Synthesis of ultrasmall superparamagnetic iron oxides using reduced polysaccharides.

Abstract

Investigation into the effect of the reducing sugar of dextran on formation and stability of dextran-coated ultrasmall superparamagnetic iron oxides (USPIO) has demonstrated that reduction of the terminal reducing sugar can have a significant effect on particle size, coating stability, and magnetic properties. Four aspects of polysaccharide-coated USPIO particle synthesis were investigated: (i) the effect reduction of the terminal polysaccharide sugar has upon polysaccharide usage, particle size, stability, and magnetic susceptibility; (ii) the effect an exogenous reducing sugar can have upon particle synthesis; (iii) the effect the molecular weight of the reduced polysaccharide has on particle synthesis; and (iv) the effectiveness of reduced and native dextrans in stabilizing a preformed magnetic sol. For low molecular weight dextrans (MW <or=10 kDa), reduction resulted in a 10 fold or greater decrease in the carbohydrate-to-iron ratio necessary during particle formation to produce the desired particle size (<20 nm). Particles prepared at the equivalent dextran-to-iron ratio using the equivalent native dextrans yielded larger particles except for a 70 kDa dextran where reduced and native dextran yielded identical particles with respect to size and magnetic properties. The stability of particle size and coating was studied using 10 kDa native and reduced dextran. Particles prepared with reduced dextran yielded a more stable coating as evidenced by stability on autoclaving. For native dextrans (MW < 70 kDa), small (<or=30 nm) particles could be obtained at much higher dextran-to-iron ratios, but only the 10 kDa dextran gave a particle with comparable magnetic properties (susceptibility > 20,000 x 10(-6) cgs). Similar results were obtained with a 12 kDa pullulan. The effect of polysaccharide molecular weight on particle size was studied, wherein higher molecular weight reduced dextrans produced larger particles. The effectiveness of the reduced and native dextrans in stabilizing a preformed magnetic sol was compared. Reduced dextrans were found to be superior for stabilizing the magnetic sol. The observed effects of reduction of the terminal sugar in dextran compared with the native dextran were modeled using the Langmuir adsorption isotherm. A good fit of experimental data with this model was found.