Ratiometric Upconversion Temperature Sensor Based on Cellulose Fibers Modified with Yttrium Fluoride Nanoparticles.

In this study, an optical thermometer based on regenerated cellulose fibers modified with YF3: 20% Yb3+, 2% Er3+ nanoparticles was developed. The presented sensor was fabricated by introducing YF3 nanoparticles into cellulose fibers during their formation by the so-called Lyocell process using N-methylmorpholine N-oxide as a direct solvent of cellulose. Under near-infrared excitation, the applied nanoparticles exhibited thermosensitive upconversion emission, which originated from the thermally coupled levels of Er3+ ions. The combination of cellulose fibers with upconversion nanoparticles resulted in a flexible thermometer that is resistant to environmental and electromagnetic interferences and allows precise and repeatable temperature measurements in the range of 298-362 K. The obtained fibers were used to produce a fabric that was successfully applied to determine human skin temperature, demonstrating its application potential in the field of wearable health monitoring devices and providing a promising alternative to thermometers based on conductive materials that are sensitive to electromagnetic fields.

Generation of Pure Green Up-Conversion Luminescence in Er3+ Doped and Yb3+-Er3+ Co-Doped YVO4 Nanomaterials under 785 and 975 nm Excitation.

Materials that generate pure, single-color emission are desirable in the development and manufacturing of modern optoelectronic devices. This work shows the possibility of generating pure, green up-conversion luminescence upon the excitation of Er3+-doped nanomaterials with a 785 nm NIR laser. The up-converting inorganic nanoluminophores YVO4: Er3+ and YVO4: Yb3+ and Er3+ were obtained using a hydrothermal method and subsequent calcination. The synthesized vanadate nanomaterials had a tetragonal structure and crystallized in the form of nearly spherical nanoparticles. Up-conversion emission spectra of the nanomaterials were measured using laser light sources with λex = 785 and 975 nm. Importantly, under the influence of the mentioned laser irradiation, the as-prepared samples exhibited bright green up-conversion luminescence that was visible to the naked eye. Depending on the dopant ions used and the selected excitation wavelengths, two (green) or three (green and red) bands originating from erbium ions appeared in the emission spectra. In this way, by changing the UC mechanisms, pure green luminescence of the material can be obtained. The proposed strategy, in combination with various single-doped UC nanomaterials activated with Er3+, might be beneficial for modern optoelectronics, such as light-emitting diodes with a rich color gamut for back-light display applications.

Improving performance of luminescent nanothermometers based on non-thermally and thermally coupled levels of lanthanides by modulating laser power.

This work sheds light on the pump power impact on the performance of luminescent thermometers, which is often underestimated by researchers. An up-converting, inorganic nanoluminophore, YVO4:Yb3+,Er3+ (nanothermometer) was synthesized using the hydrothermal method and a subsequent calcination. This nanomaterial appears as a white powder composed of small nanoparticles (≈20 nm), exhibiting a very intense, green upconverted luminescence (λex = 975 nm), visible to the naked eye. Its emission spectrum consists of four Er3+ bands (500-850 nm) and one Yb3+ band (>900 nm). The obtained compound exhibits temperature-dependent luminescence properties, hence it is used as an optical nanosensor of temperature. The determined band intensity ratios of the non-thermally coupled levels (non-TCLs) of Yb3+/Er3+ and thermally coupled levels (TCLs) of Er3+ are correlated with temperature, and they are used for ratiometric sensing of temperature. The effects of the pump (NIR laser) power on the luminescence properties of the material, including band intensity ratios, absolute and relative sensitivities and temperature resolution are analysed. It was pointed out that the applied laser power has a huge impact on the values of the aforementioned thermometric parameters, and manipulating the laser power can significantly improve the performance of optical nanothermometers.

Luminescent Nanothermometer Operating at Very High Temperature-Sensing up to 1000 K with Upconverting Nanoparticles (Yb3+/Tm3+).

Lanthanide-based luminescent nanothermometers play a crucial role in optical temperature determination. However, because of the strong thermal quenching of the luminescence, as well as the deterioration of their sensitivity and resolution with temperature elevation, they can operate in a relatively low-temperature range, usually from cryogenic to ≈800 K. In this work, we show how to overcome these limitations and monitor very high-temperature values, with high sensitivity (≈2.1% K-1) and good thermal resolution (≈1.4 K) at around 1000 K. As an optical probe of temperature, we chose upconverting Yb3+-Tm3+ codoped YVO4 nanoparticles. For ratiometric sensing in the low-temperature range, we used the relative intensities of the Tm3+ emissions associated with the 3F2,3 and 3H4 thermally coupled levels, that is, 3F2,3 → 3H6/3H4 → 3H6 (700/800 nm) band intensity ratio. In order to improve sensitivity and resolution in the high-temperature range, we used the 940/800 nm band intensity ratio of the nonthermally coupled levels of Yb3+ (2F5/2 → 2F7/2) and Tm3+ (3H4 → 3H6). These NIR bands are very intense, even at extreme temperature values, and their intensity ratio changes significantly, allowing accurate temperature sensing with high thermal and spatial resolutions. The results presented in this work may be particularly important for industrial applications, such as metallurgy, catalysis, high-temperature synthesis, materials processing and engineering, and so forth, which require rapid, contactless temperature monitoring at extreme conditions.

Surface Modification of Luminescent LnIII Fluoride Core-Shell Nanoparticles with Acetylsalicylic acid (Aspirin): Synthesis, Spectroscopic and in Vitro Hemocompatibility Studies.

Luminescent lanthanide fluoride core-shell (LaF3 :Tb3+ ,Ce3+ @SiO2 -NH2 ) nanoparticles, with acetylsalicylic acid (aspirin) coated on the surface have been obtained. The synthesized products, which combine the potential located in the silica shell with the luminescent activity of the core, were characterized in detail with the use of luminescence spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and transmission electron microscopy (TEM) methods. The in vitro effects of the modified luminescent nanoparticles on human red blood cell (RBC) membrane permeability, RBC shape, and sedimentation rate were investigated to assess the hemocompatibility of the obtained compounds. This study demonstrates that LaF3 : Tb3+ 5 %, Ce3+ 10 %@SiO2 -NH2 nanoparticles with acetylsalicylic acid (aspirin) coated on the surface are very good precursors for multifunctional drug-delivery systems or bio-imaging probes that can be used safely in potential biomedical applications.

UV-Vis-NIR absorption spectra of lanthanide oxides and fluorides.

The absorption characteristics of lanthanide-based functional materials are of key importance for many scientists and engineers, e.g. in luminescence studies, bioimaging, optical heating/cooling, Raman spectroscopy, and industrial applications such as new light sources, optical sensors, labeling and tracing techniques, etc. Here we show the absorption spectra of solid, optically active lanthanide fluorides (CeF3, PrF3, NdF3, SmF3, EuF3, GdF3, TbF3, DyF3, HoF3, ErF3, TmF3, and YbF3) and oxides (CeO2, Pr6O11, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb4O7, Dy2O3, Ho2O3, Er2O3, Tm2O3, and Yb2O3), measured in the UV-Vis-NIR range, from 200 to 2500 nm. The spectra were measured in diffused-reflectance mode using a spherical integrator. We assigned energy levels (2S+1LJ) of lanthanide ions(iii), i.e. intraconfigurational 4f-4f transitions to the observed absorption bands. In order to clearly distinguish the 4f → 4f transitions, we also pointed out other absorption bands commonly observed in the measured spectra, such as intrinsic absorption of the matrices, interconfigurational 4f → 5d and charge transfer transitions, artificial bands from absorbed water (present in most materials) and a quartz holder.

Upconverting Lanthanide Fluoride Core@Shell Nanorods for Luminescent Thermometry in the First and Second Biological Windows: ß-NaYF4:Yb3+- Er3+@SiO2 Temperature Sensor.

Upconverting core@shell type ß-NaYF4:Yb3+-Er3+@SiO2 nanorods have been obtained by a two-step synthesis process, which encompasses hydrothermal and microemulsion routes. The synthesized nanomaterial forms stable aqueous colloids and exhibits a bright dual-center emission (λex = 975 nm), i.e., upconversion luminescence of Er3+ and down-shifting emission of Yb3+, located in the first (I-BW) and the second (II-BW) biological windows of the spectral range, respectively. The intensity ratios of the emission bands of Er3+ and Yb3+ observed in the vis-near-infrared (NIR) range monotonously change with temperature, i.e., the thermalized Er3+ levels (2H11/2 → 4I15/2/4S3/2 → 4I15/2) and the nonthermally coupled Yb3+/Er3+ levels (2F5/2 → 2F7/2/4I9/2 → 4I15/2 or 4F9/2 → 4I15/2). Hence, their thermal evolutions have been correlated with temperature using the Boltzmann type distribution and second-order polynomial fits for temperature-sensing purposes, i.e., Er3+ 525/545 nm (max Sr = 1.31% K-1) and Yb3+/Er3+ 1010/810 nm (1.64% K-1) or 1010/660 nm (0.96% K-1). Additionally, a fresh chicken breast was used as a tissue imitation in the performed ex vivo experiment, showing the advantage of the use of NIR Yb3+/Er3+ bands, vs. the typically used Er3+ 525/545 nm band ratio, i.e., better penetration of the luminescence signal through the tissue in the I-BW and II-BW. Such nanomaterials can be utilized as accurate and effective, broad-range vis-NIR optical, contactless sensors of temperature.

Optical Pressure Sensor Based on the Emission and Excitation Band Width (fwhm) and Luminescence Shift of Ce3+-Doped Fluorapatite-High-Pressure Sensing.

A novel, contactless optical sensor of pressure based on the luminescence red-shift and bandwidth (full width at half-maximum, fwhm) of the Ce3+-doped fluorapatite-Y6Ba4(SiO4)6F2 powder has been successfully synthesized via a facile solid-state method. The obtained material exhibits a bright blue emission under UV light excitation. It was characterized using powder X-ray diffraction, scanning electron microscopy and luminescence spectroscopy, including high-pressure measurements of excitation and emission spectra, up to above ∼30 GPa. Compression of the material resulted in a significant red-shift of the allowed 4f → 5d and 5d → 4f transitions of Ce3+ in the excitation and emission spectra, respectively. The pressure-induced monotonic shift of the emission band, as well as changes in the excitation/emission band widths, have been correlated with pressure for sensing purposes. The material exhibits a high pressure sensitivity (dλ/d P ≈ 0.63 nm/GPa) and outstanding signal intensity at high-pressure conditions (∼90% of the initial intensity at around 20 GPa) with minimal pressure-induced quenching of luminescence.