chitosan-based membrane: (1) 3g of chitosan (molecular weight of 50kDa, degree of deacetylation of 95%) was added to a 2.5% acetic acid aqueous solution and stirred to dissolve, thus preparing a 3% chitosan solution (I); 0.8g of pyrrole-2-carboxaldehyde was added to the chitosan solution (I) and stirred at room temperature for 8h; 1 / 2 volume of 5% NaOH solution was added to the solution under constant pressure in the stirring state, and the precipitate was precipitated. The solution was filtered under reduced pressure and washed once with distilled water and once with 95% ethanol. After vacuum drying, pyrrole-2-carboxaldehyde-g-chitosan was obtained.

[0060] The prepared pyrrole-2-carboxaldehyde-g-chitosan was dissolved in a 2.5% acetic acid aqueous solution to prepare a 3% pyrrole-2-carboxaldehyde-g-chitosan solution. 0.7g of silver acetate was added, and the mixture was stirred at room temperature in the dark for 5 hours. Then, 0.8g of glycerol was added and stirred until homogeneous. The mixture was then degassed under vacuum to obtain the outer membrane solution.

[0061] (2) Add 2g of chitosan (molecular weight of 50kDa, degree of deacetylation of 95%) to a 2.5% acetic acid aqueous solution and stir to dissolve to prepare a 3% chitosan solution (II); add 0.6g of methyl 3,4-dihydroxyphenylacetate to the chitosan solution (II) and stir at room temperature in the dark for 7h; then add 0.5g of glycerol, stir evenly, and degas under vacuum to obtain the inner membrane solution.

[0062] (3) Inject the outer membrane liquid into the mold, let it stand under the blower and solidify into an outer membrane; then inject the inner membrane liquid into the mold with the prepared outer membrane, let it stand under the blower and dry to obtain a chitosan-based membrane.

[0063] Comparative Example 1 A method for preparing a chitosan membrane differs from Example 1 only in that only chitosan is used; the remaining process steps and parameters are the same as in Example 1.

[0064] The specific preparation steps are as follows: (1) Add 2g of chitosan (molecular weight of 100kDa and degree of deacetylation of 95%) to a 1% acetic acid aqueous solution, stir to dissolve, and prepare a 2% chitosan solution (I); vacuum degassing treatment is performed to obtain the outer membrane solution.

[0065] (2) Add 1.3g of chitosan (molecular weight of 100kDa and degree of deacetylation of 95%) to a 1% acetic acid aqueous solution, stir to dissolve, and prepare a 2% chitosan solution (II); vacuum degassing treatment is performed to obtain the inner membrane solution.

[0066] (3) Inject the outer membrane liquid into the mold, let it stand under the blower and solidify into an outer membrane; then inject the inner membrane liquid into the mold with the prepared outer membrane, let it stand under the blower and dry to obtain a chitosan membrane.

[0067] Comparative Example 2 A method for preparing a chitosan membrane differs from Example 1 only in that methyl 3,4-dihydroxyphenylacetic acid is omitted; all other process steps and parameters are the same as in Example 1.

[0068] The specific preparation steps are as follows: (1) 2g of chitosan (molecular weight of 100kDa, degree of deacetylation of 95%) was added to a 1% acetic acid aqueous solution and stirred to dissolve, thus preparing a 2% chitosan solution (I); 0.6g of pyrrole-2-carboxaldehyde was added to the chitosan solution (I) and stirred at room temperature for 7h; 1 / 2 volume of 4% NaOH solution was added in a constant pressure dropping funnel while stirring, and the precipitate was precipitated. The precipitate was filtered under reduced pressure and washed once with distilled water and once with 95% ethanol. After vacuum drying, pyrrole-2-carboxaldehyde-g-chitosan was obtained.

[0069] The prepared pyrrole-2-carboxaldehyde-g-chitosan was dissolved in a 1% (w / w) aqueous acetic acid solution to prepare a 2% (w / w) pyrrole-2-carboxaldehyde-g-chitosan solution. 0.5g of silver acetate was added, and the mixture was stirred at room temperature in the dark for 4 hours. Then, 0.6g of glycerol was added and stirred until homogeneous. The mixture was then degassed under vacuum to obtain the outer membrane solution.

[0070] (2) Add 1.3g of chitosan (molecular weight of 100kDa and degree of deacetylation of 95%) to a 1% acetic acid aqueous solution, stir to dissolve, and prepare a 2% chitosan solution (II); vacuum degassing treatment is performed to obtain the inner membrane solution.

[0071] (3) Inject the outer membrane liquid into the mold, let it stand under the blower and solidify into an outer membrane; then inject the inner membrane liquid into the mold with the prepared outer membrane, let it stand under the blower and dry to obtain a chitosan-based membrane.

[0072] Comparative Example 3 A method for preparing a chitosan membrane differs from Example 1 only in that pyrrole-2-carboxaldehyde is omitted; all other process steps and parameters are the same as in Example 1.

[0073] The specific preparation steps are as follows: (1) Add 2g of chitosan (molecular weight of 100kDa and degree of deacetylation of 95%) to a 1% acetic acid aqueous solution, stir to dissolve, and prepare a 2% chitosan solution (I); vacuum degassing treatment is performed to obtain the outer membrane solution.

[0074] (2) 1.3g of chitosan (molecular weight of 100kDa and degree of deacetylation of 95%) was added to a 1% acetic acid aqueous solution and stirred to dissolve, thus preparing a 2% chitosan solution (II); 0.4g of methyl 3,4-dihydroxyphenylacetate was added to the chitosan solution (II) and stirred at room temperature in the dark for 6 hours; then 0.35g of glycerol was added, stirred evenly, and vacuum degassing was performed to obtain the inner membrane solution.

[0075] (3) Inject the outer membrane liquid into the mold, let it stand under the blower and solidify into an outer membrane; then inject the inner membrane liquid into the mold with the prepared outer membrane, let it stand under the blower and dry to obtain a chitosan-based membrane.

[0076] Comparative Example 4 A method for preparing a chitosan membrane differs from Example 1 only in that methyl 3,4-dihydroxyphenylacetate is replaced with 3,4-dihydroxybenzoic acid; all other process steps and parameters are the same as in Example 1.

[0077] The specific preparation steps are as follows: (1) 2g of chitosan (molecular weight of 100kDa, degree of deacetylation of 95%) was added to a 1% acetic acid aqueous solution and stirred to dissolve, thus preparing a 2% chitosan solution (I); 0.6g of pyrrole-2-carboxaldehyde was added to the chitosan solution (I) and stirred at room temperature for 7h; 1 / 2 volume of 4% NaOH solution was added in a constant pressure dropping funnel while stirring, and the precipitate was precipitated. The precipitate was filtered under reduced pressure and washed once with distilled water and once with 95% ethanol. After vacuum drying, pyrrole-2-carboxaldehyde-g-chitosan was obtained.

[0078] The prepared pyrrole-2-carboxaldehyde-g-chitosan was dissolved in a 1% (w / w) aqueous acetic acid solution to prepare a 2% (w / w) pyrrole-2-carboxaldehyde-g-chitosan solution. 0.5g of silver acetate was added, and the mixture was stirred at room temperature in the dark for 4 hours. Then, 0.6g of glycerol was added and stirred until homogeneous. The mixture was then degassed under vacuum to obtain the outer membrane solution.

[0079] (2) Add 1.3g of chitosan (molecular weight of 100kDa, degree of deacetylation of 95%) to a 1% acetic acid aqueous solution and stir to dissolve to prepare a 2% chitosan solution (II); add 0.4g of 3,4-dihydroxybenzoic acid to the chitosan solution (II) and stir at room temperature in the dark for 6h; then add 0.35g of glycerol, stir evenly, and degas under vacuum to obtain the inner membrane solution.

[0080] (3) Inject the outer membrane liquid into the mold, let it stand under the blower and solidify into an outer membrane; then inject the inner membrane liquid into the mold with the prepared outer membrane, let it stand under the blower and dry to obtain a chitosan-based membrane.

[0081] Comparative Example 5 A method for preparing a chitosan membrane differs from Example 1 only in that pyrrole-2-carboxaldehyde is replaced with pyridine-2-carboxaldehyde; all other process steps and parameters are the same as in Example 1.

[0082] The specific preparation steps are as follows: (1) 2g of chitosan (molecular weight of 100kDa, degree of deacetylation of 95%) was added to a 1% acetic acid aqueous solution and stirred to dissolve, thus preparing a 2% chitosan solution (I); 0.6g of pyridine-2-carboxaldehyde was added to the chitosan solution (I) and stirred at room temperature for 7h; 1 / 2 volume of 4% NaOH solution was added in a constant pressure dropping funnel while stirring, and the precipitate was precipitated. The precipitate was filtered under reduced pressure and washed once with distilled water and once with 95% ethanol. After vacuum drying, pyridine-2-carboxaldehyde-g-chitosan was obtained.

[0083] The prepared pyridine-2-carboxaldehyde-g-chitosan was dissolved in a 1% (w / w) aqueous acetic acid solution to prepare a 2% (w / w) pyridine-2-carboxaldehyde-g-chitosan solution. 0.5 g of silver acetate was added, and the mixture was stirred at room temperature in the dark for 4 h. Then, 0.6 g of glycerol was added and stirred until homogeneous. The mixture was then degassed under vacuum to obtain the outer membrane solution.

[0084] (2) 1.3g of chitosan (molecular weight of 100kDa and degree of deacetylation of 95%) was added to a 1% acetic acid aqueous solution and stirred to dissolve, thus preparing a 2% chitosan solution (II); 0.4g of methyl 3,4-dihydroxyphenylacetate was added to the chitosan solution (II) and stirred at room temperature in the dark for 6 hours; then 0.35g of glycerol was added, stirred evenly, and vacuum degassing was performed to obtain the inner membrane solution.

[0085] (3) Inject the outer membrane liquid into the mold, let it stand under the blower and solidify into an outer membrane; then inject the inner membrane liquid into the mold with the prepared outer membrane, let it stand under the blower and dry to obtain a chitosan-based membrane.

[0086] (a) Performance Testing: (1) Mechanical property test: The mechanical properties of the tested thin film were tested, and the test procedure is as follows: Five points were randomly selected on the membrane to be tested, and the thickness of each sample was measured using a digital micrometer. The average value was taken with an accuracy of 0.001 mm. The sample membrane was cut into rectangles of 20 mm × 150 mm and fixed at both ends of an electronic universal testing machine (INSTRON5982, USA). The tensile speed was set to 50 mm / min, and the tensile strength and elongation at break were measured. Each test was repeated 5 times.

[0087] As shown in Table 1, the tensile strength and elongation at break of the pure chitosan film in Comparative Example 1 were 15.07±0.53 MPa and 11.35±0.46%, respectively. Compared with Comparative Example 1, the tensile strength and elongation at break of the films in Comparative Example 2 and Comparative Example 3 were improved (p<0.05) without significant difference in film thickness, but they did not achieve the best effect of synergistic effect of the double-layer film due to the omission of one component. The tensile strengths of the chitosan-based films prepared in Examples 1-3 were 38.52±0.46 MPa, 31.36±0.38 MPa, and 35.71±0.52 MPa, respectively, and their elongations at break were 67.32±1.06%, 68.55±1.13%, and 61.74±1.21%, respectively. Compared to the pure chitosan film of Comparative Example 1, the tensile strength and elongation at break of the chitosan-based films prepared in Examples 1-3 were significantly improved (p<0.01), indicating that the chitosan-based films prepared in Examples 1-3 have good mechanical properties. Compared to Examples 1-3, with no significant difference in film thickness, the tensile strength and elongation at break of the films in Comparative Examples 4 and 5 were significantly reduced (p<0.05). This indicates that replacing any component of the film would reduce its mechanical properties.

[0088] Table 1 Properties of the thin film (2) Water vapor permeability test: The water vapor transmission rate (WVP) of the test samples was measured using the cup weight gain method. The test conditions were 23℃ and 90% relative humidity. Each sample was measured three times, and the average value was taken. The formula for calculating water vapor transmission rate (WVP) is as follows: WVP=(W b -W a )×d / (A×T×Δp); In the formula, W a and W b d and A represent the weight of the cup before and after the test timing interval, respectively, in g; d and A represent the thickness (m) and test area (m²) of the sample film, respectively. 2 T is the test time, h; Δp is the water vapor pressure difference across the sample, Pa.

[0089] The low water vapor permeability of the membrane reduces excessive moisture loss from fruit and prevents the invasion of external microorganisms, which is crucial for fruit preservation. As shown in Table 1, the water vapor permeability of the pure chitosan membrane in Comparative Example 1 is (7.27±0.23)×10⁻⁶. -7 g·m -1 ·h -1 ·Pa -1 Compared to Comparative Example 1, while the water vapor permeability of Comparative Example 2 and Comparative Example 3 films was reduced (p<0.05) without significant difference in film thickness, the optimal synergistic effect of the bilayer films was not achieved due to the omission of one component. Compared to the pure chitosan film of Comparative Example 1, the water vapor permeability of the film samples prepared in Examples 1-3 was significantly reduced (p<0.01). This can be attributed to the chelating effect between silver ions and modified chitosan, the multiple physical interactions within the molecular chain caused by polyphenols, and the dense structure of the composite film resulting from the bilayer design. These results indicate that the film samples prepared in Examples 1-3 have good moisture-blocking properties. Compared to Examples 1-3, while the film thickness was not significantly different, the water vapor permeability of Comparative Example 4 and Comparative Example 5 films was significantly increased (p<0.05). This indicates that replacing any component leads to increased water vapor permeability, thereby reducing the preservation performance of the film.

[0090] (3) Antioxidant properties: The antioxidant properties of the tested films were evaluated using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging method. A 0.1 mmol / L DPPH solution was prepared by dissolving DPPH in anhydrous ethanol. 3 mg of the shredded sample film was immersed in 3 mL of the DPPH solution and incubated in the dark at room temperature for 30 min. After incubation, the mixture was centrifuged at 8000 rpm for 10 min at 4 °C, and the absorbance of the supernatant at 517 nm was measured and denoted as As. The DPPH solution without added sample was used as a blank sample, and its absorbance was denoted as Ac; ascorbic acid was used as a positive control. The DPPH free radical scavenging rate (%) was calculated using the following formula: DPPH free radical scavenging rate (%) = (Ac-As) / Ac×100%.

[0091] As a functional material, antioxidant packaging films can effectively protect food from oxidation and extend its shelf life. For example... Figure 3As shown, the pure chitosan membrane of Comparative Example 1 exhibited certain DPPH radical scavenging activity (27.35±1.31%), mainly due to the hydrogen donation capacity of the amino and hydroxyl groups on the polymer chain, which can terminate the free radical chain reaction. Compared with Comparative Example 1, the DPPH radical scavenging activity of Comparative Example 2 increased, but there was no significant difference (p>0.05), while the DPPH radical scavenging activity of Comparative Example 3 increased significantly (p<0.01), indicating that the introduction of methyl 3,4-dihydroxyphenylacetate has a decisive influence on the antioxidant properties of the film. Compared with Comparative Example 1, the DPPH radical scavenging activity of the film samples prepared in Examples 1-3 was significantly improved (p<0.01), and the DPPH radical scavenging activity reached more than 80%. This is attributed to the unique molecular structure of catechol introduced into the chitosan chain. The phenolic hydroxyl groups in its structure act as efficient hydrogen donors. When encountering DPPH radicals, they reduce DPPH radicals to a stable structural form, thereby interrupting the free radical chain reaction. These results highlight the potential of the film samples prepared in Examples 1-3 for food preservation packaging applications. Compared with Examples 1-3, the DPPH radical scavenging activity of the films in Comparative Examples 4 and 5 was significantly reduced (p<0.05), indicating that replacing any component would weaken the antioxidant properties of the films, thereby affecting their preservation effect.

[0092] (4) Antibacterial properties: Antibacterial activity was tested using a liquid culture method. Gram-positive Staphylococcus aureus (S. aureus, ATCC 6538) and Gram-negative Escherichia coli (E. coli, ATCC 25922) were used as test bacteria to evaluate the antibacterial activity of the tested samples. The experimental procedure is as follows: Two test bacteria were inoculated into sterilized Erlenmeyer flasks containing 50 mL of LB liquid medium (1% tryptone, 0.5% yeast extract, 1% sodium chloride), and incubated on a shaker at 200 rpm and 37°C for 12 h. The cultures were then diluted with LB liquid medium to prepare bacterial suspensions (10⁶ CFU / mL). A UV-sterilized film sample (20 mm × 20 mm) was immersed in an Erlenmeyer flask containing 20 mL of LB liquid medium, inoculated with 100 μL of the above bacterial suspension (10⁵ CFU / mL), and incubated on a shaker at 200 rpm and 37°C for 24 h. The absorbance of the culture medium at 600 nm was measured using a spectrophotometer. A blank control group (without sample) and a positive control group (ciprofloxacin hydrochloride) were set up. The inhibition rate of the samples was calculated using the following formula: Antibacterial rate (%) = [(A control group - A sample group) / A control group] × 100%.

[0093] In the formula, A control group and A sample group are the absorbance of the blank control group and the sample group, respectively.

[0094] Table 2 Antibacterial properties of the tested samples As shown in Table 2, at the experimental concentration of this invention, the pure chitosan film of Comparative Example 1 exhibited a certain antibacterial effect, but the effect was relatively weak. This is because although chitosan has a broad-spectrum antibacterial effect, its antibacterial effect is constrained by its own properties, environmental conditions, and the type of microorganisms, and is also affected by the formulation. Compared with Comparative Example 1, the antibacterial effects of the films of Comparative Example 2 and Comparative Example 3 were significantly increased (p<0.05), but they did not achieve the optimal synergistic antibacterial effect of the double-layer film due to the omission of one component. Compared with Comparative Example 1, the film samples prepared in Examples 1-3 all showed significant antibacterial activity and had a significant inhibitory effect on the growth of common bacteria. This characteristic can directly reduce the contamination of food by microorganisms, thus providing a reliable material basis for effectively extending the shelf life of food and ensuring its storage quality. Compared with Examples 1-3, the antibacterial activity of the films of Comparative Example 4 and Comparative Example 5 was significantly decreased (p<0.05), indicating that replacing any component would weaken the antibacterial performance of the film, thereby affecting the food storage period.

[0095] (II) Applications in fruit preservation: The sample films prepared in Example 1 and Comparative Example 1 were used in fruit preservation application tests. Taking strawberry as an example, the test process is as follows: Fresh, undamaged strawberries were selected and grouped according to similar color, size, and ripeness, and placed in clean petri dishes, with four strawberries in each dish, and completely wrapped with a thin film. A commercially available polyethylene film (PE film) was used as the control group, while the experimental groups used the chitosan-based film prepared in Example 1 and the pure chitosan film prepared in Comparative Example 1, respectively. The strawberries were stored at room temperature and a relative humidity of 45-65% for 5 days, with the weight of the strawberries measured daily.

[0096] Experimental results are as follows Figure 4 As shown, strawberries wrapped in polyethylene film and the pure chitosan film prepared in Comparative Example 1 developed lesions on the 3rd day, with mycelium forming on the surface. By the 4th day, they were severely dehydrated and rotted to varying degrees, and by the 5th day, they were almost completely moldy. This is because the high sugar content of strawberries provides a favorable environment for mold growth, resulting in a shorter shelf life. In contrast, strawberries wrapped in the chitosan-based film prepared in Example 1 showed no significant changes in appearance to the naked eye during the 5-day storage period. Furthermore, the weight loss of the strawberries during storage was minimal. Figure 5As can be seen, after 5 days of storage, the weight loss of strawberries in the polyethylene film group and the pure chitosan film group prepared in Comparative Example 1 was 14.74±0.31% and 12.14±0.19%, respectively. Compared with the polyethylene film and the pure chitosan film prepared in Comparative Example 1, the chitosan-based film prepared in Example 1 showed a significant advantage in postharvest preservation of strawberries, with a strawberry weight loss rate of only 2.07±0.22%, proving its excellent ability to block moisture loss. In particular, on the 4th and 5th days of storage, the weight loss rate of the polyethylene film group and the pure chitosan film group prepared in Comparative Example 1 increased sharply due to microbial spoilage, while the sample film group of Example 1 maintained a steady trend, indicating that the film has effective antibacterial activity and can successfully delay the fruit spoilage process caused by microbial invasion. Therefore, the chitosan-based film prepared in Example 1 shows great promise in the preservation of perishable fruits and vegetables by synergistically exerting a dual mechanism of highly efficient water blocking and active antibacterial activity.

[0097] (III) Material safety testing: To verify the safety of the thin film material, the silver ion content of strawberries wrapped in the film was detected using atomic absorption spectrometry. The experimental procedure is as follows: Fresh strawberries were wrapped in the sample film prepared in Example 1 for 5 days and then homogenized using a homogenizer. One gram of the homogenized sample was accurately weighed and digested. The silver ion content was measured using an atomic absorption spectrometer at a detection wavelength of 328.1 nm.

[0098] After testing, it was found that the concentration of silver ions in the strawberries tested was below the instrument's detection limit, and the result was "not detected," indicating that silver ions are relatively stable in the designed bilayer membrane structure. This invention not only ensures the antibacterial activity of silver ions but also greatly improves the safety of the membrane material.

[0099] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0100] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A chitosan-based membrane, characterized in that, The chitosan-based membrane has a double-layer structure consisting of an inner layer and an outer layer. The outer layer is a water-blocking and antibacterial layer formed by silver complexes, and the inner layer is an antibacterial and antioxidant layer formed by polyphenols. The silver complex is a complex formed by the reaction of pyrrole-2-carboxaldehyde-g-chitosan and silver acetate; pyrrole-2-carboxaldehyde-g-chitosan is prepared by chitosan and pyrrole-2-carboxaldehyde; The polyphenols are formed by grafting methyl 3,4-dihydroxyphenylacetic acid onto chitosan.

2. The method for preparing the chitosan-based membrane according to claim 1, characterized in that, Includes the following steps: Chitosan was dissolved in an aqueous acetic acid solution, and pyrrole-2-carboxaldehyde was added to react with it. After alkaline precipitation, washing, and drying, pyrrole-2-carboxaldehyde-g-chitosan was obtained. Pyrrole-2-carboxaldehyde-g-chitosan was dissolved in an aqueous acetic acid solution, and silver acetate was added to react with it. Glycerin was then added and stirred evenly. After degassing, the outer membrane solution was obtained. Chitosan was dissolved in an aqueous acetic acid solution, methyl 3,4-dihydroxyphenylacetic acid was added and reacted, then glycerol was added and stirred evenly to remove bubbles and obtain the inner membrane solution. The outer membrane is formed by liquid injection and curing, and then the inner membrane liquid is injected and dried to obtain the chitosan-based membrane.

3. The preparation method according to claim 2, characterized in that, The acetic acid aqueous solution contains 0.5-2.5% acetic acid by mass; the chitosan solution obtained by dissolving chitosan in the acetic acid aqueous solution contains 1-3% chitosan by mass.

4. The preparation method according to claim 3, characterized in that, The chitosan has a molecular weight of 50~300kDa and a degree of deacetylation ≥90%.

5. The preparation method according to claim 2, characterized in that, When preparing the outer membrane solution, the mass ratio of chitosan, pyrrole-2-carboxaldehyde, silver acetate and glycerol is (10~30):(4~8):(3~7):(4~8).

6. The preparation method according to claim 2, characterized in that, The reaction time after adding pyrrole-2-carboxaldehyde is 6-8 hours.

7. The preparation method according to claim 2, characterized in that, The reaction conditions for adding silver acetate are to react in the dark for 3-5 hours.

8. The preparation method according to claim 2, characterized in that, When preparing the inner membrane solution, the mass ratio of chitosan, methyl 3,4-dihydroxyphenylacetate and glycerol is (6~20):(2~6):(2~5).

9. The preparation method according to claim 2, characterized in that, The reaction conditions for adding methyl 3,4-dihydroxyphenylacetic acid are: reaction in the dark for 5-7 hours.

10. The application of the chitosan-based film according to claim 1 in fruit preservation or food packaging.

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