Water-soluble dual-frequency upconversion luminescent materials and their preparation methods

Abstract

The application provides a water-soluble dual-frequency up-conversion luminescent material and a preparation method thereof. The up-conversion luminescent material comprises an inert core, a luminescent shell, an active shell covering the luminescent shell, and a surface CTAB ligand. The chemical formula of the inert core is beta-NaYF4:Gd3+; the chemical formula of the luminescent shell is beta-NaYF4:Er3+, Gd3+; and the chemical formula of the active shell is beta-NaYF4:Yb3+. Thus, the core-shell-shell structure material of the application can emit high-brightness green up-conversion luminescence under the co-excitation of two different wavelengths of near-infrared light (840-860 nm and 1520-1550 nm). The up-conversion luminescent material with the three-layer structure of the inert core-luminescent shell-active shell can significantly improve the luminescent intensity of the dual-frequency co-excitation compared with the single-core material. The hydrophilic CTAB surfactant on the surface enables the dual-frequency up-conversion material to be dispersed in a polar solvent, so as to obtain a non-toxic and physically stable up-conversion suspension, and to realize large-size and high-resolution spatial three-dimensional suspension display.

Description

Technical Field

[0001] This invention belongs to the interdisciplinary technical fields of true 3D display, upconversion luminescence and preparation of nano-core-shell materials, specifically involving water-soluble dual-frequency upconversion luminescent materials and their preparation methods. Background Technology

[0002] Although human display technology has evolved from traditional blurry black-and-white photographs to high-definition 8K televisions, with significant improvements in image resolution and clarity, mainstream display technology remains at the two-dimensional level, regardless of the increase in resolution. However, the world we live in is indeed three-dimensional, and people have always longed to view more realistic three-dimensional images. Therefore, research into 3D displays has been booming. Because 3D display technology can reconstruct the complete object and light field information of a realistic 3D scene, it has broad application prospects in fields such as national defense, medical surgery, culture and entertainment, advertising, and education. As research into 3D display technology deepens, its applications are gradually becoming commercialized, increasingly appearing in people's lives and bringing them more and more benefits.

[0003] The objects in the real world that people see have a three-dimensional sense, mainly because the human eyes can capture two images of an object with a certain parallax. After being fused by the central nervous system of the brain, a three-dimensional sense is produced. At present, three-dimensional display can be divided into traditional three-dimensional display and true three-dimensional display. The three-dimensional display currently widely used is mainly based on the imaging principle of binocular parallax [8]. The display device sends two images with parallax to the left and right eyes of the viewer respectively. However, it is easy to cause a mismatch between the image and the convergence and accommodation of the eyes, which causes the brain to synthesize three-dimensional images in a disordered manner. At this time, long-term viewing will cause eye fatigue and dizziness. In severe cases, it will lead to the degeneration of the depth and dynamic perception functions of the eyes.

[0004] Traditional 3D displays are primarily based on the principle of binocular parallax. By wearing assistive devices, the human eye receives two images with a certain parallax, which are then fused by the brain to form a three-dimensional image. However, 3D displays based on binocular parallax have significant drawbacks, such as the need for assistive devices, limited viewing window, and discomfort after prolonged viewing. Therefore, more and more researchers are focusing on achieving ideal 3D displays—that is, obtaining the same three-dimensional stereoscopic effect as real objects without the need for assistive devices.

[0005] Spatial 3D display, which truly realizes the display of three-dimensional images within a spatial volume, has broad research significance. Spatial 3D display can be divided into spatial 3D display based on luminescent materials, spatial 3D display based on multi-layer passive arrays, and volumetric 3D display. For spatial 3D display, the requirements for the display substrate or equipment are generally high. When the performance of the display substrate is good, especially when the luminous efficiency is significantly improved, the requirements for the system equipment can be greatly reduced. Therefore, finding high-performance display substrates is of great significance for constructing spatial 3D display systems.

[0006] Compared with other fluorescent materials, upconversion materials exhibit anti-Stokes luminescence, meaning that the material can be excited by low-energy light and then emit high-energy light. In other words, when excited by long-wavelength, low-frequency light, the material can emit short-wavelength, high-frequency light.

[0007] Upconversion materials composed of rare-earth elements are typically excited by infrared light, eliminating the interference from visible light excitation. When excited by dual-frequency invisible lasers, the resulting patterns are even clearer. However, when using solid-state upconversion materials as display media, the size is often limited to the centimeter scale, preventing any increase in volume. Furthermore, the excitation efficiency is insufficient, still requiring high-power lasers for large-scale displays. The materials are mostly solid or dispersed in toxic and volatile organic solvents, posing health risks and having short shelf lives. These limitations make the use of upconversion materials for large-scale spatial 3D displays a distant dream. Summary of the Invention

[0008] The present invention aims to improve at least one of the above-mentioned technical problems to at least some extent.

[0009] To address the aforementioned problems, this invention provides a water-soluble dual-frequency upconversion luminescent material. The upconversion luminescent material comprises an inert core, a luminescent shell encapsulating the inert core, an active shell encapsulating the luminescent shell, and a surfactant CTAB. The inert core has the chemical formula β-NaYF4:Gd3+; the luminescent shell has the chemical formula β-NaYF4:Er3+,Gd3+; and the active shell has the chemical formula β-NaYF4:Yb3+. Therefore, the core-shell-shell structure of this invention can emit high-brightness green upconversion luminescence under co-excitation with near-infrared light of two different wavelengths (840~860) nm and (1520~1550) nm. The three-layer structure of the inert core-luminescent shell-active shell upconversion luminescent material significantly improves the luminescence intensity under dual-frequency co-excitation compared to mononuclear materials.

[0010] According to an embodiment of the present invention, in the inert core, the molar amount of Gd3+ is 5% to 30% of the total molar amount of β-NaYF4 and Gd3+.

[0011] According to an embodiment of the present invention, in the luminescent shell, the molar amount of Er3+ is 0.5% to 3% of the total molar amount of β-NaYF4, Er3+ and Gd3+; in the luminescent shell, the molar amount of Gd3+ is 5% to 30% of the total molar amount of β-NaYF4, Er3+ and Gd3+.

[0012] According to an embodiment of the present invention, in the active shell, the molar amount of Yb3+ is 2% to 10% of the total molar amount of β-NaYF4 and Yb3+.

[0013] This invention also provides a method for preparing the water-soluble dual-frequency upconversion luminescent material described above, the method comprising: preparing an inert core β-NaYF4:Gd3+; using the obtained inert core β-NaYF4:Gd3+ as a crystal nucleus, coating the outer layer of the inert core with a luminescent shell β-NaYF4:Er3+,Gd3+ to obtain an inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+; using the obtained inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+ as a crystal nucleus, coating the outer layer of the inert core-luminescent shell with a luminescent shell β-NaYF4:Yb3+ to obtain an inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@ Using the obtained inert core-luminescent shell-active shell structure β-NaYF4:Yb3+ as the crystal nucleus, the surfactant hexadecyltrimethylammonium bromide is coated onto the outer layer of the inert core-luminescent shell-active shell to obtain a water-soluble core-shell-shell structured upconversion luminescent material β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+. Therefore, the water-soluble dual-frequency upconversion luminescent material prepared by this method possesses all the characteristics and advantages of the water-soluble dual-frequency upconversion luminescent material described above, which will not be repeated here. Furthermore, this method has the advantages of simple operation and low production cost, making it suitable for large-scale industrial production.

[0014] According to an embodiment of the present invention, the preparation of the inert core β-NaYF4:Gd3+ includes: mixing a first rare earth salt with oleic acid and 1-octadecene, reacting to generate a first rare earth oleic acid complex, mixing the first rare earth oleic acid complex with a methanol solution containing a first sodium source and a first fluorine source, and reacting after the methanol evaporates to obtain the luminescent core β-NaYF4:Gd3+; wherein the first rare earth salt includes a Y-containing rare earth salt and a Gd-containing rare earth salt; the preparation of the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+ includes: mixing a second rare earth salt with oleic acid and 1-octadecene, reacting to generate a first rare earth oleic acid complex, and ... The reaction generates a second rare earth oleic acid complex. The inert core β-NaYF4:Gd3+ is added to the second rare earth oleic acid complex as a crystal nucleus. A methanol solution containing a second sodium source and a second fluorine source is added. After the methanol evaporates, the reaction proceeds to obtain an inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+, wherein the second rare earth salt includes a Y-containing rare earth salt, an Er-containing rare earth salt, and a Gd-containing rare earth salt. An inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ is then prepared. The process includes: mixing a third rare earth salt with oleic acid and 1-octadecene, reacting to generate a third rare earth oleic acid complex, adding the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+ as a crystal nucleus to the third rare earth oleic acid complex, adding a methanol solution containing a third sodium source and a third fluorine source, and reacting after the methanol evaporates to obtain an inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+, wherein the third rare earth salt includes Y-containing rare earth salts and Yb-containing rare earth salts; preparation The preparation of a water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ involves: mixing the surfactant CTAB with deionized water; adding the obtained inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ to the above mixture and reacting to obtain the water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+.

[0015] According to an embodiment of the present invention, when preparing the luminescent core β-NaYF4:Gd3+, the ratio of the first rare earth salt to oleic acid and 1-octadecene is 1 mmol:(5~10) mL:(10~20) mL; when preparing the luminescent core-inert shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+, the ratio of the second rare earth salt to oleic acid and 1-octadecene is 1 mmol:(5~10) mL:(10~20) mL.

[0016] According to an embodiment of the present invention, the first rare earth salt, the second rare earth salt, and the third rare earth salt are rare earth chlorides; the first sodium source, the second sodium source, and the third sodium source are NaOH; and the first fluorine source, the second fluorine source, and the third fluorine source are NH4F.

[0017] According to embodiments of the present invention, when preparing the inert core β-NaYF4:Gd3+, the ratio of the first rare earth salt to the first sodium source, the first fluorine source, and methanol is 1 mmol:(1.5~3.5) mmol:(3~5) mmol:(8~12) mL; when preparing the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+, the ratio of the second rare earth salt to the second sodium source, the second fluorine source, and methanol is 1 mmol:(1.5~3.5) mmol:(3~5) mmol:(8~12) mL; when preparing the inert core-luminescent shell-active shell structure β-NaYF4:Gd .... When preparing the water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Yb3+, the ratio of the third rare earth salt to the third sodium source, the third fluorine source, and methanol is 1 mmol:(1.5~3.5) mmol:(3~5) mmol:(8~12) mL; when preparing the water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+, the ratio of the surfactant CTAB to deionized water is (0.1g~0.2g):40mL.

[0018] According to embodiments of the present invention, when preparing the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+, the molar ratio of the second rare earth salt to the inert core β-NaYF4:Gd3+ is 1:(0.9~1.3); when preparing the upconversion luminescent material β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ with an inert core-luminescent shell-active shell structure, the molar ratio of the third rare earth salt to the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+ is 1:(0.9~1.3); The molar ratio of Gd3+ is 1:(0.9~1.3); when preparing the water-soluble inert core-luminescent shell-active shell structured upconversion luminescent material β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+, the mass ratio of the surfactant to the inert core-luminescent shell-active shell structured upconversion luminescent material β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ is 1:(0.9~1.3). Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of a water-soluble dual-frequency upconversion luminescent material in one embodiment of the present invention.

[0020] Figure 2 This is a flowchart of a method for preparing water-soluble dual-frequency upconversion luminescent materials in one embodiment of the present invention.

[0021] Figure 3 This is a morphological image of the water-soluble dual-frequency upconversion luminescent material prepared in Example 1 of this invention, taken by a scanning electron microscope.

[0022] Figure 4 This is the emission spectrum of the inert core, inert core-luminescent shell, and inert core-luminescent shell-active shell of the water-soluble dual-frequency upconversion luminescent material prepared in Example 1 of this invention under dual-frequency excitation.

[0023] Figure 5 The image is a three-dimensional display image generated based on the water-soluble dual-frequency upconversion luminescent material prepared in Example 1 of this invention. Detailed Implementation

[0024] The embodiments of this application are described in detail below. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents used, unless otherwise specified, are all conventional products that can be purchased on the market.

[0025] The water-soluble dual-frequency upconversion luminescent material provided by this invention, reference Figure 1 The water-soluble dual-frequency upconversion luminescent material comprises an inert core, a luminescent shell coating the inert core, an active shell coating the luminescent shell, and a surfactant coating the active shell. The chemical formula of the inert core is β-NaYF4:Gd3+; the chemical formula of the luminescent shell is β-NaYF4:Er3+,Gd3+; the chemical formula of the active shell is β-NaYF4:Yb3+; and the surfactant is CTAB. Therefore, the core-shell-shell structure material of this invention can emit high-brightness green upconversion luminescence under co-excitation with near-infrared light of two different wavelengths (840~860) nm and (1520~1550) nm. Compared with the core-shell structure β-NaYF4:Er3+,Gd3+@β-NaYF4, the luminescence intensity of the core-shell-shell structure material of this invention is nearly twice that of the core-shell structure material. Specifically, this invention improves upconversion luminescence intensity by encapsulating and isolating Er3+ ions spatially, thereby reducing the impact of Er3+ concentration quenching. On the other hand, it reduces surface defects in the material, lowers non-radiative transitions, and increases the absorption of near-infrared light both inside and outside the material, thus enhancing the upconversion luminescence intensity of dual-frequency co-excitation. Furthermore, the surfactant in this invention improves the water solubility of the upconversion material, allowing it to be dispersed in common non-toxic solvents, making it suitable for large-scale use.

[0026] According to an embodiment of the present invention, the molar amount of Gd3+ in the inert core is 5% to 30% of the total molar amount of β-NaYF4 and Gd3+. This allows the inert core to possess a favorable hexagonal phase morphology. When the Gd3+ doping concentration increases from 5% to 20%, the green fluorescence intensity generated by the co-excitation of (1520~1550) nm and (840~860) nm gradually increases. After the Gd3+ doping concentration exceeds 20%, the green fluorescence intensity shows a decreasing trend. The initial increase in emission intensity is related to the increase in the relative number of hexagonal phase atoms in the crystal structure. This is because when Y3+ is replaced by Gd3+, the formation energy of each atom in the hexagonal phase increases by approximately 0.07 eV, indicating that NaGdF4 is more energy-stable than NaYF4 in the hexagonal phase, and the increase in Gd3+ doping concentration is beneficial to the formation of hexagonal phase nanocrystals. The subsequent decrease in emission intensity is mainly attributed to the reduction in crystal size. This is because the size evolution of the nanocrystals can be partly attributed to the strong influence of Gd3+ dopant ions on the crystal growth rate through surface charge modification. After Gd3+ ions replace Y3+ ions in the lattice, the electron charge density on the crystal surface increases. Due to the increased charge packing, the change in electron charge density on the surface of small-sized nanocrystals can slow the diffusion of negatively charged F- ions to the surface, thus leading to a tunable reduction in the size of NaYF4 nanocrystals. Smaller nanocrystals have relatively more quenching sites on their surface, resulting in enhanced non-radiative energy relaxation at the luminescent centers, thereby suppressing upconversion luminescence.

[0027] According to an embodiment of the present invention, the molar amount of Er3+ in the luminescent shell is 0.5% to 3% of the total molar amount of β-NaYF4, Er3+, and Gd3+. This allows the luminescent shell to have a strong dual-frequency excitation upconversion luminescence intensity. If the doping ratio of Er3+ in the luminescent shell is too small, there will be too few luminescent centers, resulting in a low upconversion luminescence intensity during dual-frequency excitation. If the doping ratio of Er3+ in the luminescent shell is too large, Er3+ will undergo concentration quenching, leading to a decrease in the upconversion luminescence intensity during dual-frequency excitation.

[0028] According to embodiments of the present invention, the molar amount of Yb3+ in the active shell is 2% to 10% of the total molar amount of β-NaYF4 and Yb3+. This allows the core-shell-shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ to exhibit strong dual-frequency excitation upconversion luminescence intensity. If the doping ratio of Yb3+ in the luminescent shell is too small, there will be too little sensitizer, resulting in low dual-frequency excitation upconversion luminescence intensity. If the doping ratio of Yb3+ in the luminescent shell is too large, there will be too much sensitizer, leading to a decrease in dual-frequency excitation upconversion luminescence intensity.

[0029] The upconversion luminescent material according to the embodiments of the present invention has the following advantages: (1) The upconversion luminescent material of the present invention rationally utilizes the core-shell-shell structure to successfully passivate the surface of the mononuclear upconversion nanomaterial. The doped Er3+ ions are confined in the internal core space, and the luminescence quenching caused by the high-energy vibration generated by the reaction of surface ions with the surrounding environment is suppressed. The efficiency of the green fluorescence upconversion process of the core-shell structure is significantly improved compared with that of the mononuclear structure. (2) The inert core / luminescent shell / active shell nanostructure designed in the present invention can promote the transformation of the NaYF4:Gd3+ core from cubic phase to hexagonal phase while reducing its size to facilitate the coating of the luminescent layer and the active layer; it improves the utilization rate of the upconversion luminescent material for (1520~1550) nm near-infrared light, thereby making the upconversion luminescent material have excellent performance of high brightness and high contrast. (3) The surfactant CTAB of the present invention is used to modify the surface of oil-soluble inert core / luminescent shell / active shell nanomaterials. The surfactant can convert oil-soluble nanomaterials into water-soluble nanomaterials so that they can be dispersed in stable and non-toxic polar solvents; thus improving the feasibility and suitability of upconversion luminescent materials for spatial three-dimensional suspension display.

[0030] This invention also provides a method for preparing the water-soluble dual-frequency upconversion luminescent material described above, see reference. Figure 2 The method includes: S100, preparing inert core β-NaYF4:Gd3+.

[0031] According to an embodiment of the present invention, an inert core β-NaYF4:Gd3+ can be prepared by a solvothermal method. Specifically, step S100 includes: mixing a first rare earth salt with oleic acid and 1-octadecene, reacting to generate a first rare earth oleic acid complex, mixing the first rare earth oleic acid complex with a methanol solution containing a first sodium source and a first fluorine source, and reacting after the methanol evaporates to obtain the inert core β-NaYF4:Gd3+; wherein the first rare earth salt includes a Y-containing rare earth salt and a Gd-containing rare earth salt.

[0032] According to an embodiment of the present invention, in the reaction for generating the first rare earth oleic acid complex, the reaction is carried out under an inert gas protection, the reaction temperature is (150℃~160℃), the reaction time is 30 min, and after the reaction for generating the first rare earth oleic acid complex is completed, the method further includes: cooling the reaction system to (40℃~50℃), adding a methanol solution containing a first sodium source and a first fluorine source under vigorous stirring, and maintaining the temperature at (40℃~50℃) for 0.5 h; heating to (100℃~120℃) and maintaining the temperature for 10 min to remove methanol; rapidly heating to 294~300℃ and maintaining the temperature for 1.5 h, and cooling to room temperature; centrifuging the reaction solution with an ethanol solution at a speed of 8000 r / min for 10 min, collecting the precipitate, and washing it three times with a mixture of cyclohexane and anhydrous ethanol, and finally dispersing the product in cyclohexane to obtain β-NaYF4:Gd3+.

[0033] According to some embodiments of the present invention, the contents of each component in the Y-containing rare earth salt and the Gd-containing rare earth salt in the first rare earth salt are weighed according to the stoichiometric ratio of the inert core β-NaYF4:Gd3+. Specifically, since the molar amount of Gd3+ in the inert core is 5% to 30% of the total molar amount of β-NaYF4 and Gd3+, the first rare earth salt should satisfy the following: the percentage of the molar amount of Gd in the Gd-containing rare earth salt to the sum of the molar amounts of Y in the Y-containing rare earth salt and Gd in the Gd-containing rare earth salt is 5% to 30%.

[0034] According to an embodiment of the present invention, when preparing the inert core β-NaYF4:Gd3+, the ratio of the first rare earth salt to oleic acid and 1-octadecene is 1 mmol:(5~10) mL:(10~20) mL. Under these conditions, the rapid dissolution of the first rare earth salt and the formation of the first rare earth oleic acid complex are facilitated.

[0035] According to an embodiment of the present invention, the first rare earth salt is a rare earth chloride; the first sodium source is NaOH; and the first fluorine source is NH4F.

[0036] According to an embodiment of the present invention, when preparing the inert core β-NaYF4:Gd3+, the ratio of the first rare earth salt to the first sodium source, the first fluorine source, and methanol is 1 mmol:(1.5~3.5) mmol:(3~5) mmol:(8~12) mL. Under these conditions, the generation of the luminescent core β-NaYF4:Gd3+ is favored.

[0037] S200. Using the obtained inert core β-NaYF4:Gd3+ as the crystal nucleus, a luminescent shell β-NaYF4:Er3+,Gd3+ is coated on the outer layer of the inert core to obtain an inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+.

[0038] According to an embodiment of the present invention, an inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+ can be prepared by a solvothermal method. Specifically, step S200 includes: mixing a second rare earth salt with oleic acid and 1-octadecene, reacting to generate a second rare earth oleic acid complex, adding the inert core β-NaYF4:Gd3+ as a seed crystal to the second rare earth oleic acid complex, adding a methanol solution containing a second sodium source and a second fluorine source, and reacting after the methanol evaporates to obtain the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+, wherein the second rare earth salt includes a Y-containing rare earth salt, an Er-containing rare earth salt, and a Gd-containing rare earth salt.

[0039] According to an embodiment of the present invention, in the reaction for generating the second rare earth oleic acid complex, the reaction is carried out under an inert gas atmosphere at a temperature of 150°C to 160°C. The reaction time is 30 min. After the reaction to generate the second rare earth oleic acid complex is completed, the method further includes: cooling the reaction system to (40℃~50℃), adding seed crystal β-NaYF4:Gd3+ under vigorous stirring, stirring for 5 min, then adding a methanol solution containing a second sodium source and a second fluorine source, and maintaining the temperature at (40℃~50℃) for 0.5 h; raising the temperature to (100℃~120℃) and maintaining the temperature for 10 min to remove methanol; rapidly raising the temperature to 294~300℃ and maintaining the temperature for 1.5 h, and then cooling to room temperature; centrifuging the reaction solution with ethanol solution at 8000 r / min for 10 min, collecting the precipitate, and washing it three times with a mixture of cyclohexane and anhydrous ethanol. The final product is dispersed in cyclohexane to obtain an inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+.

[0040] According to some embodiments of the present invention, the contents of each component in the second rare earth salt containing Y rare earth salt, Er rare earth salt, and Gd rare earth salt are weighed according to the stoichiometric ratio of the inert core β-NaYF4:Er3+,Gd3+. Specifically, since the molar amount of Er3+ in the luminescent shell is 0.5% to 3% of the total molar amount of β-NaYF4, Er3+, and Gd3+, and the molar amount of Gd3+ is 5% to 30% of the total molar amount of β-NaYF4, Er3+, and Gd3+, the second rare earth salt should satisfy the following: the percentage of the molar amount of Er in the Er-containing rare earth salt to the sum of the molar amounts of Y, Er, and Gd in the Y-containing rare earth salt is 0.5% to 3%, and the percentage of the molar amount of Gd in the Gd-containing rare earth salt to the sum of the molar amounts of Y, Er, and Gd in the Y-containing rare earth salt is 5% to 30%.

[0041] According to an embodiment of the present invention, when preparing the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+, the ratio of the second rare earth salt to oleic acid and 1-octadecene is 1 mmol:(5~10) mL:(10~20) mL. Under these conditions, rapid dissolution of the second rare earth salt and the formation of the second rare earth oleic acid complex are facilitated.

[0042] According to an embodiment of the present invention, the second rare earth salt is a rare earth chloride; the second sodium source is NaOH; and the second fluorine source is NH4F.

[0043] According to an embodiment of the present invention, when preparing the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+, the ratio of the second rare earth salt to the second sodium source, the second fluorine source, and methanol is 1 mmol:(1.5~3.5) mmol:(3~5) mmol:(8~12) mL. Under these conditions, the formation of the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+ is favored.

[0044] According to an embodiment of the present invention, when preparing the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+, the ratio of the second rare earth salt to the inert core β-NaYF4:Gd3+ is 1:(0.9~1.3). Under these conditions, the formation of the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+ is favored.

[0045] S300. Using the obtained inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+ as the crystal nucleus, an active shell β-NaYF4:Yb3+ is coated on the outer layer of the luminescent shell to obtain the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+.

[0046] According to an embodiment of the present invention, an inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ can be prepared by a solvothermal method. Specifically, step S300 includes: mixing a third rare earth salt with oleic acid and 1-octadecene to generate a third rare earth oleic acid complex; adding the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+ as a seed crystal to the third rare earth oleic acid complex; adding a methanol solution containing a third sodium source and a third fluorine source; and reacting after the methanol evaporates to obtain the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+, wherein the third rare earth salt includes a Y-containing rare earth salt and a Gd-containing rare earth salt.

[0047] According to an embodiment of the present invention, in the reaction for generating the third rare earth oleic acid complex, the reaction is carried out under an inert gas atmosphere, the reaction temperature is (150℃~160℃), the reaction time is 30 min, and after the reaction for generating the third rare earth oleic acid complex is completed, the method further includes: cooling the reaction system to (40℃~50℃), and adding seed crystals β-NaYF4:Gd3+@β-NaYF4:Er under vigorous stirring. Add Gd3+, stir for 5 min, then add a methanol solution containing a third sodium source and a third fluorine source, and keep warm at (40℃~50℃) for 0.5 h; raise the temperature to (100℃~120℃) and keep warm for 10 min to remove methanol; rapidly raise the temperature to 294~300℃ and keep warm for 1.5 h, then cool to room temperature; centrifuge the reaction solution with ethanol solution at 8000 r / min for 10 min, collect the precipitate, and wash it three times with a mixture of cyclohexane and anhydrous ethanol. The final product is dispersed in cyclohexane to obtain an inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+.

[0048] According to some embodiments of the present invention, the contents of each component in the Y-containing rare earth salt and the Gd-containing rare earth salt in the third rare earth salt are weighed according to the stoichiometric ratio of β-NaYF4:Yb3+ in the active shell. Specifically, since the molar amount of Gd3+ in the active shell is 5% to 30% of the total molar amount of β-NaYF4 and Gd3+, the third rare earth salt should satisfy the following: the percentage of the molar amount of Gd in the Gd-containing rare earth salt to the sum of the molar amounts of Y in the Y-containing rare earth salt and Gd in the Gd-containing rare earth salt is 5% to 30%.

[0049] According to an embodiment of the present invention, when preparing the inert core-luminescent shell-active shell structure β-NaYF4:Gd 3+@β-NaYF4:Er 3+,Gd 3+@β-NaYF4:Yb 3+, the ratio of the third rare earth salt to oleic acid and 1-octadecene is 1 mmol:(5~10) mL:(10~20) mL. Under these conditions, rapid dissolution of the third rare earth salt and formation of the third rare earth oleic acid complex are facilitated.

[0050] According to an embodiment of the present invention, the third rare earth salt is a rare earth chloride; the third sodium source is NaOH; and the third fluorine source is NH4F.

[0051] According to an embodiment of the present invention, when preparing the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+, the ratio of the third rare earth salt to the third sodium source, the third fluorine source, and methanol is 1 mmol:(1.5~3.5) mmol:(3~5) mmol:(8~12) mL. Under these conditions, the formation of the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ is favored.

[0052] According to an embodiment of the present invention, when preparing the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+, the ratio of the third rare earth salt to the inert core-luminescent shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+ is 1:(0.9~1.3). Under these conditions, the formation of the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ is favored.

[0053] S400. Using the obtained inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ as the crystal nucleus, the surfactant CTAB is coated on the outer layer of the active shell to obtain a water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+.

[0054] According to an embodiment of the present invention, a water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ can be prepared by reverse micelle method. Specifically, step S400 includes: mixing CTAB with deionized water and reacting to generate a CTAB aqueous solution; adding the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ as a seed crystal to the CTAB aqueous solution; stirring vigorously and reacting to obtain the water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+.

[0055] According to an embodiment of the present invention, when preparing the water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+, the mass ratio of the surfactant CTAB to the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ is 1:(0.9~1.3). Under these conditions, the formation of the water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ is favored.

[0056] This invention uses a solvothermal method to coat rare earth luminescent materials layer by layer, which can make each coating layer structurally complete and uniform, and achieve directional control of the coating layer thickness and number of layers. It can flexibly adjust and optimize the structure of the upconversion luminescent material.

[0057] The present invention will be further described below with reference to specific embodiments. The raw materials used in the embodiments and comparative examples can be obtained commercially or prepared by known methods. Example 1

[0058] The water-soluble dual-frequency excitation upconversion luminescent material with a core-shell-shell structure consists of an inert core, a luminescent shell, and an active shell. The inert core structure is β-NaYF4:20%Gd3+, the luminescent shell structure is β-NaYF4:2%Er3+,20%Gd3+, and the active shell structure is β-NaYF4:Yb3+. The chemical formula of the water-soluble dual-frequency excitation upconversion luminescent material with a core-shell-shell structure is: β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+.

[0059] The preparation methods of water-soluble dual-frequency excited upconversion luminescent materials with core-shell-shell structure include:

[0060] (1) Preparation of inert core β-NaYF4:20% Gd3+; 0.8 mmol YCl3 and 0.2 mmol GdCl3·6H2O were added to a mixed solvent of 6 mL oleic acid and 15 mL 1-octadecene, and inert gas was introduced to remove oxygen. The mixture was kept at 150℃~160℃ for 30 min until the rare earth salt was completely dissolved to obtain a rare earth oleic acid complex. The temperature was then lowered to 40℃~50℃. Under vigorous stirring, 10 mL of a mixture containing 2.5 mmol NaOH and 4 mmol GdCl3·6H2O was added. The methanol solution of NH4F was kept at 40℃~50℃ for 0.5 h; the temperature was raised to 100℃~120℃ and kept at 10 min to remove methanol; the temperature was rapidly raised to 295℃ and kept at 1.5 h, and then cooled to room temperature; the reaction solution was centrifuged with ethanol solution at 8000 r / min for 10 min, the precipitate was collected, and washed three times with a mixture of cyclohexane and anhydrous ethanol. The final product was dispersed in 4 mL of cyclohexane to obtain the luminescent core β-NaYF4:20%Gd3+.

[0061] (2) Preparation of inert core-luminescent shell structure β-NaYF4:20%Gd3+@β-NaYF4:2%Er3+,20%Gd3+; using the inert core β-NaYF4:20%Gd3+ prepared in step (1) as the crystal nucleus, the inert core-luminescent shell structure β-NaYF4:20%Gd3+@β-NaYF4:2%Er3+,20%Gd3+ was prepared by solvothermal method. Specifically, the preparation of the inert core-luminescent shell structure β-NaYF4:20%Gd3+@β-NaYF4:2%Er3+,20%Gd3+ includes: adding 0.78mmol YCl3, 0.02mmol ErCl3, and 0.2mmol GdCl3·6H2O to a mixed solvent of 6mL oleic acid and 15mL 1-octadecene, purging with inert gas to remove oxygen, and maintaining the temperature at (150℃~160℃) for 30min until the rare earth salt is completely dissolved to obtain a rare earth oleic acid complex, and then cooling to (40℃~50℃); adding the crystal nucleus β-NaYF4:20%Gd3+ dissolved in 4mL cyclohexane in step (1) under vigorous stirring, stirring for 5min, then adding 10mL of methanol solution containing 2.5mmol NaOH and 4mmol NH4F, and maintaining the temperature at (40℃~50℃) for 0.5h; and then heating to (100℃~120℃). The mixture was kept at this temperature for 10 min to remove methanol; the temperature was rapidly increased to 295℃ and kept at this temperature for 1.5 h, then cooled to room temperature; the reaction solution was centrifuged with ethanol at 8000 r / min for 10 min, the precipitate was collected, and washed three times with a mixture of cyclohexane and anhydrous ethanol. The final product was dispersed in 4 mL of cyclohexane to obtain an inert core-luminescent shell structure β-NaYF4:20%Gd3+@β-NaYF4:2%Er3+,20%Gd3+.

[0062] (3) Prepare an inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+; Using the inert core-luminescent shell structure β-NaYF4:20%Gd3+@β-NaYF4:2%Er3+,20%Gd3+ prepared in step (2) as the crystal nucleus, prepare the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ by solvothermal method. Specifically, the preparation of the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ includes: adding 0.95mmol YCl3 and 0.05mmol YbCl3 to a mixed solvent of 6mL oleic acid and 15mL 1-octadecene, purging with an inert gas to remove oxygen, and maintaining the temperature at (150℃~160℃) for 30min until the rare earth salt is completely dissolved to obtain a rare earth oleic acid complex, and then cooling to (40℃~50℃); adding the crystal nuclei β-NaYF4:20%Gd3+@β-NaYF4:2%Er3+,20%Gd3+ dissolved in 4mL cyclohexane in step (2) under vigorous stirring, stirring for 5min, and then adding 10mL containing 2.5mmol NaOH and 4mmol The methanol solution of NH4F was kept at 40℃~50℃ for 0.5 h; the temperature was then increased to 100℃~120℃ and kept at 10 min to remove methanol; the temperature was then rapidly increased to 295℃ and kept at 1.5 h, and then cooled to room temperature; the reaction solution was centrifuged with ethanol at 8000 r / min for 10 min, the precipitate was collected, and washed three times with a mixture of cyclohexane and anhydrous ethanol. The final product was dispersed in 4 mL of cyclohexane to obtain the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+.

[0063] (4) Prepare a water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+; Using the inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ prepared in step (3) as the crystal nucleus, prepare a water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ using the reverse micelle method. Specifically, the preparation of the water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ includes: adding 0.2g of surfactant CTAB to 40mL of deionized water, adding the crystal nuclei β-NaYF4:20%Gd3+@β-NaYF4:2%Er3+,20%Gd3+ dissolved in 4mL of cyclohexane in step (3) under vigorous stirring to form a milky white solution, then heating at 80℃ to evaporate and remove cyclohexane until the solution is transparent and clear; cooling to room temperature; centrifuging the reaction solution at 8000r / min for 10 minutes, collecting the precipitate, and washing it three times with ultrapure water, and finally dispersing the product in 4mL of dimethyl sulfoxide solvent to obtain the water-soluble inert core-luminescent shell-active shell structure β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β- NaYF4:Yb3+.

[0064] Depend on Figure 4 It can be seen that the dual-frequency excitation luminescence intensity of the core-shell-shell structured upconversion luminescent material is greater than that of the core-shell structured material. Compared with the core-shell structured β-NaYF4:2.0%Er3+,20%Gd3+@β-NaYF4, the luminescence intensity of the core-shell-shell structured upconversion luminescent material β-NaYF4:Gd3+@β-NaYF4:Er3+,Gd3+@β-NaYF4:Yb3+ at 540 nm is nearly twice as high. Example 2

[0065] The core-shell structured dual-frequency excitation upconversion luminescent material was prepared according to the method of Example 1. The difference was that in the luminescent shell prepared in Example 2, the molar amount of Er3+ was 0.5% of the total molar amount of β-NaYF4, Er3+, and Gd3+, and the molar amount of Gd3+ was 20% of the total molar amount of β-NaYF4, Er3+, and Gd3+. The chemical formula of the core-shell-shell structured dual-frequency excitation upconversion luminescent material prepared in Example 2 was: β-NaYF4:20%Gd3+@β-NaYF4:0.5%Er3+, 20%Gd3+@β-NaYF4:5%Yb. Example 3

[0066] The core-shell structured dual-frequency excitation upconversion luminescent material was prepared according to the method of Example 1. The difference was that in the luminescent shell prepared in Example 3, the molar amount of Er3+ was 1.0% of the total molar amount of β-NaYF4, Er3+, and Gd3+, and the molar amount of Gd3+ was 20% of the total molar amount of β-NaYF4, Er3+, and Gd3+. The chemical expression of the core-shell-shell structured dual-frequency excitation upconversion luminescent material prepared in Example 3 was: β-NaYF4:20%Gd3+@β-NaYF4:1.0%Er3+, 20%Gd3+@β-NaYF4:5%Yb. Example 4

[0067] The core-shell structured dual-frequency excitation upconversion luminescent material was prepared according to the method of Example 1. The difference was that in the luminescent shell prepared in Example 4, the molar amount of Er3+ was 2.0% of the total molar amount of β-NaYF4 and Er3+, and the molar amount of Gd3+ was 5% of the total molar amount of β-NaYF4, Er3+, and Gd3+. The chemical formula of the core-shell-shell structured dual-frequency excitation upconversion luminescent material prepared in Example 4 was: β-NaYF4:20%Gd3+@β-NaYF4:2.0%Er3+,5%Gd3+@β-NaYF4:5%Yb. Example 5

[0068] The core-shell structured dual-frequency excitation upconversion luminescent material was prepared according to the method of Example 1. The difference was that in the luminescent shell prepared in Example 4, the molar amount of Er3+ was 2.0% of the total molar amount of β-NaYF4 and Er3+, and the molar amount of Gd3+ was 10% of the total molar amount of β-NaYF4, Er3+, and Gd3+. The chemical formula of the core-shell-shell structured dual-frequency excitation upconversion luminescent material prepared in Example 4 was: β-NaYF4:20%Gd3+@β-NaYF4:2.0%Er3+,10%Gd3+@β-NaYF4:5%Yb. Example 6

[0069] The core-shell structured dual-frequency excitation upconversion luminescent material was prepared according to the method of Example 1. The difference was that in the luminescent shell prepared in Example 4, the molar amount of Er3+ was 2.0% of the total molar amount of β-NaYF4 and Er3+, and the molar amount of Gd3+ was 15% of the total molar amount of β-NaYF4, Er3+, and Gd3+. The chemical formula of the core-shell-shell structured dual-frequency excitation upconversion luminescent material prepared in Example 4 was: β-NaYF4:20%Gd3+@β-NaYF4:2.0%Er3+,15%Gd3+@β-NaYF4:5%Yb. Comparative Example 1

[0070] Core-shell structured dual-frequency excitation upconversion luminescent materials were prepared according to the method of Example 1. The difference was that in the luminescent shell prepared in Comparative Example 1, the molar amount of Er3+ was 1.5% of the total molar amount of β-NaYF4, Er3+, and Gd3+, and the molar amount of Gd3+ was 20% of the total molar amount of β-NaYF4, Er3+, and Gd3+. The chemical formula of the core-shell-shell structured dual-frequency excitation upconversion luminescent material prepared in Comparative Example 1 was: β-NaYF4:20%Gd3+@β-NaYF4:1.5%Er3+, 20%Gd3+@β-NaYF4:5%Yb. Comparative Example 2

[0071] A core-shell structured dual-frequency excitation upconversion luminescent material was prepared according to the method of Example 1. The difference was that in the luminescent shell prepared in Comparative Example 1, the molar amount of Er3+ was 2.5% of the total molar amount of β-NaYF4, Er3+, and Gd3+, and the molar amount of Gd3+ was 20% of the total molar amount of β-NaYF4, Er3+, and Gd3+. The chemical formula of the core-shell-shell structured dual-frequency excitation upconversion luminescent material prepared in Comparative Example 1 was: β-NaYF4:20%Gd3+@β-NaYF4:2.5%Er3+, 20%Gd3+@β-NaYF4:5%Yb. Comparative Example 3

[0072] Core-shell structured dual-frequency excitation upconversion luminescent materials were prepared according to the method of Example 1. The difference was that in the luminescent shell prepared in Comparative Example 1, the molar amount of Er3+ was 3.0% of the total molar amount of β-NaYF4, Er3+, and Gd3+, and the molar amount of Gd3+ was 20% of the total molar amount of β-NaYF4, Er3+, and Gd3+. The chemical formula of the core-shell-shell structured dual-frequency excitation upconversion luminescent material prepared in Comparative Example 1 was: β-NaYF4:20%Gd3+@β-NaYF4:3.0%Er3+, 20%Gd3+@β-NaYF4:5%Yb. Comparative Example 4

[0073] A core-shell structured dual-frequency excitation upconversion luminescent material was prepared according to the method of Example 1. The difference was that in the luminescent shell prepared in Comparative Example 1, the molar amount of Er3+ was 2.0% of the total molar amount of β-NaYF4, Er3+, and Gd3+, and the molar amount of Gd3+ was 20% of the total molar amount of β-NaYF4, Er3+, and Gd3+. The chemical formula of the core-shell-shell structured dual-frequency excitation upconversion luminescent material prepared in Comparative Example 1 was: β-NaYF4:20%Gd3+@β-NaYF4:2.0%Er3+, 20%Gd3+@β-NaYF4:5%Yb. Comparative Example 5

[0074] Core-shell structured dual-frequency excitation upconversion luminescent materials were prepared according to the method of Example 1. The difference was that in the luminescent shell prepared in Comparative Example 1, the molar amount of Er3+ was 2.0% of the total molar amount of β-NaYF4, Er3+, and Gd3+, and the molar amount of Gd3+ was 25% of the total molar amount of β-NaYF4, Er3+, and Gd3+. The chemical formula of the core-shell-shell structured dual-frequency excitation upconversion luminescent material prepared in Comparative Example 1 was: β-NaYF4:20%Gd3+@β-NaYF4:2.0%Er3+, 25%Gd3+@β-NaYF4:5%Yb. Comparative Example 6

[0075] Core-shell structured dual-frequency excitation upconversion luminescent materials were prepared according to the method of Example 1. The difference was that in the luminescent shell prepared in Comparative Example 1, the molar amount of Er3+ was 2.0% of the total molar amount of β-NaYF4, Er3+, and Gd3+, and the molar amount of Gd3+ was 30% of the total molar amount of β-NaYF4, Er3+, and Gd3+. The chemical formula of the core-shell-shell structured dual-frequency excitation upconversion luminescent material prepared in Comparative Example 1 was: β-NaYF4:20%Gd3+@β-NaYF4:2.0%Er3+,30%Gd3+@β-NaYF4:5%Yb.

[0076] The performance of the upconversion luminescent materials prepared in Examples 1-6 and Comparative Examples 1-6 was tested.

[0077] Test method: The luminescence intensity of the upconversion luminescent material under dual-frequency excitation was measured using a self-made fluorescence spectroscopy acquisition system. The excitation source was a combination of an 852nm (1W) and a 1530nm (2W) infrared laser. The emission spectral signals were collected and recorded by a fiber optic spectrometer. The contrast ratio is the ratio of the luminescence intensity under dual-frequency excitation at 852nm and 1530nm to the luminescence intensity under single-frequency excitation at 1530nm.

[0078] The test results of Examples 1-6 and Comparative Examples 1-6 are shown in Table 1 below.

[0079] Table 1.

[0080]

[0081] The test results from Examples 1, 2, 3, Comparative Examples 1, 2, and 3 show that the prepared upconversion luminescent material exhibits good performance when the molar amount of Er3+ in the luminescent core is 2% of the total molar amount of β-NaYF4, Er3+, and Gd3+, and the molar amount of Gd3+ is 20% of the total molar amount of β-NaYF4, Er3+, and Gd3+. At this point, the prepared upconversion luminescent material can possess a high dual-frequency excitation luminescence intensity. When the Er3+ doping concentration is between 0.5% and 2.0%, the number of Er3+ ions per unit volume of solution gradually increases, and the resulting fluorescence intensity also gradually increases. When the Er3+ doping concentration is between 2.0% and 3.0%, due to the excessively high concentration of Er3+ ions, the interaction between ions is stronger, increasing the probability of non-radiative transitions at high-energy luminescent centers, resulting in a fluorescence quenching effect.

[0082] The test results from Examples 4, 5, 6, Comparative Examples 4, 5, and 6 show that when the molar amount of Gd3+ in the luminescent core is 20% of the total molar amount of β-NaYF4 and Er3+, the prepared upconversion luminescent material exhibits better performance. At this point, the upconversion luminescent material can simultaneously possess high luminescence intensity from dual-frequency excitation and high contrast. When the Gd3+ doping concentration increases from 5% to 15%, the green fluorescence intensity generated by co-excitation at 1530 nm and 852 nm gradually increases. After the Gd3+ doping concentration exceeds 15%, there are relatively more quenching sites on the smaller nanocrystal surface, leading to enhanced non-radiative energy relaxation at the luminescent center, thereby inhibiting upconversion luminescence and causing a downward trend in green fluorescence intensity.

[0083] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0084] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A water-soluble dual-frequency upconversion luminescent material, characterized in that, Comprising: an inert core having a chemical formula of β-NaYF4:Gd 3+ ; a luminescent shell coated on the surface of the inert core and having a chemical formula of β-NaYF4:Er 3+ ,Gd 3+ ; an active shell coated on the surface of the luminescent shell and having a chemical formula of β-NaYF4:Yb 3+ ; a surfactant coated on the surface of the active shell and the surfactant being cetyltrimethylammonium bromide (CTAB). 2.The water-soluble dual-frequency upconversion luminescent material of claim 1, wherein, The molar amount of Gd in the inert core is Y 3+ and the molar amount of Gd is 5% to 30% of the total molar amount of Y 3+ and Gd 3+ .

3. The upconverting luminescent material of claim 1, wherein The molar amount of Er 3+ is Y 3+ % of the total molar amount of Y 3+ , Er 3+ , and Gd 3+ ; and the molar amount of Gd 3+ is 5% to 30% of the total molar amount of Y 3+ , Er 3+ , and Gd .

4. The up-conversion luminescent material of claim 1, wherein, In the active shell, Yb 3+ The molar amount of Y 3+ and Yb 3+ 2% to 10% of the total molar amount.

5. The upconverting luminescent material of claim 1, wherein The surface active agent is CTAB, the mass ratio of CTAB and prepared β-NaYF4:Gd 3+ @β-NaYF4:Er 3+ , Gd 3+ @β-NaYF4:Yb 3+ is 1:0.9~1.

3.

6. A method of preparing the up-conversion luminescent material according to any one of claims 1 to 5, characterized in that, The first rare earth salt, the second rare earth salt, and the third rare earth salt are chloride rare earth salts; the first sodium source, the second sodium source, and the third sodium source are NaOH; and the first fluorine source, the second fluorine source, and the third fluorine source are NH4F. (1) mixing a first rare earth salt, oleic acid, 1-octadecene, a fluorine source, and a sodium source to obtain a compound of the general formula β-NaYF4:Gd 3+ inert core; (2) mixing the inert core of general formula β-NaYF4:Gd 3+ obtained in step (1) with a second rare earth salt, the oleic acid, the 1-octadecene, the fluorine source and the sodium source to form a luminescent shell of general formula β-NaYF4on the surface of the inert core, so as to obtain a luminescent core / shell nanocrystal of general formula β-NaYF4:Gd 3+ @β-NaYF4:Er 3+ ,Gd 3+ obtained in step (1) with a second rare earth salt, the oleic acid, the 1-octadecene, the fluorine source and the sodium source to form a luminescent shell of general formula β-NaYF4on the surface of the inert core, so as to obtain a luminescent core / shell nanocrystal of general formula β-NaYF4:Gd (3) mixing the inert core / luminescent shell obtained in step (2) with a third rare earth salt, said oleic acid, said 1-octadecene, said fluorine source and said sodium source, forming an active shell of general formula β-NaYF4:Yb 3+ @β-NaYF4:Er 3+ ; Gd 3+ on the surface of said inert core / luminescent shell, so as to obtain an inert core / luminescent shell / active shell nanocrystal of general formula β-NaYF4:Gd 3+ @β-NaYF4:Er 3+ ,Gd 3+ @β-NaYF4:Yb 3+ @β-NaYF4:Yb 3+ @β-NaYF4:Yb (4) The inert core / luminescent shell / active shell nanocrystals obtained in step (3) are mixed with a surfactant, CTAB, and deionized water to obtain water-soluble β-NaYF4:Gd 3+ @β-NaYF4:Er 3+ ,Gd 3+ @β-NaYF4:Yb 3+ Inert core / luminescent shell / active shell nanocrystals are mixed with a surfactant, CTAB, and deionized water to obtain water-soluble β-NaYF4:Gd 3+ @β-NaYF4:Er 3+ ,Gd 3+ @β-NaYF4:Yb 3+ Inert core / luminescent shell / active shell nanocrystals.

7. The method of claim 6, wherein, Preparation of inert core β-NaYF4:Gd 3+ When the ratio of the first rare earth salt to oleic acid, 1-octadecene is 1 mmol:5-10 mL:10-20 mL. Preparation of inert core- luminescent shell structure β-NaYF4:Gd 3 + @β-NaYF4:Er 3+ ,Gd 3+ When the ratio of the second rare earth salt to oleic acid, 1-octadecene is 1 mmol:5-10 mL:10-20 mL. Preparation of core-shell-shell structure upconversion luminescent material β-NaYF4:Gd 3+ @β-NaYF4:Er 3+ ,Gd 3+ @β-NaYF4:Yb 3+ When the ratio of the third rare earth salt to oleic acid, 1-octadecene is 1 mmol:5-10 mL:10-20 mL.

8. The method of claim 6, wherein, ​ 9. The method of claim 7, wherein, Preparation of inert core β-NaYF4:Gd 3+ At that time, the ratio of the first rare earth salt to the first sodium source, the first fluorine source, and methanol was 1 mmol: 1.5~3.5 mmol: 3~5 mmol: 8~12 mL; β-NaYF4:Gd was prepared as an inert core-luminescent shell structure. 3+ @β-NaYF4:Er 3+ ,Gd 3+ At that time, the ratio of the second rare earth salt to the second sodium source, the second fluorine source, and methanol was 1 mmol: 1.5~3.5 mmol: 3~5 mmol: 8~12 mL; the core-shell-shell structured upconversion luminescent material β-NaYF4:Gd was prepared. 3+ @β-NaYF4:Er 3+ ,Gd 3+ @β-NaYF4:Yb 3+ When the ratio of the third rare earth salt to the third sodium source, the third fluorine source, and methanol is 1 mmol: 1.5~3.5 mmol: 3~5 mmol: 8~12 mL, the ratio of the third rare earth salt to the third sodium source, the third fluorine source, and methanol is 1 mmol: 1.5~3.5 mmol: 3~5 mmol: 8~12 mL.

10. The method of claim 6, wherein, Preparation of inert core-luminescent shell structure β-NaYF4:Gd 3+ @β-NaYF4:Er 3+ ,Gd 3+ When the molar ratio of the second rare earth salt to the inert core β-NaYF4:Gd 3+ is 1:0.9~1.3, a core-shell-shell structure upconversion luminescent material β-NaYF4:Gd 3+ @β-NaYF4:Er 3+ ,Gd 3+ @β-NaYF4:Yb 3+ When the molar ratio of the third rare earth salt to the inert core-luminescent shell structure β-NaYF4:Gd 3+ @β-NaYF4:Er 3+ ,Gd 3+ is 1:0.9~1.3.

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