Tuning magnetic behavior of Fe3O4 nanoparticles through oleic acid, PEG, and agar surface modifications

In this study, magnetic nanoparticles were synthesized using co-precipitation method and, subsequently, surface modified it using oleic acid (OA), polyethylene glycol (PEG), and agar (AGAR). The novelty of this study lies in the implementation of four sequential steps of surface modification OA-PEG-AGAR-OA to enhance their suitability for biomedical applications. The key outcome is that surface modification significantly alters the magnetic behaviors of nanoparticles. First, the OA layer increased the magnetic saturation (Ms) from 23.36 emu/gr for bare Fe3O4 to 51.84 emu/gr for Fe3O4-OA—a value higher than some previous reports for Fe3O4-OA. However, subsequent coatings with PEG and AGAR led to a decrease in Ms due to the introduction of nonmagnetic domains, which reduce inter-particle magnetic interactions. Reintroducing OA at the final stage partially restored the Ms. All samples exhibited superparamagnetic-like behavior with coercive fields between 1.3 and 4.6 Oe, and near-zero coercivity and remanence, essential for preventing agglomeration. The time dependence of magnetization revealed slow relaxation times. Although slow relaxation aids magnetic targeting and control, it may prolong tissue interaction and risk overheating. For biomedical applications, nanoparticles should have a size of about 10 nm. The sizes of samples Fe3O4-OA (8.16 nm) and Fe3O4-OA/PEG (10.08 nm) are within this ideal range, while Fe3O4 (11.76 nm) is slightly above. The Fe3O4-OA/PEG/AGAR/OA sample had a significantly larger particle size of 30.21 nm. This implies an increase in magnetic anisotropy and interparticle interactions, causing its ZFC-FC branch point to shift to higher temperatures and preventing the observation of its blocking temperature below 300 K.