A preparation method for preparing nitrogen-doped carbon quantum dots based on reeling wastewater

Abstract

The application relates to a method for preparing nitrogen-doped carbon quantum dots based on reeling wastewater, mainly comprising the following steps: pretreatment of reeling wastewater, hydrothermal reaction of the reeling wastewater and L-aspartic acid, centrifugation, dialysis and drying treatment, and obtaining nitrogen-doped carbon quantum dot solid powder; the nitrogen-doped carbon quantum dots prepared by the method are spherical, the average diameter is about 9.8 nm, under ultraviolet excitation light, the nitrogen-doped carbon quantum dots obtained by the method emit blue-green fluorescence; the method is simple in operation, the nitrogen doping based on amino acids can realize the wavelength control of fluorescence emission, with the increase of the nitrogen content, the emission wavelength is continuously increased, and the fluorescence wavelength is red-shifted; the preparation technology based on the application is not only beneficial to solving the reutilization problem of the reeling wastewater, but also has potential application prospects in the fields of biomedicine, energy, environment, electronic information, textiles and the like.

Description

Technical Field

[0001] The present invention belongs to the field of preparation technology, and in particular relates to a method for preparing nitrogen-doped carbon quantum dots based on silk reeling wastewater. Background Art

[0002] my country's silk reeling technology has long been a world leader, making significant contributions to the development of the global sericulture industry. However, the environmental pollution caused by reeling wastewater generated during the reeling process cannot be underestimated. Reeling wastewater, based on the process flow, is primarily composed of cocoon boiling, vertical reeling (reeling and re-shaping), and by-product wastewater. Its primary pollutants are organic compounds such as sericin, fibroin, pupal protein, and fatty acids. By-product processing wastewater, in particular, contains a particularly high concentration of organic compounds. Reeling wastewater is high in nitrogen and phosphorus. Direct discharge into rivers, lakes, and oceans would not only cause a sharp increase in COD and BOD, leading to eutrophication, but also waste resources such as sericin.

[0003] At present, the common silk reeling wastewater treatment technologies mainly include physical and chemical treatment technology, anaerobic biological treatment technology, sequential batch activated sludge process (SBR) treatment process, aerated biological filter (BAF) treatment process, etc. The main advantage of using physical and chemical treatment technology is that the effluent water quality is relatively stable, but the investment cost is high, the energy consumption is high, and it is easy to produce secondary pollution; anaerobic biological treatment technology has many advantages, such as low power consumption, small amount of residual sludge, and the ability to degrade some organic matter that is difficult to degrade, but the start-up time of anaerobic organisms is long, and the anaerobic microbial flora needs to be cultivated and domesticated, which is very demanding on operation and High management requirements. After SBR biochemical treatment, the vast majority of chemical oxygen demand and biochemical oxygen demand in reeling wastewater are degraded, and some ammonia nitrogen undergoes denitrification, resulting in good purification effects. However, this requires high automation control and drainage equipment, large post-treatment equipment requirements, and significant head loss. After BAF biochemical treatment, most BOD and COD are effectively degraded, and some ammonia nitrogen undergoes nitrification under aerobic conditions. However, the BAF process cannot achieve biological phosphorus removal and can only be combined with chemical phosphorus removal to achieve phosphorus removal. Furthermore, it has high requirements for influent water quality and complex repeated flushing operations. Therefore, developing a method for treating and reusing reeling wastewater is of great significance for reducing the discharge of reeling wastewater and alleviating environmental pollution.

[0004] As a new type of carbon-based nanomaterial, carbon quantum dots have potential application prospects in the fields of bioimaging, sensing and catalysis due to their excellent biocompatibility, water solubility and chemical stability. At present, the preparation of carbon quantum dots based on biomass waste raw materials has attracted widespread attention. Researchers have converted biomass waste raw materials such as garden waste, kitchen waste, agricultural and forestry waste (bagasse, waste bean dregs, etc.) into high-performance carbon quantum dots through simple chemical or biological methods. This type of carbon quantum dots prepared based on biomass waste raw materials not only have good biocompatibility and excellent fluorescence properties, and can be used for biosensing, imaging, photovoltaic cells and other energy applications, but also can realize the recycling of waste and reduce environmental pollution. Therefore, seeking a technology to convert reeling wastewater into carbon quantum dots can not only reduce the discharge of reeling wastewater, but also realize the high-value utilization of reeling wastewater, which is of great significance to the realization of sustainable social development. Summary of the Invention

[0005] In order to solve the problems existing in the prior art, the present invention provides a method for preparing carbon quantum dots based on reeling wastewater, thereby achieving the purpose of the invention of preparing nitrogen-doped carbon quantum dots with good stability and excellent luminescence properties.

[0006] To solve the above technical problems, the object of the present invention is to provide a preparation method of nitrogen-doped carbon quantum dots based on silk reeling wastewater, characterized in that the preparation method includes pretreatment of silk reeling wastewater, hydrothermal reaction of silk reeling wastewater with amino acids, centrifugation, dialysis and drying treatment:

[0007] The silk reeling wastewater pretreatment is to remove floating matter, suspended matter and some large particle impurities in the silk reeling wastewater;

[0008] The hydrothermal reaction of the silk reeling wastewater and amino acid comprises the following steps:

[0009] Step 1: Prepare sodium hydroxide solutions of different concentrations;

[0010] Step 2: mixing the pretreated reeling wastewater with the sodium hydroxide solution described in step 1 in a certain proportion to obtain a mixed solution, and transferring the mixed solution into a polytetrafluoroethylene-lined stainless steel reactor;

[0011] Step 3: adding a certain amount of amino acids to the mixed solution described in step 2, dissolving them to obtain a reaction solution, sealing the stainless steel reactor, and heating to perform a hydrothermal reaction to obtain a solution after the hydrothermal reaction;

[0012] The centrifugation, dialysis and drying treatment, the obtained supernatant containing carbon quantum dots after the hydrothermal reaction solution cooling, centrifugation treatment in step 3, the supernatant is placed in a dialysis bag for dialysis, and the supernatant after dialysis is dried to obtain a carbon quantum dot solid powder.

[0013] Further, in step 1, the concentration of the sodium hydroxide solution is 0.8 mol / L to 1 mol / L.

[0014] Further, in step 2, the mass ratio of the reeling wastewater to the sodium hydroxide solution is 1:1 to 1:2.

[0015] As preferred, in step 3, the amino acid in the reaction solution is one or more of L-aspartic acid, histidine, arginine and lysine.

[0016] As preferred, in step 3, the amino acid in the reaction solution is L-aspartic acid with a mass fraction of 0.1% to 0.5%.

[0017] Further, the heating reaction temperature of the hydrothermal reaction in step 3 is 190 to 250 DEG C.

[0018] Further, the reaction time of the heating reaction in step 3 is 4 to 8 hours.

[0019] Further, in S3, the molecular weight cut-off of the dialysis bag is 1000.

[0020] Thanks to the above technical solutions, the present application achieves the following technical effects:

[0021] 1. The hydrothermal method is a green chemical synthesis method. In the hydrothermal reaction process, phosphorus is directly converted or doped into carbon quantum dots, which can realize biological phosphorus removal function, and no harmful organic solvent is needed, thereby reducing environmental pollution and resource consumption.

[0022] 2. The hydrothermal method for synthesizing carbon quantum dots can prepare high-purity carbon quantum dots in a short time, thereby improving the synthesis efficiency of carbon quantum dots.

[0023] 3. The amino acid, especially L-aspartic acid, is a biocompatible amino acid. The use of L-aspartic acid as a nitrogen source can introduce more nitrogen atoms in the synthesis process of carbon quantum dots, thereby changing the electronic structure of carbon quantum dots, adjusting the optical properties thereof, and realizing the nitrogen doping of carbon quantum dots, the adjustment of the fluorescence intensity of carbon quantum dots, and the adjustment of the fluorescence emission wavelength by using the synthesis precursor of L-aspartic acid, which has important significance for expanding the application of carbon quantum dots.

[0024] 4. Based on the preparation method of the present invention, the reuse technology of silk reeling wastewater can be enriched, which can not only reduce the discharge of silk reeling wastewater and reduce environmental pollution, but also realize the high-value utilization of silk reeling wastewater, which is of great significance to the realization of sustainable social development. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the preparation process of carbon quantum dots based on silk reeling wastewater.

[0026] Figure 2 (ab) Transmission electron microscopy (TEM) images of carbon quantum dots at different magnifications (c) Corresponding particle size distribution diagram of carbon quantum dots.

[0027] Figure 3 Infrared spectra of carbon quantum dots obtained under different synthesis conditions.

[0028] Figure 4 Dynamic light scattering test data of carbon quantum dot solutions obtained at different hydrothermal times.

[0029] Figure 5 Dynamic light scattering test data of carbon quantum dot solutions obtained with different sodium hydroxide concentrations.

[0030] Figure 6 This is the dynamic light scattering test data of the carbon quantum dot solution obtained at different mass fractions of L-aspartic acid.

[0031] Figure 7 (a) Excitation spectra of carbon quantum dots obtained at different hydrothermal times; (b) Emission spectra of carbon quantum dots obtained at different hydrothermal times; (c) Color comparison of carbon quantum dots dispersed in water obtained at different hydrothermal times under sunlight, from left to right, 4h, 5h, 6h, 7h, 8h; (d) Color comparison of carbon quantum dots dispersed in water obtained at different hydrothermal times under ultraviolet light, from left to right, 4h, 5h, 6h, 7h, 8h.

[0032] Figure 8 (a) Excitation spectra of carbon quantum dots obtained at different sodium hydroxide concentrations; (b) Emission spectra of carbon quantum dots obtained at different sodium hydroxide concentrations; (c) Color comparison of carbon quantum dots obtained at different sodium hydroxide concentrations dispersed in water under sunlight, from left to right, 0.1M, 0.2M, 0.4M, 0.6M, 0.8M; (d) Color comparison of carbon quantum dots obtained at different sodium hydroxide concentrations dispersed in water under ultraviolet light, from left to right, 0.1M, 0.2M, 0.4M, 0.6M, 0.8M.

[0033] Figure 9(a) Excitation spectra of carbon quantum dots obtained at different L-aspartic acid mass fractions; (b) Emission spectra of carbon quantum dot solutions obtained at different L-aspartic acid mass fractions; (c) Color comparison of carbon quantum dots dispersed in water at different L-aspartic acid mass fractions under sunlight, from left to right, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5%; (d) Color comparison of carbon quantum dots dispersed in water at different L-aspartic acid mass fractions under UV light, from left to right, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5%.

[0034] Figure 10 (a) Ultraviolet absorption spectra of the prepared carbon quantum dots dispersed in buffer solutions of different pH values; (b) Emission spectra of the prepared carbon quantum dots dispersed in buffer solutions of different pH values; (c) Color comparison of the prepared carbon quantum dots dispersed in buffer solutions of different pH values ​​under sunlight, from left to right, pH=2 to pH=12; (d) Color comparison of the prepared carbon quantum dots dispersed in buffer solutions of different pH values ​​under ultraviolet light. DETAILED DESCRIPTION

[0035] The following is a detailed description of the embodiments and Figure 1-10 , further illustrating the present invention.

[0036] Example 1 A method for preparing carbon quantum dots based on silk reeling wastewater

[0037] S1. First, assemble the vacuum filtration device, connect the vacuum pump to the filtration bottle, then take a piece of polytetrafluoroethylene microporous filter membrane and place it in the Büchner funnel. Use the filter membrane to cover all the micropores in the Büchner funnel, then turn on the vacuum pump switch of the filtration device, then pour the reeling wastewater into the funnel, and use the filter membrane to filter out floating matter, suspended matter and some large particles of impurities in the reeling wastewater to obtain pretreated reeling wastewater.

[0038] S2. Weigh 0.08 g of sodium hydroxide powder and pour it into 20 mL of deionized water. Stir with a glass rod to fully dissolve the sodium hydroxide to obtain a sodium hydroxide solution with a concentration of 0.1 mol / L. Then, use a graduated cylinder to measure 20 mL of filtered reeling wastewater and pour it into the sodium hydroxide solution. Use a magnetic stirrer to thoroughly mix the reeling wastewater and the sodium hydroxide solution.

[0039] S3. Transfer 40 mL of the uniformly mixed sodium hydroxide and reeling solution to a polytetrafluoroethylene-lined stainless steel reactor. Add 40 mg (0.1% by mass) of L-aspartic acid and use a magnetic stirrer to fully dissolve the L-aspartic acid in the mixed solution. Seal the reactor and place it in an oven set to 190°C. After 6 hours of heating, stop heating and allow the reactor to cool naturally to room temperature.

[0040] S4. Pour the solution in the reactor that has been naturally cooled to room temperature into the inner tube of a centrifuge, and centrifuge at a speed of 12,000 r / min for 15 minutes to remove non-fluorescent precipitates and obtain a supernatant of carbon quantum dots.

[0041] S5. Pour the supernatant after centrifugation into a dialysis bag with a molecular weight cutoff of 1000, place it in deionized water for 24 hours to remove residual sodium hydroxide, and finally place the dialyzed solution in a vacuum drying oven at 40°C to obtain carbon quantum dot solid powder.

[0042] Example 2 A method for preparing carbon quantum dots based on silk reeling wastewater

[0043] The difference between this embodiment and embodiment 1 is that the concentration of sodium hydroxide in step S2 is 0.2 mol / L.

[0044] Example 3 A method for preparing carbon quantum dots based on silk reeling wastewater

[0045] The difference between this embodiment and embodiment 1 is that the concentration of sodium hydroxide in step S2 is 0.4 mol / L.

[0046] Example 4 A method for preparing carbon quantum dots based on silk reeling wastewater

[0047] The difference between this embodiment and embodiment 1 is that the concentration of sodium hydroxide in step S2 is 0.6 mol / L.

[0048] Example 5 A method for preparing carbon quantum dots based on silk reeling wastewater

[0049] The difference between this embodiment and embodiment 1 is that the concentration of sodium hydroxide in step S2 is 0.8 mol / L.

[0050] Example 6 A method for preparing carbon quantum dots based on silk reeling wastewater

[0051] The difference between this embodiment and embodiment 1 is that the heating reaction time in step S3 is 4 hours.

[0052] Example 7 A method for preparing carbon quantum dots based on silk reeling wastewater

[0053] The difference between this embodiment and embodiment 1 is that the heating reaction time in step S3 is 5 hours.

[0054] Example 8 A method for preparing carbon quantum dots based on silk reeling wastewater

[0055] The difference between this embodiment and embodiment 1 is that the heating reaction time in step S3 is 7 hours.

[0056] Example 9 A method for preparing carbon quantum dots based on silk reeling wastewater

[0057] The difference between this embodiment and embodiment 1 is that the heating reaction time in step S3 is 8 hours.

[0058] Example 10 A method for preparing carbon quantum dots based on silk reeling wastewater

[0059] The difference between this embodiment and embodiment 1 is that the amount of L-aspartic acid added in step S3 is 80 mg (mass fraction 0.2%).

[0060] Example 11 A method for preparing carbon quantum dots based on silk reeling wastewater

[0061] The difference between this embodiment and embodiment 1 is that the amount of L-aspartic acid added in step S3 is 120 mg (mass fraction 0.3%).

[0062] Example 12 A method for preparing carbon quantum dots based on silk reeling wastewater

[0063] The difference between this embodiment and embodiment 1 is that the amount of L-aspartic acid added in step S3 is 160 mg (mass fraction 0.4%).

[0064] Example 13 A method for preparing carbon quantum dots based on silk reeling wastewater

[0065] The difference between this embodiment and embodiment 1 is that the amount of L-aspartic acid added in step S3 is 200 mg (mass fraction 0.5%).

[0066] The concentration of sodium hydroxide in step S2 in Examples 1-5, the hydrothermal reaction time in step S3 in Examples 6-9, and the mass fraction of L-aspartic acid in step S3 in Examples 10-13 will all affect the structural characteristics of carbon quantum dots, the dynamic light scattering of carbon quantum dot solutions, and the luminescence properties of carbon quantum dots.

[0067] Figure 2 (a-b) are TEM images of carbon quantum dots obtained by hydrothermal heating in 0.1 M sodium hydroxide for 6 h at different magnifications. The TEM results show that the synthesized carbon quantum dots are composed of dispersed spherical carbon particles. Figure 2 (c) is the particle size distribution of carbon quantum dots. From the distribution diagram, it can be seen that the particle size of carbon quantum dots is about 5nm-16nm, the average diameter is about 9.8nm, and the size distribution is uniform.

[0068] Figure 3 The infrared absorption spectra of carbon quantum dots obtained under different conditions are shown in Figure 2. When the hydrothermal reaction conditions are 5 hours and the sodium hydroxide concentration is 0.1M, the infrared absorption spectrum of carbon quantum dots prepared shows a wave number of 1558 cm -1 The absorption peak at 1491 cm is due to the skeleton vibration of C=C. -1 The absorption peak is also caused by the skeleton vibration of C=C; the wave number is 1409cm -1 The absorption peak at corresponds to the bending vibration of CH-CH2; the wave number is 1258cm- 1 The absorption peak corresponds to the stretching vibration of CO, which can also be attributed to the bending vibration of CH. The wave number is 2359cm -1 The peak value corresponds to the cumulative double bond stretching vibration region of C≡C. When the hydrothermal time is increased to 6h, the wave number is 2359cm -1 The peak disappears. As the reaction time increases, the structure of the carbon quantum dots changes, so the peak shifts slightly. Under the conditions of hydrothermal treatment for 6h, 0.1M, and then nitrogen doping, the wave number is 2358cm -1 The peaks appear, corresponding to the cumulative double bond stretching vibration region of C≡C. The wave number ranges from 1399 to 1420 cm -1 , which is the stretching vibration of CN. The results show that the carbon quantum dots have more nitrogen atoms, which should be the product of the hydrothermal decomposition of proteins in the reeling wastewater, making the carbon quantum dots have excellent water solubility.

[0069] Figure 4 It is the dynamic light scattering intensity curve of the carbon quantum dot solution obtained under different hydrothermal times. As can be seen from the figure, with the increase of the hydrothermal reaction time, the particle size of the carbon quantum dot solution obtained by hydrothermal reaction first increases, then decreases, and then continues to increase. Its particle size distribution is narrower when the hydrothermal reaction time is short. Increasing the hydrothermal time will increase the size of the particle aggregates in the solution, and the particle size distribution will increase at the same time. This may be because when the reaction time is short, the hydrothermal reaction is not sufficient, carbonization does not occur completely, only a small amount of carbon quantum dots are synthesized, and the unreacted organic matter in the reeling wastewater is filtered out. At this time, the particle size distribution of the carbon quantum dot aggregates is relatively narrow. When the reaction time is extended, the organic matter in the reeling wastewater fully reacts with the sodium hydroxide, and the hydrothermal kettle contains not only a large number of carbon quantum dots but also residual impurities. The aggregate particle size in the solution increases and the distribution becomes wider.

[0070] like Figure 5It is the dynamic light scattering intensity curve of the carbon quantum dot solution obtained under different sodium hydroxide concentrations. As can be seen from the figure, with the increase of sodium hydroxide concentration, the particle size of the carbon quantum dot solution obtained by hydrothermal reaction first increases and then decreases. Its particle size distribution gradually becomes wider with the increase of sodium hydroxide concentration. The main reason is that when the sodium hydroxide concentration is low, the hydrothermal reaction is not sufficient, only a small amount of carbon quantum dots are synthesized, the unreacted organic matter is filtered out, and there are relatively few water-soluble impurities. Therefore, at this time, it is mainly carbon quantum dots and their aggregates, and the distribution at this time is also relatively narrow. When the sodium hydroxide concentration increases, the silk reacts more fully with sodium hydroxide, producing more carbon quantum dot aggregate particles and impurity particles with better dispersibility in water, so the size gradually increases.

[0071] like Figure 6 The following are the dynamic light scattering intensity curves of the carbon quantum dot solutions obtained under different aspartic acid addition amounts (different mass fractions). As can be seen from the figure, with the increase of the amount of aspartic acid added, the particle size of the carbon quantum dot solution obtained by hydrothermal treatment shows a slow increasing trend. Its particle size distribution shows a trend of first decreasing and then increasing with the increase of the amount of aspartic acid added. When the amount of aspartic acid added is low, aspartic acid is directly hydrothermally generated into aggregate particles with uneven size distribution. When the amount of aspartic acid added increases, aspartic acid is fully mixed with the organic matter in the reeling wastewater to obtain uniform carbon quantum dot aggregate particles, and the distribution and size are significantly increased.

[0072] Figure 7 (a) is the excitation spectrum of carbon quantum dots obtained at different hydrothermal times. As can be seen from the figure, the excitation wavelength of carbon quantum dots obtained after 4 hours of hydrothermal reaction is located at 374nm, the excitation wavelength of carbon quantum dots obtained after 5 hours is located at 376nm, the excitation wavelength of carbon quantum dots obtained after 6 hours is located at 377nm, the excitation wavelength of carbon quantum dots obtained after 7 hours is located at 326nm, and the excitation wavelength of carbon quantum dots obtained after 8 hours is located at 325nm. As time increases, the excitation wavelength shows a trend of first increasing and then gradually decreasing. Figure 7(b) is the emission spectrum of carbon quantum dots obtained under different hydrothermal times. It can be seen from the figure that the emission position of carbon quantum dots obtained after 4h is located at 440nm, the emission position of carbon quantum dots obtained after 5h is located at 451nm, the emission position of carbon quantum dots obtained after 6h is located at 441nm, the emission position of carbon quantum dots obtained after 7h is located at 404nm, and the emission position of carbon quantum dots obtained after 8h is located at 403nm. The overall fluorescence emission peak shows a trend of first red shift and then blue shift, which corresponds to the trend of excitation wavelength change. In addition, with the increase of hydrothermal reaction time, the fluorescence intensity shows a trend of first decreasing and then increasing. When the hydrothermal reaction time is increased from 4 hours to 6 hours, the fluorescence intensity of the carbon quantum dot solution continues to decrease. From 6 hours to 7 hours, the fluorescence intensity of the carbon quantum dot solution increases significantly. From 7 hours to 8 hours, the fluorescence intensity weakens again. Its fluorescence emission is mainly controlled by the quantum size effect. With the increase of reaction time, the size and number of carbon quantum dots continue to increase. This is because the carbon quantum dots aggregate, causing self-quenching and resulting in reduced intensity.

[0073] Figure 7 Figure c shows the color comparison of carbon quantum dots under sunlight, from left to right, at 4h, 5h, 6h, 7h, and 8h. First, under sunlight, the color of the carbon quantum dot solution changes from yellow to brownish-yellow as the reaction time increases from 4 to 6 hours. As the reaction time continues to increase, the carbon quantum dot solution exhibits a subtle light yellow or even transparent color. This trend, consistent with the change in excitation light, is consistent with the principle of complementary colors. Figure 7 d is the color contrast of the carbon quantum dot solution under ultraviolet light, which corresponds to the fluorescence emission wavelength of the solution. First, with the increase of reaction time, the emission wavelength increases. When the reaction time increases from 4 hours to 5 hours, the fluorescence wavelength red-shifts and appears blue-green under ultraviolet light; when the reaction time is long enough to 8 hours, the fluorescence emission wavelength shows an obvious blue shift. At this time, the emission wavelength is 403nm, and it appears blue light under ultraviolet light.

[0074] Figure 8 (a) is the excitation spectrum of carbon quantum dots obtained at different sodium hydroxide concentrations. As can be seen from the figure, the excitation wavelength of carbon quantum dots obtained at 0.1 mol / L is located at 368 nm, the excitation wavelength of carbon quantum dots obtained at 0.2 mol / L is located at 367 nm, the excitation wavelength of carbon quantum dots obtained at 0.4 mol / L is located at 367 nm, the excitation wavelength of carbon quantum dots obtained at 0.6 mol / L is located at 366 nm, and the excitation wavelength of carbon quantum dots obtained at 0.8 mol / L is located at 368 nm. As time increases, the excitation wavelength shows a trend of first decreasing and then increasing. Figure 8(b) is the emission spectrum of carbon quantum dots obtained under different hydrothermal times. It can be seen from the figure that the emission position of carbon quantum dots obtained at 0.1mol / L is located at 442nm, the emission position of carbon quantum dots obtained at 0.2mol / L is located at 441nm, the emission position of carbon quantum dots obtained at 0.4mol / L is located at 452nm, the emission position of carbon quantum dots obtained at 0.6mol / L is located at 446nm, and the emission position of carbon quantum dots obtained at 0.8mol / L is located at 461nm. The overall fluorescence emission peak shows a trend of first blue shift and then red shift, which corresponds to the trend of excitation wavelength change. In addition, with the increase of particle concentration, the fluorescence intensity shows a trend of gradually decreasing. This is because the increase of carbon quantum dot particle concentration, the distance between particles is reduced, and they interact with each other, inhibiting the luminescence behavior of a single quantum dot, increasing the possibility of energy quenching, and superimposing carbon quantum dots at high concentrations, causing aggregation-induced quenching and weakening of fluorescence intensity.

[0075] Figure 8 (c) shows the color comparison of carbon quantum dots under sunlight, from left to right, at 0.1 mol / L, 0.2 mol / L, 0.4 mol / L, 0.6 mol / L, and 0.8 mol / L. First, under sunlight, as the sodium hydroxide concentration increases from 0.1 mol / L to 0.6 mol / L, the color of the carbon quantum dot solution changes from brown to light. Then, as the concentration increases to 0.8 mol / L, the color darkens again. This trend of change is consistent with that of the excitation light, according to the principle of complementary colors. Figure 9 (d) is the color comparison of the carbon quantum dot solution under ultraviolet light, which corresponds to the fluorescence emission wavelength of the solution. First, with the increase of concentration, the emission wavelength first decreases and then increases. When the reaction concentration increases from 0.1mol / L to 0.2mol / L, the fluorescence wavelength blue-shifts and appears blue light under ultraviolet light; when the reaction concentration reaches 0.8mol / L, the fluorescence emission wavelength shows an obvious red shift. At this time, the emission wavelength is 461nm, and it shows obvious blue-green light under ultraviolet light.

[0076] Figure 9 (a) is the excitation spectrum of carbon quantum dots obtained by adding different amounts of L-aspartic acid (mass fraction). As can be seen from the figure, the excitation light position of the carbon quantum dots obtained by 40mg (0.1%) is located at 360nm, the excitation light position of the carbon quantum dots obtained by 80mg (0.2%) is located at 365nm, the excitation light position of the carbon quantum dots obtained by 120mg (0.3%) is located at 365nm, the excitation light position of the carbon quantum dots obtained by 160mg (0.4%) is located at 368nm, and the excitation light position of the carbon quantum dots obtained by 200mg (0.5%) is located at 370nm. As time increases, the excitation wavelength shows a trend of gradually increasing. Figure 9(b) is the emission spectrum of carbon quantum dots obtained at different hydrothermal times. It can be seen from the figure that the emission position of carbon quantum dots obtained by 40mg (0.1%) is located at 448nm, the emission position of carbon quantum dots obtained by 80mg (0.2%) is located at 450nm, the emission position of carbon quantum dots obtained by 120mg (0.3%) is located at 452nm, the emission position of carbon quantum dots obtained by 160mg (0.4%) is located at 455nm, and the emission position of carbon quantum dots obtained by 200mg (0.1%) is located at 460nm. The overall fluorescence emission peak shows a red-shift trend, which may be caused by different emission sites on the surface of carbon dots or uneven particle size distribution, corresponding to the trend of change in excitation wavelength. In addition, with the increase of nitrogen doping amount, the fluorescence intensity shows a trend of continuous increase. This is because with the increase of nitrogen content, the transition energy gap in carbon quantum dots increases, the chemical stability and resistance to photodecomposition are improved, especially when a certain nitrogen content is reached, the fluorescence intensity increases rapidly.

[0077] Figure 9 (c) shows the color comparison of carbon quantum dots under sunlight. From left to right, the concentrations are 40mg (0.1%), 80mg (0.2%), 120mg (0.3%), 160mg (0.4%), and 200mg (0.5%). First, under sunlight, the color of the carbon quantum dot solution gradually darkens with increasing nitrogen content. This trend, consistent with the increasing excitation light level, is consistent with the principle of complementary colors. Figure 9 (d) is the color comparison of the carbon quantum dot solution under ultraviolet light, which corresponds to the fluorescence emission wavelength of the solution. First, with the increase of nitrogen content, the emission wavelength continues to increase, and the fluorescence wavelength red-shifts, showing blue-green light under ultraviolet light.

[0078] Figure 10 (a) is the ultraviolet absorption spectrum of carbon quantum dots in solutions with different pH values. As can be seen from the figure, the position of the obtained ultraviolet light is all located at 295 nm, and the emission wavelength has not changed, indicating that the properties of carbon quantum dots have not changed in buffer solutions with different pH values ​​and are not affected by pH. Figure 10 (b) is the emission spectrum of carbon quantum dots in solutions with different pH values. As can be seen from the figure, the emission light position is all located at 450 nm, and the fluorescence intensity shows a trend of first increasing and then decreasing with the change of pH. This is because the pH value causes the carbon quantum dots to undergo different degrees of hydrolysis and aggregation. Under acidic conditions, the fluorescence intensity reaches the highest, and under alkaline conditions, the fluorescence intensity is the lowest. This may be because there are many amino groups on the nitrogen-doped surface, and alkaline substances will affect the charge of the surface groups. Figure 10(c) shows the color change of carbon quantum dots in a buffer solution with a pH range of 2-12 under sunlight. As can be seen from the figure, there is no significant color change under different pH conditions in sunlight, indicating that pH has no significant effect on the luminescence properties of carbon quantum dots. Figure (d) shows the color change of carbon quantum dots in a buffer solution with a pH range of 2-12 under UV light, corresponding to the change in emission wavelength, with the solution exhibiting blue fluorescence.

[0079] Unless otherwise specified, the ratios and percentages described in the present invention are all by mass; all raw materials are commercially available.

[0080] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing nitrogen-doped carbon quantum dots based on silk reeling wastewater, characterized in that: The preparation method includes pretreatment of reeling wastewater, hydrothermal reaction of reeling wastewater with amino acids, centrifugation, dialysis and drying. The silk reeling wastewater pretreatment is to remove floating matter, suspended matter and some large particle impurities in the silk reeling wastewater; The hydrothermal reaction of the silk reeling wastewater and amino acid comprises the following steps: Step 1: Prepare sodium hydroxide solutions of different concentrations; Step 2: mixing the pretreated reeling wastewater with the sodium hydroxide solution described in step 1 in a certain proportion to obtain a mixed solution, and transferring the mixed solution into a polytetrafluoroethylene-lined stainless steel reactor; Step 3: adding a certain amount of amino acids to the mixed solution described in step 2, dissolving them to obtain a reaction solution, sealing the stainless steel reactor, and heating to perform a hydrothermal reaction to obtain a solution after the hydrothermal reaction; The centrifugation, dialysis and drying treatment, the solution after the hydrothermal reaction obtained in step 3 is cooled and centrifuged to obtain a supernatant containing carbon quantum dots, the supernatant is placed in a dialysis bag for dialysis, and the dialyzed supernatant is dried to obtain a carbon quantum dot solid powder.

2. The method for preparing nitrogen-doped carbon quantum dots based on silk reeling wastewater according to claim 1, characterized in that: In step 1, the concentration of the sodium hydroxide solution is 0.8 mol / L to 1 mol / L.

3. The method for preparing nitrogen-doped carbon quantum dots based on silk reeling wastewater according to claim 1, characterized in that: In step 2, the mass ratio of the reeling wastewater and the sodium hydroxide solution is 1:1 to 1:

2.

4. The method for preparing nitrogen-doped carbon quantum dots based on silk reeling wastewater according to claim 1, wherein: In step 3, the amino acid in the reaction solution is one or more of L-aspartic acid, histidine, arginine, and lysine.

5. The method for preparing nitrogen-doped carbon quantum dots based on silk reeling wastewater according to claim 4, characterized in that: In step 3, the amino acid in the reaction solution is L-aspartic acid, with a mass fraction of 0.1% to 0.5%.

6. The method for preparing nitrogen-doped carbon quantum dots based on silk reeling wastewater according to claim 1, characterized in that: The heating reaction temperature of the hydrothermal reaction in step 3 is 190-250°C.

7. The method for preparing nitrogen-doped carbon quantum dots based on silk reeling wastewater according to claim 1, characterized in that: The reaction time of the heating reaction in step 3 is 4 to 8 hours.

8. The method for preparing nitrogen-doped carbon quantum dots based on silk reeling wastewater according to claim 1, characterized in that: The molecular weight cut-off of the dialysis bag is 1000.

Images