Synthesis And Bio-applications Of Upconversion Nanoparticles Under 808 Nm Excitation

Lanthanide doped upconversion nannoparticals(UCNPs) have received many attentions over the past few years. UCNPs havemany excellent properties including large anti-Stokes shift, narrow-band emission, no photobleaching and no photoblinking, low toxicity and so on. In addition, near-infrared light was employed as excitation light which brings about high penetration depth, and low biological autofluorescence background. Based on these superior physicochemical features, UCNPs have been successfullyused in biolabeling, bioimaging and photodynamic therapy, photovoltaics. However, 980 nm light was used to trigger Yb3+-sensitized upconversion nanoparticals, which also absorbed by the water component in biological tissues thus limits penetration depth and caused severe overheating effects. To avoid this overheating effect, shifting the excitation wavelength to 808 nm is an elegant way because water has little absorption around this range. Moreover, Nd3+ ions can be used as new sensitizers due to Nd3+ions feature a sharp absorption band centered at 808 nm. The main contents of this dissertation are about synthesis and bio-applications of UCNPs under 808 nm excitation, which include five parts:(1) we synthesized Na YF4:Nd/Yb/Ho codoped upconversion nanoparticals and studied their upconversion luminescence(UCL) under 808 nm excitation. Optimal doped concentration of different ions was explored. High upconversion luminescence was achieved by modulating Nd3+, Yb3+ and Ho3+ concentrations. Especially, we found that two peaks appeared when plotting luminescence intensity against Yb3+ concentration, and explained this phenomena.(2) We synthesized Nd3+ ions sensitized core-shell nanostructure and optimized their UCL intensity under 808 nm excitation. To avoid quenching effect, we spatially separated Nd3+ ions in the shell and Ho3+ ions in the core. Compared with codoped structure, the concentration of Nd3+ was elevated to 20%, and Yb3+ was elevated to 20%~25%. In particular, the UCL of core-shell structure remarkably increased 37.8 times.(3) We constructed near-infrared organic dye(IR-806) mediate Nd3+-sensitized core-shell upconversion nanoparticals to enhance UCL under 808 nm excitation. Upconversion emission of Na YF4:Yb/Er@Na YF4:Yb@Na YF4:Nd core-shell-shell nanocrystals increased largely when bonded with near-infrared dye(IR-806), We proved that energy transfer efficiency from IR-806 to Nd3+ more efficient than IR-806 to Yb3+ due tolarge spectral overlap between emission of IR-806 and absorption of Nd3+. We also transferred these IR-806 mediate Nd3+-sensitized core shell UCNPs to water to perform bioimaging.(4) We performed photodynamic therapy(PDT) experiments under 808 nm excitation. In vivo biological application of upconversion nanoparticles(UCNPs) prefers excitation of 700-850 nm, instead of 980 nm, due to the less absorption of 808 nm for water. Recently approach in constructing robust Nd3+doped UCNPs subject to 808 nm excitation relies on a thick Nd3+ sensitized shell. However, for the very important and popular F?rster resonance energy transfer(FRET)-based applications, such as photodynamic therapy(PDT) or switchable biosensor, this structure has its restriction resulting in a poor energy transfer. We have in this work designed the Na YF4:Yb/Ho@Na YF4:Nd@Na YF4 core-shell-shell nanostructure. It is proved that this optimal structure balances the robustness of the upconversion emission and the FRET efficiency for FRET-based bio-application. A proof of the concept was demonstrated for photodynamic therapy and simultaneous fluorescence imaging of Hela cell triggered by 808 nm light, where low heating and high PDT efficiency were reached.It was well known that UCNPs excited by 808 nm is a new material of UCL developed in the last two years. This new material will take great effort for solving the challenging problems in the field of biomedical research.