SYNTHESIS AND CHARACTERIZATION OF MAGNETIC NANOPARTICLES IN SPONTANEOUSLY GENERATED VESICLES

Unilamellar vesicles, formed spontaneously by mixing single-tailed ani onic and cationic surfactants (dodecylbenzenesulfonic acid (HDBS) and cetyltrimethylammonium bromide (CTAB), respectively), have been used a s reactors for the synthesis of magnetic nanoparticles. The micellar c ationic surfactant solution containing ferrous chloride was mixed with the micellar anionic surfactant solution, resulting in the formation of defect-free unilamellar vesicles, with ferrous chloride within the cores as well as in the extravesicular spaces. The external ferrous io ns were replaced with sodium ions by gel permeation chromatography. So dium hydroxide was then added to the extravesicular region. Hydroxyl i ons penetrated the vesicle cores and reacted with the available ferrou s ions to initiate particle formation. The presence of intravesicular particles was confirmed by cryogenic transmission electron microscopy. Absorbance measurement showed that the reaction proceeded over a peri od of several minutes. To form the magnetic nanoparticles, the suspens ion was heated to about 70 degrees C for 1 min, and then cooled back t oroom temperature. The resulting particles had a mean diameter of appr oximately 2.6 nm and displayed superparamagnetic behavior. Wide-area e lectron diffraction showed the particles to be either gamma-ferrite or magnetite. Magnetization measurements yielded a least upper bound for the magnetic diameter of these particles of 0.61 nm. These results ar e consistent with the presence of a magnetically disordered surface la yer on the order of 1 nm thick. (C) 1994 Academic Press, Inc.