Ligand Exchange Using Alkosilanes on Oleic Acid-Cobalt Ferrite Nanoparticles

Victoria L. Calero-DiazdelCastillo and Carlos Rinaldi. Chemical Engineering, University of Puerto Rico, 636 Manuel Ramos, Mayaguez, PR 00682

Monodisperse cobalt ferrite nanoparticles have been synthesized by the thermal decomposition method. Controlling the temperature in the nucleation and growth stages between 150 - 250 C and 300 - 330 C respectively, monodisperse particles were obtained with various diameters. Particles with ~12 nm diameter and a geometric deviation of 0.11 were obtained when temperatures of 150C and 300C were used in the nucleation and growth stages. X-ray diffraction showed all peaks characteristic of the crystalline structure of cobalt ferrite. EDAX confirmed the incorporation of cobalt in the ferrite. Magnetic properties were determined using a SQUID magnetometer, at 5 K and 300 K. Some samples had superparamagnetic behaviour at 300 K. As synthesized., the particle surface is covered with oleic acid which does not permit suspending the particles in polar solvents. In order to suspend the particles in water, ligand exchange was accomplished using an alkoxysilane with carboxylic functional groups. FTIR confirmed the successful exchange between oleic acid and the alkoxysilane. A decrease in the relative intensity of vibration bands ascribed to CH2 (2900-2800 cm-1) was observed, which was attributed to exchange of the long chain of oleic acid by the short chain of the alkoxysilane. TEM shows the particles to be singly dispersed after exchange, which is an indication that the alkoxysilane provides sufficient steric and electrostatic barrier against agglomeration. The distance between particles in hexagonal arrays decreases from ~4 nm to ~2 nm, also confirming ligand exchange. The alkoxysilane-cobalt ferrite nanoparticles could be suspended in deionized water and the solution is stable for at least two months. COO-nanoparticles will be functionalized with Biotinyl-3,6-dioxaoctadiamine to use as a model sensor for avidin.