The electrostatic complexes of single-stranded deoxyribonucleic acid (ssDNA) and a cationic conjugated polyelectrolyte (CPE), poly{9,9-di[3-(1-ethyl-1,1-dimethyl ammonio)propyl]-2,7-fluorenyl-alt-1,4-phenylene dibromide} (PFN), were investigated. Fluorescence emission of PFN solution (10 mu mol/L) can be drastically quenched to about one fourth of its original intensity in the presence of a trace amount (2.6 mu mol/L) of ssDNA. The effect of oligonucleotide length on the fluorescence quenching behavior was also investigated. In contrast to single-stranded DNA with 20 bases (ssDNA-20), ssDNA with 40 bases (ssDNA-40) induces a relatively higher quenching efficiency and larger red-shift of PFN emission maximum. The binding constant of ssDNA-20 and PFN is estimated to be 1.12 x 10(21). At extremely low concentration (10 nmol/L), PFN can respond to 0.2 nmol/L (or 2 x 10(-10) mol/L) of ssDNA-20 by significant enhancement of its emission intensity. The result is contrary to the observation in the relative higher concentration, and its mechanism is postulated. Based on the high binding ability of ssDNA with cationic CPE, a label-free method for ssDNA detection is designed. It uses an electrostatic complex of cationic PFN and an anionic CPE, which exhibits fluorescence resonance energy transfer (FRET) between the two components. Addition of ssDNA improves the FRET extent, indicated by obvious change of fluorescence spectra of the conjugated polyelectrolyte complex. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 1278-1286, 2009