Cold plasma in-situ synthesized carbon dot / nanocellulose membrane as well as preparation method and food preservation application thereof

Abstract

The invention discloses a cold plasma in-situ synthesized carbon dot / nanocellulose membrane as well as a preparation method and food preservation application thereof. The method comprises the following steps: (1) preparing a cellulose nanofiber dispersion liquid, and activating carboxyl groups on the surfaces of cellulose nanofibers by adopting a 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide / N-hydroxysuccinimide system; (2) introducing o-phenylenediamine into the activated dispersion liquid; and (3) carrying out cold plasma treatment to realize in-situ synthesis of the carbon dots on the surface of the cellulose nanofiber so as to obtain the composite film. The method breaks through the limitation of step-by-step implementation of carbon dot synthesis and material compounding in the traditional process, the process flow is remarkably shortened, and the energy consumption is reduced. And meanwhile, the growth of the carbon nucleus is strictly limited on the surface of the cellulose nanofiber through cold plasma induction, so that relatively uniform growth of the carbon dots on the surface of the cellulose nanofiber is realized. The fiber membrane disclosed by the invention has excellent mechanical and anti-oxidation properties and can be applied to fresh keeping of fruits and vegetables.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanocellulose modification, and specifically relates to a carbon dot / nanocellulose film synthesized in situ by cold plasma, a preparation method thereof, and food preservation application thereof. Background Art

[0002] Nanocellulose is a nano-scale fiber material extracted from natural biomass, with remarkable properties such as large specific surface area, environmental friendliness and biodegradability. It is widely available and sustainable, and exhibits excellent performance in applications such as catalysis, adsorption and sensing. In addition, the biodegradability of nanocellulose enables it to completely decompose in the natural environment, reducing the burden on the environment. These properties make it an ideal green material, widely used in packaging, composite materials, biomedicine and other fields. As an emerging zero-dimensional nanomaterial, carbon dots, with their excellent optical properties, chemical properties and chemical stability, can be combined with nanocellulose to form a composite material with both optical and mechanical properties, providing new ideas for the high-value-added applications of both.

[0003] In recent years, with the continued pursuit of environmentally friendly material synthesis methods, research on the efficient and green synthesis of carbon dots in liquid phases using cold plasma technology has increased. By introducing cold plasma into a liquid environment, this technology enables the rapid synthesis of carbon dots at relatively low temperatures while avoiding the use of hazardous chemical reagents commonly found in traditional methods. This demonstrates excellent environmental performance and high synthesis efficiency.

[0004] Traditional methods for preparing carbon dot / nanocellulose membranes have many shortcomings, such as low efficiency and serious environmental pollution, and it is difficult to achieve a perfect balance between high strength and high toughness. These methods usually require the separation and purification of carbon dots, which not only increases the complexity of the process, but also leads to the generation of a large amount of waste liquid. The post-processing process is cumbersome and easily burdens the environment. In addition, the combination of carbon dots and nanocellulose is not tight enough, and the interfacial compatibility is poor, making it difficult for the mechanical properties of the composite membrane to reach an ideal state. Patent application 202311004777.0 discloses "A method for preparing ultraviolet-absorbing carbon quantum dot nanocellulose film". The carbon dots synthesized by hydrothermal synthesis are mixed with nanocellulose to prepare a film. The hydrothermal synthesis of carbon dots has a high temperature and a long time. For example, the hydrothermal reaction temperature is 160-180°C and the reaction time is 4-6h. Therefore, there is an urgent need to develop a method for preparing carbon dots / nanocellulose that is efficient, low-consumption, and environmentally friendly. Summary of the Invention

[0005] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the object of the present invention is to provide a carbon dot / nanocellulose membrane prepared in situ by cold plasma, and a preparation method and application thereof.

[0006] The purpose of the present invention is achieved through the following technical solutions.

[0007] A method for preparing a carbon dot / nanocellulose membrane synthesized in situ by cold plasma comprises the following steps:

[0008] (1) TEMPO-CNF (cellulose nanofibers prepared by TEMPO oxidation) was dispersed in an acetic acid-sodium acetate buffer solution, and EDC (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide) and NHS (N-hydroxysuccinimide) were added to activate the carboxyl groups on the surface of the cellulose nanofibers.

[0009] (2) introducing o-phenylenediamine, a carbon dot precursor, into the dispersion activated in step (1) and mixing thoroughly;

[0010] (3) The mixed solution obtained in step (2) is subjected to cold plasma treatment to achieve in-situ synthesis of carbon dots on the surface of cellulose nanofibers, and after centrifugal washing, a vacuum filtration process is used to form a carbon dot / nanocellulose membrane.

[0011] Preferably, in step (1), TEMPO-CNF is dispersed in an acetic acid-sodium acetate buffer solution with a pH of 3-4, and the concentration of TEMPO-CNF is 0.1-1.5 wt%.

[0012] Further preferably, in step (1), TEMPO-CNF is dispersed in an acetic acid-sodium acetate buffer solution with a pH of 3, and the concentration of TEMPO-CNF is 1 wt%.

[0013] Preferably, in step (1), TEMPO-CNF is subjected to carboxyl activation treatment. The specific process is as follows: EDC and NHS are added to the TEMPO-CNF suspension, and the reaction is stirred at room temperature for 30-60 minutes. The molar ratio of the amount of EDC and NHS added to the carboxyl content in the suspension is 1:1:(4-5). After the reaction, the cellulose nanofibers are centrifuged and washed to remove unreacted activator and by-products, and then redispersed in an acetic acid-sodium acetate buffer solution.

[0014] Further preferably, in step (1), the molar ratio of the added amount of EDC and NHS to the carboxyl content in the suspension is 1:1:4.

[0015] Preferably, in step (1), the cellulose nanofibers are centrifugally washed after the reaction at a centrifugal speed of 6000-8000 rpm for 10-15 min.

[0016] Preferably, the concentration of TEMPO-CNF in the dispersion after activation in step (2) is 0.1-0.2 wt %, o-phenylenediamine is added, and magnetic stirring is carried out at room temperature for 10-20 min.

[0017] Preferably, in step (2), the amount of o-phenylenediamine added is 0.2-0.4 wt % of the activated dispersion.

[0018] Preferably, in step (3), the cold plasma treatment time is 4-10 min, the operating voltage is 40-60 kV, and the frequency is 50-70 Hz.

[0019] Preferably, in step (3), the centrifugal washing is performed at a speed of 6000-8000 rpm for 10-15 min.

[0020] A carbon dot / nanocellulose membrane prepared by any of the preparation methods described above.

[0021] The application of the carbon dot / nanocellulose film in food preservation mentioned above comprises the following steps: covering the surface of the food with the carbon dot / nanocellulose film as a fresh-keeping package.

[0022] Mechanism of the present invention:

[0023] In situ synthesis of carbon dots (Cdots) on cellulose nanofibers is achieved by exciting the reaction solution with active particles from a cold plasma. An EDC / NHS system is used to activate the carboxyl groups on the surface of TEMPO-CNFs. The active ester (-COO-NHS) on the CNF surface reacts with the amino group of o-phenylenediamine to form an amide bond, confining the carbon nucleus growth to the CNF surface. Electron collisions rupture the π bond of the benzene ring, generating o-phenylenediamine free radicals, which initiate a free radical chain reaction. Particles such as ·OH and ·O2- trigger redox reactions such as decarboxylation and dehydration, promoting the condensation of the carbon nucleus. The cold plasma-induced interfacial bonding strengthens the bond between the nanocellulose and Cdots, resulting in a composite film with both high strength and toughness. The active groups formed during the synthesis of the Cdots, such as phenolic hydroxyl, amino, and carboxyl groups, act as electron or hydrogen donors, directly scavenging free radicals. Furthermore, plasma synthesis allows the Cdots to be directly anchored to the cellulose nanofiber surface, increasing the exposure of the active sites.

[0024] The role played by cold plasma in the present invention includes:

[0025] 1. Efficient and controllable synthesis of carbon dots. The synthesis of carbon dots at room temperature and pressure is much more time-consuming than the hydrothermal method.

[0026] 2. Precise anchoring and uniform growth of carbon dots. Cold plasma induction strictly limits the growth of carbon nuclei to the surface of cellulose nanofibers, preventing the agglomeration of free carbon dots and achieving relatively uniform growth of carbon dots on the surface of cellulose nanofibers.

[0027]

[0028] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0029] 1) The present invention uses cold plasma technology to directly synthesize carbon dots in situ on the surface of nanocellulose at room temperature and pressure, replacing the traditional hydrothermal method of high temperature and high pressure (160–180°C / 4–6h). The reaction time is shortened to less than 20 minutes, and energy consumption is reduced by more than 90%, achieving green and efficient production.

[0030] 2) The present invention uses cold plasma to relatively evenly anchor carbon dots to cellulose nanofibers, significantly improving the mechanical strength of the composite membrane while also having good antioxidant properties, breaking through the limitations of traditional materials that are difficult to coordinate strength and function.

[0031] 3) The carbon dot / nanocellulose film prepared by the present invention can extend the shelf life of cherry tomatoes to more than 15 days through the synergistic effects of moisture barrier and antioxidant properties. The material is also completely biodegradable, solving the problem of plastic pollution and providing a solution for fruit and vegetable preservation and green packaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Flow chart of the preparation of carbon dot / nanocellulose membrane in the present invention.

[0033] Figure 2 This is a physical picture of the carbon dot / nanocellulose membrane prepared in the present invention.

[0034] Figure 3 Graph showing the tensile strength of different films prepared in Examples 1-3 of the present invention and Comparative Examples 1-3.

[0035] Figure 4 This is a comparison chart of the DPPH radical scavenging rates of different membranes prepared in Examples 1-3 of the present invention and Comparative Examples 1-3.

[0036] Figure 5 This is a comparison chart of ABTS radical scavenging rates of different membranes prepared in Examples 1-3 of the present invention and Comparative Examples 1-3.

[0037] Figure 6 This is a comparison chart of the weight loss rates of cherry tomatoes packaged in Example 3 of the present invention, Comparative Examples 2-3, and without film covering.

[0038] Figure 7 This is a comparison chart of the hardness changes of cherry tomatoes packaged in Example 3 of the present invention, Comparative Examples 2-3, and without film covering. DETAILED DESCRIPTION

[0039] The TEMPO-oxidized cellulose nanofiber suspension used in the following examples was prepared using existing technology. The typical preparation method is as follows: 1g of absolutely dry cellulose nanofibers is dispersed in 100mL of deionized water and stirred continuously to form a uniform suspension; 0.0125g of TEMPO oxidant (2,2,6,6-tetramethylpiperidine-1-oxyl free radical), 0.125g of sodium bromide, and a sodium hypochlorite buffer solution (5mmol of sodium hypochlorite is added per g of cellulose nanofiber) are then added. The pH is maintained at 10 with a 1mol / L hydrochloric acid solution. After the solution pH no longer changes, the reaction is continued for 90min, and then the pH is adjusted to 7.0 with a 1mol / L hydrochloric acid solution to terminate the reaction. Repeat three times of centrifugation and washing (8000rpm, 15min), take the precipitate and disperse it in deionized water to make a suspension and refrigerate. Some of the hydroxyl groups on the surface of the cellulose nanofibers are oxidized to carboxyl groups, thereby introducing carboxyl functional groups. The TEMPO oxidized cellulose nanofiber suspension (TEMPO-CNF) prepared by the method has a solid content of 1.02% and a carboxyl content of 1.76 mmol / g.

[0040] The flow chart of the present invention for preparing carbon dot / nanocellulose membrane is shown in Figure 1 .

[0041] Example 1

[0042] TEMPO-CNF was dispersed in acetic acid-sodium acetate buffer (pH = 3, same below) to form a 1 wt% suspension (10 mL). 8.43 mg of EDC and 5.06 mg of NHS were added and stirred at room temperature for 30 minutes. After centrifugation and washing to remove unreacted activator, the suspension was redispersed in 100 mL of acetic acid-sodium acetate buffer. 0.2 g of o-phenylenediamine was added and magnetic stirring was applied for 10 minutes to achieve uniform dispersion. The suspension was then treated with a cold plasma at 40 kV and 60 Hz for 5 minutes, followed by three centrifugal washes (8000 rpm for 15 minutes). The suspension was redispersed in deionized water and vacuum filtered to form a membrane. The resulting membrane was then applied to cherry tomatoes for packaging. After 15 days of storage at 20°C and 75% relative humidity, the tomatoes showed no signs of mold, experienced a weight loss of approximately 4%, and decreased in hardness by approximately 16%.

[0043] Example 2

[0044] TEMPO-CNF was dispersed in acetic acid-sodium acetate buffer (pH = 3, same below) to form a 1 wt% suspension (20 mL). 16.8 mg of EDC and 10 mg of NHS were added and stirred at room temperature for 30 minutes. After centrifugation and washing to remove unreacted activator, the suspension was redispersed in 100 mL of acetic acid-sodium acetate buffer. 0.3 g of o-phenylenediamine was added and magnetic stirring was applied for 10 minutes to achieve uniform dispersion. Cold plasma treatment was then performed at 50 kV and 60 Hz for 6 minutes. Unreacted small molecules were removed by centrifugation three times (8000 rpm for 15 minutes). The suspension was redispersed in deionized water and vacuum filtered to form a membrane. The resulting membrane was then coated with cherry tomatoes and packaged. After 15 days of storage at 20°C and 75% relative humidity, the tomatoes showed no signs of mold, experienced a weight loss of approximately 4.5%, and had a hardness reduction of approximately 17%.

[0045] Example 3

[0046] TEMPO-CNF was dispersed in acetic acid-sodium acetate buffer (pH 3, same below) to form a 1 wt% suspension in 30 mL. 25.3 mg of EDC and 15.18 mg of NHS were added and stirred at room temperature for 30 minutes. After centrifugation and washing to remove unreacted activator, the suspension was redispersed in 300 mL of acetic acid-sodium acetate buffer. 1.2 g of o-phenylenediamine was added and magnetically stirred for 10 minutes to achieve uniform dispersion. Cold plasma treatment was then performed at 60 kV and 60 Hz for 10 minutes, followed by three centrifugal washes (8000 rpm for 15 minutes). The suspension was redispersed in deionized water and vacuum filtered to form a membrane. The resulting membrane was then applied to cherry tomatoes for packaging. After 15 days of storage at 20°C and 75% relative humidity, the tomatoes showed no signs of mold, experienced a weight loss of approximately 4%, and had a hardness reduction of approximately 15%.

[0047] Comparative Example 1

[0048] TEMPO-CNF was dispersed in acetic acid-sodium acetate buffer (pH = 3, the same below) to form a 1wt% suspension in 30mL. 25.3mg EDC and 15.18mg NHS were added and stirred at room temperature for 30min. After centrifugation and washing to remove unreacted activator, the suspension was redispersed in 300mL of acetic acid-sodium acetate buffer. The suspension was then treated with cold plasma at 60kV and 60Hz for 10min, followed by three centrifugation washes (8000rpm, 15min). The suspension was redispersed in deionized water and vacuum filtered to form a membrane. The resulting membrane was then coated with cherry tomatoes and packaged. After 15 days of storage at 20°C and 75% relative humidity, the tomatoes showed slight mildew, a weight loss of approximately 7%, and a decrease in hardness of approximately 35%.

[0049] Comparative Example 2

[0050] TEMPO-CNF was dispersed in deionized water to form a 0.1 wt% suspension (300 mL). The suspension was then vacuum filtered to form a membrane. The resulting nanocellulose membrane was then coated on cherry tomatoes for packaging. After 15 days of storage at 20°C and 75% relative humidity, the tomatoes showed slight signs of mold, lost approximately 6.3% of their weight, and decreased in firmness by approximately 36%.

[0051] Comparative Example 3

[0052] 1.2 g of o-phenylenediamine was dissolved in 50 mL of deionized water and ultrasonically dispersed for 10 minutes. The mixture was then transferred to a polytetrafluoroethylene reactor and hydrothermally reacted at 180°C for 6 hours. The resulting solution was cooled, centrifuged (8000 rpm, 15 minutes), filtered, and dialyzed to obtain a tan carbon dot solution.

[0053] TEMPO-CNF was dispersed in acetic acid-sodium acetate buffer (pH 3) to prepare a 1 wt% suspension in 30 mL. 25.3 mg of EDC and 15.18 mg of NHS were added and stirred at room temperature for 30 min. The suspension was washed by centrifugation to remove any unreacted activator and then redispersed in 300 mL of acetic acid-sodium acetate buffer.

[0054] The resulting brown carbon dot solution was mixed with the activated TEMPO-CNF suspension and magnetically stirred for 10 minutes to achieve uniform dispersion. The resulting membrane was then vacuum filtered to form a film. The resulting membrane was then coated on cherry tomatoes and packaged. After 15 days of storage at 20°C and 75% relative humidity, the tomatoes showed no signs of mold, lost approximately 6% in weight, and decreased in firmness by approximately 30%.

[0055] Comparative Example 4

[0056] Commercially available polyethylene (PE) plastic film. Cherry tomatoes were randomly placed in fresh-keeping boxes, covered with PE film, and stored at 20°C and 75% relative humidity. After six days of storage, all PE-wrapped cherry tomatoes were infected with mold. PE film has poor air permeability, and after covering, the humidity inside the box becomes oversaturated, condensation accumulates, creating conditions for mold growth. PE film only acts as a physical barrier and lacks antibacterial and antioxidant properties. Microorganisms on the cherry tomatoes multiply rapidly in high temperature and humidity.

[0057] Depend on Figure 3 It can be seen that the tensile strength of the carbon dot / nanocellulose membrane synthesized by cold plasma in situ is significantly improved compared to the pure nanocellulose membrane (Comparative Example 2). Comparative Example 1 (without the addition of o-phenylenediamine) shows that cold plasma treatment alone can increase the membrane strength, but the increase is lower than that of the carbon dot / nanocellulose membrane. On the one hand, the cold plasma treatment itself can improve the mechanical properties by modifying the nanocellulose. On the other hand, the cold plasma induces the carbon dots to grow uniformly on the surface of the nanocellulose and combine with the nanocellulose interface to produce a synergistic enhancement effect. In addition, compared with the traditional hydrothermal non-in situ preparation method (Comparative Example 3), the cold plasma in situ synthesis also has more advantages in tensile strength.

[0058] Figure 4 Figure 5 This indicates that the carbon dot / nanocellulose membrane synthesized in situ by cold plasma exhibits excellent free radical scavenging capabilities, with significantly higher scavenging efficiency than that of carbon dot / nanocellulose membranes synthesized ex situ by hydrothermal methods. This is because the carbon dots synthesized in situ by cold plasma are stably anchored to the nanocellulose through chemical bonds, resulting in uniform distribution. However, carbon dots synthesized ex situ by hydrothermal methods rely solely on physical adsorption, resulting in easy detachment and low loading capacity.

[0059] Figure 6 Figure 7 The results showed that in the storage and preservation of cherry tomatoes, the cherry tomatoes packaged with cold plasma in situ synthesized carbon dots / nanocellulose membrane had the lowest weight loss rate and no significant decrease in hardness compared with unpackaged, simple nanocellulose membrane packaged and traditional hydrothermal non-in situ synthesis membrane packaged. This indicates that cold plasma in situ synthesized carbon dots / nanocellulose membrane packaging significantly extended the shelf life of cherry tomatoes and has advantages over traditional hydrothermal non-in situ synthesis.

[0060] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing carbon dot / nanocellulose membrane synthesized in situ by cold plasma, characterized in that: The following steps are involved: (1) TEMPO-CNF was dispersed in acetic acid-sodium acetate buffer solution, and EDC and NHS were added to activate the carboxyl groups on the surface of cellulose nanofibers; (2) introducing o-phenylenediamine, a carbon dot precursor, into the dispersion activated in step (1) and mixing thoroughly; (3) The mixed solution obtained in step (2) is subjected to cold plasma treatment to achieve in-situ synthesis of carbon dots on the surface of cellulose nanofibers, and after centrifugal washing, a vacuum filtration process is used to form a carbon dot / nanocellulose membrane.

2. The preparation method according to claim 1, characterized in that In step (1), TEMPO-CNF is dispersed in an acetic acid-sodium acetate buffer solution with a pH of 3-4, and the concentration of TEMPO-CNF is 0.1-1.5 wt%.

3. The preparation method according to claim 1, characterized in that In step (1), TEMPO-CNF is subjected to carboxyl activation treatment. The specific process is as follows: EDC and NHS are added to the TEMPO-CNF suspension, and the reaction is stirred at room temperature for 30-60 minutes. The molar ratio of the amount of EDC and NHS added to the carboxyl content in the suspension is 1:1:(4-5). After the reaction, the cellulose nanofibers are centrifuged and washed to remove unreacted activator and by-products, and then redispersed in an acetic acid-sodium acetate buffer solution.

4. The preparation method according to claim 1, characterized in that In step (1), after the reaction, the cellulose nanofibers are centrifuged and washed at a speed of 6000-8000 rpm for 10-15 minutes.

5. The preparation method according to claim 1, characterized in that The concentration of TEMPO-CNF in the activated dispersion in step (2) is 0.1-0.2 wt %, o-phenylenediamine is added, and magnetic stirring is carried out at room temperature for 10-20 minutes.

6. The preparation method according to claim 1, characterized in that In step (2), the amount of o-phenylenediamine added is 0.2-0.4 wt % of the activated dispersion.

7. The preparation method according to claim 1, characterized in that In step (3), the cold plasma treatment time is 4-10 minutes, the operating voltage is 40-60 kV, and the frequency is 50-70 Hz.

8. The preparation method according to claim 1, characterized in that In step (3), the centrifugal washing is performed at a speed of 6000-8000 rpm for 10-15 minutes.

9. A carbon dot / nanocellulose membrane prepared by the preparation method according to any one of claims 1 to 8.

10. The use of the carbon dot / nanocellulose film according to claim 9 in food preservation, characterized in that: The following steps are involved: The carbon dot / nanocellulose film is covered on the surface of food as fresh-keeping packaging.

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