Detection of immunocomplex formation by enhanced photoluminescence of antibody-functionalized diatom biosilica

Abstract

Diatoms are single-celled photosynthetic algae that make silica shells or "frustules" with intricate features patterned at the nano and microscales. In this study, antibody-functionalized diatom biosilica frustules serve as a biosensor platform for selective and label free antibody-antigen immunocomplex formation by enhanced photoluminescence. Biosilica frustules of 10 micron diameter were isolated from cells of the centric marine diatom <i>Cyclotella</i> sp. They were then mounted on glass and covalently functionalized with the model antibody Rabbit Immunoglobulin G (IgG) to yield a uniform nanostructured surface that selectively binds to its complimentary antigen, Goat anti-Rabbit IgG. Diatom frustules possess an intrinsic capacity to emit blue light when excited with a UV laser light source, a property called photoluminescence. Binding the antibody-functionalized diatom frustule with its complimentary antigen selectively enhanced the intrinsic photoluminescence intensity of the diatom frustule by a factor of three, whereas challenging the antibody-functionalized diatom frustule with a non-complimentary antigen, Goat anti-human IgG did not change the intrinsic photoluminescence intensity. The nucleophilic immunocomplex increases the photoluminescence by donating electrons to non-radiative sites on the photoluminescent diatom biosilica, thereby decreasing non-radiative electron decay and increasing radiative emission. The intensified photoluminescence intensity is correlated to the antigen, goat anti-rabbit IgG concentration, with a binding constant of 2.8 &plusmn; 0.7x10^-7 M.