A double-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite film with pH indication and photothermal antibacterial property, and a preparation method and application thereof

Abstract

The application discloses a double-response carboxymethyl cellulose / polyvinyl alcohol composite film modified by nitrogen-doped carbon dots with pH indication and photothermal antibacterial properties as well as a preparation method and application thereof. Nitrogen-doped carbon dots are prepared by a hydrothermal method, and the carbon dots are purified by ethanol deposition; carvacrol is loaded into beta-cyclodextrin, and then the nitrogen-doped carbon dots are blended and added into carboxymethyl cellulose / polyvinyl alcohol to prepare a double-response composite film. The composite film has multiple functions such as photothermal antibacterial property, pH indication and slow-release antibacterial property, and can effectively preserve fruits and vegetables and monitor the freshness of the fruits and vegetables in real time.

Description

Technical Field

[0001] This invention belongs to the field of food packaging materials technology, specifically relating to a dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite film with pH indication and photothermal antibacterial properties, its preparation method, and its application. Background Technology

[0002] With global population growth and consumption upgrades, consumers are placing higher demands on the quality, safety, and shelf life of fresh agricultural products. Traditional technologies often face numerous limitations when dealing with complex scenarios. For example, in the field of food preservation, traditional packaging materials have limited functions and cannot meet the needs for long-term antibacterial effects and quality monitoring, leading to rapid deterioration of fruits and vegetables after harvest due to microbial infection, moisture loss, and respiratory metabolic imbalance. In environmental monitoring and bioimaging, existing probe materials often suffer from insufficient response sensitivity and poor stability. Against this backdrop, nitrogen-doped carbon dots (N,CDs) and cyclodextrin metal-organic frameworks (CD-MOFs) have attracted widespread attention from the scientific community as two highly promising functional materials.

[0003] N,CDs are synthesized via a hydrothermal method using citric acid and urea as precursors. With precisely controllable particle size, abundant surface functional groups, and heteroatom-doped structures, they exhibit excellent pH indication, photothermal responsiveness, and antibacterial properties, providing a new carrier for intelligent sensing, photothermal therapy, and composite material reinforcement. However, their antibacterial properties still need further improvement, and their pH indication may be compromised due to their combination with other materials.

[0004] In addition, the natural antibacterial agent carvacrol exhibits a broad-spectrum antibacterial effect against foodborne pathogens, but its volatile and photothermal unstable properties limit its long-term application in packaging films.

[0005] Therefore, how to further enhance the antibacterial properties of food packaging films with added N,CDs by utilizing the natural antibacterial agent carvacrol, and how to utilize the pH indication and photothermal responsiveness of N,CDs to achieve color indication and long-term effects of food packaging films, remains to be studied. Summary of the Invention

[0006] To address the shortcomings and deficiencies of existing technologies, the primary objective of this invention is to provide a method for preparing a dual-response nitrogen-doped carbon dot-modified carboxymethyl cellulose / polyvinyl alcohol composite membrane with pH indication and photothermal antibacterial properties. Based on the sustained-release characteristics of cyclodextrin, this invention loads carvacrol into the composite membrane. Through multiple mechanisms including photothermal synergistic sterilization, pH response indication, and sustained-release antibacterial agents, it achieves an intelligent upgrade of packaging functions while maintaining the storage quality of fruits and vegetables, providing new design ideas and experimental basis for the development of active intelligent packaging materials for fruits and vegetables.

[0007] Another object of the present invention is to provide a dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite membrane with pH indication and photothermal antibacterial properties prepared by the above preparation method.

[0008] Another object of the present invention is to provide the application of the above-mentioned dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite film with pH indication and photothermal antibacterial properties in the preservation of fruits and vegetables.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a method for preparing a dual-response nitrogen-doped carbon dot-modified carboxymethyl cellulose / polyvinyl alcohol composite membrane with pH indication and photothermal antibacterial properties, comprising the following steps:

[0011] (1) Mix citric acid, urea and water, and perform hydrothermal reaction. After centrifugation, take the supernatant, filter and let stand, add ethanol for alcohol precipitation to obtain purified nitrogen-doped carbon dots.

[0012] (2) A base film solution is prepared by mixing carboxymethyl cellulose, polyvinyl alcohol and plasticizer in a solvent;

[0013] (3) The purified nitrogen-doped carbon dots, cyclodextrin and carvacrol (CAR) were added to the base membrane solution and mixed evenly. After degassing and drying, a dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite membrane with pH indication and photothermal antibacterial properties was obtained.

[0014] Preferably, the mass ratio of citric acid and urea in step (1) is 1:1 to 1:2.

[0015] Preferably, the ratio of citric acid to water in step (1) is 1-2g: 40-50mL.

[0016] Preferably, the temperature of the hydrothermal reaction in step (1) is 160-180°C and the time is 6-8 hours.

[0017] Preferably, the centrifugation speed in step (1) is 8000-10000 rpm and the time is 8-10 min.

[0018] Preferably, the filtration in step (1) refers to passing through a 0.22μm microporous filter membrane.

[0019] Preferably, the temperature for standing in step (1) is room temperature, and the time is 3 to 5 days.

[0020] Preferably, the nitrogen-doped carbon dot solution obtained after standing in step (1) and ethanol are mixed in a volume ratio of 1:2 to 1:4.

[0021] Preferably, the alcohol precipitation in step (1) is carried out at room temperature for 1 to 5 days.

[0022] Preferably, the alcohol precipitation in step (1) is followed by drying at 30–50°C to constant weight to obtain purified nitrogen-doped carbon dots.

[0023] Preferably, the mass ratio of carboxymethyl cellulose to polyvinyl alcohol in step (2) is 3:1 to 4:1.

[0024] Preferably, the plasticizer in step (2) is glycerol.

[0025] Preferably, the mass ratio of the plasticizer to polyvinyl alcohol in step (2) is 2:1 to 4:1.

[0026] Preferably, the solvent in step (2) is water.

[0027] Preferably, the mass ratio of the solvent to polyvinyl alcohol in step (2) is 200:1 to 300:1.

[0028] Preferably, in step (2), the temperature at which the mixture is dissolved in the solvent is 90-100°C and the time is 4-6 hours, and the stirring speed is 800-1000 rpm.

[0029] Preferably, the ratio of purified nitrogen-doped carbon dots, cyclodextrin, carvacrol and base film solution in step (3) is (0.01~0.05) g: 0.3±0.05 g: 0.5±0.05 g: 100±1 mL; more preferably, it is 0.03±0.005 g: 0.3±0.005 g: 0.5±0.005 g: 100±5 mL.

[0030] Preferably, the cyclodextrin in step (3) includes at least one of β-cyclodextrin and γ-cyclodextrin.

[0031] Preferably, the specific conditions for mixing in step (3) are: stirring at room temperature for 3±1h to obtain a blended solution.

[0032] Preferably, the specific steps of drying in step (3) are: air drying for 2 to 3 days under conditions of relative humidity ≤30% and temperature 20℃.

[0033] Secondly, the present invention provides a dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite membrane with pH indication and photothermal antibacterial properties prepared by the above preparation method.

[0034] The composite membrane of the present invention contains nitrogen-doped carbon dots, which have excellent photothermal conversion performance and pH-responsive color-changing performance. Furthermore, a multifunctional system is constructed by introducing carvacrol and β / γ-cyclodextrin for the slow-release of the antibacterial agent carvacrol.

[0035] Thirdly, the present invention provides the application of the above-mentioned dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite film with pH indication and photothermal antibacterial properties in the preservation of fruits and vegetables.

[0036] Furthermore, the method of using the nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite film is as follows: the fruits and vegetables are sealed and packaged with the nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite film and stored in an environment of 7±1℃ and relative humidity of 75~85%.

[0037] The principle of this invention: Carbon dot materials, with their abundant carboxyl and amino functional groups on their surface, can achieve regular color changes. Furthermore, carbon dots can absorb photons and transition to an excited state, subsequently converting the excitation energy into thermal energy through an efficient non-radiative relaxation process. Cyclodextrin, with its internally hydrophobic and externally hydrophilic cavity structure, can form supramolecular inclusion complexes with hydrophobic carvacrol. This effectively inhibits the volatilization and oxidation of carvacrol through physical shielding, and allows for the slow release of carvacrol via a competitive substitution mechanism under moisture or thermal induction.

[0038] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0039] (1) The dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite membrane with pH indication and photothermal antibacterial properties prepared by the method of the present invention utilizes the excellent optical sensitivity of nitrogen-doped carbon dots to enable the composite membrane to have a sensitive pH response indication function, and can monitor the food spoilage process in real time through color or fluorescence changes; at the same time, it leverages the efficient photothermal conversion performance of carbon dots to generate local high temperature under near-infrared light irradiation, thereby achieving active physical disinfection of pathogenic bacteria.

[0040] (2) The dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite film with pH indication and photothermal antibacterial properties prepared by the method of the present invention effectively solves the defects of antibacterial agents being volatile and poorly soluble in water through the molecular inclusion of carvacrol by cyclodextrin; combined with the heat energy driven by the photothermal effect of carbon dots, intelligent temperature-controlled release of carvacrol inside the packaging is realized, and a dual synergistic antibacterial system of photothermal physical sterilization and slow release of antibacterial agents is constructed.

[0041] (3) The dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite film with pH indication and photothermal antibacterial properties prepared by the method of the present invention and the control example can effectively extend the shelf life of blueberries by 12 days and 8 days, respectively. Attached Figure Description

[0042] Figure 1 The color of the nitrogen-doped carbon dots prepared in Example 1 (A) and Comparative Example 5 (B) of this invention in a buffer solution of pH 2–13.

[0043] Figure 2 The image shows the XRD pattern of the purified nitrogen-doped carbon dots prepared in Example 1 of this invention.

[0044] Figure 3 This is the XPS spectrum of the purified nitrogen-doped carbon dots prepared in Example 1 of the present invention.

[0045] Figure 4 This is the UV-Vis absorption spectrum of the purified nitrogen-doped carbon dots prepared in Example 1 of the present invention.

[0046] Figure 5 This is a TEM spectrum of the purified nitrogen-doped carbon dots prepared in Example 1 of the present invention.

[0047] Figure 6 These are scanning electron microscope (SEM) images of the composite membranes and PVA / CMC membranes prepared in Comparative Examples 2, 10 and Examples 2-3 of this invention.

[0048] Figure 7 The graphs show the photothermal response behavior of the composite films and PVA / CMC films prepared in Comparative Examples 1-4, 10 and Examples 2-3 of this invention.

[0049] Figure 8 The images show color indicators of the composite membranes and PVA / CMC membranes prepared in Comparative Examples 1-4, 10 and Examples 2-3 of this invention in different pH buffer solutions.

[0050] Figure 9 The images show the color changes of the packaging film and the gas composition analysis of the packaging in different experimental groups in Examples 4 and 5 of this invention.

[0051] Figure 10 The appearance changes of blueberries treated in different experimental groups in Examples 4, 5 and Comparative Examples 6-9 of this invention are shown.

[0052] Figure 11 The changes in weight loss rate of blueberries treated in different experimental groups in Examples 4 and 5 and Comparative Examples 6 to 9 of this invention are shown.

[0053] Figure 12 The changes in soluble solids content of blueberries treated in different experimental groups in Examples 4, 5 and Comparative Examples 6-9 of this invention are shown.

[0054] Figure 13 The changes in total phenolic content of blueberries treated in different experimental groups in Examples 4, 5 and Comparative Examples 6-9 of this invention are shown.

[0055] Figure 14 The changes in anthocyanin content in blueberries treated with different experimental groups in Examples 4 and 5 and Comparative Examples 6 to 9 of this invention are shown.

[0056] Figure 15The color indicator diagrams show the ED,CDs / Cur and ED,CDs / CA base membrane solutions prepared in Comparative Example 11 of this invention in different pH buffer solutions. Detailed Implementation

[0057] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the implementation of the present invention is not limited thereto.

[0058] Unless otherwise specified in the embodiments of this invention, the conditions shall be performed according to conventional conditions or conditions recommended by the manufacturer. All raw materials and reagents used, unless otherwise specified, are commercially available conventional products.

[0059] Example 1

[0060] Accurately weigh 2g each of citric acid and urea and dissolve them thoroughly in 50mL of distilled water. Stir until completely dissolved to obtain a homogeneous precursor solution. Then, transfer the solution to a polytetrafluoroethylene-lined reactor and perform a hydrothermal reaction at 170℃ for 6 hours. After the reaction is completed and the mixture is naturally cooled to room temperature, the product is centrifuged at 10000r / min for 10 minutes to remove insoluble impurities. The supernatant is then vacuum filtered through a 0.22μm microporous membrane to obtain a pure nitrogen-doped carbon dot liquid. Finally, the liquid is allowed to stand at room temperature for 3 days for natural oxidation.

[0061] Take 10 mL of nitrogen-doped carbon dot solution and slowly add 30 mL of anhydrous ethanol (final concentration 75%, v / v), stirring thoroughly to ensure homogeneous mixing. Then, allow the mixture to stand at room temperature for 24 h to induce complete precipitation of the nitrogen-doped carbon dots. After standing, centrifuge at 10000 r / min, discarding the supernatant containing impurities and collecting the bottom precipitate. Finally, dry the obtained precipitate in a vacuum drying oven at 40 °C to constant weight to obtain ethanol-precipitated nitrogen-doped carbon dots.

[0062] Example 2

[0063] The nitrogen-doped carbon dot preparation process is the same as in Example 1.

[0064] 16g of carboxymethyl cellulose (CMC) and 4g of polyvinyl alcohol (PVA) were dissolved in 1000mL of distilled water, and 9g of glycerol was used as a plasticizer to mix thoroughly (total dry weight 29g). The mixture was stirred in a water bath at 96℃ for 4h until completely dissolved to obtain a CMC / PVA base film solution. 100mL of this solution was taken, and 0.03g of purified nitrogen-doped carbon dots (N,CDs), 0.5g of CAR and 0.3g of β-cyclodextrin were added. The mixture was stirred at room temperature for 3h to obtain a blend solution. 75mL of this solution was poured into a 15cm×20cm mold and air-dried for 3d at a relative humidity ≤30% and a temperature of about 20℃. After film formation, the film was carefully peeled off to obtain a PVA / CMC / 0.3EN,CDs / CAR / β composite membrane.

[0065] Example 3

[0066] The nitrogen-doped carbon dot preparation process is the same as in Example 1.

[0067] 16g of carboxymethyl cellulose (CMC) and 4g of polyvinyl alcohol (PVA) were dissolved in 1000mL of distilled water, and 9g of glycerol was used as a plasticizer to mix thoroughly (total dry weight 29g). The mixture was stirred in a water bath at 96℃ for 4h until completely dissolved to obtain a CMC / PVA base film solution. 100mL of this solution was taken, and 0.03g of purified nitrogen-doped carbon dots (N,CDs), 0.5g of CAR and 0.3g of γ-cyclodextrin were added. The mixture was stirred at room temperature for 3h to obtain a blend solution. 75mL of this solution was poured into a 15cm×20cm mold and air-dried for 3d at a relative humidity ≤30% and a temperature of about 20℃. After film formation, the film was carefully peeled off to obtain a PVA / CMC / 0.3EN,CDs / CAR / γ composite film.

[0068] Example 4

[0069] Take 80±2g of blueberries and pre-cool them at 4℃ for 6 hours. Cover the container with the film as described in Example 3. Store the entire container in a constant temperature and humidity chamber at 25±1℃ and 50%RH. Regularly measure the preservation indicators. Regularly photograph and record the color change of the packaging film and the appearance of the fruit. Evaluate the fruit based on the color of the peel and stem, rot and mold, and the degree of skin wrinkling.

[0070] Example 5

[0071] Take 80±2g of blueberries, pre-cool them at 4℃ for 6 hours, cover them with the film in Example 3, and irradiate them with 808nm infrared light for 3 minutes daily. Store them in a constant temperature and humidity chamber at 25±1℃ and 50%RH, and measure the preservation indicators regularly. Take photos regularly to record the color change of the packaging film and the appearance of the fruit, and evaluate them based on the color of the peel and stem, rot and mold, and the degree of skin wrinkling.

[0072] Comparative Example 1

[0073] The nitrogen-doped carbon dot preparation process is the same as in Example 1.

[0074] 16g of carboxymethyl cellulose (CMC) and 4g of polyvinyl alcohol (PVA) were dissolved in 1000mL of distilled water, and 9g of glycerol was used as a plasticizer to mix thoroughly (total dry weight 29g). The mixture was stirred in a water bath at 96℃ for 4h until completely dissolved to obtain a CMC / PVA base film solution. 100mL of this solution was taken, and 0.01g of purified nitrogen-doped carbon dots (N,CDs) was added. The mixture was stirred at room temperature for 3h to obtain a blend solution. 75mL of this solution was poured into a 15cm×20cm mold and air-dried for 3d under conditions of relative humidity ≤30% and temperature of about 20℃. After film formation, the film was carefully peeled off to obtain a PVA / CMC / 0.1EN,CDs composite film.

[0075] Comparative Example 2

[0076] The nitrogen-doped carbon dot preparation process is the same as in Example 1.

[0077] 16g of carboxymethyl cellulose (CMC) and 4g of polyvinyl alcohol (PVA) were dissolved in 1000mL of distilled water, and 9g of glycerol was used as a plasticizer to mix thoroughly (total dry weight 29g). The mixture was stirred in a water bath at 96℃ for 4h until completely dissolved to obtain a CMC / PVA base film solution. 100mL of this solution was taken, and 0.03g of purified nitrogen-doped carbon dots (N,CDs) was added. The mixture was stirred at room temperature for 3h to obtain a blend solution. 75mL of this solution was poured into a 15cm×20cm mold and air-dried for 3d under conditions of relative humidity ≤30% and temperature of about 20℃. After film formation, the film was carefully peeled off to obtain a PVA / CMC / 0.3EN,CDs composite film.

[0078] Comparative Example 3

[0079] The nitrogen-doped carbon dot preparation process is the same as in Example 1.

[0080] 16g of carboxymethyl cellulose (CMC) and 4g of polyvinyl alcohol (PVA) were dissolved in 1000mL of distilled water, and 9g of glycerol was used as a plasticizer to mix thoroughly (total dry weight 29g). The mixture was stirred in a water bath at 96℃ for 4h until completely dissolved to obtain a CMC / PVA base film solution. 100mL of this solution was taken, and 0.05g of purified nitrogen-doped carbon dots (N,CDs) was added. The mixture was stirred at room temperature for 3h to obtain a blend solution. 75mL of this solution was poured into a 15cm×20cm mold and air-dried for 3d under conditions of relative humidity ≤30% and temperature of about 20℃. After film formation, the film was carefully peeled off to obtain a PVA / CMC / 0.5EN,CDs composite film.

[0081] Comparative Example 4

[0082] The nitrogen-doped carbon dot preparation process is the same as in Example 1.

[0083] 16g of carboxymethyl cellulose (CMC) and 4g of polyvinyl alcohol (PVA) were dissolved in 1000mL of distilled water, and 9g of glycerol was used as a plasticizer to mix thoroughly (total dry weight 29g). The mixture was stirred in a water bath at 96℃ for 4h until completely dissolved to obtain a CMC / PVA base film solution. 100mL of this solution was taken, and 0.03g of purified nitrogen-doped carbon dots (N,CDs) and 0.5g of CAR were added. The mixture was stirred at room temperature for 3h to obtain a blend solution. 75mL of this solution was poured into a 15cm×20cm mold and air-dried for 3d under conditions of relative humidity ≤30% and temperature of about 20℃. After film formation, the film was carefully peeled off to obtain a PVA / CMC / 0.3EN,CDs / CAR composite film.

[0084] Comparative Example 5

[0085] Accurately weigh 2g each of citric acid and urea and dissolve them thoroughly in 50mL of distilled water. Stir until completely dissolved to obtain a homogeneous precursor solution. Then, transfer the solution to a polytetrafluoroethylene-lined reactor and perform a hydrothermal reaction at 170℃ for 6 hours. After the reaction is completed and the mixture is naturally cooled to room temperature, the product is centrifuged at 10000r / min for 10 minutes to remove insoluble impurities. The supernatant is then vacuum filtered through a 0.22μm microporous membrane to obtain a pure nitrogen-doped carbon dot liquid. Finally, allow it to stand at room temperature for 3-5 days for natural oxidation.

[0086] The nitrogen-doped carbon dot solution was dialyzed for 24 hours using a dialysis bag (MD77-300). The dialyzed solution was then dried in a vacuum drying oven at 40°C until constant weight was obtained to obtain nitrogen-doped carbon dots.

[0087] Comparative Example 6

[0088] Blueberries (80±2g) were pre-cooled at 4℃ for 6 hours and covered with a preservation box using the pure PVA / CMC film prepared in Comparative Example 10. The entire batch was stored in a constant temperature and humidity chamber at 25±1℃ and 50%RH, and preservation indicators were measured periodically. Changes in the color of the packaging film and the appearance of the fruit were photographed periodically, and the results were evaluated based on the color of the peel and stem, the degree of rot and mold, and the extent of skin wrinkling.

[0089] Comparative Example 7

[0090] Take 80±2g of blueberries, pre-cool them at 4℃ for 6 hours, and cover them with PE film in a preservation box. Store the entire batch in a constant temperature and humidity chamber at 25±1℃ and 50%RH, and measure preservation indicators regularly. Regularly photograph and record changes in the color of the packaging film and the appearance of the fruit, and evaluate them based on the color of the skin and stems, rot and mold, and the degree of skin wrinkling.

[0091] Comparative Example 8

[0092] Take 80±2g of blueberries, pre-cool them at 4℃ for 6 hours, and store them in an uncovered container. Store the entire batch in a constant temperature and humidity chamber at 25±1℃ and 50%RH, and regularly measure preservation indicators. Regularly photograph and record changes in the color of the packaging film and the appearance of the fruit, and evaluate the fruit based on the color of the skin and stems, the degree of rot and mold, and the extent of skin wrinkling.

[0093] Comparative Example 9

[0094] Blueberries (80±2g) were pre-cooled at 4℃ for 6 hours, placed in an uncovered preservation box, and irradiated with 808nm infrared light for 3 minutes daily. The entire batch was stored in a constant temperature and humidity chamber at 25±1℃ and 50%RH, with preservation indicators measured periodically. Changes in the color of the packaging film and the appearance of the fruit were regularly photographed and recorded. Evaluation was based on the color of the skin and stems, the degree of rot and mold, and the extent of skin wrinkling.

[0095] Comparative Example 10

[0096] Dissolve 16g of carboxymethyl cellulose (CMC) and 4g of polyvinyl alcohol (PVA) in 1000mL of distilled water, and mix thoroughly with 9g of glycerol as a plasticizer (total dry weight 29g). Stir in a water bath at 96℃ for 4 hours until completely dissolved to obtain a CMC / PVA base film solution. Pour 75mL of this solution into a 15cm×20cm mold and air dry for 2-3 days at a relative humidity ≤30% and a temperature of about 20℃. After film formation, carefully peel off to obtain a pure PVA / CMC film.

[0097] Comparative Example 11

[0098] The nitrogen-doped carbon dot preparation process is the same as in Example 1.

[0099] 16g of carboxymethyl cellulose (CMC) and 4g of polyvinyl alcohol (PVA) were dissolved in 1000mL of distilled water, and 9g of glycerol was used as a plasticizer to mix thoroughly (total dry weight 29g). The mixture was stirred in a water bath at 96℃ for 4h until completely dissolved to obtain a CMC / PVA base membrane solution. 100mL of this solution was taken, and 0.03g of purified nitrogen-doped carbon dots (N,CDs) and 0.5g of curcumin were added. The mixture was stirred at room temperature for 3h to obtain an ED,CDs / Cur base membrane solution.

[0100] 16g of carboxymethyl cellulose (CMC) and 4g of polyvinyl alcohol (PVA) were dissolved in 1000mL of distilled water, and 9g of glycerol was used as a plasticizer to mix thoroughly (total dry weight 29g). The mixture was stirred in a water bath at 96℃ for 4h until completely dissolved to obtain a CMC / PVA base film solution. 100mL of this solution was taken, and 0.03g of purified nitrogen-doped carbon dots (N,CDs) and 0.5g of cinnamaldehyde were added. The mixture was stirred at room temperature for 3h to obtain an ED,CDs / CA base film solution.

[0101] Performance Analysis

[0102] Figure 1 Photographs of nitrogen-doped carbon dot solutions purified by different methods are shown in the pH range of 2–13. Figure 1 The color of solution A shows a clear pattern with pH changes. Under acidic conditions of pH=2 to 5, it is light gray or light brown. Under neutral to weakly alkaline conditions of pH=6 to 11, it gradually turns into a clear blue. Under strongly alkaline conditions of pH=12 to 13, the blue color weakens and turns into a light blue-green. Figure 1 The color of solution B does not change with pH.

[0103] Figure 2 The results showed that the broadened and diffuse peaks of the carbon dot core were preserved during the ethanol precipitation process, while the sharp impurity peaks remaining from the precursor were almost completely eliminated, and the spectrum was smoother and purer. This proves that the post-treatment of ethanol precipitation can efficiently remove small molecule crystalline impurities such as CA and Urea without destroying the amorphous / graphite-like framework structure of the carbon dots.

[0104] Figure 3 XPS was used to characterize the surface chemical composition of EN and CDs. Characteristic peaks for C1s, N1s, and O1s appeared at 285.0 eV, 401.0 eV, and 532.0 eV, respectively, indicating that the samples are mainly composed of carbon, nitrogen, and oxygen, with C, N, and O atomic percentages of 67.38%, 8.06%, and 24.56%, respectively. The surface is rich in oxygen- and nitrogen-containing functional groups. Nitrogen-doped carbon dots maintain an amorphous carbon and graphite-like microcrystalline structure, with intact surface functional groups, exhibiting significant characteristic absorption in the visible light region. The results indicate that ethanol precipitation treatment did not destroy the core framework and chemical bonding structure of the carbon dots.

[0105] Figure 4 The results show that as the pH increases from acidic to alkaline, the absorption peak at 335 nm exhibits a regular rightward red shift: under acidic conditions, the peak is concentrated at 320–325 nm; under neutral to weakly alkaline conditions, it red-shifts to 330–340 nm; and under strongly alkaline conditions, the peak position remains relatively stable. The change in absorption intensity at 648 nm is consistent with the color evolution of the solution: absorption is weaker under acidic conditions, significantly increases under neutral to weakly alkaline conditions and reaches a peak at pH 8–10, and slightly decreases under strongly alkaline conditions. This change is consistent with the color evolution of the carbon point solution, from near-brownish-yellow to blue, and then to pale yellow-green.

[0106] Figure 5The study showed that after ethanol precipitation, nitrogen-doped carbon dots significantly agglomerated, forming spherical aggregates with a wide size distribution. Particle size analysis revealed an average diameter of 66.201 ± 26.331 nm, indicating ethanol-induced agglomeration. After ultrasonic dispersion in water, the agglomerates completely dissociated, and EN,CDs reverted to uniformly dispersed spherical single particles with good dispersibility. The magnified lattice spacing of a single EN,CDs particle was 0.09 nm, consistent with the structure of composite carbon quantum dots. Particle size analysis showed that the EN,CDs particle size was mainly distributed between 1 and 3 nm, with an average diameter of 1.984 ± 0.4047 nm, consistent with the size range of carbon quantum dots. This demonstrates that ethanol precipitation is a reversible purification method that preserves the intrinsic structure of the carbon dots.

[0107] Figure 6 The surface and cross-sectional SEM morphology of the PVA / CMC-based composite membrane were displayed. The PVA / CMC membrane surface was smooth and dense, without obvious defects, and the cross-sectional structure was uniform with tight interlayer bonding. After adding 0.3 mg / mL EN and CDs, a few fine protrusions appeared on the membrane surface, but the cross-section remained dense and uniform, without obvious pores or delamination. This indicates that nitrogen-doped carbon dots form hydrogen bonds with the matrix through hydrophilic groups such as -COOH and -OH, effectively improving the compactness of the membrane structure. Further introduction of β-CD and CAR slightly increased the surface roughness of the membrane, and a few micropores began to appear in the cross-section. However, after adding γ-CD and CAR, a large number of rod-shaped crystal structures appeared on the membrane surface, and the number of micropores in the cross-section increased significantly, and the structure became rougher. This indicates that the cavity structure of cyclodextrin and the self-assembly behavior of the components changed the aggregation state of the membrane. Overall, the introduction of EN and CDs maintained the compactness of the membrane, while the addition of cyclodextrin made the membrane structure more porous and fibrous.

[0108] Figure 7The results showed that the PVA / CMC film maintained a constant temperature of 21℃ within 0–36 s, exhibiting no photothermal conversion capability. After introducing nitrogen-doped carbon dots, the PVA / CMC / EN,CDs composite film exhibited typical photothermal kinetic characteristics: rapid temperature rise from 0–12 s, followed by a slowdown and equilibrium after 12 s, with both the heating rate and equilibrium temperature significantly increasing with the nitrogen-doped carbon dot content. The PVA / CMC / 0.5EN,CDs film reached a surface temperature of 200.35℃ after 18 s of irradiation and exceeded 280℃ after 24 s, indicating an excessively large temperature rise, making it unsuitable for fruit and vegetable preservation. In contrast, the PVA / CMC / 0.3EN,CDs film reached a temperature of 124.04℃ after 24 s of irradiation and remained stable near this temperature, meeting the requirements for short-term high-temperature sterilization. After introducing CAR, β, and γ components into the PVA / CMC / 0.3EN,CDs membrane, each system maintained a trend of rapid initial temperature rise followed by a slowdown. The PVA / CMC / 0.3EN,CDs / CAR / γ membrane reached a temperature of 118.21℃ after 24 seconds, with a temperature rise of 98.04℃, exhibiting the optimal synergistic photothermal effect and achieving short-term high-temperature sterilization. The PVA / CMC / 0.3EN,CDs membrane achieved a maximum temperature rise of 139.21℃ and an average temperature retention rate of 94.84% during five near-infrared light irradiation cycles, indicating structural stability, minimal performance degradation, and good photothermal stability.

[0109] Figure 8The color changes and specific color parameters of different PVA / CMC / EN,CDs composite membranes in buffer solutions with different pH values ​​are shown, with pH=7 as the reference standard. The PVA / CMC membrane remains transparent grayish-white with no significant color change within the pH range of 2–13. After the introduction of EN,CDs, the pH response of the composite membrane is significantly enhanced, exhibiting high sensitivity in the acidic region, especially near the pH=5.6 carbonic acid critical value, where a distinct hue transitions from brownish-red to cool blue, showing an overall trend of changing from a warm hue in acidic conditions to a cool hue in alkaline conditions. The PVA / CMC / 0.1EN,CDs membrane shows a weak color change response and insufficient photothermal conversion efficiency, making it difficult to meet the requirements of short-term high-temperature sterilization; the PVA / CMC / 0.5EN,CDs membrane is too dark, appearing black, severely affecting the visual observation of color changes. The PVA / CMC / 0.3EN,CDs membrane exhibits a deep brown color within the pH range of 2–6. At pH 5.6, it transitions from a warm to a cool hue, and under strongly alkaline conditions, the membrane appears deep blue. The PVA / CMC / 0.3EN,CDs membrane shows significant color change and suitable photothermal properties, making it the optimal base system for subsequent applications. After incorporating CAR into the PVA / CMC / 0.3EN,CDs membrane, the composite membrane gradually changes from its original blue to a greenish hue. This is likely due to the interaction between CAR and the functional groups on the carbon dot surface, altering the surface electronic states and optical response of the carbon dots, causing a redshift in their absorption and emission spectra, resulting in a yellowish-green hue. Further introduction of β-CD or γ-CD gradually restores the membrane's hue to blue. This is likely because cyclodextrin can form inclusion complexes with CAR through hydrophobic cavities, reducing its impact on the optical properties of the carbon dots and allowing the system to regain the intrinsic blue optical characteristics of the carbon dots. In summary, all carbon dot-modified membranes exhibit highly sensitive pH indication in acidic environments, and all show significant hue abrupt changes near pH 5.6. Considering both photothermal performance and color indication, 0.3 EN, CDs is the optimal system.

[0110] Figure 9 The results showed that during 20 days of blueberry storage, both the PVA / CMC / 0.3EN,CDs / CAR / γ and PVA / CMC / 0.3EN,CDs / CAR / γNIR+ membranes exhibited significant pH-responsive color changes, which were closely related to the increase in CO2 concentration within the packaging. On day 4 of storage, both the PVA / CMC / 0.3EN,CDs / CAR / γ and PVA / CMC / 0.3EN,CDs / CAR / γNIR+ membranes changed from their initial blue color to gray. At this time, the CO2 concentration within the packaging reached its peak (9.16–9.98%), and the CO2 dissolved to form carbonic acid, causing a decrease in the pH inside the packaging. This prompted the chromogenic groups within the membranes to undergo initial protonation, thus initiating the color response. By day 16 of storage, the color had completely changed from a cool blue-green tone to a warm reddish-yellow tone, demonstrating a significant color response behavior.

[0111] Figure 10 The results showed significant differences in the protective effects of different packaging films on the appearance quality of blueberries during a 20-day storage period. The control group (Blank, BlankNIR+) and the PE film group exhibited the fastest deterioration rate, with severely shrunken fruit by 12 days of storage. The PE film group showed virtually no shrunkenness, but exhibited more pronounced mold and moisture condensation, resulting in the worst preservation effect. The PVA / CMC / 0.3EN, CDs / CAR / γ film group showed moderate preservation efficacy, maintaining fruit with only slight shrunkenness up to 16 days, and a small amount of rot appearing after 20 days, effectively delaying fruit deterioration. The PVA / CMC / 0.3EN, CDs / CAR / γNIR+ film group demonstrated the best preservation effect, keeping the fruit plump throughout the first 12 days of storage, and only experiencing slight shrunkenness due to the film's hydrophilicity from 16 to 20 days, without significant rot, showing a significantly lower degree of deterioration than other groups. The weight loss rates of the PVA / CMC, PVA / CMC / 0.3EN, CDs / CAR / γ membrane groups and the PVA / CMC / 0.3EN, CDs / CAR / γNIR+ membrane groups after 20 days of preservation were 19.33%, 20.70%, and 19.64%, respectively, which were significantly lower than those of the control group. Figure 11 This indicates that near-infrared irradiation did not exacerbate moisture loss, and the photothermal effect of EN and CDs only served to sterilize and regulate the sustained release of carvacrol, without damaging the membrane structure and barrier properties. The SSC of the PVA / CMC / 0.3EN,CDs / CAR / γ membrane and the PVA / CMC / 0.3EN,CDs / CAR / γNIR+ membrane groups initially increased slowly and then stabilized, reaching approximately 13.16% and 13.10% after 20 days, respectively, significantly better than the PE membrane group. Figure 12 This is because the CAR loading in the PVA / CMC / 0.3EN,CDs / CAR / γ membrane effectively inhibits fruit respiration intensity and delays carbohydrate decomposition and consumption; while reducing water loss, it maximizes the retention of soluble solids and other flavor substances. The PVA / CMC / 0.3EN,CDs / CAR / γNIR+ membrane group effectively maintained a high phenolic level in the early stage of storage. Although some degradation occurred in the later stage, its overall performance was significantly better than other control groups. The PVA / CMC / 0.3EN,CDs / CAR / γNIR+ membrane group effectively maintained anthocyanin levels in the middle stage of storage, delaying peel fading. Although there was a decrease in the later stage, the overall retention rate was higher than the blank group, proving that the composite membrane can reduce mold stress and effectively inhibit pigment degradation and maintain the appearance quality of blueberries through the synergistic effect of carbon point photothermal sterilization and CAR.

[0112] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a dual-response nitrogen-doped carbon dot-modified carboxymethyl cellulose / polyvinyl alcohol composite membrane with pH indication and photothermal antibacterial properties, characterized in that, Includes the following steps: (1) Mix citric acid, urea and water, and perform hydrothermal reaction. After centrifugation, take the supernatant, filter and let stand, add ethanol for alcohol precipitation to obtain purified nitrogen-doped carbon dots. (2) A base film solution is prepared by mixing carboxymethyl cellulose, polyvinyl alcohol and plasticizer in a solvent; (3) The purified nitrogen-doped carbon dots, cyclodextrin and carvacrol were added to the base membrane solution and mixed evenly. After degassing and drying, a dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite membrane with pH indication and photothermal antibacterial properties was obtained.

2. The preparation method according to claim 1, characterized in that, The nitrogen-doped carbon dot solution obtained after standing in step (1) is mixed with ethanol at a volume ratio of 1:2 to 1:

4. And / or, the alcohol precipitation in step (1) is carried out at room temperature for 1 to 5 days.

3. The preparation method according to claim 1 or 2, characterized in that, The ratio of purified nitrogen-doped carbon dots, cyclodextrin, carvacrol, and base film solution in step (3) is (0.01-0.05) g: 0.3±0.05 g: 0.5±0.05 g: 100±1 mL; more preferably, it is 0.03±0.005 g: 0.3±0.005 g: 0.5±0.005 g: 100±5 mL; And / or, the cyclodextrin in step (3) includes at least one of β-cyclodextrin and γ-cyclodextrin.

4. The preparation method according to claim 1 or 2, characterized in that, The mass ratio of citric acid to urea in step (1) is 1:1 to 1:2; And / or, the temperature of the hydrothermal reaction in step (1) is 160-180°C and the time is 6-8h.

5. The preparation method according to claim 1 or 2, characterized in that, The mass ratio of carboxymethyl cellulose to polyvinyl alcohol in step (2) is 3:1 to 4:1; And / or, the mass ratio of the plasticizer to polyvinyl alcohol in step (2) is 2:1 to 4:1; And / or, the plasticizer in step (2) is glycerol.

6. The preparation method according to claim 1 or 2, characterized in that, The solvent used in step (2) is water; And / or, the mass ratio of the solvent to polyvinyl alcohol in step (2) is 200:1 to 300:

1.

7. The preparation method according to claim 1 or 2, characterized in that, The specific mixing conditions in step (3) are: stirring at room temperature for 3±1h to obtain a blended solution; And / or, the specific steps of drying in step (3) are: air drying for 2 to 3 days under conditions of relative humidity ≤30% and temperature 20℃.

8. The preparation method according to claim 1 or 2, characterized in that, The ratio of citric acid to water in step (1) is 1-2g: 40-50mL; And / or, the centrifugation speed in step (1) is 8000-10000 rpm and the time is 8-10 min; And / or, the filtration in step (1) refers to passing the filter through a 0.22 μm microporous membrane; And / or, the temperature for standing in step (1) is room temperature, and the time is 3 to 5 days.

9. A dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite membrane with pH indication and photothermal antibacterial properties, prepared by the method according to any one of claims 1 to 8.

10. The application of the dual-response nitrogen-doped carbon dot modified carboxymethyl cellulose / polyvinyl alcohol composite film with pH indication and photothermal antibacterial properties as described in claim 9 in the preservation of fruits and vegetables.

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