Reactivity of Oxidized Porous Silicon with Calcium Ion in Aqueous Media: Stability, pH Behavior, and Host-Guest...

Oxidized mesoporous silicon, composed of a silicon skeletal core surrounded by a silicon oxide shell, reacts with aqueous calcium ions to generate a modified shell comprised of a mixed silicon oxide-calcium silicate phase. The performance of the metal silicate phase as a condenser for the xanthene dye calcein and as a modulator for the dissolution of the porous silicon host and the release of calcein into aqueous media is studied using a chip-based format. The reaction conditions for oxide shell formation (thermal, ozone, or hydrogen peroxide oxidation) and for its conversion to calcium silicate (reaction time and pH) are compared. The optimal conditions are determined, and dissolution of the composite material is studied as a function of pH and buffer composition. The composite shell is stable under neutral aqueous conditions, but it dissolves 30 times faster at pH 4. In phosphate-buffered saline (PBS), incorporation of phosphorus into the shell is observed, which suppresses the rate of dissolution through secondary mineralization. Loading experiments using a nanoparticle formulation of the material show efficient encapsulation of calcein (21% by mass). The dye is released more rapidly at pH 4 than at pH 7, and the presence of phosphate in the pH 7 buffer further slows release.