Sr₂LuF₇:Yb³⁺–Ho³⁺–Er³⁺ Upconverting Nanoparticles as Luminescent Thermometers in the First, Second, and Third Biological Windows

Abstract

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.

Primary Luminescent Nanothermometers for Temperature Measurements Reliability Assessment

“…[1,[4][5][6][7][8][9][10][11][12] Ratiometric thermometers are held as very reliable temperature sensors, as they are not affected by local intensity fluctuations (i.e., concentration of emitting centers or excitation source fluctuations) while requiring simple instrumentation. [3,4] However, there are a few effects, including experimental artifacts (e.g., the numerical aperture of the optics used for excitation and emission detection [13] ), the use of high laser power densities (P D ), [14] and unverified assumptions about the thermal dependence of the optical transmittance of biological tissues, [15][16][17] that can induce spectral distortions on the emission of the chromophores, altering the relative intensity of the transitions and, consequently, the value of the thermometric parameter. This, ultimately, results in false temperature readouts, compromising the reliability of the thermometers.…”

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

“…In order to generate efficient UC luminescence, the inorganic matrices co-doped with rare earth ions (typically Ho 3+ , Er 3+ and Tm 3+ ) are commonly used, such as vanadates, phosphates, borates, fluorides, oxides etc. [1][2][3][6][7][8][9][10][11][12][13][14][15][16]. Currently, up-converting (nano)materials are commonly studied in terms of the components of solar cells [17], bioimaging [18,19], nanothemrometry [3,[20][21][22][23][24][25][26], forensics [27], composite membranes etc.…”

All near-infrared multiparametric luminescence thermometry using Er3+, Yb3+-doped YAG nanoparticles

“…Therefore, to circumvent this limitation the Yb 3+ co-dopant can be used, which is well known for its high absorption cross section at 10 638 cm À1 as a sensitizer of Er 3+ NIR luminescence. 25,[33][34][35][36] For this study, we have prepared luminescent nanothermometers based on yttrium aluminium garnet (Y 3 Al 5 O 12 , YAG) nanocrystals doped with Yb 3+ and Er 3+ using the modied Pechini method. [37][38][39] YAG is well-known host material which possess high photo resistance, chemical stability, and ease of emission tuning.…”