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396 Handbook on the Physics and Chemistry of Rare Earths physiological values by Cadiau et al. using a similar ligand, bdc, and a 1/100 Eu/Tb ratio (Cadiau et al., 2013). In this case, Sm 1⁄4 0.37% at 318 K, and the size of the thermometric crystals was reduced to the nanometric range, so they could be suspended in a liquid, opening the way for biological applications. Since then, most of the newly developed MOF nanothermometers have been proposed for this application. Later on Cui et al. reported the use of Eu0.0069Tb0.9931-dmbdc regarding the improvement relative sensitivity value in the physiological gamut, presenting a Sm 1⁄4 16% K1 at 300 K, by means of a simultaneous decrease of the 5D4 ! 7F5 transition intensity and an increase of the 5D0 ! 7F2 transitions intensity (Cui et al., 2014). MOF thermometers can be obtained with a maximum absolute sensitivity Sa$30%K1 between 20 and 65K (Sm1⁄47.14%K1 at 65K, calculated by us) using dsb ligands and combinations of Eu0.02Gd0.98 or Eu0.05Tb0.09Gd0.86 ions (D’Vries et al., 2013). This thermometer, however, is only operative in the 10–300 K range, not covering physiological temperatures. In this system, temperature-dependent emission is the ligand triplet blue emission that decreases with the temperature, and the reference emission is that of the Eu3+ (or Tb3+) ion. In this case, the structure is one-dimensional and the ions are arranged in the form of singular heptanuclear [Ln7(OH)9]+12 metallic- cored secondary building unit surrounded by 4 dsb ligands. Ratiometric mixed lanthanide-organic dye thermometers can also be con- structed by absorption of organic dye emitters in the pores of the MOF struc- ture. A proof-of-concept can be found in Cui et al. (2015a). The MOF consists of Eu3+ ions and qptc ligands, the absorbed dye is perylene that has an emis- sion peak at 473 nm that decreases with the temperature. The thermometer’s working range (388 nm excitation) is 293–353 K with an absolute sensitivity (based on the I615/I473 ratio) constant and equal to Sa 1⁄4 1.28% K1 (corresponding to Sm 1⁄4 0.86% K1 at 301 K, calculated by us). The advan- tages introduced by this approach are to expand the available wavelength range for thermometry and to increase Sr near room temperature by dye-to- Ln3+ energy transfer. A postfunctionalization method (Zhou et al., 2014d) has been used to produce MOF thermometers that show a good sensitivity (Sa1⁄44.97%K1 and r2 1⁄4 0.996 that corresponds to Sm 1⁄4 1.11% K1 at 283 K, calculated by us), even in the physiological range. The thermometers are prepared from In(OH)(bpydc) crystals with a three-dimensional framework structure con- sisting of chains of corner-sharing InO6-octahedra interconnected by bpydc ligands. The Eu3+ and Tb3+ ions in a 0.005 ratio are introduced after syn- thesis by immersion in a solution of the lanthanides salts. The Ln3+ ions are fixed to the framework structure by coordination to the bipyridine (bpy) ligands. The excitation wavelength (315 nm) is well in the UV range, how- ever, it can be increased to 487 nm with only a slight decrease in the absolute sensitivity to 3.69% K1 and r2 1⁄4 0.9993 (corresponding to Sm 1⁄4 1.13% K1 at 283 K). It is also shown that Sa can be improved by decreasing thePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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