Effects of organic polymer addition in magnetite synthesis on the crystalline structure

Magnetite (Fe₃O₄) nanoparticles and magnetite-based inorganic-organic hybrids are attracting attention in biomedical fields as thermoseeds for hyperthermia and a contrast medium in magnetic resonance imaging. Size control of Fe₃O₄ thermoseeds is important as the particle size affects the heat generation properties. Fe₃O ₄ can be easily synthesized via aqueous processes and the presence of organic substances during synthesis can affect the size and crystalline phase of the Fe₃O₄ formed. In this study, various polymers with different functional groups and surface charges were added to the precursor solution of Fe₃O₄ to clarify the relationship between the chemical structure of the organic substances and the crystal structure of Fe₃O₄. At first, coexistence effects of the organic substances in the solutions were clarified. As a result, crystalline Fe ₃O₄ was precipitated even after addition of neutral polyethylene glycol and cationic poly(diallyldimethylammonium chloride). The poly(sodium-4-styrene sulfonate) addition significantly decreased the particle size, while polyacrylic acid addition inhibited Fe₃O₄ nucleation to afford an amorphous phase. These differences were related to the ease of complex formation from iron ions and coexisting organic polymers. In order to clarify this assumption, a modified experimental procedure was applied for the polyacrylic acid. Namely, the iron oxide precipitation by the NaOH solution was followed by the polyacrylic acid addition. Notably, Fe ₃O₄ nucleation was not inhibited. Hence, the size and crystalline phase of the iron oxide prepared by the aqueous process were drastically affected by organic polymers.