Nitrogen-doped carbon quantum dot composite film and preparation method thereof

Abstract

The invention relates to the technical field of composite materials, in particular to a nitrogen-doped carbon quantum dot composite film and a preparation method thereof. According to the specific technical scheme, the preparation method of the nitrogen-doped carbon quantum dot composite film comprises the following steps: (1) carrying out hydrothermal reaction by taking polyethylene polyamines as a nitrogen source and combining with a carbon source to complete preparation of carbon quantum dots; and (2) adding 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and N-hydroxysuccinimide or N-hydroxythiosuccinimide into the hydrophilic modified high-molecular polymer membrane, carrying out a reaction to obtain the nitrogen-doped carbon dot modified composite membrane after the reaction is completed, and soaking the nitrogen-doped carbon dot modified composite membrane in water for preservation. The nitrogen-doped design enables the nitrogen-doped carbon dot composite membrane to have a good effect in capturing and enriching persistent organic pollutants such as perfluorinated compounds, and the nitrogen-doped carbon dot composite membrane has a good application value in treatment of new pollutants in the environment.

Description

Technical Field

[0001] The invention relates to the technical field of composite materials, and in particular to a nitrogen-doped carbon quantum dot composite film based on polyethylene polyamine functionalization regulation and a preparation method thereof. Background Art

[0002] Carbon quantum dots (CQDs) are a new type of nanoscale carbon-based materials with good biocompatibility. According to different reaction precursors and conditions, they can be divided into graphene quantum dots, carbon nanodots and polymer carbon quantum dots (PCDs). Among them, polymer carbon quantum dots have a core-shell nested structure, and the shell surface of the carbon core is covered by polymer chains and active groups. Therefore, this type of polymer carbon dots has both nano properties and the functional integration of polymers. They not only show good biocompatibility, environmental protection, low toxicity, and fluorescence properties, but also have the advantages of structural controllability and easy functional modification. In actual industrial applications, for example, the fluorescence properties of PCDs can be used to analyze and detect heavy metal ions such as cadmium ions, mercury ions, copper ions, trivalent iron ions, and carcinogens such as nitrite in the environment. In addition to using the fluorescence properties of PCDs to achieve fluorescence detection and bioimaging, the rich active functional groups of PCDs themselves also provide ideas for their functional modules as materials.

[0003] The synthesis of carbon quantum dots can be designed and implemented through two strategies: top-down and bottom-up. The top-down synthesis strategy uses physical or chemical methods to peel off large-sized carbon sources into small-sized carbon quantum dots. The top-down strategy usually selects carbon sources such as carbon nanotubes, carbon fibers, graphite rods and activated carbon, and uses arc discharge, laser ablation, electrochemical synthesis and other methods to peel off large-sized carbon skeletons to obtain small-sized carbon quantum dots. The bottom-up synthesis strategy uses organic small molecules or oligomers as carbon sources to synthesize carbon quantum dots. The bottom-up strategy usually selects glucose, polyethylene glycol, biomass, ionic liquids and the like as carbon sources, and uses hydrothermal method, microwave method, solvothermal method, strong acid oxidation method and other methods to achieve the synthesis of carbon quantum dots. The bottom-up strategy is the preferred strategy for preparing polymer carbon quantum dots. The bottom-up synthesis strategy has a variety of precursors, so the functional properties of the synthesized polymer carbon quantum dots are richer.

[0004] With the continuous development and innovation of the functional properties of polymer carbon dots, the preparation of new adsorption and separation composite materials based on PCDs has received more and more attention. Carbon quantum dot composite membrane materials are an important type of carbon-based functional composite materials, especially the research and development of nitrogen-doped polymer carbon quantum dot composite membrane materials. Such materials can achieve unique high oxygen and nitrogen-rich properties and show great potential in adsorption and extraction. Using them as membrane materials in the membrane extraction process can achieve efficient extraction and enrichment of new pollutants such as perfluorinated compounds.

[0005] Membrane extraction technology is a sample processing method that uses polymer membrane materials as carriers to separate, purify, enrich, capture, etc. complex matrices. Membrane extraction technology only requires a small amount of solvent or even no solvent to complete the collection, purification and extraction of target compounds. Therefore, it is of great significance in the treatment and research of environmental pollutants, and can solve the current urgent problem of new pollutant analysis and detection. Membrane extraction technology has the advantages of low organic solvent usage, environmental protection and high efficiency, high degree of purification, high enrichment multiple, small and portable device, and easy online connection with analytical instruments. Therefore, it is widely used in wastewater treatment, enrichment of environmentally harmful organic matter, drug extraction, heavy metal ion recovery, etc. The key to membrane extraction lies in the membrane material. The performance of the membrane material directly determines whether the extraction process is feasible and efficient. In recent years, functional composite membrane materials have become a key area of ​​the new materials industry, and the demand for domestic substitution is urgent. Quantum dot composite membranes are new materials that have attracted much attention in the scientific and technological community in recent years. Carbon quantum dot composite membranes have the characteristics of both quantum dot materials and membrane materials, and have shown excellent performance and application prospects in many fields. The key link in the preparation of carbon quantum dot composite membrane materials lies in the synthesis of carbon quantum dots and the modification of membrane materials. Polymer carbon quantum dots can be prepared through the bottom-up synthesis strategy. By screening suitable carbon source precursors and nitrogen source precursors, rich functions such as high oxygen and nitrogen-rich properties can be achieved. The choice of precursors is not only diverse and controllable, but also can make the raw materials cheap, easily available, green and non-toxic. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a nitrogen-doped carbon quantum dot composite membrane and a preparation method thereof, wherein green, safe and cheap carbon sources such as citric acid and polyethylene polyamine nitrogen sources such as diethylenetriamine and triethylenetetramine are used as precursors to prepare nitrogen-doped carbon quantum dots through a "bottom-up" synthesis strategy, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and N-hydroxysuccinimide or N-hydroxythiosuccinimide are added, and the membrane is coupled to a polyvinylidene fluoride membrane through EDC / NHS interfacial cross-linking polymerization to obtain a nitrogen-doped carbon quantum dot functionalized modified composite membrane material, and the application of the nitrogen-doped carbon quantum dot composite membrane material in the enrichment and extraction of new pollutants such as perfluorinated compounds, which is an excellent membrane extraction material.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0008] The present invention discloses a method for preparing a nitrogen-doped carbon quantum dot composite film, comprising the following steps:

[0009] (1) Using polyethylene polyamines as a nitrogen source and a carbon source to carry out a hydrothermal reaction to complete the preparation of carbon quantum dots;

[0010] (2) 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and N-hydroxysuccinimide or N-hydroxysulfosuccinimide are added to the hydrophilic modified polymer membrane to react. After the reaction is completed, a nitrogen-doped carbon dot modified composite membrane is obtained, which is then immersed in water for storage.

[0011] Preferably, the polyethylene polyamine is any one of urea, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, hexaethyleneheptamine, polyethyleneimine, and polyamide-amine dendrimer.

[0012] Preferably, the carbon source is any one of citric acid, vitamin C, glucose and biomass.

[0013] Preferably, in step (1), the reaction medium is deionized water, and the molar ratio of the polyethylene polyamine to the carbon source is greater than or equal to 1.5; during the hydrothermal reaction, the reaction temperature is 160-220° C., and the reaction time is 3-6 hours.

[0014] Preferably, the amount of the polyethylene polyamines used is greater than or equal to 1.5 times the equivalent of the carbon source.

[0015] Preferably, in step (1), the prepared carbon quantum dots are dialyzed using a 3500 equivalent dialysis bag, and the dialyzed solution is then purified by freeze-drying for later use; the particle size of the carbon quantum dots is 3.0-7.2 nm.

[0016] Preferably, in step (2), the preparation process of the hydrophilic modified polymer membrane is as follows: the polymer membrane is ultrasonically cleaned, immersed in an ethanol solution, then added to an alkaline potassium permanganate oxidant hydrophilic modification system, and then transferred to an acrylic acid solution, azobisisobutyronitrile is added, and a polymerization reaction is carried out. After the reaction is terminated, the polymer membrane is washed with deionized water and dried to obtain the hydrophilic modified polymer membrane.

[0017] Preferably, the base in the alkaline potassium permanganate oxidant hydrophilic modification system is potassium hydroxide, the molar concentration of potassium hydroxide is 2.5 mol / L, and the mass fraction of potassium permanganate is 5%; during the modification process, the modification temperature is 40-60° C. and the reaction time is 1-2 hours.

[0018] Preferably, in step (2), the reaction is carried out under an argon protection environment, the reaction temperature is 0-8° C., the reaction time is 1-2 hours, and the reaction pH is controlled at 4.0-6.0.

[0019] Correspondingly, the nitrogen-doped carbon dot modified composite membrane prepared by the above preparation method.

[0020] The present invention has the following beneficial effects:

[0021] The present invention uses ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, hexaethyleneheptamine, polyethyleneimine, polyamide-amine type dendrimers, etc. as nitrogen source precursors to prepare nitrogen-doped carbon quantum dots, so that the surface of the carbon quantum dot core-shell structure has a large number of amine functional groups, and the carbon source can be selected from biomass such as citric acid, vitamin C, glucose and green vegetable leaves, which is in line with the environmental protection concept of green, low-toxicity, cheap and easy to obtain. Nitrogen-doped carbon quantum dots are synthesized from bottom to top by a one-step hydrothermal method, and the size of the quantum dots is uniform, and the particle size is 3.0-7.2nm. By selecting different polyene polyamine molecules, such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, etc., different nitrogen-doped carbon quantum dot structures can be designed, and the amine functional group content of the nitrogen-doped carbon quantum dots can be regulated by controlling the nitrogen source and carbon source ratio. Then, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and N-hydroxysuccinimide or N-hydroxysulfosuccinimide are used to trigger the EDC / NHS reaction, completing the specific connection between the carboxyl group on the polymer membrane and the amine group on the carbon quantum dots, and finally obtaining the carbon dot-loaded membrane. The carbon dot-loaded membrane is used as a membrane extraction material, which has a good capture and adsorption effect on toxic and harmful molecules such as perfluorinated compounds, and can effectively treat new pollutants in the environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The preparation process of nitrogen-doped carbon quantum dots loaded membrane for preparing nitrogen-doped carbon quantum dots using diethylenetriamine as nitrogen source;

[0023] Figure 2 Actual picture and characterization results of nitrogen-doped carbon quantum dots loaded membrane materials prepared for nitrogen-doped carbon quantum dots using urea as nitrogen source

[0024] Figure 3 The preparation process of nitrogen-doped carbon quantum dots loaded membrane for preparing nitrogen-doped carbon quantum dots using polyethyleneimine as nitrogen source;

[0025] Figure 4 A diagram of a "sandwich" type membrane extraction device based on a nitrogen-doped carbon quantum dot-loaded modified membrane;

[0026] Figure 5 Application of membrane extraction device based on nitrogen-doped carbon quantum dots loaded modified membrane in the analysis and detection of perfluorinated compounds. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0029] The invention discloses a method for preparing a nitrogen-doped carbon quantum dot composite membrane. The method comprises the following steps: using diethylenetriamine, triethylenetetramine and other polyethylene polyamine nitrogen sources and biomass such as citric acid, vitamin C and green vegetable leaves as carbon sources, and using a "bottom-up" synthesis strategy and a one-step hydrothermal method to efficiently and quickly prepare nitrogen-doped fluorescent carbon quantum dots; using a potassium permanganate / potassium hydroxide alkaline oxidant hydrophilic system to hydrophilize a polymer membrane by a surface chemical modification method, and then using EDC / NHS interface cross-linking polymerization to specifically connect the nitrogen-doped carbon quantum dots and the polymer membrane, thereby obtaining a nitrogen-doped carbon quantum dot composite membrane having an adsorption and capture effect on new pollutants such as perfluorinated compounds, and using a membrane separation method to achieve adsorption and extraction of new pollutants such as perfluorinated compounds.

[0030] The specific steps include:

[0031] (1) Synthesis of functionally controllable nanoscale nitrogen-doped carbon quantum dots using a bottom-up strategy

[0032] Using polyethylene polyamines as nitrogen sources and carbon sources, and deionized water as a medium, a one-step hydrothermal procedure is set up, a hydrothermal reaction is initiated in a polytetrafluoroethylene autoclave, and the preparation of carbon quantum dots is completed from bottom to top through the hydrothermal reaction, and dialyzed for later use. The particle size of the carbon quantum dots is 3.0-7.2nm, the particle size distribution is uniform, and the fluorescence performance is long-lasting and stable; the preparation of carbon quantum dots is completed; the prepared carbon quantum dots are dialyzed using a 3500 equivalent dialysis bag, and the solution after dialysis is purified by freeze-drying for later use.

[0033] The polyethylene polyamine is any one of urea, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, hexaethyleneheptamine, polyethyleneimine, and polyamide-amine dendrimers. The carbon source is any one of citric acid, vitamin C, glucose, and biomass (such as green vegetable leaves). The molar ratio of the polyethylene polyamine to the carbon source is greater than or equal to 1.5, or the amount of the polyethylene polyamine is greater than or equal to 1.5 times the equivalent of the carbon source; during the hydrothermal reaction, the total amount of the reaction liquid shall not exceed 2 / 3 of the volume of the autoclave, the reaction temperature is 160-220°C, and the reaction time is 3-6 hours.

[0034] (2) The surface chemical modification method is used to hydrophilize the polymer membrane so that hydrophilic groups are bonded to the membrane surface, thereby improving the hydrophilicity of the polymer membrane surface.

[0035] The polymer membrane (taking polyvinylidene fluoride membrane as an example) is ultrasonically cleaned with deionized water, dried and transferred to an ethanol solution for full immersion after cleaning, and then added to an alkaline oxidant (such as alkaline potassium permanganate) hydrophilic system for phase transfer, removing the surface hydrophobic bonds, and modifying the membrane surface with hydrophilic groups such as -OH, and then the membrane is transferred to an acrylic acid solution, and azobisisobutyronitrile is added as an initiator to initiate a polymerization reaction, so that the membrane surface is modified with hydrophilic groups such as -COOH, and after the reaction is terminated, it is fully cleaned with deionized water and dried. Among them, the acrylic acid concentration is 0.2-5mol / L, and the amount of azobisisobutyronitrile is 0.1%-1% of the weight of the polymer membrane.

[0036] The polyvinylidene fluoride membrane has a membrane size of 5cm×8cm. The modifier system used is an alkaline potassium permanganate oxidant system. The alkaline reagent used is a potassium hydroxide solution with a molar concentration of 2.5mol / L and a mass fraction of 5% of the potassium permanganate solution. The hydrophilic modification degree of the membrane material can be controlled by the modification time and temperature of the alkaline potassium permanganate oxidant hydrophilic system. The longer the modification time, the stronger the hydrophilicity. The modification temperature is 40-60℃ and the reaction time is 1-2 hours.

[0037] (3) The nitrogen-doped carbon quantum dots are loaded onto the modified polymer membrane by surface grafting method to obtain nitrogen-doped carbon quantum dots composite membrane material that can be used for membrane extraction.

[0038] Under argon protection, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and N-hydroxysuccinimide or N-hydroxythiosuccinimide are added to the hydrophilic modified polymer membrane to trigger the EDC / NHS reaction to form an O-acylisourea active intermediate, which undergoes an amidation reaction with the carboxyl group. The carboxyl group and the amine group on the carbon quantum dot are specifically connected by EDC / NHS interfacial cross-linking polymerization, thereby realizing the loading and modification of the carbon quantum dots on the polymer membrane, and finally preparing a nitrogen-doped carbon dot polymer membrane with good adsorption and capture effects on pollutants such as perfluorinated compounds. The membrane extraction method can achieve adsorption and enrichment of perfluorinated compounds in the environment. After cleaning, soak it in water for preservation to obtain a nitrogen-doped carbon dot composite membrane with surface amino functionalization that emits blue fluorescence. Among them, the EDC / NHS reaction is carried out in a buffer system under neutral conditions, NHS can be N-hydroxysuccinimide or N-hydroxysulfosuccinimide, the reaction medium is an aqueous phase, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC) must be prepared and used immediately to avoid deactivation, and the reaction pH is controlled at 4.0-6.0. The interfacial polymerization process of carbon quantum dots loaded and modified onto a polymer membrane can also be achieved using trimesoyl chloride.

[0039] (4) The washed and dried nitrogen-doped carbon dot composite membrane is installed in a membrane extraction device using a sandwich assembly method for the capture, enrichment and extraction of new pollutants such as perfluorinated compounds. A double-layer method, solid phase microextraction fiber membrane type, etc. can also be used. Among them, 95% ethanol and deionized water are used for washing, and finally deionized water is used for soaking and storage, and the storage temperature is 4°C to room temperature.

[0040] The amine loading of the nitrogen-doped carbon dot composite membrane of the present invention can be controlled by the content of the loaded carbon quantum dots, and can also be controlled by the amount or type of the nitrogen source used in the carbon quantum dot synthesis process. The nitrogen source is selected from polyethylene polyamine molecules. The nitrogen-doped carbon dot composite membrane adopts a "sandwich" membrane separation component mode to capture and enrich harmful substances such as perfluorinated compounds. The high oxygen and nitrogen-rich characteristics of the nitrogen-doped carbon dot composite membrane can be controlled by controlling the type and proportion of the nitrogen source, and the active amine groups on the shell surface of the nitrogen-doped carbon dot core-shell structure can be achieved by controlling the molecular chain length of the nitrogen source, and the excess reactants can be removed by dialysis through a dialysis membrane or the like.

[0041] The present invention will be further described below in conjunction with specific embodiments.

[0042] Example 1

[0043] refer to Figure 1 As shown, the preparation process of the nitrogen-doped carbon dot composite film based on citric acid and diethylenetriamine is as follows, and the steps are:

[0044] (1) A bottom-up strategy was used to synthesize functionally controllable nanoscale nitrogen-doped carbon quantum dots.

[0045] Diethylenetriamine molecules were selected as nitrogen source precursors, anhydrous citric acid was selected as carbon source precursors, a one-step hydrothermal program was set up, a hydrothermal reaction was initiated in a polytetrafluoroethylene autoclave, and the preparation of carbon quantum dots was completed from bottom to top through a hydrothermal reaction. The molar ratio of citric acid and diethylenetriamine was set to 1:1.5, the reaction temperature was 180°C, and the reaction time was 3 hours.

[0046] (2) After the reaction is completed and the reactor is cooled naturally, the product solution is placed in a dialysis bag with a molecular weight cutoff of 3500 for dialysis purification and impurity removal. After the dialysis is completed, the product solution is freeze-dried and stored for later use.

[0047] (3) The polyvinylidene fluoride membrane is hydrophilized by using an alkaline potassium permanganate oxidant hydrophilic system, and then the membrane is transferred to an acrylic acid solution, and azobisisobutyronitrile is added as an initiator to initiate a polymerization reaction, so that the membrane surface is modified with hydrophilic groups such as -COOH. After the reaction is terminated, the membrane is thoroughly washed with deionized water and dried.

[0048] (4) 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and N-hydroxysuccinimide were added to trigger the EDC / NHS reaction, and the carboxyl groups and the amine groups on the carbon quantum dots were specifically connected by EDC / NHS interfacial cross-linking polymerization to obtain a carbon dot-loaded membrane.

[0049] Among them, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide is prepared into a 100 mg / mL solution with MES buffer solution; N-hydroxysuccinimide is prepared into a 100 mg / mL solution with MES buffer solution; the dosage ratio of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and N-hydroxysuccinimide is not less than 1:1, and the upper limit of the dosage is until precipitation is produced.

[0050] (5) After cleaning, the mixture is immersed in water and stored to obtain a nitrogen-doped carbon dot composite film with surface amino functionalization that emits blue fluorescence.

[0051] Example 2

[0052] The preparation process of nitrogen-doped carbon dot composite film based on citric acid and urea is as follows, and the steps are as follows:

[0053] The steps are the same as (1), (2), (3), (4) and (5) of Example 1, except that the nitrogen source in step (1) is changed to urea. The actual picture and characterization results of the preparation of nitrogen-doped carbon quantum dots composite film material using urea as the nitrogen source are shown in Figure 2 shown.

[0054] Example 3

[0055] refer to Figure 3As shown, the preparation process of the nitrogen-doped carbon dot composite film based on citric acid and polyethyleneimine is as follows, and the steps are:

[0056] The steps are the same as (1), (2), (3), (4) and (5) of Example 1, except that the nitrogen source in step (1) is changed to polyethyleneimine; and the grafting method in step (4) is changed to trimesoyl chloride (TMC) interfacial polymerization, i.e., N-hydroxysuccinimide is changed to trimesoyl chloride.

[0057] Example 4

[0058] The application of membrane extraction device based on nitrogen-doped carbon quantum dot-loaded modified membrane in the capture, enrichment and analytical detection of harmful substances such as perfluorinated compounds.

[0059] like Figure 4 , Figure 5 As shown, the nitrogen-doped carbon quantum dot composite membranes synthesized in Example 1, Example 2, and Example 3 were used as membrane materials, a membrane extraction device was constructed in a sandwich mode, and the device was combined with a chromatographic instrument through column switching to finally complete the extraction and analysis of perfluorinated compounds in the sample.

[0060] The results show that the "sandwich" membrane extraction device based on the nitrogen-doped carbon quantum dot-loaded modified membrane provided by the present invention can realize the efficient online and automated enrichment of perfluorinated compounds in environmental samples. The device can be used in conjunction with a liquid chromatography-mass spectrometry detector to detect new perfluorinated compound pollutants. The device was used in environmental water samples from 20 different waters, and perfluorinated compounds were detected in 14 samples, with a content of 12-105.4 ng / L.

[0061] The chromatographic instrument is equipped with a mass spectrometer detector, and the specific operating conditions are as follows:

[0062] Instrument: Liquid chromatography = tandem mass spectrometry (LC-MS / MS)

[0063] Mobile phase: 5mmol / L ammonium acetate aqueous solution + methanol

[0064] Stationary phase: C18 reverse phase column

[0065] Sample: Environmental water sample (self-collected)

[0066] Flow rate: 0.3mL / min

[0067] Column temperature: 40°C

[0068] Mode: Negative ion mode

[0069] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for preparing a nitrogen-doped carbon quantum dot composite film, characterized in that: The following steps are involved: (1) Using polyethylene polyamines as a nitrogen source and a carbon source to carry out a hydrothermal reaction to complete the preparation of carbon quantum dots; (2) 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and N-hydroxysuccinimide or N-hydroxysulfosuccinimide are added to the hydrophilic modified polymer membrane to react. After the reaction is completed, a nitrogen-doped carbon dot modified composite membrane is obtained, which is then immersed in water for storage.

2. The preparation method according to claim 1, characterized in that: The polyethylene polyamine is any one of urea, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, hexaethyleneheptamine, polyethyleneimine, and polyamide-amine type dendrimers.

3. The preparation method according to claim 1 or 2, characterized in that: The carbon source is any one of citric acid, vitamin C, glucose and biomass.

4. The preparation method according to claim 1, characterized in that: In step (1), the reaction medium is deionized water, and the molar ratio of the polyethylene polyamine to the carbon source is greater than or equal to 1.5; during the hydrothermal reaction, the reaction temperature is 160-220° C., and the reaction time is 3-6 hours.

5. The preparation method according to claim 4, characterized in that: The amount of the polyethylene polyamines used is greater than or equal to 1.5 times the equivalent of the carbon source.

6. The preparation method according to claim 1, characterized in that: In step (1), the prepared carbon quantum dots are dialyzed using a 3500 equivalent dialysis bag, and the dialyzed solution is then purified by freeze-drying for later use; the particle size of the carbon quantum dots is 3.0-7.2 nm.

7. The preparation method according to claim 1, characterized in that: In step (2), the preparation process of the hydrophilic modified polymer membrane is as follows: the polymer membrane is ultrasonically cleaned, immersed in an ethanol solution, then added to an alkaline potassium permanganate oxidant hydrophilic modification system, and then transferred to an acrylic acid solution, azobisisobutyronitrile is added, and a polymerization reaction is carried out. After the reaction is terminated, the polymer membrane is washed with deionized water and dried to obtain the hydrophilic modified polymer membrane.

8. The preparation method according to claim 7, characterized in that: The base in the alkaline potassium permanganate oxidant hydrophilic modification system is potassium hydroxide, the molar concentration of potassium hydroxide is 2.5 mol / L, and the mass fraction of potassium permanganate is 5%; During the modification process, the modification temperature is 40-60°C and the reaction time is 1-2 hours.

9. The preparation method according to claim 1, characterized in that: In step (2), the reaction is carried out under an argon protection environment, the reaction temperature is 0-8° C., the reaction time is 1-2 hours, and the reaction pH is controlled at 4.0-6.

0.

10. A nitrogen-doped carbon dot modified composite membrane prepared by the preparation method according to any one of claims 1 to 9.

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