The Influence Of Local Environment On The Luminescence Properties Of Upconversion Nanomaterials

Rare earth doped upconversion nanomaterials have many unique properties,such as narrow emssion bandwidth,large anti-Stokes shift,controllable emission color,good photochemical stability,long emission lifetime,deep tissue penetration depth and low damage to bio-tissues.Therefore,they are promising in various fields,including biomedicine,display,lighting,solar cells and so on.To provide guidance for the design and applications of upconversion nanomaterials,it is crucial to understand the modulation mechanism of upconversion luminescence thoroughly and systematically.To this end,we carried out a series of researches centered on the influence of local environment on the luminescence properties of the upconversion nanoparticles?UCNPs?,which involves the local environment of the host lattice,the local environment on the surface of the UCNPs and reabsorption-resonance energy transfer processes.The relevant influencing mechanism was discussed,and high-sensitive cell imaging was realized using magnetic-upconversion luminescent nanocomposites,which were constructed based on the luminescence modulation model utilizing reabsorption-resonance energy transfer processes.The details are as follows:?1?To further understand the effect of the local environment of the host lattice on the upconversion luminescence,we studied the influencing mechanism of the host type and ionic radius of the dopant on the luminescence properties of the upconversion nanomaterials.The results showed that compared with cubic phase,the doping of Zn2+ into hexagonal NaYF4:Yb³⁺,Er³⁺ UCNPs reduced the local lattice symmetry around the rare earth ions to a larger extent,and the resultant upconversion luminescence enhancement was thus one order of magnitude higher that of the cubic phase.Compared with Zn2+ and Cd2+,the doping of Ca2+,which has a larger ionic radius,reduced the local lattice symmetry around the rare earth ions in hexagonal UCNPs to a larger extent.The Ca2+ doped NaYF4:Yb³⁺,Er³⁺UCNPs also have a larger size than Zn2+ and Cd2+ doped UCNPs.It could weaken the surface quenching effect of the nanoparticles.Therefore,the upconversion luminescence enhancement of Ca2+ doped hexagonal NaYF4:Yb³⁺,Er³⁺ UCNPs was one order of magnitude higher that of Zn2+ doped UCNPs.The results provide important guidance for the design,luminescence modulation and modulation mechanism study of the upconversion nanomaterials.?2?To study the effects of the local environment on the surface of the UCNPs on upconversion luminescence properties,we analyzed the luminescence properties of NaYF4:Yb³⁺,Er³⁺ UCNPs dispersed in water and various non-aqueous solvents.It was found that the absorption of the excitation and emission light of the UCNPs by the solvents,as well as the nonradiative relaxation processes of the excited states of Er³⁺ ions induced by the high-frequency vibrational groups of the solvents,such as-OH,-CH2 and-CH3 groups,were responsible for the decrease in the upconversion luminescence intensities and increase in the red to green emission intensity ratios.Similar changes in upconversion luminescence properties were also triggered by increasing the water,OH-ion or DMF contents in mixed solvent,which would increase the content of-OH or-CH3 groups.Further study indicates that the effects of solvents on upconversion luminescence properties cannot be wiped off by just changing the dispersive solvents.The results can be extended to other luminescent nanomaterials and solvent systems,and provide guidance for the relevant applications.?3?To study the influences of reabsorption-resonance energy transfer processes on the upconversion luminescence properties,we constructed magnetic-upconversion nanocomposites based on the base pairing interaction of DNA molecules.The distance between Fe3O4 and NaYF4:Yb³⁺,Er³⁺ nanoparticles was controlled by adjusting the chain length of DNA,which further affected the reabsorption-resonance energy transfer efficiency.Based on this,the luminescence properties of the nanocomposites were modulated efficiently.Besides,the luminescent nanocomposites with suitable chain length of DNA were chosen as multi-functional bio-probes,and highly-sensitive magnetic targeted cell imaging was realized.The results will facilitate the expansion of the versatility and application fields of upconversion nanomaterials.