A highly selective protein assay was created which combines the fluorescent ratiometric technique based on FRET with the light-harvesting properties of conjugated polymers. the cationic poly[9,9-bis(6'-N,N,N-trimethylammonium)-hexyl-fluorene phenylene]-bromide (PFP-NMe3+) and the negatively charged biotinylated fluorescein probe (Fl-B) were used to detect the target protein streptavidin optically. the strong, electrostatic interactions between PFP-NMe3+ and fluorescein result in efficient FRET from PFP-NMe3+ to fluorescein. In the presence of streptavidin, however, the biotin moiety of Fl-B specifically associates with streptavidin and the fluorescein molecule is burned deeply in the adjacent vacant binding sites. this separates the fluorescein spatially from the PFP-NMe3+ moiety, resulting in inefficient FRET from PFP-NMe3+ to fluorescein. Although a nonspecific protein, such as BSA, shows nonspecific interactions with PFP-NMe3+, it does not affect the fluorescent ratio value of PFP-NMe3+ to fluorescein. Hence, the charged neutral complex of two oppositely charged conjugated polymers can eliminate the nonspecific interactions, and thus optimize their application in protein assays.