Method for green and large-scale synthesis of carbon quantum dots and carbon quantum dots

Abstract

The invention relates to the technical field of nano material preparation, and particularly provides a green and large-scale carbon quantum dot synthesis method and a carbon quantum dot, the method comprises the following steps: drying a biomass raw material, and crushing to obtain biomass powder; uniformly spreading the biomass powder in a high-temperature-resistant tray; under the normal pressure condition, the tray is placed in an oven to be subjected to heat treatment, the biomass powder is subjected to thermal decomposition and carbonization reaction within the temperature range of 200-250 DEG C, and a carbonized product is obtained; and dispersing the carbonized product in water, and separating and purifying the obtained dispersion liquid to obtain the water-phase dispersed carbon quantum dots. Uniform heating is achieved in a tray tiling mode, the process is simple, amplification is easy, the method is suitable for batch and large-scale production, and the obtained carbon dots have good water dispersibility and stable fluorescence emission performance.

Description

Technical Field

[0001] This invention relates to the field of nanomaterial preparation technology, and in particular provides a green, large-scale method for synthesizing carbon quantum dots and carbon quantum dots. Background Technology

[0002] Quantum dots (QDs) are a class of zero-dimensional materials with dimensions in the nanoscale range. Due to the quantum confinement effect, they exhibit unique optical and electrical properties, particularly the tunable fluorescence emission wavelength achieved through size control, making them valuable for applications in light-emitting displays, bio-imaging, and optoelectronic devices. Fundamental research on the size-controllable synthesis and quantum confinement effect of quantum dots has profoundly impacted the development of nanomaterials science, with related findings earning the 2023 Nobel Prize in Chemistry, further highlighting the significant scientific value of quantum dot materials. However, most current mainstream quantum dots are group II–VI or III–V semiconductor nanocrystals, whose preparation processes often rely on heavy metals and organic solvents, posing potential biotoxicity and environmental risks, thus limiting their applications in food, agriculture, and biomedicine.

[0003] To overcome the aforementioned problems, carbon dots (CDs) have gradually attracted attention as a novel carbon-based quantum dot material. Carbon dots typically consist of a carbon core and abundant functional groups on their surface, exhibiting excellent fluorescence properties, biocompatibility, and chemical stability. Their raw materials are widely available, and they can be prepared from biomass or small organic molecules, thus showing promising application prospects in fields such as smart food packaging, sensing and detection, and functional materials. Existing technologies for preparing carbon dots mainly include hydrothermal methods, solvothermal methods, microwave methods, electrochemical methods, and high-temperature pyrolysis methods. Among these, hydrothermal and solvothermal methods typically require high-pressure reactors, resulting in high equipment costs, low yields, and complex and time-consuming operations; microwave methods have high requirements for equipment and reaction conditions; while high-temperature pyrolysis methods usually require temperatures above 350°C, consuming large amounts of energy and hindering green, safe, and large-scale production.

[0004] Recent studies have shown that carbonization of biomass precursors and formation of carbon dots can also be achieved at lower temperatures (approximately 180–250℃) by rationally controlling the heat treatment method and reaction time. However, existing low-temperature preparation methods mostly rely on closed reactors or high-pressure hydrothermal systems, resulting in relatively complex processes that are difficult to meet the demands for low cost, visualized operation, and large-scale production. Therefore, developing a low-temperature carbon dot preparation method that does not require high-pressure equipment, is easy to operate, and is suitable for large-scale production remains of significant research and application value. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing biomass carbon dots based on low-temperature, atmospheric-pressure baking in a conventional oven, so as to overcome the problems of high carbon dot preparation temperature, complex equipment and difficulty in large-scale production in the prior art, and to achieve low-cost, green and scalable preparation of carbon dots.

[0006] This invention provides a green, large-scale method for synthesizing carbon quantum dots, comprising the following steps: 1) The biomass raw material is dried and pulverized to obtain biomass powder; 2) Spread the biomass powder evenly in a high-temperature resistant tray; 3) Under normal pressure, the tray is placed in an oven for heat treatment, so that the biomass powder undergoes thermal decomposition and carbonization reaction in the temperature range of 200–250 °C to obtain carbonized products. 4) Disperse the carbonization product in water, and separate and purify the resulting dispersion to obtain aqueous carbon quantum dots.

[0007] Preferably, in step 1), the biomass raw material is plant-derived biomass.

[0008] Preferably, the plant is selected from one or a combination of golden ear grass, red ginseng, American ginseng, black ginseng, pomelo peel, tea, and plant rhizomes.

[0009] Preferably, in step 2), the thickness of the biomass powder spread in the tray is 0.5 cm, and the particle size is 0.5–3.5 nm according to Imagej particle size statistics.

[0010] Preferably, in step 3), the heat treatment temperature of the biomass powder is 210-230℃.

[0011] Preferably, in step 3), the heat treatment time is 1-4 hours.

[0012] Preferably, in step 4), the separation and purification steps include at least one of filtration, centrifugation, microfiltration, and dialysis.

[0013] Preferably, a dialysis membrane with a molecular weight cutoff range of 500-3500 Da is used when dialyzing the dispersion.

[0014] A carbon quantum dot is provided, prepared using the method described above. The carbon quantum dot has a quantum yield of 20-25% and exhibits considerable fluorescence emission intensity under ultraviolet light irradiation.

[0015] Compared with the prior art, the advantages of the present invention are as follows: 1) Low-temperature heat treatment is carried out in a regular oven under normal pressure, without the need for a high-pressure reactor or high-temperature pyrolysis equipment, thus requiring less equipment. 2) The carbonization temperature is significantly lower than that of traditional high-temperature pyrolysis methods, reducing energy consumption and safety risks; 3) Uniform heating is achieved by laying the tray flat, the process is simple, easy to scale up, and suitable for batch and large-scale production; 4) The obtained carbon dots have good water dispersibility and stable fluorescence emission properties. Attached Figure Description

[0016] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments: Figure 1 This is a TEM image of carbon dots from *Gynostemma pentaphyllum* in Example 1; Figure 2 This is a particle size distribution diagram of carbon dots in *Gynostemma pentaphyllum* from Example 1; Figure 3 Fourier transform infrared image of carbon dots from *Gynostemma pentaphyllum* in Example 1; Figure 4 The image shows the XRD pattern of carbon dots from *Gynostemma pentaphyllum* in Example 1. Figure 5 The fluorescence ultraviolet spectrum of the carbon dots of *Gynostemma pentaphyllum* in Example 1; Figure 6 XPS (C, N, O) plot of carbon dots from *Gynostemma pentaphyllum* in Example 1; Figure 7 Images of the carbon dots of *Gynostemma pentaphyllum* under ultraviolet and visible light irradiation in Example 1; Figure 8 This is an image of the application of carbon dots from *Gynostemma pentaphyllum* in a composite film in Example 1, under ultraviolet light. Figure 9 The fluorescence intensity at different temperatures and times in Example 2 is shown on the left and right. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0018] Example 1: A method for preparing carbon dots from *Hymenochloa crus-galli* based on low-temperature baking This embodiment provides a method for preparing biomass carbon dots by low-temperature baking in a conventional oven, using *Gynostemma pentaphyllum* as the raw material. However, the present invention is not limited to this specific biomass precursor.

[0019] First, the raw material of *Hymenochloa crus-galli* is washed and dried at a temperature controlled between 60 and 100°C until most of the moisture is removed. The dried *Hymenochloa crus-galli* is then pulverized using a grinding machine and sieved to obtain uniformly sized biomass powder for later use.

[0020] Subsequently, 100g of *Hypericum perforatum* powder was evenly spread in a high-temperature resistant tray, with the thickness controlled within 0.5cm to ensure uniform heating. The tray was placed in a regular electric oven and heated under normal pressure without the need for inert gas protection. The baking temperature was set to 200–230℃, preferably about 220℃, and maintained for 1–2 hours. Under these temperature conditions, the biomass precursor underwent thermal decomposition and carbonization reactions, forming dark-colored carbonized products.

[0021] After baking, allow the product to cool naturally to room temperature. Then, add the resulting carbonized solid to deionized water for dispersion and sonicate for 30–60 min to promote the stripping and dispersion of carbon dots in the carbonized product. Subsequently, filter or centrifuge the dispersion to remove large particles of impurities that are not fully carbonized. The preferred centrifugation conditions are 8000–12000 rpm for 10–20 min, and collect the supernatant.

[0022] Further, the supernatant was subjected to microfiltration and dialysis purification to remove small molecule impurities and inorganic salts. The microfiltration membrane had a pore size of 0.05–0.45 μm, the dialysis membrane had a molecular weight cutoff of 500 Da, and the dialysis time was 24–72 h, with the dialysis water being changed periodically. After dialysis, a biomass carbon dot solution stably dispersed in the aqueous phase was obtained, which was then freeze-dried to yield approximately 3 g of carbon quantum dots.

[0023] The carbon dots processed using the above method, based on TEM results ( Figure 1 It can be seen that they exhibit an approximately circular or elliptical morphology, with good dispersibility and no obvious agglomeration; the particle size is mainly concentrated in the range of 0.5–3.5 nm. Figure 2 The size distribution is relatively uniform; HRTEM images show ( Figure 1 The illustration shows a lattice spacing of approximately 0.22 nm, corresponding to the (100) crystal plane of graphitic carbon, indicating that it possesses the structural characteristics of graphitized carbon. Infrared spectrum ( Figure 3Analysis results show that its surface is rich in oxygen and nitrogen functional groups, including –OH, –NH, C=O, C–N, and C–O–C. The broad absorption peak at 3200–3500 cm⁻¹ corresponds to the stretching vibrations of –OH and –NH, the peak at approximately 2920 cm⁻¹ is attributed to the stretching vibration of aliphatic C–H, the characteristic peak at approximately 1700 cm⁻¹ can be attributed to the stretching vibration of C=O, while the absorption peaks at 1600–1500 cm⁻¹ and 1400–1300 cm⁻¹ are related to the vibrations of C=C and C–N, respectively. XRD results show ( Figure 4 A broad diffraction peak appears at 2θ≈21.94°, consistent with the interlayer spacing characteristics of graphite, indicating that this peak corresponds to the (002) crystal plane of graphite. This result shows that the synthesized HM-CDs exhibit a low-degree graphitization or amorphous carbon structure, indicating that the carbon dots formed a graphite-like layered structure during the low-temperature carbonization process, thus further verifying the successful preparation of HM-CDs from *Hypericum esculentum*. The UV-vis absorption spectrum shows a strong absorption peak in the ultraviolet region, which can be attributed to the π–π transition of the C=C structure in the carbon core. Simultaneously, an n–π transition absorption shoulder peak related to the surface C=O functional groups appears at a longer wavelength, indicating the presence of abundant luminescence-related functional groups on the carbon dot surface. Furthermore, the PL spectrum of HM-CDs shows that its maximum excitation wavelength (Ex) and emission wavelength (Em) are 373 nm and 493 nm, respectively. Figure 6 XPS spectroscopy ( Figure 6 The results showed that the surface of the carbon dots was mainly composed of C, N, and O elements, with contents of 63.8%, 1.5%, and 34.7%, respectively. C1s spectral analysis revealed C–C / C=C / C–N (≈284.6 eV), sp³ carbon (285.2 eV), C–O / C–OH (286.2 eV), and C=O (287.8 eV) components, indicating that the carbon dot surface possesses both a graphitized structure and oxygen-containing functional groups, providing sites for water solubility and chemical modification. The N1s peak corresponded to pyrrole nitrogen (399.5 eV), graphitic nitrogen (400.8 eV), and pyridine nitrogen (401.6 eV), with pyrrole nitrogen being dominant, indicating that nitrogen is doped into the carbon framework in multiple forms, contributing to enhanced fluorescence and electron transfer properties. The O1s peaks include C–O / C–OH / O–C=O (533.5 eV) and C=O / O–C–N (531.5 eV), indicating that the surface is rich in hydroxyl, carbonyl, and amide functional groups, which is beneficial to water solubility, film compatibility, and antibacterial properties. The prepared carbon dots were irradiated with ultraviolet and visible light. Figure 6 As can be seen, the carbon dots irradiated with 365nm ultraviolet light exhibit blue fluorescence. Figure 7 To apply the prepared carbon dots to the composite film, the composite film also showed blue fluorescence after ultraviolet irradiation, further confirming the synthesis of the carbon dots.

[0024] The carbon dots prepared by the above method have the advantages of low preparation temperature, simple process, low equipment requirements and easy scale-up production, and are suitable for food packaging materials, functional films and fluorescent sensing.

[0025] Example 2: Effect of different baking temperatures and times on the carbon dot fluorescence properties of red ginseng Based on Example 1, this embodiment uses red ginseng as a biomass precursor to study the changes in fluorescence properties of carbon dots obtained under different baking temperatures and baking times, in order to verify the applicability of the method of the present invention and the influence of process parameters on the fluorescence properties of carbon dots.

[0026] First, the red ginseng raw material is washed and dried at a temperature controlled between 60 and 100°C until most of the moisture is removed. The dried red ginseng is then pulverized and sieved to obtain uniformly sized powder. The powder is then evenly spread on a high-temperature resistant tray, with a spreading thickness controlled within 0.5 cm.

[0027] A tray containing red ginseng powder was placed in a regular electric oven for heating. Different combinations of baking temperatures and times were set for the experiment without introducing inert gas. The baking temperatures were set to 180℃, 190℃, 200℃, 210℃, and 220℃, and the baking times were set to 10-100 minutes. After baking, the product was allowed to cool naturally to room temperature. The carbonized product was then processed according to the dispersion, ultrasonication, centrifugation, microfiltration, and dialysis steps described in Example 1 to obtain an aqueous solution of red ginseng carbon dots.

[0028] Fluorescence properties of red ginseng carbon dot samples prepared under different temperature and time conditions were characterized. Fluorescence emission spectra were measured under the same excitation wavelength, and the corresponding changes in fluorescence intensity were recorded. Figure 9 The results showed that baking temperature and time significantly affected the fluorescence properties of the carbon dots in red ginseng: at lower temperatures or shorter times, the degree of carbonization was insufficient, resulting in lower fluorescence intensity of the carbon dots; with increasing baking temperature and time, the fluorescence intensity of the red ginseng carbon dots gradually increased. When the baking temperature was close to 220 °C and the baking time was about 80 min, the obtained red ginseng carbon dots exhibited the strongest fluorescence emission intensity and good photostability; when the temperature was further increased or the baking time was extended to 100 min, the fluorescence intensity of the carbon dots showed a decreasing trend, which is presumably related to the reduction of surface luminescent functional groups due to excessive carbonization.

[0029] The above results show that the fluorescence performance of carbon dots in red ginseng can be effectively controlled by adjusting the baking temperature and time parameters. The low-temperature baking condition of about 200-220℃ has obvious advantages in balancing carbonization efficiency and fluorescence performance, further verifying the feasibility of the method of the present invention in preparing high fluorescence carbon dots under low-temperature conditions.

Claims

1. A green, large-scale method for synthesizing carbon quantum dots, characterized in that, Includes the following steps: 1) The biomass raw material is dried and pulverized to obtain biomass powder; 2) Spread the biomass powder evenly in a high-temperature resistant tray; 3) Under normal pressure, the tray is placed in an oven for heat treatment, so that the biomass powder undergoes thermal decomposition and carbonization reaction in the temperature range of 200–250 °C to obtain carbonized products. 4) Disperse the carbonization product in water, and separate and purify the resulting dispersion to obtain aqueous carbon quantum dots.

2. The method for green, large-scale synthesis of carbon quantum dots according to claim 1, characterized in that, In step 1), the biomass raw material is plant-derived biomass.

3. The method for green, large-scale synthesis of carbon quantum dots according to claim 2, characterized in that, The plant is selected from one or a combination of the following: golden ear grass, red ginseng, American ginseng, black ginseng, pomelo peel, tea leaves, and plant root materials.

4. The method for green, large-scale synthesis of carbon quantum dots according to claim 1, characterized in that, In step 2), the biomass powder is spread to a thickness of 0.5 cm in the tray, and the particle size is 0.5–3.5 nm.

5. The method for green, large-scale synthesis of carbon quantum dots according to claim 1, characterized in that, In step 3), the heat treatment temperature of the biomass powder is preferably 210-230℃.

6. The method for green, large-scale synthesis of carbon quantum dots according to claim 1, characterized in that, In step 3), the heat treatment time is 1-4 hours.

7. The method for green, large-scale synthesis of carbon quantum dots according to claim 1, characterized in that, In step 4), the separation and purification steps include at least one of filtration, centrifugation, microfiltration and dialysis.

8. The method for green, large-scale synthesis of carbon quantum dots according to claim 7, characterized in that, When dialyzing the dispersion, a dialysis membrane with a molecular weight cutoff range of 500-3500 Da is used.

9. A carbon quantum dot, characterized in that, It is prepared by the method described in any one of claims 1-8.

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