Fluorescent carbon dots based on ultrasonic suspension as well as preparation method and application of fluorescent carbon dots

Abstract

The invention belongs to the technical field of fluorescent carbon dots, and relates to a fluorescent carbon dot based on ultrasonic suspension and a preparation method and application thereof, the preparation method comprises the following steps: 1, respectively adding an aromatic compound containing an electron-donating group and citric acid into an organic solvent, and stirring until the aromatic compound and the citric acid are completely dissolved to obtain a mixed solution; 2, starting an ultrasonic suspension instrument, injecting the mixed solution into an ultrasonic suspension working window by using an injector, and stably suspending; and 3, suspending for 1-5 minutes, and sucking liquid drops to obtain a target product. The preparation of the fluorescent carbon dots is realized at normal temperature and normal pressure within a short time by utilizing an ultrasonic suspension technology, and the prepared fluorescent carbon dots have the characteristic of long-wavelength emission and excellent fluorescence property, and can be used as a fluorescent probe for screening coal dissolving strains and detecting microbial coal dissolving products.

Description

Technical Field

[0001] The present invention belongs to the technical field of fluorescent carbon dots, and relates to a fluorescent carbon dot based on ultrasonic suspension, a preparation method thereof, and an application thereof. Background Art

[0002] Due to their excellent optical properties, low cytotoxicity, abundant synthesis raw materials, environmental friendliness and other characteristics, carbon dots have wide applications in detection and sensing, bioimaging, fluorescence anti-counterfeiting, photoelectrocatalysis and other aspects.

[0003] Common carbon dot synthesis methods mainly include the top-down method and the bottom-up method. Among them, the top-down method mainly includes high-temperature pyrolysis, laser ablation, quenching, electrolysis, acidolysis, etc. These methods have relatively harsh conditions and low preparation efficiency. The bottom-up method mainly includes hydrothermal and solvothermal methods, microwave method, reflux method, etc. Among them, the most widely used are hydrothermal and solvothermal methods, which are simple to operate and have a wide range of applications, and are widely favored. However, the hydrothermal and solvothermal methods have the following defects in the preparation process: the hydrothermal method and the solvothermal method are contact reactions carried out in a reaction vessel, and the reaction vessel generally uses high-temperature-resistant steel. However, the prepared fluorescent carbon dots are sensitive materials, so that the reaction vessel damages or pollutes the prepared fluorescent carbon dots, resulting in a decrease in the performance of the fluorescent carbon dots; the existing hydrothermal and solvothermal methods generally take a long time, and the reaction needs to be carried out under high-temperature and high-pressure conditions, which not only has harsh reaction conditions, but also requires high reaction equipment, resulting in an increase in equipment cost; in addition, the hydrothermal and solvothermal methods are carried out under closed conditions. When heated at high temperature, the fluid volume in the closed reaction kettle expands, which can generate extremely high pressure and there are great safety hazards. Summary of the Invention

[0004] Aiming at the technical problems that the performance of the fluorescent carbon dots is reduced, the time consumption is long, and the reaction conditions are harsh during the preparation of the fluorescent carbon dots, the present invention provides a fluorescent carbon dot based on ultrasonic suspension, a preparation method thereof, and an application thereof.

[0005] The present invention utilizes ultrasonic suspension technology to prepare fluorescent carbon dots at normal temperature and pressure within a short time; the prepared fluorescent carbon dots have the characteristics of long-wavelength emission and excellent fluorescence performance; therefore, they can be used as fluorescent probes for screening of coal-dissolving strains and detection of coal-dissolving products of microorganisms.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A preparation method of a fluorescent carbon dot based on ultrasonic suspension, comprising the following steps:

[0008] Step 1: Add the aromatic compound containing an electron-donating group and citric acid into an organic solvent respectively, and stir until completely dissolved to obtain a mixed solution; the mass-volume ratio of the aromatic compound containing an electron-donating group, citric acid and the organic solvent is (1-3) g : (1-3) g : 10 mL; the aromatic compound containing an electron-donating group is aromatic amine or aromatic phenol;

[0009] Step 2: Turn on the ultrasonic suspension instrument and set the working parameters; use a syringe to inject the mixed solution in Step 1 into the ultrasonic suspension working window, and adjust the ultrasonic suspension instrument during the injection process so that the droplets formed by the mixed solution are completely and stably suspended;

[0010] Step 3: After suspending for 1 min - 5 min, use another syringe to suck the droplets, which are the target products.

[0011] Further limitation: In Step 1, the boiling point of the organic solvent is above 100 °C.

[0012] Further limitation: The organic solvent is N,N-dimethylformamide or dimethyl sulfoxide.

[0013] Further limitation: In Step 2, the working parameters are: power is 350 W - 450 W, and the resonance frequency is 18 KHz - 20 KHz.

[0014] Further limitation: In Step 2, the mixed solution in Step 1 is injected into the ultrasonic suspension working window at a rate of 15 μL each time.

[0015] The fluorescent carbon dots prepared by the method for preparing fluorescent carbon dots based on ultrasonic suspension as described above.

[0016] Further limitation: The average particle size of the fluorescent carbon dots is 2.93 ± 0.69 nm; the lattice fringe spacing of the fluorescent carbon dots is 0.10 nm; the average fluorescence lifetime of the fluorescent carbon dots is 2.950 ns.

[0017] Further limitation: The potential of the fluorescent carbon dots is -0.061 mV; the optimal excitation wavelength of the fluorescent carbon dots is 434 nm, and the optimal emission wavelength of the fluorescent carbon dots is 517 nm.

[0018] The application of the fluorescent carbon dots as described above as a fluorescent probe in the screening of coal-dissolving strains.

[0019] The application of the fluorescent carbon dots as described above as a fluorescent probe in the detection of coal-dissolving products of microorganisms.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1. The present invention utilizes ultrasonic suspension technology to prepare fluorescent carbon dots at normal temperature and pressure within a short time. The prepared fluorescent carbon dots have the characteristics of long-wavelength emission, high quantum yield, and excellent fluorescence performance.

[0022] 2. Compared with the traditional preparation method provided by the present invention, the preparation method is simple in operation, simple in equipment, mild in reaction conditions without the need for high temperature and high pressure, and the reaction time is significantly shortened. It can be rapidly prepared within the range of 1 - 5 minutes, improving the preparation efficiency.

[0023] 3. The fluorescent carbon dots prepared by the present invention are quasi-spherical, well-dispersed, and uniform in size, with an average particle size of 2.93 ± 0.69 nm. The surface of the fluorescent carbon dots has abundant functional groups such as amino, hydroxyl, and carboxyl groups, and the Zeta potential is -0.061 mV. Further, the ultraviolet-visible absorption spectrum shows that the fluorescent carbon dots exhibit π-π* transitions of C═C and n-π* transitions of C═O on the conjugated aromatic ring at 269 nm and 301 nm. The fluorescence spectrum shows that the optimal excitation wavelength of the fluorescent carbon dots is 434 nm, the optimal emission wavelength is 517 nm, the emission wavelength is long, and it does not depend on excitation, with excellent photostability.

[0024] 4. The fluorescent carbon dots prepared by the present invention have good fluorescence performance. Therefore, in the bioconversion of oil-rich coal, the fluorescent carbon dots can be used as fluorescent probes for the screening of coal-dissolving strains and the selective qualitative and quantitative detection of microbial coal-dissolving products (such as simple alkanes like methane and ethane, as well as long-chain alkanes). Description of the Drawings

[0025] Figure 1 Comparison diagrams of carbon dots prepared with different solvents before and after ultrasonic suspension;

[0026] Figure 2 Photos of carbon dots prepared with different ultrasonic suspension durations under ultraviolet light;

[0027] Figure 3 Fluorescence intensity diagrams of carbon dots prepared with different ultrasonic suspension durations (5 - 30 minutes);

[0028] Figure 4 Fluorescence intensity diagrams of carbon dots prepared with different ultrasonic suspension durations (1 - 5 minutes);

[0029] Figure 5 Transmission electron microscope image of the fluorescent carbon dots prepared in Example 1;

[0030] Figure 6 Particle size distribution diagram of the fluorescent carbon dots prepared in Example 1;

[0031] Figure 7 High-resolution transmission electron microscope image of the fluorescent carbon dots prepared in Example 1;

[0032] Figure 8 Infrared spectrum of the fluorescent carbon dots prepared in Example 1;

[0033] Figure 9 Zeta potential diagram of the fluorescent carbon dots prepared in Example 1;

[0034] Figure 10 Optimal excitation and optimal emission wavelength diagram of the fluorescent carbon dots prepared in Example 1;

[0035] Figure 11 Three-dimensional fluorescence spectrum diagram of the fluorescent carbon dots prepared in Example 1;

[0036] Figure 12 Ultraviolet-visible absorption spectrum diagram of the fluorescent carbon dots prepared in Example 1;

[0037] Figure 13 Transient fluorescence absorption spectrum diagram of the fluorescent carbon dots prepared in Example 1;

[0038] Figure 14 Fluorescence intensity diagram of the fluorescent carbon dots prepared in Example 1 under different pH conditions;

[0039] Figure 15 Fluorescence intensity diagram of the fluorescent carbon dots prepared in Example 1 in different concentrations of NaCl solution;

[0040] Figure 16 Fluorescence intensity diagram of the fluorescent carbon dots prepared in Example 1 at different low temperatures;

[0041] Figure 17 Fluorescence intensity diagram of the fluorescent carbon dots prepared in Example 1 at different high temperatures;

[0042] Figure 18 Fluorescence intensity diagram of the fluorescent carbon dots prepared in Example 1 under ultraviolet lamp irradiation for different durations;

[0043] Figure 19 Photos of the fluorescent carbon dots (A, a) prepared in Example 1 and the fluorescent carbon dots prepared by other methods (solvothermal (B, b), reflux (C, c), (conventional ultrasound (D, d)) under visible light and ultraviolet light;

[0044] Figure 20 Three-dimensional fluorescence spectra of the carbon dots (b) prepared from the fluorescent carbon dots in Example 1 and the carbon dots prepared by other methods (solvothermal (a), reflux (c), (conventional ultrasound (d));

[0045] Figure 21 Synthesis route diagram of the fluorescent carbon dots prepared in Example 1. Detailed implementation method

[0046] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings and embodiments, but the present invention is not limited to the following embodiments.

[0047] Unless otherwise defined, the technical terms or scientific terms used in the present invention should have the ordinary meaning understood by those of ordinary skill in the field to which the present invention pertains.

[0048] For technologies, methods, and devices known to those of ordinary skill in the relevant fields, they may not be discussed in detail, but where appropriate, the said technologies, methods, and devices should be regarded as part of the specification.

[0049] It should also be understood that the specific embodiments described above are only used to explain the present invention, and the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered within the protection scope of the present invention / invention.

[0050] The technical idea of the present invention is to use the ultrasonic suspension method to prepare fluorescent carbon dots.

[0051] The ultrasonic suspension technology is a technology that uses an ultrasonic field to generate an acoustic field gradient force, and under the action of gas flow, suspends an object in the air. Compared with traditional mechanical manipulation methods, the ultrasonic suspension technology does not require contact with other objects, avoiding the friction and wear caused by contact in traditional mechanical suspension, and can achieve very high manipulation accuracy and speed, enabling more flexible and precise operations, and having the advantages of good stability and strong controllability. The present invention applies the ultrasonic suspension technology to the synthesis of fluorescent carbon dots, which has the advantages of containerless, safe, and controllable; the non-contact operation of ultrasonic suspension does not require a reaction container, which helps to avoid damage or contamination to sensitive materials; and ultrasonic suspension can be carried out at room temperature and atmospheric pressure, ensuring the safety of the experiment; on the other hand, using the ultrasonic suspension method can change the size, morphology, and optical properties of carbon quantum dots by adjusting the frequency, power, and processing time of ultrasonic waves, which helps to prepare near-infrared carbon dots with long-wavelength emission and high quantum yield.

[0052] The method for preparing fluorescent carbon dots based on ultrasonic suspension of the present invention includes the following steps:

[0053] Step 1: Add an aromatic compound containing an electron-donating group and citric acid to an organic solvent respectively, and stir until completely dissolved to obtain a mixed solution.

[0054] The mass-volume ratio of the aromatic compound containing an electron-donating group, citric acid, and the organic solvent is (1-3) g: (1-3) g: 10 mL.

[0055] The aromatic compound containing an electron-donating group is aromatic amine or aromatic phenol.

[0056] An aromatic amine refers to an amine having an aromatic substituent, that is, -NH 2 , -NH or a nitrogen-containing group is attached to an aromatic hydrocarbon.

[0057] In implementation, the aromatic amine can be an o-phenylenediamine compound. Preferably, the aromatic amine is o-phenylenediamine.

[0058] The aromatic phenol can be a catechol compound. Preferably, the aromatic phenol is catechol.

[0059] In the present invention, to ensure the stability of the liquid droplets under ultrasonic suspension conditions, it is required that the boiling point of the organic solvent is relatively high. Once the boiling point is low, the organic solvent is likely to volatilize, resulting in unstable liquid droplets during ultrasonic suspension. Preferably, the boiling point of the organic solvent is above 100 °C. At the same time, the viscosity of the organic solvent should be greater than the viscosity of water (at 25 °C) to enhance the stability of the liquid droplets during ultrasonic suspension.

[0060] In the present invention, the organic solvent is N,N-dimethylformamide, dimethyl sulfoxide or ethylene glycol. It can also be an aqueous solution containing N,N-dimethylformamide, dimethyl sulfoxide or ethylene glycol.

[0061] Step 2: Turn on the ultrasonic suspension instrument and set the working parameters; use a syringe to inject the mixed solution in Step 1 into the ultrasonic suspension working window, and adjust the ultrasonic suspension instrument during the injection process so that the liquid droplets formed by the mixed solution are complete and stably suspended.

[0062] In Step 2 of the present invention, the working parameters are: power 350W - 450W, resonance frequency 18KHz - 20KHz.

[0063] Preferably, the working parameters are power 395W and resonance frequency 19775Hz.

[0064] In Step 2 of the present invention, the mixed solution in Step 1 is injected into the ultrasonic suspension working window at a rate of 15 μL each time.

[0065] Step 3: After suspending for 1 min - 5 min, use another syringe to aspirate the liquid droplets, which are the target products.

[0066] The present invention also provides fluorescent carbon dots prepared by using the above-mentioned preparation method of fluorescent carbon dots based on ultrasonic suspension. The average particle size of the fluorescent carbon dots is 2.93 ± 0.69 nm; the lattice fringe spacing of the fluorescent carbon dots is 0.10 nm; the average fluorescence lifetime of the fluorescent carbon dots is 2.950 ns. The potential of the fluorescent carbon dots is -0.061 mV; the optimal excitation wavelength of the fluorescent carbon dots is 434 nm, and the optimal emission wavelength of the fluorescent carbon dots is 517 nm. It can be seen that the fluorescent carbon dots prepared by the method of the present invention have excellent fluorescence properties and can be used as fluorescent probes, showing good application prospects in the screening of coal-dissolving strains and the detection of coal-dissolving products of microorganisms.

[0067] The following is a detailed description of the present invention with specific examples.

[0068] It should be noted that, unless otherwise specified, the chemical drugs, reagents, etc. used in the following examples are all commercially available products known in the art.

[0069] It should be noted that, unless otherwise specified, the operations used in the following examples are all conventional operations in the art. For example, if there is no special specification for the reaction temperature, pressure, etc., it refers to normal temperature and pressure.

[0070] Example 1

[0071] Refer to Figure 21 , this example provides a preparation method of fluorescent carbon dots based on ultrasonic suspension, including the following steps:

[0072] Step 1: Add an aromatic compound containing an electron-donating group (o-phenylenediamine) and citric acid into an organic solvent (N,N-dimethylformamide) respectively, and stir until completely dissolved to obtain a mixed solution.

[0073] Specifically, accurately weigh 0.165 g of o-phenylenediamine and 0.375 g of citric acid using an analytical balance, and add the weighed citric acid and o-phenylenediamine into a beaker in sequence; measure 10 mL of N,N-dimethylformamide, pour the N,N-dimethylformamide into the beaker containing the drugs, and stir with a glass rod until completely dissolved to obtain a mixed solution.

[0074] Step 2: Turn on the power of the ultrasonic suspension instrument, set the parameters of power to 395 W and resonance frequency to 19775 Hz, press the operation switch, and when a sharp high-frequency noise is heard, it indicates successful startup. Before injecting the solution, align the emitting end with the receiving end. Use a syringe to suck the prepared mixed solution, and then inject the mixed solution into the ultrasonic suspension working window. Each time, inject about 15 μL of the mixed solution droplets. During the injection process, adjust the height of the lower rod with a knob to make the droplets complete and stably suspended. During the suspension process, always adjust the height of the lower rod to keep the droplets stable.

[0075] Step 3: After 3 minutes of timing, use another syringe to aspirate the liquid droplets. During aspiration, adjust the height of the lower rod to prevent atomization and splashing of the unaspirated part of the liquid droplets, collect the liquid, and obtain orange-yellow fluorescent carbon dots, which are the target products.

[0076] Example 2

[0077] This example provides a method for preparing fluorescent carbon dots based on ultrasonic levitation, including the following steps:

[0078] Step 1: Add an aromatic compound containing an electron-donating group (catechol) and citric acid to an organic solvent (N,N-dimethylformamide) respectively, and stir until completely dissolved to obtain a mixed solution.

[0079] Specifically, accurately weigh 0.165 g of catechol and 0.375 g of citric acid using an analytical balance, and add the weighed citric acid and catechol to a beaker in sequence; measure 10 mL of N,N-dimethylformamide, pour the N,N-dimethylformamide into the beaker containing the drugs, and use a glass rod to stir until completely dissolved to obtain a mixed solution.

[0080] Step 2: Turn on the power of the ultrasonic levitator, set the parameters of power to 395 W and resonance frequency to 19775 Hz, press the operation switch, and when a sharp high-frequency noise is heard, it indicates successful startup. Before injecting the solution, align the transmitting end with the receiving end. Use a syringe to aspirate the prepared mixed solution, and then inject the mixed solution into the ultrasonic levitation working window. Each time, inject about 15 μL of the mixed solution droplets. During the injection process, use the knob to adjust the height of the lower rod to make the droplets complete and stably suspended. During the suspension process, constantly adjust the height of the lower rod to keep the droplets stable.

[0081] Step 3: After 3 minutes of timing, use another syringe to aspirate the liquid droplets. During aspiration, adjust the height of the lower rod to prevent atomization and splashing of the unaspirated part of the liquid droplets, collect the liquid, and obtain orange-yellow fluorescent carbon dots, which are the target products.

[0082] Through the following experimental studies, the performance of the fluorescent carbon dots prepared in the above examples was studied.

[0083] Experiment 1: Comparison of different suspension systems

[0084] Experimental group: Use N,N-dimethylformamide as the solvent and perform ultrasonic levitation with reference to the preparation method of Example 1.

[0085] Control group 1: Use distilled water as the solvent and perform ultrasonic levitation with reference to the preparation method of Example 1.

[0086] Control group 2: Use ethanol as the solvent and perform ultrasonic levitation with reference to the preparation method of Example 1.

[0087] Then observe the suspension processes of the experimental group and the control group, and the results are shown in Figure 1.

[0088] See Figure 1 , in the system with N,N-dimethylformamide as the solvent as shown in Figure 1 (a) and (b), the droplets remain stable during ultrasonic suspension. After 10 min of suspension, the droplet volume remains unchanged and the fluorescence effect is excellent. In the system with distilled water as the solvent as shown in Figure 1 (c) and (d), the droplets can also remain stable during ultrasonic suspension and can be suspended for a long time, but the fluorescence effect is not good; in the system with ethanol as the solvent as shown in Figure 1 (e) and (f), the droplets in ultrasonic suspension are unstable and the distance of the lower rod needs to be manually adjusted to maintain the stability of the droplets. This is because the surface tension and boiling point of ethanol are both relatively low, resulting in an extremely fast evaporation rate of the droplets, so long-term ultrasonic suspension cannot be carried out. Considering comprehensively, an organic solvent with a boiling point above 100 °C and a viscosity greater than that of water (at 25 °C) is selected for the synthesis of carbon dots by ultrasonic suspension. Preferably, N,N-dimethylformamide is a better organic solvent.

[0089] Experiment 2: Comparison of different suspension times

[0090] The experimental process is as follows: Referring to the ultrasonic suspension system of Example 1, ultrasonic suspension is carried out for 0 min, 1 min, 2 min, 3 min, 4 min, 5 min, 10 min, 15 min, 20 min, 25 min and 30 min respectively, and then the fluorescent carbon dots obtained under the corresponding ultrasonic suspension times are collected. See Figure 2 , Figure 3 and Figure 4 as shown.

[0091] See Figure 2 , after ultrasonic suspension, the fluorescence properties of the solution change, and the fluorescent carbon dots change from the original yellow-green color to orange-yellow, that is, a red shift occurs. With the change of the ultrasonic time, the fluorescence intensity also changes accordingly.

[0092] See Figure 3 , for each group with ultrasonic suspension time of 5 min - 30 min, the fluorescence intensity of the solution (fluorescent carbon dots) weakens with the increase of the suspension time. Among them, the fluorescence intensity of the group with an ultrasonic suspension time of 5 min is the best.

[0093] See Figure 4 , for each group of solutions (fluorescent carbon dots) with ultrasonic suspension time of 1 min - 5 min, the fluorescence intensity shows a trend of first increasing and then decreasing, and the fluorescence intensity of the group with an ultrasonic suspension time of 3 min is the strongest. Therefore, considering comprehensively, the best ultrasonic suspension time is 3 min.

[0094] Experiment 3: Physical property testing

[0095] Take the fluorescent carbon dots prepared in Example 1 to obtain a transmission electron micrograph and particle size distribution, and the results are as Figure 5 , Figure 6 and Figure 7 shown.

[0096] See Figure 5 . It can be seen from the figure that the morphology of the fluorescent carbon dots is a regular sphere, and they show good dispersibility in N,N-dimethylformamide solution.

[0097] See Figure 6 . The average particle size of the fluorescent carbon dots is 2.93 ± 0.69 nm.

[0098] See Figure 7 . The lattice fringe spacing of the fluorescent carbon dots is about 0.10 nm, which corresponds to the crystal plane of graphite. It is proved that the fluorescent carbon dots prepared by the ultrasonic suspension method have high crystallinity.

[0099] Experiment 4. Infrared test

[0100] Take the fluorescent carbon dots prepared in Example 1 for infrared test to obtain an infrared spectrum, and the results are as Figure 8 shown.

[0101] See Figure 8 . The strong absorption peak at 3483 cm -1 is the stretching vibration of O-H / N-H; the peak appearing at 2929 cm -1 is the stretching vibration of C-H; the peak at 1662 cm -1 is due to the C=O bond; the peak appearing at 1390 cm -1 is due to the stretching vibration of -CH 2 -; the absorption peak at 1255 cm -1 is caused by the stretching vibration of C-O; the peak appearing at 661 cm -1 is caused by the stretching vibration of C-H on the benzene ring. It shows that the fluorescent carbon dots synthesized by ultrasonic suspension using citric acid and o-phenylenediamine as precursors have an aromatic ring structure and surface functional groups such as hydroxyl, carboxyl, and amino groups.

[0102] Experiment 5. Electrochemical performance test

[0103] Take the fluorescent carbon dots prepared in Example 1 for electrochemical test to obtain a Zeta potential diagram, and the results are as Figure 9 shown.

[0104] See Figure 9 . The potential of the fluorescent carbon dots is -0.061 mV, indicating that the surface of the fluorescent carbon dots not only contains positively charged functional groups but also has rich negatively charged functional groups.

[0105] Experiment 6: Fluorescence Performance Test

[0106] Take the fluorescent carbon dots prepared in Example 1 for fluorescence performance test, and obtain the optimal excitation and optimal emission wavelength diagram, three-dimensional fluorescence spectrum diagram, ultraviolet-visible absorption spectrum diagram and transient fluorescence absorption spectrum diagram. The results are as Figure 10 、 Figure 11 、 Figure 12 and Figure 13 shown.

[0107] See Figure 10 , the optimal excitation wavelength of the fluorescent carbon dots is 434 nm, and the optimal emission wavelength of the fluorescent carbon dots is 517 nm.

[0108] See Figure 11 , the emission wavelength of the fluorescent carbon dots does not depend on excitation and has excellent photostability.

[0109] See Figure 12 , for the fluorescent carbon dots synthesized by ultrasonic suspension for 3 min, the ultraviolet-visible absorption spectrum has obvious absorption peaks at 269 nm and 301 nm in the ultraviolet region. The absorption peak at 269 nm can be attributed to the π-π* transition of the aromatic ring structure, and the broad absorption peak at 290 - 310 nm is due to the n-π* transition of the functional group C=O.

[0110] See Figure 13 , according to the analysis of the transient fluorescence absorption spectrum, the average fluorescence lifetime of the fluorescent carbon dots obtained by ultrasonic suspension is 2.950 ns.

[0111] Experiment 7: Stability Test

[0112] Take the fluorescent carbon dots prepared in Example 1 for stability test, and obtain the fluorescence intensity diagrams under different pH conditions, the fluorescence intensity diagrams under different concentrations of NaCl solutions, the fluorescence intensity diagrams under different temperatures (low temperature), the fluorescence intensity diagrams under different temperatures (high temperature) and the fluorescence intensity diagrams under ultraviolet lamp irradiation for different durations. The results are as Figure 14 、 Figure 15 、 Figure 16 、 Figure 17 and Figure 18 shown.

[0113] See Figure 14 , when the fluorescent carbon dots are in a strongly acidic or strongly alkaline environment, their fluorescence intensity changes significantly, but they are relatively stable in the pH range of 2 - 10. It shows that the fluorescent carbon dots have good stability in the pH range of 2 - 10.

[0114] See Figure 15 , when the fluorescent carbon dots are in sodium chloride solutions with different ionic strengths, they can maintain a stable fluorescence intensity.

[0115] See Figure 16 , the fluorescence intensity of the fluorescent carbon dots did not change significantly at low temperatures (-15°C - 10°C) and could remain stable.

[0116] See Figure 17 , the fluorescence intensity of the fluorescent carbon dots did not change significantly at higher temperatures (25°C - 45°C) and could remain stable.

[0117] See Figure 18 , the fluorescence intensity of the fluorescent carbon dots remained basically unchanged under ultraviolet lamp irradiation for different durations. This indicates that the synthesized fluorescent carbon dots are not easily affected by ultraviolet lamp irradiation and have good stability.

[0118] In summary, the fluorescent carbon dots prepared by the present invention are stable in the pH range of 2 - 10 and in sodium chloride solution; in the temperature range of -15°C to 45°C and within 120 min of ultraviolet lamp irradiation, they have high stability.

[0119] Experiment 8. Comparison of different methods

[0120] Experimental group: Fluorescent carbon dots prepared by ultrasonic suspension in Example 1.

[0121] Control group 1: Referring to the raw materials and dosage ratios in Example 1, a fluorescent carbon dot solution was prepared by conventional ultrasonic treatment. Specifically: The reaction raw materials (o-phenylenediamine and citric acid) were dissolved in 10 mL of N,N-dimethylformamide (DMF solvent), stirred with a glass rod until completely dissolved, and the resulting mixed solution was poured into a clean centrifuge tube. The centrifuge tube was placed in an ultrasonic cleaner and fixed, with a temperature of 25°C and a power of 5 KW. The switch was turned on, and it was taken out after 3 minutes of timing.

[0122] Control group 2: Referring to the raw materials and dosage ratios in Example 1, a fluorescent carbon dot solution was prepared by the solvothermal method. Specifically, the reaction raw materials (o-phenylenediamine and citric acid) were dissolved in 10 mL of DMF solvent, stirred with a glass rod until completely dissolved, and the dissolved mixed solution was transferred to a reaction kettle. The reaction kettle was placed in a forced-air drying oven, with the temperature set at 180°C and the time set at 12 h for heating reaction. After 12 h, the power of the drying oven was turned off, and the reaction kettle was taken out after it cooled to room temperature.

[0123] Control Group 3: Referring to the raw materials and dosage ratios in Example 1, a fluorescent carbon dot solution was prepared by the reflux method. Specifically, the reaction raw materials (o-phenylenediamine and citric acid) were dissolved in 10 mL of DMF solvent, and stirred with a glass rod until completely dissolved. The resulting mixed solution was poured into a round-bottom flask and a magnetic stir bar was added, and then the round-bottom flask was placed in an oil bath. The temperature of the oil bath was set to 180 °C, the magnetic stir bar was rotated at a moderate speed, and after heating to 180 °C and reacting for 12 h, the reaction was terminated.

[0124] Photos of the fluorescent carbon dots prepared by the above different preparation methods were obtained under visible light and ultraviolet light respectively, and the results are as Figure 19 shown; then the three-dimensional fluorescence spectra of the fluorescent carbon dots prepared by the above different preparation methods were obtained respectively, and the results are as Figure 20 shown.

[0125] Refer to Figure 19 , the fluorescent carbon dots synthesized by different methods were compared under visible light and ultraviolet light. Under visible light, the fluorescent carbon dots prepared by the ultrasonic suspension method showed brown color, the fluorescent carbon dots prepared by the solvothermal method showed reddish-brown color, the fluorescent carbon dots prepared by the reflux method showed dark yellow color, while the fluorescent carbon dots obtained by the conventional ultrasonic method were light yellow; under ultraviolet light, the fluorescent carbon dots obtained by the ultrasonic suspension method, the solvothermal method, the reflux method and the conventional ultrasonic method showed orange-yellow, blue-violet, blue-green and yellow-green colors respectively.

[0126] Refer to Figure 20 , the optimal excitation wavelengths of the fluorescent carbon dots obtained by the ultrasonic suspension method and the fluorescent carbon dots obtained after conventional ultrasonic treatment were both 434 nm, and the emission wavelengths were both 517 nm; however, the fluorescence intensity of the conventional ultrasonic method was 30, and the fluorescence intensity of the ultrasonic suspension was 726 ( Figure 20 b and Figure 20 d); the optimal emission wavelength of the solvothermal method was located at 425 nm, and the fluorescence intensity was 5220 ( Figure 20 a); the optimal emission wavelength of the reflux method was 430 nm, but the fluorescence intensity was 3560 ( Figure 20 c). From the comparison of fluorescence intensities, the fluorescence intensity of the solvothermal method was the strongest, and the fluorescence intensity of the reflux method was the second; however, compared with the ultrasonic suspension method and the conventional ultrasonic method, the solvothermal method and the reflux method had a long preparation time, a high reaction temperature, and the optimal emission wavelengths of the obtained fluorescent carbon dots both showed a blue shift, and the optimal emission wavelength decreased; although the optimal emission wavelengths of the ultrasonic suspension method and the conventional ultrasonic method were the same, compared with the conventional ultrasonic method, the fluorescence intensity of the ultrasonic suspension was significantly improved; it can be seen that the ultrasonic suspension method of the present invention enables the fluorescent carbon dots to not only have a large emission wavelength but also have a high fluorescence intensity.

[0127] As can be seen from the above various preparation methods and the properties of each fluorescent carbon dot solution, compared with the traditional carbon dot preparation methods (solvothermal method, reflux method and conventional ultrasonic method), the fluorescent carbon dots prepared by the ultrasonic suspension method of the present invention have the characteristics of long-wavelength emission and high quantum yield, and excellent fluorescence performance; and the preparation of fluorescent carbon dots is achieved in a very short time under normal temperature and high pressure.

[0128] Furthermore, since the fluorescent carbon dot solution prepared by ultrasonic suspension of the present invention presents a brown color under sunlight and an orange-yellow color under ultraviolet light, and the optimal excitation wavelength is 434 nm and the emission wavelength is 517 nm, the application range of the fluorescent carbon dot material is relatively wide; for example, in the bioconversion of oil-rich coal, fluorescent carbon dots can be used as fluorescent probes for the screening of coal-dissolving strains; they can also be used as fluorescent probes for the selective qualitative and quantitative detection of microbial coal-dissolving products (such as simple alkanes such as methane and ethane and long-chain alkanes).

[0129] The above experimental verification was carried out using the fluorescent carbon dot solution prepared in Example 1. When the fluorescent carbon dot solution of Example 2 was tested according to the above experimental method, it was found that the fluorescent carbon dot solution prepared using catechol as a raw material exhibited the same fluorescence performance as the fluorescent carbon dot solution of Example 1. When the organic solvents in Example 1 and Example 2 were replaced with dimethyl sulfoxide or ethylene glycol, and the raw material ratio, suspension conditions, etc. were all replaced, the prepared fluorescent carbon dot solutions also exhibited the same fluorescence performance as the fluorescent carbon dot solution of Example 1.

[0130] In summary, the present invention proposes a method for preparing fluorescent carbon dots by ultrasonic suspension, which has the advantages of simple operation method, short reaction time, simple equipment, mild reaction conditions and low energy consumption. In addition, the fluorescent carbon dots obtained by ultrasonic suspension have more excellent fluorescence performance, and the fluorescence emission wavelength is more biased towards the long-wavelength direction. They not only have a high quantum yield but also excellent photostability, which provides a convenient, fast and efficient new way and new method for the subsequent large-scale synthesis of high-quality near-infrared carbon dots.

[0131] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the preferred embodiments. Those of ordinary skill in the art should understand that they can still modify or equivalently replace the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing fluorescent carbon dots based on ultrasonic suspension, characterized in that: The following steps are involved: Step 1, adding an aromatic compound containing an electron-donating group and citric acid to an organic solvent respectively, stirring until completely dissolved, to obtain a mixed solution; the mass volume ratio of the aromatic compound containing an electron-donating group, citric acid and organic solvent is (1-3) g: (1-3) g: 10 mL; the aromatic compound containing an electron-donating group is an aromatic amine or an aromatic phenol; Step 2: Turn on the ultrasonic suspension instrument and set the working parameters; use a syringe to inject the mixed solution of step 1 into the ultrasonic suspension working window, and adjust the ultrasonic suspension instrument during the injection process so that the droplets formed by the mixed solution are complete and stably suspended; Step 3: After suspending for 1-5 minutes, use another syringe to absorb the droplets, which are the target product.

2. The method for preparing fluorescent carbon dots based on ultrasonic suspension according to claim 1, characterized in that: In the step 1, the boiling point of the organic solvent is above 100°C.

3. The method for preparing fluorescent carbon dots based on ultrasonic suspension according to claim 2, characterized in that: The organic solvent is N,N-dimethylformamide, dimethyl sulfoxide or ethylene glycol.

4. The method for preparing fluorescent carbon dots based on ultrasonic suspension according to claim 1, characterized in that: In the step 2, the operating parameters are: power 350W-450W, resonant frequency 18KHz-20KHz.

5. The method for preparing fluorescent carbon dots based on ultrasonic suspension according to claim 1, characterized in that: In the step 2, the mixed solution of step 1 is injected into the ultrasonic suspension working window at a rate of 15 μL each time.

6. Fluorescent carbon dots prepared by the fluorescent carbon dots preparation method based on ultrasonic suspension according to claim 1.

7. The fluorescent carbon dots according to claim 6, characterized in that: The average particle size of the fluorescent carbon dots is 2.93±0.69 nm; the lattice fringe spacing of the fluorescent carbon dots is 0.10 nm; and the average fluorescence lifetime of the fluorescent carbon dots is 2.950 ns.

8. The fluorescent carbon dots according to claim 6, characterized in that: The potential of the fluorescent carbon dots is -0.061 mV; the optimal excitation wavelength of the fluorescent carbon dots is 434 nm, and the optimal emission wavelength of the fluorescent carbon dots is 517 nm.

9. Use of the fluorescent carbon dots as claimed in claim 6 as fluorescent probes in the screening of coal-dissolving bacteria.

10. Use of the fluorescent carbon dots as claimed in claim 6 as fluorescent probes in the detection of microbial coal dissolution products.

Images