Assessing magnetic iron oxide nanoparticle properties under different thermal treatments

Magnetic nanoparticle structures have been examined as potential carrier vehicles and substrates in a wide range of applications where they undergo mechanical, chemical, and/or thermal manipulation to allow for their modification, conjugation, and transport. For safe and effective use, it is imperative to not only measure the initial physicochemical and structural properties of nanomaterials, but also identify and quantify any property changes related to a loss of chemical and/or physical integrity during processing and usage conditions. In this study, an assessment of iron oxide magnetic nanoparticle thermal stability using modulated differential scanning calorimetry (mDSC) and a controlled heating system is conducted on two types of iron oxide nanoparticles: maghemite (Fe 2 O 3 ; 500 nm) with silanol surface functional groups and magnetite (Fe 3 O 4 ; 200 nm) with primary amine-terminated alkoxysilane surface functional groups. mDSC results revealed an endothermic peak at 388 K for both types of nanoparticles indicating possible molecular rearrangement within the structure. To confirm this result, iron oxide nanoparticles in aqueous suspensions were heated at discrete temperatures from 303 to 403 K. Calorimetry, FTIR spectroscopy, and dynamic light scattering measurements were used to examine changes in the chemical and physical stability of the suspensions. Morphological characteristics were evaluated using optical microscopy, transmission electron microscopy, and atomic force microscopy. Results showed that the chemical and morphological structure of the nanocomposite is critical in determining the thermal performance of the iron oxide nanoparticles. Amine-terminated silane-functionalized magnetite nanoparticles were highly susceptible to morphological and surface chemistry changes starting at ca. 353 K. Conversely, silanol-functionalized maghemite nanoparticles were shown to be stable in terms of morphology and chemical structure up to 403 K. Micrographs demonstrated variations in magnetic domains distribution after exposing the nanoparticles to thermal treatments, confirming the results obtained through mDSC and FTIR measurements.