Solid multicolor carbon dots@MOF sensor and preparation method and application thereof

Abstract

The application provides a solid-state multi-color carbon dot@MOF sensor and a preparation method and application thereof, belongs to the technical field of heavy metal ion detection, and is used for solving the problems of poor anti-interference performance and selectivity of existing fluorescent sensors. The application comprises the following steps: mixing nitrogen-containing natural polysaccharide, carboxyl-containing polysaccharide and water, performing stepwise temperature rising polymerization under nitrogen protection by using a hydrothermal method to obtain carbon dots; mixing rare earth salt, polycarboxylic acid ligand, a first solvent and hollow silica template, performing ultrasonic treatment, mixing the obtained solid precipitate with an aqueous hydrogen fluoride solution to dissolve the hollow silica template, and obtaining a rare earth-MOF material; and encapsulating the carbon dots and a red light-emitting fluorescent dye in the rare earth-MOF material to obtain the solid-state multi-color carbon dot@MOF sensor. The solid-state multi-color carbon dot@MOF sensor has high sensitivity, high selectivity and excellent anti-interference performance, and can realize naked-eye visual semi-quantitative detection.

Description

Technical Field

[0001] This invention relates to the field of heavy metal ion detection technology, specifically to a solid-state multicolor carbon dot@MOF sensor, its preparation method, and its application. Background Technology

[0002] Developing highly sensitive and selective heavy metal detection technologies is of great significance. Currently, fluorescence sensing technology is widely used for heavy metal detection due to its advantages such as ease of operation, rapid response, and low cost. Among them, carbon dots (CDs), as an emerging fluorescent nanomaterial, have the characteristics of good water solubility, high biocompatibility, and tunable fluorescence. However, carbon dots are prone to fluorescence quenching (i.e., "aggregation quenching effect") in the solid or aggregated state, which limits their application in solid-state sensors. In addition, traditional fluorescence sensors are mostly based on a single fluorescence signal, which is easily affected by environmental factors (such as light source fluctuations and pH changes), leading to a decrease in detection accuracy. At the same time, most sensors lack selectivity for specific heavy metal ions, making it difficult to achieve accurate detection in real-world complex water samples. Summary of the Invention

[0003] The technical problem to be solved by this invention is to provide a solid-state multicolor carbon dot@MOF sensor, its preparation method and application, and the solid-state multicolor carbon dot@MOF sensor for Pb 2+ Cd 2+ Hg 2+ It features high sensitivity, high selectivity, and excellent anti-interference performance, and can achieve naked-eye visual semi-quantitative detection, meeting the need for rapid detection of trace heavy metal ions in environmental water bodies.

[0004] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0005] In a first aspect, the present invention provides a method for fabricating a solid-state multicolor carbon dot@MOF sensor, comprising:

[0006] Step (1): Nitrogen-containing natural polysaccharide, carboxyl-containing polysaccharide and water are mixed and polymerized by step heating under nitrogen protection using a hydrothermal method to obtain carbon dots;

[0007] Step (2): Mix rare earth salt, polycarboxylic acid ligand, first solvent and hollow silica template, sonicate, mix the obtained solid precipitate with hydrofluoric acid aqueous solution, dissolve hollow silica template, and obtain rare earth-MOF material;

[0008] Step (3): The carbon dots are mixed with the rare earth-MOF material and calcined under nitrogen protection. The carbon dots are embedded in the pores of the rare earth-MOF material under nitrogen protection to obtain carbon dots@rare earth-MOF material.

[0009] Step (4): Mix the carbon dot@rare earth-MOF material, the red-emitting fluorescent dye, and the second solvent, and perform solution diffusion adsorption so that the red-emitting fluorescent dye diffuses and adsorbs into the pores of the rare earth-MOF material to obtain a solid-state multicolor carbon dot@MOF sensor.

[0010] Optionally, in step (1), the nitrogen-containing natural polysaccharide is at least one of chitin, cellulose amine derivative, and chitosan, and the carboxyl-containing polysaccharide is sodium alginate and / or carboxymethyl cellulose;

[0011] The mass ratio of the nitrogen-containing natural polysaccharide to the carboxyl-containing polysaccharide is 2-3:1.

[0012] Optionally, in step (1), the stepped heating includes:

[0013] In the first stage, the reaction system is heated from room temperature to 120-130℃ at a rate of 3-4℃ / min, and the nitrogen flow rate is 50-60ml / min for 5-8min.

[0014] In the second stage, the temperature is increased from 120-130℃ to 180-200℃ at a rate of 2-3℃ / min, with a nitrogen flow rate of 100-110ml / min and a duration of 5-8min.

[0015] In the third stage, maintain a temperature of 180-200℃ and a nitrogen flow rate of 50-60 ml / min for 10-15 minutes.

[0016] Optionally, in step (2), the rare earth salt is a cerium salt;

[0017] The polycarboxylic acid ligand is at least one of biphenyl-3,3',5,5'-tetracarboxylic acid, pyromellitic acid, and 2,6-naphthalenedicarboxylic acid.

[0018] Optionally, in step (2), the molar ratio of the rare earth salt to the polycarboxylic acid ligand is 1:1-1.2.

[0019] Optionally, in step (3), the mass ratio of the carbon dots to the rare earth-MOF material is 1:10-20.

[0020] Optionally, in step (4), the fluorescent dye capable of emitting red light is at least one of Nile Red, Rhodamine B, and Rhodamine 6G.

[0021] Optionally, in step (4), the mass ratio of the carbon dot@rare earth-MOF material to the fluorescent dye that emits red light is 100-110:1.

[0022] Secondly, the present invention provides a solid-state multicolor carbon dot@MOF sensor prepared by the above-mentioned method for preparing a solid-state multicolor carbon dot@MOF sensor.

[0023] Thirdly, this invention also provides the above-mentioned solid-state multicolor carbon dot@MOF sensor for detecting heavy metal ions Pb. 2+ or Cd 2+ or Hg 2+ Applications in [the field].

[0024] The above-described solution of the present invention has at least the following beneficial effects:

[0025] The above-described solution of the present invention introduces rare earth elements into MOF materials, forming a rare earth-MOF material with a rigid framework structure, which can effectively prevent the molecular aggregation of carbon dots and red-emitting fluorescent dyes, and effectively prevent fluorescence quenching caused by aggregation. The rare earth-MOF material has molecular sieving and pre-enrichment functions, enhancing the selective adsorption of target heavy metals. Encapsulating carbon dots and red-emitting fluorescent dyes in the rare earth-MOF material forms a dual-emission system. By measuring the ratio of the two emission peaks as a signal, common interference factors can be canceled. Heavy metals interact specifically with carbon dots or red-emitting fluorescent dyes, resulting in differential changes in their fluorescence intensity, achieving visualized semi-quantitative detection. Attached Figure Description

[0026] Figure 1 This is an XPS image of the carbon dot CAB-CDs prepared in Example 1 of this invention;

[0027] Figure 2 This is the XPS image of the Ce-MOF prepared in Example 1 of this invention;

[0028] Figure 3 This is an XPS image of CAB-CDs@Ce-MOF / NR prepared in Example 1 of this invention. Detailed Implementation

[0029] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0030] Existing carbon dot-based fluorescence sensors, especially composites encapsulating carbon dots in organic framework materials (such as MOFs), typically employ a single fluorescence signal (e.g., changes in the fluorescence intensity of the carbon dot) or a single dye as an energy acceptor. These technologies fail to effectively address the fluorescence quenching problem in solid-state applications of carbon dots. Specifically, carbon dots readily undergo π-π stacking or aggregation in solid-state or high-concentration conditions, leading to fluorescence quenching and reduced sensor sensitivity and stability. While existing technologies attempt to encapsulate carbon dots with polymers or matrices, aggregation cannot be completely avoided and may affect the fluorescence performance of the carbon dots. Furthermore, existing technologies lack ratiometric self-calibration capabilities, resulting in poor anti-interference capabilities. Additionally, the selectivity of these sensors for heavy metal ions often depends on the pore size sieving of the MOF, but they often cannot distinguish between multiple ions (such as Pb). 2+ Cd 2+ Hg 2+ Furthermore, the lack of specific interaction mechanisms results in poor anti-interference capabilities; the visualization effect is limited, with most sensors unable to achieve obvious color changes, relying on instrument readings, which makes it difficult to meet the needs of rapid on-site detection or naked-eye qualitative judgment.

[0031] Based on this, in a first aspect, the present invention provides a method for fabricating a solid-state multicolor carbon dot@MOF sensor, comprising:

[0032] Step (1): Nitrogen-containing natural polysaccharide, carboxyl-containing polysaccharide and water are mixed and polymerized by step heating under nitrogen protection using a hydrothermal method to obtain carbon dots;

[0033] Step (2): Mix rare earth salt, polycarboxylic acid ligand, first solvent and hollow silica template, sonicate, mix the obtained solid precipitate with hydrofluoric acid aqueous solution, dissolve hollow silica template, and obtain rare earth-MOF material;

[0034] Step (3): The carbon dots are mixed with the rare earth-MOF material and calcined under nitrogen protection. The carbon dots are embedded in the pores of the rare earth-MOF material under nitrogen protection to obtain carbon dots@rare earth-MOF material.

[0035] Step (4): Mix the carbon dot@rare earth-MOF material, the red-emitting fluorescent dye, and the second solvent, and perform solution diffusion adsorption so that the red-emitting fluorescent dye diffuses and adsorbs into the pores of the rare earth-MOF material to obtain a solid-state multicolor carbon dot@MOF sensor.

[0036] The solid-state multicolor carbon dot@MOF sensor prepared by the method of this invention can detect heavy metal ions (Pb) by means of changes in fluorescence color. 2+ Cd 2+ Hg 2+The principle behind its fluorescence color change is as follows: The solid-state multicolor carbon dot@MOF sensor itself is red fluorescence (carbon dots are green fluorescence, and fluorescent dyes that emit red light are red fluorescence; in the absence of heavy metal ion interference, the red fluorescence is significantly brighter than the green fluorescence, causing the sensor itself to display red fluorescence). When it reacts with Hg... 2+ Upon contact, Hg 2+ When it reacts with a fluorescent dye that emits red light, it enhances red fluorescence; when it reacts with Cd... 2+ Upon contact, Cd 2+ It interacts with carbon dots to enhance their green light; when it interacts with Pb... 2+ Upon contact, Pb 2+ It reacts simultaneously with carbon dots and red-emitting fluorescent dyes, quenching them both, but the red-light quenching is more significant. Based on this principle, qualitative and quantitative detection of heavy metal ions can be performed.

[0037] Specifically, the solid-state multicolor carbon dot@MOF sensor prepared by the method of the present invention has the following advantages:

[0038] (1) Solving the problem of solid-state fluorescence quenching of carbon dots:

[0039] This invention encapsulates carbon dots within the rigid channels of a rare-earth-metal-magnetic-foil (MOF) structure. High-temperature calcination embeds the carbon dots into the MOF, with the carbon dots acting as energy donors (green light, 550 nm). Simultaneously, a red-emitting fluorescent dye (e.g., Nile Red, red light, 620 nm) is introduced as an energy acceptor, both encapsulated within the rare-earth-MOF channels. The rare-earth-MOF provides a rigid framework with a highly ordered mesoporous structure (average pore size 3.0-3.5 nm). This rigid framework effectively prevents molecular aggregation of carbon dots and the red-emitting fluorescent dye, avoiding fluorescence quenching caused by π-π stacking. The stable microenvironment of the rare-earth-MOF also protects the carbon dots, allowing them to maintain high fluorescence quantum yield in the solid state. Thermogravimetric analysis (TGA) confirms that the solid-state multicolor carbon dot@MOF sensor maintains framework integrity at 300°C, ensuring the sensor's thermal stability.

[0040] (2) Achieve ratio-based self-calibration and overcome environmental interference:

[0041] The sensor employs a dual-emission system consisting of a carbon dot (donor, emission peak at 550 nm) and a red-emitting fluorescent dye (acceptor, such as Nile Red, emission peak at 620 nm). The fluorescence intensity ratio (R=I) is measured. 550 / I 620Quantitative detection is performed using ratiometric fluorescence detection, which utilizes the ratio of two emission peaks as a signal to cancel out common interference factors (such as light source fluctuations and sensor concentration changes). When heavy metal ions enter the rare earth-MOF channels, they undergo specific interactions with carbon dots or red-emitting fluorescent dyes (such as coordination and electron transfer), leading to differential changes in fluorescence intensity (e.g., Hg). 2+ Enhance the fluorescence of red-emitting fluorescent dyes, Cd 2+ Enhanced carbon dot fluorescence, Pb 2+ It can simultaneously quench carbon dots and red-emitting fluorescent dyes to varying degrees, and distinguish between different ions by monitoring their ratio fluorescence changes; it can also measure the fluorescence intensity ratio R (e.g., I0). 550 / I 620 ), and based on standard curves established for different ions, quantitative analysis of heavy metal ions is achieved; by establishing R values ​​(I 550 / I 620 The standard curve of ion concentration is used to achieve self-calibration detection.

[0042] (3) Enhance selectivity and differentiation capabilities:

[0043] Rare earth-MOFs not only serve as carriers, but their porous structure also provides size sieving and pre-enrichment functions. Furthermore, the functional groups of carbon dots and red-emitting fluorescent dyes (such as amino and carboxyl groups) selectively coordinate with specific heavy metal ions, enhancing the selective adsorption of target heavy metals. XPS analysis shows that Ce in the composite material... 3+ / Ce 4+ A suitable ratio is beneficial for forming coordination bonds with heavy metal ions. Different ions interact differently with carbon dots or red-emitting fluorescent dyes due to differences in size and electronegativity: Hg 2+ It preferentially interacts with fluorescent dyes that emit red light, enhancing the red light; Cd 2+ It interacts with carbon dots to enhance green light; Pb 2+ Simultaneous quenching occurs, but red light quenching is more significant. This differential response enables the differentiation of multiple ions, and the type of heavy metal ions can be determined by the change in fluorescence color.

[0044] (4) Achieve multi-color visual detection:

[0045] The sensor achieves visual qualitative judgment by detecting changes in fluorescence color; heavy metal ions cause a change in the fluorescence intensity ratio of carbon dots and red-emitting fluorescent dyes, resulting in an overall shift in fluorescence color. For example, Hg 2+ To change the color to red, Cd 2+ To make the color turn yellowish-green, Pb 2+It changes the color from red to green. This color change is visible to the naked eye under ultraviolet light (365nm), enabling visual color change and supporting qualitative judgment without the naked eye. It is suitable for the development of test strips or portable sensors.

[0046] For example, in step (1), the nitrogen-containing natural polysaccharide is at least one of chitin, cellulose amine derivative, and chitosan, and the carboxyl-containing polysaccharide is sodium alginate and / or carboxymethyl cellulose;

[0047] Optionally, the mass ratio of the nitrogen-containing natural polysaccharide to the carboxyl-containing polysaccharide is 2-3:1.

[0048] For example, in step (1), the volume ratio of water to the total mass of nitrogen-containing natural polysaccharides and carboxyl-containing polysaccharides is 50 mL: 1 g.

[0049] To improve the stability of carbon points, step (1) of this invention employs a stepped heating method, which includes:

[0050] In the first stage, the reaction system is heated from room temperature to 120-130℃ at a rate of 3-4℃ / min, and the nitrogen flow rate is 50-60ml / min for 5-8min.

[0051] In the second stage, the temperature is increased from 120-130℃ to 180-200℃ at a rate of 2-3℃ / min, with a nitrogen flow rate of 100-110ml / min and a duration of 5-8min.

[0052] In the third stage, maintain a temperature of 180-200℃ and a nitrogen flow rate of 50-60 ml / min for 10-15 minutes.

[0053] For example, step (1) also includes a purification process for the carbon dots, including: cooling the product after the hydrothermal reaction, adding ultrapure water to dilute it, removing the large particle aggregates present by centrifugation, collecting the supernatant, filtering it with a 0.22µm microporous membrane and dialyzing it with a 1000Da dialysis bag for 24 hours (changing the water every 8 hours), and concentrating it to obtain a fluorescent carbon dot solution, which is used as an energy donor for subsequent steps.

[0054] For example, the centrifugation speed is 8000-9000 rpm and the time is 15-20 min.

[0055] For example, in step (2), the rare earth salt is a cerium salt.

[0056] For example, in step (2), the rare earth salt is a rare earth nitrate. Further, the rare earth salt is cerium nitrate.

[0057] For example, the polycarboxylic acid ligand is at least one of biphenyl-3,3',5,5'-tetracarboxylic acid, pyromellitic acid, and 2,6-naphthalenedicarboxylic acid.

[0058] For example, in step (2), the molar ratio of the rare earth salt to the polycarboxylic acid ligand is 1:1-1.2.

[0059] For example, in step (2), the first solvent is N,N-dimethylformamide (DMF).

[0060] For example, in step (2), the ratio of the volume of the first solvent to the total moles of rare earth salt and polycarboxylic acid ligand is 20-40 mL: 1 mmol.

[0061] For example, in step (2), the ratio of the mass of the hollow silica template to the total mass of the rare earth salt and polycarboxylic acid ligand is 0.8-2:1.

[0062] For example, in step (2), rare earth salts and polycarboxylic acid ligands are first dissolved in the first solvent and stirred for 20-30 minutes to form a precursor solution. Then, a hollow silica template is added and gently stirred for 10-15 minutes to ensure that the hollow silica template shell is fully wetted by the precursor solution. At this time, the precursor will be drawn into the cavity inside the shell through capillary action and attached to the surface of the shell to form a mixed suspension. The mixed suspension is then subjected to ultrasonic treatment.

[0063] For example, in step (2), the ultrasonic treatment temperature is 25-30℃, the frequency is 20-30kHz, and the power density is 500-600W / cm². 2 It adopts a pulse mode (working for 2 seconds, with a 1-second interval), and the total time is 5-6 minutes.

[0064] For example, in step (2), before the ultrasonically treated solid precipitate is mixed with hydrofluoric acid aqueous solution, the solid precipitate is washed and purified, including: washing the solid precipitate twice with DMF to remove unreacted precursors and impurities adsorbed on the product surface, and then washing it three times with ethanol to replace the DMF in the pores.

[0065] For example, in step (2), the mass concentration of the hydrofluoric acid aqueous solution is 50-60%.

[0066] For example, in step (2), the mixing process of the solid precipitate and the hydrofluoric acid aqueous solution is as follows: gently stir in an ice-water bath for 4-5 hours to safely dissolve the silica template without destroying the MOF framework structure, and then centrifuge to collect the solid until the supernatant is neutral to obtain the rare earth-MOF material.

[0067] For example, in step (3), the mass ratio of the carbon dots to the rare earth-MOF material is 1:10-20.

[0068] For example, in step (3), before mixing the carbon dots with the rare earth-MOF material, the rare earth-MOF material is activated in a vacuum drying oven at 120-130°C for 8-10 hours.

[0069] For example, in step (3), the mixing is a grinding mixture.

[0070] For example, in step (3), the calcination temperature is 300-350℃, the heating rate is 5-7℃ / min, and the calcination time is 8-10h. Under high temperature, the carbon dots have a certain fluidity and enter the channels of the rare earth-MOF under capillary action to achieve encapsulation.

[0071] For example, in step (3), the flow rate of nitrogen is 50-60 mL / min.

[0072] For example, in step (3), the calcined product is ultrasonically dispersed with sulfuric acid solution, diluted with ultrapure water, filtered with a 0.22µm microporous membrane, the obtained solid is washed with water again, washed three times, the solid on the filter membrane is washed off with ethanol, the white precipitate is collected by high-speed centrifugation, dried, and a white powder of carbon dot@rare earth-MOF material is obtained.

[0073] The framework of rare-earth-MOFs provides excellent protection for the fluorescent carbon dots encapsulated within them and can selectively adsorb heavy metal ions (Pb) through their channels. 2+ Cd 2+ Hg 2+ The adsorbed heavy metal ions specifically interact with the red-emitting fluorescent dye and carbon dots co-encapsulated in the pores, resulting in differential quenching or enhancement of their fluorescence. Quantitative detection is achieved by measuring the ratio R of their fluorescence intensities, and the color change is visible to the naked eye, making it ideal for developing test strips or for use in sensor devices.

[0074] For example, in step (4), the fluorescent dye capable of emitting red light is at least one of Nile Red, Rhodamine B, and Rhodamine 6G.

[0075] For example, in step (4), the mass ratio of the carbon dot@rare earth-MOF material to the fluorescent dye that can emit red light is 100-110:1.

[0076] For example, the second solvent is dichloromethane. The volume ratio of the second solvent to the mass of the carbon dot@rare earth-MOF material is 100-300 mL: 1 g.

[0077] For example, in step (4), the conditions for solution diffusion adsorption include: being carried out under light-protected conditions, at a temperature of 40-50°C, for a time of 12-15 hours. This allows the fluorescent dye capable of emitting red light to diffuse and adsorb into the rare earth-MOF channels as an energy acceptor.

[0078] For example, in step (4), the mixture after diffusion adsorption in the solution is centrifuged to collect the white precipitate, and the white precipitate is washed and purified.

[0079] For example, in step (4), the washing and purification includes: washing the supernatant three times by vortex dispersion-centrifugation with methanol until the supernatant is clear and has no fluorescence. During this process, the fluorescent dye that can emit red light will be loaded into the pores of the composite material and sealed in the rare earth-MOF material together with the carbon dots. The washed solid is placed in a vacuum drying oven and dried at 60-70°C for 6-8 hours to obtain a dried solid multicolor carbon dot@MOF sensor.

[0080] Secondly, the present invention also provides a solid-state multicolor carbon dot@MOF sensor prepared by the above-mentioned method for preparing a solid-state multicolor carbon dot@MOF sensor.

[0081] This solid-state multicolor carbon dot@MOF sensor uses the rigid structure of rare-earth-MOF material as a framework, encapsulating carbon dots and red-emitting fluorescent dyes within the framework. The rigid structure of the rare-earth-MOF prevents the aggregation and quenching of the carbon dots and the red-emitting fluorescent dyes. By monitoring the intensity ratio of the emission peaks of the carbon dots and the red-emitting fluorescent dyes, ratiometric self-calibration is achieved, effectively overcoming factors such as light source fluctuations and environmental interference, thus improving detection accuracy. Furthermore, the rare-earth-MOF not only serves as a carrier, but its porous structure also provides size screening and pre-enrichment for specific molecules, endowing the sensor with excellent selectivity.

[0082] Thirdly, this invention also provides the above-mentioned solid-state multicolor carbon dot @MOF sensor for detecting heavy metal ions Pb. 2+ or Cd 2+ or Hg 2+ Applications in [the context of the text].

[0083] This solid-state multicolor carbon dot@MOF sensor was used to detect the heavy metal ion Pb. 2+ or Cd 2+ or Hg 2+In practice, ratio fluorescence standard curves for different heavy metal ions can be established in advance, which can effectively counteract environmental interference and achieve high-precision, self-calibrated detection of target ions. Specifically, this includes: preparing heavy metal ion solutions of different concentrations, detecting the ratio R (ratio fluorescence) of the emission peak intensity of carbon dots to the emission peak intensity of a red-emitting fluorescent dye at different concentrations, establishing a standard curve between R and the heavy metal ion concentration, and when using the solid-state multicolor carbon dot@MOF sensor to detect heavy metal ions in actual water, the actual detected ratio fluorescence R value can be compared with the pre-established standard curve to obtain the actual heavy metal ion concentration in the detected water.

[0084] The following specific embodiments further illustrate the solid-state multicolor carbon dot@MOF sensor of the present invention, its preparation method, and its application.

[0085] Example 1

[0086] (1) Preparation of CAB-CDs carbon dots: Chitosan and sodium alginate were used in a mass ratio of 2:1, with 0.2 g of chitosan and 0.1 g of sodium alginate as precursors. They were dissolved in 15 ml of ultrapure water and subjected to gas flow-assisted stepwise thermal polymerization in a 50 ml high-pressure reactor. Nitrogen was used as the protective gas. In the first stage, the reaction system was heated from room temperature to 120 °C at a rate of 3 °C / min and the nitrogen flow rate was 50 ml / min for 5 min. In the second stage, the temperature was increased from 120 °C to 180 °C at a rate of 2 °C / min and the nitrogen flow rate was 100 ml / min for 5 min. In the third stage, the temperature was maintained at 180 °C and the nitrogen flow rate was 50 ml / min for 10 min. After the reaction was completed and cooled, ultrapure water was added to dilute to 50 ml. The mixture was first centrifuged (8000 rpm, 15 min) to remove large particle aggregates. The supernatant was collected, filtered using a 0.22 µm microporous membrane, and dialyzed with a 1000 Da dialysis bag for 24 hours (with water changed every 8 hours). The solution was then concentrated to obtain a fluorescent carbon dot solution as an energy donor.

[0087] (2) Preparation of Ce-MOF material: 436 mg of cerium nitrate nonahydrate and 180 mg of biphenyl-3,3',5,5'-tetracarboxylic acid were dissolved in 30 ml of N,N-dimethylformamide (DMF) and magnetically stirred for about 20 min at room temperature to form a clear solution. 200 mg of hollow silica spheres were added, and gentle stirring was continued for 10 min. The mixed suspension was transferred to a sonochemical reactor with a cooling jacket, and the cooling circulation system was turned on to stabilize the reaction temperature at 25 °C (this temperature was only used to maintain the reactor). The generator was turned on, using a frequency of 20 kHz and a power density of 500 W / cm². 2The liquid was sonicated for 5 minutes using a pulse mode (2 seconds of operation followed by a 1-second interval). The resulting turbid suspension was centrifuged (8000 rpm, 5 min), and the solid was collected. The precipitate was washed twice with DMF and then three times with ethanol. The washed solid was dispersed in 50 ml of 50% hydrofluoric acid aqueous solution and gently stirred in an ice-water bath for 4 h to safely dissolve the silica template without damaging the MOF framework structure. Finally, the solid was collected by centrifugation (8000 rpm, 10 min) until the supernatant was neutral, yielding the Ce-MOF material.

[0088] (3) Preparation of CAB-CDs@Ce-MOF: The obtained Ce-MOF material was activated in a vacuum drying oven at 120℃ for 8h. The CAB-CDs carbon dots and Ce-MOF material were thoroughly ground and mixed (the mass ratio of carbon dots to Ce-MOF material was 1:15), placed in a quartz dish, and calcined in a high-temperature calcination furnace (heating rate: 5℃ / min, final temperature: 300℃, duration: 8h). Nitrogen gas was used as a protective gas (gas flow rate: 50ml / min). During this process, under the action of high temperature, the carbon dots have a certain fluidity and enter the pores of Ce-MOF under capillary action to achieve encapsulation. The obtained solid was washed with sulfuric acid and then completely dispersed by sonication. It was diluted with ultrapure water to 1000 ml and filtered through a 0.22 µm microporous membrane. The obtained solid was washed again with 1000 ml of water. After washing three times, the solid on the filter membrane was washed off with 20 ml of ethanol. The white precipitate was collected by high-speed centrifugation and allowed to stand in an oven for 24 h to obtain the product CAB-CDs@Ce-MOF white powder.

[0089] (4) Preparation of CAB-CDs@Ce-MOF / NR sensor: 50 mg of CAB-CDs@Ce-MOF white powder was dispersed in 10 ml of dichloromethane solution containing 0.5 mg of Nile Red. The mixture was stirred at 40 °C in the dark for 12 h to allow Nile Red to diffuse and adsorb into the Ce-MOF channels as an energy acceptor. The white precipitate was collected by centrifugation at 8000 rpm for 10 min. The precipitate was washed three times with methanol by vortex dispersion-centrifugation until the supernatant was clear and free of fluorescence. During this process, Nile Red was loaded into the pores of the composite material and sealed in the Ce-MOF material together with the carbon dots. The washed solid was placed in a vacuum drying oven and dried at 60 °C for 6 h to obtain the CAB-CDs@Ce-MOF / NR sensor.

[0090] Example 2

[0091] (1) Preparation of CAB-CDs carbon dots: Chitosan and sodium alginate were used in a mass ratio of 3:1, with 0.3g of chitosan and 0.1g of sodium alginate as precursors. They were dissolved in 20ml of ultrapure water and subjected to gas flow-assisted stepwise thermal polymerization in a 50ml high-pressure reactor. Nitrogen was used as the protective gas. In the first stage, the reaction system was heated from room temperature to 130℃ at a rate of 4℃ / min and the nitrogen flow rate was 60ml / min for 8min. In the second stage, the temperature was raised from 130℃ to 200℃ at a rate of 3℃ / min and the nitrogen flow rate was 110ml / min for 8min. In the third stage, the temperature was maintained at 200℃ and the nitrogen flow rate was 60ml / min for 15min. After the reaction was completed and cooled, ultrapure water was added to dilute to 60 ml. The mixture was first centrifuged (9000 rpm, 20 min) to remove large particle aggregates. The supernatant was collected, filtered using a 0.22 µm microporous membrane, and dialyzed with a 1000 Da dialysis bag for 24 hours (water was changed every 8 hours). The solution was then concentrated to obtain a fluorescent carbon dot solution as an energy donor.

[0092] (2) Preparation of Ce-MOF material: 436 mg of cerium nitrate nonahydrate and 180 mg of biphenyl-3,3',5,5'-tetracarboxylic acid were dissolved in 30 ml of N,N-dimethylformamide (DMF) and magnetically stirred for about 20 min at room temperature to form a clear solution. 924 mg of hollow silica spheres were added, and gentle stirring was continued for 10 min. The mixed suspension was transferred to a sonochemical reactor with a cooling jacket, and the cooling circulation system was turned on to stabilize the reaction temperature at 25 °C (this temperature was only used to maintain the reactor). The generator was turned on, using a frequency of 30 kHz and a power density of 600 W / cm². 2 The liquid was sonicated for 6 minutes using a pulse mode (2 seconds of operation followed by a 1-second interval). The resulting turbid suspension was centrifuged (8000 rpm, 5 min), and the solid was collected. The precipitate was washed twice with DMF and then three times with ethanol. The washed solid was dispersed in 50 ml of 50% hydrofluoric acid aqueous solution and gently stirred in an ice-water bath for 4 h to safely dissolve the silica template without damaging the MOF framework structure. Finally, the solid was collected by centrifugation (8000 rpm, 10 min) until the supernatant was neutral, yielding the Ce-MOF material.

[0093] (3) Preparation of CAB-CDs@Ce-MOF: The obtained Ce-MOF material was activated in a vacuum drying oven at 130℃ for 10h. The CAB-CDs carbon dots and Ce-MOF material were thoroughly ground and mixed (the mass ratio of carbon dots to Ce-MOF material was 1:10), placed in a quartz dish, and calcined in a high-temperature calcination furnace (heating rate: 7℃ / min, final temperature: 300℃, duration: 8h). Nitrogen was used as the protective gas (gas flow rate: 60ml / min). During this process, under the action of high temperature, the carbon dots have a certain fluidity and enter the pores of Ce-MOF under capillary action to achieve encapsulation. The obtained solid was washed with sulfuric acid and then completely dispersed by sonication. It was diluted with ultrapure water to 1000 ml and filtered through a 0.22 µm microporous membrane. The obtained solid was washed again with 1000 ml of water. After washing three times, the solid on the filter membrane was washed off with 20 ml of ethanol. The white precipitate was collected by high-speed centrifugation and allowed to stand in an oven for 24 h to obtain the product CAB-CDs@Ce-MOF white powder.

[0094] (4) Preparation of CAB-CDs@Ce-MOF / NR sensor: 55 mg of CAB-CDs@Ce-MOF white powder was dispersed in 10 ml of dichloromethane solution containing 0.5 mg of Nile Red. The mixture was stirred at 40 °C in the dark for 12 h to allow Nile Red to diffuse and adsorb into the Ce-MOF channels as an energy acceptor. The white precipitate was collected by centrifugation at 8000 rpm for 10 min. The precipitate was washed three times with methanol by vortex dispersion-centrifugation until the supernatant was clear and free of fluorescence. During this process, Nile Red was loaded into the pores of the composite material and sealed in the Ce-MOF material together with the carbon dots. The washed solid was placed in a vacuum drying oven and dried at 60 °C for 6 h to obtain the CAB-CDs@Ce-MOF / NR sensor.

[0095] Test case

[0096] 1. The pore structure and specific surface area of ​​the Ce-MOF material and CAB-CDs@Ce-MOF material prepared in Example 1 were detected using the nitrogen adsorption-desorption isotherm (BET) method. The results showed that Ce-MOF exhibited a typical type IV isotherm, indicating that it has a mesoporous structure and a specific surface area of ​​1680 m². 2 / g, with an average pore size of approximately 3.2 nm. The specific surface area of ​​CAB-CDs@Ce-MOF decreased slightly (approximately 1520 m² / g). 2 Although the porosity is still relatively high, it is still conducive to the adsorption and diffusion of heavy metal ions.

[0097] 2. The X-ray photoelectron spectroscopy (XPS) of the CAB-CDs carbon dots, Ce-MOF material, and CAB-CDs@Ce-MOF / NR sensor prepared in Example 1 was analyzed. Figures 1 to 3 As shown in the figure, CAB-CDs carbon dots were successfully introduced into Ce-MOF materials, while retaining nitrogen / oxygen functional groups. The Ce-MOF material contains Ce... 3+ / Ce 4+ A suitable ratio is beneficial for the coordination and sensing of metal ions.

[0098] 3. Thermal stability analysis: Through thermogravimetric analysis (TGA), the Ce-MOF of Example 1 is structurally stable below 300℃. The CAB-CDs@Ce-MOF / NR sensor still maintains the integrity of the skeleton after calcination at 300℃, indicating that it has good thermal stability and is suitable for high-temperature packaging processes.

[0099] 4. Detection of heavy metal ions Pb by the CAB-CDs@Ce-MOF / NR sensor prepared in Example 1 2+ Cd 2+ Hg 2+ Detection performance:

[0100] (1) Detection conditions and procedures: Take 2.0 mg of the CAB-CDs@Ce-MOF / NR sensor powder prepared in Example 1, disperse it in 4 mL of the test aqueous solution, and vortex for 10 s to suspend it uniformly. Use a fluorescence spectrometer, fix the excitation wavelength at 365 nm, and scan the emission spectrum from 400 to 700 nm. Record the fluorescence intensity (IL) of the carbon dots (donors) at 550 nm. 550 The fluorescence intensity of Nile Red (receptor) at 620 nm (I) 620 ), and calculate its fluorescence intensity ratio R=I 550 / I 620 .

[0101] (2) Quantitative detection performance: For the three target heavy metal ions, the carbon point emission peak intensity (I) is uniformly measured. 550 ) and Nile red emission peak intensity (I 620 The ratio of ) is defined as R 标 =I 550 / I 620 This forms the basis for constructing the standard lines for all three ions. The respective fluorescence intensity ratios (R0) are then established. 测 The standard curve between ion concentration (C) and ion concentration (C).

[0102] For Hg 2+ Detection:

[0103] Response behavior: Hg 2+ The addition of I620 (Red light) was significantly enhanced, while I 550 (Green light) remains essentially unchanged, and the fluorescence intensity ratio R 测 The value decreased.

[0104] Linear range and detection limit: In the concentration range of 0.01-5 µM, R 测 With Hg 2+ Logarithm of concentration (lg C_Hg) 2+ The equation shows a good linear relationship, and the linear equation is R. 测 =-0.35lg C_Hg 2+ +1.02, R 2 =0.996. Based on 3σ / slope, the limit of detection (LOD) is 3.2 nM. It should be noted that Hg... 2+ The affinity is high, which is manifested in a lower detection limit and a larger dynamic range of response. It approaches saturation at low concentrations, and using the direct concentration will result in a non-linear relationship. Therefore, the logarithm of the concentration is used here.

[0105] Uncorrected visual detection limit: when Hg 2+ When the concentration reaches 0.5µM, the fluorescence color of the suspension under ultraviolet light (365nm) changes from dark red to bright orange-red, indicating that if the concentration is less than 0.5µM, it is difficult to detect the red enhancement with the naked eye. Therefore, the detection limit for the naked eye is 0.5µM.

[0106] For Cd 2+ Detection:

[0107] Response behavior: Cd 2+ The addition of I 550 (Green light) gradually intensifies, while I 620 (Red light) decreases accordingly, and the fluorescence intensity ratio R 测 The value increased.

[0108] Linear range and detection limit: In the concentration range of 0.05-10 µM, R 测 Value and Cd 2+ Concentration (C_Cd) 2+ The relationship is linear, and the linear equation is R. 测 =0.28C_Cd 2+ +0.75, R 2 =0.993. The limit of detection (LOD) is 12 nM. Cd 2+ The response is not that strong, and at low concentrations, a linear relationship can be directly established using concentration.

[0109] Visualizing the changes: as Cd 2+ As the concentration increased from 0 to 10 µM, the fluorescence color changed from red through orange-yellow and finally to yellow-green.

[0110] For Pb 2+ Detection:

[0111] Response behavior: Pb 2+ The addition of I simultaneously quenched 550 and I 620 But for I 620 The quenching efficiency of (red light) is much higher than that of I. 550 (Green light) leads to a fluorescence intensity ratio R 测 The value increased significantly.

[0112] Linear range and detection limit: In the concentration range of 0.1-8 µM, R 测 Value and Pb 2+ Concentration (C_Pb) 2+ The relationship is linear, and the linear equation is R. 测 =0.41C_Pb 2+ +0.85, R 2 =0.998. The limit of detection (LOD) is 25 nM. Pb 2+ The response is not that strong, and at low concentrations, a linear relationship can be directly established using concentration.

[0113] Visualizing the changes: with Pb 2+ As the concentration increases, the fluorescence color changes from red to orange, then green, and finally darkens.

[0114] (3) Selectivity and anti-interference test:

[0115] Selectivity experiment: Under the same conditions, the CAB-CDs@Ce-MOF / NR sensor of Example 1 was compared with a 10µM ionomer containing other common metal ions as interfering ions (including K+). + Na + Ca 2+ Mg 2+ Zn 2+ Cu 2+ Fe 3+ Co 2+ Ni 2+ The aqueous solution of ) was mixed. The results showed that only the target ion Pb was present. 2+ Cd 2+ Hg 2+ Can cause R 标 Significant changes in value (ΔR>50%, ΔR=(R)) 离子 -R 空白 ) / R 空白 The changes in R value caused by other ions were all less than 5%, which proves the high selectivity of the sensor for the three target heavy metal ions.

[0116] Anti-interference experiment: In the presence of 5µM target ions (Pb) 2+ Cd 2+ or Hg 2+ In a solution containing only the target ions, 50 µM of the aforementioned interfering ion mixture was added simultaneously. The detection results showed that the rate of change of the R value was less than ±4% compared to when only the target ions were present, indicating that the sensor has good anti-interference capability in real-world complex water samples.

[0117] (4) Actual water sample testing: To evaluate the practicality of the sensor, the spiked recovery method was used to test tap water and river water samples. Low, medium, and high concentrations (Hg) were added to the water samples respectively. 2+ :0.05µM, 1.0µM, 5.0µM; Cd 2+ :0.1µM, 2.0µM, 10.0µM; Pb 2+ Standards (0.5µM, 3.0µM, 8.0µM) were used, and each concentration was tested in triplicate.

[0118] Recovery results: The recovery rates of the three target heavy metal ions were all between 95% and 105%. There may be systematic biases or fluctuations in the detection method itself. This recovery rate range proves that the scheme is an accurate and reliable method.

[0119] Relative standard deviation (RSD): all less than 3.5%.

[0120] The results demonstrate that the CAB-CDs@Ce-MOF / NR sensor prepared in this invention has high accuracy and reliability in the detection of real-world environmental samples.

[0121] The above tests demonstrate that the CAB-CDs@Ce-MOF / NR sensor is effective against Pb. 2+ Cd 2+ Hg 2+ It features high sensitivity, high selectivity, and excellent anti-interference performance, and can achieve naked-eye visual semi-quantitative detection, fully meeting the needs for rapid detection of trace heavy metal ions in environmental water bodies.

[0122] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for fabricating a solid-state multicolor carbon dot@MOF sensor, characterized in that, include: Step (1): Nitrogen-containing natural polysaccharide, carboxyl-containing polysaccharide and water are mixed and polymerized by step heating under nitrogen protection using a hydrothermal method to obtain carbon dots; Step (2): Mix rare earth salt, polycarboxylic acid ligand, first solvent and hollow silica template, sonicate, mix the obtained solid precipitate with hydrofluoric acid aqueous solution, dissolve hollow silica template, and obtain rare earth-MOF material; Step (3): The carbon dots are mixed with the rare earth-MOF material and calcined under nitrogen protection. The carbon dots are embedded in the pores of the rare earth-MOF material under nitrogen protection to obtain carbon dots@rare earth-MOF material. Step (4): Mix the carbon dot@rare earth-MOF material, the fluorescent dye that can emit red light, and the second solvent, and perform solution diffusion adsorption so that the fluorescent dye that can emit red light diffuses and adsorbs into the pores of the rare earth-MOF material to obtain a solid-state multicolor carbon dot@MOF sensor. In step (1), the nitrogen-containing natural polysaccharide is at least one of chitin, cellulose amine derivative, and chitosan, and the carboxyl-containing polysaccharide is sodium alginate and / or carboxymethyl cellulose; In step (1), the stepped heating includes: In the first stage, the reaction system is heated from room temperature to 120-130℃ at a rate of 3-4℃ / min, and the nitrogen flow rate is 50-60ml / min for 5-8min. In the second stage, the temperature is increased from 120-130℃ to 180-200℃ at a rate of 2-3℃ / min, with a nitrogen flow rate of 100-110ml / min and a duration of 5-8min. The third stage involves maintaining a temperature of 180-200℃ and a nitrogen flow rate of 50-60 ml / min for 10-15 minutes. In step (2), the rare earth salt is a cerium salt; The polycarboxylic acid ligand is at least one of biphenyl-3,3',5,5'-tetracarboxylic acid, pyromellitic acid, and 2,6-naphthalenedicarboxylic acid; In step (4), the fluorescent dye capable of emitting red light is at least one of Nile Red, Rhodamine B, and Rhodamine 6G.

2. The method for fabricating a solid-state multicolor carbon dot@MOF sensor according to claim 1, characterized in that, In step (1), the mass ratio of the nitrogen-containing natural polysaccharide to the carboxyl-containing polysaccharide is 2-3:

1.

3. The method for fabricating a solid-state multicolor carbon dot@MOF sensor according to claim 1, characterized in that, In step (2), the molar ratio of the rare earth salt to the polycarboxylic acid ligand is 1:1-1.

2.

4. The method for fabricating a solid-state multicolor carbon dot@MOF sensor according to claim 1, characterized in that, In step (3), the mass ratio of the carbon dots to the rare earth-MOF material is 1:10-20.

5. The method for fabricating a solid-state multicolor carbon dot@MOF sensor according to claim 1, characterized in that, In step (4), the mass ratio of the carbon dot@rare earth-MOF material to the fluorescent dye that emits red light is 100-110:

1.

6. A solid-state multicolor carbon dot@MOF sensor prepared by the method of any one of claims 1 to 5.

7. The solid-state multicolor carbon dot@MOF sensor according to claim 6 for detecting heavy metal ions Pb 2+ or Cd 2+ or Hg 2+ Applications in [the context of the text].

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