Effect of H2O and D2O Thermal Anomalies on the Luminescence of Eu3+ Aqueous Complexes

Abstract

Aqueous solutions of luminescent metal–ion complexes, in particular, those of lanthanide ions, can play an essential role in biomedical applications. For all of these applications, the knowledge about the influence of temperature variations within the physiological range (20–60 °C) 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 water properties present remarkable physicochemical changes above a certain temperature, which ranges between 30 and 50 °C. In this work, we systematically investigate the effect of temperature on the luminescence properties of Eu3+ ions when dissolved in either H2O or D2O. An anomalous thermal behavior, manifested as a bilinear trend, with crossover at around 35 °C for H2O and 38 °C for D2O, is found in a variety of Eu3+ optical spectroscopic properties (branching ratio, luminescence lifetime, and emission band shape). The observed changes are tentatively explained here in terms of different aggregation states of H2O and D2O molecules below and above the 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 Eu3+ 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.