A double rare earth element doped carbon quantum dot and its preparation method and application

Abstract

The invention discloses a carbon quantum dot doped with double rare earth elements, a preparation method and an application thereof, and belongs to the technical field of nanomaterials. The preparation method of the carbon quantum dot doped with double rare earth elements comprises the following steps: (1) weighing carbon source citric acid, nitrogen source urea, neodymium source and cerium source, mixing them evenly and placing them in a microwave reactor, adding deionized water to fully dissolve the above raw materials, then adding carbon source propylene glycol, mixing evenly and ensuring that the raw material liquid has no precipitation, and reacting the raw material liquid at 680-720W under the action of a microwave field; (2) adding water to quench the product after the reaction in step (1), cooling it to room temperature, further dialyzing, centrifuging, washing and drying to obtain the carbon quantum dot doped with double rare earth elements; the carbon quantum dot doped with double rare earth elements can effectively realize up-conversion luminescence, and has a high fluorescence quantum efficiency; and can realize effective biological imaging.

Description

Technical Field

[0001] The present invention relates to the technical field of nanomaterials, and in particular to a double rare earth element doped carbon quantum dot and a preparation method and application thereof. Background Art

[0002] Carbon quantum dots are a new type of zero-dimensional carbon nanomaterial with significant fluorescent properties that has emerged in recent years. They are also called carbon dots. They have good biocompatibility and water solubility, are environmentally friendly, and have a wide source of raw materials. The current synthesis methods include arc discharge method, electrochemical synthesis method, hydrothermal synthesis method, microwave synthesis method, template method, etc. The unique structure of carbon quantum dots can produce size effects, macroscopic quantum tunneling effects, surface effects and quantum confinement effects, making them widely used in biomedical fields such as biological detection and sensing, in vivo and in vitro imaging, drug delivery and diagnostic drugs.

[0003] Bioimaging can quickly locate specific organs, tissues or substances in organisms so that they can be observed by the naked eye or optical instruments. However, most of the fluorescent substances currently used for bioimaging are organic substances, which are toxic to organisms. Carbon quantum dots have strong and stable fluorescence properties and can be used for cell imaging, bacterial imaging and in vivo imaging, but improving the stability of carbon quantum dots is still one of the technical problems that need to be solved urgently in existing technologies.

[0004] Rare earth elements have abundant electronic energy levels and long-life excited states, and there are a large number of energy level transition channels, thus showing good optical properties and being able to produce a variety of radiation absorption and emission. In order to optimize the performance of carbon quantum dots, rare earth elements are usually doped in carbon quantum dots. For example, Chinese patent document No. CN110437835A discloses a method for preparing a cerium sulfide-doped carbon quantum dot nano fluorescent material, which uses itaconic acid as a carbon source, thioacetamide as a homogeneous sulfur source, and cerium chloride as a cerium source, and adopts a hydrothermal method to prepare a cerium sulfide-doped carbon quantum dot nano fluorescent material. Cerium sulfide is a red powdery substance with a yellowish inorganic pigment, containing rare earth components. By doping carbon quantum dots with cerium sulfide, pink carbon quantum dots can be obtained, and the emission wavelength of fluorescence is 540nm, and the maximum wavelength of its absorption spectrum is about 510nm, which can also be used for spectrophotometry in the visible light wavelength range.

[0005] The Chinese patent document with publication number CN112920793A discloses a rare earth luminescent material with enhanced emission in the visible light / near infrared region. The invention uses NaErF4:10%Yb,0.5%Tm as the crystal core to construct a core-shell-shell structure of NaErF4:10%Yb,0.5%Tm@NaGdF4:20%Yb,X%Ce@NaGdF4 rare earth luminescent material, in which NaErF4:10%Yb,0.5%Tm as the core material can produce single-band red light emission. And Ce is doped in the first shell layer. 3+ It can not only effectively enhance Er 3+ The characteristic emission in the second near-infrared region (about 1550nm) also effectively avoids Er 3+ The luminescence quenching in the visible light region and the construction of the NaGdF4 inert shell are beneficial to improving the overall luminescence intensity of the material.

[0006] A Chinese patent document with publication number CN108822840A discloses a method for preparing a composite material of neodymium doped carbon quantum dots using hemp as a carbon source. The invention firstly uses neodymium chloride to react with quercetin to prepare a quercetin-neodymium complex, and then uses 3-methylimidazole chloride ionic liquid as a solvent. In a reactor, the following composition weight percentages are added: 3-methylimidazole chloride: 93-96%, hemp powder: 3-6%, and quercetin-neodymium complex: 0.5-1.5%. The reaction is carried out at a constant temperature of 110±2°C for 6 hours to obtain a neodymium doped carbon quantum dot composite material.

[0007] However, the steps of the above-mentioned prior art are relatively complicated, involving many reagents, and the optical performance of carbon quantum dots after doping needs to be further improved. Summary of the invention

[0008] The present invention provides a method for preparing carbon quantum dots doped with double rare earth elements. The synthesis method has simple steps and high efficiency. The prepared carbon quantum dots doped with double rare earth elements can emit fluorescence with high sensitivity in pure water medium and in vivo, have high fluorescence quantum efficiency and up-conversion luminescence ability, and can realize biological imaging of living organisms or in vitro tissues and cells.

[0009] The specific technical solutions adopted are as follows:

[0010] A method for preparing double rare earth element doped carbon quantum dots comprises the following steps:

[0011] (1) Weighing carbon source citric acid, nitrogen source urea, neodymium source and cerium source and mixing them evenly, then adding deionized water to fully dissolve the above raw materials, then adding carbon source glycerol, mixing evenly and ensuring that there is no precipitation in the raw material solution, transferring the raw material solution to a microwave reactor, and reacting at 680-720W under the action of a microwave field;

[0012] (2) quenching the product after the reaction in step (1) with water, cooling it to room temperature, further dialyzing, centrifuging, washing, and drying to obtain the double rare earth element-doped carbon quantum dots;

[0013] The neodymium source is a water-soluble neodymium source selected from neodymium chloride hexahydrate; the cerium source is a water-soluble cerium source selected from cerium chloride; the mass ratio of the neodymium source to the cerium source is 4-8:1.

[0014] The method of the present invention uses citric acid and glycerol as carbon sources. Glycerol and citric acid contain abundant alcohol hydroxyl groups and carboxyl groups, while the abundant amine groups of urea can be condensed to form a network polymer under the action of microwave field. The structure of these network polymers sways under the action of high temperature and disintegrates when the energy reaches the peak. The carbon atoms are connected by sp 2 Hybrid nucleation, amide groups and other groups formed by polymerization cover the surface of carbon atoms and produce coordination with rare earth ions, and then cover the surface of carbon quantum dots with rare earth ions to prepare double rare earth element doped carbon quantum dots. Neodymium ions and cerium ions have rich spectral terms and long-life excited states. They can absorb infrared light and accumulate photon energy, complete upconversion luminescence through multi-photon processes, and give carbon quantum dots the ability to upconvert luminescence. At the same time, these two ions can improve the downconversion luminescence fluorescence quantum efficiency of carbon quantum dots, have a synergistic effect, and will not cause concentration quenching.

[0015] Preferably, the ratio of citric acid, urea and glycerol is 8-10 g: 1 g: 40-55 mL. Under the above preferred ratio, the interaction between the three and the generation of carbon quantum dots are beneficial.

[0016] Preferably, the ratio of citric acid, urea, neodymium source, cerium source, deionized water and glycerol is 8-10g: 1g: 3-3.5g: 0.6-0.7g: 26-28mL: 50-55mL. The reaction under the above preferred ratio can shorten the reaction time and reduce energy consumption while ensuring the high fluorescence quantum efficiency of carbon quantum dots, so that the prepared carbon quantum dots have good water solubility, and the distribution of double rare earth elements on the surface of carbon quantum dots is more uniform, and no concentration quenching effect occurs.

[0017] Preferably, in step (1), the reaction end temperature is controlled at 210-230° C. If the reaction end temperature is too high, the raw materials will be over-carbonized, the particle size of the carbon quantum dots will be increased, the fluorescence intensity of the carbon quantum dots will be weakened, or even the photoluminescence ability will be lost. At the same time, the over-carbonized carbon quantum dot solution will be brown, with poor light transmittance, and the excitation light can only pass through the surface of the solution to excite the surface solution, which will reduce the luminescence ability of the carbon quantum dot solution and reduce the application ability of the carbon quantum dot solution. If the reaction end temperature is too low, the reactants do not reach the mutation point, the raw materials are still in the polymer form, and have not disintegrated to form carbon quantum dots, and have no luminescence ability. The carbon quantum dot solution still appears as a viscous transparent solution, and does not emit light under 365nm wavelength ultraviolet light excitation.

[0018] Preferably, in step (2), a 1000Da dialysis bag is used for dialysis in ultrapure water for 48-72 hours to remove impurities such as unreacted raw material molecules.

[0019] Preferably, in step (2), the centrifugation conditions are a rotation speed of 10000-11000 r / min and a time of 8-13 min.

[0020] The present invention also provides double rare earth element doped carbon quantum dots prepared by the preparation method of double rare earth element doped carbon quantum dots.

[0021] The average particle size of the double rare earth element doped carbon quantum dots is 15-40nm, and the carbon core has sp 2 Graphite configuration, but the degree of graphitization is not high, doped with some amorphous carbon, the surface of carbon quantum dots contains more amide groups and amino groups, a small amount of carboxyl groups or hydroxyl groups, which makes carbon quantum dots have good water solubility. Amide groups have a coordination effect on rare earth elements, and rare earth elements can be wrapped on the surface of carbon quantum dots through the coordination of amide groups to form sensitization centers and activation centers. At the same time, the melody restriction of rare earth element ff transition is changed through the perturbation of the crystal field, thereby enhancing its luminescence ability.

[0022] Carbon quantum dots, as ligands with a larger radius, perturb the 4f orbital of rare earth ions, causing energy level splitting. After the energy level splitting, the rare earth ions are no longer restricted by the melodic prohibition, and are partially lifted to allow transitions. The sensitizing ions serve as sensitizing centers, absorbing energy and transferring it to the activation centers near the surface. Since the double rare earth element doped carbon quantum dots prepared by the method of the present invention have low toxicity, good biosafety, and excellent luminescence performance, they can be used as good bioimaging materials.

[0023] The present invention also provides the application of the double rare earth element doped carbon quantum dots in the field of biological imaging; specifically, microorganisms (such as Escherichia coli, etc.) are co-incubated with the double rare earth element doped carbon quantum dot solution and then observed under a microscope.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The double rare earth element doped carbon quantum dots in the present invention are synthesized in one step by microwave method, which consumes less energy, takes less time and is simple in method; the main raw materials are citric acid, urea, propylene glycol, neodymium source and cerium source (chloride), which are non-biologically toxic; compared with other synthesis methods, the method of the present invention has low cost, simple impurity removal, no toxic side effects caused by heavy metal ions, and good economic benefits.

[0026] (2) The present invention utilizes a one-step reaction method to synthesize nitrogen-doped rare earth ion coordinated carbon quantum dots, without the need for a surface functionalization reaction process; compared with carbon quantum dots not doped with rare earth elements, the fluorescence quantum efficiency of the carbon quantum dots synthesized by the present invention is increased from 0.43 to 69.04; compared with carbon quantum dots doped with a single rare earth element, the fluorescence quantum efficiency of the carbon quantum dots synthesized by the present invention is increased from 3.55 and 52.00 to 69.04, respectively; the carbon quantum dots synthesized by the present invention have upconversion luminescence under excitation at 740nm and 800nm, which is not possessed by carbon quantum dots not doped with rare earth elements. Compared with carbon quantum dots doped with a single rare earth element, the carbon quantum dots synthesized by the present invention exhibit a richer emission spectrum in upconversion luminescence.

[0027] (3) The double rare earth element doped carbon quantum dots synthesized in the present invention can realize up-conversion luminescence using near-infrared light, have good tissue penetration effect, and can better realize in vivo imaging of organisms. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Transmission electron microscope characterization image (A) and particle size distribution image (B) of the double rare earth element doped carbon quantum dots prepared in Example 1.

[0029] Figure 2 FTIR image (A) and UV-Vis-IR absorption spectrum (B) of the double rare earth element doped carbon quantum dots prepared in Example 1.

[0030] Figure 3 This is the fluorescence spectrum of the double rare earth element-doped carbon quantum dots prepared in Example 1; the upper right illustrations a and b are the colors of the carbon quantum dot solution under natural light and 365 nm wavelength light, respectively (colorless under natural light, blue under 365 nm wavelength light).

[0031] Figure 4 Up-conversion fluorescence spectrum (A) and fluorescence quantum efficiency analysis diagram (B) of the double rare earth element doped carbon quantum dots prepared in Example 1.

[0032] Figure 5The up-conversion luminescence fluorescence spectra of carbon quantum dots (CDs) without rare earth element doping, carbon quantum dots containing cerium (C-CDs) with single rare earth element doping, and carbon quantum dots containing neodymium (N-CDs) at 740nm and 800nm.

[0033] Figure 6 Bright field (A) and fluorescence microscopy (B) characterization images of a solution of double rare earth element-doped carbon quantum dots used for bioimaging of Escherichia coli. DETAILED DESCRIPTION

[0034] In view of the defects of the prior art, the inventor of this case has proposed the technical solution of the present invention after long-term research and extensive practice. The technical solution of the present invention will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] The experimental materials used in the examples were purchased from Shanghai MacLean Biochemical Technology Co., Ltd. and were of analytical grade.

[0036] Example 1 Double rare earth element doped carbon quantum dots

[0037] 1.5 g of citric acid, 0.15 g of urea, 0.5 g of neodymium chloride hexahydrate, and 0.1 g of cerium chloride were added to a reaction container and mixed evenly. 4 mL of water was added to fully dissolve the above raw materials, and then 8 mL of propylene glycol was added. The mixture was stirred and mixed thoroughly to ensure that the raw material solution had no precipitation. The raw material solution was transferred to a microwave reactor, and reacted at 700 W for 57 seconds under the action of a microwave field, and the temperature after the reaction was 220° C.; 10 mL of water was added to quench the reaction, and after naturally cooling to room temperature, the mixture was placed in a 1000 Da dialysis bag, and dialyzed in ultrapure water for 48 hours to obtain a colorless to light pink solution; the mixture was further centrifuged in a centrifuge at 10,000 r / min for 10 minutes, and the supernatant was taken for natural evaporation or placed in an oven not exceeding 60° C. for drying. The obtained solid product was placed in a solvent filter and washed with anhydrous ethanol, and the double rare earth element-doped carbon quantum dots were obtained after natural drying.

[0038] Example 2 Double rare earth element doped carbon quantum dots

[0039] Add 2.0 g of citric acid, 0.2 g of urea, 0.5 g of neodymium chloride hexahydrate, and 0.1 g of cerium chloride into a reaction container and mix them evenly, add 4 mL of water to fully dissolve the above raw materials, then add 8 mL of propylene glycol, stir and mix thoroughly to ensure that the raw material liquid has no precipitation, transfer the raw material liquid to a microwave reactor, react at 700 W for 57 seconds under the action of the microwave field, and the temperature after the reaction is 220°C; add 10 mL of water to quench the reaction, cool naturally to room temperature, put it into a 1000Da dialysis bag, dialyze in ultrapure water for 48 hours to obtain a colorless to light pink solution; further centrifuge in a centrifuge at 10000 r / min for 10 minutes, take the supernatant and evaporate it naturally or place it in an oven not exceeding 60°C for drying, and place the obtained solid product in a solvent filter and wash it with anhydrous ethanol, and obtain the double rare earth element doped carbon quantum dots after natural drying.

[0040] Example 3 Double rare earth element doped carbon quantum dots

[0041] Add 1.5 g of citric acid, 0.15 g of urea, 0.5 g of neodymium chloride hexahydrate, and 0.1 g of cerium chloride into a reaction container and mix them evenly. Add 3 mL of water to fully dissolve the above raw materials, then add 8 mL of propylene glycol, stir and mix thoroughly to ensure that the raw material liquid has no precipitation, transfer the raw material liquid to a microwave reactor, react at 700 W for 52 seconds under the action of the microwave field, and the temperature after the reaction is 220°C; add 20 mL of water to quench the reaction, cool naturally to room temperature, put it into a 1000 Da dialysis bag, dialyze in ultrapure water for 48 hours to obtain a colorless to light pink solution; further centrifuge at 11000 r / min for 8 minutes in a centrifuge, take the supernatant and evaporate it naturally or place it in an oven not exceeding 60°C for drying, and place the obtained solid product in a solvent filter and wash it with anhydrous ethanol, and obtain the double rare earth element doped carbon quantum dots after natural drying.

[0042] Comparative Example 1 Carbon Quantum Dots

[0043] Add 1.5 g of citric acid and 0.15 g of urea to the reaction container and mix well. Add 4 mL of water to fully dissolve the above raw materials, then add 8 mL of propylene glycol, stir and mix well to ensure that the raw material liquid has no precipitation, transfer the raw material liquid to a microwave reactor, react at 700 W for 57 seconds under the action of the microwave field, and the temperature after the reaction is 220°C; add 10 mL of water to quench the reaction, cool naturally to room temperature, put it into a 1000Da dialysis bag, dialyze in ultrapure water for 48 hours to obtain a colorless to light yellow solution; further centrifuge at 10000 r / min for 10 minutes in a centrifuge, take the supernatant and evaporate it naturally or place it in an oven not exceeding 60°C to dry, and place the obtained solid product in a solvent filter, wash it with anhydrous ethanol, and obtain carbon quantum dots after natural drying.

[0044] Comparative Example 2 Single rare earth element doped carbon quantum dots

[0045] Add 1.5 g of citric acid, 0.15 g of urea, and 0.5 g of neodymium chloride hexahydrate into a reaction container and mix well, add 4 mL of water to fully dissolve the above raw materials, then add 8 mL of propylene glycol, stir and mix well to ensure that the raw material solution has no precipitation, transfer the raw material solution to a microwave reactor, react at 700 W for 57 seconds under the action of the microwave field, and the temperature after the reaction is 220°C; add 10 mL of water to quench the reaction, cool naturally to room temperature, put it into a 1000Da dialysis bag, dialyze in ultrapure water for 48 hours to obtain a colorless to light pink solution; further centrifuge at 10000 r / min for 10 minutes in a centrifuge, take the supernatant and evaporate it naturally or place it in an oven not exceeding 60°C to dry, and place the obtained solid product in a solvent filter and wash it with anhydrous ethanol, and obtain single rare earth element doped carbon quantum dots after natural drying.

[0046] Comparative Example 3 Single rare earth element doped carbon quantum dots

[0047] Add 1.5 g of citric acid, 0.15 g of urea, and 0.1 g of cerium chloride into a reaction container and mix well, add 4 mL of water to fully dissolve the above raw materials, then add 8 mL of propylene glycol, stir and mix well to ensure that the raw material liquid has no precipitation, transfer the raw material liquid to a microwave reactor, react at 700 W for 57 seconds under the action of the microwave field, and the temperature after the reaction is 220°C; add 10 mL of water to quench the reaction, cool naturally to room temperature, put it into a 1000Da dialysis bag, dialyze in ultrapure water for 48 hours to obtain a colorless to light yellow solution; further centrifuge at 10000 r / min for 10 minutes in a centrifuge, take the supernatant and evaporate it naturally or place it in an oven not exceeding 60°C to dry, and place the obtained solid product in a solvent filter and wash it with anhydrous ethanol, and obtain single rare earth element doped carbon quantum dots after natural drying.

[0048] Application Example 1

[0049] The double rare earth element doped carbon quantum dots prepared in Example 1 are used for biological imaging, and the specific method steps are as follows:

[0050] (1) Prepare a 30 mg / mL solution of double rare earth element-doped carbon quantum dots for later use.

[0051] (2) Take 17.0 g of trypticase, 3.0 g of soybean papain hydrolyzate, 5.0 g of sodium chloride, 2.5 g of potassium dihydrogen phosphate, and 2.5 g of glucose, dissolve them in ultrapure water and make the volume to 1 L, heat and boil until completely dissolved, dispense into conical flasks, seal with sealing film, and place them in an autoclave at 121°C for 15 min to prepare trypticase soytone liquid culture medium.

[0052] (3) After the culture medium has cooled to room temperature, dispense it into 6-well bacterial culture plates, 15 mL per plate; use an inoculation loop to pick up a loop (about 10 μL) of E. coli culture solution and add it to the culture medium for inoculation.

[0053] (4) Add 25 μL of the carbon quantum dot solution prepared in step (1) to each well, place the well on a shaker and incubate at 37°C for 24 h, then make a smear of the E. coli and observe it under a phase contrast microscope and a fluorescence microscope.

[0054] Sample analysis

[0055] The double rare earth element doped carbon quantum dots prepared in Example 1 were characterized by high resolution transmission electron microscopy (HRTEM). Figure 1 As shown in A, the particle size distribution diagram is as follows Figure 1 As shown in B, it can be seen that most of the particle sizes of the carbon quantum dots are distributed between 15-40 nm, with an average particle size of 27 nm.

[0056] The double rare earth element doped carbon quantum dots prepared in Example 1 were subjected to infrared spectrum test and ultraviolet spectrum test. The FTIR graph is as follows: Figure 2 As shown in A, it is shown at 3383.01cm -1 The broad peak of 2086.60cm is the strongest peak of FTIR, which is the characteristic absorption peak of NH stretching vibration of free primary amine and amide, and alcohol hydroxyl group. -1 The weak broad peak is the characteristic peak of carbonyl; at 1634.86cm -1 The shoulder peak is the stretching vibration of the C=O complex of the primary amide and the characteristic peak of the NH plane shear deformation vibration of the primary amine; at 1406.82cm -1 The weak peak is the characteristic absorption peak of CN stretching vibration of primary amide; 558.77cm -1 There are many impurity peaks near the center, which are the characteristic absorption peaks of the out-of-plane swinging deformation vibration of the primary amine NH. Based on this, it can be judged that the surface of the prepared carbon quantum dots contains more amide groups and amino groups; a small amount of carboxyl groups or hydroxyl groups; this makes the carbon quantum dots have rich water solubility; the amide group has a coordination effect on rare earth elements, and the rare earth elements can be wrapped on the surface of the carbon quantum dots through the coordination of the amide group.

[0057] UV-Vis-IR absorption spectrum Figure 2As shown in B, carbon quantum dots often have strong light absorption at 230-320nm, and often tail to the visible light region; this can be attributed to the transition of the C=C structure of the carbon quantum dots or the contribution of the transition produced by the amide group as a chromophore. Therefore, it can be inferred that this is the absorption peak of the carbon core of the carbon quantum dots doped with double rare earth elements. It is possible that the absorption wavelength is red-shifted due to the presence of auxochromophores such as hydroxyl groups on the surface of the carbon quantum dots; the carbon quantum dots have several small absorption peaks in the visible light region, which are of little significance for the practical application of up-conversion and down-conversion luminescence; two absorption peaks appear at 740nm and 800nm ​​in the near-infrared region, corresponding to the characteristic absorption peaks of neodymium; this shows that neodymium-coated carbon quantum dots can achieve infrared absorption at 740nm and 800nm; and have the ability to achieve up-conversion luminescence.

[0058] Under 365nm ultraviolet excitation, the fluorescence spectrum of the double rare earth element doped carbon quantum dots prepared in Example 1 was measured, and the results were as follows: Figure 3 As shown, it can be found that the fluorescence emission of the carbon quantum dots synthesized in the present invention presents a single-peak curve; the peak value is 440nm, located in the center of the blue light band; this proves that the carbon quantum dots synthesized in the present invention have good light purity (a and b are the colors of the carbon quantum dot solution under natural light and 365nm wavelength light, respectively, colorless under natural light and blue under 365nm wavelength light).

[0059] Under 740nm and 800nm ​​infrared excitation, the upconversion fluorescence spectra of carbon quantum dots were measured ( Figure 4 A); it can be found that the carbon quantum dots doped with double rare earth elements prepared in Example 1 emit light under two wavelengths of excitation, wherein 740nm excitation shows a single peak curve with a peak at 436nm; 800nm ​​excitation shows a shoulder peak curve with a main peak at 400nm and a secondary peak at 446nm. Both are blue light emission; the fluorescence quantum efficiency analysis diagram of carbon quantum dots measured under 365nm excitation and 440nm emission ( Figure 4 B); it can be found that the quantum yield of carbon quantum dots is 69.04%.

[0060] like Figure 5As shown, the carbon quantum dots not doped with rare earth elements in Comparative Example 1 have no valuable fluorescence emission under 740nm and 800nm ​​infrared light excitation. Under 740nm excitation, N-CDs contribute to the 436nm emission that constitutes the emission peak center of L-CDs (double rare earth element doped carbon quantum dots prepared in Example 1). C-CDs have a broad peak emission centered at 460nm, corresponding to the tailing of the emission peak of the L-CDs upconversion emission spectrum under 740nm excitation light to the red light region. Under 800nm ​​excitation, N-CDs exhibit high-purity and extremely strong emission centered at 400nm, and the emission peak is very narrow. Corresponding to the main peak of the shoulder peak of the L-CDs upconversion emission spectrum under 800nm ​​excitation light. C-CDs show a broad peak with a center of 480nm and a long tail in the visible red region, which cooperates with the weak broad peak of N-CDs at about 460nm to form a secondary peak of the shoulder peak of the L-CDs upconversion emission spectrum under 800nm ​​excitation light; this shows that the upconversion luminescence of the double rare earth element doped carbon quantum dots is the result of the combined action of neodymium and cerium. Neodymium and cerium are indispensable for the material prepared by the present invention to achieve upconversion luminescence.

[0061] The fluorescence quantum efficiency of the carbon quantum dots in Comparative Examples 1-3 and Example 1 was measured at the same time. The results showed that the fluorescence quantum efficiency of the carbon quantum dots in Comparative Example 1 that did not contain any rare earth elements was very low, which was related to the fact that the carbon quantum dots that did not contain any rare earth elements had a high degree of carbonization under the same preparation conditions, the solution was brown, and the light transmittance was poor. The fluorescence quantum efficiency of the carbon quantum dots containing cerium in Comparative Example 3 was 3.55%, which was several times higher than that of the carbon quantum dots that did not contain any rare earth elements. The fluorescence quantum efficiency of the carbon quantum dots containing neodymium in Comparative Example 2 was 52.00%, but it was still lower than that in Example 1; this shows that the fluorescence quantum efficiency of carbon quantum dots co-doped with neodymium and cerium is the strongest, and the synergy of the two rare earth elements can effectively promote the improvement of the fluorescence quantum efficiency of carbon quantum dots.

[0062] The bright field microscope image obtained by phase contrast microscope observation in Application Example 1 is as follows Figure 6 As shown in A, the fluorescence microscopy image is as follows Figure 6 As shown in B, the Escherichia coli that takes up carbon quantum dots emits bright blue fluorescence under a fluorescence microscope, which proves the application of the double rare earth element doped carbon quantum dots prepared in the present invention in the field of biological imaging.

[0063] The embodiments described above provide a detailed description of the technical solutions of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements or similar substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing double rare earth element doped carbon quantum dots, characterized in that: The following steps are involved: (1) Weighing carbon source citric acid, nitrogen source urea, neodymium source and cerium source and mixing them evenly, then adding deionized water to fully dissolve the above raw materials, then adding carbon source glycerol, mixing evenly and ensuring that the raw material liquid has no precipitation, transferring the raw material liquid to a microwave reactor, reacting at 680-720W under the action of a microwave field, and controlling the reaction end temperature at 210-230°C; the ratio of citric acid, urea, neodymium source, cerium source, deionized water and glycerol is 8-10g: 1g: 3-3.5g: 0.6-0.7g: 26-28mL: 50-55mL; (2) quenching the product after the reaction in step (1) with water, cooling it to room temperature, further dialyzing, centrifuging, washing, and drying to obtain the double rare earth element-doped carbon quantum dots; The neodymium source is a water-soluble neodymium source selected from neodymium chloride hexahydrate; the cerium source is a water-soluble cerium source selected from cerium chloride; the mass ratio of the neodymium source to the cerium source is 4-8:

1.

2. The method for preparing double rare earth element doped carbon quantum dots according to claim 1, characterized in that: In step (2), dialyze in ultrapure water using a 1000Da dialysis bag for 48-72 hours.

3. The method for preparing double rare earth element doped carbon quantum dots according to claim 1, characterized in that: In step (2), the centrifugal conditions are a rotation speed of 10000-11000 r / min and a time of 8-13 min.

4. Double rare earth element-doped carbon quantum dots prepared according to the method for preparing double rare earth element-doped carbon quantum dots according to any one of claims 1 to 3.

5. The double rare earth element doped carbon quantum dots according to claim 4, characterized in that: The average particle size of the double rare earth element doped carbon quantum dots is 15-40nm.

6. Application of the double rare earth element doped carbon quantum dots according to claim 4 or 5 in the field of biological imaging.

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