A method for determining melamine in dairy products using molecular imprinting fluorescence sensor

Abstract

The present invention discloses a method for determining melamine in dairy products using a molecular imprinting fluorescence sensor, and belongs to the technical field of melamine detection. The present invention combines molecular imprinting technology with carbon quantum dot technology to prepare highly sensitive carbon quantum dots, and synthesizes molecular imprinting polymers with good selectivity for melamine. 2+ A method for rapidly detecting melamine content in dairy products is proposed based on the principle that the fluorescence of molecular imprinted polymers adsorbed with carbon quantum dots is quenched and then the fluorescence is restored by adding melamine. The method of the invention has high sensitivity, good selectivity, simple operation, fast detection speed, low cost, and is suitable for the determination of melamine in various dairy products.

Description

Technical Field

[0001] The invention belongs to the technical field of melamine detection, and particularly relates to a method for detecting melamine in dairy products by utilizing a molecular imprinting fluorescence sensor. Background Art

[0002] Melamine (Chemical formula C3H6N6), also known as melamine and protein essence, is named "1,3,5-triazine-2,4,6-triamine" by IUPAC. It is a triazine nitrogen-containing heterocyclic organic compound and an important nitrogen-containing heterocyclic organic chemical raw material, with a nitrogen content of up to 66%. Melamine is a white monoclinic crystal, almost odorless, slightly soluble in water (3.1g / L at room temperature), soluble in methanol, formaldehyde, acetic acid, hot ethylene glycol, glycerol, pyridine and other solvents, and insoluble in acetone and ethers.

[0003] At present, the main methods for melamine detection include high performance liquid chromatography, liquid chromatography-mass spectrometry, etc. In addition, enzyme-linked immunosorbent assay, capillary electrochromatography, mass spectrometry, and infrared absorption spectroscopy have also been reported in the literature. These methods generally require complex sample pretreatment, expensive instruments, and complex methods. The cost of detection is high and requires professional personnel to operate. They cannot fully meet the needs of rapid screening of melamine residues in milk. Therefore, it is necessary to propose a melamine detection method that is simple to operate, has a rapid response, and has a low detection limit. Summary of the invention

[0004] The present invention aims at the defects of melamine detection in the prior art. The present invention combines molecular imprinting technology with carbon quantum dot technology to prepare highly sensitive carbon quantum dots and synthesize molecular imprinting polymers with good selectivity for melamine. 2+ Based on the principle of quenching the fluorescence of molecular imprinting polymers adsorbed with carbon quantum dots and restoring the fluorescence by adding melamine, a method for rapidly detecting the melamine content in dairy products was proposed.

[0005] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0006] A method for determining melamine in dairy products using a molecular imprinting fluorescence sensor comprises the following steps:

[0007] (1) Synthesis of melamine molecularly imprinted polymer;

[0008] (2) Molecularly imprinted polymer adsorption of carbon quantum dots: 2.0 g of citric acid and 0.8 g of urea were added to a 100 mL round-bottom flask, and the temperature was gradually raised to 200 °C under continuous stirring. The solid reagent began to melt in about 5 minutes, and the color gradually turned yellow. It turned into a dark brown slightly viscous solution in about 12 minutes. During this period, the speed of the stirrer needed to be adjusted according to the viscosity of the solution to keep it in a molten state and not stick to the wall of the flask. The NaOH suspension of the melamine molecularly imprinted polymer obtained in step (1) was added dropwise at room temperature, and the pH value was adjusted to 7.0. The mixture was stirred for 30 minutes to allow the polymer to fully adsorb the carbon quantum dots. The mixture was allowed to stand, and the product was collected in a 10 mL centrifuge tube and centrifuged. The upper layer of liquid was discarded, and the obtained solid was washed with water and vacuum dried to obtain a molecularly imprinted polymer adsorbing carbon quantum dots.

[0009] (3) Dissolve the molecular imprinting polymer adsorbing carbon quantum dots obtained in step (2) in water and prepare 0.5 mg mL -1 SiO2-CD S -NH2 solution;

[0010] (4) Prepare a standard curve: Add 3 mL of pH 7.0 buffer solution and 10 uL SiO2-CD to a 10 mL colorimetric tube. S -NH2 solution and 250uL of 1ug·mL -1 Hg 2+ solution, mix thoroughly, and add different volumes of 3.0 g·L -1 melamine solution, and then dilute it to 6 mL with a buffer solution with a pH value of 7.0. After fully mixing, scan its fluorescence emission spectrum at an excitation wavelength of 350 nm, and make a working curve according to the degree of fluorescence recovery;

[0011] (5) Actual dairy product processing: Weigh 5.00 g of milk sample into a 50 mL centrifuge tube, add 5 mL of 5% trichloroacetic acid aqueous solution, vortex shake for 30 min, ultrasonic extraction for 20 min, take the supernatant into a 10 mL centrifuge tube, and centrifuge at 10,000 r / min. -1 Centrifuge at high speed for 15 minutes, retain the supernatant, and complete the treatment after the supernatant passes through a 0.45 um microporous filter membrane to obtain a sample solution;

[0012] (6) Sample detection: Add 3 mL of pH 7.0 buffer solution and 10 uL SiO2-CD to a 10 mL colorimetric tube. S -NH2 solution and 250uL of 1ug·mL -1 Hg 2+solution, shake well, add 50uL of the treated sample solution, and then make up to 6mL with a buffer solution with a pH value of 7.0. After sufficient oscillation, scan its fluorescence emission spectrum at an excitation wavelength of 350nm, measure the fluorescence recovery degree of the system, and calculate the melamine content in the actual sample according to the standard curve.

[0013] Furthermore, the method for synthesizing the melamine molecularly imprinted polymer in step (1) is as follows: 0.7 mL of 2.0 mol·L - 1 NaOH and 100mL5.5×10 -3 mol·L -1 CTAB was mixed and heated to 70°C in a 250mL round-bottom flask, and then 1.0mL LTEOS and 0.1 mmol melamine were added, and heated in a water bath at 70°C. After reacting for 3 hours, the mixture was cooled to room temperature. The product was collected in a 10mL centrifuge tube and centrifuged, the supernatant was discarded, and the mixture was washed twice with water and three times with ethanol. After drying, the product was the melamine molecular imprinting polymer.

[0014] Furthermore, in step (1), the molecularly imprinted polymer is dried by burning or air-drying.

[0015] Furthermore, the mass concentration of the NaOH suspension of melamine molecularly imprinted polymer in step (2) is 10 mg·mL -1

[0016] Furthermore, in step (5), when the sample is milk powder, the sample treatment method is as follows: weigh 2.00 g of the milk powder sample, dissolve it in 3 mL of deionized water, mix it, add 5 mL of a 5% trichloroacetic acid aqueous solution, vortex and shake it for 30 minutes, perform ultrasonic extraction for 20 minutes, take the supernatant into a 10 mL centrifuge tube, and centrifuge it at 10000 r•min. -1 Centrifuge at high speed for 15 minutes, retain the supernatant, and complete the treatment after the supernatant passes through a 0.45um microporous filter membrane to obtain a sample solution.

[0017] Furthermore, the buffer solution in step (4) and step (6) is a sodium dihydrogen phosphate and sodium hydrogen phosphate buffer solution with a pH value of 7.0. Beneficial Effects

[0018] (1) The present invention uses citric acid and urea as raw materials to prepare nitrogen-doped carbon quantum dots by thermal decomposition. The fluorescence intensity of the carbon dots is stable, the raw materials are simple and easy to obtain, and the preparation method is simple. The melamine molecular imprinting polymer with regular appearance is prepared by using tetraethyl orthosilicate and melamine as raw materials. The melamine molecular imprinting polymer has a good recognition effect on melamine.

[0019] (2) The present invention utilizes the molecular imprinting polymer adsorbed with carbon quantum dots to emit strong fluorescence, through Hg2+ Based on the principle that the fluorescence is quenched by occupying the holes on the imprinted polymer, and the fluorescence can be restored by "pulling" mercury ions after adding melamine, a method for measuring the melamine content in dairy products was designed; the maximum excitation wavelength and maximum emission wavelength of the melamine molecular imprinted polymer solution adsorbed with carbon quantum dots are located at 350nm and 441nm, respectively; melamine in the range of 2.5~100ug·mL -1 The concentration range is linearly related to the degree of fluorescence recovery of the system (R 2 =0.9967), the detection limit of the present invention is 2.1ug·mL -1 , the spiked recovery rate of milk powder is 93%, and the spiked recovery rate of liquid milk is 92%~99%;

[0020] (3) The method of the present invention has high sensitivity, good selectivity, simple operation, fast detection speed and low cost, and is suitable for the determination of melamine in various dairy products. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is the relationship diagram between the fluorescence recovery degree and the melamine concentration and the standard curve;

[0022] Figure 2 This is the effect diagram of coexistence on melamine determination;

[0023] Figure 3 The following are photos of three materials under fluorescent light, where tube 1 contains a melamine molecular imprinted polymer suspension without carbon quantum dots adsorbed thereon, tube 2 contains a carbon quantum dot solution prepared by thermal decomposition, and tube 3 contains a melamine molecular imprinted polymer suspension adsorbed with carbon quantum dots;

[0024] Figure 4 The following are photos of three materials under ultraviolet light, where tube 1 contains a melamine molecularly imprinted polymer suspension without carbon quantum dots adsorbed thereon, tube 2 contains a carbon quantum dot solution prepared by thermal decomposition, and tube 3 contains a melamine molecularly imprinted polymer suspension adsorbed with carbon quantum dots;

[0025] Figure 5 This is the transmission electron microscopy image of melamine molecular imprinting polymer after burning;

[0026] Figure 6 This is a transmission electron microscopy image of naturally dried melamine molecularly imprinted polymer;

[0027] Figure 7 This is a transmission electron microscopy image of melamine molecular imprinted polymer adsorbed with carbon quantum dots. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is further described below in conjunction with specific embodiments, but is not limited thereto. Example 1

[0029] A method for determining melamine in dairy products using a molecular imprinting fluorescence sensor comprises the following steps:

[0030] (1) Synthesis of melamine molecularly imprinted polymer: Take 0.7 mL of 2.0 mol·L -1 NaOH and 100mL5.5×10 -3 mol·L -1 CTAB was mixed and heated to 70°C in a 250mL round-bottom flask, and then 1.0mL LTEOS and 0.1 mmol melamine were added, and heated in a water bath at 70°C. After reacting for 3 hours, the mixture was cooled to room temperature. The product was collected in a 10mL centrifuge tube and centrifuged, the upper layer was discarded, and the mixture was washed with water twice and with ethanol three times. The product was dried to obtain the melamine molecular imprinting polymer.

[0031] (2) Molecularly imprinted polymer adsorption of carbon quantum dots: 2.0 g of citric acid and 0.8 g of urea were added to a 100 mL round-bottom flask, and the temperature was gradually raised to 200 °C under continuous stirring. The solid reagent began to melt in about 5 min, and the color gradually turned yellow. It turned into a dark brown slightly viscous solution in about 12 min. During this period, the speed of the stirrer needed to be adjusted according to the viscosity of the solution to keep it in a molten state and not stick to the wall of the flask. The NaOH suspension (10 mg mL) of the melamine molecularly imprinted polymer obtained in step (1) was added dropwise at room temperature. -1 ), adjust the pH value to 7.0, stir for 30 min to allow the polymer to fully adsorb carbon quantum dots, and let stand; collect the product in a 10 mL centrifuge tube and centrifuge, discard the upper liquid, wash the obtained solid with water, and vacuum dry to obtain a molecular imprinting polymer adsorbing carbon quantum dots;

[0032] (3) Dissolve the molecular imprinting polymer adsorbing carbon quantum dots obtained in step (2) in water and prepare 0.5 mg mL -1 SiO2-CD S -NH2 solution;

[0033] (4) Prepare a standard curve: Add 3 mL of pH 7.0 buffer solution and 10 uL SiO2-CD to a 10 mL colorimetric tube. S -NH2 solution and 250uL of 1ug·mL -1 Hg 2+ solution, mix thoroughly, and add different volumes of 3.0 g·L -1 melamine solution, and then dilute to 6 mL with sodium dihydrogen phosphate and disodium hydrogen phosphate buffer solution with a pH value of 7.0. After fully mixing, scan its fluorescence emission spectrum at an excitation wavelength of 350 nm, and make a working curve according to the degree of fluorescence recovery;

[0034] (5) Actual dairy product processing: Weigh 5.00 g of milk sample into a 50 mL centrifuge tube, add 5 mL of 5% trichloroacetic acid aqueous solution, vortex shake for 30 min, ultrasonic extraction for 20 min, take the supernatant into a 10 mL centrifuge tube, and centrifuge at 10,000 r / min. -1 Centrifuge at high speed for 15 minutes, retain the supernatant, and complete the treatment after the supernatant passes through a 0.45 um microporous filter membrane to obtain a sample solution;

[0035] (6) Sample detection: Add 3 mL of sodium dihydrogen phosphate and disodium hydrogen phosphate buffer solution with a pH value of 7.0, 10 uL SiO2-CD S -NH2 solution and 250uL of 1ug·mL -1 Hg 2+ solution, shake well, add 50uL of the treated sample solution, and then make up to 6mL with a buffer solution with a pH value of 7.0. After sufficient oscillation, scan its fluorescence emission spectrum at an excitation wavelength of 350nm, measure the fluorescence recovery degree of the system, and calculate the melamine content in the actual sample according to the standard curve. Example 2

[0036] A method for determining melamine in dairy products using a molecular imprinting fluorescence sensor comprises the following steps:

[0037] (1) Synthesis of melamine molecularly imprinted polymer: Take 0.7mL 2.0mol·L -1 NaOH and 100mL5.5×10 -3 mol·L -1 CTAB was mixed and heated to 70°C in a 250mL round-bottom flask, and then 1.0mL LTEOS and 0.1 mmol melamine were added, and heated in a water bath at 70°C. After reacting for 3 hours, the mixture was cooled to room temperature. The product was collected in a 10mL centrifuge tube and centrifuged, the upper layer was discarded, and the mixture was washed with water twice and with ethanol three times. The product was dried to obtain the melamine molecular imprinting polymer.

[0038] (2) Molecularly imprinted polymer adsorption of carbon quantum dots:

[0039] Add 2.0 g of citric acid and 0.8 g of urea to a 100 mL round-bottom flask. Gradually raise the temperature to 200 °C while stirring continuously. The solid reagent begins to melt in about 5 minutes, and the color gradually turns yellow. It turns into a dark brown slightly viscous solution in about 12 minutes. During this period, the speed of the stirrer needs to be adjusted according to the viscosity of the solution to keep it in a molten state and not stick to the wall of the flask. At room temperature, add dropwise the NaOH suspension (10 mg•mL) of the melamine molecular imprinting polymer obtained in step (1).-1 ), adjust the pH value to 7.0, stir for 30 min to allow the polymer to fully adsorb carbon quantum dots, let stand, collect the product in a 10 mL centrifuge tube and centrifuge, discard the upper liquid, wash the obtained solid with water, and vacuum dry to obtain a molecular imprinting polymer adsorbing carbon quantum dots;

[0040] (3) Dissolve the molecular imprinting polymer adsorbing carbon quantum dots obtained in step (2) in water and prepare 0.5 mg mL -1 SiO2-CD S -NH2 solution;

[0041] (4) Prepare a standard curve: Add 3 mL of pH 7.0 buffer solution and 10 uL SiO2-CD to a 10 mL colorimetric tube. S -NH2 solution and 250uL of 1ug·mL -1 Hg 2+ solution, mix thoroughly, and add different volumes of 3.0 g·L -1 melamine solution, and then dilute to 6 mL with sodium dihydrogen phosphate and disodium hydrogen phosphate buffer solution with a pH value of 7.0. After fully mixing, scan its fluorescence emission spectrum at an excitation wavelength of 350 nm, and make a working curve according to the degree of fluorescence recovery;

[0042] (5) Actual dairy product processing: Weigh 2.00 g of milk powder sample, dissolve it in 3 mL of deionized water, mix well, add 5 mL of 5% trichloroacetic acid aqueous solution, vortex and shake for 30 min, ultrasonically extract for 20 min, take the supernatant into a 10 mL centrifuge tube, and centrifuge at 10,000 r•min. -1 Centrifuge at high speed for 15 minutes, retain the supernatant, and complete the treatment after the supernatant passes through a 0.45 um microporous filter membrane to obtain a sample solution;

[0043] (6) Sample detection: Add 3 mL of sodium dihydrogen phosphate and disodium hydrogen phosphate buffer solution with a pH value of 7.0, 10 uL SiO2-CD S -NH2 solution and 250uL of 1ug·mL -1 Hg 2+ solution, shake well, add 50uL of the treated sample solution, and then make up to 6mL with a buffer solution with a pH value of 7.0. After sufficient oscillation, scan its fluorescence emission spectrum at an excitation wavelength of 350nm, measure the fluorescence recovery degree of the system, and calculate the melamine content in the actual sample according to the standard curve.

[0044] Figure 3-4The following are photos of carbon quantum dot solution, melamine molecular imprinted polymer suspension and melamine molecular imprinted polymer suspension without carbon quantum dots adsorbed under sunlight and ultraviolet light. Tube 1 contains melamine molecular imprinted polymer suspension without carbon quantum dots adsorbed, tube 2 contains carbon quantum dot solution prepared by thermal decomposition, and tube 3 contains melamine molecular imprinted polymer suspension adsorbed with carbon quantum dots. Figure 4 It can be seen that the melamine molecular imprinting polymer without adsorbed carbon quantum dots has no fluorescence, and tubes 2 and 3 show strong blue fluorescence. According to the comparison between tubes 1 and 3, the molecular imprinting polymer of melamine effectively adsorbs carbon quantum dots, thus showing strong fluorescence, which lays the foundation for the subsequent quenching of fluorescence by mercury ions and the restoration of fluorescence by melamine.

[0045] Furthermore, the present invention performs transmission electron microscopy analysis on the molecular imprinted polymer adsorbing carbon quantum dots, such as Figure 5-7 As shown in the figure, there is no significant difference in the shapes of melamine molecular imprinted polymers obtained by the two drying methods. They are all spherical and regular in appearance. Figure 7 As shown, the surface of the melamine molecular imprinted polymer adsorbed with carbon quantum dots is no longer clear, indicating that there are indeed a large number of quantum dots adsorbed on the polymer surface.

[0046] Standard curve and detection limit

[0047] Melamine molecularly imprinted polymer-Hg adsorbed with carbon quantum dots 2+ Different amounts of melamine were added to the system to prepare standard systems containing different concentrations of melamine (the concentrations were 2.5, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 ug·mL -1 ), and the degree of fluorescence recovery of all systems was measured. The results are as follows: the maximum fluorescence emission wavelength of carbon quantum dots has nothing to do with the concentration of melamine; but as the concentration of melamine increases, the fluorescence intensity of the system gradually increases, and the degree of fluorescence recovery gradually increases. Figure 1 As shown, when the melamine concentration is between 2.5 and 100 ug·mL -1 In the range of , the fluorescence recovery degree (F-F0) of the melamine molecular imprinting polymer solution adsorbed with carbon quantum dots has a good linear relationship with the melamine concentration, and the regression equation is F-F0=2.950C+3.304, and the linear correlation coefficient is R 2 =0.9967, the detection limit of this method is 2.1ug·mL -1 .

[0048] The influence of coexistence

[0049] This study investigated the effect of possible interfering substances on the analytical results when determining the content of melamine (Mel) in actual samples. -1 The melamine content of 50ug•mL was used as the judgment standard. -1 The coexisting substances have an influence on the fluorescence recovery degree of the molecular imprinting polymer suspension system after fluorescence quenching. Figure 2 As shown in the figure, the effects of six possible coexisting substances, including L-lysine (L-Lys), glucose (Glu), ovalbumin (OVA), L-glutamic acid (L-Glu), L-tyrosine (L-Tyr), and leucine (Leu), were measured. Figure 2 As shown, compared with melamine at the same concentration, Glu, L-Glu, and L-Lys can slightly restore the fluorescence, but not as good as melamine; OVA, L-Tyr, and Leu can slightly quench the fluorescence, and the relative errors of the measurement results are all within +5%. Therefore, these substances do not affect the determination of the melamine content in the actual sample, indicating that the method of the present invention has good selectivity.

[0050] Determination of actual samples

[0051] Milk powder, pure milk, i.e. liquid milk 1-3 (purchased from local supermarkets), water for preparing reagents was double distilled water, and the test was carried out according to the method of Example 1-2.

[0052] Add 3 mL of pH 7.0 buffer solution and 10 uL SiO2-CD to a 10 mL colorimetric tube. S -NH2 solution and 250uL of 1 ug·mL -1 Hg 2+ Solution, shake well, add 50 uL of the treated sample solution, and then dilute to 6mL with a buffer solution with a pH value of 7.0. After sufficient oscillation, scan its fluorescence emission spectrum at an excitation wavelength of 350nm. Determine the fluorescence recovery degree (F-F0) of the system, and then calculate the content of melamine in the actual sample according to the standard curve. Add 100uL of melamine standard solution and further calculate the recovery rate. The specific data are shown in Table 1. The experiment shows that the spiked recovery rate of milk powder is 93%, and the spiked recovery rate of liquid milk is 92%-99%. Melamine was not detected in the four samples, which meets the national limit standards for melamine. The method of the present invention is suitable for the determination of melamine content in dairy products.

[0053] Table 1 Determination of melamine content in dairy products (n=5)

[0054]

[0055] In summary, the present invention uses citric acid and urea as raw materials and hydrolysis method to prepare nitrogen-doped carbon quantum dots, which have stable fluorescence intensity, simple and easy-to-obtain raw materials, and simple preparation method. A melamine molecular imprinted polymer with regular appearance is prepared using tetraethyl orthosilicate and melamine as raw materials, which has a good recognition effect on melamine. The present invention utilizes the melamine molecular imprinted polymer adsorbed with carbon quantum dots to emit strong fluorescence, and through Hg 2+ Based on the principle that the fluorescence is quenched by occupying the holes on the imprinted polymer, and the fluorescence can be restored by "pulling" mercury ions after adding melamine, a method for measuring the melamine content in dairy products was designed. The experiment showed that the maximum excitation wavelength and maximum emission wavelength of the melamine molecular imprinted polymer solution adsorbed with carbon quantum dots are located at 350nm and 441nm respectively. Melamine is 2.5~100ug·mL -1 The concentration range is linearly related to the degree of fluorescence recovery of the system (R 2 =0.9967), the detection limit of this method is 2.1ug·mL -1 The spiked recovery rates of milk powder and liquid milk were 93% and 92% to 99%, respectively. This method has high sensitivity and good selectivity and is suitable for the determination of melamine in dairy products.

[0056] It should be noted that the above embodiments are only partial embodiments of the preferred methods of implementing the present invention, rather than all embodiments. Obviously, based on the above embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

Claims

1. A method for determining melamine in dairy products using a molecular imprinting fluorescence sensor, characterized in that: The following steps are involved: (1) Synthesis of melamine molecularly imprinted polymer; (2) Molecularly imprinted polymer adsorbing carbon quantum dots: 2.0 g of citric acid and 0.8 g of urea were added to a 100 mL round-bottom flask, and the temperature was gradually raised to 200° C. under continuous stirring. The rotation speed was continuously adjusted to keep the mixture in a molten state and not stick to the wall of the flask. The mixture was then cooled to room temperature. The NaOH suspension of the melamine molecularly imprinted polymer obtained in step (1) was added dropwise at room temperature. The pH value was adjusted to 7.

0. The mixture was stirred for 30 min to allow the polymer to fully adsorb carbon quantum dots. The mixture was allowed to stand. The product was collected in a centrifuge tube and centrifuged. The upper layer of liquid was discarded. The obtained solid was washed with water and dried under vacuum to obtain a molecularly imprinted polymer adsorbing carbon quantum dots. (3) Dissolve the molecular imprinting polymer adsorbing carbon quantum dots obtained in step (2) in water and prepare 0.5 mg mL -1 SiO2-CD S -NH2 solution; (4) Prepare a standard curve: Add 3 mL of pH 7.0 buffer solution and 10 uL SiO2-CD to a 10 mL colorimetric tube. S -NH2 solution and 250uL of 1ug·mL -1 Hg 2+ solution, mix thoroughly, and add different volumes of 3.0 g·L -1 melamine solution, and then dilute to 6 mL with a buffer solution with a pH value of 7.

0. After fully mixing, scan its fluorescence emission spectrum at an excitation wavelength of 350 nm, and make a working curve according to the degree of fluorescence recovery; (5) Actual dairy product processing: Weigh 5.00 g of milk sample into a 50 mL centrifuge tube, add 5 mL of 5% trichloroacetic acid aqueous solution, vortex shake for 30 min, ultrasonic extraction for 20 min, take the supernatant into a 10 mL centrifuge tube, and centrifuge at 10,000 r / min. -1 Centrifuge at high speed for 15 minutes, retain the supernatant, and complete the treatment after the supernatant passes through a 0.45 um microporous filter membrane to obtain a sample solution; (6) Sample detection: Add 3 mL of pH 7.0 buffer solution and 10 uL SiO2-CD to a 10 mL colorimetric tube. S -NH2 solution and 250uL of 1ug·mL -1 Hg 2+ solution, shake well, add 50uL of the treated sample solution, and then dilute to 6mL with a buffer solution with a pH value of 7.

0. After sufficient shaking, scan its fluorescence emission spectrum at an excitation wavelength of 350nm to measure the fluorescence recovery degree of the system, and calculate the content of melamine in the actual sample according to the standard curve; Step (1) The method for synthesizing melamine molecular imprinting polymer is as follows: take 0.7 mL 2.0 mol·L -1 NaOH and 100mL5.5×10 -3 mol·L -1 CTAB was mixed and heated to 70°C in a 250mL round-bottom flask, and then 1.0mL LTEOS and 0.1mmol melamine were added, and heated in a water bath at 70°C. After reacting for 3 hours, the mixture was cooled to room temperature. The product was collected in a 10mL centrifuge tube and centrifuged, the supernatant was discarded, and the mixture was washed with water twice and with ethanol three times. After drying, the product was the melamine molecular imprinting polymer.

2. The method for determining melamine in dairy products using a molecular imprinting fluorescence sensor according to claim 1, characterized in that: Step (2) The mass concentration of the NaOH suspension of melamine molecularly imprinted polymer is 10 mg mL -1 .

3. The method for determining melamine in dairy products using a molecular imprinting fluorescence sensor according to claim 1, characterized in that: Step (5), when the sample is milk powder, the sample treatment method is as follows: weigh 2.00 g of the milk powder sample, dissolve it in 3 mL of deionized water, mix it, add 5 mL of a 5% trichloroacetic acid aqueous solution, vortex and shake it for 30 minutes, perform ultrasonic extraction for 20 minutes, take the supernatant into a 10 mL centrifuge tube, and centrifuge it at 10000 r•min. -1 Centrifuge at high speed for 15 minutes, retain the supernatant, and complete the treatment after the supernatant passes through a 0.45 um microporous filter membrane to obtain a sample solution.

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