Cationic water-soluble dendritic poly(fluorene)s with positively charged an-Lines on the exterior (PFP-G(0-2)) can be prepared by a macromonomer (generation-by-generation) approach. The charge densities of PFP-G(0-2) can control their distances to DNA by electrostatic interactions. The PFP-G(2) with higher generation of dendritic side chains possesses a higher charge density, and gives rise to efficient fluorescence resonance energy transfer (FRET) to a signaling fluorescein labeled at the terminus of DNA. These new conjugated polymers form less aggregates and yield more efficient FRET in a tetrahydrofuran/H2O mixed solvent as compared to in pure water. The sensitivity of the DNA sensor is improved by utilizing highly dendritic conjugated polymers.