The thesis includes literature and experimental parts. The first part introduces the reader to the topic while the later part contains synthesis and characterization details, results and publications. Up-conversion (UPC) luminescence materials are very interesting and they have plenty of potential applications of which bioanalytical ones are probably the most promising. 

A lot of research has been done to study and develop UPC luminescence materials and lanthanide (Ln) doped NaYF4 is maybe the most popular - especially Yb3+,Er3+ doped NaYF4. The trivalent lanthanides are very good for UPC luminescence due to their ladder-like energy level structures and long excited state lifetimes. Er3+, Tm3+ and Ho3+ activators are the best ones and often they are used together with an Yb3+ sensitizer which enhances the UPC efficiency. Moreover, NaYF4 is considered as the best host. 

The aim of this work was to prepare, study and improve UPC luminescence materials based on NaYF4. Yb3+ was used as the sensitizer and Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+ and Tm3+ were used as activators. The materials were prepared with co-precipitation synthesis and studied with TG-DSC, XPD, TEM, TOF-SIMS, XPS and EXAFS. UPC luminescence was studied under 976 nm excitation. 

The synthesis was optimized so that the obtainable luminescence intensity is now even 2 orders of magnitude stronger than before. Several factors contributing to differences between the old and the new improved material were found: The cubic-to-hexagonal phase transition temperature is ca. 100 °C lower. The crystal structure is always hexagonal. The particles are covered with sodium. There are less lattice strains. The Na site is partly occupied with Yb3+ ions which enhances the Yb3+-Ln3+ energy transfer. In the end, UPC luminescence is obtained from all the lanthanides except Sm3+ and Dy3+