A chitosan-polysaccharide-based composite film containing two-dimensional COFs, a preparation method thereof and application thereof in olive preservation

Abstract

The application discloses a preparation method of a chitosan-polysaccharide-based composite film containing two-dimensional COFs. The method uses chitosan as a base material, compounding polysaccharide substances to prepare a chitosan-polysaccharide composite film, and then loading a two-dimensional COF film to prepare the chitosan-polysaccharide-based composite film containing two-dimensional COFs. The application also discloses application of the chitosan-polysaccharide-based composite film containing two-dimensional COFs prepared by the method in olive preservation. The chitosan-polysaccharide-based composite film containing two-dimensional COFs has good film-forming performance, biocompatibility, mechanical properties, barrier properties and antibacterial properties. The two-dimensional COF film can controllably regulate the flow of oxygen, carbon dioxide, water vapor and other gases in the process of packaging and storage of olives, reduces the occurrence of oxidation reactions, significantly prolongs the storage period of olives in olive preservation, can be stored for 28 days at 25 DEG C, and the color, moisture and nutrients of the olives are well maintained.

Description

Technical Field

[0001] This invention belongs to the field of fruit and vegetable preservation, specifically relating to a chitosan-polysaccharide composite film containing two-dimensional COFs, its preparation method, and its application in olive preservation. Background Technology

[0002] Olives are a specialty fruit resource in southern my country, rich in nutrients and possessing various health benefits, resulting in a huge market potential. However, the vigorous physiological activity of olives after harvest leads to rapid water loss, accelerated browning, and a rapid decline in quality. They are also susceptible to microbial infection, significantly shortening their shelf life and commercial value. Currently, there is an urgent need in the market for a preservation technology that can effectively delay olive aging and inhibit spoilage while ensuring their freshness and nutritional value.

[0003] Edible films are a common method for preserving fruits and vegetables. They form a barrier to prevent moisture loss and oxidation, thus slowing down the aging process and maintaining freshness and color. Simultaneously, they act as a barrier, regulating the entry and exit of gases from fruits and vegetables, preventing the loss of moisture, oil, and flavor compounds, and enhancing the commercial value of the food. Polysaccharides are natural polymers commonly used in the preparation of edible films, possessing excellent film-forming properties, antibacterial properties, stability, and barrier properties, which can improve food protection performance and extend shelf life. Chitosan, a natural polysaccharide derived from shrimp and crab shells, is widely used in food packaging and preservation due to its biodegradability, non-toxicity, antibacterial properties, and good film-forming properties. However, pure chitosan films suffer from low mechanical strength, insufficient gas barrier properties, weak moisture resistance and antioxidant properties, a narrow antibacterial spectrum, poor heat-sealing properties, crystallinity, and low porosity, reducing their selective adsorption capacity for gases and limiting their application in the packaging field.

[0004] Two-dimensional covalent organic frameworks (2D COFs), as emerging porous materials, possess highly ordered pore structures, large specific surface areas, and tunable chemical properties, effectively improving the physical and functional characteristics of thin film materials, such as enhancing gas selective permeability, adsorption, or catalytic activity. COF materials are insoluble in water and common organic solvents, preventing migration into fruits and vegetables and thus avoiding safety issues. Furthermore, they exhibit minimal adsorption capacity loss after repeated adsorption-desorption processes, allowing for recycling and reuse. In recent years, composite materials of 2D COFs and biopolymers have become a research hotspot in high-performance packaging materials. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a chitosan-polysaccharide composite film containing two-dimensional COFs.

[0006] The present invention also aims to provide a chitosan-polysaccharide composite film containing two-dimensional COFs prepared by the above method.

[0007] The final objective of this invention is to provide the application of the chitosan-polysaccharide composite film containing two-dimensional COFs in olive preservation.

[0008] The first objective of this invention can be achieved by the following technical solution: a method for preparing a chitosan-polysaccharide composite film containing two-dimensional COFs, comprising the following steps:

[0009] (1) Preparation of two-dimensional COFs membranes by surfactant-assisted interfacial polymerization:

[0010] (1.1) Dissolve monomer a in an aqueous solution of p-toluenesulfonic acid (PTSA) and sonicate to dissolve it completely to obtain a p-toluenesulfonic acid solution of monomer a, wherein monomer a is an amine substance;

[0011] (1.2) Add monomer b and organic acid to ultrapure water and sonicate to dissolve them completely to obtain an organic acid solution of monomer b, wherein monomer b is an aldehyde.

[0012] (1.3) Dissolve the surfactant in the solvent, place the resulting surfactant solution on the water-air interface of ultrapure water, let it stand, wait for the solvent to evaporate completely, inject the toluenesulfonic acid solution of monomer a prepared in step (1.1) into the resulting solution, let it stand, then inject the organic acid solution of monomer b prepared in step (1.2) according to the molar ratio of aldehyde group in monomer b to amino group in added monomer a, react at room temperature for a period of time, and obtain a two-dimensional COFs film on the air-water interface;

[0013] (2) Preparation of chitosan-polysaccharide composite membrane:

[0014] Chitosan (CS) was dissolved in an aqueous solution of glacial acetic acid and stirred to obtain a chitosan solution. The polysaccharide was mixed with the chitosan solution and stirred to obtain a chitosan-polysaccharide composite emulsion. The chitosan-polysaccharide composite emulsion was cast, dried, cooled, and peeled off to obtain a chitosan-polysaccharide-based composite film.

[0015] (3) Set the two-dimensional COFs film prepared in step (1) on the chitosan-polysaccharide composite film prepared in step (2) to obtain the chitosan-polysaccharide composite film containing two-dimensional COFs.

[0016] In the above method for preparing chitosan-polysaccharide-based composite films containing two-dimensional COFs:

[0017] Optionally, the amine substance mentioned in step (1.1) is one or more of tetra(4-aminophenyl)methane, 1,3,6,8-tetra(4-aminophenyl)pyrene, 1,1,2,2-tetra(4-aminophenyl)ethylene, N,N,N',N'-tetra(p-aminophenyl)p-phenylenediamine and 1,2,4,5-phenyltetramine tetrahydrochloride.

[0018] Optionally, the concentration of the p-toluenesulfonic acid solution of monomer a in step (1.1) is 1 to 2 mg / mL.

[0019] Optionally, the concentration of the p-toluenesulfonic acid aqueous solution in step (1.1) is 0.1 to 2 mol / L.

[0020] Optionally, the aldehydes mentioned in step (1.2) are one or more of 2,5-dihydroxyterephthalaldehyde, 2,5-dimethoxybenzene-1,4-dicarboxaldehyde, 2,3-dihydroxyterephthalaldehyde, 2,6-hydroxy-1,5-dialdehydenaphthalene, 2,5-dialdehydepyrazine, 2,2'-bipyridine-5,5'-dicarboxaldehyde, and 1,4-bis(4-aldehydephenyl)benzene.

[0021] Optionally, the concentration of the organic acid solution of monomer b in step (1.2) is 0.05 to 0.1 mg / mL.

[0022] Optionally, the organic acid mentioned in step (1.2) is p-toluenesulfonic acid, benzenesulfonic acid, trifluoromethanesulfonic acid, perchloric acid, or tetrafluoroboric acid.

[0023] Optionally, the concentration of the organic acid in step (1.2) is 0.1 to 2 mol / L.

[0024] Optionally, the surfactant mentioned in step (1.3) is sodium dodecyl sulfonate (SDS), sodium tetradecyl sulfonate (STS), sodium hexadecyl sulfonate (SHS) or sodium octadecyl sulfonate (SOS).

[0025] Optionally, the solvent in step (1.3) is water, dichloromethane (DCM), chloroform, isopropanol, or ethyl acetate.

[0026] Optionally, in step (1.3), the p-toluenesulfonic acid solution of monomer a prepared in step (1.1) is injected into the resulting solution and allowed to stand for 55-65 min (preferably 60 min). Then, the organic acid solution of monomer b prepared in step (1.2) is injected in an amount equal to the molar ratio of the aldehyde group in monomer b to the amino group in monomer a. The mixture is reacted at room temperature for 6-8 days (preferably 7 days) to obtain a two-dimensional COFs membrane at the air-water interface.

[0027] Optionally, the simplified synthesis formula of COFs in step (1.3) is:

[0028]

[0029] Optionally, the volume percentage of the glacial acetic acid aqueous solution in step (2) is 0.8-1.2%, more preferably 1.0%.

[0030] Optionally, the concentration of the chitosan solution in step (2) is 0.8-1.2%, w / v.

[0031] Optionally, the polysaccharide in step (2) is sodium alginate, modified starch, gum arabic, gelatin, pectin, pullulan, hemicellulose, or nanocellulose.

[0032] Optionally, the mass ratio of chitosan to polysaccharide in step (2) is 1:1 to 5.

[0033] Further, in step (3), a layer of two-dimensional COFs film prepared in step (1) is placed on the chitosan-polysaccharide composite film prepared in step (2) and air-dried naturally to obtain a chitosan-polysaccharide composite film containing two-dimensional COFs.

[0034] The second objective of the present invention can be achieved by the following technical solution: a chitosan-polysaccharide composite film, prepared by the above method.

[0035] The last objective of the present invention can be achieved by the following technical solution: the application of the above-mentioned chitosan-polysaccharide composite film in olive preservation.

[0036] Alternatively, the chitosan-polysaccharide composite film mentioned above can be used as a preservation material for olive preservation.

[0037] Chitosan, a high-performance edible preservative, can be directly sprayed or dipped into fruits and vegetables to form a colorless, transparent, edible semi-permeable membrane on their surface. This effectively inhibits the respiration and water evaporation of fruits and vegetables, reduces cell membrane damage caused by lipid peroxidation, delays aging and spoilage, and maintains their freshness. However, chitosan films alone have problems such as low mechanical strength, insufficient gas barrier properties, weak moisture resistance and antioxidant properties, narrow antibacterial spectrum, poor heat-sealing properties, crystallinity, and low porosity, which reduce their selective adsorption capacity for gases and limit their application in the packaging field.

[0038] In this invention, polysaccharides are high-molecular polymers composed of multiple monosaccharide molecules linked by glycosidic bonds. They can form films through hydrogen bonding, and the polysaccharide films exhibit excellent barrier properties, effectively preventing the entry of gases such as oxygen. Furthermore, polysaccharides are widely available and diverse, commonly including various plant starches, alginate, and cellulose derivatives. The addition of polysaccharides significantly improves the performance of chitosan films, including enhanced mechanical strength, improved gas barrier properties, improved moisture resistance, enhanced antioxidant properties, broadened antibacterial spectrum, improved heat-sealing performance, and regulation of crystallinity and porosity. These improvements broaden the application prospects of chitosan films in the packaging field, enhancing their practicality and effectiveness in areas such as food packaging.

[0039] Chitosan-polysaccharide composite emulsions, when coated on the surface of fruits and vegetables, form a protective film that delays aging and spoilage, maintaining freshness and nutritional value. The chitosan-polysaccharide composite film possesses moisturizing, mechanical, and flexible properties, reducing moisture evaporation, preventing dryness, and protecting fruits and vegetables from stress and damage. While chitosan-polysaccharide composite films offer numerous advantages in protecting fruits and vegetables, they also have drawbacks such as poor gas permeability, antioxidant properties, and antibacterial activity. To improve performance, two-dimensional COF materials are added to enhance gas barrier properties, antioxidant properties, and antibacterial activity, thereby delaying fruit and vegetable aging, maintaining freshness and nutritional value, and improving the overall preservation effect.

[0040] Two-dimensional COFs materials have advantages such as controllable structure, high specific surface area, large pore size, and high crystallinity. By regulating the micro-gas environment through their nanoporous structure, they can reduce the cellular respiration rate of olive fruits, reduce transpiration and ethylene effects, and slow down the decay process of olives and other fruits.

[0041] Existing food preservation film materials, while extending the shelf life of food, often struggle to simultaneously meet multiple requirements such as breathability, moisture retention, and inhibition of microbial growth. This invention addresses this issue by designing a chitosan-polysaccharide composite film containing two-dimensional COFs. The aim is to combine the biocompatibility of chitosan, the excellent physicochemical properties of polysaccharides, and the superior physicochemical properties of two-dimensional COFs to overcome the limitations of traditional chitosan films and achieve highly efficient preservation of perishable foods such as olives.

[0042] Therefore, this invention develops a chitosan-polysaccharide composite film containing two-dimensional COFs by using chitosan as a base material, compounding it with polysaccharide substances to prepare a chitosan-polysaccharide composite film, and then loading a two-dimensional COFs film onto the chitosan-polysaccharide composite film. This solves the problems existing in the practical application of single chitosan films and makes full use of the advantages of all three materials. On the one hand, by utilizing the excellent film-forming properties, biocompatibility, and antibacterial activity of chitosan, the shelf life of olive fruits can be extended, protecting them from external environmental pollution and oxidation. On the other hand, by combining it with other polysaccharide substances (such as sodium alginate, gelatin, starch, cellulose, etc.), the moisture-proof performance, antioxidant properties, and mechanical properties of the film can be improved, thereby enhancing the stability and durability of the packaging material. Furthermore, by utilizing the advantages of two-dimensional COFs materials, such as controllable structure, high specific surface area, and large pore size, the micro-gas environment can be regulated through its nanoporous structure, thereby reducing the cellular respiration rate of olives and other fruits, reducing transpiration and ethylene effects, and slowing down the spoilage process of olives and other fruits. This study aims to provide a scientific basis and practical guidance for the application of composite preservatives obtained by combining COFs with chitosan and polysaccharides in the storage and preservation of olive fruits, and to further provide theoretical and technical support for promoting the development of the olive industry and expanding the sales market.

[0043] Compared with the prior art, the present invention has the following advantages:

[0044] (1) Composite material design: This invention introduces two-dimensional COFs into the chitosan-polysaccharide composite membrane system for the first time. By optimizing the preparation process, the uniform dispersion and stable combination of two-dimensional COFs in the chitosan-polysaccharide matrix are successfully achieved, forming an integrated high-performance composite membrane.

[0045] (2) Performance improvement: The composite membrane obtained has significantly improved the mechanical strength, selective permeability of oxygen and carbon dioxide and antibacterial properties of the membrane while maintaining the original biological activity of chitosan, thereby effectively controlling the respiration of olive fruit, slowing down water loss and inhibiting the reproduction of harmful microorganisms.

[0046] (3) Practical application verification: The present invention applies this novel composite film to the preservation of olive fruit. The test results prove that it can effectively extend the shelf life of olives, maintain their color, taste and nutritional value, and provide a green and efficient solution for food preservation. Attached Figure Description

[0047] Figure 1 The results of antibacterial tests on chitosan-polysaccharide composite membranes containing two-dimensional COFs in Comparative Examples 1-2 and Examples 1-8 are as follows;

[0048] Figure 2 The changes in browning index of olive fruits during storage were compared with those of chitosan-polysaccharide composite membranes containing two-dimensional COFs in Comparative Examples 1-2 and Examples 1-8 and the control group.

[0049] Figure 3 The changes in the rate of good fruit of olives during storage were compared between the chitosan-polysaccharide composite membranes containing two-dimensional COFs used in Comparative Examples 1-2 and Examples 1-8 and the control group.

[0050] Figure 4 The changes in weight loss rate of olive fruits during storage were compared with those of chitosan-polysaccharide composite membranes containing two-dimensional COFs in Comparative Examples 1-2 and Examples 1-8 and the control group.

[0051] Figure 5 The changes in total color difference ΔE of chitosan-polysaccharide composite films containing two-dimensional COFs in Comparative Examples 1-2 and Examples 1-8 and the control group olive fruits during storage;

[0052] Figure 6 The changes in respiration intensity of olive fruits during storage were compared with those of chitosan-polysaccharide composite membranes containing two-dimensional COFs in Comparative Examples 1-2 and Examples 1-8 and the control group.

[0053] Figure 7 The changes in relative conductivity of chitosan-polysaccharide composite membranes containing two-dimensional COFs in Comparative Examples 1-2 and Examples 1-8 and the control group olive fruits during storage were compared.

[0054] Figure 8 The study investigated the changes in soluble solids content of olive fruits during storage using chitosan-polysaccharide composite membranes containing two-dimensional COFs from Comparative Examples 1-2 and Examples 1-8, and the control group.

[0055] Figure 9 The changes in the acid content of olive fruits during storage were determined using chitosan-polysaccharide composite membranes containing two-dimensional COFs from Comparative Examples 1-2 and Examples 1-8, and from the control group. Detailed Implementation

[0056] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0057] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods, and the experimental materials used in the following embodiments are all available through conventional commercial channels.

[0058] This invention tests the mechanical properties, barrier properties, and antibacterial properties of chitosan-polysaccharide composite films containing two-dimensional COFs, using the following methods:

[0059] (1) Mechanical properties

[0060] According to GB / T1040.1-2018, the composite film is first cut into rectangular strips of 50mm×20mm, then its thickness is measured using a thickness gauge, and finally the mechanical properties of the composite film are tested using an electronic tensile testing machine GBH-1500N.

[0061] (2) Barrier performance

[0062] According to the national standard GB / T 1037-2021, the moisture permeability of the membrane is determined using the moisture permeation cup method and a water vapor transmission rate meter. According to GB / T 1038-2000, the gas permeability of the membrane is determined using the pressure difference method and a gas transmission rate meter.

[0063] (3) Antibacterial properties

[0064] The antibacterial experiment employed the film-based inhibition test method. 0.1 mL of bacterial suspension was transferred onto sterilized and coagulated agar medium and spread evenly. Each sample film was then placed on the agar surface, positioned in the center of the plate, and gently pressed flat. For the bacterial inhibition test, the petri dish was inverted and incubated at 37°C for 18–24 hours; the formation of inhibition zones was then observed.

[0065] Example 1

[0066] The method for preparing chitosan-polysaccharide-based composite films containing two-dimensional COFs provided in this embodiment includes the following steps:

[0067] (1) Preparation of two-dimensional COFs membranes by surfactant-assisted interfacial polymerization:

[0068] (1.1) Dissolve 20 mg of tetrakis(4-aminophenyl)methane in 20 mL of p-toluenesulfonic acid (PTSA) aqueous solution (0.1 mol / L), sonicate for 30 min, and obtain a 1 mg / mL p-toluenesulfonic acid solution of tetrakis(4-aminophenyl)methane.

[0069] (1.2) Dissolve 50 mg of 2,5-dihydroxyterephthalaldehyde in 1000 mL of p-toluenesulfonic acid (0.1 mol / L) and sonicate for 30 min to obtain a p-toluenesulfonic acid solution of 0.05 mg / mL 2,5-dihydroxyterephthalaldehyde.

[0070] (1.3) Sodium dodecyl sulfonate (SDS) was dissolved in water, and the resulting solution (20 μL, 1 mg / mL) was slowly spread on the air-water interface of ultrapure water. After standing for 30 min, the solvent was allowed to evaporate completely. Then, a solution of tetra(4-aminophenyl)methane in p-toluenesulfonic acid (200 μL, 1 mg / mL) was slowly injected into the bottom of the solution. After standing for 1 h, a solution of 2,5-dihydroxyterephthalaldehyde in p-toluenesulfonic acid (4166 μL, 0.05 mg / mL) was slowly injected into the water. The reaction was carried out at room temperature for one week, and a two-dimensional COF1 membrane was obtained at the air-water interface.

[0071] (2) Preparation of chitosan-modified starch-based composite emulsion:

[0072] Weigh 1 mL of glacial acetic acid and 100 mL of ultrapure water and mix well to prepare a 1% glacial acetic acid solution. Weigh 1 g of chitosan and dissolve it in 100 mL of the 1% glacial acetic acid solution. Stir magnetically until completely dissolved to prepare a chitosan solution with a concentration of 1.0% w / v. Then, weigh 1 g of modified starch, mix it with the chitosan CS solution and stir to obtain a chitosan CS / modified starch composite emulsion. Cast the CS / modified starch composite emulsion, dry it, cool it and peel off the film.

[0073] (3) The chitosan CS-modified starch composite membrane obtained in step (2) is loaded with a two-dimensional COF1 membrane obtained in step (1) to obtain a two-dimensional COF1-CS / modified starch-based composite membrane.

[0074] Example 2

[0075] The method for preparing chitosan-polysaccharide-based composite films containing two-dimensional COFs provided in this embodiment includes the following steps:

[0076] (1) Preparation of two-dimensional COFs membranes by surfactant-assisted interfacial polymerization:

[0077] (1.1) Dissolve 25 mg of 1,3,6,8-tetra(4-aminophenyl)pyrene in 20 mL of p-toluenesulfonic acid (PTSA) aqueous solution (0.5 mol / L), sonicate for 30 min, and completely dissolve to obtain a p-toluenesulfonic acid solution of 1.25 mg / mL of 1,3,6,8-tetra(4-aminophenyl)pyrene;

[0078] (1.2) Dissolve 60 mg of 2,5-dimethoxybenzene-1,4-dicarboxaldehyde in 1000 mL of benzenesulfonic acid aqueous solution (0.5 mol / L) and sonicate for 30 min to obtain a benzenesulfonic acid solution of 0.06 mg / mL of 2,5-dimethoxybenzene-1,4-dicarboxaldehyde.

[0079] (1.3) Sodium tetradecyl sulfonate (STS) was dissolved in dichloromethane (DCM), and the resulting solution (20 μL, 1 mg / mL) was slowly spread on the air-water interface of ultrapure water. After standing for 30 min, the solvent was allowed to evaporate completely. Then, a p-toluenesulfonic acid solution of 1,3,6,8-tetra(4-aminophenyl)pyrene (200 μL, 1.25 mg / mL) was slowly injected into the bottom of the solution. After standing for 1 h, a benzenesulfonic acid solution of 2,5-dimethoxybenzene-1,4-dicarboxaldehyde (2854 μL, 0.06 mg / mL) was slowly injected into the water. The reaction was carried out at room temperature for one week, and a two-dimensional COF2 membrane was obtained at the air-water interface.

[0080] (2) Preparation of chitosan-arabinol-based composite emulsion:

[0081] Weigh 1 mL of glacial acetic acid and 100 mL of ultrapure water and mix well to prepare a 1% glacial acetic acid solution. Weigh 1 g of chitosan and dissolve it in 100 mL of the 1% glacial acetic acid solution, stirring magnetically until completely dissolved to prepare a chitosan solution with a concentration of 1.0% w / v. Then, weigh 1.5 g of gum arabic, mix it with the CS solution and stir to obtain a CS / gum arabic composite emulsion. Cast the CS / gum arabic composite emulsion, dry it, cool it, and peel off the film.

[0082] (3) Load the CS / gum arabic composite membrane obtained in step (2) with a layer of two-dimensional COF2 membrane obtained in step (1) to obtain a two-dimensional COF2-CS / gum arabic composite membrane.

[0083] Example 3

[0084] The method for preparing chitosan-polysaccharide-based composite films containing two-dimensional COFs provided in this embodiment includes the following steps:

[0085] (1) Preparation of two-dimensional COFs membranes by surfactant-assisted interfacial polymerization:

[0086] (1.1) Dissolve 30 mg of 1,1,2,2-tetra(4-aminophenyl)ethylene in 20 mL of p-toluenesulfonic acid (PTSA) aqueous solution (1 mol / L), sonicate for 30 min, and completely dissolve to obtain a p-toluenesulfonic acid solution of 1.5 mg / mL 1,1,2,2-tetra(4-aminophenyl)ethylene;

[0087] (1.2) Dissolve 70 mg of 2,3-dihydroxyterephthalaldehyde in 1000 mL of trifluoromethanesulfonic acid aqueous solution (1 mol / L) and sonicate for 30 min to obtain a trifluoromethanesulfonic acid solution of 0.07 mg / mL of 2,3-dihydroxyterephthalaldehyde.

[0088] (1.3) Sodium hexadecyl sulfonate (SHS) was dissolved in chloroform (trichloromethane), and the resulting solution (20 μL, 1 mg / mL) was slowly spread on the air-water interface of ultrapure water and allowed to stand for 30 min to allow the solvent to evaporate completely. Then, a p-toluenesulfonic acid solution of 1,1,2,2-tetra(4-aminophenyl)ethylene (200 μL, 1.5 mg / mL) was slowly injected into the bottom of the solution and allowed to stand for 1 h. Then, a trifluoromethanesulfonic acid solution of 2,3-dihydroxyterephthalaldehyde (3628 μL, 0.07 mg / mL) was slowly injected into the water. The reaction was carried out at room temperature for one week, and a two-dimensional COF3 membrane was obtained at the air-water interface.

[0089] (2) Preparation of chitosan-gelatin-based composite emulsion:

[0090] Weigh 1 mL of glacial acetic acid and 100 mL of ultrapure water and mix well to prepare a 1% glacial acetic acid solution. Weigh 1 g of chitosan and dissolve it in 100 mL of the 1% glacial acetic acid solution, stirring magnetically until completely dissolved to prepare a chitosan solution with a concentration of 1.0% w / v. Then, weigh 2 g of gelatin, mix it with the CS solution and stir to obtain a CS / gelatin composite emulsion. Cast the CS / gelatin composite emulsion, dry it, cool it, and peel off the film.

[0091] (3) Load the CS / gelatin composite membrane obtained in step (2) with a layer of two-dimensional COF3 membrane obtained in step (1) to obtain a two-dimensional COF3-CS / gelatin composite membrane.

[0092] Example 4

[0093] The method for preparing chitosan-polysaccharide-based composite films containing two-dimensional COFs provided in this embodiment includes the following steps:

[0094] (1) Preparation of two-dimensional COFs membranes by surfactant-assisted interfacial polymerization:

[0095] (1.1) Weigh 35 mg of N,N,N',N'-tetra(p-aminophenyl)p-phenylenediamine and dissolve it in 20 mL of p-toluenesulfonic acid (PTSA) aqueous solution (2 mol / L). Sonicate for 30 min to fully dissolve and obtain a p-toluenesulfonic acid solution of N,N,N',N'-tetra(p-aminophenyl)p-phenylenediamine.

[0096] (1.2) Dissolve 80 mg of 2,6-hydroxy-1,5-dialdehyde naphthalene in 1000 mL of perchloric acid aqueous solution (2 mol / L) and sonicate for 30 min to obtain a perchloric acid solution of 0.08 mg / mL 2,6-hydroxy-1,5-dialdehyde naphthalene;

[0097] (1.3) Sodium octadecyl sulfonate (SOS) was dissolved in isopropanol, and the resulting solution (20 μL, 1 mg / mL) was slowly spread on the air-water interface of ultrapure water. After standing for 30 min, the solvent was allowed to evaporate completely. Then, a p-toluenesulfonic acid solution of N,N,N',N'-tetra(p-aminophenyl)p-phenylenediamine (200 μL, 1.75 mg / mL) was slowly injected into the bottom of the solution. After standing for 1 h, a perchloric acid solution of 2,6-hydroxy-1,5-dialdehyde naphthalene (4003 μL, 0.08 mg / mL) was slowly injected into the water. The reaction was carried out at room temperature for one week, and a two-dimensional COF4 membrane was obtained at the air-water interface.

[0098] (2) Preparation of chitosan-pectin-based composite emulsion:

[0099] Weigh 1 mL of glacial acetic acid and 100 mL of ultrapure water and mix well to prepare a 1% glacial acetic acid solution. Weigh 1 g of chitosan and dissolve it in 100 mL of the 1% glacial acetic acid solution, stirring magnetically until completely dissolved to prepare a chitosan solution with a concentration of 1.0% w / v. Then, weigh 2.5 g of pectin, mix it with the CS solution and stir to obtain a CS / pectin composite emulsion. Cast the CS / pectin composite emulsion, dry it, cool it, and peel off the film.

[0100] (3) Load the CS / pectin composite membrane obtained in step (2) with a layer of two-dimensional COF4 membrane obtained in step (1) to obtain a two-dimensional COF4-CS / pectin-based composite membrane.

[0101] Example 5

[0102] The method for preparing chitosan-polysaccharide-based composite films containing two-dimensional COFs provided in this embodiment includes the following steps:

[0103] (1) Preparation of two-dimensional COFs membranes by surfactant-assisted interfacial polymerization:

[0104] (1.1) Dissolve 40 mg of 1,2,4,5-phenyltetramine tetrahydrochloride in 20 mL of p-toluenesulfonic acid (PTSA) aqueous solution (2 mol / L), sonicate for 30 min, and completely dissolve to obtain a p-toluenesulfonic acid solution of 2 mg / mL 1,2,4,5-phenyltetramine tetrahydrochloride;

[0105] (1.2) Dissolve 90 mg of 2,5-dialdehyde pyrazine in 1000 mL of tetrafluoroboric acid aqueous solution (2 mol / L) and sonicate for 30 min to obtain a tetrafluoroboric acid solution of 0.09 mg / mL 2,5-dialdehyde pyrazine.

[0106] (1.3) Sodium octadecyl sulfonate (SOS) was dissolved in ethyl acetate, and the resulting solution (20 μL, 1 mg / mL) was slowly spread onto the air-water interface of ultrapure water and allowed to stand for 30 min to allow the solvent to evaporate completely. Then, a p-toluenesulfonic acid solution of 1,2,4,5-phenyltetramine tetrahydrochloride (200 μL, 2 mg / mL) was slowly injected into the bottom of the solution and allowed to stand for 1 h. Then, a tetrafluoroboric acid solution of 2,5-dialdehyde pyrazine (6928 μL, 0.09 mg / mL) was slowly injected into the water. The reaction was carried out at room temperature for one week to obtain a two-dimensional COF5 membrane at the air-water interface.

[0107] (2) Preparation of chitosan-pullulan polysaccharide composite emulsion:

[0108] Weigh 1 mL of glacial acetic acid and 100 mL of ultrapure water and mix well to prepare a 1% glacial acetic acid solution. Weigh 1 g of chitosan and dissolve it in 100 mL of the 1% glacial acetic acid solution, stirring magnetically until completely dissolved to prepare a chitosan solution with a concentration of 1.0% w / v. Then, weigh 3 g of pullulan, mix it with the CS solution and stir to obtain a CS / pullulan composite emulsion. Cast the CS / pullulan composite emulsion, dry it, cool it, and peel off the film.

[0109] (3) Load the CS / pullulan polysaccharide composite membrane obtained in step (2) with a two-dimensional COF5 membrane obtained in step (1) to obtain a two-dimensional COF5-CS / pullulan polysaccharide composite membrane.

[0110] Example 6

[0111] The method for preparing chitosan-polysaccharide-based composite films containing two-dimensional COFs provided in this embodiment includes the following steps:

[0112] (1) Preparation of two-dimensional COFs membranes by surfactant-assisted interfacial polymerization:

[0113] (1.1) Dissolve 20 mg of tetrakis(4-aminophenyl)methane in 20 mL of p-toluenesulfonic acid (PTSA) aqueous solution (1 mol / L), sonicate for 30 min, and completely dissolve to obtain a 1 mg / mL p-toluenesulfonic acid solution of tetrakis(4-aminophenyl)methane;

[0114] (1.2) Dissolve 100 mg of 2,2'-bipyridine-5,5'-dicarboxaldehyde in 1000 mL of p-toluenesulfonic acid aqueous solution (1 mol / L) and sonicate for 30 min to obtain a p-toluenesulfonic acid solution of 0.1 mg / mL 2,2'-bipyridine-5,5'-dicarboxaldehyde;

[0115] (1.3) Sodium hexadecyl sulfonate (SHS) was dissolved in chloroform (trichloromethane), and the resulting solution (20 μL, 1 mg / mL) was slowly spread on the air-water interface of ultrapure water and allowed to stand for 30 min to allow the solvent to evaporate completely. Then, a solution of tetra(4-aminophenyl)methane in p-toluenesulfonic acid (200 μL, 1 mg / mL) was slowly injected into the bottom of the solution and allowed to stand for 1 h. Then, a solution of 2,2'-bipyridine-5,5'-dicarboxaldehyde in p-toluenesulfonic acid (2231 μL, 0.1 mg / mL) was slowly injected into the water. The reaction was carried out at room temperature for one week to obtain a two-dimensional COF6 membrane at the air-water interface.

[0116] (2) Preparation of chitosan-sodium alginate-based composite emulsion:

[0117] Weigh 1 mL of glacial acetic acid and 100 mL of ultrapure water and mix well to prepare a 1% glacial acetic acid solution. Weigh 1 g of chitosan and dissolve it in 100 mL of 1% acetic acid, stirring magnetically until completely dissolved to prepare a chitosan solution with a concentration of 1.0% w / v. Then, weigh 3.5 g of sodium alginate, mix it with the CS solution and stir to obtain a CS / sodium alginate composite emulsion. Cast the CS / sodium alginate composite emulsion, dry it, cool it, and peel off the film.

[0118] (3) Load the CS / sodium alginate composite membrane obtained in step (2) with a layer of two-dimensional COF6 membrane obtained in step (1) to obtain a two-dimensional COF6-CS / sodium alginate-based composite membrane.

[0119] Example 7

[0120] The method for preparing chitosan-polysaccharide-based composite films containing two-dimensional COFs provided in this embodiment includes the following steps:

[0121] (1) Preparation of two-dimensional COFs membranes by surfactant-assisted interfacial polymerization:

[0122] (1.1) Dissolve 30 mg of 1,3,6,8-tetra(4-aminophenyl)pyrene in 20 mL of p-toluenesulfonic acid (PTSA) aqueous solution (0.5 mol / L), sonicate for 30 min, and obtain a p-toluenesulfonic acid solution of 1.5 mg / mL of 1,3,6,8-tetra(4-aminophenyl)pyrene.

[0123] (1.2) Dissolve 50 mg of 1,4-bis(4-aldehyde phenyl)benzene in 1000 mL of benzenesulfonic acid aqueous solution (0.5 mol / L) and sonicate for 30 min to obtain a benzenesulfonic acid solution of 0.05 mg / mL 1,4-bis(4-aldehyde phenyl)benzene;

[0124] (1.3) Sodium tetradecyl sulfonate (STS) was dissolved in dichloromethane (DCM), and the resulting solution (20 μL, 1 mg / mL) was slowly spread on the air-water interface of ultrapure water and allowed to stand for 30 min to allow the solvent to evaporate completely. Then, a p-toluenesulfonic acid solution of 1,3,6,8-tetra(4-aminophenyl)pyrene (200 μL, 1.5 mg / mL) was slowly injected into the bottom of the solution and allowed to stand for 1 h. Then, a benzenesulfonic acid solution of 1,4-bis(4-aldehydephenyl)benzene (6063 μL, 0.05 mg / mL) was slowly injected into the water. The reaction was carried out at room temperature for one week, and a two-dimensional COF7 membrane was obtained at the air-water interface.

[0125] (2) Preparation of chitosan-hemicellulose-based composite emulsion:

[0126] Weigh 1 mL of glacial acetic acid and 100 mL of ultrapure water and mix well to prepare a 1% glacial acetic acid solution. Weigh 1 g of chitosan and dissolve it in 100 mL of the 1% glacial acetic acid solution, stirring magnetically until completely dissolved to prepare a chitosan solution with a concentration of 1.0% w / v. Then, weigh 4 g of hemicellulose, mix it with the CS solution and stir to obtain a CS / hemicellulose composite emulsion. Cast the CS / hemicellulose composite emulsion, dry it, cool it, and peel off the film.

[0127] (3) Load the CS / hemicellulose composite membrane obtained in step (2) with a layer of two-dimensional COF7 membrane obtained in step (1) to obtain a two-dimensional COF7-CS / hemicellulose composite membrane.

[0128] Example 8

[0129] The method for preparing chitosan-polysaccharide-based composite films containing two-dimensional COFs provided in this embodiment includes the following steps:

[0130] (1) Preparation of two-dimensional COFs membranes by surfactant-assisted interfacial polymerization:

[0131] (1.1) Dissolve 40 mg of 1,1,2,2-tetra(4-aminophenyl)ethylene in 20 mL of p-toluenesulfonic acid (PTSA) aqueous solution (2 mol / L), sonicate for 30 min, and completely dissolve to obtain a p-toluenesulfonic acid solution of 2 mg / mL 1,1,2,2-tetra(4-aminophenyl)ethylene.

[0132] (1.2) Dissolve 80 mg of 2,5-dihydroxyterephthalaldehyde in a trifluoromethanesulfonic acid aqueous solution (2 mol / L) and sonicate for 30 min to obtain a trifluoromethanesulfonic acid solution of 0.08 mg / mL 2,5-dihydroxyterephthalaldehyde.

[0133] (1.3) Sodium dodecyl sulfonate (SDS) was dissolved in water, and the resulting solution (20 μL, 1 mg / mL) was slowly spread on the air-water interface of ultrapure water and allowed to stand for 30 min to allow the solvent to evaporate completely. Then, a p-toluenesulfonic acid solution of 1,1,2,2-tetra(4-aminophenyl)ethylene (200 μL, 2 mg / mL) was slowly injected into the bottom of the solution and allowed to stand for 1 h. Then, a trifluoromethanesulfonic acid solution of 2,5-dihydroxyterephthalaldehyde (4232 μL, 0.08 mg / mL) was slowly injected into the water. The reaction was carried out at room temperature for one week, and a two-dimensional COF8 membrane was obtained at the air-water interface.

[0134] (2) Preparation of chitosan-nanocellulose-based composite emulsion:

[0135] Weigh 1 mL of glacial acetic acid and 100 mL of ultrapure water and mix well to prepare a 1% glacial acetic acid solution. Weigh 1 g of chitosan and dissolve it in 100 mL of the 1% glacial acetic acid solution, stirring magnetically until completely dissolved to prepare a chitosan solution with a concentration of 1.0% w / v. Then, weigh 5 g of nanocellulose, mix it with the CS solution and stir to obtain a CS / nanocellulose composite emulsion. Cast the CS / nanocellulose composite emulsion, dry it, cool it, and peel off the film.

[0136] (3) Load the CS / nanocellulose composite membrane obtained in step (2) with a layer of two-dimensional COF8 membrane obtained in step (1) to obtain a two-dimensional COF8-CS / nanocellulose-based composite membrane.

[0137] Comparative Example 1

[0138] Unlike Example 1, step (1) was not set, that is, no two-dimensional COF membrane was added, and no other polysaccharide substances were added in step (2).

[0139] Weigh 1 mL of glacial acetic acid and 100 mL of ultrapure water and mix well to prepare a 1% glacial acetic acid solution. Weigh 1 g of chitosan and dissolve it in 100 mL of the 1% glacial acetic acid solution. Stir magnetically until completely dissolved to prepare a chitosan solution with a concentration of 1.0% w / v, thus obtaining a chitosan emulsion. Cast the chitosan emulsion, dry it, cool it, and peel off the film.

[0140] Comparative Example 2

[0141] Unlike Example 1, step (1) was not set, that is, no two-dimensional COFs membrane was added.

[0142] Weigh 1 mL of glacial acetic acid and 100 mL of ultrapure water and mix well to prepare a 1% glacial acetic acid solution. Weigh 1 g of chitosan and dissolve it in 100 mL of the 1% glacial acetic acid solution. Stir magnetically until completely dissolved to prepare a chitosan solution with a concentration of 1.0% w / v. Then, weigh 1 g of sodium alginate, mix it with the CS solution and stir to obtain a CS / sodium alginate composite emulsion. Cast the CS / sodium alginate composite emulsion, dry it, cool it, and peel off the film.

[0143] The mechanical properties, barrier properties, and antibacterial properties of the prepared two-dimensional COFs-chitosan / polysaccharide composite membrane were tested according to the corresponding test methods. The test results are shown in Table 1 and... Figure 2 .

[0144] Table 1: Test and characterization results of relevant samples from Examples 1-8 and Comparative Examples 1-2

[0145]

[0146] Table 1 shows that the two-dimensional COFs-chitosan / polysaccharide composite membrane of the present invention exhibits good tensile strength and elongation at break. Compared with the chitosan monolayer membrane of Comparative Example 1 and the chitosan-sodium alginate composite membrane of Comparative Example 2, the tensile strength and elongation at break of the two-dimensional COFs-chitosan / polysaccharide composite membrane are significantly improved. The barrier properties show that the water vapor permeability of each membrane is approximately 15-16 g / (m·24h), which is about 3 times lower than that of the chitosan monolayer membrane, indicating good water resistance. The oxygen permeability data also demonstrate that the two-dimensional COFs-chitosan / polysaccharide composite membrane exhibits good oxygen barrier properties.

[0147] The antibacterial test results of the relevant samples from Examples 1-8 and Comparative Examples 1-2 are shown in the figure. Figure 1 .

[0148] from Figure 1 It can be seen that the chitosan monolayer membrane in Comparative Example 1 did not show a significant antibacterial effect against Escherichia coli and Staphylococcus aureus, with only a slight inhibition zone. The chitosan-sodium alginate composite membrane in Comparative Example 2 showed a significant antibacterial effect, with the inhibition zone clearly visible. Examples 1-8, containing two-dimensional COFs-chitosan / polysaccharide composite membranes, all exhibited significant inhibition zones against Escherichia coli and Staphylococcus aureus, indicating that the antibacterial effect of the three composite membranes was better than that of the chitosan monolayer membrane and the chitosan-polysaccharide membrane.

[0149] To investigate the preservation effect of two-dimensional COFs-chitosan / polysaccharide composite films, preservation experiments were conducted on olive fruits using different films.

[0150] 1. Experimental Design:

[0151] (1) Selection of olives: Select 600 olives that are uniform in size, normal in shape, without defects, without any physical damage, and with saturated color;

[0152] (2) Pretreatment of olives: The selected olives are placed in alkaline water (sodium bicarbonate) for washing;

[0153] (3) Drying treatment of olives: After pre-cooling treatment, olives are dried at room temperature so that no water droplets drip from the surface of the fruit.

[0154] (4) Packaging and storage of olives: The treated olives were randomly divided into 11 groups of 50 each. The olives were wrapped with commercially available PE preservation film and two-dimensional COF-chitosan / polysaccharide composite film (i.e., Examples 1-8 and Comparative Examples 1-2), respectively. The remaining group was left untreated as a blank control. The fruits of the above 12 groups were stored in a climate chamber at 25°C for 28 days. Samples were taken out every 7 days to detect the weight loss rate, color difference, browning rate, good fruit rate, respiration rate, relative cell membrane permeability, soluble solids content and titratable acid content of each group of olives, so as to study the quality changes of olives during the preservation process.

[0155] 1.1 Determination of olive browning index and good fruit rate

[0156] Twenty olives were randomly selected each time, and the degree of browning of the peel was divided into 6 levels according to the size of the browned area on the surface of the olives:

[0157] Grade 1 fruit: No browning;

[0158] Grade 2 fruit: Browning area < 1 / 4 of the fruit surface area;

[0159] Grade 3 fruit: 1 / 4 of the fruit surface area ≤ browning area < 1 / 2 of the fruit surface area;

[0160] Grade 4 fruit: 1 / 2 of the fruit surface area ≤ browning area < 3 / 4 of the fruit surface area;

[0161] Grade 5 fruit: 3 / 4 of the fruit surface area ≤ browning area < total fruit surface area;

[0162] Grade 6 fruit: The entire surface area of ​​the fruit turns brown.

[0163] Browning index = Σ(browning grade × number of fruits in that grade) / total number of fruits sampled.

[0164] Good fruit rate = (Number of Grade 1 fruits + Number of Grade 2 fruits / Total number of fruits) × 100%.

[0165] 1.2 Determination of olive weight loss rate

[0166] The weight loss rate of olive fruits was determined by a weighing method. Before storage, a sample mass m0 was taken, and samples were taken and weighed every 72 hours to obtain the mass m. n The mass loss value is m0 - m n Perform three repeated experiments. The calculation formula is as follows:

[0167]

[0168] 1.3 Determination of olive color difference

[0169] The measurements were performed using a CS-R precision colorimeter. The most representative color area was selected from the equatorial region of the olive fruit skin for measurement. The L*, a*, and b* values ​​of the olive fruit were measured using the colorimeter. Each group was measured in triplicate, and the average value was taken.

[0170] 1.4 Determination of Olive Respiration Intensity

[0171] Three olive fruits were randomly selected from each replicate, weighed, and placed into the breathing chamber of the breath intensity meter. The airflow pump was started, and the CO2 volume of the breathing chamber was read after the airflow rate reached 0.4 L·min⁻¹ and the reading stabilized. The CO2 volume was expressed as mg / (kg·h).

[0172] 1.5 Determination of relative permeability of olive cell membranes

[0173] Take 2.0 g of fruit slices (5 mm in diameter) from 10 olive fruits, add 25 mL of distilled water, and place at 20℃ for 3 h. After stirring evenly, measure the conductivity of the extract using a conductivity meter (C1). Then, reflux and boil the fruit slices and extract for 30 min, cool, and measure the total osmotic conductivity of the fruit slices (C2). The relative permeability of the fruit cell membrane is expressed as the relative permeability of the cell membrane. Relative cell membrane permeability = (C1 / C2) × 100%.

[0174] 1.6 Determination of soluble solids in olives

[0175] Accurately weigh 5g of pulp from 10 random olive fruits, grind it thoroughly into a paste in a mortar, bring the volume to 50mL, and determine the soluble solids content using a handheld refractometer.

[0176] 1.7 Determination of titratable acid content in olives

[0177] The titratable acid content of olives was tested using a fully automated potentiometric titrator.

[0178] 2. Preservation effect analysis:

[0179] Figure 2 and Figure 3This is a graph showing the browning index and good fruit rate of olives during storage. The control group had a browning index of 3 and a good fruit rate of 0 on the 7th day of storage, and was completely browned by the 14th day, with a browning index of 5.

[0180] The browning index of PE plastic wrap reached 1.65 on day 14, and the good fruit rate dropped to 50%; however, the good fruit rate remained at 15% by day 28. This is because PE plastic wrap can effectively prevent the entry of oxygen and water vapor compared to the control group, slowing down the rate of fruit decay and water loss, thereby maintaining the freshness of olives.

[0181] In Comparative Example 1, the chitosan single membrane resulted in a good fruit rate of 30% and a browning index of 1.9 after 28 days of storage.

[0182] The chitosan-sodium alginate composite film in the two comparative groups showed a good fruit rate of 40% and a browning index of 1.85 after 28 days of storage. This is because the chitosan single film and the chitosan-sodium alginate composite film can form a protective layer that isolates the olives from the external environment, reducing external damage and pollution to the fruit. At the same time, it has certain antibacterial properties, which can inhibit the growth of microorganisms on the surface and inside of the olives, reduce fruit decay and deterioration, and thus maintain the integrity and quality of the olives.

[0183] Examples 1-8 show that with prolonged storage, the browning index of olive peel gradually increased and the rate of good fruit gradually decreased, but the browning index remained below 1.45 and the rate of good fruit remained above 60%. This may be because the addition of two-dimensional COFs enhanced the membrane's air permeability. Two-dimensional COFs have a highly ordered pore structure, enabling free gas transfer, thereby maintaining the balance of oxygen and carbon dioxide inside the olive and preventing fruit suffocation and decay. Combined with the chitosan / polysaccharide composite membrane, it can more effectively maintain the rate of good fruit. In Example 6, the browning index was still 1.05 on day 28, and the rate of good fruit was 75%, significantly higher than the blank control group and the PE preservation group.

[0184] Figure 4As can be seen, the weight loss rate gradually increases with the extension of storage time. Within the same time period, the weight loss rate of the composite films containing two-dimensional COFs-chitosan / polysaccharide in Examples 1-8 was significantly lower than that of the blank group (P<0.05) and PE preservation film, and then lower than that of the chitosan single film in Comparative Example 1 and the chitosan-sodium alginate composite film in Comparative Example 2. This is because, compared to the blank group and PE preservation film, chitosan, as the matrix of the composite film, has good moisturizing properties, can absorb and retain the moisture inside the olives, and reduce moisture loss; the addition of sodium alginate improves the antibacterial properties of the film, inhibits the growth of bacteria and fungi, and extends the shelf life of olives. Compared to Comparative Example 1 and Comparative Example 2, the addition of two-dimensional COFs, with its highly ordered pore structure, enables free gas transfer, maintaining the balance of oxygen and carbon dioxide inside the olives. The composite film containing two-dimensional COFs-chitosan / polysaccharide can more effectively retain the moisture of olives, thereby reducing weight loss.

[0185] Figure 5 As can be seen, the total color difference ΔE value of olive fruits decreased with the extension of storage time, indicating that the browning of olive fruits intensified with the extension of storage time. The color difference change was most obvious in the blank group, followed by the PE preservation film, then the chitosan single film of Comparative Example 1 and the chitosan-sodium alginate composite film of Comparative Example 2; while the color difference change was the smallest in Examples 1-8 containing two-dimensional COFs-chitosan / polysaccharide composite films. This indicates that the good barrier properties of the two-dimensional COFs-chitosan / polysaccharide composite films can reduce oxygen penetration and the possibility of pigment oxidation in olives. Moreover, the polysaccharides have antioxidant and antibacterial properties, which are more conducive to maintaining the color brightness and stability of olives.

[0186] The intensity of respiration is closely related to the ripening and senescence of fruits and vegetables, changes in quality, and shelf life. From... Figure 6 As can be seen, all sample groups experienced a respiratory peak on day 7, which gradually decreased with the extension of storage time. The rate of decrease in the blank group, PE preservation film, comparative example 1 (chitosan single film), and comparative example 2 (chitosan-sodium alginate composite film) was significantly faster than that in Examples 1-8 (containing two-dimensional COFs-chitosan / polysaccharide composite film). After 28 days of storage, the respiratory intensity of the blank group was 1.42 mg / (kg·h), the PE preservation film was 3.22 mg / (kg·h), the comparative example 1 (chitosan single film) was 4.47 mg / (kg·h), and the comparative example 2 (chitosan-sodium alginate composite film) was 5.59 mg / (kg·h). The respiratory intensity of the two-dimensional COFs-chitosan / polysaccharide composite films remained above 9.5 mg / (kg·h), with Example 6 showing a respiratory intensity of 10.57 mg / (kg·h). This indicates that the respiration of olives can be effectively inhibited and the respiration intensity reduced under the protection of a two-dimensional COFs-chitosan / polysaccharide composite membrane, thus providing a certain degree of protection for the olives.

[0187] Cell membrane permeability reflects the integrity of fruit cell membrane structure. Cell membrane integrity can be represented by the relative permeability of the cell membrane, which is determined by measuring relative conductivity. Figure 7 It can be seen that the relative conductivity of each sample group increased with the extension of storage time. The blank group was significantly higher than other sample groups throughout the storage process, followed by the PE preservation film, then the chitosan single film of Comparative Example 1, and the chitosan-sodium alginate composite film of Comparative Example 2. The change trend of the two-dimensional COFs-chitosan / polysaccharide composite film in Examples 1-8 was relatively slow. When olives are first stored, the physiologically active substances inside the fruit begin to metabolize, leading to cell wall rupture and release of intracellular substances. The relative conductivity increases with the increase of soluble substances. Under the treatment of olive fruit with two-dimensional COFs-chitosan / polysaccharide composite film, the decomposition process of intracellular substances is slowed down, which maintains the integrity of the cell membrane structure of olive fruit to a certain extent and delays the spoilage of olive fruit.

[0188] Depend on Figure 8 It can be seen that the soluble solids content of each sample group generally increased with the extension of storage time. After 7 days of storage, the soluble solids content of the blank group was significantly higher than that of other sample groups. The soluble solids content of the two-dimensional COFs-chitosan / polysaccharide composite membranes in Examples 1-8 changed slowly and remained at a low level. This is because different treatments effectively inhibited the respiration of olive fruits, controlling the increase in the soluble solids content of olive fruits. This indicates that the two-dimensional COFs-chitosan / polysaccharide composite membranes can effectively slow down changes in olive quality, delay the ripening and senescence process of fruits to a certain extent, and improve the storage quality of fruits.

[0189] Changes in titratable acid content can reflect the degree of nutrient depletion in olives during storage. Figure 9 It can be seen that in the early stage of storage, the organic acids in olives, whose metabolism is not yet fully mature, maintain a titratable acid content of around 12.5%. However, as the storage time of olives progresses, the metabolic activity within the fruit gradually slows down, and the organic acids are gradually consumed or transformed into other substances, leading to a gradual decrease in the titratable acid content. The decrease was more significant in the control group, while the decrease was slower in the composite membrane containing two-dimensional COFs-chitosan / polysaccharide. This is because two-dimensional COFs can promote oxygen exchange and carbon dioxide release inside the olives, which helps maintain the freshness and respiration of the olives and reduces the occurrence of oxidation reactions. At the same time, the polysaccharides in the composite membrane can also increase the membrane's permeability and antibacterial properties, further extending the shelf life of olives and reducing the decrease in titratable acid content.

[0190] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a chitosan-polysaccharide-based composite film containing two-dimensional COFs, characterized in that... Includes the following steps: (1.1) Dissolve monomer a in an aqueous solution of p-toluenesulfonic acid and sonicate to dissolve it completely to obtain a p-toluenesulfonic acid solution of monomer a, wherein monomer a is an amine substance; The amine substance mentioned in step (1.1) is one or more of tetra(4-aminophenyl)methane, 1,3,6,8-tetra(4-aminophenyl)pyrene, 1,1,2,2-tetra(4-aminophenyl)ethylene, N,N,N',N'-tetra(p-aminophenyl)p-phenylenediamine and 1,2,4,5-phenyltetramine tetrahydrochloride; (1.2) Add monomer b and organic acid to ultrapure water and sonicate to dissolve them completely to obtain an organic acid solution of monomer b, wherein monomer b is an aldehyde. The aldehydes mentioned in step (1.2) are one or more of 2,5-dihydroxyterephthalaldehyde, 2,5-dimethoxybenzene-1,4-dicarboxaldehyde, 2,3-dihydroxyterephthalaldehyde, 2,6-hydroxy-1,5-dialdehydenaphthalene, 2,5-dialdehydepyrazine, 2,2'-bipyridine-5,5'-dicarboxaldehyde and 1,4-bis(4-aldehydephenyl)benzene; (1.3) Dissolve the surfactant in the solvent, place the resulting surfactant solution on the water-air interface of ultrapure water, let it stand, wait for the solvent to evaporate completely, inject the p-toluenesulfonic acid solution of monomer a prepared in step (1.1) into the resulting solution, let it stand, and then inject the organic acid solution of monomer b prepared in step (1.2) according to the molar ratio of aldehyde group in monomer b to amino group in added monomer a, react at room temperature for 6 to 8 days, and obtain a two-dimensional COFs membrane at the air-water interface; The surfactant mentioned in step (1.3) is sodium dodecyl sulfonate, sodium tetradecyl sulfonate, sodium hexadecyl sulfonate or sodium octadecyl sulfonate; (2) Preparation of chitosan-polysaccharide composite membrane: Chitosan was dissolved in glacial acetic acid aqueous solution and stirred to obtain a chitosan solution. Polysaccharides were mixed with the chitosan solution and stirred to obtain a chitosan-polysaccharide composite emulsion. The chitosan-polysaccharide composite emulsion was cast, dried, cooled and peeled off to obtain a chitosan-polysaccharide-based composite film. (3) Load the two-dimensional COFs membrane prepared in step (1) onto the chitosan-polysaccharide composite membrane prepared in step (2) to obtain the chitosan-polysaccharide composite film containing two-dimensional COFs.

2. The method for preparing chitosan-polysaccharide composite films containing two-dimensional COFs according to claim 1, characterized in that: In step (1.1), the concentration of the p-toluenesulfonic acid solution of monomer a is 1~2 mg / mL, and the concentration of the p-toluenesulfonic acid aqueous solution is 0.1~2 mol / L.

3. The method for preparing chitosan-polysaccharide composite films containing two-dimensional COFs according to claim 1, characterized in that: The concentration of the organic acid solution of monomer b in step (1.2) is 0.05~0.1 mg / mL; the organic acid is p-toluenesulfonic acid, benzenesulfonic acid or trifluoromethanesulfonic acid, and the concentration of the organic acid is 0.1~2 mol / L.

4. The method for preparing chitosan-polysaccharide composite films containing two-dimensional COFs according to claim 1, characterized in that: The solvent mentioned in step (1.3) is water, dichloromethane, chloroform, isopropanol or ethyl acetate.

5. The method for preparing chitosan-polysaccharide composite films containing two-dimensional COFs according to claim 1, characterized in that: In step (1.3), the p-toluenesulfonic acid solution of monomer a prepared in step (1.1) is injected into the resulting solution and allowed to stand for 55-65 minutes.

6. The method for preparing chitosan-polysaccharide composite films containing two-dimensional COFs according to claim 1, characterized in that: The volume percentage of the glacial acetic acid aqueous solution in step (2) is 0.8-1.2%, the concentration of the chitosan solution is 0.8-1.2% w / v, and the polysaccharide is sodium alginate, modified starch, gum arabic, gelatin, pectin, pullulan, hemicellulose, or nanocellulose.

7. The method for preparing chitosan-polysaccharide composite films containing two-dimensional COFs according to claim 1, characterized in that: In step (2), the mass ratio of chitosan to polysaccharide is 1:1~5.

8. The method for preparing chitosan-polysaccharide composite films containing two-dimensional COFs according to claim 1, characterized in that: In step (3), a layer of two-dimensional COFs film prepared in step (1) is placed on the chitosan-polysaccharide composite film prepared in step (2) and air-dried naturally to obtain a chitosan-polysaccharide composite film containing two-dimensional COFs.

9. A chitosan-polysaccharide composite film, characterized in that: It is prepared by the method described in any one of claims 1-8.

10. The application of the chitosan-polysaccharide composite film according to claim 9 in olive preservation.

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