A fluorescent temperature-sensitive film based on humic acid carbon quantum dots and a preparation method and application thereof

Abstract

This invention discloses a fluorescent temperature-sensitive thin film based on humic acid carbon quantum dots, its preparation method, and its applications, belonging to the field of waste resource utilization technology. The invention involves mixing biomass waste with an acidic solution for a first hydrothermal reaction; after alkali treatment, a second hydrothermal reaction is carried out under oxidizing conditions. After the reaction, solid-liquid separation is performed to collect humic acid-containing liquids with different oxidation degrees. The humic acid is precipitated by acid addition and then washed and dried sequentially to obtain high-purity humic acid solids with different oxidation degrees. These humic acid solids are then dissolved to form a humic acid solution. The humic acid solution is mixed with a polyvinyl alcohol aqueous solution, ultrasonically dispersed, and then drop-coated onto a substrate and dried at room temperature to form a thin film. The resulting thin film is the fluorescent temperature-sensitive thin film based on humic acid carbon quantum dots. The film obtained by this invention exhibits high sensitivity, high stability, and reversibility, with fluorescence spectral intensity varying with temperature within the range of 20-180℃. Therefore, it can be used to prepare fluorescent temperature probes for applications in biomedical and life science fields such as smart packaging and cell and tissue temperature monitoring, as well as in industrial and engineering sensing fields such as non-contact temperature detection systems and flexible electronics and wearable devices.

Description

Technical Field

[0001] This invention belongs to the field of waste resource utilization technology, and particularly relates to a fluorescent thermosensitive film based on humic acid carbon quantum dots, its preparation method and application. Background Technology

[0002] Humic acid is a class of large-molecule natural organic mixtures formed from the decomposition of plant and animal remains by microorganisms and geochemical transformation. It possesses a complex aromatic-aliphatic structure and abundant oxygen-containing functional groups (such as carboxyl and phenolic hydroxyl groups). Due to its unique physicochemical properties, such as ion exchange capacity, complexation, colloidal characteristics, and bioactivity, humic acid shows broad application potential in agriculture, environment, materials, and biomedicine. In agriculture, it acts as a soil conditioner and plant growth stimulant, improving soil structure, water and fertilizer retention, and promoting nutrient absorption. In the environmental field, it can be used as a heavy metal adsorbent and a solubilizer for organic pollutants. Industrially, it can be used as a drilling mud additive, ceramic dispersant, and precursor for battery electrode materials.

[0003] However, the traditional industrial production of high-purity humic acid mainly relies on non-renewable mineral resources such as lignite, weathered coal, or peat. This process not only consumes valuable fossil resources, but its extraction (often using strong acids and alkalis) and purification steps are also often accompanied by high energy consumption and potential environmental pollution. Furthermore, the structure and performance of the obtained product are limited by the quality of the original coal and are difficult to control in a targeted manner, which has significant limitations from the perspective of resource sustainability and green manufacturing. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a fluorescent thermosensitive thin film based on humic acid carbon quantum dots, its preparation method, and its application.

[0005] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots, comprising the following steps: The biomass waste is mixed with the first acid to carry out the first hydrothermal reaction; After the first hydrothermal reaction is completed, the solid-liquid mixture after the reaction is separated into solid and liquid components. The resulting solid product, hydrothermal carbon, is mixed with alkaline solution and subjected to a second hydrothermal reaction under oxidizing conditions. After the second hydrothermal reaction is completed, solid-liquid separation is performed, the liquid containing humic acid is collected, acid is added to precipitate humic acid, and the mixture is washed and dried in sequence to obtain high-purity humic acid solid. The high-purity humic acid solid is dissolved into a humic acid solution, mixed with a polyvinyl alcohol aqueous solution, ultrasonically dispersed, placed on a substrate, and dried at room temperature to form a thin film. The resulting film is the fluorescent thermosensitive film based on humic acid carbon quantum dots.

[0006] This invention first involves mixing biomass waste with an acidic solution in a certain proportion and placing the mixture in a hydrothermal reactor. A first-step hydrothermal carbonization reaction is then carried out under specific hydrothermal conditions. After the first-step reaction, the resulting solid-liquid mixture is directly treated with alkali (i.e., alkali solution is added directly to the mixture), or the solid product collected after solid-liquid separation is mixed with alkali solution in a certain proportion, and a second-step alkaline hydrothermal humification reaction is carried out under specific hydrothermal oxidation conditions. After the second-step reaction, solid-liquid separation is performed to collect humic acid-containing liquids with different oxidation degrees. Acid is added to precipitate the humic acid, and the mixture is then washed and dried sequentially to obtain high-purity humic acid solids with different oxidation degrees. These humic acid solids are then dissolved into a humic acid solution. The humic acid solution is mixed with a polyvinyl alcohol aqueous solution, ultrasonically dispersed, and then drop-coated onto a substrate. The solution is dried at room temperature to form a thin film, which is a fluorescent thermosensitive thin film based on humic acid carbon quantum dots. According to the temperature-controlled fluorescence testing platform, the fluorescence spectral intensity of the thin film obtained by this invention can change with temperature in the range of 20-180℃, exhibiting high sensitivity, high stability and reversibility. Therefore, it can be used to prepare fluorescent temperature probes for application in biomedical and life science fields such as smart packaging and cell and tissue temperature monitoring, as well as industrial and engineering sensing fields such as non-contact temperature detection systems and flexible electronics and wearable devices.

[0007] This invention is based on the directional conversion of biomass waste under hydrothermal conditions and the controllability of humic acid structure. Through stepwise hydrothermal carbonization and oxidation reactions, components such as lignocellulose in biomass are converted into humic acid carbon quantum dots with fluorescent properties. The fused aromatic structure and oxygen-containing functional groups on the surface of the humic acid carbon quantum dots generate stable fluorescence under photoexcitation, and the fluorescence intensity exhibits a reversible response to temperature changes. By controlling the hydrothermal oxidation conditions (such as oxygen pressure), humic acids with different oxidation degrees and molecular structures can be prepared, thereby affecting their fluorescence temperature-sensitive behavior. A thin film is prepared by composited humic acid solution with polyvinyl alcohol, and the fluorescence temperature response characteristics of humic acid carbon quantum dots are utilized to achieve highly sensitive, reversible, and stable detection of external temperatures.

[0008] Traditional humic acid is a complex mixture of macromolecules, but this invention uses a two-step hydrothermal method (first step carbonization, second step oxidative humification) to directionally transform components such as cellulose and lignin in biomass into an aromatic polyphenol-rich substance. 2Fluorescent materials with abundant oxygen-containing functional groups on their carbon domains and surfaces. The first step, acidic hydrothermal carbonization, promotes the dehydration and aromatization of biomass, forming tiny carbon nuclei composed of fused aromatic rings. When these carbon nuclei are small enough (typically <10 nm), a quantum confinement effect occurs, confining electrons and holes within a tiny space. This widens and discretizes the band gap, allowing the absorption of specific wavelengths of light and the emission of longer-wavelength (lower-energy) fluorescence. The second step, alkaline hydrothermal oxidation, not only "tailors" and oxidizes these carbon nuclei, making their oxidation degree tunable, but more importantly, introduces a large number of carboxyl, hydroxyl, and carbonyl functional groups onto the carbon nuclei surface, forming abundant surface states. When excitons (electron-hole pairs) generated by photoexcitation of the carbon nuclei are captured by these surface states, they release energy through non-radiative transitions or emit fluorescence of specific wavelengths (surface state fluorescence) through radiative transitions. Different oxidation degrees correspond to different surface chemical environments, thus resulting in different emitted fluorescence colors (wavelengths) and intensities. Within the macromolecule of humic acid, intramolecular charge transfer can occur between the electron-rich aromatic nuclei and the electron-deficient oxygen-containing functional groups. Excitation light can promote this charge transfer, the efficiency of which is influenced by molecular conformation and the surrounding environment, and is also an important mechanism for fluorescence generation and change. The fluorescence of pure humic acid solutions or solid powders may not be sensitive enough or irreversible with temperature changes. Composite humic acid with polyvinyl alcohol (PVA) to form thin films is key to achieving high-performance, reversible temperature-sensitive sensing. PVA can uniformly disperse humic acid, effectively preventing fluorescence quenching caused by aggregation, while also imparting good mechanical strength and flexibility to the film, facilitating practical applications. In this composite film, the inherent fluorescence temperature response characteristics of humic acid are stably and reversibly exhibited. Temperature changes affect the conformation, surface state distribution, or intramolecular charge transfer efficiency of humic acid molecules, thereby altering their fluorescence intensity and achieving high-sensitivity temperature detection. Therefore, this invention can directionally prepare humic acids with different oxidation degrees, molecular weights, and fluorescence properties to meet sensing needs in different temperature ranges.

[0009] Furthermore, the biomass waste was crushed before being mixed with the first acid.

[0010] Furthermore, the liquid-to-solid ratio of the biomass waste to the first acid is (1-20) mL:1g.

[0011] Furthermore, the biomass waste is selected from corn stalks, wheat stalks, kitchen waste, or shrimp shells.

[0012] For example, the substrate is a glass plate.

[0013] Furthermore, the first acid is selected from hydrochloric acid or sulfuric acid.

[0014] Furthermore, the temperature of the first hydrothermal reaction is 160-250℃, and the time is 3-8h.

[0015] Furthermore, the alkaline solution is at least one of sodium hydroxide solution and potassium hydroxide solution; after performing the alkaline treatment or mixing the solid product after solid-liquid separation with the alkaline solution, the alkaline concentration in the resulting solid-liquid mixture is 0.1 mol / L-1 mol / L.

[0016] Furthermore, the temperature of the second hydrothermal reaction is 160℃-200℃, the time is 1-8 h, and the liquid-to-solid ratio is (1-20) mL:1g; the oxidation conditions are that the second hydrothermal reaction is carried out under oxygen conditions, and the oxygen pressure is 5 bar-20 bar.

[0017] Furthermore, when adding acid to precipitate humic acid, the amount of acid added is sufficient to adjust the pH of the humic acid liquid to below 2; the purity of the high-purity humic acid solid is greater than 95%.

[0018] The present invention also provides a fluorescent thermosensitive thin film based on humic acid carbon quantum dots prepared by the above preparation method.

[0019] The present invention also provides a fluorescent temperature probe, which is prepared from the above-mentioned fluorescent temperature-sensitive thin film based on humic acid carbon quantum dots.

[0020] This invention also provides applications of the above-mentioned fluorescent temperature-sensitive thin film based on humic acid carbon quantum dots and the above-mentioned fluorescent temperature probe in the fields of flexible electronics, smart packaging and biomedical non-contact temperature measurement.

[0021] This invention utilizes a fluorescent thermosensitive thin film based on humic acid carbon quantum dots in a hydrothermal reactor. Using only water as the medium, and under relatively mild temperatures (180-300℃) and autogenous pressure, it can efficiently convert biomass wastes such as lignocellulosic materials into humic acid-rich products within hours. A core advantage of the hydrothermal process is that by precisely controlling the reaction temperature, time, pH, and oxidation environment (such as introducing oxygen or oxidants), the condensation, aromatization, and functional group evolution pathways of humic acid can be actively manipulated. This allows for the customized preparation of humic acid with target chemical structures, molecular weight distributions, and reactivity to meet the needs of specific downstream applications (such as as a carbon precursor with specific functions, an adsorbent material, or a bioactive substance).

[0022] Achieving efficient preparation of humic acid while expanding its applications from traditional low-value-added fields such as soil improvement to higher-value advanced functional materials is key to enhancing the overall economic efficiency and competitiveness of this technology. Based on this, this invention conducts in-depth research on the fine structure and derived functions of hydrothermal humic acid, and for the first time systematically reveals its inherent fluorescent carbon quantum dot characteristics, namely, that the fused aromatic clusters and surface functional groups in the humic acid molecule can generate stable and tunable fluorescence under photoexcitation. Therefore, this invention enables the fabrication of fluorescent temperature probes based on humic acid carbon quantum dots in fluorescent thermosensitive films.

[0023] In view of this, the present invention aims to develop a complete, green, and economically feasible technological process system: First, using an optimized hydrothermal humification technology for biomass waste, structurally tunable humic acid is prepared on a large scale; then, this humic acid is used as a natural, functionalized carbon nanomaterial precursor, and combined with a biocompatible polymer (such as polyvinyl alcohol) through a simple process to form a uniform, flexible fluorescent film. The fluorescence intensity or wavelength of this film exhibits high sensitivity, reversible response, and excellent stability to temperature changes, thus making it suitable for use as a high-performance temperature sensor. Compared to traditional fluorescent temperature sensors that rely on rare-earth-doped inorganic materials (high cost and complex preparation) or carbon-based nanomaterials requiring fine synthesis (such as graphene quantum dots), this technology uses all biomass waste as raw material and employs green processes such as hydrothermal treatment throughout the entire process. This not only significantly reduces raw material costs but also avoids the use of toxic reagents, demonstrating overwhelming advantages in terms of economy, environmental friendliness, and sustainability. It has significant industrial application prospects and value in fields such as flexible electronics, smart packaging, and non-contact temperature measurement in biomedicine.

[0024] Compared with the prior art, the present invention has the following advantages and technical effects: 1. Green and sustainable raw materials: Using biomass waste such as agricultural waste and forestry waste as raw materials, it replaces the traditional humic acid extraction method that relies on coal resources, realizes the resource utilization of waste, reduces raw material costs and is environmentally friendly.

[0025] 2. Simple and efficient process: Humic acid is prepared stepwise by hydrothermal method, without the need for complicated synthesis steps or toxic reagents. The reaction conditions are mild and the energy consumption is low, making it suitable for large-scale production.

[0026] 3. High structural controllability: By adjusting the hydrothermal oxidation conditions (such as oxygen pressure, temperature, time, etc.), humic acids with different oxidation degrees, molecular weights and fluorescence properties can be prepared in a targeted manner to meet the sensing needs of different temperature ranges.

[0027] 4. Excellent fluorescence temperature sensing performance: The prepared humic acid-polyvinyl alcohol composite film exhibits high sensitivity, reversibility and good cycle stability in the range of 20-180℃, making it suitable for sensing applications with a wide temperature range.

[0028] 5. Wide range of applications: This fluorescent temperature probe is both flexible and biocompatible, and can be applied to many cutting-edge fields such as cell and tissue temperature monitoring, non-contact temperature detection systems, flexible electronics and wearable devices.

[0029] 6. Low overall cost: The entire process from raw materials to preparation is green and economical, avoiding the high cost problems of rare earth materials or finely synthesized carbon materials, and has significant industrialization potential and market competitiveness. Attached Figure Description

[0030] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 The fluorescence spectrum changes of the fluorescent thermosensitive film based on humic acid carbon quantum dots in Example 2 during the heating process; Figure 2 The temperature-sensing characteristics of the fluorescent thermosensitive film based on humic acid carbon quantum dots in Example 2; Figure 3 The fluorescence spectrum changes of the fluorescent thermosensitive film based on humic acid carbon quantum dots in Example 3 containing straw humic acid during the heating process; Figure 4 The temperature sensing characteristics of the fluorescent temperature probe based on humic acid carbon quantum dots in Example 3 are shown. Figure 5 The fluorescence spectrum changes of the fluorescent thermosensitive film based on humic acid carbon quantum dots in Example 4 during the heating process; Figure 6 The temperature-sensing characteristics of the fluorescent thermosensitive film based on humic acid carbon quantum dots in Example 4; Figure 7 The results show the temperature-sensing cycle test performance of the fluorescent thermosensitive film based on humic acid carbon quantum dots in Example 4. Detailed Implementation

[0031] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0032] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0033] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0034] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0035] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0036] An embodiment of the present invention provides a method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots, comprising the following steps: First, the biomass waste is crushed and mixed with the first acid in a certain proportion and placed in a hydrothermal reactor. The first step of hydrothermal carbonization reaction is carried out under certain hydrothermal conditions. After the first step of hydrothermal carbonization reaction is completed, the solid-liquid mixture after the reaction is separated into solid and liquid. The collected solid product hydrothermal carbon is mixed with alkaline solution and carried out in the second step of alkaline hydrothermal humification reaction under certain hydrothermal oxidation conditions. After the second step of alkaline hydrothermal humification reaction is completed, solid-liquid separation is performed to collect humic acid-containing liquids with different oxidation degrees. Acid is added to precipitate humic acid, and the liquids are washed and dried in sequence to obtain high-purity humic acid solids with different oxidation degrees. The solid humic acid was then dissolved to form a humic acid solution. This solution was mixed with a polyvinyl alcohol aqueous solution at a specific mass ratio, ultrasonically dispersed, and then drop-coated onto a clean substrate (such as a clean glass plate). After drying at room temperature for two days, a thin film was formed. The resulting film is a fluorescent temperature-sensitive film based on humic acid carbon quantum dots. Measurements using a temperature-controlled fluorescence testing platform showed that the fluorescence intensity of this film varied with temperature within the range of 20-180℃, exhibiting high sensitivity, high stability, and reversibility. Therefore, it can be used as a fluorescent temperature probe and applied in biomedical and life science fields such as cell and tissue temperature monitoring, as well as in industrial and engineering sensing fields such as non-contact temperature detection systems and flexible electronics and wearable devices.

[0037] In embodiments of the present invention, the specific operating conditions for crushing biomass waste are not particularly limited. For example, the biomass waste can be crushed to 0.1-0.3 mm to facilitate subsequent processing.

[0038] In a preferred embodiment of the present invention, the biomass waste is selected from corn stalks, wheat stalks, kitchen waste, or shrimp shells.

[0039] In a preferred embodiment of the present invention, the liquid-to-solid ratio of biomass waste to the first acid is (1-20) mL:1g.

[0040] In a preferred embodiment of the present invention, the first acid is selected from hydrochloric acid or sulfuric acid.

[0041] In a preferred embodiment of the present invention, the temperature of the first hydrothermal reaction (i.e., the first step of hydrothermal carbonization reaction) is 160-250°C and the time is 3-8h.

[0042] In a preferred embodiment of the present invention, the alkaline solution is at least one of sodium hydroxide solution and potassium hydroxide solution; after performing alkali adjustment treatment or mixing the solid product after solid-liquid separation with the alkaline solution, the alkali concentration in the resulting solid-liquid mixture is 0.1 mol / L-1 mol / L.

[0043] In a preferred embodiment of the present invention, the temperature of the second hydrothermal reaction (i.e., the second alkaline hydrothermal humification reaction) is 160℃-200℃, the time is 1-8 h, and the liquid-solid ratio is (1-20) mL:1g; the second hydrothermal reaction under oxidizing conditions is carried out under oxygen conditions, and the oxygen pressure is 5 bar-20 bar.

[0044] In a preferred embodiment of the present invention, when adding acid to precipitate humic acid, the amount of acid added is adjusted to adjust the pH of the humic acid liquid to below 2; high-purity humic acid solids with different oxidation degrees refer to humic acid with a purity greater than 95% and an oxygen content ranging from 25% to 45%.

[0045] In a preferred embodiment of the present invention, the acid used for precipitating humic acid is hydrochloric acid with a concentration of 6 M.

[0046] In a preferred embodiment of the present invention, dissolving solid humic acid into a humic acid solution refers to dissolving solid humic acid in water or sodium hydroxide or potassium hydroxide solution to form a humic acid solution.

[0047] In a preferred embodiment of the present invention, the concentration of the polyvinyl alcohol aqueous solution is 1 wt%-10 wt%, and it is mixed with the humic acid solution at a mass ratio of (1-20):1.

[0048] An embodiment of the present invention also provides a fluorescent thermosensitive thin film based on humic acid carbon quantum dots prepared by the above preparation method.

[0049] Embodiments of the present invention also provide a fluorescent temperature probe, which is prepared from the above-described fluorescent temperature-sensitive thin film based on humic acid carbon quantum dots. This fluorescent temperature probe can be applied in biomedical and life science fields such as cell and tissue temperature monitoring, as well as in industrial and engineering sensing fields such as non-contact temperature detection systems and flexible electronics and wearable devices.

[0050] The embodiments of the present invention also provide the application of the above-mentioned fluorescent temperature-sensitive thin film based on humic acid carbon quantum dots and the above-mentioned fluorescent temperature probe in the fields of flexible electronics, smart packaging and biomedical non-contact temperature measurement.

[0051] This invention utilizes a temperature-controlled fluorescence testing platform to determine the sensitivity, stability, and reversibility of a humic acid carbon quantum dot-based fluorescent thermosensitive thin film. In the following embodiments of this invention, the testing conditions of the temperature-controlled fluorescence testing platform are as follows: fluorescence spectroscopy is used to determine the absorption / emission characteristics of the thin film; the fluorescence intensity of the thin film as a function of temperature is measured within the range of 20℃-180℃; and the stability of the thin film for recycling within the range of 20℃-80℃ is measured, with at least 5 cycles.

[0052] Unless otherwise specified, the room temperature in this invention is 25±2℃.

[0053] All raw materials used in the embodiments of the present invention were obtained through commercial purchase.

[0054] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0055] The technical solution of the present invention will be further illustrated by the following embodiments.

[0056] Example 1 This embodiment uses different biomass wastes as raw materials to prepare different hydrothermal charcoals. The specific process is as follows: Dry biomass waste powder (0.3 mm particle size) was added to a 100 mL PTFE liner, along with 10 mL of hydrochloric acid or sulfuric acid. The liner was then placed in a stainless steel reactor and subjected to hydrothermal reaction in an oven. After the reaction was completed, the reactor was allowed to cool naturally to room temperature. The solid-liquid mixture was separated by vacuum filtration using a 0.45 μm filter membrane. The resulting solid product was thoroughly washed with deionized water and dried in an oven at 80 °C until constant weight, yielding hydrothermal carbon. The yields of hydrothermal carbon obtained under different raw materials and hydrothermal carbonization conditions are shown in Table 1.

[0057] Table 1. Experimental results of hydrothermal carbonization of typical biomass waste As shown in Table 1, the present invention can prepare hydrothermal carbon by using different biomass wastes (such as corn stalks, wheat stalks, kitchen waste, and shrimp shells) as raw materials, adding hydrochloric acid or sulfuric acid with a pH of 0-7, and conducting hydrothermal reactions at 160-250℃ for 3-8 hours.

[0058] Example 2 This embodiment provides a method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots, the steps of which are as follows: 0.5 g of hydrothermal char derived from corn stalks (group 8 in Example 1) was added to a 10 mL PTFE liner, along with 5 mL of sodium hydroxide solution at pH 13 (i.e., the final alkali concentration in the solid-liquid mixture was 0.1 mol / L). The liner was placed in an atmosphere-controlled reactor for hydrothermal oxidative humification. The specific process was as follows: the oxygen pressure in the reactor was set to 5 bar, and then the reactor was kept at 190°C for 3 h for the humification reaction. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. The solid-liquid mixture was first centrifuged at 5000 rpm for 5 minutes, and then the liquid product was separated by vacuum filtration using a 0.45 μm filter membrane. The pH of the liquid product was adjusted to 1 with concentrated hydrochloric acid (6 M) to precipitate the humic acid. The liquid product was then centrifuged at 5000 rpm for 5 minutes. Solid humic acid was separated by centrifugation at rpm for 3 minutes. The solid powder was then dried in an 80°C oven to constant weight. It was washed once with water to remove surface salts and dried again to constant weight, yielding humic acid powder (purity 96.5%, oxygen content 30.1%), with a yield of 49.5%. This humic acid powder was dissolved in 5 mL of sodium hydroxide solution at pH 13 and named HA-5 bar (it is a liquid). 1 mL of the HA-5 bar humic acid liquid was mixed uniformly with 10 mL of 4 wt% polyvinyl alcohol (PVA) aqueous solution. After ultrasonic dispersion, the mixture was drop-coated onto a clean glass plate and dried at room temperature for 48 hours to form a composite film, which is a fluorescent thermosensitive film based on humic acid carbon quantum dots, denoted as HA-5 bar film.

[0059] Using a temperature-controlled fluorescence testing platform, the fluorescence intensity of the above-mentioned thin film as a function of temperature was measured within the temperature range of 20℃ to 180℃. Figure 1 As shown. Experimental results show that the fluorescence intensity of the film changes significantly with temperature, exhibiting thermosensitive response characteristics. Further... Figure 2 It can be seen that the fluorescence intensity of the film in the low temperature range of 20℃ to 100℃ is not significantly related to the temperature response, while the fluorescence intensity in the high temperature range of 100℃ to 180℃ is linear with the temperature response, that is, the higher the ambient temperature, the lower the fluorescence intensity of the film.

[0060] Example 3 This embodiment provides a method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots, the steps of which are as follows: 0.5 g of hydrothermal char derived from corn stalks (group 8 in Example 1) was added to a 10 mL PVC liner, along with 5 mL of sodium hydroxide solution at pH 13. The liner was placed in an atmosphere-controlled reactor for hydrothermal oxidative humification. The specific process was as follows: the oxygen pressure in the reactor was set to 10 bar, and the reactor was kept at 190°C for 3 h for the humification reaction. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. The solid-liquid mixture was first centrifuged at 5000 rpm for 5 minutes, and then the liquid product was separated by vacuum filtration using a 0.45 μm filter membrane. The pH of the liquid product was adjusted to 1 with concentrated hydrochloric acid (6 M) to precipitate the humic acid. The liquid product was then centrifuged at 5000 rpm for 5 minutes. Solid humic acid was separated by centrifugation at rpm for 3 minutes. The powder was then transferred to an 80°C oven and dried to constant weight. The solid powder was washed once with water to remove surface salts, and then dried again to constant weight to obtain humic acid powder (purity 97.5%, oxygen content 35.9%), with a yield of 41.4%. This humic acid powder was dissolved in 5 mL of sodium hydroxide solution with a pH of 13 and named HA-10 bar. 1 mL of the HA-10 bar humic acid liquid was mixed uniformly with 10 mL of 4 wt% PVA aqueous solution (mass ratio of humic acid liquid to PVA aqueous solution as in Example 2). After ultrasonic dispersion, the mixture was drop-coated onto a clean glass plate and dried at room temperature for 48 hours to form a composite film, which is a fluorescent thermosensitive film based on humic acid carbon quantum dots. This film is denoted as HA-10 bar film.

[0061] Using a temperature-controlled fluorescence testing platform, the fluorescence intensity of the above-mentioned thin film as a function of temperature was measured within the temperature range of 20℃ to 180℃. Figure 3 As shown. Experimental results show that the fluorescence intensity of the film changes significantly with temperature, exhibiting thermosensitive response characteristics. Further... Figure 4 It can be seen that the fluorescence intensity of the film in the temperature range of 20℃ to 150℃ shows a linear relationship with temperature, that is, the higher the ambient temperature, the lower the fluorescence intensity of the film; and as the ambient temperature continues to increase, the fluorescence intensity of the film weakens faster.

[0062] Example 4 This embodiment provides a method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots, the steps of which are as follows: 0.5 g of hydrothermal char derived from corn stalks (group 8 in Example 1) was added to a 10 mL PVC liner, along with 5 mL of sodium hydroxide solution at pH 13. The liner was placed in an atmosphere-controlled reactor for hydrothermal oxidative humification. The specific process was as follows: the oxygen pressure in the reactor was set to 20 bar, and the reactor was kept at 190°C for 3 h for the humification reaction. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. The solid-liquid mixture was first centrifuged at 5000 rpm for 5 minutes, and then the liquid product was separated by vacuum filtration using a 0.45 μm filter membrane. The pH of the liquid product was adjusted to 1 with concentrated hydrochloric acid (6 M) to precipitate the humic acid. The liquid product was then centrifuged at 5000 rpm for 5 minutes. Solid humic acid was separated by centrifugation at rpm for 3 minutes. The powder was then transferred to an 80°C oven and dried to constant weight. The solid humic acid powder was washed once with water to remove surface salts, and then dried to constant weight to obtain humic acid powder (purity 97.1%, oxygen content 40.1%), with a yield of 31.3%. This humic acid powder was dissolved in 5 mL of sodium hydroxide solution with a pH of 13 and named HA-20bar. 1 mL of HA-20bar humic acid liquid was mixed uniformly with 10 mL of 4 wt% PVA aqueous solution (mass ratio of humic acid liquid to PVA aqueous solution as in Example 2). After ultrasonic dispersion, the mixture was drop-coated onto a clean glass plate and dried at room temperature for 48 hours to form a composite film, which is a fluorescent thermosensitive film based on humic acid carbon quantum dots, denoted as HA-20bar film.

[0063] Using a temperature-controlled fluorescence testing platform, the fluorescence intensity of the above-mentioned thin film as a function of temperature was measured within the temperature range of 20℃ to 180℃. Figure 5 As shown. Experimental results show that the fluorescence intensity of the film changes significantly with temperature, exhibiting thermosensitive response characteristics. Further... Figure 6 It can be seen that the fluorescence intensity of the film in the entire temperature range of 20℃ to 180℃ exhibits a semi-quadratic function relationship with temperature, that is, the higher the ambient temperature, the lower the fluorescence intensity of the film.

[0064] As can be seen from Examples 2-4, the fluorescent thermosensitive films based on humic acid carbon quantum dots prepared by the method of this invention all exhibit a significant fluorescence-temperature dependence within the test range of 20-180℃. In particular, the film prepared with HA-20 bar (Example 4) shows the best temperature-sensitive response. Therefore, this invention conducted 9 cycles of recycling tests on the HA-20 bar film in the temperature range of 20-80℃ (based on the consensus that carbon quantum dots are used in temperature sensing, when the temperature rises to a certain range (usually above 150℃), adverse phenomena appear, and in some studies, even below 100℃, adverse phenomena appear, meaning that the fluorescence performance of carbon quantum dots will undergo irreversible decay or quenching). Figure 7 As shown, the results demonstrate that the fluorescent thermosensitive film based on humic acid carbon quantum dots prepared by the method of the present invention has excellent recycling performance, overcoming the aforementioned biases.

[0065] Furthermore, the thermosensitive characteristics of humic acid (HA-0 bar) prepared under oxygen-free conditions were tested using the same method as in Examples 2-4 above. The results showed that the composite film had no thermosensitive effect.

[0066] The above results confirm that by controlling the hydrothermal oxidation conditions (oxygen pressure), humic acid-based fluorescent probes with different temperature response characteristics and excellent temperature-sensing stability can be directionally prepared, thereby meeting the sensing requirements of different temperature ranges. This study successfully verifies the feasibility of the technical route for preparing high-performance, low-cost fluorescent temperature-sensing films using biomass-derived humic acid.

[0067] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots, characterized in that, Includes the following steps: The biomass waste is mixed with the first acid to carry out the first hydrothermal reaction; After the first hydrothermal reaction is completed, the solid-liquid mixture after the reaction is separated into solid and liquid components. The resulting solid product, hydrothermal carbon, is mixed with alkaline solution and subjected to a second hydrothermal reaction under oxidizing conditions. After the second hydrothermal reaction is completed, solid-liquid separation is performed, the liquid containing humic acid is collected, acid is added to precipitate humic acid, and the mixture is washed and dried in sequence to obtain high-purity humic acid solid. The high-purity humic acid solid is dissolved into a humic acid solution, mixed with a polyvinyl alcohol aqueous solution, ultrasonically dispersed, placed on a substrate, and dried at room temperature to form a thin film. The resulting thin film is the fluorescent thermosensitive thin film based on humic acid carbon quantum dots.

2. The method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots according to claim 1, characterized in that, The liquid-to-solid ratio of the biomass waste to the first acid is (1-20) mL:1g.

3. The method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots according to claim 2, characterized in that, The biomass waste is selected from corn stalks, wheat stalks, kitchen waste, or shrimp shells; The first acid is selected from hydrochloric acid or sulfuric acid.

4. The method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots according to claim 1, characterized in that, The temperature of the first hydrothermal reaction is 160-250℃, and the time is 3-8h.

5. The method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots according to claim 1, characterized in that, The alkaline solution is at least one of sodium hydroxide solution and potassium hydroxide solution; after performing the alkaline treatment or mixing the solid product after solid-liquid separation with the alkaline solution, the alkaline concentration in the resulting solid-liquid mixture is 0.1 mol / L-1 mol / L.

6. The method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots according to claim 1, characterized in that, The temperature of the second hydrothermal reaction is 160℃-200℃, the time is 1-8 h, and the liquid-to-solid ratio is (1-20) mL:1g; the oxidation conditions are that the second hydrothermal reaction is carried out under oxygen conditions, and the oxygen pressure is 5 bar-20 bar.

7. The method for preparing a fluorescent thermosensitive thin film based on humic acid carbon quantum dots according to claim 1, characterized in that, When adding acid to precipitate humic acid, the amount of acid added is sufficient to adjust the pH of the humic acid liquid to below 2; the purity of the high-purity humic acid solid is greater than 95%.

8. A fluorescent thermosensitive thin film based on humic acid carbon quantum dots, characterized in that, It is prepared according to any one of claims 1-7.

9. A fluorescent temperature probe, characterized in that, The fluorescent thermosensitive film based on humic acid carbon quantum dots as described in claim 8 is prepared.

10. The application of a fluorescent thermosensitive thin film based on humic acid carbon quantum dots as described in claim 8 and a fluorescent temperature probe as described in claim 9 in the fields of flexible electronics, smart packaging, and non-contact temperature measurement in biomedicine.

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