Preparation method of coal-based red fluorescent carbon dots

Abstract

The application discloses a preparation method of coal-based red fluorescent carbon dots, and belongs to the technical field of coal-based carbon nanomaterials. The preparation method of the coal-based red fluorescent carbon dots comprises the following steps: brown coal is added into a mixed solution of concentrated sulfuric acid and concentrated nitric acid, a first mixing reaction is carried out, ammonia water is used to adjust the pH to neutral, a second mixing reaction is carried out, and a coal sample is obtained; the coal sample is added into water to prepare a brown coal mixed solution, and then cavitation stripping treatment is carried out on the brown coal mixed solution, so that the coal-based red fluorescent carbon dots are obtained. The coal-based carbon dots with the red fluorescent characteristics are prepared, the common problem that the fluorescent colors of the coal-based carbon dots prepared by using the mainstream chemical oxidation stripping method are generally blue is solved, and a new technical path and solution are provided for expanding the fluorescent wavelength range of the coal-based carbon dots and enriching the polychromaticity of the coal-based carbon dots.

Description

Technical Field

[0001] This invention relates to the field of coal-based carbon nanomaterials technology, and in particular to a method for preparing coal-based red fluorescent carbon dots. Background Technology

[0002] Carbon dots not only possess excellent biocompatibility but also exhibit outstanding fluorescence properties and electron transport capabilities, demonstrating broad application potential in fields such as panoramic high-definition in vivo imaging, flexible quantum dot displays, warm white LED light sources, and full-spectrum catalysis. Most of these applications are based on the pleochroism of carbon dots, with red carbon dots being particularly representative. Therefore, screening for structurally suitable and stable industrial raw materials, and developing industrial preparation processes that match the structure, are the core keys to promoting the large-scale application of multicolor carbon dots.

[0003] Coal, as a common industrial product, is not only abundant and inexpensive, but its microstructure also gives it unique advantages. Structurally, coal consists of conjugated sp... 2 Hybridized carbon graphite crystallites form three-dimensional cross-linked macromolecular polymers through interactions such as short aliphatic hydrocarbon bonds and ether bonds. These graphite crystallites mostly exist as zero-dimensional carbon nanoparticles, highly similar to the conjugated carbon core structure of carbon dots. Furthermore, the chemical bonds in the three-dimensional cross-linked network and the functional groups formed after their breakage provide abundant amorphous surface groups for carbon dots. Based on these characteristics, coal becomes an ideal raw material for the industrial production of carbon dots.

[0004] Currently, the mainstream method for preparing coal-based carbon dots involves using various oxidants to break the cross-linking bonds between graphite microcrystals in coal, causing them to release and form carbon nanoparticles (i.e., carbon dots). Examples include mixed acids (H₂SO₄ / HNO₃), HNO₃, and H₂O₂. In addition, some existing technologies have proposed new oxidation strategies, such as Fenton's reagent and ferrate pre-oxidation followed by hydrogen peroxide. However, while these oxidation methods break the cross-linking bonds and release carbon nanoparticles, they also indiscriminately oxidize various functional groups and dangling bonds on the surface of graphite microcrystals. This non-selective oxidation significantly reduces the types and numbers of surface groups on the carbon dots, thus severely weakening their surface fluorescence. Consequently, the fluorescence of existing coal-based carbon dots mainly originates from the conjugated carbon core. Due to the small size of its conjugated π-domain, the energy band gap caused by the quantum confinement effect is relatively wide, resulting in high radiation fluorescence energy, with the spectrum concentrated in the short-wavelength region, exhibiting a predominantly blue fluorescence characteristic. This short-wavelength blue fluorescence faces a dual dilemma in its application: on the one hand, it is easily interfered with by the autofluorescence of biological tissues, and on the other hand, it may cause damage to the normal matrix, which greatly limits the application expansion of coal-based carbon dots in scenarios such as multicolor fluorescence imaging and long-wavelength detection. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing coal-based red fluorescent carbon dots to solve the problems existing in the prior art.

[0006] To achieve the above objectives, the present invention provides the following solution:

[0007] One of the technical solutions of the present invention: a method for preparing coal-based red fluorescent carbon dots, comprising the following steps:

[0008] Lignite was added to a mixed solution of concentrated sulfuric acid and concentrated nitric acid to carry out the first mixing reaction. Then, the pH was adjusted to neutral using ammonia water to carry out the second mixing reaction, and a coal sample was obtained.

[0009] The coal sample was added to water to prepare a lignite mixture, and then the lignite mixture was subjected to cavitation stripping treatment to obtain the coal-based red fluorescent carbon dots.

[0010] Furthermore, the particle size of the lignite is ≤200 mesh.

[0011] Furthermore, the mass ratio of the lignite to the mixed solution of concentrated sulfuric acid and concentrated nitric acid is 1:1 to 1:4.

[0012] Furthermore, the mass ratio of the lignite to the mixed solution of concentrated sulfuric acid and concentrated nitric acid is 1:2 to 1:4.

[0013] Furthermore, the volume ratio of the concentrated sulfuric acid to the concentrated nitric acid is 1:2 to 1:10.

[0014] Furthermore, the rotation speed of the first mixing reaction is 600-1500 r / min, and the time is 8-12 h.

[0015] Furthermore, the second mixing reaction is carried out at a speed of 600-1000 r / min for 8-10 h.

[0016] Furthermore, the volume ratio of the coal sample to water is 5-50 g: 1 L.

[0017] Furthermore, the cavitation stripping treatment has a flow rate of 1-200 L / h and a duration of 8-10 h.

[0018] Furthermore, the flow rate of the cavitation stripping treatment is 25-75 L / h.

[0019] Furthermore, the process includes a filtration step after the second mixing reaction; the filtration includes filtration using a microporous membrane with a pore size of 0.1-0.6 μm.

[0020] Furthermore, the process includes a filtration step to remove large particles after cavitation stripping (i.e., hydrocavitation technology); the filtration includes filtration using a microporous membrane with a pore size of 0.1-0.6 μm.

[0021] The main function of filtration is to remove large coal particles remaining after the reaction, while carbon dots remain in the aqueous solution.

[0022] Furthermore, the cavitation stripping process is carried out in a cavitation device containing a venturi tube.

[0023] The cavitation stripping process (i.e., hydraulic cavitation technology) used in this invention generates cavitation bubbles that release phase transition energy at the moment of collapse through physical and chemical actions such as shock waves, microjets, local hot spots, and hydroxyl radicals. This significantly enhances the physicochemical processes in the fluid, thereby effectively breaking the cross-linking bonds between graphite microcrystals in the coal and releasing them as carbon dots.

[0024] The cavitation stripping treatment (i.e., hydraulic cavitation technology) employed in this invention has significant advantages: firstly, it eliminates the need for any chemical reagents, completely avoiding environmental burden; secondly, the equipment is simple in structure, low in cost, easy to operate, and readily scalable for industrial application, demonstrating enormous industrialization potential. More importantly, unlike chemical methods that cleave cross-links through redox reactions, hydraulic cavitation technology primarily relies on physical actions such as high-pressure shock waves and microjets to mechanically disrupt cross-links. This maximizes the preservation of aliphatic hydrocarbon bonds, ether bonds, and other functional groups on the carbon dot surface, providing abundant surface-state fluorescence for the carbon dots.

[0025] This invention uses lignite with low coalification degree, abundant surface functional groups, and few aromatic ring structures as raw material (lignite elemental content: carbon 68.94%, oxygen 25.40%, silicon 2.82%, aluminum 2.19%, iron 0.65%; among which, the carbon content is significantly lower than that of bituminous coal (82.40%), while the oxygen content is significantly higher than that of bituminous coal (12.46%), indicating that the surface of lignite contains abundant oxygen-containing functional groups). First, the coal sample is pretreated with a reagent combination of mixed acid and ammonia water. Then, the treated coal sample is stripped by hydraulic cavitation technology, and finally, coal-based carbon dots with red fluorescent properties are prepared.

[0026] The specific principle is as follows:

[0027] Lignite has a low degree of coalification, retaining a large amount of incompletely decomposed plant residues (such as lignin and cellulose). Its molecular chains are relatively short and contain abundant functional groups such as hydroxyl (-OH), carboxyl (-COOH), and methoxy (-OCH3). Simultaneously, its aromatic ring structure is underdeveloped, mostly consisting of low-polymerization cyclic structures with relatively long side chains. These structural features (abundant surface functional groups and low-polymerization aromatic ring systems) provide a diverse material basis for the surface-state luminescence of carbon dots.

[0028] When lignite is pretreated using a combination of mixed acid and ammonia, the mixed acid opens the aromatic ring structure through oxidation, while the ammonia, through nitrogen doping, attaches surface groups with red fluorescence properties to the coal surface. These groups form new surface state energy levels in the coal-based carbon dots, which not only increases the number of energy levels in a single energy band but also narrows the band gap, causing the energy of the radiative fluorescence to decrease, ultimately endowing the coal-based carbon dots with red fluorescence emission characteristics.

[0029] When the lignite mixture flows through a venturi tube, it triggers a cavitation reaction and releases a huge amount of phase transition energy (approximately 1-10). 18 kW / m 3 Specifically, this occurs through shock waves (propagation speed 2000 m / s), microjets (450 MPa high pressure and 3.5 kPa high shear force), and localized thermal effects (hot spots at 2000-6000 K). These physical and mechanical effects effectively sever the three-dimensional cross-linking bonds in the coal mass, causing graphite microcrystals to exfoliate and release into the water, forming carbon dots. Simultaneously, these physical effects have minimal impact on the functional groups on the coal powder surface, preserving existing functional groups or bonds on the carbon dot surface, thereby enabling the control of the carbon dot band structure.

[0030] The second technical solution of the present invention: a coal-based red fluorescent carbon dot prepared by the above preparation method.

[0031] The third technical solution of the present invention: the application of the above-mentioned coal-based red fluorescent carbon dots in the preparation of fluorescent materials or electron transport materials.

[0032] The present invention discloses the following technical effects:

[0033] (1) The present invention has prepared coal-based carbon dots with red fluorescence characteristics, which solves the common problem that the fluorescence of coal-based carbon dots prepared by the current mainstream chemical oxidation stripping method is generally blue, and provides a new technical path and solution for expanding the fluorescence wavelength range of coal-based carbon dots and enriching their polychromaticity.

[0034] (2) The present invention uses a mixed acid + ammonia water mixed reagent to pretreat lignite samples to make them doped or attached with specific red fluorescent groups. Then, using hydrocavitation technology, coal-based red fluorescent carbon dots are obtained by stripping while retaining these specific groups. This provides a method with industrial production potential for applications in fields such as full-view high-definition live imaging, flexible quantum dot display, warm white LED light source, and full-spectrum catalysis.

[0035] (3) The preparation method of the present invention has the advantages of being green, environmentally friendly and low cost, and has the potential for industrial application. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 Fluorescence curves of the aqueous solution of coal-based red fluorescent carbon dots prepared in Example 1 under different excitation light and photographs under 356nm ultraviolet light (illustrations);

[0038] Figure 2 Transmission electron microscope image of coal-based red fluorescent carbon dots prepared in Example 1;

[0039] Figure 3 Fluorescence curves of the aqueous solution of coal-based fluorescent carbon dots prepared for Comparative Example 1 under different excitation light and photographs under 360 nm ultraviolet light (inset);

[0040] Figure 4 The image shown is a fluorescence confocal microscope image of Shewanella bacteria with carbon dot labeling prepared in Example 1, as shown in Example 1. Detailed Implementation

[0041] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0042] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0043] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0044] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0045] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0046] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0047] In the specific embodiments of the present invention, the concentration of concentrated sulfuric acid is 98.3 wt% and the concentration of concentrated nitric acid is 68 wt%.

[0048] The elemental content of lignite used in the specific embodiments of the present invention is as follows: carbon 68.94%, oxygen 25.40%, silicon 2.82%, aluminum 2.19%, and iron 0.65%. The carbon content is significantly lower than that of bituminous coal (82.40%), while the oxygen content is significantly higher than that of bituminous coal (12.46%), indicating that the surface of lignite contains abundant oxygen-containing functional groups.

[0049] Example 1

[0050] A method for preparing coal-based red fluorescent carbon dots:

[0051] (1) Dry the lignite to constant weight, grind it and pass it through a 200-mesh sieve to obtain a primary coal sample.

[0052] (2) Weigh 5g of initial coal sample, mix the initial coal sample with a mixed acid solution of concentrated sulfuric acid and concentrated nitric acid (volume ratio of concentrated sulfuric acid and concentrated nitric acid is 1:2) at a mass ratio of 1:1, stir at 600r / min for 8h, add 28% ammonia water to adjust the pH of the solution to 7, continue stirring at 600r / min for 8h, and then filter using a microporous membrane to obtain 3g of treated coal sample.

[0053] (3) Place the 3g of treated coal sample obtained in step (2) into a beaker, add 1000mL of deionized water, and stir at a stirring speed of 600r / min for 8h to obtain lignite mixture.

[0054] (4) Place the lignite mixture in a cavitation device equipped with a venturi tube, start the cavitation program, adjust and maintain the flow rate at 100 L / h, collect the circulating liquid in the cavitation device after 8 hours of cavitation stripping, filter it with a microporous membrane with a pore size of 0.45 μm to remove residual large particles, and obtain an aqueous solution of coal-based red fluorescent carbon dots.

[0055] The fluorescence curves of the aqueous solution of coal-based red fluorescent carbon dots prepared in this embodiment under different excitation lights and the photographs under 356nm ultraviolet light (illustrations) are shown below. Figure 1 .

[0056] from Figure 1 As can be seen, under different excitation wavelengths, the fluorescence peak of the carbon dots prepared in this embodiment is located in the red light region, and the carbon dot solution appears red under 540nm excitation light.

[0057] The aqueous solution of the coal-based red fluorescent carbon dots prepared in this embodiment was dropped onto a copper mesh containing a film, dried, and then analyzed by transmission electron microscopy. The results are shown in the figure. Figure 2 .

[0058] from Figure 2 As can be seen from the data, the carbon dots prepared in this embodiment are spherical with an average size of 3 nm, exhibit good dispersibility, and no agglomeration was observed.

[0059] Example 2

[0060] A method for preparing coal-based red fluorescent carbon dots:

[0061] (1) Dry the lignite to constant weight, grind it and pass it through a 200-mesh sieve to obtain a primary coal sample.

[0062] (2) Weigh 5g of raw coal sample and mix the raw coal sample with a mixed acid solution of concentrated sulfuric acid and concentrated nitric acid (volume ratio of concentrated sulfuric acid and concentrated nitric acid is 1:5) at a mass ratio of 1:2. Stir at 600r / min for 8h and then add 28% ammonia water to adjust the pH of the solution to 7. Continue stirring at 600r / min for 8h and then filter using a microporous membrane to obtain 3g of treated coal sample.

[0063] (3) Place the 3g of treated coal sample obtained in step (2) into a beaker, add 1000mL of deionized water, and stir at a stirring speed of 600r / min for 8h to obtain lignite mixture.

[0064] (4) Place the lignite mixture in a cavitation device equipped with a venturi tube, start the cavitation program, adjust and maintain the flow rate at 100 L / h, collect the circulating liquid in the cavitation device after 8 hours of cavitation stripping, filter it with a microporous membrane with a pore size of 0.45 μm to remove residual large particles, and obtain an aqueous solution of coal-based red fluorescent carbon dots.

[0065] Example 3

[0066] A method for preparing coal-based red fluorescent carbon dots:

[0067] (1) Dry the lignite to constant weight, grind it and pass it through a 200-mesh sieve to obtain a primary coal sample.

[0068] (2) Weigh 5g of initial coal sample, mix the initial coal sample with a mixed acid solution of concentrated sulfuric acid and concentrated nitric acid (volume ratio of concentrated sulfuric acid and concentrated nitric acid is 1:4) at a mass ratio of 1:2, stir at 600r / min for 8h, add 28% ammonia water to adjust the pH of the solution to 7, continue stirring at 600r / min for 8h, and then filter using a microporous membrane to obtain 3g of treated coal sample.

[0069] (3) Place the 3g of treated coal sample obtained in step (2) into a beaker, add 1000mL of deionized water, and stir at a stirring speed of 600r / min for 8h to obtain lignite mixture.

[0070] (4) Place the lignite mixture in a cavitation device equipped with a venturi tube, start the cavitation program, adjust and maintain the flow rate at 200 L / h, collect the circulating liquid in the cavitation device after 8 hours of cavitation stripping, filter it with a microporous membrane with a pore size of 0.45 μm to remove residual large particles, and obtain an aqueous solution of coal-based red fluorescent carbon dots.

[0071] The fluorescence properties of the carbon dots prepared in Examples 2 and 3 are similar to those in Example 1.

[0072] Comparative Example 1

[0073] Preparation of coal-based fluorescent carbon dots:

[0074] (1) Dry the lignite to constant weight, grind it and pass it through a 200-mesh sieve to obtain a primary coal sample.

[0075] (2) Weigh 5g of primary coal sample and add it to concentrated sulfuric acid (the mass ratio of primary coal sample to concentrated sulfuric acid is 1:2). Stir at 800r / min for 8h and then add 28% ammonia water to adjust the pH of the solution to 7. Continue stirring at 800r / min for 8h and then filter using a microporous membrane to obtain 3g of treated coal sample.

[0076] (3) Place the 3g of treated coal sample obtained in step (2) into a beaker, add 1000mL of deionized water, and stir at a stirring speed of 800r / min for 8h to obtain lignite mixture.

[0077] (4) Place the lignite mixture in the cavitation device, start the cavitation program, adjust and maintain the flow rate at 200 L / h, collect the circulating liquid in the cavitation device after 8 hours of cavitation stripping, filter it with a microporous membrane with a pore size of 0.45 μm to remove residual large particles, and obtain an aqueous solution of coal-based fluorescent carbon dots.

[0078] The fluorescence curves of the aqueous solution of the coal-based fluorescent carbon dots prepared in this comparative example under different excitation lights and the photographs under a 360nm ultraviolet lamp (illustrations) are shown below. Figure 3 .

[0079] from Figure 3 As can be seen from the data, the fluorescence peak of the carbon dots prepared in this comparative example is concentrated in the blue light region; the aqueous solution of the carbon dots also appears blue under 360nm ultraviolet light excitation.

[0080] Comparative Example 2

[0081] Preparation of coal-based fluorescent carbon dots:

[0082] (1) Dry the anthracite to constant weight, grind it and pass it through a 200-mesh sieve to obtain a primary coal sample.

[0083] (2) The initial coal sample was mixed with a mixed acid solution of concentrated sulfuric acid and concentrated nitric acid (volume ratio of concentrated sulfuric acid to concentrated nitric acid was 1:2) at a mass ratio of 1:4. After stirring at 800 r / min for 8 h, 28% ammonia was added to adjust the pH of the solution to 7. The mixture was stirred at 800 r / min for another 8 h. The mixture was then filtered through a microporous membrane to obtain 4 g of the treated coal sample.

[0084] (3) Place the 4g of treated coal sample obtained in step (2) into a beaker, add 1L of deionized water, and stir at a stirring speed of 600r / min for 8h to obtain lignite mixture.

[0085] (4) Place the lignite mixture in the cavitation device, start the cavitation program, adjust and maintain the flow rate at 200 L / h, collect the circulating liquid in the cavitation device after 8 hours of cavitation stripping, filter it with a microporous membrane with a pore size of 0.45 μm to remove residual large particles, and obtain an aqueous solution of coal-based fluorescent carbon dots.

[0086] Under different wavelengths of excitation light, the fluorescence peaks of the coal-based fluorescent carbon dots prepared in this comparative example are concentrated in the blue light region.

[0087] Example 1

[0088] The red fluorescent carbon dots based on bituminous coal prepared in Example 1 were used to biolabel microorganisms (Shewanella), and the results are as follows: Figure 4 As shown.

[0089] The specific method is as follows: The aqueous solution of coal-based red fluorescent carbon dots prepared in Example 1 and Shewanella bacteria are added to a liquid culture medium, and then both are placed together in a temperature-controlled shaker and cultured for 2 days at 25°C. Fresh liquid culture medium and the aqueous solution of coal-based green fluorescent carbon dots are added daily.

[0090] from Figure 4As can be seen, Shewanella bacteria are rod-shaped, several micrometers in length; the red fluorescence is uniformly distributed within the rod-shaped bacteria, indicating that the carbon dots are also uniformly distributed within the microorganism. This also demonstrates that the carbon dots prepared in this invention can be applied to the fields of microbial biomarking and imaging.

[0091] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing coal-based red fluorescent carbon dots, characterized in that, Includes the following steps: Lignite was added to a mixed solution of concentrated sulfuric acid and concentrated nitric acid to carry out the first mixing reaction. Then, the pH was adjusted to neutral using ammonia water to carry out the second mixing reaction, and a coal sample was obtained. The coal sample was added to water to prepare a lignite mixture, and then the lignite mixture was subjected to cavitation stripping treatment to obtain the coal-based red fluorescent carbon dots.

2. The preparation method according to claim 1, characterized in that, The lignite has a particle size ≤200 mesh.

3. The preparation method according to claim 1, characterized in that, The mass ratio of the lignite to the mixed solution of concentrated sulfuric acid and concentrated nitric acid is 1:1 to 1:

4.

4. The preparation method according to claim 1, characterized in that, The volume ratio of concentrated sulfuric acid to concentrated nitric acid is 1:2 to 1:

10.

5. The preparation method according to claim 1, characterized in that, The rotation speed of the first mixing reaction is 600-1500 r / min, and the time is 8-12 h.

6. The preparation method according to claim 1, characterized in that, The second mixing reaction is carried out at a speed of 600-1000 r / min for 8-10 h.

7. The preparation method according to claim 1, characterized in that, The mass-to-volume ratio of the coal sample to water is 5-50 g: 1 L.

8. The preparation method according to claim 1, characterized in that, The cavitation stripping process has a flow rate of 1-200 L / h and a duration of 8-10 h.

9. A coal-based red fluorescent carbon dot prepared by the preparation method according to any one of claims 1-8.

10. The application of the coal-based red fluorescent carbon dot of claim 9 in the preparation of fluorescent materials or electron transport materials.

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