Oleate coating of iron oxide nanoparticles in aqueous systems: the role of temperature and surfactant concentration

Abstract

Coating magnetic nanoparticles (MNPs) with sodium oleate (SO) is known to be an excellent method to create biocompatible, stable colloids with a narrow size distribution. However, the mechanism of oleate adsorption on the MNP surface in aqueous systems, as well as its influence on colloidal stability, is not yet fully understood. In this context, we present here a physico-chemical study to provide a deeper understanding of surfactant interaction mechanisms with nanoparticles. We examined the effect of temperature and the SO/MNP ratio (w/w) on the adsorption process in water and observed the existence of a maximum for the adsorbed oleate amount at lower temperatures, whereas at higher temperatures, the isotherm can be adapted to the Langmuir model with constant capacity after saturation. The oleate load on the MNP surface was quantified using reversed-phase high-performance liquid chromatography measurements of samples in solution. The thermogravimetric analyses of the solid residues together with infrared spectroscopy analyses indicate a bilayer-similar structure at the MNP/water interface even for low oleate loads. The oleate interacts with the iron oxide surface through a bidentate coordination of the carboxyl group. Zeta potential measurements demonstrate the high stability of the coated system. The maximal oleate load per unit mass of MNPs reaches approximately 0.35 g oleate g MNP -1 .

Oxidation of magnetite nanoparticles: impact on surface and crystal properties

“…While acidic groups are often reported to replace surface hydroxides we cannot confirm this behaviour for the maghemite nanoparticles. 15,23,63 The multi-analytical characterisation of magnetite oxidation under harsh and mild conditions, presented in this study illustrates two different oxidation routes and final nanoparticles. Whereas magnetite nanoparticles oxidise almost completely to maghemite under harsh conditions, the mild oxidation leads to particles in a transition state between magnetite and maghemite.…”

“…21 On the other hand, for catalytic applications, such as Fenton chemistry, a high reactivity of MNP is desired. 22 While most investigations focus on the modification and functionalisation of MNP, [23][24][25][26][27] in depth fundamental understanding of the formation, phase transition and factors influencing the surface chemistry is still required. Many interesting studies focusing on the formation pathways of iron oxide nanoparticles exist for different routes such as polyol, 28,29 hydrothermal, 30 electrochemical, 31 pyrolytic 32 and coprecipitative synthesis.…”

Gold-iron oxide nanohybrids: insights into colloidal stability and surface-enhanced Raman detection

“…1). 49 Here, multiple peaks around 1558 and 1423 cm -1 corresponding to the asymmetric and symmetric stretching vibrations of COO -, respectively, can be observed. 49,51 Furthermore, the C-H stretch vibrations of oleate are visible at 2960 and 2870 cm -1 .…”

“…Oleate stabilization leads to a lipid bilayer formation around the nanoparticles and therefore to a steric and electrostatic stabilization. 49,50 The electrostatic stabilization through the carboxy group which protrudes out of the lipid bilayer leads to a negatively charged surface at neutral pH and therefore to repulsion between nanoparticles. 49 The oleate molecules are anchored via a bidentate binding state, which can be observed with FTIR spectroscopy (Fig.…”

Experimental characterization and simulation of amino acid and peptide interactions with inorganic materials

“…Another interesting way of studying the interaction of amino acids and peptides with inorganic surfaces are vibrational spectroscopy methods. The force constants of molecular vibrations are influenced by electrostatic interactions, hydrogen bonding and by direct coordination to the surface . The most common method for characterizing bio‐inorganic interactions is IR spectroscopy, where the wavelength can be easily modulated and a high spectral resolution can be achieved.…”