Study of energy transfer processes between the rare earth ions in NaGdF4 nanoparticles tri-doped with rare earth ions Yb3+, Er3+ and Ho3+ or Tm3+ was carried out. The luminescence spectra in short-wave infrared and visible ranges were investigated. It was shown that Er3+ → Ho3+ energy transfer leads to Ho3+ luminescence increase. Both Er3+ and Ho3+ luminescence peaks were observed in short-wave infrared range. For Er3+ and Tm3+ co-doped nanoparticles it was hard to separate luminescence peaks in SWIR. However, both Er3+ and Tm3+ luminescence peaks were observed in visible range. We attribute this effects to Tm3+ → Er3+ energy transfer which occurs due to overlap of Er3+ and Tm3+ luminescence bands in short-wave infrared range which leads to Tm3+ luminescence decrease. This hypothesis was confirmed by study of β-NaGdF4 tri-doped with Yb3+, Er3+ and Tm3+ luminescence spectra during heating. The intensity of Tm3+ luminescence increased during heating due to non-resonant nature of Yb3+ →Tm3+ energy transfer and the shape of spectra changed.
The Yb3+-Tm3+-doped NaGdF4 upconversion nanoparticles were studied as contactless nanothermometers for the first biological tissue transparency window under 980 nm excitation. The single hexagonal phase NaGdF4:Yb3+-Tm3+ nanoparticles were synthesized by solvothermal technique. The influence of dopants concentration and pumping power density on thermal sensitivity and temperature resolution of obtained nanoparticles was analyzed. It was shown, that an increase of Yb3+ doping concentration leads to a strong increase in near-infrared Tm3+ luminescence intensity, which corresponds to transitions 3F2-3H6, 3F3-3H6, 3H4-3H6 and could be used for thermometry. The measured efficiency of upconversion luminescence for 80% of Yb3+ and 2% of Tm3+ doped nanoparticles was 5.0% compared to 0.2% efficiency for 30% of Yb3+ and 0.5% of Tm3+ doped nanoparticles. Laser induced heating of synthesized nanoparticles with ratiometric temperature measurement was studied. The increase of pumping power density negatively affected the sensitivity, but increased the accuracy of measurement due to the increased near-infrared luminescence. In addition, the comparison of different wavelengths for ratiometric thermal calibration was performed. It was shown, that the use of 680-720 nm luminescence peak to 730-750 nm valley intensity ratio for thermometry promotes significant enhancement of thermal sensitivity and temperature resolution. Thermal sensitivity of 4%∙C-1 and temperature resolution of 0.6˚C in 30-36˚C region were obtained for NaGdF4 nanoparticles doped with 80% of Yb3+ and 2% of Tm3+.
For clinical application in photothermal therapy the nanoparticles should be efficient light-to-heat converters and luminescent markers. In this work, we investigate upconversion nanoparticles with NaYxGd1-xF4 (x=0-1) host lattice as self-monitored thermo-agents for bioimaging and local laser hyperthermia with real-time temperature control.
The ability of non-contact temperature sensing using NaYxGd1-xF4 on one hand and laser induced heating on the other hand was shown. It was found, that the heat conversion luminescence efficiency is strongly affected by the concentration ratio of Gd3+ to Y3+ ions in host lattice. The optimal composition among the studied is NaY0.4Gd0.4Yb0.17Er0.03 with luminescence efficiency of 3.5% under 1 W/cm2 pumping power. Higher Gd3+ concentrations lead to higher heating temperature, but also to the decrease of the luminescence intensity and the accuracy of the ratiometric temperature determination. It was also shown that the optimization of Yb3+ doping concentration is one of the possible ways for optimization of the conditions of laser induced photothermal effects.
Experimental in vitro study of hyperthermia with use of upconversion nanoparticles on HeLa and C6 cell lines was performed. The investigated nanoparticles are capable of in vitro photothermal heating, luminescent localization and thermal sensing.
The great interest in upconversion nanoparticles exists due to their high efficiency under multiphoton excitation. However, when these particles are used in scanning microscopy, the upconversion luminescence causes a streaking effect due to the long lifetime. This article describes a method of upconversion microparticle luminescence lifetime determination with help of modified Lucy–Richardson deconvolution of laser scanning microscope (LSM) image obtained under near-IR excitation using nondescanned detectors. Determination of the upconversion luminescence intensity and the decay time of separate microparticles was done by intensity profile along the image fast scan axis approximation. We studied upconversion submicroparticles based on fluoride hosts doped with Yb3+-Er3+ and Yb3+-Tm3+ rare earth ion pairs, and the characteristic decay times were 0.1 to 1.5 ms. We also compared the results of LSM measurements with the photon counting method results; the spread of values was about 13% and was associated with the approximation error. Data obtained from live cells showed the possibility of distinguishing the position of upconversion submicroparticles inside and outside the cells by the difference of their lifetime. The proposed technique allows using the upconversion microparticles without shells as probes for the presence of OH− ions and CO2 molecules.
The structurization of holographic planes in holograms recorded in CaF2 crystal with color centers was found. The structurization is apparent in the formation of spiral bundles, which pierce the holographic planes. It testifies to self-organization of color centers in these planes. This process is believed to be linked with colloidal centers, 2D metal islets in the crystal lattice, whose formation and decay during hologram recording at temperatures of 150–190 °C may be considered as a dynamic phase transition that facilitates the generation of stable spatially inhomogeneous (dissipative) structures in the form of bundles. The bundles arise during hologram recording process and remain frozen on cooling of the crystal after the process is finished.
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