Electron paramagnetic resonance (EPR) spectroscopy complemented with X‐ray diffraction, X‐ray fluorescence, and optical spectroscopy was used to study nanocrystalline CeO2 powder samples that exhibit weak room‐temperature ferromagnetism. EPR lines assigned to the Ce3+ trigonal sites were found for the first time in cerium dioxide that contains a trace impurity of Mn2+. This finding indicates that manganese dopant facilitates the conversion of the oxidation state of Ce4+ to Ce3+ in nanocrystalline CeO2. Our results support the view that Ce3+/Ce4+ pairs along with defects on the surface of nanoparticles are responsible for the ferromagnetism in CeO2. The EPR study reveals that the charge‐transfer mechanism proposed recently is more suitable to explain the origin of room‐temperature ferromagnetism in CeO2 than the F+‐centers exchange interactions.

The main goal of this study was creating multifunctional nanoparticles based on rare-earth doped LaF 3 nanocrystals, which can be used as fluorescence thermal sensors operating over the 80-320 K temperature range including physiological temperature range (10-50 ∘ C). The Pr 3+ :LaF 3 ( Pr = 1%) microcrystalline powder and the Pr 3+ :LaF 3 ( Pr = 12%, 20%) nanoparticles were studied. It was proved that all the samples were capable of thermal sensing into the temperature range from 80 to 320 K. It was revealed that the mechanisms of temperature sensitivity for the microcrystalline powder and the nanoparticles are different. In the powder, the 3 P 1 and 3 P 0 states of Pr 3+ ion share their electronic populations according to the Boltzmann and thermalization of the 3 P 1 state takes place. In the nanoparticles, two temperature dependent mechanisms were suggested: energy migration within 3 P 0 state in the temperature range from 80 K to 200 K followed by quenching of 3 P 0 state by OH groups at higher temperatures. The values of the relative sensitivities for the Pr 3+ :LaF 3 ( Pr = 1%) microcrystalline powder and the Pr 3+ :LaF 3 ( Pr = 12%, 20%) nanoparticles into the physiological temperature range (at 45 ∘ C) were 1, 0.5, and 0.3% ∘ C −1 , respectively.

The Pr3+:LaF3 (CPr = 3, 7, 12, 20, 30%) nanoparticles were characterized by means of high-resolution transmission electron microscopy, X-ray diffraction, optical spectroscopy, energy dispersive X-ray spectroscopy, dynamic light scattering, and MTT assay. It was revealed that the average diameter of all the NPs is around 14–18 nm. The hydrodynamic radius of the Pr3+:LaF3 (CPr = 7%) nanoparticles strongly depends on the medium. It was revealed that hydrodynamic radii of the Pr3+:LaF3 (CPr = 7%) nanoparticles in water, DMEM, and RPMI-1640 biological mediums were 18 ± 5, 41 ± 6, and 186 ± 8 nm, respectively. The Pr3+:LaF3 (CPr = 7%) nanoparticles were nontoxic at micromolar concentrations toward COLO-320 cell line. The lifetime curves were fitted biexponentially, and for the Pr3+:LaF3 (CPr = 7%) NPs, the luminescence lifetimes of Pr3+ ions were 480 ± 2 and 53 ± 5 nanosec.

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