Applications Of Functional Fluorescence Frameworks In The Detection Of Organic Small Molecules And Heavy Metal Irons

Fluorescent sensing is widely used in the field of chemical analysis and detection due to its high sensitivity,quick response and easy operation.The design and synthesis of luminescent porous frameworks has always been one of the focuses in the research of fluorescence sensing.In recent years,metal-organic frameworks（MOFs）and covalent organic frameworks（COFs）,which belong to the porous frameworks materials,have many advantages,such as large specific surface area,high porosity,diverse structure and easy functionalization.Therefore,the study of their fluorescence characteristics and sensing applications has aroused great interest of researchers.From the existing research results and practical applications,MOFs and COFs materials have played an important role in the detection of organic small molecules and heavy metal ions.With the improvement of detection requirements and the diversification of detection objects,the pristine MOFs/COFs and other single materials could not meet today’s requirements,which leads researchers in this field to explore functional design methods to make up for the shortcomings of pristine/single materials in detection applications,such as the selection of appropriate organic ligands or metal nodes,or the introduction of other functional materials（nanoparticles,dye molecules,etc.）into the cavity structure through post-modification or doping.MIL-101,UIO-66,ZIF-8 and COF-LZU 1,four representative parent framework materials in MOFs and COFs,were selected to design and synthesize four kinds of functionalized fluorescent materials in this paper,which were amino functionalized MIL-101（Al）-NH₂,Ui O-66-NH₂@Ru complex encapsulating[Ru（bpy）₃]²⁺,CDs-Fluorescein@ZIF-8 complex encapsulating carbon quantum dots and fluorescein,and COF-LZU-NH₂@FITC by post-modified with thiourea group,respectively.Then,the above four founctionalized nanomaterials have been realized in sensing applications of organic small molecules（ellagic acid,melamine,lysine）and heavy metal ions(Hg²⁺,Ag⁺),thus providing some novel ideas for the development and research of fluorescence sensors based on functionalized MOFs and COFs.The main works are as follows:（1）Here,the exploration of amino functionalized MOFs of MIL-101（Al）-NH₂for rapid and sensitive sensing of EA in aqueous solution was reported initially.The porous MIL-101（Al）-NH₂was synthesized by solvent-thermal method with inexpensive 2-aminoterephthalic acid and aluminum salt,which exhibited uniform spherical crystals（～340 nm）and specific mesoporous structure（3.2 nm）.The fluorescence intensity of MIL-101（Al）-NH₂at 425 nm showed a good linear relationship with EA concentration in the range of 0.15～100μM.The detection limit was as low as 43.8 n M,the rapid response time was within 2 min,and the cost of detection was low.In addition,the turn off fluorescence probe could be utilized for visual detection of EA according to the color change under the UV lamp.Based on the Stern-Volmer equation,the quenching constants were decreased with the rise of temperature,which indicated that the probable quenching mechanism was static quenching.Hydrogen bonding between functionalized amino group and four phenolic hydroxyl groups on EA was also an inducement for quenching.The nanosensor was successfully used to detect EA in the cherry and serum samples.MIL-101（Al）-NH₂represents the first instance of MIL MOFs-based fluorescent probe in EA detection.This work not only enriches the detection method of EA,but also expands the potential application of MOFs in small molecules.（2）Here,an amino functionalized zirconium-based metal-organic frameworks（Zr-based MOFs）encapsulated tris（2,2-bipyridyl）ruthenium（II）chloride hexahydrate([Ru（bpy）₃]²⁺)composite,donated as Ui O-66-NH₂@Ru,was reported and further studied its sensing application in melamine based on ratiometric fluorescent sensor.The Ui O-66-NH₂@Ru was prepared from Zr Cl₄,2-amino-terephthalic acid（TPA-NH₂）and[Ru（bpy）₃]²⁺by one pot reaction through solvothermal method.FRET phenomenon existed between TPA-NH₂ligand and[Ru（bpy）₃]²⁺.When melamine was adsorbed on framework ligand,the melamine could strongly interact with TPA-NH₂byπ-stacking,donor-acceptor interaction and hydrogen bonding,which disrupted the energy transfer of TPA-NH₂to[Ru（bpy）₃]²⁺.Therefore,the corresponding two peaks showed different responses to melamine,that was,the obvious enhancement at 445nm of Ui O-66-NH₂and the slow decrease of[Ru（bpy）₃]²⁺at 595 nm,thus achieving the“on-off”ratiometric fluorescence detection.When the concentration of melamine was in the range of 0.27 to 110μM,the ratio of fluorescence intensity F₄₄₅/F₅₉₅had a good linear relation with the correlation coefficient R²=0.995 and the detection limit of 90 n M.The synthetic Ui O-66-NH₂@Ru probe implemented rapid determination of melamine in real sample of milk powder with high sensitivity and selectivity.Compared with the reported carbon dots,quantum dots,metal nanoclusters,nanocomposites and other fluorescent sensors,MOFs-based sensor have broadened the application of nanomaterials in melamine detection.This study also demonstrates the Zr-based MOFs as a platform to encapsulate fluorescent dye for constructing ratiometric sensor in the determination of melamine is feasible and exciting.（3）A MOFs-based composite（CDs-Fluorescein@ZIF-8）containing carbon quantum dots（CDs）and fluorescein encapsulated in imidazole zeolite framework crystal（ZIF-8）was designed and synthesized for the simultaneous detection of lysine and Hg²⁺.Under the excitation of a single 365 nm,CDs prepared from spinach had a blue emission at 413 nm and a red emission at 668 nm,and fluorescein had a green emission peak at 511 nm.After the addition of lysine,the fluorescence intensity of CDs at 413 nm was enhanced,while the peak at 511 nm and 668 nm remained basically unchanged.There was a good linear relationship between the ratio of F₄₁₃/F₅₁₁and lysine concentration in the range of 0.3～373μM.Therefore,CDs-Fluorescein@ZIF-8 could be used as ratiometric sensor（turn on type）for the determination of lysine with a detection limit of 114 n M.In addition,after the addition of Hg²⁺,it quenched the fluorescence intensity of CDs at 668 nm,while the other two peaks basically had no change.There was a good linearity between the ratio of F₅₁₁/F₆₆₈and the concentration of Hg²⁺in the range of 0.04～28.625μM.Therefore,CDs-Fluorescein@ZIF-8 could be used as a turn-off sensor for the determination of Hg²⁺with a detection limit of 13.8 n M.To our knowledge,this is the first report of ratiometric detection of lysine and Hg²⁺targets based on the three-emission of ZIF-8composite sensor,which has good stability and water solubility,and can be used in the analysis of actual water、serum and grapes samples.The results indicate that it has potential practical application value in the diagnosis of lysine and Hg²⁺related diseases and disorders,food safty detection,heavy metal ion pollution and so on.（4）Here,an innovative fluorescence sensor,COF-LZU-NH₂@FITC,was rationnally designed by coupling fluorescent dye fluorescein isothiocyanate（FITC）into a porous amino-based covalent organic framework（COF-LZU-NH₂）for trace Ag⁺detection.The COF-LZU-NO₂was firstly synthesized from2-nitro-1,4-diaminobenzene and 1,3,5-benzenetricarboxaldehyde,and then it was reduced to an amino-functionalized COF-LZU-NH₂by using of Sn Cl₂,and finally FITC was grafted on COF-LZU-NH₂through covalent bonding.The COF-LZU-NH₂@FITC showed a strong emission peak originating from FITC at 514nm under 455 nm excitation wavelength,and the green fluorescence was significantly quenched by Ag⁺.The thiourea groups in the functionalized COFs could be used as the cheating point to recognize Ag⁺,that is,the coordination of Ag⁺with S and N will affect the p-πconjugation degree of FITC,resulting in the decrease of fluorescence.The fluorescence changes of I/I₀had a good linear relationship toward Ag⁺in the range of 0.05～10.625μM with the detection limits of 16.3 n M（S/N=3）,which were far lower than the maximum allowable amount of Ag⁺（460 n M）in drinking water.Therefore,the the unique design of COF-LZU-NH₂@FITC not only enables the non-fluorescent COFs to have strong fluorescence from FITC and be used as a fluorescence sensor,but also provides the specific recognition sites for the sensitive and selective identification of Ag⁺,which will give guidance to design more post-modified COFs for the"target-specific"applications in heavy metal irons.