Synthesis Of Several Optical Chemosensors And Their Recognitive Sensing Properties And Binding Modes Studies On Metal Ions

Among a number of analytical methods that can be used to detect cations,optical chemosensors have displayed many advantages.The design and development of sensors has always been a research hotspot in the field of crown ether chemistry.The excellent chelating properties and structural features that can be easily modified of the azacrown ether ring make it an ideal structural component of optical chemosensors.Because of excellent photophysical properties and coordination ability,quinoline and coumarin groups can be used as suitable optical signal systems of sensors.Optical chemosensors obtained according to the above analysis,whether which are used for metal ion detection in biological and environmental systems or theoretical researches of sensing and binding mechanisms are both of great value.The multi-target analysis and detection by single sensor for multiple ions based on different optical signal is undoubtedly a more attractive research topic in this field,and this article has conducted a detailed discussion on this topic.In this paper,two kinds of optical chemosensor containing crown rings and one without crown ring were synthesized using quinoline and coumarin group derivatives as photosensitive systems,azacrown rings as acceptors and acylhydrazone or chalcone chains as linkers.Their selective recognition properties on different metal ions,ion interference properties,the binding reversibility,the linear range and the detection limit for the analytical test as well as the binding stoichiometry and stability constants were investigated by absorption and fluorescence spectroscopy methods.Moreover,possible binding modes of every sensor with responding ions were investigated using MS,IR and ¹H NMR titration methods.The specific experimental work is as follows:1.N-（8’-hydroxyquinoline-5’-carboxaldehyde acetylhydrazone）benzoaza-15-crown-5（L1）was synthesized,and the spectroscopic properties of L1were investigated in the presence of alkali and alkaline earth metal ions by means of absorption and fluorescence spectroscopy.The results of absorption spectra showed that L1 displayed an obvious selectivity for Mg²⁺in ethanol,as shown by the fact that pronounced red shifts in original absorbance peaks of L1 were observed in the presence of Mg²⁺,concomitantly resulting in the solution color change from colorless to yellow.Fluorescence spectroscopy showed that Mg²⁺caused a remarkable increasing of fluorescence peak of L1at 520nm.In addition,this sensor exhibited excellent chemical reversibility upon binding with Mg²⁺,which was supported by Na₂EDTA titration experiments.The results of detection limit tests showed that the detection limits of L1 for Mg²⁺in ethanol and acetonitrile were 2.39×10^-7 mol·L^-1 and7.95×10^-7 mol·L^-1,respectively.The binding constants（lgK_s）of L1 with Mg²⁺in ethanol and acetonitrile were found to be 4.54 and 3.57 respectively,and the binding stoichiometries were 1:1,which were obtained by the linear fitting using Benesi-Hildebrand equation.The binding modes of sensor L1 with Mg²⁺ion were further investigated by means of MS,IR and ¹H NMR titration spectroscopy.Preliminary analysis suggested that L1 might chelate Mg²⁺with a 1:1 stoichiometry through interactions with the carbonyl O atom,the imine N atom（Schiff-base）,the hydroxyl oxygen atom and the quinoline nitrogen atom.2.7-hydroxyl-4-methyl-8-（4’-phenylaza-15-crown-5-acryloyl）-coumarin（L2）was developed,and its recognition properties on common metal ions in ethanol and DMSO were investigated by UV-vis and fluorescence spectroscopy methods.The results of spectral tests in ethanol showed that Al³⁺and Cu²⁺resulted in a red shift of the absorption peak of L2 from 456 to520 nm and to 504 nm,respectively.Simultaneously,Al³⁺and Cu²⁺induced instant color changes of the solution of L2 from yellow to pink and to orange,respectively.However,Al³⁺and Mg²⁺incuced the generation of strong fluorescence emission peaks of L2 at 592 and 547 nm,respectively.In addition,L2 exhibited excellent chemical reversibility upon binding with Al³⁺,Cu²⁺and Mg²⁺,which was supported by Na₂EDTA titration experiments.Furthermore,the binding modes of L2 with Al³⁺,Cu²⁺and Mg²⁺ions were determined through Job’s plot,MS,IR and ¹H NMR titration experiments.Preliminary analysis indicated that the phenol hydroxyl,ketene and crown ring might involve in the complexation and sensor L2 chelated Cu²⁺and Mg²⁺at a 1:1 stoichiometry,while bound Al³⁺to form a 1:2 complex.The results of spectral tests in DMSO showed that L2 behaved apparently selective and recognitive abilities towards Al³⁺,Ni²⁺and Fe³⁺.L2 exhibited excellent chemical reversibility upon binding with Al³⁺,Ni²⁺and Fe³⁺,which was confirmed by Na₂EDTA titration.The detection limits of L2 for Al³⁺,Ni²⁺and Fe³⁺obtained by detection limit tests were 0.6,1.4 and 1.5μmol·L^-1,respectively.The binding constants（lg K_s）of L2 with Al³⁺,Ni²⁺and Fe³⁺were8.3,5.0 and 4.9,and the responding binding stoichiometries were 1:2,1:1 and1:1 respectively,which were obtained by the linear fitting using Benesi-Hildebrand equation.The multi-taget selective sensing properties of L2 for different ions with different optical signals showed its good application value.3.Sensor molecule 2,4-dihydroxybenzaldehyde-2-（4’-methylcoumarin-7’-oxy）acetyl hydrazone（L3）,having a keto-and enol-based tautomeric structure,was developed and its selective recognition properties on 17 kinds of metal ions in ethanol were inspected by absorption and fluorescence spectroscopy methods.The experimental results showed that the fluorescence spectroscopy of L3 only behaved specific recognition and sensitivity to Al³⁺.The addition of Al³⁺resulted in strong enhancement of the fluorescence peak of L3 at 414 nm,and the fluorescence response of L3 to Al³⁺was confirmed to be reversible by Na₂EDTA titration.the detection limit of L3 for Al³⁺obtained by detection limit test was 3.87×10^-7 mol·L^-1.The binding stability constant（lgK_s）and stoichiometry of L3 with Al³⁺obtained by the linear fitting using Benesi-Hildebrand equation were 62.4 and 2:1,respectively.Moreover,the binding mode of L3 with Al³⁺was futher studied through Job’s plot,MS,IR and ¹H NMR titration methods,which indicated that L3 might chelate Al³⁺with a 2:1 stoichiometry through interactions with the imine N atom（Schiff-base）,the enol O atom and the 2-hydroxyl O atom.