Polyacrylic Acid Ligand Exchange in Cobalt Ferrite Nanoparticles: Aqueous Stability Without Compromising Magnetic Properties

Using the thermal decomposition method, we have prepared monodisperse CoFe₂O₄ nanoparticles of average size ~ 12 nm. A ligand exchange process was carried out to replace the post-synthesis hydrophobic oleic acid (OA) coating with hydrophilic polyacrylic acid (PAA). X-ray photoelectron spectroscopy confirmed the successful ligand exchange process, and zeta potential measurements validated long-term stability in water in a broad pH range due to a significant negative charge of PPA-coated samples (−30 mV). Importantly, the magnetic behavior remains essentially unchanged with saturation magnetization (~ 65 Am2/kg) and coercivity (~ 80 mT) at 300 K for samples before and after ligand exchange. In addition, δmplots confirmed moderate dipole–dipole interparticle magnetic interactions, which remained consistent after surface modification. These findings demonstrate that surface functionalization with PAA preserves the key magnetic features of CoFe₂O₄ NPs, while providing stability in aqueous media for further biomedical applications.