Europium-activated luminescent nanoprobes: From fundamentals to bioapplications

Trivalent europium ions (Eu3+) activated inorganic materials, known as one of the most important red emitting phosphors, are emerging as an attractive class of luminescent materials over the past decades. Specifically, Eu3+ ion is considered as an ideal spectroscopic probe to decipher the local environment in view of its non-degenerate ground state of F-7(0) and excitation state of D-5(0). Benefiting from the intense long-lived luminescence from D-5(0), Eu3+-activated inorganic nanoparticles are also frequently reported as luminescent probes in biological assays and medical imaging. With the development of advanced synthesis strategies and characterization techniques, more new understanding or progress in Eu3+-activated luminescent nanoprobes have been gained with regard to their fundamental photophysics/photochemistry properties and versatile bioapplications. As such, it is urgent to renew the knowledge in this field. Rather than being exhaustive, this review aims to highlight the latest progress in the electronic structures, optical properties and emerging bioapplications of Eu3+-activated nanoprobes, which covers from spectroscopic site symmetry, excited-state dynamics, downshifting/upconversion luminescence enhancement strategies, and luminescent biosensing, with an emphasis on their heterogeneous/homogeneous biodetection as well as in vitro and in vivo bioimaging. Some future prospects and efforts toward this rapidly developing field are also discussed. (C) 2017 Elsevier B.V. All rights reserved.