Effect of physical state (solid vs. liquid) of lipid core on the rate of transport of oxygen and free radicals in solid lipid nanoparticles and emulsion

Solid lipid nanoparticles (SLNs) may have significant potential to limit oxidation of encapsulated bioactive compounds. This is based on the hypothesis that the solid core in SLNs can significantly limit the rate of transport of oxygen and free radicals into the lipid core from the aqueous phase. In this study, we have directly compared the rate of transport of oxygen and free radicals in SLNs and liquid emulsion prepared from the same lipid material (eicosane) at a fixed temperature. Eicosane nanoparticles stabilized by high melting lecithin and bile salts were used as a model system. The physical state (solid vs. liquid) of the lipid phase was engineered using the supercooling phenomenon of emulsified eicosane. Transport of oxygen and free radicals was measured based on changes in fluorescence intensity of oxygen or peroxyl radical sensitive dyes encapsulated in the lipid phase upon exposure to either air or peroxyl radicals generated in the aqueous phase. The results showed that the rate of oxygen transport was marginally reduced in SLNs as compared to liquid emulsion, while the rate of transport of peroxyl free radicals was not significantly affected by the physical state of the lipid core in SLNs and emulsion. Together, these results indicated that the solid core of SLNs does not significantly reduce the rate of transport of oxygen or free radicals as compared to the liquid core emulsion. To address this paradox, the distribution of encapsulated dye was characterized in both SLNs and emulsion using fluorescence imaging. The results showed significant redistribution of the encapsulated dye molecules with formation of the solid lipid core. In contrast to homogeneous distribution in the liquid emulsion, SLNs showed higher concentration of the dye at the periphery as compared to the center of the lipid droplet. The expulsion of encapsulated molecules to the surface of SLNs can potentially limit the ability of the solid core to protect encapsulated products from oxygen and free radicals.