Effect of pH, citrate treatment and silane-coupling agent concentration on the magnetic, structural and surface properties of functionalized silica-coated iron oxide nanocomposite particles

Superparamagnetic iron oxide nanoparticles were synthesized by coprecipitation of iron chloride salts at various pH values (9, 10, 11 and12) that were adjusted using an ammonia solution. Increasing the pH from 9 to 12 led to decreases in the size of iron oxide nanoparticles from 7.9±1.4 to 5±0.6 nm and the saturation magnetization (Ms) from 82.73 to 67.14 emu/g, respectively, when analyzed with transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). X-ray diffraction patterns as well as Ms values showed that magnetite is the dominantly synthesized phase in the examined pH values. Unmodified iron oxide nanoparticles were coated with silica via the hydrolysis and condensation of tetraethyl orthosilicate (TEOS), designated P1 particles. The size distribution diagram of P1 particles showed two regions with mean sizes of 143.3±15.4 and 216.9±13.7 nm corresponding to silica and iron oxide@silica particles, respectively. Stabilization of iron oxide nanoparticles using sodium citrate prior to coating with silica (P2 particles) resulted in nanocomposites with a mean size of 275±16.1 nm and an Ms value of 2.9 emu/g. Subsequently, the surface of P2 particles was functionalized by amine groups using N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (EDS). Results obtained from the measurement of zeta potential revealed that the highest value of isoelectric point (PI) change, indicating a more efficient surface functionalization, occurs when the EDS concentration of 90 mM is used, as compared to that for particles aminated using 25 and 180 mM EDS.