Construction And Properties Of Rare Earth Luminescent Gels Induced By Carboxyl Coordination In Nonaqueous Solvents

As one kind of soft materials,gels are widely used in many fields of daily life and production,such as nanomedicine,catalysis,drug delivery,lubricants and cosmetics.The gels with continuous structures and solid-like properties are construscted from gelators in certain solvents.Among them,supramolecular gels self-assembled by the low molecular weight gelators through non-covalent interactions have been widely concerned recently.The driving forces forming supramolecular gels mainly include electrostatic,?-? stacking,charge-transfer,or coordination interaction and hydrogen bonding,etc.The coordination interaction between metal ions and ligands not only triggers the gelation process,but also imparts unique properties of luminescence,conduction,redox,which has attracted much attention of researchers.The rare earth ions have excellent optical properties,including long lifetimes,line-like luminescence and high color purity.Therefore,the introduction of rare earth ions into the gels system through coordination interaction can not only improve the luminescent properties of rare earth ions,but also broaden the application of rare earth materials.Rare earth luminescent hydrogels have been preliminary studied.For these motivations,we choose the non-aqueous gels containing rare earth ions as the research objects.The coordination interaction between rare earth ions and carboxylic acid groups can induce the low molecular weight gelators,such as,benzenetricarboxylic acid and sodium deoxycholate,to form supramolecular gels.The microstructures,mechanical properties,formation mechanisms and fluorescence properties of the gels were studied and compared with the corresponding aqueous systems.The main points of this paper are divided into the following three parts:1.We introduce the overview of the gels,including the definition and classification of the gels,the main driving forces for the formation of supramolecular gels,luminescence principle of rare earth ions and the research status of rare earth luminescent gels.2.The organogels containing rare earth ions were obtained through mixing the rigid carboxylic acid ligand,benzenetricarboxylic acid and Tb3+ in 1:1 molar ratio in ethanol after ultrasonic treatment.The microstructures of gels system were characterized by scanning,transmission electron microscopy and X-ray energy dispersive spectroscopy.It was found that gels were mainly composed of fiber network.The fibers have branching structures,with diameters in between 100-200 nm.The Fourier transformed infrared spectroscopy results showed that the coordination interaction between Tb3+ and carboxyl groups was the main driving force for gel formation.The rheological measurements revealed a corresponding increasing in mechanical properties as increasing the concentration of Tb-BTC components.The fluorescence spectroscopy results showed that the obtained gels had an improved photophysical property compared with their corresponding aqueous solution due to the severe coordination water molecule fluorescence quenching.3.The aggregation behavior of biosurfactant sodium deoxycholate in proton ionic liquid ethylammonium nitrate was studied for the first time.It was found that the critical micelle concentration was 124.6 mM at 25 ?,and there was no gelation process.Adding Eu3+ below the critical micelle concentration of sodium deoxycholate obtained transparent or translucent ionogels.The scanning electron microscopy and X-ray energy dispersive spectroscopy results revealed that the gel microstructure was a typical fibrous grid and the diameters of fibers were in between 20-80 nm.When the concentration of sodium deoxycholate was fixed and the concentration of Eu3+ was increased,the fiber structure was closer.The Fourier transformed infrared spectroscopy and X-ray powder diffraction releaved that the ion-pairs were formed by hydrogen bond interaction between deoxycholic acid anion,and gels were formed by the coordination interaction between Eu3+ and ion-pairs.Herein,ethylammonium nitrate behaved as solvent and "bridge" to connect ion-pairs.The rheology and fluorescence spectroscopy results have revealed that lonogels also had excellent mechanical properties and improved fluorescence properties compared to corresponding hydrogels.We have further studied the system in different structures of ionic liquids.