Nanoscale microcavities for biomedical sensor applications

Porous silicon nanostructures are ideal hosts for sensor applications because of their large internal surface area, which implies strong adsorbate effects. The average pore size can be easily adjusted to accommodate either small or large molecular species. When porous silicon is fabricated into a structure consisting of two high reflectivity multilayer mirrors separated by an active layer, a microcavity is formed. Multiple narrow and visible luminescence peaks are observed with a fun width at half the maximum value of 3 nm. These multiple peak microcavity resonators are very sensitive structures. Any slight change in the effective optical thickness induces a change in the reflectivity spectra, causing a shift in the interference peaks. We demonstrate the usefulness of this microcavity resonator structure as a biosensor. Biosensors are devices that exploit the powerful recognition capability of bioreceptors. We have fabricated a DNA biosensor based on a porous silicon multiple peak microcavity structure. An initial strand of DNA is first immobilized in a porous silicon substrate and then subsequently exposed to its complementary DNA strand. Shifts in the luminescence spectra are observed and detected for DNA concentrations less than 1 mu M. When exposed to a non-complementary DNA strand, no shifts are observed. A detailed study on the selectivity and sensitivity issues of porous silicon microcavity biosensors is presented.