Richard Harris

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.

Qualitative analysis of some alkanethiols on Au nanoparticles during SERS

The surface enhanced Raman spectroscopy enhancement factors (SERS EFs) for different AuNP–surfactant systems are measured and the observed trend is theoretically and qualitatively investigated. Four thiolated coumarin derivatives (HS–(CH2)11–NHCO–coumarin, HS–(CH2)11–NHCO–indole, HS–(CH2)11–triphenylimidazole and HS–(CH2)11–hydroquinone respectively) together with a stabilizing surfactant, HS–(CH2)11–PEG-COOH, were adsorbed in ratios of 1% and 50% onto different sized Au nanoparticles. Similar simulation experiments were done to determine the influence of the nanoparticle–surfactant binding energy (Eb) on SERS EFs. Other parameters that were qualitatively investigated were the dependency of the Raman enhancement on (a) the size of Au nanoparticles, (b) the relative surface-to-molecule orientation, (c) the molecular orbitals (HOMOs and LUMOs), (d) the molecule electro-and nucleophilic centres and cross-sectional areas as well as (e) the molecular electrostatic potentials. It was observed that each of these parameters contribute to the observed trend and the final Raman signal enhancement.

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.