Urchin-like Ce/Tb co-doped GdPO4 hollow spheres for in vivo luminescence/X-ray bioimaging and drug delivery

Abstract

In this paper, we report a self-sacrificing route for fabrication of the Ce/Tb co-doped GdPO₄ hollow spheres under hydrothermal conditions using the Gd(OH)CO₃:Ce/Tb precursor as a template and NH₄H₂PO₄ as a phosphorus source. The X-ray diffraction (XRD) patterns show the amorphous crystal nature of the precursor and pure hexagonal phase of the hollow spheres. The microstructures of the as-prepared precursor and hollow spheres were characterized by transmission electron microscopy (TEM) and scanning TEM (STEM) assays. The results reveal the urchin-like morphology of the solid precursor and hollow spheres. Bright green emissions of the spheres have been detected using an ultraviolet (UV) lamp at 288 nm and the calculated CIE coordinates are (0.289, 0.491). The energy transfer mechanism of Ce and Tb ions in the GdPO₄ host has been investigated. The quantum efficiency of the hollow spheres was measured to be 61% and the lifetime calculated as 6.94 ms. In addition, the magnetic mass susceptibilities and magnetization of the spheres are found to be 6.39 × 10⁻⁵ emu gOe⁻¹ and 1.27 emu g⁻¹ at 20 kOe, respectively. Owing to their excellent downshift luminescence properties, the as-prepared GdPO₄:Ce/Tb hollow spheres have been successfully applied in in vivo luminescence and X-ray bioimaging for the first time. Moreover, three-dimensional (3D) in vivo X-ray bioimaging of the mouse can provide the accurate location from multiple directions. The high contrast ratio makes the spheres a promising X-ray contrast agent. Due to the hollow structure, these GdPO₄:Ce/Tb hollow spheres were also used as drug delivery systems for doxorubicin (DOX) loading and release. The drug loading efficiency was measured to be 17% at a pH value of 7.4, and the pH-dependent drug release was studied. 47% of the loaded DOX was released within 10 h when pH = 5, while there was only 30% during the same time at pH = 7.4 and it took nearly 48 h to reach a comparable level. The different release nature gives these spheres a promising application in targeted therapy of tumors.