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392 Handbook on the Physics and Chemistry of Rare Earths was implemented in a magnetic nanoheater consisting on a core–shell nano- bead with a magnetic nucleus and a copolymer shell. The magnetic nucleus consists of iron oxide NPs that can be heated at a distance with an alternating magnetic field. The thermometric Ln3+ complexes are placed on the surface of the magnetic nucleus so they are sensing the temperature of the nanoheater core. The response of the molecular thermometer depends on the experimental setup and was measured as 0.25 s. Thus, the evolution of the nanoheater tem- perature could be followed in detail during relatively long heating and cooling periods, but also at the events of switching on and switching off the field (Fig. 24). In fact, the rapidity of the response is crucial at these events and the measurements revealed unexpected features on the process of heat diffu- sion from the nanoheater to the medium that cannot be explained with present theories (Pin ̃ol et al., 2015). The molecular nature of the thermometer and its optical character permitted also a high spatial resolution when the emission was captured on the camera of a fluorescence microscope. Given the biocom- patibility of the materials used, its nanometric size, and the capacity of the object for conjugation with biovectors and other biological functionalities, it can be of great utility in biological applications. Actually, its capacity for tem- perature mapping of cells has been demonstrated (Pin ̃ol et al., 2015), and it can be a decisive tool for the development of local hyperthermia therapies of cancer. The concept of local hyperthermia is based on the generation of lethal temperature increments in small areas inside the cells instead of a global tissue temperature increase. To determine the feasibility of this concept it is necessary to use local temperature probes to measure the temperature gra- dients from the heater source and the exterior of the cell. Besides a reliable means to monitor temperature during magnetic hyperthermia treatments at a subcellular scale, these nanothermometers can potentially solving the paradox of “cold hyperthermia” (cellular death in the absence of a perceptible rise of the macroscopic temperature of the medium) (Perigo et al., 2015). 5.2.2 In Inorganic Matrices The encapsulation of molecular thermometers in inorganic matrices is rare and mostly based on the use of clays. For instance, positively charged amino- clays have been used as a host for Eu3+:Tb3+ (1/7) complexes with btc (Wang et al., 2014). The material shows a linear temperature dependence of the ITb/ IEu ratio in the 78–288 K range with Sm 1⁄4 0.74% K1 for Tb3+ and is dispers- ible in water, being therefore processed as a paint. Although proposed for pH sensing, the Eu-(tta) complex has also been encapsulated in other inorganic matrices, like laponite (Li et al., 2015). 5.2.3 In Organic–Inorganic Hybrid Matrices Hybrid materials are also versatile matrices that can expand the processability and thermometric applications of powder-based thermometers presented inPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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