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390 Handbook on the Physics and Chemistry of Rare Earths Borisov and Wolfbeis (2006) and Stich et al. (2008) presented a dual sen- sor for temperature and O2 in which the excitation maxima of tta and other b-diketonate ligands shifted into the visible region (410 nm) after addition of a N-based heterocyclic ligand, dpbt, with concomitant increase of the brightness. Ligand-to-Eu3+ energy transfer achieves visible light sensitization. When the thermometric parameter is the decay time, poly-(tert-butyl styrene) is used for complex encapsulation and poly(vinyl methyl ketone) is used as the binder, then, the working temperature range is extended to 273–343 K. The temperature-sensitive microparticles and the oxygen-sensitive microbe- ads are both dispersed in a polymer hydrogel solution and casted on a surface to give a dual temperature and oxygen sensor. The oxygen-sensitive microbe- ads were made of oxygen-sensitive complexes of palladium and porphyrin embedded in P(St-co-AN). Stich et al. (2008) suggest that the temperature can be monitored with very high spatial resolution by time-resolved fluores- cence imaging because the luminescence lifetime of the temperature indicator is 10-fold longer than that of the oxygen indicator. The crucial role of the matrix is also outlined by Sun et al. in comparing the thermometric performance of the [Tb(thba)3] complex when it is pro- cessed in several ways (Sun et al., 2010). The excitation wavelength of the complex is well in the UV (334 nm). The thermometric parameters are both intensity and lifetime and the working temperature range is 288–343 K. The [Tb(thba)3] complex was processed in three different ways: l Dispersed in a polyurethane hydrogel and subsequently casted on a glass slide. l Aqueous dispersion of precipitated NPs (10–20 nm in size) to make a colloid. l NPs dispersed in acetonitrile mixed with a 10 wt% aqueous solution of PVA, knife coated onto a glass slide and evaporated to obtain transparent films. The maximum relative sensitivity in polyurethane films is Sm1⁄42.9%K1 (298–318 K) and that of NPs in water dispersions in this range is similar, Sm 1⁄4 2.8% K1. The big difference between the two materials is that the vari- ation of the intensity with temperature is positive in the first case and negative in the second. Furthermore, the intensity/temperature ratio returned to nega- tive values when the NPs were dispersed in PVA. The mechanism behind the temperature sensing was not entirely understood (Sun et al., 2010). Polymers are also excellent matrices for dual-center emitters, despite the possibility of the energy transfer between the two Ln3+ ions quenches one of the emissions, as established by Sato et al. (1989). Dual-center emitters based on polymer matrices can also be made using nonlanthanide emitters as reference, as shown in Wang et al. (2015b). Moreover, this report is also demonstrating the capacity of polymer system for processing and multisen- sing. It presents a multisensor in the form of biocompatible sprayable thermo- gelating material (Fig. 23). In this case, the thermometric probe is [Eu(dbm)3(phen)] working in the range 298–328 K that is incorporated inPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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