A self-aggregating luminescent metal nanocluster fluorescent ink, its preparation method and application

Abstract

This invention belongs to the field of functional materials and freshness detection, and discloses a self-aggregating luminescent metal nanocluster fluorescent ink, its preparation method, and its application. The preparation method of the fluorescent ink includes the following steps: dissolving an organic ligand in water to form a first precursor solution, and dissolving a copper salt in water to form a second precursor solution; mixing the first and second precursor solutions, and performing a coordination reaction to form metal nanoclusters; freeze-drying the reaction solution to obtain a freeze-dried metal nanocluster powder; dissolving a polymer in a polar solvent to form a polymer solution; combining the obtained freeze-dried metal nanocluster powder with the polymer solution, and then ultrasonically dispersing the mixture to form the self-aggregating luminescent metal nanocluster fluorescent ink. This invention establishes a synergistic regulatory relationship between the metal nanocluster synthesis parameters, ink formulation parameters, and screen printing process parameters. The resulting ink-printed labels are highly sensitive to ammonia gas, enabling visual detection of the freshness of aquatic foods.

Description

Technical Field

[0001] This invention relates to the field of functional materials and freshness detection, specifically to a self-aggregating luminescent metal nanocluster fluorescent ink and its preparation method and application, particularly to a fluorescent ink with ammonia-responsive function, especially a smart label ink based on copper nanoclusters suitable for screen printing, and its application in the visual detection of freshness of aquatic food products. Background Technology

[0002] Currently, food freshness monitoring is a crucial issue in the field of food safety. Food freshness is a core concern for consumers and a key aspect of food safety control. Aquatic products (such as fish, shrimp, and shellfish) are rich in water and protein, making them highly susceptible to spoilage during storage, transportation, and sales due to the action of endogenous enzymes and microorganisms. The core characteristic of spoilage is the production of large amounts of alkaline nitrogenous substances, such as ammonia, trimethylamine, and dimethylamine, collectively known as volatile basic nitrogen (TVB-N). TVB-N content has been widely adopted both domestically and internationally as an objective physicochemical indicator for evaluating the freshness of aquatic products, and its value increases significantly with the degree of spoilage.

[0003] Currently, the detection of food freshness, especially TVB-N content, mainly relies on traditional laboratory instrumental analysis methods, such as gas chromatography and the Kjeldahl method. While these methods offer advantages in terms of high precision and accurate results, their drawbacks are also significant: the testing procedures are cumbersome and time-consuming, typically requiring several hours or even longer; they depend on large, expensive specialized equipment and dedicated laboratory environments; and they must be operated by highly trained technicians. These characteristics make traditional methods unable to meet the urgent needs of modern supply chains for on-site, rapid, real-time, low-cost, and non-destructive testing of food quality, resulting in regulatory lag and failing to provide ordinary consumers with immediate and intuitive information on freshness.

[0004] Therefore, developing a smart label that can be directly integrated into the interior or surface of food packaging, capable of sensitive and rapid response to TVB-N (especially ammonia), and able to convert chemical signals into visible optical signals (such as color or fluorescence changes) has become a cutting-edge research direction in the field of smart packaging. This type of label is expected to simplify complex laboratory analyses into intuitive visual readings, which has significant practical application value and broad market prospects for protecting consumer rights, reducing food waste, improving supply chain transparency, and building a modern food safety early warning system.

[0005] Currently, in the field of freshness detection, Chinese patent CN116046732A reports a ratiometric fluorescent indicator consisting of a gold-copper bimetallic nanocluster (indicator) with red fluorescence and a dual-ligand gold nanocluster (internal standard) with green fluorescence. This indicator is used to prepare fluorescent smart labels for predicting the freshness grade and total volatile basic nitrogen content of meat. Another patent, CN110057805A, discloses a metal-organic framework / copper nanocluster that can accurately and rapidly detect the ATP content in aquatic products, using this as an evaluation index of the freshness of aquatic products. While the aforementioned Chinese patents CN116046732A and CN110057805A involve the application of nanoclusters in freshness detection, they often employ complex synthesis routes or neglect the adaptability to printing processes, failing to fully leverage the advantages of screen printing in large-area, low-cost label manufacturing.

[0006] Therefore, research on the application of metal nanoclusters in functional inks still faces several challenges, especially when adapting them to screen printing processes to prepare practical smart labels. First, a core contradiction lies in balancing the high dispersion stability of clusters during ink storage with the rapid, ordered self-aggregation required after printing. Excessive dispersion, while ensuring smooth ink storage and printing, may result in weak fluorescence after drying due to isolated clusters. Conversely, insufficient dispersion, while potentially beneficial for film formation and luminescence, can lead to sedimentation and aggregation during ink storage, causing serious problems such as screen clogging and pattern discontinuity during screen printing. Second, screen printing requires inks with specific rheological properties to ensure good screen passability and pattern clarity. This means that in addition to basic factors such as cluster concentration, polymer content, and solvent composition, the rheological behavior of the ink system must be precisely controlled to ensure smooth screen passability and the formation of a uniformly thick, clearly defined film on the substrate. Furthermore, the performance stability of the ink and the final printed label in practical application environments is crucial to its practicality. This includes the ink's resistance to sedimentation, the mechanical stability of the printed film, and its specificity, sensitivity, and reliability in responding to the target analyte ammonia when in contact with food packaging environments. These all require systematic evaluation beyond the laboratory stage to meet the application needs of real-world scenarios.

[0007] In summary, the development and application of inks based on highly dispersed self-aggregating luminescent metal nanoclusters for freshness detection is a multidisciplinary research field involving materials chemistry, colloid science, fluid mechanics, and other disciplines. By precisely controlling the dispersion and aggregation behavior of metal nanoclusters, novel functional inks with excellent performance can be developed, providing innovative material solutions for related application fields, especially freshness detection. The real challenge and innovation in developing metal nanocluster fluorescent inks suitable for screen printing lies not in the simple execution of a single step, but in how to achieve deep coupling and parameter matching between material synthesis, ink formulation, and printing processes. Existing technologies have failed to reveal and utilize this cross-stage parameter correlation, resulting in inks that often have shortcomings in dispersibility, printability, or final sensing performance. There is an urgent need for a systematic design method that can coordinate and control the particle size of nanoclusters, the rheology of ink, and the precision of printed patterns to meet the diverse performance and cost requirements of smart labels in different application scenarios. This study provides new ideas and methods for solving key scientific problems in the application of metal nanoclusters in inks through systematic material design and process optimization, and offers new solutions and technologies for the field of food freshness detection. Summary of the Invention

[0008] In order to overcome the shortcomings and deficiencies of the prior art, the purpose of this invention is to provide a method for preparing self-aggregating luminescent metal nanoclusters fluorescent ink.

[0009] Another objective of this invention is to provide a self-aggregating luminescent metal nanoclusters fluorescent ink.

[0010] Another object of the present invention is to provide the application of the above-mentioned self-aggregating luminescent metal nanoclusters fluorescent ink.

[0011] Another object of the present invention is to provide a fluorescent smart tag.

[0012] The basic idea of ​​this invention is based on the recognition that to obtain fluorescent inks suitable for screen printing that possess both high dispersion stability and rapid self-aggregation and luminescence characteristics after printing, the synthesis of metal nanoclusters, ink formulation, and screen printing processes must be considered as a holistic system and designed collaboratively. The "particle size" of the nanoclusters is a key physical quantity connecting these three stages. In the synthesis stage, the initial particle size and aggregation tendency of the clusters need to be preset; in the ink preparation stage, the hydrodynamic particle size of the clusters needs to be precisely controlled to match the pore size of the target screen printing stencil through the selection of solvents and dispersants, while ensuring storage stability; in the printing stage, the printing parameters need to be adapted to the rheological properties of the ink to ensure uniform film formation and trigger the self-aggregation of clusters to achieve fluorescence enhancement.

[0013] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows:

[0014] A method for preparing a self-aggregating luminescent metal nanoclusters fluorescent ink includes the following steps:

[0015] 1) Synthesis of metal nanoclusters:

[0016] An organic ligand is dissolved in water to form a first precursor solution; the organic ligand is an organic ligand containing thiol and amino groups;

[0017] The copper salt is dissolved in water to form a second precursor solution;

[0018] The first precursor solution and the second precursor solution are mixed, and a coordination reaction is carried out to form metal nanoclusters; the reaction solution is freeze-dried to obtain freeze-dried powder of metal nanoclusters.

[0019] 2) Preparation of fluorescent ink:

[0020] The polymer is dissolved in a polar solvent to form a polymer solution; the resulting freeze-dried metal nanoclusters are combined with the polymer solution and then ultrasonically dispersed to form a uniform and stable fluorescent ink, namely, a self-aggregating luminescent metal nanoclusters fluorescent ink.

[0021] Preferably, the organic ligand is an organic ligand containing thiol and amino groups. The thiol group in the ligand coordinates with Cu²⁺ to form copper nanoclusters, and the amino group in the ligand interacts specifically with ammonia to achieve a fluorescence response.

[0022] Preferably, the organic ligand is at least one selected from L-cysteine, D-cysteine, glutathione, mercaptoethylamine, and penicillamine, with L-cysteine ​​being the most preferred; the copper salt is at least one selected from copper nitrate, copper sulfate, copper chloride, cuprous chloride, and copper acetate, and the organic ligand reacts with Cu... 2+ The molar ratio is 35:1-10:1.

[0023] Preferably, in step 1), the concentration of the first precursor solution is 0.2-0.8 M, the concentration of the second precursor solution is 0.1-0.3 M, and the volume ratio of the first precursor solution to the second precursor solution is 5:1-5:3.

[0024] The coordination reaction is carried out at a temperature of 20-30 °C, with the pH adjusted to 3-10, and for a time of 15-60 min.

[0025] Preferably, in step 1), the freeze-drying temperature is -60 to -50 °C, the vacuum degree is 5 to 15 Pa, and the drying time is 8 to 48 h.

[0026] Preferably, in step 2), the polymer is selected from at least one of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), hydroxypropyl methylcellulose (HPMC), polyacrylate (PA), and chitosan (CS), more preferably polyvinylpyrrolidone.

[0027] Preferably, the polyvinylpyrrolidone has a molecular weight of 40,000-58,000. PVP with a molecular weight of 40,000-58,000 has moderate steric hindrance, can bind to the surface of copper nanoclusters through hydrogen bonds, effectively prevents cluster aggregation, and can impart moderate viscosity and typical shear thinning behavior to the ink.

[0028] The mass-to-volume ratio of the metal nanocluster powder to the polymer solution is 0.01-0.2:1 (g / mL).

[0029] Preferably, in step 2), the polar solvent is selected from at least one of methanol, ethanol, N,N-dimethylformamide, ethyl acetate, tetrahydrofuran, and isopropanol; the mass-to-volume ratio of the polymer to the polar solvent (g / mL) is 0.05~1:1, preferably 0.06~0.1:1, and more preferably 0.09:1;

[0030] In step 2), the ultrasonic power for ultrasonic dispersion is 100-180 W, and the duration is 30-120 min.

[0031] A self-aggregating luminescent metal nanoclusters fluorescent ink was prepared by the method described above.

[0032] Preferably, the self-aggregating luminescent metal nanocluster fluorescent ink has high dispersibility, with an average hydrodynamic particle size of 90~110nm and a polydispersity index of <0.15.

[0033] The above-mentioned self-aggregating luminescent metal nanocluster fluorescent ink is used in the detection of food freshness, wherein the food freshness detection includes the detection of volatile basic nitrogen.

[0034] Preferably, the food is an aquatic product.

[0035] A fluorescent smart tag prepared using the above-mentioned self-aggregating luminescent metal nanoclusters fluorescent ink.

[0036] Preferably, the fluorescent smart tag is prepared as follows:

[0037] The above-mentioned self-aggregating luminescent metal nanocluster fluorescent ink is coated or patterned on a substrate by screen printing, and after drying, it forms a fluorescent smart label for freshness detection, which is then sealed and stored.

[0038] The screen printing uses a screen with a mesh count of 200-400, a squeegee angle of 60°-75°, and a printing pressure of 0.2-0.4 MPa; the drying process is carried out at a temperature of 30-50 ℃ for 10-40 min.

[0039] Preferably, the printed label is concealed under natural light and displays orange-red fluorescence under 365 nm ultraviolet light. The fluorescence intensity decreases as the concentration of ammonia in the environment increases, thereby achieving a visual semi-quantitative detection of freshness.

[0040] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0041] (1) Advantages of material design and synthesis: This invention successfully prepared a fluorescent ink based on aggregation-induced emission copper nanoclusters (Cu NCs@L-Cys). The material uses L-cysteine ​​as a ligand, and the synthesis method is simple, mild, and environmentally friendly. The prepared nanoclusters exhibit good fluorescence performance under ultraviolet light excitation, and their specific response to amines is not easily affected by environmental interference, laying a material foundation for sensing applications.

[0042] (2) Excellent ink performance, resolving the core contradiction: By introducing polyvinylpyrrolidone (PVP) into the system as a dispersant and thickener, and combining it with the key post-processing technology of freeze-drying-reconstitution, a stable ink with appropriate viscosity and high dispersibility was successfully prepared. This strategy perfectly balances the core contradiction between the high dispersion stability required for ink storage (>30 days without sedimentation) and the rapid self-aggregation luminescence (fluorescence enhancement of 3-5 times) achieved by utilizing the AIE effect after printing and film formation, ensuring a good performance conversion from ink to functional film.

[0043] (3) Innovative printing process with strong adaptability: This invention creatively adopts screen printing technology. Screen printing has a stronger tolerance for ink particle size and viscosity range, which significantly improves the error tolerance and reliability of the process, making it possible to prepare functional nanomaterial inks in a large area, patterned and low cost, and is highly feasible for mass production.

[0044] (4) Sensitive and reliable sensing performance: The smart label made by screen printing the above ink is sensitive to ammonia gas, and has obvious and reversible fluorescence quenching effect. Practical application shows that the label can effectively distinguish the freshness status of aquatic food at different storage stages (fresh, relatively fresh, spoiled), showing strong potential for direct and visual indication of food quality in smart packaging.

[0045] (5) Low cost and environmentally friendly: The entire technical route uses inexpensive copper as the metal source and mainly adopts aqueous synthesis. The screen printing process itself also has the advantages of simple equipment and low material consumption, which significantly reduces the overall cost. At the same time, this process reduces the use of precious metals and the consumption of organic solvents, which is in line with the concept of green manufacturing.

[0046] (6) Clear application orientation and significant comprehensive advantages: Starting from the practical needs of on-site detection of food freshness, this invention has successfully achieved rapid and low-cost production of fluorescent smart tags through integrated innovation of materials, inks, and processes. This technical solution is simple to prepare, easy to operate, and easy to scale up, providing a convenient and intuitive solution for food safety monitoring, with strong comprehensive practicality. Attached Figure Description

[0047] Figure 1 A flowchart illustrating the preparation method and application of a highly dispersed, self-aggregating, luminescent metal nanoclusters fluorescent ink for food freshness detection.

[0048] Figure 2 Images of the Cu NCs@L-Cys solution in Example 1 under natural visible light and 365 nm ultraviolet light illumination;

[0049] Figure 3 The image shows the UV absorption spectrum of Cu NCs@L-Cys in Example 1.

[0050] Figure 4 The fluorescence excitation and emission spectra of Cu NCs@L-Cys in Example 1 are shown.

[0051] Figure 5 Images of Cu NCs@L-Cys@PVP ink in Example 1 under visible light and ultraviolet light conditions;

[0052] Figure 6 The fluorescence excitation and emission spectra of Cu NCs@L-Cys@PVP ink in Example 1 are shown.

[0053] Figure 7 The histograms show the particle sizes (a) of Cu NCs@L-Cys ink and (b) of Cu NCs@L-Cys@PVP ink in Example 1.

[0054] Figure 8 The rheological curve of Cu NCs@L-Cys@PVP ink in Example 1 is shown.

[0055] Figure 9 The fluorescence spectra of Cu NCs@L-Cys@PVP ink in Example 1 and its response to different concentrations of ammonia are shown.

[0056] Figure 10 This is a photograph of the screen-printed label sample (letters: SCUT) from Example 1 under 365 nm ultraviolet light irradiation;

[0057] Figure 11 This is a photograph of the fluorescence color change of the screen-printed label sample (letter: SCUT) in Example 1 under different ammonia concentrations.

[0058] Figure 12 The fluorescent color change diagram of the screen-printed label used in Example 1 to monitor the freshness of prawns stored at 4°C;

[0059] Figure 13 Images of Cu NCs@L-Cys@PVP-9 ink in Comparative Example 1 under visible and ultraviolet light conditions.

[0060] Figure 14 Images of Cu NCs@L-Cys@PVP-10 ink in Comparative Example 2 under visible and ultraviolet light conditions.

[0061] Figure 15 Images of Cu NCs@L-Cys@PVP-11 ink in Comparative Example 3 under visible and ultraviolet light conditions.

[0062] Figure 16 Images of Cu NCs@L-Cys@PVP-12 ink in Comparative Example 4 under visible and ultraviolet light conditions. Detailed Implementation

[0063] To better understand the present invention, it will be further described below with reference to the accompanying drawings and specific embodiments. However, the implementation of the present invention is not limited thereto. The described embodiments are some, but not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0064] In the field of intelligent packaging for food freshness, developing fluorescent labels that respond to volatile basic nitrogen (TVB-N) and are suitable for large-scale printing processes is key to the practical application of the technology. Existing ammonia-responsive materials based on metal nanoclusters often suffer from the following limitations: First, nanoclusters are prone to uncontrolled aggregation in liquid systems, leading to increased hydrodynamic particle size, making it difficult to meet the ink dispersion stability requirements of printing processes; second, traditional inkjet printing technology is extremely demanding on ink particle size distribution (typically requiring <200 nm), viscosity range, and long-term stability, severely limiting the direct application of most functional nanomaterials in printed electronics.

[0065] While screen printing technology offers significant advantages in adaptability, cost, and large-area fabrication, a core challenge remains for functional inks: how to ensure high dispersion stability of nanoclusters during ink storage and printing, while simultaneously enabling rapid self-aggregation after printing to achieve strong fluorescence signal output (i.e., aggregation-induced emission, AIE). Existing technologies often compromise on one aspect while failing to achieve both simultaneously.

[0066] Therefore, there is an urgent need to develop a fluorescent ink with stable photoluminescence properties that can respond to TVB-N. This fluorescent ink needs to 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 its stability and reliability in practical applications without affecting its luminescence performance.

[0067] This invention first synthesizes Cu NCs@L-Cys with AIE properties in an aqueous phase, and obtains a powder precursor by freeze-drying. Subsequently, it is compounded with a polar solvent solution of PVP (preferably isopropanol) and ultrasonically dispersed to prepare a uniform and stable fluorescent ink. Finally, the ink is patterned onto various substrates using screen printing technology to create a smart fluorescent label for food freshness detection. This label exhibits good concealment under natural light and emits bright orange-red fluorescence under 365 nm ultraviolet light excitation. Its fluorescence intensity undergoes a sensitive quenching response as the concentration of ammonia in the environment increases, thereby achieving visualized and semi-quantitative detection of freshness.

[0068] The ink prepared by this method can be stored stably at room temperature for more than 30 days without sedimentation. The printed label pattern can still maintain more than 95% of the initial fluorescence intensity after mechanical bending test, showing excellent comprehensive performance and fully meeting the needs of practical applications.

[0069] The polyvinylpyrrolidone (PVP) used in this invention is PVP K30, with a molecular weight range of 40,000-58,000.

[0070] Example 1

[0071] 1) Preparation of copper nanoclusters (Cu NCs@L-Cys-1)

[0072] 0.606 g of L-cysteine ​​(L-Cys) was dispersed in 10 mL of ultrapure water under magnetic stirring at room temperature to obtain a clear mixed solution (0.5 M). Copper nitrate solution (0.1 M, 2 mL) was added to the clear mixed solution. The solution after adding copper nitrate was then magnetically stirred at room temperature for 30 min, and the pH was adjusted to 5. The solution gradually turned milky white, thus obtaining the CuNCs@L-Cys reaction solution. After pre-freezing at -80 °C, the reaction solution was freeze-dried at -50 °C / 10 Pa for 24 h to obtain an orange-red fluorescent powder.

[0073] The synthesized metal nanoclusters were characterized by UV-Vis spectroscopy, fluorescence spectroscopy, and optical properties, and exhibited the following characteristics: Figure 2 As shown, the Cu NCs@L-Cys-1 solution appears milky white under visible light and emits strong orange-red fluorescence under ultraviolet light; the ultraviolet-visible spectrum shows that ( Figure 3 The UV absorption curves of Cu NCs@L-Cys showed no surface plasmon resonance absorption peaks for metal ions, indicating that no large-sized copper nanoparticles were formed in the synthesized copper nanoclusters; the fluorescence spectrum ( Figure 4 As shown above, Cu NCs@L-Cys exhibits a strong fluorescence emission peak at 646 nm. In summary, the prepared Cu NCs@L-Cys possesses good fluorescence properties.

[0074] 2) Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-1)

[0075] At room temperature and with magnetic stirring, 4.5 g of PVP was dissolved in 50 mL of isopropanol and magnetically stirred for 30 min. Then, 0.5 g of Cu NCs@L-Cys lyophilized powder was added to the PVP isopropanol solution and magnetically stirred for 30 min. Following this, an ultrasonic power of 180 W and an ultrasonic time of 120 min were used to promote the dissolution of PVP and the dispersion of Cu NCs@L-Cys. The resulting Cu NCs@L-Cys@PVP-1 ink was obtained. Printing with this ink and observing the fluorescent pattern under a 365 nm UV lamp revealed a visible pattern.

[0076] The prepared fluorescent ink was characterized by ultraviolet-visible spectroscopy, fluorescence spectroscopy, and optical properties. The results are as follows: Figure 5 As shown, Cu NCs@L-Cys@PVP-1 ink appears milky white under visible light and emits strong orange-red fluorescence under ultraviolet light. Figure 6The results showed that, compared with Cu NCs@L-Cys solution, the fluorescence excitation and emission peaks did not shift, and a strong fluorescence emission peak was still present at 646 nm, proving that the ink formation process did not destroy its luminescent centers. Figure 7 As shown in (a), the synthesized CuNCs@L-Cys hydrated particle size distribution is relatively wide. However, after adding PVP to form an ink, as... Figure 7 As shown in (b), its particle size distribution is significantly narrower, with an average hydrodynamic particle size of about 100 nm and a polydispersity index (PDI) of less than 0.15, which proves the excellent dispersion effect of PVP and the high dispersibility of the ink. Figure 8 The rheological curves show that the ink exhibits typical shear-thinning behavior, with viscosity decreasing as the shear rate increases. This rheological property gives it good suitability for screen printing.

[0077] 3) Preparation and performance testing of screen-printed smart labels

[0078] Pour the above ink into the ink tank of a screen printing machine (300 mesh). Set the squeegee angle to 70° and the printing pressure to 0.3 MPa, and print the preset pattern (the letters "SCUT") on a paper substrate without a fluorescent background. Place the printed substrate in a 30 ℃ oven to dry for 15 min to obtain the smart label.

[0079] The printed pattern is almost invisible under natural light, providing excellent concealment; such as Figure 10 As shown, under 365 nm ultraviolet light illumination, it exhibits a high-contrast, bright orange-red fluorescence. Figure 11 As shown, when the tag is exposed to different concentrations of ammonia gas, its fluorescence intensity undergoes a regular quenching with increasing ammonia concentration, demonstrating excellent ammonia response sensitivity. Figure 12 As shown, when the label was actually used to monitor prawns stored at 4°C, the fluorescence color gradually changed from bright orange-red to weak and then to no fluorescence as the storage time increased (TVB-N value increased), which intuitively reflected the change in the freshness of the food.

[0080] In summary, the prepared Cu NCs@L-Cys@PVP-1 ink exhibits good fluorescence properties and excellent screen printing printability.

[0081] To clarify the synergistic relationship between material synthesis, ink preparation, and screen printing in this invention, this embodiment focuses on the matching between the nanocluster particle size and the mesh size of the screen printing stencil. By adjusting the reaction time in step 1) (15 min, 30 min, 60 min), we prepared Cu NCs@L-Cys with different initial particle sizes (denoted as samples A-small, A-medium, and A-large). After preparing them into inks using the same formulation and process, their hydrodynamic particle sizes (D_h) were measured using a laser particle size analyzer to be ~80 nm, ~100 nm, and ~150 nm, respectively. Subsequently, patterns were printed using 400 mesh (pore size approximately 38 μm), 300 mesh (pore size approximately 55 μm), and 200 mesh (pore size approximately 75 μm) screens, respectively.

[0082] To quantify the uniformity of printed patterns, this invention employs image analysis: a label printed with a solid square is photographed under 365 nm ultraviolet light, converted to a grayscale image, and the relative standard deviation (RSD) of the pixel grayscale values ​​within the entire square area is calculated. A smaller RSD value indicates a more uniform fluorescence distribution. Patterns printed using the optimized parameters of this invention (Group B: angle 70°, pressure 0.3 MPa) exhibit a fluorescence intensity RSD < 5%, demonstrating the importance of the process parameters of this invention for obtaining high-quality, uniform sensor labels.

[0083] The results are shown in Table 1:

[0084] Table 1

[0085]

[0086] This invention obtains nanoclusters of different particle sizes by actively controlling synthesis parameters, thereby enabling the "tailor-made" inks for printing needs of varying precision (such as high-precision small patterns vs. large-area coarse patterns), achieving a precise match between material properties and application scenarios. This demonstrates that the three steps are not independent but closely related through the key parameter of "particle size."

[0087] Example 2

[0088] 1) Preparation of copper nanoclusters (Cu NCs@L-Cys-2)

[0089] 0.727 g of L-cysteine ​​(L-Cys) was dispersed in 10 mL of ultrapure water under magnetic stirring at room temperature to obtain a clear mixed solution (0.6 M). Copper nitrate solution (0.1 M, 2 mL) was added to the clear mixed solution. The solution with added copper nitrate was then magnetically stirred at room temperature for 30 min, and the pH was adjusted to 5 to obtain the Cu NCs@L-Cys reaction solution. After pre-freezing at -80 °C, the reaction solution was freeze-dried at -50 °C / 10 Pa for 24 h to obtain an orange-red fluorescent powder.

[0090] 2) Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-2)

[0091] At room temperature and with magnetic stirring, 4.5 g of PVP was dissolved in 50 mL of isopropanol and stirred magnetically for 30 min. Then, 0.5 g of Cu NCs@L-Cys lyophilized powder was added to the PVP isopropanol solution and stirred magnetically for 30 min. Following this, an ultrasonic power of 180 W and an ultrasonic time of 120 min were used to promote the dissolution of PVP and the dispersion of Cu NCs@L-Cys. The resulting Cu NCs@L-Cys@PVP-2 ink was obtained. When screen-printed with this ink and observed under a 365 nm UV lamp, a fluorescent pattern was visible.

[0092] 3) Preparation and performance testing of screen-printed smart labels

[0093] Pour the above ink into the ink tank of a screen printing machine (300 mesh). Set the squeegee angle to 70° and the printing pressure to 0.3 MPa, and print the preset pattern (the letters "SCUT") on a paper substrate without a fluorescent background. Place the printed substrate in a 30 ℃ oven to dry for 15 min to obtain the smart label.

[0094] Example 3

[0095] 1) Preparation of copper nanoclusters (Cu NCs@L-Cys-3)

[0096] 0.485 g of L-cysteine ​​(L-Cys) was dispersed in 10 mL of ultrapure water under magnetic stirring at room temperature to obtain a clear mixed solution (0.4 M). Copper nitrate solution (0.1 M, 2 mL) was added to the clear mixed solution. The solution after adding copper nitrate was then magnetically stirred at room temperature for 30 min, and the pH was adjusted to 5 to obtain the Cu NCs@L-Cys reaction solution. The reaction solution was pre-frozen at -80 °C and then freeze-dried at -50 °C / 10 Pa for 24 h to obtain an orange-red fluorescent powder.

[0097] 2) Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-3)

[0098] At room temperature and with magnetic stirring, 4.5 g of PVP was dissolved in 50 mL of isopropanol and magnetically stirred for 30 min. Then, 0.5 g of Cu NCs@L-Cys lyophilized powder was added to the PVP isopropanol solution and magnetically stirred for 30 min. Following this, an ultrasonic power of 180 W and an ultrasonic time of 120 min were used to promote the dissolution of PVP and the dispersion of Cu NCs@L-Cys. The resulting Cu NCs@L-Cys@PVP-3 ink was obtained. When printed with this ink and observed under a 365 nm UV lamp, a fluorescent pattern was visible.

[0099] 3) Preparation and performance testing of screen-printed smart labels

[0100] Pour the above ink into the ink tank of a screen printing machine (300 mesh). Set the squeegee angle to 70° and the printing pressure to 0.3 MPa, and print the preset pattern (the letters "SCUT") on a paper substrate without a fluorescent background. Place the printed substrate in a 30 ℃ oven to dry for 15 min to obtain the smart label.

[0101] Example 4

[0102] 1) Preparation of copper nanoclusters (Cu NCs@L-Cys-4)

[0103] 0.363 g of L-cysteine ​​(L-Cys) was dispersed in 10 mL of ultrapure water under magnetic stirring at room temperature to obtain a clear mixed solution (0.3 M). Copper nitrate solution (0.1 M, 2 mL) was added to the clear mixed solution. The solution with added copper nitrate was then magnetically stirred at room temperature for 30 min, and the pH was adjusted to 5 to obtain the Cu NCs@L-Cys reaction solution. After pre-freezing at -80 °C, the reaction solution was freeze-dried at -50 °C / 10 Pa for 24 h to obtain an orange-red fluorescent powder.

[0104] 2) Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-4)

[0105] At room temperature and with magnetic stirring, 4.5 g of PVP was dissolved in 50 mL of isopropanol and stirred magnetically for 30 min. Then, 0.5 g of Cu NCs@L-Cys lyophilized powder was added to the PVP isopropanol solution and stirred magnetically for 30 min. Following this, an ultrasonic power of 180 W and an ultrasonic time of 120 min were used to promote the dissolution of PVP and the dispersion of Cu NCs@L-Cys. The resulting Cu NCs@L-Cys@PVP-4 ink was obtained. When screen-printed using this ink, a fluorescent pattern was observed under a 365 nm UV lamp.

[0106] 3) Preparation and performance testing of screen-printed smart labels

[0107] Pour the above ink into the ink tank of a screen printing machine (300 mesh). Set the squeegee angle to 70° and the printing pressure to 0.3 MPa, and print the preset pattern (the letters "SCUT") on a paper substrate without a fluorescent background. Place the printed substrate in a 30 ℃ oven to dry for 15 min to obtain the smart label.

[0108] Example 5

[0109] 1) Preparation of copper nanoclusters (Cu NCs@L-Cys-5)

[0110] 0.242 g of L-cysteine ​​(L-Cys) was dispersed in 10 mL of ultrapure water under magnetic stirring at room temperature to obtain a clear mixed solution (0.2 M). Copper nitrate solution (0.1 M, 2 mL) was added to the clear mixed solution. The solution with added copper nitrate was then magnetically stirred at room temperature for 30 min, and the pH was adjusted to 5 to obtain the Cu NCs@L-Cys reaction solution. After pre-freezing at -80 °C, the reaction solution was freeze-dried at -50 °C / 10 Pa for 24 h to obtain an orange-red fluorescent powder.

[0111] 2) Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-5)

[0112] At room temperature and with magnetic stirring, 4.5 g of PVP was dissolved in 50 mL of isopropanol and stirred magnetically for 30 min. Then, 0.5 g of Cu NCs@L-Cys lyophilized powder was added to the PVP isopropanol solution and stirred magnetically for 30 min. Following this, an ultrasonic power of 180 W and an ultrasonic time of 120 min were used to promote the dissolution of PVP and the dispersion of Cu NCs@L-Cys. The resulting Cu NCs@L-Cys@PVP-5 ink was obtained. When screen-printed using this ink, a fluorescent pattern was observed under a 365 nm UV lamp.

[0113] 3) Preparation and performance testing of screen-printed smart labels

[0114] Pour the above ink into the ink tank of a screen printing machine (300 mesh). Set the squeegee angle to 70° and the printing pressure to 0.3 MPa, and print the preset pattern (the letters "SCUT") on a paper substrate without a fluorescent background. Place the printed substrate in a 30 ℃ oven to dry for 15 min to obtain the smart label.

[0115] Example 6

[0116] 1) Preparation of copper nanoclusters (Cu NCs@L-Cys-6)

[0117] 0.606 g of L-cysteine ​​(L-Cys) was dispersed in 10 mL of ultrapure water under magnetic stirring at room temperature to obtain a clear mixed solution. Copper nitrate solution (0.1 M, 2 mL) was added to the clear mixed solution. The solution after adding copper nitrate was then magnetically stirred at room temperature for 30 min, and the pH was adjusted to 5 to obtain the Cu NCs@L-Cys reaction solution. The reaction solution was pre-frozen at -80 °C and then freeze-dried at -50 °C / 10 Pa for 24 h to obtain an orange-red fluorescent powder.

[0118] 2) Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-6)

[0119] Under room temperature and magnetic stirring, different doses of PVP (3.0 g, 4.5 g, and 5.0 g) were dissolved in 50 mL of isopropanol and magnetically stirred for 30 min. Then, 0.5 g of CuNCs@L-Cys lyophilized powder was added to the PVP isopropanol solutions of different concentrations and magnetically stirred for 30 min. Subsequently, ultrasonic power of 180 W and ultrasonic time of 120 min were set to promote the dissolution of PVP and the dispersion of CuNCs@L-Cys. The CuNCs@L-Cys@PVP-6 ink was obtained after these steps. When printed with this ink and observed under a 365 nm UV lamp, a fluorescent pattern was visible.

[0120] 3) Preparation and performance testing of screen-printed smart labels

[0121] Pour the above ink into the ink tank of a screen printing machine (300 mesh). Set the squeegee angle to 70° and the printing pressure to 0.3 MPa, and print the preset pattern (the letters "SCUT") on a paper substrate without a fluorescent background. Place the printed substrate in a 30 ℃ oven to dry for 15 min to obtain the smart label.

[0122] Table 2 Effect of different PVP dosages on ink performance

[0123]

[0124] Example 7: The Influence of Screen Mesh Count on Printed Pattern Quality

[0125] 1) Preparation of copper nanoclusters (Cu NCs@L-Cys-7)

[0126] 0.606 g of L-cysteine ​​(L-Cys) was dispersed in 10 mL of ultrapure water under magnetic stirring at room temperature to obtain a clear mixed solution. Copper nitrate solution (0.1 M, 2 mL) was added to this clear mixed solution. The solution after adding copper nitrate was then magnetically stirred at room temperature for 30 min, and the pH was adjusted to 5. The solution gradually turned milky white, yielding the CuNCs@L-Cys reaction solution. After pre-freezing at -80 °C, the reaction solution was freeze-dried at -50 °C / 10 Pa for 24 h to obtain an orange-red fluorescent powder.

[0127] 2) Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-7)

[0128] At room temperature and with magnetic stirring, 4.5 g of PVP was dissolved in 50 mL of isopropanol and stirred magnetically for 30 min. Then, 0.5 g of Cu NCs@L-Cys lyophilized powder was added to the PVP isopropanol solution and stirred magnetically for 30 min. Following this, an ultrasonic power of 180 W and an ultrasonic time of 120 min were used to promote the dissolution of PVP and the dispersion of Cu NCs@L-Cys. The resulting Cu NCs@L-Cys@PVP-7 ink was obtained. When screen-printed using this ink, a fluorescent pattern was observed under a 365 nm UV lamp.

[0129] 3) Preparation and performance testing of screen-printed smart labels

[0130] Ink was poured into the ink trough using polyester screens of 200 mesh, 300 mesh, and 400 mesh respectively. The squeegee angle was set to 70°, and the printing pressure to 0.3 MPa. A preset pattern (the letters "SCUT") was printed on a paper substrate without a fluorescent background. The printed substrate was then dried in a 30°C oven for 15 minutes to obtain the smart label.

[0131] The results show that all three mesh counts can achieve the technical effects of this invention. The prepared smart labels have complete patterns and uniform fluorescence, and exhibit sensitive fluorescence response to ammonia gas produced by the spoilage of aquatic products. There are no problems such as screen clogging, printing omissions, or fluorescence quenching. Among them, 300 mesh count is the optimal mesh count, which balances pattern resolution and ink layer thickness.

[0132] Example 8: The Influence of Squeegee Angle and Pressure on Printing Uniformity

[0133] 1) Preparation of copper nanoclusters (Cu NCs@L-Cys-8)

[0134] 0.606 g of L-cysteine ​​(L-Cys) was dispersed in 10 mL of ultrapure water under magnetic stirring at room temperature to obtain a clear mixed solution. Copper nitrate solution (0.1 M, 2 mL) was added to this clear mixed solution. The solution after adding copper nitrate was then magnetically stirred at room temperature for 30 min, and the pH was adjusted to 5. The solution gradually turned milky white, yielding the CuNCs@L-Cys reaction solution. After pre-freezing at -80 °C, the reaction solution was freeze-dried at -50 °C / 10 Pa for 24 h to obtain an orange-red fluorescent powder.

[0135] 2) Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-8)

[0136] At room temperature and with magnetic stirring, 4.5 g of PVP was dissolved in 50 mL of isopropanol and stirred magnetically for 30 min. Then, 0.5 g of Cu NCs@L-Cys lyophilized powder was added to the PVP isopropanol solution and stirred magnetically for 30 min. Subsequently, an ultrasonic power of 180 W and an ultrasonic time of 120 min were set to promote the dissolution of PVP and the dispersion of Cu NCs@L-Cys. The resulting Cu NCs@L-Cys@PVP-8 ink was obtained. When screen-printed using this ink, a fluorescent pattern was observed under a 365 nm UV lamp.

[0137] 3) Preparation and performance testing of screen-printed smart labels

[0138] The ink was poured into the ink tank of a screen printing machine (300 mesh). Combined experiments were conducted by adjusting the squeegee angle and pressure: Group A (angle 60°, pressure 0.2 MPa), Group B (angle 70°, pressure 0.3 MPa), and Group C (angle 75°, pressure 0.4 MPa). A preset pattern (the letters "SCUT") was printed on a paper substrate without a fluorescent background. The printed substrate was then dried in a 30°C oven for 15 minutes to obtain the smart label.

[0139] The results show that all process conditions within this parameter range can achieve uniform transfer of copper nanocluster fluorescent ink, and the printed pattern has no problems such as missing prints, ghosting, or blurred edges. The mechanical stability, fluorescence performance, and ammonia response performance of the prepared smart label all meet the requirements of this invention. Among them, the optimal parameters are 70° squeegee angle and 0.3 MPa printing pressure, which result in the best printing uniformity.

[0140] Example 9: Fluorescent smart tags for monitoring the freshness of prawns.

[0141] The fluorescent smart label (Cu NCs@L-Cys -1) for visually detecting food freshness printed in Example 1 was affixed to the top of the inside of a 100 mm petri dish (100 mm × 20 mm). A 50 g sample of prawns was stored in a constant temperature and humidity chamber at 4 ℃. The TVB-N standard value of the prawns stored for different days was determined according to the physicochemical indicators specified in the national standard GB5009.228—2016 to determine the degree of spoilage. The color change of the indicator under ultraviolet light was recorded.

[0142] Table 3. Effects of different storage times on TVB-N content and fluorescent tag color in prawns.

[0143]

[0144] Example 10: Fluorescent smart tags for monitoring the freshness of salmon.

[0145] The fluorescent smart label (Cu NCs@L-Cys -1) for visually detecting food freshness printed in Example 1 was cut and pasted onto the top of the inside of a 100 mm petri dish (100 mm × 20 mm). A 50 g salmon sample was stored with the label in a constant temperature and humidity chamber at 4 ℃. The TVB-N standard value of the salmon stored for different days was determined according to the physicochemical indicators specified in the national standard GB5009.228—2016 to determine the degree of spoilage. The color change of the indicator under ultraviolet light was recorded.

[0146] Table 4. Effects of different storage times on TVB-N content and fluorescent label color in salmon.

[0147]

[0148] Comparative Example 1: Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-9)

[0149] Step 1) is the same as in Example 1.

[0150] 2) Under room temperature and magnetic stirring, 4.5 g of PVP was dissolved in 50 mL of ethanol and magnetically stirred for 30 min. 0.5 g of Cu NCs@L-Cys-1 lyophilized powder was added to the PVP ethanol solution and magnetically stirred for 30 min. Then, the ultrasonic power was set to 180 W and the ultrasonic time was 120 min to promote the dissolution of PVP and the dispersion of Cu NCs@L-Cys. After the process was completed, Cu NCs@L-Cys@PVP-9 ink was obtained.

[0151] like Figure 13 As shown, the prepared Cu NCs@L-Cys@PVP-9 ink appears pale milky white under visible light and faintly orange-red under ultraviolet light. Compared with the Cu NCs@L-Cys@PVP-1 ink prepared in Example 1, the fluorescence intensity of the material decreased with the same PVP addition. Changes in the dispersion solvent have a significant impact on the fluorescence properties of the material.

[0152] Comparative Example 2: Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-10)

[0153] Step 1) is the same as in Example 1.

[0154] 2) Under room temperature and magnetic stirring, 4.5 g of PVP was dissolved in 50 mL of NN-dimethylformamide (DMF) and magnetically stirred for 30 min. 0.5 g of Cu NCs@L-Cys-1 lyophilized powder was added to the PVP-DMF solution and magnetically stirred for 30 min. Then, the ultrasonic power was set to 180 W and the ultrasonic time was 120 min to promote the dissolution of PVP and the dispersion of Cu NCs@L-Cys. After the process was completed, Cu NCs@L-Cys@PVP-10 ink was obtained.

[0155] like Figure 14 As shown, the prepared Cu NCs@L-Cys@PVP-10 ink appears pale milky white under visible light and a very faint orange-red under ultraviolet light. Compared with the Cu NCs@L-Cys@PVP-1 ink prepared in Example 1, the fluorescence intensity of the material decreased with the same PVP addition. Changes in the dispersion solvent have a significant impact on the fluorescence properties of the material.

[0156] Comparative Example 3: Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-11)

[0157] Step 1) is the same as in Example 1.

[0158] 2) Under room temperature and magnetic stirring, 4.5 g of PVP was dissolved in 50 mL of dimethyl sulfoxide (DMSO) and magnetically stirred for 30 min. 0.5 g of Cu NCs@L-Cys-1 lyophilized powder was added to the PVP-DMSO solution and magnetically stirred for 30 min. Then, the ultrasonic power was set to 180 W and the ultrasonic time was 120 min to promote the dissolution of PVP and the dispersion of Cu NCs@L-Cys. After the process was completed, Cu NCs@L-Cys@PVP-11 ink was obtained.

[0159] like Figure 15 As shown, the prepared Cu NCs@L-Cys@PVP-11 ink exhibits a pale yellow hue under visible light and shows almost no luminescence under ultraviolet light. Compared to the Cu NCs@L-Cys@PVP-1 ink prepared in Example 1, the fluorescence intensity of the material decreased with the same PVP addition. Changes in the dispersion solvent have a significant impact on the fluorescence properties of the material.

[0160] Comparative Example 4: Preparation of copper nanocluster ink (Cu NCs@L-Cys@PVP-12)

[0161] 0.606 g of L-cysteine ​​(L-Cys) was dispersed in 50 mL of isopropanol and dissolved by magnetic stirring to obtain a clear mixed solution one. Copper nitrate solution (0.1 M, 2 mL) was added to the clear mixed solution one. The solution after adding copper nitrate was then magnetically stirred for 30 min at room temperature, and the pH was adjusted to 5 to obtain the Cu NCs@L-Cys reaction solution. 4.5 g of PVP was added to the reaction solution, and the mixture was magnetically stirred for 30 min. Then, the ultrasonic power was set to 180 W and the ultrasonic time was 120 min to promote the dissolution of PVP and the dispersion of Cu NCs@L-Cys. After the ultrasonic treatment was completed, Cu NCs@L-Cys@PVP-12 ink was obtained.

[0162] like Figure 16 As shown, the prepared Cu NCs@L-Cys@PVP-12 ink appears pale milky white under visible light and faintly orange-red under ultraviolet light. Compared with the Cu NCs@L-Cys@PVP-1 ink prepared in Example 1, with the same PVP addition, omitting the freeze-drying and reconstitution step and directly using the stock solution to prepare the ink resulted in a decrease in the fluorescence intensity of the material. This indicates that the freeze-drying and reconstitution process has a significant impact on the fluorescence performance of the material. Freeze-drying-reconstitution is a key post-processing step that can significantly enrich and purify luminescent centers, thereby effectively improving the fluorescence quantum yield of nanoclusters in the final ink.

[0163] Table 5. Effects of different dispersants on ink performance

[0164]

[0165] Comparative Example 5: Preparation of Copper Nanocluster Ink (Cu NCs@SC@PVP-13)

[0166] 0.606 g of sodium citrate (SC) was dispersed in 50 mL of isopropanol and dissolved by magnetic stirring to obtain a clear mixed solution one. Copper nitrate solution (0.1 M, 2 mL) was added to the clear mixed solution one. The solution after adding copper nitrate was then magnetically stirred for 30 min at room temperature to obtain the Cu NCs@SC reaction solution. 4.5 g of PVP was added to the reaction solution, and the mixture was magnetically stirred for 30 min. Then, the ultrasonic power was set to 180 W and the ultrasonic time was 120 min to promote the dissolution of PVP and the dispersion of Cu NCs@SC. After the ultrasonic treatment, Cu NCs@SC@PVP-13 ink was obtained.

[0167] Compared to the Cu NCs@L-Cys@PVP-1 ink prepared in Example 1, the prepared Cu NCs@SC@PVP-13 ink showed no significant response to ammonia, and its fluorescence intensity was also lower. This indicates that the absence of key ligands affects fluorescence performance and the ammonia response mechanism. It further suggests that the thiol-amino synergistic effect of L-Cys is crucial for achieving high fluorescence and ammonia response.

[0168] Table 6. Effects of different ligands on ink performance

[0169]

[0170] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a self-aggregating luminescent metal nanoclusters fluorescent ink, characterized in that, Includes the following steps: 1) Synthesis of metal nanoclusters: The organic ligand is dissolved in water to form the first precursor solution; The copper salt is dissolved in water to form a second precursor solution; The first precursor solution and the second precursor solution were mixed, and a coordination reaction was carried out to form metal nanoclusters; the reaction solution was freeze-dried to obtain freeze-dried powder of metal nanoclusters. 2) Preparation of fluorescent ink: The polymer is dissolved in a polar solvent to form a polymer solution; the resulting freeze-dried metal nanoclusters are combined with the polymer solution and then ultrasonically dispersed to form a uniform and stable fluorescent ink, namely, a self-aggregating luminescent metal nanoclusters fluorescent ink.

2. The method for preparing the self-aggregating luminescent metal nanoclusters fluorescent ink according to claim 1, characterized in that, In step 1), the organic ligand is selected from at least one of L-cysteine, D-cysteine, glutathione, mercaptoethylamine, and penicillamine; the copper salt is selected from at least one of copper nitrate, copper sulfate, copper chloride, cuprous chloride, and copper acetate. The organic ligand and Cu 2+ The molar ratio is 35:1-10:

1.

3. The method for preparing the self-aggregating luminescent metal nanoclusters fluorescent ink according to claim 1, characterized in that, In step 1), the concentration of the first precursor solution is 0.2-0.8 M, the concentration of the second precursor solution is 0.1-0.3 M, and the volume ratio of the first precursor solution to the second precursor solution is 5:1-5:

3. The coordination reaction is carried out at a temperature of 20-30 °C, with the pH adjusted to 3-10, and for a time of 15-60 min.

4. The method for preparing the self-aggregating luminescent metal nanoclusters fluorescent ink according to claim 1, characterized in that, In step 1), the freeze-drying temperature is -60 to -50 °C, the vacuum degree is 5 to 15 Pa, and the drying time is 8 to 48 h.

5. The method for preparing the self-aggregating luminescent metal nanoclusters fluorescent ink according to claim 1, characterized in that, In step 2), the polymer is selected from at least one of polyvinylpyrrolidone, polyethylene glycol, hydroxypropyl methylcellulose, polyacrylate, and chitosan; The mass-to-volume ratio of the metal nanocluster powder to the polymer solution is 0.01-0.2:1 (g / mL).

6. The method for preparing the self-aggregating luminescent metal nanoclusters fluorescent ink according to claim 1, characterized in that, In step 2), the polar solvent is selected from at least one of methanol, ethanol, N,N-dimethylformamide, ethyl acetate, tetrahydrofuran, and isopropanol; the mass-to-volume ratio of the polymer to the polar solvent is 0.05~1:1 (g / mL). The ultrasonic dispersion has an ultrasonic power of 100-180 W and a duration of 30-120 min.

7. A self-aggregating luminescent metal nanoclusters fluorescent ink, characterized in that, It is prepared by the method described in any one of claims 1 to 6.

8. The application of the self-aggregating luminescent metal nanocluster fluorescent ink of claim 7 in food freshness detection, wherein the food freshness detection includes the detection of volatile basic nitrogen.

9. A fluorescent smart tag prepared using the self-aggregating luminescent metal nanoclusters fluorescent ink of claim 7.

10. The fluorescent smart tag according to claim 9, characterized in that, The preparation method of the fluorescent smart tag is as follows: The self-aggregating luminescent metal nanocluster fluorescent ink of claim 7 is coated or patterned on a substrate by screen printing, and after drying, a fluorescent smart label is formed and sealed for storage. The screen printing uses a screen with a mesh count of 200-400, a squeegee angle of 60°-75°, and a printing pressure of 0.2-0.4 MPa; the drying process is carried out at a temperature of 30-50 ℃ for 10-40 min.

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