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Lanthanides in Luminescent Thermometry Chapter 281 353 Lifetime-based sensing methods do not suffer from the above-mentioned disadvantages of the single-transition intensity technique. However, compar- ing with the ratiometric intensity response to temperature changes, they require longer acquisition times, postprocessing techniques, and the complex- ity and demands on the component instrumentation increase with the decreas- ing decay times (Berthou and J€orgensen, 1990; Brites et al., 2012; Peng et al., 2010b). Moreover, lifetime-based sensing methods are less appropriate for large-area gradient temperature measurements and very inappropriate to study dynamic measurements in which temperature variations occur at time inter- vals shorter than or equal to the lifetime of the probe. According to the wide range of applications and as industry requires dedi- cated sensors for specific processes, it is highly improbable that a particular sensor can be simultaneously suitable for high (T>500 K), physiological, or cryogenic monitoring ranges. Then, both the ratiometric intensity response and the lifetime-based sensing methods show considerable potential and have been extensively studied for a wide range of materials. Although the compari- son between the performances of each method is of particular interest, it has not been done systematically. An interesting example is the pioneer work of Collins et al. for Cr3+-, Er3+-, and Pr3+-based crystals (Fig. 3) (Collins et al., 1998). In general, the ratiometric intensity response provides higher relative sensitivity values (5 larger in the Pr3+-based crystal of Fig. 3). This is more evident for cryogenic temperatures as lifetime values are generally temperature independent in this range and, thus, the relative sensitivity approaches zero. More recently, Paviolo et al. came to the same conclusion using a fluores- cent molecular thermometer based on RhB to resolve temperature distribu- tions within the volume of a single cell (Paviolo et al., 2013). The measure of temperature in the cytoplasm of an organic tissue via RhB emission inten- sity is more accurate and reliable than that performed using the RhB lifetime. Therefore, in general, the wisest luminescent thermometry choice should produce a ratiometric intensity response to temperature changes. 3 THERMOMETER PERFORMANCE In this section, we will discuss how the emission intensity should be converted into temperature and how the performance of ratiometric luminescent thermo- meters should be quantified. The comparison of the performance of distinct luminescence thermometers is made using the following parameters: l relative thermal sensitivity, l temperature uncertainty, l spatial and temporal resolution, and l repeatability (or test–retest reliability) and reproducibility.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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