A method for preparing sulfur-functionalized carbon quantum dots by using sulfonated waste sulfuric acid to fractionally convert waste cotton fibers

Abstract

The application discloses a method for preparing sulfur-functionalized carbon quantum dots by using sulfonated waste sulfuric acid to grade-convert waste cotton fibers, and belongs to the technical field of high-value utilization of waste resources and preparation of nanometer carbon materials. The application takes waste cotton fibers as main raw materials, takes sulfonated waste sulfuric acid as acidolysis medium, realizes the co-production of microcrystalline cellulose solid phase and sulfur-functionalized carbon quantum dots through selective acidolysis, solid-liquid separation and liquid-phase hydrothermal carbonization, and thus a new path for the collaborative high-value utilization of textile solid waste and textile-related acidic waste is constructed. In addition, the sulfur-functionalized carbon quantum dots obtained by the application show stronger fluorescence emission intensity compared with the carbon quantum dots obtained by the ordinary sulfuric acid control system.

Description

Technical Field

[0001] This invention relates to the field of high-value utilization of waste resources and preparation of nano-carbon materials, specifically to a method for preparing sulfur-functionalized carbon quantum dots by graded conversion of waste cotton fibers using sulfonated waste sulfuric acid. Background Technology

[0002] With the development of the textile industry and the accelerated pace of consumer textile replacement, the amount of waste textiles, especially waste cotton fiber fabrics, continues to increase. Waste cotton fiber, with cellulose as its main component, is characterized by its wide availability, renewability, and high carbon content. Direct incineration or landfilling not only wastes resources but also puts pressure on the environment. Therefore, the high-value utilization of waste cotton fiber has become one of the important research directions in the field of textile solid waste resource utilization.

[0003] Meanwhile, large quantities of sulfonated waste sulfuric acid are generated during the production of dyes, dye intermediates, textile auxiliaries, and related fine chemicals. This type of waste acid is typically high in acidity, complex in composition, and contains a certain amount of organic residues and sulfur-containing components, making it a challenging industrial acidic waste to treat. Current technologies mostly employ neutralization, dilution, incineration for regeneration, or recycling as a low-value-added byproduct acid. This not only results in high treatment costs but also a low level of resource utilization, failing to fully realize its potential value. Therefore, developing new resource utilization pathways for sulfonated waste sulfuric acid that balance pollution control and high-value utilization is of great significance.

[0004] Carbon quantum dots are a class of nanomaterials with particle sizes typically less than 10 nm, exhibiting good fluorescence properties, water dispersibility, low biotoxicity, and good chemical stability. In recent years, they have shown promising applications in fluorescence sensing, bioimaging, photocatalysis, anti-counterfeiting, and functional composite materials. Traditional carbon quantum dots are mostly prepared using small-molecule chemical reagents such as citric acid, glucose, and ethylenediamine as raw materials. While the preparation routes are relatively mature, they generally suffer from high raw material costs, limited resource utilization, and insufficient green attributes. In contrast, preparing carbon quantum dots from waste biomass or industrial waste not only helps reduce costs but also facilitates the development of new pathways for the high-value utilization of waste, thus becoming an important research direction in recent years.

[0005] For waste cotton fibers, the main component, cellulose, can undergo hydrolysis, chain breakage, and structural reconstruction under acidic conditions. Amorphous regions are more readily degraded and enter the liquid phase, while crystalline regions are relatively preserved. Therefore, appropriate acid hydrolysis holds promise for the graded transformation of waste cotton fibers, yielding both a solid-phase product rich in crystalline cellulose and a liquid or suspension phase containing oligosaccharides, small-molecule organic matter, and other carbonizable components. Furthermore, hydrothermal carbonization of this liquid or suspension phase could potentially produce carbon quantum dots. If the acid hydrolysis medium itself contains sulfur-containing components that can participate in surface modification or functionalization, it is possible to introduce sulfur-containing surface functional groups during the formation of carbon quantum dots, thereby regulating their surface chemical properties and fluorescence performance.

[0006] Although some progress has been made in the research on the preparation of carbon quantum dots from waste biomass and the resource utilization of industrial waste acid, existing technologies still have significant limitations. First, the resource utilization of waste cotton fibers is mostly concentrated on respinning, regenerated cellulose materials, or the preparation of single carbon materials, lacking a graded conversion method that simultaneously utilizes solid and liquid phases for high-value applications. Second, the utilization of industrial waste acid, especially sulfonated waste sulfuric acid, remains primarily at the level of traditional recycling or low-value disposal, lacking a technical route that simultaneously uses it as an acidolysis medium and a source of functionalization for the preparation of nano-carbon materials. Third, while ordinary sulfuric acid can serve as an acidolysis medium and may introduce small amounts of sulfate esters or sulfur-containing oxidized structures, its main function remains acid-catalyzed hydrolysis and dehydration. In contrast, sulfonated waste sulfuric acid, in addition to sulfuric acid, contains sulfonated organic matter and other sulfur-containing residual components, which can simultaneously provide an acidic environment, carbonizable organic components, and a source of sulfur-containing functionalization during the acidolysis of waste cotton fibers and liquid-phase hydrothermal carbonization, thus facilitating the formation of surfaces rich in C-SO₄. X Carbon quantum dots containing sulfur-containing oxidizing functional groups such as -SO3H.

[0007] Therefore, developing a method for the co-production of microcrystalline cellulose solid phase and sulfur-functionalized carbon quantum dots through selective acidolysis, solid-liquid separation, and liquid-phase hydrothermal carbonization using waste cotton fiber as the main raw material and sulfonated waste sulfuric acid as the acidolysis medium is of great significance for improving the resource utilization level of waste textiles and industrial waste acid, reducing processing costs, simplifying the process flow, and improving the fluorescence performance of carbon quantum dots. It also provides a new technical path for the synergistic high-value utilization of related wastes in the textile industry. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention provides the following technical solution:

[0009] In a first aspect, the present invention provides a method for preparing sulfur-functionalized carbon quantum dots by graded conversion of waste cotton fibers using sulfonated waste sulfuric acid, comprising the following steps:

[0010] a. Pretreatment of waste cotton fibers

[0011] Waste cotton fibers are cleaned, washed and dried to reduce their size to 1-10 mm, thus obtaining pretreated waste cotton fibers.

[0012] b. Pretreatment and acidity adjustment of sulfonated waste sulfuric acid

[0013] Remove insoluble impurities from sulfonated waste sulfuric acid and adjust the effective acid concentration (calculated as H2SO4) in the acidolysis system to be controlled at 5 wt%–20 wt%; preferably 8 wt%–15 wt%.

[0014] c. Selective acid hydrolysis

[0015] The pretreated waste cotton fibers are mixed with the acid hydrolysis system of sulfonated waste sulfuric acid after acidity adjustment, and the acid hydrolysis reaction is carried out under certain temperature and time conditions.

[0016] d. Solid-liquid separation

[0017] The acid-hydrolyzed system was subjected to solid-liquid separation to obtain a solid phase and a liquid phase product; the solid phase was washed, neutralized and dried to obtain a microcrystalline cellulose solid product.

[0018] e. Hydrothermal carbonization

[0019] The liquid phase is placed in a closed reactor and subjected to hydrothermal treatment to form sulfur-functionalized carbon quantum dots.

[0020] f. Purification treatment

[0021] The system after the hydrothermal reaction was centrifuged, filtered, neutralized, dialyzed, desalted, and freeze-dried to obtain sulfur-functionalized carbon quantum dots.

[0022] In this invention, the graded conversion refers to first recovering the microcrystalline cellulose in the solid phase after acid hydrolysis, and then processing and recovering the liquid phase product.

[0023] Furthermore, the waste cotton fiber includes one or more of the following: waste pure cotton fabrics, cotton spinning scraps, waste cotton yarn, waste cotton cloth, and waste home textile cotton fabrics.

[0024] Furthermore, the sulfonated waste sulfuric acid includes waste acid from the sulfonation process in the production of dyes, dye intermediates, textile auxiliaries, or surfactants. The total acidity of the sulfonated waste sulfuric acid, calculated as H2SO4, is 20 wt% to 80 wt%, preferably 25 wt% to 65 wt%.

[0025] Furthermore, in the selective acid hydrolysis step, the liquid-to-solid ratio of waste cotton fibers to the acid hydrolysis system is 30:1 to 60:1 mL / g.

[0026] Furthermore, the temperature of the selective acidolysis step is 50–90 °C, preferably 65–85 °C; the acidolysis time is 0.5–8 h, preferably 3–5 h.

[0027] Furthermore, the reaction temperature of the hydrothermal carbonization step is 160–240 °C, preferably 190–230 °C; the reaction time is 1–10 h, preferably 2–6 h; the closed reactor can be a reaction vessel, and the filling amount of the reaction vessel is 20%–80%, preferably 40%–60%.

[0028] Further, in the purification process, the centrifugation speed is 6000–12000 rpm, and the centrifugation time is 5–30 min; filtration is performed using a 0.22–0.45 μm filter membrane; the neutralization filtrate is prepared using a 0.05–0.5 M NaOH solution, preferably a 0.1 M NaOH solution; dialysis is performed using a dialysis bag with a molecular weight cutoff of 500–5000 Da, and the dialysis time is 12–48 h. Freeze-drying is performed at -50 ℃ for 48 h; the obtained powder can be redispersed in deionized water to obtain a sulfur-functionalized carbon quantum dot dispersion. If necessary, ethanol washing or alcohol precipitation can be used to assist in desalination. Further, the average particle size of the sulfur-functionalized carbon quantum dots is 1–10 nm, with an optional average particle size of approximately 5 nm, and they exhibit blue or blue-green fluorescence under ultraviolet light excitation.

[0029] Furthermore, the surface of the sulfur-functionalized carbon quantum dots contains -C-SO x The substance contains a sulfur-containing functional group, where x is 2, 3, or 4. The -C-SO group... x Sulfur-containing functional groups can be characterized by high-resolution XPS spectroscopy of sulfur.

[0030] Furthermore, under the same testing conditions, compared with the carbon quantum dots obtained from the ordinary sulfuric acid control system, the sulfur-functionalized carbon quantum dots obtained in this invention exhibit stronger fluorescence emission intensity.

[0031] In a second aspect, the present invention also provides sulfur-functionalized carbon quantum dots prepared by the method described in the first aspect.

[0032] It should be understood that, within the scope of this invention, the above-described technical features of this invention and the technical features specifically described below (such as in the embodiments) can be combined with each other to form new or preferred technical solutions. Due to space limitations, they will not be described in detail here.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] This invention uses waste cotton fiber as the main raw material and sulfonated waste sulfuric acid as the acidolysis medium. Through selective acidolysis, solid-liquid separation, and liquid-phase hydrothermal carbonization, it achieves the co-production of microcrystalline cellulose solid phase and sulfur-functionalized carbon quantum dots. Furthermore, the sulfur-functionalized carbon quantum dots obtained by this invention exhibit stronger fluorescence emission intensity compared to carbon quantum dots obtained from a conventional sulfuric acid control system under the same testing conditions. Attached Figure Description

[0035] Figure 1 The effect of acidolysis temperature on the degradation rate of cotton fibers, the yield of microcrystalline cellulose, and the yield of carbon quantum dots.

[0036] Figure 2 SEM image (a) of the microcrystalline cellulose solid product after acid hydrolysis of waste cotton fibers and TEM image (b) of the prepared sulfur-functionalized carbon quantum dots.

[0037] Figure 3 High-resolution S element spectrum of the prepared sulfur-functionalized carbon quantum dots.

[0038] Figure 4 XPS full spectrum comparison (a) and fluorescence spectrum comparison (b) of carbon quantum dots prepared using sulfonated waste sulfuric acid and commercially available sulfuric acid as acidolysis media.

[0039] Figure 5 TEM image of sulfur-functionalized carbon quantum dots formed under hydrothermal reaction conditions of 160 °C and 2 h. Detailed Implementation

[0040] The present invention will now be described in further detail with reference to specific embodiments. The given embodiments are merely illustrative of the invention and not intended to limit its scope. The embodiments provided below can serve as a guide for further improvements by those skilled in the art and do not constitute a limitation of the invention in any way.

[0041] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0042] Example:

[0043] Example 1

[0044] A method for converting waste cotton fibers into sulfur-functionalized carbon quantum dots using sulfonated waste sulfuric acid in a graded manner includes the following steps:

[0045] a. Pre-treatment of waste cotton cloth

[0046] Waste cotton fabric is cleaned, dried, and then cut to a size of 1-10 mm to obtain pre-treated waste cotton fibers.

[0047] b. Pretreatment and acidity adjustment of waste sulfuric acid from dye sulfonation

[0048] Remove insoluble impurities from the waste sulfuric acid from dye sulfonation and adjust the effective acid concentration (calculated as H2SO4) in the acidolysis system of the waste sulfuric acid to be controlled at 15 wt%.

[0049] c. Selective acid hydrolysis

[0050] Pretreated waste cotton fibers were mixed with a sulfonated waste sulfuric acid acidolysis system adjusted for acidity, at a liquid-to-solid ratio of 60:1 mL / g, and the acidolysis reaction was carried out at 80 °C for 4 h. For comparison, results at other temperatures are shown below. Figure 1 As the acidolysis temperature increased from 65 ℃ to 90 ℃, the degradation rate of waste cotton fibers gradually increased, indicating that increasing the temperature is beneficial for destroying the long fiber structure of cotton fibers and promoting the breakage of cellulose chain segments. The yield of microcrystalline cellulose showed a trend of first increasing and then decreasing, reaching a relatively high level at 80 ℃; when the temperature was further increased to 90 ℃, some microcrystalline cellulose continued to hydrolyze and enter the liquid phase, resulting in a decrease in the yield of solid-phase products. The yield of sulfur-functionalized carbon quantum dots increased with increasing temperature, but the increase was limited, indicating that although excessively high temperatures are beneficial for the generation of liquid-phase carbon dot precursors, they also increase small molecule byproducts and non-recyclable losses. Considering the degradation rate of cotton fibers, the yield of microcrystalline cellulose, and the yield of sulfur-functionalized carbon quantum dots, the optimal acidolysis temperature is 80 ℃.

[0051] The cotton fiber degradation rate R is calculated using the following formula:

[0052] R = (m0-m r ) / m0×100%

[0053] The yield of microcrystalline cellulose Y1 is calculated using the following formula:

[0054] Y1 = m1 / m0 × 100%

[0055] The carbon quantum dot yield Y2 is calculated using the following formula:

[0056] Y2 = (m2-m b ) / m0×100%

[0057] Where m0 is the dry weight of the pretreated waste cotton fiber; m r m1 is the dry weight of undeconstructed cotton fibers that retain a continuous long fiber morphology after acid hydrolysis; m2 is the dry weight of microcrystalline cellulose obtained after separation, washing, neutralization, and drying; m3 is the dry weight of carbon quantum dots obtained after hydrothermal carbonization, centrifugation, filtration, dialysis desalination, and freeze-drying; m4 is the dry weight of undeconstructed cotton fibers that retain a continuous long fiber morphology after acid hydrolysis; m5 is the dry weight of undeconstructed cotton fibers that retain a continuous long fiber morphology after acid hydrolysis; m6 is the dry weight of undeconstructed cotton fibers that retain a continuous long fiber morphology after acid hydrolysis; m7 is the dry weight of undeconstructed cotton fibers that retain a continuous long fiber morphology after acid hydrolysis; m8 is the dry weight of undeconstructed cotton fibers that retain a continuous long fiber morphology after acid hydrolysis; m9 is the dry weight of undeconstructed cotton fibers that retain a continuous long fiber morphology bThe residual dry mass of the blank system after undergoing the same hydrothermal carbonization, centrifugation, filtration, dialysis, and freeze-drying treatments without the addition of waste cotton fibers.

[0058] d. Solid-liquid separation

[0059] The acid-hydrolyzed system is subjected to solid-liquid separation to obtain a solid phase and a liquid or suspended phase; the solid phase is then washed, neutralized, and dried to obtain a microcrystalline cellulose solid product. Figure 2 a)

[0060] e. Hydrothermal carbonization

[0061] The liquid or suspended phase is placed in a closed reactor with a filling amount of 60%, a reaction temperature of 220 °C, and a reaction time of 5 h to form sulfur-functionalized carbon quantum dots.

[0062] f. Purification treatment

[0063] The system after the hydrothermal reaction was centrifuged at 12,000 rpm for 10 min, and the supernatant was collected. After filtration through a 0.22 μm filter membrane, the solution was adjusted to neutral with 0.1 M NaOH solution. Then, the solution was dialyzed for 24 h using a dialysis bag with a molecular weight cutoff of 3500 Da to remove Na₂SO₄, residual acid, and small molecule byproducts. The dialyzed carbon quantum dot dispersion was freeze-dried at -50 °C for 48 h to obtain sulfur-functionalized carbon quantum dot powder. The obtained powder can be redispersed in deionized water to obtain a sulfur-functionalized carbon quantum dot dispersion. If necessary, ethanol washing or alcohol precipitation can be used to assist in desalination. Figure 2 b is a TEM image of the obtained carbon quantum dots, illustrating the successful preparation of carbon quantum dots. Figure 3 The image shows the high-resolution XPS spectrum of sulfur on the surface of carbon quantum dots, indicating that sulfur-containing functional groups were successfully introduced into the surface of carbon quantum dots.

[0064] Comparative Example 1

[0065] The steps for preparing carbon quantum dots using a standard sulfuric acid control system are as follows:

[0066] a. Pre-treatment of waste cotton cloth

[0067] Waste cotton fabric is cleaned, dried, and then cut to a size of 1–10 mm to obtain pretreated waste cotton fibers.

[0068] b. Preparation and acidity adjustment of ordinary sulfuric acid acidolysis system

[0069] Commercially available ordinary sulfuric acid was used as the acid source and mixed with deionized water to prepare an ordinary sulfuric acid acidolysis system with an effective acid concentration of 15 wt% (calculated as H2SO4).

[0070] c. Selective acid hydrolysis

[0071] The pretreated waste cotton fibers were mixed with a common sulfuric acid hydrolysis system with acidity adjusted, at a liquid-to-solid ratio of 60:1 mL / g, and the hydrolysis reaction was carried out at 80 ℃ for 4 h.

[0072] d. Solid-liquid separation

[0073] The acid-hydrolyzed system is subjected to solid-liquid separation to obtain a solid phase and a liquid or suspended phase; the solid phase is washed, neutralized and dried to obtain a microcrystalline cellulose solid product.

[0074] e. Hydrothermal carbonization

[0075] The liquid or suspended phase is placed in a closed reactor with a filling volume of 60%, a reaction temperature of 220 °C, and a reaction time of 5 h to form carbon quantum dots.

[0076] f. Purification treatment

[0077] The system after the hydrothermal reaction was centrifuged at 12,000 rpm for 10 min, and the supernatant was collected. After filtration through a 0.22 μm filter membrane, the supernatant was adjusted to neutral with 0.1 M NaOH solution. Then, it was dialyzed for 24 h using a dialysis bag with a molecular weight cutoff of 3500 Da to remove Na₂SO₄, residual acid, and small molecule byproducts. The dialyzed carbon quantum dot dispersion was freeze-dried at -50 °C for 48 h to obtain sulfur-functionalized carbon quantum dot powder. The obtained powder can be redispersed in deionized water to obtain a carbon quantum dot dispersion prepared from a common sulfuric acid system. If necessary, ethanol washing or alcohol precipitation can be used to assist in desalination.

[0078] Compared with Example 1, the S 2p signal of the carbon quantum dots obtained in Comparative Example 1 was significantly weakened or lower than the detection limit. Figure 4 (a) indicates that the ordinary sulfuric acid system has a significantly weaker effect on introducing sulfur-containing functional groups onto the surface of carbon quantum dots than the sulfonated waste sulfuric acid system. Furthermore, the carbon quantum dots obtained in Comparative Example 1 have lower fluorescence intensity ( Figure 4 (b) indicates that, under the same effective acidity conditions, sulfonated organic residues and other sulfur-containing components in sulfonated waste sulfuric acid can serve as additional sources of sulfur functionalization, promoting the formation of sulfur-containing oxidized functional groups on the surface of carbon quantum dots and enhancing their fluorescence emission intensity.

[0079] Comparative Example 2

[0080] A method for converting waste cotton fibers into sulfur-functionalized carbon quantum dots using sulfonated waste sulfuric acid in a graded manner includes the following steps:

[0081] a. Pre-treatment of waste cotton cloth

[0082] Waste cotton fabric is cleaned, dried, and then cut to a size of 1-10 mm to obtain pre-treated waste cotton fibers.

[0083] b. Pretreatment and acidity adjustment of waste sulfuric acid from dye sulfonation

[0084] Remove insoluble impurities from the waste sulfuric acid from dye sulfonation and adjust the effective acid concentration (calculated as H2SO4) in the acidolysis system of the waste sulfuric acid to be controlled at 15 wt%.

[0085] c. Selective acid hydrolysis

[0086] The pretreated waste cotton fibers were mixed with the acid hydrolysis system of sulfonated waste sulfuric acid after acidity adjustment, with a liquid-to-solid ratio of 60:1 mL / g, and the acid hydrolysis reaction was carried out at 80 ℃ for 4 h.

[0087] d. Solid-liquid separation

[0088] The acid-hydrolyzed system is subjected to solid-liquid separation to obtain a solid phase and a liquid or suspended phase; the solid phase is washed, neutralized and dried to obtain a microcrystalline cellulose solid product.

[0089] e. Hydrothermal carbonization

[0090] The liquid or suspended phase is placed in a closed reactor with a filling amount of 60%, a reaction temperature of 160 °C, and a reaction time of 2 h to form sulfur-functionalized carbon quantum dots.

[0091] f. Purification treatment

[0092] The system after the hydrothermal reaction was centrifuged at 12000 rpm for 10 min, and the supernatant was collected. After filtration through a 0.22 μm filter membrane, the solution was adjusted to neutral with 0.1 M NaOH solution. Then, it was dialyzed for 24 h using a dialysis bag with a molecular weight cutoff of 3500 Da to remove Na₂SO₄, residual acid, and small molecule byproducts. The dialyzed carbon quantum dot dispersion was freeze-dried at -50 °C for 48 h to obtain sulfur-functionalized carbon quantum dot powder. The obtained powder could be redispersed in deionized water to obtain a sulfur-functionalized carbon quantum dot dispersion. If necessary, ethanol washing or alcohol precipitation could be used to assist in desalination. Due to the low hydrothermal temperature, the yield of carbon quantum dots was low. Figure 5 The TEM image shows that the distribution of carbon quantum dots is relatively sparse.

[0093] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A method for graded conversion of waste cotton fibers using sulfonated waste sulfuric acid and preparation of sulfur-functionalized carbon quantum dots, characterized in that, Includes the following steps: a. Pretreatment of waste cotton fibers Waste cotton fibers are cleaned, washed and dried to reduce their size to 1-10 mm, thus obtaining pretreated waste cotton fibers. b. Pretreatment and acidity adjustment of sulfonated waste sulfuric acid Remove insoluble impurities from sulfonated waste sulfuric acid and adjust the effective acid concentration (calculated as H2SO4) in the sulfonated waste sulfuric acid acidolysis system to be controlled between 5 wt% and 20 wt%. c. Selective acid hydrolysis The pretreated waste cotton fibers are mixed with the acid hydrolysis system of sulfonated waste sulfuric acid after acidity adjustment, and the acid hydrolysis reaction is carried out under certain temperature and time conditions. d. Solid-liquid separation The acid-hydrolyzed system is subjected to solid-liquid separation to obtain a solid phase and a liquid or suspended phase; The solid phase was washed, neutralized and dried to obtain a microcrystalline cellulose solid product. e. Hydrothermal carbonization The liquid or suspended phase is placed in a closed reactor and hydrothermally treated to form sulfur-functionalized carbon quantum dots. f. Purification treatment The system after the hydrothermal reaction was centrifuged, filtered, neutralized, dialyzed, and dried to obtain sulfur-functionalized carbon quantum dots.

2. The method according to claim 1, characterized in that, The waste cotton fibers include one or more of the following: waste pure cotton fabrics, cotton spinning scraps, waste cotton yarn, waste cotton cloth, and waste home textile cotton fabrics.

3. The method according to claim 1, characterized in that, The sulfonated waste sulfuric acid includes waste acid from the sulfonation process in the production of dyes, dye intermediates, textile auxiliaries, or surfactants.

4. The method according to claim 1, characterized in that, In the selective acid hydrolysis step, the liquid-to-solid ratio of waste cotton fibers to the acid hydrolysis system is 30:1 to 60:1 mL / g.

5. The method according to claim 1, characterized in that, The temperature of the selective acidolysis step is 50–90 °C; the acidolysis time is 0.5–8 h.

6. The method according to claim 1, characterized in that, The reaction temperature of the hydrothermal carbonization step is 180–240°C; the reaction time is 1–10 h; the closed reactor can be a reaction vessel, and the filling amount of the reaction vessel is 20%–80%.

7. The method according to claim 1, characterized in that, In the purification process, the centrifugation speed is 6000-12000 rpm and the centrifugation time is 5-30 min; filtration is carried out using a 0.22-0.45 μm filter membrane; the neutralization filtrate is prepared using a 0.05-0.5 M NaOH solution; dialysis is performed using a dialysis bag with a molecular weight cutoff of 500-5000 Da for 12-48 h; the freeze-drying temperature is -50 ℃ and the time is 48 h; the obtained powder can be redispersed in deionized water to obtain a sulfur-functionalized carbon quantum dot dispersion.

8. The method according to claim 1, characterized in that, The average particle size of the sulfur-functionalized carbon quantum dots is 1–10 nm.

9. The method according to claim 1, characterized in that, The sulfur-functionalized carbon quantum dots contain -C-SO on their surface. x The sulfur-containing functional group, wherein x is 2, 3 or 4.

10. Sulfur-functionalized carbon quantum dots prepared by any one of claims 1 to 9.

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