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400 Handbook on the Physics and Chemistry of Rare Earths 85 K of 9.7% K1, corresponding to Sm 1⁄4 1.61% K1, computed by us (Lojpur et al., 2013). Nevertheless, Sa decreases abruptly below and above this value. At 300 K, the value has decreased to 2%K1, corresponding to Sr1⁄40.14%K1, as computed by us. According to the same authors, the maximum absolute sensitivity of Y2O3:Yb3+/Tm3+ although lower is found at higher tempera- tures; near room temperature it is 7.8%K1 (or Sm1⁄40.55%K1, computed by us). The feasibility of in vivo upconversion imaging has been demonstrated by subcutaneously injection of PA-coated Yb/Er/Mo-doped nanocrystal suspen- sions into the back of nude mice, and imaging with a 976 nm laser as the exci- tation light (power density $ 0.2 W cm2) (Dong et al., 2012). However, the Er3+ emissions lie in the visible range, and to improve the penetration depth, both the excitation and the emission should be located within the first biological window (650–950 nm) (Cero ́n et al., 2015). An example is the two-photon NIR-to-NIR conversion observed in Yb3+/Tm3+-doped CaF2 NPs (Dong et al., 2011). The penetration depth of Tm3+ fluorescence signal at 790 nm in phantom tissue was close to 2 mm, more than 4 higher than that of Er3+ fluorescence visible signal at 655 nm in similar Yb3+/Er3+-doped CaF2 NPs. Most of UCNPs proposed for thermometry of biological applications are based on the Yb3+ sensitization at 980 nm. This excitation wavelength is over- lapped with the maximum absorption band of water molecules (obviously abundant in biological tissues) and, therefore, the penetration depth in the tis- sues at this wavelength is reduced, with the associated increase of the local heating (McNichols et al., 2004). The optimal wavelength within the first biological window is around 800 nm (Kobayashi et al., 2010) that shows the lowest water absorption. Consequently, one has now the tendency to use exci- tation wavelengths around this value, see Shen et al. (2013) and Fig. 27. A very intriguing example concerning UCNPs is the work of Zheng et al. that proposes a ratiometric temperature sensor based on core/shell NPs formed by a NaGdF4:Yb3+/Tm3+ core and an NaGdF4:Eu3+/Tb3+ shell (Zheng et al., 2014). The Yb3+/Tm3+ pair harvests NIR photons at 980 nm exciting then the Gd3+ ions to the UV-lying levels via Yb3+–Tm3+–Gd3+ energy transfer process. Subsequently, energy is transferred from the Gd3+ ions in the core to the Gd3+ ions in the shell where, through another energy transfer process, it reaches the Tb3+ and Eu3+ ions resulting in green (545nm) and red (615 nm) emissions, respectively. The thermal ratiometric parameter is the ratio between the intensities of these two emissions and Sm1⁄40.50%K1 at 300 K is obtained. What is uncommon in this example is that the Tb3+ and Eu3+ excitation is reached through a two-step mechanism involving firstly a Yb3+–Tm3+–Gd3+ NIR-to-UV upconversion multiphoton process followed by energy migration between Gd3+ ions for a wide range of Ln3+ activators without long-lived intermediary energy states (Wang et al., 2011b). This design of core–shell nanostructures with different dopants in each layer,PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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