A rare earth doped nanocrystal with core-shell structure and its preparation method and application

Abstract

The present invention relates to the technical field of nanocrystal materials, and in particular to a rare earth doped nanocrystal with a core-shell structure and a preparation method and application thereof. The rare earth doped nanocrystal with a core-shell structure includes a NaYF4 core, a NaYbF4:Er,Ce intermediate shell layer, and a NaYF4:Ca outer shell layer from the inside to the outside. The present invention uses the NaYF4 core as a template to enable the nanoparticles to grow uniformly, thereby ensuring the controllability of the particle morphology and size; then, the content of the sensitizer Yb is increased in the intermediate shell layer, effectively avoiding the long-range energy transfer generated by the excitation light between the luminescent center ions Er; at the same time, Ce ions are doped in the intermediate shell layer to suppress the up-conversion luminescence process of the rare earth doped nanoparticles, thereby more efficiently transferring the excitation light energy to the emission energy level in the near-infrared light region; finally, by coating the outermost layer with an inert shell layer, the surface defects caused by the high-doping sensitizer and the interference of the external environment on the luminescent center Er ions in the intermediate shell layer are effectively suppressed.

Description

Technical Field

[0001] The present invention relates to the technical field of nanocrystal materials, and in particular to a rare earth doped nanocrystal with a core-shell structure and a preparation method and application thereof. Background Art

[0002] Since visible light (400-700nm) has serious scattering and shallow penetration depth in biological tissues, the application of rare earth-doped inorganic nanomaterials with emission peaks in the visible light region in the fields of biological in vivo imaging and sensing is severely limited. The emission light in the near-infrared (700-1700nm) band can effectively solve the problem of rapid attenuation of visible light in biological tissues, especially in the near-infrared second region (1000-1700nm), the scattering and absorption coefficients of biological tissues are lower, which is conducive to improving the penetration depth of tissues.

[0003] In recent years, Zhang et al. synthesized NaGdF4:Nd rare earth doped inorganic nanoparticles with an emission peak at 1060 nm by thermal decomposition and successfully applied them to imaging-mediated surgical treatment. However, their emission peak is short and it is difficult to achieve biological imaging at a greater depth. Subsequently, Dai et al. synthesized NaGdF4:Nd rare earth doped inorganic nanoparticles with an emission peak at 1060 nm by thermal decomposition and successfully applied them to imaging-mediated surgical treatment. However, their emission peak is short and it is difficult to achieve biological imaging at a greater depth. 3+ Rare earth-doped nanocrystals with longer emission peaks in the second near-infrared region (1500-1700 nm) were synthesized, which effectively improved the penetration depth of biological imaging. However, due to the structure of the nanomaterial and the doping type, the fluorescence emission efficiency of the nanoparticles is relatively limited.

[0004] Therefore, the prior art still needs to be improved and developed. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a rare earth-doped nanocrystal with a core-shell structure and a preparation method and application thereof, aiming to solve the problems such as the loss of excitation light caused by the long-range energy transfer between rare earth ions in the existing rare earth-doped inorganic nanoparticles, resulting in limited fluorescence emission efficiency.

[0006] The technical solution of the present invention is as follows:

[0007] A rare earth doped nanocrystal with a core-shell structure comprises, from the inside to the outside, a NaYF4 core, a NaYbF4:Er,Ce intermediate shell layer, and a NaYF4:Ca outer shell layer.

[0008] The rare earth doped nanocrystals with core-shell structure, wherein the particle size of the NaYF4 core is 3-17nm; and the particle size of the rare earth doped nanocrystals is 5-35nm.

[0009] The rare earth-doped nanocrystals with a core-shell structure, wherein the mass ratio of the NaYF4 core, the NaYbF4:Er,Ce intermediate shell and the NaYF4:Ca outer shell is (20-40):(30-50):(45-60).

[0010] A method for preparing rare earth-doped nanocrystals with a core-shell structure comprises the following steps:

[0011] Mixing oleic acid and 1-octadecene, and then adding yttrium acetate aqueous solution to obtain a first rare earth oleic acid complex;

[0012] Adding a methanol solution containing sodium hydroxide and ammonium fluoride to the first rare earth oleic acid complex, and obtaining a NaYF4 core after a first reaction;

[0013] Mixing oleic acid and 1-octadecene, and then adding a rare earth acetate aqueous solution to obtain a second rare earth oleic acid complex;

[0014] Adding a methanol solution containing sodium hydroxide and ammonium fluoride and the NaYF4 core to the second rare earth oleic acid complex, and obtaining NaYF4 / NaYbF4:Er,Ce nanoparticles after a second reaction;

[0015] Mixing oleic acid and 1-octadecene, and then adding yttrium acetate aqueous solution and calcium acetate aqueous solution to obtain a third rare earth oleic acid complex;

[0016] adding a methanol solution containing sodium hydroxide and ammonium fluoride to the third rare earth oleic acid complex to obtain a shell precursor;

[0017] Oleic acid, 1-octadecene and the NaYF4 / NaYbF4:Er,Ce nanoparticles are mixed and then added to the shell layer precursor, and after the third reaction, rare earth doped nanocrystals with a core-shell structure are obtained.

[0018] The method for preparing rare earth-doped nanocrystals with a core-shell structure, wherein in the first rare earth oleic acid complex, the molar ratio of oleic acid to 1-octadecene is (3-15):(7-35); and / or, in the second rare earth oleic acid complex, the molar ratio of oleic acid to 1-octadecene is (3-15):(7-35); and / or, in the third rare earth oleic acid complex, the molar ratio of oleic acid to 1-octadecene is (3-15):(7-35); and / or, in the shell layer precursor, the molar ratio of oleic acid to 1-octadecene is (3-15):(7-35).

[0019] The method for preparing rare earth-doped nanocrystals with a core-shell structure, wherein the rare earth acetate aqueous solution is composed of ytterbium acetate aqueous solution, erbium acetate aqueous solution, and cerium acetate aqueous solution in a molar ratio of (0.78-3.9):(0.02-0.1):(0.2-1).

[0020] The method for preparing rare earth-doped nanocrystals with a core-shell structure, wherein a methanol solution containing sodium hydroxide and ammonium fluoride is added to the first rare earth oleic acid complex, and after a first reaction, a NaYF4 core is obtained, comprising the steps of:

[0021] When the temperature of the first rare earth oleic acid complex drops to 40-45° C., a methanol solution containing sodium hydroxide and ammonium fluoride is added and stirred at 45-55° C. for 1.5-2 hours to obtain a first reaction solution;

[0022] The first reaction liquid is heated to 100-120° C., vacuum degassed for 10-12 minutes, and then heated to 300-330° C. in an inert atmosphere and kept warm for 1-2 hours. After washing, a NaYF4 core is obtained.

[0023] The method for preparing rare earth doped nanocrystals with a core-shell structure, wherein a methanol solution containing sodium hydroxide and ammonium fluoride and the NaYF4 core are added to the second rare earth oleic acid complex, and after a second reaction, NaYF4 / NaYbF4:Er,Ce nanoparticles are obtained, including the steps of:

[0024] When the temperature of the second rare earth oleic acid complex drops to 40-45° C., a methanol solution containing sodium hydroxide and ammonium fluoride and the NaYF4 core are added and stirred at 45-55° C. for 1.5-2 hours to obtain a second reaction solution;

[0025] The second reaction liquid is heated to 100-120° C., vacuum degassed for 10-12 minutes, and then heated to 300-330° C. in an inert atmosphere and kept warm for 1-2 hours. After washing, NaYF4 / NaYbF4:Er,Ce nanoparticles are obtained.

[0026] The method for preparing rare earth doped nanocrystals with a core-shell structure, wherein oleic acid, 1-octadecene and the NaYF4 / NaYbF4:Er,Ce nanoparticles are mixed and then added to the shell precursor, and after the third reaction, rare earth doped nanocrystals with a core-shell structure are obtained, including the steps of:

[0027] After mixing oleic acid, 1-octadecene and the NaYF4 / NaYbF4:Er,Ce nanoparticles, the temperature is raised to 75-85°C and kept warm for 25-35 minutes, then continued to be raised to 100-120°C, vacuum degassed for 10-12 minutes, and then heated to 290-320°C in an inert atmosphere, and then the shell precursor is added and kept warm at 290-320°C for 9-12 minutes to obtain rare earth doped nanocrystals with a core-shell structure.

[0028] The invention discloses an application of rare earth doped nanocrystals with a core-shell structure in medical imaging equipment in the near-infrared band.

[0029] Beneficial effects: The present invention provides a rare earth doped nanocrystal with a core-shell structure and a preparation method and application thereof, wherein the rare earth doped nanocrystal with a core-shell structure includes a NaYF4 core, a NaYbF4:Er,Ce intermediate shell layer, and a NaYF4:Ca outer shell layer from the inside to the outside. The present invention uses the NaYF4 core as a template to enable the nanoparticles to grow uniformly, thereby ensuring the controllability of the particle morphology and size; then the content of the sensitizer Yb is increased in the intermediate shell layer, effectively avoiding the long-range energy transfer generated by the excitation light between the luminescent center ions Er; at the same time, Ce ions are doped in the intermediate shell layer to inhibit the up-conversion luminescence process of the rare earth doped nanoparticles, thereby more efficiently transferring the excitation light energy to the emission energy level in the near-infrared light region; finally, the surface defects caused by the high-doped sensitizer and the interference of the external environment on the luminescent center Er ions in the intermediate shell layer are effectively suppressed by coating the inert shell layer in the outermost layer; the core-shell structure can effectively solve the technical problem of low fluorescence efficiency of rare earth doped nanocrystals, and effectively improve the luminescence efficiency of the material in the near-infrared second region. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The fluorescence enhancement mechanism diagram of core-shell nanocrystals NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca;

[0031] Figure 2 It is a transmission electron microscope image of NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca nanocrystals in Example 1;

[0032] Figure 3 This is the fluorescence emission diagram after the intermediate shell sensitizer ion Yb is regulated in Example 2;

[0033] Figure 4 This is the fluorescence emission diagram after regulating the Ce ions in the middle shell in Example 3;

[0034] Figure 5The fluorescence emission comparison diagram of NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca nanocrystals and NaYF4:Yb,Er@NaYF4 prepared in Example 1;

[0035] Figure 6 This is a non-invasive near-infrared second-zone imaging image of the NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca nanocrystals of Example 1 used for the brain of a living mouse. DETAILED DESCRIPTION

[0036] The present invention provides a rare earth doped nanocrystal with a core-shell structure and a preparation method and application thereof. To make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0037] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless specifically defined as herein.

[0038] Rare earth doped inorganic nanomaterials are mainly composed of inorganic crystal matrix and doped rare earth ions. Among them, the inorganic crystal matrix provides a suitable crystal field environment for rare earth ions, and the matrix material needs to meet the requirements of lattice matching with doped rare earth ions, good chemical stability and low lattice vibration phonon energy. Rare earth elements include 15 lanthanide elements in the periodic table and two elements with similar properties to lanthanide elements, scandium (Sc) and yttrium (Y). Rare earth elements have unique electronic layer structures and rich energy level transitions, so they can maintain the energy level of free ions in various matrix materials, so that rare earth doped inorganic nanomaterials can achieve fluorescence emission from ultraviolet, visible light to near-infrared bands. At the same time, compared with organic fluorescent materials, rare earth doped inorganic nanoparticles can adjust the position of emission peaks by changing doping ions, and have excellent optical properties such as strong photostability, large Stokes / anti-Stokes shift, long fluorescence lifetime, and narrow spectral line width, showing broad application prospects in the fields of bioimaging, treatment, luminescent display, photovoltaic devices, etc.

[0039] However, existing rare earth-doped inorganic nanoparticles have problems such as long-range energy transfer between rare earth ions causing loss of excitation light, which leads to limited fluorescence emission efficiency.

[0040] Based on this, the present invention provides a rare earth doped nanocrystal with a core-shell structure, which includes a NaYF4 core, a NaYbF4:Er,Ce intermediate shell layer, and a NaYF4:Ca outer shell layer from the inside to the outside.

[0041] In this embodiment, firstly, by using the NaYF4 core as a template, the nanoparticles can grow uniformly, ensuring the controllability of the particle morphology and size; secondly, by increasing the content of the sensitizer Yb in the NaYbF4:Er,Ce intermediate shell, the long-range energy transfer of the excitation light between the luminescent center ions Er is effectively avoided; at the same time, Ce ions are doped in the intermediate shell to suppress the upconversion luminescence process of the rare earth-doped nanoparticles, thereby more efficiently transferring the excitation light energy to the emission energy level in the near-infrared light region; finally, by coating the outermost layer with an inert shell, the surface defects caused by the highly doped sensitizer and the interference of the external environment on the intermediate luminescent center Er ions are effectively suppressed; this doping structure can effectively solve the technical problem of low fluorescence efficiency of rare earth nanoparticles and achieve an effective improvement in the luminescence efficiency of the material in the near-infrared region II.

[0042] Specifically, the rare earth doped nanocrystals with the core-shell structure have a stable high fluorescence efficiency in the near-infrared region II (1000-1700nm) window, and the core-shell structure can improve the luminescence efficiency of rare earth doped inorganic nanocrystals in the near-infrared region II, which is suitable for deep imaging of biological tissues. The fluorescence enhancement mechanism of the rare earth doped nanocrystals with the core-shell structure is as follows Figure 1 As shown in the figure, the fluorescence enhancement principle of core-shell structured nanoparticles mainly includes: the inner core template maintains the uniformity of the nanoparticle morphology, the middle shell adds sensitizer Yb and competitive absorption Ce to enhance the fluorescence emission in the second near-infrared region, and the outermost inert shell suppresses the fluorescence quenching of the luminescent center Er by the external environment.

[0043] In some embodiments, the rare earth-doped nanocrystals of the core-shell structure include, from the inside to the outside, a NaYF4 core, a NaYbF4: (1-2)% Er, (16-24)% Ce middle shell, and a NaYF4: (12-18)% Ca outer shell.

[0044] In a preferred embodiment, the rare earth-doped nanocrystals of the core-shell structure include, from the inside to the outside, a NaYF4 core, a NaYbF4:2%Er,20%Ce intermediate shell, and a NaYF4:15%Ca outer shell.

[0045] In some embodiments, the particle size of the NaYF4 core is 3-17 nm; the particle size of the rare earth-doped nanocrystals is 5-35 nm.

[0046] In some embodiments, the mass ratio of the NaYF4 core, the NaYbF4:Er,Ce intermediate shell and the NaYF4:Ca outer shell is (20-40):(30-50):(45-60).

[0047] In addition, the present invention also provides a method for preparing rare earth-doped nanocrystals with a core-shell structure, comprising the steps of:

[0048] Step S10: mixing oleic acid and 1-octadecene, and then adding an aqueous solution of yttrium acetate to obtain a first rare earth oleic acid complex;

[0049] Step S20: adding a methanol solution containing sodium hydroxide and ammonium fluoride to the first rare earth oleic acid complex, and obtaining a NaYF4 core after a first reaction;

[0050] Step S30: mixing oleic acid and 1-octadecene, and then adding a rare earth acetate aqueous solution to obtain a second rare earth oleic acid complex;

[0051] Step S40: adding a methanol solution containing sodium hydroxide and ammonium fluoride and the NaYF4 core to the second rare earth oleic acid complex, and obtaining NaYF4 / NaYbF4:Er,Ce nanoparticles after a second reaction;

[0052] Step S50: mixing oleic acid and 1-octadecene, and adding yttrium acetate aqueous solution and calcium acetate aqueous solution to obtain a third rare earth oleic acid complex;

[0053] Step S60: adding a methanol solution containing sodium hydroxide and ammonium fluoride to the third rare earth oleic acid complex to obtain a shell precursor;

[0054] Step S70: oleic acid, 1-octadecene and the NaYF4 / NaYbF4:Er,Ce nanoparticles are mixed and then added to the shell precursor, and after the third reaction, rare earth doped nanocrystals with a core-shell structure are obtained.

[0055] In this embodiment, a core template, an intermediate shell layer and an inert protective shell layer are prepared by using oleic acid, 1-octadecene and rare earth acetate in different molar ratios, and rare earth doped nanocrystals with the core-shell structure are prepared by a co-precipitation method. This method can effectively improve the luminescence efficiency of rare earth doped inorganic nanocrystals in the near-infrared region II, and is suitable for deep-depth imaging of biological tissues.

[0056] In some embodiments, in the first rare earth oleic acid complex, the molar ratio of oleic acid to 1-octadecene is (3-15):(7-35); and / or, in the second rare earth oleic acid complex, the molar ratio of oleic acid to 1-octadecene is (3-15):(7-35); and / or, in the third rare earth oleic acid complex, the molar ratio of oleic acid to 1-octadecene is (3-15):(7-35); and / or, in the shell precursor, the molar ratio of oleic acid to 1-octadecene is (3-15):(7-35). At this molar doping ratio, the nanoparticles have uniform morphology and good monodispersity, which is conducive to their further use in subsequent biological imaging.

[0057] In a preferred embodiment, in the first rare earth oleic acid complex, the molar ratio of the oleic acid to the 1-octadecene is 3:7; in the second rare earth oleic acid complex, the molar ratio of the oleic acid to the 1-octadecene is 3:7; in the third rare earth oleic acid complex, the molar ratio of the oleic acid to the 1-octadecene is 3:7; in the shell precursor, the molar ratio of the oleic acid to the 1-octadecene is 3:7.

[0058] In some embodiments, in step S10, oleic acid and 1-octadecene are mixed and then an aqueous yttrium acetate solution is added to obtain a first rare earth oleic acid complex, which includes the steps of: fully stirring oleic acid (OA), 1-octadecene (1-ODE) and an aqueous yttrium acetate (Y(AC)3) solution with a molar ratio of 3:7:1, and then placing it on a heating table and setting the temperature to 110°C under an argon atmosphere. After the water is fully evaporated, the temperature is raised to 165°C and maintained at this temperature for 1 hour to form a rare earth oleic acid mixture.

[0059] In some embodiments, in step S20, adding a methanol solution containing sodium hydroxide (NaOH) and ammonium fluoride (NH4F) to the first rare earth oleic acid complex, and obtaining a NaYF4 core after a first reaction comprises the following steps:

[0060] Step S21: when the temperature of the first rare earth oleic acid complex drops to 40-45° C., a methanol solution containing sodium hydroxide and ammonium fluoride is added and stirred at 45-55° C. for 1.5-2 hours to obtain a first reaction solution;

[0061] Step S22: The first reaction liquid is heated to 100-120°C, vacuum degassed for 10-12 minutes, and then heated to 300-330°C in an inert atmosphere and kept warm for 1-2 hours. After washing, a NaYF4 core is obtained.

[0062] Specifically, the sodium hydroxide and ammonium fluoride are fully mixed with the first rare earth oleic acid complex to complete the nucleation of nanoparticles, and the nucleation step helps to form uniform nanoparticle morphology. Then the temperature is increased, vacuum degassing is performed, and the temperature is increased again, so that the nanoparticles can grow uniformly, the particle morphology and size can be controlled, and the nanoparticles are grown. Finally, cyclohexane and anhydrous ethanol are used for multiple washings with equal proportions to remove excess oleic acid and 1-octadecene, and the NaYF4 core obtained after washing is resuspended in cyclohexane and stored at 4°C.

[0063] In some embodiments, the rare earth acetate aqueous solution is composed of ytterbium acetate aqueous solution, erbium acetate aqueous solution, and cerium acetate aqueous solution in a molar ratio of (0.78-3.9):(0.02-0.1):(0.2-1). The rare earth acetate aqueous solution composed of this molar ratio can make the content of sensitizer Yb added to the middle shell layer effectively avoid the long-range energy transfer generated by the excitation light between the luminescent center ions Er, and at the same time dope Ce ions in the middle shell layer to suppress the up-conversion luminescence process of the rare earth-doped nanoparticles, so as to more efficiently transfer the excitation light energy to the emission energy level in the near-infrared light region.

[0064] In a preferred embodiment, the rare earth acetate aqueous solution is composed of ytterbium acetate aqueous solution, erbium acetate aqueous solution, and cerium acetate aqueous solution in a molar ratio of 0.78:0.02:0.2. After the rare earth acetate aqueous solution composed of this molar ratio is prepared into an intermediate shell, the excitation light energy can be more efficiently transferred to the emission energy level in the near-infrared light region.

[0065] In some embodiments, in step S30, oleic acid and 1-octadecene are mixed and then rare earth acetate aqueous solution is added to obtain a second rare earth oleic acid complex, which includes the following steps: oleic acid and 1-octadecene with a molar ratio of 3:7 are fully stirred, and then a rare earth acetate aqueous solution with a molar ratio of ytterbium acetate (Yb(AC3)): erbium acetate (Er(AC)3): cerium acetate (Ce(AC)3) = 0.78:0.02:0.2 is added, and the mixture is placed on a heating table, and the system temperature is raised to 110°C to fully evaporate the water, and then the temperature is continued to be raised to 165°C and maintained at this temperature for 1 hour to form a second rare earth oleic acid complex.

[0066] In some embodiments, in step S40, adding a methanol solution containing sodium hydroxide and ammonium fluoride and the NaYF4 core to the second rare earth oleic acid complex, and obtaining NaYF4 / NaYbF4:Er,Ce nanoparticles after a second reaction comprises the following steps:

[0067] Step S41: when the temperature of the second rare earth oleic acid complex drops to 40-45° C., a methanol solution containing sodium hydroxide and ammonium fluoride and the NaYF4 core are added and stirred at 45-55° C. for 1.5-2 hours to obtain a second reaction solution;

[0068] Step S42: The second reaction liquid is heated to 100-120°C, vacuum degassed for 10-12 minutes, and then heated to 300-330°C in an inert atmosphere and kept warm for 1-2 hours. After washing, NaYF4 / NaYbF4:Er,Ce nanoparticles are obtained.

[0069] In some embodiments, in step S50, oleic acid and 1-octadecene are mixed and then yttrium acetate aqueous solution and calcium acetate aqueous solution are added to obtain a third rare earth oleic acid complex, which includes the steps of: fully stirring oleic acid and 1-octadecene with a molar ratio of 3:7, adding yttrium acetate (Y(AC)3) aqueous solution and calcium acetate (Ca(AC)3) aqueous solution with a molar ratio of 0.85:0.15, and then placing on a heating table, raising the system temperature to 110°C to fully evaporate the water, and then continuing to heat to 165°C and maintaining the temperature for 1 hour to form a third rare earth oleic acid complex.

[0070] In some embodiments, in step S60, adding a methanol solution containing sodium hydroxide and ammonium fluoride to the third rare earth oleic acid complex to obtain a shell precursor comprises the following steps:

[0071] Step S61: when the temperature of the third rare earth oleic acid complex drops to 40-45° C., a methanol solution containing sodium hydroxide and ammonium fluoride is added and stirred at 45-55° C. for 1.5-2 hours to obtain a third reaction solution;

[0072] Step S62: heating the third reaction liquid to 100-120° C. and vacuum degassing for 18-25 min to obtain a shell precursor.

[0073] In some embodiments, in step S70, oleic acid, 1-octadecene and the NaYF4 / NaYbF4:Er, Ce nanoparticles are mixed and then the shell precursor is added. After the third reaction, rare earth doped nanocrystals with a core-shell structure are obtained, which includes the steps of mixing oleic acid, 1-octadecene and the NaYF4 / NaYbF4:Er, Ce nanoparticles, heating the mixture to 75-85°C and keeping the mixture warm for 25-35 minutes, then continuing to heat the mixture to 100-120°C, vacuum degassing for 10-12 minutes, heating the mixture to 290-320°C under an inert atmosphere, then adding the shell precursor and keeping the mixture warm at 290-320°C for 9-12 minutes to obtain rare earth doped nanocrystals with a core-shell structure.

[0074] Specifically, oleic acid and 1-octadecene are mixed in a molar ratio of 3:7, and then 0.4 mmol NaYF4 / NaYbF4:Er,Ce nanoparticles are added dropwise, and then the temperature is raised to 80°C and maintained for 30 minutes to fully evaporate cyclohexane to obtain a mixed solution; the temperature of the mixed solution is further raised to 110°C, and after vacuum degassing for 10 minutes, the temperature of the mixed solution is raised to 300°C under an argon atmosphere, 2 mL of the shell precursor is extracted using a syringe and quickly injected into the mixed solution, after reacting at 300°C for 10 minutes, the step of injecting the shell precursor is repeated until the precursor is completely injected into the mixed solution, after the last injection, the system is maintained at 300°C for 10 minutes and then cooled to room temperature, the obtained product is washed with equal proportions of cyclohexane and anhydrous ethanol, resuspended in cyclohexane, and stored at 4°C; the product is a rare earth doped nanocrystal NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca nanocrystal with a core-shell structure.

[0075] In some embodiments, in step S20, step S40, and step S60, the molar ratio of the sodium hydroxide to the ammonium fluoride is independently controlled between (2.5-12.5):(4-20).

[0076] In addition, the present invention also provides an application of rare earth doped nanocrystals with a core-shell structure in medical imaging equipment in the near-infrared band.

[0077] The following examples are further given to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention belong to the scope of protection of the present invention.

[0078] Example 1

[0079] In order to verify the imaging performance of the core-shell structure NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca nanocrystals in the near-infrared region II, the prepared nanocrystals need to be hydrophilically modified, and then the animal blood vessels are marked by tail vein injection, and the near-infrared imaging system is used to collect fluorescent photos. The specific operation steps are as follows:

[0080] Step 1: Add 3 mmol oleic acid (OA) and 7 mmol 1-octadecene (1-ODE) to a three-necked flask, stir well, then add 1 mmol yttrium acetate (Y(AC)3) aqueous solution in a 100 mL three-necked flask, place on a heating table, set the temperature of the heating table to 120°C, and in an argon atmosphere, after the water is fully evaporated, raise the temperature to 165°C and maintain the temperature for 1 hour to form a first rare earth oleic acid complex.

[0081] Step 2: When the first rare earth oleic acid complex is cooled to 45°C, a methanol solution of 2.5 mmol sodium hydroxide (NaOH) and 4 mmol ammonium fluoride (NH4F) is added, and the system is maintained at about 50°C and stirred for 2 h, and the first rare earth oleic acid complex formed in step 1 is fully reacted to complete the nucleation of nanoparticles. This nucleation step helps to uniform the morphology of nanoparticles. Then the system is adjusted to 110°C, vacuum degassed for 10 min, and the system temperature is raised to 310°C under argon atmosphere protection, and maintained at this temperature for 1 h. After the system temperature is lowered to room temperature, the product is washed several times with equal proportions of cyclohexane and anhydrous ethanol to remove excess oleic acid and 1-octadecene, and the final product is resuspended in cyclohexane and stored at 4°C; the product is the NaYF4 core.

[0082] Step 3: Take a clean three-necked flask again, add 3 mmol oleic acid (OA) and 7 mmol 1-octadecene (1-ODE) into the three-necked flask, stir thoroughly, then add 0.78 mmol ytterbium acetate (Yb(AC)3), 0.02 mmol erbium acetate (Er(AC)3) and 0.2 mmol cerium acetate (Ce(AC)3) rare earth acetate aqueous solution into a 100 mL three-necked flask, place it on a heating table, raise the system temperature to 110°C to fully evaporate the water, then continue to raise the temperature to 165°C and maintain the temperature for 1h to form a second rare earth oleic acid complex;

[0083] Step 4: When the temperature of the second rare earth oleic acid complex is reduced to 45°C, a methanol solution of 2.5 mmol NaOH and 4 mmol NH4F and the NaYF4 core prepared in step 1 are added, and the system is maintained at about 50°C and stirred for 2 h to complete the nucleation of the nanoparticles. The system is then temperature adjusted to 110°C, vacuum degassed for 10 min, and then the system temperature is raised to 310°C under argon atmosphere protection and maintained at this temperature for 1 h. After the system temperature is reduced to room temperature, the product is washed several times with equal proportions of cyclohexane and anhydrous ethanol, and the final product is resuspended in cyclohexane and stored at 4°C to complete the coating of the intermediate shell layer to prepare NaYF4 / NaYbF4:Er,Ce nanoparticles.

[0084] Step 5: Take a clean three-necked flask again, add 3 mmol oleic acid (OA) and 7 mmol 1-octadecene (1-ODE) into the three-necked flask, stir well, add 0.85 mmol yttrium acetate (Y(AC)3) and 0.15 mmol calcium acetate (Ca(AC)3) rare earth acetate aqueous solution, place on a heating table, raise the system temperature to 110°C to fully evaporate the water, then continue to raise the temperature to 165°C and maintain the temperature for 1h to form a third rare earth oleic acid complex;

[0085] Step 6: When the temperature of the third rare earth oleic acid complex is reduced to 45°C, a methanol solution of 2.5 mmol NaOH and 4 mmol NH4F is added, and the system is maintained at about 50°C and stirred for 2 h to complete the nucleation of the nanoparticles. The system is then temperature-adjusted to 110°C, vacuum-degassed for 20 min, and the system is cooled to room temperature to obtain a shell precursor. The obtained shell precursor is stored at room temperature for later use;

[0086] Step 7: Add 3 mmol oleic acid (OA) and 7 mmol 1-octadecene (1-ODE) to the three-necked flask again, add the two-layer structure NaYF4 / NaYbF4:Er,Ce prepared in step 4 dropwise into the three-necked flask, heat the system to 80°C and maintain the temperature for 30 min to fully evaporate cyclohexane, continue to raise the system temperature to 110°C, vacuum degas for 10 min, raise the system temperature to 300°C under argon atmosphere, use a syringe to extract 2 mL of the prepared shell precursor solution, quickly inject it into the system, react at 300°C for 10 min, repeat the step of injecting the precursor until the precursor is completely injected into the system, after the last injection, maintain the system at 300°C for 10 min and then cool to room temperature, wash the product with equal proportions of cyclohexane and anhydrous ethanol, resuspend it in cyclohexane, and store it at 4°C. The product is core-shell structured NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca nanocrystals.

[0087] The morphology and size of the nanocrystals were characterized by transmission electron microscopy. Figure 2 is a transmission electron microscope image of NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca nanocrystals, where Figure 2 The figure includes transmission electron microscopy (left), high-resolution transmission electron microscopy (upper right) and dark-field transmission electron microscopy (lower right). The transmission electron microscopy shows that the prepared core-shell nanoparticles have a uniform morphology, the high-resolution transmission electron microscopy shows that the nanoparticles have a regular lattice structure, and the dark-field transmission electron microscopy verifies the core-shell structure of the nanoparticles.

[0088] Example 2

[0089] The molar amount of ytterbium acetate (Yb(AC)3) in step 3 of Example 1 was adjusted, and the remaining steps were the same as in Example 1 to obtain nanocrystals with different yttrium contents; the fluorescence emission graph after regulating the intermediate shell sensitizer ion Yb is as shown in Figure 3 As shown, UC represents up-conversion luminescence and DC represents down-conversion luminescence; the figure shows that when the content of sensitizer Yb increases to 98%, the fluorescence in both the visible light region and the near-infrared region II is significantly enhanced, with the fluorescence in the visible light region enhanced by 5.3 times and the fluorescence in the near-infrared region II enhanced by 6.5 times.

[0090] Example 3

[0091] The molar amount of cerium acetate (Ce(AC)3) in step 3 of Example 1 was adjusted, and the remaining steps were the same as in Example 1 to obtain nanocrystals with different cerium contents; the fluorescence emission graph after adjusting the Ce ions in the intermediate shell is as shown in Figure 4 As shown, UC represents up-conversion luminescence and DC represents down-conversion luminescence. The figure shows that after doping Ce ions in the intermediate shell, the fluorescence intensity in the visible light region decreases by 12.7 times, and the fluorescence intensity in the near-infrared region increases by 5.3 times.

[0092] Example 4

[0093] The conventional core-shell structure NaYF4:Yb,Er@NaYF4 is prepared by existing technology, such as Figure 5 As shown, it is a fluorescence emission comparison diagram of NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca nanocrystals and NaYF4:Yb,Er@NaYF4 prepared in Example 1, and the inset is a fluorescence photo of the two. The figure shows that the fluorescence of the near-infrared region II is enhanced by 24 times compared with the traditional core-shell structure by the doping method of the core-shell structure. The fluorescence photos of the two also show that the fluorescence of the core-shell structure nanoparticles is much higher than that of the conventional core-shell structure.

[0094] Example 5

[0095] Weigh 50 mg of nitrosotetrafluoroborate (NOBF4) and mix it with 1 ml of NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca nanocrystals prepared in Example 1, place it on a magnetic stirrer, and stir it for 2 hours to remove the oleic acid ligands on the surface of NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca. Wash the resulting product with ultrapure water for 3 times, resuspend the product in deionized water, and store it in a refrigerator at 4°C.

[0096] Weigh 200 mg of polyacrylic acid (PAA) powder and dissolve it in water. Place it on a magnetic stirrer and add dropwise NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca nanocrystals whose surface oleic acid ligands have been removed. After stirring at room temperature overnight, wash the product three times with deionized water, resuspend it in 1X phosphate buffer (PBS), and store it in a refrigerator at 4°C.

[0097] Six-week-old male nude mice were used as imaging targets. Before imaging, the mice were placed in a small animal anesthesia machine with a flow rate of 2 L min -1 Mice were fully anesthetized with 2.5% isoflurane mixed with 1% oxygen.

[0098] The anesthetized mice were placed on a heating pad and maintained at 37°C. The tail vein of the mice was fully disinfected with 75% alcohol cotton balls, and 200 uL of NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca nanocrystals resuspended in PBS were injected into the mice using a 1 mL sterile insulin syringe.

[0099] The mouse brain injected with nanoparticles was placed under a 980 nm continuous light laser, and the power density at the sample surface was set to 50 mW cm -2 The generated fluorescence emission was collected by an InGaAs camera with a collection band of 900-1700nm after passing through a 1300nm long-pass filter, and the exposure time of the image collection was set to 200ms.

[0100] Figure 6 This is a non-invasive near-infrared zone II imaging image of the brain of a living mouse using NaYF4 / NaYbF4:Er,Ce / NaYF4:Ca. This image shows that when no craniotomy is performed on the mouse, 980nm excitation is used to collect 1532nm fluorescence, achieving clear imaging of superficial venous vessels in the brain, indicating that the prepared core-shell nanoparticles have excellent fluorescence properties.

[0101] The obtained non-invasive near-infrared fluorescence photographs of the mouse brain showed extremely high signal-to-noise ratio and penetration depth, indicating that the core-shell structured rare earth doped nanocrystals proposed in this invention can effectively enhance the fluorescence emission in the near-infrared second region and have been successfully applied to wide-field imaging of living organisms. Its excellent fluorescence properties make it possible for the probe to be used in macroscopic, microscopic and endoscopic medical imaging equipment in the near-infrared band.

[0102] In summary, the present invention provides a rare earth doped nanocrystal with a core-shell structure and a preparation method and application thereof, wherein the rare earth doped nanocrystal with a core-shell structure includes a NaYF4 core, a NaYbF4:Er,Ce intermediate shell layer, and a NaYF4:Ca outer shell layer from the inside to the outside. The present invention uses the NaYF4 core as a template to enable the nanoparticles to grow uniformly, thereby ensuring the controllability of the particle morphology and size; then the content of the sensitizer Yb is increased in the intermediate shell layer, effectively avoiding the long-range energy transfer generated by the excitation light between the luminescent center ions Er; at the same time, Ce ions are doped in the intermediate shell layer to suppress the up-conversion luminescence process of the rare earth doped nanoparticles, thereby more efficiently transferring the excitation light energy to the emission energy level in the near-infrared light region; finally, the surface defects caused by the high-doped sensitizer and the interference of the external environment on the luminescent center Er ions in the intermediate shell layer are effectively suppressed by coating the inert shell layer in the outermost layer; the core-shell structure can effectively solve the technical problem of low fluorescence of rare earth doped nanocrystals, and effectively improve the luminescence efficiency of the material in the near-infrared second region.

[0103] It should be understood that the application of the present invention is not limited to the above examples. For ordinary technicians in this field, improvements or changes can be made based on the above description. All these improvements and changes should fall within the scope of protection of the claims attached to the present invention.

Claims

1. A rare earth doped nanocrystal with a core-shell structure, characterized in that: From the inside to the outside, it includes the NaYF4 core, the NaYbF4: (1-2)% Er, (16-24)% Ce middle shell, and the NaYF4: (12-18)% Ca outer shell.

2. The rare earth doped nanocrystals with core-shell structure according to claim 1, characterized in that: The particle size of the NaYF4 core is 3-17nm; the particle size of the rare earth doped nanocrystal is 5-35nm.

3. The rare earth doped nanocrystals with core-shell structure according to claim 1, characterized in that: The mass ratio of the NaYF4 core, the NaYbF4: (1-2)% Er, (16-24)% Ce intermediate shell and the NaYF4: (12-18)% Ca outer shell is (20-40): (30-50): (45-60).

4. A method for preparing rare earth-doped nanocrystals with a core-shell structure as claimed in any one of claims 1 to 3, characterized in that: Includes steps: Mixing oleic acid and 1-octadecene, and then adding yttrium acetate aqueous solution to obtain a first rare earth oleic acid complex; Adding a methanol solution containing sodium hydroxide and ammonium fluoride to the first rare earth oleic acid complex, and obtaining a NaYF4 core after a first reaction; Mixing oleic acid and 1-octadecene, and then adding a rare earth acetate aqueous solution to obtain a second rare earth oleic acid complex; Adding a methanol solution containing sodium hydroxide and ammonium fluoride and the NaYF4 core to the second rare earth oleic acid complex, and after a second reaction, obtaining NaYF4 / NaYbF4: (1-2)% Er, (16-24)% Ce nanoparticles; Mixing oleic acid and 1-octadecene, and then adding yttrium acetate aqueous solution and calcium acetate aqueous solution to obtain a third rare earth oleic acid complex; adding a methanol solution containing sodium hydroxide and ammonium fluoride to the third rare earth oleic acid complex to obtain a shell precursor; Oleic acid, 1-octadecene and the NaYF4 / NaYbF4: (1-2)% Er, (16-24)% Ce nanoparticles are mixed and then added to the shell precursor, and after the third reaction, rare earth doped nanocrystals with a core-shell structure are obtained.

5. The method for preparing rare earth-doped nanocrystals with a core-shell structure according to claim 4, characterized in that: In the first rare earth oleic acid complex, the molar ratio of oleic acid to 1-octadecene is (3-15):(7-35); and / or, in the second rare earth oleic acid complex, the molar ratio of oleic acid to 1-octadecene is (3-15):(7-35); and / or, in the third rare earth oleic acid complex, the molar ratio of oleic acid to 1-octadecene is (3-15):(7-35); and / or, after mixing oleic acid, 1-octadecene and the NaYF4 / NaYbF4:(1-2)%Er,(16-24)%Ce nanoparticles, the molar ratio of oleic acid to 1-octadecene added to the shell precursor is (3-15):(7-35).

6. The method for preparing rare earth-doped nanocrystals with a core-shell structure according to claim 4, characterized in that: Adding a methanol solution containing sodium hydroxide and ammonium fluoride to the first rare earth oleic acid complex, and obtaining a NaYF4 core after a first reaction, comprises the following steps: When the temperature of the first rare earth oleic acid complex drops to 40-45° C., a methanol solution containing sodium hydroxide and ammonium fluoride is added and stirred at 45-55° C. for 1.5-2 hours to obtain a first reaction solution; The first reaction liquid is heated to 100-120° C., vacuum degassed for 10-12 minutes, and then heated to 300-330° C. in an inert atmosphere and kept warm for 1-2 hours. After washing, a NaYF4 core is obtained.

7. The method for preparing rare earth-doped nanocrystals with a core-shell structure according to claim 4, characterized in that: Adding a methanol solution containing sodium hydroxide and ammonium fluoride and the NaYF4 core to the second rare earth oleic acid complex, and after a second reaction, obtaining NaYF4 / NaYbF4: (1-2)% Er, (16-24)% Ce nanoparticles comprises the following steps: When the temperature of the second rare earth oleic acid complex drops to 40-45° C., a methanol solution containing sodium hydroxide and ammonium fluoride and the NaYF4 core are added and stirred at 45-55° C. for 1.5-2 hours to obtain a second reaction solution; The second reaction liquid is heated to 100-120°C, vacuum degassed for 10-12 minutes, and then heated to 300-330°C in an inert atmosphere and kept warm for 1-2 hours. After washing, NaYF4 / NaYbF4: (1-2)% Er, (16-24)% Ce nanoparticles are obtained.

8. The method for preparing rare earth-doped nanocrystals with a core-shell structure according to claim 4, characterized in that: The steps of mixing oleic acid, 1-octadecene and the NaYF4 / NaYbF4: (1-2)% Er, (16-24)% Ce nanoparticles and adding the shell precursor, and obtaining rare earth doped nanocrystals with a core-shell structure after the third reaction include: After mixing oleic acid, 1-octadecene and the NaYF4 / NaYbF4: (1-2)% Er, (16-24)% Ce nanoparticles, the mixture is heated to 75-85°C and kept warm for 25-35 minutes, then continued to heat to 100-120°C, vacuum degassed for 10-12 minutes, and then heated to 290-320°C in an inert atmosphere, and then the shell precursor is added and kept warm at 290-320°C for 9-12 minutes to obtain rare earth doped nanocrystals with a core-shell structure.

9. Use of the rare earth doped nanocrystals with a core-shell structure as claimed in any one of claims 1 to 3 in medical imaging equipment in the near-infrared band.

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