Food freshness indicator label and preparation method thereof

Abstract

This application relates to a food freshness indicator label and its preparation method, comprising waterborne polyurethane, carbon quantum dots, silicon quantum dots, a film-forming agent, and water. The indicator label of this application uses carbon quantum dots and silicon quantum dots as composite freshness indicators, which are characterized by low toxicity and good stability. The carbon quantum dots and silicon quantum dots specifically bind and react with volatile substances produced by food spoilage, causing changes in fluorescent color, thereby reflecting the freshness of the food in real time. It has the advantages of sensitive response, low production cost, and mass production. Compared with single-component indicators, the composite indicator of this application has the characteristics of a wide detection range and adaptability to a wide range of conditions, making it an economical and practical smart food freshness indicator label.

Description

Technical Field

[0001] The present application relates to the technical field of food freshness detection, and in particular to a food freshness indicator label and a preparation method thereof. Background Art

[0002] "Food is the most important thing for the people, and food safety comes first." With the rapid development of my country's economy and the improvement of people's living standards, people's attention to food quality has gradually increased. Consequently, food packaging technology, which is directly related to food quality, has been given higher expectations. However, the following problems still exist in the food packaging field: First, frequent food safety issues and delayed transmission of food freshness information lead to widespread food spoilage and waste, resulting in significant resource waste and economic losses for consumers and producers. Second, currently, food quality testing in my country typically relies on sampling. While relatively advanced testing equipment, the testing process is cumbersome, expensive, and time-consuming. Testing can only be performed before packaging, making it impossible to monitor food quality in real time. Third, the three functions of traditional packaging—protection, circulation, and storage—can no longer meet the new demands of the food industry. Therefore, improving real-time monitoring of food quality is urgent.

[0003] At present, colorimetric indicator labels are often used as real-time detection media for food freshness. However, many problems still exist: First, the ingredients of most indicator labels are not environmentally friendly. They use industrial reagents, which are toxic and inedible. Second, most natural pigment indicator labels change greatly with the environment and have poor stability and sensitivity. Third, most food indicators are expensive, have complex preparation steps, and are limited in practical application, which is not conducive to widespread use. Fourth, the active ingredients are single and the detection range is small. Summary of the Invention

[0004] The embodiments of the present application provide a food freshness indicator label and a preparation method thereof to solve the problems of low environmental protection, poor stability, low sensitivity, high cost and small detection range of food freshness indicator labels in related technologies.

[0005] The technical solutions provided in this application are as follows:

[0006] In a first aspect, the present application provides a food freshness indicator label comprising waterborne polyurethane, carbon quantum dots, silicon quantum dots, a film-forming agent, and water.

[0007] In some embodiments, the components are as follows by weight:

[0008] 10-99.98 parts of waterborne polyurethane, 0.01-1 part of carbon quantum dots, 0.01-1 part of silicon quantum dots, 10-99.98 parts of film-forming agent, and 10-50 parts of water.

[0009] In some embodiments, the film-forming agent includes one or more of hydroxymethyl cellulose, hydroxyethyl cellulose, ethyl cellulose, hydroxyethyl propyl cellulose, chitosan, agarose, gelatin, acrylic resin, polyvinyl alcohol, and epoxy resin.

[0010] In a second aspect, the present application provides a method for preparing the food freshness indicator label, comprising the following steps:

[0011] Evenly mix waterborne polyurethane, carbon quantum dots, silicon quantum dots and film-forming agent, and stir in water to obtain indicator label film-forming liquid;

[0012] The food freshness indicator label is obtained by forming the indicator label film-forming liquid into a film.

[0013] In some embodiments, the steps of preparing carbon quantum dots are further included:

[0014] dissolving o-phenylenediamine in anhydrous ethanol to obtain a mixed solution;

[0015] The mixed solution was transferred to a reactor and reacted at 100-200°C for 2-24 hours. The product was cooled to room temperature to obtain a crude product;

[0016] The crude product was purified by silica gel column chromatography to obtain a carbon quantum dot mixture, which was then rotary evaporated to obtain the carbon quantum dots.

[0017] In some embodiments, the following steps are further included for preparing silicon quantum dots:

[0018] Dissolve citric acid and N-aminoethyl-3-aminopropylmethyldimethoxysilane in water and mix well to obtain a clear solution;

[0019] The clarified liquid was transferred to an autoclave and reacted at 100-200° C. for 2-24 hours. The product was cooled to room temperature to obtain a crude product;

[0020] The crude product is centrifuged, the supernatant is taken and dried to obtain silicon quantum dots.

[0021] In some embodiments, the method further comprises the following steps of preparing waterborne polyurethane:

[0022] (1) Under a nitrogen atmosphere, reacting pre-dehydrated polytetrahydrofuran with 0.001-0.1 mol of isophorone diisocyanate to obtain a first intermediate;

[0023] (2) adding 0.001-0.1 mol of 2,2-dihydroxymethylpropionic acid and 1-20 mol of acetone to the first intermediate to react to obtain a second intermediate;

[0024] (3) adding 0.001-0.1 mol of 1,4-butanediol to the second intermediate to react and obtain a third intermediate;

[0025] (4) adding dibutyltin dilaurate to the third intermediate to react and obtain a homogeneous mixed solution;

[0026] (5) The homogeneous system mixture is cooled to room temperature, and 0.005-0.1 mol of a pH regulator is added to react to obtain the waterborne polyurethane.

[0027] In some embodiments, the polytetrahydrofuran is pre-dehydrated by the following steps:

[0028] Polytetrahydrofuran was heated at a constant temperature of 85°C for 3 hours, during which time the gas was evacuated. The heating was stopped and the temperature was lowered to 60°C.

[0029] In some embodiments, the pH adjuster comprises HI.

[0030] In some embodiments, the reaction temperature of step (1) is 85° C. and the reaction time is 1-5 h;

[0031] And / or, the reaction temperature of step (2) is 60° C. and the reaction time is 1-10 h;

[0032] And / or, the reaction temperature of step (3) is 60° C. and the reaction time is 1 h;

[0033] And / or, the reaction temperature of step (4) is 60° C. and the reaction time is 1 h;

[0034] And / or, the reaction time of step (5) is 30 min

[0035] The beneficial effects brought about by the technical solution provided by this application include: the indicator label of this application uses carbon quantum dots and silicon quantum dots as composite freshness indicators, which have the characteristics of low toxicity and good stability. The carbon quantum dots and silicon quantum dots specifically bind to and react with volatile substances produced by food spoilage, and cause changes in fluorescent color, thereby reflecting the freshness of the food in real time. It has the advantages of sensitive response, low production cost, and batch production. Compared with single-component indicators, the composite indicator of this application has the characteristics of a wide detection range and wide adaptability conditions. It is an economical and practical food freshness intelligent indicator label. DETAILED DESCRIPTION

[0036] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0037] In a first aspect, an embodiment of the present application provides a food freshness indicator label, comprising water-based polyurethane, carbon quantum dots, silicon quantum dots, a film-forming agent, and water.

[0038] During the spoilage process of common foods such as meat and milk, the decomposition of proteins and amino acids typically produces volatile substances such as biogenic amines and sulfides. Therefore, biogenic amines and sulfides are often considered characteristic compounds for evaluating the freshness of meat. This application uses carbon quantum dots and silicon quantum dots as indicators of food freshness. The principle is as follows:

[0039] The fluorescence of the two quantum dots changes differently in response to different pH environments: silicon quantum dots experience fluorescence quenching in acidic conditions, while carbon quantum dots experience fluorescence quenching in alkaline conditions. Leveraging this property, a dual-quantum-dot fluorescent probe was prepared by mixing the two quantum dots in varying proportions, enabling pH detection in multiple acidic and alkaline environments. This probe exhibits a fluorescence color shift from green to blue as the external pH increases from 1 to 14, offering non-destructive testing, visual identification, low cost, high sensitivity, and energy-saving and environmentally friendly features.

[0040] In some embodiments, the components are as follows by weight:

[0041] 10-99.98 parts of waterborne polyurethane, 0.01-1 part of carbon quantum dots, 0.01-1 part of silicon quantum dots, 10-99.98 parts of film-forming agent, and 10-50 parts of water.

[0042] The food freshness indicator label provided in this application has low-cost raw materials and high economic benefits. The water-based polyurethane and film-forming agent are mainly used to form the matrix of the food freshness indicator label. The carbon quantum dots and silicon quantum dots have good compatibility and dispersibility in the water-based polyurethane and film-forming agent.

[0043] Furthermore, it is preferred that the water is deionized water.

[0044] In some embodiments, the film-forming agent includes one or more of hydroxymethyl cellulose, hydroxyethyl cellulose, ethyl cellulose, hydroxyethyl propyl cellulose, chitosan, agarose, gelatin, acrylic resin, polyvinyl alcohol, and epoxy resin.

[0045] The film-forming agent has the advantages of low toxicity and good stability.

[0046] In a second aspect, the present invention also provides a method for preparing the food freshness indicator label, comprising the following steps:

[0047] Evenly mix waterborne polyurethane, carbon quantum dots, silicon quantum dots and film-forming agent, and stir in water to obtain indicator label film-forming liquid;

[0048] The food freshness indicator label is obtained by forming the indicator label film-forming liquid into a film.

[0049] The food freshness indicator label provided in this application has a simple and easy production method and can be produced in large quantities.

[0050] Specifically, the "film-forming liquid for the indicator label" includes the following steps:

[0051] The indicator label film-forming liquid is deaerated, transferred to a polytetrafluoroethylene mold, and cured for 24 hours to obtain the food freshness indicator label.

[0052] In some embodiments, the step of preparing carbon quantum dots is further included:

[0053] dissolving o-phenylenediamine in anhydrous ethanol to obtain a mixed solution;

[0054] The mixed solution was transferred to a reactor and reacted at 100-200°C for 2-24 hours. The product was cooled to room temperature to obtain a crude product;

[0055] The crude product was purified by silica gel column chromatography to obtain a carbon quantum dot mixture, which was then rotary evaporated to obtain the carbon quantum dots.

[0056] In the present embodiment, o-phenylenediamine is used as a carbon source to prepare carbon quantum dots that can emit green fluorescence and have good optical properties and chemical stability.

[0057] Furthermore, the inner lining of the reactor is preferably polytetrafluoroethylene.

[0058] Specifically, ethyl acetate was used as the eluent when the crude product was purified by silica gel column chromatography, and the obtained carbon quantum dots were brown solid powder.

[0059] In some embodiments, the process further includes the following steps of preparing silicon quantum dots:

[0060] Dissolve citric acid and N-aminoethyl-3-aminopropylmethyldimethoxysilane in water and mix well to obtain a clear solution;

[0061] The clarified liquid was transferred to an autoclave and reacted at 100-200° C. for 2-24 hours. The product was cooled to room temperature to obtain a crude product;

[0062] The crude product is centrifuged, the supernatant is taken and dried to obtain silicon quantum dots.

[0063] In the embodiment of the present application, citric acid is used as a reducing agent and N-aminoethyl-3-aminopropylmethyldimethoxysilane is used as a silicon source to prepare silicon quantum dots that can emit blue fluorescence. The silicon quantum dots have uniform size and stable optical properties. They are water-soluble and can exist stably in ultrapure water. They have good pH stability and can exist stably in different pH environments.

[0064] Furthermore, it is preferred that the inner lining of the autoclave is polytetrafluoroethylene.

[0065] Specifically, the parameters for centrifugal separation of the crude product can be reasonably selected according to actual conditions, for example, the centrifugal speed can be 10,000 r / min, and the centrifugal time can be 10 min.

[0066] Furthermore, the supernatant can be dried by freeze drying to obtain silicon quantum dots in the form of white solid powder.

[0067] In some embodiments, the step of preparing waterborne polyurethane is further included:

[0068] (1) Under a nitrogen atmosphere, reacting pre-dehydrated polytetrahydrofuran with 0.001-0.1 mol of isophorone diisocyanate to obtain a first intermediate;

[0069] (2) adding 0.001-0.1 mol of 2,2-dihydroxymethylpropionic acid and 1-20 mol of acetone to the first intermediate to react to obtain a second intermediate;

[0070] (3) adding 0.001-0.1 mol of 1,4-butanediol to the second intermediate to react and obtain a third intermediate;

[0071] (4) adding dibutyltin dilaurate to the third intermediate to react and obtain a homogeneous mixed solution;

[0072] (5) The homogeneous system mixture is cooled to room temperature, and 0.005-0.1 mol of a pH regulator is added to react to obtain the waterborne polyurethane.

[0073] In some embodiments, the polytetrahydrofuran is pre-dehydrated using the following steps:

[0074] Polytetrahydrofuran was heated at a constant temperature of 85°C for 3 hours, during which time the gas was evacuated. The heating was stopped and the temperature was lowered to 60°C.

[0075] Furthermore, a three-necked flask can be used to remove water from the polytetrahydrofuran. A vacuum pump can be provided at the mouth of the three-necked flask to remove air from the flask to dehydrate the polytetrahydrofuran.

[0076] In some embodiments, the pH adjusting agent comprises HI.

[0077] In some embodiments, the reaction temperature of step (1) is 85° C. and the reaction time is 1-5 h;

[0078] And / or, the reaction temperature of step (2) is 60° C. and the reaction time is 1-10 h;

[0079] And / or, the reaction temperature of step (3) is 60° C. and the reaction time is 1 h;

[0080] And / or, the reaction temperature of step (4) is 60° C. and the reaction time is 1 h;

[0081] And / or, the reaction time of step (5) is 30 min.

[0082] The present application is further described below through specific examples.

[0083] Example 1

[0084] This embodiment 1 provides a food freshness indicator label and a preparation method thereof.

[0085] The steps include:

[0086] (1) Preparation of carbon quantum dots

[0087] 101: dissolving o-phenylenediamine in anhydrous ethanol to obtain a mixed solution;

[0088] 102: Transfer the mixed solution to a reaction kettle lined with polytetrafluoroethylene, react at 200° C. for 24 hours, and then remove;

[0089] 103: The product is cooled to room temperature to obtain a crude product;

[0090] 104: The crude product is purified by silica gel column chromatography using ethyl acetate as an eluent to obtain a carbon quantum dot mixture. The carbon quantum dot mixture is subjected to rotary evaporation to remove ethyl acetate to obtain brown-yellow solid powder carbon quantum dots.

[0091] (2) Preparation of silicon quantum dots

[0092] 101: Dissolve 1.0 g of citric acid and 1 mL of N-aminoethyl-3-aminopropylmethyldimethoxysilane in water, stir and mix well to obtain a clear solution;

[0093] 102: Transfer the clarified liquid to an autoclave lined with polytetrafluoroethylene, react at 200° C. for 24 h, and then remove;

[0094] 103: The product is cooled to room temperature, and then centrifuged at 10,000 rpm for 10 minutes. The supernatant is collected and freeze-dried to obtain yellow solid powder silicon quantum dots.

[0095] (3) Preparation of waterborne polyurethane

[0096] 101: Add 0.05 mol of PTMG (polytetrahydrofuran, Mn=1000, soft segment) into a three-necked flask and stir in an 85°C oil bath for 3 h while using an air pump to remove water. Stop heating and lower the temperature to 60°C.

[0097] 102: Add 0.1 mol IPDI (isophorone diisocyanate, hard segment) to polytetrahydrofuran, heat the oil bath to 85°C and continue stirring for 5 h. At the same time, remove the air pump and replace the atmosphere with nitrogen.

[0098] 103: After the previous step, add 0.1 mol DMPA (2,2-dihydroxymethylpropionic acid, hydrophilic chain extender) and 20 ml acetone, connect a condenser reflux tube to the three-necked flask, and react at 60°C for 1 h;

[0099] 104: After the previous step, 0.1 mol BDO (1,4-butanediol, chain extender) was added and the reaction was continued for 1 h.

[0100] 105: Add 10 drops of DBTDL (dibutyltin dilaurate, catalyst) and continue the reaction for 1 hour to form a homogeneous system, then cool naturally to room temperature;

[0101] 106: Add 0.1 mol TEA (HI, pH adjustment) and react for 30 minutes to obtain waterborne polyurethane.

[0102] (4) Preparation of food freshness indicator labels

[0103] 101: Slowly add 50g of waterborne polyurethane to 20g of deionized water solvent and stir magnetically until the mixture is evenly mixed. Then slowly add 29.98g of hydroxymethyl cellulose powder and stir magnetically for 10h until the hydroxymethyl cellulose powder is completely dissolved. Then, add 0.01g of silicon quantum dots and 0.01g of carbon quantum dots in sequence and continue stirring magnetically for 6h to ensure that the mixture is fully mixed.

[0104] 102: Place the mixture in a vacuum drying oven to degas for 10 hours, transfer it into a polytetrafluoroethylene mold, and cure it at 30°C for 24 hours to obtain a food freshness indicator label.

[0105] Example 2

[0106] This embodiment 2 provides a food freshness indicator label and a preparation method thereof.

[0107] The steps of (1) carbon quantum dot preparation, (2) silicon quantum dot preparation, and (3) waterborne polyurethane preparation are the same as those in Example 1, except that:

[0108] (4) Preparation of food freshness indicator labels

[0109] 101: Slowly add 30g of waterborne polyurethane to 30g of deionized water solvent and stir magnetically until the mixture is evenly mixed. Then slowly add 39.8g of hydroxyethyl cellulose powder and stir magnetically for 10h until the hydroxyethyl cellulose powder is completely dissolved. Then, add 0.1g of silicon quantum dots and 0.1g of carbon quantum dots in sequence and continue stirring magnetically for 6h to ensure that the mixture is fully mixed.

[0110] 102: Place the mixture in a vacuum drying oven to degas for 10 hours, transfer it into a polytetrafluoroethylene mold, and cure it at 20°C for 24 hours to obtain a food freshness indicator label.

[0111] Example 3

[0112] This embodiment 3 provides a food freshness indicator label and a preparation method thereof.

[0113] The steps of (1) carbon quantum dot preparation, (2) silicon quantum dot preparation, and (3) waterborne polyurethane preparation are the same as those in Example 1, except that:

[0114] (4) Preparation of food freshness indicator labels

[0115] 101: Slowly add 40g of waterborne polyurethane to 50g of deionized water solvent and stir magnetically until evenly mixed. Then slowly add 5g of chitosan powder and 4.94g of hydroxyethyl propyl cellulose powder and stir magnetically for 10h until the powder is completely dissolved. Then, add 0.05g of silicon quantum dots and 0.01g of carbon quantum dots in sequence and continue stirring magnetically for 6h to fully mix the mixture.

[0116] 102: Place the mixture in a vacuum drying oven to degas for 10 hours, transfer it into a polytetrafluoroethylene mold, and cure it at 25°C for 24 hours to obtain a food freshness indicator label.

[0117] Example 4

[0118] This embodiment 4 provides a food freshness indicator label and a preparation method thereof.

[0119] The steps of (1) carbon quantum dot preparation, (2) silicon quantum dot preparation, and (3) waterborne polyurethane preparation are the same as those in Example 1, except that:

[0120] (4) Preparation of food freshness indicator labels

[0121] 101: Slowly add 70g of waterborne polyurethane to 10g of deionized water solvent and stir magnetically until evenly mixed. Then slowly add 8g of polyvinyl alcohol powder and 11.8g of epoxy resin and stir magnetically for 10h until the powder is completely dissolved. Then, add 0.1g of silicon quantum dots and 0.1g of carbon quantum dots in sequence and continue stirring magnetically for 6h to ensure that the mixture is fully mixed.

[0122] 102: Place the mixture in a vacuum drying oven to degas for 10 hours, transfer it into a polytetrafluoroethylene mold, and cure it at 40°C for 24 hours to obtain a food freshness indicator label.

[0123] Example 5

[0124] This embodiment 5 provides a food freshness indicator label and a preparation method thereof.

[0125] The steps of (1) carbon quantum dot preparation, (2) silicon quantum dot preparation, and (3) waterborne polyurethane preparation are the same as those in Example 1, except that:

[0126] (4) Preparation of food freshness indicator labels

[0127] 101: Slowly add 40g of waterborne polyurethane to 20g of deionized water solvent and stir magnetically until evenly mixed. Then slowly add 15g of hydroxymethyl cellulose, 5g of hydroxyethyl cellulose, and 19.9g of ethyl cellulose and stir magnetically for 10h until the powder is completely dissolved. Then, add 0.03g of silicon quantum dots and 0.07g of carbon quantum dots in sequence and continue stirring magnetically for 6h to fully mix the mixture.

[0128] 102: Place the mixture in a vacuum drying oven to degas for 10 hours, transfer it into a polytetrafluoroethylene mold, and cure it at 35°C for 24 hours to obtain a food freshness indicator label.

[0129] Example 6

[0130] This embodiment 6 provides a food freshness indicator label and a preparation method thereof.

[0131] The steps of (1) carbon quantum dot preparation, (2) silicon quantum dot preparation, and (3) waterborne polyurethane preparation are the same as those in Example 1, except that:

[0132] (4) Preparation of food freshness indicator labels

[0133] 101: Slowly add 30g of waterborne polyurethane to 40g of deionized water solvent and stir magnetically until evenly mixed. Then slowly add 10g of hydroxyethyl propyl cellulose, 10g of agarose, and 9.8g of gelatin and stir magnetically for 10h until the powder is completely dissolved. Then, add 0.1g of silicon quantum dots and 0.1g of carbon quantum dots in sequence and continue stirring magnetically for 6h to fully mix the mixture.

[0134] 102: Place the mixture in a vacuum drying oven to degas for 10 hours, transfer it into a polytetrafluoroethylene mold, and cure it at 35°C for 24 hours to obtain a food freshness indicator label.

[0135] [Test example]

[0136] A simulation test was conducted using the food freshness indicator label prepared in Example 1 as a representative example to illustrate that the food freshness indicator label provided by the present application is used for meat freshness detection.

[0137] First, the prepared polyurethane film was cut into 1*1 cm squares for use as fluorescent labels;

[0138] The purchased pork was cut into two pieces of 20g each with similar surface area, and placed in a glass culture dish. The two films were fixed to the top of the culture dish with tape, covered with plastic wrap, and fixed with a rubber band to obtain an experimental sample for pork freshness detection.

[0139] The samples were placed in a UV light box and irradiated with a 365nm UV lamp for 1 minute every hour. A camera was installed in the UV light box to capture the color change of the fluorescent label on the sample each time the UV lamp was activated. The experimental temperature was controlled at 25°C and the experiment lasted for a total of 96 hours.

[0140] The foregoing is merely a list of specific embodiments of the present application, intended to enable those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the broadest scope consistent with the principles and novel features of the present application.

Claims

1. A method for preparing a food freshness indicator label, characterized in that: The food freshness indicator label includes water-based polyurethane, carbon quantum dots, silicon quantum dots, a film-forming agent, and water, and is used for pH detection in multiple acidic and alkaline environments. The preparation method includes the following steps: Evenly mix waterborne polyurethane, carbon quantum dots, silicon quantum dots and film-forming agent, and stir in water to obtain indicator label film-forming liquid; forming a film with the indicator label film-forming liquid to obtain the food freshness indicator label; The invention also includes the following steps of preparing waterborne polyurethane: (1) Under a nitrogen atmosphere, reacting pre-dehydrated polytetrahydrofuran with 0.001-0.1 mol of isophorone diisocyanate to obtain a first intermediate; (2) adding 0.001-0.1 mol of 2,2-dihydroxymethylpropionic acid and 1-20 mol of acetone to the first intermediate to react to obtain a second intermediate; (3) Add 0.001-0.1 mol of 1,4-butanediol to the second intermediate to react and obtain a third intermediate; (4) adding dibutyltin dilaurate to the third intermediate to react and obtain a homogeneous mixed solution; (5) cooling the homogeneous system mixture to room temperature, adding 0.005-0.1 mol of a pH regulator to react, thereby obtaining the waterborne polyurethane; The polytetrahydrofuran was pre-dehydrated by the following steps: the polytetrahydrofuran was heated at 85°C for 3 h, during which time the air was evacuated, the heating was stopped, and the temperature was lowered to 60°C; The method also includes the following steps of preparing carbon quantum dots: dissolving o-phenylenediamine in anhydrous ethanol to obtain a mixed solution; The mixed solution was transferred to a reactor and reacted at 100-200°C for 2-24 hours. The product was cooled to room temperature to obtain a crude product; The crude product was purified by silica gel column chromatography to obtain a carbon quantum dot mixture, which was then rotary evaporated to obtain carbon quantum dots; The method also includes the following steps of preparing silicon quantum dots: Dissolve citric acid and N-aminoethyl-3-aminopropylmethyldimethoxysilane in water and mix well to obtain a clear solution; The clarified liquid was transferred to an autoclave and reacted at 100-200°C for 2-24 hours, and the product was cooled to room temperature to obtain a crude product; The crude product is centrifuged, the supernatant is taken and dried to obtain silicon quantum dots.

2. The method for preparing a food freshness indicator label according to claim 1, wherein: The components are as follows by mass: 10-99.98 parts of waterborne polyurethane, 0.01-1 part of carbon quantum dots, 0.01-1 part of silicon quantum dots, 10-99.98 parts of film-forming agent, and 10-50 parts of water.

3. The method for preparing a food freshness indicator label according to claim 1, wherein: The film-forming agent includes one or more of hydroxymethyl cellulose, hydroxyethyl cellulose, ethyl cellulose, hydroxyethyl propyl cellulose, chitosan, agarose, gelatin, acrylic resin, polyvinyl alcohol, and epoxy resin.

4. The method for preparing a food freshness indicator label according to claim 1, wherein: The pH adjuster includes HI.

5. The method for preparing a food freshness indicator label according to claim 1, wherein: The reaction temperature of step (1) is 85°C and the reaction time is 1-5h; And / or, the reaction temperature of step (2) is 60°C and the reaction time is 1-10h; And / or, the reaction temperature of step (3) is 60°C and the reaction time is 1h; and / or, the reaction temperature of step (4) is 60° C. and the reaction time is 1 h; And / or, the reaction time of step (5) is 30 min.