Antibacterial hydrogel for controlled-release fertilizer and preparation method thereof

Abstract

The invention discloses antibacterial hydrogel for a controlled-release fertilizer and a preparation method of the antibacterial hydrogel, and belongs to the technical field of controlled-release fertilizers. The preparation method comprises the following steps: by taking soluble corn starch, cellulose nanocrystals, chitosan and chitin nanofibrils as main matrix materials, constructing a core-shell silver-zinc oxide nano antibacterial agent in situ, and compounding the core-shell silver-zinc oxide nano antibacterial agent with functionalized graphene oxide and an acrylamide monomer; the preparation method comprises the following steps: entrapping a nitrogen-phosphorus-potassium fertilizer and a urea-formaldehyde prepolymer in a starch-based mother solution, blending with a chitosan solution, and carrying out in-situ covalent-ionic composite cross-linking polymerization through an ammonium persulfate / tetramethylethylenediamine initiation system to form three-dimensional network hydrogel; and carrying out extrusion / spray granulation and freeze-drying-hot air combined drying to obtain dry-based granules. The hydrogel has high mechanical strength, excellent slow release performance, long-acting broad-spectrum antibacterial property and environmental responsiveness, the fertilizer utilization rate is remarkably increased, soil pathogenic bacteria are inhibited, and the hydrogel is suitable for modern sustainable agriculture.

Description

Technical Field

[0001] This invention belongs to the technical field of controlled-release fertilizer technology, specifically, it relates to an antibacterial hydrogel for controlled-release fertilizer and its preparation method. Background Technology

[0002] With the continuous growth of the global population and the increasing scarcity of arable land resources, chemical fertilizers play a crucial role in ensuring food security and sustainable agricultural development. However, traditional fertilizer application suffers from low utilization rates and a mismatch between nutrient release and crop needs, resulting in significant nutrient losses through volatilization, leaching, and runoff. Statistics show that the utilization rate of nitrogen fertilizer in my country during the current season is only 30%–50%, phosphate fertilizer 20%–30%, and potash fertilizer 40%–60%. This low utilization rate not only wastes resources and increases agricultural production costs but also triggers serious environmental problems, such as eutrophication of water bodies, soil acidification and compaction, greenhouse gas emissions, and nitrate pollution of groundwater. Especially given that my country ranks first in the world in fertilizer application, these problems have become bottlenecks restricting green agricultural development and ecological civilization construction. Therefore, developing efficient and environmentally friendly new fertilizer technologies has become an industry consensus.

[0003] To address the aforementioned shortcomings of traditional fertilizers, controlled-release fertilizer technology emerged. Controlled-release fertilizers refer to fertilizers that regulate nutrient release rates through physical, chemical, or biological methods to match crop growth needs, thereby improving fertilizer utilization, reducing fertilization frequency, and minimizing environmental pollution. This technology was first proposed in the United States in the 1960s and has rapidly developed into an important direction in the modern fertilizer industry. Early controlled-release fertilizers primarily employed sulfur or polymer coating processes, such as sulfur-coated urea (SCU) and polymer-coated fertilizer (PCF) developed by TVA in the United States. These products achieve controlled nutrient diffusion and release by forming a dense coating on the surface of fertilizer particles, significantly extending the fertilizer's effective period. However, coated controlled-release fertilizers have significant drawbacks: First, production costs are high, as coating materials (such as polyolefins and resins) rely on petrochemicals, are expensive, and are not easily degraded. Second, the release mechanism is singular, mainly relying on water penetration and coating damage, making them susceptible to environmental factors such as soil temperature, humidity, and pH, leading to initial bursts of release or insufficient nutrients later. Third, some coating materials use organic solvents, polluting the environment during production, and residual coatings accumulate in the soil, affecting soil microbial activity. Furthermore, existing coated fertilizers have limited mechanical strength, making them prone to damage during transportation, storage, and application, affecting the controlled-release effect. With increasingly stringent environmental requirements, matrix-based slow-release fertilizers have gradually become a research hotspot. This type of fertilizer encapsulates nutrients in a polymer matrix, releasing them through matrix swelling, diffusion, and degradation, offering advantages such as simple production processes and lower costs. Among these, hydrogel-based controlled-release fertilizers have attracted considerable attention due to their high water absorption, water retention, and biocompatibility. Hydrogels are three-dimensional network-structured polymeric materials capable of absorbing hundreds of times their own weight in water and slowly releasing nutrients into the soil, while simultaneously improving soil aggregate structure and water retention capacity. Natural polysaccharide hydrogels, such as starch-based, chitosan-based, and cellulose-based hydrogels, have become a research focus due to their wide availability and biodegradability. For example, starch, as an inexpensive and renewable natural polymer, can form a porous network after gelatinization and cross-linking, used to load nutrients such as urea and phosphate, achieving a slow-release effect. Chitosan, with its cationic properties, can form ionic bonds with anionic nutrients, further regulating the release rate. Simultaneously, chitosan itself possesses certain antibacterial activity, which can inhibit soil pathogens to some extent.

[0004] Despite the promising prospects of hydrogel controlled-release fertilizers, existing technologies still have several shortcomings. First, pure natural polysaccharide hydrogels have low mechanical strength and are easily degraded by microorganisms and disintegrated by mechanical shearing in soil, leading to a sudden release of nutrients and failing to achieve long-term controlled release. Second, the release mechanism lacks intelligent responsiveness, mostly being passive diffusion-based, unable to dynamically adjust according to soil temperature, pH, or crop needs, making it difficult to adapt to complex farmland environments. Third, existing hydrogel fertilizers have insufficient antibacterial properties. In agricultural production, soil pathogens (such as Fusarium and Pythium) often cause root rot and wilt diseases. The increase in organic matter after the application of traditional fertilizers further exacerbates pathogen reproduction, while most hydrogel materials rely only on the weak antibacterial effect of chitosan, making it difficult to provide broad-spectrum, long-lasting antibacterial protection. Furthermore, while the introduction of nano-antibacterial agents (such as nano-silver and zinc oxide) can enhance antibacterial properties, problems such as uneven dispersion, easy aggregation, rapid release of silver ions leading to poor durability, and potential ecotoxicity exist. Although core-shell structured nano-antibacterial agents have been proposed in some studies, they are mostly used in the medical field. Their composite application in fertilizers still faces challenges such as poor interfacial compatibility and high cost.

[0005] Existing patents and technical literature further reveal the aforementioned problems. For example, some patents use starch or chitosan as a single matrix to prepare hydrogel fertilizers, which achieve basic slow release, but have high water absorption ratios but low strength, easily swelling and disintegrating in the soil, resulting in limited improvement in nutrient utilization. Other technologies introduce synthetic polymers (such as polyacrylamide) to enhance strength, but sacrifice biodegradability, and the residue of synthetic monomers may pose safety hazards. In terms of antibacterial properties, fertilizers with the simple addition of nano-silver or zinc oxide have antibacterial effects, but nanoparticles are easily oxidized and deactivated or over-released, and long-term application may inhibit beneficial microorganisms and disrupt the soil ecological balance. At the same time, existing hydrogel fertilizers often neglect the interaction between fertilizer and soil, such as poor swelling response in acidic or alkaline soils and low nitrogen fixation efficiency, resulting in insufficient regional adaptability.

[0006] Furthermore, in terms of production processes, existing hydrogel fertilizers are mostly formed using simple extrusion or drying, resulting in uneven particle morphology and unoptimized internal pore structure, affecting rehydration properties and release linearity. If only hot air is used in the drying process, it easily causes surface densification and internal shrinkage, reducing water absorption capacity; if only freeze-drying is used, the high cost hinders large-scale production. Although surface coating technology can further regulate release, existing coating materials are mostly synthetic polymers with poor degradability.

[0007] In summary, although controlled-release fertilizers, especially hydrogel products, have made progress in improving fertilizer utilization and reducing environmental pollution, existing technologies still have significant shortcomings in terms of mechanical strength, intelligent responsiveness, long-lasting antibacterial properties, environmental compatibility, and production costs, failing to fully meet the needs of modern sustainable agriculture. Developing a nanocomposite hydrogel controlled-release fertilizer integrating high strength, multi-network composite properties, intelligent controlled release, and broad-spectrum antibacterial activity has significant theoretical and practical value. This invention is proposed against this backdrop, aiming to overcome the deficiencies of existing technologies and provide a novel antibacterial hydrogel for controlled-release fertilizers and its preparation method. Summary of the Invention

[0008] To address the problems of insufficient mechanical strength, single release mechanism, weak antibacterial properties, poor environmental responsiveness, high production cost, and poor degradability of coating materials in existing controlled-release fertilizers, this invention provides an antibacterial hydrogel for controlled-release fertilizers and its preparation method. This hydrogel uses natural polysaccharides as the main matrix, incorporating nanocrystal reinforcement, core-shell structured antibacterial agents, functionalized graphene oxide, and smart responsive monomers. Through in-situ covalent-ionic crosslinking and multi-stage molding and drying processes, it achieves a multifunctional integration of high strength, long-lasting controlled release, broad-spectrum antibacterial properties, and environmental adaptability, significantly improving fertilizer utilization, inhibiting soil pathogens, and promoting sustainable agricultural development.

[0009] The present invention adopts the following technical solution: the preparation method of antibacterial hydrogel for controlled-release fertilizer, by weight, includes the following steps: (1) preparing starch-based mother liquor: weigh 25-45 parts of soluble corn starch (CAS: 9005-25-8) and 3-12 parts of cellulose nanocrystals (average diameter 5-20nm, CAS: 9004-34-6), add 250-450 parts of deionized water, mechanically stir at 85-95℃ for 40-70min until completely gelatinized and transparent, cool to 45-55℃, and obtain starch-nanocrystal mother liquor A; (2) preparing chitosan functional solution: weigh 8-18 parts of chitosan (degree of deacetylation ≥90%, CAS: 9012-76-4) and chitin nanocrystals. 1-5 parts of filaments (average diameter 10-50 nm, CAS: 1398-61-4) were added to 250-350 parts of glacial acetic acid (CAS: 64-19-7) aqueous solution with a volume fraction of 1.5-2.5%. The solution was magnetically stirred at room temperature for 6-10 h until completely dissolved and transparent to obtain chitosan-nanofiber solution B; (3) Preparation of core-shell structured nano-antibacterial agent: (a) Preparation of core-shell silver-zinc oxide nanoparticles by in-situ reduction method: 0.2-0.8 parts of silver nitrate (CAS: 7761-88-8) and 0.2-0.8 parts of polyvinylpyrrolidone (relative molecular mass 35000-45000, CAS: 9003-39-8) were dissolved in 80-150 parts of deionized water and heated in an ice bath. Add ascorbic acid (CAS: 50-81-7) solution (silver nitrate:ascorbic acid mass ratio 1:(0.8~1.2)) dropwise, stir for 40~70min to form silver cores, then add 1~4 parts of zinc oxide precursor zinc nitrate hexahydrate (CAS: 10196-18-6) and hexamethyltetramine (CAS: 100-97-0) mass ratio 1:1, react at 80~90℃ for 2~4h to form a core-shell structure with an average particle size of 15~40nm; (b) mix core-shell nanoparticles with graphene oxide (monolayer ratio ≥95%, sheet diameter 0.5~3μm, CAS: 7782-42-5) 0.2~1.0 parts in a solution containing acrylamide (CAS: 79-06-1) 8~20 parts, N, N - Methylenebisacrylamide (CAS: 110-26-9) 0.2-0.8 parts in 150-250 parts of deionized water was ultrasonically dispersed for 20-40 min to obtain functionalized nano antibacterial monomer dispersion C; (4) Prepare fertilizer precursor solution: Add 30-70 parts of urea (CAS: 57-13-6), 8-25 parts of ammonium dihydrogen phosphate (CAS: 7722-76-1), and 8-25 parts of potassium sulfate (CAS: 7778-80-5) to mother liquor A, so that the mass ratio of nitrogen: phosphorus pentoxide: potassium oxide is (2-4): (1-2): (1-2). After complete dissolution, add urea-formaldehyde slow-release prepolymer (molar ratio of urea to 37% formaldehyde solution 1.2:1, pH 8.0-8).5. Prepolymerize at 60℃ for 3 hours to obtain 5-15 parts, then slowly add 15-40 parts of dispersion C, stir at 45-55℃ for 40-80 minutes to obtain fertilizer nano-functional precursor liquid D; (5) In-situ crosslinking polymerization: Under nitrogen protection, mix solution B and precursor liquid D at a mass ratio of (1.2-2.5): (4-7), heat to 55-65℃, add 0.2-0.7 parts of ammonium persulfate (CAS: 7727-54-0) and N,N,N',N'-tetramethyl 0.1-0.3 parts of ethylenediamine (CAS: 110-18-9) were reacted at a constant temperature for 2-4 hours to form a covalent-ionic composite cross-linked three-dimensional network hydrogel E; (6) Molding and drying: the hydrogel E was extruded or dripped into particles with a particle size of 1.0-4.0 mm, pre-frozen at -45--25℃ for 4-8 hours, freeze-dried under vacuum of ≤30 Pa for 16-28 hours, and then dried with hot air at 40-50℃ for 6-14 hours until the moisture content was ≤12%, to obtain antibacterial hydrogel particles for controlled-release fertilizer.

[0010] The preparation details of the urea-formaldehyde slow-release prepolymer are as follows: Using urea and 37% formaldehyde solution as raw materials, calculate the required amounts based on a urea to formaldehyde molar ratio of 1.2:1. For example, based on 1 mol of formaldehyde, approximately 72.07 g of urea and 81.16 g of 37% formaldehyde solution are needed. First, add urea to a three-necked flask, then slowly add the formaldehyde solution while stirring at 200–300 rpm to dissolve. If the solution is too thick, add a small amount of deionized water to dilute it. Adjust the pH to 8.0–8.5 using a 5%–10% sodium hydroxide solution to promote the alkali-catalyzed addition reaction. Then, place the mixture in a constant temperature and humidity environment. In a warm water bath, the temperature is slowly raised to 60°C (the heating rate should not exceed 5°C / min), and the mixture is stirred at this temperature for 3 hours (stirring speed 300-400 rpm). During this period, the solution gradually thickens and turns light yellow, indicating that urea and formaldehyde have undergone hydroxymethylation to form a low-polymerization degree hydroxymethylurea prepolymer. After the reaction is complete, the mixture is allowed to cool naturally to room temperature and stirred for another 10-15 minutes to homogenize it, resulting in a viscous and transparent prepolymer solution (solid content of about 60%-70%), which can be used directly in subsequent fertilizer formulations or vacuum dried into a dry basis form. The entire process must be carried out in a fume hood.

[0011] Preferably, in step (1), 4 to 15 parts of carboxymethyl chitosan (degree of substitution 0.8 to 1.2, CAS: 83512-85-0) are further added at a mass ratio of (3 to 6): 10 with corn starch to enhance the swelling response and structural integrity of the hydrogel in acidic soil.

[0012] Preferably, after the core-shell structure is formed in step (3) (a), 0.3 to 1.5 parts of titanium dioxide nanoparticles (anatase type, average particle size 10 to 25 nm, CAS: 13463-67-7) are added to make the mass ratio of silver-zinc oxide to titanium dioxide (2 to 5): 1, so as to form a ternary core-shell synergistic photocatalytic antibacterial system.

[0013] Preferably, in step (3) (b), the graphene oxide is pre-modified with KH-550 silane coupling agent (CAS: 919-30-2) (mixed and stirred at a constant temperature for 8 hours). The amount of the modifier is 3 to 8% of the mass of the graphene oxide, so as to improve its dispersibility in acrylamide monomers and covalent bonding with polymer networks.

[0014] Preferably, in step (4), 3 to 12 parts of potassium humate (humic acid content ≥60%, CAS: 68514-28-3) are further added, with a mass ratio of 1: (5 to 10) to urea, so as to form an organic-inorganic composite slow-release layer and improve the nitrogen fixation efficiency in alkaline soil.

[0015] Preferably, in step (5), 2 to 10 parts of N-isopropylacrylamide (N-isopropylacrylamide, CAS: 2210-25-5) are added simultaneously with the crosslinking polymerization, with a mass ratio of (1 to 4): 10 with acrylamide, to form a temperature-responsive shrinkage-expansion network, thereby achieving intelligent controlled release of fertilizer at temperatures above 30 to 35°C.

[0016] Preferably, in step (6), the molding process uses a dual-fluid nozzle spray granulation method. The mixed precursor liquid is sprayed through a 0.5-1.0 mm nozzle at a flow rate of 5-15 mL / min into a coagulation bath containing 2-6% calcium chloride (CAS: 10043-52-4) and 0.5-2% sodium pimecrolate (CAS: 27825-99-6) by a drop distance of 15-35 cm, forming core-shell structured particles with a dense surface and porous interior.

[0017] Preferably, in step (6), after freeze-drying, the surface is further sprayed with an ethanol solution (mass concentration 3-8%) of polylactic acid-polyethylene glycol copolymer (molecular weight 20000-40000, CAS: 34346-01-5) to form a semi-permeable outer membrane, thereby further controlling the moisture-triggered release rate.

[0018] An antibacterial hydrogel for controlled-release fertilizer, wherein the controlled-release antibacterial hydrogel is a dry granular material obtained by the preparation method described above.

[0019] Compared to existing technologies, this invention has the following significant advantages and innovations: Mechanism innovation for significantly improved mechanical properties: This invention achieves multi-scale synergistic toughening by constructing a dual interpenetrating nanoreinforcement network of "starch-cellulose nanocrystals" and "chitosan-chitin nanofibers." Cellulose nanocrystals (5-20 nm in diameter) are uniformly dispersed as rigid fillers in the starch gelatinized matrix, forming a physically entangled network through hydrogen bonds and van der Waals forces, thus improving the matrix modulus. Chitin nanofibers (10-50 nm in diameter) form nanofiber bridges with chitosan molecular chains, enhancing the ionic crosslinking density. Simultaneously, silane-modified graphene oxide (GO) introduces aminopropyl groups through KH-550, copolymerizing with acrylamide monomers to form covalent bonds, further chemically anchoring the dual network and forming an "organic-inorganic hybrid" three-dimensional structure. This multi-level reinforcement mechanism enables the hydrogel to achieve a compressive strength of 240–260 kPa, far exceeding the brittle structure of traditional single polysaccharide hydrogels (typically <150 kPa). It maintains long-term integrity under soil mechanical shear and microbial degradation conditions, preventing sudden nutrient release. The innovative mechanism for superior controlled-release performance: This invention designs a multi-level, multi-mechanism synergistic barrier to achieve "linear + intelligent" nutrient release. First, chitosan cations form ionic bonds with phosphate / humate ions, and urea-formaldehyde prepolymer encapsulates urea to form an organic slow-release layer, physically blocking initial diffusion. Second, the porous structure of the cross-linked polyacrylamide network controls water penetration and nutrient diffusion pathways. Third, optional N-isopropylacrylamide (PNIPAM) copolymerization introduces a temperature-sensitive phase transition (LCST approximately 32°C), causing the network to shrink and expel nutrients at high temperatures, achieving synchronized accelerated release with the crop's vigorous growth period (30–35°C). Finally, optional polylactic acid-polyethylene glycol (PLA-PEG) semi-permeable outer membrane and a core-shell particle structure formed by two-fluid spraying further create a water-triggered valve. This synergistic mechanism results in a nitrogen cumulative release rate of <45% on day 30 and <65% on day 60, with a release curve approaching an S-shape or linear pattern. Fertilizer utilization is improved by more than 30% compared to traditional fertilizers, significantly outperforming the passive diffusion mechanism of single-coated or matrix-based products. The innovative mechanism of long-lasting broad-spectrum antibacterial action involves in-situ construction of core-shell silver-zinc oxide (Ag@ZnO) nanoparticles (particle size 15–40 nm). The silver core provides strong bactericidal activity, while the ZnO shell slowly releases Ag through a heterojunction interface. + And prevents oxidative deactivation, while Zn 2+This invention independently releases and interferes with bacterial enzyme systems, achieving a synergistic dual-mode approach of contact sterilization and ion release. An optional ternary Ag@ZnO / TiO2 structure introduces anatase TiO2, generating hydroxyl radicals and superoxide anions under light, endowing it with photocatalytic antibacterial capabilities, effective in both shallow (with light) and deep (without light) soil layers. This core-shell design avoids the rapid release and toxicity accumulation of pure nano-silver, extending antibacterial durability by 2-3 times. The antibacterial rate against Escherichia coli and Staphylococcus aureus is >99%, effectively inhibiting the reproduction of rhizosphere pathogens and reducing pesticide use by more than 20%. The innovative mechanism of intelligent environmental response: This invention precisely regulates pH / temperature dual responsiveness through functional components. Carboxymethyl chitosan (degree of substitution 0.8-1.2) introduces carboxyl groups, which, under acidic soil (pH < 6), protonate to weaken electrostatic repulsion, promoting network swelling and accelerating nutrient release, adapting to acidic red soils in the south. Potassium humate provides polyphenol / carboxyl sites, reacting with NH4+ in alkaline soil (pH > 8). 4+ The process involves complexation and fixation to reduce nitrogen volatilization; the thermosensitive segments of PNIPAM shrink and squeeze out pore water and nutrients at high temperatures, achieving accelerated release during hot summer months. This multi-response mechanism allows the hydrogel to adapt to diverse soil and climate conditions in my country, with release behavior dynamically matching the environment, overcoming the shortcomings of existing technologies in terms of poor regional adaptability. The innovative mechanism for green environmental protection and economy: Over 90% of the components of this invention are natural renewable polysaccharides (starch, chitosan, cellulose / chitin nanocrystals) and bio-based additives (potassium humate, urea-formaldehyde prepolymer). The final product degrades into CO2, water, and humus under the action of soil microorganisms, with no petroleum-based coating residue accumulation; the core-shell antibacterial agent dosage is low (<6 parts), avoiding the ecological risks of nanomaterials; the process adopts a "one-pot" in-situ composite crosslinking + combined drying (freeze-drying to preserve pores + low-cost hot air), eliminating the need for organic solvents and high-pressure equipment, and facilitating large-scale production. This mechanism ensures environmental protection throughout the product's entire life cycle, and the production cost is 30%–50% lower than that of polymer-coated fertilizers.

[0020] In summary, this invention, through the deep integration of mechanisms such as nanoscale enhancement, multi-network hybridization, core-shell synergistic antibacterial action, multi-mechanism controlled release, and intelligent response, systematically solves the core pain points of existing hydrogel controlled-release fertilizers, such as low strength, single release, weak antibacterial effect, and poor adaptability, achieving a qualitative leap in performance and demonstrating significant originality and industrialization prospects. Attached Figure Description

[0021] Figure 1 This is the infrared spectrum of starch-nanocrystalline mother liquor A prepared in Example 1.

[0022] Figure 2 This is the infrared spectrum of chitosan-nanofibers solution B prepared in Example 1.

[0023] Figure 3 This is a scanning electron microscope image of the covalent-ionic composite crosslinked three-dimensional network hydrogel E prepared in Example 1. Detailed Implementation

[0024] The present invention will be described in detail below through specific embodiments. However, the uses and purposes of these illustrative embodiments are only for illustrating the invention and do not constitute any limitation on the actual scope of protection of the invention, nor are they intended to limit the scope of protection of the invention to these embodiments. The selection of embodiments is based on the coverage of the patent claims and aims to demonstrate the typical implementation, technical effects and repeatability of the invention. For those skilled in the art, reasonable adjustments, optimizations or equivalent substitutions can be made according to actual needs without departing from the concept of the invention. All such modifications should be considered to fall within the scope of protection defined by the claims of the present invention. For parameter ranges not explicitly mentioned or specifically listed in the embodiments, their intermediate values ​​are selected as representative operating conditions. At the same time, for mass ratios not explicitly stated or mentioned, they generally refer to the mass ratio of each component after addition. In addition, in the present invention, the unit of mass is uniformly grams (g), unless otherwise specified. Common or well-known steps (such as basic stirring, heating, pH adjustment, centrifugation, vacuum drying, etc.) are not mentioned in detail. They can generally be implemented by referring to common knowledge and standard methods in the art to ensure the operability and repeatability of the experiment.

[0025] Test methods and standards: In the following examples and comparative examples, the properties of the prepared materials were characterized using the following test methods: Swelling Ratio (SR): A certain mass (W) of water was weighed... d The dried gel particles were soaked in deionized water at pH 7.0 and swollen at room temperature for 48 hours to reach equilibrium. After filtering off the surface water, they were weighed (W). s Calculation formula: SR = (W) s -W d ) / W dCompressive Strength: Fully swollen hydrogel samples were cut into cylinders (10 mm in diameter and 10 mm in height) and compressed using a universal testing machine at a compression rate of 5 mm / min. The compressive stress at 60% strain was recorded. Cumulative Release: The static water immersion method was used. 1.0 g of dried fertilizer granules was placed in 200 mL of deionized water and placed in a 25°C constant-temperature shaker. Samples were taken on days 1, 3, 5, 7, 10, 15, 20, 30, 45, and 60. The nitrogen content in the water was determined using the Kjeldahl method, and the cumulative release rate was calculated. Antibacterial Activity: The inhibition zone method and the shaking flask method were used. *Escherichia coli* (ATCC25922) was used as the test species. The inhibition rate was calculated as: Rate (%) = (AB) / A, where A is the colony count in the control group and B is the colony count in the experimental group. Encapsulation integrity and morphology: The surface and internal pore structure were observed using scanning electron microscopy (SEM).

[0026] Example 1

[0027] This embodiment provides an antibacterial hydrogel for controlled-release fertilizer that integrates all preferred technical features. The specific preparation steps are as follows: (1) Prepare modified starch-based mother liquor: Weigh 35g of soluble corn starch, 8g of cellulose nanocrystals (average diameter 10nm), and 10g of carboxymethyl chitosan (degree of substitution 1.0), and add 350g of deionized water. Mechanically stir at 90℃ for 55min until completely gelatinized and transparent, and cool to 50℃ to obtain starch-nanocrystal mother liquor A, whose infrared spectrum is shown below. Figure 1 As shown. (2) Preparation of functionalized chitosan solution: Weigh 12g of chitosan (95% degree of deacetylation) and 3g of chitin nanofibers (average diameter 30nm), add 300g of 2.0% glacial acetic acid aqueous solution, and stir magnetically at room temperature for 8h until completely dissolved and transparent to obtain chitosan-nanofiber solution B, whose infrared spectrum is shown. Figure 2As shown. (3) Preparation of modified core-shell structured nano-antibacterial agent and dispersion: (a) Preparation of ternary core-shell particles: Dissolve 0.5g of silver nitrate and 0.5g of polyvinylpyrrolidone in 100g of deionized water, add ascorbic acid solution (silver nitrate:ascorbic acid mass ratio 1:1) dropwise under ice bath, and stir for 55min to form silver core. Add 2.5g of zinc nitrate hexahydrate and 2.5g of hexamethyltetramine, and react at 85℃ for 3h. Then add 0.8g of titanium dioxide nanoparticles and continue the reaction to form a silver-zinc oxide / titanium dioxide ternary structure. (b) Preparation of functionalized dispersion: Take the above core-shell nanoparticles and 0.6g of graphene oxide modified with KH-550 silane coupling agent (the amount of modifier is 5% of the mass of GO), and ultrasonically disperse them for 30min in 200g of deionized water containing 15g of acrylamide and 0.5g of N,N-methylenebisacrylamide (MBA) to obtain functionalized nano antibacterial monomer dispersion C. (4) Preparation of fertilizer-containing organic-inorganic composite precursor solution: Add 50g of urea, 15g of ammonium dihydrogen phosphate and 15g of potassium sulfate (N:P:K ratio meets the requirements) to mother liquor A, and after complete dissolution, add 10g of urea-formaldehyde slow-release prepolymer and then add 8g of potassium humate (mass ratio of 1:6.25 with urea). Slowly add 28g of dispersion C and stir at 50℃ for 60min to obtain fertilizer-containing nano functional precursor solution D. (5) Intelligent response in-situ crosslinking polymerization: Under nitrogen protection, solution B and precursor solution D were mixed at a mass ratio of 1.8:5.5. The temperature was raised to 60℃, and 5g of N-isopropylacrylamide (mass ratio of N-isopropylacrylamide to acrylamide was approximately 3.3:10) was added, along with 0.5g of ammonium persulfate and 0.2g of N,N,N',N'-tetramethylethylenediamine. The mixture was reacted at a constant temperature for 3h to form a covalent-ionic composite crosslinked three-dimensional network hydrogel E. Its scanning electron microscope image is shown below. Figure 3 As shown. (6) Two-fluid molding and combined drying: Hydrogel E was sprayed through a two-fluid nozzle (nozzle diameter 0.8 mm, flow rate 10 mL / min) into a coagulation bath containing 4% calcium chloride and 1% sodium pimecrolate (drop distance 25 cm) to form particles. The particles were pre-frozen at -35℃ for 6 h, freeze-dried under vacuum of 20 Pa for 24 h, and then dried with hot air at 45℃ for 10 h. Finally, a polylactic acid-polyethylene glycol copolymer ethanol solution (concentration 5%, spray amount 4% of the dry particle mass) was sprayed to obtain the finished product.

[0028] To fully verify the scope of protection of the claims, embodiments 1-12 were designed. Embodiment 1 is the preferred example of the above-described full-component combination.

[0029] Table 1: Parameter table for starch-based mother liquor A in step (1) and chitosan solution B in step (2) (unit: g)

[0030]

[0031] Table 2: C-parameter table of nano-antibacterial dispersion in step (3) (unit: g)

[0032]

[0033] Table 3: Precursor solution D in step (4) and post-treatment parameters in steps (5)-(6) (unit: g)

[0034]

[0035] The comparative examples are based on the basic formulation of Example 1, but with key components omitted or parameters adjusted to outside the range to demonstrate the technical advantages of the present invention.

[0036] Table 4: Comparative Scale Design Description

[0037]

[0038] Table 5: Performance test results of the examples and comparative examples (1) (Note: All data are the average of three parallel measurements)

[0039]

[0040] Table 6: Performance test results and antibacterial properties of the examples and comparative examples (2)

[0041]

[0042] Results Analysis: Mechanical Properties and Structural Stability: Comparison of Example 1 with Comparative Examples 1, 2, 4, and 7 shows that the dual network of "starch / bacterial cellulose" and "chitosan / chitin nanofibers" constructed in this invention, combined with the cross-linking effect of graphene oxide, significantly improves the compressive strength of the hydrogel (from ~110 kPa in the comparative examples to 245.6 kPa). The absence of any nano-reinforcing component (Comparative Examples 1 and 2) leads to a significant decrease in gel strength, making it impossible to maintain long-term structural integrity in the soil, resulting in uncontrolled nutrient release in the later stages. Slow-Release Performance: Example 1 showed a cumulative release rate of 45.3% on day 30 and approximately 65% ​​(estimated) on day 60, demonstrating excellent linear release characteristics. Comparative Example 5 (without prepolymer) exhibited a significant initial burst release. Comparative Example 6 (without cross-linking agent) rapidly disintegrated in water, completely losing its controlled-release ability. Example 11 (containing NIPAM and coating) showed the best slow-release effect, indicating that the thermal switch and semi-permeable membrane acted as a synergistic barrier. Antibacterial performance: Examples 1-12 all exhibited excellent antibacterial rates >99%. Comparative Example 3 (pure Ag, without ZnO shell) showed an antibacterial rate decreasing to ~85%, and rapid silver oxidation and discoloration were observed, indicating that the core-shell structure effectively protected the silver core and enhanced durable antibacterial properties through Zn / Ag synergy. A comparison between Examples 1 and 8 showed that the addition of TiO2 (Example 1) further maintained high antibacterial activity under light conditions (simulating shallow soil). Component synergistic effect: Comparative Example 8 showed that the unmodified GO had poor dispersibility, resulting in lower strength than Example 1, indicating that the silane coupling agent achieved chemical bonding between the inorganic filler and the organic network. Although Example 10 (with added potassium humate) had little effect on strength, its soil improvement effect was superior to other groups according to agronomic tests (data not listed). In summary, the controlled-release antibacterial hydrogel for fertilizers prepared by this invention achieves high strength, intelligent controlled release, and long-lasting antibacterial effect through the synergistic effect of polysaccharide matrix, nanocrystal reinforcement, inorganic-organic hybrid crosslinking, and core-shell antibacterial agent, and has significant application value.

[0043] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered to fall within the scope of protection defined by the claims submitted herein.

Claims

1. A method for preparing an antibacterial hydrogel for controlled-release fertilizers, characterized in that: The steps include the following steps by weight: (1) Weigh 25-45 parts of soluble corn starch and 3-12 parts of cellulose nanocrystals, add 250-450 parts of deionized water, stir mechanically, and cool to obtain starch-nanocrystal mother liquor A; (2) Weigh 8-18 parts of chitosan and 1-5 parts of chitin nanofibers, add 250-350 parts of 1.5-2.5% glacial acetic acid aqueous solution, stir magnetically at room temperature for 6-10 hours until completely dissolved and transparent to obtain chitosan-nanofiber solution B; (3) Prepare core-shell structured nano-antibacterial agent: (a) Prepare core-shell structured nano-antibacterial agent by in-situ reduction method. Silver-zinc oxide nanoparticles: 0.2-0.8 parts silver nitrate and 0.2-0.8 parts polyvinylpyrrolidone were dissolved in 80-150 parts deionized water. Ascorbic acid solution was added dropwise under ice bath conditions, and the mixture was stirred for 40-70 min to form a silver core. Then, 1-4 parts zinc nitrate hexahydrate, a zinc oxide precursor, and hexamethyltetramine were added at a mass ratio of 1:

1. The mixture was reacted at 80-90℃ for 2-4 h to form a core-shell structure. (b) Core-shell nanoparticles and 0.2-1.0 parts graphene oxide were reacted in 15 parts deionized water containing 8-20 parts acrylamide and 0.2-0.8 parts N,N-methylenebisacrylamide. (3) Disperse 0-250 parts of the solution in an ultrasonic bath for 20-40 min to obtain functionalized nano-antibacterial monomer dispersion C; (4) Add 30-70 parts of urea, 8-25 parts of ammonium dihydrogen phosphate, and 8-25 parts of potassium sulfate to starch-nanocrystalline mother liquor A, dissolve them, add 5-15 parts of urea-formaldehyde slow-release prepolymer, and then add 15-40 parts of functionalized nano-antibacterial monomer dispersion C, stir to obtain fertilizer-containing nano-functional precursor liquid D; (5) Under nitrogen protection, mix chitosan-nanofibers solution B and fertilizer-containing nano-functional precursor liquid D at a mass ratio of (1.2-2.5):(4-7), and heat. Temperature the temperature to 55-65℃, add 0.2-0.7 parts of ammonium persulfate and 0.1-0.3 parts of N,N,N',N'-tetramethylethylenediamine, and crosslink at a constant temperature for 2-4 hours to form a covalent-ionic composite crosslinked three-dimensional network hydrogel E; (6) Extrude or drop the covalent-ionic composite crosslinked three-dimensional network hydrogel E into particles with a particle size of 1.0-4.0 mm, pre-freeze at -45--25℃ for 4-8 hours, freeze-dry at a vacuum degree ≤30 Pa for 16-28 hours, and then dry with hot air at 40-50℃ for 6-14 hours until the moisture content is ≤12% to obtain antibacterial hydrogel particles for controlled-release fertilizer.

2. The method for preparing the controlled-release fertilizer antibacterial hydrogel according to claim 1, characterized in that: In step (1), 4 to 15 parts of carboxymethyl chitosan are added, with a mass ratio of (3 to 6): 10 with corn starch.

3. The method for preparing the controlled-release fertilizer antibacterial hydrogel according to claim 1, characterized in that: The parameters for mechanical stirring in step (1) are as follows: mechanical stirring at 85-95℃ for 40-70 minutes until completely gelatinized and transparent; the temperature after cooling in step (1) is 45-55℃.

4. The method for preparing the controlled-release fertilizer antibacterial hydrogel according to claim 1, characterized in that: After the core-shell structure is formed in step (3) (a), 0.3 to 1.5 parts of titanium dioxide nanoparticles are added; the mass ratio between silver nitrate and ascorbic acid solution in step (3) (a) is 1: (0.8 to 1.2).

5. The method for preparing the controlled-release fertilizer antibacterial hydrogel according to claim 1, characterized in that: In step (3) (b), the graphene oxide is pre-modified with KH-550 silane coupling agent, and the amount of the modifier is 3 to 8% of the mass of the graphene oxide.

6. The method for preparing the controlled-release fertilizer antibacterial hydrogel according to claim 1, characterized in that: In step (4), 3-12 parts of potassium humate are added, with a mass ratio of 1:(5-10) to urea. The preparation of the urea-formaldehyde slow-release prepolymer in step (4) is as follows: the molar ratio of urea to 37% formaldehyde solution is 1.2:1, pH 8.0-8.5, and prepolymerization is carried out at 60℃ for 3 hours. The stirring parameters in step (4) are as follows: stirring at 45-55℃ for 40-80 minutes.

7. The method for preparing the controlled-release fertilizer antibacterial hydrogel according to claim 1, characterized in that: In step (5), 2 to 10 parts of N-isopropylacrylamide are added simultaneously to the isothermal crosslinking reaction, with a mass ratio of (1 to 4):10 with acrylamide, to form a temperature-responsive shrinkage-expansion network.

8. An antibacterial hydrogel for controlled-release fertilizers, characterized in that: The controlled-release fertilizer antibacterial hydrogel is obtained by the preparation method described in any one of claims 1-7.

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