Prospects for application of upconversion particles NaYF$_4$: Er, Yb for phototherapy

Background and Objectives: Functionalized upconversion particles allow for photodynamic and photothermal therapy of tumor with simultaneous temperature monitoring and visualization of the area of treatment. Upconversion particles can increase the depth of therapeutic effects due to the high penetration depth of the required excitation radiation. That is why they are a promising material for the combined phototherapy and simultaneous monitoring of biological tissue heating. The purpose of the paper is to review the operating principle of upconversion particles, methods of synthesis and therapeutic applying. Results: The features of the hydrothermal synthesis of upconversion particles NaYF$_4$: Er, Yb, which seems to be the most promising, we considered. The hydrothermal method allows us to vary parameters of synthesized particles by adjusting the temperature and duration of synthesis, concentration of passivating and fluorinating agents. Thus, particles of different morphology, from rods to plates, and with different sizes, 10 nm - 5 $\mu$m, can be obtained. The subsequent functionalization of the particles by photosensitizer provides the photodynamic activity. The results of in vitro studies on marker substances and cells are presented. They confirm the efficiency of the generation of oxygen toxic forms in the presence of functionalized upconversion particles and their cytotoxic effect. Conclusion: It is shown that the use of such particles enables one to increase the depth of therapeutic effect. This leads to a higher effectiveness of PDT and expansion of the boundaries of its applicability. The described processes may give grounds for the development of photodynamic therapy methods aimed at expanding the field and functionality of the therapy.