A high-yield preparation method and application of carbon quantum dots

Abstract

This invention relates to the field of carbon nanomaterial preparation technology, specifically a high-yield preparation method for carbon quantum dots and its application. The preparation method includes: adding citric acid, ethylenediamine, EDC, and NHS to deionized water, stirring at 60-90°C for a preset time to obtain a mixed solution; subjecting the mixed solution to a hydrothermal reaction, cooling to room temperature after the reaction is complete, and then centrifuging, filtering, and dialyzing; freeze-drying the dialysate to obtain carbon quantum dots. This invention combines EDC and NHS pretreatment with a hydrothermal method to prepare carbon quantum dots. The solvent used is water, which is non-toxic, harmless, safe, and reliable. The process is simple, safe, and environmentally friendly. Simple pretreatment can greatly improve the yield of carbon quantum dots while ensuring quantum efficiency. The preparation equipment is simple, the process is safe, and it consumes fewer resources, which is conducive to large-scale production.

Description

Technical Field

[0001] This invention relates to the field of carbon nanomaterial preparation technology, and in particular to a high-yield preparation method for carbon quantum dots and its application. Background Technology

[0002] Carbon quantum dots, carbon nanoparticles with a particle size of nanometers, are a novel type of fluorescent nanomaterial. While possessing optical properties similar to traditional semiconductor quantum dots, they also exhibit excellent environmental friendliness and biocompatibility, thus showing broad research and application prospects in fields such as photocatalysis, biosensing, drug delivery, environmental monitoring, and fine chemicals. As carbon quantum dots have gradually become a research hotspot, research on their optical and chemical properties has become increasingly sophisticated.

[0003] Carbon quantum dots are typically obtained through two main synthetic routes: top-down and bottom-up. The former is based on breaking down large graphite materials into smaller carbon-based materials. However, the top-down approach usually requires harsh reaction conditions, expensive materials or equipment, and long processing times. In contrast, the bottom-up approach typically involves the dehydration and carbonization of powdered or small molecule dissolved forms under pyrolysis to form carbon quantum dots. The bottom-up strategy has advantages such as suitability for large-scale production, environmental friendliness, and low cost. Among these, the hydrothermal method is widely used in the synthesis of carbon quantum dots due to its mild conditions and simple equipment.

[0004] Several scholars have proposed methods for preparing carbon quantum dots with high yields. For example, some researchers have used a microwave solid-state method to prepare carbon quantum dots with a yield of 35% [Yong W, et al. Chemistry A European Journal, 2015, 21: p. 13004–13011]; others have obtained carbon quantum dots with a yield of 46% by polymerizing nitrosoacetic acid under high temperature and pressure [Renbing T, et al. Journal of Materials Chemistry C, 2017.5: p. 9174-9180]. However, further research is needed to develop high-yield methods for preparing carbon quantum dots.

[0005] Nitrogen-containing citric acid carbon quantum dots have been a research hotspot due to their environmental friendliness, ease of preparation, and high quantum yield. Existing studies have achieved yields of up to 58% [Zhu S, et al. Angewandte Chemie International Edition, 2013.52(14):3953-3957]. However, researchers have found that a large number of luminescent small molecule byproducts are generated in their preparation system [Fang Q, et al. Carbon, 2017.118:p.319-326.]. This limits the yield of carbon quantum dots, increases the difficulty of post-processing, and also affects the industrial application of carbon quantum dots. This will lead to high economic costs in actual production, limiting their large-scale production and thus affecting the practical application of such carbon quantum dots.

[0006] Therefore, it is necessary to provide an improved method for the high-yield preparation and application of carbon quantum dots to solve the above problems. Summary of the Invention

[0007] The purpose of this invention is to provide a high-yield preparation method and application of carbon quantum dots. By combining EDC and NHS pretreatment with a hydrothermal method, the generation of luminescent small molecule byproducts is reduced, thereby improving the yield of carbon quantum dots and solving the problems of low yield, many byproducts, and low economic benefits of carbon quantum dots in existing technologies.

[0008] To achieve the above objectives, in a first aspect, the present invention provides a method for preparing carbon quantum dots, comprising the following steps:

[0009] Citric acid, ethylenediamine, EDC (1-(3-dimethylaminopropyl)-3-ethylcarbodiimide) and NHS (N-hydroxysuccinimide) were added to deionized water and stirred at 60-90℃ for a preset time to obtain a mixed solution.

[0010] The mixed solution was subjected to a hydrothermal reaction. After the reaction was completed, it was cooled to room temperature, then centrifuged, filtered, and dialyzed. The dialysate was freeze-dried to obtain carbon quantum dots.

[0011] This invention improves the yield of carbon quantum dots by adding EDC / NHS before hydrothermal treatment, which allows citric acid and ethylenediamine to undergo pre-amide condensation. This effectively reduces the possibility of luminescent small molecules being generated as byproducts during the hydrothermal reaction. Furthermore, by controlling the content of each substance, quantum efficiency can be improved while maintaining the yield.

[0012] Furthermore, the mass ratio of citric acid to ethylenediamine is 6:(0.5-3.6), preferably 6:(0.9-1.8).

[0013] The formation of carbon quantum dots mainly comes from the dehydration and nucleation of citric acid in the system. The addition of excessive ethylenediamine will consume excessive amounts of citric acid required for the formation of carbon nuclei in the system, resulting in most of the product becoming luminescent small molecules. After sample dialysis, most of the luminescent small molecules are dialyzed off, thus reducing the yield of the obtained sample.

[0014] Furthermore, the mass ratio of EDC to NHS is (1.8 to 2.2):1, preferably 2:1.

[0015] Furthermore, the mass ratio of citric acid to EDC is 6:(0.5-1.5), preferably 6:(0.5-1).

[0016] Appropriately increasing the amount of EDC / NHS can promote the formation of carbon nuclei from citric acid in the system, and also help more luminescent small molecules to connect to the surface of the carbon quantum dots formed in the system by chemical bonds. This ensures that the sample has both high yield and high quantum efficiency after dialysis.

[0017] Without the addition of EDC / NHS, the system undergoes an amidation reaction with increasing temperature. Further, some of the amidation products form luminescent small molecules, while others promote the formation of carbon nuclei from citric acid. However, due to the slow amidation rate in an aqueous environment, the carbon nucleus formation reaction of citric acid can still proceed normally under conditions of relative excess ethylenediamine, resulting in moderate yield and quantum efficiency. When ethylenediamine is in excess and EDC / NHS is added simultaneously, the amidation reaction rate is greatly accelerated, significantly increasing the consumption of citric acid in the system and hindering the formation of carbon nuclei from citric acid, thus greatly affecting the yield. Simultaneously increasing the amount of EDC / NHS allows the luminescent small molecules to chemically bond with carbon quantum dots, thereby mitigating the yield reduction and improving the quantum efficiency.

[0018] In one embodiment, the mass ratio of citric acid, ethylenediamine, EDC, and NHS is 6:0.9:0.3:0.15. In another embodiment, the mass ratio of citric acid, ethylenediamine, EDC, and NHS is 6:0.6:0.5:0.25.

[0019] In another embodiment, the mass ratio of citric acid, ethylenediamine, EDC, and NHS is 6:0.9:0.5:0.25. In yet another embodiment, the mass ratio of citric acid, ethylenediamine, EDC, and NHS is 6:0.9:0.8:0.4.

[0020] In another embodiment, the mass ratio of citric acid, ethylenediamine, EDC, and NHS is 6:1.8:0.5:0.25. In yet another embodiment, the mass ratio of citric acid, ethylenediamine, EDC, and NHS is 6:1.8:1:0.5.

[0021] In another specific embodiment, the mass ratio of citric acid, ethylenediamine, EDC, and NHS is 6:2.7:0.5:0.25. In yet another specific embodiment, the mass ratio of citric acid, ethylenediamine, EDC, and NHS is 6:2.7:1:0.5.

[0022] Furthermore, the temperature of the hydrothermal reaction is 180–220°C, preferably 200°C; the time is 5–15 h, preferably 8 h; the hydrothermal reaction is preferably carried out in a polytetrafluoroethylene-lined hydrothermal reactor.

[0023] Furthermore, the preset stirring time is 0 to 72 hours, with a preferred time of 8 to 24 hours.

[0024] Furthermore, the concentration of citric acid in the mixed solution is 0–200 g / L, preferably 80–120 g / L.

[0025] Furthermore, the concentration of ethylenediamine in the mixed solution is 0–100 g / L, preferably 20–50 g / L.

[0026] Furthermore, the concentration of EDC in the mixed solution is 0–100 g / L, preferably 5–20 g / L.

[0027] Furthermore, the concentration of NHS in the mixed solution is 0–50 g / L, preferably 2–10 g / L.

[0028] Furthermore, the centrifugation speed is 3000-8000 r / min, and the time is 5±1 min.

[0029] Furthermore, the filtration uses an aqueous phase filter membrane with a pore size of 0.22 μm.

[0030] Furthermore, the dialysis uses dialysis bags with a density of 500–1000D.

[0031] Furthermore, the freeze-drying temperature is -50 to -30°C.

[0032] As one specific embodiment of the present invention, the high-yield preparation method of the carbon quantum dots includes: adding citric acid, ethylenediamine, EDC, and NHS to deionized water, and stirring at 80°C for 8–24 h to obtain a mixed solution. The solution is then transferred to a hydrothermal reactor, sealed, and reacted at 200°C for 8 h. Afterward, it is naturally cooled to room temperature, centrifuged, filtered, and the filtrate is collected. The obtained filtrate is dialyzed for 24 h, and then the dialysate is freeze-dried at -40°C to obtain carbon quantum dot powder.

[0033] Secondly, the present invention provides a carbon quantum dot, which is prepared by any of the preparation methods described above.

[0034] Furthermore, the yield of the carbon quantum dots is 25% to 80%, and / or the quantum dot efficiency is 30% to 85%;

[0035] Preferably, at least one of the carbon quantum dot yield and quantum dot efficiency satisfies ≥60%;

[0036] More preferably, the yield of the carbon quantum dots is 65% to 80%; the quantum dot efficiency is 60% to 85%.

[0037] Thirdly, the present invention provides an application of the carbon quantum dots described above in the fields of photocatalysis, biosensing, drug delivery, environmental monitoring, and fine chemicals.

[0038] The beneficial effects of this invention are as follows:

[0039] 1. The carbon quantum dot preparation method provided by the present invention, by adding EDC / NHS at a relatively low temperature before hydrothermal treatment, allows citric acid and ethylenediamine to undergo pre-amide condensation, which effectively reduces the possibility of generating by-product luminescent small molecules during the hydrothermal reaction stage, thereby improving the yield of carbon quantum dots.

[0040] 2. The carbon quantum dots prepared by this invention exhibit high yield, few byproducts, excellent dispersibility, water solubility, and strong fluorescence, enabling large-scale production of carbon quantum dots and their application in photocatalysis, biosensing, drug delivery, environmental monitoring, and fine chemicals. This invention features a simple process, requires minimal equipment, is easy to operate, has easily achievable process conditions, low production costs, and a wide range of applications, facilitating large-scale production. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0042] Figure 1 Images of the carbon quantum dot aqueous solution prepared for Example 1 under sunlight (left) and ultraviolet light (right).

[0043] Figure 2 Transmission electron microscopy (TEM) image of the carbon quantum dots prepared in Example 1.

[0044] Figure 3 The particle size distribution diagram is shown for the carbon quantum dots prepared in Example 1.

[0045] Figure 4 The UV-Vis absorption spectrum and excitation and emission spectra of the carbon quantum dots prepared in Example 1 are shown. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention are described clearly and completely below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0047] Example 1

[0048] This embodiment provides a method for preparing carbon quantum dots, including:

[0049] (1) Add 6g citric acid, 0.9g ethylenediamine, 0.5g EDC and 0.25g NHS to 60g deionized water and stir at 80℃ for 12h to obtain a brownish-yellow mixed solution;

[0050] (2) Transfer the mixed solution obtained in step (1) to a hydrothermal reactor, seal it, and heat it at 200℃.

[0051] The mixture was reacted for 8 hours under the given conditions, allowed to cool naturally to room temperature, centrifuged at 8000 r / min for 5 min, and then filtered through a 0.22 μm filter membrane to collect the filtrate.

[0052] (3) Dialyze the filtrate obtained in step (2) using a dialysis bag with a specification of 500-1000D. After 24 hours, the dialysis solution is obtained.

[0053] (4) The dialysate obtained in step (3) was freeze-dried at -40°C for 24 hours to obtain carbon quantum dots with a quantum efficiency of 37.6% and a yield of 77.61%.

[0054] Figure 1 Images of the xylene-encapsulated carbon quantum dot aqueous solution prepared in Example 1 under sunlight (left) and ultraviolet light (right) irradiation. As can be seen from the images, the carbon quantum dots obtained in this example exhibit strong blue fluorescence under 365nm ultraviolet light irradiation.

[0055] Figure 2 The image shows a transmission electron microscope (TEM) image of the carbon quantum dots prepared in Example 1. As can be seen from the image, the interplanar spacing is 0.21 nm, corresponding to the (100) crystal plane of graphite.

[0056] Figure 3 This is a particle size distribution diagram of the carbon quantum dots prepared in Example 1. As can be seen from the figure, the average particle size of the obtained carbon quantum dots is 3.0 nm.

[0057] Figure 4 The UV-Vis absorption and excitation / emission spectra of the carbon quantum dots prepared in Example 1 are shown. The results indicate that the optimal absorption wavelength is 365 nm and the emission wavelength is 450 nm.

[0058] Example 2

[0059] This embodiment provides a high-yield preparation method for carbon quantum dots, including:

[0060] (1) Add 6g citric acid, 1.8g ethylenediamine, 0.5g EDC and 0.25g NHS to 60g deionized water and stir at 80℃ for 12h to obtain a brownish-yellow mixed solution;

[0061] (2) Transfer the mixed solution obtained in step (1) to a hydrothermal reactor, seal it, and heat it at 200℃.

[0062] The mixture was reacted for 8 hours under the given conditions, allowed to cool naturally to room temperature, centrifuged at 8000 r / min for 5 min, and then filtered through a 0.22 μm filter membrane to collect the filtrate.

[0063] (3) Dialyze the filtrate obtained in step (2) using a dialysis bag with a specification of 500-1000D. After 24 hours, the dialysis solution is obtained.

[0064] (4) The dialysate obtained in step (3) was freeze-dried at -40°C for 24 hours to obtain carbon quantum dots with a quantum efficiency of 26.2% and a yield of 70.30%.

[0065] Example 3

[0066] This embodiment provides a high-yield preparation method for carbon quantum dots, including:

[0067] (1) Add 6g citric acid, 2.7g ethylenediamine, 0.5g EDC and 0.25g NHS to 60g deionized water and stir at 80℃ to obtain a brownish-yellow mixed solution;

[0068] (2) Transfer the mixed solution obtained in step (1) to a hydrothermal reactor, seal it, and heat it at 200℃.

[0069] The mixture was reacted for 8 hours under the given conditions, allowed to cool naturally to room temperature, centrifuged at 8000 r / min for 5 min, and then filtered through a 0.22 μm filter membrane to collect the filtrate.

[0070] (3) Dialyze the filtrate obtained in step (2) using a dialysis bag with a specification of 500-1000D. After 24 hours, the dialysis solution is obtained.

[0071] (4) The dialysate obtained in step (3) was freeze-dried at -40°C for 24 hours to obtain carbon quantum dots with a quantum efficiency of 81.6% and a yield of 21.63%.

[0072] Example 4

[0073] This embodiment provides a high-yield preparation method for carbon quantum dots, including:

[0074] (1) Add 6g citric acid, 3.6g ethylenediamine, 0.5g EDC and 0.25g NHS to 60g deionized water and stir at 80℃ for 12h to obtain a brownish-yellow mixed solution;

[0075] (2) Transfer the mixed solution obtained in step (1) to a hydrothermal reactor, seal it, and heat it at 200℃.

[0076] The mixture was reacted for 8 hours under the given conditions, allowed to cool naturally to room temperature, centrifuged at 8000 r / min for 5 min, and then filtered through a 0.22 μm filter membrane to collect the filtrate.

[0077] (3) Dialyze the filtrate obtained in step (2) using a dialysis bag with a capacity of 500-1000 μm.

[0078] D, the dialysate was obtained after 24 hours;

[0079] (4) The dialysate obtained in step (3) was freeze-dried at -40°C for 24 hours to obtain carbon quantum dots with a quantum efficiency of 64.7% and a yield of 15.56%.

[0080] Example 5

[0081] This embodiment provides a high-yield preparation method for carbon quantum dots, including:

[0082] (1) Add 6g citric acid, 0.9g ethylenediamine, and 1g EDC to 60g deionized water.

[0083] 0.5 g of NHS was stirred at 80 °C for 12 h to obtain a brownish-yellow mixed solution;

[0084] (2) Transfer the mixed solution obtained in step (1) to a hydrothermal reactor, seal it, and heat it at 200℃.

[0085] The mixture was reacted for 8 hours under the given conditions, allowed to cool naturally to room temperature, centrifuged at 8000 r / min for 5 min, and then filtered through a 0.22 μm filter membrane to collect the filtrate.

[0086] (3) Dialyze the filtrate obtained in step (2) using a dialysis bag with a capacity of 500-1000 μm.

[0087] D, the dialysate was obtained after 24 hours;

[0088] (4) The dialysate obtained in step (3) was freeze-dried at -40°C for 24 hours to obtain carbon quantum dots with a quantum efficiency of 41.8% and a yield of 69.37%.

[0089] Example 6

[0090] This embodiment provides a high-yield preparation method for carbon quantum dots, including:

[0091] (1) Add 6g citric acid, 1.8g ethylenediamine, and 1g EDC to 60g deionized water.

[0092] 0.5 g of NHS was stirred at 80 °C for 12 h to obtain a brownish-yellow mixed solution;

[0093] (2) Transfer the mixed solution obtained in step (1) to a hydrothermal reactor, seal it, and heat it at 200℃.

[0094] The mixture was reacted for 8 hours under the given conditions, allowed to cool naturally to room temperature, centrifuged at 8000 r / min for 5 min, and then filtered through a 0.22 μm filter membrane to collect the filtrate.

[0095] (3) Dialyze the filtrate obtained in step (2) using a dialysis bag with a capacity of 500-1000 μm.

[0096] D, the dialysate was obtained after 24 hours;

[0097] (4) The dialysate obtained in step (3) was freeze-dried at -40°C for 24 hours to obtain carbon quantum dots with a quantum efficiency of 67.1% and a yield of 71.00%.

[0098] Example 7

[0099] This embodiment provides a high-yield preparation method for carbon quantum dots, including:

[0100] (1) Add 6g citric acid, 2.7g ethylenediamine, and 1g EDC to 60g deionized water.

[0101] 0.5 g of NHS was stirred at 80 °C for 12 h to obtain a brownish-yellow mixed solution;

[0102] (2) Transfer the mixed solution obtained in step (1) to a hydrothermal reactor, seal it, and heat it at 200℃.

[0103] The mixture was reacted for 8 hours under the given conditions, allowed to cool naturally to room temperature, centrifuged at 8000 r / min for 5 min, and then filtered through a 0.22 μm filter membrane to collect the filtrate.

[0104] (3) Dialyze the filtrate obtained in step (2) using a dialysis bag with a capacity of 500-1000 μm.

[0105] D, the dialysate was obtained after 24 hours;

[0106] (4) The dialysate obtained in step (3) was freeze-dried at -40℃ for 24h to obtain carbon quantum dots with a quantum efficiency of 68.8% and a yield of 27.63%.

[0107] Example 8

[0108] This embodiment provides a high-yield preparation method for carbon quantum dots, including:

[0109] (1) Add 6g citric acid, 2.7g ethylenediamine, 0.5g EDC and 0.25g NHS to 60g deionized water and stir at 80℃ for 48h to obtain a brownish-yellow mixed solution;

[0110] (2) Transfer the mixed solution obtained in step (1) to a hydrothermal reactor, seal it, and heat it at 200℃.

[0111] The mixture was reacted for 8 hours under the given conditions, allowed to cool naturally to room temperature, centrifuged at 8000 r / min for 5 min, and then filtered through a 0.22 μm filter membrane to collect the filtrate.

[0112] (3) Dialyze the filtrate obtained in step (2) using a dialysis bag with a capacity of 500-1000 μm.

[0113] D, the dialysate was obtained after 24 hours;

[0114] (4) The dialysate obtained in step (3) was freeze-dried at -40°C for 24 hours to obtain carbon quantum dots with a quantum efficiency of 71.8% and a yield of 26.93%.

[0115] Comparative Example 1

[0116] This embodiment provides a conventional method for preparing carbon quantum dots, including:

[0117] (1) Add 6g of citric acid and 1.8g of ethylenediamine to 60g of deionized water and stir at room temperature for 24h to obtain a mixed solution;

[0118] (2) Transfer the mixed solution obtained in step (1) to a hydrothermal reactor, seal it, and heat it at 200℃.

[0119] The mixture was reacted for 8 hours under the given conditions, allowed to cool naturally to room temperature, centrifuged at 8000 r / min for 5 min, and then filtered through a 0.22 μm filter membrane to collect the filtrate.

[0120] (3) Dialyze the filtrate obtained in step (2) using a dialysis bag with a specification of 500-1000D. After 24 hours, the dialysis solution is obtained.

[0121] (4) The dialysate obtained in step (3) was freeze-dried at -40°C for 24 hours to obtain carbon quantum dots with a yield of 54.10%.

[0122] Comparative Example 2

[0123] This embodiment provides a conventional method for preparing carbon quantum dots, including:

[0124] (1) Add 6g of citric acid and 2.7g of ethylenediamine to 60g of deionized water and stir at room temperature for 24h to obtain a mixed solution;

[0125] (2) Transfer the mixed solution obtained in step (1) to a hydrothermal reactor, seal it, and heat it at 200℃.

[0126] The mixture was reacted for 8 hours under the given conditions, allowed to cool naturally to room temperature, centrifuged at 8000 r / min for 5 min, and then filtered through a 0.22 μm filter membrane to collect the filtrate.

[0127] (3) Dialyze the filtrate obtained in step (2) using a dialysis bag with a specification of 500-1000D. After 24 hours, the dialysis solution is obtained.

[0128] (4) The dialysate obtained in step (3) was freeze-dried at -40°C for 24 hours to obtain carbon quantum dots with a yield of 54.94%.

[0129] Comparative Example 3

[0130] This embodiment provides a conventional method for preparing carbon quantum dots, including:

[0131] (1) Add 6g of citric acid and 0.9g of ethylenediamine to 60g of deionized water and stir at room temperature for 24h to obtain a mixed solution;

[0132] (2) Transfer the mixed solution obtained in step (1) to a hydrothermal reactor, seal it, react at 200℃ for 8 hours, cool it naturally to room temperature, centrifuge it at 8000 r / min for 5 minutes, and collect the filtrate after filtering it through a 0.22 μm filter membrane.

[0133] (3) Dialyze the filtrate obtained in step (2) using a dialysis bag with a specification of 500-1000D. After 24 hours, the dialysis solution is obtained.

[0134] (4) The dialysate obtained in step (3) was freeze-dried at -40°C for 24 hours to obtain carbon quantum dots with a yield of 33.33%.

[0135] Table 1. Test results of the examples and comparative examples.

[0136]

[0137] As shown in Table 1, the yield of carbon quantum dots gradually decreases while the quantum efficiency gradually increases with increasing ethylenediamine content. This is likely because excess ethylenediamine consumes a large amount of citric acid required for carbon nucleus formation, resulting in most of the product becoming luminescent small molecules. After sample dialysis, most of these luminescent small molecules are dialyzed out, leading to a decrease in the yield and an increase in the quantum efficiency. Simultaneously increasing the EDC / NHS content improves the yield but slightly decreases the quantum efficiency. This is likely because more EDC / NHS promotes carbon nucleus formation from citric acid and further facilitates the chemical bonding of more luminescent small molecules to the surface of the carbon quantum dots formed in the system, ensuring both high yield and high quantum efficiency after dialysis. Appropriately extending the pretreatment stirring time can also improve the yield to some extent.

[0138] The yield of carbon quantum dots decreased when EDC / NHS was not added. Without EDC / NHS, an amidation reaction occurred in the system with increasing temperature. Further, some of the amidation products formed reflective small molecules, while others promoted the formation of carbon nuclei from citric acid. However, due to the slow amidation rate in an aqueous environment, the carbon nuclei formation reaction of citric acid could still proceed normally even with a relative excess of ethylenediamine. But the addition of EDC / NHS greatly accelerated the amidation reaction rate, thus increasing the consumption of citric acid in the system and relatively hindering the formation of carbon nuclei from citric acid, thereby affecting the yield. Therefore, the yield of Example 3 was lower than that of Comparative Example 2.

[0139] In summary, by adjusting the mass ratio of citric acid, ethylenediamine, EDC, and NHS, this invention can synergistically regulate the yield and quantum efficiency, thereby obtaining the desired product.

[0140] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing carbon quantum dots, characterized by, The method comprises the following steps: adding citric acid, ethylenediamine, EDC and NHS into deionized water, stirring at 60-90℃ for a preset time to obtain a mixed solution; subjecting the mixed solution to hydrothermal reaction, cooling to room temperature after the reaction is completed, then centrifuging, filtering and dialyzing, and freeze-drying the dialyzed solution to obtain carbon quantum dots; the mass ratio of the citric acid to the ethylenediamine is 6:(0.5-3.6); the mass ratio of the EDC to the NHS is (1.8-2.2):1; the mass ratio of the citric acid to the EDC is 6:(0.5-1.5).

2. The method for preparing carbon quantum dots according to claim 1, characterized in that, the mass ratio of the citric acid to the ethylenediamine is 6:(0.9-1.8).

3. The method for preparing carbon quantum dots according to claim 1, characterized in that, the mass ratio of the EDC to the NHS is 2:1; the mass ratio of the citric acid to the EDC is 6:(0.5-1).

4. The method for preparing carbon quantum dots according to claim 1, characterized in that, the temperature of the hydrothermal reaction is 180-220℃, and the time is 5-15h; the hydrothermal reaction is preferably carried out in a polytetrafluoroethylene-lined hydrothermal reaction kettle; and / or, the preset time of the stirring is 0-72h. 5.The method of claim 4, wherein the carbon quantum dots are prepared by the method of claim 1 or 2. the preset time of the stirring is 8-24h.

6. The method for preparing carbon quantum dots according to claim 1, characterized in that, the concentration of the citric acid in the mixed solution is 0-200g / L; and / or, the concentration of the ethylenediamine in the mixed solution is 0-100g / L. 7.The method of claim 6, wherein the carbon quantum dots are prepared by the method of claim 1. the concentration of the citric acid in the mixed solution is 80-120g / L; and / or, the concentration of the ethylenediamine in the mixed solution is 10-50g / L. 8.The method of claim 1, wherein the carbon quantum dots are prepared by the method comprising: preparing a solution of a carbon source and a polymerization initiator; and polymerizing the solution to prepare the carbon quantum dots. the concentration of the EDC in the mixed solution is 0-100g / L; and / or, the concentration of the NHS in the mixed solution is 0-50g / L. 9.The method of claim 8, wherein the carbon quantum dots are prepared by the method of claim 1. the concentration of the EDC in the mixed solution is 5-20g / L; and / or, the concentration of the NHS in the mixed solution is 2-10g / L. 10.The method of claim 1, wherein the carbon quantum dots are prepared by the method comprising: preparing a solution of a carbon source and a polymer; and heating the solution to prepare the carbon quantum dots. the rotation speed of the centrifuging is 3000-8000r / min, and the time is 5±1min; and / or, the filtering adopts a water-phase filter membrane with a pore size of 0.22um; and / or, the dialysis adopts a dialysis bag with a molecular weight of 500-1000D; and / or, the temperature of the freeze-drying is-50--30℃.

11. A carbon quantum dot, characterized by, obtained by the preparation method in any one of claims 1-10. 12.The method of claim 10, wherein the carbon quantum dots are prepared by the method comprising: mixing a carbon source and a polymerization initiator; and polymerizing the carbon source and the polymerization initiator to prepare the carbon quantum dots. the yield of the carbon quantum dots is 25%-80%, and / or, the quantum dot efficiency is 30%-85%. 13.The method of claim 12, wherein the carbon quantum dots are prepared by the method comprising: mixing a carbon source and a polymerization initiator to prepare a mixture; and polymerizing the mixture to prepare the carbon quantum dots. at least one of the yield and the quantum dot efficiency of the carbon quantum dots satisfies ≥60%. 14.The method according to claim 13, characterized in that, the yield of the carbon quantum dots is 65%-80%, and the quantum dot efficiency is 60%-85%.

15. Application of the carbon quantum dots in claim 11 in the fields of photocatalysis, biosensing, drug delivery, environmental detection and fine chemical industry.

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