Structural, optical and magnetic properties of Y3-0.02-xEr0.02Yb x Al5O12 (0 < x < 0.20) nanocrystals: effect of Yb content

The paramagnetic Y_3-0.02-x Er_0.02Yb _x Al₅O₁₂ (x = 0.02, 0.06, 0.10, 0.12, 0.18, 0.20) nanocrystals (NCs) were synthesized by the microwave-induced solution combustion method. The XRD, TEM and SEM techniques were applied to determine the NCs' structures and sizes. The XRD patterns confirmed that the NCs have for the most part a regular structure of the Y₃Al₅O₁₂ (YAG) phase. The changes of the distance between donor Yb³⁺ (sensitizer) and acceptor Er³⁺ (activator) were realized by changing the donor's concentration with a constant amount of acceptor. Under 980 nm excitation, at room temperature, the NCs exhibited strong red emission near 660 and 675 nm, and green upconversion emission at 550 nm, corresponding to the intra 4f transitions of Er³⁺ (⁴F_9/2, ²H_11/2, ⁴S_3/2) → Er³⁺ (⁴I_15/2). The strongest emission was observed in a sample containing 18% Yb³⁺ ions. The red and green emission intensities are respectively about 5 and 12 times higher as compared to NCs doped with 2% of Yb³⁺. In order to prove that the main factor responsible for the increase of the upconversion luminescence efficiency is reduction of the distance between Yb³⁺ and Er³⁺, we examined, for the first time the influence of hydrostatic pressure on luminescence and luminescence decay time of the radiative transitions inside donor ion. The decrease of both luminescence intensity and luminescence decay times, with increasing hydrostatic pressure was observed. After applying hydrostatic pressure to samples with e.g. 2% and 6% Yb³⁺, the distance between the donor and acceptor decreases. However, for higher concentrations of the donor, this distance is smaller, and this leads to the effective energy transfer to Er³⁺ ions. With increasing pressure, the maximum intensity of near infrared emission is observed at 1029, 1038 and 1047 nm, what corresponds to ²F_5/2 → ²F_7/2 transition of Yb³⁺.

Luminescent-Magnetic Cellulose Fibers, Modified with Lanthanide-Doped Core/Shell Nanostructures

Novel luminescent-magnetic cellulose microfibers were prepared by a dry-wet spinning method with the use of N-methylmorpholine-N-oxide. The synthesized luminescent-magnetic core/shell type nanostructures, based on the lanthanide-doped fluorides and magnetite nanoparticles (NPs)-Fe₃O₄/SiO₂/NH₂/PAA/LnF₃, were used as nanomodifiers of the fibers. Thanks to the successful incorporation of the bifunctional nanomodifiers into the cellulose structure, the functionalized fibers exhibited superior properties, that is, bright multicolor emission under UV light and strong magnetic response. By the use of the as-prepared fibers, the luminescent-magnetic thread was fabricated and used to sew and make a unique pattern in the glove material, as a proof of concept for advanced, multimodal cloths'/materials' protection against counterfeiting. The presence and uniform distribution of the modifier NPs in the polymer matrix were confirmed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis (EDX). The concentration of the modifier NPs in the fibers was determined by inductively coupled plasma mass spectrometry, EDX, and magnetic measurements. The luminescence characteristics of the materials were examined by photoluminescence spectroscopy, and their magnetic field-responsive behavior was investigated by a superconducting quantum interference device.

Mammalian cell defence mechanisms against the cytotoxicity of NaYF4:(Er,Yb,Gd) nanoparticles

Water-soluble upconversion nanoparticles (UCNPs), based on polyvinylpyrrolidone (PVP)-coated NaYF₄:Er³⁺,Yb³⁺,Gd³⁺, with various concentrations of Gd³⁺ ions and relatively high upconversion efficiencies, were synthesized. The internalization and cytotoxicity of the thus obtained UCNPs were evaluated in three cell lines (HeLa, HEK293 and astrocytes). No cytotoxicity was observed even at concentrations of UCNPs up to 50 μg ml^-1. The fate of the UCNPs within the cells was studied by examining their upconversion emission spectra with confocal microscopy and confirming these observations with transmission electron microscopy. It was found that the cellular uptake of the UCNPs occurred primarily by clathrin-mediated endocytosis, whereas they were secreted from the cells via lysosomal exocytosis. The results of this study, focused on the mechanisms of the cellular uptake, localization and secretion of UCNPs, demonstrate, for the first time, the co-localization of UCNPs within discrete cell organelles.