A biomass multi-emission carbon dot, a preparation method thereof, a fluorescent sensor and a construction method thereof, and a portable detection system

Abstract

The application discloses a kind of biomass multi-emission carbon dots and preparation method thereof, fluorescent sensor and construction method thereof, portable detection system, and relates to functional nanomaterials and food safety detection technical field.The carbon dots are prepared by solvothermal method with no-petals Galium aparine as carbon source and urea as nitrogen source;The carbon dots have fluorescence emission peaks at 485nm, 610nm and 655nm under 420nm excitation light.The application can be used for rapid, sensitive and on-site detection of aureomycin hydrochloride and hematin chloride in food, and has advantages of green environmental protection, simple operation, low cost and strong anti-interference.

Description

Technical Field

[0001] This invention relates to the field of functional nanomaterials and food safety detection technology, specifically to a biomass multi-emission carbon dot and its preparation method, a fluorescence sensor and its construction method, and a portable detection system. Background Technology

[0002] Monitoring antibiotic and additive residues in food is crucial for protecting public health. Tetracycline antibiotic chlortetracycline hydrochloride (CTC) and nutritional supplement hemin chloride (Hemin) are particularly notable examples. CTC may induce the proliferation of drug-resistant bacteria, trigger allergies and endocrine disorders, and migrate along the food chain; excessive intake of Hemin is associated with the risk of iron poisoning and metabolic disorders. According to the GB 31650-2019 standard, the maximum residue limits for CTC in pork and poultry muscle, liver, and kidney are 200 µg / kg, 600 µg / kg, and 1200 µg / kg, respectively.

[0003] Currently used detection methods such as chromatography, mass spectrometry, electrochemical sensing, and Raman spectroscopy often rely on complex pretreatment and expensive equipment, and suffer from limitations such as poor reproducibility, insufficient stability, and high operational expertise requirements. In recent years, fluorescence sensing has attracted attention due to its ease of operation, rapid response, high sensitivity, and visualization potential. Fluorescence sensors based on nanomaterials have further improved detection performance; their large specific surface area, tunable optical properties, and easily modifiable surfaces contribute to enhanced selectivity and stability, and can support multi-target analysis. Carbon dots (CDs) have become ideal materials for constructing high-performance ratio fluorescence sensors due to their good biocompatibility, low toxicity, ease of synthesis, and tunable emission. However, the synthesis of CD precursors still faces challenges: organic precursors have limited fluorescence performance, inorganic precursors pose toxicity concerns, biomass precursors often have insufficient nitrogen doping and weak luminescence, and chemical reagents may cause sustainability and biocompatibility issues. In addition, traditional single-emission sensors are susceptible to interference from environmental and concentration fluctuations, resulting in poor reproducibility; although dual-emission ratio sensors improve stability through self-calibration, they are still insufficient for multi-target detection of complex samples due to the limited number of emission channels.

[0004] Furthermore, current portable devices and platforms for food safety testing include test strips, microfluidic chips, handheld spectrometers, and wearable sensors. However, existing portable food safety testing technologies have inherent limitations in terms of quantitative accuracy, operational complexity, cost control, and environmental friendliness. Therefore, this invention aims to provide a visual sensing platform based on green biomass-derived fluorescent carbon dots and RGB color recognition, enabling rapid and accurate quantitative screening of target substances on-site. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a novel detection method that combines biomass multi-emission carbon dots and their preparation method, a fluorescence sensor and its construction method, and a portable detection system. This approach creatively constructs a novel detection method that integrates the specific fluorescence response of biomass carbon dots, the visualization of color changes under ultraviolet excitation, and a miniaturized RGB signal acquisition and conversion module. This method offers significant comprehensive advantages over current mainstream portable methods.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: One object of the present invention is to provide a biomass multi-emission carbon dot, wherein the carbon dot is prepared by a solvothermal method using *Chilodon apetala* as the carbon source and urea as the nitrogen source; the carbon dot has fluorescence emission peaks at 485 nm, 610 nm and 655 nm under 420 nm excitation light.

[0007] Another object of the present invention is to provide a method for preparing the biomass multi-emission carbon dots, comprising the following steps: mixing *Chilodon apetala* with urea, and reacting the mixture using a solvothermal method to obtain multi-emission N-doped red carbon dots, i.e., the biomass multi-emission carbon dots, specifically: S1: Clean the barkless chickweed, dry it with hot air, grind it into powder, and sieve it to obtain barkless chickweed powder; S2: Add anhydrous ethanol to the Chilodon apetala powder, extract by ultrasonication, and then vacuum filter to obtain the filtrate. Transfer the filtrate to a high-pressure reactor lined with polytetrafluoroethylene. Separately, add urea powder to anhydrous ethanol, stir to dissolve, and then transfer to a high-pressure reactor for reaction. S3: After the reaction is complete, the reaction solution is cooled to room temperature naturally, centrifuged, the supernatant is collected, and filtered through a 0.22µm microfiltration membrane to obtain a biomass multi-emission carbon dot solution.

[0008] Furthermore, in step S1, the hot air drying conditions are 60°C for 40 minutes; the sieve size is 20 mesh. In step S2, the solid-liquid ratio of the chitosan powder to anhydrous ethanol is 2:25 (g:mL); the ultrasonic conditions are: 100kHz, 25℃ for 60min; the volume of the polytetrafluoroethylene liner is 100mL; the mass ratio of the chitosan powder to the urea powder is 1:1; the solid-liquid ratio of the urea powder to the anhydrous ethanol is 2:5 (g:mL); the reaction temperature is 180℃; and the reaction time is 6h. In step S3, the centrifugation speed is 8000 rpm and the centrifugation time is 15 min.

[0009] Another object of the present invention is to provide a fluorescence sensor whose sensing element comprises the biomass multi-emitting carbon dots; the sensor detects the fluorescence intensity of the carbon dots at 485 nm, 610 nm, and 655 nm before and after the addition of the target substance under 420 nm excitation, and calculates (F... 485 +F 610 ) / F 655 The change in the ratio enables the detection of the target object.

[0010] Furthermore, the target substance is chlortetracycline hydrochloride or heme chloride.

[0011] Another objective of this invention is to provide a method for constructing the fluorescence sensor, comprising the following steps: mixing a 100-fold diluted biomass multi-emission carbon dot solution with BR buffer, adding a test solution containing the target analyte, and incubating at room temperature to obtain the sensor; when detecting the target analyte, measuring the fluorescence intensity at 485nm, 610nm, and 655nm under 420nm excitation, and establishing a quantitative relationship for the target analyte concentration based on the ratio of (F485+F610) / F655.

[0012] Furthermore, the volume ratio of the 100-fold diluted biomass multi-emission carbon dot solution, BR buffer, and test solution is 1:1:1; the target substance is chlortetracycline hydrochloride or heme chloride; when the target substance is chlortetracycline hydrochloride, the incubation time is 5 min, and when the target substance is heme chloride, the incubation time is 15 min.

[0013] Another object of the present invention is to provide a portable detection system for implementing the fluorescence sensor detection, including a processor module, a display driver module, a running driver module, an optical signal acquisition and processing module, and a data transmission module.

[0014] Furthermore, the processor module uses an 8-bit surface-mount microcontroller (MCU) chip, the display driver module is an LED display screen, the operation driver module is a power supply and a 365nm ultraviolet lamp, and the light signal acquisition and processing module is an AD620 amplifier, a miniature camera, and a color recognition sensor.

[0015] Another object of the present invention is to provide the application of the fluorescent sensor in the preparation of a detection product for detecting chlortetracycline hydrochloride and / or heme chloride in food.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Green design and precise synthesis of specific fluorescent carbon dots: This invention is the first to utilize Chilodon aloescens as a biomass carbon source, combined with urea nitrogen doping, to controllably synthesize carbon dots with multiple emission (F) via a solvothermal method. 485 F 610 F 655Nitrogen-doped red carbon dots (R-CDs) with three emission characteristics have been transformed from untapped to high-value functional materials, possessing the advantages of being both environmentally friendly and cost-effective, laying the material foundation for subsequent high-selectivity ratiometric fluorescence sensing. (2) Construction of a ratiometric sensing mechanism based on multiple emission signals: This invention innovatively utilizes the multiple emission characteristics of R-CDs, and through precise experiments, determines the "(F 485 +F 610 ) / F 655 "This optimal fluorescence intensity ratio is used as an internal reference signal. This design transforms the concentration information of the target substance (CTC / Hemin) into a stable ratio change, rather than a single intensity change, which in principle gives the sensor a strong anti-interference capability. (3) Precise correlation model between fluorescence color and digital RGB signal: This invention establishes a complete correlation model of "target substance concentration → R-CDs fluorescence color change → RGB value captured by the device → calculation of (R+G) / B ratio". This is the core of transforming abstract fluorescence spectral signals into intuitive, visual and digitally quantifiable signals, and is a key bridge connecting chemical sensing and information technology. (4) Highly integrated dedicated portable detection device: This invention Ming designed a hardware system integrating a specific wavelength ultraviolet light source (365nm), a micro fluorescence capture module, a dedicated color recognition sensor, a low-power microprocessor, and a wireless transmission module. This system is not a patchwork of general equipment, but a deep optimization and integration for the specific task of "fluorescence colorimetric recognition". It achieves speed, multi-channel, low cost, convenient data acquisition, and integrated configuration, which is suitable for on-site food safety and quality control. (5) A rapid pretreatment and detection combination scheme for complex meat products: This scheme effectively removes the interference of complex matrices in meat products, verifies the feasibility and reliability of the sensor in real and complex samples, and is a key link for the technology to become practical. Attached Figure Description

[0017] Figure 1 A schematic diagram of the preparation of R-CDs and its fluorescence color changes in response to CTC and Hemin; Figure 2 The graph shows the spectral changes of the R-CDs fluorescence sensor with CTC and Hemin responses; Figure 3 To optimize the detection conditions of CTC and Hemin for the R-CDs fluorescence sensor, including (a) dilution factor; (b) pH value; (c) incubation time; (d) incubation temperature; and (e) the effect of UV light on fluorescence stability. Figure 4 The graph shows the detection of CTC and Hemin by the R-CDs fluorescence sensor, where (a) is the fluorescence spectrum of the R-CDs fluorescence sensor for different concentrations of CTC; and (b) is the fluorescence spectrum of R-CDs (F...). 485 +F610 ) / F 655 (c) Linear relationship curve between the ratio and CTC concentration; (d) CIE chromaticity diagrams of R-CDs fluorescence sensors at different CTC concentrations; (e) Linear relationship curve between (R+G) / B value and CTC concentration; (f) Fluorescence spectra of R-CDs fluorescence sensors at different Hemin concentrations; (c) CIE chromaticity diagrams of R-CDs fluorescence sensors at different CTC concentrations; (d) Linear relationship curve between (R+G) / B value and CTC concentration; (e) Fluorescence spectra of R-CDs fluorescence sensors at different Hemin concentrations; (f) (F ... 485 +F 610 ) / F 655 (g) CIE chromaticity diagrams of R-CDs fluorescence sensors at different Hemin concentrations; (h) Linear relationship between (R+G) / B value and Hemin concentration. Figure 5 Construction diagram of portable visual inspection equipment; Figure 6 Diagram of device module composition; Figure 7 A schematic diagram of CTC and Hemin based on R-CDs fluorescence sensors and using a portable detection system; Figure 8 The selectivity and anti-interference of the R-CDs fluorescence sensor to CTC, Hemin and other interfering substances are evaluated (the inset shows the color change and fluorescence spectrum effect of the R-CDs solution when CTC and Hemin are added). Detailed Implementation

[0018] To make the objectives and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention.

[0019] Unless otherwise specified, the instruments, reagents, and materials used in the following embodiments are all conventional instruments, reagents, and materials already available in the prior art and can be obtained through legitimate commercial channels. Unless otherwise specified, the experimental methods and detection methods used in the following embodiments are all conventional experimental methods and detection methods already available in the prior art.

[0020] Example 1 Sensor Construction 1. Preparation of biomass multi-emission carbon dot R-CDs Using *Chilodon apetala*, a biomass material, as the main carbon source and urea as the external nitrogen source, multi-emission N-doped red carbon dots (R-CDs) were prepared by a simple solvothermal method. A schematic diagram of the preparation process is shown below. Figure 1The specific preparation process is as follows: *Stellaria media apetala* was thoroughly washed three times with purified water to ensure the cleanliness of the raw material. It was then dried with hot air (60℃, 40 min). The dried *Stellaria media apetala* was then ground into powder using a mortar and pestle and passed through a 20-mesh sieve. 2.00 g of the dry powder was weighed into a 100 mL beaker, and 25.00 mL of anhydrous ethanol was added. After ultrasonic extraction (100 kHz, 25℃, 60 min), the mixture was vacuum filtered to obtain the filtrate. The filtrate was transferred to a high-pressure autoclave lined with 100 mL of polytetrafluoroethylene (PTFE). 2.00 g of urea (powder) was placed in a beaker, and 5.00 mL of anhydrous ethanol was added. The mixture was stirred with a glass rod until fully dissolved and then poured into the liner. The autoclave was then reacted at 180℃ for 6 h. After the reaction, the mixture was allowed to cool naturally to room temperature. The resulting solution was centrifuged at 8000 rpm for 15 min, and the supernatant was collected. The supernatant was filtered through a 0.22 µm microfiltration membrane to obtain the R-CDs solution, which was stored at 4℃ for subsequent experiments. This strategy effectively optimizes the fluorescence properties of CDs by adjusting their elemental composition and surface state. A schematic diagram of the fluorescence response of R-CDs to CTC and Hemin is shown below. Figure 2 .

[0021] 2. Construction of R-CDs ratio fluorescence sensor 1.00 mL of a 100-fold diluted R-CDs solution was mixed with an equal volume of BR buffer (pH 8.0), and 1.00 mL of CTC at different concentrations was added. After incubation at room temperature for 5 min, three distinct emission peaks were observed under 420 nm excitation: 520 nm, 610 nm, and 655 nm. However, since the 100-fold diluted R-CDs solution produces three stable emission peaks under 420 nm excitation: 485 nm, 610 nm, and 655 nm, to ensure peak stability throughout the detection process, the fluorescence emission peaks of the R-CDs solution itself were selected to detect CTCs. The fluorescence intensity at each emission wavelength (F1, F2, F3, F4, F5, F6, F7, F8, F9 ... 485 F 610 and F 655 By plotting the fluorescence ratio (F...) 485 +F 610 ) / F 655 The relationship with CTC concentration was used to generate a calibration curve.

[0022] Similarly, 1.00 mL of Hemin solution of different concentrations was added to the prepared R-CDs solution and BR buffer mixture. After incubation at room temperature for 15 min, the concentrations were measured (F... 485 +F 610 ) / F 655 The linear relationship between the value and the Hemin concentration.

[0023] To ensure the accuracy of the tests, all experiments were performed three times.

[0024] The test results are as follows: Under optimal detection conditions (R-CDs diluted 100-fold, BR buffer at pH 8.0, room temperature, CTC incubation time 5 min, Hemin incubation time 15 min, fluorescence stability is less affected by prolonged UV exposure) Figure 3 The R-CDs ratiometric fluorescence sensor showed good linearity with CTC in the range of 0–160.00 μM (y = 0.00668x + 2.71527, R). 2 =0.993), detection limit LOD = 13.00 × 10 -3 μM, the visualization method also shows a good linear relationship ((R+G) / B=0.05953x+0.72446, R 2 =0.998), LOD=1.50×10 -3 μM.

[0025] The R-CDs ratiometric fluorescence sensor showed good linearity with Hemin in the range of 0–5.00 μM (y = 0.12137x + 3.46054, R 2 =0.998), LOD=0.74×10 -3 μM, the visualization method also shows a good linear relationship ((R+G) / B=-0.16765C+2.53628, R 2 =0.9996), LOD=0.53×10 -3 μM, both methods showed high sensitivity and low detection limit for CTC and Hemin (μM, both methods showed high sensitivity and low detection limit). Figure 4 ).

[0026] Example 2 Portable Detection System This embodiment provides a portable detection system based on fluorescence colorimetric changes. By optimizing the color recognition module, it achieves rapid, visualized detection. This system boasts advantages such as high detection speed, low cost, and high sensitivity. It can also be integrated with a smartphone to achieve semi-quantitative visual detection of multiple targets. Its key hardware components are as follows: Figure 5 As shown. This system integrates multiple functional modules, including a processor module (MCU), a display driver module (LED display), a runtime driver module (power supply and light source driver), a light signal acquisition and processing module (for signal amplification, acquisition, and conversion), and a data transmission module (supporting USB, Bluetooth, and Wi-Fi). Figure 6 ).

[0027] Specifically, an 8-bit surface-mount microcontroller (MCU) chip is used, which saves motherboard space while offering advantages such as low cost, high compatibility, and low power consumption, making it suitable for control tasks in compact sensing devices. The LED display (model 12864G-086) shows clear black text against a green background, with rapid response and no visible delay. The light source uses a 365 nm ultraviolet lamp to match the excitation spectrum of various fluorescent materials. The signal amplification module mainly uses an AD620 amplifier, capable of amplifying millivolt-level signals with an adjustable gain between 1.5 and 1000 times, supporting zero-point adjustment to improve measurement accuracy. The signal acquisition module uses an OV5640 miniature camera, operating at 3.3V, featuring high sensitivity, strong anti-interference, low noise, and automatic image calibration. The signal conversion module is a GY-33 TCS34725 color recognition sensor, characterized by low power consumption, small size, and easy installation. During operation, the target solution is excited by ultraviolet light to produce fluorescence, which is captured by the camera. The color sensor then processes the information and extracts the corresponding RGB values.

[0028] The specific operating procedure is as follows: The well-mixed detection system solution is transferred into the cuvette of the portable detection system and sealed. Fluorescence is generated by UV excitation, and the RGB values ​​corresponding to the fluorescence are captured by the built-in miniature camera and color recognition sensor. The collected data is transmitted to a smartphone for display in real time, and the (R+G) / B ratio is calculated. Figure 7 This ratio shows a linear relationship with CTC and Hemin concentrations and can be used for semi-quantitative analysis. The reliability of the RGB values ​​output by the platform can be verified by the trajectory of data points in the chromaticity diagram. This chromaticity diagram is drawn based on the R and G values ​​obtained from analysis using mobile software, further enhancing the credibility of the results.

[0029] Example 3: Evaluation of Selectivity and Interference Immunity of R-CDs Sensors To evaluate the high selectivity of the R-CDs ratiometric fluorescence sensor for CTC and Hemin, interference tests were conducted on other antibiotics and food additives. Interfering substances detected included antibiotics tetracycline (TC), oxytetracycline (OTC), erythromycin (EM), norfloxacin (NOR), ofloxacin (OFLX), and ciprofloxacin (CPFX), as well as food additives glutathione (GSH), citric acid, malic acid, tartaric acid, sodium citrate, EDTA-2Na, and sodium sulfite. Each antibiotic solution was at a concentration of 0.50 μM, and 0.5 mL of each solution was taken to measure the fluorescence intensity of each mixture. The fluorescence intensity was determined based on the ratio (F... 485 +F 610 ) / F 655Evaluate the selectivity of the sensor.

[0030] The results showed that the R-CDs ratio fluorescence sensor exhibited high selectivity and specificity for both CTC and Hemin, and had significant anti-interference ability. Figure 8 ).

[0031] Example 4 Spiked Recovery Test To evaluate the reliability of the R-CDs ratiometric fluorescence sensor in detecting CTC and Hemin in real samples, we conducted spiked recovery experiments on chicken, duck, pork, ham, bacon, and sausage samples from a local market. The accuracy and practical value of the method were verified by calculating the recovery rates.

[0032] The specific experimental steps are as follows: Take 2.00 g of homogenized sample and aliquot it into two 50.00 mL centrifuge tubes. Add 2.00 mL of CTC solution of different concentrations (0, 5.50, and 11.00 μM) to the first tube, and add 2.00 mL of Hemin solution of different concentrations (0, 1.00, and 2.00 μM) to the second tube. Then, add 30.00 mL of acetonitrile to each sample, vortex to mix, and let stand overnight at room temperature. The next day, centrifuge the sample at 8000 rpm for 15 min, collect the supernatant, and filter it through a 0.22 μm organic microporous membrane. Take 2.00 mL of the filtrate, dilute with 5.00 mL of anhydrous ethanol, and then add 1.00 mL of R-CDs solution diluted 100 times. Vortex to mix for 10 min. To improve data reliability and control error, all experimental operations were repeated 3 times (n=3). Finally, fluorescence spectroscopy was used to analyze the mixed solution to evaluate the performance of the R-CDs ratio fluorescence sensor in detecting CTC and Hemin in real samples.

[0033] The experimental results are shown in Tables 1 and 2 below: For CTC detection, the ratiometric fluorescence sensor (R-CDs) showed a recovery rate of 98.49%–102.25% (RSD 0.26%–0.63%), while the visualization method showed a recovery rate of 99.51%–102.17% (RSD 0.69%–1.12%). For Hemin detection, the ratiometric fluorescence method showed a recovery rate of 99.18%–117.65% (RSD 0.12%–0.51%), while the visualization method showed a recovery rate of 93.79%–110.85% (RSD 0.27%–0.97%). Both methods can accurately and stably detect CTC residues and Hemin in fresh meat samples, demonstrating good accuracy and repeatability.

[0034] Table 1. Detection of CTCs in real samples using R-CDs ratiometric fluorescence and visualization methods (n = 3)

[0035] Table 2. Detection of Hemin in real samples using R-CDs ratiometric fluorescence method and visualization method (n = 3)

[0036] In summary, the present invention has the following advantages: (1) Green and sustainable materials: The carbon dots are synthesized using green biomass, which completely avoids the toxicity of traditional inorganic precursors and the biocompatibility problems of chemical synthesis routes, and achieves environmental protection and safety of the detection carrier from the source, which is superior to existing methods that rely on precious metals or complex chemical modifications. (2) Qualitative and quantitative integration: It combines the intuitiveness of visual rapid detection with the objective quantitative capability of RGB digital recognition. Color changes can be read by the naked eye under ultraviolet irradiation, and a concentration curve can be established through RGB value conversion to realize semi-quantitative to quantitative analysis, combining the accuracy of handheld spectrometers with the simplicity of test strips; (3) Extremely simple operation and low cost: The process is simplified to three steps: "mixing-irradiation-reading", without the need for long-term incubation or complex fluid control. The hardware only requires an ultraviolet light source, a reaction vessel and an RGB acquisition module, avoiding the precision processing of microfluidic chips and the high-priced optical components of spectrometers, which significantly reduces costs and operating threshold; (4) Strong anti-interference ability: Based on the ratio response mechanism of fluorescent carbon dots, the concentration of the target substance is reflected by the change of the ratio of the two emission peaks. The chromaticity shift of RGB recognition can correct the common interference of ambient light, light source fluctuation, etc., and improve the reproducibility and accuracy in complex samples, which is better than the traditional method of single intensity signal.

[0037] In summary, this invention is not a simple improvement on existing single technologies, but a systematic and integrated innovation for rapid on-site monitoring of food safety. It successfully integrates the advantages of green materials chemistry, visual sensing, and lightweight digital technologies, providing an integrated on-site solution that is environmentally friendly, extremely easy to operate, provides intuitive and accurate quantitative results, is low-cost, and highly resistant to interference. This solution effectively fills the technological gap between high-end precision instruments and simple qualitative test strips, and is particularly suitable for a wide range of scenarios such as on-site enforcement by regulatory authorities, monitoring of food production processes, rapid market screening, and home self-inspection. It has significant practical significance and application prospects for improving the timeliness, coverage, and effectiveness of food safety risk early warning and control.

[0038] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A biomass multi-emitting carbon dot, characterized in that: The carbon dots were prepared by a solvothermal method using Chilodon apetalis as the carbon source and urea as the nitrogen source. The carbon dots exhibited fluorescence emission peaks at 485 nm, 610 nm and 655 nm under 420 nm excitation light.

2. The method for preparing biomass multi-emitting carbon dots as described in claim 1, characterized in that: Includes the following steps: S1: Clean the barkless chickweed, dry it with hot air, grind it into powder, and sieve it to obtain barkless chickweed powder; S2: Add anhydrous ethanol to the Chilodon apetala powder, extract by ultrasonication, and then vacuum filter to obtain the filtrate. Transfer the filtrate to a high-pressure reactor lined with polytetrafluoroethylene. Separately, add urea powder to anhydrous ethanol, stir to dissolve, and then transfer to a high-pressure reactor for reaction. S3: After the reaction is complete, the reaction solution is cooled to room temperature naturally, centrifuged, the supernatant is collected, and filtered through a 0.22µm microfiltration membrane to obtain a biomass multi-emission carbon dot solution.

3. The preparation method according to claim 2, characterized in that: In step S1, the hot air drying conditions are 60℃ for 40 minutes; the sieve size is 20 mesh. In step S2, the solid-liquid ratio of the chitosan powder to anhydrous ethanol is 2:25 (g:mL); the ultrasonic conditions are: 100kHz, 25℃ for 60min; the volume of the polytetrafluoroethylene liner is 100mL; the mass ratio of the chitosan powder to the urea powder is 1:1; the solid-liquid ratio of the urea powder to the anhydrous ethanol is 2:5 (g:mL); the reaction temperature is 180℃; and the reaction time is 6h. In step S3, the centrifugation speed is 8000 rpm and the centrifugation time is 15 min.

4. A fluorescence sensor, characterized in that: Its sensing element comprises the biomass multi-emission carbon dots as described in claim 1; the sensor detects the fluorescence intensity of the carbon dots at 485 nm, 610 nm, and 655 nm before and after the addition of the target substance under 420 nm excitation, and calculates (F... 485 +F 610 ) / F 655 The change in the ratio enables the detection of the target object.

5. The fluorescence sensor according to claim 4, characterized in that: The target substance is chlortetracycline hydrochloride or heme chloride.

6. The method for constructing a fluorescence sensor as described in claim 4, characterized in that: The process includes the following steps: A 100-fold diluted biomass multi-emission carbon dot solution is mixed with BR buffer, and the test solution containing the target analyte is added. After incubation at room temperature, the sensor is obtained. When detecting the target analyte, the fluorescence intensity at 485 nm, 610 nm, and 655 nm is measured under 420 nm excitation, based on (F... 485 +F 610 ) / F 655 The ratio is used to establish a quantitative relationship between the concentration of the target substance and the concentration of the target substance.

7. The construction method according to claim 6, characterized in that: The volume ratio of the 100-fold diluted biomass multi-emission carbon dot solution, BR buffer, and test solution is 1:1:1; the target substance is chlortetracycline hydrochloride or heme chloride; when the target substance is chlortetracycline hydrochloride, the incubation time is 5 min, and when the target substance is heme chloride, the incubation time is 15 min.

8. A portable detection system for implementing the fluorescence sensor of claim 4, characterized in that: It includes a processor module, a display driver module, a runtime driver module, an optical signal acquisition and processing module, and a data transmission module.

9. The portable detection system according to claim 8, characterized in that: The processor module uses an 8-bit surface-mount microcontroller (MCU) chip, the display driver module is an LED display screen, the operation driver module is a power supply and a 365nm ultraviolet lamp, and the light signal acquisition and processing module is an AD620 amplifier, a miniature camera and a color recognition sensor.

10. The use of the fluorescence sensor as described in claim 4 in the preparation of a detection product for detecting chlortetracycline hydrochloride and / or heme chloride in food.

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