Growth phase diagram and X-ray excited luminescence properties of NaLuF4:Tb3+ nanoparticles

Abstract

X-rays are energy sources exhibiting extended penetration depths, and they have attracted increasing attention in industry and for clinical application. With the rapid development of nanomaterials and X-ray excited luminescent nanoparticles (XLNPs), new modalities for bioimaging and cancer therapy have been developed, such as X-ray luminescent computed tomography (XLCT) and X-ray excited photodynamic therapy (X-PDT). To meet the requirements of biomedical applications, XLNPs must exhibit high luminescence intensities, appropriate size distributions (less than100 nm) and negligible cytotoxicity. Due to the optical properties associated with f-electrons, rear earth (RE) elements are highly suitable for creating XLNPs. NaREF4 nanoparticles (NPs) have been shown to be suitable hosts with high luminescence intensities, controllable sizes, and biocompatibility for X-ray-based biomedical applications. Syntheses of NaLuF4 NPs doped with rare earth elements for upconversion applications have been systematically studied. However, for X-ray excited applications, the doping levels of the NPs must be totally different, which greatly affects the morphologies and sizes of the NaLuF4 NPs. Thus, in this paper, nucleation, phase transitions, morphologies and sizes, and luminescence properties of Tb3+-doped NaLuF4 NPs were systematically studied. OA-capped NaLuF4:Tb3+ NPs were synthesized via coprecipitation processes with different reaction temperatures and reaction times to study the nucleation mechanism systematically, and the morphologies, size distributions and crystal phases were characterized with TEM and XRD. The morphologies, size distributions and crystal phases of these NPs were seriously influenced by the reaction temperature and reaction time. At 295 ℃, the NP sizes increased with prolonged reaction time, and the crystalline phase was a mixture of cubic and hexagonal phases. At 300 ℃ and 310 ℃, the pure hexagonal phase was obtained after 20 min and 35 min reaction times, respectively. The luminescence strengths of these NPs were associated with the particle sizes, crystalline phases, and Tb3+ doping levels. Stronger luminescence was achieved with larger particle sizes and purer hexagonal crystal phases. In addition, the 15 % doping level for Tb3+ provided the maximum luminescence intensity. The present work provides insights into the mechanism of NaLuF4:Tb3+ nanocrystal growth.