Preparation method and application of double-sided heterostructure humidity triggering fresh-keeping pad based on MOFs confinement effect

Abstract

The invention belongs to the technical field of intelligent controlled-release antibacterial packaging, and particularly relates to a double-sided heterostructure humidity triggering fresh-keeping pad based on the MOFs confinement effect and a preparation method and application of the double-sided heterostructure humidity triggering fresh-keeping pad. According to the design of the fresh-keeping pad, high-sensitivity sensing of microenvironment humidity fluctuation and intelligent reversible and accurate controlled release of active ingredients are achieved, and the long-term stability of the active ingredients is remarkably improved. A double-sided heterogeneous fiber membrane design is adopted, and a stable hydrogen bond and physical cross-linked network is constructed by utilizing the interface complementary characteristic of different fiber layers, so that the material is endowed with excellent reversible swelling-shrinking capability in a wide humidity range; the structure collapse easily caused by a single material is effectively avoided, the problem of active ingredient explosive release caused by the structure collapse is thoroughly solved, and the repeatability and long-term stability of intelligent response are remarkably improved; due to the heterostructure design, one-way efficient transmission of water is ensured, water management is optimized, the overall fresh-keeping performance is further improved, and wide application prospects are achieved.

Description

Technical Field

[0001] This invention belongs to the field of intelligent controlled-release antibacterial packaging technology, specifically relating to a double-sided heterogeneous structure humidity-triggered preservation pad based on MOF confinement effect, its preparation method and application. Background Technology

[0002] As consumers increasingly demand higher levels of freshness, nutritional value, and sensory quality from fresh food, extending shelf life and preventing quality deterioration during storage, transportation, and sales have become core challenges in the food preservation industry. On the one hand, with the continuous improvement of cold chain systems, the market circulation of fresh foods such as fruits, vegetables, meat, aquatic products, and semi-processed foods has increased significantly. On the other hand, during storage, transportation, and display, packaging materials often experience condensation or liquid seepage due to mechanical compression, temperature and humidity fluctuations, and uneven gas exchange, providing an ideal environment for microbial growth. Simultaneously, increased water activity accelerates the catalytic activity of oxidases in food and promotes free radical chain reactions, leading to the oxidative loss of nutrients such as vitamins and unsaturated fatty acids. Liquid seepage also easily causes softening of the product texture and dulling of its color, significantly reducing the sensory quality and added value of the product.

[0003] Currently, commonly used food preservation packaging technologies include modified atmosphere packaging, high-barrier sealing packaging, and ethylene adsorption packaging. These technologies mainly rely on adjusting the gas composition within the packaging or improving barrier properties to slow down the respiration and aging process of food. However, they are mostly "static" protections, lacking dynamic response capabilities to changes in internal humidity. They also have insufficient coupling control over moisture fluctuations and biological metabolic processes within the microenvironment, making it difficult to achieve intelligent and precise preservation release. In recent years, the shift from static controlled release to intelligent controlled release has become a research hotspot, and humidity-responsive materials have received widespread attention due to their high sensitivity to changes in moisture within the packaging microenvironment. For example, patent CN202410800286.5 proposes using the swelling-shrinkage properties of water-soluble films to construct humidity-responsive release preservation cards. While this can extend the shelf life of fruits to some extent, its swelling behavior is irreversible. Under dynamic humidity fluctuations, it is prone to "sudden" or "delayed" release of active ingredients, leading to premature depletion or delayed response of the preservative. In addition, existing humidity trigger thresholds are high, and response sensitivity is limited by the reconstruction efficiency of hydrogen bond networks, making it difficult to achieve real-time synchronization with the physiological metabolic rhythm of food. Commonly used active components such as essential oils are prone to volatility and failure, and nano-active substances are prone to agglomeration and deposition. Traditional loading methods often embed active sites, reducing free radical scavenging and antibacterial efficiency, which restricts the stability and reusability of controlled release performance. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention aims to solve the technical bottlenecks of existing humidity-responsive preservation materials, such as excessively high trigger thresholds, irreversible release behavior, and poor stability of active components. This invention proposes a double-sided heterogeneous structure humidity-responsive preservation pad based on the MOF confinement effect. This design aims to achieve highly sensitive detection of microenvironmental humidity fluctuations and intelligent, reversible, and precise controlled release of active ingredients, while significantly improving the long-term stability of active components. Employing a double-sided heterogeneous fiber membrane design, the complementary interfacial properties of different fiber layers are utilized to construct a robust hydrogen bond and physical cross-linking network, endowing the material with excellent reversible swelling-shrinkage capabilities over a wide humidity range. This effectively avoids the structural collapse that easily occurs with single materials, completely solving the problem of explosive release of active ingredients caused by this, and significantly improving the repeatability and long-term stability of the intelligent response. This heterogeneous structure design also ensures efficient unidirectional moisture transport, optimizing moisture management and further enhancing overall preservation performance. Utilizing the hydrophilic properties and controllable swelling characteristics of ethylene-vinyl alcohol copolymer (EVOH), it can make a rapid and reversible swelling-shrinkage response to changes in microenvironmental relative humidity. This characteristic is the core mechanism for precisely regulating the humidity-triggered release threshold, enabling highly sensitive sensing of humidity fluctuations. Copper-based MOFs are loaded into the polymer fibers, fully leveraging their confinement effect. This achieves efficient adsorption and confinement of small-molecule antibacterial agents, antioxidants, and other active substances, precisely "locking" the active components into the MOF nanopores, greatly improving their stability during storage and transportation, and preventing degradation or escape. Furthermore, a dual humidity-controlled release mechanism is achieved. Under low humidity conditions, the MOF pore structure remains "closed," effectively locking in the active ingredients. When the ambient humidity increases, combined with the overall swelling response of the material, the MOF pore structure undergoes a controllable slight expansion, thereby "opening" the precise release channel and achieving intelligent and controllable release of the active ingredients. This invention, through the synergistic innovation of a double-sided heterogeneous structure design, the reversible response mechanism of EVOH, and the intelligent confined release function of CuBTC, achieves for the first time in a single material system highly sensitive humidity sensing, reversible and precise controlled release, efficient stabilization of active components, and unidirectional moisture management. This overcomes the core deficiencies of existing technologies and significantly improves the performance of intelligent preservation materials. Currently, no publicly disclosed technology can achieve intelligent humidity sensing and reversible controlled release while effectively improving the stability and cyclic response capability of active ingredients.

[0005] The present invention achieves the above-mentioned technical objectives through the following technical means:

[0006] A method for preparing a humidity-triggered food preservation pad with a double-sided heterogeneous structure based on the MOF confinement effect includes the following steps:

[0007] Step 1: Preparation of CQDs@CuBTC nanomaterials;

[0008] S1. Add bio-based material powder to deionized water, mix and transfer to a polytetrafluoroethylene reactor for hydrothermal reaction; after the reaction is completed, cool to room temperature, centrifuge and collect the supernatant; filter the supernatant through a microporous membrane, freeze dry and then re-dissolve the product in deionized water to obtain CQDs solution.

[0009] S2. Next, dissolve CQDs solution, Cu(NO3)2·3H2O and PVP in DMF to obtain a mixed solution, denoted as solution A; dissolve H3BTC in DMF to obtain a mixed solution, denoted as solution B; add solution B to solution A, stir thoroughly, and then transfer to a polytetrafluoroethylene reactor for hydrothermal reaction. After the reaction is completed, wash the product with DMF (to remove impurities), centrifuge to recover the solid after washing, and dry to obtain CQDs@CuBTC.

[0010] Furthermore, in step S1, the bio-based material is a bio-based material containing polyphenols; the ratio of the amount of bio-based material powder to deionized water and the amount of deionized water required for resolution is 2-3g: 50mL: 5-10mL; the temperature of the hydrothermal reaction is 180-200℃, and the treatment time is 4-6h; the microporous membrane is a 0.22μm microporous filter membrane.

[0011] Furthermore, in step S2 of step one, the ratio of CQDs solution, Cu(NO3)2·3H2O, DMF and PVP is 1-2 mL: 1.5-2 g: 20-25 mL: 0.5-1 g; the ratio of H3BTC to DMF is 0.5-1 g: 20-25 mL; the volume ratio of solution B added to solution A is 1:1; the temperature of the hydrothermal reaction is 70-130℃, and the treatment time is 4-6 h.

[0012] Step 2: Preparation of spinning solution for double-sided heterogeneous structure humidity-responsive food preservation pad;

[0013] S1. Dissolve ethylene-vinyl alcohol copolymer (EVOH) in N,N-dimethylacetamide (DMAc), heat in a water bath and stir until completely dissolved to obtain EVOH solution; add CQDs@CuBTC prepared in step one to EVOH solution, mix evenly and then ultrasonically disperse for degassing treatment, and the humidity-responsive inner layer spinning solution can be obtained after treatment;

[0014] S2. Dissolve cellulose acetate (CA) in glacial acetic acid, heat and stir in a water bath until completely dissolved, and then ultrasonically disperse to remove bubbles. After treatment, a hydrophobic protective outer spinning solution can be obtained.

[0015] Furthermore, in step S1 of step two, the ratio of EVOH to DMAc and CQDs@CuBTC is 2-4g: 15-25mL: 0.5-3g; the water bath heating temperature is 90-100℃; the ultrasonic power is 50-60W, the working time interval is 1s, and the processing time is 0.5-1h.

[0016] Furthermore, in step S2 of step two, the ratio of cellulose acetate (CA) to glacial acetic acid is 2-3 g: 15-25 mL; the water bath heating temperature is 50-80°C; the ultrasonic power is 50-60 W, the working time interval is 1 s, and the processing time is 0.5-1 h.

[0017] Step 3: Preparation of a double-sided heterogeneous structure humidity-responsive food preservation pad;

[0018] To construct a double-layer fiber mat, the aforementioned humidity-responsive inner layer spinning solution and hydrophobic protective outer layer spinning solution are continuously spun layer by layer using electrospinning to achieve interfacial cohesion between the inner and outer layers, thus obtaining a humidity-responsive food preservation mat with a double-sided heterogeneous fiber membrane structure based on the MOF confinement effect. The electrospinning can be performed layer by layer in either a forward or reverse order: first, the inner layer is spun with the humidity-responsive inner layer spinning solution, and then the outer layer is spun with the hydrophobic protective outer layer spinning solution; or first, the outer layer is spun with the hydrophobic protective outer layer spinning solution, and then the inner layer is spun with the humidity-responsive inner layer spinning solution.

[0019] The specific operation steps are as follows.

[0020] The humidity-responsive inner layer spinning solution obtained in step two is loaded into a syringe and installed in an electrospinning machine; the high voltage power supply voltage and feed rate are set to perform electrospinning; the spun nanofibers are received by the oil paper on the drum, and the humidity-responsive inner layer fiber membrane is obtained after the solution is fed.

[0021] After completing the spinning of the humidity-responsive inner layer to obtain the humidity-responsive inner layer fiber membrane, the hydrophobic protective outer layer spinning solution prepared in step two is loaded into a syringe, and the high voltage power supply voltage and advance rate are set to perform electrospinning of the hydrophobic protective layer; the humidity-responsive inner layer fiber membrane prepared in the above steps is used to receive the spun nanofibers. After spinning is completed, a double-sided heterostructure humidity-responsive food preservation pad containing MOF confined material is obtained.

[0022] Furthermore, in step three, the high-voltage power supply voltage for electrospinning the humidity-responsive inner spinning solution is 14–18 kV, the feed rate is 0.4–0.5 mL / h, and the electrospinning needle type is 18G; the high-voltage power supply voltage for electrospinning the hydrophobic protective outer spinning solution is 18–25 kV, the feed rate is 0.2–0.4 mL / h, and the electrospinning needle type is 20G; the spinning temperature is 20℃ and the relative humidity is 40% in both cases.

[0023] The prepared double-sided heterogeneous humidity-responsive preservation pad has a double-layer structure, consisting of a humidity-responsive inner fiber membrane (humidity-responsive inner layer) and a hydrophobic protective outer fiber membrane (hydrophobic protective layer) from the inside out. It can be used for intelligent humidity-responsive preservation of fresh food.

[0024] The humidity-responsive preservation pad based on the prepared double-sided heterogeneous fiber membrane structure is applied to the antibacterial and preservation treatment of fresh food.

[0025] Furthermore, this preservation mat is widely applicable to fresh meat (pork, beef, chicken, fish, etc.), aquatic products (shrimp, crab, shellfish, etc.), fruits and vegetables (strawberries, cherries, watermelons, tomatoes, cucumbers, lettuce, etc.), poultry and eggs, and various semi-processed foods (cured meats, sliced ​​fruits, pre-made salads, etc.). By placing it in direct contact with the inner wall of the packaging, it achieves precise and efficient preservation.

[0026] The prepared double-sided heterogeneous fiber membrane structure humidity-responsive food preservation pad is composed of an inner humidity-responsive membrane and an outer hydrophobic protective membrane. The inner layer is a humidity-responsive fiber membrane made of EVOH matrix and MOF confinement effect material, which can rapidly swell or shrink when the humidity of the packaging microenvironment changes. When the humidity increases, the EVOH molecular chains form microchannels, which significantly reduce the release resistance of active components and accelerate the on-demand release of antibacterial / antioxidant substances, thereby inhibiting microbial growth in a timely manner during high-risk periods. The confinement channels of MOFs undergo reversible deformation with humidity fluctuations, which not only locks in the active molecules for stable storage, but also precisely controls the release rate, avoiding premature depletion or delayed response. The outer layer is a hydrophobic protective layer made of CA matrix hydrophobic fibers, which comes into direct contact with fresh food and can effectively block the seepage of liquid inside the packaging and external microbial contamination. While ensuring structural stability, it does not adsorb or weaken the active molecules released from the inner layer, ensuring that the antibacterial agent can fully exert its effect. In addition, the unique asymmetric wettability of the bilayer fiber membrane's heterogeneous structure creates a unidirectional moisture transport gradient of "hydrophobic outer layer → hydrophilic inner layer", which can quickly expel excess water vapor and maintain suitable humidity inside the packaging.

[0027] The beneficial effects of this invention are:

[0028] (1) This invention employs a confined synergistic design of a bilayer heterogeneous fiber membrane and metal-organic frameworks (MOFs) to achieve low-threshold triggered release of antibacterial active substances within a specific relative humidity range, thereby significantly improving the flexibility and release accuracy of the food preservation pad. The inner layer is made of ethylene-vinyl alcohol copolymer (EVOH), whose molecular backbone is rich in hydrophilic hydroxyl groups. When the relative humidity of the environment increases slightly from below the threshold to above the threshold, EVOH rapidly absorbs water and swells. Based on hydration, the hydrogen bonds between polymer chains are broken, forming a hydrated layer and expanding the free volume, increasing the spacing between polymer chains and opening microchannels, thus providing the packaging film with instantaneous humidity-controlled release channels. When the ambient humidity decreases, the adsorbed water in EVOH is desorbed, the polymer chains reassociate, the hydrogen bonds are restored, the microchannels close and cut off the release channels, realizing the humidity-driven precise "on / off" controlled release function. Meanwhile, MOFs loaded with CQDs construct a two-dimensional confined space. Under high humidity conditions, the water adsorbed within the MOF channels enhances the polarization of the pore walls, reduces the interaction forces between active molecules and the pore walls, and promotes molecular diffusion and release. Under low humidity conditions, the reduced moisture causes the pores to "contract" and strengthens the interaction between molecules and the MOF framework, inhibiting the escape of antibacterial agents, "locking in" active substances, and achieving precise cutoff. This enables rapid humidity sensing, confined amplification, and precise quantification of active ingredients.

[0029] (2) This invention combines a highly elastic EVOH fiber layer with porous MOFs, relying on the rapid swelling-shrinkage of EVOH and the opening and closing of the microscopic pores of MOFs to achieve multiple intelligent releases of active ingredients without collapse during wet-dry cycles. In high humidity, the macroscopic channels of EVOH open first, significantly increasing the molecular permeation rate; simultaneously, the pores of MOFs widen their confinement, promoting the diffusion of active substances; when humidity decreases, EVOH immediately closes the macroscopic channels, and the pores of MOFs simultaneously "lock" the drug loading, achieving precise interception. This synergistic design of a dual-layer, dual-mechanism not only significantly reduces the humidity trigger threshold but also ensures reversible recycling and high stability under fluctuating humidity conditions through the complementary relationship between molecules and structures.

[0030] (3) This invention achieves efficient dispersion and long-term stability of quantum dots by in-situ confining carbon quantum dots (CQDs) within the ordered nanopores of metal-organic frameworks (MOFs). The CQDs and metal centers within the pores enhance electron-hole separation efficiency and prolong free radical lifetime through hydrogen bonds or coordination bonds, thereby significantly improving antioxidant performance. In high humidity environments, the opening of MOF pores promotes the synergistic diffusion and release of reactive oxygen species (ROS) and metal ions, rapidly destroying bacterial membrane structures and proteins. In low humidity environments, the pores close to inhibit excessive release, achieving a continuous and controllable antibacterial effect. The humidity-induced release of metal ions and the dual bactericidal mechanism of ROS generated by CQDs photoexcitation further improve antibacterial efficiency, constructing a humidity-responsive precision control mode for antibacterial and antioxidant effects, significantly extending the shelf life of food and showing broad application prospects.

[0031] (4) The electrospinning technology used in this invention is not only mature and widely available, but also highly modular and scalable. It can meet the continuous production needs of batch manufacturing on conventional electrospinning and multi-unit, online pretreatment and cutting modular production lines, significantly improving capacity and consistency. The spun product is a nanoscale random fiber nonwoven fabric, compatible with a variety of substrates, and can be arbitrarily cut to suit various packaging forms such as fresh meat, fish and shrimp, fruits and vegetables, poultry and eggs and pre-made foods, thus achieving comprehensive coverage of "one pad for multiple uses". By adjusting the fiber composition of the inner and outer layers and the MOF loading online, the respiration rate and water activity requirements of different foods can be precisely matched, supporting personalized customization design. In addition, electrospinning equipment has been widely deployed in medical, industrial and laboratory fields, and the overall preparation cost is lower than that of traditional membrane materials and lamination processes. It is easy to integrate with existing components such as cling film and packaging boxes, showing excellent process feasibility and promotion value. Attached Figure Description

[0032] Figure 1 This is a schematic diagram showing the composition and structure of the humidity-responsive food preservation pad with a double-sided heterogeneous fiber membrane structure prepared in Example 1 of the present invention.

[0033] Figure 2 This is a graph showing the release rate of active ingredients in different humidity microenvironments for the double-sided heterogeneous fiber membrane structure humidity-responsive food preservation pad prepared in Example 1 of the present invention.

[0034] Figure 3 This is a dynamic water droplet contact angle diagram of the inner and outer layers of the humidity-responsive food preservation pad with a double-sided heterogeneous fiber membrane structure prepared in Example 2 of the present invention.

[0035] Figure 4 This is a SEM image of the fiber morphology and MOF distribution of the humidity-responsive double-sided heterogeneous fiber membrane structure food preservation pad prepared in Example 3 of the present invention.

[0036] Figure 5Detection of total volatile basic nitrogen (TVB-N) content in a humidity-responsive double-sided heterogeneous fiber membrane structure preservation pad for the preservation of chilled pork. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0038] like Figure 1 As shown, the present invention provides a double-sided heterogeneous structure humidity-triggered food preservation pad based on MOF confinement effect, which consists of an inner EVOH humidity-responsive film and an outer CA hydrophobic protective film from the inside out.

[0039] Example 1:

[0040] A method for preparing a humidity-triggered food preservation pad with a double-sided heterogeneous structure based on the MOF confinement effect includes the following steps:

[0041] Step 1: Preparation of CQDs@CuBTC nanomaterials, with grapefruit peel chosen as the biomaterial substrate;

[0042] Take 2g of grapefruit peel powder, add it to 50mL of deionized water, mix, and transfer it to a polytetrafluoroethylene reactor; hydrothermally react at 180℃ for 4h. After the reaction is complete, cool to room temperature, centrifuge and collect the supernatant; filter the supernatant through a 0.22μm microporous membrane, freeze-dry the obtained dried product, and redissolve it in 5mL of deionized water to obtain a CQDs solution.

[0043] The above 1 mL CQDs solution, 1.5 g Cu(NO3)2·3H2O, and 0.5 g PVP were dissolved in 20 mL DMF, denoted as solution A. 0.5 g H3BTC was dissolved in 20 mL DMF, denoted as solution B. Solution B was quickly added to solution A at a 1:1 volume ratio, and after thorough stirring, the mixture was transferred to a polytetrafluoroethylene (PTFE) reactor and hydrothermally reacted at 70 °C for 4 h. After the reaction was complete, the product was washed three times with DMF to remove impurities, the solid was recovered by centrifugation, and dried in an oven to obtain CQDs@CuBTC powder.

[0044] Step 2: Preparation of spinning solution for double-sided heterogeneous structure humidity-triggered food preservation pad;

[0045] Dissolve 2g of ethylene-vinyl alcohol copolymer (EVOH) in 15mL of DMAc, heat in a 90℃ water bath and stir until completely dissolved to obtain an EVOH solution; add 0.5g of CQDs@CuBTC, mix evenly and then ultrasonically disperse for degassing treatment. The ultrasonic power is 50W, the working time interval is 1s, and the treatment time is 0.5h. After treatment, the humidity-responsive inner layer spinning solution can be obtained.

[0046] Dissolve 2g of cellulose acetate (CA) in 15mL of glacial acetic acid, heat and stir in a 50℃ water bath until completely dissolved, and then perform ultrasonic dispersion for degassing treatment. The ultrasonic power is 50W, the working time interval is 1s, and the treatment time is 0.5h. After treatment, the hydrophobic protective outer spinning solution can be obtained.

[0047] Step 3: Preparation of a double-sided heterogeneous structure humidity-triggered food preservation pad;

[0048] The double-sided heterogeneous structure humidity-triggered food preservation pad consists of a humidity-responsive inner layer and a hydrophobic protective outer layer.

[0049] First, the humidity-responsive inner layer spinning solution prepared in step two is loaded into a syringe, installed on an electrospinning machine, and connected to an 18G needle; the high voltage power supply voltage is set to 14kV, the feed rate is 0.4mL / h, and the distance between the needle and the collector is 15cm, and electrospinning is performed; the spun nanofibers are received by the oil paper on the drum, and the humidity-responsive inner layer fiber membrane is obtained after the solution is pushed out.

[0050] After completing the humidity-responsive inner fiber membrane, the hydrophobic protective outer spinning solution prepared in step two is loaded into a syringe and connected to a 20G needle. The high-voltage power supply is set to 18kV, the injection rate is 0.2mL / h, and the distance between the needle and the collector is 15cm. Electrospinning of the hydrophobic protective outer layer is then performed. Using the humidity-responsive inner fiber membrane as a receiving substrate, it is spun onto the surface of the humidity-responsive inner fiber membrane to form a firmly adhered hydrophobic protective fiber layer. After spinning is completed, a double-sided heterogeneous structure humidity-triggered food preservation pad is obtained.

[0051] To quantitatively evaluate the release behavior of CQDs@CuBTC in a double-sided heterogeneous structure food preservation pad under different relative humidity conditions, this study constructed a bilayer fiber membrane using electrospinning technology and placed it in a constant temperature environment at 4℃. Release experiments were conducted under set RH conditions of 33%, 75%, 85%, and 100%. The concentration of CQDs@CuBTC in the supernatant was determined by timed sampling and UV-Vis spectrophotometry to plot the cumulative release curve. Figure 2 As shown, the material exhibits significantly enhanced release performance under increasing humidity conditions: as RH increases from 33% to 100%, the cumulative release amount continuously rises, with both the release rate and total release showing a positive correlation. Particularly under 100% RH conditions, the cumulative release amount within 7 days is nearly 60% or more, approximately four times higher than the 15% at 33% RH, fully validating the high responsiveness of the food preservation pad to humidity changes and its excellent release regulation capability. The fitted curves and the 95% confidence interval and prediction interval further confirm the reliability and trend stability of the experimental data, indicating that the system possesses stable and efficient triggered release behavior in high humidity environments.

[0052] Example 2:

[0053] A method for preparing a humidity-triggered food preservation pad with a double-sided heterogeneous structure based on the MOF confinement effect includes the following steps:

[0054] Step 1: Preparation of CQDs@CuBTC nanomaterials, with orange peel as the selected biomaterial substrate;

[0055] Take 3g of orange peel powder, add it to 50mL of deionized water, mix, and transfer it to a polytetrafluoroethylene reactor; hydrothermally react at 200℃ for 6h. After the reaction is complete, cool to room temperature, centrifuge and collect the supernatant; filter the supernatant through a 0.22μm microporous membrane, freeze-dry, and redissolve in 10mL of deionized water to obtain the CQDs solution.

[0056] 2 mL of the above CQDs solution, 2 g of Cu(NO3)2·3H2O, and 1 g of PVP were dissolved in 25 mL of DMF, denoted as solution A. 1 g of H3BTC was dissolved in 25 mL of DMF, denoted as solution B. Solution B was quickly added to solution A at a 1:1 volume ratio, and after thorough stirring, the mixture was transferred to a polytetrafluoroethylene (PTFE) reactor and hydrothermally reacted at 130 °C for 6 h. After the reaction was completed, the product was washed three times with DMF to remove impurities, the solid was recovered by centrifugation, and dried in an oven to obtain CQDs@CuBTC powder.

[0057] Step 2: Preparation of spinning solution for double-sided heterogeneous structure humidity-triggered food preservation pad;

[0058] 4g of ethylene-vinyl alcohol copolymer (EVOH) was dissolved in 25mL of DMAc, heated in a 90℃ water bath and stirred until completely dissolved to obtain an EVOH solution; 3g of CQDs@Cu-MOF was added to the solution, mixed evenly and then ultrasonically dispersed for degassing treatment. The ultrasonic power was 60W, the working time interval was 1s, and the treatment time was 1h. After treatment, the humidity-responsive inner layer spinning solution can be obtained.

[0059] Dissolve 3g of cellulose acetate (CA) in 25mL of glacial acetic acid, heat and stir in an 80℃ water bath until completely dissolved, and then degas using ultrasonic dispersion. The ultrasonic power is 60W, the working time interval is 1s, and the treatment time is 1h. After treatment, the hydrophobic protective outer spinning solution can be obtained.

[0060] Step 3: Preparation of a double-sided heterogeneous structure humidity-triggered food preservation pad;

[0061] The double-sided heterogeneous structure humidity-triggered food preservation pad consists of a humidity-responsive inner layer and a hydrophobic protective outer layer.

[0062] First, the humidity-responsive inner layer spinning solution prepared in step two is loaded into a syringe, installed on an electrospinning machine, and connected to an 18G needle; the high voltage power supply voltage is set to 18kV, the feed rate is 0.5mL / h, and the distance between the needle and the collector is 15cm, and electrospinning is performed; the spun nanofibers are received by the oil paper on the drum, and the humidity-responsive inner layer fiber membrane is obtained after the solution is pushed out.

[0063] After completing the humidity-responsive inner fiber membrane, the hydrophobic protective outer spinning solution prepared in step two is loaded into a syringe and connected to a 20G needle. The high-voltage power supply is set to 25kV, the injection rate is 0.4mL / h, and the distance between the needle and the collector is 15cm. Electrospinning of the hydrophobic protective outer layer is then performed. Using the humidity-responsive inner fiber membrane as a receiving substrate, it is spun onto the surface of the humidity-responsive inner fiber membrane to form a firmly adhered hydrophobic protective fiber layer. After spinning is completed, a double-sided heterogeneous structure humidity-triggered food preservation pad is obtained.

[0064] To verify the asymmetric behavior of the prepared Janus-structured food preservation pad in terms of interfacial wettability and to systematically evaluate its surface diffusion and permeation characteristics under droplet action, this study used contact angle testing to analyze the differences in hydrophilic and hydrophobic properties of the bilayer fiber membrane. The membrane was laid flat on a horizontal platform, ensuring that the inner layer (EVOH) and the outer layer (CA) were exposed to air. 5 μL of deionized water was dropped onto the surface of each layer, and the spreading behavior and contact angle changes of the water droplets within 0–120 s were recorded using side-view imaging. Figure 3 A significant Janus interface effect was observed. The inner EVOH layer exhibited high hydrophilicity, with an initial contact angle of 47.8°, which rapidly decreased to 37.4° within 10 s and further to 12.5° by 120 s. Water droplets quickly spread into a thin film but did not penetrate to the back side of the membrane, indicating good surface wettability and intralayer liquid retention capacity. In stark contrast, the outer CA layer had an initial contact angle as high as 123.1°, and the droplets remained almost spherical for the first 30 s, indicating good initial hydrophobic barrier properties. However, as time progressed, the contact angle gradually decreased to 30.5° at 120 s, and the water droplet morphology slowly collapsed from a hemispherical shape, eventually leading to slow liquid penetration into the membrane. This behavior indicates that although the CA layer has a certain initial water barrier function, its water-blocking performance exhibits a delayed release characteristic under continuous droplet loading. In summary, this Janus structure achieves temporal and dimensional control of droplet behavior by constructing an interface gradient of inner hydrophilicity and outer delayed permeation hydrophobicity: the inner layer rapidly adsorbs, spreads, and retains moisture, while the outer layer delays permeation. While maintaining structural integrity, it slowly releases excess moisture, providing structural support and functional basis for moisture control and preservation in high-humidity food packaging.

[0065] Example 3:

[0066] A method for preparing a humidity-triggered food preservation pad with a double-sided heterogeneous structure based on the MOF confinement effect includes the following steps:

[0067] Step 1: Preparation of CQDs@CuBTC nanomaterials, with lemon peel chosen as the biomaterial substrate;

[0068] Take 2.5g of lemon peel powder, add it to 50mL of deionized water, mix, and transfer it to a polytetrafluoroethylene reactor; hydrothermally react at 190℃ for 5h. After the reaction is complete, cool to room temperature, centrifuge and collect the supernatant; filter the supernatant through a 0.22μm microporous membrane, freeze-dry, and redissolve in 8mL of deionized water to obtain the CQDs solution.

[0069] Dissolve 1.5 mL of the above CQDs solution, 1.7 g of Cu(NO3)2·3H2O, and 0.7 g of PVP in 23 mL of DMF, denoted as solution A. Dissolve 0.7 g of H3BTC in 23 mL of DMF, denoted as solution B. Quickly add solution B to solution A, stir thoroughly, and transfer to a polytetrafluoroethylene reactor. React hydrothermally at 100 °C for 5 h. After the reaction, wash the product three times with DMF to remove impurities, centrifuge to recover the solid, and dry in an oven to obtain CQDs@CuBTC powder.

[0070] Step 2: Preparation of spinning solution for double-sided heterogeneous structure humidity-triggered food preservation pad;

[0071] Dissolve 3g of ethylene-vinyl alcohol copolymer (EVOH) in 23mL of DMAc, heat in a 90℃ water bath and stir until completely dissolved to obtain an EVOH solution; add 1.5g of CQDs@CuBTC, mix evenly and then ultrasonically disperse for degassing treatment. The ultrasonic power is 55W, the working time interval is 1s, and the treatment time is 0.7h. After treatment, the humidity-responsive inner layer spinning solution can be obtained.

[0072] Dissolve 2.5g of cellulose acetate (CA) in 23mL of glacial acetic acid, heat and stir in a 70℃ water bath until completely dissolved, and then degas using ultrasonic dispersion. The ultrasonic power is 55W, the working time interval is 1s, and the treatment time is 0.7h. After treatment, the hydrophobic protective outer spinning solution can be obtained.

[0073] Step 3: Preparation of a double-sided heterogeneous structure humidity-triggered food preservation pad;

[0074] The double-sided heterogeneous structure humidity-triggered food preservation pad consists of a humidity-responsive inner layer and a hydrophobic protective outer layer.

[0075] First, the humidity-responsive inner layer spinning solution prepared in step two is loaded into a syringe, installed on an electrospinning machine, and connected to an 18G needle; the high voltage power supply voltage is set to 15kV, the feed rate is 0.45mL / h, and the distance between the needle and the collector is 15cm, and electrospinning is performed; the spun nanofibers are received by the oil paper on the drum, and the humidity-responsive inner layer fiber membrane is obtained after the solution is pushed out.

[0076] After completing the humidity-responsive inner fiber membrane, the hydrophobic protective outer spinning solution prepared in step two is loaded into a syringe and connected to a 20G needle. The high-voltage power supply is set to 20kV, the injection rate is 0.3mL / h, and the distance between the needle and the collector is 15cm. Electrospinning of the hydrophobic protective outer layer is then performed. Using the humidity-responsive inner fiber membrane as a receiving substrate, it is spun onto the surface of the humidity-responsive inner fiber membrane to form a firmly adhered hydrophobic protective fiber layer. After spinning is completed, a double-sided heterogeneous structure humidity-triggered food preservation pad is obtained.

[0077] Figure 4 This paper presents the microstructure morphology of the Janus bilayer fiber membrane in Example 3 and the distribution characteristics of MOF particles within it. The left image is a cross-sectional scanning electron microscope (SEM) image of the fiber membrane, clearly showing the distinct boundaries between the upper and lower fiber layers, corresponding to the outer CA layer and the inner EVOH layer, respectively. The CA layer exhibits a dense arrangement, with relatively coarse fibers and a compact structure, effectively performing its hydrophobic barrier function. The EVOH layer, on the other hand, is composed of finer hydrophilic fibers with a loose texture, facilitating water molecule permeation and the release of active substances, achieving functional separation and synergistic response in the bilayer heterogeneous structure. The right image is a magnified SEM image of the MOF particles, showing regular cubic particles with uniform size and an average particle size of approximately 100–200 nm, indicating that the MOFs have good dispersibility and crystallinity in the support. This morphological feature helps to provide a high specific surface area and porous structure, thereby enhancing the loading and controlled release capability of active substances. Figure 4 The microstructure observation results verified the rationality of the Janus bilayer membrane constructed by electrospinning in this embodiment in terms of interface structure, material distribution and functional integration, and provided morphological evidence to support its application in a humidity-responsive controlled-release preservation system.

[0078] Applications in fresh food preservation:

[0079] Taking Example 3 as an example, the humidity-responsive preservation pad based on the prepared double-sided heterogeneous fiber membrane structure is applied to the preservation of fresh food, and the operation is as follows:

[0080] A humidity-responsive double-sided heterogeneous fiber membrane structure food preservation pad was placed together with fresh pork in a food preservation box as the experimental group; the control group did not add a humidity-responsive double-sided heterogeneous fiber membrane structure food preservation pad. Both groups of samples were sealed and refrigerated at 4℃, and the total volatile basic nitrogen (TVB-N) content was measured periodically to quantify the preservation effect.

[0081] The results are as follows Figure 5 As shown in the figure, the change in TVB-N content over storage time clearly reflects the significant protective effect of the humidity-responsive preservation pad on the freshness of pork. The TVB-N content in the experimental group slowly increased from an initial 3.25 mg / 100g to 9.27 mg / 100g on day 4, while the control group had already reached 15.36 mg / 100g at the same time. This indicates that the rate of increase in TVB-N in the experimental group was significantly lower than that in the control group, and it took approximately 3 days longer to reach the acceptable limit. This phenomenon suggests that the humidity-responsive preservation pad significantly extends the shelf life of pork by dynamically regulating the moisture environment within the packaging and synergistically releasing antibacterial active ingredients, thereby achieving dual inhibition of protein degradation and the accumulation of volatile alkaline compounds.

[0082] Note: The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. Therefore, although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention. All technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a double-sided heterogeneous structure humidity-triggered food preservation pad based on MOF confinement effect, characterized in that, Includes the following steps: Step 1: Preparation of CQDs@CuBTC nanomaterials; S1. Add bio-based material powder to deionized water, mix and transfer to a polytetrafluoroethylene reactor for hydrothermal reaction; after the reaction is completed, cool to room temperature, centrifuge and collect the supernatant; filter the supernatant through a microporous membrane, freeze dry and then re-dissolve the product in deionized water to obtain CQDs solution. S2. Next, dissolve CQDs solution, Cu(NO3)2·3H2O and PVP in DMF to obtain a mixed solution, denoted as solution A; dissolve H3BTC in DMF to obtain a mixed solution, denoted as solution B; add solution B to solution A, stir thoroughly, and then transfer to a polytetrafluoroethylene reactor for hydrothermal reaction. After the reaction is complete, wash the product with DMF, centrifuge to recover the solid, and dry it to obtain CQDs@CuBTC; Step 2: Preparation of spinning solution for double-sided heterogeneous structure humidity-responsive food preservation pad; S1. Dissolve EVOH in DMAc, heat in a water bath and stir until completely dissolved to obtain an EVOH solution; add CQDs@CuBTC prepared in step one to the EVOH solution, mix evenly and then ultrasonically disperse for degassing treatment, and the humidity-responsive inner layer spinning solution can be obtained after treatment. S2. Dissolve cellulose acetate (CA) in glacial acetic acid, heat and stir in a water bath until completely dissolved, and then ultrasonically disperse to remove bubbles. After treatment, a hydrophobic protective outer spinning solution can be obtained. Step 3: Preparation of a double-sided heterogeneous structure humidity-responsive food preservation pad; The humidity-responsive inner spinning solution obtained in step two is loaded into a syringe and installed in an electrospinning machine. Set the high voltage and feed rate of the high voltage power supply to perform electrospinning; use oil paper on the drum to receive the spun nanofibers, and obtain the humidity-responsive inner fiber membrane after the solution is pushed out. After completing the spinning of the humidity-responsive inner layer to obtain the humidity-responsive inner layer fiber membrane, the hydrophobic protective outer layer spinning solution prepared in step two is loaded into a syringe, and the high voltage power supply voltage and propulsion rate are set to perform electrospinning of the hydrophobic protective layer; the humidity-responsive inner layer fiber membrane prepared in the above steps is used to receive the spun nanofibers. After spinning is completed, a double-sided heterostructure humidity-responsive food preservation pad containing MOF confined material is obtained.

2. The method for preparing a double-sided heterogeneous structure humidity-triggered food preservation pad based on MOF confinement effect according to claim 1, characterized in that, In step S1, the bio-based material is a bio-based material containing polyphenols; the ratio of the amount of bio-based material powder to deionized water and the amount of deionized water required for reconstitution is 2-3g: 50mL: 5-10mL.

3. The method for preparing a double-sided heterogeneous structure humidity-triggered food preservation pad based on MOF confinement effect according to claim 1, characterized in that, The hydrothermal reaction in step S1 is carried out at a temperature of 180–200°C for 4–6 hours; the microporous membrane is a 0.22 μm microporous filter membrane.

4. The method for preparing a double-sided heterogeneous structure humidity-triggered food preservation pad based on MOF confinement effect according to claim 1, characterized in that, In step S2 of step one, the ratio of the amount of CQDs solution, Cu(NO3)2·3H2O, DMF and PVP is 1-2 mL: 1.5-2 g: 20-25 mL: 0.5-1 g; the ratio of the amount of H3BTC to DMF is 0.5-1 g: 20-25 mL; and the volume ratio of solution B added to solution A is 1:

1.

5. The method for preparing a double-sided heterogeneous structure humidity-triggered food preservation pad based on MOF confinement effect according to claim 1, characterized in that, The hydrothermal reaction in step S2 of step one is carried out at a temperature of 70–130°C for 4–6 hours.

6. The method for preparing a double-sided heterogeneous structure humidity-triggered food preservation pad based on MOF confinement effect according to claim 1, characterized in that, In step S1 of step two, the ratio of EVOH to DMAc and CQDs@CuBTC is 2-4g: 15-25mL: 0.5-3g; the water bath heating temperature is 90-100℃; the ultrasonic power is 50-60W, the working time interval is 1s, and the processing time is 0.5-1h.

7. The method for preparing a double-sided heterogeneous structure humidity-triggered food preservation pad based on MOF confinement effect according to claim 1, characterized in that, In step S2 of step two, the ratio of cellulose acetate to glacial acetic acid is 2-3g:15-25mL; the water bath heating temperature is 50-80℃; the ultrasonic power is 50-60W, the working time interval is 1s, and the processing time is 0.5-1h.

8. The method for preparing a double-sided heterogeneous structure humidity-triggered food preservation pad based on MOF confinement effect according to claim 1, characterized in that, In step three, the high-voltage power supply voltage for electrospinning the humidity-responsive inner spinning solution is 14–18 kV, the feed rate is 0.4–0.5 mL / h, and the electrospinning needle type is 18G; the high-voltage power supply voltage for electrospinning the hydrophobic protective outer spinning solution is 18–25 kV, the feed rate is 0.2–0.4 mL / h, and the electrospinning needle type is 20G; the spinning temperature is 20℃ and the relative humidity is 40% for both processes.

9. A double-sided heterogeneous structure humidity-triggered food preservation pad based on the MOF confinement effect prepared by the method according to any one of claims 1-8, characterized in that, The food preservation pad is composed of an inner humidity-responsive membrane and an outer hydrophobic protective membrane.

10. The application of the double-sided heterogeneous structure humidity-triggered preservation pad based on MOF confinement effect as described in claim 9 for antibacterial preservation of fresh food.

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