The Study Of Scientific Problems Related With Near Infrared Light Excited Photodynamic Therapy

In recent years,rare earth nanocrystals have been widely studied in fluorescence imaging and photodynamic therapy,which is mainly because rare earth nanocrystals have a lot of features: near infrared(NIR)light excitation,non-photobleaching and non-photoblinking,weak autofluorescence,without photodamage,sufficient emission,narrow emission bands,and so on.As we all know,a main part of biomedicine application of rare earth nanocrystal is to develop photodynamic therapy(PDT).It is attributed to the introduction of rare earth nanocrystals adjusting the excitation wavelength of photodynamic therapy to NIR region.In this way,it is possible to realize the treatment for solid tumors.However,there are several servious problems limiting applications for rare earth upconversion nanocrystal combined photosensitizers(UCNC-PS)in PDT,including low reactive oxygen species(ROS)efficiency,high excitation power density,uncontrollable therapeutic effect,and so on.To solve these problems mentioned above,we have carried out the studies in upconversion luminescence efficiency to improve ROS generation efficiency,fluorescence imaging diagnosis to realize imaging guided PDT and development of a new type NIR light excited photosensitizer.The influence of surroundings to fluorescence properties of the rare earth centers has been investigated.The special multilayer core/shell nanostructure NaYF4/NaYF4:Yb3+,Er3+/Na YF4 was designed and synthesized.The special nanostructure contains three parts: one part in the center is supporting Na YF4 core;another part in the middle is a thin emitting Na YF4:Yb3+,Er3+ shell;the last part in the outer is shielding Na YF4 shell.The special nanostructure can guarantee every emitting center with the same influence from surroundings,and accurate influence to emitting centers from surroundings can be obtained by adjusting the thickness of shielding shell.The surrounding effects to luminescence properties of rare earth centers were explored by fluorescence spectra and time resolved spectra.It demonstrates that the green upconversion fluorescence intensity of the NaYF4/NaYF4:Yb3+,Er3+/Na YF4 nanocrystal has an exponential relationship with the thickness d of the shielding shell.When d decreases to 0,the fluorescence intensity is ~356 times smaller than intrinsic fluorescence intensity.Our experimental results also indicate that ~90% of the severe effect of the surroundings can be eliminated by coating with a 4 nm-thick Na YF4 shielding shell.Furthermore,the reason of the surrounding effects on fluorescence properties has been explored by the radiation properties of energy levels.Fluorescence probes with monochromatic fluorescence in NIR region have been developed.The monochromatic 800 nm fluorescence can be obtained in NaYF4:Yb3+,Tm3+ by adjusting Tm3+ doping concentration.The ratio of I800 to I470 is used to describe the monochromaticity.By the analysis of the spectrum,the monochromaticity of 800 nm emission shows exponential growth with the increase of Tm3+ doping concentration.When Tm3+ doping concentration reaches 4%,the I800/I470 can be as high as 757.In order to develop fluorescence imaging,the dependent of the absolute quantum efficiency with Tm3+ doping concentration has been investigated.At Tm3+ doping concentration of 4%,the absolute quantum efficiency of 470 nm decreases to 10-7 and 800 nm still reach 3.9× 10-3.Subsequently,the fluorescence imaging with monochromatic 800 nm upconversion emission has been carried out,realizing high contrast fluorescence imaging.A novel NIR excited photosensitizer Na Yb F4 nanocrystals has been proposed.NaYb F4 nanocrystal can be excited by NIR light and transfer energy directly to oxygen with a high efficiency.Therefore,NaYb F4 nanocrystals can be expected to realize effective treatment for solid tumors in deep tissue.In Na Yb F4 nanocrystals,Yb3+ plays an important role.The Yb3+ ions in Na Yb F4 absorb NIR and then transfer energy directly to oxygen to generate ROS.The generation of ROS by NaYb F4 nanocrystal has been demonstrated by chemical probe method.Furthermore,by the the response of Yb3+ decay time with oxygen concentration,the energy transfer rate between Yb3+ and O2 has been obtained,which presents a high energy transfer rate bewteen them.Meanwhile,the comparision of the ROS generation rate between NaYb F4 nanocrystals and UCNC-PS has been conducted,which shows NaYb F4 nanocrystals with a greater ROS generation rate.The photodynamic therapy based on the photosensitizer Na Yb F4 nanocrystals has been investigated.Before PDT was carried out,the treatment conditions were explored,including drug incubation time,drug incubation concentrations,the power densities,irradiation time,drug toxicity.PDT in vitro shows obvious cell cytotoxicity.When the cells are treated with drug concentration 10 ?g/m L and power density 575 m W/cm2 for 0.5 h,the cell survival rate still can decrease to 40%.Meanwhile,the cell viability decreases exponentially with the increase of therapeutic dose,which can guide to perform controllable PDT.Furthermore,the mechanism of cell death has been analyzed by flow cytometry.The result indicates that the cell death results from apoptosis and necrosis.Photodynamic therapy in vivo has also been carried out,which shows obvious decrease in volume of tumor.