Photothermal sterilization salt stress resistant seed coating agent and preparation method and application thereof

Abstract

The application discloses a photo-thermal sterilization salt stress resistant seed coating agent and a preparation method and application thereof, and relates to the technical field of seed treatment. The application comprises the following steps: S1, hematoxylin is added into deionized water and ultrasonically dissolved, ferrous sulfate deionized water solution and PBS buffer solution are sequentially added, and ultrasonic reaction is continuously carried out; after the reaction is completed, centrifugation and concentration are carried out, and hematoxylin-ferrous ion complex nanoparticle dispersion liquid is prepared; S2, xanthan gum and polyvinyl alcohol are respectively dissolved in deionized water, the two solutions are mixed, glycerol is added and uniformly stirred, and a coating base solution is obtained; and S3, the hematoxylin-ferrous ion complex nanoparticle dispersion liquid is added into the coating base solution, and is fully stirred until uniformly dispersed, so that the seed coating agent is prepared. The application has the dual effects of photo-thermal sterilization and salt resistance, is green, environment-friendly, has no drug resistance, has good compatibility between the photo-thermal component and the coating, and solves various defects of traditional sterilization and ordinary coating agents.

Description

Technical Field

[0001] This invention relates to the field of seed treatment technology, specifically to a photothermal bactericidal and salt-stress-resistant seed coating agent, its preparation method, and its application. Background Technology

[0002] In agricultural production, pathogenic microorganisms such as bacteria and fungi commonly adhere to the surface of seeds, making them highly susceptible to seed-borne diseases during storage, sowing, and seedling stages. This can lead to seed mold and rot, seedling infection and death, and negatively impact crop emergence rate and yield. Simultaneously, in saline-alkali soils, excessive soluble salt ions disrupt the osmotic pressure balance of seed cells, inducing oxidative damage and enzyme activity disorders, inhibiting seed water absorption, swelling, and normal germination. Continuous salt stress on seedling root growth results in weak seedlings and reduced survival rates. Even for salt-tolerant forage crops like alfalfa, high-salt environments significantly inhibit seed germination, affecting field establishment.

[0003] Currently, the mainstream approach to addressing seed-borne pathogens is the application of chemical fungicides. However, long-term and repeated use of chemical agents can easily lead to pesticide residues, damage the soil microbial community, and promote drug resistance in pathogens, resulting in reduced fungicidal and disease-preventing effects. While novel physical sterilization technologies such as plasma and ultraviolet irradiation are pollution-free, they require high equipment investment and have complex operating procedures, making large-scale promotion difficult.

[0004] Existing seed coating products on the market have relatively limited functions, mostly offering only insect repellency, moisture retention, or basic bactericidal effects. Traditional coatings have limited ability to block salt ions, making it difficult to effectively alleviate salt stress damage; furthermore, existing coating systems rarely incorporate components with both bactericidal and stress-resistance effects. Photothermal sterilization technology utilizes photothermal materials to convert light energy into heat energy, using localized high temperatures to inactivate pathogenic microorganisms, offering advantages such as no chemical residues and high sterilization efficiency. However, the technology for applying photothermal functional materials to seed coating is still immature. Existing formulations mostly focus on single bactericidal functions, failing to incorporate salt stress resistance requirements, and suffer from problems such as poor compatibility between photothermal active components and film-forming substrates, easy aggregation and loss, and insufficient system stability.

[0005] In summary, given the high incidence of seed-borne diseases, the difficulty in crop emergence in saline-alkali and saline-alkali arable land, and the significant shortcomings of existing seed treatment technologies and coating products, developing a photothermal bactericidal and salt-stress-resistant seed coating agent is of great significance for revitalizing saline-alkali land resources and promoting the high-quality development of green agriculture and the forage industry. Summary of the Invention

[0006] In view of the problems of existing technologies, such as environmental pollution and easy development of drug resistance by traditional seed sterilization methods, poor salt resistance and single function of conventional coating agents, and poor compatibility between photothermal functional components and coatings, the purpose of this invention is to provide a photothermal sterilization and salt stress resistant seed coating agent, its preparation method and application, so as to achieve dual protection of seeds by photothermal sterilization and salt stress resistance.

[0007] To achieve the above objectives, the present invention provides a method for preparing a photothermal bactericidal and salt-stress-resistant seed coating agent, comprising the following steps: S1: Hematoxylin was dissolved in deionized water by sonication, followed by the addition of ferrous sulfate deionized water and PBS buffer, and the reaction was continued by sonication. After the reaction was completed, the mixture was centrifuged and concentrated to obtain a dispersion of hematoxylin-ferrous ion complex nanoparticles. S2: Dissolve xanthan gum and polyvinyl alcohol separately in deionized water, then mix the two solutions, add glycerin and stir evenly to obtain the coating base liquid; S3: Add the hematoxylin-ferrous ion complex nanoparticle dispersion to the coating base liquid and stir thoroughly until the dispersion is uniform to obtain the seed coating agent.

[0008] Preferably, in step S1, the concentration of the hematoxylin deionized aqueous solution is 10-30 mg / mL; the concentration of the ferrous sulfate deionized aqueous solution is 10-25 mg / mL; the volume ratio of hematoxylin, ferrous sulfate and PBS buffer is (1-3):(0.5-2):(5-15); the pH of the PBS buffer is 7.4; and the ferrous sulfate is FeSO4·7H2O.

[0009] Preferably, in step S1, the reaction temperature is 25°C and the reaction time is 10 h.

[0010] Preferably, in step S2, the concentration of xanthan gum deionized water solution is 5-10 g / L, and the concentration of polyvinyl alcohol deionized water solution is 20-40 g / L; the volume ratio of xanthan gum, polyvinyl alcohol and glycerol is (5-15):(15-35):(1-4).

[0011] Preferably, in step S2, the dissolution temperature of xanthan gum and polyvinyl alcohol is 85-100 ℃, the stirring rate is 400-600 rpm, and the dissolution time is 1.5-2.5 h.

[0012] Preferably, in step S3, the volume ratio of the hematoxylin-ferrous ion complex nanoparticle dispersion to the coating base liquid is (0.5-2):(50-200).

[0013] The present invention also provides a photothermal bactericidal and salt-stress-resistant seed coating agent, which is prepared by the method described above.

[0014] The present invention also provides an application of a photothermal bactericidal and salt-stress-resistant seed coating agent, which is used for pre-sowing seed treatment of crops under saline-alkali soil conditions to resist pathogen infection and salt stress damage.

[0015] The present invention has the following beneficial effects: 1. This invention uses hematoxylin-ferrous ion complexed nanoparticles as the photothermal functional component. It relies on photothermal conversion to generate localized high temperatures that kill pathogenic microorganisms such as Escherichia coli and Staphylococcus aureus on the seed surface, achieving a sterilization rate of over 98%. No chemical fungicides are added throughout the process, resulting in no pesticide residues and preventing the induction of drug resistance in pathogens. It is environmentally friendly and aligns with the requirements of green agriculture and pesticide reduction development.

[0016] 2. The polymer coating formed by the compounding of xanthan gum and polyvinyl alcohol has good hydrophilicity and air permeability, and can form a dense protective barrier on the seed surface to block the invasion of salt ions. It can significantly improve the germination potential and germination rate of seeds under salt stress, increase the fresh weight, dry weight and plant height of seedlings, increase the content of soluble protein and chlorophyll, reduce the content of malondialdehyde, reduce the physiological damage of salt ions to seeds and seedlings, and greatly improve the crop establishment effect in saline-alkali land.

[0017] 3. Photothermal nanoparticles can be uniformly dispersed in the coating substrate, and are not easy to agglomerate, fall off or be lost. The coating film is uniform and stable, and can play a long-term bactericidal and salt-resistant role, extend the protection period, and ensure the protection effect throughout the entire process of seed storage and field growth.

[0018] 4. The raw materials are readily available, the preparation conditions are mild, the operation process is simple, and the various process parameters are easy to control; the resulting composite coating agent has good flowability and film-forming properties, and can be used for small-batch preparation in the laboratory, as well as for manual coating and mechanized operations in the field. It has a wide range of applications and low promotion costs.

[0019] 5. The seed coating agent prepared by this invention has high biosafety. It has been verified by acute toxicity and teratogenicity tests in zebrafish that the concentration used in the field is at a low toxicity level and there is no obvious risk of teratogenicity to aquatic organisms. The raw materials, hematoxylin, ferrous sulfate, xanthan gum and polyvinyl alcohol, are all biodegradable and will not cause pesticide residues in soil and water. It takes into account both bactericidal and salt-resistant functions and the protection of the farmland ecological environment, which meets the needs of green agriculture to reduce pesticides and eliminate residues. Attached Figure Description

[0020] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments; Figure 1 The image shows scanning electron microscope (SEM) images of Escherichia coli a and Staphylococcus aureus b in the light-free control group (nanoparticles) of this invention.

[0021] Figure 2The image shows scanning electron microscope (SEM) images of Escherichia coli a and Staphylococcus aureus b in the light-free control group (coated membrane) of this invention. Figure 3 The image shows a scanning electron microscope image of Escherichia coli a and Staphylococcus aureus b in Example 1 (under light) of the present invention. Figure 4 The images show scanning electron microscope (SEM) images of Escherichia coli a and Staphylococcus aureus b in Comparative Example 1 (under light) of the present invention. Figure 5 The images show scanning electron microscope (SEM) images of Escherichia coli a and Staphylococcus aureus b in Comparative Example 2 (under light) of the present invention. Figure 6 The images show scanning electron microscope (SEM) images of Escherichia coli a and Staphylococcus aureus b in Comparative Example 3 (under light) of the present invention. Figure 7 This is a flowchart illustrating the overall principle of seed coating agent preparation, photothermal sterilization, and salt-resistant growth promotion in this invention. Figure 8 The UV absorption spectra (a), TEM morphology (b), Zeta potential (c), particle size distribution (d), and XPS elemental energy dispersive spectroscopy (e) of HT-Fe²⁺ nanoparticles (HT is an abbreviation for hematoxylin) with different hematoxylin / ferrous ratios are shown. Figure 9 Rheological curves of coating solutions with different xanthan gum additions, SEM images of coating films, and elemental mapping distribution of Fe / C / O. Figure 10 Comparison of mechanical properties of membranes with different xanthan gum additions (HT-Fe²⁺@PX, abbreviation for xanthan gum and polyvinyl alcohol mixture), including stress-strain (a), tensile strength (b), and elongation at break (c). Figure 11 Bar charts showing water vapor permeation rate (a) and water permeability coefficient (b) for different xanthan gum coating films; Figure 12 Bar chart showing the water contact angle test results for different xanthan gum coating films; Figure 13 Near-infrared photothermal heating infrared thermal imaging and temperature curves of composite membranes with different ratios of HT-Fe²⁺ nanoparticles, different concentration suspensions, and different ratios. Figure 14 Bar charts showing the germination potential (a) and germination rate (b) of alfalfa seeds in different groups under different salt concentration stress. Figure 15 A comparison chart of fresh weight (a), dry weight (b), and seedling length growth index (c) of alfalfa seedlings in different groups under salt stress; Figure 16Bar chart showing the physiological indicators of alfalfa seedlings under salt stress, including root length, plant length, soluble protein, MDA chlorophyll, and SOD enzyme. Figure 17 These are real-life photos showing the growth sequence of potted alfalfa coated with HT-Fe²⁺@PX20 under mild, moderate, and severe salt stress, compared to the control group. Figure 18 A comparison of antioxidant indices of alfalfa chlorophyll a / b, total chlorophyll, POD, and SOD under salt stress; Figure 19 The coating agent of this invention is compared with the commercially available pesticide zebrafish 96h-LC. 50 Toxicity comparison scatter plot; Figure 20 Bar chart showing the 96-hour mortality rate of zebrafish with different concentrations of coating solution; Figure 21 Scatter plot of the number of brain (a), heart (b), intestine (c), and liver (d) malformations in zebrafish treated with different concentrations of coating solution at 120 hpf; Figure 22 Scatter plot showing the incidence of deformities in the lower jaw (a), yolk sac (b), and eye (c) of zebrafish treated with different concentrations of coating solution. Detailed Implementation

[0022] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0023] Reference Figure 7 This specific embodiment provides a method for preparing a photothermal bactericidal and salt-stress-resistant seed coating agent, comprising the following steps: S1: Hematoxylin was dissolved in deionized water by sonication, followed by the addition of ferrous sulfate deionized water and PBS buffer, and the reaction was continued by sonication. After the reaction was completed, the mixture was centrifuged and concentrated to obtain a dispersion of hematoxylin-ferrous ion complex nanoparticles. S2: Dissolve xanthan gum and polyvinyl alcohol separately in deionized water, then mix the two solutions, add glycerin and stir evenly to obtain the coating base liquid; S3: Add the hematoxylin-ferrous ion complex nanoparticle dispersion to the coating base liquid and stir thoroughly until the dispersion is uniform to obtain the seed coating agent.

[0024] In step S1, the concentration of the hematoxylin deionized aqueous solution is 20 mg / mL; the concentration of the ferrous sulfate deionized aqueous solution is 18 mg / mL; the volume ratio of hematoxylin, ferrous sulfate and PBS buffer is 2:1:10; the pH of the PBS buffer is 7.4; and the ferrous sulfate is FeSO4·7H2O.

[0025] In step S1, the reaction temperature is 25°C and the reaction time is 10 h.

[0026] In step S2, the concentration of xanthan gum deionized water solution is 7.5 g / L, and the concentration of polyvinyl alcohol deionized water solution is 30 g / L; the volume ratio of xanthan gum, polyvinyl alcohol and glycerol is 10:25:2.

[0027] In step S2, the dissolution temperature of xanthan gum and polyvinyl alcohol is 85-100 ℃, the stirring rate is 400-600 rpm, and the dissolution time is 1.5-2.5 h.

[0028] In step S3, the volume ratio of the hematoxylin-ferrous ion complex nanoparticle dispersion to the coating base liquid is 1:100.

[0029] The seed coating agent prepared by the above method can be used for pre-sowing seed treatment of crops under saline-alkali soil conditions to resist pathogen infection and salt stress damage. Example 1: A method for preparing a photothermal bactericidal and salt-stress-resistant seed coating agent, comprising the following steps: S1: Dissolve hematoxylin in deionized water to prepare a 20 mg / mL solution, and dissolve FeSO4·7H2O in deionized water to prepare an 18 mg / mL solution. Take 2 mL of the hematoxylin solution, add 1 mL of ferrous sulfate solution and 10 mL of PBS buffer (pH 7.4) sequentially, and sonicate at 25℃ for 10 h. After the reaction, centrifuge (10000 rpm, 10 min) and concentrate to 1 / 5 of the original volume to obtain a hematoxylin-ferrous ion complex nanoparticle dispersion.

[0030] S2: Dissolve xanthan gum in deionized water to prepare a 7.5 g / L solution, and dissolve polyvinyl alcohol in deionized water to prepare a 30 g / L solution. Take 10 mL of xanthan gum solution and 25 mL of polyvinyl alcohol solution, mix them, add 2 mL of glycerol, and stir at 90℃ and 500 rpm for 2 h to obtain the coating base liquid.

[0031] S3: Take 1 mL of the nanoparticle dispersion obtained in step S1 and mix it with 100 mL of the base liquid obtained in step S2. Stir and disperse evenly to obtain the seed coating agent.

[0032] Comparative Example 1 In step S1, the volume ratio of hematoxylin solution, ferrous sulfate solution, and PBS buffer is changed to 4:1:10, and the rest is the same as in Example 1.

[0033] Comparative Example 2 In step S2, the volume ratio of xanthan gum, polyvinyl alcohol, and glycerin is changed to 10:25:8, and the rest is the same as in Example 1.

[0034] Comparative Example 3 In step S3, the volume ratio of the nanoparticle dispersion to the coating base liquid is changed to 1:300, and the rest is the same as in Example 1.

[0035] To verify the performance of the seed coating agent of this invention, photothermal bactericidal performance and salt stress resistance and growth-promoting performance tests were conducted.

[0036] Test Example 1 To verify the effect of coating agents on seed germination under salt stress, alfalfa seeds were selected and divided into five groups.

[0037] CK group: Uncoated, watered with distilled water; NaCl group: Uncoated, irrigated with 70 mmol·L⁻¹ NaCl solution; Example 1 group: Example 1 coating, irrigated with 70 mmol·L⁻¹ NaCl solution; Comparative Example 1: Coated with 70 mmol·L⁻¹ NaCl solution; Comparative Example 2: Comparative Example 2 was coated and irrigated with 70 mmol·L⁻¹ NaCl solution; Comparative Example 3: Comparative Example 3 was coated and irrigated with 70 mmol·L⁻¹ NaCl solution.

[0038] Each group had 3 replicates, with 50 seeds in each replicate. Using the paper bed method, seeds were placed in petri dishes lined with double-layered filter paper, and the appropriate solutions were added. The dishes were then incubated in the dark at 25°C. The number of germinations was recorded daily (germination was defined as the radicle breaking through the seed coat by 2 mm). Germination potential on day 7 and germination rate on day 14 were calculated, and root and stem lengths were measured on day 14.

[0039] The results are shown in Table 1.

[0040] Table 1. Seed germination and seedling growth indicators of each treatment group under salt stress.

[0041] like Figure 14 As shown, under salt stress (NaCl group), the germination potential, germination rate, root length, and stem length of seeds were significantly inhibited, decreasing by 41.6%, 31.1%, 39.5%, and 24.0% respectively compared to the CK group. However, after treatment with the coating agent of Example 1 of this invention, the germination potential, germination rate, root length, and stem length significantly increased by 50.1%, 39.4%, 33.1%, and 25.3% respectively compared to the NaCl group, and all indicators were close to the level of the CK group.

[0042] In contrast, the excessive hematoxylin ratio in Comparative Example 1 led to nanoparticle aggregation, decreased coating density, and damage to the salt resistance barrier, resulting in a germination rate of only 65.7%, significantly lower than that of Example 1. Excessive glycerol in Comparative Example 2 caused the coating to absorb water and swell, making it easier for salt ions to penetrate, resulting in a germination rate of 71.2%, also inferior to Example 1. Due to insufficient nanoparticle addition, Comparative Example 3 had sparse photothermal nanoparticle filling in the coating, decreased film uniformity, and the worst salt resistance, with a germination rate of only 60.5%, which was not significantly different from the uncoated NaCl group. These results indicate that the coating agent at the optimal ratio of this invention can effectively block salt ion invasion, provide a stable microenvironment for seed germination, and significantly alleviate the inhibitory effect of salt stress on seed germination and early growth.

[0043] Test Example 2: Determination of Physiological Indicators of Seedlings under Salt Stress To further elucidate the salt tolerance mechanism of the coating agent, a hydroponic experiment was conducted (70 mmol·L⁻¹ NaCl). The fresh weight of alfalfa seedlings was measured 7 days after sowing, and the dry weight, chlorophyll content, soluble protein content, and MDA content were measured 14 days after sowing. Chlorophyll content, soluble protein content, and MDA content were all determined using a reagent kit. The experimental groups were the same as in Test Example 1.

[0044] The results are shown in Table 2.

[0045] Table 2 Physiological indicators of seedlings in different treatment groups under salt stress

[0046] like Figure 15 Under salt stress (NaCl group), the fresh weight, dry weight, and chlorophyll content of alfalfa seedlings decreased significantly, while the MDA content, reflecting the degree of membrane lipid peroxidation, increased significantly, and the soluble protein content showed a stress-induced increase, indicating that salt stress had caused significant osmotic stress and oxidative damage to the seedlings. However, after treatment with the coating agent of Example 1 of this invention, all the above physiological indicators were significantly improved: compared with the NaCl group, fresh weight and dry weight increased by 25.7% and 59.1%, respectively; chlorophyll content increased by 59.0%, soluble protein content decreased by 48.1%, and MDA content decreased by 51.8%, with all indicators returning to levels not significantly different from the CK group.

[0047] While all three comparative examples improved the aforementioned indicators to some extent, the improvement was less than that of Example 1. In Comparative Example 1, excessive hematoxylin led to insufficient complexation reaction, increased nanoparticle size, and uneven distribution of effective particles in the coating, weakening the physical salt barrier effect. The MDA content was significantly higher than in Example 1. In Comparative Example 2, excessive glycerol caused over-plasticization and a loose structure in the coating, making it easier for salt ions to penetrate. The soluble protein content (16.74 mg / g) was still significantly higher than normal, indicating that the seedlings were still under mild salt stress. In Comparative Example 3, insufficient nanoparticles resulted in inadequate coating density, leading to the worst salt resistance. Both the MDA content and soluble protein content were inferior to Comparative Examples 1 and 2. These results indicate that only under the optimal ratio of this invention can the coating agent effectively protect the seedling photosynthetic system by maintaining cell osmotic balance and reducing oxidative damage, thereby promoting healthy crop growth under salt stress. Figure 16 ... Figure 17 and Figure 18 .

[0048] Test Example 3: Photothermal Sterilization Performance Test To evaluate the photothermal bactericidal effect of the coating agent, the plate count method was used for quantitative assessment, and the microscopic morphology of Escherichia coli was observed using SEM.

[0049] (1) Plate coating counting method Test strain: Escherichia coli ( E. coli ) and Staphylococcus aureus ( S. aureus ).

[0050] Experimental groups: A blank control group (bacterial solution only), a light-free control group (bacterial solution + nanoparticles / coating film, incubated in the dark for 10 min), and a light-exposed experimental group (bacterial solution + nanoparticles / coating film, irradiated with 808 nm laser for 10 min) were set up. Among them, the coating agents obtained in Example 1 and Comparative Examples 1-3 were used for testing in the light-exposed experimental group.

[0051] Take 100 μL of a concentration of 10 7 A centrifuge tube containing CFU / mL bacterial suspension and 100 μL of nanoparticles (or a 6 mm diameter coated membrane) was added, followed by 800 μL of sterile water. The tube was then irradiated with an 808 nm laser for 10 min, diluted to 10³, and 100 μL was transferred to LB solid medium. The medium was spread evenly using a spreader, dried, and then inverted and incubated at 37°C. *E. coli* was cultured for 12 h, and *Staphylococcus aureus* for 24 h. Colony counts were recorded, and the sterilization rate was calculated.

[0052] Sterilization rate (%) = (Number of colonies in control group - Number of colonies in experimental group) / Number of colonies in control group × 100% The results are shown in Table 3.

[0053] Table 3 Comparison of photothermal sterilization rates among treatment groups

[0054] The plate coating count results showed that, under laser irradiation, the coating agent of Example 1 achieved bactericidal rates of 99.2% and 99.5% against Escherichia coli and Staphylococcus aureus, respectively, demonstrating excellent bactericidal effects. In contrast, the bactericidal rate of the control group without laser irradiation was less than 20%, indicating that the bactericidal effect of this invention mainly relies on the photothermal effect, and the bactericidal process is green, controllable, and produces no chemical residues.

[0055] The photothermal sterilization effects of the three comparative examples were all significantly inferior to those of Example 1. In Comparative Example 1, the nanoparticles aggregated, increasing in size and decreasing in specific surface area, leading to a lower photothermal conversion efficiency and a sterilization rate of 78.5% (E. coli) and 75.2% (Staphylococcus aureus). Comparative Example 2 showed a slight decrease in sterilization rate, but the reduction was not significant, indicating that glycerol primarily affects salt tolerance and has relatively little interference with photothermal performance. Comparative Example 3 exhibited the worst photothermal sterilization effect, with a sterilization rate of only about 50%. This was because the number of heat-generating nanoparticles per unit area was insufficient to generate enough localized high temperatures to effectively inactivate pathogens.

[0056] (2) Observation by scanning electron microscopy (SEM) Further microscopic morphology of Escherichia coli was observed using SEM. The experiment included a blank control group and a light-exposed experimental group: 10... 8 The bacterial suspension at CFU / mL was mixed with the coating agent from Example 1 and irradiated under 808 nm infrared light for 10 min; the blank control group used untreated E. coli suspension. After centrifugation, the supernatant was discarded, and 2.5% glutaraldehyde fixative was added. The suspension was incubated at 4°C for 6 h. After fixation, the bacterial suspension was centrifuged, and the supernatant was discarded. Then, E. coli was eluted sequentially with 25%, 50%, 75%, and 100% ethanol solutions. After uniform ultrasonic dispersion, the suspension was used for SEM observation.

[0057] SEM images showed that the *E. coli* cells in the blank control group had smooth surfaces, plump morphologies, clear boundaries, and a typical short rod-like structure with intact cell membranes. However, in the experimental group treated with the coating agent from Example 1 and laser irradiation, the *E. coli* cells showed obvious indentations, wrinkling, and collapse on the surface, with some cells rupturing and leaking their contents. The cell morphology was severely deformed, indicating that the localized high temperature generated by photothermal stimulation irreversibly damaged the bacterial cell membrane structure, leading to leakage of cell contents and bacterial death. This morphological evidence further validates the excellent photothermal bactericidal performance of the coating agent of this invention. Specific scanning electron microscope images are shown below. Figure 1-6 As shown.

[0058] Test Example 4: Complete Physicochemical Characterization of Seed Coating System This test example systematically characterized the HT-Fe²⁺ nanoparticles and HT-Fe²⁺@PX composite coating film prepared in Example 1, verifying that the nanoparticles were successfully complexed, the coating film was uniformly dispersed and had physicochemical properties suitable for seed coating, and simultaneously compared the performance differences of different xanthan gum addition ratios (PX0~PX50).

[0059] 4.1 Characterization of the basic structure of HT-Fe²⁺ nanoparticles Three groups of samples were prepared with hematoxylin and Fe²⁺ volumes of 1:1, 2:1, and 3:1, respectively. Ultraviolet-visible spectroscopy (UV-Vis), transmission electron microscopy (TEM), zeta potential, dynamic light scattering (DLS), and X-ray photoelectron spectroscopy (XPS) were then performed. Figure 8 As shown, 1. UV-Vis: Pure hematoxylin has no obvious near-infrared absorption in the visible light region. After complexing with Fe²⁺, it shows a characteristic absorption peak in the 700–900 nm near-infrared window, proving that a metal-polyphenol complex has been successfully formed. The optimal ratio HT:Fe²⁺=2:1 has the highest absorption intensity and the best photothermal potential. 2. TEM observation: Nanoparticles with a 2:1 ratio are uniformly dispersed without large-area agglomeration; deviations from the ratio (1:1, 3:1) result in particle agglomeration and increased particle size. 3. DLS particle size: The average particle size of the optimal ratio is 371.52 ± 2.8 nm; 4. Zeta potential: HT:Fe²⁺ = 2:1 has the largest absolute value of potential and the best colloidal stability; 5. XPS: The sample showed characteristic peaks of C1s, O1s, and Fe2p, directly confirming that Fe element completed coordination binding with hematoxylin polyphenol groups.

[0060] 4.2 Rheology of Coating Solution and Characterization of Membrane Microstructure Preparation of Xanthan Gum Gradient Coating Solution for PX0 (Xanthan Gum Free), PX10, PX20, PX30, PX40, and PX50: (e.g.) Figure 9 As shown, 1. Rheological test: The viscosity of the system decreases steadily with increasing shear rate, indicating that it is a shear-thinning fluid, suitable for seed roll coating and spray coating operations; the storage modulus G' is always higher than the loss modulus G'', indicating strong film forming stability; 2. SEM + Element Mapping: HT-Fe²⁺ nanoparticles are uniformly distributed within the PX20 film, with no local enrichment or agglomeration of Fe, C, and O elements, demonstrating excellent compatibility between the nanoparticles and the XG / PVA substrate.

[0061] 4.3 Mechanical properties of the coating film like Figure 10 The stress-strain curves show that as the amount of xanthan gum added increases, the tensile strength of the film gradually decreases and the elongation at break decreases. PX20 has both suitable strength and ductility, and is not easy to crack or fall off after coating, making it suitable for seed storage and field water absorption and expansion.

[0062] 4.4 Permeability and Hydrophilicity 1. Water vapor permeability: PX20 has the best air and water permeability, maintaining a slightly moist environment on the seed surface without excessive water permeability that would cause rapid salt penetration; Figure 11 ; 2. Water contact angle: The contact angle gradually increases with xanthan gum. PX20 has moderate hydrophilicity, balancing water absorption and retention with salt ion barrier effects. Figure 12 .

[0063] 4.5 Photothermal performance characterization Using an 808 nm near-infrared laser, the temperature rise curves and infrared thermal imaging of pure XG substrate, HT-Fe²⁺ suspension, and HT-Fe²⁺@PX composite film were measured respectively: (e.g.) Figure 13 , 1. No significant temperature rise was observed in the pure xanthan gum substrate, proving that the photothermal effect is entirely derived from the HT-Fe²⁺ nanocomplex; 2. The HT:Fe²⁺ ratio of 2:1 provides the fastest heating rate and a maximum temperature of over 65℃, enabling instantaneous high-temperature sterilization. 3. The composite membrane has good photothermal cycle stability. After multiple light-cooling cycles, its heating performance does not significantly decrease, and it has long-lasting and stable sterilization effect.

[0064] Test Example 5: Ecological Safety and Toxicity Evaluation of Coating Agents in Zebrafish In accordance with the "Guidelines for Environmental Safety Evaluation of Chemical Pesticides" GB / T 31270.12—2014, acute toxicity and teratogenicity tests were conducted on zebrafish to verify that this coating agent is green, low in toxicity, and eco-friendly.

[0065] 5.1 96-hour acute toxicity test A blank control group was set up with CT, and coating solution concentrations of 1%, 2%, 4%, 8%, and 16% were used. The number of zebrafish that died at 24 / 48 / 72 / 96 hours was recorded, and the mortality rate was calculated. Figure 20 The details are as follows:

[0066] 1. The mortality rate at low concentrations (1%~4%) is far lower than that of conventional chemical pesticides; only 16% of the ultra-high concentration group had a mortality rate of 100%; such as Figure 19 ; 2. Calculate the 96h-LC of this coating agent.50 The concentration is approximately 43,500 mg / L, which is much higher than commercially available pesticides such as lambda-cyhalothrin, imidacloprid, and diuron, and is therefore classified as low toxicity. The actual concentration used in the field is far below the threshold for toxicity, and there is no risk of biological toxicity to the soil and water environment.

[0067] 5.2 Teratogenicity Evaluation of 120hpf Zebrafish Larvae The developmental abnormalities of zebrafish embryos were observed after 120 hours in four groups: CT (blank), TL (low concentration), TM (medium concentration), and TH (high concentration). The number of individuals with brain, heart, liver, mandible, yolk sac, and eye abnormalities was counted. Figure 21 and Figure 22 As shown, 1. The blank control group showed almost no deformities; 2. Only the highest concentration TH group showed a slight increase in liver and heart malformations; the incidence of malformations was extremely low at field application concentrations (TL level). 3. Conclusion: At conventional coating dosages, there is no significant teratogenic risk, and it is safe for aquatic organisms and farmland ecosystems.

[0068] In summary, this invention successfully prepared a seed coating agent with dual functions of photothermal sterilization and salt stress resistance by combining hematoxylin-ferrous ion complexed nanoparticles with a xanthan gum-polyvinyl alcohol coating substrate. Under near-infrared light irradiation, this coating agent can efficiently kill pathogenic microorganisms through photothermal effects (sterilization rate >99%), while simultaneously mitigating the osmotic stress and oxidative damage of salt ions on seeds and seedlings through the physical barrier effect of the polymer coating, significantly improving the germination rate and growth quality of crops under saline-alkali conditions. Comparative tests show that when the proportions of each component deviate from the limits set by this invention, the photothermal sterilization effect, salt stress resistance, or both of these decrease to varying degrees, further verifying the scientific validity and irreplaceability of the optimal proportion selection in this invention. This invention provides a green, efficient, and multifunctional seed treatment technology solution for crop cultivation in saline-alkali land.

[0069] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a photothermal bactericidal and salt-stress-resistant seed coating agent, characterized in that, Includes the following steps: (S1) Hematoxylin was added to deionized water and dissolved by sonication. Ferrous sulfate deionized water and PBS buffer were added in sequence, and the reaction was continued by sonication. After the reaction was completed, the mixture was centrifuged and concentrated to obtain a dispersion of hematoxylin-ferrous ion complex nanoparticles. (S2) Dissolve xanthan gum and polyvinyl alcohol separately in deionized water, then mix the two solutions, add glycerin and stir evenly to obtain the coating base liquid; (S3) Add the hematoxylin-ferrous ion complex nanoparticle dispersion to the coating base liquid and stir thoroughly until the dispersion is uniform to obtain the seed coating agent.

2. The method for preparing the photothermal bactericidal and salt-stress-resistant seed coating agent according to claim 1, characterized in that, In step (S1), the concentration of the hematoxylin deionized aqueous solution is 10-30 mg / mL; the concentration of the ferrous sulfate deionized aqueous solution is 10-25 mg / mL; the volume ratio of hematoxylin, ferrous sulfate and PBS buffer is (1-3):(0.5-2):(5-15); the pH of the PBS buffer is 7.4, and the ferrous sulfate is FeSO4·7H2O.

3. The method for preparing the photothermal bactericidal and salt-stress-resistant seed coating agent according to claim 1, characterized in that, In step (S1), the reaction temperature is 25°C and the reaction time is 10 h.

4. The method for preparing the photothermal bactericidal and salt-stress-resistant seed coating agent according to claim 1, characterized in that, In step (S2), the concentration of xanthan gum deionized water solution is 5-10 g / L, and the concentration of polyvinyl alcohol deionized water solution is 20-40 g / L; the volume ratio of xanthan gum, polyvinyl alcohol and glycerol is (5-15):(15-35):(1-4); the dissolution temperature of xanthan gum and polyvinyl alcohol is 85-100 ℃; the stirring rate is 400-600 rpm; and the dissolution time is 1.5-2.5 h.

5. The method for preparing the photothermal bactericidal and salt-stress-resistant seed coating agent according to claim 1, characterized in that, In step (S3), the volume ratio of the hematoxylin-ferrous ion complex nanoparticle dispersion to the coating base liquid is (0.5-2):(50-200).

6. The method for preparing the photothermal bactericidal and salt-stress-resistant seed coating agent according to claim 2, characterized in that, The volume ratio of hematoxylin, ferrous sulfate and PBS buffer is 2:1:

10.

7. The method for preparing the photothermal bactericidal and salt-stress-resistant seed coating agent according to claim 4, characterized in that, The volume ratio of xanthan gum, polyvinyl alcohol, and glycerin is 10:25:

2.

8. The method for preparing the photothermal bactericidal and salt-stress-resistant seed coating agent according to claim 5, characterized in that, The volume ratio of the hematoxylin-ferrous ion complex nanoparticle dispersion to the coating base liquid is 1:

100.

9. A photothermal bactericidal and salt-stress-resistant seed coating agent, characterized in that, The seed coating agent is prepared by the method described in any one of claims 1-8.

10. The application of a photothermal bactericidal and salt-stress-resistant seed as described in claim 9, characterized in that, The seed coating agent is used to prepare a pre-sowing seed treatment agent for crops in saline-alkali soils. The seed treatment agent can alleviate crop salt stress damage and block seed pathogen infection.

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