Photothermal Conversion And Luminescent Thermal Properties Of Upconversion Nanoparticles

Upconversion nanoparticles?UCNPs?can absorb near-infrared?NIR?and emit visible light via multiphoton mechanism,and have unique upconversion luminescent?UCL?properties.Moreover,UCNPs have good photothermal conversion properties due to the non-radiative transition of the different energy level or the quenching.These make it possible to achieve multifunctional integration on a single UCNPs.In addition,the temperature change also has an important influence on UCL,which provides a new idea and method to obtain tunable multicolor emissions.In this thesis,small-sized UCNPs based on NaGdF₄ nanoparticles have been prepared by co-precipitation method,and their multifunctional integration based on photothermal conversion,thermal-induced luminescence and tunable multicolor were studied.Firstly,small-sized?<10nm?NaGdF₄:Yb,Er@NaYbF₄ UCNPs were prepared by successive layer-by-layer strategy.The coating of the active shell not only increases the absorption cross section of the excitation light but also protects the nano-nucleus,simultaneously improves the photothermal conversion and luminescence properties,and also has a good temperature sensing property,realizing the multifunctional integration on a single particle.NaGdF₄:Yb,Er@NaGdF₄:Yb,Nd UCNPs were prepared by introducing 50mol%Nd³⁺into the shell,and the excitation wavelength was adjusted to 808 nm to avoid laser damage to biological tissues.The water-soluble UCNPs are obtained through surface treatment,and still have good photothermal conversion and temperature sensing in the water system,laying the foundation for its multifunctional integration application in the biological field.Secondly,the luminescent thermal behavior of small-sized UCNPs sensitized by Yb³⁺and Nd³⁺were studied.It was found that NaGdF₄:Yb,Tm/Ho sensitized by Yb³⁺has an abnormal thermal-induced luminescence enhancement.This may be due to the multiphonon deactivation of Yb³⁺caused by the stretching of-OH originated the water molecular absorbed on the surface of nanoparticles.However,NaGdF₄:Yb,Ho@NaGdF₄:Yb,Nd sensitized by Nd³⁺is contrary to the above phenomenon.This may be due to the fact that the energy level structure of Nd³⁺is different from Yb³⁺ion,and the-OH of the water molecule cannot affect the excited state energy level of Nd³⁺.Finally,the effects of activation ions,core-shell structure,temperature and laser power on the emission color of UCNPs were studied.By changing the type of the luminescent center and introducing the Ce³⁺ion to annihilate the green light of Ho³⁺,the UCNPs that emit blue,green,and red light were obtained.Then based on the temperature dependence of the luminescence transition of Ho³⁺and Tm³⁺,the core-shell structure of Tm@Gd@Ho@Yb is constructed,the color of UCL was changed from blue to white to green with the increasing of temperature.And the multicolor tuning by thermal induced was realized on a single nanoparticles.Furthermore,in combination with their inherent photothermal conversion properties,the color shifting of the core-shell UCNPs could also be simply achieved by prolonging the laser irradiation time,making it safer for laser anti-counterfeiting applications and has a simple identification method.