Heavy metal color developing test paper based on carbon quantum dot composite material as well as preparation method and application of heavy metal color developing test paper

Abstract

The invention discloses heavy metal developing test paper based on a carbon quantum dot composite material as well as a preparation method and application thereof, and belongs to the technical field of heavy metal detection. The heavy metal chromogenic test paper comprises a quantitative filter paper substrate, a composite indicator and a colloid protection layer, wherein the composite indicator is prepared from a silk fibroin-anthocyanin compound, fluorescent carbon-based quantum dots and rich-pore carbon nitride, and is loaded on the substrate through mesoporous silicon balls; and the colloid protection layer is a mixed colloid solution of a masking agent and a sensitizer. The preparation method comprises the following steps: independently preparing the components, constructing the composite indicator, loading the meso-porous silicon balls and coating the protective layer. According to the invention, on-site rapid semi-quantitative detection of cadmium ions and ferric ions in soil or water can be realized, the operation is simple and convenient, the cost is low, and on-site screening requirements are met.

Description

Technical Field

[0001] This invention relates to the field of heavy metal detection technology, specifically to a heavy metal colorimetric test paper based on carbon quantum dot composite material, its preparation method, and its application. Background Technology

[0002] Heavy metal pollution, due to its persistence, bioaccumulation, and high toxicity, has become a significant threat to ecological environment security and human health. Therefore, there is an urgent need to establish an efficient and convenient heavy metal detection technology system.

[0003] Currently, mainstream heavy metal detection technologies such as atomic absorption spectrometry (AAS) and inductively coupled plasma mass spectrometry (ICP-MS) have high detection accuracy, but they rely on large precision instruments and professional technicians for operation. They also have inherent drawbacks such as high detection costs and long analysis times, making them difficult to meet the actual needs of rapid on-site screening.

[0004] To achieve rapid on-site detection, chemical colorimetric test strips have attracted attention due to their advantages such as ease of operation and low cost. However, existing technologies still have significant shortcomings: on the one hand, commonly used colorimetric reagents are mostly artificially synthesized organic reagents, and their characteristics of being difficult to degrade and toxic can easily cause secondary pollution if not handled properly after use; on the other hand, traditional colorimetric test strips generally suffer from problems such as low sensitivity, poor selectivity, and unstable color development, making it difficult to achieve accurate qualitative and semi-quantitative detection of heavy metal ions.

[0005] Therefore, developing a visual rapid test strip for heavy metals that combines the characteristics of simple preparation, environmental friendliness, non-toxicity, rapid and sensitive color development, and strong selectivity is of great technical value and application prospect for solving the pain points of existing detection technologies and improving the efficiency of on-site screening for heavy metal pollution. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the above-mentioned technologies and provide a heavy metal colorimetric test paper based on carbon quantum dot composite material, its preparation method and application, to achieve rapid on-site visual detection of cadmium ions and iron ions, which is environmentally friendly, highly sensitive and has strong anti-interference ability.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A heavy metal colorimetric test paper based on carbon quantum dot composite material, characterized in that it comprises: The substrate is quantitative filter paper; A composite indicator is loaded onto the surface of the substrate via mesoporous silica spheres. The composite indicator is composed of a silk fibroin-anthocyanin complex, fluorescent carbon-based quantum dots, and porous carbon nitride. The silk fibroin-anthocyanin complex is coupled with the fluorescent carbon-based quantum dots and then loaded onto the porous carbon nitride. A colloidal protective layer, wherein the colloidal protective layer is a mixed colloidal solution of a masking agent and a sensitizer, and the colloidal protective layer is applied to the surface of a substrate loaded with a composite indicator.

[0008] A further improvement of the present invention is that the mass ratio of the silk fibroin-anthocyanin complex to the fluorescent carbon-based quantum dots is 1:3.

[0009] A further improvement of the present invention is that the masking agent is selected from one or more combinations of thiourea, potassium sodium tartrate, glycine and hydroxylamine hydrochloride; the sensitizer comprises Triton X-100 and xanthan gum, wherein the mass-to-volume ratio of xanthan gum to Triton X-100 is (0.1-0.3) g / mL; and the mass ratio of the masking agent to the sensitizer is 1:(1-3).

[0010] A method for preparing a heavy metal colorimetric test paper based on carbon quantum dot composite material, characterized by comprising the following steps: (a) Preparation of fluorescent carbon-based quantum dots: Trisodium citrate dihydrate and urea were ground and mixed at a mass ratio of 1:1, pyrolyzed at 180°C for 1 hour, cooled and purified to obtain fluorescent carbon-based quantum dots; (b) Preparation of porous carbon nitride: Sodium chloride and melamine were ground and mixed at a mass ratio of 30:1, calcined at 550°C for 4 hours, cooled and purified to obtain porous carbon nitride. (c) Preparation of silk fibroin-anthocyanin complex: Silk fibroin and cyanidin were mixed at a mass ratio of (480–520):1 to form silk fibroin-anthocyanin complex. (d) Preparation of composite indicator: The fluorescent carbon-based quantum dots obtained in step (a) and the silk fibroin-anthocyanin complex obtained in step (c) are mixed at a mass ratio of 3:1, dissolved in deionized water, and the anthocyanin concentration is adjusted to 0.4 mg / mL and the silk fibroin concentration is 0.2 g / mL. The pH value is adjusted to 4.5 to form a composite indicator solution. Porous carbon nitride obtained in step (b) is added to the composite indicator solution at a mass-volume ratio of 10 mg / mL. The mixture is then subjected to ultrasonication, stirring, centrifugation, washing, drying, and pulverization to prepare the composite indicator. (e) Preparation of a mixed colloidal solution of masking agent and sensitizer: Weigh each component of the masking agent, Triton X-100 and xanthan gum according to a preset ratio, dissolve them in pure water, heat and stir evenly at 60-80℃ to obtain a mixed colloidal solution; the preset ratio is that the mass-volume ratio of Triton X-100 to xanthan gum is (0.1-0.3) g / mL, and the mass ratio of masking agent to sensitizer is 1:(1-3); (f) Preparation of mesoporous silica spheres: Hexadecyltrimethylammonium chloride, triethanolamine and pure water were mixed at a mass ratio of 20:1:200 and stirred at 75°C for 2 h. Tetraethyl orthosilicate was added dropwise and stirring was continued for 3 h. The solid was collected by centrifugation, washed with ethanol, stirred in 1 wt% NaCl / methanol solution for 3 h, centrifuged again and freeze-dried to obtain mesoporous silica spheres. (g) Test strip forming: Weigh the mesoporous silica spheres obtained in step (f) and the composite indicator obtained in step (d) at a mass ratio of (30-35):1, disperse them in an ethanol-water mixed solvent with a volume ratio of 1:1, and sequentially perform stirring, centrifugation, water washing, resolution, and ultrasonic dispersion treatment under a light-protected environment. Filter the ultrasonically dispersed solution using quantitative filter paper to allow the composite indicator to pass through the mesoporous silica spheres and be loaded onto the filter paper. Then, coat the mixed colloidal solution obtained in step (e) as a colloidal protective layer to fix the composite indicator. After drying, cut the solution to obtain the test strip.

[0011] A further improvement of the present invention is that the purification process in step (a) includes: after the pyrolysis product is cooled, it is dissolved in distilled water and centrifuged at 4000 r / min for 15 min; the supernatant is filtered through a 0.22 μm microporous membrane, and the filtrate obtained after filtration is placed in a dialysis bag with a molecular weight cutoff of 1000 and dialyzed for 24 h to remove impurities and obtain a pure fluorescent carbon-based quantum dot solution; the fluorescent carbon-based quantum dot solution is freeze-dried to obtain a powdered fluorescent carbon-based quantum dot.

[0012] A further improvement of the present invention is that the purification process in step (b) includes: after the calcined product is cooled, it is washed several times by centrifugation with distilled water and ethanol to remove residual sodium chloride. After washing, it is dried to obtain a powdered porous carbon nitride.

[0013] A further improvement of the present invention is that the ultrasonication, stirring, centrifugation, washing, drying and pulverization process in step (d) includes: adding porous carbon nitride into the composite indicator solution, ultrasonicating for 10 minutes, and then stirring for 2 hours; after stirring, centrifuging is performed, the precipitated solid is taken, and washed several times with deionized water; the washed product is freeze-dried and pulverized into a powdered composite indicator.

[0014] A further improvement of the present invention is that the stirring, centrifugation, washing, resolution, and ultrasonic dispersion treatment in step (g) includes: dispersing the mesoporous silica spheres and the composite indicator in an ethanol-water mixed solvent, stirring for 32 hours in a light-protected environment, centrifuging at 5000 rpm for 40 minutes after stirring, taking the precipitate, washing it several times with water to remove impurities, and then resolution in ultrapure water; after resolution, ultrasonically dispersing it evenly, and then filtering it using quantitative filter paper in a Buchner funnel.

[0015] An application of a heavy metal colorimetric test strip based on carbon quantum dot composite material is characterized by its use in detecting cadmium ions (Cd²⁺) and ferric ions (Fe³⁺) in soil or water. During detection, the test strip is brought into contact with the sample to be tested, and the semi-quantitative detection of Cd²⁺ and Fe³⁺ in soil or water is achieved by comparing the color change of the test strip with a standard color scale.

[0016] A further improvement of the present invention is that the detection limit of the cadmium ion is 11.9 mg / L.

[0017] The beneficial effects of this invention are as follows: 1. The core raw materials of the composite indicator of this invention, anthocyanins and silk fibroin, are natural products. Fluorescent carbon-based quantum dots (g-CNQDs) are non-toxic, and porous carbon nitride is eco-friendly, avoiding secondary pollution caused by traditional organic colorimetric agents and meeting environmental protection and green requirements.

[0018] 2. This invention introduces fluorescent carbon-based quantum dots (g-CNQDs), which are rich in hydrophilic groups such as hydroxyl, carbonyl / cyano groups on their surface. They have excellent water solubility and reactivity, which can enhance the photoluminescence performance of the composite indicator and amplify the colorimetric signal. At the same time, they can assist in the identification of heavy metal ions through their own optical properties, improve the accuracy of low-concentration ion detection, achieve high-sensitivity detection of the test strip, shorten the color development time by about 10%, and reduce the detection limit of cadmium ions to as low as 11.9 mg / L, which can meet the needs of rapid on-site screening.

[0019] 3. The porous carbon nitride in this invention achieves efficient loading and uniform dispersion of fluorescent carbon-based quantum dots and silk fibroin-anthocyanin complex through its porous structure, which not only ensures the full exposure of the indicator active sites, but also improves the stability and anti-interference ability of the composite system.

[0020] 4. The masking agent in the colloidal protective layer of this invention can effectively shield common interfering ions such as sodium, calcium, and barium in soil or water, ensuring the specificity of cadmium ion and ferric ion detection; Triton X-100 in the sensitizer can reduce the interfacial tension of the system, promote the uniform dispersion of the composite indicator and the contact reaction between heavy metal ions and the indicator, and shorten the color development time; xanthan gum can adjust the colloidal viscosity, so that the protective layer is evenly coated on the surface of the test paper, which not only fixes the composite indicator to prevent it from falling off or oxidizing, but also maintains the air permeability and sample permeability of the test paper. The combination of the two not only improves the color development sensitivity and detection efficiency of the test paper, but also enhances the storage stability of the test paper, providing a reliable guarantee for rapid semi-quantitative detection on site.

[0021] 5. The mesoporous silica spheres of this invention, with their unique porous structure, provide ample and uniform loading sites for the composite indicator, effectively preventing indicator particle aggregation and ensuring its uniform distribution on the surface of the quantitative filter paper substrate. This results in a more complete colorimetric reaction and a more uniform colorimetric effect. Simultaneously, their excellent adsorption properties stably immobilize the composite indicator, preventing it from falling off or leaking during detection, thus improving the storage and use stability of the test paper. Furthermore, the mesoporous structure facilitates the rapid diffusion of heavy metal ions from soil or water to the active sites of the indicator, shortening the reaction time. The composite indicator, dually immobilized by the mesoporous silica spheres and colloidal protective layer, is stable in air and can be stored for a long time. Moreover, detection requires no additional buffer solution, further reducing usage costs and environmental impact.

[0022] 6. The preparation process of this invention does not require complex and precision equipment, and the steps are simple and controllable; no professional technicians are required to operate during detection. The test strip can develop color within 3 minutes after contacting the sample. Semi-quantitative detection can be achieved by comparing with the standard color scale, which is suitable for on-site rapid screening scenarios. Attached Figure Description

[0023] Figure 1 The diagram shows the experimental products of the present invention, wherein (a) is porous carbon nitride, (b) is silk fibroin-anthocyanin complex, (c) is fluorescent carbon-based quantum dot, (d) is composite indicator, (e) is a uniform suspension of composite indicator and mesoporous silica spheres, (f) is a colloidal solution of masking agent and sensitizer, (g) is mesoporous silica spheres, and (h) is the appearance of the test strip. Figure 2 This is a schematic diagram showing the color development effect of the composite indicator on different metal ions. It shows that the composite indicator has a specific color development effect only on cadmium ions (blue) and ferric ions (pale yellow), and has no obvious color development reaction on sodium, calcium and barium ions. Figure 3 The image shows a SEM image of carbon-based fluorescent quantum dot powder, revealing that the quantum dots have a bulk structure and are composed of stacked layered structures. Figure 4SEM images comparing the silk fibroin-anthocyanin complex before and after coupling with carbon-based fluorescent quantum dots, where (A) is the composite indicator without coupled fluorescent carbon-based quantum dots and (B) is the composite indicator with coupled fluorescent carbon-based quantum dots, confirming the coupling of fluorescent carbon-based quantum dots with the silk fibroin-anthocyanin complex. Figure 5 The diagram shows a comparison of the color development effects of the blank group and the experimental group on cadmium metal solution, demonstrating that the color development effect of the experimental group coupled with fluorescent carbon-based quantum dots is better than that of the uncoupled blank group. Figure 6 The diagram shows a comparison of the color development effects of the blank group and the experimental group on the trivalent iron metal solution, illustrating the promoting effect of coupled fluorescent carbon-based quantum dots on the color development effect. Figure 7 The graph shows the relationship between absorbance and metal ion concentration, indicating a good linear relationship between absorbance and concentration when cadmium ion concentration is in the range of 10-60 mg / L. Figure 8 The diagram illustrates the color development process in an interference experiment, showing that even in the presence of copper, calcium, sodium, potassium, and barium, the ferric metal solution can still develop normal color after the addition of a masking agent. Detailed Implementation

[0024] The technical solution of the present invention will be further described in detail below through specific embodiments and with reference to the accompanying drawings: Example 1

[0025] This embodiment provides a heavy metal colorimetric test paper based on carbon quantum dot composite materials, and its preparation process is as follows: (a) Preparation of fluorescent carbon-based quantum dots: 10 g each of trisodium citrate dihydrate and urea were weighed in equal mass ratio, ground into powder in a mortar, and mixed evenly. The mixture was then transferred to a crucible and pyrolyzed in a muffle furnace at 180 °C for 1 h. After cooling, a brownish-black solid powder was obtained and dissolved in distilled water. The solution was centrifuged at 4000 r / min for 15 min, and the supernatant was filtered through a 0.22 μm microporous membrane. The filtrate was dialyzed in a dialysis bag for 24 h, and then freeze-dried to obtain fluorescent carbon-based quantum dot powder. The product is brownish-black, as shown in the schematic diagram below. Figure 1 As shown in (c); its SEM image is as follows: Figure 3 As shown, the whole structure is blocky, and the interior is composed of stacked layers.

[0026] (b) Preparation of porous carbon nitride: 30g of sodium chloride and 1g of melamine were weighed, thoroughly ground and mixed, and then placed in a muffle furnace. The temperature was increased to 550℃ at 2℃ / min and calcined for 4 hours. After cooling, the mixture was washed 5 times with distilled water and 2 times with ethanol. The residual sodium chloride was removed by centrifugation, and the product was dried to obtain porous carbon nitride powder. It is a pale yellow to orange-yellow powder with a clear porous structure. A schematic diagram of the product is shown below. Figure 1 As shown in (a).

[0027] (c) Preparation of the silk fibroin-anthocyanin complex: Weigh 1g of silk fibroin and 0.002g of cyanidin, and mix to form the silk fibroin-anthocyanin complex. The product is purplish-red, as shown in the schematic diagram. Figure 1 As shown in (b).

[0028] (d) Preparation of the composite indicator: Take 3g of the fluorescent carbon-based quantum dot powder prepared in step (a), mix it with the silk fibroin-anthocyanin complex from step (c), dissolve it in 5ml of deionized water, and adjust the pH to 4.5 with acetic acid and sodium hydroxide. Add 50mg of the porous carbon nitride prepared in step (b) to the solution, sonicate for 10min, stir for 2h, centrifuge to collect the precipitate, wash it three times with deionized water, freeze-dry it, and then pulverize it to obtain the composite indicator powder. The product is purplish-gray, and a schematic diagram of the product is shown below. Figure 1 As shown in (d); its SEM image is as follows. Figure 4 As shown in (B), fluorescent carbon-based quantum dots can be observed to be successfully coupled to the surface of the silk fibroin-anthocyanin complex, and to the uncoupled complex ( Figure 4 (A) shows a clear difference.

[0029] (e) Preparation of a mixed colloidal solution of masking agent and sensitizer: Weigh 1g each of thiourea, potassium sodium tartrate, glycine, and hydroxylamine hydrochloride, add 10mL of Triton X-100 and 100mg of xanthan gum, dissolve in 90mL of ultrapure water, heat and stir at 70℃ until homogeneous to obtain a mixed colloidal solution. The total mass of the masking agent is 4g, and the total mass of the sensitizer is 10.8g, with a mass ratio of 1:2.7. The product is a transparent viscous substance, as shown in the schematic diagram below. Figure 1 As shown in (f).

[0030] (f) Preparation of mesoporous silica spheres: Weigh 2g of hexadecyltrimethylammonium chloride and 0.1g of triethanolamine, dissolve them in 20mL of pure water (i.e., 20g of pure water), stir at 75℃ for 2h, add 1.5mL of tetraethyl orthosilicate dropwise, and continue stirring for 3h. Collect the solid by centrifugation, wash three times with ethanol, stir in 1wt% NaCl / methanol solution for 3h, centrifuge again, and freeze-dry to obtain mesoporous silica spheres. The product is a white powder, as shown in the schematic diagram below. Figure 1 As shown in (g).

[0031] (g) Test strip preparation: Weigh 50 mg of the mesoporous silica spheres prepared in step (f) and 1.5 mg of the composite indicator prepared in step (d), add them to a mixed solvent of 5 mL ethanol and 5 mL water, stir in the dark for 32 h, centrifuge at 5000 rpm for 40 min, wash the precipitate three times with water, redissolve it in 10 mL of ultrapure water, and ultrasonically disperse it evenly. Filter the dispersion through a Buchner funnel onto quantitative filter paper, spread it onto the mixed colloidal solution from step (e), dry it, and cut it into 1 cm × 3 cm test strips to obtain the target heavy metal colorimetric test strip. The product's appearance is as follows... Figure 1 As shown in (h), the surface is smooth and there is no obvious particle agglomeration. Example 2

[0032] This embodiment provides a colorimetric test paper for heavy metals without coupled fluorescent carbon-based quantum dots, and its preparation process is as follows: (a) Preparation of fluorescent carbon-based quantum dots: Since fluorescent carbon-based quantum dots are not coupled in this embodiment, the preparation steps of fluorescent carbon-based quantum dots are omitted.

[0033] (b) Preparation of porous carbon nitride: same as step (b) in Example 1.

[0034] (c) Preparation of silk fibroin-anthocyanin complex: Same as step (c) in Example 1.

[0035] (d) Preparation of composite indicator: without adding fluorescent carbon-based quantum dots, the composite indicator was constructed only with silk fibroin-anthocyanin complex and porous carbon nitride. The remaining steps were the same as step (d) in Example 1.

[0036] (e) Preparation of the mixed colloidal solution of masking agent and sensitizer: Same as step (e) in Example 1.

[0037] (f) Preparation of mesoporous silica spheres: Same as step (f) in Example 1.

[0038] (g) Test strip formation: Same as step (g) in Example 1, to obtain a test strip without coupled fluorescent carbon-based quantum dots. Test case

[0039] The heavy metal colorimetric test strip prepared in Example 1 was used as the experimental group, and the heavy metal colorimetric test strip prepared in Example 2 was used as the blank group for testing.

[0040] (1) Colorimetric specificity test Standard solutions of cadmium ions, ferric ions, sodium ions, calcium ions, and barium ions (all at a concentration of 1 mg / mL) were taken and contacted with the test strips prepared in the experimental group, respectively, and allowed to stand for 3 minutes at pH=5.0. The results are as follows: Figure 2As shown, the test strip only shows a blue color for cadmium ions and a pale yellow color for ferric ions, and no obvious color reaction for sodium ions, calcium ions, and barium ions, indicating that it has good specificity.

[0041] (2) Color development time and sensitivity test Cadmium ion solution and ferric ion solution with a concentration of 0.5 mg / mL were respectively brought into contact with the test strips prepared in the experimental group, and the color development and stabilization time were recorded. The results showed that the color development and stabilization time of cadmium ion was 2.92 min, and that of ferric ion was 3.15 min. Compared with the blank group without carbon-based fluorescent quantum dots (cadmium ion color development time 3.28 min, ferric ion color development time 3.45 min), the color development time was shortened by about 10%. Figure 5 , Figure 6 As shown, the color development effect of the experimental group was better than that of the blank group.

[0042] (3) Detection limit test Cadmium ion solutions with concentrations of 10, 20, 30, 40, 50, and 60 mg / L were prepared. After contacting the test strips prepared in the experimental group, the absorbance was measured at a wavelength of 530 nm. The relationship between absorbance and concentration was plotted as follows: Figure 7 As shown in the figure. The results showed that within the cadmium ion concentration range of 10-60 mg / L, the absorbance and concentration had a good linear relationship (linear equation y=0.0003x+0.0246, correlation coefficient R²=0.9996), and the detection limit was 11.9 mg / L, which meets the requirements for rapid on-site screening.

[0043] (4) Anti-interference test Equal concentrations of copper, calcium, sodium, potassium, and barium ions were added to a ferric ion solution (1 mg / mL). This solution was then brought into contact with the test strips prepared in the experimental group and allowed to stand for 5 minutes. The results are as follows: Figure 8 As shown, the test strip can still develop color normally after the masking agent is added, with no obvious interference, indicating good anti-interference performance.

[0044] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A heavy metal colorimetric test paper based on carbon quantum dot composite material, characterized in that, include: The substrate is quantitative filter paper; A composite indicator is loaded onto the surface of the substrate via mesoporous silica spheres. The composite indicator is composed of a silk fibroin-anthocyanin complex, fluorescent carbon-based quantum dots, and porous carbon nitride. The silk fibroin-anthocyanin complex is coupled with the fluorescent carbon-based quantum dots and then loaded onto the porous carbon nitride. A colloidal protective layer, wherein the colloidal protective layer is a mixed colloidal solution of a masking agent and a sensitizer, and the colloidal protective layer is applied to the surface of a substrate loaded with a composite indicator.

2. The heavy metal colorimetric test paper based on carbon quantum dot composite material according to claim 1, characterized in that, The mass ratio of the silk fibroin-anthocyanin complex to the fluorescent carbon-based quantum dots is 1:

3.

3. The heavy metal colorimetric test paper based on carbon quantum dot composite material according to claim 1, characterized in that, The masking agent is selected from one or more combinations of thiourea, potassium sodium tartrate, glycine and hydroxylamine hydrochloride; the sensitizer comprises Triton X-100 and xanthan gum, wherein the mass-to-volume ratio of xanthan gum to Triton X-100 is (0.1-0.3) g / mL; the mass ratio of the masking agent to the sensitizer is 1:(1-3).

4. A method for preparing a heavy metal colorimetric test paper based on carbon quantum dot composite material as described in any one of claims 1-3, characterized in that, Includes the following steps: (a) Preparation of fluorescent carbon-based quantum dots: Trisodium citrate dihydrate and urea were ground and mixed at a mass ratio of 1:1, pyrolyzed at 180°C for 1 hour, cooled and purified to obtain fluorescent carbon-based quantum dots; (b) Preparation of porous carbon nitride: Sodium chloride and melamine were ground and mixed at a mass ratio of 30:1, calcined at 550°C for 4 hours, cooled and purified to obtain porous carbon nitride. (c) Preparation of silk fibroin-anthocyanin complex: Silk fibroin and cyanidin were mixed at a mass ratio of (480–520):1 to form silk fibroin-anthocyanin complex. (d) Preparation of composite indicator: The fluorescent carbon-based quantum dots obtained in step (a) and the silk fibroin-anthocyanin complex obtained in step (c) are mixed at a mass ratio of 3:1, dissolved in deionized water, and the anthocyanin concentration is adjusted to 0.4 mg / mL and the silk fibroin concentration is 0.2 g / mL. The pH value is adjusted to 4.5 to form a composite indicator solution. Porous carbon nitride obtained in step (b) is added to the composite indicator solution at a mass-volume ratio of 10 mg / mL. The mixture is then subjected to ultrasonication, stirring, centrifugation, washing, drying, and pulverization to prepare the composite indicator. (e) Preparation of a mixed colloidal solution of masking agent and sensitizer: Weigh each component of the masking agent, Triton X-100 and xanthan gum according to a preset ratio, dissolve them in pure water, heat and stir evenly at 60-80℃ to obtain a mixed colloidal solution; the preset ratio is that the mass-volume ratio of Triton X-100 to xanthan gum is (0.1-0.3) g / mL, and the mass ratio of masking agent to sensitizer is 1:(1-3); (f) Preparation of mesoporous silica spheres: Hexadecyltrimethylammonium chloride, triethanolamine and pure water were mixed at a mass ratio of 20:1:200 and stirred at 75°C for 2 h. Tetraethyl orthosilicate was added dropwise and stirring was continued for 3 h. The solid was collected by centrifugation, washed with ethanol, stirred in 1 wt% NaCl / methanol solution for 3 h, centrifuged again and freeze-dried to obtain mesoporous silica spheres. (g) Test strip forming: Weigh the mesoporous silica spheres obtained in step (f) and the composite indicator obtained in step (d) at a mass ratio of (30-35):1, disperse them in an ethanol-water mixed solvent with a volume ratio of 1:1, and sequentially perform stirring, centrifugation, water washing, resolution, and ultrasonic dispersion treatment under a light-protected environment. Filter the ultrasonically dispersed solution using quantitative filter paper to allow the composite indicator to pass through the mesoporous silica spheres and be loaded onto the filter paper. Then, coat the mixed colloidal solution obtained in step (e) as a colloidal protective layer to fix the composite indicator. After drying, cut the solution to obtain the test strip.

5. The method for preparing heavy metal colorimetric test paper based on carbon quantum dot composite material according to claim 4, characterized in that, The purification process in step (a) includes: after the pyrolysis product is cooled, it is dissolved in distilled water and centrifuged at 4000 r / min for 15 min; the supernatant is filtered through a 0.22 μm microporous membrane, and the filtrate is placed in a dialysis bag with a molecular weight cutoff of 1000 and dialyzed for 24 h to remove impurities and obtain a pure fluorescent carbon-based quantum dot solution; the fluorescent carbon-based quantum dot solution is freeze-dried to obtain powdered fluorescent carbon-based quantum dots.

6. The method for preparing heavy metal colorimetric test paper based on carbon quantum dot composite material according to claim 4, characterized in that, The purification process in step (b) includes: after cooling the calcined product, washing it several times by centrifugation with distilled water and ethanol to remove residual sodium chloride, and drying it after washing to obtain a powdered porous carbon nitride.

7. The method for preparing heavy metal colorimetric test paper based on carbon quantum dot composite material according to claim 4, characterized in that, The ultrasonic, stirring, centrifugation, washing, drying and pulverizing processes in step (d) include: adding porous carbon nitride into the composite indicator solution, ultrasonicating for 10 min, and then stirring for 2 h; after stirring, centrifuging is performed, the precipitated solid is taken, and washed several times with deionized water; the washed product is freeze-dried and pulverized into a powdered composite indicator.

8. The method for preparing heavy metal colorimetric test paper based on carbon quantum dot composite material according to claim 4, characterized in that, The stirring, centrifugation, washing, resolution, and ultrasonic dispersion treatment in step (g) include: dispersing mesoporous silica spheres and composite indicator in an ethanol-water mixed solvent, stirring for 32 hours in a dark environment, centrifuging at 5000 rpm for 40 minutes after stirring, taking the precipitate, washing it several times with water to remove impurities, and then resolution in ultrapure water; after resolution, ultrasonically dispersing it evenly, and then filtering it using quantitative filter paper in a Buchner funnel.

9. An application of the heavy metal colorimetric test paper based on carbon quantum dot composite material as described in any one of claims 1-3, characterized in that, This test strip is used to detect cadmium and ferric ions in soil or water. During testing, the test strip is brought into contact with the sample to be tested, and the color change of the test strip is compared with the standard color scale to achieve semi-quantitative detection of cadmium and ferric ions in soil or water.

10. The application of the heavy metal colorimetric test paper based on carbon quantum dot composite material according to claim 9, characterized in that, The detection limit for cadmium ions was 11.9 mg / L.

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