Study On Preparation And Applications In Cancer Photodynamic Therapy Of Rare Earth Doped Gd₂O₃ Upconversion Nanoparticles

For a long time,cancer has always been one of the greatest killer for human beings.The number of deaths caused by cancer accounts for a quarter of total global deaths each year.Many methods have been developed to deal with cancer.Recently,photodynamic therapy?PDT?has attracted an increasing attention due to its advantages of less injury,strong targeting,little toxic side-effects and no damage to the hematopoietic system and immune system.However,in most cases,PDT is triggered by visible light,as a result of that most photosensitizers could only be activated by visible light.So near-infrared?NIR?-light-triggered PDT has become a hot topic due toNIR-light is rarely absorbed by biological tissues.Herein,in order to solve above problem,rare earth doped Gd₂O₃ nanoparticles were fabricated,which were anticipated to act as nanotranducers in NIR-triggered PDT.In this article,precursor of Yb³⁺ and Tm³⁺ codoped Gd₂O₃?Gd₂O₃:Yb,Tm?nanoparticles were synthesized by a simple hydrothermal method firstly,followed by a calcination process to obtain the final products of Gd₂O₃:Yb,Tm that exhibited excellent upconverting fluorescent properties.Experimental results show that the prepared nanoparticles emitted bright blue upconversion fluorescence?470-520 nm?when excited by 980 nm laser.The doping ratio of Yb element and Tm element has a significant effect on the emission intensity of the product.The sample that has a doping ratio of Gd:Yb:Tm = 0.959:0.039:0.002 emitted the brightest UC fluorescence.Human cervical cancer cells?HeLa cells?and hepatoma cells?HepG2cells?were used as cell models to assess cytotoxicity of the prepared Gd₂O₃:Yb,Tm nanomaterials.The results showed that cell viability measured with a co-culturing time of 24 h remained at 78-82.3% even with a high concentration of 500 ?g m L-1,indicating the low cytotoxicity of Gd₂O₃:Yb,Tm.In order to fulfill the role of Gd₂O₃:Yb,Tm nanoparticles as light tranducers in PDT,a well-selected PDT drug of merocyanine 540?MC540?,which shows absorption maximum just within the emission band of the upconverting fluorescence of Gd₂O₃:Yb,Tm nanoparticles,was loaded onto the nanoparticles to obtain Gd₂O₃:Yb,Tm-MC540.MC540 has absorption maximum within 495-540 nm band.Under the irradiation of 980 nm NIR-light MC540 could utilize the upconverting fluorescence emitting from Gd₂O₃:Yb,Tm nanoparticles to the utmost extent,and then generate a large quantity of singlet oxygen that can kill cancer cells.HeLa cells and HepG2 cells were used as cell models to study the applications of Gd₂O₃:Yb,Tm nanoparticles in NIR-triggered PDT.Prominent PDT killing effect on HeLa cells and HepG2 cells was accomplished without obvious hyperthermia damage under 980 nm laser irradiation with a very low laser dosage,i.e.,a low power density of 0.65W?cm-2 and a short irradiation time of 5 minutes.Experimental results of subcellular localization of Gd₂O₃:Yb,Tm-MC540 demonstrated that the prepared Gd₂O₃:Yb,Tm-MC540 not only could be endocytosed by HeLa cells and HepG2 cells,but they were also lysosome-targeted for both cell types.Cell death mode was further studied in details via an osmium aceticum section staining method to analyze cell morphologic change before and after PDT.The results provided substantial evidences that the Gd₂O₃:Yb,Tm-MC540 preferentially entered lysosomes of the cancer cells.Subsequent photodamage of the lysosomes triggered by 980 nm laser resulted in lysosome destruction and release of lysosomal enzymes into cytosol,which caused apoptosis and autophagy of the cancer cells.In the excess of damage,the cells were efficiently killed although autophagy was found to be initiated right after laser irradiation finished.As a result,secondary necrosis happened and finally led to remarkable decrease in cell viability.In this section occurrence of apoptosis was identified via fluorescent labeling of mitochondrion by rhodamine 123 and condensed chromatin observed in osmium aceticum stained cells.Autophagy was recognized by a large number ofautophagosomes?AP?and autophagic compartments?AC?observed in osmium aceticum stained cells,necrosis by lost of membrane integrity in the stained cells.Taken above all the results,the prepared Gd₂O₃:Yb,Tm upconverting fluorescent nanomaterials show great application prospects in NIR-triggered tumor photodynamic therapy.