Preparation method and application of high-branched carbon dot-mediated intelligent response film material

Abstract

The present application relates to the technical field of fresh-keeping materials, and particularly relates to a preparation method and application of a high-branched carbon dot-mediated intelligent response film material. The specific technical scheme is as follows: taking degradable polymer materials such as polyvinyl alcohol, polylactic acid or polycaprolactone as a base material, a 'dynamic borate ester bond cross-linking- hyperbranched hydrogen bond entanglement' composite network is constructed through p-benzene diboronic acid and hyperbranched polyester; NH2-NCDs are uniformly fixed and loaded in the base material; and a high-branched carbon dot-mediated intelligent response film is prepared through a flow casting film forming and annealing strengthening process. The high-branched carbon dot-mediated intelligent response film has a sensitive fluorescent response to food state changes (pH, humidity changes), and has a high-efficiency inhibitory effect on pathogenic bacteria such as escherichia coli and staphylococcus aureus, and can be used as an intelligent packaging material for poultry egg preservation, and can also be applied to the preservation of fruits and vegetables, chilled meat, aquatic products and the like.

Description

Technical Field

[0001] This invention relates to the field of preservation materials technology, specifically to a method for preparing and applying a highly branched carbon dot-mediated smart response membrane material. Background Technology

[0002] Carbon quantum dots (CQDs) are a class of zero-dimensional carbon-based nanomaterials with good biocompatibility and sizes ranging from 1 to 10 nm. Classified according to the functional groups they carry, they can be divided into nitrogen-doped carbon quantum dots, hydroxylated carbon quantum dots, carboxylated carbon quantum dots, and thiol-functionalized carbon quantum dots. With their excellent fluorescence stability and high quantum yield, they need to have multifunctional integrated properties such as environmental stimulus responsiveness (e.g., humidity response, pH response), antibacterial properties, and gas regulation. By adjusting their preservation behavior in real time in response to environmental changes, they can achieve precise and efficient preservation and monitoring.

[0003] The performance of smart responsive food preservation films hinges on the choice of substrate and the modification load of functional modules. Currently, the substrates of smart preservation films mainly include natural polymers (starch, chitosan, gelatin, etc.) and synthetic polymers (polyethylene, polylactic acid, polycaprolactone, etc.). Natural polymer substrates have good biodegradability and high safety, but weak mechanical properties and insufficient stability. Synthetic polymer substrates have excellent mechanical properties and strong barrier properties, but their biocompatibility and degradability need improvement. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for preparing amino-terminated nitrogen-doped carbon dots (NH2-NCDs) using a bottom-up synthesis strategy. Furthermore, through a synergistic technique of dynamic crosslinking and hyperbranching toughening, the carbon dots are composited with a biodegradable polymer substrate to prepare a zero-dimensional amino-terminated nitrogen-doped carbon dot-mediated smart responsive membrane. This membrane is then used as a food preservation film, possessing intelligent responsiveness, high-efficiency antibacterial properties, excellent mechanical properties, and environmental friendliness. It enables the dual functions of real-time monitoring of food freshness and extended shelf life.

[0005] To achieve the above objectives, the present invention provides the following technical solution: This invention uses citric acid, glucose, vitamin C, or dried and pulverized biomass (such as sugar-rich biomass like leafy greens) as carbon sources, and different generations of polyamide-amine dendrimers (PAMAM) or dendritic polymers such as polypropylene imide (PPI) and polyethyleneimine (PEI) as nitrogen source precursors to prepare amino-terminated nitrogen-doped carbon dots (NH2-NCDs) with topological structures via hydrothermal synthesis. Using biodegradable polymers such as polyvinyl alcohol (PVA), polylactic acid (PLA), or polycaprolactone (PCL) as substrates, a "dynamic borate ester crosslinking-hyperbranched hydrogen bond entanglement" composite network is constructed using terephthalic acid (PBA) as a dynamic crosslinking agent and hyperbranched polyester (HBPE) as a toughening agent. NH2-NCDs are uniformly fixed and loaded into the substrate. A highly branched carbon dot-mediated smart responsive membrane (i.e., food preservation film) is prepared through a casting and annealing strengthening process. This type of plastic wrap has a sensitive fluorescent response to changes in food condition (pH, humidity changes), and also has a highly effective inhibitory effect on pathogenic bacteria such as Escherichia coli and Staphylococcus aureus. It can be used as a smart packaging material for the preservation of poultry and eggs, and can also be applied to the preservation of fruits, vegetables, chilled meat, aquatic products and other products.

[0006] This invention provides a method for preparing a highly branched carbon dot-mediated smart responsive membrane material, comprising the following steps: (1) Add the biodegradable polymer substrate to deionized water or organic solvent, heat and stir until completely dissolved, then add terephthalic acid (PBA) and hyperbranched polyester (HBPE) in sequence, and continue stirring to form a transparent and uniform substrate premix. (2) Disperse amino-terminated nitrogen-doped carbon dots (NH2-NCDs, whose preparation is described in CN202510172280.2) powder in a solvent and sonicate to obtain a quantum dot dispersion. Then slowly drop it into the substrate premixed liquid, stir at a constant temperature and introduce nitrogen gas to form a composite film liquid without agglomeration. The amino-terminated nitrogen-doped carbon dots are prepared by reacting a nitrogen source precursor with a carbon source precursor. The nitrogen source precursor is one of polyamide-amine dendrimer, polypropyleneimide, and polyethyleneimine. The generation number of the polyamide-amine dendrimer is an integer number of 1.0G-3.0G.

[0007] Preferably, in step (1), the biodegradable polymer substrate is one of polyvinyl alcohol, polylactic acid and polycaprolactone, and the organic solvent is one of chloroform, dichloromethane, ethyl acetate and dioxane.

[0008] Preferably, in step (2), the solvent is one of chloroform, water, ethanol, and dioxane.

[0009] Preferably, in step (2), the amount of amino-terminated nitrogen-doped carbon dots added is 1-2% of the mass of the biodegradable polymer substrate.

[0010] Accordingly, a highly branched carbon dot-mediated smart response membrane material prepared by the preparation method described above.

[0011] Accordingly, the application of a highly branched carbon dot-mediated smart responsive membrane material prepared by the aforementioned preparation method in food preservation.

[0012] The present invention has the following beneficial effects: This invention uses biodegradable polymers such as polyvinyl alcohol (PVA), polylactic acid (PLA), or polycaprolactone (PCL) as the substrate; uses PAMAM dendrimers of different generations as the nitrogen source and green substances such as citric acid, glucose, and biomass as the carbon source to synthesize amino-terminated nitrogen-doped carbon dots (NH2-NCDs) via a bottom-up hydrothermal method; uses terephthaloboric acid (PBA) as a dynamic crosslinking agent and hyperbranched polyester (HBPE) as a toughening agent, and the two work synergistically to form a "dynamic crosslinking-hyperbranched entanglement" composite network to fix NH2-NCDs in the substrate; and prepares a food preservation film with intelligent response indication function through casting and annealing strengthening.

[0013] The highly branched carbon dot-mediated smart response membrane provided by this invention has uniform size, stable mechanical properties, and strong water and weather resistance. It exhibits a sensitive fluorescence response to changes in food quality and also possesses highly efficient antibacterial and antioxidant functions. It can be adapted to different food preservation needs by adjusting the substrate type and the amount of NH2-NCDs added. Furthermore, the raw materials are biodegradable and the product is biocompatible, which aligns with the trend of green production.

[0014] The highly branched carbon dot-mediated smart responsive membrane of this invention differs from traditional single-function food preservation films. It can serve as a highly efficient and sensitive food preservation material, suitable for the preservation and packaging of various foods such as poultry eggs, fruits and vegetables, chilled meat, and aquatic products, possessing both practical value and environmental significance. The highly branched carbon dot-mediated smart responsive membrane provided by this invention can adjust the amino content and fluorescence response sensitivity of NH2-NCDs by controlling the generation of PAMAM dendritic macromolecules, achieving controllable preparation of functionalized materials. Furthermore, the combination of annealing and air-drying processes shortens the preparation cycle by 50%, reducing industrialization costs and demonstrating significant technical advantages and application prospects.

[0015] The membrane material prepared by this invention has sensitive fluorescent intelligent response characteristics to changes in food quality, enabling real-time in-situ monitoring of food freshness. It also has a highly effective inhibitory effect on pathogenic bacteria such as Escherichia coli and Staphylococcus aureus, delaying food oxidation and deterioration and extending the shelf life of food. As an intelligent preservation material, it can be widely used in preservation scenarios for poultry eggs, fruits and vegetables, chilled meat, aquatic products, and other types of food. Attached Figure Description

[0016] Figure 1Transmission electron microscopy (TEM) image of amino-terminated nitrogen-doped carbon dots prepared in Example 1; Figure 2 The images show physical representations of the smart response membrane ((a) is the smart response membrane prepared in Example 1, and (b) is the pure membrane without mixed carbon quantum dots). Figure 3 The preservation effect of plastic wrap on fresh chilled meat and halved fresh grapes; Figure 4 Comparison of the preservation effects of smart responsive membranes prepared for different generations of PAMAM on halved grapes. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Unless otherwise specified, the technical means used in the implementation examples are conventional means well known to those skilled in the art.

[0019] Amino-terminated nitrogen-doped carbon dots, as functional mediators, possess unique value in the preparation of smart food preservation films. The amino groups on the surface of these carbon dots can form stable chemical bonds with polymer substrates, enhancing interfacial compatibility. Simultaneously, the highly active sites introduced by nitrogen doping endow the material with functions such as antibacterial and gas adsorption, and its excellent optical properties enable visual monitoring of the preservation process. Therefore, this invention proposes a smart responsive film mediated by zero-dimensional amino-terminated nitrogen-doped carbon dots. By setting key parameters and effectively designing the carbon dot structure and composite processing technology, precise control of the intelligent response of the highly branched carbon dot-mediated smart responsive film is achieved. Through multifunctional synergistic design, it can effectively compensate for the performance defects of traditional food preservation films and meet the urgent needs of the food industry for high-quality preservation materials.

[0020] The invention provides a highly branched carbon dot-mediated smart response membrane that combines intelligent response and efficient preservation. It uses biodegradable polymers as substrates and synthesizes amino-terminated nitrogen-doped carbon dots (NH2-NCDs) through a bottom-up strategy. Combined with dynamic crosslinking and hyperbranching toughening technology, a composite membrane structure is constructed to achieve dynamic sensing and spoilage warning of the food storage environment. At the same time, the antibacterial and antioxidant effects of NH2-NCDs extend the shelf life of food.

[0021] This invention provides a method for preparing a highly branched, zero-dimensional carbon dot-mediated smart responsive membrane material, comprising the following steps: 1) Preparation of amino-terminated nitrogen-doped carbon dots (NH2-NCDs) using a bottom-up strategy: The nitrogen source precursor and the carbon source precursor are mixed in proportion, dissolved in deionized water by ultrasound, and transferred to a polytetrafluoroethylene high-pressure reactor. The mixture is reacted at a constant temperature of 180-220℃ for 4-6 hours, preferably at 200℃ for 5 hours, to ensure that the amino content of NH2-NCDs is ≥2mmol / g and the fluorescence quantum yield is ≥35%.

[0022] After the reaction was completed, the product solution was placed in a dialysis bag with a molecular weight cutoff of 3500 and dialyzed with deionized water for 3 days (changing the dialysate 3 times a day). After freeze-drying for 48 hours, a light blue-yellow NH2-NCDs powder was obtained with a particle size controlled between 2.0 and 9.0 nm.

[0023] The reaction of the nitrogen source precursor and the carbon source precursor is carried out at a mass ratio of not less than 1:1.5. The nitrogen source precursor is a polyamide-amine dendritic macromolecule of different generations (PAMAM) or a dendritic polymer such as polypropylene imine (PPI) or polyethyleneimine (PEI). The carbon source precursor is citric acid, glucose, vitamin C, or biomass that has been dried, pulverized, and passed through an 80-mesh sieve.

[0024] For details on the preparation of amino-terminated nitrogen-doped carbon dots (NH2-NCDs) and polyamide-amine dendrimers (PAMAMs) of different generations, please refer to the invention patent application CN202510172280.2 (A nitrogen-doped carbon dot-grafted propyl methacrylate-divinylbenzene copolymer microsphere adsorbent and its preparation method). The generation of the polyamide-amine dendrimer is 1.0G-3.0G, and must be an integer generation, i.e., 1.0G, 2.0G, or 3.0G.

[0025] The amino content of NH2-NCDs is controlled by adjusting the ratio of nitrogen source to carbon source: the higher the nitrogen source ratio, the higher the amino content and the stronger the antibacterial performance; the fluorescence response sensitivity of quantum dots is adjusted by changing the PAMAM generation. For example, NH2-NCDs prepared with 3.0G PAMAM are most sensitive to pH changes, that is, the fluorescence intensity change rate is ≥50% in the pH range of 4-7.

[0026] 2) Preparation of substrate premix: A biodegradable polymer substrate is added to deionized water or an organic solvent and heated and stirred until completely dissolved. Then, terephthalic acid (PBA) and hyperbranched polyester (HBPE) are added sequentially, and stirring continues to form a transparent and homogeneous substrate premix. The organic solvent is one of chloroform, dichloromethane, ethyl acetate, or dioxane. HBPE has hydroxyl-terminated end groups, with branching parameters of a branching factor (g value) of 0.60-0.70, a branching point density of 25-35 molecular chains, and 16-24 hydroxyl groups at the end groups.

[0027] The biodegradable polymer substrate is selected according to the preservation requirements: PVA is suitable for fruit and vegetable preservation (high air permeability), PLA is suitable for chilled meat preservation (high barrier properties), and PCL is suitable for aquatic product preservation (water resistance). The addition amount of PBA and HBPE must be strictly controlled at 3%-25% of the substrate weight to avoid excessive addition which will lead to a decrease in membrane hardness or insufficient toughness. For example, the addition amount of PBA is 2-5% of the substrate weight, and the addition amount of HBPE is 20-30% of the substrate weight. This addition amount can ensure that the mechanical properties and dynamic response of the membrane are optimally synergistic.

[0028] 3) Composite intelligent response system: NH2-NCDs powder was dispersed in a solvent and ultrasonically treated to obtain a quantum dot dispersion. This dispersion was then slowly dripped into a substrate premix, stirred at a constant temperature, and nitrogen gas was introduced to prevent oxidation, thus forming a non-agglomerated composite film.

[0029] The amount of NH2-NCDs added is 1-2% of the substrate mass; less than 1% will lead to insufficient response sensitivity, while more than 2% is prone to agglomeration. The ultrasonic treatment power is 300W and the time is 15 minutes to avoid excessive ultrasonication that may damage the quantum dot structure. The solvent is one of chloroform, water, ethanol, and dioxane.

[0030] The composite membrane solution may selectively contain 0.1% vitamin E (antioxidant) or 0.05% chitosan (enhanced antibacterial properties) without affecting the mechanical properties and response characteristics of the membrane, and in compliance with the food additive usage standards (GB2760).

[0031] 4) Film formation and annealing strengthening: The composite membrane solution was poured into a polytetrafluoroethylene mold, allowed to stand at room temperature to remove bubbles, and then placed in an oven to anneal at 120°C for 1 hour to form a synergistic structure. The annealing temperature was strictly controlled at 120°C; temperatures below 110°C would result in insufficient crystallization, while temperatures above 130°C would cause thermal degradation of the membrane. The mold was sterilized by wiping it with ethanol before use to prevent contamination of the membrane surface.

[0032] 5) Post-processing and storage: After annealing, the membrane is peeled off, air-dried, cut to the required size, sterilized with ultraviolet light, and then sealed and stored away from light until use. Air drying must be carried out in a cleanroom (Class 10,000 cleanliness) to avoid dust adhesion; after ultraviolet sterilization, the total bacterial count on the membrane surface must be ≤10 CFU / cm³. 2 Ensure compliance with hygiene standards for food contact materials.

[0033] The highly branched carbon dot-mediated smart response membrane prepared by this invention is suitable for preservation scenarios such as poultry eggs, fruits and vegetables, chilled meat, and aquatic products, and can realize the dual functions of real-time monitoring of food freshness and extension of shelf life.

[0034] The present invention will be further described below with reference to specific embodiments.

[0035] Example 1 The preparation of amino-terminated nitrogen-doped carbon dots (NH2-NCDs) based on 2.0G PAMAM and the preparation of smart responsive films are as follows: (1) Preparation of amino-terminated nitrogen-doped carbon dots (1.1) Accurately weigh 2g of PAMAM dendrimer (2.0g) as the nitrogen source for carbon quantum dots and 3g of anhydrous citric acid as the carbon source for carbon quantum dot synthesis, and set aside; (1.2) Add the above raw materials to 20 mL of deionized water and place them in an ultrasonic cleaner (power 300 W, frequency 40 kHz) for ultrasonic treatment for 20 minutes until a homogeneous mixed solution is formed. (1.3) Transfer the mixed solution to a 50 mL polytetrafluoroethylene high-pressure reactor, seal it and place it in a forced-air drying oven. Set the heating program: increase the temperature from room temperature to 200 °C at a rate of 5 °C / min, and keep the temperature constant for 5 hours. After the reaction is completed, let it cool naturally to room temperature and remove the reactor. (1.4) Pour the reaction product into a dialysis bag with a molecular weight cutoff of 3500, and use deionized water as the dialysis solution for dialysis purification. Change the dialysis solution 3 times a day and continue dialysis for 3 days to remove unreacted small molecule impurities. (1.5) The dialyzed solution was transferred to a freeze dryer, and the temperature was set to -50℃ and the vacuum degree to 10 Pa. The solution was freeze-dried for 48 hours to obtain a light yellow, fluffy NH2-NCDs powder. Transmission electron microscopy (TEM) showed that the particles were uniformly dispersed (see...). Figure 1 The particle size distribution is 3-9 nm; thermogravimetric analysis shows that the nitrogen content is 3.6 mmol / g; fluorescence spectrophotometry shows that the fluorescence quantum yield is 38% at an excitation wavelength of 365 nm.

[0036] (2) Preparation of smart response membranes: (2.1) Accurately weigh 5g PVA (degree of polymerization 1750±50), add 50mL of deionized water, place in a constant temperature water bath (90℃), and magnetically stir (500rpm) for 1.5 hours until the PVA is completely dissolved and a transparent PVA solution is formed; then add 0.15g terephthalic acid (PBA) and 1.25g hyperbranched polyester (HBPE, number average molecular weight 5000, branching factor (g value) 0.66, branching point density of 30 molecular chains, number of end groups of 19 hydroxyl groups) in sequence, lower the water bath temperature to 60℃, and continue stirring for 30 minutes to obtain a homogeneous substrate premix; (2.2) Weigh 0.08g of the NH2-NCDs powder prepared in step (1) above, add 5mL of deionized water, and sonicate for 15 minutes (power 300W) to form a stable quantum dot dispersion; slowly drop the dispersion into the substrate premix, with the dropping speed controlled at 1 drop / second, while continuously introducing nitrogen gas (flow rate 50mL / min) to prevent oxidation, and stir at 60℃ for 1 hour to obtain a yellow-blue composite film liquid without obvious agglomeration; (2.3) Take 20 mL of composite membrane solution and slowly pour it into a 10 cm × 10 cm polytetrafluoroethylene mold that has been wiped and sterilized with ethanol in advance. Place it on a water platform and let it stand at room temperature for 30 minutes to remove any residual air bubbles in the membrane solution. Then place the mold in a forced-air drying oven and anneal at 120 °C for 1 hour to promote PVA crystallization and rearrangement of borate ester bonds to form a synergistic structure of “crystallization zone - dynamic cross-linking zone - hyperbranched toughening zone”. (2.4) Peel the annealed film from the mold and air dry it in a Class 10,000 cleanroom for 12 hours; then irradiate it in an ultraviolet sterilization chamber (wavelength 254nm) for 30 minutes; after sterilization, seal it in a sterile polyethylene bag and store it at room temperature away from light until use (see actual product image). Figure 2 (As shown).

[0037] (3) Verification of fluorescence response and preservation performance: (3.1) Sample preparation: Fresh, undamaged, and uniformly sized strawberries (the variety is "Hongyan") were selected and randomly divided into two groups of 30 strawberries each. The experimental group was wrapped with the smart response film prepared in step (2), and the control group was wrapped with commercially available traditional PE preservation film. Both groups were stored in a refrigerated environment at 4°C (relative humidity 85%). (3.2) Intelligent response detection: During storage, the fluorescence intensity of the plastic wrap was detected daily using a fluorescence spectrophotometer (excitation wavelength 365nm); on the first day of storage, the fluorescence intensity of both groups of plastic wrap was 1250 a.u.; on the fourth day, the fluorescence intensity of the experimental group decreased to 1180 a.u., while the control group showed no significant change; on the seventh day, due to the slight metabolism of strawberries producing acidic substances, the fluorescence intensity of the experimental group decreased to 870 a.u. (change rate 30.4%), while the strawberries in the control group showed mold spots, but the plastic wrap showed no fluorescence response; (3.3) Evaluation of preservation effect: Sensory evaluation: On the 7th day of storage, the strawberries in the experimental group maintained their bright red color, plump texture, and no off-odor; the strawberries in the control group darkened in color, softened in flesh, and 6 strawberries developed mold spots. Microbial count: Following GB4789.2-2022, the total bacterial count on the surface of strawberries was measured; the total bacterial count in the experimental group was 1.5 × 10⁻⁶. 3 CFU / g, control group was 4.2×10 5The antibacterial rate reached 99.6% with CFU / g. Moreover, no common foodborne pathogens such as coliform bacteria and Staphylococcus aureus were detected in the experimental group, and there were no signs of mold spore germination. In the control group, mold (mainly Penicillium and Aspergillus) was detected in 6 strawberries with mold spots, and coliform bacteria (32 CFU / g) were also detected, but Staphylococcus aureus was not detected.

[0038] Physicochemical indicators: The weight loss rate and vitamin C content of strawberries were detected; the weight loss rate of the experimental group was 4.2%, and the vitamin C content was 58 mg / 100g; the weight loss rate of the control group was 8.5%, and the vitamin C content was 32 mg / 100g; it can be seen that the plastic wrap of the present invention can effectively reduce the loss of moisture and nutrients in strawberries.

[0039] Example 2 The preparation of amino-terminated nitrogen-doped carbon dots based on 3.0 G PAMAM and the fabrication of a smart responsive film are as follows: (1) Preparation of amino-terminated nitrogen-doped carbon dots: (1.1) Accurately weigh 2g of PAMAM dendritic macromolecules (3.0g) as a nitrogen source and 3g of anhydrous citric acid as a carbon source, add them to 20mL of deionized water, and sonicate at 300W for 20 minutes until a homogeneous solution is formed. (1.2) Transfer the mixed solution to a 50 mL polytetrafluoroethylene high-pressure reactor, raise the temperature to 200 °C at a rate of 5 °C / min, keep the temperature constant for 5 hours, and let it cool naturally to room temperature; (1.3) The product was poured into a dialysis bag with a molecular weight cutoff of 3500 and dialyzed with deionized water for 3 days (the dialysate was changed 3 times a day). Then, it was freeze-dried at -50℃ and vacuum degree of 10Pa for 48 hours to obtain light blue-yellow NH2-NCDs powder.

[0040] The powder was tested and found to have a particle size distribution of 4-9 nm, a nitrogen content of 4.8 mmol / g, a fluorescence quantum yield of 41% at an excitation wavelength of 365 nm, and a fluorescence intensity change rate of 56% in the pH range of 4-7.

[0041] (2) Preparation of smart response membranes: (2.1) Accurately weigh 5g PVA (degree of polymerization 1750±50), add 50mL deionized water, and stir magnetically at 90℃ for 1.5 hours until completely dissolved; add 0.15g terephthalic acid and 1.25g hyperbranched polyester (number average molecular weight 5000, branching factor (g value) 0.66, branching point density of 30 molecular chains, number of end groups of 19 hydroxyl groups) in sequence, and stir at 60℃ for 30 minutes to obtain the substrate premix; (2.2) Weigh 0.08g of the NH2-NCDs powder prepared in step (1) above, add 5mL of deionized water, sonicate at 300W for 15 minutes to form a dispersion, slowly drip it into the substrate premix at a rate of 1 drop / second, continuously introduce nitrogen gas, and stir at 60℃ for 1 hour to obtain the composite film liquid. (2.3) Pour 20 mL of composite membrane solution into a 10 cm × 10 cm polytetrafluoroethylene mold sterilized with ethanol, let it stand at room temperature for 30 minutes to remove bubbles, and then anneal it in an oven at 120 °C for 1 hour. (2.4) Peel the annealed film from the mold, place it in a Class 10,000 cleanroom to air dry for 12 hours, sterilize it with 254nm ultraviolet light for 30 minutes, and store it in a sealed, light-proof container for later use.

[0042] (3) Verification of fluorescence response and preservation performance: Fresh red strawberries were selected and divided into an experimental group (the smart response membrane of this embodiment) and a control group (the smart response membrane of Example 1), with 30 strawberries in each group. They were refrigerated at 4°C and 85% relative humidity. On the first day of storage, the initial fluorescence intensity of both groups of preservation films was 1250 a.u. On the seventh day of storage, the fluorescence intensity of the experimental group decreased to 730 a.u. (a change rate of 42.1%), while that of the control group decreased to 870 a.u. (a change rate of 30.4%). The fluorescence response sensitivity of the experimental group was significantly improved.

[0043] The total bacterial count of strawberries in the experimental group was 1.2 × 10⁻⁶ during the same period. 3 The CFU / g, weight loss rate of 3.8%, and vitamin C content of 61mg / 100g are the average values ​​of three repeated experiments, with a deviation of no more than 5% between experiments. The preservation effect is better than the control group. Experiments show that replacing 3.0G of PAMAM dendrimers can significantly modulate the fluorescence response sensitivity of NH2-NCDs. Therefore, by replacing the PAMAM generation from 2.0G to 3.0G, the nitrogen content of carbon dots and the fluorescence response sensitivity can be significantly increased, thereby achieving functional tunability of the smart responsive membrane material.

[0044] Example 3 The preparation of amino-terminated nitrogen-doped carbon dots and smart responsive films based on 1.0 G PAMAM involves the following steps: (1) Preparation of amino-terminated nitrogen-doped carbon dots: (1.1) Accurately weigh 2g of PAMAM dendritic macromolecules (1.0G) as a nitrogen source and 3g of anhydrous citric acid as a carbon source, add them to 20mL of deionized water, and sonicate at 300W for 20 minutes until a homogeneous solution is formed. (1.2) Transfer the mixed solution to a 50 mL polytetrafluoroethylene high-pressure reactor, raise the temperature to 200 °C at a rate of 5 °C / min, keep the temperature constant for 5 hours, and let it cool naturally to room temperature; (1.3) The product was poured into a dialysis bag with a molecular weight cutoff of 3500 and dialyzed with deionized water for 3 days (the dialysate was changed 3 times a day). Then, it was freeze-dried at -50℃ and vacuum degree of 10Pa for 48 hours to obtain a light yellow NH2-NCDs powder.

[0045] The powder was tested and found to have a particle size distribution of 2-8 nm, an amino content of 2.2 mmol / g, a fluorescence quantum yield of 32% at an excitation wavelength of 365 nm, and a fluorescence intensity change rate of 28% in the pH range of 4-7.

[0046] (2) Preparation of smart response membrane: (2.1) Accurately weigh 5g PVA (degree of polymerization 1750±50), add 50mL deionized water, and stir magnetically at 90℃ for 1.5 hours until completely dissolved; add 0.15g terephthalic acid and 1.25g hyperbranched polyester (number average molecular weight 5000, branching factor (g value) 0.66, branching point density of 30 molecular chains, number of end groups of 19 hydroxyl groups) in sequence, and stir at 60℃ for 30 minutes to obtain the substrate premix; (2.2) Weigh 0.08g of the NH2-NCDs powder prepared in step (1) above, add 5mL of deionized water, sonicate at 300W for 15 minutes to form a dispersion, slowly drip it into the substrate premix at a rate of 1 drop / second, continuously introduce nitrogen gas, and stir at 60℃ for 1 hour to obtain the composite film liquid. (2.3) Pour 20 mL of composite membrane solution into a 10 cm × 10 cm polytetrafluoroethylene mold sterilized with ethanol, let it stand at room temperature for 30 minutes to remove bubbles, and then anneal it in an oven at 120°C for 1 hour. (2.4) After the annealed film is peeled off, it is air-dried in a Class 10,000 cleanroom for 12 hours, sterilized with 254nm ultraviolet light for 30 minutes, and then sealed and stored in the dark for later use.

[0047] (3) Verification of fluorescence response and preservation performance: Fresh red strawberries were selected and divided into an experimental group (the smart response membrane of this embodiment) and a control group (the smart response membrane of Example 1), with 30 strawberries in each group. They were refrigerated at 4°C and 85% relative humidity. On the first day of storage, the initial fluorescence intensity of both groups of preservation films was 1250 a.u. On the seventh day of storage, the fluorescence intensity of the experimental group decreased to 950 a.u. (a change rate of 24.0%), and that of the control group decreased to 870 a.u. (a change rate of 30.4%). The fluorescence response of the experimental group was positively correlated with the PAMAM algebraic gradient.

[0048] The total bacterial count of strawberries in the experimental group was 2.2 × 10⁻⁶ during the same period. 3The CFU / g, weight loss rate of 4.8%, and vitamin C content of 54 mg / 100g are the average values ​​of three repeated experiments, with a deviation of no more than 5% between experiments, demonstrating stable preservation effects. Experiments have shown that 1.0 G of whole-generation PAMAM dendrimers can successfully prepare NH2-NCDs that meet the requirements of this invention. By replacing 1.0 G / 2.0 G / 3.0 G of different whole-generation PAMAMs, the amino content of carbon dots and fluorescence response sensitivity can be graded and controlled.

[0049] Example 4 Preparation of highly branched carbon dot-mediated smart responsive membranes based on PLA substrates: (1) Accurately weigh 5g PLA (number average molecular weight 80000), add 50mL chloroform (analytical grade), place in a constant temperature water bath (40℃), and stir magnetically (600rpm) for 2 hours until PLA is completely dissolved; add 0.15g PBA and 1.25g HBPE in sequence, and continue stirring for 30 minutes to obtain a transparent substrate premix; (2) Weigh 0.08g of NH2-NCDs powder prepared in Example 1, add 5mL of chloroform, and sonicate for 15 minutes (power 300W) to form a quantum dot dispersion; drop the dispersion into the substrate premix, stir at 60℃ for 1 hour, and introduce nitrogen to prevent oxidation to obtain the composite film liquid; (3) Pour 20 mL of composite membrane solution into a 10 cm × 10 cm polytetrafluoroethylene mold and let it stand at room temperature for 20 minutes to remove bubbles; then put it in a fume hood and dry it at 40 °C for 2 hours to remove chloroform. (4) After drying, place the film in an oven at 120°C for annealing for 1 hour, and peel it off after cooling naturally to room temperature; after air drying in a clean room for 6 hours and ultraviolet sterilization for 30 minutes, seal and store it; the film has high oxygen barrier properties and is suitable for the preservation of protein foods such as poultry eggs.

[0050] Application of smart responsive membranes in quail egg preservation: (1) Sample processing: 20 fresh quail eggs were selected and divided into two groups; 10 eggs were vacuum-wrapped with the PLA substrate preservation film prepared above (experimental group), and 10 eggs were vacuum-wrapped with traditional PE preservation film (control group). Both were stored in the refrigerator at 4±1℃. (2) Intelligent response detection: On the 15th day of storage, the experimental group produced volatile sulfides and carbon dioxide due to slight metabolism and microbial activity of quail eggs, which caused the local environment pH to drop from the initial 7.2 to 6.5, and the fluorescence intensity of the plastic wrap to drop from the initial 1250 a.u. to 800 a.u. (the change rate was about 36%). The quail eggs in the control group showed visible signs of quality decline, and no fluorescence response was observed in the plastic wrap. (3) Evaluation of preservation effect: Physicochemical indicators: Referring to GB2749-2015 "Eggs and Egg Products", on the 15th day of storage, the Huff unit value of the experimental group quail eggs was 72 and the yolk index was 0.40, both of which met the first-grade fresh egg standard; the Huff unit value of the control group dropped to 55 and the yolk index was 0.29, which was close to the second-grade egg limit. Microbiological indicators: Salmonella and coliform bacteria were not detected in the experimental group; coliform bacteria were detected in the control group (12 CFU / g). Sensory indicators: The experimental group had clean eggshells, small air cells, clear egg whites, and clear yolk outlines; the control group had slightly blemishes on the eggshells, slightly cloudy egg whites, and slightly flattened yolks.

[0051] Example 5 Preparation of highly branched carbon dot-mediated smart response membranes based on PCL substrates: (1) Accurately weigh 5g of PCL (number average molecular weight 100,000), add 50mL of dichloromethane (analytical grade), place in a constant temperature water bath (45℃), and stir magnetically (600rpm) for 2 hours until the PCL is completely dissolved; add 0.2g of PBA and 1.25g of HBPE in sequence, and continue stirring for 30 minutes to obtain a stable substrate premix; (2) Weigh 0.08g of NH2-NCDs powder prepared in Example 1, add 5mL of dichloromethane, and sonicate for 15 minutes (power 300W) to form a quantum dot dispersion; drop the dispersion into the substrate premix, stir at 45℃ for 1 hour, and introduce nitrogen to prevent oxidation to obtain the composite film liquid; (3) Pour 20 mL of composite membrane solution into a 10 cm × 10 cm polytetrafluoroethylene mold and let it stand at room temperature for 25 minutes to remove bubbles; then put it in a fume hood and dry it at 40 °C for 3 hours to remove dichloromethane.

[0052] (4) After drying, place the film in an oven at 120°C for annealing for 1 hour, and peel it off after it cools naturally to room temperature. After air drying in a clean room for 8 hours and ultraviolet sterilization for 30 minutes, seal it and store it away from light for later use.

[0053] Application of smart response membranes in shrimp preservation: (1) Sample preparation: 20 fresh, uniform-sized (10-12cm in length) whiteleg shrimp were selected. The shrimp whiskers and rostrum were removed. After rinsing with sterile water and draining the surface water, the shrimp were divided into two groups: 10 shrimp were vacuum-wrapped with the PCL substrate preservation film prepared above (experimental group) and 10 shrimp were vacuum-wrapped with traditional PE preservation film (control group). All shrimp were stored in the refrigerator at 4±1℃. (2) Intelligent response detection: On the first day of storage, the fluorescence intensity of the plastic wrap in both groups was 1250 a.u. and the pH of the environment was 7.0. On the third day, due to the slight metabolism of shrimp, the pH of the experimental group dropped to 6.7 and the fluorescence intensity dropped to 1050 a.u. (change rate 16%). The pH of the control group also dropped to 6.7, but the plastic wrap did not show a fluorescence response. On the eighth day, the metabolism of shrimp in the experimental group intensified and the pH further dropped to 6.2, and the fluorescence intensity dropped to 780 a.u. (total change rate 37.6%). The shrimp in the control group had sticky body surfaces and the plastic wrap did not show a fluorescence response.

[0054] (3) Evaluation of preservation effect: Sensory evaluation: On the 8th day of storage, the shrimp in the experimental group had a bright, bluish-gray color, firm and elastic flesh, and no off-odor; the shrimp in the control group had a dark, blackish color, soft and sticky flesh, a distinct ammonia smell, and the shells of 3 shrimp were easy to fall off and the shrimp meat was mushy.

[0055] Microbial count: Following GB4789.2-2022, the total bacterial count in shrimp was determined; the total bacterial count in the experimental group was 2.3 × 10⁻⁶. 3 CFU / g, no Escherichia coli or Vibrio parahaemolyticus detected, and the total bacterial count in the control group was 5.7 × 10⁻⁶. 5 The test results showed that Escherichia coli (23 CFU / g) and Vibrio parahaemolyticus (11 CFU / g) were detected, both exceeding the safety limits for fresh products.

[0056] Physicochemical indicators: Volatile basic nitrogen (TVBN) of shrimp was tested according to GB 2733-2015; the TVBN of the experimental group was 18mg / 100g (which meets the standard of ≤30mg / 100g for fresh products), and the water loss rate was 3.8%; the TVBN of the control group was 38mg / 100g (exceeding the standard), and the water loss rate was 7.2%.

[0057] Example 6: Comparative experiment of the smart responsive membrane prepared in Example 1 in the preservation of pork and grapes. (1) Sample processing: Fresh chilled meat and fresh grapes (whole grapes cut in half) were selected and divided into two groups: fresh chilled meat and halved fresh grapes wrapped with PVA substrate preservation film (i.e. smart response film) prepared in Example 1 (experimental group), and fresh chilled meat and halved fresh grapes wrapped with PVA substrate film without quantum dot mixing (control group). Two parallel samples were set up in each group and stored at 4±1℃ for 10 days. The index was tested once a day.

[0058] (2) Intelligent response detection: On day 1, the fluorescence intensity of all plastic wraps was 1250 a.u., and the initial pH of the storage environment for pork and grapes was 7.1; on day 5, the pH of the environment in the pork experimental group decreased to 6.6, and the fluorescence intensity of the plastic wrap decreased to 1080 a.u. (change rate 13.6%); the pH of the grape experimental group decreased to 6.5, and the fluorescence intensity decreased to 1050 a.u. (change rate 16%); the pH of the environment in all control groups changed synchronously, but the plastic wraps showed no fluorescence response; on day 8, the fluorescence intensity of the pork experimental group decreased to 820 a.u. (total change rate 34.4%), and the fluorescence intensity of the grape experimental group decreased to 790 a.u. (total change rate 36.8%); the pork and grapes in the control groups showed obvious signs of spoilage, but the plastic wraps still showed no fluorescence response (e.g. Figure 3 (As shown).

[0059] (3) Evaluation of preservation effect: Sensory evaluation: On the 10th day of storage, the pork in the control group was sticky on the surface, dull in color, and had a slight off-odor, with some fat oxidized and turned yellow; the pork in the experimental group was bright in color, firm in texture, without stickiness, and had no obvious off-odor, with a significantly lower degree of fat oxidation than the control group; the grape control group had severely shrunken berries, lost their luster, and had some bacterial spots; the grape in the experimental group had slightly shrunken berries, the berries still maintained a certain luster, and no bacterial spots were produced.

[0060] Microbial count: Following GB4789.2-2022, the total bacterial count on the surface of pork and grapes was measured; the total bacterial count in the pork control group was 3.8 × 10⁻⁶. 5 CFU / g, coliform bacteria were detected (21 CFU / g); the total bacterial count in the pork experimental group was 2.1 × 10⁻⁶. 3 CFU / g, no pathogenic bacteria such as coliforms and Staphylococcus aureus were detected; the total bacterial count in the grape control group was 4.5 × 10⁻⁶. 5 CFU / g, mold was detected (8 CFU / g); the total bacterial count in the grape experimental group was 1.8 × 10⁻⁶. 3 CFU / g, no mold or coliform bacteria detected.

[0061] Physicochemical properties: The weight loss rate data for each group in 10 tests are shown in the table below:

[0062] As shown in the table above, the weight loss rate of the carbon dot film group was lower than or close to that of the pure film control group during most of the storage periods. In particular, at the final storage node (10th time), the loss rate of the carbon dot film was significantly lower in the pork 1 and grape 2 groups. Combined with the core preservation logic of food moisture loss, it can be proved that the carbon dot film has a better preservation effect on pork and grapes than the pure PVA film.

[0063] Combining microbiological and sensory indicators, the carbon dot-mediated PVA substrate preservation film of this invention can significantly delay the spoilage of pork and grapes, and its preservation effect is superior to that of carbon-free quantum dot pure PVA substrate preservation film.

[0064] Comparative Example 1 The preparation of amino-terminated nitrogen-doped carbon dots based on 2.5G half-generation PAMAM and the corresponding smart response film are carried out through the following steps: (1) Parallel preparation of amino-terminated nitrogen-doped carbon dots of 2.5G half-generation PAMAM To clarify the decisive influence of PAMAM end-group structure on carbon dot performance and the final membrane material function, this embodiment prepared and compared carbon dots based on whole-generation (2.0G, amino-terminated, Example 1) and half-generation (2.5G, carboxyl-terminated) PAMAM while maintaining a consistent mass ratio of PAMAM to citric acid (2g:3g). It should be noted that the molecular weights of 2.0G and 2.5G PAMAM are different (approximately 1,500Da and 3,000Da, respectively). The same mass ratio was used here instead of a molar ratio to highlight the fundamental difference in end-group chemical properties. The results are sufficient to determine the technical feasibility.

[0065] (1.1) Accurately weigh 2g of PAMAM dendritic macromolecules and 3g of anhydrous citric acid, add 20mL of deionized water respectively, and sonicate at 300W for 20 minutes until a homogeneous mixed solution is obtained; (1.2) The mixed solution was transferred to a 50 mL polytetrafluoroethylene high-pressure reactor and reacted at 200 °C for 5 hours. After natural cooling, it was dialyzed for 3 days using a 3500 molecular weight dialysis bag and then freeze-dried at -50 °C and 10 Pa vacuum for 48 hours to obtain NH2-NCDs powder.

[0066] The obtained carbon dots had a particle size of 4-12 nm, exceeding the controllable range of 2.0-9.0 nm of this invention, exhibiting poor particle dispersibility, an amino content of 0.3 mmol / g, a fluorescence quantum yield of 9% under 365 nm excitation, and a fluorescence intensity change rate of 6% within the pH range of 4-7. The carbon dots obtained in Example 1 had a particle size of 3-9 nm, an amino content of 3.6 mmol / g, a fluorescence quantum yield of 38% under 365 nm excitation, and a fluorescence intensity change rate of 30.4% within the pH range of 4-7.

[0067] (2) Preparation of smart response membrane (2.1) Accurately weigh 5g PVA (degree of polymerization 1750±50), add 50mL deionized water, and stir magnetically at 90℃ for 1.5 hours until completely dissolved; add 0.15g terephthalic acid and 1.25g hyperbranched polyester (number average molecular weight 5000, branching factor g value 0.66) in sequence, and stir at 60℃ for 30 minutes to obtain the substrate premix.

[0068] (2.1) Weigh 0.08g of the NH2-NCDs powder prepared in step (1) above, add 5mL of deionized water, sonicate at 300W for 15 minutes to form a dispersion, slowly drip 1 drop / second into the substrate premix, continuously purge with nitrogen, and stir at 60℃ for 1 hour to obtain the composite film liquid. (2.2) Take 20 mL of composite membrane liquid, cast it through a 10 cm × 10 cm polytetrafluoroethylene mold, let it stand at room temperature for 30 minutes to remove bubbles, anneal at 120 °C for 1 hour, air dry in a Class 10,000 clean room for 12 hours, and sterilize it with 254 nm ultraviolet light for 30 minutes to obtain a smart response membrane, and store it in a sealed, light-proof container.

[0069] (3) Comparative verification of the preservation performance of halved grapes Fresh Kyoho grapes of the same bunch, with uniform maturity and size, were selected and cut in half under aseptic conditions. They were randomly divided into two groups, with three parallel samples in each group. The cut surfaces were completely covered with the smart response membranes prepared in step (2) and Example 1, respectively. The samples were then refrigerated at 4°C and 85% relative humidity for 8 days. The appearance and fluorescence intensity of the membranes were observed on day 1 and day 8. The total bacterial count and weight loss rate were measured on day 8. The preservation effects were compared in the following figures. Figure 4 .

[0070] The results show: On day 1 of storage, the pulp of both groups of grapes was plump and translucent, with no significant difference; on day 8 of storage (see...), Figure 4 In contrast, the grape pulp of the smart-response membrane prepared in Example 1 became soft and mushy, with significant juice leakage and a total bacterial count as high as 4.2 × 10⁻⁶. 5 The CFU / g weight loss rate was 8.1%; the membrane fluorescence intensity decreased from an initial 1250 a.u. to 1185 a.u., a change rate of only 5.2%, indicating that the membrane had no effective fluorescence response to the grape spoilage process and did not possess intelligent monitoring capabilities; the grape pulp of the intelligent response membrane group prepared in Example 1 still maintained good plumpness and luster, with only slight juice seepage on the cut surface, no obvious softening or mold, and a total bacterial count of 1.8 × 10⁻⁶. 3 The CFU / g loss rate was 4.1%; the membrane fluorescence intensity decreased from an initial 1250 a.u. to 790 a.u., a change rate of up to 36.8%, demonstrating sensitive pH response and excellent preservation performance.

[0071] In summary, 2.5G and other half-generation PAMAMs have carboxyl-terminated structures, which cannot be used to prepare nitrogen-doped carbon dots with high amino content and high fluorescence response sensitivity required by this invention. Consequently, the corresponding membrane material cannot achieve the core invention objectives of intelligent fluorescence response and efficient preservation.

[0072] Next, NH2-NCDs based on 0.5G, 1.5G, and 3.5G PAMAM were prepared using the same method as Comparative Example 1. Their performance was consistent with Comparative Example 1, and none of them could achieve the effect of the present invention.

[0073] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. The present invention is not limited to the embodiments described herein. Any improvements and modifications made to the present invention by those skilled in the art based on the disclosure of the present invention should be within the scope of protection of the present invention. For example, adjusting the fluorescence response sensitivity of NH2-NCDs by replacing PAMAM dendrimers of different generations; optimizing the mechanical properties of the membrane by adjusting the addition ratio of PBA and HBPE; and adapting to different biodegradable substrates to meet the preservation needs of different foods such as fruits, vegetables, meat, and seafood are all within the scope of protection of the present invention.

Claims

1. A method for preparing a highly branched carbon dot-mediated smart response membrane material, characterized in that: Includes the following steps: (1) Add the biodegradable polymer substrate to deionized water or organic solvent, heat and stir until completely dissolved, add terephthalic acid and hyperbranched polyester in sequence, and continue stirring to form a transparent and uniform substrate premix. (2) Disperse the amino-terminated nitrogen-doped carbon dot powder in a solvent and sonicate it to obtain a quantum dot dispersion. Then slowly drop it into the substrate premixed liquid, stir at a constant temperature and introduce nitrogen gas to form a composite film liquid without agglomeration. The amino-terminated nitrogen-doped carbon dots are prepared by reacting a nitrogen source precursor with a carbon source precursor. The nitrogen source precursor is one of polyamide-amine dendrimer, polypropyleneimide, and polyethyleneimine. The generation number of the polyamide-amine dendrimer is an integer number of 1.0G-3.0G.

2. The preparation method according to claim 1, characterized in that: In step (1), the biodegradable polymer substrate is one of polyvinyl alcohol, polylactic acid and polycaprolactone, and the organic solvent is one of chloroform, dichloromethane, ethyl acetate and dioxane.

3. The preparation method according to claim 1, characterized in that: In step (2), the solvent is one of chloroform, water, ethanol, and dioxane.

4. The preparation method according to claim 1 or 3, characterized in that: In step (2), the amount of amino-terminated nitrogen-doped carbon dots added is 1-2% of the mass of the biodegradable polymer substrate.

5. A highly branched carbon dot-mediated smart response membrane material prepared by the preparation method according to any one of claims 1-4.

6. The application of a highly branched carbon dot-mediated smart responsive membrane material prepared by the preparation method according to any one of claims 1-4 in food preservation.

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