Formation of iron oxides in unilamellar vesicles

Abstract

The precipitation of nanometer-size iron oxide particles within unilamellar phosphatidylcholine vesicles of ca. diameter 30 nm has been studied by electron microscopy, energy dispersive X-ray analysis, and electron diffraction. Addition of NaOH to vesicles containing intravesicular solutions of Fe(III), Fe(II), and Fe(II)/Fe(III) resulted in the formation of membrane-bound discrete particles of goethite (α-FeOOH), magnetite (Fe 3O 4), and ferrihydrite (5Fe 2O 3·9H 2O), respectively. The particles were spherical or disk shaped and had dimensions in the range 1.5–12 nm depending on the extent of aging. These products were different from precipitates formed in bulk solution in the absence of vesicles. The intravesicular oxidation of Fe(II) was followed with time by measurement of the Fe(III) absorbance band at 420 nm and shown to exhibit overall first-order diffusion-limited kinetics. The role of the vesicle membrane in the mediation of intravesicular precipitation of iron oxides is discussed in terms of the kinetic and spatial restrictions inherent in the reaction environment.