Composite nanofiltration membrane, preparation method thereof and application thereof in quantum dot purification

Abstract

The application relates to a composite nanofiltration membrane and a preparation method thereof and application in quantum dot purification, and belongs to the technical field of composite nanofiltration membranes. The preparation method comprises the following steps: (1) dissolving structural unit molecules in a solvent A to obtain a structural unit molecule solution; (2) dissolving crosslinking molecules in a solvent B to obtain a crosslinking molecule solution; (3) the structural unit molecules in the structural unit molecule solution in the step (1) and the crosslinking molecules in the crosslinking molecule solution in the step (2) are subjected to a polymerization reaction at a two-phase solution interface, and then are subjected to heat treatment to obtain the composite nanofiltration membrane; the solvent A and the solvent B are mutually insoluble. The composite nanofiltration membrane can concentrate a quantum dot crude product solution, remove impurities such as a raw reaction solvent, a ligand and unreacted precursors, and replace a solvent in the quantum dot crude product solution into a target solvent, and the purification efficiency is high, and the original morphology and light emitting characteristics of the quantum dots are not affected.

Description

Technical Field

[0001] This invention belongs to the field of composite nanofiltration membrane technology, and particularly relates to a composite nanofiltration membrane, its preparation method, and its application in quantum dot purification. Background Technology

[0002] Quantum dots are important low-dimensional semiconductor materials, generally spherical or near-spherical in shape, with diameters typically between 2 nm and 20 nm. Common quantum dots are composed of elements IV, II-VI, IV-VI, or III-V. In recent years, with the rapid development of nanotechnology, quantum dots have attracted widespread attention due to their excellent luminescent properties, showing broad application prospects in fields such as light-emitting diode devices, information transmission, optical imaging, and biomedicine. Currently, the main synthesis method for quantum dots is chemical solution growth. In 1993, Professor Bawendi's research group at MIT first used this method to synthesize uniformly sized quantum dots in an organic solution (J. Am. Chem. Soc. 1993, 115, 8706). They dissolved three oxalate group elements—sulfur, selenium, and tellurium—in tri-n-octylphosphine oxide, and then reacted them with dimethylcadmium in an organic solution at 200 to 300 degrees Celsius to generate the corresponding cadmium sulfide, cadmium selenide, and cadmium telluride quantum dot materials. Researchers have subsequently improved and optimized this method, and most quantum dot materials can now be synthesized using chemical solution growth. However, after the reaction, the resulting quantum dot solution still contains a large number of ligands (such as trioctylphosphine, trioctyloxyphosphine, oleic acid, or oleylamine) and unreacted precursors, among other impurities. Therefore, purification of quantum dots is an essential pretreatment process before their use. Currently, the common method for purifying quantum dots is precipitation centrifugation. Because quantum dots are extremely small, they cannot be directly separated from the system by centrifugation. Typically, a certain amount of unsuitable solvent is first added to the solution to aggregate the quantum dots, then centrifugation is used to remove the upper layer of the original solvent, and finally, the quantum dots are dispersed in a good solvent system to achieve purification. Although this method has relatively high separation and purification efficiency, in practice, at least the following problems have been found:

[0003] When unsuitable solvents are used to induce quantum dot aggregation, the quantum dots exhibit severe agglomeration, making it difficult to restore them to their initial single-particle dispersed state during subsequent processing. This leads to luminescence quenching, affecting the quality of light-emitting devices or the effectiveness of fluorescence imaging. Furthermore, the addition of unsuitable solvents also impacts the original structure and morphology of the quantum dots. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the problems of quantum dot aggregation and luminescence quenching that easily occur in the purification process of quantum dots in the prior art.

[0005] To address the aforementioned technical problems, this invention provides a composite nanofiltration membrane, its preparation method, and its application in quantum dot purification.

[0006] The first objective of this invention is to provide a method for preparing a composite nanofiltration membrane, comprising the following steps:

[0007] (1) Dissolve the structural unit molecules in solvent A to obtain a solution of structural unit molecules;

[0008] (2) Dissolve the cross-linked molecules in solvent B to obtain a cross-linked molecule solution;

[0009] (3) The structural unit molecules in the structural unit molecule solution described in step (1) and the cross-linking molecules in the cross-linking molecule solution described in step (2) undergo a polymerization reaction at the interface of the two-phase solution, and then the composite nanofiltration membrane is obtained by heat treatment.

[0010] Solvent A and solvent B are immiscible.

[0011] In one embodiment of the present invention, in step (1), the structural unit molecule is one or more of piperazine, m-phenylenediamine, p-phenylenediamine, 5,5',6,6'-tetrahydroxy-3,3',3,3'-tetramethyl-1,1'-spirobisindane (TTSBI) and hematoxylin; the concentration of the structural unit molecule solution is 0.5 g / L-20 g / L.

[0012] In one embodiment of the present invention, in step (2), the crosslinking molecule is one or more of pyromellitic methyl methacrylate, terephthaloyl chloride and terephthalic diisocyanate; the concentration of the crosslinking molecule solution is 1 g / L-50 g / L.

[0013] In one embodiment of the present invention, in step (3), the reaction time of the polymerization reaction is 5s-240s; the reaction temperature is 15℃-35℃.

[0014] In one embodiment of the present invention, in step (3), the temperature of the heat treatment is 60°C-80°C and the time is 28 min-32 min.

[0015] In one embodiment of the present invention, the immiscible solvent system is (1) or (2):

[0016] (1) Solvent A is water, and solvent B is n-hexane;

[0017] (2) Solvent A is water and solvent B is xylene.

[0018] In one embodiment of the present invention, in step (1) or step (2), the solution further includes a base film.

[0019] In one embodiment of the present invention, the material of the base film is polysulfone, polyethersulfone, polyacrylonitrile, polyimide or nylon.

[0020] In one embodiment of the invention, solvent A and solvent B are immiscible, allowing a stable interfacial layer to form at their interface. The structural unit molecules in solvent A and solvent B possess high reactivity; when they diffuse to the interfacial layer, they react rapidly to form a cross-linked network. After a certain degree of reaction, the resulting cross-linked network structure becomes sufficiently dense, preventing the diffusion of structural and cross-linked molecules, and the reaction stops. Then, heat treatment promotes further reaction of unreacted active functional groups, enhancing the cross-linking degree and structural stability of the nanofiltration membrane. This interfacial polymerization reaction produces a nanofiltration membrane that ensures both the density of the selected layer and the ultrathin selective layer with a thickness of 10 nm to 200 nm. Therefore, in the quantum dot purification and separation process, the nanofiltration membrane can maintain both high separation flux and high-precision separation.

[0021] The second objective of this invention is to provide a composite nanofiltration membrane prepared by the method described above. The composite nanofiltration membrane has a cross-linked network structure, a thickness of 10 nm to 200 nm, and a pore size of 0.5 nm to 2.0 nm, allowing solvent molecules or certain low molecular weight solutes to pass through, while being able to trap quantum dots with a diameter greater than 2.0 nm.

[0022] A third objective of this invention is to provide an application of the aforementioned composite nanofiltration membrane in quantum dot purification, comprising the following steps:

[0023] (1) Under pressure-driven action, the crude quantum dot product solution is passed through the composite nanofiltration membrane to obtain a quantum dot-containing retentate;

[0024] (2) Add a good quantum dot solvent to the retentate containing quantum dots described in step (1), filter multiple times, and obtain a purified quantum dot solution.

[0025] In one embodiment of the present invention, in step (1), the crude quantum dot product solution is obtained by chemical solution growth method; the crude quantum dot product solution contains impurities such as the original reaction solution, ligands and unreacted precursors.

[0026] In one embodiment of the present invention, in step (2), the good solvent for the quantum dots is n-hexane, cyclohexane, or n-octane. The composite nanofiltration membrane can maintain structural stability for a long time in these solvents without dissolving or swelling.

[0027] The technical solution of the present invention has the following advantages compared with the prior art:

[0028] (1) The composite nanofiltration membrane of the present invention is prepared by interfacial polymerization. During the reaction, when the structural unit molecules and crosslinking molecules diffuse to the interfacial layer, they react rapidly to form a crosslinked network of molecules. After a certain period of reaction, the generated crosslinked network structure becomes sufficiently dense, preventing the diffusion of structural molecules and crosslinking molecules, and the reaction stops. The nanofiltration membrane prepared by this interfacial polymerization reaction can ensure the density of the generated selective layer and obtain an ultrathin selective layer with a thickness of 10 nm-200 nm.

[0029] (2) The composite nanofiltration membrane of the present invention has a pore size of 0.5nm-2nm and a narrow pore size distribution. Under the separation mechanism of pore size sieving, high-precision separation can be achieved.

[0030] (3) The composite nanofiltration membrane of the present invention has an ultrathin selective layer and high porosity, which can maintain a high separation flux. The flux of n-hexane during the separation process is 20 L / h.

[0031] (4) The composite nanofiltration membrane described in this invention can concentrate the crude quantum dot product solution, remove impurities such as the original reaction solvent, ligands, and unreacted precursors quickly, and can also replace the solvent in the crude quantum dot product solution with the target solvent, resulting in high purification efficiency and no impact on the original morphology and luminescence properties of the quantum dots.

[0032] (5) The quantum dot purification application described in this invention has advantages such as not introducing impurities, not causing quantum dot aggregation, not affecting the original morphology and luminescence properties of quantum dots, and has short purification time and high purification efficiency, making it suitable for large-scale purification processes.

[0033] (6) Compared with the traditional quantum dot coagulation-centrifugation purification process, the quantum dot purification application described in this invention optimizes the coagulation and centrifugation steps required for traditional purification, and focuses on using nanofiltration membranes as the core key to the quantum dot purification process. Compared with the traditional quantum dot purification steps that require a lot of time and complex electrically driven devices, this invention innovatively uses nanofiltration membranes for quantum dot retention and concentration, and phase conversion purification, achieving simple and efficient quantum dot purification. Attached Figure Description

[0034] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein:

[0035] Figure 1 This is a schematic diagram of the quantum dot purification process using the composite nanofiltration membrane of Example 1 in this invention.

[0036] Figure 2 This is a transmission electron microscope image of CdSe quantum dots before purification in this invention.

[0037] Figure 3 This is a transmission electron microscope image of CdSe quantum dots purified by nanofiltration membrane according to the present invention.

[0038] Figure 4 This is a transmission electron microscope image of CdSe quantum dots purified by precipitation and centrifugation according to the present invention. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0040] In this invention, unless otherwise specified, the preparation method of the CdSe quantum dot solution to be treated is as follows: First, 5 mmol of Se is added to 10 mL of tri-n-butylphosphine (TBP), and stirred at room temperature for 3 h to obtain a Se-TBP solution. Second, 0.02 mmol of cadmium acetate, 2 mL of 1-dodecyl mercaptan, and 8 mL of oleylamine are added to a 100 mL three-necked flask, and heated to 110 °C under a nitrogen atmosphere to obtain a clear cadmium acetate mixed solution. Third, the above cadmium acetate mixed solution is heated to 170 °C, and 0.1 mL of a 0.05 mmol Se-TBP solution is rapidly injected under vigorous stirring. This temperature is maintained for 1 h, and then slowly cooled to room temperature to obtain the CdSe quantum dot solution to be treated.

[0041] Example 1

[0042] A composite nanofiltration membrane and its preparation method, specifically including the following steps:

[0043] (1) Place the polyethersulfone ultrafiltration membrane in a 2.4 g / L piperazine aqueous solution and soak for 60 s.

[0044] (2) Remove excess piperazine solution from the surface of the ultrafiltration membrane, and use the remaining piperazine as a monomer for interfacial polymerization reaction.

[0045] (3) A 2.0 g / L solution of trimesoyl chloride and hexane was brought into contact with the ultrafiltration membrane treated in step (2) to initially form a polyamide nanofiltration membrane on the surface; the reaction time was 30 s and the reaction temperature was 27 °C.

[0046] (4) The membrane after the reaction in step (3) is immersed in hexane for 30 seconds to remove residual trimesoyl chloride monomer, and then heat-cured at 60°C for 30 minutes.

[0047] (5) There are unreacted acyl chloride groups in the active layer network of the membrane after step (4). In order to promote the hydrolysis of acyl chloride, the membrane is stored in water at 4°C to obtain a composite nanofiltration membrane.

[0048] In this embodiment, a polyethersulfone ultrafiltration membrane serves as the support layer for the interfacial polymerization reaction, and piperazine, as the reactive monomer, is dispersed in the aqueous phase. The interfacial polymerization reaction allows the piperazine monomer to cross the aqueous phase and enter the organic phase, reacting with trimesoyl chloride monomer to form a polyamide selective separation layer. The piperazine monomer, dispersed in the aqueous phase, undergoes interfacial polymerization upon contact with TMC. The polyamide active layer of the polyamide nanofiltration membrane, after heat treatment, exhibits a high degree of cross-linking, which improves the separation accuracy of the polyamide nanofiltration membrane. The prepared polyamide nanofiltration membrane was tested and found to have an effective separation pore size of 1 nm-2 nm and a thickness of 25 nm.

[0049] Example 2

[0050] A composite nanofiltration membrane and its preparation method, specifically including the following steps:

[0051] (1) Place the polyethersulfone ultrafiltration membrane in a 1.0 g / L alkaline aqueous solution of 5,5',6,6'-tetrahydroxy-3,3',3,3'-tetramethyl-1,1'-spirobisindane (TTSBI) for 60 s.

[0052] (2) Remove excess TTSBI solution from the surface of the ultrafiltration membrane, and use the remaining TTSBI as a monomer for interfacial polymerization reaction.

[0053] (3) The ultrafiltration membrane after step (2) is initially formed on the surface by contact reaction with a 2.0 g / L solution of trimesoyl chloride and hexane; the reaction time is 2 min and the reaction temperature is 27 °C.

[0054] (4) The membrane after the reaction in step (3) is immersed in hexane for 30 seconds to remove residual trimesoyl chloride monomer, and then heat-cured at 80°C for 30 minutes to obtain a composite nanofiltration membrane.

[0055] In this embodiment, an ultrathin cross-linked polyaryl ester nanomembrane with enhanced micropores was obtained through interfacial polymerization of alkali-activated phenolic hydroxyl groups and trimesoyl chloride monomer at the interface between the aqueous and organic phases. The prepared polyester nanofiltration membrane exhibits excellent performance in organic solvent nanofiltration, characterized by high permeability and high selectivity. The effective pore size of the prepared polyester nanofiltration membrane was measured to be 0.5 nm-1.5 nm, and its thickness was 50 nm.

[0056] Example 3

[0057] A composite nanofiltration membrane and its preparation method, specifically including the following steps:

[0058] (1) Place the polyethersulfone ultrafiltration membrane in a 2.0 g / L hematoxylin alkaline aqueous solution (pH 14) and soak for 60 s.

[0059] (2) Remove excess hematoxylin solution from the surface of the ultrafiltration membrane, and use the remaining hematoxylin as a monomer for interfacial polymerization reaction.

[0060] (3) A 2.0 g / L xylene solution of terephthalic diisocyanate was brought into contact with the ultrafiltration membrane treated in step (2) to initially form a polyurethane nanofiltration membrane on the surface; the reaction time was 120 s and the reaction temperature was 25 °C.

[0061] (4) The membrane after the reaction in step (3) is immersed in n-hexane for 30s to remove residual terephthalic diisocyanate monomer, and then heat-cured at 80°C for 30min to obtain a composite nanofiltration membrane.

[0062] In this embodiment, an interfacial polymerization reaction is used. The aqueous phase is an alkaline solution containing hematoxylin monomer, and the organic phase is a xylene solution containing terephthalic diisocyanate. The reaction is carried out on a polyethersulfone ultrafiltration membrane support layer to obtain a polyurethane layer, thereby improving the filtration performance of the polyurethane nanofiltration membrane. The prepared polyurethane nanofiltration membrane was tested and found to have an effective pore size of 1.5 nm-2.0 nm and a thickness of 75 nm.

[0063] Comparative Example 1

[0064] (1) Place the polyethersulfone ultrafiltration membrane in a 2.0 g / L hematoxylin aqueous solution (pH 7) and soak for 60 s.

[0065] (2) Remove excess hematoxylin solution from the surface of the ultrafiltration membrane, and use the remaining hematoxylin as a monomer for interfacial polymerization reaction.

[0066] (3) A 2.0 g / L xylene solution of terephthalic diisocyanate was brought into contact with the ultrafiltration membrane treated in step (2) to initially form a polyurethane nanofiltration membrane on the surface; the reaction time was 120 s and the reaction temperature was 25 °C.

[0067] (4) Immerse the membrane after the reaction in step (3) in n-hexane for 30 seconds to remove residual terephthalic diisocyanate monomer.

[0068] Scanning electron microscopy revealed that no polyurethane nanofiltration membrane formed on the surface of the polyethersulfone ultrafiltration membrane. This indicates that the unactivated hematoxylin molecules have relatively weak activity and cannot react rapidly with terephthalic diisocyanate to form a polyurethane nanofiltration membrane.

[0069] Comparative Example 2

[0070] (1) Place the polyethersulfone ultrafiltration membrane in a 2.0 g / L hematoxylin alkaline aqueous solution (pH 14) and soak for 60 s.

[0071] (2) Remove excess hematoxylin solution from the surface of the ultrafiltration membrane, and use the remaining hematoxylin as a monomer for interfacial polymerization reaction.

[0072] (3) A 2.0 g / L xylene solution of terephthalic diisocyanate was brought into contact with the ultrafiltration membrane treated in step (2) to initially form a polyurethane nanofiltration membrane on the surface; the reaction time was 120 s and the reaction temperature was 25 °C.

[0073] (4) The membrane after the reaction in step (3) is immersed in n-hexane for 30s to remove residual terephthalic diisocyanate monomer, and then dried at 25°C for 30min to obtain a composite nanofiltration membrane.

[0074] In this embodiment, the prepared polyurethane nanofiltration membrane was found to have an effective pore size greater than 4.0 nm and a thickness of approximately 75 nm. During the separation process, most of the quantum dots permeated through the separation membrane, failing to achieve the desired separation and purification.

[0075] Application Example 1

[0076] Reference Figure 1 As shown, the composite nanofiltration membrane prepared in Example 3 was used to purify CdSe quantum dots, specifically including the following steps:

[0077] (1) Pour 20 mL of the CdSe quantum dot solution to be treated into the membrane cell, and introduce a high-pressure gas with a pressure of 5 atmospheres above the solution. Under the pressure drive, the CdSe quantum dot solution to be treated is filtered through a composite nanofiltration membrane to concentrate the CdSe quantum dot solution to be treated to 2 mL.

[0078] The composite nanofiltration membrane used in this step has a highly cross-linked active layer, which improves the separation accuracy and high permeability of the composite nanofiltration membrane. After filtration, the CdSe quantum dots in the raw solution are effectively retained, enter the retentate, and are concentrated and returned to the membrane tank.

[0079] (2) Add 18 mL of n-hexane to the concentrate obtained in step (1), pressurize it with a pressure pump, and filter it again through a composite nanofiltration membrane to concentrate the solution to 2 mL.

[0080] This step utilizes the different dispersion properties of quantum dots in different solvents to transfer CdSe quantum dots from the original solution to n-hexane solvent, thereby achieving the effect of concentrating and purifying CdSe quantum dots.

[0081] (7) Repeat step (2) 3 times, collect the retentate, and obtain the purified quantum dot solution.

[0082] This step reduces the amount of residual permeate in the original solution and increases the amount of hexane solvent, causing the quantum dots to transfer from the original solution to the hexane solvent, thus achieving the effect of quantum dot purification.

[0083] Comparative Application Example 1

[0084] The purification of CdSe quantum dots using a coagulation-precipitation method includes the following steps: the CdSe quantum dot solution to be treated is mixed with acetone and centrifuged at 10,000 r / min for 30 min using a high-speed centrifuge to remove the supernatant solution. Then, the lower layer solution is redispersed in n-hexane to obtain the purified quantum dot solution.

[0085] Test case

[0086] The CdSe quantum dots before purification, the CdSe quantum dots purified by the composite nanofiltration membrane in Application Example 1, and the CdSe quantum dots purified by the coagulation-precipitation method in Comparative Application Example 1 were characterized, and the results are shown in 2-4.

[0087] contrast Figure 2 and Figure 3 It can be seen that the CdSe quantum dots purified by the composite nanofiltration membrane have a narrower size distribution, because CdSe quantum dots with a diameter of less than 2 nm are removed during the purification process. Furthermore, the CdSe quantum dots purified by the composite nanofiltration membrane still maintain good dispersion, without any quantum dot aggregation. (Comparison) Figure 4 Conventional coagulation-precipitation methods involve adding a polar solvent (such as acetone, ethanol, or isopropanol) to the CdSe quantum dot solution. This causes the CdSe quantum dots to aggregate, forming large clusters that are easier to centrifuge. The supernatant is then removed, and a fresh non-polar solvent is added. The aggregated quantum dots are then redispersed under ultrasonication. However, during aggregation, CdSe quantum dots possess very high surface energy, and some may permanently aggregate. This aggregation significantly impacts the emission wavelength and luminescence efficiency. Therefore, CdSe quantum dots purified using a composite nanofiltration membrane avoid the problem of quantum dot aggregation and remove small-sized quantum dots, resulting in a more uniform particle size distribution.

[0088] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. An application of a composite nanofiltration membrane in quantum dot purification, characterized in that, Includes the following steps, S1. Under pressure, the crude quantum dot product solution is passed through a composite nanofiltration membrane to obtain a quantum dot-containing filtration solution; the composite nanofiltration membrane has a cross-linked network structure; its thickness is 10nm-200nm; and its pore size is 0.5nm-2.0nm. S2. Add a good quantum dot solvent to the retentate containing quantum dots described in step S1, and filter multiple times to obtain a purified quantum dot solution. The method for preparing the composite nanofiltration membrane includes the following steps: (1) Dissolve the structural unit molecules in solvent A to obtain a solution of structural unit molecules; (2) Dissolve the cross-linked molecules in solvent B to obtain a cross-linked molecule solution; (3) The structural unit molecules in the structural unit molecule solution described in step (1) and the cross-linking molecules in the cross-linking molecule solution described in step (2) undergo a polymerization reaction at the interface of the two-phase solution, and then the composite nanofiltration membrane is obtained by heat treatment. Solvent A and solvent B are immiscible.

2. The application according to claim 1, characterized in that, In step (1), the structural unit molecule is one or more of piperazine, m-phenylenediamine, p-phenylenediamine, 5,5',6,6'-tetrahydroxy-3,3',3,3'-tetramethyl-1,1'-spirobine and hematoxylin; the concentration of the structural unit molecule solution is 0.5 g / L-20 g / L.

3. The application according to claim 1, characterized in that, In step (2), the crosslinking molecule is one or more of pyromellitic methyl methacrylate, terephthaloyl chloride and terephthalic diisocyanate; the concentration of the crosslinking molecule solution is 1 g / L-50 g / L.

4. The application according to claim 1, characterized in that, In step (3), the reaction time of the polymerization reaction is 5s-240s; the reaction temperature is 15℃-35℃.

5. The application according to claim 1, characterized in that, The immiscible solvent system is (1) or (2): (1) Solvent A is water, and solvent B is n-hexane; (2) Solvent A is water and solvent B is xylene.

6. The application according to claim 1, characterized in that, In step (1) or step (2), the solution also includes a base membrane; the material of the base membrane is polysulfone, polyethersulfone, polyacrylonitrile, polyimide or nylon.

7. The application according to claim 1, characterized in that, In step (3), the heat treatment temperature is 60℃-80℃ and the time is 28min-32min.

8. The application according to claim 1, characterized in that, In step S1, the good solvent for quantum dots is n-hexane, cyclohexane, or n-octane.

Images