Red-Light-Activated Red-Emitting Persistent Luminescence for Multicycle Bioimaging: A Case Study of CaS:Eu2+,Dy3+

In recent years, a wide range of persistent phosphors with different emission colors have been developed. Most of these compounds require excitation in the ultraviolet or blue part of the spectrum, which limits their use to specific applications. In this work, the red-emitting persistent luminescent CaS:Eu2+,Dy3+ phosphor, chargeable by red light, is investigated. The broadband persistent luminescence centered at 650 nm coming from the 5d-4f transition of Eu2+ can be effectively obtained upon a wide range of charging wavelengths from UV to red light. By using a red-light-emitting diode for charging, the samples exhibit intense persistent luminescence, which indicates that the charge transfer process occurs from the lowest 5d level of Eu2+ to the trapping centers. The charging process to fill traps is proved to be thermally activated in CaS:Eu2+,Dy3+. The feature of red-light-activated red-emitting persistent luminescence could facilitate the concept of reactivation by in situ external irradiation to realize long-term and multicycle bioimaging application and beyond.