Method for promoting microalgae carbon sequestration by using polyethylene glycol passivated nitrogen-doped carbon dot conversion spectrum

Abstract

The invention relates to a microalgae photosynthetic carbon sequestration technology, and aims to provide a method for promoting microalgae carbon sequestration by a polyethylene glycol passivated nitrogen-doped carbon dot conversion spectrum. The method comprises the following steps: passivating the surfaces of nitrogen-doped carbon dots by using polyethylene glycol, and then directly adding the passivated nitrogen-doped carbon dots into a microalgae suspension, ultraviolet light with the wavelength of 280-390 nm in external light is converted into blue light with the wavelength of 400-500 nm by passivating the nitrogen-doped carbon dots so as to be directly utilized by microalgae, so that growth and carbon sequestration are promoted while the stress resistance of algae cells is improved. According to the method, damage to algae cells can be greatly reduced by converting an ultraviolet spectrum, and meanwhile, microalgae growth and carbon sequestration can be further promoted by converting ultraviolet light into blue light; the spectrum conversion effect is enhanced, the overall light energy utilization efficiency of microalgae carbon sequestration is improved, light inhibition is relieved, and the stress resistance of microalgae cells is improved, so that microalgae growth carbon sequestration is promoted, and the development of the microalgae carbon sequestration industry is promoted.

Description

Technical Field

[0001] This invention relates to the field of microalgal photosynthetic carbon fixation technology, and particularly to a method for promoting microalgal carbon fixation by using polyethylene glycol to passivate the nitrogen-doped carbon point conversion spectrum. Background Technology

[0002] Microalgae, as the oldest organisms on Earth, possess advantages such as high photosynthetic efficiency, extremely rapid reproduction rate, strong environmental adaptability, and ease of integration with other engineering technologies. Using industrial waste gas as a CO2 source in microalgae cultivation demonstrates high feasibility in both economic benefits and technical implementation. This significant advantage makes microalgae cultivation a promising area for the resource utilization of industrial waste gas.

[0003] Microalgal carbon fixation technology not only contributes to achieving carbon neutrality but also generates biomass energy. However, its limited photosynthetic efficiency restricts the biomass yield of microalgal biomass. Microalgal photosynthesis mainly involves chlorophyll absorption of red and blue light, accounting for approximately 43% of incident sunlight. The narrow absorption band of microalgae in the solar spectrum and the shading effect between microalgae result in energy utilization efficiency far below theoretical values. Currently, materials science and bioengineering have explored many strategies to improve biomass yield and photosynthetic efficiency while minimizing light energy loss. Photonics engineering increases the biomass yield and photosynthetic efficiency of microalgae by improving and optimizing light energy utilization efficiency. Some key applications of photonics engineering include: using fluorescent and phosphorescent materials to convert specific wavelengths in sunlight to better match the spectrum required for microalgae growth, while removing wavelengths that are detrimental to microalgae growth, mitigating photoinhibition, reducing ineffective energy loss, thereby improving light energy utilization, promoting microalgae cell growth and lipid accumulation. Overcoming the physical limitations of light energy utilization has become a key scientific issue in improving the efficiency of large-scale microalgae cultivation.

[0004] Studies have shown that carbon dots, as promising light-harvesting nanomaterials, exhibit fluorescence emission spectra that highly match the light absorption spectra of plant photosynthesis, primarily concentrated in the visible light region. Quantum dots, due to their unique optical, electrical, and magnetic properties, have demonstrated broad application potential in various fields in recent years; for example, the interaction between nanomaterials and microalgae has significantly impacted the growth, metabolism, and photosynthetic efficiency of microalgae.

[0005] Despite this, most nanomaterials still suffer from unstable fluorescence emission and quenching, limiting their practical applications. Exposure of metal-based carbon quantum dots to different microalgae may induce physiological or molecular changes, which could stimulate growth or enhance defense mechanisms. Currently, the common technical starting point for carbon-dot-based photosynthetic materials is to enhance the absorption of red light by microalgae, with limited absorption of blue light. Furthermore, traditional carbon dots, while broadening the utilization spectrum of microalgae, suffer from significant impacts on algal cell survival, relatively high toxicity, and poor biocompatibility. Moreover, carbon dots are prone to nonradiative recombination losses due to surface defects (vacancy bonds, unsaturated bonds, uneven distribution of functional groups in the carbon skeleton), reducing their fluorescence quantum yield and spectral conversion efficiency, thus affecting their role in promoting microalgal photosynthesis. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for promoting carbon fixation in microalgae by using polyethylene glycol to passivate nitrogen-doped carbon point conversion spectra.

[0007] To solve the above-mentioned technical problems, the solution of the present invention is:

[0008] This invention provides a method for promoting microalgal carbon fixation by converting the spectrum of nitrogen-doped carbon dots through polyethylene glycol passivation. The method involves passivating the surface of nitrogen-doped carbon dots with polyethylene glycol, and then directly adding the passivated carbon dots to a microalgal suspension. By passing off the nitrogen-doped carbon dots, ultraviolet light (280nm–390nm) is converted into blue light (400–500nm) for direct use by the microalgae, thereby improving the algal cell's stress resistance and promoting carbon fixation growth.

[0009] The passivated nitrogen-doped carbon dots were prepared by the following method: a triethylenetetraminehexaacetic acid (TTHA) precursor solution was added to a hydrothermal reactor and reacted at 170–190°C for 4–6 h; after the reaction product was cooled to room temperature, the supernatant was separated to obtain a crude extract of nitrogen-doped carbon dots; the crude extract of nitrogen-doped carbon dots was mixed with polyethylene glycol and deionized water, stirred until completely dissolved, transferred to a hydrothermal reactor, and reacted at 170–190°C for 4–6 h; after the reaction product was cooled to room temperature, the supernatant was separated, purified by dialysis, and freeze-dried to obtain brown passivated nitrogen-doped carbon dot solid particles.

[0010] As a preferred embodiment of the present invention, the precursor solution is an aqueous solution of triethylenetetraminehexaacetic acid with a concentration of 25-30 mg / mL.

[0011] As a preferred embodiment of the present invention, the reaction product is loaded into a centrifuge tube and centrifuged at 6000-8000 rpm for 8-10 min to separate the solid precipitate from the supernatant.

[0012] As a preferred embodiment of the present invention, the volume ratio of nitrogen-doped carbon crude extract, polyethylene glycol, and deionized water is 1-10:1:2, and the mixture is stirred under ultrasonic conditions for 10-30 minutes to achieve uniform mixing.

[0013] As a preferred embodiment of the present invention, a 0.22 μm filter membrane is used for filtration and dialysis purification; the molecular cutoff of the dialysis bag is 1000, the dialysis solution is deionized water, the dialysis solution is replaced after each dialysis of 4 to 8 hours, and the dialysis is continued for 48 hours.

[0014] As a preferred embodiment of the present invention, the freeze-drying refers to transferring the dialysis-purified liquid to a vacuum freeze dryer and treating it at a temperature of -80 to -60°C for 24 to 48 hours to obtain passivated nitrogen-doped carbon points.

[0015] As a preferred embodiment of the present invention, passivated nitrogen-doped carbon dots are directly added to the microalgae suspension, and the amount added is controlled at 1 to 15 mg / L.

[0016] Description of the invention principle:

[0017] 1. This invention utilizes polyethylene glycol (TTHA) to passivate a triethylenetetraminehexaacetic acid (TTHA) precursor solution via a hydrothermal reaction to prepare nitrogen-doped carbon dots. Testing revealed that these carbon dots possess a core-shell structure, exhibiting nitrogen-doped bonding of pyrrole nitrogen, pyridine nitrogen, and graphitic nitrogen. Based on covalent coupling, PEG chains are connected to the nitrogen-doped carbon dot surface via active groups such as carboxyl and amino groups. The combined electronic structure and surface states of the various nitrogen dopants trap excitons and emit light through radiative recombination, resulting in high fluorescence intensity. Furthermore, they possess the photoluminescence characteristic of converting ultraviolet light to blue light, and the uniformity and fluorescence properties of the carbon dots are improved. Therefore, this polyethylene glycol-passivated nitrogen-doped carbon dot, at an addition amount of 1–15 mg / L, can improve its fluorescence quantum yield and broaden the spectral conversion effect when utilizing the spectrum of microalgae.

[0018] In contrast, existing literature on polyethylene glycol passivated carbon dots typically exhibits better water solubility and fluorescence intensity, and can convert ultraviolet light into red and blue light bands, making them suitable for applications in biomedicine, photocatalysis, and fluorescent probes.

[0019] 2. In traditional technologies, when photoluminescent nanomaterials are applied to microalgae, the reactive oxygen species generated by the active groups on the material surface can cause cytotoxicity, and the fluorescence stability and quantum yield are poor, affecting the optimal conditions for microalgae growth. This invention aims to reduce surface defects and optimize optical properties and dispersibility by passivating nitrogen-doped carbon dots with polyethylene glycol through physical coating and chemical modification, thereby enhancing its auxiliary role in energy transfer within the microalgae photosynthetic system.

[0020] 3. This invention utilizes polyethylene glycol passivated nitrogen-doped carbon dots to synergistically convert and utilize CO2 in a microalgal system. Combined with DIA proteomics analysis, it reveals the differential effects of different concentrations of PEG-CNDs on the physiological activity of algal cells during carbon fixation, and elucidates the cellular response mechanism of algal cells when functionalized carbon dots are added.

[0021] 4. Traditional quantum dots are prone to aggregation and inactivation, exhibiting high biotoxicity, and their band gaps do not match the energy levels of microalgal photosynthetic system II (PSII). This invention improves the carbon fixation rate of microalgae by developing a surface-passivated nitrogen-doped carbon dot conversion spectrum that matches the utilization spectrum of microalgae.

[0022] 5. This invention improves the water solubility, biocompatibility, and stability of carbon dots by passivating and modifying their surface with polyethylene glycol, while reducing non-specific adsorption and enhancing their biological application characteristics.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] 1. In this invention, the ultraviolet spectrum introduced into the photobioreactor by passivating nitrogen-doped carbon dots with polyethylene glycol can greatly reduce damage to algal cells. Simultaneously, converting ultraviolet light into blue light further promotes microalgal growth and carbon fixation.

[0025] 2. This invention utilizes the direct addition of polyethylene glycol-passivated nitrogen-doped carbon dots to microalgal suspensions to improve the uniformity of light source distribution within the algal solution. Because algal cells near the light source (usually the surface) absorb a large amount of visible light (especially red and blue light), while ultraviolet light and some blue light may penetrate too deeply or not be effectively utilized. The carbon dots can absorb the ultraviolet light that is not effectively absorbed by the algal cells and convert it into blue light wavelengths that the microalgae can directly utilize, thereby improving the distribution and utilization rate of light in the depth direction of the suspension.

[0026] 3. This invention uses polyethylene glycol to passivate nitrogen-doped carbon dots, improving the uniformity of carbon dot distribution and fluorescence characteristics, thereby enhancing the spectral conversion effect when used in microalgae culture systems, improving the overall light energy utilization efficiency of microalgae carbon fixation, reducing photoinhibition, and improving the stress resistance of algal cells, thus promoting microalgae growth and carbon fixation.

[0027] 4. Through extensive experiments and production practice, the method of this invention has been verified to increase the yield of microalgae carbon fixation biomass by more than 26% and the carbon fixation rate by more than 32%. Therefore, this invention not only proposes a new method for material preparation and carbon dot modification, but also promotes the development of the microalgae carbon fixation industry. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the preparation process of passivated nitrogen-doped carbon dots in this invention.

[0029] Figure 2This is a schematic diagram illustrating the application method of passivating nitrogen-doped carbon dots in this invention.

[0030] Figure 3 Transmission electron microscopy (TEM) image of passivated nitrogen-doped carbon dots.

[0031] Figure 4 This is a schematic diagram showing the particle size of passivated nitrogen-doped carbon dots.

[0032] Figure 5 The X-ray diffraction (XRD) pattern of passivated nitrogen-doped carbon dots.

[0033] Figure 6 The infrared spectrum of the passivated nitrogen-doped carbon point.

[0034] Figure 7 The photoluminescence spectrum of the passivated nitrogen-doped carbon dots is shown.

[0035] Figure 8 The image shows the three-dimensional fluorescence of passivated nitrogen-doped carbon dots.

[0036] Figure 9 Comparison of biomass dry weight in microalgae culture experiments.

[0037] Figure 10 Comparison of maximum quantum yield in microalgae culture experiments.

[0038] Figure 11 This is a comparison of the nutrient content in the products of microalgae culture experiments.

[0039] Figure 12 This is a comparison of the average carbon fixation rates in microalgae culture experiments. Detailed Implementation

[0040] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0041] 1. Overview of the implementation scheme of the present invention

[0042] The method for promoting microalgal carbon fixation by passingivating nitrogen-doped carbon point conversion spectroscopy with polyethylene glycol in this invention includes:

[0043] The surface of nitrogen-doped carbon dots was passivated using polyethylene glycol, and then the passivated nitrogen-doped carbon dots were directly added to the microalgae suspension at a concentration controlled between 1 and 15 mg / L. By passivating the nitrogen-doped carbon dots, ultraviolet light (280 nm–390 nm) in the external light spectrum is converted into blue light (400–500 nm) for direct use by the microalgae, thereby improving the algal cells' stress resistance and promoting carbon fixation during growth.

[0044] The passivated nitrogen-doped carbon dots are prepared by the following method:

[0045] (1) Triethylenetetraminehexaacetic acid (TTHA) precursor was added to deionized water to obtain a precursor solution with a concentration of 25-30 mg / mL; then it was transferred to a hydrothermal reactor and reacted at 170-190℃ for 4-6 h; after the reaction product was cooled to room temperature, it was placed in a centrifuge tube and centrifuged at 6000-8000 rpm for 8-10 min to separate the solid precipitate from the supernatant. The supernatant was taken to obtain a nitrogen-doped carbon spot crude extract; the nitrogen-doped carbon spot crude extract was mixed with polyethylene glycol and deionized water at a volume ratio of 1-10:1:2 and stirred under ultrasonic conditions for 10-30 min to ensure uniform mixing and complete dissolution; then it was transferred to a hydrothermal reactor and reacted at 170-190℃ for 4-6 h.

[0046] (2) After the reaction product is cooled to room temperature, it is placed into a centrifuge tube and centrifuged at 6000-8000 rpm for 8-10 min to separate the solid precipitate from the supernatant. The supernatant is then filtered and purified by dialysis using a 0.22 μm filter membrane. The molecular cutoff of the dialysis bag is 500-1000, and the dialysis solution is deionized water. The dialysis solution is replaced after each dialysis of 4-8 h, and the dialysis is continued for 48 h. The purified liquid is then transferred to a vacuum freeze dryer and dried at -80 to -60 °C for 24-48 h to obtain brown passivated nitrogen-doped carbon dot solid particles.

[0047] 2. Examples and Comparative Examples

[0048] Example 1

[0049] (1) Weigh 0.75 g of triethylenetetraminehexaacetic acid (TTHA) as a precursor, add it to 30 mL of deionized water, and sonicate for 30 min to ensure thorough mixing, obtaining a precursor solution with a concentration of 25 mg / mL. Transfer the precursor solution to a 50 mL hydrothermal reactor, seal it tightly, and place it in an oven to react at 170 °C for 6 h. After the reaction is complete, cool to room temperature, pour the solution into centrifuge tubes, and centrifuge at 6000 rpm for 10 min. Filter the supernatant after centrifugation through a 0.22 μm filter membrane to obtain a crude extract containing nitrogen-doped carbon dots.

[0050] (2) Using the nitrogen-doped carbon dot crude extract obtained in step (1), the nitrogen-doped carbon dot (CNDs) aqueous solution, polyethylene glycol (PEG), and deionized water were prepared in a volume ratio of 1:1:2 in a 250 mL flat-bottomed beaker. The mixture was stirred under ultrasonic conditions for 10 min until the raw materials were completely dissolved and mixed. Then, it was transferred to a hydrothermal reactor and heated at 170 °C for 6 h. After the reaction was completed, it was cooled to room temperature. The sample was taken out as a yellow-brown liquid. The reaction product solution was poured out and placed into a centrifuge tube and centrifuged at 6000 rpm for 10 min.

[0051] (3) The supernatant after centrifugation was collected and purified by dialysis using a 0.22 μm filter membrane. After dialysis, the yellow aqueous solution was freeze-dried using a vacuum freeze dryer at -80 to -60 °C for 24 h to obtain a yellow solid product, namely polyethylene glycol passivated nitrogen-doped carbon dots (PEG-CNDs). The filter membrane used for dialysis was made of regenerated cellulose (molecular cutoff not exceeding 1000, high acid and alkali resistance, and recyclable), and the dialysis solution was deionized water. The dialysis solution was replaced every 4 to 8 h, and dialysis was continued for 48 h to effectively remove impurities other than polyethylene glycol passivated nitrogen-doped carbon dots.

[0052] (4) After passivation of polyethylene glycol, nitrogen-doped carbon dots are directly added to the microalgae suspension, and the addition amount is controlled to be 1 mg / L. This allows the carbon dots to not only act as a spectral converter to broaden the spectral range utilized by microalgae, but also to act as a secondary light source to improve the uniformity of light intensity distribution in the longitudinal and transverse directions of the suspension and reduce the difference between strong light and weak light areas.

[0053] (5) Taking a cylindrical photobioreactor made of glass with a diameter of 65 mm and a height of 145 mm as an example, during a cycle (7 days) of microalgae culture of Chlorella species, outdoor natural irradiation was simulated at 24-hour intervals to observe the carbon fixation effect of microalgae using the spectrum after adding polyethylene glycol passivated nitrogen-doped carbon dots.

[0054] Example 2

[0055] (1) Weigh 0.8 g of triethylenetetraminehexaacetic acid (TTHA) as a precursor, add it to 30 mL of deionized water, and sonicate for 30 min to ensure thorough mixing, obtaining a precursor solution with a concentration of 26.7 mg / mL. Transfer the precursor solution to a 50 mL hydrothermal reactor, seal it tightly, and place it in an oven to react at 180 °C for 5 h. After the reaction is complete, cool to room temperature, pour the solution into centrifuge tubes, and centrifuge at 7000 rpm for 9 min. Filter the supernatant after centrifugation through a 0.22 μm filter membrane to obtain a nitrogen-doped carbon point crude extract.

[0056] (2) Using the nitrogen-doped carbon dot crude extract obtained in step (1), the nitrogen-doped carbon dot (CNDs) aqueous solution, polyethylene glycol (PEG), and deionized water were prepared in a volume ratio of 5:1:2 in a 250 mL flat-bottomed beaker. The mixture was stirred under ultrasonic conditions for 20 min until the raw materials were completely dissolved and mixed. Then, it was transferred to a hydrothermal reactor and heated at 180 °C for 5 h. After the reaction was completed, it was cooled to room temperature. The sample was taken out as a yellow-brown liquid. The reaction product solution was poured out and placed into a centrifuge tube and centrifuged at 7000 rpm for 9 min.

[0057] (3) The supernatant after centrifugation was collected and purified by dialysis using a 0.22 μm filter membrane. After dialysis, the yellow aqueous solution was freeze-dried using a vacuum freeze dryer at -80 to -60 °C for 36 h to obtain a yellow solid product, namely polyethylene glycol passivated nitrogen-doped carbon dots (PEG-CNDs). The filter membrane used for dialysis was made of regenerated cellulose (molecular cutoff not exceeding 1000, high acid and alkali resistance, and recyclable). The dialysis solution was deionized water. The dialysis solution was replaced every 4 to 8 h, and dialysis was continued for 48 h to effectively remove impurities other than polyethylene glycol passivated nitrogen-doped carbon dots.

[0058] (4) After passivation of polyethylene glycol, nitrogen-doped carbon dots are directly added to the microalgae suspension, and the addition amount is controlled at 5 mg / L. This allows the carbon dots to not only act as a spectral converter to broaden the spectral range utilized by microalgae, but also to act as a secondary light source to improve the uniformity of light intensity distribution in the longitudinal and transverse directions of the suspension and reduce the difference between strong light and weak light areas.

[0059] (5) Taking a cylindrical photobioreactor made of glass with a diameter of 65 mm and a height of 145 mm as an example, during a cycle (7 days) of microalgae culture of Chlorella species, outdoor natural irradiation was simulated at 24-hour intervals to observe the carbon fixation effect of microalgae using the spectrum after adding polyethylene glycol passivated nitrogen-doped carbon dots.

[0060] Example 3

[0061] (1) Weigh 0.9 g of triethylenetetraminehexaacetic acid (TTHA) as a precursor, add it to 30 mL of deionized water, and sonicate for 30 min to ensure thorough mixing, obtaining a precursor solution with a concentration of 30 mg / mL. Transfer the precursor solution to a 50 mL hydrothermal reactor, seal it tightly, and place it in an oven to react at 190 °C for 4 h. After the reaction is complete, cool to room temperature, pour the solution into centrifuge tubes, and centrifuge at 8000 rpm for 8 min. Filter the supernatant after centrifugation through a 0.22 μm filter membrane to obtain a nitrogen-doped carbon point crude extract.

[0062] (2) Using the nitrogen-doped carbon dot crude extract obtained in step (1), the nitrogen-doped carbon dot (CNDs) aqueous solution, polyethylene glycol (PEG), and deionized water were prepared in a volume ratio of 10:1:2 in a 250 mL flat-bottomed beaker. The mixture was stirred under ultrasonic conditions for 30 min until the raw materials were completely dissolved and mixed. Then, it was transferred to a hydrothermal reactor and heated at 190 °C for 4 h. After the reaction was completed, it was cooled to room temperature. The sample was taken out as a yellow-brown liquid. The reaction product solution was poured out and placed into a centrifuge tube and centrifuged at 8000 rpm for 8 min.

[0063] (3) The supernatant after centrifugation was collected and purified by dialysis using a 0.22 μm filter membrane. After dialysis, the yellow aqueous solution was freeze-dried using a vacuum freeze dryer at -80 to -60 °C for 48 h to obtain a yellow solid product, namely polyethylene glycol passivated nitrogen-doped carbon dots (PEG-CNDs). The filter membrane used for dialysis was made of regenerated cellulose (molecular cutoff not exceeding 1000, high acid and alkali resistance, and recyclable), and the dialysis solution was deionized water. The dialysis solution was replaced every 4 to 8 h, and dialysis was continued for 48 h to effectively remove impurities other than polyethylene glycol passivated nitrogen-doped carbon dots.

[0064] (4) After passivation of polyethylene glycol, nitrogen-doped carbon dots are directly added to the microalgae suspension, and the addition amount is controlled at 15 mg / L. This allows the carbon dots to not only act as a spectral converter to broaden the spectral range utilized by microalgae, but also to act as a secondary light source to improve the uniformity of light intensity distribution in the longitudinal and transverse directions of the suspension and reduce the difference between strong light and weak light areas.

[0065] (5) Taking a cylindrical photobioreactor made of glass with a diameter of 65 mm and a height of 145 mm as an example, during a cycle (7 days) of microalgae culture of Chlorella species, outdoor natural irradiation was simulated at 24-hour intervals to observe the carbon fixation effect of microalgae using the spectrum after adding polyethylene glycol passivated nitrogen-doped carbon dots.

[0066] Comparative Example 1

[0067] Taking a cylindrical glass photobioreactor with a diameter of 65 mm and a height of 145 mm as an example, during a 7-day cultivation cycle of Chlorella microalgae, outdoor natural irradiation was simulated at 24-hour intervals. The effect of microalgae utilizing the spectrum to promote carbon fixation was observed in the case of no polyethylene glycol passivation of nitrogen-doped carbon dots.

[0068] Comparative Example 2

[0069] Taking a conical glass flask photobioreactor with a bottom diameter of 75 mm, a neck outer diameter of 27 mm, and a height of 130 mm as an example, outdoor natural irradiation was simulated at 24-hour intervals during a 7-day cultivation cycle of *Chlorella* microalgae. Following the literature-documented method of adding graphene oxide quantum dots to cultivate microalgae proposed by the You research group, 100 mg / L of graphene oxide quantum dots were added to the microalgae suspension. The carbon fixation-promoting effect of microalgae utilizing the spectrum was observed.

[0070] Comparative Example 3

[0071] Taking a conical glass flask photobioreactor with a bottom diameter of 75 mm, a neck outer diameter of 27 mm, and a height of 130 mm as an example, outdoor natural irradiation was simulated at 24-hour intervals during a 7-day cultivation cycle of *Chlorella* microalgae. Following the literature-documented method of adding graphene oxide quantum dots to cultivate microalgae proposed by the You research group, a concentration of 1000 mg / L of graphene oxide quantum dots was added to the microalgae suspension. The effect of microalgae utilizing the spectrum to promote carbon fixation was observed.

[0072] Part Three: Product Testing and Results Analysis

[0073] from Figure 1 The schematic diagram of the preparation process of nitrogen-doped carbon dots in the present invention illustrates the carbon dot passivation synthesis method, which mainly involves hydrothermal passivation of nitrogen-doped carbon dots by adding polyethylene glycol solution. Figure 2 A schematic diagram illustrating the application method of passivated nitrogen-doped carbon dots is shown. This diagram primarily describes the experiment of directly adding passivated nitrogen-doped carbon dots to a microalgae suspension for microalgae cultivation, and includes data measurement and discussion between the experimental and control groups. Figure 3 Transmission electron microscopy (TEM) images of passivated nitrogen-doped carbon dots show that the morphology of polyethylene glycol passivated nitrogen-doped carbon dots is a spherical dot structure with a crystal plane spacing of 0.204 nm, corresponding to the (101) crystal plane of graphene. Figure 4 The analysis and statistics show that the average particle size of the passivated nitrogen-doped carbon dots is 2.25 nm, with a diameter distribution range of 1.25-3.75 nm. The particle size distribution is relatively uniform, indicating that the carbon dots are better dispersed after passivation. Figure 5 The X-ray diffraction (XRD) pattern of passivated nitrogen-doped carbon dots shows that they have an amorphous carbon structure, which corresponds to a core-shell structure of carbon dots, namely the vortex carbon phase.

[0074] from Figure 6The infrared spectrum of the passivated nitrogen-doped carbon dots shows peaks similar to those of polyethylene glycol (PEG), confirming the successful passivation of the nitrogen-doped surface by PEG. Infrared spectroscopy results show that PEG exhibits an absorption peak at 3373 cm⁻¹ corresponding to the stretching vibration of OH / NH, and secondary strong characteristic peaks of NH at 1454 cm⁻¹ and 886 cm⁻¹, originating from the in-plane and out-of-plane rocking vibrations of the NH group, respectively, indicating the presence of amine functional groups in this carbon dot. The strong characteristic peaks at 2873 cm⁻¹ and 1352 cm⁻¹ are attributed to the bending vibration of CH, while the absorption peak at 1660 cm⁻¹ is attributed to the characteristic absorption band of the stretching vibration of the C=O / C=N group. The characteristic peak at 97 cm⁻¹ is attributed to the characteristic absorption band of C=C stretching vibration. The continuous absorption peaks near this peak are attributed to the skeletal vibration of the benzene ring. The strong characteristic peak at 1111 cm⁻¹ corresponds to the stretching vibration of CO, and the characteristic peak at 951 cm⁻¹ is attributed to the stretching vibration of C=C. The continuous characteristic peaks near 600 cm⁻¹ are mainly attributed to the out-of-plane bending vibration of cyclic amines. This indicates that nitrogen-doped carbon dots and polyethylene glycol can successfully introduce more O, H, and N functional groups onto the surface of nitrogen-doped carbon dots through dehydration, polymerization, and carbonization, increasing their hydroxyl, carboxyl, and amine content. These functional groups reduce the nonradiative recombination loss of passivated nitrogen-doped carbon dots, improving their fluorescence quantum yield and fluorescence properties. Figure 7 and Figure 8 The photoluminescence and three-dimensional fluorescence spectra of passivated nitrogen-doped carbon dots show that they possess photoluminescence properties. The excitation spectrum of the carbon dots indicates the excitation wavelength region for photoluminescence and fluorescence, while the emission spectrum indicates the wavelength range of fluorescence emitted after excitation by irradiation. The color distribution of the three-dimensional fluorescence region tends towards red, indicating that the fluorescence conversion effect between excitation and emission is strongest in this region. The optimal excitation and emission wavelengths are 360 ​​nm and 443 nm, respectively, enabling the conversion of ultraviolet light from 280 nm to 390 nm into blue light from 400 nm to 500 nm.

[0075] Figure 9-12 The document presents observation data on microalgae culture for each embodiment and comparative example.

[0076] Figure 9 The comparison of biomass dry weight in the microalgae culture experiments shows the differences in microalgae culture biomass among the various examples and comparative examples. Due to the different initial accumulated biomass under different passivation nitrogen-doped carbon point addition concentrations, the difference in microalgae biomass dry weight gradually increases with microalgae growth. Figure 9It can be seen that as the concentration of polyethylene glycol passivated nitrogen-doped carbon dots increases, the accumulation of microalgal biomass dry weight first gradually increases and then decreases. When the concentration of polyethylene glycol passivated nitrogen-doped carbon dots is 5 mg / L, the accumulation of microalgal biomass dry weight on the seventh day of cultivation reaches 1.66 g / L, which is 26.43% higher than the biomass dry weight of Comparative Example 1 (1.32 g / L).

[0077] Figure 10 The comparison of maximum quantum yield in microalgae culture experiments demonstrates the differences in biomass production across various examples and comparative studies. In the early stages of microalgae carbon fixation, the maximum photochemical quantum yield (Fv / Fm) under different conditions showed little difference. This is because, although biocompatibility was improved upon initial addition of polyethylene glycol-passivated nitrogen-doped carbon dots, the addition of foreign matter could still trigger a stress response in the microalgae, leading to short-term metabolic inhibition (such as ROS) accumulation. At this point, the algal cells preferentially activate antioxidant mechanisms. Furthermore, the passivated nitrogen-doped carbon dots require time to disperse upon addition to the culture medium; initially, they may aggregate, resulting in a low effective concentration, but gradually disperse evenly, thus promoting photosynthesis. Overall, the Fv / Fm ratio generally decreased. This is because, in the later stages of growth, the increased algal cell density caused shading between cells, reducing light transmittance in the reactor and decreasing light energy absorption by the algal cells. On the 7th day, the Fv / Fm value of the experimental group with a polyethylene glycol passivated nitrogen-doped carbon dot concentration of 5 mg / L was higher than that of other conditions, and was 1.18 times that of the control group. In addition, the Fv / Fm value of the experimental group with polyethylene glycol passivated nitrogen-doped carbon dot was higher than that of the control group. This indicates that the addition of polyethylene glycol passivated nitrogen-doped carbon dot significantly improved the photochemical reaction, ensured the activity of the photochemical reaction, and promoted the photosynthesis.

[0078] Figure 11 The comparison of nutrient content in the products of microalgae culture experiments demonstrates the differences in biomass from various examples and comparative examples. Under different concentrations of polyethylene glycol (PEG) passivated nitrogen-doped carbon dots, the proportions of protein and carbohydrates in microalgae cultured initially increased and then decreased with the addition of PEG passivated nitrogen-doped carbon dots. This is because lower concentrations of PEG passivated nitrogen-doped carbon dots generally result in lower toxicity, while higher concentrations may induce oxidative stress in algal cells, inhibiting cell growth and leading to the degradation of intracellular components. Figure 11 The protein content in the experimental groups was higher than that in the control group. The highest protein content was observed in the 5 mg / L polyethylene glycol (PEG) passivated nitrogen-doped carbon dopant group, which was 36.38% higher than the control group, indicating that introducing an appropriate concentration of PEG passivated nitrogen-doped carbon dopant can better promote protein synthesis in algal cells. Regarding carbohydrate content, the 5 mg / L PEG passivated nitrogen-doped carbon dopant group resulted in the highest carbohydrate content, exceeding the control group by 22.15%. This demonstrates that an appropriate concentration of PEG passivated nitrogen-doped carbon dopant can promote microalgal cell growth and nutrient content.

[0079] Figure 12 The comparison of average carbon fixation rates in microalgae culture experiments demonstrates the differences in microalgae culture biomass among the various examples and comparative examples. The growth and carbon fixation rates of microalgae are similar to the microalgae biomass accumulation patterns, such as... Figure 12 As shown, when the concentration of polyethylene glycol passivated nitrogen-doped carbon dots was 5 mg / L, the average carbon fixation rate of microalgae was also the highest at 0.43 g / L / d, which was 32.11% higher than that of control group 1. This indicates that as the concentration of polyethylene glycol passivated nitrogen-doped carbon dots gradually increases, the carbon fixation efficiency of microalgae growth improves. However, when the concentration exceeds 5 mg / L, the excessive photocatalytic reaction of the carbon dots induces excessive ROS accumulation in algal cells, triggering oxidative stress in algal cells, which leads to a gradual decrease in microalgal biomass accumulation. However, the average carbon fixation rate under the condition of 15 mg / L polyethylene glycol passivated nitrogen-doped carbon dots is still higher than that of control group 1. Therefore, it can be concluded that polyethylene glycol passivated nitrogen-doped carbon dots at appropriate concentrations are more conducive to algal cells fixing CO2 and thus accumulating biomass.

[0080] The data comparison in the charts shows that, in this invention, passivating nitrogen-doped carbon dots with polyethylene glycol not only improves the uniformity of carbon dot distribution and avoids their easy aggregation through physical coating, thus reducing their physiological toxicity to algal cells, but also improves their fluorescence quantum yield and fluorescence characteristics, thereby enhancing the spectral conversion effect when broadening the utilization spectrum range of microalgae. Compared with Comparative Example 1 without adding a polyethylene glycol passivated nitrogen-doped carbon dot coating and the microalgae culture scheme with graphene oxide quantum dots reported in the literature, the microalgae biomass in the microalgae culture scheme in the literature decreased by 20% at 1000 mg / L and increased by 17% at 100 mg / L. However, Example 2 of this invention, with the addition of an appropriate amount of polyethylene glycol passivated nitrogen-doped carbon dots, showed the highest biomass dry weight increase efficiency of 26.4% compared to Comparative Example 1, and the average carbon fixation rate increased by 32.1% compared to Comparative Example 1.

[0081] Therefore, the present invention has the ability to improve the biological application characteristics and fluorescence characteristics of nitrogen-doped carbon dots by surface passivation and functional group modification with polyethylene glycol (PEG), promote spectral conversion and thus improve the CO2 fixation efficiency of microalgae, and can be applied to the scenario of microalgae carbon fixation based on novel nanomaterials.

[0082] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for promoting microalgal carbon fixation by passingivating nitrogen-doped carbon point conversion spectroscopy with polyethylene glycol, characterized in that, The surface of nitrogen-doped carbon dots was passivated using polyethylene glycol, and then these passivated carbon dots were directly added to a microalgae suspension. By passivating the nitrogen-doped carbon dots, ultraviolet light (280nm–390nm) was converted into blue light (400–500nm) for direct use by the microalgae, thus improving the algal cells' stress resistance and promoting carbon fixation during growth. The passivated nitrogen-doped carbon dots were prepared by the following method: a triethylenetetraminehexaacetic acid (TTHA) precursor solution was added to a hydrothermal reactor and reacted at 170–190°C for 4–6 h; after the reaction product was cooled to room temperature, the supernatant was separated to obtain a crude extract of nitrogen-doped carbon dots; the crude extract of nitrogen-doped carbon dots was mixed with polyethylene glycol and deionized water, stirred until completely dissolved, transferred to a hydrothermal reactor, and reacted at 170–190°C for 4–6 h; after the reaction product was cooled to room temperature, the supernatant was separated, purified by dialysis, and freeze-dried to obtain brown passivated nitrogen-doped carbon dot solid particles.

2. The method according to claim 1, characterized in that, The precursor solution is an aqueous solution of triethylenetetraminehexaacetic acid with a concentration of 25–30 mg / mL.

3. The method according to claim 1, characterized in that, The reaction product was loaded into centrifuge tubes and centrifuged at 6000-8000 rpm for 8-10 minutes to separate the solid precipitate from the supernatant.

4. The method according to claim 1, characterized in that, The volume ratio of nitrogen-doped carbon crude extract, polyethylene glycol, and deionized water is 1–10:1:

2. The mixture is stirred under ultrasonic conditions for 10–30 minutes to ensure uniform mixing.

5. The method according to claim 1, characterized in that, Purification was performed by filtration and dialysis using a 0.22 μm filter membrane; the molecular cutoff of the dialysis bag was 1000; the dialysis solution was deionized water; the dialysis solution was replaced after each dialysis session of 4–8 hours; and dialysis was continued for 48 hours.

6. The method according to claim 1, characterized in that, The freeze-drying process involves transferring the purified liquid after dialysis to a vacuum freeze dryer and treating it at a temperature of -80 to -60°C for 24 to 48 hours to obtain passivated nitrogen-doped carbon points.

7. The method according to claim 1, characterized in that, Passivated nitrogen-doped carbon dots were directly added to the microalgae suspension, with the addition amount controlled at 1–15 mg / L.

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