Jet printing visual food freshness label and manufacturing method and application thereof

Abstract

The invention discloses a jet printing visual food freshness label and a manufacturing method and application thereof. The manufacturing method of the visual food freshness label comprises the following steps: firstly, preparing the metal nanocluster; the obtained metal nanocluster dispersion system is compounded with polyvinylpyrrolidone, and the ink is formed through ultrasonic dispersion and heat treatment; mixing a solvent with polyvinylpyrrolidone to obtain a regulating agent; the ink and the regulating agent are injected into independent ink boxes of an ink-jet printer respectively, patterns are deposited on a low-fluorescence background substrate through preset printing parameters, and the patterns are sealed and stored after being dried. Compared with other intelligent indication labels, the fluorescent intelligent label prepared by the invention has the remarkable advantages of low cost, high stability, uniform luminescence and capability of realizing large-scale and rapid preparation; and in the preparation process of the label, the use amount of the metal nanocluster solution can be greatly reduced, so that a more environment-friendly and green production mode is realized.

Description

Technical Field

[0001] The present invention relates to a food freshness label, in particular to a food with a jet-printed visual freshness label and a production method and application thereof. Background Art

[0002] In the modern food industry, monitoring food freshness is crucial for ensuring food quality and safety. During food storage, proteins degrade under the action of enzymes and microorganisms, producing alkaline nitrogen-containing substances such as ammonia and amines. These substances are volatile and are typically classified as total volatile basic nitrogen (TVB-N). A higher TVB-N level indicates a product's age. Consuming stale food can cause symptoms of food poisoning, such as diarrhea and vomiting, and in severe cases, death.

[0003] Currently, meat freshness testing primarily relies on methods such as physical and chemical analysis and sensory evaluation. However, these methods are not only time-consuming and labor-intensive, but also destructive, making it difficult to monitor all products in real time. Furthermore, due to a lack of necessary theoretical knowledge, consumers struggle to independently assess meat freshness using these methods. With consumers' increasing emphasis on food safety and quality, smart packaging technologies that can visually display food freshness are emerging, with visual freshness indicators becoming a research focus. These indicators detect gas molecules released during meat spoilage, particularly unpleasant-smelling gases such as ammonia, dimethylamine, and trimethylamine. By generating different monitoring signals, these indicators provide real-time insights into the meat's condition and visually convey freshness information to consumers through color changes and pattern appearances, thereby helping consumers make more informed purchasing decisions.

[0004] Fluorescence sensors are sensors that use fluorescence for detection. Their core technology is fluorescent probes. By designing suitable fluorescent probes that bind to target substances and produce fluorescence changes, the concentration of the target substance can be inferred.

[0005] Chinese invention patent application CN118443660A discloses a method for detecting the freshness of meat. This method uses a fluorescent indicator that binds to volatile biogenic amines to induce a fluorescent color change, enabling freshness testing of shrimp and meat. While the color change is clearly visible to the naked eye, it cannot quantify the biogenic amine content. Furthermore, the chemically synthesized indicator used in this method is somewhat toxic.

[0006] Chinese invention patent application CN117723524A discloses a method for preparing a porous and transparent meat freshness ratiometric fluorescent indicator film and its application. The indicator film uses carbon quantum dots and fluorescein to construct ratiometric fluorescence for meat freshness detection. In addition, metal nanoclusters with aggregation-induced fluorescence (AIE) characteristics are also used in food freshness detection. Chinese invention patent application CN116046732A discloses a ratiometric fluorescent smart label for visual detection of meat freshness and its preparation method and application. The ratiometric detection label constructed using gold-copper alloy clusters (DPA-AuCuNCs) and gold nanoclusters (GSH-AuNCs@His) with AIE characteristics responds to food freshness. Compared with traditional fluorescent probes, metal nanoclusters have low toxicity, easy synthesis and excellent photoluminescence properties, and have luminescent groups with aggregation-induced emission properties, which overcome the difficulties faced by the aggregation-induced quenching (ACQ) effect of traditional fluorescent molecules in practical applications and have a natural advantage in creating solid-state sensors.

[0007] As can be seen from the above-mentioned prior art, the technology for using fluorescent indicators to detect food freshness is relatively mature. However, the preparation process for fluorescent labels currently relies primarily on traditional production methods. These traditional methods not only suffer from low production efficiency and difficulty in large-scale industrial production, but also suffer from resource waste and high production costs, limiting the widespread application of fluorescent indicators in the field of food freshness detection. Currently, traditional fluorescent indicator production processes include electrospinning, immersion, and film formation. However, each of these methods has limitations. While effective, electrospinning and film-forming techniques are complex and require high equipment costs, hindering widespread adoption for large-scale production. While the immersion method is relatively simple—a fluorescent label is prepared by immersing a paper substrate in a solution of copper nanoclusters (e.g., GSH-CuNCs with AIE properties, as disclosed in Chinese invention patent CN115171515B) and drying it—it still suffers from deficiencies in precision, uniformity, and environmental adaptability. Edge effects easily occur when the pretreatment solution dries during the immersion process, and uneven liquid distribution occurs during film transfer, leading to component concentration gradients. This manifests itself macroscopically as uneven fluorescence intensity distribution, which can lead to fluctuations in the detection signal or misjudgments. This ultimately limits the label's stability and reliability in complex environments, making it difficult to meet the multi-scenario adaptability requirements of practical applications.

[0008] Inkjet printing technology demonstrates significant advantages in producing fluorescent labels, overcoming many of the shortcomings of traditional production processes. Inkjet printing enables highly precise and uniform material deposition, ensuring the quality and performance of fluorescent labels. Its high-precision printing capabilities allow for precise control of the distribution of the fluorescent material, improving the uniformity and stability of the label. Furthermore, inkjet printing is fast and efficient, enabling large-scale production and significantly reducing unit costs. This technology not only improves production efficiency but also further reduces production costs by precisely controlling material usage.

[0009] However, existing inkjet printing technology still has limitations in the application of fluorescent materials. For example, Chinese invention patent application CN111662704A discloses a method for preparing a new type of organic phosphorescent material and its application in inkjet printing ink and writing ink, which achieves uniform and precise deposition of fluorescent materials on paper, thereby printing different fluorescent patterns. Chinese invention patent application CN118562337A proposes a pink fluorescent inkjet printing ink based on the NaYF4 system and a method for preparing the same, and provides a solution for making nanomaterial inks. Although these technologies have shown advantages in the deposition of fluorescent materials, the materials mentioned lack specific recognition groups in their design and therefore cannot specifically recognize TVB-N, which limits their application in the field of food freshness detection. Summary of the Invention

[0010] The main purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art and provide a printed visual food freshness label and its production method that has stable photoluminescent properties and can respond to TVB-N, ensures uniform dispersion of fluorescent material in ink, and is used to manufacture a visual food freshness indicator for visually monitoring food freshness using a printing method.

[0011] Another object of the present invention is to provide an application of the inkjet-printed visual food freshness label in visual monitoring of meat food freshness.

[0012] The purpose of the present invention is achieved through the following technical solutions:

[0013] A method for printing a visual food freshness label comprises the following steps:

[0014] 1) Synthesis of metal nanoclusters:

[0015] dissolving 4-mercaptobenzoic acid in a polar solvent to form a first precursor solution;

[0016] dissolving copper nitrate in an acidic medium to form a second precursor solution;

[0017] The first precursor solution and the second precursor solution are mixed to form metal nanoclusters with aggregation-induced emission properties through coordination reaction; after centrifugal purification, a metal nanocluster dispersion system dispersed in an organic solvent is obtained;

[0018] 2) Preparation of fluorescent ink and regulating agent:

[0019] The obtained metal nanocluster dispersion system is compounded with polyvinyl pyrrolidone, and subjected to ultrasonic dispersion and heat treatment to form ink;

[0020] mixing the solvent with polyvinyl pyrrolidone to obtain a regulating agent;

[0021] 3) Printing preparation of smart labels:

[0022] The ink and the regulating agent are respectively injected into independent ink cartridges of an inkjet printer, and a pattern is deposited on a low-fluorescence background substrate using preset printing parameters. The pattern is then sealed and stored after drying.

[0023] To further achieve the object of the present invention, preferably, in step 1), the polar solvent is a mixed solution of water and tetrahydrofuran, and the volume ratio of water to tetrahydrofuran is 1:6.67 to 1:20; the molar concentration ratio of the copper nitrate to the organic ligand is 1:1 to 1:3; and the acidic medium is an aqueous nitric acid solution with a pH ≤ 3.

[0024] Preferably, in step 1), the concentration range of the first precursor solution is 0.05-0.3M, the concentration range of the second precursor solution is 0.1-0.3M, wherein the volume ratio of the first precursor solution to the second precursor solution is 1:1~1:3, the temperature of the coordination reaction is 60~80℃, and the time is 5~60min; the speed of the centrifugal purification is 5000~10000rpm, the time is 3~10min, the organic solvent is ethanol, and the mass volume ratio of the metal nanoclusters to the organic solvent is 3.265~13.06mg / mL.

[0025] Preferably, in step 2), the mass volume ratio of the polyvinyl pyrrolidone to the metal nanocluster dispersion system is 0.5 to 3.5 g / mL; the ultrasonic power of the ultrasonic dispersion is 100 to 180 W, and the time is 15 to 30 min; the temperature of the heat treatment is 60 to 80 ° C, and the time is 30 to 120 min; the solvent of the regulating agent is ethanol, and the mass volume ratio of the polymer to ethanol is 0.5 to 3.5 g / mL.

[0026] Preferably, in step 3), the D65 fluorescence brightness of the low-fluorescence background substrate is ≤5.0%, and the low-fluorescence background substrate is non-fluorescent paper or polymer film; the printing parameters are regulated by the CMYK color mode, and the ink deposition amount is adapted to the resolution setting of the inkjet printer; the drying temperature is 25±2°C; and the sealed storage temperature is 4-8°C.

[0027] A printed visual food freshness label, prepared using the above-mentioned method, has a fluorescence response characteristic that is linearly correlated with the concentration of volatile amines produced by food spoilage, enabling visual monitoring of freshness through label color changes.

[0028] The application of the printed visual food freshness label in the visual monitoring of meat food freshness.

[0029] The method for applying the printing visual food freshness label in the visual monitoring of meat food freshness preferably comprises the following steps:

[0030] 1) The printed visual food freshness label and fresh meat products were stored in the same space for n days, where n is a positive integer. The TVB-N standard value of the meat was measured according to the national standard GB5009.228-2016. Fluorescent photos of the freshness fluorescent intelligent indicator label under ultraviolet light at different storage times were recorded regularly using a smartphone. The fluorescent photos at different storage times were converted into RGB values ​​using a mobile phone color picker app. The red channel values ​​of the RGB at different time points were fitted with the TVB-N values ​​at the corresponding storage time points to obtain the linear regression equation: Y = k × C + b, where k is the slope, b is the intercept, C is the TVB-N content, and Y is the value of the red channel of the label.

[0031] 2) A fluorescent smart label for visually detecting food freshness is affixed to the inner top of a fresh-keeping box. After the label and the food sample are stored in the same space, a fluorescent color image of the smart indicator label inside the packaging bag is obtained. The fluorescent color image is converted into RGB values ​​using a smartphone color picker app. The value of the red channel in RGB is substituted into the linear regression equation established in step 1) to obtain the TVB-N content of the food at the test time point.

[0032] Preferably, the slope k represents the change in the red channel value (Y) per unit increase in TVB-N content (C), and the positive / negative sign thereof reflects the increasing or decreasing trend of the fluorescence color with the accumulation of TVB-N; the intercept b is the theoretical initial value of the red channel of the label when the TVB-N content is zero.

[0033] Preferably, the meat comprises chicken, beef, pork or fish.

[0034] Compared with the existing basis, the advantages and beneficial effects of the present invention are:

[0035] (1) The present invention successfully prepared a fluorescent ink based on copper nanoclusters (Cu NCs@p-MBA). The copper nanoclusters are aggregation-induced emission metal nanocluster molecules with a simple synthesis method and are non-toxic. They have good fluorescence properties under ultraviolet excitation and are not easily affected by the environment when monitoring amine substances. By adding polyvinylpyrrolidone (PVP) to the Cu NCs@p-MBA suspension, the prepared nanocluster ink has an appropriate viscosity, and the nanomaterials are evenly distributed in the solution without agglomeration or precipitation, and have good luminescence and printing effects.

[0036] (2) The present invention utilizes inkjet printing technology to achieve rapid, low-cost, and green production of fluorescent labels. Inkjet printing technology, with its high-precision and uniform material deposition capabilities, can precisely control the distribution of fluorescent materials, thereby improving the uniformity and stability of the label.

[0037] (3) This invention applies the prepared nanomaterial ink to food freshness detection. By printing fluorescent smart labels on paper, this intelligent indicator of food freshness is achieved. Compared with other smart indicator labels, this indicator label has significant advantages: low cost, high stability, uniform luminescence, and can be produced in large quantities and quickly.

[0038] (4) The present invention's printed visual food freshness label is fast and efficient, enabling large-scale production and significantly reducing unit costs. Furthermore, the precise control of material usage further reduces production costs.

[0039] (5) Thanks to the application of advanced inkjet printing technology, the printed visual food freshness label of the present invention can significantly save the amount of metal nanocluster solution used during the preparation process, thereby achieving a more environmentally friendly and green production method.

[0040] (6) The present invention achieves rapid, low-cost, and green production of fluorescent labels through the optimized preparation of fluorescent ink and the efficient application of inkjet printing technology, and is successfully applied to food freshness detection, providing a convenient and intuitive solution for food safety monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 A flowchart for the preparation and application of fluorescent smart labels for visual detection of food freshness based on inkjet printing;

[0042] Figure 2 These are images of the Cu NCs@p-MBA solution in Example 1 under visible light and ultraviolet light conditions;

[0043] Figure 3 The fluorescence excitation and emission spectra of Cu NCs@p-MBA in Example 1;

[0044] Figure 4 This is the ultraviolet absorption spectrum of Cu NCs@p-MBA in Example 1;

[0045] Figure 5 Images of the Cu NCs@p-MBA@PVP ink in Example 2 under visible light and ultraviolet light conditions;

[0046] Figure 6 Fluorescence excitation and emission spectra of Cu NCs@p-MBA@PVP ink in Example 1;

[0047] Figure 7 This is the ultraviolet absorption spectrum of the Cu NCs@p-MBA@PVP ink in Example 1;

[0048] Figure 8 The histogram and fitting curve of the Cu NCs@p-MBA particle size in Example 1;

[0049] Figure 9 The histogram and fitting curve of the particle size of Cu NCs@p-MBA@PVP-1 in Example 1;

[0050] Figure 10 Viscosity diagram of Cu NCs@p-MBA alcohol solution and Cu NCs@p-MBA@PVP-1 ink in Example 1;

[0051] Figure 11 The fluorescence emission spectra of the Cu NCs@p-MBA@PVP-1 ink in Example 1 after reacting with ammonia water at different concentrations;

[0052] Figure 12 The fitting curves of the fluorescence intensity of the Cu NCs@p-MBA@PVP-1 ink at 650 nm and different concentrations of ammonia water in Example 1 are established;

[0053] Figure 13 This is a graph showing the freshness of shrimps detected at 4°C using the fluorescent smart tag for visually detecting food freshness in Example 5;

[0054] Figure 14 This is a graph showing the changing trend of TVB-N content in shrimp during storage in Example 5;

[0055] Figure 15 The fitting curve established for the R value of the fluorescent smart label for visually detecting food freshness in Example 5 and the TVB-N content in shrimp during storage;

[0056] Figure 16 This is a graph showing the freshness of chicken breast detected at 4°C using the fluorescent smart label for visually detecting food freshness in Example 6;

[0057] Figure 17 This is a graph showing the changing trend of TVB-N content in chicken breast during storage in Example 6;

[0058] Figure 18 This is a fitting curve diagram established for the R value of the fluorescent smart label for visually detecting food freshness and the TVB-N content in chicken breast during storage in Example 6;

[0059] Figure 19 Images of the Cu NCs@p-MBA@PVP-5 ink in Comparative Example 1 under visible light and ultraviolet light conditions;

[0060] Figure 20 These are images of the Cu NCs@p-MBA@PVP-6 ink in Comparative Example 2 under visible light and ultraviolet light conditions. DETAILED DESCRIPTION

[0061] For a better understanding of the present invention, the present invention is further described below with reference to the accompanying drawings and specific embodiments, but the embodiments of the present invention are not limited thereto. The embodiments described are part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0062] Regarding the application of existing fluorescent indicators and existing inkjet printing technology in fluorescent materials, there are also some technologies that can be used as reference in the existing technology, such as the Cu nanoclusters disclosed in the prior art (Lin-Lei Liu, Lian Xiang, Ya-Qin Chai, Ruo Yuan, Confinement-enhanced electrochemiluminescence of copper nanoclusters on 3D layered double hydroxide for ultrasensitive detection of GFAP, Biosensors and Bioelectronics, 2024, 265.116685). Although the NCs@3D-LDH composite material improves the electrochemiluminescence performance through the confinement effect of layered double hydroxide (3D-LDH), the micron-scale layered structure of 3D-LDH and the host-guest electrostatic interaction lead to a significant increase in the hydrodynamic particle size of the composite material (>500nm), far exceeding the tolerance threshold of the inkjet printing nozzle (<400nm), and its hydrophilic surface easily induces secondary aggregation in organic solvents (such as ethanol and ethylene glycol), destroying the uniform dispersion of the ink, causing nozzle clogging and patterned printing failure; and this technology relies on Zn 2+ / Al 3+ The synthesis of 3D-LDH using a binary metal system and soft template method requires high-purity metal salt raw materials and a hydrothermal reaction lasting up to 24 hours. In addition, the loading efficiency of CuNCs is limited by the strength of electrostatic interaction, resulting in low material yield and high energy consumption, which makes it difficult to meet the large-scale, low-cost production needs of inkjet printing inks.

[0063] Moreover, the metal nanoclusters with ammonia response function in the existing technology tend to spontaneously aggregate, resulting in their fluid dynamic particle size exceeding the tolerance threshold of the inkjet printing nozzle (>200nm), and the dynamic desorption characteristics of the surface ligands are prone to chemical stability degradation (oxidative inactivation or ligand shedding), causing the ink to have problems such as micropore blockage, jet trajectory deviation, and long-term storage phase separation. At the same time, because the fluorescence response performance of the material decays with the degree of aggregation, it cannot meet the core requirements of inkjet printing technology for particle size uniformity, environmental tolerance and signal stability of functional inks.

[0064] Therefore, there is an urgent need to develop a fluorescent material ink with stable photoluminescent properties that reacts with TVB-N, and to combine it with inkjet printing technology to enable the mass, low-cost, and green production of fluorescent labels. This fluorescent ink must achieve uniform distribution and improved stability of the fluorescent material. This requires not only ensuring uniform dispersion of the fluorescent material in the ink but also improving stability and reliability in practical applications without compromising its luminescent properties.

[0065] The present invention has found that by using a 4-mercaptobenzoic acid (p-MBA) and polar solvent coordination system, a step-by-step precursor solution mixing method, and a polyvinylpyrrolidone (PVP) dynamic coating technology, a copper nanocluster (Cu NCs) fluorescent ink with TVB-N specific response, stable photoluminescence properties, and inkjet printing compatibility has been successfully developed. Specifically, Figure 1 As shown, p-MBA acts as a bifunctional ligand. Its thiol group stabilizes the electronic structure of the copper nanoclusters through a strong coordination bond (Cu-S), while the carboxylic acid group specifically binds to the amino group in TVB-N through electrostatic interactions, achieving a controllable fluorescence signal response. A polar solvent (a mixed solvent system consisting of THF and ultrapure water) regulates the ligand dissolution kinetics and the metal redox state, ensuring a slow and uniform nanocluster nucleation process, resulting in ultrasmall clusters and significantly improving material stability. A step-by-step mixing strategy for the precursor solution inhibits premature hydrolysis of copper ions in a nitric acid environment and achieves diffusion-controlled growth in a light-proof water bath, avoiding particle agglomeration and size unevenness caused by traditional one-pot methods. This meets the stringent requirements of inkjet printing for ink particle size (<400nm) and dispersion uniformity. During the ink preparation stage, PVP selectively coats the hydrophobic regions (benzene rings) of the nanoclusters through van der Waals forces to form a dynamic "core-shell" structure (CuNCs@p-MBA@PVP), which not only retains the TVB-N recognition activity of the carboxylic acid group but also gives the ink shear-thinning properties, adapting to the high-shear jetting requirements of inkjet printing. At the same time, it ensures long-term stability by inhibiting the aggregation of nanoclusters and secondary agglomeration caused by solvent volatilization.

[0066] To this end, the present invention first prepares copper nanoclusters with orange-red fluorescence; then adds polyvinyl pyrrolidone to the prepared copper nanoclusters to form a stable and uniform nanocluster ink; the ink is injected into a commercial printer to prepare a fluorescent label for visually monitoring the freshness of meat. Combined with a smartphone, the total volatile basic nitrogen content of meat can be predicted and the freshness of the food can be inferred.

[0067] On the basis of the above, the present invention provides a method for printing a visual food freshness label, comprising the following steps (eg Figure 1 ):

[0068] 1) Synthesis of metal nanoclusters:

[0069] dissolving 4-mercaptobenzoic acid in a polar solvent to form a first precursor solution;

[0070] dissolving copper nitrate in an acidic medium to form a second precursor solution;

[0071] The first precursor solution and the second precursor solution are mixed to form metal nanoclusters with aggregation-induced emission properties through coordination reaction; after centrifugal purification, a metal nanocluster dispersion system dispersed in an organic solvent is obtained;

[0072] 2) Preparation of fluorescent ink and regulating agent:

[0073] The obtained metal nanocluster dispersion system is compounded with polyvinyl pyrrolidone, and subjected to ultrasonic dispersion and heat treatment to form ink;

[0074] mixing the solvent with polyvinyl pyrrolidone to obtain a regulating agent;

[0075] 3) Printing preparation of smart labels:

[0076] The ink and the regulating agent are respectively injected into independent ink cartridges of an inkjet printer, and a pattern is deposited on a low-fluorescence background substrate using preset printing parameters. The pattern is then sealed and stored after drying.

[0077] In step 1), according to the dissolution requirements, the polar solvent is preferably a mixed solution of water and tetrahydrofuran, and the volume ratio of water to tetrahydrofuran is 1:6.67 to 1:20; according to the amount ratio of the reactants, the molar concentration ratio of copper nitrate to organic ligand is preferably 1:1 to 1:3; the acidic medium is preferably a nitric acid aqueous solution with a pH of ≤3. The concentration of the first precursor solution is 0.05-0.3M, and the concentration of the second precursor solution is 0.1-0.3M, wherein the volume ratio of the first precursor solution to the second precursor solution is 1:1 to 1:3, the temperature of the coordination reaction is 60-80°C, and the time is 5-60min; the speed of centrifugal purification is preferably 5000-10000rpm, the time is 3-10min, the organic solvent is ethanol, and the mass volume ratio of metal nanoclusters to organic solvent is preferably 3.265-13.06mg / mL.

[0078] In step 2), the mass volume ratio of polyvinyl pyrrolidone to the metal nanocluster dispersion system is preferably 0.5 to 3.5 g / mL; the ultrasonic power of ultrasonic dispersion is preferably 100 to 180 W, and the time is 15 to 30 min; the temperature of heat treatment is preferably 60 to 80° C., and the time is 30 to 120 min; the preferred solvent is ethanol, and the mass volume ratio of polymer to ethanol is 0.5 to 3.5 g / mL.

[0079] In step 3), as required, the D65 fluorescence brightness of the low-fluorescence background substrate is ≤5.0%, and the low-fluorescence background substrate is preferably non-fluorescent paper or polymer film; the printing parameters are controlled by the CMYK color mode, and the ink deposition amount is adapted to the resolution setting of the inkjet printer; the drying temperature is preferably 25±2°C; and the sealed storage temperature is preferably 4-8°C.

[0080] The fluorescence response characteristics of the inkjet-printed visual food freshness label of the present invention are linearly correlated with the concentration of volatile amine substances generated by food spoilage, and visual monitoring of freshness is achieved through label color changes.

[0081] The present invention also provides the application of printing a visual food freshness label in visual monitoring of meat food freshness. The application method preferably includes the following steps:

[0082] 1) The printed visual food freshness label and fresh meat products were stored in the same space for n days, where n is a positive integer. The TVB-N standard value of the meat was measured according to the national standard GB5009.228-2016. Fluorescent photos of the freshness fluorescent intelligent indicator label under ultraviolet light at different storage times were recorded regularly using a smartphone. The fluorescent photos at different storage times were converted into RGB values ​​using a mobile phone color picker app. The red channel values ​​of the RGB at different time points were fitted with the TVB-N values ​​at the corresponding storage time points to obtain the linear regression equation: Y = k × C + b, where k is the slope, b is the intercept, C is the TVB-N content, and Y is the value of the red channel of the label.

[0083] 2) A fluorescent smart label for visually detecting the freshness of food is attached to the top of the inner storage box and stored in the same space as the food sample. Then, a fluorescent color image of the smart indicator label in the packaging bag is obtained, and the fluorescent color image is converted into RGB value through the smartphone color picker APP. The value of the red channel in RGB is substituted into the linear regression equation established in step 1) to obtain the TVB-N content value of the food at the test time point. The slope k represents the change in the red channel value (Y) for each unit increase in TVB-N content (C), and its positive / negative sign reflects the trend of increasing or decreasing fluorescent color with TVB-N accumulation; the intercept b is the theoretical initial value of the label red channel when the TVB-N content is zero.

[0084] The meat of the present invention includes chicken, beef, pork or fish.

[0085] The inkjet printing technology of this invention, with its high-precision and uniform material deposition capabilities, ensures the quality and performance of fluorescent labels. Its high-precision printing capability allows precise control of the distribution of the fluorescent material, thereby improving the uniformity and stability of the label. Furthermore, the inkjet printing technology of this invention is fast and efficient, enabling large-scale production and improving production efficiency. Furthermore, by precisely controlling material usage, it significantly reduces unit costs.

[0086] It can be seen that the present invention has stable photoluminescent properties, can respond to TVB-N, can be inkjet printed on fluorescent material ink of non-fluorescent paper, and has the characteristics of low cost, small particles, good stability, safety and environmental protection.

[0087] Example 1

[0088] 1) Preparation of copper nanoclusters (Cu NCs@p-MBA-1)

[0089] At room temperature and under magnetic stirring, 0.0463 g of 4-mercaptobenzoic acid (p-MBA) was dispersed in 3 mL of ultrapure water. 4 mL of tetrahydrofuran (THF) solution was added and stirred to dissolve the 4-mercaptobenzoic acid, yielding a clear mixed solution. A 0.1 M copper nitrate solution (3 mL) in 0.1 M nitric acid was added to the clear mixed solution. The copper nitrate-added solution was then heated in a 70°C water bath for 30 minutes to yield the Cu NCs@p-MBA reaction solution. The resulting reaction solution was centrifuged at 8000 rpm and washed three times with ethanol. The resulting precipitate was then resuspended in 10 mL of ethanol to yield the Cu NCs@p-MBA suspension-1.

[0090] The synthesized metal nanoclusters were characterized by UV-visible spectroscopy, fluorescence spectroscopy and optical properties, as follows: Figure 2 As shown in the figure, the ethanol solution of Cu NCs@p-MBA-1 appears yellow under visible light and emits strong orange-red fluorescence under ultraviolet light; it can be seen from the UV-visible spectrum ( Figure 3 ), the UV absorption curve of Cu NCs@p-MBA does not have any metal ion surface plasmon resonance absorption peak, indicating that no large-sized copper nanoparticles are generated in the synthesized copper nanoclusters; the absorption peak at 274nm is attributed to the absorbance of the ligand, while the absorption peak at 330nm is attributed to the characteristic absorbance of Cu NCs generated by the electronic transition between the ligand and the metal core; from the fluorescence spectrum ( Figure 4 ) It can be seen from the above that Cu NCs@p-MBA has a strong fluorescence emission peak at 650nm. In summary, the prepared Cu NCs@p-MBA has good fluorescence properties.

[0091] 2) Preparation of copper nanocluster ink (Cu NCs@p-MBA@PVP-1)

[0092] At room temperature, take 5 mL of Cu NCs@p-MBA-1 suspension, add 1.25 g of PVP, then set the ultrasonic power to 180 W and the ultrasonic time to 30 minutes to promote the dissolution of PVP and the dispersion of Cu NCs@p-MBA. Then place it in a 70°C stirring water bath for 3 hours with a stirring rate of 400 rpm / min to obtain Cu NCs@p-MBA@PVP-1 ink.

[0093] The prepared fluorescent ink was characterized by UV-visible spectroscopy, fluorescence spectroscopy and optical properties, and the results are as follows: Figure 5As shown in the figure, Cu NCs@p-MBA@PVP-1 ink appears yellow under visible light and emits strong orange-red fluorescence under ultraviolet light; compared with Cu NCs@P-MBA alcohol solution, the fluorescence excitation and emission peaks do not shift, and it still has a strong fluorescence emission peak at 650nm, but the peak intensity is slightly lower than that of Cu NCs@p-MBA-1 alcohol solution ( Figure 6 ), PVP promotes the dispersion of the metal nanoclusters, inhibits the aggregation of metal nanoclusters, and reduces the fluorescence intensity; Figure 4 and Figure 7 It can be seen that the UV absorption curve of Cu NCs@p-MBA@PVP-1 ink is similar to that of the alcohol solution of Cu NCs@p-MBA-1, but the absorption peak at 274nm is blue-shifted to 269nm. This is because the particle size decreases and the energy gap becomes wider, which causes the absorption band to move toward the short-wave direction. Figure 8 and Figure 9 The particle size distribution of the two materials can also be observed. After adding PVP, the particle size of Cu NCs@p-MBA is significantly reduced, from the Z-Average of 635nm of the original material to 348.7nm. Figure 10 As shown in the data, when γ = 1000 1 / s, the viscosity of the ink containing Cu NCs@p-MBA@PVP-1 is 5.2 mPa·s, while that of the Cu NCs@p-MBA alcohol solution is only 2.7 mPa·s. The Cu NCs@p-MBA@PVP-1 ink precisely fits the lower limit of the printing window recommended by the Ohnesorge number (5-15 mPa·s), indicating that the addition of PVP improves the viscosity of the ink and enhances the printability of the ink.

[0094] In summary, the prepared Cu NCs@p-MBA@PVP-1 ink has good fluorescence properties and good printability.

[0095] Detection of amines of different concentrations and types using fluorescent ink

[0096] Take 300μL of Cu NCs@p-MBA@PVP-1 ink in a micro-cuvette, then add 300μL of ammonia water with concentrations of 0, 50, 100, 150, 200, 250, and 300ppm respectively in the cuvette to react for 5 minutes, and set the excitation slit and emission slit of the fluorescence spectrophotometer to 5nm. The excitation wavelength is 350nm, and the scanning range is 600nm~750nm. The fluorescence intensity at the fluorescence emission peak of 650nm is recorded to explore whether the synthesized Cu NCs@p-MBA@PVP-1 has a good response to ammonia water. The results are shown in Figure 2. Figure 11As shown in the figure, as the concentration of ammonia increases, the emission peak fluorescence intensity of Cu NCs@p-MBA@PVP-1 at 650nm gradually decreases, indicating that Cu NCs@p-MBA@PVP-1 has good response characteristics to biogenic amines. Figure 12 The fitted linear equation shows that the fluorescence intensity at 650 nm is linearly related to the ammonia concentration, and the determination coefficient is R 2 =0.996.

[0097] 3) Preparation of polyvinylpyrrolidone (PVP) regulator

[0098] At room temperature, take 10 mL of anhydrous ethanol, add 2.5 g of polyvinylpyrrolidone (PVP), and then place it in a 70°C stirring water bath for 3 hours at a stirring rate of 400 rpm / min to obtain a PVP-1 regulator.

[0099] 4) Preparation of fluorescent smart labels for shrimp freshness by inkjet printing

[0100] First, 5 mL of the obtained Cu NCs@p-MBA@PVP-1 ink was ultrasonically dispersed at 180 W for 5 minutes and then injected into the "Yellow" ink reservoir of a clean printer. The PVP-1 regulator prepared in step 3) was injected into the "Black" ink reservoir of a clean printer. The design pattern colors were set to (0, 0, 80, 20) on a computer and printed to adjust the concentration of Cu NCs@p-MBA@PVP-1 and ensure the accuracy and stability of the detection. After the ink was naturally dried at 23-27°C, a Cu NCs@p-MBA-1 fluorescent smart label was obtained. The label was sealed in a sealing bag and stored at 4-8°C.

[0101] Example 2

[0102] 1) Preparation of copper nanoclusters (Cu NCs@p-MBA-2)

[0103] At room temperature and with magnetic stirring, 0.0926 g of 4-mercaptobenzoic acid was dispersed in 3 mL of ultrapure water. 4 mL of tetrahydrofuran was added and stirred to dissolve the 4-mercaptobenzoic acid, yielding a clear mixed solution (1). A 0.1 M copper nitrate solution (3 mL) dissolved in 0.1 M nitric acid was added to the clear mixed solution. The copper nitrate-added solution was then heated in a 70°C water bath for 40 minutes to yield the Cu NCs@p-MBA reaction solution. The resulting reaction solution was centrifuged at 8000 rpm and washed twice with ethanol. The resulting precipitate was then resuspended in 10 mL of ethanol to yield the Cu NCs@p-MBA suspension (2).

[0104] 2) Preparation of copper nanocluster ink (Cu NCs@p-MBA@PVP-2)

[0105] At room temperature, take 5 mL of Cu NCs@p-MBA-2 suspension, add 1 g of PVP, then set the ultrasonic power to 180 W and the ultrasonic time to 30 minutes to promote the dissolution of PVP and the dispersion of Cu NCs@p-MBA-2. Then place it in a 70°C stirring water bath for 3 hours with a stirring rate of 400 rpm / min to obtain Cu NCs@p-MBA@PVP-2 ink.

[0106] 3) Preparation of polyvinylpyrrolidone (PVP) regulator

[0107] At room temperature, take 10 mL of anhydrous ethanol, add 2.0 g of polyvinylpyrrolidone (PVP), and then place it in a 75°C stirring water bath for 3 hours at a stirring rate of 300 rpm / min to obtain the PVP-2 regulator.

[0108] 4) Preparation of chicken breast freshness fluorescent smart label by inkjet printing

[0109] First, 5 mL of the Cu NCs@p-MBA@PVP-2 ink prepared in Example 5 was ultrasonically dispersed, the ultrasonic power was set to 180 W, the ultrasonic time was set to 5 min, and the ink was injected into the "Yellow" ink tank of a clean printer; the prepared PVP-2 regulator was injected into the "Black" ink tank of a clean printer; and the design pattern color was set to (0, 0, 75, 10) on the computer and printed to adjust the concentration of Cu NCs@p-MBAp-MBA-2 to ensure the accuracy and stability of the detection. After the ink was naturally dried at 23-27°C, a Cu NCs@p-MBA-2 fluorescent smart label was obtained, and the label was sealed in a sealing bag and stored at 4-8°C.

[0110] Example 3

[0111] 1) Preparation of copper nanoclusters (Cu NCs@p-MBA-3)

[0112] At room temperature and under magnetic stirring, 0.0926 g of 4-mercaptobenzoic acid was dispersed in 6 mL of ultrapure water. 8 mL of tetrahydrofuran was added and stirred to dissolve the 4-mercaptobenzoic acid, yielding a clear mixed solution II. A 0.1 M copper nitrate solution in 0.1 M nitric acid (6 mL) was added to the clear mixed solution. The copper nitrate-added solution was then heated in a 65°C water bath for 50 minutes to yield the Cu NCs@p-MBA reaction solution. The resulting reaction solution was centrifuged at 8000 rpm and washed five times with ethanol. The resulting precipitate was then resuspended in 10 mL of ethanol to yield the Cu NCs@p-MBA suspension III.

[0113] 2) Preparation of copper nanocluster ink (Cu NCs@p-MBA@PVP-3)

[0114] At room temperature, take 5 mL of Cu NCs@p-MBA-3 suspension, add 1.5 g of PVP, then set the ultrasonic power to 180 W and the ultrasonic time to 30 minutes to promote the dissolution of PVP and the dispersion of Cu NCs@p-MBA-3. Then place it in a 70°C stirring water bath for 3 hours with a stirring rate of 400 rpm / min to obtain Cu NCs@p-MBA@PVP-3 ink.

[0115] 3) Preparation of polyvinylpyrrolidone (PVP) regulator

[0116] At room temperature, take 10 mL of anhydrous ethanol, add 1.25 g of polyvinylpyrrolidone (PVP), and then place it in a 70°C stirring water bath for 3 hours at a stirring rate of 400 rpm / min to obtain a PVP-3 regulator.

[0117] 4) Preparation of fluorescent smart labels for beef freshness by inkjet printing

[0118] First, 5 mL of the Cu NCs@p-MBA@PVP-3 ink prepared in Example 6 was ultrasonically dispersed, the ultrasonic power was set to 180 W, the ultrasonic time was set to 5 min, and the ink was injected into the "Yellow" ink tank of a clean printer; the prepared PVP-3 regulator was injected into the "Black" ink tank of a clean printer; and the design pattern color was set to (0, 0, 80, 10) on the computer and printed to adjust the concentration of Cu NCs@p-MBAp-MBA-3 to ensure the accuracy and stability of the detection. After the ink was naturally dried at 23-27°C, a Cu NCs@p-MBA-3 fluorescent smart label was obtained, and the label was sealed in a sealing bag and stored at 4-8°C.

[0119] Example 4:

[0120] 1) Preparation of copper nanoclusters (Cu NCs@p-MBA-4)

[0121] At room temperature and under magnetic stirring, 0.0426 g of 4-mercaptobenzoic acid was dispersed in 6 mL of ultrapure water. 8 mL of tetrahydrofuran was added and stirred to dissolve the 4-mercaptobenzoic acid, yielding a clear mixed solution (3). A 0.1 M copper nitrate solution (6 mL) dissolved in 0.1 M nitric acid was added to the clear mixed solution. The copper nitrate-added solution was then heated in a 70°C water bath for 40 minutes to yield the Cu NCs@p-MBA reaction solution. The resulting reaction solution was centrifuged at 8000 rpm and washed four times with ethanol. The resulting precipitate was then resuspended in 8 mL of ethanol to yield the Cu NCs@p-MBA suspension (4).

[0122] 2) Preparation of copper nanocluster ink (Cu NCs@p-MBA@PVP-4)

[0123] At room temperature, take 5 mL of Cu NCs@p-MBA-4 suspension, add 1 g of PVP, then set the ultrasonic power to 180 W and the ultrasonic time to 30 minutes to promote the dissolution of PVP and the dispersion of Cu NCs@p-MBA-4. Then, place it in a 70°C stirring water bath for 3 hours with a stirring rate of 400 rpm / min to obtain Cu NCs@p-MBA@PVP-4 ink.

[0124] 3) Preparation of polyvinylpyrrolidone (PVP) regulator

[0125] At room temperature, take 15 mL of anhydrous ethanol, add 2.5 g of polyvinylpyrrolidone (PVP), and then place it in a 70°C stirring water bath for 3 hours at a stirring rate of 400 rpm / min to obtain the PVP-4 regulator.

[0126] 4) Preparation of salmon freshness fluorescent smart label by inkjet printing

[0127] First, 5 mL of the Cu NCs@p-MBA@PVP-4 ink prepared in Example 7 was ultrasonically dispersed, the ultrasonic power was set to 180 W, the ultrasonic time was set to 5 min, and the ink was injected into the "Yellow" ink tank of a clean printer; the prepared PVP-4 regulator was injected into the "Black" ink tank of a clean printer; and the design pattern color was set to (0, 0, 90, 10) on the computer and printed to adjust the concentration of Cu NCs@p-MBAp-MBA-4 to ensure the accuracy and stability of the detection. After the ink was naturally dried at 23-27°C, a Cu NCs@p-MBA-4 fluorescent smart label was obtained, and the label was sealed in a sealing bag and stored at 4-8°C.

[0128] Example 5: Food Freshness Fluorescent Smart Label for Monitoring the Freshness of Shrimp

[0129] The fluorescent smart (Cu NCs@p-MBA-1) label for visually detecting food freshness printed in Example 1 was affixed to the inner top of a 100 mm culture dish (size 100 mm × 20 mm) and stored in a constant temperature and humidity chamber at 4°C with a 50 g sample of shrimp. The TVB-N standard value of shrimp stored for different days was measured according to the physical and chemical indicators specified in the national standard GB5009.228-2016 to determine the degree of spoilage of the shrimp, and the color change of the indicator under ultraviolet light was recorded.

[0130] In addition, during the storage period, a smartphone was used to obtain fluorescent color images of the smart indicator label in the packaging bag under ultraviolet light every day as the storage time changed ( Figure 13 ); When the shrimp were stored for 0, 1, 2, 3, and 4 days, the collected fluorescent color pictures were arranged in ascending order according to the storage days, and recorded as F0, F1, F2, F3, and F4, respectively. The RGB values ​​of the fluorescent color pictures arranged in ascending order of storage days were extracted through the smartphone color picker APP, and the value of the red channel (Rvalue) (Y) in RGB was compared with the TVB-N value (C) during storage ( Figure 14 ) to perform fitting calculations, and the results are as follows Figure 15 As shown, the fitting equation is Y = -12.706C + 373.82, and the determination coefficient R 2 =0.932. Using this equation, consumers can calculate the TVB-N content of mud shrimp by extracting the RGB information from the image, thereby accurately determining the freshness level of the mud shrimp.

[0131] Example 6: Food Freshness Fluorescent Smart Label for Monitoring Chicken Breast Freshness

[0132] The fluorescent smart label (Cu NCs@p-MBA-2) for visually detecting food freshness printed in Example 2 was cut and pasted on the inner top of a 100mm culture dish (specifications: 100mm×20mm) and stored in a constant temperature and humidity chamber at 4°C together with a 50g chicken breast sample. The TVB-N standard value of chicken breast stored for different days was measured according to the physical and chemical indicators specified in the national standard GB5009.228-2016 to determine the degree of spoilage of the chicken breast, and the color change of the indicator under ultraviolet light was recorded.

[0133] During storage, a smartphone was used to obtain fluorescent color images of the smart indicator label in the packaging bag under ultraviolet light every day as the storage time changed ( Figure 16); When the chicken breast is stored for 0, 1, 3, 5, and 7 days, the collected fluorescent color pictures are arranged in ascending order according to the storage days, and are recorded as F0, F1, F3, F5, and F7, respectively. The RGB values ​​of the fluorescent color pictures arranged in ascending order of storage days are extracted through the smartphone color picker APP, and the red channel value (Rvalue) (Y) is compared with the TVB-N value (C) during storage ( Figure 17 ) to perform linear regression calculation, such as Figure 18 As shown, the fitting equation is Y = -15.369C + 387.261, and the determination coefficient R 2 =0.938. Using this fitting equation, consumers can calculate the TVB-N content of chicken breast by extracting the RGB information from the image, thereby accurately judging the freshness level of the chicken breast.

[0134] Example 7: Food Freshness Fluorescent Smart Label for Monitoring Beef Freshness

[0135] The fluorescent smart label (Cu NCs@p-MBA-3) for visually detecting food freshness printed in Example 3 was cut and pasted on the inner top of a 100mm culture dish (specifications: 100mm×20mm) and stored in a constant temperature and humidity chamber at 4°C together with a 50g beef sample. The TVB-N standard value of beef stored for different days was measured according to the physical and chemical indicators specified in the national standard GB5009.228-2016 to determine the degree of spoilage of the chicken breast, and the color change of the indicator under ultraviolet light was recorded.

[0136] During the storage period, a smartphone was used to obtain fluorescent color images of the smart indicator label in the packaging bag under ultraviolet light every day. When the beef was stored for 0, 3, 6, 9, and 12 days, the collected fluorescent color images were arranged from small to large according to the storage days, and recorded as F0, F3, F6, F9, F10, F11, F12, F13, F14, F15, F16, F17, F18, F19, F20, F21, F22, F23, F24, F25, F26, F27, F28, F29, F30, F31, F32, F33, F41, F42, F51, F52, F6 12 The RGB values ​​of the fluorescent color images arranged in ascending order of storage days were extracted using a smartphone color picker APP. The red channel value (R value) (Y) was linearly regressed with the TVB-N value (C) during storage. As shown in Table 1, the fitting equation is Y = -12.7C + 333.41, and the determination coefficient R 2 =0.934. Using this equation, consumers can calculate the TVB-N content of beef by extracting the RGB information from an image, thereby accurately determining the freshness level of the beef.

[0137] Table 1 Effects of different storage times on beef TVB-N content and fluorescent label R value

[0138]

[0139] Example 8: Food Freshness Fluorescent Smart Label for Monitoring Salmon Freshness

[0140] The fluorescent smart label (Cu NCs@p-MBA-4) for visually detecting food freshness printed in Example 4 was cut and pasted on the inner top of a 100mm culture dish (specifications: 100mm×20mm) and stored in a constant temperature and humidity chamber at 4°C with a 50g salmon sample. The TVB-N standard value of chicken breast stored for different days was measured according to the physical and chemical indicators specified in the national standard GB5009.228-2016 to determine the degree of salmon spoilage, and the color change of the indicator under ultraviolet light was recorded.

[0141] During the storage period, a smartphone was used to obtain fluorescent color images of the smart indicator label in the packaging bag under ultraviolet light every day. When the salmon was stored for 0, 1, 2, 3, and 4 days, the collected fluorescent color images were arranged in ascending order according to the storage days, and recorded as F0, F1, F2, F3, and F4. The RGB values ​​of the fluorescent color images arranged in ascending order according to the storage days were extracted using the smartphone color picker APP. The red channel value (R value) (Y) was fitted with the TVB-N value (C) during storage, as shown in Table 2. The fitting equation is Y = -11.711C + 384.2, and the determination coefficient R 2 =0.945. Using this equation, consumers can calculate the TVB-N content of salmon by extracting the RGB information from an image, thereby accurately determining the freshness level of the salmon.

[0142] Table 2 Effects of different storage times on salmon TVB-N content and fluorescent label R value

[0143]

[0144]

[0145] Comparative Example 1: Preparation of copper nanocluster ink (Cu NCs@p-MBA@PVP-5)

[0146] At room temperature, 5 mL of the Cu NCs@p-MBA-1 suspension was ultrasonically treated at 180 W for 30 minutes to promote the dispersion of the Cu NCs@p-MBA. 1.25 g of polyvinylpyrrolidone (PVP) was then added. The mixture was then placed in a 70°C stirred water bath at 400 rpm / min for 3 hours to obtain a Cu NCs@p-MBA@PVP ink.

[0147] like Figure 19As shown in the figure, the prepared Cu NCs@p-MBA@PVP-5 ink appears light yellow under visible light and a faint orange-red under ultraviolet light. Compared with the Cu NCs@p-MBA@PVP-1 ink prepared in Example 2, the fluorescence intensity of the material decreases under the same PVP addition. The Cu NCs@p-MBA is subjected to ultrasound, and the cavitation effect of ultrasound promotes the dispersion of the material, forming smaller particles, resulting in a decrease in the fluorescence intensity of the prepared Cu NCs@p-MBA@PVP-5 ink. It can be seen that the change in the order of ultrasonic treatment on the dispersion of copper nanoclusters and the addition of PVP has an important influence on the fluorescence properties of the material.

[0148] Comparative Example 2: Preparation of copper nanocluster ink (Cu NCs@p-MBA@PVP-6)

[0149] At room temperature, 5 mL of ethanol solution was added with 1.25 g of polyvinyl pyrrolidone (PVP), and then placed in a 70°C water bath for 3 h. Subsequently, 5 mg of Cu NCs@p-MBA-1 freeze-dried powder was added, and then ultrasonicated at a power of 180 W to obtain Cu NCs@p-MBA@PVP-6 ink.

[0150] like Figure 20 As shown, the prepared Cu NCs@p-MBA@PVP-6 ink exhibits distinct stratification. The Cu NCs@p-MBA-1 at the bottom appears bright yellow under fluorescent light and orange-red under UV light, while the supernatant above is nonluminescent. Heating PVP forms polymers, enhancing intermolecular forces and increasing its viscosity. The subsequently added lyophilized Cu NCs@p-MBA-1 powder cannot be evenly coated by the PVP and therefore settles at the bottom of the solution. This demonstrates the importance of varying the order of PVP dissolution and Cu NCs@p-MBA-1 addition.

[0151] Comparative Example 3: Comparison of the preparation of fluorescent labels prepared by traditional process and inkjet printing technology

[0152] 1) Preparation of fluorescent tags by dropwise addition: Take the ethanol suspension of Cu NCs@p-MBA-1 prepared in Example 1 and dropwise add 200 μL of the Cu NCs@p-MBA suspension onto a circular piece of cotton fiber paper (1 cm in diameter). Dry at room temperature (25°C) for 15 min to completely evaporate the solvent. Finally, place the dried tag in a vacuum bag and store in a refrigerator at 4°C. Prepare 100 fluorescent tags according to the above preparation process and calculate the material dosage.

[0153] 2) Preparation of fluorescent labels by immersion method: Take the Cu NCs@p-MBA-1 ethanol suspension prepared in Example 1, and then soak the neatly cut fiber discs with a diameter of 1 cm in the prepared solution. Next, vortex the fiber discs for 10 minutes to fully disperse the fluorescent aggregates on the surface of the fiber cotton. Dry the soaked fluorescent labels at room temperature (25°C) for 30 minutes, then put them into a vacuum bag and store them at 4°C for subsequent experiments. Prepare 100 fluorescent labels according to the above preparation process and calculate the amount of materials used.

[0154] 3) Inkjet Printing of Fluorescent Labels: The Cu NCs@p-MBA@PVP-1 fluorescent ink prepared in Example 2 was injected into an inkjet printer and printed onto paper to create fluorescent labels with a diameter of 1 cm. The labels were then dried at room temperature (25°C) for 5 minutes and stored in a vacuum bag at 4°C for subsequent experiments. 100 fluorescent labels were prepared according to the above process and the material dosage was calculated.

[0155] Fluorescent labels prepared using three different methods were placed under ultraviolet light. A smartphone was used to capture fluorescent color images of the labels. The smartphone color picker app extracted the RGB values ​​of the fluorescent color images, arranged in ascending order of storage days, and the mean and deviation of the red channel value (R value) for each label were calculated.

[0156] Table 3 Comparison of the preparation of fluorescent labels prepared by traditional process and inkjet printing technology

[0157]

[0158] Based on the data shown in Table 3, a comparative analysis of three different fluorescent label preparation methods was conducted. The results show that among the three methods of dripping, immersion, and inkjet printing, inkjet printing technology exhibits significant advantages. Specifically, the inkjet printing method has the shortest preparation time, requiring only 600 seconds to complete the preparation of 100 labels, which is much lower than the 3600 seconds of the dripping method and the 5000 seconds of the immersion method. At the same time, inkjet printing technology is also the most economical in terms of the amount of indicator solution used. On average, each circular fluorescent label with a diameter of 1 cm consumes only 50 mL of indicator solution, while the consumption of the dripping method and the immersion method is 20 mL and 100 mL, respectively.

[0159] Furthermore, the fluorescent labels produced using inkjet printing exhibited greater stability in their R value distribution. As shown in Table 3, the R value range for 100 fluorescent labels prepared using inkjet printing was 241.2 ± 2.1, significantly superior to the 249.4 ± 5.9 obtained using the dropwise addition method and the 251.8 ± 7.5 obtained using the immersion method. This result demonstrates that inkjet printing not only offers advantages in production efficiency and material utilization, but also excels in ensuring product quality stability.

[0160] First, organic ligand-protected copper nanoclusters responsive to TVB-N were synthesized as fluorescent probes. A polymer was then added to the fluorescent probe solution, acting as a dispersant and thickener, to create a nanomaterial ink. Finally, this ink was added to a printer cartridge and programmed to evenly deposit the ink onto paper. This resulted in a food freshness indicator for monitoring food freshness, providing a more efficient, cost-effective, and reliable solution for applications such as food freshness testing.

[0161] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements 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 printing a visual food freshness label, characterized in that The following steps are involved: 1) Synthesis of metal nanoclusters: dissolving 4-mercaptobenzoic acid in a polar solvent to form a first precursor solution; dissolving copper nitrate in an acidic medium to form a second precursor solution; The first precursor solution and the second precursor solution are mixed to form metal nanoclusters with aggregation-induced emission properties through coordination reaction; after centrifugal purification, a metal nanocluster dispersion system dispersed in an organic solvent is obtained; 2) Preparation of fluorescent ink and regulating agent: The obtained metal nanocluster dispersion system is compounded with polyvinyl pyrrolidone, and subjected to ultrasonic dispersion and heat treatment to form ink; mixing the solvent with polyvinyl pyrrolidone to obtain a regulating agent; 3) Printing preparation of smart labels: The ink and the regulating agent are respectively injected into independent ink cartridges of an inkjet printer, and a pattern is deposited on a low-fluorescence background substrate using preset printing parameters. The pattern is then sealed and stored after drying.

2. The method for producing a printed visual food freshness label according to claim 1, characterized in that: In step 1), the polar solvent is a mixed solution of water and tetrahydrofuran, the volume ratio of water to tetrahydrofuran is 1:6.67 to 1:20; the molar concentration ratio of the copper nitrate to the organic ligand is 1:1 to 1:3; and the acidic medium is an aqueous nitric acid solution with a pH of ≤3.

3. The method for producing a printed visual food freshness label according to claim 1, characterized in that: In step 1), the concentration of the first precursor solution is 0.05-0.3M, the concentration of the second precursor solution is 0.1-0.3M, wherein the volume ratio of the first precursor solution to the second precursor solution is 1:1 to 1:3, the temperature of the coordination reaction is 60-80°C, and the time is 5-60min; the speed of the centrifugal purification is 5000-10000rpm, the time is 3-10min, the organic solvent is ethanol, and the mass volume ratio of the metal nanoclusters to the organic solvent is 3.265-13.06mg / mL.

4. The method for producing a printed visual food freshness label according to claim 1, characterized in that: In step 2), the mass volume ratio of the polyvinyl pyrrolidone to the metal nanocluster dispersion system is 0.5 to 3.5 g / mL; the ultrasonic power of the ultrasonic dispersion is 100 to 180 W, and the time is 15 to 30 min; the temperature of the heat treatment is 60 to 80° C., and the time is 30 to 120 min; the solvent is ethanol, and the mass volume ratio of the polymer to ethanol is 0.5 to 3.5 g / mL.

5. The method for producing a printed visual food freshness label according to claim 1, characterized in that: In step 3), the D65 fluorescence brightness of the low-fluorescence background substrate is ≤5.0%, and the low-fluorescence background substrate is non-fluorescent paper or polymer film; the printing parameters are regulated by the CMYK color mode, and the ink deposition amount is adapted to the resolution setting of the inkjet printer; the drying temperature is 25±2°C; and the sealed storage temperature is 4-8°C.

6. A printed visual food freshness label, characterized by: The invention is prepared by the preparation method according to any one of claims 1 to 5.

7. Use of the inkjet-printed visual food freshness label according to claim 6 in visual monitoring of meat food freshness.

8. The application of the inkjet printing visual food freshness label according to claim 7 in visual monitoring of meat food freshness is characterized in that The steps include: 1) The printed visual food freshness label and fresh meat products were stored in the same space for n days, where n is a positive integer. The TVB-N standard value of the meat was measured according to the national standard GB5009.228-2016. Fluorescent photos of the freshness fluorescent intelligent indicator label under ultraviolet light at different storage times were recorded regularly using a smartphone. The fluorescent photos at different storage times were converted into RGB values ​​using a mobile phone color picker app. The red channel values ​​of the RGB at different time points were fitted with the TVB-N values ​​at the corresponding storage time points to obtain the linear regression equation: Y = k × C + b, where k is the slope, b is the intercept, C is the TVB-N content, and Y is the value of the red channel of the label. 2) A fluorescent smart label for visually detecting food freshness is affixed to the inner top of a fresh-keeping box. After the label and the food sample are stored in the same space, a fluorescent color image of the smart indicator label inside the packaging bag is obtained. The fluorescent color image is converted into RGB values ​​using a smartphone color picker app. The value of the red channel in RGB is substituted into the linear regression equation established in step 1) to obtain the TVB-N content of the food at the test time point.

9. Application of the inkjet-printed visual food freshness label in meat freshness detection according to claim 8, characterized in that: The slope k represents the change in the red channel value (Y) per unit increase in TVB-N content (C), and its positive / negative sign reflects the increasing or decreasing trend of the fluorescence color with the accumulation of TVB-N; the intercept b is the theoretical initial value of the label red channel when the TVB-N content is zero.

10. Application of the inkjet-printed visual food freshness label in meat freshness detection according to claim 7, characterized in that: The meat includes chicken, beef, pork or fish.

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