Anti-Stokes luminescence of up-converting nanocrystals SrF:Yb,Er can be used as a high pressure optical sensor alternative to the ruby fluorescence-scale. In nanocrystalline SrF:Yb,Er, high pressure reversibly shortens the emission lifetimes nearly linearly up to 5.29 GPa at least. Its advantage is the use of NIR (≈980 nm) radiation, highly penetrable for many materials. The shortening of up-conversion lifetimes has been attributed mainly to the changes in energy transfer rates, caused by decreased interatomic distances and increased overlap integrals between 4f electrons and the valence shells of ligand ions. The origin of high-pressure effects on the luminescence intensity, band ratio and their spectral position has been explained by the increased interactions and distortions of the crystal-field symmetry around the emitting ions in the compressed structure.

The concept of optical temperature sensing, based mainly on the band intensity ratio, line shift, or luminescence lifetimes, is utilized for noninvasive and rapid detection of local temperature values in the inspected systems. For these purposes, lanthanide-doped nanoparticles (NPs) seem to be the most promising luminescence nanothermometers, because they can use temperature-dependent emission lifetimes, as well as band ratios of alike thermally coupled levels (TCLs) and non-TCLs. Such thermal effects are especially well pronounced in the case of upconverting NPs, whose optical response is strongly affected by temperature. Here, we use the multidoped, colloidal Sr 2 LuF 7 :Yb 3+ −Ho 3+ −Er 3+ NPs, exhibiting bright yellow-green upconversion (λ ex = 975 nm) luminescence for multimode optical temperature sensing (≈20−56 °C) in a very broad spectral range (≈400−1700 nm), covering the visible, first, second, and third biological windows. Temperature sensing is realized based on the upconversion emission lifetimes, and numerous band intensity ratios of the rarely used and underestimated non-TCLs of Yb 3+ / Er 3+ and Yb 3+ /Ho 3+ , which results in exceptionally high thermal sensitivity values, up to 5−9% K −1 . Finally, the performed ex vivo experiments allow mimicking of biological conditions (tissue barrier) and optically monitoring of temperature changes of the aqueous system, using band intensity ratios and luminescence lifetimes in the NIR ranges.

For a number of years nanomaterials have been continuously devised and comprehensively investigated because of the growing demand for them and their multifarious applications, especially in medicine. This paper reports on the properties of SrF 2 nanoparticles (NPs) for applications in biomedicine, showing effective ways of their synthesis and luminescence under near infrared radiation - upconversion. NPs doped with lanthanide, Ln 3+ ions (where Ln = Yb, Ho, Er, Tm) were prepared by the hydrothermal method and subjected to comprehensive studies, from determination of their structure and morphology, revealing small, 15 nm structures, through spectroscopic properties, to cytotoxicity in vitro . The effects of such factors as the reaction time, type and amount of precipitating compounds and complexing agents on the properties of products were characterized. The cytotoxicity of the synthesized and functionalized NPs was investigated, using human fibroblast cell line (MSU-1.1). The synthesized structures may decrease cells’ proliferation in a dose-dependent manner in the measured concentration range (up to 100 µg/mL). However, the cells remain alive according to the fluorescent assay. Moreover, the treated cells were imaged using confocal laser scanning microscopy. Cellular uptake was confirmed by the presence of upconversion luminescence in the cells.

Ultrasmall (9-30 nm) Yb/Er-doped, upconverting alkaline rare-earth fluorides that are promising for future applications were synthesized by the microwave-assisted hydrothermal method. The formation mechanism was proposed, indicating the influence of the stability of metal ions complexes with ethylenediaminetetraacetic acid on the composition of the product and tendency to form MREF (MREF) cubic compounds in the M-RE-F systems. Their physicochemical properties (structure, morphology, and spectroscopic properties) are compared and discussed. The obtained nanoparticles exhibited emission of light in the visible spectra under excitation by 976 nm laser radiation. Excitation and emission spectra, luminescence decays, laser energy dependencies, and upconversion quantum yields were measured to determine the spectroscopic properties of prepared materials. The Yb/Er pair of ions used as dopants was responsible for an intense yellowish-green emission. The upconversion quantum yields determined for the first time for MREF-based materials were 0.0192 ± 0.001% and 0.0176 ± 0.001% for SrLuF:Yb,Er and BaLuF:Yb,Er respectively, the two best emitting samples. These results indicated the prepared materials are good and promising alternatives for the most studied NaYF:Yb,Er nanoparticles.

Core@shell nanoparticles showing up-conversion under 808, 975, 1208 or 1532 nm excitation were synthesised. Nanoparticles revealed their multifunctionality and emission bands covering the visible to near-infrared range (475 to 1625 nm).