The Preparation Of Functionalized Fluorescent Metal-organic Framework Composites And Their Application For Visual Quantitative Detection Of Trace Analytes

Objective:（1）Fluorescent polydopamine nanoparticles（PDANPs）were synthesized by a rapid,simple and green method,which could be selectively quenched by Fe³⁺and applied to quantitative and semi-quantitative visual detection of Fe³⁺in environmental water samples.（2）A novel ratiometric fluorescence（RF）sensor was synthesized to improve the selective and sensitivity of the guest material for Ag⁺owing to the excellent performance of the carrier material.Combined with the hydrogel visual sensing strategy,the ultra-sensitive and naked-eye quantitative detection of Ag⁺was realized.（3）A novel composite fluorescent nanomaterial was successfully prepared for the detection of F^-and ALP based on their selective fluorescent response to different wavelengths of RF sensors.Combined with the customized pseudocolor-assisted strategy,the prevention and diagnosis of osteofluorosis were realized.Methods:（1）Dopamine（DA）was used as the carbon source,and PDANPs were formed by accelerating the oxidation and polymerization rate of DA through a large amount of reaction heat generated by the redox reaction system composed of sodium iodate（SP）and sodium borohydride（Na BH₄）.The morphology and particle size of PDANPs were characterized by transmission electron microscopy（TEM）and dynamic light scattering（DLS）.UV-vis absorption spectrum and fluorescence spectrum were used to record the optical properties of PDANPs.FT-IR and XPS were used to analyze the functional groups and elements of PDANPs,which were used for the quantitative determination of Fe³⁺in environmental water samples because Fe³⁺could selectively quench their fluorescence.The quenching mechanism of PDANPs by Fe³⁺was demonstrated by measuring the fluorescence lifetime,calculating the quenching rate（k_q）and Stern-Volmer formula.Finally,the PDANPs-based paper sensor was prepared to realize the visual and semi-quantitative detection of Fe³⁺.（2）A novel RF sensor（FA-CDs@Eu-MOFs）was synthesized by encapsulating formamide derived N-doped carbon dots（FA-CDs）in the cavities of Eu（III）organic framework（Eu-MOFs）.The morphology and particle size distribution of FA-CDs,Eu-MOFs and their composites were obtained by TEM and HR-TEM.The optical characteristics of the product were described by UV-vis absorption spectrum and fluorescent spectrum.FT-IR,XPS and XRD were used to analyze the functional groups,elemental composition and structural characteristics of the products,respectively.Based on the special coordination between the C=N group and Ag⁺,the synthesized FA-CDs showed ultrasensitive and selective fluorescent response to Ag⁺,which were successfully applied for the quantitative determination of Ag⁺in environment water samples and commercial antibacterial Ag⁺solution.The detection results were verified by inductively coupled plasma mass spectrometry（ICP-MS）.Finally,FA-CDs@Eu-MOFs were combined with hydrogel to obtain a visual sensor to realize the visual quantitative detection of Ag⁺in antibacterial Ag⁺solution.（3）The red-emissive Cd Te QDs were encapsulated into the cavitives of blue-emissive hierarchically porous metal-organic framework（HP-MOFs）HP-Ui O-66-NH₂ to obtain dual-emission RF sensor（HP-Ui O-66-NH₂@Cd Te QDs）.The encapsulation efficiency of Ui O-66-NH₂ and HP-Ui O-66-NH₂ for Cd Te QDs was compared by TEM,HR-TEM and EDS.The structure and composition of the product were characterized by nitrogen adsorption and desorption,pore size distribution,XRD,FT-IR and XPS.Based on the fluorescence enhancement effect of F^-on HP-Ui O-66-NH₂ and the fluorescence quenching effect of alkaline phosphatase（ALP）on Cd Te QDs,the simultaneous sensing of F^-and ALP in human serum could be realized and then used for the prevention and diagnosis of osteofluorosis.Finally,HP-Ui O-66-NH₂@Cd Te QDs-based hydrogels were successfully prepared,whose visual sensing performance was enhanced by customized pseudo-color strategy for the accurate quantitative detection of F^-and ALP.Results:（1）A rapid,simple,green and large-scale synthesis strategy of fluorescent PDANPs was proposed.The large amount of reaction heat generated by the redox reaction system composed of SP and Na BH₄ accelerated the oxidation and polymerization rates of DA,which contributed to form PDANPs within 1 min without external heating,greatly reducing the preparation time and cost.TEM images showed that the synthesized PDANPs were quasi-spherical particles with an average particle size of about 30 nm.FT-IR and XPS showed that there were a large number of C=N groups on the surface of PDANPs,which were beneficial to improve their fluorescent performance.The PDANPs under the optimum preparation conditions showed high quantum yield（6.3%）,yield（21.0%）,good water dispersion and photostability.Fe³⁺could selectively quench the fluorescence of PDANPs with a wide linear range（0.3-100μM）and a low LOD（0.1μM）.The quenching mechanism between Fe³⁺and PDANPs was proved to be static quenching.Based on these,the prepared PDANPs were successfully used for the quantitative detection of Fe³⁺in real water samples.Moreover,a paper-based sensor was prepared by loading PDANPs onto filter paper,which had been used for the visual and semi-quantitative detection of Fe³⁺.（2）The FA-CDs emitting bright blue fluorescence were encapsulated into the cavities of Eu-MOFs emitting red fluorescence to obtain RF sensor.TEM,HR-TEM,XRD and FT-IR were used to prove that this encapsulation was successful.Based on the special coordination between the C=N group and Ag⁺,the synthesized FA-CDs could sensitively and selectively detect Ag⁺.At the same time,Eu-MOFs were used as the carrier and internal standard to improve the selectivity and sensitivity of FA-CDs to Ag⁺because of their excellent adsorption capacity.Based on these,the FA-CDs@Eu-MOFs sensor showed a good selectivity for Ag⁺and a detection linear range of 0.3-100 n M.The LOD was calculated to be 80 p M,which was the lowest value of all reported Ag⁺sensors based on CDs and MOFs.Based on their excellent sensing performance for Ag⁺,FA-CDs@Eu-MOFs were used for the determination of Ag⁺in environmental water samples and antibacterial Ag⁺solution.The detection results were comparable to those of ICP-MS,proving their reliability in real sample detection.A visual sensor was obtained by loading the FA-CDs@Eu-MOFs on hydrogel,which could accurately quantify trace concentration of Ag⁺by dosage-dependent color responses for the visual quantitative detection of Ag⁺.（3）Cd Te QDs were successfully encapsulated into the cavities of HP-Ui O-66-NH₂ and firmly connected by EDC and NHS.The morphology and structure of Cd Te QDs,Ui O-66-NH₂,HP-Ui O-66-NH₂ and their complexes were characterized by TEM,EDS,XRD and pore size distribution,which proved that Cd Te QDs were adhere to the surface of Ui O-66-NH₂instead of being encapsulating into its cavities because of the microporous structure.On the other hand,HP-Ui O-66-NH₂ possessed multiple pore sizes and enabled Cd Te QDs to enter its cavities,greatly improving the encapsulation efficiency.Moreover,the stronger adsorption capacity of HP-Ui O-66-NH₂ for analytes compared with Ui O-66-NH₂ also enhanced the sensing performance of itself and guest materials.Hydrogen bonds were formed between the amino groups of HP-Ui O-66-NH₂ and F^-,which facilitated the transfer of electrons from the ligand to the Zr-cluster to enhance the blue fluorescence of MOFs.At the same time,ALP could catalyze the decomposition of p-nitrophenol phosphate disodium salt（p NPP）to p-nitrophenol（p NP）,which could quench the red fluorescence of Cd Te QDs due to the inner filtration effect.Based on these,HP-Ui O-66-NH₂@Cd Te QDs could simultaneously detect F^-and ALP with the LOD of 0.15μM and 0.045 U/L,respectively.The method had been applied to the quantitative determination of F^-in groundwater and simultaneous detection of F^-and ALP in human serum,whose detection results were consistent with that of ion chromatography（IC）and colorimetric method,proving its reliability for practical sample detection.Finally,agarose hydrogel was used to prepare the visual sensor.Based on the enhanced ability of human eyes to distinguish color changes by the customized pseudo-color strategy,the HP-Ui O-66-NH₂@Cd Te QDs-based RF sensor could realize the accurate visual detection of F^-and ALP.Conclusion:（1）In this study,we designed a rapid,simple and green method for large-scale synthesis of fluorescent PDANPs at room temperature.Compared with the previously reported DA-based synthesis strategy for fluorescent organic nanoparticles,the proposed self-exothermic reaction preparation method was faster,more energy efficient,and easier to large-scale production.The synthesis of PDANPs in this work also had a broad application prospect in the fields of sensing,clinical diagnosis and environmental monitoring.（2）In this work,a novel RF sensor was prepared by encapsulating FA-CDs in the cavities of Eu-MOFs and used for visual ultra-sensitive and quantitative detection of Ag⁺.This study also expanded the application potential of Ln-MOFs and provided an effective pathway for the development of visual fluorescent sensors from qualitative or semi-quantitative to quantitative detection without expensive instruments or complex processes.（3）In this work,Cd Te QDs were encapsulated into the cavities of HP-Ui O-66-NH₂to form a stable RF sensor for the quantitative detection of F^-and ALP.HP-MOFs were applied to the field of fluorescent sensing and overcame the previous size limitation of micropores MOFs.In addition,the fluorescent response at dual wavelength of RF sensor was used for the simultaneous quantitative detection of F^-and ALP by mutual internal standard method,which was used to realize the prevention and diagnosis of osteofluorosis.This work not only broadened the application of HP-MOFs,but also provided a paradigm for the application of fluorescent nanomaterials in the field of disease diagnosis.