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.
Upconversion nanoparticles (UCPNs) capable of emitting near-infrared (NIR) light under NIR excitation attract much attention for bioapplications. However, the low upconversion efficiency and emission intensity significantly limits the progress in utilizing UCPNs. The present work is dedicated to the enhancement of upconversion efficiency and NIR luminescence intensity.The theoretical model that describes the changes in the populations of the excited states participating in the upconversion process was developed using a rate equations system for Yb 3+ -Tm 3+ ions in hexagonal NaGdF 4 . According to the modeling results, by increasing of Yb 3+ content to 80%, the intensity of NIR upconversion luminescence corresponding to 3 H 4 -3 H 6 transition could be increased up to 3.5 times in comparison to 30:1.5 Yb-Tm concentration ratio which was calculated to be optimal and up to 9.5 times in comparison to often used 30:0.5 Yb-Tm concentration. The optimal doping content regions were determined. For 80% Yb 3+ content the optimal Tm 3+ content will be 2.5%, for 30% Yb 3+ content-1.5%.Modeling results were confirmed experimentally by the synthesis of NaGdF 4 nanoparticles doped with Yb 3+ and Tm 3+ with the concentration ratios of 80:2, 80:3, 80:4, and 80:5. To obtain single-phase hexagonal samples at high doping concentrations, higher synthesis temperatures were required. A single-phase sample NaGdF 4 : Yb 3+ -Tm 3+ 80:5% was obtained at 320 • C synthesis temperature. According to the spectroscopic study of the obtained nanoparticles, the optimal concentration ratio for intense NIR luminescence obtaining was 80:3.The modeling results were in good agreement with the literature data and the results of our experiments. The developed model allows determining the optimal doping concentrations for obtaining effective NIR-to-NIR converters, based on NaGdF 4 :Yb 3+ -Tm 3+ nanoparticles.
Appropriate analysis of biological tissue deep regions is important for tumor targeting. This paper is concentrated on photons’ paths analysis in such biotissue as brain, because optical probing depth of fluorescent and excitation radiation differs. A method for photon track reconstruction was developed. Images were captured focusing on the transparent wall close and parallel to the source fibres, placed in brain tissue phantoms. The images were processed to reconstruct the photons most probable paths between two fibres. Results were compared with Monte Carlo simulations and diffusion approximation of the radiative transfer equation. It was shown that the excitation radiation optical probing depth is twice more than for the fluorescent photons. The way of fluorescent radiation spreading was discussed. Because of fluorescent and excitation radiation spreads in different ways, and the effective anisotropy factor,geff, was proposed for fluorescent radiation. For the brain tissue phantoms it were found to be0.62±0.05and0.66±0.05for the irradiation wavelengths 532 nm and 632.8 nm, respectively. These calculations give more accurate information about the tumor location in biotissue. Reconstruction of photon paths allows fluorescent and excitation probing depths determination. Thegeffcan be used as simplified parameter for calculations of fluorescence probing depth.
Upconversion materials have several advantages for many applications due to their great potential in converting infrared light to visible. For practical use, it is necessary to achieve high intensity of UC luminescence, so the studies of the optimal synthesis parameters for upconversion nanoparticles are still going on. In the present work, we analyzed the synthesis temperature effect on the efficiency and luminescence decay of β-NaGd0.78Yb0.20Er0.02F4 (15-25 nm) upconversion nanoparticles with hexagonal crystal structure synthesized by anhydrous solvothermal technique. The synthesis temperature was varied in the 290—320°C range. The synthesis temperature was shown to have a significant influence on the upconversion luminescence efficiency and decay time. The coherent scattering domain linearly depended on the synthesis temperature and was in the range 13.1—22.3 nm, while the efficiency of the upconversion luminescence increases exponentially from 0.02 to 0.10% under 1 W/cm2 excitation. For a fundamental analysis of the reasons for the upconversion luminescence intensity dependence on the synthesis temperature, it was proposed to use the maximum entropy method for luminescence decay kinetics processing. This method does not require a preliminary setting of the number of exponents and, due to this, makes it possible to estimate additional components in the luminescence decay kinetics, which are attributed to different populations of rare-earth ions in different conditions. Two components in the green luminescence and one component in the red luminescence decay kinetics were revealed for nanoparticles prepared at 290-300°C. An intense short and a weak long component in green luminescence decay kinetics could be associated with two different populations of ions in the surface quenching layer and the crystal core volume. With an increase in the synthesis temperature, the second component disappears, and the decay time increases due to an increase in the number of ions in the crystal core volume and a more uniform distribution of dopants.
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