Polyvinyl alcohol composite film, preparation method thereof and application of polyvinyl alcohol composite film in food coloring detection

Abstract

The invention discloses a polyvinyl alcohol composite film as well as a preparation method and application thereof in food coloring detection. The polyvinyl alcohol composite film is prepared from raw materials including phenylboronic acid modified carbon dots and polyvinyl alcohol through a thermocuring reaction, a phenylboronic acid group on the surface of the phenylboronic acid modified carbon dot and polyvinyl alcohol form a reversible covalent bond crosslinking point based on boric acid ester so as to form a dynamic three-dimensional network, and the structural formula of the phenylboronic acid modified carbon dot is # imgabs 1 #. The phenylboronic acid modified carbon dot is simple in process, easy to process and form and simple and convenient to use; compared with the prior art, the carbon dots modified by phenylboronic acid groups and the polyvinyl alcohol are added, the crosslinking effect between the carbon dots modified by the phenylboronic acid groups and the polyvinyl alcohol is remarkably enhanced, the mechanical strength and water resistance of the composite film are improved, the food coloring detection capacity of the composite film is improved, and in addition, the composite film further has the characteristic of being capable of being reprocessed, so that the composite film has wide application value.

Description

Technical Field

[0001] The present invention belongs to the technical field of pigment detection materials, and particularly relates to a polyvinyl alcohol composite film, a preparation method thereof, and an application thereof in the detection of edible pigments. Background Art

[0002] With the improvement of people's living standards and the enhancement of living requirements, food safety issues have attracted increasing attention. Among them, the safety of edible pigments is particularly prominent. As a food additive, edible pigments are mainly used to improve the appearance of food and enhance its attractiveness. According to the source, edible pigments can be divided into natural pigments and synthetic pigments. The preparation process of natural pigments is complex, the cost is high, and the stability is poor, while synthetic pigments have the advantages of low price, good stability, bright colors, etc. Therefore, in daily applications, synthetic pigments have gradually replaced natural pigments. For example, synthetic pigments such as amaranth, tartrazine, and sunset yellow are often added to foods, especially beverages, to make the products more brightly colored. Although edible pigments are safe in most cases, excessive use or addition of non-compliant pigments may pose potential hazards to human health and may even cause serious problems such as allergies and carcinogenesis. Therefore, how to effectively detect the types and concentrations of edible pigments in foods has become a key topic for ensuring food safety.

[0003] As a new member of nanocarbon materials, carbon dots have many advantages such as good biocompatibility, simple production process, low cost, and environmental friendliness. In addition, carbon dots also have unique fluorescence properties. In the field of pigment detection, carbon dots can be used as fluorescent probes to produce specific responses to pigment molecules, so they have broad application prospects in food safety detection. However, at present, most of the detection applications based on the fluorescence mechanism of carbon dots use carbon dot solutions. In the solution state, due to the small particle size and large surface energy of carbon dots, they are prone to aggregation, resulting in changes in fluorescence and affecting the detection performance. In order to overcome the aggregation problem of carbon dots in the solution state and improve their application effect in the detection field, an effective strategy is to fix carbon dots in a polymer matrix to prepare polymer / carbon dot composites. For example, a study (Spectrochimica Acta Part A:Molecularand Biomolecular Spectroscopy 281(2022)121581) used a hydrothermal method to prepare nitrogen- and sulfur-doped carbon dots and their polyvinyl alcohol hydrogel composites, and this composite material can accurately identify the target pigments in food matrices. However, once the hydrogel material loses water, it is difficult to return to its initial state, and some products using hydrogels are difficult to be reprocessed and reused, which not only causes waste but may also lead to environmental problems. At present, polymer carbon dot composite detection materials still face many challenges, especially there are no reports on polymer carbon dot detection membranes. Summary of the Invention

[0004] The object of the present invention is to overcome the defects of the prior art and provide a polyvinyl alcohol composite film.

[0005] Another object of the present invention is to provide a method for preparing the above polyvinyl alcohol composite film.

[0006] Still another object of the present invention is to provide the application of the above polyvinyl alcohol composite film in the detection of food coloring.

[0007] The technical solution of the present invention is as follows:

[0008] A polyvinyl alcohol composite film is prepared by a thermal curing reaction from raw materials including phenylboronic acid modified carbon dots and polyvinyl alcohol. The structural formula of the phenylboronic acid modified carbon dots is

[0009] The phenylboronic acid groups on the surface of the phenylboronic acid modified carbon dots form reversible covalent bond crosslinking points based on borate esters with polyvinyl alcohol, thereby forming a dynamic three-dimensional network, and its structural formula is .

[0010] In a preferred embodiment of the present invention, the preparation method of the phenylboronic acid modified carbon dots comprises the following steps:

[0011] (1) A piperazine derivative containing an amino group, a pyrazine derivative containing a carboxylic acid group and water are fully mixed and reacted at 185 - 195 °C for 10 - 15 h. Then, after cooling to room temperature, it is successively centrifuged, filtered through a membrane and dialyzed to obtain a carbon dot solution;

[0012] (2) After the carbon dot solution obtained in step (1) is rotary evaporated, it is mixed with a phenylboronic acid compound containing an aldehyde group and methanol, and reacted at room temperature in the dark for 20 - 25 h. Then, it is rotary evaporated to obtain a solid, and the solid is washed with acetone and then vacuum dried to obtain the phenylboronic acid modified carbon dots.

[0013] Further preferably, the piperazine derivative containing an amino group is N-aminoethylpiperazine, and the pyrazine derivative containing a carboxylic acid group is pyrazine-2,3-dicarboxylic acid.

[0014] Even more preferably, the molar ratio of N-aminoethylpiperazine to pyrazine-2,3-dicarboxylic acid is 4:1.

[0015] Further preferably, the phenylboronic acid compound containing an aldehyde group is 4-formylphenylboronic acid.

[0016] The preparation method of the above-mentioned polyvinyl alcohol composite film includes: dissolving polyvinyl alcohol in a mixed solvent of water and methanol to obtain a PVA solution; dissolving phenylboronic acid-modified carbon dot powder in a mixed solvent of water and methanol, and then dropping it into the above-mentioned PVA solution, stirring and refluxing at 130-140 °C for 2-4 h, then performing rotary evaporation and concentration, and then pouring it into a mold, and obtaining the polyvinyl alcohol composite film after standing, drying and thermal curing.

[0017] In a preferred embodiment of the present invention, in the mixed solvent of water and methanol for dissolving phenylboronic acid-modified carbon dot powder, the volume ratio of water to methanol is 3:7; in the mixed solvent of water and methanol for dissolving polyvinyl alcohol, the volume ratio of water to methanol is 1:1.

[0018] The application of the above-mentioned polyvinyl alcohol composite film in the preparation of an edible pigment detection component.

[0019] In a preferred embodiment of the present invention, the edible pigments include sunset yellow, amaranth and tartrazine.

[0020] An edible pigment detection method is carried out by using the above-mentioned polyvinyl alcohol composite film.

[0021] The beneficial effects of the present invention are:

[0022] 1. In the present invention, through a simple and feasible aldehyde-amine condensation reaction, phenylboronic acid groups are grafted onto the surface of carbon dots. This phenylboronic acid-modified molecule not only realizes the passivation of the carbon dot surface and reduces defects, but also the doping effect of boron elements can greatly improve the quantum yield of carbon dots, thereby reducing the detection limit for pigments and enhancing the detection ability.

[0023] 2. In the present invention, the modified carbon dots are blended with polyvinyl alcohol to prepare a PVA / carbon dot composite film. Among them, the carbon dots are fixed in the polymer matrix, effectively avoiding the aggregation of carbon dots; in addition, the phenylboronic acid groups on the surface of the carbon dots can form reversible covalent bond cross-linking points based on borate esters with polyvinyl alcohol. The three-dimensional network constructed thereby provides a stable environment for the carbon dots, and further endows the composite film with better mechanical properties and improves the long-term stability of the composite film.

[0024] 3. The polyvinyl alcohol composite film prepared in the present invention can be used as a fluorescence sensor for detecting a variety of pigments, including sunset yellow, amaranth, tartrazine, etc., showing good universality.

[0025] 4. There is a reversible dynamic chemical covalent bond between the polyvinyl alcohol and the phenylboronic acid group-modified carbon dots in the present invention, which enables the composite film to have the characteristics of being reshaped and reprocessed. Therefore, the production cost of the composite film is low, the production process is environmentally friendly, and it has good industrial application prospects. Description of the Drawings

[0026] Figure 1 Transmission electron microscopy images of (a) unmodified carbon dots C-CDs in Example 1 and (b) modified carbon dots PBA-CDs in Example 2 of the present invention.

[0027] Figure 2 Physical images of the polyvinyl alcohol composite films prepared in the examples of the present invention: (a) PVA / PBA-CDs composite film; (b) PVA / C-CDs composite film; (c) fluorescence emission photographs of the composite films PVA / PBA-CDs and (d) PVA / C-CDs under ultraviolet light irradiation at 365 nm; (e) infrared spectra of C-CDs, PBA-CDs and PVA / PBA-CDs obtained in some examples of the present invention.

[0028] Figure 3 Showing ultraviolet absorption and fluorescence spectra: (a) unmodified carbon dots C-CDs; (b) modified carbon dots PBA-CDs; (c) PVA films, including pure PVA, PVA / C-CDs and PVA / PBA-CDs composite films.

[0029] Figure 4 Fluorescence emission spectra of (a) PVA / C-CDs composite film prepared in Example 1 of the present invention for detecting different concentrations of sunset yellow and (b) linear fitting curve of PVA / C-CDs.

[0030] Figure 5 Fluorescence emission spectra of (a) PVA / PBA-CDs prepared in Example 2 of the present invention for detecting different concentrations of sunset yellow and (b) linear fitting curve of PVA / PBA-CDs.

[0031] Figure 6 Detection comparison chart of sunset yellow by the polyvinyl alcohol composite films prepared in the examples of the present invention: (a) PVA / PBA-CDs; (b) PVA / C-CDs; (c) reprocessing effect diagram of the reshaped PVA / PBA-CDs composite film.

[0032] Figure 7 Detection fitting curves and detection effect diagrams of other pigments by the PVA / PBA-CDs composite film prepared in Example 2 of the present invention: (a, c) tartrazine; (b, d) detection fitting curves of amaranth. Detailed implementation manners

[0033] The technical solutions of the present invention will be further described and illustrated below through specific implementation manners in conjunction with the accompanying drawings.

[0034] Example 1

[0035] (1) Weigh 5 g of pyrazine-2,3-dicarboxylic acid and 3.9 mL of N-aminoethylpiperazine, and add 150 mL of H 2O. After stirring and ultrasonicating until completely dissolved, transfer it to a 250 mL reaction kettle and place it in a blast drying oven. React at 190 °C for 12 h. After the temperature cools to room temperature, take it out, and then successively perform centrifugation, filtration through a membrane filter, and dialysis. The resulting dark yellow solution is the C-CDs solution;

[0036] (2) Dissolve PVA (type 1799) in a mixed solvent of H 2 O and methanol (H 2 O:methanol = 1:1, volume ratio) at 95 °C to obtain a 5 wt% PVA solution; Concentrate the C-CDs solution to 7.5 wt% of its original mass and fully dissolve it in a mixed solvent of H 2 O and methanol (H 2 O:methanol = 3:7, volume ratio), and then slowly drop it into the above PVA solution. Carry out mechanical stirring and condensation reflux reaction at 135 °C for 3 h; The resulting reaction solution is rotary evaporated and concentrated at 60 °C to a solid mass percentage of 10 wt%. Then pour it into a mold of 6 cm × 6 cm × 2 cm (length × width × thickness) and let it stand at room temperature for 24 h. Then dry it in a vacuum drying oven at 60 °C for 24 h, and finally thermally cure it in a blast drying oven at 135 °C for 2 h to obtain a PVA / C-CDs composite film with a thickness of 90 μm ± 10 μm;

[0037] (3) Cut the PVA / C-CDs composite film into 2 cm × 2 cm pieces, then immerse it in sunset yellow pigment solutions with different concentrations. After immersing for 10 min, take out the PVA / C-CDs composite film. Wait for 30 min until the film is completely dry, and then conduct tests.

[0038] Record the fluorescence intensities of the PVA / C-CDs composite film before and after immersion in the sunset yellow pigment solution. Use the fluorescence response intensity (F0 / F) to characterize the quenching ability of sunset yellow on the PVA / C-CDs composite film. F0 and F represent the fluorescence intensities of the PVA / C-CDs composite film in the absence and presence of the pigment, respectively. All fluorescence spectra are obtained at an excitation wavelength of 365 nm.

[0039] Example 2

[0040] (1) Dissolve the oily liquid obtained by rotary evaporation of the C-CDs solution prepared in Example 1 and 4.75 g of 4-formylphenylboronic acid (4-FPBA) in 120 mL of methanol. React for 21 h under dark conditions at room temperature to obtain a brown-yellow solution; Rotary evaporate the brown-yellow solution at 60 °C to obtain a yellow solid. Then wash it three times with acetone to remove the excess 4-FPBA, and then dry it in a vacuum at 60 °C for 4 h. The resulting yellow powder is PBA-CDs, and its structural formula is; Figure 2(e) FT-IR spectra of PBA-CDs, indicating that PBA-CDs contain -COOH, B-OH and -C=N.

[0041] (2) Dissolve PVA (type 1799) in a mixed solvent of H 2 O and methanol (H 2 O:methanol = 1:1, volume ratio) at 95 °C to obtain a 5 wt% PVA solution; dissolve PBA-CDs in a mixed solvent of H 2 O and methanol (H 2 O:methanol = 3:7, volume ratio) and then slowly add it dropwise to the above PVA solution. The mass ratio of PBA-CDs to PVA is 0.25:1. Carry out mechanical stirring and condensation reflux reaction at 135 °C for 3 h; concentrate the obtained reaction solution by rotary evaporation at 60 °C until the mass percentage of the solid is 10 wt%. Then pour it into a mold of 6 cm × 6 cm × 2 cm (length × width × thickness) and let it stand at room temperature for 24 h. Then dry it in a vacuum oven at 60 °C for 24 h. Finally, thermally cure it in a forced-air oven at 135 °C for 2 h to obtain a PVA / PBA-CDs composite film with a thickness of 90 μm ± 10 μm. In this composite film, phenylboronic acid groups form reversible covalent bond cross-linking points based on borate esters with polyvinyl alcohol, thereby forming a dynamic three-dimensional network. Its structural formula is Figure 2 (e) In the FT-IR spectrum of PVA / PBA-CDs, the PVA / PBA-CDs composite film contains -C=N and O-B-O groups, indicating the existence of borate ester bonds between PBA-CDs and PVA.

[0042] (3) Cut the PVA / PBA-CDs composite film into pieces of 2 cm × 2 cm size, then immerse it in sunset yellow solutions with different concentrations. After immersion for 10 min, take out the PVA / PBA-CDs composite film. Wait for 30 min until the film is completely dry and then conduct tests.

[0043] Record the fluorescence intensities of the PVA / PBA-CDs composite film before and after immersion in the sunset yellow solution. Use the fluorescence response intensity (F0 / F) to characterize the quenching ability of sunset yellow on the PVA / PBA-CDs composite film. F0 and F represent the fluorescence intensities of the PVA / PBA-CDs composite film in the absence and presence of the pigment, respectively. All fluorescence spectra are obtained at an excitation wavelength of 365 nm.

[0044] Figure 1 TEM images of C-CDs and PBA-CDs. C-CDs and PBA-CDs are spherical particles and are uniformly dispersed, and there are clear and obvious lattice fringes with a lattice spacing of about 0.22 nm.

[0045] Figure 2Figure 0 shows the physical pictures of the polyvinyl alcohol composite films, indicating that the PVA / PBA-CDs composite film is yellow itself and presents yellowish green under ultraviolet light irradiation, while the PVA / C-CDs composite film is colorless and transparent and presents blue green under ultraviolet light irradiation. (e) shows the infrared spectra of C-CDs, PBA-CDs and PVA / PBA-CDs obtained in the examples of the present invention, indicating that the surface of C-CDs contains functional groups such as carboxyl and amino groups, PBA-CDs contains B-OH groups, and the PVA / PBA-CDs composite film contains -C=N and O-B-O groups, indicating that there is a borate covalent bond between PBA-CDs and PVA.

[0046] Figure 3 Figures 4(a) and (b) show the ultraviolet, fluorescence excitation and emission spectra of C-CDs and PBA-CDs respectively. In the ultraviolet-visible absorption spectrum, there is a strong absorption peak at 320 nm, which belongs to the n-π* transition of C=O on -COOH in C-CDs and PBA-CDs, and the absorption peak at 270 nm belongs to the π-π* transition of the heteroaromatic ring in C-CDs and PBA-CDs. The fluorescence spectrum of C-CDs shows that its optimal excitation wavelength is 360 nm and the optimal emission wavelength is 470 nm. The C-CDs solution exhibits obvious blue emission under ultraviolet light at 365 nm. The fluorescence excitation spectrum and emission spectrum of PBA-CDs show that the optimal excitation wavelength of PBA-CDs is 435 nm and the optimal emission wavelength is 515 nm. Therefore, under ultraviolet light irradiation at 365 nm, PBA-CDs emit green fluorescence. (c) shows the fluorescence spectra of PVA films including pure PVA, PVA / C-CDs and PVA / PBA-CDs composite films, indicating that C-CDs and PBA-CDs endow the PVA composite films with fluorescence properties.

[0047] Figure 4 Figures 8(a) and (b) show the fluorescence emission spectrum of the PVA / C-CDs composite film for detecting different concentrations of sunset yellow and the linear fitting curve of PVA / C-CDs. It can be seen that as the concentration of sunset yellow (1-80 μM) increases, the fluorescence emission intensity of the PVA / C-CDs composite film gradually weakens. By the 3σ rule (σ = S0 / S, where S0 is the standard deviation of the blank measurement and S is the slope of the calibration curve), the detection limit of sunset yellow for the PVA / C-CDs composite film is calculated to be 1.56 μM.

[0048] Figure 5 Figure 12 shows the influence of different concentrations of sunset yellow on the fluorescence emission of the PVA / PBA-CDs composite film. As the concentration of sunset yellow (0.1-100 μM) increases, the fluorescence emission intensity of the PVA / PBA-CDs composite film gradually weakens. By the 3σ rule, the detection limit of sunset yellow is calculated to be 40.46 nM.

[0049] Figure 6 Detection comparison chart of sunset yellow by polyvinyl alcohol composite film: (a) PVA / PBA-CDs; (b) PVA / C-CDs. It shows that for the sunset yellow solution with the same concentration, the detection effect of the PVA / PBA-CDs composite film is more obvious than that of the PVA / C-CDs composite film, and the dynamic borate ester bond formed by the phenylboronic acid group on the surface of PBA-CDs and polyvinyl alcohol endows the PVA / PBA-CDs composite film with reprocessing performance.

[0050] Figure 7 Detection fitting curves and detection effect diagrams of other pigments by the PVA / PBA-CDs composite film: (a, c) Detection fitting curves of tartrazine; (b, d) Detection fitting curves of amaranth. By the 3σ rule, the detection limits of the PVA / PBA-CDs composite film for tartrazine and amaranth are calculated to be 0.184 μM and 0.76 μM, respectively.

[0051] Table 1 below shows the tensile strength, Young's modulus and glass transition temperature data of two composite films, PVA / C-CDs and PVA / PBA-CDs. The phenylboronic acid group on the surface of the modified carbon dots PBA-CDs forms a cross-linking effect with polyvinyl alcohol, and the resulting cross-linking network endows the PVA / PBA-CDs composite film with better mechanical properties and thermal stability.

[0052] Table 1

[0053]

[0054]

[0055] As mentioned above, it is only the preferred embodiment of the present invention, so the scope of implementation of the present invention cannot be limited thereby. That is, equivalent changes and modifications made according to the scope of the present invention patent and the content of the specification should still fall within the scope covered by the present invention.

Claims

1. A polyvinyl alcohol composite film, characterized in that: The carbon dots are prepared by thermal curing reaction of raw materials including phenylboronic acid-modified carbon dots and polyvinyl alcohol. The structural formula of the phenylboronic acid-modified carbon dots is The phenylboronic acid groups on the surface of the phenylboronic acid-modified carbon dots form reversible covalent crosslinking points based on borate esters with polyvinyl alcohol, thereby forming a dynamic three-dimensional network, the structural formula of which is:

2. A polyvinyl alcohol composite film according to claim 1, characterized in that: The preparation method of phenylboronic acid modified carbon dots comprises the following steps: (1) fully mixing an amino group-containing piperazine derivative, a carboxylic acid group-containing pyrazine derivative and water, reacting at 185-195° C. for 10-15 hours, then cooling to room temperature, centrifuging, filtering with a filter membrane and dialyzing in sequence to obtain a carbon dot solution; (2) After rotary evaporation of the carbon dot solution obtained in step (1), it is mixed with an aldehyde-containing phenylboronic acid compound and methanol, and reacted at room temperature in the dark for 20-25 hours, followed by rotary evaporation to obtain a solid, which is then washed with acetone and vacuum dried to obtain phenylboronic acid-modified carbon dots.

3. A polyvinyl alcohol composite film as claimed in claim 2, characterized in that: The piperazine derivative containing an amino group is N-aminoethylpiperazine, and the pyrazine derivative containing a carboxylic acid group is pyrazine-2,3-dicarboxylic acid.

4. A polyvinyl alcohol composite film as claimed in claim 3, characterized in that: The molar ratio of the N-aminoethylpiperazine to pyrazine-2,3-dicarboxylic acid is 4:

1.

5. A polyvinyl alcohol composite film as claimed in claim 2, characterized in that: The phenylboronic acid compound containing an aldehyde group is 4-formylphenylboronic acid.

6. A method for preparing a polyvinyl alcohol composite film according to any one of claims 1 to 5, characterized in that: include: dissolving polyvinyl alcohol in a mixed solvent of water and methanol to obtain a PVA solution; The phenylboric acid modified carbon dot powder is dissolved in a mixed solvent of water and methanol, and then added dropwise to the above PVA solution, stirred and condensed under reflux at 130-140°C for 2-4 hours, and then concentrated by rotary evaporation, and then poured into a mold, and after standing, drying and thermal curing, a polyvinyl alcohol composite film is obtained.

7. The preparation method according to claim 6, characterized in that: In the mixed solvent of water and methanol for dissolving the phenylboronic acid modified carbon dot powder, the volume ratio of water to methanol is 3:7; in the mixed solvent of water and methanol for dissolving polyvinyl alcohol, the volume ratio of water to methanol is 1:

1.

8. Use of the polyvinyl alcohol composite film according to any one of claims 1 to 5 in preparing an edible pigment detection component.

9. The use according to claim 8, characterized in that: The food coloring includes sunset yellow, amaranth and tartrazine.

10. A method for detecting food coloring, characterized in that: The method is carried out using the polyvinyl alcohol composite film according to any one of claims 1 to 5.

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