Carbon quantum dot / metal organic framework composite material as well as preparation method and application thereof

Abstract

The invention discloses a carbon quantum dot / metal organic framework composite material as well as a preparation method and application thereof, and the method is characterized in that carbon quantum dots derived from waterborne polyurethane are embedded into a zeolite imidazate framework 8 to prepare the carbon quantum dot / metal organic framework composite material. The composite material can be used as a ratio type fluorescent probe for trace detection of tetracycline, a dual-emission fluorescence signal of the probe is derived from tetracycline which quenches blue fluorescence of puCDs at 450 nm through an inner filter effect, and the coordination effect between Zn < 2 + > and tetracycline triggers an aggregation-induced emission effect, so that new fluorescence emission is generated, and the detection sensitivity is improved. The intensity of the two signals changes in proportion to the concentration of tetracycline, the wide linear concentration detection range and the low detection limit are achieved, the probe has the high recovery rate and the large adsorption capacity in detection, the problems that a traditional detection method is insufficient in sensitivity, poor in anti-interference capacity, high in cost and the like are solved, and the application prospect is wide. A new thought is provided for constructing a ratio type fluorescent probe with a removal function.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ratio fluorescence detection of tetracycline, and more particularly to a carbon quantum dot / metal-organic framework composite material, a preparation method and applications thereof. BACKGROUND

[0002] Tetracycline (TC) is a broad-spectrum antibiotic widely used in the treatment of infectious diseases in human and veterinary fields. Although tetracycline plays an important role in medical and public health fields due to its low cost and strong antibacterial properties, tetracycline is difficult to degrade naturally in the environment and is prone to accumulate in animal tissues, eventually being transmitted to the human body through the food chain. In order to ensure food and environmental safety, it is particularly important to establish a rapid and sensitive tetracycline residue detection method. At present, there are various tetracycline detection technologies such as microbial assay, immunoassay and high performance liquid chromatography, although these technologies have high analysis accuracy, they usually require complex sample pretreatment, are time-consuming and expensive, which limits their widespread application in routine analysis.

[0003] Fluorescent probes are the core component of fluorescence sensing methods. Among the various materials used to construct such probes, carbon quantum dots (CDs) have become a promising class of fluorescent nanomaterials due to their excellent biocompatibility, high quantum yield, low toxicity and low preparation cost. In CN119931653A, a kind of cordyceps militaris polysaccharide fluorescent carbon dots for detecting and degrading tetracycline antibiotics, a preparation method and applications thereof are disclosed, which utilizes cordyceps militaris polysaccharide and zinc chloride to perform high-temperature reaction to obtain cordyceps militaris polysaccharide fluorescent carbon dots. The carbon quantum dots prepared by the patent can realize the integration of tetracycline antibiotic detection and degradation, and have a series of advantages such as simple operation, high selectivity, low detection limit and environmental friendliness. However, fluorescent probes based on a single emission signal often have the defect of insufficient stability, which are easily disturbed by environmental factors, probe concentration fluctuations and instrument parameter changes, ultimately affecting the detection accuracy.

[0004] In order to improve the accuracy of detection, CN118879314B discloses a kind of double-emission metal-organic framework composite nanomaterial and its application in ratio detection of tetracycline antibiotics, which prepares a UiO-66 metal-organic framework material loaded with blue carbon quantum dots, adenosine-5'-monophosphate sodium salt and Eu 3+ The patent uses adenosine-5'-monophosphate sodium salt to modify the metal-organic framework, which increases the rigidity of the material and ensures the stability of Eu 3+Stable loading. Carbon quantum dots are loaded into metal organic framework materials, the pre-enrichment of metal organic framework is utilized, the signal of blue carbon quantum dots is used as a reference signal, and the signal of red europium is used as a response signal, so that the ratio detection of tetracycline antibiotics is realized, and the method has the advantages of high sensitivity and fast response speed. 3+ ) is introduced for construction, which undoubtedly increases the complexity and cost of probe synthesis, and the chelation system (ligand-lanthanide ion) constructed has low fluorescence efficiency of lanthanide ions, is not stable enough in a complex environment, is significantly affected by pH value, and is easily dissociated or coordinated by some anions, thereby interfering with the detection signal or causing signal failure, which greatly limits its application in actual sample detection. SUMMARY

[0005] The main technical problem to be solved by the present application is to provide a carbon quantum dot / metal organic framework composite material and a preparation method in view of the insufficient sensitivity, poor anti-interference ability, inability to be used for complex actual sample detection, and high cost of existing tetracycline detection probes.

[0006] The object of the present application is achieved by the following technical solutions: A preparation method of a carbon quantum dot / metal organic framework composite material, comprising the following steps: S1. Mix waterborne polyurethane, ethylenediamine and deionized water in a mass ratio of 1.2-6:0.2-1:40-80, react by a hydrothermal method, and obtain a carbon quantum dot solution after purification; S2. Dissolve zinc nitrate and 2-methylimidazole in the carbon quantum dot solution in a molar ratio of 1:40-100 respectively to obtain solution A and solution B in sequence. Mix solution A and solution B in a volume ratio of 1:4-10 quickly, stir and react, and dry to obtain a carbon quantum dot / metal organic framework composite material.

[0007] Further, the preparation steps of the waterborne polyurethane comprise: A catalyst is added to an acetone solution containing polycarbonate diol, then isophorone diisocyanate is added dropwise under constant temperature stirring conditions at 40-50 DEG C, the pre-polymer is formed by continuous stirring reaction, dimethylolpropionic acid is added to the pre-polymer for chain extension reaction, the reaction is carried out at 40-50 DEG C for 0.5-1.5 hours, the temperature is raised to 60-70 DEG C for 4.5-5.5 hours, hydroxyethyl acrylate is added for end-capping after cooling to 45-55 DEG C, and finally isopropyl alcohol is added for stirring, and a colorless transparent waterborne polyurethane is obtained after cooling.

[0008] Further, the mass fractions of the polycarbonate diol, isophorone diisocyanate, dimethylol propionic acid and hydroxyethyl acrylate are 35%-65%, 25%-40%, 2%-12% and 3%-15% respectively.

[0009] Further, the catalyst is dibutyl tin dilaurate, and the added amount of the catalyst is 0.1-1.5% of the total mass of the solid.

[0010] Further, the volume fractions of the isopropyl alcohol and acetone are 5%-10% and 20-35% respectively.

[0011] Further, the temperature of the hydrothermal reaction in step S1 is 150-250℃, and the reaction time is 4-8 hours.

[0012] Further, the purification in step S1 is dialysis purification with a molecular weight cut-off of 1000 Da.

[0013] A carbon quantum dot / metal organic framework composite material is obtained according to the above preparation method, and the carbon quantum dot / metal organic framework composite material is applied in tetracycline detection.

[0014] Further, the detection step comprises: (1) mixing the carbon quantum dot / metal organic framework composite material with a PBS buffer solution to prepare a suspension with a concentration of 0.5-2.5 mg mL -1 , then mixing 400 μL of the suspension with 1 mL of the PBS buffer solution, adding 100 μL of a tetracycline solution with different concentrations to obtain a mixed solution, collecting a photoluminescence spectrum in the range of 390-640 nm under an excitation wavelength of 360 nm as a standard control spectrum, and drawing a standard curve of the fluorescence intensity ratio (F 450 / F 530 ) versus the tetracycline concentration; (2) preparing a to-be-detected solution containing the tetracycline solution according to step (1), collecting a fluorescence spectrum and intensity in the range of 390-640 nm under an excitation wavelength of 360 nm, and combining the linear equation of the standard curve to determine the concentration of the tetracycline antibiotic in the to-be-detected sample.

[0015] Further, the pH value of the mixed solution is 5-8.

[0016] Further, the reaction time of the to-be-detected solution is 0.5-2 minutes.

[0017] Compared with the prior art, the beneficial effects are: The present application synthesizes carbon quantum dots (puCDs) with waterborne polyurethane (WPU) as carbon source, and embeds the puCDs in zeolitic imidazolate framework-8 (ZIF-8), and designs and synthesizes a puCDs@ZIF-8 composite material, wherein the structure of the waterborne polyurethane is rich in polar functional groups such as carboxyl and amino groups, which provides high-density reaction sites for surface modification and fluorescence regulation of the CDs. 2+ Tetracycline quenches the blue fluorescence of puCDs at 450 nm through the inner filter effect, and the coordination between Zn 2+ -TC in ZIF-8 triggers the aggregation-induced emission effect, thereby generating new fluorescence emission at 530 nm. The puCDs@ZIF-8 prepared in the present application can be used as a ratio fluorescent probe, and realizes sensitive ratio detection of tetracycline (TC) based on the fluorescence intensity ratio (F 450 / F 530 ) of the aggregation-induced emission (AIE) effect of the puCDs@ZIF-8 complex. -1 The linear concentration detection range is 0-60 μg mL -1 , and the detection limit is as low as 0.0089 μg mL -1 . In addition, the equilibrium adsorption capacity of the composite material for tetracycline in aqueous solution reaches 409.45 mg g -1 , which can realize efficient removal of tetracycline, and when used for milk detection, a good recovery rate of 95.6% to 106.3% is obtained. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a schematic diagram of synthesis of puCDs@ZIF-8 and its use for TC detection; Figure 2 is a TEM and SEM detection diagram, wherein (a) is a TEM image and particle size distribution diagram of puCDs; (b) is a high-resolution TEM image of puCDs; (c) is an XRD spectrum of puCDs; (d) is a SEM image of ZIF-8; (e) is a SEM image of puCDs@ZIF-8; (f) is an XRD comparison spectrum of ZIF-8, puCDs@ZIF-8 and simulated ZIF-8; Figure 3 is an infrared spectrum diagram of WPU, puCDs, ZIF-8 and puCDs@ZIF-8; Figure 4 (a) is a fluorescence emission spectrum of puCDs, ZIF-8 and puCDs@ZIF-8 and a real object diagram of puCDs@ZIF-8 in PBS buffer, wherein the left is under natural light and the right is under 365 nm ultraviolet light; (b) is a photoluminescence spectrum of ZIF-8 after adsorbing TC; (c) is a photoluminescence emission spectrum of puCDs and (d) is a photoluminescence emission spectrum of puCDs@ZIF-8; Figure 5 The fluorescence intensity changes of (a) puCDs and (b) puCDs@ZIF-8 at different concentrations of TC, and their corresponding linear relationships are shown in (c) and (d), respectively. The insets are the real photos of puCDs and puCDs@ZIF-8 under 365 nm UV irradiation at different concentrations of TC; Figure 6 (a) UV-Vis absorption spectra of TC and excitation and emission spectra of puCDs and puCDs@ZIF-8; (b) Fluorescence decay curves of puCDs and puCDs@ZIF-8 before and after adding 10 μg·mL -1 TC under 360 nm excitation; (c) Fluorescence emission spectra of ZIF-8 after interacting with different concentrations of TC under 360 nm excitation; Figure 7 The infrared spectra of puCDs@ZIF-8 before and after adsorbing TC; Figure 8 The XPS spectra of puCDs@ZIF-8, (a, b) are the total spectra before and after adsorbing TC; (c, d), (e, f), (g, h) and (i, j) are the high-resolution spectra of C1s, N1s, O1s and Zn2p, respectively; Figure 9 (a) The influence of solution pH on the F 450 / F 530 ratio; (b) The influence of incubation time on the F 450 / F 530 ratio; Figure 10 The fluorescence response of puCDs@ZIF-8 to TC (blue) and without TC (red) in the presence of (a) metal cations, (b) anions / small molecules and (c) other antibiotics; (d) The influence of storage time on the fluorescence intensity of puCDs@ZIF-8. DETAILED DESCRIPTION

[0019] The present application will be further explained and illustrated by the following examples, but the specific examples do not limit the present application in any form.

[0020] In this embodiment of the invention, polycarbonate diol (PCDL) was purchased from Jining Huakai Resin Co., Ltd. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%) was purchased from Guangdong Guanghua Technology Co., Ltd. Other components included: dibutyltin dilaurate (95%, DBTDL), isophorone diisocyanate (99%, IPDI), 2,2-dimethylolpropionic acid (98%, DMPA), hydroxyethyl acrylate (96%, HEA), isopropanol (99.5%, IPA), ethylenediamine (98%), tetracycline (96%, TC), acetonitrile (99%, MeCN), 2-methylimidazole (98%, Hmim), citric acid (99.5%, CA), L-cysteine ​​(98%, L-Cys), L-valine (99%, L-Val), and L-malic acid (99%, L...). The following reagents were supplied by China National Pharmaceutical Group Chemical Reagent Co., Ltd.: sodium carbonate (99.8%, Na2CO3), potassium chloride (99%, KCl), ferrous chloride (98%, FeCl2), ferric chloride (98%, FeCl3), magnesium chloride hexahydrate (99.5%, MgCl2·6H2O), copper chloride hexahydrate (99.5%, CuCl2·6H2O), aluminum nitrate nonahydrate (99%, Al(NO3)3·9H2O), chromium nitrate nonahydrate (99%, Cr(NO3)3·9H2O), and sulfadiazine (99%, SMZ). Manganese chloride (99%, MnCl2), L-arginine (99%, L-Arg), L-lysine (98%, L-Lys), L-leucine (99%, L-Leu), L-tryptophan (99%, L-Trp), L-serine (99%, L-Ser), amoxicillin (95%, AMX), enrofloxacin (98%, ENR), erythromycin (85%, ERY), gentamicin sulfate (98%, GEN), and chloramphenicol (98%, CHL) were all purchased from Beijing Inocare Technology Co., Ltd. Acetone (99.55%, AC) was supplied by Hunan Huihong Reagent Co., Ltd. Lead nitrate (Pb(NO3)2) standard solution was obtained from Shenzhen Bolinda Technology Co., Ltd.

[0021] Example 1 This embodiment provides a method for preparing carbon quantum dot / metal-organic framework composite materials, such as... Figure 1 As shown, the steps include: S1. Preparation of waterborne polyurethane (WPU); S11. Under nitrogen protection, 50 g of polycarbonate diol (PCDL) was dissolved in 33 mL of acetone, and 0.09 g of dibutyltin dilaurate (DBTDL) was added as a catalyst. Then, under constant temperature of 45°C and mechanical stirring at 500 rpm, 30 g of isophorone diisocyanate (IPDI) was slowly added dropwise. After rinsing the inner wall of the dropping funnel with 7 mL of acetone, the solvent was added to the reaction system, and the mixture was stirred continuously for 1 hour to form a prepolymer.

[0022] S12. Add 6 g of dimethylolpropionic acid (DMPA) and 7 mL of acetone to the prepolymer to carry out the chain extension reaction. After continuing the reaction at 45°C for 1 hour, raise the temperature to 65°C and react for 2 hours. Add 0.04 g of DBTDL catalyst and continue the reaction for 3 hours.

[0023] S13. After cooling the system to 50°C, add 11 g of hydroxyethyl acrylate (HEA) and carry out the end-capping reaction for 2 hours. Finally, add 10 mL of isopropanol and stir for 30 minutes. After naturally cooling to room temperature, a colorless and transparent WPU is obtained.

[0024] S2. Preparation of carbon quantum dots (puCDs); 3.6 g of aqueous polyurethane, 600 μL of ethylenediamine, and 60 mL of deionized water were mixed in a polytetrafluoroethylene-lined high-pressure reactor and reacted at 200°C for 6 hours. After natural cooling to room temperature, the resulting solution was yellow-brown, indicating successful formation of puCDs. The solution was then filtered through a 0.22 μm filter to remove large particulate impurities and further purified for 24 hours using a dialysis bag with a molecular weight cutoff of 1000 Da to obtain puCDs.

[0025] S3. Preparation of carbon quantum dot / metal-organic framework composite material (puCDs@ZIF-8); S31. Dissolve 1.17 g of zinc nitrate (Zn(NO3)2) in 20 mL of puCDs solution to obtain a puCDs solution containing zinc nitrate; S32. Dissolve 22.70 g of 2-methylimidazole (Hmim) in 80 mL of puCDs solution to obtain a puCDs solution containing Hmim.

[0026] S33. At room temperature, a puCDs solution containing Hmim was rapidly poured into a puCDs solution containing zinc nitrate. The mixture immediately turned pale yellow. After stirring for 30 minutes, the precipitate was collected by high-speed centrifugation at 12,000 rpm for 10 minutes. The obtained pale yellow solid was washed three times with deionized water and ethanol, and then dried under vacuum at 60°C for 24 hours. The final product was gently ground to obtain a pale yellow puCDs@ZIF-8 powder.

[0027] In addition, the synthesis of pure ZIF-8 follows the same process, except that the puCDs solution in the system is replaced with an equal volume of deionized water.

[0028] (1) TEM and SEM were used to characterize the microstructure and particle size distribution.

[0029] like Figure 2 As shown in Figure a, puCDs exhibit an approximately spherical morphology and are well dispersed, indicating that their average particle size is approximately 1.58 nm.

[0030] Figure 2 As shown in b, the HRTEM image of puCDs reveals a lattice fringe spacing of 0.219 nm, corresponding to sp 2 The (100) crystal plane of graphite carbon.

[0031] Figure 2 Figures d and 2e show the morphological characteristics of ZIF-8 and puCDs@ZIF-8, respectively. The ZIF-8 crystal exhibits a uniform rhombic dodecahedral structure with an average grain size of approximately 117.43 nm. In contrast, the grain boundary profile of puCDs@ZIF-8 is more rounded, with an average grain size of approximately 118.65 nm, indicating that the introduction of puCDs has a certain influence on the growth of ZIF-8 crystals.

[0032] Figure 2 c shows that the XRD pattern of puCDs exhibits a broad diffuse diffraction peak near 21.05°, which is a typical feature of the inherent amorphous carbon structure of puCDs.

[0033] Figure 2 The XRD patterns of ZIF-8 and puCDs@ZIF-8 in f are highly consistent and match the diffraction peaks of simulated ZIF-8, confirming that the crystal structure is maintained despite the morphological change. No obvious puCDs characteristic peaks were observed in this composite material, which may be due to its small particle size and low concentration, making the diffraction signal undetectable.

[0034] (2) The surface functional groups of WPU, puCDs, ZIF-8 and puCDs@ZIF-8 were analyzed by Fourier transform infrared (FT-IR) spectroscopy.

[0035] like Figure 3 As shown, WPU at 3360 cm -1 A weak absorption band appears at 2958 cm⁻¹, attributed to the stretching vibrations of the NH and OH groups; -1 and 1745 cm -1 Stretching vibration absorption bands of CH and C=O groups were detected at 1534 cm⁻¹. -1The absorption peak at 1130 cm⁻¹ is related to the bending vibration of NH bonds and the stretching vibration of CN bonds, while the absorption peak at 1130 cm⁻¹ is related to the bending vibration of NH bonds and the stretching vibration of CN bonds. -1 The broad absorption bands nearby correspond to the stretching vibrations of the COC bond. These characteristic peaks collectively confirm the successful synthesis of WPU.

[0036] puCDs at 3576 cm -1 The broad absorption band in the vicinity is attributed to the stretching vibrations of the NH and OH bonds; 2937 cm⁻¹ -1 The absorption peak at 1650 cm⁻¹ corresponds to the CH stretching vibration. -1 The peak at 1145 cm⁻¹ belongs to the C=O stretching vibration. -1 The peak at this point is related to the stretching vibration of the CO bond. This confirms that the synthesized puCDs have hydrophilic groups such as -NH2, -COOH, and -OH on their surface, thus endowing them with excellent water solubility.

[0037] ZIF-8 exhibits unique absorption characteristics at 426 cm⁻¹. -1 The absorption peak at 600-1500 cm⁻¹ corresponds to the stretching vibration of the Zn-N bond; -1 Multiple absorption bands in the region are attributed to in-plane and out-of-plane bending vibrations of the imidazole ring; 1590 cm -1 The peak at 2932 cm⁻¹ originates from the stretching vibration of the C=N bond; while the peak at 2932 cm⁻¹ originates from the stretching vibration of the C=N bond. -1 With 3140 cm -1 The nearby peaks correspond to the stretching vibrations of the CH and NH bonds, respectively. These spectroscopic features collectively confirm the successful synthesis of ZIF-8.

[0038] puCDs@ZIF-8 at 3635cm -1 A new weak absorption peak appeared nearby, which originated from the OH stretching vibration of puCDs, thus verifying the successful preparation of the composite material.

[0039] (3) The specific surface area of ​​ZIF-8 and puCDs@ZIF-8 composite material was analyzed by using N2 adsorption-desorption isotherms.

[0040] Compared to the original ZIF-8, the BET specific surface area of ​​puCDs@ZIF-8 increased from 1476.61 m². 2 / g decreased to 993.56 m 2 The / g command confirms that puCDs has been successfully embedded into the ZIF-8 framework.

[0041] (4) Fluorescence property analysis like Figure 4As shown in Figure a, puCDs@ZIF-8 exhibits strong fluorescence emission under 360 nm excitation, while pure ZIF-8 shows no fluorescence signal, confirming that the fluorescence originates from the embedded puCDs. Compared with pure puCDs, the emission peak of the composite material shows a slight redshift. Since Zn-MOFs have a strong affinity for nitrogen-containing antibiotics, ZIF-8 can adsorb TC and generate aggregation-induced emission, thus producing fluorescence after binding TC.

[0042] Figure 4 b indicates that ZIF-8 produces stable and wavelength-independent emission after adsorbing TC. This characteristic, combined with the blue fluorescence of puCDs@ZIF-8, provides a basis for ratio sensing.

[0043] Figure 4 c and 4d images show that both puCDs and puCDs@ZIF-8 exhibit wavelength-dependent emission behavior, stemming from the structural heterogeneity and surface defects of the carbon core, further confirming the successful encapsulation of puCDs. Although puCDs@ZIF-8 shows strong fluorescence when excited near 330 nm, the emission of ZIF-8 after binding TC at this wavelength is negligible. Therefore, 360 nm was chosen as the optimal excitation wavelength, ensuring both signal strength and the reliability of ratiometric detection.

[0044] Example 2 This embodiment provides fluorescence response detection of tetracycline, including: (1) Fluorescence response of puCDs to TC To 1 mL of different concentrations (1-60 μg mL) -1 Add 200 μL of puCDs solution to a test tube containing TC solution, bring the volume to 3 mL with PBS buffer, and vortex for 1 minute. Record the emission spectrum in the wavelength range of 390–640 nm under 360 nm excitation conditions.

[0045] (2) Fluorescence response of puCDs@ZIF-8 to TC Take 20 mg of finely ground puCDs@ZIF-8 powder and place it in a clean glass bottle. Add 10 mL of PBS buffer solution (pH 7.4). Sonicate for 15 minutes to ensure uniform suspension. Transfer 400 μL of the suspension to a test tube and add 1 mL of PBS buffer and 100 μL of different concentrations (0-60 μg / mL). -1 The TC solution was mixed. Photoluminescence spectra in the range of 390-640 nm were collected at an excitation wavelength of 360 nm.

[0046] The fluorescence response characteristics of puCDs and puCDs@ZIF-8 in the TC sensing process are as follows: Figure 5 a shows that as the TC concentration increased from 0 to 60 μg / mL, -1 The fluorescence intensity of puCDs gradually decreases, exhibiting a good linear relationship. The relationship between the fluorescence response value (F0 / F) and TC concentration conforms to the Stern-Volmer equation: F0 / F = K SV C TC +1 in, F 0 and F The values ​​represent the fluorescence intensity of puCDs before and after the addition of TC, respectively. K SV The quenching constant is C TC This represents the TC concentration. The fitted linear calibration curve is F0 / F = 0.02487C. TC +1, linear correlation coefficient R 2 The value reached 0.9969. The linear detection range was 0-60 μg / mL. -1 The detection limit was 0.343 μg / mL. -1 (Calculated based on the 3σ criterion: 3σ / S, where σ is the standard deviation of the blank and S is the slope of the linear equation).

[0047] Figure 5 b demonstrates the ratiometric fluorescence response of puCDs@ZIF-8 to TC: as the TC concentration in the sensing system increases, the fluorescence intensity at 450 nm gradually decreases, while the emission peak at 530 nm continuously increases. (0-60 μg·mL⁻¹) -1 Within the concentration range, F 450 / F 530 The ratio exhibits a bilinear relationship with TC concentration, specifically represented by two calibration curves: 0-25 μg / mL -1 The interval conforms to F 450 / F 530 = 2.5794 - 0.04331C TC (R² = 0.9939), 25-60 μg mL -1 The interval conforms to F 450 / F 530 = 1.8803 -0.01455C TC (R² = 0.9912). The calculated limit of detection is 0.0089 μg / mL. -1 The concentration of TC was significantly lower than that of pure puCDs, indicating superior sensitivity and analytical performance. This bilinear characteristic can be attributed to a concentration-dependent TC adsorption mechanism: at low concentrations (0-25 μg / mL), the TC adsorption rate was significantly lower than that of pure puCDs, indicating superior sensitivity and analytical performance. -1TC easily enters the pores and effectively interacts with the fluorescent sites; at high concentrations (25-60 μg / mL) -1 The steric hindrance within the confined channels leads to adsorption saturation, which in turn slows down the fluorescence response rate.

[0048] To investigate the fluorescence sensing mechanism of puCDs and puCDs@ZIF-8 for TC, the UV-Vis absorption spectrum of TC and the fluorescence spectra of puCDs and puCDs@ZIF-8 were first analyzed.

[0049] Figure 6 As shown in figure a, the TC absorption spectrum significantly overlaps with the excitation spectra of puCDs and puCDs@ZIF-8, indicating that the weakening of the blue fluorescence intensity of puCDs and puCDs@ZIF-8 may originate from fluorescence resonance energy transfer (FRET) or infrafluorescence efflux (IFE). FRET is achieved through nonradiative energy transfer from the excited-state donor fluorophore to the acceptor molecule, which leads to a shortened donor fluorescence lifetime; while IFE is not a true quenching process, but rather a decrease in apparent fluorescence intensity caused by the absorption of excitation or emission light by other substances, and does not significantly affect the fluorescence lifetime.

[0050] The fluorescence lifetime of puCDs and puCDs@ZIF-8 at 450 nm before and after the introduction of TC was further examined. Figure 6 b shows that the fluorescence lifetime of puCDs and puCDs@ZIF-8 did not change significantly after the addition of TC, indicating that no excited-state energy transfer occurred during the quenching process, thus confirming that IFE is the main quenching mechanism.

[0051] Notably, fluorescence spectroscopy analysis revealed the enrichment behavior of TC within the puCDs@ZIF-8 framework. Figure 6 As shown in Figure c, ZIF-8, which originally emitted no fluorescence, exhibited a new emission peak at 530 nm after the addition of different concentrations of TC, and the fluorescence intensity was positively correlated with the TC concentration. This phenomenon indicates that the accumulation of TC within the ZIF-8 framework induces significant luminescence behavior.

[0052] To investigate the interaction mechanism between puCDs@ZIF-8 and TC, FTIR and XPS were used for characterization. Figure 7 The FTIR spectrum shown indicates that the sample after TC adsorption is at 3423 cm⁻¹. -1 A broad absorption peak related to the OH / NH vibration in the TC molecule appears at 600-1500 cm⁻¹, while the characteristic peak intensity of the Zn-N bond decreases, indicating that TC has been successfully adsorbed onto the material surface; simultaneously, the absorption peak at 600-1500 cm⁻¹... -1 The absorption peaks in the range showed no significant change, indicating that the framework structure remained stable.

[0053] Figure 8As shown, XPS analysis further confirmed the adsorption of TC. A new C=O characteristic peak appeared in C1s, and the binding energies of N1s and Zn2p were slightly positively shifted and decreased, indicating that the nitrogen-containing groups in the TC molecule coordinated with the Zn(Ⅱ) site in puCDs@ZIF-8.

[0054] Example 3 This embodiment provides the effect of solution pH and incubation time on the performance of fluorescent probes in the fluorescence response detection of tetracycline.

[0055] First, adjust the pH of the solution to 5, 6, 7, 8, 9, and 10 respectively. The test results are as follows: Figure 9 As shown in a, the F of puCDs@ZIF-8 450 / F 530 The ratio remained relatively stable within the pH range of 5-10; however, the addition of 25 μg mL -1 After TC solution was applied, the ratio remained stable in the pH range of 5-8, but gradually increased when the pH rose to 9-10. This phenomenon is attributed to the decomposition of TC under alkaline conditions, which leads to a decrease in fluorescence intensity at 530 nm. Therefore, PBS buffer at pH 7.4 was chosen as the optimal reaction condition.

[0056] The time response characteristics of puCDs@ZIF-8 to TC were further analyzed, such as... Figure 9 As shown in b, when 25 μg mL was added to puCDs@ZIF-8 -1 After TC solution, F 450 / F 530 The ratio decreased rapidly within 1 minute and remained stable between 1 and 16 minutes. This indicates that the reaction was nearly complete within the first minute. Therefore, 1 minute was ultimately chosen as the optimal reaction time.

[0057] Example 4 This embodiment provides the detection of the adsorption performance of antibiotics by puCDs@ZIF-8.

[0058] To evaluate the adsorption capacity of puCDs@ZIF-8 for TC, 1 mg mL -1 The puCDs@ZIF-8 adsorbent was mixed with a TC solution of known concentration at room temperature for 24 h, and the concentration of residual TC in the supernatant was checked using a UV-Vis spectrophotometer. The equilibrium adsorption capacity was calculated using the following formula:

[0059] In the formula, C 0 and C e The initial and equilibrium concentrations of TC (mg / L) -1 ), Vis the volume of the equilibrium solution (L), and m is the mass (g) of the adsorbent puCDs@ZIF-8.

[0060] Experimental results show that the equilibrium adsorption capacity of puCDs@ZIF-8 for TC in aqueous solution can reach 409.45 mg g. -1 Table 1 compares the adsorption performance of puCDs@ZIF-8 with other porous materials for TCs, further verifying its application potential in TC adsorption and removal.

[0061] Table 1

[0062] Note: [1]Chen, JT, Zhang, H., Chen, ST, Chen, WB, Yang, M., Lin,YY,&Dong, W. (2025). Insights into the mechanism of enhanced tetracyclineadsorption performance using the amino functionalization copper (II)-basedmetal-organic frameworks. Microporous and Mesoporous Materials, 113727. [2]Çavu o lu, FC,&Bayazit, SS (2025). Evaluation of wastepolyethylene terephthalate bottles as ligands for the synthesis of manganese-based metal-organic framework and removal of tetracycline antibiotics from aqueous solutions. Journal of Environmental Chemical Engineering, 13(3),116402. [3]Li, N., Zhou, L., Jin, X., Owens, G.,&Chen, Z. (2019).Simultaneous removal of tetracycline and oxytetracycline antibiotics fromwastewater using a ZIF-8 metal organic-framework. Journal of hazardousmaterials, 366, 563-572. [4]Li, Y., Wang, J., Huang, Z., Qian, C., Tian, Y.,&Duan, Y. (2021).An Eu-doped Zr-metal-organic framework for simultaneous detection and removalof antibiotic tetracycline. Journal of Environmental Chemical Engineering, 9(5), 106012. [5]Chen, X., Xu, J., Gong, S., Zhang, L., Bi, N., Gou, J., ...&Jia,L. (2025). Portable fluorescent film and gel microspheres for sensitivemulti-color detection and efficient adsorption of tetracycline. Separationand Purification Technology, 352, 128258. [6]Zhou, L., Zhang, G., Zeng, Y., Bao, X., Liu, B.,&Cheng, L. (2024). Endogenous iron-enriched biochar derived from steel mill wastewater sludge for tetracycline removal: Heavy metals stabilization, adsorption performance and mechanism. Chemosphere, 359, 142263. Example 5 This embodiment applies the method to the detection of total toxicity (TC) in milk. 1 mL of acetonitrile is mixed with 1 mL of commercially available milk and sonicated for 15 minutes. The sample is then centrifuged to remove denatured protein precipitates. The resulting supernatant is filtered through a 0.22 μm microporous membrane to remove residual proteins. Quantitative analysis is performed using the standard addition method: different concentrations of TC standard solution are added to equal volumes of diluted milk samples to prepare spiking levels of 0.5, 1.0, and 1.5 μg / mL. -1 The detection system was developed. After thorough mixing, the fluorescence intensity was measured, and the TC content in the sample was calculated based on the standard curve. Finally, the detection results were verified by ultraviolet-visible spectrophotometry. Three parallel determinations were performed, and the results showed that the spiked recovery rate of this method was 95.6-106.3%, and the relative standard deviation (RSD) was 2.9-6.4%. Compared with other TC detection methods, the performance advantages of this method were further evaluated, as shown in Table 2.

[0063] Table 2

[0064] Note: [7]Li, J., Dai, Y., Cui, J., Abrha, H., Kang, N.,&Liu, X. (2023). Dye-encapsulated Zr-based MOFs composites as a sensitive platform forratiometric luminescent sensing of antibiotics in water. Talanta, 251,123817; [8]Zhang, J., Li, Y., Teng, L., Cao, Y., Hu, X., Fang, G.,&Wang, S.(2023). A molecularly imprinted fluorescence sensor for sensitive detectionof tetracycline using nitrogen-doped carbon dots-embedded zinc-based metal-organic frameworks as signal-amplifying tags. Analytica chimica acta, 1251,341032; [9]Chang, Y., He, R., Wei, Y.,&Wang, L. (2023). Polyethyleneimine-sulfur quantum dot composites for dual-channel detection of tetracycline bycolorimetric and fluorescence sensing.ACS Applied Nano Materials, 6(9), 7414-7421;

[10] Yang, Q., Hong, H.,&Luo, Y. (2020). Heterogeneous nucleation andsynthesis of carbon dots hybrid Zr-based MOFs for simultaneous recognitionand effective removal of tetracycline. Chemical Engineering Journal, 392,123680;

[11] Lu, Y., Zeng, D., Zhao, J., Chang, Z., Deng,

[12] Sun, X., Xin, X., He, W., Cao, The detection results are in high agreement with the quantitative results of traditional UV-Vis spectrophotometry, confirming its reliable applicability for the detection of TC in actual samples.

[0065] Example 6 This embodiment provides performance testing of the selectivity, anti-interference, and stability of fluorescent probes.

[0066] Various interfering substances may coexist in the detection system, including metal cations (Na+). + K + Fe 2+ Mg 2+ Mn 2+ Cu 2+ Pb 2+ Al 3+ Cr 3+ Fe 3+ ), common anions (Cl) - NO3 - CO3 2- SO4 2-Small molecule organic compounds (citric acid (CA), L-malic acid (L-MA) and amino acids: L-cysteine ​​(L-Cys), L-valine (L-Val), L-arginine (L-Arg), L-lysine (L-Lys), L-leucine (L-Leu), L-tryptophan (L-Trp), L-serine (L-Ser)) and other antibiotics that may be present in milk samples (amoxicillin (AMX), enrofloxacin (ENR), erythromycin (ERY), sulfadiazine (SMZ), gentamicin (GEN), chloramphenicol (CHL).

[0067] Figure 10 As shown in ac, when these interfering substances were at a concentration of 300 μg / mL... -1 In the presence of a certain concentration, the fluorescence intensity ratio of the sensor (F) 450 / F 530 The changes are negligible compared to the unaffected system; however, the addition of 60 μg mL -1 The ratio changed significantly after TC, indicating that the sensor has a specific response to TC. More importantly, when the interfering substance (300 μg mL) -1 ) and TC (60 μg mL -1 When these substances coexisted, the fluorescence response of the probe was essentially the same as when TC was added alone, demonstrating that the presence of other substances had minimal impact, highlighting the excellent selectivity and anti-interference performance of puCDs@ZIF-8 for TC detection. Furthermore, by monitoring the fluorescence intensity of puCDs@ZIF-8 in PBS buffer at 4℃ for 18 consecutive days, with two-day intervals between each day, the fluorescence response was further improved. Figure 10 d) The fluorescence signal remained stable, confirming that the material has excellent storage stability.

[0068] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a carbon quantum dot / metal-organic framework composite material, characterized in that the steps include... include: S1. Aqueous polyurethane, ethylenediamine and deionized water are mixed at a mass ratio of 1.2~6: 0.2~1: 40~80, reacted by hydrothermal method, and purified to obtain a carbon quantum dot solution; S2. Zinc nitrate and 2-methylimidazole were dissolved in carbon quantum dot solution at a molar ratio of 1:40~100 to obtain solution A and solution B respectively. Solution A and solution B were rapidly mixed at a volume ratio of 1:4~10, stirred and reacted, and dried to obtain carbon quantum dot / metal-organic framework composite material.

2. The method for preparing the carbon quantum dot / metal-organic framework composite material according to claim 1, characterized in that, The preparation steps of the waterborne polyurethane include: A catalyst is added to an acetone solution containing polycarbonate diol. Then, isophorone diisocyanate is added dropwise under constant temperature stirring at 40-50°C, and the reaction is continued with stirring to form a prepolymer. Dimethylolpropionic acid is then added to the prepolymer for chain extension reaction. The reaction is carried out at 40-50°C for 0.5-1.5 hours, then the temperature is raised to 60-70°C for 4.5-5.5 hours. After cooling to 45-55°C, hydroxyethyl acrylate is added for end-capping. Finally, isopropanol is added and stirred. After cooling, a colorless and transparent waterborne polyurethane is obtained.

3. The method for preparing the carbon quantum dot / metal-organic framework composite material according to claim 2, characterized in that, The mass fractions of polycarbonate diol, isophorone diisocyanate, dimethylolpropionic acid and hydroxyethyl acrylate are 35%~65%, 25%~40%, 2%~12% and 3%~15%, respectively; the catalyst is dibutyltin dilaurate, and the amount of catalyst added is 0.1~1.5% of the total solid mass.

4. The method for preparing the carbon quantum dot / metal-organic framework composite material according to claim 2, characterized in that, The volume fractions of isopropanol and acetone are 5%-10% and 20-35%, respectively.

5. The method for preparing the carbon quantum dot / metal-organic framework composite material according to claim 1, characterized in that, In step S1, the hydrothermal reaction temperature is 150~250℃ and the reaction time is 4~8 hours.

6. The method for preparing the carbon quantum dot / metal-organic framework composite material according to claim 1, characterized in that, In step S1, purification is performed by dialysis with a molecular weight cutoff of 1000 Da.

7. A carbon quantum dot / metal-organic framework composite material, characterized in that, Obtained by the preparation method according to any one of claims 1 to 6.

8. The application of the carbon quantum dot / metal-organic framework composite material according to claim 7 in tetracycline detection.

9. The application of the carbon quantum dot / metal-organic framework composite material according to claim 8 in tetracycline detection, characterized in that, The testing steps include: (1) Mix the carbon quantum dot / metal-organic framework composite material with PBS buffer solution to prepare a solution with a concentration of 0.5~2.5 mg / mL. -1 The suspension was prepared, and 400 μL of the suspension was mixed with 1 mL of PBS buffer. Then, 100 μL of tetracycline solution of different concentrations was added to obtain mixed solutions. Photoluminescence spectra in the range of 390-640 nm were collected at an excitation wavelength of 360 nm as standard control spectra, and fluorescence intensity ratios (F0) were plotted. 450 / F 530 The standard curve of tetracycline concentration change; (2) Prepare the test solution containing tetracycline solution according to step (1), collect the fluorescence spectrum and intensity in the range of 390-640 nm under the excitation wavelength of 360 nm, and determine the concentration of tetracycline antibiotics in the test sample by combining the linear equation of the standard curve.

10. The application of the carbon quantum dot / metal-organic framework composite material according to claim 9 in tetracycline detection, characterized in that, The pH value of the mixed solution is 5-8.

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