Metal ion cross-linked nanocellulose composite film and preparation method and application thereof

Abstract

The invention provides a metal ion cross-linked nano cellulose composite film as well as a preparation method and application thereof, and belongs to the technical field of cellulose film preparation. The preparation method of the metal ion cross-linked nanocellulose composite film comprises the following steps: mixing nanocellulose dispersion liquid and nanosheet dispersion liquid to obtain mixed dispersion liquid; applying a metal ion aqueous solution to the filter membrane with the lyophilic surface to form a liquid membrane on the lyophilic surface; pouring the mixed dispersion liquid on the surface of the liquid film and standing for a period of time, super-spreading the nanosheets on the surface of the liquid film and performing oriented arrangement, and then performing cross-linking curing on the metal ions, the nanocellulose and the nanosheets to obtain a composite film; and drying the composite film to obtain the metal ion cross-linked nano cellulose composite film.

Description

Technical Field

[0001] The present invention belongs to the technical field of cellulose film preparation, and in particular relates to a metal ion cross-linked nano-cellulose composite film and a preparation method and application thereof. Background Art

[0002] Cellulose, one of the most abundant renewable resources on Earth, is a natural polymer material with a broad range of sources, a stable structure, and complete biodegradability. In recent years, due to its excellent sustainability, cellulose has been widely used in a variety of fields, including sustainable materials, biomedicine, food packaging, and functional films. With the continuous development of nanotechnology, nanocellulose materials such as cellulose nanocrystals and cellulose nanofibers have attracted widespread attention due to their high specific surface area and high strength.

[0003] However, the films constructed from nanocellulose still have problems such as insufficient mechanical strength and weak interfacial bonding, which makes it difficult to meet the actual needs of engineering applications. In order to improve the performance of such films, relevant studies have introduced inorganic nanosheets to construct a layered structure and used metal ions for cross-linking treatment to enhance the interfacial stability of the film. Despite this, the existing process still faces problems such as low cross-linking efficiency and poor dispersibility in practical applications. In view of this, it is particularly urgent to develop a preparation strategy that can achieve the coordinated construction of orderly arrangement of nanosheets and efficient ionic cross-linking for the construction of high-performance nanocellulose composite films. Summary of the Invention

[0004] In response to the above technical problems, the present invention provides a metal ion cross-linked nanocellulose composite film and its preparation method and application, in order to at least partially solve the above technical problems. The specific technical solutions provided by the present invention are as follows.

[0005] As a first aspect of the present invention, a method for preparing a metal ion cross-linked nanocellulose composite film is provided, comprising: mixing a nanocellulose dispersion and a nanosheet dispersion to obtain a mixed dispersion; applying an aqueous metal ion solution to a filter membrane having a liquid-philic surface to form a liquid film on the liquid-philic surface; pouring the mixed dispersion on the surface of the liquid film and allowing it to stand for a period of time, wherein the nanosheets are super-spread and oriented on the surface of the liquid film, and then the metal ions are cross-linked and solidified with the nanocellulose and nanosheets to obtain a composite film; and drying the composite film to obtain a metal ion cross-linked nanocellulose composite film.

[0006] As a second aspect of the present invention, a metal ion cross-linked nano-cellulose composite film is provided, which is prepared by the above preparation method.

[0007] As a third aspect of the present invention, a metal ion cross-linked nanocellulose composite film is provided for use in the fields of food packaging, biomedicine, and flexible electronics and sensors.

[0008] In embodiments of the present invention, the preparation method of "nanodispersion mixing - lyophilic filter membrane liquid film construction - superspreading orientation and ionic crosslinking" achieves multiple technical advantages: the liquid film on the lyophilic filter membrane surface induces superspreading of nanosheets, giving the film a highly ordered layered orientation structure and effectively optimizing stress transmission pathways; metal ions simultaneously crosslink with the nanocellulose and nanosheets, forming a strong interaction network and significantly improving the film's mechanical properties (tensile strength, toughness, etc.). The resulting metal ion-crosslinked nanocellulose composite film (hereinafter referred to as the composite film) combines excellent mechanical stability with structural regularity. In the food packaging field, its high strength and biodegradability can replace traditional plastic packaging, ensuring food freshness and reducing environmental residues. In the biomedical field, its excellent biocompatibility and mechanical support make it suitable for wound dressings, tissue engineering scaffolds, and other applications. In the field of flexible electronics and sensors, the ordered structure facilitates electron transport, and the metal ion crosslinking enhances stability, making it suitable for use in flexible electrodes, stress sensors, and other fields. This provides high-performance, environmentally friendly material solutions for multiple fields and promotes the engineering application of green functional materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 This is a flow chart of the preparation method of the metal ion cross-linked nanocellulose composite film of the present invention;

[0010] Figure 2 This is an optical photograph of the composite film in Example 3 of the present invention;

[0011] Figure 3 is a scanning electron microscope image of the composite film in Example 3 of the present invention;

[0012] Figure 4 This is an X-ray energy spectrum analysis diagram of the composite film in Example 3 of the present invention;

[0013] Figure 5 Graph showing tensile stress-strain curves of composite films with different sodium alginate contents in Example 3 of the present invention;

[0014] Figure 6 This is a graph showing the degradation of a composite film having a sodium alginate content of 29.6 wt % in soil in Example 3 of the present invention;

[0015] Figure 7 Graphs showing tensile stress-strain curves of the composite films of Examples 1-3 of the present invention;

[0016] Figure 83 and 1 are tensile stress-strain curves of the composite films in Example 3 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0017] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0018] In the process of realizing the concept of the present invention, it was found that nanocellulose film has broad application prospects as a green and sustainable material. However, in the existing technology, nanosheets are easy to agglomerate and arrange disorderly in the film, resulting in a loose structure. At the same time, metal ion crosslinking often has low crosslinking efficiency and weak interfacial bonding due to uneven dispersion, which ultimately results in insufficient mechanical properties and poor stability of the film, making it difficult to meet the needs of practical applications. Based on this, the present invention provides a method for preparing a metal ion crosslinked nanocellulose composite film. By regulating the synergistic effect of the ordered arrangement of nanosheets and efficient crosslinking, the lyophilic surface is used to induce the super-spreading of nanosheets to achieve orientation, and at the same time, the uniform crosslinking of metal ions is promoted to solve the above problems, thereby constructing a high-performance metal ion crosslinked nanocellulose composite film.

[0019] Figure 1 This is a flow chart of the preparation method of the metal ion cross-linked nanocellulose composite film in the present invention.

[0020] As a first aspect of the present invention, a method for preparing a metal ion cross-linked nanocellulose composite film is provided, such as Figure 1 As shown, it includes steps S1 to S4.

[0021] Step S1: mixing the nanocellulose dispersion and the nanosheet dispersion to obtain a mixed dispersion.

[0022] Step S2: applying a metal ion aqueous solution to a filter membrane having a lyophilic surface to form a liquid film on the lyophilic surface.

[0023] Step S3: pouring the mixed dispersion onto the surface of the liquid film and allowing it to stand for a period of time, wherein the nanosheets are super-spread and oriented on the surface of the liquid film, and then the metal ions are cross-linked and solidified with the nanocellulose and nanosheets to obtain a composite film.

[0024] Step S4: drying the composite film to obtain a metal ion cross-linked nanocellulose composite film.

[0025] In an embodiment of the present invention, the preparation method of the metal ion cross-linked nano-cellulose composite film provided by the present invention realizes the highly oriented arrangement of nanosheets inside the composite film through the super spreading strategy, and at the same time constructs a strong network with the help of metal ion cross-linking, giving the composite film higher mechanical properties. Specifically, the nanocellulose and nanosheets are mixed to form a dispersion, which is poured onto a filter membrane impregnated with metal ions; the shear force generated during the spreading process induces the orientation of the nanosheets, and the metal ions on the filter membrane are simultaneously cross-linked with the nanosheets and nanocellulose, fixing the oriented structure in time, and significantly reducing agglomeration and maintaining a high degree of orientation when drying. Finally, an all-natural composite film with high strength, excellent thermal stability and degradability is obtained through simple steps, which provides a very promising approach for the design of high-performance, biodegradable nanocellulose composite films.

[0026] According to an embodiment of the present invention, the nanocellulose is selected from at least one of cellulose nanocrystals and cellulose nanofibers. The nanosheets include montmorillonite nanosheets.

[0027] In this embodiment, at least one of cellulose nanocrystals and cellulose nanofibers is selected as the nanocellulose, and combined with montmorillonite nanosheets to create a composite system. This system leverages the advantages of each component: the high surface area and strength of the cellulose nanocrystals / fibers provide mechanical support for the composite film, while the layered structure of the montmorillonite nanosheets enhances barrier properties and stability. The synergistic effect of these two components, coupled with metal ion crosslinking, further strengthens interfacial interactions and structural integrity, significantly enhancing the overall performance of the composite film.

[0028] According to an embodiment of the present invention, the mass ratio of nanocellulose to nanosheets in the mixed dispersion is 2-4:1, for example, it can be 2:1, 2.5:1, 3:1, 3.5:1, or 4:1; the mass fraction of nanocellulose in the nanocellulose dispersion is 0.5-1.5wt%, for example, it can be 0.5wt%, 0.75wt%, 1wt%, 1.25wt%, or 1.5wt%; the mass fraction of nanosheets in the nanosheet dispersion is 0.5-1.5wt%, for example, it can be 0.5wt%, 0.75wt%, 1wt%, 1.25wt%, or 1.5wt%.

[0029] In the embodiments of the present invention, uniform dispersion and synergistic effects of the nanocellulose and nanosheets in the mixed system can be achieved by regulating the mass ratio of the nanocellulose and nanosheets and the concentration of their respective dispersions. At the right ratio, the nanosheets can be fully embedded in the nanocellulose network, forming a structurally stable composite system. This not only leverages the reinforcing effect of the nanosheets but also relies on the matrix support of the nanocellulose, laying a good foundation for subsequent super-spreading orientation and ionic crosslinking, thereby improving the mechanical properties and structural integrity of the composite film, and ensuring its performance stability and repeatability.

[0030] According to an embodiment of the present invention, a sodium alginate solution is further added to the mixed dispersion. During the super-spreading process, the metal ions cross-link and solidify with the nanocellulose, nanosheets, and sodium alginate. The solid content of sodium alginate in the mixed dispersion is 5-50wt%, for example, 5wt%, 10wt%, 20wt%, 30wt%, 40wt%, and 50wt%. The mass fraction of sodium alginate in the sodium alginate solution is 1-3wt%, for example, 1wt%, 1.5wt%, 2wt%, 2.5wt%, and 3wt%.

[0031] In an embodiment of the present invention, a sodium alginate solution is added to the mixed dispersion, and the cross-linking network of the composite system can be further strengthened by means of its cross-linking reaction with metal ions. The introduction of sodium alginate can form a synergistic effect with nanocellulose and nanosheets, enhance the interfacial bonding force between the components, and optimize the mechanical properties and structural stability of the composite film. By regulating the content of sodium alginate and the concentration of the solution, the degree of cross-linking can be flexibly adjusted, so that the composite film has good degradability and functionality while maintaining excellent mechanical properties, expanding its application potential in multiple fields.

[0032] According to an embodiment of the present invention, the metal ion is selected from Ca 2+ Mg 2+ 、Cu 2+ 、Zn 2+ 、Fe 3+ 、Al 3+ The concentration of the metal ion aqueous solution is 0.25-1.25 mol / L, for example, 0.25 mol / L, 0.5 mol / L, 0.75 mol / L, 1 mol / L, or 1.25 mol / L.

[0033] In an embodiment of the present invention, a variety of metal ions are selected and their aqueous solution concentrations are regulated to form a stable bond with nanocellulose, nanosheets, and sodium alginate through ionic crosslinking, thereby strengthening the network structure of the composite film. Different metal ions, with their unique coordination capabilities, can optimize crosslinking efficiency and interfacial interactions, improving the mechanical properties, thermal stability, and functionality of the film. By adjusting the ion concentration, the degree of crosslinking can be flexibly controlled, allowing the film to maintain structural integrity while adapting to the performance requirements of different application scenarios, expanding its applicability in multiple fields.

[0034] According to an embodiment of the present invention, the filter membrane is selected from any one of a cellulose acetate filter membrane, a nylon filter membrane, a polyethersulfone filter membrane, and a polytetrafluoroethylene filter membrane; when the filter membrane is selected from a polyethersulfone filter membrane and a polytetrafluoroethylene filter membrane, the filter membrane is subjected to a lyophilic treatment to obtain a lyophilic surface.

[0035] In an embodiment of the present invention, selecting a specific type of filter membrane and performing a lyophilization treatment as needed ensures the formation of a stable liquid film on the membrane surface, providing an ideal interfacial environment for super-spreading of the mixed dispersion and oriented alignment of the nanosheets. This lyophilization treatment can enhance the lyophilicity of the membrane surface, strengthen its compatibility with aqueous metal ion solutions, promote uniform spreading of the liquid film, avoid discontinuities in the liquid film due to lyophobicity, and ensure the orderly orientation of the nanosheets during super-spreading. Lyophilization treatment methods include surface grafting of hydrophilic groups, plasma treatment, and coating modification, which can be used to specifically improve the membrane surface to adapt it to system requirements.

[0036] As a second aspect of the present invention, a metal ion cross-linked nano-cellulose composite film is provided, which is prepared by the above preparation method.

[0037] In embodiments of the present invention, the metal ion-crosslinked nanocellulose composite film produced by the above-described method exhibits excellent mechanical properties, thermal stability, and biodegradability due to the highly oriented, ordered structure formed by the nanosheets during super-spreading, coupled with the efficient cross-linking of the metal ions with the various components, which creates a strong network system. This structural regularity and the synergistic interaction between the components give the composite film outstanding overall performance, meeting the demands of high-intensity use while adhering to green environmental protection concepts. It is widely applicable to a wide range of applications, including food packaging, biomedicine, and flexible electronics.

[0038] As a third aspect of the present invention, a metal ion cross-linked nanocellulose composite film is provided for use in the fields of food packaging, biomedicine, and flexible electronics and sensors.

[0039] In embodiments of the present invention, this composite film demonstrates significant technical advantages in a variety of applications: in food packaging, its high strength and biodegradability allow it to replace traditional plastics, reducing pollution while ensuring food freshness; in biomedical applications, its biocompatibility and mechanical support make it suitable for wound dressings and tissue engineering scaffolds; and in flexible electronics and sensors, its ordered structure facilitates electron transport, while metal ion crosslinking enhances stability, making it suitable for flexible electrodes and stress sensors. Its high performance and environmentally friendly properties provide green material solutions for various fields and promote industrial upgrading.

[0040] The present invention is further illustrated below by examples and related test experiments. In the detailed description below, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present invention. However, it is obvious that one or more embodiments can also be implemented without these specific details. Moreover, in the case of no conflict, the details in the following embodiments can be arbitrarily combined into other feasible embodiments. All instruments, consumables and reagents in the following examples, unless otherwise specified, can be obtained from commercial sources.

[0041] Example 1

[0042] In this Example 1, a metal ion cross-linked nanocellulose composite film was prepared by the following method.

[0043] (1) 1 wt% montmorillonite nanosheet (MMT) dispersion and 1 wt% cellulose nanocrystal (CNC) dispersion were mixed at a mass ratio of 1:3, and then ultrasonicated for 5 min to remove bubbles to obtain a uniform mixed dispersion.

[0044] (2) Place a nylon filter membrane (with a pore size of 0.22 μm) on a glass plate, and then add 1 mL of calcium chloride aqueous solution (the concentration of calcium ions in the solution is 1 mol / L) onto the filter membrane to completely soak the nylon filter membrane, thereby forming a liquid film on the surface of the filter membrane.

[0045] (3) Take 5 mL of the mixed dispersion prepared in step (1) and pour it onto the liquid film of the nylon filter membrane. The shear force generated at this time will promote the orientation of the montmorillonite nanosheets. At the same time, the calcium ions will cross-link and solidify with the montmorillonite nanosheets and cellulose nanocrystals to obtain a composite membrane.

[0046] (4) The composite film was placed in an environment of 40 °C and dried for 6 h. Finally, the film was peeled off from the filter membrane to obtain a calcium ion cross-linked montmorillonite / cellulose nanocrystal composite film (denoted as MMT / CNC).

[0047] Example 2

[0048] In this Example 2, a metal ion cross-linked nanocellulose composite film was prepared by the following method.

[0049] (1) 1 wt% of montmorillonite nanosheet dispersion, 1 wt% of cellulose nanocrystal dispersion, and 1 wt% of cellulose nanofiber (CNF) dispersion were mixed in a mass ratio of 1:2:1 for montmorillonite nanosheets, cellulose nanocrystals, and cellulose nanofibers, and then ultrasonically treated for 5 min to remove bubbles to obtain a uniform mixed dispersion.

[0050] (2) Place a nylon filter membrane (with a pore size of 0.22 μm) on a glass plate, and then add 1 mL of calcium chloride aqueous solution (the concentration of calcium ions in the solution is 1 mol / L) onto the filter membrane to completely soak the nylon filter membrane, thereby forming a liquid film on the surface of the filter membrane.

[0051] (3) Take 5 mL of the mixed dispersion prepared in step (1) and pour it onto the liquid film of the above-mentioned nylon filter membrane. The shear force generated at this time will promote the orientation of the montmorillonite nanosheets. At the same time, the calcium ions will cross-link and solidify with the montmorillonite nanosheets, cellulose nanocrystals, and cellulose nanofibers to obtain a composite membrane.

[0052] (4) The composite film was placed in an environment of 40 °C and dried for 6 h. Finally, the film was peeled off from the filter membrane to obtain a calcium ion cross-linked montmorillonite / cellulose nanocrystal / cellulose nanofiber composite film (denoted as MMT / CNC / CNF).

[0053] Example 3

[0054] In this Example 3, a metal ion cross-linked nanocellulose composite film was prepared by the following method.

[0055] (1) 1 wt% of montmorillonite nanosheet dispersion, 1 wt% of cellulose nanocrystal dispersion, and 1 wt% of cellulose nanofiber dispersion were mixed in a mass ratio of 1:2:1 for montmorillonite nanosheets, cellulose nanocrystals, and cellulose nanofibers. 2 wt% of sodium alginate (SA) solution was then added (the amount of SA added was 10-50 wt% of the mixed system). The mixture was then ultrasonically treated for 5 min to remove bubbles, resulting in a uniform mixed dispersion.

[0056] (2) Place a nylon filter membrane (with a pore size of 0.22 μm) on a glass plate, and then add 1 mL of calcium chloride aqueous solution (the concentration of calcium ions in the solution is 1 mol / L) onto the filter membrane to completely soak the nylon filter membrane, thereby forming a liquid film on the surface of the filter membrane.

[0057] (3) Take 5 mL of the mixed dispersion prepared in step (1) and pour it onto the liquid film of the above-mentioned nylon filter membrane. The shear force generated at this time will promote the orientation of the montmorillonite nanosheets. At the same time, the calcium ions will cross-link and solidify with the montmorillonite nanosheets, cellulose nanocrystals, cellulose nanofibers, and sodium alginate to obtain a composite membrane.

[0058] (4) The composite film was placed in an environment of 40 °C and dried for 6 h. Finally, the film was peeled off from the filter membrane to obtain a calcium ion cross-linked montmorillonite / cellulose nanocrystal / cellulose nanofiber / sodium alginate composite film (denoted as MMT / CNC / CNF / SA).

[0059] Figure 2 This is an optical photograph of the composite film in Example 3 of the present invention; Figure 3 is a scanning electron microscope image of the composite film in Example 3 of the present invention; Figure 4 This is an X-ray energy spectrum analysis diagram of the composite film in Example 3 of the present invention.

[0060] from Figure 2-Figure 4 It can be seen that the MMT / CNC / CNF / SA composite film has a regular layered stacking structure, each layer is tightly bonded, and the nanosheets are orderly oriented, providing structural support for the film performance; Figure 4It clearly shows the distribution of C, O, Si, and Ca elements in the composite film, verifies the effective existence and uniform dispersion of montmorillonite, cellulose, sodium alginate, and calcium ion crosslinker. The two together demonstrate the successful realization of the ordered assembly of nanocomponents and the synergistic effect of ionic crosslinking in the preparation process of the composite film from the microstructure and elemental composition levels.

[0061] Figure 5 3 is a tensile stress-strain curve of composite films with different sodium alginate contents in Example 3 of the present invention.

[0062] from Figure 5 It can be seen that with the increase of strain, the stress of each curve shows an upward trend, reflecting the mechanical response of the composite film; the corresponding curves of different sodium alginate contents (9.5%, 19.1wt%, 29.6wt%, 40.6wt%, 50.0wt%) are significantly different, reflecting that the sodium alginate content has a significant effect on the tensile properties of the composite film. The appropriate content (such as the corresponding curve of 40.6wt%) can enable the film to obtain better stress-strain performance, indicating that by regulating the sodium alginate content, the mechanical properties of the composite film can be effectively optimized.

[0063] The degradation performance of an MMT / CNC / CNF / SA composite film containing 29.6 wt% sodium alginate was further tested. The composite film was cut into 2 cm x 2 cm squares and buried at a depth of 3-5 cm in natural soil. Degradability was assessed by tracking the changes in sample size over time, with observations recorded every five days. At least three samples were randomly arranged and tested under identical conditions to ensure reproducibility.

[0064] Figure 6 This is a diagram showing the degradation of the composite film with a sodium alginate content of 29.6 wt % in soil in Example 3 of the present invention.

[0065] from Figure 6 It can be seen that the degradation residual rate of the MMT / CNC / CNF / SA composite film in the soil environment shows a clear downward trend over time. Initially (day 0), the degradation residual rate is close to 100%. This rate then gradually decreases with increasing days, such as 5 and 10 days, reaching nearly 0 by day 30. This demonstrates that the composite film exhibits good biodegradability in the soil environment, and the degradation process is regular and continuous.

[0066] Figure 7 Graph showing the tensile stress-strain curves of the composite films in Examples 1-3 of the present invention.

[0067] from Figure 7It can be seen that with the addition of cellulose nanofibers and sodium alginate, the tensile properties of the composite film can be significantly improved. In particular, the addition of sodium alginate can enable the film to obtain higher stress and strain, indicating that the addition of two types of nanocellulose and sodium alginate can effectively improve the mechanical properties of the composite film.

[0068] Comparative Example 1

[0069] In this comparative example 1, a metal ion cross-linked nanocellulose composite film was prepared by a solution casting method. The specific preparation method is as follows.

[0070] (1) 1 wt% of montmorillonite nanosheet dispersion, 1 wt% of cellulose nanocrystal dispersion, and 1 wt% of cellulose nanofiber dispersion were mixed in a mass ratio of 5:10:4. Then, 2 wt% of sodium alginate (SA) solution was added (the amount of SA added was 10-50 wt% of the mixed system), and the mixture was ultrasonically treated for 5 min to remove bubbles, thereby obtaining a uniform mixed dispersion.

[0071] (2) Take 5 mL of the mixed dispersion and pour it into a culture dish. Dry it at room temperature for 5 days, and then remove the formed film from the culture dish.

[0072] (3) The film obtained in step (2) was immersed in a calcium chloride solution with a concentration of 1 mol / L for 24 h, and then dried at room temperature for 24 h to obtain a montmorillonite / cellulose nanocrystal / cellulose nanofiber / sodium alginate composite film prepared by a solution casting method.

[0073] Figure 8 3 and 1 are tensile stress-strain curves of the composite films in Example 3 of the present invention and Comparative Example 1.

[0074] from Figure 8 As can be seen, the stress of the composite film prepared by the super-spreading method in Example 3 shows a continuous and significant upward trend with increasing strain, ultimately reaching a relatively high stress value. In contrast, the composite film prepared by the solution casting method in Comparative Example 1 shows a relatively limited increase in stress and reaches a stress plateau or decrease earlier. This clearly demonstrates that the super-spreading method can more effectively promote synergy between components when constructing composite film structures, imparting superior tensile mechanical properties to the composite film, providing a more advantageous technical path for the preparation of high-strength nanocellulose composite films.

[0075] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a metal ion cross-linked nanocellulose composite film, characterized in that: include: mixing the nanocellulose dispersion and the nanosheet dispersion to obtain a mixed dispersion; applying a metal ion aqueous solution to a filter membrane having a lyophilic surface to form a liquid film on the lyophilic surface; pouring the mixed dispersion onto the surface of the liquid film and allowing it to stand for a period of time, wherein the nanosheets are super-spread and oriented on the surface of the liquid film, and then the metal ions are cross-linked and solidified with the nanocellulose and the nanosheets to obtain a composite film; The composite film is dried to obtain a metal ion cross-linked nanocellulose composite film.

2. The preparation method according to claim 1, characterized in that The nanocellulose is selected from at least one of cellulose nanocrystals and cellulose nanofibers.

3. The preparation method according to claim 2, characterized in that The nanosheets include montmorillonite nanosheets.

4. The preparation method according to claim 1, characterized in that The mass ratio of nanocellulose to nanosheets in the mixed dispersion is 2-4:1; The mass fraction of nanocellulose in the nanocellulose dispersion is 0.5-1.5wt%; The mass fraction of the nanosheets in the nanosheet dispersion is 0.5-1.5 wt %.

5. The preparation method according to claim 1, characterized in that A sodium alginate solution is also added to the mixed dispersion. During the super-spreading process, the metal ions are cross-linked and solidified with the nanocellulose, the nanosheets and the sodium alginate.

6. The preparation method according to claim 5, characterized in that The solid content of sodium alginate in the mixed dispersion is 5-50wt%; The mass fraction of sodium alginate in the sodium alginate solution is 1-3 wt %.

7. The preparation method according to claim 1, characterized in that The metal ion is selected from Ca 2+ Mg 2+ 、Cu 2+ 、Zn 2 + 、Fe 3+ 、Al 3+ Any of the following; The concentration of the metal ion aqueous solution is 0.25-1.25 mol / L.

8. The preparation method according to any one of claims 1 to 7, characterized in that The filter membrane is selected from any one of cellulose acetate filter membrane, nylon filter membrane, polyethersulfone filter membrane, and polytetrafluoroethylene filter membrane; When the filter membrane is selected from polyethersulfone filter membrane and polytetrafluoroethylene filter membrane, the filter membrane is subjected to lyophilization treatment to obtain a lyophilic surface.

9. A metal ion cross-linked nanocellulose composite film, characterized in that: Prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the metal ion cross-linked nanocellulose composite film according to claim 9 in the fields of food packaging, biomedicine, flexible electronics and sensors.

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