Theoretical and Experimental Studies of Aliovalent Effects in Color Tuning and Biocompatibility of Eu ^{3 <b>+</b>} -Doped SrZrO ₃ Perovskite

The development of biocompatible luminescent materials with high stability and tunable emission remains a critical challenge for fluorescence bioimaging. Conventional fluorescent probes and halide perovskites often suffer from toxicity, poor photostability, and uncontrolled incorporation of dopants, limiting their biomedical applicability. To address these challenges, europium-doped strontium zirconate (SrZrO₃:Eu³⁺) particles were synthesized via a controlled hydrothermal method and systematically investigated through experimental and first-principles approaches. X-ray diffraction analysis confirmed the formation of a phase-pure orthorhombic perovskite structure and observed a peak shift toward higher angles, indicating lattice contraction resulting from Eu³⁺ incorporation. A first-principles study validated dual-site occupation of Eu³⁺ ions at both Sr²⁺(8-coordinate) and Zr⁴⁺(6-coordinate) sites, with a negative formation energy of -1.02 eV, signifying thermodynamic stability. Morphological analyses revealed monodisperse spherical particles, promoting colloidal stability and efficient cellular uptake for bioimaging applications. UV-vis diffuse reflectance spectroscopy revealed a narrowing of the band gap upon doping relative to the host, improving light absorption in the visible region. XPS confirmed the presence of all elements and both Eu²⁺ and Eu³⁺ oxidation states, indicating successful lattice substitution. Photoluminescence spectra exhibited tunable emission from orange (⁵D₀ → ⁷F₁ at 595 nm) to red (⁵D₀ → ⁷F₂ at 614 nm) with increasing Eu³⁺ concentration, correlating to local symmetry variations and dual-site occupancy. Since this perovskite structure has two coordination sites, increasing the dopant concentration results in occupation at both sites. Anticancer activity of 5 mol % Eu³⁺-doped SrZrO₃ on A549 cells was demonstrated by <i>in vitro</i> cell viability assay, followed by hemolysis assay and fluorescence imaging. Eu³⁺-doped SrZrO₃ exhibits anticancer activity, good hemocompatibility, and brighter intracellular red fluorescence that penetrates cells efficiently. The excellent structural features, tunable emission, biocompatibility, and <i>in vitro</i> cytotoxicity of SrZrO₃:Eu³⁺ particles confirm their potential as a dual-functional probe for effective <i>in vitro</i> fluorescence imaging and promising anticancer efficacy, indicating their potential for future <i>in vivo studies</i>.