Preparation method and application of polysaccharide-based fluorescent carbon quantum dots with adjustable emission wavelength

Abstract

The invention relates to a preparation method and application of polysaccharide-based fluorescent carbon quantum dots with adjustable emission wavelength, and belongs to the technical field of preparation of fluorescent carbon nanomaterials. Polysaccharide is respectively mixed with different nitrogen dopants o-phenylenediamine, m-phenylenediamine and p-phenylenediamine, then the mixture is subjected to a hydrothermal reaction to respectively prepare blue, green and yellow green fluorescent carbon quantum dots, and then the blue, green and yellow green fluorescent carbon quantum dots are applied to nitrite detection. The carbon quantum dots with different emission wavelengths and different colors are obtained by adjusting the types of the nitrogen doping agents, the problems that the emission wavelengths of the carbon quantum dots cannot be adjusted and the range is narrow are solved, and reference is provided for subsequent design and opening of the generator-based multicolor fluorescent carbon quantum dots. Meanwhile, the biological polysaccharide is used as a raw material, deionized water is used as a solvent, the whole technological process is easy to operate and low in cost, and the method has far-reaching significance on further high-value utilization of biomass resources and promotion of sustainable utilization of the biomass resources.

Description

Technical Field

[0001] The present invention relates to a preparation method and application of polysaccharide-based fluorescent carbon quantum dots with adjustable emission wavelength, belonging to the technical field of preparation of fluorescent carbon nanomaterials. Background Art

[0002] Nitrite (NO2 - ) is a commonly used food additive. It can not only effectively prevent food oxidation and degradation, but also inhibit the growth of food bacteria and improve the color, aroma and taste of food, playing an essential role in food preservation. However, NO2 - will be absorbed by the human body after ingestion, and a large amount of NO2 - ingested in a short time will cause poisoning due to hypoxia and increase the risk of cancer. Bleaching, cooking, freezing, sterilization and storage time, etc. will all affect the content of NO2 - in food. To avoid health problems, the addition amount of NO2 - is strictly controlled. According to the regulations of the World Health Organization (WHO), the maximum limits of NO2 - in food and drinking water are 30 mg·kg -1 and 3 mg·L -1 respectively. Therefore, designing an accurate and sensitive NO2 - detection strategy is of great significance for ensuring food quality and safety.

[0003] Carbon quantum dots are a new type of "zero-dimensional" fluorescent carbon nanomaterials with a size less than 10 nm. Compared with traditional semiconductor quantum dots and organic dyes, carbon quantum dots have excellent photoluminescence, stable physical and chemical properties, easy surface functionalization, excellent water solubility, low toxicity and good biocompatibility, etc. Therefore, they are widely used in sensing, light-emitting devices, cell imaging, anti-counterfeiting and other fields. At present, the research on synthesizing carbon quantum dots using small molecule organic compounds, foods, waste and natural resources, etc. has been widely reported. Among many carbon sources, polysaccharide is a natural polymer material with rich reserves, renewable, good biocompatibility and degradability, and has attracted much attention in the research of carbon quantum dots. However, most of the reported carbon quantum dot preparation methods at present focus on the blue light region. The obtained carbon quantum dots generally emit blue fluorescence under ultraviolet light irradiation, and the emission wavelength is relatively narrow, which will be interfered by the matrix spontaneous blue light widely existing in the water environment and cell environment, and it is difficult to meet the demand for adjustable emission wavelength in different fields, thus restricting the application of carbon quantum dots in sensing, probes, biomedicine, light energy conversion fields and limiting the utilization value and development prospect of carbon quantum dots. Therefore, developing an effective strategy to prepare multi-color carbon quantum dots with adjustable emission wavelength is of great significance for improving their application value and promoting the high-value utilization of biomass resources. Summary of the Invention

[0004] The present invention provides a method for preparing polysaccharide-based fluorescent carbon quantum dots with adjustable emission wavelength and its application. The present invention uses polysaccharides, which are rich in reserves and renewable, as carbon sources, and then different nitrogen dopants (o-phenylenediamine, m-phenylenediamine or p-phenylenediamine) are added respectively. Among them, blue carbon quantum dots can be obtained when o-phenylenediamine is added, green carbon quantum dots with long emission wavelength can be obtained when m-phenylenediamine is added, and yellow-green carbon quantum dots with long emission wavelength can be obtained when p-phenylenediamine is added. Then the obtained green carbon quantum dots are applied to the detection of NO2 - The detection of. By adjusting the types of nitrogen dopants, the present invention obtains carbon quantum dots with different emission wavelengths and different colors, solves the problems of non-adjustable emission wavelength and narrow range of carbon quantum dots, and provides a reference for the subsequent design and development of biomass-based multicolor fluorescent carbon quantum dots.

[0005] A method for preparing polysaccharide-based fluorescent carbon quantum dots with adjustable emission wavelength includes the following steps: The aqueous solution of polysaccharide is mixed with different nitrogen dopants o-phenylenediamine, m-phenylenediamine and p-phenylenediamine respectively, stirred evenly in a closed environment at room temperature, and then hydrothermal reaction is carried out to obtain a carbon dot solution, which is filtered, dialyzed and dried to obtain blue, green and yellow-green nitrogen-doped polysaccharide-based fluorescent carbon quantum dot powders respectively. Among them,

[0006] The polysaccharide is one or more of sodium carboxymethyl cellulose, carboxymethyl cellulose I, carboxymethyl cellulose IV, carboxymethyl cellulose V, hydroxyethyl cellulose, ethyl cellulose, starch or xylan;

[0007] The mass ratio of the polysaccharide to each nitrogen dopant is 1:1.

[0008] Furthermore, when the nitrogen dopant is o-phenylenediamine, the obtained polysaccharide-based fluorescent carbon quantum dots are blue carbon quantum dots, with uniform particle size and spherical shape, the best emission wavelength is 414-423 nm, and the quantum efficiency is 14.1-25.0%.

[0009] Furthermore, when the nitrogen dopant is m-phenylenediamine, the obtained polysaccharide-based fluorescent carbon quantum dots are green carbon quantum dots, with uniform particle size and spherical shape, the best emission wavelength is 501-511 nm, and the quantum efficiency is 15.7-20.6%.

[0010] Furthermore, when the nitrogen dopant is p-phenylenediamine, the obtained polysaccharide-based fluorescent carbon quantum dots are yellow-green carbon quantum dots, with uniform particle size and spherical shape, the best emission wavelength is 519-523 nm, and the quantum efficiency is 4.6-7.2%.

[0011] In the above technical solution, the hydrothermal reaction condition is to react at 220 °C for 24 h.

[0012] In the above technical solution, the stirring conditions are stirring at a stirring rate of 600-1000 rpm for 15-20 min.

[0013] In the above technical solution, the carbon dot solution is filtered through a 0.22 μm aqueous filter membrane and dialyzed with deionized water in a dialysis membrane with a molecular weight cut-off of 1000 Da for 48 h.

[0014] In the method of the present invention, the main function of using the nitrogen dopant is to improve the quantum efficiency of the obtained carbon quantum dots and adjust the wavelength range. Different nitrogen dopants result in different colors and emission wavelengths of the obtained carbon quantum dots.

[0015] Compared with other types of nitrogen dopants, the nitrogen dopant of the present invention has an amino group and a benzene ring structure. The amino group can react with the carboxyl group in the polysaccharide, and then introduce the benzene ring at the same time. The amount of benzene ring introduced during the synthesis of carbon dots can be inferred by measuring the element content and the size of the carbon dots. The more benzene rings are introduced, the stronger its ability to adjust the wavelength.

[0016] Another object of the present invention is to provide the green polysaccharide-based fluorescent carbon quantum dots prepared by the above method or the application of the green polysaccharide-based fluorescent carbon quantum dots in the detection of nitrite.

[0017] Further, the fluorescence intensity of the green polysaccharide-based fluorescent carbon quantum dots is linearly related to the concentration of NO2 - with a detection limit of 0.42 μM and a detectable reliable concentration range of 2-250 μmol / L.

[0018] A detection method for nitrous acid based on the obtained green polysaccharide-based fluorescent carbon quantum dots includes the following steps: After mixing the sample solution to be measured, the aqueous solution of green carbon quantum dots and the acetic acid buffer solution with a pH of 3.5, standing in the dark for 15 min, testing its fluorescence intensity at an excitation wavelength of 397 nm and an emission wavelength of 503 nm, and calculating the concentration of NO2 in the sample solution to be measured. - concentration.

[0019] Further, the volume ratio of the sample solution to be measured, the green carbon quantum dot solution, and the acetic acid buffer solution is 5:12:283.

[0020] Further, the concentration of the aqueous solution of green carbon quantum dots is 1 mg / mL.

[0021] Further, the concentration of NO2 in the sample solution to be measured is calculated according to the formula F0 / F = 0.0399C + 1.1351, where C is the concentration of NO2 in the sample solution to be measured, F0 is the fluorescence intensity of the blank sample, and F is the fluorescence intensity of the carbon quantum dot solution after adding the sample solution to be measured. - concentration, F0 is the fluorescence intensity of the blank sample, and F is the fluorescence intensity of the carbon quantum dot solution after adding the sample solution to be measured. - concentration, F0 is the fluorescence intensity of the blank sample, and F is the fluorescence intensity of the carbon quantum dot solution after adding the sample solution to be measured.

[0022] Another object of the present invention is to provide a method for preparing a test strip for detecting nitrite, comprising the following steps: Soaking qualitative filter paper in a 0.2 mol·L -1 HCl solution for 30 min, then washing it 3 times with ultrapure water to remove excess hydrochloric acid, and then soaking it in a mixed solution containing an ethanol / water solution and 3-aminopropyltriethoxysilane for 40 min to graft amino groups. Subsequently, an aqueous solution of green carbon quantum dots, an aqueous solution of N-hydroxysuccinimide, and a mixed aqueous solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide are added in sequence, and shaken for 5 h to obtain the product.

[0023] Further, the green carbon quantum dots are the green polysaccharide-based fluorescent carbon quantum dots prepared by the above method.

[0024] Further, the volume ratio of the ethanol / water solution, 3-aminopropyltriethoxysilane, the aqueous solution of green carbon quantum dots, N-hydroxysuccinimide, and the mixed aqueous solution of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide is 20:1:16:12.

[0025] Further, the concentration of the aqueous solution of green carbon quantum dots is 2 mg / mL.

[0026] Further, in the mixed aqueous solution of N-hydroxysuccinimide and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, the concentration of N-hydroxysuccinimide is 10 mg / mL, the concentration of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide is 20 mg / mL, and the mass ratio of N-hydroxysuccinimide to 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide is 1:2.

[0027] Further, the volume ratio of ethanol to water in the ethanol / water solution is 1:1.

[0028] An application method of a test strip for detecting nitrite obtained based on the above preparation method, comprising the following steps: Mixing a test sample solution and an acetic acid buffer solution with a pH of 3.5, and uniformly dropping the mixture onto the surface of the test strip, standing for 15 min, testing the fluorescence intensity at an excitation wavelength of 397 nm and an emission wavelength of 503 nm, and calculating the NO2 - concentration in the test sample solution.

[0029] Further, the volume ratio of the test sample solution to the acetic acid buffer solution is 1:4.

[0030] Further, the NO2 - concentration in the test sample solution is calculated according to the formula F0 / F = 0.0181C + 1.1567, where C is the NO2 -The concentration, where F0 is the fluorescence intensity of the blank sample and F is the fluorescence intensity of the carbon quantum dot solution after adding the sample solution to be measured.

[0031] Furthermore, the detection limit of the test strip for detecting nitrite obtained by the above preparation method is 1.99 μM, and the reliable detection concentration range is 2 - 60 μmol / L.

[0032] In the above method for detecting nitrite, when the sample to be measured is a liquid, it can be directly measured; when the sample to be measured is a solid, the solvent of the sample solution to be measured is water.

[0033] Advantages of the present invention:

[0034] 1. The preparation method provided by the present invention can effectively adjust the emission wavelength of polysaccharide-based fluorescent carbon quantum dots. By adding different nitrogen dopants, carbon quantum dots with different emission wavelengths and colors are obtained. The optimal emission wavelengths are 414 - 423 nm, 501 - 511 nm, and 519 - 523 nm respectively, and the fluorescence colors are blue, green, and yellowish-green respectively. The present invention establishes a method for hydrothermally preparing carbon quantum dots with adjustable fluorescence colors using biological polysaccharides as raw materials. Using deionized water as the solvent and without using any strong acids, strong bases, or toxic reagents, the entire process flow is simple to operate; by changing different carbon sources, it is confirmed that this method is universal for polysaccharide raw materials, providing a reference for the subsequent design and development of biomass-based multicolor fluorescent carbon quantum dots.

[0035] 2. The nitrogen-doped polysaccharide-based fluorescent carbon quantum dots prepared by the present invention can be applied to the detection of NO2 - . Nitrite is detected by the fluorescence quenching of green fluorescent carbon quantum dots when the functional groups on the carbon dots react with nitrite ions. The method is simple, easy to operate, highly sensitive, and selective. Its fluorescence intensity has a linear relationship with the concentration of NO2 - , and the detection limit is 0.42 μM.

[0036] 3. The test strip for detecting nitrite provided by the present invention has a linear relationship between the fluorescence intensity and the concentration of NO2 - , with a detection limit of 1.99 μM, high specificity, simple operation, and has application value. Description of the Drawings

[0037] Figure 1 It is the transmission electron microscope image and particle size distribution diagram of the blue fluorescent carbon quantum dots prepared in Example 1.

[0038] Figure 2 It is the transmission electron microscope image and particle size distribution diagram of the green fluorescent carbon quantum dots prepared in Example 1.

[0039] Figure 3Transmission electron microscopy image and particle size distribution diagram of the yellow-green fluorescent carbon quantum dots prepared in Example 1.

[0040] Figure 4 UV irradiation physical images of the blue, green, and yellow-green fluorescent carbon quantum dots prepared in Example 1.

[0041] Figure 5 Excitation / emission spectrum of the blue fluorescent carbon quantum dots prepared in Example 1.

[0042] Figure 6 Excitation / emission spectrum of the green fluorescent carbon quantum dots prepared in Example 1.

[0043] Figure 7 Excitation / emission spectrum of the yellow-green fluorescent carbon quantum dots prepared in Example 1.

[0044] Figure 8 For the green fluorescent carbon quantum dots prepared in Example 1 with 2- Fluorescence spectra varying with NO concentration.

[0045] Figure 9 For the test strip based on green fluorescent carbon quantum dots prepared in Example 1 with 2- Fluorescence spectra varying with NO concentration. Detailed implementation manners

[0046] The following non-limiting examples can enable those of ordinary skill in the art to understand the present invention more comprehensively, but do not limit the present invention in any way.

[0047] In the following examples, the test methods are conventional methods unless otherwise specified; the reagents and materials can be obtained from commercial sources unless otherwise specified.

[0048] Example 1

[0049] Add 0.2 g of sodium carboxymethylcellulose to 10 mL of ultrapure water, continuously stir on a magnetic stirrer until completely dissolved, and then add 0.2 g of o-phenylenediamine, m-phenylenediamine, or p-phenylenediamine (each diamine needs to be experimented separately) to the above solution. Stir at room temperature and seal for 15 min at a rotation speed of 600 rpm; then carry out a hydrothermal reaction at 220 °C for 24 h to obtain a carbon dot solution. Then filter the carbon dot solution through a 0.22 μm aqueous filter membrane and dialyze it in a dialysis membrane (MWCO 1000 Da) with deionized water for 48 h, and then dry the dialyzed filtrate to obtain carbon quantum dots of different colors.

[0050] When o - phenylenediamine was added, blue polysaccharide - based fluorescent carbon quantum dot powder was obtained. Its diameter was 3.01 nm, the best emission peak was 420 nm, and the fluorescence quantum efficiency was 19.2%; when m - phenylenediamine was added, green polysaccharide - based fluorescent carbon quantum dot powder was obtained. Its diameter was 4.12 nm, the best emission peak was 509 nm, and the fluorescence quantum efficiency was 20.0%; when p - phenylenediamine was added, yellow - green polysaccharide - based fluorescent carbon quantum dot powder was obtained. Its diameter was 4.63 nm, the best emission peak was 522 nm, and the fluorescence quantum efficiency was 5.7%. The TEM images of the three kinds of carbon quantum dots obtained in this example are shown in Figures 1 to 3 , combined with the above particle size results, it can be seen that when o - phenylenediamine, m - phenylenediamine or p - phenylenediamine was added, the diameter of the obtained carbon quantum dots increased, which in turn led to a red - shift of their emission wavelength.

[0051] Example 2

[0052] 0.2 g of carboxymethyl cellulose Ⅰ was added to 10 mL of ultrapure water and continuously stirred on a magnetic stirrer until completely dissolved. Then 0.2 g of o - phenylenediamine, m - phenylenediamine or p - phenylenediamine (each diamine needed to be experimented separately) was added to the above solution. It was stirred at room temperature and sealed for 15 min at a rotation speed of 600 rpm; then hydrothermal reaction was carried out at 220 °C for 24 h to obtain a carbon dot solution. Then the carbon dot solution was filtered through a 0.22 - μm aqueous filter membrane and dialyzed in a dialysis membrane (MWCO 1000 Da) with deionized water for 48 h. After that, the dialyzed filtrate was dried to obtain carbon quantum dots of different colors.

[0053] When o - phenylenediamine was added, blue polysaccharide - based fluorescent carbon quantum dot powder was obtained, the best emission peak was 419 nm, and the fluorescence quantum efficiency was 18.7%; when m - phenylenediamine was added, green polysaccharide - based fluorescent carbon quantum dot powder was obtained, the best emission peak was 511 nm, and the fluorescence quantum efficiency was 17.4%; when p - phenylenediamine was added, yellow - green polysaccharide - based fluorescent carbon quantum dot powder was obtained, the best emission peak was 521 nm, and the fluorescence quantum efficiency was 4.7%.

[0054] Example 3

[0055] 0.2 g of carboxymethyl cellulose Ⅳ was added to 10 mL of ultrapure water and continuously stirred on a magnetic stirrer until completely dissolved. Then 0.2 g of o - phenylenediamine, m - phenylenediamine or p - phenylenediamine (each diamine needed to be experimented separately) was added to the above solution. It was stirred at room temperature and sealed for 15 min at a rotation speed of 600 rpm; then hydrothermal reaction was carried out at 220 °C for 24 h to obtain a carbon dot solution. Then the carbon dot solution was filtered through a 0.22 - μm aqueous filter membrane and dialyzed in a dialysis membrane (MWCO 1000 Da) with deionized water for 48 h. After that, the dialyzed filtrate was dried to obtain carbon quantum dots of different colors.

[0056] When o-phenylenediamine was added, blue polysaccharide-based fluorescent carbon quantum dot powder was obtained, with the best emission peak at 421 nm and a fluorescence quantum efficiency of 19.3%; when m-phenylenediamine was added, green polysaccharide-based fluorescent carbon quantum dot powder was obtained, with the best emission peak at 511 nm and a fluorescence quantum efficiency of 15.7%; when p-phenylenediamine was added, yellowish-green polysaccharide-based fluorescent carbon quantum dot powder was obtained, with the best emission peak at 520 nm and a fluorescence quantum efficiency of 7.2%.

[0057] Example 4

[0058] Carboxymethyl cellulose Ⅳ was replaced with carboxymethyl cellulose Ⅴ, and other operations were the same as in Example 3. When o-phenylenediamine was added, blue polysaccharide-based fluorescent carbon quantum dots were obtained, with the best emission peak at 421 nm and a fluorescence quantum efficiency of 25.0%; when m-phenylenediamine was added, green carbon quantum dots were obtained, with the best emission peak at 511 nm and a fluorescence quantum efficiency of 16.6%; when p-phenylenediamine was added, yellowish-green carbon quantum dots were obtained, with the best emission peak at 520 nm and a fluorescence quantum efficiency of 4.6%.

[0059] Example 5

[0060] Carboxymethyl cellulose Ⅳ was replaced with hydroxyethyl cellulose, and other operations were the same as in Example 3. When o-phenylenediamine was added, blue carbon quantum dots were obtained, with the best emission peak at 423 nm and a fluorescence quantum efficiency of 15.0%; when m-phenylenediamine was added, green carbon quantum dots were obtained, with the best emission peak at 505 nm and a fluorescence quantum efficiency of 16.8%; when p-phenylenediamine was added, yellowish-green carbon quantum dots were obtained, with the best emission peak at 519 nm and a fluorescence quantum efficiency of 6.2%.

[0061] Example 6

[0062] Carboxymethyl cellulose Ⅳ was replaced with starch, and other operations were the same as in Example 3. When o-phenylenediamine was added, blue carbon quantum dots were obtained, with the best emission peak at 415 nm and a fluorescence quantum efficiency of 14.1%; when m-phenylenediamine was added, green carbon quantum dots were obtained, with the best emission peak at 505 nm and a fluorescence quantum efficiency of 18.6%; when p-phenylenediamine was added, yellowish-green carbon quantum dots were obtained, with the best emission peak at 520 nm and a fluorescence quantum efficiency of 6.8%.

[0063] Example 7

[0064] Replace carboxymethyl cellulose Ⅳ with ethyl cellulose, and keep other operations the same as in Example 3. When o-phenylenediamine is added, the obtained blue carbon quantum dots have an optimal emission peak at 423 nm and a fluorescence quantum efficiency of 21.0%; when m-phenylenediamine is added, the obtained green carbon quantum dots have an optimal emission peak at 504 nm and a fluorescence quantum efficiency of 20.6%; when p-phenylenediamine is added, the obtained yellowish-green carbon quantum dots have an optimal emission peak at 521 nm and a fluorescence quantum efficiency of 5.5%.

[0065] Example 8

[0066] Replace carboxymethyl cellulose Ⅳ with xylan, and keep other operations the same as in Example 3. When o-phenylenediamine is added, the obtained blue carbon quantum dots have an optimal emission peak at 414 nm and a fluorescence quantum efficiency of 22.9%; when m-phenylenediamine is added, the obtained green carbon quantum dots have an optimal emission peak at 501 nm and a fluorescence quantum efficiency of 18.0%; when p-phenylenediamine is added, the obtained yellowish-green carbon quantum dots have an optimal emission peak at 523 nm and a fluorescence quantum efficiency of 4.8%.

[0067] Example 9

[0068] Application of the green fluorescent carbon quantum dots obtained based on Example 1 in the detection of NO2 - :

[0069] Prepare NO2 solutions with different concentrations (0 - 300 μM) using deionized water, prepare an acetic acid buffer solution with a pH of 3.5, and prepare a 1 mg / mL carbon quantum dot solution from the green fluorescent carbon quantum dot powder obtained in Example 1. Add 120 μL of the prepared carbon quantum dot solution, 2830 μL of the acetic acid buffer solution, and 50 μL of the NO2 solution into a centrifuge tube, let it stand for 15 min, and then use a fluorescence spectrophotometer to measure the fluorescence intensity of the obtained mixed solution at an excitation wavelength of 397 nm and an emission wavelength of 503 nm. The results are shown in - Figure 8 - , and it can be seen that: the presence of NO2 will cause quenching of the carbon quantum dots and a decrease in fluorescence intensity; under the experimental conditions, when the actual concentration of NO2 in the mixed solution is 30 μM, the fluorescence intensity decreases by about 50%, indicating that NO2 has a significant quenching effect on the carbon quantum dots prepared by this method. At the same time, the data is fitted, and the fitting results show that as the concentration of the NO2 solution increases, the fluorescence intensity of the carbon quantum dots gradually decreases. When the actual concentration of NO2 is 2 - 250 μmol / L, the fluorescence intensity and the concentration of NO2 Figure 8 show that - the presence of NO2 - will cause quenching of the carbon quantum dots and a decrease in fluorescence intensity; under the experimental conditions, when the actual concentration of NO2 in the mixed solution is 30 μM, the fluorescence intensity decreases by about 50%, indicating that NO2 - has a significant quenching effect on the carbon quantum dots prepared by this method. At the same time, the data is fitted, and the fitting results show that as the concentration of the NO2 solution increases, the fluorescence intensity of the carbon quantum dots gradually decreases. When the actual concentration of NO2 is 2 - 250 μmol / L, the fluorescence intensity and the concentration of NO2 - show that - the presence of NO2 -showed a good linear relationship, and the fitting equation was F0 / F = 0.0399C + 1.1351, R 2 was 0.9949, and the detection limit was 0.42 μM, meeting the requirements of practical applications.

[0070] Example 10

[0071] A preparation method and application process of a test strip for detecting nitrite based on the green fluorescent carbon quantum dots obtained in Example 1:

[0072] Soak qualitative filter paper (25 mm × 25 mm) in 0.2 mol·L -1 HCl solution for 30 min, and then wash it 3 times with ultrapure water to remove excess hydrochloric acid. Then put the above-treated qualitative filter paper into a beaker containing 30 mL ethanol / water (v / v = 1:1) solution and 1.5 mL 3-aminopropyltriethoxysilane and soak for 40 min to graft amino groups. Subsequently, dissolve 0.05 g of green fluorescent carbon quantum dots (G-CDs) powder in 24 mL of deionized water and add it to the above beaker. Finally, add 18 mL of a mixed aqueous solution of N-hydroxysuccinimide and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (the preparation process is: take 180 mg of N-hydroxysuccinimide and 360 mg of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and dissolve them in 18 mL of deionized water to make the concentrations of N-hydroxysuccinimide and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide 10 mg / mL and 20 mg / mL respectively), shake for 5 h to graft G-CDs onto the filter paper surface, and thus obtain a test strip for detecting nitrite.

[0073] Drop 200 μL of the sample solution to be tested and 800 μL of acetic acid buffer solution with a pH of 3.5 evenly onto the surface of the test strip, let it stand for 15 min, and then use a fluorescence spectrophotometer to measure its fluorescence intensity at an excitation wavelength of 397 nm and an emission wavelength of 503 nm. The results are shown in Figure 9 , and it can be seen that the presence of NO2 - will cause the fluorescence intensity of the test strip to decrease. When the actual concentration of NO2 - is 30 μM, the fluorescence intensity decreases by about 50%, indicating that NO2 - has a significant quenching effect on the carbon quantum dot test strip prepared by this method. At the same time, the data is fitted, and the fitting results show that as the concentration of the NO2 - solution increases, the fluorescence intensity of the carbon quantum dots gradually decreases. When the actual concentration of NO2 - is 2 - 60 μmol / L, the fluorescence intensity and the concentration of NO2 - showed a good linear relationship, and the fitting equation was F0 / F = 0.0181C + 1.1567, R2 It is 0.9908, and the detection limit is 1.99 μM, meeting the requirements of practical applications.

[0074] The fitting equations obtained from Example 9 and Example 10 can be used to calculate the concentration of NO2 in the sample solution to be measured. - concentration.

[0075] The above experiments show that the carbon quantum dots prepared by this method have good detection performance for NO2 - and have an extremely low detection limit, showing good application prospects for detecting NO2 in food. -

[0076] Table 1 Optimal excitation, emission wavelengths and quantum yields of the carbon quantum dots obtained from Examples 1 - 8

[0077]

[0078] ​

Claims

1. A method for preparing polysaccharide-based fluorescent carbon quantum dots with adjustable emission wavelength, characterized in that: The polysaccharide aqueous solution was mixed with different nitrogen doping agents o-phenylenediamine, m-phenylenediamine and p-phenylenediamine, respectively, and then stirred evenly in a closed environment at room temperature for hydrothermal reaction to obtain a carbon dot solution, which was filtered, dialyzed and dried to obtain blue, green and yellow-green nitrogen-doped polysaccharide-based fluorescent carbon quantum dot powders, respectively. The polysaccharide is one or more of sodium carboxymethyl cellulose, carboxymethyl cellulose I, carboxymethyl cellulose IV, carboxymethyl cellulose V, hydroxyethyl cellulose, ethyl cellulose, starch or xylan; The mass ratio of the polysaccharide to the nitrogen dopant is 1:

1.

2. The preparation method according to claim 1, characterized in that: The hydrothermal reaction conditions are 220° C. for 24 h.

3. The preparation method according to claim 1, characterized in that: The carbon dot solution was filtered through a 0.22 μm water filter membrane and dialyzed against deionized water in a dialysis membrane with a molecular weight cutoff of 1000 Da for 48 h.

4. The preparation method according to claim 1, characterized in that: When the nitrogen dopant is o-phenylenediamine, the obtained polysaccharide-based fluorescent carbon quantum dots are blue carbon quantum dots with uniform particle size and spherical shape, the best emission wavelength is 414-423 nm, and the quantum efficiency is 14.1-25.0%.

5. The preparation method according to claim 1, characterized in that: When the nitrogen dopant is m-phenylenediamine, the obtained polysaccharide-based fluorescent carbon quantum dots are green carbon quantum dots with uniform particle size and spherical shape, the best emission wavelength is 501-511 nm, and the quantum efficiency is 15.7-20.6%.

6. The preparation method according to claim 1, characterized in that: When the nitrogen dopant is p-phenylenediamine, the obtained polysaccharide-based fluorescent carbon quantum dots are yellow-green carbon quantum dots with uniform particle size and spherical shape, the best emission wavelength is 519-523 nm, and the quantum efficiency is 4.6-7.2%.

7. Use of the green polysaccharide-based fluorescent carbon quantum dots prepared by the method according to any one of claims 1 to 3 or the green polysaccharide-based fluorescent carbon quantum dots according to claim 5 in nitrite detection, characterized in that: The fluorescence intensity of the green polysaccharide-based fluorescent carbon quantum dots is related to NO2 - The concentration showed a linear relationship, the detection limit was 0.42μM, and the detection credible concentration range was 2-250μmol / L.

8. The use according to claim 7, characterized in that: After mixing the sample solution, green carbon quantum dot aqueous solution and acetic acid buffer solution with a pH of 3.5, the mixture was kept in dark for 15 minutes, and the fluorescence intensity was tested at an excitation wavelength of 397 nm and an emission wavelength of 503 nm. The NO2 in the sample solution was calculated. - Concentration, wherein the volume ratio of the sample solution to be tested, the green carbon quantum dot aqueous solution, and the acetic acid buffer solution is 5:12:283; the concentration of the green carbon quantum dot aqueous solution is 1 mg / mL.

9. A method for preparing a test strip for detecting nitrite, characterized in that: The qualitative filter paper was placed at 0.2 mol·L -1 After soaking in HCL solution for 30 minutes, it was washed with ultrapure water three times to remove excess hydrochloric acid, and then soaked in a mixed solution containing ethanol / water solution and 3-aminopropyltriethoxysilane for 40 minutes to graft amino groups, and then green carbon quantum dot aqueous solution, N-hydroxysuccinimide and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide mixed aqueous solution were added in sequence, and shaken for 5 hours to obtain, wherein, The green carbon quantum dots are green polysaccharide-based fluorescent carbon quantum dots prepared by the method according to any one of claims 1 to 3 or the green polysaccharide-based fluorescent carbon quantum dots according to claim 5, and the concentration of the green carbon quantum dot aqueous solution is 2 mg / mL; The volume ratio of the ethanol / water solution, 3-aminopropyltriethoxysilane, green carbon quantum dot aqueous solution, and N-hydroxysuccinimide and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide mixed aqueous solution is 20:1:16:12; In the mixed aqueous solution of N-hydroxysuccinimide and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, the concentration of N-hydroxysuccinimide is 10 mg / mL, the concentration of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide is 20 mg / mL, and the mass ratio of N-hydroxysuccinimide to 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide is 1:2; The volume ratio of ethanol to water in the ethanol / water solution is 1:

1.

10. Use of the test strip for detecting nitrite prepared by the method of claim 9, characterized in that: The sample solution to be tested and the acetate buffer solution with a pH of 3.5 were mixed and evenly added to the surface of the test strip. After standing for 15 minutes, the fluorescence intensity was tested at an excitation wavelength of 397 nm and an emission wavelength of 503 nm. The NO2 in the sample solution to be tested was calculated. - Concentration, where The volume ratio of the sample solution to be tested to the acetate buffer solution is 1:4; The fluorescence intensity of the test strip is related to NO2 - The concentration showed a linear relationship, the detection limit was 1.99μM, and the detection credible concentration range was 2-60μmol / L.

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