Role of citric acid in ultrasonically assisted hydroxyapatite@Fe₃O₄ functional nanohybrid formation: A detailed characterization study

Hydroxyapatite (HAp) and Fe₃O₄ nanoparticles are widely valued for biomedical and catalytic applications owing to their biocompatibility, magnetic properties, and stability. However, integrating them into a uniform nanohybrid with long-term colloidal stability remains challenging due to particle aggregation and phase separation. In this study, citric acid was employed as a multifunctional stabilizing and chelating agent in the ultrasonic-assisted synthesis of HAp@Fe₃O₄ nanohybrids. Systematic variation of citric acid concentration (0.2–0.8 wt%) revealed its critical role in interfacial binding, electrostatic stabilization, and dispersion control. Characterization by Fourier transform infrared and Raman spectroscopy confirmed citric acid adsorption through C=O and carboxylate coordination with Ca²⁺ in HAp and Fe²⁺/Fe³⁺ in Fe₃O₄, accompanied by phosphate peak shifts and Fe–O band attenuation. Zeta potential and dynamic light scattering analyses demonstrated that dispersion stability was highly dependent on concentration. The control sample in water rapidly sedimented, while 0.5 % citric acid provided the most stable colloidal system, balancing electrostatic repulsion (zeta potential −13.1 mV post-sonication), reducing aggregation, and uniform particle size (∼1604 nm). Stability at 0.2 % citric acid was moderate, whereas 0.8 % citric acid initially improved dispersion but later induced re-agglomeration due to excessive ionic interactions. X-ray diffraction confirmed phase retention of HAp and Fe₃O₄ with progressive peak broadening at higher citric acid content, suggesting surface modification and crystallite size reduction. Scanning electron microscopy imaging demonstrated that citric acid, in synergy with ultrasonic cavitation, reduced agglomerate size and improved dispersion, particularly at 0.2–0.5 % citric acid. Notably, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy confirmed a homogeneous distribution of Fe, Ca, and P in citric acid–modified hybrids, in sharp contrast to the strong clustering observed for pristine Fe₃O₄ in water. The findings establish citric acid as an effective molecular bridge between HAp and Fe₃O₄, enhancing colloidal stability and structural integration without post-synthetic modification. The optimized hybrid exhibits characteristics desirable for biomedical use, such as uniform dispersion, tunable surface chemistry, and retained crystallinity, laying the groundwork for future applications in magnetic hyperthermia, drug delivery, and bone tissue engineering.