Carbon nano-based edible fruit fresh-keeping hydrogel as well as preparation method and application thereof

Abstract

The invention discloses carbon nano-based edible fruit fresh-keeping hydrogel and a preparation method and application thereof.The method comprises the following steps that S1, 5-fluorouracil and D-arginine are dissolved in deionized water, and carbon dots are prepared through a hydrothermal reaction; and S2, adding the carbon dots into the gelatin solution, and uniformly stirring to obtain the carbon nano-based edible fruit fresh-keeping hydrogel. The invention provides an edible fruit fresh-keeping hydrogel (CDs / Gel) based on carbon dots, and the carbon dots are synthesized by using gallic acid (GA), 5-fluorouracil (5-FU) and D-arginine (D-Arg) as precursors through a hydrothermal method; wherein the GA provides antioxidant activity, the 5-FU enhances the penetrating power of a bacterial membrane, and the D-Arg promotes an antibacterial effect; in combination with the excellent performance of the gelatin matrix, the shelf life of the fruits can be effectively prolonged by the CDs / Gel hydrogel; according to the invention, a safe and efficient solution can be provided for fresh-keeping of fruits and vegetables.

Description

Technical Field

[0001] The present invention relates to the fields of chemistry and nanomaterials, and in particular to a carbon nano-based edible fruit-keeping hydrogel, a preparation method and application thereof. Background Art

[0002] Postharvest deterioration of grain, primarily caused by microbial contamination, oxidative stress, and water loss, not only reduces fruit quality but also shortens its shelf life. Despite the high nutritional and economic value of fruit, existing preservation methods (such as chemical preservatives) often rely on harmful additives, are costly, or pose environmental risks. Therefore, the development of safe, efficient, and environmentally friendly preservation technologies is crucial.

[0003] Although current preservation strategies (such as cold chain logistics, chemical preservatives, and modified atmosphere packaging) can extend the shelf life of agricultural products, they all have limitations. Cold chain systems have high energy consumption and large carbon emissions; chemical preservatives (such as sulfites and benzoates) may be harmful to health; and modified atmosphere packaging relies on plastic materials, exacerbating environmental pollution. In contrast, bio-based edible coatings based on natural polymers (proteins, polysaccharides, etc.) are more promising. They can form a semi-permeable membrane on the surface of fruits, regulate gas exchange, inhibit microbial growth, and reduce water loss, and are both biodegradable and safe. However, traditional coatings still face problems such as poor adhesion, weak mechanical properties, and limited antibacterial ability. There is an urgent need to develop more stable and multifunctional improved formulas.

[0004] In recent years, nanomaterial-enhanced hydrogel coatings have attracted much attention due to their potential to impart antibacterial and antioxidant properties. For example, metal nanoparticles (Ag, ZnO), carbon-based materials (graphene oxide, carbon dots), and plant-derived antioxidants have been introduced into gelatin, chitosan, or alginate matrices to construct functional coatings, demonstrating promising results in antibacterial and reactive oxygen species (ROS) scavenging. However, such systems still face key challenges: (1) the potential toxicity or inedibility of some nanofillers limits their food contact applications; (2) the compatibility and stability of multicomponent systems are poor; and (3) uniform adhesion of the coating to the hydrophobic peel is difficult, especially without the use of chemical crosslinkers. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a carbon nano-based edible fruit preservative hydrogel, a preparation method and application thereof, in view of the deficiencies in the above-mentioned prior art.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows: In a first aspect of the present invention, a method for preparing a carbon nano-based edible fruit preservative hydrogel is provided, comprising the following steps:

[0007] S1, dissolving 5-fluorouracil, D-arginine and arginine in deionized water, and preparing carbon dots by hydrothermal reaction;

[0008] S2. Add the carbon dots into the gelatin solution and stir evenly to obtain a carbon nano-based edible fruit preservative hydrogel.

[0009] Preferably, step S1 is specifically:

[0010] 5-Fluorouracil, D-arginine and gallic acid were dissolved in deionized water and stirred until completely dissolved. The resulting solution was transferred to an autoclave for a hydrothermal reaction. After the reaction was completed and naturally cooled to room temperature, the product was centrifuged, the supernatant was filtered, and the filtrate was freeze-dried to obtain carbon dots.

[0011] Preferably, step S1 is specifically:

[0012] 0.05-0.2 g of 5-fluorouracil, 0.05-0.2 g of D-arginine, and 0.1-0.4 g of gallic acid were dissolved in 10-40 mL of deionized water and stirred until completely dissolved. The resulting solution was transferred to a stainless steel autoclave lined with polytetrafluoroethylene fiber and hydrothermally reacted at 160-200°C for 3-12 hours. After naturally cooling to room temperature, the product was centrifuged at 5000-22000 rpm for 5-20 minutes. The supernatant was filtered through a 0.1-0.4 μm membrane, and the filtrate was freeze-dried to obtain carbon dots.

[0013] Preferably, step S1 is specifically:

[0014] 0.1 g of 5-fluorouracil, 0.1 g of D-arginine, and 0.2 g of gallic acid were dissolved in 20 mL of deionized water and stirred until completely dissolved. The resulting solution was transferred to a stainless steel autoclave lined with polytetrafluoroethylene fiber and hydrothermally reacted at 180°C for 6 hours. After naturally cooling to room temperature, the product was centrifuged at 11,000 rpm for 10 minutes, the supernatant was filtered through a 0.22 μm membrane, and the filtrate was freeze-dried to obtain carbon dots.

[0015] Preferably, in step S2, the concentration of carbon dots added to the gelatin solution is 25-200 μg / mL.

[0016] Preferably, step S2 is specifically:

[0017] Add gelatin at a mass concentration of 1-4% to deionized water, stir at 30-70°C and 100-400 rpm for 5-30 minutes, until a clear gelatin solution is obtained;

[0018] The carbon dots were added to the gelatin solution, the concentration of the carbon dots was controlled to be 25-200 μg / mL, and the solution was stirred evenly to obtain a carbon nano-based edible fruit preservative hydrogel.

[0019] Preferably, step S2 is specifically:

[0020] Gelatin was added to deionized water at a mass concentration of 2%, and stirred at 50°C and 200 rpm for 15 minutes until a clear gelatin solution was obtained;

[0021] The carbon dots were added to the gelatin solution, the concentration of the carbon dots was controlled to be 25-200 μg / mL, and the solution was stirred evenly to obtain a carbon nano-based edible fruit preservative hydrogel.

[0022] A second aspect of the present invention provides a carbon nano-based edible fruit-preserving hydrogel, which is prepared by the method described above.

[0023] A third aspect of the present invention provides a use of the hydrogel described above in preserving fruits.

[0024] Preferably, the application method is: evenly coating the carbon nano-based edible fruit preservative hydrogel on the surface of the fruit or soaking the fruit in the carbon nano-based edible fruit preservative hydrogel and then taking it out and drying it naturally to form a hydrogel film on the surface of the fruit.

[0025] The beneficial effects of the present invention are:

[0026] The present invention provides an edible fruit preservation hydrogel (CDs / Gel) based on carbon dots (CDs). The carbon dots are synthesized by a hydrothermal method using gallic acid (GA), 5-fluorouracil (5-FU) and D-arginine (D-Arg) as precursors. GA provides antioxidant activity, 5-FU enhances bacterial membrane penetration, and D-Arg promotes antibacterial effects. Combined with the excellent performance of the gelatin matrix, the CDs / Gel hydrogel has an inhibition rate of over 99.9% against common pathogens at a concentration of 100 μg / mL and a free radical scavenging rate of 70%, effectively extending the shelf life of fruits. The present invention can provide a safe and efficient solution for preserving fruits and vegetables. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a transmission electron microscopy photograph of the carbon dots prepared in step S1;

[0028] Figure 2 The performance characterization results of the carbon dots prepared in step S1;

[0029] Figure 3 Demonstrate the antibacterial properties of the hydrogel film;

[0030] Figure 4 The antioxidant performance test results of the hydrogel film;

[0031] Figure 5 The test results of the protective performance of hydrogel film for fruits. DETAILED DESCRIPTION

[0032] The present invention is further described in detail below with reference to the embodiments so that those skilled in the art can implement the invention with reference to the description.

[0033] It should be understood that terms such as “having”, “including” and “comprising” used herein do not preclude the existence or addition of one or more other elements or combinations thereof.

[0034] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Materials and reagents used in the following examples are commercially available unless otherwise specified. In the following examples, where specific conditions are not specified, the experiments were conducted under conventional conditions or those recommended by the manufacturer. Reagents and instruments used, where the manufacturer is not specified, are commercially available conventional products.

[0035] Example 1

[0036] A carbon nano-based edible fruit-preserving hydrogel is prepared by the following method:

[0037] S1. Dissolve 0.1 g of 5-fluorouracil, 0.1 g of D-arginine, and 0.2 g of gallic acid in 20 mL of deionized water and stir until completely dissolved. The resulting solution is transferred to a stainless steel autoclave lined with polytetrafluoroethylene fiber and subjected to hydrothermal reaction at 180°C for 6 hours. After naturally cooling to room temperature, the product is centrifuged at 11,000 rpm for 10 minutes to remove insoluble residues. The supernatant is filtered through a 0.22 μm membrane to eliminate residual aggregates. The filtrate is freeze-dried to obtain carbon dots (CDs).

[0038] S2. Add gelatin at a mass concentration of 2% to deionized water, stir at 50°C and 200 rpm for 15 minutes, until a clear gelatin solution is obtained; add carbon dots to the gelatin solution, control the concentration of carbon dots to 25-200 μg / mL, and stir evenly to obtain a carbon nano-based edible fruit preservative hydrogel.

[0039] In this example, several groups of hydrogel samples with different carbon dot concentrations were prepared: 25 μg / mL (CDs / Gel-25), 50 μg / mL (CDs / Gel-50), 100 μg / mL (CDs / Gel-100), and 200 μg / mL (CDs / Gel-200). The hydrogels were then cast onto a clean mold and naturally dried at ambient temperature to form a hydrogel film. The dried hydrogel film was carefully peeled off and stored at -4°C for subsequent experimental use.

[0040] Performance Testing

[0041] Reference Figure 1, is a transmission electron micrograph of the carbon dots prepared in step S1. Transmission electron microscopy (TEM) shows uniformly dispersed spherical nanoparticles with an average diameter of about 1.5 nm and a lattice fringe of 0.27 nm, which corresponds to the lattice spacing of graphitic carbon.

[0042] Reference Figure 2 , are the performance characterization results of the carbon dots prepared in step S1. Fourier transform infrared spectroscopy (FTIR) is used to characterize the functional groups of the synthesized CDs and compare them with precursors such as gallic acid, 5-fluorouracil and D-arginine; Figure 2 As shown in a, the FTIR spectrum of CDs reveals typical signals related to -OH / -NH2, C=O and COC groups. These features indicate that CDs retain most of the characteristic chemical structure of the original material and retain rich surface functions during the carbonization process, which is conducive to hydrophilicity and potential biological interactions. After hydrogel (Gel) encapsulation, the Zeta potential changes from -15mV of the original CDs to +3.2mV of CDs / Gel, indicating the presence of strong electrostatic interactions within the composite material ( Figure 2 b). Notably, E. coli and S. aureus exhibited negatively charged surfaces, which favored electrostatic attraction of positively charged CDs / Gel and may enhance antibacterial adhesion. X-ray photoelectron spectroscopy (XPS) wide scan revealed the presence of C, N, O, and F elements in CDs ( Figure 2 c). High-resolution spectroscopy reveals the main CO and CN bonds in the C1s spectrum, C=O and CO / COC in the O1s spectrum, and functional groups such as pyridinic-N, graphitic-N, and pyrrolic-N in the N1s spectrum ( Figure 2 df), which contributes to the photoluminescence and antibacterial properties.

[0043] Reference Figure 3 , which is the result of demonstrating the antibacterial performance of the hydrogel film, this example tests the antibacterial effect of the CDs / Gel coating on Escherichia coli, Staphylococcus aureus, methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa. Figure 3 As shown in a, the plate count results showed that bacterial growth was inhibited in a CDs / Gel concentration-dependent manner. When the hydrogel film contained CDs at a concentration of 100 μg / mL (CDs / Gel-100), no visible bacterial colonies were detected on the agar plates of all test strains. At this concentration, the corresponding antibacterial efficiency exceeded 99.9%, confirming the strong bactericidal activity. In contrast, at concentrations of 25 μg / mL and 50 μg / mL, colony formation was reduced but not completely eliminated. By monitoring the optical density (OD 600) values ​​were used to further evaluate the antibacterial effect. Within 24 hours, CDs / Gel-100 effectively inhibited the growth of all four bacterial strains throughout the entire time course, while the effect was weakened at lower concentrations of CDs / Gel ( Figure 3 b). To further investigate the effects of CDs / Gel on bacterial survival and membrane integrity, three-dimensional fluorescence imaging was performed using Calcein AM and propidium iodide (PI) staining. Figure 3 As shown in c, after CDs / Gel treatment, the concentration of most bacterial cells was greatly reduced or they showed strong red fluorescence, indicating cell membrane rupture and cell death. Quantitative analysis of biofilm structure showed that its thickness decreased from 25.5μm, 20.0μm, 23.7μm and 22.8μm to 4.3μm, 4.5μm, 5.0μm and 7.3μm ( Figure 3 d). Morphological analysis by SEM showed extensive membrane rupture, collapse, and leakage of intracellular material in bacteria treated with CDs / Gel coating ( Figure 3 e). In addition, crystal violet staining confirmed that biofilm formation was significantly reduced in all groups ( Figure 3 f), these results support the efficacy of the hydrogel film in biofilm inhibition. Overall, the CDs / Gel hydrogel film exhibited strong concentration-dependent antibacterial activity by destroying bacterial membranes and biofilms, and could effectively protect against microbial-induced fruit spoilage.

[0044] Reference Figure 4 , is the test result of the antioxidant property of the hydrogel film. Oxidative stress is another key factor leading to fruit spoilage and inflammation-related cell damage. In order to evaluate the antioxidant property of CDs / Gel hydrogel film, in vitro free radical scavenging test and intracellular ROS imaging study were carried out. In the in vitro analysis, the antioxidant property of CDs / Gel at different concentrations was determined using DPPH, ABTS and hydroxyl radical (·OH) assays. Figure 4 As shown in a, the free radical scavenging rate increased in a concentration-dependent manner. When the CDs concentration in CDs / Gel reached 200 μg / mL, the scavenging efficiencies of DPPH, ABTS, and ·OH reached peak values ​​of 86.3%, 82.4%, and 78.1%, respectively, indicating that it has a strong free radical neutralization ability. In order to investigate the intracellular antioxidant and anti-inflammatory potential of CDs / Gel hydrogel membranes, RAW 264.7 macrophages were treated with LPS to simulate the pro-inflammatory environment in vivo. When cells were incubated with 50 and 100 μg / mL of CDs / Gel, a significant decrease in fluorescence intensity was shown after DCFH-DA staining, indicating that the generation of intracellular ROS was effectively inhibited ( Figure 4b). Notably, the ROS scavenging behavior observed in cells was consistent with the results of free radical elimination in vitro, confirming that the antioxidant capacity of CDs / Gel was retained in the biological environment.

[0045] Reference Figure 5 The following are the test results of the protective performance of the hydrogel film on fruits. To comprehensively evaluate the fruit preservation performance of the developed CDs / Gel coating, cherry tomatoes and strawberries were selected as model fruits for demonstration. The hydrogel prepared in step S2 of Example 1 was first evenly coated on the surface of the fruit and naturally dried to form a hydrogel film. All fruits were then stored at room temperature and observed for 10 days. Figure 5 As shown in (a), at day 0, there were no significant differences in the surface appearance of all groups. Microbial colonization first appeared in the untreated and CDs-treated samples on day 4, while visible spoilage occurred in the calcium lactate (CaLac) and gel (Gel) groups on days 6 and 8, respectively. Notably, even after 10 days, the fruit treated with CDs / Gel remained visually intact, with minimal signs of microbial contamination or structural collapse.

[0046] Quantitative evaluation further supported these observations. After 10 days of storage, the fruits in the CDs / Gel group showed the lowest weight loss rate, the slowest pH increase, and the most stable texture among all groups ( Figure 5 ch). In contrast, the untreated sample experienced significant weight loss, rapid softening, and a significant increase in pH, indicating physiological degradation and deterioration. These results clearly demonstrate that the CDs / Gel hydrogel coating provides excellent preservation by effectively inhibiting microbial growth, reducing water loss, and delaying fruit senescence.

[0047] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the description and implementation methods. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to specific details.

Claims

1. A method for preparing a carbon nano-based edible fruit preservative hydrogel, characterized in that: The following steps are involved: S1, dissolving 5-fluorouracil, D-arginine and arginine in deionized water, and preparing carbon dots by hydrothermal reaction; S2. Add the carbon dots into the gelatin solution and stir evenly to obtain a carbon nano-based edible fruit preservative hydrogel.

2. The method for preparing the carbon nano-based edible fruit fresh-keeping hydrogel according to claim 1, wherein: Step S1 is specifically as follows: 5-Fluorouracil, D-arginine and gallic acid were dissolved in deionized water and stirred until completely dissolved. The resulting solution was transferred to an autoclave for a hydrothermal reaction. After the reaction was completed and naturally cooled to room temperature, the product was centrifuged, the supernatant was filtered, and the filtrate was freeze-dried to obtain carbon dots.

3. The method for preparing the carbon nano-based edible fruit fresh-keeping hydrogel according to claim 2, wherein: Step S1 is specifically as follows: 0.05-0.2 g of 5-fluorouracil, 0.05-0.2 g of D-arginine, and 0.1-0.4 g of gallic acid were dissolved in 10-40 mL of deionized water and stirred until completely dissolved. The resulting solution was transferred to a stainless steel autoclave lined with polytetrafluoroethylene fiber and hydrothermally reacted at 160-200°C for 3-12 hours. After naturally cooling to room temperature, the product was centrifuged at 5000-22000 rpm for 5-20 minutes. The supernatant was filtered through a 0.1-0.4 μm membrane, and the filtrate was freeze-dried to obtain carbon dots.

4. The method for preparing the carbon nano-based edible fruit preservative hydrogel according to claim 3, wherein: Step S1 is specifically as follows: 0.1 g of 5-fluorouracil, 0.1 g of D-arginine, and 0.2 g of gallic acid were dissolved in 20 mL of deionized water and stirred until completely dissolved. The resulting solution was transferred to a stainless steel autoclave lined with polytetrafluoroethylene fiber and hydrothermally reacted at 180°C for 6 hours. After naturally cooling to room temperature, the product was centrifuged at 11,000 rpm for 10 minutes, the supernatant was filtered through a 0.22 μm membrane, and the filtrate was freeze-dried to obtain carbon dots.

5. The method for preparing the carbon nano-based edible fruit preservative hydrogel according to claim 1, wherein: In step S2, the concentration of carbon dots added to the gelatin solution is 25-200 μg / mL.

6. The method for preparing the carbon nano-based edible fruit preservative hydrogel according to claim 5, wherein: Step S2 is specifically as follows: Add gelatin at a mass concentration of 1-4% to deionized water, stir at 30-70°C and 100-400 rpm for 5-30 minutes, until a clear gelatin solution is obtained; The carbon dots were added to the gelatin solution, the concentration of the carbon dots was controlled to be 25-200 μg / mL, and the solution was stirred evenly to obtain a carbon nano-based edible fruit preservative hydrogel.

7. The method for preparing the carbon nano-based edible fruit preservative hydrogel according to claim 6, wherein: Step S2 is specifically as follows: Gelatin was added to deionized water at a mass concentration of 2%, and stirred at 50°C and 200 rpm for 15 minutes until a clear gelatin solution was obtained; The carbon dots were added to the gelatin solution, the concentration of the carbon dots was controlled to be 25-200 μg / mL, and the solution was stirred evenly to obtain a carbon nano-based edible fruit preservative hydrogel.

8. A carbon nano-based edible fruit preserving hydrogel, characterized in that: It is prepared by the method according to any one of claims 1 to 7.

9. Use of the hydrogel according to claim 8 in preserving fruits.

10. The use according to claim 9, characterized in that The application method is: evenly coating the carbon nano-based edible fruit preservative hydrogel on the surface of the fruit or soaking the fruit in the carbon nano-based edible fruit preservative hydrogel and then taking it out and drying it naturally to form a hydrogel film on the surface of the fruit.

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