An assay for a target single strand 20-base sequence of DNA coding for the anthrax lethal factor, based on conjugated polymer fluorescence superquenching, is reported. The assay employs a platform in which the receptor (a biotinylated complementary sequence "capture strand") and polymer (two components: an anionic poly(phenylene ethynylene) (PPE) and a biotinylated -PPE) are co-located on streptavidin-derivatized polystyrene microspheres. A conjugate of the target strand with the energy transfer quencher QSY-7 (DNA-QTL) is used to construct competition assays for the target. A direct competition assay between the target-DNA and DNA-QTL for the microsphere-bound capture is only marginally successful due evidently to greater kinetic affinity of the polymer-capture ensemble for the conjugate. However a sequential addition of target, followed by DNA-QTL affords a quantitative assay for the target by attenuation of PPE fluorescence quenching by the DNA-QTL. Likewise a direct competition in solution between the target and DNA-QTL for the biotinylated capture strand followed by addition of microspheres provides a sensitive and quantitative assay for the target single strand DNA.