Diatoms are single-celled algae that make microscale silica shells called "frustules", which possess intricate nanoscale features imbedded within periodic two-dimensional pore arrays. In this study, antibody-functionalized diatom biosilica frustules serve as a microscale biosensor platform for selective and label-free photoluminescence (PL)-based detection of immunocomplex formation. The model antibody rabbit immunoglobulin G (IgG) is covalently attached to the frustule biosilica of the disk-shaped, 10-mu m diatom Cyclotella sp. by silanol amination and crosslinking step to a surface site density of 3948 +/- 499 IgG molecules mu m(-2). Functionalizatio of the diatom biosilica with the nucleophilic IgG antibody amplifies the intrinsinc blue PL of diatom biosilica by a factor of six. Furthermore, immunocomplex formation with the complimentary antigen anti-rabbit IgG further increases the peak PL intensity by at least a factor of three, whereas a non-complimentary antigen (goat anti-human IgG) does not. The nucleophilic immunocomplex increases the PL intensity by donating electrons to non-radiative defect sites on the photoluminescent diatom biosilica, thereby decreasing non-radiative electron decay and increasing radiative emission. This unique enhancement in PL emission is correlated to the antigen (goat anti-rabbit IgG) concentration, where immunocomplex binding follows a Langmuir isotherm with binding constant of 2.8 +/- 0.7 X 10(-7) M.