A biotin-tetramethylrhodamine (biotin-TMR) quenchei-ligand interacts with a (phenylene-ethynylene) based helical conjugated polyelectrolyte (poly-l) via intercalation of the TMR unit into the helix The interaction is signaled by efficient fluorescence resonance energy transfer (FRET) from the polymer to the TMR chromophore Avidin addition to the poly-l/biotin-TMR intercalation complex does not interrupt FRET, instead resulting in the formation of avidin-biotin "cross-links" Mixing of biotin-TMR with avidin prior to addition of the polymer efficiently disrupts the FRET signal, giving rise to a sensor with a detection limit of 100 pM for avidin Study of the FRET response as a function of biotin-TMR and avidin concentration affords insight into the interaction of the protein with the poly-l/biotin-TMR intercalation complex