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Some of the old and unrealizable dreams of biomedicine have become possible thanks to the appearance of novel advanced materials such as luminescent nanothermometers, nanoparticles capable of providing a contactless thermal reading through their light emission properties. Luminescent nanothermometers have already been demonstrated to be capable of in vivo subcutaneous punctual thermal reading but their real application as diagnosis tools still requires demonstrating their actual capacity for the acquisition of in vivo, time-resolved subcutaneous thermal images. The transfer from 1D to 2D subcutaneous thermal sensing is blocked in the last years mainly due to the lack of high sensitivity luminescent nanothermometers operating in the infrared biological windows. This work demonstrates how core/shell engineering, in combination with selective rare earth doping, can be used to develop supersensitive infrared luminescent nanothermometers. Erbium, thulium, and ytterbium core-shell LaF 3 nanoparticles, operating within the biological windows, provide thermal sensitivities as large as 5% °C −1 . This "record" sensitivity has allowed for the final acquisition of subcutaneous thermal videos of a living animal. Subsequent analysis of thermal videos allows for an unequivocal determination of intrinsic properties of subcutaneous tissues, opening the venue to the development of novel thermal imaging-based diagnosis tools.

the measured spectroscopic parameter is usually a pressure-induced line shift, i.e., spectral shift of the emission bands of Cr 3+ (ruby) or Sm 2+ . [14,15,20,22] Whereas, in the case of temperature the commonly measured parameter is the luminescence intensity ratio (LIR), i.e., band ratio of two thermally coupled levels (TCLs; separated by ≈200-2000 cm −1 ) of, e.g., Nd 3+ , Er 3+ , or Tm 3+ , which is directly related to the local temperature of the system (probe), and conforms Boltzmann distribution. [3,[23][24][25] A great number of optically active functional materials is based on the Ln 2+/3+ , because of their unique spectroscopic properties, such as multicolor photoluminescence induced by UV or near-infrared (NIR) (energy up-conversion) irradiation, narrow absorption/emission bands, large spectral shift of the emission bands in relation to the absorption ones, long emission lifetimes, etc. [26][27][28][29][30][31][32][33][34][35] Matrices hosting Ln 3+ ions are usually fluorides, oxides, vanadates, phosphates, and borates. [3,4,11,12,[19][20][21][22][23][24][25][26] This is mainly because of their resistance to photobleaching and high temperature treatment, as well as relatively low phonon energy in contrast to organic compounds. [3][4][5][19][20][21][22][23][24][25][26] Moreover, the Ln 3+ -doped inorganic materials may exhibit up-conversion (UC) phenomena, i.e., anti-Stokes emission of higher-energy photons, generated by the absorption of two or more lowerenergy photons. [32,[36][37][38][39] Thanks to the high absorption cross-section of Yb 3+ in the NIR range, and the presence of a ladder-like structure of Ln 3+ energy levels, the upconverting materials codoped with Yb 3+ / Ln 3+ (Ln 3+ = Ho 3+ , Er 3+ , Tm 3+ ) may work not only as temperature sensors, but also as optical "heaters," as during their irradiation with a high-power NIR lasers they locally heat up. [40][41][42][43][44][45][46][47][48] This is due to the occurrence of various nonradiative processes between the Ln 3+ ions, quenching luminescence of the material and leading to heat generation. [43][44][45][46][47][48] Thanks to the efficient light-to-heat conversion, the optical heating phenomenon can be utilized in photothermal therapies, thermophotovoltaics, formation of new materials under extreme conditions, etc. [43][44][45][46][47][48][49] Currently, temperature of the system can be optically monitored in a relatively broad range, starting from cryogenic up to around ≈10 3 K, whereas pressure could be monitored only in the "high-pressure" range (≈10 2 -10 6 bar). These limitations are associated with the fundamental concept of pressure sensing, i.e., measurements of physical parameters directly Currently the lowest optically determinable pressure values are around 10 2 bar, making the pressure below inaccessible for optical detection. This work shows for the first time how to overcome these limitations, and optically monitor the low pressure values in a vacuum region (from ≈10 −5 to 10 −2 bar), utilizing the light-induced and pressure-g...

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 Yb 3+ −Tm 3+ codoped YVO 4 nanoparticles. For ratiometric sensing in the low-temperature range, we used the relative intensities of the Tm 3+ emissions associated with the 3 F 2,3 and 3 H 4 thermally coupled levels, that is, 3 F 2,3 → 3 H 6 / 3 H 4 → 3 H 6 (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 Yb 3+ ( 2 F 5/2 → 2 F 7/2 ) and Tm 3+ ( 3 H 4 → 3 H 6 ). 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.

The potential use of CdTe quantum dots as luminescence nano-probes for lifetime fluorescence nano-thermometry is demonstrated. The maximum thermal sensitivity achievable is strongly dependent on the quantum dot size. For the smallest sizes (close to 1 nm) the lifetime thermal sensitivity overcomes those of conventional nano-probes used in fluorescence lifetime thermometry.

Broadband and fluorescence line narrowing optical spectroscopic studies have been used to investigate the local environments of Eu 3ϩ ions in lithium fluoroborate glasses. From the vibronic spectra, different borate groups coupled with the Eu 3ϩ ions have been identified. A pulsed tunable dye laser has been used to selectively excite the 5 D 0 level of the Eu 3ϩ ion and the subsequent 5 D 0 → 7 F 1 fluorescence spectra have been monitored as a function of the exciting wavelength. From these FLN studies, three 7 F 1 Stark levels have been identified and a C 2v orthorhombic symmetry has been assumed in the subsequent calculation of the crystal-field parameters for the different environments occupied by the Eu 3ϩ ions in the glass. The second rank crystal-field parameters have been systematically analyzed for the Eu 3ϩ :lithium fluoroborate glass from the site dependent behavior of the 7 F 1 level splitting. The importance of the J-mixing in the crystal-field analysis has been emphasized. An appropriate method for comparing the crystal-field interactions in different glasses has been proposed by analyzing the 7 F 1 level. Thus, results obtained for the Eu 3ϩ :fluoroborate have been compared with recalculated results in other Eu 3ϩ doped fluoride, borate, silicate, and borosilicate glasses. An intermediate behavior between Eu 3ϩ :oxide and Eu 3ϩ :fluoride glasses is observed for the local structure of the Eu 3ϩ ions in the fluoroborate glass, indicating the active participation of fluorine ions in the immediate environments of the lanthanide ion in this glass.

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.

The optically active dopant distribution in a Tm(3+)-Yb(3+) doped silica based glass ceramic sample has been investigated. A systematic analysis of the upconversion fluorescence of the Tm(3+)-Yb(3+) codoped glass and glass ceramic has been performed at room temperature. Tm(3+) and Yb(3+) single doped glass and glass ceramics have also been included in the study. Upon infrared excitation at 790 nm into the (3)H(4) level of the Tm(3+) ions a blue upconversion emission is observed, which is drastically increased in the Yb(3+) codoped samples. A rate equation model confirmed the energy transfer upconversion mechanism. Based on these results, the temporal dynamic curves of the levels involved in the upconversion process, (3)H(4), (2)F(5/2), and (1)G(4) were interpreted in the glass ceramic samples. The contribution of the optically active Tm(3+) and Yb(3+) ions in the crystalline and in the vitreous phase of the glass ceramic was distinguished and the ratio of Tm(3+) ions in the crystalline phase could be quantified for the 1 mol % Tm(3+)-2.5 mol % Yb(3+) glass ceramic. A surprising result was obtained for that concentration: the main contribution to the upconversion emission of the glass ceramic is due to Tm(3+)-Yb(3+) ions in the vitreous phase.