Construction,Properties And Sensing Applications Of Upconversion Luminescent Materials In LiErF₄:Tm System

Rare earth-doped upconversion nanoparticles(UCNPs)have a wide range of applications in bioimaging and therapy,anti-counterfeiting,detection,photovoltaic devices and photocatalysis due to their unique optical properties such as near-infrared excitation,narrow-band emission,large anti-Stokes shifts,superior photostability and biobackground-free autofluorescence.Although considerable progress has been made in the last few decades,the limitations of single excitation wavelength and the low upconversion luminescence(UCL)efficiency have hampered the further practical application of UCNPs.Therefore,the development of novel rare earth-doped UCL materials,the expansion of excitation wavelength selectivity,the improvement of UCL efficiency and the exploration of new UCL mechanisms are of great significance and research value to promote the applications of rare earth doped UCL materials.In this paper,LiErF₄:Tm system core-shell structure UCNPs with high concentration doping of luminescence centre Er³⁺were designed and prepared to obtain strong near-monochromatic upconversion red emission under various near-infrared wavelength excitations(?808 nm,?980 nm and?1530 nm).In addition,the UCL enhancement has been achieved by the modulation of local light field and high-pressure technique.Then UCNPs were used for water and cholesterol sensing,demonstrating the promising prospects of LiErF₄:Tm system UCNPs for sensing applications.The main results are as follows:1.Highly activator Er³⁺-doped LiErF₄:0.5%Tm³⁺@Li Ln F₄(Ln=Y,Lu,Gd)core-shell UCNPs were successfully prepared,the effect of heterogeneous inert shell coating on the UCL of LiErF₄:0.5%Tm³⁺was investigated,and the UCL mechanism of the structures was clarified.A series of LiErF₄:0.5%Tm³⁺@LiLnF₄(Ln=Y,Lu,Gd)and LiErF₄@Li YF₄core-shell UCNPs with controllable morphology and size were prepared by chloride solvothermal method.Due to the large amount of defects on the surface of LiErF₄:0.5%Tm³⁺bare core,it is easy to cause concentration quenching.The coating of heterogeneous inert shell layer can passivate the surface of LiErF₄:0.5%Tm³⁺UCNPs and suppress or even eliminate the concentration quenching effect to restore the UCL.In addition,we have systematically investigated the effect of epitaxial growth of LiErF₄:0.5%Tm³⁺UCNPs with different types of heterogeneous inert shell Li Ln F₄(Ln=Y,Lu,Gd)on the luminescence performance.The results show that the strongest UCL can be obtained by coating the Li YF₄ inert shell,whose mismatch to the LiErF₄ lattice is minimal.Furthermore,a comparative study shows that a doping of 0.5%Tm³⁺as the excitation energy capture center in LiErF₄ bare core can effectively suppress the luminescence quenching caused by energy migration to internal lattice defects and eventually promote the red emission of Er³⁺through energy transfer upconversion.The red UCL intensity is enhanced by an order of magnitude compared to that without Tm³⁺doping.Based on the intrinsic ladder-like energy levels of Er³⁺,LiErF₄:0.5%Tm³⁺@Li Ln F₄(Ln=Y,Lu,Gd)core-shell UCNPs exhibit the unique property of near-monochromatic red emission(?660 nm)under multi-wavelength excitations(?808 nm,?980 nm and?1530 nm).They are prospective for applications in biomedicine,sensing,solar cells and photodetectors.2.Pressure-induced UCL enhancement of LiErF₄:0.5%Tm³⁺@LiYF₄ UCNPs was achieved by high-pressure technique.In this chapter,we use the lattice distortion caused by pressure to regulate the local symmetry and the spatial distance between rare earth ions to enhance the UCL intensity of LiErF₄:0.5%Tm³⁺@Li YF₄ UCNPs under 980 nm excitation.A 2.6-fold UCL enhancement was obtained when the applied pressure was 6.15 GPa.The enhancement remained even after two compression-release cycles.The breaking of local symmetry during compression increases the transition probability of the luminous center.At present,the distance between Er³⁺and Tm³⁺is shortened and the energy transfer rate is accelerated during compression,which further inhibits the energy loss in lattice.The above two reasons lead to the enhancement of UCL intensity during compression.In addition,the local lattice distortion increases the splitting of the energy levels during compression,resulting in a broadening and red-shift of the emission band.The dependence of the energy transfer between Er³⁺and Tm³⁺in LiErF₄:0.5%Tm³⁺@Li YF₄ UCNPs on the non-radiation transition differs at different wavelength excitations,and the high pressure affects this non-radiation transition process,causing the material to exhibit excited wavelength-dependent high-pressure luminescence enhancement.The above results indicate that this UCL material has great potential in near infrared(NIR)activated high pressure responsive optical sensors and provides a viable method for enhancing UCL.3.The LiErF₄:0.5%Tm³⁺@LiYF₄ upconversion nanoprobe for trace water sensing has been constructed and achieved the ultra-sensitive detection of water content in organic solvents.The O-H phonons of water molecules easily resonate with the ⁴F_9/2?⁴I_9/2,⁴I_11/2?⁴I_13/2 and ⁴I_13/2?⁴I_15/2 energy gaps of Er³⁺and quench the red emission of LiErF₄:0.5%Tm³⁺@Li YF₄ upconversion nanoprobes.Based on the above property,LiErF₄:0.5%Tm³⁺@Li YF₄ upconversion nanoprobe was constructed for the detection of water content in various organic solvents.The excitation wavelength used for the detection is 808 nm,in comparison to 980 nm excitation,the water molecules absorb light at 808 nm weakly,which avoids the thermal effect caused by the absorption of excitation light and improves the sensitivity and accuracy of the detection.The results show that the limit of detection(LOD)of water is 30 ppm in acetonitrile,50 ppm in dimethylsulfoxide(DMSO)and 58 ppm in N,N-dimethylformamide(DMF).The relationship between the lifetime of each energy level of Er³⁺and the water content has been analyzed to reveal the quenching mechanism by water molecules of UCL.Due to the superior chemical and physical stability of UCNPs,these nanoprobes have excellent anti-interference and recoverability,making them suitable for real-time and long-term water monitoring.4.UCL enhancement of LiErF₄:0.5%Tm³⁺@LiYF₄ UCNPs was successfully achieved by photonic crystals effect,and was eventually applied to cholesterol sensing.We have successfully compounded LiErF₄:0.5%Tm³⁺@LiYF₄ UCNPs with poly(methyl methacrylate)(PMMA)photonic crystals(OPCs).The multi-wavelength excitation property of LiErF₄:0.5%Tm³⁺@Li YF₄ was utilized to enhance the UCL by selectively modulating the coupling between the photonic band gap(PBG)and the excitation field.A 48.5-fold red UCL enhancement was achieved at 980 nm excitation.Based on the principle that 3,3?,5,5?-tetramethylbenzidine(TMB)oxidation products(Ox TMB)generated from the cholesterol cascade reaction can quench the red emission of LiErF₄:0.5%Tm³⁺@Li YF₄ UCNPs,we constructed a cholesterol sensor with OPCs/UCNPs composite structure.The results showed that the LOD of cholesterol was 1.6?M with good specificity and stability.In particular,the cholesterol levels in serum measured by the designed sensor were highly consistent with clinical diagnostic data,providing a sensitive,reliable,reusable and interference-free assay for clinical cholesterol detection.