Synthesis of Nanoceramic Particles by Intravesicular Precipitation
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SYNTHESIS OF NANOCERAMIC PARTICLES BY INTRAVESICULAR PRECIPITATION SUHAS BHANDARKAR,* ISKANDAR YAACOB AND ARIJIT BOSE Department of Chemical Engineering, University of Rhode Island, Kingston, RI 02881 *AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974
ABSTRACT Nanometer sized magnetic particles have been fabricated by aqueous phase coprecipitation inside single compartment vesicles. These vesicles were generated by sonicating egg yolk phosphatidylcholine molecules in an appropriate ionic solution containing the reactant cations. Using cobalt and ferric nitrate as the starting solution, the reaction product was the desired cobalt ferrite; direct formation of the oxide is potentially a very important step. For the barium and ferric nitrate systems, the reaction product could not be identified easily. However, we have not been able to produce barium ferrite directly. In an attempt to compare the effect of drastic changes in the microenvironment, we have also completed aqueous phase precipitation in multilamellar vesicles. For the aluminum nitrate system, precipitation takes place only in the outermost "annular" space. Multiple particles are formed, and the resultant diffraction pattern shows a close match with an aluminum hydroxide chloride complex. This differs significantly from the product in single compartment vesicles, where f-Al 2 O3 was formed, and in free precipitation which resulted in Al(OH) 3 . INTRODUCTION Phospholipid vesicles form an organically constrained nanometer sized aqueous core that can serve as unique reactors for the synthesis of nanometer sized ceramic particles. The organic membrane is permeable to anions but does not permit transport of cations. Utilizing this key property, reaction can be confined to the intravesicular space only, producing both single component and composite nanoparticles [1-3]. Precipitation within vesicles also offers other important pathways for the mediation of final particle size and phases. These include polar head group mediated organization of the reactant cations that lead to unexpected phases [3], fine control of the intravesicular supersaturation as well as concentration with a concomitant control on the final particle size, phase and morphology. In fact, precipitation within these highly constrained domains gives product that is significantly different in crystallinity, morphology, size and phase from the same reaction in free solution. In this paper, we explore aqueous phase reactions in both single compartment and multilamellar vesicles (MLV). The aqueous regions in MLV form a microenvironment for precipitation that is quite different even from the encapsulated regions in single compartment vesicles because of the extreme proximity of the polar head groups in neighboring bilayers. These head groups can act as parallel templates predisposing the organization of the reactant cations, and restricting the range of phases that are possible in the final product. Because of their practical utility, the experiments reported here for the single compa
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