A fluorescent film and a preparation method and application thereof

Abstract

The application relates to the field of food detection, in particular to a fluorescent film and a preparation method and application thereof. The fluorescent film comprises PVA, N-GQDs and MMT. The fluorescent film provided by the application can detect ammonia by fluorescence intensity and can detect the freshness of fish meat, has the advantages of fast response speed, simple operation, high detection precision and the like, and can solve the problem that the freshness is difficult to be quickly and nondestructively detected during fish meat storage and transportation, and provides a more effective technical means for quality monitoring and tracking of fish meat.

Description

Technical Field

[0001] This invention relates to the field of detection technology, and in particular to a fluorescent thin film, its preparation method, and its application. Background Technology

[0002] Fish is an important source of protein, and due to its low price and high nutritional value, it occupies an important part of the diet. However, because fish tissue cells are more porous than those of livestock meat, bacteria can easily invade the muscle through the gaps in the tissue, thus accelerating spoilage. Besides the loss of nutritional value, spoiled fish can also harm human health. Therefore, assessing the freshness of fish to prevent spoilage during storage and transportation is crucial.

[0003] Currently, there are many methods for detecting fish freshness, including electrochemical, optical, and biosensing methods, which are gradually replacing physicochemical and microbiological analyses to achieve simple and rapid detection. Ultraviolet-visible spectroscopy provides relatively intuitive observation results, but it is easily affected by natural light and is difficult to implement in portable applications. Electrochemical methods are widely used due to their low cost and high precision; however, their sensitivity is limited at low temperatures, making it difficult to detect ammonia at low temperatures. Fluorescence methods, due to their inherent excited-state response and high sensitivity to the environment, exhibit high detection sensitivity, making them an important tool in the sensor field.

[0004] Zhang et al. (DIO: 10.1016 / j.foodchem.2022.132521) developed an ammonia-sensitive fluorescent sensor based on an indium phosphide / zinc sulfide core-shell quantum dot and sodium roserate (InP / ZnS-SR) system to determine the freshness of fish meat by observing changes in fluorescence intensity and color. However, the InP / ZnS quantum dots in solution are prone to photobleaching when exposed to harsh excitation light sources, resulting in poor long-term stability. Khan et al. (DIO: 10.1016 / j.colsurfa.2023.131242) developed a pH-sensitive indicator using filter paper and green tea powder (GLP)-derived carbon dots (CDs) (CLP-CDs). They detected shrimp freshness and spoilage based on pH changes caused by total volatile basic nitrogen (TVB-N) concentration. However, the compounds produced during food spoilage are diverse, including acidic volatile compounds in addition to amines, which interfere with the pH-sensitive fluorescent membrane. Although there are many inventions for non-destructive testing of freshness, few of these sensors are insensitive to pH. Currently, there are no reports on using a novel hybrid membrane to effectively reduce quantum dot aggregation, increase the fluorescence intensity of the hybrid membrane, and achieve dynamic quenching of ammonia gas through photoinduced electron transfer for the detection of fish freshness. Summary of the Invention

[0005] This invention provides a fluorescent thin film, its preparation method, and its application.

[0006] Specifically, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides a fluorescent thin film comprising PVA, N-GQDs and MMT.

[0008] The fluorescent film provided by this invention involves polyvinyl alcohol (PVA), nitrogen-doped graphene quantum dots (N-GQDs), and montmorillonite (MMT). The layered structure of MMT provides loading sites for N-GQDs, enhancing their dispersibility. The N-GQDs interact and bind with MMT, improving the fluorescence intensity of the fluorescent film (PVA-N-GQDs / MMT hybrid film). In this fluorescent film, N-GQDs are intercalated into MMT, and N-GQDs / MMT are dispersed in PVA, exhibiting excellent fluorescence intensity and ammonia response. The fluorescence of the PVA-N-GQDs / MMT hybrid film of this invention can be quenched by ammonia via photoinduced electron transfer (PET). Due to the presence of electron acceptor groups (-COOH and -CONH2) on the surface of GQDs, the N-GQDs / MMT nanocomposite material becomes an electron acceptor. Ammonia molecules donate electrons through the lone pair of electrons on the nitrogen atom, thus causing the N-GQDs / MMT nanocomposite to undergo quenching upon photoexcitation, such as... Figure 3 As shown in the diagram. Under this mechanism, the hybrid membrane can more accurately reflect the freshness of fish meat through the fitting relationship between fluorescence intensity and ammonia. It can quantitatively detect the ammonia content during changes in fish meat quality. This method has superior fluorescence performance and accuracy, providing a more effective technical means for monitoring and tracking fish meat quality.

[0009] According to the fluorescent thin film provided by the present invention, the N-GQDs are intercalated in the MMT.

[0010] In this invention, N-GQDs aggregate in the solid state, which leads to a decrease in the fluorescence intensity of the film. By intercalating N-GQDs into the MMT structure through hydrogen bonding, the dispersibility can be improved, the aggregation of N-GQDs can be effectively reduced, and the fluorescence performance of the PVA-N-GQDs / MMT hybrid film can be significantly improved.

[0011] Preferably, the raw materials for the fluorescent film include N-GQDs solution, acidified MMT powder, and PVA solution.

[0012] Preferably, the ratio of the N-GQDs solution, acidified MMT powder, and PVA solution is 1-30 mL: 2-30 mg: 10-40 mL, and more preferably 1-20 mL: 2-26 mg: 10-30 mL.

[0013] More preferably, the concentration of the PVA solution is 2% to 10%, and the concentration of the N-GQDs solution is 1 to 20 mg / mL.

[0014] Further preferably, the MMT powder is acidified MMT powder treated with sulfuric acid.

[0015] In this invention, by using the above-mentioned N-GQDs solution, acidified MMT powder, and PVA solution, especially at the above-mentioned proportions, the dispersion of N-GQDs / MMT in PVA can be improved, thereby significantly improving the fluorescence intensity and response characteristics of the fluorescent film to ammonia compared with existing materials, further improving its fluorescence performance and accuracy, and enabling better quantitative detection of ammonia content during changes in fish meat quality.

[0016] Secondly, the present invention provides a method for preparing the above-mentioned fluorescent thin film, comprising:

[0017] 1) Mix the acidified MMT powder with the N-GQDs solution to obtain an N-GQDs / MMT suspension;

[0018] 2) The N-GQDs / MMT suspension is mixed with a PVA solution and dried to obtain a fluorescent film. The fluorescent film synthesized in this invention, namely the PVA-N-GQDs / MMT hybrid membrane, can be used as a freshness detection sensor in fish storage and transportation environments to achieve rapid and non-destructive detection of fish quality.

[0019] Preferably, in the preparation method of the fluorescent thin film provided by the present invention, in step 1), the ratio of the acidified MMT powder to the N-GQDs solution is 2-26 mg: 1-30 mL, preferably 8 mg: 10 mL.

[0020] Preferably, in step 1), the concentration of the N-GQDs solution is 1-20 mg / mL, more preferably 5 mg / mL; the pH of the N-GQDs solution is 6-8, more preferably 7.

[0021] Preferably, in step 1), the acidified MMT powder is acidified MMT powder treated with sulfuric acid.

[0022] Preferably, in step 2), the volume ratio of the N-GQDs / MMT suspension to the PVA solution is 10:1 to 20, preferably 1:1; the concentration of the PVA solution is 2% to 10%, preferably 6%.

[0023] Preferably, in step 2), the mixing is carried out by shaking, the shaking temperature is 70-90℃, preferably 80℃, and the shaking time is 20-100min, preferably 40min.

[0024] Preferably, in step 2), the drying is carried out in a petri dish at a temperature of 30–60°C, preferably 55°C.

[0025] Preferably, the thickness of the fluorescent film obtained in step 2) is 0.1 to 3.0 mm, and preferably the fluorescent film is cut to a size of 1 cm × 2 cm.

[0026] In this invention, by optimizing the acidified MMT powder, N-GQDs solution, PVA solution, and their proportions and conditions, the fluorescence intensity of the fluorescent film and its response characteristics to ammonia can be further improved.

[0027] Further preferably, the preparation of the N-GQDs solution includes: mixing an aqueous solution of salicylic acid and 2,6-pyridinedicarboxylic acid and reacting it under ultraviolet light irradiation, and then cooling it to obtain the N-GQDs solution;

[0028] The molar ratio of salicylic acid to 2,6-pyridinedicarboxylic acid in the aqueous solution is 5 to 15:1, preferably 45:4; the concentration of salicylic acid is 40 to 300 nM, preferably 90 nM; and the concentration of 2,6-pyridinedicarboxylic acid is 2 to 40 nM, preferably 8 nM.

[0029] Preferably, the ultraviolet lamp irradiation temperature is 30-90℃, and more preferably 55℃.

[0030] Preferably, the reaction time is 3 to 8 hours, and more preferably 2.5 hours.

[0031] Preferably, the cooling temperature is 2 to 15°C, and more preferably 6°C.

[0032] In this invention, by using the above-described reaction conditions and methods for preparing N-GQDs, and especially by controlling the reaction temperature and time, the synthesized quantum dots exhibit better fluorescence properties and water solubility within the above-described conditions.

[0033] Further preferably, the preparation of the acidified MMT powder includes: mixing MMT with sulfuric acid, heating and stirring, washing with water, and drying.

[0034] Preferably, the concentration of the sulfuric acid is 1 to 5 mol / L, and more preferably 2 mol / L.

[0035] Preferably, the ratio of MMT to sulfuric acid is 1-20g:1-2000mL, and more preferably 10g:1000mL.

[0036] Preferably, the heating temperature is 40-80℃, and more preferably 55℃.

[0037] Preferably, the stirring time is 38-52 hours, and more preferably 48 hours.

[0038] Preferably, the washing is performed until the pH value is 6-8, more preferably 7.

[0039] Preferably, the drying temperature is 20–60°C, and more preferably 40°C.

[0040] In this invention, sulfuric acid-acidified MMT powder, especially acidified MMT powder prepared using the above-mentioned method and conditions, can interact better with the above-mentioned N-GQDs compared with other materials, and at the same time, combined with polyvinyl alcohol, it further improves the fluorescence performance of the fluorescent film.

[0041] Thirdly, the present invention provides applications of the fluorescent film described above or the fluorescent film prepared by the method described above, particularly in the detection of ammonia or the freshness of fish.

[0042] Fourthly, the present invention provides a method for detecting ammonia, comprising using a fluorescent film prepared by the above-described fluorescent film or the above-described method for preparing a fluorescent film, contacting the ammonia to be detected with the fluorescent film, measuring the fluorescence emission peak intensity of the fluorescent film, and obtaining the content of the ammonia to be detected based on the fluorescence emission peak intensity.

[0043] In some specific embodiments, the ammonia gas to be detected is introduced into the fluorescent film, and the fluorescence emission peak intensity of the fluorescent film is obtained after 1 to 5 minutes.

[0044] According to an embodiment of the present invention, the method for detecting ammonia further includes the step of obtaining a calibration curve showing the correlation between the fluorescence emission peak intensity and the ammonia content to be detected.

[0045] According to an embodiment of the present invention, the ammonia gas to be detected can be obtained using conventional methods in the art during the process of obtaining the calibration curve.

[0046] According to an embodiment of the present invention, the fluorescence emission intensity of the fluorescent film can be obtained using a fluorescence spectrophotometer.

[0047] Fifthly, the present invention provides a method for detecting the freshness of fish meat, comprising: using the fluorescent film prepared by the above-mentioned method or the method for preparing the fluorescent film, placing the fluorescent film and the fish meat sample to be tested in a sealed container, obtaining the fluorescence emission peak intensity of the fluorescent film, and obtaining the freshness of the fish meat sample to be tested based on the fluorescence emission peak intensity.

[0048] Preferably, the fish sample to be tested is a freshwater fish, including carp and trout; or a saltwater fish, including salmon and tuna.

[0049] Preferably, the fish sample to be tested and the fluorescent film are kept out of contact in a sealed container.

[0050] Preferably, the sealed container is placed in a constant temperature environment, preferably -5 to 5°C or 20 to 30°C, more preferably -2°C or 20°C, such as using a constant temperature chamber.

[0051] Preferably, the intensity of the fluorescence emission peak of the fluorescent film is acquired at regular intervals, preferably every 1 to 5 days or 2 to 10 hours, such as every 4 days or 3 hours.

[0052] Further preferably, in the method for detecting the freshness of fish meat, the detection conditions for obtaining the fluorescence emission peak intensity include: the excitation light λex is 350-370 nm, preferably 365 nm; the slit width is 2-7 nm, preferably 3 nm.

[0053] Preferably, the process also includes the calibration of the fluorescent film, using an ammonia concentration of 0–70 ppm and a reaction time of 0–5 min, preferably 3 min.

[0054] The beneficial effects of this invention are at least as follows: The fluorescent film prepared by this invention has advantages such as good fluorescence performance, high detection accuracy, and simple preparation. It can detect ammonia through fluorescence intensity and can quantitatively and qualitatively detect the freshness of fish. The fluorescent film reacts with volatile components produced by fish spoilage, resulting in changes in the intensity of the fluorescence emission peak. During the storage and transportation of fish, the freshness of the fish can be accurately assessed based on the changes in the intensity of the fluorescence emission peak, enabling rapid and non-destructive detection of fish freshness. Compared with existing fluorescent films, this fluorescent film utilizes hydrogen bonding to load N-GQDs onto the MMT surface, effectively reducing their aggregation and significantly improving fluorescence performance. The hybrid membrane is insensitive to pH, eliminating interference from acidic volatile compounds during fish spoilage, and possesses high sensitivity and accuracy, providing an effective technical means for fish quality monitoring and tracking. Attached Figure Description

[0055] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0056] Figure 1 The image shows the particle size distribution (left) and the lattice division (right) of the N-GQDs in the embodiment of the present invention via transmission electron microscopy.

[0057] Figure 2 The images show the ultraviolet spectra of N-GQDs / MMT and N-GQDs in the embodiments of the present invention.

[0058] Figure 3This is a schematic diagram of the PVA-N-GQDs / MMT hybrid film in an embodiment of the present invention;

[0059] Figure 4 The fluorescence intensity of N-GQDs at different pH values ​​in the embodiments of the present invention (λex = 365 nm);

[0060] Figure 5 This is a stability curve of the PVA-N-GQDs / MMT hybrid film in an embodiment of the present invention;

[0061] Figure 6 The fluorescence spectrum (λex = 365 nm) of the PVA-N-GQDs / MMT hybrid film in the embodiments of the present invention after reacting for 3 min in ammonia samples of different concentrations;

[0062] Figure 7 This is a linear relationship curve between the fluorescence intensity and ammonia concentration of the PVA-N-GQDs / MMT hybrid membrane in this embodiment of the invention after different concentrations of ammonia gas are introduced;

[0063] Figure 8 This is a fluorescence intensity diagram of the PVA-N-GQDs fluorescent film in an embodiment of the present invention after different concentrations of ammonia gas were introduced;

[0064] Figure 9 The image shows the fluorescence intensity of the PVA-N-GQDs / 12mgMMT hybrid membrane in the comparative example of this invention after different concentrations of ammonia gas were introduced. Detailed Implementation

[0065] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0066] Unless otherwise specified, specific techniques or conditions in the embodiments of this invention shall be performed in accordance with the techniques or conditions described in the literature in this field, or in accordance with the product instructions. Devices, instruments, reagents, etc., whose manufacturers are not specified are all conventional products that can be purchased through legitimate channels. All raw materials used in this invention are readily available in the domestic market.

[0067] Example 1

[0068] This embodiment provides a PVA-N-GQDs / MMT hybrid film. The optimization of the synthesis conditions for the PVA-N-GQDs / MMT hybrid film includes:

[0069] 1. Prepare an aqueous solution of N-GQDs with a pH of 4 to 11. Add 50 μL of 1 mol / L ammonia solution to the solution. Under the condition of excitation wavelength of 365 nm, record the changes in the fluorescence emission spectrum of GQDs every 1 min, as follows: Figure 4 As shown, the fluorescence intensity is optimal at pH=7, so pH=7 was chosen as the experimental environment.

[0070] 2. Under pH=7 conditions, prepare N-GQDs solutions with a concentration of 1-20 mg / mL, and record the fluorescence emission spectrum of the solutions. The fluorescence intensity is maximum at 5 mg / mL, which is the optimal concentration of the N-GQDs solution.

[0071] 3. By recording the fluorescence emission spectra (λex = 365 nm) of three fluorescent films, namely PVA-N-GQDs / 6 mg MMT, PVA-N-GQDs / 8 mg MMT and PVA-N-GQDs / 12 mg MMT, the fluorescence intensity of PVA-N-GQDs / 8 mg MMT was the strongest, that is, the optimal MMT content in the PVA-N-GQDs / MMT hybrid film is 8 mg.

[0072] Example 2

[0073] This embodiment provides the synthesis steps of the PVA-N-GQDs / MMT hybrid film used:

[0074] 1) A mixed solution of salicylic acid (90 nM) and 2,6-pyridinedicarboxylic acid (8 nM) was irradiated with UV light at 55 °C for 2 hours at a molar ratio of 45:4, and then rapidly cooled to 5 °C to obtain a pale yellow nitrogen-doped graphene quantum dot (N-GQDs) solution (5 mg / mL, pH = 7). Figure 1 As shown in the left image, the N-GQDs are spherical and disperse well in aqueous solution. The particle size distribution of the N-GQDs ranges from 4.5 to 7.0 nm, with the largest proportion (over 90%) being quantum dots with a diameter of 6.0-6.5 nm, exhibiting a normal distribution. Figure 1 As shown in the right image, the N-GQDs have a distinct lattice with a lattice spacing of 0.28 nm.

[0075] 2) Add 10g of MMT to 1000mL of sulfuric acid (2.0M) and stir at 55°C for 48 hours. Subsequently, the etched MMT is washed with deionized water until the pH reaches neutral. Finally, the sample is dried at 35°C and powdered.

[0076] 3) Add 8 mg of etched MMT powder to 10 mL of N-GQDs solution (5 mg / mL, pH=7), and mix to obtain N-GQDs / MMT suspension.

[0077] 4) Add the above N-GQDs / MMT suspension to 10 mL of 6% PVA solution, and then shake well at 80 °C for 40 min. Subsequently, transfer the mixture to a petri dish and dry at 55 °C to obtain the PVA-N-GQDs / MMT hybrid membrane. Figure 2 As shown, a new absorption peak was observed at a wavelength of 314 nm. This phenomenon is thought to be caused by hydrogen bonding. This result further confirms the successful synthesis of N-GQDs / MMT.

[0078] Example 3

[0079] This embodiment provides a calibration method for the PVA-N-GQDs / MMT hybrid film, including the following steps:

[0080] 1. Take 1×2 prepared fluorescent films and place them in a petri dish. Introduce ammonia of different concentrations into the petri dish and detect the changes in fluorescence intensity of the fluorescent films. The concentrations of ammonia are 0, 20, 30, 40, 60, and 70 ppm.

[0081] 2. Measurement of fluorescence intensity: Using an Edinburgh FS5 fluorescence spectrophotometer with an excitation slit width of 3.0 nm and an emission slit width of 3.0 nm, the fluorescence emission spectra of the fluorescent film under different concentrations of ammonia were recorded. Figure 6 And the peak fluorescence emission intensity Imax at the optimal emission spectrum.

[0082] 3. Fit the peak fluorescence intensity of the fluorescent film to the ammonia concentration, such as... Figure 7 As shown in the figure, the fluorescent film of Example 2 exhibits a good linear response to ammonia.

[0083] Calibration curve: y = -14.32x + 1928.55;

[0084] Where y represents the fluorescence intensity after the addition of ammonia, and x represents the concentration of ammonia.

[0085] The fluorescent film was placed in a petri dish, and 10 ppm ammonia was introduced. After 3 min, the fluorescence intensity Ia was recorded as 1779 (au).

[0086] Substituting Ia into the calibration curve described in Example 3, the concentration of ammonia was calculated to be 10.44 ppm. The calculation showed that the error of the method proposed in this invention was within 5.0%. Figure 5 It can be seen that the PVA-N-GQDs / MMT hybrid film has good stability.

[0087] Example 4

[0088] This embodiment provides a method for detecting the freshness of fish meat, including the following steps:

[0089] 1) Fish samples were collected from fresh trout with proper quality assurance.

[0090] TVB-N testing of fish meat: The determination was carried out according to the semi-micro nitrogen determination method in the micro diffusion method of GB 5009.228-2016 "National Food Safety Standard - Determination of Volatile Basic Nitrogen in Food".

[0091] 2) Place the fish meat sample and the fluorescent film from Example 1 together in a sealed container and place them at different temperatures (20°C and -2°C).

[0092] 3) The fluorescence intensity (λex = 365 nm, λem = 440 nm) and total volatile basic nitrogen (TVB-N) content were measured every 4 hours and 3 days, respectively.

[0093] The specific operation is as follows: Measurement of fluorescence emission spectrum intensity I: The fluorescence emission intensity I of the sample at the excitation wavelength of λex = 365 nm and the optimal emission spectrum λem = 440 nm was recorded using a fluorescence spectrophotometer (same as in Example 3).

[0094] 4) Plot the peak intensity Imax of the fluorescence emission spectrum of the fluorescent film against the correction curve of TVB-N content in trout.

[0095] 5) The fluorescence intensity of the hybrid membrane is linearly correlated with the TVB-N content accumulated in trout during storage at different temperatures (20℃ and -2℃). The freshness of fish can be detected by detecting fluorescence intensity instead of traditional physicochemical indicators.

[0096] Example 5

[0097] Fluorescent films (PVA-N-GQDs / MMT hybrid films) were prepared using the same method as in Example 2, except that step 3) was performed by increasing the MMT powder from 8 mg to 12 mg and adding different concentrations of ammonia (0-70 ppm) to the pre-cut PVA-N-GQDs / MMT films, and then measuring their fluorescence intensity.

[0098] The results show that, compared with Example 2, the accuracy of the MMT excess film is poor, such as... Figure 8 As shown.

[0099] Example 6

[0100] Fluorescent films (PVA-N-GQDs / MMT) were prepared using the same method as in Example 2. The fluorescence intensity of the films was measured continuously for 6 weeks, and the change rate was consistently within 10%, demonstrating that the fluorescent film exhibits good stability. Figure 5As shown. Due to the layered structure of MMT, it possesses high porosity and surface area, providing numerous binding sites for N-GQDs for adsorption and immobilization, effectively enhancing the stability of quantum dots.

[0101] Example 7

[0102] The fluorescent film (PVA-N-GQDs / MMT) was prepared using the same method as in Example 2, except that in step 3), 8 mg of etched MMT powder was added to 10 mL of N-GQDs solution (5 mg / mL, pH = 9), and after mixing, an N-GQDs / MMT suspension was obtained.

[0103] The results showed that, compared with Example 2, the fluorescence intensity of PVA-N-GQDs / MMT was poor at pH=9.

[0104] Comparative Example 1

[0105] This comparative example provides a method for detecting fish freshness based on indium phosphide / zinc sulfide core-shell quantum dots and sodium roserite (CN114397281A). The method for detecting fish freshness provided in invention CN114397281A uses an MPA-InP / ZnS QDs-SR aqueous solution to detect ammonia. However, liquid fluorescence sensors suffer from poor portability and face challenges in integration with electronic devices. The fluorescent thin-film sensor proposed in this invention overcomes the limitations of liquid fluorescence sensors.

[0106] Comparative Example 2

[0107] Fluorescent films (PVA-N-GQDs) were prepared using the same method as in Example 1, except that they contained only N-GQDs quantum dots. Ammonia water of different concentrations (0-70 ppm) was added to the pre-cut PVA-N-GQDs films, and their fluorescence intensity was measured.

[0108] The results showed that only graphene quantum dot fluorescent films exhibited poor fluorescence intensity and low precision, such as... Figure 9 As shown.

[0109] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fluorescent thin film, characterized in that, The fluorescent film comprises PVA, N-GQDs, and MMT; the fluorescent film is prepared by the following method, including: 1) mixing acidified MMT powder with an N-GQDs solution to obtain an N-GQDs / MMT suspension; 2) mixing the N-GQDs / MMT suspension with a PVA solution and drying to obtain the fluorescent film; the preparation of the N-GQDs solution includes: mixing an aqueous solution of salicylic acid and 2,6-pyridinedicarboxylic acid and reacting it under ultraviolet light irradiation, then cooling to obtain the N-GQDs solution; wherein, the molar ratio of salicylic acid to 2,6-pyridinedicarboxylic acid in the aqueous solution is 5~15:1, the concentration of salicylic acid is 40~300 nM, the concentration of 2,6-pyridinedicarboxylic acid is 2~40 nM; the ultraviolet light irradiation temperature is 30~90℃; the reaction time is 3~8h; and the cooling temperature is 2~15℃.

2. The fluorescent thin film according to claim 1, characterized in that, The N-GQDs are intercalated in the MMT; And / or, the raw materials of the fluorescent film include N-GQDs solution, acidified MMT powder, and PVA solution; wherein, the ratio of N-GQDs solution, acidified MMT powder, and PVA solution is 1~30mL:2~30mg:10~40mL; the concentration of PVA solution is 2%~10%, the concentration of N-GQDs solution is 1~20mg / mL, and the acidified MMT powder is sulfuric acid-treated acidified MMT powder.

3. The method for preparing the fluorescent thin film according to claim 1 or 2, characterized in that, include: 1) Mix the acidified MMT powder with the N-GQDs solution to obtain an N-GQDs / MMT suspension; 2) The N-GQDs / MMT suspension was mixed with a PVA solution and dried to obtain a fluorescent film; The preparation of the N-GQDs solution includes: mixing an aqueous solution of salicylic acid and 2,6-pyridinedicarboxylic acid and reacting it under ultraviolet light irradiation, followed by cooling to obtain the N-GQDs solution; wherein, the molar ratio of salicylic acid to 2,6-pyridinedicarboxylic acid in the aqueous solution is 5~15:1, the concentration of salicylic acid is 40~300 nM, and the concentration of 2,6-pyridinedicarboxylic acid is 2~40 nM; the ultraviolet light irradiation temperature is 30~90℃; the reaction time is 3~8h; and the cooling temperature is 2~15℃.

4. The method for preparing a fluorescent thin film according to claim 3, characterized in that, In step 1), the ratio of acidified MMT powder to N-GQDs solution is 2~26mg:1~30mL; And / or, in step 1), the concentration of the N-GQDs solution is 1~20 mg / mL; the pH of the N-GQDs solution is 6~8; And / or, in step 1), the acidified MMT powder is sulfuric acid-treated acidified MMT powder; And / or, in step 2), the volume ratio of the N-GQDs / MMT suspension to the PVA solution is 10:1~20; the concentration of the PVA solution is 2%~10%; And / or, in step 2), the mixing is carried out by shaking, the shaking temperature is 70~90℃, and the shaking time is 20~100min; And / or, in step 2), the drying is carried out in a petri dish at a temperature of 30~60℃.

5. The method for preparing a fluorescent thin film according to claim 3 or 4, characterized in that, The preparation of the acidified MMT powder includes: mixing MMT with sulfuric acid, heating and stirring, washing with water, and drying; Wherein, the concentration of sulfuric acid is 1~5 mol / L; and / or, the ratio of MMT to sulfuric acid is 1~20 g: 1~2000 mL; and / or, the heating temperature is 40~80℃; and / or, the stirring time is 38~52 h; and / or, the washing is carried out until the pH value is 6~8; and / or, the drying temperature is 20~60℃.

6. The application of the fluorescent thin film prepared by the method of claim 1 or 2 or any one of claims 3-5, characterized in that, Applications in ammonia detection or fish freshness detection.

7. A method for detecting ammonia, characterized in that, include: A fluorescent film prepared by the method of claim 1 or 2 or any one of claims 3-5 is used to contact the ammonia gas to be detected with the fluorescent film to obtain the fluorescence emission peak intensity of the fluorescent film, and the content of the ammonia gas to be detected is obtained based on the fluorescence emission peak intensity.

8. A method for detecting the freshness of fish meat, comprising: A fluorescent film prepared by the method of claim 1 or 2 or any one of claims 3-5 is used to prepare a fluorescent film. The fluorescent film and the fish meat sample to be tested are placed in a sealed container to obtain the fluorescence emission peak intensity of the fluorescent film. The freshness of the fish meat sample to be tested is obtained based on the fluorescence emission peak intensity.

9. The method for detecting the freshness of fish meat according to claim 8, characterized in that, The detection conditions for the fluorescence emission peak intensity include: excitation light λex of 350~370nm; slit width of 2~7nm.

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