Controlled Synthesis And Multifunctional Integration Of NAF₄:Yb³⁺, Er³⁺ Upconversion Nanoparticles

As an important type of rare earth luminescent materials, upconversion nanoparticles (UCNPs) can convert low frequency light into high frequency light via a multiphoton mechanism. This unique anti-Stokes luminescence property endows them great potential for applications in various fields, such as solid-state lasers, three-dimensional displays, solar cells and laser security, etc. UCNPs have many advantages, such as low toxicity, good chemical stability, high light stability, and large Stokes shift, etc. In addition, the near-infrared excitation light of UCNPs is right in the light transmission window of biological tissue (630-1350nm), making them suitable to be used in biological imaging, drug delivery and release, and photodynamic therapy, due to their unique advantages including low radiation damage, improved tissue penetration depth, and high signal-to-noise ratios. In recent years, a lot of fruitful work has been made in terms of chemical synthesis, performance optimization and biomedical applications of UCNPs. However, there are still many challenges in dimensional control of UCNPs, temperature sensing properties and the improvement of upconversion luminescence efficiency. In addition, the multifunctional integrated performances of UCNPs also need further efforts to achieve their optimal combination. This thesis will conduct targeted research on the basis of the problems that UCNPs have meet in biomedical applications, and the major achievements are as follows:NaYF4:Yb3+, Er3+upconversion nanocrystals have been prepared using rare earth acetates as precursor. Based on the evolution of the morphology and crystal structure of UCNPs at different synthesis conditions, we revealed the phase change mechanism of NaYF4:Yb3+, Er3+ upconversion nanocrystals, and thereby establishing a theoretical model by using rare earth acetates to synthesize UCNPs. Furthermore, the particle size and morphology were effectively controlled by adjusting the raw materials and solvent ratios, respectively.The nucleation and growth process of P-NaYF4:Yb3+, Er3+upconversion nanocrystals were effectively controlled by adjusting the reactant content and changing the heating procedures, and ultrasmall β-NaYF4:Yb3+, Er3+ UCNPs have been successfully synthesized due to the improved nucleation rate and the reduced growth rate of upconversion nanocrystals.The effects of particle morphology and surface structure on the temperature sensing performa- nee of NaYF4:Yb3+, Er3+ UCNPs were studied, and an excellent nanothermometer with high reliability, strong adaptability and good stability has been designed. An anomalous size-dependent upconversion luminescence enhancement phenomena was observed with the increase of temperature, by studying the temperature-dependent luminescence performance of Yb3+/Ln3+ (Ln=Er, Tm, Ho) co-doped UCNPs with different particle sizes. Furthermore, the novel applications of UCNPs in multicolor temperature indication and anti-counterfeiting technologies were developed.Based on the preparation of NaYF4:Yb3+, Er3+ @Si02@Au composite nanostructures, the influences of Au particles adsorption and Au shell coating on the upconversion luminescence intensities of NaYF4:Yb3+, Er3+ UCNPs were investigated. A significant fluorescence quenching effect was observed for NaYF4:Yb3+, Er3+@Si02@Au composite nanostructures. The multifunction integrated performance of the composite nanostructures was systematically studied, and found various functions including upconversion luminescence, temperature sensing and photothermal conversion can be achieved simultaneously using NaYF4:Yb3+, Er3+@Si02@Au composite nanostructures. Therefore, these composite nanostructures have great potential applications in biomcdical imaging, cell temperature monitoring and photothermal therapy.