Optical Properties And Imaging Of Er-based Upconversion Nanocrystals

Lanthanide-doped upconversion nanoparticles,transforming near-infrared energy to visible and ultraviolet emission,have been widely used in deep tissue biological imaging,nanomedicine,optogenetics,safety labeling and so on.To date,upconversion nanoparticles with surface modifications have been widely explored in the imaging of mammalian cells,while imaging in plant cells has rarely been reported.This is mainly due to the fact that plants have cell walls that are different from animal cells.Plant cell walls are mainly composed of cellulose and pectin,and thick cellulose acts as a barrier to prevent large molecules from entering plant cells.The pore size of plant cell wall is about 3?10 nm.When the diameter of up-converted nanoparticles is larger than the pore size of plant cell wall,it is difficult to penetrate into plant cells.Therefore,nanoparticles used in plant cell imaging should be small enough.Unfortunately,when the size of up-converted nanoparticles decreases,the defects on the surface of the particles increase significantly,leading to the increase of surface quenching sites,and ultimately deteriorating the resolution of the imaging in cells for the reduced luminous efficiency.Therefore,it is still a difficult task to achieve high resolution of plant cells by using up-converted nanoparticles.Here,bipyramidal LiErF₄:1%Tm³⁺@LiYF₄ core-shell upconversion nanoparticles were synthesized by co-precipitation method.The internal and surface defects of the nanoparticles were modified by introducing Tm³⁺ions as the energy capture centers and the core-shell structure,which contributes to a significantly improved upconversion output.The as-obtained bipyramidal upconversion nanoparticles could readily puncture plant cell walls and further penetrate into cell membrane,facilitating an improved tissue imaging of cellular internalization,as demonstrated with the luminescence images obtained by the multiphoton laser-scanning microscopy.The main research contents of this paper are as follows:Firstly,LiYF₄:x%Er³⁺(x=10,20,50,80,100)nanoparticles were synthesized by thermal pyrolysis with particle size of 8 nm.It is found that the quenching degree of nanoparticles increases along with the increased concentration of Er³⁺.By introducing Tm³⁺as the energy capture centers,the energy loss of energy migration to the defect sites within the grain is reduced greatly,and the luminescence intensity of LiErF₄upconversion nanoparticles at 650 nm is significantly enhanced.Secondly,in order to passivate the defects on the surface of LiErF₄ upconversion nanoparticles for further enhance the up-conversion luminescence,we selected LiYF₄with a bipyramidal shell layer for coating.The bipyramidal structure has a high aspect ratio,which could easily penetrate the cell wall of a plant cell.We found that the nanoparticles coated with a LiYF₄ shell achieved super red light output at 980 nm,locating in the biological window with strong tissue penetration,which is beneficial to the imaging of plant cells.Finally,the bipyramidal LiErF₄:1%Tm³⁺@LiYF₄ upconversion nanoparticles were surface treated and incubated into onion epidermis cells.The upconversion emission is presented in the cells under multi-photon laser scanning microscope.Compared with spherical Na Gd F₄:Yb³⁺,Er³⁺counterpart,the bipyramidal LiErF₄:1%Tm³⁺@LiYF₄ nanoparticles are inclined to penetrate the cell wall and further penetrate the cell membrane,which significantly improved the cell-internal capability of imaging.This division of labor opens up a new way to explore the upconversion nanoparticles to achieve high resolution imaging of plant tissues.