Scheme 1.

The process by which luminescent porous silicon nanoparticles dissolve in aqueous media involves a combination of hydrolysis and oxidation. In the hydrolysis step the SiO₂ shell surrounding the silicon skeleton dissolves (top). Removal of this oxide exposes elemental silicon from the skeleton core, which then is oxidized by water to replace the SiO₂ shell (bottom). The result of this continuous cycle is the gradual shrinking in size of the quantum-confined Si domains, generating a blue shift in the photoluminescence spectrum. Eventually the porous Si nanoparticles dissolve completely, resulting in total loss of photoluminescence.