Optimizing the Binding Energy of the Surfactant to Iron Oxide Yields Truly Monodisperse Nanoparticles.

Despite the great progress in the synthesis of iron oxide nanoparticles (NPs) using a thermal decomposition method, the production of NPs with low polydispersity index is still challenging. In a thermal decomposition synthesis, oleic acid (OAC) and oleylamine (OAM) are used as surfactants. The surfactants bind to the growth species, thereby controlling the reaction kinetics and hence playing a critical role in the final size and size distribution of the NPs. Finding an optimum molar ratio between the surfactants oleic OAC/OAM is therefore crucial. A systematic experimental and theoretical study, however, on the role of the surfactant ratio is still missing. Here, we present a detailed experimental study on the role of the surfactant ratio in size distribution. We found an optimum OAC/OAM ratio of 3 at which the synthesis yielded truly monodisperse (polydispersity less than 7%) iron oxide NPs without employing any post synthesis size-selective procedures. We performed molecular dynamics simulations and showed that the binding energy of oleate to the NP is maximized at an OAC/OAM ratio of 3. The optimum OAC/OAM ratio of 3 is allowed for the control of the NP size with nanometer precision by simply changing the reaction heating rate. The optimum OAC/OAM ratio has no influence on the crystallinity and the superparamagnetic behavior of the Fe3O4 NPs and therefore can be adopted for the scaled-up production of size-controlled monodisperse Fe3O4 NPs.

Engineered Inorganic/Organic-Core/Shell Magnetic FexOy Nanoparticles with Oleic Acid and/or Oleylamine As Capping Agents.

Magnetic nanoparticles with tailored surface chemistry are widely used for a number of different in vivo applications, ranging from tissue repair and magnetic cell separation through to cancer-hyperthermia, drug delivery and magnetic resonance imaging contrast enhancement. A major requirement for all these biomedical applications is that these nanoparticles must have high magnetization values and sizes smaller than 100 nm with a narrow particle size distribution. Thus nanoparticles must have uniform physical and chemical properties. For these applications, a tailored surface coating/shell needs to be engineered, which has to be non-toxic, biocompatible and make allowance for targetable drug delivery with particle localization in a targeted area. Most work in this field has been done on improving the biocompatibility of the nanoparticles. Only a few scientific investigations have been carried out on improving the quality of magnetic nanoparticles with specific focus on the nanoparticle's surface chemistry, size distribution and shape (which directly influences the magnetic properties). All these particles also need to be properly characterized in order to get a protocol for the quality control of these particles, the nature of the surface coatings and their subsequent geometric arrangement. This will ultimately determine the overall size of the colloids and also plays a significant role in biokinetics and biodistribution of nanoparticles in the body. This review highlights recent advances in the synthetic chemistry, magnetic characterization and biological applications of inorganic/organic - core/shell FexOy based magnetic nanoparticles with specific focus on using the two popular surfactants for producing MNPs namely oleic acid and/or oleylamine as capping agents. Although the main nano-magnets under discussion are magnetite (Fe3O4) nanoparticles, maghemite (γ-Fe2O3) is also briefly mentioned.

Analysis of the interaction of surfactants oleic acid and oleylamine with iron oxide nanoparticles through molecular mechanics modeling.

The interface interactions between surfactants oleic acid and oleylamine and magnetic nanoparticles are studied via molecular mechanics and dynamics. Mixtures of these two surfactants are widely advocated in the chemical synthesis of nanoparticles. However, the exact dynamic mechanism remains unclear. Here we report, for the first time, a comprehensive qualitative model showing the importance of acid-base complex formation between oleic acid and oleylamine as well as the presence of free protons in the engineering of nanoparticles of specific shapes and sizes. We show why critical parameters such as surfactant concentration may modify iron oxide nanoparticle shape and size and how this can be understood in the light of acid-base complex pair formation. We report on the influence these parameters have on both the in situ nanoparticle surface charge and zeta potential. Transmission electron microscopy (TEM), FTIR, and pH studies are used to confirm the validity of the calculated binding energies and number of acid-base pairs.