Methods for engineering a nucleic acid sensor molecule are provided. Biosensors comprise a plurality of nucleic acid sensor molecules labeled with a first signaling moiety and a second signaling moiety. The nucleic acid sensor molecules recognizes target molecules which do not naturally bind to DNA. Binding of a target molecule to the sensor molecules triggers a change in the proximity of the signaling moieties which leads to a change in the optical properties of the nucleic acid sensor molecules on the biosensor. Reagents and systems for performing the method are also provided. The method is useful in diagnostic applications and drug optimization.