A biodegradable coated urea with zirconium-based MOF nanocrystals having phosphatase activity and a method of preparation and use

Abstract

The application discloses a biodegradable coated urea added with zirconium-based MOF nanocrystals with phosphatase activity and a preparation method and application thereof, and belongs to the technical field of fertilizers. The zirconium-based MOF nanocrystals with phosphatase activity are prepared, a solution of the zirconium-based MOF nanocrystals is mixed with urea and dried, polypropylene carbonate is used as a coating material, and then the biodegradable coated urea added with the zirconium-based MOF nanocrystals with phosphatase activity is prepared through a fluidized bed solvent method. The coated urea prepared in the application can realize nitrogen control release, hydrolyze soil organic phosphorus through the phosphatase activity of the Zr-MOF nanocrystals, realize the dual functions of continuous supply of nitrogen and in-situ activation of soil phosphorus, simultaneously meet the demand of crops for two core nutrient elements of nitrogen and phosphorus, significantly relieve the low-phosphorus stress of crops, promote the growth of the root system and the aboveground part of crops, and improve the biomass of crops.

Description

Technical Field

[0001] This invention relates to a biodegradable coated urea with zirconium-based MOF nanocrystals containing phosphatase activity, its preparation method, and its application, belonging to the field of fertilizer technology. Background Technology

[0002] Phosphorus is one of the three essential nutrients for plant growth, playing a crucial role in regulating physiological metabolism and synthesizing genetic material. Plants can directly utilize soluble phosphates, primarily derived from soil humus and phosphorus-containing minerals. However, over 50% of phosphorus in the soil exists in organic phosphorus form, which cannot be directly absorbed and utilized by plants. Low phosphorus stress is widespread in flooded / burnt land, sandy soils, and intensively cultivated areas, significantly inhibiting plant growth and reducing crop yield and quality. Plants can typically alleviate mild phosphorus stress by releasing organic acids and phosphatases to decompose organic phosphorus in the soil; however, their self-regulation is insufficient in severely phosphorus-deficient environments. Currently, agricultural production mainly alleviates phosphorus stress by applying phosphate fertilizers. However, soil phosphorus fixation results in a utilization rate of only 10-20% for phosphate fertilizers in the current season, and excessive application of phosphate fertilizers can easily lead to environmental problems such as eutrophication of water bodies. Therefore, there is an urgent need to develop economical and ecological methods for alleviating phosphorus stress.

[0003] Currently, nanozymes have attracted much attention in the field of enzyme mimicry due to their structural stability and strong adaptability to complex environments. However, existing research mainly focuses on oxidoreductase nanozymes, with limited research on phosphatase nanozymes capable of cleaving organophosphorus phosphate bonds. Metal-organic frameworks (MOFs), as a highly diverse class of porous crystalline materials, are ideal materials for constructing nanozymes due to their highly tunable and ordered structures. The metal nodes in MOFs act as active centers, while ligands act as molecular bridges modifying the electronic environment of the central metal, thus jointly mimicking a pattern similar to the catalytic center and cofactors of natural enzymes. However, the cleavage efficiency of nanozymes for soil organophosphorus varies significantly, making it difficult to meet the activation requirements of diverse soil phosphorus pools in agricultural production. Currently, there are no systematic reports on the use of nanozymes combined with fertilizer carriers to alleviate plant phosphorus stress.

[0004] Meanwhile, traditional urea fertilizer releases nutrients quickly, which can easily lead to nitrogen loss. Coated urea can achieve controlled release of nitrogen nutrients, but most existing coated urea only have the function of supplying nitrogen and cannot solve the problem of soil phosphorus deficiency at the same time; moreover, some coating materials are not biodegradable, which can easily cause secondary soil pollution.

[0005] Therefore, developing a biodegradable coated urea that combines nitrogen controlled release and soil organic phosphorus activation is of great significance for the green development of agriculture. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a biodegradable coated urea with zirconium-based MOF nanocrystals containing phosphatase activity, a preparation method, and applications. This coated urea can simultaneously achieve controlled release of nitrogen nutrients and activation of soil organic phosphorus, thereby increasing the supply of available phosphorus in the soil and promoting crop growth in low-phosphorus stress soils.

[0007] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows: A biodegradable coated urea with added zirconium-based MOF nanocrystals exhibiting phosphatase activity is disclosed. The coated urea has a core-shell layered structure, comprising a fertilizer core, an active layer, and a biodegradable coating layer. The fertilizer core is composed of large-particle urea. The active layer comprises zirconium-based MOF nanocrystals. The biodegradable coating layer comprises polypropylene carbonate.

[0008] The biodegradable coated urea of ​​this invention utilizes a core-shell layered structure to organically combine a large-particle urea slow-release nitrogen source, a zirconium-based MOF nanocrystal active layer, and a PPC biodegradable coating layer. The zirconium-based MOF nanocrystals possess phosphatase activity, catalyzing the hydrolysis of soil organic phosphorus to increase available phosphorus content and promoting the biodegradation of the PPC coating. The PPC coating, in turn, regulates the slow-release rate of urea nitrogen while protecting the catalytic activity of the zirconium-based MOF. This synergistic effect of the three components achieves efficient nitrogen utilization, soil phosphorus activation, and simultaneous, long-term release of nitrogen and phosphorus.

[0009] The method for preparing biodegradable coated urea with added zirconium-based MOF nanocrystals exhibiting phosphatase activity includes the following steps: (1) Mix anhydrous ethanol and desalinated water, add zirconium-based MOF nanocrystals, and obtain a Zr-MOF nanocrystal solution; (2) After preheating the large granular urea, quickly pour it into a constant temperature mixer, spray the Zr-MOF nanocrystal solution onto the large granular urea while stirring, and then heat and dry it to obtain large granular urea coated with Zr-MOF nanocrystals. (3) Add polypropylene carbonate to ethyl acetate and stir until completely dissolved to obtain a coating solution; (4) Add the large-particle urea coated with Zr-MOF nanocrystals to the fluidized bed for preheating, add liquid paraffin, introduce the coating liquid into the fluidized bed through a peristaltic pump and atomize it through the nozzle, and spray it evenly on the surface of the fluidized large-particle urea. After the spraying is completed, continue fluidization to obtain biodegradable coated urea with zirconium-based MOF nanocrystals with phosphatase activity.

[0010] Furthermore, in step (1), the method for preparing the zirconium-based MOF nanocrystals includes the following steps: (a) Dissolve zirconium sulfate in a mixed solution of formic acid and methanol, stir to react, centrifuge, wash, and vacuum dry to obtain an acidified zirconium source; (b) Add the acidified zirconium source to acetic acid, stir until homogeneous, then add deionized water and stir until the solution is transparent. Then add it and 3-bromophthalic acid to methanol, stir the reaction, centrifuge, wash, and vacuum dry to obtain zirconium-based MOF nanocrystals (Zr-MOF).

[0011] Zr with empty 4d orbits 4+ As the core site of zirconium-based metal-organic framework materials, it possesses excellent affinity for phosphate groups. By attracting the lone pairs of electrons from the phosphate ester bonds in organophosphorus substrates, it achieves polarization activation of the P=O bonds. The phosphatase activity of zirconium-based MOF nanocrystals is fundamentally dependent on Zr. 4+ The appropriate Lewis acidity of the active center needs to be sufficient to bind to and polarize the phosphate ester bonds of organophosphorus compounds for cleavage, but not so strong as to prevent substrate dissociation due to excessive binding to the phosphate group. Simultaneously, it must ensure the bonding strength of the metal-ligand (MO) bond and the spatial confinement effect of crystal growth. The ligand's interference with the charge density near the central metal ion determines the key electronic properties of the metal-organic framework (MOF), which directly affects the MOF's affinity for the substrate. Therefore, this invention selects 3-bromophthalic acid to modulate Zr through its inductive effect. 4+ The electronic structure of the site enhances the polarization of the Zr-O bond, promotes the hydrolysis of phosphate ester bonds, and converts soil organic phosphorus into available phosphorus that can be utilized by plants.

[0012] Furthermore, in step (a), the ratio of zirconium sulfate, formic acid, and methanol is 1g:6mL:10mL.

[0013] Furthermore, in step (a), the stirring reaction is carried out at a temperature of 100°C for 120 min; the centrifugation speed is 700 rpm; the washing method is to wash twice with acetone; and the vacuum drying temperature is 50°C for 3 h.

[0014] Furthermore, in step (b), the ratio of the acidified zirconium source, acetic acid, demineralized water, 3-bromophthalic acid, and methanol is 3g:20mL:50mL:2g:320mL; the particle size of the zirconium-based MOF nanocrystals is <10nm. The small particle size of the zirconium-based MOF nanocrystals endows them with a high specific surface area and more exposed active sites, which is beneficial for sufficient contact with soil organophosphorus substrates.

[0015] Furthermore, in step (b), the stirring speed is 500 rpm and the time is 2 h; the centrifugation speed is 7000 rpm and the time is 1 h; the washing method is to wash the product twice with a mixed solution of methanol and acetone with a volume ratio of 1:1; the vacuum drying temperature is 50°C and the time is 3 h.

[0016] Furthermore, in step (1), the volume ratio of anhydrous ethanol to deionized water is 1:1; the concentration of the Zr-MOF nanocrystal solution is 5 g / L.

[0017] Furthermore, in step (2), the ratio of the large-particle urea to the Zr-MOF nanocrystal solution is 10g:1mL; the diameter of the large-particle urea is 2-4mm, the nitrogen content is 46%, and it is purchased from Shandong Ruixing Group Co., Ltd.

[0018] Furthermore, in step (2), the preheating temperature is 100℃ and the time is 30min; the temperature of the constant temperature mixer is 90℃; the stirring speed is 50r / min; and the heating and drying time is 30min.

[0019] Furthermore, in step (3), the ratio of polypropylene carbonate to ethyl acetate is 1g:25g; the stirring time is 2h. Polypropylene carbonate (PPC) is a biodegradable aliphatic polyester that coats the surface of urea particles to form a dense film. Water molecules need to permeate through the film to contact the urea core and dissolve the urea. The dissolved urea then diffuses through the film to be released into the soil, thereby achieving controlled release.

[0020] Furthermore, in step (4), the preheating temperature is 40±5℃ and the time is 10min; the liquid inlet rate of the peristaltic pump is 20 mL / min; the pressure of the nozzle atomization is 0.4 MPa; and the fluidization time is 5 min.

[0021] Furthermore, in step (4), the amount of liquid paraffin added is 0.02% of the mass of the large-particle urea; the mass ratio of the coating solution to the large-particle urea coated with Zr-MOF nanocrystals is 2:1. Urea particles have a polar surface and are highly hydrophilic. However, the coating solution formed by polypropylene carbonate (PPC) dissolved in ethyl acetate is an organic phase, and the two have poor interfacial compatibility. When directly sprayed, the coating solution is difficult to spread evenly on the urea surface, and droplet shrinkage is easily formed, resulting in incomplete coating and uneven thickness. Liquid paraffin is a non-polar hydrophobic substance. After being coated on the urea surface, it forms an extremely thin hydrophobic transition layer. This transition layer can effectively improve the wetting and spreading performance of the coating solution on the urea surface, reduce interfacial tension, prevent the solvent from penetrating too quickly, and at the same time reduce the adhesion between particles, improve the uniformity and density of the coating layer, thereby ensuring the stable controlled-release performance of the coated urea.

[0022] An application of biodegradable coated urea with added zirconium-based MOF nanocrystals containing phosphatase activity: This coated urea is applied to low-phosphorus-stressed soils with available phosphorus content below 3 mg / kg to provide nitrogen nutrients to crops, activate soil organic phosphorus, increase the supply of available phosphorus in the soil, and promote crop growth.

[0023] Furthermore, the application dosage is 1g of biodegradable coated urea with zirconium-based MOF nanocrystals containing phosphatase activity per 1kg of soil.

[0024] Furthermore, the crop in question is rapeseed.

[0025] The principle of this invention is: This invention constructs zirconium-based MOF nanocrystals with phosphatase activity, wherein Zr 4+ As the core metal node of zirconium-based MOF nanocrystals, it possesses excellent Lewis acidity and a high selective affinity for phosphate groups, serving as an active center for phosphate ester bond hydrolysis. By binding to the phosphate groups of organophosphorus substrates, it achieves polarization activation of P=O bonds, providing a basis for phosphate ester bond cleavage. Using 3-bromophthalic acid as an organic ligand, its strong electron-withdrawing inductive effect effectively modulates Zr. 4+ The electron density at the site causes a redshift in the MO bond formed between Zr and the ligand, thus optimizing Zr. 4+ The binding affinity to organophosphorus substrates significantly enhances the cleavage efficiency of organophosphorus phosphate bonds; simultaneously, the regulation of Zr... 4+ Exhibiting moderate Lewis acidity, it effectively polarizes phosphate ester bonds without binding too strongly to the hydrolyzed phosphate groups, ensuring the continuity of substrate dissociation and catalytic cycling. Furthermore, Zr-MOF nanocrystals possess an extremely large specific surface area, allowing for ample contact with organophosphorus substrates in the soil, significantly improving catalytic efficiency. This effectively hydrolyzes organophosphorus compounds in the soil that plants cannot directly utilize into soluble phosphates, making them available for plant absorption and alleviating low-phosphorus stress in the soil.

[0026] Furthermore, this invention organically combines Zr-MOF nanocrystals with PPC-coated urea. First, Zr-MOF nanocrystals are uniformly coated onto the surface of urea particles, and then a PPC coating is applied to form an outer protective layer. The PPC film ensures a stable bond between the Zr-MOF nanocrystals and the urea core material, slowly releasing nutrients from the inner layer, replacing the rapid dissolution and release of traditional urea. PPC is an aliphatic polyester that can naturally biodegrade under the action of soil microorganisms. During the slow degradation of the PPC layer, Zr-MOF nanocrystals are gradually released into the soil, continuously exerting their phosphorus activation effect. Simultaneously, nitrogen is also gradually released, achieving a synchronous and synergistic supply of nitrogen and phosphorus.

[0027] Compared with the prior art, the beneficial effects of the present invention are: (1) The biodegradable coated urea with zirconium-based MOF nanocrystals with phosphatase activity prepared by the present invention can achieve controlled release of nitrogen and hydrolyze soil organic phosphorus through the phosphatase activity of Zr-MOF nanocrystals, thus realizing the dual functions of continuous nitrogen supply and in-situ activation of soil phosphorus. It can simultaneously meet the crop's demand for the two core nutrients of nitrogen and phosphorus, significantly alleviate crop low phosphorus stress, promote the growth of crop roots and aboveground parts, and increase crop biomass. At the same time, the PPC coating layer can slowly degrade and gradually release Zr-MOF nanocrystals, avoiding their loss and improving utilization.

[0028] (2) The utilization rate of chemical phosphate fertilizers in existing agricultural production is low in the current season and is easily wasted due to soil phosphorus fixation. After applying the coated urea of ​​this invention, there is no need to apply additional phosphate fertilizer. Phosphorus supply is achieved by activating the original organic phosphorus in the soil, thereby improving the utilization rate of phosphorus resources from the source and greatly improving the utilization efficiency of soil phosphorus, avoiding the cost and resource waste caused by the inefficient application of phosphate fertilizer.

[0029] (3) This invention systematically integrates the catalytic function of nanocrystals with the nutrient controlled release function of coated fertilizers, and proposes a fertilizer design concept of nutrient supply + in-situ activation. A single application of fertilizer can achieve dual nitrogen and phosphorus supply, greatly simplifying field fertilization operations, reducing labor and management costs of fertilization, and the preparation process is simple and suitable for large-scale production. Attached Figure Description

[0030] Figure 1 This is a transmission electron microscope (TEM) image of zirconium-based MOF nanocrystals; Figure 2 This is the X-ray diffraction (XRD) pattern of zirconium-based MOF nanocrystals; Figure 3 It is the Fourier transform infrared (FTIR) spectrum of zirconium-based MOF nanocrystals; Figure 4 This is a graph showing the phosphatase activity of zirconium-based MOF nanocrystals; Figure 5 This is a diagram showing the hydrolysis effect of zirconium-based MOF nanocrystals on p-nitrophenyl phosphate (p-NPP). Figure 6 This is a nitrogen release rate curve of biodegradable coated urea with added zirconium-based MOF nanocrystals containing phosphatase activity. Figure 7 This is a graph showing the growth indicators of rapeseed after application of biodegradable coated urea with zirconium-based MOF nanocrystals containing phosphatase activity. Figure 8 This is a graph showing the available phosphorus content in the soil after application of biodegradable coated urea with zirconium-based MOF nanocrystals containing phosphatase activity. Figure 9This is a comparison diagram of the phosphatase activities of zirconium-based MOF nanocrystals in Example 1 and Zr metal-organic framework nanozymes in Comparative Example 6. Detailed Implementation

[0031] Example 1 The preparation method of zirconium-based MOF nanocrystals includes the following steps: (a) Dissolve 20 g of zirconium sulfate in a mixed solution of 120 mL formic acid and 200 mL methanol, stir at 100 °C for 120 min, centrifuge at 7000 rpm, wash twice with acetone, and dry under vacuum at 50 °C for 3 h to obtain the acidified zirconium source. (b) Add 15g of acidified zirconium source to 100 mL of acetic acid, stir well, then add 250 mL of deionized water and stir until the solution is clear. Then add it and 10g of 3-bromophthalic acid to 1.6L of methanol, stir at 500 rpm for 2 h, centrifuge at 7000 rpm for 1 h, wash twice with a 1:1 mixture of methanol and acetone, and vacuum dry at 50℃ for 3 h to obtain zirconium-based MOF nanocrystals (Zr-MOF).

[0032] Example 2 A method for preparing biodegradable coated urea with added zirconium-based MOF nanocrystals exhibiting phosphatase activity includes the following steps: (1) Mix anhydrous ethanol and desalinated water at a volume ratio of 1:1, add zirconium-based MOF nanocrystals, and obtain a Zr-MOF nanocrystal solution with a concentration of 5 g / L. (2) After preheating 1 kg of large granular urea at 100℃ for 30 min, quickly pour it into a constant temperature mixer at 90℃. Under stirring at 50 r / min, spray 100 mL of Zr-MOF nanocrystal solution onto the large granular urea, and then heat and dry for 30 min to obtain large granular urea coated with Zr-MOF nanocrystals. (3) Add 80g of polypropylene carbonate (PPC) to 2kg of ethyl acetate and stir for 2h until completely dissolved to obtain a coating solution; (4) Add the large-particle urea coated with Zr-MOF nanocrystals to the fluidized bed, preheat at 40±5℃ for 10 min, add 0.2g of liquid paraffin, turn on the peristaltic pump and air compressor, set the liquid inlet rate of the peristaltic pump to 20 mL / min and the pressure of the air compressor to 0.4 MPa, and spray the coating liquid evenly onto the surface of the fluidized large-particle urea through a two-fluid nozzle. After the spraying is completed, continue fluidization for 5 min to obtain biodegradable coated urea with zirconium-based MOF nanocrystals with phosphatase activity.

[0033] Comparative Example 1 The process steps are the same as in Example 2, except that zirconium-based MOF nanocrystals are not added, and polypropylene carbonate-coated urea without Zr-MOF nanocrystals is obtained.

[0034] Comparative Example 2 The process steps are the same as in Example 1, except that in step (2), 3-bromophthalic acid is replaced with 2-fluoroterephthalic acid to obtain zirconium-based MOF nanocrystals (Zr-MOF-F).

[0035] Comparative Example 3 The process steps are the same as in Example 1, except that in step (2), 3-bromophthalic acid is replaced with 2-chloroterephthalic acid to obtain zirconium-based MOF nanocrystals (Zr-MOF-Cl).

[0036] Comparative Example 4 The process steps are the same as in Example 1, except that in step (2), 3-bromophthalic acid is replaced with 2-bromoterephthalic acid to obtain zirconium-based MOF nanocrystals (Zr-MOF-Br).

[0037] Comparative Example 5 The process steps are the same as in Example 1, except that in step (2), 3-bromophthalic acid is replaced with 2-iodophthalic acid to obtain zirconium-based MOF nanocrystals (Zr-MOF-I).

[0038] Comparative Example 6 The prior art discloses a method for preparing Zr metal-organic framework nanozymes: (1) Dissolve 1 g of zirconium chloride in a mixed solution of 1.5 mL acetic acid and 2.5 mL isopropanol; react the mixture at 120 °C and 600 rpm for 60 min; cool to room temperature after the reaction is complete; centrifuge the reaction product at 10000 rpm for 10 min and collect the solid product; wash the product three times with acetone and dry it under vacuum at 60 °C for 8 h. The powder obtained after drying is Zr6 cluster. (2) 75 mg of Zr6 clusters were dispersed in a mixed solution of 0.5 mL acetic acid and 1.25 mL deionized water and stirred at 600 rpm at 25 °C until clear. This mixed solution and 55 mg of 2-fluoroterephthalic acid were added to 80 mL of ethanol. The mixture was stirred at 600 rpm at 25 °C for 120 min. The product was centrifuged at 14500 rpm for 60 min. The product was washed three times with a mixed solution of 5 mL ethanol and 5 mL acetone and then vacuum dried at 60 °C for 8 h. The powder obtained after drying was Zr metal-organic framework nanozyme.

[0039] Experimental Example 1 The zirconium-based MOF nanocrystals (Zr-MOF) of Example 1 were characterized and analyzed by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy. The test results are attached. Figure 1-3 As shown.

[0040] from Figure 1 It can be seen that the Zr-MOF nanocrystals prepared by the present invention are all smaller than 10 nm in size and exhibit a nanocrystalline morphology. This indicates that the preparation method of the present invention can control the size of Zr-MOF nanocrystals to a small nanocrystal level. Their large specific surface area is beneficial to fully contact with soil organic phosphorus and improve catalytic efficiency.

[0041] from Figure 2 It can be seen that the characteristic peaks of the Zr-MOF nanocrystals prepared in this invention are consistent with those of the analog UiO-66 (purchased from Xi'an Qiyue Biotechnology Co., Ltd.), and UiO-66 is a metal-organic framework material with Zr as the metal center and terephthalic acid as the organic ligand, indicating that the Zr-MOF nanocrystals in this invention were successfully prepared.

[0042] from Figure 3 It can be seen that the Zr-MOF nanocrystals prepared in this invention have a 3401 cm⁻¹ chromatogram. -1 The characteristic peak of the stretching vibration of -OH is present at 1556 cm⁻¹. -1 and 1378cm -1 There is a characteristic peak of stretching vibration at OC=O, 1029 cm⁻¹. -1 The characteristic peak of CO stretching vibration is present at 530 cm⁻¹. -1 The characteristic peaks are attributed to the MO bonds formed between the linker and Zr, and the MO bonds exhibit a red shift, indicating that the introduction of halogen atoms (Br) modulates the electronic structure of the Zr sites, and that this modulation occurs at 3401 cm⁻¹. -1 The peak shape at this location is wider than that of UiO-66, indicating that it has a greater number and richer distribution of surface hydroxyl groups. This demonstrates that the Zr-MOF nanocrystals prepared in this invention successfully retain the core metal-organic framework structure of UiO-66. At the same time, the introduction of 3-bromophthalic acid ligands regulates the coordination environment and the number of surface hydroxyl groups of Zr, providing a key structural and chemical basis for its efficient phosphatase activity.

[0043] Experimental Example 2: Detection of Phosphatase Activity in Zr-MOF Nanocrystals A 10 mmol / L Tris-HCl buffer solution (pH=9.0) was prepared. Zr-MOF nanocrystals and UiO-66 were weighed and dissolved in Tris-HCl buffer to prepare suspensions with a concentration of 2 mg / mL, resulting in Zr-MOF nanocrystal suspensions and UiO-66 suspensions. Then, a 0.3 mg / mL 4-MUP stock solution was prepared using Tris-HCl buffer. 30 μL of 4-MUP stock solution was taken, and 40 μL of Zr-MOF nanocrystal suspension and UiO-66 suspension were added to each. The solutions were then diluted to 1.5 mL with Tris-HCl solution and incubated in a 40℃ water bath for 30 min. The fluorescence intensity of the solutions was measured. The results are attached. Figure 4 As shown.

[0044] from Figure 4 It was found that, using 4-methylumbelliferyl ketone phosphate (4-MUP) as a substrate, the solution containing Zr-MOF nanocrystals, after incubation, exhibited strong blue fluorescence at a wavelength of 447 nm, indicating that Zr-MOF nanocrystals can hydrolyze 4-methylumbelliferyl ketone phosphate (4-MUP) to release the fluorescent product 4-methylumbelliferyl ketone (4-MU). In contrast, the solution containing UiO-66 showed very weak fluorescence intensity, indicating that UiO-66 could not hydrolyze 4-methylumbelliferyl ketone phosphate (4-MUP). Therefore, the Zr-MOF nanocrystals prepared in this invention possess strong phosphatase activity.

[0045] Experimental Example 3 The hydrolysis process of p-nitrophenyl phosphate (p-NPP) from Zr-MOF nanocrystals was determined by attenuated total reflectance (ATR)-Fourier transform infrared spectroscopy (FTIR). The method was as follows: Zr-MOF nanocrystals were dispersed in a small amount of anhydrous ethanol to prepare a suspension with a concentration of 10 mg / mL. 15 μL of this suspension was uniformly drop-coated onto the surface of the ATR crystals. The solvent was allowed to evaporate naturally at room temperature, and the nanocrystals were fixed with Nafion solution (15 μL, 0.05 mM) to obtain a Zr-MOF film. The film spectrum was collected as background spectra. Then, p-NPP solution (70 μL, 2 mM) and Tris-HCl solution (1000 μL, pH=9, 10 mM) were added dropwise, and the reaction was carried out at 37 °C. Spectral data were collected at 1 min and 10 min. The test results are attached. Figure 5 As shown.

[0046] from Figure 5 It can be seen that as the reaction time increases from 1 minute to 10 minutes, 3406 cm -1The peak at 1060 cm⁻¹ is attributed to H₂O. The increased intensity of this peak is due to the adsorption of H₂O at the reaction interface, which acts as a reaction medium and proton donor during the p-NPP activation process, providing a proton source for the subsequent hydrolysis of phosphate ester bonds and ensuring the continued progress of the catalytic reaction. -1 The decrease in peak intensity indicates the breaking of the P=O bond, suggesting that the Zr site, acting as a Lewis acid center, activates the P=O bond, creating conditions for subsequent nucleophilic attack. (1228 cm⁻¹) -1 The characteristic peak at the location corresponds to the PO linking the phosphate group to the aromatic ring of p-nitrophenol (p-NPP) in the phosphate ester bond (PO-Ar). The weakening of this peak intensity indicates that the PO-Ar bond has been broken and p-NPP has been hydrolyzed to generate p-nitrophenol (p-NP). This shows that the Zr-MOF nanocrystals prepared in this invention can activate the phosphate ester bonds in organophosphorus compounds and hydrolyze the organophosphorus compounds.

[0047] Experiment Example 4 The nitrogen release rate of coated urea in Example 2 and Comparative Example 1 was determined according to GB / T 23348-2009 using the 25℃ water immersion method. The specific steps are as follows: Ten g of uniformly sized, intact coated urea from Example 2 and Comparative Example 1 were selected and placed into 100-mesh nylon mesh bags (10*10 cm). The bags were sealed with cotton thread and placed in 250 mL plastic bottles. 200 mL of deionized water was added, the caps were tightened, and the bottles were placed in a 25°C incubator. Samples were taken on days 1, 4, 7, 10, 14, 28, 42, 56, and 63. The extract in the plastic bottles was completely poured out, and 200 mL of deionized water was added again for soaking. The coated urea group from Example 2 was designated as PPC-coated urea + Zr-MOF, and the coated urea group from Comparative Example 1 was designated as PPC-coated urea. The urea nitrogen content in the extract was determined using the p-dimethylaminobenzaldehyde spectrophotometric method, and the cumulative nitrogen release rate was calculated. The results are attached. Figure 6 As shown.

[0048] from Figure 6 It can be seen that the initial nitrogen release rate of PPC-coated urea and PPC-coated urea + Zr-MOF is 0.2%, the cumulative nitrogen release rates after 28 days are 30.0% and 33.0% respectively, and the cumulative nitrogen release rates after 63 days are 78.6% and 79.8% respectively. The difference in the cumulative nitrogen release rate between the two is very small, indicating that the addition of Zr-MOF nanocrystals has minimal impact on the nitrogen nutrient controlled release performance of PPC-coated urea. The biodegradable coated urea with zirconium-based MOF nanocrystals with phosphatase activity prepared in this invention can achieve a nitrogen controlled release period of more than 60 days, and at the same time realizes the dual-function integration of nitrogen controlled release and in-situ phosphorus activation, and the long-term synchronous supply of nitrogen and phosphorus.

[0049] Experiment Example 5: Verification of the Application Effect of Coated Urea in Low Phosphorus Stress Soils To investigate the application of the biodegradable coated urea with zirconium-based MOF nanocrystals containing phosphatase activity as described in this invention under low phosphorus stress, the following experiment was designed: (1) Simulated low phosphorus stress soil: ordinary culture soil was repeatedly rinsed with deionized water and tested by ammonium molybdate colorimetric method until the available phosphorus content of the soil was less than 3 mg / kg, thus obtaining low phosphorus stress soil; (2) Pot experiment setup: Each pot contained 5 kg of low phosphorus stress soil, and two treatment groups were set up, with 10 pots replicated in each group: Control group: 5g of polypropylene carbonate coated urea prepared in Comparative Example 1 was applied and mixed evenly with the soil, and this group was designated as the PPC coated urea group. Experimental group: 5g of biodegradable coated urea prepared in Example 2 with added zirconium-based MOF nanocrystals containing phosphatase activity was applied and mixed evenly with the soil, and was recorded as PPC coated urea + Zr-MOF group; (3) Crop planting: The rapeseed seedlings that have been cultivated for one week are planted in the soil of the control group and the experimental group, one seedling per pot, and placed in the greenhouse for conventional cultivation for 15 days. During the cultivation period, the soil moisture is kept consistent and no other fertilizers are applied.

[0050] After cultivation, the aboveground fresh weight and root fresh weight of rapeseed were measured and recorded. The available phosphorus content in the soil was determined using the sodium bicarbonate extraction-molybdenum antimony spectrophotometric method. The results are attached. Figure 7-8 As shown.

[0051] from Figure 7 It can be seen that, compared with the control group, the aboveground fresh weight of rapeseed in the experimental group (PPC-coated urea + Zr-MOF) increased by 107%, and the root fresh weight increased by 33%, indicating that the application of the biodegradable coated urea with zirconium-based MOF nanocrystals containing phosphatase activity prepared in this invention can effectively promote rapeseed growth. Figure 8 It can be seen that, compared with the control group, the soil available phosphorus content of the experimental group (PPC-coated urea + Zr-MOF) increased by 56%, indicating that the application of biodegradable coated urea with zirconium-based MOF nanocrystals with phosphatase activity prepared in this invention can increase the available phosphorus content. This shows that zirconium-based MOF nanocrystals with phosphatase activity can effectively decompose organic phosphorus in the soil and convert it into available phosphorus in the soil, which can significantly increase the available phosphorus supply in soil under low phosphorus stress. At the same time, it can continuously provide nitrogen nutrition through controlled nitrogen release and synergistically promote rapeseed growth.

[0052] Experimental Example 5 Following the method in Experimental Example 2, the phosphatase activity of zirconium-based MOF nanocrystals from Example 1 and Comparative Examples 2-5 was measured, and characterized by fluorescence intensity. The results are listed in Table 1.

[0053] Table 1. Comparison of phosphatase activities of zirconium-based MOF nanocrystals As shown in Table 1, compared with the zirconium-based MOF nanocrystals of Comparative Examples 2-5, the zirconium-based MOF nanocrystals of Example 1 exhibited higher phosphatase activity. This is because when 3-bromophthalic acid is used as a ligand, the strong electron-withdrawing effect of the ortho-bromine atom, and its non-participation in the benzene ring conjugation system, can reduce the activity of Zr. 4+ The higher electron cloud density enhances Lewis acidity, resulting in a higher affinity for phosphate groups in organophosphorus substrates, effectively polarizing P=O bonds, and leading to the highest phosphatase activity. In contrast, the phosphatase activity of comparative 2-5 zirconium-based MOF nanocrystals decreases sequentially because the electron-withdrawing ability of their ligand substituents gradually weakens, and the Lewis acidity of the Zr sites becomes increasingly weaker.

[0054] Experimental Example 6 Following the method in Experimental Example 2, the phosphatase activities of the zirconium-based MOF nanocrystals of Example 1 and the Zr metal-organic framework nanozyme of Comparative Example 6 were determined. The zirconium-based MOF nanocrystal group of Example 1 was designated Zr-MOF+4-MUP, and the Zr metal-organic framework nanozyme group of Comparative Example 6 was designated Zr-MOF(F)+4-MUP. The results are attached. Figure 9 As shown.

[0055] from Figure 9 It is evident that the phosphatase activity of the zirconium-based MOF nanocrystals in Example 1 of this invention is significantly higher than that of the Zr metal-organic framework nanozyme in Comparative Example 6. This is because the Br in 3-bromophthalic acid used in this invention exhibits a strong electron-withdrawing inductive effect, without the interference of conjugation effects. This significantly reduces the electron cloud density around the Zr center, enhancing its positive charge (i.e., Lewis acidity), thereby significantly increasing its coordination affinity to the phosphoryl substrate. Furthermore, it accelerates the catalytic cleavage of the P=O bond through polarization, resulting in stronger phosphatase activity. While the 2-fluoroterephthalic acid used in Comparative Example 6 has high electronegativity, the strong p-π conjugation between F and the carboxyl group partially offsets the electron-withdrawing inductive effect and may even influence the Zr group. 4+ The center provides an additional electron cloud, weakening Lewis acidity and resulting in low phosphatase activity. Therefore, this invention uses 3-bromoterephthalic acid as the organic ligand, and the prepared zirconium-based MOF nanocrystals exhibit higher phosphatase activity.

Claims

1. A biodegradable coated urea with added zirconium-based MOF nanocrystals possessing phosphatase activity, characterized in that: The coated urea has a core-shell layered structure, including a fertilizer core, an active layer, and a biodegradable coating layer; the fertilizer core is large-particle urea; the active layer includes zirconium-based MOF nanocrystals; and the biodegradable coating layer includes polypropylene carbonate.

2. A method for preparing biodegradable coated urea with zirconium-based MOF nanocrystals exhibiting phosphatase activity as described in claim 1, characterized in that: Includes the following steps: (1) Mix anhydrous ethanol and desalinated water, add zirconium-based MOF nanocrystals, and obtain a Zr-MOF nanocrystal solution; (2) After preheating the large granular urea, quickly pour it into a constant temperature mixer, spray the Zr-MOF nanocrystal solution onto the large granular urea while stirring, and then heat and dry it to obtain large granular urea coated with Zr-MOF nanocrystals. (3) Add polypropylene carbonate (PPC) to ethyl acetate and stir until completely dissolved to obtain a coating solution; (4) Add the large-particle urea coated with Zr-MOF nanocrystals to the fluidized bed for preheating, add liquid paraffin, introduce the coating liquid into the fluidized bed through a peristaltic pump and atomize it through the nozzle, and spray it evenly on the surface of the fluidized large-particle urea. After the spraying is completed, continue fluidization to obtain biodegradable coated urea with zirconium-based MOF nanocrystals with phosphatase activity.

3. The method for preparing biodegradable coated urea with added zirconium-based MOF nanocrystals exhibiting phosphatase activity according to claim 2, characterized in that: In step (1), the method for preparing the zirconium-based MOF nanocrystals includes the following steps: (a) Dissolve zirconium sulfate in a mixed solution of formic acid and methanol, stir to react, centrifuge, wash, and vacuum dry to obtain an acidified zirconium source; (b) Add the acidified zirconium source to acetic acid, stir until homogeneous, then add deionized water and stir until the solution is transparent. Then add it and 3-bromophthalic acid to methanol, stir the reaction, centrifuge, wash, and vacuum dry to obtain zirconium-based metal-organic framework nanocrystals (Zr-MOF).

4. The method for preparing biodegradable coated urea with added zirconium-based MOF nanocrystals exhibiting phosphatase activity according to claim 3, characterized in that: In step (b), the ratio of the acidified zirconium source, acetic acid, deionized water, 3-bromophthalic acid, and methanol is 3g:20mL:50mL:2g:320mL.

5. The method for preparing biodegradable coated urea with zirconium-based MOF nanocrystals exhibiting phosphatase activity according to claim 2, characterized in that: In step (1), the volume ratio of anhydrous ethanol to deionized water is 1:1; the concentration of the Zr-MOF nanocrystal solution is 5 g / L.

6. The method for preparing biodegradable coated urea with zirconium-based MOF nanocrystals exhibiting phosphatase activity according to claim 2, characterized in that: In step (2), the ratio of the large-particle urea to the Zr-MOF nanocrystal solution is 10g:1mL; the diameter of the large-particle urea is 2-4mm and the nitrogen content is 46%.

7. The method for preparing biodegradable coated urea with added zirconium-based MOF nanocrystals exhibiting phosphatase activity according to claim 2, characterized in that: In step (4), the preheating temperature is 40±5℃ and the time is 10min; the liquid inlet rate of the peristaltic pump is 20 mL / min; the pressure of the nozzle atomization is 0.4 MPa; and the fluidization time is 5min.

8. The method for preparing biodegradable coated urea with zirconium-based MOF nanocrystals exhibiting phosphatase activity according to claim 2, characterized in that: The amount of liquid paraffin added is 0.02% of the mass of the large-particle urea; the mass ratio of the coating solution to the large-particle urea coated with Zr-MOF nanocrystals is 2:

1.

9. The application of the biodegradable coated urea with zirconium-based MOF nanocrystals exhibiting phosphatase activity as described in claim 1, characterized in that: The coated urea was applied to low-phosphorus stress soils with an available phosphorus content of less than 3 mg / kg.

10. The application of the biodegradable coated urea with zirconium-based MOF nanocrystals exhibiting phosphatase activity as described in claim 9, characterized in that: The application dosage is 1g of biodegradable coated urea with zirconium-based MOF nanocrystals containing phosphatase activity per 1kg of soil.

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