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402 Handbook on the Physics and Chemistry of Rare Earths explored for thermal sensing in Yb3+/Tm3+- (dos Santos et al., 2001; Lojpur et al., 2013; Pereira et al., 2015) and Yb3+/Ho3+-doped (Xu et al., 2013) mate- rials. An intriguing example of a five-photon excited Yb3+/Er3+ b-NaLuF4 nanothermometer involving the highest energy states for optical thermometry known so far (the 4D7/2 and 4G9/2 Er3+ levels) is recently reported (Zheng et al., 2015). Naked crystals have limited light absorption due to the low doping concen- tration necessary to avoid quenching. However, the doping concentration can be increased without quenching problems when the excitation irradiation is increased. In this way, single crystal sensitivity has been achieved (Zhao et al., 2013). Light absorption and spectrum broadness can be greatly enhanced also by sensitization with NIR organic dye antennas (Zou et al., 2012). It has been observed that NaYF4:Yb3+/Er3+ NPs coated with the organic dye IR-806 increases their integrated spectral response in the 720–1000 nm range by more than 3000, with respect to uncoated NPs. The enhancement of upconversion efficiency occurs by absorption of NIR photons by the dye “antennae” and transfer of their excitation energy to the Yb3+ 2F5/2 level by resonance energy transfer. Then the energy is typically transferred form Yb3+ to Er3+ leading to the Er3+ emission of upconverted light. 5.5 NIR-Emitting Nanoparticles In particular, because Nd3+ has a ladder-like intra-4f energy level structure, the excitation and emission lie within the first biological window where the transparency of living tissues is high due to low optical absorption, offering much potential for deep-tissue luminescence imaging and temperature sensing (Carrasco et al., 2015; Cero ́n et al., 2015), as mentioned earlier. Only a lim- ited number of reports on luminescent thermometry involving Nd3+-doped nanocrystals are available, including LaF3:Nd3+, Sm1⁄40.10%K1 at 283K (Rocha et al., 2013, 2014a,b; Villa et al., 2014), NaYF4:Nd3+, Sm1⁄40.12% K1 at 273 K (Tian et al., 2014; Wawrzynczyk et al., 2012), YAG:Nd3+, Sm 1⁄4 0.15% K1 at 283 K (Benayas et al., 2015), and (Gd0.991Nd0.009)2O3 nanorods, Sm 1⁄4 1.75% K1 at 288 K (Balabhadra et al., 2015). Apart from their thermometric properties, UCNPs containing Nd3+ ions at high doping concentrations have also the capacity of heating by irradiation with a 808 nm laser beam (Carrasco et al., 2015). Actually, their use in controlled hyperthermia therapy has already been tested in animal tissues (Fig. 27). Mice with tumors formed by subcutaneous inoculation of cancer cells were injected with LaF3:Nd3+ NPs directly in the tumor and then they were irradiated with the laser beam. The IR fluorescence of the NPs was captured with a camera and it was transformed to a thermal image that showed the NPs were heated up to temperatures around 323 K. The heating caused a selective size reduc- tion of the tumors inoculated with the NPs, with respect to those inoculated with a phosphate-buffered saline solution.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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