Fig. 1.

Scheme showing the synthesis of bifunctional smart-dust particles. A multilayered porous Si dielectric mirror (rugate filter) first is etched into the single-crystal Si substrate. The film then is hydrosilylated to generate a chemically stable hydrophobic mirror. A second rugate filter with a different periodicity is then etched into the substrate immediately beneath the first. The entire structure is removed from the Si substrate by the application of a current pulse. Thermal oxidation then imparts hydrophilic character to the second mirror. The bifunctional, freestanding film then is placed in water and fractured into micrometer-sized particles by ultrasonication.