Trap depth optimization to improve optical properties of diopside-based nanophosphors for medical imaging

Abstract

Regarding its ability to circumvent the autofluorescence signal, persistent luminescence was recently shown to be a powerful tool for in vivo imaging and diagnosis applications in living animal. The concept was introduced with lanthanide-doped persistent luminescence nanoparticles (PLNP), from a lanthanide-doped silicate host Ca_0.2Zn_0.9Mg_0.9Si₂O₆:Eu²⁺, Mn²⁺, Dy³⁺ emitting in the near-infrared window. In order to improve the behaviour of these probes in vivo and favour diagnosis applications, we showed that biodistribution could be controlled by varying the hydrodynamic diameter, but also the surface charges and functional groups. Stealth PLNP, with neutral surface charge obtained by polyethylene glycol (PEG) coating, can circulate for longer time inside the mice body before being uptaken by the reticulo-endothelial system. However, the main drawback of this first generation of PLNP was the inability to witness long-term monitoring, mainly due to the decay kinetic after several decades of minutes, unveiling the need to work on new materials with improved optical characteristics. We investigated a modified silicate host, diopside CaMgSi₂O₆, and increased its persistent luminescence properties by studying various Ln³⁺ dopants (for instance Ce, Pr, Nd, Tm, Ho). Such dopants create electron traps that control the long lasting phosphorescence (LLP). We showed that Pr3+ was the most suitable Ln³⁺ electron trap in diopside lattice, providing optimal trap depth, and resulting in the most intense luminescence decay curve after UV irradiation. A novel composition CaMgSi₂O₆:Eu²⁺,Mn²⁺,Pr³⁺ was obtained for in vivo imaging, displaying a strong near-infrared persistent luminescence centred on 685 nm, allowing improved and sensitive detection through living tissues.