A fluorescent film and a preparation method and application thereof

Abstract

The present application relates to the technical field of detection, and particularly relates to a fluorescent film, a preparation method and application thereof. The fluorescent film is prepared from a mixed precursor solution containing carbon quantum dots and a polyimide precursor, and the carbon quantum dots are prepared by using fluorescein isothiocyanate as a carbon source. The enhanced fluorescent film provided by the present application has a significantly enhanced fluorescence intensity when exposed to ammonia, and the fluorescence intensity has a good linear response to the ammonia concentration within 0-8 ppm, so that the freshness of fish meat and the like can be well detected, and the enhanced fluorescent film has the advantages of fast response, strong anti-interference, high precision and the like, and can solve the technical problem that the freshness of fish meat is difficult to be quickly and non-destructively detected during storage and transportation, and provide 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] Aquatic products play a vital role in the human food supply. However, due to their high water activity, abundant microbial growth, and near-neutral pH, aquatic products are highly susceptible to spoilage during storage and transportation. Spoilage of fish and meat not only leads to economic losses but also seriously endangers consumer health. With increasingly sophisticated dietary demands, ensuring the safe delivery of aquatic products to consumers is a pressing issue that needs to be addressed.

[0003] Volatile basic nitrogen (TVB-N), including volatile nitrogen compounds such as ammonia (NH3), dimethylamine, and trimethylamine, is an internationally recognized indicator of fish freshness, and its rapid and non-destructive detection has always been a scientific challenge in the field of food safety. Fluorescence methods, with their flexible design and ease of integration, have been widely applied in food testing in recent years. Fluorescence methods are generally divided into fluorescence quenching and fluorescence enhancement. CN113640258B proposes a thin-film fluorescence sensor based on N,S co-doped carbon quantum dots, gelatin, and Nafion solution. The freshness of fish is determined by changes in fluorescence intensity during storage, achieving rapid and non-destructive detection. However, as a fluorescence quenching sensor, its fluorescence intensity change is not significant in the later stages, leading to reduced accuracy. While fluorescence quenching is simple to implement, potential quenching agents in the environment can cause false positives, and the fluorescence intensity continuously decreases with increasing target analyte concentration, which is detrimental to subsequent circuit detection.

[0004] Fluorescence enhancement methods utilize the fluorescence activation of target analytes to improve detection sensitivity and selectivity. In ammonia detection, fluorescein isothiocyanate (FITC) is a widely used fluorescence enhancement compound. For example, CN117430844A and CN104974344B have prepared fluorescent composite membranes based on the pH sensitivity of FITC to detect parameters such as pH. However, these fluorescent composite membranes are only highly sensitive to pH values ​​between 4 and 9. To improve the sensitivity of FITC as a fluorescence detection unit, CN117723524A and CN106085410A have mixed FITC and carbon quantum dots to construct ratiometric fluorescence detection methods, effectively improving the detection range and sensitivity. However, these sensors are complex to operate as they use two fluorescent materials. Furthermore, FITC has potential carcinogenicity and poses a risk of migration and leakage in food detection.

[0005] In summary, whether fluorescence quenching or fluorescence enhancement type, current meat freshness detection sensors suffer from problems such as complex structure, low sensitivity, and poor safety. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides an enhanced fluorescent film, its preparation method, and its applications. The (enhanced) fluorescent film prepared by this invention uses bio-nontoxic carbon quantum dots (CQDs) as the fluorescence-sensitive material, enabling better detection of fish freshness and ammonia content in water samples. It possesses advantages such as non-toxicity, rapid response, strong anti-interference ability, and simple preparation, allowing for faster and non-destructive detection of fish freshness during storage and transportation, and providing a more effective technical means for fish quality monitoring and tracking.

[0007] In a first aspect, the present invention provides a fluorescent film prepared from a mixed precursor solution containing carbon quantum dots and a polyimide precursor, wherein the carbon quantum dots are prepared using fluorescein isothiocyanate as the carbon source.

[0008] In this invention, CQDs are prepared using a specific FITC as the sole carbon source. These CQDs retain the π-electron conjugated structure of FITC well and contain a large number of phenolic hydroxyl functional groups on their surface. Therefore, under acidic conditions, the phenolic hydroxyl groups in the CQDs remain in the -OH form, limiting the electron delocalization of the π-conjugated structure and resulting in low fluorescence intensity. However, when the pH increases, the phenolic hydroxyl groups form phenoxy anions under the action of hydroxide ions, significantly expanding the π-electron delocalization range and significantly enhancing the fluorescence intensity. Moreover, this phenomenon is reversible: after re-acidification, the CQDs return to their weak fluorescence state.

[0009] NH3 is a typical Lewis basic gas that readily undergoes an electrolytic reaction with water molecules (Equation 1) to produce hydroxide ions. Therefore, when CQDs are exposed to ammonia, the hydroxide ions produced by the reaction of ammonia with water molecules in the air or aqueous solution enhance the fluorescence intensity of the CQDs. The detection principle diagram is shown below. Figure 1 As shown.

[0010]

[0011] The enhanced fluorescent film provided by this invention can significantly enhance fluorescence intensity when exposed to ammonia gas, and the fluorescence intensity has a good linear response to ammonia concentration within 0~8ppm. This enhanced fluorescent film can better detect the freshness of fish meat, providing a more effective technical means for fish meat quality monitoring and tracking. The fluorescent film has the advantages of being non-toxic, fast-responding, highly resistant to interference, and highly accurate.

[0012] Preferably, the mixed precursor solution includes a CQDs solution and a PI precursor solution.

[0013] Preferably, the volume ratio of the CQDs solution to the PI precursor solution is 1~10:100~1000, for example 2:450, 2:300, 2:190, 4:300, 4:250, 4:200, 5:300, 5:180, 5:160, 5:240, 5:190, 5:200, 5:210, etc., more preferably 2~5:150~500, and more preferably 5:160~240.

[0014] Preferably, the pH value of the mixed precursor solution is 6.0 to 7.5.

[0015] Preferably, the concentration of the PI precursor solution is 5~25 wt%, such as 6wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 14wt%, 16wt%, 18wt%, 22wt%, 24wt%, etc.

[0016] Preferably, the concentration of the CQDs solution is 0.1~1.0 mg / mL, for example 0.1 mg / mL, 0.2 mg / mL, 0.3 mg / mL, 0.4 mg / mL, 0.5 mg / mL, 0.6 mg / mL, 0.7 mg / mL, 0.8 mg / mL, 0.9 mg / mL, 1.0 mg / mL, etc.

[0017] Preferably, the concentration of the PI precursor solution is 5-20 wt%; and / or the concentration of the CQDs solution is 0.5-0.8 mg / mL.

[0018] Preferably, the carbon quantum dots are prepared by hydrothermal reaction using fluorescein isothiocyanate as the carbon source; the hydrothermal reaction temperature is preferably 175~185℃ and the time is preferably 10~14 h.

[0019] In this invention, by using the pH value and concentration of the CQDs solution and the PI precursor solution polyamic acid with the above-mentioned mixed precursor solution, especially at the above-mentioned preferred ratio, it is helpful for the dispersion of CQDs in the PI film, significantly reduces fluorescence-induced aggregation quenching, and facilitates the fluorescence activation of subsequent ammonia detection.

[0020] Secondly, the present invention also provides a method for preparing the above-mentioned fluorescent thin film, comprising:

[0021] 1) Using FITC as the carbon source, a hydrothermal reaction was carried out to obtain a CQDs solution.

[0022] 2) Mix the CQDs solution with the PI precursor solution and adjust the pH to 6.0~7.5 to obtain the precursor solution.

[0023] 3) The precursor solution is coated onto the substrate and subjected to high-temperature treatment.

[0024] This invention uses FITC as a carbon source and prepares CQDs solution via hydrothermal method. The prepared CQDs solution is added to PI precursor solution. The precursor solution is coated on a substrate and polyimide is cured by high temperature treatment to obtain an enhanced fluorescent PI film containing CQDs. This fluorescent film has excellent properties such as fast response, strong anti-interference, and high precision.

[0025] Preferably, in step 1), FITC is mixed with a solvent to obtain a FITC solution; the FITC solution is subjected to a hydrothermal reaction and dialyzed with ethanol as the dialysate to obtain a CQDs solution; the concentration of the CQDs is 0.1~1.0 mg / mL.

[0026] Preferably, in step 1), the concentration of the FITC solution is 0.2~1.0 mg / mL; preferably, the solvent is an ethanol solution with a concentration of 60%~90%.

[0027] Preferably, the hydrothermal reaction is carried out at a temperature of 175-185°C for 10-14 hours.

[0028] Preferably, the dialysis uses a dialysis bag with a diameter of 500-1000 D, the concentration of ethanol is 50%-75%, and the dialysis solution is preferably changed every 10-14 hours, with a dialysis time of 45-50 hours.

[0029] In this invention, a 60%–90% ethanol solution is used as the solvent for the hydrothermal reaction of FITC, which improves the solubility of FITC and avoids agglomeration caused by the low solubility of FITC in water during the hydrothermal reaction. Simultaneously, ethanol is oxidized to acetaldehyde at high temperature, participating in the surface modification of CQDs, effectively inhibiting carbon core agglomeration and facilitating the formation of small-sized carbon dots. Through the above optimization of hydrothermal reaction and dialysis conditions, CQDs with excellent fluorescence properties and pH sensitivity can be better prepared. These CQDs are then uniformly dispersed in the PI precursor solution, ultimately yielding a PI film with superior fluorescence properties.

[0030] Preferably, in step 2), the volume ratio of the CQDs solution to the PI precursor solution is 1~10:100~1000; preferably, the CQDs solution and the PI precursor solution are mixed and ultrasonically treated for 15~30 min.

[0031] Preferably, the pH of the CQDs solution and the PI precursor solution is adjusted to 6.0-7.5 after mixing. More preferably, the pH is adjusted to 6.0-7.5 using 0.1 mol / L H2SO4 and NaOH solutions.

[0032] In this invention, the CQDs surface is rich in -COOH and -OH functional groups, while the PI precursor contains a large number of carboxyl groups (-COOH). The two form a stable composite system through multiple hydrogen bonds. Therefore, the enhanced fluorescent film prepared by dispersing CQDs in a PI precursor solution to form hydrogen bonds, followed by dehydration condensation of the mixture, can effectively utilize the hydrogen bonding between CQDs and the PI precursor to achieve uniform dispersion while preserving the pH sensitivity and fluorescence properties of CQDs through physical embedding. This reduces the quenching effect of CQDs' induced aggregation in the solid matrix, thereby further improving the fluorescence performance and pH sensitivity of the fluorescent film, and ultimately enhancing the sensitivity and repeatability of ammonia detection.

[0033] Preferably, the method further includes the preparation of the PI precursor solution, which can be prepared using conventional methods in the art. The method provided by this invention includes: mixing a diamine monomer, a dianhydride monomer, and an organic solvent, and stirring in a water bath at 45-50°C for 2-4 hours; wherein the molar ratio of the diamine monomer to the dianhydride monomer is 0.5-2:1; and the diamine monomer is selected from 4,4'-diaminodiphenyl ether (ODA), p-phenylenediamine (PDA), 2,2-bis(4-aminophenyl)propane (BAPP), 4, The dianhydride monomer is selected from one or more of trimellitic anhydride acyl chloride (PMDA), 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BPDA), hexafluorodianhydride (6FDA), phthalic anhydride acyl chloride (BTDA), and tetrahydrobenzoic anhydride (THPA); the organic solvent is N,N-dimethylformamide (DMF) or N-methylpyrrolidone (NMP).

[0034] Preferably, in step 3), the coating is applied by spin coating, with a preferred spin coating speed of 3000~4000 rpm and a time of 30~60 s.

[0035] Preferably, the high-temperature treatment is performed at a temperature of 210~230℃ for 1~2 hours.

[0036] Further preferably, the high-temperature treatment includes a pre-drying treatment; the pre-drying treatment is performed at a temperature of 110~130℃ for a time of 4~6 hours.

[0037] Preferably, the method further includes annealing the fluorescent film precursor obtained in step 3) to imidize the precursor polyamic acid.

[0038] Further preferably, the annealing conditions include: heating from 20-30°C to 75-85°C for 25-35 minutes; then heating to 110-130°C for 25-35 minutes; then heating to 210-230°C for 110-130 minutes, followed by holding at this temperature for 5-7 hours, and finally allowing natural cooling. In this invention, by pre-baking, high-temperature treatment, annealing, and optimizing the treatment conditions, the fluorescent film possesses superior mechanical properties and stability, which helps to further extend the film's lifespan and reduce detection costs.

[0039] Thirdly, the present invention provides applications of the fluorescent film described above or the fluorescent film prepared by the above method, particularly in the detection of ammonia content in water or the detection of freshness of preferred aquatic fish meat.

[0040] The beneficial effects of this invention are at least as follows: The fluorescent film prepared by this invention uses FITC as the sole carbon source to prepare CQDs, which has advantages such as good fluorescence performance, high detection accuracy, simple preparation, and non-toxicity. It can detect ammonia by fluorescence intensity and can quantitatively and qualitatively detect the freshness of fish meat. Compared with existing fluorescent films, this fluorescent film detects ammonia based on fluorescence "on," which can eliminate the influence of other interfering factors and has the advantage of strong anti-interference. In addition, this fluorescent film detects ammonia by pH change, which is reversible and can be reused, reducing detection costs. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in this 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 some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0042] Figure 1 This is a schematic diagram illustrating the reaction principle of the CQDs synthesized in Example 1 of the present invention for detecting ammonia.

[0043] Figure 2 This is a transmission electron microscope image of CQDs in Embodiment 1 of the present invention.

[0044] Figure 3 This is a particle size distribution diagram of CQDs in Example 1 of the present invention.

[0045] Figure 4 This is the optical characterization spectrum of the fluorescent PI film in Example 2 of the present invention.

[0046] Figure 5 This is a linear relationship curve between the fluorescence intensity of the PI film and the ammonia concentration after adding different concentrations of ammonia in Example 3 of the present invention.

[0047] Figure 6 This is a graph showing the fluorescence intensity changes of the PI membrane cyclically exposed to ammonia and HCl vapor in Example 6 of the present invention.

[0048] Figure 7 This is a linear relationship curve between the fluorescence intensity of the PI membrane and the ammonia concentration after exposure to different concentrations of ammonia gas in Comparative Example 2 of the present invention.

[0049] Figure 8 This is a graph showing the change in fluorescence intensity of the PI film in Comparative Example 3 of the present invention after exposure to different concentrations of ammonia gas.

[0050] Figure 9 This is a graph showing the change in fluorescence intensity of the PI membrane in Comparative Example 4 of the present invention after exposure to different concentrations of ammonia gas.

[0051] Figure 10 This is a graph showing the change in fluorescence intensity of the PI membrane in Comparative Example 5 of the present invention after exposure to different concentrations of H2S. Detailed Implementation

[0052] 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.

[0053] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0054] Unless otherwise specified, the techniques or conditions described in the literature of this invention shall apply, or the product instructions shall be followed. Devices, instruments, reagents, etc., whose manufacturers are not specified, are all conventional products that can be purchased from legitimate channels. All experimental reagents and raw materials involved are commercially available, and all reagents are analytical grade products.

[0055] Example 1

[0056] The CQDs provided in this embodiment are synthesized as follows: FITC is dissolved in ethanol (75%) to prepare a solution with a mass concentration of 0.5 mg / mL. This solution is placed in a 100 mL hydrothermal reactor and reacted at 180°C for 12 hours. After cooling, the solution is placed in a 500-1000 D dialysis bag, using ethanol (55%) as the dialysate. The dialysate is replaced every 12 hours, and dialyzing is performed for 48 hours to remove unreacted substances, yielding a high-purity CQDs solution. The transmission electron microscopy (TEM) image of the CQDs is shown below. Figure 2 As shown, it is uniformly dispersed in the aqueous solution, and the particle size distribution is statistically significant (…). Figure 3 The average particle size of the CQDs is 2.25 nm. The UV-Vis spectrum, fluorescence excitation and emission patterns of the CQDs are shown below. Figure 4 As shown, the CQDs appear pale yellow under sunlight and emit yellow-green fluorescence under ultraviolet light. Fluorescence spectroscopy tests indicate that the optimal excitation wavelength for these CQDs is 270 nm, and the optimal emission wavelength is 520 nm.

[0057] Example 2

[0058] This embodiment provides a fluorescent PI film, the preparation method of which is as follows:

[0059] 1) Dissolve 10 g of 6FDA and ODA in 90 g of DMF at a mass ratio of 1:1. Stir slowly in a water bath at 50°C for 3 h until completely dissolved into a transparent and homogeneous solution to obtain a 10% PI precursor solution.

[0060] 2) Using the CQDs prepared in Example 1 as raw material, a solution with a concentration of 0.5 mg / mL was prepared and added to the PI precursor solution at a volume ratio of 5:200. The pH of the above mixed solution was adjusted to 7.0 with 0.1 mol / L H2SO4 and NaOH solution. The mixture was stirred for 30 min to disperse it evenly, and then placed in a vacuum degassing machine for 10 min to remove bubbles.

[0061] 3) The mixed solution was coated onto a cleaned glass substrate using a spin coating method at a speed of 3500 rpm for 45 seconds. The substrate was pre-baked at 120°C for 5 hours, and then rapidly heated to 220°C for 2 hours to obtain the fluorescent film precursor.

[0062] 4) Annealing the fluorescent film precursor. The fluorescent film precursor was placed in a muffle furnace, and the temperature was set to start from room temperature (25°C), rise to 80°C after 30 minutes, then rise to 120°C after another 30 minutes, and then rise to 220°C within 120 minutes. The temperature was then maintained at this temperature for 6 hours, and the film was then allowed to cool naturally to room temperature.

[0063] Example 3

[0064] This embodiment provides the plotting of fluorescent PI film standard curves:

[0065] Ammonia standard solutions of different concentrations (0, 2, 4, 6, 8 ppm) were prepared. The fluorescent film prepared in Example 2 was fixed in the detection device and exposed to ammonia gas of different concentrations, and the changes in fluorescence intensity were recorded. A standard curve was plotted with ammonia concentration on the x-axis and fluorescence intensity change on the y-axis. Figure 5 ).

[0066] Calibration curve: y =5.4 x +228.4.

[0067] in, y Indicates the fluorescence intensity after the addition of ammonia. x This indicates the concentration of ammonia.

[0068] Example 4

[0069] Application of fluorescent PI membrane for detecting ammonia content in water samples:

[0070] 15 μL of water sample was added to the center of the fluorescent PI membrane prepared in Example 2, and the membrane was placed in a fluorescence spectrometer to detect its fluorescence intensity Ia, which was 240.7 (au) (Ex=270nm, Em=520nm). The fluorescence intensity was then substituted into the standard curve obtained in Example 3, and the ammonia content in the water sample was calculated to be 2.03 ppm. The calculation showed that the error of the method proposed in this invention was within 15.0%.

[0071] Example 5

[0072] The fluorescent PI membrane obtained in Example 2 was used to detect the freshness of fish meat.

[0073] Fish samples were placed in a sealed container. As the spoilage process proceeded, volatile ammonia was released and came into contact with a fluorescent film. The freshness of the fish was inferred by recording changes in fluorescence intensity and referring to the standard curve in Example 3.

[0074] The method for detecting the freshness of fish meat provided in this embodiment includes the following steps:

[0075] 1) Place the fish meat sample and the fluorescent film from Example 2 into a sealed container and place them at 25°C and -4°C, respectively.

[0076] 2) The fluorescent PI film was removed every 4 hours and 3 days and placed in a fluorescence spectrometer to measure the fluorescence intensity (Ex=270nm, Em=520nm).

[0077] 3) As storage time increases, the fluorescence of the fluorescent PI film turns on, and under ultraviolet light (360nm) irradiation, it has a bright yellow-green color, indicating that the fish meat is rotten at this time.

[0078] The fluorescent film of this invention achieves sensitive detection of ammonia through fluorescence enhancement effect. It has the advantages of simple preparation method and excellent detection performance, and can be widely used in non-destructive testing of the freshness of aquatic products.

[0079] Example 6

[0080] The fluorescent PI film obtained in Example 2 was placed in 6 ppm ammonia gas, and its fluorescence intensity was measured. Subsequently, the film was exposed to HCl vapor for 3 minutes, and its fluorescence intensity was measured again. The above steps were repeated 5 times, and the results were as follows: Figure 6 As shown in the figure. The results indicate that the film prepared by this invention is reusable, and its accuracy remains as high as 90% after five cycles of use.

[0081] Comparative Example 1

[0082] This comparative example provides a thin-film fluorescent sensor and its preparation and application methods (CN113640258A).

[0083] The method for determining ammonia and fish freshness provided in CN113640258A is based on N,S-CQDs, gelatin, and Nafion solution, and the fluorescence intensity gradually decreases with increasing ammonia concentration. This gradual decrease in fluorescence intensity with increasing analyte concentration is detrimental to fluorescence spectrometry detection, especially at high concentrations, leading to significant detection errors. Furthermore, the presence of other interfering factors in the environment can cause fluorescence quenching, resulting in false positives. In contrast, the fluorescent film proposed in this invention exhibits a gradual increase in fluorescence intensity with increasing analyte concentration, overcoming the limitations of large detection errors and the potential for false positives.

[0084] Comparative Example 2

[0085] This comparative example uses the same method as Example 2 to prepare fluorescent films, except that the CQDs prepared in Example 1 are used as raw materials, and a solution with a concentration of 5.0 mg / mL is prepared and added to the PI precursor solution at a volume ratio of 5:200 to prepare PI films.

[0086] The detection was performed according to the method in Example 3, and the detection results are as follows: Figure 7 As shown.

[0087] The initial fluorescence intensity of the fluorescent film constructed in this comparative example was weaker than that at a CQDs concentration of 0.5 mg / mL. Furthermore, with increasing ammonia concentration, the fluorescence intensity of the fluorescent film constructed in this comparative example remained low within the ammonia concentration range of 0–8 ppm (slope of 3.0, less than the slope of 5.4 of the standard curve in Example 2). This is due to the induced aggregation quenching phenomenon caused by excessively high concentrations of CQDs.

[0088] Comparative Example 3

[0089] This comparative example uses the same method as Example 2 to prepare fluorescent films, except that the pH of the CQDs and PI precursor mixed solution is adjusted to 9.0 with 0.1 mol / L H2SO4 and NaOH solution to prepare the PI film.

[0090] The detection was performed according to the method in Example 3, and the detection results are as follows: Figure 8 As shown.

[0091] The fluorescent film constructed in this comparative example initially exhibited stronger fluorescence intensity than the film at pH 7.0. However, with increasing ammonia concentration, the fluorescence intensity of the fluorescent film constructed in this comparative example did not significantly increase. This is because the detection principle of ammonia by CQDs is essentially based on pH changes. Under alkaline conditions, an increase in ammonia concentration has little effect on the pH of the film, therefore, the fluorescence enhancement is not significant.

[0092] Comparative Example 4

[0093] This comparative example uses the same method as Example 2 to prepare fluorescent films, except that the pH of the CQDs and PI precursor mixture was adjusted to 4.0 with 0.1 mol / L H2SO4 and NaOH solutions to prepare the PI film.

[0094] The detection was performed according to the method in Example 3, and the detection results are as follows: Figure 9 As shown.

[0095] The fluorescent film constructed in this comparative example had a weaker initial fluorescence intensity than the film at pH 7.0, almost zero. This is because, under acidic conditions, the phenolic hydroxyl groups of CQDs remain in the -OH form, which restricts the electronic delocalization of the π-conjugated structure, resulting in low fluorescence intensity.

[0096] Within the ammonia concentration range of 0–8 ppm, the fluorescence intensity increased, but the sensitivity remained poor. Within the ammonia concentration range of 10–50 ppm, the film fluorescence intensity significantly increased. This is because at ammonia concentrations of 0–8 ppm, the film's pH was still less than 7, resulting in weak fluorescence. As the ammonia concentration increased, the film's pH became greater than 7, and the phenolic hydroxyl groups formed phenoxy anions under the action of hydroxide ions, significantly expanding the π-electron delocalization range and resulting in a significant increase in fluorescence intensity.

[0097] Comparative Example 5

[0098] This comparative example uses the same method as Comparative Example 3 to prepare the fluorescent thin film and performs detection according to the method in Example 3, the difference being that the gas detected is H2S. The detection results are as follows: Figure 10As shown, the fluorescence intensity of the film gradually decreases with increasing H2S gas concentration, indicating that the film constructed in this invention can detect acidic gases at an appropriate pH value (>7).

[0099] 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 is prepared from a mixed precursor solution containing carbon quantum dots and polyimide precursors, wherein the carbon quantum dots are prepared using fluorescein isothiocyanate as the sole carbon source. The method for preparing the fluorescent thin film includes: 1) Using FITC as the carbon source, a hydrothermal reaction was carried out to obtain a CQDs solution; 2) Mix the CQDs solution with the PI precursor solution and adjust the pH to 6.0~7.5 to obtain the precursor solution; 3) The precursor solution is coated onto the substrate and subjected to high-temperature treatment; In step 1), FITC is mixed with a solvent to obtain a FITC solution; the FITC solution is subjected to a hydrothermal reaction, and dialyzed with ethanol as the dialysate to obtain a CQDs solution; the concentration of the CQDs solution is 0.1~1.0 mg / mL; the concentration of the FITC solution is 0.2~1.0 mg / mL; the solvent is an ethanol solution with a concentration of 60%~90%; the temperature of the hydrothermal reaction is 175~185℃, and the time is 10~14h; the dialysis is performed using a 500~1000D dialysis bag, the concentration of the ethanol is 50%~75%, the dialysate is changed every 10~14h, and the dialysis time is 45~50h.

2. The fluorescent thin film according to claim 1, characterized in that, The mixed precursor solution includes a CQDs solution and a PI precursor solution; the volume ratio of the CQDs solution to the PI precursor solution is 1~10:100~10000.

3. The method for preparing the fluorescent thin film according to claim 1 or 2, characterized in that, include: 1) Using FITC as the carbon source, a hydrothermal reaction was carried out to obtain a CQDs solution; 2) Mix the CQDs solution with the PI precursor solution and adjust the pH to 6.0~7.5 to obtain the precursor solution; 3) The precursor solution is coated onto the substrate and subjected to high-temperature treatment; In step 1), FITC is mixed with a solvent to obtain a FITC solution; The FITC solution was subjected to a hydrothermal reaction, and dialyzed with ethanol as the dialysate to obtain a CQDs solution; the concentration of the CQDs solution was 0.1~1.0 mg / mL; In step 1), the concentration of the FITC solution is 0.2~1.0 mg / mL; the solvent is an ethanol solution with a concentration of 60%~90%; the temperature of the hydrothermal reaction is 175~185 ℃, and the time is 10~14 h; the dialysis uses a dialysis bag of 500~1000D, the concentration of the ethanol is 50%~75%, the dialysis solution is changed every 10~14 h, and the dialysis time is 45~50 h.

4. The method for preparing a fluorescent thin film according to claim 3, characterized in that, In step 2), the volume ratio of the CQDs solution to the PI precursor solution is 1~10:1000~10000.

5. The method for preparing a fluorescent thin film according to claim 4, characterized in that, In step 2), the CQDs solution is mixed with the PI precursor solution and ultrasonically treated for 15-30 minutes.

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

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