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Background: Highly photochemically stable nanoparticles, in which upconversion luminescence can be excitedso-called upconversion nanocrystals (UC-NCs) -exhibit widely separated (up to 500 nm) narrow luminescence bands in the visible (VIS) region located far from the excitation near-infrared (NIR) laser radiation, and thus can be more easily identified compared to organic luminophores and semiconductor nanoparticles. Due to a deep penetration of exciting infrared (IR) radiation, the absence of parasitic fluorescence of biomolecules and the absence of phototoxicity and photobleaching upon near IR excitation, UC-NCs can be efficiently used as fluorescent probes in biological studies and fluorescence diagnostics (FD). The doping of such nanoparticles with Gd 3 + ions provides the additional possibility of combining fluorescence visualization with magnetic resonance imaging, which will considerably improve the sensitivity of diagnostics of cancer tumors even in the early stages. Materials and methods: We studied the upconversion characteristics of inorganic nanoparticles made of different materials doped with rare-earth ion (REI) pairs Yb 3 + -Er 3 + and Yb 3 + -Tm 3 + as functions of the concentration and composition of a dopant at different excitation intensities. Matrices chosen for doping were complex polycrystalline oxide rare earth compounds Gd 2 GeMoO 8 , La 4 Gd 10 B 6 Ge 2 O 34 , and Gd 11 SiP 3 O 26 which permit the introduction of significant concentrations of the activator luminescence ions (Yb 3 + , Er 3 + and Tm 3 + ), synthesized by solid-phase reaction methods from corresponding oxides. The final product was obtained by combined precipitation of initial components from aqueous solutions followed by the annealing of hydroxide mixtures and grinding. The redistribution of the intensity of the 550 nm 2 H 11/2 , 4 S 3/2 → 4 I 15/2 and 650 nm 4 F 9/2 → 4 I 15/2 upconversion luminescence bands in Er 3 + was investigated depending on matrices, dopants, and the laser power density. The quantum yield and lifetime of upconversion luminescence were determined for individual electronic transitions, which were used to optimize the composition of dopants in matrices. Results: Based on the results obtained, the matrix La 4 Gd 10 B 6 -Ge 2 O 34 is most effective for the upconversion process in the VIS spectrum. Doping nanoparticles by REI pairs Yb 3 + -Er 3 + and Yb 3 + -Tm 3 + , each has its advantages. REI pair Yb 3 + -Er 3 + is good for energy transfer in the green and/or red part of the spectrum as well as for the FD and can be used for a further energy transfer to the photosensitizers at photodynamic therapy (PDT). REI pair Yb 3 + -Tm 3 + transform the infrared radiation in the blue region of the spectrum, which is also suitable for FD and PDT and additional intensive energy conversion in NIR will allow for deep tissue imaging. Conclusion: The investigated complex polycrystalline oxide compounds are promising as diagnostic agents for biological tissues visualization by fluorescence, light scattering, and nuclear m...

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