Nanoparticle probes can unlock the potential for multimodal biomedical imaging (in vivo and in vitro) with enhanced spatial resolution and penetration depth and targeted visualization of complex organisms. This thesis demonstrates synthesis and characterization of magnetic upconversion Gd2O3 nanoparticles that can serve as bimodal probes for optical imaging in near infrared (NIR) biomedical window, where minimal tissue auto uorescence is expected, as well as magnetic resonance imaging. More speci cally, Gd2O3:Yb3+,Tm3+ and Gd2O3:Yb3+,Er3+ nanoparticles are synthesized using urea-based homogeneous precipitation method (UHP) as well as Y2O3:Yb3+/Er3+ micro and nanoparticles using conventional combustion and thermal synthesis methods. The morphological and compositional properties of nanoparticles as well as their photonic and magnetic responses are systematically analyzed to understand the role of synthesis methods and initial synthesis materials including the concentrations of Tm and Er dopants and urea solution on the properties of the synthesized particles. The upconversion nanoparticles synthesized using UHP method are spherical and monodisperse with a size distribution in the range of 60 to 150 nm and controllable dopant concentration through manipulation of initial synthesis chemistry. When excited with 975 nm NIR radiation, Gd2O3:Yb3+,Tm3+ nanoparticles show a pure near infrared emission centered at around 810 nm (i.e., NIR-to-NIR upconversion) in NIR biological window with potential for high depth optical imaging while Er3+ doped particles emit light mainly in visible red centred at around 661 nm. The photoluminescence and transient optical decay measurements demonstrate distinctly di erent energy transfer mechanisms for Er and Tm doped samples. While these measurements signify a dominant role for Yb3+ dopants in strong upconversion emission of Tm3+ samples with a double exponential decay behaviour, they show less important role of Yb3+ in Er3+ samples with a single exponential decay. Systematic magnetic characterization demonstrate strong paramagnetic behaviour for the optically active upconversion nanoparticles, demonstrating their potential for bimodal optical and magnetic resonant imaging
