Structural, Optical, and Neutron Sensitivity Properties of Yb³⁺/Tb³⁺ Codoped Alkaline Earth Tetraborates

This study explores the synthesis, structural properties, and multifunctional potential of Yb³⁺/Tb³⁺ codoped alkaline earth tetraborates (CaB₄O₇, MgB₄O₇, and SrB₄O₇) synthesized via solid-state (SS), solution combustion (SC), and combustion (C) methods. The multifunctional potential of the synthesized materials arises from their unique combination of properties in one material. The first property is related to their high boron content and thus to their high neutron capture cross section, providing effective functionality for boron neutron capture therapy (BNCT) and shielding. This enables the particles to be selective tumor cell destruction agents under neutron irradiation. The second property is their strong upconversion luminescence that allows for near-infrared light-triggered photodynamic therapy. This offers deep-tissue treatment capability along with BCNT. The third property is their excellent photochemical stability and low biotoxicity. This combination of therapeutic modalities within a single platform holds promise for synergistic and minimally invasive cancer treatment strategies, along with imaging capabilities and drug delivery. X-ray diffraction analysis confirmed the phase purity and crystal structures of CaB₄O₇ (CBO), MgB₄O₇ (MBO), and SrB₄O₇ (SBO). CBO crystallizes in a monoclinic structure with a P2₁/n(14) space group, while both MBO and SBO adopt orthorhombic structures with Pcba and Pnm2₁ space groups, respectively. Scanning electron microscopy revealed particle sizes ranging from 100 to 200 nm, with more uniform and smaller particles achieved by SC. Strong upconversion luminescence, driven by cooperative energy transfer between Yb³⁺ and Tb³⁺ ions, was observed in all materials, particularly in CBO and MBO codoped with 5% Yb³⁺ and 5% Tb³⁺. The quantum yields (QY) for CBO and MBO reached approximately 0.35. Furthermore, neutron sensitivity was significantly enhanced with ¹⁰B isotope enrichment, making these materials promising candidates for BNCT and neutron shielding applications. Cytotoxicity tests confirmed the biocompatibility of the materials, especially for CBO and MBO, which have maintained high cell viability.