Preparation Of Composite Fluorescent Sensors Consisting Of Metal Organic Frameworks@nanocellulose For Detection Of Uranyl Ions

Uranyl ions(UO₂²⁺),a significant radionuclide that escapes into natural water mostly,can be detrimental to human health due to their radiant light.Therefore,for the preservation of the environment and the wise use of uranium resources,the detection and recovery of UO₂²⁺are of utmost significance.The use of lanthanide metal-organic frameworks（Ln-MOF）as a new porous material for the fluorescence detection of UO₂²⁺has been demonstrated to be extremely promising.Ln-MOF,on the other hand,are all powder materials with nano-and micro-scale dimensions that are challenging to work with and shape for actual applications.Ln-MOF may be made into one-dimensional（1D）,two-dimensional（2D）,and three-dimensional（3D）multi-scale,polymorphic,and flexible composites by mixing it with nanocellulose,which has high specific surface area,high aspect ratio,high mechanical strength,and biodegradable qualities.In addition,nanocellulose is an excellent carrier for functionalization due to its surface modifiability and ultra-fine template structure.A more effective use of the MOF active sites is made possible by its templating action,which stimulates the nucleation development of individual MOF particles and modifies the size morphology and loading distribution of MOF.By using TEMPO-oxidized nanocellulose（TOCNF）and bacterial nanocellulose（BC）as substrates,Ln-MOF and Ln-MOF@nanocellulose composite nanofibres were prepared in this thesis work.This method controlled the size of micron-sized rod-shaped Eu-MOF to nanoscale spherical shape,greatly enhancing its fluorescence properties.Then,the 2D nanopaper and 3D aerogel-based Ln-MOF fluorescence sensors with high sensitivity and quick response time were built and the enrichment of UO₂²⁺was successfully detected.The details of the study are as follows:（1）High fluorescence intensity Eu-MOF@TOCNF 1D nanocomposite fibers were prepared by rapid solvothermal in situ growth of Eu-MOF using nanocellulose as a carrier.The traditional solvothermal process requires along time and high-temperature heating,to obtain the micron scale synthesized MOF particles.And during the development phase,the fluorescence properties of the smaller size MOF particles are not studied.In this thesis work,the fluorescence intensity and surface morphology of Eu-MOF obtained over different reaction durations were characterized,and it was demonstrated that nanoscale spherical Eu-MOF could be constructed rapidly and efficiently within 30 min.Further,TEMPO oxidation was used to prepare TOCNF with high carboxyl content,and Eu-MOF was grown in situ on the TOCNF surface through the complexation of Eu³⁺with surface carboxyl groups.The presence of TOCNF significantly improved the size and morphology of Eu-MOF,and the diameter of Eu-MOF grown in situ on TOCNF was reduced from 88.4 nm to6.8 nm by transmission electron microscopy,which greatly improved its dispersion stability and binding sites.Due to the increased light accessibility of the nanoscale MOF,the fluorescence intensity of the Eu-MOF prepared by this method was found to be 25 times higher than that of the conventional solvothermal method by fluorescence spectroscopy,and the fluorescence intensity of the Eu-MOF@TOCNF nanocomposite fibers was found to be 225 times higher than that of the Eu-MOF prepared by the conventional solvothermal method.A new MOF preparation strategy is proposed for the first time to modulate the MOF nanosize and significantly enhance its fluorescence performance by controlling the short-time fast reaction.（2）Construction of a nanopaper-based fluorescent sensor based on Eu-MOF@TOCNF and its UO₂²⁺detection performance.The Eu-MOF@TOCNF composite 2D nanopaper was constructed by vacuum filtration,combining the good template structure and self-supporting properties of TOCNF.Since the Lewis basic sites on the Eu-MOF@TOCNF nanofibres can specifically recognize UO₂²⁺and lead to the quenching of Eu-MOF fluorescence,the 2D nanopaper can achieve sensitive detection of UO₂²⁺by fluorescence quenching.The sensing performance of the Eu-MOF@TOCNF nanopaper was investigated in the presence of different ionic interferences and it was found that it was still able to detect UO₂²⁺without interference from other ions in the solution.Quantitative analysis showed that the quenching constant K_SV value of the 2D nanopaper-based sensor for UO₂²⁺detection was 8.21×10⁴ M^-1 and the low of detection（LOD）was reduced from 1.32μM for Eu-MOF to 0.66μM for Eu-MOF@TOCNF.In addition,the paper-based sensor has good fluorescence stability and cyclic detection performance,enabling fast and convenient detection of UO₂²⁺in the aqueous phase.（3）Construction of a composite porous gel fluorescence sensor based on Tb-MOF@BC and sodium alginate（SA）and its UO₂²⁺detection performance.To determine the optimal reaction time for Tb-MOF synthesis,the changes in the morphology and fluorescence properties of Tb-MOF grown on BC at different times were investigated.The best performance was found to be obtained for the Tb-MOF@BC composite structures constructed on BC substrates at 5 h.Moreover,the fluorescence intensity of Tb-MOF@BC was enhanced to 12.5 times that of pure Tb-MOF,and its LOD to UO₂²⁺was reduced from 2.24μM to 0.42μM.To solve the problem that MOF particles are difficult to be recycled,Tb-MOF@BC was embedded in SA solution and cross-linked with Tb3+to prepare Tb-MOF@BC/SA gel microspheres for UO₂²⁺sensing detection.The fluorescence intensity of Tb-MOF@BC/SA gel microspheres remained stable after 20 days of immersion in water.In addition,the high mass transfer rate of the porous gel material allows for a rapid response burst to UO₂²⁺within 15 s and maintains good selectivity in the presence of other interfering ions.The value of the quenching constant KSV was found to be 8.87×10⁴ M^-1 in the quantitative detection of UO₂²⁺concentration,and the LOD for UO₂²⁺was reduced from 2.24μM to 0.61μM for Tb-MOF,making it a fast,sensitive,and stable 3D gel fluorescence sensor.