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Lanthanides in Luminescent Thermometry Chapter 281 349 (Wolfbeis, 2008; Yu et al., 2010), on b-diketonate complexes (Brites et al., 2011, 2012), on organic–inorganic hybrids (Brites et al., 2012; Milla ́ n et al., 2016), and on NPs (Bettinelli et al., 2015; Fischer et al., 2011; Quintanilla et al., 2016; Wang and Zhang, 2015). Luminescent thermometers based on the transient emission intensity response of Ln3+-activated phosphors (eg, Y2O2S:Eu3+ and CaS:Eu3+) were firstly proposed by Kusama et al. in 1976 (Kusama et al., 1976) and by Samulski & Shrivastava four years later (Samulski and Shrivastava, 1980; Samulski et al., 1982). In 2002, the seminal paper of Wang et al. discussed the concept of using luminescent NPs for thermometry (Wang et al., 2002). The temperature-dependent emission characteristics of distinct semiconductor NPs were used as a proof of concept and ratiometric luminescent thermo- meters were proposed for the first time based on ZnS:Mn2+, Eu3+ NPs, in which the ratio of the emission intensities of the two dopants—so-called fluo- rescence intensity ratio, FIR—provides a robust temperature measurement approach. A few years later, this same FIR concept was generalized to NPs doped only with Ln3+ ions, BaTiO3:Er3+ UCNPs (Alencar et al., 2004). In Er3+-based systems, the FIR method involves measurements of the fluores- cence intensities from two closely spaced electronic energy levels (2H11/2 and 4S3/2) that are thermally coupled (eg, in a thermodynamically quasi equi- librium state) (Collins et al., 1998; Shinn et al., 1983), see Section 4.1.1. We proposed a more recent breakthrough on luminescent nanothermome- try in 2010 with the development of siloxane-based hybrid magnetic nanoclusters doped with Eu3+ and Tb3+ chelates (Brites et al., 2010). The key point in this approach is the temperature dependence of the energy transfer and back transfer rates between the host, the ligands, and the Ln3+ ions (Ln = Tb and Eu). The temperature dependence of the 5D4 (Tb3+) and 5D0 (Eu3+) emissions is essentially determined by the temperature dependence of the balance between i) host-to-Ln3+ and ligand-to-Ln3+ energy transfer rates, including pathways through excited singlet and triplet states; ii) Ln3+- to-host and Ln3+-to-ligand energy back transfer rates, and iii) non-radiative deactivations from upper excited 4f levels to the emitting 5D4 and 5D0 states. Since the Tb3+ and Eu3+ ions are well dispersed within the hybrid host (typi- cally in concentrations lower than 5%), ion-ion energy transfer is unimportant. The 5D4 ! 7F5/5D0 ! 7F2 relative intensity ratio guarantees the absolute mea- surement of temperature, with spatial resolution adjustable by the size of the nanoclusters to which the luminescent probes are anchored. We remark that similar energy resonance schemes may be obtained using other hybrid or polymer hosts or b-diketonate chelates with different ligands (Brites et al., 2013b, 2016). This mechanism involving the global balance between host- to-ions and ligand-to ions energy transfer and back transfer rates is different than that usually observed in Tb3+/Eu3+-doped materials in which the temper- ature dependence of the 5D4 ! 7F5/5D0 ! 7F2 intensity ratio is determined byPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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