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Aqueous solutions of luminescent metal-ion complexes, in particular those of lanthanide ions, can play an essential role in biomedical applications. For all these applications, the knowledge about the influence of temperature variations within the physiological range (20-60 ºC) on their optical properties becomes essential. At variance with other liquids, water is unique as it does present an anomalous temperature dependent behavior. In particular, most of water properties present remarkable physico-chemical changes above a certain temperature, which ranges between 30 and 50 ºC. In this work we systematically investigate the effect of temperature on the luminescence properties of Eu 3+ ions when dissolved either in H2O or D2O. An anomalous thermal behavior, manifested as a bilinear trend, with crossover at around 35 ºC for H2O and 38 ºC for D2O, is found in a variety of Eu 3+ optical spectroscopic properties (branching ratio, luminescence lifetime, and emission band shape). The observed changes are here tentatively explained in terms of a different aggregation state of H2O and D2O molecules below and above crossover temperature. Such changes in the molecular clustering lead to a temperature induced change in the relative concentrations of the 8-fold and 9-fold coordinated Eu 3+ complexes. Finally, we have observed that the pH of the aqueous solution plays an essential role in defining the temperature at which the anomaly takes place, so that the differences in the values reported in the literature for the crossover temperature are accounted for.

Persistent phosphorescence nanoparticles emitting in the red and near-infrared spectral regions are strongly demanded as contrast nanoprobes for autofluorescence free bioimaging and biosensing. In this work, we have developed Sr4Al14O25:Eu2+, Cr3+, Nd3+ nanopowders that produce persistent red phosphorescence peaking at 694 nm generated by Cr3+ ions. This emission displays temperature sensitivity in the physiological temperature range (20–60 °C), which makes these nanoparticles potentially useful as fluorescence (contactless) nanothermometers operating without requiring optical excitation. Nd3+ ions, which act as shallow electron traps for the red Cr3+ persistent emission, also display infrared emission bands, extending the fluorescence imaging capability to the second biological window. This unique combination of properties makes these nanoparticles multifunctional luminescent probes with great potential applications in nanomedicine.

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