Enteromorpha biomass-based strong red light-emitting carbon dots and preparation method and application thereof

Abstract

The application discloses a preparation method and application of Enteromorpha biomass-based strong red light emitting carbon dots, and belongs to the field of biomass-based nanometer fluorescent materials and plant light supplementing technology. The application takes marine waste Enteromorpha as raw material, and after pretreatment, the raw material is placed in a KOH-anhydrous ethanol mixed solvent system, gradient power microwave is adopted for carbonization treatment, and strong red light emitting carbon dots with a fluorescent emission peak in the wavelength range of 630-670 nm are obtained; the gradient power microwave comprises a low-temperature preheating stage and a high-temperature carbonization stage. In the application, natural N and S heteroatoms of Enteromorpha are used for in-situ synergic doping, the N element retention rate is greater than or equal to 80%, and the S element retention rate is greater than or equal to 80%; the microwave reaction time is short, and the preparation period is less than or equal to 26 hours; the quantum yield of the prepared carbon dots is 24.7%-32.8%, and the carbon dots can be directly sprayed on the plant leaf surface for photosynthetic light supplementing.

Description

Technical Field

[0001] This invention belongs to the field of biomass-based nano-fluorescent material preparation and plant supplemental lighting technology. Specifically, it relates to a method for preparing strong red light emitting carbon dots using marine waste seaweed as raw material through gradient microwave synergistic carbonization, and the application of these carbon dots in supplemental lighting for photosynthetic plants in facility agriculture. Background Technology

[0002] The information disclosed in this background section is intended only to enhance some understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art.

[0003] Carbon dots, as novel fluorescent nanomaterials, possess advantages such as excellent optical stability, good biocompatibility, low toxicity, and controllable preparation costs, showing broad application prospects in fields such as bioimaging, fluorescence sensing, photocatalysis, and plant lighting. Plant photosynthesis mainly relies on chlorophyll a and chlorophyll b absorbing red and blue light energy. Among them, red light directly regulates photosynthetic efficiency, dry matter accumulation, and growth cycle, and is the core wavelength for plant supplemental lighting. High quantum yield and strong emission red carbon dots are key materials in the field of plant lighting.

[0004] Current methods for synthesizing red carbon dots mainly include chemical synthesis, hydrothermal synthesis, solvothermal synthesis, and microwave synthesis. Chemical synthesis uses toxic reagents such as nitroaniline, resulting in significant pollution and poor biocompatibility, which contradicts the principles of green agriculture. Hydrothermal / solvothermal methods require high temperatures and pressures, with reaction cycles of 6-24 hours, leading to high energy consumption, poor product dispersibility, and complex purification processes. None of these methods can simultaneously achieve both environmental friendliness and high-efficiency synthesis.

[0005] Although microwave methods are recognized for their rapid reaction and low energy consumption, existing microwave carbonization processes are mainly designed for chemically synthesized raw materials. When applied directly to biomass, there are significant technical obstacles: fixed power mode can easily lead to local overheating of biomass, resulting in over-carbonization or insufficient carbonization. This makes it impossible to effectively retain and dope natural heteroatoms in biomass, and the quantum yield of the product is generally less than 10%. Furthermore, the emission wavelength is concentrated in the blue-green light region, making it difficult to obtain the efficient red light emission required for plant photosynthesis.

[0006] As a marine green algae, *Ulva prolifera* has an extremely high reproductive capacity, easily erupting to form "green tides," causing serious damage to marine ecosystems, fisheries production, and coastal landscapes. Hundreds of thousands of tons of *Ulva prolifera* are harvested annually from the Yellow Sea and East China Sea in my country. Current landfill and incineration methods easily cause secondary pollution and have not achieved high-value utilization. In terms of composition, *Ulva prolifera* is rich in polysaccharides, proteins, and natural nitrogen (N) and sulfur (S) elements, theoretically serving as a high-quality carbon framework and heteroatom source for carbon dot synthesis. However, because N and S heteroatoms easily escape in gaseous form during carbonization, current technologies struggle to effectively retain these heteroatoms, resulting in a natural N / S utilization rate of less than 40%, failing to achieve in-situ synergistic doping effects. Therefore, the transformation of *Ulva prolifera* into high-value-added red-light carbon dot materials has long been unsuccessful.

[0007] The closest existing technology, such as CN105502338A, discloses a method for preparing carbon dots using onions as raw material and fixed-power microwaves. However, this technology still has the following essential differences and shortcomings: (1) The N and S content of onions is much lower than that of Ulva prolifera, making it impossible to achieve N / S synergistic doping; (2) Using fixed-power microwaves (500-800W continuous reaction for 3-6 minutes) without a preheating gradient design easily leads to uneven carbonization; (3) Using dry microwaves without a solvent system makes it difficult to control the functional groups on the surface of carbon dots; (4) The product mainly emits blue-green light, which cannot match the 630-670nm red light band required for plant photosynthesis. The above defects indicate that the existing microwave biomass carbon dot technology has not yet solved the core problems of high-value utilization of Ulva prolifera and preparation of carbon dots with strong red light emission.

[0008] Existing preparation technologies have several key drawbacks: First, biomass utilization efficiency is low, with effective component conversion and utilization rates below 40%. Second, microwave processes are poorly adaptable, and fixed power modes easily lead to over-carbonization or under-carbonization, resulting in quantum yields generally below 10%. Third, synthesis and purification costs are high, reactions are time-consuming, and purification processes are complex, hindering large-scale applications. Fourth, plant supplemental lighting adaptability is insufficient, existing red light materials have low spectral matching, and carbon dot supplemental lighting materials mostly rely on chemical raw materials and LED devices, failing to achieve a closed loop of "biomass-synthesis-application".

[0009] Existing similar technologies all have shortcomings: the non-microwave biomass method has a long reaction cycle and weak red light intensity; the non-biomass microwave method relies on chemical reagents and has poor biocompatibility; the plant-based supplemental lighting carbon dots need to be coated with LED devices, and the luminous efficiency is easily attenuated. None of the above solutions have solved the core problems of high-value utilization of *Ulva prolifera*, efficient and green synthesis of red-light carbon dots, and insufficient adaptability to plant supplemental lighting. Therefore, the key challenge is how to efficiently prepare strong red-light emitting carbon dots using the natural N / S elements of *Ulva prolifera* via microwave method without adding external dopants, and achieve precise matching with the photosynthetic spectrum of plants. Summary of the Invention

[0010] To address the shortcomings of existing technologies, this invention aims to provide a method for preparing biomass-based strong red light emitting carbon dots from Ulva prolifera. Specific objectives include: (1) achieving biomass-microwave synergistic optimization, utilizing the natural N / S ratio of Ulva prolifera. The synergistic doping effect of heteroatoms, combined with gradient microwave power control technology, eliminates the need for external dopants, with N element retention rate of 82.6% and S element retention rate of 81.2%, improving the environmental friendliness of the product and biomass utilization efficiency; (2) Efficiently synthesize red light carbon dots, by optimizing microwave reaction parameters, the reaction is completed within 2 minutes (including preheating), and strong red light emitting carbon dots with quantum yield of 24.7%~32.8%, conductivity <50μS / cm, and emission wavelength of 630-670nm are prepared, precisely matching the red light band required for plant photosynthesis; (3) Simplify the preparation process and reduce costs, by one-step microwave and two-stage purification process, the carbon dots are directly converted from seaweed biomass to high-purity products, with a dissolution rate of 62.8%~69.7%, and the total preparation cycle is shortened to less than 26 hours; (4) Construct a full-process environmental protection system, using marine waste seaweed as raw material, the synthesis process does not add toxic reagents, the product can be naturally degraded, and a closed loop of waste resource utilization, green synthesis and ecological application is realized.

[0011] The technical solution adopted in this invention is as follows: In a first aspect, the present invention provides a method for preparing strong red light emitting carbon dots based on Ulva prolifera biomass, comprising the following steps: using Ulva prolifera as raw material, placing the Ulva prolifera in an alkaline organic solvent system, and performing carbonization treatment using gradient power microwave to obtain carbon dots with fluorescence emission peaks in the wavelength range of 630~670nm; The alkaline organic solvent system is a mixture of an alkaline aqueous solution and an organic solvent, wherein the alkaline aqueous solution is a KOH aqueous solution and the organic solvent is anhydrous ethanol; The gradient power microwave includes a low-temperature preheating stage and a high-temperature carbonization stage. The power of the low-temperature preheating stage is lower than that of the high-temperature carbonization stage. The power of the low-temperature preheating stage is 50~150W and the time is 0.5~3 minutes. The power of the high-temperature carbonization stage is 150~800W and the time is 0.5~6 minutes.

[0012] In this invention, KOH provides an alkaline environment while K... + The ion size is moderate, which is conducive to the intercalation reaction; the dielectric constant and boiling point characteristics of anhydrous ethanol are matched with the microwave gradient power.

[0013] Preferably, the concentration of the KOH aqueous solution is 1.0~5.0M, and the volume ratio of the KOH aqueous solution to anhydrous ethanol is 1:2~2:1.

[0014] More preferably, the concentration of the KOH aqueous solution is 4.0M, and the volume ratio of the KOH aqueous solution to anhydrous ethanol is 1:1.

[0015] Preferably, the power of the low-temperature preheating stage is 80W and the time is 1 minute, and the power of the high-temperature carbonization stage is 240W and the time is 1 minute.

[0016] Preferably, the nitrogen retention rate and sulfur retention rate of the carbon dots derived from seaweed are ≥80%.

[0017] Preferably, the quantum yield of the carbon dots is 24.7% to 32.8%.

[0018] Preferably, the process also includes a pretreatment step for the seaweed: washing with a deionized water-ethanol mixture, ultrasonic treatment, drying, grinding, and sieving.

[0019] Secondly, the present invention provides a strong red light emitting carbon dot prepared by the method described in the first aspect.

[0020] Thirdly, the application of the carbon dots described in the second aspect in supplemental lighting for plant photosynthesis is provided.

[0021] Preferably, the application involves spraying the carbon dots directly onto the plant leaves.

[0022] Compared with the related technologies known to the inventors, one of the technical solutions of the present invention has the following beneficial effects: Maximizing biomass utilization efficiency: Fully activating the synergistic doping effect of natural N and S heteroatoms in Ulva prolifera without the need for additional chemical dopants, the N element retention rate is 82.6% and the S element retention rate is 81.2%, and the conversion rate of effective biomass components reaches 62.8%~69.7%, realizing a green synthesis mode based on biomass and supplemented by chemical reagents, and significantly reducing the environmental risks of the products.

[0023] 2. Significant advantages in synthesis efficiency and energy consumption: The entire microwave reaction process (including preheating) takes only 2 minutes, which significantly improves efficiency and reduces energy consumption. Furthermore, it eliminates the need for complex temperature control equipment, further reducing production energy consumption. At the same time, the purification process is simplified, shortening the total preparation cycle to less than 26 hours, reducing production costs, and possessing the potential for large-scale production.

[0024] 3. Excellent product performance and strong adaptability: The prepared carbon dots have strong red light emission characteristics in the 630-670 nm wavelength range, with the maximum emission intensity at 665 nm under 365 nm light source excitation and a quantum yield ≥24.7%; the carbon dot solution has good dispersibility and high spectral matching with the photosynthetic requirements of plants, and can achieve supplemental lighting effect by direct spraying without relying on auxiliary devices such as LEDs.

[0025] 4. Significant environmental and economic benefits: Using marine waste seaweed as raw material, hundreds of thousands of tons of seaweed waste can be consumed annually, solving the problem of secondary pollution caused by traditional treatment methods; the product can be widely used in facility agriculture plant supplemental lighting, greenhouse vegetable cultivation and other fields, and can be directly sprayed for plant photosynthesis supplemental lighting to improve plant photosynthetic efficiency and achieve quality improvement and yield increase to a certain extent. Attached Figure Description

[0026] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0027] Figure 1 The image shows the emission of carbon dots under a 365nm excitation source. Figure 2 Transmission electron microscopy (TEM) and particle size distribution of carbon dots; Figure 3 The X-ray diffraction (XRD) pattern of carbon dots; Figure 4 X-ray photoelectron spectroscopy (XPS) images of the *Ulva prolifera* sample and after carbon dots were formed; Figure 5 The image shows the UV-Vis spectrophotometry of carbon dots. Figure 6 The fluorescence spectrum (PL) of the carbon dots is shown. Figure 7 The emission patterns of carbon dots formed with different KOH concentrations under a 365 nm excitation source; Figure 8 UV-Vis images of carbon dots formed with different KOH concentrations; Figure 9 PL diagrams of carbon dots formed at different KOH concentrations; Figure 10 Fourier transform infrared (FTIR) spectra of carbon dots formed with different KOH concentrations; Figure 11 The emission patterns of carbon dots formed by different microwave powers under a 365nm excitation source; Figure 12 UV-Vis images of carbon dots formed at different microwave powers; Figure 13 PL plots of carbon dots formed at different microwave powers; Figure 14 FTIR images of carbon dots formed at different microwave powers; Figure 15 The emission patterns of carbon dots formed at different reaction times under a 365nm excitation source; Figure 16UV-Vis images of carbon dots formed at different reaction times; Figure 17 PL plots of carbon dots formed at different reaction times; Figure 18 The emission patterns of carbon dots formed in different solvents under a 365nm excitation source; Figure 19 UV-Vis images of carbon dots formed in different solvents; Figure 20 PL diagrams of carbon dots formed in different solvents; Figure 21 This is the emission diagram of Comparative Example 1 under a 365nm excitation source; Figure 22 This is the UV-Vis plot for Comparative Example 1; Figure 23 The PL diagram is for Comparative Example 1; Figure 24 The image shows the emission of Comparative Example 2 under a 365nm excitation source. Detailed Implementation

[0028] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0029] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, and / or combinations thereof.

[0030] In some typical embodiments of the present invention, a method for preparing strong red light emitting carbon dots based on *Ulva prolifera* biomass is provided, comprising the following steps: 1. Pretreatment of *Ulva prolifera* biomass: Fresh *Ulva prolifera* was selected and washed with a deionized water-ethanol mixture (volume ratio 3:1) at a solid-liquid ratio of 1:10. The mixture was then ultrasonically treated at 80 W for 15 minutes, repeated three times. This process, through the synergistic effect of ethanol and water, effectively removes salt, mud, and other impurities adhering to the surface of the *Ulva prolifera*, while avoiding protein dissolution caused by simple water washing. This preserves the natural N and S elements in the *Ulva prolifera*, laying the foundation for subsequent heteroatom doping. The washed *Ulva prolifera* was then dried in a 60℃ oven for 2 hours until constant weight, and ground until it passed through a 200-mesh sieve to remove coarse fibrous impurities, enhance the powder's microwave absorption efficiency, and avoid uneven local carbonization caused by uneven microwave reflection.

[0031] 2. Construction of the microwave reaction system: Weigh 0.1~0.2g of pretreated *Ulva prolifera* powder and add 10~30mL of a 1.0~5.0M KOH-anhydrous ethanol mixed solution (KOH solution to anhydrous ethanol volume ratio 0~1:0~1, excluding 0) as a polar solvent. Stir magnetically for 5~10 minutes until homogeneous. The KOH-anhydrous ethanol mixed solution not only provides an alkaline reaction environment, promoting the activation of functional groups such as hydroxyl and amino groups in *Ulva prolifera* molecules, but also enhances the dielectric constant of the reaction system, improves the microwave coupling effect, and accelerates molecular vibration and reaction process.

[0032] 3. Gradient Microwave Carbonization Reaction: The mixed system is placed in a microwave reactor and the reaction is carried out in gradient power mode: First, it is preheated at 80W for 1 minute to allow the *Ulva prolifera* polysaccharide molecules to fully swell in the polar solvent, providing a uniform reaction environment for the subsequent carbonization reaction; then, the power is switched to 160~800W for 0~6 minutes (excluding 0). Under effective microwave power, dehydration condensation, cyclization and conjugation of *Ulva prolifera* molecules can be promoted, and a stable C=N / SC conjugated system is finally constructed to ensure red light emission performance; during the reaction, the temperature is automatically locked at 60~200℃ by the solvent boiling point, eliminating the need for additional temperature control equipment and avoiding excessive decomposition of biomass or insufficient carbonization.

[0033] 4. Product Purification: Transfer the product from the microwave reaction to a centrifuge tube and centrifuge at 8000-10000 rpm for 10-20 minutes to quickly remove unreacted *Ulva prolifera* residue, preventing residue from adhering to plant leaves and affecting absorption. Take the supernatant and permeate it through a 0.02 μm aqueous PES membrane. Then, take 5-15 mL and inject it into a 1000 Da dialysis bag. Dialyze with 500-1000 mL of deionized water for 6-12 hours, changing the deionized water 2-6 times during this period to effectively remove small molecule metabolites and potassium from the reaction system. + Impurities such as carbon dots are removed to reduce the conductivity of the carbon dot solution to <50μS / cm, ultimately yielding high-purity, high-red-light-emitting carbon dots.

[0034] 5. Preparation of powder samples: The prepared carbon dot solution was placed in a freeze dryer and freeze-dried for 6 hours to obtain solid powder.

[0035] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0036] Example 1: A method for preparing strong red light emitting carbon dots based on *Ulva prolifera* biomass, the specific steps of which are as follows: Select 50g of fresh seaweed, wash it with a mixture of deionized water and ethanol (volume ratio 3:1) at a solid-liquid ratio of 1:10, sonicate at 80W for 15 minutes, repeat 3 times; dry in an oven at 60℃ to constant weight, grind, sieve through a 200-mesh sieve, and collect the seaweed powder that passes through the sieve for later use.

[0037] First, weigh 0.1g of *Ulva prolifera* powder and add 20mL of a 4.0M KOH-anhydrous ethanol mixed solution (KOH solution to anhydrous ethanol volume ratio of 1:1). Stir magnetically for 10 minutes until homogeneous. Then, place the mixture into a microwave reactor, preheat at 80W for 1 minute, and then react at 240W for 1 minute. The reaction temperature is locked at 78.2℃ by the solvent boiling point.

[0038] Next, the reacted solution was centrifuged at 10,000 rpm for 15 minutes, and the supernatant was injected into a 1000 Da dialysis bag. Dialysis with deionized water was performed for 12 hours, with the deionized water changed 6 times, to obtain a carbon dot solution with a concentration of 3.56 mg / mL. Subsequently, the solution was freeze-dried for 6 hours to obtain a solid carbon dot powder.

[0039] Testing showed that the carbon dots prepared in this embodiment had a dissolution rate of 69.7%, an emission wavelength of 655 nm, and a quantum yield of 32.8%. When the KOH concentration reached 4.0 M, the degree of carbonization and the formation of conjugated structures reached an optimal balance, significantly improving the quantum yield. The solution conductivity was 24.7 μS / cm, and it exhibited significant strong red light emission under 365 nm ultraviolet light irradiation. Figure 1 The carbon dots exhibited good dispersibility. Surprisingly, when KOH-anhydrous ethanol mixed solvent was used in conjunction with gradient microwaves, the resulting carbon dots showed extremely strong red light emission characteristics, and their quantum yield was significantly higher than that of the single solvent system (Examples 19-20) and the constant power mode (Comparative Example 2). This synergistic effect was difficult to predict from the prior art.

[0040] TEM and particle size distribution ( Figure 2 The figure shows carbon quantum dots ranging from 0.21 to 0.22 nm, indicating the successful preparation of carbon dots. XRD ( Figure 3 The figure shows that the carbon dots have a broad peak at 34.2°, indicating that the carbon dots are well carbonized.

[0041] Combination Figure 4XPS full spectrum and high-resolution spectral analysis of (a~f) show that both the raw material of Ulva prolifera and the prepared carbon dots (CDs) have characteristic peaks of C 1s, O 1s, N 1s, and S 2p, indicating that magnesium-containing components such as chlorophyll in Ulva prolifera have been effectively removed during carbonization. The red light emission of the carbon dots is due to the fluorescence characteristics of the carbon dots themselves rather than the residual interference of chlorophyll in the raw material. The C 1s spectrum shows that both *Ulva prolifera* and carbon dots contain CC / C=C (284.8 eV), CN / CO (~286 eV), and C=O (~288 eV) functional groups. The carbon dot peaks are more concentrated and regular, indicating that a stable carbon skeleton was formed after carbonization and the CN bonds were preserved. In the O 1s spectrum, carbon dots retain CO / OH and C=O functional groups, ensuring the water solubility of the material. In the N 1s spectrum, both *Ulva prolifera* and carbon dots contain graphitic N (401.5 eV) and pyrrolic N (399.8 eV). In the S 2p spectrum, carbon dots retain the original -SO3- of *Ulva prolifera*. - Based on the peaks of 169.8 eV and O=S=O (168.2 eV), a new peak of CS / SH (163.8 eV) was added, indicating that N and S heteroatoms were successfully incorporated into the carbon framework in the form of covalent bonds, forming a conjugated structure that is conducive to red light emission. Elemental content comparison calculations showed that the N element retention rate reached 82.6% and the S element retention rate reached 81.2%, indicating that the natural N and S heteroatoms of *Ulva prolifera* were not significantly lost during the gradient microwave carbonization process, achieving efficient in-situ doping.

[0042] Table 1. XPS elemental content data of carbon dots sample C N S Enteromorpha powder 75.24% 6.35% 5.70% Example 1 Carbon Dots 73.13% 5.25% 4.63% Proportion 97.20% 82.6% 81.2% Figure 5 It exhibits a significant π-π* transition characteristic absorption peak around 365 nm, corresponding to sp in the carbon framework. 2 The conjugated electronic transition of hybrid carbon, the absorption peak of which is highly matched with the subsequent fluorescence excitation source (365nm), provides an optical basis for efficient excitation of red light emission; at the same time, no obvious absorption is observed in the 630~670nm red light band, avoiding the interference of self-absorption on fluorescence emission and ensuring the high efficiency of carbon dot red light emission.

[0043] Figure 6 Photofluorescence spectroscopy showed that the carbon dots exhibited a strong and broad red light emission peak in the 630-670 nm band, with a maximum emission wavelength of 655 nm, which precisely matched the red light absorption band (630-660 nm) required for plant photosynthesis, demonstrating excellent adaptability to plant supplemental lighting. The emission peak had a narrow half-width and symmetrical peak shape, indicating good uniformity of carbon dot luminescence, and no obvious blue shift or red shift, reflecting the excellent optical stability of carbon dots prepared by the gradient microwave carbonization process.

[0044] Example 2: In Example 2, the concentration of KOH in Example 1 was changed to 1.0M, and other substances were prepared using the same method for carbon dot preparation.

[0045] Example 3: In Example 3, the concentration of KOH in Example 1 was changed to 2.0M, and other substances were prepared using the same method for carbon dot preparation.

[0046] Example 4: In Example 4, the concentration of KOH in Example 1 was changed to 3.0M, and other substances were prepared using the same method for carbon dot preparation.

[0047] Example 5: In Example 5, the concentration of KOH in Example 1 was changed to 5.0M, and other substances were prepared using the same method for carbon dot preparation.

[0048] The quantum yields of Examples 2, 3, 4, and 5 were 25.1%, 25.9%, 26.2%, and 29.3%, respectively; the electrical conductivities were 25.7 μS / cm, 24.8 μS / cm, 28.3 μS / cm, and 29.1 μS / cm, respectively; and the dissolution rates were 63.1%, 63.4%, 62.5%, and 64.3%, respectively.

[0049] like Figure 7 As shown, the emission intensity continuously increases with increasing KOH concentration, reaching a peak at 4.0 M; however, when the concentration continues to rise to 5.0 M, the emission intensity decreases, demonstrating the dual influence of KOH concentration on the degree of carbonization and the formation of conjugated structures.

[0050] like Figure 8 As shown, the UV-Vis absorption spectroscopy results indicate that increasing the KOH concentration effectively promotes the π-π* transition absorption of carbon dots around 365 nm. With increasing alkali concentration, the absorption peak intensity of carbon dots at this wavelength gradually increases, reflecting the gradual improvement of the conjugated structure of the carbon skeleton and the sp... 2 Increased carbon content.

[0051] like Figure 9 As shown in the fluorescence emission spectrum, the red light emission intensity of the carbon dots first increases and then decreases with the change of KOH concentration. The fluorescence intensity is the highest and the luminescence performance is the best under the 4.0M KOH system. When the concentration is too low, carbonization is insufficient, and when the concentration is too high, over-carbonization is easily caused, both of which will lead to a decrease in red light emission intensity.

[0052] like Figure 10 As shown, infrared spectroscopy analysis confirmed that changes in KOH concentration directly affect the composition and content of functional groups on the carbon dot surface. As the alkali concentration increases, the intensity of characteristic absorption peaks of oxygen-containing functional groups such as CO bonds first increases and then tends to stabilize, indicating that a suitable KOH concentration is beneficial to retaining hydrophilic functional groups and improving the water solubility and dispersibility of carbon dots.

[0053] Example 6: In Example 6, the reaction power of 240W in Example 1 was changed to 80W, and other substances were prepared using the same method for carbon dot preparation.

[0054] Example 7: In Example 7, the reaction power of 240W in Example 1 was changed to 160W, and other substances were prepared using the same method for carbon dot preparation.

[0055] Example 8: In Example 8, the reaction power of 240W in Example 1 was changed to 320W, and other substances were prepared using the same method to prepare carbon dots.

[0056] Example 9: In Example 9, the reaction power of 240W in Example 1 was changed to 400W, and other substances were prepared using the same method for carbon dot preparation.

[0057] Example 10: In Example 10, the reaction power of 240W in Example 1 was changed to 480W, and other substances were prepared using the same method for carbon dot preparation.

[0058] Example 11: In Example 11, the reaction power of 240W in Example 1 was changed to 560W, and other substances were prepared using the same method to prepare carbon dots.

[0059] Example 12: In Example 12, the reaction power of 240W in Example 1 was changed to 640W, and other substances were prepared using the same method for carbon dot preparation.

[0060] The quantum yields of Examples 6, 7, 8, 9, 10, 11, and 12 were measured to be 29.7%, 31.2%, 28.2%, 28.1%, 27.6%, 26.4%, and 26.0%, respectively; the electrical conductivities were 29.1 μS / cm, 29.2 μS / cm, 30.3 μS / cm, 30.4 μS / cm, 27.4 μS / cm, 28.9 μS / cm, and 29.6 μS / cm, respectively; and the dissolution rates were 67.5%, 68.4%, 69.2%, 67.8%, 67.0%, 66.3%, and 65.7%, respectively.

[0061] like Figure 11 As shown, different microwave powers significantly modulate the red light emission intensity of carbon dots. Under 365nm excitation, as the microwave power increases from 80W to 640W, the brightness of the red light of carbon dots first increases and then decreases. The luminescence intensity reaches its peak at 240W, exhibiting the strongest red light emission. Too low a power results in insufficient carbonization, while too high a power easily leads to over-carbonization, both of which reduce the luminescence performance.

[0062] like Figure 12As shown in the UV-Vis absorption spectrum, the absorption intensity of the π-π* transition at 365 nm of carbon points first increases and then decreases with increasing microwave power, with the strongest absorption signal at 240 W, indicating that the sp* transition in the carbon framework is at this power. 2 The formation of hybrid carbon and conjugated structure is most complete.

[0063] like Figure 13 As shown, the fluorescence emission spectrum indicates that the red light emission wavelength of the carbon dots is concentrated in the range of 645~668nm under different microwave powers, which all match the red light band required for plant photosynthesis. Moreover, the emission intensity first increases and then decreases with the change of power, and the red light emission efficiency is the highest and the fluorescence signal is the most stable at 240 W.

[0064] like Figure 14 As shown, the infrared spectroscopy results indicate that microwave power directly affects the formation and content of functional groups such as CO, OH, and CH on the surface of carbon dots. Under the condition of 240W, the characteristic peak intensity of functional groups is moderate and the proportion is optimal, which not only ensures the water solubility and dispersibility of carbon dots, but also facilitates the stable construction of red light conjugated structures.

[0065] Example 13: In Example 13, the reaction time of 240W for 1 minute in Example 1 was changed to 2 minutes, and other substances were prepared by the same method to prepare carbon dots.

[0066] Example 14: In Example 14, the reaction time of 240W for 1 minute in Example 1 was changed to 3 minutes, and other substances were prepared by the same method to prepare carbon dots.

[0067] Example 15: In Example 15, the reaction time of 240W for 1 minute in Example 1 was changed to 4 minutes, and other substances were prepared by the same method to prepare carbon dots.

[0068] Example 16: In Example 16, the reaction time of 240W for 1 minute in Example 1 was changed to 5 minutes, and other substances were prepared by the same method to prepare carbon dots.

[0069] Example 17: In Example 17, the reaction time of 240W for 1 minute in Example 1 was changed to 6 minutes, and other substances were prepared by the same method to prepare carbon dots.

[0070] Example 18: In Example 18, the reaction time of 240W for 1 minute in Example 1 was changed to 0 minutes, and other substances were prepared using the same method to prepare carbon dots.

[0071] The quantum yields of Examples 13, 14, 15, 16, 17, and 18 were 31.9%, 30.7%, 30.1%, 28.2%, 27.7%, and 26.5%, respectively; the electrical conductivities were 30.2 μS / cm, 31.5 μS / cm, 32.6 μS / cm, 31.2 μS / cm, 29.2 μS / cm, and 28.7 μS / cm, respectively; and the dissolution rates were 68.4%, 66.5%, 65.4%, 64.6%, 64.2%, and 63.5%, respectively.

[0072] like Figure 15 As shown, different microwave reaction times directly determine the red light emission effect of carbon dots. Under 365nm excitation, as the reaction time increases from 0 min to 6 min, the red light brightness of carbon dots first increases rapidly and then gradually decreases. The red light emission is brightest at 1 min. At 0 min, carbonization is not completed and there is no obvious red light. After 1 min, excessive carbonization leads to a continuous decrease in luminescence.

[0073] like Figure 16 As shown, the UV-Vis absorption spectroscopy indicates that the microwave reaction time significantly modulates the absorption intensity of the π-π* transition of carbon dots at 365 nm. The absorption intensity first increases and then decreases with time, reaching its strongest peak at 1 min, indicating that the sp transition of the carbon skeleton at this time... 2 The conjugated structure is most fully formed and the degree of carbonization is optimal.

[0074] like Figure 17 As shown in the fluorescence emission spectrum, the red light emission wavelength of the carbon dots remained stable at 655~667 nm under different microwave reaction times, which all met the red light requirements of plant photosynthetic supplementation. The emission intensity first increased and then decreased with the reaction time, and the fluorescence intensity was the highest at 1 min. Excessive reaction time would destroy the luminescent conjugate structure and reduce the red light efficiency.

[0075] Example 19: In Example 19, the volume ratio of KOH solution to anhydrous ethanol in Example 1 was changed from 1:1 to 1:0, and other substances were prepared using the same method for carbon dot preparation.

[0076] Example 20: In Example 20, the volume ratio of KOH solution to anhydrous ethanol in Example 1 was changed from 1:1 to 0:1, and other substances were prepared using the same method for carbon dot preparation.

[0077] The quantum yields of Examples 19 and 20 were 16.8% and 11.2%, respectively; the electrical conductivityes were 32.9 μS / cm and 32.4 μS / cm, respectively; and the dissolution rates were 69.2% and 67.4%, respectively.

[0078] like Figure 18As shown, different solvent systems have a significant impact on the red light emission intensity of carbon dots. Under 365 nm excitation, carbon dots prepared by KOH-anhydrous ethanol mixed solvent (1:1) exhibit the brightest red light emission, followed by the pure water system. The red light brightness of anhydrous ethanol as the single solvent is the weakest, indicating that mixed solvents are more conducive to the carbonization of Ulva prolifera and the construction of conjugated luminescent structures.

[0079] like Figure 19 As shown in the UV-Vis absorption spectroscopy, there are significant differences in the absorption intensity of the π-π* transition of carbon dots at 365 nm under different solvent systems. The mixed solvent group has the strongest absorption peak, followed by the pure water group, and the anhydrous ethanol group has the weakest absorption peak. This reflects that the mixed solvent can more effectively promote the sp transition of carbon skeletons. 2 The formation of hybrid carbon and conjugated structures.

[0080] like Figure 20 As shown in the fluorescence emission spectra, the carbon dots prepared by the three solvent systems all emitted in the red light band of 648–655 nm. Among them, the mixed solvent group had the highest fluorescence emission intensity and the best luminescence performance, while the emission intensity of the pure water and anhydrous ethanol single solvent groups was significantly reduced. This proves that the KOH-anhydrous ethanol mixed solvent is the optimal solvent system for preparing Ulva prolifera-based red-light carbon dots. In addition, the carbon dot solution prepared by the mixed solvent system has better dispersibility and storage stability, making it more suitable for plant supplemental lighting applications.

[0081] Example 21: In Example 21, the volume of the 20 mL 4.0 M KOH-anhydrous ethanol mixed solution in Example 1 was changed to 16 mL, and the carbon dots of other substances were prepared in the same way.

[0082] Example 22: In Example 22, the volume of the 20 mL 4.0 M KOH-anhydrous ethanol mixed solution from Example 1 was 12 mL, and the other substances were prepared using the same method to prepare carbon dots.

[0083] Example 23: In Example 23, the volume of the 20 mL 4.0 M KOH-anhydrous ethanol mixed solution from Example 1 was 8 mL, and the other substances were prepared using the same method to prepare carbon dots.

[0084] Example 24: In Example 24, the volume of the 20 mL 4.0 M KOH-anhydrous ethanol mixed solution from Example 1 was 4 mL, and the other substances were prepared using the same method to prepare carbon dots.

[0085] Comparative Example 1: In Comparative Example 1, the seaweed in Example 1 was replaced with laver, and the carbon dots of other substances were prepared in the same way.

[0086] Tests showed that the quantum yield of Comparative Example 1 was 12.4%, and the dissolution rate was 20.6%.

[0087] like Figure 21 As shown, Comparative Example 1 (porphyria-based carbon dots) exhibits blue fluorescence under 365 nm ultraviolet light excitation, which is significantly different from the red light emission characteristics of the porphyria-based carbon dots.

[0088] like Figure 22 As shown, the UV-Vis absorption spectrum of Comparative Example 1 (porphyrin carbon dots) shows a weak absorption peak at 365 nm, and the absorption intensity gradually decreases with increasing wavelength.

[0089] like Figure 23 As shown, Comparative Example 1 (porphyria-based carbon dots) exhibits a fluorescence emission peak in the ~450 nm blue light band under 365 nm excitation, with no obvious red light emission, showing a significant difference in red light emission performance compared to the porphyria-based carbon dots.

[0090] Comparative Example 2: Comparative Example 2 changed the gradient power mode in Example 1 to a constant 240W power (the preheating stage was canceled, and the reaction was carried out directly at 240W for 2 minutes), while other conditions were the same as in Example 1.

[0091] like Figure 24 As shown, Comparative Example 2 exhibits weaker red absorption compared to Example 1. The carbon dots prepared in this comparative example emitted light at a wavelength of 662 nm, with a quantum yield of 26.5%, lower than the 32.8% of Example 1. This indicates that the gradient power mode (preheating + carbonization) achieves higher quantum yield and better red light emission performance compared to the constant power mode.

[0092] Application example: Used for photosynthetic supplementation of lettuce seedlings, a typical greenhouse crop. 1. Experimental Materials Test plants: Lettuce seedlings (variety: fast-growing lettuce) with uniform growth and in the three-leaf-one-heart stage were selected as test materials.

[0093] Test carbon dots: Take the strong red light emitting carbon dot solution of Ulva prolifera biomass prepared in Example 1 of this invention and dilute it with deionized water to the experimental design concentration (concentration: 50 mg / L).

[0094] 2. Experimental Methods Application method: Foliar spraying, with a dosage of 5 mL per plant, ensuring even coverage on both sides of the leaves. Spray once every 7 days, for a total of 3 treatments.

[0095] Cultivation conditions: All tested plants were placed in an artificial climate chamber with identical environments. Basic white light illumination was provided by LED plant growth panels with a spectral range of 400-700 nm and a photon flux density (PPFD) of 100 μmol·m⁻²·s⁻¹, with a photoperiod of 12 hours light / 12 hours darkness to simulate natural light conditions. Temperature was controlled at 25 ± 2°C (day) / 18 ± 2°C (night), and relative humidity was 60-70%.

[0096] Processing settings: Blank control group (CK): Sprayed with an equal amount of deionized water, without any supplemental lighting (only receiving basic white light illumination).

[0097] Traditional LED supplemental lighting system (LED): Spray an equal amount of deionized water, and in addition to receiving basic white light illumination, use commercial plant supplemental LED lights (red light wavelength 655 nm ± 10 nm) for supplemental lighting. The additional photon flux density provided is 52 μmol·m⁻²·s⁻¹, and the illumination duration is 12 hours / day.

[0098] Carbon dot supplemental lighting (CDs): Spray with 50 mg / L carbon dot solution, without providing additional artificial light source (only accepts basic white light illumination).

[0099] Sampling and measurement: Seven days after the last treatment, the second to third functional leaves from the top of each treatment group were selected for various index measurements. Six plants were set up for each treatment.

[0100] Measurement Indicators and Methods Table 2. Measurement Indicators and Methods for Lettuce Seedlings

[0101] 4. Results and Discussion Effects on seedling growth: As shown in Table 3, compared with the blank control group (CK), the growth indicators of lettuce seedlings, such as plant height, stem diameter, and aboveground dry weight, were significantly increased by 35.0%, 38.6%, and 39.3% respectively after spraying with the carbon dots of this invention (CDs group). Compared with the traditional LED supplemental lighting group, the CDs group also showed better results in plant height, stem diameter, and aboveground dry weight. The results indicate that carbon dot spraying can effectively promote plant vegetative growth.

[0102] Effects on photosynthetic efficiency: The chlorophyll content and net photosynthetic rate (Pn) of the carbon dot treatment group were significantly higher than those of the CK group. P< 0.05). Specifically, the chlorophyll a+b content in the CDs group reached 2.31 mg / g FW, an increase of 26.9% compared to the CK group. Simultaneously, the Fv / Fm value (0.83) was also higher than that of the CK group (0.78), indicating that carbon dot treatment helps maintain a high light energy conversion efficiency. This advantage is directly related to the strong red light emission characteristics of carbon dots in the 630-670 nm range, a wavelength that can be efficiently absorbed by chlorophyll, thus compensating for the deficiency of red light components in natural light.

[0103] Impact on quality and yield: The soluble sugar content of lettuce seedlings in the CDs group accumulated significantly, reaching 12.4 mg / g, which was 45.9% higher than that in the CK group, indicating that carbon point spraying is beneficial to improving the nutritional quality of lettuce seedlings. At the same time, the yield per plant (fresh weight) reached 12.81 g / plant, which was 56.1% higher than that in the CK group and 11.3% higher than that in the LED group, showing a good yield-increasing effect.

[0104] Table 3 Effects of different treatments on growth and photosynthetic parameters of lettuce seedlings

[0105] 5. Conclusion The *Ulva prolifera*-based high-red-light-emitting carbon dots prepared in Example 1 of this invention, through simple foliar spraying (50 mg / L, once every 7 days, for a total of 3 sprays), can effectively improve the photosynthetic efficiency of greenhouse lettuce seedlings, resulting in significant quality and yield improvements. Compared with the blank control, the net photosynthetic rate increased by 38.9%, and the aboveground dry weight increased by 39.3%; compared with traditional LED supplemental lighting, the net photosynthetic rate increased by 8.7%, and the yield per plant increased by 11.3%; the overall effect is superior to the traditional LED supplemental lighting solution. This application method avoids the complex integration of LED supplemental lighting devices, providing a low-cost, easy-to-operate, green, and efficient new plant supplemental lighting strategy for greenhouse agriculture.

[0106] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing strong red light emitting carbon dots based on *Ulva prolifera* biomass, characterized in that, Includes the following steps: Using *Ulva prolifera* as raw material, the *Ulva prolifera* is placed in an alkaline organic solvent system and carbonized using gradient power microwaves to obtain carbon dots with fluorescence emission peaks in the wavelength range of 630-670 nm. The alkaline organic solvent system is a mixture of an alkaline aqueous solution and an organic solvent, wherein the alkaline aqueous solution is KOH aqueous solution and the organic solvent is anhydrous ethanol. The gradient power microwaves include a low-temperature preheating stage and a high-temperature carbonization stage, wherein the power of the low-temperature preheating stage is lower than that of the high-temperature carbonization stage. The power of the low-temperature preheating stage is 50-150 W and the time is 0.5-3 minutes, while the power of the high-temperature carbonization stage is 150-800 W and the time is 0.5-6 minutes.

2. The preparation method according to claim 1, characterized in that, The concentration of the KOH aqueous solution is 1.0~5.0M, and the volume ratio of the KOH aqueous solution to anhydrous ethanol is 1:2~2:

1.

3. The preparation method according to claim 2, characterized in that, The concentration of the KOH aqueous solution is 4.0 M, and the volume ratio of the KOH aqueous solution to anhydrous ethanol is 1:

1.

4. The preparation method according to claim 1, characterized in that, The power of the low-temperature preheating stage is 80W and the time is 1 minute, while the power of the high-temperature carbonization stage is 240W and the time is 1 minute.

5. The preparation method according to claim 1, characterized in that, The carbon dots contain ≥80% nitrogen and ≥80% sulfur derived from Ulva prolifera.

6. The preparation method according to claim 1, characterized in that, The quantum yield of the carbon dots is 24.7% to 32.8%.

7. The preparation method according to claim 1, characterized in that, It also includes a pretreatment step for the seaweed: washing with a deionized water-ethanol mixture, ultrasonic treatment, drying, grinding, and sieving.

8. The strong red light emitting carbon dots prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the carbon dots as described in claim 8 in supplemental lighting for plant photosynthesis.

10. The application according to claim 9, characterized in that, The carbon dots are sprayed directly onto the plant leaves.

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