Intelligent gel label integrating freshness monitoring and preservation as well as preparation method and application of intelligent gel label

Abstract

The invention relates to an intelligent gel label integrating freshness monitoring and preservation as well as a preparation method and application of the intelligent gel label. The intelligent gel label is composed of a porous gel substrate, a plant source preservative eugenol and an aggregation-induced emission (AIE) copper nanocluster carried on the gel substrate. The cerium ions serve as a bridge between the gel and the copper nanoclusters, the copper nanoclusters are anchored to the gel, and indicator leakage is prevented. Besides, the porous gel structure realizes an embedding effect on eugenol, and cerium ions induce the copper nanoclusters to gather on the surface of the gel, so that eugenol is further blocked in a gel substrate. When the freshness of the aquatic product is reduced, the copper nano aggregate responds to the TVB-N, the structure disintegrates, the fluorescence intensity of the label is reduced, and the visual detection of the freshness is realized. In the aspect of preservation, a multi-factor trigger eugenol release mechanism is designed, so that the gel label can quickly respond to food quality change, and integration of food freshness monitoring and intelligent preservation is realized.

Description

Technical Field

[0001] The present invention relates to the fields of non-destructive testing of food and food preservation, and particularly relates to an intelligent gel sensor for visualizing the freshness of aquatic products and controlled release of plant preservatives, and a preparation method and application thereof. Background Art

[0002] During the processes of production, transportation, processing, storage and sales, aquatic products are prone to spoilage and deterioration due to autolysis of the endogenous enzyme system and microbial contamination. Consumers eating spoiled aquatic products will cause adverse physiological reactions such as headache, low blood pressure, arrhythmia and digestive disorders, and may even lead to neurotoxicity. In order to address this challenge, it is necessary to achieve rapid monitoring of the freshness of aquatic products and intelligent preservation.

[0003] Conventional detection methods for the freshness of aquatic products mainly include: microbiological method, physicochemical detection method, and sensory evaluation method. However, the above methods have the disadvantages of cumbersome operation, long detection period, high cost, and sample destruction caused by pretreatment. Therefore, it is necessary to develop a rapid non-destructive detection technology for food freshness. Gas sensing labels, which can quickly sense the changes in the atmosphere inside the aquatic product packaging, have high sensitivity and good selectivity, and have received wide attention. During the spoilage process of aquatic products, proteins in aquatic products are gradually decomposed into peptides and amino acids under the action of bacteria and enzymes, and then degraded into volatile basic nitrogen such as methyl indole, ammonia, and amines. Therefore, developing a gas sensing label for rapid detection of total volatile basic nitrogen (TVB-N) is an important means to monitor the freshness of aquatic products. However, traditional labels have the problem of indicator leakage, posing a food safety hazard. Therefore, fixing the indicator on the label substrate to avoid the migration of the indicator into the food is an important means to improve the safety of the sensing label. Fluorescent sensing labels have received more attention due to their high sensitivity, good selectivity, and strong anti-interference ability. However, traditional fluorescent molecular dyes are prone to aggregation-induced fluorescence quenching (ACQ) due to Π-Π stacking in the aggregated state, which hinders their application in solid-state labels. On the contrary, fluorescent materials with aggregation-induced emission (AIE) properties show enhanced fluorescence due to restricted intramolecular motion and inhibited non-radiative relaxation in the aggregated state. Therefore, developing fluorescent materials with AIE properties can effectively overcome the research and development difficulties of fluorescent gas sensing labels.

[0004] Fresh preservation technology can inhibit the actions of microorganisms and enzymes, enabling food to maintain its edible quality for a longer time and reducing economic losses during food transportation and storage. Common fresh preservation technologies mainly include physical fresh preservation, chemical fresh preservation, and new fresh preservation technologies. Among them, physical fresh preservation technologies mainly include low-temperature preservation, vacuum packaging, modified atmosphere packaging, high-pressure processing, radiation fresh preservation technology, etc., but they have limitations such as high costs and dependence on large equipment. Chemical fresh preservation technology mainly involves adding chemical preservatives, but there is a risk of chemical residues. Currently, the research and application of some new fresh preservatives are gradually emerging. Among them, plant essential oils are a kind of natural biological fresh preservative with low cost, antibacterial, and antioxidant properties. However, plant essential oils have limitations such as being photo-thermally unstable and volatile, which restricts their development in the field of food fresh preservation. Therefore, enhancing the stability of plant essential oils and slowing down their release rate are the keys to achieving long-term fresh preservation of plant essential oils.

[0005] Chinese patent application CN117337874A discloses "A Sustained-Release Type Intelligent Fresh Preservative and a Method for Directly Evaluating Its Fresh Preservation Effect". This invention prepares a porous hydrogel loaded with graphene oxide and citral and applies it to the field of mango fresh preservation. During storage, due to the respiration of mangoes, the humidity inside the package gradually increases, and the humidity will trigger the release of citral to achieve the intelligent fresh preservation of mangoes. At the same time, after the gel absorbs moisture, its appearance will change significantly, and the freshness of mangoes can be directly evaluated by colorimetry. However, the essential oil in this gel is triggered to release only by one factor, humidity, with limited sensitivity and accuracy. At the same time, compared with fluorescent labels, colorimetric labels have lower accuracy and are vulnerable to interference from the external light environment.

[0006] Therefore, there is an urgent need to create a new type of intelligent gel label that can not only monitor the changes in freshness indicators of meat but also accurately and quickly respond to the changes in the quality of aquatic products according to the freshness information captured inside the package, and timely inhibit the process of spoilage and deterioration of aquatic products. Summary of the Invention

[0007] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the primary object of the present invention is to prepare a sensing label that can sensitively detect TVB-N in the aquatic product packaging system to real-time monitor the freshness information of aquatic products. At the same time, based on copper nanoclusters responsive to TVB-N and a humidity-sensitive gel substrate, a multi-factor (TVB-N, moisture) synergistic triggering fresh preservative release strategy is designed to achieve the rapid response of the gel composite label to changes in food quality and delay the spoilage and deterioration of food.

[0008] The object of the present invention is achieved through the following technical solutions:

[0009] An intelligent gel label for integrated freshness monitoring and preservation, which is composed of a humidity-sensitive porous gel substrate, eugenol, and aggregation-induced emission copper nanoclusters. Among them, the copper nanoclusters form stable nanoaggregates on the gel surface through coordination with cerium ions, locking eugenol within the porous gel substrate.

[0010] Preferably, the humidity-sensitive porous gel substrate is composed of a polyacrylic acid-carboxymethyl cellulose-polyvinyl alcohol three-component composite gel.

[0011] Preferably, the copper nanoclusters are prepared by dispersing N-acetyl-L-cysteine in a sodium hydroxide solution and adding a copper nitrate solution. And these copper nanoclusters can be induced to aggregate by cerium ions to enhance the fluorescence quantum yield.

[0012] Preferably, eugenol enters the gel porous network through the solute diffusion method, and is locked within the gel pores by spraying a solution of copper nanoclusters and cerium nitrate on the gel surface.

[0013] A method for preparing the above-mentioned intelligent gel label for integrated freshness monitoring and preservation includes the following steps:

[0014] (1) The preparation steps of the humidity-sensitive sensing gel are as follows:

[0015] Add polyvinyl alcohol (PVA) to an aqueous sodium hydroxide solution, heat and stir in a water bath for a period of time until the solid is completely dissolved, cool to room temperature, then add a certain amount of carboxymethyl cellulose (CMC), acrylic acid monomer (AA), N,N-methylenebisacrylamide (MBA) and cerium nitrate solution (Ⅲ), heat and stir in a water bath for a period of time, cool to room temperature, and then add a certain amount of ammonium persulfate (APS), and mix evenly. Pour the above sol into a mold, and after thermal polymerization in a water bath, a polyacrylic acid-carboxymethyl cellulose-polyvinyl alcohol (PAA-CMC-PVA) three-component hydrogel is formed. The gel is impregnated in distilled water and ethanol solution to remove unreacted acrylic acid monomers. The soaked and washed gel is freeze-dried to make PAA-CMC-PVA porous aerogel.

[0016] (2) The preparation steps of the AIE copper nanoclusters sensitive to TVB-N are as follows:

[0017] Under room temperature conditions, disperse a certain amount of N-acetyl-L-cysteine (NAC) in a sodium hydroxide solution with a certain concentration, add a certain amount of copper nitrate solution, shake well, and prepare a copper nanoclusters (CuNCs) solution.

[0018] (3) The steps of locking eugenol in the gel porous network are as follows:

[0019] The porous gel prepared in step (1) is impregnated in an ethanol solution of eugenol. After a period of time, it is taken out, and the excess liquid on the surface of the gel is removed with filter paper. The CuNCs solution prepared in step (2) is sprayed on the surface of the gel using a spray bottle, and then a certain amount of cerium nitrate solution is sprayed. Finally, the gel is freeze-dried to obtain a gel label with freshness indication and preservation efficacy.

[0020] Preferably, in step (1), the mass ratio of polyvinyl alcohol, carboxymethyl cellulose, acrylic monomer: N,N - methylenebisacrylamide: ammonium sulfate is 0.1 - 2.0: 0.05 - 0.4: 3 - 5: 0.03 - 0.05: 0.10 - 0.50; the concentration of the sodium hydroxide aqueous solution is 0.5 - 5 mol / L; the concentration of the cerium nitrate solution is 0.1 - 1 mol / L, and the mass ratio of the cerium nitrate solution to the mass of the acrylic monomer is 0.025 - 0.2.

[0021] More preferably, in step (1), the mass of CMC is 0.05 - 0.4 g, the mass of PVA is 0.1 - 2.0 g, the concentration of the sodium hydroxide solution is 0.5 - 5 mol / L, the volume of the sodium hydroxide solution is 5 - 20 mL, the mass of AA is 3 - 5 g; the mass of MBA is 0.03 - 0.05 g, the concentration of the cerium nitrate solution is 0.1 - 1 mol / L, the volume is 0.5 - 1 mL, the mass of APS is 0.10 - 0.50 g; the water bath heating time is 1 - 2 h, the water bath heating temperature is 60 - 90 °C, and the rotation speed of the magnetic stirring in the water bath is 500 - 1000 rpm / min.

[0022] Preferably, in step (2), the concentration of NAC is 0.2 - 1.0 mol / L, the concentration of the copper nitrate solution is 0.02 - 0.1 mol / L, the volume of the sodium hydroxide solution is 5 - 10 mL, the concentration of the sodium hydroxide solution is 0.1 - 1 mol / L, and the oscillation time is 0.5 - 5 min.

[0023] Preferably, in step (3), the concentration of eugenol is 1 - 5 μg / mL, the mass fraction of the ethanol solution is 20% - 95%, the impregnation time is 2 - 5 h, and the gel weight increases to 100 - 150 times the initial mass; the spraying amount of the CuNCs solution is 2 - 5 times the initial gel mass, the concentration of the cerium nitrate solution is 0.1 - 1 mol / L, and the spraying amount of the cerium nitrate solution is 0.5 - 1 times the initial gel mass.

[0024] Establishment of a method for monitoring the freshness of aquatic products using a multifunctional gel label

[0025] Place the prepared intelligent gel tags in a petri dish. Put the petri dish containing the gel tags and a certain mass of freshly purchased aquatic products at the bottom of a sealed box, and place the sealed box in a thermostatic and humidistatic chamber for storage. At regular intervals, under ultraviolet light, use a mobile phone to take pictures of the fluorescence changes of the gel tags. Import the pictures into a computer, call the OpenCV library for image preprocessing, and use functions to extract the color information of the region of interest (ROI) in the HSV color space of the pictures. At the same time, refer to GB5009.228-2016 to measure the TVB-N value of the aquatic products at the moment when the photos are taken. Taking the V value of the picture in the HSV color space as the independent variable and the TVB-N value as the dependent variable, establish a linear regression prediction model for the freshness of aquatic products.

[0026] Preferably, the mass of the aquatic products is 200-500 g, the volume of the sealed box is 700-1000 mL, the temperature of the thermostatic and humidistatic chamber is 4°C and 25°C, and the interval time is 2-3 h.

[0027] Verification of the release mechanism triggered by moisture and TVB-N for eugenol

[0028] (1) Verification of the release mechanism triggered by moisture: According to the ASTM E 104-14 standard, prepare different types of saturated salt solutions. The relationship between the types of saturated salt solutions and the standard humidity is shown in Table 1. Measure a certain volume of saturated salt solution into a petri dish, and place the petri dish in a sealed container. Place the sealed container in a 25°C thermostatic and humidistatic chamber to construct different standard humidity systems. Hang the gel loaded with eugenol and CuNCs in the above sealed containers with different humidity values. At regular intervals, take the same mass of gel samples each time. Immerse the taken gel in a centrifuge tube containing an extractant, and use ultrasonic-assisted treatment to fully extract the residual eugenol in the gel. Pipette a certain volume of the extract into a cuvette, and use a UV spectrophotometer to measure the absorbance value of the extract at 281 nm. Refer to the pre-established standard curve of the eugenol concentration and the absorbance value at 281 nm to calculate the concentration of eugenol in the extract. Calculate the eugenol residue rate (R%) according to formula 1, and draw the residue rate-time curve of eugenol at different humidities.

[0029] (2) Verification of ammonia-triggered release mechanism: Prepare ammonia water solutions with different concentrations. Add a certain amount of ammonia water solution to a petri dish, and place the petri dish containing ammonia water and a certain amount of saturated salt solution in a sealed container. Then place the sealed container in a constant temperature and humidity chamber at 25°C to construct a system with the same humidity but different ammonia concentrations. Hang the gel loaded with eugenol and CuNCs in the above systems with different ammonia concentrations. At intervals, take out gel samples of the same mass each time. Immerse the taken-out gel in a centrifuge tube containing an extractant, and use ultrasonic assistance to fully extract the residual eugenol in the gel. Pipette a certain volume of the extract into a cuvette, and use a UV spectrophotometer to measure the absorbance value of the extract at 281 nm. Refer to the pre-established standard curve of eugenol concentration and absorbance value at 281 nm to calculate the concentration of eugenol in the extract. Calculate the residual rate (R%) of eugenol according to Formula 1, and draw the residual rate-time curve of eugenol under different ammonia concentrations.

[0030] Preferably, the relationship table between the saturated salt solution and humidity in step (1) is as follows:

[0031] Table 1 Standard humidity constructed by saturated salt solution at 25°C

[0032]

[0033] Preferably, the volume of the saturated salt solution in step (1) is 5 - 10 mL.

[0034] Preferably, the ammonia concentration in step (2) is 5 - 5000 ppm, the volume of ammonia water is 0.5 - 2 mL, and the volume of the saturated salt solution is 5 - 10 mL.

[0035] Preferably, Formula 1 in steps (1) and (2) is as follows:

[0036] Calculation formula for the residual rate (R%) of eugenol:

[0037]

[0038] Where: R% --- Residual rate of eugenol, C t -- Concentration of eugenol in the gel extract taken out at time t, C0 — Concentration of eugenol in the gel extract at the initial time.

[0039] Preferably, in steps (1) and (2), the culture dish has a diameter of 60 - 90 mm, the sealed container has a volume of 1 - 3 L; the release experiment period is 48 - 180 h; the sampling interval time is 2 - 3 h, and the mass of the gel taken out is 0.1 - 0.5 g; the extractant is ethanol, petroleum ether, ether, n - hexane, cyclohexane, and dichloromethane, the volume of the extractant is 30 - 50 mL, and the extraction time is 12 - 24 h; the ultrasonic time is 0.5 - 3 h, the ultrasonic power is 100 - 300 W, and the water bath temperature during ultrasonic treatment is 25 - 30 °C.

[0040] Fresh - keeping application test of multifunctional gel labels

[0041] The experiment is divided into three groups, A, B, and C. Each group contains multiple sealed boxes, and the same mass of fish fillets is placed at the bottom. In addition, a blank gel substrate is placed at the bottom of each sealed box in group B; a gel label loaded with eugenol is placed at the bottom of each sealed box in group C. The above - mentioned three groups of sealed boxes are placed in a constant - temperature and constant - humidity chamber. Every once in a while, 30 g of fish fillets are taken out from the sealed boxes, and the TVB - N value of the fish fillets is measured with reference to GB5009.228 - 2016. The difference in the TVB - N values of the fish fillets in groups A and B is compared to verify the fresh - keeping effect of the moisture - absorbing function of the gel label substrate. The difference in the TVB - N values of the fish fillets in the packages of groups B and C is compared to verify the fresh - keeping effect of the gel label releasing eugenol.

[0042] Preferably, the mass of the fish fillets is 50 - 200 g, the mass of the gel is 0.2 - 2.0 g, the temperature of the constant - temperature and constant - humidity chamber is 4 °C and 25 °C, and the time interval is 3 - 6 h;

[0043] The essential - oil fresh - keeping agent in the present invention can be triggered to release by both humidity and TVB - N, with a more sensitive and adjustable response. At the same time, by introducing cerium ions, copper nanoclusters with AIE properties are anchored on the gel substrate. By observing the fluorescence change of the gel label, intelligent monitoring of the freshness of aquatic products is realized. First, the present invention uses a thermal polymerization technique to prepare a polyacrylic acid - carboxymethyl cellulose - polyvinyl alcohol composite gel substrate. Then, through a solution replacement method, the fresh - keeping agent is loaded into the gel porous network. After that, copper nanoclusters and cerium nitrate solution are sequentially sprayed on the surface of the gel. Through the coordination of cerium ions, the copper nanoclusters are anchored on the gel surface, effectively avoiding indicator leakage. In addition, cerium ions induce the formation of a layer of nano - aggregates of copper nanoclusters on the gel surface, encapsulating the fresh - keeping agent in the gel network and reducing the evaporation rate of the fresh - keeping agent. Finally, through freeze - drying, a multifunctional gel sensing label with freshness indication and fresh - keeping functions is prepared.

[0044] During the application process, the above-mentioned gel label is placed inside the aquatic product packaging. In terms of freshness indication: the copper nanoaggregates on the label respond to TVB-N, the structure of the nanoaggregates collapses, and the fluorescence intensity of the label decreases. By regularly collecting label pictures with a smartphone and measuring the TVB-N value of the aquatic product at this time, the relationship between the picture information and the physical and chemical indicators of the freshness of the aquatic product is established to achieve real-time monitoring of the freshness of the aquatic product. In terms of preservation: the copper nanoaggregates covering the gel surface respond to TVB-N in the packaging, and the structure disintegrates, which is conducive to the release of the preservative encapsulated in the gel. At the same time, during the storage process of the aquatic product, due to the volatilization of meat moisture and the respiration of microorganisms, the humidity in the packaging gradually increases. The humidity-sensitive gel absorbs the moisture in the packaging, the gel polymer network expands, and the pores enlarge, further promoting the release of the preservative from the pores of the gel network. Compared with the single-factor triggering mechanism, the multi-factor triggering mechanism of the synergistic action of TVB-N and humidity is more adjustable. The triggering factors and thresholds can be adjusted according to the characteristics and preservation requirements of different foods, providing a new strategy for customized preservation technologies in the future. In addition, the gel label achieves the hurdle preservation effect through the dual actions of reducing the humidity in the packaging and releasing the preservative.

[0045] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0046] (1) The copper nanoclusters prepared in the present invention use N-acetyl-L-cysteine as a ligand. The preparation steps are simple, only 30 s are required, no organic reagents are involved, and no heating is required, which is green and environmentally friendly. The copper nanoclusters have AIE properties, overcoming the quenching (ACQ) characteristics caused by aggregation of traditional fluorescent dyes, and can be applied to the preparation of portable solid-state sensors.

[0047] (2) The ligand on the surface of the copper nanoclusters prepared in the present invention coordinates with cerium ions to form copper nanoaggregates on the gel surface. The fluorescence intensity and stability after aggregation of the copper nanoclusters are both significantly improved. At the same time, the copper nanoclusters can be anchored on the gel network through cerium ions to prevent the leakage of the indicator.

[0048] (3) The present invention uses the porous structure of the gel to encapsulate the plant essential oil, and coordinates through cerium ions and carboxyl groups on the gel substrate to enhance the crosslinking degree on the gel surface and limit the volatilization of the plant essential oil. In addition, cerium ions induce the formation of copper nanoaggregates on the gel surface, further locking the plant essential oil in the gel substrate.

[0049] (4) The gel label prepared by the present invention can trigger the collapse of the copper nanocluster structure on the gel by TVB-N, causing eugenol to be released from the gel. In addition, the gel substrate material has strong hygroscopicity. The polymer network of the gel absorbs moisture and swells, and the pores in the gel expand, promoting the release of the plant-derived preservative. A dual-factor triggering mechanism (TVB-N, moisture) is designed to achieve the intelligent slow release of eugenol.

[0050] (5) The present invention prepares a composite gel label based on AIE copper nanoclusters, which can rapidly and intelligently regulate fresh-keeping according to the monitored information on the change of aquatic product freshness, realizing the integration of the monitoring and fresh-keeping of aquatic product freshness. Description of the Drawings

[0051] Figure 1 Scanning electron micrograph of the cross-section of the polyacrylic acid-carboxymethyl cellulose-polyvinyl alcohol ternary gel prepared in Example 1;

[0052] Figure 2a UV-visible absorption spectra of the copper nanoclusters prepared in Example 2 before and after being induced by cerium ions; Figure 2b Fluorescence emission and excitation spectra;

[0053] Figure 3 Fluorescence quantum yield test graphs of the copper nanoclusters prepared in Example 2 before and after aggregation induced by cerium ions;

[0054] Figure 4 Scanning electron micrograph of the aggregation of the copper nanoclusters on the gel surface after being induced by cerium ions in Example 4;

[0055] Figure 5 Test graphs of the sensitivity and selectivity of the paper label loaded with copper nanoclusters prepared in Example 5 to ammonia;

[0056] Figure 6 Absorption humidity curves of the porous gel substrate prepared in Example 1 in different humidity environments;

[0057] Figure 7a UV absorption spectra of different eugenol concentrations in Example 7; Figure 7b Standard curve of eugenol concentration and absorbance value at 281 nm established in Example 7;

[0058] Figure 8 Graph of the change of the residual rate of eugenol in the gel with time under different humidity and ammonia concentration environments for the gel label prepared in Example 4;

[0059] Figure 9 Graph of the relationship between the V value of the gel label picture prepared in Example 4 and the TVB-N content in black carp fillets;

[0060] Figure 10 It is the relationship curve between the TVB-N value of black carp slices in each group of packages in Example 9 and the storage time;

[0061] Figure 11 It is a schematic flow chart of preparing an intelligent gel label integrating freshness monitoring and preservation according to the present invention. Specific embodiments

[0062] The present invention will be further described in detail below in conjunction with embodiments, but the embodiments of the present invention are not limited thereto.

[0063] Example 1: Synthesis of humidity-sensitive gel substrate

[0064] Add 0.50 g of polyvinyl alcohol (PVA) to 20 mL of sodium hydroxide aqueous solution (7 wt%), and heat and stir for 1 h under the condition of a 90°C water bath. After cooling to room temperature, add 0.2 g of carboxymethyl cellulose (CMC), 4 g of acrylic acid monomer (AA), 0.04 g of N,N-methylenebisacrylamide, and 1 mL of cerium nitrate solution (0.1 mol / L), and heat and stir for 1 h under the condition of a 70°C water bath. After cooling to room temperature, add 0.15 g of ammonium persulfate, mix evenly, and perform ultrasonic treatment for 10 min to remove the bubbles in the sol. Then pour the sol into a cylindrical glass mold and carry out hydrothermal polymerization at 70°C for 30 min to generate a polyacrylic acid-carboxymethyl cellulose-polyvinyl alcohol (PAA-CMC-PVA) three-component hydrogel. After the gel is demolded, it is cut into circular pieces with a thickness of 5 mm, and then impregnated in a 20% ethanol solution to remove the unreacted acrylic acid monomer. The washed gel is stored in a -80°C refrigerator, pre-frozen for 12 h, and then placed in a vacuum freeze dryer with a trap temperature of -60°C for 48 h of drying treatment to make a PAA-CMC-PVA gel substrate. As Figure 1 shown, from the scanning electron microscope image of the gel cross-section, it can be observed that the gel contains a large number of pores, and a porous gel substrate is successfully prepared.

[0065] Example 2: Preparation of copper nanocluster solution and copper nanoaggregates

[0066] Add 0.2613 g of N-acetyl-L-cysteine to a 10 mL centrifuge tube, then add 4 mL of sodium hydroxide solution (0.25 mol / L) and 1 mL of copper nitrate solution (0.1 mol / L), and oscillate at room temperature for 30 s to prepare a CuNCs solution. Pipette 100 μL of cerium nitrate solution (1 mol / L) into 1 mL of the above CuNCs solution and oscillate at room temperature for 2 min to prepare cerium ion-induced aggregated CuNCs-Ce 3+ nanoaggregates.

[0067] For the above-prepared CuNCs and CuNCs-Ce 3+ Perform ultraviolet-visible absorption (UV-vis) spectroscopy, fluorescence spectroscopy, and fluorescence quantum yield (QY) characterization. As Figure 2a shown, in the UV-vis spectrum of CuNCs, the characteristic peak of Cu 2+ at 301 nm does not appear, indicating the successful preparation of CuNCs. In the UV-vis spectrum of CuNCs-Ce 3+ the characteristic peak of Ce 3+ at 297 nm does not appear, indicating that Ce 3+ is successfully bound to CuNCs. As Figure 2b shown, after adding Ce 3+ to induce the aggregation of CuNCs, the fluorescence intensity of CuNCs-Ce 3+ is 8.1 times that before aggregation. As Figure 3 shown, in the aqueous dispersion system, the QY of CuNCs is 0.01%, and after adding Ce 3+ to generate CuNCs-Ce 3+ aggregates, the QY increases to 8.51%.

[0068] Example 3: Preparation of a gel label with a freshness indication function

[0069] Evenly spray 1 mL of the CuNCs solution prepared in Example 2 and 0.5 mL of cerium nitrate solution (0.1 mol / L) on the surface of the gel prepared in Example 1. Place the gel in a refrigerator at -80 °C and freeze it for 12 h, then perform freeze-drying for 48 h to obtain a gel label with a freshness indication function.

[0070] Example 4: Preparation of a gel label with freshness indication and antibacterial and fresh-keeping effects

[0071] Add 0.5 mg of eugenol and 40 mL of ethanol solution (20 wt%) to a 50 mL centrifuge tube and shake for 5 min. Weigh 0.20 g of the gel prepared in Example 1 and add it to the centrifuge tube. Fix the centrifuge tube in a water bath shaker and shake for 3 h at a temperature of 37 °C and a rotation speed of 200 rpm / min. Take out the gel embedded with eugenol and place it on a clean glass plate. First, dry the excess solution on the surface with filter paper. Evenly spray 1 mL of the CuNCs solution prepared in Example 2 and 0.5 mL of cerium nitrate solution (1 M) on the surface of the gel. Place the gel in a refrigerator at -80 °C and freeze it for 12 h, then perform freeze-drying for 48 h to obtain a gel label with freshness indication and antibacterial and fresh-keeping effects. Figure 4 Figure [ID] is a scanning electron micrograph of the gel label, and a large number of CuNCs-Ce 3+ nanometer aggregates induced by cerium ions can be observed to be distributed on the surface of the gel.

[0072] Example 5: CuNCs-Ce 3+ Preparation of paper labels and CuNCs-Ce 3+ Responsiveness test to ammonia.

[0073] Use a punch to prepare a circular hole with a diameter of 1 cm on a polytetrafluoroethylene film, paste a circular cotton sheet with a diameter of 1.2 cm between two films to construct a paper sensing label. Add 100 μL of the CuNCs-Ce 3+ aggregate solution prepared in Example 2 dropwise to the cotton paper area of the label. Place the paper label in an oven and dry it at 60 °C for 4 h to obtain a paper sensing label. Paste a paper label on the top of 13 2-L sealed barrels. Divide the sealed barrels into two groups, A and B, with 5 in group A and 8 in group B.

[0074] Add different volumes of ammonia aqueous solution to the bottom of each sealed barrel in group A to construct ammonia concentration environments of 200 ppm, 400 ppm, 600 ppm, 800 ppm, and 1000 ppm. Every 10 min, take a fluorescence picture of the paper label under a 365-nm ultraviolet lamp to verify the responsiveness sensitivity of CuNCs-Ce 3+ aggregates to ammonia. As Figure 5 shown, after the label is exposed to 200 ppm ammonia for 10 min, the fluorescence intensity of the label changes significantly, indicating that CuNCs-Ce 3+ aggregates have high responsiveness sensitivity to ammonia.

[0075] Add 0.5 mL of hydrogen sulfide, ethyl acetate, distilled water, methanol, ethanol, and isopropanol solutions to the bottom of the sealed barrels in group B respectively, and let the paper label be exposed to different spoiled gas environments. After the label responds to different spoiled gases for 30 min, take a fluorescence picture of the label under a 365-nm ultraviolet lamp. As Figure 5 shown, after the label responds to ammonia for 30 min, the fluorescence intensity of the label decreases significantly. After the label is treated with hydrogen sulfide, ethyl acetate, water vapor, and alcohol gases, the change in fluorescence intensity is weak and almost invisible to the naked eye, indicating that CuNCs-Ce 3+ aggregates have high selectivity to ammonia response.

[0076] Example 6: Moisture absorption performance test of the porous gel substrate prepared in Example 1.

[0077] As shown in Table 1, five saturated salt solutions of lithium chloride, magnesium chloride, sodium bromide, sodium chloride, and copper sulfate were prepared with distilled water to construct humidity environments with relative humidities of 11.30%, 33.70%, 57.60%, 75.30%, and 97.60%. First, 10 mL of the above-mentioned saturated salt solutions were aspirated into different petri dishes, and a hygrometer and a petri dish containing the saturated salt solution were placed in a 700 mL sealed box. The sealed box was placed in an incubator at 25 °C for a period of time to balance the humidity in the sealed box to the theoretical value. Then, a gel prepared in Example 1 was placed in each sealed box. After an interval of time, the gel was weighed until the gel reached a constant weight. With time as the abscissa and the moisture absorption rate as the ordinate, the moisture absorption curve of the gel in different humidity environments was plotted. Among them, the moisture absorption rate (W%) was calculated according to Formula 2. As Figure 6 shown, in an environment with a lower humidity value, the W% of the gel is lower and the time to reach humidity equilibrium is short. For example, in an environment with a humidity value of 75.3%, the W% of the gel is 36.8% and the humidity equilibrium is reached in 24 h. While in an environment with a humidity value of 97.6%, the W% of the gel can reach 162% and the time to reach humidity equilibrium is 168 h. Therefore, the gel has good moisture absorption performance in an environment with a higher humidity and can continuously absorb moisture from the environment for a long time.

[0078]

[0079] Where: W% --- moisture absorption rate, m0 --- mass of the gel at the initial moment, m t --- mass of the gel at time t.

[0080] Example 7: Influence of humidity and TVB-N on the release of eugenol

[0081] Saturated lithium chloride and copper sulfate solutions were prepared. 10 mL of the saturated salt solution was measured and added to petri dishes with a diameter of 90 mm, and the petri dishes were respectively placed in sealed barrels with a volume of 2 L. For the sealed barrel containing the saturated copper sulfate solution, the corresponding standard humidity is 97.60%; for the sealed barrel containing the saturated lithium chloride solution, the corresponding standard humidity is 11.30%. Then, the sealed barrels were placed in a constant temperature and humidity incubator at 25 °C until the humidity reached the standard humidity value. Then, 0.2 g of the gel sample prepared in Example 4 was weighed and placed in a breathable mesh bag. 30 mesh bags containing 0.2 g of the gel sample were respectively hung in the above two sealed barrels for a slow-release experiment. After an interval of time, one gel sample was taken and immersed in a centrifuge tube containing 60 mL of ethanol solution (20 wt%) for 3 h, and ultrasonic-assisted treatment was carried out for 30 min with 150 W to extract the residual eugenol in the gel. 600 μL of the extract was aspirated into a 700 μL cuvette, and the ultraviolet absorbance value at 281 nm was measured. AsFigure 7a and 7b As shown in 7b , a standard curve of eugenol concentration and absorbance value at 281 nm was established in advance. Referring to the standard curve, the concentration of eugenol in the extract was calculated. The residual rate (R%) of eugenol in the gel was calculated according to Formula 1, and a relationship curve between the residual rate of eugenol and time was plotted to evaluate the effect of humidity on the sustained release of eugenol.

[0082] In the experiment to evaluate the effect of TVB-N on the sustained release of eugenol, 10 mL of saturated copper sulfate solution and 0.5 mL of ammonia water solution were added into the sealed barrel, and the remaining steps were the same as above.

[0083] As Figure 8 shown, compared with the environment with a humidity of 11.30%, in the environment with a humidity of 97.60%, the R% value of eugenol in the gel decreased faster, indicating that the release rate of eugenol was faster in the environment with a higher humidity. At the same time, in the environment with a humidity of 97.60%, after adding the ammonia water solution, the R% value of eugenol in the gel decreased even faster. It indicates that humidity and TVB-N jointly promoted the release of eugenol from the gel.

[0084] Example 8: Establishment of the relationship between the gel label picture information prepared in Example 4 and the freshness of aquatic products.

[0085] Weigh 200 g of freshly purchased black fish fillets, place the fish fillets and the gel label prepared in Example 4 at the bottom of a 700 mL packaging box as the sample for gel label picture collection; weigh 5 portions of 200 g black fish fillets and place them in 700 mL sealed packaging boxes respectively as the samples for TVB-N test of the fish meat. Place the above sealed boxes in a constant temperature and humidity incubator at 25°C. Every 3 h, take a photo of the gel label with a mobile phone under an ultraviolet lamp, and take out 30 g of fish fillets from the remaining sealed boxes. Refer to GB5009.228-2016 to test the TVB-N content of the fish meat. Upload the taken gel pictures to the computer folder, and name the pictures in the folder with the TVB-N value of the fish meat measured when the gel pictures were taken. Traverse all the pictures in the specified folder, call the OpenCV library for image preprocessing and eigenvalue extraction. The preprocessing steps include: resizing the image, converting the color space, noise reduction, enhancing the contrast, image segmentation, etc. For the preprocessed pictures, use the cv2.split() function to extract the V value of the region of interest (ROI) of the picture in the HSV space, and store the V value and the corresponding TVB-N value into a CSV file to establish a linear regression model of the picture V value and the TVB-N value. The results are as Figure 9 shown. As the storage time of the fish meat extends, the TVB-N value of the fish meat in the package increases, and the V value of the gel label decreases. The R 2 of the linear regression model of the two can reach 0.9391.

[0086] Example 9: Test on the effect of the gel label prepared in Example 4 on the freshness of fish

[0087] 200 g of snakehead fillet and 0.5 g of the eugenol and CuNCs-Ce prepared in Example 4 were placed in 12 sealed boxes. 3+ The sealed boxes were placed in a constant temperature and humidity chamber at 4°C. At intervals, 30g of fish fillets were taken out from different sealed boxes and the TVB-N values of the fish fillets were determined according to GB 5009.228-2016.

[0088] Comparative Example 1: Test on the impact of paper labels on fish freshness

[0089] A sheet of CuNCs-Ce prepared in Example 5 was pasted on the top of the 12 sealed boxes. 3+ A 200g snakehead fillet was placed at the bottom of the sealed box. The sealed box was placed in a constant temperature and humidity chamber at 4°C. At intervals, 30g of fish fillets were taken out from different sealed boxes and the TVB-N value of the fish fillets was determined with reference to GB 5009.228-2016.

[0090] Comparative Example 2: Test on the effect of gel label on fish freshness

[0091] 200 g of black fish fillet and 0.5 g of CuNCs-Ce prepared in Example 3 were placed in 12 sealed boxes. 3+ The sealed boxes were placed in a constant temperature and humidity chamber at 4°C. At intervals, 30g of fish fillets were taken out from different sealed boxes, and the TVB-N values of the fish fillets were determined according to GB5009.228-2016.

[0092] According to GB 2733-2015, if the TVB-N value of freshwater fish exceeds 20mg / 100g, the fish meat is not edible. Figure 10 As shown, the TVB-N content of the snakehead fish fillets in Comparative Example 1 exceeded 20 mg / 100 g on the 10th day of storage; the TVB-N content of the snakehead fish fillets in Comparative Example 2 exceeded 20 mg / 100 g on the 12th day of storage; and the TVB-N content of the snakehead fish fillets in Example 10 exceeded 20 mg / 100 g on the 14th day of storage. These results demonstrate that, compared to the paper label in Comparative Example 1, the moisture-absorbing effect of the gel base in Comparative Example 2 delayed fish spoilage. Compared to the gel label in Comparative Example 2, the eugenol preservative loaded in the gel label in Example 9 also delayed fish spoilage. Compared to traditional paper labels, the gel label in Example 9 exhibited superior freshness-preserving properties, extending the shelf life of the snakehead fish fillets by four days.

[0093] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. An intelligent gel label for integrated freshness monitoring and preservation, characterized in that, The intelligent gel label consists of a humidity-sensitive porous gel substrate, eugenol, and aggregation-induced emission copper nanoclusters. Among them, the copper nanoclusters form stable nanoaggregates on the gel surface through coordination with cerium ions, locking eugenol within the porous gel substrate.

2. The intelligent gel label according to claim 1, wherein The humidity-sensitive porous gel substrate is composed of a polyacrylic acid-carboxymethyl cellulose-polyvinyl alcohol three-component composite gel.

3. The intelligent gel label according to claim 1, wherein The copper nanoclusters are prepared by dispersing N-acetyl-L-cysteine in a sodium hydroxide solution and adding a copper nitrate solution. And these copper nanoclusters can be induced to aggregate by cerium ions to enhance the fluorescence quantum yield.

4. The intelligent gel label according to claim 1, wherein The eugenol enters the gel porous network through the solute diffusion method, and is locked within the gel pores by spraying the copper nanoclusters and cerium nitrate solution on the gel surface.

5. A method for preparing the intelligent gel label according to any one of claims 1 to 4, characterized in that, It includes the following steps: (1) Prepare a humidity-sensitive porous gel substrate: Add polyvinyl alcohol to an aqueous sodium hydroxide solution, heat and stir in a water bath until the solid is completely dissolved, cool to room temperature, then add carboxymethyl cellulose, acrylic acid monomer, N,N-methylenebisacrylamide, and cerium nitrate solution, heat and stir in a water bath, cool to room temperature, then add ammonium persulfate, mix evenly, and then carry out thermal polymerization in a water bath to generate a polyacrylic acid-carboxymethyl cellulose-polyvinyl alcohol three-component hydrogel. Then immerse it in distilled water and ethanol solution to remove unreacted acrylic acid monomers, and then freeze-dry to make a polyacrylic acid-carboxymethyl cellulose-polyvinyl alcohol porous aerogel; (2) Prepare a copper nanocluster solution sensitive to TVB-N: Disperse N-acetyl-L-cysteine in a sodium hydroxide solution, and then add a copper nitrate solution. After shaking well, a copper nanocluster solution is prepared; (3) Prepare an intelligent gel label with freshness monitoring and preservation functions: Immerse the polyacrylic acid-carboxymethyl cellulose-polyvinyl alcohol porous aerogel prepared in step (1) in an eugenol-ethanol solution, take it out and remove the excess liquid on the gel surface with filter paper. Then spray the copper nanocluster solution and cerium nitrate solution prepared in step (2) on the gel surface, and finally freeze-dry to obtain an intelligent gel label with freshness monitoring and preservation functions.

6. The method according to claim 5, characterized in that, In step (1), The mass ratio of the polyvinyl alcohol, carboxymethyl cellulose, acrylic acid monomer: N,N-methylenebisacrylamide: ammonium sulfate is 0.1-2.0: 0.05-0.4: 3-5: 0.03-0.05: 0.10-0.50; The concentration of the aqueous sodium hydroxide solution is 0.5-5 mol / L; The concentration of the cerium nitrate solution is 0.1-1 mol / L, and the mass ratio of the cerium nitrate solution to the acrylic acid monomer is 0.025-0.2; The time for water bath heating is 1-2 h, and the temperature for water bath heating is 60-90 °C.

7. The method according to claim 5, characterized in that In step (2), The concentration of N-acetyl-L-cysteine is 0.2-1.0 mol / L, the concentration of the copper nitrate solution is 0.02-0.1 mol / L, the concentration of the sodium hydroxide solution is 0.1-1 mol / L, and the oscillation time is 0.5-5 min.

8. The method according to claim 5, wherein In step (3), The concentration of eugenol is 1 - 5 μg / mL, the mass concentration of the ethanol solution is 20% - 95%, the impregnation time is 2 - 5 h, the gel weight gain is 100 - 150 times the initial mass, the spraying amount of the copper nanocluster solution is 2 - 5 times the initial gel mass, the concentration of the cerium nitrate solution is 0.1 - 1 mol / L, and the spraying amount of the cerium nitrate solution is 0.5 - 1 times the initial gel mass.

9. A method for monitoring the freshness of aquatic products using the intelligent gel label according to any one of claims 1 to 4, characterized in that, It includes the following steps: (1) Place the intelligent gel label inside the aquatic product packaging; (2) Regularly collect label pictures through a smartphone and measure the TVB-N value of the aquatic product at this time; (3) Establish the relationship between the picture information and the physicochemical indexes of the freshness of aquatic products to realize the real-time monitoring of the freshness of aquatic products.

10. A method for preserving aquatic products using the intelligent gel label according to any one of claims 1 to 4, characterized in that, It includes the following steps: (1) The copper nanoaggregates covered on the gel surface respond to TVB-N, and the structure disintegrates, promoting the release of the preservative encapsulated in the gel; (2) During the storage of aquatic products, the humidity-sensitive gel absorbs the moisture inside the packaging, the gel polymer network expands, and the pores enlarge, further promoting the release of the preservative from the pores of the gel network.

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