A method for preparing fluorescent carbon quantum dots by pre-pressurized microwave-dilute acid etching

Abstract

This invention discloses a method for preparing fluorescent carbon quantum dots using a pre-pressurized microwave-dilute acid etching process. The method involves digesting a carbon material sample with nitric acid solution, followed by ultrasonic treatment. 440 mL of high-purity water and 10 mL of concentrated HNO3 are added to a TFM (tissue-mixed vapor chamber) digestion vessel as the microwave medium, which is then placed in the stainless steel cavity of the UltraCLAVE super microwave chemistry platform. A digestion tube containing the suspended carbon material sample is fixed in the TFM vessel, with its bottom immersed in the microwave medium. The stainless steel cavity lid is then closed and locked, and the pressure within the cavity is adjusted to 40-70 Bar. The instrument cover is closed, and the microwave pressurization reaction is then performed. The solution in the digestion tube is transferred to a centrifuge tube and centrifuged at 8000 rpm for 1 min. The supernatant is collected, and the collected supernatant is purified to obtain fluorescent carbon quantum dots. Compared to the traditional strong acid oxidation method, this invention requires only dilute acid, uses a smaller amount of acid, operates under milder conditions, and is more environmentally friendly. It also improves work efficiency and significantly shortens the carbon quantum dot preparation time.

Description

Technical Field

[0001] This invention relates to the field of nanoluminescent material preparation, and in particular to a method for preparing fluorescent carbon quantum dots by pre-pressurized microwave-dilute acid etching. Background Technology

[0002] Currently, carbon quantum dot synthesis strategies are divided into "bottom-up" and "top-down" strategies. The "bottom-up" strategy refers to synthesizing carbon quantum dots from small to large using small organic molecules or oligomers as carbon sources. The "top-down" strategy refers to synthesis methods that exfoliate large carbon materials into small carbon particles and modify their surfaces (to improve luminescence efficiency) through various means. The "top-down" strategy is suitable for preparing carbon quantum dots with precise structures and high quantum efficiency. However, the strong acid oxidation method requires a large amount of corrosive strong acid, which has a negative impact on the environment.

[0003] CN201911174821.6 discloses a method for synthesizing green carbon quantum dots and their application in the detection of nitrite. The preparation process is as follows: (1) dissolve m-aminophenol in anhydrous ethanol, then add concentrated nitric acid and concentrated hydrochloric acid to obtain a mixture; (2) transfer the mixture to a reaction vessel, perform a solvothermal reaction, and then cool to obtain a reactant; (3) elute, concentrate, dry, and grind the reactant into a solid powder to obtain the target product, green fluorescent carbon quantum dots; in the mixture of step (1), the concentrated nitric acid and concentrated hydrochloric acid... The volume ratio of the acid is 1:3, and the addition ratio of m-aminophenol to anhydrous ethanol is (0.2-0.4) g: (20-40) mL; the reaction temperature in step (2) is 80℃-140℃, and the reaction time is 11-13 h; in step (3), the reactants are eluted using a silica gel column, and the eluent is a mixed solution of methanol and dichloromethane with a volume ratio of 1:5; there is an internal filtration effect between the green fluorescent carbon quantum dots and nitrosothiols, which can lead to the fluorescence quenching of the green fluorescent carbon quantum dots.

[0004] CN201410778272.4 discloses a microwave method for preparing multicolor fluorescent graphene quantum dots, using carbon materials as reactants, and obtaining the product through a two-step reaction. Step 1: The carbon material is dispersed in a mixture of water, concentrated nitric acid, and concentrated sulfuric acid and heated for oxidation. The resulting mixture is then dispersed in water, neutralized, desalted, and dried to obtain carbon oxide material. Step 2: The carbon oxide material obtained in Step 1 is ultrasonically dispersed in N,N-dimethylformamide. The resulting dispersion is reacted in a microwave environment, then filtered, desolventized, redispersed, and dialyzed to obtain graphene quantum dots.

[0005] CN201911174821.6 describes the preparation of graphene quantum dots using concentrated nitric acid and concentrated sulfuric acid etching. Although CN201410778272.4 uses a microwave method to prepare multicolor fluorescent graphene quantum dots, both methods suffer from problems such as complex operation, long processing time, and lack of environmental friendliness. Summary of the Invention

[0006] To address the problems of complex operation, long time consumption, and lack of environmental friendliness in the preparation of carbon quantum dots using the "top-down" strategy in existing technologies, this invention provides a method for preparing fluorescent carbon quantum dots using a pre-pressurized microwave-dilute acid etching method.

[0007] To achieve the above objectives, the present invention is implemented according to the following technical solution:

[0008] One objective of this invention is to provide a method for preparing fluorescent carbon quantum dots using a pre-pressurized microwave-dilute acid etching method, comprising the following steps:

[0009] S1. Sample addition: Weigh 5 mg of carbon material sample and add it to a quartz or polytetrafluoroethylene digestion tube. Then, use a pipette to add 2 mL of nitric acid solution with a volume concentration of 5%-25% to the digestion tube and cover it with a dust cap.

[0010] S2, Ultrasonication: The digestion tube is placed in an ultrasonic instrument for ultrasonic treatment to make the carbon material sample dispersed evenly and obtain a suspended carbon material sample.

[0011] S3. Sample loading: Add 440 mL of high-purity water and 10 mL of concentrated HNO3 as microwave medium to the TFM digestion vessel and load it into the stainless steel chamber of the UltraCLAVE super microwave chemistry platform; fix the digestion tube of the suspended carbon material sample in the TFM vessel, with the bottom of the digestion tube immersed in the microwave medium; then close and lock the stainless steel chamber cover, introduce nitrogen gas to adjust the pressure of the stainless steel chamber to 40-70 Bar, and close the instrument cover;

[0012] S4. Microwave Pressurization Reaction: The microwave conditions within the stainless steel cavity of the UltraCLAVE super microwave chemical platform are as follows: 1. Temperature rises from 0℃ to 70℃ within 10 minutes, power is 1000W, protection temperature is 60℃, and pressure is 100bar; 2. Temperature rises from 70℃ to 120℃ within 10 minutes, power is 1000W, protection temperature is 60℃, and pressure is 130bar; 3. Temperature rises from 120℃ to 180℃-200℃ within 10 minutes, power is 1000W, protection temperature is 60℃, and pressure is 150bar; 4. Temperature remains constant at 180℃-200℃ for 30 minutes, power is 1200W, protection temperature is 60℃, and pressure is 150bar; After the heat preservation period, the cavity is allowed to cool naturally, and the pressure is automatically released and the chamber door opens.

[0013] S5. Post-processing: Transfer the solution in the digestion tube to a centrifuge tube, centrifuge at 8000 rpm for 1 min, and collect the supernatant.

[0014] S6. Purification: The collected supernatant is purified to obtain fluorescent carbon quantum dots.

[0015] Furthermore, in step S1, the carbon material is one of carbon black, coal, graphene, or monolayer graphene oxide.

[0016] Furthermore, in step S2, the ultrasonic frequency of the ultrasonic instrument is 5000Hz-25000Hz, and the ultrasonic treatment lasts for 5 minutes.

[0017] Further, step S6 includes: adding the collected supernatant into a 1000 Da dialysis bag and dialysis in pure water for purification, changing the pure water every 3 hours, dialysis for 24 hours, and obtaining fluorescent carbon quantum dots.

[0018] The second objective of this invention is to provide a fluorescent carbon quantum dot prepared using the above method.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This invention prepares fluorescent carbon quantum dots based on a "top-down" strategy. The pre-pressurized microwave-dilute acid etching method is simpler to operate and requires less manpower, making it a more suitable method for preparing carbon quantum dots.

[0021] This invention utilizes the pre-pressurized cavity of the UltraCLAVE super microwave chemistry platform to achieve internal and external pressure balance in each reaction tube, reducing the requirements for the reaction tubes and significantly lowering the cost of carbon quantum dot preparation compared to traditional microwave methods. It can simultaneously digest multiple digestion tubes, improving work efficiency and facilitating the mass production of carbon quantum dots.

[0022] Compared with the traditional strong acid oxidation method, the pre-pressurized microwave-dilute acid etching method only requires dilute acid, uses less acid, has milder conditions, and is more environmentally friendly.

[0023] The pre-pressurized microwave-dilute acid etching method of this invention enables reactions at high temperatures and ultra-high pressures, improving reaction efficiency and significantly shortening the preparation time of carbon quantum dots. Attached Figure Description

[0024] Figure 1 Optical images of solutions before and after digestion of different carbon materials (digestion temperature 160℃, reaction time 30min).

[0025] Figure 2 Fluorescence spectra of carbon quantum dots after digestion of different carbon materials (digestion temperature 160℃, reaction time 30 min).

[0026] Figure 3 Optical images of solutions before and after digestion of different carbon materials (digestion temperature 180℃, reaction time 30 min).

[0027] Figure 4 Fluorescence spectra of carbon quantum dots after digestion of different carbon materials (digestion temperature 180℃, reaction time 30 min).

[0028] Figure 5 Optical images of solutions before and after digestion of different carbon materials (digestion temperature 200℃, reaction time 30min).

[0029] Figure 6 Fluorescence spectra of carbon quantum dots after digestion of different carbon materials (digestion temperature 200℃, reaction time 30 min).

[0030] Figure 7 Optical images of solutions before and after wet treatment of different carbon materials (heating temperature 110℃, reaction time 2h).

[0031] Figure 8 Fluorescence spectra of carbon quantum dots after wet treatment (heating temperature 110℃, reaction time 2h) with different carbon materials.

[0032] Figure 9 Fluorescence spectra and intensity bar graphs of monolayer graphene oxide with different volume fractions of acid are shown below: (a) Fluorescence spectra at heating temperatures of 110℃ and 160℃; (b) Fluorescence spectra at heating temperatures of 110℃ and 180℃; (c) Fluorescence spectra at heating temperatures of 110℃ and 200℃; (d) Comparison of fluorescence intensity at heating temperatures of 110℃ and 160℃; (e) Comparison of fluorescence intensity at heating temperatures of 110℃ and 180℃; (f) Comparison of fluorescence intensity at heating temperatures of 110℃ and 200℃.

[0033] Figure 10 Optical images of monolayer graphene oxide samples after acid treatment with different volume fractions (irradiated by a 365nm UV lamp): (a) under heating temperatures of 110℃ and 160℃; (b) under heating temperatures of 110℃ and 180℃; (c) under heating temperatures of 110℃ and 200℃.

[0034] Figure 11Fluorescence spectra and intensity comparisons of monolayer graphene oxide at different microwave treatment temperatures are shown below: (a) Fluorescence spectra at heating temperatures of 110℃ and 160℃; (b) Fluorescence spectra at heating temperatures of 110℃ and 180℃; (c) Fluorescence spectra at heating temperatures of 110℃ and 200℃; (d) Fluorescence intensity comparison at heating temperatures of 110℃ and 160℃; (e) Fluorescence intensity comparison at heating temperatures of 110℃ and 180℃; (f) Fluorescence intensity comparison at heating temperatures of 110℃ and 200℃.

[0035] Figure 12 Optical images of monolayer graphene oxide samples at different microwave processing temperatures (irradiated with a 365nm UV lamp): (a) under heating temperatures of 110℃ and 160℃; (b) under heating temperatures of 110℃ and 180℃; (c) under heating temperatures of 110℃ and 200℃. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0037] The following examples demonstrate the preparation of fluorescent carbon quantum dots using graphene, coal, and monolayer graphene oxide as carbon materials.

[0038] Example 1

[0039] S1. Sample addition: Weigh 5 mg of one of the following: graphene, coal, or single-layer graphene oxide, and add it to a quartz or polytetrafluoroethylene digestion tube. Then, add 2 mL of nitric acid solution with a volume concentration of 5%, 12.5%, or 25% to the digestion tube, respectively, and cover with a dust cover.

[0040] S2. Ultrasound: Place the digestion tube into an ultrasonic instrument for ultrasonic treatment. The ultrasonic frequency of the ultrasonic instrument is 5000Hz-25000Hz, and the ultrasonic treatment lasts for 5 minutes to make the carbon material sample dispersed evenly and obtain a suspended carbon material sample.

[0041] S3. Sample loading: Add 440 mL of high-purity water and 10 mL of concentrated HNO3 as microwave medium to the TFM digestion vessel and load it into the stainless steel chamber of the UltraCLAVE super microwave chemistry platform; fix the digestion tube of the suspended carbon material sample in the TFM vessel, with the bottom of the digestion tube immersed in the microwave medium; then close and lock the stainless steel chamber cover, and introduce nitrogen into the stainless steel chamber to adjust the pressure of the stainless steel chamber to 40-70 Bar, and close the instrument cover;

[0042] S4. Microwave Pressurization Reaction: The microwave conditions within the stainless steel cavity of the UltraCLAVE super microwave chemical platform are as follows: 1. Temperature rises from 0℃ to 70℃ within 10 minutes, power is 1000W, protection temperature is 60℃, and pressure is 100bar; 2. Temperature rises from 70℃ to 120℃ within 10 minutes, power is 1000W, protection temperature is 60℃, and pressure is 130bar; 3. Temperature rises from 120℃ to 160℃, 180℃, and 200℃ within 10 minutes, power is 1000W, protection temperature is 60℃, and pressure is 150bar; 4. Temperature remains constant at 160℃, 180℃, and 200℃ within 30 minutes, power is 1200W, protection temperature is 60℃, and pressure is 150bar; After the holding period, the cavity is allowed to cool naturally, and the pressure is automatically released and the door opens.

[0043] S5. Post-processing: Transfer the solution in the digestion tube to a centrifuge tube, centrifuge at 8000 rpm for 1 min, and collect the supernatant.

[0044] S6. Purification: Add the collected supernatant to a 1000 Da dialysis bag and dialyze it in pure water for purification. Change the pure water every 3 hours and dialyze for 24 hours to obtain fluorescent carbon quantum dot samples.

[0045] Comparative Example 1

[0046] Weigh 5 mg of one of the following: graphene, coal, or monolayer graphene oxide, and add it to a quartz or polytetrafluoroethylene digestion tube. Then, add 2 mL of nitric acid solution with volume concentrations of 5%, 12.5%, and 25% to the digestion tube, respectively. Treat at 110℃ for 2 h to obtain fluorescent carbon quantum dot samples.

[0047] In Example 1, optical images of solutions before and after digestion of different carbon materials (digestion temperature 160℃, reaction time 30 min) are shown below. Figure 1 As shown, the fluorescence spectra of carbon quantum dots after digestion (digestion temperature 160℃, reaction time 30 min) using different carbon materials are as follows: Figure 2 As shown; optical images of solutions before and after digestion (digestion temperature 180℃, reaction time 30 min) using different carbon materials are shown below. Figure 3 As shown, the fluorescence spectra of carbon quantum dots after digestion (digestion temperature 180℃, reaction time 30 min) using different carbon materials are as follows: Figure 4 As shown; optical images of solutions before and after digestion (digestion temperature 200℃, reaction time 30 min) using different carbon materials are shown below. Figure 5 As shown, the fluorescence spectra of carbon quantum dots after digestion (digestion temperature 200℃, reaction time 30 min) using different carbon materials are as follows: Figure 6 As shown.

[0048] from Figure 1It can be seen that the solution color of monolayer graphene oxide samples digested at 160℃ with different amounts of acid becomes light yellow, and the solution color gradually darkens with increasing acid concentration; Figure 2 It can also be seen that the fluorescence intensity gradually increases with the increase of acid dosage. The solutions of coal samples digested at 160℃ with different acid dosages showed a distinctly deep yellow color, while... Figure 2 It can be seen that the fluorescence intensity gradually decreases with increasing acid concentration. Graphene samples with different acid concentrations showed almost no color change after digestion at 160℃, and the supernatant did not exhibit a yellow solution state. Figure 2 It can be seen that graphene samples with different amounts of acid did not produce fluorescence peaks after digestion. Therefore, 160℃ is not enough to digest graphene samples to produce fluorescent carbon quantum dots.

[0049] from Figure 3 It can be seen that the solutions of monolayer graphene oxide samples digested at 180℃ with different amounts of acid are yellow, a phenomenon more pronounced than that of samples digested at 160℃. The degree of color darkness of the sample solutions after digestion with different amounts of acid is 12.5% ​​> 25% > 5%, suggesting that there may be a balance between sample digestion and oxidation. Figure 4 It can also be seen that the fluorescence intensity gradually increases with the increase of acid dosage. Figure 3 It can be seen that the solutions after digestion of coal samples with different amounts of acid exhibit a distinctly deep yellow color, while from... Figure 4 It can be seen that the fluorescence intensity gradually decreases with increasing acid concentration, with the sample using 5% acid (volume fraction) exhibiting the highest fluorescence intensity. The graphene samples with different acid concentrations showed almost no color change after digestion, and the supernatant did not exhibit a yellow solution state. Figure 4 It can be seen that graphene samples with different amounts of acid did not produce fluorescence peaks after digestion. Therefore, 180℃ is not enough to digest graphene samples to produce fluorescent carbon quantum dots.

[0050] from Figure 5 It can be seen that the solutions of monolayer graphene oxide samples digested at 200℃ with different amounts of acid are yellow. The yellow color is not as pronounced as that after digestion at 180℃, but it is more obvious than that after digestion at 160℃. After digestion at 200℃, the solution color gradually lightens with increasing acid concentration. Figure 6 It can also be seen that the fluorescence intensity 12.5% ​​> 5% > 25%. From... Figure 5 It can be seen that the solutions after digestion of coal samples with different amounts of acid are yellow, and the color becomes lighter than that at 160℃ and 180℃. Furthermore, the solution color gradually lightens with increasing acid concentration. From... Figure 6 It can be seen that the fluorescence intensity of samples with different acid concentrations is 12.5% ​​> 5% > 25%, with the sample using 12.5% ​​acid exhibiting the highest fluorescence intensity. Figure 5 As can be seen from sample tube #7, the graphene sample can be digested at 200℃ with 25% (v / v) acid, and the solution turns yellow; from Figure 6 It can also be seen that the graphene sample also showed a fluorescence peak at this time. Compared with the results of digestion at 160℃ and 180℃, this indicates that the graphene sample can be digested at a temperature of 200℃.

[0051] In Comparative Example 1, optical images of solutions before and after wet treatment (heating temperature 110℃, reaction time 2h) with different carbon materials are shown below. Figure 7 As shown, the fluorescence spectra of carbon quantum dots after wet treatment (heating temperature 110℃, reaction time 2h) with different carbon materials are as follows: Figure 8 As shown.

[0052] from Figure 7 It can be seen that the solutions of monolayer graphene oxide samples with different amounts of acid showed a very light yellow color after wet treatment at 110℃. Figure 8 It can also be seen that the fluorescence intensity of samples 1-1 (5% v / v nitric acid) and 1-2 (25% v / v nitric acid) is very low. From... Figure 7 It can be seen that the solutions from coal samples digested with different amounts of acid exhibit varying degrees of yellow color. Figure 8 It can be seen that the coal sample solution with an acid volume fraction of 25% is the deepest yellow and has the highest fluorescence intensity; however, its fluorescence intensity decreases with increasing reaction time. From... Figure 7 It can be seen that the supernatant of the graphene sample did not turn yellow after wet treatment at 110℃. Figure 8 The fluorescence spectra also proved that samples 3-1 and 3-2 did not have fluorescence peaks, indicating that monolayer graphene oxide and graphene samples are not suitable for wet processing to prepare carbon quantum dots. Moreover, this non-pre-pressurized acid etching method has a long reaction time and low yield of fluorescent carbon quantum dots.

[0053] Taking monolayer graphene oxide samples as an example, the fluorescence properties of samples pre-treated with acid at different volume fractions and microwave pressure were studied. The fluorescence spectra and intensity bar graphs of monolayer graphene oxide with different volume fractions of acid are shown below. Figure 9 As shown, Figure 9In the image: (a) Fluorescence spectra at heating temperatures of 110℃ and 160℃; (b) Fluorescence spectra at heating temperatures of 110℃ and 180℃; (c) Fluorescence spectra at heating temperatures of 110℃ and 200℃; (d) Comparison of fluorescence intensity at heating temperatures of 110℃ and 160℃; (e) Comparison of fluorescence intensity at heating temperatures of 110℃ and 180℃; (f) Comparison of fluorescence intensity at heating temperatures of 110℃ and 200℃. Optical images of monolayer graphene oxide samples with different volume fractions and acid concentrations (irradiated by a 365nm UV lamp) are shown below. Figure 10 As shown, Figure 10 In the middle: (a) under heating temperatures of 110℃ and 160℃; (b) under heating temperatures of 110℃ and 180℃; (c) under heating temperatures of 110℃ and 200℃.

[0054] from Figure 9 and Figure 10 It can be seen that:

[0055] 1) Under wet treatment at 110℃, the highest fluorescence peak wavelength of the samples appeared at 430nm, and the fluorescence intensity of the samples treated under 5% v / v conditions was even higher. However, after pre-pressurized microwave treatment, the highest fluorescence peak wavelength of the samples with 5% and 12.5% ​​acid concentrations appeared at 510nm, while that of the sample with 25% acid concentration appeared at 530nm. This indicates that under microwave treatment at 160℃, the carbon quantum dot fluorescence peak exhibited a red shift with increasing acid concentration. Overall, the samples treated at 160℃ had higher fluorescence intensity, especially the sample with 25% acid concentration, whose fluorescence intensity was 2-4 times that of the other samples. It can be seen that only the sample with 25% acid concentration showed a light green fluorescence under UV light irradiation; the other samples were difficult to distinguish with the naked eye due to their weak fluorescence.

[0056] 2) Under wet treatment at 110℃, the highest fluorescence peak wavelength of the sample appeared at 430nm. However, under microwave treatment at 180℃, the fluorescence intensity of the sample was significantly greater under microwave treatment than under wet treatment. The sample treated by wet treatment did not show fluorescence to the naked eye under ultraviolet light irradiation.

[0057] 3) Under wet processing at 110℃, the highest fluorescence peak wavelength of the sample appeared at 430nm. However, under microwave processing at 200℃, the fluorescence intensity of the sample was significantly greater under microwave treatment than under wet processing. Specifically, after microwave treatment at 200℃, the highest fluorescence peak wavelength of samples with different acid concentrations all appeared at 515nm, indicating that temperature is the key factor determining the fluorescence wavelength. Samples treated at 200℃ under UV light showed a slightly pale bluish-green light (compared to those treated at 180℃). Samples treated by wet processing did not show fluorescence to the naked eye. This demonstrates that the pre-pressurized microwave method for preparing carbon quantum dots is superior to conventional methods.

[0058] Fluorescence spectra and fluorescence intensity comparisons of monolayer graphene oxide at different microwave treatment temperatures are shown in the figure below. Figure 11 As shown, Figure 11 In the image: (a) Fluorescence spectra at heating temperatures of 110℃ and 160℃; (b) Fluorescence spectra at heating temperatures of 110℃ and 180℃; (c) Fluorescence spectra at heating temperatures of 110℃ and 200℃; (d) Comparison of fluorescence intensity at heating temperatures of 110℃ and 160℃; (e) Comparison of fluorescence intensity at heating temperatures of 110℃ and 180℃; (f) Comparison of fluorescence intensity at heating temperatures of 110℃ and 200℃. Optical images of monolayer graphene oxide samples at different microwave treatment temperatures (irradiated by a 365nm UV lamp) are shown below. Figure 12 As shown, Figure 12 In the middle: (a) under heating temperatures of 110℃ and 160℃; (b) under heating temperatures of 110℃ and 180℃; (c) under heating temperatures of 110℃ and 200℃.

[0059] from Figure 11 and Figure 12 It can be seen that:

[0060] 1) Under the condition of 5% acid volume fraction, the highest fluorescence peak wavelength of the sample treated by wet processing at 110℃ appeared at 430nm, while the highest fluorescence peak wavelength of the sample treated by microwave processing at 160℃ appeared at 510nm. Under this acid volume condition, the sample treated by wet processing at 110℃ had a higher fluorescence intensity than the sample treated by microwave processing at 160℃. Under microwave processing at 180℃ and 200℃, the highest fluorescence peak wavelength of the samples appeared at 530nm, and the intensity was greatly enhanced. Among them, the sample treated by microwave processing at 180℃ had the strongest fluorescence intensity, indicating that 180℃ is a more suitable digestion temperature at 5% acid volume. Furthermore, under UV irradiation, the fluorescence intensity observed for the sample with 5% acid volume was greater at 180℃ than at 200℃, while the fluorescence of the samples at 160℃ and 110℃ was almost invisible.

[0061] 2) Under the condition of 12.5% ​​acid by volume, the highest fluorescence peak wavelength of the sample treated with microwave at 160℃ appeared at 510nm, the highest fluorescence peak wavelength of the sample treated with microwave at 180℃ appeared at 540nm, and the highest fluorescence peak wavelength of the sample treated with microwave at 200℃ appeared at 515nm. This indicates that different microwave treatment temperatures cause a red shift or blue shift in the fluorescence wavelength of carbon quantum dots. Furthermore, the fluorescence intensity under UV irradiation was 180℃ > 200℃ > 160℃.

[0062] 3) Under conditions of 25% acid volume fraction and wet treatment at 110℃, the highest fluorescence peak wavelength of the sample appeared at 430nm, with the weakest fluorescence intensity. The highest fluorescence peak wavelength of the sample treated with microwave at 160℃ appeared at 530nm, the sample treated with microwave at 180℃ appeared at 540nm, and the sample treated with microwave at 200℃ appeared at 515nm. The fluorescence intensity comparison is 180℃ > 200℃ > 160℃ > 110℃. The sample treated with microwave at 180℃ showed a distinct bluish-green light under UV light irradiation, while the fluorescence of other samples was difficult to distinguish with the naked eye.

[0063] Therefore, the carbon materials available for wet processing in the preparation of carbon quantum dots are limited; in Comparative Example 1, only coal samples showed good processing results. In contrast, the pre-pressurized microwave-dilute acid etching method exhibits corresponding etching effects on monolayer graphene oxide, coal, and graphene under different conditions, resulting in a wider variety of carbon materials suitable for carbon quantum dot preparation. Secondly, the stability of the sample processing is more conducive to the mass production of carbon quantum dots. In wet processing, we found that the sample solution is prone to volatility, leading to instability in the preparation conditions. Finally, the pre-pressurized microwave-dilute acid etching method is faster (0-30 min) than wet processing (1-2 h), making it a more efficient method for preparing carbon quantum dots.

[0064] The technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made in accordance with the technical solutions of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing fluorescent carbon quantum dots using a pre-pressurized microwave-dilute acid etching method, characterized in that, Includes the following steps: S1. Sample addition: Weigh 5 mg of carbon material sample and add it to a quartz or polytetrafluoroethylene digestion tube. Then add 2 mL of nitric acid solution with a volume concentration of 5%-25% to the digestion tube and cover it with a dust cover. S2, Ultrasonication: The digestion tube is placed in an ultrasonic instrument for ultrasonic treatment to make the carbon material sample dispersed evenly and obtain a suspended carbon material sample. S3. Sample loading: Add 440 mL of high-purity water and 10 mL of concentrated HNO3 as microwave medium to the TFM digestion vessel and load it into the stainless steel chamber of the UltraCLAVE super microwave chemistry platform; fix the digestion tube of the suspended carbon material sample in the TFM vessel, with the bottom of the digestion tube immersed in the microwave medium; then close and lock the stainless steel chamber cover, introduce nitrogen gas to adjust the pressure of the stainless steel chamber to 40-70 Bar, and close the instrument cover; S4. Microwave Pressurization Reaction: The microwave conditions within the stainless steel cavity of the UltraCLAVE super microwave chemical platform are as follows:

1. Temperature rises from 0℃ to 70℃ within 10 minutes, power is 1000W; 2. Temperature rises from 70℃ to 120℃ within 10 minutes, power is 1000W; 3. Temperature rises from 120℃ to 180℃-200℃ within 10 minutes, power is 1000W; 4. Temperature remains constant at 180℃-200℃ within 30 minutes, power is 1200W; After the heat preservation is completed, the cavity cools naturally, and the pressure is automatically released and the chamber door opens. S5. Post-processing: Transfer the solution in the digestion tube to a centrifuge tube, centrifuge at 8000 rpm for 1 min, and collect the supernatant. S6. Purification: The collected supernatant is purified to obtain fluorescent carbon quantum dots.

2. The method for preparing fluorescent carbon quantum dots by pre-pressurized microwave-dilute acid etching according to claim 1, characterized in that, In step S1, the carbon material is one of carbon black, coal, graphene, or monolayer graphene oxide.

3. The method for preparing fluorescent carbon quantum dots by pre-pressurized microwave-dilute acid etching according to claim 1, characterized in that, In step S2, the ultrasonic frequency of the ultrasonic instrument is 5000Hz-25000Hz, and the ultrasonic treatment lasts for 5 minutes.

4. The method for preparing fluorescent carbon quantum dots by pre-pressurized microwave-dilute acid etching according to claim 1, characterized in that, Step S6 includes: adding the collected supernatant into a 1000 Da dialysis bag and dialysis in pure water for purification, changing the pure water every 3 hours, dialysis for 24 hours, and obtaining fluorescent carbon quantum dots.

5. A fluorescent carbon quantum dot prepared by the method according to any one of claims 1-4.

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