CN115418223A - Preparation method of electromagnetic shielding material with fluorescence - Google Patents
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Abstract
A preparation method of an electromagnetic shielding material with fluorescence relates to a preparation method of an electromagnetic shielding material. The invention aims to solve the problems that the electromagnetic shielding material prepared by the prior art has single performance and cannot simultaneously have electromagnetic shielding performance and fluorescence performance. The method comprises the following steps: 1. preparing a precursor solution; 2. heating; 3. freeze drying; 4. purifying and vacuum drying; 5. preparing a metal salt solution; 6. and adding the carbon dot powder into a metal salt solution, and heating for reaction to obtain the fluorescent electromagnetic shielding material. The electromagnetic shielding material with fluorescence prepared by the invention is spherical particles with the particle size of 5-15nm and uniform distribution, the optimal reflectivity is-22.6 dB, and the emission wavelength peak is 400-540nm when 320-480nm is used as the excitation wavelength. The electromagnetic shielding material with fluorescence prepared by the invention has both fluorescence performance and electromagnetic shielding performance.
Description
Technical Field
The invention relates to a preparation method of an electromagnetic shielding material.
Background
The rapid development of 5G communication technology and the large popularity of Wi-Fi devices overcrowded electromagnetic waves in the space that can interfere with different communication channels. Meanwhile, electromagnetic radiation generated by widely used communication and electronic devices may adversely affect device performance as well as the surrounding environment and human health. In this context, electromagnetic wave shielding materials are produced, and the electromagnetic wave shielding materials refer to materials that can strongly reflect or absorb electromagnetic waves and further attenuate the electromagnetic waves. However, the conventional electromagnetic shielding material only considers improvement and update of relevant parameters of electromagnetic shielding, and development of the electromagnetic shielding material with multiple performances is still in a standstill phase. In fact, the need for electromagnetic shielding materials with multiple performances is more urgent in actual production life. For example, the electromagnetic shielding material used in hospitals needs to warn irrelevant personnel to take precautions and keep away from the shielding area while shielding electromagnetic waves. However, the conventional electromagnetic shielding material does not satisfy the above requirements, and related research and development is still in the blank. Therefore, it is necessary to develop an electromagnetic shielding material having a warning function.
Carbon materials have found applications in the field of electromagnetic shielding. For example, chinese patent "an efficient wave-absorbing ultrathin carbon fiber reinforced composite material and design optimization method thereof" (publication No. CN 111125861A) discloses an efficient wave-absorbing ultrathin carbon fiber reinforced composite material, which consists of two layers of carbon fiber reinforced planar structures; the invention can provide a continuous conductive network, effectively absorb electromagnetic waves; optimized electromagnetic characteristics are realized; the microwave absorption efficiency is improved; the periodic embedding of the fibers in the ceramic matrix helps to increase microwave absorption. In addition, the chinese patent "a high absorption type electromagnetic shielding composite film" (publication No. CN 113583273A) discloses a high absorption type electromagnetic shielding composite film, which is a double-layer composite film formed by compounding a porous carbon material layer and a graphene layer, and has ultrahigh absorption efficiency for electromagnetic waves and excellent electromagnetic shielding performance. From the above background, the popularity of carbon materials in the field of electromagnetic shielding is seen.
Carbon Dots (CDs) have been widely noticed by researchers since being discovered as a quasi-zero-dimensional carbon-based nano fluorescent material. The carbon dots can emit visible light with the wavelength of 300-700nm under special illumination, so that the carbon dots are widely applied to the fields of information transmission, anti-counterfeiting and the like as an information encryption material. Then, as research on CDs has been advanced, the inherent luminescence properties of CDs have failed to meet practical requirements, and multifunctional nanocomposites based on the luminescence properties of CDs have been pursued. For example, in the Chinese patent "a near-infrared carbon dot/molybdenum disulfide composite material and its application" (publication No. CN 114887060A), a quaternary ammonium salt modified copper-doped near-infrared carbon dot is combined with modified molybdenum disulfide by electrostatic adsorption, so as to obtain a near-infrared carbon dot/molybdenum disulfide composite material with multiple antibacterial properties, excellent biocompatibility and high-efficiency bactericidal property. The composite material combines the advantages of near infrared carbon dots and molybdenum disulfide and can be used in the fields of biological imaging and multiple synergistic antibacterial. As can be seen from the above examples, the carbon dot-based composite material can give new life to the material development, and the development of the carbon dot-based composite material has great significance for the multi-field application of the carbon dot and even the zero-dimensional carbon-based material. But at present, the carbon material in the form of carbon quantum dots has less reports on the application of the carbon material in the field of electromagnetic shielding. Meanwhile, on the basis of electromagnetic shielding, the relevant reports of other properties (such as fluorescence) of the material are better that of the phoenix unicorn.
Under the background, the method provided by the invention enables carbon dots to be mixed with MnFe 2 O 4 The prepared material has electromagnetic shielding performance and fluorescence performance, and the research has important significance and practical application value.
Disclosure of Invention
The invention aims to solve the problems that the electromagnetic shielding material prepared by the prior art has single performance and cannot have electromagnetic shielding performance and fluorescent performance at the same time, and provides a preparation method of a composite material with fluorescent performance and electromagnetic shielding performance.
A preparation method of an electromagnetic shielding material with fluorescence is completed according to the following steps:
1. preparing a precursor solution:
adding organic acid and a protein denaturant into a solvent, and performing ultrasonic treatment to obtain a precursor solution;
2. heating the precursor solution from room temperature to 90-210 ℃, and then heating at 90-210 ℃ to obtain a suspension;
3. freeze-drying the suspension to obtain crude carbon dot solid powder;
4. dissolving the crude carbon dot solid powder into the mixed solution by taking the mixed solution as eluent, purifying by using a silica gel chromatographic column, removing the mixed solution, and finally drying in vacuum to obtain carbon dot powder;
5. adding divalent manganese salt and trivalent ferric salt into a solvent, and adjusting the pH to 11 by using an alkali liquor to obtain a metal salt solution;
6. adding the carbon dot powder into a metal salt solution, heating the solution from room temperature to 100-130 ℃, and reacting at 100-130 ℃ to obtain a black suspension; and cleaning the black suspension, and drying to obtain the fluorescent electromagnetic shielding material.
The electromagnetic shielding material with fluorescence prepared by the invention is spherical particles with the particle size of 5-15nm and uniform distribution, the optimal reflectivity is-22.6 dB, and the emission wavelength peak is 400-540nm when 320-480nm is used as the excitation wavelength.
The principle of the invention is as follows:
the carbon dots are spherical-like nanoparticles which take carbon atoms as a skeleton structure and have fluorescent property and the size of less than 10 nm. The spinel nano composite material is developed on the basis of the carbon dots, and the obtained material has fluorescence and electromagnetic shielding effects.
The invention has the advantages that:
(1) The invention provides a method for preparing a fluorescent electromagnetic shielding material;
(2) The electromagnetic shielding material with fluorescence prepared by the invention has excellent chemical stability;
(3) The electromagnetic shielding material with fluorescence prepared by the invention is suitable for being applied in the actual field;
(4) The preparation method is simple in preparation process, easy to operate, low in preparation cost and easy to popularize.
(5) The electromagnetic shielding material with fluorescence prepared by the invention has both fluorescence performance and electromagnetic shielding performance;
(6) The electromagnetic shielding material with fluorescence prepared by the invention plays a role in obvious identification through natural fluorescence, thereby achieving the warning function of electromagnetic shielding.
(7) The optimal reflectivity of the electromagnetic shielding material with fluorescence prepared by the invention is-22.6 dB.
The invention can obtain the electromagnetic shielding material with fluorescence.
Drawings
FIG. 1 is a transmission electron microscope photograph of the electromagnetic shielding material having fluorescence prepared in example 1;
FIG. 2 is an X-ray diffraction pattern of the electromagnetic shielding material having fluorescence prepared in example 1;
FIG. 3 is a fluorescence spectrum of the electromagnetic shielding material having fluorescence prepared in example 1;
FIG. 4 is a graph showing the absorption of electromagnetic waves at different thicknesses of the electromagnetic shielding material having fluorescence prepared in example 1.
Detailed Description
The following examples further illustrate the present invention but are not to be construed as limiting the invention. Modifications and substitutions to methods, procedures, or conditions of the invention may be made without departing from the spirit of the invention.
The first embodiment is as follows: the preparation method of the electromagnetic shielding material with fluorescence is specifically completed according to the following steps:
1. preparing a precursor solution:
adding organic acid and a protein denaturant into a solvent, and performing ultrasonic treatment to obtain a precursor solution;
2. heating the precursor solution from room temperature to 90-210 ℃, and then heating at 90-210 ℃ to obtain a suspension;
3. freeze-drying the suspension to obtain crude carbon dot solid powder;
4. dissolving the crude carbon dot solid powder into the mixed solution by taking the mixed solution as eluent, purifying by using a silica gel chromatographic column, removing the mixed solution, and finally drying in vacuum to obtain carbon dot powder;
5. adding divalent manganese salt and trivalent ferric salt into a solvent, and adjusting the pH to 11 by using an alkali liquor to obtain a metal salt solution;
6. adding the carbon dot powder into a metal salt solution, heating the solution from room temperature to 100-130 ℃, and reacting at 100-130 ℃ to obtain a black suspension; and cleaning the black suspension, and drying to obtain the fluorescent electromagnetic shielding material.
The second embodiment is as follows: the first difference between the present embodiment and the present embodiment is: the organic acid in the step one is one or a mixture of two of citric acid and tartaric acid; the concentration of the organic acid in the precursor solution in the first step is 0.1-0.5 mol/L; the protein denaturant in the step one is urea or guanidine hydrochloride; the concentration of the protein denaturant in the precursor solution in the first step is 0.3-1.5 mol/L. Other steps are the same as in the first embodiment.
The third concrete implementation mode: the present embodiment differs from the first or second embodiment in that: the solvent in the step one is deionized water, absolute ethyl alcohol or formamide; the time of the ultrasound in the first step is as follows: 30 min-60 min; the temperature of the ultrasound in the first step is 10-40 ℃. The other steps are the same as in the first or second embodiment.
The fourth concrete implementation mode is as follows: the difference between this embodiment and one of the first to third embodiments is: the heating time in the step two is 8-24 h; the temperature rise rate in the second step is 10 ℃/min; the temperature of the freeze drying in the third step is-40 ℃ to-10 ℃, and the time is 6h to 36h. The other steps are the same as those in the first to third embodiments.
The fifth concrete implementation mode is as follows: the difference between this embodiment and one of the first to fourth embodiments is: the mixed solution in the fourth step is a mixed solution of dichloromethane and methanol, a mixed solution of dichloromethane and absolute ethyl alcohol, a mixed solution of petroleum ether and ethyl acetate or a mixed solution of petroleum ether and absolute ethyl alcohol; the volume ratio of dichloromethane to methanol in the mixed solution of dichloromethane and methanol is (1-20): 1, the volume ratio of dichloromethane to absolute ethanol in the mixed solution of dichloromethane and absolute ethanol is (1-20): 1, the volume ratio of petroleum ether to ethyl acetate in the mixed solution of petroleum ether and ethyl acetate is (1-20): 1, and the volume ratio of petroleum ether to absolute ethanol in the mixed solution of petroleum ether and absolute ethanol is (1-20): 1; the mass ratio of the crude carbon dot solid powder in the fourth step to the volume ratio of the mixed solution is 1g (200 mL-1000 mL). The other steps are the same as those in the first to fourth embodiments.
The sixth specific implementation mode: the difference between this embodiment and one of the first to fifth embodiments is: the specification of the silica gel in the silica gel chromatogram in the step four is as follows: 60 to 100 meshes, 100 to 200 meshes, 200 to 300 meshes or 300 to 400 meshes; in the fourth step, the mixed solution is removed by using a rotary evaporator at the temperature of between 50 and 90 ℃; the temperature of the vacuum drying in the fourth step is 50-90 ℃. The other steps are the same as those in the first to fifth embodiments.
The seventh embodiment: the difference between this embodiment and the first to sixth embodiments is: the divalent manganese salt in the step five is Mn (NO) 3 ) 2 ·4H 2 O、MnCl 2 ·4H 2 O or MnSO 4 ·4H 2 O; the ferric salt in the step five is Fe (NO) 3 ) 3 ·9H 2 O、FeCl 3 ·6H 2 O or Fe 2 (SO 4 ) 3 ·9H 2 O; the solvent in the step five is deionized water; and fifthly, the alkali liquor is one or a mixed solution of NaOH solution and KOH solution. The other steps are the same as those in the first to sixth embodiments.
The specific implementation mode eight: the difference between this embodiment and one of the first to seventh embodiments is: the molar concentration of the divalent manganese salt in the metal salt solution in the step five is 0.1-0.3 mol/L; the molar concentration of the trivalent ferric salt in the metal salt solution in the step five is 0.2-0.6 mol/L; the mass ratio of the divalent manganese salt to the trivalent ferric salt in the step five is 1; and the molar concentration of the alkali liquor in the step five is 0.5-2 mol/L. The other steps are the same as those in the first to seventh embodiments.
The specific implementation method nine: the difference between this embodiment and the first to eighth embodiments is: the mass ratio of the carbon dot powder to the volume ratio of the metal salt solution in the sixth step is (0.1 g-1.0 g): 10 mL-100 mL; the reaction time in the sixth step is 6-10 h. The other steps are the same as those in the first to eighth embodiments.
The specific implementation mode is ten: the difference between this embodiment and the first to ninth embodiments is: the temperature rise rate in the sixth step is 10 ℃/min; and in the sixth step, the black suspension is washed for 3 to 10 times by using deionized water and then dried for 12 to 48 hours at the temperature of between 50 and 70 ℃. The other steps are the same as those in the first to ninth embodiments.
The following examples were used to demonstrate the beneficial effects of the present invention:
example 1: a preparation method of an electromagnetic shielding material with fluorescence is specifically completed according to the following steps:
1. preparing a precursor solution:
adding citric acid and guanidine hydrochloride into formamide, and performing ultrasonic treatment at 25 deg.C and ultrasonic power of 80W for 30min to obtain precursor solution;
the concentration of citric acid in the precursor solution in the first step is 0.2mol/L;
the concentration of guanidine hydrochloride in the precursor solution in the step one is 0.4mol/L;
2. heating the precursor solution from room temperature to 180 ℃, and then heating at 180 ℃ for 10h to obtain a suspension;
the temperature rising rate in the step two is 10 ℃/min;
3. freeze-drying the suspension for 36h at-40 deg.C to obtain brown crude carbon dot solid powder;
4. dissolving the crude carbon dot solid powder into the mixed solution by taking the mixed solution as eluent, purifying by using a silica gel chromatographic column, removing the mixed solution by using a rotary evaporator at 75 ℃, and finally drying in vacuum at 90 ℃ to obtain carbon dot powder;
the mixed solution in the fourth step is a mixed solution of dichloromethane and methanol, and the volume ratio of dichloromethane to methanol is 20;
the mass ratio of the crude carbon dot solid powder in the fourth step to the volume ratio of the mixed solution is 1g;
the specification of the silica gel in the silica gel chromatogram in the step four is as follows: 300-400 meshes;
5. adding Mn (NO) 3 ) 2 ·4H 2 O and Fe (NO) 3 ) 3 ·9H 2 Adding O into deionized water, and adjusting the pH to 11 by using a KOH solution of 1mol/L to obtain a metal salt solution;
mn (NO) in the metal salt solution described in the fifth step 3 ) 2 ·4H 2 The molar concentration of O is 0.2mol/L;
fe (NO) in the metal salt solution described in step five 3 ) 3 ·9H 2 The molar concentration of O is 0.4mol/L;
mn (NO) described in step five 3 ) 2 ·4H 2 O and Fe (NO) 3 ) 3 ·9H 2 The mass ratio of O is 1;
6. adding the carbon dot powder into a metal salt solution, heating the solution from room temperature to 110 ℃, and reacting the solution at 110 ℃ for 6 hours to obtain a black suspension; and washing the black suspension with deionized water for 5 times, and drying at 70 ℃ for 24 hours to obtain the fluorescent electromagnetic shielding material.
The volume ratio of the mass of the carbon dot powder to the metal salt solution in the sixth step is 0.5g;
and the temperature rising rate in the sixth step is 10 ℃/min.
FIG. 1 is a TEM image of the fluorescent EM shielding material prepared in example 1;
as can be seen from FIG. 1, the size of the electromagnetic shielding material with fluorescence prepared in example 1 is below 15 nm.
FIG. 2 is an X-ray diffraction pattern of the electromagnetic shielding material having fluorescence prepared in example 1;
as can be seen from FIG. 2, the fluorescent electromagnetic shielding material prepared in example 1 found strong peaks at 9.59 °, 34.88 °, 42.40 °, 56.02 ° and 61.52 ° 2 θ, which are attributed to single-phase spinel MnFe 2 O 4 . However, 2 θ =25.4 ° does not have the feature (002) of graphite, which is caused by the low content and high dispersion of carbon dots in the electromagnetic shielding material having fluorescence.
FIG. 3 is a fluorescence spectrum of the electromagnetic shielding material having fluorescence prepared in example 1;
as can be seen from FIG. 3, the fluorescent electromagnetic shielding material prepared in example 1 exhibits the strongest emission at 450nm when irradiated at an excitation wavelength of 360 nm.
The fluorescent electromagnetic shielding material prepared in example 1 was mixed with 90# paraffin 3 in a weight ratio to prepare a hollow cylinder, and then a vector network analyzer was used to perform an electromagnetic wave absorption performance test, as shown in fig. 4;
FIG. 4 is an electromagnetic wave absorption graph of the electromagnetic shielding material with fluorescence prepared in example 1 at different thicknesses;
as can be seen from fig. 4: except for the highest absorption value of different frequency bands at different thicknesses, the reflectivity below-10 (more than 90 percent of absorption rate) appears at all the thicknesses, and the optimal reflectivity at the thickness of 3mm is-22.6 dB.
Example 2: a preparation method of an electromagnetic shielding material with fluorescence is specifically completed according to the following steps:
1. preparing a precursor solution:
adding tartaric acid and guanidine hydrochloride into deionized water, and performing ultrasonic treatment at 25 deg.C and ultrasonic power of 180W for 30min to obtain precursor solution;
the concentration of tartaric acid in the precursor solution in the first step is 0.1mol/L;
the concentration of guanidine hydrochloride in the precursor solution in the first step is 0.3mol/L;
2. heating the precursor solution from room temperature to 160 ℃, and then heating at 160 ℃ for 12 hours to obtain a suspension;
the temperature rising rate in the step two is 10 ℃/min;
3. freeze-drying the suspension for 24h at-40 deg.C to obtain brown crude carbon dot solid powder;
4. dissolving the crude carbon dot solid powder into the mixed solution by taking the mixed solution as eluent, purifying by using a silica gel chromatographic column, removing the mixed solution by using a rotary evaporator at 75 ℃, and finally drying in vacuum at 90 ℃ to obtain carbon dot powder;
the mixed solution in the fourth step is a mixed solution of dichloromethane and methanol, and the volume ratio of dichloromethane to methanol is 15;
the mass ratio of the crude carbon dot solid powder in the step four to the volume ratio of the mixed solution is 1g;
the specification of the silica gel in the silica gel chromatogram in the step four is as follows: 200-300 meshes;
5. will be provided withMnCl 2 ·4H 2 O and FeCl 3 ·6H 2 Adding O into deionized water, and adjusting the pH to 11 by using a KOH solution of 1mol/L to obtain a metal salt solution;
the MnCl in the metal salt solution in the step five 2 ·4H 2 The molar concentration of O is 0.2mol/L;
FeCl in the metal salt solution in the step five 3 ·6H 2 The molar concentration of O is 0.4mol/L;
MnCl described in step five 2 ·4H 2 O and FeCl 3 ·6H 2 The mass ratio of O is 1;
6. adding the carbon dot powder into a metal salt solution, heating the solution from room temperature to 110 ℃, and reacting the solution at 110 ℃ for 6 hours to obtain a black suspension; and washing the black suspension with deionized water for 5 times, and drying at 70 ℃ for 24 hours to obtain the fluorescent electromagnetic shielding material.
The mass ratio of the carbon dot powder to the metal salt solution in the sixth step is 0.2g;
and the temperature rising rate in the sixth step is 10 ℃/min.
The size of the electromagnetic shielding material with fluorescence prepared in the example 2 is below 15 nm; when the prepared electromagnetic shielding material is irradiated by the excitation wavelength of 360nm, the strongest emission exists at 449 nm; the optimum reflectivity is-21.5 dB.
Example 3: a preparation method of an electromagnetic shielding material with fluorescence is specifically completed according to the following steps:
1. preparing a precursor solution:
adding tartaric acid and urea into deionized water, and performing ultrasonic treatment at 25 deg.C and 240W for 30min to obtain precursor solution;
the concentration of tartaric acid in the precursor solution in the first step is 0.2mol/L;
the concentration of urea in the precursor solution in the first step is 0.3mol/L;
2. heating the precursor solution from room temperature to 180 ℃, and then heating at 180 ℃ for 12h to obtain a suspension;
the temperature rise rate in the second step is 10 ℃/min;
3. freeze-drying the suspension for 24h at-40 deg.C to obtain brown crude carbon dot solid powder;
4. dissolving the crude carbon dot solid powder into the mixed solution by taking the mixed solution as eluent, purifying by using a silica gel chromatographic column, removing the mixed solution by using a rotary evaporator at 75 ℃, and finally drying in vacuum at 90 ℃ to obtain carbon dot powder;
the mixed solution in the fourth step is a mixed solution of dichloromethane and methanol, and the volume ratio of dichloromethane to methanol is 20;
the mass ratio of the crude carbon dot solid powder in the fourth step to the volume ratio of the mixed solution is 1 g;
the specification of the silica gel in the silica gel chromatogram in the step four is as follows: 200-300 meshes;
5. adding Mn (NO) 3 ) 2 ·4H 2 O and FeCl 3 ·6H 2 Adding O into deionized water, and adjusting the pH to 11 by using a KOH solution of 1mol/L to obtain a metal salt solution;
mn (NO) in the metal salt solution described in step five 3 ) 2 ·4H 2 The molar concentration of O is 0.2mol/L;
FeCl in the metal salt solution in the fifth step 3 ·6H 2 The molar concentration of O is 0.4mol/L;
MnCl mentioned in the fifth step 2 ·4H 2 O and FeCl 3 ·6H 2 The mass ratio of O is 1;
6. adding the carbon dot powder into a metal salt solution, heating the solution from room temperature to 110 ℃, and reacting the solution at 110 ℃ for 6 hours to obtain a black suspension; and (3) washing the black suspension for 5 times by using deionized water, and drying at 70 ℃ for 24 hours to obtain the fluorescent electromagnetic shielding material.
The mass ratio of the carbon dot powder to the metal salt solution in the sixth step is 0.2g;
and the temperature rising rate in the sixth step is 10 ℃/min.
The size of the electromagnetic shielding material with fluorescence prepared in the embodiment 3 is below 15 nm; when the prepared electromagnetic shielding material is irradiated by the excitation wavelength of 360nm, the strongest emission exists at 452 nm; the optimum reflectivity is-22.0 dB.
Claims (10)
1. A preparation method of an electromagnetic shielding material with fluorescence is characterized by comprising the following steps:
1. preparing a precursor solution:
adding organic acid and a protein denaturant into a solvent, and performing ultrasonic treatment to obtain a precursor solution;
2. heating the precursor solution from room temperature to 90-210 ℃, and then heating at 90-210 ℃ to obtain a suspension;
3. freeze-drying the suspension to obtain crude carbon dot solid powder;
4. dissolving the crude carbon dot solid powder into the mixed solution by taking the mixed solution as eluent, purifying by using a silica gel chromatographic column, removing the mixed solution, and finally drying in vacuum to obtain carbon dot powder;
5. adding divalent manganese salt and trivalent ferric salt into a solvent, and adjusting the pH to 11 by using an alkali liquor to obtain a metal salt solution;
6. adding the carbon dot powder into a metal salt solution, heating the solution to 100-130 ℃ from room temperature, and reacting at 100-130 ℃ to obtain a black suspension; and cleaning the black suspension, and drying to obtain the fluorescent electromagnetic shielding material.
2. The method of claim 1, wherein the organic acid in step one is one or a mixture of citric acid and tartaric acid; the concentration of the organic acid in the precursor solution in the first step is 0.1-0.5 mol/L; the protein denaturant in the step one is urea or guanidine hydrochloride; the concentration of the protein denaturant in the precursor solution in the first step is 0.3-1.5 mol/L.
3. The method according to claim 1, wherein the solvent in the first step is deionized water, absolute ethanol or formamide; the ultrasonic time in the step one is as follows: 30 min-60 min; the temperature of the ultrasound in the first step is 10-40 ℃.
4. The method according to claim 1, wherein the heating time in step two is 8-24 h; the temperature rising rate in the step two is 10 ℃/min; the temperature of the freeze drying in the third step is-40 ℃ to-10 ℃, and the time is 6h to 36h.
5. The method according to claim 1, wherein the mixed solution in step four is a mixed solution of dichloromethane and methanol, a mixed solution of dichloromethane and absolute ethyl alcohol, a mixed solution of petroleum ether and ethyl acetate, or a mixed solution of petroleum ether and absolute ethyl alcohol; the volume ratio of dichloromethane to methanol in the mixed solution of dichloromethane and methanol is (1-20): 1, the volume ratio of dichloromethane to absolute ethanol in the mixed solution of dichloromethane and absolute ethanol is (1-20): 1, the volume ratio of petroleum ether to ethyl acetate in the mixed solution of petroleum ether and ethyl acetate is (1-20): 1, and the volume ratio of petroleum ether to absolute ethanol in the mixed solution of petroleum ether and absolute ethanol is (1-20): 1; the mass ratio of the crude carbon dot solid powder in the fourth step to the volume ratio of the mixed solution is 1g (200 mL-1000 mL).
6. The method according to claim 1, wherein the specification of the silica gel in the silica gel chromatography in the step four is as follows: 60 to 100 meshes, 100 to 200 meshes, 200 to 300 meshes or 300 to 400 meshes; in the fourth step, the mixed solution is removed by using a rotary evaporator at the temperature of between 50 and 90 ℃; the temperature of the vacuum drying in the fourth step is 50-90 ℃.
7. The method according to claim 1, wherein the salt of manganese (Mn) in step five is Mn (NO) 3 ) 2 ·4H 2 O、MnCl 2 ·4H 2 O or MnSO 4 ·4H 2 O; the ferric salt in the step five is Fe (NO) 3 ) 3 ·9H 2 O、FeCl 3 ·6H 2 O or Fe 2 (SO 4 ) 3 ·9H 2 O; the solvent in the fifth step is deionized water; and fifthly, the alkali liquor is one or a mixed solution of NaOH solution and KOH solution.
8. The method according to claim 1, wherein the molar concentration of the divalent manganese salt in the metal salt solution in the fifth step is 0.1mol/L to 0.3mol/L; the molar concentration of the ferric iron salt in the metal salt solution in the step five is 0.2-0.6 mol/L; the mass ratio of the divalent manganese salt to the trivalent iron salt in the step five is 1; and the molar concentration of the alkali liquor in the step five is 0.5-2 mol/L.
9. The method according to claim 1, wherein the ratio of the mass of the carbon dot powder to the volume of the metal salt solution in the sixth step is (0.1 g-1.0 g): 10 mL-100 mL); the reaction time in the sixth step is 6-10 h.
10. The method according to claim 1, wherein the temperature raising rate in the sixth step is 10 ℃/min; and in the sixth step, the black suspension is washed for 3 to 10 times by using deionized water and then dried for 12 to 48 hours at the temperature of between 50 and 70 ℃.
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