Water-soluble, cationic conjugated polymer binds single-stranded DNA with higher affinity than it binds double-stranded or otherwise "folded" DNA. This stronger binding results from the greater hydrophobicity of single-stranded DNA. Upon reducing the strength of the hydrophobic interactions, the electrostatic attraction becomes the important interaction that regulates the binding between the water-soluble conjugated polymer and DNA. The different affinities between the cationic conjugated polymer and various Forms of DNA (molecular beacons and its open state; single-stranded DNA and double-stranded DNA and single-stranded DNA and complex DNA folds) can be used to design a variety of biosensors.