A fluorescence emission-independent excitation carbon quantum dot and its preparation method

Abstract

This invention discloses a fluorescence emission-independent excitation carbon quantum dots and its preparation method. The method includes: Step 1, mixing o-phenylenediamine and boric acid at a molar ratio of 4 mmol:(1.2~4 mmol), grinding the mixture, transferring it to a square ceramic boat, and placing it in an oven. Under air atmosphere, the temperature is increased from room temperature to 100~135℃ at a heating rate of 5℃ / min, and the solid-phase reaction is carried out for 4~6 h. The mixture is then naturally cooled to room temperature to obtain a brownish-red solid; Step 2, dissolving the brownish-red solid in deionized water and sonicating it to obtain a yellowish-brown liquid; Step 3, filtering the yellowish-brown liquid to obtain a pure carbon quantum dot aqueous solution; Step 4, freeze-drying the carbon quantum dot aqueous solution to obtain carbon quantum dot powder. This invention simplifies the process flow, reduces energy consumption and post-processing complexity, and provides controllable reaction process. The prepared carbon quantum dots exhibit fluorescence emission-independent characteristics and long-wavelength orange fluorescence.

Description

Technical Field

[0001] This invention relates to carbon nanomaterials, specifically a carbon quantum dot whose fluorescence emission is independent of excitation and its preparation method. Background Technology

[0002] Carbon quantum dots, as an emerging fluorescent nanomaterial, have attracted much attention due to their low toxicity, good biocompatibility, and ease of functionalization. However, the practical application of existing carbon quantum dots is constrained by two major problems: First, most reported carbon quantum dots exhibit strong excitation-dependent fluorescence, meaning that the color (wavelength) of their emitted light changes significantly with the wavelength of the excitation source. This phenomenon stems from the complex luminescent sites and non-uniform energy level structure within the material, leading to unstable fluorescence signals and impure colors, which severely limits their application in scenarios requiring high color fidelity and quantitative accuracy, including multicolor biolabeling, ratiometric sensing, and high-precision display technologies. Second, achieving efficient and stable long-wavelength fluorescence (especially orange and red light) emission remains a technical challenge. Common carbon quantum dots mostly emit blue and green light, while long-wavelength fluorescence usually requires precise bandgap engineering, such as introducing specific heteroatoms (e.g., rare earth metals, nitrogen, and sulfur), constructing large sp² carbon conjugated domains, or precisely controlling the surface chemical state. These synthetic strategies are often cumbersome, demanding, and have poor reproducibility of product performance.

[0003] Currently, the hydrothermal method is the main method for synthesizing carbon quantum dots. However, the water-mediated reaction environment is precisely the root cause of many bottlenecks. While water can promote the reaction under high temperature and pressure, it also leads to the "chaos" of the process. The dissolution, hydrolysis, and reaction rates of precursors in water vary, causing the carbonization, polymerization, and surface functionalization steps to become intertwined and difficult to separate and control in the liquid phase. This uncontrollability results in the non-uniform structure and luminescence properties of the final product, which is the inherent reason why traditional carbon quantum dots have a strong wavelength dependence, broad fluorescence peaks, and instability. In addition, the hydrothermal reaction needs to be carried out in a closed high-pressure reactor, which poses safety risks for large-scale preparation of carbon quantum dots. Furthermore, it usually requires complex purification steps (such as long-term dialysis, multiple centrifugations, column chromatography, etc.) to remove unreacted precursors and byproducts, resulting in a low product yield of carbon quantum dots, which limits the large-scale preparation and application of carbon quantum dots. Summary of the Invention

[0004] The purpose of this invention is to provide a carbon quantum dot whose fluorescence emission is independent of excitation and its preparation method. This method simplifies the process, reduces energy consumption and post-processing complexity, and allows for controllable reaction. The prepared carbon quantum dots exhibit the characteristics of fluorescence emission independent of excitation and long-wavelength orange fluorescence.

[0005] This invention is achieved through the following technical solution: A method for preparing carbon quantum dots whose fluorescence emission is independent of excitation includes the following steps: Step 1: According to the molar ratio of o-phenylenediamine to boric acid 4 mmol : (1.2~4 mmol), o-phenylenediamine and boric acid were mixed, ground, transferred to a porcelain boat and placed in an oven. In an air atmosphere, the temperature was increased from room temperature to 100~135℃ at a heating rate of 5℃ / min. The solid-phase reaction was carried out for 4~6 h. After natural cooling to room temperature, a brown solid was obtained. Step 2: Dissolve the brownish-brown solid in deionized water and sonicate to obtain a yellowish-brown liquid; Step 3: Filter the yellow-brown liquid to obtain a pure aqueous solution of carbon quantum dots; Step 4: After freeze-drying the carbon quantum dot aqueous solution, carbon quantum dot powder is obtained.

[0006] Further, the molar ratio of o-phenylenediamine to boric acid in step 1 is 4 mmol:1.2 mmol, 4 mmol:2 mmol, 4 mmol:2.8 mmol, or 4 mmol:4 mmol.

[0007] Furthermore, the grinding in step 1 is performed using an agate mortar and pestle for 20-40 minutes.

[0008] Furthermore, the ultrasonic treatment in step 2 uses an ultrasonic wave with a power of 300 W and is performed for 20-40 minutes.

[0009] Furthermore, in step 3, the yellow-brown liquid is filtered using an organic phase filter membrane with a pore size of 50 nm, and the yellow-brown liquid is filtered three times.

[0010] A carbon quantum dot whose fluorescence emission is independent of excitation has an optimal excitation wavelength of 468 nm and a fluorescence emission peak of 580 nm, exhibiting orange fluorescence.

[0011] The present invention has the following beneficial technical effects: 1) The carbon quantum dots prepared by this invention exhibit a stable fluorescence emission peak in the long-wavelength orange region of 580 nm over a wide excitation range, shifting with the excitation wavelength. This demonstrates the characteristic of being independent of the excitation wavelength, solving the core problems of emission spectrum shifting with excitation wavelength and color impurity caused by multiple luminescent centers in traditional carbon quantum dots. This provides an ideal material for application fields requiring high color fidelity and stable optical signals (e.g., multicolor bioimaging and precision sensing), fundamentally achieving a breakthrough in the optical performance of carbon quantum dots.

[0012] 2) This invention employs a solvent-free solid-phase grinding and medium-temperature heat treatment method, which not only eliminates the dependence on solvent media in traditional hydrothermal methods, simplifying the process flow, reducing energy consumption and post-processing complexity, and embodying the concept of green chemistry, but more importantly, the solid-phase reaction environment is conducive to the directional and uniform condensation of precursor molecules, promoting the formation of carbon cores or polymer-like structures with uniform structures and clear luminescent centers from the source of synthesis, thus effectively ensuring the product performance and batch-to-batch reproducibility. In addition, the product yield of carbon quantum dots reached 31.5%, exceeding the current product yield of carbon quantum dots prepared using o-phenylenediamine as a raw material via hydrothermal methods. It is evident that the synthesis method of this invention has significant innovation and practicality, achieving precise control and stable reproducibility of the optical properties of carbon quantum dots.

[0013] 3) This invention directly and efficiently obtains long-wavelength orange fluorescence by precisely controlling the precursor molar ratio, breaking the limitation of the complex doping or modification steps usually required for the synthesis of long-wavelength carbon quantum dots. Moreover, the solid-phase reaction is carried out directly in an air atmosphere, and the prepared carbon quantum dots not only have abundant oxygen, nitrogen and boron elements on their surface, but also have bright and stable luminescence color. Furthermore, it solves the problems of complex post-processing steps and uneven luminescence performance of products in hydrothermal reactions.

[0014] 4) The preparation process of this invention does not require complex equipment and high-pressure reactors, and has the advantages of mild conditions, safety, high efficiency, controllability and good repeatability, laying a solid foundation for the large-scale application of carbon quantum dots in biomedicine (such as deep tissue imaging), optoelectronic devices and fluorescent labeling. Attached Figure Description

[0015] Figure 1 Photographs of the carbon quantum dot solution prepared in Example 3 of this invention and its appearance under 365 nm ultraviolet light irradiation; Figure 2 TEM image of carbon quantum dot powder prepared in Example 3 of this invention; Figure 3 Example 1 of this invention: Excitation spectrum and fluorescence emission spectrum at different excitation wavelengths of carbon quantum dot solution prepared; Figure 4 Example 2 of this invention: Excitation spectrum and fluorescence emission spectrum at different excitation wavelengths of carbon quantum dot solution prepared; Figure 5 Example 3 of this invention: Excitation spectrum and fluorescence emission spectrum at different excitation wavelengths of carbon quantum dot solution prepared; Figure 6 Example 4 of this invention shows the excitation spectrum and fluorescence emission spectrum of the carbon quantum dot solution prepared at different excitation wavelengths. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0017] Example 1 Step 1: Take 4 mmol of o-phenylenediamine and 1.2 mmol of boric acid, mix them in an agate mortar, grind for 20 min at room temperature to obtain a uniform solid powder, transfer the solid powder to a square porcelain boat and put it in an oven, and heat it from room temperature to 110℃ at a heating rate of 5℃ / min in air atmosphere for 5 h for solid-phase reaction. After the reaction is completed, cool it naturally to room temperature to obtain a brown solid. Step 2: Place the brown solid in a beaker, add 50 mL of deionized water, and sonicate it at room temperature using an ultrasonic wave with a power of 300 W for 30 min to fully dissolve and disperse it, thus obtaining a yellowish-brown liquid. Step 3: Using an organic phase filter membrane with a pore size of 50 nm, filter the yellow-brown liquid three times to obtain a pure carbon quantum dot aqueous solution. Step 4: After freeze-drying the carbon quantum dot aqueous solution, carbon quantum dot powder is obtained.

[0018] Example 2 Step 1: Take 4 mmol of o-phenylenediamine and 2 mmol of boric acid, mix them in an agate mortar, grind for 30 min at room temperature to obtain a uniform solid powder, transfer the solid powder to a square porcelain boat and put it in an oven, and heat it from room temperature to 135℃ in air atmosphere at a heating rate of 5℃ / min for 4 h for solid-phase reaction. After the reaction is completed, cool it naturally to room temperature to obtain a brown solid. Step 2: Place the brown solid in a beaker, add 50 mL of deionized water, and sonicate it at room temperature using an ultrasonic wave with a power of 300 W for 20 min to fully dissolve and disperse it, thus obtaining a yellowish-brown liquid. Step 3: Using an organic phase filter membrane with a pore size of 50 nm, filter the yellow-brown liquid three times to obtain a pure carbon quantum dot aqueous solution. Step 4: After freeze-drying the carbon quantum dot aqueous solution, carbon quantum dot powder is obtained.

[0019] Example 3 Step 1: Take 4 mmol of o-phenylenediamine and 2.8 mmol of boric acid, mix them in an agate mortar, grind for 30 min at room temperature to obtain a uniform solid powder, transfer the solid powder to a square porcelain boat and put it in an oven, and heat it from room temperature to 100℃ in air atmosphere at a heating rate of 5℃ / min for 6 h for solid-phase reaction. After the reaction is completed, cool it naturally to room temperature to obtain a brown solid. Step 2: Place the brown solid in a beaker, add 50 mL of deionized water, and sonicate it at room temperature using an ultrasonic wave with a power of 300 W for 40 min to fully dissolve and disperse it, thus obtaining a yellowish-brown liquid. Step 3: Using an organic phase filter membrane with a pore size of 50 nm, filter the yellow-brown liquid three times to obtain a pure carbon quantum dot aqueous solution. Step 4: After freeze-drying the carbon quantum dot aqueous solution, carbon quantum dot powder is obtained.

[0020] Example 4 Step 1: Take 4 mmol of o-phenylenediamine and 4 mmol of boric acid, mix them in an agate mortar, grind at room temperature for 40 min to obtain a uniform solid powder, transfer the solid powder to a square porcelain boat and put it in an oven, and heat it from room temperature to 120℃ in air atmosphere at a heating rate of 5℃ / min for 5 h for solid-phase reaction. After the reaction is completed, cool it naturally to room temperature to obtain a brown solid. Step 2: Place the brown solid in a beaker, add 50 mL of deionized water, and sonicate it at room temperature using an ultrasonic wave with a power of 300 W for 30 min to fully dissolve and disperse it, thus obtaining a yellowish-brown liquid. Step 3: Using an organic phase filter membrane with a pore size of 50 nm, filter the yellow-brown liquid three times to obtain a pure carbon quantum dot aqueous solution. Step 4: After freeze-drying the carbon quantum dot aqueous solution, carbon quantum dot powder is obtained.

[0021] Example 5 Step 1: Take 4 mmol of o-phenylenediamine and 2.6 mmol of boric acid, mix them in an agate mortar, grind at room temperature for 40 min to obtain a uniform solid powder, transfer the solid powder to a square porcelain boat and put it in an oven, and heat it from room temperature to 115℃ in air atmosphere at a heating rate of 5℃ / min for 5 h of solid-phase reaction. After the reaction is completed, cool it naturally to room temperature to obtain a brown solid. Step 2: Place the brown solid in a beaker, add 50 mL of deionized water, and sonicate it at room temperature using an ultrasonic wave with a power of 300 W for 25 min to fully dissolve and disperse it, thus obtaining a yellowish-brown liquid. Step 3: Using an organic phase filter membrane with a pore size of 50 nm, filter the yellow-brown liquid three times to obtain a pure carbon quantum dot aqueous solution. Step 4: After freeze-drying the carbon quantum dot aqueous solution, carbon quantum dot powder is obtained.

[0022] Figure 1(Left) A photograph of the carbon quantum dot solution prepared in Example 3 under natural light shows that it appears as a transparent brownish-yellow color. Figure 1 (Right) shows an image of the carbon quantum dot solution prepared in Example 3 under 365 nm ultraviolet light irradiation. It can be seen that it exhibits bright orange fluorescence.

[0023] Figure 2 (a) The TEM image shows that the carbon quantum dot particles prepared in Example 3 are completely dispersed without any agglomeration, and the particle size of the carbon quantum dots is 3-4 nm. Figure 2 (b) The high-resolution TEM image shows that there are obvious lattice fringes on the surface of the carbon quantum dot particles prepared in Example 3, with a fringe spacing of 0.22 nm, corresponding to the diffraction fringes of the graphitic carbon (100) crystal plane.

[0024] The optical properties of the carbon quantum dot aqueous solutions prepared in Examples 1-4 were characterized using a steady-state / transient fluorescence spectrometer (Edinburgh FLS980, UK). The results are as follows: Figures 3-6 As shown, it can be seen that the carbon quantum dot solutions prepared in Examples 1 to 4 of the present invention exhibit the strongest fluorescence intensity under excitation at 468 nm, indicating that the strongest excitation wavelength is 468 nm; at the same time, the fluorescence emission center of the carbon quantum dot solutions prepared in Examples 1 to 3 of the present invention is always located at 580 nm under different excitation wavelengths from 360 to 500 nm, and the fluorescence emission center of the carbon quantum dot solution prepared in Example 4 is also always located at 580 nm under different excitation wavelengths from 390 to 500 nm. Through analysis Figures 3-6 It can be seen that the optimal excitation wavelength of the carbon quantum dots prepared in Examples 1 to 4 is 468 nm, and they all exhibit fluorescence characteristics excited by a single, high-intensity fluorescence emission peak near 580 nm. By changing the excitation wavelength (from 360 nm to 500 nm), the position and shape of their fluorescence emission peak remain basically unchanged, proving that the prepared carbon quantum dots are wavelength-independent and all emit bright orange fluorescence. This is attributed to the solid-phase grinding and medium-temperature heat treatment method used in this invention, which effectively avoids problems such as uneven reaction, excessive hydrolysis, and by-product generation caused by the water medium in the traditional hydrothermal synthesis method. Moreover, the solid-phase environment promotes directional and uniform condensation and carbonization between precursor molecules, which is conducive to the formation of a carbon core structure with a single and clear luminescent center. The carbon quantum dots prepared in this way have a highly uniform size distribution (3-4 nm) and a clear crystal structure, thereby achieving high uniformity and stability of luminescent performance.

Claims

1. A method for preparing carbon quantum dots whose fluorescence emission is independent of excitation, characterized in that, Includes the following steps: Step 1: According to the molar ratio of o-phenylenediamine to boric acid 4 mmol : (1.2~4 mmol), o-phenylenediamine and boric acid were mixed, ground, transferred to a porcelain boat and placed in an oven. In an air atmosphere, the temperature was increased from room temperature to 100~135℃ at a heating rate of 5℃ / min. The solid-phase reaction was carried out for 4~6 h. After natural cooling to room temperature, a brown solid was obtained. Step 2: Dissolve the brownish-brown solid in deionized water and sonicate to obtain a yellowish-brown liquid; Step 3: Filter the yellow-brown liquid to obtain a pure aqueous solution of carbon quantum dots; Step 4: After freeze-drying the carbon quantum dot aqueous solution, carbon quantum dot powder is obtained.

2. The method for preparing carbon quantum dots whose fluorescence emission is independent of excitation according to claim 1, characterized in that, The molar ratio of o-phenylenediamine to boric acid in step 1 is 4 mmol:1.2 mmol, 4 mmol:2 mmol, 4 mmol:2.8 mmol, or 4 mmol:4 mmol.

3. The method for preparing carbon quantum dots whose fluorescence emission is independent of excitation according to claim 1, characterized in that, The grinding in step 1 is carried out using an agate mortar and pestle for 20-40 minutes.

4. The method for preparing carbon quantum dots whose fluorescence emission is independent of excitation according to claim 1, characterized in that, The ultrasonic treatment in step 2 uses an ultrasonic wave with a power of 300 W and lasts for 20 to 40 minutes.

5. The method for preparing carbon quantum dots whose fluorescence emission is independent of excitation according to claim 1, characterized in that, In step 3, the yellow-brown liquid is filtered three times using an organic phase filter membrane with a pore size of 50 nm.

6. A carbon quantum dot with fluorescence emission independent of excitation prepared by the method according to any one of claims 1 to 5, characterized in that, The optimal excitation wavelength is 468 nm, and the fluorescence emission peak is at 580 nm, exhibiting orange fluorescence.

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