Fig. 8. (Top) A sandwich immunoassay is performed by exposing antibody-functionalized microbeads to three solutions: (1) an antigen containing sample, (2) biotinylated detection antibodies, and (3) streptavidin modified with a Ru(bpy)₃²⁺ complex (streptavidin(SA)-Ru). The beads are housed in microwells created from an etched gold-coated fiber-optic bundle. The gold coated fiber bundle acts as the working electrode (WE) for ECL. CE and RE refer to the counter electrode and reference electrode. Reproduced from ref. 83 with permission from the American Chemical Society, copyright [2009].