Evidence of iron (III) reduction in gamma -Fe sub(2)O sub(3) nanoparticles due to meso-2,3-dimercaptosuccinic acid functionalization

In this study we report on the meso-2,3-dimercaptosuccinic acid (DMSA) surface functionalization of nanosized maghemite particles which were obtained from oxidation of freshly-precipitated magnetite nanoparticles. Stable magnetic sols were produced while using [DMSA]/[Fe] in a wide range (2 to 90%) of values for the surface functionalization protocol. We found experimental evidence of Fe (III) reduction down to Fe (II) in the whole range of [DMSA]/[Fe] values employed, though presenting differences for lower and higher values of DMSA/Fe molar ratio. At lower (up to 10%) [DMSA]/[Fe] values the DMSA-functionalized iron oxide core remains essentially maghemite while the reduced Fe (II) ions move out to the bulk solution as soluble species. In contrast, at higher (20% and above) [DMSA]/[Fe] values the DMSA-functionalized iron oxide core holds the reduced Fe (II) on its crystal structure. The thiol group oxidation, via disulfide bridge formation, plays a key role in the Fe (III) reduction to Fe (II) during the surface functionalization process. We hypothesize that at higher [DMSA]/[Fe] values (20% and above) intermolecular disulfide bridge formation dominates, leading to the onset of a network at the nanoparticle's surface, thus preventing the surface reduced Fe (II) ions moving out into the bulk solution. Experimental evidence based on visual inspection and different techniques (UV-vis-IR spectroscopy, chemical analysis, x-ray diffraction, and Raman spectroscopy) are present to support the model picture herein introduced.