A method for detecting the total amount of rare earth ions based on a paper-based technology of upconversion fluorescence resonance energy transfer

Abstract

The present invention discloses a method for detecting the total amount of rare earth ions based on a paper-based technology of upconversion fluorescence resonance energy transfer, belonging to the technical fields of analytical chemistry and nanomaterials. In this method, filter paper is used as the detection matrix, polyethyleneimine (PEI)-modified upconversion nanoparticles (UCNPs) are used as the energy donor, and DNAzyme-modified gold nanoparticles (AuNPs) are used as the energy acceptor. When rare earth ions are present, the DNAzyme undergoes a cleavage reaction, causing the AuNPs to approach the UCNPs, resulting in resonance energy transfer (FRET), which weakens the green fluorescence excited by 980 nm infrared light. The luminescence image is captured by a charge-coupled device (CCD) and processed by RGB image software to obtain the value of the luminescence intensity, thereby determining the content of rare earth ions in the sample. The method constructed by the present invention has good sensitivity, with a lowest detection limit of 1.3 μmol / L, high selectivity, simple operation, and can be used for the determination of rare earth ions in water samples.

Description

Technical Field

[0001] The invention belongs to the technical field of analytical chemistry and nanomaterials, and in particular relates to a method for detecting the total amount of rare earth ions based on a paper-based technology of up-conversion fluorescence resonance energy transfer. Background Art

[0002] At present, the detection of rare earth elements mostly adopts high performance liquid chromatography (HPLC), atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectrometry (ICP-AES) and other technologies. Among them, in HPLC, ion exchange chromatography is the main method for detecting rare earth elements. It is a chromatographic method that mainly uses ion exchange resin or chemically bonded ion exchange agent as the stationary phase and utilizes the difference in ion exchange capacity or selectivity coefficient of the separated components to achieve separation; AAS is based on the absorption of the characteristic spectral line of the ground state atomic vapor of the element to be measured, and the characteristic of the characteristic spectral line and the degree of attenuation of the spectral line are used to perform qualitative and quantitative analysis of the element to be measured; and ICP-AES is an atomic emission spectroscopic analysis method that uses plasma as the excitation light source, which can perform simultaneous determination of multiple elements.

[0003] The above experimental techniques mainly rely on corresponding experimental instruments. Since these instruments require specific experimental temperatures and stable experimental environments, they are mainly collected on site and transported to the laboratory, and then go through experimental processes such as sample pretreatment, and finally draw conclusions through experimental instruments. The experiment relies on bulky experimental instruments and a stable experimental environment, and it is impossible to carry out detection on site. In addition, the experimental process requires the experimenters to have corresponding technical capabilities and perform complex experimental processing, which is costly and greatly increases the difficulty of the experiment and the stability and accuracy of the test results.

[0004] Since the fluorescence method is simple to operate, highly sensitive and quick to respond, combined with cheap and portable filter paper, the stability of upconversion nanomaterials and the portability of corresponding technologies, it can realize on-site detection of rare earth elements, and is therefore suitable for solving the shortcomings of existing technologies. Summary of the invention

[0005] The purpose of the present invention is to provide a method for detecting the total amount of rare earth ions based on paper-based technology of upconversion fluorescence resonance energy transfer in view of the defects of the above-mentioned prior art. The method is based on the visualization paper-based technology of fluorescence resonance energy transfer and realizes the detection of the total amount of rare earth ions through the change of upconversion fluorescence intensity.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for detecting the total amount of rare earth ions based on a paper-based technology of upconversion fluorescence resonance energy transfer. Using filter paper as the detection matrix, polyethyleneimine (PEI)-modified UCNPs as the energy donor, and DNAzyme-modified gold nanoparticles (AuNPs) as the energy acceptor. When rare earth ions are present, DNAzyme is cleaved and broken, causing AuNPs to approach UCNPs, and resonance energy transfer (FRET) occurs, resulting in the weakening of the green fluorescence excited by 980 nm infrared light. The luminescence image is captured by a CCD, processed by RGB image software, and the value of the luminescence intensity is obtained. The content of rare earth ions in the sample is determined by measuring through the standard curve of the fluorescence quenching rate and the rare earth ion concentration.

[0008] A method for detecting the total amount of rare earth ions based on a paper-based technology of upconversion fluorescence resonance energy transfer, comprising the following steps:

[0009] (1) Preparation of polyethyleneimine (PEI)-modified UCNPs: First, synthesize rare earth stearates containing erbium, ytterbium, and yttrium. Then, using oleic acid (OA) and octadecene (ODE), synthesize rare earth stearates containing erbium, ytterbium, and yttrium, and use NaF as the raw material to synthesize UCNPs. Finally, modify UCNPs with PEI to obtain polyethyleneimine (PEI)-modified UCNPs;

[0010] (2) Preparation of DNAzyme-modified gold nanoparticles (AuNPs): React chloroauric acid with sodium citrate to synthesize AuNPs. React the mixture of substrate and DNAzyme-SH with AuNPs to obtain DNAzyme-modified gold nanoparticles (AuNPs);

[0011] (3) Preparation of the filter paper detection matrix: First, fix the polyethyleneimine (PEI)-modified UCNPs prepared in step (1) on the oxidized and activated filter paper, and then fix the DNAzyme-modified gold nanoparticles (AuNPs) on the oxidized and activated filter paper as well;

[0012] (4) Detection of the total amount of rare earth ions: Drop the rare earth ion solution suspended on the filter paper detection matrix prepared in step (4), react at room temperature for a period of time, and generate green fluorescence by exciting with 980 nm infrared light (0.5 W / cm 2 ) in a dark box. The luminescence image is captured by a CCD, processed by RGB image software, and the value of the luminescence intensity is obtained. The concentration of rare earth ions in the sample is determined by measuring through the standard curve of the fluorescence quenching rate and the rare earth ion concentration.

[0013] The preparation of the above-mentioned polyethyleneimine (PEI)-modified UCNPs in step (1) specifically includes:

[0014] Synthesis of UCNPs: Take a clean three-necked flask, add 12 mL of oleic acid (OA) and 8 mL of octadecene (ODE), add 0.8 mmol of rare earth stearates (Y, Yb, Er) and 28 mmol of NaF. Heat under reflux to 135 - 145 °C and maintain for 30 min for dehydration and degassing to form a clear liquid. Then quickly raise the temperature and maintain the reaction temperature at 312 - 314 °C for 45 min. After the reaction, cool to room temperature. After centrifugation, discard the supernatant and retain the centrifuged precipitate. Wash it three times with a solution of cyclohexane:ethanol at 3:1, and then wash it three times with deionized water. Dry it in vacuum at 60 °C to obtain oleic acid-coated nanoparticles UCNPs for standby.

[0015] PEI-modified UCNPs: Mix 10 mL of oleic acid-coated nanoparticles UCNPs (10 mg / mL) dispersed in hexane and 20 mL of a 0.01 mol / L NOBF4 DMF solution, stir at room temperature for 2 h, remove the upper hexane layer, purify and centrifuge through a mixture of toluene and hexane (1:1, v / v), and wash twice with DMF. Subsequently, disperse it in 5 mL of DMF solution, mix it with 1 g of PEI dispersed in 15 mL of DMF solution, stir overnight, wash several times with deionized water, and dry it in vacuum at 60 °C to obtain PEI-modified UCNPs for standby.

[0016] The preparation of DNAzyme-modified gold nanoparticles AuNPs in the above step (2) specifically includes:

[0017] Synthesis of AuNPs: Add 49.75 mL of pure water to a 100 mL three-necked flask, add 250 μL of 2% chloroauric acid, heat to boiling, then add 3 mL of 1% sodium citrate solution, continue to stir for 15 min until the solution turns purple-red. After cooling, store it at 4 °C to obtain a sodium citrate-coated AuNPs solution for standby.

[0018] Preparation of AuNPs-DNAzyme: Mix substrate and DNAzyme-SH in equal proportions in HEPES buffer (50 mM, 300 mM NaCl, pH 7.0), heat at 90 °C for 5 min, and anneal to room temperature to prepare a DNAzyme complex (substrate and DNAzyme-SH hybrid complex); then mix the DNAzyme complex with the prepared gold colloid (AuNPs) solution at a molar ratio of 200:1, freeze at -20 °C for 2 h, add sodium chloride solution (final concentration 0.3 M) to thaw, and centrifuge to remove excess DNA; finally, redisperse AuNPs-DNAzyme in HEPES buffer to obtain DNAzyme-modified gold nanoparticles AuNPs and store them at 4 °C for standby.

[0019] Furthermore, the base sequence of the substrate is: ACGAGTCACTATrAGGAAGATGGC; the base sequence of the DNAzyme-SH is: SH-TTTTTTTTTTCGCCATCTTGACGCATATCGTTTTCGATAGCACGTGTTAGTGACTCGTGAC-NH2.

[0020] The preparation of the filter paper detection matrix in the above step (3) specifically includes:

[0021] Oxidation treatment of filter paper: Wates Grade 1 filter paper is prepared into small pieces with a diameter of 3 mm using a punch, placed in a conical flask, and activated with 14% NaOH solution for 24 h; after activation, it is washed several times with distilled water, and then added to the filter paper oxidation modification reaction solution (26 mM NaIO4, 47 mM LiCl, pH 2), and oxidized at 35 °C for 2 days. After the reaction, it is washed several times with distilled water and dried at 40 °C for standby.

[0022] Fixation of fluorescent probe: PEI-modified UCNPs are prepared into a 0.1 mg / mL solution with HEPES buffer (100 mM, 200 mM NaBH3CN, PH 7.2) for standby. It is suspended and dropped on the oxidized filter paper, 3 μL per piece, and reacted at room temperature for 1 h. After the reaction, the unfixed PEI-modified UCNPs are washed away with 0.2% Tween washing solution and ultrapure water and dried for standby. Then, AuNPs-DNAzyme is suspended and dropped on the small filter paper pieces, 3 μL per piece, and after reacting at room temperature for 1 h, the filter paper detection matrix is prepared and reserved.

[0023] The detection of the total rare earth ion content in the above step (4) specifically includes:

[0024] Take 3 μL of rare earth ion solutions with different concentrations and suspend them on the filter paper, react at room temperature for 2 h, and generate green fluorescence by excitation with 980 nm infrared light (0.5 W / cm 2 ) in a dark box. The luminescence image is captured by a CCD and processed by RGB image software to obtain the value of the luminescence intensity. It is determined by the standard curve of the fluorescence quenching rate and the rare earth ion concentration, so as to determine the rare earth ion concentration in the sample.

[0025] The linear range for the determination of rare earth ions used is 5 - 100 μmol / L, and the lowest detection limit is 1.3 μmol / L.

[0026] Advantages of the present invention:

[0027] (1) Select filter paper, a material that is inexpensive, easy to handle, and disposable, as the reaction interface;

[0028] (2) Select UCNPs as the luminescent matrix. Due to their unique anti-Stokes luminescence properties, they have photochemical stability, no autofluorescence, and at the same time avoid the absorption of 980 nm excitation light by water in the homogeneous system in upconversion, reducing the heat generation effect;

[0029] (3) This method is based on fluorescence energy resonance transfer between UCNPs and AuNPs, and uses DNAzyme as a highly selective recognition element, making the monitoring of rare earth ions accurate and visual. The lowest detection limit of rare earth ions reaches 1.3 μmol / L. Description of the Drawings

[0030] Figure 1 It is a detection mechanism diagram of rare earth ions by a paper-based fluorescence sensor.

[0031] Figure 2 It is a characterization diagram of PEI-UCNPs. In the figure, a is the TEM image; b in the figure is the XRD pattern; c in the figure is the FT-IR spectrum.

[0032] Figure 3 It is the TEM image of AuNPs.

[0033] Figure 4 It is the UV-Vis spectra of unmodified AuNPs and AuNPs-DNAzyme;

[0034] Figure 5 It is the SEM image of filter paper: a in the figure is the untreated filter paper; b in the figure is the oxidized filter paper.

[0035] Figure 6 It is the SEM image of the filter paper fixed with PEI-UCNPs. In the figure, a is the filter paper fixed with PEI-UCNPs; b in the figure is the filter paper fixed with PEI-UCNPs.

[0036] Figure 7 It is the fluorescence intensity diagram of PEI-UCNPs fixed on the filter paper at different times.

[0037] Figure 8 It is the quenching change diagram of different concentrations of AuNPs-DNAzyme (when the rare earth ion concentration is 100 nM).

[0038] Figure 9 It is the optimization diagram of the quenching time when AuNPs-DNAzyme is 6 nM.

[0039] Figure 10It is a selectivity graph of AuNPs-DNAzyme for different metal ions.

[0040] Figure 11 They are target rare earth ion solutions with different concentrations, their quenching rates, and the corresponding standard curves. Specific implementation manners

[0041] Example 1: Synthesis of UCNPs and modification of PEI

[0042] Synthesis of rare earth stearates: Weigh 0.2 mmol of Er2O3, 2 mmol of Yb2O3, and 7.8 mmol of Y2O3 respectively; mix them and place them in a 250 mL three-necked flask. Add 100 mL of sufficient dilute nitric acid (concentrated nitric acid: water = 1:1). Seal it with a plastic wrap, heat it with a magnetic stirrer in a water bath at 90 - 100 °C. After it becomes clear, open the plastic wrap and continue heating until the excess nitric acid completely volatilizes to obtain rare earth nitrate solids; after natural cooling, add 100 mL of ethanol, and then add 60 mmol of stearic acid to the 250 mL three-necked flask; heat it under reflux at 70 °C until the liquid becomes clear, gradually add 10 mL of 119 g / L NaOH solution to adjust the pH of the solution to between 5 and 6; after dropping, raise the temperature to 78 °C and continue reflux stirring for 30 min; after cooling to room temperature, filter it under reduced pressure, wash it with water twice and then wash it with ethanol twice; transfer the obtained filter cake to an oven and dry it at 60 °C for 12 h to obtain white powdery rare earth stearates (Y, Yb, Er).

[0043] Synthesis of UCNPs: Take a clean three-necked flask, add 12 mL of oleic acid (OA) and 8 mL of octadecene (ODE). Add 0.8 mmol of rare earth stearates (Y, Yb, Er) and 28 mmol of NaF. Heat it under reflux to 135 - 145 °C and keep it for 30 min for dehydration and degassing to form a clear liquid. Then quickly raise the temperature and maintain the reaction temperature at 312 - 314 °C for 45 min. After the reaction is completed, cool it to room temperature. After centrifugation, discard the supernatant and retain the centrifuged precipitate. Wash it three times with cyclohexane:ethanol = 3:1, and then wash it three times with deionized water. Dry it in vacuum at 60 °C for standby to prepare rare earth-doped upconversion nanoparticles (UCNPs).

[0044] PEI-modified (PEI-UCNPs): 10 mL of oleic acid-coated nanoparticles dispersed in hexane (concentration: 10 mg / mL) were mixed with 20 mL of a 0.01 mol / L nitrosyl tetrafluoroborate (NOBF4) solution in N,N-dimethylformamide (DMF). The mixture was stirred at room temperature for 2 h. The upper hexane layer was removed, and the product was purified by toluene and hexane (1:1, v / v) and centrifuged, then washed twice with DMF. Subsequently, it was dispersed in 5 mL of DMF solution to prepare BF4 - -UCNPs. The BF4

[0045] -UCNPs were mixed with 1 g of polyethyleneimine (PEI) dispersed in 15 mL of DMF solution, stirred overnight, washed several times with deionized water, and dried in vacuum at 60 °C to obtain PEI-modified (PEI-UCNPs). Figure 2 . Figure 2 In it, a is the TEM image. The particle size is uniform and well-dispersed. The average particle size is about 30 nm, and an obvious PEI shell can be observed on the periphery of the particles; b in the figure is the XRD pattern. By comparing with the standard spectrum of NaYF4 (JCPDS: 28-1192), it is found that the synthesized nanocrystal is hexagonal phase (β); c in the figure is the FT-IR pattern. The characteristic peak at 3436 cm -1 is the stretching vibration peak of O-H or N-H. The peaks at 2923 cm -1 and 2856 cm -1 correspond to the asymmetric and symmetric stretching vibrations of -CH2 in PEI respectively. The peaks at 1638 cm -1 and 1538 cm -1 correspond to the bending peaks of N-H groups in PEI, and the peak at 1164 cm -1 corresponds to the C-N stretching vibration, indicating the successful preparation of PEI-UCNPs.

[0046] Example 2 Preparation of DNAzyme-modified gold colloid (AuNPs-DNAzyme)

[0047] Synthesis of gold colloid (AuNPs): 49.75 mL of pure water was added to a 100 mL three-necked flask, 250 μL of 2 vol% chloroauric acid was added, and the mixture was heated to boiling. Subsequently, 3 mL of 1 wt% sodium citrate solution was added, and stirring was continued for 15 min until the solution turned purple-red. After cooling, it was stored at 4 °C to obtain the gold colloid (AuNPs) solution. The TEM image of AuNPs is as shown in Figure 3 .

[0048] Preparation of DNAzyme-modified gold nanoparticles (AuNPs-DNAzyme): The base sequence of the enzyme substrate is: 5’-ACGAGTCACTATrAGGAAGATGGC-3’, and the base sequence of DNAzyme-SH is: 5’-SH-TTTTTTTTTTCGCCATCTTGACGCATATCGTTTTCGATAGCACGTGTTAGTGACTCGTGAC-NH2-3’. The substrate and DNAzyme-SH are mixed at the same molar mass ratio in HEPES buffer (50 mM, 300 mM NaCl, pH 7.0), heated at 90 °C for 5 min, and annealed to room temperature to prepare a DNAzyme complex (a hybrid complex of the substrate and DNAzyme-SH); then the DNAzyme complex is mixed with the prepared gold nanoparticle (AuNPs) solution at a molar ratio of 200:1, frozen at -20 °C for 2 h, thawed by adding sodium chloride solution (final concentration 0.3 M), and centrifuged to remove excess DNA; the DNAzyme complex-modified gold nanoparticles (AuNPs-DNAzyme) are obtained, and the AuNPs-DNAzyme is redispersed in HEPES buffer and stored at 4 °C. Figure 4 UV-Vis spectra of unmodified AuNPs and AuNPs-DNAzyme. AuNPs have a characteristic ultraviolet absorption at 518 nm. After binding DNA, its characteristic absorption peak redshifts to 523 nm, indicating that DNAzyme-SH has successfully bound to the surface of AuNPs through Au-S bonds; the results in the figure show that AuNPs-DNAzyme has been successfully prepared.

[0049] Example 3 Treatment of filter paper and immobilization of fluorescent probe

[0050] Treatment of filter paper: Wates Grade 1 filter paper is punched into small pieces with a diameter of 3 mm, placed in a conical flask, and activated with 14 wt% NaOH solution for 24 h. After activation, it is washed three times with distilled water, and then added to the filter paper oxidation modification reaction solution (26 mM NaIO4, 47 mM LiCl, pH = 2), and oxidized at 35 °C for 2 days. After the reaction, it is washed three times with distilled water and dried at 40 °C for standby. Figure 5 SEM image of filter paper: Figure 5 a in the figure is the untreated filter paper; b in the figure is the oxidized filter paper. The results show that the oxidized filter paper has been prepared.

[0051] Fixation of the fluorescent probe: The PEI-UCNPs prepared in Example 1 were configured into a 0.1 mg / mL solution with HEPES buffer (100 mM, 200 mM NaBH3CN, pH 7.2) for standby. It was suspended and dropped on the filter paper, 3 μL per piece, and reacted at room temperature for 1 h. After the reaction, the unfixed PEI-UCNPs were washed away with 0.2 vol% Tween 80 washing solution and ultrapure water, and then dried; then 6 nM AuNPs-DNAzyme was suspended and dropped on the small filter paper pieces, 3 μL per piece. After reacting at room temperature for 1 h, a paper-based filter paper based on upconversion fluorescence resonance energy transfer was prepared. Figure 6 SEM image of the filter paper fixed with PEI-UCNPs Figure 6 a is the oxidized filter paper successfully modified with PEI-UCNPs. PEI-UCNPs are fixed and well-dispersed on the cellulose units of the oxidized filter paper matrix. Figure 6 b is the oxidized filter paper fixed with PEI-UCNPs modified with AuNPs-DNAzyme. It can be observed that AuNPs-DNAzyme is evenly dispersed on the cellulose units modified with PEI-UCNPs.

[0052] Figure 7 Fluorescence intensity diagram of PEI-UCNPs at different fixation times on the filter paper. The upconversion luminescence (UCL) intensity of the light spot changes with the reaction time. When the reaction time reaches 60 min, the UCL spot is the brightest and completely uniform, and the reaction reaches saturation, indicating that after reacting for 60 min, PEI-UCNPs are completely fixed on the oxidized filter paper.

[0053] Example 4 Optimization of conditions and selectivity for rare earth ion detection using paper-based technology based on upconversion fluorescence resonance energy transfer

[0054] Figure 8 Quenching change diagram of different concentrations of AuNPs-DNAzyme (when the rare earth ion concentration is 100 nM). The rare earth ion concentration was fixed at 100 nM, and the modification concentration of AuNPs-DNAzyme was gradually increased to observe the quenching effect of AuNPs-DNAzyme on UCL. As the concentration of AuNPs-DNAzyme increased to 6 nM, the quenching of UCL reached the saturation state, and the optimal concentration of AuNPs-DNAzyme was 6 nM.

[0055] Figure 9 Optimization diagram of quenching time when AuNPs-DNAzyme is 6 nM. The concentration of AuNPs-DNAzyme was fixed at 6 nM. As time goes by, the UCL intensity gradually decreases, and the reaction reaches saturation at 120 min. Therefore, 120 min is the optimal reaction time.

[0056] Figure 10 Selectivity graph of AuNPs-DNAzyme for different metal ions. To evaluate the specificity of the sensor for rare earth ions, the concentration of rare earth ions is 10 μM·L -1 , interfering ions such as K + , Na + , Mg 2+ , Mn 2+ , Ca 2+ , Fe 3+ , Y 3+ , La 3+ , Lu 3+ is 100 μM·L -1 . The detection results are as Figure 8 shown. The designed sensor has high selectivity and can be used for the detection of the total amount of heavy rare earth ions in complex water bodies in the environment.

[0057] Example 5 Detection of total rare earth ions using upconversion fluorescence resonance energy transfer-based paper technology

[0058] Figure 11 Different concentrations of target rare earth ion solutions, their quenching rates, and the corresponding standard curves. Take 3 μL of target rare earth ion solutions with different concentrations (Gd 3+ , Tb 3+ , Dy 3+ in equimolar mass ratio), with a concentration range of 5 - 50 μmol·L -1 , suspend and drop them on the filter paper prepared in Example 3, react at room temperature for 2 h, generate green fluorescence by excitation with 980 nm infrared light (0.5 W / cm 2 ) in a dark box, capture the luminescence image using a CCD, process it with RGB image software to obtain the magnitude of the luminescence intensity, and determine it through the standard curve of the fluorescence quenching rate and the rare earth ion concentration, so as to determine the rare earth ion concentration in the sample. The results are as Figure 11 shown. In the range of 5 - 50 μmol·L -1 , there is a good linear relationship between the UCL intensity and the rare earth ion concentration. The fitting regression equation is y = 0.5444logc - 0.2312, the correlation coefficient is 0.9967, and the LOD is 1.26 μmol·L -1 .

[0059] The above are only the preferred embodiments of the present invention, and all equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the present invention.

Claims

1. A method for detecting the total amount of rare earth ions based on a paper-based technology of upconversion fluorescence resonance energy transfer, characterized in that: Using filter paper as the detection matrix, polyethyleneimine (PEI)-modified rare earth-doped upconversion nanoparticles (UCNPs) as the energy donor, and DNAzyme-modified gold nanoparticles (AuNPs) as the energy acceptor. When rare earth ions are present, DNAzyme undergoes cleavage, causing AuNPs to approach UCNPs and resulting in resonance energy transfer (FRET), which weakens the green fluorescence excited by 980 nm infrared light. The luminescence image is captured using a CCD, processed by RGB image software to obtain the value of the luminescence intensity, and the content of rare earth ions in the sample is determined by measuring through the standard curve of the fluorescence quenching rate and the rare earth ion concentration. The DNAzyme-modified AuNPs are prepared by reacting a mixture of enzyme substrate and DNAzyme-SH with AuNPs. The base sequence of the substrate is: 5’-ACGAGTCACTATrAGGAAGATGGC-3’; the base sequence of DNAzyme-SH is: 5’-SH-TTTTTTTTTTCGCCATCTTGACGCATATCGTTTTCGATAGCACGTGTTAGTGACTCGTGAC-NH2-3’.

2. The method for detecting the total amount of rare earth ions based on a paper-based technology of upconversion fluorescence resonance energy transfer according to claim 1, characterized in that: Including the following steps: (1) Preparation of PEI-modified UCNPs: Using oleic acid (OA), octadecene (ODE), rare earth stearates containing erbium, ytterbium, and yttrium, and NaF as raw materials to synthesize UCNPs; modifying UCNPs with PEI to obtain PEI-modified UCNPs. (2) Preparation of DNAzyme-modified AuNPs: Reacting chloroauric acid with sodium citrate to synthesize AuNPs; reacting a mixture of enzyme substrate and DNAzyme-SH with AuNPs to obtain DNAzyme complex-modified AuNPs. (3) Preparation of the filter paper detection matrix: First, fix the PEI-modified UCNPs prepared in step (1) on the oxidized and activated filter paper, and then fix the DNAzyme-modified AuNPs on the oxidized and activated filter paper as well. (4) Detection of the total amount of rare earth ions: Drop the rare earth ion solution suspended on the filter paper detection matrix prepared in step (4), react at room temperature for a period of time, generate green fluorescence by excitation with 980 nm infrared light in a dark box, capture the luminescence image using a CCD, process it by RGB image software to obtain the value of the luminescence intensity, and determine the concentration of rare earth ions in the sample by measuring through the standard curve of the fluorescence quenching rate and the rare earth ion concentration.

3. The method for detecting the total amount of rare earth ions based on a paper-based technology of upconversion fluorescence resonance energy transfer according to claim 2, characterized in that: The preparation of the PEI-modified UCNPs in step (1) specifically includes the following steps: Synthesis of UCNPs: Take a clean three-necked flask, add 12 mL of oleic acid (OA) and 8 mL of octadecene (ODE), add 0.8 mmol of rare earth stearate containing erbium, ytterbium, and yttrium, and 28 mmol of NaF. Reflux and heat to 135 - 145 °C for 30 min to dehydrate and degas, forming a clear liquid. Then quickly raise the temperature and maintain the reaction temperature at 312 - 314 °C for 45 min. After the reaction, cool to room temperature. After centrifugation, discard the supernatant, retain the centrifuged precipitate, and wash it three times with a solution of cyclohexane:ethanol at a ratio of 3:1, and then wash it three times with deionized water. Dry it in vacuum at 60 °C to obtain oleic acid-coated nanoparticles UCNPs for standby; PEI-modified UCNPs: Mix 10 mL of 10 mg / mL oleic acid-coated nanoparticles UCNPs dispersed in hexane and 20 mL of 0.01 mol / L NOBF4 in DMF solution, stir at room temperature for 2 h, remove the upper hexane layer, purify and centrifuge through a mixed solution of toluene and hexane at a volume ratio of 1:1, and wash twice with DMF; Subsequently, disperse it in 5 mL of DMF solution, mix it with 1 g of PEI dispersed in 15 mL of DMF solution, stir overnight, wash several times with deionized water, and dry it in vacuum at 60 °C to obtain PEI-modified UCNPs, namely PEI-UCNPs, for standby.

4. The method for detecting the total amount of rare earth ions based on a paper-based technology of upconversion fluorescence resonance energy transfer according to claim 2, characterized in that: Preparation of DNAzyme-modified gold nanoparticles AuNPs described in step (2) specifically includes: Synthesis of AuNPs: Add 49.75 mL of pure water to a 100 mL three-necked flask, add 250 μL of 2% chloroauric acid, heat to boiling, then add 3 mL of 1 wt% sodium citrate solution, continue to stir for 15 min until the solution turns purple-red, cool and store at 4 °C to obtain a sodium citrate-coated AuNPs solution for standby; Preparation of AuNPs-DNAzyme: Mix the enzyme substrate and DNAzyme-SH in an equal ratio in a HEPES buffer solution at pH 7.0, heat at 90 °C for 5 min, and anneal to room temperature to prepare a DNAzyme complex; Then mix the DNAzyme complex with the prepared AuNPs solution at a molar ratio of 200:1, freeze at -20 °C for 2 h, add a sodium chloride solution with a final concentration of 0.3 M to thaw, and centrifuge to remove excess DNA to obtain DNAzyme-modified gold nanoparticles AuNPs-DNAzyme; Finally, redisperse AuNPs-DNAzyme in a HEPES buffer solution and store the DNAzyme-modified gold nanoparticles AuNPs at 4 °C for standby.

5. The method for detecting the total amount of rare earth ions based on a paper-based technology of upconversion fluorescence resonance energy transfer according to claim 2, characterized in that: Preparation of the filter paper detection matrix described in step (3) specifically includes: Oxidation treatment of filter paper: Wates Grade 1 filter paper was prepared into small pieces with a diameter of 3 mm using a hole puncher, placed in a conical flask, activated with 14 wt% NaOH solution for 24 h. After activation, it was washed several times with distilled water, and then added to a filter paper oxidation modification reaction solution containing 26 mM NaIO4, 47 mM LiCl, and pH 2. It was oxidized at 35 °C for 2 days. After the reaction, it was washed several times with distilled water and dried at 40 °C for standby; Fixation of fluorescent probe: UCNPs modified with PEI were prepared into a 0.1 mg / mL solution with HEPES buffer for standby. It was suspended and dropped onto the oxidized filter paper, 3 μL per piece, and reacted at room temperature for 1 h. After the reaction, the unfixed PEI-modified UCNPs were washed away with 0.2 vol% Tween washing solution and ultrapure water, and dried for standby. AuNPs-DNAzyme was suspended and dropped onto the small filter paper pieces, 3 μL per piece. After reacting at room temperature for 1 h, the filter paper detection matrix was prepared for standby.

6. The method for detecting the total amount of rare earth ions based on a paper-based technology of upconversion fluorescence resonance energy transfer according to claim 2, characterized in that: The detection of the total amount of rare earth ions described in step (4) above specifically includes: Take 3 μL of rare earth ion solution and suspend it on the filter paper, react at room temperature for 2 h, generate green fluorescence by excitation with 980 nm infrared light in a dark box, capture the luminescence image using a CCD, and process it with RGB image software to obtain the value of the luminescence intensity. It was determined by the standard curve of the fluorescence quenching rate and the rare earth ion concentration, so as to determine the rare earth ion concentration in the sample.

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