Injection-controlled Synthesis And Bioimaging Applications Of Sandwiched Structure NaYF₄@NaYF₄:Yb/Tm@NaYF₄ Upconversion Nanoparticles

Fluorescence imaging is rapidly developing as a promising analytical bio-detection method.Traditional fluorescence biomarkers?e.g.,organic dyes and quantum dots?,have serious shortcomings such as biological toxicity and photobleaching,which limit their applications in biology.Rare-earth doped upconversion nanoparticles are emerging as new type of biomarker materials,which have high detection sensitivity and low toxicity due to their excellent optical properties and unique structure.In particular,when excited at 980 nm,Yb³⁺and Tm³⁺co-doped nanoparticles are able to emit an intense emission at800 nm in the first near infrared?NIR?biological window?NIR-I?.Since light absorption and scattering are significantly lowered in the NIR-I window,optical bioimaging in this window allows low imaging background,high penetration depth,and low biological damage.As a result,Yb³⁺and Tm³⁺co-doped upconversion nanoparticles hold tremendous promise as in vivo bioimaging contrast agents.However,these nanoparticles suffer from low fluorescence emission intensities or upconversion quantum yields.To solve this problem,we propose to use a sandwiched core-shell-shell structure,whereby the Yb³⁺and Tm³⁺active domain are sandwiched by two inert NaYF₄ domains.Optimization of the structure and coating were then performed.Subsequently,these nanoparticles were transferred from organic phase to aqueous phase,whose stability and biological toxicity were evaluated in detail.The in vivo imaging and metabolic processes in mice were also explored,and the ability to imaging these nanoparticles through human skulls were demonstrated.First,we proposed a sandwich structure upconversion nanoparticles,which were synthesized by injection of precursors at high temperature in a precisely controlled way.By adjusting the parameters of the injection?injection time,injection interval,the amount of precursors?,Yb³⁺and Tm³⁺co-doped NaYF₄:Yb³⁺,Tm³⁺active layers with varying thicknesses?0-5 nm?were successfully grown onto the NaYF₄ template seeds with a monolayer growth precision.Subsequently,the injection parameters were adapted to allow growing a uniform epitaxial inert shell outside the core/active shell nanoparticles.Then,the core/active shell nanoparticles were coated with different types of inert coating materials such as NaYF₄,NaLuF₄ and NaGdF₄,and the upconversion luminescence enhancement induced by different coating materials were correspondingly investigated.The results showed that the NaYF₄ inert shell layer had the greatest enhancement possibly due to having the identical host material to the NaYF₄ template seeds.After choosing NaYF₄ as the inert coating layer,we investigated the effects of the concentration of Yb³⁺and Tm³⁺on the fluorescence emission intensity,reaching an optimized doping concentration of 99%Yb³⁺and 1%Tm³⁺.On this basis,we studied the effect of the thickness of the outmost NaYF₄ inert layer on the enhancement of the upconversion luminescence intensity,showing the optimized coating thickness to be over3 nm.The upconversion quantum yield of the optimized nanoparticles was measured to be about⁸.7%,about three times greater than the highest one reported in literature.Subsequently,after phase transfer into water via polyacrylic acid?PAA?treatment,the high thermodynamic stability and low biological toxicity were verified by dynamic light scattering?DLS?and Hela cell viability test.And the bioimaging of these hydrated nanoparticles in mouse was studied by tail vein injection with ppm level concentrations.Then,we investigated the metabolic processes of these nanoparticles in mouse via ex vivo imaging and elemental content analysis of major isolated organs.Microscopic investigations of organic issues through Hematoxylin and eosin stain?H&E?were further implemented.All results showed no toxic effects to mouse after exposure to these upconversion nanoparticles.Finally,we studied the imaging ability and lateral imaging resolution of these optimized upconverting nanocrystals through a human skull,and a set of clear patterned images were obtained with a lateral resolution determined to be¹-2mm.