A pH-responsive antibacterial coating for fruit and vegetable cartons and its preparation method

Abstract

This invention discloses a pH-responsive antibacterial coating for fruit and vegetable cartons and its preparation method. The preparation method includes the following steps: adding a chelated titanate coupling agent and petroleum ether to a hydrophobic chitosan solution, adding nano-MgO, and obtaining an antibacterial agent by water bath heating, centrifugation, washing, and drying; dissolving a host α-cyclodextrin-modified amide-amine dendritic macromolecule and a guest adamantane-modified polyamide-amine dendritic macromolecule in PBS buffer, stirring, filtering, and drying to obtain a carrier product; adding an aqueous solution containing the antibacterial agent to an aqueous solution containing the carrier product, stirring, centrifuging, and drying the precipitate to obtain the antibacterial material CS / MgO@PAMAM; preparing a spraying solution containing the antibacterial material using PBS as the liquid phase medium, spraying it onto the surface of the carton substrate, and drying to obtain a pH-responsive antibacterial coating for fruit and vegetable cartons. This invention is the first to prepare a nano-magnesium oxide-chitosan@dendritic macromolecular antibacterial material, which can achieve responsive release of antibacterial carriers according to the pH stimulation of the microenvironment, improve the utilization efficiency of antibacterial agents and the ability to precisely inactivate spoilage organisms, enhance the long-term anti-rot effect of packaging cartons on harvested fruits and vegetables, and effectively prevent fruit and vegetable spoilage.

Description

Technical Field

[0001] This invention relates to the field of antibacterial packaging technology for harvested fruits and vegetables, and in particular to a pH-responsive antibacterial coating for fruit and vegetable cartons and its preparation method. Background Technology

[0002] During the storage, long-distance transportation, and sale of harvested fruits, vegetables, and other agricultural products, the malignant proliferation of spoilage microorganisms is one of the most important factors causing their deterioration, greatly reducing their nutritional and commercial value.

[0003] To address the aforementioned key issues, existing technologies have focused on designing green, efficient, and broad-spectrum antibacterial agents. Green, natural polymeric antibacterial agents (such as chitosan, essential oils, antimicrobial peptides, cellulose, and their derivatives) have been successfully used in antibacterial packaging for fruits and vegetables. Among these, chitosan and its derivatives have attracted considerable attention due to their broad-spectrum bactericidal activity, good safety profile, biodegradability, and ease of modification. Furthermore, nanomaterials with unique physicochemical properties and antibacterial capabilities (such as Ag, Cu, TiO2, MgO, ZnO, SiO2, and graphene) are also being used in the packaging field for fruit and vegetable preservation and freshness maintenance.

[0004] However, traditional antibacterial agents are basically insensitive to changes in the external environment and cannot intelligently control the effective release of their bactericidal components, which leads to a decrease in the utilization rate of antibacterial agents and makes it impossible to effectively prevent the fruits and vegetables in the cardboard box from rotting and maintain their freshness. Summary of the Invention

[0005] This invention provides a pH-responsive antibacterial coating for fruit and vegetable cartons and its preparation method to solve the above-mentioned problems.

[0006] This invention provides a method for preparing a pH-responsive antibacterial coating for fruit and vegetable cartons, comprising the following steps:

[0007] S1: Add 0.75 wt% of chelating titanate coupling agent and 1.0 wt% of petroleum ether to a hydrophobic chitosan solution and mix thoroughly to obtain a mixture. Add 5.0-8.0 wt% of ultrasonically dispersed nano-MgO to the mixture, and then successively heat in a water bath, centrifuge, wash and dry to obtain an antibacterial agent.

[0008] S2: According to the host-guest molar ratio of 1:10, the host α-cyclodextrin-modified amide-amine dendritic molecule and the guest adamantane-modified polyamide-amine dendritic molecule were respectively dissolved in PBS buffer at pH 7.0, stirred at room temperature for 24 h, and then filtered and dried to obtain the carrier product.

[0009] S3: Add the aqueous solution containing the antibacterial agent obtained in S1 to the aqueous solution containing the carrier product obtained in S2, and stir at room temperature for 24 hours to obtain a mixed solution, wherein the molar ratio of the antibacterial agent to the carrier product is 1:10 to 1:15;

[0010] After centrifuging the mixed solution and drying the precipitate, a pH-smart responsive antibacterial material CS / MgO@PAMAM for fruit and vegetable cardboard boxes is obtained.

[0011] S4: Using PBS with pH=7.0 as the liquid medium, prepare a spray solution containing 10-25 wt% of the antibacterial material CS / MgO@PAMAM obtained in S3. Then, spray the entire antibacterial material CS / MgO@PAMAM onto the clean surface of the cardboard substrate and dry it at room temperature for 12 hours. After the surface of the cardboard substrate is free of liquid, a pH-responsive antibacterial coating for fruit and vegetable cardboard boxes is obtained.

[0012] Further, in S1, the water bath heating method is as follows: after adding 5.0 to 8.0 wt% of ultrasonically dispersed nano-MgO to the mixture, water bath heating is carried out in a constant temperature water bath at 40-60℃ with mechanical stirring for 40 minutes.

[0013] Furthermore, in S3, the centrifugation conditions are: centrifugation at 6000 rpm for 5 minutes.

[0014] Furthermore, in S2, the filtration method is as follows: filtration for 48 hours using a dialysis bag with a molecular weight cutoff of 10,000.

[0015] Furthermore, in S1, the mixing time of the chelating titanate coupling agent and petroleum ether with the hydrophobic chitosan solution is 60 min.

[0016] Furthermore, in S4, the carton substrate is sandwich paperboard, corrugated paperboard, or corrugated paperboard.

[0017] Furthermore, in S4, the spraying method is to use an air compressor for spraying.

[0018] Another aspect of the present invention provides a pH-responsive antibacterial coating for fruit and vegetable cartons, which is prepared by the aforementioned method for preparing a pH-responsive antibacterial coating for fruit and vegetable cartons.

[0019] The beneficial effects of this invention are:

[0020] (1) The present invention discloses a method for preparing a pH-responsive antibacterial coating for fruit and vegetable cartons. It introduces a low-cost, safe, non-toxic, and antibacterial nano-MgO material into the chitosan natural antibacterial agent. This method improves the antimicrobial growth ability of the natural antibacterial agent in essence, while also ensuring that it does not harm the safety of agricultural products, human health, or the ecological environment. Compared with chitosan alone, the novel chitosan-based composite material has more obvious antibacterial advantages and effectively solves the core problem of insufficient antibacterial ability.

[0021] (2) This invention is the first to prepare nano-magnesium oxide-chitosan@dendritic macromolecular antibacterial material, which can achieve responsive release of antibacterial carrier according to the pH stimulation of the microenvironment, thereby improving the utilization efficiency of antibacterial agent;

[0022] (3) By utilizing the non-covalent complexation-dissociation effect of the antibacterial carrier, when the pH changes, the non-covalent bonds undergo dissociation to release the loaded antibacterial agent. Compared with stable covalent bonding, the reversibility of non-covalent complexation-dissociation is more suitable for responding to micro-changes in environmental conditions, providing a new approach for constructing pH-intelligent antibacterial carriers.

[0023] (4) By releasing chitosan-based composite materials through pH intelligent response, the precise inactivation ability of spoilage organisms is significantly improved, the ineffective use of antibacterial agents is significantly reduced, the utilization rate of antibacterial agents is maximized, and the long-term anti-rot effect of packaging cartons on harvested fruits and vegetables is enhanced. This is of great significance for effectively preventing the spoilage of fruits and vegetables in cartons and maintaining their fresh quality. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 The antibacterial effect of the antibacterial agent prepared in Example 1 of the present invention and the antibacterial agents prepared in Comparative Examples 1-2 on Escherichia coli is shown in the figure.

[0026] Figure 2 The graph shows the antibacterial rate results of the antibacterial agent prepared in Example 1 and the antibacterial system of Comparative Example 3 against Escherichia coli, Staphylococcus aureus and Penicillium spp. in different pH environments.

[0027] Figure 3 The image shows the 7-day rot rate of grapes inside an antibacterial cardboard box coated with antibacterial materials prepared with different amounts of nano-MgO in Example 2 of this invention.

[0028] Figure 4 The image shows the 7-day rot rate of grapes inside an antibacterial cardboard box coated with the antibacterial material prepared by stirring in a constant temperature water bath at different temperatures in Example 3 of this invention.

[0029] Figure 5 The graph shows the 7-day rot rate of grapes inside an antibacterial cardboard box coated with antibacterial materials prepared by different molar ratios of antibacterial agents and carrier products in Example 4 of the present invention.

[0030] Figure 6 This is a graph showing the 7-day rot rate of grapes inside antibacterial cardboard boxes obtained from different spraying targets in Example 6 of the present invention. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Example

[0033] Example 1:

[0034] A method for preparing a pH-smart responsive antibacterial coating for fruit and vegetable cartons includes the following steps:

[0035] S1: 0.75 wt% chelating titanate coupling agent was selected as the modifier, and 1.0 wt% petroleum ether was selected as the diluent. The modifier and diluent were added to the hydrophobic chitosan solution and mixed thoroughly for 60 min to obtain a mixture. 7 wt% nano-MgO was ultrasonically dispersed and added to the above mixture. After mechanical stirring in a 50℃ constant temperature water bath for 40 min, the mixture was centrifuged, washed, and dried to obtain antibacterial agents with different nano-MgO addition amounts.

[0036] S2: Take 1 mol of α-cyclodextrin-modified amide-amine dendritic macromolecule and 10 mol of adamantane-modified polyamide-amine dendritic macromolecule, respectively, and dissolve them in PBS buffer at pH 7.0 (host-guest molar ratio 1:10). After stirring at room temperature for 24 h, a mixed solution is obtained. The solution is placed in a dialysis bag with a molecular weight cutoff of 10,000 and filtered for 48 h. After drying, the carrier product is obtained.

[0037] S3: Add 1 mol of the aqueous solution containing the antibacterial agent in S1 to 12.5 mol of the aqueous solution containing the antibacterial carrier product in S2 and stir at room temperature for 24 h (molar ratio of 1:12.5); centrifuge the mixed solution at 6000 rpm for 5 min and then dry the precipitate to obtain the pH-responsive antibacterial material (CS / MgO@PAMAM).

[0038] S4: A CS / MgO@PAMAM spraying solution containing 25 wt% was prepared using PBS (pH=7.0) as the liquid medium. The solution was then evenly sprayed onto a clean corrugated cardboard surface using an air compressor. After drying at room temperature for 12 hours until no liquid remained on the surface, the desired pH-responsive antibacterial coating for fruit and vegetable cartons was obtained.

[0039] Subsequently, grapes were used as test samples and packaged in a closed antibacterial cardboard box system made of corrugated cardboard with an antibacterial coating to simulate actual storage scenarios.

[0040] Using blank cardboard boxes as a control, the 7-day rot rate of grapes inside the blank cardboard boxes and the antibacterial cardboard boxes coated with 7wt% nano-MgO were measured. The results are as follows: Figure 2 As shown, the 7-day decay rates were 84.9% and 1.2%, respectively.

[0041] Example 2: Determination of the antibacterial effect of antibacterial coatings prepared with antibacterial agents containing different amounts of nano-MgO.

[0042] A method for preparing a pH-smart responsive antibacterial coating for fruit and vegetable cartons includes the following steps:

[0043] S1: A chelating titanate coupling agent with an addition amount of 0.75 wt% was selected as the modifier, and petroleum ether with an addition amount of 1.0 wt% was selected as the diluent. The modifier and diluent were added to the hydrophobic chitosan solution and mixed thoroughly for 60 min to obtain a mixture. Nano MgO with addition amounts of 5 wt%, 6 wt%, and 8 wt% was ultrasonically dispersed and added to the above mixture. After mechanical stirring in a 50℃ constant temperature water bath for 40 min, the mixture was centrifuged, washed, and dried to obtain antibacterial agents with different nano MgO addition amounts.

[0044] S2: Take 1 mol of α-cyclodextrin-modified amide-amine dendritic macromolecule and 10 mol of adamantane-modified polyamide-amine dendritic macromolecule, respectively, and dissolve them in PBS buffer at pH 7.0 (host-guest molar ratio 1:10). After stirring at room temperature for 24 h, a mixed solution is obtained. The solution is placed in a dialysis bag with a molecular weight cutoff of 10,000 and filtered for 48 h. After drying, the carrier product is obtained.

[0045] S3: Add 1 mol of the aqueous solution containing the antibacterial agent in S1 to 12.5 mol of the aqueous solution containing the antibacterial carrier product in S2 and stir at room temperature for 24 h (molar ratio of 1:12.5); centrifuge the mixed solution at 6000 rpm for 5 min and then dry the precipitate to obtain 3 groups of pH-responsive antibacterial materials (CS / MgO@PAMAM).

[0046] S4: Using PBS (pH=7.0) as the liquid medium, three sets of CS / MgO@PAMAM spraying solutions containing 25wt% were prepared. The spraying solutions were then uniformly sprayed onto clean corrugated cardboard using an air compressor. After drying at room temperature for 12 hours with forced air, and until no liquid remained on the surface, the desired pH-responsive antibacterial coating for fruit and vegetable cartons was obtained.

[0047] Subsequently, grapes were used as test samples and packaged in a closed antibacterial cardboard box system made of corrugated cardboard with an antibacterial coating to simulate actual storage scenarios.

[0048] Using blank cardboard boxes as a control, and comparing them with Example 1 (where the 7-day rot rate of grapes in sprayed antibacterial cardboard boxes with 7wt% nano-MgO added was 1.2%), the 7-day rot rate of grapes in blank cardboard boxes and sprayed antibacterial cardboard boxes with 5wt%, 6wt%, and 8wt% nano-MgO added, respectively, was measured. The results are as follows: Figure 3 As shown, the 7-day decay rates were 84.9%, 2.9%, 1.6%, and 1.3%, respectively.

[0049] Example 3: Determination of the antibacterial effect of antibacterial coatings prepared with antibacterial agents under different constant temperature water bath conditions.

[0050] A method for preparing a pH-smart responsive antibacterial coating for fruit and vegetable cartons includes the following steps:

[0051] S1: Select 0.75wt% chelating titanate coupling agent as modifier and 1.0wt% petroleum ether as diluent. Add the modifier and diluent to the hydrophobic chitosan solution and mix thoroughly for 60 min to obtain a mixture. Add 7wt% nano MgO, which is ultrasonically dispersed, to the above mixture. Then, place the mixture in a constant temperature water bath at 40℃, 45℃, and 60℃ and mechanically stir for 40 min, respectively. After centrifugation, washing, and drying, the antibacterial agent is obtained.

[0052] S2: Same as Example 1;

[0053] S3: Same as Example 1;

[0054] S4: Same as Example 1;

[0055] Subsequently, grapes were used as test samples and packaged in a closed antibacterial cardboard box packaging system made of corrugated cardboard with an antibacterial coating to simulate actual storage scenarios.

[0056] Using blank cardboard boxes as a control, and comparing them with Example 1 (where the 7-day rot rate of grapes in sprayed antibacterial cardboard boxes obtained by stirring in constant temperature water baths at 50°C was 1.2%), the 7-day rot rate of grapes in blank cardboard boxes and antibacterial cardboard boxes obtained by stirring in constant temperature water baths at 40°C, 45°C, and 60°C were measured. The results are as follows: Figure 4 As shown, the 7-day decay rates were 84.9%, 2.7%, 1.6%, and 8.8%, respectively.

[0057] Example 4: Determination of the antibacterial effect of antibacterial coatings prepared from antibacterial agents with different molar ratios of antibacterial agent to carrier product.

[0058] A method for preparing a pH-smart responsive antibacterial coating for fruit and vegetable cartons includes the following steps:

[0059] S1: Select 0.75wt% chelating titanate coupling agent as modifier and 1.0wt% petroleum ether as diluent. Add the modifier and diluent to the hydrophobic chitosan solution and mix thoroughly for 60 min to obtain a mixture. Add 7wt% nano MgO, which is ultrasonically dispersed, to the above mixture. Place the mixture in a 50℃ constant temperature water bath and mechanically stir for 40 min. After centrifugation, washing, and drying, obtain the antibacterial agent.

[0060] S2: Same as Example 1;

[0061] S3: Add the aqueous solution containing the antibacterial agent to the aqueous solution containing the antibacterial carrier product and stir at room temperature for 24 hours. The molar ratios of the two are set to 1:10.0, 1:12.5, 1:14.0 and 1:15.0, respectively. After centrifuging the mixed solution at 6000 rpm for 5 minutes, the precipitate is dried to obtain the pH-responsive antibacterial material (CS / MgO@PAMAM).

[0062] S4: Same as Example 1;

[0063] Subsequently, grapes were used as test samples and packaged in a closed antibacterial cardboard box packaging system made of corrugated cardboard with an antibacterial coating to simulate actual storage scenarios.

[0064] Using blank cardboard boxes as a control and comparing them with Example 1 (where the molar ratio of antibacterial agent to carrier product was 1:12.5, the 7-day rot rate of grapes inside the sprayed antibacterial cardboard boxes was 1.2%), the 7-day rot rate of grapes inside the antibacterial cardboard boxes was measured for blank cardboard boxes and for antibacterial agent to carrier product molar ratios of 1:10.0, 1:14.0, and 1:15.0, respectively. The results are as follows: Figure 5As shown, the 7-day decay rates were 84.9%, 1.7%, 1.2%, and 1.4%, respectively.

[0065] Example 5: Determination of the antibacterial effect of antibacterial coatings prepared by different spraying amounts. A method for preparing a pH-responsive antibacterial coating for fruit and vegetable cartons includes the following steps:

[0066] S1: 0.75 wt% chelating titanate coupling agent was selected as the modifier and 1.0 wt% petroleum ether as the diluent. The modifier and diluent were added to the hydrophobic chitosan solution and mixed thoroughly for 60 min to obtain a mixture. 7 wt% nano-MgO was ultrasonically dispersed and added to the above mixture. The mixture was then placed in a 50℃ constant temperature water bath and mechanically stirred for 40 min. After centrifugation, washing, and drying, the antibacterial agent was obtained.

[0067] S2: Same as Example 1;

[0068] S3: Same as Example 1;

[0069] S4: Using PBS (pH=7.0) as the liquid medium, CS / MgO@PAMAM spraying solutions containing 10wt%, 15wt%, and 20wt% were prepared respectively. The spraying solutions were then uniformly sprayed onto clean corrugated cardboard using an air compressor. After drying at room temperature for 12 hours with a forced airflow until no liquid remained on the surface, the desired pH-responsive antibacterial coating for fruit and vegetable cartons was obtained.

[0070] Subsequently, grapes were used as test samples and packaged in a closed antibacterial cardboard box packaging system made of corrugated cardboard with an antibacterial coating to simulate actual storage scenarios.

[0071] Using blank cardboard boxes as a control and comparing them with Example 1 (where the 7-day rot rate of grapes in the antibacterial cardboard box containing 25 wt% CS / MgO@PAMAM spray solution was 1.2%), the 7-day rot rate of grapes in the antibacterial cardboard boxes with spray solution amounts of 10 wt%, 15 wt%, and 20 wt% were measured, and the 7-day rot rates were 84.9%, 16.7%, 12.0%, and 6.3%, respectively.

[0072] Example 6: Determination of the antibacterial effect of antibacterial coatings prepared on different sprayed objects

[0073] A method for preparing a pH-smart responsive antibacterial coating for fruit and vegetable cartons includes the following steps:

[0074] S1: 0.75 wt% chelating titanate coupling agent was selected as the modifier and 1.0 wt% petroleum ether as the diluent. The modifier and diluent were added to the hydrophobic chitosan solution and mixed thoroughly for 60 min to obtain a mixture. 7 wt% nano-MgO was ultrasonically dispersed and added to the above mixture. The mixture was then placed in a 50℃ constant temperature water bath and mechanically stirred for 40 min. After centrifugation, washing, and drying, the antibacterial agent was obtained.

[0075] S2: Same as Example 1;

[0076] S3: Same as Example 1;

[0077] S4: Using PBS (pH=7.0) as the liquid medium, a CS / MgO@PAMAM spraying solution containing 25wt% was prepared. The spraying solution was then uniformly sprayed onto clean sandwich paperboard, corrugated paperboard, and cardboard using an air compressor. After drying at room temperature for 12 hours with a forced air fan, the desired pH-responsive antibacterial coating for fruit and vegetable cartons was obtained when there was no liquid on the surface.

[0078] Subsequently, grapes were used as test samples and packaged in a sealed cardboard box packaging system with the aforementioned antibacterial coating to simulate actual storage scenarios.

[0079] Using blank cardboard boxes as a control and comparing them with Example 1 (where the 7-day rot rate of grapes inside antibacterial cardboard boxes obtained by spraying the coating liquid onto corrugated paper was 1.2%), the 7-day rot rate of grapes inside blank cardboard boxes and antibacterial cardboard boxes obtained using sandwich paperboard and corrugated paperboard, respectively, was measured. The results are as follows: Figure 6 As shown, the 7-day decay rates were 84.9%, 1.5%, and 1.2%, respectively.

[0080] Comparative Example

[0081] Comparative Example 1:

[0082] A method for preparing an antibacterial coating includes the following steps:

[0083] S1: 0.75 wt% chelating titanate coupling agent was selected as the modifier, and 1.0 wt% petroleum ether was selected as the diluent. The mixture was added to the hydrophobic chitosan solution and thoroughly mixed for 60 min. Subsequently, the mixture was mechanically stirred in a 50℃ constant temperature water bath for 40 min, centrifuged, washed, and dried to obtain the antibacterial agent.

[0084] S2: Prepare a spray solution containing 25 wt% antibacterial agent using PBS (pH=7.0) as the liquid medium.

[0085] S3: Using an air compressor, the coating liquid is evenly sprayed onto a clean corrugated cardboard surface. After drying at room temperature with forced air for 12 hours until no liquid remains on the surface, the desired pH-responsive antibacterial coating for fruit and vegetable cartons is obtained. Subsequently, grapes are used as test samples and packaged in a sealed cardboard box packaging system to simulate actual storage conditions.

[0086] Comparative Example 2:

[0087] A method for preparing an antibacterial coating includes the following steps:

[0088] S1: 0.75 wt% chelated titanate coupling agent was selected as the modifier, and 1.0 wt% petroleum ether was used as the diluent. The mixture was added to deionized water and thoroughly mixed for 60 min. 7 wt% nano-MgO was ultrasonically dispersed and added to the above solution. The mixture was then mechanically stirred in a 50℃ constant temperature water bath for 40 min, followed by centrifugation, washing, and drying to obtain the antibacterial agent.

[0089] S2: Prepare a spray solution containing 25 wt% antibacterial agent using PBS (pH=7.0) as the liquid medium;

[0090] S3: Using an air compressor, the coating liquid is evenly sprayed onto a clean corrugated cardboard surface. After drying at room temperature with forced air for 12 hours until no liquid remains on the surface, the desired pH-responsive antibacterial coating for fruit and vegetable cartons is obtained. Subsequently, grapes are used as test samples and packaged in a sealed cardboard box packaging system to simulate actual storage conditions.

[0091] Comparative Example 3

[0092] A method for preparing an antibacterial coating includes the following steps:

[0093] S1: 0.75 wt% chelating titanate coupling agent was selected as the modifier, and 1.0 wt% petroleum ether was used as the diluent. The mixture was added to a hydrophobic chitosan solution and thoroughly mixed for 60 min. 7 wt% nano-MgO was ultrasonically dispersed and added to the above solution. The mixture was then mechanically stirred in a 50℃ constant temperature water bath for 40 min, followed by centrifugation, washing, and drying to obtain the antibacterial agent.

[0094] S2: The host-guest molecule-free amide-amine dendritic macromolecule and polyamide-amine dendritic macromolecule were dissolved in PBS buffer at pH 7.0, stirred at room temperature for 24 h, and then placed in a dialysis bag with a molecular weight cutoff of 10,000 for 48 h. After drying, the carrier product was obtained.

[0095] S3: Add the aqueous solution containing the antibacterial agent to the aqueous solution containing the antibacterial carrier product and stir at room temperature for 24 hours. The molar ratio of the two is 1:12.5. Centrifuge the mixed solution at 6000 rpm for 5 minutes, and then dry the precipitate to obtain the antibacterial material system.

[0096] Performance testing:

[0097] (1) The antibacterial effects of the antibacterial agent prepared by adding 7wt% nano-MgO in step S1 of Example 1, the antibacterial agent in Comparative Example 2, and the antibacterial agent in Comparative Example 3 were tested using the inhibition zone method and the plate calculation method:

[0098] 300 μg / mL of the antibacterial agents from Examples 1, 2, and 3 were added to 35 mL of E. coli bacterial culture (concentration 10). 6 The bacterial suspensions containing antibacterial agents were then placed in a shaking incubator (37℃) and incubated for 24 hours. Afterward, the antibacterial concentrations of each group were diluted to 150 μg / mL and evenly spread on the surface of LB solid medium (LB solid medium components: 5g sodium chloride, 10g peptone, 5g yeast extract, 20g agar powder). After incubation, colony counting was performed to evaluate the antibacterial efficiency. The results are shown in Table 1. Figure 1 ;

[0099] Table 1. Bactericidal efficiency of the antibacterial agents prepared in Example 1 and Comparative Examples 1-2

[0100]

[0101] As shown in Table 1, the antibacterial activity of the antibacterial agent in Example 1 against Escherichia coli, Staphylococcus aureus, and Penicillium spp. was 19.7, 23.2, and 17.6 mm, and 99.3%, 99.9%, and 95.5%, respectively. The antibacterial effect was significantly higher than that of the antibacterial agents in Comparative Example 1 and Comparative Example 2.

[0102] Moreover, the combination Figure 1 It can be seen that the nano-MgO and chitosan composite antibacterial agent in Example 1 exhibits the best antibacterial activity against *Escherichia coli*, with significantly lower colony survival rates on agar plates compared to pure chitosan (Comparative Example 1) or pure nano-MgO (Comparative Example 2). This is because the positively charged chitosan and the negatively charged bacteria disrupt the bacterial outer wall through electrostatic adsorption, while the nano-MgO induces the production of ROS, enhancing lipid peroxidation damage to bacteria. In other words, chitosan and pure nano-MgO produce a synergistic effect, improving its antibacterial ability. Furthermore, tests against *Staphylococcus aureus* and *Penicillium viride* showed that this composite antibacterial agent also exhibited excellent antibacterial efficiency (greater than 95%), especially against *Staphylococcus aureus*, indicating that this composite antibacterial agent possesses good broad-spectrum bactericidal properties.

[0103] (2) Antibacterial tests were conducted on the pH-responsive antibacterial material CS / MgO@PAMAM for fruit and vegetable cartons prepared in Example 1 and the antibacterial agent system of Comparative Example 3 in buffer solutions with different pH values:

[0104] The initial pH of the PBS buffer solution (pH = 7.0) was adjusted to 6.0 and 5.0 using dilute hydrochloric acid, respectively. Bioactive Escherichia coli, Staphylococcus aureus, and Penicillium viride were added to the PBS solutions at pH 7.0, 6.0, and 5.0, respectively, to prepare 35 ml PBS-bacterial solution mixtures (where the concentration of Escherichia coli and Staphylococcus aureus was 10 μL each). 6 cfu / mL, concentration of Penicillium spores was 10 5 The antibacterial material (concentration 300 μg / mL) from Example 1 and the antibacterial agent system (concentration 300 μg / mL) from Comparative Example 3 were then added to the PBS-bacterial solution mixture, respectively. The antibacterial ability of the prepared antibacterial material was tested based on the counting method, and the results are as follows: Figure 2 As shown:

[0105] Depend on Figure 2 The results show that in a buffer solution with pH 7.0, the antibacterial material system in Comparative Example 3 exhibited bactericidal rates of 32.7%, 41.2%, and 34.6% against *Escherichia coli*, *Staphylococcus aureus*, and *Penicillium spp.*, respectively; in a buffer solution with pH 6.0, the bactericidal rates were 30.7%, 42.7%, and 31.5%; and in a buffer solution with pH 5.0, the bactericidal rates increased to 30.8%, 41.5%, and 32.9%, respectively. These results indicate that as the pH of the buffer solution decreases, the CS / MgO carrier is not responsive to pH changes, therefore the effective antibacterial components encapsulated in the carrier cannot be released. Consequently, the material in Comparative Example 3 cannot achieve pH-responsive intelligent antibacterial action.

[0106] In a buffer solution at pH 7.0, the antibacterial material CS / MgO@PAMAM prepared in Example 1 exhibited bactericidal rates of 39.9%, 42.7%, and 33.5% against *Escherichia coli*, *Staphylococcus aureus*, and *Penicillium spp.*, respectively. In a buffer solution at pH 6.0, the bactericidal rates against these bacteria were 75.3%, 79.1%, and 72.8%, respectively. In a buffer solution at pH 5.0, the bactericidal rates against *Escherichia coli*, *Staphylococcus aureus*, and *Penicillium spp.* increased to 89.6% and 92.5%, respectively. At a pH of 7.0 and 83.0%, the host molecule β-cyclodextrin modified on the surface of the dendritic macromolecule undergoes host-guest self-assembly with the guest molecule benzimidazole. When the ambient pH is less than 6.5, the acidic environment causes benzimidazole to dissociate from the cyclodextrin cavity, thereby releasing the CS / MgO antibacterial agent. The above results demonstrate that the CS / MgO@PAMAM prepared by this method achieves intelligent antibacterial response to pH changes and significantly improves the precise inactivation ability against spoilage organisms. This is of great significance for effectively preventing the spoilage of fruits and vegetables in cardboard boxes and maintaining their freshness.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a pH-responsive antibacterial coating for fruit and vegetable cartons, characterized in that, Includes the following steps: S1: Add 0.75 wt% of chelating titanate coupling agent and 1.0 wt% of petroleum ether to a hydrophobic chitosan solution and mix thoroughly to obtain a mixture. Add 5.0~8.0 wt% of ultrasonically dispersed nano-MgO to the mixture, and then heat in a water bath, centrifuge, wash and dry in sequence to obtain an antibacterial agent. S2: According to the host-guest molar ratio of 1:10, the host α-cyclodextrin-modified amide-amine dendritic molecule and the guest adamantane-modified polyamide-amine dendritic molecule were respectively dissolved in PBS buffer at pH 7.0, stirred at room temperature for 24 h, and then filtered and dried to obtain the carrier product. S3: Add the aqueous solution containing the antibacterial agent obtained in S1 to the aqueous solution containing the carrier product obtained in S2, and stir at room temperature for 24 h to obtain a mixed solution, wherein the molar ratio of the antibacterial agent to the carrier product is 1:10 to 1:15; After centrifuging the mixed solution and drying the precipitate, a pH-smart responsive antibacterial material CS / MgO@PAMAM for fruit and vegetable cardboard boxes is obtained. S4: Using PBS with pH=7.0 as the liquid medium, prepare a spray solution containing 10~25wt% of the antibacterial material CS / MgO@PAMAM obtained in S3. Then, spray the entire antibacterial material CS / MgO@PAMAM onto the clean surface of the cardboard substrate and dry it at room temperature for 12 hours. After the surface of the cardboard substrate is free of liquid, a pH-responsive antibacterial coating for fruit and vegetable cardboard boxes is obtained.

2. The method for preparing a pH-responsive antibacterial coating for fruit and vegetable cartons according to claim 1, characterized in that, In S1, the water bath heating method is as follows: after adding 5.0~8.0 wt% of ultrasonically dispersed nano-MgO to the mixture, the mixture is heated at 40-60°C. o The water bath is heated in a constant temperature water bath for 40 minutes with mechanical stirring.

3. The method for preparing a pH-responsive antibacterial coating for fruit and vegetable cartons according to claim 1, characterized in that, In S3, the centrifugation conditions are: centrifugation at 6000 rpm for 5 min.

4. The method for preparing a pH-responsive antibacterial coating for fruit and vegetable cartons according to claim 1, characterized in that, In S2, the filtration method is as follows: filtration is performed for 48 hours using a dialysis bag with a molecular weight cutoff of 10,000.

5. The method for preparing a pH-responsive antibacterial coating for fruit and vegetable cartons according to claim 1, characterized in that, In S1, the mixing time of the chelating titanate coupling agent and petroleum ether with the hydrophobic chitosan solution is 60 min.

6. The method for preparing a pH-responsive antibacterial coating for fruit and vegetable cartons according to claim 1, characterized in that, In S4, the carton substrate is sandwich cardboard or corrugated cardboard.

7. The method for preparing a pH-responsive antibacterial coating for fruit and vegetable cartons according to claim 1, characterized in that, In S4, the spraying method is to use an air compressor for spraying.

8. A pH-smart responsive antibacterial coating for fruit and vegetable cartons, characterized in that, It is prepared by the method described in any one of claims 1 to 7.

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