Preparation,Characterization And Application Of Microfibrillated Cellulose/Rare Earth Multifunctional Materials

Rare earth elements(REE)show excellent luminescence performance due to their unique inner 4f electron configurations.However,pure rare earth complexes are greatly limited in practical application.By introducing rare earth complexes into some matrix materials,the application range of rare earth complexes can be greatly expanded.Usually,the traditional method is by doping rare earth complexes into some synthetic polymer or silica matrix materials.However,these matrixes show poor biocompatibility,unstable mechanical properties and may cause serious pollution to the environment.In recent years,microfibrillated cellulose has attracted wide attention as an environmentally friendly material in the field of preparation of high-toughness hydrogels.However,there are few reports on the preparation of rare earth complex hydrogels with good mechanical properties and excellent luminescence properties by combining biological macromolecules and microfibrillated cellulose.In this thesis,five kinds of environmentally friendly cellulose-based/rare earth functional materials were prepared by a facile freezing-thawing method.These luminescent functional materials were then characterized in details.Scanning electron microscopy(SEM)showed that these materials have multilayer porous structure.EDX analysis demonstrated that the rare earth complexes were uniformly distributed in the whole materials.No aggregation of rare earth complexes was observed.The materials have good thermal stability,which can be evidenced by thermogravimetric data.The materials also showed excellent toughness and shape memory function.Finally,the fluorescence spectra indicate that these materials have good luminescence properties.These materials can be used to detect some common cations(Fe³⁺,Ni²⁺,Cr³⁺),anions(NO₂^-)and organic reagents(pyridine,benzene,DMSO).It is worth noting that the detection limit is in the range of 10^-8M,showing huge potential in the application of luminescent probe.This thesis is mainly divided into the following parts:(1)Preparation and application of sodium alginate/microfibrillated cellulose/rare earth functional materials(Alg-Eu³⁺-TTA-MFC-hydrogel/Alg-Tb³⁺-Aba-MFC-hydrogel).Firstly,the hydrogels were prepared by crosslinking alginate and microfibrillated cellulose with epichlorohydrin,then rare earth ions were coordinated to carboxyl functional groups of sodium alginate.The rare earth ions were then connected to the 3D network of hydrogels by coordination bonds.Finally,luminescent hydrogel composite materials with rare earth complexes were obtained by reacting with suitable organic ligands.The finally prepared material showed excellent luminescence properties.These materials can be used to detect Ni²⁺and Fe³⁺ions simultaneously.They showed excellent selectivity and anti-interference.In order to realize the independent detection of Ni²⁺ions,Na F was used as masking agent to mask Fe³⁺ions,since[Fe F_n]^3-n and[Ni F_n]^2-n have different stability constant.The obtained material can also be used to detect NO₂^-ions in water.It was founded that these materials showed a detection limit of 0.01?M,which is significantl y below the m aximal level limits(0.69?M)of NO₂^-in drinking water set by the WHO.Finally,these materials can also detect pyridine.The proposed detection mechanism is as follows:since the ultraviolet absorption peaks of pyridine cover most of the absorption peaks of ligands,then pyridine will compete with the ligands for absorbing energy.Owning to the competition of energy absorption,only a small fraction of the energy is absorbed by the ligands,which lead to inefficient energy transfer from ligands to rare earth ions.(2)Preparation and application of Tb³⁺/Sodium alginate/sodium lignosulfonate/microfibrillated cellulose gels(Alg-SL-Tb³⁺-Aba-MFC-hydrogel).The mechanical properties of the gels materials were improved greatly by introducing a small amount of sodium lignosulfonate into the sodium alginate/microfibrillated cellulose gels system.By adjusting the content of sodium lignosulfonate in the composite materials,the toughness of the materials can be tuned.The above prepared materials can be used to detect Fe³⁺ions.Fe³⁺ions can greatly quenched the fluorescence of the rare earth complexes.The quenching mechanism may be described as follows:rare earth ions and Fe³⁺ions have the same valence state and similar coordination chelation ability,so Fe³⁺ions can compete with rare earth ions to absorb energy transferred from the ligands.This can greatly block energy transfer between ligands and rare earth ions,leading to the quenching of the luminescence.Furthermore,these materials can be used to detect benzene.Since benzene has a UV absorption band from 224 nm to 273 nm,which covers most of the absorption band of the ligands.Then,only a small amount of excitation energy can be transferred to the rare earth ions through the ligands,which leads to the quenching of the luminescence of rare earth ions.(3)Preparation and application of carboxylate chitosan/microfibrillated cellulose/rare earth functional materials(CCS-Eu³⁺-TTA-MFC-hydrogel/CCS-Tb³⁺-Aba-MFC-hydrogel).Carboxylate chitosan and microfibrillated cellulose were cross-linked by epichlorohydrin and hydrogen bonding.Then rare earth ions were coordinated to carboxyl functional groups in the carboxylate chitosan.In order to enhance the fluorescence of the materials,“antenna effect”between ligands and rare earth ions were realized by introducing suitable ligands into the materials.Besides,the form of hydrogel materials can be designed freely according to the needs.The prepared materials showed potential application to recognize Cr³⁺ions.These materials also can be used to detect organic reagent DMSO.When the DMSO content is only about 1%,the fluorescence showed obvious enhancement.We proposed that since DMSO does not absorb energy in the range between 200-400 nm,then there is no energy competition between DMSO and ligands.DMSO does not block the energy transfer from the ligands to the rare earth ions.By ligands exchange,DMSO can replace coordination water molecules surrounding the rare earth ions,then the fluorescence enhancement can be realized because the high frequency hydroxyl vibration of water molecules is greatly reduced.