Preparation of CQDs Films Based on Biomass Bauhinia purpurea and Method for Detecting Quinoline Yellow

Abstract

The present invention is specifically to prepare a fluorescent carbon quantum dot film of Bauhinia ovata and use it for the detection of food additive quinoline yellow. The technical problem solved by the present invention is to provide a method for preparing a fluorescent carbon quantum dot film and its application in the field of food additive detection. The fluorescent carbon quantum dot film prepared by the present invention detects quinoline yellow with the advantages of high selectivity, strong anti-interference, good cycle performance and low cost. The optimal concentration range of linear detection is 8-256 μmol / L, and the detection limit is 7.6214 μmol / L, which is much lower than that specified in GB 2760-2014, the national standard for food additives for food safety. Quinoline yellow can only be added to pre-mixed wine, and the maximum usage shall not exceed 0.1 g / L (converted to 209.5 μmol / L), which has significant competitiveness compared with other detection methods, and can be used as a convenient and accurate quinoline yellow rapid detection method, with good application prospects.

Description

Technical Field

[0001] The present invention relates to the preparation of CQDs thin films and their application in the detection of quinoline yellow, belonging to the field of food additive detection. Background Art

[0002] Quinoline yellow is a water-soluble azo synthetic pigment, which is a mixture of sodium 2-(2-quinolyl)-indanyl-1,3-dione monosulfonate (QYNa) and disodium 2-(2-quinolyl)-indanyl-1,3-dione disulfonate (QYNa2). It is often used as a colorant in fields such as pharmaceuticals, foods, and cosmetics. However, due to its genotoxicity, quinoline yellow can cause adverse effects on children's nerves and respiratory systems, triggering a series of adverse reactions such as excitement, inattention, respiratory diseases, and rashes. Some countries such as Japan, the United States, and the United Kingdom have banned the use of quinoline yellow in foods, and in China, its scope of use and dosage have also been strictly restricted. It is only allowed to be added to formulated wines, and the maximum usage amount is specified as 0.1 g / L. At present, the detection method standard for quinoline yellow in foods in China has not been determined, resulting in the lack of quinoline yellow detection items. Therefore, studying a detection method for quinoline yellow has important practical significance for food safety supervision and detection.

[0003] Currently, the main detection methods for the content of quinoline yellow in foods include spectrophotometry, liquid chromatography, liquid chromatography-mass spectrometry, electrochemistry, etc. In "Karim Asadpour-Zeynali, Sara Manafi-Khoshmanesh. Simultaneous Spectrophotometric Determination of Sunset Yellow and Quinoline Yellow in a Single Step. Journal of the Chinese Chemical Society, 2015, 62(09): 772-779.", the detection of two artificial pigments, quinoline yellow and sunset yellow, in candies by ultraviolet spectrophotometry was reported. Compared with other detection methods, this method does not require complex pretreatment and is simple and convenient to operate. However, its disadvantage is that the detection limit is very high, and the detection results are relatively rough. In the case where the absorption spectra of two or more pigments seriously overlap, the analysis interference is serious.

[0004] Shi Lu, Gao Tianlanxing, Liu Ying, et al. Research on high-throughput detection technology for 12 artificial pigments in complex food matrices [J]. Journal of Food Safety and Quality, 2023, 14(07): 296-304. DOI: 10.19812 / j.cnki.jfsq11-5956 / ts.2023.07.050. discloses a method for detecting 12 artificial pigments in a variety of complex food matrices using high performance liquid chromatography. This method can be used for rapid screening and determination of the contents of 12 artificial pigments in a variety of complex food matrices such as large quantities of meat products, capsule-type health foods, pastries, and gummy candies. However, it is interfered by components with each other and requires complex technical means, resulting in a long experimental period and cumbersome operation.

[0005] Zheng Juanmei, Mo Zimei, Wang Jing, et al. Simultaneous determination of 31 food additives in pastries by liquid chromatography-tandem mass spectrometry. China Food Additives, 2021, 32(04): 70-80. An analytical method of liquid chromatography-tandem mass spectrometry was established, which can rapidly and sensitively screen 31 additives in pastry foods. The results show that the linear correlation coefficient of this method is good, R 2 = 0.992. At the same time, the average recovery rate is between 71.3% and 103.1%, and RSD ≤ 7.62%. Liquid chromatography-mass spectrometry has a wide range of applications, high analysis efficiency, both qualitative and quantitative functions, can avoid the occurrence of false positives, and can rapidly detect and screen multiple components with high sensitivity. Therefore, it has been widely used in the detection of food additives. However, the instruments of this method are relatively complex and the cost is high.

[0006] Eb A, Lh A, Mm A, et al. A 2D tungsten disulphide / diamond nanoparticles hybrid for an electrochemical sensor development towards the simultaneous determination of sunset yellow and quinoline yellow - ScienceDirect. Sensors and Actuators B: Chemical, 2020, 324. An electrochemical sensor was developed for the simultaneous analysis of quinoline yellow and sunset yellow in food additives. The sensor is based on the synergistic effect between diamond nanoparticles (DNP) and WS2 nanosheets obtained by liquid exfoliation. The glassy carbon electrode modified with WS2 and DNP achieved sensitive detection of quinoline yellow and sunset yellow, with detection limits of 3.4 μmol / L and 0.086 μmol / L respectively, relative error ≤ 8.40%, and RSD ≤ 12.6%. It was actually applied to the determination of quinoline yellow and sunset yellow in commercial throat lozenge samples, and the average recoveries were very good: 93% for quinoline yellow and 106% for sunset yellow. Compared with other detection methods, the electrochemical method has higher sensitivity, and has the advantages of simple operation, fast response, stable signal, easy miniaturization, and low cost. However, the disadvantages are also obvious, such as poor electrode selectivity, electrocatalytic activity, detection anti-interference ability, and repeatability.

[0007] Yu Yunhui. Construction of Graphene Nitride Composite Electrodes and Their Electrochemical Detection of Edible Pigments [D]. Jiangxi Agricultural University, 2020. DOI: 10.27177 / d.cnki.gjxnu.2019.000105. Designed a Au / NG composite material prepared by hydrothermal method using Au and NG as raw materials. Au was used as an auxiliary material to enhance the experiment and improve the detection performance of graphene nitride in a synergistic way. The experimental results showed that compared with Au electrodes and graphene nitride electrodes, the Au / graphene nitride composite material had a wider detection range. Under the optimal conditions, the linear range of Au / NG for the detection of quinoline yellow was from 0.005 μmol / L to 8 μmol / L, and the detection limit was as low as 0.0016 μmol / L after 120 s of accumulation. However, the construction of its electrode was relatively complex and the cost was also high.

[0008] Sivasankaran U, Radecki J, Radecka H, et al. Copper nanoclusters: an efficient fluorescence sensing platform for quinoline yellow[J]. Luminescence, 2019, 34(02). L-cysteine was dissolved in NaOH solution by ultrasound, and then the CuSO4 solution was mixed with the above solution to synthesize L-cysteine-stabilized copper nanoclusters (L-Cys-CuNCs). The results showed that L-Cys-CuNCs could be used as a fluorescence sensor, which could selectively detect quinoline yellow among other yellow colorants in the concentration range of 5.50 - 0.20 μmol / L, and the detection limit was as low as 0.11 μmol / L. However, this method has problems such as complex sample preparation and high cost.

[0009] The above methods for detecting quinoline yellow each have their own characteristics, but they all have deficiencies such as complex equipment, long detection cycle, and high cost. Therefore, it is necessary to explore an efficient, rapid, simple and suitable method for the rapid detection of quinoline yellow to ensure consumer health and food safety.

[0010] Carbon quantum dots (CQDs) films are composed of CQDs and polymer organic polymers. In addition to having excellent fluorescence properties, chemical stability, biocompatibility and other characteristics, they also have the advantages of antioxidant and antibacterial properties. At the same time, due to their adjustable geometric shape and portability, they have become an important soft material and can play an important role in fields such as sensing detection, food packaging, and optical applications.

[0011] DOHERTY M B C M, EMAILPROTECTED, EMAILPROTECTED E, et al. Fabrication of a Reusable Carbon Dot / Gold Nanoparticle / Metal-Organic Framework Film for Fluorescence Detection of Lead Ions in Water[J]. 2022. By mimicking the interaction between metals and proteins in biological systems, a metal-organic framework (MOFs) material was developed, and zirconium terephthalate-based framework materials were co-modified with Au nanoparticles and thiol-functionalized carbon dots to obtain HS-C / Au(x) / UiO-66 composites with different Au contents (x) for Pb 2+ with high sensitivity, and then a thin-film solid-state sensor was prepared. The sensor can highly sensitively detect Pb in drinking water2+ , for Pb at levels below 10 ppb in water 2+ shows obvious fluorescence intensity changes and has good Pb 2+ specific selectivity. In addition, by adjusting the Au content in the film, emission quenching can be converted into fluorescence enhancement. This change in fluorescence response is expected to develop sensors with different response modes to detect different toxic substances with high sensitivity.

[0012] YH A, RG A, SHUAI Z A, et al. A convenient fluorescence sensor of tetracycline based on B,Ncodoped carbon dots / polymer composite film, 2021. Using citric acid and 3-aminophenylboronic acid as precursors, B,N co-doped CDs were synthesized by a hydrothermal method. Then, CDs and polyvinyl alcohol (PVA) hydrogel were combined to prepare a fluorescent CDs film, and a novel tetracycline (TC) fluorescence sensor was constructed in a visual way. By immersing the CDs-PVA film in TC solutions with different concentrations, the experimental results showed that the fluorescence intensity of the CDs-PVA film was related to the TC concentration. As the TC concentration increased, the fluorescence intensity of the CDs-PVA film decreased. This indicated that the CDs-PVA film could be used to detect TC in a visual way.

[0013] ZHAO L, ZHANG M, MUJUMDAR AS, et al. Preparation of a Novel Carbon Dot / Polyvinyl Alcohol Composite Film and Its Application in Food Preservation[J]. ACS Applied Materials&Interfaces, 2022, 14(33): 37528-37539. Using bananas as a carbon source, CDs were prepared by a hydrothermal method. Then, CDs were added to the PVA matrix to prepare a CDs / PVA active food packaging film. The 0.50% CDs / PVA film was used as an active package, and its application effects on foods such as bananas, Chinese dates, and fried meatballs were experimentally tested. The results showed that it significantly extended the shelf life of the foods, which proved the feasibility of the CDs / PVA composite film as an active food packaging material and had good application prospects.

[0014] GUO H, ZHANG X, CHEN Z, et al. High-energy short-wave blue light conversion films via carbon quantum dots for preventing retinal photochemical damage[J]. Carbon, 2022, 199: 431-438. In this study, bright yellow CDs (Y-CQDs) with a quantum yield as high as 71% were synthesized by the strategy of alkali-catalyzed molecular fusion method, and showed good anti-short-wave blue light performance. 1-Amino-2-naphthol-4-sulfonic acid (0.010 g) was dissolved in ethanol, and EDA (0.4 mL) was added to prepare Y-CQDs powder. Then, using Y-CQDs and PVA as raw materials, a flexible and transparent Y-CQDs / PVA film was prepared. The film showed efficient short-wave blue light conversion characteristics. Using the Y-CQDs / PVA film (≥20 wt%) in actual WLEDs could effectively block harmful short-wave blue light, and the role of the Y-CQDs / PVA film in eliminating short-wave blue light was further verified in the biological experiment of rat retinal photochemical damage. This study achieved the conversion of high-energy short-wave blue light and provided a valuable proof for the concept of developing practical anti-short-wave blue light films from carbon-based materials.

[0015] Carbon dots can be used as functional fillers to improve the properties of thin film materials. Publication number: CN107337194A. A preparation method of a fluorescence tunable carbon dot film and its application on LEDs: Citric acid and urea were completely dissolved in an N,N-dimethylformamide solution, and a carbon dot solution was obtained by heating in a polytetrafluoroethylene hydrothermal reaction kettle; then it was poured into an ethyl acetate solution to obtain precipitated carbon dot powder; the carbon dot powder was dissolved in an organosilane solution for pre-hydrolysis reaction of the organosilane to obtain a slurry-like carbon dot / silane mixture; by the casting method, a fluorescence tunable carbon dot film was finally formed at 100 °C.

[0016] Publication number: CN103554534A. A preparation method of a lignocellulose biomass film: Using lignocellulose as raw material, it was crushed and passed through a 160-mesh sieve, and then the crushed lignocellulose biomass was dissolved in DMSO / LiCl to obtain a film-forming solution of the lignocellulose biomass; the film-forming solution was scraped on a flat plate, immersed in a coagulation bath to remove the solvent, and dried to obtain a regenerated lignocellulose biomass film.

[0017] In this invention, Bauhinia variegata tree trunks were selected as raw materials, and carbon quantum dots (CQDs) were prepared by a one-step hydrothermal method. Then, using the CQDs solution, lignin, and sodium alginate as raw materials and CaCl2 as a cross-linking agent, a CQDs film was prepared. Based on the CQDs film, a fluorescence detection method for rapid detection of quinoline yellow was constructed. Summary of the invention

[0018] The first technical problem solved by the present invention is the preparation of CQDs thin film.

[0019] The preparation method of the CQDs film of the present invention comprises the following steps:

[0020] a. Weigh an appropriate amount of Bauhinia powder into a beaker, add ultrapure water, stir evenly with a glass rod, transfer the solution into a reaction kettle, put it into an electric heating blast drying oven, and react at a set temperature;

[0021] b. After the reaction is completed, the reactor is taken out and cooled naturally to room temperature. The reaction solution is filtered with a microporous filter membrane and a circulating water vacuum pump. The collected filtrate is dialyzed and purified in a dialysis bag in ultrapure water, and then rotary evaporated using a rotary evaporator, and then dried to obtain Bauhinia fluorescent carbon quantum dot powder;

[0022] c. Weigh appropriate amounts of sodium alginate and lignin into a beaker, add CQDs solution, stir evenly with a glass rod, transfer the solution to a water bath, mix evenly, and then let it stand to remove bubbles;

[0023] d. Apply the mixed solution to the smooth surface of the culture dish to obtain a wet film, put it into an oven, and react it at a set temperature and time;

[0024] e. After the film is dried, immerse it in a CaCl2 solution, and then rinse the film surface with water several times to finally obtain the desired CQDs film.

[0025] Preferably, the specific operation of step a is: weigh 0.125g of Bauhinia hornbilosa into a beaker, add 40mL of ultrapure water, stir evenly with a glass rod, then transfer the solution to a reactor, set the reaction temperature to 210°C, the reaction time to 10h, and proceed to step b.

[0026] Preferably, the specific operation of step c is: adding 2% sodium alginate and 1% lignin (mass fraction) to 60 mL CQDs solution, and then magnetically stirring in a 60° C. water bath for 2 h to fully mix the solution. After mixing evenly, standing the solution for 24 h to remove bubbles; and proceeding to step d.

[0027] Preferably, in step d, the reaction temperature is set to 60° C. and the reaction time is set to 4 h.

[0028] Preferably, in step e, after the film is dried, it is immersed in a 0.5 mol / L CaCl2 solution for 10 min to completely crosslink it. Subsequently, the film surface is rinsed with water several times to remove excess CaCl2, and finally the desired CQDs film is obtained.

[0029] The second technical problem solved by the present invention is the application of CQDs film in the detection of quinoline yellow.

[0030] After the CQDs thin film of the present invention is prepared by the above method, when detecting quinoline yellow, it has high selectivity and high sensitivity. Quinoline yellow has a good linear relationship in the range of 8 - 256 μmol / L, and the linear fitting equation is: F / F0 = 0.808 - 0.0013C 喹啉黄 , the correlation coefficient R 2 = 0.9923. The data results are very stable, which proves that the detection data obtained by this method has practical application value, and the optimal sensing concentration of quinoline yellow for the CQDs thin film is 256 μmol / L. This indicates that this method has high accuracy for the detection of quinoline yellow in complex food matrices, and the CQDs thin film has good application prospects in the detection of quinoline yellow in food.

[0031] After the CQDs thin film of the present invention is prepared by the above method, when quinoline yellow is added, the fluorescence of the CQDs thin film is significantly quenched, while when other possible interfering substances are added, the fluorescence intensity of the CQDs thin film has no obvious change. Research shows that the prepared CQDs thin film has high selectivity for the detection of quinoline yellow.

[0032] After the CQDs thin film of the present invention is prepared by the above method, after the CQDs thin film is detected for quinoline yellow and then eluted with ultrapure water, the fluorescence intensity of the thin film can be restored and can be reused for the detection of quinoline yellow.

[0033] After the CQDs thin film of the present invention is prepared by the above method, it has high sensitivity for the detection of quinoline yellow, and the detection limit is 7.6214 μmol / L. According to the "National Food Safety Standard - Standard for the Use of Food Additives" GB 2760 - 2014, quinoline yellow can only be added to pre - mixed cocktails, and the maximum usage amount shall not exceed 0.1 g / L (converted to 209.5 μmol / L). The detection limit of the CQDs thin film obtained by the present invention is much lower than this standard. Therefore, this CQDs thin film is expected to be applied to the detection of quinoline yellow in actual samples. Description of the Drawings

[0034] Figure 1 It is the SEM images of (A) the blank thin film and (B) the CQDs thin film in the present invention.

[0035] Figure 2 It is the FTIR spectra of the blank thin film and the CQDs thin film in the present invention.

[0036] Figure 3 It is the WCA image of the CQDs thin film in the present invention.

[0037] Figure 4 It is the fluorescence sensing image of the CQDs thin film in the present invention for detecting different concentrations of quinoline yellow.

[0038] Figure 5This is the standard curve graph for the detection of quinoline yellow by the CQDs film in the present invention.

[0039] Figure 6 This is the fluorescence sensing graph within 30 minutes after adding quinoline yellow in the present invention.

[0040] Figure 7 This is the graph of the change in fluorescence intensity within 30 minutes after adding quinoline yellow in the present invention.

[0041] Figure 8 This is the fluorescence sensing ratio graph of the CQDs film with quinoline yellow and some common food additives and some metal ions in the present invention.

[0042] Figure 9 This is the fluorescence sensing ratio graph of the CQDs film + quinoline yellow and some common food additives and some metal ions in the present invention.

[0043] Figure 10 This is the SEM graph of (A) the CQDs film and (B) the CQDs film after detecting quinoline yellow in the present invention.

[0044] Figure 11 This is the FTIR graph of the CQDs film before and after detecting quinoline yellow.

[0045] Figure 12 This is the WCA graph of the CQDs film after detecting quinoline yellow. Detailed implementation manners

[0046] For a clearer understanding of the purpose, technical features, and beneficial effects of the present invention, the technical solutions of the present invention are described in detail below, but it should not be construed as a limitation on the implementable scope of the present invention.

[0047] Example 1: Preparation of the CQDs film

[0048] This Example 1 provides a method for preparing a CQDs film, which specifically includes the following steps:

[0049] Preferably, a. Prepare CQDs solutions with Bauhinia purpurea powders at concentrations of 1.5625 mg / mL, 3.125 mg / mL, 6.25 mg / mL, 12.5 mg / mL, and 25 mg / mL, stir evenly with a glass rod, and transfer the solution to a reaction kettle; more preferably, when the concentration of Bauhinia purpurea powder is 3.125 mg / mL, the fluorescence intensity of CQDs is the best; place the reaction kettle in an oven and react at 150°C to 230°C for 6 to 12 hours; more preferably, at 210°C for 10 hours.

[0050] Preferably, c. Add 2% sodium alginate and 1% lignin (mass fraction) into 40 - 80 mL of CQDs solution, perform magnetic stirring in a water bath at 40 - 80 °C, and let it stand to remove bubbles; more preferably 60 mL and 60 °C.

[0051] Preferably, d. Coat the mixture onto the surface of a smooth petri dish to obtain a wet film, and place the wet film in an oven at 40 - 80 °C for 2 - 6 h; more preferably 60 °C for 4 h.

[0052] Preferably, e. After drying the film, immerse it in 0.3 - 0.8 mol / L CaCl2 solution for 5 - 15 min, and rinse the film surface with water multiple times to finally obtain the required CQDs film; more preferably 0.5 mol / L for 10 min.

[0053] Example 2: Application of CQDs Film in the Detection of Quinoline Yellow

[0054] Trim the CQDs film prepared in Example 2 above into circular small pieces with a diameter of 1.5 cm, immerse them in ultrapure water and quinoline yellow solutions with different concentrations (2, 4, 8, 16, 32, 64, 128, 256 μmol / L) for 5 min, then take out the CQDs film and measure it by a fluorescence spectrometer using the solid method. Draw a standard curve based on the fluorescence intensity of the CQDs film immersed in quinoline yellow solutions with different concentrations. (Quinoline yellow has a good linear relationship with the CQDs film in the concentration range of 8 - 256 μmol / L, and the correlation coefficients are R 2 = 0.9923, and its detection limit is 7.6214 μmol / L);

[0055] Immerse the CQDs film in the test solution containing quinoline yellow for fluorescence detection to obtain the change in fluorescence intensity;

[0056] Calculate the concentration of quinoline yellow in the test sample solution through the standard curve.

[0057] Table 3 Selectivity of CQDS Film in Example 2 for Quinoline Yellow and Some Common Food Additives and Some Metal Ions

[0058]

[0059]

[0060] It can be seen from this that when quinoline yellow is added, the fluorescence of the CQDs film is significantly quenched, while when other possible interfering substances are added, the fluorescence intensity of the CQDs film has no obvious change. Research shows that the prepared CQDs film has high selectivity for the detection of quinoline yellow.

[0061] In this example, the effects of sensing quinoline yellow concentration, sensing reaction time, etc. on the fluorescence quenching reaction between the CQDs film and the water-soluble azo synthetic pigment - quinoline yellow were explored to obtain the characteristics of the CQDs film and the optimal detection conditions. The selectivity and anti-interference ability of the CQDs film were tested, and the fluorescence quenching mechanism between quinoline yellow and the CQDs film was studied and analyzed.

[0062] The experimental results and analysis are as follows:

[0063] (1) Characterization of the CQDs film

[0064] Figure 1 These are the SEM images of (A) the blank film and (B) the CQDs film of the present invention. From Figure 1 it can be seen that the CQDs are relatively uniformly dispersed on the film surface.

[0065] Figure 2 These are the FTIR spectra of the blank film and the CQDs film of the present invention. From Figure 2 it can be seen that the CQDs film shows stretching vibrations of C-H and -CH2 at wavenumbers around 2981 cm -1 and 2905 cm -1 . The absorption peak around 1643 cm -1 is caused by the stretching vibration of C=O, and the absorption peak around wavenumber 880 cm -1 corresponds to the bending vibration of C-H. These characteristic peaks are similar to the surface functional groups of CQDs, and other characteristic peaks are similar to those of the blank film. Thus, it can be confirmed that CQDs are successfully compounded in the film, and the CQDs film has abundant hydrophilic groups such as hydroxyl and carboxyl groups, and the surface CQDs film has hydrophilicity.

[0066] Figure 3 These are the WCA images of the CQDs film of the present invention. The CQDs film has good hydrophilicity, and the surface of CQDs has abundant hydrophilic groups, such as: hydroxyl and carboxyl groups. Therefore, the addition of CQDs makes the CQDs film show excellent hydrophilic characteristics.

[0067] (2) Detection and analysis of quinoline yellow

[0068] Figure 4 These are the fluorescence sensing images of the CQDs film for detecting different concentrations of quinoline yellow in the present invention. The fluorescence sensing spectra of the CQDs film immersed in quinoline yellow solutions with different concentrations were measured using a fluorescence spectrometer at an excitation wavelength of 375 nm. The results are shown in the figure. As the concentration of quinoline yellow increases, the fluorescence intensity of the CQDs film also decreases.

[0069] Figure 5This is the standard curve graph for the detection of quinoline yellow by the CQDs film in the present invention. Among them, F0 is the fluorescence intensity of the blank CQDs film, F is the fluorescence intensity of the CQDs film after being soaked in the quinoline yellow solution, and C represents the concentration value of the quinoline yellow solution; the experimental results show that quinoline yellow has a good linear relationship in the range of 8 - 256 μmol / L, and the linear fitting equation is: F / F0 = 0.808 - 0.0013C 喹啉黄 , the correlation coefficient R 2 = 0.9923, and the data results are very stable, proving that the detection data obtained by this method has practical application value, and the optimal sensing concentration of quinoline yellow for the CQDs film is 256 μmol / L.

[0070] Figure 6 This is the fluorescence sensing graph within 30 minutes after adding quinoline yellow in the present invention, Figure 7 This is the graph of the change in fluorescence intensity within 30 minutes after adding quinoline yellow in the present invention. As shown in the figure, after adding quinoline yellow, the fluorescence emission has tended to the equilibrium point of the entire spectrum, and the fluorescence intensity remains basically unchanged within the next 30 minutes. This indicates that the quenching effect between the CQDs film and quinoline yellow is not affected by time and is very stable within this reaction time.

[0071] Figure 8 This is the fluorescence sensing ratio graph of the CQDs film with quinoline yellow and some common food additives and some metal ions in the present invention. The selective test results of the CQDs film are shown in the figure. When quinoline yellow is added, the fluorescence of the CQDs film is significantly quenched, while when other possible interfering substances are added, the fluorescence intensity of the CQDs film does not change significantly. The research shows that the prepared CQDs film has high selectivity for the detection of quinoline yellow.

[0072] Figure 9 This is the fluorescence sensing ratio graph of the CQDs film + quinoline yellow and some common food additives and some metal ions in the present invention. The anti-interference test results of the quinoline yellow detection system of the CQDs film for the above interfering substances are shown in the figure. When the above interfering substances are added to the CQDs film / quinoline yellow detection system, adding a higher concentration of Fe2(SO4)3 causes a change in fluorescence intensity. When the remaining interfering substances are added, there is no obvious change in F / F0. This indicates that the CQDs film / quinoline yellow detection system has good anti-interference ability.

[0073] Table 4 Fluorescence intensity of 10 times blank CQDs films and after adding quinoline yellow solution

[0074]

[0075]

[0076]

[0077]

[0078] According to the data in Table 3, the detection limit DL = 7.6214 μmol / L was calculated through Formulas (1), (2), and (3). According to the "National Food Safety Standard - Standard for the Use of Food Additives" GB 2760-2014, quinoline yellow can only be added to pre-mixed cocktails, and the maximum usage amount shall not exceed 0.1 g / L (converted to 209.5 μmol / L). The detection limit of the CQDs film obtained in this article is much lower than this standard. Therefore, this CQDs film is expected to be applied to the detection of quinoline yellow in actual samples.

[0079] (3) Analysis of the fluorescence quenching mechanism of quinoline yellow on the CQDs film

[0080] As Figure 10 shown, the surface morphology of the CQDs film before and after detecting quinoline yellow did not change significantly, indicating that the CQDs film and quinoline yellow will not change the structure of the CQDs film during the detection process.

[0081] Figure 11 FTIR diagrams of the CQDs film before and after detecting quinoline yellow. As can be seen from the figure, some changes occurred in the surface functional groups of the CQDs film after detection compared with before detection. The absorption peaks of C-H, -CH2, and C-H at wavenumbers 2981 cm -1 , 2905 cm -1 , and 880 cm -1 disappeared, and the intensity of the absorption peak of C═C at wavenumber 1595 cm -1 decreased, indicating that quinoline yellow reacted with the CQDs on the surface of the CQDs film, resulting in changes in its surface functional groups.

[0082] Figure 12 Measure the WCA of the CQDs film after detecting quinoline yellow. Combining with the figure analysis, it is because quinoline yellow combines with the abundant hydrophilic groups on the surface of CQDs, such as hydroxyl and carboxyl groups, reducing the interaction between the film surface and water, resulting in a decrease in the hydrophilicity of the CQDs film. Therefore, combining the above text, it can be concluded that the fluorescence quenching effect of quinoline yellow on the CQDs film is the result of the combined action of dynamic quenching caused by the binding of quinoline yellow to the CQDs on the surface of the CQDs film and the inner filter effect.

[0083] (4) Detection of quinoline yellow in actual samples

[0084] Table 5 Detection of quinoline yellow in actual samples using CQDs thin films

[0085]

[0086] The results obtained in this paper are shown in Table 4: the recovery rates of quinoline yellow spiked samples were 101.8% - 123.9%, and the RSDs were between 1.8% and 3.0%. This indicates that this method has high accuracy for the detection of quinoline yellow in complex food matrices, and CQDs thin films have good application prospects in the detection of quinoline yellow in foods.

Claims

1. Application of a fluorescent carbon quantum dots (CQDs) film prepared from biomass Bauhinia purpurea in the detection field of food additive quinoline yellow. The fluorescent carbon quantum dots film is prepared by the following steps: a. Prepare CQDs solutions with concentrations of 1.5625 mg / mL, 3.125 mg / mL, 6.25 mg / mL, 12.5 mg / mL, and 25 mg / mL using Bauhinia purpurea powder. Stir evenly with a glass rod, transfer the solution to a reaction kettle, and then place the reaction kettle in an oven and react at 150°C to 230°C for 6 to 12 h; b. Add 2% sodium alginate and 1% lignin (mass fraction) to 40 - 80 mL of the CQDs solution, and then perform magnetic stirring in a water bath at 40 - 80°C for 2 h to make them fully mixed; c. Coat the mixed solution on the surface of a smooth petri dish to obtain a wet film, and place the wet film in an oven at 40 - 80°C for 2 - 6 h; d. After the film is dried, immerse it in a 0.3 - 0.8 mol / L CaCl2 solution for 5 - 15 min, and wash the film surface with water multiple times to finally obtain the required CQDs film.

2. The application according to claim 1, characterized in that The detection of food additive quinoline yellow includes the following steps: Immerse the finally obtained CQDs film into quinoline yellow solutions with different concentrations for fluorescence detection. According to the linear relationship between the detected fluorescence intensity and the concentration of quinoline yellow, establish a standard curve; calculate the concentration of quinoline yellow in the solution through the standard curve.

3. The application according to claim 2, wherein: When performing the fluorescence enhancement reaction, the reaction time is 0 to 30 min.

4. The application according to claim 2, characterized in that: When performing the fluorescence enhancement reaction, the linear detection range of the quinoline yellow concentration is 8 - 256 μmol / L.

5. The application according to claim 2, wherein: When performing the fluorescence enhancement reaction, after the CQDs film is eluted and treated with ultrapure water, the fluorescence intensity of the film can be restored and can be reused for detecting quinoline yellow.

6. The application according to any one of claims 2-4, characterized in that: In step a, the concentration of the CQDs solution is 3.125 mg / mL, and it is heated and reacted at 210°C for 10 h.

7. The application according to any one of claims 2-4, characterized in that: In step b, the CQDs solution is 60 mL, and it is heated in a water bath at 60°C.

8. The application according to any one of claims 2-4, characterized in that: In step c, it is heated and reacted at 60°C for 4 h.

9. The application according to any one of claims 2-4, characterized in that: In step d, the concentration of the CaCl2 solution is 0.5 mol / L, and the immersion time is 10 min.

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