A sensor is provided that detects single-stranded deoxyribonucleic acid (ssDNA) with a specific base sequence. The ssDNA sequence sensor comprises an aqueous solution containing a cationic water-soluble conjugated polymer [in this case, poly(9,9-bis(6'-N,N,N-trimethylammonium)-hexyl)-fluorene phenylene), 1] with a ssDNA labeled with a dye (in this case, fluorescein). The emission of light from the sensor solution with the wavelength characteristic of the probe oligonucleotide indicates the presence of ssDNA with a specific base sequence complementary to that of the probe ssDNA-fluorescein. Maximum energy transfer from 1 to the signaling chromophore occurs when the ratio of polymer chains to DNA strands is approximately 1:1. Energy transfer from 1 results in a fluorescein emission that is more intense than that observed by direct excitation of the chromophore. Furthermore, the decrease in energy transfer upon addition of electrolyte indicates that electrostatic forces dominate the interactions between 1 and DNA.